Non-color shifting multilayer structures and protective coatings thereon

Abstract

An omnidirectional structural color pigment having a protective coating. The pigment has a first layer of a first material and a second layer of a second material, the second layer extending across the first layer. In addition, the pigment reflects a band of electromagnetic radiation having a predetermined full width at half maximum (FWHM) of less than 300 nm and a predetermined color shift of less than 30° when the pigment is exposed to broadband electromagnetic radiation and viewed from angles between 0 and 45°. The pigment has a weather resistant coating that covers an outer surface thereof and reduces a relative photocatalytic activity of the pigment by at least 50%.

Description

FIELD OF THE INVENTION

The present invention is related to multilayer thin film structures having protective coatings thereon, and in particular to multilayer thin film structures that exhibit a minimum or non-noticeable color shift when exposed to broadband electromagnetic radiation and viewed from different angles with a protective coating thereon.

BACKGROUND OF THE INVENTION

Pigments made from multilayer structures are known. In addition, pigments that exhibit or provide a high-chroma omnidirectional structural color are also known. However, such prior art pigments have required as many as 39 thin film layers in order to obtain desired color properties.

It is appreciated that cost associated with the production of thin film multilayer pigments is proportional to the number of layers required. As such, the cost associated with the production of high-chroma omnidirectional structural colors using multilayer stacks of dielectric materials can be prohibitive. Therefore, a high-chroma omnidirectional structural color that requires a minimum number of thin film layers would be desirable.

In addition to the above, it is appreciated that pigments can exhibit fading, changing of color, etc. when exposed to sunlight, and in particular to ultraviolet light. As such, a high-chroma omnidirectional structural color pigment that is weather resistant would also be desirable.

SUMMARY OF THE INVENTION

An omnidirectional structural color pigment having a protective coating is provided. The pigment has a first layer of a first material and a second layer of a second material, the second layer extending across the first layer. In addition, the pigment reflects a band of electromagnetic radiation having a predetermined full width at half maximum (FWHM) of less than 300 nm and a predetermined color shift of less than 30° when the pigment is exposed to broadband electromagnetic radiation and viewed from angles between 0 and 45°. Also, the pigment has a weather resistant coating that covers an outer surface thereof and reduces a relative photocatalytic activity of the pigment by at least 50%.

The weather resistant coating can include an oxide layer and the oxide layer can be selected from silicon oxide, aluminum oxide, zirconium oxide, titanium oxide, and/or cerium oxide. In addition, the weather resistant coating can include a first oxide layer and a second oxide layer, the second oxide layer being different from the first oxide layer. Furthermore, the second oxide layer can be a hybrid oxide layer that is a combination of at least two different oxides. Finally, the pigment itself, i.e. the pigment without the protective coating, does not contain an oxide layer.

A process for producing omnidirectional structural color pigments with a protective coating is also disclosed. The process includes providing a plurality of pigment particles having structure and properties as stated above, and suspending the plurality of pigment particles in a first liquid to form a pigment suspension. In addition, an oxide precursor containing a second liquid and an oxide forming element such as silicon, aluminum, zirconium, titanium or cerium is provided. The pigment suspension and the oxide precursor are mixed and result in the deposition of a weather resistant oxide coating onto the plurality of pigment particles, the coating reducing a relative photocatalytic activity of the pigment particles by at least 50%.

In some instances, the first liquid is a first organic solvent and the second liquid is a second organic solvent. Also, the first and second organic solvents can be organic polar solvents such as n-propyl alcohol, isopropyl alcohol, ethanol, n-butanol and acetone. In other instances, the first organic solvent and the second organic solvent can be organic polar protic solvents.

Regarding the oxide precursor, the oxide forming element silicon can be in the form of tetraethoxysilane, the oxide forming element aluminum can be in the form of at least one of aluminum sulfate and aluminum-tri-sec-butoxide, the oxide forming element zirconium can be in the form of zirconium butoxide, the oxide forming element cerium can be in the form of at least one of cerium nitrate hexahydrate, cerium sulfate and the oxide forming element titanium can be in the form of at least one of titanium ethoxide, titanium isopropoxide and titanium-butoxide.

In other instances, the first liquid is a first aqueous liquid and the second liquid is a second aqueous liquid. In addition, the oxide forming element silicon can be in the form of sodium silicate, the oxide forming element aluminum can be in the form of at least one of aluminum sulfate, aluminum sulfate hydrate and sodium aluminate, the oxide forming element zirconium can be in the form of zirconyl chloride octahydrate, the oxide forming element cerium is in the form of cerium nitrate hexahydrate and the oxide forming element titanium can be in the form of titanium tetrachloride.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a schematic illustration of a dielectric layer (DL) reflecting and transmitting incident electromagnetic radiation;

FIG. 1B is a schematic illustration of a reflector layer (RL) reflecting incident electromagnetic radiation;

FIG. 1C is a schematic illustration of an absorbing layer (AL) absorbing incident electromagnetic radiation;

FIG. 1D is a schematic illustration of a selective absorbing layer (SAL) reflecting, absorbing and transmitting incident electromagnetic radiation;

FIG. 2 is a schematic illustration of reflectance and transmission of incident electromagnetic radiation by a 1^st generation omnidirectional structural color multilayer thin film made from a plurality of dielectric layers;

FIG. 3 is a schematic illustration of a 1^st generation omnidirectional structural color multilayer thin film made from a plurality of dielectric layers;

FIG. 4 is a graphical representation showing a comparison of the range to mid-range ratio of 0.2% for the transverse magnetic mode and transverse electric mode of electromagnetic radiation;

FIG. 5 is a graphical representation of reflectance as a function of wavelength for Case II shown in FIG. 4;

FIG. 6 is a graphical representation of the dispersion of the center wavelength in Case I, II and III shown in FIG. 4;

FIG. 7 is a schematic illustration of reflectance and absorption of incident electromagnetic radiation by a 2^nd generation omnidirectional structural color multilayer thin film made from a plurality of dielectric layers and an absorbing layer;

FIG. 8 is a schematic illustration of a 2^nd generation omnidirectional structural color multilayer thin film made from a plurality of dielectric layers and an absorbing layer and/or reflecting layer;

FIG. 9A is schematic illustration of a 2^nd generation 5-layer omnidirectional structural color multilayer thin film made from a plurality of dielectric layers and an absorbing/reflecting layer having a chroma (C*) of 100 and a reflectance (Max R) of 60%;

FIG. 9B is a graphical representation of reflectance versus wavelength for the 2^nd generation 5-layer multilayer stack thin film shown in FIG. 9A compared to a 1^st generation 13-layer multilayer thin film and for viewing angles of 0 and 45 degrees;

FIG. 10 is a schematic illustration of a 3^rd generation omnidirectional structural color multilayer thin film made from a dielectric layer, a selective absorbing layer (SAL) and a reflector layer;

FIG. 11A is a schematic illustration of a zero or near-zero electric field point within a ZnS dielectric layer exposed to electromagnetic radiation (EMR) having a wavelength of 500 nm;

FIG. 11B is a graphical illustration of the absolute value of electric field squared (|E|²) versus thickness of the ZnS dielectric layer shown in FIG. 1A when exposed to EMR having wavelengths of 300, 400, 500, 600 and 700 nm;

FIG. 12 is a schematic illustration of a dielectric layer extending over a substrate or reflector layer and exposed to electromagnetic radiation at an angle θ relative to a normal direction to the outer surface of the dielectric layer;

FIG. 13 is a schematic illustration of a ZnS dielectric layer with a Cr absorber layer located at the zero or near-zero electric field point within the ZnS dielectric layer for incident EMR having a wavelength of 434 nm;

FIG. 14 is a graphical representation of percent reflectance versus reflected EMR wavelength for a multilayer stack without a Cr absorber layer (e.g., FIG. 1A) and a multilayer stack with a Cr absorber layer (e.g., FIG. 3A) exposed to white light;

FIG. 15A is a graphical illustration of first harmonics and second harmonics exhibited by a ZnS dielectric layer extending over an Al reflector layer (e.g., FIG. 4A);

FIG. 15B is a graphical illustration of percent reflectance versus reflected EMR wavelength for a multilayer stack with a ZnS dielectric layer extending across an Al reflector layer, plus a Cr absorber layer located within the ZnS dielectric layer such that the second harmonics shown in FIG. 8A are absorbed;

FIG. 15C is a graphical illustration of percent reflectance versus reflected EMR wavelength for a multilayer stack with a ZnS dielectric layer extending across an Al reflector layer, plus a Cr absorber layer located within the ZnS dielectric layer such that the first harmonics shown in FIG. 8A are absorbed;

FIG. 16A is a graphical illustration of electric field squared versus dielectric layer thickness showing the electric field angular dependence of a Cr absorber layer for exposure to incident light at 0 and 45 degrees;

FIG. 16B is a graphical illustration of percent absorbance by a Cr absorber layer versus reflected EMR wavelength when exposed to white light at 0 and 45° angles relative to normal of the outer surface (0° being normal to surface);

FIG. 17A is a schematic illustration of a red omnidirectional structural color multilayer stack according to an embodiment of the present invention;

FIG. 17B is a graphical illustration of percent absorbance of the Cu absorber layer shown in FIG. 10A versus reflected EMR wavelength for white light exposure to the multilayer stack shown in FIG. 10A at incident angles of 0 and 45°;

FIG. 18 is a graphical comparison between calculation/simulation data and experimental data for percent reflectance versus reflected EMR wavelength for a proof of concept red omnidirectional structural color multilayer stack exposed to white light at an incident angle of 0°.

FIG. 19 is a graphical illustration of percent reflectance versus wave length for an omnidirectional structural color multilayer stack according to an embodiment of the present invention;

FIG. 20 is a graphical illustration of percent reflectance versus wave length for an omnidirectional structural color multilayer stack according to an embodiment of the present invention;

FIG. 21 is a graphical illustration of percent reflectance versus wave length for an omnidirectional structural color multilayer stack according to an embodiment of the present invention;

FIG. 22 is a graphical illustration of percent reflectance versus wave length for an omnidirectional structural color multilayer stack according to an embodiment of the present invention;

FIG. 23 is a graphical representation of a portion of an a*b* color map using the CIELAB color space in which the chroma and hue shift are compared between a conventional paint and a paint made from pigments according to an embodiment of the present invention (Sample (b));

FIG. 24 is a schematic illustration of an omnidirectional structural color multilayer stack according to an embodiment of the present invention;

FIG. 25 is a schematic illustration of an omnidirectional structural color multilayer stack according to an embodiment of the present invention;

FIG. 26 is a schematic illustration of an omnidirectional structural color multilayer stack according to an embodiment of the present invention;

FIG. 27 is a schematic illustration of a four layer omnidirectional structural color pigment according to an embodiment of the present invention;

FIG. 28 is a schematic illustration of a seven layer omnidirectional structural color pigment according to an embodiment of the present invention;

FIG. 29 is a schematic illustration of a seven layer omnidirectional structural color pigment having a protective coating according to an embodiment of the present invention;

FIG. 30 is a schematic illustration of a protective coating containing two or more layers according to an embodiment of the present invention;

FIG. 31 is a graphical plot illustrating normalized relative photolytic activity for an omnidirectional structural color pigment having a number of different protective coatings;

FIG. 32A is one of a pair of scanning electron microscopy (SEM) images for a plurality of seven layer omnidirectional structural color pigments without a protective coating; and

FIG. 32B is one of a pair of scanning electron microscopy (SEM) images for a plurality of seven layer omnidirectional structural color pigments having a silicon oxide and zirconium oxide-aluminum oxide protective coating.

