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J. Phys. Chem. C 2007, 111, 9060-9065
Influence of Surfactant Micelles on Morphology and Photoluminescence of Zinc Oxide Nanorods Prepared by One-Step Chemical Synthesis in Aqueous Solution Hiroyuki Usui* Venture Business Laboratory (VBL), Kobe UniVersity, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan ReceiVed: February 19, 2007; In Final Form: April 17, 2007
Single-crystalline ZnO nanorods with different morphologies of radial-shaped needles and single needle were prepared by one-step chemical synthesis in aqueous solutions with and without the surfactant sodium dodecyl sulfate (SDS). The ZnO nanorods prepared in the SDS solution have narrower average width, smaller aspect ratio, and wider optical band gap of about 3.36 eV compared with those prepared in water. In both cases, ZnO nanorods exhibit visible photoluminescence by various lattice defects. These results indicate that a confinement of growing ZnO crystals by spherical micelles leads to more uniform growth, and thus the ZnO nanorods in the SDS solution have narrower average width and more uniform morphology than those in water. It is suggested that a band gap narrowing does not occur for ZnO nanorods in SDS solution because dodecyl sulfate ions effectively passivate lattice defects of neutral oxygen vacancies which produce shallow donor levels.
1. Introduction Zinc oxide (ZnO) is a versatile material of compound semiconductors with excellent properties and extensive applications in electronics, photoelectronics, sensors, and catalyses. The remarkable properties of ZnO are its wide direct-band gap of 3.37 eV1,2 and high-exciton-binding energy of 60 mV at room temperature.3 Nanomaterials of ZnO with one-dimensional structures such as nanowires,4 nanowhiskers,5,6 nanotubes,7 nanobelts,8 and nanorods9,10 have been intensively studied for applications of opto-electronic devices in nanoscale because the optical and electronic properties in these nanostructures are enhanced by the quantum confinement effect.11 These days, many synthetic techniques using vapor-phase deposition such as thermal evaporation,7,12 molecular beam epitaxy,4,13 plasmaenhanced chemical vapor deposition,14 and pulsed laser ablation2,15 have been used to fabricate one-dimensional ZnO nanomaterials. However, these techniques require multiple steps for purification of products, high-temperature conditions, and special equipments including laser and vacuum chamber. A simple and one-step synthetic technique should be developed for a mass-production of one-dimensional ZnO nanomaterials. The chemical synthesis of ZnO nanorods with a one-step process was most recently found to be attainable in basic aqueous solutions containing Zn(OH)42- ions and anionic surfactants under a mild condition of low temperatures below 90 °C without any special equipment.16,17 It is suggested that species of ZnO nanorods are surrounded by micelles of the surfactant molecules in the aqueous solutions and that the micelles act as a rod-shaped nanoreactor for ZnO.18,19 However, well-dispersed ZnO nanorods have been reported to form even in an aqueous solution without surfactant.20 This result indicates that a template consisted of surfactant molecules is not necessary for a formation of ZnO nanorods. Thus, there is still a controversial matter with a growth process of ZnO nanorods in this technique. Morphology and photoluminescence of nano* To whom correspondence should be addressed. E-mail: usui@ denjiken.ne.jp. Phone: +81-22-245-8027. Fax: +81-22-245-8031.
