Tuning Solvated Electrons by Polar-Nonpolar Oxide Heterostructure

Efficient Photoexcited Charge Separation at the Interface of a Novel 0D/2D Heterojunction: A Time-Dependent Ultrafast Dynamic Study

To achieve efficient conversion and avoid loss of solar energy, ultrafast charge separation and slow electron-hole recombination are desired. Combining time-dependent density functional theory (TD-DFT) with nonadiabatic molecular dynamics, Au₉(PH₃)₈/MoS₂, as a prototype for zero-dimensional/two-dimensional (0D/2D) heterojunction, has been demonstrated to present excellent light absorption capacity and effective charge separation characteristics. In the heterojunction, photoexcitation of the Au₉(PH₃)₈ nanocluster drives an ultrafast electron transfer from Au₉(PH₃)₈ to MoS₂ within 20 fs, whereas photoexcitation of the MoS₂ nanosheet leads to hole transfer from MoS₂ to Au₉(PH₃)₈ within 680 fs. The strong nonadiabatic coupling and prominent density overlap are responsible for the faster electron separation relative to hole separation. In competition with the charge separation, electron-hole recombination requires 205 ns, ensuring an effective carrier separation. Our atomistic TD-DFT simulation provides valuable insights into the photocarrier dynamics at the Au₉(PH₃)₈/MoS₂ interface, which would stimulate the exploration of 0D/2D hybrid materials for photovoltaic and optoelectronic devices.

Suppression of Electron-Hole Recombination by Intrinsic Defects in 2D Monoelemental Material

The Shockley-Read-Hall (SRH) model, in which the deep trap defect states in the band gap are proposed as nonradiative electron-hole (e-h) recombination centers, has been widely used to describe the nonradiative e-h recombination through the defects in semiconductor. By using the ab initio nonadiabatic molecular dynamics method, we find that the SRH model fails to describe the e-h recombination behavior for defects in 2D monoelemental material such as monolayer black phosphorus (BP). Through the investigation of three intrinsic defects with shallow and deep defect states in monolayer BP, it is found that, surprisingly, none of these defects significantly accelerates the e-h recombination. Further analysis shows that because monolayer BP is a monoelemental material, the distinct impurity phonon, which often induces fast e-h recombination, is not formed. Moreover, because of the flexibility of 2D material, the defects scatter the phonons present in pristine BP, generating multiple modes with lower frequencies compared with the pristine BP, which further suppresses the e-h recombination. We propose that the conclusion can be extended to other monoelemental 2D materials, which is important guidance for the future design of functional semiconductors.

Ultrafast dynamics of solvated electrons at anatase TiO2/H2O interface

Solvated electrons are known to be the lowest energy charge transfer pathways at oxide/aqueous interface and the understanding of the electron transfer dynamics at the interface is fundamental for photochemical and photocatalytic processes. Taking anatase TiO₂/H₂O interface as a prototypical system, we perform time-dependent ab initio nonadiabatic molecular dynamics calculations to study the charge transfer dynamics of solvated electrons. For the static electronic properties, we find that the dangling H atoms can stabilize solvated electrons. A solvated electron band can be formed with one monolayer H₂O adsorption. The energies of the solvated electron band minimum (SEBM) decrease when H₂O adsorbs dissociatively. Moreover, the surface oxygen vacancies are also helpful for stabilizing the solvated electron band. For the dynamics behaviour, we find that the ultrafast charge transfer from SEBM to anatase TiO₂ (1 0 1) surface at 100 K is mainly contributed by nonadiabatic mechanism. Comparing with rutile TiO₂ (1 1 0) surface, the lifetime of solvated electron on anatase TiO₂ (1 0 1) surface is longer, suggesting a better photocatalytic properties. Our results provide essential insights into the understanding of the charge transfer dynamics and the possible photocatalytic mechanism at oxide/aqueous interface.