Liquid Crystals at Nanopatterned Surfaces

1
Liquid Crystals at Nanopatterned Surfaces
2
AFM Contact Nanolithography
AFM used as a probe of the surface topography
3
Instead, use AFM to scratch substrate

• The cantilever is very stiff
• Tip diameter lt 20 nm (silicon)
• Use contact mode for scribing surface

Example of hard writing on a polyimide.
4

• 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 x for
  the relevant phase

5
AFM Dip-Pen Nanolithography

• 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

Tip moves into the screen
6
Cesium perfluorodecanoate lines width 150
nm period 1 mm substrate ITO-coated
glass purpose produce localized
homeotropic alignment
7
Near field scanning optical microscopy (NSOM)
Polarizer
Thin optical fiber (tip lt 50 nm diameter)
x-y translation
Collection optics (w/analyzer)
PMT
Computer
8
Defects in thin layer of 5CB. (Smallest structure
150 nm)
5 mm
5 mm
7OCB in crystalline phase
Thin layer of 5CB in nematic phase with imposed
herringbone director orientation. Period 1 mm
2.5 mm
9
Multi-domain Alignment
200 mm period

• Tradeoff among
• Elastic
• Anchoring
• and Nematic Order Parameter (Landau) energies

10
Gratings

• No zero order diffraction peak
• Only odd order diffraction peaks

Polarization-independent switchable blazed grating
Blazed grating
a
11
Controlling Pretilt

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

12

• Many uses
• Create large pretilt (from homeotropic direction)
  in nematic phase
• Surface-induced polarization for chiral nematic
  tilted at substrate Phys. Rev. E 67, 041707
  (2003).
• Create patterned polar tilt in nematic phase.
• Create ultra-soft polar anchoring for
  continuous pretilt alignment.
• Phys. Rev. E 67, 041706 (2003)
• Create subpixels with different anchoring
  strengths ? step-wise Fréedericksz transition.

13
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)