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Materials Chemistry and Physics 129 (2011) 887– 891

Contents lists available at ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

tudying trivalent/bivalent metal ion doped TiO2 as p-TiO2 in bipolareterojunction devices

oumen Dasa, Dae-Young Kima, Han-Seok Choia, Yoon-Bong Hahna,b,∗

School of Semiconductor and Chemical Engineering, Chonbuk National University, 567 Baekje-daero Deokjin-gu, Jeonju-si 561-756, Republic of KoreaWCU Department of BIN Fusion Technology, and Nanomaterials Processing Research Center, Chonbuk National University, 567 Baekje-daero Deokjin-gu,

eonju-si 561-756, Republic of Korea

r t i c l e i n f o

rticle history:eceived 2 October 2010eceived in revised form 27 April 2011ccepted 13 May 2011

a b s t r a c t

Trivalent/bivalent metal ions doped TiO2 thin films (MxTi1−xO2, M = Cr3+, Fe3+, Ni2+, Co2+, Mn2+ and x = 0.01,0.05, 0.1, 0.15, 0.2) were deposited on Indium–tin oxide (ITO) coated glass substrates by spin coating tech-nique. X-ray photoelectron spectroscopy (XPS) showed Ti4+ oxidation state of the Ti2p band in the doped

eywords:et chemical synthesis

hin films-TiO2

lectrical characterization

p-TiO2. The homogenous MxTi1−xO2 was used to support n-ZnO thin films with thickness ∼40–80 nmand vertically aligned n-ZnO nanorods (NR) with length ∼300 nm and 1.5 �m. Current (I)–voltage (V)characteristics for the Ag/n-ZnO/MxTi1−xO2/ITO/glass assembly showed rectifying behavior with smallturn-on voltages (V0) < 1 V. The ideality factor (�) and the resistances in both forward and reverse biaswere calculated. The temperature dependence performance of these bipolar devices was performed andvariation of the parameters with temperature was studied.

. Introduction

Until recently, much of the researches in dye-sensitized-solar-ells (DSSCs) have been focused on the sensitization of n-typeemiconductors, such as titanium dioxide. In a p-type DSSC the pho-oexcited sensitizer is reductively quenched by the hole injectionnto the valence band of a p-type semiconductor. The study of p-SSC is important both to understand the factors that control the

ate of hole photoinjection and to aid to rational design of efficientSSCs [1].

In a review article [2], Zaleska pointed at the several approachesor TiO2 modification such as metal ion implanted TiO2 (using tran-ition metals like Cu, Co, Ni, Mn, Mo, Nb, V, Fe, Ru, Au, As, Pt) or,on-metal doped TiO2 (N, S, C, B, P, I, F) [3–7]. In fact, bivalent andrivalent elements are used as dopants to create donor or accep-or like impurity in pure TiO2. The process can be expressed in theollowing manner

O → MTi + Uo + Oo∗ (1)

here MTi is the bivalent/trivalent ‘M’ substitution in Ti sites, Uo ishe oxygen vacancy and Oo* denotes the missing oxygen. Therefore,he technique of cation doping provides a shortest route to alter the

∗ Corresponding author at: School of Chemical Engineering and Technology,honbuk National University, 567 Baekje-daero Deokjin-gu, Jeonju-si 561-756,epublic of Korea. Tel.: +82 63 270 2439; fax: +82 63 270 2306.

E-mail address: ybhahn@chonbuk.ac.kr (Y.-B. Hahn).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.05.039

© 2011 Elsevier B.V. All rights reserved.

electronic and catalytic properties for gas interaction at the inter-face. Bhatnagar and co-workers [8] observed that by addition oftrivalent dopant the concentration of oxygen vacancies increasesin titania lattice. It is also suggested that with increasing doping theoxidation state of Ti increases indicating a charge transfer from Tito Cr site. In photochemical solar cell cation doping provides addi-tional bands within the band gap of TiO2 for easy transportation ofcharge. The roles of doped TiO2 as catalysis are well documented[9,10]. TiO2 layer is also used to optimized charge injection andtransport for efficient organic light emitting devices (OLED) [11,12],or as hole-injection layer in OLED [13–15]. There are reports on thenonlinear I–V behavior of doped TiO2 [16–20]. However, there isa complete lack of information on p-TiO2 thin films as an activematerial in heterojunction devices. An elaborate work on this topicwould be interesting as p-type conductivity in TiO2 can improvethe photocurrent conversion efficiency and can lower the trans-port resistance of the photo-induced electrons through the overallprocess of power generation.

