Home
Search
Collections
Journals
About
Contact us
My IOPscience
Highly transparent and conductive Al-doped ZnO nanoparticulate thin films using direct write processing
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 195301 (http://iopscience.iop.org/0957-4484/25/19/195301) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 66.194.72.152 This content was downloaded on 16/06/2014 at 02:04
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 195301 (9pp)
doi:10.1088/0957-4484/25/19/195301
Highly transparent and conductive Al-doped ZnO nanoparticulate thin films using direct write processing S Vunnam1, K Ankireddy2, J Kellar2 and W Cross2 1
Program of Nanoscience and Nanoengineering, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA 2 Program of Materials Engineering and Science, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA E-mail: [email protected] Received 31 December 2013, revised 7 March 2014 Accepted for publication 24 March 2014 Published 23 April 2014 Abstract
Solution processable Al-doped ZnO (AZO) thin films are attractive candidates for low cost transparent electrodes. We demonstrate here an optimized nanoparticulate ink for the fabrication of AZO thin films using scalable, low-cost direct write processing (ultrasonic spray deposition) in air at atmospheric pressure. The thin films were made via thermal processing of as-deposited films. AZO films deposited using the proposed nanoparticulate ink with further reducing in vacuum and rf plasma of forming gas exhibited optical transparency greater than 95% across the visible spectrum, and electrical resistivity of 0.5 Ω cm and it drops down to 7.0 × 10−2 Ω cm after illuminating with UV light, which is comparable to commercially available tin doped indium oxide colloidal coatings. Various structural analyses were performed to investigate the influence of ink chemistry, deposition parameters, and annealing temperatures on the structural, optical, and electrical characteristics of the spray deposited AZO thin films. Optical micrographs confirmed the presence of surface defects and cracks using the AZO NPs ink without any additives. After adding N-(2-Aminoethyl)-3-aminopropylmethyldimethoxy silane to the ink, AZO films exhibited an optical transparency which was virtually identical to that of the plain glass substrate. Keywords: direct write processing, transparent conducting oxides, Al-doped ZnO, optical transparency, electrical resistivity (Some figures may appear in colour only in the online journal)
1. Introduction
oxide, zinc indium oxide, zinc indium gallium oxide, etc [14, 15]. Al-doped zinc oxide (AZO) has been considered as a promising alternative to indium tin oxide (ITO) [16–21]. In conventional sol–gel methods, nucleation of crystalline particles from precursor solution occurs only at elevated temperatures (>500 °C), whereas solution processed films prepared from crystalline nanoparticles will sinter to form crystalline films at lower temperatures [22]. Among the solution deposition methods, ultrasonic spray deposition gives the desired thickness in a single step, which is an
The solution-based deposition of functional materials is an attractive technique for the fabrication of high performance, low-cost electronics [1–6]. Transparent conductive oxides (TCOs) are widely used in many electronic and optoelectronic devices such as light emitting diodes, liquid crystal displays, and photovoltaic devices [7–13]. More recently, there has been tremendous interest in the use of transparent transistors based on semiconducting transparent oxides, such as zinc 0957-4484/14/195301+09$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 195301
S Vunnam et al
alternative to the time consuming repeated steps (coating and drying) involved in traditional solution deposition routes such as spin coating, dip coating and chemical bath deposition [23–25]. Ultrasonic spray deposition holds several key advantages over more widely employed pressure-driven spraying methods. Among these are lower materials usage and improved uniformity due to a narrower size distribution of droplets in the atomized material [26–29]. To the best of our knowledge, there have been no reports of the optical and electrical properties of ultrasonic spray coated AZO films made from nanoparticulate inks. The major challenge in colloidal-based thin films is cracking during drying. This can be eliminated by adjusting the ink chemistry or manipulating the drying mechanism [30]. Ink chemistry can be altered by adding polymeric binders, plasticizers, surfactants and sol–gel precursors to the ink. Organosilanes are an alternative to organic polymer binders [31, 32]. The application of organosilanes in colloidal thin films has been reported by several researchers [33]. In particular, amino silane coupling agents enhances the silver nanoparticle adhesion to different substrates [34]. Oxygen vacancies play a vital role in the conduction of colloidal and polycrystalline zinc oxide (ZnO) films [35, 36]. In ZnO nanoparticle synthesis, there will be lowest concentration of oxygen vacancies in this crystal structure if potassium hydroxide (KOH) to zinc acetate (Zn(Ac)2) mole ratio is near to its stoichiometric value. Low concentration of oxygen vacancies causes low conductivity because oxygen vacancies behave as deep donors [37]. It is anticipated that AZO films fabricated from non-stoichiometric nanoparticles exhibit higher conductivity than films made from stoichiometric particles. Higher concentrations of oxygen vacancies provide extra electrons as carriers making the AZO films an n-type transparent conducting oxide. The amount of charge carriers is controlled by oxygen-vacancy formation via annealing under a reducing atmosphere; in particular, hydrogen can act as both a shallow donor and a defect passivator in ZnO-based material [38]. However, charge carriers generated by reduction are sensitive to process conditions, which make it difficult to precisely control their concentration. In this paper, we studied the structural, optical and electrical properties of AZO thin films fabricated from nanoparticulate inks using an ultrasonic spray deposition method. We also report that increased transparency of AZO thin films by adding a N-(2-Aminoethyl)-3-aminopropylmethyldimethoxy (APMDE) silane to the AZO nanoparticulate ink.
For AZO nanoparticles synthesis, 90.25 mmol Zn(Ac)2 · 2 H2O and 2.50 mL of water was added into a flask containing 420 mL of methanol. Doping was obtained by adding AlCl3 with appropriate dopant concentrations. The solution was stirred magnetically and heated to 60 °C. Subsequently, 72.2 mmol KOH was dissolved into 230 mL of methanol and was dropped into the flask within 10–15 min. The solution was stirred at 60 °C for 2 h 15 min to obtain AZO nanoparticles. The final solution was centrifuged for 15 min at 6000 rpm and the supernatant solution was carefully decanted to separate the nanoparticles. Size distribution of these particles was analyzed using JEOL JEM-2100 LaB6 transmission electron microscope (TEM). Optical absorption spectra were obtained using a HP 8452A Diode Array Spectrometer. Asprepared AZO nanoparticles were dispersed in ethylene glycol butyl ether and then monoethanalomine (100 μl/10 ml ink) was added to the solution to serve as a capping agent. To obtain relatively uniform dispersion of the nanoparticles, inks were vortex mixed and ultrasonicated for 30 min each. By using ethylene glycol butyl ether and monoethanalomine as solvent and stabilizer, respectively AZO nanoparticles concentrations in ink were achieved up to 40 wt%. For ultrasonic spray deposition, 15 wt% of AZO nanoparticulate inks were used. To enhance the transparency and adhesion of the printed films, APMDE silane was added to the ink (3 wt% of the ink).
