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Nucleation and growth of epitaxial metal-oxide films based on polymer-assisted deposition T. M. McCleskey,*a P. Shi,b E. Bauer,a M. J. Highland,c J. A. Eastman,c Z. X. Bi,a P. H. Fuoss,c P. M. Baldo,c W. Ren,b B. L. Scott,a A. K. Burrellc and Q. X. Jia*a Polymer-assisted deposition (PAD) is one of the chemical solution deposition methods which have been successfully used to grow films, form coatings, and synthesize nanostructured materials. In comparison with other conventional solution-based deposition techniques, PAD differs in its use of water-soluble polymers in the solution that prevent the metal ions from unwanted chemical reactions and keep the solution stable. Furthermore, filtration to remove non-coordinated cations and anions in the PAD process ensures well controlled nucleation, which enables the growth of high quality epitaxial films with desired structural and physical properties. The precursor solution is prepared by mixing water-
Received 1st August 2013 DOI: 10.1039/c3cs60285k
soluble polymer(s) with salt(s). Thermal treatment of the precursor films in a controlled environment leads to the formation of desired materials. Using BaTiO3 grown on SrTiO3 and LaMnO3 on LaAlO3 as model systems, we show the effect of filtration on the nucleation and growth of epitaxial complex metal-oxide films based on the PAD process.
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Key learning points (1) (2) (3) (4)
Unique features of polymer-assisted deposition. Broad and expanding range of materials synthesized by polymer-assisted deposition. Critical roles of precursor solution, filtration, de-polymerization, and crystallization in the synthesis of high quality epitaxial films. Potential applications of polymer-assisted deposition for thin films, coatings, and nanostructured materials.
1. Introduction Thin films, coatings, and nanostructured materials have found extensive applications in virtually all technological and scientific areas. Products using these materials cover an extraordinarily wide range from nanoscale electronics to gas turbine engines. Tremendous advances have been made to synthesize and process materials in different formats with desired structural and functional properties using either physical- or chemical-vapor deposition techniques.1 Equally impressive is the progress towards improving and optimizing the solution deposition techniques for the synthesis and processing of a broad and constantly expanding range of materials of interest to research and industry. Chemical solution deposition (CSD) has the advantages of low capital investment and the ability to coat large areas and a
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. E-mail: [email protected], [email protected] b School of Electronic and Information Eng., Xi’an Jiaotong University, Xian, China c Argonne National Laboratory, Argonne, Illinois 60439, USA
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3D objects. Both sol–gel2 and metal–organic deposition (MOD) are classical examples of CSD routes. Extensive reviews on the chemical solution deposition of functional oxide films for targeted applications can be found elsewhere.3–6 Recently, we have developed a polymer-assisted deposition (PAD)7–9 method that is becoming a promising alternative CSD method for the synthesis and processing of high quality films and coatings for specific applications. In particular, its ability to grow high quality epitaxial films such as metals,10 metal-oxides,11,12 metal-nitrides,13,14 and metal-carbides15 without modifying the chemistry of the precursor solution has made PAD the choice for not only practical applications but also the fundamental investigation of growth and nucleation involved in the solution deposition of thin films. In this article, we start by describing the important considerations of the PAD process that are critical for the growth of high quality epitaxial films. We then use two different examples to illustrate the interplay between processing parameters and the resulting microstructures of epitaxial metaloxide films.
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2. Polymer-assisted deposition As PAD itself is a solution deposition process, it has the traditional advantages of CSD. In terms of processing steps, PAD is similar to sol–gel and other solution-based alternatives. However, the use of a specific polymer in solution that binds directly with metal ions has led to the growth of highly crystalline epitaxial films10–15 not commonly observed when conventional CSD processes are used. The main differences between PAD and conventional CSD processes are the unique chemistry of the precursor solution, the use of filtration, and the distinctive process during de-polymerization and crystallization. Using the same chemistry or precursor solution, one can grow materials such as metals, metal-oxides, metal-nitrides, and metalcarbides, simply by controlling the environment during the thermal treatment. Detailed experimental procedures to grow metals, metal-nitrides, and metal-carbides can be found in published articles.10,13–15
T. Mark McCleskey received his BS from Harvey Mudd College in 1987 and his PhD from the California Institute of Technology in 1993 for work on electron transfer with Dr Harry Gray. He then did postdoctoral research at Los Alamos National Laboratory, where he is currently Deputy Division Leader of the Chemistry Division. He has over 100 publications, 1000 citations and 20 patents. His research at Los T. M. McCleskey Alamos has focused on metal– ligand interactions and most recently has involved a new thin-film coating technique – polymer assisted deposition.
A. K. Burrell
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Anthony K. Burrell received his PhD in chemistry in 1990 from the University of Auckland, New Zealand. He is currently the Head of Department of the CSE Electrochemical Energy Storage Department at Argonne National Laboratory. His research interests include thin film materials, advanced lithium ion batteries, structural changes of lithium– manganese cathode materials and beyond lithium ion battery chemistries.
