Measuring unoccupied bands in van der Waals materials

Speaker: Sense Jan van der Molen – Huygens-Kamerlingh Onnes Laboratorium, Universiteit Leiden

Host: Ingmar Swart

Abstract:

We study van der Waals (vdW) systems using low-energy electron microscopy (LEEM) and spectroscopy. Specifically, spectra of reflection vs. energy, i.e. R(E), give us direct information on unoccupied electron states. If an incoming electron is resonant with a state, it can be transmitted into the sample, leading to a minimum in R(E). For graphene and hBN an unoccupied interlayer state is added with each additional layer. Hence, the local R(E) curve gives direct information on the number of layers and their stacking [1-3]. We have taken this concept one step further and can even study the 2D-dispersion relations of these unoccupied bands. For that, we have developed ‘angle-resolved reflected-electron spectroscopy’ (ARRES) [2,3].

Interestingly, whereas at resonance reflection minima are expected, maxima are anticipated in transmission. To test this, we created eV-TEM, i.e. transmission EM at very low energies (0-100 eV) [1]. Indeed, the transmission vs. energy curves, T(E), for freestanding graphene show maxima at the interlayer state energies. Moreover, the combination of T(E) and R(E) allows us to study inelastic path lengths for electrons of these energies [1].

Summarizing, we are able to measure the unoccupied band structure of vdW materials (above Evac) directly, and we can do that at the nanoscale. The latter is a necessity when studying TMDs as well as heterogeneous and/or twisted vdW systems, e.g. twisted bilayer graphene [4-6].

[1] D. Geelen et al. PRL 123, 086802 (2019).
[2] J. Jobst et al., Nat. Comm. 6, 8926 (2015)
[3] J. Jobst et al., Nat. Comm. 7, 13621 (2016)
[4] S. Lisi et al., Nature Phys. 17, 189 (2021)
[5] T.A. de Jong et al., Nature Comm. 13, 70 (2022)
[6] P. Neu et al. PRB 107, 075427 (2023)