Measurement of the Azimuthal Anchoring Energy of Liquid Crystals in Contact with Oligo(ethylene glycol)-Terminated Self-Assembled Monolayers Supported on Obliquely Deposited Gold Films Brian H. Clare, Orlando Guzma ´n, Juan J. de Pablo, and Nicholas L. Abbott* Department of Chemical and Biological Engineering, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706 ReceiVed December 29, 2005. In Final Form: March 8, 2006 We report measurements of the orientations and azimuthal anchoring energies of the nematic liquid crystal 4-cyano- 4′-pentylbiphenyl (5CB) on polycrystalline gold films that are deposited from a vapor at an oblique angle of incidence and subsequently decorated with organized monolayers of oligomers of ethylene glycol. Whereas the gold films covered with monolayers presenting tetra(ethylene glycol) (EG4) lead to orientations of 5CB that are perpendicular to the plane of incidence of the gold, monolayers presenting tri(ethylene glycol) (EG3) direct 5CB to orient parallel to the plane of incidence of the gold during deposition of the gold film. We also measure the azimuthal anchoring energy of the 5CB to be smaller on the surfaces presenting EG3 (3.2 ( 0.8 µJ/m 2 ) as compared to EG4 (5.5 ( 0.9 µJ/m 2 ). These measurements, when combined with other results presented in this paper, are consistent with a physical model in which the orientation and anchoring energies of LCs on these surfaces are influenced by both (i) short-range interactions of 5CB with organized oligomers of ethylene glycol at these surfaces and (ii) long-range interactions of 5CB with the nanometer-scale topography of the obliquely deposited films. For surfaces presenting EG3, these short- and long-range interactions oppose each other, leading to small net values of anchoring energies that we predict are dependent on the level of order in the EG3 SAM. These measurements provide insights into the balance of interactions that control the orientational response of LCs to biological species (proteins, viruses, cells) on these surfaces. Introduction Nematic liquid crystals (LCs) are materials that can exhibit orientational order over distances that are much greater (mi- crometer) than the sizes of their molecular components (na- nometer). 1 Near the surface of a structured solid, a LC will typically assume a preferred average orientation due to interactions with the solid. 2,3 The orientation of the director of the LC near such an interface, in the absence of an additional external field, is defined as the easy axis, η 0 . A variety of approaches have been investigated to prepare structured solid surfaces that lead to changes in the orientation of η 0 , including the mechanical shearing of polymers and the use of microfabricated surfaces with micrometer- and nanometer-scale topography. 2,3 An equally important aspect of the phenomenon of the anchoring of LCs at surfaces is the energy of interaction between the substrate and LC. The energy of interaction leading to a particular azimuthal orientation can be characterized by the so-called azimuthal anchoring energy, defined as where τ is the magnitude of torque applied to the LC at the surface (surface anchoring torque) that leads to a departure of the azimuthal orientation of the director of the LC from the easy axis by an angle of . Knowledge of W az permits prediction of the responses of LCs to external perturbations (e.g., electrical or magnetic fields) and also provides fundamental insights into the nature of the interactions between the LCs and surfaces that define the preferred orientations of LCs. In this paper, we report measurements of the azimuthal anchoring energy of the nematic LC 4-cyano-4′-pentylbiphenyl (5CB) with structured interfaces that present oligomers of ethylene glycol. A number of past studies have reported measurements of the azimuthal anchoring energy of mechanically sheared, polymeric surfaces. 4-7 It is now understood that mechanical shearing of polymeric surfaces simultaneously introduces two elements of surface structure that can influence the anchoring energy of a LC: (1) micro- and nanogrooves (topography) 8 and (2) preferred alignments of the polymer chains in the near-surface region of the substrate. 9,10 Several groups have isolated the contributions that surface topography can make to azimuthal anchoring energies by studying LCs in contact with substrates having feature sizes that can be systematically controlled (e.g. surface gratings 11,12 or periodic microrelief structures prepared by the deformation of hard coatings supported on soft polymeric substrates 13 ). Distortion of the director of the LC over the topography of the surface can create an “elastic contribution” to the anchoring energy. 8 In addition, the influence of the molecular-level organization of substrate molecules on azimuthal anchoring energies of LCs have been studied independently of topography * To whom correspondence should be addressed. Phone: (608) 265- 5278. Fax: (608) 262-5434. E-mail: abbott@engr.wisc.edu. (1) de Gennes, P. G. The Physics of Liquid Crystals, 1st ed.; Oxford University Press: London, 1974. (2) Cognard, J. Mol. Cryst. Liq. Cryst. Suppl. 1982, 78, 1. (3) Jerome, B. Rep. Prog. Phys. 1991, 54, 391. (4) Sato, Y.; Sato, K.; Uchida, T. Jpn. J. Appl. Phys. 2 1992, 31, L579. (5) Lee, E. S.; Vetter, P.; Miyashita, T.; Uchida, T. Jpn. J. Appl. Phys. 2 1993, 32, L1339. (6) Ban, B. S.; Kim, Y. B. J. Phys. Chem. B 1999, 103, 3869. (7) Oka, S.; Mitsumoto, T.; Kimura, M.; Akahane, T. Phys. ReV.E 2004, 69, 061711. (8) Berreman, D. W. Phys. ReV. Lett. 1972, 28, 1683. (9) Toney, M. F.; Russell, T. P.; Logan, J. A.; Kikuchi, H.; Sands, J. M.; Kumar, S. K. Nature 1995, 374, 709. (10) Stohr, J.; Samant, M. G.; Luning, J.; Callegari, A. C.; Chaudhari, P.; Doyle, J. P.; Lacey, J. A.; Lien, S. A.; Purushothaman, S.; Speidell, J. L. Science 2001, 292, 2299. (11) Newsome, C. J.; O’Neill, M.; Farley, R. J.; Bryan-Brown, G. P. Appl. Phys. Lett. 1998, 72, 2078. (12) Wood, E. L.; Bradberry, G. W.; Cann, P. S.; Sambles, J. R. J. Appl. Phys. 1997, 82, 2483. (13) Belyaev, V.; Misnik, V.; Trofimov, S.; Volynsky, A.; Konovalov, V.; Muravski, A. Appl. Phys. Lett. 2005, 86, 011904. τ ) W az sin 2/2 (1) 4654 Langmuir 2006, 22, 4654-4659 10.1021/la0535126 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/15/2006