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Connors, Benjamin James. "Simulation of current crowding mitigation in GaN core-shell nanowire led designs." Thesis, Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/41206.
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Core-shell nanowire LEDs are light emitting devices which, due to a high aspect
ratio, have low substrate sensitivity, allowing the possibility of low defect density GaN
light emitting diodes. Current growth techniques and physical non-idealities make the
production of high conductivity p-type GaN for the shell region of these devices difficult.
Due to the structure of core-shell nanowires and the difference in conductivity between ntype and p-type GaN, the full junction area of a core-shell nanowire is not used
efficiently. To address this problem, a series of possible doping profiles are applied to
the core of a simulated device to determine effects on current crowding and overall
device efficiency. With a simplified model it is shown that current crowding has a
possible dependence on the doping in the core in regions other than those directly in
contact with the shell. The device efficiency is found to be improved through the use of
non-constant doping profiles in the core region with particularly large efficiency
increases related to profiles which modify portions of the core not in contact with the
shell
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Fickenscher, Melodie A. "Optical and Structural Characterization of Confined and Strained Core/Multi-Shell Semiconducting Nanowires." University of Cincinnati / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1329936272.
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Küpers, Hanno. "Growth and properties of GaAs/(In,Ga)As core-shell nanowire arrays on Si." Doctoral thesis, Humboldt-Universität zu Berlin, 2018. http://dx.doi.org/10.18452/19402.
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Diese Arbeit präsentiert Untersuchungen zum Wachstum von GaAs Nanodrähten (NWs) und (In,Ga)As Hüllen mittels Molekularstrahlepitaxie (MBE) mit sekundärem Fokus auf den optischen Eigenschaften solcher Kern-Hülle Strukturen. Das ortsselektive Wachstum von GaAs NWs auf mit Oxidmasken beschichteten Si Substraten wird untersucht, wobei der entscheidende Einfluss der Oberflächenpreparation auf die vertikale Ausbeute von NW Feldern aufgedeckt wird. Basierend auf diesen Ergebnissen wird ein zweistufiger Wachstumprozess präsentiert der es ermöglicht NWs mit dünner und gerade Morphologie zu erhalten ohne die vertikale Ausbeute zu verringern. Für die detaillierte Beschreibung der NW Form wird ein Wachstumsmo- dell entwickelt, das die Einflüsse der Veränderung der Tropfen Größe während des Wachstums sowie direktes des Wachstums auf den NW Seitenwänden umfassend beschreibt. Dieses Wachstumsmodell wird benutzt für die Vorhersage der NW Form über einen großen Parameterraum um geeignete Bedingungen für die Realisierung von erwünschten NW Formen und Dimensionen zu finden. Ausgehend von diesen NW Feldern werden die optimalen Parameter für das Wachstum von (In,Ga)As Hüllen untersucht und wir zeigen, dass die Anordnung der Materialquellen im MBE System die Materialqualität entscheidend beeinflusst. Die dreidimensionale Struktur der NWs in Kombination mit der Substratrotation und der Richtungsabhängigkeit der Materialflüsse in MBE resultieren in unterschiedlichen Flusssequenzen auf der NW Seitenfacette welche die Wachstumsdynamik und infolgedessen die Punktde- fektdichte bestimmen. An Proben mit optimaler (In,Ga)As Hülle und äußerer GaAs Hülle zeigen wir, dass thermionische Emission mit anschließender nichtstrahlender Rekombination auf der Oberfläche zu einem starken thermischen Verlöschen der Lumineszenz Intensität führt, welches durch das Hinzufügen einer AlAs Barrierenhülle zur äußeren Hüllenstruktur erfolgreich unterdrückt werden kann. Abschließend wird ein Prozess präsentiert der das ex-situ Tempern von NWs bei hohen Temperaturen ermöglicht, was in der Reduzierung von Inhomogenitäten in den (In,Ga)As Hüllenquantentöpfen führt und in beispiellosen optischen Eigenschaften resultiert.This thesis presents an investigation of the growth of GaAs nanowires (NWs) and (In,Ga)As shells by molecular beam epitaxy (MBE) with a second focus on the optical properties of these core-shell structures. The selective-area growth of GaAs NWs on Si substrates covered by an oxide mask is investigated, revealing the crucial impact of the surface preparation on the vertical yield of NW arrays. Based on these results, a two-step growth approach is presented that enables the growth of thin and untapered NWs while maintaining the high vertical yield. For a detailed quantitative description of the NW shape evolution, a growth model is derived that comprehensively describes the NW shape resulting from changes of the droplet size during elongation and direct vapour-solid growth on the NW sidewalls. This growth model is used to predict the NW shape over a large parameter space to find suitable conditions for the realization of desired NW shapes and dimensions. Using these GaAs NW arrays as templates, the optimum parameters for the growth of (In,Ga)As shells are investigated and we show that the locations of the sources in the MBE system crucially affect the material quality. Here, the three-dimensional structure of the NWs in combination with the substrate rotation and the directionality of material fluxes in MBE results in different flux sequences on the NW sidefacets that determine the growth dynamics and hence, the point defect density. For GaAs NWs with optimum (In,Ga)As shell and outer GaAs shell, we demonstrate that thermionic emission with successive nonradiative recombination at the surface leads to a strong thermal quenching of the luminescence intensity, which is succesfully suppressed by the addition of an AlAs barrier shell to the outer shell structure. Finally, a process is presented that enables the ex-situ annealing of NWs at high temperatures resulting in the reduction of alloy inhomogeneities in the (In,Ga)As shell quantum wells and small emission linewidths.
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Alqarni, Fahad, and Fahad Dhafer Alqarni. "Study of Piezo-phototronic Effect on Type-II Heterojunction ZnO/ZnSe Core/Shell Nanowire Array." ScholarWorks@UNO, 2015. http://scholarworks.uno.edu/td/2034.
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Yang, Li. "First-principles Calculations on the Electronic, Vibrational, and Optical Properties of Semiconductor Nanowires." Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/14133.
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The first part of my PhD work is about the lattice vibrations in silicon nanowires. First-principles calculations based on the linear response are performed to investigate the quantum confinement effect in lattice vibrations of silicon nanowires (SiNW). The radial breathing modes (RBM) are found in our calculations, which have a different size-dependent frequency shift compared with the optical modes. They are well explained by the elastic model. Finally, the relative activity of the Raman scattering in the smallest SiNW is calculated. The RBM can be clearly identified in the Raman spectrum, which can be used to estimate the size of nanowires in experiment.
In the second part of my PhD work, we focus on the electron-hole pair (exciton) in semiconductor nanowires and its influence on the optical absorption spectra. First-principles calculations are performed for a hydrogen-passivated silicon nanowire with a diameter of 1.2 nm. Using plane wave and pseudopotentials, the quasiparticle states are calculated within the so-called GW approximation, and the electron-hole interaction is evaluated with the Bethe-Salpeter Equation (BSE). The enhanced excitonic effect is found in the absorption spectrum.
The third part of my work is about the electronic structure in Si/Ge core-shell nanowires. The electronic band structure is studied with first-principles methods. Individual conduction and valence bands are found in the core part and the shell part, respectively. The band offsets are determined, which give rise to the spatial separation of electron and hole charge carriers in different regions of the nanowires. This allows for a novel-doping scheme that supplies the carriers into a separate region in order to avoid the scattering problem. This is the key factor to create high-speed devices. With the confinement effect, our results show important correction in the band offset compared with the bulk heterostructure. Finally, an optimum doping strategy is proposed based on our band-offset data.
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Klankowski, Steven Arnold. "Hybrid core-shell nanowire electrodes utilizing vertically aligned carbon nanofiber arrays for high-performance energy storage." Diss., Kansas State University, 2015. http://hdl.handle.net/2097/27651.
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Doctor of PhilosophyDepartment of Chemistry
Jun Li
Nanostructured electrode materials for electrochemical energy storage systems have been shown to improve both rate performance and capacity retention, while allowing considerably longer cycling lifetime. The nano-architectures provide enhanced kinetics by means of larger surface area, higher porosity, better material interconnectivity, shorter diffusion lengths, and overall mechanical stability. Meanwhile, active materials that once were excluded from use due to bulk property issues are now being examined in new nanoarchitecture. Silicon was such a material, desired for its large lithium-ion storage capacity of 4,200 mAh g[superscript]-1 and low redox potential of 0.4 V vs. Li/Li[superscript]+; however, a ~300% volume expansion and increased resistivity upon lithiation limited its broader applications. In the first study, the silicon-coated vertically aligned carbon nanofiber (VACNF) array presents a unique core-shell nanowire (NW) architecture that demonstrates both good capacity and high rate performance. In follow-up, the Si-VACNFs NW electrode demonstrates enhanced power rate capabilities as it shows excellent storage capacity at high rates, attributed to the unique nanoneedle structure that high vacuum sputtering produces on the three-dimensional array. Following silicon’s success, titanium dioxide has been explored as an alternative highrate electrode material by utilizing the dual storage mechanisms of Li+ insertion and pseudocapacitance. The TiO[subscript]2-coated VACNFs shows improved electrochemical activity that delivers near theoretical capacity at larger currents due to shorter Li[superscript]+ diffusion lengths and highly effective electron transport. A unique cell is formed with the Si-coated and TiO[subscript]2-coated electrodes place counter to one another, creating the hybrid of lithium ion battery-pseudocapacitor that demonstrated both high power and high energy densities. The hybrid cell operates like a battery at lower current rates, achieving larger discharge capacity, while retaining one-third of that capacity as the current is raised by 100-fold. This showcases the VACNF arrays as a solid platform capable of assisting lithium active compounds to achieve high capacity at very high rates, comparable to modern supercapacitors. Lastly, manganese oxide is explored to demonstrate the high power rate performance that the VACNF array can provide by creating a supercapacitor that is highly effective in cycling at various high current rates, maintaining high-capacity and good cycling performance for thousands of cycles.
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Messinese, Danilo. "Morphological instability analysis of a misfit strained core-shell nanowire for the growth of quantum dots." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2014. http://amslaurea.unibo.it/7159/.
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Nell'ambito delle nanostrutture, un ruolo primario è svolto dai punti quantici. In questo lavoro siamo interessati all'analisi teorica del processo di creazione dei punti quantici: esso può avvenire per eteroepitassia, in particolare secondo il metodo studiato da Stranski-Krastanov. Un film di Germanio viene depositato su un substrato di Silicio in modo coerente, cioè senza dislocazioni, e, a causa del misfit tra le maglie dei due materiali, c'è un accumulo di energia elastica nel film. A una certa altezza critica questa energia del film può essere ridotta se il film si organizza in isole (punti quantici), dove la tensione può essere rilassata lateralmente. L'altezza critica dipende dai moduli di Young (E, υ), dal misfit tra le maglie (m) e dalla tensione superficiali (γ).
Il trasporto di materiale nel film è portato avanti per diffusione superficiale.
Il punto focale nell'analisi delle instabilità indotte dal misfit tra le maglie dei materiali è la ricerca delle caratteristiche che individuano il modo di crescita più rapido dei punti quantici. In questo lavoro siamo interessati ad un caso particolare: la crescita di punti quantici non su una superficie piana ma sulla superficie di un nanofilo quantico a geometria cilindrica. L'analisi delle instabilità viene condotta risolvendo le equazioni all'equilibrio: a tal fine sono state calcolate le distribuzioni del tensore delle deformazioni e degli sforzo di un nanofilo core-shell con una superficie perturbata al primo ordine rispetto all'ampiezza della perturbazione.
L'analisi è stata condotta con particolari condizioni al contorno ed ipotesi geometriche, e diverse scelte dello stato di riferimento del campo degli spostamenti.
Risolto il problema elastico, è stata studiata l'equazione dinamica di evoluzione descrivente la diffusione di superficie.
