J. Phys. Chem. C 2008, 112, 10385–10388
10385
Defect Control and Its Influence on the Exciton Emission of Electrodeposited ZnO Nanorods Jingbiao Cui Department of Physics and Astronomy, UniVersity of Arkansas at Little Rock, Arkansas 72204 ReceiVed: February 29, 2008; ReVised Manuscript ReceiVed: April 8, 2008
Growth of zinc oxide nanorods with controlled defect densities was achieved in a low-temperature electrochemical process. The concentration of the defects was found to play an important role in the exciton emission in the ZnO nanorods, leading to shifts in the peak position and changes in the spectral shape of the band edge emission at room temperature. A high concentration of defects suppressed both free and bound exciton emissions, likely resulting from the exciton scattering by defects. This investigation indicates that the exciton emission can be tuned by the control of defect density, which is important for optoelectronic applications that require a well-defined exciton emission. 1. Introduction ZnO nanostructured materials are excellent II-VI compound semiconductors and have attracted much interest because of their various remarkable physical properties and potential applications in a number of emerging areas such as low-voltage and shortwavelength optoelectronics and photonics. Although high-quality ZnO nanostructures have been synthesized by a variety of techniques,1–9 defects are often observed in the as-grown samples. These defects likely have a negative impact on the p-type doping of ZnO due to a charge compensation effect. The appearance of green and/or yellow bands in photoluminescence (PL) is a fingerprint of the presence of the defects in ZnO.10–12 The origin of the defect emissions is still a matter of debate, although some researchers attributed the green band to oxygen vacancies13,14 and the yellow emission to oxygen interstitials.6,15 Progress has been made in understanding the band edge emission of ZnO. However, the reported peak positions are controversial. The as-grown ZnO were reported to have an identical wurtzite crystal structure, but the peak position of their band edge emission exhibited large variations in different samples.6,16–21 Models involving bound exciton (DX), free exciton (FX), and phonon-assisted FX transitions have been proposed to explain the band edge emission.17–22 Charge carrier concentration was reported to affect the FX energy in ZnO.23 Since defects in ZnO are commonly observed and their concentrations often vary from sample to sample, their role in band edge emission may not be trivial. Therefore, growth of ZnO with the controlled density of defects is important for both fundamental understanding and practical applications in optoelectronics. So far, however, it is still a challenge to grow ZnO nanomaterials with a desired concentration of defects. Here we report on the growth of zinc oxide nanorod arrays with controlled defect densities in a low-temperature electrochemical process. The defect density in ZnO was controlled by adjusting the concentration of chemical reagents in the growth solution, providing a unique opportunity to investigate the influence of defects on exciton emission and optoelectronic applications of ZnO. We found that the defects strongly affect the peak position and spectral shape of band edge emission of ZnO nanorods. A high concentration of defects suppressed both FX and DX emissions. This finding helps explain the reported variations in the peak position and spectral shape. The controlled
growth of defects and their influence on exciton emission in ZnO are important for optoelectronic applications such as lightemitting diodes and UV lasers. 2. Experiment Synthesis of ZnO Nanorods. Single crystal zinc oxide nanorods were synthesized using zinc nitrate hydrate (Zn(NO3)2 · 6H2O) and hexamine (C6H12N4) in an electrochemical process. The source materials were dissolved in deionized water at room temperature and then heated to 90 °C on a hot plate. Gold wires were used as working and counter electrodes. During the ZnO nanostructure growth, a dc electric potential as referred to a gold reference electrode was applied to the substrate. Zinc oxide nanostructures were deposited at a constant potential of -0.8 V using different concentrations of hexamine, while zinc nitrate was kept at 6.3 mM in the source solution. At the end of the growth, the samples were taken out of the aqueous solution and immediately rinsed in flowing deionized water in order to remove any residual salt on the surface. Characterization. The morphology of the as-grown samples was analyzed by a high-resolution scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDX). X-ray diffraction pattern was measured by a Rigaku X-ray diffractometer (XRD; Cu Ka radiation, λ ) 1.5418 Å at 35 kV and 30 mA). PL was excited by a He-Cd laser with a 325 nm line. The emitted light was transferred onto a monochromator and detected by a charge-coupled device (CCD) camera. Low-temperature PL was performed by cooling the samples in a cryostat. 3. Results and Discussion Figure 1 displays SEM images of ZnO nanorods grown with different concentrations of hexamine in the source solution while the concentration of zinc nitrate was kept constant (6.3 mM). The growth time was 1.5 h. The average diameter of the ZnO nanorods is about 100 nm in the different samples. The growth behavior looks similar in the four samples, with no noticeable differences in their nucleation density and average diameter. However, subtle differences are noted as shown in Figure 1a,d. Hexagonal nanorods with a well-defined smooth surface were obtained when 6.3 mM hexamine was used. As hexamine was
10.1021/jp801803u CCC: $40.75 2008 American Chemical Society Published on Web 06/21/2008
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Figure 1. SEM images of ZnO nanorods grown at different concentrations of hexamine: (a) 6.3 mM, (b) 12.5 mM, (c) 25 mM, and (d) 43 mM.
