WO2020046211A1 - Quantum dot for phosphor film - Google Patents

Quantum dot for phosphor film Download PDF

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Publication number
WO2020046211A1
WO2020046211A1 PCT/SG2019/050430 SG2019050430W WO2020046211A1 WO 2020046211 A1 WO2020046211 A1 WO 2020046211A1 SG 2019050430 W SG2019050430 W SG 2019050430W WO 2020046211 A1 WO2020046211 A1 WO 2020046211A1
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quantum dot
ligands
polar solvent
suspension
coating
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PCT/SG2019/050430
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French (fr)
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Darren Chi Jin NEO
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Agency For Science, Technology And Research
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Publication of WO2020046211A1 publication Critical patent/WO2020046211A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
    • H01L31/055—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means where light is absorbed and re-emitted at a different wavelength by the optical element directly associated or integrated with the PV cell, e.g. by using luminescent material, fluorescent concentrators or up-conversion arrangements
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B19/00—Selenium; Tellurium; Compounds thereof
    • C01B19/002—Compounds containing, besides selenium or tellurium, more than one other element, with -O- and -OH not being considered as anions
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G11/00—Compounds of cadmium
    • C01G11/006—Compounds containing, besides cadmium, two or more other elements, with the exception of oxygen or hydrogen
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
    • C09K11/02—Use of particular materials as binders, particle coatings or suspension media therefor
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
    • C09K11/08—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials
    • C09K11/88—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing selenium, tellurium or unspecified chalcogen elements
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/84—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by UV- or VIS- data
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/64—Nanometer sized, i.e. from 1-100 nanometer
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/80—Particles consisting of a mixture of two or more inorganic phases
    • C01P2004/82—Particles consisting of a mixture of two or more inorganic phases two phases having the same anion, e.g. both oxidic phases
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/48—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor body packages
    • H01L33/50—Wavelength conversion elements
    • H01L33/501—Wavelength conversion elements characterised by the materials, e.g. binder
    • H01L33/502—Wavelength conversion materials
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/52—PV systems with concentrators

Definitions

  • This invention relates to a quantum dot for a phosphor film, a method of forming the quantum dot and a method of forming the phosphor film.
  • a stable and efficient solution-processed phosphor thin film is fabricated using a metal chalcogenide colloidal quantum dot.
  • the thin film is processed to be optimal as a down conversion phosphor to improve the efficiency of solar cells, and possesses properties of high emissivity, stable operation at high temperature and long operation lifetime.
  • quantum dot comprising: a core comprising a gradient alloy of at least one metal chalcogenide; a shell around the core; and thermally combustible ligands on the shell; the quantum dot being soluble in a polar solvent having a higher vapour pressure than water.
  • the gradient alloy may have the formula I:
  • values of x and y decrease radially from a centre to a circumference of the core; and wherein the shell comprises a layer of ZnS around the core.
  • Thickness of the shell may range from 0.3nm to l .5nm.
  • the thermally combustible ligands may comprise tetramethylammonium hydroxide (TMAH) bound to 3-mercaptoproprionic acid (3 -MPA).
  • TMAH tetramethylammonium hydroxide
  • 3 -MPA 3-mercaptoproprionic acid
  • step (e) adding a non-polar solvent to the solution obtained in step (d) to obtain a third suspension;
  • the method may further comprise the step of (g) dissolving the quantum dot having thermally combustible ligands in solid form in a polar solvent having a higher vapour pressure than water.
  • the polar solvent may be ethanol.
  • a phosphor film comprising: an inert substrate; a coating comprising micro voids and the quantum dot of the first aspect without the thermally combustible ligands on the inert substrate; and a layer of AI2O3 conformally deposited on the coating.
  • a fourth aspect there is provided a method of forming the phosphor film of the third aspect, the method comprising the steps of:
  • step (b) performing atomic layer deposition of AI2O 3 on the coating.
  • the thermally combustible ligands may be decomposed to form the micro voids in the coating.
  • FIG. 1 is a schematic illustration of an exemplary quantum dot formed from a preliminary quantum dot.
