Dissertations / Theses on the topic 'Silicène'

Le but de ce travail est la croissance du silicène sur Gr. J'ai décrit le substrat en fonction des conditions d’élaboration par CVD. Lorsque la proportion de H2 est faible il est possible d’obtenir du Gr homogène sur couche tampon (BL) sur SiC. Le STM et LEED montrent la superposition de la maille du Gr et de la reconstruction de la BL représentatif du Gr épitaxié. Lorsque la proportion de H2 est élevée la couche de Gr obtenue est totalement hydrogénée. Ceci est un résultat nouveau car aucun procédé d’intercalation d’hydrogène n’avait permis jusqu’à présent d’hydrogéner totalement les échantillons de (6x6)Gr épitaxié sur BL. Pour des proportions intermédiaires de H2/Ar, la coexistence de (6x6)Gr et H-Gr est observée. En fonction de la proportion de H2 dans le mélange gazeux, soit la surface du SiC reste passivée pendant toute la croissance du Gr et on obtient du H-Gr, soit le H2 désorbe partiellement, ou totalement et on obtient soit la coexistence des deux structures soit du (6x6)Gr pleine plaque. J’ai étudié la croissance par MBE de Si-ene sur (6x6)Gr. J’ai démontré qu'il est possible de former des flaques de Si-ene pour des épaisseurs de dépôt &lt;0.5MC. Nous observons la présence de zones planes d’une épaisseur de 0.2-0.3nm correspondant à une monocouche de Si-ene, entourées d’îlots dendritiques 3D de Si. Les spectres Raman mettent en évidence un pic allant jusqu’à 563cm-1 ce qui est la valeur la plus proche du Si-ene FS jamais obtenue. Ces démontrent la formation de Si-ene quasi-FS. Ce travail contribue à une meilleure compréhension du mécanisme de croissance CVD du Gr et à l’avancement des recherches dans le domaine de la croissance épitaxiale des matériaux 2D<br>The topic of this thesis deals with the study of the growth and properties of silicene (Si-ene) on graphene (Gr) on 6H-SiC(0001) with the final goal of forming free-standing (FS) Si-ene on an insulating or semiconductor substrate. I have described the substrate as a function of the CVD processing conditions. When the proportion of H2 is low it is possible to obtain homogeneous Gr on buffer layer (BL) on SiC. The STM and LEED show the superposition of the Gr mesh and the BL reconstruction representative of the epitaxial Gr. When the proportion of H2 is high, the resulting Gr layer is fully hydrogenated. This is a new result as no hydrogen intercalation process has been able to fully hydrogenate (6x6)Gr samples epitaxial on BL until now. For intermediate proportions of H2/Ar, the coexistence of (6x6)Gr and H-Gr is observed. Depending on the proportion of H2 in the gas mixture, either the SiC surface remains passivated during the entire Gr growth and H-Gr is obtained, or the H2 partially or totally desorbs and either both structures coexist or full plate (6x6)Gr is obtained. I have studied the MBE growth of Si-ene on (6x6)Gr. I have shown that it is possible to form Si-ene puddles for deposit thicknesses &lt;0.5MC. We observe the presence of flat areas of 0.2-0.3nm thickness corresponding to a Si-ene monolayer, surrounded by 3D dendritic islands of Si. The Raman spectra show a peak up to 563cm-1 which is the closest value to Si-ene FS ever obtained. This demonstrates the formation of quasi-FS Si-ene. This work contributes to a better understanding of the CVD growth mechanism of Gr and to the advancement of research in the field of epitaxial growth of 2D materials

This dissertation works out the vibrational properties of epitaxial silicene, which was discovered by Vogt et al. in 2012 by the epitaxial synthesis on the silver substrate. Its two-dimensional (2D) character is modified in comparison to the free-standing silicene due to its epitaxial nature, since the underlying substrate alters the physical properties of silicene as a result of the strong hybridization of the electronic levels of the substrate and adlayer. The growth of silicene layers is complicated by the sensitivity of the Si structures to the experimental conditions, mainly temperature, resulting in the formation of several seemingly different surface reconstructions. Another Si structure appears on the Ag surface at a supramonolayer coverage. The Raman spectroscopy was utilized to understand the relation between different Si structures and reveal their origin as well as to investigate the phonon-related physical properties of two-dimensional Si sheets. The central core of this work is the growth and characterization of these 2D silicene monolayers on the Ag (111) surface as well as the formation of silicene multilayer structures. The characterization of these materials was performed using in situ surface-sensitive measurement methods such as Raman spectroscopy and low-energy electron diffraction under ultra-high vacuum conditions due to high chemical reactivity of epitaxial silicene. Additional characterization was done ex situ by means of scanning force microscopy. The experimentally determined spectral signature of the prototypical epitaxial (3x3)/(4x4) silicene structure was confirmed by ab initio calculations, in collaboration with theory groups. The Raman signatures of the other 2D and 3D Si phases on Ag (111) were determined which allowed us to provide a clear picture of their formation depending on the preparation conditions. The monitoring of the silicene multi-layer growth yielded the vibrational signature of the top layer, reconstructed in a (√3x√3) fashion. It was compared to the inverse, (√3x√3)-Ag/Si(111), system showing the vast amount of similarities, which suggest that the (√3x√3) reconstruction belong to the silver layer. The chemical and physical properties of this surface structure additionally strengthen this equivalence. The possibility of functionalization of epitaxial silicene was demonstrated via exposure to the atomic hydrogen under UHV conditions. The adsorbed hydrogen covalently bonds to the silicene lattice modifying it and reducing its symmetry. As shown by Raman spectroscopy, such modification can be reversed by thermal desorption of hydrogen. The excitation-dependent Raman measurements also suggest the change of the electronic properties of epitaxial silicene upon hydrogenation suggesting that its originally semi-metallic character is modified into a semiconducting one<br>Die experimentellen Forschungsarbeiten zum Thema Silicen basieren auf den 2012 von Vogt et al. durchgeführten Untersuchungen zu dessen Synthese auf Silbersubstraten. Diese Untersuchungen lieferten die Grundlage, auf der zweidimensionales (2D) epitaktisches Silicen sowie weitere 2D Materialien untersucht werden konnten. In den anfänglichen Arbeiten konnte dabei gezeigt werden, dass sich die Eigenschaften von epitaktischem Silicen gegenüber den theoretischen Vorhersagen von frei-stehendem Silicen unterscheiden. Darüber hinaus verkomplizieren sich die experimentellen Untersuchungen dieses 2D Materials, da auf dem Ag(111) Wachstumssubstrat sechs verschiedene 2D Si Polytypen existieren. Eine detaillierte Darstellung dieser Untersuchungen findet sich in dem einführenden Kapitel der vorliegen Promotionsschrift. Der zentrale Kern dieser Arbeit beschäftigt sich mit dem Wachstum und der Charakterisierung dieser 2D Silicen Monolagen auf Ag(111) Oberflächen sowie der Bildung von Silicen- Multilagen Strukturen. Die Charakterisierung dieser Materialien wurde in situ mit oberflächenempfindlichen Messmethoden wie der Raman Spektroskopie und der niederenergetischen Elektronenbeugung unter Ultrahochvakuum-Bedingungen durchgeführt. Eine zusätzliche Charakterisierung erfolgte ex situ mittels Raster-KraftMikroskopie. Die experimentell bestimmte spektrale Raman-Signatur der prototypischen epitaktischen (3x3)/(4x4) Silicene Struktur wurde durch ab initio Rechnungen, in Zusammenarbeit mit Theoriegruppen, bestätigt. Durch diesen Vergleich wir die zweidimensionale Natur der epitaktischen Silicen-Schichten vollständig bestätigt, wodurch andere mögliche Interpretationen ausgeschlossen werden können. Darüber hinaus wurden die Ramans-Signaturen der weiteren 2D und 3D Siliziumphasen auf Ag(111) bestimmt, wodurch sich ein klares Bild der Bildung dieser Strukturen in Abhängigkeit von den Präparationsbedingungen ergibt. Um die Möglichkeit der Funktionalisierung von Silicen und der weiteren 2D Si Strukturen zu testen, wurden diese unter UHV Bedingungen atomarem Wasserstoff ausgesetzt. Durch die Bindung zu den Wasserstoffamen wird die kristalline Struktur der Silicen-Schichten modifiziert und die Symmetrie reduziert, was sich deutlich in der spektralen Raman-Signatur zeigt. Wie mittels Raman Spektroskopie gezeigt werden konnte, kann diese Modifikation durch thermische Desorption des Wasserstoffs rückgängig gemacht werden, ist also reversibel. Raman Messungen mit verschiedenen Anregungswellenlängen deuten darüber hinaus auf die Änderung der elektronischen Eigenschaften der Silicen-Schichten durch die Hydrierung hin. Der ursprüngliche halbmetallische Charakter der epitaktischen Silicen-Schicht geht möglicherweise in einen halbleitenden Zustand über. Das Wachstum von Silicen Multilagen wurde ebenfalls mit in situ Ramanspektroskopie verfolgt. Die sich dabei ergebene Raman-Signatur wurde mit der Raman-Signatur von Ag terminiertem Si(111) verglichen. Hier zeigen sich große Ähnlichkeiten, die auf eine ähnliche atomare Struktur hindeuten und zeigen, dass Ag Atome für die Ausbildung der Oberflächenstruktur während des Wachstums der Si-Lagen verantwortlich sind. Die chemischen und physikalischen Eigenschaften dieser Struktur bestärken zusätzlich diese Äquivalenz

This dissertation works out the vibrational properties of epitaxial silicene, which was discovered by Vogt et al. in 2012 by the epitaxial synthesis on the silver substrate. Its two-dimensional (2D) character is modified in comparison to the free-standing silicene due to its epitaxial nature, since the underlying substrate alters the physical properties of silicene as a result of the strong hybridization of the electronic levels of the substrate and adlayer. The growth of silicene layers is complicated by the sensitivity of the Si structures to the experimental conditions, mainly temperature, resulting in the formation of several seemingly different surface reconstructions. Another Si structure appears on the Ag surface at a supramonolayer coverage. The Raman spectroscopy was utilized to understand the relation between different Si structures and reveal their origin as well as to investigate the phonon-related physical properties of two-dimensional Si sheets. The central core of this work is the growth and characterization of these 2D silicene monolayers on the Ag (111) surface as well as the formation of silicene multilayer structures. The characterization of these materials was performed using in situ surface-sensitive measurement methods such as Raman spectroscopy and low-energy electron diffraction under ultra-high vacuum conditions due to high chemical reactivity of epitaxial silicene. Additional characterization was done ex situ by means of scanning force microscopy. The experimentally determined spectral signature of the prototypical epitaxial (3x3)/(4x4) silicene structure was confirmed by ab initio calculations, in collaboration with theory groups. The Raman signatures of the other 2D and 3D Si phases on Ag (111) were determined which allowed us to provide a clear picture of their formation depending on the preparation conditions. The monitoring of the silicene multi-layer growth yielded the vibrational signature of the top layer, reconstructed in a (√3x√3) fashion. It was compared to the inverse, (√3x√3)-Ag/Si(111), system showing the vast amount of similarities, which suggest that the (√3x√3) reconstruction belong to the silver layer. The chemical and physical properties of this surface structure additionally strengthen this equivalence. The possibility of functionalization of epitaxial silicene was demonstrated via exposure to the atomic hydrogen under UHV conditions. The adsorbed hydrogen covalently bonds to the silicene lattice modifying it and reducing its symmetry. As shown by Raman spectroscopy, such modification can be reversed by thermal desorption of hydrogen. The excitation-dependent Raman measurements also suggest the change of the electronic properties of epitaxial silicene upon hydrogenation suggesting that its originally semi-metallic character is modified into a semiconducting one.<br>Die experimentellen Forschungsarbeiten zum Thema Silicen basieren auf den 2012 von Vogt et al. durchgeführten Untersuchungen zu dessen Synthese auf Silbersubstraten. Diese Untersuchungen lieferten die Grundlage, auf der zweidimensionales (2D) epitaktisches Silicen sowie weitere 2D Materialien untersucht werden konnten. In den anfänglichen Arbeiten konnte dabei gezeigt werden, dass sich die Eigenschaften von epitaktischem Silicen gegenüber den theoretischen Vorhersagen von frei-stehendem Silicen unterscheiden. Darüber hinaus verkomplizieren sich die experimentellen Untersuchungen dieses 2D Materials, da auf dem Ag(111) Wachstumssubstrat sechs verschiedene 2D Si Polytypen existieren. Eine detaillierte Darstellung dieser Untersuchungen findet sich in dem einführenden Kapitel der vorliegen Promotionsschrift. Der zentrale Kern dieser Arbeit beschäftigt sich mit dem Wachstum und der Charakterisierung dieser 2D Silicen Monolagen auf Ag(111) Oberflächen sowie der Bildung von Silicen- Multilagen Strukturen. Die Charakterisierung dieser Materialien wurde in situ mit oberflächenempfindlichen Messmethoden wie der Raman Spektroskopie und der niederenergetischen Elektronenbeugung unter Ultrahochvakuum-Bedingungen durchgeführt. Eine zusätzliche Charakterisierung erfolgte ex situ mittels Raster-KraftMikroskopie. Die experimentell bestimmte spektrale Raman-Signatur der prototypischen epitaktischen (3x3)/(4x4) Silicene Struktur wurde durch ab initio Rechnungen, in Zusammenarbeit mit Theoriegruppen, bestätigt. Durch diesen Vergleich wir die zweidimensionale Natur der epitaktischen Silicen-Schichten vollständig bestätigt, wodurch andere mögliche Interpretationen ausgeschlossen werden können. Darüber hinaus wurden die Ramans-Signaturen der weiteren 2D und 3D Siliziumphasen auf Ag(111) bestimmt, wodurch sich ein klares Bild der Bildung dieser Strukturen in Abhängigkeit von den Präparationsbedingungen ergibt. Um die Möglichkeit der Funktionalisierung von Silicen und der weiteren 2D Si Strukturen zu testen, wurden diese unter UHV Bedingungen atomarem Wasserstoff ausgesetzt. Durch die Bindung zu den Wasserstoffamen wird die kristalline Struktur der Silicen-Schichten modifiziert und die Symmetrie reduziert, was sich deutlich in der spektralen Raman-Signatur zeigt. Wie mittels Raman Spektroskopie gezeigt werden konnte, kann diese Modifikation durch thermische Desorption des Wasserstoffs rückgängig gemacht werden, ist also reversibel. Raman Messungen mit verschiedenen Anregungswellenlängen deuten darüber hinaus auf die Änderung der elektronischen Eigenschaften der Silicen-Schichten durch die Hydrierung hin. Der ursprüngliche halbmetallische Charakter der epitaktischen Silicen-Schicht geht möglicherweise in einen halbleitenden Zustand über. Das Wachstum von Silicen Multilagen wurde ebenfalls mit in situ Ramanspektroskopie verfolgt. Die sich dabei ergebene Raman-Signatur wurde mit der Raman-Signatur von Ag terminiertem Si(111) verglichen. Hier zeigen sich große Ähnlichkeiten, die auf eine ähnliche atomare Struktur hindeuten und zeigen, dass Ag Atome für die Ausbildung der Oberflächenstruktur während des Wachstums der Si-Lagen verantwortlich sind. Die chemischen und physikalischen Eigenschaften dieser Struktur bestärken zusätzlich diese Äquivalenz.

