CN111175876A

CN111175876A - Omnidirectional high-chroma red-structure colorants with semiconductor absorber layer

Info

Publication number: CN111175876A
Application number: CN202010092655.1A
Authority: CN
Inventors: D·班纳尔吉
Original assignee: Toyota Motor Engineering and Manufacturing North America Inc
Current assignee: Toyota Motor Corp
Priority date: 2015-07-07
Filing date: 2016-06-07
Publication date: 2020-05-19
Anticipated expiration: 2036-06-07
Also published as: JP6643192B2; JP2017021333A; CN111175876B; CN106338790A; JP2020046692A; JP6817407B2; DE102016110192A1; CN106338790B

Abstract

The invention relates to an omnidirectional high-chroma red structural colorant having a semiconductor absorber layer. An omnidirectional high-chroma red structural color pigment. The omnidirectional structural color pigment is in the form of a multilayer stack having a reflective core layer, a semiconductor absorber layer extending across the reflective core layer, and a high refractive index dielectric layer extending across the semiconductor absorber layer. The multilayer stack reflects a single band of visible light having a hue on the a x b Lab color map between 0-40 °, and preferably between 10-30 °. The single band visible light has a hue shift of less than 30 ° on an a x b Lab color map when viewed from all angles between 0-45 ° normal to the outer surface of the multilayer stack.

Description

Omnidirectional high-chroma red-structure colorants with semiconductor absorber layer

The present application is a divisional application of an invention patent application having an application date of 2016, 6/7, an application number of 201610397388.2, and an invention name of "omnidirectional high-chroma red structural coloring material having a semiconductor absorber layer".

Cross Reference to Related Applications

The present application continues as part of (CIP) U.S. patent application Ser. No. 14/607,933 filed on day 28/1/2015, U.S. patent application Ser. No. 14/607,933, CIP of U.S. patent application Ser. No. 14/471,834 filed on day 28/8/2014, CIP of U.S. patent application Ser. No. 14/471,834, U.S. patent application Ser. No. 14/460,511 filed on day 15/8/2014, U.S. patent application Ser. No. 14/460,511 filed on day 1/4/2014, CIP of U.S. patent application Ser. No. 14/242,429, U.S. patent application Ser. No. 14/138,499 filed on day 23/2013, U.S. patent application Ser. No. 13/402,913 filed on day 8/6/2013, U.S. patent application serial No. 13/913,402, in turn, CIP of U.S. patent application serial No. 13/760,699 filed on 6.2.2013, and CIP of U.S. patent application serial No. 13/760,699 filed on 10.8.2012, all of which are incorporated herein by reference in their entirety.

Technical Field

The present invention relates to multilayer stack structures exhibiting high chroma red colors with minimal or insignificant color shift when exposed to broadband electromagnetic radiation and viewed from different angles.

Background

Pigments made of multilayer structures are known. Furthermore, pigments which exhibit or provide high chroma omnidirectional structural colors are also known. However, such prior art pigments require up to 39 film layers in order to obtain the desired color properties.

It will be appreciated that the cost associated with the preparation of thin film multilayer pigments is proportional to the number of layers required. As such, the costs associated with using multi-layer dielectric material stacks to produce high chroma omnidirectional structural colors may be prohibitive. Thus, a high chroma omnidirectional structure colorant that requires a minimum number of film layers may be desirable.

In addition to the above, it is also understood that the design of pigments having a red color faces additional difficulties relative to pigments of other colors (e.g., blue, green, etc.). In particular, the control of the angular independence of the red color is difficult because a thicker dielectric layer is required, which in turn leads to a higher harmonic design, i.e. the presence of a second and possibly a third harmonic is unavoidable. Moreover, the dark red color hue space is very narrow. As such, the red color multilayer stack has a higher angular dispersion (angular variance).

For the reasons described above, high chroma red omnidirectional structured color pigments with a minimum number of layers may be desirable.

Disclosure of Invention

An omnidirectional high chroma red structural color pigment is provided. The omnidirectional structural color pigment is in the form of a multilayer stack having a reflective core layer, a semiconductor absorber layer extending across the reflective core layer, and a high refractive index dielectric layer extending across the semiconductor absorber layer. The multilayer stack reflects a single band of visible light having a hue on the a x b Lab color map between 0-40 °, and preferably between 10-30 °. Further, the single band visible light has a hue shift of less than 30 ° on the a × b Lab color map when viewed from all angles between 0-45 ° normal to the outer surface of the multilayer stack and thereby provides a color shift that is not significant to the human eye.

The reflective core layer has a thickness between 50-200 nanometers (nm), inclusive, and may be made of a reflective metal, such as aluminum (Al), silver (Ag), platinum (Pt), tin (Sn), combinations thereof, and the like. The reflective core layer may also be made of colored (colorful) metals such as gold (Au), copper (Cu), brass, bronze, etc.

