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Solar Energy 81 (2007) 529–534

Electrodeposition of TiO2/SiO2 nanocompositefor dye-sensitized solar cell

The-Vinh Nguyen 1, Hyun-Cheol Lee, M. Alam Khan, O-Bong Yang *

School of Environmental and Chemical Engineering, Chonbuk National University, Jeon-Ju, South Korea

Received 2 March 2006; received in revised form 17 June 2006; accepted 11 July 2006Available online 14 September 2006

Communicated by: Associate Editor Sam-Shajing Sun

Abstract

For the working electrode of dye-sensitized solar cell (DSC), TiO2/SiO2 nanocomposite materials were electrodeposited on transpar-ent fluorine doped tin oxide-coated glass by cathodic electrodeposition at room temperature. The electrode and DSC fabricated withTiO2/SiO2 nanocomposite were characterized with photocurrent density, X-ray diffraction (XRD), field emission-scanning electronmicroscopy (FE-SEM) and a photovoltaic performance test. On the electrodeposition, the addition of an appropriate amount ofSiO2 in the bath containing TiO2 slurry was essential to achieve the superior crystallinity, photocurrent density and photovoltaic perfor-mance of the resulting TiO2/SiO2 electrode, which was significantly superior to a bare TiO2 electrode. This enhanced performance ofoptimized TiO2/SiO2 electrode was ascribed to the role of SiO2 as an energy barrier, increasing the physical separation of injected elec-trons and oxidized dyes/redox couple, and thereby retarding the recombination reactions in the resulting DSC.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Dye-sensitized solar cell; Electrodeposition; TiO2/SiO2 nanocomposite; Energy barrier

1. Introduction

Dye-sensitized solar cell (DSC) has been known as apromising photovoltaic device to achieve moderate effi-ciency at ultra-low cost (O’Regan and Gratzel, 1991).The use of large surface area semiconductor materials inDSC is necessary to provide sufficient light absorption withonly one adsorbed mono-layer of dye. For the workingelectrode in DSC, nanoporous TiO2 thin film had been pre-pared by coating the hydrolyzed titanium alkyloxidesunder acidic conditions on transparent conducting oxide(TCO) glass and calcining at 450 �C for ca. 30 min toremove organic compounds. The latter process decreasesthe level of the specific surface area of TiO2 substrate at

0038-092X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.solener.2006.07.008

* Corresponding author. Tel.: +82 63 270 2313; fax: +82 63 270 2306.E-mail address: obyang@chonbuk.ac.kr (O.-B. Yang).

1 Present address: Faculty of Environment, Hochiminh City Universityof Technology, Hochiminh City, Viet Nam.

the high temperature calcination. In order to avoid hightemperature calcination of TiO2 thin film, Pichot et al.reported on the low temperature calcination of TiO2 col-loid films at around 100 �C for DSC (Pichot et al., 2000).And Lindstrom et al. reported a new method of ‘‘non-sin-tered nanostructured porous electrode’’ preparation as aneffort to prepare TiO2 substrate at room temperature with-out calcination step (Lindstrom et al., 2002).

In the present study, TiO2 and TiO2/SiO2 thin films areprepared at room temperature by using a cathodic electro-deposition method. This technique has been well known toprovide simple and inexpensive alternative routes to syn-thesize nanoparticulate materials and is also feasible to pre-pare non-flat substrates for flexible DSCs (Therese andKamath, 2000). In this work, the electrodeposition condi-tions were optimized and the properties of electrodepositedelectrodes were elucidated in terms of their photocurrentdensities and the photovoltaic performance of resultingDSCs. Optimized TiO2/SiO2 nanocomposite thin films

Nomenclature

FF fill factorI current density (mA/cm2)ISC short-circuit current density (mA/cm2)g overall conversion efficiencyx, n mol of SiO2 and TiO2 in the complex

[(SiO2)x(TiO2)n](n�x)+; n P x

M SiO2/TiO2 molar ratioV voltage (V)VOC open-circuit voltage (V)PZC point of zero charge

530 T.-V. Nguyen et al. / Solar Energy 81 (2007) 529–534

could be prepared by introducing SiO2 as a wide band-gapmaterial, which resulted in a significant improvement of theperformance of corresponding DSC. In the optimizedTiO2/SiO2 electrode, SiO2 seemed to act as an energy bar-rier to suppress recombination at the TiO2/electrolyteinterface.

