Time-Domain ab Initio Modeling of Electron-Phonon Relaxation in High-Temperature Cuprate Superconductors

Control of Charge Carrier Dynamics in Plasmonic Au Films by TiOx Substrate Stoichiometry

Plasmonic excitations in noble metals have many fascinating properties and give rise to a broad range of applications. We demonstrate, using nonadiabatic molecular dynamics combined with time-domain density functional theory, that the chemical composition and stoichiometry of substrates can have a strong influence on charge dynamics. By changing oxygen content in TiO₂, including stoichiometric, oxygen rich, and oxygen poor phases, and Ti metal, one can alter lifetimes of charge carriers in Au by a factor of 5 and control the ratio of electron-to-hole relaxation rates by a factor of 10. Remarkably, a thin TiO_x substrate greatly alters charge carrier properties in much thicker Au films. Such large variations stem from the fact that the Ti and O atoms are much lighter than Au, and their vibrations are much faster at dissipating the energy. The control over a particular charge carrier and an energy range depends on the Au and TiO_x level alignment, and the interfacial interaction strength. These factors are easily influenced by the TiO_x stoichiometry. In particular, oxygen rich and poor TiO₂ can be used to control holes and electrons, respectively, while metallic Ti affects both charge carriers. The detailed atomistic analysis of the interfacial and electron-vibrational interactions generates the fundamental understanding of the properties of plasmonic materials needed to design photovoltaic, photocatalytic, optoelectronic, sensing, nanomedical, and other devices.

Covalent Functionalized Black Phosphorus Greatly Inhibits Nonradiative Charge Recombination: A Time Domain Ab Initio Study

Mono- or few-layer black phosphorus (BP) has emerged as a promising photovoltaic and optoelectronic material with realistic applications subjected to instability and short charge carrier lifetime. Experiments show that covalent functionalization can improve the stability, but the underlying mechanism for the prolonged lifetime remains elusive. By performing spin-polarized time domain density functional theory combined with nonadiabatic (NA) molecular dynamics simulations, we demonstrate that BP passivated with both phenyl and nitrophenyl can suppress the nonradiative electron-hole recombination by a factor of 2 and 3, respectively, relative to the pristine system. The slow recombination is due to the interplay between energy gap, NA coupling, and decoherence time, which happens either through a hole-trap-assisted process or in a direct way between a free electron and hole in the spin-up channel. The observations hold in the spin-down channel. The study suggests that the passivating strategy should work for BP and other two-dimensional materials.

Disorder-induced localisation and suppression of superconductivity in YSr2Cu3O6+x

By means of ab initio calculations within the local density approximation to density functional theory, we investigate the electronic structure of the 60 K superconductor YSr₂Cu₃O_6+x (YSCO). We focus on the effects of the Sr/Ba substitution and on the main structural modifications induced by this substitution experimentally found in the Sr compound, namely the tetragonal symmetry and the oxygen disorder in the basal plane. In the calculations, this disorder is simulated by using a supercell approach. Due to band structure effects, we find a larger stabilisation free energy of the orthorhombic structure in YBa₂Cu₃O_6+x (YBCO). In YSCO, the tetragonal disordered phase is found to be stabilized by oxygen overdoping (x  >  1) and by sufficiently large mass-enhancement factors, [Formula: see text]. The analysis of the atomic site projected density of states suggests that oxygen disorder in the CuO basal planes of YSCO induces hole localisation, which accounts for the large 30 K reduction of [Formula: see text] with respect to YBCO.

Strong Influence of Oxygen Vacancy Location on Charge Carrier Losses in Reduced TiO2 Nanoparticles

Oxygen vacancies in TiO₂ nanoparticles are important for charge carrier dynamics, with recent studies reporting contradictory results on TiO₂ nanoparticle photocatalytic activity. We demonstrate that ground state multiplicity, defect levels, and formation energies depend strongly on vacancy location. Quantum dynamics simulations show that charges are trapped within several picoseconds and recombine over a broad range of time scales from tens of picoseconds to nanoseconds. Specifically, nanoparticles with missing partially coordinated surface oxygens showed fast recombination, while nanoparticles with missing highly coordinated subsurface oxygens or singly coordinated oxygens at tips showed slow recombination, even slower than in the pristine system. The results are rationalized by energy gaps and electron-hole localization, the latter determining nonadiabatic coupling and quantum coherence time. The diverse charge recombination scenarios revealed by the nonadiabatic dynamics simulations rationalize the contradictory experimental results for photocatalytic activity and provide guidelines for rational design of nanoscale metal oxides for solar energy harvesting and utilization.

Grain Boundary Facilitates Photocatalytic Reaction in Rutile TiO2 Despite Fast Charge Recombination: A Time-Domain ab Initio Analysis

TiO₂ is an excellent photocatalytic and photovoltaic material but suffers low efficiency because of deep trap states giving rise to fast charge and energy losses. Using a combination of time-domain density functional theory and nonadiabatic molecular dynamics, we demonstrate that grain boundaries (GBs), which are common in polycrystalline TiO₂, accelerate nonradiative electron-hole recombination by a factor of 3. Despite GBs increase the band gap without creating deep trap states, and accelerate coherence loss, they enhance nonadiabatic electron-phonon coupling, and facilitate the relaxation. Importantly, electrons accumulated at the boundaries together with the relatively long-lived excite state favor photocatalytic reaction. Our study rationalizes the experimental observations and provides valuable perspectives for improving the device performance by defect engineering.

