No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Shear-induced diamondization of multilayer graphene structures: A computational study
Abstract
Diamond is the hardest superhard material with excellent optoelectronic, thermomechanical, and electronic properties. Here, we have investigated the possibility of a new synthesis technique for diamane and diamond thin films from multilayer graphene at pressures far below the graphite → diamond transformation pressure. We have used the Molecular Dynamics technique with reactive force fields. Our results demonstrate a significant reduction (by a factor of two) in the multilayer graphene → diamond transformation stress upon using a combined shear and axial compression. The shear deformation in the multilayer graphene lowers the phase transformation energy barrier and plays the role of thermal fluctuations, which itself promotes the formation of diamond. We revealed a relatively weak temperature dependence of the transformation strain and stresses. The transformation stress vs. strain curve for the bulk graphite drops exponentially for finite temperatures.
... Copyright 2020 Elsevier.) molecular dynamics, and ab initio calculations are some other models that have been used to understand diamane [113,129]. ...
... Diamane can be used in applications in the coatings and defense industries [129]. Ultra-thin coatings produced from low frictional hydrogenated diamanes can help to improve wearability in mechanical parts. ...
The constant changes in the field of nanotechnology need to be analyzed to obtain
an idea of the present state of these technologies. One branch of nanotechnology,
nanocarbons, experiences constant changes and growth for effective, new, robust,
and efficient applications that can fill the voids left by newer technologies. In this
sense, the status of different nanocarbons should be assessed regularly and
continuously so that the current state can be viewed and future outcomes can be
predicted. We have found that chemical vapor deposition, laser ablation, and arc
discharge are the most prominent methods for the synthesis of nanocarbons. In
terms of properties, excellent electrical, mechanical, optical, and thermal properties
have been shown by all the nanocarbons that are discussed here. Enhanced electrical
and thermal conductivity, mechanical strength and transmissivity are some of the
properties that are observed in these nanocarbons, which provide applications in
electronics, energy storage, drug carriers, biosensors, biomedicine, aerospace,
thermal management, etc. In this chapter we review five different types of nano�carbons, namely carbon nanotubes, fullerenes, graphene, diamane, and diamanoids.
Analysis of their methods of synthesis and properties are provided, which can
subsequently be used in the brilliant applications of nanotechnology
... [12][13][14] The transformation to diamond from glassy carbon 12 and graphite 13,14 was found to be thermodynamically favored with the application of shear. As to graphene, the occurrence of shear-induced diamonization has been predicted by molecular dynamics simulations, 15 while the effect of high shear stress on graphene has not been sufficiently studied by experimental work. ...
... 9,25,26 In contrast, the diamond phase transited from graphite under shear could be preserved to ambient condition, 13,14 and the diamondization of graphene under shear was also predicted theoretically. 15 This makes the rDAC experiments a hopeful approach to synthesize the long pursued "diamane," the atomically thin diamond. However, in our experiment, although the indentation (ring crack) of the anvil culets (Fig. S4) and the decrease in optical conductivity supported the formation of superhard phases 27,28 and concentration of sp 3 bonding in the sheared material, neither cubic nor hexagonal diamond phase could be found in the recovered sample. ...
A key factor that determines the mechanical and electrical performance of graphene-based materials and devices is how graphene behaves under extreme conditions, yet the response of few-layer graphene to high shear stress has not been investigated experimentally. Here we applied high pressure and shear to graphene powder using a rotational diamond anvil cell and studied the recovered sample with multiple means of characterization. Sustaining high pressure and shear, graphene breaks into nanometer-long clusters with generation of large number of defects. At a certain stress level, it transforms to amorphous state and carbon onions. The reduction of infrared reflectivity in the severely sheared phase indicates the decrease in conductivity. Our results unveil the shear sensitive nature of graphene, point out the effects of shear on its physical properties, and provide a potential method to manipulate this promising material.
... Interestingly, Gao et al. [41] results indicate that the pressure threshold for the transformation into a diamond-like structure could be reduced with the application of shear. The same conclusion was reached by Paul et al. [38] using Molecular Dynamics (MD) simulations with Reactive Force Fields (ReaxFF). Furthermore, Bakharev et al. [42] reported that chemical vapor deposited bilayer graphene could be converted into a fluorinated single-layer diamond by fluorine chemisorption. ...
... Compared with recent theoretical predictions of diamond-like phases [30,51] and previously reported structures available in the Samara Carbon Allotrope Database (SACADA) [52] from LA1 to LA10, we find that the obtained diamond-like structures are similar to the cubic diamond structure (LA1). Interestingly, Paul et al. [38] and Gao et al. [41] reported that the transformation of multi-layer graphene into diamond could be facilitated by shear loading. Our results suggest that shear-induced stacking fault formation may be the underlying mechanism for these observations. ...
Recent results suggest a pressure-induced transformation of bilayer graphene to a diamond-like structure. Intriguingly, the reported transformation is absent or incomplete in multi-layer graphene beyond two layers, implying the suppression of the transformation by an unclear mechanism. In this work, we use reactive molecular dynamics simulations to describe the pressure-induced structural evolution of multi-layer graphene with two to six layers to pressures up to 200 GPa. The results show that bilayer graphene transforms into a diamond-like film, i.e., more than 75% sp³ hybridized atoms. In contrast, three-to six-layer graphene with AB layer stacking fail to form a diamond-like film up to 200 GPa. Bond formation analysis indicates that the transformation and its suppression is related to the generation of a bond chain structure across layers, connecting sp³ hybridized atoms. The result suggests that the ABA layer stacking is directly related to the hindering of the chain structure formation and the suppression of the transformation. Surprisingly, the bond-chain-structure transformation mechanism is effective in three-layer graphene with ABC layer stacking, as well as five-layer graphene with ABCAB layer stacking. That suggests that stacking faults are effective catalysts for the high-pressure transformation of multi-layer graphene to diamond.
... Efficient computational tools have been developed to perform simulations within timespans not accessible to experimental studies [22,23]. We may refer to large-scale continuum simulations , phase-field simulations for capturing the microstructure [48][49][50][51][52][53][54][55], MD simulations to capture the atomistic mechanisms [2,47,[56][57][58][59][60][61][62][63][64][65][66][67], and multiscale simulations to capture the broad spectrum of materials and processes response [24,25,46,[68][69][70][71][72][73]. Out of several computational methods, molecular dynamics simulation allows the capturing of materials evolution with atomistic accuracy, including the radiation damage mechanism. ...
Ferritic-martensitic steels, such as T91, are candidate materials for high-temperature applications, including superheaters, heat exchangers, and advanced nuclear reactors. Considering these alloys’ wide applications, an atomistic understanding of the underlying mechanisms responsible for their excellent mechano-chemical properties is crucial. Here, we developed a modified embedded-atom method (MEAM) potential for the Fe-Cr-Si-Mo quaternary alloy system—i.e., four major elements of T91—using a multi-objective optimization approach to fit thermomechanical properties reported using density functional theory (DFT) calculations and experimental measurements. Elastic constants calculated using the proposed potential for binary interactions agreed well with ab initio calculations. Furthermore, the computed thermal expansion and self-diffusion coefficients employing this potential are in good agreement with other studies. This potential will offer insightful atomistic knowledge to design alloys for use in harsh environments.
... In this work, the thicknesses of the boundary layer and thermostatic layer are both 0.5 nm. The long-range bond-order (LCBOP) potential proposed by J. H. Los [38] was selected to describe the interatomic interaction, which has successfully been used to simulate the elastic and plastic deformations of diamond [39] or nt-diamond [25] under indenting [40,41] or shear [42] conditions. Periodic boundary condition was applied along the Y-direction, and fixed boundary conditions were set in the X-and Z-directions. ...
... Furthermore, ABC can be converted into cubic diamonds under high pressure [31,32]. However, these simulations failed to produce other types (4H, 9R, and 15R), which could be attributed to the absence of additional influencing factors, such as the complexity of the raw material structure and compression method [35,36] in high-pressure experiments. ...
