64 research outputs found
Spectroscopic characterization of charge carrier anisotropic motion in twisted few-layer graphene
Graphene, a layer of carbon atoms in a honeycomb lattice, captures enormous interest as probably the most promising component of future electronics thanks to its mechanical robustness, flexibility, and unique charge carrier quasiparticles propagating like massless high energy Dirac fermions. If several graphene layers form a stack, the interaction between them is, on the one hand, weak, allowing realization of various registries between the layers and, on the other hand, strong enough for a wide range tuning of the electronic properties. Here we grow few layer graphene with various number of layers and twist configurations and address the electronic properties of individual atomic layers in single microscopic domains using angle-resolved photoelectron spectromicroscopy. The dependence of the interlayer coupling on the twist angle is analyzed and, in the domains with tri-layers and more, if different rotations are present, the electrons in weaker coupled adjacent layers are shown to have different properties manifested by coexisting van Hove singularities, moir\ue9 superlattices with corresponding superlattice Dirac points, and charge carrier group velocity renormalizations. Moreover, pronounced anisotropy in the charge carrier motion, opening a possibility to transform strongly coupled graphene bilayers into quasi one-dimensional conductors, is observed
Is graphene on copper doped?
Angle-resolved photoemission spectroscopy (ARPES) and X-ray photoemission spectroscopy have been used to characterise epitaxially ordered graphene grown on copper foil by low-pressure chemical vapour deposition. A short vacuum anneal to 200 °C allows observation of ordered low energy electron diffraction patterns. High quality Dirac cones are measured in ARPES with the Dirac point at the Fermi level (undoped graphene). Annealing above 300 °C produces n-type doping in the graphene with up to 350 meV shift in Fermi level, and opens a band gap of around 100 meV.
Dirac cone dispersion for graphene on Cu foil after vacuum anneals (left: 200 °C, undoped; right: 500 °C, n-doped). Centre: low energy electron diffraction from graphene on Cu foil after 200 °C anneal. Data from Antares (SOLEIL)
Moiré Superlattice Effects and Band Structure Evolution in Near-30-Degree Twisted Bilayer Graphene
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
Determination of band offsets, hybridization, and exciton binding in 2D semiconductor heterostructures
Combining monolayers of different two-dimensional semiconductors into heterostructures creates new phenomena and device possibilities. Understanding and exploiting these phenomena hinge on knowing the electronic structure and the properties of interlayer excitations. We determine the key unknown parameters in MoSe2/WSe2 heterobilayers by using rational device design and submicrometer angle-resolved photoemission spectroscopy (μ-ARPES) in combination with photoluminescence. We find that the bands in the K-point valleys are weakly hybridized, with a valence band offset of 300 meV, implying type II band alignment. We deduce that the binding energy of interlayer excitons is more than 200 meV, an order of magnitude higher than that in analogous GaAs structures. Hybridization strongly modifies the bands at Γ, but the valence band edge remains at the K points. We also find that the spectrum of a rotationally aligned heterobilayer reflects a mixture of commensurate and incommensurate domains. These results directly answer many outstanding questions about the electronic nature of MoSe2/WSe2 heterobilayers and demonstrate a practical approach for high spectral resolution in ARPES of device-scale structures
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
Robustness of momentum-indirect interlayer excitons in MoS2/WSe2 heterostructure against charge carrier doping
Monolayer transition-metal dichalcogenide (TMD) semiconductors exhibit strong
excitonic effects and hold promise for optical and optoelectronic applications.
Yet, electron doping of TMDs leads to the conversion of neutral excitons into
negative trions, which recombine predominantly non-radiatively at room
temperature. As a result, the photoluminescence (PL) intensity is quenched.
Here we study the optical and electronic properties of a MoS2/WSe2
heterostructure as a function of chemical doping by Cs atoms performed under
ultra-high vacuum conditions. By PL measurements we identify two interlayer
excitons and assign them to the momentum-indirect Q-Gamma and K-Gamma
transitions. The energies of these excitons are in a very good agreement with
ab initio calculations. We find that the Q-Gamma interlayer exciton is robust
to the electron doping and is present at room temperature even at a high charge
carrier concentration. Submicrometer angle-resolved photoemission spectroscopy
(micro-ARPES) reveals charge transfer from deposited Cs adatoms to both the
upper MoS2 and the lower WSe2 monolayer without changing the band alignment.
This leads to a small (10 meV) energy shift of interlayer excitons. Robustness
of the momentum-indirect interlayer exciton to charge doping opens up an
opportunity of using TMD heterostructures in light-emitting devices that can
work at room temperature at high densities of charge carriers
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
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