DETAILED DESCRIPTION OF THE INVENTION

An omnidirectional structural color is provided. The omnidirectional structural color has the form of a multilayer thin film (also referred to as a multilayer stack herein) that reflects a narrow band of electromagnetic radiation in the visible spectrum and has a small or non-noticeable color shift when the multilayer thin film is viewed from angles between 0 to 45 degrees. The multilayer thin film can be used as pigment in a paint composition, a continuous thin film on a structure and the like.

The multilayer thin film includes a multilayer stack that has a first layer and a second layer extending across the first layer. In some instances, the multilayer stack reflects a narrow band of electromagnetic radiation that has a FWHM of less than 300 nm, preferably less than 200 nm and in some instances less than 150 nm. The multilayer thin film also has a color shift of less than 50 nm, preferably less than 40 nm and more preferably less than 30 nm, when the multilayer stack is exposed to broadband electromagnetic radiation, e.g. white light, and viewed from angles between 0 and 45 degrees. Also, the multilayer stack may or may not have a separate reflected band of electromagnetic radiation in the UV range and/or the IR range.

The overall thickness of the multilayer stack is less than 2 μm, preferably less than 1.5 μm, and still more preferably less than 1.0 μm. As such, the multilayer stack can be used as paint pigment in thin film paint coatings.

The first and second layers can be made from dielectric material, or in the alternative, the first and/or second layer can be made from an absorbing material. Absorbing materials include selective absorbing materials such as Cu, Au, Zn, Sn, alloys thereof, and the like, or in the alternative colorful dielectric materials such as Fe₂O₃, Cu₂O, combinations thereof, and the like. The absorbing material can also be a non-selective absorbing material such as Cr, Ta, W, Mo, Ti, Ti-nitride, Nb, Co, Si, Ge, Ni, Pd, V, ferric oxides, combinations or alloys thereof, and the like. The thickness of an absorbing layer made from selective absorbing material is between 20-80 nm whereas the thickness of an absorbing layer made from non-selective absorbing material is between 5-30 nm.

The multilayer stack can also include a reflector layer which the first layer and the second layer extend across, the reflector layer made from metals such as Al, Ag, Pt, Cr, Cu, Zn, Au, Sn, alloys thereof, and the like. The reflector layer typically has a thickness between 30-200 nm.

The multilayer stack can have a reflected narrow band of electromagnetic radiation that has the form of a symmetrical peak within the visible spectrum. In the alternative, the reflected narrow band of electromagnetic radiation in the visible spectrum can be adjacent to the UV range such that a portion of the reflected band of electromagnetic radiation, i.e. the UV portion, is not visible to the human eye. In the alternative, the reflected band of electromagnetic radiation can have a portion in the IR range such that the IR portion is likewise not visible to the human eye.

Whether the reflected band of electromagnetic radiation that is in the visible spectrum borders the UV range, the IR range, or has a symmetrical peak within the visible spectrum, multilayer thin films disclosed herein have a reflected narrow band of electromagnetic radiation in the visible spectrum that has a low, small or non-noticeable color shift. The low or non-noticeable color shift can be in the form of a small shift of a center wavelength for a reflected narrow band of electromagnetic radiation. In the alternative, the low or non-noticeable color shift can be in the form of a small shift of a UV-sided edge or IR-sided edge of a reflected band of electromagnetic radiation that borders the IR range or UV range, respectively. Such a small shift of a center wavelength, UV-sided edge and/or IR-sided edge is typically less than 50 nm, in some instances less than 40 nm, and in other instances less than 30 nm when the multilayer thin film is viewed from angles between 0 and 45 degrees.

In addition to the above, the omnidirectional structural color in the form of a multilayer thin film can be in the form of a plurality of pigment particles with a protective coating thereon. As such, a weather resistant pigment is provided. The protective coating can include one or more oxide layers that reduce the relative photocatalytic activity of the pigment particles. The oxide layer can be any oxide layer known to those skilled in the art, illustratively including a silicon oxide layer, an aluminum oxide layer, a zirconium oxide layer, a titanium oxide layer, a cerium oxide, combinations thereof and the like. In some instances, the protective coating includes a first oxide layer and a second oxide layer. In addition, the first oxide layer and/or the second oxide layer can be a hybrid oxide layer, i.e. an oxide layer that is a combination of two different oxides. Also, and as noted above, the pigment itself can be in the form of a multilayer thin film that does not contain an oxide layer.

A process for producing the omnidirectional structural color pigment may or may not include the use of an acid, an acidic compound, acidic solution, and the like. Stated differently, the plurality of omnidirectional structural color pigment particles may or may not be treated in an acidic solution. Additional teachings and details of the omnidirectional structural color pigment and a process for manufacturing the pigment are discussed later in the instant document.

Turning now to FIG. 1, FIGS. 1A-1D illustrate the basic components of an omnidirectional structural color design. In particular, FIG. 1A illustrates a dielectric layer exposed to incident electromagnetic radiation. In addition, the dielectric layer (DL) reflects a portion of the incident electromagnetic radiation and transmits a portion thereof. In addition, the incident electromagnetic radiation is equal to the transmitted portion and the reflected portion and typically the transmitted portion is much greater than the reflected portion. Dielectric layers are made from dielectric materials such as SiO₂, TiO₂, ZnS, MgF₂, and the like.

In sharp contrast, FIG. 1B illustrates a reflective layer (RL) in which all of the incident electromagnetic radiation is reflected and essentially has zero transmittance. Reflector layers are typically made from materials such as aluminum, gold, and the like.

FIG. 1C illustrates an absorbing layer (AL) in which incident electromagnetic radiation is absorbed by the layer and not reflected or transmitted. Such an absorbing layer can be made from, for example, graphite. Also, the total incident electromagnetic radiation is absorbed and transmission and reflectance is approximately zero.

FIG. 1D illustrates a partial or selective absorbing layer (SAL) in which a portion of the incident electromagnetic radiation is absorbed by the layer, a portion is transmitted, and a portion is reflected. As such, the amount of electromagnetic radiation transmitted, absorbed, and reflected equals the amount of incident electromagnetic radiation. In addition, such selective absorbing layers can be made from material such as a thin layer of chromium, layers of copper, brass, bronze, and the like.

With respect to the present invention, three generations of design and manufacture of omnidirectional structural color thin films are disclosed.

First Generation

Referring now to FIG. 2, a schematic illustration of a multilayer thin film having a plurality of dielectric layers is shown. In addition, the reflectance and transmittance of an incident electromagnetic radiation is schematically shown. As stated above, typical transmission of the incident electromagnetic radiation is much greater than reflectance thereof and thus many layers are required.

FIG. 3 shows part of a multilayer thin film made from dielectric layers having a first index of refraction (DL₁) and a second index of refraction (DL₂). It should be appreciated that the double lines between the layers simply represent an interface between the different layers.

Not being bound by theory, one method or approach for designing and manufacturing a desired multilayer stack is the following.

When electromagnetic radiation impacts a material surface, waves of the radiation can be reflected from or transmitted through the material. Furthermore, when electromagnetic radiation impacts the first end 12 of the multilayer structure 10 at the angle θ₀, the reflected angles the electromagnetic waves make with the surface of the high and low refractive index layers are θ_H and θ_L, respectively. Using Snell's law:n₀ Sin θ₀=n_L Sin θ_L=n_H Sin θ_H  (1)the angles θ_H and θ_L can be determined if the refractive indices n_H and n_L are known.

Regarding omnidirectional reflectivity, a necessary but not sufficient condition for the TE mode and the TM mode of electromagnetic radiation requires the maximum angle of refraction (θ_H,MAX) inside the first layer to be less than the Brewster angle (θ_B) of the interface between the first layer and the second layer. If this condition is not satisfied, the TM mode of the electromagnetic waves will not be reflected at the second and all subsequent interfaces and thus will transmit through the structure. Using this consideration:

$\begin{array}{cc}\mathrm{Sin}{\theta }_{H,\mathrm{Max}}=\frac{n_0}{n_H}\mathrm{and}& \left(2\right)\\ \mathrm{Tan}{\theta }_B=\frac{n_L}{n_H}& \left(3\right)\end{array}$ Thereby requiring:

$\begin{array}{cc}{n}_0<\frac{n_H{n}_L}{\sqrt{n_H^2+n_L^2}}& \left(4\right)\end{array}$

In addition to the necessary condition represented by Equation 4, if electromagnetic radiation of wavelength λ falls on a multilayer structure with an angle θ₀, and the individual bi-layers of the multilayer structure have thicknesses d_H and d_L with respective refractive indices n_H and n_L, the characteristic translation matrix (F_T) can be expressed as:

$\begin{array}{cc}{F}_T=\frac{1}{1+{\rho }_T^2}\begin{array}{cc}{e}^{i{\delta }_L}& {\rho }_T{e}^{-i{\delta }_L}\\ {\rho }_T{e}^{i{\delta }_L}& e^{-i{\delta }_L}\end{array}\times \frac{1}{1-{\rho }_T}\begin{array}{cc}{e}^{i{\delta }_H}& {\rho }_T{e}^{-i{\delta }_H}\\ {\rho }_T{e}^{i{\delta }_H}& e^{-i{\delta }_H}\end{array}& \left(5\right)\end{array}$ which can also be expressed as:

$\begin{array}{cc}{F}_T=\frac{1}{1-{\rho }_T}\begin{array}{cc}{e}^{i\left({\delta }_L+{\delta }_H\right)}-{\rho }_T^2{e}^{-i\left({\delta }_H-{\delta }_L\right)}& -2i{\rho }_T{e}^{-i{\delta }_H}\mathrm{Sin}{\delta }_L\\ 2i{\rho }_T{e}^{i{\delta }_H}\mathrm{Sin}{\delta }_L& e^{-i\left({\delta }_L+{\delta }_H\right)}-{\rho }_T^2{e}^{-i\left({\delta }_H-{\delta }_L\right)}\end{array}& \left(6\right)\end{array}$ and where:

$\begin{array}{cc}{\delta }_H=\frac{2\pi }{\lambda }{n}_H{d}_H\mathrm{Cos}{\theta }_H& \left(7\right)\\ {\delta }_L=\frac{2\pi }{\lambda }{n}_L{d}_L\mathrm{Cos}{\theta }_L& \left(8\right)\\ \mathrm{Cos}{\theta }_H=\sqrt{1-\frac{n_o^2{\mathrm{Sin}}^2{\theta }_0}{n_H^2}}\mathrm{and}& \left(9\right)\\ \mathrm{Cos}{\theta }_L=\sqrt{1-\frac{n_o^2{\mathrm{Sin}}^2{\theta }_0}{n_L^2}}& \left(10\right)\end{array}$ In addition,