Le silicène est l’équivalent du graphène pour le silicium avec une structure bidimensionnelle (2D). Il est supposé avoir des propriétés électroniques intéressantes comme les fermions de Dirac sans masse et présentant une grande mobilité des électrons. L’existence du silicène a été montrée récemment sur des substrats de métaux nobles comme l’argent ou l’or. Cependant les résultats montrent des interactions fortes entre la couche de silicène et le substrat métallique, ce qui a pour conséquence de détruire les propriétés électroniques intrinsèques du silicène. Dans le but de résoudre ce problème, nous proposons dans ce travail d’explorer d’autres substrats potentiels présentant de faibles interactions avec le silicène. Nous avons étudié la croissance de couches 2D de silicium sur un film mince isolant de NaCl. En effet, les métaux alcalins halogénés tel que NaCl offrent une solution avantageuse comme surface alternative puisqu'ils se comportent comme une couche diélectrique, permettant la caractérisation du silicène. Nous avons étudié les propriétés structurales et électroniques des couches de silicium 2D déposées sur un film mince de NaCl, lui-même déposé sur un substrat d’Ag(110). Une étude expérimentale a été réalisée combinant un grand nombre de techniques utilisées en science des surfaces telles que : « low energy electron diffraction » (LEED), « Auger electron spectroscopy » (AES), « scanning tunneling microscopy and spectroscopy » (STM/STS), «extended x-ray absorption fine structure » (EXAFS), « x-ray photoelectron spectroscopy » (XPS) et « angle resolved photoemission spectroscopy (ARPES) ». L’absorption d’atomes de silicium sur les films de NaCl révèle l’existence d’une couche de silicium 2D superficielle avec une structure très ordonnée en forme de nids d’abeilles. Cette couche présente une interaction faible avec le substrat tout en étant analogue au silicène. Enfin, des expériences préliminaires sur la croissance de silicene sur des films de NaCl dissociés sont présentées. L’effet de l’irradiation électronique du film de NaCl ainsi que des mesures d’ARPES sur le silicène intercalé sur Na sont présentées<br>Silicene, the silicon-based analog of graphene which has a two-dimensional (2D) structure. It is expected to have attractive electronic properties such as massless Dirac fermions and high electron mobility. The existence of silicene has been shown recently on noble metal substrates such as Ag and Au. The results present strong interactions between the silicene adlayer and the metallic substrate which destroy the intrinsic electronic properties of silicene. In order to solve this problem, we propose in this work to explore other potential substrates that have weaker interactions with silicene. We studied the growth of a 2D silicon layer on insulating NaCl thin film. Indeed, Alkali metal halides such as NaCl offer a great solution as an alternative surface because they behave as a dielectric layer, allowing characterization of silicene material. We studied the structural and electronic properties of 2D silicon layer grown on a NaCl film deposited over Ag(110) substrate. A combined experimental investigation was performed with a large number of techniques which are used in surface science such as: low energy electron diffraction (LEED), auger electron spectroscopy (AES), scanning tunneling microscopy and spectroscopy (STM/STS), extended x-ray absorption fine structure (EXAFS), x-ray photoelectron spectroscopy (XPS) and angle resolved photoemission spectroscopy (ARPES). The adsorption of silicon atoms on NaCl films reveals the existence of a 2D silicon sheet adlayer with a highly ordered honeycomb-like structure. The silicon ad-layer has weak interactions with the substrate and it mimics the structure of silicene. Finally, preliminary experiments on the growth of silicene on dissociated NaCl films are presented. The effect of electron irradiation on the NaCl film and initial ARPES measurement on the silicone intercalated-Na atoms system are presented

With the emergence of nanotechnology, mankind has obtained the capability to manipulate materials at nanoscale and this led to the invention of a new group of novel materials like carbon nanotubes, graphene and quantum nanodots. Silicene nanoribbons (SiNRs) are one of the newest members of this nanomaterial family which has been synthesized very recently by deposition on silver substrates. A SiNR sheet is made up of a layer of two dimensional honeycomb structure solely composed of silicon atoms. In this thesis, structural and mechanical properties of SiNR are being investigated with the help of classical empirical molecular dynamics simulation technique. In the first part of this thesis, structural properties of SiNR sheets have been investigated. This investigation has been carried out by performing classical molecular dynamics simulations using atomistic many-body potential energy functions at many different SiNR sheet lengths and widths, at low and room temperatures with and without periodic boundaries. It has been found that SiNR sheets do not have perfectly flat honeycomb geometry. It has also been found that finite length models are more likely to form tubular structures resembling distorted silicon nanotubes at room temperature. In the second part of this thesis, mechanical properties of SiNRs have been investigated. Mechanical properties of silicene nanoribbons of varying width have been investigated under 5% and 10% uniaxial strain via classical Molecular-Dynamics simulations at 1 K&deg<br>and 300 K&deg<br>temperatures by the aid of atomistic many-body potential energy functions. It has been found that under strain, SiNRs show such material properties: they are very ductile, they have considerable toughness and despite their low elasticity, they have a very long plastic range before fragmentation.

Nel presente lavoro espongo i risultati degli esperimenti svolti durante la mia internship all’Institut des NanoSciences de Paris (INSP), presso l’Università Pierre et Marie Curie (Paris VI), nel team "Phisico-Chimie et Dynamique des Surfaces", sotto la supervisione del Dott. Geoffroy Prévot. L’elaborato è stato redatto e in- tegrato sotto la guida del Dott. Pasquini, del dipartimento di Fisica e Astronomia dell’Università di Bologna. La tesi s’inserisce nel campo di ricerca del silicene, i.e. l’allotropo bidimensionale del silicio. Il cosidetto free-standing silicene è stato predetto teoricamente nel 2009 utilizzando calcoli di Density Functional Theory, e da allora ha stimolato un’intensa ricerca per la sua realizzazione sperimentale. La sua struttura elettronica lo rende particolarmente adatto per eventuali appli- cazioni tecnologiche e sperimentali, mentre lo studio delle sue proprietà è di grande interesse per la scienza di base. Nel capitolo 1 presento innanzitutto la struttura del silicene e le proprietà previste dagli studi pubblicati nella letteratura scientifica. In seguito espongo alcuni dei risultati sperimentali ottenuti negli ultimi anni, in quanto utili per un paragone con i risultati ottenuti durante l’internship. Nel capitolo 2 presento le tecniche sperimentali che ho utilizzato per effettuare le misure. Molto tempo è stato investito per ottenere una certa dimistichezza con gli apparati in modo da svolgere gli esperimenti in maniera autonoma. Il capitolo 3 è dedicato alla discussione e analisi dei risultati delle misure, che sono presentati in relazione ad alcune considerazioni esposte nel primo capitolo. Infine le conclusioni riassumono brevemente quanto ottenuto dall’analisi dati. A partire da queste considerazioni propongo alcuni esperimenti che potrebbero ulteriormente contribuire alla ricerca del silicene. I risultati ottenuti su Ag(111) sono contenuti in un articolo accettato da Physical Review B.

De nombreuses propriétés intéressantes sont attendues des simulations théoriques pour le silicène et le germanène autoportants. Leur synthèse reste cependant controversée. Ce travail présente une étude expérimentale par microscopie à effet tunnel (STM) et par diffraction des rayons X en incidence rasante (GIXD) de la croissance du germanium et du silicium sur des surfaces métalliques. Trois systèmes ont été étudiés : Ge/Al(111), Ge/Ag(111) et Si/Ag(110). Différentes reconstructions ordonnées ont été observées par STM en fonction de la température de croissance. La STM en temps réel me permet de déterminer les transitions entre ces structures. Dans tous les cas, les images STM acquises pendant la croissance montrent que les atomes de surface échangent avec les atomes de Si ou de Ge pendant le dépôt. Les trois structures ordonnées ont été analysées quantitativement par GIXD. Les facteurs de structure expérimentaux obtenus ont ensuite été comparés à ceux simulés à partir des modèles DFT. Pour Ge/Al(111), j’ai constaté que la reconstruction (3×3) correspond à un alliage de surface Ge-Al à deux couches, et non au germanène comme proposé précédemment. Pour Ge/Al(111), je démontre la formation d’un alliage de surface Ag2Ge avec une structure similaire à la reconstruction (22×√3) de Au(111). Cependant, j’ai remarqué qu’un dépôt supplémentaire de Si sur les pentamères de Si précédemment observés sur Ag(110) conduit à la formation d’une couche ordonnée en nid d’abeille de silicène en haltère<br>While numerous interesting properties are expected from theoretical calculations for free-standing silicene and germanene, their synthesis remain controversial. This thesis presents an experimental study by scanning tunneling microscopy (STM) and grazing incidence X-ray diffraction (GIXD) of the growth of germanium and silicon on metal surfaces. Three systems have been studied: Ge/Al(111), Ge/Ag(111) and Si/Ag(110). Various ordered reconstructions are observed by STM, depending on the growth temperature. Real-time STM allows me to determine the transitions between these structures. For all cases, STM images acquired during growth show that surface atoms exchange with Si or Ge atoms during deposition. Three ordered structures have been quantitatively analyzed by GIXD. The experimental structure factors obtained have been further compared with the ones simulated from DFT models. For Ge/Al(111), I show that the (3×3) reconstruction corresponds to a two-layer Ge-Al surface alloy, and not to germanene as previously proposed. For Ge/Al(111), I demonstrate the formation of a Ag2Ge surface alloy with a structure similar to the (22×√3) reconstruction of Au(111). However, I demonstrate that further Si deposition on the Si pentamers previously observed on Ag(110) lead to the formation of a dumbbell silicene honeycomb ordered layer

Dissertação (mestrado)—Universidade de Brasília, Instituto de Física, Programa de Pós-Graduação em Física, 2014.<br>Submitted by Ana Cristina Barbosa da Silva (annabds@hotmail.com) on 2014-10-14T17:46:05Z No. of bitstreams: 1 2014_JorgeLuisHuamaniCorrea.pdf: 20726420 bytes, checksum: 750343fd3e2b899cfc4174ccc2d0b5bc (MD5)<br>Approved for entry into archive by Tania Milca Carvalho Malheiros(tania@bce.unb.br) on 2014-10-14T19:53:11Z (GMT) No. of bitstreams: 1 2014_JorgeLuisHuamaniCorrea.pdf: 20726420 bytes, checksum: 750343fd3e2b899cfc4174ccc2d0b5bc (MD5)<br>Made available in DSpace on 2014-10-14T19:53:11Z (GMT). No. of bitstreams: 1 2014_JorgeLuisHuamaniCorrea.pdf: 20726420 bytes, checksum: 750343fd3e2b899cfc4174ccc2d0b5bc (MD5)<br>Neste trabalho apresentamos um estudo sistemático sobre as propriedades eletrônicas de dois novos materiais bidimensionais - Grafeno e Siliceno. Efeitos de interações spin-órbita (ISO), strain uniaxial e potencial elétrico sobre as estruturas eletrônicas de grafeno, siliceno e suas correspondentes nanofitas foram investigadas através de cálculos de tight-binding. Encontramos que o strain induz a mudanças nos pontos de Dirac e uma distorção da primeira zona de Brillouin. Como resultado, o strain possibilita o aparecimento de gaps nos pontos de Dirac K e K^' e modula a estrutura de banda do siliceno em outros pontos k da rede recíproca. A intensidade dos efeitos de strain dependem fortemente da direção de aplicação do strain. Em adição, potenciais do tipo staggered pode ser utilizado para controlar estados de spin polarizado. A combinação de efeitos de spin-órbita intrínseco e campos externos aplicados podem induzir uma transição de fase topológica na nanofita de siliceno. Também realizamos um estudo sobre o transporte eletrônico de nanofitas de siliceno pelo método das funções de Green e fórmula de Landauer-Bütikker. A densidade de estados e condutância calculadas mostram a existência de estados de borda de energia zero para nanofitas do tipo zigzag, que são consideravelmente afetados por efeitos de ISO. Nossos resultados demonstram a grande aplicabilidade dessas nanoestruturas em dispositivos baseados no grau de liberdade do spin do elétron, na denominada spintrônica. ______________________________________________________________________________ ABSTRACT<br>We have performed a systematic study on the electronic structures of novel two-dimensional materials – Graphene and Silicene. Effects of spin-orbit interactions (SOI), uniaxialstrain and staggered potential on electronic structures of graphene, silicene and their correspondent nanoribbons have been investigated by means of tight-binding calculation.We found that the strain induces shifts of Dirac points and a distortion of the first Brillionzone. As a result, an applied tensile strain opens gaps at Dirac points K and K0 andmodulates the band structure of silicene in other k-points. The magnitude of these straineffects depends strongly on direction of applied strain. In addition, staggered potentialcan be used to control both the band gap and the polarized spin-states. Furthermore,the combination of SOI and applied external fields may drive the silicene nanoribbonto a topological phase transition. On the other hand, we have also carried out study onthe electronic transport of silicene nanoribbons by using the Green’s function methodand Landauer-Büttiker formula. The density of states and conductance clearly show anexistence of the zero-energy edge states for zigzag nanoribbons, which are considerablyaffected by SOI. Our results demonstrate the feasibility of these nanostructures in devicesbased on the spin degree of freedom of the electron, in the so-called spintronics.