Il risultato dell'analisi di instabilità è il tasso di crescita in funzione del numero d'onda q, con diversi valori del raggio del core, spessore dello shell e modo normale n, al fine di trovare il più veloce modo di crescita della perturbazione.
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Day, Robert Watson. "Crystal Growth on One-Dimensional Substrates: Plateau-Rayleigh Crystal Growth and Other Opportunities for Core/Shell Nanowire Synthesis." Thesis, Harvard University, 2015. http://nrs.harvard.edu/urn-3:HUL.InstRepos:17464133.
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Nanowires hold significant promise for both fundamental studies and technological applications ranging from energy conversion to electronics to biological sensing. The detailed understanding of nanowire synthesis and the realization of new synthetic approaches have enabled precise control over their size, morphology, and composition, and, consequently, their material properties. While much of the work on synthesis in the literature relates to axial nanowire growth, where growth proceeds in the direction of its long axis, this thesis has focused on probing the unique opportunities of shell growth, where material deposits radially around a nanowire core. To this end, I will show, first, that faceted Si core/shell nanowires can be synthesized with embedded pn junctions and that these structures can function as efficient photovoltaic devices with enhanced light absorption properties distinct from bulk Si devices. Second, through choice of reactants and reaction conditions used for shell growth, we demonstrate fine control over the size and morphology of these nanowires, which, in turn, drastically enhances their light absorption at particular wavelengths. Finally, we report for the first time a growth phenomenon that is unique to one-dimensional materials and which combines the underlying physics of the Plateau-Rayleigh instability with crystal growth. By exploiting this phenomenon, which we term Plateau-Rayleigh crystal growth, we demonstrate the growth of periodic shells on one-dimensional substrates. Specifically, we show that for conditions near the Plateau-Rayleigh instability the deposition of Si onto uniform-diameter Si cores, Ge onto Ge cores, and Ge onto Si cores can generate diameter-modulated core/shell nanowires. Rational control of deposition conditions enabled tuning of distinct morphological features, including diameter-modulation periodicity, amplitude and cross-sectional anisotropy. More generally, Plateau-Rayleigh crystal growth highlights the opportunities in understanding the thermodynamics and kinetics unique to crystal growth on nanowires and other low dimensional systems.Chemistry and Chemical Biology
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Küpers, Hanno [Verfasser], Henning [Gutachter] Riechert, Christoph [Gutachter] Koch, and Stefano [Gutachter] Sanguinetti. "Growth and properties of GaAs/(In,Ga)As core-shell nanowire arrays on Si / Hanno Küpers ; Gutachter: Henning Riechert, Christoph Koch, Stefano Sanguinetti." Berlin : Humboldt-Universität zu Berlin, 2018. http://d-nb.info/1185578552/34.
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Sahu, Gayatri. "Investigating the Electron Transport and Light Scattering Enhancement in Radial Core-Shell Metal-Metal Oxide Novel 3D Nanoarchitectures for Dye Sensitized Solar Cells." ScholarWorks@UNO, 2012. http://scholarworks.uno.edu/td/1478.
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Dye-sensitized solar cells (DSSCs) have attained considerable attention during the last decade because of the potential of becoming a low cost alternative to silicon based solar cells. Electron transport is one of the prominent processes in the cell and it is further a complex process because the transport medium is a mesoporous film. The gaps in the pores are completely filled by an electrolyte with high ionic strength, resulting in electron-ion interactions. Therefore, the electron transport in these so called state-of-the-art systems has a practical limit because of the low electron diffusion coefficient (Dn) in this mesoporous film photoanode. This work focuses on the influence of the advanced core-shell nanoarchitecture geometry on electron transport and also on the influence of electron-ion interactions. In order to achieve the proposed goals, DSSCs based on ordered, highly aligned, 3D radial core-shell Au-TiO2 hybrid nanowire arrays were fabricated, using three different approaches. J-V, IPCE, and EIS characteristics were studied. The efficiency, light scattering and charge transport properties of the core-shell nanowire based devices were compared to TiO2 nanotube as well as TiO2 mesoporous film based DSSCs. The Au nanowires inside the crystalline TiO2 anatase nanoshell provided a direct conduction path from the TiO2 shell to the TCO substrate and improved transport of electrons between the TiO2 and the TCO. The optical effects were studied by IPCE measurement which demonstrated that Au-TiO2 nanowires showed an improved light harvesting efficiency, including at longer wavelengths where the sensitizer has weak absorption. The metal nanostructures could enhance the absorption in DSSCs by either scattering light enabling a longer optical path-length, localized surface plasmon resonance (LSPR) or by near-field coupling between the surface plasmon polariton (SPP) and the dye excited state. Rapid, radial electron collection is of practical significance because it should allow alternate redox shuttles that show relatively fast electron-interception dynamics to be utilized without significant sacrifice of photocurrent. A combination of improved electron transport and enhanced light harvesting capability make Au-TiO2 core-shell nanowire arrays a promising photoanode nanoarchitecture for improving photovoltaic efficiency while minimizing costs by allowing thinner devices that use less material in their construction.
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ALBANI, MARCO GIOCONDO. "Modeling of 3D heteroepitaxial structures by continuum approaches." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2019. http://hdl.handle.net/10281/241273.
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I semiconduttori sono una categoria di materiali fondamentali per lo sviluppo di molteplici dispositivi. Negli ultimi decenni, l’evoluzione dell’industria dei semiconduttori ha seguito la nota legge di Moore. Tuttavia, questo straordinario processo di innovazione va incontro a un ostacolo nei prossimi anni, in quanto il processo di miniaturizzazione sta raggiungendo la scala atomica. Per questo motivo, è necessario sviluppare strategie alternative. In particolare, metodi di crescita bottom-up sono attualmente studiati per lo sviluppo di nanostrutture 3D.
In questa Tesi, per riprodurre la dinamica di crescita 3D, abbiamo sviluppato una tecnica modellistica che possa trattare la crescita verticale di nanostrutture. Un approccio cinetico, legato alla dinamica di incorporazione degli adatomi, deve essere utilizzato per simulare questo regime di crescita, che non può essere correttamente spiegato con un approccio termodinamico standard, basato sulle densità di energia superficiale. La simulazione di crescite verticali è un risultato complicato non solo per la definizione di un modello appropriato, ma richiede anche una tecnica numerica specifica. In particolare, in questa Tesi, abbiamo adottato un modello phase-field applicato allo studio della crescita di nanomembrane di GaAs, sfruttando il metodo a elementi finiti per la risoluzione numerica delle equazioni di evoluzione del sistema.
Per lo sviluppo di dispositivi, è spesso necessario ricorrere a etero-strutture, che combinano diversi tipi di semiconduttori, per esempio per le applicazioni optoelettroniche dove spesso si utilizzano delle giunzioni p-n. Inoltre, la crescita eteroepitassiale può essere sfruttata anche per trasferire la struttura cristallina da un materiale a un altro. In questa Tesi, ci siamo focalizzati sullo studio di nanofili core/shell e abbiamo effettuato un’accurata caratterizzazione delle deformazioni elastiche della struttura cristallina che si verificano in questi sistemi. In particolare, il rilassamento elastico è stato studiato con un modello continuo, basato sul metodo a elementi finiti. In particolare, abbiamo studiato il fenomeno di piegamento di nanowire GaP/InGaP e abbiamo correlato questo fenomeno con la distribuzione delle deformazioni elastiche all’interno della struttura. Inoltre, abbiamo investigato il ruolo del rilassamento elastico nei nanofili Ge/GeSn in riferimento al fenomeno di incorporazione di Sn nella shell.
L’evoluzione di nanostrutture può essere determinata anche dall’effetto combinato di energia di superficie ed energia elastica. L’esempio più studiato in letteratura è la crescita eteroepitassiale di isole su substrati planari, secondo la modalità di crescita di tipo Stranski-Krastanov. Per le applicazioni tecnologiche, è fondamentale poter controllare la distribuzione spaziale e l’uniformità della taglia delle isole.
In questa Tesi, presentiamo un modello di crescita phase-field, che combina la descrizione della dinamica di diffusione superficiale con la caratterizzazione tramite elementi finiti del rilassamento elastico, al fine di simulare la crescita ordinata di isole su substrati patternati con pit. In particolare, ci focalizziamo sul sistema prototipico di Ge cresciuto su Si. Il vantaggio del modello phase-field basato sul metodo a elementi finiti è la possibilità di risolvere in modo esatto le equazioni di evoluzione, senza la necessità di adottare approssimazioni di ordine superiore nella formulazione delle equazioni, pur considerando con precisione la geometria patternata del substrato.Semiconductors are the main building block for a variety of devices in our life. The semiconductor industry, in the last decades, has evolved by following the Moore's law. However, this incredible innovation process is going to reach an end in the next years, as the miniaturization process is getting too close to the atomistic size, which hinders the development of smaller devices. Therefore, alternative ways to evolve the current technologies have to been exploited. In particular, bottom-up approaches are currently being studied for the growth of 3D nanostructures. In this Thesis, to deal with the 3D growth dynamics, we develop a modeling technique that can reproduce the vertical growth of nanostrucutures. A kinetic approach, related to the incorporation dynamics of adatoms on the surface, has to be adopted to model the peculiar growth of 3D nanostructures, which cannot be explained by the standard thermodynamic arguments based on the surface energy densities. The simulation of the vertical growth is not just challenging for the definition of a proper model, but it requires also a dedicated technique for the numerical solution of the evolution dynamics. In particular, in this Thesis, we exploit a phase field model to simulate the growth on GaAs nanomembranes, based on a finite element method for the solution of the evolution equations. For the development of devices, it is often required to build heterostructures which combine different semiconductors, for instance for optoelectronic applications where a p-n junction is required. Furthermore, the heteroepitaxial growth can be exploited also to transfer some structural material properties, such as the hexagonal lattice structure, from a material to another. In this Thesis, we focus on the core/shell nanowire heteroepitaxial system and we provide a detailed characterization of the elastic deformations in the crystal structure. The elastic relaxation is studied in a continuum elasticity framework by finite element method. In particular, we study the bending of GaP/InGaP nanowires and we correlate this phenomenon with the partitioning of the elastic deformation within the nanostructure. Moreover, we investigate the role of the elastic relaxation in Ge/GeSn core/shell nanowires with respect to the incorporation of Sn in the shell. The evolution of nanostructures can be driven also by the combined effect of surface energy and elastic energy contributions. One of the most studied examples of this is the heteroepitaxial growth of islands on planar substrates, following the Stranski-Krastanov growth mode. For technological applications it is fundamental to control the spatial distribution and the size-uniformity of the islands. In this Thesis, we propose a phase-field model which combines the description for the surface diffusion dynamics and the finite element characterization of the strain field to study the ordered growth of islands on pit-patterned substrates. In particular, we choose the prototypical system where Ge islands are grown on a Si substrate. The advantage of the phase-field model based on finite element method is the possibility to exactly solve the evolution equations of the system, without the need of higher order approximations and with the possibility to precisely consider the effect on the elastic relaxation which is provided by the substrate morphology.
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Tang, Ming Y. 1979. "Modeling analysis of core-shell Si/SiGe nanowires." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/28734.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.Includes bibliographical references (p. 111-113).
(cont.) a composition that results in a high mobility has a very promising thermoelectric performance. Lastly, the thermoelectric-related transport properties for a Si/SiGe core-shell nanowire are compared with the related properties for a Si nanowire and a SiGe nanowire. The Si/SiGe core-shell nanowire shows a better thermoelectric performance than its Si nanowire counterpart. On the other hand, by relaxing the harsh conditions imposed on the carrier mobility of the Si/SiGe core-shell nanowire structure in this thesis, the Si/SiGe core-shell nanowire structure is also expected to have a better thermoelectric performance than its SiGe nanowire counterpart.
