Surface Modification of Indium Tin Oxide by Phenoxytin Complexes

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Notes Surface Modification of Indium Tin Oxide by Phenoxytin Complexes Amelia R. Span, Eric L. Bruner, Steven L. Bernasek,* and Jeffrey Schwartz* Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009 Received September 18, 2000. In Final Form: November 14, 2000

Modern organic light-emitting diodes (OLEDs) commonly use indium tin oxide, ITO, as the device anode.1-4 It is known that surface treatment of the ITO can affect its electrode function, and two broad classes of processing have been described. The first involves reaction of the ITO surface with “inorganic” species, such as oxygen plasma,5-10 ozone/UV,6 hydrogen peroxide,11 nitric acid,12 or aqua regia.10 The second uses organics or organometallics, whose structure can be systematically varied, and reactions of ITO have been described with metallophthalocyanines,13 carboxylic14,15 or phosphonic acids,14,16 amines,17 and thiols.18 One current model19 suggests that organization of a dipole at the ITO electrode surface can increase its work function and hence effect changes in its hole injection behavior in a device. However, although various inorganic treatments have been shown to increase the work function of ITO,5,7,20-25 these procedures do not (1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Campbell, I. H.; Davids, P. S.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Appl. Phys. Lett. 1998, 72, 1863. (3) Antoniadis, H.; Miller, J. N.; Roitman, D. B.; Campbell, I. H. IEEE Trans. Electron Devices 1997, 44, 1289. (4) Burrows, P. E.; Gu, G.; Bulovic´, V.; Shen, Z.; Forrest, S. R.; Thompson, M. E. IEEE Trans. Electron Devices 1997, 44, 1188. (5) Milliron, D. J.; Hill, I. G.; Shen, C.; Kahn, A.; Schwartz, J. J. Appl. Phys. 2000, 87, 572. (6) Wu, C.; Wu, C. I.; Sturm, J. C.; Kahn, A. Appl. Phys. Lett. 1997, 70, 1348. (7) Kim, J. S.; Granstro¨m, M.; Friend, R. H.; Johansson, N.; Salaneck, W. R.; Daik, R.; Feast, W. J.; Cacialli, F. J. Appl. Phys. 1998, 84, 6859. (8) Sto¨ssel, M.; Staudigel, J.; Steuber, F.; Simmerer, J.; Wittmann, G.; Kanitz, A.; Klausmann, H.; Rogler, W.; Roth, W.; Schumann, J.; Winnacker, A. Phys. Chem. Chem. Phys. 1999, 1, 1791. (9) Steuber, F.; Staudigel, J.; Sto¨ssel, M.; Simmerer, J.; Winnacker, A. Appl. Phys. Lett. 1999, 74, 3558. (10) 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. (11) Osada, T.; Kugler, T.; Bro¨ms, P.; Salaneck, W. R. Synth. Met. 1998, 96, 77. (12) Nu¨esch, F.; Kamara´s, K.; Zuppiroli, L. Chem. Phys. Lett. 1998, 283, 194. (13) Hill, I. G.; Kahn, A. J. Appl. Phys. 1999, 86, 2116. (14) Gardner, T. J.; Frisbie, C. D.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 6927. (15) Nu¨esch, F.; Rotzinger, F.; Si-Ahmed, L.; Zuppiroli, L. Chem. Phys. Lett. 1998, 288, 861. (16) Appleyard, S. F. J.; Day, S. R.; Pickford, R. D.; Willis, M. R. J. Mater. Chem. 2000, 10, 169. (17) Oh, S.-Y.; Yun, Y.-J.; Kim, D.-Y.; Han, S.-H. Langmuir 1999, 15, 4690. (18) Yan, C.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Langmuir 2000, 6208. (19) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Adv. Mater. 1999, 11, 605. (20) Mori, T.; Fujikawa, I.; Tokito, S.; Taga, Y. Appl. Phys. Lett. 1998, 73, 2763. (21) Rajagopal, A.; Kahn, A. J. Appl. Phys. 1998, 84, 355.

