Molecular-dynamics simulations of carbon nanotubes as gigahertz oscillators

Reaction mechanism and sensitivity enhancement of energetic materials doped with carbon nanotubes under electric fields by molecular dynamics simulations

Energetic materials (EM) can be remotely, uniformly and rapidly excited by electromagnetic radiation. Mastering the response mechanism of EM to electromagnetic radiation and promoting the efficient utilization of electromagnetic energy are fundamental to the development of electromagnetic radiation-induced explosive technologies. EM are generally non-magnetic, so the effect of magnetic fields in the system can usually be negligible; instead, the focus is on the interaction between electric fields and EM. In this paper, ReaxFF-lg reactive molecular dynamics simulations were performed to investigate the reaction process of pure RDX systems and RDX systems doped with single-walled carbon nanotubes (SWCNTs) under different electric fields and temperatures, and the response mechanisms of the two systems under electric fields were obtained. A method for estimating the field enhancement factor of SWCNTs through the initial decomposition time of RDX molecules was proposed. Compared to high-temperature thermal decomposition, the decomposition mechanisms and reaction pathways of some RDX molecules were different under electric fields. Compared to pure RDX systems, the addition of SWCNTs makes the charge values of some atoms in systems abnormal, weakening the bond energies of them and affecting the stability of the RDX system under electric fields, which is a key reason for enhancement of sensitivities of the electric field.

Modeling and Vibration Analysis of Carbon Nanotubes as Nanomechanical Resonators for Force Sensing

Carbon nanotubes (CNTs) have attracted considerable attention as nanomechanical resonators because of their exceptional mechanical properties and nanoscale dimensions. In this study, a novel CNT-based probe is proposed as an efficient nanoforce sensing nanomaterial that detects external pressure. The CNT probe was designed to be fixed by clamping tunable outer CNTs. By using the mobile-supported outer CNT, the position of the partially clamped outer CNT can be controllably shifted, effectively tuning its resonant frequency. This study comprehensively investigates the modeling and vibration analysis of gigahertz frequencies with loaded CNTs used in sensing applications. The vibration frequency of a partially clamped CNT probe under axial loading was modeled using continuum mechanics, considering various parameters such as the clamping location, length, and boundary conditions. In addition, the interaction between external forces and CNT resonators was investigated to evaluate their sensitivity for force sensing. Our results provide valuable insights into the design and optimization of CNT-based nanomechanical resonators for high-performance force sensing applications.

Design of High-Frequency Carbon Nanotube-Carbon Nanotorus Oscillators for Energy Harvesting: A Molecular Dynamics Study

The objective of this study is to examine the feasibility of using carbon-based nanostructures as nano-oscillators for future nanoelectromechanical applications such as energy harvesting devices and vibration sensing. The proposed nano-oscillator is comprised of a carbon nanotube (CNT) oscillating through a fixed carbon nanotorus molecule. For the first time in the literature, molecular dynamics (MD) simulations in conjunction with the Tersoff-Brenner (TB) and 6-12 Lennard-Jones (LJ) potential functions are adopted to determine the molecular interactions of the introduced nanodevice. To simulate the oscillatory behavior, two different schemes, namely, rigid and flexible, are considered. A detailed parametric study is performed to investigate the effects of rigidity, flexibility, and size of nanostructures as well as initial velocity on the force distribution and time histories of displacement and velocity of the core. Numerical results reveal that unlike the rigid oscillators, the flexible oscillators damp out within a few cycles. It is shown that the escape velocity of the flexible scheme is ∼6 times greater than that of the rigid scheme. The operating frequency and the generated power of rigid and flexible schemes under different system parameters are also calculated and compared. It is demonstrated that with increasing the ratio of nanotube-to-nanotorus diameter, the operating frequencies of both schemes decrease, while the generated powers do not behave monotonically. For a determined system parameter, it is observed that the flexible scheme provides higher operating frequencies compared to the rigid one. Moreover, considering that the initial velocity of the system is identical to the escape velocity, the generated power of the flexible scheme is calculated to be ∼14 times greater than that of the rigid scheme.

