Search

In the present study we investigate the irradiation-defects hybridized graphene scaffold as one potential building material for the anode of Li-ion batteries. Designating the Wigner V22 defect as a representative, we illustrate the interplay of Li atoms with the irradiation-defects in graphene scaffolds. We examine the adsorption energetics and diffusion kinetics of Li in the vicinity of a Wigner V22 defect using density functional theory calculations. The equilibrium Li adsorption sites at the defect are identified and shown to be energetically preferable to the adsorption sites on pristine (bilayer) graphene. Meanwhile the minimum energy paths and corresponding energy barriers for Li migration at the defect are determined and computed. We find that while the defect is shown to exhibit certain trapping effects on Li motions on the graphene surface, it appears to facilitate the interlayer Li diffusion and enhance the charge capacity within its vicinity because of the reduced interlayer spacing and characteristic symmetry associated with the defect. Our results provide critical assessment for the application of irradiated graphene scaffolds in Li-ion batteries., 23 pages, 5 figures

The thermal transport properties of hybrid graphene/h-BN heterostructures are investigated using atomistic simulations. While the thermal conductivity is observed to be significantly limited perpendicular to the graphene/h-BN interface, it is tunable via a composition parallel to the interface. In particular we show that the thermal transport parallel to the interface can be understood by viewing the hybrid system as a series of individual embedded graphene nanoribbons (GNRs) constrained by neighboring h-BN. A mechanistic model is proposed to relate the thermal conductivities of the embedded and free-standing GNRs through a linear function of the composition. The model predictions are demonstrated to be in good agreement with the simulation results.

Atomic-scale imaging and first-principles modeling are applied to the heterophase interface between the Al-Cu solid solution (${\ensuremath{\alpha}}_{\mathrm{Cu}}$) and ${\ensuremath{\theta}}^{\ensuremath{'}}$ (${\mathrm{Al}}_{2}\mathrm{Cu}$) phases. Contrary to recent studies, our observations reveal a diffuse interface of complex but well-defined structure that enables the progression from ${\ensuremath{\alpha}}_{\mathrm{Cu}}$ to ${\ensuremath{\theta}}^{\ensuremath{'}}$ over a distance of $\ensuremath{\approx}1\text{ }\text{ }\mathrm{nm}$. We demonstrate that, surprisingly, the observed interfacial structure is not preferred on energetic grounds. Rather, the excess in interfacial energy is compensated by efficient atomic-scale kinetics of the ${\ensuremath{\alpha}}_{\mathrm{Cu}}\ensuremath{\rightarrow}{\ensuremath{\theta}}^{\ensuremath{'}}$ phase transformation.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) can withstand a large deformation without fracture or inelastic relaxation, making them attractive for application in novel strain-engineered and flexible electronic and optoelectronic devices. In this study, we characterize the mechanical response of monolayer group VI TMDs to large elastic deformation using first-principles density functional theory calculations. We find that the ultimate strength and the overall stress response of these 2D materials is strongly influenced by their chemical composition and loading direction. We demonstrate that differences in the observed mechanical behavior can be attributed to the spatial redistribution of the occupied hybridized electronic states in the region between the transition metal atom and the chalcogens. In spite of the strong covalent bonding between the transition metal and the chalcogens, we find that a simple linear relationship can be established to describe the dependence of the mechanical strength on the charge transfer from the transition metal atom to the chalcogens.

The edge stresses, which are responsible for all physical properties of nanoribbons, always involve two sided non-equivalent edge effects for zigzag honeycomb nanoribbons of two dimensional binary compounds such as BN and MoS 2 . In present work, we develop an approach to determine the edge stresses of individual non-equivalent zigzag edges. For prototypical system of BN zigzag nanoribbon, the edge stresses of N-terminated and B-terminated zigzag edge are determined. Using this approach, even the edge stresses for particular passivated edges observed in MoS 2 nanoribbons are able to be obtained as well. The key results presented here are published in Ref. [1].

Separation of molecules based on molecular size in zeolites with appropriate pore aperture dimensions has given rise to the definition of "molecular sieves" and has been the basis for a variety of separation applications. We show here that for a class of chabazite zeolites, what appears to be "molecular sieving" based on dimension is actually separation based on a difference in ability of a guest molecule to induce temporary and reversible cation deviation from the center of pore apertures, allowing for exclusive admission of certain molecules. This new mechanism of discrimination permits "size-inverse" separation: we illustrate the case of admission of a larger molecule (CO) in preference to a smaller molecule (N(2)). Through a combination of experimental and computational approaches, we have uncovered the underlying mechanism and show that it is similar to a "molecular trapdoor". Our materials show the highest selectivity of CO(2) over CH(4) reported to date with important application to natural gas purification.

