Surface Defect-Related Luminescence Properties of SnO2 Nanorods

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Surface Defect-Related Luminescence Properties of SnO2 Nanorods and Nanoparticles Arik Kar, Simanta Kundu, and Amitava Patra* Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed: August 24, 2010; ReVised Manuscript ReceiVed: NoVember 23, 2010

We demonstrate the surface defect-related luminescence properties of SnO2 nanorods and nanoparticles using steady-state and time-resolved spectroscopy. Defect-related bands are identified by Raman and EPR spectroscopy. On the basis of the experimental results, we propose a schematic model for different relaxation processes in SnO2 nanocrystals upon photoexcitation. Analysis suggests that the visible emission of SnO2 nanocrystals is due to a transition of an electron from a level close to the conduction band edge to a deeply trapped hole in the bulk (V••0 ) of the SnO2 nanocrystals. Analysis suggests that the surface-related defects are more prominent in smaller nanocrystals than in nanorods. It is found that the PL emission and decay time strongly depend on the shape of the nanocrystals. This proposed model is further confirmed by time-resolved spectroscopy. 1. Introduction Interest in the fabrication of the nanostructures with tunable size and morphology is increasing because of their numerous potential applications in various areas in materials sciences,1 electronics,2 and optics.3,4 Tin oxide (SnO2) is an important metal-oxide, n-type wide band gap (3.6 eV at 300 K) semiconductor.5-7 Because of its outstanding electrical, optical, and electrochemical properties, SnO2 offers a wide range of applications in solar cells,8,9 catalytic support materials,10 transparent electrodes,11,12 and solid-state chemical sensors.13-17 These unique properties have stimulated the search for new synthetic methodologies for well-controlled SnO2 nanostructures. There are many different synthesis methods available in the literature such as discharge,18 laser ablation,19 solution,20 vapor-liquid-solid (VLS),21 hydrothermal reaction,22-26 etc., to prepare low-dimensional nanostructured SnO2 such as nanoparticles,27 nanorods,28 nanodisks,29 nanosheets,30 nanobelts,31 nanowires,32 etc. However, less attention has been paid to synthesizing SnO2 nanocrystals by microwave irradiation techniques. Here we use a microwave synthesis technique to prepare small SnO2 nanoparticles having a diameter of about 2-3 nm. Optical measurement such as photoluminescence (PL) is a very useful tool to determine the structure, defects, and impurity in the nanocrystals, and there have been several reports on the luminescence of SnO2 nanocrystals33-37 for which the luminescence is generally observed in the range 350-550 nm (UV and visible region both). The luminescence is generally believed to come from defects such as oxygen vacancies and tin interstitial or dangling bonds.7,38-42 Similarly, defect-related luminescence of ZnO nanoparticles has been reported.43-45 It is believed that generation of defects in nanocrystals vary with the shape of nanocrystals, which would influence the emission properties. Feng Gu et al.33 have proposed a model for blue emission from SnO2 nanocrystals. The main goal of our work is to synthesize different shapes of SnO2 and study their defect-related luminescence properties. Here we use a microwave irradiation method and hydrothermal method to control nanostructured materials. Reports are available in the literature on the shape* Author to whom correspondence should be addressed. E-mail: [email protected]. Phone: (91)-33-2473-4971. Fax: (91)-33-2473-2805.

