Origin of Visible Photoluminescence of ZnO Quantum Dots: Defect

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Origin of Visible Photoluminescence of ZnO Quantum Dots: Defect-Dependent and Size-Dependent Luyuan Zhang, Longwei Yin,* Chengxiang Wang, Ning lun, Yongxin Qi, and Dong Xiang Key Laboratory for Liquid-Solid Structural EVolution & Processing of Materials, Ministry of Education, School of Materials Scinece & Engineering, Shandong UniVersity, Jinan 250061, People’s Republic of China

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ReceiVed: February 11, 2010; ReVised Manuscript ReceiVed: April 24, 2010

To get a real understanding on the complexity of origin and mechanism of visible emission for ZnO quantum dots (QDs), we systematically property of visible emission of ZnO QDs with tunable diameters in a range of 2.2-7.8 nm synthesized via a sol-gel route using self-made zinc-oleate complex as a precursor. It is indicated that the visible emission of ZnO QDs can be ascribed to singly ionized oxygen vacancies, which is associated with the paramagnetic centers with electron paramagnetic resonance (EPR) value of g ) 2.0056. The visible emission property of the ZnO QDs displays highly size-dependent behavior. With ZnO QDs size decreasing, the visible emission peaks blue-shift to the positions with shorter wavelength due to quantum size effect, however, is different from that of band gap. Quantitative investigation shows that the visible emission can correspond to a transition of holes from the valence band to the preexisting deep donor energy level, which is different from the well-known conclusion that the visible emission is due to the transition of an electron from the conduction band to a deep trap. Two important points can be obtained: the defects of singly ionized oxygen vacancies determine the origin and intensity of visible emission of ZnO QDs; and the visible emission peak position of ZnO QDs is decided by their size, and a transition of holes from the valence band to the preexisting deep donor energy level is responsible for the visible emission of the ZnO QDs. Introduction As the first kind of synthetic low-dimensional semiconductor nanoparticles clearly showing “quantum size effect”, ZnO quantum dots (QDs) are of great importance because of their unique electrical and optical properties.1,2 Comparing with the traditionally II-VI group QDs, such as CdSe and CdTe QDs, ZnO QDs are cheap and biocompatible to the biological systems,3 and have inspired great interest in biological labeling and photocatalytic applications. However, the photoluminescence properties (especially their visible emission) and the associated mechanism of ZnO QDs are not as clear as that of CdSe and CdTe QDs. For CdSe and CdTe QDs, their photoluminescence is well size-dependent, so the desired emission colors can be obtained by just changing their size.4,5 For wurtzite ZnO, there are normally two photoluminescent emission bands. One is centered in the UV region, the other is centered in the visible region. The origin of UV emission is well-known to be associated with the radiative recombination of electron from conduction band with hole from valence band,6 and because of their direct association with band gap, it is sizedependent emission due to quantum confinement. While the situation of visible emission is so complicated that the origin and property of ZnO QDs have not been fully understood, the visible emission is usually thought to be some defect induced emission, and many point defects have been proposed to be responsible for the emission, such as oxygen vacancies (Vo),7-15 zinc vacancies (VZn),16,17 oxygen interstitials (Oi),16,18,19 zinc interstitials (Zni),20,21 and antisite oxygen (OZn).20 Among them, oxygen vacancies are the most cited hypothesis, and different mechanisms have been given. It is believed that the recombina* To whom correspondence should be addressed. Phone: + 86 531 88396970. Fax: + 86 531 88396970. E-mail: [email protected] (Longwei Yin).