As the pursuit for more powerful electronic devices progresses, individual components have had to be produced at ever smaller dimensions. Today, conventional technologies are at the edge of feasibility as they approach a fundamental limit at the atomic scale. Much research is aimed at overcoming the barrier to atomic scale devices, and indeed some of the explosion of interest into two-dimensional materials over the past decade has its roots in this goal. Graphene, the first atomically thin two-dimensional material, has since been followed by a growing number of intriguing materials with a wide variety of interesting properties, many of which may prove useful in technological devices. One of these two-dimensional materials is silicene, the silicon analogue to graphene that shares many of its properties. Moreover, owing to it being made of silicon, it may be more easily integrated into existing industrial processes. In this thesis, silicene grown upon conductive zirconium diboride is investigated by scanning tunnelling microscopy and spectroscopy. It is found that the structural and electronic properties of epitaxial silicene can be fine-tuned by depositing small amounts of silicon on its surface. However, with continued silicon deposition a significant change is observed: the additional silicon leads to the formation of a layered metallic silicon nanostructure that could be utilised as an atomically precise metallic contact to silicene. Beyond this, the magnetic interactions of individual cobalt atoms on the silicene surface are investigated and it is found that the combination of the semiconducting silicene surface with the metallic zirconium surface yields an unusual spatially distributed Kondo effect. When cobalt atoms are in close proximity to one another on the silicene surface, they exhibit an incredibly strong indirect exchange (RKKY) interaction even at significant separations above 1 nm. The results in this thesis highlight the rich array of phenomena that can manifest in two-dimensional materials and point towards potential future developments for atomic scale electronic and spintronic devices.

The main objective of this work is to research and obtain surface protected topological states in nano-ribbons created from the leaves of Germanene and Silicene. These sheets belong to the class of Topological Insulators and correspond to monolayers of germanium and silicon atoms in a hexagonal arrangement that is similar to the graphene sheet. For this investigation, we conducted a study of the electronic and structural properties of these sheets, as well as their respective nano-ribbons through first-principles calculations based on density functional theory (DFT). In this methodology we use the generalized gradient approximation (GGA) for estimating the exchange and correlation term, and the PAW method for the effective potential and the expansion of plane waves of the Kohn-Sham. We conducted a computer simulation with the aid of the package VASP (Vienna ab-initio Simulation Package). As a starting point for our research, we used the methodology of solid state physics in order to describe the crystalline structure of the leaves as well as their mutual space. Subsequently we analyze the band structure, from which many of its properties can be visualized. For this task, we initially proceeded to investigate the stability of these systems via total energy calculations, in turn obtaining the network parameters that minimizes the energy of the system. We also obtained the energy cutoff, ECUT used in our calculations, or in other words, determining the number of plane waves needed to expand the electronic wave functions on the DFT formalism. We continued our study, with the creation and analysis of two different configurations of nano-ribbons, one that corresponds to a straightforward cut of the sheet with the armchair termination pattern, and the other based on a reconstruction of those edges, which provide an energetically more stable system. Subsequently we obtained electronic structures, and conducted a study of its variation due to the change of the width of the nano-ribbon and ionic relaxation of its edges. In a way, we modified the above parameters in order to obtain a system that would give us a zero gap, or at least insignificant, as well as a specific configuration for the spin texture, in order to verify the evidence of surface protected topological states in these nano-ribbons.<br>O objetivo principal deste trabalho é a investigação e obtenção dos estados topologicamente protegidos de superfície em nano-fitas criadas a partir das folhas de Germaneno e Siliceno. Estas folhas pertencem a classe dos Isolantes Topológicos e correspondem a monocamadas de átomos de Germânio e Silício, em um arranjo hexagonal que se assemelha a folha do Grafeno. Para esta investigação, realizamos um estudo das propriedades eletrônicas e estruturais destas folhas, bem como de suas respectivas nano-fitas, através de cálculos de primeiros princípios fundamentados na teoria do funcional da densidade (DFT). Nesta metodologia utilizamos a aproximação do gradiente generalizado (GGA) para a estimativa do termo de troca e correlação, e o método PAW para o potencial efetivo e a expansão em ondas planas dos orbitais de Kohn-Sham. Realizamos a simulação computacional com o auxílio do pacote VASP (Vienna ab-initio Simulation Package). Como ponto de partida para nossa pesquisa, utilizamos a metodologia da física do estado sólido com o intuito de descrever a estrutura cristalina das folhas, bem como seu espaço recíproco. Posteriormente analisamos as estruturas de bandas, a partir das quais muitas de suas propriedades podem ser visualizadas. Para esta tarefa, inicialmente procedemos à investigação da estabilidade destes sistemas via cálculos de energia total, obtendo o parâmetro de rede a que minimiza a energia do sistema. Obtivemos também a energia de corte ECUT utilizada em nossos cálculos, ou em outras palavras, a determinação do número de ondas planas necessárias para expandir as funções de onda eletrônicas no formalismo da DFT. Prosseguimos nosso estudo, com a criação e análise de duas distintas configurações de nano-fitas, uma que corresponde a um corte simples e direto da folha com terminação no padrão armchair, e a outra baseada em uma reconstrução destas bordas, que acaba por fornecer um sistema mais estável energeticamente. Posteriormente obtivemos as estruturas eletrônicas, e realizamos um estudo de sua variação em função da alteração da largura da nano-fita e a relaxação iônica de suas bordas. De certa maneira, modificamos os parâmetros acima, de forma a obter um sistema que nos fornecesse um gap nulo, ou pelo menos desprezível, bem como uma determinada configuração para a textura de spin, de modo a verificarmos a evidência de uma proteção topológica nos estados de superfície nestas nano-fitas.<br>Mestre em Física

La possibilità di controllare la materia alle dimensioni tipiche di qualche atomo ha permesso negli ultimi decenni enormi progressi in campo tecnologico. Questa miniaturizzazione è stata resa possibile dallo sviluppo di adeguate tecniche sperimentali di sintesi e controllo dei materiali, ma anche dall’introduzione di modelli teorici per descrivere i fenomeni fisici tramite il formalismo della meccanica quantistica. Questi modelli hanno permesso di descrivere la materia tramite approcci numerici, in modo da supportare, espandere ed anticipare i risultati sperimentali. Questo lavoro di tesi è concentrato sull’investigazione di interfacce create dall’accoppiamento di superfici metalliche con materiali a diversa dimensionalità. Si sono identificati i modelli più appropriati per descrivere tali sistemi ed estratte diverse quantità fisiche indicative della natura dei processi attivi alle interfacce e che potessero essere confrontate con risultati di natura sperimentale. Si sono considerati quattro casi, prototipi di materiali a diversa dimensionalità: una singola molecola organica, il pentacene, rappresentativo di un sistema isolato; catene di atomi di carbonio, rappresentativi di sistemi monodimensionali; un foglio di silicene, rappresentativo di sistemi puramente bidimensionali ed infine la ricostruzione di atomi di silicio su un substrato come caso limite per un sistema esteso. Parte dello studio è stato dedicato ad identificare le proprietà di questi sistemi in assenza di altre interazioni, ma soprattutto si è indirizzato a mostrare le modifiche a tali proprietà in presenza di diversi substrati metallici, rispettivamente Pt(111), Au(111), Ag(111) e Ag(110), caso tipico nel caso delle maggiori applicazioni nel campo dell’optoelettronica. Dal punto di vista teorico, la struttura geometrica e le proprietà elettroniche di ogni sistema sono state costruite a partire dalla teoria del funzionale densità. Tale metodo però non è sufficiente, visto che così viene costruito lo stato fondamentale del sistema: è necessario estendere i modelli per poter costruire gli stati eccitati, che tipicamente sono quelli investigati dalle tecniche sperimentali. Ogni tecnica, d’altra parte, richiede un diverso approccio per costruire e risolvere il relativo modello. In dettaglio, si è studiato lo spettro di assorbimento dei raggi x per classificare l’ interazione all’interfaccia pentacene/Pt(111) come chemisorbimento o per verificare l’impronta dell’assorbimento di catene di carbonio nel caso di molecole organiche caratterizzate da ibridazione mista di tipo sp1 e sp2. Si è investigato lo spettro di assorbimento ottico e la relativa riflettività nel caso del silicene cresciuto su Ag(111), in relazione alla struttura elettronica del sistema e delle sue modifiche indotte dalla funzionalizzazione della superficie tramite adsorbimento di singoli atomi di H o F a diversi ricoprimenti. Si è mostrato come la caratteristiche tipiche del silicene vengano perse non solo a causa del substrato, ma a causa della reattività del silicio. Come ultimo caso si sono considerati atomi di Si depositati su un film sottile di NaCl cresciuto su una superficie di Ag(110), con lo scopo di recuperare la struttura elettronica dovuta al cono di Dirac. Gli indizi contrastanti forniti da una caratterizzazione sperimentale del sistema riguardo la geometria e le proprietà elettroniche hanno richiesto un metodo diversificato per ricercare il modello più appropriato di ricostruzione degli atomi di Si. In particolare si è proceduto sia staticamente, ricercando le strutture più in accordo con la topografia emersa dagli esperimenti, ma anche tramite un approccio dinamico, volto a caratterizzare il processo stesso di aggregazione degli adatomi. Infine, si è considerata anche una ricerca ottimizzata di configurazioni casuali, basata su algoritmi genetici, ovvero ispirati al processo di selezione naturale osservato in biologia.<br>In the last decades, the ability to control the behavior of the matter at the nanoscale has yielded many technological breakthroughs. Such miniaturization steps have been triggered not only by the development of novel experimental techniques of synthesis and characterization of the materials, but also by the introduction of advanced theoretical models based on quantum mechanics. Through these models, it has been possible to simulate the behavior of the matter in order to support, expand and also predict experimental results. The work included in this thesis is focused on investigating the interfaces created by coupling metallic surfaces with materials with different dimensionality. The models more appropriate for describing such systems have been identified in order to extract the physical properties more relevant to describe the active processes at the interfaces but also to be compared with and fully exploit the experimental data. Four cases have been taken into account, as prototypes of materials with different dimensionality: a single organic molecule, pentacene, representing the isolated system, chains of carbon atoms, representing mono-dimensional systems, a silicene sheet, representing a two-dimensional system and the reconstruction of Si atoms on a substrate as a limit case of higher dimensionality. Part of the study has been dedicated to identify the properties of the pristine materials, but the main effort has been focused on the changes of such properties induced by the presence of metallic surfaces as Pt(111), Au(111), Ag(111) and Ag(110) respectively, commonly found in opto-electronic devices. From the atomistic point of view, the geometry and the electronic properties of each system have been derived within the framework of density function theory. Such an approach is suited for the ground state only: therefore it has been necessary to extend the theoretical models to describe the excited states too, which are those commonly probed by experimental techniques. Every technique, however requires a dedicated approach to deal with and solve the related model. In particular, in this thesis the x-rays absorption spectrum has been investigated to gather insights on the interactions between pentacene and Pt(111) and to verify the contribution of carbon chains in the absorption from organic molecules with mixed sp1, sp2 hybridization states. The optical absorption has been then investigated along with the reflectivity for silicene grown on Ag(111), in relation with the electronic structure of the system and its modifications induced by the functionalization of the surface through the adsorption of H or F at different coverages. It has been shown that silicene peculiar properties are lost not only because of the presence of a substrate, but because of the reactivity itself of Si atoms. As a last case, the deposition of Si on the substrate made of a thin film of NaCl grown on Ag(110) has been investigated in order to understand whether is possible to recover the Dirac cone structure. As experimental results do not provide a solid evidence about the actual formation of a silicene overlayer, additional insight was indeed required about the structure of the reconstruction of Si atoms. Both a static and a dynamic approach have been followed, in order to find the best structure matching the experimental topography and to tailor the self-assembly process itself. Finally, the most probable configuration has been searched through a novel approach based on genetic algorithms, which are inspired by the natural selection process observed in biology to optimize the evaluation of random configurations.

Recent advances in experimental and computational methods have opened up new directions in graphene fundamental studies. In addition to understanding the basic properties of this material and its quasi-one dimensional structures, significant efforts are devoted to describing their long ranged dispersive interactions. Other two-dimensional materials, such as silicene, germanene, and transition metal dichalcogenides, are also being investigated aiming at finding complementary to graphene systems with other "wonder" properties. The focus of this work is to utilize first principles simulations methods to build our basic knowledge of structure-interaction relations in two-dimensional materials and design their properties. In particular, mechanical folding and extended defects in zigzag and armchair graphene nanoribbons can be used to modulate their electronic and spin polarization characteristics and achieve different stacking patterns. Our simulations concerning zigzag silicene nanoribbons show width-dependent antiferromagnetic-ferromagnetic transitions unlike the case of zigzag graphene nanoribbons, which are always antiferromagnetic. Heterostructures, build by stacking graphene, silicene, and MoS$_2$, are also investigated. It is found that hybridization alters the electronic properties of the individual layers and new flexural and breathing phonon modes display unique behaviors in the heterostructure compositions. Anchored to SiC substrate graphene nanoribbons are also proposed as possible systems to be used in graphene electronics. Our findings are of importance not only for fundamental science, but they could also be used for future experimental developments.

