WO2019032051A1

WO2019032051A1 - A multilayer coating, a method of fabricating the same and use(s) thereof

Info

Publication number: WO2019032051A1
Application number: PCT/SG2018/050404
Authority: WO
Inventors: Goutam Kumar Dalapati; Santiranjan Shannigrahi; Mohit Sharma
Original assignee: Agency For Science, Technology And Research
Priority date: 2017-08-10
Filing date: 2018-08-08
Publication date: 2019-02-14

Abstract

There is provided a multilayer coating comprising at least one metal layer disposed between at least two doped metal chalcogenide layers. There is also provided a method of fabricating a multilayer coating comprising: providing a metal layer; and depositing at least two doped metal chalcogenide layers on at least two surfaces of the metal layer. There is also provided use of the multilayer coating for coating a substrate. In a preferred embodiment, the multilayer coating is coated on a glass window, the multilayer coating may result in energy savings and be used on smart window applications.

Description

A Multilayer Coating, A Method of Fabricating the Same and Use(s) Thereof

References To Related Applications

This application claims priority to Singapore application number 10201706551R filed on 10 August 2017, the disclosure of which is hereby incorporated by reference.

Technical Field

The present invention relates to a multilayer coating, a method for preparing the multilayer coating and uses of the same.

Background Art

Transparent heat regulating (THR) materials and coating for energy saving window applications include transparent heat reflecting mirror, thermo-chromic, transparent solar cells, and luminescent based materials. The coating performance primarily depends on the selection of materials, surface and structural morphology, dielectric passivation growth process and architecture of the multi-layered structure.

Currently, thin film silver and tin oxide are widely used for low-emission glass coating. Metal oxide and sulfide can be used as a dielectric material, and there can be multiple dielectric layers within the glass coating. The metal oxides have been deposited using metal as a target in oxygen ambient by conventional sputter deposition system. However, low emissivity glass prepared by such conventional methods suffers in terms of performance and stability. Formation of thin interfacial oxide in between metal and dielectric significantly reduced the performance of the low-emission glass. To overcome the above issue, thin metal layer particularly nickel-chromium (NiCr) alloy or metal nitride have been introduced, but overall thickness of the metal would be increased to affect visible transmittance and heat reflection.

In another approach to make the low-emission glass, the dielectric layer was deposited using metal oxide targets instead of metal targets. The deposition of the dielectric layer on the low emissivity film (metal layer) usually was done in an inert gas ambient using metal oxide targets. However, even by using metal oxide targets, the low emissivity film can also be partially oxidized due to the presence of residual oxygen in the chamber and some oxygen radical from the targets. Moreover, there are some dielectrics where oxygen is able to diffuse through easily. Thus, during the growth/deposition of dielectric, oxygen can diffuse into the metal layer and form an interfacial layer that reduces the durability of the low emissive layer.

Conventional glass emits 84% of the radiant heat falling upon it by absorbing and transmitting infrared radiation (IR) radiation while reflecting only 16% of IR radiation. For smart window applications, not only does the infrared rejection need to be maximized, the multilayer film should also be able to achieve high transmission in the visible range of the spectrum and filter out ultra-violet (UV) radiation as much as possible. There is therefore a need to provide a multilayer coating that overcomes or at least ameliorates, one or more of the disadvantages described above.

Summary

In one aspect, there is provided a multilayer coating comprising at least one metal layer disposed between at least two doped metal chalcogenide layers.

Advantageously, the doped metal chalcogenide layers play a significant role to enhance the visible light transmission and color tuning of the multilayer coating. The visible light transmission can be tuned by using a metal dopant in the doped metal chalcogenide layer and by controlling the thickness of the doped metal chalcogenide layer. By controlling the type of metal dopant, the refractive index of the doped metal chalcogenide layer can be modulated. Further advantageously, the doped metal chalcogenide layers function to improve the infrared reflectance of the multilayer coating as well. The multilayer coating may be developed at room temperature to achieve the purpose of energy saving.

