Brief communication

A novel way of modifying nano grain size by solution concentration in the growth

of ZnAl2O4 thin films

K. Kumar1,2,*, K. Ramamoorthy1, P. M. Koinkar3, R. Chandramohan4 and K. Sankaranarayanan11Crystal Research Centre, Alagappa University, Karaikudi, 630 003, India; 2C.R.C., SO PL KumarappanAPM, Head Post Office, Devakottai, Tamilnadu, 630 302, India; 3Department of Electronics Engineering,Nanotubes and Nanodevices Laboratory, Korea University, 5-1 Anam dong, Seongbuk-Ku, Seoul, 136-701,South Korea; 4Department of Physics, Sree Sevugan Annamalai College, Devakottai, 630 302, India; *Authorfor correspondence (Tel.: +91-4561-261620; E-mail: secltkumar@military.com, kumardrf@rediffmail.com)

Received 9 November 2005; accepted in revised form 3 April 2006

Key words: liquid phase deposition, compound semiconductor, nano-crystalline material, X-ray diffrac-tion, surface morphology, nanoengineering

Abstract

A novel route to the growth of thin films of ZnAl2O4 in nano-scale order was developed and nano-thin filmsof ZnAl2O4 are grown. The variation of grain size with solution concentration is reported. The thin film wasdeposited by modified liquid-phase deposition (LPD) technique using a novel acid based chemical reactionfor the first time to ternary system. This modified LPD is based on a novel reaction that favours theformation of nanostructures during the treatment of a precursor (here ZnO) and a metal foil (Al) in dilutedHF acid. The acid serves both as a solvent and catalyst. Usually, in wet process synthesis of binary systems,the metal foil will act as F) ion scavenger. In this method, formation of a ternary compound as well asgrowth of thin film nanostructures of that compound was achieved by the same chemical reaction at roomtemperature. The role of acid concentration in the nanostructure formation is discussed. The relationshipbetween HF concentration and grain size were also graphically enumerated. Structural, compositional andsurface morphological properties of thin films were studied using Philips, Xpert-MPD: X-ray diffractometerand Philips, ESEM-TMP + EDAX, Nanoscope-III: AFM. The technique is a novel, simple and low costroute for the growth of nano-thin films of ternary oxide material.

Introduction

ZnO and its related oxides have emerged aspromising nanostructured materials for futureelectronic and photonic applications (Mathuret al., 2001; Wang, 2004). Spinel oxideslike ZnAl2O4, ZnGa2O4 have been identified asnew ultraviolet transparent conducting oxides(UV-TCO) systems. They are actively studied for

application in numerous solid state photo elec-tronic devices (Omata et al., 1994). ZnAl2O4 is awell-known spinel system, widely used as ceramic,electronic, catalytic, catalytic support material andemerging as one of the best wide band gap com-pound semiconductor (Eg = 3.8 eV) for variousoptoelectronic applications. ZnAl2O4 is alsotransparent to light of wavelengths above 320 nmand hence is suitable for UV optoelectronic

Journal of Nanoparticle Research (2007) 9:331–335 � Springer 2006DOI 10.1007/s11051-006-9108-3

application and thermal control coatingsfor spacecrafts (Pandey et al., 1999; Mathuret al., 2001).At present, thin-film deposition at low temper-

atures from aqueous solutions techniques such aschemical bath deposition (CBD), microwave acti-vated chemical bath deposition(MW-CBD), pho-tochemical deposition (PCD), liquid phasedeposition (LPD), electroless deposition(ELD),aqueous chemical growth technique (ACG), etcare widely used for growth II–VI compoundsemiconductors (Niesen & De Guire, 2002). Thesesimple techniques were also highly suitable forgrowth of metal oxide nanostructured thin films(Vayssieres, 2004). Generally, ZnAl2O4 powderwas synthesized by sintering process at high tem-peratures and then thin films were deposited byvacuum techniques such as PVD, CVD, PLD andsputtering (Fang et al., 2002) using the synthesizedmaterials.In this work, we have developed a novel, low

cost acid based chemical reaction route- an in situmethod for the synthesis and growth of ZnAl2O4

thin-film deposition. This is a modified LPDmethod. This modified LPD is based on a novelreaction that favours the formation of nanostruc-tures during the treatment of a precursor (hereZnO) and a metal foil (Al) in diluted HF acid. Theacid serves both as a solvent and catalyst. Usually,in wet process synthesis of binary systems, themetal foil will act as a F scavenger. However, internary systems when the metal component isneeded as a constituent of that system, at appro-priate concentration of the acid the metal getsdissolved leading to the formation of that ternarycompound as well as growth of thin film nano-structures. ZnAl2O4 nano-thin films were grownfor the first time at room temperature and theeffect of HF concentration on the grain size of thefilms are discussed in detail.

