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Inorganica Chimica Acta 361 (2008) 188–194

Deposition and characterization of ZnO thin films from a novelhexanuclear zinc precursor

Mazhar Hamid a, Asif A. Tahir a, Muhammad Mazhar a,*, Fiaz Ahmad a,Kieran C. Molloy b, Gabriele Kociok-Kohn b

a Department of Chemistry, Quaid-I-Azam University, Islamabad 45320, Pakistanb Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK

Received 3 December 2006; received in revised form 26 April 2007; accepted 8 July 2007Available online 20 July 2007

Abstract

The high nuclearity zinc complex, Zn6(OAc)8(l-OH)2(dmae)2(dmaeH)2 (1) (OAc = acetate and dmaeH = N,N 0-dimethylaminoetha-nol), having a low decomposition temperature and sufficiently high solubility in non-polar solvents, was synthesized by a simple chemicaltechnique in high yield and analyzed by melting point, elemental analysis, FTIR, NMR, single crystal X-ray crystallography and thermalanalysis. Aerosol-assisted chemical vapor deposition technique was used to deposit a high-quality thin film with good adhesion to theglass substrate at relatively low temperature (320 �C). Scanning electron microscopy of the film shows clearly distinct crystallites of uni-form shape with 2.4–2.9 lm size. Powder X-ray diffraction measurements have indicated the deposition of a crystalline phase of hexag-onal ZnO with space group P63mc.� 2007 Elsevier B.V. All rights reserved.

Keywords: Hexanuclear complex; ZnO film

1. Introduction

ZnO has tremendous applications in the fields of UVlight emitters, varistors [1], surface acoustic wave devices[2], piezoelectric transducers, transparent high power elec-tronics, solar cells [3], Zn/Ag high energy electrochemicalcells, gas and chemical sensors and a window material fordisplay. It has a direct energy band gap of 3.37 eV [4]and hence a promising material for light emitting devicesin blue, violet and ultraviolet regions. Recent improve-ments in the quality and control of conductivity in bulkand epitaxial ZnO have increased the interest in the useof this material for short wavelength light emitters andtransparent electronics [5–7]. As a wide band gap semicon-ductor, ZnO is a candidate host for solid-state blue to UVoptoelectronics, including laser development. It has impor-

0020-1693/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2007.07.013

* Corresponding author. Tel.: +92 3005193859.E-mail address: mazhar42pk@yahoo.com (M. Mazhar).

tant applications in high-density data storage systems,solid-state lighting, secure communications and biodetec-tors [8]. For device fabrication, high-quality crystals withsmooth specular surfaces and free from cracks are requiredwhich are not easy to grow because of large lattice mis-match of surfaces and difference in their thermal expansioncoefficients [9]. Transparency to visible light providesopportunities to develop transparent electronics, UV opto-electronics, and integrated sensors, all from the same mate-rial system.

The specific chemical, surface and nanostructured prop-erties of zinc oxide make it a potential candidate especiallyfor catalysis and gas sensing applications, where the expos-ing surface area of the particle to the target gas is veryimportant [10]. It has been reported [11] that the sensitivityof solid-state gas sensors increases with decreasing grainsize. Physical vapor deposition (PVD), chemical vapordeposition (CVD) and thermal pyrolysis techniques aregenerally used to synthesize ZnO particles and films

M. Hamid et al. / Inorganica Chimica Acta 361 (2008) 188–194 189

[12,13]. A number of techniques such as metal–organicchemical vapor deposition (MOCVD) [14–16], rf magne-tron sputtering [17–19], reactive sputtering [20], and spraypyrolysis [21] have been exercised for the deposition of zincoxide films on various substrates. Dimethyl zinc[Zn(CH3)2] and diethyl zinc [Zn(C2H5)2] are commonlyused precursors in metal–organic chemical vapor deposi-tion that is used for large scale coatings at high growthrates. Dimethyl zinc is reactive enough towards oxygen atnormal atmospheric conditions to coat large substrateareas at high growth rates [22,23] but it has high cost andreacts violently with air, water or alcohol even at low tem-perature. The use of dimethyl zinc with water or alcoholsolvent is therefore restricted to deposit uniform film ofgood quality over large areas at 300 �C in flow reactor.The problem of requirement of oxygen and reactivity withsolvent of alkyl zinc complexes with alcohol prior to depo-sition makes them unfit. Diethyl zinc has advantage of lowcost over dimethyl zinc and has been widely used to depositboth doped and undoped ZnO films at different tempera-tures by atmospheric [15] and low-pressure [16] chemicalvapor deposition. The oxidation reaction of diethyl zincis more favorable and leads to the premature reaction withpowder formation even at low temperature and under mildreaction conditions.