... The long-range bond-order (LCBOP) potential proposed by J. H. Los [38] was selected to describe the interatomic interaction, which has successfully been used to simulate the elastic and plastic deformations of diamond [39] or nt-diamond [25] under indenting [40,41] or shear [42] conditions. Periodic boundary condition was applied along the Y-direction, and fixed boundary conditions were set in the X-and Z-directions. ...
... However, they are generally focused on one aspect of the growth and do not capture the complete picture of the synthesis process. We may refer to classical Wulff construction 36 , atomistic simulations [37][38][39][40][41] , and diffuse interface methods [42][43][44][45][46][47][48][49] as examples of these efforts. However, so far, precise control of the uniformity and coverage of as-grown 2D materials is considered a formidable challenge due to intrinsic complexities associated with the growth involving multiple physics across a wide range of length and time scales. ...
Reproducible wafer-scale growth of two-dimensional (2D) materials using the Chemical Vapor Deposition (CVD) process with precise control over their properties is challenging due to a lack of understanding of the growth mechanisms spanning over several length scales and sensitivity of the synthesis to subtle changes in growth conditions. A multiscale computational framework coupling Computational Fluid Dynamics (CFD), Phase-Field (PF), and reactive Molecular Dynamics (MD) was developed – called the CPM model – and experimentally verified. Correlation between theoretical predictions and thorough experimental measurements for a Metal-Organic CVD (MOCVD)-grown WSe2 model material revealed the full power of this computational approach. Large-area uniform 2D materials are synthesized via MOCVD, guided by computational analyses. The developed computational framework provides the foundation for guiding the synthesis of wafer-scale 2D materials with precise control over the coverage, morphology, and properties, a critical capability for fabricating electronic, optoelectronic, and quantum computing devices.
... Then the product of gaseous reactions deposits on the substrate, forming the 2D monolayer material. Various computational methods such as finite element analysis, 22−25 atomistic simulation, 26,27 and the firstprinciple method 28 have been employed to guide the synthesis of 2D materials. 10,29−31 Although most of these studies focus on first principal/DFT calculations, these simulations cannot analyze larger atomic systems involved in TMDs with certain rotation angles. ...
... Isolated modeling techniques have been used to investigate the growth of 2D materials [8][9][10][11][12]. Molecular dynamics (MD) simulations based on empirical [13,14] and reactive [15][16][17] potentials are utilized to study the growth of these monolayered materials. Ab initio MD simulations are utilized to study nucleation mechanisms [18]. ...
The exotic properties of 2D materials made them ideal candidates for applications in quantum computing, flexible electronics, and energy technologies. A major barrier to their adaptation for industrial applications is their controllable and reproducible growth at a large scale. A significant effort has been devoted to the chemical vapor deposition (CVD) growth of wafer-scale highly crystalline monolayer materials through exhaustive trial-and-error experimentations. However, major challenges remain as the final morphology and growth quality of the 2D materials may significantly change upon subtle variation in growth conditions. Here, we introduced a multiscale/multiphysics model based on coupling continuum fluid mechanics and phase-field models for CVD growth of 2D materials. It connects the macroscale experimentally controllable parameters, such as inlet velocity and temperature, and mesoscale growth parameters such as surface diffusion and deposition rates, to morphology of the as-grown 2D materials. We considered WSe 2 as our model material and established a relationship between the macroscale growth parameters and the growth coverage. Our model can guide the CVD growth of monolayer materials and paves the way to their synthesis-by-design.
Graphic abstract
... Since experimentally the transition at 300 K occurs at p = 26.6 GPa [60], it must be assisted by extrinsic factors such as lattice defects [69][70][71][72], dislocations [73][74][75][76][77][78][79][80], grain boundaries [81], surfaces [69][70][71] or nonhydrostatic pressure [82,83]. This observation is similar to the one found for nucleation of melting [79] and crystallisation of ice [84]. ...
We propose here an NPT metadynamics simulation scheme using coordination number and volume as collective variables and apply it to the reconstructive structural transformation B1/B2 in NaCl. Studying systems with size up to 64000 atoms we reach regime beyond the collective mechanism, observe transformation proceeding via nucleation and growth and show the size-dependence of the transition pathway. The scheme is likely to be applicable to a broader class of pressure-induced structural transitions allowing to study complex nucleation effects and bring simulations closer to realistic conditions.
... The authors state that their investigations indicate a novel, surface-to-bulk phase transition mechanism that hints at diamondene formation. In [58], the authors propose and theoretically show the shear-induced diamondization of multilayer graphene at pressures much lower than the high pressure used for the transformation of graphite into diamond. Preparation schemes for diamane-like structures are still under development. ...
Diamanes are 2D diamond-like films that are nanometers in thickness. Diamanes can exist as bilayer or multilayer graphene with various modes of stacking and interlayer covalent sp3 bonds. The term “diamane” is used broadly for a variety of diamond-like materials at the nanoscale, from individual diamond clusters to nanocrystal films. A short overview of recent progress in the investigation of diamanes, starting from the first theoretical predictions to practical realization, is presented. The results of both theoretical and experimental studies on diamanes with various atomic structures and types of functionalization are considered. It is shown that diamanes are stronger than graphene and graphane and have wide bandgaps ranging from 3.1 to 4.5 eV depending on the structure. Diamane-like structures have been obtained using different experimental techniques, and their structures have been determined by Raman spectroscopy. The potential applications of these carbon nanostructures are briefly reviewed.
This research text presents the synthesis, characterisation, and advanced technological applications of novel nanocarbons, with a focus on graphane, graphyne, graphdiyne, graphene nanoribbon, onion like carbons and carbon nanotori. The book covers the various applications of novel nanocarbons, such as in solar cells and flexible electronics, provides updates on the effects of doping and electronic properties of electrode materials based on novel nanocarbons, and references further research and application-oriented innovations using these new carbon nanostructures. It also provides new updates on the effect of doping and structural twist on the electrochemical performance of electrode materials based on novel nanocarbons. This book will appeal to researchers and graduate students working with novel nanocarbon allotropes, including disciplines such as physics, chemistry, materials science, and chemical engineering.
Key features
• Presents the synthesis, characterisation, and advanced technological applications of novel nanocarbons
• Focuses on graphane, graphyne, graphdiyne, graphene nanoribbon, onion like carbons and carbon nanotori
• Covers the applications of novel nanocarbons in solar cells, flexible electronics, supercapacitor batteries and next-generation energy storage devices
• Includes cutting-edge research on new carbon nanomaterials (2D nanosystems) challenging graphene
• Covers the effect of doping and electronic properties of nanocarbons for quantum devices
• Discusses the commercialisation, shortcomings and future prospects of nanocarbons
Supercritical CO2 (SC CO2 ), as one of the unique fluids that possess fascinating properties of gas and liquid, holds great promise in chemical reactions and fabrication of materials. Building special nanostructures via SC CO2 for functional applications has been the focus of intense research for the past two decades, with facile regulated reaction conditions and a particular reaction field to operate compared to the more widely used solvent systems. In this review, the significance of SC CO2 on fabricating various functional materials including modification of 1D carbon nanotubes, 2D materials, and 2D heterostructures is stated. The fundamental aspects involving building special nanostructures via SC CO2 are explored: how their structure, morphology, and chemical composition be affected by the SC CO2 . Various optimization strategies are outlined to improve their performances, and recent advances are combined to present a coherent understanding of the mechanism of SC CO2 acting on these functional nanostructures. The wide applications of these special nanostructures in catalysis, biosensing, optoelectronics, microelectronics, and energy transformation are discussed. Moreover, the current status of SC CO2 research, the existing scientific issues, and application challenges, as well as the possible future directions to advance this fertile field are proposed in this review.
