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The room-temperature photoluminescence (PL) spectra of hydrothermal grown ZnO film and nanowires coated with Al are investigated, which exhibit much less UV emission enhancement ratio as against that of nanowires fabricated by thermal evaporation method. A model is suggested at last to interpret the experimental results considering the influence of the defect on the contact property between metal and ZnO, which is further evidenced by the weak PL enhancement ratio of thermal evaporation grown ZnO nanowires with H2O2 treatment.
©2013 Optical Society of America
1. Introduction
Over the past decade, there have been numerous investigations of metal-mediated enhancement of near band edge (NBE) emission of ZnO, owing to its promising prospects in enhancing the luminescence efficiency of ZnO-based light emitting devices. Plenty of metals have been demonstrated to be able to increase the NBE emission of ZnO [1–4]. However, a lot of contradictory results have been shown in the literature regarding the effect of metals on the NBE emission of ZnO. For example, Cheng et al [1] reported the enhanced NBE luminescence of ZnO nanorods after capping Au nanoparticles, while Lai et al [3] showed that Au had negligible effect on the emission enhancement of ZnO film. Richters et al [5] observed that the deposition of five different metals on ZnO nanowires would all reduce their relative intensity of NBE emission. These inconsistent results indicate that the mechanism behind the luminescence enhancement of metal-coated ZnO is quite complicated. In most cases, the emission enhancement of ZnO can be attributed to the coupling between surface plasmons of metals and excitons of ZnO [1, 3]. Many other factors, however, have also been reported to take effect in some situations, such as the diffusion of metals into ZnO [6], the unintentional hydrogen incorporation in ZnO [7], and the separation distance between metals and ZnO [8]. Recently, we reported that metals with low work functions, like Al, Zn, and Ti, could enhance considerably the UV emission of ZnO nanowires fabricated by thermal evaporation method, and interpreted this phenomenon by considering the Ohmic contact formed between metals and ZnO and hence the electron transfer between them [2]. But, when replacing the thermal evaporation grown ZnO nanowires with hydrothermal grown ZnO films, we found that the UV emission enhancement ratio was quite marginal after metal coating. Since hydrothermal method is also commonly used to fabricated ZnO nanostructures due to its merits of low temperature, large scale, and economical synthesis [9], it is highly desirable to investigate why the fabrication methods of ZnO would affect its photoluminescence (PL) property after metal coating. Besides, Song et al [4] recently reported that the surface/volume ratios would affect the UV emission of Ti-capped ZnO nanostructures. However, it is noteworthy that the ZnO samples with different surface/volume ratios in their report were fabricated by three different methods, therefore we believe that the real factors for the enhancement of PL intensity should be further investigated and figured out.
In this letter, ZnO nanowires and film with quite different surface/volume ratios were both fabricated by hydrothermal method. The room-temperature and temperature-dependent PL spectra of both bare and Al-coated hydrothermal grown ZnO were studied in details, and compared with our previous results of metal coated ZnO nanowires fabricated by thermal evaporation method [2]. The mechanism responsible for the observed phenomenon was speculated considering the surface defects, which was verified by the result of weak PL enhancement ratio of thermal evaporation grown ZnO with H2O2 treatment.
2. Experiment
The substrates on which the ZnO nanowires and film are grown are n-type silicon (100) wafers and are coated with a very thin ZnO seed layer according to the method mentioned in Ref [10]. For the preparation of ZnO nanowires, the substrate is leaned upside down against the wall of the bottle, which is filled with solution of zinc nitrate hexahydrate (Zn(NO3)2•6H2O) and methenamine (C6H12N4) in equal concentration of 0.01 M. The reaction is kept at 95 °C for 6 h in an oven. After the growth, the sample is annealed at 400 °C in the forming gas (10% H2 in N2 atmosphere) for 1 h. The fabrication process of ZnO film is the same as the nanowires, except that the reactive solution is composed of zinc acetate dehydrate (Zn(CH3COO)2•2H2O) and methenamine (C6H12N4) in equal concentration of 0.1 M, and that the reaction time is 2 h. The synthesis process of ZnO nanowires via thermal evaporation method is mentioned in our previous literature [11]. ZnO structures grown by hydrothermal and thermal evaporation methods are denoted as H-ZnO and T-ZnO, respectively. The H2O2 treatment of nanowires is performed by immerging the nanowires in 30% H2O2 for 20 min. The deposition of metals on the ZnO nanowires and film is carried out in a radio-frequency magnetron sputtering system at room temperature. The morphology of the as-grown samples is characterized by a field-emission scanning electron microscope (SEM, Hitachi S-4800). X-ray diffraction (XRD) data of the samples were collected from a PANalytical X’Pert PRO diffractometer using Cu Kα radiation. Transmission electron microscopy (TEM) measurements were carried out on a CM-200 electron microscope, and high resolution TEM (HRTEM) were performed on a Jeol JEM-2011 electron microscope. The PL measurement is performed on a luminescence spectrometer (Edinburgh Instruments FLS 920) with Xe lamp emitting at 300 nm as the excitation source. A cycle refrigerator is used to control the temperature of the samples during the temperature-dependent PL measurement.