2.2. Ultrasonic spray deposition and characterization of AZO films
The ultrasonic spray coater was obtained from Sono-Tek Corporation and includes an Impact EDGE print head system which contains conical spray nozzle. Deposition parameters were infusing rate 1.0 ml min−1, process speed 75 to 150 mm s−1, and process pressure 3.0 bars. Corning display grade 1737 pre-cleaned glass substrates from Delta Technologies, LTD were used for spray deposition. For all the depositions, the substrate temperature was kept at room temperature. In the first heat treatment, as-printed thin films were dried on a hot plate at 350 °C for 20 min then crystallized at 700 °C for 1 h in a tube furnace in air. In the second heat treatment, furnace annealed films were further reduced by vacuum annealing then immersed in a forming gas (95% N2–5% H2) rf plasma at a fixed input plasma power of 29.6 W for 15 min processing time intervals using Harrick plasma cleaner. The crystallinity of each AZO film was measured using a Rigaku Ultima Plus theta-theta diffractometer. The surface morphology and crosssection of the films were observed with a Zeiss Supra 40VP field-emission scanning electron microscope (SEM). The electrical resistance was measured by a manual four-point probe system with cylindrical probe head from MDC Corporation, USA. Optical transmittance measurements were carried out using a HP 8452A Diode Array Spectrometer. Surface topography was analyzed using Bruker Multimode 8 atomic force microscope (AFM).
2. Experimental 2.1. AZO nanoparticles synthesis and ink formulation
AZO nanoparticles were synthesized using zinc acetate dihydrate (Zn(Ac)2 · 2H2O) and aluminum chloride (AlCl3) as precursors and KOH as a alkali base according to a literature method developed by Sun et al with some modification [39]. 2
Nanotechnology 25 (2014) 195301
S Vunnam et al
Figure 1. TEM image of AZO nanoparticles and inset figure shows the size distribution.
3. Results and discussion 3.1. Structural properties
TEM image shows the typical AZO NPs synthesized using the simple hydrothermal growth. As-synthesized AZO nanoparticles have narrow size distribution (inset figure 1) with mean diameter 6.6 nm. In order to obtain uniform dispersion of nanoparticles in the solvent, as-synthesized wet AZO particles were dispersed in ethylene glycol butyl ether. Since there is no encapsulant for nanoparticles, the dried particles agglomerated and did not disperse as well as wet nanoparticles. Monoethanalomine acted as an encapsulant and increased the stability of the AZO nanoparticulate ink. Post processing of the AZO ink using ultrasonication and vortex mixing induced highly transparent ink (figure 2). As-synthesized AZO NPs are crystalline and are single phase ZnO hexagonal wurtzite structure having random crystal orientation (figure 3) [40]. It is well known that the (002) direction i.e., the c-axis, is the preferential orientation for ZnO and doped ZnO films prepared by various deposition techniques such as solution based thin film deposition, sputtering, pulsed laser deposition, and chemical vapor deposition [41–44]. However, solution deposition processes have been shown to yield a finely-grained, poly crystalline ZnO and AZO films [45–47]. In the synthesis process, the mole ratio of KOH to Zn(Ac)2 · 2H2O was maintained around 0.8 to obtain nonstoichiometric AZO nanocrystals to enhance the conductivity of the films [39]. The presence of a very small (002) peak at diffraction angle ∼34o could be due to the non-stoichiometry of AZO nanocrystals [48]. The Al composition in AZO NPs was tuned by the amount of AlCl3 used in the reaction mixture. With increase in Al concentration, the AZO NPs ink color changed from
Figure 2. AZO ink (a) without and (b) with monoethanalomine.
transparent to light whitish color (figure 4). The absorption also increased from pure ZnO to AZO with 5 mol% Al ink (figure 5). To estimate the relative amounts of Al and Zn among the synthesized AZO NPs, SEM-EDX compositional analysis was employed. The data have been averaged on at least three different measurements per sample, and the error bars reflect the standard deviation between the different measurements and the noise of the EDX signals. The Aldoping content in AZO NPs increased with the increase of the Al input molar ratio, as determined by EDX (figure 6). The 3
Nanotechnology 25 (2014) 195301
S Vunnam et al
Figure 3. XRD pattern of a typical AZO sample; theoretical
diffraction peaks position for wurtzite-ZnO are shown as vertical lines.
Figure 5. Elemental composition measured by EDX of AZO NPs.
Figure 4. Photograph of a series of AZO NPs dispersions in ethylene glycol butyl ether with different Al compositions.
Figure 6. Optical absorption spectra of AZO films with different Al concentrations.
error associated with the EDX evaluation is fairly large, but nonetheless, a clear trend can be observed. As the Al-doping increased the absorbance is increased slightly for AZO films, this is consistent with the increased electron concentration provided by a greater Al-doping, as reported previously also for other colloidal TCOs nanocrystalline coatings [49, 50].
nanoparticle concentrations greater than 20 wt% resulted in cracks in the sintered films even after adding APMDE silane. AZO ink with 15 wt% of AZO nanoparticles and 3 wt% APMDE silane was determined to be a good ink to obtain uniform smooth films without cracks and pores. AFM image of AZO film sintered at 700 °C showed the smooth surface morphology composed of closely packed particles with a particle size in tens of nanometers (figure 8(a)). The optical transparency of solution deposited films mainly depends on film morphology defects such as cracks, pores, and surface roughness. For UV–Visible transmittance measurements, reference spectrum was taken using plain glass substrate to avoid the reflection from glass due to its higher refractive index. The film from AZO ink without APMDE silane has low transmittance due to scattering (figure 9). In contrast, AZO films which were deposited from AZO ink with 3 wt% APMDE silane exhibited optical transmittance greater than
3.2. Effect of ink chemistry on morphology and optical properties of films
The additives to the ink not only influence the ink chemistry but also influence the ink wetting and the microstructure of the sintered films. Microstructure of spray coated AZO films strongly depended on the concentrations of nanoparticles and APMDE silane. APMDE silane is used as a binder and drastically decreased the crack density in the sintered AZO films (figure 7). AZO films fabricated using inks having 4
Nanotechnology 25 (2014) 195301
S Vunnam et al
Figure 7. Optical micrographs of annealed AZO film fabricated from AZO nanoparticulate ink (a) with out and (b) with APMDE.
Figure 8. (a) AFM height image of sintered AZO film showing the location of the line along which the height profile (b) was measured.
95% in the visible region. These phenomena are attributed to the change of the microstructure [51]. The low transmittance ascribed to light scattering of the AZO film through cracks and pores whereas, the dense and continuous structures with smooth surfaces exhibits less surface scattering. In fact, if we neglect the reflection from the AZO coating, the transmission of the AZO film is virtually identical to that of the plain glass substrate, being comparable with the properties obtained for gallium-doped ZnO and even other TCOs such as AZO and ITO dense coatings deposited with well established physical techniques [52–54]. Spray deposited films were dried on the sample stage in air at room temperature with a slow evaporation of the solvent. This led to better spreading of the ink and higher uniformity in the film thickness. The thickness of the printed films mainly depends on ink composition and printing parameters, such as process speed and deposition rate. In order to make the solution covering the surface with a full merged
Figure 9. Optical transmittance spectra of transparent AZO films.
