Nano-indentation of circular graphene flakes are studied by using Molecular Dynamics simulation. We show that the theory of continuum elasticity based on nonlinear Föpple-Hencky equations is applicable on a circular suspended graphene flake. Our simulation results are plenty compatible to those results calculated by the nonlinear elasticity theory. We find the force-displacement curves in good agreement with the recent experimental measurements and conclude they are temperature independent. In addition, we find the vibration frequency for such a system and monitor that it behaves as a sinusoidal manner at small circular graphene size.
Nanoindentation is an effective way of finding mechanical properties at nanoscale. They are especially useful for thin films where elimination of the substrate effect is required. The mechanism is based upon depth sensing indentation based on Oliver and Pharr modeling. Nanoindentation was used to evaluate the nanomechanical and fractographic properties of hard coatings deposited by magnetron sputtering. Radial and lateral cracks accompanied by delamination of the coatings were observed during the indentation tests. A novel phenomenon of stress induced crystallization was observed in case of th…
Coupling of nano-indentation and crystal plasticity finite element (CPFE) simulations is widely used to quantitatively probe the small-scale mechanical behaviour of materials. Earlier studies showed that CPFE can successfully reproduce the load-displacement curves and surface morphology for different crystal orientations. Here, we report the orientation dependence of residual lattice strain patterns and dislocation structures in tungsten. For orientations with one or more Burgers vectors close to parallel to the sample surface, dislocation movement and residual lattice strains are confined to …
Graphene nano-flakes (GNFs) are predicted to host spin-polarized metallic edge states, which are envisioned for exploration of spintronics at the nanometer scale. To date, experimental realization of GNFs is only in its infancy because of the limitation of precise cutting or synthesizing methods at the nanometer scale. Here, we use low temperature scanning tunneling microscope (STM) to manipulate coronene molecules on a Cu(111) surface to build artificial triangular and hexagonal GNFs with either zigzag or armchair type of edges. We observe that the metallic edge states only exist in the GNFs …
We investigate the electronic transport properties of a bilayer graphene flake contacted by two monolayer nanoribbons. Such a finite-size bilayer flake can be built by overlapping two semiinfinite ribbons or by depositing a monolayer flake onto an infinite nanoribbon. These two structures have a complementary behavior, that we study and analyze by means of a tight-binding method and a continuum Dirac model. We have found that for certain energy ranges and geometries, the conductance of these systems oscillates markedly between zero and the maximum value of the conductance, allowing for the des…
The yield surface in crystal plasticity can be approached from various directions during mechanical loading. We consider the competition between nanoindentation and tensile loading towards plastic yielding. For this purpose, we develop a two-dimensional discrete dislocation model that is then utilized to investigate the hardness and pop-in event statistics during nanoindentation of single crystal under tensile pre-stress. Indentation is performed by using cylindrical (circular in 2D) indentation with varying radius and under both displacement and load control. Tensile in-plane stress, varying …
In this paper, we investigate the residual deformation field in the vicinity of nano-scratch tests using two orientations of a Berkovich tip on an (001) Cu single crystal. We compare the deformation with that from indentation, in an attempt to understand the mechanisms of deformation in tangential sliding. The lattice rotation fields are mapped experimentally using high-resolution electron backscatter diffraction (HR-EBSD) on cross-sections prepared using focused ion beam (FIB). A physically-based crystal plasticity finite element model (CPFEM) is used to simulate the lattice rotation fields, …
We have measured the mechanical properties of few-layer graphene and graphite flakes that are suspended over circular holes. The spatial profile of the flake's spring constant is measured with an atomic force microscope. The bending rigidity of and the tension in the membranes are extracted by fitting a continuum model to the data. For flakes down to eight graphene layers, both parameters show a strong thickness-dependence. We predict fundamental resonance frequencies of these nanodrums in the GHz range based on the measured bending rigidity and tension.