Where the multilayer coating is coated onto a glass substrate, the multilayer coating may function as a heat rejection coating that eliminates heat transfer through the glass substrate. Thus, where the glass substrate is a window of a building, the indoor temperature of the building will be lower compared with the outdoor temperature of the building. This may aid in reducing the usage of cooling devices such as air conditioners or fans so as reduce energy consumption and lead to energy savings. In another aspect, there is provided a method of fabricating a multilayer coating comprising: (a) providing a metal layer; and (b) depositing at least two doped metal chalcogenide layers on at least two surfaces of the metal layer.

In another aspect, there is provided use of a multilayer coating as described herein for coating a substrate. Advantageously, the multilayer coating may be used as a transparent heat regulating coating that optionally has UV filtration properties.

Definitions The following words and terms used herein shall have the meaning indicated:

The term "dopant" as used herein refers to a trace element that is inserted into a substance to alter the optical property of the substance. Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value. Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub- ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub -ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Certain embodiments may also be described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the disclosure. This includes the generic description of the embodiments with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

Detailed Disclosure of Embodiments

Exemplary, non-limiting embodiments of a multilayer coating will now be disclosed.

The multilayer coating comprises at least one metal layer disposed between at least two doped metal chalcogenide layers.

The metal layer may comprise a metal. The metal may be selected from Group 11, Group 12 or Group 13 of the Periodic Table of Elements. The metal may be selected from the group consisting of copper, gold, silver, aluminium, zinc, alloys and mixtures thereof. The metal layer may be one that has low emissivity, excellent heat reflecting properties and/or optically transparent at least to visible light. The metal layer may have a thickness in the range of about 10 nm to about 30 nm, wherein the thickness may be about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, about 15 nm, about 16 nm, about 17 nm, about 18 nm, about 19 nm, about 20 nm, about 21 nm, about 22 nm, about 23 nm, about 24 nm, about 25 nm, about 26 nm, about 27 nm, about 28 nm, about 29 nm, about 30 nm, or sub-ranges there between.

The material for the metal chalcogenide layer may be a sulfide. The sulfide may be a sulfide of an element selected from Group 4, Group 11, Group 12, Group 13 or Group 14 of the Periodic Table of Elements. The element may be selected from the group consisting of aluminium, copper, zinc, silicon, titanium and mixture thereof. The sulphide layer may comprise zinc sulfide (ZnS).

The material for the metal chalcogenide layer may be an oxide. The oxide may be an oxide of an element selected from Group 4, Group 5, Group 6 or Group 14 of the Periodic Table of Elements. The element may be selected from the group consisting of zirconium, tantalum, niobium, titanium, hafnium, tin, tungsten, molybdenum and mixtures thereof. The oxide layer may comprise zirconium oxide (Zr0₂). The metal chalcogenide layer may have a thickness in the range of about 20 nm to about 100 nm, about 30 nm to about 100 nm, about 40 nm to about 100 nm, about 50 nm to about 100 nm, about 60 nm to about 100 nm, about 80 nm to about 100 nm, about 20 nm to about 80 nm, about 20 nm to about 60 nm, about 20 nm to about 40 nm, about 20 nm to about 30 nm, about 30 nm to about 40 nm, or sub-ranges there between. The thickness of the barrier layer may be about 40 nm.

The doped metal chalcogenide layer may comprise a metal dopant. The metal dopant may be a metal selected from Group 4, Group 11, Group 12, or Group 13 of the Periodic Table of Elements. The metal dopant may be selected from the group consisting of aluminium, copper, zinc, silicon, titanium and mixture thereof.

The amount of metal dopant in the metal chalcogenide layer may be in the range of about 0.01% to about 2%, 0.01% to about 1%, about 0.01% to about 0.5%, about 0.01% to about 0.3%, about 0.01% to about 0.2%, about 0.01% to about 0.1%, about 0.01% to about 0.08%, about 0.01% to about 0.05%, about 0.01% to about 0.03%, about 0.03% to about 1%, about 0.05% to about 1%, about 0.08% to about 1%, about 0.1% to about 1%, about 0.2% to about 1%, about 0.3% to about 1%, or about 0.5% to about 1%.