In this thesis, I present a theoretical model for the Si core/SiGe shell core-shell nanowire system. A model for the single carrier pocket core-shell nanowire is first developed, along with the boundary conditions of a circular wire and sharp interfaces between the two media. A numerical scheme is then developed for the core-shell nanowire system, along with educated approximations for the numerical boundary conditions. The numerical model is designed such that low energy levels have higher accuracy than the high energy levels. The core-shell nanowire model is applied to a Si core/SiGe alloy shell structure, which is considered as a core-shell nanowire building block containing multiple carrier pockets. Based on the 2D band structure of strained SiGe on a Si substrate, the strained SiGe layer of the Si core/SiGe shell core-shell nanowire is modeled. The effect of different parameters (the interface offset energy V, the total core-shell diameter e, and the core diameter d) on the energy levels of the Si/SiGe core-shell nanowire system is investigated. It is found that the core-shell nanowire system with the greatest quantum mechanical effect is the one with a small e, a relatively small magnitude V, and a d that results in a secondary confinement effect in the lower potential energy region. A 1D semi-classical transport model for the core-shell nanowire structure based on the Boltzmann transport equation is developed. Applying the 1D semi-classical model to the Si core/SiGe shell core-shell nanowire system, the thermoelectric properties of this particular system and the effect of doping on these properties are investigated. It is found that the system with an optimal doping concentration (n[opt] or p[opt]), a small V, a small e, a small d, and a shell
by Ming Y. Tang.
S.M.
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Guénolé, Julien. "Étude par simulations à l'échelle atomique de la déformation de nanofils de silicium." Thesis, Poitiers, 2012. http://www.theses.fr/2012POIT2321/document.
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L'étude des nano-objets en matériau semi-conducteur a révélé des propriétés mécaniques exceptionnelles, différentes de celles observées dans le massif. Outre l'intérêt technologique majeur qu'ils représentent à travers la miniaturisation toujours plus poussée des systèmes électroniques, leurs caractéristiques intrinsèques en font des objets particulièrement bien adaptés pour des études fondamentales. Dans ce contexte, nous avons étudié le déclenchement de la plasticité dans les nano-fils de silicium, les premiers stades de la plasticité étant en effet déterminants pour l'évolution ultérieure du système. Le silicium est ici considéré comme un semi-conducteur modèle. Pour cette étude, nous avons utilisé des simulations atomistiques qui sont parfaitement appropriées à l'analyse détaillée de la structure atomique des nano-objets. Après avoir contextualisé notre étude tant du point de vue de l'expérience que de celui des simulations, nous présentons les techniques numériques que nous avons utilisées. Nous décrivons ensuite l'étude de la déformation de nano-fils monocristallins, révélant notamment le rôle majeur des surfaces et l'activation d'un système de glissement jamais observé dans le silicium massif. Ce système de glissement est analysé en détail, et son activation est expliquée notamment au moyen de calculs ab initio. Enfin, nous avons considéré la déformation de nano-fils coeur-coquille cristal-amorphe et mis en évidence un comportement différent de celui observé pour les nano-fils monocristallins. Ainsi, des défauts natifs à l'interface cristal-amorphe semblent agir comme des germes favorisant la nucléation de la première dislocation qui va initier la plasticitéThe study of semiconductor nano-objets has revealed amazing mechanical properties, different from what is commonly observed in bulk. Besides the technological interest of these objects, due to the ever more pronounced miniaturization of electronic devices, their intrinsic specificities make them particularly well suited for fundamental studies. During this thesis, we have thus studied the onset of plasticity in silicon nanowires, the first stages of plasticity being indeed deciding for the subsequent evolution of the system. Silicon is here considered as a model semiconductor. For that study, we have used atomistic simulations, which are well appropriate for the detailed analysis of the nano-objects atomic structure. We first recall the context of that study, both from the experiments and simulations points of view. We then present the numerical methods used. Thestudy of the deformation of monocrystalline nanowires is then described; it reveals in particular the deciding role of surfaces, and the activation of one slip system never observed in bulk silicon. This slip system is analyzed in details, and its activation is explained notably thanks to ab initio calculations. Finally, crystalline-amorphous core-shell silicon nanowires are considered; and shownto exhibit a different behavior from that of monocrystalline nanowires. Indeed, native defects at the crystalline/amorphous interface seem to act as seeds, favoring the nucleation of the first dislocation which gives rise to the plasticity
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Kockert, Maximilian Emil. "Thermoelectric transport properties of thin metallic films, nanowires and novel Bi-based core/shell nanowires." Doctoral thesis, Humboldt-Universität zu Berlin, 2021. http://dx.doi.org/10.18452/23001.
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Thermoelektrische Phänomene können in Nanomaterialien im Vergleich zum Volumenmaterial stark modifiziert werden. Die Bestimmung der elektrischen Leitfähigkeit, des absoluten Seebeck-Koeffizienten (S) und der Wärmeleitfähigkeit ist eine wesentliche Herausforderung für die Messtechnik in Hinblick auf Mikro- und Nanostrukturen aufgrund dessen, dass die Transporteigenschaften vom Volumenmaterial sich durch Oberflächen- und Einschränkungseffekte verändern können.
Im Rahmen dieser Abschlussarbeit wird der Einfluss von Größeneffekten auf die thermoelektrischen Eigenschaften von dünnen Platinschichten untersucht und mit dem Volumenmaterial verglichen. Dafür wurde eine Messplattform als standardisierte Methode entwickelt, um S einer dünnen Schicht zu bestimmen. Strukturelle Eigenschaften wie Schichtdicke und Korngröße werden variiert. Grenz- und Oberflächenstreuung reduzieren S der dünnen Schichten im Vergleich zum Volumenmaterial.
Außerdem wird eine Methode demonstriert um S von einzelnen metallischen Nanodrähten zu bestimmen. Für hochreine und einkristalline Silber-Nanodrähte wird der Einfluss von Nanostrukturierung auf die Temperaturabhängigkeit von S gezeigt.
Ein Modell ermöglicht die eindeutige Zerlegung des temperaturabhängigen S von Platin und Silber in einen Thermodiffusions- und Phononen-Drag-Anteil.
Des Weiteren werden die thermoelektrischen Transporteigenschaften von einzelnen auf Bismut-basierenden Kern/Hülle-Nanodrähten untersucht. Der Einfluss des Hüllenmaterials (Tellur oder Titandioxid) und der räumlichen Dimension des Nanodrahts auf die Transporteigenschaften wird diskutiert. Streuung an Oberflächen, Einkerbungen und Grenzflächen zwischen dem Kern und der Hülle reduzieren die elektrische und thermische Leitfähigkeit. Eine Druckverformung induziert durch die Hülle kann zu einer Bandöffnung bei Bismut führen, sodass S gesteigert werden kann. Das Kern/Hülle-System zeigt in eine Richtung, um die thermoelektrischen Eigenschaften von Bismut erfolgreich anzupassen.Thermoelectric phenomena can be strongly modified in nanomaterials compared to the bulk. The determination of the electrical conductivity, the absolute Seebeck coefficient (S) and the thermal conductivity is a major challenge for metrology with respect to micro- and nanostructures because the transport properties of the bulk may change due to surface and confinement effects. Within the scope of this thesis, the influence of size effects on the thermoelectric properties of thin platinum films is investigated and compared to the bulk. For this reason, a measurement platform was developed as a standardized method to determine S of a thin film. Structural properties, like film thickness and grain size, are varied. Boundary and surface scattering reduce S of the thin films compared to the bulk. In addition, a method is demonstrated to determine S of individual metallic nanowires. For highly pure and single crystalline silver nanowires, the influence of nanopatterning on the temperature dependence of S is shown. A model allows the distinct decomposition of the temperature-dependent S of platinum and silver into a thermodiffusion and phonon drag contribution. Furthermore, the thermoelectric transport properties of individual bismuth-based core/shell nanowires are investigated. The influence of the shell material (tellurium or titanium dioxide) and spatial dimension of the nanowire on the transport properties are discussed. Scattering at surfaces, indentations and interfaces between the core and the shell reduces the electrical and the thermal conductivity. A compressive strain induced by the shell can lead to a band opening of bismuth increasing S. The core/shell system points towards a route to successfully tailor the thermoelectric properties of bismuth.
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Rogstad, Espen. "Fabrication and Characterization of GaAs/AlGaAs Core-Shell Photonic Nanowires." Thesis, Norwegian University of Science and Technology, Department of Electronics and Telecommunications, 2009. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-10001.
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GaAs/AlGaAs core-shell nanowires (NWs) were grown on GaAs(111)B substrates by Au-assisted molecular beam epitaxy (MBE) to investigate how different Al compositions in the shell influences the structural and optical properties of the NWs. Investigations with a secondary electron microscope (SEM) revealed that an increase in Al content leads to an increase in radial growth rate and a decrease in the axial growth rate of the AlGaAs shell. Low temperature μ-photoluminescence (PL) measurements showed that there was great improvement in the luminescence for the GaAs/AlGaAs core-shell NWs compared to GaAs NWs without shell.
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Tambe, Michael Joseph. "Controlled growth and doping of core-shell GaAs-based nanowires." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/59237.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.Includes bibliographical references (p. 151-158).
The use of compound semiconductor heterostructures to create electron confinement has enabled the highest frequency and lowest noise semiconductor electronics in existence. Modem technology uses two-dimensional electron gasses and there is considerable interest to explore one-dimensional electron confinement. This thesis develops the materials science toolkit needed to fabricate, characterize, and control the compositional, structural and electronic properties of core-shell GaAs/AlGaAs nanowires towards studying quasi-one-dimensional confinement and developing high mobility electronics First, nanowire growth kinetics were studied to optimize nanowire morphology. Variations in nanowire diameter were eliminated by understanding the role Ga adatom diffusion on sidewall deposition and vertical growth was enabled by understanding the importance of Ga and As mass-transport to nanowire nucleation. These results demonstrate that arrays of vertically-aligned GaAs nanowires can be produced. Then, the deposition of epitaxial AlGaAs shells on GaAs nanowires was demonstrated. By reducing the nanowire aerial density the stability of the nanowire geometry was maintained. A variety of analytical electron microscopy techniques confirmed the shell deposition to be uniform, epitaxial, defect-free, and nearly atomic sharp. These results demonstrate that core-shell nanowires possess a core-shell interface free of many of the imperfections that lithographically-defined nanowires possess. Finally, the adverse effect of the Au seed nanoparticle during n-type doping was identified and n-type doping was achieved via the removal of the Au nanoparticle prior to doping. A combination of energy dispersive X-ray spectroscopy, current-voltage, capacitance-voltage, and Kelvin probe force microscopy demonstrated that if the Au seed nanoparticle is present during the shell deposition, Au diffuses from the seed nanoparticle and creates a rectifying IV behavior. A process was presented to remove the Au nanoparticle prior to shell deposition and was shown to produce uniform n-type doping. The conductivity of GaAs/n-GaAs nanowires was calculated as a function of donor concentration and geometric factors taking into account the effects of Fermi level pinning. The control demonstrated over all of these parameters is sufficient enough for core-shell nanowires to be considered candidates for high mobility electronics.
by Michael Joseph Tambe.
Ph.D.
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Wang, Kai. "II-VI Core-Shell Nanowires: Synthesis, Characterizations and Photovoltaic Applications." ScholarWorks@UNO, 2012. http://scholarworks.uno.edu/td/1533.
