Precursor-Dependent Blue-Green Photoluminescence Emission of

Dec 5, 2011 - We report on properties of ZnO nanoparticles synthesized via non-aqueous sol–gel routes. The role of the hydrates in the zinc precurso...
0 downloads 9 Views 2MB Size
Download PDF
ARTICLE pubs.acs.org/JPCC

Precursor-Dependent Blue-Green Photoluminescence Emission of ZnO Nanoparticles Erwan Rauwel,*,†,‡,§ Augustinas Galeckas,§,|| Protima Rauwel,§,|| Martin Fleissner Sunding,§,|| and  †,§ Helmer Fjellvag Department of Chemistry, ‡SFI-inGaP, and §SMN, University of Oslo, N-0315 Oslo, Norway Department of Physics, University of Oslo, N-0316 Oslo, Norway

)

†

bS Supporting Information ABSTRACT: We report on properties of ZnO nanoparticles synthesized via non-aqueous solgel routes. The role of the hydrates in the zinc precursor (Zn(acac)2, xH2O) on the structure and surface termination during the synthesis is studied for the first time. The structural and chemical properties of the ZnO nanoparticles were studied by standard structural and optical characterization methods. A broad luminescence was observed from the nanoparticles stretching throughout the visible region of the spectrum and comprising characteristic blue and green emission bands commonly associated with intrinsic defects in ZnO. A tentative model is proposed to explain differences in the luminescence of nanoparticles synthesized using different routes by taking into account the role of oxygen vacancies and other native defects: most likely being zinc vacancies and interstitials, located near the surface of the nanoparticles.

1. INTRODUCTION The engineering of materials at the nanometer scale is of fundamental importance as size reduction and shape modification allow tuning of their fundamental properties. Research targeted toward the preparation and the characterization of nanostructured ZnO-based materials has gained particular significance over the past decade1 and has been the subject of numerous studies. ZnO due to its high refractive index, high thermal conductivity, and wide direct bandgap (Eg ≈ 3.37 eV)2 has a high potential in many micro-/nanoelectronic,35 magnetic,6 photovoltaic,7 and optical applications,8,9 where the latter is also considered as most challenging. Even though electroluminescence in ZnO was demonstrated over a decade ago,10 the integration of ZnO in light-emitting devices has been inhibited by problems related to p-type doping of ZnO.11 As mentioned above, the fundamental material properties can be modified by size reduction and also depend on the particular method through which nanostructuring is realized.3,1216 For instance, an improved crystallinity is commonly reported for ZnO nanopillars as compared to the bulk material, based on considerably quenched defect-related emission in luminescence spectra. On the other hand, any substantial size reduction also means a consequential increase of surface-to-volume ratio and a subsequent stronger surface-defect-related emission. In the case of unsupported nanometer-size particles, the surface effects may start to dominate the bulk properties, turning the defect-related luminescence into a potential light source. The native (intrinsic) defects in the crystalline structure are believed to be responsible r 2011 American Chemical Society

for the broad visible luminescence in ZnO, although certain controversy in particular assignments remains.17 The green emission is commonly associated with oxygen vacancies,4,18,19 whereas the blueviolet band is linked to zinc vacancies or interstitials.16 While the efficient red light emitters needed for many practical applications can be realized from cadmium sulfide or selenide by controlling the particle size,2022 stable green- and blue-emitting nanocrystals are more difficult to achieve. In this respect, ZnO nanoparticles (NPs) are among the most promising candidates because of the nontoxicity, stability in air, and tendency to aggregate due to high surface energy; more specifically, surface modification has proven an effective way to stabilize luminescence of NPs at room temperature.23 It was also shown by Meulenkamp that aging and the size of the ZnO nanoparticles prepared with zinc acetate dihydrate in an alcoholic solution are governed by the water content and the presence of reaction products. In fact, it is demonstrated that water has many roles like increasing the concentration of dissolved ZnII species and enhancing the activity of all species.23 This study has therefore reinforced our motivation to outline the role of water molecules in the case of the non-aqueous solgel route. Recently, preparation of ionic liquid crystals was shown to be a feasible way to realize stable green and blue light emitters.24 In the present study, we address the possibility to Received: September 2, 2011 Revised: November 4, 2011 Published: December 05, 2011 25227

dx.doi.org/10.1021/jp208487v | J. Phys. Chem. C 2011, 115, 25227–25233

The Journal of Physical Chemistry C

ARTICLE

Figure 1. X-ray diffraction pattern of ZnO nanoparticles (a) produced using zinc acetylacetonate hydrate (type-A NPs) and (b) produced using zinc acetate (type-B NPs).

