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J. Phys. Chem. C 2011, 115, 58–64
Size-Dependent Surface Effects on the Photoluminescence in ZnO Nanorods Haiping He,* Qian Yang, Chao Liu, Luwei Sun, and Zhizhen Ye State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China ReceiVed: July 26, 2010; ReVised Manuscript ReceiVed: NoVember 19, 2010
One-dimensional semiconductor nanostructures are expected to show significant surface effects, which results in remarkable modification of the optical properties. In this work, an experimental study of surface effects on the photoluminescence (PL) of ZnO nanorods with different sizes is reported. A thin shell layer of Al2O3 is used to passivate the nanorods’ surface, which allows one to compare the PL spectra before and after dielectric coating. It is found that strong surface exciton recombination is present in ZnO nanorods with average diameter as large as ∼500 nm. Coating the nanorods by Al2O3 significantly reduces the surface state-related emissions, indicating that surface passivation rather than surface band-bending mechanism dominates. We also provide evidence that the long controversial 3.31 eV emission in ZnO is not related to surface states but a free-tobound transition involving an unknown acceptor level of ∼125 meV. In the visible spectral region, an orange emission around 2.1 eV together with the normal green emission is observed in the thick nanorods. The little change in intensity after Al2O3 coating allows us to conclude that the visible emissions are unlikely from the surface. Based on a DAP-like transition model, we are able to interpret the blue shift of the orange emission with increasing temperature and attribute the emission to zinc vacancy defects. 1. Introduction Owing to their unique optical and electrical properties, onedimensional (1D) ZnO nanostructures such as nanowires and nanorods have received great attention in the past decade. Photoluminescence (PL) properties of ZnO nanowires have been extensively investigated due to the potential application of these materials as optoelectronic nanodevices, in particular, lightemitting devices.1,2 Due to the large surface-to-volume ratio in 1D ZnO nanostructures, it is expected that the surface states play a crucial role in the luminescence properties. In fact, emission from surface excitons (SX), generally appearing at around 3.366 eV, has been frequently observed in lowtemperature PL spectra of ZnO nanostructures with various sizes and morphologies.3,4 This emission is characterized by fast quenching with increasing temperature and intensity saturation with increasing excitation density. After the coating of a polymer or insulating shell, the SX is enhanced due to the reduced spatial separation of electrons and holes in the surface region based on a model of surface band bending proposed by Richters et al.5,6 However, the SX can also be quenched by surface passivation, as reported by Yang et al., who treated the ZnO surface by Ti plasma immersion ion implantation.7 Moreover, SX emission is not necessarily observed8 in ZnO nanostructures. It is well accepted that the PL features of ZnO nanostructures depend strongly on growth method and morphology, although the detailed mechanisms remain unclear. An emission line around 3.31 eV in ZnO nanocrystals has also been attributed to SX by some researchers.9,10 However, the origin of this band in ZnO has been a long controversial topic,11-14 and Schirra et al.14 argued that the band is more likely the free-to-neutral acceptor (FA) transition in nature, in which the acceptor is stacking faults. The very close energy of the * To whom correspondence should be addressed. E-mail: hphe@ zju.edu.cn.
first LO phonon replica of free exciton (FX) recombination to this band further complicates the accurate assignment.12,15 Moreover, ZnO nanostructures generally show greatly enhanced green emission. Many researchers believe that the green emission is related to surface states. For example, Shalish et al.16 have observed a linear relationship between the green emission intensity and the nanowire diameter and developed a simple model to describe the contribution of surface. There is also a contrary viewpoint which suggests the green emission is not originated from the nanowire surface but from within the “bulk”, namely, the tunneling of holes accumulated in the surface region into the bulk deep levels, such as oxygen vacancy.5,17 It is thus of prime importance to study the surface effects on the PL of ZnO nanostructures to better understand the physics behind it. In this work, we study temperature-dependent PL of thick (diameter ∼ 500 nm) and thin (diameter ∼ 50 nm) ZnO nanorods. A thin layer of Al2O3 is used to coat the nanorods to evaluate the surface effects. Both the near band edge (NBE) and the visible spectral range are surveyed. Our results show that the PL features depend strongly on the nanorod diameter. Coating thick ZnO nanorods by Al2O3 dramatically reduces the low-temperature excitonic emission but slightly enhances the intensity of room temperature PL in both the NBE and visible spectral region. The 3.31 eV emission is not necessarily related to surface states and could be assigned to the FA transition. A less-frequently observed orange emission at ∼2.1 eV is presented in thick nanorods, which shows negative thermal quenching and abnormal blue shift with increasing temperature. Taking these results altogether, we discuss the surface effects on the PL features and the origin of the orange emission. 2. Experimental Details The synthesis of ZnO nanorods with different diameters was carried out by a vapor transport method in a horizontal quartz tube furnace. High-purity Zn powder (99.999%) and oxygen
10.1021/jp106990a 2011 American Chemical Society Published on Web 12/10/2010
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Figure 2. Typical HREM image of the ZnO nanorod coated with Al2O3 layer.
