U.S. patent application number 16/164660 was filed with the patent office on 2020-12-31 for tuning the polar anchoring strength by doping graphene flakes and resulting accelerated electro-optic switching in liquid crystal devices.
This patent application is currently assigned to The Government of the United States of America, as represented by the Secretary of the Navy. The applicant listed for this patent is The Government of the United States of America, as represented by the Secretary of the Navy, The Government of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Rajratan Basu.
Application Number | 20200409194 16/164660 |
Document ID | / |
Family ID | 1000005272700 |
Filed Date | 2020-12-31 |
United States Patent
Application |
20200409194 |
Kind Code |
A9 |
Basu; Rajratan |
December 31, 2020 |
Tuning the polar anchoring strength by doping graphene flakes and
resulting accelerated electro-optic switching in liquid crystal
devices
Abstract
A method of achieving higher polar anchoring strength of liquid
crystal (LC) using monolayer graphene flakes in an LC device and
attaining faster electro-optic switching in an LC device comprising
the steps of providing graphene in an ethanol solvent, adding a
liquid crystal to the graphene and ethanol solution, forming a
liquid crystal graphene ethanol solution, evaporating the ethanol,
and forming a pure liquid crystal graphene mixture. A liquid
crystal device with faster electro-optic switching and higher polar
anchoring strength comprising an LC cell having a polyimide (PI)
alignment layer, the liquid crystal graphene mixture, wherein the
graphene flakes preferentially attach to the PI alignment layer;
wherein the effective polar anchoring energy in the LC cell is
enhanced by an order of magnitude and wherein the electro-optic
response of the LC is accelerated.
Inventors: |
Basu; Rajratan; (Annapolis,
MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Government of the United States of America, as represented by
the Secretary of the Navy |
Arlington |
VA |
US |
|
|
Assignee: |
The Government of the United States
of America, as represented by the Secretary of the Navy
Arlington
VA
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20190384083 A1 |
December 19, 2019 |
|
|
Family ID: |
1000005272700 |
Appl. No.: |
16/164660 |
Filed: |
October 18, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62685409 |
Jun 15, 2018 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F 2001/133776
20130101; G02F 1/133711 20130101; G02F 1/13306 20130101; G02F
1/133703 20130101; G02F 1/0045 20130101; C01B 32/184 20170801 |
International
Class: |
G02F 1/133 20060101
G02F001/133; C01B 32/184 20060101 C01B032/184; G02F 1/1337 20060101
G02F001/1337; G02F 1/00 20060101 G02F001/00 |
Claims
1. A method of achieving higher polar anchoring strength of liquid
crystal (LC) using monolayer graphene flakes in an LC device and
attaining faster electro-optic switching in an LC device comprising
the steps of: providing a graphene sample in an ethanol solvent;
wherein the graphene sample comprises monolayer flakes; mixing the
graphene sample and the ethanol solvent and forming a graphene and
ethanol solution; adding a liquid crystal to the graphene and
ethanol solution; allowing the liquid crystal to dissolve into the
graphene and ethanol solution; forming a liquid crystal graphene
ethanol solution; evaporating the ethanol from the liquid crystal
graphene ethanol solution; forming a liquid crystal graphene
mixture; degassing the liquid crystal graphene mixture; and forming
a pure liquid crystal graphene mixture.
2. The method of achieving higher polar anchoring strength of
liquid crystal (LC) using monolayer graphene flakes in an LC device
and attaining faster electro-optic switching in an LC device of
claim 1 wherein the liquid crystal comprises liquid crystal E7
comprising nematic-isotropic phase transition temperature
(T.sub.NI) of 60.5.degree. C.
3. The method of achieving higher polar anchoring strength of
liquid crystal (LC) using monolayer graphene flakes in an LC device
and attaining faster electro-optic switching in an LC device of
claim 2 wherein the monolayer flakes have an average thickness of
0.35 nm and an average lateral size of 550 nm and wherein the
liquid crystal graphene mixture is liquid crystal E7 and pristine
graphene mixture.
4. The method of achieving higher polar anchoring strength of
liquid crystal (LC) using monolayer graphene flakes in an LC device
and attaining faster electro-optic switching in an LC device of
claim 3 wherein the graphene concentration in the liquid crystal E7
is 2.5.times.10.sup.-3 wt %.
