WO1998059015A1

WO1998059015A1 - Highly emissive white phosphor compounds

Info

Publication number: WO1998059015A1
Application number: PCT/US1998/011855
Authority: WO
Inventors: Michael J. Sailor; William H. Green; Khoa P. Le
Original assignee: The Regents Of The University Of California University Of California At San Diego
Priority date: 1997-06-20
Filing date: 1998-06-19
Publication date: 1998-12-30
Also published as: AU8062298A

Abstract

Highly emissive broadband phosphores with external quantum yields exceeding 35 % at 365 nm excitation wavelength can be synthesized from an alkoxysilane sol-gel precursor and a carboxyxlic acid. Aluminum activators may be included by mixing an alkoxyaluminate with the alkoxysilane. Other activators may be substituted. The silica-based phosphors are synthesized at temperatures of less than 650 °C and display broad visible photoluminescence spectra which appear white to the eye. Alkoxysilanes such as TMOS (tetramethoxysilane) and TEOS (tetraethoxysilane) can be used to produce a rigid porous silicate material, while APTES (3-aminopropyltriethoxysilane) and APT MS (3-aminopropyltriethoxysilane) can be used to create water-soluble phosphors which can be drawn into fibers or cast into thick films.

Description

HIGHLY EMISSIVE WHITE PHOSPHOR COMPOUNDS

RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 1 1 9(e) to U.S. provisional application Serial No. 60/050,400 to Michael J. Sailor, William H. Green and Khoa Le, filed June 20, 1 997, and entitled HIGHLY EMISSIVE WHITE PHOSPHOR. The subject matter of U.S. provisional application Serial No. 60/050,400 is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to highly emissive white phosphor compounds, methods of their preparation, and applications therefor. In particular, the present invention relates to carboxysilicate derivatives.

BACKGROUND OF THE INVENTION

Luminescent materials that are commonly used for display and lighting applications comprise an inorganic host, or precursor, doped with various activator metals. The host material is selected to be chemically and physically inert and yet provide a suitable coordination geometry and redox environment for the activator metal. By choosing the appropriate host and activator, phosphors can be made that emit virtually any color of light upon absorption of higher energy radiation.

The phosphors used in modern fluorescent light bulbs have quantum efficiencies approaching unity when excited at 254 nm such that further gains in phosphor efficiency are not likely to improve the power conversion efficiency of the bulb. The most commonly used plasma material for fluorescent light bulbs is mercury vapor, which is, as yet, unmatched in its efficiency in generating high-energy UV light (254 and 1 85 nm) from electrical excitation. The overall efficiency of converting electrical power into visible light in any fluorescent light source is limited by the process of electrically stimulating ultraviolet (UV) emission from the plasma contained within the bulb. Limitations arise from the fact that, with higher level of UV radiation such as needed for higher output intensity, heat is generated, representing an inefficiency in conversion of the excitation energy, as well as contributing to the degradation of the phosphor, matrix. The UV light generated by the mercury plasma has a relatively short wavelength and a relatively high energy, and thus is particularly subject to such thermal losses. In addition, the mercury vapor raises significant issues in manufacture and disposal of the light bulb due to the toxicity of mercury and its negative impact on the environment. If a lower energy, longer wavelength light could be effectively used, there would be diminished loss of heat as well as less damage to the phosphor matrix from the increased temperatures. Potential alternatives to mercury, such as xenon, which generates short wavelength UV light, have been used in specialized situations, but are unlikely to replace mercury in more general applications when used in combination with currently available phosphors. As a result, the electrical efficiency of mercury vapor-based fluorescent light bulbs is unlikely to improve until a better plasma source is found. Other considerations involved in the selection of materials used in the manufacture of fluorescent light bulbs is that, while the host materials that make up most phosphors are relatively benign to the environment, the activator metals used to dope the phosphors, including lead, europium and terbium, raise health and environmental issues similar to those relating to mercury. Again, procedures for disposal of spent fluorescent bulbs, as well as for handling of the materials used in the manufacture of the bulbs, must include consideration of potential toxicity of the materials if allowed to accumulate in the environment. Another issue is cost, with the rare earth activators used in most fluorescent bulbs representing a significant fraction of the total cost of the bulb. Accordingly, motivating factors remain for the development of new materials, including lowering the manufacturing cost and making lighting products more environmentally friendly.

