U.S. patent application number 13/264411 was filed with the patent office on 2013-06-13 for method of creating colored materials by fixing ordered structures of magnetite nanoparticles within a solid media.
This patent application is currently assigned to THE REGENTS OF THE UNIVERSITY OF CALIFORNIA. The applicant listed for this patent is Jianping Ge, Sunghoon Kwon, Yadong Yin. Invention is credited to Jianping Ge, Sunghoon Kwon, Yadong Yin.
Application Number | 20130146788 13/264411 |
Document ID | / |
Family ID | 42982994 |
Filed Date | 2013-06-13 |
United States Patent
Application |
20130146788 |
Kind Code |
A1 |
Yin; Yadong ; et
al. |
June 13, 2013 |
METHOD OF CREATING COLORED MATERIALS BY FIXING ORDERED STRUCTURES
OF MAGNETITE NANOPARTICLES WITHIN A SOLID MEDIA
Abstract
Compositions and methods wherein ordered structures of photonic
nanocrystals are created in a liquid medium and then such
structures are fixed by converting the liquid medium to a solid. In
addition, compositions and methods of reversibly fixing such
structures, so that ordered structures can be reversibly created in
a liquid medium, converted to solid, and then converted back to
liquid, wherein new ordered structures can be created and again
fixed.
Inventors: |
Yin; Yadong; (Riverside,
CA) ; Ge; Jianping; (Shanghai, CN) ; Kwon;
Sunghoon; (Gwanak-gu, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Yin; Yadong
Ge; Jianping
Kwon; Sunghoon |
Riverside
Shanghai
Gwanak-gu |
CA |
US
CN
KR |
|
|
Assignee: |
THE REGENTS OF THE UNIVERSITY OF
CALIFORNIA
Oakland
CA
|
Family ID: |
42982994 |
Appl. No.: |
13/264411 |
Filed: |
April 14, 2010 |
PCT Filed: |
April 14, 2010 |
PCT NO: |
PCT/US10/01105 |
371 Date: |
June 1, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61169260 |
Apr 14, 2009 |
|
|
|
Current U.S.
Class: |
250/492.1 ;
252/62.51R; 264/1.1; 264/1.36; 359/280; 359/298 |
Current CPC
Class: |
H01F 1/0063 20130101;
C08K 3/22 20130101; G02F 1/29 20130101; G02B 1/005 20130101; C09C
1/24 20130101; C01P 2006/42 20130101; G02B 2207/101 20130101; G02F
1/09 20130101; B82Y 20/00 20130101; C09D 7/62 20180101; B01J 19/12
20130101; C08K 9/08 20130101; C08K 2201/01 20130101; C09C 3/10
20130101; B01J 19/123 20130101; Y10T 428/24802 20150115; C08K
2201/011 20130101; C09D 7/69 20180101; B82Y 25/00 20130101; C09D
11/50 20130101; G02B 5/201 20130101; H01F 1/344 20130101 |
Class at
Publication: |
250/492.1 ;
252/62.51R; 264/1.1; 264/1.36; 359/280; 359/298 |
International
Class: |
B01J 19/12 20060101
B01J019/12; G02F 1/29 20060101 G02F001/29; G02F 1/09 20060101
G02F001/09 |
Claims
1. A method of creating colored materials, comprising: fixing
ordered structures of magnetically responsive nanoparticles within
a media, such that the ordered structures diffract light to create
colors.
2. The method of claim 1, further comprising creating the ordered
structures of magnetically responsive nanoparticles with an
external magnetic field.
3. The method of claim 2, wherein the ordered structures of
magnetically responsive nanoparticles are created in a liquid media
and the ordered structures are fixed by converting the liquid media
to a solid media.
4. The method of claim 3, wherein the liquid media is a
photocurable solution.
5. The method of claim 4, further comprising fixing the ordered
structures of magnetically responsive nanoparticles with an UV
source having a wavelength of approximately 240 nm to approximately
365 nm.
6. The method of claim 1, wherein the ordered structures are
created in a reversible media, wherein the reversible media is
reversible from a solid to a liquid, such that the color can be
changed.
7. A method of generating multicolored patterns comprising: fixing
a structural color from a superparamagnetic colloidal nanocrystal
clusters (CNC); and introducing a high resolution patterning of
multiple structural colors using a single material.
8. The method of claim 7, further comprising repetitive tuning and
fixing of the structural color from a mixture of superparamagnetic
photonic crystals and photocurable resin.
9. The method of claim 7, wherein the superparamagnetic photonic
crystals consists of a plurality of domain magnetite nanoparticles,
which are coated.
10. The method of claim 7, further comprising adding an external
magnetic field to the photonic crystals, and wherein the external
magnetic field assembles the photonic crystals in a chain-like
structures along the magnetic field lines.
11. The method of claim 7, wherein the attractive magnetic force
due to the superparamagnetic core is balanced with repulsive
solvation force, both of which determine the inter-particle
distance under any given magnetic field strength.
12. The method of claim 7, wherein the inter-particle distance in a
chain determines the color of the light diffracted from the
chain.
13. The method of claim 7, wherein the color can be tuned by simply
varying the inter-particle distance using external magnetic
fields.
14. The method of claim 7, wherein once the desired color is
obtained, the desired color is fixed by solidifying the
photocurable resin through UV exposure.
15. The method of claim 7, wherein the particle chains are frozen
in the solidified polymer network without distorting its periodic
arrangements, thus retaining the structural color.
16. The method of claim 7, further comprising adding a hydrogen
bonding solvent, which forms a solvation layer around the particle
surface, and which provides a strong repulsion when two solvation
layers overlap.
17. The method of claim 16, wherein the hydrogen bonding solvent is
an alkanol.
18. The method of claim 17, further comprising adding ethanol to
the system.
19. (canceled)
20. (canceled)
21. (canceled)
22. (canceled)
23. (canceled)
24. (canceled)
25. (canceled)
26. (canceled)
27. (canceled)
28. (canceled)
29. (canceled)
30. (canceled)
31. A method of forming magnetochromatic microspheres comprising:
coating a plurality of magnetite nanocrystals with a surface
medium; dispersing the plurality of coated magnetite nanocrystals
in a curable solution; placing the magnetite nanocrystals and
curable solution in an immiscible solution to form an emulsion;
exposing the emulsion to an external magnetic field, which aligns
the coated magnetite nanocrystals in one-dimensional chains within
emulsion droplets within the curable solution; and curing the
emulsion droplets within the curable solution into magnetochromatic
microspheres.
32. The method of claim 31, wherein the step of curing the emulsion
droplets is by exposing the curable solution to a UV illumination
source.
33. The method of claim 32, wherein the step of exposing the
curable solution to the UV illumination source fixes the ordered
structures within the microspheres.
34. The method of claim 31, wherein the plurality of magnetite
nanocrystals have a chemical composition of
.gamma.-Fe.sub.2O.sub.3Fe.sub.2O.sub.3 and/or Fe.sub.3O.sub.4.
35. (canceled)
36. (canceled)
37. (canceled)
38. (canceled)
39. The method of claim 31, further comprising microspheres
immersed in a phase-changeable matrix, the phase-changeable matrix
having a liquid phase and a solid phase.
40. The method of claim 39, wherein when the matrix is the liquid
phase, adjusting an angle of the external magnetic field to change
an orientation of the microspheres.
41. (canceled)
42. (canceled)
43. (canceled)
44. (canceled)
45. (canceled)
46. (canceled)
47. The method of claim 31, wherein the immiscible liquid is a
viscous non-polar solvent, mineral oil and/or silicone oil and/or
paraffin oil.
48. (canceled)
49. (canceled)
50. (canceled)
51. (canceled)
52. The method of claim 39, wherein the phase-changeable matrix is
a polyethylene glycol (PEG) film, paraffin, long-chain alkanes,
esters, primary alcohols and/or a non-crosslinked polymers such as
polyethylene, poly(ethylene oxide),
polyethylene-block-poly(ethylene glycol) and/or polyesters.
53. (canceled)
54. (canceled)
55. (canceled)
56. (canceled)
57. (canceled)
58. (canceled)
59. A method of forming magnetochromatic microspheres comprising: a
simultaneous magnetic assembly and UV curing process of an emulsion
system comprised of superparamagnetic Fe.sub.3O.sub.4@SiO.sub.2
colloidal particles, which are self-organized into ordered
structures inside emulsion droplets of UV curable resin.
60. The method of claim 59, wherein the ordered structures are
fixed by an immediate UV curing process to polymerize the
droplets.
61. The method of claim 59, further comprising rotating the
microspheres using an external magnetic field to change the
orientation of the magnetic chains and thereby the diffractive
colors of the microspheres.
62. A display comprising: microspheres containing ordered
structures of photonic crystals, which are rotated, which changes
the angle of diffraction of light passing through the
microspheres.
63. The display of claim 62, further comprising rotating the
microspheres, which changes the angle of diffraction of light
passing through the microspheres, which changes a first color to a
second color.
64. The display of claim 62, further comprising microspheres
immersed in a phase-changeable matrix, the phase-changeable matrix
having a liquid phase and a solid phase.
65. The display of claim 64, wherein when the matrix is the liquid
phase, adjusting an angle of the external magnetic field to change
an orientation of the microspheres.
66. The display of claim 65, wherein the orientation of the
microspheres are fixed when the matrix goes to the solid phase.
