Spatially Resolved Electronic Properties of Single-Layer WS₂ on Transition Metal Oxides

Abstract

There is a substantial interest in the heterostructures of semiconducting transition metal dichalcogenides (TMDCs) amongst each other or with arbitrary materials, through which the control of the chemical, structural, electronic, spintronic, and optical properties can lead to a change in device paradigms. A critical need is to understand the interface between TMDCs and insulating substrates, for example high-κ dielectrics, which can strongly impact the electronic properties such as the optical gap. Here we show that the chemical and electronic properties of the single-layer (SL) TMDC, WS 2 , can be transferred onto high-κ transition metal oxide substrates TiO 2 and SrTiO 3 . The resulting samples are much more suitable for measuring their electronic and chemical structures with angle-resolved photoemission than their native-grown SiO 2 substrates. We probe the WS 2 on the micron scale across 100-micron flakes, and find that the occupied electronic structure is exactly as predicted for freestanding SL WS 2 with a strong spin-orbit splitting of 420 meV and a direct band gap at the valence band maximum. Our results suggest that TMDCs can be combined with arbitrary multi-functional oxides, which may introduce alternative means of controlling the optoelectronic properties of such materials.

Imaging microscopic electronic contrasts at the interface of single-layer WS2 with oxide and boron nitride substrates

“…EDC fits lead to a value for the spin-orbit splitting of 0.42(6) eV for WS 2 /hBN as demonstrated in Fig. 4(d), which is in agreement with other studies [9,19]. The linewidth broadening masks the spin-orbit splitting to such an extent that the EDC fits for SiO 2 and TiO 2 in Fig.…”

“…The area-selective XAS spectra over the entire edge shown in Fig. 2(f) resemble typical pristine TiO 2 spectra, indicating the cleanliness of the interface [9]. Having established the characteristic real space electronic contrasts, we collect distinct microARPES spectra with k-resolved PEEM from clean areas of the three vertical interfaces WS 2 /SiO 2 (Fig.…”

“…Here, we use the SPECS PEEM P90 microscope installed at the Microscopic And Electronic STRucture Observatory (MAESTRO) at the Advanced Light Source to investigate the electronic properties of vertical stacks * address correspondence to ulstrup@phys.au.dk † address correspondence to jkatoch@andrew.cmu.edu based on SL WS 2 transferred on oxide and hexagonal boron nitride (hBN) substrates. The thickness of WS 2 is checked before and after transfer using photoluminescence and Raman spectroscopy as shown in our earlier works [9,18]. The influence of the dielectric environment on the electronic properties of SL WS 2 is studied using insulating 300 nm SiO 2 on Si (SiO 2 /Si) with relative permittivity SiO2 = 3.9 and 0.5 wt % Nb-doped rutile TiO 2 (100) (Shinkosha Co., Ltd) with TiO2 = 113 as supporting substrate.…”

“…By shorting the WS 2 flake on hBN to the TiO 2 we avoid using a metal electrode in this system. The rationale of using SiO 2 , hBN and TiO 2 as substrates for SL WS 2 is three-fold: (i) These materials are commonly used in devices where they are known to exhibit strong variations in interfacial quality with other 2D materials [9,20,21], (ii) the dielectric properties vary strongly across the interfaces, potentially affecting the electronic bandstructure of the adjacent SL WS 2 [22,23], and (iii) the quasiparticle band gaps and electron affinities are very different and thus give rise to substantially different band alignments. Here, we address these key points by first presenting PEEM measurements of electronic contrasts to identify the three types of interfaces and investigate their quality from a photoemission perspective.…”

Giant spin-splitting and gap renormalization driven by trions in single-layer WS2/h-BN heterostructures

“…The upper valence band of single-layer WS 2 is located inside the bandgap of h-BN and the single-layer WS 2 VBM is characterized by a spin-orbit splitting of 430 meV (see Fig. 1j), in agreement with theoretical predictions 12 and previous experiments 23,24 . The clear electronic states and lack of band hybridization reveal a weak interlayer interaction between the two materials.…”