The dielectric constant or relative permittivity of a dielectric material, which describes how the net electric field in the medium is reduced with respect to the external field, is a parameter of critical importance for charging and screening in electronic devices. Such a fundamental material property is intimately related to not only the polarizability of individual atoms, but also the specific atomic arrangement in the crystal lattice. In this letter, we present both experimental and theoretical investigations on the dielectric constant of few-layer In2Se3 nano-flakes grown on mica substrat…
Decoupling Strain-Rate Sensitivity and Deformation Length Scale Effects in Neutron-Irradiated Tungsten: A Coupled Nano-Indentation, HR-EBSD and Crystal Plasticity Study
Indian Institute of Science（印度科学学院）；University of Oxford（牛津大学）；UK Atomic Energy Authority（英国原子能管理局）
We investigate quantum transport properties of triangular graphene flakes with zigzag edges by using first principles calculations. Triangular graphene flakes have large magnetic moments which vary with the number of hydrogen atoms terminating its edge atoms and scale with its size. Electronic transmission and current-voltage characteristics of these flakes, when contacted with metallic electrodes, reveal spin valve and remarkable rectification features. The transition from ferromagnetic to antiferromagnetic state under bias voltage can, however, terminate the spin polarizing effects for speci…
We study the rotational motion of nano-flake ferromagnetic discs suspended in a Newtonian fluid, as a potential material owing the vortex-like magnetic configuration. Using analytical expressions for hydrodynamic, magnetic and Brownian torques, the stochastic angular momentum equation is determined in the dilute limit conditions under applied magnetic field. Results are compared against experimental ones and excellent agreement is observed. We also estimate the uncertainty in the orientation of the discs due to the Brownian torque when an external magnetic field aligns them. Interestingly, thi…
We demonstrate the local oxidation nanopatterning of graphene films by an atomic force microscope. The technique provides a method to form insulating trenches in graphene flakes and to fabricate nanodevices with sub-nm precision. We demonstrate fabrication of a 25-nm-wide nanoribbon and sub-micron size nanoring from a graphene flake. We also found that we can write either trenches or bumps on the graphene surface depending on the lithography conditions. We attribute the bumps to partial oxidation of the surface and incorporation of oxygen into the graphene lattice.
Piezoelectric and ferroelectric properties in the two dimensional (2D) limit are highly desired for nanoelectronic, electromechanical, and optoelectronic applications. Here we report the first experimental evidence of out-of-plane piezoelectricity and ferroelectricity in van der Waals layered $α$-In2Se3 nano-flakes. The non-centrosymmetric R3m symmetry of the $α$-In2Se3 samples is confirmed by scanning transmission electron microscopy, second-harmonic generation, and Raman spectroscopy measurements. Domains with opposite polarizations are visualized by piezo-response force microscopy. Single-p…
The effect of graphene flakes on the strength of Ni-graphene composites is investigated using molecular dynamics simulation. Rather than introducing flakes as flat structures into the Ni matrix, as it is common in available studies, we introduce them into the heated liquid Ni and let the structures equilibrate in a 14-ns molecular-dynamics run; these structures are then quenched to obtain the composites. By varying the interaction of flake edge atoms with the Ni matrix, two different flake morphologies -- wrinkled vs flat -- are obtained. The mechanical properties, and in particular the compos…
In this paper, we study the thermal rectification in asymmetric U-shaped graphene flakes by using nonequilibrium molecular dynamics simulations. The graphene flakes are composed by a beam and two arms. It is found that the heat flux runs preferentially from the wide arm to the narrow arm which indicates a strong rectification effect. The dependence of the rectification ratio upon the heat flux, the length and the width of the beam, the length and width of the two arms are studied. The result suggests a possible route to manage heat dissipation in U-shaped graphene based nanoelectronic devices.
