Minimum Energy Barrier Research Articles

Storing-hydrogen processes on graphene activated by atomic-vacancies

We investigated the minimum energy pathways and energy barriers of reversible reaction (V111+H2↔V221) based upon calculations using density functional theory. We find a comparable activation barrier of around 1.3 eV for both the dissociative chemisorption and desorption processes. The charge transfer rate from a reacting hydrogen atom to the graphene is around 0.18 e per hydrogen atom in the final state. A subsequent reaction path to recover the initial structure of V111 is realized by the migration of hydrogen atoms from V221 onto the graphene surface. The comparable energy barrier of 1.3 eV for both adsorption and desorption suggests that this novel storage and release concept has the potential to act as a hydrogen storage system for certain applications.

Absorption and Diffusion of Oxygen in Ti-Al Bulk Alloys

The oxygen absorption and diffusion properties are studied in γ-TiAl and TiAl3 alloys within density functional theory using projector augmented wave method in the plane-wave basis. It is shown that the octahedral site inside the Ti-rich octahedron is preferable for oxygen in case of γ-TiAl alloy whereas the Al-rich octahedron is more favorable environment for oxygen in TiAl3. It is shown that the energy barriers for oxygen jumps between different sites in bulk alloys depend significantly on the local environments of oxygen and increase for its jump from Ti-rich sites. The trajectories with minimum energy barriers are determined for both Ti-Al alloys. It is shown that the increase of Al content in alloy leads to the decrease of barriers for oxygen jumps.

Metadynamics-Biased ab Initio Molecular Dynamics Study of Heterogeneous CO2 Reduction via Surface Frustrated Lewis Pairs

The recent discovery of frustrated Lewis pairs (FLPs) capable of heterolytically splitting hydrogen gas at the surface of hydroxylated indium oxide (In2O3–x(OH)y) nanoparticles has led to interesting implications for heterogeneous catalytic reduction of CO2. Although the role of surface FLPs in the reverse water-gas shift (RWGS) reaction (CO2 + H2 → CO + H2O) has been experimentally and theoretically demonstrated, the interplay between surface FLPs and temperature and their consequences for the reaction mechanism have yet to be understood. Here we use well-tempered metadynamics-biased ab initio molecular dynamics to obtain the free energy landscape of the multistep RWGS reaction at finite temperatures. The reaction is simulated at 20 and 180 °C, and the minimum energy reaction pathways and energy barriers corresponding to H2 dissociation and CO2 reduction are obtained. The reduction of CO2 at the surface FLP catalytically active site, where H2 is heterolytically dissociated and bound, is found to be the r...

Dissociative Adsorption of H2 on PtRu Bimetallic Surfaces

We present a theoretical study of the dissociative adsorption of molecular hydrogen on PtRu bimetallic surfaces based on density functional theory (DFT) calculations. We focus on the reactivity of a pseudomorphic Pt monolayer deposited on Ru(0001), Pt1ML/Ru(0001), for which we have obtained a minimum activation energy barrier for H2 dissociation, Eb = 0.32 eV, i.e., ∼0.3 and 0.26 eV higher than on Ru(0001) and Pt(111), respectively. Accordingly, the initial sticking probability for low energy impinging molecules (Ei ≲ 0.1 eV) derived from classical trajectory calculations is various orders of magnitude smaller than on Ru(0001) in apparent contradiction with available experimental data. However, undercoordinated Pt atoms in the borders of Pt pseudomorphic islands and isolated Ru atoms or small Ru aggregates in a Pt-rich two-dimensional PtxRu1–x surface alloy provide active sites allowing nonactivated H2 dissociative adsorption. These two possible defects in a full pseudomorphic Pt monolayer deposited on Ru...

Influence of methyl functional groups on the stability of cubane carbon cage

We present a quantum-chemical study to elucidate the structure, energetics and stability of isolated polymethylcubane molecules C8H8–q(CH3)q. The results obtained by means of originally developed nonorthogonal tight-binding approach are in good agreement with the existed experimental data for solid octamethylcubane C8(CH3)8. The isomerization mechanisms for polymethylcubane family are studied in detail and the minimum energy barriers' heights preventing the decomposition are calculated. The temperature dependence of octamethylcubane molecule lifetime to the decomposition moment was determined by direct molecular dynamics simulation. It is shown that methyl groups destabilize the cubic carbon cage, but less than nitro groups.