Present work is a continuation of a previous finding about thefabrication of heterojunction devices with p-TiO2 and verticallygrown n-ZnO NRs [21]. A possible combination of trivalent/bivalentdoping (M = Cr3+, Fe3+, Ni2+, Co2+ and Mn2+) were used to obtain p-TiO2 through a much simpler and straightforward wet chemicalsynthesis method. The homogenous p-TiO2 was used to sup-

port n-ZnO thin films and vertically aligned n-ZnO NRs. Thisreport elaborates the fabrication and electrical properties of Ag/n-ZnO films/MxTi1−xO2/ITO and aligned Ag/n-ZnO NR/MxTi1−xO2/ITObipolar assembly. The thrust of the present manuscript is the elec-

888 S. Das et al. / Materials Chemistry and Physics 129 (2011) 887– 891

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ig. 1. FESEM images of the cross-sectional n-ZnO/p-TiO2 bipolar assembly fabricalms, and with ∼300 nm long (c) and ∼1.5 �m long (d) vertically-aligned n-ZnO NR

rical response of the heterojunction device and the altering theiO2 from n- to p-type conductivity with acceptor doping.

. Experimental details

The experimental details are given elsewhere [21]. In summary, TiO2 based solu-ion was produced by mixing 0.001 mole titanium (IV) isopropoxide (TIPO) with 5 mLthanol. The mixture immediately produced white precipitate of titanium hydrox-de. In another glass beaker, required amount of M(NO3) (M = Cr, Fe, Ni, Co, Mn) wasissolved in 2 mL ethanol. These two solutions were mixed together and 2 mL aceticcid was added. The solution was stirred for 6 h. After that, a clear solution was pro-uced. The concentration of M+ was varied as 1, 5, 10, 15 and 20 molar percentage%) (x = 0.01, 0.05 0.1, 0.15 and 0.2) in the total 100% M+:Ti4+ (MxTi1−xO2) intake (seeig. 1 of the supporting information). MxTi1−xO2 films were deposited on ITO coatedlass substrates by spin coating technique at 4000 rpm for 20 s and were dried at0 ◦C for 10 min. Finally, the films were annealed at 450 ◦C for 1 h in vacuum andere cooled naturally.

A ZnO seed layer with a thickness of ∼30 nm was deposited using plasmanhanced atomic layer deposition (PEALD) technique at temperature of 220 ◦C bysing Diethyl zinc (Zn(C2H5)2:DEZn) and oxygen plasma (5 sccm) as source for zincnd oxygen, respectively [22,23]. The ZnO layer was annealed at 300 ◦C for 30 min.he ZnO NRs were grown on MxTi1−xO2/ITO/glass substrates by seed-layer assistedrowth technique. For this, an aqueous solution was prepared using 0.05 M equimo-ar zinc nitrate hexahydrate and hexamethylenetetramine precursors. The detail ofhe preparation of aligned ZnO NRs was described elsewhere [24–28].

The cross-sectional images of the bipolar assembly were studied by field emis-ion scanning electron microscope (FESEM, JSM S4800). The interfacial character ofhe doped TiO2 layer was characterized by X-ray photoelectron spectra (XPS) whichere recorded with a Thermo K-alpha ESKA System with a monochromatic Al K�

ource and a charge neutralizer. The electrical (I–V) property of the samples was car-ied out utilizing 4-probe station (HP 4156 C semiconductor parameter analyzer) inhe range from −2 to 2 V at room temperature. For this 1 mm × 1 mm Ag electrodeas deposited on the top of the assembly by thermal evaporation method.

In the next section, the samples will be referred as follows. Sample S1 (Cr3+

oped), sample S2 (Fe3+ doped), sample S3 (Ni2+ doped), sample S4 (Co2+ doped)nd sample S5 (Mn2+ doped) are annealed at 450 ◦C for 1 h. Samples marked as “A”,B” and “C” represent x = 0.01, 0.05 and 0.1 in S1–S5, respectively.

