Grain Boundaries Are Benign and Suppress Nonradiative Electron–Hole Recombination in Monolayer Black Phosphorus: A Time-Domain Ab Initio Study

Using time-domain density functional theory combined with nonadiabatic molecular dynamics, we demonstrate that both symmetrical (GB_s) and asymmetrical grain boundaries (GB_a) significantly extend charge-carrier lifetime compared with monolayer black phosphorus. Boundaries create no deep trap states, which decrease electron–phonon coupling. As a result, GB_s increases carrier lifetime by a factor of 22, whereas GB_a extends the lifetime by a factor of 4. More importantly, the interplay between the immobile electron localized at the boundaries in the GB_s and extended excited-state lifetime facilitates a chemical reaction, which is beneficial for photocatalysts. In contrast, GB_a separates electron and hole spatially in different locations, which forms a long-lived charge-separated state and is favorable for photovoltaics. Our simulations demonstrate that grain boundaries are benign and retard nonradiative electron–hole recombination in monolayer black phosphorus, suggesting a route to reduce energy losses via rational choice of defect to realize high-performance photovoltaic and photocatalytic devices

Defects Slow Down Nonradiative Electron–Hole Recombination in TiS₃ Nanoribbons: A Time-Domain Ab Initio Study

Layered TiS₃ materials hold appealing potential in photovoltaics and optoelectronics due to their excellent electronic and optical properties. Using time domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we show that the electron–hole recombination in pristine TiS₃ nanoribbons (NRs) occurs in tens of picoseconds and is over 10-fold faster than the experimental value. By performing an atomistic ab initio simulation with a sulfur vacancy, we demonstrate that a sulfur vacancy greatly reduces electron–hole recombination, achieving good agreement with experiment. Introduction of a sulfur vacancy increases the band gap slightly because the NR’s highest occupied molecular orbital is lowered in energy. More importantly, the sulfur vacancy partially diminishes the electron and hole wave functions’ overlap and reduces NA electron–phonon coupling, which competes successfully with the longer decoherence time, slowing down recombination. Our study suggests that a rational choice of defects can control nonradiative electron–hole recombination in TiS₃ NRs and provides mechanistic principles for photovoltaic and optoelectronic device design

Nonadiabatic Molecular Dynamics Simulation of Charge Separation and Recombination at a WS₂/QD Heterojunction

Two-dimensional transition metal dichalcogenides (TMDs), such as WS₂, are appealing candidates for optoelectronics and photovoltaics. The strong Coulomb interaction in TMDs is however known to prevent electron–hole pairs from dissociating into free electron and hole. The experiment demonstrates that combination of WS₂ and quantum dots (QDs) can achieve efficient charge separation and enhance photon-to-electron conversion efficiency. Using real-time time-dependent density functional theory combined with nonadiabatic molecular dynamics, we model electron and hole transfer dynamics at a WS₂/QD heterojunction. We demonstrate that both electron and hole transfer are ultrafast due to strong donor–acceptor coupling. The photoexcitation of the WS₂ leads to a 75 fs electron transfer, followed by a 0.45 eV loss within 90 fs. The photoexcitation of QD results in 240 fs hole transfer, but loses only 0.15 eV of energy within 1 ps. The strong charge–phonon coupling and a broad range of phonon modes involved in electron dynamics are responsible for the faster electron transfer than the hole transfer. The electron–hole recombination across the WS₂/QD interface occurs in several 100 ps, ensuing a long-lived charge-separated state. Particularly, the hole transfer is threefold magnitude faster than the electron–hole recombination inside QD, ensuing that QD can be an excellent light-harvester. The detailed atomistic insights into the photoinduced charge and energy dynamics at the WS₂/QD interface provide valuable guidelines for the optimization of solar light-harvesting and photovoltaic efficiency in modern nanoscale materials

Tunnel-Structured K_<i>x</i>TiO₂ Nanorods by in Situ Carbothermal Reduction as a Long Cycle and High Rate Anode for Sodium-Ion Batteries

The low electronic conductivity and the sluggish sodium-ion diffusion in the compact crystal structure of Ti-based anodes seriously restrict their development in sodium-ion batteries. In this study, a new hollandite K_<i>x</i>TiO₂ with large (2 × 2) tunnels is synthesized by a facile carbothermal reduction method, and its sodium storage performance is investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses illustrate the formation mechanism of the hollandite K_<i>x</i>TiO₂ upon the carbothermal reduction process. Compared to the traditional layered or small (1 × 1) tunnel-type Ti-based materials, the hollandite K_<i>x</i>TiO₂ with large (2 × 2) tunnels may accommodate more sodium ions and facilitate the Na⁺ diffusion in the structure; thus, it is expected to get a large capacity and realize high rate capability. The synthesized K_<i>x</i>TiO₂ with large (2 × 2) tunnels shows a stable reversible capacity of 131 mAh g^–1 (nearly 3 times of (1 × 1) tunnel-structured Na₂Ti₆O₁₃) and superior cycling stability with no obvious capacity decay even after 1000 cycles, which is significantly better than the traditional layered Na₂Ti₃O₇ (only 40% of capacity retention in 20 cycles). Moreover, the carbothermal process can naturally introduce oxygen vacancy and low-valent titanium as well as the surface carbon coating layer to the structure, which would greatly enhance the electronic conductivity of K_<i>x</i>TiO₂ and thus endow this material high rate capability. With a good rate capability and long cyclability, this hollandite K_<i>x</i>TiO₂ can serve as a new promising anode material for room-temperature long-life sodium-ion batteries for large-scale energy storage systems, and the carbothermal reduction method is believed to be an effective and facile way to develop novel Ti-based anodes with simultaneous carbon coating and Ti­(III) self-doping