Chunyang Sheng

Enhanced charge recombination due to surfaces and twin defects in GaAs nanostructures

Power conversion efficiency of gallium arsenide (GaAs) nanowire (NW) solar cells is severely limited by enhanced charge recombination (CR) at sidewall surfaces, but its atomistic mechanisms are not well understood. In addition, GaAs NWs usually contain a high density of twin defects that form a twin superlattice, but its effects on CR dynamics are largely unknown. Here, quantum molecular dynamics (QMD) simulations reveal the existence of an intrinsic type-II heterostructure at the (110) GaAs surface. Nonadiabatic quantum molecular dynamics (NAQMD) simulations show that the resulting staggered band alignment causes a photoexcited electron in the bulk to rapidly transfer to the surface. We have found orders-of-magnitude enhancement of the CR rate at the surface compared with the bulk value. Furthermore, QMD and NAQMD simulations show unique surface electronic states at alternating (111)A and (111)B sidewall surfaces of a twinned [111]-oriented GaAs NW, which act as effective CR centers. The calculated large surface recombination velocity quantitatively explains recent experimental observations and provides microscopic understanding of the underlying CR processes.

Nanocarbon synthesis by high-temperature oxidation of nanoparticles

High-temperature oxidation of silicon-carbide nanoparticles (nSiC) underlies a wide range of technologies from high-power electronic switches for efficient electrical grid and thermal protection of space vehicles to self-healing ceramic nanocomposites. Here, multimillion-atom reactive molecular dynamics simulations validated by ab initio quantum molecular dynamics simulations predict unexpected condensation of large graphene flakes during high-temperature oxidation of nSiC. Initial oxidation produces a molten silica shell that acts as an autocatalytic ‘nanoreactor’ by actively transporting oxygen reactants while protecting the nanocarbon product from harsh oxidizing environment. Percolation transition produces porous nanocarbon with fractal geometry, which consists of mostly sp2 carbons with pentagonal and heptagonal defects. This work suggests a simple synthetic pathway to high surface-area, low-density nanocarbon with numerous energy, biomedical and mechanical-metamaterial applications, including the reinforcement of self-healing composites.

Facile Five-Step Heteroepitaxial Growth of GaAs Nanowires on Silicon Substrates and the Twin Formation Mechanism.

Monolithic integration of III-V semiconductors with Si has been pursued for some time in the semiconductor industry. However, the mismatch of lattice constants and thermal expansion coefficients represents a large technological challenge for the heteroepitaxial growth. Nanowires, due to their small lateral dimension, can relieve strain and mitigate dislocation formation to allow single-crystal III-V materials to be grown on Si. Here, we report a facile five-step heteroepitaxial growth of GaAs nanowires on Si using selective area growth (SAG) in metalorganic chemical vapor deposition, and we further report an in-depth study on the twin formation mechanism. Rotational twin defects were observed in the nanowire structures and showed strong dependence on the growth condition and nanowire size. We adopt a model of faceted growth to demonstrate the formation of twins during growth, which is well supported by both a transmission electron microscopy study and simulation based on nucleation energetics. Our study has led to twin-free segments in the length up to 80 nm, a significant improvement compared to previous work using SAG. The achievements may open up opportunities for future functional III-V-on-Si heterostructure devices.

Gel phase in hydrated calcium dipicolinate

The mineralization of dipicolinic acid (DPA) molecules in bacterial spore cores with Ca2+ ions to form Ca-DPA is critical to the wet-heat resistance of spores. This resistance to “wet-heat” also depends on the physical properties of water and DPA in the hydrated Ca-DPA-rich protoplasm. Using reactive molecular dynamics simulations, we have determined the phase diagram of hydrated Ca-DPA as a function of temperature and water concentration, which shows the existence of a gel phase along with distinct solid-gel and gel-liquid phase transitions. Simulations reveal monotonically decreasing solid-gel-liquid transition temperatures with increasing hydration, which explains the experimental trend of wet-heat resistance of bacterial spores. Our observation of different phases of water also reconciles previous conflicting experimental findings on the state of water in bacterial spores. Further comparison with an unmineralized hydrated DPA system allows us to quantify the importance of Ca mineralization in decreasi...

