J. Phys. Chem. B 2006, 110, 4099-4104
4099
Nanocrystalline ZnO with Ultraviolet Luminescence Y. S. Wang,† P. John Thomas, and P. O’Brien* School of Chemistry, The UniVersity of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom ReceiVed: NoVember 16, 2005; In Final Form: January 6, 2006
Octylamine capped ZnO nanoparticles with mean diameters in the range of 4-14 nm were obtained by thermolysis of single source metal-organic precursors. The nanocrystals were characterized by X-ray diffraction, transmission electron microscopy, and UV-visible, luminescence, and excitation spectroscopies. The nanocrystals are sufficiently defect-free to exhibit band edge luminescence. The size dependence of the changes in the band gap have been calculated by different methods and compared with the Kayanuma model.
Introduction ZnO is a wide band gap semiconductor that has been extensively studied because of its properties and its potential uses in devices such as field effect transistors, resonators, gas sensors, solar cells, and as a catalyst.1 The possibility of engineering the band gap and influencing the physical, chemical, and electronic properties has provided a strong impetus to study semiconducting material on the nanoscale.2 Nanocrystals of ZnO with various diameters have been obtained using different chemical routes including arrested precipitation and sol-gel techniques based on the hydrolysis of zinc salts.3-6 Studies on ZnO nanocrystals prepared by the above methods have yielded useful results. For example, the variation of band gap in ZnO nanocrystals is strongly size-dependent below a diameter of ∼7.0 nm.7,8 However, the ionic species used to influence growth often get adsorbed on the surface of the nanocrystals along with a hydroxylated layer, preventing effective capping and introducing surface states and other defects in the nanocrystals.9,10 Further, these sols possess limited stability. Non-hydrolytic routes employing metal-organic precursors have, therefore, become important. Shim and Guyot-Sionnest11 have synthesized ZnO by thermolysis of diethylzinc in trioctylphosphine oxide in the presence of oxygen. The nanocrystals obtained above have high defect densities. In fact, the fluorescence spectra of these nanocrystals only consist of defect emission bands. It is generally believed that synthesis carried out using single source precursors that contain preformed Zn-O bonds could yield ZnO nanocrystals with low defect densities. Thus, Zn(CH3CH2OO)2, EtZnOiPr, and [MeZnOSiMe3]4 have been decomposed under various conditions to yield ZnO nanocrystals.12-14 However, defect related emission still dominates the emission from such particles. There has been some limited success recently in synthesizing ZnO nanocrystals with low defects.15-17 ZnO nanocrystals synthesized by decomposition of zinc acetate in 1-octadecene15 or by reacting zinc stearate with alcohols16 have been shown to emit in the UV region with little or no defect emission. Gamelin and Norberg17 have studied the effect of capping agent on emissive properties of ZnO nanocrystals. Despite these advances, synthesis of ZnO nanocrystals with low defect densities poses significant challenges. * To whom correspondence should be addressed. E-mail: paul.obrien@ manchester.ac.uk. † Permanent address: Department of Physics, Beijing Normal University, Beijing 1000875, People’s Republic of China.
Figure 1. Metal-organic precursors used in this study.
We have sought to prepare ZnO nanocrystals from the single source precursors: zinc cupferronate (Zn(C6H5N2O2)2, the Zn(II) salt of N-nitroso-N-phenylhydroxylamine) and the ketoacidoximate (C8H16N2O8Zn, diaquabis[2-(methoxyimino)propanoato]zinc(II)). The structures of the precursors used are shown in Figure 1. The precursors were decomposed at different temperatures, and the structure, optical properties, and morphology of nanocrystals obtained were examined. The nanocrystals were characterized by X-ray diffraction, transmission electron microscopy (TEM), and UV-visible, emission, and excitation spectroscopies. The size dependence of the band gap of the nanocrystals has been examined and compared with the results obtained from the Kayanuma model. Experimental Section Zinc cupferronate was precipitated by adding 150 mL of 4 mM aqueous cupferron to a vigorously stirred 50 mL solution of 11.2 mM Zn(CH3CH2OO)2‚2H2O maintained at 5 °C.18 The precipitate obtained was washed with water and dilute ammonia to remove unreacted cupferron. The cupferronate was characterized by mass spectrometry, X-ray diffraction, and thermogravimetric analysis (TGA). (Found (%): C, 42.2; H, 2.9; N, 16.5; Zn, 19.2. Expected (%): C, 42.4; H, 2.9; N, 16.5; Zn, 19.2.) The zinc ketoacidoximate precursor was synthesized adapting a previously reported procedure.19 Briefly, 30 mmol aqueous sodium carbonate was added dropwise to a stirred solution of sodium pyruvate (30 mmol) and methoxylamine hydrochloride (30 mmol) in 50 mL of water maintained at 5 °C, and the stirring continued until all gas evolution had ceased. Zn(NO3)2‚6H2O (15 mmol) was then added, and the mixture was stirred for 24 h, after which time a white precipitate was obtained. The zinc ketoacidoximate (C8H16N2O8Zn) obtained was characterized by
