Probing thermal expansion coefficients of monolayers using surface enhanced Raman scattering

Abstract

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 Â 10 À6 K À1, 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 directbandgap 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 Â 10 À6 K À1 ) 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 variations but require consistency, such as for nano-electromechanical 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 nega...

Temperature dependence of the double‐resonance Raman bands in monolayer MoS2

“…Temperature changes quite dramatically the electronic and vibrational properties of 2D materials and is responsible for the behavior of thermal conduction and thermal expansion of graphene, black phosphorous, and TMDs . For TMDs specifically, both the excitonic transition energies and phonon wavenumbers decrease with increasing temperature.…”

A sensitive, uniform, reproducible and stable SERS substrate has been presented based on MoS2@Ag nanoparticles@pyramidal silicon

“…10 Layered transition metal dichalcogenides (LTMDs) have received much attention due to their remarkable optoelectronic properties. [11][12][13] Molybdenum disulde (MoS 2 ) belongs to the family of LTMDs. The layered MoS 2 has unique physical, optical and electrical properties, making it very interesting for applying as a promising supporting material to stabilize metal nanoparticles, forming hierarchical composites.…”

Surface enhanced resonant Raman scattering in hybrid MoSe2@Au nanostructures

“…have reported a strong plasmon-induced surface enhanced Raman scattering (SERS) of TMDs layers [13][14][15][16]. The reported studies to date deal with plasmonic nanoparticles randomly deposited on top of the TMD layer [13][14][15][16][17][18]. In these works, SERS has been utilized to probe the local strain imposed to the TMD layer by the metal nanoparticles [15,18].…”

“…In these works, SERS has been utilized to probe the local strain imposed to the TMD layer by the metal nanoparticles [15,18]. Thermal measurements at the monolayer scale are made accessible owing to the SERS, as shown by Zhang et al in the case of MoS 2 and WS 2 monolayers hybridized with Ag nanoparticles [14].…”