Nanotube Slidetronics

One-dimensional slidetronics is predicted for double-walled boron-nitride nanotubes. Local electrostatic polarization patterns along the body of the nanotube are found to be determined by the nature of the two nanotube walls, their relative configuration, and circumferential faceting modulation during coaxial interwall sliding. By careful choice of chiral indices, chiral polarization patterns can emerge that spiral around the nanotube circumference. The potential usage of the discovered slidetronic effect for low-dimensional nanogenerators is briefly discussed.

Toward steering the motion of surface rolling molecular machines by straining graphene substrate

The surface rolling molecular machines are proposed to perform tasks and carrying molecular payloads on the substrates. As a result, controlling the surface motion of these molecular machines is of interest for the design of nano-transportation systems. In this study, we evaluate the motion of the nanocar on the graphene nanoribbons with strain gradient, through the molecular dynamics (MD) simulations, and theoretical relations. The nanocar indicates directed motion from the maximum strained part of the graphene to the unstrained end of the substrate. The strain gradient induced driving force and diffusion coefficients of nanocars are analyzed from the simulation and theoretical points of view. To obtain the optimum directed motion of nanocar, we consider the effects of temperature, strain average, and magnitude of strain gradient on the directionality of the motion. Moreover, the mechanism of the motion of nanocar is studied by evaluating the direction of the nanocar's chassis and the rotation of wheels around the axles. Ultimately, the programmable motion of nanocar is shown by adjusting the strain gradient of graphene substrate.

Self-assembly for preparing nanotubes from monolayer graphyne ribbons on a carbon nanotube

Graphyne nanotube (GNT), as a promising one-dimensional carbon material, attracts extensive attention in recent years. However, the synthesis of GNT is still challenging even in the laboratory. This study reveals the feasibility of fabricating a GNT by self-assembling a monolayer graphyne (GY) ribbon on a carbon nanotube (CNT) via theoretical and numerical analysis. Triggered by the van der Waals force from the CNT, a GY ribbon near the tube first winds upon the tube and then conditionally self-assembles to form a GNT. The self-assembly process and result are heavily influenced by the ambient temperature, which indicates the thermal vibration of the nanosystem. Molecular dynamic simulation results address the temperature range conducive to successful self-assembly. Different types of GNTs, e.g.α-,β-, andγ-GNTs with specified chirality (armchair, zigzag, and chiral), length, and radius, can be obtained via self-assembly by controlling the geometry of the GY ribbons and temperature. The present theoretical understanding is helpful for fabricating GNTs with predefined morphology.

Temperature-dependent mechanical properties and the microscopic deformation mechanism of bilayerγ-graphdiyne under tension

γ-graphdiyne (γ-GDY) is a new two-dimensional carbon allotrope that has received increasing attention in scientific and engineering fields. The mechanical properties ofγ-GDY should be thoroughly understood for realizing their practical applications. Althoughγ-GDY is synthesized and employed mainly in their bilayer or multilayer forms, previous theoretical studies mainly focused on the single-layer form. To evaluate the characteristics of the multilayer form, the mechanical properties of the bilayerγ-GDY (γ-BGDY) were tested under uniaxial tension using the molecular dynamics simulations. The stress-strain relation ofγ-BGDY is highly temperature-dependent and exhibits a brittle-to-ductile transition with increasing temperature. When the temperature is below the critical brittle-to-ductile transition temperature,γ-BGDY cracks in a brittle manner and the fracture strain decreases with increasing temperature. Otherwise, it exhibits ductile characteristics and the fracture strain increases with temperature. Such a temperature-dependent brittle-to-ductile transition is attributed to the interlayer cooperative deformation mechanism, in which the co-rearrangement of neighboring layers is dominated by thermal vibrations of carbon atoms in diacetylenic chains. Furthermore, the brittle-to-ductile transition behavior ofγ-BGDY is independent of loading direction and loading rate. The ultimate stress and Young's modulus decrease at higher temperatures. These results are beneficial for the design of advancedγ-GDY-based devices.