Predicting the dramatic changes in material properties caused by irradiation damage is key for the design of future nuclear fission and fusion reactors. Self-ion implantation is an attractive tool for mimicking the effects of neutron irradiation. However, the damaged layer of implanted samples is only few microns thick, making it difficult to estimate macroscopic properties. Here we address this challenge using a combination of experimental and modelling techniques. We concentrate on self-ion-implanted tungsten, the front-runner for fusion armour components and a prototypical bcc material. To …
We report the observation and characterization of field emission current from individual single- and few-layer graphene flakes laid on a flat SiO2/Si substrate. Measurements were performed in a scanning electron microscope chamber equipped with nanoprobes, used as electrodes to realize local measurements of the field emission current. We achieved field emission currents up to 1 μA from the flat part of graphene flakes at applied fields of few hundred V/μm. We found that emission process is stable over a period of several hours and that it is well described by a Fowler-Nordheim model for curren…
The electronic structure and functionality of 2D materials is highly sensitive to structural morphology, opening the possibility for manipulating material properties, but also making predictable and reproducible functionality challenging. Black phosphorus (BP), a corrugated orthorhombic 2D material, has in-plane optical absorption anisotropy critical for applications such as directional photonics, plasmonics, and waveguides. Here, we use polarization-dependent photoemission electron microscopy to visualize the anisotropic optical absorption of BP with 54 nm spatial resolution. We find the edge…
We demonstrate a new approach for forming hybrid metal/carbonaceous nanostructures in a controlled direct laser planting process. Au-Ag nanoclusters in amorphous or crystalline carbonaceous matrices are formed with different morphology: nanoparticles, nanoflakes, and nanoflowers. In contrast to other generation techniques our approach is simple, involving only a single laser-induced process transforming supramolecular complexes dissolved in solvent such as acetone, acetophenone, or dichloroethane into hybrid nanostructures in the laser-affected area of the substrate. The morphology of the hybr…
Using ab initio density functional theory and quantum transport calculations based on nonequilibrium Green's function formalism we study structural, electronic, and transport properties of hydrogen-terminated short graphene nanoribbons (graphene flakes) and their functionalization with vanadium atoms. Rectangular graphene flakes are stable, having geometric and electronic structures quite similar to that of extended graphene nanoribbons. We show that a spin-polarized current can be produced by pure, hydrogenated rectangular graphene flakes by exploiting the spatially-separated edge states of t…
We report the frictional response of few-layer graphene (FLG) flakes obtained by liquid phase exfoliation (LPE) of pristine graphite. To this end, we inkjet print FLG on bare and hexamethyldisilazane-terminated SiO2 substrates, producing micrometric patterns with nanoscopic roughness that are investigated by atomic force microscopy. Normal force spectroscopy and atomically resolved morphologies indicate reduced surface contamination by solvents after a vacuum annealing procedure. Notably, the printed FLG flakes show ultralow friction comparable with micromechanically exfoliated graphene flakes…
Using molecular-dynamics simulation and finite-element modelling, we simulate nanoindentation into the three principal surfaces -- the (100), (110) and (111) surface -- of Cu and Al. In the elastic regime, the simulation data agree fairly well with the linear elastic theory of indentation into an elastically anisotropic substrate. With increasing indentation, the effect of pressure hardening becomes visible. When the critical stress for dislocation nucleation is reached, even the elastically isotropic Al shows a strong dependence of the surface orientation on the force-displacement curves. Aft…
The electronic properties of graphene nano-flakes (GNFs) with different edge passivation is investigated by using density functional theory. Passivation with F and H atoms are considered: C$_{N_c}$ X$_{N_x}$ (X=F or H). We studied GNFs with $10<N_c<56$ and limit ourselves to the lowest energy configurations. We found that: i) the energy difference $Δ$ between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases with $N_c$, ii) topological defects (pentagon and heptagon) break the symmetry of the GNFs and enhance the electric polarization, i…
Density-functional tight-binding and classical molecular dynamics simulations are used to investigate the structural deformations and melting of planar carbon nano-clusters $C_{N}$ with N=2-55. The minimum energy configurations for different clusters are used as starting configuration for the study of the temperature effects on the bond breaking/rotation in carbon lines (N$<$6), carbon rings (5$<$N$<$19) and graphene nano-flakes. The larger the rings (graphene nano-flake) the higher the transition temperature (melting point) with ring-to-line (perfect-to-defective) transition structures. The m…
The impact of the edges and presence of dopants to the work function (WF) of graphene nanoribbons (GNR) and nanoflakes was studied by an ab initio approach. The strong dependence of the WF upon the GNR structure was found and a promising character for the field emission by the donor type impurities was observed. Basing on the predominant impact of the nanostructure edges to the emission properties, the small graphene flakes were investigated as a possible source for the electron emission. The obtained weak dependence of the low WF values of the graphene flakes on their size and shape allows to…
ZrTe$_5$ is a newly discovered topological material. Shortly after a single layer ZrTe$_5$ had been predicted to be a two-dimensional topological insulator, a handful of experiments have been carried out on bulk ZrTe$_5$ crystals, which however suggest that its bulk form may be a three-dimensional topological Dirac semimetal. We report the first transport study on ultra thin ZrTe$_5$ flakes down to 10 nm. A significant modulation of the characteristic resistivity maximum in the temperature dependence by thickness has been observed. Remarkably, the metallic behavior, occurring only below about …
Rational combination holds the key for non-precious metal oxides and economic carbon materials to the creation of high performance air electrode materials. Carbon nanotubes (CNTs)-supported manganese oxides composite catalysts (CMnCs) were obtained by reacting commercial CNTs with MnO$_4^-$ in diluted H$_2$SO$_4$ with cancelling conventional hydrothermal processing or adding surfantant templates. CMnCs were then assembled into commercial air cathodes equipped in metal air fuel cells. CNTs demonstrated structurally much defective. Outer CNT walls were cracked into graphene nano pieces yet remai…
The growth of single and multilayer graphene nano-flakes on MgO and ZrO2 at low temperatures is shown through transmission electron microscopy. The graphene nano-flakes are ubiquitously anchored at step edges on MgO (100) surfaces. Density functional theory investigations on MgO (100) indicate C2H2 decomposition and carbon adsorption at step-edges. Hence, both the experimental and theoretical data highlight the importance of step sites for graphene growth on MgO.
