Optical and Electrical Properties of Eu3+-Doped ... - ACS Publications

J. Phys. Chem. C 2009, 113, 4375–4380

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Optical and Electrical Properties of Eu3+-Doped SnO2 Nanocrystals Arik Kar and Amitava Patra* Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed: December 8, 2008; ReVised Manuscript ReceiVed: January 19, 2009

Here, we report the preparation of pure and Eu3+-doped SnO2 nanocrystals by microwave synthesis. The size dependence of the band gap energies of the quantum-confined SnO2 particles agrees very well with the confinement regime. The PL intensity, decay time, and quantum efficiency are found to be sensitive to the particle size. The calculated quantum efficiencies are 22.0%, 31.0%, and 26.0% for 300, 400, and 800 °C heated samples, respectively, because minimum nonradiative decay rate is observed at 400 °C heated sample. Analysis suggests that the crystallite size plays an important role in tuning the quantum efficiency, emission intensity, and decay time of Eu3+-doped SnO2 nanocrystals. Results show that the conductivity for doped sample is higher than that for pure SnO2 nanocrystals and pure SnO2 nanocrystals showed a typical rectifying behavior. Introduction Tin oxide (SnO2) is a well-known wide band gap n-type semiconductor with potential applications as gas sensors, dyebased solar cells, catalytic supports, and optoelectronic devices.1-4 The optical property of doped semiconductor nanoparticles has given rise to intriguing science in nanoresearch in the new millennium.5-9 Recently, attention has been paid to rare-earth (RE) ion doped semiconductors for photonic applications. The confinement effects of semiconductor nanoparticles create photogenerated carriers that may have an interaction with f-electrons which has important manifestations in influencing the optical properties. It is already reported that the spontaneous emission probability of optical transitions (luminescence lifetime) from rare-earth ions doped in nanoparticles can be significantly modified by changing the particle size, shape, and surrounding medium.10-13 The luminescence lifetime may change owing to modifications in the (i) photon emission probability and/or (ii) phonon emission probability. The electronic wave functions of rare-earth 4f-4f transitions are strongly localized, and they are not affected by quantum confinement. On the other hand, the phonon density-of-states of the host material can be significantly modified according to the size and shape of the nanoparticles and, consequently, these changes of nonradiative relaxation probabilities (multiphonon emission) affect the luminescence lifetime. Nogami et al.14 found that the Eu3+ emission is strongly enhanced by energy transfer from the SnO2 nanocrystals in a glass matrix. In our previous work, we reported the role of nanoenvironment on luminescence of Eu3+-activated SnO2 nanocrystals.10d A few papers15-19 have been recently reported on rare-earth ions containing SnO2 nanoparticles, which is the topic of the paper. Fabrication of nanodevices with enhanced performance still possesses a key challenge for the realization of new nanometerscale devices, which, in turn, depends on the better understanding of the electrical properties of nanomaterials.20-23 Depending on the doping level, the nanoparticles offer many interesting electrical properties. Generally, undoped SnO2 nanowire shows Schottky contacts with a metal electrode which is suitable for photodetector applications. Recently, Wan et al.24 reported that * Author to whom correspondence should be addressed: electronic mail, [email protected]; phone, (91)-33-2473-4971; fax, (91)-33-2473-2805.

Figure 1. X-ray powder diffraction patterns of 1.0 mol % Eu3+-doped SnO2 nanocrystal at different temperatures prepared by the microwaveassisted method.

Sb doping has significant influenced on the electrical properties of SnO2 nanowires. However, there is no study on the influence of rare-earth ions on electrical properties of SnO2 nanoparticles. Here, we use a novel microwave synthesis method for preparing SnO2:Eu nanocrystals. Microwave heating provides an effective and rapid method for the preparation of nanocrystalline materials. In recent years, microwave synthesis is an interesting technique for the synthesis of oxide nanomaterials because of several advantages; i.e., it takes a few minutes to complete the reaction and it prevents the agglomeration.25-29 Furthermore, this method can be used for large scale synthesis of nanomaterials. It is also found that microwave reactions in solutions can yield nanoparticles to a greater extent than solidstate reactions. The effect of the heating is created by the interaction of the dipole moment of the molecules with highfrequency electromagnetic radiation (2.45 GHz). Water has a very high dipole moment, which makes it one of the best solvents for microwave-assisted reactions. With microwave irradiation of the liquid sample, temperature and concentration gradients can be avoided, providing a uniform environment for nucleation.

