Field-induced band-shifts in two-dimensional heterostructures

Out-of-plane electric fields are used in van der Waals heterostructures to engineer band-structure changes for more efficient device performance. Here, we use spatially-resolved angle-resolved photoemission spectroscopy (µARPES) with in-situ electrostatic gating to probe band-alignment changes in heterostructures of 2D materials as the electric field perpendicular to the layers is varied. We focus on heterostructures containing semiconducting monolayer transition metal dichalcogenides (MX2). With a graphene top-contact, a graphite back-electrode, and a boron nitride (BN) dielectric, the electric-field across the heterostructure shifts the MX2 band edges by up to around 1 eV. Most of this shift is due to tuning the work function of the graphene top contact, but a significant portion is due to the electric field across the MX2, demonstrating the importance of back-gating with a top contact for efficient charge injection in MX2.
Mechanical exfoliation and dry transfer were used to fabricate the devices, combining an MX2 layer with graphene top contact, BN dielectric and graphite back-gate electrode as shown schematically in Fig. 1a and in the optical micrograph of Fig. 1b. The devices were inserted in ceramic chip carriers and mounted on custom-made sample plates that allow in-situ electrical connections at the Elettra Spectromicroscopy beamline, enabling in operando momentum-resolved band structure measurements.
With the EUV beam fixed on the heterostructure, µARPES spectra were acquired whilst the back-gate voltage (VG) was varied in-situ. ARPES is sensitive to the top few atomic layers of the sample, hence the spectra integrated photoemitted electrons from both the graphene and the underlying MX2 layer. The hemispherical analyzer was positioned to acquire spectra around the symmetry points of the Brillouin zones of the graphene and MX2 layers, see Fig. 1c, independently analyzing the valence band dispersion in each, see schematic in Fig. 1d. From a graphene on WSe2 heterostructure, the photoemission intensity as a function of energy and momentum (energy-momentum) slices were acquired at the zone center (Γ, red line) and the zone corners of WSe2 (KW, blue line) and of graphene (Kgr, green line), as a function of VG, Fig. 1e-g. These three-dimensional data cubes, from which the band edge energies were determined, contain the photoemission intensity as a function of energy, momentum and VG.
As VG is applied, the bands shift due to the induced electric field across the heterostructure. The WSe2 upper valence band shifts rigidly, with no evidence for change in the band dispersion. The graphene Dirac points shift by ΔED= 0.65 ±0.04 eV in a non-linear manner, as expected for the field-effect tuning of the work function of graphene. In contrast, the shift in the valence band offset of the WSe2 is significantly greater, ΔEK= 0.83 ±0.05 eV.

 figure 1

Figure 1.  Gate-dependent valence band measurements in a 2D heterostructure. (a) Schematic and (b) optical microscope image of a back-gated 2D device, scale bar 50 µm. (c) First Brillouin zones of the monolayer WSe2 (blue hexagon) and graphene (green hexagon), with the positions of the energy-momentum slices marked as solid lines. (d) Energy-momentum schematic of the band energies. Photoemission intensity as a function of energy, momentum and gate voltage around the Brillouin zone center, (e), and the zone corner of WSe2, (f), and graphene, (g).

These band shifts can be understood by considering the band diagram schematics shown in Fig. 2a and b. At VG= 0, the graphene top contact is undoped and its work function similar to the graphite back-gate electrode. When VG is applied, the heterostructure behaves like a parallel plate capacitor, with a field across the BN dielectric and charge accumulation in the graphene. Due to the low density of states in graphene, charge accumulation changes the chemical potential relative to the Dirac points, changing the work-function and giving a potential drop (sometimes described as a quantum capacitance of graphene). For quantitative analysis, we assume that the BN and monolayer WSe2 can be treated as ideal dielectrics without free charge carriers or traps. An equivalent circuit can then be constructed of three effective capacitances in series, corresponding to the BN, MX2and graphene, which predicts the voltage drop across the WSe2 to be proportional to the voltage drop across the BN. The data confirms this, Fig. 2c. Band shifts from a similar heterostructure, with a monolayer of MoSe2 rather than WSe2, give a similar linear dependence, Fig. 2d. This confirms a surprising result, the atomically thin layer of semiconductor, sandwiched in a heterostructure, can be described as a dielectric slab.
The results demonstrate that ARPES with in-situ gating can probe device Physics in 2D heterostructures, isolating the effect of electric field on band alignments across consecutive atomically thin layers. The large band shifts illustrate the potential for band-engineering in all 2DM heterostructures.


Figure 2

Figure 2.  Gate-dependent electrostatic potential drop across the MX2 layer, Δ. Schematics of the band edge energies across the heterostructure without applied gate voltage VG = 0 (a), and with applied gate voltage, VG, (b). The electrostatic potential difference between states on the MX2 and top graphene layer is plotted as a function of the electrostatic potential drop across the BN for WSe2 (c) and MoSe2 (d) heterostructures. The dashed lines are linear fits to the data.



This research was conducted by the following research team:

Paul V Nguyen,1 Natalie C Teutsch,Nathan Wilson,1 Joshua Kahn,1 Xue Xia,2 Abigail J Graham,2 Viktor Kandyba,3 Alexei Barinov,3 Xiaodong Xu,1,4 David H Cobden,1 Neil R Wilson2


1 Department of Physics, University of Washington, Seattle, Washington, USA

2 Department of Physics, University of Warwick, Coventry, UK

3 Elettra - Sincrotrone Trieste, S.C.p.A., Trieste, Italy

4 Department of Material Science and Engineering, University of Washington, Seattle, Washington, USA

Contact persons:

Neil Wilson, e-mail:




P.V. Nguyen, N.C. Teutsch, N.P. Wilson, J. Kahn, X. Xia, A.J. Graham, V. Kandyba, A. Barinov, X. Xu, D.H. Cobden, and N.R. Wilson,“Field-Dependent Band Structure Measurements in Two-Dimensional Heterostructures”, Nano Letters 21, 1053 (2021), DOI: 10.1021/acs.nanolett.1c04172

Last Updated on Monday, 24 January 2022 10:13