U.S. patent application number 15/193210 was filed with the patent office on 2017-03-23 for processing of superhydrophobic, infrared transmissive, anti-reflective nanostructured surfaces.
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 Ishwar D. Aggarwal, Shyam S. Bayya, Darryl A. Boyd, Lynda E. Busse, Jesse A. Frantz, Woohong Kim, Jasbinder S. Sanghera.
Application Number | 20170082783 15/193210 |
Document ID | / |
Family ID | 58282306 |
Filed Date | 2017-03-23 |
United States Patent
Application |
20170082783 |
Kind Code |
A1 |
Boyd; Darryl A. ; et
al. |
March 23, 2017 |
PROCESSING OF SUPERHYDROPHOBIC, INFRARED TRANSMISSIVE,
ANTI-REFLECTIVE NANOSTRUCTURED SURFACES
Abstract
Methods for producing nanostructured, hydrophobic,
superhydrophobic, or hydrophilic, transmissive, anti-reflective
surfaces are described. The method for providing a hydrophilic
surface includes steps of providing a substrate that is
transmissive at at least one wavelength in the infrared to
ultraviolet range of the electromagnetic spectrum and comprises at
least one surface including nanostructures of a size smaller than
the at least one wavelength; and functionalizing the at least one
surface with hydroxyl groups thereon. A hydrophobic or
superhydrophobic surface can be provided by contacting the at least
one surface with a hydrophobic fluoropolymer for a time sufficient
to apply at least a monolayer of fluorine-containing material to
the at least one surface. These methods provide devices having
excellent transmittance and anti-reflectance properties and which
are resistant to seawater.
Inventors: |
Boyd; Darryl A.;
(Alexandria, VA) ; Frantz; Jesse A.; (Landover,
MD) ; Bayya; Shyam S.; (Ashburn, VA) ; Busse;
Lynda E.; (Alexandria, VA) ; Sanghera; Jasbinder
S.; (Ashburn, VA) ; Kim; Woohong; (Lorton,
VA) ; Aggarwal; Ishwar D.; (Waxhaw, NC) |
|
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 |
|
|
Family ID: |
58282306 |
Appl. No.: |
15/193210 |
Filed: |
June 27, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62184269 |
Jun 25, 2015 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C03C 17/30 20130101;
C03C 2217/732 20130101; C03C 2217/76 20130101; C03C 17/32 20130101;
G02B 1/118 20130101; B05D 3/142 20130101; B05D 1/18 20130101; B05D
1/185 20130101; B05D 5/083 20130101; G02B 1/18 20150115 |
International
Class: |
G02B 1/18 20060101
G02B001/18; G02B 1/118 20060101 G02B001/118; B05D 5/00 20060101
B05D005/00; B05D 3/14 20060101 B05D003/14; B05D 1/18 20060101
B05D001/18 |
Claims
1. A method of forming a hydrophobic, anti-reflective, transmissive
material, comprising: providing a substrate that is transmissive at
at least one wavelength in the infrared to ultraviolet range of the
electromagnetic spectrum and comprising at least one surface
including nanostructures of a size smaller than the at least one
wavelength; functionalizing the at least one surface to provide
hydroxyl groups thereon; and contacting the at least one surface
with a solution comprising a hydrophobic fluoropolymer for a
sufficient time to apply at least a monolayer of a
fluorine-containing material on the at least one surface.
2. The method of claim 1, wherein the at least one hydrophobic
fluoropolymer comprises fluorine proximate to or at a first end and
a hydroxyl-reactive group on a second end.
3. The method of claim 1, wherein the hydroxyl-reactive group
contains a trichlorosilane group.
4. The method of claim 3, wherein the at least one hydrophobic
fluoropolymer is selected from the group consisting of,
1H,1H,2H,2H-perfluorooctyl trichlorosilane,
1H,1H,2H,2H-perfluorodecyl trichlorosilane,
1H,1H,2H,2H-perfluorodecyl acrylate, an amorphous
polytetrafluoroethylene resin, and an alkyl or fluoroalkyl
thiol.
5. The method of claim 1, wherein the substrate is selected from
the group consisting of, fused silica, quartz-containing materials,
germanium-containing materials, alumina-containing materials, and
other optical and non-optical glasses, crystals and ceramics.
6. The method of claim 1, wherein the nanostructures occur in a
non-random or periodic pattern.
7. The method of claim 1, wherein the nano-structures occur in a
random pattern.
8. The method of claim 1, wherein the nano-structures have
topological features selected from the group consisting of: jagged,
pointed, cylindrical pillars, pointed cones, truncated cones, and a
honeycomb pattern.
9. The method of claim 1, wherein the nanostructures are patterned
into the at least one surface of said substrate.
10. The method of claim 1, wherein the contacting step is carried
out for a period of 10 seconds to 3 minutes with a solution of the
hydrophobic fluoropolymer.
11. The method of claim 1, wherein the functionalizing step
comprising plasma etching in an oxygen atmosphere.
12. The method of claim 1, further comprising a step of curing at a
temperature of at least 80.degree. C. after the contacting
step.
13. The method of claim 1, wherein the at least one wavelength
comprises a range of wavelengths in the infrared range of the
electromagnetic spectrum.
14. A method of forming an anti-reflective, transmissive,
superhydrophilic material, comprising: providing a substrate that
is transmissive at at least one wavelength in the infrared to
ultraviolet range of the electromagnetic spectrum and comprising at
least one surface including nanostructures of a size smaller than
the at least one wavelength; and functionalizing the at least one
surface with hydroxyl groups thereon.
15. The method of claim 14, wherein the substrate is selected from
the group consisting of, fused silica, quartz-containing materials,
germanium-containing materials, alumina-containing materials, and
other optical and non-optical glasses, crystals and ceramics.
16. The method of claim 14, wherein the at least one wavelength
comprises a range of wavelengths in the infrared range of the
electromagnetic spectrum.
17. The method of claim 14, wherein the nanostructures are
patterned into the at least one surface of said substrate and have
topological features selected from the group consisting of: jagged,
pointed, cylindrical pillars, pointed cones, truncated cones, and a
honeycomb pattern.
18. The method of claim 14, wherein the functionalizing step
comprising plasma etching in an oxygen atmosphere.
19. The method of claim 14, wherein the substrate is fused
silica.
