Illuminated signage using quantum dots

Abstract

An illuminated sign has a primary light source in spaced apart relation to a transparent or translucent substrate having quantum dot phosphors printed or coated thereon. The primary light source may be a blue LED, a white LED or an LED having a significant portion of its emission in the ultraviolet region of the spectrum. The LED may be a backlight for the transparent or translucent substrate and/or an edge light, a down light or an up light.

Claims

1. An illuminated sign comprising: an enclosure having at least one transparent or translucent surface; a light source within the enclosure configured to illuminate the transparent or translucent surface with primary light; a light diffuser located between the light source and the transparent or translucent surface; and, a plurality of quantum dots adhered to the transparent or translucent surface in a preselected pattern, said quantum dots emitting secondary light in response to excitation by the primary light, wherein the sign illumination is a blend of the primary light and the secondary light.

2. An illuminated sign as recited in claim 1 wherein the preselected pattern comprises alphanumeric characters.

3. An illuminated sign as recited in claim 1 wherein the preselected pattern comprises a graphics pattern.

4. An illuminated sign as recited in claim 1 wherein the light source comprises light-emitting diodes that emit predominately in the blue portion of the visible spectrum or in the ultraviolet portion of the electromagnetic spectrum.

5. An illuminated sign as recited in claim 1 wherein the quantum dots comprise a core of II-VI, II-V, III-V, IV or IV-VI semiconductor material.

6. An illuminated sign as recited in claim 5 wherein the quantum dots comprise a core of heavy metal-free semiconductor material.

7. An illuminated sign as recited in claim 6 wherein the heavy metal-free quantum dots comprise a core comprising indium and phosphorus and optionally comprising one or more elements selected from the group consisting of zinc, sulphur, and selenium.

8. An illuminated sign as recited in claim 5 wherein the heavy metal-free quantum dot cores are shelled with one or more layers comprised of heavy metal-free II-VI and/or III-V semiconductor material and/or their ternary and quaternary alloys.

9. An illuminated sign as recited in claim 1 wherein the quantum dots are adhered to the translucent surface as a component of dried ink.

10. An illuminated sign as recited in claim 9 further comprising an oxygen barrier coating applied to the dried ink.

11. An illuminated sign as recited in claim 10 wherein the oxygen barrier coating comprises butyl rubber.

12. An illuminated sign as recited in claim 1 wherein the quantum dots are contained within polymer beads.

13. An illuminated sign as recited in claim 1 wherein the quantum dots are contained within the pores of a porous polymer.

14. An illuminated sign as recited in claim 1 comprising a solid-state light emitting diode and a quantum dot phosphor at a location remote from the light-emitting diode.

15. The illuminated sign recited in claim 1 wherein the transparent or translucent surface is a surface of the light diffuser.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

(1) FIG. 1 is a diagram an embodiment of QD phosphor signage as disclosed herein.

(2) FIG. 2 is a diagram showing methods of color mixing of red and green QDs using QD beads, either incorporating red and green QDs into the same bead (FIG. 2A) or preparing separate red and green QD beads, which may be printed on the same QD phosphor sheet (FIG. 2B).

(3) FIG. 3 shows bottom-lit QD signage using UV solid-state LEDs with a diffuser and a QD phosphor encased in a suitable housing unit.

DETAILED DESCRIPTION

(4) FIG. 1 illustrates an embodiment 100 of QD-based illuminated signage, as disclosed herein. Signage 100 includes one or more primary light sources 101, which emits light of a first color 102. For example, primary light source(s) 101 can be a solid-state LED that emits ultraviolet or blue light 102. The primary light impinges on a diffuser 103 upon which a QD phosphor layer 104 is disposed. Alternatively, element 103 of FIG. 1 may be simply transparent or translucent substrate rather than a diffuser. According to another embodiment, element 103 may include both a transparent or translucent substrate and a diffuser. In either case, the QD phosphor layer absorbs primary light 102 and emits secondary light 105. QD phosphor layer 104 may be patterned into sections 104a, 104b, . . . 104n, the sections having different mixtures of QD phosphors. For example, section 104a of QD phosphor layer 104 may include QDs that absorb primary light 102 and emit light 105a. Section 104b of QD phosphor layer 104 may include QDs that absorb primary light 102 and emit light 105b. For example, 105a may be green light and 105b may be red light. Some amount of primary light 102 may also be transmitted through the diffuser and QD phosphor layers and may be blended with the emitted light from the QD phosphors.

