Adatom Clusters Research Articles

Enhanced H2 storage in silicene through lithium decoration and single vacancy and Stone‐Wales defects: A density functional theory investigation

AbstractDefect engineering and metal decoration onto 2‐D materials have gained major attention as a means of creating viable hydrogen storage materials. This Density Functional Theory (DFT) based study presents lithium decorated single vacancy (SV) and Stone‐Wales (SW) defective silicene as a viable media for storing hydrogen via physisorption. Introducing defects increases the Li adatom's binding energy from −2.36 eV in pristine silicene to −3.44 and −2.73 eV in SV and SW silicene, respectively, preventing Li adatom clustering. The presence of defects and Li adatom further aid hydrogen adsorption onto the substrates with binding energies present between the US‐DOE set range of −0.2 to −0.7 eV/H2 with the highest binding energy measured to be −0.389 eV/H2. The enhanced H2 binding energies are a result of a combined contribution of the Li(p) and Li(s) orbitals with the H(s) orbital with an indirect electronic transfer from the silicene substrate to the Li adatom. Upon double side Li decoration, both the Li‐decorated defective systems were able to effectively store multiple H2 molecules up to 28 H2 with the highest gravimetric density being 5.97 wt%. Ab Initio molecular dynamic simulations conducted at 300 K and 310 K confirm the stability of the Li adatom as well as the adsorbed H2 molecules at room temperature and establish the viability of these systems as effective, high gravimetric density, physisorption‐based hydrogen storage media.

Kinetic Monte Carlo simulation study of the early stages of epitaxial SiC (0001) growth

A kinetic Monte Carlo (kMC) simulation with a cluster-multiple labeling technique was carried out to study the dynamic behavior of Si-C clusters during the early stage of epitaxial growth. In the model, a crystal lattice was established to fix the physical location of atoms and interatomic bonding. Events, such as adatoms adsorption, diffusion on the planar surface, and attachment to and detachment from clusters of adatoms were considered in the model. The Si and C atoms were treated individually to achieve more elaborate information for dynamic behavior of the clusters in the crystal surface. Moreover, a cluster multiple labeling technique, also known as the Hoshen - Kopelman algorithm was used to identify Si-C clusters. Then the properties of the clusters were calculated and analyzed by the algorithm. The simulation results showed that the number of Si-C clusters gradually decreases with increasing substrate temperature and decreasing deposition rate, and that the average size of Si-C clusters at high temperatures is much larger than that at low temperatures. Furthermore, the Si:C ratio on the growing surface has a significant effect on the surface morphology. At Si-rich conditions, the cluster density is relatively high, and the cluster shape is not homogeneously distributed. While with the increase of C mole fraction, the cluster density gradually decreases and the cluster shape becomes more uniform. Therefore, a higher concentration of C in deposition flux is beneficial to the early stage of nucleation. These findings are consistent with predictions for the case of the epitaxial growth of SiC.

Assessing factors that determine adatom migration and clustering on a thin film oxide; Pt1 and Rh1 on the “29” CuxO/Cu(1 1 1) surface

The dynamical nature of single-site catalysts under non-equilibrium conditions poses significant challenges in the full characterization of their active sites. The “29” oxide is a thin CuxO film grown on Cu(111) which provides a well-defined surface on which to study the structure and chemistry of atomically dispersed precious metals. A combination of experimental and first-principles approaches is used to investigate the factors that influence the mobility of metal adatoms and their clustering tendencies. Our work elucidates how a homogenous coordination environment of neutral single-atoms can be obtained when its tendency to break oxide bonds within its vicinity is low. We demonstrate that this in turn affects the chemical activity of atomically dispersed species on thin-film oxides, as defects on the oxide would in fact allow such species to be more accessible to CO. We thus highlight the importance of understanding the relationship between defects and atomically dispersed active sites upon designing single-site catalysts.

Coupling between bulk thermal defects and surface segregation dynamics

Surface segregation is a phenomenon that depends on the delicate interplay between thermodynamic driving forces and kinetic obstacles, for which elevated temperature is often needed to enhance the atom mobility and reach equilibrium. Using the classic system of ${\mathrm{Cu}}_{3}\mathrm{Au}(100)$ under nonisothermal conditions, herein, we show an adatom process underlying transient surface segregation dynamics through the temperature-change-driven creation and annihilation of thermal vacancies in the bulk and the resulting bulk-surface mass exchanges. This is demonstrated by monitoring the surface composition evolution of ${\mathrm{Cu}}_{3}\mathrm{Au}(100)$ with temperature changes between 250 and $500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, showing that the increase in temperature decreases monotonically the surface Au concentration as a result of the transfer of more Cu than Au from the bulk to the surface to form Cu-rich clusters of adatoms. Such a bulk thermal defect effect is expected to be universal in inducing the disparity in the bulk-surface mass exchanges of dissimilar atoms in multicomponent materials because of the inherent differences in the vacancy formation energies of the constituent atoms.

