U.S. patent application number 11/473289 was filed with the patent office on 2007-12-27 for methods for producing coated phosphor and host material particles using atomic layer deposition methods.
Invention is credited to Karen J. Buechler, Steven M. George, Jarod McCormick, Joseph A. Spencer, Alan W. Weimer.
Application Number | 20070298250 11/473289 |
Document ID | / |
Family ID | 38873890 |
Filed Date | 2007-12-27 |
United States Patent
Application |
20070298250 |
Kind Code |
A1 |
Weimer; Alan W. ; et
al. |
December 27, 2007 |
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.
Inventors: |
Weimer; Alan W.; (Niwot,
CO) ; George; Steven M.; (Boulder, CO) ;
Buechler; Karen J.; (Westminster, CO) ; Spencer;
Joseph A.; (Longmont, CO) ; McCormick; Jarod;
(Boulder, CO) |
Correspondence
Address: |
GARY C. COHN, PLLC
1147 NORTH FOURTH STREET, UNIT 6E
PHILADELPHIA
PA
19123
US
|
Family ID: |
38873890 |
Appl. No.: |
11/473289 |
Filed: |
June 22, 2006 |
Current U.S.
Class: |
428/336 ;
427/212; 427/248.1; 428/688; 428/690 |
Current CPC
Class: |
Y10T 428/2991 20150115;
C09K 11/025 20130101; C23C 16/403 20130101; C23C 16/45555 20130101;
Y10T 428/265 20150115; C23C 16/45525 20130101; Y10T 428/2993
20150115; C09K 11/56 20130101; C23C 16/405 20130101; C23C 16/442
20130101; C23C 16/4417 20130101; Y10T 428/2982 20150115 |
Class at
Publication: |
428/336 ;
428/688; 428/690; 427/212; 427/248.1 |
International
Class: |
B32B 19/00 20060101
B32B019/00; G11B 5/64 20060101 G11B005/64; B32B 9/00 20060101
B32B009/00; B05D 7/00 20060101 B05D007/00; C23C 16/00 20060101
C23C016/00 |
Claims
1. A method comprising forming a layer of an inorganic material on
the surface of a phosphor particle via an atomic layer deposition
process.
2. The method of claim 1, wherein the atomic layer deposition
process is conducted at a temperature of no greater than
475.degree.K.
3. The method of claim 2, wherein the inorganic material has a
thickness of about 5 to about 100 nm.
4. The method of claim 3, wherein the inorganic material is a
passivating layer.
5. The method of claim 3, wherein the inorganic material is alumina
or titania.
6. The method of claim 3, wherein the phosphor includes a host
material that contains luminescent centers.
7. The method of claim 6, wherein the host material is at least one
of ZnS, CaS, (ZnCd)S, CaS, (CaSrGa)S, CaGa.sub.2S.sub.4,
ZnGa.sub.2O.sub.4, Zn.sub.2SiO.sub.4, Y.sub.2O.sub.2S,
Y.sub.2SiO.sub.5, SrS, SrGa.sub.2S.sub.4, ZnO and
Y.sub.2O.sub.3.
8. The method of claim 3, further comprising depositing a layer of
a hydrophobic material atop the layer of the inorganic
material.
9. A method comprising forming a luminescent layer on the surface
of a particle of a host material, wherein the luminescent layer is
formed via an atomic layer deposition process.
10. The method of claim 9, wherein the luminescent layer is formed
by intermittently and sequentially introducing into a reaction zone
containing the host material particles, a first precursor mixture
that includes a compound containing a metal that forms a
luminescent center and a second compound that does not contain a
metal that forms a luminescent center, and a second precursor
material that reacts with the first precursor mixture at the
surface of the host material particles.
11. The method of claim 9, wherein the luminescent layer is formed
in a series of reaction cycles by intermittently and sequentially
introducing into a reaction zone containing the host material
particles a metal precursor and a reducing agent for the metal
precursor material, wherein the metal precursor is in some but not
all reaction cycles a compound of a metal that forms a luminescent
center.
12. The method of claim 11 wherein the host material is at least
one of ZnS, CaS, (ZnCd)S, CaS, (CaSrGa)S, CaGa.sub.2S.sub.4,
ZnGa.sub.2O.sub.4, Zn.sub.2SiO.sub.4, Y.sub.2O.sub.2S,
Y.sub.2SiO.sub.5, SrS, SrGa.sub.2S.sub.4, ZnO and
Y.sub.2O.sub.3.
13. The method of claim 11 wherein a passivating layer is deposited
via an ALD process atop the luminescent layer.
14. The method of claim 13 wherein a hydrophobic layer is deposited
atop the passivating layer.
15. The method of claim 10 or 11 wherein a hydrophobic layer is
deposited atop the luminescent layer.
16. A phosphor particle having a passivating film of 5-100 nm
thickness which has been deposited by the method of claim 1.
17. A host material having a luminescent layer of 5-100 nm in
thickness.
18. A host material of claim 17 wherein the luminescent layer is
deposited via an ALD process.
19. The host material of claim 18, wherein the luminescent layer
contains at least two different luminescent centers.
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 panel displays commonly include a phosphor layer which
is sandwiched between two insulator layers. A reflecting electrode
such as aluminum and a transparent electrode such as indium tin
oxide bracket the phosphor/insulator sandwich. In the standard
MISIM (metal-insulator-semiconductor (phosphor)-insulator-metal)
construction, a glass or other transparent substrate lies atop the
transparent electrode. Various filter layers may be incorporated
into the structure to assist with color production.
[0004] The phosphor material is commonly printed as a thin film
onto an adjacent layer. There are several reasons that make it
preferable to use a powdered phosphor. Chief among these is
cost--powdered phosphors can be used in very small amounts and so
the amount of phosphor that is needed can be significantly reduced.
In addition, light loss through internal reflection can be
minimized using particles and there is no loss in brightness due to
light lost at edges, as in thin phosphor films. Efficiency (light
emitted/unit applied power) is also higher for powders. The use of
powders also makes it possible to produce all colors in a single
phosphor plane, as a particulate mixture of different
color-emitting phosphors can be formed as a single layer.
[0005] The phosphor particles typically are composites of a host
material that in the case of electroluminescent particles provides
a necessary set of electrical properties, and one or more
"luminescent centers". The "luminescent centers" are usually metal
cations and sometimes anions which are "doped" or otherwise
combined with the host material. These ions usually become
incorporated into the crystalline lattice of the host material, or
dispersed as discrete domains within the host material. The
luminescent centers provide the desired optical emission properties
to the phosphor particles. Again, a wide variety of these materials
are known, which differ in their composition according to the
specific application and desired emitted color. Phosphors that emit
white, yellow, red, green and blue wavelengths of visible light are
commonly used in display and monitor applications.
[0006] It is often necessary to coat the surface of the phosphor
particles. Reasons for doing this include (1) particle protection,
often against reaction with water, but also against reaction with
air, other oxidants, or contaminants; (2) improving screening
characteristics and (3) improving contrast or pigmentation. Among
the coating materials used for these purposes are ZnO, MgO,
In.sub.2O.sub.3, Al.sub.2O.sub.3 and SiO.sub.2, and CuS. Chemical
vapor deposition (CVD) and sol-gel methods have been used to
provide coatings of these types.
[0007] To be effective, the applied coating needs to be as uniform
and as thin as possible. It is also beneficial that the coating
process does not cause individual particles to agglomerate to form
larger aggregates. 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. Quite often, material applied by these
processes form separate particles instead of forming films on the
surface of the phosphor particles.
[0008] For various reasons, it is desired to develop phosphor
particles that are smaller than those commonly used now.
Commercially available phosphor particles usually have diameters in
the 1-50 micron range. Phosphor particles having diameters of less
than 1 micron, and in particular less than 100 nm, potentially
offer advantages in screen design and performance. CVD and sol-gel
coating methods are particularly unsuitable for coating these
smaller particles.
[0009] It is therefore desirable to provide a method by which thin,
conformal coatings can be applied to the surface of phosphor
particles. Such a method preferably will result in minimal
agglomeration of the particles, and provide coatings with minimal
defects. The method also desirably permits very thin coatings to be
applied, and further allows for close control over coating
thickness. Even more preferably, the method would allow for
sequential deposition of multiple coatings of different materials
onto the phosphor. For example, a preferred method would permit one
to deposit a luminescent layer atop a particle of the host
material, and then to deposit a protective layer atop the
luminescent layer.
SUMMARY OF THE INVENTION
[0010] This invention is a method comprising forming a layer of an
inorganic material on the surface of a phosphor particle via an
atomic layer deposition process.
[0011] This invention is also a method comprising forming a
luminescent layer on the surface of a particle of a host material,
wherein the luminescent layer is formed via an atomic layer
deposition process.
[0012] This invention is also a phosphor particle having a
passivating film of 5-100 nm thickness which has been deposited by
an ALD process.
[0013] This invention is also a base particle having a luminescent
layer of 5-100 nm thickness. The base particle is in preferred
embodiments a host material, which is capable of capturing an
excitation energy and transferring it to a luminescent center in
the luminescent layer.
[0014] The process of the invention permits the deposition of an
extremely thin layer onto the surface of the phosphor particle or
host material particle. In some embodiments, the applied layer can
act as a passivating layer, protecting the base particle against
corrosion or attack from ambient agents such as water or air. The
ALD process permits effective protective layers to be formed even
at thicknesses of less than 100 nm. The resulting phosphor
particles often perform better, in that they often emit more light
(per unit applied energy) than phosphors coated using CVD or
sol-gel processes. This increases the energy demands of a display
made using the phosphor.
[0015] By applying a luminescent layer (as described more fully
below) over a base particle (or an inert material), it is possible
to use lower cost materials as the base particle, and thus reduce
phosphor cost.
[0016] This invention also provides the ability to prepare a wide
variety of specialized phosphor particles. In one aspect of the
invention, at least one applied layer is of a material that can
capture all or a part of the light emitted by an underlying
phosphor particle, and re-emit that light at a different (typically
lower energy) wavelength. It is also possible to use the applied
coatings to produce particles that produce multiple wavelengths of
emitted light.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The atomic layer deposition process is characterized in that
at least two different reactants are needed to form the coating
layer. The reactants are introduced into the reaction zone
individually, sequentially and in the gas phase. Excess amounts of
reactant are removed from the reaction zone before introducing the
next reactant. Reaction by-products are removed as well, between
introductions of the reagents. This procedure ensures that
reactions occur at the surface of the phosphor particles, rather
than in the gas phase. Gas phase reactions, such as occur in
chemical vapor deposition processes, are undesirable for several
reasons. CVD reactions tend to cause particle agglomeration, form
uneven and non-conformal coatings, and use greater amounts of raw
materials than desired.
[0018] A purge gas is typically introduced between the alternating
feeds of the reactants, in order to further help to remove excess
reactants. A carrier gas, which is usually but not necessarily the
same as the purge gas, generally is introduced during the time each
reactant is introduced. The carrier gas may perform several
functions, including (1) facilitating the removal of excess
reactant and reaction by-products, (2) distributing the reactant
through the reaction zone, thereby helping to expose all particle
surfaces to the reactant and (3) fluidizing the phosphor particles
so that all particle surfaces become exposed to the reactant.
[0019] A typical pattern of introducing reactants (in a two-reagent
ALD reaction scheme) is:
[0020] 1. Introduce purge/fluidizing gas.
[0021] 2. Introduce mixture of carrier gas and first reagent.
[0022] 3. Introduce purge/fluidizing gas and/or pull a high vacuum
to remove excess quantities of the first reagent as well as
reaction by-products.
[0023] 4. Introduce mixture of carrier gas and second reagent.
[0024] 5. Introduce purge/fluidizing gas and/or pull a high vacuum
to remove excess quantities of the second reagent and reaction
by-products.
[0025] 6. Repeat steps 2-5 until desired coating thickness is
obtained.
[0026] As mentioned, the same material may be used as the
purge/fluidizing gas and each carrier gas. It is also possible to
use different materials.
[0027] Analogous patterns are used when the film-forming reaction
involves more than two reagents, or when a catalyzed reaction
system is used. An example of a catalyzed reaction system is
described below.
[0028] Such atomic layer controlled growth techniques permit the
formation of deposits of up to about 0.3 nm in thickness per
reaction cycle, and thus provide a means of extremely fine control
over deposit thickness. The reactions are self-limited, and in most
instances can be repeated to sequentially deposit additional layers
of the deposited material until a desired thickness is
achieved.
[0029] It is preferred to treat the particles before initiating the
reaction sequence to remove volatile materials that may be absorbed
onto the particle surface. This is readily done by exposing the
particles to elevated temperatures and/or vacuum. Also, in some
instances a precursor reaction may be performed to introduce
desirable functional groups onto the surface of the particle.
[0030] Reaction conditions are selected mainly to meet two
criteria. The first criterion is that the reagents are gaseous
under the conditions of the reaction. Therefore, temperature and
pressure conditions are selected such that the reactants are
volatilized. The second criterion is one of reactivity. Conditions,
particularly temperature, are selected such that the desired
reaction between the film-forming reagents (or, at the start of the
reaction, the first-introduced reagent and the particle surface)
occur at a commercially reasonable rate.
[0031] The temperature of the reactions may range from
250-700.degree. K. The temperature is preferably no greater than
about 475.degree. K and more preferably no greater than 425.degree.
K when the particle being coated is a phosphor rather than simply a
particle of the host material. Temperatures in excess of these tend
to cause diffusion of the luminescent centers from the crystalline
lattice of the host material, which destroys or diminishes the
ability of the particle to emit light.
[0032] Specific temperature and pressure conditions will depend on
the particular reaction system, as it remains necessary to provide
gaseous reactants. Subatmospheric pressures will normally be
required.
[0033] A suitable apparatus for conducting the ALD reaction is one
which permits the particles to become separated so that all
particle surfaces become exposed to the reagents. One convenient
method for exposing the base particles to the reagents is to form a
fluidized bed of the particles, and then pass the various reagents
in turn through the fluidized bed under reaction conditions.
Methods of fluidizing particulate materials are well known, and
generally include supporting the particles on a porous plate or
screen. A fluidizing gas is passed upwardly through the plate or
screen, lifting the particles somewhat and expanding the volume of
the bed. With appropriate expansion, the particles behave much as a
fluid. The reagents can be introduced into the bed for reaction
with the surface of the particles. In this invention, the
fluidizing gas also can act as an inert purge gas for removing
unreacted reagents and volatile or gaseous reaction products.
[0034] In addition, the reactions can be conducted in a rotating
cylindrical vessel or a rotating tube. A rotating reactor comprises
a hollow tube that contains the base particles. The reactor may be
held at an angle to the horizontal, so that the particles pass
through the tube through gravitational action. In such a case, the
reactor angle determines the flow rate of the particulate through
the reactor. The reactor can be rotated in order to distribute
individual particles evenly and expose all particles to the
reactants. The reactor design permits the substrate particles to
flow in a near plug-flow condition, and is particularly suitable
for continuous operations. The rotating cylindrical vessel can also
be sealed on both ends and have porous metal walls that allow the
gases to flow in and out of the rotating cylindrical vessel. This
rotary reactor is convenient for static reactant exposures and
batch processing of phosphor particles.
[0035] The progress of the reaction can be monitored using
techniques such as transmission Fourier transform infrared
techniques, transmission electron spectroscopy, scanning electron
microscopy, Auger electron spectroscopy, x-ray fluorescence, X-ray
photoelectron spectroscopy and x-ray diffraction.
[0036] The particles used as substrates in this invention are
either (1) phosphor particles, i.e. particles that include both
host material and luminescent centers and emit photons in response
to the application of a particular type of excitation energy, or
(2) particles of a host material.
[0037] The composition of the phosphor or host material is not
particularly limited. All that is necessary is that the particle
contains some surface functional group which can serve as a site
through which the first-applied ALD reagent can become bonded to
the substrate particle. Useful functional groups are typically an
M-H, M-O--H, M-S--H or M-N--H group, where M represents an atom of
a metal or semi-metal. If necessary, these sites can be introduced
onto the particle surface through various preparative methods.
[0038] Suitable host materials are materials that can capture one
or more forms of excitation energy and transfer the captured energy
to the luminescent center. Examples of phosphor host materials
include, for example, ZnS, CaS, (ZnCd)S, CaS, (CaSrGa)S,
CaGa.sub.2S.sub.4, ZnGa.sub.2O.sub.4, Zn.sub.2SiO.sub.4,
Y.sub.2O.sub.2S, Y.sub.2SiO.sub.5, SrS, SrGa.sub.2S.sub.4, ZnO,
Y.sub.2O.sub.3 and the like. Materials that are susceptible to
oxidation and/or hydrolysis when exposed to air are of particular
interest, as it is usually necessary to provide a passivating film
on the surface of such materials.
[0039] The term "luminescent center" refers to ions contained
within the crystalline lattice of the host material, which render
the phosphor capable of emitting a photon in response to the
excitation energy that is captured by the host material. The
luminescent center typically includes a metal cation. In some
cases, an anion such as a halide forms the luminescent center in
conjunction with the metal cation. Luminescent centers each
typically produce photons of one or more characteristic
wavelengths. The selection of luminescent center is therefore
dictated mainly by the color of light that is to be produced and
the ability to function together with the host material. In cases
where the substrate particle is the phosphor itself (rather than
simply the host material), the selection of luminescent center is
not considered critical to the invention, provided that the
substrate particle has some functional groups as described
before.
[0040] Luminescent centers that include manganese, samarium,
europium, terbium, cerium, selenium, silver and like atoms are of
particular interest in forming electroluminescent materials for
flat screen displays. In some instances, combinations of different
metals may constitute the luminescent center. For instance a zinc
sulfide host material containing both europium and cerium as
luminescent centers forms a phosphor that emits white light, rather
than the red (europium) and blue-green (cerium) colors emitted by
the individual metals alone.
[0041] In most cases, the luminescent center is incorporated into
the crystalline lattice of the host material. However, it is also
possible that the luminescent center forms domains within or upon
the host material. It is not considered critical to the invention
that the luminescent center have any particular structural or
spatial relationship to the host material, provided that the
phosphor particle as a whole performs the desired function.
[0042] A combination of host material and luminescent center is
often described using the notation XX:YY, where XX designates the
host material and YY designates the luminescent center. Some
particular phosphor materials of interest for use in this invention
include ZnS:Mn (which produces yellow light), ZnS:Sm,Cl (red),
CaS:Eu (red), CaSSe:Eu (red), ZnS:TbOF (green), ZnS:Tb (green),
CaS:Ce,Cl, (green), ZnGa.sub.2O.sub.4:Mn (green), ZnSiO.sub.4:Mn
(green), SrS:Ce,F (blue-green), SrS:Ce,Mn,Cl (blue-green), SrS;Ce
(blue-green), ZnS:Tm,F (blue), SrGa.sub.2S.sub.4:Ce (blue),
CaGa.sub.2S.sub.4:Ce (blue), SrS:Ce,Eu (white) and ZnS:Mn/SrS:Ce
(white).
[0043] The host material or phosphor is in the form of a
particulate. The size of the particles may range to up about 500
.mu.m, with preferred particle sizes ranging from the nanometer
range (e.g. about 0.001 .mu.m) to about 100 .mu.m, more preferred
particle sizes ranging from 0.005 to about 50 .mu.m, even more
preferred particle sizes ranging from about 0.05 to 35 .mu.m and
most preferred particle sizes ranging from about 0.4 to about 25
.mu.m. The invention is particularly suitable for coating extremely
fine phosphor particles having a particle size of 1 micron or less,
especially less than 100 nm. Particle size can also be expressed in
terms of the surface area of the particles. Preferred particulate
materials have surface areas in the range from about 0.1 to about
200 m.sup.2/g or more.
[0044] The ALD coating may be any coating that can be applied via
an ALD process. Preferred ALD coatings are those which can be
applied via an ALD process at a temperature no higher than
475.degree. K, and especially 425.degree. K or less. Several types
of coatings are of particular interest. The first type is a
passivating film that protects the underlying particle from
undesired reactions, such as with air, ambient humidity, or other
materials. The second type is a film that constitutes or contains a
luminescent layer. By "luminescent layer", it is meant a layer that
contains luminescent centers which emit photons in response to
excitation energy captured by the host material (or other
components of the luminescent layer). The luminescent layer may and
preferably does comprise a host material as described above, which
contains the luminescent centers.
[0045] Suitable films of the first type include metal or semi-metal
oxides, sulfides, phosphides and nitrides, in which the metal ion
does not function as a luminescent center. The film is preferably
transparent or nearly transparent to the photons emitted by the
underlying phosphor particle, and forms a barrier to air, moisture
and/or other ambient materials from which the underlying particle
is to be protected. Films of particular interest include oxides
such as silicon dioxide, zirconia, alumina, silica, yttria, zinc
oxide, magnesium oxide, TiO.sub.2 and the like; nitrides such as
silicon nitride, AlN and BN; and sulfides such as gallium sulfide,
tungsten sulfide and molybdenum sulfide.
[0046] Suitable films of the second type (luminescent layers)
include ions that function as luminescent centers. These ions are
typically present in small quantities, the remainder of the
luminescent layer being a non-luminescent inorganic material that
is either a host material (i.e., one that captures an excitation
energy and transfers the captured energy to the luminescent
center), or some other material that permits excitation energy to
be transferred from the underlying host material to the luminescent
center in the luminescent layer. Metal cations or anions that
function as luminescent centers, and which can be used as such in
the luminescent layer, are as described above. All that is
necessary is that they are capable of being deposited via an ALD
process.
[0047] A luminescent layer can be formed in at least two ways. In
one method, a non-luminescent material is applied onto the particle
surface via repeated half-reactions, and the luminescent center is
formed by occasionally substituting into the ALD reaction sequence
a precursor that contains the luminescent metal. This method forms
extremely thin layers containing the luminescent centers within a
thicker luminescent layer. Thus, for example, a ZnS layer may be
deposited by alternately exposing a host material particle with
zinc-containing precursor and a reducing agent (typically hydrogen
sulfide) in a reaction sequence as described before. Occasionally,
one or more of the exposures substitutes a precursor that contains
the luminescent center metal for the zinc-containing precursor.
This introduces the luminescent center metal into the deposited
coating. Generally it is only necessary to provide the luminescent
center metal in amounts up to 100 ppm or less of the weight of the
luminescent layer.
[0048] In the second method, a precursor that contains the
luminescent center is mixed with one of the precursors for the
non-luminescent material. In this case, both precursors should be
able to react with the same co-reactant in the ALD process. For
example, a precursor containing the luminescent center metal may be
introduced simultaneously with a zinc-containing precursor. Each is
then reduced simultaneously with a single reducing agent (for
example, hydrogen sulfide) to form a layer containing both the zinc
and the luminescent center metal.
[0049] When a luminescent layer is applied, a film of the first
type may be applied over it to provide a passivating coating.
[0050] A third type of film is a material that captures energy
captured and transformed by the underlying particle (such as light
emitted by a phosphor particle that serves as the base particle),
and emits light at some specialized wavelength. In this embodiment,
the base particle may be a host material (which captures and
transfers the energy to the applied film) or a phosphor (which will
capture energy and emit light, which is then capture by the applied
film and re-emitted at a different (usually lower energy)
wavelength). This method permits the development of phosphor
particles that emit specialized wavelengths of light or specialized
combinations of wavelengths of light.
[0051] The second and third types of film may contain multiple
types of luminescent centers, i.e., different cations or anions
that each can act as a luminescent center to produce its own
characteristic wavelength of emitted light. The ability to easily
apply luminescent layers having a wide range of possible light
emission characteristics makes it possible to "tailor" the coated
particle to produce a very wide variety of light emissions.
Specific, characteristic combinations of emitted light wavelengths
can be produced, which can act, for example, as "fingerprints"
which enable identification of the source of the emitted light.
[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. M.sup.1 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*+M.sup.1X.sub.n.fwdarw.M-Z-M.sup.1X*+HX (A1)
M-Z-M.sup.1X*+H.sub.2O.fwdarw.M-Z-M.sup.1 OH*+HX (B1)
In reaction A1, reagent M.sup.1X.sub.n reacts with one or more
M*-Z-H groups on the surface of the particle to create a new
surface group having the form -M.sup.1-X. M.sup.1 is bonded to the
particle through one or more Z atoms. The -M.sup.1-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 M.sup.1 atoms. Note that in some cases (such as, e.g., when
M.sup.1 is silicon, zirconium, titanium, yttrium or aluminum)
hydroxyl groups can be eliminated as water, forming
M.sup.1-O-M.sup.1 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 M.sup.1 is silicon, are described more fully in J.
W. Klaus et al, "Atomic Layer Controlled Growth of SiO.sub.2 Films
Using Binary Reaction Sequence Chemistry", Appl. Phys. Lett. 70,
1092 (1997) and O. Sheh et al., "Atomic Layer Growth of SiO.sub.2
on Si(100) and H.sub.2O 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 M.sup.1 is aluminum, are described in A. C. Dillon et al,
"Surface Chemistry of Al.sub.2O.sub.3 Deposition using
Al(CH.sub.3).sub.3 and H.sub.2O in a Binary reaction Sequence",
Surface Science 322, 230 (1995) and A. W. Ott et al.,
"Al.sub.2O.sub.3 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 SiO.sub.2 and Al.sub.2O.sub.3 coatings on particulate
materials in accordance with this invention.
[0058] A specific reaction sequence of the Al/B1 type that produces
alumina is:
Al--(CH.sub.3)*+H.sub.2O.fwdarw.Al--OH*+CH.sub.4 (A1A)
Al--OH*+Al(CH.sub.3).sub.3.fwdarw.Al--O--Al(CH.sub.3).sub.2*+CH.sub.4
(B1A)
This particular sequence of reactions is particularly preferred to
deposit alumina, as the reactions proceed well even at temperature
below 350.degree. K. This particular reaction sequence tends to
deposit Al.sub.2O.sub.3 ALD at a rate of .about.1.2 .ANG. per AB.
Triethyl aluminum (TEA) can be used in place of trimethyl
aluminum.
[0059] Analogous reaction sequences can be performed to produce
nitride and sulfide deposits. An illustrative reaction sequence for
producing a nitride coating is:
M-Z-H*+M.sup.1X.sub.n.fwdarw.M-Z-M.sup.1X*+HX (A2)
M-Z-M.sup.1X*+NH.sub.3.fwdarw.M-Z-M.sup.1 NH*+HX (B2)
Ammonia can be eliminated to form M.sup.1-N-M.sup.1 bonds within or
between layers. This reaction can be promoted if desired by, for
example, annealing at elevated temperatures and/or reduced
pressures.
[0060] An illustrative reaction sequence for producing sulfide
deposits is:
M-Z-H*+M.sup.1X.sub.n.fwdarw.M-Z-M.sup.1X*+HX (A3)
M-Z-M.sup.1X*+H.sub.2S.fwdarw.M-Z-M.sup.1 SH*+HX (B3)
Hydrogen sulfide can be eliminated to form M.sup.1-S-M.sup.1 bonds
within or between layers. As before, this reaction can be promoted
by annealing at elevated temperatures and/or reduced pressures.
This reaction scheme is particularly useful for forming a
luminescent layer, as sulfides such as Zn, Sr and Ga sulfide are
good host materials. To form a luminescent layer, a portion of the
M atoms will be metals that form luminescent centers as described
above.
[0061] In the foregoing reaction sequences, preferred metals
M.sup.1 include silicon, aluminum, yttrium, titanium, zinc,
magnesium and zirconium. Suitable replaceable nucleophihc groups
will vary somewhat with M.sup.1, but include, for example,
fluoride, chloride, bromide, alkoxy, alkyl, acetylacetonate, and
the like. Specific compounds having the structure M.sup.1X.sub.n
that are of particular interest are silicon tetrachloride, tungsten
hexafluoride, tetramethylorthosilicate (Si(OCH.sub.3).sub.4),
tetraethyl-orthosilicate (Si(OC.sub.2H.sub.5).sub.4), trimethyl
aluminum (Al(CH.sub.3).sub.3), triethyl aluminum
(AM(C.sub.2H.sub.5).sub.3), other trialkyl aluminum compounds,
yttrium acetylacetonate, cobalt acetylacetonate, and the like.
[0062] In addition, catalyzed binary reaction techniques such as
described in U.S. Pat. No. 6,818,250, incorporated by reference,
are suitable for depositing inorganic materials, in particular
oxide coatings. Reactions of this type can be represented as
follows:
M-F.sub.1+C.sub.1.fwdarw.M-F.sub.1 . . . C.sub.1 (A4a)
M-F.sub.1 . . .
C.sub.1+F.sub.2-M.sub.1-F.sub.2.fwdarw.M-M.sup.1-F.sub.2+F.sub.1--F.sub.2-
+C.sub.1 (A4b)
M-M.sup.1-F.sub.2+C.sub.2.fwdarw.M-M.sup.1-F.sub.1 . . . C.sub.2
(B4a)
M-M.sup.1-F.sub.1 . . .
C.sub.2+F.sub.1-M-F.sub.1.fwdarw.M-M.sup.1-M-F.sub.1+F.sub.1--F.sub.2+C.s-
ub.2 (B4b)
C.sub.1 and C.sub.2 represent catalysts for the A4b and B4b
reactions, and may be the same or different. F.sub.1 and F.sub.2
represent functional groups, and M and M.sup.1 are as defined
before, and can be the same or different. Reactions A4a and A4b
together constitute the first part of a binary reaction sequence,
and reactions B4a and B4b together constitute the second half of
the binary reaction sequence. An example of such a catalyzed binary
reaction sequence is:
Si--OH*(particle)+C.sub.5H.sub.5N.fwdarw.Si--OH . . .
C.sub.5H.sub.5N*
Si--OH . . .
C.sub.5H.sub.5N*+SiCl.sub.4.fwdarw.Si--O--SiCl.sub.3*+C.sub.5H.sub.5N+HCl
Si--O--SiCl.sub.3*+C.sub.5H.sub.5N.fwdarw.Si--O--SiCl.sub.3 . . .
C.sub.5H.sub.5N*
Si--O--SiCl.sub.3 . . .
C.sub.5H.sub.5N*+H.sub.2O.fwdarw.Si--O--SiOH*+C.sub.5H.sub.5N+HCl
where the asterisks (*) again denote atoms at the surface of the
particle. This general method is applicable to depositing various
other materials, including zirconia or titania.
[0063] Suitable binary reaction schemes for depositing metals
include those described in the U.S. patent application No.
6,958,174, which is incorporated herein by reference. A specific
reaction scheme described therein involves sequential reactions of
a substrate surface with a metal halide followed by a metal halide
reducing agent. The metal of the metal halide is preferably one
that forms a luminescent center in a luminescent layer that is
deposited on a particle of a host material.
[0064] Another binary reaction scheme suitable for depositing a
metal (M.sup.2) on a particle having surface hydroxyl or amine
groups can be represented as:
M*-Z-H+M.sup.2X.sub.n.fwdarw.M-Z-M.sup.2*X+HX (precursor
reaction)
M-Z-M.sup.2X*+H.sub.2.fwdarw.M-Z-M.sup.2-H*+HX (B6)
M-Z-M.sup.2-H*.sub.+M.sup.2(acac).fwdarw.M-Z-M.sup.2-M.sup.2*(acac)
(A6)
"Acac" refers to acetylacetonate ion, and X, Z and M are as defined
before. Also as before, the asterisk (*) refers to an atom residing
at the surface of the particle. By heating to a sufficient
temperature, hydrogen bonded to the surface as M.sup.2-H will
thermally desorb from the surface as H.sub.2, thereby generating a
final surface composed of M.sup.2 atoms.
[0065] In another embodiment, the coated particles are treated with
a hydrophobic compound in order to increase their hydrophobic
nature and provide further protection against hydrolysis or
reaction with water-soluble materials. The hydrophobic compound is
conveniently applied as a final step in the ALD process. A suitable
hydrophobic compound includes one or more alkyl or, preferably,
fluoroalkyl groups and at least one functional group that can react
with a surface species on the surface of the particle and form a
bond to the particle surface. Aminosilanes containing alkyl and
especially fluoroalkyl groups are examples of such hydrophobic
compounds. A specific example of such a compound is
tridecafluoro-1,1,2,2-tetrahydrooctylmethyl-bis(dimethylamino)silane
(FOMB(DMA)S,
C.sub.8F.sub.13H.sub.4(CH.sub.3)Si(N(CH.sub.3).sub.2).sub.2).
[0066] The following examples are provided to illustrate the
invention, but are not intended to limit its scope. All parts and
percentages are by weight unless otherwise indicated.
EXAMPLE 1
[0067] Zinc sulfide-based phosphor particles having a particle size
of approximately 26.+-.1 micron are placed into a fluidized bed
reactor. The fluidized bed column is tubular and constructed of
stainless steel. It is heated using an external furnace that
surrounds the reactor. The column has an inside diameter of 6 cm
and a height of 60 cm. It is equipped with a distributor
constructed of 10 micron average pore size porous stainless steel.
The upper head of the fluidized bed reactor contains four internal
10 micron pore size porous metal filters that prevent phosphor
particles from being entrained out of the fluidized bed. Protruding
from the center of the fluidized bed upper head is a long stirring
device that extends to the distributor plate where it sweeps across
the distributor plate surface and up the outside wall. It is
connected to an external drive via a magnetic coupling. Four
internal filters are on a manifold that connects to a pipe leading
to a vacuum pump (Alcatel model 2063) that provides system vacuum.
A plenum section below the gas distributor is connected to a mass
flow controller, which controls the fluidizing gas flow. Separate
injection loops are provided for the introduction of the reactants,
each with a needle valve controlling each loop's final conductance.
Reactant vapor pressure is used as the driving force for flow. A
purge setup is achieved by connecting the system to both a mass
flow controller that provides a nitrogen source and a separate
vacuum pump, one for each loop (Alcatel model 2010). Valves that
control the system flows are all pneumatically operated and
remotely controlled via a LabVIEW.TM. program running on a personal
computer. The fluidized bed reactor is mounted to a plate which is
vibrated using two Martin Engineering vibratory motors. The
mechanical vibration is useful for improving the fluidizability of
ultrafine particles.
[0068] The particles require a minimum fluidization velocity at 1.4
Torr pressure of 8 cm/s, or 25 sccm N.sub.2@ STP. Dry nitrogen is
supplied to the reactor at that rate from a liquid nitrogen source
dewar. Trimethylaluminum and water vapor are separately admitted
into the system via two separate injection loops. The loops are
constructed to ensure that both loops have the same conductance,
with a needle valve controlling each loop's final conductance.
Reactant vapor pressure is used as the driving force for flow.
[0069] 100 grams of the phosphor particles are placed in the
stainless steel tubular fluidized bed reactor. This height of the
bed is approximately 3 cm above the distributor. The bed is
vibrated and system pressure is brought down to an operating
pressure of approximately 2 Torr. Fluidizing/purging gas flow is
started at 25 sccm. The reactor is then heated to a temperature of
350K. Trimethyl aluminum is admitted into the reactor for 20
seconds, followed by a 50 second flow of the purge gas. Water vapor
is then admitted into the reactor for 20 seconds, followed by
another 50 second flow of the fluidization gas. This sequence is
repeated 20 times. A film of approximately 1 angstrom thickness is
deposited each cycle. Film thickness at the end of the 20 cycles is
about 2.5 nanometers.
[0070] XPS analysis of the surface of these particles indicates
that the surface has become coated with alumina, as evidenced by
the appearance of Al 2p peaks in the spectrum at binding energy
values of approximately 72-77 eV. A particle size distribution
analysis of the alumina coated particles indicates that the average
particle size is 27.0.+-.1.0 micron, thus indicating that
essentially no agglomeration of phosphor particles has
occurred.
[0071] The coated ZnS phosphor particles are placed in a silver
nitrate solution at room temperature. This is a simple test of the
integrity of the applied coating, as silver nitrate that contacts
the zinc sulfide will form silver sulfide, which causes the
particles to turn black. The particles turn black in less than 1
hour.
[0072] A lighting test lamp device is fabricated using the coated
particles. The lamp shines more brightly than a similar lamp coated
with ZnS based phosphor particles coated via a CVD process. The
improved brightness is attributed to (1) the films being thinner
(approximately 50 nanometers, instead of 500 to 1000 nanometers as
is typical of a CVD process) and (2) the film being conformal and
non-granular.
EXAMPLE 2
[0073] The method of Example 1 is repeated with the exception that
the number of reaction cycles is increased to 175. An alumina film
thickness of approximately 20 nanometers thickness is produced.
Some of the coated particles are placed in a silver nitrate
solution. The particles turn black after 2 days, thus indicating
that the silver nitrate penetrates the 20 nanometer alumina film
much more slowly than it does the 2.5 nanometer film of Example 1.
A lighting test lamp device fabricated of some of the freshly
coated particles shines more brightly than a like lamp made using
conventionally coated ZnS based phosphor materials.
EXAMPLE 3
[0074] The method of Example 1 is repeated with the exception that
the number of reaction cycles is increased to 550. An alumina film
thickness of approximately 60 nanometers thickness is produced on
the surface of the phosphor particles. Some of the coated particles
are placed in a silver nitrate solution as before, but show no
evidence of reaction even after one month. As before, a lamp
produced from these particles shines more brightly than one made
using conventionally coated phosphor particles.
EXAMPLE 4
[0075] A rotary fluidized bed reactor is used to coat zinc
sulfide-based phosphor particles in this example. The rotary
fluidized bed reactor is similar to that used in Example 1, except
that the particles are contained in a porous metal cylinder located
in a vacuum system. This porous metal has an average pore size of
20 microns. The porous metal cylinder has an inside diameter of 7.5
cm and a length of 20 cm. The particles are rotated in the cylinder
at 90 rpm using a magnetically coupled rotary motion
feedthrough.
[0076] 200 grams of 25 micron average diameter zinc sulfide based
phosphor particles are placed in the stainless steel porous metal
cylinder. This fills the porous metal cylinder to approximately 15%
of the total volume. The system is pumped to a base pressure of 30
mTorr. The reactor is then heated to 450 K. This deposition
temperature allows for anatase phase titania to be deposited,
though other deposition temperatures can be used if desired to
produce rutile or amorphous titania films. Titanium tetrachloride
is introduced to the reactor to a pressure of 8 Torr and allowed to
react statically at the particle surface for 60 seconds. The system
is then evacuated to base pressure and 30 Torr of nitrogen fills
the chamber. This is held statically for 5 seconds. The reactor is
then evacuated to base pressure again. Next, water vapor is
introduced to the reactor to a pressure of 8 Torr, allowed to react
statically for 60 seconds and evacuated to base pressure. Nitrogen
is again introduced and evacuated for another purge step. This
sequence is repeated 125 times. An average titania (TiO.sub.2)
growth rate of approximately 0.4 Angstroms per cycle is achieved,
resulting in a final film thickness of approximately 5
nanometers.
[0077] XPS analysis of the surface of these particles indicates
that the surface is coated with titania (TiO.sub.2), as evidenced
by the appearance of a Ti 2p.sup.3/2 peak in the spectrum at
binding energy values of approximately 458 eV. The powder has a
surface area of 0.6 m.sup.2 g.sup.-1. A particle size distribution
analysis of the titania coated particles indicates an average
particle size of 25 microns, thus indicating that no measurable
agglomeration of phosphor particles occurs during the coating
process.
[0078] The coated phosphor particles are placed in a silver nitrate
solution as described before. The particles turn black in during
the fifth day of testing. A lighting test lamp device is fabricated
as described in Example 1. A lamp fabricated from the coated
particles shines more brightly than a control lamp containing a
conventionally-coated phosphor.
* * * * *