Modification of the electrical properties of PEDOT:PSS by the incorporation of ZnO nanoparticles synthesized by laser ablation

Chemical Physics Letters 484 (2010) 283–289

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Modification of the electrical properties of PEDOT:PSS by the incorporation of ZnO nanoparticles synthesized by laser ablation N.G. Semaltianos a,*, S. Logothetidis a, N. Hastas a, W. Perrie b, S. Romani b, R.J. Potter b, G. Dearden b, K.G. Watkins b, P. French c, M. Sharp c a b c

Department of Physics, Aristotle University of Thessaloniki, Thessaloniki GR-54124, Greece Department of Engineering, University of Liverpool, Brownlow Hill, Liverpool L69 3GH, UK General Engineering Research Institute, Liverpool John Moores University, Byrom Street, Liverpool L3 3AF, UK

a r t i c l e

i n f o

Article history: Received 3 September 2009 In final form 24 November 2009 Available online 26 November 2009

a b s t r a c t Laser ablation of a solid target in a liquid environment offers an easy, fast, and environmentally friendly method for the generation of nanoparticles with desired properties. In this Letter we report modification of the electrical properties of PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)) by the incorporation into it of ZnO nanoparticles which were synthesized by laser ablation. By forming the nanocomposite, change of the chemical structure of the polymer from a mixture of benzoid and quinoid to a mostly quinoid and a conformational change of the polymer chains from coil to expanded-coil or linear was observed. Furthermore, these changes result in an increase by almost twice of the electrical conductivity of the polymer. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction PEDOT:PSS (poly(3,4-ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate)) is an organic material which is commonly used as the anode electrode (hole injection/transport layer) in organic electronic devices such as light emitting diodes, field effect transistors and photovoltaic cells [1,2]. It is easily processable from aqueous solution, has high strength and flexibility, excellent thermal, mechanical and environmental stability and high transparency in the visible range. It replaces the traditionally used indium tin oxide (ITO) usually in the case of flexible organic electronic devices or it can be used as a buffer layer on top of ITO to smooth its surface and improve device stability [2,3]. The electrical conductivities of thin films spin-coated from commonly used types of PEDOT:PSS such as pure PH500 were reported to have values of up to 1 S/cm which is by more than two orders of magnitude lower than the conductivity of ITO (8000 S/cm) [3,4]. Traditional methods to increase the conductivity of PEDOT included in the past the addition of DMSO (dimethyl sulfoxide) or DMF (N,N-dimethylformamide) to the polymer [3,5,6], a compound with two or more polar groups such as glycols (EG, DEG or TEG), meso-erythritol, methyl sulfoxide, 1-methyl-2-pyrrolidinone or 2-nitroethanol [7–10] or anionic surfactants [11]. Lately a maximum conductivity of 570 S/cm was reported for the PEDOT:PSS PH750 modified with 5% DMSO and 13% isopropanol [12] while the PEDOT:PSS PH500 modified with 5% DMSO and 13% isopropa* Corresponding author. E-mail address: [email protected] (N.G. Semaltianos). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.11.054

nol showed a conductivity of 330 S/cm [4,12]. Very recently H.C. Starck has also produced a PEDOT:PSS (CLEVIOS™ PH1000) with a minimum conductivity of 900 S/cm (after again the addition of 5% of DMSO) [13]. In the evolution of the field of organic electronics, incorporation into polymer matrices of inorganic or metallic nanoparticles (NPs), in general, with the resulting formation of polymer–NPs composites (hybrid materials, nanocomposites) seems as an attractive method for the modification of the properties of polymers which are commonly used in the fabrication of organic optoelectronic devices, for improved performance [14]. This method combines the large variety of organic polymers which are solution-processable with the excellent electronic and optical properties of NPs. Due to its unique optical and electrical properties as an inorganic material for optoelectronic applications combined with its intoxicity and relatively large abundance at low cost, ZnO in the form of NPs has been used for the formation of hybrid materials which are used mainly as active layers in organic devices [15– 18] and in the form of nanorods, in a smaller degree, for the modification of the properties of the device anode layer (PEDOT:PSS) [19]. The ZnO NPs or nanorods which have been utilized so far for the fabrication of nanocomposites, have been usually synthesized by sol–gel colloidal chemical methods involving hydrolysis and condensation of zinc acetate dihydrate by potassium hydroxide in alcoholic solution under basic conditions [20–22] or mixing zinc acetate dihydrate with the polymer polyvinylpyrrolidone followed by water evaporation and finally calcination of the dried mixture [19,23]. These methods usually require high temperatures and long reaction times for the synthesis of nanomaterials.