2.1
Precursor solution
PAD is both solution-based and polymer-assisted.7–9 It is solutionbased in that, when used for deposition, it employs direct application of a water-based chemical solution to the surface to be coated. The solution contains water, salt(s), and polymer(s). The particular salt selected, i.e. the metal element it contains, determines which metal element will be present in the compound synthesized. It is polymer-assisted in that the solution contains specific organic polymers such as polyethyleneimine (PEI). Other polymers can also be used.16,17 It is noted that ethylenediaminetetraacetic acid (EDTA) has been advantageously used in PAD solution since EDTA can bind with many different metal ions, and the EDTA complexes thereafter bind to PEI via a combination of hydrogen bonding and electrostatic attraction.7 One of the most important characteristics of PAD is the interactions between metal ions and polymers. The formation of covalent complexes from the lone pairs on the nitrogen atoms of the polymers and the metal cations of the salts provides a stable network to remove non-coordinated species. A detailed description of transition metals bound to polymers can be found elsewhere.8 It should be emphasized that the polymers used in the PAD process provide the following functions: (i) prevent the metal ions from engaging in unwanted chemical reactions; (ii) maintain a homogeneous distribution of metal ions in solution (due to filtration), thus yielding an even and uniform coating; (iii) help maintain the desired viscosity of the solution, enabling good control of thickness; and (iv) keep the solution stable in air. 2.2
Filtration
Filtration has been widely used in the concentration or purification of molecular solutions and in water treatment. One of the key features of the precursor solution of PAD is the use of Amicons filtration to remove non-coordinated cation and anion species. The solution is passed through a filter or membrane to
Quanxi Jia is a Thrust Leader at the Center for Integrated Nanotechnologies, a DOE Nanoscale Science Research Center operated jointly by Los Alamos and Sandia National Laboratories. He is a Fellow of Los Alamos National Laboratory and a Fellow of the following societies: The American Physical Society, American Ceramic Society, and American Association for the Advancement of Q. X. Jia Science. He received BS and MS in Electronic Engineering from Xian Jiaotong University, China, and PhD in the same field from the State University of New York, Buffalo. He has authored/co-authored over 400 refereed journal articles and has been awarded 45 US patents.
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Fig. 1 (a) Schematic illustration of filtration used in the PAD process, where metal ions are coordinated with polymers. Cations and anions are washed out via filtration if they are not coordinated with polymers. (b) Photograph of an Amicon Ultrafiltration cell used in the experiment.
wash out cations and anions not coordinated with polymers. Fig. 1(a) shows a schematic illustration of the filtration process. This is possible in the PAD process because of the large differences in molecular weight and size between polymers and cations–anions. Fig. 1(b) shows a photograph of the filtration apparatus used in our experiment. In a typical filtration process, a pressure of 60 psi (Ar gas) is applied to make the filtration more efficient. Filtering the solution prevents precipitation and therefore yields a homogeneous solution (and ultimately a homogeneous coating) at the molecular level. Our experimental results have illustrated that filtration plays an important role in the growth of high quality epitaxial films (to be discussed in the given examples). This step is especially critical in the growth of epitaxial complex materials where two or more cations are involved. Whether the targeted metal concentration in a given precursor solution has been achieved can be determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). 2.3
De-polymerization and crystallization
Once the precursor film is applied, it is heated in a controlled environment at the desired temperature for the desired time to depolymerize and crystallize the film with the desired crystal structure. Fig. 2 shows a schematic drawing of the
Tutorial Review typical temperature profiles used in the PAD process. The precursor film is first heated to a moderate temperature (B120 1C) to drive out water. Different from sol–gel and other solution-based alternatives, the PAD process involves a higher temperature (350–500 1C or slightly above) exposure in a controlled environment to depolymerize the polymer, releasing the polymer from the precursor film. In other words, the polymer such as PEI in the precursor film does not undergo combustion, but rather a thermal de-polymerization back to NH2CHQCH2. In the case of EDTA, it decomposes to acetic acid, formic acid, and ethylenediamine.8 It should be noted that this non-combustion process can lead to less carbon contamination in the synthesized films. Crystallization takes place at temperatures above 600 1C. High temperature and longer time at the temperature will normally improve the crystallinity of the film. A temperature of above 800 1C is often required if one needs to achieve high quality epitaxial metaloxide films. Depending on the temperature used during the thermal treatment and the substrate materials used, the formed films can be epitaxial, polycrystalline, nanocrystalline, or amorphous. For example, polycrystalline metal-oxide films have been formed on Si3N4 at an annealing temperature of 600 1C.18 It should be noted that the temperature profile (ramping rate, temperature, and time) needs to be optimized for a given material composition in order to accomplish high crystallinity and phase-pure films. Note that it is the combination of the metal precursor solution, the temperature, and the environment at the temperature that determines the chemical composition of the converted films. For example, thermal treatment of a precursor film containing niobium ions in a reducing atmosphere such as argon mixed with hydrogen will result in pure niobium; the precursor film will be converted to Nb2O3 if the thermal treatment is done in pure oxygen; the same precursor film will be transformed to NbN if the thermal treatment is carried out in an ammonia atmosphere (which can be considered as a reducing nitrogen source);19 and the very same precursor film will be converted to NbC if the thermal treatment is carried out in an environment containing ethylene (which contains carbon).20 Using a lattice matched substrate and optimal temperature profiles, researchers have demonstrated that PAD provides a powerful technique to grow high performance epitaxial films.10–15,21–23
3. Examples 3.1 Effect of filtration on the nucleation and growth of complex metal-oxide films
Fig. 2 Schematic drawing of the thermal treatment profile used in the PAD process. The annealing time at high temperature (for crystallization) depends on the materials and the microstructures (epitaxial, polycrystalline, amorphous). It ranges from minutes to 3–4 hours. Polymer PEI is depolymerized at a temperature around 500 1C.