Controlling CNT-Based Nanorotors via Hydroxyl Groups

Nanomotor systems have attracted extensive attention due to their applications in nanorobots and nanodevices. The control of their response is crucial but presents a great challenge. In this work, the rotating and braking processes of a carbon nanotube (CNT)-based rotor system have been studied using molecular dynamics simulation. The speed of response can be tuned by controlling the ratio of hydroxyl groups on the edges. The ratio of hydroxyl groups is positively correlated with the speed of response. The mechanism involved is that the strong hydrogen bonds formed between interfaces increase the interface interaction. Incremental increase in the hydroxyl group concentration causes more hydrogen bonds and thus strengthens the interconnection, resulting in the enhancement of the speed of response. The phonon density of states analysis reveals that the vibration of hydroxyl groups plays the key role in energy dissipation. Our results suggest a novel routine to remotely control the nanomotors by modulating the chemical environment, including tuning the hydroxyl groups concentration and pH chemistry.

Torsional Fracture of Carbon Nanotube Bundles: A Reactive Molecular Dynamics Study

Carbon nanotubes individually show excellent mechanical properties, being one of the strongest known materials. However, when assembled into bundles, their strength drops dramatically. It still limit the understanding of their...

Position effects of the graphene-origami actuators on the rotation of a CNT nanomotor

This study designs a carbon nanotube (CNT)-based rotary nanomotor actuated by four graphene origami (G-ori) drivers with adjustable positions. When the drivers' tips have different contact states with the CNT rotor at a finite temperature, the rotor has different rotational states due to different interaction strength between the rotor and the tips. Using the molecular dynamics simulation approach, we study the effects of the drivers' position, such as the gaps between the rotor and the drivers' tips and their layout angles. Numerical results indicate that both the stable rotational frequency (SRF) and the rotational direction change with the layout angles. In an interval from -40° to -25°, the SRF increases monotonously. There also exists an angle interval in which the G-ori drivers fail to actuate the rotor's rotation. The gap offset leads to different SRF of the same rotor. Hence, one can design a rotary nanomotor with controllable rotation, which is critical for its applications in a nanomachine.

Controlling Movement at Nanoscale: Curvature Driven Mechanotaxis

Locating and manipulating nano-sized objects to drive motion is a time and effort consuming task. Recent advances show that it is possible to generate motion without direct intervention, by embedding the source of motion in the system configuration. In this work, an alternative manner to controllably displace nano-objects without external manipulation is demonstrated, by employing spiral-shaped carbon nanotube (CNT) and graphene nanoribbon structures (GNR). The spiral shape contains smooth gradients of curvature, which lead to smooth gradients of bending energy. It is shown that these gradients as well as surface energy gradients can drive nano-oscillators. An energy analysis is also carried out by approximating the carbon nanotube to a thin rod and how torsional gradients can be used to drive motion is discussed. For the nanoribbons, the role of layer orientation is also analyzed. The results show that motion is not sustainable for commensurate orientations, in which AB stacking occurs. For incommensurate orientations, friction almost vanishes, and in this instance, the motion can continue even if the driving forces are not very high. This suggests that mild curvature gradients, which can already be found in existing nanostructures, could provide mechanical stimuli to direct motion.

Ideal Oscillation of a Hydrogenated Deformable Rotor in a Gigahertz Rotation-Translation Nanoconverter at Low Temperatures

It was discovered that large-amplitude axial oscillation can occur on a rotor with an internally hydrogenated deformable part (HDP) in a rotation-translation nanoconverter. The dynamic outputs of the system were investigated using molecular dynamics simulations. When an input rotational frequency (100 GHz > ω > 20 GHz) was applied at one end of the rotor, the HDP deformed under the centrifugal and van der Waals forces, which simultaneously led to the axial translation of the other end of the rotor. Except at too high an input rotational frequency (e.g., >100 GHz), which led to eccentric rotation and even collapse of the system, the present system could generate a periodic axial oscillation with an amplitude above 0.5 nm at a temperature below 50 K. In other ranges of temperature and amplitude, the oscillation dampened quickly due to the drastic thermal vibrations of the atoms. Furthermore, the effects of the hydrogenation scheme and the length of HDP on the equilibrium position, amplitude, and frequency of oscillation were investigated. The conclusions can be applied to the design of an ideal nano-oscillator based on the present rotation-translation converter model.