10.1021/jp0566313 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/04/2006
4100 J. Phys. Chem. B, Vol. 110, No. 9, 2006 mass spectrometry, X-ray diffraction, and TGA. (Found (%): C, 28.9; H, 4.4; N, 8.0; Zn, 19.4. Expected (%): C, 28.8; H, 4.4; N, 8.4; Zn 19.6.) These syntheses of ZnO nanocrystals were performed using standard air-free techniques. Trioctylamine (10 mL) was degassed by heating to 120 °C in a vacuum and repeatedly flushing with N2. The solution was then heated to temperatures chosen for decomposing the precursors. Subsequently, 0.05 mmol of the precursors were injected as solutions in octylamine (1 mL). Following injection, a temperature drop of ∼5 °C was observed. The reaction mixture was allowed to rise back to the original temperature and was maintained at that temperature for a further 60 min. At the end of the reaction, the mixture was cooled to room temperature, and the nanocrystals were precipitated by adding ethanol. The precipitate was isolated by centrifugation. The octylamine coated nanocrystals obtained were freely dispersible in organic solvents such as CHCl3. Injection temperatures of 180, 200, 230, and 245 °C were used for the cupferronate while the oximate was injected at 110, 130, 160, 180, and 200 °C. Absorption spectra were recorded on a ThermoSpectronic Heλios β UVB v4.55 spectrometer using quartz cuvettes with a 1 cm path length. X-ray diffraction measurements were carried out using a Bruker AXS D8 diffractometer using monochromated Cu KR radiation. Samples for X-ray diffraction were prepared by spreading the precipitate containing nanocrystalline matter on a glass plate. TEM measurements were carried out on Philips CM200 microscope fitted with an EDAX-DX4 EDS, operating at 200 keV. Samples for TEM were prepared by placing a drop of a dilute dispersion of nanocrystals on a carbon coated Cu grid, followed by drying in a vacuum overnight. Fluorescence and excitation spectra were measured on a Fluorolog-3 FL3-22 spectrometer with two double grating monochromators, a silicone photodiode reference detector and an ozone-free xenon lamp. Microanalyses were carried out at the microanalysis facility at the School of Chemistry, University of Manchester.
Wang et al.
Figure 2. X-ray diffraction patterns of ZnO nanocrystals obtained using zinc ketoacidoximate at temperatures of (a) 200, (b) 180, (c) 160, (d) 130, and (e) 110 °C.
Results and Discussion Oximes such as the ketoacidoximate are known to decompose, yielding metal oxides, and liberate water, carbon dioxide, and nitriles. TiO2 and ZnO have been prepared by decomposition of oximes. Recently, Hill et al.19 synthesized several volatile zinc oximes, including the one used in this study, and have suggested that these oximes are especially suited as precursors for chemical vapor deposition. Similarly, metal cupferron complexes, originally used for extraction of metal ions, have recently found use in the synthesis of metal oxide nanocrystals such as CoO, NiO, MnO, Cu2O, and Fe2O3.20-22 Zinc cupferronate has been used to prepare ZnO nanocrystals solvothermally.23 We believed that capped nanocrystals can be obtained from these precursors in a coordinating medium because the organic moieties in these molecules appear volatile. We find that this is indeed the case when these organometallic complexes are decomposed in a mixture of trioctylamine and octylamine. Nanocrystals Grown from Zinc Ketoacidoximate. The X-ray diffraction patterns of ZnO nanocrystals obtained by thermal decomposition of the ketoacidoximate are shown in Figure 2. The patterns can be indexed to the hexagonal wurtzite form of ZnO with a ) 0.5193 nm and c ) 0.3252 nm, closely matching with the standard values of a ) 0.5207 nm and c ) 0.3250 nm. TEM images of the samples obtained at different decomposition temperatures show nanoparticulates that are nearly spherical (Figure 3). Diameters of the individual particles
Figure 3. TEM images of nanocrystals obtained by decomposing zinc ketoacidoximate at (a) 200, (b) 180, (c) 160, (d) 130, and (e) 110 °C.
