j phys chem c 113-2009-13643

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J. Phys. Chem. C 2009, 113, 1364313650

13643

Synthesis and Luminescence Properties of (N-Doped) ZnO Nanostructures from a Dimethylformamide Aqueous SolutionBrigitte Sieber,*, Hongqin Liu,, Gaelle Piret, Jacky Laureyns,| Pascal Roussel, Bernard Gelloz,# Sabine Szunerits, and Rabah Boukherroub*,Laboratoire de Structure et Proprietes de lEtat Solide, UMR CNRS 8008, UniVersite Lille 1, Batiment C6, 59655 VilleneuVe dAscq, France, Institut de Recherche Interdisciplinaire (IRI), USR CNRS 3078, Parc de la Haute Borne, 50 aVenue de Halley, BP 70478, 59658 VilleneuVe dAscq, France, Institut dElectronique, de Microelectronique et de Nanotechnologie (IEMN), UMR CNRS 8520, Cite Scientique, AVenue Poincare, BP 60069, 59652 VilleneuVe dAscq, France, The Institute for Chemical Physics, School of Science, Beijing Institute of Technology, Beijing, 100081, Peoples Republic of China, Laboratoire de Spectrochimie Infrarouge et Raman (LASIR), Batiment C5 - UMR CNRS 8516, UniVersite Lille 1, 59655 VilleneuVe dAscq, France, UCCS, Equipe de Chimie du Solide, UMR CNRS 8181, ENSCL et UniVersite Lille 1, BP 90108, 59652 VilleneuVe dAscq, France, and DiVision of Electrical and Electronic Engineering, Faculty of Technology, Tokyo UniVersity of Agriculture and Technology, Koganei-shi, Tokyo 184 8588, Japan ReceiVed: April 16, 2009; ReVised Manuscript ReceiVed: June 10, 2009

Downloaded by UNIV LILLE 1 on August 19, 2009 Published on July 1, 2009 on http://pubs.acs.org | doi: 10.1021/jp903504w

The paper reports on the optical properties of ZnO nanostructures elaborated on a zinc foil substrate by a simple chemical approach. The doping type and density of the ZnO nanostructures were evaluated using electrochemical impedance spectroscopy. XRD diffraction patterns and Raman spectroscopy were used to study the structural properties evolution upon thermal annealing at 300 C for 1 h in air. Their optical properties, probed by low temperature photoluminescence and room temperature cathodoluminescence (CL), are correlated to their electronic and structural properties. The luminescence of the nanorods is dominated by a broad near band edge emission located in the blue-violet region of the optical spectrum. Analysis of the CL spectra and monochromatic CL images show that the main luminescence has an extrinsic origin, which is tentatively assigned to nitrogen impurities.I. Introduction Zinc oxide (ZnO) is one of the most studied semiconductors due to its outstanding properties. It is a direct, wide bandgap semiconductor (Egap ) 3.36 eV) with a free exciton binding energy large enough (60 meV) to allow excitons to be stable at room temperature. ZnO is used in blue/UV optoelectronics,1 transparent electronics,2 piezoelectric transducers,3 photovoltaic applications,4 varistors,5 spintronic devices,6 and gas sensors.7 The preparation of ZnO nanostructures such as nanorods and nanowires has already been described in several reports. They have been elaborated by many different techniques such as reactive sputtering,8 thermal evaporation,9 spray pyrolysis,10 oxidation of Zn,11 pulsed laser deposition,12 chemical vapor transport and condensation,13 and metal organic chemical vapor deposition.14 Among these various techniques, wet chemical approaches have been increasingly used in the last years15-20 because they require neither sophisticated equipment nor vigorous experimental conditions. Thus, they became more widely used next to vapor-phase deposition techniques. Their optical properties have been explored by means of photoluminescence (PL),21-23 spatially and spectrally resolved cathodoluminescence (CL),* To whom correspondence should be addressed. E-mail: brigitte.sieber@ univ-lille1.fr (B.S.); rabah.boukherroub@iemn.univ-lille1.fr (R.B.). UMR CNRS 8008. USR CNRS 3078 and UMR CNRS 8520. Beijing Institute of Technology. | Batiment C5 - UMR CNRS 8516. UMR CNRS 8181. # Tokyo University of Agriculture and Technology.

which offers a higher spatial resolution,24-28 or by the combination of both PL and CL.29 In this paper, we report on the synthesis, electronic, morphological, and structural characterizations, and optical properties of ZnO nanostructures elaborated by a simple chemical approach. The synthesis was performed following a slightly modied approach of the simple and mild strategy for largearea fabrication of high-quality ZnO nanorod arrays proposed by Zhang et al.30 Different positions of the blue-violet band have been observed for different nanostructures.31 Because not all of them correspond to the expected excitonic transition, they are likely related to defects.31 In this study, we put a special focus on the analysis of the shape and the origin of the dominant blue-violet emission band. We show that the intense blue-violet band is not always related to an excitonic transition. The inuence of postannealing at 300 C in air has also been studied. II. Experimental Section A. Materials. Zinc foils (99.9%, 0.25 mm thick), dimethylformamide (DMF), carbonate propylene, and lithium perchlorate (LiClO4) were obtained from Aldrich and used without further purication. B. Preparation of ZnO Nanostructures. Zinc substrates (zinc foil cut into 1 1 cm2) were ultrasonically degreased in ethanol, propanol, and water before use. The clean zinc foil was immersed in a 5% dimethylformamide (DMF) aqueous solution and heated up to 95 C (oil bath) for 24 h, washed with water, and nally dried in an oven at 130 C for 1 h. Some of the samples were annealed afterward at 300 C in air during 1 h.32