tion of electrons trapped in Vo• (singly charged oxygen vacancy) with photoexcited holes results in the visible emission,7 while van Dijken et al. gave a different explanation that the photogenerated holes are first trapped at a surface system (probably O2+/O-) and then return back into the particle where they recombine with electrons trapped at Vo• center, instead of emitting a visible light which results in forming double charged oxygen vacancy (Vo2•). The recombination of holes trapped at Vo2• center with electrons in conduction band is the origin of visible emission.9 Additionally, copper impurities,22 surfacebonded hydroxides (OH-s),23 and donor-acceptor complexes24,25 are also proposed to be responsible for the visible emission. In the earlier reports, some approaches have been given to tune the visible photoluminescence of ZnO QDs, such as by adjusting the molar ratio of [LiOH]/[Zn],26-28 or changing the doping content of magnesium.29 Meanwhile, the effect of particle size on the visible emission was also pointed out. But, previous research on the influence of size on the visible emission of ZnO QDs was always qualitative or semiquantitative,8,12 few quantitative investigations have been reported. There exists arguing on the mechanism of the size-dependent visible photoluminescence properties. For example, the ZnO QDs synthesized by different groups are of the same size (diameter ) 3.7 nm), but display different emission peaks (one is 440 nm, one is 507 nm).29,30 So, the mechanisms behind this visible emission are still a matter of controversy, and we cannot say that the visible emission of ZnO QDs is exactly size-dependent yet. Recent works reveal that surface plays a key role in the visible emission of ZnO QDs,31 and meanwhile when investigating the origin of ultraviolet photoluminescence in ZnO QDs, Fonoberov et al. revealed that UV emission was also related to the surface,32 but the real role of the surface is not clear. It is claimed that the visible emission arises from nothing but surface-bonded materials like oleic acid (OA).30 It is believed that surface

10.1021/jp101324a  2010 American Chemical Society Published on Web 05/11/2010

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modification, such as cationic and anionic surface binding sites,23 can markedly affect the luminescence of ZnO QDs. Until now, a real understanding of visible emission of ZnO QDs has not been achieved. The complexity of the visible emission of ZnO comes from at least two sides: one is the diversiform origins about their visible emission, the other is the size factor caused by quantum size effect. Herein, a sol-gel method with self-made zinc-oleate complex as a precursor was used to tune the size of ZnO QDs in a range of 2.2-7.8 nm. Compared with the traditional sol-gel route based on hydrolysis of zinc acetate in alcohol, for the present growth of ZnO QDs, no sonic stirring and zero temperature were needed, and the growth can be finished in a relatively short time. Examination with XRD, UV-visible spectra, high-resolution transmission elctron microscopy (HRTEM), and photoluminescence (PL) spectra were used to investigate the structure and optical property of ZnO QDs. We found that the visible emission property of the synthesized ZnO QDs is well size-dependent. But the size-dependence has a condition that the origin associated with this visible emission does not change. Electron paramagnetic resonance (EPR) technique was applied to investigate the paramagnetic centers of ZnO QDs, and it is found an EPR signal with g ) 2.0056 appears in all samples. The obtained EPR signal at g ) 2.0056 can be attributed to the oxygen vacancies.7,15,33,34 Very interestingly, the EPR signal intensity becomes weaker with the size of ZnO QDs increasing. Considering the fact that the specific surface area of ZnO QDs declines rapidly with their size increasing, therefore it is reasonable to think that the defects come from their surface. Meanwhile, as the EPR signal becomes weak, the visible photoluminescence intensity becomes weak as well. Systematical investigations show that the visible emission of the ZnO QDs corresponds to the transition of holes from the valence band to the preexisting donor energy level. This is different form the result reported by Dijken et al. that the visible emission is due to the transition of an electron from the conduction band to a deep trap. Experimental Section Preparation of Zinc-Oleate Complex Precursor.35 In a typical synthesis, 5.45 g of zinc chloride (ZnCl2, 40 mmol) and 24.35 g of sodium oleate (80 mmol) were dissolved in a mixed solvent composed of 80 mL of ethanol, 60 mL of distilled water, and 140 mL of hexane. This solution was added into a flask and refluxed at 70 °C for 4 h. When the reaction was completed, the upper organic layer containing the zinc-oleate complex was washed three times with 30 mL of distilled water in a separatory funnel. The residue is the zinc-oleate complex after the hexane evaporated. Preparation of ZnO QDs. In a typical preparation, 6.28 g of zinc-oleate complex and 100 mL of ethanol were added into a flask equipped with a condenser. Then the system was heated to 70 °C, and the zinc-oleate complex was dissolved in ethanol under constant magnetic stirring. At a given molar ratio of Zn2+/LiOH, LiOH · H2O was first dissolved in 100 mL of ethanol with the aid of ultrasonic irradiation, and then the obtained solution was added into the above zinc oleate solution. At given intervals, the reaction solution was withdrawn in order to obtain ZnO QDs with different sizes. To obtain very pure ZnO QDs, the obtained QDs were washed thoroughly by hexane and ethanol in turn. Physical Measurement. X-ray powder diffraction (XRD) data were collected on a Rigaku D/max-kA diffractometer with Cu KR radiation (60 kV,40 mA), using the synthesized samples