Role of the Structure and Reactivity of Cu and Ag Surfaces in the Formation of a 2D Metal–Hexahydroxytriphenylene Network

We have investigated the role played by the atomic structure and reactivity of the supporting Ag(111) and Cu(111) surfaces on the formation of 2D metal–organic networks (2D-MONs) involving metal adatom clusters spawned by these two surfaces. While the hexahydroxytriphenylene (HHTP) molecule forms complexes with Ag3 and Cu3 adatom clusters on, respectively, the Ag(111) and Cu(111) surfaces, an extended order is only observed in the 2D-MON on Ag(111). By combining scanning tunneling microscopy measurements, density functional theory calculations, and microscopy image simulations, we show that the formation of Ag–HHTP metal–organic complexes is structurally compatible with a periodic arrangement of HHTP on the Ag(111) surface. In contrast, on Cu(111), tightly bonded and localized Cu–HHTP motifs are stabilized by the interaction of Cu adclusters with HHTP. However, they cannot match the surface structure of Cu(111) to form an extended 2D-MON. We observed that the formation of large 2D-MON domains on a metallic surface is only possible when the periodicity of the adsorbed surface assembly is weakly perturbed by the addition of metal adclusters that reinforced the bonding.

Real-space observation of surface structuring induced by ultra-fast-laser illumination far below the melting threshold

Intense short laser pulses are an intriguing tool for tailoring surface properties via ultra-fast melting of the surface layer of an irradiated target. Despite extensive studies on the interaction of femto-second laser interaction with matter, the initial steps of the morphological changes are not yet fully understood. Here, we reveal that substantial surface structure changes occur at energy densities far below the melting threshold. By using low-temperature scanning tunneling microscopy we resolve atomic-scale changes, i.e. the creation of nanosized adatom and vacancy clusters. The two cluster types have distinct non-linear fluence-dependencies. A theoretical analysis reveals their creation and motion to be non-thermal in nature. The formation of these atomistic changes, individually resolved here for the first time, recast our understanding of how surfaces respond to low-intensity ultra-short laser illumination. A visualization and control of the initial morphological changes upon laser illumination are not only of fundamental interest, but pave the way for the designing material properties through surface structuring.

CO2 adsorption on graphene supported Ni catalysts

We perform density functional theory (DFT) study of CO 2 adsorption on graphene supported Ni catalyst. We implement ordinary PBE functional as well as van der Waals density functional (vdW-DF) to accommodate weakly interaction between CO 2 and graphene. Based on vdW-DF calculations, we obtained that CO 2 is physisorbed on pristine graphene, in contradiction with PBE results. The adsorption energy further increases when we introduce single Ni adatom and small Ni cluster on graphene. The adsorbed CO 2 bond angle and bond length on graphene with single Ni adatom and cluster are deformed from its gas phase condition, indicating that CO 2 is chemically adsorbed on the decorated graphene area. Our results provide useful insight into appropriate design of graphene supported metal catalysts.

Dynamic behavior of clusters in the early stage of SiC (0001) epitaxial growth: A Kinetic Monte Carlo study

A Kinetic Monte Carlo (KMC) model with a cluster-multiple labeling technique was proposed to study the effect of growth parameters such as temperature, deposition flux and Si/C ratio on Si–C clusters in the initial stage of SiC (0001) surface nucleation. In this model, Si and C atoms were treated individually and a crystal lattice was established to fix the physical location of atoms and interatomic bonding. The adsorption and diffusion of adatoms on the planar surface, attachment to and detachment from clusters of adatoms, and their diffusion along the edge of clusters were considered in the KMC model. Cluster-multiple labeling technique was used to identify Si–C clusters and the properties of clusters were analyzed. The results showed that larger and less clusters were constructed on growing surface with the increase of temperature and decrease of deposition rate. The saturated clusters density was exponentially related to the ratio of effective diffusion capacity to deposition rate, and the relationship between saturated density of clusters and average growth rate of clusters was obtained. Finally, Si/C ratio affected the characteristics of nucleation, and at C-rich condition, the binding energy of nucleus was larger than that at Si-rich condition, leading to the formation of fewer but larger clusters.