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The emergence of semiconducting nanowires as the new building blocks for photovoltaic (PV) devices has drawn considerable attention because of the great potential of achieving high efficiency and low cost. In special, nanowires with a coaxial structure, namely, core-shell structures have demonstrated significant advantages over other device configurations in terms of radial charge collection and cost reduction. In this dissertation, several core-shell nanowire structures, including ZnO/ZnSe, ZnO/ZnS, and CdSe/ZnTe, have been synthesized and the photovoltaic devices processed from a ZnO/ZnS core-shell nanowire array and a single CdSe/ZnTe core-shell nanowire have been demonstrated.
By combining the chemical vapor deposition and pulsed laser deposition (PLD) techniques, type-II heterojunction ZnO/ZnSe and ZnO/ZnS core-shell nanowire array were synthesized on indium-tin-oxide substrates. Their structures and optical properties have been investigated in detail, which revealed that, despite highly mismatched interfaces between the core and shell, both systems exhibited an epitaxial growth relationship. The quenching in photoluminescence but enhancement in photocurrent with faster response upon coating the core with the shell provides the evidence that the charge separation and collection in the type II core-shell nanowire is greatly improved. This demonstration brings much greater flexibility in designing next generation PV devices in terms of material selection and device operation mechanisms for achieving their maximum energy conversion efficiencies at a low cost and in an environmentally friendly manner.
In order to achieve a high quality interface in the core-shell nanowire, CdSe and ZnTe, which have close lattice parameters and thermal expansion coefficients, were chosen to fabricate nanowire solar cells. ZnTe and CdSe nanowires were first synthesized by thermal evaporation and the shells were subsequently deposited by PLD. ZnTe/CdSe nanowires represented an inhomogeneous coating while the CdSe/ZnTe core-shell exhibited a conformal coating with obvious ZnTe eptilayer. The final PV device based on an individual CdSe/ZnTe nanowire demonstrated an efficiency of ~1.7%. In addition, a controllable synthesis of CdSe nanowire array on muscovite mica substrate was presented, providing the possibility to harvest hybrid energies in an all-inorganic nanowire array.
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Gulla, Stian. "Advanced Micro Photoluminescence Spectroscopy of Single GaAs/AlGaAs Core-Shell Nanowires." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for materialteknologi, 2013. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-22788.
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Self-catalyzed (SC) growth is a relatively new approach to growing GaAs nanowires (NWs). It has several advantages to Au-catalyzed growth, such as integration on Si platforms. Many of the growth parameters, including growth rate and V/III ratio, and their influence on the growth process, are not fully understood. Three samples of SC GaAs/AlGaAs core-shell NWs were grown with different V/III ratios (17.4, 23.8 and 30.1), in order to study how this parameter influences the optoelectronic properties of the NWs. The molecular beam epitaxy vapor-liquid-solid approach was used to grow the samples. In this work they were investigated with excitation power-, temperature-, polarization- and time-resolved micro photoluminescence (u-PL) spectroscopy.Only the sample of highest V/III ratio, sample "sigma", exhibited free exciton emission coming from zinc blende (ZB) GaAs in the core of the NWs. The other two samples were dominated by type-II transitions, likely originating from the tip regions with interchanging ZB-wurtzite (WZ) layers. Sigma-NWs appear to have defects, however, as shown by its low emission intensity at room temperature. A reference sample, sample "alpha", was also studied with the same methods. This sample, which was grown with a medium V/III ratio (20.0), but a lower growth rate, also showed signs of free exciton emission, possibly originating from WZ regions in the tip.It is concluded that a higher V/III ratio is beneficial to optoelectronic applications, and that the number of defects are lowered, mostlikely due to fewer Ga antisites in the core. It is proposed that samples sigma and alpha are studied by high-resolution transmission electron microscopy(HR-TEM) to further determine their structure. Also, new samples should be grown with higher V/III ratios, some of which should have a lower growth rate. Efforts to passivate the defect-rich tip from the core, such as axial AlGaAs inserts, are proposed.
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Ollivier, Maelig. "Elaboration de nanostructures à une dimension à base de carbure de silicium." Thesis, Grenoble, 2013. http://www.theses.fr/2013GRENI095/document.
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Le carbure de silicium est pressenti comme un matériau prometteur dans plusieurs domaines de l’électroniquetels que la nano-électronique, l’électronique de puissance ou les capteurs travaillant en milieuxhostiles (hautes températures, milieux corrosifs, milieux biologiques) du fait de ses propriétés physicochimiquessupérieures à celles du silicium, notamment. Cependant, parmi les différentes méthodesd’élaboration par voie descendante ou ascendante permettant de fabriquer des nano-objets à 1D enSiC, aucune n’a pour l’instant permis d’obtenir du SiC d’excellente qualité cristalline.Le travail de cette thèse a porté sur la démonstration de l’élaboration de nanostructures 1D àbase de SiC, à savoir nanofils coeur-coquille Si-SiC, nanofils de SiC et nanotubes de SiC, par unprocédé original de carburation de nanofils de silicium, eux-mêmes élaborés par gravure plasma. Cettedémonstration a été possible grâce au contrôle de la pression de carburation, ce qui permet la maîtrisede l’exodiffusion des atomes de silicium à travers le carbure de silicium.À pression atmosphérique l’exodiffusion des atomes de silicium est restreinte ce qui permet d’élaborerdes nanofils coeur-coquille Si-SiC avec une coquille de SiC monocristalline et entièrement recouvrante.En se servant de la biocompatibilité du SiC et du bon contrôle électronique dans le silicium, ilest possible d’envisager l’utilisation de ces nanofils coeur-coquille Si-SiC pour des bio-nano-capteurs.En diminuant la pression au cours de la carburation, il est possible d’augmenter l’exodiffusion etainsi d’obtenir des nanotubes de SiC cubique de très bonne qualité cristalline avec des parois denses.Ces nanotubes de SiC sont largement modulables en termes de dimensions, et la faisabilité de leurouverture a été démontrée, permettant ainsi l’utilisation du fort rapport surface sur volume de telsnano-objets pour des capteurs électroniques notamment.Un premier pas a été franchi vers les applications des nanofils coeur-coquille Si-SiC et des nanotubesde SiC, puisque les mesures électriques réalisées sur des nano-transistors à effet de champ utilisant cesdeux types de nano-objets comme canal sont prometteursDue to their superior physical and chemical properties —such as high breakdown field, high thermalconductivity and biocompatibility— compared to other semiconductors, silicon carbide is forseento be a promising materials for power electronics, bio-nano-sensors and nano-electronics in harsh environments.However, among the numerous top-down or bottom-up methods used to synthesise siliconcarbide 1D nano-objects, none has been able yet to produce SiC with a high cristalline quality.The aim of this project is to demonstrate the synthesis of silicon carbide- based 1D nanostructures—e.g. core-shell Si-SiC nanowires, SiC nanowires and SiC nanotubes— through an original processbased on the carburization of plasma-etched silicon nanowires. This demonstration is based on thecontrol of the pressure during the carburization process, which leads to the monitoring of the outdiffusionof silicon atoms through silicon carbide.Thus if the pressure is kept at the atmospheric pressure, the out-diffusion of silicon is limited andSi-SiC core-shell nanowires can be synthesized with a single-crystalline cubic SiC shell. Thanks to thebiocompatibility of the SiC shell and the good electronic transport into the Si core, bio-nano-sensorscan be considered.If the pressure is decreased during the carburization process, the outdiffusion of silicon atomsthrough SiC is enhanced, and leads to SiC nanotubes synthesis. SiC nanotubes sidewalls are dense,with an excellent crystalline quality. These original SiC nanotubes have a high surface to volume ratioand thus can be used for sensors or storage devices.The first step for direct applications has also been demonstrated since first results on electricalperformances of nano-field effect transistors, with these nano-objects as channel, are promising
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Guan, Xin. "Growth of semiconductor ( core) / functional oxide ( shell) nanowires : application to photoelectrochemical water splitting." Thesis, Lyon, 2017. http://www.theses.fr/2017LYSEC057/document.
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L’objectif de cette thèse est de développer un réseau de nanofils GaAs (coeur) / oxyde (coquille) pour la photoélectrolyse de l'eau. Pour cela, la géométrie des nanofils GaAs a été d’abord optimisée en ajustant différents paramètres expérimentaux de la croissance auto-catalysée de ces nanofils par Épitaxie par Jets Moléculaires. Nous avons ensuite étudié systématiquement l'oxydation de surface des nanofils GaAs et son effet négatif sur la croissance de la coquille. Nous avons donc développé une méthode dite d'encapsulation / désencapsulation d'une couche d'arsenic (As) amorphe qui protège les facettes des NFs de l'oxydation. Une étude physico-chimique a montré l'effet bénéfique d'une telle méthode sur la croissance de la coquille. La croissance d'une coquille de SrTiO3 sur des nanofils de GaAs a ensuite été réalisée. Des caractérisations approfondies de la croissance de la coquille de SrTiO3 sur les NFs de GaAs ont été réalisées. La plus grande partie de la structure pérovskite SrTiO3 était en relation d'épitaxie avec le réseau cristallin de GaAs. La dernière partie de cette thèse concerne l’utilisation de tels réseaux de nanofil GaAs / oxyde pour les dispositifs PEC où l'oxyde sert de couche de passivation. L'influence du dopage et de la morphologie des nanofils GaAs a d'abord été étudiée. Les propriétés des réseaux de nanofils de GaAs / SrTiO3 et de GaAs / TiO2 servant de photoélectrodes dans des dispositifs PEC sont étudiéesThe objective of this PhD is to develop the network of GaAs (core) / oxide (shell) nanowires for solar water splitting. The geometry of the GaAs nanowires was firstly optimized by adjusting different experimental parameters of the self-catalyzed growth of these nanowires by molecular beam epitaxy. We then systematically studied the surface oxidation of the GaAs nanowires and its negative effect on the growth of the shell. We have therefore developed a method called the arsenic (As) capping / decapping method that protects the facets of nanowires from the oxidation. A physico-chemical study has shown the beneficial effect of such a method on the growth of the shell. The growth of a SrTiO3 shell on GaAs nanowires was then performed. In-depth characterizations of SrTiO3 shell growth on GaAs nanowires were carried out. Most of the SrTiO3 perovskite structure was in epitaxial relationship with the GaAs crystalline lattice. The last part of this thesis concerns the application of such GaAs / oxide nanowire networks to PEC devices where the oxide serves as a passivation layer. The influence of the doping and the morphology of GaAs nanowires was first studied. The properties of GaAs / SrTiO3 and GaAs / TiO2 nanowire networks used as photoelectrodes in PEC devices are finally studied
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Kockert, Maximilian Emil [Verfasser]. "Thermoelectric transport properties of thin metallic films, nanowires and novel Bi-based core/shell nanowires / Maximilian Emil Kockert." Berlin : Humboldt-Universität zu Berlin, 2021. http://d-nb.info/123689698X/34.
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Slåttnes, Patrick Rene Tollefsen. "Photoluminescence study of as-grown self-catalyzed GaAs/AlGaAs core-shell nanowires." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for elektronikk og telekommunikasjon, 2012. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-18921.
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A macro-photoluminescence study of self-catalyzed nanowires, using a specializedsetup for this purpose, has been performed. Several samples were even comparedto single nanowire micro-photoluminescence measurements, on the samesamples, done by other students in the nanowire group at NTNU. The macrophotoluminescence measurements were done by changing parameters such as temperature and excitation power. A 532 nm laser was used to excite the wires and thephotoluminescence was measured by a CCD detector attached to a spectroscope.Using macro-photoluminescence measurements in a qualitative comparison withmeasurements done on single nanowires by micro-photoluminescence spectroscopycan give the researcher a decent understanding of the quality of the sample and itscontributions to photoluminescence. Measurements done in this thesis show probablecontributions from most types of transitions expected in zinc-blende nanowires,consisting of such defects as dense zinc-blende/wurtzite heterostructures, highlytwinned regions, but also from nanowires of higher purity.As nanowires move closer to the realization of e.g. nanowire based solar cells,such macro-photoluminescence measurements will become increasingly importantin deciding the overall quality of the grown structures.
