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

Abstract

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‐ReS 2 [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, MoS 2 and ReS 2 at nanoscale. Areas with mono, bi, tri and multilayers of MoS 2 and ReS 2 have been identified using a electron microscope. The atomic resolution image of mono and multilayer MoS 2 and ReS 2 have been given in figure 2. For monolayer MoS 2 , 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 MoS 2 . For ReS 2 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 MoS 2 going from monolayer to bilyer but no such transition is observen in ReS 2 The results demonstrate the power of HREELS technique as a nanoscale optical absorption spectroscopy tool.