Preparation of Al 2 O 3 and AlN Nanotubes by Atomic Layer Deposition
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Preparation of Al2O3 and AlN Nanotubes by Atomic Layer Deposition
Cagla Ozgit1, Fatma Kayaci1, Inci Donmez1, Engin Cagatay1, Tamer Uyar1, and Necmi Biyikli1 1
UNAM – Institute of Materials Science and Nanotechnology, Bilkent University, Ankara 06800, Turkey.
ABSTRACT Al2O3 and AlN nanotubes were fabricated by depositing conformal thin films via atomic layer deposition (ALD) on electrospun nylon 66 (PA66) nanofiber templates. Depositions were carried out at 200°C, using trimethylaluminum (TMAl), water (H2O), and ammonia (NH3) as the aluminum, oxygen, and nitrogen precursors, respectively. Deposition rates of Al2O3 and AlN at this temperature were ~1.05 and 0.86 Å/cycle. After the depositions, Al2O3- and AlN-coated nanofibers were calcinated at 500°C for 2 h in order to remove organic components. Nanotubes were characterized by using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). AlN nanotubes were polycrystalline as determined by high resolution TEM (HR-TEM) and selected area electron diffraction (SAED). TEM images of all the samples reported in this study indicated uniform wall thicknesses. INTRODUCTION Atomic layer deposition (ALD) is a special type of low-temperature chemical vapor deposition, in which the substrate is exposed to sequential pulses of two or more precursors separated by purging periods [1]. Unless decomposition of the precursor occurs, each pulse leads to surface reactions that terminate after the adsorption of a single monolayer. Film growth mechanism of ALD is therefore self-limiting, which gives rise to unique properties such as high uniformity and conformality, as well as sub-nanometer thickness control. In 2007, Peng et al. [2] combined ALD with electrospinning in order to fabricate long and uniform metal-oxide microtubes with precise wall thickness control. In their study, Al2O3 was deposited on electrospun poly(vinyl alcohol) microfibers, which were then removed by calcination. This approach was followed by other researchers, who synthesized tubes of various sizes, materials (Al2O3 [3,4], TiO2 [5,6], ZnO [3,4,7-9]), and structures (e.g. core-shell nanofibers) using different electrospun templates (nylon-6 [4], poly(vinyl acetate) [7-9], poly(vinyl alcohol) [3], poly(vinyl pyrrolidone) [5,6]). Here we report on the fabrication of Al2O3 and AlN nanotubes by depositing conformal thin films via ALD on electrospun nylon 66 nanofiber templates. Chemical composition and bonding states of the nanotubes were investigated by XPS, whereas structural characterization was carried out by electron microscopy and diffraction.
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EXPERIMENTAL DETAILS Nylon 66 (PA66) nanofiber templates were prepared by electrospinning of formic acid and hexafluoroisopropanol (HFIP) solutions. Different polymer concentrations were used according to solvent systems in order to obtain nanofibers having different diameters (see table I). For all the polymer solutions studied; feed rate, applied voltage and tip-to-collector distance were 1 ml/h, 15 kV
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