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Langmuir 2007, 23, 6498-6500
Control of Alq3 Wetting Layer Thickness via Substrate Surface Functionalization Shufen Tsoi,*,† Bryan Szeto,†,§ Michael D. Fleischauer,† Jonathan G. C. Veinot,‡ and Michael J. Brett†,§ Department of Electrical and Computer Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2V4 Canada, Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta T6G 2G2 Canada, and National Institute for Nanotechnology, 11421 Saskatchewan DriVe, Edmonton, Alberta T6G 2M9 Canada ReceiVed March 12, 2007. In Final Form: April 18, 2007 The effects of substrate surface energy and vapor deposition rate on the initial growth of porous columnar tris(8-hydroxyquinoline)aluminum (Alq3) nanostructures were investigated. Alq3 nanostructures thermally evaporated onto as-supplied Si substrates bearing an oxide were observed to form a solid wetting layer, likely caused by an interfacial energy mismatch between the substrate and Alq3. Wetting layer thickness control is important for potential optoelectronic applications. A dramatic decrease in wetting layer thickness was achieved by depositing Alq3 onto alkyltrichlorosilane-derivatized Si/oxide substrates. Similar effects were noted with increasing deposition rates. These two effects enable tailoring of the wetting layer thickness.
Controlled-morphology thin films have recently been the focus of fundamental and technological investigation as a result of their unique optical, chemical, and mechanical properties. Porous inorganic and organic nanostructured thin films fabricated using the glancing angle deposition (GLAD) technique1,2 are of particular interest because of the wide range of possible morphologies and material choices. GLAD combines limited surface diffusion, self-shadowing, and advanced substrate motion to produce porous isolated columnar nanostructures. Separate control of the incident vapor angle and the rotational speed allows for the fabrication of numerous thin film morphologies, including vertical pillars, helices, square spirals, and graded-density columns.1,3,4 Porous inorganic structures prepared in this manner are well-suited for many different applications, including threedimensional photonic band gap crystals,3 humidity sensors,5 and optoelectronic devices.6,7 Recently, Hrudey et al.8 reported a direct deposition of porous chiral organic tris(8-hydroxyquinoline)aluminum (Alq3) nanostructures at glancing angles using thermal evaporation. Alq3 is commonly used as an electron transport layer and for emission in organic light-emitting diodes (OLEDs). The GLAD-fabricated Alq3 nanostructures show a self-ordered periodicity and do not broaden or bifurcate,8 unlike their inorganic counterparts.9-11 A solid wetting layer was also observed to form below the Alq3 * To whom correspondence should be addressed. Tel: 1-780-4927926. Fax: 1-780-492- 2863. E-mail:
[email protected]. † Department of Electrical and Computer Engineering, University of Alberta. ‡ Department of Chemistry, University of Alberta. § National Institute for Nanotechnology. (1) Robbie, K.; Sit, J. C.; Brett, M. J. J. Vac. Sci. Technol., B 1998, 16, 1115. (2) Messier, R.; Venugopal, V. C.; Sunal, P. D. J. Vac. Sci. Technol., A 2000, 18, 1538. (3) Jensen, M. O.; Brett, M. J. Opt. Express 2005, 13, 3348. (4) Robbie, K.; Cui, Y.; Elliott, C.; Kaminska, K. Appl. Opt. 2006, 45, 8298. (5) Steele, J. J.; Gospodyn, J.; Sit, J. C.; Brett, M. J. IEEE Sens. 2006, 6, 24. (6) Xu, J.; Lakhtakia, A.; Liou, J.; Chen, A.; Hodgkinson, I. J. Opt. Commun. 2006, 264, 235. (7) Wang, S.; Fu, X.; Xia, G.; Wang, J.; Shao, J.; Fan, Z. Appl. Surf. Sci. 2006, 252, 8734. (8) Hrudey, P. C. P.; Westra, K. L.; Brett, M. J. AdV. Mater. 2006, 18, 224. (9) Malac, M.; Egerton, R. F. J. Vac. Sci. Technol., A 2001, 19, 158. (10) Vick, D.; Smy, T.; Dick, B.; Kennedy, S.; Brett, M. J. Materials Research Society Symposium Proceedings; Materials Research Society: Boston, MA, 2001; Vol. 648. (11) Kesapragada, S. V.; Gall, D. Appl. Phys. Lett. 2006, 89, 203121.