2. Experimental

2.1. Materials

TiO2 powder (P25, Degussa), fumed silica (SiO2, specificsurface area: 335 m2/g, Aldrich), Eosin Y (Aldrich) as a dyesensitizer were used as received. The supporting substrateswere optically transparent fluorine doped tin oxide-coatedglass supplied by Libbey-Owens-Ford (TEC-8, FTO-coated glass, 8 X/sq, 80% transmittance in the visible).

2.2. Preparation of TiO2 thin film

Cathodic electrodeposition of TiO2 nanoparticles on theoptically transparent fluorine doped tin oxide-coated(FTO) glass was carried out in an electrochemical bath(100 ml). Negative potentials (V vs. Ag/AgCl) were appliedon FTO glass plate (15 mm · 15 mm) by the potentiostat inan aqueous slurry containing 0.8 M TiO2. The electrode-position bath was purged continuously with ultra-purenitrogen gas under mild magnetic stirring. The bath pHwas controlled by 35% HCl (for acid range) or Na2SO3

(for base range). The scanning time of potentiogalvanostatwas fixed at 600 s for every experiment. After electrodepos-ition, the resulting TiO2 electrode film was gently rinsedwith distilled and de-ionized water and then dried in airat 75 �C for 30 min. For comparison, dried films were sub-ject to calcining in air at 150 �C, 250 �C, 350 �C, 450 �Cand 550 �C for 30 min.

2.3. Preparation of TiO2/SiO2 nanocomposite thin film

For the preparation of TiO2/SiO2 nanocomposite elec-trode thin films, the same procedures for TiO2 thin filmpreparation were repeated except for the addition of fumedsilica with the SiO2/TiO2 molar ratios of 0.27, 0.40, 0.53,0.67 and 0.80. The resulting film was denoted as TiO2/SiO2–M (where M indicates the SiO2/TiO2 molar ratio inthe bath). For comparison, electrode thin films of TiO2

and TiO2/SiO2 were prepared by doctor-blade method withthe SiO2/TiO2 molar ratios of 0.07, 0.13 and 0.20, whichwere represented as Db–TiO2 or Db–TiO2/SiO2–M (Ngu-yen et al., 2006).

2.4. Fabrication of DSC

The sandwich DSC of two electrodes was fabricated bythe typically known method with the dye (Eosin Y)adsorbed working electrode of TiO2 or TiO2/SiO2, a Pt-sputtered counter electrode and an organic electrolyte(0.3 M LiI, 15 mM I2, and 0.2 M tert-butyl pyridine in ace-tonitrile) as described elsewhere (Park et al., 2000). Theactive area of the resulting cell was approximately 0.25 cm2.

2.5. Characterization of electrode thin film and DSC

Electrode thin films were characterized with XRD (Rig-aku, Cu Ka radiation) and FE-SEM (S-4700, Hitachi)equipped energy-dispersive X-ray (EDX) element analysissystem (EMAX, Horiba). The photocurrent densities ofTiO2-based films were measured at pH 9 by using a scan-ning potentiostat (EG&G 273) with the working electrodeof TiO2-based film, the counter electrode of the Pt wire andthe reference electrode of Ag/AgCl/3 M NaCl in an electro-chemical cell (100 ml) containing 0.1 M Na2SO3 electrolyte.Photocurrent–voltage (I–V) curves of DSCs were measuredby using two computerized digital multimeters (Model2000, Keithley) and a variable load. A 150 W Xe lamp(Hamamatsu) with a visible-band pass filter served as alight source and its intensity was adjusted for AM-1.5irradiation.

