2,463 research outputs found
Effect of a skin-deep surface zone on formation of two-dimensional electron gas at a semiconductor surface
Two dimensional electron gases (2DEGs) at surfaces and interfaces of
semiconductors are described straightforwardly with a 1D self-consistent
Poisson-Schr\"{o}dinger scheme. However, their band energies have not been
modeled correctly in this way. Using angle-resolved photoelectron spectroscopy
we study the band structures of 2DEGs formed at sulfur-passivated surfaces of
InAs(001) as a model system. Electronic properties of these surfaces are tuned
by changing the S coverage, while keeping a high-quality interface, free of
defects and with a constant doping density. In contrast to earlier studies we
show that the Poisson-Schr\"{o}dinger scheme predicts the 2DEG bands energies
correctly but it is indispensable to take into account the existence of the
physical surface. The surface substantially influences the band energies beyond
simple electrostatics, by setting nontrivial boundary conditions for 2DEG
wavefunctions.Comment: 9 pages, 7 figures, 2 table
Determination of the photoelectron reference plane in nanostructured surfaces
In angle-resolved photoemission (ARPES) from crystalline solids, wave-vector conservation applies to the two-dimensional (2D) surface, which may thus be defined as the reference plane in ARPES. We investigate whether such reference varies for photoemitted electrons in nanometer-sized systems that expose different crystal planes. To this aim, we exploit the structural tunability of the Ag/Cu(223) system which is capable of offering surfaces with periodic arrays of nanofacets of varying size and orientation. A thorough, photonenergy- dependent analysis of the surface states confined to such nanostructures is performed comparing different reference planes for photoemitted electrons. Assuming the premise that k|| must be a good quantum number for 2D states, we conclude that the (final state) photoelectron reference direction is not the average optical direction but the local facet that confines the (initial state) surface electrons. Moreover, in the general case of nanostructured systems with uneven surfaces, we show how the photoelectron reference plane can be empirically determined through such a photon-energy-dependent ARPES analysisJLC and AM acknowledge the Spanish Ministerio de Ciencia e Innovación (MICINN) for financial support through the research program Ramón y Cajal. JEO acknowledges support from the Spanish MICINN (MAT2010-21156-C03-01), the Basque Government (IT-257-07) and the DIPC (sabbatical program). EGM acknowledges support from the Spanish MICINN (FIS2008-00399
Effect of photoelectron mean free path on the photoemission cross-section of Cu(111) and Ag(111) Shockley states
The photoemission cross-section of Shockley states of Cu(111) and Ag(111) surfaces is studied over a wide range of photon energies. The constant initial-state spectra are very different for the two surfaces and show rich structure that does not follow the generally accepted nearly free electron model for the final state. Angle resolved photoemission data are interpreted within a one-step ab initio theory, revealing a multiple Bloch wave structure of photoemission final states. The inelastic scattering parameter-optical potential-is determined, and the energy dependence of the mean free path of the outgoing electron is calculated, which turns out to be the key for the understanding of the photoemission cross-section curve. These are essential steps for future exploration of wave function perturbations in the presence of surface nanostructures. 2011 American Physical Society.This work was supported by the Spanish Ministerio de Ciencia e Innovación (Grants No. FIS2010-19609-C02-02, FIS2008-00399, MAT2010-21156-C03-01, and MAT2010-21156-C03-02 and through the Research Program Ramón y Cajal) and the Basque Government (IT-257-07). The SRC is funded by the National Science Foundation (Award No. DMR-0084402).Peer Reviewe
Accurate band mapping via photoemission from thin films
Electron bands in solids can be determined in angle-resolved photoemission experiments from thin films, where the perpendicular wave vector (k⊥) uncertainty that characterizes photoemission from bulk crystals is removed. However, the comparison with state-of-the-art quasiparticle band-structure calculations has never been done. In this work we have mapped both initial-state (occupied) and final-state (empty) E(k⊥) bands along the A axis of aluminum, from photon-energy- and thickness-dependent quantum-well spectra of aluminum films. For final states the best fit is obtained with inverse low-energy electron diffraction band structure calculations. For initial-state bands of Cu and Al, thin-film data display excellent agreement with bulk quasiparticle theory, suggesting the use of thin films as model systems to investigate fine effects in the crystal band structure.This work has been supported in part by the Universidad del País Vasco (A.Mu. and J.E.O. under Contract No. 00057.240-EA-13668/2001, and A.Ma., A.R., and F.J.G.A. under Contract No. 00206.215-13639/2001), the European Community (EC) research training network NANOPHASE [A.Ma. and A.R. (Grant No. HPRN-CT-2000-00167)], and the Spanish Ministerio de Ciencia y Tecnología [A.Ma., A.R., and F.J.G.A. (Grant No. MAT2001-0946), and J.L. and E.G.M. (BFM2001-0244)].Peer Reviewe
Tunable Graphene Electronics with Local Ultrahigh Pressure
We achieve fine tuning of graphene effective doping by applying ultrahigh
pressures (> 10 GPa) using Atomic Force Microscopy (AFM) diamond tips. Specific
areas in graphene flakes are irreversibly flattened against a SiO2 substrate.