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Du, Sichao. "Atom probe microscopy of III-V semiconductor nanowires." Thesis, The University of Sydney, 2013. http://hdl.handle.net/2123/10219.
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Group III-V semiconductor nanowires, showing novel properties, are promising building blocks for future applications. However, characterisation of individual NWs at the atomic level, which remains challenging, is critical to understand the detail growth mechanism. Therefore, this doctoral research aims, to develop a characterisation technique amenable to the specific configuration of a single nanowire, and to apply this technique to understand III-V nanowire growth. First, we developed a unique atom probe microscopy technique for directly measuring pristine nanowires on the growth substrate, which allows avoiding any damage introduced during sample preparation. Both experimental results and electrostatic simulations demonstrate the viability of this measuring approach to image the entire cross-section of a specimen, termed full tip imaging. Second, we characterised the distribution and relative concentration of dopants. The reconstruction parameters were determined based on crystallographic features. Background noises were removed based on multiple detector events. The concentration profile shows the dopants distributing radially inhomogeneous. Third, we investigated the vapour-liquid-solid and vapour-solid growth components in individual ternary InGaAs nanowires. A Ga-enriched core and In-enriched shell structure in the InGaAs nanowire has been found, which can essentially be related to the different growth mechanisms. This doctoral research advanced the atom probe microscopy characterisation of III-V nanowires. It developed a way to measure the pristine nanowire and increased the field-of-view by using the unique specimen preparation method. The atom probe microscopy results offer important compositional and structural information for the understanding of nanowire growth in order to realize applications in the future.
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Qu, Jiangtao. "Atom-Scale Insights into III-V Semiconductor Nanowires." Thesis, The University of Sydney, 2017. http://hdl.handle.net/2123/17851.
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As the feature size of MOSFET is scaling down to nano-size, series of problems need to be overcome to continue Moore’ Law, which seems an impossible task with traditional bulk Si technology due to the physical limitation and various negative effects being subject to small feature size. The critical issues for both further improving the devices’ performance and lowering their cost lie in the exploration of substitutions for Si and the control of morphological and compositional properties of materials. group III-V nanowires due to its unique properties are considered as the building block for next-generation electronic devices. To fulfill these commercial applications with group III-V nanowires, a fundamental and quantitative understanding of growth-structure-property relationships is central to applications where nanowires exhibit clear advantages. Therefore, this doctoral research systematically investigates three different semiconductor nanowires: Au-seeded, self-seed and planar nanowires, in terms of elemental, morphological and structural aspects by taking advantage of cutting-edge technique atom probe tomography, and endeavor to unveil the correlation between nanowires’ intrinsic properties and performance. Based on the atom probe findings, the growth mechanism of Au-seeded and self-seeded nanowire have been systematically discussed, and new model has been proposed to explain the phenomena on the basis of density functional calculation. Moreover, the doping distribution in planar nanowires has also been carefully investigated, and the results demonstrate that the dopants can diffuse into the substrate which subsequently degrade the device performance due to parasitic channel effect, and accordingly, suggestions have been given to optimize the planar nanowire growth for improved dopant distribution. The outcomes of this project are expected to theoretically support high-quality nanowire synthesis for specific applications.
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Alzahrani, Hanan Yahya S. "Non linear piezoelectricity in wurtzite semiconductor core-shell nanowires : an atomistic modelling approach." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/non-linear-piezoelectricity-in-wurtzite-semiconductor-coreshell-nanowires-an-atomistic-modelling-approach(b4be879a-b85f-4e58-81d7-79f304baa23d).html.
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Piezotronics is a new field, as first explored by Professor Zhong Lin Wang (Georgia Institute of Technology, Atlanta, USA), which describes the exploitation of the piezoelectric polarization and internal electric field inside semiconductor nanostructures by applying strain, to develop electronic devices with new functionality. Such concepts find applications in both III-V and II-VI semiconductor compounds, in optics, optoelectronics, catalysis, and piezoelectricity, sensors, piezoelectric transducers, transparent conductor and nanogenerators. In this work I explore the strain dependence of the piezoelectric effect in wurtzite ZnO crystals. The Linear and quadratic piezoelectric coefficients of III-V (GaP, InP, GaAs and InAs) wurtzite semiconductors are also calculated using ab-initio density functional theory. The polarization in terms of the internal anion–cation displacement, the ionic and dipole charges is written and the ab initio Density Functional Theory is used to evaluate the dependence of all quantities on the strain tensor. The piezoelectric effect of III–V semiconductors are nonlinear in the strain tensor. The quadratic piezoelectric coefficients and a revised value of the spontaneous polarization are reported. Furthermore, the ZnO nanowires is found to be non-linear piezoelectric effect and leads to predictions in some cases opposite to those obtained using the widely used linear model. The predicted magnitude of such coefficients are much larger than previously reported and of the same order of magnitude as those of III-N semiconductors. We also model the bending distortion created on a III-V wurtzite nanowire by an atomic force microscope tip induced deflection to calculate the piezoelectric properties of both homogenous and core shell structures. A number of combinations of III-V materials for the core and the shell of the nanowires, are shown a favour much increased voltage generation. The largest core voltages in core/shell combinations of InAs/GaP, InP/GaP, GaP/ InAs and GaP/InP are observed which can be theoretically 3 orders of magnitude larger than the typical values of ±3V in homogenous nanowires. Also considering properties such as bandgap discontinuity and mobility, III-V wurtzite core shell nanowires are candidates for high performance components in piezotronics and nanogeneration.
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Haas, Fabian [Verfasser], Hans [Akademischer Betreuer] Lüth, and Hendrik [Akademischer Betreuer] Bluhm. "Quantum transport in GaAs/InAs core/shell nanowires / Fabian Haas ; Hans Lüth, Hendrik Bluhm." Aachen : Universitätsbibliothek der RWTH Aachen, 2016. http://d-nb.info/1129876160/34.
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Iglesias, Irene [Verfasser], and Michael [Akademischer Betreuer] Farle. "Towards the miniaturization of core-shell cylindrical structures: fabrication and characterization of ferromagnetic core-shell micro- and nanowires / Irene Iglesias ; Betreuer: Michael Farle." Duisburg, 2020. http://d-nb.info/1221960245/34.
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Bohorquez, Ballen Jaime. "Thermal transport in low dimensional semiconductor nanostructures." OpenSIUC, 2014. https://opensiuc.lib.siu.edu/dissertations/798.
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We have performed a first principles density functional theory (DFT) calculations to study the thermal conductivity in ZnO nanotubes, ZnO nanowires, and Si/Ge shell-core nanowires. We found the equilibrium configuration and the electric band structure of each nanostructure using DFT, the interatomic force constants and the phonon dispersion relations were calculated using DFPT as implemented in Quantum Espresso. In order to fundamentally understand the effect of atomic arrangements, we calculated the phonon conductance in a ballistic approach using a Green's function method. All ZnO nanostructures studied exhibit semiconducting behavior, with direct bandgap at the Gamma point. The calculated values for the bandgaps were larger than the value of the bandgap of the bulk ZnO. We were able to identify phonon modes in which the motion of Zn atoms is significant when it is compared with the motion of oxygen atoms. The thermal conductivity depends on the diameter of the nanowires and nanotubes and it is dramatically affected when the nanowire or nanotube is doped with Ga. For Si/Ge nanowires, the slope and the curvature of acoustic modes in the phonon dispersion relation increases when the diameter increases. For nanowires with the same number of atoms, the slope and curvature of acoustic modes depends on the concentration of Si atoms. We were able to identify phonon modes in which the motion of core atoms is significant when it is compared with motion of atoms on the nanowire's shell. The thermal conductivity in these nanostructures depends on the nanowire's diameter and on the Si atoms concentration.
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Kohen, David. "Etude des nanofils de silicium et de leur intégration dans des systèmes de récupération d'énergie photovoltaïque." Phd thesis, Université de Grenoble, 2012. http://tel.archives-ouvertes.fr/tel-00859825.
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L'objectif de cette thèse porte sur la fabrication et la caractérisation de cellules solaires à jonction radiale à base d'assemblée de nanofils de silicium cristallin. Une étude sur la croissance des nanofils à partir de deux catalyseurs métalliques (cuivre et aluminium) dans une machine de dépôt chimique en phase vapeur (CVD) à pression réduite est présentée. L'influence des conditions de croissance sur la morphologie, le dopage et la contamination des nanofils par le catalyseur est analysée par des mesures électriques, chimiques (SIMS, Auger) et structurales (SEM, TEM, Raman). Le cuivre est utilisé pour la fabrication d'une cellule solaire avec des nanofils de type p et une jonction radiale créée avec du silicium amorphe de type n. Les performances photovoltaïques de la cellule solaire sont ensuite mesurées et interprétées. Un rendement de conversion de 5% est mesuré sur une cellule avec des nanofils de hauteur 1,5µm.
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Nilsen, Julie Stene. "Position controlled Growth of GaAs/AlGaAs core-shell Nanowires - more uniform in their structural and optical Properties?" Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for materialteknologi, 2014. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-26841.
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Semiconducting nanowires (NWs), especially those with a direct band gap, could be promising building blocks in future optoelectronic devices. Position controlled, self catalyzed GaAs/AlGaAs core-shell NWs with an AlGaAs axial insert, grown using molecular beam epitaxy, are studied structurally and optically, and compared to similar, but random grown, core-shell NWs as well as position controlled bare-core NWs. The characterization tools used include transmission electron microscopy, photoluminescence, scanning (transmission) electron microscopy, high angle annular dark field scanning transmission electron microscopy and energy dispersive spectrometry. In addition to the effect of growing NWs in regular, predefined patterns, the structure and morphology of the NWs are explained through the growth parameters, and the optical properties studied on the exact same NWs are related to their structure. Position controlled NWs are found to be highly uniform both structurally and optically. The NWs show excitonic emission related to a pure, defect free zinc blende segments. Small, defect related, structural variations seen in the tip and bottom, induced by the growth and consumption of the Ga catalyst droplet, are believed to cause variations in the emission below the free exciton energy. Shell growth is found to cause variations in morphology as well as a varying high energy emission in the range 1.6 - 1.8 eV, which is hypothesized to be due to confined GaAs in the AlGaAs shell.
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Bhandavat, Romil. "Molecular precursor derived SiBCN/CNT and SiOC/CNT composite nanowires for energy based applications." Diss., Kansas State University, 2013. http://hdl.handle.net/2097/15347.