Table 1. X-ray Diffraction Peak Values Used for Scherrer Calculation Measurements acetylacetonate, hydrate precursor (A) acetate precursor (B) 100

002

100

002

2θ (deg)

31.77

34.40

31.93

34.58

fwhm (deg)

0.299

0.226

0.212

0.210

Ø (nm)

27.62

36.81

38.98

39.62

produce ZnO nanoparticles using a non-aqueous solgel method with an ultimate goal of integrating them in thermal imaging applications. ZnO NPs synthesized using benzylamine and Zn acetylacetonate have already been reported in the literature;25 however, nothing was mentioned on their physical properties. Per contra, in this report we present structural and optical properties of ZnO NPs synthesized using the benzylamine and Zn acetylacetonate route along with a second synthesis based on an anhydrous precursor (Zn acetate). In this way, the effects of the hydrate groups on the structural and optical properties can be directly examined and interrelated. First, we address the outcome of hydrate added to the precursor on the structure and composition of the NPs. Then, PL properties of the NPs are compared with those of bulk ZnO material and discussed considering the luminescence of different near-surface intrinsic defects created during the growth of the NPs with or without hydrate.

2. MATERIALS AND METHODS Synthesis. The procedure for synthesizing ZnO NPs was carried out in a glovebox (O2 and H2O < 1 ppm). In a typical synthesis, zinc acetylacetonate hydrate (1.52 mmol) (99.995%, Aldrich) or zinc acetate (2.73 mmol) (99.99%, Aldrich) was added to 20 mL (183 mmol) of benzyl amine (purified by redistillation (g99.5%), Aldrich). The reaction mixture was transferred into a stainless steel autoclave and carefully sealed.

Thereafter, the autoclave was taken out of the glovebox and heated in a furnace at 200 C for 2 days. The resulting milky suspensions were centrifuged and the precipitates thoroughly washed with ethanol and dichloromethane and subsequently dried in air at 60 C. Characterization. X-ray diffraction (XRD) patterns were collected using a Philips X’Pert powder diffractometer equipped with a Cu Kα1 radiation source (λ = 0.15406 nm). Scanning Electron Microscopy (SEM) images were recorded on a FEI Quanta 200FEG. Transmission electron microscopy (TEM) was carried out on JEOL 2000FS and JEOL-2010FS Transmission Electron Microscopes both operating at 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Kratos Analytical Axis UltraDLD photoelectron spectrometer equipped with a monochromated Al Kα X-ray source. The analyzer settings employed for the narrow scans allow for 0.57 eV energy resolution as determined by the full width at half-maximum of the Ag 3d5/2 photoelectron peak. Low-energy electrons were used for charge compensation, and the energy scale was calibrated based on the C 1s peak for adventitious carbon at 285.0 eV binding energy (BE). Manufacturer’s sensitivity factors were used for quantification. Thermogravimetric (TG) analyses were carried out in flowing N2 atmosphere (15 mL/min) with a heating rate of 5 C/min, using a Rheometric Scientific STA 1500 instrument. Optical absorption properties were derived from the diffuse reflectance measurements performed at room temperature using a ThermoScientific EVO-600 UVvis spectrophotometer. Photoluminescence (PL) was investigated at 8 K temperature by employing 325 nm wavelength of a cw HeCd laser with an output power of 10 mW as an excitation source. The emission was collected by a microscope and directed to a fiber optic spectrometer (Ocean Optics USB4000, spectral resolution 2 nm). Low-temperature measurements were realized using a closedcycle He refrigerator (Janis, Inc. CCS450). 25228

dx.doi.org/10.1021/jp208487v |J. Phys. Chem. C 2011, 115, 25227–25233

The Journal of Physical Chemistry C

ARTICLE

Figure 2. TEM micrographs of ZnO nanoparticles produced using zinc acetylacetonate hydrate (type-A NPs) (a) and (b) with low magnification and (c) electron diffraction pattern indicating pure hexagonal wurtzite-analogous (P63mc) crystal structure.

