Jourrnal of New Teechnology and d Materials J JNTM Vol. 01, N°00 N (2011) 244-28

OEB Univ. Publlish. Co.

Low w Copper Doped d CdO Nanowi N ires Gro own by S Sol-Gel Route R M. Ben nhalilibaa*, C.E. Benouissa*, A. Tiburrcio Silverb a

Physics Depaartment, Scienc nces Faulty, Orran Universityy of Sciences and an Technologgy USTOMB, BP1505 Oran an, Algeria. b IT TT-DIEE, Apddo, Postal 20, Metepec M 3, 522176, Estado de d Mexico, M Mexico. *Correspondiing author

b bmost_31@yah hoo.fr, Tel. +2137722114491

Recei eived: 23 May 2011, 2 accepted ed: 30 Septembber 2011 Absttract In thhe current worrk, pure and copper c dopedd cadmium oxiide (Cd1-x Cux O, O x=0, 0.02, 0.03) 0 thin film ms are grown by b sol-gel spin coating route te. Optical trannsmittance is measured m in UV, U VIS andd IR spectra; itt is revealed thhat the copper er improves th he transmittancce. The opticcal band gap increased i withh the doping. The room teemperature ele lectrical resistaance was affeccted by coppeer doping. Thhe AFM morpphology reveal als that pure CdO Cd and Cu doped do thin film ms are nanostruuctured. Keywordss: Sol-gel spinn coating, Grai ain size, CdO, Cu C level dopin ing, Nanowiress, Optical propperties, AFM investigation. i 1. Introducttion Currrently, Cadm mium oxide (C CdO) is amo ong of interessting cond ducting oxide (TCO) group p such as tin oxide (SnO2) [1], indiu um oxide (In2O3) [2], indium m tin oxide (IT TO) [3] and zinc z oxidee ZnO [4-5]] due to itss high electrical and opttical prop perties. Cadmiium oxide (C CdO) is n typ pe semiconducctor with direct band gaap found to bee 2.3 eV and iti exhibits a cu ubic system and the laattice parameeter is 4.695 Å [6]. Thin-ffilm depo osition techniques are eith her purely physical, p such h as evaporative metho ods, or simplly chemical, such as gas and liquid d phase chem mical processess like sol-gel [7]. [ Furthermo ore, CdO O has been deeposited also by chemical bath (CBD) [8], sprayy pyrolysis [99], sputtering [10] and therrmal evaporattion [11]. Cadmium oxxide is one off the promisin ng II-VI familyy of semiiconductors which wh has a greeat potential for fo optoelectro onic devicces [12]. CdO O was doped with w many elements such ass Li [13], Al [14], Fe [115], Ga [16], Sm S [17] and Eu E [18, 19]. Up p to our knowledge, there is no o works pu ublished on the preparation and structural, UV-VIS-IR optical, AFM A morp phological an nd electrical properties p invvestigation of Cu dopeed CdO. Ou ur work conssists on the preparation and charaacterization off CdO doped d with transitio on metal (Cu)), in the proportion p 2 % and 3 %, to stand up their characterizattion applications. The influencee of for th he advanced technological t Cu doping d level on n structural, op ptical, morpho ological, electrrical prop perties of CdO O synthesized d by facile soll-gel spin coaating routee is investigated. 2.

hen the stirrring continued d for 1hour.. Consequenttly, the th solution was aged a for 24 hours until the t gel formaation at ambient a (see sccheme 1 and 2 [20]).

