704 research outputs found
Simultaneous quantization of bulk conduction and valence states through adsorption of nonmagnetic impurities on Bi2Se3
Exposing the (111) surface of the topological insulator Bi2Se3 to carbon
monoxide results in strong shifts of the features observed in angle-resolved
photoemission. The behavior is very similar to an often reported `aging' effect
of the surface and it is concluded that this aging is most likely due to the
adsorption of rest gas molecules. The spectral changes are also similar to
those recently reported in connection with the adsorption of the magnetic
adatom Fe. All spectral changes can be explained by a simultaneous confinement
of the conduction band and valence band states. This is only possible because
of the unusual bulk electronic structure of Bi2Se3. The valence band
quantization leads to spectral features which resemble those of a band gap
opening at the Dirac point.Comment: 5 pages, 4 figure
Free-standing graphene membranes on glass nanopores for ionic current measurements
A method is established to reliably suspend graphene monolayers across glass nanopores as a simple, low cost platform to study ionic transport through graphene membranes. We systematically show that the graphene seals glass nanopore openings with areas ranging from 180 nm2 to 20 μm2, allowing detailed measurements of ionic current and transport through graphene. In combination with in situ Raman spectroscopy, we characterise the defects formed in ozone treated graphene, confirming an increase in ionic current flow with defect density. This highlights the potential of our method for studying single molecule sensing and filtration.The authors would like to thank S. Purushothaman and K. Göpfrich for careful reading of the manuscript and V. Thacker for useful discussions. This work was supported by the EPSRC Cambridge NanoDTC, EP/G037221/1, and EPSRC grant GRAPHTED, EP/K016636/1. R.S.W. acknowledges a Research Fellowship from St. John's College, Cambridge. N.A.W.B. acknowledges an EPSRC doctoral prize award.This is the accepted manuscript. Copyright 2015 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The final version is available in Applied Physics Letters 106, 023119 (2015); doi: 10.1063/1.490623
Measuring the proton selectivity of graphene membranes
By systematically studying the proton selectivity of free-standing graphene
membranes in aqueous solutions we demonstrate that protons are transported by
passing through defects. We study the current-voltage characteristics of
single-layer graphene grown by chemical vapour deposition (CVD) when a
concentration gradient of HCl exists across it. Our measurements can
unambiguously determine that H+ ions are responsible for the selective part of
the ionic current. By comparing the observed reversal potentials with positive
and negative controls we demonstrate that the as-grown graphene is only weakly
selective for protons. We use atomic layer deposition to block most of the
defects in our CVD graphene. Our results show that a reduction in defect size
decreases the ionic current but increases proton selectivity.This is the author accepted manuscript. The final version is available from AIP via http://dx.doi.org/10.1063/1.493633
Intra- and Interband Electron Scattering in the Complex Hybrid Topological Insulator Bismuth Bilayer on BiSe
The band structure, intra- and interband scattering processes of the
electrons at the surface of a bismuth-bilayer on BiSe have been
experimentally investigated by low-temperature Fourier-transform scanning
tunneling spectroscopy. The observed complex quasiparticle interference
patterns are compared to a simulation based on the spin-dependent joint density
of states approach using the surface-localized spectral function calculated
from first principles as the only input. Thereby, the origin of the
quasiparticle interferences can be traced back to intraband scattering in the
bismuth bilayer valence band and BiSe conduction band, and to interband
scattering between the two-dimensional topological state and the
bismuth-bilayer valence band. The investigation reveals that the bilayer band
gap, which is predicted to host one-dimensional topological states at the edges
of the bilayer, is pushed several hundred milli-electronvolts above the Fermi
level. This result is rationalized by an electron transfer from the bilayer to
BiSe which also leads to a two-dimensional electron state in the
BiSe conduction band with a strong Rashba spin-splitting, coexisting
with the topological state and bilayer valence band.Comment: 11 pages, 5 figure
Evidence for a direct band gap in the topological insulator Bi2Se3 from theory and experiment
Using angle-resolved photoelectron spectroscopy and ab-initio GW
calculations, we unambiguously show that the widely investigated
three-dimensional topological insulator Bi2Se3 has a direct band gap at the
Gamma point. Experimentally, this is shown by a three-dimensional band mapping
in large fractions of the Brillouin zone. Theoretically, we demonstrate that
the valence band maximum is located at the Brillouin center only if many-body
effects are included in the calculation. Otherwise, it is found in a
high-symmetry mirror plane away from the zone center.Comment: 8 pages, 4 figure
Absence of superconductivity in ultra-thin layers of FeSe synthesized on a topological insulator
The structural and electronic properties of FeSe ultra-thin layers on
BiSe have been investigated with a combination of scanning
tunneling microscopy and spectroscopy and angle-resolved photoemission
spectroscopy. The FeSe multi-layers, which are predominantly 3-5 monolayers
(ML) thick, exhibit a hole pocket-like electron band at \bar{\Gamma} and a
dumbbell-like feature at \bar{M}, similar to multi-layers of FeSe on
SrTiO. Moreover, the topological state of the Bi2Se3 is preserved beneath
the FeSe layer, as indicated by a heavily \it{n}-doped Dirac cone. Low
temperature STS does not exhibit a superconducting gap for any investigated
thickness down to a temperature of 5 K
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