The multilayer coating in the present disclosure may further comprise a surface protector layer. The surface protector layer may comprise a film coating material. The film coating material may be an acrylic material. The surface protector layer may additionally comprise a UV absorber material within the film coating material. The surface protector layer may enhance the scratch resistance and/or reduce UV reflectance of the multilayer coating. The surface protector layer may be disposed on one of the at least two doped metal chalcogenide layer. The surface protector layer may be thin so that it will only eliminate UV radiation without compromising visible light transmission. The surface protector layer may have a thickness in the range of about 10 to 100 nm, about 15 to 100 nm, about 20 to 100 nm, about 30 to 100 nm, about 40 to 100 nm, about 50 to 100 nm, about 60 to 100 nm, about 70 to 100 nm, about 80 to 100 nm, about 90 to 100 nm, about 10 to 90 nm, about 10 to 80 nm, about 10 to 70 nm, about 10 to 60 nm, about 10 to 50 nm, about 10 to 40 nm, about 10 to 30 nm, about 10 to 20 nm, about 10 to 15 nm.

The UV absorber material may be a niobate, a tantalate or an antimonate of at least an element selected from Group 1 of the Periodic Table of Elements. The UV absorber materials may be doped with at least an element selected from Group 3 of the Periodic Table of Elements. The UV absorber materials may be potassium sodium niobate with 5 mol% lanthanum doping (KLNN).

The multilayer coating in the present disclosure may further comprise a dielectric layer. The dielectric layer may comprise metal oxides and a dielectric alloy. The dielectric layer may be used to enhance the adhesion and durability of the multilayer coating. The dielectric layer may be selected from the group consisting of Ti0₂, Zr0₂, A1₂0₃, Hf0₂, Zr0₂+Ti0₂, Al₂0₃+Ti0₂, or Hf0₂+Al₂0₃. The dielectric layer may be disposed on the other of the at least two doped metal chalcogenide layer. The dielectric layer may have a thickness in the range of about 2 nm to about 10 nm, about 3 nm to about 10 nm, about 5 nm to about 10 nm, about 7 nm to about 10 nm, about 9 nm to about 10 nm, about 2 nm to about 3 nm, about 2 nm to about 5 nm, about 2 nm to about 7 nm, or about 2 nm to about 9 nm.

The multilayer coating in the present disclosure may have excellent transparency to visible light in the sense that visible light may be transmitted across the multilayer coating to the substrate that the multilayer coating is coated on. The transparency of the multilayer coating may be measured using UV-Vis spectrometry. The transparency of the multilayer coating for visible light may be at least about 30%, at least about 40%, at least about 50%, at least about 60%, at least about 70% or at least about 80%.

The multilayer coating in the present disclosure may maintain excellent transparency to visible light, after annealing at a temperature of at least about 100°C, at least about 200°C or at least about 300°C. The transparency of the multilayer coating, after annealing at the above temperature, as measured using UV-Vis spectrometry, may be at least about 30%, at least about 40%, at least about 50%, at least about 60%, at least about 70% or at least about 80%. The multilayer coating in the present disclosure may be developed under ambient conditions (such as in room temperature (about 20°C to about 25°C) and atmospheric pressure). Thus, the multilayer coating may work suitably well with transparent substrates such as, but are not limited to, materials that can be coated at ambient conditions. Transparent substrates that may be coated with the multilayer coating at ambient conditions may comprise glass, plastics, quartz or diamond.