materials are strongly influenced by growth process and crystal structure of the nanomaterials. Therefore, an investigation of these properties should be very helpful for understanding the formation mechanism of the ZnO nanorods in the chemical synthesis. In this study, the author directly prepared ZnO nanorods by the one-step chemical synthesis in a basic aqueous solution with and without an anionic surfactant, and investigated the morphology, crystal structure, and photoluminescence of the ZnO nanorods to discuss an influence of surfactant micelles on the morphology and photoluminescence of ZnO nanorods. 2. Experimental Section Aqueous solutions used in this experiment are 0.31 mol/L zinc sulfate heptahydrate (ZnSO4‚7H2O) with the volume of 32.5mL for a source of Zn2+ ions, 4.0 mol/L sodium hydroxide (NaOH) with 15 mL for an adjustment of pH value, and 0.2 mol/L sodium dodecyl sulfate (SDS, C12H25SO4Na) with 2.5 mL. The amount of SDS surfactant was set so that a nominal SDS concentration of 1.0 × 10-2 mol/L in a mixed solution with a total volume of 50 mL exceeds its critical micelle concentration of 0.81 × 10-2 mol/L.21 All reagents were analytical grade and were used without further purification in the experiment. First, the transparent solutions of ZnSO4‚7H2O and NaOH were mixed in an open flask, and the SDS solution or distilled water of 2.5 mL were added in the mixed solutions under a vigorous stirring. The pH value of the mixed solution was around 13.6. Next, the mixed solutions were stirred for 60 min at the temperature of 5 °C, and subsequently were stirred for 90 min at room temperature. After that, the mixed solutions were stirred for 5 h at 85 °C in a sealed flask, and then milkywhite colloidal suspensions were obtained. To remove the surfactant molecules from the products prepared in SDS solution, the colloidal suspensions were centrifuged for 30 min at 2500 rpm and were washed with distilled water, which was repeated several times. The centrifuge and washing were performed at the same conditions for products prepared in water solution for comparison.
10.1021/jp071388o CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007
ZnO Nanorods Prepared by One-Step Chemical Synthesis
Figure 1. TEM images of ZnO nanorods prepared in water without surfactant at (a) lower and (b) higher magnifications. The directions of the major axis in all nanorods are parallel to the [0001] direction. (c) The corresponding selected area electron diffraction pattern of ZnO nanorods is shown in (b).
The washed colloidal suspensions were dropped on a copper grid covered by amorphous carbon film for specimens of a transmission electron microscopic (TEM) observation. The microscope used (Hitachi H-7000) was operated at an accelerating voltage of 125 kV. The morphology, size, and crystal structure of the products were investigated by bright field image and selected area electron diffraction (SAED). The camera length of the microscope was calibrated by an observation of a single-crystalline silicon wafer as a reference specimen at the same objective lens setting to determine an accurate lattice constant of the products by SAED patterns. The chemical composition of the products was studied by an energy dispersive X-ray spectroscopy (EDX) analysis. The photoluminescence spectra and photoluminescence excitation spectra of the washed colloidal suspensions were measured by a fluorophotometer (Jasco FP-6600) at the excitation wavelength of 320 nm (3.88 eV) and 370 nm (3.35 eV) and the emission wavelength of 551 nm (2.25 eV). The measurements of photoluminescence and photoluminescence excitation spectra were carried out at room temperature without a cutoff filter. 3. Results 3.1. Morphology and Crystal Structure. Figure 1a,b depicts typical TEM images of the products prepared in water without SDS surfactant at lower and higher magnifications. Nanorods with the morphology of single straight needle were observed on the amorphous carbon film. The average width of nanorods is 32.2 nm. The nanorods have sharp edges as shown in Figure 1b. Figure 1c reveals the corresponding SAED pattern of the nanorods shown in Figure 1b. A transmission electron diffraction pattern from a single crystal can be obviously recognized. The diffraction pattern can be indexed as the [011h0] zone axis pattern of a hexagonal closed-pack structure. The lattice constants of
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9061
Figure 2. TEM images of ZnO nanorods prepared in an aqueous solution of SDS surfactant at (a) lower and (b) higher magnifications. The directions of the rod axis are parallel to the direction of [0001] and mainly radiated toward the center of the radial-shape aggregates. (c) The corresponding selected area electron diffraction pattern of ZnO nanorods is shown in (b).