3. RESULTS AND DISCUSSION Structural and Chemical Characterizations. Zinc oxide NPs of about 30 and 40 nm diameter were synthesized using zinc acetylacetonate, hydrate (hereafter referred to as type-A NPs), and zinc acetate (type-B NPs) with benzylamine, respectively. This non-aqueous solgel process was recently developed as a potential alternative to conventional hydrolytic routes.26 The most important advantage of this method is the possibility to synthesize extremely pure and highly crystalline oxide NPs at low temperatures and without any surfactant.25,27,28 An XRD study showed that ZnO NPs have the hexagonal wurtzite-analogous (P63mc) crystal structure (a = 3.25 Å and c = 5.20 Å) with a high crystallinity.29 No signs of secondary phase are observed using XRD or TEM, whereas TEM analysis indicates the high purity of the ZnO NPs. Figure 1 exhibits the XRD pattern for both samples A and B. The analysis of spectra illustrate that in the case of acetylacetonate precursor (type-A NPs) the NPs seem to be smaller due to broader XRD peaks. In fact, the Scherrer method30 applied to the 100 and 002 reflections (Table 1) estimated the crystallite size of 37 and 27 nm for ZnO synthesized using acetylacetonate (type-A) and a diameter of 39 nm for ZnO synthesized using acetate (type-B). The mean size of the crystallites, T, was estimated from the Scherrer equation

T ¼

0:9λ β cos θ

where λ is the Kα radiation (λ = 0.15406 nm); θ is the Bragg diffraction angle; and β is the width of the peak at half the maximum intensity in radian. It appears that in the case of type-A NPs two values are found and correspond to two different growth rates for two different directions, implying a nonspherical particle shape. The chemical composition of ZnO NPs was analyzed using energy-dispersive spectroscopy (EDS) and demonstrates a low carbon content for both types of nanoparticles, estimated to be even lower for type-A NPs, suggesting that some organic species linger even after rinsing the NPs. Figures 2 and 3 depict the general morphology of the ZnO NPs revealed by using TEM and HRTEM. As previously calculated using the Scherrer equation, type-A ZnO NPs are comprised of “bean-shaped” particles with an average length of 30 nm and a width of 1520 nm (Figure 2a and 2b). These

nanoparticles illustrate a good degree of monodispersion. Electron diffraction (ED) performed on a selected area (Figure 2c) confirms the high crystallinity and the hexagonal P63mc structure. XRD measurements, via the Scherrer equation, indicated that the growth direction is (002). The TEM study performed on type-B NPs showed that the shape is mostly hexagonal with sharp edges. Figures 3a, 3b, and 3c are transmission electron micrographs taken at different magnifications. The aim was to first acquire a closer look at how these nanoparticles are disposed on the carbon grid. In the lower magnification image of Figure 3a, one observes monodispersed particles along with particles tending to slightly agglomerate. The higher magnification image of a cluster of particles in Figure 3b again displays agglomerated and monodispersed particles. At an even higher magnification as in Figure 3c, the sharp facets of the particles are discernible. The selected area diffraction pattern (Figure 3f) from one of these zones confirms the crystallinity and the hexagonal hcp structure of the material. The high crystalline quality of these ZnO nanostructures is represented in the HRTEM image of Figure 3e which is part of a larger nanoparticle represented in Figure 3d. The inset of Figure 3e is the power spectrum indicating that the nanoparticles are oriented along the Æ001æ zone axis. Lattice parameters of a = 3.25 Å and b = 5.20 Å were measured for both samples. The difference in particle size was attributed to the precursors’ ligand. In fact, longer ligands will promote the production of smaller NPs. In the present case, the acetyl acetonate ligand has not only decreased the particle diameter but also promoted the synthesis of the elongated “bean” like shape. Thermogravimetric analysis (TGA) was performed on both powders to characterize the thermal stability of these NPs and detect the presence of organic contamination, if any, that could adhere as a result of the synthesis. The typical measurements performed on both ZnO synthesized using zinc actetylacetonate hydrate and acetate are shown in Figures 1aS and 1bS, respectively (Supporting Information). The TGA measurements performed on type-A NPs (Figure 1aS, Supporting Information) are typical from oxide NPs synthesized using this method.31 However, only a total weight loss of about 1.80% was measured up to 800 C speaking for the high purity of the samples. The weight loss that corresponds to water desorption was estimated to be about 0.06%. In the case of type-B NPs (Figure 1bS, Supporting Information), the curve is slightly different, and a sharp drop corresponding to a weight loss of about 30% is visible 25229