  • FIG. 2 is a schematic illustration of formation of an exemplary phosphor film formed from the quantum dot of FIG. 1.
  • FIG. 3 is a plot of normalised photoluminescence vs wavelength of the exemplary
  • FIG. 4 is a plot of normalised photoluminescence vs wavelength of a phosphor film formed from as-prepared quantum dots at different temperatures.
  • FIG. 5 is a flow chart of an exemplary method of forming the quantum dot of FIG. 1.
  • FIG. 6 is a flow chart of an exemplary method of forming the phosphor film shown in FIG.
  • a quantum dot 10 for a phosphor film 20 a method of forming the quantum dot 100 and a method of forming the phosphor film 200 will be described below with reference to FIGS. 1 to 6.
  • the same reference numerals are used throughout the figures for the same or similar parts.
  • the quantum dot 10 has a core-shell structure with thermally combustible ligands 13.
  • the core 11 comprises a metal chalcogenide gradient alloy having the formula I:
  • the shell 12 comprises a layer of ZnS around the core 11.
  • the shell 12 may have a thickness ranging from 0.3 nm to 1.5 nm, which corresponds to 1 to 5 monolayers of ZnS, with l.5nm being an exemplary optimum thickness.
  • the quantum dot 10 may comprise core/shell nanocrystals of other gradient alloys of at least one metal chalcogenide, such as PbS/CdS, InP/ZnS and CuInS2/ZnS.
  • the quantum dot 10 also has thermally combustible ligands 13 provided on the shell 13, allowing the quantum dot 10 to be soluble in a polar solvent with a lower boiling point and higher vapour pressure than water, such as ethanol, isopropanol and methanol.
  • a solution of the quantum dot 10 in the polar solvent may be used to form a down conversion phosphor films using solution-processed coating techniques such as spin coating or dip coating to improve the efficiency of solar cells, as will be described in greater detail below.
  • the thermally combustible ligands 13 comprise tetramethylammonium hydroxide (TMAH) bound to 3-mercaptoproprionic acid (3-MPA).
  • TMAH tetramethylammonium hydroxide
  • 3-MPA 3-mercaptoproprionic acid
  • Other exemplary combustible ligands may include short chain acids such as formic acid.
  • the preliminary quantum dot 80 may be synthesized by first synthesizing a core 11 that comprises an alloyed basic quantum dot with a gradient composition of at least one metal chalcogenide.
  • the gradient alloy has the formula CdxZni-xSe y Si- y , where the values of x and y are higher in value in the core regions 11 and smaller nearer the circumference.
  • a further synthesis step introduces a certain thickness of a ZnS shell material around the basic quantum dot or core 11, thus synthesizing the shell 12, which confers a higher photoluminescence quantum yield due to increased surface passivation and chemical stability.
  • Synthesis of the core 11 and shell 12 may be performed using known methods [1, 2], resulting in formation of a preliminary quantum dot 80 having native ligands 83 on the shell 12, as shown in FIG. 1, where the native ligands may comprise oleic acid ligands, for example.
  • the preliminary quantum dot (that is dissolved in an organic solvent such as hexane, toluene, chloroform or octane), is reacted with 3-MPA to obtain a first suspension comprising a dispersion of the preliminary quantum dot (103).
  • 3-MPA a dispersion of the preliminary quantum dot
  • 5ml of 10 mg/ml of a solution of the preliminary quantum dot in hexane 0.2 ml of 3-MPA may be added.
  • Reaction of the gradient alloyed preliminary quantum dot with 3-MPA results in precipitation of the preliminary quantum dot from the hexane, which can be separated and the preliminary quantum dot collected by centrifugation.
  • Immediate flocculation of the preliminary quantum dot upon addition of the 3-MPA is due to ligand exchange of long native stabilizing ligands of the preliminary quantum dot with short 3-MPA ligands.
  • the first suspension is centrifuged to obtain the preliminary quantum dot having surface-bound 3-MPA ligands in solid form (104).