20. The method of 14, wherein the substrate is germanium.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. provisional
application No. 62/184,269, filed Jun. 25, 2015.
TECHNICAL FIELD
[0002] This disclosure pertains to a method of making
superhydrophobic, or superhydrophilic materials having nanoscale
features that impart anti-glare properties and enhanced
transmittance in the ultraviolet (UV), visible and infrared (IR)
region of the light spectrum. Such materials may also maintain
their wettability characteristics in seawater.
BACKGROUND
[0003] Nature has provided numerous examples of materials with
surfaces having nanoscale features that serve a practical survival
purpose for living species. The material that comprises a moth's
eye is one such surface. The surface of moth eye provides
advantageous transmission and anti-reflection ("AR") properties
that allow a moth to see in dim light and prevents reflection of
light off of the moth eye in order to help evade potential
predators.
[0004] Upon examination, the structure of the moth's eye has been
determined to contain an array of nanoscopic features that are
responsible for its AR character. These features allow the surface
to transmit a large fraction of incident light, allowing the moth
to see in dimly lit areas while at the same time limiting
reflection so that the moth's eye can go visually undetected by
predators.
[0005] There are many well-known moth eye mimic technologies that
work well for AR for wavelengths from the visible into the mid-IR.
An array of different materials have been modified to contain
periodic and randomly nanopatterned surfaces. The application of
the nanopattern onto a surface has often been accomplished by
coating a surface with a material to impart the AR properties.
[0006] When nanoscale features are patterned into the surface of a
transparent substrate, the shape and dimensions of those features
determine the light transmission as a function of wavelength. These
features may be ordered or random as long as the features are
sub-wavelength in scale.
[0007] Fused silica is commonly used as a substrate material that
transmits in the near-infrared (NIR) region of the light spectrum.
Additionally, various other materials have been used as substrate
materials based on their ability to transmit energy in specific
regions of the electromagnetic spectrum. A surface of each of these
materials may be patterned to provide AR technology to the surface.
However, fused silica and the other materials used in these
approaches tend to be hydrophilic, which presents obstacles for use
of the technologies in certain applications. Moreover, the
application of nanostructures to a surface may increase the
hydrophilicity of a material. Specifically, a nanopatterned surface
of fused silica may be more hydrophilic when imprinted with
nanostructures.
[0008] What is needed is a surface modification process for
altering the surface of a highly transmissive, AR nanopatterned
material to make them superhydrophobic, or to control their wetting
properties such that they can also be made superhydrophilic. Such
modification would broaden the scope of applications in which this
technology can be used (e.g. windows, goggles, lenses, etc.).
SUMMARY OF THE INVENTION
[0009] The present disclosure relates to a method of forming a
hydrophobic, anti-reflective material, which is transmissive at at
least one wavelength in the ultraviolet to infrared range of the
electromagnetic spectrum. The method includes steps providing a
substrate having a transmissive wavelength at at least one
wavelength in the ultraviolet to infrared range of the
electromagnetic spectrum, the substrate comprising at least one
surface including nanostructures of a size smaller than the at
least one wavelength, functionalizing at least one surface to
provide hydroxyl groups thereon, and contacting the at least one
surface with a solution comprising a hydrophobic fluoropolymer for
a sufficient time to apply at least a monolayer of a
fluorine-containing material on the at least one surface.
[0010] In the foregoing methods, at least one hydrophobic
fluoromonomer may comprise fluorine proximate to or at a first end
and a hydroxyl-reactive group on the second end. The
hydroxyl-reactive group is preferably trichlorosilane. The
fluoromonomer is preferably selected from the group of
perfluoroalkyl trichlorosilanes, such as 1H,1H,2H,2H-perfluorooctyl
trichlorosilane and 1H,1H,2H,2H-perfluorodecyl trichlorosilane.
Other fluorinated monomers, such as fluoroalkyl thiol, may also be
used. In each of the foregoing embodiments, the contacting step may
be carried out for a period of 10 seconds to 3 minutes with a
solution of the fluoromonomer. In each of the foregoing
embodiments, the functionalizing step may comprise plasma etching
in an oxygen atmosphere. In each of the foregoing embodiments, the
method may further comprise a step of curing at a temperature of at
least 80.degree. C. after the contacting step.
[0011] Also disclosed is a method of forming an anti-reflective,
transmissive, superhydrophilic material. The method includes steps
of providing a substrate, which is transmissive at at least one
wavelength in the ultraviolet to infrared range of the
electromagnetic spectrum. The substrate comprises at least one
surface including nanostructures of a size smaller than the at
least one wavelength of the substrate that provides less Fresnel
reflection loss; and functionalizing at least one surface with
hydroxyl groups thereon.
[0012] In each of the foregoing embodiments, the nanostructures may
occur in a non-random pattern, a periodic pattern or in a random
pattern. The nanostructures may have topological features selected
from the group consisting of: jagged, pointed, cylindrical pillars,
pointed cones, truncated cones, and a honeycomb pattern. The
nanostructures are patterned into at least one surface of said
substrate.
[0013] In each of the foregoing embodiments the substrate may be
formed from any material having the desired transmissive
properties. The substrate may be selected from the group consisting
of, fused silica, quartz-containing materials, germanium-containing
materials, alumina-containing materials, sesquioxide based
materials, yttrium aluminum garnet materials and other optical and
non-optical glasses, crystals and ceramics. In each of the
foregoing embodiments, the substrate may be transmissive at a range
of wavelengths in the infrared range of the electromagnetic
spectrum. Preferably, the substrate may be fused silica or
germanium.
[0014] Additional details and advantages of the disclosure will be
set forth in part in the description which follows, and/or may be
learned by practice of the disclosure. The details and advantages
of the disclosure may be realized and attained by means of the
elements and combinations particularly pointed out in the appended
claims. It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are not restrictive of the disclosure, as
claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1A is an edge-on scanning electron microscope (SEM)
image of an embodiment showing nanostructures patterned (i) into a
fused silica substrate (ii).
[0016] FIG. 1B is a SEM image of the structures of FIG. 1A taken
top down.
[0017] FIG. 1C is a SEM image of the structures of FIG. 1A taken at
a 30.degree. angle.
[0018] FIG. 1D is an enlarged SEM image of the structures of FIG.