Effects of Aluminum on Hydrogen Solubility and Diffusion in Deformed Fe-Mn Alloys

We discuss hydrogen diffusion and solubility in aluminum alloyed Fe-Mn alloys. The systems of interest are subjected to tetragonal and isotropic deformations. Based onab initiomodelling, we calculate solution energies and then employ Oriani’s theory which reflects the influence of Al alloying via trap site diffusion. This local equilibrium model is complemented by qualitative considerations of Einstein diffusion. Therefore, we apply the climbing image nudged elastic band method to compute the minimum energy paths and energy barriers for hydrogen diffusion. Both for diffusivity and solubility of hydrogen, we find that the influence of the substitutional Al atom has both local chemical and nonlocal volumetric contributions.

Combined theoretical and experimental approaches for development of squaraine dyes with small energy barrier for electron injection

A series of far-red sensitizing squaraine dyes has been systematically designed and synthesized in order to correlate the theoretically calculated values with their corresponding experimental parameters. Efforts have been directed towards determining the minimum thermodynamic energy barrier for the electron injection in the nanoporous TiO2 by logical molecular design. Theoretical calculations using Gaussian program package were performed for ground and excited states in both of the isolated gaseous state as well as in solution including the solvent effect using a self-consistent reaction field polarizable continuum model (PCM). Implementation of the PCM model or use of LSDA functional under TD-DFT calculations gives much better results for energetics as well as absorption maximum for all of the sensitizers used in this work. Newly designed symmetrical squaraine dye SQ-5 exhibits a minimum energy barrier of 0.16eV for electron injection and shows photon harvesting behavior in far-red region with external photoconversion efficiency of 2.02% under simulated solar irradiation.

First Principles Study on the Electronic Structure and Interface Stability of Hybrid Silicene/Fluorosilicene Nanoribbons.

The interface stability of hybrid silicene/fluorosilicene nanoribbons (SFNRs) has been investigated by using density functional theory calculations, where fluorosilicene is the fully fluorinated silicene. It is found that the diffusion of F atoms at the zigzag and armchair interfaces of SFNRs is endothermic, and the corresponding minimum energy barriers are respectively 1.66 and 1.56 eV, which are remarkably higher than the minimum diffusion energy barrier of one F atom and two F atoms on pristine silicene 1.00 and 1.29 eV, respectively. Therefore, the thermal stability of SFNRs can be significantly enhanced by increasing the F diffusion barriers through silicene/fluorosilicene interface engineering. In addition, the electronic and magnetic properties of SFNRs are also investigated. It is found that the armchair SFNRs are nonmagnetic semiconductors, and the band gap of armchair SFNRs presents oscillatory behavior when the width of silicene part changing. For the zigzag SFNRs, the antiferromagnetic semiconducting state is the most stable one. This work provides fundamental insights for the applications of SFNRs in electronic devices.

Boron Diffusion in Silicon in the Presence of Grain Boundaries and Voids

We have investigated the behavior of one B impurity in the presence of dislocations, nanovoids and grain boundaries (GBs) in Si. Our study has been motivated by recent experiments on highly B-doped polycrystalline Si where a very significant thermoelectric power factor enhancement was observed. We study the influence of nanovoids on B diffusivity and activation in Si. We compare with the grain-boundary of pure tilt Si[001](120) poly-Si, where the presence of dislocations is dominant, similarly as in the case of nanovoids in c-Si. We performed DFT calculations to investigate minimum energy configurations, energy barriers and activation or deactivation of one B impurity in the proximity of the above structures. We found that, B preferably occupies substitutional lattice sites at the edge of the cavities. Thus the presence of small voids reduces its diffusivity and mobility, but the electrical activation of B is not significantly influenced. In the case of Si[001](120) GB, B is found to form Boron interstitials clusters in the close vicinity of a GB. The formation energy of those clusters is lower when the cluster is closer to center of the GB. Our results are in agreement with previously reported experimental data. It is provided insight to the effect of structural inhomogeneity on the dopant properties in Si structures that are currently of interest for thermoelectric applications.

Density functional theory study on the electronic properties and stability of silicene/silicane nanoribbons

The minimum energy barriers for the diffusion of hydrogen atoms at the zigzag and armchair interfaces of SSNRs are respectively 1.54 and 1.47 eV.

