Colloidal 2D Layered SiC Quantum Dots from a Liquid Precursor: Surface Passivation, Bright Photoluminescence, and Planar Self-Assembly.

We report the bottom-up synthesis of colloidal two-dimensional (2D) layered silicon carbide (SiC) quantum dots with a cubic structure, lateral size of 5-10 nm, ⟨110⟩ exfoliation to few atomic layers, and surface passivation with 1-dodecene. Samples shielded from oxygen and plasma-annealed for purity exhibit narrow blue photoluminescence (PL) with quantum yields (QYs) over 60% in exceptional cases, while unshielded nanocrystals (NCs) exhibit broad blue/green/white PL with 10-15% QY. The latter scenario is attributed to excess surface carbon and oxygen accrued during synthesis and processing, with size separation through ultracentrifugation revealing size-dependent impurity emission.

Effects of Surface Defects on Performance and Dynamics of CsPbIBr Perovskite: First-Principles Nonadiabatic Molecular Dynamics Simulations.

Inorganic mixed-halogen perovskites exhibit excellent photovoltaic properties and stability; yet, their photoelectric conversion efficiency is limited by inherent surface defects. In this work, we study the impact of defects on properties of CsPbIBr slabs using first-principles calculations, focusing on specific defects such as I vacancy (V), I interposition (I), and I substitution by Pb (Pb). Our findings reveal that these defects affect the geometric and optoelectronic properties as well as dynamics of charge carriers of slabs.

Formation and Luminescence of Single Oxygen Impurities on the Surface of SiC Nanocrystals.

Impurities that hinder luminescence are a common problem in the synthesis of nanocrystals, and controlling the synthesis reaction could provide a way to avoid or use impurities beneficially. Excited state molecular dynamics is used to determine how oxygen impurities appear in the plasma synthesis of silicon carbide nanocrystals (SiC NCs). Formation of impurities is studied by considering the intermediate structures in the simulated photoreaction.

Photoabsorbance of supported metal clusters: density matrix and model studies of large Ag clusters on Si surfaces.

Metal clusters with 10 to 100 atoms supported by a solid surface show electronic structure typical of molecules and require treatments starting from their atomic structure, and they also can display collective electronic phenomena similar to plasmons in metal solids. We have employed electronic structure results from two different density functionals (PBE and the hybrid HSE06) and a reduced density matrix treatment of the dissipative photodynamics to calculate light absorbance by the large Ag clusters Ag, = 33, 37(open shell) and = 32, 34 (closed shell), adsorbed at the Si(111) surface of a slab, and forming nanostructured surfaces. Results on light absorption are quite different for the two functionals, and are presented here for light absorbances using orbitals and energies from the hybrid functional giving correct energy band gaps.

First-Principles Study on Optoelectronic Properties of Fe-Doped Montmorillonite Clay.

A theoretical investigation is conducted to describe optoelectronic properties of Fe-doped montmorillonite nanoclay under spin states of low spin (LS), intermediate spin (IS), and high spin (HS). Ground state electronic properties are studied using spin-polarized density functional theory calculations. The nonradiative and radiative relaxation channels of charge carriers are studied by computing nonadiabatic couplings (NACs) using an "on-the-fly" approach from adiabatic molecular dynamics trajectories.

Electronic relaxation of photoexcited open and closed shell adsorbates on semiconductors: Ag and Ag on TiO.

A theoretical treatment based on the equations of motion of an electronic reduced density matrix, and related computational modeling, is used to describe and calculate relaxation times for nanostructured TiO(110) surfaces, here for Ag and Ag adsorbates. The theoretical treatment deals with the preparation of a photoexcited system under two different conditions, by steady light absorption with a cutoff and by a light pulse, and describes the following relaxation of electronic densities. On the computational modeling, results are presented for electronic density of states, light absorbance, and relaxation dynamics, comparing results for Ag and Ag adsorbates.

Hot Carrier Dynamics at Ligated Silicon(111) Surfaces: A Computational Study.

We provide a case-study for thermal grafting of benzenediazonium bromide onto a hydrogenated Si(111) surface using molecular dynamics (AIMD) calculations. A sequence of reaction steps is identified in the AIMD trajectory, including the loss of N from the diazonium salt, proton transfer from the surface to the bromide ion that eliminates HBr, and deposition of the phenyl group onto the surface. We next assess the influence of the phenyl groups on photophysics of hydrogen-terminated Si(111) slabs.

Magnetic-Field-Driven Electron Dynamics in Graphene.

Graphene exhibits unique optoelectronic properties originating from the band structure at the Dirac points. It is an ideal model structure to study the electronic and optical properties under the influence of the applied magnetic field. In graphene, electric field, laser pulse, and voltage can create electron dynamics which is influenced by momentum dispersion.

