In recent years, graphene and other two-dimensional (2D) materials, such as graphene allotrope, silicene, boron nitride, molybdenum disulfide, have drawn great research interest. Using molecular dynamics (MD) simulations, we studied the mechanical properties and fracture behavior of some 2D materials, including graphene, graphene allotrope, and silicene. We found that fracture strength of single-crystalline graphene greatly depends on the tension direction, with the zigzag direction having higher fracture strength than that of the armchair direction. In contrast, this anisotropic fracture behavior is not obvious in polycrystalline graphene due to the random distribution of lattice orientation in different grains. However, polycrystalline graphene has much lower fracture strength. The smaller the average grain size in polycrystalline graphene, the lower the fracture strength. The fracture strength of polycrystalline graphene with grain size of 10 nm is only about 30% of single-crystalline graphene. We also studied the mechanical properties and fracture behavior of four graphene allotropes: α-, β-, γ-, and 6612-graphyne. It is found that the presence of acetylenic linkages in graphynes leads to a significant reduction in fracture stress and Young’s modulus with the degree of reduction being proportional to the percentage of the acetylenic linkages. This deterioration in mechanical properties stems from the low atom density in graphynes and weak single bonds in the acetylenic linkages where the facture is initiated.

The conduction of heat in two dimensions displays a wealth of fascinating phenomena of key relevance to the scientific and technological applications of novel layered materials. Here, we use third order density-functional perturbation theory and an exact, variational solution of the Boltzmann transport equation to study fully from first-principles phonon transport and heat conductivity in graphene and related materials (boron nitride, functionalized derivatives, transition-metal dichalcogenides...). Very good agreement is obtained with experimental data, where available, together with a microscopic understanding of the collective character of heat-carrying excitations, and the unusual length scales involved. Last, and at variance with typical three dimensional solids, normal processes dominate over Umklapp scattering well above cryogenic conditions, extending to room temperature and more. As a result, novel hydrodynamics regimes, hitherto typically confined to ultra-low temperatures, become readily apparent.

Silk fibroin has attracted great attention due to its superior mechanical properties such as ultra-high strength and stretchability, biocompatibility, as well as its versatile biodegradability and processability. It can be made into various morphologies such as sponges, hydrogels, films, mats and particles, facilitate their wide applications as apparel/medical textiles, surgical sutures, tissue engineering scaffolds, drug/gene carriers, optics, sensors, etc. Great efforts are demanded in order to further enhance the mechanical properties of silk fibroin. In this study, large scale molecular dynamics simulations were carried out on interactions between graphene substrate and model peptides with different sizes extracted from different domains of silk fibroin. The simulation result on secondary structure component of peptides agrees well with the experimental data. Our study shows that graphene substrate has different impact on structural properties of different domains of silk fibroin. Tensile tests were also carried out on representative peptides to measure the mechanical properties of the peptides related to strength and resilience. It was found that the strength of the peptides are enhanced upon adsorption to the graphene surface. These results provide in-depth understandings in molecular structure-mechanical property correlation of protein upon adsorption to the substrate, and will be significant help to future design of bio-inspired materials.

Graphene is a promising candidate for the novel carbon-based nanoelectronic devices primarily due to its extremely high carrier mobility at room temperature. For device fabrication, however, graphene needs to be placed on top of an insulating substrate and the interactions between graphene and the underlying substrate can substantially change the carrier mobility and concentration of the adsorbed graphene layer, having strong influence on its electrical performances. Therefore, it is of great importance to develop a detailed understanding regarding the electronic property changes of graphene via dopant and the relevant induced point defects in the underlying substrates to control or further improve its performance. In this presentation, we will introduce our recent first-principles study on the effects of the Sn-doped SiO₂ substrate on the electronic structure changes of monolayer graphene. Unlike boron and phosphorus, Sn atoms are possible to form substitutional point defects, Sn metal clusters, and Sn oxides in the SiO₂ substrate. Using ab initio molecular dynamic simulations, we have identified several possible point defects induced by Sn-doping, and generated the realistic structure models of the Sn metal/oxide clusters embedded in the SiO₂ matrix. Our calculated results show that the threefold Sn-E’ center is the only point defect that can induce charge transfer between graphene and the SiO₂ substrate. Although the fourfold and fivefold coordinated Sn atoms cannot cause any doping effect, they were found to induce charge transfer from graphene to the underlying substrate as they grow into Sn oxide clusters due to the lowering of the Sn-O antibonding states in the SiO₂ band gap. For Sn metal clusters, they were able to induce sizable p-type doping on graphene monolayer. However, this doping effect may become smaller as the size of metal clusters increases, and eventually turn into n-type doping as the β-Sn bulk structure is formed.