easily lend themselves to adjustable modification of the ITO surface. Organic or organometallic surface modification can be envisaged as a means to control the dipole moment at the ITO electrode surface by means of a bound overlayer. In this general context, changes in the work function of a gold26 or copper27 electrode by self-assembly of substituted organic thiols have been described. However, thiols may not be appropriate for modification of ITO in a practical context because of weak interactions with that mixed oxide,18 and the use of organic acids may be problematic because of the presence of mobile protons.28 We have described29 the reaction of surface hydroxyl groups of ITO with simple metal alkoxides as a means to create a reactive interface. Replacement of residual alkoxide ligation in the coordination sphere of the surfacebound complex is easily accomplished by ligand metathesis;30,31 we showed that such surface modification does not introduce an insulator between the ITO and a third material near the surface.30 Herein, we report that ITO surface-bound tin phenoxides can be readily synthesized through the sequence of surface deposition and ligand metathesis (Scheme 1). Because phenoxide ligands can be synthesized with a range of substitution patterns,32 this methodology allows for systematic variation of organic functionality and hence dipolar properties of the overlayer, which is covalently bound to the ITO surface via a tin complex interface. Experimental Section General. The UHV chamber33 is equipped with a Mattson Research Series FT-IR spectrometer, a quadrupole mass spectrometer (QMS), and an X-ray photoelectron spectrometer (XPS). Indium tin oxide on glass (Colorado Concept Coatings, 15 Ω/0, 1500 Å) was cleaned by sonication in detergent and then by rinsing first with water and then with hot trichloroethylene, acetone, and methanol. Treated slides were stored under vacuum, but samples prepared in this manner were not adequate for surface complex analysis in ultrahigh vacuum (UHV) because of organic carbon and oxygen surface contamination.29 Samples of ITO/ glass were cleaned, dried, and then Ar+ ion bombarded in UHV (22) Hill, I. G.; Rajagopal, A.; Hu, Y.; Kahn, A. Appl. Phys. Lett. 1998, 73, 662. (23) Lee, S. T.; Hou, X. Y.; Mason, M. G.; Tang, C. W. Appl. Phys. Lett. 1998, 72, 1593. (24) Hill, I. G.; Rajagopal, A.; Kahn, A. J. Appl. Phys. 1998, 84, 3236. (25) Sugiyama, K.; Ishii, H.; Ouchi, Y.; Seki, K. J. Appl. Phys. 2000, 87, 295. (26) Zehner, R. W.; Parsons, B. F.; Hsung, R. P.; Sita, L. R. Langmuir 1999, 15, 1121. (27) Campbell, I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Appl. Phys. Lett. 1997, 71, 3528. (28) Nu¨esch, F.; Forsythe, E. W.; Le, Q. T.; Gao, Y.; Rothberg, L. J. J. Appl. Phys. 2000, 87, 7973. (29) Purvis, K. L.; Lu, G.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc. 2000, 1808. (30) VanderKam, S. K.; Gawalt, E. S.; Schwartz, J.; Bocarsly, A. B. Langmuir 1999, 15, 6598. (31) VanderKam, S. K.; Lu, G.; Bernasek, S. L.; Schwartz, J. J. Am. Chem. Soc. 1997, 119, 11639. (32) Yonezawa, S.; Komurasaki, T.; Kawada, K.; Tsuri, T.; Fuji, M.; Kugimiya, A.; Haga, N.; Mitsumori, S.; Inagaki, M.; Nakatani, T.; Tamura, Y.; Takechi, S.; Taishi, T.; Ohtani, M. J. Org. Chem. 1998, 63, 5831. (33) Miller, J. B.; Bernasek, S. L.; Schwartz, J. Langmuir 1994, 10, 2629.

