Langmuir 2007, 23, 11089-11099
11089
Surface Composition and Electrical and Electrochemical Properties of Freshly Deposited and Acid-Etched Indium Tin Oxide Electrodes Michael Brumbach, P. Alex Veneman, F. Saneeha Marrikar,† Thomas Schulmeyer,‡ Adam Simmonds,§ Wei Xia,| Paul Lee, and Neal R. Armstrong* Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721 ReceiVed June 13, 2007. In Final Form: July 24, 2007 We compare the near-surface composition and electroactivity of commercial indium tin oxide (ITO) thin films, activated by plasma cleaning or etching with strong haloacids, with ITO films that have been freshly deposited in high vacuum, before and after exposure to the atmosphere or water vapor. Conductive-tip AFM, X-ray photoelectron spectroscopy (XPS), and the electrochemistry of probe molecules in solution were used to compare the relative degrees of electroactivity and the near-surface composition of these materials. Brief etching of commercial ITO samples with concentrated HCl or HI significantly enhances the electrical activity of these oxides as revealed by C-AFM. XPS was used to compare the composition of these activated surfaces, focusing on the intrinsically asymmetric O 1s line shape. Energy-loss processes associated with photoemission from the tin-doped, oxygen-deficient oxides complicate the interpretation of the O 1s spectra. O 1s spectra from the stoichiometric indium oxide lattice are accompanied by higher-binding-energy peaks associated with hydroxylated forms of the oxide (and in some cases carbonaceous impurities) and overlapping photoemission associated with energy-loss processes. Characterization of freshly sputter-deposited indium oxide (IO) and ITO films, transferred under high vacuum to the surface analysis environment, allowed us to differentiate the contributions of tin doping and oxygen-vacancy doping to the O 1s line shape, relative to higherbinding-energy O 1s components associated with hydroxyl species and carbonaceous impurities. Using these approaches, we determined that acid activation and O2 plasma etching create an ITO surface that is still covered with an average of one to two monolayers of hydroxide. Both of these activation treatments lead to significantly higher rates of electron transfer to solution probe molecules, such as dimethyferrocene in acetonitrile. Solution electron-transfer events appear to occur at no more than 4 × 107 electroactive sites per cm2 (each with diameters of ca. 50-200 nm) (i.e., a small fraction of the geometric area of the electrode). Electron-transfer rates correlate with the near-surface tin dopant concentration, suggesting that these electroactive sites arise from near-surface tin enrichment.
Introduction The optimization of transparent conducting oxides (TCO) as indicator electrodes for chemical sensors and as the transparent bottom contact electrode for electrochromic displays, organic light-emitting diodes (OLEDs), and organic photovoltaic cells (OPVs) is a critical part of the development of these new technologies.1-26 One of the most important limitations of TCOs * To whom correspondence should be addressed. E-mail: nra@ u.arizona.edu. Tel: (520) 621-8242. † Current address: Huntsman Chemical Co., Longview, Texas. ‡ Current address: Advanced Mask Technology Center, Dresden, Germany. § Current address: Sion Power Corp., Tucson, Arizona. | Current address: Veeco/Digital Instruments, Santa Barbara, California. (1) Kim, J. S.; Ho, P. K. H.; Thomas, D. S.; Friend, R. H.; Cacialli, F.; Bao, G. W.; Li, S. F. Y. Chem. Phys. Lett. 1999, 315, 307. (2) Kim, J. S.; Friend, R. H.; Cacialli, F. J. Appl. Phys. 1999, 86, 2774. (3) Kim, J. S.; Lagel, B.; Moons, E.; Johansson, N.; Baikie, I. D.; Salaneck, W. R.; Friend, R. H.; Cacialli, F. Synth. Met. 2000, 111, 311. (4) Kim, J. S.; Cacialli, F.; Friend, R. Thin Solid Films 2003, 445, 358. (5) Popovich, N. D.; Wong, S. S.; Yen, B. K. H.; Yeom, H. Y.; Paine, D. C. Anal. Chem. 2002, 74, 3127. (6) Popovich, N. D.; Wong, S. S.; Ufer, S.; Sakhrani, V.; Paine, D. J. Electrochem. Soc. 2003, 150, H255. (7) Swint, A. L.; Bohn, P. W. Langmuir 2004, 20, 4076. (8) Swint, A. L.; Bohn, P. W. Appl. Phys. Lett. 2004, 84, 61. (9) Gassenbauer, Y.; Klein, A. J. Phys. Chem. B 2006, 110, 4793. (10) Gassenbauer, Y.; Schafranek, R.; Klein, A.; Zafeiratos, S.; Havecker, M.; Knop-Gericke, A.; Schlogl, R. Phys. ReV. B 2006, 73, 245312. (11) Gassenbauer, Y.; Klein, A. Solid State Ionics 2004, 173, 141. (12) Armstrong, N. R.; Lin, A. W. C.; Fujihira, M.; Kuwana, T. Anal. Chem. 1976, 48, 741. (13) Armstrong, N. R.; Carter, C.; Donley, C.; Simmonds, A.; Lee, P.; Brumbach, M.; Kippelen, B.; Domercq, B.; Yoo, S. Thin Solid Films 2003, 445, 342. (14) Donley, C.; Dunphy, D.; Paine, D.; Carter, C.; Nebesny, K.; Lee, P.; Alloway, D.; Armstrong, N. R. Langmuir 2002, 18, 450.