5
Nanotechnology 25 (2014) 195301
S Vunnam et al
Figure 10. SEM cross-sectional images of AZO thin film printed with 100 mm s
−1
process speed (a) with APMDE (b) without APMDE.
Figure 11. SEM cross-sectional images of AZO thin film printed with process speed (a) 75 mm s−1 (b) 125 mm s−1.
collection of droplets, a deposition rate at least 1.0 ml min−1 was needed. AZO films deposited with ink containing APMDE have thickness around 500 nm, whereas films deposited without APMDE have thicknesses around 900 nm (figure 10). Therefore, adding APMDE gives thinner AZO films compared to the ink without any binder. Also, the APMDE containing films are more uniform in thickness. SEM cross-sectional images of AZO films deposited with different process speed show the thickness dependence on process speed (figure 11). AZO films with thickness around 400 nm were obtained with a process speed of 125 mm s−1 and a deposition rate of 1 ml min−1 without any cracks and pores and it was reproducible. Unlike other solution deposition routes such as spin coating, the thickness was highly uniform throughout the film which is highly desirable for device fabrication [28].
releases localized electrons through oxygen absorption [55, 56]. Reduction annealing is vital for solution derived films because adsorption of oxygen at the grain boundaries leads to poor electrical conductivity. Crystallinity was obtained by annealing in a tube furnace in air gradually heating the thin film to 700 °C, whereas reduction annealing was carried out by vacuum annealing followed by plasma annealing. The AZO films deposited using inks without APMDE have very high sheet resistances beyond the detection limit. This is likely caused by cracks in the sintered film (figures 7(a) and 10(b)). However, films deposited using inks containing APMDE were conductive. Therefore, electrical resistivities of the films containing APMDE are discussed further in detail. The films annealed in vacuum prior to the plasma treatment showed lower resistivity compared to films not annealed in vacuum. AZO film resistivity decreased with increasing the crystallization temperature (figure 12). Films annealed at 300 °C exhibited resistivity beyond the detection limit even after both the vacuum annealing and plasma exposure. AZO films annealed at 400 °C further reduced with vacuum annealing and plasma exposure also showed high resistivity ∼9.2 ± 1.2 KΩ cm, while the films annealed at
3.3. Electrical resistivity
The electrical conductivity of solution processed AZO films can be enhanced by crystallization of films which provides higher mobility of electrons and by reduction annealing which 6
Nanotechnology 25 (2014) 195301
S Vunnam et al
Figure 12. Resistivity of AZO (1.6 mol%) films annealed in air at different temperatures.
Figure 14. Resistivity of AZO films with respect to thermal treatment
(H1-vacuum annealing, H2-forming gas plasma annealing, H3-UV light illumination).
prepared by other liquid chemistry based techniques and the films exhibited outstanding optical transparency values with low dopant concentrations [58, 59]. After plasma annealing, the sheet resistance values were 5–10 KΩ sq−1 range. These values are directly comparable with the sheet resistance of recently published ITO colloidal coatings, even using commercially available ITO nanoparticles [50, 60]. Further UVlight illumination reduced the resistivity to ∼7.0 × 10−2 Ω cm (figure 14). Transparency and conductivity of the AZO films prepared in this study are greater than that of the ink jet deposited AZO films reported by Yan Wu et al [61] and colloidal and sol–gel AZO films deposited using spin coating [62, 63]. In particular, ultrasonic spray deposited AZO films’ transparency is greater than AZO films deposited from other spray deposition techniques [64]. The developed AZO nanoparticulate inks can thus be used for fabricating transparent conducting films that are highly transparent in the visible region. It has proven that ultrasonic spray deposition technique is effective to deposit high quality films, even on large area substrates. This ink can be readily used to print desired patterns using different printing techniques such as inkjet and aerosol jet printers on various kinds of substrates.
Figure 13. Resistivity of ZnO films according to Al amount.
700 °C have the lowest resistivity 0.5 ± 0.1 Ω cm. The electrical resistivity of the doped films is lower than that of the undoped films (figure 13). The lowest electrical resistivity was obtained with dopant concentration of 1.6 mol%. However, the increase of the electrical resistivity of doped films with increasing doping concentration is likely due to the large number crystal defects at the grain boundaries arising in solution processed films [57]. Doped aluminum is acting as an electrical dopant at initial doping concentration but as an impurity at higher doping concentrations, it causes a decrease in mobility of the carriers caused by the segregation of the dopant at the grain boundary. The conduction electrons from the substitutional Al3+ ions are most likely to be trapped by defects in the growing film which resulted in diminished electrical performances. The resistivity values are at least one order of magnitude smaller than the AZO colloidal films
4. Conclusions Highly transparent and conductive AZO thin films were fabricated using AZO nanoparticulate inks and an ultrasonic spray coating method. Making use of direct write technology for thin film fabrication is an attractive and simple route for controlling the thickness and electrical properties of AZO films fabricated from nanoparticulate inks. Careful control of ink composition such as concentrations of nanoparticles and APMDE silane coupling agent is crucial for achieving crack free films with good optical and electrical properties. In 7
Nanotechnology 25 (2014) 195301
S Vunnam et al
particular, Sono-Tek spray coated AZO thin films were highly transparent (>95%) in the visible region with an electrical resistivity as low as 7.0 × 10−2 Ω cm. The proposed AZO film fabrication is easily scalable to large dimensions and offers great promise for future industrial applications in thin films technology such as consumer electronics, photovoltaics and glazing technology.
