Vijaya Kayastha

Formation of single crystalline ZnO nanotubes without catalysts and templates

Oxide and nitride nanotubes have gained attention for their large surface areas, wide energy band gaps, and hydrophilic natures for various innovative applications. These nanotubes were either grown by templates or multistep processes with uncontrollable crystallinity. Here the authors show that single crystal ZnO nanotubes can be directly grown on planar substrates without using catalysts and templates. These results are guided by the theory of nucleation and the vapor-solid crystal growth mechanism, which is applicable for transforming other nanowires or nanorods into nanotubular structures.

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.

High-density vertically aligned multiwalled carbon nanotubes with tubular structures

Ammonia (NH3) gas was thought to be essential for the growth of vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) and led to the formation of bamboo-like structures. Here, we show that VA-MWCNTs with ideal tubular structures can be grown on substrates by various mixed gases with or without NH3 gas. The growth of these VA-MWCNTs is guided by a growth model that combined the dissociative adsorption of acetylene molecules (C2H2) and the successive vapor-liquid-solid growth mechanism. Results indicate that the key factor for growing these VA-MWCNTs is a balance between the decomposition rate of the C2H2 molecules on the iron catalyst and the subsequent diffusion and segregation rates of carbon.

Controlling dissociative adsorption for effective growth of carbon nanotubes

Dissociative adsorption has been widely simplified as part of the vapor–liquid–solid (VLS) growth model. We found that the addition of specific carrier gases can critically modify the growth rate and growth density of multiwall carbon nanotubes (MWNTs). These results were explained by dissociative adsorption of C2H2 molecules and a solid-core VLS growth model. Based on these integrated mechanisms, vertically aligned MWNTs were grown with an initial growth rate as high as ∼800μm∕h. This efficient growth process results at temperature and C2H2 partial pressures at which the decomposition and segregation rates of carbon are balanced. Appropriate use of carrier gas is one of the factors that could facilitate efficient and continuous growth of carbon nanotubes in the future.

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.

Investigation of the humidity-dependent conductance of single-walled carbon nanotube networks

In this paper, we investigate the conductance of single walled carbon nanotube (SWCNT) networks at different humidity levels and various device temperatures. The carrier transport processes are analyzed by performing a temperature-dependent conductance study. It is found that the conductance of the SWCNT networks is dominated by the thermal activation carrier hopping over the barriers between CNTs. The average separation between the SWCNTs is found to vary linearly with the humidity levels. The humidity-dependent conductance of the SWCNT network is modeled and compared with the experimental data. The model agrees well with the experimental data.

A Printable CNT-Based FM Passive Wireless Sensor Tag on a Flexible Substrate With Enhanced Sensitivity

In this paper, we report a frequency-modulated (FM) passive wireless sensor tag for ammonia (NH3) sensing. The passive wireless sensor tag consists of a single-walled carbon nanotube (SWCNT) network based NH3 sensor, a radio frequency antenna, a ring oscillator, and other supporting circuits. The SWCNT network-based NH3 sensor is fabricated on a flexible plastic substrate through printable processes. The printable SWCNT-based NH3 sensor shows an enhanced sensitivity of 0.76% per part per million primarily due to the large surface area of the SWCNT network. The sensor also exhibits a high linearity between the resistance of the sensor and logarithm of the NH3 concentration (referred to as log [NH3] henceforth). A simple FM circuit is designed to convert the resistance change of the sensor to the oscillating frequency shift of the circuit. By properly designing the circuit, we have obtained a linear response between the frequency shift and log [NH3]. The linear response allows one to precisely predict the NH3 concentration by measuring the frequency shift of the FM wireless sensor tag. Such an FM-modulated passive wireless sensor tag with linear response and enhanced sensitivity is promising for power-less stand-alone low-level NH3 sensing and monitoring with high accuracy.

Introduction to B–C–N Materials

B–C–N is an emerging material system consisting of novel nanostructures of boron (B), carbon (C), boron nitride (BN), carbon nitride (CN x ), boron-carbon nitride (B x C y N z ), and boron carbide (B x C y ). These B–C–N materials are sometimes called as frontier carbon materials, because of their flexibility in forming materials of various types of hybridizations similar to those in the pure carbon system. This chapter provides a concise introduction on all these materials. Readers are referred to various references and other chapters compiled in this book for further reading.

All-Printed Thin-Film Transistor Based on Purified Single-Walled Carbon Nanotubes with Linear Response

We report an all-printed thin-film transistor (TFT) on a polyimide substrate with linear transconductance response. The TFT is based on our purified single-walled carbon nanotube (SWCNT) solution that is primarily consists of semiconducting carbon nanotubes (CNTs) with low metal impurities. The all-printed TFT exhibits a high ON/OFF ratio of around 103 and bias-independent transconductance over a certain gate bias range. Such bias-independent transconductance property is different from that of conventional metal-oxide-semiconductor field-effect transistors (MOSFETs) due to the special band structure and the one-dimensional (1D) quantum confined density of state (DOS) of CNTs. The bias-independent transconductance promises modulation linearity for analog electronics.

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.