(5) The quantum dot phosphorescent material, illuminated with UV or blue LEDs as the primary light source, as illustrated in FIG. 1, produces brighter secondary light than that from white light with color filters. Energy losses are typically 10-20% in comparison to 50-90% using color filters. Energy losses and power consumption are also lower than other lighting systems, such as neon and fluorescent tubes, since little heat is produced.

(6) The QD signage display system described herein is inexpensive to power in comparison to signage displays utilizing gas discharge tubes, for which there are high energy losses as heat. Multiple pure colors may be emitted using a single solid-state lighting (SSL) backlight, reducing the cost associated with installing multiple LEDs, along with the associated cost of the increased circuitry required for multiple illumination sources.

(7) The QD phosphor layer may be illuminated with UV or blue LEDs, which are considerably less expensive than the white LEDs required for color filter-based signage. The QD phosphor down-converts the UV or blue light to a longer wavelength, tuned by the particle size, which is emitted as bright, narrow bandwidth light. Thus, strong, intense color is produced. Using Cd-based QDs, the emission may be tuned to any desired color by manipulating the particle size. Further, using heavy metal-free quantum dots (for example, CFQD quantum dots, available from Nanoco Group PLC, Manchester, UK), emission may be tuned across the visible spectrum from blue to red using non-toxic materials. Producing a range of colors is much more facile than for solid-state LEDs, which require either a range of different colored solid-state LEDs or different phosphors. In addition, QDs require less specific excitation wavelengths relative to many other phosphors. Since the entire visible spectrum of colors may be emitted by QD material, all of the color requirements of the UK Health and Safety legislation of 1996 are achievable.

(8) The QD phosphor layer may be printed onto the substrate using ink containing the QD materials. The QD materials disclosed herein are soluble in a range of organic solvents and the resulting inks are printable by many methods, including screen printing, inkjet printing and doctor blading. The ease of processability enables signs to be produced and replaced inexpensively and quickly. This is particularly advantageous for emergency signage, such as fire exits, where the signs must be easily replaced if they encounter damage.

(9) QD-containing inks are described in co-owned patent application publication no. 2013/0075692, filed Sep. 21, 2012, the entire contents of which are incorporated herein by reference. Particularly suitable ink formulations include QDs or QD-containing beads, disbursed in a polystyrene/toluene mixture. Other suitable ink matrices include acrylates.

(10) Using remote phosphor architecture, rather than a system where the phosphor is in physical contact with the backlight, provides enhanced longevity. Thermal quenching of the phosphor is reduced as it is less exposed to heat emitted from the primary light source. This assists in maintenance of the color frequency and intensity throughout the lifetime of the device.

(11) Some of the disadvantages of the illuminated display technologies currently in use for signage applications revolve around their safety. Safety is a key consideration throughout the lifetime of a sign. It is necessary that a sign may be maintained safely, and that potential damage or failure of system does not pose a significant risk. This is particularly important for signage in public places, which could potentially harm passers-by. The QD phosphor signage aims to minimise many of the existing safety concerns associated with existing display technologies. Since the QD phosphor signage utilizes solid-state LED backlights, little heat is generated. Thus, the signage may be touched while in operation without the risk of being burnt. This is particularly advantageous for low level signage in public places. The QD phosphor layer does not emit appreciable heat. The lighting arrangement does not involve elevated pressures or vacuum, therefore there is no risk of explosion or implosion if the device is damaged.

(12) The signage disclosed herein will gradually fail over time. Failure may either be from the LED backlights, or from decay of the photoluminescence of the QD phosphor. Both will result in gradual dimming of the signage display, while the latter may also result in a progressive shift in the emission wavelength as a higher proportion of the LED backlighting is transmitted. These gradual changes in performance are more favourable for signage applications than the instant failures associated with discharge lighting. A gradual change provides warning that the signage may be coming to the end of its lifespan and allows time for replacement, whereas an instant illumination failure may give no warning and may have potentially dangerous consequences, for instance if being used for safety signs.

(13) The QDs used herein are optimally made from core-shell semiconductor nanoparticles.

(14) The core material may be made from:

(15) Group II-VI compounds including a first element from group 12 (II) of the periodic table and a second element from group 16 (VI) of the periodic table, as well as ternary and quaternary materials including, but not restricted to: CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe.

(16) Group II-V compounds incorporating a first element from group 12 of the periodic table and a second element from group 15 of the periodic table, and also including ternary and quaternary materials and doped materials. Nanoparticle material includes, but is not restricted to: Zn.sub.3P.sub.2, Zn.sub.3As.sub.2, Cd.sub.3P.sub.2, Cd.sub.3As.sub.2, Cd.sub.3N.sub.2, Zn.sub.3N.sub.2.

