Liquid Crystals at Nanopatterned Surfaces. AFM Contact Nanolithography AFM used as a probe of the...
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Transcript of Liquid Crystals at Nanopatterned Surfaces. AFM Contact Nanolithography AFM used as a probe of the...
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)