  • the supernatant, containing excess 3- MPA ligands and native ligands, is discarded.
  • a polar solvent is then added to the obtained preliminary quantum dot with 3-MPA ligands to form a second suspension (105): for example, 5ml of ethanol may be added to the solid preliminary quantum dot obtained from the centrifugation and agitated to form the second suspension (105).
  • TMAH is next added dropwise to the second suspension with stirring, thereby forming a solution of the quantum dots 10 having ligands 13 of TMAH bound to 3-MPA in the polar solvent (106), as shown in FIG. 1.
  • This may be achieved by agitating the vessel in which the second suspension is contained by stirring, and concurrently, adding drops of methanolic TMAH (i.e., TMAH in methanol) to the second suspension until the mixture turns from turbid to the clear solution.
  • TMAH binds to surface bound 3-MPA via acid-base reaction to form a new ionic ligand 13 that is thermally combustible and also allows the quantum dot 10 to be soluble in polar solvents with a lower boiling point and higher vapour pressure than water.
  • a washing step is then employed to remove excess TMAH, in which a non-polar solvent (such as toluene) is added to the solution to obtain a third suspension comprising the quantum dot dispersed in the solution (107).
  • a non-polar solvent such as toluene
  • the toluene is added until the solution turns turbid, indicating formation of the third suspension.
  • the third suspension is then centrifuged to obtain the quantum dot in solid form (108). This may be achieved by centrifugation and collecting the solid, discarding the supernatant and drying the collected solid to obtain the quantum dot in dried solid form.
  • the obtained quantum dot 10 in solid form may subsequently be dissolved in a polar solvent such as ethanol for use in phosphor film 20 formation.
  • a polar solvent such as ethanol
  • a coating comprising a thin film 20 of the quantum dots 10 may be formed on a substrate 21 via a solution processing technique (201) such as spin coating or dip coating, using the quantum dot 10 dissolved in a high vapour pressure polar solvent such as ethanol.
  • the substrate 21 may be made of glass or silicon, for example.
  • Post-processing via atomic layer deposition (ALD) of alumina (AI2O3) 23 on the matrix 21 coated with the quantum dots 10 is then performed in a heated chamber (202), for example, at a temperature ranging from 150 °C to 200 °C).
  • a heated chamber for example, at a temperature ranging from 150 °C to 200 °C.
  • the combustible ligands 13 on the quantum dots 10 are combusted by heating to leave behind micro voids 22 in the quantum dot coating 15 due to decomposition of the combustible ligands 13, while alumina 23 is layered directly onto the film of quantum dots without ligands 15 as well as the voids 22, resulting in in-situ infilling of the voids 22 with alumina 23 and forming the phosphor film 20.
  • the layer of alumina 23 protects and stabilises the phosphor film 20 against environmental factors such as oxygen and humidity, thus assisting in lengthening shelf life of the phosphor film 20.
  • a total of 25 nm of AI2O3 is deposited via ALD.
  • the layer of deposited alumina may range from 20 to 50 nm.
  • decomposition of the combustible ligands 13 during the post-processing creates micro voids 25 which creates unevenness that scatters light. Creating a roughened surface on the quantum dot coating thus helps to minimize specular reflection.
  • ALD is a conformal coating technique, the coating of alumina preserves this microporous or rough surface structure which reduces the need for an anti-reflection coating to be implemented on top of the quantum dot phosphor coating when the phosphor film 20 is used as down convertors in solar cells.
  • the alumina coated phosphor film 20 made with ligand-exchanged quantum dots 10 dissolved in ethanol has the ability to survive high temperature conditions and with little drop in emission intensity.
  • FIG. 3 in which high temperature in-situ normalized photoluminescence (PL) vs wavelength of the phosphor film 20 was plotted for the phosphor film 20 under room temperature, 50 °C, 100 °C, 150 °C and 200 °C, the drop in PL when temperature increased is significantly less (from 1.0 to above 0.6) when compared to FIG. 4 which plots PL vs wavelength for phosphor films made using as- prepared preliminary quantum dots in a hexane solution without ligand exchange (from 1.0 to below 0.4).