67. The display of claim 62, wherein the field strength required to
rotate the microspheres is dependent on an amount magnetite
nanocrystals in each of the microspheres.
68. The display of claim 64, wherein the phase-changeable matrix is
a polyethylene glycol (PEG) film.
69. The display of claim 64, wherein the phase-changeable matrix is
paraffin, long-chain alkanes, esters, primary alcohols and/or a
non-crosslinked polymers such as polyethylene, poly(ethylene
oxide), polyethylene-block-poly(ethylene glycol) and/or polyesters.
Description
FIELD OF THE INVENTION
[0001] This invention relates to methods of creating colored
materials by fixing ordered structures of magnetite nanoparticles
within a solid media, such that the ordered structures diffract
light to create colors.
BACKGROUND
[0002] Superparamagnetic nanocrystals, or photonic crystals, which
are capable of forming ordered structures that diffract light to
create colors, have been previously described. For example, Yin et
al, Superparamagnetic Magnetite Colloidal Nanocrystal Clusters,
Angwantde Chemie, 46:4342 (2007), Magnetically responsive colloidal
photonic crystals, Journal of Material Chemistry 18: 5041 (2008),
Self-Assembly and Field-Responsive Optical Diffractions of
Superparamagnetic Colloids, Langmuir 24:3671 (2008), Assembly of
Magnetically Tunable Photonic Crystals in Nonpolar Solvents, JACS
131: 3484 (2009), and WO2009/017525, all incorporated herein by
reference, all describe the synthesis of magnetite nanocrystals, or
photonic crystals, which can be induced to form ordered structures
when exposed to a magnetic field. Furthermore, these ordered
structures can be tuned by varying the strength of the magnetic
field such that different diffractive patterns and colors are
created. However these previous efforts have required a constant
magnetic field in order to maintain the ordered structure and thus
the color.
[0003] Accordingly, it would be desirable to have materials and
methods comprising photonic nanocrystals which can be tuned in a
liquid medium to create ordered structures which impart color, and
which such ordered structures can be fixed by converting the liquid
medium to solid. It would further be desirable to create ordered
structures of photonic crystals in a medium which can be reversibly
converted from liquid to solid, such that the color can be
changed.
SUMMARY
[0004] In its broadest scope, the invention described herein
comprises compositions and methods wherein ordered structures of
photonic nanocrystals are created in a liquid medium and then such
structures are fixed by converting the liquid medium to a solid.
Further provided are methods of reversibly fixing such structures,
so that ordered structures can be reversibly created in a liquid
medium, converted to solid, and then converted back to liquid,
wherein new ordered structures can be created and again fixed.
[0005] In accordance with an exemplary embodiment, a method of
creating colored materials, comprises: fixing ordered structures of
magnetite nanoparticles within a media, such that the ordered
structures diffract light to create colors.
[0006] In accordance with another exemplary embodiment, a method of
generating multicolored patterns comprises: fixing a structural
color from a superparamagnetic collidal nanocrystal clusters (CNC
or CNCs); and introducing a high resolution patterning of multiple
structural colors using a single material.
[0007] In accordance with a further exemplary embodiment, a full
color printing and particle encoding based on artificial structural
colors from a magnetically tunable photonic crystal, the printing
and particle encoding comprises: a plurality of magnetite
nanoparticles; ethanol; and a photocurable resin.
[0008] In accordance with another exemplary embodiment, a method of
forming magnetochromatic microspheres comprises: coating a
plurality of magnetite nanocrystals with a medium; dispersing the
plurality of coated magnetite nanocrystals in a curable solution;
placing the magnetite nanocrystals and curable solution in an
immiscible solution to form an emulsion; exposing the emulsion to
an external magnetic field, which aligns the coated magnetite
nanocrystals in one-dimensional chains within emulsion droplets
within the curable solution; and curing the emulsion droplets
within the curable solution into magnetochromatic microspheres.
[0009] In accordance with a further exemplary embodiment, a
magnetochromatic composition formed by the method as recited above,
and wherein the composition is used for a color display, signage,
bio and chemical detection and/or magnetic field sensing.
[0010] In accordance with another exemplary embodiment, a method of
forming magnetochromatic microspheres comprises: a simultaneous
magnetic assembly and UV curing process of an emulsion system
comprised of superparamagnetic Fe.sub.3O.sub.4@SiO.sub.2 colloidal
particles, which are self-organized into ordered structures inside
emulsion droplets of UV curable resin.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1(a) is a schematic illustration of the mechanism for
generating multiple structural colours with a single material,
wherein FIG. 1(a) shows the main concept of immobilization of
structure of CNCs in photocurable resin having a superparamagnetic
core and ethanol solvation layer allows the stable dispersion of
the CNCs in the liquid resin, and upon the application of an
external magnetic field, CNCs are assembled to form chain-like
photonic crystal, and UV exposure instantaneously fixes the ordered
structure in polymeric matrix.
[0012] FIGS. 1(b)-1(g) are schematic illustrations of the
multicolour patterning of structural colour with single material by
a sequential action of "tuning and fixing", and wherein the
diffraction wavelength is tuned by varying the strength of magnetic
fields, and spatially patterned UV light polymerizes the
photocurable resin and fixes the position of ordered CNCs; and
wherein after polymerization, remnant liquid resin is washed away
with unreacted PEG-DA monomer solution; and wherein FIG. 1(h) shows
the mechanism for creation of various colours from a single ink;
and wherein the UV curing of the M-ink under magnetic fields with
different strengths can freeze the chain-like assemblies with
different inter-particle distances which determine the diffracted
wavelength of light: shorter diffracted wavelength for shorter
interparticle distance.
[0013] FIG. 2(a) is a reflection micrograph of multicoloured
structural colour generated by gradually increasing magnetic
fields, and wherein the microstructure (i) is generated under no
magnetic field, and microstructures (ii) to (viii) are generated
under gradually increasing strength of magnetic field from 130 G to
700 G.
[0014] FIG. 2(b) is a transmission micrograph of the same sample of
FIGS. 2(a) and 2(c), and the corresponding spectra of the
microstructures, and wherein the microstructure (i) does not show
any diffraction peak in the visible range, and wherein the
microstructures (ii) to (viii) show the shift of the diffraction
peak to the shorter wavelength, and wherein the scale bars is as
follows: 100 .mu.m in FIGS. 2(a), 2(b), 2(e), 2(f); 1 .mu.m in FIG.
2(d), and 250 .mu.m in FIGS. 2(g)-2(i).
[0015] FIG. 2(d) is an SEM image of the sliced cross section of a
photocured sample, and wherein the dimpled surface profile shows
the traces of chain-like ordering of CNCs.
[0016] FIG. 2(e) is an SEM image of concentric patterns of a
triangle, a square, a pentagon and a circle.
[0017] FIG. 2(f) is an SEM image of multicoloured barcodes.
[0018] FIG. 2(g) is an SEM image of a composite pattern of strip
and polygon.
[0019] FIGS. 2(h) and 2(i) are reflection and transmission
micrographs, respectively of a tree.
[0020] FIGS. 3(a)-3(g) show reflection intensity modulation and
spatial colour mixing of structural colour, and having a scale bars
as follows: 250 .mu.m in FIGS. 3(a) and 3(d), and 100 .mu.m in FIG.
3(g).
[0021] FIG. 3(a) is a 4-bit reflection intensity modulation by the
varying number of monotone structural colour dots, and wherein each
of the red dotted lines stands for a pixel which shows distinct
level of reflection intensity.
[0022] FIG. 3(b) is a reflectance spectrum of the corresponding 16
pixels of FIG. 3(a).
[0023] FIG. 3(c) is a monotone 4-bit image of Mona Lisa which
consists of 4800 pixels.
[0024] FIG. 3(d) is a spatial colour mixing of structural colour,
wherein each pixel of 4.times.4 matrix consists of different colour
dots, and each of which is size of approximately 25 .mu.m.
[0025] FIG. 3(e) is a corresponding reflectance spectra of selected
pixels in FIG. 3(d) (inset), and wherein the green line in the
spectra stands for the spectrum of (1,1) component of the pixel,
orange line for the (1,2) component, gray line for the mathematical
addition of the green and orange line, and blue line for the
normalized spectrum of full pixel.
[0026] FIG. 3(f) is a reproduction of butterfly, Papilo Palinurus,
and wherein the colour of wings in the reproduced image shows
structural colour mixing by mixing blue and yellow-green.
[0027] FIG. 3(g) is a magnification of wing area of FIG. 3(f),
which consists of blue and yellow-green dots, and wherein each dot
is the size of 16.7.times.16.7 .mu.m.sup.2 (.about.1500 DPI).
[0028] FIGS. 4(a)-4(f) show colour and shape encoded particles
fabricated in microfluidic environment using a single ink, wherein
FIGS. 4(a)-4(c) are schematic diagrams for generating encoded
particles using M-Ink in PDMS microfluidic channels; FIG. 4(d) is a
free floating encoded particle with various shape and colour around
the PDMS anchor area; FIG. 4(e) is an enlarged micrograph of FIG.
4(d), showing closely packed particles with various colour and
shape; and FIG. 4(f) are heterogeneously encoded particles embedded
with small colour dots. Scale bars: 200 .mu.m in FIG.
4(d)-4(f).