the nanorods estimated from the pattern was confirmed to coincide with those of wurtzite ZnO (P63mc, JCPDS No. 361451, a ) 0.325 nm, c ) 0.521 nm). Some diffuse streaks around the diffraction spots were observed along reciprocal [2h110] and [21h1h0] directions. The directions of major axes in all nanorods are parallel to the [0001] direction of each ZnO crystal. Figure 2a,b shows typical TEM images of the products prepared in the solution of SDS surfactant at lower and higher magnifications. Aggregated nanorods were observed, and the edges of almost all the nanorods were shared with other nanorods. A unique morphology of radial-shaped needles is recognized as shown in Figure 2a. The nanorods exhibit a narrower average width of 22.1 nm compared with that prepared in water. The nanorods prepared in SDS solution also have sharp edges as shown in Figure 2b. Figure 2c presents the corresponding SAED pattern of the nanorods shown in Figure 2b. It is confirmed that the pattern is the [011h0] zone axis pattern of the hexagonal closed-pack structure, and the lattice constants of the nanorods correspond with those of the wurtzite ZnO as in the case of the nanorods prepared in water. The diffuse streaks appeared around the diffraction spots along the reciprocal [2h110] and [21h1h0] directions. The directions of rod axis were parallel to the direction of [0001], and mainly radiated outward from the center of the radial-shaped needles. As a result of EDX analysis, it was confirmed that the nanorods consist of only oxygen and zinc, and that impurities such as sodium and sulfur are not detected in the nanorods. Therefore, these results clearly reveal that nanorods of singlecrystalline ZnO with the wurtzite structure are formed both in water and in SDS solution. The diffuse streaks along the reciprocal [2h110] and [21h1h0] directions indicate that the ZnO
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Figure 4. Photoluminescence spectra measured at room temperature for colloidal suspensions of ZnO nanorods prepared in water and the SDS solution. The wavelength of excitation light is 320 nm (3.88 eV). The visible photoluminescence peaks are recognized in orange-green, blue, and violet light regions at around 2.0∼2.3, 2.67, and 2.95 eV, respectively. Another blue photoluminescence peaks with lower intensity appear at around 2.76 eV. The UV photoluminescence can be recognized at ∼3.05 and ∼3.15 eV in water and the SDS solution.
Figure 3. Aspect ratio distributions of ZnO nanorods prepared in (a) water and (b) SDS solution. The aspect ratio was defined as a ratio of the length to width measured as described by insets. The average width and aspect ratio are 32.2 nm and 25.4 in water and those are 22.1 nm and 8.3 in the SDS solution.
nanorods have dense stacking faults that are parallel with (2h110) and (21h1h0) planes. These stacking faults are suggested to produce high-density point defects in ZnO crystal.22 The aspect ratio of ZnO nanorods was defined as a ratio of the length to the width measured from the bright field images of TEM observations. In nanorods with the morphology of the radial-shaped needles, the length was measured between the center of the aggregate and the outer edge of the nanorod. Figure 3 compares the aspect ratio distributions of the ZnO nanorods prepared in water and the SDS solution. The ZnO nanorods prepared in water have a much broader distribution of aspect ratio compared with that prepared in the SDS solution even though the average width of 32.2 nm in the case of water is not so drastically wider than that of 22.1 nm in case of SDS solution. The average aspect ratios of the ZnO nanorods prepared in water and SDS solution are 25.4 and 8.3, respectively. These results reveal that the ZnO nanorod prepared in the SDS solution have more uniform morphology than that prepared in water. 3.2. Photoluminescence and Photoluminescence Excitation. Figure 4 depicts photoluminescence spectra measured at room temperature for the colloidal suspensions of the ZnO nanorods prepared in water and the SDS solution. The wavelength of the excitation light was 320 nm (3.88 eV). In the ZnO nanorods prepared in water, an overlapping emission band consisting of several photoluminescence peaks was found to appear in the visible light region below the photon energy of ∼3.0 eV. The photoluminescence peaks are mainly recognized in orange-green, blue, and violet light regions around 2.0∼2.3, 2.67, and 2.95 eV, respectively. In addition, one can see another photoluminescence peak with lower intensity in the blue light region at around ∼2.76 eV. It is well known for photoluminescence of ZnO that visible emissions at various energies are caused by