10.1021/jp810777f CCC: $40.75  2009 American Chemical Society Published on Web 02/23/2009

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Kar and Patra

Figure 2. TEM micrographs of (a) 200 °C heated, (b) 400 °C heated, and (c) 800 °C heated Eu-doped SnO2 nanocrystals. (d) EDX data for the Eu3+-doped SnO2 nanocrystals.

Figure 3. UV-vis spectra of 1.0 mol % Eu3+-doped SnO2 nanocrystal at different temperatures, (a) 100 °C, (b) 200 °C, (c) 400 °C and (d) 800 °C. Inset shows (Rhυ) vs. photon energy plot of pure and Eu3+doped SnO2 nanocrystals).

Here, we address the role of size on the optical and electrical properties of SnO2:Eu3+ nanocrystals derived from microwave synthesis. 2. Experimental Section Synthesis of SnO2:Eu Nanoparticles. A 0.5 mmol (0.176 g) portion of SnCl4 · 5H2O was dissolved in 1 mL of water in a beaker. Five millimoles (0.2 g) of NaOH was also dissolved in 1 mL of water, and to this solution 1 mL of ethanol was added to make a basic mixture of alcohol and water (1:1). Now this basic mixture was added dropwise to SnCl4 · 5H2O solution under continuous stirring. A white cloudy suspension slowly begins to form. For europium-doped SnO2 nanoparticles, the same experimental procedure was repeated with 1 mol % Eu(NO3)3 · 6H2O (0.0046 g). Now the pH was adjusted to 5-6.

Figure 4. Emission spectra of pure and Eu3+-doped SnO2 nanocrystals prepared at 400 °C with excitation wavelength 280 nm.

The total volume of the solution was adjusted to 5 mL by adding ethanol. Now this 5 mL of white cloudy suspension (pH ) 5.5) was transferred to a microwave tube of 5 mL capacity. Then the tube was inserted in a monomode microwave reactor (CEM, Discover, USA) with power set to 300 W at 100 °C for 5 mins. After the sample was subjected to microwaves for the desired time, the synthesized precipitate was centrifuged (rpm ) 8000-9000) and washed with ethanol two times and dried in an oven at 60 °C. The final white powder was collected for characterization. These samples were heated at 200, 300, 400, 600, 700, and 800 °C for 1 h with a rate of heating of 2 °C /min in a furnace. The crystalline phases of annealed powders were identified by X-ray diffraction (XRD) using a Philips model PW-1730,

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TABLE 1: Lifetime, Judd-Ofelt Parameter (Ω2), Radiative Decay Rate, Nonradiative Decay Rate, and Luminescence Quantum Efficiency (η %) of 1.0 mol % Eu3+-Doped SnO2 Nanocrystals 1.0 mol % Eu3+-doped SnO2 nanoparticles (°C) 200 300 400 700 800

lifetime (ms)

Judd-Ofelt parameter (10-20 cm2)

radiative (ms-1)

nonradiative (ms-1)

quantum efficiency (%)

0.39 0.56 0.77 0.66 0.65

10.56 13.88 14.54 14.25 14.32

0.324 0.392 0.403 0.400 0.400

2.24 1.39 0.89 1.12 1.13

12.6 22.0 31.0 26.4 26.0

powder X-ray diffractometer using a Cu KR source (1.5418 Å radiation). Crystallite sizes (D, in Å) were estimated from Scherrer’s equation

D ) Kλ/β cos θ

(1)

where λ is the wavelength of Cu KR radiation, β is the corrected half-width of the diffraction peak, θ is the angle, and K is equal to 0.9. The excitation and emission spectra of SnO2:Eu3+ ions (pure and doped) were measured using a fluoro Max-P (Horiba Jobin Yvon) luminescence spectrometer. All measurements were done at room temperature using solid sample holder. All samples were excited at 280 nm, under the same conditions. The optical absorption coefficient R of a semiconductor close to the band edge can be expressed by the following equation