The multilayer coating may comprise additional layers of a metal chalcogenide layer, which may be undoped or doped as stated above. Each additional metal chalcogenide layer may be made from the same or different material as the preceding metal chalcogenide layer. Hence, the multilayer coating may have one layer of the metal layer, at least two doped metal chalcogenide layers disposed on both sides of the metal layer with additional undoped or doped metal chalcogenide layers. Hence, the multilayer coating may have at least three layers (of the metal layer and the doped metal chalcogenide layers) or at least five layers (with two additional undoped or doped metal chalcogenide layers). Where the multilayer coating includes the surface protector layer and the dielectric layer, the multilayer coating may then have at least five layers or at least seven layers respectively. It is to be noted that the number of multi-layers is not limited to the above and may be any number of layers as required as long as the transmittance and reflectance of the multilayer coating is not compromised. Further, given these conditions are met, the number of metal chalcogenide layers can be increased to enhance the functionalities of the multilayer coating such as self-cleaning and anti-bacterial protection.

Exemplary, non-limiting embodiments of a method for fabricating a multilayer coating will now be disclosed. The method of fabricating a multilayer coating may comprise the steps of: (a) providing a metal layer, (b) depositing at least two doped metal chalcogenide layers on at least two surfaces of the metal layer. The depositing step may comprise a sputtering step.

During the sputtering step, an ambient inert atmosphere may be provided using nitrogen gas, argon gas or any mixture thereof.

The method may further comprise the step of depositing a surface protector layer on one of the at least two doped metal chalcogenide layers. This depositing step may comprise the step of sputtering, spin coating and/or dip coating. The depositing step may comprise the step of adding viscosity modifiers so as to achieve a uniform and tenacious layer.

The method may further comprise the step of depositing a dielectric layer on the other of the at least two doped metal chalcogenide layers. Metal doped chalcogenide may be deposited using co-sputtering of metal and chalcogenide. DC sputter may be used for metal and RF sputter may be used for Chalcogenide. Sputtering power for chalcogenide may be varied from 30 W to 200 W; whereas sputtering power for metal may be varied from 2W to 30 W. Exemplary, non-limiting embodiments of a use of a multilayer coating will now be disclosed.

The use of the multilayer coating may be as a coating on a substrate. The substrate may be a glass, a plastic, a packaging material or a transparent substrate that can be coated at ambient conditions. The multilayer coating may confer heat reflection, UV absorber and/or scratch resistance properties to the substrate.

Brief Description of Drawings

The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

Fig.l

[Fig. 1A] shows a schematic diagram of a multilayer coating on a substrate. [Fig. IB] is a graph showing the transmittance at different wavelengths of the multilayer coating of Fig. 1A, whereby the transmittance was measured for multilayer coatings having different thickness of the metal layer.

Fig.2

[Fig. 2A] is a graph showing the reflectance at different wavelengths of the multilayer coating of Fig. 1A, whereby the reflectance was measured for multilayer coatings having different thickness of the metal layer. Fig. 2B is a graph showing the reflectance at different wavelengths of the multilayer coating of Fig. 1A, whereby the reflectance was measured for multilayer coatings having different types of doped metal chalcogenide layers. Fig.3

[Fig. 3] is a number of photographs showing the varying colours for various multilayer coatings.

Fig.4

[Fig. 4A] is a graph showing the UV-vis spectra of a Zr02/Cu/Zr02 multilayer coating while [Fig. 4B] is a graph showing the UV-vis spectra of a ZnS/Cu/ZnS multilayer coating.

Fig.5

[Fig. 5] is a schematic diagram of a multilayer coating with a metal layer, at least two doped metal chalcogenide layer, a surface protector layer and a dielectric layer.

Fig.6

[Fig. 6] is a schematic process showing the method of fabricating a multilayer coating according to one disclosed embodiment.

Fig.7

[Fig. 7] is a graph showing the UV-vis spectra of a sulfide/Cu/sulfide multilayer coating after thermal treatment at different temperatures for 2 hours in atmospheric pressure. Fig.8

[Fig. 8] is a series of photographs where Fig. 8A is an uncoated glass substrate, Fig. 8B is a sulfide/Cu/sulphide multilayer coating on the glass substrate where the thickness of the Cu layer was 12 nm and Fig. 8C is a sulfide/Cu/sulphide multilayer coating on the glass substrate where the thickness of the Cu layer was 18 nm.