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Doctor of PhilosophyDepartment of Mechanical and Nuclear Engineering
Gurpreet Singh
Molecular precursor derived ceramics (also known as polymer-derived ceramics or PDCs) are high temperature glasses that have been studied for applications involving operation at elevated temperatures. Prepared from controlled thermal degradation of liquid-phase organosilicon precursors, these ceramics offer remarkable engineering properties such as resistance to crystallization up to 1400 °C, semiconductor behavior at high temperatures and intense photoluminescence. These properties are a direct result of their covalent bonded amorphous network and free (-sp2) carbon along with mixed Si/B/C/N/O bonds, which otherwise can not be obtained through conventional ceramic processing techniques. This thesis demonstrates synthesis of a unique core/shell type nanowire structure involving either siliconboroncarbonitride (SiBCN) or siliconoxycarbide (SiOC) as the shell with carbon nanotube (CNT) acting as the core. This was made possible by liquid phase functionalization of CNT surfaces with respective polymeric precursor (e.g., home-made boron-modified polyureamethylvinylsilazane for SiBCN/CNT and commercially obtained polysiloxane for SiOC/CNT), followed by controlled pyrolysis in inert conditions. This unique architecture has several benefits such as high temperature oxidation resistance (provided by the ceramic shell), improved electrical conductivity and mechanical toughness (attributed to the CNT core) that allowed us to explore its use in energy conversion and storage devices. The first application involved use of SiBCN/CNT composite as a high temperature radiation absorbant material for laser thermal calorimeter. SiBCN/CNT spray coatings on copper substrate were exposed to high energy laser beams (continuous wave at 10.6 μm, 2.5 kW CO2 laser, 10 seconds) and resulting change in its microstructure was studied ex-situ. With the aid of multiple techniques we ascertained the thermal damage resistance to be 15 kW/cm2 with optical absorbance exceeding 97 %. This represents one order of magnitude improvement over bare CNTs (1.4 kW/cm2) coatings and two orders of magnitude over the conventional carbon paint (0.1 kW/cm2) currently in use. The second application involved use of SiBCN/CNT and SiOC/CNT composite coatings as energy storage (anode) material in a Li-ion rechargeable battery. Anode coatings (~1mg/cm2) prepared using SiBCN/CNT synthesized at 1100 °C exhibited high reversible (useable) capacity of 412 mAh/g even after 30 cycles. Further improvement in reversible capacity was obtained for SiOC/CNT coatings with 686 mAh/g at 40 cycles and approximately 99.6 % cyclic efficiency. Further, post cycling imaging of dissembled cells indicated good mechanical stability of these anodes and formation of a stable passivating layer necessary for long term cycling of the cell. This improved performance was collectively attributed to the amorphous ceramic shell that offered Li storage sites and the CNT core that provided the required mechanical strength against volume changes associated with repeated Li-cycling. This novel approach for synthesis of PDC nanocomposites and its application based testing offers a starting point to carry out further research with a variety of PDC chemistries at both fundamental and applied levels.
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Benkouider, Abdelmalek. "Fabrication and characterization of sige-based core-shell nanostructures." Thesis, Aix-Marseille, 2015. http://www.theses.fr/2015AIXM4345.
Full textAbstract:
Du fait de leur facilité de fabrication et de leurs propriétés physiques uniques, les nanofils (NFs) de semi-conducteurs présentent des potentialités d’application importantes elles pouvaient être comme briques élémentaires de nombreux dispositifs nano- et opto-électroniques. Différents procédés de fabrication ont été développés pour fabriquer et organiser ces nanofils en épitaxie sur silicium. Cependant, un des principaux problèmes réside dans le manque de reproductibilité des NFs produits naturellement. Pour obtenir un meilleur contrôle de leur périodicité, localisation, forme et taille, différents types de gravure ont été mis au point. Aujourd’hui, des incertitudes importantes persistent quant à leurs propriétés fondamentales, en raison d’un manque de corrélation entre les propriétés électroniques et optiques et les détails microscopiques (composition, structure, chimie ...etc.). L’objectif de ce travail est de développer deux types de procédés de fabrication : le premier "top-down" est basé sur la nanogravure directe par faisceau d’ions focalisés (FIB)de couches bi-dimonsionnelles de SiGe. Ce procédé permet de contrôler la taille des NFs, les déformations, et leur localisation précise. Il permet de fabriquer des réseaux de larges piliers. Les NFs réalisés par cette technique sont peu denses et de diamètre important. Le second procédé est de type "Bottom-Up" ; il s’appuie sur la croissance VLS à partir de catalyseurs métalliques (AuSi). Les NFs réalisés ont étudiés à l’échelle locale afin de mesurer la taille moyenne de contrainte ainsi que leur effet sur le confinement quantique et sur la structure de bande des NFsSiGe/Si core/shell nanowires (NWs) and nanodots (NDs) are promising candidates for the future generation of optoelectronic devices. It was demonstrated that the SiGe/Si heterostructure composition, interface geometry, size and aspect ratios can be used to tune the electronic properties of the nanowires. Compared to pure Si or Ge nanowires, the core-shell structures and exhibit extended number of potential configurations to modulate the band gap by the intrinsic strain. Moreover, the epitaxial strain and the band-offsets produce a better conductance and higher mobility of charge carriers. Recent calculations reported that by varying the core-shell aspect ratio could induce an indirect to direct band gap transition. One of the best configurations giving direct allowed transitions consists of a thin Si core embedded within wide Ge shell. The Germanium condensation technique is able to provide high Ge content (> 50%) shell with Si core whom thickness of core and shell can be accurately tuned. The aim of this work is to develop two types of synthesis processes: the first "top-down" will be based on direct nanoetching by focused ion beam (FIB) of 2D SiGe layer. This process allows the control of the size of NWs, and their precise location. The NWs achieved by this technique are not very dense and have a large diameter. The second processes called "bottom-up"; are based on the VLS growth of NWs from metal catalysts (AuSi). Grown NWs have been studied locally in order to measure the mean size and the strain and their effects on the quantum confinement and band structure of NWs
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Vettori, Marco. "Growth optimization and characterization of regular arrays of GaAs/AIGaAs core/shell nanowires for tandem solar cells on silicon." Thesis, Lyon, 2019. http://www.theses.fr/2019LYSEC010/document.
Full textAbstract:
L'objectif de cette thèse est de réaliser l'intégration monolithique de nanofils (NFs) à base de l’alliage Al0.2Ga0.8As sur des substrats de Si par épitaxie par jets moléculaires via la méthode vapeur-liquide-solide (VLS) auto-assistée et de développer une cellule solaire tandem (TSC) à base de ces NFs.Pour atteindre cet objectif, nous avons tout d'abord étudié la croissance de NFs GaAs, étape clé pour le développement des NFs p-GaAs/p.i.n-Al0.2Ga 0.8As coeur/coquille, qui devraient constituer la cellule supérieure de la TSC. Nous avons montré, en particulier, l'influence de l'angle d'incidence du flux de Ga sur la cinétique de croissance des NFs GaAs. Un modèle théorique et des simulations numériques ont été réalisées pour expliquer ces résultats expérimentaux.Nous avons ensuite utilisé le savoir-faire acquis pour faire croître des NFs p-GaAs/p.i.n-Al0,2Ga0,8As coeur/coquille sur des substrats de Si prêts pour l'emploi. Les caractérisations EBIC réalisées sur ces NFs ont montré qu'ils sont des candidats potentiels pour la réalisation d’une cellule photovoltaïque. Nous avons ensuite fait croître ces NFs sur des substrats de Si patternés afin d'obtenir des réseaux réguliers de ces NFs. Nous avons développé un protocole, basé sur un pré-traitement thermique, qui permet d'obtenir des rendements élevés de NFs verticaux (80-90 %) sur une surface patternée de 0,9 x 0,9 mm2.Enfin, nous avons consacré une partie de notre travail à définir le procédé de fabrication optimal pour la TSC, en concentrant notre attention sur le développement de la jonction tunnel de la TSC, l'encapsulation des NFs et le contact électrique supérieur du réseau de NFsThe objective of this thesis is to achieve monolithical integration of Al0.2Ga0.8As-based nanowires (NWs) on Si substrates by molecular beam epitaxy via the self-assisted vapour-liquid-solid (VLS) method and develop a NWs-based tandem solar cell (TSC).In order to fulfil this purpose, we firstly focused our attention on the growth of GaAs NWs this being a key-step for the development of p-GaAs/p.i.n-Al0.2Ga0.8As core/shell NWs, which are expected to constitute the top cell of the TSC. We have shown, in particular, the influence of the incidence angle of the Ga flux on the GaAs NW growth kinetic. A theoretical model and numerical simulations were performed to explain these experimental results.Subsequently, we employed the skills acquired to grow p-GaAs/p.i.n-Al0.2Ga0.8As core/shell NWs on epi-ready Si substrates. EBIC characterizations performed on these NWs have shown that they are potential building blocks for a photovoltaic cell. We then committed to growing them on patterned Si substrates so as to obtain regular arrays of NWs. We have developed a protocol, based on a thermal pre-treatment, which allows obtaining high vertical yields of such NWs (80-90 %) on patterned Si substrates (on a surface of 0.9 x 0.9 mm2).Finally, we dedicated part of our work to define the optimal fabrication process for the TSC, focusing our attention to the development of the TSC tunnel junction, the NW encapsulation and the top contacting of the NWs
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Chia-ChunLin and 林嘉峻. "Study of Core-Shell Diode Characteristic with CuInSe2 Nanowire." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/2dgu6t.
Full textAbstract:
碩士國立成功大學
微電子工程研究所
104
CuInSe2 nanowire photovoltaic has been the subject of research with a view to enhancing the photo absorption efficiency and reducing the material consumption compared with bulk and thin-film PV. However, its surface recombination is a problem. Therefore, we use the core-shell structure to solve the problem. In this study, we employ the annealing process and tune the temperature to improve the crystallization of CIS nanowire. In I-V measurement, we obtained leakage current Io near 49.1nA. Then, we fabricated core-shell structure with PEDOT/CIS material to reduce the defects near the surface of nanowire. In I-V measurement, we obtained leakage current Io near 36.9nA.
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Ting, Chi-Feng, and 丁啟峰. "Synthesis of ITO nanowire and ITO/TiO2 core-shell nanowire Arrays for Dye- sensitized solar cells." Thesis, 2008. http://ndltd.ncl.edu.tw/handle/39690232312700698192.
Full textAbstract:
碩士中原大學
化學研究所
96
In this study, researches of dye-sensitized solar cells using ITO nanowires as photoelectrode were carried out. Compare with those of pure anatase TiO2 film DSSC, enhanced short-current (Jsc) and energy conversion efficiency (η%) were observed. Fabrication of dye-sensitized solar cell with ITO nanowires was accomplished in two methods. Method one is synthesis of ITO nanowires as photoelectrode by template and electrophoretic deposition, then coated with TiO2 gel by doctor blade method. Method two is the use of ITO/TiO2 core-shell nanowires as photoelectrode, which was fabricated by coating the channels of PC membrane with TiO2 solution. Due to the capillarity and hydrophilicity of PC membrane, TiO2 nanotubes were formed. ITO/TiO2 core-shell nanowires were achieved by electrophoretic deposition ITO inside these TiO2 nanotubes. Method one results in better conversion efficiency than those of method two. The best conversion efficiency we obtained in this study is 4.3% using ITO nanoelectrode coated with 12μm TiO2 film.The component and microstructure of nanowires was characterized by XRD, FE-SEM, TEM, and EDS. The energy conversion efficiency of dye-sensitized solar cell was characterized by AM 1.5 simulated solar light of 100mA/cm2 intensity. The enhancement of electrical property using ITO nanowires was confirmed by characterizing in cyclic voltammetry (C.V).
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Dillen, David Carl. "Confined electron systems in Si-Ge nanowire heterostructures." Thesis, 2011. http://hdl.handle.net/2152/ETD-UT-2011-08-4360.
Full textAbstract:
Semiconductor nanowire field-effect transistors (NWFET) have been recognized as a possible alternative to silicon-based CMOS technology as traditional scaling limits are neared. The core-shell nanowire structure, in particular, also allows for the enhancement of carrier mobility through radial band engineering.
In this thesis, we have evaluated the possibility of electron confinement in strained Si-Si1-xGex core-shell nanowire heterostructures. Cylindrical strain distribution was calculated analytically for structures of various dimensions and shell compositions. The strain-induced conduction band edge shift of each region was found using k•p theory coupled with a coordinate system shift to account for strain. A positive conduction band offset of up to 200 meV was found for a Si-Si0.2Ge0.8 structure.
