Nanocystalline ZnO films prepared via polymeric precursor method (Pechini)

Physica B 405 (2010) 3679–3684

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Nanocystalline ZnO films prepared via polymeric precursor method (Pechini) C. Sa´nchez a, J. Doria a, C. Paucar a, M. Hernandez a, A. Mo´squera b, J.E. Rodrı´guez b, A. Go´mez c, E. Baca d, O. Mora´n a,n a

´micos y Vı´treos, Universidad Nacional de Colombia, Sede Medellı´n, A.A. 568, Medellı´n, Colombia Laboratorio de Materiales Cera ´n, Colombia Grupo CYTEMAC, Universidad del Cauca, Calle 5 No 4-70, Popaya c Departamento de Ingenierı´a de Materiales, Universidad Nacional de Colombia, sede Medellı´n, A.A. 568, Medellı´n, Colombia d Grupo de Ingenierı´a de Nuevos Materiales, Universidad del Valle, A.A. 25360 Cali, Colombia b

a r t i c l e in fo

abstract

Article history: Received 28 January 2010 Received in revised form 19 May 2010 Accepted 21 May 2010

The polymeric precursor method (Pechini) was employed to prepare high-quality nanocrystalline zinc oxide (ZnO) films. Briefly, the process started off with the preparation of a coating solution by the Pechini process followed by a coating of the glass substrates by a dip-coating technique and subsequent heat-treatment of the as-deposited films up to 550 1C for 30 min. The Rietveld profile analysis of the ¨ X-ray diffraction (XRD) spectra revealed the wurzite structure as expected for ZnO with a P63mc symmetry. No additional peaks were observed that would correspond to any secondary crystalline phase. The average crystallites size was 20 nm as calculated by Sherrer’s equation. UV–vis spectroscopy showed sharp ultraviolet absorption edges at  380 nm. The absorption edge analysis yielded optical band gap energy of 3.24 eV with electronic transition of the direct transition type. The Fourier transform infrared (FTIR) analysis showed asymmetric and symmetric stretching modes of the carboxyl group (CQO). Scanning electron microscope (SEM) analysis revealed a crack-free surface morphology indicating that coating of the amorphous glass substrates was homogeneous on large surface areas. The temperature dependent conductivity featured a typical semiconducting-like behavior with resistivity approaching 3  10  1 O cm at 220 K. & 2010 Elsevier B.V. All rights reserved.

Keywords: Oxides Chemical synthesis Monolayers Optical properties

1. Introduction During the last decade enormous efforts have been made to realize a new class of spintronics devices such as spin valve transistors, spin light emitting diodes, non-volatile memory, logic devices, optical insulators and ultra-fast optical switches [1,2]. Such spintronics devices exploit both charge and spin to carry information data. As a matter of course, ferromagnetic properties at room temperature should be introduced in semiconducting materials. Among the materials reported so far, Mn-doped GaAs has been found to be ferromagnetic with the highest reported Curie temperature TC  172 K [3]. Recently, semiconducting ZnO has attracted enormous research attention because of its interesting electrical, optical, magnetic and piezoelectric properties [4–7]. For example, ZnO has a direct band gap (Eg ¼3.37 eV) and a large exciton binding energy (60 meV). Hence, it is a potential candidate material for technological applications as ultraviolet light emitting devices [8] or UV lasers. Many of such practical applications demand the fabrication of high quality ZnO thin films. Although physical methods as molecular-beam epitaxy

n

Corresponding author. Tel.: + 57 4306341; fax: + 57 4309327. E-mail address: [email protected] (O. Mora´n).

