Eric W. Wong

Probing electrical transport in nanomaterials: Conductivity of individual carbon nanotubes

A general approach has been developed to determine the conductivity of individual nanostructures while simultaneously recording their structure. Conventional lithography has been used to contact electrically single ends of nanomaterials, and a force microscope equipped with a conducting probe tip has been used to map simultaneously the structure and resistance of the portion of the material protruding from the macroscopic contact. Studies of individual carbon nanotubes demonstrate that the structurally most perfect nanotubes have resistivities an order of magnitude lower than those found previously and that defects in the nanotube structure cause substantial increases in the resistivity.

fMRI of Monkey Visual Cortex

While functional magnetic resonance imaging (fMRI) is now used widely for demonstrating neural activity-related signals associated with perceptual, motor, and cognitive processes in humans, to date this technique has not been developed for use with nonhuman primates. fMRI in monkeys offers a potentially valuable experimental approach for investigating brain function, which will complement and aid existing techniques such as electrophysiology and the behavioral analysis of the effects of brain lesions. There are, however, a number of significant technical challenges involved in using fMRI with monkeys. Here, we describe the procedures by which we have overcome these challenges to carry out successful fMRI experiments in an alert monkey, and we present the first evidence of activity-related fMRI signals from monkey cerebral cortex.

Substitutional doping in nanocrystal superlattices

Doping is a process in which atomic impurities are intentionally added to a host material to modify its properties. It has had a revolutionary impact in altering or introducing electronic, magnetic, luminescent, and catalytic properties for several applications, for example in semiconductors. Here we explore and demonstrate the extension of the concept of substitutional atomic doping to nanometre-scale crystal doping, in which one nanocrystal is used to replace another to form doped self-assembled superlattices. Towards this goal, we show that gold nanocrystals act as substitutional dopants in superlattices of cadmium selenide or lead selenide nanocrystals when the size of the gold nanocrystal is very close to that of the host. The gold nanocrystals occupy random positions in the superlattice and their density is readily and widely controllable, analogous to the case of atomic doping, but here through nanocrystal self-assembly. We also show that the electronic properties of the superlattices are highly tunable and strongly affected by the presence and density of the gold nanocrystal dopants. The conductivity of lead selenide films, for example, can be manipulated over at least six orders of magnitude by the addition of gold nanocrystals and is explained by a percolation model. As this process relies on the self-assembly of uniform nanocrystals, it can be generally applied to assemble a wide variety of nanocrystal-doped structures for electronic, optical, magnetic, and catalytic materials.

Turning on resonant SERRS using the chromophore-plasmon coupling created by host-guest complexation at a plasmonic nanoarray.

An active molecular plasmonics system is demonstrated where a supramolecular chromophore generated in a host-guest binding event couples with the localized surface plasmon resonance (LSPR) arising from gold nanodisc gratings. This coupling was achieved by wavelength-matching the chromophore and the LSPR with the laser excitation, thus giving rise to surface-enhanced resonance Raman scattering (SERRS). The chromophore is a broad charge-transfer (CT) band centered at 865 nm (epsilon = 3500 M(-1) cm(-1)) generated by the complexation of cyclobis(paraquat-p-phenylene) (CBPQT(4+)) and the guest molecule tetrathiafulvalene (TTF). The substrates consist of sub-1-microm gold nanodisc arrays which display dimension-tunable plasmon wavelengths (600-1000 nm). The vibrational spectra of the complex arising from SERRS (lambda(exc) = 785 nm) were generated by irradiating an array (lambda(LSPR) = 765 nm) through the solution to give a chromophore-specific signature with the intensities surface enhanced by approximately 10(5). Surface adsorption of the empty and complexed CBPQT(4+) is also implicated in bringing the chromophore into the electric field arising from the surface-localized plasmon. In a titration experiment, the SERRS effect was then used to verify the role of resonance in turning on the spectrum and to accurately quantify the binding between surface-adsorbed CBPQT(4+) and TTF. The use of a nonpatterned gold substrate as well as a color mismatched complex did not show the enhancement, thus validating that spectral overlap between the chromophore and plasmon resonance is key for resonance surface enhancement. Simulations of the electric fields of the arrays are consistent with interdisc plasmon coupling and the observed enhancement factors. The creation of a responsive plasmonic device upon the addition of the guest molecule and the subsequent coupling of the CT chromophore to the plasmon presents favorable opportunities for applications in molecular sensing and active molecular plasmonics.

Molecular logic gates using surface-enhanced Raman-scattered light.