The use of thin film phosphors in electroluminescent displays, such as high resolution flat panel displays for computers and in interactive commercial systems such as automatic teller machines (ATMs), has been the subject of development efforts due to several significant drawbacks in clarity and durability. For example, a problem with ATMs, which are commonly located outdoors, is that the display must produce relatively high intensity light in order to avoid being washed out by strong ambient light. Such electroluminescent displays consist of a phosphor, typically zinc sulfide (ZnS) doped with manganese, mixed within a binder and sandwiched between two transparent electrodes. When the voltage applied to the electrodes exceeds a threshold, the phosphor breaks down and conducts current, exciting the manganese ions to produce light. One solution to the intensity problem is to increase the energy input to the phosphor material, however, this can accelerate degradation of the adhesion of the phosphor film, often causing it to separate from the electrodes. To prevent separation, additional binder can be added at the expense of diluting the phosphor and detracting from the intensity.

The foregoing examples are indication that improvement needs to be made in phosphor technology. Such improvements should address the issues of cost reduction, environmental impact, durability and/or intensity in lighting and display applications.

SUMMARY OF THE INVENTION The object of the present invention is to provide a method for producing highly emissive white phosphor compounds suitable for use in displays, fluorescent lighting, and other applications of photoluminescence. Another object of the present invention is to provide a highly emissive phosphor which uses non-toxic activators.

Sti.ll another object of the present invention is to provide a light emissive silicate compound which can be excited using a plasma source other than mercury vapor.

Yet another advantage of the present invention is to provide a silicate-based phosphor formed by a sol-gel route using relatively inexpensive liquid precursors and a simple heat treatment to activate the luminescence. Highly emissive broadband phosphors with external quantum yields exceeding 35% at 365 nm excitation wavelength can be synthesized from an alkoxysilane sol-gel precursor and a carboxylic acid. Aluminum activators may be included by mixing an alkoxyaluminate with the alkoxysilane. The silica-based phosphors are synthesized at temperatures of less than 650°C and display broad visible photoluminescence spectra which appear white to the eye. Alkoxysilanes such as TMOS (tetramethoxysilane) and TEOS (tetraethoxysilane) can be used to produce a rigid porous silicate material, while APTES (3- aminopropyltriethoxysilane) and APTMS (3-aminopropyltrimethoxysilane) can be used to create water-soluble phosphors which can be drawn into fibers or cast into thick films.

In an exemplary embodiment, carbon-activated or aluminum and carbon-activated phosphors are formed using a host silicate (SiO₂) containing carbon, hydrogen and/or aluminum impurities. Aluminum is introduced into the sol-gel by mixing a trialkoxyaluminate with a tetraalkoxysilane, while carbon is introduced by reacting the latter mixture with a simple carboxylic acid. The resulting liquid typically gels within 8 hours and is ready for a heat treatment after an aging period of 24-48 hours at room temperature. The luminescence is activated by heating the aged gel in flowing air for 2-3 hours. Variation of heating parameters can produce materials with different physical, optical and light- emitting characteristics. In one form, a liquid or gel can be applied to the surface which is desired to luminesce, or incorporated into a matrix of another material. The surface may be the inner wall of a fluorescent tube, a clear conductive substrate for a flat panel display, an exterior surface of a watch dial or a toy, or incorporated into paper or ink for use in detection of counterfeit currency or documents. Alternatively, the gel may be treated to remove the alcohol and water by-products to create a rigid porous silicate network. When the rigid material is formed as a sheet it may be used, for example, as the phosphor in a flat panel display by sandwiching the sheet of material between appropriate electrodes.

