Letteratura scientifica selezionata sul tema "XPS/ARPES"

Epitaxial graphene (EG) grown on SiC(0001) resides on the so-called buffer layer. This carbon rich (6√3×6√3)R30° reconstruction is covalently bound to the topmost silicon atoms of the SiC. Decoupling the graphene buffer layer from the SiC interface is a well studied topic since successful intercalation has been shown for hydrogen [1-3]. Recently, intercalation was also shown for oxygen [4, 5]. We present ARPES, XPS and Raman spectroscopy studies to determine the quality of oxygen intercalated buffer layer samples in terms of decoupling and integrity of the transformed graphene layer. The decoupling effect is demonstrated by ARPES measurements showing a graphene-like π band. XPS shows whether the oxidation takes place in the buffer layer or at the interface. Raman spectroscopy is well suited to investigate oxygen induced defects in graphene-like material.

Research on ultrathin quantum materials requires full control of the growth and surface quality of the specimens in order to perform experiments on their atomic structure and electron states leading to ultimate analysis of their intrinsic properties. We report results on epitaxial FeSe thin films grown by pulsed laser deposition (PLD) on CaF2 (001) substrates as obtained by exploiting the advantages of an all-in-situ ultra-high vacuum (UHV) laboratory allowing for direct high-resolution surface analysis by scanning tunnelling microscopy (STM), synchrotron radiation X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES) on fresh surfaces. FeSe PLD growth protocols were fine-tuned by optimizing target-to-substrate distance d and ablation frequency, atomically flat terraces with unit-cell step heights are obtained, overcoming the spiral morphology often observed by others. In-situ ARPES with linearly polarized horizontal and vertical radiation shows hole-like and electron-like pockets at the Γ and M points of the Fermi surface, consistent with previous observations on cleaved single crystal surfaces. The control achieved in growing quantum materials with volatile elements such as Se by in-situ PLD makes it possible to address the fine analysis of the surfaces by in-situ ARPES and XPS. The study opens wide avenues for the PLD based heterostructures as work-bench for the understanding of proximity-driven effects and for the development of prospective devices based on combinations of quantum materials.

Beamline 13 of the Photon Factory has been in operation since 2010 as a vacuum ultraviolet and soft X-ray undulator beamline for X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and angle-resolved photoelectron spectroscopy (ARPES) experiments. The beamline and the end-station at branch B have been recently upgraded, enabling microscopic XPS, XAS, and ARPES measurements to be performed. In 2015, a planar undulator insertion device was replaced with an APPLE-II (advanced planar polarized light emitter II) undulator. This replacement allows use of linear, circular, and elliptical polarized light between 48 and 2000 eV with photon intensities of 109–1013 photons s−1. For microscopic measurements, a toroidal post-mirror was renewed to have more focused beam with profile sizes of 78 µm (horizontal) × 15 µm (vertical) and 84 µm × 11 µm at photon energies of 100 and 400 eV, respectively. A high-precision sample manipulator composed of an XYZ translator, a rotary feedthrough, and a newly developed goniometer, which is essential for microscopic measurements, has been used to control a sample specimen in six degrees of freedom, i.e. translation in the X, Y, and Z directions and rotation in the polar, azimuthal, and tilt directions. To demonstrate the performance of the focused beams, one- and two-dimensional XPS and XAS scan measurements of a copper grid have been performed. It was indicated from analysis of XPS and XAS intensity maps that the actual spatial resolution can be determined by the beam size.

Graphene samples were grown on the C-face of SiC, at high temperature in a furnace and an Ar ambient, and were investigated using LEEM, XPEEM, LEED, XPS and ARPES. Formation of fairly large grains (crystallographic domains) of graphene exhibiting sharp 1x1 patterns in m-LEED was revealed and that different grains showed different azimuthal orientations. Selective area constant initial energy photoelectron angular distribution patterns recorded showed the same results, ordered grains and no rotational disorder between adjacent layers. A grain size of up to a few mm was obtained on some samples.