$\begin{array}{cc}{\rho }_T=\frac{n_{\mathrm{HT}}-n_{\mathrm{LT}}}{n_{\mathrm{HT}}+n_{\mathrm{LT}}}\mathrm{where}& \left(11\right)\\ n_{\mathrm{HT}}=\{\begin{array}{c}\frac{n_H}{\mathrm{Cos}{\theta }_H}\\ n_H\mathrm{Cos}{\theta }_H\end{array}\left(\mathrm{for}\mathrm{TM}\mathrm{and}\mathrm{TE}\mathrm{polarization}\mathrm{respectively}\right)\mathrm{and}& \left(12\right)\\ n_{\mathrm{LT}}=\{\begin{array}{c}\frac{n_L}{\mathrm{Cos}{\theta }_L}\\ n_L\mathrm{Cos}{\theta }_L\end{array}\left(\mathrm{for}\mathrm{TM}\mathrm{and}\mathrm{TE}\mathrm{polarization}\mathrm{respectively}\right)& \left(13\right)\end{array}$ Solving ρ_T explicitly for TE and TM:

$\begin{array}{cc}{\rho }_{\mathrm{TM}}=\frac{n_H\mathrm{Cos}{\theta }_L-n_L\mathrm{Cos}{\theta }_H}{n_H\mathrm{Cos}{\theta }_L+n_L\mathrm{Cos}{\theta }_H}\mathrm{and}& \left(14\right)\\ {\rho }_{\mathrm{TE}}=\frac{n_H\mathrm{Cos}{\theta }_H-n_L\mathrm{Cos}{\theta }_L}{n_H\mathrm{Cos}{\theta }_H+n_L\mathrm{Cos}{\theta }_L}& \left(15\right)\end{array}$

A viewing angle dependent band structure can be obtained from a boundary condition for the edge, also known as the bandedge, of the total reflection zone. For the purposes of the present invention, bandedge is defined as the equation for the line that separates the total reflection zone from the transmission zone for the given band structure.

A boundary condition that determines the bandedge frequencies of the high reflectance band can be given by:Trace|F_T|=−1  (16)Thus, from equation 3:

$\begin{array}{cc}\frac{\mathrm{Cos}\left({\delta }_H+{\delta }_H\right)-{\rho }_T^2\mathrm{Cos}\left({\delta }_H-{\delta }_L\right)}{1-{\rho }_T^2}=-1& \left(17\right)\end{array}$ or expressed differently:

$\begin{array}{cc}{\mathrm{Cos}}^2\left(\frac{{\delta }_H+{\delta }_L}{2}\right)={\rho }_T^2{\mathrm{Cos}}^2\left(\frac{{\delta }_H-{\delta }_L}{2}\right)& \left(18\right)\end{array}$ Combining equations 15 and 7, the following bandedge equation is obtained:

$\begin{array}{cc}\mathrm{Cos}\left(\frac{\pi L_{+}}{\lambda }\right)=\pm {\rho }_T\mathrm{Cos}\left(\frac{\pi L_{-}}{\lambda }\right)& \left(19\right)\end{array}$ Where:L₊=n_Hd_H Cos θ_H+n_Ld_L Cos θ_L  (20)and:L=n_Hd_H Cos θ_H−n_Ld_L Cos θ_L  (21)The + sign in the bandedge equation shown above represents the bandedge for the long wavelength (λ_long) and the − sign represents the bandedge for the short wavelength (λ_short). Recompiling equations 20 and 21:

$\begin{array}{cc}\mathrm{Cos}\left(\frac{\pi L_{+}}{{\lambda }_{\mathrm{long}}}\right)=+{\rho }_{\mathrm{TE}}\mathrm{Cos}\left(\frac{\pi L_{-}}{{\lambda }_{\mathrm{long}}}\right)\mathrm{and}\mathrm{Cos}\left(\frac{\pi L_{+}}{{\lambda }_{\mathrm{short}}}\right)=-{\rho }_{\mathrm{TE}}\mathrm{Cos}\left(\frac{\pi L_{-}}{{\lambda }_{\mathrm{short}}}\right)& \left(22\right)\end{array}$ for the TE mode, and:

$\begin{array}{cc}\mathrm{Cos}\left(\frac{\pi L_{+}}{{\lambda }_{\mathrm{long}}}\right)=+{\rho }_{\mathrm{TM}}\mathrm{Cos}\left(\frac{\pi L_{-}}{{\lambda }_{\mathrm{long}}}\right)\mathrm{and}\mathrm{Cos}\left(\frac{\pi L_{+}}{{\lambda }_{\mathrm{short}}}\right)=-{\rho }_{\mathrm{TM}}\mathrm{Cos}\left(\frac{\pi L_{-}}{{\lambda }_{\mathrm{short}}}\right)& \left(23\right)\end{array}$ for the TM mode.

An approximate solution of the bandedge can be determined by the following expression:L₋=n_Hd_H Cos θ_H−n_Ld_L Cos θ_L˜0  (24)This approximate solution is reasonable when considering a quarter wave design (described in greater detail below) and optical thicknesses of the alternating layers chosen to be equal to each other. In addition, relatively small differences in optical thicknesses of the alternating layers provide a cosine close to unity. Thus, equations 23 and 24 yield approximate bandedge equations:

$\begin{array}{cc}{\lambda }_{\mathrm{long}}\left({\theta }_0\right)=\frac{\pi L_{+}\left({\theta }_0\right)}{{\mathrm{Cos}}^{-1}{\rho }_{\mathrm{TE}}\left({\theta }_0\right)}\mathrm{and}{\lambda }_{\mathrm{short}}\left({\theta }_0\right)=\frac{\pi L_{+}\left({\theta }_0\right)}{{\mathrm{Cos}}^{-1}\left(-{\rho }_{\mathrm{TE}}\left({\theta }_0\right)\right)}& \left(25\right)\end{array}$ for the TE mode and:

$\begin{array}{cc}{\lambda }_{\mathrm{long}}\left({\theta }_0\right)=\frac{\pi L_{+}\left({\theta }_0\right)}{{\mathrm{Cos}}^{-1}{\rho }_{\mathrm{TM}}\left({\theta }_0\right)}\mathrm{and}{\lambda }_{\mathrm{Short}}\left({\theta }_0\right)=\frac{\pi L_{+}\left({\theta }_0\right)}{{\mathrm{Cos}}^{-1}\left(-{\rho }_{\mathrm{TM}}\left({\theta }_0\right)\right)}& \left(26\right)\end{array}$ for the TM mode.

Values for L₊ and ρ_TM as a function of incident angle can be obtained from equations 7, 8, 14, 15, 20 and 21, thereby allowing calculations for λ_long and λ_short in the TE and TM modes as a function of incident angle.

The center wavelength of an omnidirectional reflector (λ_c), can be determined from the relation:λ_c=2(n_Hd_H Cos θ_H+n_Ld_L Cos θ_L)  (27)The center wavelength can be an important parameter since its value indicates the approximate range of electromagnetic wavelength and/or color spectrum to be reflected. Another important parameter that can provide an indication as to the width of a reflection band is defined as the ratio of range of wavelengths within the omnidirectional reflection band to the mid-range of wavelengths within the omnidirectional reflection band. This “range to mid-range ratio” (η) is mathematically expressed as:

$\begin{array}{cc}{\eta }_{\mathrm{TE}}=2\frac{{\lambda }_{\mathrm{long}}^{\mathrm{TE}}\left({\theta }_0={90}^{°}\right)-{\lambda }_{\mathrm{Short}}^{\mathrm{TE}}\left({\theta }_0=0^{°}\right)}{{\lambda }_{\mathrm{long}}^{\mathrm{TE}}\left({\theta }_0={90}^{°}\right)+{\lambda }_{\mathrm{Short}}^{\mathrm{TE}}\left({\theta }_0=0^{°}\right)}& \left(28\right)\end{array}$ for the TE mode, and:

$\begin{array}{cc}{\eta }_{\mathrm{TM}}=2\frac{{\lambda }_{\mathrm{long}}^{\mathrm{TM}}\left({\theta }_0={90}^{°}\right)-{\lambda }_{\mathrm{Short}}^{\mathrm{TM}}\left({\theta }_0=0^{°}\right)}{{\lambda }_{\mathrm{long}}^{\mathrm{TM}}\left({\theta }_0={90}^{°}\right)+{\lambda }_{\mathrm{Short}}^{\mathrm{TM}}\left({\theta }_0=0^{°}\right)}& \left(29\right)\end{array}$ for the TM mode. It is appreciated that the range to mid-range ratio can be expressed as a percentage and for the purposes of the present invention, the term range to mid-range ratio and range to mid-range ratio percentage are used interchangeably. It is further appreciated that a ‘range to mid-range ratio’ value provided herein having a ‘%’ sign following is a percentage value of the range to mid-range ratio. The range to mid-range ratios for the TM mode and TE mode can be numerically calculated from equations 28 and 29 and plotted as a function of high refractive index and low refractive index.

It is appreciated that to obtain the narrow omnidirectional band that the dispersion of the center wavelength must be minimized. Thus, from equation 27, the dispersion of the center wavelength can be expressed as:

$\begin{array}{cc}{\mathrm{\Delta \lambda }}_c={\lambda }_c{{}_{{\theta }_0=0^{°}}-{\lambda }_c}_{{\theta }_0={90}^{°}}=2\left(\frac{n_H{d}_H}{1}+\frac{n_L{d}_L}{1}-\frac{n_H{d}_H}{\sqrt{1-\frac{n_0^2}{n_H^2}}}-\frac{n_L{d}_L}{\sqrt{1-\frac{n_0^2}{n_L^2}}}\right)& \left(31\right)\end{array}$ where:

$\begin{array}{cc}\Delta {\lambda }_c=\frac{{\lambda }_0}{4}{F}_c& \left(32\right)\end{array}$ and F_c, the center wavelength dispersion factor can be expressed as:

$\begin{array}{cc}{F}_c=\left(2-\frac{1}{\sqrt{1-\frac{n_0^2}{n_H^2}}}-\frac{1}{\sqrt{1-\frac{n_0^2}{n_L^2}}}\right)& \left(33\right)\end{array}$

Given the above, a multilayer stack with a desired low center wavelength shift (Δλ_c) can be designed from a low index of refraction material having an index of refraction of n_L and one or more layers having a thickness of d_L and a high index of refraction material having an index of refraction of n_H and one or more layers having a thickness of d_H.

In particular, FIG. 4 provides a graphical representation of a comparison of the range to midrange ratio of 0.2% for the transverse magnetic mode and transverse electric mode of electromagnetic radiation plotted as a function of high refractive index versus low refractive index. As shown in the figure, three cases are illustrated in which Case I refers to a large difference between the transverse magnetic mode and the transverse electric mode, Case II refers to a situation for a smaller difference between the transverse magnetic mode and transverse electric mode, and Case III refers to a situation for a very small difference between the transverse magnetic mode and transverse electric mode. In addition, FIG. 5 illustrates a percent reflectance versus wavelength for reflected electromagnetic radiation for a case analogous with Case II.

