Changhong Ke

Numerical Analysis of Nanotube Based NEMS Devices — Part II: Role of Finite Kinematics, Stretching and Charge Concentrations

In this paper ci nonlinear analysis of nanotuhe based nano-electromechanical systems is reported. Assuming continuum mechanics, the complete nonlinear equation of the elastic line of the nanotube is derived and then numerically solved. In particular, we study singly and doubly clamped nanotubes under electrostatic actuation. The analysis emphasizes the importance of nonlinear kinematics effects in the prediction of the pull-in voltage of the device, a key design parameter. Moreover, the nonlinear behavior associated with finite kinematics (i.e., large deformations), neglected in previous studies, as well as charge concentrations at the tip of singly clamped nanotuhes, are investigated in detail. We show that nonlinear kinematics results in an important increase in the pull-in voltage of doubly clamped nanotube devices, but that it is negligible in the case of singly damped devices. Likewise, we demonstrate that charge concentration at the tip of singly clamped devices results in a significant reduction in pull-in voltage. By comparing numerical results to analytical predictions, closed form formulas are verified. These formulas provide a guide on the effect of the various geometrical variables and insight into the design of novel devices.

Numerical Analysis of Nanotube-Based NEMS Devices—Part I: Electrostatic Charge Distribution on Multiwalled Nanotubes

The charge distribution on the surface of a biased conductive, finite-length, cylindrical nanotube, free standing above an infinite grounded plane, is investigated. The diameter range of the cylinder tube under study is 20-60 nm, which is much larger than the screening length, meaning the quantum and statistical effects on the charge distribution are negligible. The relationship between the charge distribution and the geometry of the nanotube is examined in detail by classical electrostatics using full three-dimensional numerical simulations based on the boundary element method. A model of the concentrated charge at the end of nanotubes is proposed. The charge distribution for a clamped cantilever nanotube is also computed and discussed. The findings here reported are of particular usefulness in the design and modeling of electrostatic actuated nanotube/ nanowire based nano-electromechanical systems.

Feedback controlled nanocantilever device

A switchable carbon-nanotube-based nanoelectromechanical systems (NEMS) with close-loop feedback is examined. The device consists of a cantilever carbon nanotube clamped to a top electrode and actuated by a bottom electrode. The actuation circuit includes a source and a feedback resistor. The pull-in/pull-out and tunneling characteristics of the system are investigated by means of an electromechanical analysis. The model includes the concentration of electrical charge, at the end of the nanocantilever, and the van der Waals force. The analysis shows that the device has two well-defined stable equilibrium positions as a result of tunneling and the incorporation of a feedback resistor to the circuit. Potential applications of the device include NEMS switches, random-access memory elements, logic devices, electron counters, and gap sensing devices.

Mechanical Peeling of Free‐Standing Single‐Walled Carbon‐Nanotube Bundles

An in situ electron microscopy study is presented of adhesion interactions between single-walled carbon nanotubes (SWNTs) by mechanically peeling thin free-standing SWNT bundles using in situ nanomanipulation techniques inside a high-resolution scanning electron microscope. The in situ measurements clearly reveal the process of delaminating one SWNT bundle from its originally bound SWNT bundle in a controlled-displacement manner and capture the deformation curvature of the delaminated SWNT bundle during the peeling process. A theoretical model based on nonlinear elastica theory is employed to interpret the measured deformation curvatures of the SWNTs and to quantitatively evaluate the peeling force and the adhesion strength between bundled SWNTs. The estimated adhesion energy per unit length for each pair of neighboring tubes in the peeling interface based on our peeling experiments agrees reasonably well with the theoretical value. This in situ peeling technique provides a potential new method for separating bundled SWNTs without compromising their material properties. The combined peeling experiments and modeling presented in this paper will be very useful to the study of the adhesion interactions between SWNTs and their nonlinear mechanical behaviors in the large-displacement regime.

Analysis of Doubly Clamped Nanotube Devices in the Finite Deformation Regime

In this paper, a nonlinear theory applicable to the design of nanotube based devices is presented. The role of finite kinematics for a doubly clamped nanotube device is investigated. In particular, we analyze the continuous deformation and instability (pull in) of a clamped-clamped nanotube suspended over an electrode from which a potential differential is imposed. The transformation of an applied voltage into a nanomechanical deformation indeed represents a key step toward the design of innovative nanodevices. Likewise, accurate prediction of pull-in/pull-out voltages is highly needed. We show that an energy-based method can be conveniently used to predict the structural behavior and instability corresponding to the ON/OFF states of the device at the so-called pull-in voltage. The analysis reveals that finite kinematics effects can result in a significant increase of the pull-in voltage. This increase results from a ropelike behavior of the nanotube as a consequence of the stretching imposed by the actuation.