1C.
[0019] FIG. 2A shows photographic images of substrates made from
germanium having various shapes of nanostructures patterned on a
germanium substrate according to embodiments of the invention.
[0020] FIG. 2B is a top down SEM image of the embodiments of
nanostructures patterned on a germanium substrate of FIG. 2A.
[0021] FIG. 2C is a SEM image of the structures of FIG. 2B taken at
a 30.degree. angle.
[0022] FIG. 3 is a drawing representing a testing pattern used to
test the water contact angle of the invention.
[0023] FIG. 4A is a representative image of a surface water contact
angle on a flat silica substrate.
[0024] FIG. 4B is a representative image of a surface water contact
angle on a flat silica substrate with chemical modification.
[0025] FIG. 5A is a representative image of a surface water contact
angle on a fused silica substrate modified according to an
embodiment of the present invention.
[0026] FIG. 5B is a picture of a fused silica substrate made
according to an embodiment of the present invention.
[0027] FIG. 6 is a graph of % transmittance versus wavelength of
various different fused silica substrates.
[0028] FIG. 7 is a graph of % reflectance versus wavelength of
various different nanostructured fused silica substrates.
[0029] FIG. 8A and FIG. 8B are representative images of a surface
water contact angle on a fused silica substrate modified according
to an embodiment of the present invention related to submersion and
incubation in seawater.
[0030] FIG. 9 is a graph of % transmission versus wavelength of
various different germanium substrates.
[0031] FIG. 10A shows pictures of a drop of water on unmodified
germanium surfaces containing the nanostructures of FIG. 2A.
[0032] FIG. 10B shows representative images of a surface water
contact angles on the germanium substrates of FIG. 10A modified
according to an embodiment of the present invention.
[0033] FIG. 10C shows representative images of a surface water
contact angles on germanium substrates of FIG. 10B related to
submersion and incubation in seawater.
[0034] FIG. 10D shows representative images of a surface water
contact angles on germanium substrates of FIG. 10B related to
sonication in deionized water after submersion and incubation in
seawater.
[0035] FIG. 11 is a graph of % transmission versus wavelength of
germanium substrates having nanostructures of various different
shapes.
[0036] FIG. 12 is a graph of % transmission versus wavelength of
various different germanium substrates having honeycomb-shaped
nanostructures.
DETAILED DESCRIPTION
[0037] Various embodiments of the disclosure provide a method for
chemically modifying a light-transmissive, anti-reflective surface
including nanostructures, such that the wetting properties of the
surface can be altered as desired. Specifically, the surface may be
made hydrophobic, superhydrophobic, or superhydrophilic, depending
on the desired end use of the product.
[0038] The device created by the present method includes a
substrate having at least one surface including a nanopatterned
structure. Controlled nanopatterning of the substrate surface
increases the transmission of the substrate by decreasing the
surface reflectance, both of which are desirable qualities. The
nanopatterned surface is then subjected to chemical modification to
produce substrates that have beneficial surface wetting properties,
desirable transmission in the ultraviolet, visible and infrared
region of the optical spectrum, and advantageous light reflection
properties.
[0039] The resulting chemically modified patterned surface may have
decreased reflectance as compared with a non-chemically modified
smooth surface of the same substrate. The resulting nanostructured
surface after chemical modification may have increased transmission
properties for certain target wavelengths, as compared with a
non-chemically modified substrate material with the same
nanostructured surface.
[0040] Wetting properties of a surface may be determined by
observing the behavior of liquid droplets on the surface and
measuring the contact angle formed at the location where the
liquid-vapor interface contacts the solid-liquid interface. A
hydrophilic material is a material having a contact angle for water
on the surface of less than 90.degree.. A superhydrophilic material
is a material having a contact angle for water on the surface of
less than 10.degree.. A hydrophobic material has a water contact
angle between 90.degree. and 150.degree., and a superhydrophobic
material has a water contact angle of 150.degree. or greater.
[0041] Various different substrate materials are known to provide
antireflective (AR) properties when they are modified to have
nanostructures on at least one surface. Surface modification to
provide nanostructures may be carried out by application of a
coating to the surface, or by direct patterning of the surface of
the substrate to include the nanostructure. Coatings have the
potential drawback that delamination may occur. Therefore, directly
patterning at least one surface of the substrate is preferable as
it eliminates the possibility of coating delamination. Preferably,
the nanostructures are patterned on one of the surfaces of the
substrate, but the nanostructures may also be patterned on two, or
more of the surfaces of a substrate material, if desired. The
nanostructures are intentionally formed on at least one surface and
they are not the same as surface scratches. Such unintentional
surface scratches could reduce the transmission properties of a
substrate material.
[0042] The substrate may be made of any material having
transmissive properties for at least one wavelength of the
electromagnetic spectrum. Fused silica is known to have good
transmissive properties from the visible to near infrared (NIR, 1-2
.mu.m) wavelengths. Optical materials having transmissive
properties in other wavelength regions, such as the mid-wave
infrared (MWIR, 2-5 .mu.m) and long-wave infrared (LWIR, 8-14
.mu.m), may be used as substrate materials. Additionally, optical
materials having transmissive properties in the visible (400-1000
nm) and ultraviolet (UV, 10-400 nm) wavelength ranges may also be
used as a substrate. Preferable materials for use as the substrate
include, fused silica, quartz-containing materials,
germanium-containing materials, alumina-containing materials,
sesquioxide based materials, yttrium aluminum garnet materials,
other optical and non-optical glasses, crystals, including, but not
limited to diamonds, ceramics, as well as other transmissive
materials. Preferably, the substrate is selected from fused silica
and germanium, and even more preferably, the substrate is fused
silica.
[0043] Each substrate material has transmissive properties within a
known wavelength range, based on the inherent properties of the
material. The nanostructures patterned onto the surface of the
substrate may be formed in specific, sizes, shapes, and/or
patterns, or may be randomized in order to provide the best
transmissive and AR properties for each specific material. For
example, FIG. 1A and FIG. 1B show a randomized nanostructure
patterned on the surface of a substrate made of fused silica. FIG.