[0029] FIG. 5(a) is a schematic of a synthetic procedure for the
magnetochromatic microspheres, where when dispersed as emulsion
droplets, superparamagnetic Fe.sub.3O.sub.4@SiO.sub.2 core-shell
particles self-organize under the balanced interaction of repulsive
and attractive forces to form one-dimensional chains, each of which
contains periodically arranged particles diffracting visible light
and displaying field-tunable colors, and UV initiated
polymerization of the oligomers in emulsion droplets fixes the
periodic structures inside the microspheres and retains the
diffraction property.
[0030] FIG. 5(b) is an SEM image of Fe.sub.3O.sub.4@SiO.sub.2
particle chains embedded in a PEGDA matrix.
[0031] FIG. 5(c) are schematic illustrations and optical microscopy
images for the magnetochromatic effect caused by rotating the
chain-like photonic structures in magnetic fields.
[0032] FIG. 6(a) is a schematic illustration of the experimental
setup for studying the angular dependence of the diffraction
property of the magnetochromatic microspheres.
[0033] FIG. 6(b) is a reflection spectrum and corresponding digital
photo recorded from a single Fe.sub.3O.sub.4@SiO.sub.2/PEGDA
microsphere at different tilting angles.
[0034] FIGS. 7(a)-7(f) are optical microscopy images (500.times.)
of magnetochromatic microspheres with diffractions switched between
"on" (a, c, e) and "off" (b, d, f) states by using external
magnetic fields, and wherein these microspheres were prepared using
(a, b) 127, (c, d) 154, and (e, f) 197 nm Fe.sub.3O.sub.4@SiO.sub.2
colloids.
[0035] FIG. 8(a) are dark-field optical microscopy images of a
series of Fe.sub.3O.sub.4@SiO.sub.2/PEGDA microspheres with
diameters from approximately 150 .mu.m to 4 .mu.m, and wherein the
larger microspheres were fabricated in mineral oil and smaller ones
in silicon oil.
[0036] FIGS. 8(b)-8(d) are top view to side view SEM images of the
microspheres, showing some of the Fe.sub.3O.sub.4@SiO.sub.2
particle chains aligned on the surface along the longitudinal
direction, and wherein it should be noted that a plurality of
particle chains are embedded inside the microspheres, with only
ends occasionally observable in the top view image (b).
[0037] FIGS. 9(a)-9(b) are statistical diagrams showing the turning
threshold of field strength for Fe.sub.3O.sub.4@SiO.sub.2/PEGDA
microspheres with different loadings of magnetic particles, wherein
FIG. 9(a) is 8 and FIG. 9(b) is 6 mg Fe.sub.3O.sub.4/ml PEGDA, and
wherein the diagrams show the percentage of viewable area which is
turned on at certain field strengths, and the corresponding
accumulative curves.
[0038] FIGS. 10(a)-10(d) are schematic diagrams of the optical
response of Fe.sub.3O.sub.4@SiO.sub.2/PEGDA microspheres in a (a,
b) 1.22 and (c, d) 3.33 Hz vertical/horizontal alternating magnetic
field, wherein Hs/Ho is the ratio of reflection with H field to
that without H field.
[0039] FIGS. 11(a)-11(i) are digital photos and reflection spectra
of three types of Fe.sub.3O.sub.4@SiO.sub.2/PEGDA microspheres
loaded in 1.8.times.1.8.times.0.1 cm glass cells filled with PEG
(Mw=1500), wherein the diffraction is switched on (a, d, g) or off
(b, e, h) by melting the PEG matrix, rotating the microspheres with
a magnetic field, and finally cooling down the PEG matrix to lock
the sphere orientation such that bistable states can therefore be
maintained in the absence of magnetic fields, and the corresponding
reflection spectra (c, f, i) display diffraction peaks at the "on"
stage and none at the "off" stage.
DETAILED DESCRIPTION
[0040] The invention described herein provides various methods of
fixing the ordered structure by (1) using an external magnetic
field to create ordered structures of photonic crystals in a liquid
medium, and (2) converting the liquid medium to a solid medium to
preserve the ordered structure, such that it remains when the
external magnetic field is removed.
[0041] In accordance with an exemplary embodiment, the media (or
medium) of the invention can be any media or medium capable of
phase change from a liquid to a solid phase. Transparent,
semi-transparent, or translucent medium is preferred. Exemplary
media include, but are not limited to UV curable resins, such as
polyethyleneglycol diacrylate (PEGDA) oligomers in combination with
trace amount of photo initiator 2,2-Dimethoxy-2-phenylacetophenone
(DMPA), acrylic, epoxy, polyester, stereolithography resins, or
other liquid media capable of being converted to a solid upon
exposure to UV light. The media of the invention further comprise
light-curable, temperature-curable, air-curable, and energy-curable
liquid media capable of being converted to solid form. In
accordance with an exemplary embodiment, the invention further
comprises media which can be reversibly converted from liquid to
solid and back to liquid, such as that described in "CARIVERSE
resin: a thermally reversible network polymer for electronic
applications" Chang, et al, Electronic Components and Technology
Conference, 1999. 1999 Proceedings. 49.sup.th Volume, Issue, 1999
Page(s):49-55 herein incorporated by reference, Polyethelene glycol
films (polyethylene glycol films), and/or paraffin.
[0042] In accordance with an exemplary embodiment, the media (or
medium) of the invention can comprise a film, beads, microspheres,
and any 3-dimensional shape which is desired.
[0043] The invention consists of ordering the photonic crystals
within the media (or medium) using an external magnetic field to
attain a desired spacing which will create a desirable color by
diffracting light, and then subjecting the medium to conditions
which cause it to convert to a solid, which solidifies and fixes
the photonic crystals in the ordered structure such that the color
is preserved.
[0044] In some embodiments, the solidification of the media (or
medium) results is done in bulk, in other embodiments the
solidification is performed on very small scales to create and fix
local regions of color, creating fine features and the ability to
create multi-colored patterns.
[0045] Provided herein are two exemplary embodiments of the
invention. The first is a method of creating detailed multicolored
patterns by local tuning and fixing of ordered structures. The
second is a method of creating microspheres containing fixed
ordered structures. Further provided is a method of creating a
display using ordered structure containing microspheres.
[0046] Full Colour Printing and Particle Encoding Based on
Artificial Structural Colours from a Magnetically Tunable Photonic
Crystal
[0047] It can be appreciated that many creatures in nature, such as
butterflies, beetles, and peacocks display unique iridescent and
metallic colors (or colours), known as "structural colors" or
"structural colours", which result from the light interaction with
periodic nanostructures on their surface.sup.1-6. Without relying
on multiple pigments or dyes, various colored patterns are cleverly
produced using a single structural material by simply altering the
dimension of the nanostructures. Cost-effective and scalable
implementation of this feature in manufacturing would greatly
simplify production of multicolored goods such as electronics,
displays, and vehicles. There have been many attempts to produce
artificial structural colored patterns using various bottom-up and
top-down techniques in a variety of research fields.sup.7-22.
However, mimicking such nanostructures found in the natural world
requires state-of-the-art nanofabrication techniques that are
expensive and not scalable. Especially, production of multi-colors
and patterning of such structure were not possible with a single
structural material.
[0048] A high resolution patterning and artificial production of
multiple structural colors based on successive tuning and fixing
the structural color of a single structural material is
demonstrated in accordance with an exemplary embodiment. In
accordance with another exemplary embodiment, a color tunable
structural material, whose color is magnetically tunable and
lithographically fixable is disclosed. Using photonic crystals, a
curable resin, and a specially designed lithographic instrument,
fine nanostructures for scalable production of a structural color
can be generated, tuned the color through the entire visible
spectrum by magnetically changing the dimension of the
nanostructures, and immobilized the nanostructures lithographically
to produce patterns with arbitrary spatial arrangements of color.
In addition, in accordance with a further embodiment, two
applications of the disclosed system including high resolution
color patterning for printing and micro-scale particle encoding for
bio-assays are demonstrated. With the superior scalability and
simplicity, the multicolor production scheme as disclosed herein
can have a significant impact on the color production for both
special instruments and general consumer goods.
[0049] It can be appreciated that structural colors in nature such
as butterfly wings, beetle cuticles and peacock feathers have
attracted considerable attention in diverse research areas.sup.1-6.
Structural color shows many characteristics different from chemical
pigments or dyes. For example, as can be found in the feathers of a
peacock, various colors result from the interaction of light with a
single biological material, melanin rods, and its iridescent colors
can be determined by the lattice spacing of the rods.sup.5. In
nature, a single biological material with different physical
configurations displays various colors and it greatly simplifies
the manufacturing process to produce multiple colors. The unique
colors originating from the physical structures are iridescent and
metallic, and cannot be mimicked by chemical dyes or pigments.
Also, structural color is free from photobleaching unlike
traditional pigments or dyes.
[0050] Due to its unique characteristics, there have been many
attempts to make artificial structural color through various
technological approaches such as colloidal
crystallization.sup.7-18, dielectric layer stacking.sup.19,20 and
direct lithographic pattering.sup.21,22. Colloidal crystallization
technique is most frequently employed to make a photonic crystal,
which blocks a specific wavelength of light in the crystal and
therefore displays the corresponding color. Gravitational
force.sup.7, centrifugal force.sup.8, hydrodynamic flow.sup.9,
electrophoretic deposition.sup.10 and capillary force from the
evaporation of solvents.sup.11-18 are utilized to assemble the
colloidal crystals. Although these methods produce structural
colors with large-area, the growth of colloidal crystals usually
takes a long time for better crystallization and fewer defects.