Diamond has high hardness and chemical inertness and is extensively used in friction materials. However, its wear resistance is weak in high-temperature environments and cannot meet actual requirements. In this study, the tribological properties between diamond and SiC balls were investigated at three different temperatures. Its microscopic friction and wear properties were investigated using molecular dynamics, and the simulation findings demonstrated that the diamond's wear ratio was increasing with the increase in temperature. The atomic interactions between diamond and SiC balls are understood by examining the change in stress–strain of diamond and SiC balls. This simulation findings hope to offer some theoretical reference for the industrial application of diamonds.
Transition metal dichalcogenides (TMDCs) are potential materials for future optoelectronic devices. Grain boundaries (GBs) can significantly influence the optoelectronic properties of TMDC materials. Here, we have investigated the mechanical characteristics of tungsten diselenide (WSe2) monolayers and failure process with symmetric tilt GBs using ReaxFF molecular dynamics simulations. In particular, the effects of topological defects, loading rates, and temperatures are investigated. We considered nine different grain boundary structures of monolayer WSe2, of which six are armchair (AC) tilt structures, and the remaining three are zigzag (ZZ) tilt structures. Our results indicate that both tensile strength and fracture strain of WSe2 with symmetric tilt GBs decrease as the temperature increases. We revealed an interfacial phase transition for high-angle GBs reduces the elastic strain energy within the interface at finite temperatures. Furthermore, brittle cracking is the dominant failure mode in the WSe2 monolayer with tilted GBs. WSe2 GB structures showed more strain rate sensitivity at high temperatures than at low temperatures.
The ability to change material properties through phase engineering has long been sought, with the goal of ad hoc tunability of the physical and chemical properties of the transformed phases. The synthesis and study of graphene have made it possible to explore the mechanisms of 2D phase transformations, opening up paths towards the formation of 2D diamond and diamond thin films. This Review examines the state-of-the-art phase transformations in 2D graphitic systems and beyond. The theoretical models formulated to describe the sp2-to-sp3 transitions from graphene to 2D diamond and the experimental processes developed to induce the transition to 2D diamond are discussed, focusing on the transformations induced by chemical functionalization and pressure. The effects of different structural and environmental factors on the evolution of the phase transformations and on the properties of the transformed diamond phases are explored. Without comprehensively reviewing phase transitions in all 2D materials, we briefly mention hexagonal boron nitride, phosphorene, transition metal dichalcogenides and MXenes systems. Finally, the Review delves into the technologies and applications of phase transformations in 2D materials and the opportunities for this field. Mechanically and chemically induced phase transitions in 2D materials offer access to new properties, opening up paths towards future applications. This Review summarizes the theoretical models and experimental processes for inducing phase transformations in 2D materials, especially graphene to 2D diamond, and examines the associated applications.
New materials are the backbone of their technology-driven modern civilization and at present carbon nanostructures are the leading candidates that have attracted huge research activities. Diamanes and diamanoids are the new nanoallotropes of sp 3 hybridized carbon which can be fabricated by proper functionalization, substitution, and via Birch reduction under controlled pressure using graphitic system as a precursor. These nanoallotropes exhibit outstanding electrical, thermal, optical, vibrational, and mechanical properties, which can be an asset for new technologies, especially for quantum devices, photonics, and space technologies. Moreover, the features like wide bandgap, tunable thermal conductivity, excellent thermal insulation, etc. make diamanes and diamanoids ideal candidates for nano-electrical devices, nano-resonators, optical waveguides, and the next generation thermal management systems. In this review, diamanes and diamanoids are discussed in detail in terms of its historical prospect, method of synthesis, structural features, broad properties, and cutting-edge applications. Additionally, the prospects of diamanes and diamanoids for new applications are carefully discussed. This review aims to provide a critical update with important ideas for a new generation of quantum devices based on diamanes and diamanoids which are going to be an important topic in the future of carbon nanotechnology.
Here we propose an NPT metadynamics simulation scheme for pressure-induced structural phase transitions, using coordination number and volume as collective variables, and apply it to the reconstructive structural transformation B1−B2 in NaCl. By studying systems with size up to 64 000 atoms we reach a regime beyond collective mechanism and observe transformations proceeding via nucleation and growth. We also reveal the crossover of the transition mechanism from Buerger-like for smaller systems to Watanabe-Tolédano for larger ones. The scheme is likely to be applicable to a broader class of pressure-induced structural transitions, allowing study of complex nucleation effects and bringing simulations closer to realistic conditions.
Shock compression subjects materials to a unique regime of high quasi-hydrostatic pressure and coupled shear stresses for durations on the order of 1–10 nanoseconds for laser-driven loading of samples. There is, additionally, an attendant temperature increase due to the shock and the mechanisms of plastic deformation in metals whereby dislocations, twins, and phase transitions nucleate and propagate at velocities near the sound speed. Covalently bonded materials have, by virtue of the directionality of their bonds, great difficulty in responding by conventional plastic deformation to this extreme regime of shock compression. We propose that the shear from shock compression induces amorphization, as observed in Si, Ge, B4C, SiC, and olivine ((Mg, Fe)2SO4) and that this is a general deformation mechanism in a broad class of covalently bonded materials. The crystalline structure transforms to amorphous along regions of maximum shear stress, forming nanoscale bands, and thereby relaxing the shear component of the imposed shock stress. This process is usually preceded by the emission and propagation of a critical concentration of dislocations.
Multilayer graphene oxide(MGO) highly stacked nature restricts its application in the sensor field, and obtaining high-quality 3D graphene remains a significant challenge. Herein, we report the direct use of MGO to synthesize highly dispersed and rich ordered mesoporous 3D reduced graphene oxide frameworks(rGOFs) through a novel improved KOH activation method, which is based on the pre-treatment process of fully adsorbing and fixing the appropriate amount of K⁺ by using the limited interlayer space of MGO and freeze-drying crystallization technology. Surprisingly, different from previous reports that only the micropores and individual macropores have been obtained by conventional KOH activation, we fully confirmed the dense and ordered mesoporous defects on the surface of rGOFs by SEM for the first time, as well as the multilayer stacked structure of rGOFs is in line with the typical 3D structure. Besides, rGOFs have the excellent properties of reduced graphene oxide (rGO) and a particular 3D framework structure, making it an excellent functionalized modification platform. In order to verify the performance of rGOFs as a framework platform, in which amination modification and small-size Ag NPs loading were successfully realized, the constructed [email protected]2/GCE sensor detects chloride ions over a wide linear concentration range of 5μmol L⁻¹-10μmol L⁻¹, with the detection limit as low as 0.1 μmol L⁻¹ (S/N=3). This work would not only open a new path to synthesize 3D graphene with excellent performance, and the concept of graphene frameworks nanomaterial is proposed for the first time, but also promotes its excellent performance as an ideal matrix platform for other nanomaterials in the field of electrochemical sensing.
The advent of two‐dimensional transition metal dichalcogenides has triggered an interest in exploring a new class of high‐performance materials with intriguing physicochemical attributes. Molybdenum and tungsten disulfides have attracted significant attention due to surface excitons and trions and the large spin‐orbit effects of these compounds. Moreover, WS2 is especially intriguing due to large relativistic effects, which result in bound excitons at the edge and biexciton formation in the bilayers. Hence, we present a relativistic topological model for the characterization of these two types of metal disulfides. We have employed relativistically weighted graph‐theoretical methods for obtaining structural descriptors of these compounds through the inclusion of different shapes on the boundaries and employing the topological cut techniques. Molybdenum and tungsten disulfides are 2D transition metal dichalcogenides with intriguing physicochemical attributes that exhibit ideal magnetic, electrical, and optical applications. This paper presents a relativistic topological model for the characterization of these metal disulfides in their two significant structural phases. As this model includes relativistic quantum effects, it is anticipated that the results obtained will enable greater insights into the geometries and behaviors of these materials.