Effect of the structure and morphology of carbon nanotubes on the vibration damping characteristics of polymer-based composites

The structure and morphology of the reinforcing material play an important role in the vibration damping characteristics of polymer composites. In this work, multiwalled carbon nanotubes (MWCNTs) with different structures and morphologies are incorporated into a polymer matrix. The vibration damping characteristics of the nanocomposites, in Oberst beam configuration, are studied using a free vibration test in cantilever mode. Inner tube oscillation is established as the vibration damping mechanism by correlating the extent of the loss factor obtained from the two nanocomposites with the dissimilarities in the structure and morphology of the two varieties of MWCNTs. Inner tube oscillation is simulated using molecular dynamics (MD). Since the open-ended double walled CNT (DWCNT) models used in earlier studies over predict the damping, we propose a capped DWCNT model. This can simulate the atomic interactions at the end caps of the tube. This study indicates that the contributions to the observed damping have their origins in the interaction between atoms that constitute the inner and outer tubes rather than the inter-tube frictional energy loss.

Recoverability of a gigahertz rotation-translation nanoconvertor with hydrogenated deformable rotor at room temperature

To design a rotor with recoverable deformation for conversion between rotation and translation in a nanodevice, an internally hydrogenated deformable part (HDP) was introduced in the carbon nanotube-based rotor. Initially, under van der Waals (vdW) force, the hydrogenated areas on the HDP curved toward the rotating axis. When a rotational frequency was exerted on the rotor, the hydrogenated parts on the HDP were separated under strong centrifugal force. Translational motion of the free edge of the rotor was generated synchronously during deformation of the HDP. Once removing the input rotation, the rotor would stop rotating by friction from the stators, and the HDP shrank back by strong vdW force but weakening centrifugal force. Hence, the nanoconvertor has recoverability, which was verified by molecular dynamics simulations with considering the effects of hydrogenation schemes and input rotational frequency at room temperature. Conclusions were drawn for a design of a nanodevice based on the present rotation-translation nanoconvertor model.

Defect-driven rotating system based on a double-walled carbon nanotube and graphene

A nanoscale rotating system that consists of a double-walled carbon nanotube (DWCNT) and graphene and is driven by a defect in the graphene is proposed, and its rotating dynamics and driving mechanism are investigated through molecular dynamics simulations. A potential energy difference caused by the presence of the vacancy defect on the graphene substrate causes the outer tube in the DWCNT to stably rotate in a specific direction. The rotational speed of the outer tubem initially increases before reaching a stable speed. This phenomenon indicates that the driving torque is a difference between the sides of the outer tube in the van der Waals potential; this difference in potential is caused by the presence of the defect in the graphene. In addition, the effects of the system temperature, the radius and chiral vectors of the DWCNT, and the location of the defect in the graphene are investigated. The theoretical work reported here should provide a reference for the design of motion systems based on carbon nanotubes and graphene and their applications.

Critical Output Torque of a GHz CNT-Based Rotation Transmission System Via Axial Interface Friction at Low Temperature

It was discovered that a sudden jump of the output torque moment from a rotation transmission nanosystem made from carbon nanotubes (CNTs) occurred when decreasing the system temperature. In the nanosystem from coaxial-layout CNTs, the motor with specified rotational frequency (ω_M) can drive the inner tube (rotor) to rotate in the outer tubes. When the axial gap between the motor and the rotor was fixed, the friction between their neighbor edges was stronger at a lower temperature. Especially at temperatures below 100 K, the friction-induced driving torque increases with ω_M. When the rotor was subjected to an external resistant torque moment (M_r), it could not rotate opposite to the motor even if it deformed heavily. Combining molecular dynamics simulations with the bi-sectioning algorithm, the critical value of M_r was obtained. Under the critical torque moment, the rotor stopped rotating. Accordingly, a transmission nanosystem can be designed to provide a strong torque moment via interface friction at low temperature.

Detection of molecular behavior that characterizes systems using a deep learning approach

Molecular dynamics (MD) simulation is a powerful computational method to observe molecular behavior. Although the detection of molecular behavior that characterizes systems is an important task in the study of MD, it is typically difficult and depends on human expert knowledge. Therefore, we propose a novel analysis scheme for MD data using deep neural networks. A key aspect of our scheme is the estimation of statistical distances between different ensembles that are probability distributions over the possible states of systems. This allows us to build low-dimensional embeddings of ensembles to visualize differences between systems in a compact metric space. Furthermore, the molecular behavior that contributes to the differences between systems can also be detected using the trained function of deep neural networks. The applicability of our scheme is demonstrated using three types of MD data. Our scheme could be a powerful tool to clarify the underlying physics in the molecular systems.