Stability and mobility of tungsten clusters on tungsten (110) surface: Ab initio and atomistic simulations

Quantifying the surface transport properties of tungsten (W) is of prime importance to understand the formation of nano-fuzz in fusion plasma-facing conditions. The stability and mobility of W adatom clusters (Wn, n = 2-9) on the W(110) surface has been investigated by computer simulations, including ab initio calculations using density functional theory (DFT) and molecular statics (MS) simulations with multiple W interatomic potentials. The DFT results demonstrate that the sequential binding energy generally increases with number of W adatoms, except for the 5th and 7th W adatoms. The most common elemental migration steps of Wn (n>2) clusters are observed to consist of monomer and dimer hops, while larger Wn clusters can also diffuse by dissociation and recombination of smaller clusters. The threshold migration energy of W9 is the highest, then followed by W8, W4, and W6, while W3, W5, and W7 have similar migration energies. Compared to DFT, each interatomic potential evaluated overestimates the binding energies of Wn clusters. The embedded-atom potential developed by Juslin and Wirth adequately predicts the threshold migration energy of Wn (n>2) clusters, although it predicts different underlying migration mechanisms. The results show that interaction mechanism between W adatoms controls the stability and mobility of Wn clusters on the W(110) surface.

Effect of conductivity on the electromigration-induced morphological evolution of islands with high symmetries of surface diffusional anisotropy

We report on the electromigration-induced morphological evolution of islands (vacancies, precipitates, and homoepitaxial adatom clusters) using a phase-field method with high symmetries of surface diffusional anisotropy. The analysis emphasizes on islands migrating in the {100} and {111} planes of the face-centered-cubic crystal, which resembles fourfold and sixfold symmetries, respectively. The numerical results intend to elaborate on the role of conductivity contrast between the island and the matrix and the misorientation of the fast diffusion direction with respect to the applied electric field on the morphological evolution. Based on numerical results, a morphological diagram is constructed in the plane of misorientation angle and conductivity contrast delineating a rich variety of morphologies, which includes steady-state, time-periodic, zigzag oscillations, and island breakup. While the shape of the island is primarily dictated by the conductivity contrast, the migration modes depend on the misorientation. The various migration modes are further distinguished based on the shape of the island such as a faceted wedge or seahorse morphology, an oscillatory characteristic such as standing wave or traveling wave time-periodic oscillations, and different breakup features. The steady-state kinetics obtained from the fourfold and sixfold symmetries are critically compared with the twofold symmetry, isotropic analytical, and numerical findings. Our result suggests that the steady-state velocity decreases with the symmetry fold of the island. Furthermore, the influence of variation in conductivity contrast and misorientation on kinetics in the time-periodic oscillations are discussed. Finally, the numerically obtained stable facets are compared with the analytically derived orientations. The observed results have direct repercussions in terms of the fabrication of nanopatterns and the performance of thin-film interconnects.

Parallel algorithms for hyperdynamics and local hyperdynamics.

Hyperdynamics (HD) is a method for accelerating the timescale of standard molecular dynamics (MD). It can be used for simulations of systems with an energy potential landscape that is a collection of basins, separated by barriers, where transitions between basins are infrequent. HD enables the system to escape from a basin more quickly while enabling a statistically accurate renormalization of the simulation time, thus effectively boosting the timescale of the simulation. In the work of Kim et al. [J. Chem. Phys. 139, 144110 (2013)], a local version of HD was formulated, which exploits the intrinsic locality characteristic typical of most systems to mitigate the poor scaling properties of standard HD as the system size is increased. Here, we discuss how both HD and local HD can be formulated to run efficiently in parallel. We have implemented these ideas in the LAMMPS MD code, which means HD can be used with any interatomic potential LAMMPS supports. Together, these parallel methods allow simulations of any size to achieve the time acceleration offered by HD (which can be orders of magnitude), at a cost of 2-4× that of standard MD. As examples, we performed two simulations of a million-atom system to model the diffusion and clustering of Pt adatoms on a large patch of the Pt(100) surface for 80 μs and 160 µs.