Figure 1. SEM images of the (a) thin and (b) thick ZnO nanorods.
gas (99.999%) were used as the zinc and oxygen sources, respectively. Zn powder was loaded in the center of a quartz boat which was then inserted into the center of the furnace tube. Si(100) substrate was placed about 9 cm downstream of the Zn powder. Before the reaction was started, the quartz tube was evacuated to about 10-1 torr using a rotation pump. Then the temperature of the furnace was ramped to the growth temperature under a nitrogen flow rate of 100 sccm (standard-state cubic centimeter per minute) and maintained for 30 min. To achieved thin ZnO nanorods with average diameter of ∼50 nm, we kept the growth temperature at 550 °C for 30 min and maintained the pressure at 10-1 torr with N2 (50 sccm) and O2 (30 sccm). When we changed the growth conditions to 650 °C and 10 torr with N2 (20 sccm) and O2 (50 sccm), thick nanorods with average diameter of ∼500 nm were obtained. The morphology of the samples was examined by a field emission scanning electron microscope (SEM, HITACHI S-4800). Both the thick and thin nanorods are quasi-aligned and possess well-defined top and side facets (Figure 1). The Al2O3 layer on ZnO nanorod arrays was grown by atomic layer deposition (ALD) at 200 °C, using trimethylaluminum (Al(CH3)3) and water as the Al precursor and oxygen reactant sources, respectively. The coating of Al2O3 was confirmed by using a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30 S-Twin), as shown in Figure 2. The Al2O3 shell is uniform with a thickness of ∼10 nm. The growth direction of ZnO nanorods is along the c-axis as determined from Figure 2. PL measurements were performed on a FLS920 fluorescence spectrometer (Edinburgh Instruments) at temperature ranging from 15 to 300 K with a closed cycle helium cryostat using 300 nm light from a xenon lamp as the excitation source. The incident angle of the excitation light beam was set to 45°. X-ray photoelectron spectroscopy (XPS) was used to determine the
chemical environment of Zn element in the as-grown and Al2O3coated ZnO nanorods. The measurements were performed on a PHI5000 VersaProbe spectrometer with a base pressure of 6.7 × 10-8 Pa, using Al KR radiation as the excitation source. To obtain the information in the Al2O3-ZnO interfacial region, the coated alumina was peeled off by Ar+ ion sputtering for durations of 0.5 and 0.7 min. 3. Results and Discussion Room temperature PL spectra, plotted in the full spectral range, of the thick and thin nanorods both without and with Al2O3 coating, are illustrated in Figure 3. There are two significant differences between the thick and thin nanorods. First, the NBE emission of the thick nanorods, peaked at 3.30 eV, displays a clear shoulder around 3.24 eV. Second, the visible emission of the thick nanorods is composed of the commonly observed green emission near 2.5 eV and an orange emission near 2.1 eV. For both samples, it is found that the line shapes of the PL spectra are little affected by the coating of Al2O3 except for a slight increase in intensity. In the following, we address the temperature-dependent NBE and visible emission successively. A. Excitonic Emission. Low-temperature (15 K) and temperature-dependent PL spectra of the thick nanorods (both asgrown and Al2O3-coated) are plotted in Figures 4 and 5. The as-grown sample shows a strong line at 3.372 eV (line width ∼ 10 meV) and a shoulder at 3.381 eV (more clearly seen in the 40 K spectrum in Figure 5). The 3.381 eV line is assigned to free exciton (FX) recombination with a slight blue shift of ∼4 meV, and the 3.372 eV line can be attributed to surface exciton (SX) recombination with a localization energy of 9 meV according to the literature.3 Interestingly, the FX energy shifts back to the bulk value, 3.377 eV, after Al2O3 coating, together with the SX energy back to 3.367 eV. Moreover, the SX intensity is greatly reduced. Temperature-dependent PL spectra (Figure 5) reveal that the SX quenches completely at 120 K for the as-grown nanorods. For the coated sample, it is at 80 K. For thin nanorods, however, the features of SX emission are significantly different from the thick ones. Within the whole temperature range (Figure 6), both the as-grown and coated nanorods are dominated by the broad emission at 3.370 and 3.366 eV (line width ∼ 20 meV), respectively, with the FX
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Figure 3. RT PL full spectra of the (a) thick and (b) thin ZnO nanorods. Both the as-grown and Al2O3-coated spectra are plotted for both samples. The dashed lines represent Gaussian fit.