10.1021/jp903504w CCC: $40.75 2009 American Chemical Society Published on Web 07/01/2009

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J. Phys. Chem. C, Vol. 113, No. 31, 2009

Sieber et al.

C. Characterizations of the Nanostructured ZnO Substrates. Scanning Electron Microscopy (SEM). SEM images were obtained using an electron microscope ULTRA 55 (Zeiss) equipped with a thermal eld emission emitter and a high efciency In-lens SE detector. Raman Spectroscopy. Raman measurements were carried out at room temperature using a microspectrometer LABRAM Jobin-Yvon. The 1 m spot diameter on the sample surface was produced by the 514.5 nm line of a 10 mW Ar+ ion laser. Raman spectra are recorded in backscattering geometry with the incident and scattered light (not polarized) propagating parallel to the c-axis. Electrochemical Impedance Spectroscopy (EIS). EIS experiments were performed using an Autolab potentiostat 30 (Eco Chemie, Utrecht, The Netherlands). The Zn/ZnO nanowires interface was sealed against the bottom of a single compartment electrochemical cell (V ) 5 mL) by means of a rubber O-ring (the electrical contact was made to a copper plate through the Zn). A platinum sheet and an AgCl-modied Ag wire were used as counter and reference electrodes, respectively. EIS was performed using the following parameters: amplitude of 20 mV; frequency range of 10 kHz-1 Hz, potential range: -0.8-0.8 V. The electrolyte was carbonate propylene/LiClO4 (0.1 M) to avoid ZnO decomposition.32,33 X-ray Diffraction. The XRD patterns were obtained from - scans in a Bruker D8 XRD operating at 50 kV and 30 mA with a Cu anticathode ( ) 1.5418 ). Photoluminescence. PL spectra were measured while the samples were in a cryostat under vacuum. The temperature was varied from 10 to 300 K. An optical multichannel analyzer (resolution: 1 nm) and the fourth harmonic line (266 nm) of a YAG laser (pulse duration: 12 ps; repetition rate: 10 Hz; power: 4 mW; spot diameter: 6 mm) were used for detection and excitation, respectively. Each spectrum was acquired during 10 s in order to average over many laser pulses. The area of the sample probed by the detector was about 200 m in diameter. Cathodoluminescence. The CL experiments were performed at 300 K in a Hitachi 4700 FESEM equipped with a Gatan parabolic mirror. The accelerating voltage of the electron beam was 8 kV which corresponds to an electron penetration depth of 0.3 m.34 The beam current is in the range 100-200 pA and the working distance is equal to 12.4 mm. This corresponds to a focused beam spot size close to 30-50 nm. The spectral resolution of the CL system is equal to 10 meV. III. Results and Discussion A. Morphology of the ZnO Nanostructures. ZnO nanostructures investigated in this work were prepared by chemical oxidation of Zn foils in a 5% dimethylformamide (DMF) aqueous solution at 95 C as reported recently.30,32 The oxidation of metallic Zn by naturally dissolved oxygen in water is slow due to the formation of a passive oxide layer. The presence of DMF in the aqueous solution accelerates signicantly the oxidation process of metallic Zn. The underlying mechanism for the ZnO nanostructures formation can be rationalized in eqs 1-2.

Zn2+ + 2OH- f Zn(OH)2 9 ZnO + H2O 8

(2)

Downloaded by UNIV LILLE 1 on August 19, 2009 Published on July 1, 2009 on http://pubs.acs.org | doi: 10.1021/jp903504w

Zn2+ ions produced under these conditions are continuously released in the DMF aqueous solution, leading to zinc hydroxide Zn(OH)2 precipitation on the Zn surface. Working at an elevated temperature (95 C in the present work) ensures that a homogeneous nucleation process in solution and Zn(OH)2 does not block the further dissolution of zinc and allows the conversion of the hydroxide into ZnO. Figure 1A shows a SEM image of a freshly grown ZnO sample. ZnO nanostructures appear as hexagonal prisms with a faceted hexagonal end face or faceted pyramids with a diameter ranging from 60-600 nm. Annealing the ZnO nanostructures up to 300 C in air for 1 h does not induce any signicant change in their morphology (Figure 1B). B. Determination of the Charge Carrier Concentration. The charge carrier concentration was determined using electrochemical impedance spectroscopy (EIS) in a carbonate propylene electrolyte (0.1 M LiClO4) to avoid ZnO decomposition as described recently.32 An equivalent circ

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