arXiv subjects

Moneesh Upmanyu

Publications and source records attributed to Moneesh Upmanyu.

18 recordsLinked to original sources

arXiv2022

Emergent grain boundary phases in stressed polycrystalline thin films

The grain boundary (GB) microstructure influences and is influenced by the development of residual stresses during synthesis of polycrystalline thin films. Recent studies have shown that the frustration between the preferred growth direction and rotations of abutting crystals to local cusps in GB energies leads to internal stresses localized within nanoscopic surface layers around the valleys and ridges that form at emergent boundaries (eGBs). Using a combination of continuum frameworks, numerical analyses and all-atom simulations of bicrystal $\langle 111\rangle$ copper films, we show that eGBs tune their surface morphology and rotation extent in response to external strains. Compression favors rotation to and growth of low energy GB phases (complexions) at eGB valleys while tension favors the transitions at eGB ridges, a reflection of the stress-induced mass efflux/influx that changes the energetic balance between interfacial and deformation energies. Molecular dynamics simulations of strained and growing bicrystal films reveal that the eGB phase transition is coupled to island formation at the surface triple junctions, providing a direct link between eGB phases and surface step flow. The interplay between eGB structure, morphology and mechanics emerges as a crucial ingredient for predictive understanding of stress and morphological evolution during film growth, with broad implications for multifunctional response of polycrystalline surfaces in a diverse range of surface phenomena such as surface mediated deformation, interfacial embrittlement, thermal grooving, stress corrosion, surface catalysis and topological conduction.

cond-mat.mtrl-sci↗

arXiv2021

Microgroove convexity is critical for robust gaseous layers on hierarchically-structured superhydrophobic surfaces

Gaseous layers (plastrons) trapped on the surfaces of immersed hydrophobic surfaces are critical for their function. Fibrillar morphologies offer a natural pathway, yet they are limited to a narrow range of liquid-surface systems and are vulnerable to pressure fluctuations that irreversibly destroy the plastron. Inspired by the convexly grooved surfaces of water fern (Salvinia) leaves that support their fibrous outgrowths, we study the plastron formation on 3D-printed dual-scale surfaces with elliptical interconnected microgrooves. The groove curvature stabilizes a seed gas layer (SGL) that facilitates plastron formation and restoration for all immersed hydrophobic surfaces. Computations and theoretical calculations reveal that the SGL storage capacity that sets the plastron robustness follows from the liquid menisci adaption to the groove geometry and pressure, and it can be further tuned using separated grooves. Our study highlights groove convexity as a key morphological feature for the design of multi-scale immersed surfaces for robust superhydrophobicity.

cond-mat.soft↗

arXiv2020

Solute drag forces from equilibrium interface fluctuations

The design of polycrystalline alloys hinges on a predictive understanding of the interaction between the diffusing solutes and the motion of the constituent crystalline interfaces. Existing frameworks ignore the dynamic multiplicity of and transitions between the interfacial structures and phases. Here, we develop a computationally-accessible theoretical framework based on short-time equilibrium fluctuations to extract the drag force exerted by the segregating solute cloud. Using three distinct classes of computational techniques, we show that the random walk of a solute-loaded interface is necessarily non-classical at short time-scales as it occurs within a confining solute cloud. The much slower stochastic evolution of the cloud allows us to approximate the short-time behavior as an exponentially sub-diffusive Brownian motion in an external trapping potential with a stiffness set by the average drag force. At longer time-scales, the interfacial and bulk forces lead to a gradual recovery of classical random walk of the interface with a diffusivity set by the extrinsic mobility. The short-time response is accessible via {\it ab-initio} computations, offering a firm foundation for high throughput, rational design of alloys for controlling microstructural evolution in polycrystals, and in particular for nanocrystalline alloys-by-design.

cond-mat.mtrl-sci↗

arXiv2020

Stability and energetics of 2D surface crystals in liquid AuSi thin films and nanoscale droplets