Also, since the band gap of a photonic crystal is dependent on the
size of colloids, different sizes of colloidal suspensions are
needed to produce multicolored structures. Furthermore, there have
been great technological difficulties in assembling colloids of
different sizes to form multicolored patterns with fine
resolutions.
[0051] Dielectric layer stacking and lithographic pattering of
periodic dielectric material generate structural color by directly
controlling the submicrometer structure of the surface. Diverse
fabrication processes were reported such as replicating natural
substrates.sup.19, depositing materials layer by layer.sup.20 and
etching substrate using various lithographic techniques.sup.21,22.
These approaches are advantageous in that they accurately fabricate
periodic dielectric structure on the surface, which controls the
desired photonic band gap. However, in spite of the advantage of
sculpting sophisticated nanostructures in a well controlled manner,
a cost-effective manufacturing scheme to generate multicolored
structures over a large area is hard to achieve due to the
requirement of a vacuum process. Moreover, great effort is
necessary to produce multicolored patterns on a substrate since
different pitches of dielectric stacks are required for different
colors.
[0052] Recently, dynamic tuning of structural color with a single
material has been demonstrated by exerting an external magnetic
field on a solution of photonic crystals. This magnetically tunable
photonic crystal shows broad tunability in its photonic band gap
covering the whole visible spectrum and has a fast response
time.sup.23,24. However, the color of this material cannot be fixed
permanently because the external magnetic field is required to
maintain structural order. If one could instantaneously `freeze`
the photonic crystal structure of the photonic crystals with great
spatial resolution, the artificial patterning of various structural
colors with a single material would be possible.
[0053] In accordance with an exemplary embodiment, an instantaneous
fixing of structural color from photonic crystals and introduce
high resolution patterning of multiple structural colors using a
single material, is described herein. Both material system and
special instrumentation are developed to overcome the limitations
of the previous approaches to produce artificial structural colors.
In accordance with an exemplary embodiment, the applications of
this promising technology: structural color printing for design
materials and structural color encoded particles for biochemical
assay are disclosed.
[0054] It can be appreciated that it would desirable to generate
multicolored patterns with high resolution using a single material
by repetitive tuning and fixing the structural color from the
mixture of superparamagnetic photonic crystals and photocurable
resin (FIG. 1). In accordance with an exemplary embodiment, the
superparamagnetic photonic crystals, each consisting of many single
domain magnetite nanoparticles, which are capped in a shells, which
is preferably a silica shell.sup.24. It can be appreciated that the
superparamagnetic photonic crystals are any composition which can
form ordered structures when exposed to an external magnetic field,
such that the ordered structures diffract light to create color.
preferably the photonic crystals are composed of magnetite
(Fe.sub.3O.sub.4). In addition, the magnetite nanoparticles can be
coated in shells of other suitable mediums, including but not
limited to silica, titania (titanium oxide), and/or polymers such
as polystyrene and polymethylmethacrylate. The coating process
provides a means to obtain good dispersibility and promotes
solvation repulsion in the photocurable solution or resin. The
polymers such as polystyrene and polymethylmethacrylate can be used
after a necessary surface modification. In accordance with an
exemplary embodiment, the thickness of the silica coating can be
controlled by controlling the amount of silane precursors or the
catalyst. The thickness control can be found in (1) Ge, J. and Yin.
Y., "Magnetically Tunable Colloidal Photonic Structures in Alkanol
Solutions", Adv. Mater., 2008, 20, 3485-3491. (2) Yin, Y.; Lu, Y.;
Sun, Y. and Xia, Y., "Silver Nanowires Can Be Directly Coated with
Amorphous Silica to Generate Well-Controlled Coaxial Nanocables of
Silver/Silica", Nano Lett. 2002, 2, 427-430, both herein
incorporated by reference.
[0055] It can be appreciated that in accordance with an exemplary
embodiment, in solution, the magnetite particles are attracted to
each other and will aggregate unless treated to create balancing
repulsive forces. Such balancing forces can be created by solvating
the particles in a solution with a positive charge, which will
repel neighboring positively charged particles. Alkanols, ethanol,
and other solvation solvents can be used for this function.
Alternatively, coatings can be applied to the particles to create
optimal repulsive forces to balance the attraction the magnetite
particles will have for each other. For example, the compositions
and methods described in U.S. Provisional Patent Application Ser.
No. 61/154,717, "Assembly of magnetically tunable photonic crystals
in nonpolar solvents," herein incorporated by reference, can be
employed to produce particles with the proper balance of attractive
and repulsive forces.
[0056] Without an applied external magnetic field, the photonic
crystals are randomly dispersed in the photocurable resin and
display a brown color which is the intrinsic color of magnetite.
Under the external magnetic field, the photonic crystals are
assembled to form chain-like structures along the magnetic field
lines.sup.25,26. Attractive magnetic force due to the
superparamagnetic core is balanced with repulsive solvation force,
both of which determine the inter-particle distance. The
inter-particle distance in a chain determines the color of the
light diffracted from the chain, which can be explained by Bragg
diffraction theory. Thus, the color can be tuned by simply varying
the inter-particle distance using external magnetic fields of
varying strength. Once the desired color is obtained, it can be
fixed by solidifying the photocurable resin through UV exposure.
The particle chains can be frozen in the solidified polymer network
without distorting its periodic arrangements, thus retaining the
structural color.
[0057] However, the above fabrication scheme previously could not
be achieved because of the difficulty of maintaining the tunability
of photonic crystals in photocurable resin and the instantaneous
immobilization of the chain structure. A simple dispersion of CNCs
in photocurable resin does not possess strong and long-range
repulsive inter-particle forces that can cooperate with the
magnetically induced attractive force to allow dynamic tuning.
Without a strong repulsion, the photonic crystals irreversibly
aggregate with each other when they are pushed together upon
application of the external magnetic fields.sup.27,28. A strong
hydrogen bonding solvent such as Alkanols can form a relatively
thick solvation layer around the particle surface which can provide
strong repulsion when two solvation layers overlap.sup.24. In
accordance with an exemplary embodiment, the problem of aggregation
and dynamic assembly in photocurable resin has been solved by
adding a small amount of ethanol to the system. It can be
appreciated that this three phase system, composed of photonic
crystals, ethanol, and photocurable resin, can successfully
stabilize the photonic crystals and maintain the color tunability
(FIG. 1(a)). Once the photonic structures are fixed, the gradual
evaporation of ethanol will not disturb the structural color.
[0058] The second challenge was to develop a rapid solidification
process to prevent distortion of photonic nanostructure.sup.29. In
accordance with an exemplary embodiment, a photopolymerization can
be used to achieve lithographic high resolution patterning of the
photonic crystals. In comparison to the other solidification
methods such as thermal curing, photocuring is instantaneous and
can rapidly fix the color of the photonic crystals achieved by
tuning the external magnetic field. Because of its instantaneous
nature, photocuring also allows localized solidification for high
resolution patterning by avoiding significant free-radical
diffusion during polymerization.sup.30, making it possible to use
techniques such as optofluidic maskless lithography (OFML).sup.31
for creating desired microscale patterns. Any UV or directed energy
system capable of creating localized polymerization or curing of
liquid media to solid can be used. In accordance with an
embodiment, poly(ethylene glycol) diacrylate (PEG-DA or PEGDA) with
a photoinitiator (2,2-dimethoxy-2-phenylacetophenone) can be used
as the photocurable resin. Other suitable photocurable resins
include ethoxylated trimethylolpropane triacrylate (ETPTA), PEG-DA
of various molecular weights (Mw: 258, 575, 700), 2-hydroxyethyl
methacrylate (HEMA), methylmethacrylate (MMA), acrylamide (AAm),
allyamine (AM), and/or any combination thereof.
[0059] In accordance with another exemplary embodiment, the
instantaneous illumination of focused UV energy has been achieved
by exploiting the previously reported OFML system, a versatile tool
for dynamically generating heterogeneous microstructures by in-situ
photopolymerization in microfluidic environment. Fast
microelectromechanical system (MEMS) based spatial light modulator
inside the system provides instantaneous illumination (less than
(<) 80 ms) of patterned UV light to the photocurable
resin.sup.31,32. Using this system, the chain structure can be
preserved without distortion. Compared with the traditional methods
for generating structural color by the slow growth of colloidal
photonic crystals, the magnetic assembly followed by
photopolymerized immobilization can be accomplished within seconds
with a high degree of spatial control.
[0060] Various multicolored patterns can be generated with a single
material by a sequential process involving cooperative actions of
magnetic field modulation and spatially controlled UV exposure
(FIG. 1(b)-(g). In accordance with an exemplary embodiment, a PEG
coated glass slide was used as a substrate to avoid adhesion of the
photonic crystals onto the surface of a bare glass slide. A thin
layer of photonic crystals in curable liquid resin is then
deposited on the substrate (FIG. 1(b). Once a desired color of the
photonic crystals is obtained by exerting a magnetic field, the
patterned UV exposure fixes the color locally, producing a colored
pattern at specific regions (FIG. 1(c)). Then, the color of uncured
liquid resin is changed by simply varying the strength of magnetic
field. Subsequent controlled UV exposure produces another colored
pattern in a different location (FIG. 1(d)). As illustrated by FIG.
1(b)-1(g), micropatterns with different structural colors (FIG.