[22] Morfa A J, Beane G, Mashford B, Singh B, Gaspera E D, Martucci A and Mulvaney P 2010 J. Phys. Chem. C 114 19815–21 [23] Nayak P K, Yang J, Kim J, Chung S, Jeong J, Lee C and Hong Y 2009 J. Phys. D: Appl. Phys. 42 035102 [24] Zhu M, Huang H, Gong J and Sun C 2007 J. Appl. Phys. 102 043106 [25] Wu Z Y, Cai J H and Ni G 2008 Thin Solid Films 516 7318–22 [26] Berger H L 2006 Ultrasonic Liquid Atomization: Theory and Application (Hyde Park, NY: Partridge Hill) p 177 [27] Tenent R C, Gillaspie D T, Miedaner A, Parilla P A, Curtis C J and Dillon A C 2010 J. Electrochem. Soc. 157 H318–22 [28] Krebs F C 2009 Sol. Energy Mater. Sol. Cells 93 394–412 [29] Pham N P, Burghartz J N and Sarro P M 2005 J. Micromech. Microeng. 15 691–7 [30] Prosser J H, Brugarolas Lee T S, Nolte A J and Lee D 2012 Nano Lett. 12 5287–91 [31] Carreras E S, Chabert F, Dunstan D E and Franks G V 2007 J. Colloid Interface Sci. 313 160–8 [32] Lewis J A 2000 J. Am. Ceram. Soc. 59 2341–59 [33] Chu L, Daniels M W and Francis L F 1997 Chem. Mater. 9 2577–82 [34] Joo Sand B D F 2010 Nanotechnology 21 055204 [35] Morfa A J, Kirkwood N, Karg M, Singh T B and Mulvaney P 2011 J. Phys. Chem. C 115 8312–5 [36] Ellmer K 2001 J. Phys. D: Appl. Phys. 34 3097–108 [37] Van de Walle C G 2000 Phys. Rev. Lett. 85 1012–5 [38] Cai P F, You J B, Zhang X W, Dong J J, Yang X L, Yin Z G and Chen N F 2009 J. Appl. Phys. 105 083713 [39] Sun B and Sirringhaus H 2005 Nano Lett. 5 2408–13 [40] Powder Diffraction Files 1967 Joint Committee on Powder Diffraction Standards (Philadelphia, PA: ASTM) pp 36–1451 [41] Li C, Li Y, Wu Y, Ong B S and Loutfy R O 2009 J. Mater. Chem. 19 1626–34 [42] Li Q H, Zhu D L, Liu W J, Liu Y and Ma X C 2008 Appl. Surf. Sci. 254 2922–6 [43] Mass J, Bhattacharya P and Katiyar R S 2003 Mater. Sci. Eng. B 103 9–15 [44] Lu J G et al 2007 J. Appl. Phys. 101 083705 [45] Yao P C, Hang S T, Lin Y S, Yen W T and Lin Y C 2010 Appl. Surf. Sci. 257 1441–8 [46] Castaneda L, Maldonado A, Escobedo-Morales A, Avendano-Alejo M, Gomez H, Vega-Perez J, Olvera M and de la L 2011 Mater. Sci. Semicond. Process. 14 114–9 [47] Chen K J, Fang T H, Hung F Y, Ji L W, Chang S J, Young S J and Hsiao Y J 2008 Appl. Surf. Sci. 254 5791–5 [48] Pacholski C, Kornowski A and Weller H 2002 Angew. Chem. Int. Ed. 41 1188–91 [49] Buonsanti R, Llordes A, Aloni S, Helms B A and Milliron D J 2011 Nano Lett. 11 4706−4710 [50] Garcia G, Buonsanti R, Runnerstrom E L, Mendelsberg R J, Llordes A, Anders A, Richardson T J and Milliron D J 2011 Nano Lett. 11 4415–20 [51] Gupta V and Mansingh A 1996 J. Appl. Phys. 80 1063 [52] Assuncao V, Fortunato E, Marques A, Aguas H, Ferreira I, Costa M E V and Martins R 2003 Thin Solid Films 427 401−5 [53] Kim K H, Park K C and Ma D Y 1997 J. Appl. Phys. 81 7764 [54] Sittinger V, Ruske F, Werner W, Jacobs C, Szyszka B and Christie D J 2008 Thin Solid Films 516 5847−59 [55] Lin J P and Wu J M 2008 Appl. Phy. Lett. 92 134103 [56] McCluskey M D, Jokela S J, Zhuravlev K K, Simpson P J and Lynn K G 2002 Appl. Phys. Lett. 81 3807 [57] Graetzel M, Janssen R A J, Mitzi D B and Sargent E H 2012 Nature 488 304–12 [58] Hartner S, Ali M, Schulz C, Winterer M and Wiggers H 2009 Nanotechnology 20 445701
Acknowledgements We would like to thank E F Duke for his help in SEM experiments and J Randle for his help in spray deposition of AZO films. This material is based upon work supported by the National Science Foundation/EPSCoR Grant No. 0903804 and by the State of South Dakota. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
References [1] Pasquarelli R M, Curtis C J, Miedaner A, Van Hest M F A M, O’Hayre R P and Ginley D S 2010 Inorg. Chem. 49 5424–31 [2] Subramanian V 2010 Solution-processed electronics based on transparent conductive oxides Transparent Electronics: From Synthesis to Applications ed A Facchetti and T J Marks (Chichester, UK: Wiley) p 1 [3] Ma W et al 2007 Nano Lett. 7 1649–53 [4] Ankireddy K, Vunnam S, Kellar J and Cross W 2013 J. Mater. Chem. C 1 572–9 [5] Vunnam S, Ankireddy K, Kellar J and Cross W 2013 Thin Solid Films 531 294–301 [6] Ankireddy K, Iskander M, Vunnam S, Anagnostou D E, Kellar J and Cross W 2013 J. Appl. Phys. 114 124303 [7] Zhu L, Tang H, Harima Y, Yamashita K, Aso Y and Otsubo T 2002 J. Mater. Chem. 12 2250–4 [8] Granqvist C G 2007 Sol. Energy Mater. Sol. Cells 91 1529–98 [9] Thomas G 1997 Nature 389 907–8 [10] Kumar A and Zhou C 2010 ACS Nano 4 11–4 [11] Bamiduro O, Mustafa H, Mundle R, Konda R B and Pradhan A K 2007 Appl. Phys. Lett. 90 252108 [12] Subramanian V, Fréchet J M J, Chang P C, Huang D, Lee J B, Molesa S E, Murphy A R, Redinger D R and Volkman S K 2005 Proc. IEEE 93 1330–8 [13] Subramanian V, Bakhishev T, Redinger D and Volkman S K 2009 J. Display Technol. 5 525–30 [14] Song K, Kim D, Li X S, Jun T, Jeong Y and Moon J 2009 J. Mater. Chem. 19 8881–6 [15] Jeong S, Jeong Y and Moon J 2008 J. Phys. Chem. C 112 11082–5 [16] Wu K Y, Wang C C and Chen D H 2007 Nanotechnology 18 305604 [17] Zhou H, Yi D, Yu Z, Xiao L and Li J 2007 Thin Solid Films 515 6909–14 [18] Lee J H and Park B O 2003 Thin Solid Films 426 94–9 [19] Natsume Y and Sakata H 2000 Thin Solid Films 372 30–6 [20] Kuo S Y, Chen W C, Lai F I, Cheng C P, Kuo H C, Wang S C and Hsieh W F 2006 J. Cryst. Growth 287 78–84 [21] Minami T 2008 Thin Solid Films 516 5822–8 8
Nanotechnology 25 (2014) 195301
S Vunnam et al
[62] Tarasov T K and Raccurt O 2011 J Nanopart. Res. 13 6717–24 [63] Nam G M and Kwon M S 2009 J. Info. Disp. 10 24–7 [64] Rahman M M, Khan M, Islam M R, Halim M, Shahjahan M, Hakim M M, Shaha D K and Khan J U 2012 J. Mater. Sci. Technol. 28 329–35
[59] Tarasov K and Raccurt O 2011 J. Nanopart. Res. 13 6717–24 [60] Chung C H, Song T B, Bob B, Zhu R, Duan H S and Yang Y 2012 Adv. Mater. 24 5499–504 [61] Wu Y, Tamaki T, Voit W, Belova L and Rao K V 2009 Mater. Res. Soc. Symp. Proc. 1161 I03–22
9
Search
Collections
Journals
About
Contact us
My IOPscience
Highly transparent and conductive Al-doped ZnO nanoparticulate thin films using direct write processing
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 195301 (http://iopscience.iop.org/0957-4484/25/19/195301) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 66.194.72.152 This content was downloaded on 16/06/2014 at 02:04
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 195301 (9pp)
doi:10.1088/0957-4484/25/19/195301
Highly transparent and conductive Al-doped ZnO nanoparticulate thin films using direct write processing S Vunnam1, K Ankireddy2, J Kellar2 and W Cross2 1
Program of Nanoscience and Nanoengineering, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA 2 Program of Materials Engineering and Science, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA E-mail: [email protected] Received 31 December 2013, revised 7 March 2014 Accepted for publication 24 March 2014 Published 23 April 2014 Abstract