The semiconductor absorber layer may have a thickness between 5-500nm, inclusive, and may be made of such materials as amorphous silicon (Si), germanium (Ge), and combinations thereof. The high refractive index dielectric layer has a thickness greater than 0.1 times a quarter wave thickness (QW) of a target wavelength having a predetermined hue on an a × b × Lab color map within 0-40 ° and less than or equal to 4 times the QW. The high refractive index dielectric layer may be formed of a dielectric material (e.g., zinc sulfide (ZnS), titanium dioxide (TiO)₂) Hafnium oxide (HfO)₂) Niobium oxide (Nb)₂O₅) Tantalum oxide (Ta)₂O₅) And combinations thereof).

The reflective core layer and the semiconductor absorber layer may be dry deposited layers and the high refractive index dielectric layer may be a wet deposited layer. Further, the reflective core layer may be a central reflective core layer and the semiconductor absorber layer may be a pair of semiconductor absorber layers extending across opposite sides of the central reflective core layer, i.e., the central reflective core layer is sandwiched between the pair of semiconductor absorber layers. Also, the high refractive index dielectric layers may be a pair of high refractive index dielectric layers such that the central reflective core layer and the pair of semiconductor absorber layers are sandwiched between the pair of high refractive index dielectric layers.

A method of making such an omnidirectional high-chroma red structural colorant includes making a multilayer stack by dry depositing a reflective core layer and dry depositing a semiconductor absorber layer extending across the reflective core layer. A high refractive index dielectric layer extending across the semiconductor absorber layer is then wet deposited thereon. In this manner, hybrid manufacturing methods are used to produce omnidirectional high chroma red structural colorants that can be used in paints, coatings, and the like.

Drawings

FIG. 1 is a schematic illustration of an omnidirectional structural colorant multilayer stack made of dielectric layers, Selective Absorbing Layers (SALs), and reflector layers;

FIG. 2A 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. 2B is the square of the absolute value of the electric field (| E! of the ZnS dielectric layer shown in FIG. 2A when exposed to EMR having wavelengths of 300, 400, 500, 600 and 700nm²) A graphical representation of the thickness;

FIG. 3 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 an outer surface of the dielectric layer;

FIG. 4 is a schematic illustration of a ZnS dielectric layer having a Cr absorber layer located at or near the zero electric field point within the ZnS dielectric layer for incident EMR at a wavelength of 434 nm;

FIG. 5 is a graphical representation of the percent reflectance versus the wavelength of reflected EMR for a multilayer stack without a Cr absorber layer (e.g., FIG. 2A) and a multilayer stack with a Cr absorber layer (e.g., FIG. 4) exposed to white light;

FIG. 6A is a graphical representation of a first harmonic and a second harmonic exhibited by a ZnS dielectric layer extending over an Al reflector layer (e.g., FIG. 2A);

FIG. 6B is a plot of the percent reflectance versus the wavelength of reflected EMR for a multilayer stack having a ZnS dielectric layer extending across the Al reflector layer plus a Cr absorber layer located within the ZnS dielectric layer (thereby absorbing the second harmonic shown in FIG. 6A);

FIG. 6C is a plot of the percent reflectance versus the wavelength of reflected EMR for a multilayer stack having a ZnS dielectric layer extending across the Al reflector layer plus a Cr absorber layer located within the ZnS dielectric layer (thereby absorbing the first harmonic shown in FIG. 6A);

FIG. 7A is a plot of the square of the electric field versus the dielectric layer thickness showing the dependence of the electric field angle for the Cr absorber layer when exposed to incident light at 0 and 45 degrees;

FIG. 7B is a plot of the percent absorption of a Cr absorber layer versus the wavelength of reflected EMR when exposed to white light at 0 and 45 degrees relative to the normal to the outer surface (0 degrees being normal to the surface);

fig. 8A is a schematic illustration of a red omnidirectional structural colorant multilayer stack in accordance with an aspect disclosed herein;

FIG. 8B is a plot of the percent absorption of the Cu absorber layer shown in FIG. 8A versus the wavelength of reflected EMR when white light is exposed to the multilayer stack shown in FIG. 8A at incident angles of 0 and 45;

FIG. 9 is a graph of calculated/simulated data and experimental data of percent reflectance versus wavelength of reflected EMR when a conceptually verified multilayer stack of red omnidirectional structural colorants is exposed to white light at an angle of incidence of 0 °;

fig. 10 is a graphical representation of percent reflectance versus wavelength for an omnidirectional structured colorant multilayer stack in accordance with an aspect disclosed herein;

FIG. 11 is a graphical representation of percent reflectance versus wavelength for an omnidirectional structured colorant multilayer stack in accordance with an aspect disclosed herein;

fig. 12 is a graphical representation of a portion of a b color mapping using CIELAB (Lab) color space, comparing the chroma and hue shift of a conventional coating with a coating prepared from a pigment according to an aspect disclosed herein (sample (b));

fig. 13A is a schematic illustration of a red omnidirectional structured colorant multilayer stack, according to another aspect disclosed herein;

fig. 13B is a schematic illustration of a red omnidirectional structural colorant multilayer stack, according to another aspect disclosed herein;

FIG. 14A is a graphical representation of percent reflectance versus wavelength for the aspect shown in FIG. 13A;

FIG. 14B is a graphical representation of percent reflectance versus wavelength for the aspect shown in FIG. 13B;

FIG. 15 is a graphical representation of percent absorbance versus wavelength for the aspect shown in FIG. 13A;

FIG. 16 is a plot of percent reflectance versus wavelength versus viewing angle for the aspect shown in FIG. 13A;

FIG. 17 is a plot of chromaticity and hue versus viewing angle for the aspect shown in FIG. 13A;

fig. 18 is a graphical representation of color versus a B Lab color mapping reflected by the aspects shown in fig. 13A and 13B; and

fig. 19 is a schematic illustration of a method for making an omnidirectional red structural colorant multilayer stack, according to one aspect disclosed herein.

