Anisotropic atomistic shock response mechanisms of aramid crystals.

Aramid fibers composed of poly(p-phenylene terephthalamide) (PPTA) polymers are attractive materials due to their high strength, low weight, and high shock resilience. Even though they have widely been utilized as a basic ingredient in Kevlar, Twaron, and other fabrics and applications, their intrinsic behavior under intense shock loading is still to be understood. In this work, we characterize the anisotropic shock response of PPTA crystals by performing reactive molecular dynamics simulations. Results from shock loading along the two perpendicular directions to the polymer backbones, [100] and [010], indicate distinct shock release mechanisms that preserve and destroy the hydrogen bond network. Shocks along the [100] direction for particle velocity Up < 2.46 km/s indicate the formation of a plastic regime composed of shear bands, where the PPTA structure is planarized. Shocks along the [010] direction for particle velocity Up < 2.18 km/s indicate a complex response regime, where elastic compression shifts to amorphization as the shock is intensified. While hydrogen bonds are mostly preserved for shocks along the [100] direction, hydrogen bonds are continuously destroyed with the amorphization of the crystal for shocks along the [010] direction. Decomposition of the polymer chains by cross-linking is triggered at the threshold particle velocity Up = 2.18 km/s for the [010] direction and Up = 2.46 km/s for the [100] direction. These atomistic insights based on large-scale simulations highlight the intricate and anisotropic mechanisms underpinning the shock response of PPTA polymers and are expected to support the enhancement of their applications.

Electric-field-induced crossover of polarization reversal mechanisms in Al1-xScxN ferroelectrics.

Scandium-doped aluminum nitride, Al1-xScxN, represents a new class of displacive ferroelectric materials with high polarization and sharp hysteresis along with high-temperature resilience, facile synthesizability and compatibility with standard CMOS fabrication techniques. The fundamental physics behind the transformation of unswitchable piezoelectric AlN into switchable Al-Sc-N ferroelectrics depends upon important atomic properties such as local structure, dopant distributions and the presence of competing mechanism of polarization switching in the presence of an applied electric-field that have not been understood. We computationally synthesize Al1-xScxN to quantify the inhomogeneity of Sc distribution and phase segregation, and characterize its crystal and electronic structure as a function of Sc-doping. Nudged elastic band calculations of the potential energy surface and quantum molecular dynamics simulations of direct electric-field-driven ferroelectric switching reveal a crossover between two polarization reversal mechanisms-inhomogeneous nucleation-and-growth mechanism originating near Sc-rich regions in the limit of low applied fields and nucleation-limited-switching in the high-field regime. Understanding polarization reversal pathways for these two mechanisms as well as the role of local Sc concentration on activation barriers provides design rules to identify other combinations of dopant elements, such as Zr, Mg etc. to synthesize superior AlN-based ferroelectric materials.

Growth Kinetics and Atomistic Mechanisms of Native Oxidation of ZrSxSe2-x and MoS2 Crystals.

A thorough understanding of native oxides is essential for designing semiconductor devices. Here, we report a study of the rate and mechanisms of spontaneous oxidation of bulk single crystals of ZrSxSe2-x alloys and MoS2. ZrSxSe2-x alloys oxidize rapidly, and the oxidation rate increases with Se content. Oxidation of basal surfaces is initiated by favorable O2 adsorption and proceeds by a mechanism of Zr-O bond switching, that collapses the van der Waals gaps, and is facilitated by progressive redox transitions of the chalcogen. The rate-limiting process is the formation and out-diffusion of SO2. In contrast, MoS2 basal surfaces are stable due to unfavorable oxygen adsorption. Our results provide insight and quantitative guidance for designing and processing semiconductor devices based on ZrSxSe2-x and MoS2 and identify the atomistic-scale mechanisms of bonding and phase transformations in layered materials with competing anions.

Photoexcitation-Induced Nonthermal Ultrafast Loss of Long-Range Order in GeTe.

Nonadiabatic quantum molecular dynamics is used to investigate the evolution of GeTe photoexcited states. Results reveal a photoexcitation-induced picosecond nonthermal path for the loss of long-range order. A valence electron excitation threshold of 4% is found to trigger local disorder by switching Ge atoms from octahedral to tetrahedral sites and promoting Ge-Ge bonding. The resulting loss of long-range order for a higher valence electron excitation fraction is achieved without fulfilling the Lindemann criterion for melting, therefore utilizing a nonthermal path. The photoexcitation-induced structural disorder is accompanied by charge transfer from Te to Ge, Ge-Te bonding-to-antibonding, and Ge-Ge antibonding-to-bonding change, triggering Ge-Te bond breaking and promoting the formation of Ge-Ge wrong bonds. These results provide an electronic-structure basis to understand the photoexcitation-induced ultrafast changes in the structure and properties of GeTe and other phase-change materials.