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Figure 1. XRD pattern of samples obtained at different times.

dried at 4 °C. High-resolution electron microscopy (HRTEM) was obtained with a Phillips Tecnai 20U-Twin high-resolution transmission electron microscope (HRTEM). UV-vis absorption spectra were collected with a TU-1901 spectrophotometer and the PL spectra were measured with an F-4600 spectrofluorometer with an excitation wavelength of 325 nm at room temperature. A Bruker EMX-10/12 electron paramagnetic resonance spectrometer (EPR) was used to study the defects in ZnO QDs solid powders dried at 4 °C. Results and Discussion Figure 1 shows X-ray diffraction patterns of the ZnO QDs samples obtained for different times. All the reflection peaks can be indexed to wurtzite ZnO. No other diffraction peaks from impurities and residues were detected, indicating that the synthesized products are pure ZnO QDs. With synthesis time increasing, the broad diffraction peaks tend to be narrower and sharper. The broadening of the diffraction peaks is due to the small particle size. The diameter (D) of ZnO QDs can be calculated by using the Debye-Scherrer formula:36

D ) 0.9λ/B cos θB

(1)

where λ is the X-ray wavelength (0.15418 nm), θB is the Bragg diffraction angle, and B is the full width at half-maximum. According to the diffraction peak positions and the width at half-maximum, the mean size of ZnO QDs can be obtained, and the results are shown in Table 1. The size of ZnO QDs can be controlled in a range from 2.17 to 4.25 nm by adjusting the synthesis time in a range of 5 min to 50 h. For the ZnO QDs synthesized for 4 days, the size is 7.8 nm. Figure 2a shows typical UV-visible absorption spectra of the synthesized ZnO QDs. As the size of the ZnO QDs increases with the growth time lengthening, the characteristic absorption peaks for the synthesized ZnO QDs red-shift. An empirical formula based on UV-vis absorption spectra has been given by Meulenkamp to calculate the size of ZnO QDs, which is applicable in the size range from 2.5 to 4.33 nm.37 The formula is

1240/λ1/2)a + b/D2 - c/D

(2)

Visible Photoluminescence of ZnO Quantum Dots

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TABLE 1: Property Comparison of ZnO QDs with Different Sizes

e

synthesis time

sizea (nm)

sizeb (nm)

∆Ec [eV]

∆Ed [eV]

emission peak [eV]

band gape [eV]

band gapf [eV]

5 min 5h 10 h 30 h 50 h 4 days

2.17 2.31 2.48 3.06 4.25 7.80

× 2.50 2.62 3.18 4.33 ×

1.12 0.91 0.81 0.54 0.29 0.09

0.53 0.43 0.38 0.26 0.14 0.04

2.68 2.60 2.55 2.47 2.38 2.29

3.96 3.77 3.72 3.56 3.45 3.32

4.72 4.44 4.30 4.00 3.64 3.40

a By XRD. b By UV-vis spectra. c Increased Eg values associated with valence band. d Increased Eg values associated with conduction band. Measured values by using UV-vis spectra. f Calculated values via eq 3.

Figure 2. (a) UV-visible absorption spectra of the obtained samples. (b) (Rhν)2 versus hν plot obtained from curves shown in panel a.

where λ1/2 is the wavelength at which the absorption is half of that at the excitonic peak (or shoulder). The size values obtained by using this formula are given in Table 1. The results are comparable with those obtained by XRD. Transmission electron microscopy was used to further investigate the microstructures of the synthesized ZnO QDs. Figure 3 depicts HRTEM images of the ZnO QDs prepared for different times. It is available to judge the size of ZnO QDs intuitionally by using the HRTEM images. The 0.28 nm as illustrated corresponds well with that of d-spacing of (100) planes of wurtzite ZnO. The diameter of the ZnO QDs synthesized for 4 days can be up to 7.8 nm. The results can be generally in agreement with the data calculated from XRD patterns and UV-vis absorption spectra. To investigate the potential presence and type of paramagnetic centers, electron paramagnetic resonance (EPR) measurements were performed at room temperature. As seen from Figure 4, an EPR signal with g ) 2.0056 appears in all samples, which suggests that all the samples possess the same type of paramagnetic centers. Importantly, the relative intensity of the EPR signal becomes weaker with size increasing, and the decline

