The properties of alloyed materials are a fundamental issue in Materials Science. For years, layered semiconductors of the TX₂ type (T=Mo, W; X=S, Se, Te) have been the subject of a varied range of studies due to their interesting electric, optical, catalytic and structural properties. In this sense, Mo_xW_(1-x)S₂ alloys have been recently reported [1,2]; but most of their properties haven’t been delved into yet. In this contribution, we focus on the local optoelectronic properties of such atomically thin (up to 6-8 layers) of Mo_xW_(1-x) S₂ . These properties have been probed by low-loss EELS measurements [3] and we have examined the bandgap behavior for different alloying degrees of such nanomaterials as well as a function of the number of layers. 

These works have been carried out using a FEI Titan Cs probe-corrected microscope equipped with a monochromator (working at 80 KV and with an energy resolution of ~180 meV).

Figure 1 displays three HRSTEM-HAADF micrographs of three different monolayers of MoS_2, Mo_0.5W_0.5S₂ and WS₂ samples, respectively. The alloying effects of such materials can be easily distinguished from the HAADF image corresponding to the Mo_0.5W_0.5S₂ sample (Fig. 1(a) second image from the left). The areas of reduced number of layers have been selected via optical and low-magnification TEM images and identified for the low-loss measurements, see Fig. 1(b).

            Low-loss EEL spectra, using the spectrum-line mode, have been recorded in regions where different stacks of a few layers can be noticed, see Fig. 2. In each of these stacks, easily recognisable for being the flat regions in the HAADF intensity profile (Figure 2(b)), several spectra have been integrated over a window of 10 to 12 nm. After zero loss peak (ZLP) extraction, the different spectra are fitted to obtain the band-gap value in each of these zones. In parallel, the thickness of these areas has been estimated, using the standard procedure [4]. Finally, these results show a relation between the band-gap of the material and the number of layers for every composition (Figure 2(c)).

All these results will be deeply discussed in the framework of previous experimental (photoluminescence) and theoretical (DFT calculations) works carried out in these material [1,2]. In conclusion, the present studies improve our knowledge of the optoelectronic properties of atomically thin layered alloys of dichalcogenides and provides further insight into the potential applications of these materials.

[1] D.O. Dumcenco, H. Kobayashi, Z. Liu, Y.S. Huang, K. Suenaga, Nature Comm. 4, 1351 (2013).

[2] Y. Chen, J. Xi, D.O. Dumcenco, Z. Liu, K. Suenaga, et al., Acs Nano 7, 4610-4616 (2013).

[3] R Arenal, O Stephan, M Kociak, D Taverna, A Loiseau, C Colliex, Phys. Rev. Lett. 95, 127601 (2005).

[4] R.F. Egerton, Electron energy-loss spectroscopy in the electron microscope, Springer, New York, 2011.

Acknowledgements:

This work was supported by the project ESTEEM2 (Integrated Infrastructure Initiative – I3, Grant Agreement 312483), the Spanish MINECO (FIS2013-46159-C3-3-P) and from the EU under Grant Agreement 604391 Graphene Flagship. Low-loss EELS studies were developed at the Advanced Microscopy Laboratory (LMA) of Institute of Nanoscience of Aragon (INA) – U. of Zaragoza (Spain).

Figures:

Figure 1. (a) HRSTEM-HAADF images of three different monolayers of MoS2, Mo0.5W0.5 S2 and WS2 samples, respectively. (b) Optical image and STEM-HAADF micrograph of the Mo0.5W0.5 S2 sample, where the low-loss measurements have been developed.

Figure 2. (a) Low-loss EEL spectra, after zero-loss peak removal, acquired in MoS2, Mo0.5W0.5S2 and WS2 samples, respectively. (b) HAADF images, intensity profiles of the HAADF micrographs obtained from the regions where the spectra-lines have been recorded, and the EEL spectra taken for different numbers of layers stacked. From left to right, MoS2, Mo0.5W0.5S2 and WS2, respectively. (c) Plotting of the relation between the number of layers and the gap measured. Noticeable differences in the gap can be appreciated.

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EMC Abstracts - https://emc-proceedings.com/abstract/gap-measurements-via-low-loss-eels-on-atomically-thin-moxw1-xs2-nanoflakes/