An Analytical Model for Adsorption and Diffusion of Atoms/Ions on Graphene Surface

Theoretical investigations are made on adsorption and diffusion of atoms/ions on graphene surface based on an analytical continuous model. An atom/ion interacts with every carbon atom of graphene through a pairwise potential which can be approximated by the Lennard‐Jones (L‐J) potential. Using the Fourier expansion of the interaction potential, the total interaction energy between the adsorption atom/ion and a monolayer graphene is derived. The energy‐distance relationships in the normal and lateral directions for varied atoms/ions, including gold atom (Au), platinum atom (Pt), manganese ion (Mn2+), sodium ion (Na1+), and lithium‐ion (Li1+), on monolayer graphene surface are analyzed. The equilibrium position and binding energy of the atoms/ions at three particular adsorption sites (hollow, bridge, and top) are calculated, and the adsorption stability is discussed. The results show that H‐site is the most stable adsorption site, which is in agreement with the results of other literatures. What is more, the periodic interaction energy and interaction forces of lithium‐ion diffusing along specific paths on graphene surface are also obtained and analyzed. The minimum energy barrier for diffusion is calculated. The possible applications of present study include drug delivery system (DDS), atomic scale friction, rechargeable lithium‐ion graphene battery, and energy storage in carbon materials.

Peierls-Nabarro barrier and protein loop propagation.

When a self-localized quasiparticle excitation propagates along a discrete one-dimensional lattice, it becomes subject to a dissipation that converts the kinetic energy into lattice vibrations. Eventually the kinetic energy no longer enables the excitation to cross over the minimum energy barrier between neighboring sites, and the excitation becomes localized within a lattice cell. In the case of a protein, the lattice structure consists of the C(α) backbone. The self-localized quasiparticle excitation is the elemental building block of loops. It can be modeled by a kink that solves a variant of the discrete nonlinear Schrödinger equation. We study the propagation of such a kink in the case of the protein G related albumin-binding domain, using the united residue coarse-grained molecular-dynamics force field. We estimate the height of the energy barriers that the kink needs to cross over in order to propagate along the backbone lattice. We analyze how these barriers give rise to both stresses and reliefs, which control the kink movement. For this, we deform a natively folded protein structure by parallel translating the kink along the backbone away from its native position. We release the transposed kink, and we follow how it propagates along the backbone toward the native location. We observe that the dissipative forces that are exerted on the kink by the various energy barriers have a pivotal role in determining how a protein folds toward its native state.

Theoretical investigations on Zundel cation present inside boron-nitride nanotubes: Effect of confinement and hydrogen bonding

The properties of protonated water systems in nano-confinement differ significantly from its bulk counterparts mainly due to the geometric constraint and nature of water–medium interactions. Herein, we have investigated the structure, vibrational spectra and proton transfer energetics of Zundel cation (ZC) under the confinement of boron–nitride nanotubes (BNNTs) by varying the degree of confinement in a systematic manner. Our results based on dispersion-corrected density functional theory (DFT) based methods reveal that the structure of ZC is significantly modified under the confinement and the nature of interaction largely depends on the diameter of BNNT. The ZC effectively forms hydrogen bonds through Hwater⋯NBNNT with BNNT. Among the BNNTs considered in the present study, the maximum interaction energy of ZC and minimum energy barrier for proton transfer are observed for the case of BNNT(5,5). This implies that this particular nanotube can facilitate the proton to easily hop between water molecules of a confined protonated water system.

Ag and Au atoms intercalated in bilayer heterostructures of transition metal dichalcogenides and graphene

The diffusive motion of metal nanoparticles Au and Ag on monolayer and between bilayer heterostructures of transition metal dichalcogenides and graphene are investigated in the framework of density functional theory. We found that the minimum energy barriers for diffusion and the possibility of cluster formation depend strongly on both the type of nanoparticle and the type of monolayers and bilayers. Moreover, the tendency to form clusters of Ag and Au can be tuned by creating various bilayers. Tunability of the diffusion characteristics of adatoms in van der Waals heterostructures holds promise for controllable growth of nanostructures.

Numerical optimization of writer geometries for bit patterned magnetic recording

A fully-automated pole-tip shape optimization tool, involving write head geometry construction, meshing, micromagnetic simulation and evaluation, is presented. Optimizations have been performed for three different writing schemes (centered, staggered and shingled) for an underlying bit patterned media with an areal density of 2.12 Tdots/in$^2$ . Optimizations were performed for a single-phase media with 10 nm thickness and a mag spacing of 8 nm. From the computed write field and its gradient and the minimum energy barrier during writing for islands on the adjacent track, the overall write error rate is computed. The overall write errors are 0.7, 0.08, and 2.8 x 10$^{-5}$ for centered writing, staggered writing, and shingled writing.