Figure 2. Growth rate in length as a function of the concentration of hexamine in the source solution.
increased up to 43 mM, the nanorods were found to have a corn-like shape with a rough surface. These ZnO nanocones may have potential applications in cold cathodes due to their high efficiency of electron emission resulting from the geometric field enhancement at the tips. The rough surface of the nanocones shown in Figure 1d is composed of a large amount of nanocrystals with an average size of 5-10 nm. These nanocrystals start to appear on the nanorods grown at 25 mM hexamine. This growth behavior may be associated with the microscopic variation of hexamine attachment to the nonpolar facets of ZnO nanowires at different concentrations of hexamine,24 resulting in an inhomogeneous growth of ZnO on the nanorod surfaces. Further study is needed to clarify the underlying mechanism. Figure 2 shows the growth rate as a function of the concentration of hexamine. The nanorod length was obtained by SEM measurements. One may see that the growth rate in length decreases by a factor of 4 as hexamine is increased from 6.3 mM to 43 mM. Note that the nucleation density does not seem to depend on the concentration of hexamine. It is worth noting that the concentration of zinc nitrate in the solution was kept constant while that of hexamine was varied. It is reasonable to assume that the amount of zinc ions available for ZnO nanorod growth is unchanged in the solution since hexamine does not decompose in aqueous media, even upon boiling, and complexation of zinc with hexamine is unlikely.24 Therefore, the changes in the morphology and growth rate of ZnO nanorods merely result from the increase of hexamine in the source solution.
Cui
Figure 3. X-ray diffraction patterns of ZnO nanorods grown at different concentrations of hexamine. The intensities of (002) peaks have been normalized for comparison. The spectra are vertically offset for clarification.
