The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
New Nano-materials: Growth and Characterization I
Chair: Peyman Servati, University of British Columbia
Synthesis of Carbon Nanotubes on Ni/Si-Ni/SiO2 Substrates by Thermal CVD
EP-USP, University of São Paulo. P.O. Box 61548, 5424-970, São Paulo, Brazil
Chemical vapor deposition (CVD) has proved to be a technique with very good results in the process of synthesis of carbon nanotubes (CNT). Groups of CNT samples were grown through thermal CVD technique utilizing nickel (Ni) thin films as catalyst material deposited over silicon (Ni-Si) and silicon oxide (SiO2 – 500 nm) substrates, with three different thicknesses (15, 25, and 35 nm). The catalyst deposition was carried out by two different methods; by RF sputtering and electron beam. An important parameter in this study is the formation of Ni nanoislands as catalyst material through a thermal treatment process before CNT growth. In order to compare the effects of the catalyst morphology in the CNT formation, a third type of samples was prepared with commercial nickel nanopowder (100 nm) in isopropyl alcohol solution, deposited by spin coating technique. Different time processes (15, 30, 45 and 60 minutes) were used in the CVD reactor in order to identify the optimum one. The samples were characterized by Raman spectroscopy, scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM). The results indicate the presence of both multi walled carbon nanotubes (MWCNT) and single walled carbon nanotubes (SWCNT) on Ni thin films depending on the parameters utilized, also better results in terms of production of SWCNT were obtained by the nanopowder method.
Keywords: Carbon nanotubes, CVD, Raman spectroscopy
Design of a Nano-structured Pyroelectric Detector with Low Thermal Conductivity
1. Department of Physics and Engineering and Optical Science Center for Applied Research (OSCAR), Delaware State University, 1200 N DuPont Highway, Dover, DE, USA
2. Department of Electrical Engineering and Nanotechnology Research and Education Center, University of Texas, Arlington, TX, USA
We report the design of a state-of-the-art uncooled pyroelectric detector utilizing a nanometer sized mesh or truss to support the suspended detector. Pyroelectric detector is a class of thermal detector in which the change in temperature causes the change in the charge distribution in the sensing material. Pyroelectric detectors are free of 1/f-noise as it does not require the bias current to operate. Ca modified lead titanate (PCT) has a high pyroelectric figure of merit and was employed as the thermometer in the detector in this work. The design and performance of pyroelectric detectors has been conducted by simulating the structure with engineering design software Intellisuite™. Finite element method was used to simulate the structural and thermal properties of the device. The simulated detector had spiderweb like structure with each of the strut (ring) of spider web had a width of 100 nm. The pyroelectric detectors utilized NiCr absorber, PCT sensing layer, Ti electrodes, Al2O3 structural layer to obtain lower thermal conductivity between the detector and Si substrate. Three different types of pyroelectric detectors with various dimensions and structures were designed and analyzed. The first detector design had linear electrode and simple spider web support. The lowest value of the thermal conductivity of these detectors was found to be 3.98x10–8 W/K. The second detector design had a longer thermal path than the first one and the lowest thermal conductivity of this device was found to be 2.41x10–8 W/K. High detectivity was obtained by reducing the thermal conductance between the sensing layer and the substrate in the third design. The design was optimized for best result by modifying the shape, dimension and thickness of various layers namely absorber, electrodes, sensing layer, struts. The lowest thermal conductance between the sensor and the substrate using the third design was found to be as low as 4.57x10–9 W/K. The thicknesses of the web structure, web support, electrodes, sensing layer and absorber of the final structure were 2 μm, 1 μm, 0.5 μm, 2 μm and 0.2 μm respectively for this value of thermal conductance. The absorber diameter was 50 μm and the diameter of the spider web was 200 μm. Total 80 struts each of with a width of 100 nm were used in the design.