0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.05.065

(MBE), sputtering [9–11] or spray pyrolysis [12] have been extensively used in thin film technology, chemical methods as sol–gel processes [13,14] or polymeric precursor method (Pechini) [15] particularly adapt to produce ZnO colloids and films in a simple, low-cost and highly-controlled way. The process sol–gel has been used for ceramic and glass preparation for years [13,14]. There are several methods of preparation in the sol–gel technology, which depend on the employed inorganic alcoxides [16]. Some methods are more versatile than others and involve a determined organometallic compound in an alcoholic dissolvent followed by a series of chemical reactions of hydrolysis, condensation and curing to produce a gel which is formed by a continuous inorganic network [17]. The most important step in this route is the formation of an inorganic polymer by means of hydrolysis reactions. The hydrolysis of a solution of tetraethyl orthosilicate (TEOS) in a dissolvent as the ethanol allows for the formation of silanols which form a sol. A gel is then obtained by means of the curing by subsequent condensation [18]. This area has progressed in the last decades through the modified inorganic polymer preparation by means of the organic molecules used to obtain hybrid gels [19]. The hybrid materials have the properties of an inorganic matrix and the functionality of the organic component. The TEOS may be used to connect to organic monomers and depending on the composition and the amount

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of the organic and inorganic components, new families of materials can be prepared. A way to combine the sol–gel method, which uses alcoxides and typically produces an inorganic network, with polymeric precursors to produce a hybrid gel is to use the method of Pechini [15]. The Pechini process [15] also called as ‘‘liquid mix’’ [20], ‘‘resin intermediate’’ [21] or ‘‘polymerizable complex’’ [22] method bases upon an aqueous polyalcohol–citric acid system in which a wide range of metal salts are soluble. The original patent for the Pechini process [15] reports on the formation of a polymeric resin produced through polyesterification between metal chelate complexes using alpha-hydroxycarboxylic acid such as citric acid and polyhydroxy alcohols such as ethylene glycol. The main purpose is to obtain a polyester resin comprising randomly coiled macro-molecular chains in which the metal ions may be uniformly distributed. Many of the metal ions, except monovalent cations, form very stable chelate complexes with citric acid. Most of these metal–citric acid complexes are soluble in a mixed solvent of water and ethylene glycol, which ensures perfect mixing of metal ions at the molecular level [23]. Esterification of citric acid occurs in the presence of ethylene glycol when heated at moderate temperatures. Prolonged heating of the mixed solution promotes polyesterification yielding a transparent polymeric resin precursor. Heating of the polymeric resin at high temperatures (above 300 1C) causes a breakdown of the polymer. Generally speaking, this method of synthesis possesses some advantages on the traditional methods that involve reactions in solid state. Among others, it allows for preparing complex compositions, obtaining high homogeneity and high purity through mixing at molecular level in solution and exact control of the stoichiometry [24]. Furthermore, by means of these alternative techniques, it is possible to obtain thin films, fibers and particles of nanometric sizes. Although the main disadvantage of this method of synthesis is the lack of information related to the involved chemical reactions during the formation of the complexes, this has not prevented the method to be widely used for the synthesis of multicomponent oxide to diverse applications. In this work the structural, morphological, optical and transport properties of ZnO films fabricated by the polymeric precursor method onto glass substrates by dip-coating are reported. The achieved results indicate that high quality ZnO films may be obtained on large areas by using this chemical procedure.

2. Experiment The preparation of the coating solution is based on the Pechini method [15]. A metallo-organic precursor was mixed with stoichiometry ester in the appropriate ratio in order to complex the desired cation. Zinc acetate dihydrate [Zn(CH3COO)2  2H2O, 99.9%] was used as metallo-organic compound. Two solutions were prepared separately and then mixed together. In the first one, Zn(CH3COO)2  2H2O was dissolved in ethanol and nitric acid. In the second one, citric acid was diluted by ethylene glycol in a molar rate of 1:4 and the resulting solution was heated up to 80 1C. When this mixture was completely homogenous, it was mixed with the first one and then, the final solution was let to cool down until it reached the room temperature. Next, the resulting solution was heated up to 80 1C and maintained for 10 min. The addition of the ethylene glycol and the heating of the mixture led to the formation of a polymeric precursor solution. The viscosity of the resulting gelled system gradually increased as the solvent was evaporated. At this stage, it was observed that the times needed to get the appropriated coating solution were not too long as those involved in the traditional sol–gel procedure