Simulation of Cu Precipitation in the Fe-Cu Binary System

Cu precipitation in steel has been investigated numerous times. Still, a consistent simulation of the nucleation, growth and coarsening kinetics of Cu precipitates is lacking. Major reason for this is the fact that Cu precipitation involves complex physical interactions and mechanisms, which go beyond the classical precipitation models based on evaporation and absorption of precipitate-forming monomers (atoms). In the present work, we attempt a comprehensive modeling approach, incorporating coalescence results from Monte Carlo simulation, prediction of the nucleus composition based on the minimum energy barrier concept, diffusion enhancement from quenched-in vacancies, dislocation pipe diffusion, as well as the transformation sequence of Cu-precipitates from bcc-9R-fcc. Our simulations of number density, radius and phase fraction coincide well with experimental values. The results are consistent over a large temperature range, which is demonstrated in a TTP-plot.

Tight-binding Molecular Dynamics Simulation of the Endohedral Complex C4H4@C60

Tight-binding molecular dynamics simulations were carried out to analyze the thermal stability of the tetrahedrane molecule inside the [60]fullerene cage over the temperature range T = 400 − 800 K. The temperature dependence of the endohedral complex lifetime to the moment of tetrahedrane decay was obtained from the numerical experiment. The height of the minimum energy barrier preventing the decomposition of the encapsulated tetrahedrane molecule was found to be similar for the isolated one. Possible channels and final products of its decomposition were also studied.

DFT Simulation of the Extra Me Adatom Diffusion on the Ge(111) √3×√3 1/3 ML Me Induced Surfaces

A first-principle simulation of the surface diffusion of an extra metal (Me) adatom on the corresponding 1/3 monolayer (ML) Ge (111)√3×√3 Me induced surfaces has been performed. Using the Nudged Elastic Band (NEB) optimization method, the minimum energy paths and activation energy barrier profiles for all known Me inducing √3×√3 reconstruction on a Ge(111) surface have been obtained. The value of the activation barrier is shown to depend on the adatom formation energies and the atomic radius of the diffusing metal: 0.33 eV for Pb and 0.25 eV for Sn. The Arrhenius pre-exponential factors that were obtained in the harmonic approximation are as large as 1011-12Hz for all of the investigated surfaces, which supports the single-atomic diffusion model considered here.

A first principle study for the adsorption and absorption of carbon atom and the CO dissociation on Ir(100) surface

We employ density functional theory to examine the adsorption and absorption of carbon atom as well as the dissociation of carbon monoxide on Ir(100) surface. We find that carbon atoms bind strongly with Ir(100) surface and prefer the high coordination hollow site for all coverages. In the case of 0.75 ML coverage of carbon, we obtain a bridging metal structure due to the balance between Ir-C and Ir-Ir interactions. In the subsurface region, the carbon atom prefers the octahedral site of Ir(100) surface. We find large diffusion barrier for carbon atom into Ir(100) surface (2.70 eV) due to the strong bonding between carbon atom and Ir(100) surface, whereas we find a very small segregation barrier (0.22 eV) from subsurface to the surface. The minimum energy path and energy barrier for the dissociation of CO on Ir(100) surface are obtained by using climbing image nudge elastic band. The energy barrier of CO dissociation on Ir(100) surface is found to be 3.01 eV, which is appreciably larger than the association energy (1.61 eV) of this molecule.

Enhanced reduction of graphene oxide by means of charging and electric fields applied to hydroxyl groups

We present a first-principles study of the effects of charging and perpendicular electric fields on hydroxyl groups, both of which mediate the reduction of graphene oxide through the formation of H2O and H2O2. Starting with an investigation of the interaction between the hydroxyl groups and graphene, we determine the equilibrium binding geometry, binding energy, and the diffusion path with a minimum energy barrier and show that those equilibrium properties are strongly affected by external agents. While co-adsorbed H and O form bound OH, co-adsorbed H and OH in close proximity form H2O with almost no energy barrier. When negatively charged or subjected to a perpendicular electric field, the energy barrier between two OH co-adsorbed in close proximity is weakened or totally suppressed, forming an oxygen atom strongly bound at the bridge site, together with a water molecule. The water molecule by itself is very weakly bound to graphene and is prone to desorb from the surface, leading to the reduction of graphene oxide. It is therefore demonstrated that the reduction of graphene oxide is promoted to a large extent by negative charging or an applied perpendicular electric field, through the formation of weakly bound water molecules from hydroxyl groups.