# Patent

## Title
Nanocrystal doped matrixes

## Abstract

The present invention provides matrixes doped with semiconductor nanocrystals. In certain embodiments, the semiconductor nanocrystals have a size and composition such that they absorb or emit light at particular wavelengths. The nanocrystals can comprise ligands that allow for mixing with various matrix materials, including polymers, such that a minimal portion of light is scattered by the matrixes. The matrixes of the present invention can also be utilized in refractive index matching applications. In other embodiments, semiconductor nanocrystals are embedded within matrixes to form a nanocrystal density gradient, thereby creating an effective refractive index gradient. The matrixes of the present invention can also be used as filters and antireflective coatings on optical devices and as down-converting layers. The present invention also provides processes for producing matrixes comprising semiconductor nanocrystals.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
The present application claims the benefit of the filing dates of U.S. Provisional Patent Application No. 60/536,962, filed Jan. 15, 2004, and U.S. Provisional Patent Application No. 60/635,784, filed Dec. 15, 2004, the disclosures of which applications are incorporated by reference herein in their entireties.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to nanocomposite matrixes, including polymeric layers comprising nanocrystals, and processes for preparing nanocrystal doped matrixes.
2. Background Art
High performance down-converting phosphor technologies will play a prominent role in the next generation of visible light emission, including high efficiency solid-state white lighting (SSWL). In addition, such technologies are also applicable to near infrared (NIR) and infrared (IR) light emitting technologies. Down-conversion from ultraviolet (UV) or blue light emitting semiconductor light emitting diodes (LEDs) into blue, red and green wavelengths offers a fast, efficient and cost-effective path for delivering commercially attractive white light sources. Unfortunately, existing rare-earth activated phosphors or halophosphates, which are currently the primary source for solid-state down-conversion, were originally developed for use in fluorescent lamps and cathode ray tubes (CRTs), and therefore have a number of critical shortfalls when it comes to the unique requirements of SSWL. As such, while some SSWL systems are available, poor power efficiency (<20 light lumens/watt (lm/W)), poor color rendering (Color Rendering Index (CRI)<75) and extremely high costs (>$200/kilolumen (klm)) limit this technology to niche markets such as flashlights and walkway lighting.
Furthermore, LEDs often suffer from reduced performance as a result of internal reflection of photons at the chip/coating interface. Typically, LEDs are encapsulated or coated in a polymeric material (which may comprise phosphors) to provide stability to the light-emitting chip. Currently these coatings are made by using an inorganic or organic coating that has a very different refractive index than the base material (i.e., the chip), which results in a detrimental optical effect due to the refractive index mismatch at the interface between the two materials. In addition, the temperature of the LED can reach in excess of 100° C. To allow for the expansion and contraction that can accompany this temperature rise, a compliant polymeric layer (e.g., silicone) is often placed in contact with the chip. In order to provide additional stability to the LED, this compliant layer is often further coated with a hard shell polymer.
The resulting LED structure suffers loss of light at the chip/compliant polymer interface due to the lower refractive index of the polymer coating in relation to the LED. However, if the refractive index of the compliant layer is increased, even greater loss will occur due at the high refractive index/low refractive index interface between the compliant polymer and the hard shell polymer due to internal reflection.
There are several critical factors which result in poor power efficiencies when using traditional inorganic phosphors for SSWL. These include: total internal reflection at the LED-chip and phosphor layer interface resulting in poor light extraction from the LED into the phosphor layer; poor extraction efficiency from the phosphor layer into the surroundings due to scattering of the light generated by the phosphor particles as well as parasitic absorption by the LED chip, metal contacts and housing; broad phosphor emission in the red wavelength range resulting in unused photons emitted into the near-IR; and poor down-conversion efficiency of the phosphors themselves when excited in the blue wavelength range (this is a combination of absorption and emission efficiency). While efficiencies improve with UV excitation, additional loss due to larger Stokes-shifted emission and lower efficiencies of LEDs in the UV versus the blue wavelength range makes this a less appealing solution overall.
As a result, poor efficiency drives a high effective ownership cost. The cost is also significantly impacted from the laborious manufacturing and assembly process to construct such devices, for example the heterogeneous integration of the phosphor-layer onto the LED-chip during packaging (DOE and Optoelectronics Industry Development Association “Light emitting diodes (LEDs) for general illumination,” Technology Roadmap (2002)). Historically, blue LEDs have been used in conjunction with various band edge filters and phosphors to generate white light. However, many of the current filters allow photon emission from the blue end of the spectrum, thus limiting the quality of the white LED. The performance of the devices also suffer from poor color rendering due to a limited number of available phosphor colors and color combinations that can be simultaneously excited in the blue. There is a need therefore for efficient nanocomposite filters that can be tailored to filter out specific photon emissions in the visible (especially the blue end), ultraviolet and near infrared spectra.
While some development of organic phosphors has been made for SSWL, organic materials have several insurmountable drawbacks that make them unlikely to be a viable solution for high-efficiency SSWL. These include: rapid photodegradation leading to poor lifetime, especially in the presence of blue and near-UV light; low absorption efficiency; optical scattering, poor refractive index matching at the chip-interface, narrow and non-overlapping absorption spectra for different color phosphors making it difficult or impossible to simultaneously excite multiple colors; and broad emission spectra. There exists a need therefore for polymeric layers that aid production of high quality, high intensity, white light.
The present invention fulfills these needs by providing polymeric nanocomposites that function as down-converting layers, photon-filtering layers and/or refractive index matching layers, by taking advantage of the ability to tailor nanocrystals to maximize their emission, absorption and refractive index properties.
BRIEF SUMMARY OF THE INVENTION
The present invention provides matrix materials doped with nanocrystals that have specific emission and/or absorption characteristics and also allow for specific tailoring of refractive indexes of the nanocomposites.
In one embodiment, the present invention provides polymeric layers comprising a polymer and semiconductor nanocrystals embedded within the polymer, wherein the nanocrystals have a size and a composition such that they absorb visible, ultraviolet, near-infrared and/or infrared light, and wherein the polymeric layers scatter a minimal portion of light that enters the layers. In certain embodiments, the polymer is silicone. The polymeric layers of the present invention can be used to coat optical devices (e.g., refractive lenses or reflective elements) or can be used to encapsulate active devices, such as a light emitting diodes (LEDs). Suitably, the polymeric layers of the present invention that absorb visible light will absorb red light, blue light and/or green light.
The nanocrystals utilized throughout the embodiments of the present invention will suitably be between about 1-10 nm in size, about 1-4 nm in size or about 1-3 nm in size and can further comprise miscibility-enhancing ligands attached to their surface to aid in mixing with the polymers. The polymeric layers of the present invention can have any effective refractive index between that of the pure polymer and the pure nanocrystals, and will suitably have an effective refractive index greater than about 1.5 and in certain embodiments about 1.8. In certain embodiments, the polymeric layers of the present invention will be greater than about 0.5 mm in thickness
In another embodiment, the present invention provides polymeric layers comprising a polymer and semiconductor nanocrystals embedded within the polymer, wherein the polymeric layer has an effective refractive index greater than the polymer alone, and wherein the polymeric layer scatters a minimal portion of light that enters the polymeric layer. Suitably, the polymeric layers will scatter less than about 50%, less than about 20% or less than about 15% of light that enters the polymeric layers. In suitable embodiments, the nanocrystals will be ZnS nanocrystals and the polymeric layers will be greater than about 0.5 mm in thickness.
In another embodiment, the present invention provides polymeric layers that encapsulate an active device (e.g., an LED) that has an effective refractive index, n1. The layer comprises a polymer and semiconductor nanocrystals embedded within the polymer. The layer has an inner boundary in contact with the active device and an outer boundary in contact with a medium having an effective refractive index, n2, wherein the layer has an effective refractive index less than or equal to n1 at the inner boundary and an effective refractive index greater than or equal to n2 at the outer boundary. In certain embodiments, effective refractive index n1 will be greater than n2, suitably greater than about 1.5, and in certain embodiments about 1.8. In certain such embodiments, the layer will have a nanocrystal density gradient, being highest at the inner boundary and lowest at the outer boundary. Suitably this nanocrystal density gradient will be linear throughout the polymeric layer.
The present invention also provides processes for preparing polymeric layers, comprising mixing semiconductor nanocrystals at a first density with a solvent and a polymer to form a first mixture, coating a substrate material with the first mixture and evaporating the solvent to form the polymeric layer, wherein the polymeric layer has an effective refractive index of n1.
The processes of the present invention can be used to prepare polymeric layers for coating active devices (e.g., LEDs), or optical devices (e.g., refractive lenses or reflective elements). The processes of the present invention can utilize nanocrystals which further comprise miscibility-enhancing ligands attached to their surface.
In suitable embodiments, the processes of the present invention can further comprise mixing semiconductor nanocrystals at a second density with a solvent and a polymer to form a second mixture, coating the substrate material with the second mixture and evaporating the solvent to form a second polymeric layer, wherein the second polymeric layer has an effective refractive index of n2. In other embodiments, the processes can further comprise repeating these steps with a third through ith density of semiconductor nanocrystals to produce third through ith polymeric layers, wherein the third through ith polymeric layers have effective refractive indices, n3 through ni, respectively. In certain such embodiments, the effective refractive index n1 will be greater than n2 and the effective refractive index of the ith polymeric layer will be less than the effective refractive index of any other polymeric layer. The processes of the present invention can further comprise centrifuging the first mixture of semiconductor nanocrystals, solvent and polymer, to form a nanocrystal density gradient within the mixture prior to coating the substrate material.
In suitable embodiments of the processes of the present invention, the coating can be via spin coating or screen printing. As discussed throughout, the nanocrystals used in the processes of the present invention can have a size and a composition such that they absorb light at a particular wavelength. In other embodiments, the nanocrystals can be tuned so as to emit light at a particular wavelength. In other embodiments, the process of the present invention can utilize semiconductor nanocrystals that comprise two or more different sizes or compositions and therefore can have different properties. The polymeric layers produced by the processes of the present invention will suitably be greater than about 0.5 mm in thickness.
In another embodiment, the present invention provides solid state white lighting devices comprising a power efficiency greater than 25 lm/W, suitably greater than 50 lm/W, greater than 100 lm/W, greater than 150 lm/W, or greater than 200 lm/W.
In other embodiments, the solid state white lighting devices comprise a down converting nanocomposite that comprises two or more semiconductor nanocrystals tuned to emit light at one or more selected wavelengths. The solid state white lighting devices of the present invention will suitably provide a CRI of greater than about 80. In still other embodiments, the solid state white lighting devices comprise a matrix coupled to the two or more semiconductor nanocrystals via one or more chemical moieties.
Another embodiment the present invention provides down converting nanocomposite devices, comprising two or more semiconductor nanocrystal phosphors of two or more sizes, the nanocrystal phosphors tuned to emit light at one or more selected wavelengths, and providing a CRI of greater than about 80; a matrix with a high index of refraction, low UV degradation and/or matched thermal expansion; and a chemical structure coupling the matrix to the nanocrystal phosphors. Suitably, the two or more semiconductor nanocrystal phosphors will comprise a core-shell structure, wherein a shell (e.g., ZnS) provides a type I band gap with respect to a core. The core-shell nanocrystals of the present invention will suitably have a quantum efficiency of about 10% to about 90%.
In further embodiments of the present invention, the two or more semiconductor nanocrystal phosphors are color matched and the matrix can comprise TiO2. In yet further embodiments, the nanocomposite can be layered on an LED substrate which comprises sapphire or SiC. Suitably, the matrix will be a compliant layer that can withstand the thermal expansion that results when the LED heats up, and suitably will be silicone.
In another embodiment, the present invention provides polymeric layers, comprising a polymer; and semiconductor nanocrystals embedded within the polymer, wherein the nanocrystals have miscibility-enhancing ligands conjugated to their surface, and wherein the ligands comprise an alkane chain of between 6 and 18 carbons in length. In suitable embodiments, the ligands can comprise an alkane chain of between 12 and 18 carbons in length. The polymer will suitably be silicone, and the semiconductor nanocrystals will suitably have a size between about 1-10 nm, and in certain embodiments will be ZnS nanocrystals. In certain embodiments, the polymeric layers will scatter a minimal portion of light that enters said polymeric layer. Suitably, the layer will be greater than about 0.5 mm in thickness.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure and particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.



BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES
The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.
FIG. 1 shows absorption and emission spectra for various nanocrystal radii showing continuous tailoring of the emission and absorption wavelengths.
FIG. 2 shows a comparison of traditional thick phosphors integrated during packaging, and a nanocomposite down-converting layer integrated prior to dicing, according to one embodiment of the present invention.
FIG. 3 shows the elimination of wasted light at the edges of the visible spectrum by using phosphor nanocrystals, compared to traditional phosphor edge losses.
FIG. 4 shows the normalized intensity generated by mixing a continuum of nanocrystal sizes, creating broad-band white light.
FIG. 5 shows a three color emitting LED in accordance with one embodiment of the present invention.
FIG. 6 is a cross-sectional view of a polymeric layer in accordance with one embodiment of the present invention.
FIG. 7 is a cross-sectional view of a polymeric layer having a nanocrystal density gradient in accordance with one embodiment of the present invention.
FIG. 8 is a cross-sectional view of an optical device with a polymeric layer coating the device in accordance with one embodiment of the present invention.
FIG. 9 is a plot showing the effective refractive index of various matrixes versus volume loading ratio of ZnS nanocrystals.
FIG. 10 is a plot showing the effective refractive index of a silicone nanocomposite comprising ZnS nanocrystals as a function of wavelength.
FIG. 11 is a cross-sectional view of a light emitting diode encapsulated within a polymeric layer in accordance with one embodiment of the present invention.
FIG. 12 is a cross-sectional view of a light emitting diode encapsulated within a polymeric layer having a nanocrystal density gradient in accordance with one embodiment of the present invention.
FIG. 13 is a traditional LED chip—silicon cap assembly.
FIG. 14 is a nanocomposite—LED chip assembly in accordance with one embodiment of the present invention.
FIG. 15 is a nanocomposite—LED chip assembly in accordance with one embodiment of the present invention.
FIG. 16 is a plot of percent transmittance for a silicone nanocomposite comprising ZnS nanocrystals as a function of nanocrystals size.
FIG. 17 is a plot of percent transmission for a silicone nanocomposite comprising ZnS nanocrystals as a function of wavelength.
FIG. 18 shows a representation of a 3-part ligand, with a tail-group, a head group and a middle/body-group.
FIG. 19 is an example ligand that can be conjugated to the nanocrystals of the present invention.
FIGS. 20a-20n show examples, chemical synthesis, and NMR characterization of several example ligands in accordance with the present invention.
FIG. 21 is a flowchart depicting processes for preparing polymeric layers in accordance with the present invention.
FIG. 22 is a cross-sectional view of a polymeric layer comprising individual layers each with a different nanocrystal density gradient according to one embodiment of the present invention.
FIG. 23 shows an X-Ray diffraction analysis of ZnS nanocrystals.
FIG. 24 shows Transmission Electron Micrographs of ZnS nanocrystals.



The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.
DETAILED DESCRIPTION OF THE INVENTION
It should be appreciated that the particular implementations shown and described herein are examples of the invention and are not intended to otherwise limit the scope of the present invention in any way. Indeed, for the sake of brevity, conventional electronics, manufacturing, semiconductor devices, and nanocrystal, nanowire (NW), nanorod, nanotube, and nanoribbon technologies and other functional aspects of the systems (and components of the individual operating components of the systems) may not be described in detail herein. It should further be appreciated that the manufacturing techniques described herein can be used to create any semiconductor device type, and other electronic component types. Further, the techniques would be suitable for applications in electrical systems, optical systems, consumer electronics, industrial or military electronics, wireless systems, space applications, or any other application.
The present invention provides various polymeric nanocomposites comprising polymeric materials with embedded nanocrystals. The various properties of the nanocrystals, including their absorption properties, emission properties and refractive index properties, are utilized to create nanocomposites that can be tailored and adjusted for various applications. In one embodiment, the present invention provides applications of semiconductor nanocrystals that utilize their emission properties in down-conversion applications. In another embodiment, the present invention combines two, non-electronically active properties of the same nanocrystals, by using the high absorption coefficient and relatively sharp band edge of the nanocrystals to filter light as a cutoff filter. In another embodiment, the high refractive index of nanocrystals can also be used when mixed into low refractive index materials to create substantially transparent nanocomposites with effective refractive indexes matched to the substrates they are coating. In further embodiments, the refractive index of the nanocomposite can be matched to a second, further encapsulating material. The present invention also provides for nanocomposites that combine two or more of these various properties in different configurations into the same nanocomposite.
As used herein, the term “nanocrystal” refers to nanostructures that are substantially monocrystalline. A nanocrystal has at least one region or characteristic dimension with a dimension of less than about 500 nm, and down to on the order of less than about 1 nm. As used herein, when referring to any numerical value, “about” means a value of ±10% of the stated value (e.g. “about 100 nm encompasses a range of sizes from 90 nm to 110 nm, inclusive). The terms “nanocrystal,” “nanodot,” “dot” and “quantum dot” are readily understood by the ordinarily skilled artisan to represent like structures and are used herein interchangeably. The present invention also encompasses the use of polycrystalline or amorphous nanocrystals.
Typically, the region of characteristic dimension will be along the smallest axis of the structure. Nanocrystals can be substantially homogenous in material properties, or in certain embodiments, can be heterogeneous. The optical properties of nanocrystals can be determined by their particle size, chemical or surface composition. The ability to tailor the nanocrystal size in the range between about 1 nm and about 15 nm enables photoemission coverage in the entire optical spectrum to offer great versatility in color rendering. Particle encapsulation offers robustness against chemical and UV deteriorating agents.
Nanocrystals for use in the present invention can be produced using any method known to those skilled in the art. Suitable methods are disclosed in U.S. patent application Ser. No. 10/796,832, filed Mar. 10, 2004, U.S. patent application Ser. No. 10/656,910, filed Sep. 4, 2003 and U.S. Provisional Patent Application No. 60/578,236, filed Jun. 8, 2004, the disclosures of each of which are incorporated by reference herein in their entireties. The nanocrystals for use in the present invention can be produced from any suitable material, suitably an inorganic material, and more suitably an inorganic conductive or semiconductive material. Suitable semiconductor materials include those disclosed in U.S. patent application Ser. No. 10/796,832 and include any type of semiconductor, including group II-VI, group III-V, group IV-VI and group IV semiconductors. Suitable semiconductor materials include, but are not limited to, Si, Ge, Sn, Se, Te, B, C (including diamond), P, BN, BP, BAs, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe, MgS, MgSe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr, CuI, Si3N4, Ge3N4, Al2O3, (Al, Ga, In)2 (S, Se, Te)3, Al2CO, and an appropriate combination of two or more such semiconductors.
In certain aspects, the semiconductor nanocrystals may comprise a dopant from the group consisting of: a p-type dopant or an n-type dopant. The nanocrystals useful in the present invention can also comprise II-VI or III-V semiconductors. Examples of II-VI or III-V semiconductor nanocrystals include any combination of an element from Group II, such as Zn, Cd and Hg, with any element from Group VI, such as S, Se, Te, Po, of the Periodic Table; and any combination of an element from Group III, such as B, Al, Ga, In, and Tl, with any element from Group V, such as N, P, As, Sb and Bi, of the Periodic Table.
The nanocrystals useful in the present invention can also further comprise ligands conjugated, cooperated, associated or attached to their surface as described throughout. Suitable ligands include any group known to those skilled in the art, including those disclosed in U.S. patent application Ser. No. 10/656,910 and U.S. Provisional Patent Application No. 60/578,236. Use of such ligands can enhance the ability of the nanocrystals to incorporate into various solvents and matrixes, including polymers. Increasing the miscibility (i.e., the ability to be mixed without separation) of the nanocrystals in various solvents and matrixes allows them to be distributed throughout a polymeric composition such that the nanocrystals do not aggregate together and therefore do not scatter light. Such ligands are described as “miscibility-enhancing” ligands herein.
As used herein, the term nanocomposite refers to matrix materials comprising nanocrystals distributed or embedded therein. Suitable matrix materials can be any material known to the ordinarily skilled artisan, including polymeric materials, organic and inorganic oxides. Nanocomposites of the present invention can be layers, encapsulants, coatings or films as described herein. It should be understood that in embodiments of the present invention where reference is made to a layer, polymeric layer, matrix, or nanocomposite, these terms are used interchangeably, and the embodiment so described is not limited to any one type of nanocomposite, but encompasses any matrix material or layer described herein or known in the art.
I. Down-Converting Nanocomposites
In order to become competitive with traditional lighting from fluorescent and incandescent lights, significant improvements must be made in solid-state white lighting (SSWL). Improvements not just in the quantum efficiency of the phosphors, but in all aspects of the down-conversion system that relate to efficiency, color-rendering and overall system cost are needed. In one embodiment, the present invention provides a complete down-conversion system based on engineered nanocomposite materials for use with currently available blue LED excitation sources that dramatically improve the overall cost, performance and efficiency of SSWL. The down-converting nanocomposites of the present invention utilize the emission properties of nanocrystals that are tailored to absorb light of a particular wavelength and then emit at a second wavelength, thereby providing enhanced performance and efficiency of active sources (e.g., LEDs). As such, the nanocrystals utilized in the down-converting applications of the present invention will be constructed and tailored so as to be highly emitting. In one embodiment, this system produces SSWL that exceeds performance of the best traditional fluorescent and incandescent bulbs, with color rendering of greater than 80 and power efficiency of greater than 200 lm/W, at a cost of less than one U.S. dollar/klm.
Performance Characteristics of SSWL Devices
To evaluate the performance characteristics of solid-state white lighting (SSWL) devices, three primary attributes are commonly used: (1) luminous efficiency, (2) Correlated Color Temperature (CCT) and (3) Color Rendering Index (CRI). DOE and Optoelectronics Industry Development Association “Light emitting diodes (LEDs) for general illumination,” Technology Roadmap (2002).
The luminous efficiency (measured in lm/W) is the efficiency of the conversion from electrical power (W) to optical power (W), combined with the efficiency of the conversion from optical power (W) to the luminous flux. The luminous efficiency is influenced by a number of factors, and can, in general terms, be written as a contribution of several separate efficiencies:

Eluminous=ηwp×ηlum×ηss×ηIQE×ηpackage(Eos, Epa, ETIR, Eexp)×

where ηwp is the wall plug efficiency, ηlum is the photopic efficiency/response of the human eye, ηss is the stokes shift efficiency from converting a blue photon to a longer wavelength photon, ηIQE is the internal quantum efficiency of the phosphor, and ηpackage is overall package efficiency and accounts for losses in light extraction efficiency from optical scattering (Eos), parasitic scattering (Eps), total internal reflection (ETIR), external packaging like the lead frame and submount (Eexp), etc.