3. Results

3.1. Optimization of electrodeposition parameters

Photocurrent density correlates well with the chargetransfer property of TiO2 film in DSC (Nguyen et al.,2006). The higher the photocurrent density of TiO2 film,the higher the short-circuit current of resulting DSC (Ngu-yen et al., 2006). Thus, we evaluated and optimized theelectronic properties of electrodeposited TiO2/SiO2 filmsbased on their photocurrent densities. The electrodeposit-ion conditions were optimized by studying the effect ofdeposition parameters on the photocurrent densities of

0

50

100

150

200

250

300

0.40-3V

(c)(b)(a)

0.80

0.67

0.53

0.27

-6V

-5V

-4V

-2V

pH9TCO

pH6

pH5

pH4

pH2

Ph

oto

curr

ent

den

sity

(μA

/cm

2 )

Fig. 1. The effect of pH (a), deposition potential (b) and molar ratio ofSiO2/TiO2 (c) on the photocurrent density at zero DC bias of resultingTiO2/SiO2 film. Electrodeposition conditions: (a) deposition potential =�4 V, SiO2/TiO2 = 0.53; (b) pH 5, SiO2/TiO2 = 0.53; (c) pH 5, depositionpotential = �4 V.

Fig. 2. Cross-section FE-SEM image of TiO2/SiO2-0.53 film as-prepared.

20 30 40 50 60 70 80

R : rutileA : anatase

RATiO

2/SiO

2-0.53

bare TiO2

TCO glass

2-theta (degree)

Inte

nsi

ty (

a.u

.)

Fig. 3. XRD patterns of bare TiO2 and TiO2/SiO2-0.53 films as-prepared.

T.-V. Nguyen et al. / Solar Energy 81 (2007) 529–534 531

resulting TiO2/SiO2 films. The effects of various parametersof the deposition bath on the photocurrent densities aresummarized in Fig. 1. While a parameter is varied, the oth-ers are fixed at their preliminarily optimum values. How-ever, the trends shown in Fig. 1 are consistent whateverthe other electrodeposition conditions are chosen. Fig. 1adepicts the effect of bath pH on the photocurrent densitiesof electrodeposited TiO2/SiO2 films. The photocurrent den-sities were peaked at pH 5. At pH values higher than thepoint of zero charge (PZC) of TiO2 (5.5 (Palomareset al., 2003)), the TiO2 nanoparticles would be negativelycharged. The negatively charged TiO2 nanoparticles arerepelled electrostatically from the cathode surface owingto the negative potential applied at working electrodes.This repelling force may cause weak deposition of TiO2

nanoparticles on FTO glass and increase the resistance,resulting in the low photocurrent density of derivedTiO2/SiO2 film. At very low pH value, hydrogen gas wasevolved significantly at the working electrode, which givesrise to adverse effect on the electrodeposition. Fig. 1b illus-trates that �4 V is the optimal potential to deposit effec-tively TiO2 nanoparticles on FTO. Fig. 1c shows theeffect of SiO2 addition on the photocurrent density ofderived TiO2/SiO2 film. The highest photocurrent densitywas observed in the electrode prepared by adding silicawith a SiO2/TiO2 molar ratio of 0.53, indicating the opti-mal condition in terms of the SiO2 concentration in thebath. In summary, optimal electrodeposition parametersare �4 V of deposition potential, pH 5 and ca. 0.53 ofSiO2/TiO2 molar ratio. All of the thin films used in thecharacterization and DSC fabrication were prepared underthis optimal condition.

3.2. Characterization of electrodeposited thin films

The thin film prepared in the optimal electrodepositioncondition, TiO2/SiO2-0.53 film was characterized by FE-

SEM (Fig. 2) and EDX analysis. It showed that thecross-sectional thickness was ca. 2.3 lm and SiO2/TiO2

molar ratio was 0.1. The amount of deposited silica wasjust 10%, even though 50% of silica was contained in thebath originally. It could be explained by the fact that pos-itively charged titania particles might be deposited easily ascompared to negatively charged silica particles in the bathpH of 5 (PZCTiO2

> 5 > PZCSiO2). The morphology and

composition of the TiO2/SiO2 film were not changed aftercalcination at 450 �C. Fig. 3 shows the XRD patterns ofbare TiO2 and TiO2/SiO2-0.53 films. The intensity of ana-tase peak (2h = 25.5�) of TiO2/SiO2-0.53 film is muchhigher than that of bare TiO2 film, indicating the superiorcrystallinity of the former compared to the latter. Theintensities of the anatase peaks are almost the same beforeand after calcination at 450 �C (not shown).