In the case of the complexes having non-oxygenatedligands especially, an additional source of oxygen isrequired as in the case of halide precursors, while mostmetal carboxylates and b-diketonates have low solubilitiesin non-polar solvents. Thus, the design of better precursorseliminating the problems of solubilities, high decompositiontemperature while containing the elements of the targetmaterial is always the need of progressing technologies.Therefore, we have synthesized the hexanuclear complexZn6(OAc)8(l-OH)2(dmae)2(dmaeH)2, by taking the advan-tage of aminoalcohols in bridging (CH3COO)2Zn units in asingle molecule, thus covering the metal–oxygen core by anorganic sheath making it more soluble in non-polar sol-vents. We also report here the deposition of high-qualitythin film of ZnO at relatively low temperature on a non-crystalline substrate from this novel precursor.

2. Experimental

2.1. General considerations

All manipulations were carried out under an atmosphereof dry argon using Schlenk tube and glovebox techniques.Solvents were rigorously dried and distilled over sodiummetal/benzophenone. Zn(OAc)2 Æ 2H2O and N,N 0-dimeth-ylaminoethanol were purchased from Aldrich chemicals.All other solvents were purchased from Fluka chemicals.Elemental analysis was performed using a CHN AnalyzerLECO model CHNS-932. FTIR spectrum was recordedwith Spectrum-1000 of Perkin–Elmer. 1H NMR spectrumwas recorded on a Bruker 300 MHz NMR spectrometerin CDCl3 solution at room temperature; the CDCl3 signal

was used to lock the field, and all chemical shifts (ppm) areassigned relative to tetramethylsilane (TMS); 1H NMRspectrum was measured at 300.132 MHz.

2.2. Synthesis of Zn6(OAc)8(l-OH)2(dmae)2(dmaeH)2

Zinc acetate dihydrate, Zn(OAc)2 Æ 2H2O (1.00 g, 4.0mmol), was suspended in dry THF (15 ml) with stirring in100 ml Schlenk tube. dmaeH (0.238 g, 2.6 mmol) was addeddropwise while stirring the suspension until solution becameclear. Unreacted Zn(OAc)2 Æ 2H2O was removed by cannulafiltration. The solution was concentrated and transparentblock-like crystals suitable for single crystal X-ray analysiswere formed at room temperature after one month (80%yield, mp: 101 �C). Anal. Calc. for C36H76N4O23Zn6: C,32.6; H, 5.70; N, 4.22. C, 32.2; H, 5.99; N, 3.92% FTIR/cm�1: 3424br, 2497w, 1609s, 1405s, 1339m, 1023m, 933w,679s, 618m, 503w, 324w,284m. 1H NMR (300 MHz,20 �C, CDCl3, ppm): d = 2.04 (s, CH3COO, 24H), 2.61 (s,CH3N, 24H), 2.87 (t, CH2NMe2, 8H, J = 4.8 Hz), 3.34 (s,RO–H, 2H) 3.84 (t, OCH2, 8H, J = 5.1 Hz), 7.28 (s, OH,2H); TGA: 215–305 �C (35.0% residual mass).

2.3. X-ray crystallography

Diffraction data of complex (1) were collected at150(2) K using a Nonius Kappa image plate diffractometerat 150(2) K using monochromatic Mo Ka radiation. Usingphi and omega scans 406 1.5� images were collected [24].The unit cell was determined, the reflections were inte-grated and scaled with the HKL Denzo-Scalepack suiteof programs [25], and the data were corrected for absorp-tion using Sortav [26]. The structure of (1) was solved bydirect method using SIR97 [27] and refined by full-matrixleast squares against F2 using SHELXL97 [28]. All non-hydrogen atoms were refined anisotropically. The asym-metric unit comprises three zinc atoms and makes up halfof the complete hexa-metallic compound. Each Zn6 unitcrystallizes with one disordered molecule of THF. In gen-eral, hydrogen atoms were placed on calculated positions,whereas H(10) and H(11) (attached to dmaeH and OHrespectively) were located in the difference Fourier mapand refined at a distance of 0.89 A from their parent oxy-gen atoms. Additional crystal data and experimentaldetails are listed in Table 1.