Keywords: pyroelectric detector, PCT, micromachined detector, nanomachined detector
Transparent and Conducting Graphene-RNA Nanocomposites and their Transport Properties
1. Department of Physics & Astronomy, University of Western Ontario, London, ON, N6A 3K7, Canada
2. Department of Chemistry, University of Western Ontario, London, ON, N6A 5B7, Canada
In this study, ribonucleic acid (RNA) is proposed as a nonionic surfactant for the efficient exfoliation of graphite in thin flakes of few-layer graphene and the subsequent preparation of transparent and conducting thin films by the vacuum filtration method . Parameters such as the type of RNA used and the size of starting graphite flakes are demonstrated to be essential for obtaining graphene-RNA thin films of good quality . A model explaining the exfoliation of graphene by RNA in water is suggested. A number of post- and predeposition treatments (including thermal annealing, functionalization of the films, and the preoxidation of graphite) are critical to improve the performance of graphene-RNA nanocomposites as transparent conductors. We show that the electrical transport properties of our graphene-RNA thin films can be described by a simple model that assuming percolation between partially overlapped graphene nanoplatelets. We also show that the thermal conductivity of these nanocomposites is limited by the thermal insulation properties of RNA. At a given RNA concentration in the films, the thermal conductivity is significantly degraded in films in which RNA makes a continuous envelope around graphene platelets, but films in which a significant fraction of the surface of the platelets is free from RNA, still retain, to a certain extent, the excellent thermal transport properties of graphene-based materials. Our study establishes an ideal link between RNA and graphene, the fundamental building blocks for nanobiology and carbon-based nanotechnology.
 G. Eda, G. Fanchini, and M. Chhowalla, Nature Nanotechnology 3, 270 (2008)
 F. Sharifi, R. Bauld, M. S. Ahmed, and G. Fanchini, Small 8, 699–706 (2012)
Keywords: graphene nanoplatelets, nanocomposites, transport properties, optical properties
Laser-assisted Growth of t-Te Nanotubes and their Controlled Photo-induced Unzipping to Ultrathin core-Te/sheath-TeO2 Nanowires
1. Foundation for Research and Technology Hellas – Institute of Chemical Engineering Sciences (FORTH/ICE-HT), P.O. Box 1414, GR-26504, Rio-Patras, Greece
2. Department of Materials Science, University of Patras, GR-26504, Rio-Patras, Greece
3. Lab of Electron Microscopy and Microanalysis, University of Patras, GR-26504, Rio-Patras, Greece
One dimensional (1D) nanostructures of semiconducting oxides and elemental chalcogens culminate over the last decade in nanotechnology owing to their unique properties exploitable in several applications sectors. Whereas several synthetic strategies have been established for rational design of 1D materials using solution chemistry and high temperature evaporation methods, much less attention has been paid to the laser-assisted growth of hybrid inorganic nanostructures. In the current work, we present a novel method for the laser-assisted, template-free and surfactant-free fabrication (at ambient conditions) of low-dimensional nanostructures, e.g. trigonal Te (t-Te) nanotubes (NTs) and nanospheres and core-Te/sheath-TeO2 and TeO2 nanowires (NWs). Nanostructures are fabricated via laser ablation using visible cw lasers and are characterized by Raman scattering, Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive Spectroscopy (EDS) and High Resolution Transmission Electron Microscopy (HRTEM).
At low and moderate fluence conditions, photoinduced oxidation of the irradiated area takes place, which causes surface nanostructuring. The details of the photoinduced oxidation kinetics are monitored in-situ with Raman scattering. Oxidation saturates at certain levels depending on light fluence. Raman spectra revealed the simultaneous formation of amorphous TeO2 and amorphous Te (a-Te) providing key information on the microscopic origin of the involved photo-induced phase changes. Exposure of t-Te to intense light, by increasing the fluence by one order of magnitude, leads to mass transport caused by material ablation. A series of light-driven phase transitions is employed to controllably transform the grown Te NTs to core-Te/sheath-TeO2 nanowires. The underlying mechanism combines a bottom-up step for the growth of t-Te NTs and a top-down step for the photo-induced ripping of these NTs towards the fabrication of a-TeO2 nanowires and/or core-Te/sheath-TeO2 hybrid nanostructures. The vapor-solid mechanism describes the formation of t-Te NTs at high fluence, which grow at the periphery of the irradiated area. The second step, namely, photo-oxidation is more complex; its origin was revealed by time-resolved Raman scattering at low fluence conditions. At early stages of irradiation a-Te and amorphous TeO2 emerge simultaneously. At prolonged exposure conditions photo-oxidized zones on the surface of t-Te NTs undermine their integrity by an unzipping effect along the NT axis. The composition of the NWs formed by NT ripping varies from core-Te/sheath-TeO2 nano-hybrids to pure TeO2 nanowires at longer exposure times.
The current method involves an all-laser, solid-state materials processing offering new opportunities for the fast and spatially controlled fabrication of Te and TeO2 anisotropic nanomaterials that meet a number of applications in optics, photonics and gas sensing. The method also provides a means of simultaneous growing and integrating these nanostructures into an optoelectronic or photonic device.
Keywords: nanowires, nanotubes, core/sheath nanostructures, photo-induced oxidation