The nanoelectronic evolution, which was driven for many years by the ‘‘aggressive scaling’’ of the complementary metal-oxide-semiconductor (CMOS) devices, needs new approaches in order to face the demands for smaller, more performing, and less power-consuming integrated circuits. A few years ago, high-mobility semiconductors, e.g., germanium and III–V semiconductors, started to be investigated as possible substitutes of silicon as materials for the CMOS channel. On the other hand, dielectric materials with a higher dielectric constant (j) than the native silicon dioxide, such as HfO2, were introduced into CMOS devices a couple of years ago, in order to obtain a larger oxide capacitance, improving the performance of the devices while keeping their power consumption as low as possible. To take effective advantage of the introduction of high-mobility semiconductors and high-j dielectrics in the next generations of CMOS devices, high quality interfaces are required. In the first part of this thesis, we investigate the vibrational properties of defective HfO2 by first-principles simulations, and we compare them with experimental results from inelastic electron tunneling spectroscopy (IETS). This spectroscopic technique is very powerful for the investigation of nanoscale junctions. We also model amorphous defective GeO2, likely present at the interface of Ge/HfO2 gate stacks. Different defects, including three-folded oxygen atoms and divalent germanium centers are investigated. We show how the calculated vibrational spectra of the defective oxides, correlated to IETS measurements, can be successfully used for the investigation of high-mobility/high-j gate stacks interfaces. Recently, the interest of the physics and electronic engineering community in 2D materials, such as graphene, increased exponentially. These materials, made up of one single atomic layer, can be used to exploit quantum confinement effects, resulting in unique electronic and magnetic properties. The linear electronic dispersion observed in graphene, linked to the presence of massless Dirac fermions, was recently predicted also for its silicon and germanium counterparts, the so-called silicene and germane. This is very appealing for nanoelectronic and energy applications, in which materials with an extremely high conductivity are highly demanded. Recent experiments showed that silicene grown on metallic substrates has different structural configurations and presents a characteristic puckering of the silicon atoms, which are in contrast to graphene.In the second part of this thesis, the structural and vibrational properties of silicene on Ag(111) surfaces are calculated. Their comparison with experimental measurements, such as scanning tunneling microscopy and Raman spectroscopy, allows us to investigate the structural but also the electronic properties of different silicene reconstructions on Ag(111). Finally, the possible growth of silicene on nonmetallic templates is theoretically investigated. We show that different layered chalcogenide compounds (i.e., MoX2 and GaX, X=S, Se, Te) can be used as templates for the silicene layer. The van der Waals interaction between the silicene layer and the templates is important for avoiding strong interactions (hybridization) between the silicon atoms and the substrates. The different in-plane lattice parameters of the chalcogenide compounds can be exploited to tune the electronic properties of the silicene layer, preserving in some cases its massless Dirac fermions.

The popularity of graphene owing to its unique properties has triggered huge interest in other two-dimensional (2D) nanomaterials. Among them, silicene shows considerable promise for electronic devices due to the expected compatibility with silicon electronics. However, the high-end potential application of silicene in electronic devices is limited owing to the lack of an energy band gap. Hence, the principal objective of this research is to tune the electronic and magnetic properties of silicene related nanomaterials through first-principles models. I first explored the impact of edge functionalization and doping on the stabilities, electronic, and magnetic properties of silicene nanoribbons (SiNRs) and revealed that the modified structures indicate remarkable spin gapless semiconductor and half-metal behaviors. In order to open and tune a band gap in silicene, SiNRs were perforated with periodic nanoholes. It was found that the band gap varies based on the nanoribbon’s width, nanohole’s repeat periodicity, and nanohole’s position due to the quantum confinement effect. To continue to take advantage of quantum confinement, I also studied the electronic and magnetic properties of hydrogenated silicene nanoflakes (SiNFs). It was discovered that half-hydrogenated SiNFs produce a large spin moment that is directly proportional to the square of the flake’s size. Next, I studied the adsorption behavior of various gas molecules on SiNRs. Based on my results, the SiNR could serve as a highly sensitive gas sensor for CO and NH3 detection and a disposable gas sensor for NO, NO2, and SO2. I also considered adsorption behavior of toxic gas molecules on boron carbide (BC3) and found that unlike graphene, BC3 has good sensitivity to the gas molecules due to the presence of active B atoms. My findings divulged the promising potential of BC3 as a highly sensitive molecular sensor for NO and NH3 detection and a catalyst for NO2 dissociation. Finally, I scrutinized the interactions of CO2 with lithium-functionalized germanene. It was discovered that although a single CO2 molecule was weakly physisorbed on pristine germanene, a significant improvement on its adsorption energy was found by utilizing Li-functionalized germanene as the adsorbent. My results suggest that Li-functionalized germanene shows promise for CO2 capture.

В настоящее время среди многообразия нелинейных сред большой интерес вызывают среды с фазовым переходом (с параметром порядка) с точки зрения их практических применений. С другой стороны, возникает задача о спектроскопии параметра порядка в средах, способных выдержать экстремально сильные электромагнитные поля и в которых возможно устойчивое распространение световых пуль (СП). К ним относятся среды, содержащие силицен.

La physique des surfaces et nanosciences est une discipline qui permet la conception d’une diversité de matériaux innovants pour mieux répondre aux besoins de la technologie actuelle. Dans ce contexte, nous nous sommes intéressés à caractériser les propriétés de différentes structures 2D élaborées sur des substrats d’argent en combinant différentes techniques d'analyses de surface.D'une part nous avons étudié des films 2D auto-assemblés à base de phtalocyanine de cobalt adsorbées sur Ag(100). Au régime de la monocouche, deux phases ont été essentiellement observées : la (5x5) et la (7x7). La spectroscopie de pertes d'énergie nous a permis de mettre en évidence deux mécanismes de transfert de charge métal/molécule différents en raison des sites d'adsorption différents.D'autre part, nous avons synthétisé, par évaporation d'atomes de silicium sur de l'Ag(111), du silicene en monocouche et multicouche. Ainsi nous avons entrepris des mesures dans le but de comprendre les propriétés structurales du silicene en multicouche. Pour cela nous avons engagé des mesures par spectroscopie de photoémission et diffraction de photoélectrons, et comparé nos données avec des simulations réalisées dans le cadre la diffusion multiple. Plusieurs hypothèses concernant la nature de ce matériau ont pu être testées. Par ailleurs nous avons étudié la fonctionnalisation du silicene par hydrogénation et adsorption de molécules de F4TCNQ<br>Nanosciences and surface science are key elements in the conception of a diversity of innovative materials designed to better cope with the needs of current technology. Within this context, we have resolved to characterise the properties of different two-dimensional structures grown on silver substrates with the help of several complementary techniques of surface analysis.Firstly, we have studied auto-assembled 2D films of cobalt phthalocyanine on Ag(100) substrates. In situations with coverages close to the monolayer, two phases were observed: the (5x5) and the (7x7). The electron energy loss spectroscopy has allowed us to support the existence of two inequivalent charge transfer mechanisms between the substrate and the molecules due to differences in the adsoprtion sites. Secondly, we have synthesised both monolayer and multilayer silicene by evaporating silicon atoms on Ag(111) substrates. We have decided to delve into the characteristics of multilayer silicene as it’s less well-known than its monolayer counterpart. With this aim, the system has been subjected to experiments of photoemission spectroscopy and diffraction. In this manner, several hypotheses on the very nature of this material have been tested. On another matter also related to silicene, we have studied its functionalization by adsorption of F4TCNQ molecules and atomic hydrogen

Submitted by MARCIA ROVADOSCHI (marciar@unifra.br) on 2018-08-20T12:07:07Z No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_MirkosOrtizMartins.pdf: 14740958 bytes, checksum: 7fbd6b5923fb58ef079c134775d7e50d (MD5)<br>Made available in DSpace on 2018-08-20T12:07:07Z (GMT). No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_MirkosOrtizMartins.pdf: 14740958 bytes, checksum: 7fbd6b5923fb58ef079c134775d7e50d (MD5) Previous issue date: 2016-07-22<br>In this thesis is studied the interaction between the nitrogenous bases, adenine, cytosine, guanine and thymine with two nanostructures: the graphene and silicene nanoribbons through ab initio calculations based on DFT (Density Functional Theory) using the SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms) software. Prior to implementing the software to simulate the behavior at the atomic level of molecules all feature and variants was studied, for computer modeling and simulation; which applications using nanostructures and the best spatial con guration for construction of an initial parameter simulation. The nitrogenous bases as main constitution of the genetic code component, in the form of a double trainer strand of DNA, is the point of interest of this work, along with their behavior when the interaction, by positioning the walking distance - with nanoribbons graphene and silicene. The simulation of the nitrogenous bases and nanoribbons initially obey perpendicular position with the atoms of forming hydrogen bonds in DNA, pointing to the nanostructures. It was calculated, at the end of the simulations, the removal of nitrogenous bases of its current initial position, the di erence in the values of total energy and charge transfer. It has been shown that the interaction energy between nitrogenous bases and graphene nanoribbon are thinner than those found for the interaction with the silicene nanoribbon. When the nitrogenous bases are placed parallel to nanoribbons (both of graphene as silicene) they present chemical bonds, while when they are arranged perpendicularly to the surface of the material, they deviate in the course of its transverse path. It also follows that the silicene nanoribbon is more stable in the adsorption energy than nanoribbon graphene and the interaction of the bases is the energy bands that change xiii subtly, with respect of the gap values. For the graphene nanoribbon, the changes are associated with the position of the Fermi level. These results show, in an original way, di erent con gurations for the nitrogeneous basis on one dimensional carbon and silicon materials.<br>Nessa tese e estudada a intera c~ao entre as bases nitrogenadas, adenina, citosina, guanina e timina, com duas nanoestruturas: a nano ta de grafeno e a nano ta de siliceno, atrav es de c alculos ab initio baseados na Teoria do Funcional da Densidade (DFT), utilizando o software SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms), esse programa computacional executa c alculos de estrutura eletr^onica e simula c~oes ab initio de din^amica molecular de mol eculas e s olidos. Anterior a execu c~ao do software para simular o comportamento a n vel at^omico das mol eculas, foi estudada a caracter stica e variantes, para modelagem e simula c~ao computacional; quais as aplica c~oes utilizando nanoestruturas e a melhor con gura c~ao espacial para constru c~ao de um par^ametro inicial de simula c~ao. As bases nitrogenadas, sendo componente principal da constitui c~ao do c odigo gen etico, na forma de um duplo lamento formador do DNA (Deoxyribonucleic acid - Acido Desoxiribonucleico), e o ponto de interesse deste trabalho, juntamente com seu comportamento quando na intera c~ao, atrav es do posicionamento a pouca dist^ancia - com as nano tas de grafeno e siliceno. A simula c~ao entre as bases nitrogenadas e as nano tas inicialmente obedecem um posicionamento perpendicular com os atomos formadores das liga c~oes de hidrog^enio, no DNA, apontando para as nanoestruturas. Foi calculado, ao nal das simula c~oes, o afastamento das bases nitrogenadas da sua posi c~ao atual, a diferencia c~ao nos valores de energia total e a respectiva transfer^encia de carga. Foi demonstrado que a energia de adsor c~ao entre as bases nitrogenadas e a nano ta de grafeno s~ao mais t^enues do que aquelas encontradas para a intera c~ao com a nano ta de siliceno. Quando as bases nitrogenadas s~ao colocadas paralelas as nano tas (tanto de grafeno quanto de siliceno) elas fazem liga c~oes qu micas, enquanto se forem dispostas perpendicularmente a superf cie do material, as mesmas se xi afastam no decorrer de seu trajeto transversal. Tamb em se conclui que a nano ta de siliceno e mais est avel no sentido de energia de adsor c~ao do que a nano ta de grafeno e a intera c~ao das bases faz as bandas de energia dessa, alterar de forma sutil, via mudan ca no gap de energia. No caso da adsor c~ao na nano ta de grafeno observam-se altera c~oes na posi c~ao do n vel de Fermi, sem mudan cas nas caracter sticas met alicas do sistema original. Desta forma, este trabalho apresenta, de forma original, diferentes con gura c~oes para a intera c~ao de bases nitrogenadas em sistemas unidimensionais de carbono e sil cio, com aplica c~ao para a detec c~ao individual das bases nitrogenadas formadoras da mol ecula de DNA.