Fig.9

[Fig. 9] is a series of photographs where Fig. 9A is an uncoated glass substrate, Fig. 9B is a ZnS:Al/Cu/ ZnS:Al multilayer coating on the glass substrate and Fig. 9C is a Zr0₂/Cu/ Zr0₂ multilayer coating on the glass substrate.

Fig.10

[Fig. 10] is a graph comparing the transmittance between a multilayer coating that contains the doped metal chalcogenide layer and another multilayer coating whereby the metal chalcogenide layer is undoped.

Examples Non-limiting examples of the invention and a comparative example will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Example 1: Cu and metal doped ZnS based multilayer coating

A schematic diagram of a multilayer coating having a copper metal layer and metal doped zinc sulphide layers is shown in Fig. 1 A, where the multilayer coating is coated onto a glass substrate. Metal doped chalcogenide deposited on a glass substrate using co-sputtering of metal and chalcogenide. DC sputter was used for metal and RF sputter was used for Chalcogenide. Sputtering power for chalcogenide can be varied from 30 W to 200 W; whereas sputtering power for metal can be varied from 2W to 30 W.

For Fig. IB, it shows the transmittance spectra of aluminium doped zinc sulphide as the doped metal chalcogenide layers with copper at a thickness of 12 nm (sample A) and aluminium doped zinc sulphide as the doped metal chalcogenide layers with copper at a thickness of 18 nm (sample B) as compared to the glass without any coating layer. By increasing the copper layer thickness, visible transmittance slightly decreased.

In order to determine the relationship between UV reflectance, intra-red reflectance and the thickness of the metal layer, the thickness of the metal layer was altered. Where the metal layer used was copper, sample A was based on aluminium doped zinc sulphide as the doped metal chalcogenide layers with copper at a thickness of 12 nm as the metal layer while sample B was based on aluminium doped zinc sulphide as the doped metal chalcogenide layers with copper at a thickness of 18 nm. As seen from Fig. 2A, the infra-red reflectance increased as the thickness of the metal layer increased.

For Fig. 2B, sample C was based on zirconium oxide as the dielectric layers with copper as the metal layer. As seen from Fig. 2B, where sulphide was used as the metal chalcogenide layer, the infra-red reflectance started at a wavelength of 800 nm while when oxide was used as the overcoat layer on copper metal, the infra-red reflectance started at a wavelength of about 600 nm. In view of this, the colour of the multilayer coating changes accordingly. Furthermore, ZnS/Cu/ZnS multilayer structure shows high reflectance in the UV region (Fig. 2B), which is essential for smart windows application.

As shown in Fig. 3, neutral color can be developed with copper based multilayer coating, which is essential for smart window application. For ZnS /Cu/ ZnS structure (Fig. 3A), the multilayer coating showed light brown color with good transparency. For ZnS /Cu/ ZnS with thicker Cu structure (Fig. 3B), the multilayer coating showed dark brown color and reduced transparency as compared to Fig. 3 A. For Zr0₂/Cu/Zr0₂ structure (Fig. 3C), the multilayer coating showed light brownish color with more grey color component and similar transparency as compared to Fig. 3A. By selecting the suitable dielectric, it is possible to tune the required color. Example 2: Cu and metal doped chalcogenide based multilayer coating with surface coating

The adequate scratch resistance and film adhesion strength on substrate plays a crucial role for long term usage. For the smart windows application, it is better to eliminate UV radiation (absorb UV radiation instead reflectance). Henceforth, a transparent surface protection layer was developed on top of the dielectric layer as well as to limit UV penetration through the coating. The top coatings with addition of UV absorbing nano additives will increase scratch resistance and reduces the UV penetration through the coating without compromising the visible transmission and IR reflection. For Fig. 4A, the ZnS /Cu/ ZnS structure with surface coating showed much lower UV transmittance as compared to the ZnS /Cu/ ZnS structure without surface coating, with obvious influence for the visible transmittance. For Fig. 4B, the Zr0₂ /Cu/ Zr0₂ structure with surface coating showed lower UV transmittance as compared to the Zr0₂ /Cu/ Zr0₂ without surface coating as well. The visible transmittance of the Zr0₂ /Cu/ Zr0₂ structure with surface coating also decreased slightly as compared to the Zr0₂ /Cu/ Zr0₂ without surface coating.

