Enhanced mechanical properties of 4H-SiC by epitaxial carbon films obtained from bilayer graphene.

Graphene exhibits excellent mechanical properties under atomically thin thickness, which made it very suitable for nanoelectromechanical systems that had high requirements for the thickness of coatings. The epitaxial bilayer graphene on the 4H-SiC (0001) surface presents high stiffness and hardness comparable to diamond. However, due to structural transition occurring at the nanoscale, it is difficult to elucidate reinforcement mechanisms using experimental methods. Here, we applied molecular dynamics simulations to study nanoindentation of epitaxial carbon-film-covered 4H-SiC (0001) surfaces. Because a weak interaction potential existed between graphene layers at indentation depth (h < 0.8 Å) that far smaller than interlayer distance, the epitaxial bilayer graphene does not allow the 4H-SiC to exceed its intrinsic stiffness. When the indentation depth h ≥6.45 Å, the sp3 hybridized bonds formed on the interlayer of graphene, which leads to fewer amorphous atoms in the sample of 4H-SiC and exhibits stronger stiffness, in comparison with bare 4H-SiC. This strongly suggests the existence of sp3 bonds contributing to the surface strengthening. Meanwhile, we found that the comprehensive mechanical properties of nanocomposites with hydrogenated diamond-like films were superior to those of nanocomposites with other carbon films at high temperatures.

A nano continuous variable transmission system from nanotubes.

A nano continuous variable transmission (nano-CVT) system is proposed by means of carbon nanotubes (CNTs). The dynamic behavior of the CNT-based nanosystem is assessed using molecular dynamics simulations. The system contains a rotary CNT-motor and a CNT-bearing. The tube axes of the nanomotor and the rotor in the bearing are laid in parallel, and the distance between them is known as the eccentricity of the rotor with a diameter of d. By changing the eccentricity (e) of the rotor from 0 to d, some interesting rotation transmission phenomena are discovered, whose procedures can be used to design various nanodevices. This might include the failure of rotation transmission-i.e. the rotor has no rotation-when e ≥ d at an extremely low temperature, or when the edges of the two tubes are orthogonal at their intersections in any condition. This hints that the state of the nanosystem can be used as an on/off switch or breaker. For a system with e = d and a high temperature, the rotor rotates in the reverse direction of the motor. This means that the output signal (rotation) is the reverse of the input signal. When changing the eccentricity from 0 to d continuously, the output signal gradually decreases from a positive value to a negative value; as a result a nano-CVT system is obtained.

Significance tests on the output power of a thermally driven rotary nanomotor.

Many factors can have a significant influence on the output power of a thermally driven rotary nanomotor made of carbon nanotubes (CNTs). Making use of a computational molecular dynamics approach, we evaluate for the first time the output power of a nanomotor, considering some of the main factors including temperature, the diameter of the rotor and the number of IRD atoms (N) on the stator. When applying extra-resistant torque to the rotor to let the stable value of the rotational frequency of the rotor fluctuate near zero, the value of the resistant torque can be considered as the output power of the rotor. The effects of these factors on the output power of a motor are roughly predicted via a fitting approach. Using stepwise regression analysis, we discover that N has the greatest influence on the output power. The second and the third main factors that affect the output power of a nanomotor are the diameter of the rotor, and the interaction between N and the diameter, respectively. To improve the output power of a nanomotor, one can place more IRD atoms in the system and/or employ CNTs with larger diameters.

Fabrication of an ideal nanoring from a black phosphorus nanoribbon upon movable bundling carbon nanotubes.

As a low dimensional material, black phosphorus (BP) continues to attract much attention from researchers due to its excellent electric properties. In particular, the one-dimensional material, in the form of a ring or tube formed from BP, has been extensively studied and found to be a perfect semiconductor. But the BP ring has never been reported in laboratories. To form an ideal ring from a rectangular BP ribbon, we choose a carbon nanotube (CNT) bundle to attract the ribbon and move one or more CNTs in the bundle to induce the unsaturated ends of the BP ribbon to become covalently bonded. Numerical experiments are applied to BP ribbons with lengths either equal to, shorter, or longer than the perimeter of the CNT bundle, to investigate the formation of a BP ring. Experiments show that if one end of the BP ribbon is attracted by a CNT, moving the other CNTs away endows the ribbon with high probability of forming an ideal ring. The conclusions drawn from these results will benefit future in situ experiments involving forming a ring from a BP ribbon.