(17) Group III-V compounds including a first element from group 13 (III) of the periodic table and a second element from group 15 (V) of the periodic table, as well as ternary and quaternary materials. Examples of nanoparticle core materials include, but are not restricted to: BP, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb; InN, InP, InAs, InSb, AlN, BN, GaNP, GaNAs, InNP, InNAs, GAInPAs, GaAlPAs, GaAlPSb, GaInNSb, InAlNSb, InAlPAs, InAlPSb.

(18) Group III-VI compounds including a first element from group 13 of the periodic table and a second element from group 16 of the periodic table and also including ternary and quaternary materials. Nanoparticle material includes, but is not restricted to: Al.sub.2S.sub.3, Al.sub.2Se.sub.3, Al.sub.2Te.sub.3, Ga.sub.2S.sub.3, Ga.sub.2Se.sub.3, In.sub.2S.sub.3, In.sub.2Se.sub.3, Ga.sub.2Te.sub.3, In.sub.2Te.sub.3.

(19) Group IV elements or compounds including elements from group 14 (IV): Si, Ge, SiC, SiGe.

(20) Group IV-VI compounds including a first element from group 14 (IV) of the periodic table and a second element from group 16 (VI) of the periodic table, as well as ternary and quaternary materials including, but not restricted to: PbS, PbSe, PbTe, SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbSe, SnPbTe, SnPbSeTe, SnPbSTe.

(21) The shell layer(s) grown on the nanoparticle core may include any one or more of the following materials:

(22) Group IIA-VIB (2-16) material, incorporating a first element from group 2 of the periodic table and a second element from group 16 of the periodic table, and also including ternary and quaternary materials and doped materials. Nanoparticle material includes, but is not restricted to: MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe.

(23) Group IIB-VIB (12-16) material incorporating a first element from group 12 of the periodic table and a second element from group 16 of the periodic table, and also including ternary and quaternary materials and doped materials. Nanoparticle material includes, but is not restricted to: ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe.

(24) Group II-V material incorporating a first element from group 12 of the periodic table and a second element from group 15 of the periodic table, and also including ternary and quaternary materials and doped materials. Nanoparticle material includes, but is not restricted to: Zn.sub.3P.sub.2, Zn.sub.3As.sub.2, Cd.sub.3P.sub.2, Cd.sub.3As.sub.2, Cd.sub.3N.sub.2, Zn.sub.3N.sub.2.

(25) Group III-V material incorporating a first element from group 13 of the periodic table and a second element from group 15 of the periodic table, and also including ternary and quaternary materials and doped materials. Nanoparticle material includes, but is not restricted to: BP, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb; InN, InP, InAs, InSb, AlN, BN.

(26) Group III-IV material incorporating a first element from group 13 of the periodic table and a second element from group 14 of the periodic table, and also including ternary and quaternary materials and doped materials. Nanoparticle material includes, but is not restricted to: B.sub.4C, Al.sub.4C.sub.3, Ga.sub.4C.

(27) Group III-VI material incorporating a first element from group 13 of the periodic table and a second element from group 16 of the periodic table, and also including ternary and quaternary materials. Nanoparticle material includes, but is not restricted to: Al.sub.2S.sub.3, Al.sub.2Se.sub.3, Al.sub.2Te.sub.3, Ga.sub.2S.sub.3, Ga.sub.2Se.sub.3, In.sub.2S.sub.3, In.sub.2Se.sub.3, Ga.sub.2Te.sub.3, In.sub.2Te.sub.3.

(28) Group IV-VI material incorporating a first element from group 14 of the periodic table and a second element from group 16 of the periodic table, and also including ternary and quaternary materials and doped materials. Nanoparticle material includes, but is not restricted to: PbS, PbSe, PbTe, Sb.sub.2Te.sub.3, SnS, SnSe, SnTe.

(29) Nanoparticle material incorporating a first element from any group in the d-block of the periodic table, and a second element from any group 16 of the periodic table, and also including ternary and quaternary materials and doped materials. Nanoparticle material includes, but is not restricted to: NiS, CrS, CuInS.sub.2, CuInSe.sub.2, CuGaS.sub.2, CuGaSe.sub.2.

(30) In one particular embodiment, the QDs are made of a heavy metal-free semiconductor material. For example, the cores may comprise InP or may comprise an alloy comprising indium and phosphorous and further comprising one or more other elements, such as zinc, selenium, or sulphur. The cores may be shelled with one or more layers comprised of heavy metal-free semiconductor material such as, but not restricted to, Group II-VI materials, e.g. ZnO, ZnSe, ZnS, Group III-V materials, e.g., GaP, and/or their ternary and quaternary alloys. This method utilizes QDs that are capable of emitting across the entire visible spectrum, while being fully compliant with regulations prohibiting the use of heavy metals in electronic and electrical products.