Achieving structural superlubricity in graphitic samples of macroscale size is particularly challenging due to difficulties in sliding large contact areas of commensurate stacking domains. Here, we show the presence of macroscale structural superlubricity between two randomly stacked graphene layers produced by both mechanical exfoliation and chemical vapour deposition. By measuring the shifts of Raman peaks under strain we estimate the values of frictional interlayer shear stress (ILSS) in the superlubricity regime (mm scale) under ambient conditions. The random incommensurate stacking, the presence of wrinkles and the mismatch in the lattice constant between two graphene layers induced by the tensile strain differential are considered responsible for the facile shearing at the macroscale. Furthermore, molecular dynamic simulations show that the stick-slip behaviour does not hold for incommensurate chiral shearing directions for which the ILSS decreases substantially, supporting the experimental observations. Our results pave the way for overcoming several limitations in achieving macroscale superlubricity using graphene. Superlubricity in macro-scale graphitic samples is hampered by commensurate stacking domains that prevent facile sliding between adjacent graphene layers. Here, the authors show the presence of macroscale structural superlubricity between two randomly stacked graphene layers produced by both mechanical exfoliation and CVD upon the imposition of a tensile stress.
The successful discovery and isolation of graphene in 2004, and the subsequent synthesis of layered semiconductors and heterostructures beyond graphene have led to the exploding field of two-dimensional (2D) materials that explore their growth, new atomic-scale physics, and potential device applications. This review aims to provide an overview of theoretical, computational, and machine learning methods and tools at multiple length and time scales, and discuss how they can be utilized to assist/guide the design and synthesis of 2D materials beyond graphene. We focus on three methods at different length and time scales as follows: (i) nanoscale atomistic simulations including density functional theory (DFT) calculations and molecular dynamics simulations employing empirical and reactive interatomic potentials; (ii) mesoscale methods such as phase-field method; and (iii) macroscale continuum approaches by coupling thermal and chemical transport equations. We discuss how machine learning can be combined with computation and experiments to understand the correlations between structures and properties of 2D materials, and to guide the discovery of new 2D materials. We will also provide an outlook for the applications of computational approaches to 2D materials synthesis and growth in general.
Mono- and few-layer graphene exhibit unique mechanical, thermal, and electrical properties. However, their hardness and in-plane stiffness are still not comparable to the other allotrope of carbon, i.e. diamond. This makes layered graphene structures to be less suitable for application in harsh environments. Thus, there is an unmet need for the synthesis of atomically thin diamond films for such applications that are also stable under ambient conditions. Here, we demonstrate the possibility for the synthesis of such diamond films from multilayer graphene using the molecular dynamics approach with reactive force fields. We study the kinetics and thermodynamics of the phase transformation as well as the stability of the formed diamond thin films as a function of the layer thickness at different pressures and temperatures for pristine and hydrogenated multilayer graphene. The results indicate that the transformation conditions depend on the number of graphene layers and surface chemistry. We revealed a reduction in the transformation strain by up to 50% while transformation stress has reduced by as much as five times upon passivation with hydrogen atoms. While the multilayer pristine graphene to diamond transformation is shown to be reversible, hydrogenated multilayer graphene structures had formed a metastable diamond film. Our simulations have further revealed temperature-independence of transformation strain, while transformation stresses are strong functions of temperature.
2D growth: multiscale simulations capture MoS2 CVD growth dynamics A computational multiscale framework is capable of modelling the growth morphology during chemical vapour deposition (CVD) synthesis of MoS2. A team led by Kasra Momeni at Louisiana Tech University and Long-Qing Chen at Pennsylvania State University developed a hierarchical model capable of predicting the size and distribution of atomically thin materials synthetized by CVD in relation to several control parameters of the reactor chamber. The equations governing heat and mass transport in the reactor were solved numerically to determine the thermodynamic state of the precursor and its concentration within the chamber. The resulting concentration distributions were then coupled to the mesoscale phase-field equations describing the growth morphology. This framework can be used to explore the growth diagrams resulting in the formation of 2D materials under specific parameters, or, conversely, to find optimal growth conditions as a function of the desired material morphology.
In this study, we present a number of experiments on the transformation of graphite, diamond, and multiwalled carbon nanotubes under high pressure conditions. The analysis of our results testifies to the instability of diamond in the 55–115 GPa pressure range, at which onion-like structures are formed. The formation of interlayer sp3-bonds in carbon nanostructures with a decrease in their volume has been studied theoretically. It has been found that depending on the structure, the bonds between the layers can be preserved or broken during unloading.
Atomically thin graphene exhibits fascinating mechanical properties, although its hardness and transverse stiffness are inferior to those of diamond. So far, there has been no practical demonstration of the transformation of multilayer graphene into diamond-like ultrahard structures. Here we show that at room temperature and after nano-indentation, two-layer graphene on SiC(0001) exhibits a transverse stiffness and hardness comparable to diamond, is resistant to perforation with a diamond indenter and shows a reversible drop in electrical conductivity upon indentation. Density functional theory calculations suggest that, upon compression, the two-layer graphene film transforms into a diamond-like film, producing both elastic deformations and sp 2 to sp 3 chemical changes. Experiments and calculations show that this reversible phase change is not observed for a single buffer layer on SiC or graphene films thicker than three to five layers. Indeed, calculations show that whereas in two-layer graphene layer-stacking configuration controls the conformation of the diamond-like film, in a multilayer film it hinders the phase transformation.
Moving to nanoscale is a path to get perfect materials with superior properties. Yet defects, such as stacking faults (SFs), are still forming during the synthesis of nanomaterials and, according to common notion, degrade the properties. Here, we demonstrate the possibility of engineering defects to, surprisingly, achieve mechanical properties beyond those of the corresponding perfect structures. We show that introducing SFs with high density increases the Young’s Modulus and the critical stress under compressive loading of the nanowires above those of a perfect structure. The physics can be explained by the increase in intrinsic strain due to the presence of SFs and overlapping of the corresponding strain fields. We have used the molecular dynamics technique and considered ZnO as our model material due to its technological importance for a wide range of electromechanical applications. The results are consistent with recent experiments and propose a novel approach for the fabrication of stronger materials.
A phase-field approach for phase transformations (PTs) between three different phases at nonequilibrium temperatures is developed. It includes advanced mechanics, thermodynamically consistent interfacial stresses, and interface interactions. A thermodynamic Landau-Ginzburg potential developed in terms of polar order parameters satisfies the desired instability and equilibrium conditions for homogeneous phases. The interfacial stresses were introduced with some terms from large-strain formulation even though the small-strain assumption was utilized. The developed model is applied to study the PTs between two solid phases via highly disordered intermediate phase ($IP$) or intermediate melt (IM) hundreds of degrees below the melting temperature. In particular, the $\beta \leftrightarrow \delta$ PTs in HMX energetic crystal via IM is analyzed. The effects of various parameters (temperature, ratios of widths and energies of solid-solid (SS) to solid-melt (SM) interfaces, elastic energy, and interfacial stresses) on the formation, stability, and structure of the IM within a propagating SS interface are studied. Interfacial and elastic stresses within SS interphase and their relaxation and redistribution with appearance of partial or complete IM are analyzed. Energy and structure of the critical nucleus (CN) of IM are studied as well. In particular, the interfacial stresses increase the aspect-ratio of the CN. Although including elastic energy can drastically reduce the energy of CN of IM, the activation energy of the CN of IM within SS interface increases when interfacial tension is taken into account. The developed thermodynamic potential can also be modified to model other multiphase physical phenomena, such as multi-variant martensitic PTs, grain boundary and surface-induced pre-melting and PTs, as well as developing phase diagrams for IPs.
The effect of elastic energy on nucleation and disappearance of a nanometer size intermediate melt (IM) region at a solid-solid (S1S2) phase interface at temperatures 120 K below the melting temperature is studied using a phase-field approach. Results are obtained for broad range of the ratios of S1S2 to solid-melt interface energies, kE, and widths, kδ. It is found that internal stresses only slightly promote barrierless IM nucleation but qualitatively alter the system behavior, allowing for the appearance of the IM when kE < 2 (thermodynamically impossible without mechanics) and elimination of what we termed the IM-free gap. Remarkably, when mechanics is included within this framework, there is a drastic (16 times for HMX energetic crystals) reduction in the activation energy of IM critical nucleus. After this inclusion, a kinetic nucleation criterion is met, and thermally activated melting occurs under conditions consistent with experiments for HMX, elucidating what had been to date mysterious behavior. Similar effects are expected to occur for other material systems where S1S2 phase transformations via IM take place, including electronic, geological, pharmaceutical, ferroelectric, colloidal, and superhard materials.
