Mechanism of charge transfer and its impacts on Fermi-level pinning for gas molecules adsorbed on monolayer WS2

Abstract

Density functional theory calculations were performed to assess changes in the geometric and electronic structures of monolayer WS2 upon adsorption of various gas molecules (H2, O2, H2O, NH3, NO, NO2, and CO). The most stable configuration of the adsorbed molecules, the adsorption energy, and the degree of charge transfer between adsorbate and substrate were determined. All evaluated molecules were physisorbed on monolayer WS2 with a low degree of charge transfer and accept charge from the monolayer, except for NH3, which is a charge donor. Band structure calculations showed that the valence and conduction bands of monolayer WS2 are not significantly altered upon adsorption of H2, H2O, NH3, and CO, whereas the lowest unoccupied molecular orbitals of O2, NO, and NO2 are pinned around the Fermi-level when these molecules are adsorbed on monolayer WS2. The phenomenon of Fermi-level pinning was discussed in light of the traditional and orbital mixing charge transfer theories. The impacts of the charge transfer mechanism on Fermi-level pinning were confirmed for the gas molecules adsorbed on monolayer WS2. The proposed mechanism governing Fermi-level pinning is applicable to the systems of adsorbates on recently developed two-dimensional materials, such as graphene and transition metal dichalcogenides.

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

“…4c, and these values are tabulated in Table 1, together with values found in the literature for comparison (values were converted to eV for easier comparison). The adsorption energies obtained using TPTEM are similar to reported theoretical and experimental values of the adsorption energy for physisorbed O 2 molecules on WS 2 22 and on graphene and HOPG 19, 20, 24–26 . The chemistry and morphology of the supporting substrate significantly influence molecular interaction in graphene 43, 44 and possibly in other 2D materials as well.…”

“…Simulations show that charge transfer occurs between the O 2 molecules and carbon atoms in graphene 19 , and similarly for other 2D materials 11, 22 , supporting the possibility of the existence of these electric dipoles. If we denote the modified driving field due to the presence of adsorbates as E d ,2  =  E d ,1  +  E dipole , then the change in the THz emission from graphene/InP due to O 2 adsorbates can be obtained using equations (1) and (2) as,where the subscripts 2 and 1 represent the state with (unannealed) and without (annealed at 445 K) adsorbed O 2 , respectively.…”

“…The adsorbates were completely removed for CVD graphene around 413 K. However, for LPE graphene, longer annealing at 445 K was needed for further desorption, implying that there are more strongly bonded adsorbates in LPE graphene. For LPE WS 2 , the absolute peak value reaching zero at 413 K could mean either (a) complete O 2 desorption at this temperature, or (b) no further desorption happened during the annealing from 413 K to 445 K. Considering the relatively larger adsorption energy in WS 2 and the high possibility of the existence of chemisorbed species 21, 22 , the latter is more likely the case.

Figure 3THz emission from CVD Graphene/InP at different temperatures in vacuum (1 × 10 −2 Pa). ( a ) Pre-exposed in air, and ( b ) after annealing at 445 K for 1 hour in vacuum.

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“…e,f O 2 molecule physisorbed on graphene, obtained by spin-unrestricted density-functional calculations 18 . g O 2 molecule physisorbed on WS 2 monolayer (on the most favorable site of adsorption), obtained by first-principles calculations 22 . …”

Gold Decoration and Photoresistive Response to Nitrogen Dioxide of WS2 Nanotubes

“…At room temperature, physisorption predominates over the interaction of NO 2 and other gases with the surface of transition metal disulfides . The adsorption of various gases on WS 2 and MoS 2 monolayers, as well as single‐wall MoS 2 nanotubes, was intensively studied by first‐principles calculations . Both oxygen and nitrogen dioxide were shown to be acceptors, inducing a charge transfer from WS 2 or MoS 2 to the adsorbed O 2 or NO 2 molecules, whereas importantly, the charge transfer to NO 2 is stronger than to the oxygen molecules.…”