Benjamin Ulmen

Effect of graphitic order on field emission stability of carbon nanotubes.

We observed current density (J) dependent degradation in field emission current from multiwalled carbon nanotubes (MWCNTs). These degradations are recoverable and can be explained by emission current-induced dislocations along the MWCNTs. MWCNTs grown by thermal chemical vapour deposition (CVD) can emit stable current continuously for at least 1200 min with upper current density limits of approximately 0.5 mA cm(-2). In contrast, this upper limit is<40 microA cm(-2) for nanotubes grown by plasma-enhanced CVD (PECVD), although higher J is possible with relatively shorter stability duration. High-resolution transmission electron microscopy and Raman spectroscopy indicate higher graphitic order of the thermal CVD grown MWCNTs as compared to PECVD grown MWCNTs. Our study suggests that graphitic order affects their upper performance limits of long-term emission stability, although the effects from adsorbates cannot be completely ignored. These results indicate that field emission cannot be considered as an ideal quantum tunnelling process. The effect of electron transport along CNTs before electron tunnelling must be considered.

Structural control of vertically aligned multiwalled carbon nanotubes by radio-frequency plasmas

Plasma-enhanced chemical vapor deposition is the only technique for growing individual vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) at desired locations. Inferior graphitic order has been a long-standing issue that has prevented realistic applications of these VA-MWCNTs. Previously, these VA-MWCNTs were grown by a one-plasma approach. Here, we demonstrate the capability of controlling graphitic order and diameters of VA-MWCNTs by decoupling the functions of the conventional single plasma into a dual-plasma configuration. Our results indicate that the ionic flux and kinetic energy of the growth species are important for improving graphitic order of VA-MWCMTs.

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.

A Dual-RF-Plasma Approach for Controlling the Graphitic Order and Diameters of Vertically-Aligned Multiwall Carbon Nanotubes

Plasma enhanced chemical vapor deposition (PECVD) is a unique technique for growing vertically-aligned multiwall carbon nanotubes (VA-MWNTs) at controllable tube densities. This technique is of considerable importance for low temperature growth of VA-MWNTs at desired locations. However, the graphitic order of these MWNTs is inferior to those grown by laser ablation, arc discharge, and thermal CVD techniques. Previously, these VA-MWNTs were grown by a one-plasma approach (DC, microwave etc), either for gas decomposition or substrate biasing. Here, we describe a dual-RF plasma enhanced CVD (dual-RF-PECVD) technique that offers unique capability for controlling the graphitic order and diameters of VA-MWNTs.