R ) κ(hυ - Eg)n /hυ

(2)

where k is a constant, Eg is the band gap, and n is a value that depends on the nature of the transition. In this case, n is equal to 1/2 for this direct allowed transition. The band gap can be estimated from a plot of (Rhυ)2 versus photon energy. For the electrical I-V characterization, first metallic Au was deposited on a glass slide which has a narrow wire rounded over it. After 5 min, metallic gold was deposited with a narrow tunnel. Then one drop of the prepared sample, dispersed in ethanol was placed in the narrow tunnel. Thus Au-SnO2-Au devices are formed which was then connected with the electrometer. I-V characteristics of the devices, kept in a shielded vacuum chamber, were recorded with a Keithley 486 picoammeter and YOKOGAWA 7651 (voltage Source). Results and Discussion X-ray diffraction patterns of SnO2:Eu heated at different temperatures are shown in Figure 1. The 2θ peaks at 26.8°, 34.3°, and 52.3° are well consistent with JCPDS card 41-1445, which confirm the samples as a pure tetragonal rutile crystalline SnO2 phase. One XRD peak at 31.76° was observed for 700 °C heated samples which is due to the Eu2O3 phase (JCPDS card no. 34-72), indicating that phase separation starts at 700 °C. It is interesting to note that this peak disappeared at 800 °C and three new peaks for the Eu2Sn2O7 phase (JCPDS card 00013-0182) appeared at 800 °C. We believe that SnO2 oxide phase reacts with Eu2O3 (solid state reaction) to form Eu2Sn2O7 at 800 °C. It is clearly seen that the width of the reflection is considerably broadened, indicating a small crystalline domain size. The width of the reflections decreases with increasing the temperature of heating, suggesting the enhancement of the particle size. The crystallite sizes of these nanocrystals were calculated using Scherrer’s equation, and our estimated average crystallite sizes are 1.7, 2.5, 3.5, and 18.2 nm for 100, 200, 400, and 800 °C heated samples, respectively. Panels a-c of Figure 2 show the TEM pictures of Eu3+-doped SnO2 nanocrystals prepared at 200, 400, and 800 °C, respectively. The estimated average sizes of the particles are 3.0, 3.9, and 12.9 nm for 200, 400, and 800 °C heated samples,

respectively, in agreement with the result obtained from X-ray diffraction studies. Figure 2d shows the EDX (energy dispersion X-ray) data for the doped sample, clearly indicating the presence of Eu3+ ions into SnO2 host lattice. Figure 3 shows the absorption spectra of Eu-doped SnO2 nanoparticles heated to different temperatures. The systematic red shift in the absorption edge with increasing temperature is obviously due to the quantum size effect of nanoparticles. The inset figure shows the systematic shift of absorption edge from pure SnO2 due to doping. For semiconductor nanoparticles, the quantum confinement effect is expected and the absorption onset appears red-shifted with increasing the particle size. The effective mass model is commonly used to study the size dependence of optical properties of QD systems. The shifting of the band gap energy is described by the following equation

Egeff ) Eg +

p2π2 2µR2

(3)

where R is the particle radius, µ is the effective reduced mass, and Eg is the bulk band gap energy (3.62 eV) and Eg eff is the effective band gap energy. As the effective mass of the electrons is much smaller than that of the holes (me* ) 0.27me), the charge carrier confinement mainly affects the energetic level of the electrons. Using the particle size estimation from XRD, the effective band gaps are 3.8, 3.7, and 3.6 eV for 200, 400, and 800 °C samples, respectively. The size dependence of the band gap energies of the quantum-confined SnO2 particles agrees very well with the confinement regime. The band gap energy Eg for SnO2 nanoparticles can also be determined by extrapolation to the zero absorption coefficients which is calculated using eq 2. The estimated band gap energies are 3.9, 3.8, and 3.6 eV for 200, 400, and 800 °C samples, respectively. There is a good agreement with the band gap energy obtained from particle size. The band gap decreases from 3.71 to 3.65 eV for pure and Eu3+doped SnO2 nanoparticles, respectively, indicating Eu3+ ions may play an important role on electrical property. Photoluminescence spectra are recorded for pure and Eu3+doped SnO2 samples prepared at 400 °C as shown in Figure 4. A blue emission peak at 420 nm is observed for pure SnO2 nanoparticle under excitation at 280 nm. The prominent band at 420 nm is attributed to the recombination of the deep trapped charged and photogenerated electron from the conduction band.3c Generally, oxygen vacancies are known to be the most common defects in oxides and usually act as radiative centers in luminescence processes. These oxygen vacancies generally act as deep defect donors in semiconductors and would cause the formation of new donor levels in the band gap. After excitation of the SnO2 nanoparticle, the electron is promoted from the valence band to conduction band, leaving a hole in the valence band. The hole can be trapped at the surface of the particle or in the oxygen vacancy site.3 Then the trapped hole may transfer back into particle to recombine with an electron of conduction band and give rise to the blue emission. In the case of Eudoped SnO2 sample, additional peaks at 612 nm (5D0 f 7F2)