were obtained by averaging the dimensions along the longest axis of the particulates and an axis perpendicular to it. The average diameters of the particulates in the sample were estimated from 200 such measurements. We find that decomposition temperatures of 110, 130, 160, 180, and 200 °C produced particulates with mean diameters of 3.9 ( 0.7, 4.7 ( 1.0, 5.3 ( 1.0, 6.0 ( 1.1, and 6.4 ( 1.2 nm, respectively. Highresolution TEM images showed lattice features which can be indexed to (111) planes of the wurtzite structure. The absorption spectrum of the ZnO nanocrystals of different diameters consists of a partially resolved band with a sharp onset (see Figure 4) corresponding to the excitionic 1Sh-1Se transition. We note that the position of the observed band is lower
Nanocrystalline ZnO with UV Luminescence
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Figure 4. Absorption spectra of ZnO nanocrystals obtained by decomposition of ketoacidoximate at different temperatures. The temperatures used for decomposition are indicated. The inset shows a plot of (Rhν)2 vs hν and the extrapolation used to estimate optical band gap.
than that of the bulk and that it shifts to the blue with decrease in size. The optical band gap (the lowest exciton state energy) was derived based on the equation
(hν - E)1/2 hν
R)A
(1)
where R is the absorption coefficient, E is the band gap, and A is a constant. Band gap values were obtained by extrapolating the linear region near the onset in a plot of (Rhν)2 versus hν (see inset to Figure 4). The exciton energy increased with decrease of particle diameter. A decrease of ∼100 meV was observed as the diameter of the nanocrystals decreased from 6.4 to 3.9 nm. Such
a pronounced change in absorption onset is not unusual in ZnO as its electron and hole effective masses are small. The photoluminescence (PL) spectra of the ZnO nanocrystals of different diameters are shown in Figure 5. The emission consists of two bands, a narrow band in the UV region, close to the absorption edge, and a much broader band in the visible region. The presence of two bands is similar to what is seen in bulk ZnO and colloidal ZnO.24 The UV emission bands are slightly asymmetric and could be fitted to two Gaussian peaks. A typical fit for 5.3 nm nanocrystals grown at 160 °C is shown in Figure 5f. The UV band is made up of peaks with centers at 3.38 and 3.23 eV. The intense peak at 3.38 eV corresponds exactly to the optical band gap (3.38 eV) and is thus a genuine band edge emission. The presence of this emission band in nanocrystalline ZnO is quite remarkable, as it suggests that the nanocrystals have a low density of defects. The second peak is red shifted by ∼150 meV compared to the absorption edge. Simple electron-phonon coupling cannot result in such a redshift because the longitudinal optical phonon energy is 70 meV for ZnO. Previous studies have attributed such red-shifted emission bands in ZnO and related semiconductor nanocrystals to either shallow traps or dark excitons arising from finestructure splitting.25-28 We notice that the position of this band is almost independent of size, indicating that the species responsible for this emission does not interact with quantum confined states. Perhaps surface defects create shallow traps responsible for this emission. As the diameters of the nanocrystals are decreased, the position of the band edge emission peak shifts to the blue, reflecting the change in the exciton energy seen in the absorption spectra. The second band in the original spectrum is situated in the green region. The origin of this band has been a subject of much debate.10,14-26,29 We believe that this band can be attributed to deep traps originating from
Figure 5. PL spectra of ZnO nanocrystals by thermolysis of ketoacidoximate at (a) 200, (b) 180, (c) 160, (d) 130, and (e) 110 °C. (f) The best fit using two Gaussian peaks for the UV band of the spectrum in part c.
4102 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Figure 6. PLE spectra of ZnO nanocrystals grown from zinc ketoacidoximate at a temperatures of (a) 110, (b) 130, (c) 160, (d) 180, and (e) 200 °C. The inset shows a comparison of PLE and absorption spectra of ZnO nanocrytals grown at 160 °C.