We have also designed and characterized a modulation doping scheme for p-type, Ge-SiGe core-shell NWFETs. Finite element simulations of hole density versus radial position were done for different combinations of dopant position and concentration. Three modulation doped nanowire samples, each with a different boron doping density in the shell, were grown using a combined vapor-liquid-solid and chemical vapor deposition process. Low temperature current-voltage measurements of bottom- and top-gate samples indicate that hole mobility is limited by the proximity of charged impurities.text
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Kao, Ming-liang, and 高銘良. "The fabrication and study of ZnO/MgO core-shell nanowire transistors." Thesis, 2009. http://ndltd.ncl.edu.tw/handle/95506582439106039426.
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碩士國立成功大學
化學工程學系碩博士班
97
In this research, the highly-oriented nanowire array grown on silver film has been successfully fabricated by chemical bath deposition (CBD) method, following by rolling transferring and photolithography process, the nanowire transistor with high electron mobility can be obtained. Also, the synthesis of higher aspect ratio nanowires can be achieved by pre-heat process. The lower adhesion force between nanowires and the growth substrate facilitated the fabrication of nanowire transistor. A series of material measurements have been conducted from the aspects of morphological, structural, optical, and chemical properties. According to TEM and EDX analyses, heat treatment of nanowire coated with Mg(OAc)2 can reduce and smooth the thickness of the shell and PL analysis also showed that nanowire can perform better optical property after been coated with Mg(OAc)2. In ESCA analyses, besides, we demonstrated that Mg atoms can diffuse from the shell into the core of nanowires during the heat treatment. In the end, we also demonstrated a methodology to transfer vertically aligned nanowires from the growth substrate to target substrate via rolling process with the use of PDMS film as a transferring medium. By this unique method, dense and oriented assembly of nanowires can be obtained and high electron mobility nanowire transistor can be fabricated.
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Chu, Ji-Fan, and 褚季凡. "Synthesis and characterization of Ge/GeO2/GeOx core-shell nanowire heterostructures." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/71696224664335173316.
Full textAbstract:
碩士國立東華大學
材料科學與工程學系
104
Abstract The Ge/Ge/GeOx nanowire core-shell nanowire heterostructures were synthesiz-ed on the sapphire substrates by using a non-toxic physical vapor deposition method. First, Ge vapor were transported onto the Au coated sapphire substrates to grow Ge nanowires. By controlling the ambient oxygen (0%, 1%, 5%, and 10%) during growth, Ge/GeO2/GeOx nanowire heterostructures with different shell thickness can be obtained. The field-emission scanning electron microscope (FE-SEM) and energy dispersive spectroscopy (EDS) were used to characterize samples. The sample consi-sts of nanowires with smooth surface and their diameters were less than 80 nm. EDS results shows only Ge and O were detected on samples, confirming the high purity of sample. The X-ray diffraction results confirm the crystal structures are cubic Ge and hexagonal GeO2 with high crystallinity. The scanning transmission electron micros-cope (STEM) was used to further characterize nanowire heterostructures. The results reveals that with increasing oxygen ambient, the thickness of oxide layer increases. For samples obtained under low oxygen ambient (0%, 1%, and 5%) were Ge/GeOx core-shell nanowire heterostructures. Sample grown under high oxygen ambient (10%) was Ge/GeO2/GeOx core-shell nanowire heterostructures. Their surface valence states were further confirmed by X-ray photoelectron spectroscopy (XPS). Macro-Raman spectroscopy indicates a red shift behavior due to size effect and phonon confinement. Photoluminescence (PL) results shows Ge/GeOx core-shell nanowire heterostructures are with strong blue-green emission due to various defect transitions such as oxygen vacancy and germanium-oxygen vacancy pairs.
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Lee, Wen-Jay, and 李玟頡. "The Study of Mechanical Properties of the Helical Multi-Shell Gold Nanowire." Thesis, 2005. http://ndltd.ncl.edu.tw/handle/47920929712134985647.
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碩士國立中山大學
機械與機電工程學系研究所
93
In recent year, the quantum device has been rapid developed. The quantum conductor has been of great interest for most authors, and one of that is gold nanowire. As the diameter of the gold nanowire is smaller than 2nm, the structure arrangement is affected by surface tensor, and therefore the FCC structure will self assemble to a helical structure. However, the nanowire would be used in quantum devices, therefore, the material property must be understood and investigated. The properties of nanowire would be a significant on development of quantum device in the future. In this study, the molecular dynamics is employed to investigate the mechanical properties of the helical multi-shall gold nanowires and nanowries of the bulk FCC. The stress and strain relationship is obtained form the tensile and compressed tests. In addition, the yielding stress, maximum stress, Young’s modulus, and breaking force can be determined from the tensile test and compressed test. Moreover, the different length/diameter ratio, temperature, and strain rate effects on mechanical properties and deformation behaviors are also investigated. The structure transform from crystalline to non-crystalline is also observed by the variation of radial distribution function (RDF) and angular correlation function (ACF). In this study, the tight-binding many body potential is employed to model the interaction between gold atoms.
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Nah, Junghyo 1978. "High performance germanium nanowire field-effect transistors and tunneling field-effect transistors." Thesis, 2010. http://hdl.handle.net/2152/ETD-UT-2010-12-2268.
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The scaling of metal-oxide-semiconductor (MOS) field-effect transistors (FETs) has continued for over four decades, providing device performance gains and considerable economic benefits. However, continuing this scaling trend is being impeded by the increase in dissipated power. Considering the exponential increase of the number of transistors per unit area in high speed processors, the power dissipation has now become the major challenge for device scaling, and has led to tremendous research activity to mitigate this issue, and thereby extend device scaling limits. In such efforts, non-planar device structures, high mobility channel materials, and devices operating under different physics have been extensively investigated. Non-planar device geometries reduce short-channel effects by enhancing the electrostatic control over the channel. The devices using high mobility channel materials such as germanium (Ge), SiGe, and III-V can outperform Si MOSFETs in terms of switching speed. Tunneling field-effect transistors use interband tunneling of carriers rather than thermal emission, and can potentially realize low power devices by achieving subthreshold swings below the thermal limit of 60 mV/dec at room temperature. In this work, we examine two device options which can potentially provide high switching speed combined with reduced power, namely germanium nanowire (NW) field-effect transistors (FETs) and tunneling field-effect transistors (TFETs). The devices use germanium (Ge) – silicon-germanium (Si[subscript x]Ge[subscript 1-x]) core-shell nanowires (NWs) as channel material for the realization of the devices, synthesized using a 'bottom-up' growth process. The device design and material choice are motivated by enhanced electrostatic control in the cylindrical geometry, high hole mobility, and lower bandgap by comparison to Si. We employ low energy ion implantation of boron and phosphorous to realize highly doped contact regions, which in turn provide efficient carrier injection. Our Ge-Si[subscript x]Ge[subscript 1-x] core-shell NW FETs and NW TFETs were fabricated using a conventional CMOS process and their electrical properties were systematically characterized. In addition, TCAD (Technology computer-aided design) simulation is also employed for the analysis of the devices.text
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Huang, Ting-Kai, and 黃亭愷. "Resistive Switching Properties and Behaviors in Core-Shell Ni/NiO/HfO2 Nanowire ReRAM Devices." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/qmf35z.
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碩士國立交通大學
材料科學與工程學系所
105
Resistive Random Access Memory (RRAM) is one of the most promising nonvolatile memory because it has several advantages; for example, simple MIM (metal-insulator-metal) structure, fast operation speed, high endurance, high retention, and low energy consumption. However, the reliability is not persistent and completely switching mechanism is not fully understood. In this work, we deposited different metal oxide (HfO2, Al2O3) covering the Ni/NiO nanowire by atomic-layer-deposition (ALD). The different properties, including electrical characteristics, surface morphology, and elements distribution have been systematically investigated. The electrical characteristics were excellent, and the endurance could up to 200 cycles, which is pretty good for 1-D nanostructure. Also, we used focus-ion-beam (FIB) to prepare TEM sample subsequently following TEM observation. From the TEM analysis, we made sure the position of conducting filament (CF) and confirmed the element components of CF. The diffusion of oxygen vacancies formed the conducting filament, resulting in the change of morphology. The study enriched the understanding of the mechanism and provided a new design to enhance switching properties of RRAM.
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HsiangTan and 譚湘. "Fabrication of ZnO@ZnSnO3 Core-Shell Nanowire Arrays as Negative Electrodes for Lithium Ion Batteries." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/3a3rsh.
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Lin, Yu-Wei, and 林昱位. "Growth and Characterization of ZnO/ZnTe Core/Shell Nanowire Arrays on Transparent Conducting Oxide Glass Substrates." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/43297170583318444081.
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碩士國立臺灣大學
應用物理所
100
Type Ⅱ heterojunction core-shell nanowire arrays have attracted much attention recently because they have advantages over conventional planar devices, such as higher surface-to-volume ratio and better light-trapping effect. Moreover, because of the type-II band alignment, electrons and holes are naturally separated into different spatial regions in these nanowire structures and thus the carrier lifetime is increased in these structures as compared with the normal devices. In this thesis, we report the growth and characterization of ZnO/ ZnTe core-shell nanostructure. Vertically aligned ZnO/ZnTe core/shell nanowire arrays on transparent conducting oxide glass substrates were fabricated by using chemical vapor deposition (CVD) to grow the ZnO core and metal organic chemical vapor deposition (MOCVD) was used to deposit the ZnTe shell. The morphology of ZnO/ZnTe core/shell nanowire arrays were studied by scanning electron microscope (SEM); the detailed atomic arrangement of the ZnO/ZnTe core/shell nanowire was investigated by transmission electron microscopy (TEM). The crystal structures of ZnO/ZnTe core/shell nanowires were characterized by X-ray diffraction (XRD). The optical properties of ZnO/ZnTe core/shell nanowire arrays were investigated by photoluminescence (PL) and transmission studies. The ZnO/ZnTe core/shell nanowires arrays were then used as the active layer and carrier transport medium to fabricate a photovoltaic device. The current density versus potential curve and time-dependent photocurrent density were measured for this device. The results showed the ZnO/ZnTe core/shell nanowire array has enhanced photocurrent and good photocurrent response under light illumination, indicating the nanowire arrays can be made into a good photovoltaic device.
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Pu, Ying-Chih, and 蒲盈志. "Titanium Oxide Nanowire-based Heterostructures and Cd1-xZnxSe Core/Shell Quantum Dots: Interfacial Charge Carrier Dynamics and Photoelectric Conversion Applications." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/eth9x3.