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This Letter reports the possibility of modifying the electrical properties of PEDOT:PSS by the incorporation into it of ZnO NPs but where the NPs were synthesized by using the method of laser

Y

X-Y-Z stage sample liquid teflon ring synthesized nanoparticles cuvette

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ablation of a solid target in a liquid environment [24] instead of by other traditionally used methods. The principle of this method for NP synthesis is similar to the method of pulsed laser deposition (PLD) of thin films [25] with the exception that the laser ablated plume species condense and form NPs within the liquid which surrounds the ablated target. It is a simple, easy, fast, and ‘green’ method for NPs synthesis. The Letter consists of two parts: in the first part we present a detail characterization of the synthesized NPs and in the second part we present a characterization of the spectroscopic, surface morphological and electrical properties of the nanocomposite films.

lens 2. Experimental details

LASER mirror viewing direction Fig. 1. Schematic diagram (top view) of the experimental setup used for the synthesis of ZnO NPs by laser ablation of the target material in a liquid environment.

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Laser ablation for NPs generation was carried out by using a Ti:Sapphire femtosecond laser (Clark-MXR 2010) (wavelength (k) = 775 nm, pulse width = 180 fs, frequency = 1 kHz) based on the chirped pulse amplification (CPA) technique [26]. To achieve high ablation rates, relatively high pulse energy of 45 lJ (fluence = 11 J/cm2) was used. The target material was a pressed powder ZnO pellet (purity > 99.1%) with 1:1 ratio of the two elements. A schematic diagram of the experimental setup used for the generation of ZnO NPs by laser ablation in a liquid environment, is shown in Fig. 1. The target material in the form of a square with dimensions of 9  9 mm, thickness 2 mm was positioned inside a cuvette (inner dimensions 10  10  50 mm) (Suprasil 300 quartz) and held firmly in place with a flexible thin Teflon ring which was placed at the bottom of the cuvette, acting as a ‘spring’ and pushing the sample onto its vertical wall. The cuvette was filled with deionized water (DIW) (since water is also the solvent for PEDOT:PSS) and sealed. It was then placed onto an Aerotech moving stage (ANT-LX/V) which could be moved in the x, y and z directions with accuracy of ±0.5 lm. The laser beam was focused onto the target material surface using a lens (focal length 5 cm). Laser ablation was carried out by scanning the sample (and thus the beam on its surface) with a speed of 1 mm/s in a rectangular pattern with dimensions of 5  2 mm and pitch of 0.005 mm (meander fashion scanning). No bubbles were observed to adhere onto the sample surface during ablation which would had otherwise shielded or scattered the laser radiation. Ablation was carried out for 34 min each time and the formation of NPs could be confirmed by the change of colour of the DIW to ‘milky’ white.

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Fig. 3. TEM image of a NP with diameter of 35 nm (a) and 4.3 nm (b), and the corresponding EDX spectra measured on the NPs centres.

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Fig. 4. (a) A typical TEM image of the NPs, (b) histogram of their size distribution, (c) a typical AFM topography image of NPs assembled onto a clean silicon substrate, and (d) corresponding phase image.