(31) The coordination around the atoms on the surface of any core, core-shell or core-multishell, doped or graded nanoparticle is incomplete and the non-fully coordinated atoms have dangling bonds which make them highly reactive and may lead to particle agglomeration. This problem is overcome by passivating (capping) the bare surface atoms with protecting organic groups.

(32) The outermost layer (capping agent) of organic material or sheath material helps to inhibit particle-particle aggregation, further protecting the nanoparticles from their surrounding electronic and chemical environments. The capping agent may be selected to provide solubility in an appropriate solvent, chosen for its printability properties (viscosity, volatility, etc.). In many cases, the capping agent is the solvent in which the nanoparticle preparation is undertaken, and consists of a Lewis base compound, or a Lewis base compound diluted in an inert solvent such as a hydrocarbon. There is a lone pair of electrons on the Lewis base capping agent that is capable of a donor-type coordination to the surface of the nanoparticle and include mono- or multi-dentate ligands such as phosphines (trioctylphosphine, triphenylphosphine, t-butylphosphine, etc.), phosphine oxides (trioctylphosphine oxide, triphenylphosphine oxide, etc.), alkyl phosphonic acids, alkyl-amines (octadecylamine, hexadecylamine, octylamine, etc.), aryl-amines, pyridines, long chain fatty acids (myristic acid, oleic acid, undecylenic acid, etc.) and thiophenes but is, as one skilled in the art will know, not restricted to these materials.

(33) The outermost layer (capping agent) of a QD may also consist of a coordinated ligand with additional functional groups that may be used as chemical linkage to other inorganic, organic or biological material, whereby the functional group is pointing away from the QD surface and is available to bond/react/interact with other available molecules, such as amines, alcohols, carboxylic acids, esters, acid chloride, anhydrides, ethers, alkyl halides, amides, alkenes, alkanes, alkynes, allenes, amino acids, azide groups, etc. but is, as one skilled in the art will know, not limited to these functionalised molecules. The outermost layer (capping agent) of a QD may also consist of a coordinated ligand with a functional group that is polymerizable and may be used to form a polymer layer around the particle.

(34) The outermost layer (capping agent) may also consist of organic units that are directly bonded to the outermost inorganic layer such as via an SS bond between the inorganic surface (ZnS) and a thiol capping molecule. These may also possess additional functional group(s), not bonded to the surface of the particle, which may be used to form a polymer around the particle, or for further reaction/interaction/chemical linkage.

(35) Referring again to FIG. 1, QD phosphor layer 104 may be fabricated with bare QDs dispersed directly into an ink formulation. Alternatively, the QDs may be incorporated into microbeads prior to their dispersion into the ink formulation. QD microbeads can exhibit superior robustness and longer lifetimes than bare QDs, and can be more stable to the mechanical and thermal processing protocols of device fabrication. By incorporating the QD material into polymer microbeads, the nanoparticles become more resistant to air, moisture and photo-oxidation, opening up the possibility for processing in air that would vastly reduce the manufacturing cost. The bead size may be tuned from 20 nm to 0.5 mm, enabling control over the ink viscosity without changing the inherent optical properties of the QDs. The viscosity dictates how the QD bead ink flows through a mesh, dries, and adheres to a substrate, so thinners are not required to alter the viscosity, reducing the cost of the ink formulation. By incorporating the QDs into microbeads, the detrimental effect of particle agglomeration on the optical performance of bare encapsulated QDs is eliminated.

(36) Moreover, QD beads provide an effective method of color mixing, as illustrated in FIG. 2. FIG. 2A illustrates an embodiment wherein different colored QDs, for example, green-emitting QDs 201 and red-emitting QDs 202 are incorporated into bead 203. Beads 203 incorporating the two colors of QDs are then incorporated into QD phosphor layer 204. Alternatively, several QD beads, each containing a different single color of QDs, may be incorporated into the phosphor layer. For example, FIG. 2B illustrates an embodiment wherein beads 205 incorporate green-emitting QDs 201 and beads 206 incorporate red-emitting QDs 202. Both beads 205 and 206 can be incorporated into QD phosphor layer 207. It will be appreciated that any color-emitting QDs can be used and combinations of the approaches illustrated in FIGS. 2A and 2B can be used.

(37) Incorporation of QDs into beads is described in co-owned patent application publication no. 2010/0123155, referenced above. Briefly, one such method for incorporating QDs into microbeads involves growing the polymer bead around the QDs. A second method incorporates QDs into pre-existing microbeads.