Nonradiative Relaxation Dynamics of a Cesium Lead Halide Perovskite Photovoltaic Architecture: Effect of External Electric Fields.

Lead halide perovskites have attracted much attention as an active material in solar cells. In this first-principles study, we consider a cesium lead halide perovskite slab interfacing with electron transport and hole transport layers, relevant to the practical photovoltaic architecture. We apply external electric fields normal to the surface of the perovskite slab and explore the induced changes onto optoelectronic properties.

Brightly Luminescent CsPbBr Nanocrystals through Ultracentrifugation.

Using a combination of density-gradient and analytical ultracentrifugation, we studied the photophysical profile of CsPbBr nanocrystal (NC) suspensions by separating them into size-resolved fractions. Ultracentrifugation drastically alters the ligand profile of the NCs, which necessitates postprocessing to restore colloidal stability and enhance quantum yield (QY). Rejuvenated fractions show a 50% increase in QY compared to no treatment and a 30% increase with respect to the parent.

Synthesis of Holey Graphene Nanoparticle Compounds.

Bulk-scale syntheses of sp nanocarbon have typically been generated by extensive chemical oxidation to yield graphite oxide from graphite, followed by a reductive step. Materials generated via harsh random processes lose desirable physical characteristics. Loss of sp conjugation inhibits long-range electronic transport and the potential for electronic band manipulation.

Universal Size-Dependent Stokes Shifts in Lead Halide Perovskite Nanocrystals.

Size-dependent photoluminescence Stokes shifts (Δ) universally exist in CsPbX (X = Cl, Br, or I) perovskite nanocrystals (NCs). Δ values, which range from ∼15 to 100 meV for NCs with average edge lengths () from approximately 13 to 3 nm, are halide-dependent such that Δ(CsPbI) > Δ(CsPbBr) ≳ Δ(CsPbCl). Observed size-dependent Stokes shifts are not artifacts of ensemble size distributions as demonstrated through measurements of single CsPbBr NC Stokes shifts (⟨Δ⟩ = 42 ± 5 meV), which are in near quantitative agreement with associated ensemble ( = 6.

Bright Silicon Nanocrystals from a Liquid Precursor: Quasi-Direct Recombination with High Quantum Yield.

Silicon nanocrystals (SiNCs) with bright bandgap photoluminescence (PL) are of current interest for a range of potential applications, from solar windows to biomedical contrast agents. Here, we use the liquid precursor cyclohexasilane (SiH) for the plasma synthesis of colloidal SiNCs with exemplary core emission. Through size separation executed in an oxygen-shielded environment, we achieve PL quantum yields (QYs) approaching 70% while exposing intrinsic constraints on efficient core emission from smaller SiNCs.

First-Principles Molecular Dynamics of Monomethylhydrazine and Nitrogen Dioxide.

The exploration of chemical reactions preceding ignition is essential for the development of ideal hypergolic propellants. Unexpected reaction pathways of a hypergolic mixture composed of monomethylhydrazine and nitrogen dioxide are predicted through a cooperative combination of (i) spin-unrestricted ab initio molecular dynamics (AIMD) and (ii) wave packet dynamics of protons. Ensembles of AIMD trajectories reveal a sequence of reaction steps for proton transfer and rupture of the C-N bond.

Excited-State Dynamics of a CsPbBr Nanocrystal Terminated with Binary Ligands: Sparse Density of States with Giant Spin-Orbit Coupling Suppresses Carrier Cooling.

Fully inorganic lead halide perovskite nanocrystals (NCs) are of interest for photovoltaic and light-emitting devices due to optoelectronic properties that can be tuned/optimized via halide composition, surface passivation, doping, and confinement. Compared to bulk materials, certain excited-state properties in NCs can be adjusted by electronic confinement effects such as suppressed hot carrier cooling and enhanced radiative recombination. Here we use spinor Kohn-Sham orbitals (SKSOs) with spin-orbit coupling (SOC) interaction as a basis to compute excited-state dissipative dynamics simulations on a fully passivated CsPbBr NC atomistic model.

Unexpected high binding energy of CO on CHNHPbI lead-halide organic-inorganic perovskites via bicarbonate formation.

The adsorption kinetics of CO2 was experimentally characterized in ultra-high vacuum (UHV). Unexpectedly, high desorption temperatures (640 K, 170 kJ mol-1) were seen. Density functional theory (DFT) calculations suggest the stabilization mechanism: bicarbonate formation in the defected perovskite film due to CO2 and H2O coadsorption.