5. The method of achieving higher polar anchoring strength of
liquid crystal (LC) using monolayer graphene flakes in an LC device
and attaining faster electro-optic switching in an LC device of
claim 3 wherein the step of degassing the liquid crystal graphene
mixture occurs under vacuum.
6. The method of achieving higher polar anchoring strength of
liquid crystal (LC) using monolayer graphene flakes in an LC device
and attaining faster electro-optic switching in an LC device of
claim 3 wherein the graphene sample comprises more than 97%
monolayer flakes.
7. The method of achieving higher polar anchoring strength of
liquid crystal (LC) using monolayer graphene flakes in an LC device
and attaining faster electro-optic switching in an LC device of
claim 2 further including the steps of: filling an LC cell having
polyimide (PI) planar-alignment layers with the liquid crystal
graphene mixture; wherein the graphene flakes sediment down from
the liquid crystal graphene mixture and preferentially attach to
the PI alignment layers during the filling step of the LC cell; and
wherein the graphene modifies the anchoring property of the LC.
8. The method of achieving higher polar anchoring strength of
liquid crystal (LC) using monolayer graphene flakes in an LC device
and attaining faster electro-optic switching in an LC device of
claim 7 further including the steps of: enhancing the effective
polar anchoring energy in the LC cell by an order of magnitude via
the .pi.-.pi. electron stacking between the graphene flakes on the
PI planar-alignment layers and LC molecules; and accelerating the
electro-optic response of the LC due to higher anchoring energy in
the LC cell.
9. A Liquid Crystal device with faster electro-optic switching and
higher polar anchoring strength using monolayer graphene flakes
made from the steps of: providing a graphene sample in an ethanol
solvent; wherein the graphene sample comprises monolayer flakes;
mixing the graphene sample and the ethanol solvent and forming a
graphene and ethanol solution; adding a liquid crystal to the
graphene and ethanol solution; allowing the liquid crystal to
dissolve into the graphene and ethanol solution; forming a liquid
crystal graphene ethanol solution; evaporating the ethanol from the
liquid crystal graphene ethanol solution; forming a liquid crystal
graphene mixture; degassing the liquid crystal graphene mixture;
and forming a pure liquid crystal graphene mixture.
10. The Liquid Crystal device with faster electro-optic switching
and higher polar anchoring strength using monolayer graphene flakes
of claim 9 wherein the liquid crystal comprises liquid crystal E7
with a nematic-isotropic phase transition temperature (T.sub.NI) of
60.5.degree. C.
11. The Liquid Crystal device with faster electro-optic switching
and higher polar anchoring strength using monolayer graphene flakes
of claim 10 wherein the monolayer flakes have an average thickness
of 0.35 nm and an average lateral size of 550 nm and wherein the
liquid crystal graphene mixture is liquid crystal E7 and pristine
graphene mixture.
12. A Liquid Crystal device with faster electro-optic switching and
higher polar anchoring strength comprising: a LC cell having a
polyimide (PI) alignment layer; a liquid crystal graphene mixture
in the LC cell; wherein the graphene flakes sediment down from the
liquid crystal graphene mixture and preferentially attach to the PI
alignment layer during filling the LC cell; wherein the graphene
modifies the anchoring property of the LC; wherein the effective
polar anchoring energy in the LC cell is enhanced by an order of
magnitude via the .pi.-.pi. electron stacking between the graphene
flakes on the PI alignment layer and LC molecules; and wherein the
electro-optic response of the LC is accelerated due to the higher
anchoring energy in the cell.
13. The Liquid Crystal device with faster electro-optic switching
and higher polar anchoring strength of claim 12 wherein the liquid
crystal comprises liquid crystal E7 and wherein the liquid crystal
E7 has a nematic-isotropic phase transition temperature (T.sub.NI)
of 60.5.degree. C.
14. The Liquid Crystal device with faster electro-optic switching
and higher polar anchoring strength of claim 13 wherein the
monolayer graphene flakes have an average thickness of 0.35 nm and
an average lateral size of 550 nm; and wherein the liquid crystal
graphene mixture is liquid crystal E7 and pristine graphene
mixture.
