BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Memento EPFL//
BEGIN:VEVENT
SUMMARY:Measuring unoccupied electron states in van der Waals materials
DTSTART:20231205T101500
DTSTAMP:20260528T052710Z
UID:84881e9b45241f5dfbd1ceb7cb921eb8aaff3476a12b9ce6672d50e8
CATEGORIES:Conferences - Seminars
DESCRIPTION:Prof. Sense Jan van der Molen\n\nHuygens-Kamerlingh Onnes Labo
 ratorium\, Leiden University\, The Netherlands \nWe study van der Waals (
 vdW) systems using low-energy electron microscopy (LEEM) and spectroscopy.
  Specifically\, reflection vs. energy\, i.e. R(E) spectra\, give us direct
  information on the local layer structure with ~2 nm resolution. For graph
 ene and hBN an unoccupied interlayer state is added with each additional l
 ayer. These interlayer states hybridize\, such that the number of states s
 cales with the number of layers (See Fig. 1). If an incoming electron is r
 esonant with a state\, it can be transmitted into the sample\, leading to 
 a minimum in R(E). Hence\, the local R(E) curve gives direct information o
 n the number of layers and their stacking [1-3]. We have taken this concep
 t one step further and study the 2D-dispersion relations of these unoccupi
 ed bands. For that\, we have developed ‘angle-resolved reflected-electro
 n spectroscopy’ (ARRES) [2\,3]. \nInterestingly\, whereas at resonance 
 reflection minima are expected\, maxima are anticipated in transmission. T
 o test this\, we created eV-TEM\, i.e. transmission EM at very low energie
 s (0-100 eV) [1]. Indeed\, the transmission vs. energy T(E) curves for fre
 estanding graphene show maxima at the interlayer state energies. Moreover\
 , the combination of T(E) and R(E) allows us to study inelastic path lengt
 hs for electrons of these energies [1].\nSummarizing\, we are able to meas
 ure the unoccupied band structure of vdW materials (above Evac) directly\,
  and we can do that at the nanoscale. The latter is a necessity when study
 ing heterogeneous and/or twisted vdW systems\, such as twisted bilayer gra
 phene [4\,5].  \n\n \n\nRefs:\n[1]         D. Geelen et al. PR
 L 123\, 086802 (2019).\n[2]         J. Jobst et al.\, Nat. Comm.
  6\, 8926 (2015)\n[3]         J. Jobst et al.\, Nat. Comm. 7\, 13
 621 (2016)\n[4]         Lisi et al.\, Nature Phys. 17\, 189 (2021
 )\n[5]         T.A. de Jong et al.\, Nature Comm. 13\, 70 (2022)\
 n
LOCATION:BS 260
STATUS:CONFIRMED
END:VEVENT
END:VCALENDAR