dependent luminescence properties of SnO2 nanoparticles.25,46 In the present study, we address an important issue: how the shapes of SnO2 nanocrystals influence the surface defects and their luminescence properties. On the basis of this experimental finding, we propose a model to describe different relaxation processes in SnO2 nanocrystals and try to correlate them with decay dynamics data by time-resolved spectroscopy. 2. Experimental Section a. Synthesis of SnO2 Nanorods by a Hydrothermal Method. A 1.5 mmol (525 mg) amount of SnCl4 · 5H2O was dissolved in 10 mL of water in a beaker. About 700 mg of NaOH was also dissolved in 20 mL of water, and then 20 mL of ethanol (Merck) was added to this solution 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 began to form in the acidic pH range. The white suspension slowly disappeared with increasing pH. After the pH value reached 11-12, a clear solution was obtained. This clear solution (pH ) 12.0) was then placed in a Teflon-lined steel chamber of 110 mL capacity. Finally, this steel chamber was placed inside a box in a furnace and was heated at 180 °C for 12 h. After the reaction was complete, the steel chamber was air-cooled to room temperature. The products were then collected and washed with distilled water and ethanol several times to remove the impurities. The obtained white powdery product was then dried in an oven overnight at 60 °C to obtain the final white powder products. b. Synthesis of Small SnO2 Nanoparticles by a Microwave Irradiation Method. A 0.375 mmol (134 mg) amount of tin(IV) acetate was dissolved in 2.2 mL of oleic acid and 3.6 mL of oleylamine in a 10 mL beaker. Then the beaker was heated at 110 °C in a heating mantle for 5 min under vigorous stirring. After heating, the tin(IV) acetate was dissolved to make a yellowish viscous solution. After cooling at room temperature, this yellow viscous suspension was transferred to a microwave tube of 10 mL capacity. Then the tube was inserted in a monomode microwave reactor (CEM, Discover) with a power setting of 300 W at 120 °C for 5 min. After microwaving for the desired time, the resulting solution was diluted in chloroform after being cooled to room temperature. Then ethanol was added

10.1021/jp110313b  2011 American Chemical Society Published on Web 12/13/2010

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SCHEME 1: Synthetic Scheme for the Preparation of SnO2 Nanorods and Nanoparticles

to this clear yellow solution. A white precipitate was formed which was again dissolved in chloroform and reprecipitated again by adding ethanol. Finally the white precipitate was centrifuged and washed several times by ethanol. The obtained product was then dried in an oven for overnight at 60 °C to get the final grayish white powdery product. The reactions that occur during the synthesis of SnO2 nanorods are as follows:

SnCl4 · 5H2O + NaOH f Sn(OH)4 + NaCl + 5H2O Sn(OH)4 + 2NaOH f Na2SnO3 + 3H2O Na2SnO3 + H2O f SnO2 + 2NaOH

(1)

Here a mixed (water-alcohol) solvent is used for the formation of nanorods. This solvent mixture influences the morphology of the final product. The presence of ethanol in water reduces the reaction rate and steady formation of the SnO2 nuclei. In the microwave irradiation technique, temperature and time are important parameters to control the size of particle. We obtained small nanoparticles at 120 °C when the time duration was 5 min. Smaller nanorods were obtained when the temperature was higher than 120 °C and the time duration was 10 min. The formation of nanoparticles is due to the uniform growth environment and homogeneous heating due to microwave irradiation. Thus, very small, fine, and discrete particles can be prepared by this microwave irradiation method. A schematic representation for preparation for SnO2 nanorods and nanoparticles is given in Scheme 1. c. Characterization. The crystalline phases of the prepared powders were identified by X-ray diffraction (XRD) using a Philips model PW-1730 powder X-ray diffractometer using a Cu KR source (1.5418 Å radiation) and a longer scan time, 0.5 s per scan (slow scan). Crystallite sizes (D, Å) were estimated from Scherrer’s equation,

D ) Kλ/β cos θ

(2)

where K is the shape factor, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (fwhm) in radians, and θ is the Bragg angle. D is the mean size of the ordered (crystalline) domains, which may be smaller or equal to the grain size. The dimensionless shape factor has a typical value of about 0.9, which varies with the actual shape of the crystallite. The Scherrer equation is limited to nanoscale particles. It is not applicable to grains larger than about 0.1 µm, which precludes those observed in most metallographic and ceramographic microstructures. It is important to realize that the Scherrer formula provides a lower bound on the particle size. A variety of factors (particle size, inhomogeneous strain, and instrumental errors) can contribute to the width of a diffraction peak. If the contributions of all these factors are zero, then the peak width would be determined solely by the particle size using the Scherrer formula. Generally, the broadenings of the diffraction peaks depend upon two predominant factors, that is, strain and particle size. We calculate the strain using the Williamson and Hall theorem47