2D materials are well-known to exhibit interesting phenomena due to quantum confinement effects. In this work, we show that quantum confinement, together with structural anisotropy in 2D black phosphorus thin films, results in an electric-field induced formation of a Dirac cone in black phosphorus thin films below a critical thickness. Using density functional theory calculations, we show that an electric field, E_ext, applied normal to a 2D black phosphorus thin film, can reduce the direct band gap of few-layer black phosphorus, resulting in an insulator-to-metal transition at a critical field, E_c. Interestingly, we find that increasing E_extbeyond E_c can induce a Dirac cone in the system, and the position of the Dirac point in the Brillouin zone can be tuned by the magnitude of the field. The resulting Fermi velocities are similar in magnitude to that in graphene. We show that the Dirac cone arises from a highly anisotropic interaction term between the frontier orbitals that are spatially separated due to the applied field, on different halves of the 2D slab. The anisotropy of this interaction term stems from the structural anisotropy in black phosphorus layers, and explains why such a topological phase transition has not been observed in other more isotropic layered materials such as transition metal dichalcogenides. Furthermore, it becomes more difficult to achieve the Dirac cone when the black phosphorus film becomes so thick that the relevant interaction term becomes vanishingly small. The formation of the Dirac cone, and the associated topological phase transition, is therefore a direct result of quantum confinement and structural anisotropy in 2D black phosphorus.

Few-layer black phosphorus (BP), obtained from bulk BP by exfoliation, is an emerging candidate as a channel material in post-silicon electronics. A deep understanding of its physical properties and its full range of applications are still being uncovered. In this paper, we present theoretical and experimental investigations of phonon properties in few-layer BP, focusing on the low-frequency regime corresponding to interlayer vibrational modes. We show that the interlayer breathing mode A³_g shows a large redshift with increasing thickness; the experimental and theoretical data points agreeing very well. This thickness dependence is two times larger than that in the chalcogenide materials such as few-layer MoS₂ and WSe₂, because of the significantly larger interlayer force constant and smaller atomic mass in BP.  The derived interlayer out-of-plane force constant is about 50% larger than that in graphene and MoS₂. We show that this large interlayer force constant arises from significant covalent interaction between phosphorus atoms in adjacent layers, and that interlayer interactions are not merely of the weak van der Waals type. These significant interlayer interactions are consistent with the known surface reactivity of BP, and have been shown to be important for electric-field induced formation of Dirac cones in thin film BP.  

MoS₂, a prototypical layered dichalcogenide material, is highly regarded as a potential channel material in post-silicon electronics. Recent experiments have shown that the performance of MoS₂ based field effect transistors (FET) is largely limited by the non-trivial contact resistance at the metal-MoS₂ interface.  Here, we show, using experimental measurements on multiple FETs as well as first principles calculations, that adding graphene as an interlayer between Nickel and MoS₂ can significantly reduce the Schottky barrier at the interface, and reduce the contact resistance by 20-fold.[1]  In this presentation, we discuss in detail the origin of the reduction in contact resistance, showing that this effect is primarily due to the significantly smaller work function of graphene-covered Ni compared to bare Ni, which reduces the Schottky barrier height. The Schottky barrier height of the Ni-Graphene-MoS₂ interface is determined by charge transfer and the resulting interface dipole between Ni-Graphene and MoS₂. This is in contrast to Fermi level pinning by metal-induced gap states for bare metal electrodes.  Further improvement in the contact resistance can be achieved by using graphene edge contacts to Ni, which arises from stronger coupling rather than further reduction in the Schottky barrier. Our results are a significant step forward for the application of MoS₂ as a mainstream channel material in electronic circuits.

[1]: ACS Nano 9, 869 (2015)

Using density functional theory calculations, we have investigated Li adsorption and diffusion mechanisms on functionalized graphene nanoribbons (GNRs) and explored the origins of their enhanced storage capacity for the anode of Li-ion batteries.  Here we have investigated the Li storage behaviors of various types of functional groups located at the edge, including -H, hydroxyl, carbonyl and pyrone groups, as well as those lie on the basal plane like hydroxyl and epoxy groups within different levels of lithiation and functionalization on the armchair and zigzag nanoribbons, respectively.  For functional groups terminating the edge, we found that only carbonyl and pyrone groups can effectively enhance Li adsorption on GNRs, and the most favorable sites for Li adsorption turn out to be these edge-oxidized groups rather than the hollow sites on the basal plane.  Accordingly, Li ion was found to diffuse readily toward the edge without sizable energy barriers as compared to that on a graphene sheet.  As for the functional groups located on the basal plane, the hydroxyl group was found to induce p-type doping on GNRs, which in turn leads to the increased Li binding energy on the neighboring hollow sites.  Similarly, the epoxy group also exhibits it capability to enhance Li adsorption on GNRs, thereby increasing Li diffusion energy barrier on the basal plane in comparison with that on a graphene sheet/edge-terminated GNRs.  Very interestingly, these two functional groups can serve as the nucleation centers for Li clustering (Li_nOH/Li_nO, n=2~4), which can effectively uplift the Li storage capacity of GNRs.  Our calculations further show that these Li clusters can undergo diffusion in a concerted way with a diffusion energy barrier increasing with the cluster size.  Nevertheless, the diffusion energy barriers of these Li clusters are all smaller than that of a Li ion neighboring to the functional groups on GNRs.