1A is an edge-on scanning electron microscope (SEM) image showing
the pattern and depth of the nanostructures on the underlying fused
silica substrate. FIG. 1B shows a top down SEM image of the
nanostructures on the fused silica substrate. FIG. 1C and FIG. 1D
are 30.degree. tilted angle SEM images of the nanostructures on the
fused silica substrate. The scale bars in FIG. 1A represents 1
.mu.m and the scale bars in FIG. 1B, FIG. 1C, and FIG. 1D represent
300 nm. As seen from the SEM images of FIG. 1A, FIG. 1B, FIG. 1C,
and FIG. 1D, the random nanostructures have jagged, pointed shapes
with various, different dimensions. The random jagged structures
are a preferred surface for use with fused silica for providing
good transmission and AR properties.
[0044] The nanostructure shape and patterning need not be the same
for every type of material that may be used as a substrate. FIG. 2A
shows photographs of representative germanium substrates. FIG. 2B
and FIG. 2C show preferred nanostructure shapes and patterns for
use on a surface of a substrate made from germanium. FIG. 2B shows
top-down SEM images of germanium substrate nanostructures. FIG. 2C
shows 30.degree. tilted angle SEM images of the nanostructures
preferred for use with germanium substrates. In FIG. 2A, FIG. 2B,
and FIG. 2C, Ge represents a non-nanostructured germanium surface.
The scale bars in FIG. 2A represent 1 cm. The scale bars in FIG. 2B
represent 10 .mu.m, and the scale bars in FIG. 2C represent 1
.mu.m.
[0045] The preferred nanostructures for use with germanium
substrates have periodic patterns of pillar-like shapes (shown as
Ge.sub.P in FIG. 2A, FIG. 2B, and FIG. 2C), truncated cone shapes
(shown as Ge.sub.C in FIG. 2A, FIG. 2B, and FIG. 2C), or honeycomb
shape (shown as Ge.sub.HC in FIG. 2A, FIG. 2B, and FIG. 2C). In the
pillar and truncated cone embodiments, each of the pillars, or
cones is similar in size and shape to the other pillars, or cones
forming a substantially uniform pattern of nanostructures.
Likewise, the honeycomb-shaped structure is also a uniform pattern
of recessed spaces. Such patterned, uniform nanostructures provide
good transmission and AR properties using a germanium
substrate.
[0046] Preferable shapes for each of the various materials are
known and any shape or combination of shapes of nanostructures that
provide beneficial transmission and AR properties to the material
may be used in the present method. Such nanostructure shapes may
include cylindrical pillars, pointed cones, truncated cones,
honeycomb patterns, as well as various combinations of one or more
of these and other structural shapes. Depth, width, angle,
periodicity, degree of taper, and the geometry of the nanostructure
is carefully designed and applied to the substrate to provide the
desired performance of the device. The substrate material and the
nanostructure pattern used on at least one surface of the material
may be selected based on the desired wavelength to be transmitted
through the substrate.
[0047] Many materials with transmissive properties that are
suitable for nanostructure patterning to provide materials with
good transmissive and AR properties tend to be hydrophilic.
Moreover, the patterning of the nanostructures onto the surface of
these materials may result in the substrate becoming even more
hydrophilic, as in the case of fused silica. The hydrophilic nature
of these substrates may present obstacles for their use in certain
applications. Depending on the material used for the substrate and
the desired end use, the following process may be used to make the
substrate hydrophobic, superhydrophobic, or superhydrophilic.
[0048] After obtaining a substrate having at least one surface
including nanostructures, the substrate surface including the
nanostructures thereon is cleaned. The cleaning process utilized
for this purpose should not damage the nanoscale features of the
substrate surface. This cleaning may be carried out by using a
process that forms hydroxyl moieties on the surface. Such processes
include plasma etching under an O.sub.2(g) atmosphere, piranha
etching, and other known processes such as treatment with
KOH/Alcohol solutions. Preferably, the surface of the substrate is
cleaned using a plasma etching device under an O.sub.2(g)
atmosphere. The plasma etching process may be carried out at any
time, temperature and pressure that is commonly known in the art.
Preferably, the plasma etching process under O.sub.2(g) atmosphere
is conducted for a time of up to 10 minutes under pressure (e.g.
320 mTorr) and high power (e.g. 200 W).
[0049] As a result of the cleaning step, hydroxyl groups are formed
on the surface of the substrate that includes the nanostructures.
If not further modified, these hydroxyl groups may interact with
water to attract the water to the surface. Thus, a surface that
includes the nanostructures and is functionalized with hydroxyl
groups tends to be superhydrophilic. In some cases a
superhydrophilic surface may be desired. In such cases, the process
may be terminated at this point to obtain a material that is
anti-reflective, light transmissive and superhydrophilic.
[0050] If a hydrophobic or superhydrophobic material is desired,
the process may be continued with a step of chemically modifying
the surface with the hydroxyl groups to create a coating on the
surface. The reactive hydroxyl groups that result from the cleaning
step are used in the coating step to form a material having a
surface with the desired level of hydrophobicity.
[0051] The chemical treatment may be accomplished by applying a
self-assembled monolayer of a hydrophobic material on the surface
of the substrate. The chemical treatment may use any monomer that
is capable of binding with hydroxyl groups located on the surface
of the substrate on a first end, and contains a hydrophobic element
on a second end. The hydrophobic element is preferably fluorine and
the hydrophobic material may be a hydrophobic fluoromonomer. More
preferably, the hydrophobic fluoromonomer contains a
trichlorosilane group as the hydroxyl-reactive group, and the
hydrophobic fluoromonomer is preferably 1H,1H,2H,2H-perfluorooctyl
trichlorosilane (PFOTS), or 1H, 1H,2H,2H-perfluorodecyl
trichlorosilane. Most preferably the hydrophobic fluoromonomer is
1H,1H,2H,2H-perfluorooctyl trichlorosilane. Other hydrophobic
coatings may also be applied to the surface of the substrate,
including, 1H, 1H,2H,2H-perfluorodecyl acrylate, an amorphous
polytetrafluoroethylene resin, and a fluoroalkyl thiol.
[0052] Trichlorosilanes are preferred because they readily form
self-assembled monolayers (SAMs) on hydroxyl-functionalized
surfaces. As a consequence, when molecules having a trichlorosilane
group are introduced to the surface, the molecules self-align
forming a surface with the trichlorosilane facing the substrate and
the opposite end of the molecule facing away from the substrate. In
the case of PFOTS, the fluorine-containing moieties are located at
the air interface. The fluorine moieties individually occupy a
large spatial area, and their collective SAM alignment above the
substrate surface makes the surface hydrophobic or
superhydrophobic.