As shown in FIG. 5, a small dispersion of the center wavelength for a multilayer thin film corresponding to Case III is shown. In addition, and with reference to FIG. 6, Case II provides a shift in the center wavelength of less than 50 nm (Case II) when a multilayer thin film structure is viewed between 0 and 45 degrees and Case III provides a center wavelength shift of less than 25 nm when the thin film structure is exposed to electromagnetic radiation between 0 and 45 degrees.

Second Generation

Referring now to FIG. 7, an illustrative structure/design according to a second generation is shown. The multilayer structure shown in FIG. 7 has a plurality of dielectric layers and an underlying absorbing layer. In addition, none of the incident electromagnetic radiation is transmitted through the structure, i.e. all of the incident electromagnetic radiation is reflected or absorbed. Such a structure as shown in FIG. 7 allows for the reduction of the number of dielectric layers that are needed in order to obtain a suitable amount of reflectance.

For example, FIG. 8 provides a schematic illustration of such a structure in which a multilayer stack has a central absorbing layer made from Cr, a first dielectric material layer (DL₁) extending across the Cr absorbing layer, a second dielectric material layer (DL₂) extending across the DL₁ layer, and then another DL₁ layer extending across the DL₂ layer. In such a design, the thicknesses of the first dielectric layer and the third dielectric layer may or may not be the same.

In particular, FIG. 9A shows a graphical representation of a structure in which a central Cr layer is bounded by two TiO₂ layers, which in turn are bounded by two SiO₂ layers. As shown by the plot, the layers of TiO₂ and SiO₂ are not equal in thickness to each other. In addition, FIG. 9B shows a reflectance versus wavelength spectrum of the 5-layer structure shown in FIG. 9A and compared to a 13-layer structure made according to the first generation design. As illustrated in FIG. 9B, a shift in the center wavelength of less than 50 nm, and preferably less than 25 nm is provided when the structures are viewed a 0 and 45 degrees. Also shown in FIG. 9B is the fact that a 5-layer structure according to the second generation essentially performs equivalent to a 13-layer structure of the first generation.

Third Generation

Referring to FIG. 10, a third generation design is shown in which an underlying reflector layer (RL) has a first dielectric material layer DL₁ extending thereacross and a selective absorbing layer SAL extending across the DL₁ layer. In addition, another DL₁ layer may or may not be provided and extend across the selective absorbing layer. Also shown in the figure is an illustration that all of the incident electromagnetic radiation is either reflected or selectively absorbed by the multilayer structure.

Such a design as illustrated in FIG. 10 corresponds to a different approach that is used for designing and manufacturing a desired multilayer stack. In particular, a zero or near-zero energy point thickness for a dielectric layer is used and discussed below.

For example, FIG. 11A is a schematic illustration of a ZnS dielectric layer extending across an Al reflector layer. The ZnS dielectric layer has a total thickness of 143 nm, and for incident electromagnetic radiation with a wavelength of 500 nm, a zero or near-zero energy point is present at 77 nm. Stated differently, the ZnS dielectric layer exhibits a zero or near-zero electric field at a distance of 77 nm from the Al reflector layer for incident EMR having a wavelength of 500 nm. In addition, FIG. 11B provides a graphical illustration of the energy field across the ZnS dielectric layer for a number of different incident EMR wavelengths. As shown in the graph, the dielectric layer has a zero electric field for the 500 nm wavelength at 77 nm thickness, but a non-zero electric field at the 77 nm thickness for EMR wavelengths of 300, 400, 600 and 700 nm.

Regarding calculation of a zero or near-zero electric field point, FIG. 12 illustrates a dielectric layer 4 having a total thickness ‘D’, an incremental thickness ‘d’ and an index of refraction ‘n’ on a substrate or core layer 2 having an index of refraction n, is shown. Incident light strikes the outer surface 5 of the dielectric layer 4 at angle θ relative to line 6, which is perpendicular to the outer surface 5, and reflects from the outer surface 5 at the same angle. Incident light is transmitted through the outer surface 5 and into the dielectric layer 4 at an angle θ_F relative to the line 6 and strikes the surface 3 of substrate layer 2 at an angle θ_s.

For a single dielectric layer, θ_s=θ_F and the energy/electric field (E) can be expressed as E(z) when z=d. From Maxwell's equations, the electric field can be expressed for s polarization as:Ē(d)={u(z),0,0}exp(ikαy)|_z=d  (34)and for p polarization as:

$\begin{array}{cc}\stackrel{\rightharpoonup }{E}\left(d\right)=\left\{0,u\left(z\right),-\frac{\alpha }{\tilde{\varepsilon }\left(z\right)}v\left(z\right)\right\}\exp \left(ik\alpha y\right){}_{z=d}& \left(35\right)\end{array}$ where

$k=\frac{2\pi }{\lambda }$ and λ is a desired wavelength to be reflected. Also, α=n_s sin θ_s where ‘s’ corresponds to the substrate in FIG. 5 and {tilde over (ε)}(z) is the permittivity of the layer as a function of z. As such,|E(d)|²=|u(z)|²exp(2ikαy)|_z=d  (36)for s polarization and

$\begin{array}{cc}{{E\left(d\right)}^2=\left[{u\left(z\right)}^2+{\frac{\alpha }{\sqrt{n}}v\left(z\right)}^2\right]\exp \left(2ik\alpha y\right)}_{z=d}& \left(37\right)\end{array}$ for p polarization.

It is appreciated that variation of the electric field along the Z direction of the dielectric layer 4 can be estimated by calculation of the unknown parameters u(z) and v(z) where it can be shown that:

$\begin{array}{cc}{\left(\begin{array}{c}u\\ v\end{array}\right)}_{z=d}=\left(\begin{array}{cc}\cos \phi & \left(i\text{/}q\right)\sin \phi \\ \mathrm{iq}\sin \phi & \cos \phi \end{array}\right){\left(\begin{array}{c}u\\ v\end{array}\right)}_{z=0,\mathrm{substrate}}& \left(38\right)\end{array}$ Naturally, ‘i’ is the square root of −1. Using the boundary conditions u|_z=01, v|_z=0=q_s, and the following relations:q_s=n_s cos θ_s for s-polarization  (39)q_s=n_s/cos θ_s for p-polarization  (40)q=n cos θ_F for s-polarization  (41)q=n/cos θ_F for p-polarization  (42)φ=k·n·d cos(θ_F)  (43)u(z) and v(z) can be expressed as:

$\begin{array}{cc}\begin{array}{c}u\left(z\right){}_{z=d}=u{{}_{z=0}\cos \phi +v}_{z=0}\left(\frac{i}{q}\sin \phi \right)\\ =\cos \phi +\frac{i·q_s}{q}\sin \phi \end{array}\mathrm{and}& \left(44\right)\\ \begin{array}{c}v\left(z\right){}_{z=d}=\mathrm{iqu}{{}_{z=0}\sin \phi +v}_{z=0}\cos \phi \\ =\mathrm{iq}\sin \phi +q_s\cos \phi \end{array}& \left(45\right)\end{array}$ Therefore:

$\begin{array}{cc}\begin{array}{c}{E\left(d\right)}^2=\left[{\cos }^2\phi +\frac{q_s^2}{q^2}{\sin }^2\phi \right]e^{2ik\alpha \gamma }\\ =\left[{\cos }^2\phi +\frac{n_s^2}{n^2}{\sin }^2\phi \right]e^{2ik\alpha \gamma }\end{array}& \left(46\right)\end{array}$ for s polarization with φ=k·n·d cos(θ_F), and:

$\begin{array}{cc}\begin{array}{c}{E\left(d\right)}^2=\left[{\cos }^2\phi +\frac{n_s^2}{n^2}{\sin }^2\phi +\frac{{\alpha }^2}{n}\left(q_s^2{\cos }^2\phi +q^2{\sin }^2\phi \right)\right]\\ =\left[\left(1+\frac{{\alpha }^2{q}_s^2}{n}\right){\cos }^2\phi +\left(\frac{n_s^2}{n^2}+\frac{{\alpha }^2{q}^2}{n}\right){\sin }^2\phi \right]\end{array}& \left(47\right)\end{array}$ for p polarization where:

$\begin{array}{cc}\alpha =n_s\sin {\theta }_s=n\sin {\theta }_F& \left(48\right)\\ q_s=\frac{n_s}{\cos {\theta }_s}\mathrm{and}& \left(49\right)\\ q_s=\frac{n_s}{\cos {\theta }_F}& \left(50\right)\end{array}$

Thus for a simple situation where θ_F=0 or normal incidence, φ=k·n·d, and α=0:

$\begin{array}{cc}{E\left(d\right)}^2\mathrm{for}s\text{-}\mathrm{polarization}={E\left(d\right)}^2\mathrm{for}p\text{-}\mathrm{polarization}=\left[{\cos }^2\phi +\frac{n_s^2}{n^2}{\sin }^2\phi \right]& \left(51\right)\\ =\left[{\cos }^2\left(k·n·d\right)+\frac{n_s^2}{n^2}{\sin }^2\left(k·n·d\right)\right]& \left(52\right)\end{array}$ which allows for the thickness ‘d’ to be solved for, i.e. the position or location within the dielectric layer where the electric field is zero.

Referring now to FIG. 13, Equation 52 was used to calculate that the zero or near-zero electric field point in the ZnS dielectric layer shown in FIG. 11A when exposed to EMR having a wavelength of 434 nm is at 70 nm (instead of 77 nm for a 500 nm wavelength). In addition, a 15 nm thick Cr absorber layer was inserted at a thickness of 70 nm from the Al reflector layer to afford for a zero or near-zero electric field ZnS—Cr interface. Such an inventive structure allows light having a wavelength of 434 nm to pass through the Cr—ZnS interfaces, but absorbs light not having a wavelength of 434 nm. Stated differently, the Cr—ZnS interfaces have a zero or near-zero electric field with respect to light having a wavelength of 434 nm and thus 434 nm light passes through the interfaces. However, the Cr—ZnS interfaces do not have a zero or near-zero electric field for light not having a wavelength of 434 nm and thus such light is absorbed by the Cr absorber layer and/or Cr—ZnS interfaces and not reflected by the Al reflector layer.

It is appreciated that some percentage of light within +/−10 nm of the desired 434 nm will pass through the Cr—ZnS interface. However, it is also appreciated that such a narrow band of reflected light, e.g. 434+/−10 nm, still provides a sharp structural color to a human eye.

The result of the Cr absorber layer in the multilayer stack in FIG. 13 is illustrated in FIG. 14 where percent reflectance versus reflected EMR wavelength is shown. As shown by the dotted line, which corresponds to the ZnS dielectric layer shown in FIG. 13 without a Cr absorber layer, a narrow reflected peak is present at about 400 nm, but a much broader peak is present at about 550+ nm. In addition, there is still a significant amount of light reflected in the 500 nm wavelength region. As such, a double peak that prevents the multilayer stack from having or exhibiting a structural color is present.