The research activity described in this thesis is mainly devoted to study the fundamental properties of alternate channel materials to bulk silicon. Indeed, there is consensus, in the scientific and industrial community, that silicon is approaching its ultimate scaling limit. Today, increasing the performance of integrated circuits by scaling silicon metal-oxide-semiconductor field effect transistor (MOSFET) is becoming more and more difficult and, therefore, a systematic survey of possible alternate solutions should be considered. This purpose is taken into account by exploring two paradigmatic candidates, namely, a promising III-V semiconductor, In0.53Ga0.47As(001), and a novel two-dimensional (2D) material, silicene, the silicon counterpart of graphene, which does not exist in nature. The former represents a mid-term option fully compatible with silicon-based processing, in a More Moore strategy, while the latter represents a completely different approach that can be envisioned for ultimate device downscaling either in a More Moore perspective or even further for More than Moore prospects. Such a systematic survey has to be carried out with probes which might consider the atomic properties, since the progressive shrinking of devices dimensions is pointing towards atomic-scale. In this framework, the choice of Scanning Tunneling Microscope (STM) represents a suitable probe which can address either the morphological properties or the electronic ones through Scanning Tunneling Spectroscopy (STS), which is the most powerful STM-related capability, since it allows for atomic-scale characterization of the local density of states (LDOS) of the sample. The local electronic properties of In0.53Ga0.47As are investigated at the interface with Al2O3. Two pristine surfaces, the As-rich (2x4) reconstruction and the group-III-rich (In/Ga) (4x2) reconstruction are scrutinized by STM. STS shows that for both n-type doped reconstructions the Fermi level is initially pinned near the valence band. However, upon in situ growth of the Al2O3 thin film by Molecular Beam Epitaxy (MBE), partial unpinning occurs, while post-deposition annealing restores the original pinned condition in a different extent depending on the surface reconstruction. This behavior is rationalized in terms of an interface dipole induced by positive charges in the as-grown oxide, which are suppressed upon annealing. Hence, this comparison shows that the (2x4) reconstruction is more favorable for application-oriented perspectives of In0.53Ga0.47As as an active channel in MOSFET devices. Despite the high expectations, graphene, the nowadays best-known 2D material, has severe limitations for logic electronics due to its gapless nature. These limitations could be possibly overcome by silicene (and germanene). Here, a thoroughly study about the formation by MBE of various 2D phases of silicon on the Ag(111) surface depending on coverage and substrate temperature is reported. The resulting scenario is depicted by several silicene phases showing different periodicities and orientations with respect to the silver substrate. These structural phases stem from the intrinsic flexibility of silicene originated by its buckled structure, in contrast to graphene. The periodically modulated LDOS of silicene superstructures evidenced by STS spectra could be ascribed to the symmetry breaking in the triangular sublattices originated by the presence of buckled honeycomb lattice. The instability of silicene upon air exposure leads to the necessary encapsulation which is provided by an aluminum-based capping layer. This configuration allows the transfer of the samples for ex situ Raman spectroscopy measurements. Both experimental and theoretical Raman spectra show the presence of E2g mode, namely G peak in graphene, which is the fingerprint of honeycomb lattice, and by other vibrational modes activated by the intrinsic disorder related to the buckling. Both theoretical models and experiments proved that silicene phases have different mixture of sp2-sp3 hybridization that means different bond lengths, bond angles, and buckling parameters. Moreover these phases of silicene exhibit different electronic properties, ranging from semiconducting to semi-metal character. The perspective of future high performance logics should probably experience an intermediate step at III-V compound semiconductors, but it is very likely to expect that 2D materials will remain a hot topic in future electronics.

Dans cet ouvrage, je résume les études menées au cours de ma thèse concernant la synthèse de monocouches de silicium, de couches minces de Si sur Ag(111) et des dépôts de Si sur des matériaux lamellaires. Je présente des résultats originaux qui ont dévoilés des phénomènes physiques intéressants associés aux systèmes étudiés. Dans une première partie, je présente les résultats d’une étude combinant expériences et théorie, basée sur des mesures GIXD et des simulations DFT, visant à déterminer la disposition atomique exacte des structures de la monocouche de silicène sur Ag(111). Ensuite, je décris la structure atomique des couches minces de Si sur Ag(111). Je montre, au moyen de mesures GIXD, que le film de Si a une structure en forme de diamant avec des défauts d’empilement. Enfin, je détermine la structure atomique de la reconstruction de surface observée au-dessus du film de Si par des mesures de GIXD. Puis, par des études combinées de STM et DFT, je donne une image originale de la croissance de Si sur Ag(111) au-delà de la couverture de 1 ML de Si. Dans la dernière partie de cette thèse, je présente des études STM concernant l’évaporation de Si sur plusieurs matériaux lamellaires: HOPG, MoS2, TiTe2 et ZrSe2. Je montre que sur chacun de ces substrats la croissance de Si, à la fois pour température ambiante et pour des hautes températures, entraîne la formation de nanoclusters 3D<br>In this work I summarize the studies conducted during my PhD, concerning the synthesis of silicene layers and thin Si films on Ag(111) and Si deposition on layered materials. I present original results which unveil interesting physical phenomena associated with the system under study. In a first part, I present the outcomes of a combined experimental and theoretical study, based on GIXD measurements and DFT simulations, aimed to determine the exact atomic arrangement of the silicene monolayer structures on Ag(111). Afterwards I focus on the atomic structure of Si thin films on Ag(111). I show, by means of GIXD measurements, that the Si film has a diamond bulklike structure with stacking faults. Finally, I determine the atomic structure of the reconstruction observed on top of the aforementioned diamond bulklike Si film by menas of GIXD measurements. Then, by combined STM and DFT studies I give an original picture for Si growth on Ag(111) above 1 ML Si coverage. In the last part of this Thesis, I report STM studies regarding Si evaporation on several layered materials: HOPG, MoS2, TiTe2 and ZrSe2. I show that on each of these substrates and both for room temperature and high temperature growth, Si evaporation results in the formation 3D Si nanoclusters

L’objet de cette thèse est l’étude de la croissance de silicène sur des substrats d’argent,ainsi que l’étude de sa réactivité vis-à-vis de l’oxygène. La croissance a été réalisée sous ultra-vide et contrôlée par spectroscopie d’électrons Auger (AES) et par diffraction d’électrons lents (LEED). Les structures obtenues et leurs réactivités à l’oxygène ont été étudiées par microscopie à champ proche (STM et nc-AFM) et par spectroscopie de photoémission résolue en angle (ARPES). Nous avons étudié la structure interne des nano-rubans de silicène auto-assemblés sur un substrat d’Ag(110). Sur Ag(111) nous obtenons un feuillet de silicène qui présente différentes structures en fonction de la température du substrat. L’étude de la réactivité des rubans et des feuillets a montré que le silicène formé sur substrat d’argent est relativement stable vis-à-vis de l’oxygène ce qui ouvre des perspectives de fonctionnalisation du silicène. La dernière partie de cette thèse concerne la synthèse de feuillets de silicium par voie chimique. Nous avons mis au point une nouvelle méthode prometteuse de synthèse chimique qui nous a permis de synthétiser des feuillets de silicium de structure graphitique<br>The objective of this thesis is the study of the growth of silicene on silver substrates as well as its reactivity towards the oxygen. The growth was performed under ultra-high vacuum and controlled by Auger electrons spectroscopy (AES) and low energy electrons diffraction (LEED). The obtained structures and their relativities towards the oxygen were studied by near field microscopy (STM and nc-AFM) and by angle resolved electrons photoemission spectroscopy (ARPES). We have studied the internal structure of the selfassembled silicene nanoribbons on Ag(110) substrate. On Ag(111), we have obtained a silicene sheet presenting different structures versus the temperature of the substrate. The reactivity of silicene nanoribbons and sheets grown on silver show that silicene is relatively stable towards the oxygen which opens a new perspectives of functionalization of the silicene. The last part of this thesis concerns the synthesis of silicone sheets by chemical process. We have develpped a new promising process of chemical synthesis which allowed us to synthesize silicon sheets with graphitic structure

Ce travail expérimental porte sur la synthèse du silicène hétéroépitaxié sur des substrats Si(111) passivés par des atomes de bore ou d’argent. L’adsorption à température contrôlée d’une quantité de Si proche d’une monocouche sur les substrats B:Si(111)-(√3×√3)R30° et Ag/Si(111)-(√3×√3)R30°, mène à la formation, sur chaque substrat, d’une couche bidimensionnelle de Si, compacte et bien ordonnée, qui adopte la symétrie du substrat, que nous attribuons au silicène hétéroépitaxié. Nous avons utilisé le LEED, AES, IPES/ARIPES. La mesure du courant absorbé (TCS) et l’évolution du travail d’extraction indiquent de fortes perturbations des propriétés électroniques de la couche de charge d’espace du substrat. Celle-ci s’accompagne de la disparition des états de surface de chaque substrat, remplacés par de nouveaux états électroniques UB et U0 caractéristiques, aux profils de dispersion sans rapport avec la symétrie (√3×√3)R30°. Sur Ag/Si(111)-(√3×√3)R30°, la disparition de l’état bidimensionnel S1 d’électrons quasi-libres, à la dispersion parabolique, provoque une transition métal/isolant. Les positions énergétiques des états UB et U0, éloignées de EF, indiquent une interaction significative avec le substrat, bien que non-covalente, impliquant un transfert de charges spatialement inhomogène du silicène avec le substrat. Mesurés par ARIPES, les profils de la dispersion des états inoccupés UB et U0 caractéristiques de la couche 2D ordonnée de Si apparaissent compatibles avec la symétrie d’une monocouche de silicène orientée, qui fait correspondre la direction Γ-M√3 de la première zone de Brillouin de la reconstruction (√3×√3)R30° avec la direction Γ-KSilicene du silicène<br>We realized the heteroepitaxy of silicene on Si(111) substrates passivated in two ways, either by B or Ag atoms. We deposited Si atoms on the UHV-prepared substrates B:Si(111)-(√3×√3)R30° and Ag/Si(111)-(√3×√3)R30° kept at controlled temperatures. we used LEED, AES, IPES/ARIPES. According to LEED, the adsorption of roughly one monolayer of Si on both substrates leads to the formation of a compact Si bidimensional layer which adopts the (√3×√3)R30° symmetry of the substrates. TCS and the evolution of the work function indicate strong perturbations of the substrate space-charge layer, while IPES reveals the disappearance of the surface states characteristic of each substrate. On Ag/Si(111)-(√3×√3)R30°, the disappearance of the well-known S1 « free-electron-like » surface state induces a metal/insulator transition. Instead of these surface states, new unoccupied electronic states UB and U0 appear which are associated to silicene on each substrate, with dispersions profiles which do not show the characteristics of a (√3×√3)R30° symmetry. Their limited overall bandwidths (resp. ~0.3 and ~0.45 eV) indicate rather large effective masses for electrons and suggest possible correlation effects which could justify the rather large measured bandgaps (resp. 2 eV and ~1 eV). The positions of UB and U0, far from the Fermi level, manifest a non-covalent but sizable interaction silcene/substrate, most probably a charge transfer which may be spatially inhomogeneous. The dispersion profiles of UB and U0 measured by ARIPES are compatible with the symmetry of a silicene layer with the Γ-KSilicene direction oriented along the Γ-M√3 direction of the reconstruction (√3×√3)R30°

This thesis is devoted to the optical properties of low-dimensional structures based on such two-dimensional materials as graphene, silicene and phosphorene. We investigate optical properties of a variety of quasi-one dimensional and quasi-zero-dimensional structures, which are promising for future optoelectronics. Primarily we focus on their low-energy optical properties and how these properties are influenced by the structures’ geometry, external fields, intrinsic strain and edge disorder. As a consequence of this endeavor, we find several interesting effects such as correlation between the optical properties of tubes and ribbons whose periodic and ‘hard wall’ boundary conditions are matched and a universal value of matrix element in narrow-gap tubes and ribbons characterizing probability of transitions across the band gap opened up by intrinsic strain originating from the tube’s surface curvature or ribbon’s edge relaxation. The analytical study of the gapped 2D Dirac materials such as silicene and germanene, which have some similarity to the aforementioned quasi-one-dimensional systems in terms of physical description, reveals a valley- and polarization-dependent selection rules. It was also found that absorption coefficient should change in gapped materials with increasing frequency and become a half of its value for gap edge transitions when the spectrum is linear. Our analysis of the electronic properties of flat clusters of silicene and phosphorene relates the emergence and the number of the peculiar edge states localized at zero energy, so-called zero-energy states, which are know to be of topological origin, to the cluster’s structural characteristics such as shape and size. This allows to predict the presence and the number of such states avoiding complicated topological arguments and provides a recipes for design of metallic and dielectric clusters. We show that zero-energy states are optically active and can be efficiently manipulated by external electric field. However, the edge disorder is important to take into account. We present a new fractal-based methodology to study the effects of the edge disorder which can be applied also to modeling of composite materials. These finding should be useful in design of optoelectronic devices such as tunable emitters and detectors in a wide region of electromagnetic spectrum ranging form the mid-infrared and THz to the optical frequencies.

Après un rappel des différents types de diagraphies existants et leur méthode d'interprétation, on présente des systèmes d'analyse et de tracé de courbes automatiques. Plusieurs programmes automatiques d'étude des faciès et des séquences sédimentaires sont mentionnés. Puis, on applique ces méthodes au cas des faciès hypersiliceux de l'ouzouri et de l'anguille : opalite, chert, grès, silt et argiles siliceux, carbonates siliceux et argiles opalifères. On montre l'intérêt des diagraphies dans la détermination de la qualité des réservoirs potentiels. L'organisation spatio-temporelle des dépôts gabonais est caractéristique d'une mégaséquence d'ouverture océanique

碩士<br>國立中正大學<br>光機電整合工程研究所<br>105<br>Two-dimensional materials have special properties that make them interesting for electronic devices. Because of the many different possible types of two-dimensional materials we believe that combinations of two-dimensional materials could be used to make new and important devices. Therefore, we need a way to produce suitable two-dimensional materials. Unfortunately, such growth requires proper surface energy conditions, which makes certain two-dimensional materials hard to grow. We demonstrate a universal route for growth two-dimensional material. We research the precipitation of silicon between graphene and the substrate, an arrangement that will produce large pressure and change the surface energy. We show that this method allows to change the growth type from three-dimensional to two-dimensional geometry and form two-dimensional siliconfilms. We observe that under non-optimized conditions, silicon atoms do not have enough kinetic energy to crystallize and the silicon films are amorphous. But if we provide a seed we can change the nucleation mechanism and form crystalline fractal films. Those crystalline fractal films exhibit a new photoluminescence peak at 610nm and show novel bonding behavior that suggests the presence of silicene – a sp2-bonded silicon allotrope with exciting applications in optoelectronic devices. Finally, we find that modification of the graphene cover affects the silicene sp2 bond type which highlights the interaction of graphene and silicene.