(sometimes more than 24 h) [13]. When the coating solution reached the room temperature films were obtained by dip-coating glass substrates at a withdrawal speed of 4 cm/min. Prior to the coating, the glass substrates were treated in a standard wet cleaning procedure. After the deposition, the films were dried first at 200 1C for 30 min in air, then at 350 1C for 30 min and finally at 550 1C for 30 min with a heating rate of 2 1C/min between successive heating steps. The crystal structure of the as-prepared ZnO films was analyzed by X-ray diffraction (XRD) at room temperature in a standard y–2y configuration. The stoichiometry and thickness of the ZnO films were determined by Rutherford backscattering spectrometry (RBS) with 2 MeV 4He + beam in random geometry. Elemental analysis was carried out by energy-dispersive X-ray spectroscopy (EDX). The surface and microstructure morphology were analyzed by scanning electron microscope (SEM) operating at 7 kV and atomic force microscopy (AFM), respectively. The optical properties were characterized by UV–vis absorption spectra using an ocean optics spectrophotometer at 200–900 nm. An optical fiber was used for transmitting the optical signal continuum to the sample and to detect the transmitted signal. Fourier transform infrared spectroscopy (FTIR) absorption was measured over the frequency range 4000–400 cm  1 Electrical resistance measurements at different temperatures were performed by a standard four-probe method using an integrated PPMS system (Quantum Design). For the electric resistance measurement, an ac bias current of 10 mA (11 Hz) was fed to the samples and the corresponding voltage drop sensed. The r–T curves were recorded in the temperature range 220–300 K.

3. Results and discussion The polymeric precursor method used in this work allowed for obtaining a homogeneous metal–organic polymer in which the zinc ions are, in principle, randomly distributed along the backbone of the polymer. By the heating treatment at 140 1C, this polymeric precursor method goes through two main stages: the first one is the reaction mechanism of esterification of citric acid with ethylene glycol as indicated in Fig. 1, and the second one, the chelation of the zinc acetate by the formed ester as indicated in Fig. 2. Citric acid and ethylene glycol generate a reaction of esterification, and the formed ester acts as complexing agent of the zinc cations forming a polymer network that is resinlike or solid as viscosity increases. At this point, it is necessary that the exchange reaction of the zinc acetate with the acid group proceeds to completion. If this exchange reaction is not driven to completion, precipitation of unreacted zinc acetate would occur in the resin on cooling and this may lead to gross segregation of the zinc in the ceramic. In the considered system, the polymer solidified to a clear resin on cooling, implying that all of the zinc acetate had reacted with the polymer. This polymer is stirred to obtain a gel that allows getting films by dip coating. As it was stated above, the films were dried first at 200 1C for 30 min in air after the deposition. This temperature is high enough so as to evaporate the solvent and remove organic residuals (although some traces were still detected in the postheated films) without altering the composition of the adsorbed zinc complex. Here, it was taken into account that the thermal decomposition of the zinc acetate begins at  240 1C [25]. Successive postheating at 350 and 550 1C should stabilize the hexagonal phase of the compound and allows for the formation of surfaces with high morphological perfection. Usually, an annealing temperature of 450 1C is considered as a reference temperature for the formation of ZnO films [26]. Nevertheless, by analyzing the different reports on the

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HO

3681

O C

OH

O

HO

140°C

OH + HO

OH Ethylene Glycol

O Citric AcidO

O

H O

OH + H2O n

O C O

OH

Esther

Water

Fig. 1. Diagram of reaction mechanism for esterification of citric acid with ethylene glycol.

OH

O

O

O O

H

O

O C O

OH+ Zn n

O

H O

O O

O

C 2

OH Zinc acetate

Esther

OH

140°C OH + n

O O Zn Chelated zinc acetate

O

OH

Acetic acid

Fig. 2. Diagram of reaction mechanism of chelation of the zinc acetate by the polymer.

