Arunkumar Subramanian

In situ observation of the electrochemical lithiation of a single SnO2 nanowire electrode.

Fragile Tin Oxide Electrodes While tin oxide has a high energy density, and would thus make an attractive anode material for a Li-ion battery, it undergoes significant volume changes when Li is intercalated. The large strains cause cracking, pulverization, and a resultant loss of electrical conduction. Huang et al. (p. 1515; see the Perspective by Chiang) used in situ transmission electron microscopy on a single tin oxide nanowire to identify the physical changes that occur during intercalation and observed a moving cloud of dislocations that separated the reacted and unreacted sections. Upon completion of the electrochemical charging, the nanowire showed up to 90% elongation and a 35% increase in diameter. Transmission electron microscopy reveals dimensional changes in a tin oxide nanowire as it intercalates lithium. We report the creation of a nanoscale electrochemical device inside a transmission electron microscope—consisting of a single tin dioxide (SnO2) nanowire anode, an ionic liquid electrolyte, and a bulk lithium cobalt dioxide (LiCoO2) cathode—and the in situ observation of the lithiation of the SnO2 nanowire during electrochemical charging. Upon charging, a reaction front propagated progressively along the nanowire, causing the nanowire to swell, elongate, and spiral. The reaction front is a “Medusa zone” containing a high density of mobile dislocations, which are continuously nucleated and absorbed at the moving front. This dislocation cloud indicates large in-plane misfit stresses and is a structural precursor to electrochemically driven solid-state amorphization. Because lithiation-induced volume expansion, plasticity, and pulverization of electrode materials are the major mechanical effects that plague the performance and lifetime of high-capacity anodes in lithium-ion batteries, our observations provide important mechanistic insight for the design of advanced batteries.

Batch fabrication of carbon nanotube bearings

Relative displacements between the atomically smooth, nested shells in multiwalled carbon nanotubes (MWNTs) can be used as a robust nanoscale motion enabling mechanism. Here, we report on a novel method suited for structuring large arrays of MWNTs into such nanobearings in a parallel fashion. By creating MWNT nanostructures with nearly identical electrical circuit resistance and heat transport conditions, uniform Joule heating across the array is used to simultaneously engineer the shell geometry via electric breakdown. The biasing approach used optimizes process metrics such as yield and cycle-time. We also present the parallel and piecewise shell engineering at different segments of a single nanotube to construct multiple, but independent, high density bearings. We anticipate this method for constructing electromechanical building blocks to be a fundamental unit process for manufacturing future nanoelectromechanical systems (NEMS) with sophisticated architectures and to drive several nanoscale transduction applications such as GHz-oscillators, shuttles, memories, syringes and actuators.

Carbon nanotubes for nanorobotics

The well-defined geometry, exceptional mechanical properties, and extraordinary electrical characteristics of carbon nanotubes qualify them for structuring nanoelectronic circuits, nanoelectromechanical systems, and nanorobotic systems. Relative displacements between the atomically smooth, nested shells in multiwalled carbon nanotubes can be used as robust nanoscale motion enabling mechanisms for applications such as bearings, switches, gigahertz oscillators, shuttles, memories, syringes, and actuators. The hollow structures of carbon nanotubes can serve as containers, conduits, pipettes, and coaxial cables for storing mass and charge, or for transport. Not only can nanotubes serve as building blocks for more complex structures, tools, sensors, and actuators, but they can also be used as fundamental components for future nanorobots. We review the technological progress on carbon nanotubes related to nanorobotics.

Engineering Multiwalled Carbon Nanotubes Inside a Transmission Electron Microscope Using Nanorobotic Manipulation

This paper provides a review of recent experimental techniques developed for shell engineering individual multiwalled carbon nanotubes (MWNTs). Basic processes for the nanorobotic manipulation of MWNTs inside a transmission electron microscope are investigated. MWNTs, bamboo-structured carbon nanotubes (CNTs), Cu-filled CNTs, and CNTs with quantum dots attached are used as test structures for manipulation. Picking is realized using van der Waals forces, ldquostickyrdquo probes, electron-beam-induced deposition, and electric breakdown. Cap opening and shell shortening are presented using field emission current. Controlled peeling and thinning of the shells of MWNTs are achieved by electric breakdown, and changes in MWNT structures are correlated with electrical measurements. These processes are fundamental for the characterization of nanoscale materials, the structuring of nanosized building blocks, and the prototyping of nanoelectromechanical systems.

