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2006, 110, 22310-22312 Published on Web 10/21/2006
Blue-Emitting Copper-Doped Zinc Oxide Nanocrystals Ranjani Viswanatha,† S. Chakraborty,‡ S. Basu,‡ and D. D. Sarma*,†,‡ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, 560012, India, and Indian Association for the CultiVation of Sciences, JadaVpur, Kolkata, 700032, India. ReceiVed: August 20, 2006; In Final Form: October 5, 2006
We report the first synthesis of Cu doped in the core region of ZnO nanocrystals and fluorescing in the blue region, establishing the novel possibility of using these as fluorescent probes.
Semiconductor nanocrystals, with their continuous tunability of optical and electronic properties, have immense potential in various applications and hence have been investigated extensively for the past few decades. Specifically, transition-metaldoped semiconductor nanocrystals have attracted a lot of attention in the context of their technologically interesting fluorescence properties.1 However, a few nanocrystal systems have proved difficult to dope, one such example being Mn doping in CdSe.2 The other system that has posed a unique difficulty in terms of an effective doping is ZnO in the following sense. Obtaining a sharp dopant emission, typically emitted by various transition metals in the visible region, from ZnO nanocrystals has until now not been possible in spite of doping various transition-metal ions like Mn,3 though a similar phenomena can be observed in other chalcogenides doped with Mn.4 Specific to the present context, we note that even Cu-doping of ZnO nanocrystals has been reported in the literature.5,6 However, in none of the cases there was any experimental evidence of Cu ions being incorporated into the interior or the core region of the nanocrystal as against being attached to the surface or the subsurface region. Consequently, there has not been any report on any emission resulting from the doped Cu ions, when the system is excited at the band edge of the host ZnO matrix. In sharp contrast, we report here for the first time a striking blue emission observed in copper-doped ZnO nanocrystals. We have synthesized various percentages of Cu-doped ZnO nanocrystals and characterized them using transmission electron microscopy (TEM), X-ray diffraction (XRD), electron paramagnetic resonance (EPR), UV absorption, and fluorescence (PL) spectroscopy. Furthermore, we have also synthesized freestanding Cu-doped ZnO nanocrystals with a similarly intense PL related to the transition-metal ion (Cu2+); in contrast, such dopant-state-derived PL has not been observed so far in the few available reports of the free-standing ZnO nanocrystals doped with transition-metal ions, such as Mn,3 Co, and Ni.7 The synthesis of Cu-doped ZnO nanocrystals was carried out using copper acetate and zinc acetate as the Cu and Zn sources, respectively, and isopropanol as a solvent; this was hydrolyzed using sodium hydroxide. Polyvinyl pyrollidone (PVP) was used as a capping agent in the case of capped nanocrystals, and this * Corresponding author. E-mail:
[email protected]. † Solid State and Structural Chemistry Unit, Indian Institute of Science. ‡ Indian Association for the Cultivation of Sciences.
10.1021/jp065384f CCC: $33.50
Figure 1. (a) TEM image of ZnO particles showing the lattice fringes. (b) XRD patterns of ZnO, capped, and free-standing 2% Cu-doped ZnO nanocrystals along with the bulk ZnO and broadened pattern for 4.5 and 7.5-nm-diameter nanocrystals.
was not added for the free-standing particles. The details of the synthesis and the characterization are reported in the Supporting Information. An atomic absorption spectrometer was used to estimate the percentage of copper actually doped in the nanocrystal. It was found that the actual mole percentages of Cu doping were 0.5%, 0.9%, 1.9%, and 4% for nominal doping of 0.5%, 1%, 2%, and 5%, respectively. The sizes of the nanocrystals were obtained by measuring the size and size distribution of these nanocrystals using the TEM. A typical bright-field image of Cu-doped ZnO nanocrystal is shown in Figure 1a. The TEM image shows the abundance of spherical particles, thus allowing us to analyze the XRD pattern assuming spherical particles. From the analysis of TEM images, we find that the particles have an average size of 4.4 nm with a size dispersity of about 8%. The lattice fringes © 2006 American Chemical Society
Letters
Figure 2. EPR spectra for different x values of Zn1-xCuxO nanocrystals.