2.4. Materials chemistry

The controlled thermal analysis of the complex wasinvestigated using a Perkin–Elmer thermogravimetric ana-lyzer TGA-7 with computer interface. The measurementswere carried out in an alumina crucible in argon atmo-sphere at flow rate of 25 ml/min.

Scanning electron microscopy (SEM) was carried outusing a JEOL JSM-5910 scanning electron microscope witha beryllium window. Elemental ratio of thin film was deter-mined on Inca-200 EDX analyzer from Oxford Instrument,

Table 1Crystal data and structure refinement parameters for compound (1)

Empirical formula C36H76N4O23Zn6

Formula weight 1325.23Crystal system monoclinicSpace group C2/ca (A) 24.0210(3)b (A) 11.97000(10)c (A) c = 21.1030(3)b (�) 115.1110(10)V (A3) 5494.29(11)Z 4l (Mo Ka) (mm�1) 2.653Crystal size (mm) 0.30 · 0.30 · 0.30h Range for data collection 3.06–25.02�Independent reflections (Rint) 4831 (0.1111)Reflections observed (>2r) 4468Goodness-of-fit on F2 1.104Final R1, wR2 indices [I > 2r(I)] 0.0530, 0.1607R1, wR2 (all data) 0.0574, 0.1657Largest difference in peak and hole (e A�3) 1.604, �1.514

Table 2Growth conditions for the deposition of zinc oxide thin film from complex(1)

Precursor concentration 0.3 g/25 ml (toluene)Carrier gas (N2) flow rate (cm3/min) 25Sample solution injection (ml/min) 0.20Furnace temperature 320 �CSubstrate soda glassDeposition time 125 min

Table 3Selected metrical data for Zn6(OAc)8(l-OH)2(dmae)2(dmaeH)2 (1)

Bond distances (A)

Zn(1)–O(1) 1.988(3) Zn(2)–O(11) 2.061(3)Zn(1)–O(3) 2.043(3) Zn(3)–O(6) 1.961(3)Zn(1)–O(9) 2.022(3) Zn(3)–O(7) 1.971(3)Zn(1)–O(11) 2.047(3) Zn(3)–O(9 0) 1.929(3)Zn(2)–O(2) 2.091(3) Zn(3)–O(11) 1.967(3)Zn(2)–O(4) 2.042(3) Zn(1)–N(1) 2.190(4)Zn(2)–O(5) 2.220(3) Zn(2)–N(2) 2.185(3)Zn(2)–O(10) 2.136(3)

Bond angles(�)

O(1)–Zn(1)–O(3) 108.43(13) O(4)–Zn(2)–O(11) 94.94(11)O(1)–Zn(1)–O(9) 103.78(13) O(4)–Zn(2)–N(2) 93.85(12)O(1)–Zn(1)–O(11) 107.48(11) O(5)–Zn(2)–O(10) 83.84(12)O(1)–Zn(1)–N(1) 92.97(13) O(5)–Zn(2)–O(11) 90.41(10)O(3)–Zn(1)–O(9) 145.96(12) O(5)–Zn(2)–N(2) 91.07(12)O(3)–Zn(1)–O(11) 87.71(11) O(10)–Zn(2)–O(11) 91.05(11)O(3)–Zn(1)–N(1) 86.50(13) O(10)–Zn(2)–N(2) 80.46(12)O(9)–Zn(1)–O(11) 93.01(11) O(11)–Zn(2)–N(2) 171.18(12)O(9)–Zn(1)–N(1) 81.06(13) O(6)–Zn(3)–O(7) 111.51(12)O(11)–Zn(1)–N(1) 159.54(12) O(6)–Zn(3)–O(90) 112.30(13)O(2)–Zn(2)–O(4) 100.52(13) O(6)–Zn(3)–O(11) 115.70(12)O(2)–Zn(2)–O(5) 173.97(12) O(7)–Zn(3)–O(90) 102.04(12)O(2)–Zn(2)–O(10) 90.33(13) O(7)–Zn(3)–O(11) 104.84(12)O(2)–Zn(2)–O(11) 91.25(11) O(90)–Zn(3)–O(11) 109.34(12)O(2)–Zn(2)–N(2) 86.42(12) Zn(3)–O(11)–Zn(1) 114.60(12)O(4)–Zn(2)–O(5) 85.11(12) Zn(3)–O(11)–Zn(2) 110.83(12)O(4)–Zn(2)–O(10) 167.46(12) Zn(1)–O(11)–Zn(2) 107.45(12)