vacant oxygen sites in the ZnO lattice. The widths of the observed bands in the green region suggest that it is reasonable to assume that a deep trap level, albeit at low concentration, is present in all samples. Defect emission in the green region dominates the spectra of nanocrystals obtained at low temperature, while the band edge emission in the UV region is preponderant in the nanocrystals grown at high temperatures. It is plausible that higher temperatures yield ZnO nanocrystals with fewer oxygen site deficiencies, leading to the observed drop in the defect emission. To shed further light on the defect luminescence, photoluminescence excitation (PLE) spectra were measured with detection at a wavelength of 550 nm. The normalized spectra are shown in Figure 6. The PLE spectra consist of a single welldefined peak, whose position is dependent on the diameter of the nanocrystals. The peaks in the excitation spectra are related to the features in the absorption spectra. The PLE peak wavelength corresponds to the wavelength where the absorption intensity is half the maximum (see the inset of Figure 6 for a comparison of absorption and excitation spectra). This observation has important implications for the determination of the band gap, and the details shall be discussed later. Nanocrystals Obtained from Zinc Cupferronate. Zinc cupferronate is more thermally stable than the oximate, and, hence, higher decomposition temperatures were employed. X-ray diffraction patterns reveal that ZnO nanocrystals obtained by thermal decomposition of the zinc cupferronate possess wurtzite structure (see Figure 7). Selected area electron diffraction as well as high-resolution transmission electron microscopic images also confirm the crystalline nature of larger ZnO nanocrystals. TEM images reveal that the nanocrystals produced by decomposition of the cupferronate are larger (see Figure 8). Spherical nanocrystals with mean diameters of 7.0 ( 1.2, 8.5 ( 1.2, 13.0 ( 1.4, and 12.0 ( 1.3 nm were obtained at growth temperatures of 180, 200, 230, and 245 °C, respectively. The variation of nanocrystal diameter with temperature is not linear. Small nanocrystals (7.0, 8.5 nm) are produced at temperatures of 180 and 200 °C, while at higher temperatures larger nanocrystals with dimensions of 13.0 and 12.0 nm are formed. Figure 9 shows the absorption and PL spectra of ZnO nanocrystals obtained by the decomposition of cupferronate at different temperatures. In contrast to absorption characteristics of smaller nanocrystals obtained by decomposition of oximate, the absorption spectra of nanocrystals obtained by decomposition of cupferronates exhibit only small shifts with change in
Wang et al.
Figure 7. X-ray diffraction patterns of ZnO nanocrystals obtained using zinc cupferronate at temperatures of (a) 245, (b) 230, (c) 200, and (d) 180 °C.
Figure 8. TEM images of nanocrystals obtained by decomposing zinc cupferrate at (a) 180, (b) 200, (c) 230, and (d) 245 °C. Inset at the top in part a shows the lattice fringes typically obtained from individual ZnO nanocrystals. The interplanar spacing of 0.53 nm is attributable to the (001) plane of ZnO. The inset at the bottom in part a shows a selected area electron diffraction pattern obtained from a group of ZnO nanocrystals. The rings from the center to the brim are attributable to (100), (101), (102), (110), and (103) planes and mixture of (200), (112), and (201) planes of hexagonal ZnO.
diameter. The exciton energy varies by only ∼13 meV across the entire range, indicating that quantum confinement effects are weaker in this size regime. The luminescence spectra are largely similar. Two peaks are seen in all cases, although the peaks in the visible region are less pronounced. Defects in the ZnO lattice are manifest as a wide low intensity band in the visible region. The changes in the diameters revealed by TEM are mirrored in the optical spectra. The absorption and emission spectra of nanocrystals grown at 180 and 200 °C are similar, as are the spectra of the nanocrystals grown at 230 and 245 °C. Thus, the band edge emission peaks are at 3.28 eV for nanocrystals grown at 180 and 200 °C and at 3.30 eV for nanocrystals grown at 230 and 245 °C. A trend similar to that of nanocrystals produced from oximes is seen with respect to temperature: the samples grown at the higher temperatures exhibit a more intense emission peak at the band edge. To gain insight into the origin of luminescence in the
Nanocrystalline ZnO with UV Luminescence
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Figure 11. Plot of the band gap (lowest exciton state energy) versus the radius of nanocrystals. The open circles represent the gaps obtained from absorption onset, while the black triangles represent the gaps obtained from PLE spectra. The band gap estimates obtained using the Kayanuma model are indicated by the continuous line.