Solution processable Al-doped ZnO (AZO) thin films are attractive candidates for low cost transparent electrodes. We demonstrate here an optimized nanoparticulate ink for the fabrication of AZO thin films using scalable, low-cost direct write processing (ultrasonic spray deposition) in air at atmospheric pressure. The thin films were made via thermal processing of as-deposited films. AZO films deposited using the proposed nanoparticulate ink with further reducing in vacuum and rf plasma of forming gas exhibited optical transparency greater than 95% across the visible spectrum, and electrical resistivity of 0.5 Ω cm and it drops down to 7.0 × 10−2 Ω cm after illuminating with UV light, which is comparable to commercially available tin doped indium oxide colloidal coatings. Various structural analyses were performed to investigate the influence of ink chemistry, deposition parameters, and annealing temperatures on the structural, optical, and electrical characteristics of the spray deposited AZO thin films. Optical micrographs confirmed the presence of surface defects and cracks using the AZO NPs ink without any additives. After adding N-(2-Aminoethyl)-3-aminopropylmethyldimethoxy silane to the ink, AZO films exhibited an optical transparency which was virtually identical to that of the plain glass substrate. Keywords: direct write processing, transparent conducting oxides, Al-doped ZnO, optical transparency, electrical resistivity (Some figures may appear in colour only in the online journal)
1. Introduction
oxide, zinc indium oxide, zinc indium gallium oxide, etc [14, 15]. Al-doped zinc oxide (AZO) has been considered as a promising alternative to indium tin oxide (ITO) [16–21]. In conventional sol–gel methods, nucleation of crystalline particles from precursor solution occurs only at elevated temperatures (>500 °C), whereas solution processed films prepared from crystalline nanoparticles will sinter to form crystalline films at lower temperatures [22]. Among the solution deposition methods, ultrasonic spray deposition gives the desired thickness in a single step, which is an
The solution-based deposition of functional materials is an attractive technique for the fabrication of high performance, low-cost electronics [1–6]. Transparent conductive oxides (TCOs) are widely used in many electronic and optoelectronic devices such as light emitting diodes, liquid crystal displays, and photovoltaic devices [7–13]. More recently, there has been tremendous interest in the use of transparent transistors based on semiconducting transparent oxides, such as zinc 0957-4484/14/195301+09$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 195301
S Vunnam et al
alternative to the time consuming repeated steps (coating and drying) involved in traditional solution deposition routes such as spin coating, dip coating and chemical bath deposition [23–25]. Ultrasonic spray deposition holds several key advantages over more widely employed pressure-driven spraying methods. Among these are lower materials usage and improved uniformity due to a narrower size distribution of droplets in the atomized material [26–29]. To the best of our knowledge, there have been no reports of the optical and electrical properties of ultrasonic spray coated AZO films made from nanoparticulate inks. The major challenge in colloidal-based thin films is cracking during drying. This can be eliminated by adjusting the ink chemistry or manipulating the drying mechanism [30]. Ink chemistry can be altered by adding polymeric binders, plasticizers, surfactants and sol–gel precursors to the ink. Organosilanes are an alternative to organic polymer binders [31, 32]. The application of organosilanes in colloidal thin films has been reported by several researchers [33]. In particular, amino silane coupling agents enhances the silver nanoparticle adhesion to different substrates [34]. Oxygen vacancies play a vital role in the conduction of colloidal and polycrystalline zinc oxide (ZnO) films [35, 36]. In ZnO nanoparticle synthesis, there will be lowest concentration of oxygen vacancies in this crystal structure if potassium hydroxide (KOH) to zinc acetate (Zn(Ac)2) mole ratio is near to its stoichiometric value. Low concentration of oxygen vacancies causes low conductivity because oxygen vacancies behave as deep donors [37]. It is anticipated that AZO films fabricated from non-stoichiometric nanoparticles exhibit higher conductivity than films made from stoichiometric particles. Higher concentrations of oxygen vacancies provide extra electrons as carriers making the AZO films an n-type transparent conducting oxide. The amount of charge carriers is controlled by oxygen-vacancy formation via annealing under a reducing atmosphere; in particular, hydrogen can act as both a shallow donor and a defect passivator in ZnO-based material [38]. However, charge carriers generated by reduction are sensitive to process conditions, which make it difficult to precisely control their concentration. In this paper, we studied the structural, optical and electrical properties of AZO thin films fabricated from nanoparticulate inks using an ultrasonic spray deposition method. We also report that increased transparency of AZO thin films by adding a N-(2-Aminoethyl)-3-aminopropylmethyldimethoxy (APMDE) silane to the AZO nanoparticulate ink.
For AZO nanoparticles synthesis, 90.25 mmol Zn(Ac)2 · 2 H2O and 2.50 mL of water was added into a flask containing 420 mL of methanol. Doping was obtained by adding AlCl3 with appropriate dopant concentrations. The solution was stirred magnetically and heated to 60 °C. Subsequently, 72.2 mmol KOH was dissolved into 230 mL of methanol and was dropped into the flask within 10–15 min. The solution was stirred at 60 °C for 2 h 15 min to obtain AZO nanoparticles. The final solution was centrifuged for 15 min at 6000 rpm and the supernatant solution was carefully decanted to separate the nanoparticles. Size distribution of these particles was analyzed using JEOL JEM-2100 LaB6 transmission electron microscope (TEM). Optical absorption spectra were obtained using a HP 8452A Diode Array Spectrometer. Asprepared AZO nanoparticles were dispersed in ethylene glycol butyl ether and then monoethanalomine (100 μl/10 ml ink) was added to the solution to serve as a capping agent. To obtain relatively uniform dispersion of the nanoparticles, inks were vortex mixed and ultrasonicated for 30 min each. By using ethylene glycol butyl ether and monoethanalomine as solvent and stabilizer, respectively AZO nanoparticles concentrations in ink were achieved up to 40 wt%. For ultrasonic spray deposition, 15 wt% of AZO nanoparticulate inks were used. To enhance the transparency and adhesion of the printed films, APMDE silane was added to the ink (3 wt% of the ink).
