Band alignments within 2-dimensional van der Waals heterostructures

Since graphene’s discovery in 2004, a new world of 2-dimensional (2D) materials has been discovered. These new materials range from metals, semi-metals, semiconductors and insulators, and reveal new physics and possibilities in the world of 2D electronics. Transition metal dichalcogenides (TMDs) are a family of materials which stand out as potential candidates for 2D device design. A subgroup within this family, MX2 layers (M for the transition metal element, X for the chalcogen element) are made up of covalently bonded MX2 sheets held together vertically by weaker van der Waals bonds. They are easy to isolate as monolayers (MLs) and have a number of interesting electronic features, including being direct bandgap semiconductors in their ML form. These MLs can be stacked into van der Waals heterostructures (HSs) to form a range of functioning 2D devices. Here, in situ angle-resolved photoemission spectroscopy (ARPES) with electrostatic doping has been used to directly measure the electronic structure of MX2 MLs and HSs and study both carrier concentration and field dependent effects. The band structure of exfoliated ML MX2 materials have been characterised, along with the layer dependent electronic structure of WS2 and WSe2. Using this technique, the band alignments within a number of 2D HSs have been measured, including gr/MX2 and MX2/MX2 HSs. In situ gated ARPES measurements of electrostatically gated MX2 MLs and HSs are reported. By controlling the carrier concentration within these flakes, the conduction band of these materials has been directly resolved, enabling measurement of the bandgap for WS2, WSe2, MoS2 and MoSe2. These values have been long disputed in literature, in part due to bandgap renormalisation; an effect observed here in WSe2. By controlling the carrier concentration, direct measurements of the layer dependent transition from indirect to direct bandgap were made. textitIn situ gating probed the field dependent effects of MX2 MLs by observing the gate dependent band shifts across HSs, showing that the flakes within these HSs act as capacitors in series to each other, a fact which could help future device design. In situ ARPES was used here to study the origin and behaviour of photocurrent in 2D HSs. By measuring the current and the photon-assisted tunnelling through a functioning device, a model was formed to describe the generation and behaviour of carriers within the sample. Combining these measurements with other surface science techniques, such as Kelvin force gradient microscopy (KFGM), gave insight into the electronic behaviour within HSs and can be used to identify conductive and insulating regions, as well as study gate dependent effects and effective geometry for charge dissipation within HSs. Combining these results demonstrates that in situ gated ARPES is a powerful technique for studying 2D materials and HSs as it effectively probes their band structure, band alignments and interlayer effects. This helps to develop our understanding of the fundamental physics behind these 2D materials and could help improve future device design

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

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

Data for Electronic structure of graphene on copper substrates

Graphene growth by chemical vapour deposition (CVD) on copper foil has emerged as one of the most promising routes for large-scale production of high-quality graphene films. The electronic properties of this system can be examined directly using angle resolved photo-emission spectroscopy (ARPES). However, a direct computational study of the band structure is complicated by constraints on periodicity imposed by the simulation methods, which require large simulation cells. In this work, we present effective band structures of graphene on Cu(111) and Cu(100) surfaces obtained by band structure unfolding techniques. This allows a direct comparison with ARPES experiments. While our calculations confirm previous results of considerable n doping (0.30 eV) of graphene on Cu(111), we find significantly smaller modification of the graphene band structure on Cu(100). ARPES spectra of graphene on predominantly (100) oriented copper foil show good agreement with the calculated band structure. This demonstrates the importance of local crystallography on metal-graphene contacts. We also show that graphene doping on Cu(100) strongly depends on Cu–C separation and thus is sensitive to first-principles modelling choices

Field-dependent band structure measurements in two-dimensional heterostructures

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

Data for Atomic and electronic structure of two-dimensional Mo(1-x)WxS2 alloys

Alloying enables engineering of the electronic structure of semiconductors for optoelectronic applications. Due to their similar lattice parameters, the two-dimensional semiconducting transition metal dichalcogenides of the MoWSeS group (MX2 where M= Mo or W and X=S or Se) can be grown as high-quality materials with low defect concentrations. Here we investigate the atomic and electronic structure of Mo(1-x)WxS2 alloys using a combination of high-resolution experimental techniques and simulations. Analysis of the Mo and W atomic positions in these alloys, grown by chemical vapour transport, shows that they are randomly distributed, consistent with Monte Carlo simulations that use interaction energies determined from first-principles calculations. Electronic structure parameters are directly determined from angle resolved photoemission spectroscopy measurements. These show that the spinorbit splitting at the valence band edge increases linearly with W content from MoS2 to WS2, in agreement with linear-scaling density functional theory (LS-DFT) predictions. The spinorbit splitting at the conduction band edge is predicted to reduce to zero at intermediate compositions. Despite this, polarisation-resolved photoluminescence spectra on monolayer Mo0.5W0.5S2 show significant circular dichroism, indicating that spin-valley locking is retained. These results demonstrate that alloying is an important tool for controlling the electronic structure of MX2 for spintronic and valleytronic applications

Data for Visualizing electrostatic gating effects in two-dimensional heterostructures

Abstract: 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 understanding of the device physics and function. Here we show that submicrometre angle-resolved photoemission spectroscopy1–3 (-ARPES) applied to two-dimensional van der Waals heterostructures4 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 its energy and momentum. In the case of monolayer WSe2 we observe that the band gap is renormalized downwards by several hundred meV, approaching the exciton energy, as the electrostatic doping increases. Both optical spectroscopy and -ARPES 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 new means to study not only fundamental semiconductor physics but also intriguing phenomena such as topological transitions5 and many-body spectral reconstructions under electrical control