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Here we use BaTiO3 grown on SrTiO3 as a model system to illustrate the effect of filtration on the nucleation and growth in the PAD process. Solutions containing metal ions (either Ba+2 or Ti+4) and polymer were prepared by dissolving PEI (BASF Corporation of Clifton, NJ) and EDTA (Sigma-Aldrich Co. LLC) in water and then adding barium nitrate (Ba2(NO3)2) or titanium chloride (TiCl4). Solutions containing Ba+2 and Ti+4 were mixed in the proportion of Ba+2 : Ti+4 = 1 : 1. Ultrafiltration was
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Fig. 3 X-ray diffractions patterns of converted films on SrTiO3 substrates. The precursor films contain Ba+2 and Ti+4 with a Ba+2 : Ti+4 ratio of 1 : 1, (a) without and (b) with filtration of the solutions. The crystallization temperature was 900 1C.
performed under 60 psi Ar by using an Amicon stirred cell (illustrated in Fig. 1(b)) having a cut-off molecular weight of 3000 g mol 1. Metal analysis was conducted using a Varian Liberty 220 inductively coupled plasma-atomic emission spectrometer (ICP-AES). Precursor films, derived from the solutions with and without filtration, were coated on single crystal (001) SrTiO3 substrates by spin-coating at 3000 rpm for 30 seconds. Thermal treatment was done following the temperature profile shown in Fig. 2. Specifically, the samples were heated at 2 1C min 1 to 900 1C in air and held at this temperature for one hour. Fig. 3 shows X-ray diffraction y–2y scans of converted films from precursor solutions without and with filtration. As discussed above, the only difference between these two films is the filtration process. The nature of nucleation and growth of these two films, however, is totally different. As shown in Fig. 3(a), the converted film grown without filtration of the precursor solution shows weak diffraction peaks from different phases and orientations, where all the diffraction peaks, possibly from rutile and anatase TiO2 as well as BaO2, cannot be assigned to BaTiO3. The converted film grown with filtration of the precursor solution, on the other hand, shows clearly only (00l) BaTiO3 diffraction peaks. Not only is the film c-axis oriented, but also no other phases are detected. To gain a better understanding of the nucleation and growth of BaTiO3 films using filtrated precursor solutions, we carried out in situ synchrotron X-ray diffraction measurements. In situ X-ray scattering studies were conducted in an environmental chamber built for use at sector 12ID-D of the Advanced Photon Source.24 High energy (28.3 keV) synchrotron X-rays were used to penetrate the quartz chamber walls in a controlled pressure gas environment. A grazing incidence surface scattering geometry was used.25 Samples were heated in a 150 Torr pure O2 environment while performing repeated scans through the reciprocal space region containing the (202) Bragg peak of the substrate and the expected position of the corresponding film peak. Data were collected using a Pilatus 100K-S area detector. Fig. 4 shows integrated intensity images at various temperatures (and times) of the reciprocal space region containing the (202)
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Fig. 4 A series of synchrotron X-ray diffraction images taken during an in situ thermal treatment of the precursor film (containing Ba+2 and Ti+4 with a Ba+2 : Ti+4 ratio of 1 : 1) coated on SrTiO3. Intensity images of the reciprocal space region containing the BaTiO3 and SrTiO3 Bragg (202) peaks (integrated along the approximate K direction) were obtained as the sample was heated. Approximate temperature values of each image are indicated. Arrows denoting the approximate H and L orientations are also shown. The higher intensity spot observed in all frames is the SrTiO3 (202) Bragg peak. Starting at B540 1C an epitaxially oriented, but relaxed (not coherently strained to the substrate) BaTiO3 Bragg peak appears at lower L and H (larger lattice parameter) and increases in intensity as the sample is heated to 700 1C.
Bragg peaks of the BaTiO3 film and SrTiO3 substrate as the sample is being heated. The sample was first heated quickly, at 20 1C min 1, to 350 1C. Then the sample was heated slowly, at 0.5 1C min 1, from 350 1C to 575 1C to observe the crystallization process. Next the sample was heated from 575 1C to 600 1C to look for further changes in the BaTiO3 peak. Finally, the sample was quickly heated to 700 1C. As shown, crystallized BaTiO3 first becomes detectable during heating at B540 1C. The observed location of the BaTiO3 peak indicates that the film crystallizes epitaxially oriented with the substrate, but is relaxed (meaning that the film is not coherently strained to the substrate). The film Bragg peak intensity subsequently increases as the sample continues to be heated to 700 1C. This is consistent with BaTiO3 crystallization nucleating at the SrTiO3-precursor interface, with the crystalline BaTiO3 that forms maintaining an epitaxial relationship with the substrate from the initial stages. It is interesting to note that the formation of the BaTiO3 perovskite phase starts at a temperature of about 540 1C, the temperature at which the polymers are fully depolymerized. No other phases such as TiO2, BaO, and BaO2 were detected below this temperature. We believe that the coordination between the polymers and the metal ions prevents premature nucleation below the temperature of depolymerization of the polymers. In other words, the metal ions are inactive before the polymers are depolymerized. In the case of a converted film where no filtration was applied, the formation of other phases, depending on the reactivity of individual metal ions, might begin as soon as the thermal treatment is started. It becomes more difficult to form BaTiO3 perovskite phase at a given temperature if TiO2 and BaO or BaO2 are formed first.
