Cu-doped ZnO nanorod arrays: the effects of copper precursor and concentration
© Babikier et al.; licensee Springer. 2014
Received: 25 January 2014
Accepted: 8 April 2014
Published: 1 May 2014
Cu-doped ZnO nanorods have been grown at 90°C for 90 min onto a quartz substrate pre-coated with a ZnO seed layer using a hydrothermal method. The influence of copper (Cu) precursor and concentration on the structural, morphological, and optical properties of ZnO nanorods was investigated. X-ray diffraction analysis revealed that the nanorods grown are highly crystalline with a hexagonal wurtzite crystal structure grown along the c-axis. The lattice strain is found to be compressive for all samples, where a minimum compressive strain of −0.114% was obtained when 1 at.% Cu was added from Cu(NO3)2. Scanning electron microscopy was used to investigate morphologies and the diameters of the grown nanorods. The morphological properties of the Cu-doped ZnO nanorods were influenced significantly by the presence of Cu impurities. Near-band edge (NBE) and a broad blue-green emission bands at around 378 and 545 nm, respectively, were observed in the photoluminescence spectra for all samples. The transmittance characteristics showed a slight increase in the visible range, where the total transmittance increased from approximately 80% for the nanorods doped with Cu(CH3COO)2 to approximately 90% for the nanorods that were doped with Cu(NO3)2.
ZnO semiconductor attracted considerable research attention in the last decades due to its excellent properties in a wide range of applications. ZnO is inherently an n-type semiconductor and has a wide bandgap of approximately 3.37 eV and a large exciton binding energy of approximately 60 meV at room temperature. As mentioned above, ZnO is a promising semiconductor for various applications such as UV emitters and photodetectors, light-emitting diodes (LEDs), gas sensors, field-effect transistors, and solar cells [1–6]. Additionally, ZnO resists radiation, and hence, it is a suitable semiconductor for space technology applications. Recently, ZnO nanostructures have been used to produce short-wavelength optoelectronic devices due to their ideal optoelectronic, physical, and chemical properties that arise from a high surface-to-volume ratio and quantum confinement effect [6–8]. Among the ZnO nanostructures, ZnO nanorods showed excellent properties in different applications and acted as a main component for various nanodevices [1, 2, 9–11]. Previous research showed that the optical and structural properties of ZnO nanorods can be modified by doping with a suitable element to meet pre-determined needs [12, 13]. The most commonly investigated metallic dopants are Cu and Al [13–15]. Specifically, copper is known as a prominent luminescence activator, which can enhance the green luminescence band by creating localized states in the bandgap of ZnO [16–19]. Previous research showed that Cu has high ionization energy and low formation energy, which speedup the incorporation of Cu into the ZnO lattice [16, 20]. Experimentally, it was observed that the addition of Cu into ZnO-based systems has led to the appearance of two defective states at +0.45 eV (above the valence band maximum) and −0.17 eV (below the conduction band minimum) [21, 22]. Currently, a green emission band was observed for many Cu-doped ZnO nanostructures grown by different techniques [23, 24]. Moreover, Cu as a dopant gained more attention due to its room-temperature ferromagnetism, deep acceptor level, some similar properties to those of Zn, gas sensitivity, and enhanced green luminescence [15–17]. However, there are several points that have to be analyzed such as the effect of the copper source on the structural, morphological, and optical properties of Cu-doped ZnO. Moreover, the luminescence and the structural properties of Cu-doped ZnO nanorods are affected by different parameters such as growth conditions, growth mechanism, post growth treatments, and Cu concentration. Despite the promising properties, research on the influence of Cu precursors on Cu-doped ZnO nanorod properties remains low.
ZnO nanostructures can be synthesized by a variety of techniques including vapor-phase transport, chemical vapor deposition, sol-gel, condensation, spray pyrolysis, and hydrothermal method. Among these methods, the hydrothermal method is used to prepare ZnO nanorods due to its low cost and simplicity [16, 25, 26].
In order to improve the structural and optical properties of Cu-doped ZnO nanorods, the effect of the Cu precursor is worth clarification. In the study reported here, we have synthesized pure and Cu-doped ZnO nanorods onto a quartz substrate pre-coated with a ZnO seed layer using the hydrothermal method. The main focus has been put on the effect of the copper precursor on the morphology, structural, transmittance, and photoluminescence properties of the synthesized ZnO nanorods.
The nanorod growth was accomplished in two steps: (1) the sputtering of ZnO seed layer to achieve highly aligned Cu-doped ZnO nanorods  and (2) the nanorod growth using the hydrothermal method.