β cos θ/λ ) 1/D + η sin θ/λ

(3)

where β is the full width at half-maximum (fwhm), θ is the diffraction angle, λ is the X-ray wavelength, D is the effective particle size, and η is the effective strain. The strain is measured from the slope, and the crystallite size (D) is measured from the intercept of a plot of β cos θ/λ against sin θ/λ. Microstructural characterizations of the SnO2 nanorods and nanoparticles were carried out by scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM, JEOL, JSM-6700F). Transmission electron microscopy (TEM, JEOL Model 200) was also used to study the morphology and particle size of the resulting powders. The Raman spectrum was produced at room temperature on a LabRam HR Raman spectrometer with a helium-neon laser at an excitation wavelength of 514 nm. The UV-vis spectra were taken in UV-2401 PC Shimadzu spectrophotometer. The emission spectra of SnO2 nanocrystals were measured using a fluoro Max-P (Horiba Jobin Yvon) luminescence spectrometer. All measurements were

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Figure 1. X-ray powder diffraction patterns of SnO2 nanoparticles and nanorods prepared by the microwave-assisted method and the hydrothermal method.

Figure 3. SEM and TEM micrographs of (a, c) SnO2 nanorods prepared by the hydrothermal method and (b, d) SnO2 nanoparticles prepared by the microwave-assisted method.

Figure 2. Plot of β cos θ/λ against sin θ/λ for (a) SnO2 nanorods prepared under hydrothermal condition and (b) SnO2 nanoparticles prepared by the microwave-assisted method.

performed at room temperature, using a solid sample holder. All samples were excited at 295 nm, under the same conditions. For the time-correlated single-photon counting (TCSPC) measurement, the samples were excited at 295 nm using a picosecond NANO-LED IBH 295. 3. Results and Discussion Figure 1 depicts the XRD patterns of the as-synthesized SnO2 nanoparticle and SnO2 nanorod samples. All the diffraction peaks can be readily indexed to the tetragonal phase of SnO2 with calculated lattice parameters a ) 4.707, c ) 3.167 for the SnO2 nanoparticle and a ) 4.735, c ) 3.175 for the SnO2 nanorod. All are in good agreement with the reported values (JCPDS card no. 41-1445) having space group P42/mnm. It is clearly seen from Figure 1 that SnO2 nanorods are highly crystalline. The diffraction peaks of SnO2 nanoparticles prepared under microwave irradiation are very broad, indicating the smaller particle size compared to SnO2 nanorods. The sizes of the prepared nanocrystals were determined from Scherrer’s equation (eq 2) from the (110), (101), (112), and (211) peaks. The average crystallite size of SnO2 nanoparticles prepared by the microwave irradiation technique is about 2.8 nm, which is in good agreement with the value obtained from the TEM images. Figure 2a shows the plot of β cos θ/λ against sin θ/λ for large size SnO2 nanorods prepared under hydrothermal conditions. The slope value is -3.8%, which indicates the compressive strain. The calculated crystallite size is 28.2 nm,

which is close to the calculated value (26.6 nm) from Scherrer’s equation. Figure 2b shows the plot of β cos θ/λ against sin θ/λ for SnO2 nanoparticles prepared by microwave irradiation. The reversal of lattice strain is observed for this sample. Here we obtain a positive slope (+0.055), which suggests the presence of tensile strain (+5.5%), and the crystallite size is 2.5 nm (from intercept), which is in good agreement with the calculated value (2.8 nm) from Scherrer’s equation. In our previous work, we demonstrated the impact of lattice strain on the luminescence properties of rare-earth-doped nanomaterials and core/shell nanomaterials.48 Nie et al.49 recently reported the influence of lattice strain on the optical properties of nanocrystals. According to them, much higher strain can be tolerated in small nanocrystals than in their bulk counterpart. Therefore, the lattice strain of SnO2 nanocrystals can be tuned by changing the shape of the prepared products, which is an important observation in this study. Figure 3a and 3c depicts the SEM and TEM images of hydrothermal-derived SnO2 nanorods, respectively. Here the average diameter is calculated to be 30 nm and the length is about 200 nm. Figure 3b and 3d shows the SEM and TEM images of microwave-derived SnO2 nanoparticles, respectively. Here the average size of the particles is approximately 3-4 nm, and they are discrete in nature. 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:

Eeff g ) Eg +

p2π2 2µR2

(4)

where R is the particle radius, µ is the effective reduced mass, Eg is the bulk band gap energy (3.60 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

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Figure 4. UV-vis spectra of SnO2 nanoparticles and nanorods prepared by the microwave-assisted method and hydrothermal method. [Inset, (Rhν)2 vs photon energy plot of the respective samples.]

Figure 5. Raman spectra of SnO2 nanoparticles (a), nanorods (b), and bulk SnO2 (c).

effective band gaps are 3.60 and 3.88 eV for SnO2 nanorods and SnO2 nanoparticles, 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 are calculated using the following equation:

R ) κ(hυ - Eg)n/hν

(5)

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 (Rhν)2 versus photon energy. The estimated band gap energies are 3.61 and 3.87 eV for the abovementioned samples, respectively (Figure 4). There is a good agreement with the band gap energy obtained from particle size. The band gap decreases from 3.87 to 3.61 eV for small to large SnO2 nanocrystals, indicating that microwave-synthesized tiny SnO2 nanoparticles have some discrete properties compared to large SnO2 nanorods. Raman spectroscopy is an important spectroscopic tool to reveal the surface-related defects of nanocrystals.14 Here the SnO2 nanocrystal is rutile in nature and belongs to the space group D4h, of which the normal lattice vibration at the Γ point of the Brillouin Zone is given by the following equation:50

Γ ) 1A1g + 1A2g + 1A2u + 1B1g + 1B2g + 2B1u + 1Eg + 3Eu (6)

Figure 6. Deconvolution fitting plots of the Raman spectra from (a) SnO2 nanorods prepared under hydrothermal condition and (b) SnO2 nanoparticles prepared by the microwave-assisted method.

Among them, B1 g, Eg, A1g, and B2g are the Raman active modes. Figure 5 shows the room temperature Raman spectra of (a) as-synthesized SnO2 nanoparticles, (b) as-synthesized SnO2 nanorods, and (c) bulk SnO2 for comparison. The Raman spectrum of as-synthesized SnO2 nanorods (Figure 5b) shows the presence of four peaks at 474 cm-1, 570 cm-1, 627 cm-1, and 768 cm-1. Among the peaks, Raman peaks at 474, 627, and 768 cm-1 corresponding to the Eg, A1g, and B2g vibration modes, respectively, are observed in good agreement with those for the rutile bulk SnO2 (Figure 5c). However, the Raman peak at 570 cm-1 is not detected in the bulk rutile SnO2. The band at 575 cm-1 is assigned to surface defects of the SnO2

nanocrystals.14 For SnO2 nanoparticles, the Raman bands at 480 cm-1 (which has very low intensity but can be seen in the deconvoluted spectrum in Figure 6b), 623 cm-1, and 762 cm-1 are assigned as Eg, A1g, and B2g vibration modes, respectively (Figure 5a), and the Raman peak at 577 cm-1 is due to surface defects. When the spectrum of bulk SnO2 is compared, a small shift in the position of the Raman bands is observed. The broadening of the peaks indicates the amorphous and nanocrystalline nature of the prepared samples. It is important to note from Figure 5 that the surface-related defect band at 575 cm-1 of the SnO2 nanoparticles is higher than that of the SnO2 nanorods, which is due to larger surface area of the small SnO2

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Kar et al. SCHEME 2: Schematic Model Proposed for Different Relaxation Processes in SnO2 Nanocrystals

Figure 7. Deconvolution fitting plots of the emission spectra from (a) SnO2 nanorods prepared under hydrothermal condition and (b) SnO2 nanoparticles prepared by the microwave-assisted method.