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3.2 Growth of high quality epitaxial complex metal-oxide films The use of polymers in solution, where the polymers bind directly with metal ions, has led to the growth of very high quality epitaxial films not commonly observed in conventional CSD processes. As a demonstration, we show the epitaxial growth of pseudo-cubic LaMnO3 (a = 0.392 nm) on the pseudo-cubic (001) LaAlO3 (a = 0.379 nm) substrate. A solution containing metal ions (La+3 and Mn+2) and polymers was prepared by dissolving PEI and EDTA in water and then adding lanthanum nitrate (La3(NO3)3) and manganese chloride (MnCl2) in the proportion of La+3 : Mn+2 = 1 : 1 to the mixture. Ultrafiltration was done under 60 psi Ar by using an Amicon stirred cell having a cut-off molecular weight of 3000 g mol 1. The precursor film was spin-coated on a single crystal (001) LaAlO3 substrate at 3000 rpm for 30 seconds. Crystallization was achieved by annealing the precursor film at 830 1C in oxygen for four hours. Fig. 5 shows an X-ray y–2y scan of LaMnO3 on LaAlO3. Except for the diffraction peaks from the substrate, only (00l) peaks from the pseudo-cubic LaMnO3 were detected, indicating that the film is preferentially oriented. The inset of Fig. 5 is a rocking curve measured on the (002) peak of LaMnO3. The fullwidth at the half-maximum intensity of the rocking curve is about 0.3361. The in-plane alignment of LaMnO3 with respect to the major axes of LaAlO3 is characterized by the X-ray f-scans on LaMnO3 (202) and LaAlO3 (202) peaks as shown in Fig. 6. Based on Fig. 5 and 6, it is clear that the LaMnO3 film is epitaxially grown (cube-on-cube) on LaAlO3. Fig. 7 shows a cross-sectional transmission electron microscopy image of a LaMnO3 film grown by PAD on LaAlO3. There is no evidence of any reaction layer between the film and the substrate. The film surface is smooth. Corresponding selectedarea electron diffraction patterns (not shown here) taken from the interface confirm the epitaxial growth of LaMnO3 on LaAlO3. The growth of a high quality epitaxial film by such a simple method is best illustrated by the observation of misfit dislocations (marked with arrows pointing to the film growth
Fig. 6 X-ray diffraction f-scans on both the (202) of LaMnO3 film and the (202) of LaAlO3 substrate.
Fig. 7 Cross-section transmission electron microscopy image of the LaMnO3 film on the LaAlO3 substrate. The measured misfit dislocation spacing is around 12 nm.
direction) along the interface (marked with arrows oriented in the plane) between the LaMnO3 and the LaAlO3 substrate. The lattice misfit between the LaMnO3 and the LaAlO3 is about 3.4%. The corresponding misfit dislocation spacing, calculated according to dfds/(ds df), where df = 0.392 nm and ds = 0.379 nm, is 11.4 nm, which agrees well with the measured misfit dislocation spacing of 12 nm shown in Fig. 7.
4. Summary
Fig. 5 X-ray diffraction normal scan of a LaMnO3 film on the LaAlO3 substrate. The inset shows the rocking curve measured on the (002) peak of LaMnO3.
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Binding metal ions to polymers, a process uniquely used by PAD, provides advantages including the formation of a homogeneous distribution of metal ions in the precursor solution and the protection of metal ions from reaction at low temperatures before polymers are depolymerized. The coordination between the polymers and metal ions also makes filtration possible in the PAD process. Filtration used by PAD effectively prevents metal ions from premature condensation and/or precipitation, which leads to a real bottom-up nucleation and growth. The successful growth of a wide range of high quality epitaxial films with desired physical properties by PAD demonstrates that PAD is emerging as a versatile and powerful
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Acknowledgements This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396. The work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.
References 1 D. L. Smith, Thin Film Deposition, McGraw-Hill, New York, 1995. 2 C. J. Brinker and G. W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic, New York, 1990. 3 F. F. Lange, Science, 1996, 273, 903. 4 R. W. Schwartz, Chem. Mater., 1997, 9, 2325. 5 R. W. Schwartz, T. Schneller and R. Waser, C. R. Chim., 2004, 7, 433. 6 T. Araki and I. Hirabayashi, Supercond. Sci. Technol., 2003, 16, R71. 7 Q. X. Jia, T. M. McCleskey, A. K. Burrell, Y. Lin, G. Collis, H. Wang, A. D. Q. Li and S. R. Foltyn, Nat. Mater., 2004, 3, 529. 8 A. K Burrell, T. M. McCleskey and Q. X. Jia, Chem. Commun., 2008, 1271. 9 G. F. Zou, J. Zhao, H. M. Luo, T. M. McCleskey, A. K. Burrell and Q. X. Jia, Chem. Soc. Rev., 2013, 42, 439. 10 G. Zou, H. Luo, F. Ronning, B. Sun, T. M. McCleskey, A. K. Burrell, E. Bauer and Q. X. Jia, Angew. Chem., Int. Ed., 2010, 49, 1782.
