17 research outputs found
Emergence of metallic surface states and negative differential conductance in thin -FeSi films on Si(001)
The electronic properties of the surface of -FeSi have been
debated for a long while. We studied the surface states of -FeSi
films grown on Si(001) substrates using scanning tunnelling microscopy (STM)
and spectroscopy (STS), with the aid of density functional theory (DFT)
calculations. STM simulations using the surface model proposed by Romanyuk et
al. [Phys. Rev. B 90, 155305 (2014)] reproduce the detailed features of
experimental STM images. The result of STS showed metallic surface states in
accordance with theoretical predictions. The Fermi level was pinned by a
surface state that appeared in the bulk band gap of the -FeSi film,
irrespective of the polarity of the substrate. We also observed negative
differential conductance at 0.45 eV above the Fermi level in STS
measurements performed at 4.5 K, reflecting the presence of an energy gap in
the unoccupied surface states of -FeSi.Comment: 16 pages, 5 figure
Topological surface states hybridized with bulk states of Bi-doped PbSb2Te4 revealed in quasiparticle interference
Topological surface states of Bi-doped PbSb2Te4 [Pb(Bi0.20Sb0.80)2Te4] are
investigated through analyses of quasiparticle interference (QPI) patterns
observed by scanning tunneling microscopy. Interpretation of the experimental
QPI patterns in the reciprocal space is achieved by numerical QPI simulations
using two types of surface density of states produced by density functional
theory calculations or a kp surface state model. We found that the Dirac point
(DP) of the surface state appears in the bulk band gap of this material and,
with the energy being away from the DP, the isoenergy contour of the surface
state is substantially deformed or separated into segments due to hybridization
with bulk electronic states. These findings provide a more accurate picture of
topological surface states, especially at energies away from the DP, providing
valuable insight into the electronic properties of topological insulators.Comment: 7+8 pages, 4+5 figure
Experimental verification of band convergence in Sr and Na codoped PbTe
Scanning tunneling microscopy and transport measurements have been performed
to investigate the electronic structure and its temperature dependence in
heavily Sr and Na codoped PbTe, which is recognized as one of the most
promising thermoelectric materials. Our main findings are as follows: (i) Below
T=4.5 K, all carriers are distributed in the first valence band at the L point
(L band), which forms tube-shaped Fermi surfaces with concave curvature. With
Sr and Na doping, the dispersion of the L band changes, and the band gap
increases from 200 meV to 300 meV. (ii) At T=4.5 K, the Fermi energy is located
~100 meV below the edge of the L band for the Sr/Na codoped PbTe. The second
valence band at the Sigma point (Sigma band) is lower than the L band by 150
meV, which is significantly smaller than that of pristine PbTe (200 meV). The
decrease in the band offset, leading to band convergence, provides a desirable
condition for thermoelectric materials.(iii) With increasing temperature, the
carrier distribution to the Sigma band starts at T=100 K and we estimate that
about 50 percent of the total carriers are redistributed in the Sigma band at
T=300 K.Our work demonstrates that scanning tunneling microscopy and angular
dependent magnetoresistance measurements are particularly powerful tools to
determine the electronic structure and carrier distribution. We believe that
they will provide a bird's eye view of the doping strategy towards realizing
high-efficiency thermoelectric materials.Comment: 36+12 pages, 4+9 figures, including Supplementary Material
Atomic species identification at the (101) anatase surface by simultaneous scanning tunnelling and atomic force microscopy
Anatase is a pivotal material in devices for energy-harvesting applications and catalysis. Methods for the accurate characterization of this reducible oxide at the atomic scale are critical in the exploration of outstanding properties for technological developments. Here we combine atomic force microscopy (AFM) and scanning tunnelling microscopy (STM), supported by first-principles calculations, for the simultaneous imaging and unambiguous identification of atomic species at the (101) anatase surface. We demonstrate that dynamic AFM-STM operation allows atomic resolution imaging within the materiala € s band gap. Based on key distinguishing features extracted from calculations and experiments, we identify candidates for the most common surface defects. Our results pave the way for the understanding of surface processes, like adsorption of metal dopants and photoactive molecules, that are fundamental for the catalytic and photovoltaic applications of anatase, and demonstrate the potential of dynamic AFM-STM for the characterization of wide band gap materialsWork supported by the NIMS (AA002 and AF006 projects), by the MEXT KAKENHI Grant Number 26104540, by the Charles University (GAUK 339311) and by the Spanish MINECO (projects PLE2009-0061, MAT2011- 023627 and CSD2010-00024). Computer time was provided by the Spanish Supercomputing Network (RES, Spain) at the MareNostrum III Supercomputer (BCS, Barcelona), and by the PRACE initiative (project RA0986) at the Curie Supercomputer (CEA, France). O.S and V.M. thank the Charles University-NIMS International Cooperative Graduate School Program. J.W.R. thanks NIMS for funding through the NIMS Internship Program and ICIQ for his ICIQ Fellowshi
Active nanocharacterization of nanofunctional materials by scanning tunneling microscopy
TOPICAL REVIEW: Active nanocharacterization of nanofunctional materials by scanning tunneling microscopy
Recent developments in the application of scanning tunneling microscopy (STM) to nanofabrication and nanocharacterization are reviewed. The main focus of this paper is to outline techniques for depositing and manipulating nanometer-scale structures using STM tips. Firstly, the transfer of STM tip material through the application of voltage pulses is introduced. The highly reproducible fabrication of metallic silver nanodots and nanowires is discussed. The mechanism is thought to be spontaneous point-contact formation caused by field-enhanced diffusion to the apex of the tip. Transfer through the application of z-direction pulses is also introduced. Sub-nanometer displacement pulses along the z-direction form point contacts that can be used for reproducible nanodot deposition. Next, the discovery of the STM structural manipulation of surface phases is discussed. It has been demonstrated that superstructures on Si(001) surfaces can be reverse-manipulated by controlling the injected carriers. Finally, the fabrication of an atomic-scale one-dimensional quantum confinement system by single-atom deposition using a controlled point contact is presented. Because of its combined nanofabrication and nanocharacterization capabilities, STM is a powerful tool for exploring the nanotechnology and nanoscience fields
Ni nanocrystals on HOPG(0001): A scanning tunnelling microscope study
The growth mode of small Ni clusters evaporated in UHV on HOPG has been investigated by scanning tunnelling microscopy. The size, the size distribution, and the shape of the clusters have been evaluated for different evaporation conditions and annealing temperatures. The total coverage of the surface strongly depends on the evaporation rate and time, whereas the influence of these parameters is low on the cluster size. Subsequent stepwise annealing has been performed. This results in a reduction of the total amount of the Ni clusters accompanied by a decreasing in the overall coverage of the surface. The diameter of the clusters appears to be less influenced by the annealing than is their height. Besides this, the cluster shape is strongly influenced, changing to a quasi-hexagonal geometry after the first annealing step, indicating single-crystal formation. Finally, a reproducible methodology for picking up individual clusters is reported