(38) With regard to the first option, by way of example, hexadecylamine-capped CdSe-based semiconductor nanoparticles may be treated with at least one, more preferably two or more polymerizable ligands (optionally one ligand in excess) resulting in the displacement of at least some of the hexadecylamine capping layer with the polymerizable ligand(s). The displacement of the capping layer with the polymerizable ligand(s) may be accomplished by selecting a polymerizable ligand or ligands with structures similar to that of trioctylphosphine oxide (TOPO), which is a ligand with a known and very high affinity for CdSe-based nanoparticles. It will be appreciated that this basic methodology may be applied to other nanoparticle/ligand pairs to achieve a similar effect. That is, for any particular type of nanoparticle (material and/or size), it is possible to select one or more appropriate polymerizable surface binding ligands by choosing polymerizable ligands comprising a structural motif which is analogous in some way (e.g. has a similar physical and/or chemical structure) to the structure of a known surface binding ligand. Once the nanoparticles have been surface-modified in this way, they may then be added to a monomer component of a number of microscale polymerization reactions to form a variety of QD-containing resins and beads. Another option is the polymerization of one or more polymerizable monomers from which the optically transparent medium is to be formed in the presence of at least a portion of the semiconductor nanoparticles to be incorporated into the optically transparent medium. The resulting materials incorporate the QDs covalently and appear highly colored even after prolonged periods of Soxhlet extraction.

(39) Examples of polymerization methods that may be used to construct QD-containing beads include, but are not restricted to, suspension, dispersion, emulsion, living, anionic, cationic, RAFT, ATRP, bulk, ring-closing metathesis and ring-opening metathesis. Initiation of the polymerization reaction may be induced by any suitable method that causes the monomers to react with one another, such as by the use of free radicals, light, ultrasound, cations, anions, or heat. A preferred method is suspension polymerization, involving thermal curing of one or more polymerizable monomers from which the optically transparent medium is to be formed. Said polymerizable monomers preferably comprise methyl (meth)acrylate, ethylene glycol dimethacrylate and vinyl acetate. This combination of monomers has been shown to exhibit excellent compatibility with existing commercially available LED encapsulants and has been used to fabricate a light emitting device exhibiting significantly improved performance compared to a device prepared using essentially prior art methodology. Other preferred polymerizable monomers are epoxy or polyepoxide monomers, which may be polymerized using any appropriate mechanism, such as curing with ultraviolet irradiation.

(40) QD-containing microbeads may be produced by dispersing a known population of QDs within a polymer matrix, curing the polymer and then grinding the resulting cured material. This is particularly suitable for use with polymers that become relatively hard and brittle after curing, such as many common epoxy or polyepoxide polymers (e.g. Optocast 3553 from Electronic Materials, Inc., USA).

(41) QD-containing beads may be generated simply by adding QDs to the mixture of reagents used to construct the beads. In some instances, nascent QDs will be used as isolated from the reaction employed for their synthesis, and are thus generally coated with an inert outer organic ligand layer. In an alternative procedure, a ligand exchange process may be carried out prior to the bead-forming reaction. Here, one or more chemically reactive ligands (for example a ligand for the QDs that also contains a polymerizable moiety) are added in excess to a solution of nascent QDs coated in an inert outer organic layer. After an appropriate incubation time the QDs are isolated, for example by precipitation and subsequent centrifugation, washed and then incorporated into the mixture of reagents used in the bead forming reaction/process.

(42) Both QD incorporation strategies will result in statistically random incorporation of the QDs into the beads and thus the polymerization reaction will result in beads containing statistically similar amounts of the QDs. It will be obvious to one skilled in the art that bead size may be controlled by the choice of polymerization reaction used to construct the beads, and additionally once a polymerization method has been selected bead size may also be controlled by selecting appropriate reaction conditions, e.g. by stirring the reaction mixture more quickly in a suspension polymerization reaction to generate smaller beads. Moreover, the shape of the beads may be readily controlled by choice of procedure in conjunction with whether or not the reaction is carried out in a mould. The composition of the beads may be altered by changing the composition of the monomer mixture from which the beads are constructed. Similarly, the beads may also be cross-linked with varying amounts of one or more cross-linking agents (e.g. divinyl benzene). If beads are constructed with a high degree of cross-linking, e.g. greater than 5 mol. % cross-linker, it may be desirable to incorporate a porogen (e.g. toluene or cyclohexane) during the bead-forming reaction. The use of a porogen in such a way leaves permanent pores within the matrix constituting each bead. These pores may be sufficiently large to allow the ingress of QDs into the bead.

