Liquid Crystals at Nanopatterned Surfaces

AFM Contact Nanolithography AFM used as a probe of the surface topography

The cantilever is very stiff Tip diameter < 20 nm (silicon) Use “contact mode” for scribing surface Example of “hard” writing on a polyimide. Instead, use AFM to “scratch” substrate

Uses of AFM writing: Write azimuthal alignment patterns on nanoscopic scales Create a patterned topography Create a uniform anchoring potential on length scales shorter than the correlation length  for the relevant phase

AFM Dip-Pen Nanolithography Tip moves into the screen Reservoir of surfactant molecules Molecules deposited via water meniscus as tip moves into and out of screen Area of deposited region depends upon diffusion of molecules from meniscus

Cesium perfluorodecanoate lines width ~ 150 nm period ~ 1  m substrate: ITO-coated glass purpose: produce localized homeotropic alignment

Near field scanning optical microscopy (NSOM) Collection optics (w/analyzer) Thin optical fiber (tip < 50 nm diameter) PMT Computer Polarizer x-y translation

Defects in thin layer of 5CB. (Smallest structure ~ 150 nm) 7OCB in crystalline phase Thin layer of 5CB in nematic phase with imposed herringbone director orientation. Period ~ 1  m 5  m 2.5  m

Multi-domain Alignment 200  m period Tradeoff among: Elastic Anchoring and Nematic Order Parameter (Landau) energies

Gratings  No zero order diffraction peak  Only odd order diffraction peaks a Blazed grating Polarization-independent switchable blazed grating

Controlling Pretilt Nissan Chemicals SE-1211 designed for homeotropic alignment. Overbake and fill cell with 5CB

Many uses: Create large pretilt (from homeotropic direction) in nematic phase Surface-induced polarization for chiral nematic tilted at substrate [Phys. Rev. E 67, (2003)]. Create patterned polar tilt in nematic phase. Create “ultra-soft” polar anchoring for continuous pretilt alignment. Phys. Rev. E 67, (2003)] Create subpixels with different anchoring strengths  step-wise Fréedericksz transition.

Conclusions Scanning probe lithography using the stylus of an atomic force microscope facilitates new scientific investigations and technological advances Serves as an excellent prototyping method; scaling up via UV photoalignment or standard lithography Involving surface modification by:  Near field scanning optical microscope (NSOM)  Scanning tunneling microscope (STM)