Visualizing electrostatic gating effects in two-dimensional heterostructures

Abstract

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 (-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 -ARPES and optical spectroscopy can be applied to a single device, allowing rigorous study of the relationship between gate-controlled electronic and excitonic properties.Angle resolved photoemission spectroscopy (ARPES), in which the energy and momentum of photoemitted electrons are measured from a sample subjected to a spectrally narrow ultraviolet or X-ray excitation, is a powerful technique that yields the momentum-dependent single-electron band structure and chemical potential in a solid with essentially no assumptions. It probes only electron states near the surface, and so cannot be applied to conventional semiconductor devices. It is, however, very effective when applied to 2D materials and has been used extensively to study the bands in graphene 1 , monolayer transition metal dichalcogenides 2-7 , and others 8,9 . Furthermore, µ-ARPES (with a micron-scale beam spot) can be performed 10 on 2D heterostructures (2DHSs) 11 made of stacked exfoliated 2D materials 12-14 , suggesting the possibility of monitoring electronic structure during actual device operation. We demonstrate here that momentum-resolved electronic spectra can indeed be obtained during reversible electrostatic gating, enabling direct visualization of chemical potential shifts and band structure changes controlled by the gate electric field.A limitation of ARPES is that it probes only occupied electron states, and so a semiconductor must first be electron-doped in order to obtain a signal from the conduction band. The usual approach is to deposit alkali metal atoms 1-7,15 which act as an n-type dopant, but this has several limitations: the density cannot be controlled accurately; it can only be reversed by high-temperature annealing; it introduces disorder through the random positions of the dopants; and it chemically perturbs the electronic structure in ways that are hard to calculate. Electrostatic doping has none of these disadvantages, and the accessible carrier densities are most relevant to practical devices.We first validate our technique using graphene, and then go on to apply it to the 2D transition metal dichalcogenide (TMD) sem...

Enhanced Electron-Phonon Interaction in Multivalley Materials

“…The energy difference ΔE ΓK reported in Table I from ARPES measurements is consistent with having only the K valley occupied upon accumulation of a density of holes reaching up to about 5 × 10 13 cm −2 [64,75,76]. Similarly, although no sufficiently systematic ARPES study of the conduction band of monolayer TMDs has been reported yet, analogous measurements on the doped surface of the bulk [77] (where doping should be limited to the first few layers) or monolayer [60] WSe 2 have shown that it is relatively easy to populate both the K and Q valleys in the conduction band (i.e., ΔE KQ in monolayer WSe 2 is significantly smaller than ΔE ΓK ; see Table I). Moreover, the observation of a larger softening in W-based monolayers, associated with the K and Q valleys being closer in WS 2 and WSe 2 than in MoS 2 , is compatible with the fact that ΔE KQ is largely controlled by spin-orbit coupling (which is stronger in W-based compounds).…”

Design of van der Waals interfaces for broad-spectrum optoelectronics

“…We conclude that k-direct interlayer transitions at Γ are robust processes, as we have shown them to occur irrespective of the relative orientation of the multilayers forming the interface (in the majority of cases we did not align the crystals when assembling the structures), of a substantial lattice constant mismatch (approximatively 15 %) between the constituents, and despite the fact that the band structure of TMD multilayers changes significantly upon varying their thickness. Besides substantiating our initial strategy to engineer systems for broad-spectrum optoelectronics, the ability to detect interlayer transitions in [39] (the energy difference is at most a few tens of meV, which we neglect here), we can use the k-direct, 2.0 eV intralayer transition at K in 2L-WS 2 to determine the energy of the maximum at K of the valence band of 2L-WS 2 . The relative alignment of all relevant band edges in 2L-InSe and 2L-WS 2 is then entirely determined.…”

Observation of Electrically Tunable van Hove Singularities in Twisted Bilayer Graphene from NanoARPES

“…Finally, by combining the results for n and E D for each layer as shown in Figure 1i we find that in both cases E D scales with doping as n, confirming the isolated graphene-like behavior in each cone. [32,33] We now turn our focus towards the key features of the superlattice dispersion at zero gate voltage before we return to the electrostatic tunability of these features. , where…”