Scheme S 1. Diifferent steps of sol-gel prrocess. (Aerogeel is a manufactured m m material with th he lowest bulk density of anyy known porous p solid. It is derived from m a gel in which h the liquid com mponent of o the gel has been b replaced w with a gas), (Xerogel is a solid formed frrom a gel by drying d with unh hindered shrink kage. Xerogel iss usually reetain high porrosity (25%) an nd enormous surface area (1150–900 m /g), along with h very small poree size 1-10 nm)[20]. Scheme S 2. Soll-gel processingg and ageing steps, resin fo ormation 2

Experimeental procedurre

2 Films prep 2.1. eparation routee onto microscope glass slid des (76 x 26) mm² supplied d by objecct trager Isolaab. 0.5 Molar of cadmium acetate dihydrate (Cd (CH3COO) 2 .2(H2O)) (p purity of 99 %) supplied by Him media, was diissolved in 10 ml of 2-Methoxyetha 2 anol (C3H8O2) stirred at 60°C for 10 mn. m The dopiing precursor was copp per (II) acetatee anhydrous Cu C (CH3COO O) 2, (purity off 98 %) su upplied by Carlo C Erba reaagents and theen 0.3 ml of the Mon no-Ethanolamiine (MEA) C2H7NO (of molar m mass 611.08 g/mo ol and densitty = 1.015) as stabilizer, waas added drop p by drop p until the hom mogeneous an nd clear solutiion was obtain ned,

(rroute1) and porrous solid materrial formation (rroute 2) [20].

The T viscous solution waas homogeno ously poured d using micropipette m o the substraate sticked on on n stainless steeel spin plates p of MTI, EQ-TC-100 D Desk-top Spin n Coater. The sample

Low Copper Doped CdO Nanowires

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M. Benhaliliba et al.

analyses were performed with the software from the WSXM system

rotates for one minute at speed of 1200 rotates per minute (RPM), the sample was heated at 150 °C for 10 mn and then the process was repeated 5 times, finally the film was annealed at 400 °C for 1hour under air in furnace MF 120 Nuve.

3.

Results and discussion

3.1. Structural analysis 2.2. Films characterization

The figure 1A shows the X- rays diffraction pattern of pure and copper doped cadmium oxide where the angle ranges within 20°-80°. The grain size G is given by the well-known Scherrer’s formula (1)[4],

X-rays pattern of samples were carried out in Bruker AXS D8 Discover diffractometer, CuKα1 (λ = 1.5418 A°) is used. The films coated transmittance and reflectance were recorded by Shimadzu 3600 PC double beam UV-VIS-NIR spectrometer, the electrical resistance at room temperature was then determined by four probes method. AFM observations of the coated films were made by using a Quesant Model 250 system having an 80x80 micrometer head, in the wave mode in air. For the (10 x10) micrometer square images the resolution was (300 x 300) pixels, the scan rate was 2 Hz for all cases. All

G =

0 . 94 λ β cos θ

(1)

Where β is the full width at half medium of the peak, 2θ is the Bragg angle and λ is X- rays wavelength. The calculated values of G are listed in table1.

Table1: Some parameters are calculated for pure and copper doped CdO

Grain size (nm ) (111)

(200)

(220)

TC (111)

CdO 8.235 8.245 6.199 1.80 CdO: Cu 11.246 9.662 7.471 1.72 2% CdO: Cu 8.801 7.853 7.122 1.73 3% The comparison of the observed XRD patterns with the standard JCPDS data (05-0640) confirms the structure of CdO phase with face centered cubic crystal structure [21]. The X rays pattern reveals that all investigated coated films are polycrystalline of cubic CdO structure and Bragg position for strong reflections like (111) direction was 32.92°, 33.08° and 33.09° respectively for pure and doped (2 % and 3 %) CdO coated films, and then a slight angle shift, estimated at 0.03°0.16°, was carefully detected as sketched in fig. 1B. Others reflection positions (2θ) and their angle shifts are listed in table2. Table1: X-rays results of pure and copper doped CdO

(hkl) (111)

(200) (220) (311)

CdO 2%Cu 3%Cu CdO 2%Cu 3%Cu CdO 2%Cu 3%Cu CdO 2%Cu 3%Cu

Bragg angle 2θ (°) 32.92 32.95 33.08 38.12 38.26 38.43 55.06 55.13 55.30 65.70 66.25 66.42