1(h)) can be easily formed by repeating this "tuning and fixing"
process. No movement of substrate is required for deposition of
multiple ink materials since the photonic crystal solution is
deposited only once at the beginning of the process. Also multiple
patterns can be exposed without movement of both substrate and mask
since the OFML system dynamically controls the pattern of multiple
UV exposure without the need of changing physical photomasks.
Therefore, the methods as described herein combine the advantages
of photonic crystals and OFML, and can achieve high resolution
heterogeneous patterning rapidly by eliminating the need for
alignment and registration.
[0061] In order to demonstrate the concept of generating structural
color with a single ink, multicolor structures were fabricated by
the method as described above.
[0062] The reflective optical microscope image (FIG. 2(a)) and the
corresponding spectrum data (FIG. 2(c)) of each microstructure
shows gradual color changes from red to blue as the applied
magnetic field strength is gradually increased. This gradual
increase in external magnetic field induces increasing attractive
force between the induced magnetic dipole moment of photonic
crystals, thereby decreasing the inter-particle distance in a
chain. In agreement with the Bragg diffraction theory, the spectra
blue shift as the result of the gradual decrease in the
inter-particle distance. It is worth noting that this tuning of the
colors of photonic crystals does not suffer from hysteresis and is
very reproducible due to the paramagnetic nature of photonic
crystals. Furthermore, the wide tuning range covering the whole
visible spectrum is owing to the strong magnetic attractive force
from the superparamagnetic property of photonic crystals as well as
the repulsive forces with comparable strength. In this case, the
repulsion is composed of the relatively weak but long-range
electrostatic force and the relatively strong but short-range
solvation force resulting from the ethanol solvation layer of the
photonic crystals. Colors of the corresponding microstructures
shown in the transmission microscope (FIG. 2(b)) are all brownish,
the intrinsic color of magnetite, which are quite different from
those of the reflective optical microscope image. This unique
difference between the reflection image and the transmission image
further proves the formation of structural color, whose coloration
mechanism is not based on the absorption of light like typical
pigments and dyes. Since the photonic crystal structure can be
frozen within the polymeric matrix, the chain structures directly
were confirmed, which usually de-assembles in solution after
removal of the magnetic field. As shown in FIG. 2(d), a scanning
electron microscope (SEM) image of the sliced cross section with
laser microtome of the cured resin reveals that the diffraction of
structural color does come from the periodic arrangement of the
particles in the chain. The dimpled structures of the sliced cross
sectional plane are the traces of the ordered photonic crystals.
Also, this shows that the photopolymerization by OFML preserves the
original chain structure formed in the liquid phase.
[0063] By controlling the UV exposure pattern and magnetic field
strength as described in FIG. 1, it can be appreciated that a high
resolution patterning of multiple structural colors with different
geometries and colors can be produced. FIG. 2(e) shows four
different multicolored patterns, and each of them is fabricated
with five concentric UV patterns under various magnetic field
intensities. Furthermore, barcoded microstructures composed of
sixteen colorful strips are also fabricated by sixteen sequential
exposures (FIG. 2(f)). It can be appreciated that there is no
alignment error since there is no movement of the substrate during
the exposure. The width of the bar code is only 10 .mu.m which
shows high resolution spatial patterning of structural colors.
Spatial positioning of a smallest feature of structural color
depends on the size of diffracting unit and resolution of the
lithography. Since the size of CNCs (approximately 170 nm) is
smaller than the resolution of our optical system, the spatial
positioning of the structural color is mainly determined by the
resolution of the optical system, which can be enhanced up to the
limit of typical optical lithographic resolutions.sup.33. Colorful
heterogeneous microstructures of any desired shape and color are
easily achieved as shown in FIGS. 2(g)-2(i).
[0064] For detailed depiction of an image, not only producing
structural color of single color depth as shown in FIG. 2, but also
grayscale modulation and color mixing are required to broaden the
ability of color expression. The proposed scheme of generating
structural color can easily be merged with well developed
reprographic techniques such as halftoning and dithering.sup.34,35,
and broaden the capability of color expression. Current digital
reprographic technique expresses grayscale by varying density of
dots in a pixel which is smaller than the human eye's resolution.
In accordance with an exemplary embodiment, analogous to
traditional grayscale expression, the overall reflection intensity
can be modulated by the number of color dots, and present similar
grayscale effects. For the proof-of-concept demonstration, 16 pixel
arrays were generated, and each of them consists of 25
.mu.m.times.25 .mu.m dots whose configuration is based on the Bayer
pattern.sup.34 (FIG. 3(a)). Reflection intensity shown in FIG. 3(b)
verifies 16 distinct intensity levels of corresponding pixel
arrays. As an example for reflection intensity modulation, as shown
in FIG. 3(c) a 4-bit monotone image of Mona Lisa, a 16th century
Italian portrait by Leonardo da Vinci was reproduced. Reflection
intensity of each pixel is modulated by varying the density of dots
with 16 levels.
[0065] Besides the reflection intensity modulation, spatial color
mixing can be achieved by parallel distribution of color dots.
Quantized dot arrays composed of different colors can be seen as a
single mixed color when their size is below human eye's resolution.
To demonstrate spatial color mixing of the structural color, a 16
pixel arrays was fabricated (FIG. 3(d)), and each pixel is composed
of 16 dots of 2 or 3 different colors. Spectrum of color mixed
pixel (FIG. 3(e)) shows that simple summation of the two different
color spectrum results in the total reflection spectrum, proving
the spatial mixing of the distinct structural colors. It can be
appreciated that this simple spatial mixing scheme of structural
color exists in nature. An Indonesian butterfly, Papilo Palinurus,
shows green on its wings, which results from the spatial mixing of
structurally colored blue and yellow.sup.2. Following the scheme of
spatial color mixing, as shown in FIG. 3(f), a butterfly was
artificially reproduced, Papilo Palinurus by biomimetically mixing
structural colors from created by small dots of photonic crystals
fixed at different colors. Magnification of the printed wing area
at FIG. 3(f) shows different color dots, and each of which is the
size of 16.7.times.16.7 .mu.m.sup.2 and well below the regular
human eye's resolution so that spatially distributed dots can be
seen as a single mixed color. Spatial color mixing makes it
possible to broaden the expression range of structural color. It
can be appreciated that a realizable possibility of structural
color printing with fine resolution can be achieved with the
described technique.
[0066] By exploiting the capability of precise color and shape
patterning with a single material, producing structural colors are
not limited to the fixed structure on the substrate, but can be
expanded to the free floating microstructures in a microfluidic
environment as color and shape encoded particles. In the field of
analytical chemistry and bioscience, multiplexed assays in
microfluidic environments have attracted much attention due to
their capability for high throughput screening for drug discovery
and gene expression profiling with precise controllability of a
small volume of reactants. Various techniques to generate encoded
particles have been reported such as semiconductor quantum
dots.sup.36,37, metallic barcode.sup.38, and dot-coded
particles.sup.39. In contrast to the case of quantum dot coding
where precise loading of quantum dots of different sizes is
required to produce distinct encoded particles, encoding with the
invention has the advantage of simultaneous shape and color coding
in a single step by using a single material in a microfluidic
environment.
[0067] In microfluidic channels made of polydimethylsiloxane (PDMS)
and PDMS coated glass substrate, by virtue of an oxygen lubricating
layer, microparticles generated by free-radical photopolymerization
can move along the flow stream without being stuck to the channel
walls.sup.40. Using this property, various color and shape encoded
particles can be generated under distinct levels of magnetic field
intensity with patterned UV light using OFML (FIG. 4(a)-4(c)). To
demonstrate the concept, the liquid curable resin containing
photonic crystals was injected into the microfluidic channel, and
generated microparticles by in-situ photopolymerization guided by
patterned UV light under different magnetic fields (FIGS.
4(d)-4(e)). The encoded particles are caught at the PDMS anchors
and the remnant liquid resin is washed out with PEG-DA monomer
solution. Morphologies of these structures are not restricted to
regular polygonal shape, but can be designed to any desired shape
as displayed in FIG. 4. Heterogeneous encoded particles embedded
with smaller color dots were generated by sequential UV exposure
under various magnetic fields (FIG. 4(f)). The expression of
graphical code, similar to the pattern shown in FIG. 2, is
limitless due to the flexibility of controlling colors and
shapes.
[0068] It can be appreciated that in accordance with an exemplary
embodiment, a high resolution patterning of multiple structural
colors by a single material has been demonstrated, of which the
color is magnetically tunable and lithographically fixable. The
versatile material, is developed by magnetically assembling
superparamagentic photonic crystals into chain-like ordered
structures in photocurable resin through the balanced interaction
of magnetically induced attractive force and the repulsive forces.
A unique process for immobilization of the color of photonic
crystals is developed by taking advantage of the instantaneous
nature of the OFML system. By combining photonic crystals, curable
resin and OFML technique, two important applications in pattern
printing and microparticle encoding all based on the artificial
structural color of photonic crystals have been demonstrated. The
described approach represents a novel multicolor patterning
technique, which produces colorful patterns conveniently from a
single ink instead of using many different inks for different
colors. It can be appreciated that the photonic crystals based
system opens a door to the wide use of structural color for various
potential applications including structural colored design
materials, reflective displays, and bioanalytical assay.