碩士<br>國立交通大學<br>電子研究所<br>106<br>The tremendous advances of modern semiconductor technology has brought forth extraordinarily scaling down of transistors, while in the meantime it is approaching the bottleneck caused by the short-channel effect. One possible solution in overcoming this is to increase the gate-to-channel control or suppress the drain-side-voltage effect, such as the Fin Field-Effect Transistor. Another way to suppress the short-channel effect is to eliminate the region dominated by the drain-side electric field (v.s that from the gate), e.g. to fabricate a thinner channel, and the thinnest that can possibly be made is a single layer of atoms, the so-called two-dimensional materials. Silicene, compared to the first two-dimensional material graphene, not only has a potential advantage of being highly compatible with traditional silicon-based transistor, but also preserve the super high mobility as graphene. In fact, in 2015, a prototype of the first silicene transistor has been demonstrated to be feasible We perform density-functional calculations of both a stand-alone silicene and the interface with the silicon dioxide. Since a strain less silicene has a gapless Dirac cone, we first calculate the electronic structures of the stand-alone strained silicene, and observe its band gap opened as a function of the applied strains. We perform such calculations using a wide range of exchange-correlation functionals, including not only the standard local-density and generalized-gradient approximations but also hybrid functionals HSE06, PBE0, and B3LYP, and further go beyond density-functional theory using the GW approximation to justify which of the above exchange-correlation functionals outperforms the rest in determining the band gap. Then we place the silicene on a silicon dioxide substrate, expecting the mismatch strain to open a band gap. To avoid chemical bonding at the interface so that the silicene π-bonds and in turn the Dirac-cone high mobility are preserved, we need to carefully choose an appropriate termination atomic layer of the silicon dioxide and introduce additional passivation atoms at the interface. We find the Si-terminated silicon dioxide surface passivated by the hydrogen atoms serves as the optimal substrate to obtain a high-mobility semiconducting silicene on top of it, among all the cases in this study. Further investigation into the interface bonding reveals that the optimal property of the above surface likely results from one bond per passviated atom. This suggest that further experiments in fabricating silicene on a silicon dioxide surface may consider passivation treatments using hydrogen, halogens, or single-dangling-bond functional groups like hydroxides. Direct calculations of the mobility of the strain-free, strained but stand-alone, and on-substrate silicenes show that the strained one preserves the same order of magnitude for the mobility, and that of the on-substrate is estimated to be lowered by one order. All our computational findings can provide helpful preliminary guidance in developing transistor based on two-dimensional materials that is feasible in the industrial product lines.

碩士<br>國立交通大學<br>電子研究所<br>106<br>As the traditional silicon-based metal oxide semiconductor field-effect transistors approaching its miniaturization limit, search of alternative ultra-thin channel materials becomes an urgent issue. Silicon can form a two-dimensional single atomic layer, called “Silicene”. It has a graphene-like structure as well as in theoretical calculations both superhigh carrier mobility and controllable energy gap. These make it an ideal ultra- thin channel material. Moreover, with elements same as the present silicon electronic devices, it has a great advantage of zero interfacial contamination when being integrated towards the silicon-bulk leads. It is important to know how to grow silicene on an insulating substrate in the developments of silicene-based devices. In this work we choose the high dielectric material hafnium oxide to be the substrate and perform first-principles calculations at middle and high levels of approximation to crosscheck, where the high level refers to the GW approximation and the middle to the local density approximation, generalized gradient approximation, and hybrid functionals. We choose the hafnium oxide(111) surface to be substrate and look for its particular surface termination atomic layer that can preserve silicene’s superhigh mobility and open up a reasonable bandgap. We expect that this computational study can become the foundation of realizing an on-market transistor with a two-dimensional channel.

碩士<br>國立中興大學<br>物理學系所<br>103<br>In this thesis, we study mechanical properties of silicon using Molecular dynamics (MD) simulation. The potential energy, atom dynamics, simulation steps in our study are considered. The structure and analysis of different types of silicone are presented. Finally the mechanics for surface stress and strain are proposed.

碩士<br>國立成功大學<br>物理學系<br>103<br>We calculate the energy dispersions and the electronic excitations of doped monolayer silicene , germanium and graphene by using the generlized tight-binding model and self-consistent theorem. Monolayer silicene and germanium , with the slightly buckled honeycomb geometric structures , have a small band gap which are opened by spin-orbital effect. On the other hand , monolayer graphene possesses the flat honeycomb structure with zero-gap energy bands. The features of excitation spectra are dominated by the Fermi energy , the band structure and the transferred momentum, doping would induce free carriers, which further cause the intraband single-particle and collective excitations. Moreover , the plasmons (collective excitations) are strongly dependent in the free carriers.

碩士<br>國立交通大學<br>電子研究所<br>106<br>As the state-of-the-art technology in semiconductor industry drives devices toward remarkably tiny dimension, the traditional Si-based MOSFETs will soon approach their scaling limits. An emerging development in this field is to find a replacement for the Si channel. A two-dimensional material consists of only a single atomic layer, being a good candidate of the MOSFET channel upon the ongoing scaling down. A two-dimensional silicon allotrope, silicene has its great advantage in being significantly compatible with the Si-based process. Silicene has a mobility two order of magnitude higher than bulk semiconductors and is expected to maintain a satisfactory device on-current. Yet there are also two major challenges to make it a feasible device. A stress-free stand-alone silicene, like graphene, has a gapless Dirac-cone band dispersion and needs additional treatments to open its gap. Besides, the silicene π bonds are chemically active, making it unstable in the air. In this thesis, I perform first-principles calculations to look for the strain-induced energy gap of silicene and how its electronic structures, especially the mobility, are affected when being sandwiched between dielectric materials. I find the energy gap is opened up to 0.17eV among the strains I have applied. We also calculate the electron and hole mobility of silicene under a particular strain to be 16.6 and 14.4m2V-1s-1, respectively, preserving the same order of magnitude of the free-standing mobility. I choose ZrO2/silicene/ZrO2 to be my model sandwich structure, where ZrO2 is a widely used high-k material in conventional semiconductor devices. I have considered both unpassivated and H-passivated interfaces of the above sandwich structure, and find that the former has no Dirac cone while the latter contains a Dirac cone and have indirect gap closing. I further apply a strain to the silicene in the H-passivated case such that the silicene restores its stand-alone symmetry, and find that the gap is re-opened. The symmetry-breaking mechanism of the unstrained (relaxed) silicene in a H-passivated sandwich is very likely due to the electrostatic charge that is transferred from ZrO2 to silicene, as calculated by the Bader analysis. In summary, my first-principles calculations show that silicene can have strain-induced gaps, and the ZrO2/silicene/ZrO2 sandwich structure needs both the H passivation (preserving the π bonds) and the silicene-symmetry restoration to open the gap. The mobility is also calculated from first principles and are found to maintain a high value in the order 10m2V-1s-1. These results provide preliminary guidance to develop silicene-based transistors.

博士<br>國立成功大學<br>物理學系<br>107<br>Geometric, magnetic, and electronic properties of graphene- and silicene-related systems are investigated by the first-principles theoretical framework, including the adatom-diversified geometric structure, atom-dominated energy bands, spatial spin density distributions, spatial charge density distributions and its variations, and spin- and orbital-projected density of states (DOSs). Such physical quantities are sufficient to identify the critical chemical bondings. The essential properties are very sensitive to adatom concentration, adatom distributions, doping positions, various kinds of adatoms. The pristine monolayer structure can be deformed, buckled, non-hexagonal, and planar after the adatom adsorption and substitution. The critical orbital hybridizations in the C-adatom bondings, the Si-adatom bondings, the finite-size confinements, and the edge structure directly determine the semiconducting, semi-metallic, and metallic behaviors. The diverse spin-dependent electronic properties cover the non-magnetic, ferromagnetic, and anti-ferromagnetic metals, the non-magnetic semiconductors, and the anti-ferromagnetic semiconductors with/without spin splitting. The developed first-principles theoretical framework can fully be generalized to other 2D layered systems.

碩士<br>國立臺灣師範大學<br>物理學系<br>104<br>After depositing silver on Si (111)-(7×7) by k-cell at 100K and annealing the substrate to 300℃，we got the flatten silver films on silicon substrate. We grow silicene on these silver films at different temperature and found four different types of silicene including 4×4、√13×√13-1、√13×√13-2、2√3×2√3 structure by using scanning tunneling microscope . We found that at higher depositing temperature the rate of 4×4 structure would increase .When we deposit more than 1 ML Si atoms，part of Si atoms will form second layer silicene and 2×2 arrangement. On the other hand，we found that silver films have dislocations by low energy electron diffraction. Dislocations cause the change of silicene arrangement .The region of arrangement change have obvious boundary or spots absence .

博士<br>國立清華大學<br>物理系<br>103<br>Two-dimensional (2D) materials such as graphene exhibit peculiar and attractive properties because of the unique symmetry of honeycomb π-orbital network. A graphene-like 2D material consists of Si atom is referred to as silicene. However, since a silicene has been experimentally synthesized on Ag(111) surface, the overlayers superstructure are described as either the unit vectors of silicene based or the primitive cell vectors of Ag(111) lattice in previous studies. We conducted the low-temperature scanning tunneling microscopy (LT-STM), X-ray core-level photoemission spectroscopy (XPS), and performed ab-initio calculations based on density functional theory (DFT) to investigate the growth processes and structural evolutions of silicene on Ag(111) surface. The results confirm the existence of various overlayer structures reported previously and that the lattice parameters of several overlayer structures at sub-monolayer coverage are in some degree related to the corresponding substrate supercells. In the early stage of silicene growth on silver terrace at 230 °C, the adsorbed silicon atoms are incorporated into the upper steps and form at each edge a stripe that consist of precursor structures. The stripe areas expand into domains apparently by further ejections of Ag atoms at the upper terraces. Moreover, the detailed analysis of silicene domain boundaries and the Moiré-like superstructures in the STM images strongly suggest that the overlayer silicene sheets and silver substrate lattices have no coincident relationships. In order to clarify our supposition, first-principles calculations based on DFT show that the barrier of binding energy for the translational displacement of silicene on Ag(111) is about 0.02–0.06 eV per Si atom. The thermal energy has big possibility to overcome the energy barrier at 230 °C. In addition, the core-level photoemission spectra show apparent binding energy shifts for the sub-monolayer and multilayer silicene sheets, providing another evidences to confirm the growth evolution. This dissertation is organized into six chapters. In chapter 1, we described the background and motivations of this research firstly, and then followed by the view of major progress, findings, and challenges in the reports of silicene field recently. Chapter 2 describes the technique methods and operations of experimental apparatus, such as UHV, LT-STM, and XPS. The information of substrate and evaporation source is also presented in this chapter. Furthermore, the theoretical calculation based on DFT, including the principle, process, and applications are introduced in chapter 3 briefly. Then, the detailed discussions and results of the free-standing silicene and silicene growth upon Ag(111) substrate are described in chapter 4 and 5, respectively. In order to clarify the structure and growth evolution of silicene growth on Ag(111), we not only conducted STM and XPS experiments but also performed first-principles calculations based on DFT by VASP to investigate the various configurations. Finally, the two main results in chapter 6, the structure and phase transition depend on different coverages and the non-coincidence relationship between overlayer silicene and silver substrate are summarized clearly in the end.

博士<br>國立高雄應用科技大學<br>機械工程系<br>105<br>Molecular dynamics simulation (MD) is used to investigate the properties of the curved and planar monolayer graphene nanoribbons (GNRs) under boundary confinement. The wrinkles are formed in curved-armchair GNRs not in curved-zigzag GNRs. With temperatures of 300–900 K and curvature radii of 3–6 nm, the wavelength of wrinkles did not change obviously. In nanoindentation tests, the zigzag graphene comparing to the armchair graphene had the larger contact stiffness about 50%. Further the concave graphene comparing to the convex graphene showed the higher contact stiffness due to its higher Young's modulus. The nanomechanical properties of graphene under nanoindentation are studied using molecular dynamics simulations based on the Tersoff–Brenner many-body potential and Lennard-Jones potential. The effects of the indentation temperature, indentation velocity, and indenter size are evaluated in terms of atomic trajectories, deformation velocity, indentation force, and strain field. The simulation results show that graphene deformation increases with increasing indentation depth, indentation velocity, temperature, and indenter size. During the holding process, a slight deformation between the center and the edges of the graphene remains due to relaxation, which increases with increasing temperature and indentation velocity. Cracks easily form with high-velocity indentation due to large strain energy accumulation in the material. The area of the deformation region increases with decreasing indentation velocity. The mechanical properties of silicene nanostructures subject to tensile loading were studied via a molecular dynamics simulation. The effects of temperature on Young’s modulus and the fracture strain of silicene with armchair and zigzag types were examined. The maximum in-plane stress and the corresponding critical strain of the armchair and the zigzag silicene sheets at 300 K were 8.85 and 10.62 N/m, and 0.187 and 0.244, respectively. The in-plane stresses of the silicene sheet in the armchair direction at the temperatures of 300, 400, 500, and 600 K were 8.85, 8.50, 8.26, and 7.79 N/m, respectively. The in-plane stresses of the silicene sheet in the zigzag direction at the temperatures of 300, 400, 500, and 600 K were 10.62, 9.92, 9.64, and 9.27 N/m, respectively. The improved mechanical properties can be calculated in a silicene sheet yielded in the zigzag direction compared with the tensile loading in the armchair direction. The wrinklons and waves were observed at the shear band across the center zone of the silicene sheet. These results provide useful information about the mechanical and fracture behaviors of silicene for engineering applications.