[0053] Once the coating is formed, the chemically modified
substrate may be cured, e.g. by thermal curing, to solidify the
bonding between the substrate and the coating. The chemical
modification typically forms a monolayer of the hydrophobic
material on the surface. A monolayer of the described materials
does not cause a significant change in the optical properties of
the substrate material.
[0054] A particular method for coating the surface of a fused
silica substrate is described in more detail below. A fused silica
substrate having random, rugged, pointed nanostructures extending
approximately 800 nm into the surface of the substrate on at least
one surface is obtained by any suitable method. The size of the
nanostructures may be in any range such that the structures are
smaller than the wavelength of the electromagnetic energy that is
transmitted by the substrate. Optionally, two or more surfaces of
the substrate may also be patterned.
[0055] The fused silica substrate having the nanostructured surface
may be cleaned using a plasma etching device under an O.sub.2(g)
atmosphere. After cleaning, the substrate is passed through a
solution of hexanes containing 0.5% PFOTS in a swirling fashion for
a short period of time to contact the nanostructured surface with
the PFOTS. The short swirling or contacting period prevents the
buildup of non-specifically bound PFOTS on the surface of the
substrate, which could potentially cloud the surface, causing a
decrease in the substrate's transmissive properties. Preferably,
the short swirling or contacting period ranges from 10 seconds to 3
minutes, more preferably, the short swirling or contacting period
is from 10 second to 1 minute, and most preferably, the swirling or
contacting period is up to 30 seconds.
[0056] The substrate is then thermally cured at 120.degree. C. to
solidify bonding between the fused silica and the PFOTS molecules.
The temperature for thermal curing is selected based on the
materials used as the substrate and as the coating and is typically
at least 80.degree. C. The 120.degree. C. is preferable for use
with fused silica and PFOTS. The range of about 105.degree. C. to
about 120.degree. C. is acceptable for use with these particular
materials. Any suitable curing temperature for curing the
particular materials used for the substrate and coating may be
employed.
[0057] Amounts, times, temperatures and other variables in the
process can be altered depending on the specific substrate and
other materials being used. Overall, the disclosed process is
straightforward, scalable, and applicable in numerous markets such
as, but not limited to, the self-cleaning materials market (i.e.
materials with high contact angle, superhydrophobic surfaces that
cause the beading and rolling off of water droplets at shallow
slide angles) and the high transmission optical coatings
market.
[0058] The following examples are illustrative, but not limiting,
of the methods and compositions of the present disclosure. Other
suitable modifications and adaptations of the variety of conditions
and parameters normally encountered in the field, and which are
obvious to those skilled in the art, are within the spirit and
scope of the disclosure. All patents and publications cited herein
are fully incorporated by reference herein in their entirety.
EXAMPLES
[0059] Water contact angle measurements were taken using a
Rame-Hart instrument, model #590-F4. 10 .mu.L droplets of room
temperature, deionized water were placed on the surface of the
patterned substrates. Measurements were taken in a three by three
grid fashion, with nine contact angle values recorded for each
substrate, as illustrated in FIG. 3, which shows the spheres of 10
.mu.L water droplets depicted on a substrate divided into a three
by three grid. The best value and an average of all nine values
were determined for each substrate. The area of each of the 10
.mu.L droplets of water that was in contact with the nanostructured
surfaces was determined by the Cassie-Baxter equation:
Cos CA.sub.avg=.phi..sub.s(cos .theta..sub.s+1)-1
Where CA.sub.avg is the average water contact angle, .theta..sub.s
is the intrinsic water contact angle for unmodified, flat fused
silica, and .phi..sub.s is the area of the substrate surface in
contact with the water.
[0060] Advancing (CA.sub.Adv) and receding water contact angles
(CA.sub.Rec) were determined using the "sessile drop" method, as
described by J. T. Korhonen, T. Huhtamaki, O. Ikkala, R. H. Ras,
Langmuir: ACS J. Surf. Colloids 29 (2013) 3858-3863.
[0061] Spectroscopic data was collected for each of the samples.
UV-visible-NIR data were obtained both before and after surface
modification using an Agilent Technologies Cary 7000 Universal
Measurement Spectrophotometer in the wavelength range of 250-2300
nm. Specifically, percent transmission and percent reflectance data
were obtained.
[0062] Scanning electron micrograph images were taken using a Carl
Zeiss LEO Supra 55 scanning electron microscope. Images were taken
at 0.degree., 10.degree., 20.degree., and 30.degree. to visualize
surface topography.
Comparative Example 1
[0063] The device of Comparative Example 1 was created using a flat
UV grade fused silica substrate. The flat silica substrate was
plasma cleaned by subjecting it to an O.sub.2(g) atmosphere for 10
minutes in a March Plasma Reactive Ion Etcher under a pressure of
320 mTorr, and at a power of 200 W. The flat fused silica substrate
was tested using the testing procedures described above.
Comparative Example 2
[0064] The device of Comparative Example 2 was created using a flat
UV grade fused silica substrate. The flat silica substrate was
plasma cleaned by subjecting it to an O.sub.2(g) atmosphere for 10
minutes in a March Plasma Reactive Ion Etcher under a pressure of
320 mTorr, and at a power of 200 W. Following the plasma clean, the
flat substrate was immersed and gently agitated in a solution of
hexanes that contained 0.5 vol % PFOTS for less than a minute.
[0065] Although the application of PFOTS onto the flat fused silica
surface did not result in a superhydrophobic surface, the chemical
modification that resulted was effective to improve the
hydrophobicity of the non-nanostructured fused silica. More
specifically, the contact angle was increased from
60.9.degree..+-.5.3.degree. to 107.5.degree..+-.3.2.degree., as
shown in FIG. 4A and FIG. 4B and included in Table 1 below.
Comparative Example 3
[0066] The device of Comparative Example 3 employed a UV grade
fused-silica substrate, which was randomly pre-patterned on one
side with a nanostructure extending approximately 800 nm into the
surface of the substrate. The substrate was subjected to an
O.sub.2(g) atmosphere for 10 minutes in a March Plasma Reactive Ion
Etcher under a pressure of 320 mTorr, and at a power of 200 W.