Description
REFERENCE TO RELATED APPLICATION
[0001] This application is a non-provisional of, and claims
priority to and the benefits of, U.S. Provisional Patent
Application No. 62/685,409 filed on Jun. 15, 2018, the entirety of
which is herein incorporated by reference.
BACKGROUND
[0002] This disclosure concerns higher polar anchoring strength of
liquid crystal (LC) using monolayer graphene flakes in an LC
device.
[0003] This disclosure concerns attaining faster electro-optic
switching in an LC device by doping monolayer graphene flakes and a
new product employing graphene flakes on the polyimide alignment
layers in an LC display device for accelerated response time.
SUMMARY OF DISCLOSURE
Description
[0004] This disclosure teaches methods and the products for
achieving higher polar anchoring strength of liquid crystal (LC)
using monolayer graphene flakes in an LC device.
[0005] This disclosure teaches procedures and the devices for
attaining faster electro-optic switching in an LC device by doping
monolayer graphene flakes.
[0006] Furthermore, the disclosure teaches a new product employing
graphene flakes on the polyimide alignment layers in an LC display
device for accelerated response time.
DESCRIPTION OF THE DRAWINGS
[0007] The following description and drawings set forth certain
illustrative implementations of the disclosure in detail, which are
indicative of several exemplary ways in which the various
principles of the disclosure may be carried out. The illustrated
examples, however, are not exhaustive of the many possible
embodiments of the disclosure. Other objects, advantages and novel
features of the disclosure will be set forth in the following
detailed description when considered in conjunction with the
drawings.
[0008] FIG. 1 illustrates the Dielectric constant, .epsilon. for E7
and E7+GP as a function of V.sub.rms at T=30.degree. C. The inset
shows the Freedericksz threshold voltage, V.sub.th for the samples
and the Dielectric constant, .epsilon. for E7 and E7+GP as a
function of 1/V.sub.rms at T=30.degree. C. The solid lines
represent the linear fit in the high-voltage linear regime (as
1/V.sub.rms.fwdarw.0, V.sub.rms.fwdarw..infin.). The extrapolated
Y-intercept of the linear fit gives the value of
.epsilon..sub..parallel..sup.extP.
[0009] FIG. 2 illustrates Polar anchoring strength coefficient,
W.theta. as a function of temperature for E7 and E7+GP on the
linear scale. Typical error bars are shown. The dotted lines are
guide-to-the-eye. The inset shows W.sub..theta. as a function of
temperature for E7 and E7+GP on the logarithmic scale.
[0010] FIG. 3 illustrates a schematic representation of the
alignment of nematic LC molecules on graphene due to .pi.-.pi.
electron stacking. The ellipsoids are LCs and the black honeycomb
structure is the graphene surface. The LC molecular structure is
shown in the ellipsoid on the graphene surface. The .pi.-.pi.
electron stacking is illustrated by matching the LC's benzene rings
on the graphene-honeycomb structure. The nematic director
({circumflex over (n)}) is orientated at 45.degree. with respect to
the crossed polarizer (P) and analyzer (A). This orientation,
therefore, produces a bright state. The system is rotated through
45.degree. and the nematic director ({circumflex over (n)}) is
parallel to A--which produces a dark state. Microphotographs of a
thin layer of nematic LC on a monolayer graphene film on a copper
substrate under a reflected crossed polarized microscope, showing a
bright state, a dark state, and a bright state of the highlighted
domain at 0.degree., 45.degree., and 90.degree., respectively, with
respect to the crossed polarizers. Normalized intensity of the
highlighted domain as a function of the relative angle of rotation.
The bar in the micrograph represents 50 .mu.m.
[0011] FIG. 4 illustrates a Microphotograph of the bare PI
alignment substrate used in the E7 cell. Randomly dispersed spacer
particles with uniform size are visible. Two of them are
highlighted with dotted circles. Microphotograph of the bare PI
alignment substrate used in the E7+GP cell. The dark spots, much
smaller than the spacer particles, are graphene aggregates. These
are small aggregated graphene flakes attached to the substrate. A
spacer particle is highlighted inside a dotted circle and a few
graphene flakes are highlighted inside a solid circle. A 7.times.
magnified image of the dotted square region. A spacer particle is
highlighted inside a dotted circle at the bottom left corner. All
other smaller dark spots are graphene flakes in small aggregates.
