InN nanocrystals, nanowires, and nanotubes.

Indium nitride has attracted increasing attention in the last few years because of its properties of potential value in optoelectronic devices and other technologies. There have been several efforts to prepare InN nanostructures, but the results reported are not altogether complete and satisfactory. Nanocrystals of InN have been produced by the reaction of InCl3 and Li3N at 250 8C, but the material obtained was a mixture of the cubic and hexagonal phases. The reaction of In2S3 and NaNH2 yielded hexagonal nanocrystals with diameters in the range of 10–30 nm, with an absorption band centered around 1.9 eV. Indium halide CVD on a Si(100) substrate in the presence of NH3 has been used to produce flowerlike hexagonal crystals, but the size of the crystals is in the micrometer regime. In the decomposition of azido precursors, a solution–liquid–solid (SLS) process gave rise to 20 nm diameter InN fibers which are polycrystalline. Decomposition of another azido precursor yielded InN whiskers of 10–200 nm diameter by the vapor–liquid–solid (VLS) process, but the optical properties of the whiskers have not been reported. InN nanowires have been grown on a gold-coated silicon substrate by the thermal evaporation of pure In metal in the presence of NH3. The nanowires had a diameter in the 40–80 nm range, with a broad emission peak at 1.85 eV. By employing a VLS route, in which a mixture of In2O3 and In metal was reacted with NH3 at 700 8C, nanowires with diameters in the 10–100 nm range were obtained, but the optical spectra are not reported. CVD with a mixture of In2O3 and In along with NH3 on a Si/SiO2 substrate coated with Au (20 nm thick), gave nanowires of 15–30 nm diameter with an absorption band at 1.85 eV. We notice that some of the procedures used to produce the InN nanostructures employ relatively high temperatures, which can give rise to defects or cause decomposition. Where low-temperature reactions have been employed, the optical properties reported are not in tune with the present understanding of the material. While most of the papers on InN nanostructures report the 1.9 eV absorption band as being characteristic of the material, it is now believed that the band characteristic of InN is in the nearIR region. Because of the importance of InN as an electronic material, we considered it important to carry out a systematic investigation of the nanostructures produced at relatively low temperatures, with specific interest in the optical properties. In this communication, we report a study of the nanocrystals, nanowires, and nanotubes of InN prepared by new chemical routes. We also demonstrate that a band around 0.7 eV in the near-infrared region is truly characteristic of InN. A typical TEM image of the InN nanocrystals obtained by the solvothermal reaction of tris(N-nitroso-phenylhydroxylaminato)indium, [In(C6H5N2O2)3], and 1,1,1,3,3,3hexamethyldisilazane (HMDS) in toluene at 265 8C is shown in Figure 1a. The image reveals nanocrystals with an aver-