A voltage-activated molecular-plasmonics device was created to demonstrate molecular logic based on resonant surface-enhanced Raman scattering (SERS). SERS output was achieved by a combination of chromophore-plasmon coupling and surface adsorption at the interface between a solution and a gold nanodisc array. The chromophore was created by the self-assembly of a supramolecular complex with a redox-active guest molecule. The guest was reversibly oxidized at the gold surface to the +1 and +2 oxidation states, revealing spectra that were reproduced by calculations. State-specific SERS features enabled the demonstration of a multigate logic device with electronic input and optical output.

High-Q mechanical resonator arrays based on carbon nanotubes

We present results of the characterization of a nanoelectromechanical signal-processing device based on arrays of carbon nanotubes embedded in RF waveguides. The design, fabrication, and operation of the device will be discussed, including initial RF measurements. Preliminary tests suggest that transmission of an RF signal through the array is associated with the mechanical resonance of the carbon nanotubes.

Determination of binding strengths of a host-guest complex using resonance Raman scattering.

The detection of analyte-binding events by receptors is drawing together the fields of Raman spectroscopy and supramolecular chemistry. This study is intended to facilitate this cohering by examining a model in the solution phase. The resonance Raman scattering (RRS) spectra of the complexation between tetrathiafulvalene (TTF) and cyclobis(paraquat-p-phenylene) (CBPQT(4+)) has been used as the model system to characterize the binding event of a host-guest system. RRS spectra are generated by excitation (lambda(exc) = 785 nm) within the lowest-energy charge-transfer (CT) transition (lambda(max) = 865 nm) of the TTF subsetCBPQT(4+) complex. The paired binding curves from the RRS and UV-vis-NIR titration data agrees with prior work, and a DeltaG of -5.7 +/- 0.6 kcal mol(-1) (MeCN, 298 K) was obtained for the complexation of TTF with CBPQT(4+). Computations on the complex and its components reproduce the energy shifts and resonance enhancements of the Raman band intensities, providing a basis to identify the structural and vibrational changes occurring upon complexation. The changes in bond lengths coincide with partial depopulation of a TTF-based HOMO and population of a CBPQT(4+)-based LUMO through CT mixing in the ground state of 0.46e(-). The structural changes upon complexation generally lead to lower wavenumber vibrations and to changes in the normal mode descriptions.

Air Bridge and Vertical Carbon Nanotube Switches for High Performance Switching Applications

Carbon nanotubes are attractive for switching applications since electrostatically-actuated CNT switches have low actuation voltages and power requirements, while allowing GHz switching speeds that stem from the inherently high elastic modulus and low mass of the CNT. Our first NEM structure, the air-bridge switch, consists of suspended single-walled nanotubes (SWNTs) that lie above a sputtered Nb electrode. Electrical measurements of these air-bridge devices show well-defined ON and OFF states as a dc bias of a few volts is applied. The switches were measured to have switching times down to a few nanoseconds. Our second NEM structure, the vertical CNT switch, consists of nanotubes grown perpendicular to the substrate. Vertical multi-walled nanotubes (MWNTs) are grown directly on a heavily doped Si substrate, from 200 − 300 nm wide, ∼ 1 μm deep nano-pockets, with Nb metal electrodes to result in the formation of a vertical single-pole-double-throw CNT switch architecture.

Carbon Nanotube Switches for Communication and Memory Applications

The development of carbon nanotube-based nanoelectromechanical (NEM) switches is described in this work for their potential application in communication and memory systems. Our first NEM structure consists of single walled nanotubes (SWNTs) suspended over shallow trenches in a SiO2 layer, with a Nb pull electrode beneath. DC measurements of these devices show well-defined ON and OFF states as the tube is actuated electrostatically at a few volts. For high frequency applications, electromagnetic modeling of these devices was performed using FEMLAB to calculate the quasi-static capacitance. An equivalent circuit of our switch was developed from which the swept frequency response was simulated up to 100 GHz in the ON and OFF states. A second NEM switch structure, where the tubes are perpendicular to the substrate is also discussed, which is primarily being developed for nonvolatile memory applications. Here, the growth of multi-walled nanotubes (MWNTs) from deep nanopores is described using thermal chemical vapor deposition (CVD) and plasma-enhanced (PE) CVD with Fe and Ni catalyst, respectively, in preparation for the formation of a vertical switch architecture.

Carbon nanotube Schottky diodes for high frequency applications

We have developed Schottky diodes using semiconducting nanotubes with titanium Schottky and platinum Ohmic contacts. At low biases these diodes showed ideality factors in the range of 1.5 to 1.9. The performance prediction as room-temperature detectors at 2.5 THz resulted in NEP potentially comparable to that of the state-of-the-art gallium arsenide solid-state Schottky diodes in the range of 10/sup -13/ W//spl radic/Hz.