35 research outputs found
Visualizing electrostatic gating effects in two-dimensional heterostructures
The ability to directly observe electronic band structure in modern nanoscale
field-effect devices could transform understanding of their physics and
function. One could, for example, visualize local changes in the electrical and
chemical potentials as a gate voltage is applied. One could also study
intriguing physical phenomena such as electrically induced topological
transitions and many-body spectral reconstructions. Here we show that submicron
angle-resolved photoemission (micro-ARPES) applied to two-dimensional (2D) van
der Waals heterostructures affords this ability. In graphene devices, we
observe a shift of the chemical potential by 0.6 eV across the Dirac point as a
gate voltage is applied. In several 2D semiconductors we see the conduction
band edge appear as electrons accumulate, establishing its energy and momentum,
and observe significant band-gap renormalization at low densities. We also show
that micro-ARPES and optical spectroscopy can be applied to a single device,
allowing rigorous study of the relationship between gate-controlled electronic
and excitonic properties.Comment: Original manuscript with 9 pages with 4 figures in main text, 5 pages
with 4 figures in supplement. Substantially edited manuscript accepted at
Natur
Protecting TiS3 photoanodes for water splitting in alkaline media by TiO2 coatings
Titanium trisulfide (TiS3) nanoribbons, whencoated with titanium dioxide (TiO2), can be used for watersplitting in the KOH electrolyte. TiO2 shells can be preparedthrough thermal annealing to regulate the response of TiS3/TiO2heterostructures by controlling the oxidation time and growthatmosphere. The thickness and structure of the TiO2 layerssignificantly influence the photoelectrocatalytic properties of theTiS3/TiO2 photoanodes, with amorphous layers showing betterperformance than crystalline ones. The oxide layers should be thinenough to transfer photogenerated charge through the electrode−electrolyte interface while protecting TiS3 from KOH corrosion.Finally, the performance of TiS3/TiO2 heterostructures has beenimproved by coating them with various electrocatalysts, NiSx beingthe most effective. This research presents new opportunities to create efficient semiconductor heterostructures to be used asphotoanodes in corrosive alkaline aqueous solutionsRTI2018-099794−B-I00, PID2021-126098OB-I0
Revealing the conduction band and pseudovector potential in 2D moir\'e semiconductors
Stacking monolayer semiconductors results in moir\'e patterns that host many
correlated and topological electronic phenomena, but measurements of the basic
electronic structure underpinning these phenomena are scarce. Here, we
investigate the properties of the conduction band in moir\'e heterobilayers
using submicron angle-resolved photoemission spectroscopy with electrostatic
gating, focusing on the example of WS2/WSe2. We find that at all twist angles
the conduction band edge is the K-point valley of the WS2, with a band gap of
1.58 +- 0.03 eV. By resolving the conduction band dispersion, we observe an
unexpectedly small effective mass of 0.15 +- 0.02 m_e. In addition, we observe
replicas of the conduction band displaced by reciprocal lattice vectors of the
moir\'e superlattice. We present arguments and evidence that the replicas are
due to modification of the conduction band states by the moir\'e potential
rather than to final-state diffraction. Interestingly, the replicas display an
intensity pattern with reduced, 3-fold symmetry, which we show implicates the
pseudo vector potential associated with in-plane strain in moir\'e band
formation.Comment: Main text: 12 pages, 4 figures. Appended Supporting Information: 10
pages, 11 figure
Visualizing electrostatic gating effects in two-dimensional heterostructures
The ability to directly monitor the states of electrons in modern field-effect devices-for example, imaging local changes in the electrical potential, Fermi level and band structure as a gate voltage is applied-could transform our understanding of the physics and function of a device. Here we show that micrometre-scale, angle-resolved photoemission spectroscopy (microARPES) applied to two-dimensional van der Waals heterostructures affords this ability. In two-terminal graphene devices, we observe a shift of the Fermi level across the Dirac point, with no detectable change in the dispersion, as a gate voltage is applied. In two-dimensional semiconductor devices, we see the conduction-band edge appear as electrons accumulate, thereby firmly establishing the energy and momentum of the edge. In the case of monolayer tungsten diselenide, we observe that the bandgap is renormalized downwards by several hundreds of millielectronvolts-approaching the exciton energy-as the electrostatic doping increases. Both optical spectroscopy and microARPES can be carried out on a single device, allowing definitive studies of the relationship between gate-controlled electronic and optical properties. The technique provides a powerful way to study not only fundamental semiconductor physics, but also intriguing phenomena such as topological transitions and many-body spectral reconstructions under electrical control
Ghost anti-crossings caused by interlayer umklapp hybridization of bands in 2D heterostructures
In two-dimensional heterostructures, crystalline atomic layers with differing lattice parameters can stack directly one on another. The resultant close proximity of atomic lattices with differing periodicity can lead to new phenomena. For umklapp processes, this opens the possibility for interlayer umklapp scattering, where interactions are mediated by the transfer of momenta to or from the lattice in the neighbouring layer. Using angle-resolved photoemission spectroscopy to study a graphene on InSe heterostructure, we present evidence that interlayer umklapp processes can cause hybridization between bands from neighbouring layers in regions of the Brillouin zone where bands from only one layer are expected, despite no evidence for Moiré-induced replica bands. This phenomenon manifests itself as ‘ghost’ anti-crossings in the InSe electronic dispersion. Applied to a range of suitable two-dimensional material pairs, this phenomenon of interlayer umklapp hybridization can be used to create strong mixing of their electronic states, giving a new tool for twist-controlled band structure engineering
Direct evidence for flat bands in twisted bilayer graphene from nano-ARPES
Transport experiments in twisted bilayer graphene revealed multiple
superconducting domes separated by correlated insulating states. These
properties are generally associated with strongly correlated states in a flat
mini-band of the hexagonal moir\'e superlattice as it was predicted by band
structure calculations. Evidence for such a flat band comes from local
tunneling spectroscopy and electronic compressibility measurements, reporting
two or more sharp peaks in the density of states that may be associated with
closely spaced van Hove singularities. Direct momentum resolved measurements
proved difficult though. Here, we combine different imaging techniques and
angle resolved photoemission with simultaneous real and momentum space
resolution (nano-ARPES) to directly map the band dispersion in twisted bilayer
graphene devices near charge neutrality. Our experiments reveal large areas
with homogeneous twist angle that support a flat band with spectral weight that
is highly localized in momentum space. The flat band is separated from the
dispersive Dirac bands which show multiple moir\'e hybridization gaps. These
data establish the salient features of the twisted bilayer graphene band
structure.Comment: Submitted to Nature Materials. Nat. Phys. (2020