Bending stiffness and interlayer shear modulus of few-layer graphene

Interlayer shear deformation occurs in the bending of multilayer graphene with unconstrained ends, thus influencing its bending rigidity. Here, we investigate the bending stiffness and interlayer shear modulus of few-layer graphene through examining its self-folding conformation on a flat substrate using atomic force microscopy in conjunction with nonlinear mechanics modeling. The results reveal that the bending stiffness of 2–6 layers graphene follows a square-power relationship with its thickness. The interlayer shear modulus is found to be in the range of 0.36–0.49 GPa. The research findings show that the weak interlayer shear interaction has a substantial stiffening effect for multilayer graphene.

Radial Mechanical Properties of Single-Walled Boron Nitride Nanotubes

The radial mechanical properties of single-walled boron nitride nanotubes (SW-BNNTs) are investigated by atomic force microscopy. Nanomechanical measurements reveal the radial deformation of individual SW-BNNTs in both elastic and plastic regimes. The measured effective radial elastic moduli of SW-BNNTs are found to follow a decreasing trend with an increase in tube diameter, ranging from 40.78 to 1.85 GPa for tube diameters of 0.58 to 2.38 nm. The results show that SW-BNNTs have relatively lower effective radial elastic moduli than single-walled carbon nanotubes (SWCNTs). The axially strong, but radially supple characteristics suggest that SW-BNNTs may be superior to SWCNTs as reinforcing additives for nanocomposite applications.

Direct Measurements of the Mechanical Strength of Carbon Nanotube–Poly(methyl methacrylate) Interfaces

Understanding the interfacial stress transfer between carbon nanotubes (CNTs) and polymer matrices is of great importance to the development of CNT-reinforced polymer nanocomposites. In this paper, an experimental study is presented of the interfacial strength between individual double-walled CNTs and poly(methyl methacrylate) (PMMA) using an in situ nanomechanical single-tube pull-out testing scheme inside a high-resolution electron microscope. By pulling out individual tubes with different embedded lengths, this work reveals the shear lag effect on the nanotube-polymer interface and demonstrates that the effective interfacial load transfer occurs only within a certain embedded length. These results show that the CNT-PMMA interface possesses an interfacial fracture energy within 0.054-0.80 J/m(2) and a maximum interfacial strength within 85-372 MPa. This work is useful to better understand the local stress transfer on nanotube-polymer interfaces.

Robust Carbon-Nanotube-Based Nano-electromechanical Devices: Understanding and Eliminating Prevalent Failure Modes Using Alternative Electrode Materials

The International Technology Roadmap for Semiconductors (ITRS [ 1 ] ) identifi es emerging technologies with the potential to sustain Moore’s Law. A necessary succession from planar CMOS (complementary metal-oxide semiconductors) to nonplanar/dual-gate CMOS, and ultimately to novel architectures such as carbon nanotube (CNT)-based nano-electromechanical systems (NEMS) is envisioned. The ITRS also identifi es critical roadblocks currently precluding advances beyond CMOS. Primary among the roadblocks to NEMS are poor reliability and manufacturing challenges. Here we investigate the prevalent failure modes of CNT-based NEMS that hamper reliability through a combined experimental–computational approach. We fi rst identify their point of onset within the design space through in situ electromechanical characterization, highlighting the extremely limited region in which failure is avoided. We use dynamic multiphysics models to elucidate the underlying causes of failure, then return to the experimental characterization to show that the usable design space expands dramatically when employing novel electrode materials such as diamondlike carbon. Finally, we demonstrate the effi cacy of this solution through 100 successive actuation cycles without failure and applications to volatile memory operations. The immense potential of CNT-based NEMS is emergent in theoretical and experimental demonstrations of up to 100-GHz switching, [ 2 ] low leakage, and high ON–OFF ratios, [ 3 ] and outstanding current-carrying capacity. [ 4 , 5 ] To date however, individual demonstrations of performance such as these have been a primary focus, with limited reports of repeated actuation beyond a few cycles. [ 2 , 3 , 6 , 7 ] This is due

Radial elasticity of multi-walled boron nitride nanotubes.

We investigated the radial mechanical properties of multi-walled boron nitride nanotubes (MW-BNNTs) using atomic force microscopy. The employed MW-BNNTs were synthesized using pressurized vapor/condenser (PVC) methods and were dispersed in aqueous solution using ultrasonication methods with the aid of ionic surfactants. Our nanomechanical measurements reveal the elastic deformational behaviors of individual BNNTs with two to four tube walls in their transverse directions. Their effective radial elastic moduli were obtained through interpreting their measured radial deformation profiles using Hertzian contact mechanics models. Our results capture the dependences of the effective radial moduli of MW-BNNTs on both the tube outer diameter and the number of tube layers. The effective radial moduli of double-walled BNNTs are found to be several-fold higher than those of single-walled BNNTs within the same diameter range. Our work contributes directly to a complete understanding of the fundamental structural and mechanical properties of BNNTs and the pursuits of their novel structural and electronics applications.