300

(103)

Counts per channel

100

(200) (112) (201)

(110)

(102)

200

(100)

Intensity [counts/s]

(002) (101)

3000 Zn 2000

O

1000

0 30

40

50

60

70

2 [deg]

0

200

400

Channel Number

Fig. 3. XRD diffractogram for a ZnO film grown by chemical route onto a glass substrate. The open symbols correspond to the Rietveld fit. The indicated peak indexation corresponds to the wurzite ZnO phase.

Fig. 4. Rutherford backscattering for a ZnO film grown by chemical route onto a glass substrate.

annealing treatment of the ZnO films after deposition, it becomes clear that the hexagonal phase of ZnO forms first at annealing temperatures between 200 and 300 1C [27]. At this point, it is important to mention that to avoid quick-cooling fractures (due to thermal shock), the samples were not taken out of the oven abruptly; instead, they were kept inside it until room temperature was reached by switching off the apparatus. Shown in Fig. 3 is the XRD spectrum of an as-prepared ZnO film on a glass substrate. The Rietveld profile analysis of the ¨ spectra revealed the wurzite structure as expected for ZnO with a P63mc symmetry. No additional peaks corresponding to any secondary crystalline phase were observed in these films. The crystal lattice parameters from the present data resulted to be ˚ which are close to a¼ 3.2490 A˚ and a ¼3.2420 A˚ and c¼5.1760 A, c¼ 5.2050 A˚ of ZnO (JCPDS card No. 00-001-1136). The difference in c-axis lattice constants may be attributed to the occurrence of stress in the film. Under compression (parallel to the surface), the c-axis lattice constant will decrease, leading to a somewhat larger interplanar distance. The polycrystalline character of the postheated ZnO films (Fig. 3) is in contrast with the results obtained on ZnO films synthesized from zinc acetate/ 2-monoethanolamine precursor solutions and pre-heated at higher temperatures (200–5001 C) [28,29]. These films showed preferential orientation along the (0 0 2) plane. Chemical parameters like solvent, inorganic precursor, type and

concentration of complexing additives were established as factors influencing the texture of the films and their preferential orientations [28]. Certainly, highly mono-oriented films were produced by two different approaches: direct formation of continuous coatings and deposition of a pre-formed colloid. Two different orientations were obtained (c//n or a//n) depending on the film nature and the solvent by using zinc acetate dihydrate as precursor and monoethanolamine as complexing agent. The use of other Zn2 + precursors or different complexing agents generally resulted in precipitation or poorly oriented films. In Ref. [26], the authors used a procedure similar to that described in our work for growing ZnO films on glass and silicon wafer substrates. Particularly, zinc acetate dihydrate and ethylene glycol were chosen as precursor and complexing additive, respectively. The XRD patterns of these films showed neither increase in the degree of crystal orientation in the films as the firing temperature was increased nor a preferential orientation along the (0 0 2) plane. On the other hand, the average grain size was calculated by Sherrer’s equation using the XRD line broadening method [20]. The crystal size D ¼0.9l/(Bcos y) where l represents the wavelength of X-ray, B the FWHM and y the diffraction angle. By using the experimental data, an average grain size of 15 nm was derived, which is in accordance with the value obtained from the Rietveld refinement (  20 nm). In a complementary RBS measurement (see Fig. 4), thickness and chemical composition of