BRIEF DESCRIPTION OF DRAWINGS

Understanding of the present invention will be facilitated by consideration of the following detailed description of a preferred embodiment of the present invention taken in conjunction with the accompanying drawings, in which like numerals refer to like parts and in which:

Figure 1 is a plot of relative intensity versus wavelength showing the photoluminescence spectra (at 337 nm excitation, 298 °K) of representative carboxysilicate phosphors prepared from (A) formic acid/ TMOS, (B) citric acid/ TMOS, (C) formic acid/ APTES, and (D) lactic acid/ APTES;

Figure 2 is a plot of relative intensity versus wavelength comparing the photoluminescence and phosphorescence spectra at 298° K of a carboxysilicate prepared from formic acid and TMOS;

Figure 3 is a plot of absorbance versus wavenumbers showing the transmission FTIR spectra (KBr disk) of silicate materials synthesized from (A) APTES/ formic acid and (B) TMOS/ formic acid; and

Figure 4 illustrates an exemplary lattice structure arising from the decomposition of a silyl formate species to produce a C substitutional defect for Si, which is postulated to be the luminescent species. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Carbon-activated or aluminum- and carbon-activated phosphors are formed using a host silicate (SiO ₂) containing carbon, hydrogen and/or aluminum impurities. Aluminum is introduced into the sol-gel by mixing a trialkoxyaluminate with a tetraalkoxysilane, while carbon is introduced by reacting the latter mixture with a carboxylic acid. Other materials, such as, but not limited to, titanium, tin, magnesium, or vanadium may be substituted for, or used in combination with, the aluminum.

In a first embodiment, a carbon-activated photoluminescent silicate material according to the present invention is prepared from an alkoxysilane and a carboxylic acid via the sol-gel route. The sol-gel synthesis of silica is carried out by the aqueous hydrolysis of a tetraalkoxysilane. The reaction of alkoxysilanes with organic acids is known in the prior art. See, e.g. , "Aminosols: New Solid State Protonic Materials by the Sol-Gel Process", Y. Charbouillot, D. Ravaine, M. Armand and C. Poinsignon, J. Non-Cryst. So/ids 103:325-330 (1 988).

An exemplary detailed procedure for synthesis of a luminescent TMOS based sol-gel is as follows: In a container at about room temperature ( ~ 25°C) deaerated TMOS and formic acid are mixed slowly in a 1 :3 mole ratio under an atmosphere of nitrogen. Deaeration of the TMOS is achieved by three freeze-pump-thaw cycles. The mixture is stirred for fifteen minutes to produce a clear liquid which is allowed to gel. After three days the container is opened to the air and the excess liquid is decanted. The remaining clear glass is then air dried at about room temperature for at least one day. The glass is placed into a programmable tube furnace which is programmed with the following heating sequence. Step 1 : the temperature is ramped 1 ° C/ minute from 25° C to 1 50° C followed by a one hour isothermal hold. Step 2: the temperature is ramped at a 2° C/ minute to 400° C followed by a 4 hour isothermal hold. The resulting material is a transparent orange glass with bright photoluminescence. When the alcohol and water byproducts of this hydrolysis are removed by the appropriate temperature processing, a rigid porous silicate network remains. Such processing steps are known in the art. See, e.g. , U.S. Patent No. 5,695,809 of Chadha, et al. , and U.S. Patent No. 5,560,957 of Johnson. The silicate network can be directly coated onto clear substrates by painting the substrate with, or dipping the substrate in, the mixed solution of sol-gel. Taking special care to prevent cracking during the drying and densification stages, high-quality optical components, including flat panels, can be produced. As described in more detail below, processing variations, and different organic acids and/or silicate source combinations, can be used to produce powders, rigid structures or gels which may be used separately, applied to, or incorporated in substrates or surfaces.

Chemical species may be added to the tetraalkoxysilane solution prior to the gelation stage to impart unique characteristics, such as emission of specific colors, to the resultant gels or glasses. For example, addition of organic laser dyes, such as Coumarin and Uranin, are known to impart characteristic and generally bright colors to otherwise colorless media. Such laser dyes are generally aromatic hydrocarbons, which are well known in the art to produce fluorescent and/or phosphorescent emissions when excited by light of the appropriate wavelength. Selection of the appropriate laser dye can provide a phosphor material that will fluoresce, i.e. , will emit light of one wavelength while being excited at another wavelength, while another dye can provide a phosphor material which will absorb enough excitation energy to phosphoresce, or continue emitting light for a period of time after the excitation is discontinued. The use of an asymmetric silicate precursor can generate nonlinear optical materials, and the incorporation of enzymes or molecular catalysts can yield catalytically active silicates. The reaction of tetramethoxysilane (TMOS) or tetraethoxysilane