Example 1
[0067] UV grade fused-silica substrates, which were randomly
pre-patterned on one side with nanostructures extending
approximately 800 nm into the surface of the substrate, were
subjected to an O.sub.2(g) atmosphere for 10 minutes in a March
Plasma Reactive Ion Etcher under a pressure of 320 mTorr, and at a
power of 200 W. The substrates were then removed, immersed and
swirled for 30 seconds in hexanes that contained 0.5%
1H,1H,2H,2H-perfluorooctyl trichlorosilane (PFOTS). The substrates
were immediately rinsed with hexanes, blown dry with N.sub.2(g) and
placed in an oven to bake for 15 minutes at 120.degree. C. The
substrates were removed from the oven and allowed to cool.
Water Contact Angle Analysis
[0068] Water contact angle measurements were taken using the
devices of Comparative Examples 1-3 and Example 1. The device of
Comparative Example 1 had a contact angle indicative of a
hydrophilic substance. The contact angle of the device of
Comparative Example 3 was found to be nearly 0.degree. (i.e.
superhydrophilic) because the fluid readily spread out along the
substrate surface upon contact, rendering both the slide angle (SA)
and contact angle immeasurable.
[0069] For the substrate of Example 1, the surface exhibited
superhydrophobicity, with the greatest contact angle being
176.degree.. The average contact angle was
172.8.degree..+-.4.5.degree.. An example contact angle for a water
droplet on the surface of Example 1 is shown in FIG. 5A. To
determine the amount of water in contact with the surface, the
Cassie-Baxter equation was employed. Using this equation, it was
determined that the fraction of the water droplets in contact with
the nanostructured surface was 0.01. By comparison, the
Cassie-Baxter value for flat fused silica was determined to be
0.47, indicating that much more of a single 10 .mu.L droplet of
water was in contact with the non-nanostructured fused silica
surface of the device of Comparative Example 3 than the chemically
modified surface of the device of Example 1. The results of the
water contact analysis for the four analyzed samples are included
in Table 1.
TABLE-US-00001 TABLE 1 Slide Contact Hysteresis Cassie-Baxter
Example Angle Angle (CA) CA.sub.Adv CA.sub.Rec Value Value
Comparative 1 >10.degree. 60.9.degree. .+-. 5.3.degree.
54.4.degree. 52.4.degree. 2.degree. N/A Comparative 2
>10.degree. 107.5.degree. .+-. 3.2.degree. 105.3.degree.
85.4.degree. 19.9.degree. 0.47 Comparative 3 N/A N/A N/A N/A N/A
N/A Example 1 <1.degree. 172.8.degree. .+-. 4.5.degree.
164.4.degree. 160.9.degree. 3.5.degree. 0.01
[0070] The hysteresis value (H) was determined using the
equation:
H=CA.sub.Adv-CA.sub.Rec
[0071] For the device of Example 1, the hysteresis value was
determined to be 3.5.degree.. This small hysteresis value suggests
that the nanostructured and chemically treated substrate of Example
1 behaves as an almost ideal surface. The hysteresis value of an
ideal surface would be 0.degree..
[0072] Chemically modifying the fused silica patterned structure
with PFOTS (Ex. 1) resulted in a hydrophobicity greater than that
of either the PFOTS on a flat fused silica substrate (Comp. Ex. 2),
or the fused silica having nanostructures that were not chemically
modified with PFOTS (Comp. Ex. 3). The hydrophobicity was increased
by a much larger amount than would have been predicted. Increases
in water contact angle of greater than 60% were consistently
measured.
Transmittance and Reflectance Properties
[0073] Transmittance and reflectance properties were measured as
discussed above. With a transmittance of approximately 92% from the
UV through the NIR spectra (with the exception of the --OH overtone
peaks seen at .about.1390 nm and .about.2200 nm), uncoated UV grade
fused silica is a common standard for visible and infrared
transmission quality. When fused silica is nanopatterned on one
side, its transmittance can be improved by almost 4% for most of
this range, as seen in FIG. 6, wherein the dashed line represents
the unmodified flat fused silica of Comparative Example 1 and the
other two lines represent the non-chemically modified (Comparative
Example 3) and chemically modified (Example 1) fused silica with
nanopatterning. In FIG. 6, the red line represents Comparative
Example 3 and the blue line represents Example 1. As is typical for
this type of AR structure, transmittance decreases in the UV
wavelength range as a result of scattering due to the
nanostructures. Following the processing of the substrate, there
was less than a 1% decrease in transmittance for the majority of
the region interrogated. This demonstrates that the fused silica
substrates having nanostructures on at least one surface can
undergo chemical modification while retaining the desirable high
transmittance properties in the visible-NIR region of the
spectrum.
[0074] The transparency of the substrate of Example 1 is also shown
in FIG. 5B, which shows four colored water drops on the substrate.
The image behind the substrate can be clearly seen which
demonstrates the transparency of the substrate.
[0075] Maintaining the reflectance properties of the material is
also desirable. Data were taken using the devices of Comparative
Example 2 and Example 1. As with the transmission properties, there
was only a slight change in the reflectance properties after
chemical modification of the nanostructure fused silica substrate,
as seen in FIG. 7. The reflectance values were comparable to the
theoretical value of 3.5% for an ideal one-sided, UV grade, and AR
modified fused silica substrate, which is shown as the dashed line
in FIG. 7. The chemically modified substrate of Example 1 had a
slightly higher reflectance than the non-chemically modified
nano-structured surface. In FIG. 7, the red line represents the %
reflectance of Comparative Example 2 and the blue line represents
the % reflectance of Example 1. Ultimately, these data confirm that
the nanopatterned surface topology improved the reflectance
characteristics of the fused silica material, and that this
reflectance character is essentially maintained after chemical
modification.
[0076] In addition to exhibiting superhydrophobicity following
surface modification, the chemically modified, nanostructured,
fused silica substrate of Example 1 also maintained its
transparency and anti-reflectance character.