In contrast, the solid line in FIG. 14 corresponds to the structure shown in FIG. 13 with the Cr absorber layer present. As shown in the figure, a sharp peak at approximately 434 nm is present and a sharp drop off in reflectance for wavelengths greater than 434 nm is afforded by the Cr absorber layer. It is appreciated that the sharp peak represented by the solid line visually appears as sharp/structural color. Also, FIG. 14 illustrates where the width of a reflected peak or band is measured, i.e. the width of the band is determined at 50% reflectance of the maximum reflected wavelength, also known as full width at half maximum (FWHM).

Regarding omnidirectional behavior of the multilayer structure shown in FIG. 13, the thickness of the ZnS dielectric layer can be designed or set such that only the first harmonics of reflected light is provided. It is appreciated that this is sufficient for a “blue” color, however the production of a “red” color requires additional considerations. For example, the control of angular independence for red color is difficult since thicker dielectric layers are required, which in turn results in a high harmonic design, i.e. the presence of the second and possible third harmonics is inevitable. Also, the dark red color hue space is very narrow. As such, a red color multilayer stack has a higher angular variance.

In order to overcome the higher angular variance for red color, the instant application discloses a unique and novel design/structure that affords for a red color that is angular independent. For example, FIG. 15A illustrates a dielectric layer exhibiting first and second harmonics for incident white light when an outer surface of the dielectric layer is viewed from 0 and 45 degrees. As shown by the graphical representation, low angular dependence (small Δλ_c) is provided by the thickness of the dielectric layer, however, such a multilayer stack has a combination of blue color (1^st harmonic) and red color (2^nd harmonic) and thus is not suitable for a desired “red only” color. Therefore, the concept/structure of using an absorber layer to absorb an unwanted harmonic series has been developed. FIG. 15A also illustrates an example of the location of the reflected band center wavelength (λ_c) for a given reflection peak and the dispersion or shift of the center wavelength (Δλ_c) when the sample is viewed from 0 and 45 degrees.

Turning now to FIG. 15B, the second harmonic shown in FIG. 15A is absorbed with a Cr absorber layer at the appropriate dielectric layer thickness (e.g. 72 nm) and a sharp blue color is provided. More importantly for the instant invention, FIG. 15C illustrates that by absorbing the first harmonics with the Cr absorber at a different dielectric layer thickness (e.g. 125 nm) a red color is provided. However, FIG. 15C also illustrates that the use of the Cr absorber layer can result in more than desired angular dependence by the multilayer stack, i.e. a larger than desired Δλ_c.

It is appreciated that the relatively large shift in λ_c for the red color compared to the blue color is due to the dark red color hue space being very narrow and the fact that the Cr absorber layer absorbs wavelengths associated with a non-zero electric field, i.e. does not absorb light when the electric field is zero or near-zero. As such, FIG. 16A illustrates that the zero or non-zero point is different for light wavelengths at different incident angles. Such factors result in the angular dependent absorbance shown in FIG. 16B, i.e. the difference in the 0° and 45° absorbance curves. Thus in order to further refine the multilayer stack design and angular independence performance, an absorber layer that absorbs, e.g. blue light, irrespective of whether or not the electric field is zero or not, is used.

In particular, FIG. 17A shows a multilayer stack with a Cu absorber layer instead of a Cr absorber layer extending across a dielectric ZnS layer. The results of using such a “colorful” or “selective” absorber layer is shown in FIG. 17B which demonstrates a much “tighter” grouping of the 0° and 45° absorbance lines for the multilayer stack shown in FIG. 17A. As such, a comparison between FIG. 16B and FIG. 16B illustrates the significant improvement in absorbance angular independence when using a selective absorber layer rather than non-selective absorber layer.

Based on the above, a proof of concept multilayer stack structure was designed and manufactured. In addition, calculation/simulation results and actual experimental data for the proof of concept sample were compared. In particular, and as shown by the graphical plot in FIG. 18, a sharp red color was produced (wavelengths greater than 700 nm are not typically seen by the human eye) and very good agreement was obtained between the calculation/simulation and experimental light data obtained from the actual sample. Stated differently, calculations/simulations can and/or are used to simulate the results of multilayer stack designs according to one or more embodiments of the present invention and/or prior art multilayer stacks.

A list of simulated and/or actually produced multilayer stack samples is provided in the Table 1 below. As shown in the table, the inventive designs disclosed herein include at least 5 different layered structures. In addition, the samples were simulated and/or made from a wide range of materials. Samples that exhibited high chroma, low hue shift and excellent reflectance were provided. Also, the three and five layer samples had an overall thickness between 120-200 nm; the seven layer samples had an overall thickness between 350-600 nm; the nine layer samples had an overall thickness between 440-500 nm; and the eleven layer samples had an overall thickness between 600-660 nm

	TABLE 1
	Ave. Chroma		Max.
	(0-45)	Δh (0-65)	Reflectance	Sample Name
3 layer	90	2	96	3-1
5 layer	91	3	96	5-1
7 layer	88	1	92	7-1
	91	3	92	7-2
	91	3	96	7-3
	90	1	94	7-4
	82	4	75	7-5
	76	20	84	7-6
9 layer	71	21	88	9-1
	95	0	94	9-2
	79	14	86	9-3
	90	4	87	9-4
	94	1	94	9-5
	94	1	94	9-6
	73	7	87	9-7
11 layer	88	1	84	11-1
	92	1	93	11-2
	90	3	92	11-3
	89	9	90	11-4

Turning now to FIG. 19, a plot of percent reflectance versus reflected EMR wavelength is shown for an omnidirectional reflector when exposed to white light at angles of 0 and 45° relative to the surface of the reflector. As shown by the plot, both the 0° and 45° curves illustrate very low reflectance, e.g. less than 20%, provided by the omnidirectional reflector for wavelengths greater than 500 nm. However, the reflector, as shown by the curves, provides a sharp increase in reflectance at wavelengths between 400-500 nm and reaches a maximum of approximately 90% at 450 nm. It is appreciated that the portion or region of the graph on the left hand side (UV side) of the curve represents the UV-portion of the reflection band provided by the reflector.

The sharp increase in reflectance provided by the omnidirectional reflector is characterized by an IR-sided edge of each curve that extends from a low reflectance portion at wavelengths greater than 500 nm up to a high reflectance portion, e.g. >70%. A linear portion 200 of the IR-sided edge is inclined at an angle (β) greater than 60° relative to the x-axis, has a length L of approximately 50 on the Reflectance-axis and a slope of 1.2. In some instances, the linear portion is inclined at an angle greater than 70° relative to the x-axis, while in other instances β is greater than 75°. Also, the reflection band has a visible FWHM of less than 200 nm, and in some instances a visible FWHM of less than 150 nm, and in other instances a visible FWHM of less than 100 nm. In addition, the center wavelength λ_c for the visible reflection band as illustrated in FIG. 19 is defined as the wavelength that is equal-distance between the IR-sided edge of the reflection band and the UV edge of the UV spectrum at the visible FWHM.

It is appreciated that the term “visible FWHM” refers to the width of the reflection band between the IR-sided edge of the curve and the edge of the UV spectrum range, beyond which reflectance provided by the omnidirectional reflector is not visible to the human eye. In this manner, the inventive designs and multilayer stacks disclosed herein use the non-visible UV portion of the electromagnetic radiation spectrum to provide a sharp or structural color. Stated differently, the omnidirectional reflectors disclosed herein take advantage of the non-visible UV portion of the electromagnetic radiation spectrum in order to provide a narrow band of reflected visible light, despite the fact that the reflectors may reflect a much broader band of electromagnetic radiation that extends into the UV region.

Turning now to FIG. 20, a generally symmetrical reflection band provided by a multilayer stack according to an embodiment of the present invention and when viewed at 0° and 45° is shown. As illustrated in the figure, the reflection band provided by the multilayer stack when viewed at 0° has a center wavelength (λ_c(0°)) shifts less than 50 nm when the multilayer stack is viewed at 45° (λ_c(45°)), i.e. Δλ_c(0-45°)<50 nm. In addition, the FWHM of both the 0° reflection band and the 45° reflection band is less than 200 nm.

FIG. 21 shows a plot of percent reflectance versus reflected EMR wavelength for another omnidirectional reflector design when exposed to white light at angles of 0 and 45° relative to the surface of the reflector. Similar to FIG. 19, and as shown by the plot, both the 0° and 45° curves illustrate very low reflectance, e.g. less than 10%, provided by the omnidirectional reflector for wavelengths less than 550 nm. However, the reflector, as shown by the curves, provides a sharp increase in reflectance at wavelengths between 560-570 nm and reaches a maximum of approximately 90% at 700 nm. It is appreciated that the portion or region of the graph on the right hand side (IR side) of the curve represents the IR-portion of the reflection band provided by the reflector.

The sharp increase in reflectance provided by the omnidirectional reflector is characterized by a UV-sided edge of each curve that extends from a low reflectance portion at wavelengths below 550 nm up to a high reflectance portion, e.g. >70%. A linear portion 200 of the UV-sided edge is inclined at an angle (β) greater than 60° relative to the x-axis, has a length L of approximately 40 on the Reflectance-axis and a slope of 1.4. In some instances, the linear portion is inclined at an angle greater than 70° relative to the x-axis, while in other instances β is greater than 75°. Also, the reflection band has a visible FWHM of less than 200 nm, and in some instances a visible FWHM of less than 150 nm, and in other instances a visible FWHM of less than 100 nm. In addition, the center wavelength λ_c for the visible reflection band as illustrated in FIG. 18 is defined as the wavelength that is equal-distance between the UV-sided edge of the reflection band and the IR edge of the IR spectrum at the visible FWHM.

It is appreciated that the term “visible FWHM” refers to the width of the reflection band between the UV-sided edge of the curve and the edge of the IR spectrum range, beyond which reflectance provided by the omnidirectional reflector is not visible to the human eye. In this manner, the inventive designs and multilayer stacks disclosed herein use the non-visible IR portion of the electromagnetic radiation spectrum to provide a sharp or structural color. Stated differently, the omnidirectional reflectors disclosed herein take advantage of the non-visible IR portion of the electromagnetic radiation spectrum in order to provide a narrow band of reflected visible light, despite the fact that the reflectors may reflect a much broader band of electromagnetic radiation that extends into the IR region.

Referring now to FIG. 22, a plot of percent reflectance versus wavelength is shown for another seven-layer design omnidirectional reflector when exposed to white light at angles of 0 and 45° relative to the surface of the reflector. In addition, a definition or characterization of omnidirectional properties provided by omnidirectional reflectors disclosed herein is shown. In particular, and when the reflection band provided by an inventive reflector has a maximum, i.e. a peak, as shown in the figure, each curve has a center wavelength (λ_c) defined as the wavelength that exhibits or experiences maximum reflectance. The term maximum reflected wavelength can also be used for λ_c.