Segregation at surfaces of metal-covalent binary liquids is often non-classical and in extreme cases such as AuSi, the surface crystallizes above the melting point. In this study, we employ atomic-scale computational frameworks to study the surface crystallization of AuSi films and droplets as a function of composition, temperature and size. For temperatures in the range $T_s^\ast=765-780$K above the melting point $(T_s^\ast\approx1.3\,T_m)$, both thin film and droplet surfaces undergo a first order transition, from a 2D Au$_2$Si crystalline phase to a laterally disordered yet stratified layer. The thin film surfaces exhibit an effective surface tension that increases with temperature and decreases with Si concentration. On the other hand, for droplets in the size range $10-30$ nm, the bulk Laplace pressure alters the surface segregation as it occurs with respect to a strained bulk. Above $T_s^\ast$ the size effect on the surface tension is small, while for $T<T_s^\ast$ the surface layer is strained and composed of 2D crystallites separated by extended grain boundary scars that lead to large fluctuations in its energetics. As a specific application, all-atom simulations of AuSi droplets on Si(111) substrate subject to Si surface flux show that the supersaturation dependent surface tension destabilizes the contact line via formation of a precursor wetting film on the solid-vapor interface, and has ramifications for size selection during VLS-based routes for nanowire growth. Our study sheds light on the interplay between stability and energetics of surfaces in these unique class of binary alloys and offers pathways for exploiting their surface structure for varied applications such as catalytic nanocrystal growth, dealloying, and polymer crystallization.

cond-mat.mtrl-sci↗

arXiv2017

Phase-field model of vapor-liquid-solid nanowire growth

We present a multi-phase-field model to describe quantitatively nanowire growth by the vapor-liquid-solid (VLS) process. The free-energy functional of this model depends on three non-conserved order parameters that distinguish the vapor, liquid, and solid phases. The evolution equations for those order parameters describe basic kinetic processes including the rapid (quasi-instantaneous) equilibration of the liquid catalyst to a droplet shape with constant mean curvature, the slow incorporation of growth atoms at the droplet surface, and crystallization within the droplet. The standard constraint that the sum of the phase fields equals unity and the conservation of the number of catalyst atoms, which relates the catalyst volume to the concentration of growth atoms inside the droplet, are handled via separate Lagrange multipliers. An analysis of the model is presented that rigorously maps the phase-field equations to a desired set of sharp-interface equations for the evolution of the phase boundaries under the constraint of force-balance at three-phase junctions given by the Young-Herring relation that includes torque term related to the anisotropy of the solid-liquid and solid-vapor interface excess free energies. Numerical examples of growth in two dimensions are presented. The simulations reproduce many of the salient features of nanowire growth observed experimentally, including growth normal to the substrate with tapering of the side walls, transitions between different growth orientations, and crawling growth along the substrate.

cond-mat.mtrl-sci↗

arXiv2015

Interface-mediated thermomechanical effects during high velocity impact between monocrystalline surfaces

High velocity impact between crystalline surfaces is important for a range of material phenomena, yet a fundamental understanding of the effect of surface structure, energetics and kinetics on the underlying thermo-mechanical response remains elusive. Here, we employ non-equilibrium molecular dynamics (NEMD) simulations to describe the nanoscale dynamics of the high velocity impact between commensurate and incommensurate monocrystalline (001) copper surfaces. For impact velocities in the range 100-1200 m/s, the kinetic energy dissipation involves nucleation and emission of dislocation loops from defective sites within the rapidly forming interface, well below the bulk single-crystal yield point. At higher velocities, adiabatic dissipation occurs via plasticity-induced heating as the interface structurally melts following the impact. The adhesive strength of the reformed interface is controlled by the formation and nucleation of dislocations and point defects as they modify the interfacial energy relative to the deformed bulk. As confirmation, the excess interface energy decreases monotonically with increasing impact velocity. The relative crystal orientation of the surfaces equally important; the grain boundaries formed following incommensurate impact exhibit higher impact resistance, with smaller defect densities and interfacial enthalpies, suggesting an enhanced ability of the grain boundaries to absorb the non-equilibrium damage and therefore facilitate particle bonding. Our study highlights the key role played by the atomic-scale surface structure in determining the impact resistance and adhesion of crystalline surfaces.

cond-mat.mtrl-sci↗

arXiv2015

Chemical Vapor Deposition Synthesized Atomically Thin Molybdenum Disulfide with Optoelectronic-Grade Crystalline Quality

