Prabhakar R. Bandaru

Toxicity issues in the application of carbon nanotubes to biological systems

UNLABELLED Carbon nanotubes (CNTs) have recently emerged as a new option for possible use in methodologies of cancer treatment, bioengineering, and gene therapy. This review analyzes the potential, through possible toxicologic implications, of CNTs in nanomedicine. Generally, proven success in other fields may not translate to the use of CNTs in medicine for reasons including inconsistent data on cytotoxicity and limited control over functionalized-CNT behavior, both of which restrict predictability. Additionally, the lack of a centralized toxicity database limits comparison between research results. To better understand these problems, we seek insight from currently published toxicity studies, with data suggesting postexposure regeneration, resistance, and mechanisms of injury in cells, due to CNTs. FROM THE CLINICAL EDITOR Carbon nanotubes (CNTs) have recently emerged as a new option for cancer treatment, bioengineering, and gene therapy. Inconsistent data on cytotoxicity and limited control over functionalized-CNT behavior currently restrict predictability of such applications.

An outline of the synthesis and properties of silicon nanowires

We consider some of the significant aspects of Silicon nanowires (NWs), referring to their various modes of fabrication and their measured properties. Lithographic patterning as well as individual NW synthesis, e.g., through chemical vapor deposition based processes, has been utilized for their fabrication. It is seen that the properties of these nanostructures, to a large extent, are determined by the enhanced surface area to volume ratio and defects play a relatively major role. A diminished size also brings forth the possibility of quantum confinement effects dictating their electronic and optical properties, e.g., where NWs can possess a direct energy gap in contrast to the indirect bandgap of bulk Si. While new challenges, such as enhanced Ohmic contact resistance, carrier depletion ‐ which can severely influence electrical conduction, and surface passivation abound, there also seem to be exciting opportunities. These include, e.g., high sensitivity sensors, nanoelectromechanical systems, and reduced thermal conductivity materials for thermoelectrics. Much preliminary work has been done in these areas as well as investigating the possible use of Si NWs for transistor applications, photovoltaics, and electrochemical batteries etc., all of which are briefly reviewed. (Some figures in this article are in colour only in the electronic version)

Enhanced dielectric constants and shielding effectiveness of, uniformly dispersed, functionalized carbon nanotube composites

It was seen that composites constituted of functionalized single-walled nanotubes (SWNTs) dispersed in a reactive ethylene terpolymer (RET) matrix possess a complex dielectric permittivity an order of magnitude larger than composites composed of pristine SWNTs and two orders of magnitude larger than functionalized multiwalled nanotube-RET composites. We seek to understand such an enhancement, both in terms of uniform nanotube dispersion and through a parallel resistor-capacitor model. We subsequently show that the ac electrical conductivity is a good predictor of the electromagnetic interference shielding effectiveness of nanocomposites.

Enhanced Electromagnetic Interference Shielding Through the Use of Functionalized Carbon-Nanotube-Reactive Polymer Composites

We report on a new principle yielding enhanced electromagnetic shielding, using as an example a composite comprised of carbon nanotubes (CNTs) integrated with a reactive ethylene terpolymer (RET). Such composites were synthesized through the chemical reaction of the functional groups on the CNT with the epoxy linkage of the RET polymer. The main advantages of these composites include good dispersion with low electrical percolation volume fractions (~0.1 volume%), yielding outstanding microwave shielding efficiency for electromagnetic interference applications. The shielding effectiveness was characterized for both single-walled and multiwalled CNT-based composites and was much enhanced in the former. The specific roles of absorption and reflection in determining the total shielding, as a function of the nanotube filling fraction, is also discussed.

Giant birefringence in multi-slotted silicon nanophotonic waveguides

We demonstrate record giant birefringence, nearly twice as large as has previously been achieved (Delta n(group) = 1.5 over more than 60 nm of bandwidth near lambda= 1550 nm) using a multi-slotted silicon nanophotonic waveguide. The birefringence is optimized by the use of materials with high refractive index contrast to create a compact single-mode waveguide, and the etching of deeply sub-wavelength channels within the waveguide, which are strongly coupled in the near field and separated by narrow air channels of optimum lateral width. When used as a polarization-selective delay element, the delay-bandwidth product per unit length is 46.6/mm over a bandwidth of 8.74 T Hz. We also design and demonstrate mode shaping of both the TE and TM polarizations to achieve near-identical coupling to a macroscopic external object, such as a lensed fiber or detector.