(TEOS) with a variety of organic carboxylic acids at room temperature can produce a gel within a few minutes to a few days. Flowing air between 200° C and 500° C is used to thermally treat the resulting gel, which produces a white or yellowish solid that is photoluminescent. Variations in the preparation conditions of a gel sample produced from the reaction of formic acid (a simple carboxylic acid) with TMOS can result in an intense white photoluminescence with emission maxima between 450 nm and 600 nm when excited using a filtered 365 nm mercury arc lamp as the source of UV radiation.

A listing of exemplary carboxylic acids that can be used to make carbon-activated photoluminescent silicates of different forms and possessing different luminescent behaviors, and the respective conditions for producing the reactions, is provided in Table 1 .

Table 1

¹ PL = photoluminescence; ² PP = phosphorescence. Luminescence intensities: vs = very strong; s = strong; m = medium; w = weak.

For processing of the examples listed in Table 1 , most of the acids were used as neat liquids. Solid acids were dissolved in a minimum amount of anhydrous methanol. The reactions were carried out under a nitrogen atmosphere using an excess of acid. Heat treatments were performed in air. The excitation wavelength for luminescence determination was 365 nm.

As shown in Figure 1 , curves A and B illustrate the relative photoluminescent intensity of carboxysilicate phosphors prepared from formic acid/TMOS and citric acid/TMOS, respectively. The external luminescence quantum yield of this material, measured with 365 nm excitation, ranges from 0.30 to 0.45. The photoluminescence lifetime using a 337 nm pulsed N₂ laser as the source of UV excitation is less than 1 0 nanoseconds for all the materials studied. Elemental analysis of the formic acid/TMOS sol-gels heated to 275° C, 450° C and 800° C yielded carbon contents of 0.31 %, 0.1 3% and 0.07% by weight respectively. When heated above 650° C the transparent glassy formic acid/ TMOS material turns an opaque brown-black color. Raman spectra of this black glass display broad peaks at 1 340 cm ^"1 and 1 590 cm ^"1, suggesting the formation of a graphite carbon phase in the sol-gel when the initial reaction is subjected to temperatures above 650° C. The physical appearance and Raman spectra of this particular formic acid/TMOS reaction are similar to those reported for silicon oxycarbide, which typically forms during the pyrolysis of carbon-containing siloxane polymers. See, e.g. , G.M. Renlund, S. Prochazka, R.H. Doremus, J. Mater. Res. , 6:271 6 ( 1 991 ); and G.M. Renlund, S. Prochazka, R.H. Doremus, J. Mater. Res. , 6:2723 ( 1 991 ). In spite of its dark color and opacity, the black glass is still highly photoluminescent and, additionally, it displays a bright phosphorescence with a lifetime of several seconds at room temperature. As indicated in the plot shown in Figure 2, the photoluminescence and phosphorescence spectra of the black glass are highly structured and the spacing between the most prominent features of the graphed spectra is about 1 330 cm ^"1. The separation between the peaks decreases when ¹³C labeled formic acid is used in the initial synthesis, indicating that the luminescence mechanism may involve vibronic coupling to one or more carbon atoms derived from formic acid. A similar experiment with deuterated formic acid did not result in a detectable spectral shift.