Angle shift ∆(2θ) (°) 0.03 0.16 0.14 0.31 0.07 0.24 0.55 0.72

Eg (eV) (αhν)²

dT/dλ

dR/dλ

2.49 2.50

2.59 2.59

2.52 2.59

T (550 nm) (%) 56 68

2.56

2.62

2.59

79

Consequently, according to both (111), (220) orientations, copper doping increases the grain size, while 3 % copper level doping reduces it. As shown in figure 1B, CdO is present in coated film as confirmed by JCPDS card No. 05-0640, while a slight (2θ) shift to higher angle is caused by Cu doping. The ionic radii of Cu (II) and Cd (II) are respectively 0.73 Å and 0.95 Å and ionic radius ratio is rCu (II) / rCd (II) =0.77, then cooper has minor radius than cadmium, it may diffuse in host lattice without causing mismatch or distortion, this fact corroborates with the slight angle shift. Figure 2 illustrates grain size of samples grown by spin coating route according to (111), (200) and (220) orientations. The grain size sweeps in average of 6 nm, these values due to X rays peaks broadening are minor; this confirms the nanostructures aspect of our coated films. Furthermore grain size is increased by the Cu doping level (2 %). (111)

CdO 2CuCdO 3CuCdO

(200) (220)

(311) (222)

100 80 60 40 20 0

Intensity (CPS)

Sample

-20

These results are in well agreement with those of literature [22]. It seems that the CdO coated films have a preferential growth along the (111) direction. Most important peaks of CdO phase (111), (200), (220), (311) and (222) are shown in figure 1A. Textural coefficient is given by TC = I111/ (1/4) (I111+I200+ I220+I311) [4] and data are tabulated according to (111) direction. The discrepancy in TC parameter is very low which confirms the crystalline structure is maintained at low doping level.

20

30

40

50

60

70

80

2θ (Degree) Figure 1A. X-rays pattern of pure and copper doped CdO grown by spin coating process at 1200 RPM, Bragg angle ranges within 20°-80°.

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om dR/dλ verssus λ (not show wn here) suggeesting that the optical fro ban nd gap shifts from f 2.49 to 22.62 eV. Thesee gap values leead to a blu ue shift which h may be exp plained by Bu urnstein-Mosss effect. Ou ur results are in well agreeement with th hose of literatu ure [6]. Flu uorine has incrreased the ban nd gap as repo orted by Akyuzz [26]. 220 200 111

4 2

Grain Siz

8 6

e (nm)

12 10

0

dO uC 3C

Figurre 1B. JCPDS card c No 05-06440 of pure and copper doped d CdO are skketched (red daash lines). Sligh ht angle shift (22θ) to higher an ngle is obserrved.

dO uC 2C

1 Optical chharacterizationn 1.1.

Figgure 2. Grain size s plot againstt Cu content acccording to (1111), (200) and d (220) direction ns

The transmittancee of pure and d copper dopeed (2 % and 3 %) cadm mium oxide grrown by sol-ggel is depicted d in figure 3 within w the wavelength w ran nge 200-25000 nm. The traansmittance plot in visiblle spectrum, of o studied sam mples and puree glass, is observed insett of fig.3. As can c be seen th he transmittan nce grew up raapidly in UV V-VIS range until u 85% for 3 % Cu doped d CdO samplee and contiinues to increaase monotoniccally in the VIIS and NIR sp pectra. It reaaches a maxim mum found to o be 91 %, 90 % and 89 % in IR for th he 3 %, 2% Cu u doped CdO O and pure Cd dO respectivelly and then transmittancee decreases sllightly with co opper doping level. Simillar evolution is observed in Kumaravvel paper [9, 23]. Subrramanyam hass found the sam me trend proffile of transmitttance in VIS V and IR spectrum s for the CdO films grown byy DC magn netron sputterring for one particular p oxyygen pressure [24]. We remark that Cu ions im mprove consid derably the optical o transsmittance partiicularly in visib ble range (see inset of fig.3). The averaage transmittan nce at 550 nm m increases wiith doping levvel, as listed d in table 1, and confirm ms the previo ous statement.. We menttion that lo ow copper level dopin ng improves the transsmittance aro ound visible edge (750--800 nm) where w transsmittance of 3 % Cu doped CdO approaaches the puree glass transsparency as can be easily seen inset off fig.3. The direct d opticcal band gap iss expressed as [4],