  • PL normalized photoluminescence
  • the above description thus discloses a semiconductor quantum dot 10 and method 100 of its forming 20 that allows the quantum dot 10 to be soluble in a polar solvent such as ethanol, thereby allowing a phosphor film 20 to be formed using a method 200 comprising solution processing and post-processing with ALD of alumina that results in a phosphor film 20 of maximum stability and yet retaining a high photoluminescence efficiency even after ligand exchange and solution in ethanol of the quantum dot 10.
  • a polar solvent such as ethanol

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Abstract

A quantum dot comprising: a core comprising a gradient alloy of at least one metal chalcogenide; a shell around the core; and thermally combustible ligands on the shell; the quantum dot being soluble in a polar solvent having a higher vapour pressure than water, e.g. ethanol. The thermally combustible ligands preferably comprise tetramethylammonium hydroxide (TMAH) bound to 3-mercaptoproprionic acid (3- MPA). A method is also provided for forming a phosphor film comprising said quantum dot, wherein the thermally combustible ligands decompose to form micro voids in the film.

Description

QUANTUM DOT FOR PHOSPHOR FIUM
FIEUD
This invention relates to a quantum dot for a phosphor film, a method of forming the quantum dot and a method of forming the phosphor film.
BACKGROUND
Use of down conversion phosphor (i.e. from UV to visible light) has been shown to improve solar cell efficiency by up to 2%. However, while quantum dots are good candidates for forming down conversion phosphor, intense UV light exposure results in intense heat which leads to instability of the quantum dots. There is thus a need to provide a quantum dot phosphor that provides high emission yield while having high temperature stability and long shelf life.
SUMMARY
A stable and efficient solution-processed phosphor thin film is fabricated using a metal chalcogenide colloidal quantum dot. The thin film is processed to be optimal as a down conversion phosphor to improve the efficiency of solar cells, and possesses properties of high emissivity, stable operation at high temperature and long operation lifetime.
According to a first aspect, there is provided quantum dot comprising: a core comprising a gradient alloy of at least one metal chalcogenide; a shell around the core; and thermally combustible ligands on the shell; the quantum dot being soluble in a polar solvent having a higher vapour pressure than water.
The gradient alloy may have the formula I:
CdxZni-xSeySi-y I
where values of x and y decrease radially from a centre to a circumference of the core; and wherein the shell comprises a layer of ZnS around the core.
Thickness of the shell may range from 0.3nm to l .5nm.
The thermally combustible ligands may comprise tetramethylammonium hydroxide (TMAH) bound to 3-mercaptoproprionic acid (3 -MPA). According to a second aspect, there is provided a method of forming the quantum dot of the first aspect, the method comprising the steps of:
(a) adding 3 -MPA to a solution of a preliminary quantum dot having native ligands in an organic solvent to obtain a first suspension;
(b) centrifuging the first suspension to obtain the preliminary quantum dot having surface bound 3 -MPA ligands in solid form;
(c) adding a polar solvent to the preliminary quantum dot having surface bound 3 -MPA ligands to form a second suspension;
(d) adding TMAH to the second suspension to form a solution of the quantum dot having thermally combustible ligands in the polar solvent;
(e) adding a non-polar solvent to the solution obtained in step (d) to obtain a third suspension; and
(f) centrifuging the third suspension to obtain the quantum dot having thermally combustible ligands in solid form.
The method may further comprise the step of (g) dissolving the quantum dot having thermally combustible ligands in solid form in a polar solvent having a higher vapour pressure than water.
For both aspects, the polar solvent may be ethanol.
According to a third aspect, there is provided a phosphor film comprising: an inert substrate; a coating comprising micro voids and the quantum dot of the first aspect without the thermally combustible ligands on the inert substrate; and a layer of AI2O3 conformally deposited on the coating.