Figure 4. Low-temperature (15 K) PL spectra of the (a) thick and (b) thin ZnO nanorods. Both the as-grown and Al2O3-coated spectra are plotted.
line not well resolved. The larger line width results from the larger surface-to-volume ratio and hence the high density of surface states, and possibly also the localized states in the band tail region.18 A blue shift of 4 meV is also observed in the asgrown sample. We note that the sizes of the nanorods, both thick and thin, are far beyond the quantum confinement regime for ZnO.
He et al.
Figure 5. Temperature-dependent PL spectra of the thick ZnO nanorods: (a) as-grown, (b) Al2O3-coated.
Compressive strain is also unlikely responsible for the blue shift because a very thin (10 nm) surface coating is not expected to release the strain (if any) in the materials. We assume that such blue shift could be due to the surface resonance effect on ZnO (101j0) surface as proposed by Chen et al.19 It has been reported20 that in ZnO the oxygen 2p dangling-bond band of the nonpolar (101j0) surface lies below the valence band maximum. After coating, however, the dangling bonds on the surface are passivated, which eliminates the blue shift as observed. It is noteworthy that the blue shift increases with the inverse of the rod diameter according to ref 19, while in our work, it seems the blue shift is independent of the rod diameter. We speculate that although surface may indeed induce PL blue shift, the mechanism is not actually that as described in ref 19 and needs further study. The surface coating also affects the SX intensity but might be in a different way. Based on the surface band-bending model, the coating of a dielectric layer screens the charge carriers located in surface states and reduces the upward band bending near the ZnO/dielectric interface region, resulting in reduced electron-hole separation and eventually enhanced recombination rate of SX in the surface depletion region. This model is supported by the enhanced SX emission after Al2O3 or polymer coating reported by Richters et al.5,6 On the other hand, surface coating may passivate the surface states such as dangling bonds and lead to quenching of SX emission.7 In our experiments, it seems the latter mechanism dominates. The passivation of surface states is consistent with the elimination of blue shift. To give direct evidence for the surface passivation model, we carried out detailed characterization with respective to donor/ defect densities and interfacial chemical environment of Zn element. By fabricating single nanorod field effect transistors
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Figure 7. XPS Zn 2p core level spectra of the as-grown and Al2O3coated thick ZnO nanorods. For the coated sample, Ar+ sputtering is used to expose the ZnO surface.
Figure 6. Temperature-dependent PL spectra of the thin ZnO nanorods: (a) as-grown, (b) Al2O3-coated.
(FETs) and measuring their transfer characteristics (see the Supporting Information), we can estimate the electron density (hence shallow donor density) as ∼mid-1016 cm-3 in the thick nanorods, which is rather low. The low n-type conductivity is consistent with the high oxygen partial pressure during the growth of the thick ZnO nanorods. We note that the low density of shallow donors does not necessarily point to a low density of total defects/impurities. In contrast, the strong intensity and relatively large line width of the SX emission (and also the strong intensities of the 3.31 eV line and orange emission) are indicative of a high defect density. Surface defects are expected to be passivated by Al2O3. This is confirmed by the shift of binding energy of Zn 2p electron in the interfacial region, as illustrated in Figure 7. The nanorods without coating show a Zn 2p peak at 1021.8 eV. For the coated nanorods with 0.5 min sputtering, however, the peak shifts to 1021.1 eV. As the sputtering time increases to 0.7 min, the Zn 2p spectrum is the superposition of 1021.1 and 1021.8 eV. These results clearly reveal that the peak at 1021.1 eV is due to Zn 2p in the interfacial region. The 0.7 eV shift indicates that the chemical environment of Zn element in the Al2O3-ZnO interfacial region is changed. B. 3.31 eV Emission. The thick ZnO nanorods show a peak at 3.316 eV, together with its LO phonon replicas, within the whole temperature range (see Figure 5). As shown in Figure 4, the intensities of these peaks are almost unchanged after Al2O3 coating. For both the as-grown and Al2O3-coated samples, the emission becomes comparable to the FX in intensity with increasing temperature and significantly affects the line shape of the room temperature NBE emission. In contrast, the thin nanorods do not show the 3.316 eV emission, and only a weak peak at 3.306 eV was observed at low temperatures (
