Archana Pandey

Stable electron field emission from PMMA-CNT matrices.

We have created PMMA-CNT matrices by embedding opened-tip vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) with poly(methyl methacrylate) (PMMA). These PMMA-CNT matrices are excellent electron field emitters with an emission threshold field of 1.675 V/μm, more than 2-fold lower that that of the as-grown sample. In addition, the emission site density from these matrices is high, merely filling up the entire sample surface. Emission stability test at ∼1.35 mA/cm(2) was performed continuously for 40 h with no significant degradation. On the basis of our theoretical simulation and hypothetical modeling, we attribute these performances to the reduced screening effect and fewer Joule heatings due to the shorter effective transport distance of the electrons in MWCNTs.

Controlled Growth of Carbon, Boron Nitride, and Zinc Oxide Nanotubes

Nanotubes represent a unique class of materials in which all atoms are located near the surface. Since electrons flowing through nanotubes are confined near the surface, nanotubes are attractive for sensing biological and chemical molecules. In addition, their tubular structures enable nanofluidic devices that are useful for novel sensing applications. In this paper, we will discuss current applications and the latest advancements on the growth of carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs), and ZnO nanotubes (ZnONTs). First, CNT growth is highly controlled by regulating the effective catalysts and the dissociative adsorption of the hydrocarbon molecules during chemical-vapor deposition growth. Second, we have achieved low temperature growth of vertically aligned BNNTs at 600 degC , the first success of growing pure BNNTs directly on substrates at temperatures about half of those reported so far. Finally, we have developed an original approach for growing ZnONTs without catalyst or template. Robust, controllable growth techniques for nanotubes are necessary in order to fully realize their sensing potential.

Dielectrophoretic Deposition of Carbon Nanotubes with Controllable Density and Alignment

Controlled deposition of carbon nanotubes (CNTs) across desired electrodes is important for the fabrication of nanoelectronic devices. Dieletrophoresis (DEP) has been recognized as a convenient and affordable technique for the deposition of nanotubes and nanowires on electrodes. Although DEP has been quite well studied for dielectric particles, the application for depositing nanotubes is still at the early stage of development. Here, we show that multi-walled CNTs can be deposited by DEP with controllable density and degree of alignment.

Growth of Carbon, Boron Nitride and ZnO Nanotubes for Biosensors

Summary It was shown that the progress in growing nanostructures affects our ability to use them for applications. In CNTs, growth is easy and controllable, leading to a wealth of study on biological applications. BNNTs, being similar to CNTs but more robust, are a promising material. Until recently, the difficulty of their growth has limited their use, but we have found easier, low-temperature growth methods that should help expand the scope of their application. Finally, ZnO materials are desired for their hydrophilic natures, their tubular structures and wide energy band gaps. These nanotubes can now be grown single-crystal by conventional thermal CVD methods, and, as continuing refinements of the growth techniques take place, they will find more and more use in biological applications. Acknowledgments This work is supported by Michigan Tech Research Excellent Funds, the US Department of Army (Grant No. W911NF-04-1-0029 through the City College of New York), National Science Foundation CAREER Award (Award number 0447555, Division of Materials Research), the U.S. Army Research Laboratory and the Defense Advanced Research Projects Agency (Contract number DAAD17-03-C-0115), and the Center for Nanophase Materials Science (CNMS) sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy, under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC.

Growth of Single Crystalline ZnO Nanotubes and Nanosquids

The growth of ZnO nanotubes and nanosquids is obtained by conventional thermal chemical vapor deposition (CVD) without the use of catalysts or templates. Characterization of these ZnO nanostructures was conducted by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), Raman spectroscopy, and photoluminescence (PL). Results indicate that these ZnO nanostructures maintain the crystalline structures of the bulk wurtzite ZnO crystals. Our results show that rapid cooling can be used to induce the formation of ZnO nanotubes and ZnO nanosquids. The self-assembly of these novel ZnO nanostructures are guided by the theory of nucleation and the vapor-solid crystal growth mechanism.

Glucose Biosensors Based on Vertically-Aligned Multi-walled Carbon Nanotubes

Vertically-aligned multiwalled carbon nanotubes (VA-MWCNTs) were grown using plasma enhanced chemical vapor deposition (PECVD) technique. These VA-MWCNTs were then dip coated by Poly methyl methacrylate (PMMA) followed by annealing. Samples were then polished to expose the tips of CNTs. Biological molecules Glucose Oxidase (GOx) were then immobilized on the exposed tips of these nanoelectrode ensembles . Here we present further characterization of these devices, with results on the detection limits and measurement stability. We found that these sensors can be reused for longer than six months when kept in proper storage conditions.

Comparing Field Emission Stability of Lithography-free, Modified Multi-Walled Carbon Nanotubes

Field emission from carbon nanotubes (CNTs) has been known for more than a decade but there is no commercialized product available in the market. Apparently, we need to improve our basics understanding on stable field emission from CNTs. Here we compared the field emission properties of as grown vertically-aligned multi-walled carbon nanotubes (MWCNTs) to two types of modified MWCNTs: 1) Conical bundles of opened-tip MWCNTs, and 2) Opened-tip MWCNTs embedded in poly-methyl methacrylate (PMMA). We found that both types of modified MWCNTs have lower emission thresholds and better emission stability than the as grown samples. Among these modified samples, MCNTs embedded in PMMA has lower emission thresholds and better emission stability. We attributed these improvements to the filling of spacing between MWCNTs with PMMA that has higher dielectric constant than vacuum.