. Results and discussion

.1. Structural and XPS study of n-ZnO/p-TiO2 bipolar assembly

Fig. 1 shows the cross sectional images of the heterojunction n-nO/MxTi1−xO2 heterojunctional assembly. The thicknesses of ZnOlm are ∼40 nm (a) and ∼80 nm (b), respectively and the lengths

th ∼70 nm thick p+-TiO2 (S3-B) with ∼40 nm thick (a) and ∼80 nm thick (b) n-ZnO

of the NRs grown over ZnO seed layer (∼30 nm) are 300 nm (c)and 1.5 �m (d), respectively. The average diameters are ∼60 and110 nm, respectively. The thickness of MxTi1−xO2 film is ∼70 nm.In the previous report [21] it is observed through Fourier trans-form infrared spectroscopy that the Ti–O vibration absorption peakat about ∼510 cm−1 becomes more prominent in the doped sam-ples. In the lower wave-number region, two prominent peaks areobserved at around 424, 435 cm−1. These peaks are assigned toTi–O–Ti vibration. The effect of the acceptor (Cr3+) doping andtemperatures on these bands was also obvious from the spectra.As is revealed, the bands at 424, 435 cm−1 show highest inten-sity for pure TiO2 annealed at 800 ◦C (rutile phase) compared tothat annealed at 400 ◦C (anatase phase). After acceptor doping, theband at 435 cm−1 becomes weaker and finally disappears. In thepresent revised manuscript, however, a fresh attempt was made tocorrelate the effect of doping through XPS and I–V responses.

Fig. 2a shows the cross-sectional image of a representative Ni2+-doped p-TiO2 thin film (S3B) deposited on ITO/glass substrate usedfor XPS study. (The XRD pattern and high-resolution TEM and AFMimages of S3B are exhibited in Figs. 1 and 2 from Supporting Infor-mation) The XPS spectra in Fig. 2b–d indicate that Ti4+2p doublet forsamples S1–S5 consists of two wide symmetric peaks of Ti4+2p3/2and Ti4+2p1/2 with binding energies centered around ∼458 and∼464 eV, respectively [29]. The Ti2p band shifts to higher bindingenergies for the doped p-TiO2 samples with respect to the undopedTiO2 samples obtained under similar conditions without doping.The amount of shift increases with the dopant concentrations as areobserved for sample “S1A–S5A” (∼0.17 eV, b), “S1B–S5B” (∼0.32 eV,c) and “S1C–S5C” (∼0.38 eV, d). It is also observed that the amountof shift depends on the type of metal ion dopant, the highest isregistered for S3 (Ni2+) and the least is obtained for S5 (Mn2+).Thus, it is concluded that Ni2+ ions generate more influence on thechemical and electronic environment in p-TiO2 lattice after doping,whereas Mn2+ influences the least. Fig. 2e shows a shift of ∼0.21 eVin the Ti4+2p band towards higher binding energy side in sample S3

(Ni2+) under two extreme doping concentrations (1% (S3A) and 10%(S3C)). The experimental profiles of the O1s peak (f), is consideredas the sum of various contributions at several binding energies. Thepeak envelope for sample S3A, shows the main peak at ∼530.02 eV

S. Das et al. / Materials Chemistry and Physics 129 (2011) 887– 891 889

Fig. 2. (a) Typical cross sectional FESEM image of the p-TiO2 (S3B) thin film deposited on ITO/glass substrate. The XPS spectra of such layers show shift in the Ti4+2p bandsfor samples S1–S5 (p-TiO2) samples with (b) 1 molar%, (c) 5 molar%, and (d) 10 molar%; (e) shows the shifts in the Ti4+2p bands in sample S3 for two extreme dopingc d O1af lder ps l and

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oncentrations, i.e. 1% (S3A) and 10% (S3C); (f) shows the asymmetric O1s (markeunction reveals the main O1s peak for S3-A (∼530.02 eV, O1a) along with a shoupecies. The shift in the O1s peak in S3-C can be attributed to the changing chemica

O1a) with a shoulder at ∼531.19 eV (O1b). The deconvolution ofhe O1s peaks into two Gaussian components reveals that withigher doping the shoulder peak (O1b) shifts to ∼531.79 eV, whilehe main peak (O1a) shifts marginally to ∼530.12 eV. Literatures29,30] show that O1a must be attributed to the lattice oxygen ions2−, which are associated with Ti4+. The higher binding energy

orms (O1b) are due to surface hydration, can be ascribed to theH groups on the surface, and are assigned to Ti–O–Ti species.he shifting of this oxygen species can be attributed to the alteredhemical and electronic environment and change in compositionn p-TiO2 as this oxygen are associated with Ti4+.

