Intense Ultraviolet Photoluminescence Observed at Room Temperature from NiO Nano-porous Thin Films Grown by the Hydrothe
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Intense Ultraviolet Photoluminescence Observed at Room Temperature from NiO Nano porous Thin Films Grown by the Hydrothermal Technique Sachindra Nath Sarangi1, Dongyuan Zhang, Pratap Kumar Sahoo2, Kazuo Uchida, Surendra Nath Sahu3 and Shinji Nozaki Graduate School of Informatics and Engineering, The University of Electro-Communications, 15-1 Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan 1
On leave from Institute of Physics, Bhubaneswar, 751005, India
2
National Institute of Science Education and Research (NISER), Institute of Physics Campus,
Bhubaneswar-751005, Orissa, India 3
The National Institute of Science and Technology, Palur Hills, Berhampur 761008, India
ABSTRACT We have successfully formed high-quality nanoporous NiO films by the hydrothermal technique and observed intense ultraviolet (UV) luminescence at room temperature. The SEM image reveals nanoporous NiO films with pore diameters from 70 to 500 nm. The results of XRD, Micro Raman and FTIR characterizations confirm the cubic structure of NiO. The optical band gaps estimated from the absorption spectrum are found to be 3.86 and 4.51 eV. The former is similar to that of bulk NiO, while the latter is much higher than that of bulk NiO. The increased band gap was attributed to the quantum confinement in the NiO nanocrystals, which may be present in the nanoporous NiO film. The room-temperature photoluminescence (PL) spectrum shows a peak of intense luminescence at 3.70 eV and several other peaks in the UV and nearUVwavelength regions. The intense UV luminescence at 3.70 eV was associated with the near band-edge emission and the others with defect-related emission. The high-quality wall of nanoporous NiO with a large surface-to-volume ratio provided the intense UV emission. INTRODUCTION Some of metal-oxide semiconductors have attracted increasing attention of engineers and scientists developing ultraviolet (UV) - LEDs because of their wide band gaps and may become materials of UV-LEDs in future. Among such metal-oxide semiconductors, Nickel oxide (NiO) is one of few oxide semiconductors which can have the p-type conductivity. However, there are only few reports on intense UV luminescence observed in NiO at room temperature. It is well known that p-type conductivity of NiO can be realized by nickel vacancies and/ or oxygen interstitials. Among the p-type oxide-semiconductors, the greatest attention has been paid to NiO thin films because of their excellent optical, electrical and magnetic properties. A number of studies have been made on high dielectric permittivity [1], thermoelectric behavior [2], and ferromagnetism [3], whereas very few reports are found on its luminescence property. The technique mostly commonly employed to deposit NiO is sputter deposition. However, the plasma damage caused by sputter-deposition may form defects and oxygen deficiency in the
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deposited NiO films. Although other techniques, such as ammonia precipitation [3,4], hydrothermal method [5], sol–gel method [2,6], electrochemistry [7], microemulsions [8] methods. Among t
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