(43) QDs may also be incorporated in beads using reverse emulsion-based techniques. The QDs may be mixed with precursor(s) to the optically transparent coating material and then introduced into a stable reverse emulsion containing, for example, an organic solvent and a suitable salt. Following agitation the precursors form microbeads encompassing the QDs, which may then be collected using any appropriate method, such as centrifugation. If desired, one or more additional surface layers or shells of the same or a different optically transparent material may be added prior to isolation of the QD-containing beads by addition of further quantities of the requisite shell layer precursor material(s).

(44) In respect of the second option for incorporating QDs into beads, the QDs may be immobilized in polymer beads through physical entrapment. For example, a solution of QDs in a suitable solvent (e.g. an organic solvent) may be incubated with a sample of polymer beads. Removal of the solvent using any appropriate method results in the QDs becoming immobilized within the matrix of the polymer beads. The QDs remain immobilized in the beads unless the sample is resuspended in a solvent (e.g. organic solvent) in which the QDs are freely soluble. Optionally, at this stage the outside of the beads may be sealed. Alternatively, at least a portion of the QDs may be physically attached to prefabricated polymer beads. Said attachment may be achieved by immobilization of the portion of the semiconductor nanoparticles within the polymer matrix of the prefabricated polymeric beads or by chemical, covalent, ionic, or physical connection between the portion of semiconductor nanoparticles and the prefabricated polymeric beads. Examples of prefabricated polymeric beads comprise polystyrene, polydivinyl benzene and a polythiol.

(45) QDs may be irreversibly incorporated into prefabricated beads in a number of ways, e.g. chemical, covalent, ionic, physical (e.g. by entrapment) or any other form of interaction. If prefabricated beads are to be used for the incorporation of QDs, the solvent accessible surfaces of the bead may be chemically inert (e.g. polystyrene) or alternatively they may be chemically reactive/functionalised (e.g. Merrifield's Resin). The chemical functionality may be introduced during the construction of the bead, for example by the incorporation of a chemically functionalised monomer, or alternatively chemical functionality may be introduced in a post-bead construction treatment, for example by conducting a chloromethylation reaction. Additionally, a post-bead construction polymeric graft or other similar process, whereby chemically reactive polymer(s) are attached to the outer layers/accessible surfaces of the bead, may be used to introduce chemical functionality. More than one such post-construction derivation process may be carried out to introduce chemical functionality onto/into the bead.

(46) As with QD incorporation into beads during the bead forming reaction, i.e. the first option described above, the pre-fabricated beads may be of any shape, size and composition, may have any degree of cross-linker, and may contain permanent pores if constructed in the presence of a porogen. QDs may be imbibed into the beads by incubating a solution of QDs in an organic solvent and adding this solvent to the beads. The solvent must be capable of wetting the beads and, in the case of lightly cross-linked beads, preferably 0-10% cross-linked and most preferably 0-2% cross-linked, the solvent should cause the polymer matrix to swell in addition to solvating the QDs. Once the QD-containing solvent has been incubated with the beads, it is removed, for example by heating the mixture and causing the solvent to evaporate, and the QDs become embedded in the polymer matrix constituting the bead or alternatively by the addition of a second solvent in which the QDs are not readily soluble but which mixes with the first solvent causing the QDs to precipitate within the polymer matrix constituting the beads. Immobilization may be reversible if the bead is not chemically reactive, or else if the bead is chemically reactive the QDs may be held permanently within the polymer matrix by chemical, covalent, ionic, or any other form of interaction.

(47) Optically transparent media that are sol-gels and glasses, intended to incorporate QDs, may be formed in an analogous fashion to the method used to incorporate QDs into beads during the bead-forming process as described above. For example, a single type of QD (e.g. one color) may be added to the reaction mixture used to produce the sol-gel or glass. Alternatively, two or more types of QD (e.g. two or more colors) may be added to the reaction mixture used to produce the sol-gel or glass. The sol-gels and glasses produced by these procedures may have any shape, morphology or 3-dimensional structure. For example, the particles may be spherical, disc-like, rod-like, ovoid, cubic, rectangular, or any of many other possible configurations.

(48) By incorporating QDs into beads in the presence of materials that act as stability-enhancing additives and optionally providing the beads with a protective surface coating, migration of deleterious species, such as moisture, oxygen and/or free radicals, is reduced if not entirely eliminated, with the result of enhancing the physical, chemical and/or photo-stability of the semiconductor nanoparticles.

(49) An additive may be combined with bare semiconductor nanoparticles and precursors at the initial stages of the production process of the beads. Alternatively or additionally, an additive may be added after the semiconductor nanoparticles have been entrapped within the beads.