.2. Current–voltage relation in n-ZnO/p-TiO2 heterojunctionevices

Fig. 3 shows the current (I)–voltage (V) relationship for the n-nO/MxTi1−xO2 heterojunction assembly (see the schematic of suchssembly) at room temperature with (a) M = Cr, x = 0.01, 0.05 0.1,

.15 and 0.2, (b) x = 0.05, for M = Cr, Fe, Ni, Co, Mn, (c) x = 0.15, M = Cr,e, Ni, Co, Mn and (d)–(h) the temperature dependent response atolar concentration x = 0.05 for M = M = Cr, Fe, Ni, Co, Mn in the

ange of 25–150 ◦C. The inset of each figure shows correspond-

) peak of the same samples (S3-A and S3-C). Deconvolution with Gaussian waveeak (marked O1b) at a higher binding energy of ∼531.19 eV belonging to –Ti-OHelectronic environment in doped p-TiO2 samples.

ing ln I versus V characteristics. The plots reveal higher currentresponse in the forward bias whereas much lower response in thereverse bias, signifying decent rectifying behavior. This also indi-cates extrinsic p-type conductivity in MxTi1−xO2 as ZnO is n-typematerial. The observed p-type conductivity in MxTi1−xO2 can beascribed to the acceptor level doping in TiO2. However, the exactreason for this p-type conductivity in TiO2 is still under investiga-tion. In the previous communications [21,31], an attempt was madeto present a case of acceptor (Cr3+) doping in TiO2 lattice. It was con-cluded that acceptor doping significantly influences local chemicalenvironment of Ti ions. In the XPS spectra, a downward shifting ofthe Fermi level toward the valence edge was observed [21]. WhenCr3+ ions are incorporated into the TiO2 lattice, the binding energyof each atom changes owing to the electron transfer from Ti2pband to Cr2p. Similar phenomena may occur with other doping e.g.Fe3+, Ni2+, Co2+ and Mn2+. Thus, the electronic structure graduallyacquired more p-type conductivity with increasing doping concen-trations [21]. In case of p–n junction, the current I is expressed by

the equation, I = I0exp(qV/�kBT), where V is the voltage across thediode, I0 is the saturation current, q is the elementary charge, kB isthe Boltzmann constant and T is the temperature in Kelvin. � is theideality factor, which is determined from the slope of the ln I versus

890 S. Das et al. / Materials Chemistry and Physics 129 (2011) 887– 891

Fig. 3. Current (I)–Voltage (V) plots for the n-ZnO/p-TiO bipolar devices at room temperature (a) Cr for x = 0.01, 0.05, 0.1, 0.15 ad 0.2, (b) for x = 0.05, M = Cr, Fe, Ni, Co, Mna –V response were recorded for x = 0.05 (d) M = Cr, (e) M = Fe, (f) M = Ni, (g) M = Co and (h)M

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nd (c) for x = 0.15, M = Cr, Fe, Ni, Co, Mn. The temperature dependent (25–150 ◦C) I = Mn. A schematic of the Ag/n-ZnO/MxTi1−xO2/ITO is also shown.

plot, is a measure of how perfectly the experimental diode followshe ideal diode equation. In ideal diode recombination occurs viaand to band, in such case � = 1. However, the slope of the straight

ine ln I = ln I0 + (q/�kBT)V equals to q/�kBT from which the calcu-ated � for (a) M = Cr (x = 0.01–0.2) found to be in the range of 4–10.he derivation of the simple diode equation uses certain assump-ion about the device. In practice, there are second order effectso that the diode does not follow the simple diode equation andhe ideality factor provides a way of describing them. The resis-ances in the forward bias situation are determined to be in theange of 100–400 � and those for the reverse bias are in the rangef 6–35 k�. The reverse saturation current (I0) was calculated to ben the range of 0.03–0.004 A. On the other hand, in samples for (b)