Identifying Multinuclear Organometallic Intermediates in On‐Surface [2+2] Cycloaddition Reactions

AbstractWe investigate the on‐surface [2+2] cycloaddition reaction of 2,3,6,7,10,11‐hexabromotriphenylene (HBTP) on Ag(111), Cu(111), Au(111), and Cu‐dosed Au(111) surfaces using STM and DFT simulation focusing on the organometallic intermediates. The fully debrominated HBTP molecules form an organo‐silver framework on Ag(111) and an organo‐copper framework on Cu(111), both incorporating multinuclear metal adatom clusters. The organo‐silver framework is converted into porous covalent networks via [2+2] cycloaddition above 240 °C. In contrast, the organo‐copper framework is very stable and does not undergo [2+2] cycloaddition even at 300 °C. On Au(111), no organo‐gold intermediate of [2+2] cycloaddition is observed. After loading Cu onto Au(111), the partially debrominated HBTP molecules bind to Cu adatom dimers to form multinuclear organo‐copper complexes at 100 °C which undergo [2+2] cycloaddition at 140 °C. This study shows that the choice of surface can direct the reaction pathway.

Identifying Multinuclear Organometallic Intermediates in On-Surface [2+2] Cycloaddition Reactions.

We investigate the on-surface [2+2] cycloaddition reaction of 2,3,6,7,10,11-hexabromotriphenylene (HBTP) on Ag(111), Cu(111), Au(111), and Cu-dosed Au(111) surfaces using STM and DFT simulation focusing on the organometallic intermediates. The fully debrominated HBTP molecules form an organo-silver framework on Ag(111) and an organo-copper framework on Cu(111), both incorporating multinuclear metal adatom clusters. The organo-silver framework is converted into porous covalent networks via [2+2] cycloaddition above 240 °C. In contrast, the organo-copper framework is very stable and does not undergo [2+2] cycloaddition even at 300 °C. On Au(111), no organo-gold intermediate of [2+2] cycloaddition is observed. After loading Cu onto Au(111), the partially debrominated HBTP molecules bind to Cu adatom dimers to form multinuclear organo-copper complexes at 100 °C which undergo [2+2] cycloaddition at 140 °C. This study shows that the choice of surface can direct the reaction pathway.

Molecular dynamics simulations of self-diffusion of adatoms on tungsten surfaces

Under an extreme fusion environment, the self-diffusion behaviors of W adatoms on W surfaces are thought to have a very important influence on the modification of surface morphology of W. Thus, in the present paper, the self-diffusion behaviors of W adatom clusters on the three typical W surfaces ((0 0 1), (0 1 1) and (1 1 1)) were investigated using the parallel replica dynamics method and the conventional molecular dynamics method. The diffusion barrier was obtained by the nudged elastic band method, and the corresponding pre-factor was also calculated. The results show that adatom clusters on the (0 1 1) surface are easier to diffuse than those on the (0 0 1) surface, and the diffusion barriers of single adatoms are 0.46 eV and 1.61 eV, separately. Furthermore, for the clusters of different sizes on the (0 0 1) surface, all diffusion barriers are strongly affected by relative positions between the jump adatom and its neighboring adatoms, but little by farther adatoms; all the scales of the pre-factors are fs−1. However, on the (0 1 1) surface, the energy barrier is mainly determined by the whole structure of an adatom cluster, the diffusion pathways are much more complex, and even trimer-shearing may occur, while all the scales of the pre-factors are ns−1. On the (1 1 1) surface, surface atoms can move and cause surface undulation, the adatoms and substrate atoms are indistinguishable, so it is meaningless to just obtain diffusions of the adatoms. This work can direct the investigation of the adatom diffusion on W surfaces by larger simulation methods, such as kinetic Monte Carlo.

Cluster adsorption and migration energetics on hcp Ti (0001) surfaces via atomistic simulations

We use the modified embedded atom method in the framework of molecular statics to investigate the energetics of Ti adatom and Tin (2 ≤ n ≤ 7) cluster adsorption and migration on hcp Ti(0001) surfaces. We have examined several parameters such as the lattice constant, elastic constants, the cohesive energy and the surface energy and we have found that the modified embedded atom method gives results in good agreement with experimental and other theoretical investigations. The Ti (0001) surface multilayer relaxation results show that the distance between the first and second layer is contracted by −5.0%, which is the usual phenomenon for low index metallic surfaces. In addition, the diffusion parameters such as adsorption energies and energy barriers for single-atom and bi-dimensional clusters were derived. These calculations show that the adsorption energy and the potential barrier increase when the cluster size increases indicating their mobility decreases. Which indicates that the mobility of the cluster decreases as the cluster grows.

