WS2 Hybrid Structures

In WS₂ hybrid structures, atomic doping is one of the effective ways to change the physical and chemical properties of the material, such as band gap and optical properties. For example, Sasaki et al. demonstrated that the exciton absorption peaks at 1.94 and 2.34 eV, respectively, were broadened by Nb doping. This suggests that excitons in WS₂ monolayers are sensitive to Nb doping because of the enhancement of the inhomogeneous broadening rate.

Dopant atoms can either directly replace atoms in the lattice or form interstitial atoms in the space between lattice sites (if the dopant is well-matched in terms of size, valence, and coordination), which are called substitution doping and interstitial doping, respectively.

In the case of alternative doping, W and S can be replaced by metallic and nonmetallic atoms, respectively. To substitute W atoms, a large number of theoretical calculations have been performed to investigate possible dopants. For example, Singh et al. found that alternative doping of three-dimensional transition metal atoms (Ti, V, Cr, Mn, Fe, Co, and Ni) at the W site is feasible by calculating the binding energy based on density generalization theory. In addition, WS₂ with a large periodic table was studied using DFT by Onofrio et al.

The calculations demonstrate that the doping of early transition metals (TMs) leads to a significant decrease in the tensile strain and band gap of WS₂. The band gap widens and the strain decreases when the d-state is filled to the intermediate transition metal; the opposite trend occurs when the d-state moves to the late transition metal. To replace S atoms, nonmetallic doping was also investigated by theoretical calculations. n-type and p-type doping of WS₂ monolayers with group V (N, P, and As) and group VII (F, Cl, Br, and I) atoms in place of S atoms was studied by Zhao et al.

The numerical results show the potential of nonmetallic doping and reveal that the incorporation of these atoms into WS₂ under W-rich experimental conditions is stable and energetically favorable. Among them, the N atom exhibits the lowest formation energy than the others. Compared to alternative doping, gap doping is relatively tough because the ideal impurity atoms should have the right size, such as H and Li.

Based on a large number of theoretical studies, various synthetic methods have been explored to fabricate doped WS₂ hybrids. Among some typical methods for the synthesis of doped WS₂ hybrid structures: the most commonly used method is in situ doping in CVD. For example, Gao et al. prepared Nb-doped WS₂ by in situ CVD method using WO₃ and S as precursors and NbCl₅ as Nb dopant. when the temperature of the CVD furnace reached 900 °C, both NbCl5 and S were provided in an ultra-high purity argon atmosphere. in addition, aerosol-assisted chemical vapor deposition (AACVD) was developed for non-volatile precursors.

For example, Murtaza et al. reported the deposition of Cr-doped WS₂ films on glass and steel substrates by AACVD with a total mass of 0.2 g including W and Cr precursors dissolved in 25 mL of tetrahydrofuran (THF). Another vapor phase method is chemical vapor transfer, which has a relatively low growth temperature, high single crystal quality, and few defects.

For example, Dumcenco et al. reported Au-doped WS₂ single crystals grown by chemical vapor transport method using I₂ as a transport agent, which possesses a surface area of 10 mm × 5 mm and a thickness of 0.5 mm. In addition, chemical doping is considered as a simple method that can be effectively doped by immersing the two-dimensional material in a dopant solution. Yu et al. achieved the intercalation of small cations between the monolayer and the underlying substrate by simply immersing the substrate-supported monolayer in some acid solution.

Article Source: Sun, CB., Zhong, YW., Fu, WJ. et al. Tungsten disulfide nanomaterials for energy conversion and storage. Tungsten 2, 109–133 (2020).

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