[0069] Methods
[0070] Material
[0071] In accordance with an exemplary embodiment, the three phase
mixture of photonic crystals, solvation liquid and photocurable
resin is used. photonic crystals were synthesized based on
previously described protocols.sup.24, which were initially
dispersed in ethanol. photonic crystals were collected by magnetic
separation, and re-dispersed in photocurable resin without complete
desiccation of ethanol. Remnant ethanol is used as a solvation
liquid. In accordance with an embodiment, poly(ethylene glycol)
diacrylate (PEG-DA, Sigma-Aldrich, M.sub.n=258) with 5 wt % of
photoinitiator (2,2-dimethoxy-2-phenylacetophenone, Sigma-Aldrich)
as the photocurable resin was used. It can be appreciated that
other photocurable resins can include ethoxylated
trimethylolpropane triacrylate (ETPTA), various molecular weights
(Mw: 258, 575, 700) of PEG-DA, 2-hydroxyethyl methacrylate (HEMA),
methylmethacrylate (MMA), acrylamide (AAm), allyamine (AM) and
combinations thereof or any other material capable of being
converted from liquid to solid by exposure to energy of certain
wavelengths. Alternatively, any material capable of being converted
from liquid to solid by exposure to temperature, energy, or other
factors can be used.
[0072] Mixture of photonic crystals and photocurable resin were
vortexed for 5 min. For structural color printing, slide glass
coated with PEG layer was made by depositing poly(ethylene glycol)
diacrylate (PEG-DA, Sigma-Aldrigh, M.sub.n=258) with 5 wt % of
photoinitiator (2,2-dimethoxy-2-phenylacetophenone), and
photopolymerize with UV light. For particle encoding, microfluidic
channel was generated using the method based on standard soft
lithography. Microfluidic channel with the height of 40 .mu.m was
used.
[0073] Immobilization Setup
[0074] In accordance with an exemplary embodiment, a NdFeB
(Neodymium Iron Boron) permanent magnet was used to generate
magnetic field which was attached to the vertical stage at the
microscope. For the dynamic controlling of magnetic field, an
electromagnet coupled to the voltage controller was used. The
photopolymerization setup used in this work was based on the
optofluidic maskless lithography system.sup.31. Exposure pattern of
UV light was controlled by digital micromirror array (DMD, Texas
Instrument) synchronized with the electromagnet, pattern of DMD and
UV exposure.
[0075] Optical Characterization
[0076] Optical micrographs were acquired by true-color charge
coupled device (CCD) camera (DP71, Olympus) which is directly
aligned to the inverted microscope (IX71, Olympus). Spectrum data
was acquired by spectrometer (Acton, Princeton Instrument) which is
connected to the inverted microscope (Eclipse Ti, Nikon). Built-in
field stop shutter in the spectrometer was used for isolating
optical signal from background noise and other neighboring
particles. FIG. 3(c) and FIG. 3(f) were obtained with the
commercially available digital camera (IXUS 870 IS, Canon).
[0077] Magnetochromatic Microspheres
[0078] In accordance with an exemplary embodiment, a method of
forming magnetochromatic microspheres, and more particularly to a
method of forming magnetochromatic microspheres by a simultaneous
magnetic assembly and UV curing process of an emulsion system
comprised of superparamagnetic Fe.sub.3O.sub.4@SiO.sub.2 colloidal
particles, which are self-organized into ordered structures inside
emulsion droplets of UV curable resin.
[0079] Photonic crystal materials with band gap property responsive
to external stimuli have important applications in bio- and
chemical sensors, color paints and inks, reflective display units,
optical filters and switches, and many other active optical
components..sup.41-49 Colloidal crystals, which can be produced
conveniently by self-assembling uniform colloidal particles, have
been particularly useful for making responsive photonic materials
because active components can be incorporated into the crystalline
lattice during or after the assembly process. The majority of
research in the field therefore has been focused on tuning the
photonic properties of colloidal systems through changes in the
refractive indices, lattice constants, or spatial symmetry of the
colloidal arrays upon the application of external stimuli such as
chemical change, temperature variation, mechanical forces,
electrical or magnetic fields, or light..sup.46-66 However, wide
use of these systems in practical applications is usually hampered
by slow and complicated fabrication processes, limited tunability,
slow response to the external stimuli, and difficulty of device
integration.
[0080] Because the photonic band gap is highly dependent on the
angle between the incident light and lattice planes, an alternative
route to tunable photonic materials is to use external stimuli to
change the orientation of a photonic crystal. For easy fabrication,
actuation and broader applications, it is highly desirable that the
photonic crystals can be divided into many smaller parts whose
orientation can be controlled individually or collectively as
needed by using external stimuli. Photonic crystal microspheres, or
"opal balls", have been previously demonstrated by Velev et al. in
a number of pioneering works by using monodispersed silica or
polystyrene beads as the building blocks..sup.67,68 The brilliant
colors associated with these three-dimensional periodic structures,
however, can not be tuned due to lack of control over the
orientation of the microspheres. Xia et al. have introduced
magnetic components into a photonic microcrystal so that its
diffraction can be changed by rotating the sample using external
magnetic fields..sup.69 However, it has not been demonstrated that
one can synthesize multiple copies of such micro-photonic crystals,
align them synchronically, and collectively output uniform color
signals.
[0081] Accordingly, it would be desirable to have a synthetic
procedure for the manufacturing of solid microspheres containing
ordered structures of photonic crystals, which can be called
magnetochromatic microspheres. Provided here in is an exemplary
embodiment for the creation of such magnetochromatic microspheres,
wherein dispersed in emulsion droplets, superparamagnetic
Fe.sub.3O.sub.4@SiO.sub.2 core-shell particles self-organize under
the balanced interaction of repulsive and attractive forces to form
one-dimensional chains, each of which contains periodically
arranged particles diffracting visible light and displaying
field-tunable colors. In addition, it would be desirable to have a
method and/or process, which utilizes UV initiated polymerization
of the oligomers in the emulsion droplets to fix the periodic
structures inside the microspheres and retain the diffraction
property.
[0082] In accordance with an exemplary embodiment, magnetochromatic
microspheres can be fabricated through instant assembly of
superparamagnetic photonic crystals inside emulsion droplets of UV
curable resin followed by an immediate UV curing process to
polymerize the droplets and fix the ordered structures. When
dispersed in the liquid droplets, superparamagnetic
Fe.sub.3O.sub.4@SiO.sub.2 core-shell particles self-organize under
the balanced interaction of repulsive and attractive forces to form
one-dimensional chains, each of which contains periodically
arranged particles diffracting visible light and displaying
field-tunable colors. UV initiated polymerization of the oligomers
of the resin fixes the periodic structures inside the droplet
microspheres and retains the diffraction property. Because the
superparamagnetic chains tend to align themselves along the field
direction, it is very convenient to control the orientation of such
photonic microspheres and accordingly, their diffractive colors, by
changing the orientation of the crystal lattice relative to the
incident light using magnetic fields. The excellent stability
together with the capability of fast on/off switching of the
diffraction by magnetic fields makes the system suitable for
applications such as color display, signage, and sensing. In
accordance with an exemplary embodiment, a display unit that has
on/off bistable states can be fabricated by embedding the
magnetochromatic microspheres in a matrix that can thermally switch
between solid and liquid phases. It can be the matrix can be a
paraffin, long-chain alkanes, esters, primary alcohols,
non-crosslinked polymers such as polyethylene, poly(ethylene
oxide), polyethylene-block-poly(ethylene glycol), and/or polyesters
or any other material capable of being reversibly converted from
liquid to solid.
[0083] It can be appreciated that among potential external stimuli,
a magnetic field has the benefits of contactless control, instant
action, and easy integration into electronic devices, though it has
only been used limitedly in assembling and tuning colloidal
crystals due to the complication of the forces that are
involved..sup.70-72 In accordance with an exemplary embodiment, a
series of magnetically tunable photonic crystal systems have been
developed through the assembly of uniform superparamagnetic (SPM)
colloidal particles in liquid media with various
polarities..sup.73-77 It can be appreciated that in accordance with
an exemplary embodiment, the assembly of such photonic crystals
includes the establishment of a balance between the magnetically
induced dipolar attraction and the repulsions resulted from surface
charge or other structural factors such as the overlap of solvation
layers. This finely tuned dynamic equilibrium leads to the
self-assembly of the magnetic colloids in the form of chain
structures with defined internal periodicity along the direction of
external field, and also renders the system fast, fully reversible
optical response across the visible-near-infrared range when the
external magnetic field is manipulated.
[0084] In accordance with an exemplary embodiment, a magnetically
responsive photonic system has been developed, wherein photonic
crystal microspheres whose orientation and consequently photonic
property can be easily controlled by using external magnetic
fields. In accordance with an exemplary embodiment, the fabrication
of microspheres involves instant assembly of photonic crystals
inside emulsion droplets of UV curable resin and then an immediate
UV curing process to polymerize the droplets and fix the ordered
structures. It can be appreciated that unlike "opal balls" whose
orientation cannot be controlled, fixing of photonic crystals
chains makes microspheres magnetically "polarized" so that their
orientation becomes fully tunable as the SPM chains always tend to
align along the external field direction. In addition, it can be
appreciated that multiple copies of photonic crystal microspheres
can be fabricated in a single process, and their orientation can be
synchronically tuned to collectively display a uniform color. It
can be appreciated that the photonic microsphere system as
disclosed does not involve the nanoparticle assembly step, and
therefore has several advantages. These advantages include
long-term stability of optical response, improved tolerance to
environmental variances such as ionic strength and solvent
hydrophobicity, and greater convenience for incorporation into many
liquid or solid matrices without the need of complicated surface
modification. For example, in accordance with an exemplary
embodiment, it can be appreciated that the magnetochromatic
microspheres can be incorporated into a matrix, which can
reversibly change between liquid and solid phases, to produce a
switchable color display system whose color information can be
switched "on" and "off" multiple times by means of an applied
magnetic field.
