Steffi Woo1,Fuhui Shao1,Ashish Arora2,3,Robert Schneider2,Benjamin Carey2,Johann Preuß2,Steffen Michaelis de Vasconcellos2,F. Javier García de Abajo4,Rudolf Bratschitsch2,Andrea Konečná5,Luiz Tizei1
Université Paris-Saclay1,University of Münster2,Indian Institute of Science Education and Research3,ICFO-Institut de Ciencies Fotoniques4,Brno University of Technology5
Steffi Woo1,Fuhui Shao1,Ashish Arora2,3,Robert Schneider2,Benjamin Carey2,Johann Preuß2,Steffen Michaelis de Vasconcellos2,F. Javier García de Abajo4,Rudolf Bratschitsch2,Andrea Konečná5,Luiz Tizei1
Université Paris-Saclay1,University of Münster2,Indian Institute of Science Education and Research3,ICFO-Institut de Ciencies Fotoniques4,Brno University of Technology5
Understanding and controlling the interlayer coupling in van der Waals homo- and heterostructure devices containing transition metal dichalcogenides (TMDs) has been an ongoing effort in the two-dimensional material community. Due to the reduced Coulomb screening at atomic thicknesses, TMD optical properties are extremely sensitive to their local dielectric environment. This contributes detrimentally to the exciton linewidth due to inhomogeneous broadening [1], but also provides opportunities for tuning of the electronic gap and exciton binding energy [2] or neutralization of charged exciton emission [3] as demonstrated previously with graphene. Obtaining exciton linewidths towards the homogeneous limit for TMD monolayers in both optical absorption and emission (by photoluminescence) has been made possible by encapsulation in hexagonal boron nitride (<i>h</i>-BN).<br/><br/>A monochromated electron microscope is the ideal platform with capabilities for analogous measurement on such 2D materials at the nanoscale, including cathodoluminescence and electron energy-loss spectroscopy (EELS). Until recently [4-6], freely suspended TMD layers have long suffered broad EELS exciton linewidths and have easily succumbed to electron-beam damage. Incorporating <i>h</i>-BN encapsulation has put electron spectroscopies on a level playing field as well-known optical measurements, with supplementary structural and chemical information down to the atomic-scale [4,6].<br/><br/>In the first part of this presentation, the influence of two dielectric materials (Si<sub>3</sub>N<sub>4</sub> and <i>h</i>-BN) in various TMD monolayer configurations (suspended, supported, and encapsulated) is explored using EELS coupled with electron diffraction to measure the layer roughness [5]. Surprisingly, interfacial cleanliness and substrate-induced charge inhomogeneity/trapping predominate the influence on absorption linewidth more than monolayer flatness, which is significantly improved by the atomically flat <i>h</i>-BN. In the second part, complementary EELS (absorption) and CL (emission) measurements offer valuable insight into the coupling behavior in monolayer WSe<sub>2</sub>/graphene heterostructures (encapsulated in <i>h</i>-BN). This includes evidence for Coulomb interactions modified by the graphene, namely a redshifted neutral exciton emission energy and reduced exciton binding energy from absorption [7]. Spectral differences with mixed <i>h</i>-BN and/or graphene encapsulation with increasing graphene layer thickness will also be discussed.<br/><br/>[1] Raja, A. <i>et al</i>., Nat. Nanotechnol. <b>14</b>, 832 (2019).<br/>[2] Raja, A. <i>et al</i>., Nat. Commun. <b>8</b>, 15251 (2017).<br/>[3] Lorchat et al., Nat. Nanotechnol. 15, 283 (2020).<br/>[4] Bonnet, N. <i>et al</i>., Nano Lett. <b>21</b>, 10178–10185 (2021).<br/>[5] Shao, F. <i>et al</i>., Phys. Rev. Mater. 6(7), 074005 (2022)<br/>[6] Woo, S.Y. <i>et al</i>. Phys. Rev. B, <b>107</b>(15), 155429 (2023).<br/>[7] Woo, S.Y., Shao, F. <i>et al</i>. in preparation.