The ability to synthesize high-quality samples over large areas and at low cost is one of the biggest challenges during the developmental stage of any novel material. While chemical vapor deposition (CVD) methods provide a promising low-cost route for CMOS compatible, large-scale growth of materials, it often falls short of the high-quality demands in nanoelectronics and optoelectronics. We present large-scale CVD synthesis of single- and few- layered MoS2 using direct vapor-phase sulfurization of MoO2, which enables us to obtain extremely high-quality single-crystal monolayer MoS2 samples with field-effect mobility exceeding 30 cm2/Vs in monolayers. These samples can be readily synthesized on a variety of substrates, and demonstrate a high-degree of optoelectronic uniformity in Raman and photoluminescence mapping over entire crystals with areas exceeding hundreds of square micrometers. Owing to their high crystalline quality, Raman spectroscopy on these samples reveal a range of multi-phonon processes through peaks with equal or better clarity compared to past reports on mechanically exfoliated samples. This enables us to investigate the layer thickness- and substrate-dependence of the extremely weak phonon processes at 285 cm-1 and 487 cm-1 in 2D MoS2. The ultra-high, optoelectronic-grade crystalline quality of these samples could be further established through photocurrent spectroscopy, which clearly reveal excitonic states at room temperature, a feat that has been previously demonstrated only on device fabricated with mechanically exfoliated that were artificially suspended across trenches. Our method reflects a big step in the development of atomically thin, 2D MoS2 for scalable, high-quality optoelectronics.

cond-mat.mtrl-sci↗

arXiv2014

Carbon nanotube plectonemes: Loops of twisted helices

The relaxation of twist in elastic filaments often drives conformational changes. We explore this paradigm using all-atom computations and report the formation of novel supercoiled shapes in individual carbon nanotubes (CNTs). Decreasing the end distance of torsionally constrained CNTs leads to spontaneous nucleation and growth of a nanotube plectoneme. We develop a stability diagram and comparisons with theoretical frameworks reveal the importance of non-local van Der Waals interactions. In some cases, they stabilize the supercoiling to an extent that its tip locally kinks and then irreversibly reconstructs into a disordered yet strengthened structure that involves $sp^3$ bonding. The ability to engineer supercoiled conformations of CNTs and related nanoscale filaments opens the possibility of a unique set of tunable functional properties at the nanoscale.

cond-mat.mes-hall↗

arXiv2014

Shaping van der Waals nanoribbons via torsional constraints: Scrolls, folds and supercoils

Interplay between structure and function in atomically thin crystalline nanoribbons is sensitive to their conformations yet the ability to prescribe them is a formidable challenge. Here, we report a novel paradigm for controlled nucleation and growth of scrolled and folded shapes in finite-length nanoribbons. All-atom computations on graphene nanoribbons (GNRs) and experiments on macroscale magnetic thin films reveal that decreasing the end distance of torsionally constrained ribbons below their contour length leads to formation of these shapes. The energy partitioning between twisted and bent shapes is modified in favor of these densely packed soft conformations due to the non-local van Der Waals interactions in these 2D crystals; they subvert the formation of supercoils that are seen in their natural counterparts such as DNA and filamentous proteins. The conformational phase diagram is in excellent agreement with theoretical predictions. The facile route can be readily extended for tailoring the soft conformations of crystalline nanoscale ribbons, and more general self-interacting filaments

cond-mat.mtrl-sci↗

arXiv2013

Effect of solute segregation on shear-induced grain boundary motion

Atomic-scale simulations are performed to study the effect of solute segregation on the shear-induced motion of select grain boundaries in the classical $α$-Fe/C system. At shear rates larger than the solute diffusion rate, we observe a transition from coupled motion to sliding. Below a critical solute excess, the boundaries break away from the solute cloud and move in a coupled motion. At smaller shear rates, we observe extrinsic coupled motion at small stresses indicating that the coupling is aided by convective solute diffusion along the boundary. Our studies underscore the role of solutes in modifying the bicrystallography, temperature and rate dependence of shear accommodation at grain boundaries.

cond-mat.mtrl-sci↗

arXiv2013

Vapor-liquid-solid growth of serrated GaN nanowires: Shape selection driven by kinetic frustration

Compound semiconducting nanowires are promising building blocks for several nanoelectronic devices yet the inability to reliably control their growth morphology is a major challenge. Here, we report the Au-catalyzed vapor-liquid-solid (VLS) growth of GaN nanowires with controlled growth direction, surface polarity and surface roughness. We develop a theoretical model that relates the growth form to the kinetic frustration induced by variations in the V(N)/III(Ga) ratio across the growing nanowire front. The model predictions are validated by the trends in the as-grown morphologies induced by systematic variations in the catalyst particle size and processing conditions. The principles of shape selection highlighted by our study pave the way for morphological control of technologically relevant compound semiconductor nanowires.