According to a fourth aspect, there is provided a method of forming the phosphor film of the third aspect, the method comprising the steps of:
(a) coating the inert substrate with quantum dots of the first aspect via solution processing using a solution of the quantum dots in a polar solvent having a higher vapour pressure than water; and
(b) performing atomic layer deposition of AI2O3 on the coating. In step (b), the thermally combustible ligands may be decomposed to form the micro voids in the coating.
BRIEF DESCRIPTION OF FIGURES
In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments of the present invention, the description being with reference to the accompanying illustrative drawings.
FIG. 1 is a schematic illustration of an exemplary quantum dot formed from a preliminary quantum dot.
FIG. 2 is a schematic illustration of formation of an exemplary phosphor film formed from the quantum dot of FIG. 1.
FIG. 3 is a plot of normalised photoluminescence vs wavelength of the exemplary
phosphor film of FIG. 2 at different temperatures.
FIG. 4 is a plot of normalised photoluminescence vs wavelength of a phosphor film formed from as-prepared quantum dots at different temperatures.
FIG. 5 is a flow chart of an exemplary method of forming the quantum dot of FIG. 1.
FIG. 6 is a flow chart of an exemplary method of forming the phosphor film shown in FIG.
2.
DETAILED DESCRIPTION
Exemplary embodiments of a quantum dot 10 for a phosphor film 20, a method of forming the quantum dot 100 and a method of forming the phosphor film 200 will be described below with reference to FIGS. 1 to 6. The same reference numerals are used throughout the figures for the same or similar parts.
In general, as shown in FIG. 1, the quantum dot 10 has a core-shell structure with thermally combustible ligands 13. In an exemplary embodiment, the core 11 comprises a metal chalcogenide gradient alloy having the formula I:
CdxZn l -xS ey S l -y I where values of x and y decrease radially from a centre to a circumference of the core 11. In the exemplary embodiment, the shell 12 comprises a layer of ZnS around the core 11. The shell 12 may have a thickness ranging from 0.3 nm to 1.5 nm, which corresponds to 1 to 5 monolayers of ZnS, with l.5nm being an exemplary optimum thickness. In other embodiments, the quantum dot 10 may comprise core/shell nanocrystals of other gradient alloys of at least one metal chalcogenide, such as PbS/CdS, InP/ZnS and CuInS2/ZnS. The quantum dot 10 also has thermally combustible ligands 13 provided on the shell 13, allowing the quantum dot 10 to be soluble in a polar solvent with a lower boiling point and higher vapour pressure than water, such as ethanol, isopropanol and methanol. In this way, a solution of the quantum dot 10 in the polar solvent may be used to form a down conversion phosphor films using solution-processed coating techniques such as spin coating or dip coating to improve the efficiency of solar cells, as will be described in greater detail below. In the exemplary embodiment, the thermally combustible ligands 13 comprise tetramethylammonium hydroxide (TMAH) bound to 3-mercaptoproprionic acid (3-MPA). Other exemplary combustible ligands may include short chain acids such as formic acid.
To form the quantum dot 10 with thermally combustible ligands 13, a preliminary quantum dot 80 is used, as shown in FIGS. 1 and 5. The preliminary quantum dot 80 may be synthesized by first synthesizing a core 11 that comprises an alloyed basic quantum dot with a gradient composition of at least one metal chalcogenide. In an exemplary embodiment, the gradient alloy has the formula CdxZni-xSeySi-y, where the values of x and y are higher in value in the core regions 11 and smaller nearer the circumference. A further synthesis step introduces a certain thickness of a ZnS shell material around the basic quantum dot or core 11, thus synthesizing the shell 12, which confers a higher photoluminescence quantum yield due to increased surface passivation and chemical stability. Synthesis of the core 11 and shell 12 may be performed using known methods [1, 2], resulting in formation of a preliminary quantum dot 80 having native ligands 83 on the shell 12, as shown in FIG. 1, where the native ligands may comprise oleic acid ligands, for example.