10.1021/la0013392 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/03/2001

Notes

Langmuir, Vol. 17, No. 3, 2001 949 Scheme 1. Synthesis of Surface Tin Phenoxides

to remove organic materials. Analysis by XPS showed the Sn content for the clean ITO surface to be ca. 6% that of the In, which served as an internal standard. A small In(3p) satellite peak was also observed at 686.4 eV with an intensity of ca. 7% of the In(3d5/2) signal. Water treatment in UHV introduced surface hydroxyl groups29 by 5 cycles of dose and desorption. XPS analysis following the fifth cycle showed approximately 8% of the total detectable surface oxygen to be OH (O[1s] binding energy [BE] ) 532.8 eV; Figure 1), a somewhat lower relative OH loading than was measured for a sample which had been subjected to 10 cycles of water dose/desorption.29 Tetra(tert-butoxy)tin (1) was used as received (Strem) and was stored under nitrogen. 4-Phenylphenol, 4′-hydroxy-4-biphenylcarbonitrile, 4-nitrophenol, and pentafluorophenol (Aldrich) were used as received or sublimed before use. 4-(4-Trifluoromethylphenyl)phenol (4b).34 A solution of 4-bromobenzotrifluoride (0.450 g, 2 mmol), DME (10 mL), n-propanol (2.5 mL), [p-[(tert-butyldimethylsilyl)oxy]phenyl]boronic acid32 (0.605 g, 2.4 mmol), 2 M aqueous Na2CO3 (3.9 mL), and tetrakis(triphenylphosphine)-palladium (0.116 g, 0.1 mmol)

Figure 1. O(1s) XPS spectrum following five cycles of H2O dose/desorption on Ar+ ion bombarded ITO.

was stirred and heated to 70 °C for 20 h. The disappearance of starting material was verified by GC-MS analysis. The reaction mixture was cooled to 0 °C, diluted with ethyl acetate (50 mL), acidified with 2 M aqueous HCl (75 mL), and filtered through Celite. Standard workup gave 4b, which was sublimed (2 × 10-4 Torr, 67 °C) to give the product as a white solid (0.265 g, 1 mmol; 56%). 1H NMR (300 MHz, CDCl3): δ 7.85-8.00 (m, 4 H), 7.74 (d, J ) 8.1, 2 H), 7.18 (d, J ) 8.1, 2 H). IR (KBr): 1607, 1595, 1502, 1325, 1243, 1131, 1072, 825 cm-1. 4-(4-Methoxyphenyl)phenol (4c).35 A similar procedure used 4-bromoanisole and involved stirring for 4 days. The product was sublimed (2 × 10-4 Torr, 90 °C) to give the title compound as a white solid (0.288 g, 1.4 mmol, 72%). 1H NMR (300 MHz, CDCl3): δ 7.45 (d, J ) 8.8, 2 H), 7.41 (d, J ) 8.8, 2 H), 6.93 (d, J ) 8.9, 2 H), 6.86 (d, J ) 8.4, 2 H), 3.83 (s, 3 H). IR (KBr): 3414, 2953, 2834, 1609, 1500, 1247, 1177, 1037, 816 cm-1. 4-(4-Nitrophenyl)phenol (4d).36 A similar procedure used 1-bromo-4-nitrobenzene and involved stirring for 30 h, but the reaction mixture was not filtered. Instead, impurities were removed by sublimation (2 × 10-4 Torr) at 100 °C, and the title compound was recovered by further sublimation at 137 °C. The sublimate was recrystallized (ethyl acetate/hexanes) to yield 4c as a yellow solid (0.290 g, 1.3 mmol, 67%). IR (KBr): 3423, 1593, 1507, 1334, 1269, 1192, 1111, 834 cm-1. 4-(4-Fluorophenyl)phenol (4e).37 A similar procedure used 1-bromo-4-fluorobenzene and involved stirring for 3 days. The product was sublimed (2 × 10-4 Torr, 75 °C) to give the title compound as a white solid (0.120 g, 0.6 mmol, 32%). IR (KBr): 3395, 1888, 1600, 1502, 1249, 817 cm-1. [ITO]-[O]3-Sn(OBut)/[ITO]-[O]2-Sn(OBut)2 (2/3). An ITOon-glass slide was exposed to vapor of 1 for two cycles, each consisting of 15 min of active vacuum followed by 30 min without active vacuum. The substrate was cooled externally by dry ice during the second cycle, and a film of 1 was visible on the sample surface. Excess 1 was removed by evacuation (10-3 Torr, 1 h). [ITO]-[O]3-Sn(O-Biphenyl)/[ITO]-[O]2-Sn(O-Biphenyl)2 (5a/6a). A sample of 2/3 was treated by three cycles of vapor exposure of p-phenylphenol (10-4 Torr) and evacuation as described above. For the second and third cycles, the p-phenylphenol reservoir was warmed (82 °C), and a film of the p-phenylphenol formed on the sample. Excess phenol was (34) Lourak, M.; Vanderesse, R.; Fort, Y.; Caube`re, P. J. Org. Chem. 1989, 54, 4844. (35) Capparelli, M. P.; DeSchepper, R. E.; Swenton, J. S. J. Org. Chem. 1987, 52, 4953. (36) Ou, S. H.; Percec, V.; Mann, J. A.; Lando, J. B.; Zhou, L.; Singer, K. D. Macromolecules 1993, 26, 7263. (37) Haga, N.; Takayanagi, H. J. Org. Chem. 1996, 61, 735.

