ICMCTF2011 Session F1-1: Nanomaterials, Nanofabrication, and Diagnostics

Zinc oxide nanowire is a n-type semiconductor which exhibits wide band gap (3.3 eV) and high bonding energy (~60 meV). It has attracted great attention because of its specific optical and electric properties. In this work, AZO (Al-doped in ZnO) seed layer was prepared on the back side of PAM substrate by spin coating and annealing in vacuum at 400^oC. The average sheet resistance of 5-layers AZO films was 129.66 Ω/sq. Zinc oxide in order arrays mediated by high aspect ratio and order pores array AZO/PAM was synthesized. The ZnO nanowire array was prepared by 3-electrode electrochemical deposition in ZnSO₄ and H₂O₂ solution at various deposition potential (-0.8 ~ -1.2 V/SCE) and temperature (25~80^oC). The microstructure and chemical composition of AZO seed layer and ZnO/AZO/PAM nanowire arrays were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and Energy dispersive spectrometer (EDS), respectively. The interface between ZnO and AZO was analyzed by high resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction pattern (SAED). The photoluminescence spectra (PL) and Hall effect measurement were employed to analyze the luminescent properties and carrier concentration of ZnO/AZO/PAM nanowire arrays. The results indicated that the ZnO nanowire arrays were assembled into the nanochannel of porous alumina template with diameter of 120~140 nm. The crystalline structure of single ZnO nanowire was depended on AZO seed layer. The nucleation and growth process of ZnO/AZO/PAM nanowires were interpreted by seed layer assisted growth mechanism.

ZnO/TiO₂ hierarchical heterostructures were created as potentially effective photocatalysts to degrade organic pollutants into more environmentally friendly chemical species. ZnO/TiO₂ heterostructures, combining two of the most effective photocatalysts in the literature, were synthesized using a two-step route that consists of the hydrothermal growth of ZnO nanowires followed by the deposition of TiO₂ using unbalanced magnetron sputtering. The synthesis conditions were altered in order to control the growth process of TiO₂. The morphology of the synthesized heterostructures was evaluated using scanning electron microscopy (SEM) revealing that the growth process of TiO₂ may be controlled to produce hierarchical and core-shell structures. The crystal structure and the optical and chemical properties were determined by means of transmission electron microscopy (TEM), x-ray diffraction (XRD), and spectroscopic ellipsometry (SE). Lastly, the photodegradation of Rhodamine 6G was investigated. Enhanced catalytic activity of the hierarchical structures was found to be due to the increased contact surface area and to the enhanced interfacial charge transfer mechanism at the junction between ZnO and TiO₂.

Using radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD) method, different CNFs and CNTs were synthesized at low temperature. Base growth vertical turbostratic CNFs were grown using sputtering 8 nm amorphous Ni thin film catalyst on Si substrates at a temperature of 150^oC. Tip growth vertical platelet graphite nanofibers (PGNFs) were grown using Ni nanocatalysts in 8 nm Ni films at 180^oC. By introduction of hydrogen tip growth vertical multi-walled carbon nanotubes (MWCNTs) were also produced at 180^oC using FeNi nanocatalysts in 8 nm FeNi films on glass substrates. Low temperature growth mechanisms of CNFs and CNTs in RF-PECVD were discussed based on plasma physics, catalysts structure, substrate characteristics and temperature, and types of gases. In comparison to the most widely used thermal CVD (T-CVD) method, in which its synthesis temperature was 550-850^oC for CNFs and CNTs growth, RF-PECVD had a huge advantage in low temperature growth of CNFs and CNTs and control of other deposition parameters.

Due to their remarkable mechanical properties, single walled carbon nanotubes (“SWNTs”) are expected to eventually revolutionize structural designs of both spacecraft and air vehicles with great expectations in reducing the overall weight of these applications. As these carbon nanotubes are considered anisotropic nanoparticles with an extreme aspect ratio, they exhibit most of their remarkable properties, such as their extraordinary tensile strength (~200 GPa) in a single direction: along the tube axis. The achievement of uniform alignment is therefore a crucial condition in order to exploit these SWNT materials as potential nano-reinforcment materials for spacecraft applications. This work is aimed at development of a technique to allow a non-contact deposition and organization of SWNTs using an ink-jet spray technique to build up three dimensional (3D) structures with greater strength and toughness than possible by use of composites or monolithic materials such as steel or aluminum. We report the successful formulation of two single walled carbon nanotube (SWNT) inks which yielded a consistent, homogenous printing pattern and possessed the requisite viscosities needed for flow through the microcapillary nozzles of the ink-jet printer with fairly modest drying times.

We explored the wetting behavior of water droplets on vertically aligned carbon nanotube arrays (VANTA) of varying film density, film thickness and alignment orientation. The carbon nanotube (CNT) arrays (or forests) were synthesized with a 3 inch tube furnace through chemical vapor deposition (CVD). The synthesis environment includes the mixture of flowing ethanol, argon, and hydrogen gases at a temperature ranging from 600-750°C. Dilute aqueous solutions (5 mmol l^-1) of iron (III) nitrate and aluminum (III) nitrate have been applied on oxidized Si (100) substrates (approximately 10mm X 10mm) in an alternating fashion with the help of a micro-pipette. These nitrate solution treated substrates were dried at room temperature, and then loaded onto a quartz boat and placed into the load-lock of the tube furnace for processing. The pre-treatment procedure includes changing applied catalyst layer and base layer concentrations separately. Some of the CNT forests synthesized in this study showed super-hydrophobic behavior upon testing with deionized water, where contact angles (CA) measured were in excess of 150°. Dynamic CA measurement experiments were also performed upto 30 minutes. The results indicated that some CNT forest samples preserved their super-hydrophobic character for an extended period of exposure to the water droplet. Some retained a CA above 150° even after 30 minutes of exposure.

We investigated the effect of film density, film thickness and orientation on the measured CA and the rate of change in the CA. Graph 1-4 indicates the change in the CA for CNT array films synthesized using different pre treatment parameters. Our studies indicated an optimum CNT film density and tube length for optimizing the dynamic behavior of these surfaces. Hence, we found that synthesis parameters and catalysis application methodology may significantly affect hydrophobic properties of these CNT forests.

Nanocrystals (NCs) and SONOS nonvolatile memory (NVM) are the promising candidates for the next generation of flash memory since the discrete storage nodes can effectively alleviate the charge leakage from storage layer during scaling down.

We have previously reported the fabrication of NiSi₂/SiN_x compound nanocrystal (CNC) memory to combine the advantages of nanocrystal and SONOS memory. And the enhanced retention characteristics over control sample (NiSi₂ nanocrystal memory) are clarified to be due to the compound tunnel barrier and nitride traps.

In this paper, the retention characteristics of above nanocrystal memory are systematically measured, especially at elevated temperature aiming at the long term storage performance. The retention characteristics of both devices degrade at 350K, and the difference of charge decay rate between CNC memory and control sample is larger than that of room temperature. At 400K, the charge decay rate of control sample continues to increase due to the stimulated leakage mechanism, however, the retention window of CNC memory increases at the first thousand seconds and then reduces.

These free standing nickel nanowires with high specific surface area have shown relatively high enhancement ability, good reproducibility, long term stability as a surface enhanced Raman active material and high conversion efficiency for dry reforming of natural gas as a catalyst. Since nickel/nickel oxides have been intensively developed to act as an alternative electrode material for supercapacitors, the free standing nickel nanowires produced in this work are also considered as promising supercapacitor electrode materials owing to their high electrochemical activity.