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Atomistic modeling of ultra-thin surface oxide growth on a ternary alloy : oxidation of Al-Ni-Fe.
By employing variable-charge molecular dynamics, surface oxide film growth on aluminum-nickel-iron alloys has been studied at 300 and 600 K. The dynamics of oxidation and oxide growth is strongly dependent on the composition of the initial alloy and the ambient temperature. Higher content of Ni and Fe in Al alloys is found to reduce the oxide growth kinetics; 15% Ni + 15% Fe Al alloy yielded 30-40% less growth at 400 ps oxygen exposure compared to pure Al. We observe dopant segregation, which disrupts the interaction between O atoms and Al atoms in the alloy, leading to a nonlinear oxide growth profile in the case of ternary Al-Ni-Fe alloy. Compared to oxidation at 300 K, 30% more oxide layer was yielded at 600 K, due to the elevated temperature. The simulated oxide kinetics indicates that the growth rate of anion surpasses the cation rate with higher sensitivity to the stoichiometry of the base metal substrate. Charge state analysis provides insights into the evolution of cation and anion species as the oxide layer grows. In particular, due to higher correlation, Fe shows a high rate of oxidation when the content is high, whereas the rate of Ni oxidation is consistently low. Density profile analysis suggests the segregation of dopant atoms below the growing ultrathin oxide layer, showing the presence of a layer-by-layer mode of oxide layer even with disordered structure. Coordination number (Z, the number of oxygen atoms around an aluminum atom) of aluminum oxide has been used to identify how the initial oxidation transitions into equilibrated states. Z = 3 is dominant in the early stages of oxidation and at the interface between oxide and bulk substrate, but it transitions quickly to Z = 4 (45%) and 5 (35%) as the oxide equilibrates and approaches its self-limiting thickness. Even though growth kinetics is dependent on the base metal stoichiometry, the composition of the oxide microstructure is not significantly affected, primarily segregating dopant elements, i.e., Ni and Fe outside of the oxide layer.
Heavy boron doping effects on biaxially tensile strained germanium (>1.5%) investigated via structural characterization, effective lifetime assessment and atomistic modeling
Highly tensile strained germanium (ε-Ge) represents an essential material system for emerging electronic and photonics applications. Moreover, adjusting the doping levels to moderate or high concentrations can effectively tailor the properties of ε-Ge for specific applications. This article combines experimental characterization with a theoretical framework to examine the effects of heavy elemental boron (B) doping on pseudomorphic sub-50 nm ε-Ge. High resolution X-ray diffractometry is used to validate tensile strain levels of 1.53% and 1.68% in Ge epilayers, surpassing the indirect-to-direct band gap crossover point at ∼1.5% biaxial tensile strain. Cross-sectional transmission electron microscopy revealed visual evidence of stacking faults and surface roughening in 1.68% ε-Ge, although a coherent and abrupt Ge/III–V heterointerface is observed, devoid of interfacial misfit dislocations. Effective lifetime measurements demonstrated approximately twofold enhancement in 1.53% B-doped ε-Ge (N B ∼7 × 10 19 cm −3 ) compared to its unstrained B-doped counterpart, while no such improvement was observed in 1.68% B-doped ε-Ge. This lack of enhancement is attributed to the presence of stacking faults and surface roughness within the ε-Ge epilayer. Through density functional theory calculations, we independently demonstrate that substitutional B atoms induce local deformation of Ge–Ge bonds in both unstrained Ge and ε-Ge epilayers, resulting in an additive tensile strain. This phenomenon could potentially lead to dynamic reduction and overcoming of the critical layer thickness for the system, facilitating the nucleation and subsequent glide of 90° leading Shockley partial dislocations, thereby generating stacking faults. In essence, these findings establish an upper limit on the B-doping concentration that can be achieved in highly ε-Ge epilayers, and collectively, offer valuable insights into the significance of heavy doping in Ge-based heterostructures. As such, this study delineates a fundamental constraint for integrating heavily doped ε-Ge in high-performance optoelectronic systems, necessitating precise strain-doping co-optimization to avoid performance degradation.