The microscopic kinetics of ubiquitous solid-solid phase transitions remain poorly understood. Here, by using single-particle-resolution video microscopy of colloidal films of diameter-tunable microspheres, we show that transitions between square and triangular lattices occur via a two-step diffusive nucleation pathway involving liquid nuclei. The nucleation pathway is favoured over the direct one-step nucleation because the energy of the solid/liquid interface is lower than that between solid phases. We also observed that nucleation precursors are particle-swapping loops rather than newly generated structural defects, and that coherent and incoherent facets of the evolving nuclei exhibit different energies and growth rates that can markedly alter the nucleation kinetics. Our findings suggest that an intermediate liquid should exist in the nucleation processes of solid-solid transitions of most metals and alloys, and provide guidance for better control of the kinetics of the transition and for future refinements of solid-solid transition theory.
Carbon-based bulk nanostructures are believed to entail certain advantages over their parent low-dimensional materials and are promising candidates for supercapacitors due to their unique properties such as extremely high specific surface area and high conductivity. Herein the mechanical and structural properties of four types of carbon nanopolymorph-based nanomaterials were calculated using molecular dynamics simulations. Bulk carbon nanostructures composed of structural units of bent graphene flakes, short carbon nanotubes, fullerenes and their mixture were considered. The effect of the loading scheme and temperature was studied and constitutive relationships describing the deformation of the materials were given. The simulation results revealed that the effect of the loading scheme significantly depends on the type of structural units, and was slightly affected by the temperature especially for high densities. The constitutive equations obtained in this work can be applied to describe the mechanical behavior of new bulk carbon nanostructures.
Using classical molecular dynamics with a more reliable reactive LCBOPII potential, we have performed a detailed study on the direct graphite-to-diamond phase transition. Our results reveal a new so-called "wave-like buckling and slipping" mechanism, which controls the transformation from hexagonal graphite to cubic diamond. Based on this mechanism, we have explained how polycrystalline cubic diamond is converted from hexagonal graphite, and demonstrated that the initial interlayer distance of compressed hexagonal graphite play a key role to determine the grain size of cubic diamond. These results can broaden our understanding of the high pressure graphite-to-diamond phase transition.
Solid–solid (SS)(SS) phase transformations via nanometer-size intermediate melts (IMs)(IMs) within the SS interface, hundreds of degrees below melting temperature, were predicted thermodynamically and are consistent with experiments for various materials. A necessary condition for the appearance of IMs, using a sharp interface approach, was that the ratio of the energies of SS and solid–melt (SM)(SM) interfaces, kEkE, were >2. Here, an advanced phase-field approach coupled with mechanics is developed that reveals various new scale and interaction effects and phenomena. Various types of IM are found: (i) continuous and reversible premelting and melting; (ii) jump-like barrierless transformation to IMs, which can be kept at much lower temperature even for kE<2kE<2; (iii) unstable IMs, i.e. a critical nucleus between the SS interface and the IM. A surprising scale effect related to the ratio of widths of SS and SM interfaces is found: it suppresses barrierless IMs but allows IMs to be kept at much lower temperatures even for kE<2kE<2. Relaxation of elastic stresses strongly promotes IMs, which can appear even at kE<2kE<2 and be retained at kE=1kE=1. The theory developed here can be tailored for diffusive phase transformations, formation of intergranular and interfacial phases, and surface-induced phase transformations.
A three-dimensional Landau theory of stress-induced martensitic phase transformations is presented. It describes transformations between austenite and martensitic variants and transformations between martensitic variants. The Landau free energy incorporates all temperature-dependent thermomechanical properties of both phases. The theory accounts for the principal features of martensitic transformations in shape memory alloys and steels, namely, stress-strain curves with constant transformation strain and constant, or weakly temperature dependent, stress hysteresis, as well as nonzero tangent elastic moduli at the phase transformation point. In part I, the austenitemartensite phase transformation is treated, while transformations between martensitic variants are considered in part II.
An irreversible phase transformation (PT) from the rhombohedral phase of boron nitride rBN to cubic cBN was recently recorded at the surprisingly low pressure of 5.6 GPa at room temperature. In this paper, a very nontrivial and unexpected explanation of this phenomenon is found, based on our criterion for the PT in plastic materials and approximate solution of corresponding plastic problems. It is found that due to orientational plastic instability and rotational softening in rBN and the higher yield stress of cBN, stresses grow drastically in the transforming region during the PT (despite a volume decrease by a factor of 1.53). This allows the fulfillment of the PT criterion which takes into account the whole stress history during the transformation process. It appears that the above experimental phenomenon is connected to the mechanical behavior of the system of transforming particles+surrounding materials at the millimeter scale.
Disordered structures of boron nitride (BN), graphite, boron carbide (BC), and boron carbon nitride (BCN) systems are considered important precursor materials for synthesis of superhard phases in these systems. However, phase transformation of such materials can be achieved only at extreme pressure-temperature conditions, which is irrelevant to industrial applications. Here, the phase transition from disordered nanocrystalline hexagonal (h)BN to superhard wurtzitic (w)BN was found at room temperature under a pressure of 6.7 GPa after applying large plastic shear in a rotational diamond anvil cell (RDAC) monitored by in situ synchrotron X-ray diffraction (XRD) measurements. However, under hydrostatic compression to 52.8 GPa, the same hBN sample did not transform to wBN but probably underwent a reversible transformation to a high-pressure disordered phase with closed-packed buckled layers. The current phase-transition pressure is the lowest among all reported direct-phase transitions from hBN to wBN at room temperature. Usually, large plastic straining leads to disordering and amorphization; here, in contrast, highly disordered hBN transformed to crystalline wBN. The mechanisms of strain-induced phase transformation and the reasons for such a low transformation pressure are discussed. Our results demonstrate a potential of low pressure-room temperature synthesis of superhard materials under plastic shear from disordered or amorphous precursors. They also open a pathway of phase transformation of nanocrystalline materials and materials with disordered and amorphous structures under extensive shear.
Graphite and diamond have comparable free energies, yet forming diamond from graphite in the absence of a catalyst requires pressures that are significantly higher than those at equilibrium coexistence. At lower temperatures, the formation of the metastable hexagonal polymorph of diamond is favoured instead of the more stable cubic diamond. These phenomena cannot be explained by the concerted mechanism suggested in previous theoretical studies. Using an ab initio quality neural-network potential, we carried out a large-scale study of the graphite-to-diamond transition assuming that it occurs through nucleation. The nucleation mechanism accounts for the observed phenomenology and reveals its microscopic origins. We demonstrate that the large lattice distortions that accompany the formation of diamond nuclei inhibit the phase transition at low pressure, and direct it towards the hexagonal diamond phase at higher pressure. The proposed nucleation mechanism should improve our understanding of structural transformations in a wide range of carbon-based materials.
Based on the Green–Kubo relation from linear response theory, we calculated the thermal current autocorrelation functions from classical molecular dynamics (MD) simulations. We examined the role of quantum corrections to the classical thermal conduction and concluded that these effects are small for fairly harmonic systems such as diamond. We then used the classical MD to extract thermal conductivities for bulk crystalline systems. We find that (at 300 K) 12C isotopically pure perfect diamond has a thermal conductivity 45% higher than natural (1.1% 13C) diamond. This agrees well with experiment, which shows a 40%–50% increase. We find that vacancies dramatically decrease the thermal conductivity, and that it can be described by a reciprocal relation with a scaling as n<sub>v</sub><sup>-alpha</sup>, with alpha= 0.69±0.11 in agreement with phenomenological theory (alpha= 1/2 to 3/4). Such calculations of thermal conductivity may become important for describing nanoscale devices. As a first step in studying such systems, we examined the mass effects on the thermal conductivity of compound systems, finding that the layered system has a lower conductivity than the uniform system.
