Defect-Mediated Charge-Carrier Trapping and Nonradiative Recombination in WSe2 Monolayers.

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.

Why and How Carbon Dioxide Conversion to Methanol Happens on Functionalized Semiconductor Photoelectrodes.

Functionalization of semiconductor electrode surfaces with adsorbed 2-pyridinide (2-PyH-*) has been postulated to enable selective CO2 photoelectroreduction to CH3OH. This hypothesis is supported by recent estimates of sufficient 2-PyH-* lifetimes and low barriers for hydride transfer (HT) to CO2. However, the complete mechanism for reducing CO2 to CH3OH remained unidentified. Here, vetted quantum chemistry protocols for modeling GaP reveal a pathway involving HTs to specific CO2 reduction intermediates. Predicted barriers suggest that HT to HCOOH requires adsorbed HCOOH* reacting with 2-PyH-*, a new catalytic role for the surface. HT to HCOOH* produces CH2(OH)2, but subsequent HT to CH2(OH)2 forming CH3OH is hindered. However, CH2O, dehydrated CH2(OH)2, easily reacts with 2-PyH-*, producing CH3OH. Further reduction of CH3OH to CH4 via HT from 2-PyH-* encounters a high barrier, consistent with experiment. Our finding that the GaP surface enables HT to HCOOH* explains why the primary CO2 reduction product over CdTe photoelectrodes is HCOOH rather than methanol, as HCOOH does not adsorb on CdTe and so the reaction terminates. The stability of 2-PyH-* (vs its protonation product DHP*), the relative dominance of CH2(OH)2 over CH2O, and the required desorption of CH2(OH)2* are the most likely limiting factors, explaining the low yield of CH3OH observed experimentally.

Dependence of hot electron transfer on surface coverage and adsorbate species at semiconductor-molecule interfaces.

Developing a molecular-level understanding of how a hot electron transfer process can be enhanced at semiconductor-molecule interfaces is central to advancing various future technologies. Using first-principles quantum dynamics simulations, we investigate how surface coverage and molecular adsorbate species influence the hot electron transfer at semiconductor-molecule interfaces. Counterintuitively, hot electron transfer from the semiconductor to molecules was found to be lessened with increased surface coverage because the inter-molecular interaction changes nonadiabatic couplings across the semiconductor and adsorbed molecules. The adsorbate molecular species itself was found to be an important factor in hot electron transfer not simply because of the energy level alignments at the interface, but also because the transfer is quite sensitive to nonadiabatic couplings. Our work shows that relatively minor variations of the couplings could lead to significant changes in hot electron transfer characteristics at semiconductor-molecule interfaces. Controlling nonadiabatic couplings must be part of developing a molecular-level "design principle" for enhancing hot electron transfer in addition to the well-recognized importance of energy level alignments.

Examining the Effect of Exchange-Correlation Approximations in First-Principles Dynamics Simulation of Interfacial Charge Transfer.

We examine the extent to which the exchange-correlation (XC) approximation influences modeling interfacial charge transfer using fewest-switches surface hopping (FSSH) simulations within the single-particle description. A heterogeneous interface between a lithium ion and an extended boron-nitride sheet was considered here, being an extreme case in which wave function localization and energy level alignments are highly sensitive to the XC approximation. The PBE0 hybrid XC approximation yields nonadiabatic couplings (NACs) that are significantly smaller than the values obtained from the PBE-GGA approximation by an order of magnitude for localized electronic states. This difference between the two XC functionals for the calculated NACs was found to derive mainly from the wave function characteristics rather than from the lattice movement although first-principles molecular dynamics trajectories, along which NACs are obtained, differ noticeably between the two XC functionals. Using the NACs and single-particle energy level alignments at different levels of theory, FSSH simulations were performed to model the electron transfer dynamics at the interface. The electron transfer time scale was found to vary as much as, but not more than, 1 order of magnitude. The time scale was found to be quite sensitive to both NACs and energy level alignments. While the order of magnitude consistency for the charge transfer rate is encouraging even for this rather extreme model of heterojunction interface, continued advancement in electronic structure methods is required for quantitatively accurate determination of the transfer rate.