Detailed Description

An omnidirectional high chroma red structural color pigment is provided. The omnidirectional high chroma red structural colorant is in the form of a multilayer stack having a reflective core layer, a semiconductor absorber layer, and a high refractive index dielectric layer. The semiconductor absorber layer extends across the reflective core layer, and in some cases directly against or on top of the reflective core layer. The high refractive index dielectric layer extends across, and in some cases directly against or on top of, the semiconductor absorber layer. The multilayer stack may be a symmetrical stack, i.e. the reflective core layer is a central reflective core layer bounded by a pair of semiconductor absorber layers, and the pair of semiconductor absorber layers is bounded by a pair of high refractive index dielectric layers.

The multilayer stack reflects a single band of visible light having a red color with a hue on the a x b Lab color map of between 0-40 °, and preferably between 10-30 °. Furthermore, the hue shift of the single band visible light is less than 30 °, preferably less than 20 °, and more preferably less than 10 ° on the a × b × Lab color map when the multilayer stack is viewed from all angles between 0-45 ° perpendicular to its outer surface. Thus, the hue shift of the reflected single band visible light may be within the 15-45 ° region on the a × b × Lab map.

The reflective core layer may be a dry deposited layer having a thickness between 50-200nm, inclusive. The term "dry deposited" means a dry deposition process such as Physical Vapor Deposition (PVD), including electron beam deposition, sputtering, Chemical Vapor Deposition (CVD), plasma assisted CVD, and the like. In some cases, the reflective core layer is made of a reflective metal (e.g., Al, Ag, Pt, Sn, Cr, combinations thereof, and the like). In other cases, the reflective core layer is made of a colored metal (e.g., Au, Cu, brass, bronze, combinations thereof, and the like). It is understood that the terms "brass" and "bronze" refer to copper-zinc alloys and copper-tin alloys, respectively, known to those skilled in the art.

The semiconductor absorber layer may also be a dry deposited layer deposited onto the reflective core layer. In the alternative, a reflective core layer may be deposited onto the semiconductor absorber layer. The semiconductor absorber layer may have a thickness between 5-500nm, inclusive, and may be made of a semiconductor material, such as amorphous silicon, germanium, combinations thereof, and the like.

The high refractive index dielectric layer may be a wet deposited layer, wherein the term "high refractive index" means a refractive index greater than 1.6. And the term "wet deposited" means a wet deposition process such as a sol-gel process, a spin-on process, a wet chemical deposition process, and the like. The high refractive index dielectric layer has a thickness D that conforms to 0.1QW<D ≦ 4QW, where QW is a quarter-wavelength thickness of the target wavelength, i.e., QW ≦ λ_t/4 of whichMiddle lambda_tIs the target wavelength or the wavelength of the desired reflection. The target wavelength has a predetermined hue on the a × b Lab color map within 0-40 °, and preferably between 10-30 °. In some cases, the target wavelength is between 600-700 nanometers, and the dielectric layer is made of a dielectric material (e.g., ZnS, TiO)₂、HfO₂、Nb₂O₅、Ta₂O₅And combinations thereof, etc.).

The overall thickness of the multilayer stack may be less than 3 microns, preferably less than 2 microns, more preferably less than 1.5 microns, and still more preferably less than or equal to 1.0 micron. Further, the multilayer stack has a total number of layers less than or equal to 9, preferably a total number of layers less than or equal to 7, and more preferably a total number of layers less than or equal to 5.

Referring to FIG. 1, a design is shown in which the underlying Reflector Layer (RL) has a first layer of dielectric material DL extending across the reflector layer₁And across the DL₁A layer extended selective absorption layer SAL. In addition, another DL may or may not be provided₁And which may or may not extend across the selective absorbing layer. Also shown in this figure is an illustration of all incident electromagnetic radiation being reflected or selectively absorbed by the multilayer structure.

As illustrated in fig. 1, such a design corresponds to different approaches for designing and manufacturing a desired multilayer stack. In particular, the thickness of the zero energy point or near zero energy point for the dielectric layer is used and discussed below.

For example, fig. 2A is a schematic illustration of a ZnS dielectric layer that extends across an Al reflector core layer. The ZnS dielectric layer has a total thickness of 143nm and for incident electromagnetic radiation having a wavelength of 500nm, a zero or near zero energy point is present at 77 nm. In other words, for incident electromagnetic radiation (EMR) having a wavelength of 500nm, the ZnS dielectric layer exhibits a zero or near zero electric field at a distance of 77nm from the Al reflector layer. In addition, fig. 2B provides a graphical representation of the energy field across the ZnS dielectric layer for several different incident EMR wavelengths. As shown in the figure, the dielectric layer has a zero electric field at 77nm thickness for a wavelength of 500nm, but a non-zero electric field at 77nm thickness for EMR wavelengths of 300, 400, 600, and 700 nm.