Plastic shear significantly reduces the phase transformation (PT) pressure when compared to hydrostatic conditions. Here, a paradoxical result was obtained: PT of graphitelike hexagonal boron nitride (hBN) to superhard wurtzitic boron nitride under pressure and shear started at about the same pressure ( approximately 10 GPa) as under hydrostatic conditions. In situ x-ray diffraction measurement and modeling of the turbostratic stacking fault concentration (degree of disorder) and PT in hBN were performed. Under hydrostatic pressure, changes in the disorder were negligible. Under a complex compression and shear loading program, a strain-induced disorder was observed and quantitatively characterized. It is found that the strain-induced disorder suppresses PT which compensates the promotion effect of plastic shear. The existence of transformation-induced plasticity (TRIP) was also proved during strain-induced PT. The degree of disorder is proposed to be used as a physical measure of plastic straining. This allows us to quantitatively separate the conventional plasticity and transformation-induced plasticity. Surprisingly, it is found that TRIP exceeds the conventional plasticity by a factor of 20. The cascade structural changes were revealed, defined as the reoccurrence of interacting processes including PTs, disordering, conventional plasticity, and TRIP. In comparison with hydrostatic loading, for the same degree of disorder, plastic shear indeed reduces the PT pressure (by a factor of 3-4) while causing a complete irreversible PT. The analytical results based on coupled strain-controlled kinetic equations for disorder and PT confirm our conclusions.
In this work, we perform atomistic simulations to study the phase transformations (PT) in graphite under compression. Our major findings are: (1) when the compression is parallel to the basal plane, graphite layers buckle, kink bands form, and then the diamond nucleates at the intersection of kink bands; the initially introduced dislocations block the graphite layer slippage and promote the graphite-to-diamond PT; (2) instead, when the sample is compressed normal to the basal plane, no buckling is observed, and in this situation, the pre-existing dislocations delay the structure change; and (3) the PT is found to be controlled by local stresses from which a criterion can be formulated for detecting the graphite lattice instability. Despite the limited length scales in our atomistic models, the above results may support the search for new routes to fabricate artificial diamonds at a significantly less cost than that required by traditional techniques.
Since their modern debut in 2004, 2-dimensional (2D) materials continue to exhibit scientific and industrial promise, providing a broad materials platform for scientific investigation, and development of nano- A nd atomic-scale devices. A significant focus of the last decade's research in this field has been 2D semiconductors, whose electronic properties can be tuned through manipulation of dimensionality, substrate engineering, strain, and doping (Mak et al 2010 Phys. Rev. Lett. 105 136805; Zhang et al 2017 Sci. Rep. 7 16938; Conley et al 2013 Nano Lett. 13 3626-30; Li et al 2016 Adv. Mater. 28 8240-7; Rhodes et al 2017 Nano Lett. 17 1616-22; Gong et al 2014 Nano Lett. 14 442-9; Suh et al 2014 Nano Lett. 14 6976-82; Yoshida et al 2015 Sci. Rep. 5 14808). Molybdenum disulfide (MoS 2 ) and tungsten diselenide (WSe 2 ) have dominated recent interest for potential integration in electronic technologies, due to their intrinsic and tunable properties, atomic-scale thicknesses, and relative ease of stacking to create new and custom structures. However, to go 'beyond the bench', advances in large-scale, 2D layer synthesis and engineering must lead to 'exfoliation-quality' 2D layers at the wafer scale. This roadmap aims to address this grand challenge by identifying key technology drivers where 2D layers can have an impact, and to discuss synthesis and layer engineering for the realization of electronic-grade, 2D materials. We focus on three fundamental areas of research that must be heavily pursued in both experiment and computation to achieve high-quality materials for electronic and optoelectronic applications.
Hexagonal diamond, a potentially superhard material, forms from a glassy carbon precursor at pressures of ∼100 GPa at the relatively low temperature of 400 °C. The formation mechanism of the hexagonal diamond phase was investigated by performing microstructural analysis on cross-sections of the recovered samples. Three distinct structures have been observed, a graphitic region near the centre of the sample with low density, a hexagonal diamond region at the edge of the sample with high density, and a mixed region containing significant proportions of both the graphitic structure and hexagonal diamond. The hexagonal diamond was more likely to occur at greater radial distance from the centre of the sample with some evidence for greater amounts also near the diamond anvil faces. The observed distribution of the hexagonal phase correlates well to regions of greatest shear strain expected from modelling studies of strain fields in diamond anvil cells. The findings support the proposition that shear strain plays an important role in the formation of hexagonal diamond, and that it may be a driving force for the natural occurrence of hexagonal diamond in the shear zone of meteorite impact craters.
The variations of the structure and properties of the diamond-like materials is now of high interest because diamond-like carbon phases has a wide range of commercial applications, especially as the protective coatings with its unusual mechanical properties and strength close to the strength of the diamond. Here the deformation behavior of the stable diamond-like phases is carefully analyzed by molecular dynamics simulation. Five stable structures (A3, A7, A8, A9 and B) based on the fullerene-like molecules are considered. The structural transformations during hydrostatic tension and compression are investigated to understand the deformation mechanisms. Deformation at hydrostatic tension can be divided into three regimes depending on the deformation mechanism. It is found that diamond-like structures can be hydrostatically compressed until the diamond densities, but most of them are losing crystalline order and transform to the amorphous state. Thermal fluctuations decrease the critical strain values at hydrostatic tension but have almost no effect on critical compression strain.
Among post-graphene two dimensional (2D) materials, transition metal dichalcogenides (TMDs, such as MoS2) have attracted significant attention due to their superior properties for potential electronic, optoelectronic and energy applications. Scalable and controllable powder vapor transport (PVT) methods have been developed to synthesize 2D MoS2 with controllable morphologies (i.e. horizontal and vertical), yet the growth mechanism for the transition from horizontal to vertical orientation is not clearly understood. Here, we combined experimental and numerical modeling studies to investigate the key growth parameters that govern the morphology of 2D materials. The transition from vertical to horizontal growth is achieved by controlling the magnitude and distribution of the precursor concentration by placing the substrate at different orientations and locations relative to the source. We have also shown that the density of as-grown nanostructures can be controlled by the local precursor-containing gas flow rate. This study demonstrates the possibility for engineering the morphology of 2D materials by controlling the concentration of precursors and flow profiles, and provides a new path for controllable growth of 2D TMDs for various applications.
Understanding nucleation and growth of two-dimensional (2D) and layered materials is a challenging topic due to the complex van der Waals interactions between layers and substrate. The morphology of 2D materials is known vary depending on experimental conditions. For the case of MoS2, the morphology has been shown to vary from rounded (molybdenum rich) domains to equilateral triangular (sulfur rich) domains. These different morphologies can result in drastically different properties, which can be exploited for applications in catalytic reactions, digital electronics, optoelectronics, and energy storage. Powder vaporization (PV) synthesis of molybdenum disulfide (MoS2) can yield vertical domains, however, these domains are often ignored when the morphology evolution of MoS2 is discussed, thereby completely omitting a major part of the impact of the Mo:S ratio to the growth mode of MoS2 during PV. Combining experimental and numerical simulation methods, we reveal a vertical-to-horizontal growth mode transition for MoS2 that occurs in the presence of a molybdenum oxide partial pressure gradient. Transmission electron microscopy reveals that the growth of vertical MoS2 results from initial seeding of single crystalline molybdenum dioxide, followed by sulfurization from the substrate upward to form vertically oriented MoS2 domains.
