Visible emission from Ce-doped ZnO nanorods grown by hydrothermal method without a post thermal annealing process
© Jung et al; licensee Springer. 2012
Received: 6 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Visible light-emitting Ce-doped ZnO nanorods [NRs] without a post thermal annealing process were grown by hydrothermal method on a Si (100) substrate at a low temperature of 90°C. The structural investigations of Ce-doped ZnO NRs showed that the Ce3+ ions were successfully incorporated into the ZnO lattice sites without forming unwanted Ce-related compounds or precipitates. The optical investigation by photoluminescence spectra shows that the doped Ce3+ ions in the ZnO NRs act as an efficient luminescence center at 540 nm which corresponds to the optical transition of 5d → 4f orbitals in the Ce3+ ions. The photoluminescence intensity of the Ce-doped ZnO NRs increased with the increasing content of the Ce-doping agent because the energy transfer of the excited electrons in ZnO to the Ce3+ ions would be enhanced by increased Ce3+ ions.
Keywordsnanostructures oxides crystal growth optical properties
Zinc oxide [ZnO] nanostructures have received much attention due to their remarkable performance in electronic, piezoelectric, thermoelectric, and optoelectronic applications . High electrical conductivity and transparency in a visible wavelength spectral range of the ZnO alloys have been regarded as an efficient candidate for transparent conducting electrodes of thin films and flexible electronics . The absence of a center of symmetry in its wurtzite structure, along with a large electromechanical coupling, results in strong piezoelectric and pyroelectric properties which make it an energy recycling material system . In addition, because ZnO has a large direct bandgap of 3.37 eV and a high exciton binding energy of 60 meV, it has been much investigated for optoelectronic applications, such as light-emitting diode [LED] and UV laser diode . Recently, the ZnO nanostructures doped with rare earth elements such as Ce, Y, and Eu have achieved much attention for biological tagging as well as optoelectronic applications due to their unique optical properties [5–7]. Most of all, the Ce element possessing a unique optical characteristic may be an ideal material for visible light-emitting phosphors in display, high-power laser, and light-emitting diode [5–9]. However, few have been reported for the fabrication and optical properties of the Ce-doped ZnO nanostructures especially by hydrothermal method [7–9] even though this method has many advantages of low temperature process which makes it favorable to the integration and in-situ fabrication of various devices. Furthermore, for more compatibility of the Ce-doped ZnO nanorods [NRs] to be applicable to other devices, visible emission should be possible without post thermal annealing processes. In this report, we have fabricated visible light-emitting Ce-doped ZnO NRs by hydrothermal method without a post thermal annealing process.
Growth of undoped and Ce-doped ZnO NRs on Si (100) substrate
The Ce-doped ZnO NRs were grown on a p-type Si (100) substrate. After cleaning the Si substrate by acetone, methanol, HF, and deionized [DI] water, a seed layer for the ZnO NRs was formed by dipping the substrate into 40 mM of zinc acetate dihydrate (Zn(CH3COO)2•2H2O) dissolved in ethanol solution, followed by drying at 100°C for 5 min. The Ce-doped ZnO NRs were grown by placing the seed layer grown substrate into a mixed solution of 20 mM zinc nitrate hexahydrate (Zn(NO3)2•6H2O), 20 mM hexamethylenetetramine [HMT] ((CH2)6N4), and cerium nitrate hexahydrate (Ce(NO3)3•6H2O) in DI water at 90°C for 3 h. The amount of Ce agent was varied from 0.1 to 0.8 mM, which corresponds from 0.5% to 4% in molarity. At a solution temperature above 70°C, the solution started being cloudy, indicating that a chemical reaction started. After the reaction, the samples were cleaned in the flowing DI water for 5 min.
The structural properties of Ce-doped ZnO NRs were investigated by field emission scanning electron microscopy [FE-SEM], energy dispersive X-ray spectroscopy [EDS], and X-ray diffraction [XRD] with an excitation source of Cu Kα radiation. The chemical composition of the deposited ZnO NRs was observed using EDS attached to a FE-SEM microscope.
The optical properties of Ce-doped ZnO NRs were investigated by photoluminescence [PL] spectra which were measured by using a 24-mW power 325-nm continuous He-Cd laser at room temperature. The laser was focused onto the sample surface by an objective lens; the excitation area was estimated to be about 400 μm in diameter.
Results and discussion
HMT plays a very complicated role in the solution during the hydrothermal process , but it supplies OH- ions to the Zn2+ and Ce3+ ions to form Zn-O and Ce-O bonds here, respectively. Thereby, Ce3+ ions substitute the Zn lattice sites during the growth of ZnO NRs.
In summary, Ce-doped ZnO NRs were grown on Si substrates by using the hydrothermal method. The structural properties investigated by FE-SEM, EDS, and XRD showed that the Ce3+ ions are successfully incorporated into the ZnO lattice sites; the growth of ZnO NRs was not influenced by the doping of Ce atoms because the molar content of the dopant was too small an amount to change the morphology, and the dopant agent provided Ce3+ ions for doping. The XRD results showed that the ZnO NRs are single-phase hexagonal ZnO, and unwanted Ce-related compounds or precipitates were not formed during the growth of Ce-doped ZnO NRs. The PL results showed that the doped Ce3+ ions in the ZnO NRs act as a luminescence center for visible emission at 543 nm even though the ZnO NRs were not thermally annealed. The PL intensity at a visible range of the Ce-doped ZnO NRs increased with the increased molarity of the Ce dopant agent because the energy transfer rate of the excited electrons from the conduction band in the ZnO host to a 5d energy state in the Ce3+ ion activators could be enhanced by increased Ce3+ ions. These results demonstrate that Ce doping in ZnO NRs can be an efficient luminescence center in ZnO NRs without post thermal annealing processes.
This work was financially supported by the basic research program (11-EN-03) through the Daegu-Gyeongbuk Institute of Science and Technology (DGIST) funded by the Ministry of Education, Science and Technology (MEST) and IT/SW Creative Research Program supervised by the National IT Industry Promotion Agency (NIPA-2011-C1820-1102-0013) supported by the Ministry of Knowledge Economy (MKE).
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