The reaction between 3-aminopropyltriethoxysilane (APTES) and simple carboxylic acids yields a clear, water soluble material that can be drawn into fibers or cast into thick films or monolithic structures without shrinking or cracking. The photoluminescent properties of the APTES- derived glasses, which are shown as curves C and D of Figure 1 , are similar to the TMOS- and TEOS-derived glasses in that the steady-state emission spectra are broad. Further, in APTES-derived glasses, the emission maximum is between 390 nm and 450 nm. As in the TMOS- and TEOS- derived glasses, the APTES-derived glasses have an emission lifetime of less than 1 0 nanoseconds. However, unlike the TMOS- and TEOS-derived materials, the APTES-derived glass decomposes upon heating above 200° C. Transmission Fourier-transform infrared (FTIR) spectra (KBr disk) of the TEOS and TMOS-derived silicate phosphors, an example of which is shown as trace B of Figure 3 (formic acid/TMOS), are similar to the spectrum of pure, amorphous Si0₂ in that strong bands associated with Si-0 bond stretching and bending vibrations are apparent at 1090 cm^"1 , 800 cm^"1 , and 470 cm^"1. However, additional weak absorptions in the 3000-2700 cm^"1 and the 1 700-1400 cm^"1 regions indicate the presence of hydrocarbon and carbonyl impurities. A sharp band at 2349 cm^"1 may be attributed to the asymmetric stretching vibration of CO ₂. This peak is seen in almost all TEOS/carboxylic acid or TMOS/carboxylic acid sol-gels that have been heated over 400 °C and may arise from the thermal decomposition of the carboxylic acid and any residual alkoxy groups. The lack of evidence of rotational fine structure in the 2349 cm ¹ band indicates that the C0₂ molecule may be immobilized in the silicate lattice. In spite of the relatively small amount of carbon ( < 0.5%) present in glasses produced from TEOS/carboxylic acid and TMOS/carboxylic acid reactions heated over 400 °C, the observed luminescence is likely related to the incorporation of the carbon into the glass. The retention of organics, typically unhydrolyzed alkoxy groups, in sol-gel-derived glasses can lead to discoloration and poor light transmission. This is generally considered to be the disadvantage of using the sol-gel method of glass production over other methods of producing optical quality Si0₂. See, e.g. , P. Robinson, D. Perlmutter, J. Non-Cryst. Solids, 1 69: 1 83 (1 994). In contrast, in the present invention, it is believed that the trapped carboxylic acid leads to luminescent defect centers that are responsible for the bright photoluminescence. Weak luminescence from carbon-doped silicon and silica is known in the art, see, e.g. , T. Kitamura, et al. , "Luminescence associated with the molecular aggregation of hydrocarbons doped in amorphous silica glasses", J. Lumin. , 48-49:373 ( 1 991 ) . However, the source of the emission has not been positively identified. Using plasma to deposit amorphous carbon and diamond-like carbon films can also produce materials that exhibit photoluminescence at room temperature.

In general, the materials synthesized from TMOS or TEOS form hard, brittle glasses or powders. Using formic acid and TMOS as an example, the reaction produces silica according to the equation,

2HC(0)OH + (CH ₃0)₄Si - Si0₂ + 2CH₃OH + 2HC(0)CH₃

Note that the equation is anhydrous, involving the elimination of methyl formate rather than water to form the Si-O-Si bonds. However, under anhydrous conditions, a significant amount of formic acid may be retained in the gel as a silyl formate species. Upon heating this formate may decompose to create a carbon substitutional defect for silicon, which is postulated to be the luminescent species in the lattice structure shown in Figure 4.

The chromophore in the water-soluble, luminescent material generated from 3- aminopropyltriethoxysilane may be different from the luminescent species in the TEOS- and TMOS-based silicates. The APTES- derived materials do not require heating over 1 00° C to exhibit luminescence, and the infrared spectra, shown as trace A of Figure 3, display strong absorption bands characteristic of amide ( 1 663 cm^"1), ammonium (3277 cm^"1 , 2500-2000 cm^"1) and aliphatic (2959- 2850 cm^"1 ) functionalities. The Si-0 asymmetric stretching bands ( 1 1 26 cm^"1 and 1024 cm^"1) are characteristic of long chain, linear siloxanes. The luminescence quantum yield of the APTES/formic acid material (measured with 365 nm excitation) is 0J 5 ± 0.03 in dilute aqueous solution and 0.40 ± OJ in the solid state.

In a second embodiment, carbon- and aluminum-activated phosphors are formed by introducing a mixture of a trialkoxyaluminate and a tetraalkoxysilane into a sol-gel. As in the previous embodiment, the tetraalkoxysilane can be replaced with APTES or APTMS to provide a water-soluble phosphor. As before, carbon is introduced into the mixture using a carboxylic acid. The resulting liquid typically gels within eight hours. After aging the gelled liquid at room temperature for two to four days, the material is annealed in flowing air for two to three hours to activate the luminescence. A reducing atmosphere is not required, and the annealing temperature is less than 650°C. The stoichiometry of the resulting material is approximately (Si0₂)_{0 95}AI_{0 01}C_{0 2}H_{0 2} after heating but can vary over a range of ± 20% with respect to aluminum, carbon and hydrogen. As with the carbon-activated phosphors, to attain optimal luminscence intensity, the heating temperature, duration of heating, and initial mole ratios of carboxylic acid, tetraalkoxysilane and aluminum should be carefully controlled.