lattice defects of vacancies, interstitials, and antisites. The visible photoluminescence observed in this study indicates that the ZnO nanorods have different kinds of lattice defects. No observable peak appeared above 3.0 eV. However, a broad shoulder of the violet photoluminescence on the higher-energy side implies that a photoluminescence of ultraviolet (UV) light range occurs around ∼3.05 eV as indicated by an arrow in Figure 4. On the other hand, a peak of UV photoluminescence at ∼3.15 eV as well as the other peaks of the visible photoluminescence was observed for the ZnO nanorods prepared in the SDS solution. The UV photoluminescence is attributed to a near-band-edge transition of ZnO associated with an energy loss due to a strong electron-phonon interaction at room temperature. Figure 5a,b shows photoluminescence spectra of the ZnO nanorod colloidal suspensions prepared in water and SDS solution at the excitation wavelength of 370 nm (3.35 eV). The broad profiles of spectra between 2.0 and 2.3 eV and the small peaks located at ∼2.67 eV are observed for both cases in the orange-green and blue regions, whereas no observable peak is found at ∼2.76 eV in the blue region. A fitting analysis using the Lorentzian function reveals that the broad profile observed for the ZnO nanorods prepared in water consists of two components centered at 1.97 eV in the orange region and at 2.25 eV in the green region as shown in Figure 5a. Similarly, two components centered at 1.98 and 2.25 eV are refined by the fitting analysis of the broad profile in the ZnO nanorods prepared in the SDS solution. The peak energies at the two components of the orange and green regions in water are almost completely coincided with those in the SDS solution. The full width at half-maximums (fwhms) of lower-energy components in the water and SDS solution are 0.24 and 0.31 eV, whereas wider fwhms of 0.54 and 0.52 eV are found for higher-energy components. In both cases, the integral intensities of higherenergy components are three or four times larger than those of lower-energy components. This indicates that an origin of defect emission at lower-energy (1.97 and 1.98 eV) is different from that at higher-energy (2.25 eV) and that there are differences in an energy level variation and density of the lattice defects that contribute these defect emissions. These results reveal that the ZnO nanorods prepared in water and SDS solution exhibit the visible photoluminescence at 1.98, 2.25, ∼2.67, and ∼2.95 eV caused by the different kinds of lattice defects.
ZnO Nanorods Prepared by One-Step Chemical Synthesis
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9063 4. Discussion
Figure 5. Photoluminescence spectra of colloidal suspensions of ZnO nanorods prepared in (a) water and (b) the SDS solution. The wavelength of excitation light is 370 nm (3.35 eV). Fitting analysis using Lorentzian function reveals that broad profiles at 2.0∼2.3 eV consist of two components in orange and green regions.
Figure 6. Photoluminescence excitation spectra of colloidal suspensions of ZnO nanorods prepared in water and the SDS solution. The wavelength of emission light is 551 nm (2.25 eV). The excitation peaks are observed at ∼3.27 and ∼3.36 eV in water and the SDS solution.
Figure 6 presents photoluminescence excitation spectra of the ZnO nanorod colloidal suspensions prepared in water and the SDS solution at the emission wavelength of 551 nm (2.25 eV). The peak of the excitation spectrum of the ZnO nanorods prepared in the SDS solution is located around 3.36 eV. This indicates that the optical band gap of the ZnO nanorods is ∼3.36 eV and is almost the same as that of bulk ZnO (3.37 eV). On the other hand, in the ZnO nanorods prepared in water the excitation spectrum shows lower-peak energy of ∼3.27 eV as shown in Figure 6. This indicates that the optical band gap of ZnO nanorods prepared in water is approximately 0.1 eV narrower than that of bulk ZnO. It is suggested that dense lattice defects lead to a formation of high-density shallow donor levels below the conduction band of ZnO, and the band gap narrowing of ZnO apparently occurs by the Burstein-Moss effect.23
