Nanotubes in spray deposited Nanocrystalline HgTe: I thin films
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Nanotubes in spray deposited Nanocrystalline HgTe: I thin films A. Ranga Rao and V. Dutta* Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi-110016, India *Corresponding Author E-mail: [email protected]
Abstract HgTe nanotubes have been prepared by spray deposition of solvothermally synthesized iodine doped HgTe nanoparticles on glass substrates at low temperature (200oC). Spray deposition was done without voltage and with an externally applied voltage (700 V) to the nozzle and it is found from TEM studies that the length of the nanotubes increases from ~4µm in case of without voltage to ~ 6µm in case of the applied voltage, with an average diameter of ~ 45nm. The nanotubes are found to have cubic lattice structure having Hg:Te in stoichimetric ratio (52:48).
Introduction Synthesis and characterization of nanotubes and nanowires constitute an important part of nanotechnology. These materials are essential building units for many nano-electronic, nanophotovoltaic devices [1-3]. Nanostructured materials exhibit a wide range of electrical and optical properties that depend on both shape and size. One-dimensional (1D) nanostructures, such as nanotubes, nanowires and nanobelts, can be prepared using a variety of physical and chemical techniques [1,4-6]. Since the discovery of carbon nanotubes by Iijima in 1991, several materials have been found to exist in nanotubes form. These include AlN, CdSe, CdS, ZnO and other semiconductor oxides [1,7-9]. Most of the synthesis methods involve the use of surfactants, catalysts or require high temperatures and pressures. To our knowledge there are no reports on the method of preparation of HgTe nanotubes in thin film form at low deposition temperatures. The alloy Hg1-xCdxTe is a well known material for long-wavelength IR detector technologies. For x=0, bulk HgTe has an inverted band structure with direct negative bandgap of -0.15 eV at 295 K [10]. The Bohr exciton radius of HgTe is 40 nm, due to the large Bohr exciton radius it is easy to observe the quantum confinement effects. The band gap of HgTe changes from –0.15eV to 2.14 eV as the particle size reduces to ~ 5 nm [11,12]. The conversion of narrow bandgap HgTe to a wide bandgap material by reducing the particle size finds applications in the fabrication of nanoelectronic devices, telecommunication, optoelectronic devices and sensors [13-16]. There are reports on the synthesis of HgTe nanoparticles and nanorods with luminescence in the visible region [17,18]. However, there are no reports on HgTe in nanotube form. One possible reason can be its existence in energetically favorable cubic structure. Most of the materials in nanotube form exist in stable hexagonal structure; one of the exceptions is cubic AlN which however requires a very high preparation temperature [8]. It should be pointed out that as prepared HgTe is a p-type material and n-HgTe can be useful in creating p-n junction based electronic and optoelectronic devices. Iodine has been used a
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