(50) The additives that may be added singly or in any desirable combination during the bead formation process can be grouped according to their intended function, as follows:

(51) Mechanical sealing: fumed silica (e.g. Cab-O-Sil), ZnO, TiO.sub.2, ZrO, Mg stearate, Zn Stearate, all used as a filler to provide mechanical sealing and/or reduce porosity.

(52) Capping agents: tetradecyl phosphonic acid (TDPA), oleic acid, stearic acid, polyunsaturated fatty acids, sorbic acid, Zn methacrylate, Mg stearate, Zn stearate, isopropyl myristate. Some of these have multiple functionalities and may act as capping agents, free-radical scavengers and/or reducing agents.

(53) Reducing agents: ascorbic acid palmitate, alpha tocopherol (vitamin E), octane thiol, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), gallate esters (propyl, lauryl, octyl, etc.), a metabisulfite (e.g. the sodium or potassium salt).

(54) Free radical scavengers: benzophenones.

(55) Hydride reactive agents: 1,4-butandiol, 2-hydroxyethyl methacrylate, allyl methacrylate, 1,6-heptadiene-4-ol, 1,7-octadiene, and 1,4-butadiene.

(56) The selection of the additive(s) for a particular application will depend upon the nature of the semiconductor nanoparticle material (e.g. how sensitive the nanoparticle material is to physical, chemical and/or photo-induced degradation), the nature of the primary matrix material (e.g. how porous it is to potentially deleterious species, such as free-radicals, oxygen, moisture, etc.), the intended function of the final material or device which will contain the primary particles (e.g. the operating conditions of the material or device), and the process conditions required to fabricate the said final material or device. As such, one or more appropriate additives may be selected from the above five lists to suit any desirable semiconductor nanoparticle application.

(57) The QDs, either after incorporated into the beads or after printing a bare QD ink, may be further coated with a suitable material to provide each bead with a protective barrier to prevent the passage or diffusion of potentially deleterious species, e.g. oxygen, moisture or free radicals from the external environment, through the bead material to the semiconductor nanoparticles. As a result, the semiconductor nanoparticles are less sensitive to their surrounding environment and the various processing conditions typically required to utilize the nanoparticles in applications such as the fabrication of QD phosphors or QD-ink-printed light guides.

(58) The coating is preferably a barrier to the passage of oxygen or any type of oxidizing agent through the bead material. The coating may be a barrier to the passage of free radical species and/or is preferably a moisture barrier so that moisture in the environment surrounding the beads cannot contact the semiconductor nanoparticles incorporated within the beads.

(59) The coating may provide a layer of material on a surface of the bead of any desirable thickness, provided it affords the required level of protection. The surface layer coating may be around 1 to 10 nm thick, up to around 400 to 500 nm thick, or more. Preferred layer thicknesses are in the range of 1 nm to 200 nm, more preferably around 5 nm to 100 nm.

(60) The coating may comprise an inorganic material, such as a dielectric (insulator), a metal oxide, a metal nitride or a silica-based material (e.g. a glass).

(61) The metal oxide may be a single metal oxide (i.e. oxide ions combined with a single type of metal ion, e.g. Al.sub.2O.sub.3), or may be a mixed metal oxide (i.e. oxide ions combined with two or more types of metal ion, e.g. SrTiO.sub.3). The metal ion(s) of the (mixed) metal oxide may be selected from any suitable group of the periodic table, such as group 2, 13, 14 or 15, or may be a transition metal, d-block metal, or lanthanide metal.

(62) Preferred metal oxides are selected from the group consisting of Al.sub.2O.sub.3, B.sub.2O.sub.3, Co.sub.2O.sub.3, Cr.sub.2O.sub.3, CuO, Fe.sub.2O.sub.3, Ga.sub.2O.sub.3, HfO.sub.2, In.sub.2O.sub.3, MgO, Nb.sub.2O.sub.5, NiO, SiO.sub.2, Sn0.sub.2, Ta.sub.2O.sub.5, TiO.sub.2, ZrO.sub.2, Sc.sub.2O.sub.3, Y.sub.2O.sub.3, GeO.sub.2, La.sub.2O.sub.3, CeO.sub.2, PrO.sub.x (x=appropriate integer), Nd.sub.2O.sub.3, Sm.sub.2O.sub.3, EuO.sub.y (y=appropriate integer), Gd.sub.2O.sub.3, Dy.sub.2O.sub.3, Ho.sub.2O.sub.3, Er.sub.2O.sub.3, Tm.sub.2O.sub.3, Yb.sub.2O.sub.3, Lu.sub.2O.sub.3, SrTiO.sub.3, BaTiO.sub.3, PbTiO.sub.3, PbZrO.sub.3, Bi.sub.mTi.sub.nO (m, n=appropriate integer), Bi.sub.aSi.sub.bO (a, b=appropriate integer), SrTa.sub.2O.sub.6, SrBi.sub.2Ta.sub.2O.sub.9, YScO.sub.3, LaAlO.sub.3, NdAlO.sub.3, GdScO.sub.3, LaScO.sub.3, LaLuO.sub.3, Er.sub.3Ga.sub.5O.sub.13.

