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2006, 110, 21412-21415 Published on Web 10/11/2006
Optical Properties of ZnO Nanocrystals Doped with Cd, Mg, Mn, and Fe Ions Y. S. Wang,† P. John Thomas, and P. O’Brien* School of Chemistry and School of Materials, The UniVersity of Manchester, Oxford Road, Manchester, M13 9PL, UK ReceiVed: August 23, 2006; In Final Form: September 28, 2006
ZnO nanocrystals doped with Cd, Mg, Mn, and Fe ions were obtained by thermolysis of a family of metal cupferrates. The nanocrystals were characterized by X-ray diffraction, energy-dispersive X-ray analysis, transmission electron microscopy, UV-visible, luminescence, and excitation spectroscopy. The band gap of the nanocrystals can be tuned in the range of 2.9-3.8 eV by the use of the dopants. In most cases, the nanocrystals are sufficiently defect-free to exhibit band edge luminescence.
The possibility of engineering band gap and influencing physical, chemical, and electronic properties by varying the dimensions of the system has provided a strong impetus to study nanocrystals and other nanodimensional material.1 However, it is being realized that tuning the band gap by changing the diameters of nanocrystals is not well suited for use in some applications such as some types of fluorescent imaging and nanoelectronics.2 The rationale is that changes in diameters lead to changes in the nature of nanocrystals such as its reactivity and its ability to self-assemble. Therefore, attempts have been made to tune band gap by varying the composition of nanocrystals instead of the diameter. Thus, the band gap of CdSe nanocrystals has been varied by the introduction of Te.2 Furthermore, it is possible to increase as well as decrease the band gap by the use of the right dopants, as opposed to sizeinduced tuning, wherein the band gap can only be increased. ZnO, a wide band-gap semiconductor, has been extensively studied due to its intrinsic properties and potential for uses in devices, such as field-effect transistors, resonators, gas sensors, and solar cells, and as a catalyst.3 ZnO has been doped with Mg to enlarge the band gap and with Cd to narrow the band gap. In either case, alloys of the form MgxZn1-xO or CdyZn1-yO with hexagonal zincite structure have been formed. There have been few reports of Mg- and Cd-doped ZnO nanocrystals. Previous research has focused on growing thin films and bulk forms of MgxZn1-xO and CdyZn1-yO.4-8 Only recently, nanorods of MgxZn1-xO and CdyZn1-yO have been grown by molecular beam epitaxy9 and thermal evaporation,10 respectively. There has been a lot of interest in doping ZnO 3d transition metal ions because of potential application in spin-based electronics (spintronics) since the prediction made by Dietl et al. that Mn-doped ZnO would be ferromagnetic at room temperature.11 Studies on bulk and nanocrystalline ZnO doped with 3d transition metal ions are primarily concerned with the magnetic properties.12-15 A few studies have suggested that ZnO doped with transition metal ions could possess interesting optical properties.16-18 * Corresponding author. E-mail:
[email protected]. † Present address: Department of Physics, Beijing Normal University, Beijing 1000875, P. R. China.
10.1021/jp0654415 CCC: $33.50
Previous attempts at doping nanocrystals have been fraught with problems because the synthetic schemes used to dope frequently yield inhomogeneously doped materials. The frequent failure of doping schemes was, until recently, attributed to the expulsion of dopant ions to the surface of nanocrystals by the intrinsic process of self-annealing. Recently, Efros et al., on the basis of a study of Cd and Zn chalcogenide nanocrystals, have suggested that doping is directly related to the ability of the ions to adsorb to the exposed surfaces of the nanocrystals rather than self-annealing.19 It appears, therefore, that successful doping of nanocrystals can be achieved by involving nanocrystals of the right size and morphology and choosing surfactants that do not bind too strongly to the dopant ions. In light of above observations, we thought it would be interesting to carry out compositional variation in ZnO nanocrystals using a single synthetic scheme to bring about changes in the band gap. In this letter, we report the changes in the optical properties of ZnO nanocrystals brought about by successful doping with Mg, Cd, Fe, and Mn ions using a family of metal cupferrates. We sought to carry out such doping in ZnO nanocrystals with diameters in a range unaffected by quantum confinement effects (diameter > ∼7.0 nm).20,21 The doping of nanocrystals with different ions was accomplished using a family of metal cupferrates, following the method reported by us recently to synthesize luminescent ZnO nanocrystals.22 Briefly, trioctylamine (10 mL) was degassed by heating to 120 °C in a vacuum and repeatedly flushing with N2. The solution was then heated to 180 °C. Subsequently, a 1 mL octylamine solution containing 0.05 mmol of metal cupferrates was injected. A temperature drop of ∼5 °C was observed following injection. The reaction mixture was allowed to recover to 180 °C and was maintained at that temperature for an additional 60 min. At the end of the reaction, the mixture was cooled to room temperature and the nanocrystals were precipitated by adding ethanol. The nanocrystals were characterized by absorption and emission spectroscopy, X-ray diffraction, and high-resolution transmission electron microscopy. The composition of the nanocrystals was independently analyzed by energy-dispersive X-ray analysis and microanalysis. We found that the molar ratios of the precursors in the feed mixture © 2006 American Chemical Society
Letters
Figure 1. (a) X-ray diffraction patterns obtained from pure and Cddoped ZnO nanocrystals. The Cd doping levels are indicated alongside. (b) X-ray diffraction patterns obtained from pure and Mg-doped ZnO nanocrystals. The Mg doping levels are indicated alongside.