As shown in FIG. 22, there is shift or displacement of λ_c when an outer surface of the omnidirectional reflector is observed from an angle 45° (λ_c(45°)), e.g. the outer surface is tilted 45° relative to a human eye looking at the surface, compared to when the surface is observed from an angle of 0° ((λ_c(0°)), i.e. normal to the surface. This shift of λ_c (Δλ_c) provides a measure of the omnidirectional property of the omnidirectional reflector. Naturally a zero shift, i.e. no shift at all, would be a perfectly omnidirectional reflector. However, omnidirectional reflectors disclosed herein can provide a Δλ_c of less than 50 nm, which to the human eye can appear as though the surface of the reflector has not changed color and thus from a practical perspective the reflector is omnidirectional. In some instances, omnidirectional reflectors disclosed herein can provide a Δλ_c of less than 40 nm, in other instances a Δλ_c of less than 30 nm, and in still other instances a Δλ_c of less than 20 nm, while in still yet other instances a Δλ_c of less than 15 nm. Such a shift in Δλ_c can be determined by an actual reflectance versus wavelength plot for a reflector, and/or in the alternative, by modeling of the reflector if the materials and layer thicknesses are known.

Another definition or characterization of a reflector's omnidirectional properties can be determined by the shift of a side edge for a given set of angle refection bands. For example, and with reference to FIG. 19, a shift or displacement of an IR-sided edge (ΔS_IR) for reflectance from an omnidirectional reflector observed from 0° (S_IR(0°)) compared to the IR-sided edge for reflectance by the same reflector observed from 45° (S_IR(45°)) provides a measure of the omnidirectional property of the omnidirectional reflector. In addition, using ΔS_IR as a measure of omnidirectionality can be preferred to the use of Δλ_c, e.g. for reflectors that provide a reflectance band similar to the one shown in FIG. 19, i.e. a reflection band with a peak corresponding to a maximum reflected wavelength that is not in the visible range (see FIGS. 19 and 21). It is appreciated that the shift of the IR-sided edge (ΔS_IR) is and/or can be measured at the visible FWHM.

With reference to FIG. 21, a shift or displacement of a UV-sided edge (ΔS_IR) for reflectance from an omnidirectional reflector observed from 0° (S_UV(0°)) compared to the IR-sided edge for reflectance by the same reflector observed from 45° (S_UV(45°)) provides a measure of the omnidirectional property of the omnidirectional reflector. It is appreciated that the shift of the UV-sided edge (ΔS_UV) is and/or can be measured at the visible FWHM.

Naturally a zero shift, i.e. no shift at all (ΔS_i=0 nm; i=IR, UV), would characterize a perfectly omnidirectional reflector. However, omnidirectional reflectors disclosed herein can provide a ΔS_L of less than 50 nm, which to the human eye can appear as though the surface of the reflector has not changed color and thus from a practical perspective the reflector is omnidirectional. In some instances, omnidirectional reflectors disclosed herein can provide a ΔS_i of less than 40 nm, in other instances a ΔS_i of less than 30 nm, and in still other instances a ΔS_i of less than 20 nm, while in still yet other instances a ΔS_i of less than 15 nm. Such a shift in ΔS_i can be determined by an actual reflectance versus wavelength plot for a reflector, and/or in the alternative, by modeling of the reflector if the materials and layer thicknesses are known.

The shift of an omnidirectional reflection can also be measured by a low hue shift. For example, the hue shift of pigments manufactured from multilayer stacks according an embodiment of the present invention is 30° or less, as shown in FIG. 23 (see Δθ₁), and in some instances the hue shift is 25° or less, preferably less than 20°, more preferably less than 15° and still more preferably less than 10°. In contrast, traditional pigments exhibit hue shift of 45° or more (see Δθ₂).

In summary, a schematic illustration of an omnidirectional multilayer thin film according to an embodiment of the present invention in which a first layer 110 has a second layer 120 extending thereacross is shown in FIG. 24. An optional reflector layer 100 can be included. Also, a symmetric pair of layers can be on an opposite side of the reflector layer 100, i.e. the reflector layer 100 can have a first layer 110 oppositely disposed from the layer 110 shown in the figure such that the reflector layer 100 is sandwiched between a pair of first layers 110. In addition, a second layer 120 can be oppositely disposed the reflector layer 100 such that a five-layer structure is provided. Therefore, it should be appreciated that the discussion of the multilayer thin films provided herein also includes the possibility of a minor structure with respect to one or more central layers. As such, FIG. 24 can be illustrative of half of a five-layer multilayer stack.

The first layer 110 and second layer 120 can be dielectric layers, i.e. made from a dielectric material. In the alternative, one of the layers can be an absorbing layer, e.g. a selective absorbing layer or a non-selective absorbing layer. For example, the first layer 110 can be a dielectric layer and the second layer 120 can be an absorbing layer.

FIG. 25 illustrates half of a seven-layer design at reference numeral 20. The multilayer stack 20 has an additional layer 130 extending across the second layer 120. For example, the additional layer 130 can be a dielectric layer that extends across an absorbing layer 110. It is appreciated that layer 130 can be the same or a different material as layer 110. In addition, layer 130 can be added onto the multilayer stack 20 using the same or a different method that used to apply layers 100, 110 and/or 120 such as with a sol gel process.

FIG. 26 illustrates half of a nine-layer design at reference numeral 24 in which yet an additional layer 105 is located between the optional reflector layer 100 and the first layer 110. For example, the additional layer 105 can be an absorbing layer 105 that extends between the reflector layer 100 and a dielectric layer 110. A non-exhaustive list of materials that the various layers can be made from are shown is shown in Table 2 below.

TABLE 2
Refractive Index Materials	Refractive Index Materials
(visible region)	(visible region)
	Refractive		Refractive
Material	Index	Material	Index
Germanium (Ge)	4.0-5.0	Chromium (Cr)	3.0
Tellurium (Te)	4.6	Tin Sulfide (SnS)	2.6
Gallium Antimonite (GaSb)	4.5-5.0	Low Porous Si	2.56
Indium Arsenide (InAs)	4.0	Chalcogenide glass	2.6
Silicon (Si)	3.7	Cerium Oxide (CeO2)	2.53
Indium Phosphate (InP)	3.5	Tungsten (W)	2.5
Gallium Arsenate (GaAs)	3.53	Gallium Nitride (GaN)	2.5
Gallium Phosphate (GaP)	3.31	Manganese (Mn)	2.5
Vanadium (V)	3	Niobium Oxide (Nb2O3)	2.4
Arsenic Selenide (As2Se3)	2.8	Zinc Telluride (ZnTe)	3.0
CuAlSe2	2.75	Chalcogenide glass + Ag	3.0
Zinc Selenide (ZnSe)	2.5-2.6	Zinc Sulfide (ZnS)	2.5-3.0
Titanium Dioxide (TiO2)-	2.36	Titanium Dioxide (TiO2)-	2.43
solgel		vacuum deposited
Alumina Oxide (Al2O3)	1.75	Hafnium Oxide (HfO2)	2.0
Yttrium Oxide (Y2O3)	1.75	Sodium Aluminum Fluoride	1.6
		(Na3AlF6)
Polystyrene	1.6	Polyether Sulfone (PES)	1.55
Magnesium Fluoride	1.37	High Porous Si	1.5
(MgF2)
Lead Fluoride (PbF2)	1.6	Indium Tin Oxide nanorods	1.46
		(ITO)
Potassium Fluoride (KF)	1.5	Lithium Fluoride (LiF4)	1.45
Polyethylene (PE)	1.5	Calcium Fluoride	1.43
Barium Fluoride (BaF2)	1.5	Strontium Fluoride (SrF2)	1.43
Silica (SiO2)	1.5	Lithium Fluoride (LiF)	1.39
PMMA	1.5	PKFE	1.6
Aluminum Arsenate (AlAs)	1.56	Sodium Fluoride (NaF)	1.3
Solgel Silica (SiO2)	1.47	Nano-porous Silica (SiO2)	1.23
N,N′ bis(1naphthyl)-	1.7	Sputtered Silica (SiO2)	1.47
4,4′Diamine (NPB)
Polyamide-imide (PEI)	1.6	Vacuum Deposited Silica	1.46
		(SiO2)
Zinc Sulfide (ZnS)	2.3 +	Niobium Oxide (Nb2O5)	2.1
	i(0.015)
Titanium Nitride (TiN)	1.5 + i(2.0)	Aluminum (Al)	2.0 + i(15)
Chromium (Cr)	2.5 + i(2.5)	Silicon Nitride (SiN)	2.1
Niobium Pentoxide (Nb2O5)	2.4	Mica	1.56
Zirconium Oxide (ZrO2)	2.36	Polyallomer	1.492
Hafnium Oxide (HfO2)	1.9-2.0	Polybutylene	1.50
Fluorcarbon (FEP)	1.34	Ionomers	1.51
Polytetrafluro-Ethylene	1.35	Polyethylene (Low Density)	1.51
(TFE)
Fluorcarbon (FEP)	1.34	Nylons (PA) Type II	1.52
Polytetrafluro-	1.35	Acrylics Multipolymer	1.52
Ethylene (TFE)
Chlorotrifiuoro-	1.42	Polyethylene	1.52
Ethylene (CTFE)		(Medium Density)
Cellulose Propionate	1.46	Styrene Butadiene	1.52-1.55
		Thermoplastic
Cellulose Acetate Butyrate	1.46-1.49	PVC (Rigid)	1.52-1.55
Cellulose Acetate	1.46-1.50	Nylons (Polyamide)	1.53
		Type 6/6
Methylpentene Polymer	1.485	Urea Formaldehyde	1.54-1.58
Acetal Homopolymer	1.48	Polyethylene	1.54
		(High Density)
Acrylics	1.49	Styrene Acrylonitrile	1.56-1.57
		Copolymer
Cellulose Nitrate	1.49-1.51	Polystyrene	1.57-1.60
		(Heat & Chemical)
Ethyl Cellulose	1.47	Polystyrene	1.59
		(General Purpose)
Polypropylene	1.49	Polycarbornate (Unfilled)	1.586
Polysulfone	1.633	SnO2	2.0

Methods for producing the multilayer stacks disclosed herein can be any method or process known to those skilled in the art or one or methods not yet known to those skilled in the art. Typical known methods include wet methods such as sol gel processing, layer-by-layer processing, spin coating and the like. Other known dry methods include chemical vapor deposition processing and physical vapor deposition processing such as sputtering, electron beam deposition and the like.

The multilayer stacks disclosed herein can be used for most any color application such as pigments for paints, thin films applied to surfaces and the like.

As stated above, omnidirectional structural color pigments having a protective/weather resistant coating are provided. For example, and turning to FIGS. 27 and 28, exemplary pigments that can be coated are shown. In particular, FIG. 27 is a schematic illustration of a 3-layer pigment 12 with a core layer 100, a first non-oxide layer 112, a selective absorbing layer 114 and an additional non-oxide layer 116 extending across the selective absorbing layer 114. FIG. 28 is a schematic illustration of a pigment 12a that is similar to pigment 12 except for the addition of layers 112a, 114a as in FIG. 28A, and an additional non-oxide layer 116a. It is appreciated that the layers 112-112a, 114-114a and/or 116-116a may or may not be made from the same material and may or may not have the same thickness.