3. Results and discussions
Figures 1(a) – 1(d) displays the typical SEM images of the as-grown H-ZnO. The top and side view SEM images of ZnO film are shown in Figs. 1(a) and 1(b), respectively. They clearly show that the film is composed of dense ZnO columns, whose diameters are in the range of 100-500 nm. These columns grow vertically on the substrate and merge with each other into a columnar film with the thickness of about 1.6 μm. Figures 1(c) and 1(d) exhibit the top and side view SEM images of ZnO nanowires, respectively. These nanowires are about 50-150 nm in diameter, with the height comparable to the ZnO film. Compared with the columns of ZnO film, the nanowires are much thinner and grow separately with each other. Considering the fact that the mean diameter of ZnO nanowires is about 93 nm and their areal density is around 19/μm2, it is estimated that the surface/volume ratio of ZnO nanowires is about eight times larger than that of ZnO film. XRD analysis was utilized to characterize the crystallographic properties of the H-ZnO, and the corresponding XRD curves are displayed in Fig. 1(e). It is found that all the diffraction peaks can be indexed to the hexagonal wurtzite phase of ZnO (JCPDS card: 36-1451) with the highly preferred orientation along the (002) direction. The strong intensity and narrow width of the (002) peak also indicates the high crystal quality of the ZnO film and nanowires.
Fig. 1 Typical top and side view SEM images of ZnO film ((a) and (b)) and ZnO nanowires ((c) and (d)) grown by hydrothermal method. The scale bar is 500 nm. (e) XRD curves of hydrothermal grown (i) ZnO nanowires and (ii) ZnO film.
The TEM image of the as-grown H-ZnO nanowires is shown in Fig. 2(a) , which shows that the diameter of the nanowire is about 50 nm. The HRTEM image of the ZnO nanowire is shown in Fig. 2(b), which illustrates the single crystal structure of the nanowire. The measured d-spacing of the crystal fringes matches well with the ZnO structure. The TEM images of the Al-coated H-ZnO nanowires are also shown in Figs. 2(c) and 2(d), with the deposition time of 60 s and 90 s, respectively. It is shown that when the deposition time of Al is 60 s, discrete Al nanoparticles with the diameter of several nanometers are attached on the surface of the nanowires. With the extension of the deposition time to 90 s, continuous Al film with the thickness of around 10 nm is formed on the nanowires.
Fig. 2 (a) Typical TEM image of the hydrothermal grown ZnO nanowires. (b) The HRTEM image of the ZnO nanowire shown in (a). (c and d) TEM images of ZnO nanowires coated with Al for (c) 60 s and (d) 90 s (the deposited Al on the nanowires is indicated by the white arrows).
The PL spectra of H-ZnO film and nanowires coated with Al for different deposition times are illustrated in Figs. 3(a) and 3(b), respectively. Both of them are dominated by the UV peak centered at around 378 nm, which can be assigned to the near band edge emission of ZnO. After the deposition of Al for 30 s, both the UV emission intensities of ZnO film and nanowires show very slight increase, and the maximum UV enhancement ratio is less than 25% for both samples with further increasing the Al deposition time. This phenomenon is totally different from the case of ZnO nanowires fabricated via thermal evaporation method, whose UV emission intensity increases more than four times after the deposition of Al for 30 s as against the bare ones’ [11]. Different from the experiments of Song et al [4], both the ZnO film and nanowires are grown via the same hydrothermal method in our case, and both of them exhibit little increase of the UV emission compared to the bare ones regardless of the large difference of the surface/volume ratios between them. This indicates that the surface/volume ratio is not a key factor that determines the enhancement ratio of UV emission of metal-coated H-ZnO nanostructures.