In the exemplary method 100 of forming the quantum dot 10, the preliminary quantum dot (that is dissolved in an organic solvent such as hexane, toluene, chloroform or octane), is reacted with 3-MPA to obtain a first suspension comprising a dispersion of the preliminary quantum dot (103). For example, to 5ml of 10 mg/ml of a solution of the preliminary quantum dot in hexane, 0.2 ml of 3-MPA may be added. Reaction of the gradient alloyed preliminary quantum dot with 3-MPA results in precipitation of the preliminary quantum dot from the hexane, which can be separated and the preliminary quantum dot collected by centrifugation. Immediate flocculation of the preliminary quantum dot upon addition of the 3-MPA is due to ligand exchange of long native stabilizing ligands of the preliminary quantum dot with short 3-MPA ligands.
Accordingly, the first suspension is centrifuged to obtain the preliminary quantum dot having surface-bound 3-MPA ligands in solid form (104). The supernatant, containing excess 3- MPA ligands and native ligands, is discarded. A polar solvent is then added to the obtained preliminary quantum dot with 3-MPA ligands to form a second suspension (105): for example, 5ml of ethanol may be added to the solid preliminary quantum dot obtained from the centrifugation and agitated to form the second suspension (105).
TMAH is next added dropwise to the second suspension with stirring, thereby forming a solution of the quantum dots 10 having ligands 13 of TMAH bound to 3-MPA in the polar solvent (106), as shown in FIG. 1. This may be achieved by agitating the vessel in which the second suspension is contained by stirring, and concurrently, adding drops of methanolic TMAH (i.e., TMAH in methanol) to the second suspension until the mixture turns from turbid to the clear solution. TMAH binds to surface bound 3-MPA via acid-base reaction to form a new ionic ligand 13 that is thermally combustible and also allows the quantum dot 10 to be soluble in polar solvents with a lower boiling point and higher vapour pressure than water.
A washing step is then employed to remove excess TMAH, in which a non-polar solvent (such as toluene) is added to the solution to obtain a third suspension comprising the quantum dot dispersed in the solution (107). In the washing step, the toluene is added until the solution turns turbid, indicating formation of the third suspension. The third suspension is then centrifuged to obtain the quantum dot in solid form (108). This may be achieved by centrifugation and collecting the solid, discarding the supernatant and drying the collected solid to obtain the quantum dot in dried solid form. The obtained quantum dot 10 in solid form may subsequently be dissolved in a polar solvent such as ethanol for use in phosphor film 20 formation. To form the phosphor film 20, as shown in FIG. 2, first, a coating comprising a thin film 20 of the quantum dots 10 may be formed on a substrate 21 via a solution processing technique (201) such as spin coating or dip coating, using the quantum dot 10 dissolved in a high vapour pressure polar solvent such as ethanol. The substrate 21 may be made of glass or silicon, for example.
Post-processing via atomic layer deposition (ALD) of alumina (AI2O3) 23 on the matrix 21 coated with the quantum dots 10 is then performed in a heated chamber (202), for example, at a temperature ranging from 150 °C to 200 °C). In the post-processing, the combustible ligands 13 on the quantum dots 10 are combusted by heating to leave behind micro voids 22 in the quantum dot coating 15 due to decomposition of the combustible ligands 13, while alumina 23 is layered directly onto the film of quantum dots without ligands 15 as well as the voids 22, resulting in in-situ infilling of the voids 22 with alumina 23 and forming the phosphor film 20. The layer of alumina 23 protects and stabilises the phosphor film 20 against environmental factors such as oxygen and humidity, thus assisting in lengthening shelf life of the phosphor film 20. In an exemplary embodiment, a total of 25 nm of AI2O3 is deposited via ALD. In other embodiments, the layer of deposited alumina may range from 20 to 50 nm.
Notably, decomposition of the combustible ligands 13 during the post-processing creates micro voids 25 which creates unevenness that scatters light. Creating a roughened surface on the quantum dot coating thus helps to minimize specular reflection. As ALD is a conformal coating technique, the coating of alumina preserves this microporous or rough surface structure which reduces the need for an anti-reflection coating to be implemented on top of the quantum dot phosphor coating when the phosphor film 20 is used as down convertors in solar cells.