Wurtzite ZnO is a polar crystal whose surface of (0001) plane is entirely terminated by either positively charged zinc ions or negatively charged oxygen ions. The surface free energy of (0001) facet planes is suggested to be relatively higher than that of other low-index facet planes because the growth rate of ZnO crystal prepared by hydrothermal synthesis has been experimentally confirmed to be as follows; [0001] > [101h1] > [101h0]. In this study, zinc and oxygen ions are sourced by the decomposition of Zn(OH)42- and OH- ions existing in a strong basic solution, which can achieve a preferential growth of ZnO crystal in the [0001] direction by reflecting the difference of the surface free energies. The sharp edges of the nanorods imply that pyramidal {101h1} facet planes with the second lowest surface free energy appear on the surface of nanorod edges. It is suggested that the higher growth rate in the [0001] direction prevents (0001) planes from stacking perfectly, which causes the dense planer defects parallel to {2h110} planes indicated by the diffuse streaks in the SAED patterns. In an aqueous solution of SDS, aggregates of dodecyl sulfate ion C12H25SO4- form spherical micelles24 above the critical micelle concentration, and the spherical micelles have a property of encapsulating various salts generated in the solution. It is well-known that the spherical structure can change to onedimensional rod structure when the size and electrochemical properties of the salts are changed. Thus, an interior of the spherical micelles can be a space for nucleation and crystal growth of ZnO nanorod. The confinement of growing ZnO in the spherical micelles leads to more uniform growth, and then the ZnO nanorods show a narrower average width and more uniform morphology compared with those prepared in water. Oxygen in the hydrophilic group of dodecyl sulfate ions is negatively charged. Consequently, a chemical reaction occurs due to an electrostatic interaction between the hydrophilic group and a positive polar plane of zinc-rich surface on growing the ZnO crystal. It is suggested that chemically active sites are randomly generated on the surface of the nucleated ZnO crystal by the chemical reaction, and that new nanorods are nucleated at the active sites and grow in another [0001] direction. This speculation is supported by the morphology of the radial-shaped nanorods obtained in the SDS solution. The origin of visible photoluminescence in ZnO is still a contentious issue because many kinds of extrinsic and intrinsic lattice defects with different ionization states could be responsible for visible photoluminescence in various emission bands.25 We will inductively reason here the origin of the visible photoluminescence. Extrinsic lattice defects produce impurity levels in the band gap of ZnO, which is one of origins of the visible photoluminescence. For instance, Na-doped ZnO exhibits a yellow photoluminescence centered at 2.17 eV (572 nm).26 However, impurities such as sodium and sulfur are not detected in the ZnO nanorods prepared in this study. Thus, the origin of visible photoluminescence can be limited in the intrinsic defects of ZnO. Visible photoluminescence is attributed to different intrinsic defects, such as oxygen vacancies (VO), zinc vacancies (VZn), oxygen interstitials (Oi), zinc interstitials (Zni), and oxygen antisites (OZn). These intrinsic defects are possibly formed in ZnO nanorods prepared in water and the SDS solution because the diffuse streaks observed in SAED indicate a formation of high-density point defects in the ZnO crystal. The origin of violet photoluminescence centered at 2.92∼3.00 eV is ascribed to an electron transition from a deep donor level of neutral Zni to the valence band.27 Thus, the deep donor level of the Zni is suggested to locate at ∼0.42 eV below the conduction
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Figure 7. Energy level diagram of intrinsic defects in ZnO nanorods. Visible photoluminescence in regions of orange, green, blue, and violet is caused by radiative transition emission from deep donor levels of neutral zinc interstitials (Zni) to acceptor levels of singly ionized oxygen interstitials (Oi-), singly ionized zinc vacancies (VZn-), neutral zinc vacancies (VZn), and valence band, respectively. In the ZnO nanorods prepared in water, shallow donor levels are merged because of high-density lattice defects of neutral oxygen vacancies (VO), which causes a band gap narrowing by the Burstein-Moss effect.
band in this study because the violet photoluminescence appears at ∼2.95 eV. It is experimentally confirmed that an orange photoluminescence centered at 1.94∼1.98 eV is related to lattice defects of oxygen interstitials.14,28 Other researchers reported that the orange and green photoluminescence are attributed to radiative transitions from the deep donor level of Zni to acceptor levels caused by singly ionized Oi- and VZn-, respectively.29,30 Because the peak positions of the orange and green photoluminescence are 1.98 and 2.25 eV in this study, the acceptor levels of the ionized Oi- and VZn- locate at ∼0.97 and ∼0.70 eV above the valence band. A blue photoluminescence centered at ∼2.66 eV is reported to be a radiative