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The total mass of the material ablated and transferred into the solvent as NPs during this time was estimated to be 0.72 mg. The synthesized NPs were characterized by Transmission Electron Microscopy (TEM) by drying out solution droplets onto carbon coated copper grids and using a high resolution JEOL JEM-3010 instrument equipped with EDS Genesis 4000 system to enable also acquisition of Energy-Dispersive X-ray spectroscopy (EDX) spectra, X-ray Diffraction (XRD) by drying out droplets of the colloidal solution onto clean glass substrates by using a Rigaku (MiniFlex) diffractometer with a Cu Ka source and Atomic Force Microscopy (AFM) by drying out droplets of the colloidal solution onto clean hydrophilic silicon substrates using a NT-MDT Solver P47H instru-

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Fig. 6. Raman spectra of the NPs and of the bulk material.

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Although the energy of the laser beam is lower than the band gap of the semiconductor, upon incidence of the laser radiation onto the material surface, electrons are still excited into its conduction band via the non-linear two or three photon absorption (due

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Table 1 Vibrational modes in the Raman spectra of PEDOT:PSS–ZnO NPs nanocomposite films.

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Assignment

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Asymmetric Ca@Cb stretching Symmetric Ca@Cb (–O) stretching Cb@Cb stretching Ca@C0a (inter-ring) stretching C–O–C deformation Oxyethylene ring deformation Oxyethylene ring deformation

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3. Results and discussion

to the high laser peak intensity). Within a time less than 1 ps the highly dense electron–hole plasma transfers its energy to the lattice via electron–phonon coupling and the material is then ablated forming a plasma plume of ablated species just above the material surface. This plasma could be seen as a bright white colour ‘spot’ at the point of incidence of the laser beam onto the material surface during the experiments. From a simple calculation (by taking into account a measured ablation depth per overscan of 12.8 lm) the surface enthalpy per pulse of the material during ablation (using the femtosecond pulse energy of 45 lJ) is estimated equal to 1.2  108 J/kg which is by three orders of magnitude larger than the sublimation (and therefore vaporization) enthalpy of 6.2  106 J/mol for ZnO [28]. This indicates that the ablation of the material is via sublimation, i.e. a direct solid to vapor transition. Cooling of the plasma plume due to its adiabatic expansion causes condensation of the plume species which leads to the formation of NPs by nucleation in the vapor phase. The UV–vis absorption spectrum from the NPs colloidal solution is shown in Fig. 2. The NPs size versus energy of absorption was calculated by considering the size quantization model for quantum dots [29,30]. According to this model, the shift (DE) to lower wavelengths (higher energies) of the first excited electronic state (the fundamental absorption edge) of a quantum dot with radius R (Eg(R)) from the band gap absorption edge of the corresponding ), is given by the relation: DE  Eg(R)  Ebulk ffi bulk material (Ebulk g g (p2⁄2/2m*R2)  (1.8e2/4pee0R), where m*  memh/(me + mh) is the electron–hole pair reduced mass. R versus energy (in wavelength

intensity-polymer (arb. units)

ment in non-contact mode with Si cantilevers with a given radius of curvature less than 10 nm. Raman spectra from the NPs were acquired by using a Jobin Yvon HR800 system (pumping beam at k = 514.48 nm) with a CCD detector. The colloidal solution was also characterized by UV–vis spectrophotometry. After formation by laser ablation, the NPs were mixed with the PEDOT:PSS CLEVIOS™ FE [13]. The final solution was estimated to be 10 wt.% in ZnO NPs. The resulting nanocomposite was deposited onto glass substrates which were pre-cleaned by ultrasonication for 10 min each time in acetone, isopropanol and methanol followed by UV-irradiation for 10 min, by spin coating at 400 rpm for 6 s and subsequently at 2000 rpm for 30 s and finally dried out on a hot plate at 90 °C for 10 min. Raman spectra of the nanocomposite thin films as well as AFM images were also measured. The thicknesses of the films were determined from Spectroscopic Ellipsometry (SE) measurements in the vis–farUV using a Jobin Yvon instrument. The resistances of the nanocomposite thin films were measured by using the Van der Pauw method at four point contact configuration using a Keithley 2400 source/meter and the resistivities were calculated by multiplying with the film thickness [27] (conductivity  1/resistivity).

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Fig. 8. (a) Raman spectra of the polymer and of the nanocomposite films in the region around the band corresponding to the symmetric Ca = Cb (–O) stretching mode of the PEDOT thiophene ring and (b1), (b2) corresponding peaks fitting.

