Jun Song

Atomic mechanism and prediction of hydrogen embrittlement in iron

Hydrogen embrittlement in metals has posed a serious obstacle to designing strong and reliable structural materials for many decades, and predictive physical mechanisms still do not exist. Here, a new H embrittlement mechanism operating at the atomic scale in α-iron is demonstrated. Direct molecular dynamics simulations reveal a ductile-to-brittle transition caused by the suppression of dislocation emission at the crack tip due to aggregation of H, which then permits brittle-cleavage failure followed by slow crack growth. The atomistic embrittlement mechanism is then connected to material states and loading conditions through a kinetic model for H delivery to the crack-tip region. Parameter-free predictions of embrittlement thresholds in Fe-based steels over a range of H concentrations, mechanical loading rates and H diffusion rates are found to be in excellent agreement with experiments. This work provides a mechanistic, predictive framework for interpreting experiments, designing structural components and guiding the design of embrittlement-resistant materials.

Probing the Dynamics of the Metallic-to-Semiconducting Structural Phase Transformation in MoS2 Crystals

We have investigated the phase transformation of bulk MoS2 crystals from the metastable metallic 1T/1T phase to the thermodynamically stable semiconducting 2H phase. The metastable 1T/1T material was prepared by Li intercalation and deintercalation. The thermally driven kinetics of the phase transformation were studied with in situ Raman and optical reflection spectroscopies and yield an activation energy of 400 ± 60 meV (38 ± 6 kJ/mol). We calculate the expected minimum energy pathways for these transformations using DFT methods. The experimental activation energy corresponds approximately to the theoretical barrier for a single formula unit, suggesting that nucleation of the phase transformation is quite local. We also report that femtosecond laser writing converts 1T/1T to 2H in a single laser pass. The mechanisms for the phase transformation are discussed.

Testing continuum concepts for hydrogen embrittlement in metals using atomistics

Hydrogen embrittlement is a pervasive mode of degradation in many metallic systems that can occur via several mechanisms. Here, the competition between dislocation emission and cleavage at a crack tip is evaluated in the presence of H. At this level, embrittlement is predicted when the critical stress intensity required for emission rises above that needed for cleavage, eliminating crack tip plasticity and blunting as toughening mechanisms. Continuum predictions for emission and cleavage are made using computed generalized stacking fault energies and surface energies in a model Ni-H system, and embrittlement is predicted at a critical H concentration. An atomistic model is then used to investigate actual crack tip behavior in the presence of controlled arrays of H atoms around the crack tip. The continuum models are accurate at low H concentrations, below the embrittlement point, but at higher H concentrations the models deviate from the atomistic behavior due to alternative dislocation emission modes. Additional H configurations are investigated to understand controlling features of the emission process. In no cases does crack propagation occur in preference to dislocation emission in geometries where emission is possible, indicating that embrittlement can be more complicated than envisioned by the basic brittle-ductile transition.

Phase engineering of monolayer transition-metal dichalcogenide through coupled electron doping and lattice deformation

First-principles calculations were performed to investigate the phase stability and transition within four monolayer transition-metal dichalcogenide (TMD) systems, i.e., MX2 (Mu2009=u2009Mo or W and Xu2009=u2009S or Se) under coupled electron doping and lattice deformation. With the lattice distortion and electron doping density treated as state variables, the energy surfaces of different phases were computed, and the diagrams of energetically preferred phases were constructed. These diagrams assess the competition between different phases and predict conditions of phase transitions for the TMDs considered. The interplay between lattice deformation and electron doping was identified as originating from the deformation induced band shifting and band bending. Based on our findings, a potential design strategy combining an efficient electrolytic gating and a lattice straining to achieve controllable phase engineering in TMD monolayers was demonstrated.

Phase engineering of MoS2 through GaN/AlN substrate coupling and electron doping

The polymorphism of two dimensional MoS2 promises new possibilities for nanoelectronics. The realization of those possibilities necessitates techniques to enable flexible and controllable phase engineering of MoS2. In the present study, based on first-principles calculations, a new and flexible route to engineer the phase stability of MoS2 by interfacing it with a GaN or AlN substrate is reported. Depending on the surface termination of the underlying substrate, MoS2 may exhibit either the 2H or 1T (1T) phase. The interface coupling between MoS2 and the substrate also affects the phase transition kinetics. In addition, electron doping can act as another means to influence MoS2-substrate interactions and enable further phase engineering of MoS2. The present findings contribute to new knowledge towards phase engineering of MoS2 and the design of hybrid nanodevices comprising both 2D and 3D optoelectronic materials.