cond-mat.mes-hall↗

arXiv2013

Atomistics of vapor-liquid-solid nanowire growth

Vapor-liquid-solid (VLS) route and its variants are routinely used for scalable synthesis of semiconducting nanowires yet the fundamental growth processes remain unknown. Here, we employ atomic-scale computations based on model potentials to study the stability and growth of gold-catalyzed silicon nanowires (SiNWs). Equilibrium studies uncover segregation at the solid-like surface of the catalyst particle, a liquid AuSi droplet, and a silicon-rich droplet-nanowire interface enveloped by heterogeneous truncating facets. Supersaturation of the droplets leads to rapid 1D growth on the truncating facets and much slower nucleation-controlled 2D growth on the main facet. Surface diffusion is suppressed and the excess Si flux occurs through the droplet bulk which, together with the Si-rich interface and contact line, lowers the nucleation barrier on the main facet. The ensuing step flow is modified by Au diffusion away from the step edges. Our study highlights key interfacial characteristics for morphological and compositional control of semiconducting nanowire arrays.

cond-mat.mtrl-sci↗

arXiv2011

Bundling dynamics regulates the active mechanics and transport in carbon nanotube networks

High-density carbon nanotube networks (CNNs) continue to attract interest as active elements in nanoelectronic devices, nanoelectromechanical systems (NEMS) and multifunctional nanocomposites. The interplay between the network nanostructure and the its properties is crucial, yet current understanding remains limited to the passive response. Here, we employ a novel superstructure consisting of millimeter-long vertically aligned singe walled carbon nanotubes (SWCNTs) sandwiched between polydimethylsiloxane (PDMS) layers to quantify the effect of two classes of mechanical stimuli, film densification and stretching, on the electronic and thermal transport across the network. The network deforms easily with increase in electrical and thermal conductivities suggestive of floppy yet highly reconfigurable network. Insight from atomistically informed coarse-grained simulations uncover an interplay between the extent of lateral assembly of the bundles, modulated by surface zipping/unzipping, and the elastic energy associated with the bent conformations of the nanotubes/bundles. During densification, the network becomes highly interconnected yet we observe a modest increase in bundling primarily due to the reduced spacing between the SWCNTs. The stretching, on the other hand, is characterized by an initial debundling regime as the strain accommodation occurs via unzipping of the branched interconnects, followed by rapid re-bundling as the strain transfers to the increasingly aligned bundles. In both cases, the increase in the electrical and thermal conductivity is primarily due to the increase in bundle size; the changes in network connectivity have a minor effect on the transport. Our results have broad implications for filamentous networks of inorganic nanoassemblies composed of interacting tubes, wires and ribbons/belts.

cond-mat.mes-hall↗

arXiv2011

Saddles, Twists, and Curls: Shape Transitions in Freestanding Nanoribbons

Efforts to modulate the electronic properties of atomically thin crystalline nanoribbons requires precise control over their morphology. Here, we perform atomistic simulations on freestanding graphene nanoribbons (GNRs) to first identify the minimal shapes, and then employ a core-edge framework based on classical plate theory to quantify the width dependence in more general systems. The elastic edge-edge interactions force ultra-narrow ribbons to be flat, which then bifurcate to twisted and bent shapes at critical widths that vary inversely with edge stress. Compressive edge stresses results in twisted and saddle shapes that are energetically indistinguishable in the vicinity of the bifurcation. Increasing widths favor the saddle shapes with (longitudinal) ribbon curvatures that vary non-linearly with width and edge stress. Positive edge stresses result in a flat-to-curled transition with similar scalings. At large widths with negligible edge-edge interactions, rippling instabilities set in, i.e. edge ripples and midline dimples for compressive and tensile edge stresses. Our results highlight the utility of the core-edge framework in developing a unified understanding of the interplay between geometry and mechanics that sets the morphology of crystalline nanoribbons.