corresponded well with the analyzed compositions. The compositions reported herein are as obtained from X-ray analysis. X-ray diffraction patterns of pure and doped nanocrystals reveal the presence of a single hexagonal phase with zincite structure (JCPDS no. 80-0075) (see Figure 1a,b), indicating that phase-pure doped nanocrystals were obtained by this synthetic scheme. Magnesium ions introduced as dopants at levels below 10% shift the diffraction peaks to higher angles, suggesting that the unit cell contracts to accommodate the ions (see Figure 1a). Such a change is indeed to be expected if Mg ions replace Zn ions in the lattice, as the Mg ions have smaller ionic radii. Higher levels of Mg lead to a broadening of the peaks with shifts indicating an expansion of the unit cell, possibly due to the Mg ions preferring to occupy interstitial sites. Doping with the Cd ions also leads to expansion of the lattice depending on the doping level (see Figure 1b). Doping with Mn ions produces no appreciable shift in the lattice parameters since both ions possess similar radii. Shifts indicating a small increase in the unit cell dimensions are seen in the case of Fe. The changes to the unit cell dimensions, although discernible in many cases, are not quantified, as the peak widths introduce errors significant enough to obscure the changes. We note, however, that the changes in unit cell dimensions seen in this case are generally in line with reported observations in the case of bulk ZnO.4-8,23 However, it appears that bulk ZnO can be doped with higher levels of Mg. Transmission electron microscopic images of pure and doped ZnO nanocrystals reveal nearly spherical nanocrystals in most cases, with dimensions in the range of 9-12 nm (see Figure 2). The presence of small amounts of Mn and Mg ions (up to ∼2%) influences the growth process, resulting in elongated nanocrystals. Simply increasing the concentrations of the dopant ions yields spherical nanocrystals. It might well be possible to tune the growth conditions to yield highly anisotropic nanostructures using these ions at low concentrations, as has been demonstrated in the case of noble metal nanostructures.24 A clear shift in the absorption onset is discernible in ZnO nanocrystals doped with Mg and Cd ions (see Figure 3). Mg doping shifts the absorption onset to blue, indicating an increase
J. Phys. Chem. B, Vol. 110, No. 43, 2006 21413
Figure 2. TEM images of ZnO nanocrystals doped with different dopants: (a) pure ZnO, (b) 5% Mg, (c) 2% Cd, (d) 2% Fe, (e) 5% Fe, (f) 2% Mn.
Figure 3. (a) Absorption spectra of ZnO nanocrystals doped with different levels of Mg. The doping levels are indicated in the plot. The inset shows a plot of (Rhν)2 vs hν (see text for details). (b) Absorption spectra of ZnO nanocrystals doped with different levels of Cd. The doping levels are indicated in the plot.
in the band gap. The optical band gap was derived based on the equation
R ) A‚
(hV - E)1/2 hV
(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 3). A linear increase in the band gap from 3.30 to 3.66 eV with an increase in Mg concentration from 0 to 15% is observed. Higher doping levels result in less pronounced shifts. Doping with Cd, on the other hand, produces a red shift indicative of a decrease in the band gap (see Figure 3b). The optical band gap falls nearly linearly from 3.30 to 2.92
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Figure 5. Emission spectra of ZnO nanocrystals doped with different levels of Cd. The doping levels are indicated in the plot.
Figure 4. (a) Emission spectra of ZnO nanocrystals doped with different levels of Mg. The doping levels are indicated in the plot. The inset shows a magnified view of the emission spectra corresponding to the nanocrystals with low levels of Mg doping. (b) PLE spectra of ZnO nanocrystals doped with different levels of Mg. The doping levels are indicated in the plot.