We establish the use of dielectrophoresis for the directed parallel assembly of individual flakes and nanoribbons of few-layer graphene into electronic devices. This is a bottom-up approach where source and drain electrodes are prefabricated and the flakes are deposited from a solution using an alternating electric field applied between the electrodes. These devices are characterized by scanning electron microscopy, atomic force microscopy, Raman spectroscopy and electron transport measurements. They are shown to be electrically active and their current carrying capacity and subsequent failure…
The electronic conductance of graphene-based bilayer flake systems reveal different quantum interference effects, such as Fabry-Pérot resonances and sharp Fano antiresonances on account of competing electronic paths through the device. These properties may be exploited to obtain spin-polarized currents when the same nanostructure is deposited above a ferromagnetic insulator. Here we study how the spin-dependent conductance is affected when a time-dependent gate potential is applied to the bilayer flake. Following a Tien-Gordon formalism we explore how to modulate the transport properties of su…
A way to produce new metal-carbon nanoobjects by transformation of a graphene flake with an attached transition metal cluster under electron irradiation is proposed. The transformation process is investigated by molecular dynamics simulations by the example of a graphene flake with a nickel cluster. The parameters of the nickel-carbon potential (I. V. Lebedeva et al. J. Phys. Chem. C, 2012, 116, 6572) are modified to improve description of the balance between the fullerene elastic energy and graphene edge energies in this process. The metal-carbon nanoobjects formed are found to range from het…
Simple, yet versatile, methods to functionalize graphene flakes with metal (oxide) nanoparticles are in demand, particularly for the development of advanced catalysts. Herein, based on light-induced electrochemistry, a laser-assisted, continuous, solution route for the simultaneous reduction and modification of graphene oxide with catalytic nanoparticles is reported. Electrochemical graphene oxide (EGO) is used as starting material and electron-hole pair source due to its low degree of oxidation, which imparts structural integrity and an ability to withstand photodegradation. Simply illuminati…
The process of creating an atomically defined and robust metallic tip is described and quantified using measurements of contact conductance between gold electrodes and numerical simulations. Our experiments show how the same conductance behavior can be obtained for hundreds of cycles of formation and rupture of the nanocontact by limiting the indentation depth between the two electrodes up to a conductance value of approximately $5G_{0}$ in the case of gold. This phenomenon is rationalized using molecular dynamics simulations together with density functional theory transport calculations which…
Because of their loosely bound electrons, electrides offer physical properties useful in chemical synthesis and electronics. For these applications and others, nano-sized electrides offer advantages, but to-date no electride has been synthesized as a nanomaterial. We demonstrate experimentally that Ca$_2$N, a layered electride in which layers of atoms are separated by layers of a 2D electron gas (2DEG), can be exfoliated into two-dimensional (2D) nanosheets using liquid exfoliation. The 2D flakes are stable in a nitrogen atmosphere or in select organic solvents for at least one month. Electron…
Systematic tight-binding investigations of the electronic spectra (as a function of the magnetic field) are presented for trigonal graphene nanoflakes with reconstructed zigzag edges, where a succession of pentagons and heptagons, that is 5-7 defects, replaces the hexagons at the zigzag edge. For nanoflakes with such reczag defective edges, emphasis is placed on topological aspects and connections underlying the patterns dominating these spectra. The electronic spectra of trigonal graphene nanoflakes with reczag edge terminations exhibit certain unique features, in addition to those that are w…
We demonstrate theoretically that hydrogenation and annealing applied to nanoscale carbon structures play a crucial role in determining the final shape of the system. In particular, graphene flakes characterized by the linear and non-hydrogenated zigzag or armchair edges have high propensity to merge into a bigger flake or a nanotube (the formation of a single carbon-carbon bond lowers the total energy of the system by up to 6.22 eV). Conversely, the line of the $sp^2$ carbon bonds (common for pure carbon structures such as graphene or a carbon nanotube) converted into the $sp^3$ type by hydro…