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less than 10% of the total number of the produced NPs in the solution. Typical XRD spectra from the NPs and from the bulk material are shown in Fig. 5a. From the 2h angles where the peaks in the spectrum appear as well as from their relative intensities, it is confirmed that they correspond to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) reflections of the hexagonal (hcp) lattice of bulk ZnO (ref: JCPDS No. 36-1451) confirming the crystal structure (Wurtzite) of the synthesized NPs. The peaks in the spectrum from the NPs are also characterized by a prominent broadening. From the Debye–Scherer formula: D = (0.89k)/(FWHMcos h0), where k = 1.54 Å, the peak at 36.2° provides with particle diameter of D  6 nm which compares well with the median NPs diameter estimated from the TEM images. From high resolution TEM images of the produced NPs (a typical image is shown in Fig. 5b), the lattice spacing is determined equal to 2.85 Å which corresponds to the distances between the {0 1 0} or {1 0 0} lattice planes of the Wurtzp ite crystal structure of bulk ZnO within 1.5% error (dtheor = a 3/ 2 = 2.81 Å). In the Raman spectra from the NPs (Fig. 6) the 2LO and TO + LO modes are easily observed at 1127 and 1108 cm1 respectively [32]. This corresponds to a red shift by 16 cm1 of the same bands in the bulk material (which appear at 1143 and 1124 cm1, spectrum in the inset of Fig. 6) which is consistent with NPs with diameter again equal to 6 nm [33].

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units) predicted by the above relation is plotted in the inset of Fig. 2, considering the set of parameters me = 0.24, mh = 0.45 and e = 3.7 [31]. The absorption curve from the colloidal solution does not exhibit a ‘steep’ increase with wavelength from the infrared to visible, and this indicates that the colloidal solution contains NPs with a broad band gap distribution and therefore a broad size distribution according to the above model. The broad band centered at 345 nm implies an ensemble of NPs with an average diameter of 6 nm. This result is also confirmed by TEM imaging of the produced NPs, as it will be analyzed below. EDX analysis on individual NPs indicates that for each NP the stoichiometric ratio of Zn to O depends on its size. The larger NPs have a higher atomic percentage of Zn relatively to O (for instance it is Zn/O  1.5 for NPs with d  35 nm (Fig. 3a) while smaller NPs for instance with d  4.3 nm have Zn/O  0.8 (Fig. 3b). Detailed information about the size distribution of the produced NPs in the colloidal solution was obtained by TEM and AFM images (typical images are shown in Fig. 4a, c and d). The histogram of particle size distribution (shown in Fig. 4b by counting approximately 350 particles in images of particle ensembles obtained on different areas on the grid) is described quite well by a log-normal function with a median diameter of d0  5.4 nm and a geometrical standard deviation r  0.5. NPs with diameters from 2 up to 45 nm are observed to be formed in the colloidal solution, however the percentage of NPs with diameters above 20 nm is estimated to be

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Fig. 9. AFM topography, corresponding phase image and ellipsometry spectrum of the films spin-coated from the polymer (a), (c) and (e) and from the nanocomposite (b), (d) and (f), respectively.

