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

5 m

2.5 m

Multi-domain Alignment

AFM scribe

Rubbing direction(easy axis)

200 m period

x

y

z

(x)

AFM-scribed AFM-scribedCloth rubbing

direction

(x)

Tradeoff among:

• Elastic

• Anchoring

• and Nematic Order Parameter (Landau) energies

Gratings

-0.2 -0.1 0.0 0.1 0.20.0

0.2

0.4

0.6

0.8

1.0

0 th order

1st order

Inte

ns

ity

(a

.u.)

Angle (rad)

No zero order diffraction peak Only odd order diffraction peaks

a

Blazed grating

-5 -4 -3 -2 -1 0 1 2 3 40

10

20

30

40

50

60

70

80

90

Inte

nsity

(A

rb.)

Angle (degree)

Grating period: 33.3m=0.6328m

First peak = 1.090

Polarization-independent switchable blazed grating

Controlling Pretilt

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

0.5 1.0 1.5 2.0 2.5 3.0 3.5 x 100

5

10

15

20

25

30

35

40

6

25 oC

26 oC

28 oC

30 oC

32 oC

34 oC

Pre

tilt

Ang

le

(de

g)

Rubbing Strength (cm-1)

Many uses:

• Create large pretilt (from homeotropic direction) in nematic phaseSurface-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.

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)