The reduced growth rate at higher concentrations of hexamine seems to support the hypothesis that the role of hexamine in ZnO nanorod growth is to terminate the nonpolar surfaces (side facets) of the nanorod and allow a preferential growth along the 〈001〉 direction (c-axis). ZnO crystal exhibits partial polar characteristics with the (001) surface as the basal polar plane, which may be terminated with either Zn or O ions. In contrast, hexamine is a nonpolar chelating agent and preferentially attaches to the side facets of ZnO nanorods. Thus, it is the competition of the attachments between Zn/O and hexamine to the ZnO surfaces that determine the aspect ratio of ZnO nanorods. Increased concentration of hexamine in the growth solution increases the attachment probability of hexamine to the (001) surface, resulting in a lower growth rate along the c-axis. Therefore, the change in growth rate solely depends on the relative concentrations of hexamine and zinc nitrate in the source solution. The growth of nanocones at higher concentrations of hexamine may be understood as follows. When the concentration of hexamine is increased, the attachments of hexamine to both the (001) surface and the side walls of ZnO nanorods are increased. The areal density of hexamine on the side wall should be higher than that on the (001) surface because of a preferential attachment to the side facets. The distribution of hexamine on the (001) surface is likely inhomogeneous with less hexamine in the center, which results in a higher growth rate in the central region. Therefore, ZnO nanocones instead of nanorods with hexagonal (001) surfaces are eventually formed at higher concentrations of hexamine. The increased hexamine concentration was found to slightly decrease the pH value from 7 to 6.3 in the growth solution. We also investigated the effect of a slight change in the pH value on the ZnO nanorod growth by adding acitic acid in the solution without changing hexamine concentration. No significant change in the ZnO growth was observed. XRD patterns are plotted in Figure 3, which confirm the hexagonal wurtzite structure in the as-grown ZnO nanorods. A strong peak at 34.44° from ZnO with indices of (002) was observed in the diffraction patterns. Other diffraction peaks were also observed but with very low intensity, indicating that the ZnO nanorods grew preferentially along the 〈002〉 direction without being affected by varying the hexamine concentration in the source solution. The peak position of (002) shifted by 0.019° to the lower angle as the hexamine concentration was increased from 6.3 to 43 mM, which suggests an increase of
Role of Defect Control in Exciton Emission of ZnO
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Figure 4. Room-temperature PL spectra of ZnO nanorods grown at different concentrations of hexamine. The spectra are vertically offset for clarification. The PL spectra have been normalized to the intensity of the band edge emission in order to highlight differences in defect emission.
lattice constant by 0.05%. This lattice change may be associated with the different concentration of defects in the nanorods as observed in the PL measurements. A broader weak peak overlapping (002) diffraction was observed from the sample grown at 43 mM hexamine. This peak is attributed to the nanocrytalline ZnO on the nanorods as observed in SEM. The nanocrystals still grow preferentially along (002) direction. The average size of the nanocrystals can be determined from the width of the diffraction peak using Sherrer’s equation.25 The calculated value is about 5 nm, which is consistent with that obtained by SEM. Room temperature PL spectra of the ZnO nanorods are shown in Figure 4. Both the UV and visible emission bands were observed. The PL spectra have been normalized to the intensity of the band edge emission in order to highlight differences in defect emission. The relative intensity of the defect emission decreased as hexamine concentration was increased from 6.3 to 43 mM, reflecting a significant reduction of the defect density in the ZnO nanostructures. This yellow emission around 2.1 eV has been frequently reported in ZnO nanostructures grown in a solution route.6,26,27 Singly charged interstitial oxygen ions have been proposed as the cause of this band.15,28 Different band edge emissions were also observed in these samples. The peak position gradually shifts to low energies and the spectral shape changes from symmetric to asymmetric distributions at higher hexamine concentrations. It is unlikely that the quantum confinement effect is responsible for the shift in the peak position because the average size of the nanorods is around 100 nm. Although quantum size effect may exist in the nanocrystals of a few nanometers attached to the nanocones, these nanocrystals are not responsible for the peak shift either. The size of the nanocrystals increases as hexamine concentration is increased. This would cause a red shift in the band edge emission. We actually observed a blue shift when hexamine is increased in the source solution. It has been reported that both FX and phonon-assisted FX emissions may contribute to room-temperature band edge emission.22,29 One longitudinal-optical (LO) phonon replicas of the free excitons (FX-1LO) normally dominate the phononassisted emission process at room temperature. In this study, a superposition of FX and FX-1LO in the band edge emission was also observed. Figure 5 shows a detailed analysis of band edge emission of the samples with different concentrations of defects. One can see that the peak position and spectral shape gradually change as the concentration of hexamine increases in the growth solution. Fits to the experimental data using the
Figure 5. Band edge emission of the ZnO nanorods at room temperature. The solid lines are fits to the experimental data using two emissions from FX (green curves) and FX-1LO (blue curves). The two vertical dashed lines indicate the peak position of FX and FX-1LO for the sample grown with 43 mM hexamine. The spectra are vertically offset for clarification.