Unraveling Photodimerization of Cyclohexasilane from Molecular Dynamics Studies.

Photoinduced reactions of a pair of cyclohexasilane (CHS) monomers are explored by time-dependent excited-state molecular dynamics (TDESMD) calculations. In TDESMD trajectories, one observes vivid reaction events including dimerization and fragmentation. A general reaction pathway is identified as (i) ring-opening formation of a dimer, (ii) rearrangement induced by bond breaking, and (iii) decomposition through the elimination of small fragments.

Photoinduced Charge Transfer versus Fragmentation Pathways in Lanthanum Cyclopentadienyl Complexes.

This study compares two competing pathways of photoexcitations in gas-phase metal-organic complexes: first, a sequence of phonon-assisted electronic transitions leading to dissipation of the energy of photoexcitations and, second, a sequence of light-driven electronic transitions leading to photolysis. Phonon-assisted charge carrier dynamics is investigated by combination of the density matrix formalism and on-the-fly nonadiabatic couplings. Light-driven fragmentation is modeled by a time-dependent excited-state molecular-dynamics (TDESMD) algorithm based on Rabi theory and principles similar to the trajectory surface hopping approximation.

Photofragmentation of Tetranitromethane: Spin-Unrestricted Time-Dependent Excited-State Molecular Dynamics.

In this study, the photofragmentation dynamics of tetranitromethane (TNM) is explored by a spin-unrestricted time-dependent excited-state molecular dynamics (u-TDESMD) algorithm based on Rabi oscillations and principles similar to trajectory surface hopping, with a midintensity field approximation. The leading order process is represented by the molecule undergoing cyclic excitations and de-excitations. During excitation cycles, the nuclear kinetic energy is accumulated to overcome the dissociation barriers in the reactant and a sequence of intermediates.

Photoinduced Single- and Multiple-Electron Dynamics Processes Enhanced by Quantum Confinement in Lead Halide Perovskite Quantum Dots.

Methylammonium lead iodide perovskite (MAPbI) is a promising material for photovoltaic devices. A modification of MAPbI into confined nanostructures is expected to further increase efficiency of solar energy conversion. Photoexcited dynamic processes in a MAPbI quantum dot (QD) have been modeled by many-body perturbation theory and nonadiabatic dynamics.

Surface Chemistry of Semiconducting Quantum Dots: Theoretical Perspectives.

Colloidal quantum dots (QDs) are near-ideal nanomaterials for energy conversion and lighting technologies. However, their photophysics exhibits supreme sensitivity to surface passivation and defects, of which control is problematic. The role of passivating ligands in photodynamics remains questionable and is a focus of ongoing research.

Photofragmentation of the Gas-Phase Lanthanum Isopropylcyclopentadienyl Complex: Computational Modeling vs Experiment.

Photofragmentation of the lanthanum isopropylcyclopentadienyl complex, La(iCp), was explored through time-dependent excited-state molecular dynamics (TDESMD), excited-state molecular dynamics (ESMD), and thermal molecular dynamics (MD). Simulated mass spectra were extracted from ab initio molecular dynamics simulations through a new and simple method and compared to experimental photoionization time-of-flight (PI-TOF) mass spectra. The computational results indicate that the value of excitation energy and mechanism of excitation determine the dissociation process.

Excited State Dynamics of Ru10 Cluster Interfacing Anatase TiO2(101) Surface and Liquid Water.

Charge transfer dynamics at the interface of supported metal nanocluster and liquid water by GGA+U calculations combined with density matrix formalism is considered. The Ru10 cluster introduces new states into the band gap of TiO2 surface, narrows the band gap of TiO2, and enhances the absorption strength. The H2O adsorption significantly enhances the intensity of photon absorption, which is due to the formation of Ti-O(water) and Ru-O(water) bonds at the interfaces.

Electronic properties of nickel-doped TiO₂ anatase.

Atomistic details of electron transfer in semiconductor materials are characterized for TiO2 thin film surfaces doped with nickel. A periodic slab model of eight atomic layers exposes the (1 0 0) crystallographic surface and is covered with a monolayer of water. The density of states, absorption spectra, partial charge densities, molecular dynamics, and non-adiabatic couplings are compared between doped and undoped models.

Modeling the surface photovoltage of silicon slabs with varying thickness.

The variation with thickness of the energy band gap and photovoltage at the surface of a thin semiconductor film are of great interest in connection with their surface electronic structure and optical properties. In this work, the change of a surface photovoltage (SPV) with the number of layers of a crystalline silicon slab is extracted from models based on their atomic structure. Electronic properties of photoexcited slabs are investigated using generalized gradient and hybrid density functionals, and plane wave basis sets.