[0085] The synthetic procedure of magnetochromatic microspheres in
accordance with an embodiment is illustrated in FIG. 5(a). As shown
in FIG. 5(a), the magnetic iron oxide or magnetite
(.gamma.-Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4) SPM particles are
first coated with a thin layer of silica (i.e., a medium) to obtain
good dispersibility and certain solvation repulsion in the curable
(or photocurable) solution. It can be appreciate that besides
silica, titania (titanium oxide) and some polymer such as
polystyrene and polymethylmethacrylate might be used instead after
necessary surface modification. The thickness of the silica coating
can be controlled by controlling the amount of silane precursors or
the catalyst. The controlling of the thickness of the silica can be
found in our previous publications: (1) Ge, J. and Yin. Y.,
"Magnetically Tunable Colloidal Photonic Structures in Alkanol
Solutions", Adv. Mater., 2008, 20, 3485-3491. (2) Yin, Y.; Lu, Y.;
Sun, Y. and Xia, Y., "Silver Nanowires Can Be Directly Coated with
Amorphous Silica to Generate Well-Controlled Coaxial Nanocables of
Silver/Silica", Nano Lett. 2002, 2, 427-430, which is incorporated
herein in its entirety.
[0086] The silica coated Fe.sub.3O.sub.4SPM particles can be
dispersed in a liquid UV curable resin preferably containing mainly
polyethyleneglycol diacrylate (PEGDA) oligomers and a trace amount
of photo initiator 2,2-Dimethoxy-2-phenylacetophenone (DMPA). It
can be appreciated that other suitable photocurable resins can be
used including but not limited to ethoxylated trimethylolpropane
triacrylate (ETPTA), and/or polyethyleneglycol diacrylate (PEGDA)
of various molecular weights (i.e., Mw: 258, 575, 700),
2-hydroxyethyl methacrylate (HEMA), methylmethacrylate (MMA),
acrylamide (AAm), allyamine (AM) and/or any combination thereof.
Alternatively, any medium capable of being converted from liquid to
solid such that ordered structures of photonic crystals are fixed
within can be used.
[0087] The Fe.sub.3O.sub.4/PEGDA mixture is then dispersed in a
viscous non-polar solvent (or immiscible liquid) such as silicone
oil or mineral oil under mechanical stirring, which leads to the
formation of an emulsion. It can be appreciated that besides
silicone oil or mineral oil, the immiscible liquid can be paraffin
oil or any oil immiscible liquid with the curable solution, and
with appropriate density and inertness to polymerize.
[0088] Upon the application of an external magnetic field, the SPM
particles self-assemble into ordered structures inside the emulsion
droplets when the magnetically induced attraction reaches a balance
with repulsive interactions including electrostatic and solvation
forces..sup.76 In accordance with an exemplary embodiment, an
immediate 365-nm UV illumination quickly polymerizes the PEGDA
oligomers to transform the emulsion droplets into solid polymer
microspheres, and at the same time permanently fixes the periodic
SPM structures..sup.78 It can be appreciated that any suitable
photolithography setup with UV light preferably in the range of
approximately 240 nm (DUV) to 365 nm (1-Line) can be used with this
system to fix the photonic structures in the resin (typical aligner
or stepper). In addition, traditional mask-defined beam patterning
usually requires mechanical movement of the physical mask so that
any alignment error is inevitably incorporated. However, the
Maskless-Lithography proposed has the capability of high resolution
patterning over the lithography with the physical photomasks.
[0089] In accordance with an exemplary embodiment, microspheres
with different colors can be obtained by controlling the
periodicity of the SPM assembly through the variation of the
external magnetic field during the UV curing process. It can be
appreciated that due to the short-range nature of the solvation
force, the range of color that can be produced from a single
Fe.sub.3O.sub.4/PEGDA mixture can be limited..sup.76 However, in
accordance with an exemplary embodiment, in order to produce
microspheres with largely different colors such as red and blue,
Fe.sub.3O.sub.4 particles with different initial sizes or with
SiO.sub.2 coatings of different thicknesses can be used. In
accordance with an exemplary embodiment, the diameter of the
microspheres typically is preferably in the range of approximately
1 .mu.m to 300 .mu.m, and more preferably approximately 10 .mu.m to
100 .mu.m, depending on the type of oil and the speed of mechanical
stirring.
[0090] In accordance with an exemplary embodiment, the microspheres
are preferably large than 10 micrometer (.mu.m), which will present
a consistent color, which is mainly contributed by the straight
photonic chain structures inside the microsphere. However, it can
be appreciated that microspheres smaller than 10 .mu.m can be used.
Once made uniformly in size, it can be appreciated that each of the
microspheres should display the same color with magnetic
tunability.
[0091] The fixation of the periodic SPM particles in the cured
polymer matrix can be verified by inspecting a section that is cut
from a sample along the chain direction. As shown in the scanning
electron microscopy (SEM) image in FIG. 5(b), parallel particle
chains with regular interparticle spacing can be easily observed,
providing direct support of the one-dimensional ordering of the SPM
particles proposed in previous studies..sup.72,75,77 In accordance
with an exemplary embodiment, since the cutting is not strictly
along the chain direction, usually part of the chain is embedded
inside the polymer and part of it has been peeled off, leaving
behind regular cavities. It can be appreciated the separation
between neighboring chains is typically on the order of a few
micrometers due to the strong inter-chain repulsion induced by the
external field..sup.75
[0092] The diffraction of the microspheres dispersed in a liquid
can be conveniently switched between "on" and "off" states by using
the external magnetic field, as shown in the schematic
illustrations and optical microscopy images in FIG. 5(c). In a
vertical field, the particle chains stand straight so that their
diffraction is turned "on" and the corresponding color can be
observed from the top. Each bright green dot in the optical
microscopy image actually represents one vertically aligned
particle chain. On the contrary, when the field is switched
horizontally, the microspheres are forced to rotate 90.degree. to
lay down the particle chains so that the diffraction is turned off
and microspheres show the native brown color of iron oxide. It can
be appreciated that the particle chains can be directly observed by
careful inspection of the microspheres through optical microscopy.
The rotation of microspheres is instant, and synchronized with the
manual movement of external fields, as supported by the videos in
the supplementary information.
[0093] Depending on the direction of the external magnetic field,
the particle chains can be suspended at any intermediate stage
between the on/off states with a specific tilting angle (.theta.).
In accordance with an exemplary embodiment, the dependence of
diffraction peak wavelength (.lamda.) and intensity on the tilting
angle (.theta.) using an optical microscope coupled with a
spectrometer is shown in FIG. 6. While the magnetic field is tuned
within the plane constructed by the incident light and back
scattered light, the diffraction from an isolated microsphere is
recorded correspondingly by the spectrometer, as schematically
shown in FIG. 5(a). It can be appreciated that the diffraction peak
blue-shifts with decreasing intensity when the magnetic field
direction is manipulated away from the angular bisector of incident
light and back scattered light (.theta..apprxeq.14.5.degree.). FIG.
5(b) shows the spectra and corresponding microscopy images when the
angle .theta. is tilted from +10.degree. to -30.degree.. Such a
change in the diffraction peak position and intensity closely
resembles the characteristics of a one-dimensional Bragg photonic
crystal, as proven by the close match between the experimental
results and theoretical simulations. Beyond -30.degree., the
diffraction intensity is very low so that the photonic state of the
microsphere can be practically considered as "off". FIG. 7
demonstrates the complete on/off switching of magnetochromatic
microspheres that originally diffract blue, green and red light.
These microspheres are synthesized by starting with SPM particles
with average diameters of 127, 154, 197 nm. It can be appreciated
that by mixing of RGB (Red, Green and Blue) microspheres in various
ratios can produce a great number of colors that can be
collectively perceived by human eyes.
[0094] In accordance with an exemplary embodiment, the average size
of the microspheres can be controlled using the simple dispersing
process through the choices of the oil type and the speed of
mechanical stirring. It can be appreciated that several methods
including those using microfluidic devices are available to produce
monodispersed microdroplets..sup.79-83 In general, using high speed
stirring and viscous oils leads to the formation of smaller
emulsion droplets. The microspheres prepared in mineral oils have
average diameters above 50 .mu.m, and those prepared in silicone
oils have average diameters less than 30 .mu.m.
[0095] FIG. 8(a) shows a series of dark-field optical microscopy
images of differently sized microspheres selected from the samples
made by using the same Fe.sub.3O.sub.4/PEGDA mixture but with
either mineral oil or silicone oil as the continuous phase.
Vertical external fields are applied so that these microspheres are
all at the "on" state. Microspheres larger than 10 .mu.m containing
particle chains with spacing such that they reflect red light all
display the expected red color, which comes from the diffraction of
a plurality of vertically aligned particle chains. Bright red dots,
which contribute to the overall production of red color, can be
clearly observed inside the microspheres when they are imaged at
higher magnification. However, in the case of microspheres 10 .mu.m
and smaller containing similarly spaced particle chains, fewer red
dots can be observed in the center. Instead, contribution of the
diffraction from the edge to the overall color of the microspheres
gradually increases, with a progressive blue-shift from orange to
yellow and eventually yellow-green as the microsphere size is
reduced. This phenomenon can be explained by the unique
self-assembly behavior of SPM particles in the PEGDA droplets.