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Figure 2. DRIFT spectra of surface-modified ITO on glass: (a) [ITO]-[O]3-Sn(O-biphenyl)/[ITO]-[O]2-Sn(O-biphenyl)2 (5a/6a) (lower trace) and a control (upper trace), (b) [ITO]-[O]3-Sn(O-biphenyl-CF3)/[ITO]-[O]2-Sn(O-biphenyl-CF3)2 (5b/6b), (c) [ITO][O]3-Sn(O-biphenyl-OMe)/[ITO]-[O]2-Sn(O-biphenyl-OMe)2 (5c/6c), (d) [ITO]-[O]3-Sn(O-biphenyl-NO2)/[ITO]-[O]2-Sn(O-biphenylNO2)2 (5d/6d), (e) ITO]-[O]3-Sn(O-biphenyl-F)/[ITO]-[O]2-Sn(-O-biphenyl-F)2 (5e/6e), (f) [ITO]-[O]3-Sn(O-biphenyl-CN)/[ITO]-[O]2Sn(O-biphenyl-CN)2 (5f/6f), (g) [ITO]-[O]3-Sn(O-pentafluorophenyl)/[ITO]-[O]2-Sn(O-pentafluorophenyl)2 (5g/6g), and (h) [ITO][O]3-Sn(O-4-nitrophenyl)/[ITO]-[O]2-Sn(O-4-nitrophenyl)2 (5h/6h). removed by evacuating (10-4 Torr) overnight at room temperature and then heating at 82 °C with continued evacuation. The sample was then analyzed by diffuse reflectance Fourier transform IR

spectroscopy (DRIFT) (Figure 2a). A control experiment, in which ITO was treated with phenol in the absence of 2/3, showed no residual phenol.