Schematically shown a natural LC alignment on the PI alignment
substrate. An illustration of the graphene-modified substrate that
enhances the average anchoring mechanism of the LC to the substrate
due to the additional strong .pi.-.pi. electron stacking between
graphene and LC. The complete LC device where graphene flakes are
attached to the PI alignment layers.
[0012] FIG. 5 illustrates Splay elastic constant, K.sub.11 as a
function of temperature for E7 and E7+GP. Typical error bars are
shown.
K 11 W .theta. ##EQU00001##
as a function of temperature for E7 and E7+GP.
[0013] FIG. 6 illustrates Electro-optic switching of E7 and E7+GP
cells. The left Y-axis shows the normalized transmitted intensity
as a function of time when an applied voltage is turned off at t=0,
and then turned on at t=25 ms, for E7 and E7+GP, listed in the
legend (T=30.degree. C.). The right Y-axis shows the applied
voltage profile across the cells.
[0014] FIG. 7 illustrates Optical switching on, .tau..sub.on and
optical switching off, .tau..sub.off as a function of temperature
for E7 and E7+GP listed in the legend.
DETAILED DESCRIPTION OF THE INVENTION
[0015] This disclosure teaches methods and devices for achieving
higher polar anchoring strength of liquid crystal (LC) using
monolayer graphene flakes in an LC device.
[0016] This disclosure teaches procedures and the products for
attaining faster electro-optic switching in an LC device by doping
monolayer graphene flakes.
[0017] Furthermore, this disclosure teaches a new product employing
graphene flakes on the polyimide alignment layers in an LC display
device for accelerated response time.
[0018] The interaction of nematic LC with the polyimide (PI)
alignment substrates in an LC cell is technologically important for
their applications in electro-optical display technology. The LC
molecules align with alkyl side chains of the PI alignment film to
generate planar alignment on the LC director.
[0019] An external electric field then can deviate the director
from its initial orientation. The energy cost of this deviation
from a planar orientation to a homeotropic orientation is
characterized by the polar anchoring strength coefficient,
W.sub..theta..
[0020] The polar anchoring strength is an important parameter for
an LC cell because it influences the LC's electro-optic properties,
such as the orientational threshold voltage and response time.
[0021] We show here that when the LC cell is filled with the
graphene-doped LC, some graphene flakes are preferentially attached
to the PI alignment layer and modify the substrate's anchoring
property with the LC.
[0022] These spontaneously deposited graphene flakes promote planar
anchoring at the substrate due to the .pi.-.pi. electron stacking
between the graphene and LC, and the effective polar anchoring
energy at the PI alignment layer is enhanced by an order of
magnitude.
[0023] The enhanced anchoring energy in the LC cell accelerates the
electro-optic response of the LC.
Example 1
[0024] The pristine graphene (GP) sample in ethanol solvent was
obtained. The graphene sample contained more than 97% of monolayer
flakes (with a small fraction of multilayer flakes) of an average
thickness of 0.35 nm and an average lateral size of 550 nm.
[0025] The ethanol+GP solution was first remixed by a
micro-homogenizer tip of 5 mm diameter at 35000 rpm for 5 min,
followed by sonication for 4 h. The liquid crystal E7
(T.sub.NI=60.5.degree. C.) then was added to the ethanol+GP and
sonicated for 5 h, allowing the LC to dissolve completely into the
solution.
[0026] The ethanol was evaporated slowly at an elevated
temperature. Finally, the LC+GP was degassed under a vacuum for 1
h, leaving a pure LC+GP mixture of 2.5.times.10.sup.-3 wt %.
Example 2
[0027] For consistency, the pure LC was also treated the same way,
such as dissolving in ethanol followed by a slow evaporation and
degassing. Commercially manufactured planar and homeotropic LC
cells with a 1 cm.sup.2 semitransparent indium tin oxide (ITO)
coated area and a d=15 .mu.m spacing were used for the polar
anchoring strength measurements.
[0028] The electro-optic switching response of the LC was performed
using planar LC cells with a 0.25 cm.sup.2 semitransparent ITO
coated area and a d=5 .mu.m spacing.