With respect to the calculation of a zero or near-zero electric field point, FIG. 3 illustrates a dielectric layer 4 having a total thickness "D", an incremental thickness "D", and a refractive index "n" located at a refractive index n_sOn the substrate or core layer 2. Incident light strikes the outer surface 5 of the dielectric layer 4 at an angle theta with respect to a line 6 perpendicular to the outer surface 5 and is reflected from the outer surface 5 at the same angle theta. Incident light is transmitted through the outer surface 5 and at an angle θ relative to the line 6_FInto the dielectric layer 4 and at an angle theta_sTo the surface 3 of the substrate layer 2.

For a single dielectric layer, θ_s＝θ_FAnd the energy/electric field (E) may be denoted as E (z) when z ═ d. For s-polarization, the electric field can be expressed as:

and for p-polarization, can be expressed as:

wherein

and λ is the desired wavelength to be reflected_ssin θ_sWherein "s" corresponds to the substrate in FIG. 5, and

is the dielectric constant of the layer as a function of z. Thus, for s-polarization

|E(d)|²＝|u(z)|²exp(2ikαy)|_z＝d(3)

And for p polarization

It will be appreciated that the variation of the electric field in the Z direction along the dielectric layer 4 can be estimated by calculating the unknown parameters u (Z) and v (Z), which can be shown as:

naturally, "i" is the square root of-1. Using boundary conditions u ∞ +_z＝0＝1,v|_z＝0＝q_sAnd the following relationships:

for s polarization, q_s＝n_scosθ_s(6)

For p polarization, q_s＝n_s/cosθ_s(7)

For s-polarization, q ═ n cos θ_F(8)

For p-polarization, q ═ n/cos θ_F(9)

u (z) and v (z) can be expressed as:

and

therefore, for having

S-polarization of (c):

and for p-polarization:

wherein:

α＝n_ssinθ_s＝n sinθ_F(15)

and

thus, for θ_FEither 0 or the simple case of normal incidence,

and α ═ 0:

s-polarized | E (d) (+) luminance²P-polarised

Which allows to solve the thickness "d", i.e. the position or location of the electric field in the dielectric layer to be zero.

Referring now to FIG. 4, equation 19 is used to calculate the point of zero or near zero electric field in the ZnS dielectric layer shown in FIG. 2A when exposed to EMR at a wavelength of 434 nm. The zero or near zero electric field point is calculated to be 70nm (instead of 77nm for a wavelength of 500 nm). Furthermore, a 15nm thick Cr absorber layer is inserted at a thickness or distance of 70nm from the Al reflector core layer to provide a zero or near zero electric field ZnS-Cr interface. Such an inventive structure allows light with a wavelength of 434nm to pass through the Cr-ZnS interface, but absorbs light without a wavelength of 434 nm. In other words, the Cr-ZnS interface has a zero electric field or an electric field close to zero for light having a wavelength of 434nm, and thus 434nm light passes through the interface. However, the Cr-ZnS interface does not have a zero or near-zero electric field for light with a wavelength other than 434nm, and therefore, such light is absorbed by the Cr absorber layer and/or the Cr-ZnS interface and is not reflected by the Al reflector layer.

It will be appreciated that some percentage of light in the +/-10nm range of 434nm would be expected to pass through the Cr-ZnS interface. However, it should also be appreciated that such narrow band reflected light, e.g., 434+/-10nm, still provides a glaring structural color to the human eye.

The results for the Cr absorber layer in the multilayer stack in fig. 4 are illustrated in fig. 5, where the percent reflectivity versus reflected EMR wavelength is shown. As shown by the dashed line, which corresponds to the ZnS dielectric layer without the Cr absorber layer shown in fig. 4, a narrow reflection peak is present at about 400nm, but a much wider peak is present at about 550+ nm. In addition, in the 500nm wavelength region, there is still a large amount of reflected light. As such, there is a double peak that prevents the multilayer stack from having or exhibiting a structural color.

In contrast, the solid line in fig. 5 corresponds to the structure shown in fig. 4 in which the Cr getter layer is present. As shown in the figure, there is a sharp peak at about 434nm and a sharp drop in reflectance for wavelengths greater than 434nm is provided by the Cr absorber layer. It will be appreciated that the sharp peaks represented by the solid lines appear visually as a dazzling/structured colour. Furthermore, fig. 5 depicts the measurement of the width of the reflection peak or band, i.e. the width of the band is determined at 50% reflectivity of the maximum reflection wavelength (which is also known as the full width at half maximum (FWHM)).

With respect to the omnidirectional behavior of the multilayer structure shown in fig. 4, the thickness of the ZnS dielectric layer can be designed or set such that only the first harmonic of the reflected light is provided. It will be appreciated that this is sufficient for the "blue" colour, however, the production of the "red" colour requires other conditions. For example, the control of the angular independence of the red color is difficult because a thicker dielectric layer is required, which in turn leads to a higher harmonic design, i.e. the presence of a second and possibly a third harmonic is unavoidable. Moreover, the dark red color hue space is very narrow. As such, the red color multilayer stack has a higher angular dispersion.