Diamond Needles Actuating Triple-Walled Carbon Nanotube to Rotate via Thermal Vibration-Induced Collision

A rotary nanomotor is an essential component of a nanomachine. In the present study, a rotary nanomotor from wedged diamonds and triple-walled nanotubes was proposed with the consideration of boundary effect. The outer tubes and mid-tubes were used as nanobearing to constrain the inner tube. Several wedges of the diamond were placed near the inner tube for driving the inner tube to rotate. At a temperature lower than 300 K, the inner tube as the rotor had a stable rotational frequency (SRF). It is shown that both the rotational direction and the value of SRF of the rotor depended on the temperature and thickness of the diamond wedges. The dependence was investigated via theoretical analysis of the molecular dynamics simulation results. For example, when each diamond wedge had one pair of tip atoms (unsaturated), the rotational direction of the rotor at 100 K was opposite to that at 300 K. At 500 K, the rotating rotor may stop suddenly due to breakage of the diamond needles. Some conclusions are drawn for potential application of such a nanomotor in a nanomachine.

Thermal shrinkage and stability of diamondene nanotubes

By curving a rectangular diamondene, an sp ²/sp ³ composite carbon film, a diamondene nanotube (DNT) can be formed when the two straight edges are sewn together. In this study, thermal stabilities of DNTs are investigated using molecular dynamics simulation approaches. An interesting thermal shrinkage of damaged DNTs is discovered. Results indicate that DNTs have critical temperatures between 320 K and 350 K. At temperatures higher than the critical value, the interlayer bonds, i.e., the sp ³-sp ³ bonds, may break. The broken ratio of the interlayer bonds mainly depends on the temperature. For the DNT with a high broken ratio of interlayer bonds, it has thermal shrinkage in both the cross section and tube axis. The sp ²-sp ³ bonds in either the inner or the outer surface are much more stable. Even at 900 K, only a few sp ²-sp ³ bonds break. These properties can be used in the design of metamaterials.

Coupling effect of van der Waals, centrifugal, and frictional forces on a GHz rotation-translation nano-convertor

A nano rotation-translation convertor with a deformable rotor is presented, and the dynamic responses of the system are investigated considering the coupling among the van der Waals (vdW), centrifugal and frictional forces. When an input rotational frequency (ω) is applied at one end of the rotor, the other end exhibits a translational motion, which is an output of the system and depends on both the geometry of the system and the forces applied on the deformable part (DP) of the rotor. When centrifugal force is stronger than vdW force, the DP deforms by accompanying the translation of the rotor. It is found that the translational displacement is stable and controllable on the condition that ω is in an interval. If ω exceeds an allowable value, the rotor exhibits unstable eccentric rotation. The system may collapse with the rotor escaping from the stators due to the strong centrifugal force in eccentric rotation. In a practical design, the interval of ω can be found for a system with controllable output translation.

An intrinsic energy conversion mechanism via telescopic extension and retraction of concentric carbon nanotubes

The conversion of other forms of energy into mechanical work through the geometrical extension and retraction of nanomaterials has a wide variety of potential applications, including for mimicking biomotors. Here, using molecular dynamics simulations, we demonstrate that there exists an intrinsic energy conversion mechanism between thermal energy and mechanical work in the telescopic motions of double-walled carbon nanotubes (DWCNTs). A DWCNT can inherently convert heat into mechanical work in its telescopic extension process, while convert mechanical energy into heat in its telescopic retraction process. These two processes are nearly thermodynamically reversible. The underlying mechanism for this energy conversion is that the configurational entropy changes with the telescopic overlapping length of concentric individual tubes. We also find that the entropy effect enlarges with the decreasing intertube space of DWCNTs. As a result, the spontaneous telescopic motion of a condensed DWCNT can be switched to extrusion by increasing the system temperature above a critical value. These findings are important for fundamentally understanding the mechanical behavior of concentric nanotubes, and may have general implications in the application of DWCNTs as linear motors in nanodevices.