nanoparticles. After deconvolution these spectra, we have seen the distinct peak position and the relative intensities of the peaks (Figure 6a and 6b). Figure 6a shows the deconvoluted Raman spectrum of the SnO2 nanorods. It is seen from Figure 6a that the relative contribution of the defect-related peak (575 cm-1) is small compared to the main Raman modes for the SnO2 nanorods because of smaller surface area. However, the relative contribution of the defect-related peak at 575 cm-1 is large compared to the main Raman modes for SnO2 nanoparticles (Figure 6b) because of the larger surface area of the smaller size SnO2 nanoparticles. As a result, the surface-related band is prominent. Analysis suggests that surface-related defects are more prominent in nanocrystals than in nanorods. Figure 7a is the photoluminescence (PL) spectrum of the SnO2 nanorod under excitation at 295 nm. The spectrum is broad and asymmetric with the PL peak centered at approximately 400 nm. Deconvolution of this broad PL peak reveals three PL peaks at 371, 400, and 430 nm. The PL band at 371 nm is not due to band edge emission because it is not matched with the effective band gap calculated from UV-vis spectra. According to Luo et al.,46 the violet UV emission may be due to some near band edge emission. Dai et al.51 have observed the same near band edge emission from T-ZnO nanorods. Therefore, on the basis of previous works, we ascribed the 371 nm PL band as near band edge emission. The other peak at 430 nm could be ascribed as the visible emission (blue). The UV and visible emission are competing with each other. SnO2 nanocrystals with large size and nearly perfect crystalline structure generally show stronger UV emission. The SnO2 nanorods prepared under hydrothermal conditions are very large in size (26.6 nm) and perfectly crystalline (evidence from TEM and XRD analysis),

and the intensity ratio of the UV emission to visible emission is very high. The sample exhibits stronger UV emission and emits ultraviolet light in theUV region (inset of Figure 7a). Figure 7b represents the PL spectrum of the SnO2 nanoparticles prepared by microwave irradiation. A small red-shifted band and an asymmetric emission peak centered at approximately 420 nm is observed. The deconvolution of this broad peak reveals three emission peaks at 382, 415, and 470 nm. The PL band at 382 nm is ascribed as UV emission, and the peak at 470 nm is ascribed as visible emission. The intensity ratio of the UV emission to the visible emission is lower, which suggests a stronger visible emission (inset of Figure 7b). To gain more insight regarding the PL properties of different shaped SnO2 nanocrystals, we propose a schematic model for the different relaxation processes of photoexcited SnO2 nanocrystals (Scheme 2). In the crystal lattice of SnO2, some O2ions can escape from the host lattice, leading to formation of oxygen vacancies (defects).37,39 This oxygen vacancy center can trap an electron, leading to formation of a Vo• state which can be identified by EPR study (discussed later). There are three consequent energy levels in the model (Scheme 2), viz., valence band (VB), defect states (Vo•/V0••), and conduction band (CB). At the surface of the particle, an energy distribution of an Os2-/ Os1- system is also shown (Scheme 2). The excitation of the SnO2 nanocrystals first creates positive holes in the valence band and negative electrons in the conduction band. Next, three types of relaxation processes can occur: (1) A hole in the VB and an electron in the CB can radiatively recombine to give ultraviolet emission (UV region), which is generally assigned as band edge emission for SnO2 nanocrystals.46 For bulk SnO2, the band gap is 3.6 eV. Therefore, this transition is assigned as perfect band edge emission (left portion in Scheme 2). As the effective band gap of our prepared product is not exactly 3.6 eV, therefore, we use the term ‘near band edge emission’ according to previous works.46,51,52 The emission is in the UV region (371 and 382 nm for nanorods and nanoparticles, respectively) (Figure 7). (2) The second possibility is that the hole in the VB can trap at the oxygen vacancy sites. This surface traps hole tunnels back into the particle to recombine with an electron in a deep trap Vo• center to form the V0•• center at the surface of the particle. Then recombination of a V0•• center with an electron in the CB