A plausible mechanism for the evolution of helical forms in nanostructure growth

The observation of helices and coils in nano-tube/-fiber (NT/NF) syntheses is explained on the basis of the interactions between specific catalyst particles and the growing nanostructure. In addition to rationalizing nonlinear structure, the proposed model probes the interplay between thermodynamic quantities and predicts conditions for optimal growth. Experimental results on the effect of indium catalyst on affecting the coil pitch in NTs and NFs are presented.

Solution-Processed CoFe2O4 Nanoparticles on 3D Carbon Fiber Papers for Durable Oxygen Evolution Reaction

We report CoFe2O4 nanoparticles (NPs) synthesized using a facile hydrothermal growth and their attachment on 3D carbon fiber papers (CFPs) for efficient and durable oxygen evolution reaction (OER). The CFPs covered with CoFe2O4 NPs show orders of magnitude higher OER performance than bare CFP due to high activity of CoFe2O4 NPs, leading to a small overpotential of 378 mV to get a current density of 10 mA/cm(2). Significantly, the CoFe2O4 NPs-on-CFP electrodes exhibit remarkably long stability evaluated by continuous cycling (over 15 h) and operation with a high current density at a fixed potential (over 40 h) without any morphological change and with preservation of all materials within the electrode. Furthermore, the CoFe2O4 NPs-on-CFP electrodes also exhibit hydrogen evolution reaction (HER) performance, which is considerably higher than that of bare CFP, acting as a bifunctional electrocatalyst. The achieved results show promising potential for efficient, cost-effective, and durable hydrogen generation at large scales using earth-abundant materials and cheap fabrication processes.

Artificial introduction of defects into vertically aligned multiwalled carbon nanotube ensembles: Application to electrochemical sensors

Carbon nanotubes (CNTs) inevitably contain defects that exert a significant influence on their physical, electrical, and electrochemical properties. In this study, we subject vertically aligned multiwalled CNT ensembles to argon and hydrogen ion irradiation, to artificially introduce defects into the structure. Subsequently, Raman spectroscopy in conjunction with electrochemical analyses was used to characterize the amount and nature of disorder within the CNTs. While an increased disorder with ion irradiation was generally observed, argon and hydrogen exhibit different effects on the Raman intensity spectra. Argon irradiation seems to cause charged defects, e.g., in the form of dangling bonds, and increases the in-plane correlation length (La), while hydrogen irradiation passivates residual defects and decreases the La. It was noted that hydrogen treated CNTs could serve as electrochemical sensors with quicker response time.

InP Layer Transfer with Masked Implantation

InP layer transfer with masked implantation was investigated to eliminate ion-implantation induced damage involved in the ion-cut process. InP donor wafers were selectively implanted with hydrogen through a mask at a dose of 8.5 X 10 16 ions/cm 2 at 160 keV. The layers which were subsequently mechanically exfoliated were characterized by large pyramidal protrusions on the surface, associated with the unimplanted regions. This undesirable morphology was bypassed through the inclusion of a selective etch-stop layer. The resulting structures possessed flat surfaces suitable for further bonding. This process enables the transfer of finished devices, unaffected by ion-implantation, onto a variety of desirable substrates.

Control of carbon nanotube morphology by change of applied bias field during growth

Carbon nanotube morphology has been engineered via simple control of applied voltage during dc plasma chemical vapor deposition growth. Below a critical applied voltage, a nanotube configuration of vertically aligned tubes with a constant diameter is obtained. Above the critical voltage, a nanocone-type configuration is obtained. The strongly field-dependent transition in morphology is attributed primarily to the plasma etching and decrease in the size of nanotube-nucleating catalyst particles. A two-step control of applied voltage allows a creation of dual-structured nanotube morphology consisting of a broad base nanocone (~200 nm dia.) with a small diameter nanotube (~7 nm) vertically emanating from the apex of the nanocone, which may be useful for atomic force microscopy.