eV with the increase of Cd doping levels from 0 to 10%. The systematic and linear shifts with doping levels suggest that the introduction of dopant ions, rather than size effects, brings about these changes. Further, a number of previous experimental and theoretical investigations have suggested that size effects have little or no influence on the band structure of ZnO nanocrystals with diameters greater than 7.0 nm.20,21 The indicated band gaps are similar to that which has been observed on other forms of ZnO doped with Mg and Cd.9 We therefore attribute the origin of band gap shifts to the influence of dopant ions. Further proof follows from the emission characteristics of the nanocrystals. The photoluminescence (PL) spectra of Mg-doped ZnO nanocrystals consist of a band edge and defect-state emission bands (see Figure 4). The presence of the former is remarkable because it suggests that the nanocrystals have a low density of defects, despite the introduction of the Mg ions. The band edge emission shifts to the blue with increasing levels of Mg doping, reflecting the change in the exciton energy seen in the absorption spectra. The defect band is believed to be due to deep traps and becomes dominant at high levels of doping (above 10%). PL excitation (PLE) spectra, measured with detection at a wavelength where the deep trap emission is maximum, reveal a linear increase in the peak energy with increasing Mg doping levels up to 10% (see Figure 4b). The dependence of the optical band gap on Mg concentration derived from absorption and the PLE spectra correspond well with each other. Nanorods of Zn1-xMgxO grown by chemical vapor deposition also exhibit similar optical properties.9 The emission spectra of ZnO nanocrystals doped with different levels of Cd are strongly dominated by band edge luminescence (see Figure 5), indicative of the success of the doping scheme. The emission band shows a red shift with an increase in Cd doping levels, as can be expected from the changes in the optical band gap derived from the absorption spectra. Thus, ZnO nanocrystals can be successfully doped with Cd and Mg by this synthetic scheme. In addition to doping with Cd and Mg ions, doping was also carried out with the transition metal ions Mn and Fe. The absorption spectra of Mn-doped nanocrystals consisted of a band in the region of 2.48-3.18 eV attributable to Mn ion d-d
Figure 6. (a) Absorption spectra of ZnO nanocrystals doped with different levels of Mn. The doping levels are indicated in the plot. (b) Absorption spectra of ZnO nanocrystals doped with different levels of Fe. The doping levels are indicated in the plot.
transition between the 6A1g ground state and the excited 4T, 4E, and 4A1 states. The strength of this band shows the expected increase with increase in the doping level. The band edge and the exciton energy shift to the blue proportional to the concentration of dopant ions (see Figure 6a). Doping with Fe also produces a blue shift proportional to the concentration of the ions (see Figure 6b). The increase in the absorption in the region of 2.9-3.2 eV is due to the d-d transition of Fe ions.25 The luminescence of the Mn-doped ZnO nanocrystals is strongly quenched (see Figure 7a). Similar quenching of luminescence has been observed previously in nanocrystalline and bulk ZnO.12,16 However, the mechanism of quenching is not clear. The emission band could be resolved to Gaussian peaks with maxima at 3.32, 3.15, and 2.76 eV. We attribute the 2.76 eV band to the 4T(G) f 6A1(S) d-d transition of Mn, following previous experimental observations on Mn-doped ZnO films.18 Because the band gap of a 5% Mn-doped sample is 3.36 eV, the 3.32 eV band is attributable to band edge emission. We are unable to identify any transition that can be associated with the band at 3.15 eV. As is the case with Mn-doped nanocrystals, emission is strongly quenched in Fe-doped nanocrystals (see Figure 7b). Exciton emission is sharp in 1% Fe-doped ZnO, while it broadens and weakens with increase in the doping level. Thus, ZnO nanocrystals can be successfully doped at low levels with dopants such as Fe and Mn by our method. The shifts in the optical band gap in ZnO caused by doping with Mg, Cd, and Mn ions are summarized in Figure 8. The band gap of the ZnO nanocrystals could be tuned in the range of 2.92-3.77 eV by the use of Mg and Cd ions as dopants.
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J. Phys. Chem. B, Vol. 110, No. 43, 2006 21415 quenches the luminescence of the nanocrystals. We believe that successful doping in this case is due to the choice of precursors with similar volatility and is also due to the larger size of nanocrystals employed in this study. Acknowledgment. P.J.T. and P.O.B. thank the University of Manchester for support. Y.S.W. thanks the China Council Scholarship for support. References and Notes
Figure 7. (a) Emission spectra of ZnO nanocrystals doped with 0 and 5% Mn. The doping levels are indicated in the plot. Inset shows the Gaussian fits to the observed band. (b) Emission spectra of ZnO nanocrystals doped with Fe. The doping levels are indicated in the plot.
Figure 8. Plot showing the changes in the optical band gap in ZnO nanocrystals that accompanies doping with different ions. The ions as well as the doping levels are indicated in the plot. The lines are included as a guide to the eye.
Such tuning can be accomplished with minimal loss in the intensity of band edge emission. The energy difference of 0.85 eV between the highest and lowest band gaps in doped nanocrystals is substantially higher than that typically achieved in ZnO nanocrystals by size tuning using a single synthetic scheme. ZnO nanocrystals can also be successfully doped with transition metal ions Fe and Mn at low concentrations by the above method. The introduction of transition metal ions
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