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3.2. Nanocomposite films of PEDOT:PSS–ZnO NPs In the Raman spectra of the nanocomposite films the peak from the 2LO phonon mode from the ZnO NPs appears as a shoulder (at 1128 cm1) in the right hand side of the C–O–C deformation band of the oxyethylene ring of PEDOT (Fig. 7 which shows the Raman spectra from the polymer as well as from the nanocomposite in the whole spectral region from 200 to 2000 cm1). Other peaks in the spectra are identified in Table 1 based on reports in the literature [34,35]. The interaction of the ZnO NPs with the polymer is best seen by a careful observation of the Raman spectra in the region around the band corresponding to the symmetric Ca = Cb (–O) stretching mode of the PEDOT thiophene ring (Fig. 8a). In the spectrum from the polymer, the band (centered at 1452 cm1) appears quite broad and it can in fact be deconvoluted into two peaks at 1446.2 and 1462.6 cm1 (Fig. 8b1) while in the spectrum from the nanocomposite only one peak at 1446.2 cm1 can be fitted in the same band (Fig. 8b2). This indicates a transformation of the resonant structure of the polymer upon incorporation of the ZnO NPs, from a mixture of benzoid and quinoid in the polymer to a mostly quinoid in the nanocomposite [36,37]. Films spin-coated from the polymer exhibit the characteristic surface morphology of PEDOT:PSS films consisting of grains (PEDOT-rich regions) and large areas among them which appear almost featureless (PSS-rich regions) (typical images are shown in Fig. 9a and c) [38,39]. A distinctly different surface morphology is observed for the film spin-coated from the nanocomposite (typical images are shown in Fig. 9b and d). Its surface is characterized by larger size grains. This change of the surface morphology of the PEDOT:PSS film is due to conformational changes of the macromolecular polymer chains from a coil conformation to a linear or expanded-coil conformation induced by the ZnO NPs, consistent with the chemical structure changes observed from the Raman spectra (Fig. 8). The strongly electronegative oxygen atom of the hydroxyl groups (–OH) on the NPs surfaces [40], may form hydrogen bonds with the sulphur cation (S+) of the thiophene ring of PEDOT thus weakening the electrostatic interaction between PEDOT and PSS. There might also be a dipole–dipole type of interaction between a dipole moment which may be possible to exist in the NPs (due to an inhomogeneous and unbalanced distribution of positive and negative charges) with the dipole moment of the ethylenedioxythiophene of PEDOT. These interactions may be the driving force for the observed conformational change of the PEDOT chains. For the analysis of the spectroscopic ellipsometry spectra (Fig. 9e and f) we have used the Bruggemann Effective Medium Approximation (BEMA) [41]. For the case of the film spin-coated from the nanocomposite (Fig. 9f), the optical response of the organic and inorganic parts have been modelled by the use of the TaucLorenz oscillator model [42]. The thicknesses of the films calculated from the analysis of the spectroscopic ellipsometry spectra and their corresponding resistances measured by the Van der Pauw method are summarized in Table 2 for three samples spin-coated under the same conditions. Calculation of the resulting conductivities of the films (also shown in Table 2) shows that the films spin-

Table 2 Thicknesses, resistances and resulting electrical conductivities of PEDOT:PSS and PEDOT:PSS–ZnO NPs films. Film type

Thickness (nm)

Resistance (X)

Conductivity (S/cm)

PEDOT:PSS

180 ± 3

330 ± 7

168 ± 6

PEDOT:PSS–ZnO NPs(#1) PEDOT:PSS–ZnO NPs(#2) PEDOT:PSS–ZnO NPs(#3)