Photoexcitation Induced Ultrafast Nonthermal Amorphization in Sb2Te3.

Phase-change materials are of great interest for low-power high-throughput storage devices in next-generation neuromorphic computing technologies. Their operation is based on the contrasting properties of their amorphous and crystalline phases, which can be switched on the nanosecond time scale. Among the archetypal phase change materials based on Ge-Sb-Te alloys, Sb2Te3 displays a fast and energy-efficient crystallization-amorphization cycle due to its growth-dominated crystallization and low melting point. This growth-dominated crystallization contrasts with the nucleation-dominated crystallization of Ge2Sb2Te5. Here, we show that the energy required for and the time associated with the amorphization process can be further reduced by using a photoexcitation-based nonthermal path. We employ nonadiabatic quantum molecular dynamics simulations to investigate the time evolution of Sb2Te3 with 2.6, 5.2, 7.5, 10.3, and 12.5% photoexcited valence electron-hole carriers. Results reveal that the degree of amorphization increases with excitation, saturating at 10.3% excitation. The rapid amorphization originates from an instantaneous charge transfer from Te-p orbitals to Sb-p orbitals upon photoexcitation. Subsequent evolution of the excited state, within the picosecond time scale, indicates an Sb-Te bonding to antibonding transition. Concurrently, Sb-Sb and Te-Te antibonding decreases, leading to formation of wrong bonds. For photoexcitation of 7.5% valence electrons or larger, the electronic changes destabilize the crystal structure, leading to large atomic diffusion and irreversible loss of long-range order. These results highlight an ultrafast energy-efficient amorphization pathway that could be used to enhance the performance of phase change material-based optoelectronic devices.

Hydrogen Bond Preserving Stress Release Mechanism Is Key to the Resilience of Aramid Fibers.

Ab initio molecular dynamics simulations of shock loading on poly(p-phenylene terephthalamide) (PPTA) reveal stress release mechanisms based on hydrogen bond preserving structural phase transformation (SPT) and planar amorphization. The SPT is triggered by [100] shock-induced coplanarity of phenylene groups and rearrangement of sheet stacking leading to a novel monoclinic phase. Planar amorphization is generated by [010] shock-induced scission of hydrogen bonds leading to disruption of polymer sheets, and trans-to-cis conformational change of polymer chains. In contrast to the latter, the former mechanism preserves the hydrogen bonding and cohesiveness of polymer chains in the identified novel crystalline phase preserving the strength of PPTA. The interplay between hydrogen bond preserving (SPT) and nonpreserving (planar amorphization) shock release mechanisms is critical to understanding the shock performance of aramid fibers.

Toward organic photohydrides: excited-state behavior of 10-methyl-9-phenyl-9,10-dihydroacridine.

The excited-state hydride release from 10-methyl-9-phenyl-9,10-dihydroacridine (PhAcrH) was investigated using steady-state and time-resolved UV/vis absorption spectroscopy. Upon excitation, PhAcrH is oxidized to the corresponding iminium ion (PhAcr(+)), while the solvent (acetonitrile/water mixture) is reduced (52% of PhAcr(+) and 2.5% of hydrogen is formed). The hydride release occurs from the triplet excited state by a stepwise electron/hydrogen-atom transfer mechanism. To facilitate the search for improved organic photohydrides that exhibit a concerted mechanism, a computational methodology is presented that evaluates the thermodynamic parameters for the hydride ion, hydrogen atom, and electron release from organic hydrides.

Toward understanding the redox properties of model chromophores from the green fluorescent protein family: an interplay between conjugation, resonance stabilization, and solvent effects.

The redox properties of model chromophores from the green fluorescent protein family are characterized computationally using density functional theory with a long-range corrected functional, the equation-of-motion coupled-cluster method, and implicit solvation models. The analysis of electron-donating abilities of the chromophores reveals an intricate interplay between the size of the chromophore, conjugation, resonance stabilization, presence of heteroatoms, and solvent effects. Our best estimates of the gas-phase vertical/adiabatic detachment energies of the deprotonated (i.e., anionic) model red, green, and blue chromophores are 3.27/3.15, 2.79/2.67, and 2.75/2.35 eV, respectively. Vertical/adiabatic ionization energies of the respective protonated (i.e., neutral) species are 7.64/7.35, 7.38/7.15, and 7.70/7.32 eV, respectively. The standard reduction potentials (E(red)(0)) of the anionic (Chr•/Chr­) and neutral (Chr+•/Chr) model chromophores in acetonitrile are 0.34/1.40 V (red), 0.22/1.24 V (green), and −0.12/1.02 V (blue), suggesting, counterintuitively, that the red chromophore is more difficult to oxidize than the green and blue ones (in both neutral and deprotonated forms). The respective redox potentials in water follow a similar trend but are more positive than the acetonitrile values.