Snap-through transition of buckled graphene membranes for memcapacitor applications

Using computational and theoretical approaches, we investigate the snap-through transition of buckled graphene membranes. Our main interest is related to the possibility of using the buckled membrane as a plate of capacitor with memory (memcapacitor). For this purpose, we performed molecular-dynamics (MD) simulations and elasticity theory calculations of the up-to-down and down-to-up snap-through transitions for membranes of several sizes. We have obtained expressions for the threshold switching forces for both up-to-down and down-to-up transitions. Moreover, the up-to-down threshold switching force was calculated using the density functional theory (DFT). Our DFT results are in general agreement with MD and analytical theory findings. Our systematic approach can be used for the description of other structures, including nanomechanical and biological ones, experiencing the snap-through transition.

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.

Structure relaxation via long trajectories made stable

Molecular dynamics (MD) is appearing in increasing applications in materials science, nanotechnologies, condensed matter physics, computational physics, biochemistry, and biophysics. Finding mechanically static equilibrium configurations of molecular systems is one of the most practical tasks in MD. Most existing potential energy optimization algorithms do not permit searching equilibrium configurations through longer MD trajectories. We introduce a simple method of utilizing a microcanonical (NVE) ensemble to obtain static equilibriums of molecular systems, that is significantly faster than the standard implementations of quick-min (QM) and fast inertial relaxation engine (FIRE) optimization algorithms. The new method is based on the capability of NVE to convert potential energy to kinetic energy. The surprising efficiency of the method is illustrated using an indentation test on monolayer graphene and, in particular, the versatility of the method is illustrated using relaxation of a polystyrene chain through longer MD trajectories and large deformation. The capability of the new method in finding more stable equilibrium configurations than common optimization algorithms is demonstrated in relaxation of a pressured lubricating oil layer and a warped monolayer graphene cantilever.

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.

Critical conditions for escape of a high-speed fullerene from a BNC nanobeam after collision

For a resonator-based nano-balance, the capability of capturing a nanoparticle is essential for it to measure the mass of the particle. In the present study, a clamped-clamped nanobeam from a Boron-Nitride and Carbon (BNC) nanotube acts as the nano-balance, and a fullerene, e.g., C60, is chosen as the particle, and the capturing capability is quantitatively estimated by the minimal escape velocity (MEV) of the fullerene from the nanobeam after collision. When centrally colliding with the nanobeam, the escape of fullerene depends on both incidence of fullerene and temperature of the system. When the colliding in the Boron-Nitride (BN) area of the beam surface, the nanoball escapes easier than that at the carbon area. The MEV of the nanoball is lower at higher temperature. As the nanoball sometimes slides for a few pica-seconds on the beam surface before being bounced out, the nanoball can escape only when the beam surface can provide the nanoball enough kinetic energy to overcome the van der Waals interaction between them. The capturing capability of the nano-balance can, thus, be improved by reducing the initial kinetic energy of the system.

Conditions for escape of a rotor in a rotary nanobearing from short triple-wall nanotubes

In a short nanobearing system made from carbon nanotubes, the rotor with high rotational frequency may escape from the stator, which may cause a stability problem to the system of a nanodevice with such a nanobearing. In the present work, nanobearings with tri-walled nanotubes are investigated to reveal the conditions for the moving away of the free inner tube from the high-speed rotating middle tube. Experimental results show that the escape happens when the radii difference between the two rotors is larger than 0.34 nm and the rotational frequency of the middle tube is higher than a critical value. And before the escape occurs, the rotational frequency of the inner tube is lower than this critical value. Due to the radii difference being larger than 0.34 nm, the two rotors are non-coaxial, and the centrifugal force of the inner tube results in strong radial and axial interactions between the edges of the two rotors. When the relative sliding speed is relatively high, an edge of the inner rotor will pass through the potential barrier at the adjacent edge of the middle rotor, and further escape from the middle rotor occurs. The selection of a longer middle rotor with smaller radius can increase the critical rotational frequency of the middle rotor.