CCT or correlated color temperature refers to the human eye property of being optimally adapted to the sunlight spectral content. The relative intensities of the blue (B), red (R) and green (G) colors, for the desired white color, referred to as chromaticity coordinates, optimally reproduce those in the visible sunlight, which corresponds to a blackbody spectral distribution of 6000 Kelvin (K). For optimum illumination the chromaticity coordinates for R, G and B must fall near the black body radiation, for temperatures between 2000° C. and 8000° C. Higher or lower than “optimum” temperatures register to the eye as too “cold” or too “warm” color hues.
Color rendering has to do with the appearance of various object colors under a given source illumination, compared to that from a reference source. A collection of 14 sample colors of different saturation is customarily used for the color rendering index (CRI), which provides a quantitative measure on a scale of 1 to 100. Two sources of similar color temperature may produce widely varying CRIs. Low CRIs make colors unacceptable for illumination, while high CRI (>80) are acceptable for general illumination purposes.
Procedure for Providing an Optimized White Light Emitting Device
In one embodiment, the present invention provides processes comprising:
(1) A simulation model can be used to determine optimized nanocrystal mixtures for CRI, CCT, and luminous efficiency with targets of CRI>80, CCT about 4,000 K and efficiency of 200 lm/W.
(2) Nanocrystals and nanocrystal component mixtures are synthesized with emission peak widths, peak maximums, and intensity ratios determined by simulation.
(3) A controlled nanocrystal phosphor nanocomposite is developed, including: (a) a surface ligand capable of achieving high (about 20% or more) loading density in the selected composite is produced; (b) a ligand exchange process to incorporate a 3-part ligand onto the nanocrystal is performed; (c) a homogeneous, non-phase separated TiO2 nanocomposite with nanocrystal loading densities up to 20% by volume is produced; (d) quantum yield (QY) dependence on nanocrystal loading density in the nanocomposite is determined; (e) an index of refraction dependence on loading density in the nanocomposite and index-matching of the nanocomposite to blue LED substrate (e.g., sapphire and/or SiC) is determined; and (f) a relationship of loading density and film thickness to optimize refractive index matching and nanocomposite optical density is determined.
Simulations for Determining Optimum Nanocrystal Component Mixture for High Color Rendering, Color Temperature and High Efficiency
In order to predict and maximize CRI, CTT and luminous efficiency of nanocrystal mixtures, a dynamic and robust simulation model is used. A super-convergent, random-search, parameter optimization algorithm is used to find the maximum performance point, subject to the imposed constraints. The model allows calculation of these performance characteristics based on actual experimental colorimetric and optical characteristics of nanophosphor components and mixtures. In turn, this model is used to aid the design and fabrication of optimal nanocomposite SSWL devices.
The simulation program incorporates an algorithm to determine the optimum spectral emission characteristics of nanocrystal component mixtures for simultaneous maximization of color rendering, color temperature and overall efficacy for production of white light. The approach provides a super-convergent, random search, optimization algorithm in the phosphor parameter space. The program seeks a combination of emission wavelengths that simultaneously maximizes luminous efficacy, color rendering (CRI) and color temperature (CCT), subject to the white-light chromaticity constrains calculated using standard CIE (Commission Internationale de l'Eclairage). The measured nanocrystal quantum efficiency, peak wavelength and emission spectral width are input parameters. Performance boundaries, as for example, efficacy no less than 90%, or CRI>90, can also be applied for flexibility in the design. The number of required wavelengths (i.e., nanocrystal sizes) is a variable that allows a determination of trade-offs between performance and manufacturing cost.
A validation procedure with iteration cycles is adopted, whereby mixtures of nanocrystal components, of size, composition peak maximum, peak widths, mixture abundance, and internal quantum efficiency predicted by the simulation are manufactured. The resulting values of CRI and CCT are determined experimentally and compared with the predictions and adjustments are made as appropriate. The luminous efficiency is determined based on optical parameters including stoke shift efficiency, internal quantum efficiency and photopic response.
The output of this procedure is the optimum number of emission colors, the precise center wavelengths of each color, the precise spectral width of each color and the exact relative intensity of each and the corresponding concentration based on excitation by, for example, a selected blue LED (about 460 nm).
The simulations described throughout can determine suitable emission characteristics for the nanocrystal. In addition, it is useful to (1) synthesize the materials with the prescribed spectral characteristics and (2) use the materials to validate the model. To achieve this objective, available solution phase synthetic techniques are used to fabricate core/shell nanocrystal phosphors and characterize mixtures as determined by the theoretical model.
Based on current methods, nanocrystal batches are fabricated with spectral characteristics generated by the theoretical model. Each distinct wavelength is synthesized separately and combined to produce the final mixture. Specific attention is paid to the center wavelength and the peak-width of each sample. In particular, narrow emission in the red avoids efficiency loss in the IR. In order to accomplish this, a solution-phase mixture of nanocrystals is produced and characterized that has the appropriate composition to produce white light with CRI and CTT, matching that of the theoretical model when illuminated with blue excitation and total down-conversion efficiency comparable to that predicted by the model, assuming zero loss to other mechanisms in the process. These measurements can be made in the solution-phase using a standard visible fluorometer and fluorescence standards with excitation matching the blue-LED.
Nanocrystal Phosphors
While any method known to the ordinarily skilled artisan can be used to create nanocrystal phosphors, suitably, a solution-phase colloidal method for controlled growth of inorganic nanomaterial phosphors is used. See Alivisatos, A. P., “Semiconductor clusters, nanocrystals, and quantum dots,” Science 271:933 (1996); X. Peng, M. Schlamp, A. Kadavanich, A. P. Alivisatos, “Epitaxial growth of highly luminescent CdSe/CdS Core/Shell nanocrystals with photostability and electronic accessibility,” J. Am. Chem. Soc. 30:7019-7029 (1997); and C. B. Murray, D. J. Norris, M. G. Bawendi, “Synthesis and characterization of nearly monodisperse CdE (E=sulfur, selenium, tellurium) semiconductor nanocrystallites,” J. Am. Chem. Soc. 115:8706 (1993). This manufacturing process technology leverages low cost processability without the need for clean rooms and expensive manufacturing equipment. In these methods, metal precursors that undergo pyrolysis at high temperature are rapidly injected into a hot solution of organic surfactant molecules. These precursors break apart at elevated temperatures and react to nucleate nanocrystals. After this initial nucleation phase, a growth phase begins by the addition of monomers to the growing crystal. The result is freestanding crystalline nanoparticles in solution that have an organic surfactant molecule coating their surface.
Utilizing this approach, synthesis occurs as an initial nucleation event that takes place over seconds, followed by crystal growth at elevated temperature for several minutes. Parameters such as the temperature, types of surfactants present, precursor materials, and ratios of surfactants to monomers can be modified so as to change the nature and progress of the reaction. The temperature controls the structural phase of the nucleation event, rate of decomposition of precursors, and rate of growth. The organic surfactant molecules mediate both solubility and control of the nanocrystal shape. The ratio of surfactants to monomer, surfactants to each other, monomers to each other, and the individual concentrations of monomers strongly influence the kinetics of growth.
In suitable embodiments, CdSe is used as the nanocrystal material, in one example, for visible light down-conversion, due to the relative maturity of the synthesis of this material. Due to the use of a generic surface chemistry, it is also possible to substitute non-cadmium-containing nanocrystals.
Core/Shell Nanocrystals
In semiconductor nanocrystals, photo-induced emission arises from the band edge states of the nanocrystal. The band-edge emission from nanocrystals competes with radiative and non-radiative decay channels originating from surface electronic states. X. Peng, et al., J. Am. Chem. Soc. 30:7019-7029 (1997). As a result, the presence of surface defects such as dangling bonds provide non-radiative recombination centers and contribute to lowered emission efficiency. An efficient and permanent method to passivate and remove the surface trap states is to epitaxially grow an inorganic shell material on the surface of the nanocrystal. X. Peng, et al., J. Am. Chem. Soc. 30:7019-7029 (1997). The shell material can be chosen such that the electronic levels are type I with respect to the core material (e.g., with a larger bandgap to provide a potential step localizing the electron and hole to the core). As a result, the probability of non-radiative recombination can be reduced.
Core-shell structures are obtained by adding organometallic precursors containing the shell materials to a reaction mixture containing the core nanocrystal. In this case, rather than a nucleation-event followed by growth, the cores act as the nuclei, and the shells grow from their surface. The temperature of the reaction is kept low to favor the addition of shell material monomers to the core surface, while preventing independent nucleation of nanocrystals of the shell materials. Surfactants in the reaction mixture are present to direct the controlled growth of shell material and ensure solubility. A uniform and epitaxially grown shell is obtained when there is a low lattice mismatch between the two materials. Additionally, the spherical shape acts to minimize interfacial strain energy from the large radius of curvature, thereby preventing the formation of dislocations that could degrade the optical properties of the nanocrystal system.
In suitable embodiments, ZnS can be used as the shell material using known synthetic processes, resulting in a high-quality emission. As above, if necessary, this material can be easily substituted if the core material is modified.
Optical Properties of Core-shell Nanocrystals
Due to the finite size of the core-shell nanocrystals, they display unique optical properties compared to their bulk counterparts. The emission spectrum is defined by a single Gaussian peak, which arises from the band-edge luminescence. The emission peak location is determined by the core particle size as a direct result of quantum confinement effects. For instance, by adjusting the particle diameter in the range of 2 nm and 15 nm, the emission can be precisely tuned over the entire visible spectrum (FIG. 1). FIG. 1 represents the absorption and emission peaks for nanocrystals of increasing size (2 nm to 15 nm). The initial peak (lower wavelength) indicates the absorption wavelength and the later peak (higher wavelength) the emission wavelength in nm. With increasing size of the nanocrystals, the absorption and emission peak wavelengths shift from about 450 nm to about 700 nm, and can be tuned over this range. The vertical shaded bars on FIG. 1 indicate visible light wavelengths in the blue 100, green 102 and red 104 ranges.
The width of the emission peak is determined by the size distribution of the sample. Peak widths down to 20 nm full width at half maximum (FWHM) can be achieved. Conversely, the absorption spectrum of nanocrystals is very broad and intense, as typical of the bulk material, which is characteristically different than organic phosphors. Absorption coefficients are in excess of 55,000/cm (in the blue range of the spectrum) over the entire range of crystal sizes. In addition, core-shell nanocrystals can be made with quantum efficiencies as high as 90% (this does not take into account energy loss due to Stokes shift, but is simply the ratio of photons-in to photons-out).
In one embodiment, the present invention provides an engineerable down-converting system (see FIG. 2). Systems according to the present invention can comprise a nanocomposite down-converting layer that can be coated directly onto an LED wafer prior to dicing and packaging, eliminating the need for heterogeneous integration of the phosphor layer during packaging. The nanocomposite down-converting layer is suitably engineered from three components, including: (1) Semiconductor nanocrystal phosphors of one or more, suitably two or more, sizes tuned to emit at the required wavelength(s) and with the required spectral characteristics to optimize the color rendering index (CRI) and power conversion efficiency in the final device; (2) A host matrix (e.g., a polymer) selected for high index of refraction (generally about 1.5 or greater), low UV degradation and matched thermal expansion to the LED chip; and, (3) A unique nanocrystal surface chemistry that acts as the interface between the nanocrystals and the host matrix, allowing each element to be independently selected and tailored without impacting the other component. As shown in FIG. 2, such a down-converting nanocomposite phosphor layer 208 will take the place of phosphor 200 and phosphor encapsulant layer 206.
By independently selecting and tuning each of these three components, it is possible to simultaneously: (1) engineer a specific composite emission spectrum that can be tailored to optimize between CRI and down-conversion efficiency; (2) refractive index match the composite layer to the LED chip to reduce light-extraction losses prior to down conversion; (3) reduce scattering in the down-conversion layer, thereby minimizing light-extraction losses from the phosphor layer; (4) produce down-conversion with a quantum efficiency greater than about 20% (e.g., 40%, 60%, 80%, 100%) at any wavelength with simultaneous and efficient absorption of light (about 300 nm) (depending upon size and composition of the nanoparticles); and (5) minimize loss of efficiency due to photons emitted into the near-infra-red (near-IR) through the use of extremely sharp emission spectra in red light wavelengths (about 20 nm FWHM). This approach makes it possible to achieve overall power-conversion efficiencies greater than 200 lm/W, with CRI greater than 80, and overall chip brightness of greater than 100 Watts/chip at a cost of less than one U.S. dollar/klm.











Nanocomposite System Features
Benefit to SSWL







High quantum efficiency
No loss of photons upon down


(as high as 90%)
conversion resulting in 2-fold



increase in overall power-



conversion efficiency over



traditional phosphors.


Continuous, tunable emission
The emission peak wavelength and


spectrum
width can be precisely tuned so



mixtures of different sized



nanocrystals can be formed with



precise emission characteristics



to achieve maximum emission



efficiency, CRI, CTT.


Narrow and Sharp Emission
Sharp emission allows tailoring of



emitted light at wavelengths where



the luminous efficiency of



photopic vision of the eye is



high.


High photo- and chemical
Nanocrystals are not susceptible


stability
to bleaching effects and



environmental sensitivities (UV,



moisture, oxygen) as traditional



organic phosphors offering long



operating lifetimes.


Mixtures of nanocrystals in
Mixtures of nanocrystals can be


host matrix
embedded in a host matrix of



virtually any material at high-



loading densities (e.g., 20% by



volume) with precise control over



relative concentration ratios through



modification of surface chemistry.


Non-scattering composites
Due to the small particle size and



capability to make homogenous



dispersion of the nanocrystals,



optical scattering as well as



parasitic absorption can be



minimized or eliminated to improve



light extraction efficiency and



hence the device luminous



efficiency.


Tunable refractive index
By selecting the proper host matrix



material and tailoring the loading



density, the index of refraction of



the nanocomposite layer can be



precisely tuned from about 1.5 to



about 2.5 to minimize or even



eliminate total internal reflection



at the LED-nanocomposite interface,



potentially increasing overall



power conversion efficiency.



Loading density and thickness can



be traded-off to simultaneously



optimize index of refraction and



optical density of the composite



layer while maintaining film



thicknesses.


High absorption coefficients
At a high loading density, the


(as high as 55,000/cm)
nanocomposite down-conversion



layer can be on the order of a



single micron in thickness. This



allows direct incorporation at the



wafer-level using traditional thin



film processing, dramatically



reducing overall manufacturing



costs for SSWL relative to thick-



film phosphor layers that are



incorporated at the package level.