dx.doi.org/10.1021/jp208487v |J. Phys. Chem. C 2011, 115, 25227–25233

The Journal of Physical Chemistry C

ARTICLE

Figure 3. TEM micrographs of ZnO nanoparticles produced using zinc acetate (type-B NPs) (a) and (b) low magnification and (c) higher magnification images of ZnO nanoparticles, (d) high-resolution TEM image of a ZnO nanoparticle, (e) HRTEM image of the encircled area of (d) (inset shows power spectrum), and (f) electron diffraction pattern indicating pure hexagonal wurtzite-analogous (P63mc) crystal structure.

Figure 4. XPS survey performed on ZnO nanoparticles produced using (a) zinc acetylacetonate, hydrate (type-A NPs), and (b) zinc acetate (type-B NPs).

from 225 to 336 C. This behavior is typical of organic solvent adsorbed on the surface that desorbs abruptly at elevated temperatures. Such a drop has already been observed at higher temperature in the case of yttria-based materials,32 but in our case this concerns the fast desorption of remaining benzylamine solvent or other organic species coming from the reaction synthesis25 adsorbed on the surface of the ZnO NPs. The presence of hydrate in the acetylacetonate precursor most probably promotes the desorption of these organic species onto the surface of the NPs, coming from the solvent during the reaction process. In the case of pure acetate precursor, i.e., without any hydrate in the

solution, benzylamine ligands and other organic species remain strongly bonded to the surface on the ZnO NPs. XPS measurements were performed on both types of NPs to compare the purity of the surface and the degree of oxidation. Figure 4 presents the XPS survey spectra, showing a higher amount of carbon in type-B NPs, thus confirming the results of previous EDS analysis (not shown here). Moreover, there is a notable difference in the nitrogen content between both samples: While the nitrogen 1s peak is clearly visible on the survey spectrum of type-B NPs, it is absent in type-A NPs. This suggests that benzylamine molecules from the solvent and other organic 25230

dx.doi.org/10.1021/jp208487v |J. Phys. Chem. C 2011, 115, 25227–25233

The Journal of Physical Chemistry C

ARTICLE

Figure 5. XPS spectra from ZnO nanoparticles. The lines show the deconvoluted (a) Zn 2p3/2 and (b) O 1s component for ZnO nanoparticles synthesized using zinc acetylacetonate hydrate (type-A NPs).

Table 2. Photoelectron Peak Positions and Modified Auger Parameter (AP) Values for Zn and O, with Reference Values from the NIST XPS Database33 a Zn 2p3/2/eV BE

a

Zn AP/eV

O 1s/eV BE

O AP/eV

acetylacetonate, hydrate precursor (A)

1021.3

2010.1

530.1 (531.4)

1040.2

acetate precursor (B)

1022.1

2010.1

530.9 (532.1)

1040.3

ZnO reference

1021.2  1022.5

2009.5  2011.0

529.9  531.2

1040.4  1041.3

average = 2010.1 2009.2

average = 530.4

average = 1040.9

Zn(OH)2 reference

average = 1021.8 1022.7

The O 1s values in parentheses correspond to the high BE component of the peak.