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Chem Soc Rev 11 Y. Lin, H. Wang, M. E. Hawley, S. R. Foltyn, Q. X. Jia, G. E. Collis, A. K. Burrell and T. M. McCleskey, Appl. Phys. Lett., 2004, 85, 3426. 12 Y. Lin, J. S. Lee, H. Wang, Y. Li, S. R. Foltyn, Q. X. Jia, G. Collis, A. K. Burrell and T. M. McCleskey, Appl. Phys. Lett., 2004, 85, 5007. 13 H. M. Luo, H. Wang, Z. X. Bi, G. F. Zou, T. M. McCleskey, A. K. Burrell, E. Bauer, M. E. Hawley, Y. Lin, S. A. Baily, L. Civale, Y. Q. Wang and Q. X. Jia, Angew. Chem., Int. Ed., 2009, 48, 1490. 14 Y. Y. Zhang, N. Haberkorn, F. Ronning, H. Wang, N. A. Mara, M. Zhuo, C. Li, J. H. Lee, K. J. Blackmore, E. Bauer, A. K. Burrell, T. M. McCleskey, M. E. Hawley, R. K. Schulze, L. Civale, T. Tajima and Q. X. Jia, J. Am. Chem. Soc., 2011, 133, 20735. 15 G. Zou, H. Wang, N. Mara, H. Luo, N. Li, Z. F. Di, E. Bauer, Y. Q. Wang, T. M. McCleskey, A. K. Burrell, X. Zhang, M. Nastasi and Q. X. Jia, J. Am. Chem. Soc., 2010, 132, 2516. 16 D. Q. Li and Q. X. Jia, U. S. Pat., No. 6,589,457, 2003. 17 T. M. McCleskey, A. K. Burrell, Q. X. Jia and Y. Lin, U. S. Pat., No. 7,604,839, 2009. 18 M. N. Ali, M. A. Garcia, T. Parsons-Moss and H. Nitsche, Nat. Protocols, 2010, 5, 1440. 19 G. Zou, M. Jain, H. Zhou, H. Luo, S. A. Baily, L. Civale, E. Bauer, T. M. McCleskey, A. K. Burrell and Q. X. Jia, Chem. Commun., 2008, 6022. 20 G. Zou, H. Luo, Y. Zhang, J. Xiong, Q. Wei, M. Zhuo, J. Zhai, H. Wang, D. Williams, N. Li, E. Bauer, X. Zhang, T. M. McCleskey, Y. Li, A. K. Burrell and Q. X. Jia, Chem. Commun., 2010, 46, 7837. 21 Y. R. Patta, D. E. Wesolowski and M. J. Cima, Physica C, 2009, 469, 129. 22 R. Cobas, S. Munoz-Perez, J. M. Cadogan, T. Puig and X. Obradors, Appl. Phys. Lett., 2011, 99, 083113. 23 F. Rivadulla, Z. X. Bi, E. Bauer, B. R. Murias, J. M. Vila-Fungueirino and Q. X. Jia, Chem. Mater., 2013, 25, 55. 24 P. H. Fuoss, R.-V. Wang, J. A. Eastman, D. D. Fong, G. B. Stephenson, S. K. Streiffer, C. Thompson, F. Jiang, G.-W. Zhou, L. E. Rehn, P. M. Baldo and L. J. Thompson, J. Taiwan Vac. Tech. Soc., 2005, 18, 69. 25 T. T. Fister, P. H. Fuoss, D. D. Fong, J. A. Eastman, C. M. Folkman, S. O. Hruszkewycz, M. J. Highland, H. Zhou and P. A. Fenter, J. Appl. Crystallogr., 2013, 46, 639.
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Nucleation and growth of epitaxial metal-oxide films based on polymer-assisted deposition T. M. McCleskey,*a P. Shi,b E. Bauer,a M. J. Highland,c J. A. Eastman,c Z. X. Bi,a P. H. Fuoss,c P. M. Baldo,c W. Ren,b B. L. Scott,a A. K. Burrellc and Q. X. Jia*a Polymer-assisted deposition (PAD) is one of the chemical solution deposition methods which have been successfully used to grow films, form coatings, and synthesize nanostructured materials. In comparison with other conventional solution-based deposition techniques, PAD differs in its use of water-soluble polymers in the solution that prevent the metal ions from unwanted chemical reactions and keep the solution stable. Furthermore, filtration to remove non-coordinated cations and anions in the PAD process ensures well controlled nucleation, which enables the growth of high quality epitaxial films with desired structural and physical properties. The precursor solution is prepared by mixing water-
Received 1st August 2013 DOI: 10.1039/c3cs60285k
soluble polymer(s) with salt(s). Thermal treatment of the precursor films in a controlled environment leads to the formation of desired materials. Using BaTiO3 grown on SrTiO3 and LaMnO3 on LaAlO3 as model systems, we show the effect of filtration on the nucleation and growth of epitaxial complex metal-oxide films based on the PAD process.
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Key learning points (1) (2) (3) (4)
Unique features of polymer-assisted deposition. Broad and expanding range of materials synthesized by polymer-assisted deposition. Critical roles of precursor solution, filtration, de-polymerization, and crystallization in the synthesis of high quality epitaxial films. Potential applications of polymer-assisted deposition for thin films, coatings, and nanostructured materials.
1. Introduction Thin films, coatings, and nanostructured materials have found extensive applications in virtually all technological and scientific areas. Products using these materials cover an extraordinarily wide range from nanoscale electronics to gas turbine engines. Tremendous advances have been made to synthesize and process materials in different formats with desired structural and functional properties using either physical- or chemical-vapor deposition techniques.1 Equally impressive is the progress towards improving and optimizing the solution deposition techniques for the synthesis and processing of a broad and constantly expanding range of materials of interest to research and industry. Chemical solution deposition (CSD) has the advantages of low capital investment and the ability to coat large areas and a
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. E-mail: [email protected], [email protected] b School of Electronic and Information Eng., Xi’an Jiaotong University, Xian, China c Argonne National Laboratory, Argonne, Illinois 60439, USA
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3D objects. Both sol–gel2 and metal–organic deposition (MOD) are classical examples of CSD routes. Extensive reviews on the chemical solution deposition of functional oxide films for targeted applications can be found elsewhere.3–6 Recently, we have developed a polymer-assisted deposition (PAD)7–9 method that is becoming a promising alternative CSD method for the synthesis and processing of high quality films and coatings for specific applications. In particular, its ability to grow high quality epitaxial films such as metals,10 metal-oxides,11,12 metal-nitrides,13,14 and metal-carbides15 without modifying the chemistry of the precursor solution has made PAD the choice for not only practical applications but also the fundamental investigation of growth and nucleation involved in the solution deposition of thin films. In this article, we start by describing the important considerations of the PAD process that are critical for the growth of high quality epitaxial films. We then use two different examples to illustrate the interplay between processing parameters and the resulting microstructures of epitaxial metaloxide films.