cond-mat.mes-hall↗

arXiv2011

Rippling Instabilities in Suspended Nanoribbons

Morphology mediates the interplay between the structure and electronic transport in atomically thin nanoribbons such as graphene as the relaxation of edge stresses occurs preferentially via out-of-plane deflections. In the case of end-supported suspended nanoribbons that we study here, past experiments and computations have identified a range of equilibrium morphologies, in particular for graphene flakes, yet a unified understanding of their relative stability remains elusive. Here, we employ atomic-scale simulations and a composite framework based on isotropic elastic plate theory to chart out the morphological stability space of suspended nanoribbons with respect to intrinsic (ribbon elasticity) and engineered (ribbon geometry) parameters, and the combination of edge and body actuation. The computations highlight a rich morphological shape space that can be naturally classified into two competing shapes, bending-like and twist-like, depending on the distribution of ripples across the interacting edges. The linearized elastic framework yields exact solutions for these rippled shapes. For compressive edge stresses, the body strain emerges as a key variable that controls their relative stability and in extreme cases stabilizes co-existing transverse ripples. Tensile edge stresses lead to dimples within the ribbon core that decay into the edges, a feature of obvious significance for stretchable nanoelectronics. The interplay between geometry and mechanics that we report should serve as a key input for quantifying the transport along these ribbons.

cond-mat.mes-hall↗

arXiv2011

Ultra-thin SWNTs Films with Tunable, Anisotropic Transport Properties

Directional transport properties at the nanoscale remain a challenge primarily due to issues associated with control over the underlying anisotropy and scalability to macroscopic scales. In this letter, we develop a facile approach based on template-guided fluidic assembly of high mobility building blocks - single walled carbon nanotubes (SWNTs) - to fabricate ultra-thin and anisotropic SWNT films. A major advancement is the complete control over the anisotropy in the assembled nanostructure, realized by three-dimensional engineering of dip-coated SWNT thin films into alternating hydrophilic and hydrophobic micro-line patterns with prescribed intra/inter-line widths and line thicknesses. Variations in the contact line profile results in an evaporation-controlled assembly mechanism that leads to the formation of an alternating, and more importantly, contiguous SWNT network. Evidently, the nanoscopic thickness modulations are direct reflections of the substrate geometry and chemistry. The nanostructured film exhibits significant anisotropy in their electrical and thermal transport properties as well as optical transparency, as revealed by characterization studies. The direct interplay between the anisotropy and the 3D micro-line patterns of the substrate combined with the wafer-level scalability of the fluidic assembly allows us to tune the transport properties for a host of nanoelectronic applications.

cond-mat.mes-hall↗

arXiv2010

Topological transitions in carbon nanotube networks via nanoscale confinement

Efforts aimed at large-scale integration of nanoelectronic devices that exploit the superior electronic and mechanical properties of single-walled carbon nanotubes (SWCNTs) remain limited by the difficulties associated with manipulation and packaging of individual SWNTs. Alternative approaches based on ultra-thin carbon nanotube networks (CNNs) have enjoyed success of late with the realization of several scalable device applications. However, precise control over the network electronic transport is challenging due to i) an often uncontrollable interplay between network coverage and its topology and ii) the inherent electrical heterogeneity of the constituent SWNTs. In this letter, we use template-assisted fluidic assembly of SWCNT networks to explore the effect of geometric confinement on the network topology. Heterogeneous SWCNT networks dip-coated onto sub-micron wide ultra-thin polymer channels exhibit a topology that becomes increasingly aligned with decreasing channel width and thickness. Experimental scale coarse-grained computations of interacting SWCNTs show that the effect is a reflection of an aligned topology that is no longer dependent on the network density, which in turn emerges as a robust knob that can induce semiconductor-to-metallic transitions in the network response. Our study demonstrates the effectiveness of directed assembly on channels with varying degrees of confinement as a simple tool to tailor the conductance of the otherwise heterogeneous network, opening up the possibility of robust large-scale CNN-based devices.

cond-mat.mtrl-sci↗

arXiv2008

Morphology of epitaxial core-shell nanowires

We analyze the morphological stability against azimuthal, axial, and general helical perturbations for epitaxial core-shell nanowires in the growth regimes limited by either surface diffusion or evaporation-condensation surface kinetics. For both regimes, we find that geometric parameters (i.e., core radius and shell thickness) play a central role in determining whether the nanowire remains cylindrical or its shell breaks up into epitaxial islands similar to those observed during Stranski-Krastanow growth in thin epilayers. The combination of small cores and rapid growth of the shell emerge as key ingredients for stable shell growth. Our results provide an explanation for the different core-shell morphologies reported in the Si-Ge system experimentally, and also identify a growth-induced intrinsic mechanism for the formation of helical nanowires.

cond-mat.mtrl-sci↗