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the ZnO film were determined. A thickness of  300 nm was obtained from the peak area. The comparison of the RBS spectrum with simulation resu1ts demonstrated pretty good adjustment of the targeted stoichiometry. The SEM image of an as-deposited ZnO film (Fig. 5(a)) gives a general view of the morphology of ZnO thin films synthesized by the described chemical route on glass substrates. Interesting enough, the surface of this film features high morphological perfection, which is hardly achieved by other growing techniques [12]. The granular character of ZnO thin films is hardly observed on this image. This result suggests the formation of a dense ZnO surface. Such surfaces are desired in multilayered systems, which are the basis for novel electronic devices. A typical EDX spectrum of a ZnO film is shown in Fig. 5(b). The Si element comes from the substrate (glass). It was calculated that the composition results were almost consistent with the molar ratio of ZnO. Similar results were obtained from the analysis of other areas in the film, which speaks for a chemically homogeneous coating of the glass substrates by the ZnO solution. An AFM scan of a ZnO nanoparticulate monolayer deposited on glass, recorded over an area of 5 mm  5 mm, is reported in Fig. 6(a). It shows that the particles are mono-dispersed with spherical morphology. Such morphology is better resolved in the magnified area (Fig. 6(b)) from which particle sizes smaller than 70 nm could be determined. The particles are packed closely and welldistributed on the glass substrate and the average surface roughness is about 10 nm. The morphology of ZnO films synthesized by soft chemistry methods shall be related to the postheat treatment [29]. In the present work, the postheating treatments were performed at relatively high temperatures in order to improve the packing density of the films. It is known that

the particle size increases with increase in postheating temperature [30]. At higher temperatures, the atoms should have enough activation energy to occupy the correct sites in the crystal lattice and grains with lower surface energy become larger. In this respect, the improvement in the packing density of the films may be explained by the reduction/disappearance of gaps between particles. Indeed, the morphological studies (Fig. 5(a)) indicate that the films are dense as confirmed by the lack of significant pores and pits in the SEM micrograph. No typical surface roughening is observable in this image which was corroborated by the AFM studies (Fig. 5(b)). Postheating at temperatures higher than 550 1C led, nevertheless, to reduced packing density, which may be caused by the evaporation of ZnO [30]. SEM studies on these films (not shown) revealed crackaffected surface. Other important factor influencing the density of the films is the heating rate. ZnO films reported in the present work were thermally treated at low heating rate (2 1C/min). This rate was selected by considering the known fact that vaporization of the solvents (125 1C), decomposition of the zinc acetate (240 1C) and beginning of the crystallization of the zinc oxide (200 1C) may occur almost simultaneously when the heating rate is high [26,29]. It was demonstrated that the structural relaxation of the gel film, which is induced by the solvent vaporization and acetate decomposition, may take place only before the crystallization; the simultaneous vaporization, decomposition and crystallization may give the film a less chance to be structurally relaxed. On the other hand, when the heating rate is low, the gel film is given an enough time to be structurally relaxed before crystallization, resulting in denser ceramic films as those reported in this work. Finally, the low withdrawal speed of the substrate (4 cm/min) shall influence the density of the

Fig. 5. SEM morphology of a ZnO film onto a glass substrate (a) and (b) corresponding EDX spectrum of the chemically grown ZnO film.

Fig. 6. 0.5 mm  0.5 mm AFM image of a ZnO film grown on glass substrate. Inset: 500  500 nm magnified area showing the nanometric-sized crystallites.

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60

30

Absorbance [%]

65

(αh)2 [x106 eV2cm-2]

Transmitance [%]

90

3683

6 4 2 0 2.5

60

55 glass

3.0

h [eV]

3.5

50

0 400

1000

600

Fig. 7. Optical transmittance spectrum of a ZnO film obtained by polymeric precursor method. Inset: optical direct band of a high-quality ZnO film grown onto a glass substrate. A band gap of 3.24 eV is calculated by extrapolating the straight part of the curve to the energy axis.

2000

1500

wave number [cm-1]

 [nm]

Fig. 8. FT–IR spectra of a ZnO film recorded at different positions on the surface.