Example 3
Anchoring Strength Measurement.
[0029] The polar anchoring strength coefficient, W.sub..theta. (for
a small pretilt angle, .theta..sub.p<5.degree.) is defined
as
W .theta. = 2 V th 2 A .pi. 2 ( C HT - C PL ) 2 C PL 1 [ 1 - ( C PL
C HT ) V -> .infin. ] ( 1 ) ##EQU00002##
where V.sub.th is the Freedericksz threshold voltage of the LC in
the planar cell, A is the active area of the cells, C.sub.HT is the
capacitance of the LC filled homeotropic cell, C.sub.PL is the
capacitance of the LC filled planar cell for V<V.sub.th.
[0030] For this measurement, both the cells must have the same
active area, A and thickness, d. In the denominator,
( C PL C HT ) V -> .infin. ##EQU00003##
is the capacitance ratio of the planar cell to the homeotropic cell
for a very high voltage limit. Note that for a positive anisotropic
(+.DELTA..epsilon.) LC, C.sub.HT does not change at the high
voltage limit as the LC does not undergo Freedericksz transition in
the homeotropic cell.
[0031] For the planar cell, C.sub.PL at the high voltage limit can
be determined from the C vs V.sup.-1 curve. For a more accurate
measurement of W.sub..theta., Eq. 1 can be rewritten using the
dielectric components of the LC,
.parallel. = d C HT A o and .perp. = d C PL A o , as
##EQU00004##
W .theta. = 2 0 V th 2 d .pi. 2 ( .parallel. - .perp. ) 2 .perp. 1
[ 1 - .parallel. extp .parallel. ] ( 2 ) ##EQU00005##
where .epsilon..sub.0 the is free space permittivity,
( C PL C HT ) V -> .infin. = .parallel. extp .parallel. ,
##EQU00006##
and .epsilon..sub..parallel..sup.extp can be determined from the
extrapolated intercept of the .epsilon. vs V.sup.-1 curve for the
planar LC cell, as shown in FIG. 1.
[0032] An Automatic Liquid Crystal Tester was used to measure the
capacitance of the empty cells and filled cells at 1000 Hz. Then
the dielectric components were obtained using the ratio of the
filled cell capacitance to the empty cell capacitance:
.parallel. = C HT C HT _ empty and .perp. = C PL C PL _ empty ( for
V < V th ) . ##EQU00007##
[0033] Equation 2 eliminates the error due to any small mismatch in
d and A between the planar and homeotropic cells by introducing
their empty cell capacitances.
[0034] FIG. 1 shows the dielectric constant s for E7 and E7+GP
samples as a function of rms voltage, obtained from the planar
cells. Both the samples exhibit a typical Freedericksz transition.
The inset in FIG. 1 shows V.sub.th for both the samples. FIG. 1
shows .epsilon. vs V.sup.-1 for E7 and E7+GP samples. The solid
lines represent the linear fit in the linear regime. The
extrapolated Y-intercept of the linear fit gives the value of
.epsilon..sub..parallel..sup.extP, as shown in FIG. 1.
[0035] After calculating the dielectric components from the
appropriate LC cells, Eq. 2 was used to obtain W.sub..theta.. In
one embodiment of the invention, shown in FIG. 2, the anchoring
energy W.sub..theta. of the GP doped LC is significantly higher
than that of the pure LC. At 30.degree. C.,
W.sub..theta..sup.E7=3.8.times.10.sup.-5 J m.sup.-2 and
w.sub..theta..sup.E7+GP=4.1.times.10.sup.-4 J m.sup.-2.
[0036] Note that FIG. 2 shows W.sub..theta. on a linear scale and
the inset shows the same on a logarithmic scale. These two scales
clearly depict the dramatic increase in W.sub..theta. and its
pretransitional behavior when graphene flakes are doped in the LC.
Note that the inset in FIG. 1 shows that V.sub.th for the E7+GP is
higher than that of E7, indicating an increase in effective
W.sub..theta. for the hybrid sample.
Example 4
[0037] Planar nematic alignment on graphene film and enhanced polar
anchoring.
[0038] LC molecules can anchor to the honeycomb pattern of graphene
or carbon nanotubes, employing the .pi.-.pi. electron stacking.