The unheated material is a colorless, opaque glass that displays only weak photoluminescence under UV excitation. Upon heating, the materials turn from colorless to dark brown or black followed by a gradual lightening to a pale yellow color at the end of the heat treatment. The photoluminescence emission peaks between 550 and 580 nm and is relatively broad with a FWHM of 0.5-0.8 eV. At room temperature, the time dependence of the photoluminescence is characterized by a fast decay (τ_f < 40 nsec.) and a slow decay (τ_s < .25 sec) . The two characteristics are associated with fluorescence and phosphorescence processes, respectively. The aluminum-doped materials are efficiently excited with either 365 nm or 254 nm UV light with little change in the emission wavelength or intensity. Such a property is desirable for phosphors used in high pressure mercury vapor (HPMV) lamps, where approximately one-half of the excitation light is at a wavelength greater than 31 3 nm. HPMV bulbs also require that the phosphors maintain high efficiencies at operating temperatures between 200-250°C. Although this operating temperature is considerably greater than for low pressure mercury vapor lamps, the phosphors in the HPMV tubes are longer lived because they are not subjected to the degrading effects of vacuum UV at 1 85 nm or attack by energetic ions and electrons from the plasma. By comparison, in preliminary tests, the aluminum-doped phosphors of the present invention haved maintained high fluorescence efficiency at temperatures exceeding 300°C, with the phosphorescence component being quenched above 75°C. The quantum yields of the aluminum-doped phosphors, measured at 365 nm, are between 0.3 and 0.4 ( ± 50%), however, the intrinsic quantum efficiency is expected to be much higher.

The following examples are provided to further define various aspects and applications of the present invention. Example 1 : Flat Panel Display

Current flat panel displays, such as ATM screens, are backlit with a phosphor material dispersed within a binder. A common material used for the binder is acrylic. The phosphor material may include conductive electron-excitable phosphors, UV-excitable light-emitting phosphors, UV light emitting phosphors, or combinations thereof, all of which are well known in the art. See, e.g. , U.S. Patent 5,747, 1 00 of Petersen. The generated light intensity must be quite high in order to avoid being washed out by strong ambient light, such as may be encountered in outdoor or brightly lit indoor settings. Increasing the intensity requires more excitation energy, which may be electrical, UV, or both. However, higher energy input can degrade the phosphor, causing it to separate from the display. An increase in the amount of binder relative to the phosphor to improve adhesion dilutes the relative amount of phosphor within the matrix, resulting in a decrease in output light intensity. One application of the present invention is the augmentation of flat panel displays by incorporating a silica-based white phosphor compound in the binder material. In the case of the carbon- and aluminum-activated phosphors, the gel produced by incorporation of a trialkoxyaluminate with a tetraalkoxysilane in a sol-gel may be added in liquid form to the binder material that is coated onto a substrate patterned with a transparent thin film electrode material, such as indium-tin-oxide (ITO). The coated substrate is annealed to activate the silica-based phosphor. The silica- based phosphor is then coated with the appropriate conventional phosphors for emitting light at the desired colors and again treated to activate the conventional phosphors. Multiple layers of the silica-based phosphor and the conventional phosphor may be applied as needed, and diffusion barriers may be added to prevent diffusion of species between the different conventional phosphors. In a variation, multiple layers of silica-based phosphor and conventional phosphor may be paired. A layer of silica-based phosphor may, itself, be modified by adding, for example, laser dyes of a selected color to match the color light to be emitted by its corresponding conventional phosphor, thus permitting each color of a multi-color flat panel display to be intensified by incorporating silica-based phosphor in the binder. Example 2: Fluorescent Light Bulb Another application of the present invention is use in conjunction with fluorescent lamps. As previously stated, the majority of commercially available phosphors require excitation by high energy, short wavelength UV light, such as produced by mercury vapor plasmas. The thermal losses resulting from the use of such high energy UV light are a source of inefficiency in fluorescent bulbs. Unlike conventional phosphors, the silica-based phosphors described herein, including, for example, silicates resulting from the reaction of formic acid and TMOS or TEOS, or lactic acid with APTES or APTMS, are highly emissive, providing broadband (white) light when excited at 365 nm. Because some of the described silicate materials can respond to short wavelength excitation, the use of the silica-based, highly emissive, white phosphor material in conjunction with less toxic xenon plasma would produce the desired white light without the use of toxic activator metals currently used in conjunction with mercury plasma sources. As an added benefit, the silica- based phosphors costs less to manufacture and do not create toxic substances that can adversely impact the envronment, such as the case with current commercial fluorescent light bulbs. Example 3: White LED