In the present disclosure as shown in the schematic diagram of Fig. 5 , the developed copper based coating eliminated infra-red radiation and ultra-violet radiation, whereas it allowed visible light; furthermore, surface coating on the top dielectric protected the overall coating from harsh environment. Thus, the developed coating was multifunctional with enhanced durability. The ultra-thin dielectric (metal oxides and alloy) was deposited on a glass substrate using sputter deposition to enhance the adhesion property between substrate (glass/plastic) and chalcogenide layer. The alloy dielectric can be prepared through co-sputtering and/or sputtering of metal chalcogenide with metal. In argon gas at ambient conditions, metal doped chalcogenide was deposited using co- sputtering of metal and chalcogenide. DC sputter was used for metal and RF sputter was used for Chalcogenide. Sputtering power for chalcogenide can be varied from 30 W to 200 W; whereas sputtering power for metal can be varied from 2W to 30 W. The acrylic based surface coating was applied on the top surface of chalcogenide using spin coating and/or dip coating methods. The stickiness of the acrylic resin is controlled by mean of adding suitable viscosity modifiers so as to achieve a uniform and tenacious film over the surface. Without affecting the transparency, the external coating will acclimate to protect the as-deposited film from severe damage as well as from tropical weathering condition. With the transparent acrylic coating the addition of UV nano additives will accompany the near infrared reflection properties arise from the films. The UV blocking additive e.g. Potassium sodium niobate with 5mol% Lanthanum doping (KLNN) was added to acrylic resin and the hybrid coated films that are deposited on the glass and/or plastic substrate can achieve heat reflection, UV absorber as well as scratch resistance properties. The overall methodologies for the Cu-based coating are shown in Fig. 6. The Cu-based coating was developed using sputter deposition methods surface treatment. Metal doped sulfide/Cu/sulfide structure was grown on the glass/plastic substrate. To enhance the adhesive force between coating and substrate, a ultra-thin sulfide/oxide layer was used. Sputter deposition technique and low thermal budget was employed to enhance the visible transmittance of the coating. Metal doped sulfide based material was used to protect low emissive layer (Cu film), anti-reflection, and oxygen diffusion barrier. Color tuning of the coating done through refractive index modulation of metal doped sulfide layer. Acrylic surface coating was employed to enhance scratch resistance and reduce UV penetration. The developed coating was multifunctional with heat rejection, UV filtration and anti-scratch.

Example 3: Stability of Cu and metal doped ZnS based multilayer coating with temperature

The temperature stability of the multiplayer coating was evaluated in Fig. 7. The multilayer coating was stable up to 300°C when annealed in atmosphere for the duration of 2 hours. At 400°C, the quality of the multilayer coating was affected, and the transmittance at infra-red region increased.

Example 4: Transparency and tunable color of Cu and metal chalcogenide based multilayer

The transparency of the Cu and ZnS based multilayer can be tuned by different thickness of the metal layer as illustrated in Fig. 8. The glass substrate without coating is shown in Fig. 8A. The transparency of the film decreased when the thickness of Cu layer increased as observed by Fig. 8B (Cu layer thickness 12m) and Fig 8C (Cu layer thickness 18nm), both with ZnS /Cu/ ZnS structure.

The color of the coating depends on the multilayer design as shown in Fig. 9. The dielectric and interface between metal and dielectric played a significant role in the coating. The glass substrate without coating is shown in Fig. 9A. The ZnS:Al/Cu/ZnS:Al multilayer showed light brown color in Fig 9B. The Zr0₂/Cu/Zr0₂ multilayer showed reddish color with less transparency as shown in Fig. 9C.