[0039] Density-functional calculations suggest that this anchoring
is further strengthened with a binding energy of -2.0 eV by
electrostatic energy due to a considerable amount of charge
transfer from the LC molecule to the honeycomb pattern.
[0040] FIG. 3 illustrates the .pi.-.pi. stacking interaction that
arises due to the overlap of the LC's benzene rings on the
graphene-honeycomb structure. However, it is difficult to visualize
the effect of this strong interaction in an LC+GP colloidal system.
Therefore, we have studied the LC alignment mechanism on a
CVD-grown monolayer graphene film on a copper foil. Between two
crossed polarizers, a bright state with the maximum transmitted
intensity appears when the planar-aligned LC nematic director is at
45.degree. with the polarizer (or with the crossed-analyzer). A
dark state is achieved if the director is parallel to the polarizer
(or to the crossed-analyzer).
[0041] FIG. 3 schematically illustrates that the LC director on
graphene is orientated at 45.degree. with respect to the crossed
polarizer and analyzer--which results in a bright state. When the
system is rotated through 45.degree., the director orients parallel
to the analyzer (FIG. 3)--which leads to a dark state.
Example 5
[0042] The CVD-grown graphene film on a copper foil was obtained.
The graphene film was continuous, with irregular holes and cracks.
In addition, the graphene film was polycrystalline (i.e. the
presence of grains with different crystallographic
orientation).
[0043] A small droplet of E7 in the isotropic phase was first
placed on the graphene film. The droplet then was blown away gently
by a dust blower--which left a thin LC layer on the graphene
surface.
[0044] The LC coated graphene on the copper substrate then was
heated up in the isotropic phase to get rid of any residual order
from the coating process and then slowly cooled down to the nematic
phase.
[0045] The alignment of the LC on graphene film then was studied by
reflected crossed polarized microscopy by observing the bright to
dark transitions as the sample was rotated over 135.degree..
[0046] After coating the thin LC layer on the graphene film,
different crystallographic graphene domains with grain boundaries
became clearly visible. FIG. 3 shows the microphotographs at three
different relative angles, 0.degree., 45.degree., and 90.degree.,
with respect to the crossed polarizers. The highlighted domain
intensity was tracked as the sample was rotated over 135.degree.
under the reflected crossed polarized microscope. FIG. 3 shows that
after rotating 45.degree., the highlighted domain becomes dark.
[0047] FIG. 3 depicts that after rotating 45.degree. more (i.e. a
total of 90.degree. from the initial state), the highlighted domain
becomes bright again. FIG. 3 shows the normalized intensity of the
highlighted domain as a function of the angle of rotation.
[0048] These results suggest that the LC, on graphene, can achieve
a planar aligned state, which can transit from dark to bright when
rotated by 45.degree..
[0049] This planar aligned state is employed by the strong
.pi.-.pi. electron stacking.
[0050] This interaction still exists in the LC+GP colloidal
system.
[0051] During the filling process of the LC cell, some monolayer
graphene flakes sediment down and/or get stuck to the substrates
from the LC+GP mixture, and align themselves horizontally on the
substrates.
[0052] When some graphene flakes are preferentially attached to the
substrate, they promote planar anchoring, enhancing the overall
polar anchoring of the system.
Example 6
[0053] An experimental approach has been used to check the presence
of graphene flakes, spontaneously deposited from the LC+GP hybrid,
lying face on the substrates of the cell.
[0054] The planar cells used for E7 and E7+GP were first
disassembled and carefully washed using acetone to get rid of any
LC. The bare alignment substrates of the cells were studied using
transmitted microscopy and the microphotographs are shown in FIG.
4. The bare alignment substrate used in the E7 cell is shown in
FIG. 4. Only randomly dispersed spacer particles with uniform size
are visible under the microscope.
[0055] FIG. 4 presents the bare alignment substrate used in the
E7+GP cell. A careful observation reveals that in addition to the
spacer particles, there present many dark spots much smaller than
the spacer particles. These are small aggregated graphene flakes
attached to the substrate. FIG. 4 shows a spacer particle inside a
dotted circle and graphene flakes inside a solid circle. FIG. 4
presents a 7.times. magnified image of the dotted square region
shown in FIG. 4. The presence of graphene flakes in small
aggregates is clearly visible in FIG. 4. There are likely many more
graphene flakes present on the surface, but they are not visible
under the microscope as they are attached to the substrate in
non-aggregate forms.