High pressure and high temperature investigation of metallic perovskite SnTaO3

High pressure electronic, elastic, mechanical, and thermodynamic properties of cubic perovskite SnTaO₃ have been explored with density function theory (DFT), and the quasi-harmonic Debye model has been applied for the incorporation of high temperature. The experimental lattice constant has been used for the optimization of structure. The optimization results present the paramagnetic (PM) nature of the compound. The spin dependent electronic band structures at ambient conditions and under high pressure present the metallic nature with complete uniformity for the majority and minority spin states. The mechanical properties, such as Young's modulus and bulk modulus, have been calculated and suggest an increase in stiffness and hardness of the material under the application of pressure. The thermodynamic properties, such as specific heat and Grüneisen parameter, have been predicted in the temperature range of 0 to 1000 K and pressure range of 0 to 60 Gpa.

Disparity in Photoexcitation Dynamics between Vertical and Lateral MoS2/WSe2 Heterojunctions: Time-Domain Simulation Emphasizes the Importance of Donor-Acceptor Interaction and Band Alignment

Two-dimensional transition metal dichalcogenides (TMDs) heterojunctions are appealing candidates for optoelectronics and photovoltaics. Using time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we show that photoexcitation dynamics exhibit a significant difference in the vertical and lateral MoS₂/WSe₂ heterojunctions arising from the disparity in the donor-acceptor interaction and fundamental band alignment. The obtained electron transfer time scale in the vertical heterojunction shows excellent agreement with experiment. Hole transfer proceeds 1.5 times slower. The electron-hole recombination is 3 orders of magnitude longer than the charge separation, which favors solar cell applications. On the contrary, the lateral heterojunction shows no band offsets steering charge separation. The excited electron is localized at the interface that attracts holes to form an exciton-like state due to Coulomb interaction, suggesting potential applications in light-emitting devices. The coupled electron and hole wave functions increase NA coupling and the coherence time, accelerating electron-hole recombination by a factor of 3 compared with the vertical case. The atomistic studies advance our understanding of the photoinduced charge-phonon dynamics in TMDs heterojunctions.

Stochastic and Quasi-Stochastic Hamiltonians for Long-Time Nonadiabatic Molecular Dynamics

In the condensed-matter environments, the vibronic Hamiltonian that describes nonadiabatic dynamics often appears as an erratic entity, and one may assume it can be generated stochastically. This property is utilized to formulate novel stochastic and quasi-stochastic vibronic Hamiltonian methodologies, which open a new route to long-time excited state dynamics in atomistic solid-state systems at negligible computational cost. Using a model mimicking a typical solid-state material in noisy environment, general conclusions regarding the simulation of nonadiabatic dynamics are obtained: (1) including bath is critical to complete excited state relaxation; (2) a totally stochastic modulation of energies and couplings has a net effect of no bath and inhibits relaxation; (3) including a single or several dominant electron-phonon modes may be insufficient to complete the excited state relaxation; (4) only the multiple modes, even those that have negligible weights, can represent both the deterministic modulation of system's Hamiltonian and stochastic effects of bath.

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.

Weak Donor-Acceptor Interaction and Interface Polarization Define Photoexcitation Dynamics in the MoS2/TiO2 Composite: Time-Domain Ab Initio Simulation

To realize the full potential of transition metal dichalcogenides interfaced with bulk semiconductors for solar energy applications, fast photoinduced charge separation, and slow electron-hole recombination are needed. Using a combination of time-domain density functional theory with nonadiabatic molecular dynamics, we demonstrate that the key features of the electron transfer (ET), energy relaxation and electron-hole recombination in a MoS₂-TiO₂ system are governed by the weak van der Waals interfacial interaction and interface polarization. Electric fields formed at the interface allow charge separation to happen already during the photoexcitation process. Those electrons that still reside inside MoS₂, transfer into TiO₂ slowly and by the nonadiabatic mechanism, due to weak donor-acceptor coupling. The ET time depends on excitation energy, because the TiO₂ state density grows with energy, increasing the nonadiabatic transfer rate, and because MoS₂ sulfur atoms start to contribute to the photoexcited state at higher energies, increasing the coupling. The ET is slower than electron-phonon energy relaxation because the donor-acceptor coupling is weak, rationalizing the experimentally observed injection of primarily hot electrons. The weak van der Waals MoS₂-TiO₂ interaction ensures a long-lived charge separated state and a short electron-hole coherence time. The injection is promoted primarily by phonons within the 200-800 cm^-1 range. Higher frequency modes are particularly important for the electron-hole recombinations, because they are able to accept large amounts of electronic energy. The predicted time scales for the forward and backward ET, and energy relaxation can be measured by time-resolved spectroscopies. The reported simulations generate a detailed time-domain atomistic description of the complex interplay of the charge and energy transfer processes at the MoS₂/TiO₂ interface that are of fundamental importance to photovoltaic and photocatalytic applications. The results suggest that even though the photogenerated charge-separated state is long-lived, the slower charge separation, compared to the electron-phonon energy relaxation, can present problems in practical applications.