Seawater Incubation Tests
[0077] To evaluate the efficacy of the chemical modification in
harsher conditions, the devices were incubated in seawater. Clean
fused silica substrates prepared according to Comparative Examples
1-3 above were immersed in artificial seawater (meeting ASTM
standard D1151-98) that was purchased from Lake Products Company,
LLC, and used as received. The surface modified fused silica
nanostructured substrate, prepared according to Example 1 above was
also tested under the same conditions. Each of the substrates was
subjected to a 20 hour incubation period in the artificial
seawater. Following incubation, water contact angle measurements
were obtained using the same test discussed above. Deionized water
was used for the contact angle measurements.
[0078] Incubating the non-nanostructured fused silica without
chemical modification in seawater yielded a surface having water
contact angles of approximately 45.degree., which is significantly
less than the contact angle of approximately 60.degree. determined
for the clean sample. However, upon sonication in deionized water
for 5 minutes the contact angle could be restored to approximately
60.degree..
[0079] Incubating the nanostructured fused silica in seawater
yielded an extremely hydrophilic surface. The water contact angle
was less than 1.degree. and was essentially immeasurable.
[0080] The chemically modified non-nanostructured fused silica
substrate produced contact angles that were within the margin of
error of each other both before and after seawater incubation. This
was likely due to the fact that there were no nanoscale features on
the surface to alter the surface response to the chemical
modification.
[0081] There was a noticeable effect of seawater incubation on the
chemically modified, nanostructured fused silica substrate.
However, the surface remained superhydrophobic with an average
water contact angle of 157.1.degree..+-.9.4.degree.. The water
contact angle increased after sonication in deionized water for 5
minutes. The average contact angle after sonication was
166.5.degree..+-.5.7.degree.. These results are shown in FIG. 8A
and FIG. 8B and indicate that the chemical modification of the
nanostructured fused silica substrate was stable when exposed to
seawater.
Example 2
[0082] PFOTS (or similar functioning species) can be applied to
other optical materials with surface anti-reflective structures to
enable superhydrophobicity in other wavelength regions, such as the
mid-wave infrared (MWIR) and long-wave infrared (LWIR). The
application to substrate materials other than fused silica was
demonstrated in Example 2 using germanium substrates. Each device
using a germanium substrate had a differently shaped periodic,
nanoscale surface pattern as shown in FIG. 2A, FIG. 2B, and FIG.
2C. These non-random patterns include nano-pillars (shown as
Ge.sub.P), truncated nano-cones (shown as Ge.sub.C), and/or
honeycomb patterns (shown as Ge.sub.HC). The substrates were
chemically treated using the same process as described in Example
1.
Water Contact Angle Analysis
[0083] Following chemical modification of the germanium substrates,
the contact angle for water was measured following the procedure
included above. After chemical modification with PFTOS, each of the
germanium substrate surfaces exhibited water contact angles that
had greater hydrophobicity than its corresponding non-treated
surface. FIG. 10A shows pictures of a water drop on the different
germanium surfaces having differently shaped nanostructures prior
to chemical modification. FIG. 10B shows a representative contact
angle of a water drop with each of the germanium surfaces having
differently shaped nanostructures after the substrates were
chemically modified. The contact angle for the PFOTS treated
surface without nanostructures was approximately 110.degree., the
PFOTS treated surface with pillar-shaped nanostructures was
approximately 160.degree., the PFOTS treated surface with
truncated-cone-shaped structures was approximately 150.degree., and
the PFOTS treated surface having honeycomb shaped nanostructures
was approximately 135.degree.. A comparison of FIG. 10A with FIG.
10B shows that the chemical modification of the germanium
substrates greatly increased their hydrophobicity.
Transmittance Properties
[0084] The transmittance of substrates having nanostructures
thereon in the MWIR and LWIR wavelengths was determined as included
above, and is shown in FIG. 11. FIG. 11 shows a comparison of the %
transmittance of non-chemically modified germanium substrates
having different shapes of nanostructures on one surface. The
different shapes are shown in FIG. 2A, FIG. 2B, and FIG. 2C and
include, pillars, truncated cones, and a honeycomb shape. The
transmittance of the baseline flat non-chemically modified
germanium substrate is shown in FIG. 11 as the dashed line. In FIG.
11, the transmittance of the substrates with the pillar-shaped
nanostructures, the truncated cone-shaped nanostructures, and the
honeycomb-shaped nanostructures are shows as red, green, and blue,
lines, respectively. FIG. 11, shows that the transmittance of the
substrates including nanostructures of any of the tested shapes was
higher than the flat germanium surface at almost all wavelengths
tested.
[0085] The transmittance of a chemically modified substrate having
nanostructures thereon in the MWIR and LWIR wavelengths was
determined and is shown in FIG. 9. FIG. 9 shows a comparison of the
chemically modified and non-chemically modified germanium
substrates having nanopatterns on one surface as the two curves
that are similarly shaped and are generally in the 55-65%
transmittance range. The % transmittance of the non-chemically
modified and chemically modified germanium substrates are shown in
FIG. 9 as the blue line and the red line, respectively. The
transmittance of the baseline flat non-chemically modified
germanium substrate is also shown in FIG. 9 as the gray dashed
line, which is the lowest line and is located below the 50%
transmittance level for the entire tested spectrum. Following
chemical modification, the germanium substrate surfaces produced
water contact angles that were hydrophobic, and the transmittance
of the material was essentially unchanged.
[0086] The % transmittance of the chemically modified substrates
having nanostructures with a honeycomb shape, shown as Ge.sub.HC in
FIG. 2A, FIG. 2B, and FIG. 2C, were determined and are shown in
FIG. 12. FIG. 12 shows a comparison of the chemically modified and
non-chemically modified germanium substrates having the honeycomb
shaped nanostructures on one surface. The grey dashed line is the
baseline flat, non-chemically modified germanium substrate. The
blue line is the non-chemically modified honeycomb nanostructured
substrate and the red line is the chemically modified honeycomb
nanostructured substrate. The transmittance of the chemically
modified germanium substrate having honeycomb shaped nanostructures
on one surface was essentially unchanged from the transmittance of
the non-chemically modified germanium substrate having honeycomb
shaped nanostructures on one surface.