FIG. 3 illustrates the emission range of the down-converting nanocomposites of the present invention, in the red region of a 2-color phosphor mix, compared to that resulting from traditional inorganic phosphors for white. Emission peaks 302 and 304 represent the emission spectra of a 2-color phosphor mix according to one embodiment of the present invention. Spectrum 306 represents the emission spectrum of traditional inorganic phosphors. Not only does the narrow emission prevent photon waste at the edges of the visible spectrum by the eye, but it also allows a superior optimization of color rendering index and power conversion efficiency. Wasted light region 308 demonstrates light emitted from traditional inorganic phosphors at the edges of the visible spectrum that is cut out by using the sharp emission peak 304.
FIG. 4 illustrates the concept of fine-tuning the emission by using more than three emission colors, each with a specific, narrow, emission peak, to generate an overall emission spectrum with a superior color rendering index that can be as high as 100 for any color temperature. Between the two extremes of extremely broad emission and extremely narrow emission, however, is a balance between efficiency and CRI. The exact number of colors, center wavelengths, relative concentrations and spectral widths can be determined theoretically to optimize both parameters simultaneously.
By using standard thin-film and lithographic processing techniques, as shown in FIG. 5, green 500 and red 502 down-conversion layers can be patterned across LED chips 204 prior to dicing. This allows low-cost fabrication of 3-color emitting LEDs integrated into a single die, such that a single chip can be used to dynamically tune emission of the LED from monochromatic to white for any color temperature. As such, the present invention provides formation of an integrated chip-level 3-color mixing-based SSWL for all lighting applications, at a cost point competitive with traditional lighting, but with far superior efficiency, performance and color engineering capability.
Suitable matrixes for use in all embodiments of the present invention include polymers and organic and inorganic oxides. Suitable polymers for use in the matrixes of the present invention include any polymer known to the ordinarily skilled artisan that can be used for such a purpose. In suitable embodiments, the polymer will be substantially translucent or substantially transparent. Such polymers include, but are not limited to, poly(vinyl butyral):poly(vinyl acetate), silicone and derivatives of silicone, including, but not limited to, polyphenylmethylsiloxane, polyphenylalkylsiloxane, polydiphenylsiloxane, polydialkylsiloxane, fluorinated silicones and vinyl and hydride substituted silicones.
The nanocrystals used the present invention can be embedded in a polymeric (or other suitable material, e.g., waxes, oils) matrix using any suitable method, for example, mixing the nanocrystals in a polymer and casting a film, mixing the nanocrystals with monomers and polymerizing them together, mixing the nanocrystals in a so-gel to form an oxide, or any other method known to those skilled in the art. As used herein, the term “embedded” is used to indicate that the nanocrystals are enclosed within the polymer that makes up the majority component of the matrix.
The thickness of the layers of the present invention can be controlled by any method known in the art, such as spin coating and screen printing. Such methods are especially useful when coating optical devices such as lenses or mirrors with the polymeric layers. While the various polymeric layers of the present invention can be any thickness required, suitably, the layers will be less than about 100 mm in thickness, and down to on the order of less than about 1 mm in thickness. In other embodiments, the polymeric layers of the present invention can be on the order of 10's to 100's of microns in thickness. In one embodiment, the present invention provides nanocrystal doped layers that are greater than about 0.5 mm in thickness, and suitably will scatter only a minimal portion of light that enters the layer (see later for a discussion of scattering). In other embodiments, the layers will be between about 0.5 mm and about 50 mm in thickness. In all embodiments of the present invention, the nanocrystals can be embedded in the various matrixes at any loading ratio that is appropriate for the desired function. Suitably, the nanocrystals will be loaded at a ratio of between about 0.001% and about 75% by volume depending upon the application, matrix and type of nanocrystals used. The appropriate loading ratios can readily be determined by the ordinarily skilled artisan and are described herein further with regard to specific applications.
II. Photon-Filtering Nanocomposites
In another embodiment, the present invention provides polymeric layers comprising a polymer and nanocrystals embedded within the polymer, such that the layers act as photon-filtering nanocomposites. Suitably, the nanocrystals will be prepared from semiconductor materials, but any suitable material described throughout can be used to prepare the nanocrystals. In certain embodiments, the nanocrystals will have a size and a composition such that the nanocrystals absorb light of a particular wavelength or over a range of wavelengths. As such, the nanocrystals utilized in these embodiments are tailored such that their absorption characteristics are enhanced or maximized, while their emission characteristics are minimized, i.e. they will absorb light in a highly efficient manner, but suitably will emit only a very low level, or preferably no light. In other embodiments, however, the photon-filtering nanocomposites can also comprise nanocrystals that have high emission properties and emit light at a particular wavelength as discussed throughout. As such, the present invention provides nanocomposites that comprise different types of nanocrystals such that the nanocomposites exhibit several, or all, of the properties discussed throughout, in a layer.
A photon-filtering nanocomposite in accordance with one embodiment of the present invention is shown in FIG. 6. FIG. 6 is a cross-sectional view of a polymeric layer 600 showing nanocrystals 604 embedded in polymer 602. Note that nanocrystals 604 are not to scale and are visibly represented for illustrative purposes only. The polymeric layers and nanocomposites of the present invention can also comprise nanocrystals of different sizes and compositions within the same layer.
In suitable embodiments, the nanocrystals can be distributed homogenously throughout the polymeric layer and nanocomposites (see FIG. 6). In other embodiments, the nanocrystals can be randomly distributed. In further embodiments, the nanocrystals can be distributed throughout the layer such that they form a nanocrystal density gradient throughout the layer (as discussed further in the refractive index section below). Such an embodiment is represented in FIG. 7, which shows a cross-sectional view of a polymeric layer 700 with nanocrystals 604 embedded in polymer 602 in such a way that they form a nanocrystal density gradient from high density (lower portion of FIG. 7) to low density (upper portion of FIG. 7) within polymer 602.
The photon-filtering polymeric layers and nanocomposites of the present invention can be used to coat, encapsulate, cover, be deposited on (or any other similar arrangement known to those skilled in the art) any substrate material. Suitably, the polymeric layers of the present invention can be used to coat optical devices. In other embodiment, the polymeric layers can be used to encapsulate active devices.
In embodiments of the present invention where the photon-filtering polymeric layers are used to coat optical devices, such optical devices can be refractive (e.g., lenses) or reflective (e.g., mirrors). FIG. 8 is a cross-sectional view of an optical device 802 coated with polymer 602 comprising nanocrystals 604. Coated optical devices 800 according to such an embodiment can be used in any application where a filter or anti-reflective coating is desired on a refractive or reflective device.
In embodiments of the present invention where the photon-filtering polymeric layers are used to encapsulate active devices, such active devices can be any device known to the skilled artisan. As used herein an “active device” is one that requires a source of energy for its operation and has an output that is a function of present and past input signals. Examples of active devices include, but are not limited to, controlled power supplies, transistors, diodes, including light emitting diodes (LEDs), light detectors, amplifiers, transmitters and other semiconductor devices.
By controlling the size and composition of the nanocrystals used in the practice of the present invention, the nanocrystals will absorb light of a particular wavelength, or a particular range of wavelengths, while not scattering light. The ability to make nanocrystals out of different semiconductors, and control their size, allows for polymeric layers to be fabricated with nanocrystals that will absorb light from the UV, to visible, to near infrared (NIR), to infrared (IR) wavelengths. Nanocrystals for use in the present invention will suitably be less than about 100 nm in size, and down to less than about 2 nm in size. In suitable embodiments, the nanocrystals of the present invention absorb visible light. As used herein, visible light is electromagnetic radiation with wavelengths between about 380 and about 780 nanometers that is visible to the human eye. Visible light can be separated into the various colors of the spectrum, such as red, orange, yellow, green, blue, indigo and violet. The photon-filtering nanocomposites of the present invention can be constructed so as to absorb light that makes up any one or more of these colors. For example, the nanocomposites of the present invention can be constructed so as to absorb blue light, red light, or green light, combinations of such colors, or any colors in between. As used herein, blue light comprises light between about 435 nm and about 500 nm, green light comprises light between about 520 nm and 565 nm and red light comprises light between about 625 nm and about 740 nm in wavelength. The ordinarily skilled artisan will be able to construct nanocomposites that can filter any combination of these wavelengths, or wavelengths between these colors, and such nanocomposites are embodied by the present invention.
Polymeric layers that comprise nanocrystals that can absorb light of a particular wavelength, or range of wavelengths, will act as edge pass filters, absorbing light that is less than a certain wavelength. For example, the photon-filtering nanocomposites can be constructed so as to absorb light that is less than about 565 nm (e.g., blue and green) and allowing wavelengths of light that are longer than about 565 nm (e.g., red) to pass through the polymeric layer.
In other embodiments, the nanocrystals have a size and a composition such that they absorb photons that are in the ultraviolet, near-infrared, and/or infrared spectra. As used herein, the ultraviolet spectrum comprises light between about 100 nm to about 400 nm, the near-infrared spectrum comprises light between about 750 nm to about 100 μm in wavelength and the infrared spectrum comprises light between about 750 nm to about 300 μm in wavelength.
While nanocrystals of any suitable material can be used in the practice of the present invention, in certain embodiments, the nanocrystals can be ZnS, InAs or CdSe nanocrystals. In one embodiment, InAs nanocrystals (with a 1340 nm absorption peak) with TOP (tri-n-octylphosphine) ligands attached to their surface can be dissolved in a solvent such as toluene. Poly(vinyl butyral):poly(vinyl acetate) (PVB:PVA) polymer can also be dissolved in toluene and the two solutions can be mixed together. A substrate can then be coated or encapsulated with the mixture and the toluene evaporated off. A thin film results that is non-light-scattering due to the size of the non-aggregated nanocrystals. Polymeric layers produced in such a manner will have an effective refractive index between that of either material by itself (i.e., the polymer or the nanocrystal material), which can be adjusted by modifying the loading ratio of the nanocrystals and the density of the nanocrystals at various points in the polymeric layer (see Refractive Index section for additional disclosure). A polymeric layer comprising such nanocrystals can act as an antireflective filter, absorbing light that is less than about 1340 nm in wavelength.
In another embodiment of the present invention, CdSe nanocrystals (having an absorption peak at about 580 nm) with stearic acid ligands can be dissolved in a solvent such as toluene. In other embodiments, ZnS nanocrystals with amine, carboxylic acid, phosphonic acid, phosphonate, phosphine, phosphine oxide or sulfur ligands can be dissolved in a solvent. In the case of CdSe nanocrystals, a ligand exchange can then be performed in solution with a siloxane ligand and excess ligand can be removed. Nanocrystals can then be mixed with a polymer base, such as silicone, and a substrate material can then be coated or encapsulated. After curing, the film will have an effective refractive index between that of the polymer (e.g., silicone) and the nanocrystals, which can be adjusted by changing the loading ratio of the nanocrystals in the silicone. Such a polymeric layer will act as a filter absorbing light that is less than about 580 nm in wavelength (i.e., blue, green, yellow, orange, violet, UV light).
III. Refractive Index Matching Nanocomposites
Poor extraction caused by total internal light reflection due to index of refraction mismatches at interfaces is a problem for light emitting devices, including LEDs. It is well known that light impinging an interface between materials of index n and n′<n, at an angle θ relative to the vertical will be totally reflected, if sin θ>sin θc=n′/n. For a direct extraction from GaN with n=2.26 into air n′=1, this limits the extraction cone within a solid angle ΔΩ=2π (1−cos θc), where θc is the critical extraction angle, θc=26°, being just 10% of the total extraction angle 2π (upper half only). In the encapsulation arrangement shown in FIG. 2, overall light extraction relates to: (a) extraction from substrate 202 to phosphor encapsulant layer 206; and (b) from phosphor encapsulant layer 206 to air. The radius of the encapsulant layer is much larger than the emitting area (e.g., cm vs. mm) thus light rays inside the encapsulant layer that reach out in the radial direction impinge almost perpendicular to the encapsulant layer surface θ<<θc and are extracted. Thus the overall extraction is mainly limited by extraction from the substrate to the phosphor plane interface, taking advantage of the higher phosphor critical angle, due to the higher than air index of refraction.
In one embodiment, as shown in FIG. 2, the present invention provides nanocomposite layers 208, that can bond to LED substrate 202, that have an effective refraction index of greater than 1.5. In suitable embodiments, by increasing the substrate effective refractive index to 1.8, an angle of θc=68° is generated and the extraction efficiency is doubled to 63%. In another embodiment, the present invention provides nanocomposites combining nanocrystals with a refractive index of about 2.0 to about 3 with host matrix materials, including polymers (e.g., TiO2 with an effective refractive index 1.5 to 2, or silicone with a refractive index of about 1.49), to generate a nanocomposite material with an effective refractive index of about 2, with a critical extraction angle of about θc=77°, thereby increasing the extraction efficiency to 78%. In further embodiments, a matched-index passive layer (e.g., a hard shell polymer) can be added above the phosphor layer to take advantage of the radial incidence, thereby enhancing extraction from the phosphor layer into the air.
As used herein, the term “effective refractive index (ne)” is used to indicate that the polymeric layers of the present invention are composite materials, and thus, their effective refractive index is a result of the refractive index of all components of the layer. The term refractive index as used herein indicates the degree to which the nanocomposites of the invention bend light. The refractive index of the nanocomposites can be determined by the ratio of the speed of light in a vacuum divided by the speed of light in the nanocomposites.
The polymeric layers of the present invention, whether down-converting, photon-filtering, or refractive index matching, have an effective refractive index that can be controlled by the ratio, density, composition and size of the nanocrystals embedded within the matrix. FIG. 9 shows the effect of loading ratio of ZnS nanocrystals (n=2.35) on the effective refractive index of various materials. For all matrixes, the effective refractive index increases linearly with the loading ratio (%) up to the refractive index of the pure ZnS nanocrystals. FIG. 10 shows the effective refractive index of a silicone nanocomposite comprising 3 nm ZnS nanocrystals at 30% by volume as a function of wavelength. An effective refractive index of greater than about 1.77 is observed for all wavelengths from 300 nm to 700 nm.
Control and tailoring of the effective refractive index allows the matrixes of the present invention to be utilized in applications where a layer having either a uniform or varying effective refractive index may be desired, for example as polymeric layers encapsulating LEDs. In such applications, a polymeric layer is used to encapsulate the light emitting diode chip of an LED to provide protection to the chip. As discussed above, due to refractive index (n) differences between the high n of the LED chip and the generally low n of the polymeric encapsulant, large amounts of light are lost due to light reflecting at the chip/polymer interface. The present invention therefore provides for a polymeric layer that has a refractive index higher than pure polymer that can approach or match the refractive index of the LED chip, thereby limiting the light lost at the chip/polymer interface. Such an embodiment is represented in FIG. 11, showing a cross-sectional view of an encapsulated light emitting diode 1100, in which polymer 602 comprising embedded nanocrystals 604 encapsulates LED chip 1106. Any active device, including those discussed throughout, can be encapsulated in a similar manner. Furthermore, FIGS. 11 and 12 showing specific LED structures are presented for illustrative purposes only, and any LED structure known to those skilled in the art can be similarly encapsulated.
The effective refractive index of the polymeric layer can be any value between that of the pure matrix material (e.g., silicone at about 1.49, TiO2 at about 1.5) and the nanocrystals themselves (e.g., up to about 3). Suitably, the effective refractive index of the matrix will be greater than about 1.5, preferably between about 1.5 and about 2.5, and in certain embodiments the refractive index of the matrix will be about 1.8.
In other embodiments, in order to add further stability to an LED structure, a second polymeric layer can be added on top of the first layer. Often, this second layer will be a “hard-shell” polymer and will have a refractive index that is lower than the LED chip. Thus, if the refractive index of the first polymeric layer is matched to the refractive index of the LED chip, reflections will occur at the first polymeric layer/hard shell polymer interface. In order to overcome this problem, in another embodiment, the present invention provides for a polymeric layer or polymeric encapsulant which has a density gradient of nanocrystals such that the effective refractive index of the polymeric layer matches both the chip and the hard shell polymer at their respective interfaces.
In one such embodiment, the present invention provides polymeric layers that encapsulate an active device that has an effective refractive index, n1. The layer comprises a polymer and semiconductor nanocrystals embedded within the polymer, and has an inner boundary in contact with the active device and an outer boundary in contact with a medium that has an effective refractive index, n2. The layer has an effective refractive index less than or equal to n1 at the inner boundary and an effective refractive index greater than or equal to n2 at the outer boundary. Suitably, the active device will be an LED, though any active device, including those described throughout, can be encapsulated. In suitable embodiments, n1 will be greater than n2.
FIG. 12 shows a cross-sectional view of an LED encapsulated in such a polymeric layer. Encapsulated LED 1200 comprises polymeric layer 602 comprising embedded nanocrystals 604 encapsulating LED chip 1106. Hard shell polymer 1202 further coats polymeric layer 602 to provide additional structural integrity and protection to the LED. FIG. 12 illustrates the nanocrystal density gradient throughout the thickness of polymeric layer 602, this gradient being highest at the boundary with LED chip 1106 and lowest at the boundary with hard shell polymer 1202. In such embodiments, the effective refractive index is n1 at the boundary with LED chip 1106 and the effective refractive index is n2 at the boundary with hard shell polymer 1202. In certain embodiments, this nanocrystal density gradient will be substantially linear throughout the polymeric layer, though it can take any form throughout the thickness of the layer, e.g., cyclic, parabolic, etc. Suitably, the effective refractive index of polymeric layer 602 will be greater than about 1.5 throughout the layer, and in certain embodiments will be about 1.8 (n1) at the interface with LED chip 1106 and about 1.5 (n2) at the interface with hard shell polymer 1202.
As shown in FIG. 13, light emitting diodes often utilize LED chip 1106 covered by a drop or layer of silicone 1300 usually a few millimeters in diameter. As discussed throughout, by replacing the silicone cap in FIG. 13 with nanocrystal doped matrixes with enhanced refractive indexes, more light can be extracted from LED chip 1106. However, two issues may arise by doing so: (1) the amount of nanocrystals required for a doped matrix a few millimeters think for each LED translates to rather large quantities of nanocrystals for mass production, thereby driving up the cost; and (2) the scattering from the nanocrystals throughout a think layer may make the matrix opaque for a path-length of a few millimeters.
To resolve these issues, in another embodiment (see FIG. 14), the present invention provides for a thin film of nanocomposite 1402 formed on the surface of an LED chip 1106, this thin film is then further capped with small hemispheres 1404 of the same nanocomposite. All of the light that enters the nanocomposite hits the composite/air interface at 90° and therefore does not suffer from any internal reflection. The thickness of the film and the diameter of the small caps can be chosen to satisfy the thermal compliance and other mechanical/thermal requirements. The thickness, t, of the film, and the diameter, d, of the small hemispheres, can be in the range of 10's-100's of nms to microns to millimeters. Suitably, the thickness of the layer will be on the order of 10's of microns, for example about 10-50 microns. The diameter of the hemispheres is generally on the order of microns.
In other embodiments of the present invention, the small hemispheres 1404 of nanocomposite can be further capped with a large hemisphere of silicone 1302, as illustrated in FIG. 15. In this case, the refractive index of the large hemisphere of silicone is not required for light extraction. The critical angle is only determined by the refractive index of the LED chip 1106, n1, and that of the nanocrystal doped matrix (1402 and 1404), n3 as:




  
    θ
    critical
  
  =
  
    
      sin
      
        -
        1
      
    
    ⁢
    
      
        
          n
          1
        
        
          n
          3
        
      
      .
    
  




Preparation of nanocomposite films and hemispheres in this manner allows for the use of larger sized nanocrystals in comparison to those that can be used in conjunction with thicker pathlength films. For example, nanocrystals on the order of 5-7 nm could be used with the thin film/hemisphere embodiments of the present invention, while nanocrystals on the order of about 3-5 nm could be required for thicker pathlength nanocomposites.
As discussed throughout, the nanocrystals useful in the practice of the present invention can have a composition and a size such that they absorb light at a particular wavelength(s) and emit at a particular wavelength(s). In certain embodiments, the polymeric layers of the present invention can comprise combinations of nanocrystals that function in the various ways described herein. For example, a nanocomposite of the present invention can comprise nanocrystals having specific, enhanced emission properties, others having specific, enhanced absorption properties but low emission properties, and the entire nanocomposite can be constructed such that the layer has a specific refractive index that is matched or tailored for a specific purpose. Combined in such a way, the polymeric layers of the present invention can be used as encapsulants for active devices (e.g., LEDs) that emit light of a certain wavelength, filter out other wavelengths and have a refractive index appropriately matched to an active device and/or an additional substrate or coating.
IV. Size and Miscibility of Nanocrystals
In all embodiments of the present invention, it is desirable that the nanocrystals do not aggregate. That is, that they remain separate from each other in the polymeric layer and do not coalesce with one another to form larger aggregates. This is important, as individual crystals will not scatter light passing through the layer, while larger aggregated structures can create an opaque layer that can hinder the passage of light.
Suitably, whether functioning as down-converting layers, photon-filtering layers, refractive index matching layers, or combinations thereof, the nanocomposites of the present invention will scatter a minimal portion of light that enters the various layers. It is desirable that the nanocomposites of the present invention scatter as little light as possible, such that the layers are substantially transparent or clear.
As used herein, the phrase “scatter a minimal portion of light,” means that the amount of light that enters the various nanocomposites of the invention from the incident side (the side that light is entering) is transmitted such that less than about 50% of this incident light is scattered by the nanocomposite. In suitable embodiments, the amount of light that is scattered by the nanocomposite will be less than about 20%, less than about 15%, and approaching 0% of the light being transmitted. The factors that impact most significantly on the amount of light that is scattered by the nanocomposites are the size of the nanocrystals and their miscibility in the polymeric matrix, and hence their ability to remain separated. It should be understood that in applications of the present invention where the nanocomposites function as filters, the amount of light that is transmitted through the polymeric layer will necessarily be reduced as certain wavelengths or ranges of wavelengths will absorbed by the nanocrystals and filtered out of the incident light.
As discussed above, the size of the nanocrystals can be tailored by selecting specific semiconductor materials and then generating and processing the nanocrystals until the desired size is attained. In the various embodiments of the present invention, the nanocrystals will suitably be between about 1 nm and about 20 nm in size. More suitably, between about 1 nm and about 10 nm, between about 1 nm and about 4 nm and most suitably between about 1 nm and about 3 nm. As shown in FIG. 16, using a constant loading volume of ZnS nanocrystals (22% by volume) in silicone, the percent transmittance of light can be tailored from between about 5% to about 100% (i.e. percent that is scattered can be tailored from between about 95% to about 0%). It is a significant advantage of the present invention that by generating nanocrystals that are between about 1 nm to about 4 nm, less than about 50% of the incident light is scattered by the nanocomposites of the present invention. As shown in FIG. 16, by creating nanocrystals that are between about 1 nm and about 3 nm, scattering of less than 20%, approaching 15%, 10%, 5% and 0%, can be achieved. As demonstrated in FIG. 17, a silicone nanocomposite, comprising 3 nm ZnS nanocrystals and a layer with a 3 mm pathlength will scatter less than about 50% (i.e. transmit more than about 50%) of the incident light over the wavelength range 350 nm to 700 mn, scatter less than about 30% over the wavelength range 400 nm to 700 nm and scatter less than about 15% over the wavelength range 500 nm to 700 nm.
Controlled Surface Chemistry for High Loading Density Nanocomposites
In the formation of the nanocomposites of the present invention, two critical issues are: (1) achieving high miscibility of the nanocrystals in the host matrix, and (2) prevention of aggregation of the nanocrystals at a high concentration. Aggregation results in quenching of the emission, hence a lowering of the amount of light transmitted, as well as light scattering from the aggregates. Tuning the index of refraction of the overall composite layer also occurs at different nanocrystals loading densities. Since the nanocrystals have a refractive index of about 2.5 to about 3 and the host matrix is about 1.5 to about 2, matching of the refractive index of the LED substrate (typically sapphire or SiC) will eliminate an optical interface and losses from total internal reflection.
As part of this approach, several issues are addressed, including determination of whether the necessary loading densities in the nanocomposites as determined by simulations are achieved; whether the nanocrystals are homogenously embedded in the host matrix with no (or minimized) aggregation or phase separation so that a high quantum yield is retained and scattering is prevented; whether the index of refraction of the composite layer can be tuned by adjusting the loading density (e.g., gradient) of nanocrystals in the host matrix; whether refractive indices close to the LED substrate are achieved and what the projected effect on the light extraction efficiency is; and at the nanocrystal loading density for refractive index matching, what is the layer thickness of the composite layer necessary to reach an optical density at the excitation wavelength to yield the optimized emission profile determined by the simulations. It can also be determined whether this thickness is compatible with low cost, thin film processing (e.g., thicknesses <1-2 microns).
In order to accomplish this, a tailored, miscibility-enhancing ligand can be designed to bind, associate, coordinate, attach or conjugate to a nanocrystal, and to allow for controlled mixing and miscibility in the host matrix. The performance characteristics, including quantifying the effects on the internal quantum efficiency and light extraction efficiency are measured on nanocomposites of various loading densities and thicknesses.
Surface Chemistry Modification
Dispersion of nanocrystals in a host matrix can be controlled by minimizing phase separation and aggregation that can occur when mixing the nanocrystals into the matrixes. A basic strategy of the present invention is to design a novel 3-part ligand, in which the head-group, tail-group and middle/body-group can each be independently fabricated and optimized for their particular function, and then combined into an ideally functioning complete surface ligand (see FIG. 18; see FIG. 19 for an example ligand). As shown in FIG. 18, head group 1804 is selected to bind specifically to the semiconductor material of the nanocrystal (e.g., can be tailored and optimized for CdSe, ZnS or any other nanocrystal material). Tail group 1800 is designed to interact strongly with the matrix material and be miscible in the solvent utilized (and can, optionally, contain a linker group to the host matrix) to allow maximum miscibility and loading density in the host matrix without nanocrystal aggregation. Middle or body group 1802 is selected for specific electronic functionality (e.g., charge isolation).
This multipart ligand strategy has been used for the fabrication of high loading density, non-fluorescent, polymer-CdSe nanorod composites in the development of hybrid inorganic-organic nanocomposite solar cells. In certain embodiments of the present invention, significant modifications to the ligand are made due to differences in the two applications. Specifically, the ligand is designed to be charge insulating (rather than charge conducting) and to provide retention of nanocrystal photoluminescence as well as to be compatible with a completely different matrix type (inorganic rather than organic polymers) and nanocrystal material type and shape.
With the development of the 3-part ligand, control of the loading density of the nanocrystals in the nanocomposite can be achieved for purposes of creating the nanoparticle density gradients as described. This permits evaluation of the influence of quantum yield and optical scattering in the nanocomposite. Additionally, tuning of the refractive index of the nanocomposite is possible since the index of refraction of the host matrix is known.
A benefit of this modular approach is the ability to rapidly evaluate new tail, head, and middle/body groups. In the area of head groups (binding with the nanocrystal), there are available methods developed for the development of CdSe synthetic techniques. This includes an understanding of the binding of nanocrystals with phosphonic acids, amines, thiols, phosphines, and phosphine oxides.
A tailored ligand can be optionally designed to bind strongly to the nanocrystal and to allow for mixing in a TiO2 host medium. The new ligand allows for dispersion control (solubility and processability) to facilitate incorporation of the nanocrystals into solvents or host matrixes over a wide range of loading densities as necessary to achieve the optimal white light device performance characteristics and refractive index matching to the blue-LED.
Ligand Synthesis
The ligand molecule can be synthesized using a generalized technique allowing three separate groups to be synthesized separately and then combined. Head groups of phosphonic acid, amines, carboxylic acids or thiol moieties can be used because of their affinity for the nanocrystal surface. Tail groups can contain terminal hydroxyl groups to tether the nanocrystal in a titania sol-gel matrix, or silicon groups to match a silicone polymer matrix. The middle/body unit is selected for charge insulation (e.g., large energy gap for both electrons and holes), and possible targets are identified using computer modeling. The modeling is performed using a Density Functional Theory (DFT) to model the bandgap of various target molecular structures for ligand design. The confirmation of the chemical identity and purity will be done using mass spectrometry, NMR and FTIR analysis.
The insulating group (middle/body unit) of the ligands can be selected from long-chain alkanes of various lengths and aromatic hydrocarbons, e.g., C6-C22 alkanes. Selection of the length of the body unit will depend on the desired characteristics of the final matrix and of the polymeric base being used. For example, in applications where it is desired that the matrix possess rheologic or other properties (e.g., mechanical/electical) similar to that of the polymeric base substance, a shorter chain (e.g., C6-C18) body unit can be selected. For example, the use of a C12 body unit-based ligand on ZnS nanocrystals allows for increased loading of the ZnS nanocrystals at a ratio sufficient to achieve a refractive index of 1.7070 in a base of immersion oil (starting refractive index 1.5180), while still maintaining the grease like consistency of the oil. The use of shorter chain ligands allows for a lower volume fraction of nanocrystals to be used to achieve the same refractive index when compared to nanocrystals with longer chain ligands.
In other applications, the use of longer chain ligands (e.g., C18-C22) can be used when it is desired that the final matrix possess properties closer to that of the ligand itself, rather than the base material. In certain applications, the ligand itself could be used to form the matrix material. Longer chain ligands also allow for additional spacing between the nanocrystals to keep them from aggregating in the base substrate.
FIG. 19 shows an example of a ligand with a phosphonic acid head group 1900, an aliphatic tail 1902 and an aromatic hydrocarbon body 1904. Appropriate choice of body and/or tail components are used to provide like functionality to the matrix to afford high concentrations of nanocrystals in siloxane polymer matrices. Refractive index (RI) tuning/matching can also be affected by the ligand. Independent tuning of the tail or body components of the ligand to obtain a particular RI match with the polymer matrix can be achieved by varying the ligand chemistry appropriately.
The general design “Head-Body-Tail” affords freedom from any particular ensemble limitations. For example: a phosphonate head group for nanocrystal binding, alkane body group for length adjustment/nanocrystal spacing and dimethyl silicone tail for silicone matrix compatibility can be synthesized as shown in FIG. 20b. An example of tuning the RI (increasing the value) can be realized by incorporation of phenyl groups shown in FIG. 20a (similar to silicone polymers (from vendor Gelest Inc., 612 William Leigh Drive Tullytown, Pa. 19007-6308): DMS-H21 dimethylsiloxane vs. HPM-502 phenyl-methylsiloxane, 1.403 and 1.500 refractive index values, respectively) in the siloxane tail. FIG. 20a illustrates several non-limiting example ligands with head-body-tail design. Matrix compatibility adjustments such as branching siloxane units can also be accommodated (FIG. 20b, molecule 3). Structure verification by NMR of synthesized precursors 1 and 2 in FIG. 20b is shown in FIGS. 20c-f. 