(63) Preferred metal nitrides may be selected from the group consisting of BN, AlN, GaN, InN, Zr.sub.3N.sub.4, Cu.sub.2N, Hf.sub.3N.sub.4, SiN.sub.c (c=appropriate integer), TiN, Ta.sub.3N.sub.5, TiSiN, TiAlN, TaN, NbN, MoN, WN.sub.d (d=appropriate integer), WN.sub.eC.sub.f (e, f=appropriate integer).

(64) The inorganic coating may comprise silica in any appropriate crystalline form.

(65) The coating may incorporate an inorganic material in combination with an organic or polymeric material, e.g. an inorganic/polymer hybrid, such as a silica-acrylate hybrid material.

(66) The coating may comprise a polymeric material, which may be a saturated or unsaturated hydrocarbon polymer, or may incorporate one or more heteroatoms (e.g. O, S, N, halogen) or heteroatom-containing functional groups (e.g. carbonyl, cyano, ether, epoxide, amide, etc.).

(67) Examples of preferred polymeric coating materials include acrylate polymers (e.g. polymethyl(meth)acrylate, polybutylmethacrylate, polyoctylmethacrylate, alkylcyanoacryaltes, polyethyleneglycol dimethacrylate, polyvinylacetate, etc.), epoxides (e.g. EPOTEK 301 A and B thermal curing epoxy, EPOTEK OG112-4 single-pot UV curing epoxy, or EX0135 A and B thermal curing epoxy), polyamides, polyimides, polyesters, polycarbonates, polythioethers, polyacrylonitryls, polydienes, polystyrene polybutadiene copolymers (Kratons), pyrelenes, poly-para-xylylene (parylenes), polyetheretherketone (PEEK), polyvinylidene fluoride (PVDF), polydivinyl benzene, polyethylene, polypropylene, polyethylene terephthalate (PET), polyisobutylene (butyl rubber), polyisoprene, and cellulose derivatives (methyl cellulose, ethyl cellulose, hydroxypropylmethyl cellulose, hydroxypropylmethylcellulose phthalate, nitrocellulose), and combinations thereof.

(68) Moreover, the coatings described above may be applied as a layer on top of the QD phosphor ink layer printed on the transparent/translucent substrate.

(69) Illuminated signage incorporating the use of QDs is demonstrated in the following examples. The examples included herein are intended for the purpose of illustration and the invention is not restricted to these.

EXAMPLE 1

(70) One embodiment of signage is illustrated in FIG. 3. This illuminated signage is fabricated using a remote phosphor architecture. One or more QD inks 301 are used to form a pattern on a substrate. The QD ink(s) is printed onto, and/or encapsulated in, an appropriate medium such as a glass substrate 302. The QD-printed substrate 302, along with a diffuser plate 303 and a primary back-lighting source 304 are encased in a suitable housing unit 306. The primary back-lighting source can be one or more UV or blue solid-state LEDs, for example. The encapsulated QD resin is illuminated from, which excites the QDs in the resin(s). The patterned QD resin down-converts the primary LED emission to a longer wavelength, determined by the nanoparticle size. The down-converted light, possibly mixed with primary light, is emitted from the signage.

EXAMPLE 2

(71) In another embodiment a light guide of printed QD phosphor material is illuminated remotely by a light source that is independent from the sign. This architecture is particularly applicable to signage that does not need to be permanently illuminated.

(72) The QD ink may be printed directly onto a transparent/translucent substrate (glass, Perspex, etc. but not restricted to these). Optionally, the dried ink may be coated with an oxygen barrier such as, but not restricted to, butyl rubber, to improve the lifetime of the QD phosphor. The substrate itself may be a light guide or the substrate may be integrated with a light guide, which gathers light from the primary light source and guides it to the printed QD phosphor. The light guide may be illuminated by UV or blue solid state LEDs from any direction: e.g. from in front, behind, above, below, or from either or both sides.

(73) Although particular embodiments of the present invention have been shown and described, they are not intended to limit what this patent covers. One skilled in the art will understand that various changes and modifications may be made without departing from the scope of the present invention as literally and equivalently covered by the following claims.