Sputtering of ZnO seed layer
Prior to the nanorod growth, a 120-nm-thick seed layer of undoped ZnO was deposited onto a quartz substrate using RF magnetron sputtering at room temperature. Before the deposition of the ZnO seed layer, a surface treatment of the quartz substrate was conducted using acetone, ethanol, and deionized water for 10 min for each at RT and then dried in air. Pure ZnO (99.999%) with a 50-mm diameter and 5-mm thickness was used as the ZnO target. The seed layer sputtering was accomplished in a mixture of O and Ar gas atmosphere with the gases' flow rates of 2.5 and 35 sccm, respectively. The base pressure attained was 10−4 Pa, and the working pressure was 1 Pa during sputtering. The sputtering power was 100 W. In order to remove the contaminants from the ZnO target, pre-sputtering for 10 min was performed. Finally, the ZnO-sputtered seed layer thin films were annealed at 500°C for 30 min.
Precursors, concentrations, and crystal parameters of undoped and Cu-doped ZnO nanorods
Characterization and measurements
In order to characterize the structure of the grown nanorods, X-ray diffraction (XRD) measurements were performed using a MiniFlex-D/MAX-rb with CuKα radiation. The morphology of the hydrothermally grown nanorods was investigated by field emission scanning electron microscope (SEM) using SEM Helios Nanolab 600i (Hillsboro, OR, USA). Photoluminescence (PL) spectra were measured at room temperature with an excitation source of 325-nm wavelength using a He-Cd laser. Transmittance measurements were recorded by a UV-vis spectrophotometer (Phenix –1700 PC, Shanghai, China).
Results and discussion
Conversely, the lack of hydrolysis process in Cu(NO3)2 would lead to a low concentration of OH−, which may slowdown the growth rate .
In conclusion, we explored the effect of Cu precursors (Cu(CH3COO)2 and Cu(NO3)2) and concentration on the structural, morphological, and optical properties of the hydrothermally synthesized Cu-doped ZnO nanorods. The XRD results revealed that the slight changes in the lattice parameters have occurred due to the substitution of Zn2+ by Cu2+ and the formation of defect complexes. The nanorods doped with Cu(NO3)2 had less crystallinity than the nanorods doped with Cu(CH3COO)2, where the maximum compressive lattice strain (−0.423%) was obtained when 2 at.% of Cu was added from Cu(NO3)2. From the SEM studies, Cu(CH3COO)2 was found to be an effective precursor for the formation of Cu-doped ZnO nanorods with large diameter. Conversely, Cu-doped ZnO nanorods with a small diameter (approximately 120 nm when 2 at.% was added) can be obtained when Cu(NO3)2 is used as a Cu precursor due to the lack of hydrolysis process. UV and green emission peaks at 378 and 544 nm were observed for all samples and are attributed to the near-band edge UV emission and the defect-related emission, respectively. A redshift of approximately 6 nm in the UV emission band was seen for the Cu-doped ZnO nanorods and was attributed to the rigid shift in the valence and the conduction bands due to the coupling of the band electrons and the localized Cu2+ impurity spin. Irrespective of Cu concentration, the nanorods doped with Cu(CH3COO)2 showed a transmittance of approximately 80% in the visible range, while the nanorods doped with Cu(NO3)2 showed a rather high transmittance (approximately 90%). The obtained results are comparable with the previous results. In conclusion, by choosing a suitable Cu precursor and concentration, we can control the diameter of Cu-doped ZnO nanorods, which is important for the fabrication of nano-optoelectronic devices.
MB obtained his MSc degree in nanoscience from Lund University, Sweden. He is currently a Ph.D. student in Harbin Institute of Technology. His research interests include fabrication and properties of metal-doped ZnO nanostructures. DW is an MSc student in Harbin Institute of Technology. His research interests include fabrication and properties of ZnO thin films. JW obtained his Ph.D. degree from Jilin University. He is currently a full professor at Harbin Institute of Technology. His research interests cover pure and doped ZnO nanomaterials, solar cell, and optoelectronic devices. QL is an MSc student at Harbin Institute of Technology. Her research interests include fabrication and properties of p-type ZnO thin films. JS is an MSc student in Harbin Institute of Technology. His research interests include fabrication and properties of ZnO UV detectors. YY obtained his MSc degree in engineering from Harbin Institute of Technology. He is currently a Ph.D. student in Harbin Institute of Technology. His research interests include fabrication and properties of metal oxide solar cells. QY is currently a full professor at Harbin Institute of Technology. His research interests cover metal oxide nanomaterials, solar cell, and gas sensors. SJ is currently a full professor at Harbin Institute of Technology. Her research interests cover pure and doped ZnO nanomaterials.
This work has been partly supported by the Program for New Century Excellent Talents in University (NCET-10-0066), an 863 project grant (2013AA031502), and Project No. 2011RFLXG006.
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