U.S. patent application number 11/339010 was filed with the patent office on 2006-06-22 for phosphors.
This patent application is currently assigned to NanoGram Corporation. Invention is credited to Xiangxin Bi, Nobuyuki Kambe.
Application Number | 20060132020 11/339010 |
Document ID | / |
Family ID | 25505749 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060132020 |
Kind Code |
A1 |
Kambe; Nobuyuki ; et
al. |
June 22, 2006 |
Phosphors
Abstract
Small particles provide for improved performance as phosphors
especially in the production of display devices. Particles with a
diameter less than about 100 nm have altered band properties that
affect the emission by the particles. A collection of such small
particles with a narrow distribution around a selected average
diameter can be used to produce emission at a desired frequency.
These particles are effective for producing a wide variety of
display types including flat panel displays. Laser pyrolysis
provides an efficient process for the production of desired
particles.
Inventors: |
Kambe; Nobuyuki; (Menlo
Park, CA) ; Bi; Xiangxin; (San Ramon, CA) |
Correspondence
Address: |
Patterson, Thuente, Skaar & Christensen, P.A.
4800 IDS Center
80 South 8th Street
Minneapolis
MN
55402-2100
US
|
Assignee: |
NanoGram Corporation
|
Family ID: |
25505749 |
Appl. No.: |
11/339010 |
Filed: |
January 25, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
08962362 |
Oct 31, 1997 |
|
|
|
11339010 |
Jan 25, 2006 |
|
|
|
Current U.S.
Class: |
313/485 ;
313/467; 313/486 |
Current CPC
Class: |
C09K 11/0811 20130101;
C09K 11/565 20130101; C09K 11/0805 20130101; C09K 11/08 20130101;
C09K 11/7701 20130101 |
Class at
Publication: |
313/485 ;
313/467; 313/486 |
International
Class: |
H01J 63/04 20060101
H01J063/04; H01J 1/62 20060101 H01J001/62; H01J 29/10 20060101
H01J029/10 |
Claims
1. A display device comprising phosphor particles having an average
diameter less than about 95 nm and wherein the phosphor particles
comprise a first collection of particles having a diameter
distribution such that at least about 95 percent of the particles
have a diameter greater than about 40 percent of the average
diameter and less than about 160 percent of the average diameter
and the phosphor particles comprising a metal oxide.
2. The display device of claim 1 wherein the phosphor particles
comprise a metal compound selected from the group consisting of
ZnO, TiO.sub.2 and Y.sub.2O.sub.3.
3. The display device of claim 2 wherein the metal compound is
ZnO.
4. The display device of claim 1 wherein the phosphor particles
have an average diameter from about 5 nm to about 50 nm.
5. The display device of claim 1 wherein the phosphor particles
have a diameter distribution such that at least about 95 percent of
the particles have a diameter greater than about 60 percent of the
average diameter and less than about 140 percent of the average
diameter.
6. The display device of claim 1 wherein the light emission follows
low velocity electron excitation.
7. The display device of claim 1 wherein the phosphor particles
further comprise a second collection of particles, the second
collection of particles having a diameter distribution such that at
least about 95 percent of the particles have a diameter greater
than about 40 percent of the average diameter and less than about
160 percent of the average diameter.
8. The display device of claim 1 wherein the phosphor particles are
in contact with an anode.
9. The display device of claim 1 further comprising a liquid
crystal layer.
10. The display device of claim 1 further comprising a partially
light transparent substrate.
11. The display device of claim 1 further comprising a transparent
electrode comprising indium tin oxide.
12. The display device of claim 1 further comprising an electrode
to guide the electrons from the cathode to the anode.
13. The display device of claim 1 wherein the display is an
electroluminescent display.
14. The display device of claim 1 wherein the device is a field
emission device with the phosphor particles located between an
anode and cathode.
15. The display device of claim 14 comprising a plurality of anodes
and cathodes where each electrode pair forms an addressable
pixel.
16. The display device of claim 1 wherein the phosphor particles
are roughly spherical.
17. The display device of claim 1 wherein the phosphor particles
are excitable by low velocity electrons.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 08/962,362, filed Oct. 31, 1997, entitled
"PHOSPORS", which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates to phosphor particles that emit light
at desired wavelengths following stimulation and devices made with
these particles. The invention further relates to methods of
producing phosphor particles.
BACKGROUND OF THE INVENTION
[0003] Electronic displays often use phosphor material, which emits
visible light in response to interaction with electrons. Phosphor
materials can be applied to substrates to produce cathode ray
tubes, flat panel displays and the like. Improvements in display
devices place stringent demands on the phosphor materials, for
example, due to decreases in electron velocity and increases in
display resolution. Electron velocity is reduced in order to reduce
power demands. In particular, flat panel displays generally require
phosphors responsive to low velocity electrons.
[0004] In addition, a desire for color display requires the use of
materials or combinations of materials that emit light at different
wavelengths at positions in the display that can be selectively
excited. A variety of materials have been used as phosphors. In
order to obtain materials that emit at desired wavelengths of
light, activators have been doped into phosphor material.
Alternatively, multiple phosphors can be mixed to obtain the
desired emission. Furthermore, the phosphor materials must show
sufficient luminescence.