Experimental processes

ZnO powder of 1.2206 gm is mixed with 2.5%–5%aqueous solution of HF acid and 5 cm2 of alumi-num foil of 0.5 mm thickness was introduced intothe bath as cation (F) ion) scavenger. Hot chromicacid treated glass substrates were immediatelydipped in to the freshly prepared chemical bath.The experiment was carried out for various

HF concentrations (5 – 2.5 ml in steps of 0.5 ml) inroom temperature. Average growth and harvesttime is 15 h for thin films through out this work.The zinc-fluro complex formed reacts with alumi-num metal ions (Al+). The pre-cleaned glass sub-strates were immersed in the acid bath for 15 h andwere washed in distilled water before drying.The grown thin films were characterized

structurally, compositionally and surface morpho-logically using Philips, Xpert-MPD: X-ray diffrac-tometer and Philips, ESEM-TMP + EDAX,Nanoscope-III: AFM.

Figure 1. X-ray diffractograms (h–2h scan) of ZnAl2O4 thinfilms deposited at various HF concentrations of (a) 2.5 ml(b) 3 ml (c) 3.5 ml (d) 4 ml (e) 4.5 ml (f) 5 ml.

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Results and discussion

ZnAl2O4 thin films were deposited by a principlecalled ‘‘liquid phase deposition’’ (Hishinuma et al.,1991; Deki et al., 1997). Usually in LPD the metalfoils will not take part in the compound formationand will act only as F cation scavenger. However,in this method the concentration of HF is sochosen that the metal foil not only act as a cationscavenger but also gets dissolved in the solutionleading to the formation of the compoundZnAl2O4. The choice of solvent may also providesome control over thin film growth i.e. depositionof a new polycrystalline lattice sites followed byslow etching on the substrate surface. The filmgrowth may take place by ion by ion condensationof the micro constituents on the substrate surface.Figure 1 shows XRD spectrums for vari-

ous HF concentrations (5 – 2.5 ml of 6 trails).Non-amorphous background of thin film XRDpatterns revealed that the as-deposited films werehighly crystalline and adherent. The peaks wereindexed using JCPDS Data (Card No: 5-669).XRD spectrum revealed significant changes in thecrystalline arrangement and orientation with HFconcentration. The diffracted peaks graduallybecome intense and sharp with the decrementin HF concentration. From the spectrum finetexturing of deposition of thin film was observed.The strong existence of peaks (222), (422) and(220), (420) in repeated order shows the layer bylayer of lattice formation in particular direction

(hkl) with k = l = 2 and (hk0) with k = 2,respectively. The grain size was calculated usingScheerer’s equation (FWHM studies). The finenanostructure nature of thin films was confirmedby nanometre grain size. The grain size is in therange of 29 – 5 nm depending up on the HFconcentration. The relationship between grainsizes with HF concentration is shown in Figure 2.From Figure 2 it is observed that HF serves as a

solvent and catalyst, and plays a vital role in thenanostructure formation. Further increment in HFconcentration in deposition bath leads to the for-mation of thin films with fine nanostructure

2.5 3.0 3.5 4.0 4.5 5.0

5

10

15

20

25

30

Nan

o G

rain

Siz

e (n

m)

HF Concentration (ml)

Figure 2. Variation of nano grain size (nm) with HF con-centration (ml).

Figure 3. EDAX spectrum of ZnAl2O4 thin films depositedat various HF concentrations of (a) 5 ml (b) 3.5 ml (c)2.5 ml.

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(having grain size <10 nm). The ultra fine nano-structure of 5 nm was obtained for the high HFconcentration of 5 ml. However, low HF concen-tration in deposition bath allow low reaction ratethat slows down the reaction and reduce theevaporation rate of the solute in deposition bathleading to the formation of bulk crystallites, whichacts as basic units for the thin film construction,and hence gradual decrease in HF concentrationproduces high quality films.The EDAX spectrum for various HF concen-

trations is shown in Figure 3. The EDAX spectraconfirmed the presence of Zn, Al and O. The Sipeak is due to glass substrate. The presence ofelements like Ca and Sn may be attributed toimpurities either from the source materials or fromthe glass substrate.Figure 4 shows the AFM of thin films deposited

at HF concentration of 5 ml. The AFM studiesrevealed a uniform surface formed due to theclustering of fine nanostructured spherical grains.These grains are aligned very closely. The nanograins clustering may be due to the film formationon the HF etched surface. The grain boundariesand voids in the nano-scale order reveal that the

film is continuous and the grains are in goodcontact and relatively uniform. The surfaceroughness is estimated to be around 15 nm fromAFM studies.

Conclusion

Nanostructured thin films of ZnAl2O4 on amor-phous substrates were synthesized at room tem-perature without any post-physical treatmentsusing a novel modified LPD technique. FromXRD spectrum fine texturing (i.e layer by layer oflattice formation in particular direction) ofdeposited film was observed. AFM image revealeduniform dense, non-porous coating. The studiesrevealed that the films were highly adherent evenunder as-deposited conditions revealing the higherquality of films. The relationship between grainsize with HF concentration is graphically enu-merated. Increment in HF concentration in depo-sition bath leads to the formation of thin films withfine nanostructure nature (having grain size<10 nm). This method exhibits excellent controlover the film formation enabling us to grow films

Figure 4. Typical AFM micrograph of ZnAl2O4 thin films deposited at HF conentration of 5 ml.

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of desired grain size and is suitable for the growthof nano-thin films of ternary oxide material.

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