= 0.05 and (c) 0.15 (M = Cr, Fe, Ni, Co, Mn) the obtained values of �ere in the range of 4–8. Moreover, plots (d)–(h) show the temper-

ture dependent (25–150 ◦C) comparative features of the I–V plotsor M = Cr, Fe, Ni, Co, Mn, where x = 0.05. The plots show more con-istent and stable response of the devices when the temperatures increased; however, � remains in the range of 4–10 irrespectivef temperature and dopant. In Fig. 4, the I–V response for the sameevice at higher reverse bias show consistent stable rectificationehavior with the resistance at forward and reverse bias situationre ∼250 � and 12 k�, respectively.

In order to extend the research findings, I versus V relation-hip for the n-ZnO aligned NR/MxTi1−xO2 bipolar assembly wastudied. The cross-sectional images of such assembly are shownn Fig. 1c and d. The obtained results for two different lengths of

he aligned NRs, 300 nm and 1.5 �m, respectively, are shown inig. 5 for samples S1B–S5B. The total thickness of the individualamples is ∼400 nm and 1.6 �m, respectively, taking into accounthe thickness of the p-TiO2 layer and that of the n-ZnO seed layers.

Fig. 4. The I–V response at temperature of 150 ◦C for higher reverse bias conditionfor M = Cr, for x = 0.05.

The device showed efficient rectifying properties indicating goodp-type conductivity in the doped TiO2 samples. It is observed thatthe turn-on voltage (V0) are comparatively lowered for devices withlonger ZnO NRs (1.5 �m). It is interesting to note that in the pre-vious work with Cr0.5Ti0.5O2 thin film [21] the obtained turn-onvoltage was calculated as 0.75 eV, which is somewhat consistentwith the value of 0.80 eV, obtained in the present work. It is alsofound that that current is relatively higher by one order in such

cases. This is somewhat opposite to what was obtained for thickerZnO film (80 nm) based devices, where it is seen that the current isone order lower indicating resistive influence on the charge trans-fer through this film. However, the amount of current is higher in

S. Das et al. / Materials Chemistry an

Fig. 5. Current–voltage plots for the vertically aligned n-ZnO NR/MxTi1−xO2 bipo-lar devices with the variation of NRs length (i.e., 300 nm and 1.5 �m). The devicestructure is shown in top. The I–V plots with different dopants are: (a) Cr-doped, (b)Fg

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e-doped, (c) Ni-doped, (d) Co-doped, and (e) Mn-doped TiO2. All the devices showood rectifying behavior with small turn-on voltage.

ase of devices made with thin films than that with NRs, yet thencrease of current for longer rods seems to stem from the easierransport of charges. This property will be useful in harnessing thevailable charge in the bipolar device in future high quality devicesor using in hybrid light emitting devices and photovoltaic devices.he results clearly showed a useful behaviour of p-TiO2 thin filmhat, with aligned ZnO NRs can be a used as an active element inipolar or transistor devices.

. Conclusions

In summary, a wet chemical route was presented to obtain p-ype conductivity in TiO2 thin films. Several trivalent and bivalentopants like Cr3+, Fe3+, Ni2+, Co2+, Mn2+ have been successfully

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d Physics 129 (2011) 887– 891 891

incorporated into the TiO2 lattice sites. Consistent behavior of thedoped p-TiO2 films was well-documented thorough XPS and I–Vdata analysis. The p–n devices fabricated with vertically alignedlonger n-ZnO NRs and thin films show efficient rectifying char-acteristics with very small turn-on voltages. The finding wouldhelp in using p-TiO2 thin films in p-DSSC with improved deviceperformances replacing present devices engaging NiO as an activeelement.

Acknowledgements

This work was supported in part by the World Class Universityprogram (R31-20029) and by the Priority Research Centers pro-gram (2010-0029707) funded by the Ministry of Education, Scienceand Technology (MEST). Authors also thank KBSI Jeonju branch fortaking quality SEM images.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.matchemphys.2011.05.039.

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