observed in the micrograph are due to the 〈002〉 plane of the wurtzite structure of ZnO nanocrystals. The typical XRD pattern of the free-standing Cu-doped ZnO (plot i) and capped ZnO (plot iii) along with the capped Cudoped ZnO nanocrystals (plot iv) are shown in Figure 1b. We have also shown in the same figure the XRD of bulk ZnO (plot vi) that is known to crystallize in the wurtzite form. From the Figure, it can be seen that XRD peaks of nanocrystals also correspond to the wurtzite crystal structure with the same lattice parameters. To obtain further information about the size of the nanocrystal from the width of the peaks, we convoluted the XRD pattern of the bulk ZnO with a Gaussian function, whose FWHM, B, at diffraction angle θ when recorded using X rays with the wavelength λ, was determined by B ) 1.2λ/D cos θ, where D is the diameter of the spherical nanocrystal. A leastsquared-error approach to describe the experimental XRD patterns of the nanocrystals, with D as the fitting parameter, showed the best fit for a diameter of 7.5 nm (plot ii) for the case of free-standing particles and of 4.5 nm (plot v) for cappeddoped and undoped nanocrystals. This is also in good agreement with the size determined by TEM. Also, the sizes of the nanocrystals do not vary with doping at least for the low percentages of doping carried out here. Though the lattice parameters are expected to be slightly influenced by doping, it becomes impossible to estimate any such small changes in the doped ZnO lattice because of the broadening observed in the XRD patterns of these nanocrystals. EPR spectra were used to detect the presence of copper in the nanocrystals. Figure 2 shows the EPR spectra of various percentages of copper doped in ZnO nanocrystals. We note that the samples without Cu doping did not show any EPR signal, indicating the absence of any significant extent of oxygen vacancies in these samples.8 The spectra from the doped samples exhibit anisotropy with two dominant features similar to that of the bulk.9 However, the hyperfine splitting is visible only in the case of 1% Cu-doped ZnO nanocrystals and smeared out in the presence of a higher concentration of copper, suggesting that there may be some interaction within the Cu2+ ions. Additionally, it is known that it is not possible to distinguish between the Cu2+ ions on the surface and that in the bulk using the EPR technique. To verify the presence of Copper in ZnO nanocrystals, we etched the nanocrystals with H2O2 in order to remove a thin surface layer.10 The measured UV absorption, fluorescence, and EPR spectra of the etched sample are shown in the Supporting Information. The UV-absorption edge shows a clear blue-shift on etching the sample, supporting the notion of making the nanocrystal sample smaller by etching away the surface layer; interestingly, however, the PL and the EPR signal from copper continue to be present. This establishes that the
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22311
Figure 3. Variation of band gap of Zn1-xCuxO nanocrystals.
copper is not washed off along with the etched layer and hence Cu doping takes place in the bulk of the nanocrystal. The UV-absorption spectra of various percentages of capped Cu-doped ZnO nanocrystals are shown in the Supporting Information. To obtain a quantifiable measure, the band gap is defined by the point of inflection, which is the minimum in the first derivative curve of the absorption spectrum. The band gap of ZnO nanocrystals thus obtained is 3.44 eV (360 nm), indicating a blue shift compared to the bulk value of 3.32 eV (373 nm) because of the well-known quantum confinement effect. This shift in the absorption edge can be correlated with the size of the nanocrystal, using the realistic and accurate tight binding (TB) scheme.11 According to these calculations for ZnO, the size of the particles are obtained as 4.7 nm, in good agreement with the sizes obtained from TEM and XRD. The band gap of Cu-doped ZnO nanocrystal shows nonmonotonic dependence on the Cu concentration, as illustrated in Figure 3. The band gap increases initially, passes through a maximum at ∼1% of Cu, and then decreases with increasing Cu concentration in the nanocrystals, the total variation observed being considerably larger than the typical error bars associated with experimental uncertainties estimated from several independent measurements. It is also observed that these trends remain qualitatively and quantitatively similar irrespective of the method used to determine the band gap. Furthermore, a similar nonmonotonic trend is also observed for the free-standing 7.5 nm nanocrystals (Figure 3), with the corresponding UVabsorption spectra being shown in the Supporting Information. This kind of nonmonotonic behavior was observed even in the case of Mn-doped ZnO nanocrystals3 where the band gap exhibited the reverse trend of first a decrease and then an increase with increasing dopant concentration. It is reasonable to expect a decrease in the band gap of the nanocrystals with increasing Cu concentrations because the band gap of CuO (bulk value of 1.4 eV) is less than that of ZnO (bulk value of 3.32 eV). We attribute the initial anomalous increase in the band gap to the band gap “bowing” in very short-ranged, paramagnetic phase similar to that in the case of Mn-doped ZnO.3 This has been theoretically explained in the context of Mn doped ZnSe nanocrystals12 using second-order perturbation theory. These short-ranged exchange interactions account for the anomalous increase in the band gap at low concentrations of Cu and are expected to be stronger in these confined solids and hence more pronounced in the case of nanocrystals. In the present context, the observation of a systematic change in the band gap of a ZnO nanocrystal as a function of the Cu content strongly supports other experimental evidence already present to show that Cu is indeed incorporated into the lattice of ZnO nanocrystals.