Multi-walled boron nitride nanotubes as self-excited launchers

A self-excited launcher consisting of multi-walled boron nitride nanotubes (BNNTs) has been investigated using molecular dynamics simulation. The results show that, after a period of high frequency oscillation, the innermost BNNT can be spontaneously ejected along its central axis at a relatively fast speed. The launching is caused by the energy transfer between the nanotubes and without absorbing energy from the external environment. Most self-excited launchers could launch their innermost nanotube, although an inappropriate structure of the nanotubes contributes to a blocked or failed launch. In addition, a launch angle corrector and a nanotube receiver associated with a self-excited launcher are also manufactured to precisely control the launch angle and distance of the BNNTs. This study provides the possibility to fabricate and design self-excited launchers using multi-walled nanotubes.

Novel Superhard sp^{3} Carbon Allotrope from Cold-Compressed C_{70} Peapods

Design and synthesis of new carbon allotropes have always been important topics in condensed matter physics and materials science. Here we report a new carbon allotrope, formed from cold-compressed C_{70} peapods, which most likely can be identified with a fully sp^{3}-bonded monoclinic structure, here named V carbon, predicted from our simulation. The simulated x-ray diffraction pattern, near K-edge spectroscopy, and phonon spectrum agree well with our experimental data. Theoretical calculations reveal that V carbon has a Vickers hardness of 90 GPa and a bulk modulus ∼400  GPa, which well explains the "ring crack" left on the diamond anvils by the transformed phase in our experiments. The V carbon is thermodynamically stable over a wide pressure range up to 100 GPa, suggesting that once V carbon forms, it is stable and can be recovered to ambient conditions. A transition pathway from peapod to V carbon has also been suggested. These findings suggest a new strategy for creating new sp^{3}-hybridized carbon structures by using fullerene@nanotubes carbon precursor containing odd-numbered rings in the structures.

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.

Buckling behaviour of composites with double walled nanotubes from carbon and phosphorus

Due to weak interactions among phosphorus atoms in black phosphorene, a nanotube obtained by curling single-layer black phosphorus is not as stable as a carbon nanotube (CNT) at finite temperature. In the present work, we recommend a new 1D composite material with a double-walled nanotube (DWNT) from a black phosphorus nanotube (BPNT) and a CNT. The dynamic response of the composite DWNTs is simulated using a molecular dynamics approach. Effects of the factors including temperature, slenderness and configurations of DWNTs on dynamic behavior of the composite are discussed. Compared with a single-walled BPNT, the composite DWNTs under uniaxial compression show some unique properties. When a BPNT is embedded in a CNT which will not only isolate the BPNT from the ambient conditions, but also improve the capability of axial deformation of the BPNT, the system will not collapse rapidly even if the BPNT has been buckled.

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.

Robust rotation of rotor in a thermally driven nanomotor

In the fabrication of a thermally driven rotary nanomotor with the dimension of a few nanometers, fabrication and control precision may have great influence on rotor’s stability of rotational frequency (SRF). To investigate effects of uncertainty of some major factors including temperature, tube length, axial distance between tubes, diameter of tubes and the inward radial deviation (IRD) of atoms in stators on the frequency’s stability, theoretical analysis integrating with numerical experiments are carried out. From the results obtained via molecular dynamics simulation, some key points are illustrated for future fabrication of the thermal driven rotary nanomotor.

Finding Stable Graphene Conformations from Pull and Release Experiments with Molecular Dynamics

Here, we demonstrate that stable conformations of graphene nanoribbons can be identified using pull and release experiments, when the stretching force applied to a single-layer graphene nanoribbon is suddenly removed. As it is follows from our numerical experiments performed by means of molecular dynamics simulations, in such experiments, favorable conditions for the creation of folded structures exist. Importantly, at finite temperatures, the process of folding is probabilistic. We have calculated the transition probabilities to folded conformations for a graphene nanoribbon of a selected size. Moreover, the ground state conformation has been identified and it is shown that its type is dependent on the nanoribbon length. We anticipate that the suggested pull and release approach to graphene folding may find applications in the theoretical studies and fabrication of emergent materials and their structures.

Extracting the inner wall from nested double-walled carbon nanotube by platinum nanowire: molecular dynamics simulations

The Pt nanowire with proper diameter is able to extract the inner wall/s in nested double-walled/multi-walled carbon nanotubes. For the multi-walled carbon nanotubes, the Pt nanowire with proper diameter can only peel off the outermost wall.