We demonstrate that the energy bandgap of layered, high-dielectric α-MoO(3) can be reduced to values viable for the fabrication of 2D electronic devices. This is achieved through embedding Coulomb charges within the high dielectric media, advantageously limiting charge scattering. As a result, devices with α-MoO(3) of ∼11 nm thickness and carrier mobilities larger than 1100 cm(2) V(-1) s(-1) are obtained.

The fundamental mechanism proposed to explain surface-enhanced Raman scattering (SERS) relies on electromagnetic field enhancement at optical frequencies. In this work, we demonstrate the use of microfabricated, silver nanotextured electrode pairs to study, in situ, the influence of low frequency (5 mHz to 1 kHz) oscillating electric fields on the SERS spectra of thiophenol. This applied electric field is shown to affect SERS peak intensities and influence specific vibrational modes of the analyte. The applied electric field perturbs the polar analyte, thereby altering the scattering cross section. Peaks related to the sulfurous bond which binds the molecule to the silver nanotexture exhibit strong and distinguishable responses to the applied field, due to varying bending and stretching mechanics. Density functional theory simulations are used to qualitatively verify the experimental observations. Our experimental and simulation results demonstrate that the SERS spectral changes relate to electric field induced molecular reorientation, with dependence on applied field strength and frequency. This demonstration creates new opportunities for external dynamic tuning and multivariate control of SERS measurements.

Ion-induced surface patterns (sputter ripples) are observed to grow more rapidly than predicted by current models, suggesting that additional sources of roughening may be involved. Using a linear stability analysis, we consider the contribution of ion-induced stress in the near surface region to the formation rate of ripples. This leads to a simple model that combines the effects of stress-induced roughening with the curvature-dependent erosion model of Bradley and Harper. The enhanced growth rate observed on Cu surfaces appears to be consistent with the magnitude of stress measured from wafer curvature measurements.

Based on a combination of finite element and quadratic programming optimization methods, we have developed an efficient numerical technique to compute the equilibrium composition profiles in coherent and dislocated alloy quantum crystals. We demonstrate that the variations in composition profiles arise due to the competition between chemical mixing effects and the relaxation of composition-dependent mismatch strain as well as strain due to dislocations. This approach provides the means for a quantitative description of the factors controlling equilibrium composition profiles in various coherent and dislocated self-organized alloy systems.

Using in situ electron microscopy, we have measured the structure of $\mathrm{SiC}(0001)\mathrm{\text{\ensuremath{-}}}4H$ during annealing in vacuum. Above $1000\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, an additional SiC bilayer forms on the surface that changes the polytype from hexagonal ($4H$) to cubic ($3C$). The interaction with surface steps prevents the cubic layer from growing thicker: the new phase does not wet the steps of the underlying $4H$ substrate. Instead, the cubic layer expands laterally, accelerating step bunching in the surrounding hexagonal regions. During SiC homoepitaxy, this lack of step edge wetting leads to the growth of $3C$ twins separated by deep grooves.

Nanoscale variations in composition arising from the competition between chemical mixing effects and elastic relaxation can substantially influence the electronic and optical properties of self-assembled alloy quantum dots. Using a combination of finite element and quadratic programming optimization methods, we have developed an efficient technique to compute the equilibrium composition profiles in strained quantum dots. We find that the composition profiles depend strongly on the morphological features such as the slopes and curvatures of their surfaces and the presence of corners and edges as well as the ratio of the strain and chemical mixing energy densities. More generally, our approach provides a means to quantitatively model the interplay among the composition variations, the temperature, the strain, and the shapes of small-scale lattice-mismatched structures.

We show that 2D self-assembled domains can remain trapped in a large variety of long-lived and metastable shapes that arise from an interplay of crystalline anisotropy and relaxation of elastic strain. On commonly used cubic (111) substrates, these shapes include extended or stacked structures made up of triangular domains connected at their corners, compact shapes with both convex and concave curvatures and others with narrow and elongated arms. We show that all of these distinct experimentally observed shapes can be explained within a unified framework and present a phase diagram that systematically classifies the metastable shapes as a function of their size.