postheated films. This statement is based on the fact that when the withdrawal speed is low, the film thickness per one dipping is small [31], and the solvent may more easily evaporate from the film. This may cause the larger shrinkage of the film giving denser gel films. Similar relationship between the withdrawal speed and relative density was also reported by Brinker et al. [32] for silica films derived from polymeric silica sols. The room temperature optical transmittance spectrum in the wavelength (l) range 200–800 nm for a ZnO film prepared by using the polymeric precursor method is shown in Fig. 7. The optical spectrum for this sample featured transparency in the visible range (l 4400 nm) larger than 90% and a sharp ultraviolet absorption edge at l  380 nm, these characteristics being of high quality ZnO films, as those grown epitaxially by pulsed laser deposition (PLD) [33]. The high transmittance displayed by this film suggested low optical scattering associated with uniformity of the particle size as well as a smooth surface morphology. Such features were partially corroborated by the SEM studies. For the measured thickness, the absorption coefficient a resulted to be 2  107 m  1 in the UV range. Based on the optical absorption theory in semiconductors, the relationship between the absorption coefficients a and the photon energy hn for direct allowed transition is given by (ahn)2 ¼B(hn–Eg), where B represents a constant, which depends on states density, and Eg the optical band gap energy. Eg may be determined at the absorption edge using the last equation. In doing this, the linear fit of (ahnd)2 against hn was extrapolated to intersect the energy axis at a ¼0 (inset to Fig. 7). A value of 3.24 eV was estimated for this sample, which is identical to that reported for single crystal ZnO [34]. Fig. 8 displays the FT–IR spectra for the ZnO film recorded on different areas. Absorption bands in the range 1600–1400 cm  1 represent the asymmetric and symmetric stretching modes of the carboxyl group (CQO). Concretely, the peak located at 1583 cm  1 represents the asymmetric stretching vibration of CQO mode of zinc mono-acetate. The zinc mono-acetato is an intermediate produced during the reaction of zinc acetate in ethanol solvent [35]. The achieved results suggest that the crystal phase was formed at comparatively low temperature but the thermal treatment of the deposited films at 550 1C was not enough to eliminate the mono-acetate completely. Fig. 9 shows the electrical conductivity of a ZnO film as a function of 1/T. The intrinsic semiconducting behavior of this material is clearly seen from this plot. The sample showed low conductivity with resistivity approaching 3  10  1 O cm at 220 K. the conspicuous rise in the resistivity of the ZnO film at To200 K

Conductivity [Ω-1cm-1]

103

102

101

100 3.8

4.0

4.2

4.4

4.6

1000/T [K-1] Fig. 9. Temperature dependence of the conductivity of a ZnO film fabricated by Polymeric precursor method on a glass substrate.

and the limited voltage-reading capacity of the measuring system made the recording of reliable values of resistivity at lower temperatures difficult. The source of the n-type conductivity of undoped ZnO films has traditionally been attributed to instrinsic donor defects, such as zinc interstitials and oxygen vacancies [36]. Finally, it shall be mentioned that such a well defined semiconducting behavior of ZnO films grown by chemical methods (Fig. 9) has hardly been reported in the literature. Basically, the issue of the electrical transport in ZnO films has nicely been treated in [37]. In these works, nevertheless, the ZnO films were prepared by chemical vapor deposition (CVD) and were c-axis-oriented.

4. Conclusions Nanometric thin films of the n-type semiconducting ZnO were successfully prepared by the simple polymeric precursor method (Pechini) onto glass substrates. XRD pattern confirmed the hexagonal wurtzite structure of the ZnO films without any additional diffraction peaks stemming from impurity phase such as Zn. This suggested a complete transformation from the Zn precursors to ZnO crystals. SEM images revealed that films consisted of uniform crystallites in the size of 20–30 nm. The composition resulted to be almost consistent with the molar ratio of ZnO as corroborated by EDX and RBS measurements. FT–IR spectra for the ZnO films displayed absorption bands in the range 1600–1400 cm  1, which were attributed to asymmetric and symmetric stretching modes of the carboxyl group (CQO). A transmittance of over 90% in the visible range was achieved for

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these films. The band gap, as determined from the absorbance spectrum, amounted to 3.25 eV, which was identical to that recorded for single crystal ZnO. The temperature dependence of the conductivity showed the typical semiconducting behavior of this material with resistivity approaching 3  10  1 O cm at 220 K. The achievement of such high-quality films seemed to be related to the production conditions, as for instance, the preheating and postheating treatments and/or heating rate.

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