species remain adsorbed on the surface of the ZnO NPs in the case of type-B NPs (without hydrate). This XPS study confirms the previous result obtained from TGA and reinforces the initial hypothesis about the role of hydrate during the reaction process. The hydrate in the precursor is likely to promote the desorption of the benzylamine solvent and other organic species during the reaction. This allows obtaining very clean and pure ZnO NPs without organic molecules on the surface. The Zn 2p3/2 photoelectron peaks can be deconvoluted with a single component for both samples, as illustrated in Figure 5a for type-A NPs. Satisfactory peak fitting is achieved. The measured Zn 2p3/2 and Zn LMM peak positions differ by 0.8 eV between both samples, and the modified Auger parameter (AP) for Zn is, however, identical for both samples and matches reported values for ZnO and, combined with the single Zn 2p3/2 photoelectron peak, excludes the presence of Zn(OH)2 (Table 2). The differences in the peak positions can indicate that the charge referencing based on the C 1s peak is not adapted for these samples. Energy referencing the spectra based on the average Zn 2p3/2 BE for ZnO in the NIST XPS database,33 1021.8 eV BE, leads to C 1s peak positions of 285.5 eV BE for the main component in type-A NPs and 284.7 eV BE in type-B NPs. The relatively low BE in type-B NPs fits well with aromatic carbon from the suggested presence of organic species on the surface of the NPs in this type.34 The BE of the C 1s peak in typeA NPs might indicate partial oxidation of the organic matter present on the sample or that the use of the Zn 2p3/2 peak for charge referencing does not lead to realiable results; i.e., the Zn 2p3/2 BE is much lower than in type-B NPs. This could then

indicate the presence of a negative charge on Zn stemming from oxygen vacancies. The O 1s photoelectron peaks are similar for both samples, with a main peak at low BEs and a shoulder at higher BEs from an additional component, as shown in Figure 5b for type-A NPs. The main peak at low BE has a typical energy for oxygen in zinc oxide (Table 2). The shoulder at a higher binding energy could be attributed to the presence of OH groups or adsorbed oxygen species or of oxygen atoms related to oxygen vacancies, possibly indicating different species between both types. As no hydrate was used during the synthesis of type-B NPs, the presence of hydroxyl groups is not likely in the assynthesized NPs. The presence of oxygen vacancies and oxygen species adsorbed on the surface certainly affects the PL response of the material. The quantification of the elements from the detailed scans enables the determination of the amount of Zn compared to O element (Table 3). In the case of type-A NPs, 40 at. % of Zn with 20 at. % of O with high BE and 40 at. % of O with lower BE were measured. A lower Zn quantity was measured in the case of type-B NPs (36 at. %). This could be attributed to Zn vacancies in the sample but can also stem from the stronger absorption of the Zn 2p3/2 photoelectrons compared to the O 1s photoelectrons by the organic matter adsorbed on the surface of the ZnO NPs. A deeper insight into the above-mentioned nonstoichiometries can be attained from optical characterization of NPs in the view of the fact that most of the (intrinsic) native defects in ZnO are luminescent. Indeed, clearly contrasting optical response was observed from the NPs under UV excitation despite similarities in size, shape, and routes of synthesis. The influence of the 25231

dx.doi.org/10.1021/jp208487v |J. Phys. Chem. C 2011, 115, 25227–25233

The Journal of Physical Chemistry C

ARTICLE

Table 3. Zn and O Quantification from XPS Results in TypeA and Type-B Nanoparticles O at. %

O at. %

Zn at. %

(high energy)

(low energy)

acetylacetonate, hydrate precursor (A)

40

20

40

acetate precursor (B)

36

22

42

Figure 6. PL spectra at 8 K from ZnO nanoparticles produced using zinc acetylacetonate hydrate (curve #01, type-A NPs) and zinc acetate (curve #02, type-B NPs) along with the reference spectrum of bulk ZnO (gray curve). Inset shows optical band edges of the corresponding nanoparticles estimated from the diffuse-reflectance spectra (DRS) at 300 K.