碩士<br>國立高雄應用科技大學<br>光電與通訊工程研究所<br>104<br>We investigate hydrogen storage on metal adatoms-decorated silicene used by the simulation tool of Dmol3 of the First-principles method. We select the metal which are common alkali metal (Li、Na、K) and alkaline earth metal (Be、Mg、Ca) for metal adatoms-decorated silicene. In this study, the structures of metal adatoms-decorated silicene, are called the structures of "Silicene-X", and the X is metal atoms (ex:Silicene-Li). The type of hydrogen adsorption is classified physical adsorption for the Silicene-X structure is classified physical adsorption since it has small average adsorption energies (0.1~0.24eV) and doesn't change the properties of the hydrogen molecules. We study the “Silicene-X” structures for hydrogen storage and calculate their gravimetric density of hydrogen storage. Compared with the gravimetric density 5.5wt% of hydrogen storage standard value setting by the US Department of Energy (DOE) in 2017, and found which the Silicene-X structures are good hydrogen storage materials. Finally, we will discuss hydrogen storage of the “Silicene-X” structures using by heating desorption. The structure of this thesis is divided into the third parts: The first part is to study the best adsorption sites of metal adatoms-decorated silicene. In second part, we use the best adsorption site of Silicene-X from the results of the first part, and then study the adsorption behavier of hydrogen molecules for the Silicene-X structures. In third part, we study the structures of Silicene-X which had adsorbed one hydrogen molecule, to investigate their heating desorption of hydrogen molecules. Our results demonstrated that the gravimetric densities of hydrogen storage in the Silicene-Na (6.863wt%), Silicene-K (7.208wt%), Silicene-Mg (5.903wt%) and Silicene-Ca (6.337wt%) structures are higher than the ones of the DOE’s standard value 5.5wt%. We found that in Silicene-Li, Silicene-Na, Silicene-K and Silicene-Ca structures there are metal-atom-drift problem in the process of heating desorption, but the metal atom only drifted about 0.26Å at 500K. Therefore, this problem doesn't affect adsorption and desorption of hydrogen molecules in Silicene-X structures. In Silicene-Be and Silicene-Mg, there are no metal-atom-drift problem at 500K in the process of heating desorption. In summary, the gravimetric densities of hydrogen storage in Silicene-Na, Silicene-K, Silicene-Mg and Silicene-Ca are more than the DOE’s standard value (the gravimetric densities of hydrogen storage 5.5wt%), are 6.863wt%, 7.208wt%, 5.903wt% and 6.337wt%, respectively. In addition we found Silicene-Na, Silicene-K, Silicene-Mg and Silicene-Ca are also suitable for heating to desorption of hydrogen molecules. Therefore Silicene-Na, Silicene-K, Silicene-Mg and Silicene-Ca are good hydrogen storage materials which are applicable for (desorption) adsorption of hydrogen molecules.

博士<br>國立臺灣師範大學<br>物理學系<br>104<br>Part I Silicene growth Using scanning tunneling microscopy (STM), we studied the formation of Si monolayer grown on (√3×√3)R30° Ag-Ge(111) and Ag-Si(111) reconstructed surface, respectively. Thereafter, we also increase Ag thickness, where is formed 6~12 ML Ag(111) layer, to grow silicene. On √3Ag-Si(111), deposited Si exchange with Ag atom to form the new √3Ag-Si(111) islands without forming new Si monolayer. On √3Ag-Ge(111), the Ag-Si exchange behavior is suppressed by stable bonding of Ag and below Ge(111) substrate. We measure the isolated Si monolayer with mixing √3×√3 and 2×2 superstructures on the top layer. From the demonstrated ball model, the Si monolayer found in this study is very possible to consist of honeycomb structure. On Ag(111)/Si(111) surface, we have measured classical silicene superstructures, such as 4×4, √13×√13-I, 2√3×2√3. Unlike growing on the single crystal Ag(111), where the discontinuous silicene sheet formed between various superstructures, the continuous silicene sheet formed between various superstructures on 6~10 ML Ag(111)/Si(111) by Ag domain rotation and shift because of low Ag(111) unstable on Si(111) substrate. Part II Iron growth In this part, we investigate Fe growth on various substrate, which are Fe/Si(111), Fe/√3Ag/Si(111), Fe/Ge(111), Fe/√3Ag/Ge(111), Fe/MoS2, and Pd/(Fe/Pd/C60)x/Au/Al2O3 system. In Fe/Si(111), we measured the thermal evolution of Fe-silicide from γ- FeSi2 to β- FeSi2, and -FeSi2 at the highest temperature by STM. In particular, the growth of β-FeSi2(011)//Si(111) is different with previous Fe-silicide studies. Besides, the isolated √3Ag-Si buffer layer causes the formation of -FeSi without appearing on Si(111) substrate at the same temperature. In Fe/Ge(111), the deposited Fe formed Fe-Ge clusters at RT. When Fe ratio is increased by increasing temperature, Fe-Ge clusters evolve into (2×2) islands, which alloy ratio between Fe1.5Ge to Fe2Ge. In addition, 3D islands forming at 640 K is considered as FeGe monoclinic structure. Besides, the number of √19 ring cluster defect increase to break the order c(2x8) reconstruction by increasing the temperature and disappeared at 640 K. With Ag buffer layer, only nanoparticle growth occurred and 3D islands were formed early at 570 K. In Fe/MoS2, we measured the surface morphology, magnetism and chemical states of Fe/MoS2 by STM, MOKE, and XPS, respectively. Fe deposition on the MoS2 substrate resulted in a nanoparticle array with the particle size ranged a few nanometer (). For low-coverage Fe deposition < 6 ML, nanoparticles were well-separated and long-range magnetic anisotropy was absent at room temperature. When the Fe coverage increased, in-plane magnetic anisotropy was observed and the magnetic coercivity increased monotonically. The depth-profiling XPS measurement of Pd/2 ML Fe/MoS2 also confirmed the dominance of the pure Fe state at the interface. The increase in Fe coverage changed the morphology from a nanoparticle array to a continuous coverage, leading to the onset of the ferromagnetic ordering and the transition from a continuous surface oxidation to a bilayer structure. At last, we report on the hybridization-induced large X-ray magnetic circular dichroism (XMCD) of carbon in Pd-Fe-C60 composite thin films. The samples were prepared by repeating sequential deposition of C60, Fe and Pd for five times on Au/Al2O3(0001) substrate in an ultrahigh vacuum (UHV) chamber. The Pd-Fe-C60 composite thin films were investigated by MOKE, XMCD, and Raman spectroscopy. The composite thin films revealed in-plane anisotropy. After annealing the sample at 527 K, the Kerr signal became weak and the magnetic coercivity was decreased. Then, considerable XMCD signal was observed at the carbon K-edge, but relatively small XMCD signal appeared at Fe L2,3-edge. In contrast to the XMCD spectrum of mixing transition metal and C60 system in previous study, we observed that the carbon will induce strong XMCD signal. These observations indicate the hybridization-induced magnetic moment in carbon and possible reduction of magnetization in Fe.

碩士<br>國立成功大學<br>物理學系<br>104<br>Despite the popularity of grapheme research in recent years, silicon is still the main material used in current semiconductor industries. From a more industrial point of view, the research on silicon based nanostructure paves a more practical development path for doth compatibility and economic reasons. The electronic structures of ASiNRs adsorbed with alkali metals and halogen elements are studied using the density functional theory based on screened exchange local density approximation method. The most preferable adsorption sites are found to be hollow and top sites for alkali metals and halogen elements, respectively. As the ASiNRs remains completely flat, the maximum concentration of adsorption can reach 16.67%. All the relaxed systems with adsorbed atoms exhibit metallic or semiconducting behavior with strongly bonded Li, Na, K, Rb or F, Cl, Br, I atoms accompanied by an appreciable electron transfer from the Li, Na, K, Rb adatom to silicene nanoribbons or from the silicene nanoribbons to the F, Cl, Br, I. There are two types of charge carriers for the purpose of conducting. For the system with the alkali metals, the electrons work as the charge carrier. As to the system with halogen elements, the charge carriers are the electron holes. From the study on charge transfer, the adatoms can mostly bond with silicon by ionic bonds. Evidently, the valence bonds have their minor contributions due to the overlapping of orbital. Keywords: alkali、halogen、ASiNRs

碩士<br>國立成功大學<br>物理學系<br>104<br>Graphene, silicene, and group IV graphene-related systems have already been very popular subjects for quite some time. Especially, it has unique structural, electronic properties and chemical properties. In this thesis, we use the first principle calculations to study the properties and simulate the electron density distribution of crystal by VASP. The study focuses on aluminum-adsorbed silicene, considering the case of extreme symmetric. Geometric structures, electronic Structures, charge distributions, and density of states (DOS) strongly depend on the different configurations and the various concentrations of aluminum adatoms. We research the adsorption of atoms and trend of bondings which can be remarkably modulated by Al-concentrations, investigating serious destruction of electronic structures with complex orbital hybridizations. On the other hand, the use of electron density distribution and charge transfer can clarify the operation of bond energy. The orbital-projected density of state (PDOS) can analyze the band structures in detail. The DOS and spatial charge distributions clearly indicate that the special bonds in Si-Si and Al-Si are responsible for the diversified properties.

碩士<br>國立高雄應用科技大學<br>機械工程系<br>106<br>This study investigated the mechanical properties and crack growth behavior of silicene using molecular dynamics simulation based on the effects of crack direction, crack length and temperature. The simulation results showed that the young's modulus does not change much ,when the length of the crack increases, but increases with the increase of the crack angle. When the temperature is increased to 500 K, the young's modulus will have a low value, mainly due to softening of the material at high temperatures. Comparing the armchair and the zigzag type, it can be seen that the zigzag type structure has a higher tensile strength, and the young's modulus is also higher than the armchair structure. Under shear load, the wrinkle height increases with the increase of the crack size, and when the temperature is 200 K, the wrinkle height will have the largest drop, and the break strain will decrease with the increase of temperature. When the silyne film is tensile, the young's modulus decreases with increasing temperature. When the silyne chain length increases, the young's modulus will decrease, and the zigzag structure will be stronger than the armchair structure. The increase in the length of the silyne chain makes the original hole become larger but its ductility becomes better, resulting in a point where the material does not break.