Comparative Example 1: Cu and ZnS based multilayer with and without metal dopant

Addition of an aluminium dopant into the metal sulfide layer enhanced visible transmittance of the coating layer, as shown in above figure. The multilayer coating with aluminium doped ZnS layer not only enhanced the visible transmittance, it also improved infra-red reflectance. The coating was developed at room temperature. Thermal treatment of the coating at elevated temperature further enhanced the visible transmittance of the coating layer (Fig. 10). Industrial Applicability

The multilayer coating may be useful as a heat reflector coating on substrates. The substrates may be glass, plastic, packaging material or transparent substrates that can be coated at ambient conditions. Where the multilayer coating is coated on a glass window, the multilayer coating may result in energy savings and be used on smart window applications. The multilayer coating may be used on windows on buildings or automobiles, or on screens on electronic devices, as well as devices that require anti- scratch and wear resistance properties, leading to multiple applications in the construction, automobile or electronic industries. The transparent multilayer shows tunable wetting property. The contact angle can be tuned between super hydrophilic to super hydrophobic. Thus, it has a wide range of application in self -cleaning, driver-less car etc.

It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

1. A multilayer coating comprising at least one metal layer disposed between at least two doped metal chalcogenide layers.

2. The multilayer coating according to claim 1, wherein the metal layer comprises a metal selected from Group 11, Group 12 or Group 13 of the Periodic Table of Elements.

3. The multilayer coating according to claim 2, wherein the metal is selected from the group consisting of copper, gold, silver, aluminium, zinc, alloys and mixtures thereof.

4. The multilayer coating according to any one of the preceding claims, wherein the thickness of the metal layer is in the range of 10 nm to 30 nm.

5. The multilayer coating according to any one of the preceding claims, wherein the metal chalcogenide layer is a sulfide of an element selected from Group 4, Group 11, Group 12, Group 13 or Group 14 of the Periodic Table of Elements.

6. The multilayer coating according to claim 5, wherein the element of the sulfide is selected from the group consisting of aluminium, zinc, silicon, titanium and mixtures thereof.

7. The multilayer coating according to any one of the preceding claims, wherein the metal chalcogenide layer is zinc sulfide.

8. The multilayer coating according to any one of claims 1 to 4, wherein the metal chalcogenide layer is an oxide of a transition metal selected from the group consisting of zirconium, tantalum, niobium, titanium, hafnium, tin, tungsten, molybdenum and mixtures thereof.

9. The multilayer coating according to any one of the preceding claims, wherein the thickness of the metal chalcogenide layer is in the range of 20 nm to 100 nm.

10. The multilayer coating according to any one of the preceding claims, wherein the doped metal chalcogenide layer comprises a metal dopant therein.

11. The multilayer coating according to claim 10, wherein the metal dopant is a metal selected from Group 4, Group 11, Group 12, or Group 13 of the Periodic Table of Elements.

12. The multilayer coating according to claim 10 or 11, wherein the amount of metal dopant in the doped metal chalcogenide layer is in the range of 0.01% to 2%.

13. The multilayer coating according to any one of the preceding claims, wherein the multilayer coating further comprises a surface protector layer.

14. The multilayer coating according to claim 13, wherein said surface protector layer comprises an acrylic material.

15. The multilayer coating according to claim 13 or 14, wherein said surface protector layer comprises a UV absorber material.

16. The multilayer coating according to any one of the preceding claims, wherein the multilayer coating further comprises a dielectric layer.

17. The multilayer coating according to claim 16, wherein said dielectric layer comprises metal oxides and a dielectric alloy.

18. A method of fabricating a multilayer coating comprising:

a. providing a metal layer; and

b. depositing at least two doped metal chalcogenide layers on at least two surfaces of said metal layer.

19. The method according to claim 18, further comprising the step of depositing a surface protector layer on one of said at least two doped metal chalcogenide layers.

20. The method according to claim 19, further comprising the step of depositing a dielectric layer on the other of said at least two doped metal chalcogenide layers.

21. Use of a multilayer coating according to any of claims 1 to 15 for coating a substrate.

22. Use according to claim 21, wherein said substrate is a glass, a plastic, a packaging material a transparent substrate.