Figure 9. (a) Absorption spectra of ZnO nanocrystals obtained by decomposition of ketoacidoximate at different temperatures. The temperatures used for decomposition are indicated in the figure. (b) PL spectra of ZnO nanocrystals obtained by decomposition of ketoacidoximate at different temperatures. The temperatures used for decomposition are indicated in the figure.
full widths at half-maximum (fwhm’s) of 129, 223, 537, and 672 meV, respectively. The nanocrystals grown at 230 °C emit at 3.29, 3.17, and 2.58 eV, with fwhm’s of 109, 247, and 872 meV, respectively. The highest energy band ∼ 3.3 eV corresponds to band edge luminescence. The peaks at ∼3.17 eV correspond to shallow traps. Blue emission such as the band at 2.89 eV has been attributed to an electron transition from the level of interstitial Zn to the valence band.29 In accordance with previous assignments, the green band at 2.58 eV and the yellow band at 2.27 eV can be attributed to oxygen vacancies.29,30 On the basis of a study of ZnO nanocrystals embedded in silica, Chen et al.30 have suggested that the position of the emission band in the visible region shifts from green to yellow as the oxygen related defect density decreases. The emission spectra thus seem to indicate upon increasing the temperature from 230 to 245 °C that ZnO nanocrystals with lower oxygen deficiencies but with a higher proportion of Zn atoms trapped in interstitial sites are formed. The PL peaks obtained at lower temperature are not readily amenable to such analysis. However, shallow and deep defect levels as discernible in the emission spectra. Band Gap Variation in ZnO Nanocrystals. The lowest exciton state energies estimated from the onset of absorption spectra using eq 1 are plotted against particle diameter in Figure 11. The Kayanuma model31 which relates the band gap to particle radius is appropriate for these nanocrystals as they fall in the strong confinement regime with their radii (R) smaller than the Bohr radius (aB). The model predicts that E1s1s, the lowest excited-state energy, is given by
E1s1s ) Eg + π2
Figure 10. PL spectra of nanocrystals obtained by thermolysis of zinc cupferronate at 245 and 230 °C and the best fitting Gaussian peaks that simulate the original peak.
nanocrystals grown from cupferronate, the PL spectra of the nanocrystal grown at high temperature were fitted with Gaussian peaks (Figure 10). The nanocrystals grown at 245 °C consist of four emission bands at 3.30, 3.18, 2.89, and 2.27 eV, with
()
aB aB 2 Ry* - 3.572 Ry* - 0.248Ry* (2) R R
Eg is bulk band gap, and Ry* is the effective Rydberg energy. Despite numerous studies some uncertainties exist about the value of these parameters in the case of ZnO. Values of aB in the range of 1.5-3.0 Å and Ry* between 30 and 160 meV have been reported.8 Following the suggestion of Wood et al.,8 we have chosen an aB value of 2.4 Å and Ry* value of 30 meV. The E1s1s values obtained thus are plotted in Figure 11. The band gaps obtained by extrapolation of absorption onset are quite different from the theoretical values and vary less sharply than those predicted especially for diameters < 7 nm. We believed that the estimates based on the absorption onset underestimate the actual band gap and have estimated the band gap using the peak in the PLE spectra where possible. We also considered alternate methods to estimate the band gap from optical spectra. Among the alternate methods, the suggestion of Muelenkamp7
4104 J. Phys. Chem. B, Vol. 110, No. 9, 2006 that band gaps can be obtained by using λ1/2, the wavelength corresponding to the point in the absorption spectra where the intensity is half the maximum (λ1/2), seemed to provide a better estimate of the band gap. This method although practical seemed not to have any physical basis. We were, therefore, intrigued to find the correspondence of λ1/2 to the peak position in the PLE spectra. The correspondence lends credence to the above method of estimating band gap and provides a rationale. The band gaps estimated by this method differ from those obtained from the absorption edge and fit the predictions of the Kayanuma model much better (see Figure 11). On this basis, we suggest that this method is more appropriate for determining band gap in nanocrystalline ZnO. Conclusions Octylamine capped ZnO nanocrystals with diameters in the range of 7.0-14.0 nm were obtained by decomposition of zinc cupferronate. Smaller nanocrystals, with diameters in the range of 3.9-6.5 nm, were obtained by decomposition of zinc oximate. The ZnO nanocrystals grown by the above methods possess low defect densities and exhibit band edge luminescence. Quantum confinement effects as indicated by the blue shift of absorption onset are apparent in all nanocrystals and are particularly strong in nanocrystals with diameters less than 7 nm. The lowest excitonic state energies/optical band gap have been determined by extrapolating the absorption onset and from the peaks in the PLE spectra. The latter provide a better match with the predictions of the Kayanuma model. Acknowledgment. P.J.T and P.O.B. thank the University of Manchester for support. Y.S.W thanks China Council Scholarship for support. References and Notes (1) Wang, Z. L. J. Phys.: Condens. Matter 2004, 16, R829. (2) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem.sEur. J. 2002, 8, 28. (3) Pesika, N. S.; Hu, Z.; Stebe, K. J.; Searson, P. C. J. Phys. Chem. B 2002, 106, 6985. (4) Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826.
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