Symmetry transformation: �x + 1/2, �y + 1/2, �z.

190 M. Hamid et al. / Inorganica Chimica Acta 361 (2008) 188–194

UK. XRD peak patterns of the thin film were collectedusing a PANanalytical, X’Pert PRO diffractometer withCu Ka radiation.

Zinc oxide thin film was deposited on a soda glass sub-strate in a hot-walled reactor at 320 �C by the surface reac-tions of the precursor in an inert atmosphere of N2 atatmospheric pressure using a self-designed aerosol-assistedchemical vapor deposition (AACVD) assembly describedelsewhere [29]. The soda glass substrate of dimension1.5 · 3 cm was degreased by successive treatment with ace-tone and water, and positioned horizontally in a glass tubein a furnace fitted with an aerosol generator. Nitrogen car-rier gas flow rate was 25 cm3/min and precursor solution(0.3 g/25 ml) was injected at 0.2 ml/min. Parameters forthe growth of thin film are listed in Table 2.

3. Results and discussion

3.1. Synthesis

Zn(OAc)2 Æ 2H2O reacts with N,N 0-dimethylaminoetha-nol at room temperature in THF to yield the high nuclear-ity zinc complex (1), as shown in Eq. (1).

6ZnðOAcÞ2 � 2H2Oþ 4 dmaeH

����!THF

room temp:Zn6ðOAcÞ8ðl-OHÞ2ðdmaeÞ2ðdmaeHÞ2

þ 4 HOAcþ 10H2O ð1Þ

Progress of the reaction is indicated by the dissolution ofZn(OAc)2 Æ 2H2O in the reaction mixture. Although com-plex (1) is quite sensitive towards open atmosphere, thepresence of water of hydration seems to be necessary as asource of bridging OH as it was observed that the reactionfails to proceed with anhydrous zinc acetate.

In the IR spectra, the broad band at 3424 cm�1 is due tothe O–H absorption while carboxylate characteristic bandsappear at 1609 cm�1 for mas(CO2) and 1405 cm�1 forms(CO2). The difference D(masCO2 � msCO2) = 204 cm�1

suggests a dominant chelating or bridging – chelatingbehavior for the acetate ligands [30,31]. In proton NMR,singlet due to acetate CH3 (2.04 ppm) appears up fieldbeing attached to more electropositive carbon atom ascompared with methyl protons (2.61 ppm) attached to N-atom and triplet of methylene near N-atom is up field ascompared with that of near O-atom. Alcoholic protonappears near 3.34 ppm while signal of free OH is at7.28 ppm. Integration of all the proton signals is in reason-able agreement with proton ratio.

3.2. Molecular structure of (1)

Selected metrical data for (1) are given in Table 3. Twoasymmetric units, each having three zinc atoms, are linkedby l2-oxygen of dmae ligand and further strengthened by afascinating hydrogen bonding of monodentate acetategroup with dmaeH [O(8)–H(10)–O(10)] and OH [O(7)–H(11)–O(11)] groups of the other half of the dimmer

M. Hamid et al. / Inorganica Chimica Acta 361 (2008) 188–194 191

(Fig. 1). In the homo-hexanuclear complex (1), six zincatoms are connected by six l2-acetate, two l2-dmae andtwo l3-OH groups while the remaining two acetate andtwo dmaeH ligands act as terminal chelating ligands havingno contribution towards linkage of the Zn6 array.