Core structures analyses of (a+c)-edge dislocations in wurtzite GaN through atomistic simulations and Peierls–Nabarro model

The core structures and slip characteristics of (a+c)-edge dislocations on pyramidal planes in wurtzite GaN were investigated employing molecular dynamics simulations. Multiple stable core configurations are identified for dislocations along the glide and shuffle planes. The corresponding generalized-stacking-fault energy (GSFE) curves for the glide and shuffle slips are calculated. The GSFE curves, combined with the Peierls–Nabarro model, demonstrate that the shuffle slip is favored over the glide slip given the markedly lower Peierls energy and stress of the shuffle slip. Our findings also indicate that in general slip motions for (a+c)-edge dislocations are only possible at elevated temperature, and the necessity of further studies of thermally activated processes to better understand the dynamics of (a+c) dislocations in GaN.

Effects of Mg and Al doping on dislocation slips in GaN

First-principles density functional theory calculations were employed to systematically examine the effects of Mg and Al additions to wurtzite GaN on the generalized stacking fault energy (GSFE) curves for (11¯00)[112¯0]u2009 and (11¯00)[0001] dislocations along the glide or shuffle slip planes. It was found that for both slip systems, Mg doping leads to significant reduction of the GSFE while Al doping elevates the GSFE curve. For each dopant, the effect of doping on the GSFE was shown to scale linearly with the dopant concentration, being independent of the slip (i.e., glide or shuffle) plane. The GSFE curves were subsequently combined with the Peierls-Nabarro model to quantitatively analyze the micromechanical characteristics of dislocation slips. The implications of our findings to slip dynamics and dislocation dissociation mechanism were then discussed. Our study provides important insights towards the understanding and control of dislocation dynamics in impurity-doped GaN.

Predictions of thermal expansion coefficients of rare-earth zirconate pyrochlores: A quasi-harmonic approximation based on stable phonon modes

Rare-earth (RE) pyrochlores are considered as promising candidate materials for the thermal barrier coating. In this study, we performed first-principles calculations, augmented by quasi-harmonic phonon calculations, to investigate the thermal expansion behaviors of several RE2Zr2O7 (RE = La, Nd, Sm, Gd) pyrochlores. Our findings show that RE2Zr2O7 pyrochlores exhibit low-lying optical phonon frequencies that correspond to RE-cation rattling vibrational modes. These frequencies become imaginary upon volume expansion, preventing correct determination of the free energy versus volume relation and thereby quantification of thermal expansion using QH phonon calculations. To address this challenge, we proposed a QH approximation approach based on stable phonon modes where the RE-cation rattling modes were systematically eliminated. This approach is shown to provide accurate predictions of the coefficients of thermal expansion (CTEs) of RE2Zr2O7 pyrochlores, in good agreement with experimental measurements and data from first-principles molecular dynamics simulations. In addition, we showed that the QH Debye model considerably overestimates the magnitudes and wrongly predicts the trend for the CTEs of RE2Zr2O7 pyrochlores.

Photoelectrochemical CO2 Reduction into Syngas with the Metal/Oxide Interface

Photoelectrochemical (PEC) reduction of CO2 with H2O not only provides an opportunity for reducing net CO2 emissions but also produces value-added chemical feedstocks and fuels. Syngas, a mixture of CO and H2, is a key feedstock for the production of methanol and other commodity hydrocarbons in industry. However, it is challenging to achieve efficient and stable PEC CO2 reduction into syngas with controlled composition owing to the difficulties associated with the chemical inertness of CO2 and complex reaction network of CO2 conversion. Herein, by employing a metal/oxide interface to spontaneously activate CO2 molecule and stabilize the key reaction intermediates, we report a benchmarking solar-to-syngas efficiency of 0.87% and a high turnover number of 24u202f800, as well as a desirable high stability of 10 h. Moreover, the CO/H2 ratios in the composition can be tuned in a wide range between 4:1 and 1:6 with a total unity Faradaic efficiency. On the basis of experimental measurements and theoretical calculations, we present that the metal/oxide interface provides multifunctional catalytic sites with complementary chemical properties for CO2 activation and conversion, leading to a unique pathway that is inaccessible with the individual components. The present approach opens new opportunities to rationally develop high-performance PEC systems for selective CO2 reduction into valuable carbon-based chemicals and fuels.

Effect of topological patterning on self-rolling of nanomembranes

The effects of topological patterning (i.e., grating and rectangular patterns) on the self-rolling behaviors of heteroepitaxial strained nanomembranes have been systematically studied. An analytical modeling framework, validated through finite-element simulations, has been formulated to predict the resultant curvature of the patterned nanomembrane as the pattern thickness and density vary. The effectiveness of the grating pattern in regulating the rolling direction of the nanomembrane has been demonstrated and quantitatively assessed. Further to the rolling of nanomembranes, a route to achieve predictive design of helical structures has been proposed and showcased. The present study provides new knowledge and mechanistic guidance towards predictive control and tuning of roll-up nanostructures via topological patterning.