Graphitization of the 6H-SiC(0001) surface as a function of annealing temperature has been studied by ARPES, high resolution XPS, and LEED. For the initial stage of graphitization – the 6√3 reconstructed surface – we observe σ-bands characteristic of graphitic sp2-bonded carbon. The π-bands are modified by the interaction with the substrate. C1s core level spectra indicate that this layer consists of two inequivalent types of carbon atoms. The next layer of graphite (graphene) formed on top of the 6√3 surface at TA=1250°C-1300°C has an unperturbed electronic structure. Annealing at higher temperatures results in the formation of a multilayer graphite film. It is shown that the atomic arrangement of the interface between graphite and the SiC(0001) surface is practically identical to that of the 6√3 reconstructed layer.

Intercalation of noble metals can produce giant Rashba-type spin–orbit splittings in graphene. The spin–orbit splitting of more than 100 meV has yet to be achieved in graphene on metal or semiconductor substrates. Here, we report the p-type graphene obtained by Pt intercalation of zero-layer graphene on SiC substrate. The spin splitting of ∼200 meV was observed at a wide range of binding energies. Comparing the results of theoretical studies of different models with the experimental ones measured by spin-ARPES, XPS and STM methods, we concluded that inducing giant spin–orbit splitting requires not only a relatively close distance between graphene and Pt layer but also the presence of graphene corrugation caused by a non-flat Pt layer. This makes it possible to find a compromise between strong hybridization and increased spin–orbit interaction. In our case, the Pt submonolayer possesses nanometer-scale lateral ordering under graphene.

This work reviews the contributions of author to the developments and applications of Photoelectron Spectroscopy (PES) and X-ray Absorption Fine Structure (XAFS) to materials studies. Focusing on Angle resolved PES (ARPES) the energy distribution is discussed for angle-resolved photoemission from valence bands of single crystals. The important influence of the spectrometer angle of acceptance on the results of X-ray PES (XPS) is investigated in detail. The Plane Density of States (PDOS) is introduced as a new property of the electronic structure. Most meaningful contributions to XAFS consist of the developments of multiple-scattering and anharmonic XAFS theory. Anharmonic correlated Einstein model (ACEM) and anharmonic correlated Debye model (ACDM) have been derived to obtain Debye-Waller factors (DWF) presented in terms of cumulant expansion which describe the thermodynamic properties and anharmonic effects in XAFS of substances contributing to their accurate structural determination. The anharmonic effective potential (AEP) procedure and first shell near neighbor contributions approach have developed to include many-body effects in the one-dimensional model by a simple measure. Based on DWFs a thermodynamic lattice theory has been derived for studying melting curve and eutectic points of binary alloys. Several applications of the derived methods are performed and the good agreement of the calculated results with experiment illustrate the advantages and efficiencies of the achieved developments.

With the advent of 2D materials, the playground to study spins in dilute and non-dilute phases has expanded. This is appealing for utilizing the additional degrees of freedom of electron systems such as spin and valley and, from the fundamental point of view, to better understand atomic scale magnetic phenomena in low dimensional materials. Dilute magnetism in 2D materials can lead to complex magnetic phenomena (e.g., Kondo effect, RKKY-interactions, quantum relaxation and coherence), with potential for applications in spintronics (e.g., spin FETs) and quantum technologies (e.g., single-atom quantum memories). We are investigating how to selectively incorporate substitutional magnetic atoms (3d transition metals and 4f rare earths) in 2D materials, using ultra low energy ion implantation, and we characterize their structural, electronic, and magnetic properties [1]. Ultra-low energy (ULE) ion implantation allows us to precisely tune the kinetic energy of the ions, providing control over the form of incorporation and concentration while preserving the structural and electronic properties of graphene. Our approach is based on a wide range of characterization techniques (structural and electronic), including scanning tunneling microscopy and spectroscopy (STM/STS), Raman spectroscopy, synchrotron-based X-ray photoelectron spectroscopy (XPS), angle-resolved photoemission spectroscopy (ARPES), X-ray magnetic circular dichroism (XMCD), among others. These experimental studies are complemented by density functional theory (DFT) and molecular dynamics (MD) simulations. The new insights provided by our work establish a framework for the controlled incorporation of magnetic dopants in 2D materials, using ULE ion implantation. [1] P. C. Lin et al., ACS Nano 15(3), 5449-5458 (2021).