α h ν = ( h ν − E g ) 0 .5

O Cd

1.2. Surfacee morphology iinvestigation he 2D and 3D D AFM investiggation is show wn in figure 5, picture Th havve dimensionss 10 x 10 miccrometers. Ovverall, the surrface of Cd dO coated film m is homogeeneous with few f voids wh hich are sign ned by circless as depicted iin 2 D views (see ( fig. 5 left)). Pure Cd dO nanograin ns look like mounts witth no well defined d bou undaries; the average of ggrain size is 0.075 0 µm and d height equ uals to 742 nm m. Pure CdO O coated film reveals agglom merated reggions with diffe ferent asperity,, the bright on nes (signed byy arrows in fig. f 5A) attractt more atoms during the gro owth process than th the darrk ones which wh might demonstratee cavities. Similar mo orphology shaape was reporrted in literatu ure [23, 27]. 2 % Cu dop ped CdO saample revealss columns lik ke wires whiich are sep parated and grown g in the same directio on having an average heiight equals to 275 nm , the 3 % Cu do oped CdO sho ows the sam me shape of wires with bigg nanowires density d ( num mber of nan nowires per µm² µ ) and maj ajor height ( ~382nm). ~ Clusters of nan nowires exhib bit different ssizes 0.186 µm µ (2 Cu % doped Cd dO), 0.208 µm m (3 Cu % dop ped CdO). We W conclude that t low cop pper doping level influen nces the surfface morphollogy of cad dmium oxide coated film an nd tends to make m longer thee nanograains into nanow wires.

(2)

t optical ban nd gap, α (m 1) 1 is the absorrption Wheere Eg (eV) is the coeffficient and ν (Hz) is the photon p frequeency. Band gap Eg estim mates were derrived from thee optical transsmission specttra by extraapolating the liinear portion of o the plot of (αhν)² against hν to α=0 as plotted in figure f 4. It vaaried with dop ping level, the pure samp ple exhibits Eg equals to 2.499 eV , the sam mples 2 % and d3% Cu doped d have respectively 2.50 and 2.56 eV as sketch hed in figuree 4 and listed d in table 1. While W the op ptical band gap p was found by T.P. Gujar G is 3.188 eV [25] an nd Kumaraveel has reported a value of o Eg ~2.53 eV V [23]. Otherrs works exhib bit an averaage of Eg aroun nd 2.46 eV [24]. Our resultts are in accord dance with those obtaineed in literaturee [23, 24]. Th he estimated energy gap from (αhν)², dT/dλ and dR/dλ d (reflectaance is not shown here)) of pure and d Cu-doped CdO C films aree given in Tab ble 1. Therre is an effecct of the Cu dopant conccentrations bu ut not regullar in the stud died range on n the optical band b gap estim mated -

Fiigure 5A. AFM M topography im mage of pure CdO coated film ms. AFM Picctures are 10x100 µm², grain sizee and voids are shown in (left) 2D view and d (right) 3D view w, (height is shown at left corn ner of 3D imagge, arrow sho ows nano-moun nts.

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M. Benhaliliiba et al. -4

5.0x10

CdO 2 % Cu 3 % Cu

-4

-1 (αhν)² (m eV)²

4.0x10

-4

3.0x10

-4

2.0x10

-4

1.0x10

0.0 2.0

Figurre 5B. AFM to opography imagge of 2 % Cu doped CdO coated c films.. AFM Picturess are 10x10 µm m², grain size and voids are sho own in (left) 2D view and (rright) 3D view, (height is shown n at left corner of 3D imagee, arrow shows nanowires. n

2 2.1

2.2

2.3

2.4 4

2.5

2.6

2.7

2.8

2.9

3.0

hν (eV)

Figgure 4. Depen ndence of (αhν))² on incident photon energyy (hν) of und doped CdO , % 2 and 3 % C Cu doped CdO, (extrapolation n straight linees are depicted).