To overcome the higher angular dispersion of the red color, the present application discloses a unique and novel design/structure that provides an angle-independent red color. For example, fig. 6A illustrates a dielectric layer exhibiting a first harmonic and a second harmonic for incident white light when the outer surface of the dielectric layer is viewed from 0 and 45 ° relative to the normal to the outer surface. As shown by the illustration, the low angle dependence (small Δ λ) is provided by the thickness of the dielectric layer_c) However, such multilayer stacks have a combination of blue (first harmonic) and red (second harmonic) colors and are therefore not suitable for the desired "red only" color. Therefore, concepts/structures have been developed that use absorber layers to absorb unwanted series of harmonics. FIG. 6A also illustrates the reflection band center wavelength (λ) for a given reflection peak_c) And dispersion or shift of the center wavelength (Δ λ) when the sample is observed from 0 and 45 °_c)。

Turning now to fig. 6B, the second harmonic shown in fig. 6A is absorbed with a Cr absorber layer at the correct dielectric layer thickness (e.g., 72nm) and provides a dazzling blue color. Further, fig. 6C depicts providing a red color by absorbing the first harmonic with a Cr absorber at different dielectric layer thicknesses (e.g., 125 nm). However, fig. 6C also illustrates that the use of Cr absorber layers may result in a greater than desired angular dependence of the multilayer stack, i.e., a greater than desired Δ λ_cIs large.

It will be appreciated that for red colors, a relatively large λ is compared to blue colors_cThe shift is due to the very narrow hue space of the dark red color 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. Thus, FIG. 7A illustrates that the zero or non-zero point is different for wavelengths of light at different angles of incidence. Such factors lead to the angle-dependent absorption shown in fig. 7B, i.e. the difference in the 0 ° and 45 ° absorption curves. Thus, to further refine multilayer stack design and angle independence performance, absorption such as blue light is usedRegardless of whether the electric field is zero or non-zero.

In particular, fig. 8A shows a multilayer stack with a Cu absorber layer, instead of a Cr absorber layer, extending across the dielectric ZnS layer. The results using such "colored" or "selective" absorber layers are shown in fig. 8B, which demonstrates a "tighter" concentration of the 0 ° and 45 ° absorption lines for the multilayer stack shown in fig. 8A. As such, the comparison between fig. 8B and fig. 7B illustrates a significant improvement in the angular independence of the absorbance when a selective absorber layer is used instead of a non-selective absorber layer.

Based on the foregoing, a concept verified multilayer stack structure was designed and prepared. In addition, the calculated/simulated results and actual experimental data of the samples for concept verification were compared. In particular, and as shown by the graph in fig. 9, a brilliant red color is produced (wavelengths greater than 700nm are typically not visible to the human eye), and very good agreement is obtained between the calculations/simulations and the experimental light data obtained from the actual samples. In other words, the calculations/simulations may be used and/or used to simulate the results of a multilayer stack design and/or a prior art multilayer stack according to one or more embodiments disclosed herein.

FIG. 10 shows a plot of the percent reflectivity versus the wavelength of reflected EMR for another omnidirectional reflector design when exposed to white light at angles of 0 and 45 relative to the normal to the outer surface of the reflector. As shown in the graph, both the 0 ° and 45 ° curves illustrate the very low reflectivity provided by the omnidirectional reflector (e.g., less than 10%) for wavelengths less than 550 nm. However, as shown by this curve, the reflector provides a sharp increase in reflectivity at wavelengths between 560-570nm, and reaches a maximum of about 90% at 700 nm. It will be understood that the portion or area 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 reflectivity provided by the omnidirectional reflector is characterized by the UV side edge of each curve extending from a low reflectivity portion at wavelengths less than 550nm to a high reflectivity portiona reflectivity fraction (e.g., greater than 70%). the linear portion 200 of the UV side edge is inclined at an angle (β) greater than 60 ° with a length L of about 40 on the reflectivity axis and a slope of 1.4 with respect to the x-axis in some cases, the linear portion is inclined at an angle greater than 70 ° with respect to the x-axis and in other cases, β is greater than 75 °. additionally, the reflection band has a visible FWHM less than 200nm, and in some cases less than 150nm, and in other cases less than 100 nm-furthermore, the center wavelength λ of the visible reflection band as illustrated in fig. 10 will be as the center wavelength λ of the visible reflection band_cDefined as the wavelength equidistant between the UV side edge of the reflection band at the visible FWHM and the IR edge of the IR spectrum.

It is to be understood that the term "visible FWHM" means the width of the reflection band between the side edges of the curve UV and the edges of the IR spectral range beyond which the reflection 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 invisible IR portion of the electromagnetic radiation spectrum to provide a sparkle or structural color. In other words, despite the fact that the reflector may reflect electromagnetic radiation extending to a wider frequency band within the IR region, the omni-directional reflectors disclosed herein utilize the invisible IR portion of the electromagnetic radiation spectrum to provide a narrow band of reflected visible light.