[0056] These results clearly indicate the presence of the adsorbed
graphene layers on the alignment substrates used in the E7+GP
cell.
[0057] FIG. 4 schematically shows a natural LC alignment on the
alignment substrate.
[0058] FIG. 4 illustrates that the graphene-modified substrate
enhances the average anchoring mechanism of the LC to the substrate
due to the additional strong .pi.-.pi. electron stacking between
graphene and LC. This mechanism leads to a strong increase in
effective W.sub..theta., as shown in FIG. 2.
[0059] FIG. 4 shows the complete LC device where graphene flakes
are attached to the PI alignment layers.
Example 7
[0060] Faster electro-optic switching in an LC device due to the
enhancement in polar anchoring energy.
[0061] Now we will present the impact of this enhanced
W.sub..theta. on the nematic switching response. The two
characteristic times, rise (voltage on) and fall (voltage off), of
the nematic director, considering W.sub..theta., can be described
as:
.tau. rise = .gamma. 1 .DELTA. o V 2 - K 11 .pi. 2 ( d 2 + 4 d K 11
W .theta. ) , .tau. fall = .gamma. 1 K 11 .pi. 2 ( d 2 + 4 d K 11 W
.theta. ) ( 3 ) ##EQU00008##
where .gamma..sub.1 is the rotational viscosity, d is the cell
thickness, .DELTA..epsilon. is the dielectric anisotropy,
.epsilon..sub.0 the is free space permittivity, K.sub.11 is the
splay elastic constant, and V (>V.sub.th) is the driving applied
voltage. .tau..sub.rise is the time the nematic director takes to
rotate from planar to homeotropic configuration, when the voltage
is turned on. Similarly, .tau..sub.fall is the time the director
takes to relax back from homeotropic to planar configuration after
the voltage is turned off.
[0062] The driving voltage, V is the dominating factor for
.tau..sub.rise. When V>>V.sub.th, the effect of W.sub..theta.
on .tau..sub.rise is very small. On the other hand, if the backflow
in the cell is neglected, .tau..sub.fall is purely driven by the
elastic interaction between the LC and the alignment substrate. In
the weak anchoring limit (W.sub..theta..about.10.sup.-4 to
10.sup.-5 J m.sup.-2), .tau..sub.fall is greatly influenced by
W.sub..theta. as the term
K 11 W .theta. ##EQU00009##
in .tau..sub.fall cannot be neglected. Therefore, in the weak
anchoring limit, an increase in W.sub..theta. results in a decrease
in .tau..sub.fall.
[0063] Since the term
K 11 W .theta. ##EQU00010##
influences .tau..sub.fall, we nave studied this term for E7 and
E7+GP samples as a function of temperature. The capacitive data for
the planar cells were used to extract K.sub.11 using the
Freedericksz transition method.
[0064] FIG. 5 shows K.sub.11 as a function of temperature for E7
and E7+GP samples. Away from the transition temperature, the E7+GP
sample shows an increase in K.sub.11. The term
K 11 W .theta. ##EQU00011##
was calculated using the results shown in FIG. 2 and FIG. 5, and
then plotted as a function of temperature in FIG. 5.
[0065] Clearly,
K 11 W .theta. ##EQU00012##
is significantly reduced in the E7+GP sample, which is expected to
accelerate .tau..sub.fall.
Example 8
[0066] The field-induced nematic switching was studied from the
electro-optic response of the LC cells.
[0067] The optical setup consisted of a beam from a 5-mW He--Ne
laser at wavelength 633 nm that passed through a polarizer, the
cell, a crossed analyzer and into a nanosecond Newport
photodetector. The beam was polarized at an angle of 45.degree.
with respect to the cell's rubbing direction. The output of the
detector was fed into a digital storage oscilloscope. A dc voltage
pulse of 10 V (much higher than the threshold switching voltage) at
a pulse interval of 20 Hz was applied across the cell and, the
change in transmitted intensity as a function of time (both when
the voltage was turned on and off) was detected by the
oscilloscope.