γTi-Al alloy and Al₂O₃ have great potential applications in the fields of aerospace engineering and mechanical manufacturing industry, in which the main damage are caused by contact and wear. In this study, we have performed large-scale molecular dynamics simulations of nano-indentation and wear on γTi-Al alloy and Al₂O₃ to investigate their mechanical properties as well as the wear (both sliding and rolling)mechanism. Our simulation results show that the Young’s modulus of γTi-Al alloy calculated from nano-indentation is about 153.9 GPa, which is in good agreement with the experimental value of 165.0 GPa. In addition, the sliding wear simulation results of γTi-Al reveal that the material loss during sliding is linearly proportional to the sliding distance and the normal load, which is consistent with the empirical Archard’s law. It also is observed that the atoms worn from the γTi-Al substrate are split directly by the indenter tip and then piled up to beside the indenter tip. We are also working on the investigating of the crack propagation mechanism during wear process when initial defects exist in γTi-Al and Al₂O₃. We hope the present work could provide significant impact on the fundamental understandings of the mechanical property and wear mechanism of γTi-Al material and Al₂O₃ at the nano-scale.

In the present work, tensile strength and fracture behaviour of ultrafine grained (UFG) Al-Mg-Si has been simulated by Xtended Finite Element Method (XFEM). The UFG Al alloy was produced from its bulk alloy by cryogenic rolling.The 6061 Al alloy was produced with different amounts of strain to different thickness reductions(25%,50%,75%,85%). Electron Back Scatter Diffraction (EBSD) characterizations of the deformed samples were made to substantiate the formation of ultrafine grains in the bulk alloy. Micro indentation measurements of the deformed alloys were made to obtain its hardness and young's modulus of elasticity. Tensile and fracture toughness of the deformed samples were measured as per ASTM standards. Fractographic study was performed using Scanning Electron Microscope (SEM) to substantiate the failure by fracture in tensile test samples and 3 point bend test samples.The measured elastic modulus,yield strength, hardness and fracture toughness were used in the XFEM simulation to investigate the size dependent elastic-plastic deformation behavior of UFG Al alloys. Ramberg Osgood material model has been used to simulate the tensile behaviour of UFG Al alloy with the materials constants evaluated from experimentally measured tensile properties. The simulated tensile and fracture properties are compared with the experimental results. Ultrafine grained Al alloy has shown higher tensile and fracture strength as compared to bulk Al alloy as evident from the XFEM simulation and experimental studies in the present work.

316L Stainless Steel (SS) is widely used in biomedical implants and other surgical instruments. With the advancement of new materials like Ti, Ni, Co and their alloys, the use of 316L SS is being confined due to the corrosion activity in the human body. There are various techniques used for the corrosion resistance of 316L SS like addition of assorted non-corrosive metals, electrochemical polishing, brushing, heat treatment, mechanical working. The use of powder injection molded 316L SS sintered parts is preferred over all corrosion resistance activities due to more dimensional accuracy, shape complicity and low cost. The key objective of this study is to investigate the effects of sintered atmospheres i.e. H₂, N₂ and mixture of bothon 316L stainless steel to increase the stability, performance and biocompatibility of material for biomedical applications. The study is focused to determine the corrosion kinetics in Hanks’ balanced salt solution using weight loss method and electrochemical polarization resistance method. The kinetic model is also developed with the help of using electrochemical impedance spectroscopy (EIS). Experimental results suggest that the N₂ shows corrosion resistance high at 0.113mpy as compare to H₂ and mixture of both at 2.372 and 0.791 mpy respectively. At (98%) density, the tensile strength of N₂ sintered 316L stainless steel is 711.2 MPa which is higher than H₂ and mixture of H₂&N₂. SEM micrograph and EDX also suggest that N₂ sintered samples have protective covering while limited in other environments.