Referring now to fig. 11, a graph of percent reflectivity versus wavelength for another seven layer design omni-reflector when exposed to white light at angles of 0 and 45 deg. relative to the reflector surface is shown. Further, a definition or characterization of the omnidirectional properties provided by the omnidirectional reflectors disclosed herein is shown. In particular, and when the reflection band provided by the reflector of the invention has a maximum, i.e. peak, as shown, each curve has a central wavelength (λ)_c) Defined as the wavelength at which the maximum reflectance is exhibited or experienced. The term wavelength of maximum reflection may also be used for λ_c。

As shown in fig. 11, when the angle is 45 ° (λ)_c(45 °)) viewing the outer surface of the omnidirectional reflector, for example, when the outer surface is tilted at 45 ° with respect to the human eye viewing the surface, and is 0 ° (v) °Angle (λ) of_c(0 °)), i.e. the ratio when the surface is viewed perpendicularly to the surface, there is λ_cOffset or displacement of. Lambda [ alpha ]_cSuch a shift (Δ λ) of_c) A measure of the omnidirectional nature of an omnidirectional reflector is provided. Naturally, a zero offset, i.e. no offset at all, would be a perfect omnidirectional reflector. However, the omni-directional reflectors disclosed herein may provide a Δ λ of less than 50nm_cIt can appear to the human eye as if the surface of the reflector has not changed color, and thus from a practical perspective, the reflector is omnidirectional. In some cases, the omni-directional reflectors disclosed herein can provide a Δ λ of less than 40nm_cIn other cases, Δ λ of less than 30nm may be provided_cAnd still in other cases can provide a Δ λ of less than 20nm_cAnd yet in other cases can provide a Δ λ of less than 15nm_c。Δλ_cSuch a shift may be determined by a plot of the actual reflectivity of the reflector versus wavelength and/or, alternatively, if the material and layer thicknesses are known, by modeling the reflector.

Another definition or characterization of the omnidirectional nature of the reflector may be determined by the offset of the side edges of a given set of angularly reflected bands. For example, and referring to fig. 11, the same reflectivity (S) of the reflector as for viewing from 45 °_UV(45 °)) UV side edge compared to reflectance (S) for an omnidirectional reflector viewed from 0 ° (S)_UV(0 °)) offset or displacement of the UV side edges (Δ S)_UV) A measure of the omnidirectional nature of an omnidirectional reflector is provided. It is understood that the offset (Δ S) of the UV side edge is measured at the visible FWHM_UV) And/or the offset of the UV side edges (as) can be measured at the visible FWHM_UV)。

Naturally, zero offset, i.e. no offset at all (Δ S)_UV0nm) would characterize a perfect omnidirectional reflector. However, the omni-directional reflectors disclosed herein may provide a Δ S of less than 50nm_UVIt can appear to the human eye as if the surface of the reflector has not changed color, and thus from a practical point of view, the reflector is omnidirectional. In some casesThe omni-directional reflectors disclosed herein may provide a Δ S of less than 40nm_UVIn other cases, a Δ S of less than 30nm may be provided_UVAnd still in other cases can provide a Δ S of less than 20nm_UVAnd yet in other cases can provide a Δ S of less than 15nm_UV。ΔS_UVSuch a shift may be determined by a plot of the actual reflectivity of the reflector versus wavelength and/or, alternatively, if the material and layer thicknesses are known, by modeling the reflector.

The offset of the omni-directional reflection can also be measured by low hue offset. For example, as shown in FIG. 12 (see, e.g., Δ θ)₁) Pigments prepared from a multilayer stack according to an aspect disclosed herein have a hue shift of 30 ° or less, and in some cases, a hue shift of 25 ° or less, preferably less than 20 °, more preferably less than 15 °, and still more preferably less than 10 °. In contrast, conventional pigments exhibit a hue shift of 45 ° or more (see, e.g., Δ θ)₂). It is understood that the sum of Δ θ₁The associated hue shift generally corresponds to a red color, however, for any color reflected by the mixed omnidirectional structured color pigments disclosed herein, a low hue shift is associated.

A schematic illustration of an omnidirectional multilayer stack according to another aspect disclosed herein is shown at 10 in fig. 13A. The multilayer stack 10 has a first layer 110 and a second layer 120. An optional reflector layer 100 may be included. Exemplary materials for the reflector layer 100 (sometimes referred to as a reflector core layer) may include, but are not limited to, Al, Ag, Pt, Cr, Cu, Zn, Au, Sn, and alloys thereof. As such, the reflector layer 100 may be a metallic reflector layer, although this is not required. In addition, an exemplary thickness of the core reflector layer is between 30 to 200 nm.

A symmetrical pair of layers may be located on opposite sides of the reflector layer 100, i.e., the reflector layer 100 may have another first layer disposed opposite the first layer 110, thereby sandwiching the reflector layer 100 between the pair of first layers. In addition, another second layer 120 may be oppositely disposed to the reflector layer 100, thereby providing a five-layer structure. Thus, it should be understood that the discussion of the multilayer stack provided herein also includes the possibility of mirror image structures with respect to one or more central layers. As such, fig. 13A may be illustrative of one half of a five-layer multilayer stack.