Gaussian function are also plotted in the figure. The average phonon energy obtained from the separations between FX and FX-1LO peaks is 76 meV, which is in good agreement with the reported value of LO phonon in ZnO (72 meV).30 The peak position of FX emission constantly decreases from 3.2996 to 3.283 eV as hexamine concentration was reduced from 43 mM to 6.3 mM in the growth solution. This FX energy change may result from the variations in carrier concentration in ZnO. It has been reported that free carrier can induce the decrease of FX energy.23 It is reasonable to assume that the increase in defect density would cause the carrier concentration to increase in the ZnO nanorods. The variations in FX energy by 17 meV observed in our experiments likely results from the different concentrations of charge carrier in the nanorods. Note that the apparent shift in the peak position of band edge emission is about 90 meV. One can see from Figure 5 that the relative intensity of FX increases and that of FX-1LO decreases as the concentration of hexamine is increased in the source solution. Both FX and FX-1LO emissions contribute to the band edge emission. The change in the relative intensity of FX and FX-1LO results in a shift in the peak position and the change in the spectral shape of the band edge emission. The large amount of defects in the samples grown at low hexamine concentrations is believed to cause the relative intensity of FX emission to decrease. Figure 6 shows the band edge emissions measured at different temperatures for the samples grown at 6.3 mM and 43 mM hexamine. The band edge emissions of both samples shift to high energies, and their intensities increase at lower temperatures. However, different dependences of the emissions on temperature were observed in these samples. The sample with fewer defects deposited at 43 mM hexamine shows a much stronger temperature dependence of its band edge emission than that with more defects grown at a lower hexamine concentration. Although the overall shifts of the band edge emission are different for the two samples, their FX emissions shift similarly as temperature drops from 295 to 6 K. So far, we have seen that FX and DX emissions are strongly affected by the presence of defects in ZnO nanorods. The FX
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Cui 4. Conclusion In summary, ZnO nanorods with controlled defect densities were deposited in a low-temperature electrochemical process. PL revealed that the samples with different concentrations of defects have different band edge emission at room temperature. Both the spectral shape and the peak position of band edge emission are affected by the concentration of defects. The variations in FX emission are explained in terms of exciton scattering by defects. This investigation indicates that growth of ZnO nanostructures with controlled defect densities is important for potential applications in optoelectronics. The impact of defects on the stimulated emission in ZnO is worth further exploration.
Figure 6. Band edge emissions of ZnO nanorods grown at 6.3 mM (a) and 43 mM hexamine (b) measured at different temperatures.
Figure 7. Fits to the band edge emission of sample D measured at 6.5 K. The solid squares are experimental data, and the red solid lines are the fits. The emissions from FX, DX, FX-1LO, and FX-2LO are labeled in the figure.
emission decreases as the intensity of defects increases. It is known that the lifetime of excitons is affected by the scattering of both phonons and defects. Most hydrothermally grown ZnO samples contain large concentrations of neutral and ionized impurities.29 The exciton scattering may be dominated by defects if their concentration is high. As such, the relative intensity of FX emission is affected by the defects. Our experiments demonstrate that high concentrations of defects affect the FX and DX emissions. The defects also cause a weak temperature dependence of the PL spectra as shown in Figure 6. Therefore, the exciton emission is tunable via the control of defect density. Similar phenomena were also observed in our pulsed laser deposited ZnO thin films with different concentrations of defects. Fits to a low-temperature PL spectrum from the sample grown with 43 mM hexamine are shown in Figure 7. Four peaks at 3.384, 3.360, 3.316 and 3.248 eV, which respectively correspond to FX, DX, FX-1LO, and FX-2LO emissions, are obtained from the fits. The fine structures were not well resolved in most of the samples grown in solution, likely resulting from the defectinduced broadening. The separation between the DX and FX peaks is 24 meV. This value is comparable to the reported energy in bulk ZnO.29 The phonon energy was found to be 68 meV from the separation of FX and the phonon replicas. This value is smaller than that obtained at room temperature.
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