For developing lithium (Li)-ion batteries with large energy densities and high rate capabilities, it is crucial that electrode materials show good reactivity with charge carrying Li ions, and simultaneously allow for their fast transport. Using first principles density functional theory calculations, here we investigate the interaction of Li with the edges of monolayer as well as multilayer silicene and identify the low energy Li binding sites and the pathways for Li diffusion. Both Li binding and diffusion are found to be significantly controlled by the morphology of the silicene edges. We show that among known structural forms of silicon, monolayer silicene edges provide the strongest binding with Li. The energy barriers for Li diffusion on monolayer silicene nanoribbons are generally very low (0.14–0.26 eV), and in particular, zigzag edges can allow for up to 80 times faster diffusion than on a silicene monolayer. Our results indicate that monolayer silicene nanoribbons terminated with zigzag edges can provide promising candidate materials for the negative electrodes of Li-ion batteries.

Density functional theory calculations were carried out for Ni1−xMgxO alloys using both GGA+U method and hybrid exchange-correlation functional HSE06. We find that the band gap of Ni1−xMgxO is a nonlinear function of MgO concentration with a strong bowing behavior at high Mg content. Band edge alignment is determined using heterojunction superlattice models. The valence-band-maximum of Ni1−xMgxO is shown to be tunable within a range of 0.90 eV. By comparing with the highest-occupied-molecular-orbital levels of some of the most widely used dye molecules, we propose that Ni1−xMgxO is a promising alternate to replace NiO photocathode in dye-sensitized solar cells with an enhanced open-circuit voltage and transparency of cathode films.

The elastic properties of edges are among the most fundamental properties of finite two-dimensional (2D) crystals. Using a combination of the first-principles density functional theory calculations and a continuum elasticity model, we present an efficient technique to determine the edge stresses of non-stoichiometric orientations in multicomponent 2D crystals. Using BN and MoS2 as prototypical examples of 2D binary monolayers with threefold in-plane symmetry, we unambiguously compute unique edge stresses of commonly observed non-stoichiometric edges. Our results show that the edge stresses for these structurally distinct orientations can differ significantly from the average values that have been typically reported to date.

Strain and nanoscale variations in composition can significantly alter the electronic and optical properties of self-assembled alloy quantum systems. Using a combination of finite element and first-principles methods, we have developed an efficient and accurate technique to study the influence of strain and composition on the quantum confinement behavior in alloy quantum dots. Interestingly, we find that a nonuniform distribution of alloy components can lead to an enhanced confinement potential that allows a large quantum dot to behave electronically in a manner similar to a much smaller dot. The approach presented here provides a general means to quantitatively predict the influence of strain and composition variations on the performance characteristics of various small-scale alloy systems.

A nonlinear model using the phase-field approach is developed to study microstructural evolution during the growth of strained heteroepitaxial multilayers. The strain from the buried layers is observed to influence the nucleation of islands in subsequently deposited strained layers. The patterns obtained during the evolution of multilayers are determined by the interplay of strain relaxation and deposition flux leading to formation of coordinated, stacked islands in the low flux regime and planar growth in the high flux regime, in agreement with the experimental observations.

We analyze the evolution of equilibrium and growth shapes of anisotropically strained two-dimensional self-assembled structures using a dynamic growth model. As examples of such structures, we study the shapes of nanowires grown heteroepitaxially on cubic (001) surfaces and monolayer islands or stress domains grown homoepitaxially on Si(001) surface. In the former case, the anisotropy in the mismatch strain in the two principal directions is large, while in the latter case, the principal components of the strain are equal in magnitude and opposite in sign. In the case of nanowires, we find that the slow kinetics of growth limits the formation of wirelike shapes with constant widths as predicted by equilibrium models. In particular, the aspect ratios of nanowires during growth are considerably smaller than the equilibrium aspect ratios. For monolayer islands on Si(001), we show that the anisotropy in strain gives rise to a novel fourfold symmetry in their equilibrium shapes. This strain-induced symmetry, c...

We have used real-time low-energy electron microscopy to observe the growth and shape evolutions of self-assembled stress domains on Si(111) surfaces. We find that elastic strain leads to dramatic transformations in the shapes of large domains that are not predicted by existing theoretical models. By comparing the experimental observations on the formation of the stress domains with dynamic growth simulations, we have developed a quantitative understanding of how a self-assembling system falls out of equilibrium. Our work shows the nonequilibrium shapes that a domain adopts during growth depending very strongly on the azimuthal dependence of its boundary energy.