Each zinc atom of the three sets of the asymmetric unithas a different geometric environment with Zn(1) squarepyramidal, Zn(2) octahedral and Zn(3) tetrahedral coordi-nation spheres (Fig. 2). In each asymmetric unit all thethree Zinc atoms [Zn(1), Zn(2), Zn(3)] are linked by triplybridging oxygen of hydroxyl group [O(11)] in addition to

Fig. 1. ORTEP drawing showing the molecular structure of Zn6(OAc)8(l-OHtransformations: #1 �x + 1/2, �y + 1/2, �z. The THF solvent molecules asclarity. Dotted lines indicate hydrogen bonds.

Fig. 2. Geometric spheres of (1): (a) square pyramidal Zn1

three acetate groups chelating two metal centers [acetategroups based on C(1) and C(3) link Zn(1), Zn(2), while thatof C(5) bridges Zn(2), Zn(3)]. One acetate group [based onC(7)] is attached terminally to the Zn(3) from which noacetate oxygen contributes directly towards bridging toanother metal center.

The environment of the Zn(1) atom can be described asdistorted square-based pyramidal and the ligation set iscomprised of bridging oxygen atoms [O(1), O(3)] of twoacetate groups, a bridging l3-hydroxyl anion [O(11)], oxy-gen [O(9)] and nitrogen atoms [N(1)] of a chelating dmae.

)2(dmae)2(dmaeH)2 (1). Thermal ellipsoids are at the 30% level. Symmetrywell as all but the hydroxyl and dmaeH hydrogen atoms are omitted for

, (b) octahedral Zn2 and (c) the tetrahedral Zn3 atoms.

Fig. 3. Thermogravimetric plot showing loss in weight with increase intemperature for complex (1).

192 M. Hamid et al. / Inorganica Chimica Acta 361 (2008) 188–194

A distorted square pyramidal geometry with one of theequatorial substituents group, the doubly bridged O(9) ofdmae group, being displaced towards O(1) and away fromO(3), resulting in the angle O(3)–Zn(1)–O(9) being enlargedfrom the ideal 120� to 145.96(12)�, and O(1)–Zn(1)–O(9)being reduced to 103.78(13)�. The overall geometry aroundZn(2) is a distorted octahedron of five oxygen and onenitrogen atoms. The octahedral environment is generatedby a l3-hydroxyl anion [O(11)] bridging Zn(1), Zn(2),Zn(3), three acetate oxygen atoms [O(2), O(4), O(5)] andtwo dmaeH donor atoms [O(10), N(2)]. Zn(3) has a coordi-nation number of four with a ZnO4 core comprising of oneacetate ligand [O(6)] bridging Zn(3) with Zn(2), anotherterminal acetate [O(7)], the l3-hydroxyl anion O(11) con-necting Zn(3) with both Zn(1) and Zn(2). The ZnO4 coreis completed by O(9), a l2-oxygen of a dmae group thatbridges Zn(3) and Zn(1 0) of two asymmetric units.

The Zn–O bond distances broadly increase with increas-ing coordination number at zinc: Zn(3) 1.929(3)–1.971(3);Zn(1) 1.988(3)–2.047(3); Zn(2) 2.042(3)–2.220 A. The twocoordinating N:! Zn interactions are identical despite dif-fering coordination numbers at Zn(1) and Zn(2) withwhich they are associated.

The tetrahedral environment around Zn(3) atom isfairly regular with bond distances between 1.929(3) and1.971(3) A for Zn(3)–O(9) and Zn(3)–O(7), respectively,which are in good agreement with those observed for tetra-hedral [Zn2Al2(acac)4(OiPr)6] [32] and octahedral [Zn-(acac)2(dmaeH)] [33] Zn complexes.

Additional stability for the complex is provided by theintra-molecular hydrogen bonding (Table 4) between oxy-gen atoms [O(7), O(8)] of acetate group of one asymmetricunit and hydroxyl and dmaeH group hydrogens [H(11),H(10), respectively] of a second other asymmetric unit.