The stress transfer mechanism from a polymer substrate to a nano-inclusion, such as a graphene flake, is of extreme interest for the production of effective nanocomposites. Previous work conducted mainly at the micron scale has shown that the intrinsic mechanism of stress transfer is shear at the interface. However, since the interfacial shear takes its maximum value at the very edge of the nano-inclusion it is of extreme interest to assess the effect of edge integrity upon axial stress transfer at the submicron scale. Here, we conduct a detailed Raman line mapping near the edges of a monolaye…
We report on the superlubric sliding of monolayer tungsten disulfide (WS2) on epitaxial graphene (EG) on silicon carbide (SiC). WS2 single-crystalline flakes with lateral size of hundreds of nanometers are obtained via chemical vapor deposition (CVD) on EG and microscopic and diffraction analyses indicate that the WS2/EG stack is predominantly aligned with zero azimuthal rotation. Our experimental findings show that the WS2 flakes are prone to slide over graphene surfaces at room temperature when perturbed by a scanning probe microscopy (SPM) tip. Atomistic force field based molecular dynamics…
Two-dimensional molybdenum disulphide on graphene grown by chemical vapour deposition is a promising van der Waals system for applications in optoelectronics and catalysis. To extend the fundamental understanding of growth and intrinsic properties of molybdenum disulphide on graphene, molybdenum disulphide on highly oriented pyrolytic graphite is a suitable model system. Here we show, experimentally and by density-functional-theory calculations, that molybdenum disulphide flakes grow in two orientations. One of the orientations is energetically preferred, the other one is rotated by 30 degree.…
The first plastic event occurring in discrete intermittent plasticity, as for example a pop-in seen in nano-indentation, is evaluated with extreme value statistics. It is found that when the same deformation is repeated many times, the average of the stress at this first event is related to the deforming volume via an exponentially truncated power-law. The present work demonstrates this trend and the expected Weibull fluctuation around it. The statistical formalism is shown to apply to the nano-indentation data of Morris {\em et al}, Phys. Rev. Lett. 106, 165502 (2011), as well as to dislocati…
The fabrication of electrochemical double-layer capacitors (EDLCs) with high areal capacitance relies on the use of elevated mass loadings of highly porous active materials. Herein, we demonstrate a high-throughput manufacturing of graphene/nanotubes hybrid EDLCs. Wet-jet milling (WJM) method is exploited to exfoliate the graphite into single/few-layer graphene flakes (WJM-G) in industrial volume (production rate ~0.5 kg/day). Commercial single/double walled carbon nanotubes (SDWCNTs) are mixed with graphene flakes in order to act as spacers between the graphene flakes during their film format…
Study the effect of scratching depth and ceramic-metal ratio on the scratch behavior of NbC/Nb Ceramic/Metal nano-laminates using molecular dynamics simulation and machine learning
The use of graphene in a form of discontinuous flakes in polymer composites limits the full exploitation of the unique properties of graphene, thus requiring high filler loadings for achieving - for example - satisfactory electrical and mechanical properties. Herein centimetre-scale CVD graphene/polymer nanolaminates have been produced by using an iterative 'lift-off/float-on' process and have been found to outperform, for the same graphene content, state-of-the-art flake-based graphene polymer composites in terms of mechanical reinforcement and electrical properties. Most importantly these th…
Ultrasonication is widely used to exfoliate two dimensional (2D) van der Waals layered materials such as graphene. Its fundamental mechanism, inertial cavitation, is poorly understood and often ignored in ultrasonication strategies resulting in low exfoliation rates, low material yields and wide flake size distributions, making the graphene dispersions produced by ultrasonication less economically viable. Here we report that few-layer graphene yields of up to 18% in three hours without introduction of basal plane defects can be achieved by optimising inertial cavitation during ultrasonication.…