Until only recently, commercially-available LEDs (light emitting diodes) were limited to those which produced light within the longer wavelength visible and IR range, e.g. , red and orange. The low energy output levels of these LEDs are insufficient for exciting phosphors which might convert the LED light into more useful broadband light. In the past few years, blue LEDs (GaN - 430 nm peak) have become available, thus providing a higher energy light which might be suitable for excitation of white phosphors. Currently, phosphors such as Ce:YAG, can be used for converting the light emitted by a blue LED to produce a white LED. The implications of the availability of white LEDs are substantial in terms of applicability to not only display technologies, but to medical devices. For example, current endoscopes are designed to use incandescent light sources, such as metal halide or halogen lamps, or lasers. The light is conducted to the end of the scope using fiber optic bundles. The incandescent light sources are extremely inefficient, generating significant amounts of heat, and are subject to breakage or burnout if not handled carefully. Heretofore, LEDs were not an option for such applications due to the limited wavelengths that were available. The Ce:YAG phosphors are capable of converting light from blue LEDs to create white LEDs that may be used in medical devices. However, these phosphors raise their own problems in terms of their expense and potential toxicity. Using the silica-based phosphors disclosed herein, white LEDs can be made which are inexpensive and non-toxic. For example, a carbon- and aluminum-activated silicate phosphor can be incorporated into a lens or filter surrounding a blue LED. Example 4: Fluorescent Tags Another use for the present invention consists of fluorescent tags in biological testing. Currently, toxic dyes or radioactive substances are used in the labeling or tagging of DNA, RNA, proteins and mammalian, bacterial and insect cells in biological research. The use of silica-based fluorescent tags would allow the tracking of labeled biologies without the risk of damage to the cells or tissue that normally occur when using other labeling materials. In addition, fluorescent tagging using the silica-based phosphors described herein would be fully reversible, allowing further experimentation on the labeled substances.

In an exemplary application, chromosomal in situ hybridization enables determination of the presence and location of DNA sequences complimentary to a labeled probe. The combination of enzymatic labeling using fluorochromized nucleotides (nucleic acids directly labeled with a fluorescent tag) and improved imaging methods have made fluorescent labeling an alternative to traditional radioactive labeling for in situ hybridization. Advantages to this direct fluorochrome-labeling method include the simplification of standard protocols (because radioactive substances are absent), less background noise, and easier signal quantification. Multiple probes, each labeled with a different fluorochrome combination, allow simultaneous multitarget detection. In DNA sequencing, dideoxy nucleotides (ddA, ddG, ddC, and ddT) are each labeled with a different colored fluorescent agent created using water- soluble APTES- or APTMS-derived silica-based phosphors with different dopants. In traditional DNA sequencing using radiolabeled dideoxys, differentiation of the nucleotides was often difficult due to varying exposure times of the labeled materials. With colored fluorescent tags, the dideoxy bases are easily distinguished from one another because each emits a different color fluorescence. Example 5: Counterfeit Currency Detection

The silica-based phosphors disclosed herein can be incorporated into either the paper fibers during processing, or in the ink used to print the currency, to create a "watermark" or other distinguishing feature which is not visible under normal background lighting conditions. Upon exposure to UV light, the hidden image will fluoresce, allowing ready identification of counterfeit currency.