presence of hydrate in the precursor on optical properties is discussed in the following section. Optical Characterizations. It is known that the higher surface-to-volume ratio in NPs as compared to bulk material leads to a corresponding enhancement of surface-related emission in the overall luminescence.35 As a first approximation, the role of surface effects in the present study was deduced from the comparison of luminescence properties of NPs against those of high crystallinity bulk ZnO. A summary of PL and diffusereflectance measurements is presented in Figure 6. One can immediately notice that the band-edge luminescence, typically prevailing in the ZnO spectra, appears generally suppressed, whereas broad visible emission is considerably enhanced in the case of nanoparticles. Among the two types of nanoparticles, optical properties of type B ZnO NPs are closer to those of bulk ZnO in terms of both band edge and PL, demonstrating a distinctively direct type of optical transition in the Tauc plot with a characteristic excitonic absorption peak and the bandgap of 3.28 eV at room temperature. In contrast, the absorption edge for the type-A ZnO NPs appears red-shifted (3.2 eV) with regard to that of ZnO, also exhibiting an absorption tail stretched throughout the visible range. The origin of enhanced visible emission from ZnO nanoparticles is usually attributed to oxygen vacancies located predominantly near the surface, the ionization state of which in turn depends on a particular charge state at the surface.36 It is known that ZnO exhibits a downward band bending near the surface,37 which leads to the formation of an accumulation region for

electrons (and depletion region for holes). The barrier height and width of such a region are related to the net positive surface charge, which might be caused by donor-like surface states or adsorbed atoms.38 This means that any modification of the surface charge will cause a corresponding change in the band bending and, accordingly, can be assessed by monitoring the intensity of visible PL. The broad visible luminescence of nanoparticles is apparently composed of several sub-bands (see Figure 6). The emission around 3 eV (410 nm), which dominates in the spectra of type-B NPs, is commonly associated with optical activity of zinc-related intrinsic defects (Zni interstitials and/or VZn vacancies).17 Note that quantification results from the XPS data suggest a possible Zn depletion that would indicate the presence of Zn vacancies specifically for the type-B NPs. Conversely, the missing blue emission in PL from type-A NPs is consistent with no indications of Zn deficiency close to the surface as indicated by XPS. On the other hand, green luminescence is a common feature for both types of NPs and is comprised of two merged emission bands centered at ∼2.5 eV (500 nm) and ∼2.2 eV (550 nm), which are believed to originate from double- (VO+2) and single-charged (VO+) oxygen vacancies, respectively.39 For type-B NPs, VOrelated emission around ∼2.5 eV (500 nm) appears more prominent presumably because of more reductive environment during the synthesis in the absence of hydrate. The latter assumption is based on the fact that, if present, hydrate would certainly promote a better oxidation of the ZnO nanoparticles during the synthesis process. As already mentioned, XPS and PL measurements both point toward some nonstoichiometry near the surface of nanoparticles for the synthesis route involving zinc acetate precursor. More specifically, there is an apparent deficit of Zn atoms on the zinc sites of the ZnO lattice, and this effect is plausibly caused by the lack of hydrate, which is known to promote desorption of the benzylamine ligands. It is therefore reasonable to associate the promotion of zinc vacancy formation with some obstruction caused by benzylamine on the surface during the synthesis. As a final point, we note that besides zinc vacancies yet another potential contributor, associated to zinc interstitials, to the observed blue emission can not be ruled out, assuming that large aromatic rings adsorbed on the surface promote intersite lattice position for Zn during the synthesis.

4. CONCLUSIONS The high surface-to-volume ratio of the investigated ZnO NPs makes the luminescence of intrinsic defects (oxygen vacancies, zinc vacancies, and/or zinc interstitials) dominant in the overall emission spectra. The band-edge luminescence, typically prevailing in spectra of bulk ZnO material, appears totally suppressed in the case of nanosized particles. The employed ZnO-nanoparticle synthesis routes using acetylacetonate hydrate and zinc acetate have different effects on the final surface state of the NPs as evidenced from TGA measurements, XPS quantitative analyses, and PL. In fact, the role of hydrate is 2-fold: it promotes the optimization of the oxidation during the growth process and facilitates the desorption of the benzylamine ligands and other organic species during the reaction process. The residual benzylamine molecules on the surface promote the formation of intrinsic defects in the form of Zn vacancies and/or interstitial Zn. All measurements considered, we demonstrate the key role of the hydrate in such reactions. Finally, without using hydrate 25232

dx.doi.org/10.1021/jp208487v |J. Phys. Chem. C 2011, 115, 25227–25233

The Journal of Physical Chemistry C during the process we were able to synthesize highly crystalline ZnO NPs with a sufficient amount of luminescent centers to make these NPs promising for device applications. Such NPs will be integrated into devices engineered for thermal imaging and will be the subject of future communications.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figures 1S. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: (47) 228 54764. Fax: (47) 228 55565. E-mail: erwan. [email protected].