Tools for the cryo-EM gold rush: going from the cryo-EM map to the atomistic model
As cryo-electron microscopy (cryo-EM) enters mainstream structural biology, the demand for fitting methods is high. Here, we review existing flexible fitting methods for cryo-EM. We discuss their importance, potential concerns and assessment strategies. We aim to give readers concrete descriptions of cryo-EM flexible fitting methods with corresponding examples.
Atomistic modeling of nanoscale patterning of L1{sub 2} order induced by ion irradiation
Theoretical predictions indicate that ordered alloys can spontaneously develop a steady-state nanoscale microstructure when irradiated with energetic particles. This behavior derives from a dynamical competition between disordering in cascades and thermally activated reordering, which leads to self-organization of the chemical order parameter. We test this possibility by combining molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations. We first generate realistic distributions of disordered zones for Ni{sub 3}Al irradiated with 70 keV He and 1 MeV Kr ions using MD and then input this data into KMC to obtain predictions of steady state microstructures as a function of the irradiation flux. Nanoscale patterning is observed for Kr ion irradiations but not for He ion irradiations. We illustrate, moreover, using image simulations of these KMC microstructures, that high-resolution transmission electron microscopy can be employed to identify nanoscale patterning. Finally, we indicate how this method could be used to synthesize functional thin films, with potential for magnetic applications.
Characterization of Semicrystalline Polymeric Materials by Atomistic Models
Characterization of two crystalline phases ({delta} and {epsilon}) of syndiotactic polystyrene using molecular modeling are discussed. These two polymorphs present nanoporosity, being able to adsorb molecules of low molecular weight in their cavities ({delta}) or in their channels ({epsilon}). By means of Grand Canonical Monte Carlo molecular simulations, adsorption isotherms of nitrogen and hydrogen were calculated, exploring the possible utilization of these materials with storage purposes. Molecular Dynamics simulations were performed to determine self diffusion behavior of light gases and these results combined with a geometric method are being employed to measure the size of the nanochannels of the e polymorph.
Understanding polarization properties of InAs quantum dots by atomistic modeling of growth dynamics
A model for realistic InAs quantum dot composition profile is proposed and analyzed, consisting of a double region scheme with an In-rich internal core and an In-poor external shell, in order to mimic the atomic scale phenomena such as In-Ga intermixing and In segregation during the growth and overgrowth with GaAs. The parameters of the proposed model are derived by reproducing the experimentally measured polarization data. Further understanding is developed by analyzing the strain fields which suggests that the two-composition model indeed results in lower strain energies than the commonly applied uniform composition model.
Atomistic modeling of bending properties of oxidized silicon nanowires
In this work, we have modeled a three point bending test of monocrystalline Si nanowires using molecular dynamics simulations in order to investigate their elastic properties. Tested nanowires were about 30 nm in length and had diameters from 5 to 9 nm. To study the influence of a native oxide layer, nanowires were covered with a 1 nm thick silica layer. The bending force was applied by a carbon diamond half-sphere with a 5 nm diameter. The Si-O parametrization for the Tersoff potential was used to describe atomic interactions between Si and O atoms. In order to remove the indentation effect of the diamond half-sphere and to obtain a pure bending behavior, we have also performed a set of simulations with fixed bottoms of the nanowires. Our results show that the oxide layer reduces the nanowire stiffness when compared with a pure Si nanowire with the same number of silicon atoms—in spite of the fact that the oxidized nanowires had larger diameters.
Atomistic modeling and HAADF investigations of misfit and threading dislocations in GaSb/GaAs hetero-structures for applications in high electron mobility transistors
A detailed investigation on the misfit and threading dislocations at GaSb/GaAs interface has been carried out using molecular dynamics simulation and quantitative electron microscopy techniques. The sources and propagation of misfit dislocations have been elucidated. The nature and formation mechanisms of the misfit dislocations as well as the role of Sb on the stability of the Lomer configuration have been explained.