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Letters Cu2+ d9 state into 2D5/2 and 2D3/2 states, we suggest a schematic energy level diagram of the process, as shown in the inset of Figure 4b. The left segment of the schematic represents the excitonic transition across the band gap of the ZnO nanocrystal host. The middle segment shows a nonradiative energy transfer that converts the ground-state Cu2+ 2D5/2 state to a Cu+ 1S (d10) state. The radiative recombination of the electron and hole generated in the first photoabsorption step, gives rise to the blue emission with CIE coordinates of (0.20, 0.28) and converting back the intermediate Cu+ state to Cu2+ state, the doublet feature arising from the statistical population of the final Cu2+ d9 state into 2D5/2 and 2D3/2 states. Thus, in conclusion, we have synthesized blue-emitting Cudoped ZnO nanocrystals in both free-standing as well as capped forms, this being the first-ever example of a perceivable fluorescence in the visible region from a ZnO-based nanocrystal system. The sizes of the nanocrystals were obtained using TEM, XRD, and the absorption edge measurements. The sizes obtained from different methods agree well with each other. The presence of Cu in the nanocrystal is established using EPR spectrum. The presence was also verified by washing the nanocrystals with pyridine and etching with H2O2. The UV-absorption spectra showed a nonmonotonic variation of the band gap similar to the Mn-doped ZnO. However, unlike all other doped ZnO nanocrystal system reported so far, characteristic blue fluorescence arising from transitions involving Cu-d states could be observed from these samples.
Figure 4. (a) Fluorescence excitation (λem ) 430 nm) and emission spectra (λex ) 325 nm) of Zn1-xCuxO nanocrystals. (b) Fluorescence emission spectra of free-standing Zn1-xCuxO nanocrystals. The energylevel diagram of the process is shown in the inset.
The fluorescence emission and excitation spectra are shown in Figure 4 for various concentrations of Cu-doped, capped, and free-standing ZnO particles. Undoped ZnO capped with PVP shows a spectral feature at ∼360 nm corresponding to the near-band gap emission. It can be observed from Figure 4a that no surface-state emission is seen in any of the cases and only the near band-gap emission is observed. Hence, the nanocrystals appear to be well passivated. However, the free-standing ZnO shows a high intensity between 500 and 600 nm (Figure 4b), which may be due to surface-state-related emission or the oxygen vacancies in the core of the nanocrystals. Cu-doped ZnO nanocrystals additionally show two clear peaks at ∼410 nm and ∼430 nm (Figure 4a and 4b). These peaks increase in intensity at the cost of the band-gap emission with increase in percentage of copper. Furthermore, the excitation spectrum of these systems for an emission at 430 nm as well as at 410 nm shows a peak at ∼360 nm corresponding to the band-edge emission. Thus, it can be seen that the particles emit strongly in the blue region of the visible spectrum (430 and 410 nm) when excited near the absorption edge of the ZnO nanocrystal host with a quantum yield of about 1/10 that of the reference dye, stilbene. Noting that the energy difference (0.14 eV) between the doublet feature in the blue emission is equal to the spin-orbit splitting of the
Acknowledgment. This work was supported by the Department of Science and Technology, Government of India. Supporting Information Available: Synthesis procedure, methods of characterisation, data on Etching and UV absorption. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Norris, D. J.; Yao, N.; Chamock, F. T.; Kennedy, T. A. Nano Lett. 2001, 1, 3. (2) Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Nature 2005, 436, 91. (3) Viswanatha, R.; Sapra, S.; Sen Gupta, S.; Satpati, B.; Satyam, P. V.; Dev, B. N.; Sarma, D. D. J. Phys. Chem. B 2004, 108, 6303. (4) Sapra, S.; Prakash, A.; Ghangrekar, A.; Periasamy, N.; Sarma, D. D. J. Phys. Chem. B 2005, 109, 1663. (5) Bhargava, R. N.; Chhabra, V.; Som, T.; Ekimov, A.; Taskar, N. Phy. Status Solidi 2002, 229, 897. (6) Borgohain K.; Mahamuni, S. Semicond. Sci. Technol. 1998, 13, 1154. (7) Rodonovic, P. V.; Norberg, N. S.; McNally, K. E.; Gamelin, D. R. J. Am. Chem. Soc. 2002, 124, 15192. (8) Gorelkinskii, Y. V.; Watkins, G. D. Phys. ReV. B 2004, 69, 115212. (9) Kannappan, R; Mahalakshmy, R; Rajendiran, T. M.; Venkatesan, R.; Sambasiva Rao, P. Proc. Indian Acad. Sci. (Chem. Sci.) 2003, 115, 1 (10) Bhattaglia, D.; Blackman, B.; Peng, X. J. Am. Chem. Soc. 2005, 127, 10889. (11) Viswanatha, R.; Sapra, S.; Satpati, B.; Satyam, P. V.; Dev, B. N.; Sarma, D. D. J. Mater. Chem. 2004, 14, 661. (12) Bylsma, R. B.; Becker, W. M.; Kossut, J.; Debska, U.; YoderShort, D. Phys. ReV. B 1986, 33, 8207.