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Figure 8. EPR spectra of SnO2 nanoparticles and nanorods prepared by the microwave-assisted method and hydrothermal method.

gives rise to trap emission in visible blue light. This emission is in the visible region (430 and 470 nm for nanorods and nanoparticles, respectively) (Figure 7). (3) The third possibility is that both the hole and the electron can trap in the oxygen vacancy sites at the surface of the SnO2 nanocrystals. Then, the electron and the hole can nonradiatively recombine at the surface, leading to nonradiative (NR) emission. The electron paramagnetic resonance (EPR) measurement of the prepared SnO2 nanocrystals also supports our proposed model (Figure 8). The following equation is used to calculate the g factor:

g ) hγ/BH

(7)

where H is the static field (gauss), γ is the frequency (Hz), B is the Bohr magnetron equal to 9.274 × 10-21 erg/Gauss, and h is Planck’s constant, 6.626 × 10-27 erg-s/cycle. An EPR signal with g ) 2.25 appears for the SnO2 nanoparticle and SnO2 nanorod, suggesting both SnO2 nanocrystals possess the same type of paramagnetic centers, i.e., singly ionized oxygen vacancies (Vo•). These are assumed to be the recombination centers for the luminescence processes. However, the weaker relative intensity of the EPR signal in nanorods is due to the decrease in the concentration of paramagnetic centers. It is possible that the paramagnetic centers detected by EPR signal can be the origin of visible emission. Microwave-synthesized small SnO2 nanoparticles have alot of defects due to their high surface area and thus have greater numbers of paramagnetic centers. Therefore, visible emission is more prominent than UV emission, and this result matches with the emission spectra and also with our proposed model (Scheme 2). Thus, the visible emission (trap emission) is directly associated with the paramagnetic centers detected by EPR. Small SnO2 nanoparticles have alot of defects, such as oxygen vacancies, due to large surface area. Therefore, the trap emission increases in the visible region. However, SnO2 nanorods due to less surface defects are supposed to give higher band edge violet emission. The photoluminescence emission data exactly matches with our proposed model. Figure 9a depicts the time-resolved PL decay curves for SnO2 nanorods prepared under hydrothermal conditions at two emission wavelength peaks, 371 and 430 nm, and excited at 295 nm wavelength by a nanoLED-295 laser. The decay part could be fitted into a triexponential. The fast decay component on the order of picoseconds (0.018 ns) cannot be detected because of lack of facility. Thus, for analysis we selected a region where

Figure 9. Photoluminescence (PL) decay of SnO2 nanorods and SnO2 nanoparticles at their two different emission wavelengths (inset is the same decay time plot, for only the nanosecond region).

the lifetime is in the range of nanoseconds (inset plot of Figure 9a). The decay curve is fitted into a biexponential for the UV emission (371 nm). The two decay times are on the order of 1.22 ns (99%) and 10.28 ns (1%) for the UV emission (371 nm). For the visible emission, the decay is comparatively slow, and it is fitted into a triexponential. The three decay times are on the order of 2.36 ns (96%), 9.89 ns (3%), and 82.71 ns (1%) for the visible emission (430 nm). The fast decay component is due to the radiative recombination process proposed in our model, and the slowest part generally comes from the defectrelated emission. Figure 9b is the decay time analysis of the SnO2 nanoparticles. The decay components are 2.13 ns (98.4%) and 10.83 ns (1.6%) for the UV emission (382 nm). For the visible emission, the decay is very slow, and it is fitted into a triexponential. The decay components are 1.66 ns (53%), 7.97 ns (41%), and 93.0 ns (6.0%) for the visible emission (470 nm). When the decay time of the visible emission for both samples was compared, we found that the decay time value for the SnO2 nanoparticles was larger than that for the SnO2 nanorods. This is definitely due to the prominent trap emission in the case of SnO2 nanoparticles as proposed in our model. Therefore, we can say that the PL emission and decay time depend on the shapes of the nanocrystals. The lower value of decay time is due to radiative recombination of the electron and hole whereas the higher value indicates the trap emission. All these results match well with our PL data (Figure 7a and 7b). From the decay time measurement, we calculated the decay time ratio of visible and UV emission and found that the value is 2.25 for SnO2 nanorods and 4.11 for SnO2 nanoparticles. Thus, we conclude

