93 research outputs found
Darkness in interlayer and charge density wave states of 2H-TaS2
No full textThe wavelike nature of electrons is evident from quantum interference effects observed during the photoemission process. When there are different nuclei in the unit cell of a crystal and/or structural distortions, photoelectron wave functions can interfere, giving rise to a peculiar intensity modulation of the spectrum, which can also hide energy states in a photoemission experiment. The 2H phase of transition metal dichalcogenides, with two nonequivalent layers per unit cell and charge density wave distortion, is an optimal platform for such effects to be observed. Here, we discover undetectable states in 2H-TaS2, interpreting high-resolution angular resolved photoemission spectroscopy considering interference effects of the correlated electron wave functions. In addition, phase mismatching induced by charge density wave distortion results in an evident signature of the phase transition in the photoemission spectrum. Our results highlight the importance of quantum interference, electronic correlations, and structural distortion to understand the physics of layered materials
Elektronische Eigenschaften von Halbleiter Oberflächen und Metall/Halbleiter Grenzflächen
No full textTitle page and contents
Acknowledgements
Abstract
Kurzfassung
1\. Introduction
2\. Experimental techniques for surface physics
3\. Si(0001) cleaved surface reconstructions
4\. Thin Manganese films on Si(111)
5\. Al-Mg alloy thin films on Si(111)
BibliographyThis thesis reports investigations of the electronic properties of a
semiconductor surface (silicon carbide), a reactive metal/semiconductor
interface (manganese/silicon) and a non-reactive metal/semiconductor interface
(aluminum-magnesium alloy/silicon). The (2x1) reconstruction of the 6H-
SiC(0001) surface has been obtained by cleaving the sample along the 0001
direction. This reconstruction has not been observed up to now for this
compound, and has been compared with those of similar elemental semiconductors
of the fourth group of the periodic table. This comparison has been carried
out by making use of photoemission spectroscopy, analyzing the core level
shifts of both Si 2p and C 1s core levels in terms of charge transfer between
atoms of both elements and in different chemical environments. From this
comparison, a difference between the reconstruction on the Si-terminated and
the C-terminated surface was established, due to the ionic nature of the Si-C
bond. The growth of manganese films on Si(111) in the 1-5 ML thickness range
has been studied by means of LEED, STM and photoemission spectroscopy. By the
complementary use of these surface science techniques, two different phases
have been observed for two thickness regimes (1 ML), which exhibit
a different electronic character. The two reconstructions, the (1x1)-phase and
the (√3 x √3)R30°-phase, are due to silicide formation, as observed in core
level spectroscopy. The growth proceeds via island formation in the monolayer
regime, while the thicker films show flat layers interrupted by deep holes. On
the basis of STM investigations, this growth mode has been attributed to
strain due to lattice mismatch between the substrate and the silicide. Co-
deposition of Al and Mg onto a Si(111) substrate at low temperature (100K)
resulted in the formation of thin alloy films. By varying the relative content
of both elements, the thin films exhibited different electronic properties,
manifested by the observation of quantum well states and a surface state. The
resulting shift in binding energy of both quantum well states and surface
state has been interpreted in terms of the virtual crystal approximation model
where the main effect of the alloying process is attributed to the change of
electron density of the system. For this system, the variation of
photoemission intensity as a function of photon energy has been also
investigated and explained in terms of collective excitations.Diese Dissertation befasst sich mit der Untersuchung der elektronischen
Eigenschaften einer Halbleitersoberfläche (Siliziumcarbid), einer reaktiven
Metall/Halbleiter-Grenzfläche (Mangan/Silizium) und einer nicht-reaktiven
Metall/Halbleiter-Grenzfläche (Aluminum-Magnesium-Legierung/Silizium). Die
(2x1) Rekonstruktion der 6H-SiC(0001) Oberfläche wurde durch Spaltung der
Probe entlang der 0001 Richtung erlangt. Diese Rekonstruktion ist für diesen
Verbundhalbleiter noch nie beobachtet worden. In dieser Arbeit wird SiC mit
den Element-Halbleitern der 4. Gruppe des Periodensystems verglichen. Dieser
Vergleich wurde mittels Photoelektronenspektroskopie durchgeführt, durch die
Analyse von Verschiebungen des Si 2p und C 1s Rumpfniveaus im Sinne von
Ladungsaustausch zwischen Atomen der beiden Elemente und in unterschiedlichen
Umgebungen. Mittels dieses Vergleiches konnte ein Unterschied zwischen den
Rekonstruktionen auf der Si-terminierten und der C-terminierten Oberfläche
aufgrund der ionischen Eigenschaft der Si-C Bindung interpretiert werden. Das
Wachstum von dünnen Mangan-Schichten auf Si(111) im Bereich von 1-5 Monolagen
