Condensed Matter seminar: "Inkjet-Printed Nanoparticle Alignment Layers – Patterned Alignment for Liquid Crystal Devices"

Wed, 11/06/2013 - 16:00 - 17:00
Torsten Hegmann, Kent State University

Liquid crystal (LC) alignment layers commonly feature some type of anisotropy that induces a preferred orientation of the LC director on the surface. Unidirectionally rubbed polyimides are the most widely used alignment layers [1]. Photoalignment and obliquely evaporated inorganic materials, among others, are alternative techniques that are utilized as well [2]. Although these processes are robust and have been implemented in large-scale production environments, they require several fabrication steps requiring high processing temperatures and often high-vacuum environment. In addition, many LC applications necessitate patterned alignment of LCs to provide spatial modulation of the optical axis, for example, for wave front control applications.  To obtain patterned alignment, complex and costly photolithography techniques must be used.  With the use of photoalignment, the process can be significantly simplified, but still requires design and fabrication of photomasks as well as the deposition of a photosensitive polymer layer using spin coating, baking and UV irradiation.

Our group has recently demonstrated that doping nematic LCs with a small quantity of thiolate capped gold nanoparticles (NPs) can lead to the induction of homeotropic alignment [3]. A similar effect is achieved if NPs are deposited onto the surface before filling of the test cell with the LC material. This leads to a uniform coverage of the surface with the NPs and, in turn, uniform vertical alignment of the LC over the entire area. The homeotropic anchoring of the LC molecules on the NPs is accompanied by a contrast inversion effect, i.e. under the action of a low-frequency electric field, Δε > 0 LCs effectively acts as Δε < 0 LCs and undergo a transition from the homeotropic to the homogenous state.

To demonstrate the next level of control over NP-induced LC alignment, we established the use ink-jet printing of NPs to obtain full coverage as well as patterned alignment of nematic LCs as a quick, simple, highly versatile, and adaptable LC alignment technique. Striking features of the ink-jet printing process are the variability of the NPs and substrates, the simplicity of pattern creation, and the scalability of the process [4].

[1] (a) J. van Haaren, Nature 411, 29-30 (2001); (b) J. Hoogboom, T. Rasing, A.E. Rowan, R.J.M. Nolte, J. Mater. Chem. 16,
1305-1314 (2006).
[2] O. Yaroshchuk, Y. Reznikov, J. Mater. Chem. 22, 286-300 (2012).
[3] (a) H. Qi, B. Kinkead, T. Hegmann, Adv. Funct. Mater. 18, 212-221 (2008); (b) H. Qi, T. Hegmann, ACS Appl. Mater.
Interf. 1, 1731-1738 (2009); (c) M. Urbanski, B. Kinkead, H. Qi, T. Hegmann, H.-S. Kitzerow, Nanoscale 2, 1118-1121
(2010).
[4] M. Reznikov, A. Sharma, T. Hegmann, Part. Part. Syst. Charact. doi: 10.1002/ppsc.201300248 (2013)

Room A4, DRL