1.3. Electrical measuremeent he electrical measurement m w was carried ou ut in four pro obes set Th up as can be seen inset of figu ure 6 and the electrical e resisttance at roo om temperatu ure is depiccted in figuree 6. The ellectrical ressistance R was calculated by using [28],

R = k

V I

(3)

Wh here k is a con nstant found tto be 4.53, V is i the applied voltage and d I is the inteensity of DC ccurrent. It is observed o that copper levvel doping dim minishes greatlyy the resistance by about 5 and a 400 tim mes for 2 % an nd 3 % Cu do oped CdO resp pectively. This fact is duee to copper, which has two o level of oxiidation I and II, can sub bstitute to cad dmium sites an nd offers free electrons wh hich can imp prove the con nductivity.

Figurre 5C. AFM to opography imagge of 3 % Cu doped CdO coated c films.. AFM Picturess are 10x10 µm m², grain size and voids are sho own in (left) 2D view and (rright) 3D view, (height is shown n at left corner of 3D imagee, arrow shows nanowires. n

Electrical resistance ( Ω)

Transmittance (%)

100 C

80

B A

60

100 80 60 40

40

3k

2k

1k

20 0 400

50 00

600

700

0

800

Pure CdO (A) CdO:2 Cu (B) CdO:3 Cu (C)

20 0

4k

0

500

p u re C d O

2 C u :C d O

3 C u :C d O

Figgure 6. DC resisstance at ambieent is plotted veersus Cu amoun nt. Inset sho ows the four pro obes set up scheeme

1000 0 1500 20 000 2500 Wave elength (nm))

Co onclusion Th he pure and Cu u doped CdO O films structurral, 2D and 3D D AFM views, optical properties werre investigated d. It is revealed that Cd dO phase is ob btained and th he grain size reaches r up to 11 nm acccording to (111) direction n and TC deecreases a litttle with cop pper doping level. A peak broadening reeveals nanostrructures forrmation of ourr coated filmss. High transp parent coated films f in VIS-IR range are a obtained and low copper level doping imp proves the traansmittance m mainly in visiblee band edge from f 56 % to t 79%, and exceeds e the po oint of 90 % in n IR spectrum m. Band

Figu ure 3. Transmitttance dependen nce on photon wavelength w of pure, p 2 and 3% 3 Cu doped CdO grown by b spin coating at 1200 RPM,, inset show ws VIS transmitttance profile off pure, 2 % Cu u and 3% Cu doped d CdO and pure glass of substrate.

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[9]R. Kumaravel, K.Ramamurthi, Indra Sulania, K.Asokan, D.Kanjilal, D.K.Avasti, P.K.Kulria, Radiation Physics and Chemistry 80 (2011) 435-439. [10]Qiang Zhou, Zhenguo Ji, BinBin Hu, Chen Chen, Lina Zhao, Chao Wang, Materials Letters 61 (2007) 531-534. [11] H.B. Lu, L. Liao, H. Li, Y. Tian, D.F. Wang, J.C. Li, Q. Fu, B.P. Zhu , Y. Wu, Materials Letters 62 (2008) 3928-3930. [12]A. Gulino, G. Tabbi, Appl. Surf. Sci. 245 (2005), 322-327. [13]A.A. Dakhel, Effect of thermal annealing in different gas atmospheres on the structural, optical, and electrical properties of Li-doped CdO nanocrystalline films, Solid State Sciences (2011), doi:10.1016/j.solidstatesciences.2011.02.002. [14]K.R. Murali, A. Kalaivanan, S. Perumal, N. Neelakanda Pillai, Journal of Alloys and Compounds 503 (2010) 350-353. [15] A.A. Dakhel, Thin Solid Films 518 (2010) 1712-1715. [16] A.A. Dakhel, Solar Energy 82 (2008) 513-519. [17]A.A. Dakhel, Journal of Alloys and Compounds 475 (2009) 51-54. [18]A.A. Dakhel, Current Applied Physics 11 (2011) 11-15. [19]A.A. Dakhel, Optical Materials 31 (2009) 691-695. [20]Robert Corriu, Nguyen Trong Anh, molecular chemistry of Sol-gel derived nanomaterials, Wiley Ed. ISBN 978-0-47072117-9 (2009). [21]Salih Kose, Ferhunde Atay, Vildan Bilgin, Idris Akyuz, International Journal of Hydrogen Energy 34 (2009) 52605266. [22]A.A. Dakhel, Materials Chemistry and Physics117 (2009)284-287. [23]R. Kumaravel, S. Menaka, S. Regina Mary Snega, K. Ramamurthi, K. Jeganathan, Materials Chemistry and Physics 122 (2010) 444-448. [24] T.K. Subramanyam, S. Uthanna, B. Srinivasulu Naidu, Materials Letters 35, (1998), 214-220. [25]T.P. Gujar, V.R. Shinde, Woo-Young Kim, Kwang-Deog Jung, C.D. Lokhande, Oh-Shim Joo, Applied Surface Science 254 (2008) 3813-3818. [26]I. Akyuz, S. Kose, E. Ketenci, V. Bilgin, F. Atay, Journal of Alloys and Compounds 509 (2011) 1947-1952. [27]D.M. Carballeda-Galicia, R. Castanedo-Pérez, O. JiménezSandoval, S. Jiménez-Sandoval, G. Torres-Delgado, C.I. ZunigaRomero, Thin Solid Films 371, (2000) 105-108. [28]R. Legros, les semiconducteurs, Vol.1, Eyrolles Ed. (1974).