’ ACKNOWLEDGMENT Financial support from Research Council of Norway for financial support, project 176740/130, and Marie Curie (PERG05GA-2009-249243) is acknowledged. We thank Mrs. S. Aravinthan from inGAP, University of Oslo, for her assistance with the TGA and TDA measurements. We thank Dr. Maria Rosario from University of Aveiro, CICECO, for XRD measurements. ’ REFERENCES (1) Klingshirn, C. ZnO: From basics towards applications. Phys. Status Solidi B 2007, 244 (9), 3027–3073. (2) Meyer, B. K.; Alves, H.; Hofmann, D. M.; Kriegseis, W.; Forster, D.; Bertram, F.; Christen, J.; Hoffmann, A.; Straβburg, M.; Dworzak, M.; Haboeck, U.; Rodina, A. V. Phys. Status Solidi B 2004, 241 (2), 231. (3) Wang, Z. L. Zinc oxide nanostructures: growth, properties and applications. J. Phys.: Condens. Matter 2004, 16, R829. € ur, U.; € Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; (4) Ozg€ Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. A comprehensive review of ZnO materials and devices. J. Appl. Phys. 2005, 98, 041301. (5) Schmidt-Mende, L.; MacManus-Driscoll, J. L. ZnO - nanostructures, defects, and devices. Mater. Today 2007, 10, 40. (6) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019. (7) Beck, W. J. E.; Wienk, M. M.; Jansen, R. A. J. Efficient hybrid solar cells from zinc oxide nanoparticles and a conjugated polymer. Adv. Mater. 2004, 16 (12), 1009. (8) Tsukazaki, A.; Ohtomo, A.; Onuma, T.; Ohtani, M.; Makino, T.; Sumiya, M.; Ohtani, K.; Chichibu, S. F.; Fuke, S.; Segawa, Y.; Ohno, H.; Koinuma, H.; Kawasaki, M. Nat. Mater. 2005, 4 (1), 42. (9) Moon, T.-H.; Jeong, M.-C.; Lee, W.; Myoung, J.-M. Appl. Surf. Sci. 2005, 240 (14), 280. (10) M. H. Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (11) Jagadish, C., Pearton, S., Eds. ZnO bulk, thin films, and nanostructures; Elsevier: New York, 2006. (12) Buha, J.; Djerdj, I.; Niederberger, M. non-aqueous synthesis of nanocrystalline indium oxide and zinc oxide in the oxygen-free solvent acetonitrile. Cryst. Growth Des. 2007, 7 (1), 113. (13) Fan, J. F.; Yang, Y.; Zacharias, M. ZnO-based ternary compound nanotubes and nanowires. J. Mater. Chem. 2009, 19, 885. (14) Fan, Z. Y.; Lu, G. C. Zinc Oxide Nanostructures: Synthesis and Properties. J. Nanosci. Nanotechnol. 2005, 5, 1561. (15) Zou, G.; Yu, D.; Wang, D.; Zhang, W.; Xu, L.; Yu, W.; Qian, Y. Controlled synthesis of ZnO nanocrystals with column-, rosette- and firber-like morphologies and their photoluminescence property. Mater. Chem. Phys. 2004, 88, 150.

