Ashwin Ramasubramaniam

Two-dimensional material nanophotonics

The optical properties of graphene and emerging two-dimensional materials including transition metal dichalcogenides are reviewed with an emphasis on nanophotonic applications. Two-dimensional materials exhibit diverse electronic properties, ranging from insulating hexagonal boron nitride and semiconducting transition metal dichalcogenides such as molybdenum disulphide, to semimetallic graphene. In this Review, we first discuss the optical properties and applications of various two-dimensional materials, and then cover two different approaches for enhancing their interactions with light: through their integration with external photonic structures, and through intrinsic polaritonic resonances. Finally, we present a narrow-bandgap layered material — black phosphorus — that serendipitously bridges the energy gap between the zero-bandgap graphene and the relatively large-bandgap transition metal dichalcogenides. The plethora of two-dimensional materials and their heterostructures, together with the array of available approaches for enhancing the light–matter interaction, offers the promise of scientific discoveries and nanophotonics technologies across a wide range of the electromagnetic spectrum.

Hydrogen Bond Networks in Graphene Oxide Composite Paper: Structure and Mechanical Properties

A multilayered composite structure formed by a random stacking of graphene oxide (GO) platelets is an attractive candidate for novel applications in nanoelectromechanical systems and paper-like composites. We employ molecular dynamics simulations with reactive force fields to elucidate the structural and mechanical properties of GO paper-like materials. We find that the large-scale properties of these composites are controlled by hydrogen bond networks that involve functional groups on individual GO platelets and water molecules within the interlayer cavities. Water content controls both the extent and collective strength of these interlayer hydrogen bond networks, thereby affecting the interlayer spacing and elastic moduli of the composite. Additionally, the chemical composition of the individual GO platelets also plays a critical role in establishing the mechanical properties of the composite--a higher density of functional groups leads to increased hydrogen bonding and a corresponding increase in stiffness. Our studies suggest the possibility of tuning the properties of GO composites by altering the density of functional groups on individual platelets, the water content, and possibly the functional groups participating in hydrogen bonding with interlayer water molecules.

Tunable band gaps in bilayer graphene-BN heterostructures.

We investigate band gap tuning of bilayer graphene between hexagonal boron nitride sheets, by external electric fields. Using density functional theory, we show that the gap is continuously tunable from 0 to 0.2 eV and is robust to stacking disorder. Moreover, boron nitride sheets do not alter the fundamental response from that of free-standing bilayer graphene, apart from additional screening. The calculations suggest that graphene-boron nitride heterostructures could provide a viable route to graphene-based electronic devices.

Edge elastic properties of defect-free single-layer graphene sheets

An energetic model is proposed to describe the edge elastic properties of defect-free single-layer graphene sheets. Simulations with the adaptive intermolecular reactive empirical bond order potential are used to extract the edge stress and edge moduli for different edges structures, namely, zigzag and armchair edges, zigzag and armchair edges terminated with hydrogen, and reconstructed zigzag and armchair edges. It is found that the properties of graphene are sensitively dependent on the edge structures; armchair and zigzag edges with and without hydrogen termination are in compression, while reconstructed edges are in tension.

Three-dimensional simulations of self-assembly of hut-shaped Si–Ge quantum dots

This article presents the results of three-dimensional modeling of heteroepitaxial thin film growth with the objective of understanding recent experiments on the early stages of quantum dot formation in SiGe/Si systems. We use a continuum model, based on the underlying physics of crystallographic surface steps, to study the growth of quantum dots, their spatial ordering and coarsening behavior. Using appropriate parameters, obtained from atomistic calculations, the (100) orientation is found to be unstable under compressive strains. The surface energy now develops a minimum at an orientation that may be interpreted as the (105) facet observed in SiGe/Si systems. This form of the surface energy allows for the growth of quantum dots without any barrier to nucleation—dots are seen to start off via a surface instability as shallow stepped mounds, which steepen continuously to reach their low energy orientations. During the very initial stages of growth, mounds are seen to grow in a dense array with several of...

The Intrinsic Mechanical Properties of Rubrene Single Crystals

IO N The study of the electrical properties of rubrene single crystals ( Figure 1 ) has provided a better understanding of the charge transport mechanisms in organic semiconductors while opening the doors to a new generation of high-performance electronic devices. [ 1–4 ] Despite the high charge carrier mobilities of rubrene crystals, their fragile nature and associated handling diffi culty have limited their use to fundamental charge transport studies, where throughput and bendability are not a requirement. Recent advances in the patterning techniques of organic single crystals over large-areas, [ 5 , 6 ] bring new promise for the utilization of these materials beyond charge transport studies, especially in the fi eld of fl exible electronics. [ 7 ] However, in order to effectively take advantage of the electrical properties of rubrene single-crystals in fl exible devices, their fundamental mechanical properties need to be understood and characterized. Here we present an experimental and computational investigation of the mechanical properties of rubrene single-crystals. The in-plane elastic constants are obtained by inducing the wrinkling instability in crystals laminated on elastomeric substrates. Our results demonstrate a dependence of wrinkling wavelength on crystallographic direction resembling the well-known anisotropic charge-transport properties of rubrene. The observed elastic anisotropy can be correlated to the crystal structure of rubrene (Figure 1 b) and suggests a non-linear coupling between mechanical and electrical properties. This fi nding expands the knowledge of structure-property correlations in organic semiconductors. Furthermore, insight into the mechanical properties of organic single crystals will be useful in defi ning limits on performance, processing, and manufacturing of devices, as well as elucidating their failure under mechanical and thermal loadings. [ 8 ]