Ce manuscrit de thèse est dédié à l’étude des oxydes de silicium (SiO) et de germanium (GeO) sous forme ultramince. Elaborés par épitaxie par jet moléculaire à la surface d’un cristal de ruthénium (0001), ces systèmes peuvent exister sous deux phases stables. La première est constituée d'une monocouche connectée au substrat par des liaisons covalentes formant une interface métal oxyde. La seconde est quant à elle constituée d’une bicouche en interaction faible déconnectée du substrat. La faiblesse des interactions de Van der Waals permet son exfoliation pour l’intégrer dans des hétérostructures fonctionnelles. Dans cette thèse nous étudions la relation entre structure et propriétés électroniques de ces matériaux bidimensionnels (2D) en combinant la microscopie champ proche (STM), la photoémission X et UV résolue en angle (XPS, ARPES), la diffraction X de surface (SXRD) et la modélisation par des méthodes DFT. Une partie de nos mesures (XPS, ARPES et SXRD) ont été obtenues à l'aide du rayonnement synchrotron. Si les propriétés de la bicouche d’oxyde de silicium (SiO) sont bien comprises, la description des propriétés électroniques de l’interface métal-oxyde s'avère plus complexe avec l’impossibilité de réconcilier les calculs emph{ab initio} avec nos mesures ARPES. Pour comprendre l’origine de ce désaccord, nous avons étudié l'interface GeO/Ru(0001) dans le régime de la monocouche. Nos études STM et XPS ont permis de valider le modèle atomique proposé par la DFT, incluant la rotation des liaisons Ge-O-Ge et la présence d'un oxygène interstitiel. Des études structurales complémentaires par SXRD ont permis de valider la relation d'épitaxie proposée par le calcul. Enfin, la structure de bande mesurée se rapproche des prédictions DFT contrairement au SiO même s'il persiste un faible désaccord. Celui-ci peut s'interpréter comme une surestimation de la force de la liaison métal/oxyde par le calcul introduisant une bande interdite au point Gamma et au point K non visibles expérimentalement en ARPES dans le cas de SiO. Des mesures SXRD complémentaires à venir sur SiO permettront d'étayer cette hypothèse
This thesis manuscript is dedicated to the study of silicon (SiO) and germanium (GeO) oxides in their ultra-thin forms. Developed by molecular beam epitaxy on the surface of a ruthenium (0001) crystal, these systems can exist in two stable phases. The first one is a monolayer connected to the substrate by covalent bonds forming a metal-oxide interface. The second one is a weakly interacting bilayer disconnected from the substrate. The weakness of the Van der Waals interactions allows its exfoliation to integrate it into functional heterostructures. In this thesis we study the relationship between structural and electronic properties of these two-dimensional (2D) materials by combining scanning tunneling microscopy (STM), angle-resolved photoemission (XPS, ARPES), surface X-ray diffraction (SXRD) and modelling by DFT methods. Some of our measurements (XPS, ARPES and SXRD) were obtained using synchrotron radiation. If the properties of the silicon oxide (SiO) bilayer are well understood, the description of the electronic properties of the metal-oxide interface proves to be more complex with the impossibility of reconciling the calculations with our ARPES measurements. To understand the origin of this disagreement, we studied the GeO/Ru(0001) interface in the monolayer regime. Our STM and XPS studies validated the atomic model proposed by DFT, including the rotation of Ge-O-Ge bonds and the presence of an interstitial oxygen. Complementary structural studies by SXRD validated the epitaxial relationship proposed by the calculation. Finally, the measured band structure is close to the DFT predictions, contrary to SiO, even if a small disagreement remains. This can be interpreted as an overestimation of the metal oxide bond strength by the calculation introducing a band gap at the Gamma point and at the K point not experimentally visible in ARPES in the case of SiO. Further SXRD measurements on SiO will support this hypothesis