wurde mittels LEED, STM und Photoemission untersucht. Durch den Einsatz dieser
Methoden wurden zwei verschiedene Phasen im Bereich unterhalb einer Monolage
sowie oberhalb dieser Schichtdicke beobachtet, welche eine unterschiedliche
elektronische Struktur aufweisen. Die beiden beobachteten Rekonstruktionen,
(1x1) und (√3 x √3)R30° entstehen durch Bildung von Mangan-Silizid, wie durch
Rumpfniveau-Spektroskopie nachgewiesen wurde. Das Wachstum geschieht durch
Inselbildung im Bereich der Monolage, während die dickeren Schichten flache
Regionen zeigen, welche von tiefen Löchern unterbrochen werden. Dieser
Wachstumsmodus wird auf das Auftreten von Verspannungen, aufgrund der
Gitterfehlanpassung zwischen Substrat und Schicht, zurückgeführt, deren
Einfluß in den STM-Bildern zu beobachten ist. Durch Kodeposition von Al und Mg
auf Si(111) wurden Al/Mg-Legierungs- Schichten erzeugt. Durch Variation der
Verdampfungsraten wurden Legierungen mit unterschiedlicher Zusammensetzung
erzeugt, und deren elektronische Struktur durch Valenz Photoemission
untersucht. In den Schichten werden Quantentopf-Zustände und Oberflächen-
Zustände beobachtet. Deren Verschiebung als Funktion der Zusammensetzung wird
im Rahmen der "virtual crystal approximation" analysiert. Die Variation der
Photoemissions-Intensität als Funktion der Photonen- Energie konnte durch die
Anregung von Plasmonen erklärt werden, deren Energien von der Elektronendichte
der Legierung abhängen
Ferromagnetic resonance of Co thin films grown by atomic layer deposition on the Sb2Te3 topological insulator
No full textInterfacing ferromagnetic materials with topological insulators is an intriguing strategy in order to enhance spin-to-charge conversion mechanisms, paving the way toward highly efficient spin-based electronic devices. In particular, the use of large-scale deposition techniques is demanding for a sustainable and cost-effective industrial technology transfer. In this work, we study the magnetic properties of the Co/Sb2Te3 heterostructure, where the ferromagnetic Co layer is deposited by atomic layer deposition on top of the Sb2Te3 topological insulator, which is grown by metal organic chemical vapor deposition. In particular, broadband ferromagnetic resonance is employed to characterize the Co/Sb2Te3 system and the reference Co/Pt heterostructure. For Co/Sb2Te3, we extract an effective magnetic anisotropy constant Keff=4.26∙106ergcm3, which is an order of magnitude higher than in Co/Pt Keff=0.43∙106ergcm3. The large difference in the Keff values observed in Co/Sb2Te3 and Co/Pt is explained in terms of the different Co crystalline structures achieved on top of Sb2Te3 and Pt, respectively. Interestingly, the Co/Sb2Te3 system displays a relatively large Gilbert damping constant (α = 0.095), which we suggest as possibly due to spin pumping from the Co layer into the Sb2Te3 topological insulator
Atom-Specific Identification of Adsorbed Chiral Molecules by Photoemission
No full textThe study of chiral adsorbed molecules is important for an analysis of enantioselectivity in heterogeneous catalysis. Here we show that such molecules can be identified through circular dichroism in core-level photoemission arising from the chiral carbon atoms in stereoisomers of 2,3-butanediol molecules adsorbed on Si(100), using circularly polarized x rays. The asymmetry in the carbon 1s intensity excited by right and left circularly polarized light is readily observed, and changes sign with the helicity of the radiation or handedness of the enantiomers; it is absent in the achiral form of the molecule. This observation demonstrates the possibility of determining molecular chirality in the adsorbed phase
Surface functionalization of nanostructured Fe2O3 polymorphs: from design to light-activated applications
No full textNanostructured iron(III) oxide deposits are grown by chemical vapor deposition (CVD) at 400−500 °C on Si(100) substrates from Fe(hfa)2TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′-tetramethylethylenediamine), yielding the selective formation of α-Fe2O3 or the scarcely studied ε-Fe2O3 polymorphs under suitably optimized preparative conditions. By using Ti(OPri)4 (OPri = iso-propoxy) and water as atomic layer deposition (ALD) precursors, we subsequently functionalized the obtained materials at moderate temperatures (<300 °C) by an ultrathin titanomagnetite (Fe3−xTixO4) overlayer. An extensive multitechnique characterization, aimed at elucidating the system structure, morphology, composition and optical properties, evidenced that the photoactivated hydrophilic and photocatalytic behavior of the synthesized materials is dependent both on iron oxide phase composition and ALD surface modification. The proposed CVD/ALD hybrid synthetic approach candidates itself as a powerful tool for a variety of applications where semiconductor-based nanoarchitectures can benefit from the coupling with an ad hoc surface layer
Height distribution of atomic force microscopy images as a tool for atomic layer deposition characterization
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