Seawater Incubation Tests
[0087] To evaluate the efficacy of the chemical modification to the
germanium substrates in harsher conditions, the devices formed from
germanium substrates were also incubated in seawater. Chemically
modified flat germanium substrate (shown as Ge in FIG. 2A, FIG. 2B,
FIG. 2C, FIG. 10A, FIG. 10B, FIG. 10C, and FIG. 10D), and
chemically modified germanium substrates having one surface with
nanostructures patterned thereon were tested. Each of the different
nanostructure shapes and patterns shown in FIG. 2A, FIG. 2B, and
FIG. 2C were tested with the seawater incubation method.
Specifically, substrates having pillar-shaped (shown as Ge.sub.P in
FIG. 2A, FIG. 2B, FIG. 2C, FIG. 10A, FIG. 10B, FIG. 10C, and FIG.
10D), truncated cone-shaped (shown as Ge.sub.C in FIG. 2A, FIG. 2B,
FIG. 2C, FIG. 10A, FIG. 10B, FIG. 10C, and FIG. 10D), and
honeycomb-shaped (shown as Ge.sub.HC in FIG. 2A, FIG. 2B, FIG. 2C,
FIG. 10A, FIG. 10B, FIG. 10C, and FIG. 10D) nanostructures were
tested.
[0088] Clean germanium substrates prepared according to the method
used for the fused silica samples above were immersed in artificial
seawater and tested according to the same procedure as used for the
fused silica devices discussed above. Deionized water was used for
the contact angle measurements.
[0089] FIG. 10C shows representative water contact angles for the
substrates after seawater incubation. Incubating the chemically
modified non-nanostructured germanium in seawater yielded a surface
having water contact angles of approximately 105.degree., which is
close to the 110.degree. for the contact angle prior to seawater
incubation. As with the fused silica samples, the small change was
likely due to the fact that there were no nanoscale features on the
surface to alter the surface response to the chemical
modification
[0090] There was a noticeable decrease in the contact angle of all
of the chemically modified, nanostructured germanium substrates
after incubation in seawater. However, the surface of each of the
substrates remained hydrophobic.
[0091] After the seawater incubation, each of the samples were
sonicated in deionized water for 5 minutes. Representative contact
angle measurements for each of the germanium samples after
sonication is shown in FIG. 10D. For almost all of the samples the
average contact angle after sonication increased compared to the
samples that were incubated in seawater. These results indicate
that the much of the hydrophobicity from the chemical modification
of the nanostructured fused silica substrate was recoverable after
seawater incubation.
[0092] The data generated and shown in FIG. 9, FIG. 10A, FIG. 10B,
FIG. 10C, FIG. 10D, and FIG. 12, indicate that the process for
providing a superhydrophobic, anti-reflective optical material may
be used on materials other than fused silica that are known to have
acceptable transmissive properties in the desired wavelength range.
The similar results between substrates of fused silica and
substrates of germanium show that the process is effective on
different substrates, and is applicable to other substrates, which
have difference transmittance properties. Additionally, the data
from the Examples shows that the method is applicable to substrates
having different shapes, patterns, or randomized nanostructures on
at least one surface.
[0093] At numerous places throughout this specification, reference
has been made to a number of U.S. patents and other documents. All
such cited documents are expressly incorporated in full into this
disclosure as if fully set forth herein.
[0094] Other embodiments of the present disclosure will be apparent
to those skilled in the art from consideration of the specification
and practice of the embodiments disclosed herein. As used
throughout the specification and claims, "a" and/or "an" may refer
to one or more than one. Unless otherwise indicated, all numbers
expressing quantities of ingredients, properties such as molecular
weight, percent, ratio, reaction conditions, and so forth used in
the specification and claims are to be understood as being modified
in all instances by the term "about," whether or not the term
"about" is present. Accordingly, unless indicated to the contrary,
the numerical parameters set forth in the specification and claims
are approximations that may vary depending upon the desired
properties sought to be obtained by the present disclosure. At the
very least, and not as an attempt to limit the application of the
doctrine of equivalents to the scope of the claims, each numerical
parameter should at least be construed in light of the number of
reported significant digits and by applying ordinary rounding
techniques. Notwithstanding that the numerical ranges and
parameters setting forth the broad scope of the disclosure are
approximations, the numerical values set forth in the specific
examples are reported as precisely as possible. Any numerical
value, however, inherently contains certain errors necessarily
resulting from the standard deviation found in their respective
testing measurements. It is intended that the specification and
examples be considered as exemplary only, with a true scope and
spirit of the disclosure being indicated by the following
claims.
[0095] The foregoing embodiments are susceptible to considerable
variation in practice. Accordingly, the embodiments are not
intended to be limited to the specific exemplifications set forth
herein above. Rather, the foregoing embodiments are within the
spirit and scope of the appended claims, including the equivalents
thereof available as a matter of law.
[0096] The patentees do not intend to dedicate any disclosed
embodiments to the public, and to the extent any disclosed
modifications or alterations may not literally fall within the
scope of the claims, they are considered to be part hereof under
the doctrine of equivalents.
[0097] It is to be understood that each amount/value or range of
amounts/values for each component, compound, substituent or
parameter disclosed herein is to be interpreted as also being
disclosed in combination with each amount/value or range of
amounts/values disclosed for any other component(s), compounds(s),
substituent(s) or parameter(s) disclosed herein and that any
combination of amounts/values or ranges of amounts/values for two
or more component(s), compounds(s), substituent(s) or parameters
disclosed herein are thus also disclosed in combination with each
other for the purposes of this description. It is further
understood that each range disclosed herein is to be interpreted as
a disclosure of each specific value within the disclosed range that
has the same number of significant digits. Thus, a range of from
1-4 is to be interpreted as an express disclosure of the values 1,
2, 3 and 4.
[0098] It is further understood that each lower limit of each range
disclosed herein is to be interpreted as disclosed in combination
with each upper limit of each range and each specific value within
each range disclosed herein for the same component, compounds,
substituent or parameter. Thus, this disclosure to be interpreted
as a disclosure of all ranges derived by combining each lower limit
of each range with each upper limit of each range or with each
specific value within each range, or by combining each upper limit
of each range with each specific value within each range.
[0099] Furthermore, specific amounts/values of a component,
compound, substituent or parameter disclosed in the description or
an example is to be interpreted as a disclosure of either a lower
or an upper limit of a range and thus can be combined with any
other lower or upper limit of a range or specific amount/value for
the same component, compound, substituent or parameter disclosed
elsewhere in the application to form a range for that component,
compound, substituent or parameter.
* * * * *