Our work represents the first demonstration of local creation of very stable
effective p-doped graphene regions with nanometer precision, as unambiguously
verified by a battery of techniques. Importantly, the doping strength depends
monotonically on the applied pressure, allowing a controlled tuning of graphene
electronics. Through this doping effect, ultrahigh pressure modifications
include the possibility of selectively modifying graphene areas to improve
their electrical contact with metal electrodes, as shown by Conductive AFM.
Density Functional Theory calculations and experimental data suggest that this
pressure level induces the onset of covalent bonding between graphene and the
underlying SiO2 substrate. Our work opens a convenient avenue to tuning the
electronics of 2D materials and van der Waals heterostructures through pressure
with nanometer resolution
Iron Oxyhydroxide-Covalent Organic Framework Nanocomposite for Efficient As(III) Removal in Water
The presence of heavy metal ions in water is an
environmental issue derived mainly from industrial and mineral
contamination. Metal ions such as Cd(II), Pb(II), Hg(II), or
As(III) are a significant health concern worldwide because of their
high toxicity, mobility, and persistence. Covalent organic frameworks (COFs) are an emerging class of crystalline organic porous
materials that exhibit very interesting properties such as chemical
stability, tailored design, and low density. COFs also allow the
formation of composites with remarkable features because of the
synergistic combination effect of their components. These
characteristics make them suitable for various applications,
among which water remediation is highly relevant. Herein, we
present a novel nanocomposite of iron oxyhydroxide@COF
(FeOOH@Tz-COF) in which lepidocrocite (γ-FeOOH) nanorods are embedded in between the COF nanoparticles favoring
As(III) remediation in water. The results show a remarkable 98.4% As(III) uptake capacity in a few minutes and impressive removal
efficiency in a wide pH range (pH 5−11). The chemical stability of the material in the working pH range and the capability of
capturing other toxic heavy metals such as Pb(II) and Hg(II) without interference confirm the potential of FeOOH@Tz-COF as an
effective adsorbent for water remediation even under harsh conditionsThis work has been supported by the Spanish MINECO
(PID2019-106268GB-C32 and PCI2019-103594) and through
the “María de Maeztu” Programme for Units of Excellence in
R&D (CEX2018-000805-M
Unveiling the oxidation behavior of liquid-phase exfoliated antimony nanosheets
Antimonene, a monolayer of β-antimony, is increasingly attracting considerable attention, more than that of other monoelemental two-dimensional materials, due to its intriguing physical and chemical properties. Under ambient conditions, antimonene exhibits a high thermodynamic stability and good structural integrity. Some theoretical calculations predicted that antimonene would have a high oxidation tendency. However, it remains poorly investigated from the experimental point of view. In this work, we study the oxidation behavior of antimonene nanosheets (ANS) prepared by ultrasonication-assisted liquid-phase exfoliation. Using a set of forefront analytical techniques, a clear effect of sonication time on the surface chemistry of prepared ANS is found. A dynamic oxidation behavior has been observed, which upon annealing at moderate temperature (210 °C) resulted in a semiconducting behavior with a bandgap of approximately 1 eV measured by ultraviolet photoelectron spectroscopy. This study yields valuable information for future applications of antimonene and paves the way towards novel modification approaches in order to tailor its properties and complement its limitations
Exfoliation of Alpha-Germanium: A Covalent Diamond-Like Structure
2D materials have opened a new field in materials science with outstanding scientific and technological impact. A largely explored route for the preparation of 2D materials is the exfoliation of layered crystals with weak forces between their layers. However, its application to covalent crystals remains elusive. Herein, a further step is taken by introducing the exfoliation of germanium, a narrow-bandgap semiconductor presenting a 3D diamond-like structure with strong covalent bonds. Pure α-germanium is exfoliated following a simple one-step procedure assisted by wet ball-milling, allowing gram-scale fabrication of high-quality layers with large lateral dimensions and nanometer thicknesses. The generated flakes are thoroughly characterized by different techniques, giving evidence that the new 2D material exhibits bandgaps that depend on both the crystallographic direction and the number of layers. Besides potential technological applications, this work is also of interest for the search of 2D materials with new properties
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