Atomically thin monolayer transition metal dichalcogenides (TMDs) are a new class of two dimensional nano-material with promising optoelectronic, energy and novel device applications. One of the important features of many TMDs is that they undergo a crossover from indirect band gap in the bulk to direct band gap in the monolayer form [1]. While the monolayer properties of TMDs are unique (e. g. direct band gap), it may be illusive while fabricating practical devices because the cross over to indirect band gap occurs due to unavoidable electronic packaging and the property could change in close proximity of foreign substance. Therefore, there is an urge to stabilize such novel properties arising from the monolayer in the interface or in the bulk form. With this goal there is a candidate material already reported in the family, i.e. crystalline 1T-ReS₂ [2]. In this work we have exploited high resolution electron energy loss spectroscopy (HREELS) to perform layer specific direct measurement of optical band gaps of two important van der Waals compounds, MoS₂ and ReS₂ at nanoscale. Areas with mono, bi, tri and multilayers of MoS₂ and ReS₂ have been identified using a electron microscope. The atomic resolution image of mono and multilayer MoS₂ and ReS₂ have been given in figure 2. For monolayer MoS₂, the twin excitons (1.8 and 1.95 eV) originating at the K point of the Brillouin zone are observed. The band gap values have been deduced after plotting Tauc-like plots for both direct and indirect band gaps from the EELS absorption spectra. An indirect band gap of 1.27 eV is obtained from the multilayers regions (see figure 1). 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 are obtained for the monolayer and multilayers, respectively (see figure 1). A direct to indirect band gap transition has been observed in MoS₂ going from monolayer to bilyer but no such transition is observen in ReS₂ The results demonstrate the power of HREELS technique as a nanoscale optical absorption spectroscopy tool.
[1] K. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, Phys. Rev. Lett. 105, 136805 (2010).
[2]S. Tongay, H. Sahin, C. Ko, A. Luce, W. Fan, K. Liu, J. Zhou, Y-S Huang, H. Ching-Hwa , J. Yan, D. F. Ogletree, S. Aloni, J. Ji, S. Li, J. Li, F.M. Peeters, and J. Wu, Nat. Comm. 5, 3252 (2014).

[3] K. Dileep, R. Sahu, Sumanta Sarkar, Sebastian C. Peter, and R. Datta, J. Appl. Phys. 119, 114309 (2016).

Figures:

Example experimental Tauc-like plots corresponding to direct and indirect band gaps for (a) & (b) monolayer MoS2, (c) & (d) multilayer MoS2, (e) & (f) monolayer ReS2 and (g) & (h) multilayer ReS2, respectively.

HRTEM images of (a) multilayer 2H-MoS2 along <0001>. The intensity profile across the columns is shown in the lower right inset, Intensity from Mo and S columns are almost same in this case. (b) monolayer 2H-MoS2. Mo and S atoms are marked. The diffractogram or FT is shown in the upper right inset, the intensity profile across columns is shown in lower right inset and intensity line trace gives higher signal for Mo compared to S. HRTEM of (c) monolayer distorted 1T-ReS2. The ‘N’ shaped Re4 cluster is highlighted. The DFT calculated Re4 clusters are shown in upper right inset. (d) FT of distorted 1T-ReS2. The superstructure spots are marked with the yellow circles.

Layer specific plots of band gaps and their types for (a) MoS2, and (b) ReS2. Note the direct to indirect band gap crossover for MoS2 from monolayer to bilayers.

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EMC Abstracts - https://emc-proceedings.com/abstract/layer-specific-optical-band-gap-measurement-at-nanoscale-in-mos2-and-res2-van-der-waals-compounds-by-high-resolution-electron-energy-loss-spectroscopy/