2.2. Ultrasonic spray deposition and characterization of AZO films
The ultrasonic spray coater was obtained from Sono-Tek Corporation and includes an Impact EDGE print head system which contains conical spray nozzle. Deposition parameters were infusing rate 1.0 ml min−1, process speed 75 to 150 mm s−1, and process pressure 3.0 bars. Corning display grade 1737 pre-cleaned glass substrates from Delta Technologies, LTD were used for spray deposition. For all the depositions, the substrate temperature was kept at room temperature. In the first heat treatment, as-printed thin films were dried on a hot plate at 350 °C for 20 min then crystallized at 700 °C for 1 h in a tube furnace in air. In the second heat treatment, furnace annealed films were further reduced by vacuum annealing then immersed in a forming gas (95% N2–5% H2) rf plasma at a fixed input plasma power of 29.6 W for 15 min processing time intervals using Harrick plasma cleaner. The crystallinity of each AZO film was measured using a Rigaku Ultima Plus theta-theta diffractometer. The surface morphology and crosssection of the films were observed with a Zeiss Supra 40VP field-emission scanning electron microscope (SEM). The electrical resistance was measured by a manual four-point probe system with cylindrical probe head from MDC Corporation, USA. Optical transmittance measurements were carried out using a HP 8452A Diode Array Spectrometer. Surface topography was analyzed using Bruker Multimode 8 atomic force microscope (AFM).
2. Experimental 2.1. AZO nanoparticles synthesis and ink formulation
AZO nanoparticles were synthesized using zinc acetate dihydrate (Zn(Ac)2 · 2H2O) and aluminum chloride (AlCl3) as precursors and KOH as a alkali base according to a literature method developed by Sun et al with some modification [39]. 2
Nanotechnology 25 (2014) 195301
S Vunnam et al
Figure 1. TEM image of AZO nanoparticles and inset figure shows the size distribution.
3. Results and discussion 3.1. Structural properties
TEM image shows the typical AZO NPs synthesized using the simple hydrothermal growth. As-synthesized AZO nanoparticles have narrow size distribution (inset figure 1) with mean diameter 6.6 nm. In order to obtain uniform dispersion of nanoparticles in the solvent, as-synthesized wet AZO particles were dispersed in ethylene glycol butyl ether. Since there is no encapsulant for nanoparticles, the dried particles agglomerated and did not disperse as well as wet nanoparticles. Monoethanalomine acted as an encapsulant and increased the stability of the AZO nanoparticulate ink. Post processing of the AZO ink using ultrasonication and vortex mixing induced highly transparent ink (figure 2). As-synthesized AZO NPs are crystalline and are single phase ZnO hexagonal wurtzite structure having random crystal orientation (figure 3) [40]. It is well known that the (002) direction i.e., the c-axis, is the preferential orientation for ZnO and doped ZnO films prepared by various deposition techniques such as solution based thin film deposition, sputtering, pulsed laser deposition, and chemical vapor deposition [41–44]. However, solution deposition processes have been shown to yield a finely-grained, poly crystalline ZnO and AZO films [45–47]. In the synthesis process, the mole ratio of KOH to Zn(Ac)2 · 2H2O was maintained around 0.8 to obtain nonstoichiometric AZO nanocrystals to enhance the conductivity of the films [39]. The presence of a very small (002) peak at diffraction angle ∼34o could be due to the non-stoichiometry of AZO nanocrystals [48]. The Al composition in AZO NPs was tuned by the amount of AlCl3 used in the reaction mixture. With increase in Al concentration, the AZO NPs ink color changed from
Figure 2. AZO ink (a) without and (b) with monoethanalomine.
transparent to light whitish color (figure 4). The absorption also increased from pure ZnO to AZO with 5 mol% Al ink (figure 5). To estimate the relative amounts of Al and Zn among the synthesized AZO NPs, SEM-EDX compositional analysis was employed. The data have been averaged on at least three different measurements per sample, and the error bars reflect the standard deviation between the different measurements and the noise of the EDX signals. The Aldoping content in AZO NPs increased with the increase of the Al input molar ratio, as determined by EDX (figure 6). The 3
Nanotechnology 25 (2014) 195301
S Vunnam et al
Figure 3. XRD pattern of a typical AZO sample; theoretical
diffraction peaks position for wurtzite-ZnO are shown as vertical lines.
Figure 5. Elemental composition measured by EDX of AZO NPs.
Figure 4. Photograph of a series of AZO NPs dispersions in ethylene glycol butyl ether with different Al compositions.
Figure 6. Optical absorption spectra of AZO films with different Al concentrations.
error associated with the EDX evaluation is fairly large, but nonetheless, a clear trend can be observed. As the Al-doping increased the absorbance is increased slightly for AZO films, this is consistent with the increased electron concentration provided by a greater Al-doping, as reported previously also for other colloidal TCOs nanocrystalline coatings [49, 50].
nanoparticle concentrations greater than 20 wt% resulted in cracks in the sintered films even after adding APMDE silane. AZO ink with 15 wt% of AZO nanoparticles and 3 wt% APMDE silane was determined to be a good ink to obtain uniform smooth films without cracks and pores. AFM image of AZO film sintered at 700 °C showed the smooth surface morphology composed of closely packed particles with a particle size in tens of nanometers (figure 8(a)). The optical transparency of solution deposited films mainly depends on film morphology defects such as cracks, pores, and surface roughness. For UV–Visible transmittance measurements, reference spectrum was taken using plain glass substrate to avoid the reflection from glass due to its higher refractive index. The film from AZO ink without APMDE silane has low transmittance due to scattering (figure 9). In contrast, AZO films which were deposited from AZO ink with 3 wt% APMDE silane exhibited optical transmittance greater than
3.2. Effect of ink chemistry on morphology and optical properties of films
The additives to the ink not only influence the ink chemistry but also influence the ink wetting and the microstructure of the sintered films. Microstructure of spray coated AZO films strongly depended on the concentrations of nanoparticles and APMDE silane. APMDE silane is used as a binder and drastically decreased the crack density in the sintered AZO films (figure 7). AZO films fabricated using inks having 4
Nanotechnology 25 (2014) 195301
S Vunnam et al
Figure 7. Optical micrographs of annealed AZO film fabricated from AZO nanoparticulate ink (a) with out and (b) with APMDE.
Figure 8. (a) AFM height image of sintered AZO film showing the location of the line along which the height profile (b) was measured.
95% in the visible region. These phenomena are attributed to the change of the microstructure [51]. The low transmittance ascribed to light scattering of the AZO film through cracks and pores whereas, the dense and continuous structures with smooth surfaces exhibits less surface scattering. In fact, if we neglect the reflection from the AZO coating, the transmission of the AZO film is virtually identical to that of the plain glass substrate, being comparable with the properties obtained for gallium-doped ZnO and even other TCOs such as AZO and ITO dense coatings deposited with well established physical techniques [52–54]. Spray deposited films were dried on the sample stage in air at room temperature with a slow evaporation of the solvent. This led to better spreading of the ink and higher uniformity in the film thickness. The thickness of the printed films mainly depends on ink composition and printing parameters, such as process speed and deposition rate. In order to make the solution covering the surface with a full merged
Figure 9. Optical transmittance spectra of transparent AZO films.