FIG. 29 provides a schematic illustration of pigment 22 that shows pigment 12a having a protective coating 200 thereon. In addition, FIG. 30 illustrates that the protective coating 200 can be a single layer, or in the alternative, be two or more layers, e.g. a first layer 202 and a second layer 204. It is appreciated that the first layer 202 and/or the second layer 204 can be a single oxide layer, or in the alternative be a hybrid oxide layer made from or containing two or more oxides. For example, the protective coating 200 can be a single oxide layer such as a single layer of silicon oxide, aluminum oxide, zirconium oxide or cerium oxide. In the alternative, the protective coating 200 can include the first layer 202 and second layer 204, and the first layer 202 and second layer 204 are each a single oxide layer of silicon oxide, aluminum oxide, zirconium oxide, titanium oxide or cerium oxide. In another alternative, the first layer 202 can be a single oxide layer and the second layer 204 can be hybrid oxide layer that is a combination of at least two of silicon oxide, aluminum oxide, zirconium oxide, titanium oxide and cerium oxide. In yet another alternative, the second layer 204 can be a single oxide layer and the first layer 204 can be hybrid oxide layer that is a combination of at least two of silicon oxide, aluminum oxide, zirconium oxide, titanium oxide and cerium oxide.

In order to better teach the invention but not limit its scope in any way, examples of weather resistant omnidirectional structural color pigments and a process protocols to produce such pigments is discussed below.

Protocol 1—7-Layer Pigments Etched with Phosphoric Acid and Coated with a SiO₂ Layer

A suspension containing 10 g of 7-layer design pigments dispersed in 110 ml of acetone had 0.13 ml of phosphoric acid (85%) added thereto and was stirred for 30 minutes at room temperature. The suspension was then filtered and washed with acetone two times. Solid particles were filtered and 7-layer pigments treated with phosphoric acid were obtained. The 7-layer pigments had a structure as illustrated in FIG. 28B with a core layer of Al, a pair of ZnS layers bounding the core Al layer, a pair of Cr selective absorbing layers bounding the pair of ZnS layers and another pair of ZnS layers bounding the pair of Cr layers.

The phosphoric acid treated 7-layer pigments were then suspended in 160 ml of ethanol in a round bottom flask equipped with a reflux condenser. The suspension was heated to 65° C. after 35 g of water and 3.5 g of a 28% aqueous ammonia solution had been added. Next, a solution of 10 g of tetraethoxysilane diluted with 13 ml of ethanol was added to the heated suspension in small amounts while stirring. This reaction mixture was stirred for 14 hours at 65° C., then solid particles were filtered from the liquid, washed with ethanol and then washed with isopropyl alcohol (IPA). Seven layer SiO₂-coated pigments were obtained after the solid particles were dried at 100° C. for 24 hours.

Protocol 1A—7-Layer Pigments Coated with a SiO₂ Layer

A quantity of 10 g of 7-layer pigments were suspended in 160 ml of ethanol in a round bottom flask equipped with a reflux condenser without first being treated with phosphoric acid as in Protocol 1. The suspension was heated to 65° C. after 35 g of water and 3.5 g of a 28% aqueous ammonia solution had been added. Next, a solution of 10 g of tetraethoxysilane diluted with 13 ml of ethanol was added to the heated suspension in small amounts while stirring. This reaction mixture was stirred for 14 hours at 65° C., then solid particles were filtered from the liquid, washed with ethanol and then washed with isopropyl alcohol (IPA). Seven layer SiO₂-coated pigments were obtained after the solid particles were dried at 100° C. for 24 hours.

Protocol 2—7-Layer Pigments Coated with a SiO₂ Layer Using Aqueous Solution

Fifteen grams of 7-layer pigments were placed in a 250 ml 3-neck flask. Then, 100 ml of DI water was added and the solution stirred inside an ethylene glycol bath that was heated to 80° C. The solution was set to a pH of 7.5 by adding a few drops of 1M NaOH solution. Next, 20 ml of Na₂SiO₃ (13 wt % SiO₂) was added to the solution at a constant flow rate of 0.1 ml/min using a syringe pump. While adding the Na₂SiO₃, an aqueous 1M HCl solution was also added to maintain the pH of 7.5 using an automated pH control system. The mixture was allowed to cool to room temperature, filtered off, washed with IPA and dried at 100° C. for 24 hours. The coated material can be further annealed at 200° C. for 24 h.

Protocol 3—7 Layer Pigments Coated with a SiO₂ Layer and a Hybrid SiO₂—Al₂O₃ Layer

Two grams of pigments coated with SiO₂ per Protocol 1 or 1A were suspended in 20 ml of a water solution and having a pH of about 10 (adjust by diluted NaOH solution). The suspension was heated to 60° C. while being constantly stirred in a 100 ml round bottom flask. Then, 0.5 ml of 18 wt % Na₂Si0₃ solution and 1 ml of 0.5M Al₂(SO₄)₃ solution were both simultaneously titrated into the pigment suspension at constant rate in 1 hour. The slurry pH was not controlled. After the titration, the suspension was aged for 30 minutes under stirring. The mixture was filtered and remaining solid particles were washed with DI water and then washed with IPA. Seven layer pigments having a SiO₂ layer and an Al₂O₃ layer were obtained after the remaining solid particles were dried at 100° C. for 24 hours.

Protocol 4—7 Layer Pigments Coated with a SiO₂ Layer and a ZrO₂+Al₂O₃ Hybrid Layer

Three grams of SiO₂-coated pigments per Protocol 1 or 1A were suspended in 20 ml of ethanol in a 100 ml round bottom flask and stirred at room temperature. Also, 0.66 g of aluminum-tri-sec-butoxide and 2.47 ml of zirconium butoxide were dissolved in 15 ml of IPA. The aluminum-tri-sec-butoxide+zirconium butoxide mixture was titrated into the pigment suspension at constant rate in 2 hours. At the same time, 0.66 ml of DI water diluted in 2 ml of ethanol was metered in. After the titration, the suspension was stirred for another 30 minutes. The mixture was filtered and remaining solid particles were washed with ethanol and then washed with IPA. Seven layer pigments having a SiO₂ layer and a hybrid ZrO₂+Al₂O₃ layer were obtained after the remaining solid particles were dried at 100° C. for 24 hours, or in the alternative after being further annealed at 200° C. for 24 hours.

Protocol 5—7 Layer Pigments Coated with a SiO₂ Layer and a ZrO₂+Al₂O₃ Hybrid Layer

Three grams of SiO2-coated pigments per Protocol 1 or 1A were suspended in 20 ml of DI water with a pH of 8 (adjusted by diluted NaOH solution) in a 100 ml round bottom flask and heated to 50° C. while constantly stirring. Then, 0.5 ml of 5 wt % NaAlO₂ solution and 0.5 ml of 10 wt % ZrOCl₂ solution were both simultaneously titrated into the pigment suspension at constant rate in 30 minutes. The slurry pH was controlled at 8 by adding diluted HCl or NaOH solution. After the titration, the suspension was aged for 30 min under stirring. The mixture was filter off, washed with DI water and then washed with IPA. The coated pigments were obtained after being dried at 100° C. for 24 hours, or in the alternative after being further annealed at 200° C. for 24 hours.

Protocol 6—7 Layer Pigments Coated with a SiO₂ Layer, a CeO₂ Layer and a ZrO₂+Al₂O₃ Hybrid

Silicon oxide (SiO₂) coated pigments (3.5 g) per Protocol 1 or 1A were suspended in 26.83 ml of water in a 100 ml round bottom flask and stirred at 70° C. for 20 minutes. Then, 0.33 g of Ce(NO₃)₃.6H₂O in 1.18 ml of H₂O was titrated into the pigment suspension at constant rate of 2 mL/hr and the mixture was kept stirring for an additional 1.5 hours after titration. During the reaction, the pH of the solution was kept constant at 7.0 using a diluted NaOH solution. The mixture was filtered and the remaining solid particles were washed with water three times and then washed with IPA an additional three times. Seven layer pigments having a layer of SiO₂ and a layer of CeO₂ were obtained after the remaining solid particles were dried at 100° C. for 24 hours.

Next, 3 g of the coated pigments were suspended in 20 ml of ethanol in a 100 ml round bottom flask and stirred at room temperature. A mixture of 0.66 g of aluminum-tri-sec-butoxide and 2.47 ml of zirconium butoxide dissolved in 15 ml of IPA was titrated into the pigment suspension at constant rate in 2 hours. At the same time, 0.66 ml of DI water diluted in 2 ml of ethanol was metered in. After the titration, the suspension was stirred for another 30 minutes. The mixture was filtered and the remaining solid particles were washed with ethanol and then washed with IPA. Seven layer pigments having a layer of SiO₂, a layer of CeO₂ and a hybrid ZrO₂+Al₂O₃ layer were obtained after the remaining solid particles were dried at 100° C. for 24 hours.

Protocol 7—7 Layer Pigments Coated with a CeO₂ Layer and a ZrO₂+Al₂O₃ Hybrid Layer

Three grams of 7-layer design pigments were suspended in 20 ml of IPA in a 100 ml round bottom flask and stirred at 75° C. A solution of 0.44 g of Ce(NO₃)₃.6H₂O dissolved in 20 ml of IPA was titrated in at constant rate in 1 hour. At the same time, 0.15 ml of ethylenediamine (EDA) diluted in 0.9 ml of DI water was metered in. A further 0.15 ml of EDA diluted in 0.9 ml of DI water was subsequently metered in. After the titration, the suspension was stirred for another 15 minutes. The mixture was filtered and the remaining solid particles were washed with IPA. Seven layer pigments having a layer of CeO₂ were obtained after the remaining solid particles were dried at 100° C. for 5 hours.

The CeO₂-coated pigments were then suspended in 20 ml of ethanol in a 100 ml round bottom flask and stirred at room temperature. Next, a mixture of 0.66 g of aluminum-tri-sec-butoxide and 2.47 ml of zirconium butoxide dissolved in 15 ml of IPA was titrated into the pigment suspension at constant rate in 2 hours. At the same time, 0.66 ml of DI water diluted in 2 nil of ethanol was metered in. After the titration, the suspension was stirred for another 30 minutes. The mixture was filtered and the remaining solid particles were washed with ethanol and then washed with IPA. Seven layer pigments having a layer of CeO₂ and a hybrid layer of ZrO₂+Al₂O₃ were obtained after the remaining solid particles were dried at 100° C. for 24 hours, or in the alternative further annealed at 200° C. for 24 hours.