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2. Polymer-assisted deposition As PAD itself is a solution deposition process, it has the traditional advantages of CSD. In terms of processing steps, PAD is similar to sol–gel and other solution-based alternatives. However, the use of a specific polymer in solution that binds directly with metal ions has led to the growth of highly crystalline epitaxial films10–15 not commonly observed when conventional CSD processes are used. The main differences between PAD and conventional CSD processes are the unique chemistry of the precursor solution, the use of filtration, and the distinctive process during de-polymerization and crystallization. Using the same chemistry or precursor solution, one can grow materials such as metals, metal-oxides, metal-nitrides, and metalcarbides, simply by controlling the environment during the thermal treatment. Detailed experimental procedures to grow metals, metal-nitrides, and metal-carbides can be found in published articles.10,13–15
T. Mark McCleskey received his BS from Harvey Mudd College in 1987 and his PhD from the California Institute of Technology in 1993 for work on electron transfer with Dr Harry Gray. He then did postdoctoral research at Los Alamos National Laboratory, where he is currently Deputy Division Leader of the Chemistry Division. He has over 100 publications, 1000 citations and 20 patents. His research at Los T. M. McCleskey Alamos has focused on metal– ligand interactions and most recently has involved a new thin-film coating technique – polymer assisted deposition.
A. K. Burrell
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Anthony K. Burrell received his PhD in chemistry in 1990 from the University of Auckland, New Zealand. He is currently the Head of Department of the CSE Electrochemical Energy Storage Department at Argonne National Laboratory. His research interests include thin film materials, advanced lithium ion batteries, structural changes of lithium– manganese cathode materials and beyond lithium ion battery chemistries.
2.1
Precursor solution
PAD is both solution-based and polymer-assisted.7–9 It is solutionbased in that, when used for deposition, it employs direct application of a water-based chemical solution to the surface to be coated. The solution contains water, salt(s), and polymer(s). The particular salt selected, i.e. the metal element it contains, determines which metal element will be present in the compound synthesized. It is polymer-assisted in that the solution contains specific organic polymers such as polyethyleneimine (PEI). Other polymers can also be used.16,17 It is noted that ethylenediaminetetraacetic acid (EDTA) has been advantageously used in PAD solution since EDTA can bind with many different metal ions, and the EDTA complexes thereafter bind to PEI via a combination of hydrogen bonding and electrostatic attraction.7 One of the most important characteristics of PAD is the interactions between metal ions and polymers. The formation of covalent complexes from the lone pairs on the nitrogen atoms of the polymers and the metal cations of the salts provides a stable network to remove non-coordinated species. A detailed description of transition metals bound to polymers can be found elsewhere.8 It should be emphasized that the polymers used in the PAD process provide the following functions: (i) prevent the metal ions from engaging in unwanted chemical reactions; (ii) maintain a homogeneous distribution of metal ions in solution (due to filtration), thus yielding an even and uniform coating; (iii) help maintain the desired viscosity of the solution, enabling good control of thickness; and (iv) keep the solution stable in air. 2.2
Filtration
Filtration has been widely used in the concentration or purification of molecular solutions and in water treatment. One of the key features of the precursor solution of PAD is the use of Amicons filtration to remove non-coordinated cation and anion species. The solution is passed through a filter or membrane to
Quanxi Jia is a Thrust Leader at the Center for Integrated Nanotechnologies, a DOE Nanoscale Science Research Center operated jointly by Los Alamos and Sandia National Laboratories. He is a Fellow of Los Alamos National Laboratory and a Fellow of the following societies: The American Physical Society, American Ceramic Society, and American Association for the Advancement of Q. X. Jia Science. He received BS and MS in Electronic Engineering from Xian Jiaotong University, China, and PhD in the same field from the State University of New York, Buffalo. He has authored/co-authored over 400 refereed journal articles and has been awarded 45 US patents.
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Fig. 1 (a) Schematic illustration of filtration used in the PAD process, where metal ions are coordinated with polymers. Cations and anions are washed out via filtration if they are not coordinated with polymers. (b) Photograph of an Amicon Ultrafiltration cell used in the experiment.