Fig. 4 shows the photocurrent densities (at zero DCbias) of bare TiO2 and TiO2/SiO2-0.53 films as a functionof calcination temperature. The photocurrent densitiesare significantly improved upon calcination. In DSC, calci-nation step of TiO2 film after coating TiO2 gel or slurry onthe TCO glass is well known and commonly employed toproduce partial sintering between the nanoparticles andbetween the nanoparticles and TCO glass for establishingan electrical contact among them (O’Regan et al., 1990).

50 150 250 350 450 5500.2

0.3

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0.8

0.9

fe

d

c

b

a

3.8%20.6%

17.3%

14.6%

12.5%

19.6%

Ph

oto

curr

ent

den

sity

(m

A/c

m2 )

Temperature (ºC)

TiO2

TiO2/SiO2-0.53

Fig. 4. Photocurrent densities (at zero DC bias) of bare TiO2 and TiO2/SiO2-0.53 films as a function of calcination temperature. Numbers on thevertical lines are the percentage increase of photocurrent density of TiO2/SiO2 film compared to that of TiO2 film. Letters a–f denote the differencesin photocurrent densities between TiO2 and TiO2/SiO2-0.53 films:a = 0.04, b = 0.05, c = 0.07, d = 0.10, e = 0.15 and f = 0.03 mA/cm2.

Table 1Photovoltaic performance data of DSCs fabricated with various thin filmsand Eosin Y sensitizer

Thin film VOC

(V)ISC (mA/cm2)

FF(%)

g(%)

Photo-currenta

TiO2 as-deposited 0.43 0.15 48 0.03 0.214TiO2/SiO2-0.53 as-

deposited0.46 0.17 46 0.04 0.256

TiO2 calcined at 450 �C 0.58 0.37 54 0.12 0.708TiO2/SiO2-0.53 calcined

at 450 �C0.60 0.55 54 0.18 0.854

a Photocurrent densities (mA/cm2) of thin films at zero DC bias.

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The photocurrent densities of TiO2/SiO2-0.53 films arehigher than those of bare TiO2 films calcined at varioustemperatures as indicated in Fig. 4. The absolute differ-ences in photocurrent densities were also increased as afunction of calcination temperature except for the valueat 550 �C as presented by letters of a–f in Fig. 4.

3.3. Photovoltaic performances of DSCs

Fig. 5 and Table 1 present the photocurrent–voltage (I–V) curves and photovoltaic performance data of DSCs fab-ricated with the electrodeposited films and Eosin Y as a dyesensitizer. TiO2/SiO2 film-based DSCs show the superiorphotovoltaic performance compared to the bare TiO2 filmcounterparts. The performances of DSCs fabricated with

-1000 -500 00

1

2

3

(d)

(b)

(c)(a)

Ph

oto

curr

ent

den

sity

(m

A/c

m2 )

Voltage (mV vs. Ag/AgCl)

Fig. 5. Photocurrent densities of TiO2/SiO2 films prepared by doctor-blade method as a function of SiO2/TiO2 molar ratio: Db–TiO2/SiO2-0.20(a), Db–TiO2/SiO2-0.13 (b), Db–TiO2/SiO2-0.07 (c) and Db–TiO2 (d).

the films calcined at 450 �C are much improved in compar-ison with those fabricated with the as-prepared films.

4. Discussion

4.1. Effect of SiO2 addition on electrodeposition

Since the PZC of TiO2 is 5.5, it becomes positivelycharged in the optimal experimental conditions (pH 5).Meanwhile, SiO2 releases proton (H+) under this pH valueand becomes negatively charged owing to its low PZC of 2(Palomares et al., 2003). Consequently, positively chargedtitania and negatively charged silica attract one anotherto form a titania–silica complex, [(SiO2)x(TiO2)n](n�x)+ inthe bath (where x and n stands for the mole of SiO2 andTiO2 in the complex, respectively; n P x). Some of thesetitania–silica complexes in the bath are electrodepositedon the working electrode under applied negative potential.While the others are still in the bath and therefore, increasethe viscosity of the suspension at a certain value of SiO2

concentration. The addition of SiO2 in the bath accord-ingly increases the numbers of protons and the viscosityof the suspension. The increase of the number of protonsin the bath is beneficial to the formation of positivelycharged titania and the amount of electrodeposited titaniawould be enhanced on the working electrode. Meanwhile,the high viscosity of suspension gives rise to the suppres-sion of particles electrodeposition. Consequently, asincreasing the concentration of SiO2 in the bath, theamount of electrodeposited titania would be increaseddue to the positive effect of proton, and then decreasedowing to the negative effect of viscosity, resulting in maxi-mum photocurrent density on TiO2/SiO2-0.53 film.