Rotation-excited perfect oscillation of a tri-walled nanotube-based oscillator at ultralow temperature.

In recent years, carbon-nanotube (CNT)-based gigahertz oscillators have been widely used in numerous areas of practical engineering such as high-speed digital, analog circuits, and memory cells. One of the major challenges to practical applications of the gigahertz oscillator is generating a stable oscillation process from the gigahertz oscillators and then maintaining the stable process for a specified period of time. To address this challenge, an oscillator from a triple-walled CNT-based rotary system is proposed and analyzed numerically in this paper, using a molecular dynamics approach. In this system, the outer tube is fixed partly as a stator. The middle tube, with a constant rotation, is named Rotor 2 and runs in the stator. The inner tube acts as Rotor 1, which can rotate freely in Rotor 2. Due to the friction between the two rotors when they have relative motion, the rotational frequency of Rotor 1 increases continuously and tends to converge with that of Rotor 2. During rotation, the oscillation of Rotor 1 may be excited owing to both a strong end barrier at Rotor 2 and thermal vibration of atoms in the tubes. From the discussion on the effects of length of Rotor 1, temperature, and input rotational frequency of Rotor 2 on the dynamic response of Rotor 1, an effective way to control the oscillation of Rotor 1 is found. Being much longer than Rotor 2, Rotor 1 will have perfect oscillation, i.e., with both stable (or nearly constant) period and amplitude-especially at relatively low temperature. This discovery can be taken as a useful guidance for the design of an oscillator from CNTs.

Strength and stability analysis of a single-walled black phosphorus tube under axial compression.

Few-layered black phosphorus materials currently attract much attention due to their special electronic properties. As a consequence, a single-layer black phosphorus (SLBP) nanotube has been theoretically built. The corresponding electronic properties of such a black phosphorus nanotube (BPNT) were also evaluated numerically. However, unlike graphene formed with 2sp(2) covalent carbon atoms, SLBP is formed with 3sp(3) bonded atoms. It means that the structure from SLBP will possess lower Young's modulus and mechanical strength than those of carbon nanotubes. In this study, molecular dynamics simulation is performed to investigate the strength and stability of BPNTs affected by the factors of diameter, length, loading speed and temperature. Results are fundamental for investigating the other physical properties of a BPNT acting as a component in a nanodevice. For example, buckling of the BPNT happens earlier than fracture, before which the nanostructure has very small axial strain. For the same BPNT, a higher load speed results in lower critical axial strain and a nanotube with lower axial strain can still be stable at a higher temperature.

Quantitative control of a rotary carbon nanotube motor under temperature stimulus.

Since a double-walled carbon nanotube (DWCNT)-based rotary motor driven by a uniform temperature field was proposed in 2014, how to control quantitatively the rotation of the rotor is still an open question. In this work, we present a mathematical relationship between the rotor's speed and interaction energy. Essentially, the increment of interaction energy between the rotor and the stator(s) determines the rotor's rotational speed, whereas the type of radial deviation of an end carbon atom on the stator determines the rotational direction. The rotational speed of the rotor can be specified by adjusting temperature and radial deviation of an end carbon atom on the stator. It is promising for designing a controllable temperature-driven rotary motor based on DWCNTs with length of few nanometers only.

Oscillators based on double-walled armchair@zigzag carbon nanotubes containing inner tubes with different helical rises.

A novel approach is presented to improve the oscillatory behavior of oscillators based on double-walled carbon nanotubes containing rotating inner tubes applied with different helical rises. The influence of the helical rise on the oscillatory amplitude, frequency, and stability of inner tubes with different helical rises in armchair@zigzag bitubes is investigated using the molecular dynamics method. Our simulated results show that the oscillatory behavior is very sensitive to the applied helical rise. The inner tube with h = 10 Å has the most ideal hexagon after the energy minimization and NVT process in the armchair@zigzag bitubes, superior even to the inner tube without a helical rise, and thus it exhibits better oscillatory behavior compared with other modes. Therefore, we can apply an appropriate helical rise on the inner tube to produce a stable and smooth oscillator based on double-walled carbon nanotubes.

Rotation measurements of a thermally driven rotary nanomotor with a spring wing.