in intensity is the result of a decrease in the concentration of paramagnetic centers. So, it is obvious that the paramagnetic centers associated with the EPR line at g ) 2.0056 decrease as the ZnO QDs size increases. Generally, it is considered that the paramagnetic centers detected by EPR examination can be the origin of visible emission. To test the relationship between paramagnetic centers and visible emission, the PL spectra of these ZnO QDs solid powders used for EPR measurement were also measured and the results are shown in Figure 5. The intensity of visible emission shows almost the same changing trend as that of EPR signals. So, it reflects that the visible emission is directly associated with the paramagnetic center detected by EPR. For generic ZnO nanomaterials, UV and visible emissions are usually observed in the PL spectra. For the presently synthesized ZnO QDs, only visible emission appears in the PL spectra. When excited by UV light, the radiative process will occur in ZnO QDs,9 and it usually contains UV and visible emission processes, which compete with each other. Except for the excitation intensity,38 the intensity ratio of the UV emission to visible emission is mainly influenced by the crystalline quality of ZnO. Generally, ZnO nanostructures with large size and nearly perfect crystalline structure show stronger UV emission,39,40 while those with small size and plenty of defects show stronger visible emission,29,41 and meanwhile surface modification of the ZnO nanostructures is proved to facilitate the UV emission and suppress the visible emission.23,42 So, we can conclude that our synthesized ZnO QDs possess plenty of defects and the paramagnetic centers detected by the above EPR are these defects. If it is assumed that the defects come from bulk ZnO, it is impossible that their concentration will decrease with increasing size. Meanwhile, considering the fact that the surfaceto-volume ratio of ZnO QDs will become larger with decreasing size, most of the defects must exist on the surface of ZnO QDs. Our conclusion is consistent with the earlier reported one, and there it was thought that the recombination process in ZnO QDs is different from those in bulk ZnO mainly due to the large surface-to-volume ratio of ZnO QDs, which corresponds to a high number of exciton recombination centers near the surface.43 The assignment of these paramagnetic defects will be done later combined with the identification of transition associated with visible emission. The normalized PL spectra of ZnO QDs dispersed in ethanol are shown in Figure 6. It can be seen that with size decreasing, the visible emission peaks blue-shift to the positions with shorter wavelength. The direct photographs under UV light can vividly show the emission color difference of the ZnO QDs with different sizes (Figure 7). Both the ZnO QDs ethanol solution and dried powders show very strong luminescence. The strong luminescence can be due to the small size of ZnO QDs. On the one hand, as discussed above, small size means more surface defects, and on the other hand, small size means the small radiative lifetime and rather thick dead layer in ZnO QDs which

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Figure 3. HRTEM images of the synthesized samples. The 0.28 nm as illustrated corresponds well with that of d-spacing of (100) planes of wurtzite ZnO. Scale bar: 10 nm.

Figure 4. X-band (9.4437 GHz) EPR traces at room temperature for all samples. The g value can be calculated by the following equation: g ) hν/µBB.

Figure 6. Normalized PL emission spectra of samples dispersed in ethanol.

colors and intensities all remain the same. It is shown that the size factor can determine the emission color under the same type of defect (proved by EPR measurement). In the literature, the blue-shifts in emission spectra of ZnO QDs are generally considered to be due to the quantum confinement effect.2,45 However, whether this blue-shift can be accurately determined by their size is still an unresolved matter. When nanostructures show quantum size effect, their band gap (E*) will become broad, and the blue-shifts of their PL spectra are closely related to this band gap enlargement. So, first the band gap enlargement with decreasing size must be confirmed. There are many approaches to evaluate the band gap enlargement of ZnO QDs. Among them, a widely used formula based on effective mass approximation can be described as follows:46 Figure 5. PL spectra for the synthesized ZnO QDs.