Supermolecular switches based on multiwalled carbon nanotubes

Electrostatically actuated nanoelectromechanical switches based on intershell displacement mechanisms within batch fabricated, bidirectional multiwalled carbon nanotube (MWNT) bearings are reported. Multiple devices with a 220 nm pitch are constructed within individual MWNT supermolecules. Experimental results on performance metrics including low switching voltages (0.8 to 6 V), repeatability, hysteresis, and failure modes are presented.

Shaping Nanoelectrodes for High-Precision Dielectrophoretic Assembly of Carbon Nanotubes

To achieve high-precision dielectrophoretic (DEP) assembly of carbon nanotubes (CNTs) for nanoelectronic circuits and nanoelectromechanical systems (NEMS), a technique is investigated both theoretically and experimentally for shaping the local geometries of nanoelectrodes to control the electrohydrodynamic behavior of CNTs. Motion trajectories and positions of CNTs assembled on electrodes are predicted based on calculated DEP forces and torques. Both simulation and experimental results show that the geometries of two opposing electrodes significantly affect the precision and robustness with which CNTs can be deposited. Experimental investigation of an electrode array demonstrates that the spacing between neighboring electrode pairs should be larger than twice the width of electrodes to avoid overlapping electric fields and unstable DEP forces; otherwise, unequally distributed electric fields and DEP forces induce a significant number of assembly failures in the array.

Simulation of Rotary Motion Generated by Head-to-Head Carbon Nanotube Shuttles

A novel rotary nanomotor is described using two axially aligned, opposing chirality nanotube shuttles. Based on intershell screw-like motion of nanotubes, rotary motion is generated by electrostatically pulling the two cores together. Simulations using molecular dynamics show the generation of rotation from armchair nanotube pairs and their actuation properties. The simulation results point toward the use of these motors as building blocks in nanoelectromechanical systems and nanorobotic systems for sensing, actuation, and computation applications.

Micro and Nanorobotic Assembly Using Dielectrophoresis.

The contact phase of an assembly task involving micro and nano objects is complicated by the presence of surface and intermolecular forces such as electrostatic, surface-tension and Van der Waals forces. Assembly strategies must account for the presence of these forces in order to guarantee successful repeatable micro and nanoassemblies with high precision. A detailed model for this electrostatic interaction is developed and analyzed. Based on the results of this analysis, dielectrophoretic assembly principles of MEMS/NEMS devices are proposed and experimentally verified with microtweezers for micro Ni parts and with nanoelectrodes fabricated with electron-beam lithography for carbon nanotube assembly. The successful manipulation and assembly of single carbon nanotubes (CNTs) using dielectrophoretic forces produced by nanoelectrodes will lead to a higher integration of CNTs into both nanoelectronics and NEMS.

Electrostatic actuation and electromechanical switching behavior of one-dimensional nanostructures.

We report on the electromechanical actuation and switching performance of nanoconstructs involving doubly clamped, individual multiwalled carbon nanotubes. Batch-fabricated, three-state switches with low ON-state voltages (6.7 V average) are demonstrated. A nanoassembly architecture that permits individual probing of one device at a time without crosstalk from other nanotubes, which are originally assembled in parallel, is presented. Experimental investigations into device performance metrics such as hysteresis, repeatability and failure modes are presented. Furthermore, current-driven shell etching is demonstrated as a tool to tune the nanomechanical clamping configuration, stiffness, and actuation voltage of fabricated devices. Computational models, which take into account the nonlinearities induced by stress-stiffening of 1-D nanowires at large deformations, are presented. Apart from providing accurate estimates of device performance, these models provide new insights into the extension of stable travel range in electrostatically actuated nanowire-based constructs as compared to their microscale counterparts.

Dielectrophoretic micro/nanoassembly with microtweezers and nanoelectrodes

The contact phase of an assembly task involving micro and nano-sized building blocks is complicated by the presence of surface and intermolecular forces such as electrostatic, surface-tension and Van der Waals forces. Assembly strategies must account for the presence of these forces in order to guarantee successful repeatable micro and nanoassembly with high precision. In this paper, a detailed model is presented for the electrostatic interactions at these scales and qualitatively analyzed. Based on the results of this analysis, dielectrophoretic assembly principles of MEMS/NEMS devices are proposed and experimentally verified with microtweezers for microscale Ni parts and with nanoelectrodes fabricated by electron-beam lithography for carbon nanotube assembly. The successful manipulation and assembly of carbon nanotubes using dielectrophoretic forces produced by nanoelectrodes will lead to a higher integration of CNTs into both nanoelectronics and NEMS