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Figure 5. Emission spectra of Eu3+ ion in Eu3+-doped SnO2 nanocrystal at different temperatures with excitation wavelength 394 nm.

and 590 nm (5D0 f 7F1) are observed and the emission band to 5 D0 to 7F1 transition (at 590 nm) is more intense than 5D0 to 7F2 transition (615 nm). The 5D0 to 7F1 magnetic transition shows three well-resolved sharp lines with strong intensities, strongly suggesting that Eu3+ ions are embedded in SnO2 nanocrystals. The interband transitions of the SnO2 semiconductor are allowed. The presence of three components in the corresponding, 5D0 to 7 F1 emission spectrum indicates the existence of two sites for the Eu3+ ions, one would correspond to the substitution of Eu3+ to the Sn4+ ions in the C2h sites of SnO2, which is consistent with previous work.17,14 There are two possible channels of excitation in the Eu3+ fluorescence. One is direct excitation of the Eu3+ ion using 394 nm. The other is indirect excitation, i.e., excitation into the conduction band of SnO2 nanoparticle, followed by an energy transfer from SnO2 into the Eu3+ ions to cause the emission. It is already seen from Figure 4 that there are three well-resolved Stark splittings of 5D0 to 7F1 emission spectrum under indirect excitation (280 nm). This narrow sharp Stark splitting indicates Eu3+ ions in crystalline phase. Figure 5 shows the emission spectra of 1.0 mol % Eu3+ ion doped into SnO2 nanocrystals prepared at different temperatures. It is interesting to note that the Stark components in the emission spectrum get broader under direct excitation (394 nm), which is typical for spectra of Eu3+ ions in oxide glasses. In europium, the 5D0 f 7F1 transition is mainly magnetically allowed (a magnetic-dipole transition) while 5D0 f 7F2 is a hypersensitive forced electric-dipole transition being allowed only at low symmetries with no inversion center and it is very sensitive to the surroundings of the Eu3+ ion. Thus, the intensity ratio I(5D0 f 7F2)/I(5D0 f 7F1) serves as an effective spectroscopic probe of the site symmetry in which europium is situated, i.e., the higher the ratio, the lower the site symmetry. The calculated asymmetry ratios are 3.1 and 3.0 for 400 and 800 °C heated samples, respectively, which lie well within the range of values usually found in oxide glasses.14 It reveals that the Eu3+ ion occupies low symmetry sites. When the excitation wavelength is 394 nm, a glassy environment is found for the Eu3+ ions in the SnO2 nanocrystals. It may be due to Eu3+ ions residing close to the surface of the semiconductor nanoparticles. In this respect, the luminescence of Eu3+ depends critically on the locations of dopants in the host. Further investigation will be required for a deeper understanding of how interior and surface located Eu3+ ions influence the fluorescence properties. Again, it is interesting to note that the maximum PL intensity of Eu3+ ion is observed at 400 °C heated samples under excitation at 394 nm, i.e. direct excitation (Figure 5). The PL intensity then decreases with further increase of the temperature.29 XRD study confirms that a small impurity of Eu2Sn2O7 phase is obtained at 800 °C. It

Kar and Patra

Figure 6. Photoluminescence (PL) decays of 300, 400, and 800 °C heated of 1.0 mol % Eu3+-doped SnO2 nanocrystals prepared by microwave assisted method and monitored at the 5D0 f 7F2 transition (615 nm).

reveals that dopant ion coming out from the SnO2 crystal structure with increasing the temperature of heating and phase separation occurs and causes nonradiative relaxation. Figure 6 shows the photoluminescence (PL) decays of different temperatures of Eu-doped SnO2 nanocrystals, monitored at the 5D0 f 7F2 transition (615 nm). For all samples, biexponential decay is observed. The decay components are (τf) 181 µs (21%) and (τs) 665 µs (79%), and the average decay time is 560 µs for Eu3+-doped SnO2 nanoparticles prepared at 300 °C. Similarly the decay components are (τf) 971 µs (56.2%) and (τs) 360 µs (43.8%), and the average decay time is 700 µs for Eu3+-doped SnO2 nanoparticles prepared at 400 °C. The decay components are (τf) 336 µs (48%) and (τs) 967 µs (51.4%), and average decay time is 650 µs for Eu3+-doped SnO2 nanoparticles prepared at 800 °C. This result matches emission spectra. To gain more insight into the possible structural changes surrounding the Eu3+ ion, Judd-Ofelt parameters were calculated. The Judd-Ofelt parameter (Ω2) gives information on the intensities or nature of the hypersensitive transitions of the Eu3+ ion.30,31 The experimental intensity parameters (Ω2) were determined from the emission spectra for Eu3+ ion (Figure 5) based on the 5D0f 7F2 electric-dipole transition and the 5D0 f 7 F1 magnetic dipole transitions as the reference and they are estimated according to the equation32

