Observation of Graphite-Like and Diamond-Like Nanostructures in the Raman Spectra of Natural and Synthesized MoS2 Crystals with Small Carbon Additives

N.E Kornienko,L.M Kulikov,A.P Naumenko

doi:10.15330/pcss.21.4.598-620

Abstract

A comparative study of Raman spectra excited by laser radiation λL = 632.8 nm and 488 nm of natural crystals of 2H-MoS2 and nanocrystallites MoS2 (C) containing 0.5 and 1.0 wt.% Carbon additives. A detailed numerical analysis of the shape of observed D and G bands was performed. The complication of the spectra of graphite-like and diamond-like structures with the appearance of additional spectral components at 1440-1500 cm-1 and 1230-1270 cm-1 as a result of doubling the size of the corresponding elementary quasi-cells are analyzed. It is shown that the frequencies of D-bands of diamond-like nanostructures 1297 ÷ 1302 cm-1 don’t depend on λL in contrast to the change in the frequencies of the G (k)-bands. A significant effect of 632.8 nm resonant radiation on the electronic states and properties of MoS2 (C) NC was established. The strengthening of the D bands of the diamond-like structure and the ordering of the graphite structure with increasing carbon content in MoS2 (C) nanocrystals have been established. The change of spectral positions of D, G, and G (k) bands at strengthening the degree of disordering of a diamond- and graphite-like structures is considered. The influence of laser radiation on carbon structures is discussed.

Highlights

The unique physico-chemical properties of inorganic structural analogues of graphene and graphite ‒ transition metal dichalcogenides (TMD) 2H-MX2 (Me = Mo, W, Ti, Nb, Zr, Ta, Hf, Re; X = S, Se, Ti) are determined the peculiarities of their two-dimensional structure, and the presence of the band gap Eg ~ 1.5 - 2.5 eV, which is due to their semiconductor properties and the presence of exciton states, which are associated with their resonant properties often manifested in Raman spectra (RS)
We observed a significant increase in the intensities of the vibrational bands ν2 (E2g), ν3 (A1g) in the Raman spectra (RS) of micro- and nanocrystallites 2H-MoS2 with laser excitation λL = 488 nm, as well as a strong increase in broadband electronic background (BEB)
The existence of diamond-like and graphite-like nanostructures in crystals of natural molybdenite and synthesized 2H-MoS2 single crystal with uncontrolled small carbon impurities was shown for the first time

Summary

Introduction

We observed a significant increase in the intensities of the vibrational bands ν2 (E2g), ν3 (A1g) in the Raman spectra (RS) of micro- and nanocrystallites 2H-MoS2 with laser excitation λL = 488 nm, as well as a strong increase in broadband electronic background (BEB) These effects can be associated with changes in electronic states, the forces of chemical bonds and internal self-compression. The formation of chains of shorter M‒ M bonds and the emergence of a spatially coordinated system of chemical bonds can lead to sp hybridization for impurity C atoms and the emergence of diamond-like nanostructures within MoS2 crystallites In this regard, we have obtained and studied in detail the Raman spectra of both natural and synthesized single crystals of 2H-MoS2 and natural microcrystals (MC) of molybdenite MoS2 and synthesized nanocrystallites (NC) of MoS2(C) with small carbon additives in order to identify the formed internal carbon nanostructures

Experimental part

General analysis of Raman spectra of 2H-MoS2 single crystals

Findings

Discussion of the results

Conclusions