Full textAbstract:
博士國立交通大學
材料科學與工程學系所
102
Due to the difference in band structure between the constituents, semiconductor heterostructures exhibit remarkable charge separation property which is beneficial to solar fuel generation. On the other hand, the advantages of quantum dots-based light emitting diodes (QD-LEDs) include color tunability, high color saturation and high color rendering index (CRI) white lighting. The performance of both photon-to-electron and electron-to-photon conversions is closely related to the intrinsic charge carrier dynamics of the constitutes. In this dissertation, the correlation between the charge carrier dynamics and the photoelectric conversion efficiency for semiconductor heterostructures and core/shell Cd1-xZnxSe QDs was investigated. Three individual yet relevant projects were included in the dissertation: First, we demonstrated that Au-decorated NaxH2-xTi3O7 nanobelts (NaxH2-xTi3O7-Au NBs) may exhibit remarkable photocatalytic performance under visible light illumination due to the remarkable charge separation property. In order to further enhance the photocatalytic efficiency, a thin layer of Cu2O was deposited on the Au surface of the Au-decorated NaxH2-xTi3O7 NBs to form Z-scheme NaxH2-xTi3O7-Au-Cu2O nanoheterostructures. Because of the relative band alignment of the constituents, Au may mediate the carrier transfer of NaxH2-xTi3O7-Au NBs to render them enhanced photocatalytic performance. Time-resolved photoluminescence (PL) spectra were measured to quantitatively analyze the electron transfer in the Z-scheme NaxH2-xTi3O7-Au-Cu2O NBs. The carrier utilization efficiency of the samples was evaluated and the result was correlated with that of the charge carrier dynamics measurement, which may provide insightful information when using Z-scheme heterostructures in photoconversion applications. Second, we investigated the plasmonic effect of noble metal nanocrystals on the photocatalytic properties of semiconductor nanostructures. Since the surface plasmon resonance (SPR) of metal (e.g. Ag and Au) energizes the conduction electrons and excites them from the outermost bands to higher energy states, there is a great probability that these electrons can participate in chemical reactions. We developed a Ag-decorated SiO2 NSs, which exhibited significantly red-shifted and relatively broad SPR absorption spanned from visible to near-infrared region. The photocatalytic activity of Ag-decorated SiO2 NSs was corresponded with the SPR absoption ability. On the other hand, by acting as an antenna that localizes the optical energy by SPR, plasmonic Au can sensitize TiO2 to light with energy below the band gap, generating additional charge carriers for water oxidation. The photoactivity of Au-decorated TiO2 electrodes for photoelectrochemical water oxidation can be effectively enhanced in the entire UV-visible region from 300 nm to 800 nm, by manipulating the shape of the decorated Au nanostructures. The analysis results suggested that the enhanced photoactivity of Au NP-decorated TiO2 nanowires in UV region was attributed to effective surface passivation. Since the existence of surface states greatly affected the photoconversion performance of TiO2, we employed a facile precursor-treatment approach for effective surface passivation of rutile TiO2 nanowire photoanode to improve its performance in photoelectrochemical water oxidation. Last, we developed a single-step hot-injection process to synthesize core/shell Cd1-xZnxSe QDs with tunable emission wavelengths. Because of the higher reactivity of the Cd precursor, QDs whose composition was rich in CdSe were generated at the beginning of reaction. As the reaction proceeded, the later-formed ZnSe shell was simultaneously alloyed with the core, giving rise to a progressive alloying treatment for the grown QDs. During the reaction period, the continued blue shifiting emissioned Cd1-xZnxSe QDs were obtained. A LED composed of conducting polymer with Cd1-xZnxSe QDs was fabricated to test the electroluminescence properties, which show high color purity for the emissions from LED. The findings from this work also demonstrate the advantage of using the current single-step synthetic approach to obtain a batch of Cd1-xZnxSe QDs that may emit different colors in prototype LEDs.
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Tso, Shuen, and 左璿. "CdS-ZnO Core-Shell Nanowires and CdS-Cu2S Core-Shell Nanowires for Hydrogen Generation." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/68610241052663527116.
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碩士國立清華大學
材料科學工程學系
104
With the increasing awareness of environmental issues, the renewable energy production has become urgent. Hydrogen is considered to be the promising fuel for the next generation. In this work, we construct novel nano-structures for hydrogen generation. CdS-ZnO core-shell nanowires and CdS-Cu2S core-shell nanowires were synthesized. First, crystalline CdS nanowires were synthesized in a three heating zone diffusion furnace through a VLS growth method. The ZnO shell was deposited with RF sputtering, and the Cu2S shell was achieved with cation exchange method. The core-shell nanowires have been examined by scanning electron microcopy, transmission electron microcopy, X-ray diffraction and energy dispersive spectroscopy to confirm their crystal structure. Photoluminescence and UV-visible analysis were used to examine their band gap and optical properties. A Shimadzu GC-2014 was used to identify and measure the amount of hydrogen gas produced by CdS-ZnO core-shell nanowires and CdS-Cu2S core-shell nanowires. Compared to CdS nanowires, the hydrogen generating activity of CdS-ZnO and CdS-Cu2S core-shell nanowires improved more than 2 orders and 1 order in magnitude, respectively. This experiment verifies the importance and usefulness of band alignment in structure design and opens up a new path for photocatalyst for hydrogen generation.
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Chen, Guan-Lin, and 陳冠霖. "Fabrication of ZnO /TiO2 core-shell nanowires." Thesis, 2009. http://ndltd.ncl.edu.tw/handle/10802524181481740632.
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碩士東海大學
物理學系
97
This experiment fabricates of ZnO/TiO2 core-shell nanowires by sol – gel method for applications of optoelectronic devices. ZnO nanowires are grown by Chemical Vapor Deposition (CVD) in this experiment, and ZnO/TiO2 core-shell nanowires assembled by soaking the nanowires in gel solution of TiO2 starting material are baked at 85 ℃ in 72 hours and annealed at 400℃ in 2 hours. In the beginning, the as-grown ZnO nanowires are all over 5 mm showed in SEM. The TEM image illustrates that diameter of wire is 30 nm with single crystal indicated by TEM diffraction patterns. The XRD results indicate the elements of nanowires are ZnO. There is a peak at 379 nm in PL spectrum without any defect emission. After the core-shell structure is formed, the diameter of nanowires is about 70 nm~120 nm with 10 nm~30 nm TiO2 covering. Moreover, the TEM diffraction patterns show that the phase of TiO2 is not only Anatase but also Rutile in the core-shell structure. The XRD results indicate that the element of ZnO/TiO2 core-shell nanowires we fabricated is only ZnO, because the crystalline qualities of TiO2 are so weak that it’s hard to be detected by XRD. Nevertheless, the defect emissions appear after 430nm as the nanowires soak in TiO2 sol-gel solution. We have fabricated the TiO2 core-shell structures on ZnO nanowires by sol-gel method, and the experiment results demonstrate that these nanowires could apply to the requirements of the optoelectronic devices such as the solar cells and so on.
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Sung, Pin-Hsien, and 宋品賢. "Hydrothermal Synthesis of MnO2 Nanowires and TiO2/MnO2 Core/Shell Nanowires for Supercapacitor Applications." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/82257564678464785184.
Full textAbstract:
碩士國立中興大學
材料科學與工程學系所
102
TiO2/MnO2 and MnO2/TiO2 core/shell nanostructures were synthesized by using a two-step hydrothermal process. Single-crystalline TiO2 nanowires with diameters and lengths of 150 nm and 6 μm, respectively, and MnO2 nanowires with diameters and lengths of 50 nm and 1.5 μm, respectively, were obtained in the first step. Then, the MnO2 nanoflake shell with thickness of about 17 nm and TiO2 amorphous shells with thickness of about 45 nm were grown on the TiO2 and MnO2 nanowire cores, respectively, in the second step.We also changed the synthesis time in the second step to obtain TiO2/MnO2 core/shell nanostructures with optimal MnO2 shell thickness to achieve the best capacitance performance. The specific capacitance of TiO2 and MnO2 nanowires and TiO2/MnO2 and MnO2/TiO2 core/shell nanostructures were calculated to be 8.33, 51.67, 283.74 and 18.05, respectively, from corresponding CV curves. After cycle life test for 500 times, the specific capacitance retention compared with the initial CV stages were found to be 100 %, 90 %, 95 % and 81 % for TiO2 and MnO2 nanowires, and TiO2/MnO2 and MnO2/TiO2 core/shell nanostructures, respectively. It appears that TiO2/MnO2 core/shell nanostructures are benefited from high specific capacitance of MnO2 and excellent cycle stability of TiO2 and exhibit a balanced capacitance performance.
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Hsieh, Wen-yuan, and 謝文元. "Growth of WO3 nanowires and Ge-Si1-xGexOy core-shell nanowires via carbothermal reduction." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/56987325131401913860.
Full textAbstract:
碩士國立成功大學
材料科學及工程學系碩博士班
95
Effects of adding GeO2 to WO3 powders on enhancing the growth of WO3 nanowires (NWs) and Ge-Si1-xGexOy core-shell NWs ( Ge-SiGeO NWs) via the carbothermal reduction process in Ar at a flow rate of 25-400 sccm at a temperature of 900-1100°C were studied. Upon the thermal evaporation or carbothermal reduction of WO3 powders at 900°C no NWs were grown. The growth of WO3 NWs follows the vapor-solid (VS) process, where GeO2 powders act as an oxidizer. Introducing 1-5% O2 into flowing Ar enhanced the growth of GeO2 NWs and WO3-GeO2 core-shell NWs at 900°C. More O2 suppressed the growth of NWs because of the exhaustion of much graphite powders. At 1050-1100°C the growth of Ge-SiGeO NWs following the VS process were observed. In addition, the growth mechanisms of WO3, GeO2, and Ge-SiGeO NWs are discussed, respectively.
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Hsu, Chao Wei, and 徐肇蔚. "Synthesis and characterization of Cu-Ni core-shell nanowires." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/u22gj5.
Full textAbstract:
碩士國立清華大學
材料科學工程學系
103
Due to it’ s good conductivity and low-cost property, copper (Cu) is often used as interconnect and flexible transparent conductive film material with the introduction of nano-processing. Therefore, many studies try to analyze the properties of copper in nanoscale, such as nanowires (NWs). Twin is a common microstructure in metals. It has been demonstrated that nano-twinned copper have high mechanical strength, good conductivity, and moreover, superior electromigration resistance. In our previous study, we have successfully fabricated Cu NWs with high density of nanoscale traverse twinning structure by using pulsed electrodeposition at low temperature. The formation of nano-twinned structure can also improve the corrosion properties of Cu metallization by changing grain boundary structure. However, research report on the oxidation characteristics of nanotwinned Cu NWs is limited. Copper is prone to oxidation in nanoscale because of high surface-to-volume ratio, some researches showed that Cu-Ni NWs with core-shell structure that was prepared by coating a thin Ni layer on Cu NWs using chemical reduction method possesses good oxidation resistance. Nevertheless, these treatment were mostly done by using highly toxic reducing agent. In this study, high aspect-ratio nano-twinned Cu NWs were prepared by pulsed current (PC) electroplating, and Cu-Ni core-shell NWs were successfully prepared by the low-toxicity reducing agent, sodium borohydride. Time-and-temperature dependence of electrical resistivity for single Cu and Cu-Ni NWs prepared by micro-fabrication process has been investigated. The bamboo-like twinning structure and core-shell structure of Cu-Ni NWs have been examined by transmission electron microscopy (TEM) and UV-visible/TEM Energy Disperse X-ray analysis. According to four-probe I-V electrical measurement, the resistivity of Cu NWs with nanotwins remained almost constant after exposed in ambient air for one month, while the resistivity of Cu-Ni core-shell NWs also kept unchanged after hundreds hours of thermal aging, revealing good chemical stability.
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Lai, Chun-Yuan, and 賴俊源. "Growth of Core-shell GaAs Nanowires by MBE System." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/37285479527751930070.
Full textAbstract:
碩士國立清華大學
材料科學工程學系
98
In this work, GaAs nanowires were grown on the GaAs (111)B substrate by molecular beam epitaxy (MBE). First, Au film with a thickness of 1 nm was deposited onto the GaAs (111)B substrate through e-gun evaporation to form metal seeds for further VLS growth of GaAs nanowires. After Au film deposition, the sample was inserted into MBE chamber, heated up to the desired temperature of 600℃ in order to remove the oxide layer and to form droplets of eutectic Au-Ga liquid alloy. After annealing, GaAs nanowires were grown at a substrate temperature of 540 ℃. With various growth parameters, i.e. time, temperature and source BEP, the morphologies of nanowires were changed. Hence, the GaAs nanowire with core- shell structure could be obtained by adjusting different parameters such as Ga, As fluxes and growth temperature. The nanowires were systematically investigated using different instruments. Scanning electron microscope (SEM) demonstrates different morphologies of nanowires. The diameter of nanowires is increasing with increasing Ga flux under a constant As flux, while the axial growth rate is increasing with increasing As flux under a constant Ga flux. From the observation of Transmission electron microscopy (TEM), a great deal of defects, e.g. stacking faults and twins, were found in the nanowires with increasing growth rate. Thus, this growth condition mentioned above resulted in a worse quality of nanowires.