Le sujet de cette thèse s'inscrit dans la thématique des matériaux bidimensionnels (2D) d'épaisseur atomique. L'étude des propriétés optiques et électroniques des hétérostructures hybrides à base de dichalcogénures de métaux de transition (TMD) MX₂ (M = Mo, W ; X = S, Se, Te) est aujourd'hui considérée avec attention en raison de futures applications et d'études plus fondamentales. Plus que leurs propriétés physiques intrinsèques, en configuration multicouche, ces matériaux offrent des phénomènes physiques prometteurs tels que la modulation de la valeur de la bande interdite, la ferroélectricité pour des configurations cristallines spécifiques, etc. En particulier, ce travail se consacre aux hétérostructures hybrides à base de diséléniure de tungstène (WSe₂) sur des substrats de graphène et de phosphate de gallium (GaP). En mettant en œuvre des techniques de microscopie et de spectroscopie telles que la spectroscopie Raman et la spectroscopie de photoémission résolue en angle (ARPES), une étude des propriétés électroniques, optiques et structurelles d'hétérostructures composées de plusieurs matériaux 2D a été réalisée afin de fournir une meilleure compréhension de ces systèmes émergents. Par conséquent, les premières mesures directes de la structure de bande électronique de la phase rhomboédrique de la structure bicouche de WSe₂ déposée sur un substrat 2D de graphène sont présentées dans ce manuscrit. La croissance directe de ce matériau 2D sur un substrat 3D de GaP a été étudiée pour plusieurs épaisseurs. Ces travaux ont permis d'identifier l'effet de la nature de la phase cristalline ainsi que la méthode de croissance sur les structures de bandes électroniques, ce qui permet une meilleure compréhension de ces systèmes émergents
The subject of this thesis is two-dimensional (2D) materials of atomic thickness. The study of the optical and electronic properties of hybrid heterostructures based on MX₂ transition metal dichalcogenides (TMDs) (M = Mo, W; X = S, Se, Te) is now being carefully considered with a view to future applications and more fundamental studies. Beyond their intrinsic physical properties, in multilayer configurations, these materials offer promising physical phenomena such as modulation of bandgap values, ferroelectricity for specific crystal configurations, and so on. In particular, this work focuses on hybrid heterostructures based on tungsten diselenide (WSe₂) on graphene and gallium phosphate (GaP) substrates. Using microscopy and spectroscopy techniques such as Raman spectroscopy and angle-resolved photoemission spectroscopy (ARPES), we investigated the electronic, optical, and structural properties of heterostructures composed of several 2D materials to better understand these emerging systems. Accordingly, the first direct measurements of the electronic band structure of the rhombohedral phase of the WSe₂ bilayer structure deposited on a 2D graphene substrate are presented in this manuscript. The direct growth of this 2D material on a 3D GaP substrate has been studied for several thicknesses. This work has enabled us to identify the effect of the nature of the crystalline phase and the growth method on the electronic band structures, providing a better understanding of these emerging systems

The doping of quasi-freestanding graphene (QFG) on H-terminated, Si-face 6H-, 4H-, and 3C-SiC is studied by angle-resolved photoelectron spectroscopy (ARPES) close to the Dirac point. Using semi-insulating as well as n-type doped substrates we shed light on the contributions to the charge carrier density in QFG caused by i) the spontaneous polarization of the substrate, and ii) the band alignment between the substrate and the graphene layer. In this way we provide quantitative support for the previously suggested model of polarization doping of graphene on SiC [Phys. Rev. Lett. 108, 246104 (2012)].