Notes

Langmuir, Vol. 17, No. 3, 2001 951 [ITO]-[O] 3 -Sn(O-Biphenyl-OMe)/[ITO]-[O] 2 -Sn(O-Biphenyl-OMe)2 (5c/6c). A similar procedure using 4-(4-methoxyphenyl)phenol was employed. The phenol reservoir was heated (90 °C) for the second and third exposure cycles, and evacuating (10-4 Torr) overnight at room temperature and then heating (90 °C) with evacuation removed excess phenol. It was then analyzed by DRIFT (Figure 2c). A control experiment showed no residual phenol. [ITO]-[O] 3 -Sn(O-Biphenyl-NO 2 )/[ITO]-[O] 2 -Sn(O-Biphenyl-NO2)2 (5d/6d). A similar procedure using 4-(4-nitrophenyl)phenol was employed. The phenol reservoir was heated to 140 °C during the second and third exposure cycles, and evacuating (10-4 Torr) overnight at room temperature and then heating (140 °C) with evacuation removed excess phenol. It was then analyzed by DRIFT (Figure 2d). A control experiment showed no residual phenol. [ITO]-[O]3-Sn(O-Biphenyl-F)/[ITO]-[O]2-Sn(-O-BiphenylF)2 (5e/6e). A similar procedure using 4-(4-fluorophenyl)phenol was employed, with four cycles of exposure. The first cycle was performed at room temperature; the second through fourth, with gradual heating of the phenol reservoir at 75 °C. The sample chamber was then evacuated overnight (10-4 Torr) first at room temperature and then at 75 °C to remove excess phenol. The sample was then analyzed by DRIFT (Figure 2e). A control experiment showed no residual phenol. [ITO]-[O]3-Sn(O-Biphenyl-CN)/[ITO]-[O]2-Sn(O-BiphenylCN)2 (5f/6f). A similar procedure using 4′-hydroxy-4-biphenylcarbonitrile was employed, with five cycles of exposure. The first cycle was done at room temperature; the second through fifth, with gradual heating of the phenol reservoir to 94 °C. The sample chamber was evacuated (10-4 Torr) overnight at room temperature and then briefly at 94 °C to remove excess phenol. The sample was then analyzed by DRIFT (Figure 2f). A control experiment showed no residual phenol. [ITO]-[O]3-Sn(O-Pentafluorophenyl)/[ITO]-[O]2-Sn(OPentafluorophenyl)2 (5g/6g). A similar procedure for the synthesis of 5a/6a using pentafluorophenol was employed. The first two cycles were done at room temperature, with external dry ice cooling during the third cycle. The sample chamber was evacuated overnight (10-4 Torr) at room temperature to remove excess phenol. The sample was then analyzed by DRIFT (Figure 2g). A control experiment showed no residual phenol. [ITO]-[O]3-Sn(O-4-Nitrophenyl)/[ITO]-[O]2-Sn(O-4-Nitrophenyl)2 (5h/6h). A similar procedure for synthesis of 5a/6a using 4-nitrophenol was employed. The first two cycles were done at room temperature, with external dry ice cooling during the third cycle. The sample chamber was evacuated overnight (10-4 Torr) at room temperature to remove excess phenol. The sample was then analyzed by DRIFT (Figure 2h). A control experiment showed no residual phenol.

Results and Discussion

Figure 3. XPS spectra for 5b/6b (In[3p] satellite peak at 686.4 eV): (a) F(1s), (b) C(1s), and (c) O(1s). [ITO]-[O]3-Sn(O-Biphenyl-CF3)/[ITO]-[O]2-Sn(O-BiphenylCF3)2 (5b/6b). A similar procedure using 4-(4-trifluoromethylphenyl)phenol was employed, and evacuating (10-4 Torr) overnight at room temperature and then heating (60 °C) in vacuo removed excess phenol. The sample was then analyzed by DRIFT (Figure 2b). A control experiment showed no residual phenol.

Reaction of tetra(tert-butoxy)tin (1) with hydroxylated ITO proceeded analogously to that reported for a more heavily hydroxylated substrate.29 Treating ITO with 1 at 180 K gave a multilayer on the substrate. The Sn(3d5/2) signal measured for the multilayer was due entirely to 1; no In(3d5/2) photoemission was observed. An experimental C/Sn ratio of sensitivity factors for subsequent XPS analysis of complex stoichiometry was set according to the measured C(1s) and Sn(3d5/2) peak intensities for this material of known composition. In contrast to reaction of 1 with the more heavily hydroxylated ITO (which yields primarily monoalkoxide 2 on heating to 380 K for 10 min29), desorbing the multilayer of 1 from the relatively less hydroxylated ITO and then heating to 380 K for 60 min gave a mixture of 2 and bisalkoxide species 3 (Table 1). The lower reactivity of the less hydroxylated ITO surface toward successive protolytic reactions between surface tin alkoxide complexes and surface OH groups on thermal treatment has been noted for analogues on both hydroxylated titanium and aluminum substrates.38,39 Treating 2/3 at 180 K with 140 L of 4b gave a multilayer of that phenol

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Notes

Table 1. X-ray Photoelectron Spectra for Multilayer and Monolayer Materials XPS temp (K) 180 380 180

350

complex observed multilayer Sn(OC4H9t)4, 1 [ITO]-[O]2-Sn(OBut)2, 3, and [ITO]-[O]3-Sn(OBut)1, 2 (5:1) multilayer (4b)