Excited Electron Dynamics at Semiconductor-Molecule Type-II Heterojunction Interface: First-Principles Dynamics Simulation.

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.

Site-Selective Passivation of Defects in NiO Solar Photocathodes by Targeted Atomic Deposition.

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.

Modeling time-coincident ultrafast electron transfer and solvation processes at molecule-semiconductor interfaces.

Kinetic models based on Fermi's Golden Rule are commonly employed to understand photoinduced electron transfer dynamics at molecule-semiconductor interfaces. Implicit in such second-order perturbative descriptions is the assumption that nuclear relaxation of the photoexcited electron donor is fast compared to electron injection into the semiconductor. This approximation breaks down in systems where electron transfer transitions occur on 100-fs time scale. Here, we present a fourth-order perturbative model that captures the interplay between time-coincident electron transfer and nuclear relaxation processes initiated by light absorption. The model consists of a fairly small number of parameters, which can be derived from standard spectroscopic measurements (e.g., linear absorbance, fluorescence) and/or first-principles electronic structure calculations. Insights provided by the model are illustrated for a two-level donor molecule coupled to both (i) a single acceptor level and (ii) a density of states (DOS) calculated for TiO2 using a first-principles electronic structure theory. These numerical calculations show that second-order kinetic theories fail to capture basic physical effects when the DOS exhibits narrow maxima near the energy of the molecular excited state. Overall, we conclude that the present fourth-order rate formula constitutes a rigorous and intuitive framework for understanding photoinduced electron transfer dynamics that occur on the 100-fs time scale.

Violating the isolated pentagon rule (IPR): endohedral non-IPR C98 cages of Gd2@C98.

The geometric, electronic structure, and thermodynamic stability of large gadolinium-containing endohedral metallofullerenes, Gd(2)@C(98), have been systematically investigated by comprehensive density functional theory calculations combined with statistical mechanics treatments. The Gd(2)@C(2)(230924)-C(98) structure, which satisfies the isolated-pentagon rule (IPR), is determined to possess the lowest energy followed with some stable non-IPR isomers. In order to clarify the relative stabilities at elevated temperatures, entropy contributions are taken into account on the basis of the Gibbs energy at the B3LYP level for the first time. Interestingly, a novel non-IPR Gd(2)@C(1)(168785)-C(98) isomer which has one pair of pentagon adjacency is more thermodynamically stable than the lowest energy IPR species within a wide temperature interval related to fullerene formation. Therefore, the Gd(2)@C(1)(168785)-C(98) is predicted to be the most proper isomer obtained experimentally, which is the largest non-IPR carbon cage found so far. Our findings demonstrate that interaction between metals and carbon cages could stabilize the fused pentagons effectively, and thus, the non-IPR isomers should not be ignored in some cases of endohedral metallofullerenes. The IR features of Gd(2)@C(98) are simulated to assist its future experimental characterization.

Dangling bond-induced graphitization process on the (111) surface of diamond nanoparticles.

The intrinsic mechanism of graphitization occurring on the (111) surface of nanodiamonds (NDs) during the transformation from NDs into bucky diamonds are explored using density functional theory (DFT) computations in conjunction with density functional based tight-binding simulations. The DFT results indicate that dangling bonds (DBs) on the ND surfaces play an important role in the graphitization process, and the orientation of the DBs on different ND surfaces determines whether there will be a graphitization process or not. Moreover, a criterion is proposed to estimate rupturing of the C-C bonds between different layers on the [111] direction in the NDs and is verified to be applicable to illustrate the phase transformation from sp(3) into sp(2) bonding structures. The energy contributions of the four-coordinated carbon atoms located at different positions on the (111) surface are exhibited for the first time and discussed in detail to gain a clear picture for the transition from NDs into bucky diamonds. The outcome may provide a deeper understanding on the influence of DBs upon the transformation from sp(3) into sp(2) bonding structures.