E* ) Egbulk + are beneficial to improve luminescence and exciton separation from the surface defects.44 In the solid state, the visible emission properties of dried ZnO QDs are very stable because their growth is stopped, and even after one month their emission

(

)

h2 1 1 1.8e2 + * * 2 4πεε0R 8R me mh

(3)

where E* is band gap of ZnO QDs with a radius of R, Egbulk (3.37 eV at room temperature) is the bulk band gap, me* is the



Figure 7. Direct photographs of ethanolic solutions (upper) and dried powder (lower) of samples obtained at different intervals under irradiation by a 254 nm UV lamp.

effective mass of the electrons in conduction band, mh* is the effective mass of the holes in valence band, h is Plank’s constant, ε is the relative permittivity, ε0 is the permittivity of free space, and e is the charge on the electron. After deliberatively selecting the effective masses of the electrons and holes in ZnO as me* ) 0.28 m0 and mh* ) 0.59 m0, respectively (m0 is the free electron mass),47 we can calculate the band gap values of ZnO QDs with different sizes. The calculated results are shown in Table 1. The radius value used in the calculation is the average radius value from that of the XRD and UV-vis absorption spectra. At the same time, for the direct band gap semiconductor of ZnO, it is well-known that the relationship between the absorption coefficient (R) near the absorption edge and the excitation energy (hν) obeys the following relation:48-50

(4)

Figure 8. Curves a and b represent the declining trend of band gap values with diameter obtained by eqs 1 and 2; curve c represents changing trend of visible emission maximum with diameter.

where A is a parameter that relates to the effective mass associated with valence and conduction bands, and Eg is the bulk band gap. From this relationship, we can also obtain the band gap values of ZnO QDs with different absorption edges. Figure 2b shows the absorbance spectra for ZnO QDs replotted according to eq 4. Extrapolating the linear part until it intersects with the hν axis, the intercept is the band gap value for each sample. The band gap values obtained here are compared with those obtained from eq 3, as shown in Figure 8. Both the obtained band gap values have the same declining trend with increasing size, but as a whole the values obtained by eq 3 are bigger than those obtained by eq 4, which is in good agreement with the results obtained by others.48 The changing trend of visible emission maximum with size was also shown in Figure 8. Clearly, it possesses a similar declining trend but has a smaller declining rate. By examining these curves, we find that when the diameters of ZnO QDs reach about 4.3 nm, the declining rate of these three curves becomes obviously flat. This result agrees well with the reported fact that the bandgap shift of ZnO becomes obvious when its size is smaller than 5 nm, because the bulk ZnO excition Bohr radius, RB, is ∼2.34 nm.1,47 On the basis of the above observed fact, a conclusion can be made that

the visible emission maximum shift is the result of quantum size effect like the band gap, but its behavior is different from that of band gap. From eq 3, we can see that except for the Coulomb interaction term, the enhancement of bandgap comes from two parts. One is the enlargement associated with conduction band, and the other is the enlargement associated with the valence band. Because UV emission is a band-edge emission and corresponds to transition from conduction band to valence band, so its blueshift with size is directly linked to bad gap enlargement. However, visible emission is generally considered to be defect induced emission, and hence energy levels arising from defects are involved in its corresponding transition. Therefore, the visible emission always shows partial quantum size effect, that is to say its blue-shift with size cannot be determined by band gap enlargement. Then, if assuming that the energy level arisen from defects does not move with the conduction band or valence band, two types of transitions can be proposed to be responsible for visible emission: (1) those involving electrons from the conduction band to the preexisting energy level and (2) those involving holes from the valence band to the preexisting energy level. It is appropriate to make such an hypothesis because

(Rhν)2 ) A(hν - Eg)

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Figure 9. The symbols b represent measured visible emission maximum, g represent band gap enlargement associated with conduction band, and 0 represent band gap enlargement associated with valence band. The inset figure shows the comparison of visible emission maximum versus diameter and energy position versus diameter after g and 0 plus a certain value, respectively.

energy levels associated with visible emission are in general the deep donor or acceptor level, so, they are less influenced by the energy band movement. In fact, in the earlier reports, researchers have already given the same hypothesis.8,47 According to the above analysis, the final energy level difference corresponding to these two transitions for visible emission of ZnO QDs with different size can be determined by the following equations:

Ee ) ∆Ee +

h2 1 8R2 me*

(5)