transition of an electron from the deep donor level of Zni to an acceptor level of neutral VZn.31 It can be estimated that the acceptor level of VZn locates at ∼0.28 eV above the valence band in this study. Another blue photoluminescence was reported to appear at around 2.80 eV (∼443 nm) in other studies with ZnO nanorods and nanowhiskers.32 It is suggested that surface defects of the ZnO nanorods and nanowhiskers relate to the blue emissions although the detailed mechanism has been not clarified for the blue photoluminescence. A small blue photoluminescence at ∼2.76 eV shown in Figure 4 implies that there are a few surface defects in the ZnO nanorods prepared in water and the SDS solution. Figure 7 illustrates a diagram of energy levels caused by intrinsic defects in ZnO nanorods. All emission bands except for the small blue photoluminescence can be explained by this diagram without contradiction. The most commonly observed defect emission of ZnO is a green photoluminescence centered at ∼2.45 eV (∼506 nm) whose origin has been identified to surface oxygen defects in some reports.33-35 The defect is found to be a singly ionized VO-.36 The fact that this green photoluminescence centered at ∼2.45 eV is not clearly observed in this study indicates a much lower density of the ionized VOon surface of the ZnO nanorods. A difference between UV emission energies of the ZnO nanorods prepared in water and the SDS solution can be explained as follows. A shallow donor level is produced by neutral VO at ∼0.05 eV below the conduction band.37 In the ZnO nanorods prepared in water, it is suggested that the shallow donor levels are merged because of high-density lattice defects of the VO, and that the band gap narrowing takes place by the Burstein-Moss effect. On the other hand, the oxygen vacancies are effectively passivated by the chemical reaction between the oxygen of the hydrophilic group in dodecyl sulfate ions and the zinc-rich surface of nanorods during the growth. Therefore, the optical band gap of ZnO nanorods is the same as that of bulk ZnO. The results suggest that the surfactant micelle in the SDS solution can decrease the defect density in ZnO nanorods during the one-step chemical
synthesis although an object of passivation is limited to only oxygen vacancies. 5. Conclusion ZnO nanorods were prepared by one-step chemical synthesis in aqueous solutions with and without SDS surfactant to investigate the influence of surfactant micelles on the morphology and photoluminescence of the ZnO nanorods. Singlecrystalline ZnO nanorods with different morphologies of single needle and radial-shaped needles were obtained in water and the SDS solution. The ZnO nanorods prepared in the SDS solution have narrower average width, smaller aspect ratio, and wider optical band gap of ∼3.36 eV compared with those prepared in water. In both cases, ZnO nanorods exhibit visible photoluminescence in orange, green, blue, and violet regions caused by various lattice defects. These results indicate that the confinement of growing the ZnO crystal by the spherical micelles leads to more uniform growth and then the ZnO nanorods in SDS solution have narrower average width and more uniform morphology than those prepared in water. It is suggested that lattice defects of neutral oxygen vacancies are effectively passivated by dodecyl sulfate ions, and thus ZnO nanorods prepared in SDS solution exhibit the wider optical band gap without band gap narrowing of the Burstein-Moss effect. Acknowledgment. The author would like to thank Professor H. Yasuda and Professor T. Uchino for their helpful assistance in transmission electron microscopic observations and photoluminescence measurements. This work was, in part, supported by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) Japan under “Grant-in-Aid for Young Scientists (B)”. References and Notes (1) Ryu, Y. R.; Zhu. S.; Budai, J. D.; Chandrasekhar, H. R.; Miceli, P. F.; White, H. W. J. Appl. Phys. 2000, 88, 201. (2) Wang, L.; Giles, N. C. J. Appl. Phys. 2003, 94, 973. (3) Look, D. C. Mater. Sci. Eng., B 2001, 80, 383. (4) Kong, Y. C.; Yu, D. P.; Zhang, B.; Fang, W.; Feng, S. Q. Appl. Phys. Lett. 2001, 78, 407. (5) Qiu, Z. R.; Wong, K. S.; Wu, M. M.; Lin, W. J.; Xu, H. F. Appl. Phys. Lett. 2004, 84, 2739. (6) Wu, R.; Yang, Y.; Cong, S.; Wu, Z. G.; Xie, C. S.; Usui, H.; Kawaguchi, K.; Koshizaki, N. Chem. Phys. Lett. 2005, 406, 457. (7) Xing, Y. J.; Xi, Z. H.; Xue, Z. Q.; Zhang, X. D.; Song, J. H.; Wang, R. M.; Xu, J.; Song, Y.; Zhang, S. L.; Yu. D. P. Appl. Phys. Lett. 2003, 83, 1689. (8) Li, Y. B.; Bando. Y.; Sato, T.; Kurashima, K. Appl. Phys. Lett. 2002, 81, 144.
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