is the fact that the surface chemical and electrical properties of both indium tin oxide (ITO) and fluorine- or antimony-doped tin oxide (FTO, ATO) are quite heterogeneous.4,5,12-19,25-32 In electroanalytical applications where the TCO electrode functions as a potentiometric or amperometric sensor, enhancements in (15) Carter, C.; Brumbach, M.; Donley, C.; Hreha, R. D.; Marder, S. R.; Domercq, B.; Yoo, S.; Kippelen, B.; Armstrong, N. R. J. Phys. Chem. B 2006, 110, 25191. (16) Marrikar, F. S.; Brumbach, M.; Evans, D.; Lebron-Paler, A.; Pemberton, J.; Wysocki, R.; Armstrong, N. Langmuir 2006, 23, 1530. (17) Xia, W. Ph.D. Thesis, University of Arizona, 2005. (18) Brumbach, M. Ph.D. Thesis, University of Arizona, 2007. (19) Donley, C. L. Ph.D. Thesis, University of Arizona, 2003. (20) Milliron, D. J.; Hill, I. G.; Shen, C.; Kahn, A.; Schwartz, J. J. Appl. Phys. 2000, 87, 572. (21) Purvis, K. L.; Lu, G.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc. 2000, 122, 1808. (22) Bruner, E. L.; Koch, N.; Span, A. R.; Bernasek, S. L.; Kahn, A.; Schwartz, J. J. Am. Chem. Soc. 2002, 124, 3192. (23) Guo, J.; Koch, N.; Schwartz, J.; Bernasek, S. L. J. Phys. Chem. B 2005, 109, 3966. (24) Hanson, E. L.; Guo, J.; Koch, N.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc. 2005, 127, 10058. (25) Bermudez, V. M.; Berry, A. D.; Kim, H.; Pique, A. Langmuir 2006, 22, 11113. (26) Karsi, N.; Lang, P.; Chehimi, M.; Delamar, M.; Horowitz, G. Langmuir 2006, 22, 3118. (27) Hwang, J. H.; Edwards, D. D.; Kammler, D. R.; Mason, T. O. Solid State Ionics 2000, 129, 135. (28) Warschkow, O.; Ellis, D. E.; Gonzalez, G. B.; Mason, T. O. J. Am. Ceram. Soc. 2003, 86, 1700. (29) Harvey, S. P.; Mason, T. O.; Gassenbauer, Y.; Schafranek, R.; Klein, A. J. Phys. D: Appl. Phys. 2006, 39, 3959. (30) Warschkow, O.; Miljacic, L.; Ellis, D. E.; Gonzalez, G. B.; Mason, T. O. J. Am. Ceram. Soc. 2006, 89, 616. (31) Nakao, T.; Nakada, T.; Nakayama, Y.; Miyatani, K.; Kimura, Y.; Saito, Y.; Kaito, C. Thin Solid Films 2000, 370, 155. (32) Kobayashi, T.; Kimura, Y.; Suzuki, H.; Sato, T.; Tanigaki, T.; Saito, Y.; Kaito, C. J. Cryst. Growth 2002, 243, 143.
10.1021/la701754u CCC: $37.00 © 2007 American Chemical Society Published on Web 09/20/2007
11090 Langmuir, Vol. 23, No. 22, 2007
Brumbach et al.
Table 1. Typical Parameters for Pulsed dc Magnetron Sputter Deposition of ITO and In2O3 Thin Films Using a Kurt J. Lesker AXXIS System predeposition pressure (base
