Effect of high-temperature shock-wave compression on few-layer MoS 2 , WS 2 and MoSe 2 K. Vasu a , H.S.S.R. Matte a , Sharmila N. Shirodkar a , V. Jayaram b , K.P.J. Reddy c , Umesh V. Waghmare a , C.N.R. Rao a,⇑ a Chemistry and Physics of Materials Unit, Theoretical Sciences Unit and International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India b Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India c Department of Aerospace Engineering, Indian Institute of Science, Bangalore 560012, India article info Article history: Received 22 June 2013 In final form 19 July 2013 Available online 26 July 2013 abstract Exposure of few-layer MoS 2 , WS 2 and MoSe 2 to high-temperature shock waves causes morphological changes and a significant decrease in the interlayer separation between the (0 0 2) planes, the decrease being greatest in MoSe 2 . Raman spectra show softening of both the A 1g and the E 1 2g modes initially, fol- lowed by a slightly stiffening. Using first-principles density functional theoretical analysis of the response of few-layer MoS 2 to shock waves, we propose that a combination of shear and uniaxial compressive deformation leads to flattening of MoS 2 sheets which is responsible for the changes in the vibrational spectra. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Transition metal dichalcogenides, MX 2 (M = Mo, W; X = S, Se, Te), are well known inorganic layered 2D materials with a variety of interesting properties [1–3]. Stacked layers in these compounds consist of six-fold-bonded metal atoms sandwiched between two three-fold-bonded chalcogenide atoms with the intralayer atoms bonded by covalent forces and inter layer atoms bonded by van der Waals forces. Layer-dependent properties of transition metal dichalcogenide (MX 2 ) have recently received attention because of the novel electronic structures and properties of single and few- layers [4–10]. These layered materials also form structures in dif- ferent morphologies such as nanotubes and fullerene-like particles [11–13]. Fullerenes and nanotubes of MoS 2 and WS 2 are strong cage structures [14,15]. Because of the superior mechanical strength and low coefficient of friction, MoS 2 and WS 2 fullerene nanoparticles are used as solid lubricants. Tenne et al. [16,17] have studied the shock-wave absorption properties of WS 2 nanotubes and MoS 2 nanoparticles and reported that they survive a shock pressure up to 25GPa and a temperature of around 1000 °C without any significant structural modification. There has been no report on the effect of shock waves on graph- ene-like few-layer MoS 2 and related layered materials. In the pres- ent Letter, we have examined the effect of high-temperature shock wave treatment on few-layer MoS 2 , WS 2 and MoSe 2 . For this pur- pose, the few-layer samples were subjected to multiple shock wave compression, the temperature generated during shock wave compression reaching around 4000 K for 1 msec. We have investi- gated the structural changes in the samples as a result of shock compressions by a field emission scanning electron microscopy, transmission electron microscopy and Raman scattering. 2. Experimental Few-layer MoS 2 , WS 2 and MoSe 2 samples were prepared by employing the methods described elsewhere [4,5]. For the synthe- sis of MoS 2 and WS 2 , molybdic acid/tungstic acid was taken as the molybdenum/tungsten source and thiourea as the sulfur source (molybdic acid/tungstic acid:thiourea = 1:48). For MoSe 2 , molybdic acid was the molybdenum source and selenourea was the sele- nium source (molybdic acid: selenourea = 1:32). The mixtures were ground and heated in a N 2 atmosphere for 5 h at 600 °C in the case of MoS 2 and WS 2 and at 500 °C in the case of MoSe 2 . The products were cooled to room temperature in a nitrogen atmo- sphere and characterized by X-ray diffraction, field emission scan- ning electron microscopy (FESEM), transmission electron microscopy (TEM) and Raman spectroscopy before shock wave treatment. The few-layer samples of MoS 2 , WS 2 and MoSe 2 were deposited on Si (1 0 0) substrate by drop casting method and sub- jected to multiple shock wave treatment up to maximum 4 shots in a 21 meter long free piston driven shock tube (FPST). FPST is a unique facility to perform high-temperature shock wave experiments, consisting of a high pressure gas reservoir, compression tube (driver section) and shock tube (driven section). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.07.044 ⇑ Corresponding author. Fax: +91 80 22082760. E-mail address: cnrrao@jncasr.ac.in (C.N.R. Rao). Chemical Physics Letters 582 (2013) 105–109 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett