Methods for producing coated phosphor and host material particles using atomic layer deposition methods

Abstract

Layers of a passivating material and / or containing luminescent centers are deposited on phosphor particles or particles that contain a host material that is capable of capturing an excitation energy and transferring it to a luminescent center or layer. The layers are formed in an ALD process. The ALD process permits the formation of very thin layers. Coated phosphors have good resistance to ambient moisture and oxygen, and / or can be designed to emit a distribution of desired light wavelengths.

Description

BACKGROUND OF THE INVENTION[0001]This invention relates to phosphor particles having ultrathin coatings on their surfaces and to methods for making and using such coated particles.[0002]Phosphors are used in flat panel plasma displays (FPDs), cathode ray tubes, x-ray imaging devices, field emission devices, fluorescent lighting fixtures, and a variety of other applications to generate visual images or simply provide light. Although a wide variety of phosphor materials are known for use in these applications, those materials all have in common the ability to generate a characteristic light in response to exposure to an excitation energy source. The excitation energy source may be, for example, a photon (photoluminescence, or PL), high energy electron beam (cathodic luminescence, or CL), an applied electrical field (electroluminescence, or EL), or applied heat (thermoluminescence). Electroluminescent phosphors are of particular interest for flat panel display applications.[0003]Flat p...

AI Technical Summary

Benefits of technology

[0052]The thickness of the applied films typically will be in the range of about 1 to about 500 nm. An advantage of the invention is that the ALD process is capable of forming highly uniform films at very small thicknesses. Thus, a preferred film thickness is from about 5 to about 100 nm. It has been found that films of such thicknesses can perform very well as passivating films. A more preferred film thickness is from about 10-100 nm, and an especially preferred film thickness is from 15-75 nm. Film thickness is controlled via the number of reaction cycles that are performed.

[0053]The particulate is preferably non-agglomerated after the inorganic material is deposited. By “non-agglomerated”, it means that the particles do not form significant amounts of agglomerates during the process of coating the substrate particles. Particles are considered to be non-agglomerated if (a) the average particle size does not increase more than about 5%, preferably not more than about 2%, more preferably not more than about 1% (apart from particle size increases attributable to the coating itself) as a result of depositing the coating, or (b) if no more than 2 weight %, preferably no more than 1 weight % of the particles become agglomerated during the process of depositing the inorganic material.

[0054]In preferred embodiments, the deposits of inorganic material form a conformal coating. By “conformal” it is meant that the thickness of the coating is relatively uniform across the surface of the particle (so that, for example, the thickest regions of the coating are no greater than 3X (preferably no greater than 2X, especially no greater than 1.5X) the thickness of the thinnest regions), so that the surface shape of the coated substrate closely resembles that of the underlying substrate surface. Conformality is determined by methods such as transmission electron spectroscopy (TEM) that have resolution of 10 nm or below. Lower resolution techniques cannot distinguish conformal from non-conformal coatings at this scale. The desired substrate surface is preferably coated substantially without pinholes or defects.

[0055]A wide range of reaction schemes can be used to provide desirable films on the particle surfaces. Some exemplary classes of reaction schemes are described below.

[0056]Oxide and nitride films can be prepared on particles having surface hydroxyl or amine (M-N—H) groups using a binary (AB) reaction sequence as follows. The asterisk (*) indicates the atom that resides at the surface of the particle or coating, and Z represents oxygen or nitrogen. M1 is an atom of a metal (or semimetal such as silicon), particularly one having a valence of 3 or 4, and X is a displaceable nucleophilic group. The reactions shown below are not balanced, and are only intended to show the reactions at the surface of the particles.M-Z—H*+M1Xn→M-Z-M1X*+HX   (A1)M-Z-M1X*+H2O→M-Z-M1 OH*+HX   (B1)In reaction A1, reagent M1Xn reacts with one or more M*-Z-H groups on the surface of the particle to create a new surface group having the form -M1-X. M1 is bonded to the particle through one or more Z atoms. The -M1-X group represents a site that can react with water in reaction B1 to regenerate one or more hydroxyl groups. The hydroxyl groups formed in reaction B1 can serve as functional groups through which reactions A1 and B1 can be repeated, each time adding a new layer of M1 atoms. Note that in some cases (such as, e.g., when M1 is silicon, zirconium, titanium, yttrium or aluminum) hydroxyl groups can be eliminated as water, forming M1-O-M1 bonds within or between layers. This condensation reaction can be promoted if desired by, for example, annealing at elevated temperatures and / or reduced pressures.

[0057]Binary reactions of the general type described by equations A1 and B1, where M1 is silicon, are described more fully in J. W. Klaus et al, “Atomic Layer Controlled Growth of SiO2 Films Using Binary Reaction Sequence Chemistry”, Appl. Phys. Lett. 70, 1092 (1997) and O. Sheh et al., “Atomic Layer Growth of SiO2 on Si(100) and H2O using a Binary Reaction Sequence”, Surface Science 334, 135 (1995), both incorporated herein by reference. Binary reactions of the general type described by equations A1 and B1, where M1 is aluminum, are described in A. C. Dillon et al, “Surface Chemistry of Al2O3 Deposition using Al(CH3)3 and H2O in a Binary reaction Sequence”, Surface Science 322, 230 (1995) and A. W. Ott et al., “Al2O3 Thin Film Growth on Si(100) Using Binary Reaction Sequence Chemistry”, Thin Solid Films 292, 135 (1997). Both of these references are incorporated herein by reference. General conditions for these reactions as described therein can be adapted to construct SiO2 and Al2O3 coatings on particulate materials in accordance with this invention.

Problems solved by technology

In addition to having much larger diameters than are wanted, these aggregates often tend to break apart, revealing defects in the coating at the break areas.

The underlying particles are subject to attack from water, oxidants and other materials at the places where these defects occur.

Neither CVD nor sol-gel techniques are entirely satisfactory, as agglomerates tend to form readily in these processes.

In addition, these methods require relatively large amounts of raw materials, as only a portion of the applied reactants actually become applied to the surface of the phosphor particles.

CVD and sol-gel coating methods are particularly unsuitable for coating these smaller particles.