wash out cations and anions not coordinated with polymers. Fig. 1(a) shows a schematic illustration of the filtration process. This is possible in the PAD process because of the large differences in molecular weight and size between polymers and cations–anions. Fig. 1(b) shows a photograph of the filtration apparatus used in our experiment. In a typical filtration process, a pressure of 60 psi (Ar gas) is applied to make the filtration more efficient. Filtering the solution prevents precipitation and therefore yields a homogeneous solution (and ultimately a homogeneous coating) at the molecular level. Our experimental results have illustrated that filtration plays an important role in the growth of high quality epitaxial films (to be discussed in the given examples). This step is especially critical in the growth of epitaxial complex materials where two or more cations are involved. Whether the targeted metal concentration in a given precursor solution has been achieved can be determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). 2.3
De-polymerization and crystallization
Once the precursor film is applied, it is heated in a controlled environment at the desired temperature for the desired time to depolymerize and crystallize the film with the desired crystal structure. Fig. 2 shows a schematic drawing of the
Tutorial Review typical temperature profiles used in the PAD process. The precursor film is first heated to a moderate temperature (B120 1C) to drive out water. Different from sol–gel and other solution-based alternatives, the PAD process involves a higher temperature (350–500 1C or slightly above) exposure in a controlled environment to depolymerize the polymer, releasing the polymer from the precursor film. In other words, the polymer such as PEI in the precursor film does not undergo combustion, but rather a thermal de-polymerization back to NH2CHQCH2. In the case of EDTA, it decomposes to acetic acid, formic acid, and ethylenediamine.8 It should be noted that this non-combustion process can lead to less carbon contamination in the synthesized films. Crystallization takes place at temperatures above 600 1C. High temperature and longer time at the temperature will normally improve the crystallinity of the film. A temperature of above 800 1C is often required if one needs to achieve high quality epitaxial metaloxide films. Depending on the temperature used during the thermal treatment and the substrate materials used, the formed films can be epitaxial, polycrystalline, nanocrystalline, or amorphous. For example, polycrystalline metal-oxide films have been formed on Si3N4 at an annealing temperature of 600 1C.18 It should be noted that the temperature profile (ramping rate, temperature, and time) needs to be optimized for a given material composition in order to accomplish high crystallinity and phase-pure films. Note that it is the combination of the metal precursor solution, the temperature, and the environment at the temperature that determines the chemical composition of the converted films. For example, thermal treatment of a precursor film containing niobium ions in a reducing atmosphere such as argon mixed with hydrogen will result in pure niobium; the precursor film will be converted to Nb2O3 if the thermal treatment is done in pure oxygen; the same precursor film will be transformed to NbN if the thermal treatment is carried out in an ammonia atmosphere (which can be considered as a reducing nitrogen source);19 and the very same precursor film will be converted to NbC if the thermal treatment is carried out in an environment containing ethylene (which contains carbon).20 Using a lattice matched substrate and optimal temperature profiles, researchers have demonstrated that PAD provides a powerful technique to grow high performance epitaxial films.10–15,21–23
3. Examples 3.1 Effect of filtration on the nucleation and growth of complex metal-oxide films
Fig. 2 Schematic drawing of the thermal treatment profile used in the PAD process. The annealing time at high temperature (for crystallization) depends on the materials and the microstructures (epitaxial, polycrystalline, amorphous). It ranges from minutes to 3–4 hours. Polymer PEI is depolymerized at a temperature around 500 1C.
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Here we use BaTiO3 grown on SrTiO3 as a model system to illustrate the effect of filtration on the nucleation and growth in the PAD process. Solutions containing metal ions (either Ba+2 or Ti+4) and polymer were prepared by dissolving PEI (BASF Corporation of Clifton, NJ) and EDTA (Sigma-Aldrich Co. LLC) in water and then adding barium nitrate (Ba2(NO3)2) or titanium chloride (TiCl4). Solutions containing Ba+2 and Ti+4 were mixed in the proportion of Ba+2 : Ti+4 = 1 : 1. Ultrafiltration was
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Fig. 3 X-ray diffractions patterns of converted films on SrTiO3 substrates. The precursor films contain Ba+2 and Ti+4 with a Ba+2 : Ti+4 ratio of 1 : 1, (a) without and (b) with filtration of the solutions. The crystallization temperature was 900 1C.
performed under 60 psi Ar by using an Amicon stirred cell (illustrated in Fig. 1(b)) having a cut-off molecular weight of 3000 g mol 1. Metal analysis was conducted using a Varian Liberty 220 inductively coupled plasma-atomic emission spectrometer (ICP-AES). Precursor films, derived from the solutions with and without filtration, were coated on single crystal (001) SrTiO3 substrates by spin-coating at 3000 rpm for 30 seconds. Thermal treatment was done following the temperature profile shown in Fig. 2. Specifically, the samples were heated at 2 1C min 1 to 900 1C in air and held at this temperature for one hour. Fig. 3 shows X-ray diffraction y–2y scans of converted films from precursor solutions without and with filtration. As discussed above, the only difference between these two films is the filtration process. The nature of nucleation and growth of these two films, however, is totally different. As shown in Fig. 3(a), the converted film grown without filtration of the precursor solution shows weak diffraction peaks from different phases and orientations, where all the diffraction peaks, possibly from rutile and anatase TiO2 as well as BaO2, cannot be assigned to BaTiO3. The converted film grown with filtration of the precursor solution, on the other hand, shows clearly only (00l) BaTiO3 diffraction peaks. Not only is the film c-axis oriented, but also no other phases are detected. To gain a better understanding of the nucleation and growth of BaTiO3 films using filtrated precursor solutions, we carried out in situ synchrotron X-ray diffraction measurements. In situ X-ray scattering studies were conducted in an environmental chamber built for use at sector 12ID-D of the Advanced Photon Source.24 High energy (28.3 keV) synchrotron X-rays were used to penetrate the quartz chamber walls in a controlled pressure gas environment. A grazing incidence surface scattering geometry was used.25 Samples were heated in a 150 Torr pure O2 environment while performing repeated scans through the reciprocal space region containing the (202) Bragg peak of the substrate and the expected position of the corresponding film peak. Data were collected using a Pilatus 100K-S area detector. Fig. 4 shows integrated intensity images at various temperatures (and times) of the reciprocal space region containing the (202)
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Fig. 4 A series of synchrotron X-ray diffraction images taken during an in situ thermal treatment of the precursor film (containing Ba+2 and Ti+4 with a Ba+2 : Ti+4 ratio of 1 : 1) coated on SrTiO3. Intensity images of the reciprocal space region containing the BaTiO3 and SrTiO3 Bragg (202) peaks (integrated along the approximate K direction) were obtained as the sample was heated. Approximate temperature values of each image are indicated. Arrows denoting the approximate H and L orientations are also shown. The higher intensity spot observed in all frames is the SrTiO3 (202) Bragg peak. Starting at B540 1C an epitaxially oriented, but relaxed (not coherently strained to the substrate) BaTiO3 Bragg peak appears at lower L and H (larger lattice parameter) and increases in intensity as the sample is heated to 700 1C.