Due to the extremely small dimensions and super high frequency of the rotor in a thermally driven rotary nanomotor made from carbon nanostructures, measuring the rotational frequency of the nanomotor is still an open issue. To this end, a measuring system is constructed in which a spring wing is connected with the rotor to provide collisions with a probe tip whose deflection reflects the rotational frequency of the rotor. The spring wing is formed by connecting an end-tube from a carbon nanotube and a graphene with differently hydrogenated surfaces. Due to the alternative hydrogenation of the two surfaces, the graphene shrinks like a spring. When the rotational frequency increases, the centrifugal force applied on the wing increases and then the spring is stretched (becoming longer). As the end-tube rotates with the rotor and reaches the probe tip, a collision occurs between the end-tube and the probe tip. After collision, the probe tip undergoes a variation of vertical deflection that can be measured through atomic force microscopy. The relation between the maximal deflection of the probe tip and the rotational frequency of the rotor is determined via numerical experiments. The effects of the configuration (namely hydrogenation and length) of graphene on the rotation of the rotor are investigated. The results provide some insight into the fabrication of nanomachines.

Self-excited oscillation of rotating double-walled carbon nanotubes.

The oscillatory behavior of a double-walled carbon nanotubes with a rotating inner tube is investigated using molecular dynamics simulation. In the simulation, one end of the outer tube is assumed to be fixed and the other is free. Without any prepullout of the rotating inner tube, it is interesting to observe that self-excited oscillation can be triggered by nonequilibrium attraction of the ends of two tubes. The oscillation amplitude increases until it reaches its maximum with decrease of the rotating speed of the inner tube. The oscillation of a bitube is sensitive to the gap between two walls. Numerical results also indicate that the zigzag/zigzag commensurate model with a larger gap of >0.335 nm can act as a terahertz oscillator, and the armchair/zigzag incommensurate model plays the role of a high amplitude oscillator with the frequency of 1 GHz. An oblique chiral model with a smaller gap of <0.335 nm is unsuitable for the oscillator because of the steep damping of oscillation.

Homogenized finite element analysis on effective elastoplastic mechanical behaviors of composite with imperfect interfaces.

A three-dimensional (3D) representative volume element (RVE) model was developed for analyzing effective mechanical behavior of fiber-reinforced ceramic matrix composites with imperfect interfaces. In the model, the fiber is assumed to be perfectly elastic until its tensile strength, and the ceramic material is modeled by an elasto-plastic Drucker-Prager constitutive law. The RVE model is then used to study the elastic properties and the tensile strength of composites with imperfect interfaces and validated through experiments. The imperfect interfaces between the fiber and the matrix are taken into account by introducing some cohesive contact surfaces. The influences of the interface on the elastic constants and the tensile strengths are examined through these interface models.

Brittle-to-ductile transition and theoretical strength in a metal-organic framework glass.

Metal-organic framework (MOF) glasses, a new type of melt-quenched glass, show great promise to deal with the alleviation of greenhouse effects, energy storage and conversion. However, the mechanical behavior of MOF glasses, which is of critical importance given the need for long-term stability, is not well understood. Using both micro- and nanoscale loadings, we find that pillars of a zeolitic imidazolate framework (ZIF) glass have a compressive strength falling within the theoretical strength limit of ≥E/10, a value which is thought to be unreachable in amorphous materials. Pillars with a diameter larger than 500 nm exhibited brittle failure with deformation mechanisms including shear bands and nearly vertical cracks, while pillars with a diameter below 500 nm could carry large plastic strains of ≥20% in a ductile manner with enhanced strength. We report this room-temperature brittle-to-ductile transition in ZIF-62 glass for the first time and demonstrate that theoretical strength and large ductility can be simultaneously achieved in ZIF-62 glass at the nanoscale. Large-scale molecular dynamics simulations have identified that microstructural densification and atomistic rearrangement, i.e., breaking and reconnection of inter-atomistic bonds, were responsible for the exceptional ductility. The insights gained from this study provide a way to manufacture ultra-strong and ductile MOF glasses and may facilitate their processing toward real-world applications.

Dynamic behavior of a rotary nanomotor in argon environments.

