For nanomaterials, surface chemistry can dictate fundamental material properties, including charge-carrier lifetimes, doping levels, and electrical mobilities. In devices, surface defects are usually the key limiting factor for performance, particularly in solar-energy applications. Here, we develop a strategy to uniformly and selectively passivate defect sites in semiconductor nanomaterials using a vapor-phase process termed targeted atomic deposition (TAD). Because defects often consist of atomic vacancies and dangling bonds with heightened reactivity, we observe-for the widely used p-type cathode nickel oxide-that a volatile precursor such as trimethylaluminum can undergo a kinetically limited selective reaction with these sites. The TAD process eliminates all measurable defects in NiO, leading to a nearly 3-fold improvement in the performance of dye-sensitized solar cells. Our results suggest that TAD could be implemented with a range of vapor-phase precursors and be developed into a general strategy to passivate defects in zero-, one-, and two-dimensional nanomaterials.

Identifying efficient electrocatalysts with low overpotential and high selectivity for producing ammonia from nitrogen gas is essential for any future electrocatalytic nitrogen reduction reaction (NRR)-based ammonia synthesis. Via density functional theory calculations and the computational hydrogen electrode model, we systematically examine the prospect of using a single-transition-metal (TM)-atom-doped graphene-like GaN (g-GaN) monolayer as an electrocatalyst for artificial nitrogen reduction. Among 15 TMs investigated, the Mo-doped g-GaN (Mo@g-GaN) monolayer is the only electrocatalyst predicted to be feasible for the NRR. The Mo@g-GaN monolayer satisfies all screening criteria considered for activating the inert NN triple bond effectively, including stabilization of the adsorbed (*) NRR intermediate *NNH and destabilization of the *NH2 species. This monolayer also possesses sufficient overall stability. A complete analysis of the likely mechanisms involved in the NRR on this catalyst suggests that the Mo@g-GaN monolayer could exhibit promising NRR catalytic activity. It achieves this via one specific (distal) pathway, which has a very low onset potential of −0.33 V vs the reversible hydrogen electrode (RHE), corresponding to a low overpotential of 0.42 V vs the RHE, defined using the measured equilibrium potential for NRR of 0.09 V vs the RHE. The potential-determining step, conversion of *NH2 to *NH3, also exhibits a surmountable barrier of 0.42 eV, suggesting kinetics will be facile. Finally, the Mo@g-GaN monolayer is predicted to exhibit substantial selectivity (∼31%) toward ammonia synthesis over the competing hydrogen evolution reaction. These findings may open a potential route for artificial ammonia synthesis using a single-atom catalyst under ambient conditions.

Nonradiative charge-carrier recombination in transition-metal dichalcogenide (TMD) monolayers severely limits their use in solar energy conversion technologies. Because defects serve as recombination sites, developing a quantitative description of charge-carrier dynamics in defective TMD monolayers can shed light on recombination mechanisms. Herein we report a first-principles investigation of charge-carrier dynamics in pristine and defective WSe2 monolayers with three of the most probable defects, namely, Se vacancies, W vacancies, and SeW antisites. We predict that Se vacancies slow down recombination by nearly an order of magnitude relative to defect-free samples by breaking the monolayer’s symmetry and thereby reducing the spectral intensity of the A1g phonon mode that promotes recombination in the pristine monolayer. By contrast, we find W vacancies accelerate recombination by more than an order of magnitude, with half of the recombination events bypassing charge traps. The subsequent dynamics feature both charge trapping and charge-trap-assisted recombination. Although SeW antisites also slightly accelerate recombination, the predicted mechanism is different from the W vacancy case. First, a shallow energy level traps a photoexcited electron. Then, both shallow- and deep-trap-assisted recombination can occur simultaneously. Accelerated recombination arises for W vacancies and SeW antisites because they introduce new phonon modes that strongly couple to electron and hole dynamics. This work thus provides a detailed understanding of the mechanisms behind charge-carrier recombination in WSe2 monolayers with distinct defects. Thus, materials engineering, particularly to avoid W vacancies, could advance this technology. The insights derived are important for future design of high-performance photoactive devices based on WSe2 monolayers.

Localized trap states, which are deleterious to the performance of many solar-energy materials, often originate from the under-coordinated bonding associated with defects. Recently, the concept of targeted atomic deposition (TAD) was introduced as a process that permits the passivation of trap states using a vapor-phase precursor that selectively reacts with only the surface defect sites. Here, we demonstrate the passivation of nickel oxide (NiO) with the TAD process using diborane gas for selective, low-temperature deposition of boron (B) under continuous flow in a chemical vapor deposition (CVD) system. NiO is a ubiquitous cathode material used in dye-sensitized solar cells (DSSCs), organic photovoltaic devices, and organo-lead halide perovskite solar cells. The deposition of B at 100 °C is shown to follow first-order kinetics, exhibiting saturation at a B to Ni atomic ratio of ∼10%. Electrochemical measurements, combined with first-principles calculations, indicate that B passivates Ni vacancy defects by partially saturating the bonding of the oxygen atoms adjacent to the vacancy. p-Type DSSCs were fabricated using TAD-treated NiO and show a modest improvement in photovoltaic performance metrics. The results highlight the potential ubiquity of TAD passivation with a range of atomic precursors and vapor-phase processes.

Excited electron dynamics at semiconductor-molecule interfaces is ubiquitous in various energy conversion technologies. However, a quantitative understanding of how molecular details influence the quantum dynamics of excited electrons remains a great scientific challenge because of the complex interplay of different processes with various time scales. Here, we employ first-principles electron dynamics simulations to investigate how molecular features govern the dynamics in a representative interface between the hydrogen-terminated Si(111) surface and a cyanidin molecule. Hot electron transfer to the chemisorbed molecule was observed but was short-lived on the molecule. Interfacial electron transfer to the chemisorbed molecule was found to be largely decoupled from hot electron relaxation within the semiconductor surface. While the hot electron relaxation was found to take place on a time scale of several hundred femtoseconds, the subsequent interfacial electron transfer was slower by an order of magnitude. At the same time, this secondary process of picosecond electron transfer is comparable in time scale to typical electron trapping into defect states in the energy gap.