5
Nanotechnology 25 (2014) 195301
S Vunnam et al
Figure 10. SEM cross-sectional images of AZO thin film printed with 100 mm s
−1
process speed (a) with APMDE (b) without APMDE.
Figure 11. SEM cross-sectional images of AZO thin film printed with process speed (a) 75 mm s−1 (b) 125 mm s−1.
collection of droplets, a deposition rate at least 1.0 ml min−1 was needed. AZO films deposited with ink containing APMDE have thickness around 500 nm, whereas films deposited without APMDE have thicknesses around 900 nm (figure 10). Therefore, adding APMDE gives thinner AZO films compared to the ink without any binder. Also, the APMDE containing films are more uniform in thickness. SEM cross-sectional images of AZO films deposited with different process speed show the thickness dependence on process speed (figure 11). AZO films with thickness around 400 nm were obtained with a process speed of 125 mm s−1 and a deposition rate of 1 ml min−1 without any cracks and pores and it was reproducible. Unlike other solution deposition routes such as spin coating, the thickness was highly uniform throughout the film which is highly desirable for device fabrication [28].
releases localized electrons through oxygen absorption [55, 56]. Reduction annealing is vital for solution derived films because adsorption of oxygen at the grain boundaries leads to poor electrical conductivity. Crystallinity was obtained by annealing in a tube furnace in air gradually heating the thin film to 700 °C, whereas reduction annealing was carried out by vacuum annealing followed by plasma annealing. The AZO films deposited using inks without APMDE have very high sheet resistances beyond the detection limit. This is likely caused by cracks in the sintered film (figures 7(a) and 10(b)). However, films deposited using inks containing APMDE were conductive. Therefore, electrical resistivities of the films containing APMDE are discussed further in detail. The films annealed in vacuum prior to the plasma treatment showed lower resistivity compared to films not annealed in vacuum. AZO film resistivity decreased with increasing the crystallization temperature (figure 12). Films annealed at 300 °C exhibited resistivity beyond the detection limit even after both the vacuum annealing and plasma exposure. AZO films annealed at 400 °C further reduced with vacuum annealing and plasma exposure also showed high resistivity ∼9.2 ± 1.2 KΩ cm, while the films annealed at
3.3. Electrical resistivity
The electrical conductivity of solution processed AZO films can be enhanced by crystallization of films which provides higher mobility of electrons and by reduction annealing which 6
Nanotechnology 25 (2014) 195301
S Vunnam et al
Figure 12. Resistivity of AZO (1.6 mol%) films annealed in air at different temperatures.
Figure 14. Resistivity of AZO films with respect to thermal treatment
(H1-vacuum annealing, H2-forming gas plasma annealing, H3-UV light illumination).
prepared by other liquid chemistry based techniques and the films exhibited outstanding optical transparency values with low dopant concentrations [58, 59]. After plasma annealing, the sheet resistance values were 5–10 KΩ sq−1 range. These values are directly comparable with the sheet resistance of recently published ITO colloidal coatings, even using commercially available ITO nanoparticles [50, 60]. Further UVlight illumination reduced the resistivity to ∼7.0 × 10−2 Ω cm (figure 14). Transparency and conductivity of the AZO films prepared in this study are greater than that of the ink jet deposited AZO films reported by Yan Wu et al [61] and colloidal and sol–gel AZO films deposited using spin coating [62, 63]. In particular, ultrasonic spray deposited AZO films’ transparency is greater than AZO films deposited from other spray deposition techniques [64]. The developed AZO nanoparticulate inks can thus be used for fabricating transparent conducting films that are highly transparent in the visible region. It has proven that ultrasonic spray deposition technique is effective to deposit high quality films, even on large area substrates. This ink can be readily used to print desired patterns using different printing techniques such as inkjet and aerosol jet printers on various kinds of substrates.
Figure 13. Resistivity of ZnO films according to Al amount.
700 °C have the lowest resistivity 0.5 ± 0.1 Ω cm. The electrical resistivity of the doped films is lower than that of the undoped films (figure 13). The lowest electrical resistivity was obtained with dopant concentration of 1.6 mol%. However, the increase of the electrical resistivity of doped films with increasing doping concentration is likely due to the large number crystal defects at the grain boundaries arising in solution processed films [57]. Doped aluminum is acting as an electrical dopant at initial doping concentration but as an impurity at higher doping concentrations, it causes a decrease in mobility of the carriers caused by the segregation of the dopant at the grain boundary. The conduction electrons from the substitutional Al3+ ions are most likely to be trapped by defects in the growing film which resulted in diminished electrical performances. The resistivity values are at least one order of magnitude smaller than the AZO colloidal films
4. Conclusions Highly transparent and conductive AZO thin films were fabricated using AZO nanoparticulate inks and an ultrasonic spray coating method. Making use of direct write technology for thin film fabrication is an attractive and simple route for controlling the thickness and electrical properties of AZO films fabricated from nanoparticulate inks. Careful control of ink composition such as concentrations of nanoparticles and APMDE silane coupling agent is crucial for achieving crack free films with good optical and electrical properties. In 7
Nanotechnology 25 (2014) 195301
S Vunnam et al
particular, Sono-Tek spray coated AZO thin films were highly transparent (>95%) in the visible region with an electrical resistivity as low as 7.0 × 10−2 Ω cm. The proposed AZO film fabrication is easily scalable to large dimensions and offers great promise for future industrial applications in thin films technology such as consumer electronics, photovoltaics and glazing technology.