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that visible emission is prominent for small SnO2 nanoparticles and UV emission is prominent for large SnO2 nanorods. 4. Conclusion We have successfully synthesized SnO2 nanorods and nanoparticles by hydrothermal and microwave methods and studied their luminescence properties by steady-state and time-resolved spectroscopy. Both samples give a broad emission peak and two emission bands; one UV band and a visible band, which is assigned to trap emission, are found after deconvoluting this band. The shifting of the Raman vibration bands depends on the particle size and shape. The lattice strain of the SnO2 nanocrystals varies with the change in shape of the crystal. To understand the luminescence properties of different shaped nanocrystals, we propose a model for different relaxation processes in SnO2 nanocrystals upon photoexcitation. The model reveals that the visible emission (trap emission) is directly associated with the paramagnetic centers, which is detected by EPR. Small SnO2 nanoparticles have alot of defects such as oxygen vacancies due to large surface area. Thus, the trap emission increases in the visible region. However, SnO2 nanorods are supposed to give prominent violet emission due to less surface defects. Photoluminescence emission data match well with our proposed model. A corelation between the EPR signal g ) 2.25 and defect state Vo• has been found in our proposed model. From the decay time measurement, it is evident that the UV and visible bands of the SnO2 nanocrystals depend on the shape of the nanocrystals. Acknowledgment. The DST, CSIR, and “Ramanujan Fellowship” are gratefully acknowledged for financial support. We give special thanks to Quyen Nguyen and HORIBA Jobin Yvon S.A.S Raman division for Raman measurements of our samples. A.K. and S.K. also thank CSIR for awarding a fellowship. References and Notes (1) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D.; Lieber, C. M. Nature 2002, 415, 617. (2) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (3) Sun, X. M.; Li, Y. D. Angew. Chem., Int. Ed. 2004, 43, 3827. (4) Tang, Z. Y.; Wang, Y.; Shanbhag, S.; Giersig, M.; Kotov, N. A. J. Am. Chem. Soc. 2006, 128, 6730. (5) Kar, A.; Patra, A. J. Phys. Chem C. 2009, 113, 4375. (6) Xu, X.; Zhuang, J.; Wang, X. J. Am. Chem. Soc. 2008, 130, 12527. (7) Lu, E. J. H.; Ribeiro, C.; Giraldi, T. R.; Longo, E.; Leite, E. R.; Varela, J. A. Appl. Phys. Lett. 2004, 84, 1745. (8) Harrison, P. G.; Willet, M. J. Nature 1988, 332, 337. (9) Ferrere, S.; Zaban, A.; Gregg, B. A. J. Phys. Chem B 1997, 101, 4490. (10) Wang, W. W.; Zhu, Y. J.; Yang, L. X. AdV. Funct. Mater. 2007, 17, 50. (11) Zhu, J.; Lu, Z.; Aruna, S. T.; Aurbach, D.; Gedanken, A. Chem. Mater. 2000, 12, 2557. (12) He, Y. S.; Campbell, J. C.; Murphy, R. C.; Arendt, M. F.; Swinnea, J. S. J. Mater. Res. 1993, 8, 3131. (13) Wang, Y.; Jiang, X.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 16176. (14) Xi, G.; Ye, J. Inorg. Chem. 2010, 49, 2302. (15) Chen, Y. J.; Xue, X. Y.; Wang, Y. G.; Wang, T. H. Appl. Phys. Lett. 2005, 87, 233503. (16) Liu, Z.; Zhang, D.; Han, S.; Li, C.; Tang, T.; Jin, W.; Liu, X.; Zhou, C. AdV. Mater. 2003, 15, 1754.

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