When argon is used as a protecting gas in the fabrication or working environment of a nanodevice, absorption of some argon atoms onto the surface of the device lead to different responses. In this work, the rotation of the rotor in a carbon nanotube (CNT)-based rotary nanomotor in argon environment is investigated. In the rotary nanomotor, two outer CNTs act as the stator and are used to constrain the inner CNT (i.e., the rotor). The rotor is driven to rotate by the stator due to their collision during thermal vibration of their atoms. A stable rotational frequency (SRF) of the rotor occurs when the rotor reaches a dynamic equilibrium state. The value of the SRF decreases exponentially with an increase in the initial argon density. At dynamic equilibrium date, some of the argon atoms rotate synchronously with the rotor when they are absorbed onto either internal or external surface of the rotor. The interaction between the rest of the argon atoms and the rotor is stronger at higher densities of argon, resulting in lower values of the SRF. These principles provide insight for future experimentation and fabrication of such rotary nanomotor.

Modelling osteoblast adhesion on surface-engineered biomaterials: optimisation of nanophase grain size.

A double-layered model is proposed for numerically simulating osteoblast adhesion on surface-engineered biomaterials. The proposed model consists of molecular and cellular motions based on theoretical and experimental evidence and creates predictive simulations from sparse experimental data. The comparison of numerical solutions and experimental data reveals that the proposed model can explain the nonlinear behaviour of osteoblast adhesion on material surfaces in respect to nanophase grain size (0-100 nm). The model further provides insight into the optimisation of nanophase grain size on the surface of the biomaterial.

A method for measuring rotation of a thermal carbon nanomotor using centrifugal effect.

A thermal nanomotor is relatively easy to fabricate and regulate as it contains just a few or even no accessory devices. Since the double-wall carbon nanotube (CNT)-based rotary nanomotor was established in a thermostat, assessment of the rotation of the rotor (inner tube) in the stator (outer tube) of the nanomotor has been critical, but remains challenging due to two factors: the small size of the rotor (only a few nanometers) and the high rotational frequency (¼1 GHz). To measure the rotation of the nanomotor, in the present study, a probe test method is proposed. Briefly, the rotor is connected to an end-tube (CNT) through a graphene (GN) nanoribbon. As the CNT-probe is on the trajectory of the end-tube which rotates with the rotor, it will collide with the end-tube. The sharp fluctuation indicating the probe tip deflection can be observed and recorded. As a curly GN by hydrogenation is adopted for connecting the rotor and the end-tube, collision between the end-tube and the probe tip occurs only when the centrifugal force is higher than a threshold which can be considered as the rotational frequency of the rotor being measured by the present method.

Self-assembly dynamics and accumulation mechanisms of ultra-fine nanoparticles.

The self-assembly of nanomaterials into three-dimensional hierarchical structures is a fundamental step impacting a large number of synthetic and natural processes. These range from the scalable fabrication of nano-devices such as batteries, sensors and third generation solar cells to the uptake and accumulation of particulate pollution in the lung alveoli. Here, we show that the Dynamic behavior of ultra-fine particles (UFP < 100 nm) diverges significantly from that of sub- and micro equivalents. For freely diffusing bodies, this leads to the formation of stochastically reproducible films that approach the morphology and density of ballistically deposited ones. A novel deposition mechanism and regime are proposed that successfully capture the full spectrum of size-dependent self-assembly dynamics. These findings are a significant step toward the engineering of scalable parallel nano-fabrication approaches, and the understanding of the interaction of unbound nanostructures with their surrounding.

Optimal Mass Distribution Prediction for Human Proximal Femur with Bi-modulus Property.

Simulation of the mass distribution in a human proximal femur is important to provide a reasonable therapy scheme for a patient with osteoporosis. An algorithm is developed for prediction of optimal mass distribution in a human proximal femur under a given loading environment. In this algorithm, the bone material is assumed to be bi-modulus, i.e., the tension modulus is not identical to the compression modulus in the same direction. With this bi-modulus bone material, a topology optimization method, i.e., modified SIMP approach, is employed to determine the optimal mass distribution in a proximal femur. The effects of the difference between two moduli on the final material distribution are numerically investigated. Numerical results obtained show that the mass distribution in bi-modular bone materials is different from that in traditional isotropic material. As the tension modulus is less than the compression modulus for bone tissues, the amount of mass required to support tension loads is greater than that required by isotropic material for the same daily activities including one-leg stance, abduction and adduction.