120 ± 3 122 ± 3 120 ± 3

290 ± 6 250 ± 5 300 ± 6

287 ± 12 327 ± 13 277 ± 12

coated from the nanocomposite exhibit higher conductivities than the films spin-coated from the polymer. This is due to the fact that in the linear or expanded-coil conformation (mostly quinoid structure of the polymer in the nanocomposite) the charge carriers have a higher intrachain mobility due to the delocalization of the conjugated p-electrons over the whole PEDOT chain because of the orientation of neighboring thiophene rings in almost the same plane. 4. Conclusions We have synthesized zincblende (Wurtzite) ZnO NPs with a median diameter of 5.4 nm by femtosecond laser ablation (775 nm, 180 fs, 1 kHz, pulse energy = 45 lJ (fluence = 11 J/cm2)) of the corresponding solid target in DIW. Incorporation of the NPs into PEDOT:PSS causes a transformation of the resonant structure of the polymer from a mixture of benzoid and quinoid to a mostly quinoid in the nanocomposite as measured by Raman spectroscopy and a conformational change of the polymer chains from a coil to a linear or expanded-coil conformation as measured by AFM imaging. These result in an increase of the electrical conductivity of the polymer by almost twice (from 168 to 300 S/cm). Laser ablation is an easy, fast and ‘green’ method for NPs synthesis and this work demonstrates that ZnO NPs synthesized by using the method of laser ablation of the corresponding solid target in a liquid environment instead of by other traditionally used methods can be used to modify the electrical conductivity of the organic material PEDOT:PSS which is commonly used as an anode electrode layer in organic electronic devices. Acknowledgement N.G.S. acknowledges support by a Marie Curie European Reintegration Grant (ERG) number: PERG03-GA-2008-226029, 83573 under the project OMALANP (‘Organic MAterials Laser Ablation NanoParticles’). References [1] L.B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12 (2000) 481. [2] S. Kirchmeyer, K. Reuter, J. Mater. Chem. 15 (2005) 2077. [3] S.-I. Na, S.-S. Kim, D.-Y. Kim, Adv. Mater. 20 (2008) 4061. [4] A. Colsmann, F. Stenzel, G. Balthasar, H. Do, U. Lemmer, Thin Solid Films 517 (2009) 1750. [5] J.Y. Kim, J.H. Jung, D.E. Lee, J. Joo, Synth. Met. 126 (2002) 311. [6] A.M. Nardes, R.A.J. Janssen, M. Kemerink, Adv. Funct. Mater. 18 (2008) 865. [7] J. Ouyang, C.-W. Chu, Q. Xu, Y. Yang, Adv. Funct. Mater. 15 (2005) 203. [8] J. Ouyang, Q. Xu, C.-W. Chu, Y. Yang, G. Li, J. Shinar, Polymer 45 (2004) 8443. [9] T. Wang, Y. Qi, J. Xu, X. Hu, P. Chen, Appl. Surf. Sci. 250 (2005) 188. [10] X. Crispin et al., J. Polym. Sci. Polym. Phys. 41 (2003) 2561. [11] B. Fan, X. Mei, J. Ouyang, Macromolecules 41 (2008) 5971. [12] H. Do et al., Thin Solid Films 517 (2009) 5900. [13] . [14] B.R. Saunders, M.L. Turner, Adv. Colloid Interf. Sci. 138 (2008) 1. [15] W.J.E. Beek, M.M. Wienk, R.A.J. Janssen, Adv. Funct. Mater. 16 (2006) 1112. [16] G.D. Sharma, P. Suresh, P. Balaraju, S.K. Sharma, M.S. Roy, Synth. Met. 158 (2008) 400. [17] M. Wang, Y. Lian, X. Wang, Curr. Appl. Phys. 9 (2009) 189. [18] C.T. That, M.R. Phillips, T.-P. Ngyyen, J. Lumin. 128 (2008) 2031. [19] T. Zhang, Z. Xu, D.L. Tao, F. Teng, F.S. Li, M.J. Zheng, X.R. Xu, Nanotechnology 16 (2005) 2861. [20] W.J.E. Beek, M.M. Wienk, M. Kemerink, X. Yang, R.A.J. Janssen, J. Phys. Chem. 109 (2005) 9505. [21] C. Pacholski, A. Kornowski, H. Weller, Angew. Chem. Int. Ed. 41 (2002) 1188. [22] D.Z. Sun, M.H. Wong, L.Y. Sun, Y.T. Li, N. Miyatake, H.J. Sue, J. Sol–Gel Sci. Technol. 43 (2) (2007) 237. [23] D.L. Tao, Z.W. Qian, Y. Huang, F. Wei, J. Cryst. Growth 27 (2004) 353. [24] G.W. Wang, Prog. Mater. Sci. 52 (2007) 648. [25] Y. Yang, H. Long, G. Yang, A. Chen, Q. Zheng, P. Lu, Vacuum 83 (2009) 892. [26] D. Strickland, G. Mourou, Opt. Commun. 56 (1985) 219. [27] . [28] D.F. Anthrop, A.W. Searcy, J. Phys. Chem. 68 (1964) 2335. [29] L.E. Brus, J. Chem. Phys. 80 (1984) 4403. [30] L.E. Brus, IEEE J. Quant. Electr. 22 (9) (1986) 909.

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