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J. Phys. Chem. C 2010, 114, 9498–9502
Evidence of Unintentional n-Doping in ZnO Nanorods Artem Kovalenko,† Genevie`ve Pourroy,* Olivier Cre´gut, Mathieu Gallart, Bernd Ho¨nerlage, and Pierre Gilliot Institut de Physique et Chimie des Mate´riaux de Strasbourg, IPCMS UMR 7504, CNRS-UDS, 23 rue du Lœss, B.P. 43, 67034 Strasbourg Cedex 2, France ReceiVed: March 12, 2010; ReVised Manuscript ReceiVed: April 12, 2010
ZnO nanorods were synthesized by using a simple two-step hydrothermal approach. The nanorod diameter ranged from 40 to 60 nm and their length was about 2 µm. X-ray diffraction shows good crystalline quality with an axial alignment of the nanorods along the wurtzite c axis. Raman and visible photoluminescence demonstrate that thermal annealing suppresses structural defaults. Photoluminescence experiments performed between 3 and 200 K close to the exciton resonances evidence the presence of optical transitions related to electron-bound exciton scattering. To our knowledge, this is the first time that this feature is identified in ZnO nanorods. The occurrence of such phenomena is an indication that, despite a good crystalline quality, the samples exhibit a strong native n-type character. Introduction ZnO has been extensively studied because of its wide range of fascinating properties that lead to applications in sensors, photocatalysis, light emitting electrodes, and laser diodes in the UV-vis range.1-3 ZnO is also promising for room-temperature operating and semiconductor-based photonic applications, such as polariton lasers4 and microcavity-based devices.5,6 Various synthesis methods have been developed, allowing the elaboration of numerous and fascinating shapes:7,8 Nanocombs, nanorings, nanohelixes/nanosprings, nanobelts, nanowires, and nanocages have been obtained by using a solid-vapor phase thermal sublimation technique under controlled pressures and temperatures. ZnO emerges now as a promising candidate for hybrid solar cells due to its semiconducting properties similar to those of TiO2.9 ZnO is, in many ways, more suited in solar cell devices than TiO2 due to its excellent electron mobility and its processability into many shapes and forms. The fact that ZnO is not as good as TiO2 as a photocatalyst is particularly useful in hybrid devices based on organic materials. Thus, the successful use of ZnO for air stable solar cells was demonstrated.10 Regarding the synthesis, ZnO enables solution processing at very low temperatures, even using nanostructured material. This has been documented in the manufacture of polymer solar cells using ZnO as an electron acceptor as well as an electron transport layer.11 In this context, low-cost techniques, such as sol-gel or hydrothermal routes, are now receiving a lot of attention.12,13 A wide variety of shapes and hierarchical structures can be obtained owing to the precursor, the base, or the temperature.14 Nanoparticles are obtained by precipitation from Zn nitrate or chlorides into a base.15,16 The growing of ZnO nanostructures by hydrothermal treatment was first reported by Andres-Verges et al.17 This method received much interest when large arrays of nanorods were obtained at low temperatures (about 90 °C) on glass or silica substrates.18 They are generally obtained by thermal decomposition of zinc nitrate hexahydrate * To whom correspondence should be addressed. E-mail:
[email protected]. † Present address: Faculty of Materials Science, M. V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119992, Russia.
(Zn(NO3)2 · 6H2O) in methenamine (hexamethylenetetramine, C6H12N4). Several studies have been performed to determine the effect of the synthesis conditions on the nanorod size and on their optical properties.19 Room-temperature photoluminescence spectra recorded on nanorods obtained at low temperature exhibit a well-resolved near-band-edge emission and a weak deep-level emission.20 The latter, originating from the defects due to the low-temperature synthesis method, disappears when annealing the nanorods in an argon atmosphere. The photoluminescence of ZnO nanorods grown on silicon (100) and plastic foil (polyethylene naphtalate (PEN)) has been studied at room temperature.21,22 A systematic photoluminescence study of arrays of ZnO nanorods 70-150 nm in diameter and from 0.3 to 2 µm in length was undertaken at temperatures between 4 and 293 K.23 A large line width of the near-band-edge emission (about 10 meV), depending on the temperature as well as the absence of sharp excitonic transitions, was pointed out. ZnO nanorods with small diameters in the range of 10 ( 2 nm were fabricated.24 For the smallest rod diameters, a blue shift of the band-edge emission line was observed. It was attributed by the authors to quantum confinement of carriers, although it is expected only for very small rod diameters (