Bragg peaks of the BaTiO3 film and SrTiO3 substrate as the sample is being heated. The sample was first heated quickly, at 20 1C min 1, to 350 1C. Then the sample was heated slowly, at 0.5 1C min 1, from 350 1C to 575 1C to observe the crystallization process. Next the sample was heated from 575 1C to 600 1C to look for further changes in the BaTiO3 peak. Finally, the sample was quickly heated to 700 1C. As shown, crystallized BaTiO3 first becomes detectable during heating at B540 1C. The observed location of the BaTiO3 peak indicates that the film crystallizes epitaxially oriented with the substrate, but is relaxed (meaning that the film is not coherently strained to the substrate). The film Bragg peak intensity subsequently increases as the sample continues to be heated to 700 1C. This is consistent with BaTiO3 crystallization nucleating at the SrTiO3-precursor interface, with the crystalline BaTiO3 that forms maintaining an epitaxial relationship with the substrate from the initial stages. It is interesting to note that the formation of the BaTiO3 perovskite phase starts at a temperature of about 540 1C, the temperature at which the polymers are fully depolymerized. No other phases such as TiO2, BaO, and BaO2 were detected below this temperature. We believe that the coordination between the polymers and the metal ions prevents premature nucleation below the temperature of depolymerization of the polymers. In other words, the metal ions are inactive before the polymers are depolymerized. In the case of a converted film where no filtration was applied, the formation of other phases, depending on the reactivity of individual metal ions, might begin as soon as the thermal treatment is started. It becomes more difficult to form BaTiO3 perovskite phase at a given temperature if TiO2 and BaO or BaO2 are formed first.
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3.2 Growth of high quality epitaxial complex metal-oxide films The use of polymers in solution, where the polymers bind directly with metal ions, has led to the growth of very high quality epitaxial films not commonly observed in conventional CSD processes. As a demonstration, we show the epitaxial growth of pseudo-cubic LaMnO3 (a = 0.392 nm) on the pseudo-cubic (001) LaAlO3 (a = 0.379 nm) substrate. A solution containing metal ions (La+3 and Mn+2) and polymers was prepared by dissolving PEI and EDTA in water and then adding lanthanum nitrate (La3(NO3)3) and manganese chloride (MnCl2) in the proportion of La+3 : Mn+2 = 1 : 1 to the mixture. Ultrafiltration was done under 60 psi Ar by using an Amicon stirred cell having a cut-off molecular weight of 3000 g mol 1. The precursor film was spin-coated on a single crystal (001) LaAlO3 substrate at 3000 rpm for 30 seconds. Crystallization was achieved by annealing the precursor film at 830 1C in oxygen for four hours. Fig. 5 shows an X-ray y–2y scan of LaMnO3 on LaAlO3. Except for the diffraction peaks from the substrate, only (00l) peaks from the pseudo-cubic LaMnO3 were detected, indicating that the film is preferentially oriented. The inset of Fig. 5 is a rocking curve measured on the (002) peak of LaMnO3. The fullwidth at the half-maximum intensity of the rocking curve is about 0.3361. The in-plane alignment of LaMnO3 with respect to the major axes of LaAlO3 is characterized by the X-ray f-scans on LaMnO3 (202) and LaAlO3 (202) peaks as shown in Fig. 6. Based on Fig. 5 and 6, it is clear that the LaMnO3 film is epitaxially grown (cube-on-cube) on LaAlO3. Fig. 7 shows a cross-sectional transmission electron microscopy image of a LaMnO3 film grown by PAD on LaAlO3. There is no evidence of any reaction layer between the film and the substrate. The film surface is smooth. Corresponding selectedarea electron diffraction patterns (not shown here) taken from the interface confirm the epitaxial growth of LaMnO3 on LaAlO3. The growth of a high quality epitaxial film by such a simple method is best illustrated by the observation of misfit dislocations (marked with arrows pointing to the film growth
Fig. 6 X-ray diffraction f-scans on both the (202) of LaMnO3 film and the (202) of LaAlO3 substrate.
Fig. 7 Cross-section transmission electron microscopy image of the LaMnO3 film on the LaAlO3 substrate. The measured misfit dislocation spacing is around 12 nm.
direction) along the interface (marked with arrows oriented in the plane) between the LaMnO3 and the LaAlO3 substrate. The lattice misfit between the LaMnO3 and the LaAlO3 is about 3.4%. The corresponding misfit dislocation spacing, calculated according to dfds/(ds df), where df = 0.392 nm and ds = 0.379 nm, is 11.4 nm, which agrees well with the measured misfit dislocation spacing of 12 nm shown in Fig. 7.
4. Summary
Fig. 5 X-ray diffraction normal scan of a LaMnO3 film on the LaAlO3 substrate. The inset shows the rocking curve measured on the (002) peak of LaMnO3.
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Binding metal ions to polymers, a process uniquely used by PAD, provides advantages including the formation of a homogeneous distribution of metal ions in the precursor solution and the protection of metal ions from reaction at low temperatures before polymers are depolymerized. The coordination between the polymers and metal ions also makes filtration possible in the PAD process. Filtration used by PAD effectively prevents metal ions from premature condensation and/or precipitation, which leads to a real bottom-up nucleation and growth. The successful growth of a wide range of high quality epitaxial films with desired physical properties by PAD demonstrates that PAD is emerging as a versatile and powerful
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Acknowledgements This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396. The work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.
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