Fig. 3 The room-temperature PL spectra of (a) ZnO film and (b) ZnO nanowires grown by hydrothermal method (denoted as H-ZnO film and H-ZnO nanowires, respectively) coated with Al for different sputtering times. The low-temperature PL spectra of the bare and Al-coated (c) ZnO film and (d) ZnO nanowires measured at T = 13 K.
In order to get more insight into this issue, the low-temperature PL measurements are implemented on H-ZnO film and nanowires without and with the deposition of Al (the deposition time of Al is 30 s hereafter if not noted) at 13 K. The spectra are presented in Figs. 3(c) and 3(d), which have been normalized to the peak values. It is observed that only one broad peak dominates the PL spectra of both the bare ZnO film and nanowires. The peak values of the spectra for ZnO nanowires and film are 368.00 nm (3.369 eV) and 367.35 nm (3.375 eV), respectively. These peaks can’t be attributed to the free excitions (FX) emissions because of their strong intensities at low temperature [12]. Actually, these board and featureless peaks are good agreement with previous reported bound exciton emissions [13]. After the deposition of Al, no significant change of the low-temperature PL spectra can be observed for both the nanowires and film, except that the peaks exhibit a slight blue-shift. To better understand this phenomenon, the temperature-dependent PL measurement is implemented on the bare and Al-coated hydrothermal grown ZnO films, and the corresponding spectra are shown in Figs. 4(a) and 4(b), respectively. It is seen that one emission peak dominates the PL spectra from 13 K to room temperature for both the bare and Al-coated ZnO films. The temperature-dependent peak values of bare and Al-coated ZnO are summarized in Fig. 4(c). It shows that the Al coating will lead to a slight blue-shift of the peak at low temperature, and this shift disappears above a certain temperature. This phenomenon is also different from the case of thermal evaporation grown ZnO nanowires, in which the Al coating resulted in a 26 meV blue-shift of the emission peak at room temperature [2]. This is due to the fact that the PL spectrum of bare thermal evaporation grown ZnO is dominated by the A-line emission at lower energy site, and Al coating substantially increased its free exciton emission at higher energy site, hence the whole spectrum exhibited a blue-shift. But this enhancement effect of Al coating is negligible in the case of hydrothermal grown ZnO, so the peak shift can’t be observed at room temperature. The temperature dependence of the integrated PL intensity is also studied for the above two samples, and the results are depicted in Fig. 4(d). It is observed that the data of both samples can be well fitted by the following expression [14]:
where and stand for the activation energy of two nonradiative recombination channels. The results are meV, meV for bare ZnO film, and meV, meV for Al coated H-ZnO film. We also calculated the activation energy of thermal evaporation grown ZnO nanowires based on the data of our previous report [2], and the results are meV, meV for bare ZnO nanowires, and meV, meV for Zn coated ZnO nanowires. The activation energy describes the non-radiative recombination at lower temperature, while describes that at higher temperature. After the metal coating, there is no discernable change of , but the of thermal evaporation grown ZnO nanowires increases, whereas that of the hydrothermal grown ZnO film decreases. Wang et. al [15] also reported the increase of of ZnMgO after Al coating, which is responsible for the increase of PL emission of ZnMgO after Al deposition. Therefore metals with low work functions, could enhance considerably the UV emission of ZnO NWs fabricated by thermal evaporation, and interpreted this phenomenon by considering the Ohmic contact formed between metals and ZnO, and hence the electron transfer between them [2]. However, for the case of hydrothermal grown ZnO, the coating of Al leads to a significant decrease of , which means the increased influence of non-radiative recombination on the PL with increasing the temperature.Fig. 4 The temperature-dependent PL spectra of (a) bare and (b) Al-coated H-ZnO films measured from 13 K to 300 K. (c) The PL peak energies for the bare (black squares) and Al-coated (red circles) ZnO films at different temperatures. (d) Normalized PL intensity of H-ZnO films without (black circles) and with Al coating (red squares) as a function of reciprocal temperature. Solid lines are the best fit results using Eq. (1).
As a matter of fact, ZnO fabricated via different approaches may contain different kinds and quantities of defects, therefore will influence their physical properties. Kolkovsky et al [16] reported that bulk single-crystalline ZnO grown by three kinds of methods, i.e. hydrothermal growth, melt at high pressure and chemical vapor transport, possess different kinds of defects, and the Schottky contacts on them also behave differently. Riaz et al [17] also studied the piezoelectric power generation from ZnO nanowire arrays grown by two different methods, and demonstrated that the high defect concentration of low-temperature aqueous chemical grown ZnO nanowires resulted in the lower output voltage compared to the case of high-temperature grown ones. Pearton et al. [18] gave a review of Ohmic and Schottky contacts to n- and p-type ZnO, and pointed out that the ideal Ohmic contacts generally occur on the samples with higher the near-surface carrier concentration. It is well known that the defects in un-doped ZnO are the main sources of the carriers [19], which would affect the contact property between metals and ZnO [18].
