Layer specific optical band gap measurement at nanoscale in MoS2 and ReS2 van der Waals compounds by high resolution electron energy loss spectroscopy

Abstract

Layer specific direct measurement of optical band gaps of two important van der Waals compounds, MoS2 and ReS2, is performed at nanoscale by high resolution electron energy loss spectroscopy. For monolayer MoS2, the twin excitons (1.8 and 1.95 eV) originating at the K point of the Brillouin zone are observed. An indirect band gap of 1.27 eV is obtained from the multilayer regions. Indirect to direct band gap crossover is observed which is consistent with the previously reported strong photoluminescence from the monolayer MoS2. For ReS2, the band gap is direct, and a value of 1.52 and 1.42 eV is obtained for the monolayer and multilayer, respectively. The energy loss function is dominated by features due to high density of states at both the valence and conduction band edges, and the difference in analyzing band gap with respect to ZnO is highlighted. Crystalline 1T ReS2 forms two dimensional chains like superstructure due to the clustering between four Re atoms. The results demonstrate the power of HREELS technique as a nanoscale optical absorption spectroscopy tool.

Cleavage tendency of anisotropic two-dimensional materials:ReX2(X=S,Se) andWang1, Liu2, Wang3 et al. 2017Phys. Rev. B591470View full textAdd to dashboardCite

“…Brillioun zone was sampled using 1 [28][29][30][31], by using a semi-empirical DFT-D2 method [32], we considered the vdW correction in all calculations except the stress-strain relations of the monolayer ReX2 and WTe2.…”

Dirac Cones in Graphene, Interlayer Interaction in Layered Materials, and the Band Gap in MoS2

“…Recent layer-specific direct measurements of band gaps of in MoS 2 and ReS 2 by high resolution electron energy loss spectroscopy (HREELS) [87] have reported an indirect band gap of 1.27 eV obtained from the multilayer regions (i.e., essentially bulk MoS 2 ). For the monolayer, the band gap becomes direct (with the valence band maximum and conduction band minimum at the K point of the Brillouin zone) and increases to 1.98 eV.…”

“…with the theoretically estimated splitting, Figure 4b). It should be noted that the energies of the exciton peaks determined by HREELS [87] well agree with the energies of PL lines for the MoS 2 monolayers [12]. Then, the difference between the width of the band gap for the monolayer (1.98 eV) and the energies of PL lines (1.8 and 1.95 eV) is about 0.03-0.18 eV, which is in reasonable agreement with anticipated (i.e., usual) values of Wannier-Mott exciton binding energies [82].…”

Distinct Photoluminescence in Multilayered van der Waals Heterostructures of MoS2/WS2/ReS2 and BN

“…Sintering in the vacuum chamber prevents oxidation of compounds as well as re‐deposition of vapor species back on the pellet surface unlike sintering performed in a sealed quartz tube. The ReS 2 target pellets were prepared following a procedure already reported earlier . The epitaxial growth method of all the heterostructure was followed from the procedure based on PLD, which was already developed for epitaxial growth of MoS 2 and WS 2 thin films on c ‐plane sapphire substrate .…”