FIG. 20g shows additional examples of ligands and synthesis schemes in accordance with the present invention, including (from top to bottom) the use of trisiloxane, cyclic tetrasiloxane and branched siloxane tail groups in the generation of ligands. Carboxylic acid functionalization of these silicone surfactants is illustrated in FIG. 20g. Structure verification by NMR of a carboxylated trisiloxane ligand shown in FIG. 20g (ligand at top of page) is represented in FIGS. 20h-i. 
FIGS. 20j and 20n show further examples and synthesis schemes for the production of phosphonate functionalized silicone ligands. Structure verification by NMR of the bromide precursor shown in FIG. 20j is represented in FIG. 20k. FIGS. 20l and 20m represent NMR verification of the structure of the phosphonate ligand product.
Ligand exchange to displace the surfactants, which are used during synthesis, can be done by mass action exchange in solution. The nanocrystals and the new ligand are co-dissolved in solvent and allowed to react for a specified time at elevated temperatures. The product is precipitated with alcohol to remove any excess unbound ligands and to remove the displaced synthesis surfactants. The attachment is confirmed by NMR analysis by redissolving the product into a deuterated NMR compatible solvent. Complexation of the ligand with the nanocrystal causes a shift and broadening in the spectrum compared to the free unbound molecule due to a hindered rotation.
In another embodiment, the present invention provides polymeric layers, comprising a polymer; and semiconductor nanocrystals embedded within the polymer, wherein the nanocrystals have miscibility-enhancing ligands conjugated to their surface, and wherein the ligands comprise an alkane chain of between 6 and 18 carbons in length. In suitable embodiments, the ligands can comprise an alkane chain of between 12 and 18 carbons in length. The polymer will suitably be silicone, and the semiconductor nanocrystals will suitably have a size between about 1-10 nm, and in certain embodiments will be ZnS nanocrystals. In certain embodiments, the polymeric layers will scatter a minimal portion of light that enters said polymeric layer. Suitably, the layer will be greater than about 0.5 mm in thickness.
V. Processes for Producing Nanocomposites
In another embodiment, as represented in FIG. 21, the present invention provides processes for preparing polymeric layers, comprising (a) mixing semiconductor nanocrystals at a first density with a solvent and a polymer to form a first mixture (2100), (b) coating a substrate material with the first mixture (2102), and (c) evaporating the solvent to form the polymeric layer (2104), wherein the polymeric layer has an effective refractive index of n1.
In suitable embodiments, the processes of the present invention can be used to coat active devices or optical devices. As discussed throughout, nanocrystals useful in the processes of the present invention can comprise miscibility-enhancing ligands conjugated, coordinated, attached, bound or otherwise associated to their surface. Any of the various types of nanocrystals discussed herein can be used in the processes of the present invention. For example, high emission nanocrystals, low emission/high absorption nanocrystals and low emission/low absorption nanocrystals can be used. In certain embodiments, two or more different types of nanocrystals can be mixed with the solvent and polymer, thereby creating a composite that has several or all of the properties described herein. Refractive index matching applications can utilize any of the nanocrystals discussed throughout, depending on if the nanocomposite is also required to function as a down-converting layer or a filtering layer. In other applications, nanocrystals that have low emission/low absorption properties are useful in refractive index matching applications where refractive index effects only are desired.
In other embodiments, as shown in FIG. 21, the processes of the present invention can further comprise (d) mixing semiconductor nanocrystals at a second density with a solvent and a polymer to form a second mixture (2106), (e) coating the substrate material with the second mixture (2108), and (f) evaporating the solvent to form a second polymeric layer (2110), wherein the second polymeric layer has an effective refractive index of n2.
In other embodiments, the processes of the present invention can further comprise repeating steps (d) through (f) with a third through ith density of semiconductor nanocrystals to produce third through ith polymeric layers, wherein the third through ith polymeric layers have effective refractive indices, n3 through ni, respectively (2112). As used herein, “i” refers to an integer. The present invention encompasses processes for producing polymeric layers which comprise any number of separate layers used to produce an overall layer, coating, or encapsulant. Each individual layer, 1 through i, can comprise a different density of nanocrystals, nanocrystals of a different composition (i.e., high emission or high absorptive properties), and nanocrystals of different sizes. As such, each layer can have a different effective refractive index and can have multiple and/or different properties and characteristics.
By providing individual polymeric layers each with a potentially different effective refractive index, an overall polymeric layer (e.g., an encapsulating layer) can be generated that has a nanocrystal density gradient throughout the overall layer, and also an effective refractive index gradient throughout the overall layer. FIG. 22 illustrates that the effective refractive index of the 1st layer, n1 (2200), will be greater than any other layer (2202, 2204, 2206), and the effective refractive index of the ith layer, ni (2206), will be less than any other layer (2200, 2202, 2204). It should also be noted that the processes of the present invention can be performed in the reverse order, i.e., where the nanocrystal density and thus the effective refractive index of the ith layer is higher than any other layer, and the effective refractive index of the first layer prepared, n1, is less than any other layer. In other embodiments, the density and effective refractive index of the individual layers can be the same, or can be prepared in such a manner that the overall effective refractive index of the polymeric layer varies throughout the layer, rather than in a graded fashion, as in FIG. 22.
As discussed throughout, various known processes can be used to coat a substrate material with the polymeric layers of the present invention, as would become apparent to people having ordinary skill in the art and based on the description herein. Suitable coating processes include, but are not limited to, spin coating and screen printing.
In general, spin coating consists of four stages. The first stage is the deposition of the coating fluid onto the substrate. It can be done using a nozzle that pours the coating solution out, or can be sprayed onto the surface, etc. Usually this dispense stage provides a substantial excess of coating solution compared to the amount that will ultimately be required in the final coating thickness. The second stage is when the substrate is accelerated up to its final, desired, rotation speed. The second stage is usually characterized by aggressive fluid expulsion from the substrate surface by the rotational motion. Ultimately, the substrate reaches its desired speed and the fluid is thin enough that the viscous shear drag exactly balances the rotational accelerations. The third stage is when the substrate is spinning at a constant rate and fluid viscous forces dominate fluid thinning behavior. This stage is characterized by gradual fluid thinning. Mathematical treatments of the flow behavior show that if the liquid exhibits Newtonian viscosity (i.e., is linear) and if the fluid thickness is initially uniform across the substrate (albeit rather thick), then the fluid thickness profile at any following time will also be uniform, leading to a uniform final coating. The fourth stage is when the substrate is spinning at a constant rate and solvent evaporation dominates the coating thinning behavior. As the prior stage advances, the fluid thickness reaches a point where the viscosity effects yield only rather minor net fluid flow. At this point, the evaporation of any volatile solvent species will become the dominant process occurring in the coating.
In another embodiment, the processes of the present invention can further comprise centrifuging the mixture produced in step 2100 to form a nanocrystal density gradient within the mixture prior to the coating in 2102. The use of centrifugation creates a gradient within the polymeric layer as nanocrystals separate in accordance with their inertia. Various centrifugation speeds or accelerations can be used to produce the nanocrystal density gradient in the polymeric layers and can readily be determined by those skilled in the art. The centrifugation speed selected depends on the size of the nanocrystals and the difference in density between the nanocrystals and the polymer solution prior to polymerization, and the centrifugal approach. Centrifugation can be for a short time at high speed and generate a gradient kinetically where the centrifugation step is timed based on a calculated or measured centrifugation rate. Alternatively, an equilibrium approach can be used where the flux of the nanocrystals toward the bottom of a centrifuge tube is matched to the flux of nanocrystals toward the top of the tube (due to diffusion). The diffusional flux is proportional to the concentration gradient of the nanocrystals. Suitably, accelerations can be in the range of a few hundred times g to 100,000 times g, where g is the acceleration due to gravity (9.8 m/s2) By selecting nanocrystals of different sizes and made from different materials, the nanocrystals will spread out through the polymeric layer according to their inertia in response to the centrifugation and generate a gradient in the layer. Any other process known to those skilled in the art to generate gradients within polymers may also be used to create the polymeric layers of the present invention.
In optical lenses, the optical path length varies with distance from its center, where optical path length is defined as the product of the physical path length, thickness, and the refractive index, n, of the lens material. In the most common lenses, the refractive index, n, is fixed and the thickness, varies. However, a lens can also be created by keeping the thickness, constant and varying the refractive index as a function of distance from the axis of the lens. Such a lens is called a Graded Index lens, or sometimes abbreviated as a GRIN lens. The methods of the present invention can also be used to create GRIN lenses. Polymer/nanocrystal blends can be used to make GRIN lenses due to the dramatic refractive index difference between nanocrystals (e.g., ZnS about 2.35) and optical plastics such as poly(methyl methacrylate) (PMMA) (refractive index about 1.45). With normal glass, a difference of about 0.05 refractive index units is achievable over about 8 mm. Utilizing the methods and processes of the present application, a difference of about 0.20 refractive index units over about 8 mm can be achieved to make much more powerful lenses.
In such embodiments, a gradient pump can be used to inject a solution containing polymer monomers and nanocrystals into the center of a mold, and then nanocrystal concentration can be varied during the fill. The lens can then be cured and removed.
The polymeric nanocomposites of the present invention can be used in any application where the down-conversion, filtering, and/or refractive index characteristics of the composites are desired. Non-limiting examples of applications of polymeric nanocomposites with increased refractive indexes include:
Super High Gloss Coatings: Increasing the refractive index of a transparent coating increases gloss. The addition of nanocrystals (e.g., ZnS nanocrystals) to polymeric coatings such as waxes and other coatings (e.g., car waxes, shoe waxes, floor coatings and related products) would increase the amount of light that is reflected from the coated surface and thus increase the glossiness of its appearance. Appropriate ligands, including C18, PEG and others discussed throughout could be used so as to allow the nanocrystals to be formulated with various polymers, waxes and coatings.
Plastic Eye Glass Lenses and Contacts: The thickness of a lens is proportional to the refractive index of the material of which it made. The higher the refractive index, the thinner the lens. Normal glass has a refractive index of about 1.523 while an example plastic, such as CR39, has refractive index of 1.49. A plastic lens, although lighter in weight, is thicker than a glass lens of equivalent power.
By incorporating nanocrystals, for example ZnS nanocrystals suitably with the appropriate ligands, into a plastic lens, the refractive index can be increased beyond the level of glass to make ultra-thin lenses. In applications such as contact lenses, there is an even more pressing need to create thin lenses due to the importance of oxygen transport through the lens to the eye. The refractive index of contact lenses are about 1.40. The addition of even a small percentage of nanocrystals (e.g., about 10% ZnS) would increase the refractive index to about 1.5, therefore allowing for thinner lenses. Ligands such as those discussed throughout can be used to lock the nanocrystals in place in the polymeric layer. The addition of nanocrystals with specific absorptive properties, e.g., ultraviolet (UV) absorbing nanocrystals, would allow for the creation of UV (or other wavelength) blocking lenses.
EXAMPLES
The following examples are illustrative, but not limiting, of the method and compositions of the present invention. Other suitable modifications and adaptations of the variety of conditions and parameters normally encountered in nanocrystal synthesis, and which would become apparent to those skilled in the art, are within the spirit and scope of the invention.
Example 1
Core/shell Nanocrystal Synthesis
Suitable nanocrystal synthesis procedures include fabricating nanocrystal samples with specific spectral characteristics matched to those prescribed by the theoretical models of the present invention. This can include fabricating nanocrystals with tunable sizes and size distributions (e.g., sizes ranging from 1-20 nm in diameter producing emission peak wavelengths tunable between 460 and 640 nm with FWHM tunable from about 15 to about 100 nm). This in turn is used to synthesize nanocrystal mixtures identified by simulations that have the optimal emission characteristics. The simulation and core/shell nanocrystal procedure is typically performed in an iterative process.