In our previous report, we proposed that the electron transfer from metal to T-ZnO nanowires by considering the Ohmic contact formed between metals and ZnO accounted for their PL enhancement [2]. However, ZnO fabricated via different approaches contain different kinds and quantities of defects, and then the contact property between metal and ZnO and therefore the PL property would be influenced by the defects. Figure 5(a) shows the PL spectra of the as-grown and the annealed H-ZnO nanowires. The as-grown H-ZnO nanowires have large defect concentrations, and the oxygen interstitial defect is the most probable candidate [20], considering the hydrothermal grown environment. Moreover the defect concentrations were substantially decreased by forming gas annealing, and the oxygen interstitial is not thought to be the source of carriers [19]. Then the ideal Ohmic contact between metal and the H-ZnO would be difficult to realize. The electron transfer will be hampered by the energy barrier. Consequently, the energy barrier and the non-radiative recombination at higher temperature lead to the weak PL enhancement of metal coated H-ZnO.
Fig. 5 The room-temperature PL spectra of (a) the as-grown and the annealed H-ZnO nanowires; (b) as-grown and the H2O2-treated ZnO nanowires synthesized by thermal evaporation method (denoted as T-ZnO nanowires).
In order to verify the above speculation, the T-ZnO is treated with H2O2. Since H2O2 can provide an oxygen-rich environment [21], we think that the oxygen radicals will fill the O vacancies (Vo) to decrease the concentration of Vo obviously. Figure 5(b) shows the room-temperature PL spectra of the as-grown and the H2O2 treated T-ZnO nanowires. As we speculated, the peak located at ~510nm, which generally originates from Vo, disappears after the as-grown T-ZnO nanowires were treated in H2O2 solution. Although the origin of the free carrier in un-doped ZnO is somewhat controversial, recently, Zunger et al. thought that the Vo causes optical F-center absorption, leading to persistent photoconductivity, which may account for the observed high free carrier density in pure ZnO [19]. Agoston et al. also reported a more complex donation mechanism involving Vo in ZnO [22]. More recently, Kim et al. pointed out that a strong attractive interaction between deep donor Vo and shallow donor in ZnO made these defects an important source of n-type conductivity in ZnO [23]. Thus the quality of the Ohmic contact between the low work function metal and the H2O2 treated T-ZnO nanowires will be declined because of the obvious decrease of the Vo concentration. The electron transfer will be hampered by the energy barrier, which leads to the weaker PL enhancement than that of as-grown T-ZnO nanowires. The room-temperature PL spectra of the bare and Al coated samples treated with H2O2 T-ZnO nanowires are shown in Fig. 6(a) . In accordance with our expectation, the enhancement ratio of UV emission after Al coating is comparable to the case of hydrothermal grown ZnO. The low-temperature PL measurement is also performed on the two samples at 13 K, and the spectra are shown in Fig. 6(b), which have been normalized to the peak centered at around 379.63 nm (3.266 eV). It is observed that the intensity of the excitonic emission peak of ZnO nanowires at 367.65 nm (3.373 eV) only increases by a factor of 2.85 after Al coating compared to the bare ones’. This value is drastically lower than the case of untreated thermal evaporation grown ZnO, whose enhancement ratio was more than 35 after metal coating [2]. This result further verifies the effect of defects on the PL enhancement of metal coated ZnO.
Fig. 6 (a) The room-temperature and (b) low-temperature PL spectra of H2O2-treated T-ZnO nanowires without and with Al coating.
4. Conclusion
In conclusion, the marginal increase of PL intensity of hydrothermal grown ZnO nanostructures coated with Al, in contrast to the case of ZnO nanowires synthesized via thermal evaporation method, can be attributed to the contact quality at the ZnO/metal interface, which is influenced by the defects of ZnO. The results presented here may shed some light on the mechanism behind the inconsistent results about the PL variations of metal-coated ZnO nanostructures reported in previous literature.
Acknowledgments
This work was supported by National Natural Science Foundation of China (Nos. 60976012 and 51272232), Program for New Century Excellent Talents in University, and the Science and Technology Innovative Research Team of Zhejiang Province (2009R50010).
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