Twin superlattice-induced large surface recombination velocity in GaAs nanostructures

Semiconductor nanowires (NWs) often contain a high density of twin defects that form a twin superlattice, but its effects on electronic properties are largely unknown. Here, nonadiabatic quantum molecular dynamics simulation shows unique surface electronic states at alternating (111)A and (111)B sidewall surfaces of a twinned [111]-oriented GaAs NW, which act as effective charge-recombination centers. The calculated large surface recombination velocity quantitatively explains recent experimental observations and provides microscopic understanding of the underlying surface-recombination processes.

Multiobjective genetic training and uncertainty quantification of reactive force fields

The ReaxFF reactive force-field approach has significantly extended the applicability of reactive molecular dynamics simulations to a wide range of material properties and processes. ReaxFF parameters are commonly trained to fit a predefined set of quantum-mechanical data, but it remains uncertain how accurately the quantities of interest are described when applied to complex chemical reactions. Here, we present a dynamic approach based on multiobjective genetic algorithm for the training of ReaxFF parameters and uncertainty quantification of simulated quantities of interest. ReaxFF parameters are trained by directly fitting reactive molecular dynamics trajectories against quantum molecular dynamics trajectories on the fly, where the Pareto optimal front for the multiple quantities of interest provides an ensemble of ReaxFF models for uncertainty quantification. Our in situ multiobjective genetic algorithm workflow achieves scalability by eliminating the file I/O bottleneck using interprocess communications. The in situ multiobjective genetic algorithm workflow has been applied to high-temperature sulfidation of MoO3 by H2S precursor, which is an essential reaction step for chemical vapor deposition synthesis of MoS2 layers. Our work suggests a new reactive molecular dynamics simulation approach for far-from-equilibrium chemical processes, which quantitatively reproduces quantum molecular dynamics simulations while providing error bars.Molecular dynamics: multi-objective genetic algorithms for training and uncertainty quantificationMulti-objective genetic algorithms allow training and uncertainty quantification of force-field parameters with minimal modifications of molecular dynamics codes. A team led by Aiichiro Nakano at University of Southern California used an algorithm based on reactive molecular dynamics (RMD) simulations for the training of reactive force-field parameters and uncertainty quantification of simulated quantities of interest. Chemical vapor deposition synthesis of MoS2 monolayer was set as a specific example, and force-field parameters were trained against quantum molecular dynamics (QMD) simulations. Starting from a 128-atom MoO3–H2S system, the reaction dynamics were investigated by estimating the numbers of H–S, Mo–O, and Mo–S bonds as a function of time during the QMD simulations. By comparing these quantities of interest with those obtained in the RMD simulations, it was found that RMD can quantitatively reproduce QMD within an error bar.

Role of H transfer in the Gas-Phase Sulfidation Process of MoO3: A Quantum Molecular Dynamics Study

Layered transition metal dichalcogenide (TMDC) materials have received great attention because of their remarkable electronic, optical, and chemical properties. Among typical TMDC family members, monolayer MoS2 has been considered a next-generation semiconducting material, primarily due to a higher carrier mobility and larger band gap. The key enabler to bring such a promising MoS2 layer into mass production is chemical vapor deposition (CVD). During CVD synthesis, gas-phase sulfidation of MoO3 is a key elementary reaction, forming MoS2 layers on a target substrate. Recent studies have proposed the use of gas-phase H2S precursors instead of condensed-phase sulfur for the synthesis of higher-quality MoS2 crystals. However, reaction mechanisms, including atomic-level reaction pathways, are unknown for MoO3 sulfidation by H2S. Here, we report first-principles quantum molecular dynamics (QMD) simulations to investigate gas-phase sulfidation of MoO3 flake using a H2S precursor. Our QMD results reveal that gas-phase H2S molecules efficiently reduce and sulfidize MoO3 through the following reaction steps: Initially, H transfer occurs from the H2S molecule to low molecular weight Mo xO y clusters, sublimated from the MoO3 flake, leading to the formation of molybdenum oxyhydride clusters as reaction intermediates. Next, two neighboring hydroxyl groups on the oxyhydride cluster preferentially react with each other, forming water molecules. The oxygen vacancy formed on the Mo-O-H cluster as a result of this dehydration reaction becomes the reaction site for subsequent sulfidation by H2S that results in the formation of stable Mo-S bonds. The identification of this reaction pathway and Mo-O and Mo-O-H reaction intermediates from unbiased QMD simulations may be utilized to construct reactive force fields (ReaxFF) for multimillion-atom reactive MD simulations.