4.2. The role of SiO2 in TiO2/SiO2 electrode

In this work, we tried to introduce SiO2 as a wide band-gap material in the working electrode of DSC by electrode-position method. There are some reports on the formationof energy barrier on semiconductor electrodes by coating athin layer of insulating oxide such as Al2O3, MgO, SiO2,TiO2, ZnO, ZrO2, etc. (Palomares et al., 2003; Diamantet al., 2003; Kay and Gratzel, 2002). Energy barrier atthe semiconductor/electrolyte interface was found to

0.00

0.1 0.2 0.3 0.4 0.5 0.6

(d)

(c)

(b)

(a)

I ( A

/cm

2 )

V (V)

200

100

300

400

500

600

μ

Fig. 6. Photocurrent–voltage (I–V) curves of DSCs fabricated with EosinY sensitizer and various thin films: (a) TiO2 as-prepared, (b) TiO2/SiO2-0.53 as-prepared, (c) TiO2 calcined at 450 �C and (d) TiO2/SiO2-0.53calcined at 450 �C.

T.-V. Nguyen et al. / Solar Energy 81 (2007) 529–534 533

decrease the interaction between the photo-excited elec-trons in the semiconductor electrode and the electrolyteions (Palomares et al., 2003; Diamant et al., 2003; Kayand Gratzel, 2002). The suppression of charge recombina-tion has been known due to the tunneling effect (Kay andGratzel, 2002; Tennakone et al., 2002) that allows electroninjection across the barrier (thin insulating oxide layer) andprevents the electron leakage from the semiconductor tothe electrolyte ions.

The role of SiO2 as an energy barrier in TiO2/SiO2-0.53film could be explained by the results shown in Fig. 4. Thephotocurrent density difference between TiO2/SiO2-0.53and TiO2 film as-deposited (dried at 75 �C without calcina-tion) is 0.04 mA/cm2, which is consistent with the superiorcrystallinity of the former compared to the latter as pre-sented in Fig. 3. As increasing the calcination temperature,the photocurrent densities of both films are significantlyimproved owing to the enhancement of bonding betweenTiO2 nanoparticles and between the TiO2 nanoparticlesand TCO glass (O’Regan et al., 1990). The absolute differ-ences of photocurrent densities between TiO2/SiO2-0.53and TiO2 film become larger as the calcination temperatureincreases up to 450 �C: 0.05 mA/cm2 at 150 �C, 0.07 mA/cm2 at 250 �C, 0.10 mA/cm2 at 350 �C and 0.15 mA/cm2

at 450 �C. On the film as-deposited (dried at 75 �C), thebonding between TiO2 and SiO2 may not be chemicallystrong enough to generate the function of energy barrier.However, the chemical bonding between TiO2 and SiO2

would be formed tightly at around 450 �C, in which thefunction of SiO2 as the energy barrier might be developedsignificantly to suppress the charge recombination at theTiO2/SiO2-0.53 film/electrolyte interface. By this energybarrier function of SiO2, the superior photovoltaic perfor-mance of TiO2/SiO2-0.53 electrode could be explained well.At 550 �C, slight decreasing of photocurrent density differ-ence between TiO2/SiO2-0.53 and TiO2 film seems to bedue to the conductivity decreasing of TCO glass after cal-cination at high temperature (Meng and Placido, 2003).

To further elucidate the role of SiO2 as an energy barriermaterial in TiO2/SiO2 film, we compared the electronicproperties of electrodeposited films with those of films pre-pared by doctor-blade method. Fig. 5 shows the photocur-rent densities of Db–TiO2 and Db–TiO2/SiO2 filmsprepared with the physical mixture of TiO2 and SiO2 bydoctor-blade method and calcined at 450 �C. In contrastto the electrodeposited TiO2/SiO2 film, photocurrent den-sity of Db–TiO2/SiO2 film was decreased drastically ascompared to the bare Db–TiO2 film. The synergistic effectof silica addition was not observed in the film prepared bydoctor-blade method whatever the molar ratios of SiO2/TiO2 are, which seems to be due to the insulating natureof SiO2 [band-gap energy of 8.0–8.9 eV (Zacheis et al.,2001)] dispersed physically in TiO2 network. This randommixed structure is not advantageous to the charge trans-port in the Db–TiO2/SiO2 network and between this net-work and TCO glass, resulting in the low photocurrentdensity of Db–TiO2/SiO2 film as compared to Db–TiO2