[0096] FIGS. 8(b)-(d) show the top-view and side-view SEM images of
the typical microspheres, suggesting that the SPM particle chains
are not only embedded inside the microspheres in the form of
straight strings but also laid on the curved surface along the
longitudinal direction. The "bent" assembly of SPM particles on the
microsphere surface can be attributed to the combined effect of the
spherical confinement of the emulsion droplets and the magnetically
induced strong repulsive force perpendicular to the direction of
the external field. The bent surface assemblies can be viewed as
chains tilted from the vertical direction with the degree of
tilting determined by the curvature of the microspheres. As the
microspheres become smaller, the curvature becomes larger and the
titling angle increases, leading to a blue-shift of the
diffraction. Additionally, the higher surface to volume ratio of
smaller microspheres may also increase the ratio of surface chains
to embedded ones and eventually change the overall diffracted color
of the spheres. For microspheres larger than 10 .mu.m, the embedded
straight assemblies dominate and the bending of the surface
assemblies is small, so that the microspheres show uniform
colors.
[0097] The optical response of the microspheres to the external
magnetic field was characterized by the switching threshold of
field strength and switching frequency, which describe how strong
of an external magnetic field is required to rotate the
microspheres and how fast the microspheres respond to the changes
in the magnetic field, respectively. First, a low concentration of
microspheres dispersed in a density matched solvent--PEGDA liquid
were used to measure the switching threshold. The dispersion was
sandwiched between two hydrophobic glass slides to avoid adhesion
to the glass substrate. With increasing magnetic field strength,
the microspheres were gradually turned "on" and digital photos were
taken after approximately 5 seconds of every change in the field
strength. FIG. 9 shows the statistic diagrams of the percentage of
microspheres (counted in viewable area) that have been turned "on"
in an increasing field for two samples with different loading of
the magnetic materials. The corresponding accumulative curves are
also plotted from the diagrams. It has been found that the loading
of the magnetic materials in the microspheres, and not the sphere
size is one of the factors, which determines the switching
threshold of the field strength. For microspheres with low magnetic
loading (8 mg Fe.sub.3O.sub.4/mL PEGDA), 80% of them can be turned
on in a magnetic field of approximately 180 Gauss; while for
microspheres containing more SPM particles (16 mg
Fe.sub.3O.sub.4/mL PEGDA), only a 100 Gauss magnetic field is
required to turn on the same number of spheres.
[0098] The switching of diffraction could be accomplished rapidly
(i.e., less than approximately 1 second (<1 s)) in a
sufficiently strong magnetic field. Turning frequency of the
microspheres was measured with a test platform built with a halogen
light source, a spectrometer and a rotating magnet unit with geared
DC motor. The rotating plate with NS and SN magnets standing
alternately will produce a periodical vertical (1100-1200 Gauss)
and horizontal magnetic field (300-400 Gauss), whose frequency can
be simply controlled by the rotating speed of the plate.
[0099] FIG. 10 shows the diffraction of microspheres in a 1.22 and
3.33 Hz vertical/horizontal alternating magnetic field,
demonstrating that the photonic microspheres can be rotated
quickly. It can be noted that the rotating amplitude gradually
decreases with the increase of turning frequency, primarily due to
the relatively weak horizontal field strength. In addition, it can
be appreciated that when the frequency is higher than approximately
7 Hz, the rotation of microspheres cannot catch up with the
external field variation so that they seem to simply vibrate around
the vertical state and the diffraction remains on all the time. In
accordance with an exemplary embodiment, the switching frequency
can be further improved when the microspheres are dispersed in a
less viscous solvent or tuned in magnetic fields with higher
strengths.
[0100] In accordance with an exemplary embodiment, the
incorporation of photonic crystals into microspheres allows tuning
of the photonic property by simply controlling the sphere
orientation, making it very convenient to create bistable states
that are required for a plurality of applications such as displays.
For example, a simple switchable color display system in which the
color information can be re-written multiple times by means of the
magnetic field. The basic idea is to create bistable states by
embedding the microspheres into a matrix that can be switched
between liquid and solid states.
[0101] In accordance with an exemplary embodiment, long chain
hydrocarbons and short chain polymers, such as paraffin and
poly(ethylene glycol), have melting points slightly above room
temperature. When heated, the matrix material melts, allowing the
display of colors by aligning the microspheres using magnetic
fields. When the system is cooled to room temperature, the matrix
solidifies and the orientation of microspheres is frozen so that
the color information remains for long time without the need of
additional energy. It can be appreciated that an external magnetic
field can not alter their color once the orientation of
microspheres is fixed by the matrix. Reheating the matrix
materials, however, will erase the particular color by randomizing
the orientation of the microspheres or by magnetically reorienting
the microspheres to a completely "off" state.
[0102] FIG. 11 shows three examples of such displays fabricated by
embedding the microspheres in polyethylene glycol (PEG, Mw=1500)
films, which can be melted at approximately 46.degree. C. The
comparison of digital photos and reflection spectra clearly
demonstrates two stable diffractive states at room temperature,
suggesting the possible applications of such systems as economical
and rewriteable color display units.
[0103] It can be appreciated that the magnetochromatic microspheres
can be prepared through a simultaneous magnetic assembly and UV
curing process in an emulsion system. In accordance with an
exemplary embodiment, superparamagnetic Fe.sub.3O.sub.4@SiO.sub.2
colloidal particles are self-organized into ordered structures
inside emulsion droplets of UV curable resin, followed by an
immediate UV curing process to polymerize the droplets and fix the
ordered structures. In addition, it can be appreciated that by
rotating the microspheres, the orientation of the magnetic chains
can be controlled, and thereby the diffractive colors. In addition,
a plurality of copies of the microspheres can be produced using the
process, and can be tuned by external fields to collectively
display uniform colors. The excellent stability, good compatibility
with dispersion media, and the capability of fast on/off switching
of the diffraction by magnetic fields, also make the system
suitable for applications such as color displays, signage, bio- and
chemical detection, and magnetic field sensing.
[0104] In accordance with an exemplary embodiment, as the size of
the magnetite particle increases, the color red shifts (or the
diffraction wavelength increases). As the thickness of the silica
coating increases, the color red shifts (or the diffraction
wavelength increases). As the magnetic field strength increases,
the color blue shifts (or the diffraction wavelength decreases).
However, it can be appreciated that the color or the diffraction
wavelength is determined by not only the magnetite particle size,
the silica coating (or coating medium), and magnetic field
strength, but also many other parameters such as the chemical
nature of the resin, the surface charge of the particle surface,
and the additives.
[0105] In accordance with an exemplary embodiment, the relation of
the colors (Red, Green & Blue) to the three parameters (size of
magnetite particle, thickness of silica coating, magnetic field
strength) is as follows, as the overall size of
Fe.sub.3O.sub.4/SiO.sub.2 colloids increase from about 120 nm to
200 nm, the color shifts from blue to red. As the magnetic field
strength increase, the color would blue shift. In accordance with
an exemplary embodiment, the magnetic field preferably is in the
range of approximately 100 Gauss to approximately 400 Gauss. It can
also be appreciated that as the amount of magnetic content within a
composite, which is defined as magnetic density, the more magnetic
content (Fe.sub.3O.sub.4), less magnetic field is required to
rotate the microspheres.
[0106] In the accordance with an exemplary embodiment, the method
and systems as disclosed herein, microspheres can be incorporated
into a display device wherein very small quanta of microspheres can
be locally manipulated to change color or to create on-off color
using an integrated micromagnetic actuator to produce local
magnetic flux in the area from several to tens of micrometers. For
example, exemplary methods and devices for actuating microspheres
includes those described in Chong H. Ahn and Mark G. Allen, A Fully
Integrated Micromagnetic Actuator With A Multilevel Meander
Magnetic Core, in "Solid-State Sensor and Actuator Workshop, 1992.
5th Technical Digest., IEEE", 1992, page 14-18; and Yae Yeong Park;
Han, S. H.; Allen, M. G., Batch-fabricated microinductors with
electroplated magnetically anisotropic and laminated alloy cores,
IEEE Transactions on Magnetics, 1999, 35, 4291-4300; (3) J. Park,
S. Han, W. Taylor, and M. Allen, "Fully integrated micromachined
inductors with electroplated anisotropic magnetic cores," in IEEE
13th Applied Power Electron. Conf. Anaheim, Calif., 1998, which are
incorporated herein in their entirety disclose examples of
microscale devices.
[0107] In accordance with another embodiment, it can be appreciated
that the ordered structures in the micromagnetospheres are composed
of parallel 1D chains of magnetite crystals, their spacing
determined by the balance of the attractive and repulsive forces,
which in turn are affected by the external magnetic field. In
addition, it can be appreciated that the colors exhibited by the
magnetite crystals in solution, or fixed, are created by the
ordered structures described above (1D chains).
[0108] It will be understood that the foregoing description is of
the preferred embodiments, and is, therefore, merely representative
of the article and methods of manufacturing the same. It can be
appreciated that many variations and modifications of the different
embodiments in light of the above teachings will be readily
apparent to those skilled in the art. Accordingly, the exemplary
embodiments, as well as alternative embodiments, may be made
without departing from the spirit and scope of the articles and
methods as set forth in the attached claims.
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