Abstract
Adsorption of gas molecules on the surface of atomically layered two-dimensional (2D) materials, including graphene and transition metal dichalcogenides, can significantly affect their electrical and optical properties. Therefore, a microscopic and quantitative understanding of the mechanism and dynamics of molecular adsorption and desorption has to be achieved in order to advance device applications based on these materials. However, recent theoretical calculations have yielded contradictory results, particularly on the magnitude of the adsorption energy. Here, we have experimentally determined the adsorption energy of oxygen molecules on graphene and 2D tungsten disulfide using temperature-programmed terahertz (THz) emission microscopy (TPTEM). The temperature dependence of THz emission from InP surfaces covered with 2D materials reflects the change in oxygen concentration due to thermal desorption, which we used to estimate the adsorption energy of oxygen molecules on graphene (~0.15 eV) and tungsten disulphide (~0.24 eV). Furthermore, we used TPTEM to visualize relative changes in the spatial distribution of oxygen molecules on monolayer graphene during adsorption and desorption. Our results provide much insight into the mechanism of molecular adsorption on the surface of 2D materials, while introducing TPTEM as a novel and powerful tool for molecular surface science.
Highlights
Adsorption of gas molecules on the surface of atomically layered two-dimensional (2D) materials, including graphene and transition metal dichalcogenides, can significantly affect their electrical and optical properties
The change in the THz emission from graphene/InP shown in Fig. 1 can be semi-quantitatively modeled using the relation ET = xE1 + (1 − x)E2, where E1 and E2 are the initial and the final waveforms in Fig. 1, respectively, and x (0 ≥ x ≥ 1) is their relative weight which is proportional to the concentration of adsorbed O2 on graphene[29]
Spatial mapping of adsorbed O2 molecules in monolayer graphene under controlled conditions reveals the effects of sample non-uniformity on O2 adsorption and desorption dynamics
Summary
Adsorption of gas molecules on the surface of atomically layered two-dimensional (2D) materials, including graphene and transition metal dichalcogenides, can significantly affect their electrical and optical properties. Many other 2D materials have since been discovered with a wide range of characteristics, from metallic to semiconducting to insulating, opening up exciting new opportunities for the development of devices based on monolayers, bilayers, and heterostructures of 2D materials[5,6,7,8] Since these materials typically consist of one or a few atomic layers, their properties are extremely susceptible to perturbations from their environment. (HOPG), which is the bulk form of graphene Another drawback is that in TDS/TPD, the desorption of molecules is usually measured from a large sample area, failing to provide information on local adsorption and desorption dynamics. This is a significant disadvantage because the large surface-to-volume ratio enhances the importance of surface interactions in 2D materials
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