Atomistic modeling of the metallic-to-semiconducting phase boundaries in monolayer MoS{sub 2}
Recent experimental demonstration on the coexistence of metallic and semiconducting phases in the same monolayer MoS{sub 2} crystal has attracted much attention for its use in ultra-low contact resistance-MoS{sub 2} transistors. However, the electronic structures of the metallic-to-semiconducting phase boundaries, which appear to dictate the carrier injection in such transistors, are not yet well understood. In this letter, interfacing the 2H and 1T′ polytypes appropriately, we first model the “beta” (β) and the “gamma” (γ) phase boundaries, and demonstrate good agreement with experiential results. We then apply first-principles based density functional theory to calculate the electronic structures for those optimized geometries. We further employ non equilibrium Green's function formalism to evaluate the transmission spectra and the local density of states (LDOS) in order to assess the Schottky barrier nature of the phase boundaries. Our study reveals that while the γ boundary yields p-type Schottky barrier, the β boundary leads to the distinct symmetric Schottky barrier with an atomically sharp transition region. This understanding could be useful for designing high performance transistors using phase-engineered MoS{sub 2} crystals.
Atomistic modelling of evaporation and explosive boiling of thin film liquid argon over internally recessed nanostructured surface
Molecular dynamics (MD) simulations have been carried out to investigate evaporation and explosive boiling phenomena of thin film liquid argon on nanostructured solid surface with emphasis on the effect of solid-liquid interfacial wettability. The nanostructured surface considered herein consists of trapezoidal internal recesses of the solid platinum wall. The wetting conditions of the solid surface were assumed such that it covers both the hydrophilic and hydrophobic conditions and hence effect of interfacial wettability on resulting evaporation and boiling phenomena was the main focus of this study. The initial configuration of the simulation domain comprised of a three phase system (solid platinum, liquid argon and vapor argon) on which equilibrium molecular dynamics (EMD) was performed to reach equilibrium state at 90 K. After equilibrium of the three-phase system was established, the wall was set to different temperatures (130 K and 250 K for the case of evaporation and explosive boiling respectively) to perform non-equilibrium molecular dynamics (NEMD). The variation of temperature and density as well as the variation of system pressure with respect to time were closely monitored for each case. The heat flux normal to the solid surface was also calculated to illustrate the effectiveness of heat transfer for hydrophilic and hydrophobic surfaces in cases of both nanostructured surface and flat surface. The results obtained show that both the wetting condition of the surface and the presence of internal recesses have significant effect on normal evaporation and explosive boiling of the thin liquid film. The heat transfer from solid to liquid in cases of surface with recesses are higher compared to flat surface without recesses. Also the surface with higher wettability (hydrophilic) provides more favorable conditions for boiling than the low-wetting surface (hydrophobic) and therefore, liquid argon responds quickly and shifts from liquid to vapor phase faster in case of hydrophilic surface. The heat transfer rate is also much higher in case of hydrophilic surface.
Atomistic models of Cu diffusion in CuInSe 2 under variations in composition
Here we construct an analytic model for the composition dependence of the vacancy-mediated Cu diffusion coefficient in undoped CuInSe 2 using parameters from density functional theory. The applicability of this model is supported numerically with kinetic lattice Monte Carlo and Onsager transport tensors. We discuss how this model relates to experimental measurements of Cu diffusion, arguing that our results can account for significant contributions to the bulk diffusion of Cu tracers in non-stoichiometric CuInSe 2 .
Atomistic models of In and Ga diffusion in Cu(In,Ga)Se 2
To improve the performance of Cu(In,Ga)Se 2 thin-film photovoltaic devices, a robust understanding of the dominant diffusion pathways of the alloy species In and Ga is needed. Here, the most probable defect complexes and mechanisms for In and Ga diffusion are identified with the aid of density functional theory. The binding energies and migration barriers for these complexes are calculated in bulk CuInSe 2 and CuGaSe 2 . Analytic models and kinetic lattice Monte Carlo simulations are employed to predict the diffusivity of In and Ga under variations in composition and temperature. Here, we find that a model based on coulombic interactions between group III antisites and vacancies on the Cu-sublattice produces results that match well with experiment.
Atomistic modeling of dislocation interactions with twin boundaries in Ti
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