Mechanical properties of nanocrystals are influenced by atomic defects. Here, we demonstrate the effect of planar defects on the mechanics of ZnO nanorods using atomic force microscopy, high resolution transmission electron microscopy, and large scale atomistic simulation. We study two different conditionally grown single nanorods. One contains extended $I_{1}$ type stacking fault (SF) and another is defect free. The SF containing nanorods show buckling behaviors with reduced critical loading, whereas the other kinds show linear elastic behavior. We also studied the size dependence of elastic modulus and yield strength. The elastic modulus in both nanorods is inversely proportional to their size. Similar trend is observed for yield strength in the SF containing nanorods, however, the opposite is observed in the SF free nanorods. This first experimental and theoretical study will guide towards the development of reliable electromechanical devices.
Reducing the dimensions of materials to atomic scales results in a large portion of atoms being at or near the surface, with lower bond order and thus higher energy. At such scales, reduction of the surface energy and surface stresses can be the driving force for the formation of new low-dimensional nanostructures, and may be exhibited through surface relaxation and/or surface reconstruction, which can be utilized for tailoring the properties and phase transformation of nanomaterials without applying any external load. Here we used atomistic simulations and revealed an intrinsic structural transformation in monolayer materials that lowers their dimension from 2D nanosheets to 1D nanostructures to reduce their surface and elastic energies. Experimental evidence of such transformation has also been revealed for one of the predicted nanostructures. Such transformation plays an important role in bi-/multi-layer 2D materials.
The influence of nonhydrostatic compression of graphite on the effective driving force of the graphite → dense phase transformations was analyzed. The martensitic mechanisms and direct diffusion type transformation of hexagonal (2H) and rhombohedral (3R) modifications into lonsdaleite and diamond were considered. The comparison of driving forces were made for hydrostatic, uniaxial compression and for compression with shear. It was shown that the combination of hydrostatic and uniaxial compression is preferential for martensitic transition 2H graphite into lonsdaleite; the oneaxial compression for 3R graphite diamond transformation, and compression with the shear for 3R graphite lonsdaleite transformation.
As a potential building block for the next generation of devices or
multifunctional materials that are spreading almost every technology sector,
one-dimensional (1D) carbon nanomaterial has received intensive research
interests. Recently, a new ultra-thin diamond nanothread (DNT) has joined this
palette, which is a 1D structure with poly-benzene sections connected by
Stone-Wales (SW) transformation defects. Using large-scale molecular dynamics
simulations, we found that this sp3 bonded DNT can transit from a brittle to a
ductile characteristic by varying the length of the poly-benzene sections,
suggesting that DNT possesses entirely different mechanical responses than
other 1D carbon allotropies. Analogously, the SW defects behave like a grain
boundary that interrupts the consistency of the poly-benzene sections. For a
DNT with a fixed length, the yield strength fluctuates in the vicinity of a
certain value and is independent of the "grain size". On the other hand, both
yield strength and yield strain show a clear dependence on the total length of
DNT, which is due to the fact that the failure of the DNT is dominated by the
SW defects. Its highly tunable ductility together with its ultra-light density
and high Young's modulus makes diamond nanothread ideal for creation of
extremely strong three-dimensional nano-architectures.
An advanced three-phase phase-�eld approach (PFA) is suggested for a non-equilibrium phase interface
which contains an intermediate phase, in particular, a solid-solid interface with a nanometersized
intermediate melt (IM). Thermodynamic potential in the polar order parameters is developed,
which satis�es all thermodynamic equilibrium and stability conditions. Special form of the gradient
energy allowed us to include the interaction of two solid-melt interfaces via intermediate melt and
obtain a well-posed problem and mesh-independent solutions. It is proved that for stationary 1D
solutions to two Ginzburg-Landau equations for three phases, the local energy at each point is equal
to the gradient energy. Simulations are performed for � $ � phase transformations (PTs) via IM
in HMX energetic material. Obtained energy - IM width dependence is described by generalized
force-balance models for short- and long-range interaction forces between interfaces but not far
from the melting temperature. New force-balance model is developed, which describes phase �eld
results even 100K below the melting temperature. The e�ects of the ratios of width and energies
of solid-solid and solid-melt interfaces, temperature, and the parameter characterizing interaction
of two solid-melt interfaces, on the structure, width, energy of the IM and interface velocity are
determined by �nite element method. Depending on parameters, the IM may appear by continuous
or discontinuous barrierless disordering or via critical nucleus due to thermal
uctuations.
The IM may appear during heating and persist during cooling at temperatures well below than it
follows from sharp-interface approach. On the other hand, for some parameters when IM is expected,
it does not form, producing an IM-free gap. The developed PFA represents a quite general
three-phase model and can be extended to other physical phenomena, such as martensitic PTs,
surface-induced premelting and PTs, premelting/disordering at grain boundaries, and developing
corresponding interfacial phase diagrams.
To make practical the molecular dynamics simulation of large scale reactive chemical systems (1000s of atoms), we developed ReaxFF, a force field for reactive systems. ReaxFF uses a general relationship between bond distance and bond order on one hand and between bond order and bond energy on the other hand that leads to proper dissociation of bonds to separated atoms. Other valence terms present in the force field (angle and torsion) are defined in terms of the same bond orders so that all these terms go to zero smoothly as bonds break. In addition, ReaxFF has Coulomb and Morse (van der Waals) potentials to describe nonbond interactions between all atoms (no exclusions). These nonbond interactions are shielded at short range so that the Coulomb and van der Waals interactions become constant as R-ij --> 0. We report here the ReaxFF for hydrocarbons. The parameters were derived from quantum chemical calculations on bond dissociation and reactions of small molecules plus heat of formation and geometry data for a number of stable hydrocarbon compounds. We find that the ReaxFF provides a good description of these data. Generally, the results are of an accuracy similar or better than PM3, while ReaxFF is about 100 times faster. In turn, the PM3 is about 100 times faster than the QC calculations. Thus, with ReaxFF we hope to be able to study complex reactions in hydrocarbons.
Amorphous carbon is one of the most lubricious materials known, but the mechanism is not well understood. It is counterintuitive that such a strong covalent solid could exhibit exceptional lubricity. A prevailing view is that lubricity of amorphous carbon results from chemical passivation of dangling bonds on surfaces. Here we show instead that lubricity arises from shear induced strain localization, which, instead of homogeneous deformation, dominates the shearing process. Shear localization is characterized by covalent bond reorientation, phase transformation and structural ordering preferentially in a localized region, namely tribolayer, resulting in shear weakening. We further demonstrate an anomalous pressure induced transition from stick-slip friction to continuous sliding with ultralow friction, due to gradual clustering and layering of graphitic sheets in the tribolayer. The proposed shear localization mechanism sheds light on the mechanism of superlubricity, and would enrich our understanding of lubrication mechanism of a wide variety of amorphous materials.
One-dimensional (1D) zinc oxide nanostructures are the main components of nanogenerators and central to the emerging field of nanopiezotronics. Understanding the underlying physics and quantifying the electromechanical properties of these structures, the topic of this research study, play a major role in designing next-generation nanoelectromechanical devices. Here, atomistic simulations are utilized to study surface and size-scale effects on the electromechanical response of 1D ZnO nanostructures. It is shown that the mechanical and piezoelectric properties of these structures are controlled by their size, cross-sectional geometry, and loading configuration. The study reveals enhancement of the piezoelectric and elastic modulus of ZnO nanowires (NW) with diameter d > 1 nm, followed by a sudden drop for d < 1 nm due to transformation of NWs to nanotubes (NTs). Degradation of mechanical and piezoelectric properties of ZnO nanobelts (NBs) followed by an enhancement in piezoelectric properties occurs when their lower dimension is reduced to <1 nm. The latter enhancement can be explained in the context of surface reconfiguration and formation of hexagon-tetragon (HT) pairs at the intersection of (21[combining macron]1[combining macron]0) and (011[combining macron]0) planes in NBs. Transition from a surface-reconstructed dominant to a surface-relaxed dominant region is demonstrated for lateral dimensions <1 nm. New phase-transformation (PT) kinetics from piezoelectric wurtzite to nonpiezoelectric body-centered tetragonal (WZ → BCT) and graphite-like phase (WZ → HX) structures occurs in ZnO NWs loaded up to large strains of ∼10%.