A nanoengine governor based on the end interfacial effect

A conceptual design is presented for a nanoengine governor based on the end interfacial effect of two rotary nanotubes. The governor contains a thermal-driven rotary nanomotor made from double-walled carbon nanotubes (DWCNTs) and a coaxially laid out rotary nanotube near one end of the nanomotor rotor. The rotation of the rotor in the nanomotor can be controlled by two features. One is the stator (the outer tube of DWCNTs) which has some end atoms with inward radial deviation (IRD) on the stator. The other is the relative rotation of the neighboring rotary tube of the rotor. As the configuration of the stator is fixed, the end interfacial interaction between the two rotors will govern the dynamic response of the rotor in the nanomotor system. The obtained results demonstrate that the relative rotational speed between the two rotors provides friction on the rotor in the nanomotor system. In particular, higher relative rotational speed will provide lower friction on rotor 1, which is opposite to that between neighboring shells in DWCNTs.

Sustaining GHz oscillation of carbon nanotube based oscillators via a MHz frequency excitation

There have been intensive studies to investigate the properties of gigahertz nano-oscillators based on multi-walled carbon nanotubes (MWCNTs). Many of these studies, however, revealed that the unique telescopic translational oscillations in such devices would damp quickly due to various energy dissipation mechanisms. This challenge remains the primary obstacle against its practical applications. Herein, we propose a design concept in which a GHz oscillation could be re-excited by a MHz mechanical motion. This design involves a triple-walled CNT, in which sliding of the longer inner tube at a MHz frequency can re-excite and sustain a GHz oscillation of the shorter middle tube. Our molecular dynamics (MD) simulations prove this design concept at ∼10 nm scale. A mathematical model is developed to explore the feasibility at a larger size scale. As an example, in an oscillatory system with the CNT's length above 100 nm, the high oscillatory frequency range of 1.8-3.3 GHz could be excited by moving the inner tube at a much lower frequency of 53.4 MHz. This design concept together with the mechanical model could energize the development of GHz nano-oscillators in miniaturized electro-mechanical devices.

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.

Dynamic behavior of curved double-wall carbon nanotubes with rotating inner tube

Molecular dynamics simulation is adopted to study the dynamic behavior of an oscillator from curved double-walled carbon nanotubes with rotating inner tube. Effects of curvature and temperature on its dynamic response are investigated.

Electronic and optical properties of pristine and boron–nitrogen doped graphyne nanotubes

The band gaps and optical responses of graphyne nanotubes can be engineered through the selection of the BN doping site and the chirality.

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.

Observation of high-speed microscale superlubricity in graphite

A sheared microscopic graphite mesa retracts spontaneously to minimize interfacial energy. Using an optical knife-edge technique, we report first measurements of the speeds of such self-retracting motion (SRM) from the mm/s range at room temperature to 25 m/s at 235°C [corrected]. This remarkably high speed is comparable with the upper theoretical limit found for sliding interfaces exhibiting structural superlubricity. We observe a strong temperature dependence of SRM speed which is consistent with a thermally activated mechanism of translational motion that involves successive pinning and depinning events at interfacial defects. The activation energy for depinning is estimated to be 0.1-1 eV.

Mechanics of Ellipsoidal Carbon Onions Inside Multiwalled Carbon Nanotubes

On the basis of the continuum approximation along with Lennard–Jones potential function, new semi-analytical expressions are presented to evaluate the van der Waals interactions between an ellipsoidal fullerene and a semi-infinite single-walled carbon nanotube. Using direct method, these expressions are also extended to model ellipsoidal carbon onions inside multiwalled carbon nanotubes. In addition, acceptance and suction energies which are two noticeable issues for medical applications such as drug delivery are determined. Neglecting the frictional effects and by imposing some simplifying assumptions on the van der Waals interaction force, a simple formula is given to evaluate the oscillation frequency of ellipsoidal carbon onions inside multiwalled carbon nanotubes. Also, the effects of the number of tube shells and ellipsoidal carbon onion shells on the oscillatory behavior are examined. It is shown that there exists an optimal value for the number of tube shells beyond which the oscillation frequency remains unchanged.