Mechanical properties of irradiated single-layer graphene

The mechanical properties of irradiated single-layer graphene sheets are determined as a function of inserted vacancy concentration. We find that the vacancy-induced crystalline-to-amorphous transition is accompanied by a brittle-to-ductile transition in the failure response of irradiated graphene sheets for inserted vacancy concentrations of 8%–12%. While point defects and larger voids appreciably degrade the strength of pristine graphene, we find that even heavily damaged samples (∼20% vacancies) exhibit tensile strengths of ∼30 GPa, in significant excess of those typical of engineering materials. Our results suggest that defect engineering of graphene is feasible without incurring a complete loss of its desirable mechanical properties.

Effect of atomic scale plasticity on hydrogen diffusion in iron: Quantum mechanically informed and on-the-fly kinetic Monte Carlo simulations

We present an off-lattice, on-the-fly kinetic Monte Carlo (KMC) model for simulating stress-assisted diffusion and trapping of hydrogen by crystalline defects in iron. Given an embedded atom (EAM) potential as input, energy barriers for diffusion are ascertained on the fly from the local environments of H atoms. To reduce computational cost, on-the-fly calculations are supplemented with precomputed strain-dependent energy barriers in defect-free parts of the crystal. These precomputed barriers, obtained with high-accuracy density functional theory calculations, are used to ascertain the veracity of the EAM barriers and correct them when necessary. Examples of bulk diffusion in crystals containing a screw dipole and vacancies are presented. Effective diffusivities obtained from KMC simulations are found to be in good agreement with theory. Our model provides an avenue for simulating the interaction of hydrogen with cracks, dislocations, grain boundaries, and other lattice defects, over extended time scales, albeit at atomistic length scales.

A variational approach to nonlinear dynamics of nanoscale surface modulations

In this paper, we propose a variational formulation to study the singular evolution equations that govern the dynamics of surface modulations on crystals below the roughening temperature. The basic idea of the formulation is to expand the surface shape in terms of a complete set of basis functions and to use a variational principle equivalent to the continuum evolution equations to obtain coupled nonlinear ordinary differential equations for the expansion coefficients. Unlike several earlier approaches that rely on ad hoc regularization procedures to handle the singularities in the evolution equations, the only inputs required in the present approach are the orientation dependent surface energies and the diffusion constants. The method is applied to study the morphological equilibration of patterned unidirectional and bidirectional sinusoidal modulations on semiconductor surfaces through surface diffusion.Abstract In this paper, we propose a variational formulation to study the singular evolution equations that govern the dynamics of surface modulations on crystals below the roughening temperature. The basic idea of the formulation is to expand the surface shape in terms of a complete set of basis functions and to use a variational principle equivalent to the continuum evolution equations to obtain coupled nonlinear ordinary differential equations for the expansion coefficients. Unlike several earlier approaches that rely on ad hoc regularization procedures to handle the singularities in the evolution equations, the only inputs required in the present approach are the orientation dependent surface energies and the diffusion constants. The method is applied to study the morphological equilibration of patterned unidirectional and bidirectional sinusoidal modulations through surface diffusion. In the case of bidirectional modulations, particular attention is given to the analysis of the profile decay as a function of ratio of the modulating wavelengths in the coordinate directions. A key question that we resolve is whether the one-dimensional decay behavior is recovered as one of the modulating wavelengths of the two-dimensional profiles diverges, or whether one-dimensional decay has qualitatively distinct features that cannot be described as a limiting case of the two-dimensional behavior. In contrast to some earlier suggestions, our analytical and numerical studies clearly show that the former situation is true; we find that the one-dimensional profiles, like the highly elongated two-dimensional profiles, decay with formation of facets. While our results for the morphological equilibration of symmetric one-dimensional profiles are in agreement with the free-boundary formulation of Spohn, the present approach can also be used to study the evolution of asymmetric profile shapes where the free-boundary approach is difficult to apply. The variational method is also used to analyze the decay of unidirectional modulations in the presence of steps that arise in most experimental studies due to a small misorientation from the singular surface.

Orbital-free density functional theory simulations of dislocations in aluminum

The core structure of screw and edge dislocations in fcc Al was investigated using orbital-free density functional theory (OF-DFT). Detailed calibrations of kinetic energy density functionals (KEDFs) and local pseudopotentials were performed to reproduce accurately the energies of several phases of bulk Al, as well as the elastic moduli and stacking fault energies of fcc Al. Thereafter, dislocations were modeled with both periodic and non-periodic cells containing a few thousand atoms, and the widths of the dissociated cores were extracted. The results are in good agreement with previous estimates from experiment and theory, further validating OF-DFT with non-local KEDFs as a seamless and accurate tool for simulating large features in main group, nearly-free-electron-like metals at the mesoscale.