ARTICLE

(16) Bitenc, M.; Podbrscek, P.; Crnjak, O.; Cleveland, M. A.; Paramo, J. A.; Peters, R. M.; Strzhemechny, Y. M. Correlation between morphology and defect luminescence in precipitated ZnO nanorod powders. Cryst. Growth Des. 2009, 9, 997. (17) Roro, K. T.; Dangbegnon, J. K.; Sivaraya, S.; Leitch, A. W. R.; Botha, J. R. J. Appl. Phys. 2008, 103, 053516. (18) Norberg, N. S.; Gamelin, D. R. Influence of surface modification on the luminescence of colloidal ZnO nanocrystals. J. Phys. Chem. B 2005, 109 (44), 20810. (19) Leiter, F. H.; Alves, H. R.; Hofstaetter, A.; Hoffmann, D. M.; Meyer, B. K. The oxygen vacancy as the origin of a green emission in undoped ZnO. Phys. Status Solidi B 2001, 226 (1), R4. (20) Qu, L.; Peng, X. J. Am. Chem. Soc. 2002, 124, 2049–2055. (21) Wang, Y.; Meng, G.; Zhang, L.; Liang, C.; Zhang, J. Chem. Mater. 2002, 14, 1773–1777. (22) Neves, M. C.; Martins, M. A.; Soares-Santos, P. C. R.; Rauwel, P.; Ferreira, R. A. S.; Monteiro, T.; Carlos, L. D.; Trindade, T. Photoluminescent, transparent and flexible di-ureasil hybrids containing CdSe/ZnS quantum dots. Nanotechnology 2008, 19, 155601. (23) Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 5566. (24) Wang, Q.-T.; Wang, X.-B.; Lou, W.-J.; Hao, J.-C. Stable Blueand Green-Emitting Zinc Oxide from Ionic Liquid Crystal Precursors. ChemPhysChem 2009, 10 (18), 3201–3203. (25) Pinna, N.; Garnweitner, G.; Antonietti, M.; Niederberger, M. A General non-aqueous Route to Binary Metal Oxide Nanocrystals Involving a C-C Bond Cleavage. J. Am. Chem. Soc. 2005, 127 (15), 5608–5612. (26) Pinna, N.; Niederberger, M. Surfactant-free non-aqueous Synthesis of Metal Oxide Nanostructures. Angew. Chem., Int. Ed. 2008, 47, 5292. (27) Pinna, N.; Neri, G.; Antonietti, M.; Niederberger, M. nonaqueous Synthesis of Nanocrystalline Semiconducting Metal Oxides for Gas Sensing. Angew. Chem., Int. Ed. 2004, 43 (33), 4345. (28) Rauwel, E.; Galeckas, A.; Rauwel, P.; Fjellvag, H. Unusual photoluminescence of CaHfO3 and SrHfO3 nanoparticles. Adv. Funct. Mater. 2011, DOI: 10.1002/adfm.201101013. (29) JCP2.CAT:000361451. (30) Scherrer, P. G€ottinger Nachr. Gesell. 1918, 2, 98. (31) Niederberger, M.; Pinna, N. Metal Oxide Nanoparticles in Organic Solvents: Synthesis, Formation, Assembly and Application; Springer: New York, 2009. (32) Pinna, N.; Garnweitner, G.; P., B.; Niederberger, M. Antonietti, M., Synthesis of yttria-based crystalline and lamellar nanostructures and their formation mechanism. Small 2005, 1, 112. (33) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble, J. R. J. (Eds.), NIST X-ray Photoelectron Spectroscopy Database; National Institute of Standards and Technology: Gaithersburg, 2003. (34) Beamson, G.; Briggs, D. High resolution XPS of organic polymers: the Scienta ESCA300 database; Wiley: Chichester [England]; New York, 1992. (35) Gong, Y.; Andelman, T.; Neumark, G. F.; O’Brien, S.; Kukovsky, I. L. Origin of defect-related green emission from ZnO nanoparticles: effect of surface modification. Nanoscale Res. Lett. 2007, 2, 297. (36) Vanheusden, K.; Seager, C. H.; Warren, W. L.; Tallant, D. R.; Voigt, J. A. Appl. Phys. Lett. 1996, 68, 403. (37) Coppa, B. J.; Fulton, C. C.; Kiesel, S. M.; Davis, R. F.; Pandariath, C.; Burnette, J. E.; Nemanich, R. J.; Smith, D. J. J. Appl. Phys. 2005, 97, 103517. (38) M€onch, W. Semiconductor Surfaces and Interfaces, 3rd ed.; Springer: Berlin, 2001. (39) Ye, J. D.; Gu, S. L.; Qin, F.; Zhu, S. M.; Liu, S. M.; Zhou, X.; Liu, W.; Hu, L. Q.; Zhang, R.; Shi, Y.; Zheng, Y. D. Appl. Phys. A: Mater. Sci. Process 2005, 81, 759.

25233

dx.doi.org/10.1021/jp208487v |J. Phys. Chem. C 2011, 115, 25227–25233