Field-Dependent Band Structure Measurements in Two-Dimensional Heterostructures

Abstract

In electronic and optoelectronic devices made from van der Waals heterostructures, electric fields can induce substantial band structure changes which are crucial to device operation but cannot usually be directly measured. Here, we use spatially resolved angle-resolved photoemission spectroscopy to monitor changes in band alignment of the component layers, corresponding to band structure changes of the composite heterostructure system, that are produced by electrostatic gating. Our devices comprise graphene on a monolayer semiconductor, WSe2 or MoSe2, atop a boron nitride dielectric and a graphite gate. Applying a gate voltage creates an electric field that shifts the semiconductor bands relative to those in the graphene by up to 0.2 eV. The results can be understood in simple terms by assuming that the materials do not hybridize.

ARPES Signatures of Few-Layer Twistronic Graphenes

“…19−25 Due to the short mean free path of the photoexcited electrons, ARPES is sensitive to the top few atomic layers, enabling the study of layer-dependent effects, while in situ back gating of 2D heterostructures during ARPES allows the study of band structure changes with carrier concentration 26−28 and with transverse displacement fields. 29,30 ARPES has previously been applied to the study of twisted graphenes, initially studying multilayer graphene grown on SiC or copper, where twisted regions can be found by chance. 31−35 However, interactions with the substrate cause complications such as inhomogeneous doping, increased screening, and additional moiréperiodicities.…”

“…Angle-resolved photoemission spectroscopy (ARPES) gives unique insight into the momentum-resolved electronic band structure of 2DMs and 2D heterostructures. − Due to the short mean free path of the photoexcited electrons, ARPES is sensitive to the top few atomic layers, enabling the study of layer-dependent effects, while in situ back gating of 2D heterostructures during ARPES allows the study of band structure changes with carrier concentration − and with transverse displacement fields. , ARPES has previously been applied to the study of twisted graphenes, initially studying multilayer graphene grown on SiC or copper, where twisted regions can be found by chance. − However, interactions with the substrate cause complications such as inhomogeneous doping, increased screening, and additional moiré periodicities. Instead, mechanical exfoliation and stacking on boron nitride can be used to fabricate twisted-graphene samples at defined twist angles for ARPES, for example, showing that moiré superlattice effects persist even in large-angle twisted-bilayer graphene where the moiré period is short .…”

Momentum-resolved view of highly tunable many-body effects in a graphene/hBN field-effect device

“…The mesoscopic sizes and intrinsic inhomogeneities of such devices have posed the biggest challenges precluding conventional ARPES studies. These issues can be circumvented by using a microscopically focused beam of photons as demonstrated in recent microARPES experiments performed on 2D material based heterostructures and devices [20][21][22][23]. We apply this approach here to investigate the Coulomb interaction in graphene on hBN (graphene/hBN) at a relatively small interlayer twist angle of 2.0 • .…”

Elettra-Sincrotrone Trieste: present and future

“…After pioneering operando experiments with electronic devices, where the single 2D material channel is only a few µm 2 in size, the NanoARPES at Spectromicroscopy is endorsed as an attractive tool for understanding functionality of wide range 2D devices and other heterostructures. For the first studied ‘real’ devices, consisting of graphene on monolayers of MoS 2 or WSe 2 atop of a BN dielectric, the shifts of semiconductor bands relative to those in the graphene when applying a gate voltage indicated that these two materials do not hybridize [ 30 ].…”