Type I core-shell nanocrystals of CdSe/ZnS (core/shell) can be synthesized by a two step process using a solution phase method, first with the fabrication of the core material followed by growth of the shell.
Core Synthesis
Stock solutions are prepared of Se powder dissolved in tri-n-butylphosphine (TBP), and Cd(CH3)2 dissolved in TBP. In an air-free environment, the Cd stock solution is added drop-wise to a mixture of trioctylphosphine oxide (TOPO), and trioctylphosphine (TOP), which was previously degassed at 120° C. The temperature is raised to 300° C., followed by a quick injection of the Se precursor. After injection, the temperature drops to around 260° C., which is held constant for a period of time to control the size of the particle. By controlling the temperature profile and starting reagents and conditions, the center-wavelength and size-distribution can be tuned independently. The identity of the product is confirmed using XRD and TEM analysis.
Shell Synthesis
Core CdSe nanocrystals are dispersed in TOPO and TOP to which a mixture of ZnEt2 and (TMS)2S will be added at a temperature between 140° C. to 220° C. ZnS shell coating thickness will be varied by changing precursor ratios and growth temperatures to obtain a uniform surface coverage and to improve the quantum efficiency. The confirmation of shell growth will be done using XRD, EDX and TEM analysis.
The optical properties of the individual nanocrystals are characterized by measurement of the UV-Vis absorption and photoluminescence spectra using a commercial UV-Vis spectrophotometer and a fluorometer. The excitation wavelength is matched to the blue LED (about 460 nm). Internal quantum efficiency of the nanocrystals in solution are calculated using internal reference standards. Nanocrystal component mixtures (solution phase) are formed by mixing the appropriate concentration ratios to match the predictions from the theoretical model. The emission and absorption information of these actual mixtures is then back-fed as an input into the simulation to validate (and to refine, if necessary) the model.
The output of this procedure is a solution-phase mixture of nanocrystals that has the appropriate composition to produce white light with CRI and CTT matching that of the theoretical model when illuminated with blue excitation and total down-conversion efficiency comparable to that predicted by the model, assuming zero loss to other mechanisms in the process.
While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
Example 2
ZnS Nanocrystal Synthesis
In the order listed, add the following to a 50 mL 3-neck round bottom flask:
1. Zn(acetate)2: 76.5 mg Lot# 12727BC
2. Stearic Acid: 484 mg Lot# 06615MA
3. Tri-n-octylphosphine oxide (TOPO): 4.07 g Lot# 21604LA
In a glove box prepare the following:
3.9 g of distilled tri-n-octylphosphine (TOP) (#35-111) in 5 mL syringe;
116.4 mg of stock solution 02-190 (bis(trimethylsilyl)sulfide (TMS2S):TOP) in 1 mL syringe; and
One 40 mL septa cap vial with 5.0 mL of MeOH
Place reactor under vacuum
Heat to 120° C.
Once at 120° C., allow to sit for 20 minutes
Place reactor under argon
Slowly inject TOP from 5 mL syringe
Change set point temperature to 250° C.
Once at 250° C., immediately inject the stock solution 02-190 (bis(trimethylsilyl)sulfide (TMS2S):TOP) from 1 mL syringe
Grow with temperature at 250° C. for 2 minutes
Remove the heating mantle and allow reaction to cool to 50° C.
At 50° C., use a syringe to remove the growth solution and inject it into the 40 mL vial with MeOH.
FIG. 23 shows an X-Ray diffraction scan of ZnS nanocrystals produced according the present invention. The scan shows the presence of Zinc Sulfide with a mixture of wurtzite and zinc blend (spaelite) crystals.
FIG. 24 shows a Transmission Electron Micrograph (TEM) of ZnS nanocrystals (about 4 nm diameter) produced according the present invention.
The ZnS nanocrystals can be produced using any chain length hydrocarbon, for example C6-C22 alkane, depending on the application and desired properties.
Example 3
Carboxylic Acid-Silicone Ligand Synthesis
General Methods
All manipulations were carried out with strict exclusion of air and moisture by using Schlenk technique under an atmosphere of dry nitrogen, unless otherwise stated. THF, toluene, chloroform-d1 and toluene-d8 were dried over activated 4 A Molecular Sieves and de-gassed by three freeze-pump-thaw cycles. 4-pentenoic acid and 1,1,1,3,5,5,5-heptamethyltrisiloxane were purchased from Aldrich (St. Louis, Mo.), distilled and stored in a storage flask using Schlenk technique before use. Heptamethyl cyclotetrasiloxane and 1,1,1,3,3,5,5-heptamethyl trisiloxane were purchased from Gelest (Morrisville, Pa.), distilled and stored in a storage flask using Schlenk technique before use. Karstedt's catalyst or platinum divinyl tetramethyl disiloxane complex, 2.1 to 2.4% in xylenes, was purchased from Gelest, stored in the glove box and used without further purification. All products were stored in the glove box. NMR chemical shift data were recorded with a Bruker FT NMR at 400 MHz for 1H, 100 MHz for 13C{1H}, 162 MHz for 31P{1H} and 79.5 MHz for 29Si{1H} and are listed in ppm.
General Synthesis Procedure (See FIG. 20g)
Synthesis of HO2C(CH2)4(SiMe2O)2SiMe3 
In a glove box, the following reaction was set up in a 100 mL Schlenk flask by addition of Karstedt's catalyst (2.66 g solution, 0.300 mmol) followed by dilution in THF, 60 mL, on the Schlenk line. Then to the clear colorless solution, 1,1,1,3,3,5,5-heptamethyltrisiloxane (8.13 mL, 6.67 g, 30.0 mmol) was added by syringe over about 90 seconds and in about 30 seconds turned the solution clear green. The solution was stirred at room temperature for about 15 minutes. Then, with the reaction flask surrounded by a room temperature water bath, 4-pentenoic acid (3.07 mL, 3.00 g, 30.0 mmol) was added by syringe over about 90 seconds which slowly turned the solution light brown and produced a small amount of heat. After about 2 hours, the water bath was heated to 35° C. using a heater controlled by a thermostat and stirred overnight.
The volatiles were removed from the clear brown solution by rotational evaporator leaving an opaque brown oil. The product was distilled from the mixture by short path apparatus collecting the fraction with vapor temperature between 80 and 95° C. and pressure<20 mtorr. The product is a clear colorless oil (5.55 g or approximately 17 mmol and 57% yield) that typically contains about 50% acid and 50% anhydride. Complete conversion to the acid was accomplished by dissolution of the product mixture (2.00 g or approximately 6.2 mmol) in acetonitrile, 40 mL, followed by addition of pyridine (5.00 mL, 5.11 g, 64.6 mmol) and water (6.20 mL, 6.20 g, 33.4 mmol). The solution was stirred overnight in air. The volatiles were removed from the solution by rotational evaporator until the residue was reduced to an oil. Next, toluene, 100 mL, was added and the volatiles removed by rotational evaporator until the residue was reduced to an oil. The water removal using toluene azeotrope was performed twice. The resulting clear colorless oil was transferred into a beaker producing a layer about 3 mm thick and the product dried in a desiccator over phosphorous pentoxide under static vacuum of <10 mtorr overnight. The product was a clear colorless oil (1.35 g, 4.18 mmol, 67% yield) and was stored in the glove box.
Additional carboxylic acid-silicone ligands such as those shown in FIG. 20g and disclosed throughout the present specification, can be prepared using a procedure similar to that above.
Analysis of HO2C(CH2)4(SiMe2O)2SiMe3 1H NMR (chloroform-d1, δ): 0.10, 0.13, 0.14 (s, SiMe), 0.52, 1.39, 1.67 (m, CH2), 2.35 (t, 2H, CH2). 13C{1H} NMR (chloroform-d1, δ): 1.5, 2.0, 2.0 (s, SiMe), 18.1, 23.1, 28.5, 34.1 (s, CH2), 180.5 (s, C═O). 29Si{1H} (1:1 CDCl3/Et3N, 0.02 M Cr(acac)3, δ): −20.9, 7.1 (s, 1:2). IR (cm−1, diamond): 1050 s (Si—O—Si), 1700 m (C═O), 3030 w (CH aromatic), 2956 sh, 2928 s, 2854 m (CH aliphatic), 3400 to 2700 v br (acid). Mass Spec ESI (m/z): 345 (MNa+).
Data for Synthesis and Analysis of HO2C(CH2)4SiMeO(SiMe2)3 (cyclic tetrasiloxane)
The boiling point of the anhydride/acid mixture was 95 to 110° C. at a pressure of <10 mbar. The yield for synthesis of the acid/anhydride mixture was about 64% and the conversion to acid was 63%.
1H NMR (chloroform-d1, δ): 0.10, 0.12, 0.13 (s, SiMe), 0.48, 1.39, 1.65 (m, 2H, CH2), 2.35 (t, 2H, CH2). 13C{1H} NMR (chloroform-d1, δ): −0.1, 1.9, 2.0 (s, SiMe), 17.5, 22.9, 28.3, 34.1 (s, CH2), 180.4 (s, C═O). 29Si{1H} (1:1 CDCl3/Et3N, 0.02 M Cr(acac)3, δ): −20.3, −19.1, −19.0 (s, 1:2:1). IR (cm−1, diamond): 1050 s (Si—O—Si), 1700 m (C═O), 3030 w (CH aromatic), 2956 sh, 2928 s, 2854 m (CH aliphatic), 3400 to 2700 v br (acid).
Data for Synthesis and Analysis of HO2C(CH2)4SiMe(OSiMe3)2 
The boiling point of the anhydride/acid mixture was 78 to 95° C. at a pressure of <10 mbar. The yield for synthesis of the acid/anhydride mixture was 63% and the conversion to acid was 62%.
1H NMR (chloroform-d1, δ): 0.10, 0.12, 0.13 (s, SiMe), 0.53, 1.43, 1.68 (m, 2H, CH2), 2.35 (t, 2H, CH2). 13C {1H} NMR (chloroform-d1, δ): 0.9, 1.0 (s, SiMe), 16.9, 22.7, 28.1, 34.0 (s, CH2), 180.0 (s, C═O). 29Si{1H} (1:1 CDCl3/Et3N, 0.02 M Cr(acac)3, δ): −22.0, −7.1 (s, 1:2). IR (cm−1, diamond): 1050 s (Si—O—Si), 1700 m (C═O), 3030 w (CH aromatic), 2956 sh, 2928 s, 2854 m (CH aliphatic), 3400 to 2700 v br (acid). Mass Spec ESI TOF 381 (MH+) and ESI TOF 379 (M−H).
Example 4
Phosphonic Acid—Silicone Ligand Synthesis
General Synthesis Procedure
Synthesis of (EtO)2P(O)(CH2)4(SiMe2O)2SiMe3 
In a glove box, Karstedt's catalyst (0.450 g solution, 0.052 mmol) was added to a 250 mL Schlenk flask. On the Schlenk line, THF, 100 mL, was added and followed by 1,1,1,3,5,5,5-heptamethyl trisiloxane (14.0 mL, 11.5 g, 51.8 mmol) by syringe over about 90 seconds. The clear colorless solution turned clear green in about 30 seconds. The reaction solution was stirred for about 15 minutes before addition of diethyl 3-butenyl phosphonate (10.0 mL, 9.95 g, 51.8 mmol) by syringe over about 90 seconds. The reaction solution then slowly turned light brown and produced a small amount of heat. After about 2 hours, the reaction flask was surrounded by a thermostat controlled water bath that was heated to 35° C. The reaction solution was heated overnight.
The volatiles were removed from the clear brown solution by rotational evaporator leaving an opaque brown oil. A column was packed with silica (230-400 mesh) in hexanes that was 30 mm in diameter and 150 mm long. After placing the crude product on the column, the column was eluted with hexane, 250 mL, followed by a mixed solvent of 1:1 ratio of ethyl acetate to hexane, 1500 mL. The elutant was collected in one fraction. Next the volatiles were then removed by rotational evaporator leaving a light brown oil. The product was then distilled using a simple distillation at pressure of <20 mtorr and pot temperature of 120° C. The product was a clear colorless oil (17.6 g, 42.5 mmol, 82.1% yield).
Additional phosphonic acid-silicone ligands such as those shown in FIGS. 20a, 20j and 20n and disclosed throughout the present specification can be prepared using a procedure similar to that above.
Analysis of (EtO)2P(O)(CH2)4(SiMe2O)2SiMe3 1H NMR (chloroform-d1, δ): 0.00 (s, 6H, SiMe), 0.05 (s, 6H, SiMe), 0.07 (s, 9H, SiMe), 0.53, 1.39, 1.60, 1.70 (m, 2H, CH2), 1.30 (t, 6H, CH2CH3), 4.06 (m, 4H, CH2CH3). 13C {1H} NMR (chloroform-d1, δ): −0.34, 1.46, 2.01 (s, SiMe), 16.68, 61.52 (d, JP—C=6 Hz, CH2CH3O), 18.05 (s, CH2), 24.62 (d, JP—C=18 Hz, CH2), 26.19 (d, JP—C=5 Hz, CH2), 25.69 (d, JP—C=140 Hz, CH2P). 31P {1H} NMR (chloroform-d1, δ): 32. 29Si{1H} (1:1 CDCl3, 0.02 M Cr(acac)3, δ): −22.00, 7.12 (s, 1:2). IR (cm−1, diamond): 1030 (s, Si—O—Si), 1260 (m, Si—Me), 1380, 1400 1430 (w, Et-O—P).
Data for Synthesis and Analysis of (EtO)2P(O)(CH2)4SiMe(OSiMe3)2 
The pot was heated to 120° C. at a pressure of <20 mtorr to distill the product as a clear colorless oil in 81% yield.
1H NMR (chloroform-d1, δ): −0.32 (s, 3H, SiMe), 0.06 (s, 18H, SiMe), 0.44, 1.37, 1.60, 1.70 (m, 2H, CH2), 1.30 (t, 6H, CH2CH3), 4.08 (m, 4H, CH2CH3). 13C {1H} NMR (chloroform-d1, δ): −0.15, 2.01 (s, SiMe), 16.65, 61.49 (d, JP—C=6 Hz, CH2CH3O), 17.38 (s, CH2), 24.48 (d, JP—C=18 Hz, CH2), 25.97 (d, JP—C=5 Hz, CH2), 25.71 (d, JP—C=140 Hz, CH2P). 31P {1H} NMR (chloroform-d1, δ): 33. 29Si{1H} (1:1 CDCl3, 0.02 M Cr(acac)3, δ): −17.96, 9.94, 10.00 (s, 1:1:1). IR (cm−1, diamond): 1030 (s, Si—O—Si), 1250 (m, Si—Me), 1380, 1400, 1430 (w, Et-O—P).
Data for Synthesis and Analysis of (EtO)2P(O)(CH2)4SiMeO(SiMe2)3 (Cyclic Tetrasiloxane)
For the distillation the vapor temperature was 84 to 96° C. at a pressure of <10 mtorr. The product was isolated as a clear colorless oil in 44% yield.
1H NMR (chloroform-d1, δ): 0.50, 0.70 (s, 21H total, SiMe), 0.51 1.41, 1.61, 1.69 (m, 2H each, CH2), 1.30 (t, 6H, CH2CH3), 4.08 (m, 4H, CH2CH3). 13C {1H} NMR (chloroform-d1, δ): −0.57, 0.91, 0.94 (s, SiMe), 16.66, 61.50 (d, JP—C=6 Hz, CH2CH3O), 16.86 (s, CH2), 24.29 (d, JP—C=18 Hz, CH2), 25.88 (d, JP—C=5 Hz, CH2), 25.70 (d, JP—C=140 Hz, CH2P). 31P {1H} NMR (chloroform-d1, δ): 33. 29Si{1H} (1:1 CDCl3, 0.02 M Cr(acac)3, δ): −20.39, −19.17, −19.08 (s, 1:2:1). IR (cm−1, diamond): 1015, 1050 (s, Si—O—Si), 1250 (m, Si—Me), 1380, 1400, 1430 (w, Et-O—P).
General Synthesis Procedure for the Phosphonic Acid, (HO)2P(O)(CH2)4(SiMe2O)2SiMe3 
In a 50 mL Schlenk flask, CH2Cl2, 15 mL, was added followed by (EtO)2P(O)(CH2)4(SiMe2O)2SiMe3 (1.00 g, 2.42 mmol) and the solution stirred until homogenous. Then trimethylsilyl bromide (0.671 mL, 0.778 g, 5.08 mmol) was added and the solution was stirred for 15 minutes.
The volatiles were removed by vacuum transfer and 10.0 mL of methanol was added followed by 0.25 mL of water. After stirring for 30 minutes, the volatiles were removed by vacuum transfer and 10.0 mL of toluene was added and the solution was stirred for 1 minute. The volatiles were removed by vacuum transfer and 10 ml of toluene was added, the solution stirred and the volatiles removed again, as before. The product was a slightly cloudy viscous oil.
Analysis of (HO)2P(O)(CH2)4(SiMe2O)2SiMe3 ESI (m/z): 359 (MH+) and 381 (MNa+).
All publications, patents and patent applications mentioned in this specification are indicative of the level of skill of those skilled in the art to which this invention pertains, and are herein incorporated by reference to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference.

