Probing thermal expansion coefficients of monolayers using surface enhanced Raman scattering Duan Zhang,† a Ye-Cun Wu,† a Mei Yang, a Xiao Liu, a Cormac ´ O Coile ´ ain, abc Hongjun Xu, ab Mourad Abid, b Mohamed Abid, b Jing-Jing Wang, c Igor V. Shvets, c Haonan Liu, a Zhi Wang, a Hongxing Yin, a Huajun Liu, d Byong Sun Chun, e Xiangdong Zhang a and Han-Chun Wu * a Monolayer transition metal dichalcogenides exhibit remarkable electronic and optical properties, making them candidates for application within flexible nano-optoelectronics, however direct experimental determination of their thermal expansion coefficients (TECs) is difficult. Here, we propose a non-destructive method to probe the TECs of monolayer materials using surface-enhanced Raman spectroscopy (SERS). A strongly coupled Ag nanoparticle over-layer is used to controllably introduce temperature dependent strain in monolayers. Changes in the first-order temperature coefficient of the Raman shift, produced by TEC mismatch, can be used to estimate relative expansion coefficient of the monolayer. As a demonstration, the linear TEC of monolayer WS 2 is probed and is found to be 10.3  10 6 K 1 , which would appear support theoretical predictions of a small TEC. This method opens a route to probe and control the TECs of monolayer materials. Two dimensional (2D) materials, such as transition metal dichalcogenides (TMDs), have attracted much attention due to their outstanding electronic and optical attributes. 1–10 For integration with existing semiconductor technology 2D TMDs have a natural advantage over graphene, in that they typically possess an energy bandgap, and yet can display high carrier mobilities. The bandgaps of TMDs are thickness dependent, typically displaying a transition from an indirect to direct- bandgap when the thickness is reduced to a monolayer. 2,3,11,12 However, a key physical consideration for the application of 2D materials is their thermal expansion coefficient (TEC), which relates changes in dimension to temperature. While many of the optical and electronic properties of TMDs have been well characterized, the thermal properties of many 2D materials remain less explored due to the difficulties associated with experimental measurements. Most materials exhibit positive thermal expansion, expanding when heated and contracting when cooled. However some materials do exhibit negative thermal expansion, and an interesting few exhibit very low (less than 2  10 6 K 1 ) or zero thermal expansion within specic temperature ranges. 13 A small TEC is highly desirable for applications where there is little tolerance for dimensional change or for systems that experience rapid temperature vari- ations but require consistency, such as for nano-electro- mechanical devices 14 or nanosensors. 15 It is well known that the origin of thermal expansion is anharmonic atomic lattice interactions, where the average interatomic distances increase as higher vibrational energy levels become available and are occupied. Therefore, crystal structure can greatly affect the TEC, for example, diamond is a positive TEC material, 16 graphite exhibits negative in-plane but positive out-of-plane TECs, 17 and from experiment and theoretical predictions, graphene is recognized as having a negative TEC over a wide range of temperatures. 18–23 Other 2D materials such as monolayer hexagonal boron nitride are also predicted to exhibit a negative TEC. 21,22 On the other hand, 2D TMDs are generally believed to demonstrate positive TECs. 23,24 Specically, recent rst princi- ples calculations have indicated that the linear TEC of mono- layer WS 2 is very small. 24 Bulk WS 2 has an indirect bandgap of 1.3 eV, whereas a monolayer has direct bandgap of 2.1 eV, 12 and another notable feature is intense photoluminescence (PL) found for monolayer WS 2 . 25 These properties suggest that such monolayers have potential for applications within exible 2D nano-optoelectronics. However, the claim of a small TEC is yet to be experimentally conrmed due to the difficulties associated with measuring expansion at the length scales associated with 2D materials. For monolayer materials the thermal expansion a Beijing Key Lab of Nanophotonics and Ultrane Optoelectronic Systems, School of Physics, Beijing Institute of Technology, Beijing 100081, P. R. China. E-mail: wuhc@ bit.edu.cn b KSU-Aramco Center, King Saud University, Riyadh 11451, Saudi Arabia c CRANN, School of Physics, Trinity College, University of Dublin, Dublin 2, Ireland d Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China e Division of Industrial Metrology, Korea Research Institute of Standards and Science, Daejeon 305-340, South Korea † These authors contributed equally to this work. Cite this: RSC Adv. , 2016, 6, 99053 Received 16th August 2016 Accepted 13th October 2016 DOI: 10.1039/c6ra20623a www.rsc.org/advances This journal is © The Royal Society of Chemistry 2016 RSC Adv. , 2016, 6, 99053–99059 | 99053 RSC Advances PAPER Published on 13 October 2016. Downloaded by Trinity College Dublin on 28/02/2017 15:53:19. View Article Online View Journal | View Issue