[0068] Transmitted responses for voltage on and voltage off were
studied as a function of temperature for pure E7 and E7+GP samples.
The setup was computer controlled and data acquisition was
performed using LabVIEW.RTM. software.
Example 9
[0069] In another embodiment of the invention, shown in FIG. 6, the
electro-optic response time is significantly accelerated when the
LC is doped with monolayer graphene flakes.
[0070] FIG. 6 represents the normalized transmitted intensity
response (left Y-axis) as a function of time for voltage off and
voltage on for E7 and E7+GP at T=30.degree. C. The right Y-axis
shows the applied voltage profile across the cells. The transmitted
intensity increases as a function of time for E7 and E7+GP, as the
applied voltage is turned off at time t=0.
[0071] It is apparent that E7+GP sample relaxes faster than pure E7
when the voltage is turned off. The time the transmitted intensity
takes to rise from 10% to 90% of the maximum intensity, after the
voltage is turned off, is defined as the optical switching off,
.tau..sub.off. When the applied voltage is turned on at t=25 ms,
the transmitted intensity drops as a function of time for E7 and
E7+GP. The time the transmitted intensity takes to drop from 90% to
10% of the maximum intensity, after the voltage is turned on, is
defined as the optical switching on, .tau..sub.on. Note that
.tau..sub.rise and .tau..sub.fall are not equal to the
electro-optical responses--.tau..sub.on and .tau..sub.off,
respectively. However, the optical response is mainly due to the
director's rotation after the voltage is turned on or off.
Therefore, neglecting the backflow in the cell, one can write
.tau..sub.rise.varies..tau..sub.on and
.tau..sub.fall.varies..tau..sub.off.
[0072] FIG. 7 shows .tau..sub.on and .tau..sub.off, respectively,
as a function of temperature for E7 and E7+GP. Even though
.tau..sub.on does not show any significant difference,
.tau..sub.off shows a dramatic decrease for the E7+GP sample.
[0073] This accelerated switching response is attributed to the
significant enhancement in W.sub..theta. due to the presence of
graphene in the LC and on the alignment substrate.
[0074] The hexagonal 2D layer is not necessarily limited to
graphene flakes, but includes any other 2D hexagonal material with
a propensity to align LC materials and mixtures. Potential
materials include, but are not limited to, hexagonal boron nitride,
borophene, germanine, phosphorene, silicone, and molybdenite.
[0075] The monolayer graphene flakes may be chemically modified
directly to promote uniaxial LC alignment on the surface. Uniaxial
alignment of LC on graphene may be promoted by adding an additional
photoalignment step, whereby polarized light is used to absorb or
chemically bond LC-compatible molecules to the graphene
surface.
[0076] Uniaxial alignment of LC on graphene may be promoted by
non-surface means including, but not limited to flow alignment,
photo-orientation and slow cooling from the isotropic phase.
[0077] The LC layer can be comprised of a single molecular
component or mixtures of components with a liquid crystal phase,
including but not limited to nematic, cholesteric, smectic and
discotic phases. The LC may possess a positive or negative
dielectric anisotropy and a permanent molecular dipole to
facilitate molecular switching in response to an applied voltage.
The graphene can be multi-layered.
[0078] This invention discloses the use of monolayer graphene
flakes as a planar anchoring amplifier on the polyimide (PI)
substrate and for faster electro-optic switching response in a
nematic LC.
[0079] The invention enables a new product employing graphene
flakes on the polyimide alignment layers in an LC display device
for accelerated response time.
[0080] The above examples are merely illustrative of several
possible embodiments of various aspects of the present disclosure,
wherein equivalent alterations and/or modifications will occur to
others skilled in the art upon reading and understanding this
specification and the annexed drawings. In addition, although a
particular feature of the disclosure may have been illustrated
and/or described with respect to only one of several
implementations, such feature may be combined with one or more
other features of the other implementations as may be desired and
advantageous for any given or particular application. Also, to the
extent that the terms "including", "includes", "having", "has",
"with", or variants thereof are used in the detailed description
and/or in the claims, such terms are intended to be inclusive in a
manner similar to the term "comprising".
* * * * *