A)

4e2ω3 1 χ 3pc3 2J + 1

∑ Ω2〈5D0|U(2)| 7F2〉2

(4)

where A0λ is the coefficient of spontaneous emission, e is the electronic charge, ω is the angular frequency of the transition, p is Planck’s constant, c is the velocity of light, χ is the Lorentz local field correction and is expressed as χ ) η(η2 + 2)2/9, where η is the refractive index of the sample which is experimentally determined, 〈5D0|U(2)|7F2〉2 are the squared reduced matrix elements whose values33 are independent of the chemical environment of the ion, and it is 0.0039 for J ) 2. Since the magnetic dipole 5D0 f 7F1 transition is relatively insensitive to the chemical environment around the Eu3+ ion, it can be considered as a reference for the whole spectrum and the coefficient of spontaneous emission was calculated according to the relation35

A0J ) A01(I0J /I01)(γ01 /γ0J)

(5) 5

where γ01 and γ0J are the energy baricenters of the D0 f 7F1 and 5D0 f 7F2 transitions, respectively. A01 is Einstein’s coefficient between 5D0 f 7F1 levels and it is calculated using

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Figure 8. Current vs voltage characteristics for pure and Eu3+-doped SnO2 nanocrystal when the voltage was applied between Au and Au terminals for forward bias direction. Figure 7. Plot of quantum efficiency (%) and nonradiative decay rate (ms-1) vs temperature (°C) of 1.0 mol % Eu3+-doped SnO2 nanocrystals at different temperatures.

A01 ) η3(A0-1)vac, where η is the refractive index of the sample (1.92 for SnO2) and (A0-1)vac ) 14.65 s-1. Radiative (Arad), nonradiative (Anrad) transition, and average decay time are related through the following equation

Atot ) 1/τ ) Arad + Anrad

(6)

32

whereas Arad can be expressed as

Arad ) A01

γ01 2 I0J ) I01 J)0 γ0J

∑

∑ A0J

(7)

J

As quantum efficiency is expressed as the ratio between the number of photons emitted by the Eu3+ ion and the number of photons absorbed by the Eu3+ ion and it is a balance between radiative and nonradiative process, quantum efficiency can be expressed as

η)

Arad Arad + Anrad

(8)

The Judd-Ofelt (JO) parameter (Ω2) is calculated by the above explained method. The values of the JO parameter (Ω2) are 13.88 × 10-20, 14.54 × 10-20, and 14.32 × 10-20 cm2 for 300, 400, and 800 °C heated samples under excitation at 394 nm. Liu et al.35 reported the JO intensity parameter (Ω2) is 12.39 × 10-20 cm2 for Eu3+ in Gd2O3 nanocrystals, which is nicely matched with our results. It also indicates that the Eu3+ environment in SnO2 is very similar to Gd2O3. The larger Ω2 value for the 400 °C heated sample suggests that Eu3+ ions reside at a more asymmetric environment, which is consistent with emission spectra and lifetime data. The calculated quantum efficiencies are 22.0%, 31.0%, and 26.0% for 300, 400, and 800 °C samples, respectively. Again, we calculate the radiative and nonradiative decay rate by using eqs 6 and 7, and it is found that there is little change in radiative decay rate but a change in nonradiative decay rate is observed (Table 1) which is shown in Figure 7. A minimum nonradiative decay rate is found for the 400 °C heated sample. Analysis suggests that the crystallite size plays important role on tuning the quantum efficiency, emission intensity, and decay time of Eu3+-doped SnO2 nanocrystals. Figure 8 shows different I-V characteristics for pure and Eu3+-doped SnO2 nanocrystals when the voltage was applied between Au and Au terminals for forward direction of applied voltage. Current versus voltage characteristics are asymmetric

Figure 9. Current vs voltage characteristics for pure and Eu3+-doped SnO2 nanocrystals when the voltage was applied between Au and Au terminals for reverse bias direction.