It was also found that the alumina coated phosphor film 20 made with ligand-exchanged quantum dots 10 dissolved in ethanol has the ability to survive high temperature conditions and with little drop in emission intensity. As can be seen in FIG. 3 in which high temperature in-situ normalized photoluminescence (PL) vs wavelength of the phosphor film 20 was plotted for the phosphor film 20 under room temperature, 50 °C, 100 °C, 150 °C and 200 °C, the drop in PL when temperature increased is significantly less (from 1.0 to above 0.6) when compared to FIG. 4 which plots PL vs wavelength for phosphor films made using as- prepared preliminary quantum dots in a hexane solution without ligand exchange (from 1.0 to below 0.4).
The above description thus discloses a semiconductor quantum dot 10 and method 100 of its forming 20 that allows the quantum dot 10 to be soluble in a polar solvent such as ethanol, thereby allowing a phosphor film 20 to be formed using a method 200 comprising solution processing and post-processing with ALD of alumina that results in a phosphor film 20 of maximum stability and yet retaining a high photoluminescence efficiency even after ligand exchange and solution in ethanol of the quantum dot 10.
Whilst there has been described in the foregoing description exemplary embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations and combination in details of design, construction and/or operation may be made without departing from the present invention.
REFERENCES
1. Y. Wei, J. Yang and J.Y. Ying, Chem. Commun., 2010, 46, 3179-3181.
S. Jun, E. Jang, J. E. Lim, Nanotechnology, 2006, 17, 3892.

Claims

1. A quantum dot comprising:
a core comprising a gradient alloy of at least one metal chalcogenide;
a shell around the core; and
thermally combustible ligands on the shell;
the quantum dot being soluble in a polar solvent having a higher vapour pressure than water.
2. The quantum dot of claim 1, wherein the gradient alloy has the formula I:
CdxZni-xSeyS i-y I
where values of x and y decrease radially from a centre to a circumference of the core;
and wherein the shell comprises a layer of ZnS around the core.
3. The quantum dot of claim 1 or claim 2, wherein thickness of the shell ranges from 0.3nm to l.5nm.
4. The quantum dot of any one of the preceding claims, wherein the thermally combustible ligands comprise tetramethylammonium hydroxide (TMAH) bound to 3- mercaptoproprionic acid (3 -MPA).
5. The quantum dot of any one of the preceding claims, wherein the polar solvent is ethanol.
6. A method of forming the quantum dot of any one of claims 1 to 5, the method comprising the steps of:
(g) adding 3 -MPA to a solution of a preliminary quantum dot having native ligands in an organic solvent to obtain a first suspension;
(h) centrifuging the first suspension to obtain the preliminary quantum dot having surface bound 3 -MPA ligands in solid form;
(i) adding a polar solvent to the preliminary quantum dot having surface bound 3 -MPA ligands to form a second suspension;
(j) adding TMAH to the second suspension to form a solution of the quantum dot having thermally combustible ligands in the polar solvent;
(k) adding a non-polar solvent to the solution obtained in step (d) to obtain a third suspension; and
(l) centrifuging the third suspension to obtain the quantum dot having thermally combustible ligands in solid form.
7. The method of claim 6, further comprising the step of (g) dissolving the quantum dot having thermally combustible ligands in solid form in a polar solvent having a higher vapour pressure than water.
8. The method of claim 7, wherein the polar solvent is ethanol.
9. A phosphor film comprising:
an inert substrate;
a coating comprising micro voids and the quantum dot of any one of claims 1 to 5 without the thermally combustible ligands on the inert substrate; and a layer of AI2O3 conformally deposited on the coating.
10. A method of forming a phosphor film of claim 9, the method comprising the steps of:
(a) coating the inert substrate with quantum dots of any one of claims 1 to 5 via solution processing techniques using a solution of the quantum dots in a polar solvent having a higher vapour pressure than water; and
(b) performing atomic layer deposition of AI2O3 on the coating.
11. The method of claim 10, wherein in step (b), the thermally combustible ligands are decomposed to form the micro voids in the coating.
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