3.3. Thermal decomposition studies and characterization of

thin films

The thermal behavior (Fig. 3) of complex (1) has beenexamined by thermogravimetric analysis (TGA), per-formed under an inert atmosphere of flowing nitrogengas (25 ml/min) and a heating rate of 10 �C/min. The TGpattern of (1) shows a single weight loss step starting atabout 215 �C with a maximum weight loss from 215 �Cto 305 �C. The decomposition ends at about 305 �C toleave a residue of 35.0% of the initial mass, slightly lessthan 36.9% calculated for complete conversion to a ZnOceramic material. Residual weight remains same from305 �C to higher temperatures, which indicates theexpected stable phase of zinc oxide.

Table 4Hydrogen bond geometry (A, �)

D–H� � �A D–H H� � �A D� � �A D–H� � �AO(10)–H(10)� � �O(8) 0.90(8) 1.72(8) 2.607(5) 169(9)O(11)–H(11)� � �O(7) 0.89(3) 1.87(4) 2.728(4) 161(5)

Using the above TGA data, the AACVD reactor tem-perature was adjusted at 320 �C. The film deposited exhib-ited excellent adhesion properties to the substrate asverified by the scotch tape test and is stable towards airand moisture. It reflected light in multi-shaded colorsdepending on the particle size and thickness of the film.

The electron micrographic image (Fig. 4) showed thatzinc oxide thin film has a compact, continuous and smoothmorphology. The particles with spherical appearance havegood orientation and clearly well defined grain boundaries.The thickness of the film and crystallite size is controlled bythe time of deposition and concentration of precursor solu-tion [34]. The film deposition parameters of conc. (0.3 g/25 ml of solvent) and time (125 min) are suitable for thecrystallites to grow to the size from 2.4 to 2.9 lm as calcu-lated from electron micrograph scanned at 5k. The energydispersive X-ray (EDX) analysis of the thin film shows

Fig. 4. The SEM micrograph scanned at 5k of oxide film deposited fromcomplex (1) at 300 �C.

Fig. 5. X-ray diffraction pattern of the thin film obtained from complex (1).

M. Hamid et al. / Inorganica Chimica Acta 361 (2008) 188–194 193

clean decomposition of complex (1) to ZnO without anycarboneous impurity from the precursor or from solvent.The atomic ratio of zinc and oxygen shows presence ofthese elements with some impurities of Al(0.63%) andSi(1.09%) from the glass substrate. Thus this precursorhas advantage over the previously applied organozinc pre-cursors such as dimethyl zinc and diethyl zinc [35].

The crystalline nature of the material is further strength-ened by powder patterns of the film (Fig. 5), which wasconfirmed to be ZnO as suggested by the TGA studies.The powder X-ray diffraction pattern of the thin filmgrown from complex (1) matches with ICDD data ofZnO [01-089-0511] [36]. ZnO crystallizes in hexagonal crys-tal system with space group P63mc and cell parametersa = b = 3.2490, c = 5.2052 A.

4. Conclusion

A novel ZnO precursor has been successfully designed,synthesized and characterized by its melting point, ele-mental analysis, FTIR, 1H NMR, single crystal X-rayanalysis and verified for its decomposition temperatureby TGA. The precursor has sufficient solubility in non-polar solvent i.e. toluene and was found to be suitablefor the deposition of good quality thin film on a non-crystalline substrate by AACVD. The film was character-ized by SEM, EDX and XRD for its compact morphol-ogy, composition and crystalline nature. The grain sizefrom 2.4 to 2.9 lm seems to be dependent on the precur-sor conc. and time of deposition. Further improvementcan lead to the customization of film morphology andproperties. This route should be useful for the develop-ment of high performance miniaturized devices for futuretechnologies.

Acknowledgement

M.H. and M.M. acknowledge the Higher EducationCommission of Pakistan for financial support throughthe ‘‘Merit Scholarship Scheme for PhD Studies in Science& Technology (200 Scholarships)’’.

Appendix A. Supplementary material

CCDC 629381 contains the supplementary crystallo-graphic data for Zn6(C2H3O2)8(C4H10NO)2 (C4H11NO)2-(OH)2. These data can be obtained free of charge viahttp://www.ccdc.cam.ac.uk/conts/retrieving.html, or fromthe Cambridge Crystallographic Data Centre, 12 UnionRoad, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementarydata associated with this article can be found, in the onlineversion, at doi:10.1016/j.ica. 2007.07.013.

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