[ITO]-[O]2-Sn(OC6H4-C6H4-CF3)2, 6b, and [ITO]-[O]3-Sn(OC6H4-C6H4-CF3)1, 5b (5:1)

atomic species Sn(3d5/2)b C(1s) Sn(3d5/2)b

binding energy (eV)a 488.9 (488.7c) 285.5, 287.0 (285.3, 286.9c) 487.4

X:Sn 16d

284.7, 286.3 284.1 (Caromatic), 293.2 (CF3);e 12.9:1 (calculated 12:1)

7.5 ( 0.3

F(1s) O(1s) C(1s)

284.4, 289.3, 292.0

23.5 ( 0.3

F(1s) O(1s) Sn(3d5/2)b

689.4 533.7 487.2

5.7 ( 0.3

C(1s) C(1s)

a Calibrated against In(3d ). b Corrected for substrate inorganic Sn. c From ref 28. d Experimentally derived sensitivity factor ratios 5/2 based on this known stoichiometry. e This difference in binding energies is comparable to that reported for an aliphatic CF3 carbon versus benzene (9.9 eV). See: Gelius, U.; Heden, P. F.; Hedman, J.; Lindberg, B. J.; Manne, R.; Nordberg, R.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 2, 70-80.

(see Table 1). Thermal desorption of excess 4b was accomplished at 350 K over a period of 45 min and yielded the corresponding surface tin phenoxides 5b/6b in about the same ratio as was found for 2/3 (Figure 3 and Table 1). Because the stoichiometry of chemisorption of 1 on ITO (2:3 ) ca. 7:1) determines the composition of 5:6 (observed ratio ) ca. 7:1 based on C), it is reasonable to assume that the ratios of any tin phenoxides, 5/6, prepared using other members of a series of comparable phenols will be similar. A parallel series of experiments was performed at 10-4 Torr to obtain a series of surface-modified ITO electrodes. Samples of ITO-on-glass were cleaned and dried under conventional conditions and were placed in a horizontal tube which was connected via stopcocks to reservoirs of 1 and 4. This tube could be evacuated and either heated with external heating tape or cooled by externally applied dry ice. A multilayer of 1 was deposited on an ITO substrate at -78 °C (10-4 Torr). Excess 1 was removed in vacuo at room temperature to give 2/3. The sample was then exposed to vapor of a particular phenol (either with or without substrate cooling depending on the phenol volatility) to give a multilayer, which was then desorbed in vacuo (at room temperature or with heating, depending (38) Lu, G.; Purvis, K. L.; Schwartz, J.; Bernasek, S. Langmuir 1997, 13, 5791. (39) Purvis, K. L.; Lu, G.; Schwartz, J.; Bernasek, S. L. Langmuir 1999, 15, 7092. (40) Lu, G.; Schwartz, J.; Bernasek, S. L. Langmuir 1998, 14, 1532.

on the volatility of the particular phenol). In each case, ligand metathesis was followed by DRIFT spectroscopy, noting the loss in intensity of the signal at 2970 cm-1 (attributed to tert-butoxy groups29,40) and the introduction of bands indicative of phenoxy ligation (Figure 2). In control experiments, it was found that no IR intensity attributed to surface phenols could be observed in the absence of 2/3. Conclusions The simple expedient of surface tin complex synthesis enables strong bonding of a variety of substituted phenols (as surface tin phenoxides) to the ITO surface. Ultraviolet photoelectron spectroscopy measurements are in progress to determine work function changes imparted to ITO as a result of our surface modifications, and OLEDs are also being constructed in UHV using these materials for device characteristic determinations. Acknowledgment. The authors acknowledge support for this research given by the National Science Foundation (CHE-9901224) and the New Jersey Center for Optoelectronic Materials. Supporting Information Available: The C(1s) XPS spectrum of the multilayer of 4-(4-trifluoromethylphenyl)phenol on ITO and the Sn(3d5/2) XPS spectrum of [ITO]-[O]3-Sn(Obiphenyl-CF3)/[ITO]-[O]2-Sn(O-biphenyl-CF3)2. This material is available free of charge via the Internet at http://pubs.acs.org. LA0013392