and nonlinear for forward direction of applied voltage, indicating the formation of Schottky barrier in the nanocomposite. An interesting feature, i.e., higher current at starting voltage (-20 V) for doped sample than pure sample, is also observed. It is known that intrinsic SnO2 is an n-type semiconductor with a work function of 4.5 eV, and the work function of Au is 5.1 eV. Thus, an Au electrode is made to facilitate the electron flow from SnO2 nanocrystals to the Au electrode since the Fermi level of the SnO2 nanoparticle is initially higher than that of the Au metal before the contact was made.23 This process results in the formation of an electron depletion region at the surface of SnO2 nanoparticles that corresponds to a Schottky barrier between the SnO2 nanoparticles and Au electrode. Therefore, the Au electrode forms Schottky barrier contacts with SnO2 nanoparticles. According to Ohm’s law, resistance (R) in doped sample is lower as current (I) is greater at starting voltage (-20 V). Therefore, the conductivity is found to be higher in doped sample than in pure SnO2 sample. Thus, a greater number of electrons can flow in doped sample. Therefore, the band gap in doped sample is lower than that in a pure sample, which matches with the calculated band gap from the UV measurement (Eg ) 3.65 eV (doped), Eg ) 3.71 eV (pure)). Figure 9 shows the I-V characteristics for pure and Eu3+doped SnO2 nanocrystals when the reverse bias voltage was applied (+20 to 0 V) between Au and Au terminals. Here, a typical rectifying behavior is observed in pure SnO2 sample, which is not observed in doped sample.23 The Schottky barrier is increased when the reverse-bias voltage is applied between the Au electrode and SnO2 nanoparticle (pure). Therefore, the electron flow from the SnO2 nanoparticle conduction band to

4380 J. Phys. Chem. C, Vol. 113, No. 11, 2009 the Au metal will be diminished or negligible. As a result, the rectifying behavior is observed from the Au-pure SnO2 nanoparticle-Au structure. In the case of Eu3+-doped SnO2 sample, it is believed that some of the Eu3+ ions substitute Sn4+ ions which lead to formation of oxygen vacancies. This oxygen vacancy center may trap electrons leading to electron accumulation. Therefore, there is negative charge accumulation (electron) in doped sample which increases the electron flow from the doped SnO2 nanoparticle conduction band to the Au metal and lost the rectifying properties. Conclusions In conclusion, chemical synthesis through a microwaveassisted method is a promising route for the preparation of pure and doped SnO2:Eu nanocrystals. The systematic shifting in the absorption edge is observed with increasing the heating temperature in the presence of dopant and the band gap decreases in the presence of Eu3+ ions. It is interesting to note that the maximum PL intensity and decay time of Eu3+ ions are observed at 400 °C samples. The larger Ω2 value for 400 °C samples suggests that Eu3+ ions reside at a more asymmetric environment which is consistent with emission spectra and lifetime data. The quantum efficiency value can be tuned with changing the particle size, and a minimum nonradiative decay rate is found at 400 °C. From the electrical study, the conductivity is found to be higher in doped sample than in pure SnO2 sample and a typical rectifying behavior is observed in pure SnO2 sample, which is not observed in doped sample. Acknowledgment. The Department of Science and Technology (NSTI) and “Ramanujan Fellowship” are gratefully acknowledged for financial support. A.K. thanks CSIR for awarding fellowship. The authors thank Professor A. J. Pal, Department of Solid State Physics, I.A.C.S., for providing electrical measurement facility and Mr. Asim Guchait for kind help. References and Notes (1) Wang, Y.; Jiang, X.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 16176. (2) Cheng, B.; Russell, J. M.; Shi, W.; Zhang, L.; Samulski, E. T. J. Am. Chem. Soc. 2004, 126, 5972. (3) (a) Liu, Y. X.; Yang, Q. B.; Xu, C. F. J. Appl. Phys. 2008, 104, 064701. (b) Zhu, J.; Lu, Z.; Aruna, S. T.; Aurbach, D.; Gedanken, A. Chem. Mater. 2000, 12, 2557. (c) Gu, F.; Wang, S. F.; Lu, M. K.; Zhou, G. J.; Xu, Dong; Yuan, D. R. J. Phys. Chem. B 2004, 108, 8119. (4) (a) Du, Y. P.; Zhang, Y. W.; Sun, L. D.; Yan, C. H. J. Phys. Chem. C 2008, 112, 12234. (b) Wang, Y.; Lee, J. Y.; Deivaraj, T. C. J. Phys. Chem. B 2004, 108, 13589. (c) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930. (d) Sun, J. Q.; Wang, J. S.; Wu, X. C.; Zhang, G. S.; Wei, J. Y.; Zhang, S. Q.; Li, H.; Chen, D. R. Cryst. Growth Des. 2006, 6, 1584.

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