With respect to the aspects discussed above, the first layer 110 may be an absorber layer, for example, a semiconductor absorber layer having a thickness between 5-500nm, inclusive. The semiconductor absorber layer 110 may be made of amorphous Si or Ge and absorbs electromagnetic radiation as shown in fig. 14A such that wavelengths less than approximately 550-575nm have a reflectivity of less than 15-20%. The second layer 120 may be a high refractive index dielectric layer having a thickness that provides reflection of wavelengths greater than approximately 575-600 nm, which corresponds to a hue on an a x b Lab color space map of between 0-40 °, and preferably between 10-30 °. Further, the chromaticity of the reflected band of visible light is greater than 70, preferably greater than 80, and more preferably equal to or greater than 90. The reflectance spectra of such a multilayer stack as shown in fig. 13A and having the layer thicknesses listed in table 1 below are exemplarily shown in fig. 14A for viewing angles of 0 ° and 45 °. As shown, the shift in center wavelength is less than 50nm, preferably less than 30nm, and even more preferably less than 20 nm. Furthermore, it should be understood that the UV side of the reflection band also has a very small shift. In combination with the width of the bands in the visible spectrum, the shift in the reflected band between

angles

0 and 45 ° corresponds to a color change that is not significant for the human eye.

TABLE 1

Layer(s)	Material	Thickness (nm)
			100	Al	80
110	Amorphous Si	300
			120	ZnS or TiO ₂	44

Another aspect of an omnidirectional high chroma red structural colorant in the form of a multilayer stack is shown at reference numeral 12 in fig. 13B. This aspect 12 is similar to the aspect 10 shown in fig. 13A, except that an additional absorber layer 112 extends across the high refractive index dielectric layer 120 and an additional high refractive index dielectric layer 122 extends across the second absorber layer 112. Absorber layer 112 may be the same or different from absorber layer 110, i.e., layer 112 may be made of amorphous Si, Ge, etc. Also, the second dielectric layer 122 may be the same as or different from the first dielectric layer 120. It should be understood that fig. 13B may be illustrative of one half of a 9 layer stack, where the reflector layer 100 is a central or core reflector layer sandwiched between the

layers

110, 120, 112, 122 as shown and another set of

layers

110, 120, 112, 122 disposed opposite thereto.

The reflectance spectrum of the multilayer stack as shown in fig. 13B and with the layer thicknesses shown in table 2 below is shown in fig. 14B. As shown in this figure, the wavelength shift of the viewing angle between 0 and 45 ° perpendicular to the outer surface of the multilayer stack (which shows a reflectivity of 70%) is relatively small. For example, the wavelength shift may be less than 50nm, preferably less than 30nm, and more preferably less than 20 nm. Likewise, this shift in the visible spectrum occurs as a color shift that is not significant to the human eye. In the alternative, a low or small shift (Δ S) in the UV-side edge of the reflection spectrum shown in the figure is passed_UV) To show or describe the omnidirectional nature.

TABLE 2

Layer(s)	Material	Thickness (nm)
			100	Al	80
110	Amorphous Si	155
			120	ZnS or TiO₂	35

FIG. 15 shows the absorption versus wavelength for the design shown in FIG. 13A. As shown in this figure, multilayer stack 10 absorbs more than 80% of the visible spectrum for wavelengths up to about 550 nm. Further, this aspect 10 absorbs more than 40% of all wavelengths up to 600 nm. As such, the combination of the absorbing layer 110 and the dielectric layer 120 provides a visible reflection band having a hue on the a × b × Lab color space of between 0-40 °, and preferably between 10-30 °, i.e. the wavelength of reflection in the red color spectrum.

FIG. 16 shows a plot of this aspect 10 as a function of percent reflectivity, wavelength reflected, and angle of observation. As shown by the 3D contour plot, the reflectivity is very low, i.e., less than 20% for wavelengths between 400-550-575nm and at viewing angles between 0 and 45-50 deg.. However, there is a sharp increase in percent reflectance at wavelengths of about 600 nm.

Another method or technique to describe the omnidirectional nature of the multilayer stacks of the present invention disclosed herein is a plot of chromaticity and hue versus viewing angle as shown in fig. 17. FIG. 17 illustrates the reflective characteristics of the aspect shown in FIG. 13A, wherein the hue of the angle between 0 and 45 is between 20-30 with a change or offset of less than 10. Further, the chromaticity is between 80-90 for all viewing angles between 0-45 °, where chromaticity (C) is defined as

a and b are coordinates on the Lab color space or map of the color reflected by the multilayer stack when exposed to broadband electromagnetic radiation (e.g., white light).

Fig. 18 shows or plots the hue on the a × B Lab color space map for the aspect shown in fig. 13A (labeled "a") and 13B (labeled "B"). Also shown on the map is a region between 15-40 deg.. It will be appreciated that these two points are used to illustrate a 0 viewing angle relative to the normal to the outer surface of the multilayer stack. In addition, it is understood that the hue in this aspect as shown in FIGS. 13A and 13B does not shift outside the 15-40 hue region between viewing angles of 0-45. In other words, this aspect indicates a low hue shift, for example less than 30 °, preferably less than 20 °, and still more preferably less than 10 °. It should be further understood that the aspect shown in fig. 13A and 13B may also be designed to provide a single band of visible light having a hue between 0-40 deg., and may be plotted in fig. 18, and preferably a single band of visible light having a hue between 10-30 deg..