## Claims

1. A polymeric layer in combination with an optical device, comprising:
(a) a polymer;
(b) semiconductor nanocrystals;
(c) a ligand composition attached to the nanocrystals wherein the ligand composition is different from the polymer, wherein the nanocrystals with ligand composition attached thereto are mechanically embedded within said polymer, the ligand composition comprising a body structure comprising an alkane chain of 4 carbons or between 6 and 22 carbons in length, a head group chemically coupled to the nanocrystals and to the body structure at a first position on the alkane chain and a tail group chemically coupled to the body structure at a second position on the alkane chain, wherein the tail group comprises one or more silicon groups or one or more terminal hydroxyl groups; and
(d) an optical device, wherein said polymeric layer encapsulates the optical device and scatters less than about 50% of incident light that enters said layer from said optical device.

2. The polymeric layer of claim 1, wherein said nanocrystals have a size and a composition such that said nanocrystals absorb ultraviolet light.

3. The polymeric layer of claim 1, wherein said nanocrystals have a size and a composition such that said nanocrystals absorb near-infrared light.

4. The polymeric layer of claim 1, wherein said nanocrystals have a size and a composition such that said nanocrystals absorb infrared light.

5. The polymeric layer of claim 1, wherein said optical device is a refractive lens or a reflective element.

6. The polymeric layer of claim 1, wherein said optical device is a light emitting diode.

7. The polymeric layer as in claim 1, wherein said polymer is silicone.

8. The polymeric layer as in claim 1, wherein a size of said nanocrystals is between about 1-10 nm.

9. The polymeric layer of claim 8, wherein said size of said nanocrystals is between about 1-4 nm.

10. The polymeric layer of claim 8, wherein said size of said nanocrystals is between about 1-3 nm.

11. The polymeric layer as in claim 1, wherein said polymeric layer has an effective refractive index of about 1.5.

12. The polymeric layer of claim 11, wherein said polymeric layer has an effective refractive index of about 1.8.

13. The polymeric layer as in claim 1, wherein said layer is greater than about 0.5 mm in thickness.

14. The polymeric layer of claim 1, wherein said nanocrystals absorb blue light.

15. The polymeric layer of claim 1, wherein said nanocrystals absorb green light.

16. The polymeric layer of claim 1, wherein said nanocrystals absorb red light.

17. A polymeric layer in combination with an active device, wherein the polymeric layer encapsulates the active device, said active device having an effective refractive index, n1, said layer comprising:
(a) a polymer;
(b) semiconductor nanocrystals;
(c) a ligand composition attached to the nanocrystals wherein the ligand composition is different from the polymer, wherein the nanocrystals with the ligand composition attached thereto are mechanically embedded within said polymer, the ligand composition comprising a body structure comprising an alkane chain of 4 carbons or between 6 and 22 carbons in length, a head group chemically coupled to the nanocrystals and to the body structure at a first position on the alkane chain and a tail group chemically coupled to the body structure at a second position on the alkane chain, wherein the tail group comprises one or more silicon groups or one or more terminal hydroxyl groups; and
(d) an active device, wherein said polymeric layer having an inner boundary in contact with said active device and an outer boundary in contact with a medium having an effective refractive index, n2, wherein said polymeric layer has an effective refractive index less than or equal to n1 at said inner boundary and an effective refractive index greater than or equal to n2 at said outer boundary.

18. The polymeric layer of claim 17, wherein n1 is greater than n2.

19. The polymeric layer of claim 17, wherein said layer has a nanocrystal density gradient, said gradient being highest at said inner boundary and lowest at said outer boundary.

20. The polymeric layer of claim 19, wherein said nanocrystal density gradient is substantially linear throughout said layer.

21. The polymeric layer of claim 17, wherein said active device is a light emitting diode.

22. The polymeric layer of claim 17, wherein said nanocrystals have a size between about 1-3 nm.

23. A polymeric layer in combination with an optical device, comprising:
(a) a polymer;
(b) semiconductor nanocrystals;
(c) miscibility-enhancing ligands conjugated to a surface of the nanocrystals wherein the ligands are different from the polymer, wherein the nanocrystals with ligands conjugated thereto are mechanically embedded within said polymer and wherein said ligands comprise a body structure comprising an alkane chain of between 18 and 22 carbons in length, a head group chemically coupled to the nanocrystals and to the body structure at a first position on the alkane chain and a tail group chemically coupled to the body structure at a second position on the alkane chain, and
(d) an optical device, wherein said polymeric layer encapsulates the optical device and scatters less than about 50% of incident light that enters said layer from said optical device.

24. The polymeric layer of claim 23, wherein said semiconductor nanocrystals have a size between about 1-3 nm.

25. The polymeric layer of claim 23, wherein said layer scatters less than about 20% of light that enters said polymeric layer.

26. The polymeric layer of claim 23, wherein said layer scatters less than about 15% of light that enters said polymeric layer.