Free energy of hydration and heat capacity of calcium dipicolinate in Bacillus spore cores

Wet heat treatments are widely used sterilization techniques for inactivating dangerous and resistant sporulating bacteria. The effectiveness of such treatments depends upon the thermodynamics of water uptake by the spore as well as the kinetics of phase transformations in the hydrated spore core. The mechanism behind these chemical and physical processes remains unknown because the thermodynamic properties of the spore core constituents are not well understood. Here, we use reactive molecular dynamics simulations to calculate the vibrational density of states and specific heat of hydrated calcium dipicolinate as well as the free energy of hydration based on Jarzynskis inequality. These two quantities are used to construct a phase diagram of hydrated calcium dipicolinate, indicating the extent of hydration at different pressures and temperatures, which can be used to identify potential regimes for wet-heat sterilization of bacterial spores.Wet heat treatments are widely used sterilization techniques for inactivating dangerous and resistant sporulating bacteria. The effectiveness of such treatments depends upon the thermodynamics of water uptake by the spore as well as the kinetics of phase transformations in the hydrated spore core. The mechanism behind these chemical and physical processes remains unknown because the thermodynamic properties of the spore core constituents are not well understood. Here, we use reactive molecular dynamics simulations to calculate the vibrational density of states and specific heat of hydrated calcium dipicolinate as well as the free energy of hydration based on Jarzynskis inequality. These two quantities are used to construct a phase diagram of hydrated calcium dipicolinate, indicating the extent of hydration at different pressures and temperatures, which can be used to identify potential regimes for wet-heat sterilization of bacterial spores.

Anisotropic frictional heating and defect generation in cyclotrimethylene-trinitramine molecular crystals

Anisotropic frictional response and corresponding heating in cyclotrimethylene-trinitramine molecular crystals are studied using molecular dynamics simulations. The nature of damage and temperature rise due to frictional forces is monitored along different sliding directions on the primary slip plane, (010), and on non-slip planes, (100) and (001). Correlations between the friction coefficient, deformation, and frictional heating are established. We find that the friction coefficients on slip planes are smaller than those on non-slip planes. In response to sliding on a slip plane, the crystal deforms easily via dislocation generation and shows less heating. On non-slip planes, due to the inability of the crystal to deform via dislocation generation, a large damage zone is formed just below the contact area, accompanied by the change in the molecular ring conformation from chair to boat/half-boat. This in turn leads to a large temperature rise below the contact area.Anisotropic frictional response and corresponding heating in cyclotrimethylene-trinitramine molecular crystals are studied using molecular dynamics simulations. The nature of damage and temperature rise due to frictional forces is monitored along different sliding directions on the primary slip plane, (010), and on non-slip planes, (100) and (001). Correlations between the friction coefficient, deformation, and frictional heating are established. We find that the friction coefficients on slip planes are smaller than those on non-slip planes. In response to sliding on a slip plane, the crystal deforms easily via dislocation generation and shows less heating. On non-slip planes, due to the inability of the crystal to deform via dislocation generation, a large damage zone is formed just below the contact area, accompanied by the change in the molecular ring conformation from chair to boat/half-boat. This in turn leads to a large temperature rise below the contact area.