In view of the low cost, ease of manufacture, many possible forms of the material (liquid, powder, or solid), and the fact that they are non- toxic, applications of the silica-based phosphors described herein are virtually unlimited. In addition to the examples specifically stated above, other uses include incorporation into toys, household products, printed paper articles, such as product packaging, art prints and posters, fabrics and clothing. It will be apparent to those skilled in the art that various modifications and variations may be made in the apparatus and process of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modification and variations of this invention provided they come within the scope of the appended claims and their equivalents.

We claim:

Claims

1 . A method for preparation of a highly emissive white phosphor compound, the method comprising: combining an alkoxide sol-gel precursor with a substantially anhydrous chemical reagent; heat treating the combination at a pre-determined temperature for a reaction time sufficient to form a luminescent material.

2. The method of claim 1 , wherein the alkoxide sol-gel precursor comprises a first alkoxide selected from the group consisting of mono-, di-, tri-, and tetra-substituted alkoxides.

3. The method of claim 1 , wherein the alkoxide sol-gel precursor is an alkoxysilane.

4. The method of claim 1 , wherein the alkoxide sol-gel precursor is selected from the group consisting of tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), silicon tetrachloride (SiCI₄), 3- aminopropyltriethoxysilane (APTES), and aminopropyltrimethoxysilane (APTES) .

5. The method of claim 1 , further comprising combining the alkoxide sol-gel precursor with a second alkoxide containing an element selected from the group consisting of aluminum, titanium, tin, magnesium and vanadium.

6. The method of claim 5, wherein the second alkoxide comprises trialkoxyaluminate.

7. The method of claim 1 , wherein the substantially anhydrous chemical reagent selected from the group consisting of organic acids, aldehydes and ketones.

8. The method of claim 7, wherein the organic acids comprise carboxylic acids.

9. The method of claim 8, where the carboxylic acid is selected from the group consisting of formic acid, lactic acid, acetic acid, trifluoroacetic acid, cyclopropanoic acid, malonic acid, citric acid, tartaric acid, glyoxylic acid and oxalic acid.

10. The method of claim 1 , wherein the step of heat treating comprises ramping a starting temperature of the combination to the predetermined temperature at a predetermined ramp rate and maintaining the combination at the predetermined temperature for one hour or longer.

1 1^'. The method of claim 1 0, wherein the sol-gel precursor is TMOS or TEOS, the starting temperature is room temperature, the predetermined temperature is between 200┬░C and 800┬░C, and the predetermined ramp rate is 1 ┬░C/min or greater.

12. The method of claim 1 , wherein the step of heat treating comprising exposing the combination to flowing air at the predetermined temperature for the reaction time, wherein the predetermined temperature is equal to or less than 650┬░C.

1 3. The method of claim 1 2, wherein the sol-gel precursor is APTES or APTMS and the reaction time is at least one day.

1 4. The method of claim 1 , wherein the alkoxide sol-gel precursor comprises a mixture of tetraalkoxysilane and trialkoxyaluminate, and substantially anhydrous reagent is a carboxylic acid and further comprising aging the combination at room temperature for two to four days prior to the step of heat treating in a flowing air, wherein the predetermined temperature is less than 650┬░C.

1 5. A highly emissive white phosphor comprising: a substantially silyl-carbonate lattice, wherein the lattice contains silicon selected from a mono-, di-, tri-, or tetra-substituted alkoxide group consisting of aluminum, silicon, titanium, tin, magnesium and vanadium, said lattice further comprising carbon, oxygen and hydrogen, wherein the carbon is a member of a group consisting of organic acids, aldehydes and ketones, wherein the lattice decomposes at a temperature sufficient to create a carbon substitutional defect in the lattice to generate a luminescent species.

16. The white phosphor of claim 1 5, wherein the silicon is selected from the group consisting of tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS) and the lattice further contains aluminum from a trialkoxyaluminate, wherein decomposition of the lattice further creates an aluminum substitutional defect.

1 7. The white phosphor of claim 1 5, wherein the silicon is selected from the the group consisting of aminopropyltriethoxysilane (APTES) and aminopropyltrimethoxysilane (APTES), wherein the lattice is water-soluble.