gap increases with copper level doping and demonstrates a blue shift. Low copper doping level maintains the crystalline structure, extends the grains and transforms them to high transparent nanowires and reduces the ambient resistance. These characteristics of high VIS-IR transparency and low resistive nanowires can provide to our coated films, produced by facile sol-gel route, various applications in material sciences and optoelectronics devices which can be investigated in next work. Acknowledgements This work, under contract number D 01920080054, is included in project “CNEPRU2009-2012” supported by the Algerian High Level Teaching and Scientific Research Ministry MESRS and Oran Sciences and Technology University USTOMB. The author would like to acknowledge the generous assistance of Pr F. Yakuphanoglu, Physics Dpt., Firat University Turkey, for his help in optical measurements, Mr. A. Avila-García, A. Tavira, Cinvestav-IPN, “Dept. Ingeniería Eléctrica”, and R. R. Trujillo, “Centro de investigación en dispositivos semiconductors”, Mexico for their fruitful help in performing AFM and X-rays pattern observations. References [1] C.E. Benouis, M. Benhaliliba, F. Yakuphanoglu, A. Tiburcio Silver , M.S. Aida, A. Sanchez Juarez, Synthetic Metals (2011). D.O.I. 10.1016/j.synthmet.2011.04.017. [2]G. Korotcenkov , A. Cerneavschi , V. Brinzari ,A. Vasiliev , M. Ivanov, A. Cornet , J. Morante , A. Cabot , J. Arbiol, Sensors and Actuators B 99 (2004) 297-303. [3] M.K. Fung, Y.C. Sun, A.M.C. Ng, A.B. Djurisic, W.K. Chan, Current Applied Physics 11 (2011) 594-597. [4]M. Benhaliliba, C. E. Benouis, M. S. Aida, F. Yakuphanoglu, A. Sanchez Juarez, J. Sol-Gel Sci. Technol. 55, 3, (2010)335-342, doi 10.1007/s10971-010-2258- x. [5]M. Benhaliliba, C.E. Benouis, M.S. Aida, A. Sanchez Juarez, F. Yakuphanoglu , A. Tiburcio Silver, J. Alloys Compd. 506 (2010) 548-553. [6] Xiaofei Han, Run Liu, Zhude Xu, Weixiang Chen, Yifan Zheng, Electrochemistry Communications 7 (2005) 1195-1198. [7]Seval Aksoy, Yasemin Caglar, Saliha Ilican, Mujdat Caglar, International journal of hydrogen energy 34 (2009) 5191-5195. [8] A.S. Kamble, R.C. Pawar, N.L. Tarwal, L.D. More, P.S. Patil, Materials Letters 65 (2011) 1488-1491.

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