Eh ) ∆Eh +

h2 1 8R2 mh*

(6)

where, Ee and Eh are the final energy level difference corresponding to transitions 1 and 2, respectively, ∆Ee is the energy level difference between the preexisting energy level of bulk ZnO and the conduction band, and ∆Eh is the energy level difference between the preexisting energy level and valence band. Now, the question concentrates on one point that one transition or both transitions can be responsible for the visible emission after all. In the earlier reports, both transitions have been cited to be responsible for visible emission by different groups.8,47,51 The main difference between the two transitions focuses on the different band edge broadening with decreasing size due to the effective mass difference between electrons in the conduction band and holes in the valence band. In Figure 9, the band edge enlargements arising from the conduction band and the valence band are shown, and their increasing trends are compared with that of the visible emission maximum. It can be seen that when the diameter is bigger than 4.6 nm, both curves a and b are flat and hence the increased values in the band edge with decreasing size are very small, and the quantum size effect is not obvious. So, it is not appropriate to investigate the size-dependent property of visible emission of ZnO when the diameters are bigger than about 4.6 nm. When the diameter is smaller than 4.6 nm, both curves a and b become steep. It is obviously that curve b is much steeper than curve a. If it is assumed that the visible emission corresponds to one of these

Figure 10. Schematic illustration of band gap enlargement with QD size and possible transitions corresponding to different emissions: (a) UV emission, (b) visible emission corresponding to eq 3, and (c) visible emission corresponding to eq 4.

two transitions, the changing trend of visible emission maxima (curve c) must be similar to such a transition.8,47 To facilitate a comparison, we construct values obtained by the conduction band plus 1.9 eV and values obtained by the valence band plus 2.2 eV, and the results are compared with the values of visible emission maxima. The inset in Figure 9 shows the results after such an operation. It can be clearly seen that curve c is very similar to the curve obtained by curve a plus 2.2 eV and very different from the curve obtained by curve b plus 1.9 eV. This indicates the visible emission corresponds well to the transition of holes from the valence band to the preexisting donor energy level. In fact, the added values (1.9 and 2.2 eV) here are the assumed preexisting energy level values. Figure 10 illustrates the two possible transitions and band gap enlargement with size decreasing of ZnO QDs. Two energy levels are supposed to locate in the band gap and they are the donor level and the acceptor level, respectively. The acceptor level is 2.2 eV above the valence band, while the donor level is 1.9 eV below the conduction band when ZnO shows no quantum size effect. Transitions a, b, and c in Figure 9 represent band-to-band transition, transition 1, and transition 2, respectively. When the band edges shift with decreasing size, the energy level differences corresponding to the three transitions all become large. Reflected in the photoluminescence spectra, this shows that the emissions associated with these transitions blue-shift to short wavelength. For the identification of the transition responsible for visible emission, the visible emission of ZnO QDs is due to the transition of holes from the valence band to the preexisting donor energy level. It is noted that we obtained different results from that obtained by van Dijken et al.8 that the visible emission is due to the transition of an electron from the conduction band to a deep trap. On the basis of the above analyses, a preexisting energy level exists in our hypothesis. According to EPR measurement results, this energy level may arise from the paramagnetic center with an EPR value of g ) 2.0056. Now, identification of this paramagnetic center must be done. There are a series of g values observed in earlier experiments and different types of defects have been cited to result in these g values.42,52-54 The previously reported EPR signal with a g value in the range of 2.0015 to 2.0075 was mostly considered to be attributed to Zn vacancies;53,54 however, theoretical calculations show that the energy level associated with the Zn vacancy is a shallow acceptor level,55,56



Figure 11. (a) EPR spectra of zinc-oleate complex (S1) and oleic acid modified ZnO QDs (S2). (b) PL spectra of ZnO QDs before (blue curve) and after (red curve) being modified by oleic acid, which are dispersed in ethanol.