[22] Morfa A J, Beane G, Mashford B, Singh B, Gaspera E D, Martucci A and Mulvaney P 2010 J. Phys. Chem. C 114 19815–21 [23] Nayak P K, Yang J, Kim J, Chung S, Jeong J, Lee C and Hong Y 2009 J. Phys. D: Appl. Phys. 42 035102 [24] Zhu M, Huang H, Gong J and Sun C 2007 J. Appl. Phys. 102 043106 [25] Wu Z Y, Cai J H and Ni G 2008 Thin Solid Films 516 7318–22 [26] Berger H L 2006 Ultrasonic Liquid Atomization: Theory and Application (Hyde Park, NY: Partridge Hill) p 177 [27] Tenent R C, Gillaspie D T, Miedaner A, Parilla P A, Curtis C J and Dillon A C 2010 J. Electrochem. Soc. 157 H318–22 [28] Krebs F C 2009 Sol. Energy Mater. Sol. Cells 93 394–412 [29] Pham N P, Burghartz J N and Sarro P M 2005 J. Micromech. Microeng. 15 691–7 [30] Prosser J H, Brugarolas Lee T S, Nolte A J and Lee D 2012 Nano Lett. 12 5287–91 [31] Carreras E S, Chabert F, Dunstan D E and Franks G V 2007 J. Colloid Interface Sci. 313 160–8 [32] Lewis J A 2000 J. Am. Ceram. Soc. 59 2341–59 [33] Chu L, Daniels M W and Francis L F 1997 Chem. Mater. 9 2577–82 [34] Joo Sand B D F 2010 Nanotechnology 21 055204 [35] Morfa A J, Kirkwood N, Karg M, Singh T B and Mulvaney P 2011 J. Phys. Chem. C 115 8312–5 [36] Ellmer K 2001 J. Phys. D: Appl. Phys. 34 3097–108 [37] Van de Walle C G 2000 Phys. Rev. Lett. 85 1012–5 [38] Cai P F, You J B, Zhang X W, Dong J J, Yang X L, Yin Z G and Chen N F 2009 J. Appl. Phys. 105 083713 [39] Sun B and Sirringhaus H 2005 Nano Lett. 5 2408–13 [40] Powder Diffraction Files 1967 Joint Committee on Powder Diffraction Standards (Philadelphia, PA: ASTM) pp 36–1451 [41] Li C, Li Y, Wu Y, Ong B S and Loutfy R O 2009 J. Mater. Chem. 19 1626–34 [42] Li Q H, Zhu D L, Liu W J, Liu Y and Ma X C 2008 Appl. Surf. Sci. 254 2922–6 [43] Mass J, Bhattacharya P and Katiyar R S 2003 Mater. Sci. Eng. B 103 9–15 [44] Lu J G et al 2007 J. Appl. Phys. 101 083705 [45] Yao P C, Hang S T, Lin Y S, Yen W T and Lin Y C 2010 Appl. Surf. Sci. 257 1441–8 [46] Castaneda L, Maldonado A, Escobedo-Morales A, Avendano-Alejo M, Gomez H, Vega-Perez J, Olvera M and de la L 2011 Mater. Sci. Semicond. Process. 14 114–9 [47] Chen K J, Fang T H, Hung F Y, Ji L W, Chang S J, Young S J and Hsiao Y J 2008 Appl. Surf. Sci. 254 5791–5 [48] Pacholski C, Kornowski A and Weller H 2002 Angew. Chem. Int. Ed. 41 1188–91 [49] Buonsanti R, Llordes A, Aloni S, Helms B A and Milliron D J 2011 Nano Lett. 11 4706−4710 [50] Garcia G, Buonsanti R, Runnerstrom E L, Mendelsberg R J, Llordes A, Anders A, Richardson T J and Milliron D J 2011 Nano Lett. 11 4415–20 [51] Gupta V and Mansingh A 1996 J. Appl. Phys. 80 1063 [52] Assuncao V, Fortunato E, Marques A, Aguas H, Ferreira I, Costa M E V and Martins R 2003 Thin Solid Films 427 401−5 [53] Kim K H, Park K C and Ma D Y 1997 J. Appl. Phys. 81 7764 [54] Sittinger V, Ruske F, Werner W, Jacobs C, Szyszka B and Christie D J 2008 Thin Solid Films 516 5847−59 [55] Lin J P and Wu J M 2008 Appl. Phy. Lett. 92 134103 [56] McCluskey M D, Jokela S J, Zhuravlev K K, Simpson P J and Lynn K G 2002 Appl. Phys. Lett. 81 3807 [57] Graetzel M, Janssen R A J, Mitzi D B and Sargent E H 2012 Nature 488 304–12 [58] Hartner S, Ali M, Schulz C, Winterer M and Wiggers H 2009 Nanotechnology 20 445701
Acknowledgements We would like to thank E F Duke for his help in SEM experiments and J Randle for his help in spray deposition of AZO films. This material is based upon work supported by the National Science Foundation/EPSCoR Grant No. 0903804 and by the State of South Dakota. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
References [1] Pasquarelli R M, Curtis C J, Miedaner A, Van Hest M F A M, O’Hayre R P and Ginley D S 2010 Inorg. Chem. 49 5424–31 [2] Subramanian V 2010 Solution-processed electronics based on transparent conductive oxides Transparent Electronics: From Synthesis to Applications ed A Facchetti and T J Marks (Chichester, UK: Wiley) p 1 [3] Ma W et al 2007 Nano Lett. 7 1649–53 [4] Ankireddy K, Vunnam S, Kellar J and Cross W 2013 J. Mater. Chem. C 1 572–9 [5] Vunnam S, Ankireddy K, Kellar J and Cross W 2013 Thin Solid Films 531 294–301 [6] Ankireddy K, Iskander M, Vunnam S, Anagnostou D E, Kellar J and Cross W 2013 J. Appl. Phys. 114 124303 [7] Zhu L, Tang H, Harima Y, Yamashita K, Aso Y and Otsubo T 2002 J. Mater. Chem. 12 2250–4 [8] Granqvist C G 2007 Sol. Energy Mater. Sol. Cells 91 1529–98 [9] Thomas G 1997 Nature 389 907–8 [10] Kumar A and Zhou C 2010 ACS Nano 4 11–4 [11] Bamiduro O, Mustafa H, Mundle R, Konda R B and Pradhan A K 2007 Appl. Phys. Lett. 90 252108 [12] Subramanian V, Fréchet J M J, Chang P C, Huang D, Lee J B, Molesa S E, Murphy A R, Redinger D R and Volkman S K 2005 Proc. IEEE 93 1330–8 [13] Subramanian V, Bakhishev T, Redinger D and Volkman S K 2009 J. Display Technol. 5 525–30 [14] Song K, Kim D, Li X S, Jun T, Jeong Y and Moon J 2009 J. Mater. Chem. 19 8881–6 [15] Jeong S, Jeong Y and Moon J 2008 J. Phys. Chem. C 112 11082–5 [16] Wu K Y, Wang C C and Chen D H 2007 Nanotechnology 18 305604 [17] Zhou H, Yi D, Yu Z, Xiao L and Li J 2007 Thin Solid Films 515 6909–14 [18] Lee J H and Park B O 2003 Thin Solid Films 426 94–9 [19] Natsume Y and Sakata H 2000 Thin Solid Films 372 30–6 [20] Kuo S Y, Chen W C, Lai F I, Cheng C P, Kuo H C, Wang S C and Hsieh W F 2006 J. Cryst. Growth 287 78–84 [21] Minami T 2008 Thin Solid Films 516 5822–8 8
Nanotechnology 25 (2014) 195301
S Vunnam et al
[62] Tarasov T K and Raccurt O 2011 J Nanopart. Res. 13 6717–24 [63] Nam G M and Kwon M S 2009 J. Info. Disp. 10 24–7 [64] Rahman M M, Khan M, Islam M R, Halim M, Shahjahan M, Hakim M M, Shaha D K and Khan J U 2012 J. Mater. Sci. Technol. 28 329–35
[59] Tarasov K and Raccurt O 2011 J. Nanopart. Res. 13 6717–24 [60] Chung C H, Song T B, Bob B, Zhu R, Duan H S and Yang Y 2012 Adv. Mater. 24 5499–504 [61] Wu Y, Tamaki T, Voit W, Belova L and Rao K V 2009 Mater. Res. Soc. Symp. Proc. 1161 I03–22
9