A generalization of a virial theorem for a bounded system is given. Also a detailed discussion starting from a virial theorem of the derivation of a microscopic expression of the stress tensor is presented. Two applications are briefly indicated.
Diamonds were reproducively synthesized from graphite in the presence of Na2SO4, MgSO4, CaSO4\cdot1/2H2O, Mg(OH)2 and Ca(OH)2 at high pressure of 7.7 GPa and a temperature of 2150°C. Although starting graphite was completely transformed to diamond in the presence of the sulfates or hydroxides, no transformation to diamond could be detected from graphite only at the same pressure and temperature condition. Therefore, we concluded that sulfates and hydroxides have a strong catalytic effect on the transformation of graphite to diamond.
The dependence of the pressure threshold of martensitic transformations on the disordering degree of starting structures is considered for graphite-like BN into diamond-like BN modifications transitions. The effect of loading conditions on transformation mechanisms of rhombohedral BN into zinc blende or wurtzite modifications is analyzed also. Analytical relations obtained allow to explain the experimental data and to predict a behavior of various graphite-like structures under different p, T conditions.
In molecular dynamics, the pressure in a homogeneous system in equilibrium may be calculated by two different methods. The first is based on the virial theorem of Clausius and gives the pressure at the boundary of the system. The second is based on the notion of stress, which is the sum of the appropriate components of the interatomic forces intercepted by an area, and of the components of momentum flux across the area, averaged over the area and over time. We show by means of a detailed comparison of the forces involved that the two methods are equivalent in the thermodynamic limit. In a small system with arbitrary boundary conditions, the neglect of a part of the interactions between the system and the wall results in some error in the pressure calculated by the virial method. In the special case of a system with periodic boundaries, there is no external ''wall,'' and the internal pressures calculated by the two methods are the same. However, with comparable effort in computation, the stress method makes more efficient use of the data and yields a result of greater precision than does the virial method. In a system not in equilibrium or not homogeneous, the stress method remains valid but the virial method leads to ambiguous results. These considerations indicate that the method of stress calculation is more general than the virial method.
The size scale effect on the piezoelectric response of bulk ZnO and ZnO nanobelts has been studied using molecular dynamics simulation. Six molecular dynamics models of ZnO nanobelts are constructed and simulated with lengths of 150.97 Å and lateral dimensions ranging between 8.13 and 37.37 Å. A molecular dynamics model of bulk ZnO has also been constructed and simulated using periodic boundary conditions. The piezoelectric constants of the bulk ZnO and each of the ZnO nanobelts are predicted. The predicted piezoelectric coefficient of bulk ZnO is 1.4 C m−2, while the piezoelectric coefficient of ZnO nanobelts increases from 1.639 to 2.322 C m−2 when the lateral dimension of the ZnO NBs is reduced from 37.37 to 8.13 Å. The changes in the piezoelectric constants are explained in the context of surface charge redistribution. The results give a key insight into the field of nanopiezotronics and energy scavenging because the piezoelectric response and voltage output scale with the piezoelectric coefficient.
We propose a bond order potential for carbon with built-in long-range interactions. The potential is defined as the sum of an angular and coordination dependent short-range part accounting for the strong covalent interactions and a radial long-range part describing the weak interactions responsible, e.g., for the interplanar binding in graphite. The short-range part is a Brenner type of potential, with several modifications introduced to get an improved description of elastic properties and conjugation. Contrary to previous long-range extensions of existing bond order potentials, we prevent the loss of accuracy by compensating for the additional long-range interactions by an appropriate parametrization of the short-range part. We also provide a short-range bond order potential. In Monte Carlo simulations our potential gives a good description of the diamond to graphite transformation. For thin (111) slabs graphitization proceeds perpendicular to the surface as found in ab initio simulations, whereas for thick layers we find that graphitization occurs layer by layer.
A potential function is presented that can be used to model both chemical reactions and intermolecular interactions in condensed-phase hydrocarbon systems such as liquids, graphite, and polymers. This potential is derived from a well-known dissociable hydrocarbon force field, the reactive empirical bond-order potential. The extensions include an adaptive treatment of the nonbonded and dihedral-angle interactions, which still allows for covalent bonding interactions. Torsional potentials are introduced via a novel interaction potential that does not require a fixed hybridization state. The resulting model is intended as a first step towards a transferable, empirical potential capable of simulating chemical reactions in a variety of environments. The current implementation has been validated against structural and energetic properties of both gaseous and liquid hydrocarbons, and is expected to prove useful in simulations of hydrocarbon liquids, thin films, and other saturated hydrocarbon systems. © 2000 American Institute of Physics.
The conditions and mechanism of formation of nano-polycrystalline diamonds directly from graphite and non-graphitic carbon (carbon black, glassy carbon, C60 and carbon nanotubes) at high pressure and high temperature have been investigated. The onset temperature for diamond formation at P≥q 15 GPa is 1500–1600 °C for all carbon materials, although the required temperature conditions for pure polycrystalline diamond are T≥q 2200 °C for graphite and T≥q 1600 °C for non-graphitic carbon. Polycrystalline diamond forms as a result of simultaneous diffusion and two-step martensitic processes from graphite, whereas it forms only due to diffusion without graphitization or formation of intermediate phases from non-graphitic carbon. Nano-polycrystalline diamonds consisting only of very fine particles (<10 nm in size) can be obtained from non-graphitic carbon at T 1600–2000 °C under pressures≥q15 GPa.
Uniaxial stress measurements are reported on two commonly occurring radiation damage centres in diamond. Their zero-phonon lines, at 17380 and 23530 cm-1; are shown to be electric-dipole transitions between E (ground) and A1, or A2, (excited) states at trigonal centres (C3, C3v of D3d). Other non-degenerate states are found approximately 100 cm-1 above the ground level. The data are consistent with dynamic Jahn-Teller distortions occurring in the ground states of the centres. On bringing together information on all the known degenerate electronic states at trigonal centres in diamond, two simple generalisations emerge for the occurrence of Jahn-Teller effects at these centres.
The Open Visualization Tool (OVITO) is a new 3D visualization software designed for post-processing atomistic data obtained from molecular dynamics or Monte Carlo simulations. Unique analysis, editing and animations functions are integrated into its easy-to-use graphical user interface. The software is written in object-oriented C++, controllable via Python scripts and easily extendable through a plug-in interface. It is distributed as open-source software and can be downloaded from the website http://ovito.sourceforge.net/.
We have used transmission electron microscopy to study the initial stages of diffusionless formation of lonsdaleite and diamond crystals when pyrolytic graphite is subjected to static compression. We consider the effect of plastic deformation of the matrix phase on development of the transformation. We propose a dislocation model for nucleation of dense phases, making it possible to explain the reason for the formation of a metastable lonsdaleite phase, the nature of its structural disordering, and also the possibility of diffusionless nucleation of diamond directly from hexagonal graphite.
A detailed analysis of in situ optical measurements of elastic scattering of light and reflectivity during diamond film synthesis is presented. From the results we propose a growth kinetic model for the very first stages when isolated tiny particles are formed and until they coalesce to form a continuous diamond film.
Although transition metals such as Fe, Co, Ni and their alloys have been used as diamond-producing solvent-catalysts under very high temperature and pressure, non-metallic compounds such as carbonates and oxides have also been claimed in the patents as the catalysts. In the present study, to make clear the catalytic effect of carbonates on the formation of diamond, high pressure experiments were carried out in the mixture of graphite and the carbonates of Li, Na, Mg, Ca and Sr. Diamond could reproducibly be synthesized from graphite in the presence of these carbonates at high pressure and temperature of 7.7 GPa and 2150°C. Although starting graphite was completely transformed to diamond in the presence of the carbonates, no transformation to diamond could be detected from graphite only at the same pressure and temperature condition. Therefore, it can be concluded that the carbonates have strong solvent-catalytic effect on the transformation of graphite to diamond.