Nous étudions l'adsorption de molécules de C60 sur deux reconstructions riches en silicium du 6H-SiC(0001) par IPES, UPS et XPS. Nous mettons en évidence que l'adsorption de C60 sur (3*3) est singulière et définit un nouveau type de liaison entre C60 et substrat : liaison covalente forte avec désorption par recuit à haute température et récupération de la reconstruction de surface. Ces expériences illustrent la complexité de la liaison Si-C60 et permettent une nouvelle mise en perspective.En combinant ARPES à basse énergie de photon et DFT sur une monocouche de ZnPc sur Ag(110), nous prouvons que l'effet de "Umklapp de surface" est effectif pour un réseau de molécules organiques organisé à grande distance. C'est à dire que les conditions de sortie des photoélectrons de volume sont modifiées par la présence du réseau.Nous démontrons aussi que l'HREELS est une technique de choix pour l'étude de l'adsorption d'hydrogène sur graphène, et l'étude de l'interaction d'un plan de graphène sur un substrat, ici le SiC. En effet l'adsorption (réversible) d'atomes d'hydrogène sur du graphène permet à l'HREELS d'être sensible sous le plan de graphène
We study by IPES, UPS and XPS the adsorption of fullerene on two silicon-rich reconstructions of 6H-SiC(0001). We show that adsorption of C60 on the (3*3) is singular and defines a new bonding type between C60 and a substrate: covalent bond accompanied by the desorption of molecules and the reconstruction's recovery. Our experiments shed a new light on the Si-C60 bounding complexity and provide new insights.By combining low photon energy ARPES and DFT on a monolayer of ZnPc on Ag(110), we provide a direct evidence that the "surface Umklapp'" effect is effective for long-range ordered organic films. Namely, the photoelectrons escape conditions are modified by the bare presence of the molecular lattice.We show that HREELS is a convenient tool to investigate the adsorption of hydrogen on graphene and the interaction of graphene with a substrate, SiC in our study. Indeed, the reversible adsorption of hydrogen on graphene permits the HREELS to gain sensitivity below the graphene layer

Le premier chapitre de cette thèse présente l’intérêt et la problématique de la fonctionnalisation du graphène. L’état de l’art actuel de cette thématique est présenté. Dans un deuxième chapitre, nous discutons de façon détaillée des techniques expérimentales. Le chapitre 3 est centré sur la modification du graphène par réaction de cycloaddition par molécules dérivées de maleimides. Dans cette étude, nous démontrons le greffage covalent de molécules sur graphène épitaxié sans défaut sur SiC, ainsi qu’une tendance d’ouverture de bande interdite à l’aide de caractérisations par spectroscopie Raman, XPS, ARPS et STM. L’augmentation du rapport ID /IG des pics Raman et des liaisons sp3 sur l’échantillon en fonction de la durée de réaction chimique confirme le greffage. Par analogie avec les bords de marche de type « zigzag » ou « armchair », l’étude des ondes de densité de charge générées sur le graphène par les molécules permet de déterminer la nature des sous-réseaux mis en jeu lors du greffage. Dans le chapitre 4, nous étudions l’intercalation du terbium dans le graphène épitaxié. Après intercalation, l’ARPES montre une structure de bande complexe dont une composante correspond à une monocouche de graphène fortement dopée n. Nous avons pu isoler cette composante et montrer qu’elle provient du découplage de la couche tampon du substrat par le Terbium. Ces résultats sont confirmés par les données XPS. Le graphène avec Terbium intercalé produit également un réseau de lignes visibles par imagerie STM, qui a l’échelle atomique à basse tension montrent les 6 atomes de carbone de la structure en nid d’abeille, confirmant ainsi la transformation de la couche tampon en graphène
The first chapter of this thesis explains the general motivation and problematic of graphene functionalization. It presents the state of the art of current research in this field. In the second chapter we discuss the experimental techniques in detail. Chapter 3 of this thesis work focuses on covalent modification of graphene by cycloaddition reaction of maleimide derivative molecules. In these studies we have confirmed the grafting of molecules on epitaxial defect free graphene on SiC and a tendency to open a gap with the help of Raman spectroscopy, XPS, ARPES and STM studies. An increase in the ID /IG ratio for Raman signature and sp3 bonding on the sample with increasing reaction time confirmed the reaction of molecules. By drawing an analogy with the standing waves obtained on armchair step edges of graphene and standing waves generated by molecules it was possible to determine the location of grafted molecules on the graphene lattice. In chapter 4, studies on terbium intercalation of epitaxial graphene are discussed. After intercalation a complex band structure was observed by ARPES with one spectra corresponding to highly n-doped graphene monolayer. We were able to isolate this highly n-doped graphene and confirmed its origin from decoupling of buffer layer and making it graphene like. These results are also supported by the XPS data. STM images on Terbium intercalated on buffer layer samples showed an interesting pattern of lines, atomic resolution scans at low bias voltage on these lines showed 6 atoms of hexagon confirming the transformation of buffer layer into graphene layer