film. However, in electrodeposition process, the electrostat-ical attractions between TiO2 and SiO2 nanoparticlesformed a titania–silica complex, [(SiO2)x(TiO2)n](n�x)+

which has the intimate contacts between them in compari-son with doctor-blade method. Consequently, the presenceof SiO2 in TiO2/SiO2 films prepared by electrodepositionmethod gives rise to the decrease of charge recombinationowing to the efficient role of SiO2 as an energy barrier.

The role of SiO2 as an energy barrier in TiO2/SiO2-0.53film is well consistent and supported by the photovoltaicperformance data of DSCs in Fig. 5 and Table 1. TiO2/SiO2 film-based DSCs show the superior photovoltaic per-formance compared to the bare TiO2 film counterparts.The superior performance is significant on the DSC fabri-cated with TiO2/SiO2-0.53 calcined at 450 �C. Consistencybetween the short-circuit current and the photocurrentdensity is observed on all of the cells, suggesting that theelectronic property of thin film is correlated well with thephotovoltaic performance of resulting DSC. The open-cir-cuit potential (VOC) is theoretically determined by the dif-ference between the potential of redox electrolytes andthe quasi-Fermi level of electrons in TiO2 in the light,which strongly depends on the density of photo-excitedelectrons injected from the adsorbed dye, Eosin Y, intoTiO2 network. When the TiO2-based films are calcined at450 �C, the nanoparticles in TiO2-based network are sub-jected to partial sintering that renders them chemicallybonded. This process not only increases the charge trans-port rate due to the enhanced number of interconnectionsbetween the particles (Frank et al., 2004) but also improvesthe dispersion of photo-excited electrons in the TiO2 net-work. The latter could suppress the charge recombinationthat brings about the increase in the density of photo-excited electrons in the TiO2-based network and there-fore negatively shifts its quasi-Fermi level (Frank et al.,2004). Consequently, DSCs fabricated with the calcined

534 T.-V. Nguyen et al. / Solar Energy 81 (2007) 529–534

TiO2-based films exhibit very high VOC in comparison tothose fabricated with the as-prepared counterparts (Fig. 6).

5. Conclusions

A novel cathodic electrodeposition of TiO2/SiO2 nano-composite film was elucidated and optimized for the prep-aration of electrode materials of DSC. The optimalelectrodeposition condition was �4 V of deposition poten-tial and bath pH 5. The addition of SiO2 around the SiO2/TiO2 molar ratio of 0.53 was essential to obtain the supe-rior TiO2/SiO2 electrode (TiO2/SiO2-0.53). The resultingTiO2/SiO2-0.53 nanocomposite electrode facilitated theincrease of ca. 20% of photocurrent density and ca. 30%of photovoltaic efficiency in comparison with the bareTiO2 electrode. The insulating tunneling role of SiO2 asan energy barrier was observed in the electrodepositedTiO2/SiO2-0.53 film, which may suppress the chargerecombination at the TiO2/electrolyte interface. This syner-gistic effect of silica addition was not recognized in thephysically mixed TiO2/SiO2 thin film prepared by doctor-blade method. Introduction of SiO2 as an energy barrierby electrodeposition consequently resulted in the improve-ment of photocurrent density of the derived TiO2/SiO2-0.53 film and the photovoltaic performance of corre-sponding DSC. This electrodeposition method can beapplied as an advanced microscopic way to control theelectrochemical properties of the electrodes.

Acknowledgements

We gratefully acknowledge the financial support fromthe Korea Industry Technology Foundation (KITF) bythe program of Human Resources Development for Regio-nal Innovation and the Center for Ultra Micro ChemicalProcess Systems (CUPS) sponsored by Korea Science andEngineering Foundation (KOSEF). T.-V. Nguyen andM. Alam Khan thanks for the financial support from theKOSEF through the foreigner scholarship program.

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