structures whose thickness varied with deposition angle.12 It is very important to note that this wetting layer has never been observed for inorganic films produced using the GLAD technique. Many optoelectronic devices such as OLEDs and organic solar cells require thin layers (on the order of tens of nanometers)13 or nanostructures to be in a specific pattern, that is, depositing structures onto seeded substrates.14,15 A better understanding of the wetting layer formation and control over its thickness is critical to the realization of optoelectronic devices utilizing GLAD-engineered Alq3 nanostructures. In the present contribution, we investigate the wetting layer formation of thermally evaporated Alq3 nanostructures by examining the effects of substrate wettability and deposition rate on wetting layer thickness. It has been shown that the combination of straightforward siloxane-based surface functionalization and SiO2-GLAD films allows for the exploitation of the synergistic effects of the film morphology and surface chemistry and provides hydrophobic and superhydrophobic surfaces.16 Surface modification of OLED anode surface chemistry has also been shown to increase OLED thermal stability via moderation of the surface energy mismatch of hydrophilic ITO and hydrophobic hole transport materials.17,18 Here, we demonstrate that similar siloxane-based surface functionalization in conjunction with deposition rate can be used to tailor Alq3 wetting layer thickness. Silicon (100) substrates passivated with thermal oxide (Evergreen Semiconductor Materials) for functionalization were cleaned by submerging them into piranha solution (3:1 vol/vol H2SO4/H2O2) for 15 min, subsequently rinsed with deionized water, and dried using compressed N2. Water vapor adsorption was limited by baking the substrates at 130 °C in a vacuum of (12) Szeto, B.; Hrudey, P. C. P.; Taschuk, M.; Brett, M. J. In Liquid Crystal Materials, DeVices, and Applications XI, Proceedings of SPIE; Chien, L.-C., Ed.; SPIE: Bellingham, WA, 2006; Vol. 6135. (13) Song, Q. L.; Wang, M. L.; Obbard, E. G.; Sun, X. Y.; Ding, X. M.; Hou, X. Y.; Li, C. M. Appl. Phys. Lett. 2006, 89, 251118. (14) Jensen, M. O.; Brett, M. J. IEEE Trans. Nanotech. 2005, 4, 269. (15) Kennedy, S. R.; Brett, M. J.; Toader, O.; John, S. Nano Lett. 2002, 2, 59. (16) Tsoi, S.; Fok, E.; Sit, J. C.; Veinot, J. G. C. Langmuir 2004, 20, 10771. (17) Malinsky, J. E.; Veinot, J. G. C.; Jabbour, G. E.; Shaheen, S. E.; Anderson, J. D.; Lee, P.; Richter, A. G.; Burin, A. L.; Ratner, M. A.; Marks, T. J.; Armstrong, N. R.; Kippelen, B.; Dutta, P.; Peyghambarian, N. Chem. Mater. 2002, 14, 3054. (18) Veinot, J. G. C.; Marks, T. J. Acc. Chem. Res. 2005, 38, 632.
10.1021/la700722b CCC: $37.00 © 2007 American Chemical Society Published on Web 05/05/2007
Letters
Langmuir, Vol. 23, No. 12, 2007 6499
Table 1. Advancing Contact Angle (θ) and rms Surface Roughness (R) of As-Supplied, RIE-Treated, and Alkyltrichlorosilane-Derivatized Si Substrates and the Corresponding Wetting Layer Thickness (WLT) of Thermally Evaporated Alq3 Thin Films WLT (nm, ( 8 nm) substrate surface treatments θ (deg.)a R (nm) Si RIE-treated and baked Si TFP-SiCl3 hexyl-SiCl3 propyl-SiCl3 OTS
20 48 90 93 99 101
0.2 1.0 0.8 1.1 0.2
4 Å/s
1 Å/s
147 126 59 50 53 44
300 283 182 176 165 100
a All contact angles reported are an average of three individual measurements taken at different locations of the same substrate.
11.18 kPa. Following the cleaning protocol, all substrates were treated with oxygen plasma reactive ion etching (RIE) using a Trion Phantom II RIE prior to surface functionalization. This technique is known to saturate the Si surface with hydroxyl moieties and hence maximizes reactivity between the substrates and the trichlorosilane reagents.16,17 All cleaned Si substrates were treated with O2 plasma for 90 s in a pre-cleaned chamber. Following standard Schlenk line protocol, all RIE-treated substrates were immersed in a dry, deoxygenated 20 mM toluene solution of various alkyltrichlorosilanes supplied by Aldrich Chemical. The substrates submerged in propyltrichlorosilane (propyl-SiCl3), hexyltrichlorosilane (hexyl-SiCl3), octadecyltrichlorosilane (OTS), and (3,3,3-trifluoropropyl)trichlorosilane (TFP-SiCl3) solutions were allowed to react for 2 h under argon atmosphere. The alkyltrichlorosilane reaction solution was removed, and the substrates were rinsed three times with dry toluene and finally acetone (ca. 0.5% H2O, Fisher Scientific) to hydrolyze any residual Si-Cl functionalities. Substrates were finally annealed in air for 12 h at 150 °C. Alq3 (Gelest, Inc., >99%) was deposited via thermal evaporation at a base pressure of 1500 °C) led to increases in the substrate temperature of ∼100 K after 90 min of deposition.23 It is expected that Alq3 deposition at much lower source temperatures (∼300 °C) will exhibit correspondingly lower substrate temperature increases. At a higher deposition rate, adsorbed molecules and impinging molecules interact more frequently; self-shadowing becomes more dominant, and wetting layer thickness decreases. For both deposition rates, the manner in which the wetting layer thickness varies with substrate wettability is very similar (Figure 2). In general, the wetting layer thickness decreases for substrates with higher advancing contact angle measurements and lower surface roughness. The minimum wetting layer thickness was achieved using OTS-derivatized substrates, as they exhibit the greatest advancing contact angle and lowest rms roughness. For simplicity, all films described here were deposited at a flux incidence angle of 76°, although all angles between 0° and ∼89° are possible. Higher deposition angle has been shown to lead to reduced wetting layer thickness for a given substrate treatment and deposition rate.12 In summary, we have demonstrated that surface wettability and deposition rate can be used to tailor the wetting layer thickness of Alq3 nanostructures deposited using the GLAD technique. The wetting layer thickness was noted to decrease significantly with increasing surface hydrophobicity and deposition rates. In both cases, surface diffusion is limited and the self-shadowing effect dominates, allowing the formation of GLAD nanostructures at an earlier stage. Varying surface wettability in conjunction with deposition rate offers the ability to control the wetting layer thickness, which is critical for the realization of optoelectronic devices. Acknowledgment. The authors acknowledge G. D. Braybrook for his excellent SEM imaging and Luc Gervais for AFM analysis. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the University of Alberta Nanofab, the Informatics Circle of Research Excellence (iCORE), and Micralyne, Inc. M.F. acknowledges NSERC, Alberta Ingenuity, and the Killam Trusts for fellowship support. LA700722B (23) Seto, M. W. M. Mechanical Response of Microspring Thin Films. Ph.D. Thesis, University of Alberta, Edmonton, AB, Canada, 2004.