La réalisation de nouveaux matériaux bidimensionnels est un domaine en plein essor de la matière condensée, à la fois pour les aspects fondamentaux, avec les propriétés exotiques émergeant de la dimensionnalité réduite, et pour les applications technologiques potentielles, avec des promesses telles que des courants sans dissipation et des hétérostructures 2D plus performantes que la technologie actuelle à base de silicium à une fraction de la taille. Dans ce travail, nous avons adopté une approche expérimentale pour la réalisation et la caractérisation de matériaux prédits pour accueillir des lignes nodales de Dirac (DNL), qui malgré de nombreuses prédictions théoriques ont vu peu de réalisations expérimentales rapportées jusqu'à présent. Ces matériaux appartiennent à la classe récemment mise en évidence des semi-métaux topologiques, dont la spécificité est un croisement de bandes protégé par symétrie entre les bandes de valence et de conduction le long d'une ligne dans l'espace réciproque, avec une dispersion linéaire. Dans un premier temps, nous nous sommes concentrés sur Cu2Si, le premier matériau 2D dans lequel des DNL ont été mis en évidence lorsqu'il est préparé sur un substrat Cu(111). Après avoir reproduit avec succès les résultats existants, nous avons montré à l'aide de l'ARPES et du XPS que, contrairement aux attentes, la structure électronique et les DNL étaient préservées après le dépôt de Pb sur la surface. Nous avons ensuite étudié Cu2Si/Si(111), et constaté que malgré une structure atomique fortement liée, le substrat Si(111) interagit assez fortement avec les orbitales hors plan de la couche Cu2Si pour empêcher l'existence des lignes nodales. Nous nous sommes ensuite penchés sur le système 2D Cu2Ge, prédit pour accueillir la DNL, et avons tenté de le synthétiser en déposant du Ge sur Cu(111). En combinant nos résultats LEED, XPS et ARPES, nous avons constaté que toutes les mesures correspondaient étroitement à ce que l'on attendait d'une monocouche de Cu2Ge libre, ce qui montre l'absence presque totale d'interactions entre le substrat Cu(111) et la couche de Cu2Ge superficielle formée sur celui-ci. Il s'agit de la première réalisation expérimentale rapportée de Cu2Ge. Dans une étude miroir, nous avons déposé Cu sur Ge(111) et observé une structure de bande dissemblable. À l'aide du STM, nous avons expliqué ces différences par une structure atomique différente, résultant d'un substrat à forte interaction. Nous soulignons par ce travail l'influence du substrat, qu'il soit métallique ou semi-conducteur, sur les propriétés électroniques des systèmes 2D à DNL
The realization of new two-dimensional materials is a booming field of condensed matter, at once for the fundamental aspects, with the exotic properties emerging from the reduced dimensionality, and for the potential technological applications, with promises such as dissipationless currents and 2D heterostructures outperforming the current silicon-based technology at a fraction of the size. In this work, we took an experimental approach to the realization and characterization of materials predicted to host Dirac nodal lines (DNLs), which despite many theoretical predictions have seen few experimental realizations reported so far. These materials belong to the recently evidenced class of topological semimetals, whose specificity is a symmetry-protected band crossing of the valence and conduction bands along a line in momentum space, with linear dispersion. As a first step, we focused on Cu2Si, the first 2D material in which DNLs have been evidenced when prepared on a Cu(111) substrate. After successfully reproducing existing results, we showed using ARPES and XPS that contrary to expectations, the DNLs were preserved after deposition of Pb on the surface without any gap, and that a band splitting occurred. We followed by the investigation of Cu2Si/Si(111), and found that despite a strongly related atomic structure, the Si(111) substrate interacts strongly enough with the out-of-plane orbitals of the Cu2Si layer to prevent the existence of the nodal lines. We then looked at the 2D Cu2Ge system, predicted to host DNL, and attempted to synthesize it by depositing Ge on Cu(111). By combining our LEED, XPS and ARPES results we found that all measurements matched closely what was expected from a free-standing Cu2Ge monolayer, showing the almost complete absence of interactions between the Cu(111) substrate and the surface Cu2Ge layer grown on it. This is the first reported experimental realization of the two-dimensional Dirac nodal line semimetal Cu2Ge. In a mirroring study, we deposited Cu on Ge(111) and observed a dissimilar band structure. Helped by STM, we explained those differences by a different atomic structure, and by a strongly interacting substrate. We highlight through this work the influence of the substrate, whether metallic or semiconductor, on the electronic properties of 2D DNL systems