while according to our analysis, the energy level in our case for the synthesized ZnO QDs is a deep donor level. Oxygen vacancies have three charged states, but only the singly charged state is paramagnetic and so can be detected by EPR. The observed EPR peak centered at g ) 1.96 is believed to arise from the singly charged oxygen vacancies or ionized impurity atoms in the crystal lattice of ZnO.7,52 In this paper, our obtained g values are very similar to those reported by Ischenko et al.,52 and there they attributed the EPR signal with g ≈ 2.006 to oxygen vacancies with a single trapped electron (Vo•). Herein, we also make the hypothesis that the EPR signal with g ) 2.0056 is ascribed to single charged oxygen vacancies. This hypothesis can be proved in many ways. A first-principle calculation by Zhang et al. shows that oxygen vacancy (Vo), zinc interstitial (Zni), and Zn-on-O antisite (Zno) have the lower formation enthalpies and hence are produced first and in abundance.57 Energy levels associated with these defects are all donor levels but have different energy positions. Generally, energy levels associated with Zni and Zno are shallow donor levels, but our above discussion indicates a deep donor level is present in our case for the synthesized ZnO QDs. On the basis of experimental data, this deep donor level in our case is located about 1.1 eV below the conduction band, which is in good agreement with the calculated results about Vo reported in earlier literature.57,58 So, the such hypothesis can be acceptable. To exclude the possibility of the obtained EPR signal from surface-bound radicals, such as •OH and •CH3,52 and zinc-oleate complex precursor, the EPR spectra of the precursor of the zinc-oleate complex and ZnO QDs sample modified by oleic acid have been measured and the result is depicted in Figure 11a. It can be clearly seen that neither of them shows an obvious EPR line at g ) 2.0056. Hereby, this possibility can be excluded. Figure 11b shows the PL spectra of ZnO QDs before and after being modified by oleic acid. After being modified by oleic acid, the visible emission disappears and the UV emission shows up again. Combining with the disappearance of the EPR signal with g ) 2.0056 in Figure 11a, we can further confirm that surface oxygen vacancies are the origin of visible emission because oleic acid may heal the oxygen vacancy. This also proves that surfacebound material cannot be responsible for visible emission. Conclusions We studied the origin and property of visible emisison of ZnO quantum dots (QDs) synthesized via a sol-gel route using self-made zinc-oleate complex as a precursor. The size of the synthesized ZnO QDs can be tuned in a range of 2.2-7.8 nm by adjusting the synthesis time. The prepared ZnO QDs were

characterized with XRD, UV-vis absorption spectroscopy, HRTEM, EPR, and PL spectroscopy. With ZnO QDs size decreasing, the visible emission peaks blue-shift to the positions with shorter wavelength due to the quantum size effect; however, the visible emission size-dependent behavior is different from that of band gap. A correlation between the EPR signal g ) 2.0056 and the visible emission has been found in our prepared ZnO QDs powders. It is indicated that the visible emission of ZnO QDs is associated with the paramagnetic centers with electron paramagnetic resonance (EPR) value of g ) 2.0056, which can be identified to singly ionized oxygen vacancies in the synthesized ZnO QDs. Because all the samples have the same EPR signal, this shows that although with different size the prepared ZnO QDs possess the same paramagnetic defects. Under such a condition, the visible emission peak positions of ZnO QDs can be determined by the size. It is shown that the transition of holes from the valence band to the preexisting energy level is responsible for the visible emission. This is different from the result reported by Dijken et al.8 that the visible emission is due to the transition of an electron from the conduction band to a deep trap. Acknowledgment. We acknowledge support from the National Nature Science Foundation of China (Nos. 50872071 and 50972079), the Shandong Natural Science Fund for Distinguished Young Scholars (JQ200915), Nature Science Foundation of Shandong Province (Y2007F03 and Y2008F26), Foundation of Outstanding Young Scientists in Shandong Province (No. 2006BS04030), Tai Shan Scholar Foundation of Shandong Province, and Gong Guan Foundation of Shandong Province (2008GG10003019). References and Notes (1) Bahnemann, D. W.; Kormann, C.; Hoffmann, M. R. J. Phys. Chem. 1987, 91, 3789–3798. (2) Spanhel, L. J. Sol-Gel Sci. Technol. 2006, 39, 7–24. (3) Xiong, H.-M.; Xu, Y.; Ren, Q.-G.; Xia, Y.-Y. J. Am. Chem. Soc. 2008, 130, 7522–7523. (4) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343–5344. (5) Qu, L.; Peng, X. J. Am. Chem. Soc. 2002, 124, 2049–2055. (6) Kahn, M. L.; Cardinal, T.; Bousquet, B.; Monge, M.; Jubera, V.; Chaudret, B. ChemPhysChem 2006, 7, 2392–2397. (7) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. J. Appl. Phys. 1996, 79, 7983–7990. (8) van Dijken, A.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Meijerink, A. J. Lumin. 2000, 90, 123–128. (9) van Dijken, A.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Meijerink, A. J. Phys. Chem. B 2000, 104, 1715–1723. (10) van Dijken, A.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Meijerink, A. J. Phys. Chem. B 2000, 104, 4355–4360.

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JP101324A

Origin of Visible Photoluminescence of ZnO Quantum Dots: Defect

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