Insight into the diffusion mechanism of Cu cluster over Cu(111) surface: Effect of syngas and H2S atmosphere on Cu diffusion

Understanding the diffusion mechanism of Cu cluster over Cu surface is one of the important issues to predict and improve the stability of Cu-based catalyst. The diffusion of Cu adatom, Cu2 and Cu3 cluster over Cu(111) surface under different atmospheres (vacuum, CO, H2 and H2S atmosphere) at different temperatures have been investigated using density functional theory calculations. The results suggest that CO accelerates Cu surface diffusion at different temperatures, while H2 atmosphere has little effect depending on the temperature. Further, under H2S atmosphere, the negative formation energy of Cu-S complexes indicate that Cu-S species are more facile to form on Cu(111) surface. The diffusivity of CuS complex increases significantly with the decreasing of temperature and the increasing of H2S concentration, which suggests that ppm H2S results in the fast sintering of Cu surface significantly. This is consistent with the experimental result.

First-principles hydrogen adsorption properties of Li-decorated ThMoB4-type graphene

The hydrogen storage (1–10H2) properties of single- and double-side lithium decorated ThMoB4-type graphene (Li/ThMoB4C) are systematically investigated by density functional first-principles calculations within Dmol3 package. After well-converged geometry optimizations, it is found that the binding energy of Li adatom is higher enough, and there is no adatom clustering. The average adsorption energies of 1–6 H2 deviate in 0.20–0.27 eV/H2 range, which is providing a convenient physical adsorption-desorption cycle. A detailed examination of the binding mechanism between the constituents of the H2 adsorbed Li-decorated systems is presented by density of states, Mulliken charge analysis, electron density and density difference calculations. For further decoration and adsorption with 12Li adatom and 72H2 molecules, the computation yields a high gravimetric density of 14.5 wt % with the acceptable adsorption energy. In this way, it is concluded that Li/ThMoB4 system can be considered as a promising hydrogen storage medium.

Diffusion of alkali metal adatoms decorated on graphene dominates its electrical response

The temperature-dependent electrical transport behavior of Li- and K-doped graphene reveals, through DFT calculations and experimental observations, the dominant role of adatom cluster formation.

Germanium growth on electron beam lithography patterned Si3N4/Si(001) substrate using molecular beam epitaxy

It is important to investigate the growth dynamics of Ge adatoms under different surface stress regimes of the patterned dielectric to control the selective growth of self-assembled Ge nanostructures on silicon. In the present work, we have studied the growth of Ge by molecular beam epitaxy on nanometer scale patterned Si3N4/Si(001) substrates generated using electron beam lithography. The pitch of the patterns has been varied to investigate its effect on the growth of Ge in comparison to un-patterned Si3N4. For the patterned Si3N4 film, Ge did not desorbed completely from the Si3N4 film and hence no site selective growth pattern is observed. Instead, depending upon the pitch, Ge growth has occurred in different growth modes around the openings in the Si3N4. For the un-patterned substrate, the morphology exhibits the occurrence of uniform 3D clustering of Ge adatoms on Si3N4 film. This variation in the growth modes of Ge is attributed to the variation of residual stress in the Si3N4 film for different pitch of holes, which has been confirmed theoretically through Comsol Multiphysics simulation. The variation in stress for different pitches resulted in modulation of surface energy of the Si3N4 film leading to the different growth modes of Ge.

Poly(4-styrenesulfonate)-induced sulfur vacancy self-healing strategy for monolayer MoS2 homojunction photodiode

We establish a powerful poly(4-styrenesulfonate) (PSS)-treated strategy for sulfur vacancy healing in monolayer MoS2 to precisely and steadily tune its electronic state. The self-healing mechanism, in which the sulfur vacancies are healed spontaneously by the sulfur adatom clusters on the MoS2 surface through a PSS-induced hydrogenation process, is proposed and demonstrated systematically. The electron concentration of the self-healed MoS2 dramatically decreased by 643 times, leading to a work function enhancement of ∼150 meV. This strategy is employed to fabricate a high performance lateral monolayer MoS2 homojunction which presents a perfect rectifying behaviour, excellent photoresponsivity of ∼308 mA W−1 and outstanding air-stability after two months. Unlike previous chemical doping, the lattice defect-induced local fields are eliminated during the process of the sulfur vacancy self-healing to largely improve the homojunction performance. Our findings demonstrate a promising and facile strategy in 2D material electronic state modulation for the development of next-generation electronics and optoelectronics.