The research presented in this thesis concerns experimental studies of thin manganese silicide and germanide layers, grown by solid phase epitaxy on the Si(111)7×7 and the Ge(111)c(2×8) surfaces, respectively. The atomic and electronic structures, as well as growth modes of the epitaxial Mn-Si and Mn-Ge layers, were investigated by low-energy electron diffraction (LEED), angle-resolved photoelectron spectroscopy (ARPES), core-level spectroscopy (CLS), and scanning tunneling microscopy and spectroscopy (STM and STS). The magnetic properties of the Mn-Ge films were investigated by X-ray magnetic circular dichroism (XMCD). The Mn-Si layers, annealed at 400 °C, showed a √3×√3 LEED pattern, consistent with the formation of the stoichiometric monosilicide MnSi. Up to 4 monolayers (ML) of Mn coverage, island formation was observed. For higher Mn coverages, uniform film growth was found. Our results concerning morphology and the atomic and electronic structure of the Mn/Si(111)-√3×√3 surface, are in good agreement with a recent theoretical model for a layered MnSi structure and the √3×√3 surface structure. Similar to the Mn-Si case, the grown Mn-Ge films, annealed at 330 °C and 450 °C, showed a √3×√3 LEED pattern. This indicated the formation of the ordered Mn5Ge3 germanide. A strong tendency to island formation was observed for the Mn5Ge3 films, and a Mn coverage of about 32 ML was needed to obtain a continuous film. Our STM and CLS results are in good agreement with the established model for the bulk Mn5Ge3 germanide, with a surface termination of Mn atoms arranged in a honeycomb pattern. Mn-Ge films grown at a lower annealing temperature, 260 °C, showed a continuous film at lower coverages, with a film structure that is different compared to the structure of the Mn5Ge3 film. XMCD studies showed that the low-temperature films are ferromagnetic for 16 ML Mn coverage and above, with a Curie temperature of ~250 K.

碩士
國立中央大學
物理學系
102
We have studied the adsorption and desorption of atomic hydrogen on the surfaces of Ag thin films. Atomically flat Ag films were grown on Au(111), exposed to a flux of thermally generated hydrogen atoms at a low sample temperature, and annealed for the desoprtion of hydrogen atoms. During annealing, the evolutions of the surfaces of the thin films were characterized on monitoring their surface states with angle-resolved photoemission spectroscopy. Our experimental results showed that, with the exposure of hydrogen atoms, the surfaces states of thin Ag films deteriorated severely, and the surface states recovered only partially during annealing. In addition, we also investigated the relationship between the deterioration of the surface states and the thickness of the thin films.