Turning now to fig. 19, a method for making an omnidirectional high chroma red structural colorant is shown generally at 20. The method 20 includes dry depositing a reflective core layer at step 202 and then dry depositing a semiconductor absorber layer onto the dry deposited reflective core layer at step 210. A high index of refraction dielectric layer is then wet deposited onto the dry deposited semiconductor absorber layer in step 220. It should be understood that

steps

210 and 220 may be repeated to create additional layers on the dry deposited reflective core layer. Furthermore, a dry deposited reflective core layer may be deposited onto the semiconductor absorber layer, and a wet deposited dielectric layer may also be deposited onto the semiconductor absorber layer.

Can be made into dry deposited n_hA non-exhaustive list of materials for the dielectric layers is shown in table 3 below.

TABLE 3

The foregoing embodiments and aspects are for illustrative purposes only and variations, changes, etc. will be apparent to those skilled in the art and still fall within the scope of the invention. Thus, the scope of the invention is defined by the claims and all equivalents thereof.

Claims

1. An omnidirectional high chroma red structural colorant comprising:

a multilayer stack having:

a reflective core layer;

a semiconductor absorber layer extending across the reflective core layer; and

a high refractive index dielectric layer extending across the semiconductor absorber layer;

the multilayer stack reflects a single band of visible light having a predetermined hue on an a x b Lab color map between 0-40 ° and a hue shift on the a x b Lab color map within the predetermined hue between 0-40 ° when viewed from all angles between 0-45 ° normal to an outer surface of the multilayer stack.

2. The omnidirectional high chroma red structural colorant of claim 1, wherein the high refractive index dielectric layer has a thickness D that follows a relationship of 0.1QW < D ≦ 4QW, wherein QW is a quarter-wavelength thickness of a target wavelength having the predetermined hue within 0-40 ° on the a x b Lab color map.

3. The omnidirectional high chroma red structural colorant of claim 1, wherein the reflective core layer has a thickness between 50 and 200 nanometers, inclusive.

4. The omnidirectional high chroma red structural colorant of claim 3, wherein the reflective core layer is made of a reflective metal selected from the group consisting of: al, Ag, Pt, Sn, Cr, and combinations thereof.

5. The omnidirectional high chroma red structural colorant of claim 3, wherein the reflective core layer is made of a colored metal selected from the group consisting of: au, Cu, brass, bronze, and combinations thereof.

6. The omnidirectional high chroma red structural colorant of claim 3, wherein the semiconductor absorber layer has a thickness between 5 and 500 nanometers, inclusive; and is made of the following materials: amorphous Si, Ge, and combinations thereof.

7. The omnidirectional high chroma red structural colorant of claim 1, wherein the reflective core layer is a central reflective core layer and the semiconductor absorber layer is a pair of semiconductor absorber layers extending across opposite sides of the central reflective core layer, the central reflective core layer being sandwiched between the pair of semiconductor absorber layers.

8. The omnidirectional high chroma red structural colorant of claim 7, wherein the high refractive index dielectric layers are a pair of high refractive index dielectric layers, the central reflective core layer and the pair of semiconductor absorber layers being sandwiched between the pair of high refractive index dielectric layers.

9. A method for preparing an omnidirectional high chroma red structural colorant, the method comprising:

a multilayer stack is made by:

dry-depositing a reflective core layer;

dry depositing a semiconductor absorber layer extending across the reflective core layer; and

wet depositing a high refractive index dielectric layer extending across the semiconductor absorber layer;

the multilayer stack reflects visible light having a predetermined hue on an a x b Lab color map between 0-40 ° and the visible light has a hue shift on the a x b Lab color map within the predetermined hue between 0-40 ° when viewed from all angles between 0-45 ° normal to an outer surface of the multilayer stack.

10. The method of claim 9 wherein the high refractive index dielectric layer has a thickness D that follows a 0.1QW < D ≦ 4QW relationship, wherein QW is a quarter-wave thickness of a target wavelength having a predetermined hue within 0-40 ° on an a x b Lab color map.

11. The method of claim 9, wherein the reflective core layer has a thickness between 50-200 nanometers, inclusive, and is made of a reflective metal selected from the group consisting of: al, Ag, Pt, Sn, and combinations thereof.

12. The method of claim 11, wherein the semiconductor absorber layer has a thickness between 5-500 nanometers, inclusive.

13. The method of claim 9, wherein the high refractive index dielectric layer is made of a dielectric material selected from the group consisting of: ZnS, TiO₂、HfO₂、Nb₂O₅、Ta₂O₅And combinations thereof.

14. The method of claim 9, further comprising dry depositing a pair of semiconductor absorber layers on opposite sides of the reflective core layer, the reflective core layer sandwiched between the pair of dry deposited semiconductor absorber layers.

15. The method of claim 14, further comprising wet depositing a pair of high refractive index dielectric layers, a pair of dry deposited semiconductor absorber layers sandwiched between the pair of wet deposited high refractive index dielectric layers.