Nonadiabatic Molecular Dynamics with Non-Condon Effect of Charge Carrier Dynamics

Interplay of Ultrafast Electron-Phonon and Electron-Electron Scattering in Ti3C2Tx MXenes: Ab Initio Quantum Dynamics

Nonthermal electrons are vital in solar energy and optoelectronics, yet their relaxation pathways are not fully understood. Ab initio quantum dynamics reveal that in Ti3C2O2 electron-phonon (e-ph) relaxation is faster than electron-electron (e-e) scattering due to strong coupling with the A1g phonon at 190 cm-1 and the presence of light C and O atoms. Nuclear quantum effects are minimal; vibrations influence e-e scattering only indirectly, and the A1g mode' zero-point energy is much lower than thermal energy at ambient conditions. Substituting O with heavier S in Ti3C2OS slows e-ph relaxation and enhances e-e scattering, making it a faster process. However, both channels proceed concurrently, challenging the e-e and e-ph time scale separation often used for metals. These results underscore the need for atomistic-level understanding of nonthermal electron dynamics, especially in light-element systems such as MXenes, and provide guidance for optimizing electronic relaxation in advanced optoelectronic materials and devices.

Formation and Recombination Dynamics of Polarons in Goethite: A Time-Domain Ab Initio Study

The temperature and the coordination environment significantly affect polaron dynamics. Using goethite (FeOOH) as a model, our study examines polaron formation and recombination behavior under various conditions, including electron injection, photoexcitation, and heterovalent doping. Ab initio and nonadiabatic molecular dynamics (NAMD) simulations reveal that polaron formation in FeOOH is dependent on temperature via an adiabatic mechanism with higher temperatures leading to shorter formation times. Only electron polarons form in FeOOH, regardless of the formation method. NAMD simulations indicate that photoexcited electron polaron recombination is significantly faster in FeOOH than in Fe2O3. This difference arises from the distinct coordination environments, resulting in higher inelastic charge-phonon scattering and stronger nonadiabatic coupling in FeOOH. Our findings highlight the crucial roles of temperature and coordination environment in polaron dynamics, offering valuable insights for designing materials to optimize carrier dynamics.

Nonadiabatic Molecular Dynamics in Momentum Space Beyond Harmonic Approximation: Hot Electron Relaxation in Photoexcited Black Phosphorus

We simulated hot-electron relaxation in black phosphorus using the nonadiabatic molecular dynamics (NA-MD) approach with a non-Condon effect in momentum space beyond the harmonic approximation. By comparing simulations at the Γ point in a large supercell with those using a few k-points in a smaller supercell─while maintaining the same number of electronic states within the same energy range, we demonstrate that both setups yield remarkably consistent energy relaxation times, regardless of the initial state energy. This consistency arises from the complementary effects of supercell size in real space and the number of k-points in the reciprocal space. This finding confirms that simulations at a single k-point in large size supercells are an effective approximation for NA-MD with a non-Condon effect. This approach offers significant advantages for complex photophysics, such as intervalley scattering and indirect bandgap charge recombination, and is particularly suitable for large systems without the need for a harmonic approximation.

Two-Dimensional Molybdenum Boride (MBene) Mo4/3B2Tx with Broadband and Termination-Dependent Ultrafast Nonlinear Optical Response

Two-dimensional molybdenum borides (MBenes) comprise a new class of 2D transition metal borides that exhibit potential photonics applications. Recently, the synthesis of individual single-layer Mo4/3B2Tx (T = O, F, OH) MBene sheets has been realized, which attracted considerable attention in optoelectronics. However, there is still a lack of understanding and regulation of the photophysical processes of Mo4/3B2Tx MBene. Here, we demonstrate that Mo4/3B2Tx MBene exhibits a surface termination-dependent electronic structure, carrier dynamics, and nonlinear optical response over a wide wavelength range (500-1550 nm). As prepared 2D Mo4/3B2F2 MBene possesses a semimetal material property that exhibits a shorter intraband scattering process (<100 ps) and a considerable nonlinear optical response at a broadband cover optical communication C band at 1550 nm. These thrilling results are confirmed theoretically and experimentally. The analysis of these results adds to the regulating and understanding of the basic photophysical processes, which is anticipated to be beneficial for the further design of MBene-based photonics and nanoelectronics devices.