自從2004年人們在實驗室上發現石墨烯以來，IV族二維半導體材料，例如石墨烯、硅烯等，由於其優異的電學、力學、光學、以及熱力學性質，受到學術界的廣泛關注。為了使IV族二維半導體材料得到廣泛引用，穩定地生長高質量的石墨烯、硅烯二維半導體材料以及透徹的理解石墨烯、硅烯二維半導體材料和襯底之間的界面特性成為至關重要的研究方向。本文對在銅表面用多環芳香烴形成石墨烯的生長機理以及石墨烯、硅烯和襯底之間的界面電子學特性進行了詳細的分析和研究。希望以此能對IV族二維半導體材料的廣泛應用具有促進作用，並且對合理的設計電子器件結構具有新的啟示。<br>首先，我們用密度泛函理論對在銅表面用多環芳香烴形成石墨烯的生長機理進行了研究。理論計算表明在銅表面多環芳香烴形成石墨烯的生長過程主要包括：（1）在銅表面的誘導下多環芳香烴脫氫，（2）這些已經脫氫的多環芳香烴在銅表面相互結合形成石墨烯。由於銅和碳的相互作用非常弱，所以在銅表面這些已經脫氫的多環芳香烴並不會進一步分解成更小的碳團簇或者單個的碳原子。因此多環芳香烴的空間幾何構型對於最終形成的石墨烯的質量以及最低成長溫度有至關重要的影響。提高生長溫度可以提升脫氫多環芳香烴的活性和熱運動性，從而提高最終生成的石墨烯的質量。六苯并苯由於具有和石墨烯相同的六重對稱性和晶格結構，所以其在低溫生長高質量石墨烯方面最具有優勢。<br>其次，我們就石墨烯和（0001）二氧化硅表面所組成的界面的電子學特性進行了研究。結果表明石墨烯在（0001）二氧化硅表面的電子學特性主要有二氧化硅表面的性質以及氫化程度決定。如果用末端為甲基的分子修飾（0001）二氧化硅表面，可以進一步減弱二氧化硅表面氧原子對石墨烯電子學特性的影響，從而提高在二氧化硅表面石墨烯的載流子遷移率。此外，當石墨烯物理吸附在二氧化硅表面上時，垂直於石墨烯和二氧化硅界面的外加電場可以調製石墨烯和二氧化硅表面的電荷轉移。這一效應可以增強雙層石墨烯之間的電場，從而有效改變雙層石墨烯的能帶結構。我們的結果有助於更好的地認識和理解石墨烯吸附在二氧化硅表面所表現的實驗現象。<br>基於以上兩個結論，我們用三亚苯合成了高質量的單層石墨烯，並對其在普通二氧化硅表面上以及十八烷基鏈三甲氧基硅烷所修飾的二氧化硅表面上，所體現出的不同電子學性質和散射機理進行了詳細研究。用三亚苯作為石墨烯的生長源可以避免傳統氣象化學沉積方法在初期成核過程中所產生的缺陷，從而得到高質量的石墨烯。電學測量表明，石墨烯在普通二氧化硅表面上的載流子遷移率約為5090 cm²V⁻¹s⁻¹。而在十八烷基鏈三甲氧基硅烷所修飾的二氧化硅表面上，其遷移率可以提高到大約9080 cm²V⁻¹s⁻¹。此外，通過這兩種不同結構的電子器件進行定量的分析和對比，我們發現在室溫下，普通二氧化硅表面上的石墨烯電子器件的平均自由程主要由電離雜質所引起的長程散射所決定，電離雜質散射源密度約為5.34×10¹¹ cm⁻²。而對於十八烷基鏈三甲氧基硅烷所修飾的二氧化硅表面上的石墨烯電子器件的平均自由程主要由甲基以及石墨烯中的缺陷和晶界所引起的共振散射所決定，共振散射源密度為9.77×10¹° cm⁻²。我們的研究結果有助於揭示通過界面修飾來提升石墨烯電子器件性能的內在原理。<br>最後， 我們對單層石墨烯和硅烯封裝在金剛石薄膜和硅薄膜結構的電子學性質，以及其隨壓強的變化，進行了系統的理論研究。結果表明，當單層石墨烯和硅烯封裝在金剛石薄膜和硅薄膜中時，通過改變壓強和堆疊結構，單層石墨烯和硅烯在狄拉克點處的能隙和電子有效質量可以被有效地調製。電子有效質量和壓強成正比。硅烯的能隙對於壓強的變化比石墨烯更加敏感。並且異質封裝結構比同質封裝結構更有利於調製石墨烯和硅烯在狄拉克點處的能隙和電子有效質量。利用封裝技術和改變壓強的方法，石墨烯和硅烯的蜂窩狀結構不會被破壞，所以其小的載流子有效質量和高的載流子遷移率將會保持。所以對於構造高性能的納米電子學器件，這種方法有明顯的應用前景。<br>Group IV two Dimensional Semiconductor Materials, such as graphene, silicene and so on, composed of an atomically thin layer of carbon and silicon atoms arranged in a honeycomb lattice, have received considerable attention, as their extraordinary electronic, mechanical, optical, and thermal properties arise from their unique 2D energy dispersions, since their representive, graphene, experimentally discovered in 2004. Reliable fabrication of high-quality graphene and silicene two dimensional layers and understanding the properties of interface between graphene or silicene two dimensional layers and substrates play an indispensable role for realizing their potential applications in nanoelectronics. This thesis attempts to paint a clear picture about the growth mechanism of graphene from Polycyclic aromatic hydrocarbons (PAHs) on Cu(111) surface and interfacial electronic properties of graphene and silicene to promote application of Two Dimensional Group IV Semiconductor and shed light on rational design of functional devices.<br>Firstly, in order to obtain insights into the reaction mechanism, the bottom-up growth of graphene from PAHs on Cu(111) surface has been systematically analyzed by means of large-scale ab initio simulation in a density functional theory (DFT) framework. Theoretical calculation shows that the underlying growth mechanism, which mainly involves surface-mediated nucleation process of dehydrogenated PAHs rather than segregation or precipitation process of small carbon clusters decomposed from the precursors. The quality of the synthesized graphene sheets and minimum growth temperature strongly depends on the structures of PAHs as well as the molecular activities. Increasing the growth temperature will augment the activity of carbon clusters, so as to increase the probability in formation of prefect graphene sheets. Coronene, having 6-fold rotational symmetry and the same lattice as graphene, has the highest probability in forming high quality graphene, especially at relatively low growth temperature.<br>Secondly, the electronic properties of graphene supported by (0001) SiO₂ surface are theoretically studied using the density functional theory. It is found that the electronic attributes of graphene on (0001) SiO₂ strongly depend on the underlying SiO₂ surface properties and the percentage of hydrogen-passivation. By applying methyl to passivate oxygen-terminated (0001) SiO₂ surface one can further reduce the interaction between the graphene sheet and oxygen-terminated surface. This can improve the charge carrier mobility of graphene supported by SiO₂ substrate and reduce the influence by residual interfacial molecules. In addition, the external electric field modulates the charge transfer between graphene and the SiO₂ surface, when graphene layers are physisorbed on the oxide surface. This phenomenon will enhance the built-in electric field of bilayer graphene so as to effectively modify its band structure. Our results shed light on a better atomistic understanding of the recent experiments on graphene supported by SiO₂.<br>Based on the above two conclusions, the graphene/substrate interface properties and engineering of bottom-gated, large-scale triphenylene-derived graphene transistors by applying octadecyltrimethoxysilane (OTMS) self-assembled monolayers (SAM) onto the gate dielectric surface are studied. To meet the challenge that the isolated carbon monomers are likely to form defective carbon clusters with pentagons, at the initial stage of CVD graphene growth, triphenylene (C₁₈H₁₂) (pentagon-free with only C and H) was used as the solid precursor for high-quality and large-scale graphene synthesis. Transport measurements performed on back-gated graphene field-effect transistors (GFETs) with large channel lengths (~25 μm) show a carrier mobility up to ~5090 cm²V⁻¹s⁻¹ on SiO₂/Si substrate at room temperature under vacuum. Furthermore we show that in virtue of the ultrasmooth SAM surface and reduced interfacial impurity scattering as well as attenuated surface polar phonon scattering, the GFET carrier mobility on octadecyltrimethoxysilane (OTMS) passiviated SiO₂ surface is consistently improved up to ~9080 cm²V⁻¹s⁻¹, whose graphene active layer has been grown with triphenylene precursor. This makes it promising for practical applications. In addition, in comparison with the devices without interface engineering, triphenylene-derived GFETs with OTMS-SAM modified SiO₂/Si substrate exhibit the marked carrier-density-dependent field-effect mobility. Quantitative analyses reveal that at ambient temperature, the predominant scattering sources affect the carrier mean free path for graphene devices on bare SiO₂ substrates and for those on OTMS passivated SiO₂ substrates are charged impurity induced long-range scattering (~5.34×10¹¹ cm⁻² in carrier density) and resonant scattering (short-range scattering ~9.77×10¹° cm⁻² carrier in density), respectively. Our findings elucidate the underlying dominate factors for achieving the significantly improved device performance of GFETs at room temperature.<br>Finally, by exploiting first-principles calculations, we show that the band gap and electron effective mass (EEM) of various confined graphene and silicene (D-X/G/H-D, Si-X/S/H-Si and D-X/S/H-D) can be effectively modulated by tuning the pressure (interlayer spacing) and stacking arrangement. The electron effective mass (EEM) is proportional to the band gap. The band gap of confined silicene is more sensitive to pressure than that of confined graphene. Moreover, heterogeneous interface would be beneficial to effectively control the band gap and carrier effective masses of confined graphene and silicene. Using the confined technique and pressure, the integrity of the honeycomb structure of graphene and silicene will be preserved, so the small effective masses and high mobility of graphene and silicene will remain during compression. The tunable band gap and high carrier mobility of the sandwich structures are promising for building high-performance nanodevices.<br>The aforementioned four sub-topics form the mechanistic understanding of graphene growth by PAHs and interfacial electronic properties of graphene and silicene down to the molecular level.<br>Detailed summary in vernacular field only.<br>Detailed summary in vernacular field only.<br>Detailed summary in vernacular field only.<br>Detailed summary in vernacular field only.<br>Detailed summary in vernacular field only.<br>Chen, Kun.<br>Thesis (Ph.D.)--Chinese University of Hong Kong, 2013.<br>Includes bibliographical references.<br>Abstracts also in Chinese.<br>Abstract --- p.II<br>博士學位論文摘要： --- p.VI<br>Acknowledgements --- p.X<br>Chapter Chapter 1 --- Introduction to Growth Methods and Electronic Properties of Graphene and Silicene --- p.1<br>Chapter 1.1 --- Electronic Properties of Graphene --- p.2<br>Chapter 1.1.1 --- The Direct Lattice and the Reciprocal Lattice --- p.2<br>Chapter 1.1.2 --- Electronic Band Structure --- p.6<br>Chapter 1.1.3 --- Tight-Binding Energy Dispersion --- p.7<br>Chapter 1.1.4 --- Massless Dirac Fermions --- p.15<br>Chapter 1.1.5 --- Carrier Density and Effective Mass --- p.21<br>Chapter 1.1.6 --- The Tight-Binding Model of Bilayer Graphene --- p.24<br>Chapter 1.1.7 --- The Two-Component Hamiltonian of Bilayer Graphene --- p.29<br>Chapter 1.1.8 --- Trigonal Warping in Graphene --- p.32<br>Chapter 1.1.9 --- Tunable Band Gap in Bilayer Graphene --- p.36<br>Chapter 1.2 --- Synthesis of Graphene --- p.38<br>Chapter 1.2.1 --- Exfoliation and Cleavage --- p.39<br>Chapter 1.2.2 --- Thermal Decomposition of SiC --- p.40<br>Chapter 1.2.3 --- Chemical Vapor Deposition of Graphene --- p.42<br>Chapter 1.3 --- Electronic Properties at Graphene/Substrate Interface --- p.55<br>Chapter 1.3.1 --- Graphene on SiO₂/Si Substrates --- p.56<br>Chapter 1.3.2 --- Graphene on Hexagonal Boron Nitride (h-BN) --- p.60<br>Chapter 1.3.3 --- Graphene on Organic Self-Assembled Monolayer (SAM) Passivation of Bared SiO₂/Si --- p.61<br>Chapter 1.4 --- Synthesis and Electronic Properties of Silicene --- p.63<br>Chapter 1.4.1 --- Synthesis of Silicene --- p.64<br>Chapter 1.4.2 --- Electronic Properties of Silicene --- p.65<br>Chapter References --- p.67<br>Chapter Chapter 2 --- Introduction to Density Functional Theory --- p.75<br>Chapter 2.1 --- Many-Particle Hamiltonian --- p.75<br>Chapter 2.2 --- Born-Oppenheimer Approximation --- p.76<br>Chapter 2.3 --- Hartree-Fock Method --- p.77<br>Chapter 2.4 --- Density Functional Theory (DFT) --- p.77<br>Chapter 2.4.1 --- Hohenberg-Kohn Theorems --- p.77<br>Chapter 2.4.2 --- Kohn-Sham Method --- p.79<br>Chapter 2.4.3 --- Kohn-Sham Equation --- p.80<br>Chapter 2.4.4 --- Solution of Kohn-Sham Equation --- p.80<br>Chapter 2.5 --- Electron Density Approximation --- p.80<br>Chapter 2.5.1 --- Local Density Approximation (LDA) --- p.80<br>Chapter 2.5.2 --- Generalized Gradient Approximation (GGA) --- p.82<br>Chapter 2.5.3 --- Hybrid Functionals --- p.82<br>Chapter 2.6 --- Plane Waves Expansion --- p.83<br>Chapter 2.7 --- Pseudopotentials --- p.84<br>Chapter 2.7.1 --- Ultrasoft Pseudopotentials (USPP) --- p.86<br>Chapter 2.7.2 --- Projector Augmented Wave Potentials (PAW) --- p.87<br>Chapter 2.8 --- DFT+U --- p.88<br>Chapter References --- p.89<br>Chapter Chapter 3 --- ab initio Study of Growth Mechanism of Graphene from Polycyclic Aromatic Hydrocarbons --- p.91<br>Chapter 3.1 --- Introduction --- p.91<br>Chapter 3.2 --- Experimental Results --- p.93<br>Chapter 3.3 --- Calculation Method --- p.94<br>Chapter 3.4 --- Calculation Results and Discussion --- p.96<br>Chapter 3.5 --- Conclusion --- p.109<br>Chapter References --- p.109<br>Chapter Chapter 4 --- Electronic Properties of Graphene Altered by Substrate Surface Chemistry and Externally Applied Electric Field --- p.113<br>Chapter 4.1 --- Introduction --- p.113<br>Chapter 4.2 --- Calculation Method --- p.115<br>Chapter 4.3 --- Results and Discussion --- p.116<br>Chapter 4.4 --- Conclusions --- p.133<br>Chapter References --- p.134<br>Chapter Chapter 5 --- High Performance Devices Based on Large-Scale Triphenylene Derived Graphene and Interface Engineering --- p.138<br>Chapter 5.1 --- Introduction --- p.138<br>Chapter 5.2 --- Experimental Section --- p.140<br>Chapter 5.3 --- Results and Discussion --- p.144<br>Chapter 5.4 --- Conclusion --- p.163<br>Chapter References --- p.164<br>Chapter Chapter 6 --- Controllable Modulation of Electronic Properties of Graphene and Silicene by Interface Engineering and Pressure --- p.169<br>Chapter 6.1 --- Introduction --- p.169<br>Chapter 6.2 --- Modeling and Methods --- p.171<br>Chapter 6.3 --- Results and Discussion --- p.174<br>Chapter 6.4 --- Conclutions --- p.200<br>Chapter References --- p.201<br>Chapter Chapter 7 --- Conclusions and Future Plans --- p.204<br>Chapter 7.1 --- Conclusions --- p.204<br>Chapter 7.2 --- Future Plans --- p.206<br>List of Publications during Ph.D. Study --- p.207

This thesis covers the structural, electronic, magnetic, and vibrational properties of graphene and silicene. In Chapter I, we will start with an introduction to graphene and silicene. In Chapter II, we will briefly discuss about the methodology (i. e. density functional theory)In Chapter III, we will introduce band gap opening in graphene either by introducing defects/doping or by creating superlattices with h-BN substrate. In Chapter IV, we will focus on the structural and electronic properties of K and Ge-intercalated graphene on SiC(0001). In addition, the enhancement of the superconducting transition temperature in Li-decorated graphene supported by h-BN substrate will be discussed. In Chapter V, we will discuss the vibrational properties of free-standing silicene. In addition, superlattices of silicene with h-BN as well as the phase transition in silicene by applying an external electric field will be discussed. The electronic and magnetic properties transition metal decorated silicene will be discussed, in particular the realization of the quantum anomalous Hall effect will be addressed. Furthermore, the structural, electronic, and magnetic properties of Mn decorated silicene supported by h-BN substrate will be discussed. The conclusion is included in Chapters VI. Finally, we will end with references and a list of publications for this thesis.