Adsorption energy of oxygen molecules on graphene and two-dimensional tungsten disulfide

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.

Figure 1

THz emission waveforms from Graphene/InP measured by TPTEM. Initially, when O₂ molecules are adsorbed in graphene, modification in the surface depletion field causes the photoexcited current to move towards the surface of InP, generating THz radiation with waveform containing a dip at ~2 ps and a peak at ~2.5 ps. Desorption of O₂ by IR irradiation and/or vacuum pumping causes both features to gradually vanish, followed by the appearance of a positive peak at ~2 ps. Subsequent annealing removes more adsorbates, and the THz emission from graphene/InP becomes very similar to the emission from bare InP, signifying that the photoexcited current flows towards the substrate. Exposure of sample to air allows O₂ to be adsorbed on graphene, which reverts the THz emission back to the initially observed waveform.

Figure 2

Temporal evolution of the THz peak amplitude at different conditions. The peak values (first peak at ~2 ps) are normalized with respect to the first peak of the initial THz emission measured in ambient conditions (I). The THz emission is recorded at 2-minute intervals after changing the environmental condition. Increase (decrease) in the peak value means net desorption (adsorption).

Figure 3

THz emission from CVD Graphene/InP at different temperatures in vacuum (1 × 10⁻²Pa). (a) Pre-exposed in air, and (b) after annealing at 445 K for 1 hour in vacuum. (c) ${\mathit{E}}_{\mathbf{u}\mathbf{n}\mathbf{a}\mathbf{n}\mathbf{n}\mathbf{e}\mathbf{a}\mathbf{l}\mathbf{e}\mathbf{d}}-{\mathit{E}}_{\mathbf{a}\mathbf{n}\mathbf{n}\mathbf{e}\mathbf{a}\mathbf{l}\mathbf{e}\mathbf{d}}\cong {\mathit{E}}_{{\mathbf{O}}_2}$ at different temperatures for CVD graphene/InP.

Figure 4

Determination of the desorption energy. (a) Peak versus temperature of waveforms (${\mathit{E}}_{\mathbf{u}\mathbf{n}\mathbf{a}\mathbf{n}\mathbf{n}\mathbf{e}\mathbf{a}\mathbf{l}\mathbf{e}\mathbf{d}}-{\mathit{E}}_{\mathbf{a}\mathbf{n}\mathbf{n}\mathbf{e}\mathbf{a}\mathbf{l}\mathbf{e}\mathbf{d}}\cong {\mathit{E}}_{{\mathbf{O}}_2}$) for all samples. (b) Concentration (N _ad) of O₂ adsorbates on surface of samples as a function of temperature. (c) In(N _ad) versus $1/\mathit{T}$ with linear fit for calculating desorption energy.

Figure 5

THz amplitude mapping during the O₂ adsorption process. (a) THz amplitude mapping for CVD graphene/InP after annealing at 445 K in vacuum. THz mapping of the same area in sample after exposure to air for a few minutes (b) without and (c) with UV illumination. The difference in THz mapping images before and after exposure to air gives a visualization of adsorbed O₂ molecules in the graphene surface. (d) Spatial map of adsorbed O₂ molecules in graphene after exposure to air without UV illumination which reveals regions with different “natural” affinities to O₂ molecules. Regions 1 and 2 (5) show the least (highest) amount of adsorbates after exposure to air, whereas regions 3 and 4 are areas with a fair amount of adsorbates. (e) Spatial map of adsorbed O₂ molecules after exposure to air under UV illumination, showing significantly higher concentration of O₂ adsorbates in the UV exposed part. In the THz amplitude maps (a–c), the blue (red) end of the scale signifies less (more) O₂ molecules on the graphene surface, while in the ΔTHz amplitude maps (d and e), the blue (red) end of the scale signifies less (more) O₂ molecules adsorbed/added on graphene during exposure to air.