Protocol 8—7-Layer Pigments Coated with a ZrO₂ Layer

Two grams of 7-layer design pigments were suspended in 30 ml of ethanol in a 100 ml round bottom flask and stirred at room temperature. A solution of 2.75 ml of zirconium butoxide (80% in 1-Butanol) dissolved in 10 ml of ethanol was titrated in at constant rate in 1 hour. At the same time, 1 ml of DI water diluted in 3 ml of ethanol was metered in. After the titration, the suspension was stirred for another 15 minutes. Seven layer pigments having a layer of ZrO₂ were obtained after the remaining solid particles were filter from the solution, washed with ethanol and dried at 100° C. for 5 hours, or in the alternative further annealed at 200° C. for 24 hours.

Protocol 9—7-Layer Pigments Coated with a SiO₂ Layer and an Al₂O₃ Layer

Two grams of SiO₂-coated pigments per Protocol 1 or 1A were suspended in 20 ml of water solution having a pH of about 8 (adjusted by diluted NaOH solution) in a 100 ml round bottom flask and heated to 50° C. while constantly stiffing. Then, 0.5 ml of 5 wt % NaAlO₂ solution was titrated into the pigment suspension at constant rate in 30 minutes. The slurry pH was controlled at 8 using a HCl 1M solution. After the titration, the suspension was aged for 30 minutes while stiffing. The mixture was filtered off, washed with DI water and then washed with IPA. The coated pigments are obtained after being dried at 100° C. for 24 hours.

Protocol 10—7-Layer Pigments Coated with a SiO₂ Layer and an TiO₂ Layer

A 250 ml 3-neck round bottom flask was placed in an ethylene glycol oil bath with a temperature set as 80° C. Then, 15 g of SiO₂-coated flakes per Protocol 1 or 1A, and 100 ml of DI water, were put into the flask and stirred at 400 rpm. The solution was set to a pH of 2 by adding a few drops of concentrated HCl solution. A pre-diluted 35% TiCl₄ solution was then titrated into the mixture at a constant flow rate of 0.1 ml/min via a syringe pump. To maintain the pH constant, base solution NaOH solution (8M) was titrated into the flask through an automated pH control system. During the deposition, flake samples were extracted at specific time intervals to determine the layer thickness. The mixture was allowed to cool to room temperature, then filtered, washed with IPA and dried at 100° C. for 24 hours, or in the alternative further annealed at 200° C. for 24 hours.

The weather resistant properties of the coated pigments were tested in the following manner Seven cylindrical pyrex flask (capacity ca. 120 mL) were used as photoreactor vessels. Each flask contained 40 ml of a fluorescent red dye (eosin B) solution (1×10⁻⁵M) and 13.3 mg of a pigment to be tested. The pigment-eosin B solution was magnetically stirred in the dark for 30 min, and then exposed to light from a solar simulator (Oriel® Sol2A™ Class ABA Solar Simulators). For each pigment, the same type pigment wrapped with aluminum foil was used as a direct control. Also, commercial TiO₂ (Degussa P25) was used as a reference with which to compare photocatalytic activity under the same experimental conditions. UV/Vis absorption spectra were recorded after 65 hours of light exposure to monitor the photocatalytic activity of each sample.

The results of the testing were plotted as relative photocatalytic activity versus pigment type as shown in FIG. 31. In addition, the 7-layer pigments with no protective coating was set as exhibiting 100% of photocatalytic activity and used as a comparison for the coated pigment samples. As shown in FIG. 31, all of the coated pigment samples exhibited a decrease in photocatalytic activity compared to the uncoated sample. In addition, the 7-layer pigment with a SiO₂ coating (labeled P/S), the 7-layer pigment with a SiO₂ coating layer and a hybrid ZrO₂+Al₂O₃ coating layer (labeled P/S/Z-A), the 7-layer pigment with a SiO₂ coating layer, a CeO₂ coating layer and a hybrid ZrO₂+Al₂O₃ coating layer (labeled P/S/C/Z-A) and the 7-layer pigment with a CeO₂ coating layer and a hybrid ZrO₂+Al₂O₃ coating layer (labeled P/C/Z-A) exhibited a reduction of photocatalytic activity of at least 50% when compared to the uncoated pigments. In contrast, the 7-layer pigment with a SiO₂ coating layer and a hybrid SiO₂+Al₂O₃ coating layer exhibited a reduction in photocatalytic activity of only 33.8%.

Scanning electron microscopy images of the 7-layer pigments before and after being coated with a SiO₂ layer and a ZrO₂—Al₂O₃ layer (protocol 4) are shown in FIGS. 32A and 32B, respectively. As shown by the images, the surfaces of the pigments are smooth and the physical shape and structural integrity of the pigments after being coated is the same as the pre-coated pigments. In addition, EDX dot map images of a 7-layer omnidirectional structural color pigment having an outer ZnS layer coated with a first SiO₂ layer and a second hybrid ZrO₂—Al₂O₃ layer (protocol 4) protective coating showed relatively high concentrations of Zn, S, Si, Zr and Al in the coating structure as expected.

A summary of coatings, the process used to produce a coating, coating thickness, coating thickness uniformity and photocatalytic activity is shown in Table 3 below.

TABLE 3
				Coating	Thickness		Photocatalytic
Sample	Core*	Layer	Material	Protocol	(nm)	Uniformity	Activity**
1	P7	1st	SiO2	1, 1A	80	G	51%
2	P7	1st	SiO2	1, 1A	80	G
3	P7	1st	SiO2	1, 1A	80	G
4	P7	1st	SiO2	1, 1A	30	G
5	P7	1st	SiO2	1, 1A	40	G
6	P7	1st	SiO2	1, 1A	50	G
7	P7	1st	SiO2	1, 1A	15	G
8	P7	1st	SiO2	1, 1A	10	G
9	P7	1st	SiO2	1, 1A	30	G	50%
10	P7	1st	SiO2	1, 1A	30	G
		2nd	Al2O3	9	<10	G
11	P7	1st	SiO2	1, 1A	30	G
		2nd	ZrO2—Al2O3	5	15	NG
12	P7	1st	SiO2	1, 1A	30	G
		2nd	ZrO2—Al2O3	6	15	G
13	P7	1st	SiO2	1, 1A	80	G
		2nd	CeO2	7		NG
14	P7	1st	SiO2	1, 1A	80	G	66%
		2nd	SiO2—Al2O3	3	10	G
15	P7	1st	SiO2	1, 1A	80	G
		2nd	SiO2—Al2O3	3	10	G
16	P7	1st	SiO2	1, 1A	80	G
		2nd	SiO2—Al2O3	3	10	G
17	P7	1st	SiO2	1, 1A	80	G
		2nd	Al2O3	9	10	G
18	P7	1st	SiO2	1, 1A	80	G	33%
		2nd	ZrO2—Al2O3	6	15	G
19	P7	1st	SiO2	1, 1A	80	G	33%
		2nd	CeO2	6	3	G
		3rd	ZrO2—Al2O3	6	15	G
20	P7	1st	CeO2	7	3	G	21%
		2nd	ZrO2—Al2O3	6	15	G
21	P7	1st	CeO2	7	3	G
		2nd	ZrO2—Al2O3	6	15	G
22	P7	1st	CeO2	7	3	G
		2nd	ZrO2—Al2O3	6	15	G
27	P7	1st	ZrO2	11	~20	G
*P7 = 7-layer pigment
**compared to non-coated 7-layer pigment

Given the above, Table 4 provides a listing of various oxide layers, substrates that can be coated and ranges of coating thickness included within the instant teachings.

	TABLE 4
			Range of Coating
	Oxide Layer	Substrate	Thickness (nm)
	SiO2	Mica, P7, metal, oxides	10-160
	TiO2	Mica, P7, metal, oxides	20-100
	ZrO2	Mica, P7, metal, oxides	20-100
	Al2O3	Mica, P7, metal, oxides	5-30
	CeO2	Mica, P7, Oxides	~5-40
	SiO2—Al2O3	Mica, P7, oxides	20-100
	ZrO2—Al2O3	Mica, P7, metal, oxides	10-50

In addition to the above, the omnidirectional structural color pigments with a protective coating can be subjected to an organo-silane surface treatment. For example, one illustrative organo-silane protocol treatment suspended 0.5 g of pigments coated with one or more of the protection layers discussed above in a 10 ml of EtOH/water (4:1) solution having pH about 5.0 (adjusted by diluted acetic acid solution) in a 100 ml round bottom flask. The slurry was sonicated for 20 seconds then stirred for 15 minutes at 500 rpm. Next, 0.1-0.5 vol % of an organo-silane agent was added to the slurry and the solution was stirred at 500 rpm for another 2 hours. The slurry was then centrifuged or filter using DI water and the remaining pigments were re-dispersed in 10 ml of a EtOH/water (4:1) solution. The pigment-EtOH/water slurry was heated to 65° C. with reflux occurring and stirred at 500 rpm for 30 minutes. The slurry was then centrifuged or filtered using DI water and then IPA to produce a cake of pigment particles. Finally, the cake was dried at 100° C. for 12 hours.

The organo-silane protocol can use any organo-silane coupling agent known to those skilled in the art, illustratively including N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (APTMS), N-[3-(Trimethoxysilyl)propyl]ethylenediamine 3-methacryloxypropyltrimethoxy-silane (MAPTMS), N-[2(vinylbenzylamino)-ethyl]-3-aminoproplyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane and the like.

The above examples and embodiments are for illustrative purposes only and changes, modifications, and the like will be apparent to those skilled in the art and yet still fall within the scope of the invention. As such, the scope of the invention is defined by the claims and all equivalents thereof.

Claims

1. An omnidirectional structural color pigment having a protective coating comprising: a pigment having a reflector layer, a dielectric layer with a thickness of less than 2.0 quarter wave (QW) of a desired reflected center wavelength extending across said reflector layer and an absorber layer extending across said dielectric layer, said reflector layer, dielectric layer and absorber layer being substantially planar layers, said pigment reflecting a single band of visible electromagnetic radiation having a predetermined full width at half maximum (FWHM) of less than 200 nm and a predetermined color shift of less than 30° when said pigment is exposed to broadband electromagnetic radiation and viewed from angles between 0 and 45°;a weather resistant coating comprising an oxide layer covering an outer surface of said pigment and reducing a photocatalytic activity of said pigment by at least 50% compared to said pigment not having said weather resistant coating.

2. The omnidirectional structural color pigment having a protective coating of claim 1, wherein said oxide layer is selected from the group consisting of silicon oxide, aluminum oxide, zirconium oxide, titanium oxide and cerium oxide. wherein said weather resistant coating has an oxide layer.

3. The omnidirectional structural color pigment having a protective coating of claim 2, wherein said weather resistant coating has a first oxide layer and a second oxide layer, said second oxide layer being different from said first oxide layer.

4. The omnidirectional structural color pigment of claim 3, wherein said second oxide layer is a hybrid oxide layer that is a combination of two different oxides.

5. The omnidirectional structural color pigment of claim 4, wherein said first oxide layer is silicon oxide and said second oxide layer from at least two of the group consisting of silicon oxide, aluminum oxide, zirconium oxide, titanium oxide and cerium oxide.

6. The omnidirectional multilayer structural color pigment of claim 1, wherein said pigment does not contain an oxide layer.