SUMMARY OF THE INVENTION
[0005] Small, nanoscale particles provide improved performance as
phosphors. For example, particles with average diameters less than
about 100 nm have altered band gaps with emission frequencies that
are functions of the particle diameters. Therefore, collections of
these particles with a narrow distribution of diameters can be used
to provide selected emission frequencies without necessarily
altering the particle composition. The small size of the particles
also results in high luminescence, responsiveness to low velocity
electrons as well as processing advantages. Laser pyrolysis
provides an efficient method for the production of highly pure
nanoscale particles with a narrow distribution of particle
sizes.
[0006] In a first aspect, the invention features a display device
comprising phosphor particles having an average diameter selected
to yield light emissions in a desirable portion of the
electromagnetic spectrum following excitation and the phosphors
particles having an average diameter less than about 100 nm. The
phosphor particles can comprise a metal compound such as ZnO, ZnS,
TiO.sub.2 and Y.sub.2O.sub.3. The phosphor particles preferably
have an average diameter from about 5 nm to about 50 nm and a
diameter distribution such that at least about 95 percent of the
particles have a diameter greater than about 60 percent of the
average diameter and less than about 140 percent of the average
diameter. In certain embodiments, the excitation of the phosphors
is accomplished with low velocity electrons.
[0007] In another aspect, the invention features a composition for
application by photolithography comprising phosphor particles and a
curable polymer, the phosphor particles having an average diameter
and a distribution of diameters selected to yield light emissions
in a selected portion of the electromagnetic spectrum following
excitation and the phosphor particles having an average diameter
less than about 100 nm. The curable polymer can be curable by UV
radiation or by electron beam radiation. The phosphor particles
preferably have an average diameter from about 5 nm to about 50
nm.
[0008] In another aspect, the invention features a method for
producing zinc oxide particles comprising the step of pyrolyzing a
molecular stream comprising a zinc precursor, an oxidizing agent
and a radiation absorbing gas in a reaction chamber, where the
pyrolysis is driven by heat absorbed from a laser beam. The zinc
oxide particles preferably have an average diameter less than about
150 nm and more preferably an average diameter from about 5 nm to
about 50 nm. In practicing the method, the laser beam preferably is
produced by a CO.sub.2 laser and the molecular stream preferably is
elongated in one dimension. Suitable zinc precursor include
ZnCl.sub.2.
[0009] In another aspect, the invention features a method for
producing zinc sulfide particles comprising the step of pyrolyzing
a molecular stream comprising a zinc precursor, a sulfur source and
a radiation absorbing gas in a reaction chamber, where the
pyrolysis is driven by heat absorbed from a laser beam.
[0010] Other features and advantages of the invention are apparent
from the following description of the preferred embodiments, and
from the claims.
BRIEF DESCRIPTION OF THE DRAWING
[0011] FIG. 1 is a schematic, sectional view of an embodiment of a
laser pyrolysis apparatus taken through the middle of the laser
radiation path. The upper insert is a bottom view of the injection
nozzle, and the lower insert is a top view of the collection
nozzle.
[0012] FIG. 2 is a schematic, perspective view of a reaction
chamber of an alternative embodiment of the laser pyrolysis
apparatus, where the materials of the chamber are depicted as
transparent to reveal the interior of the apparatus.
[0013] FIG. 3 is a sectional view of the reaction chamber of FIG. 2
taken along line 3-3.
[0014] FIG. 4 is a schematic, sectional view of an oven for heating
particles, in which the section is taken through the center of the
quartz tube.
[0015] FIG. 5 is a sectional view of an embodiment of display
device incorporating a phosphor layer.
[0016] FIG. 6 is a sectional view of an embodiment of a liquid
crystal display incorporating a phosphor for illumination.
[0017] FIG. 7 is a sectional view of an electroluminescent
display.
[0018] FIG. 8 is a sectional view of an embodiment of a flat panel
display incorporating field emission display devices.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] Small scale particles can be used as improved phosphor
particles. In particular, particles on the order of 100 nm or less
have superior processing properties to produce displays, and they
have good luminescence. Significantly, the band gap of these
materials is size dependent at diameters on the order of 100 nm or
less. Therefore, particles with a selected, narrow distribution of
diameters can serve as a phosphor at one color (wavelength) while
particles of the same or different material with similarly selected
average diameter and narrow distribution of sizes can serve as a
phosphor at a different color. In addition, the small size of the
particles can be advantageous for the production of higher
resolution displays.
[0020] Appropriate particles generally are chalcogenides,
especially ZnO, ZnS, TiO.sub.2, and Y.sub.2O.sub.3. Preferred
particles have a desired emission frequency and are highly
luminescent. In addition, preferred particles have persistent
emission, i.e., there is a significant time for the emission to
decay following stimulation of the material. Specifically, there
should be sufficient persistence of the emission to allow for human
perception. Suitable particles generally are semiconductors, and
their emission frequency is determined by the band gap. Preferably,
the luminescing state has an energy reasonably close to the
excitation energy such that little energy is wasted as heat.
[0021] Laser pyrolysis, as described below, is an excellent way of
efficiently producing ZnO, ZnS, TiO.sub.2 and Y.sub.2O.sub.3
particles with narrow distributions of average particle diameters.
A basic feature of successful application of laser pyrolysis for
the production of appropriate small scale particles is production
of a molecular stream containing a metal precursor compound, a
radiation absorber and a reactant serving as an oxygen or sulfur
source, as appropriate. The molecular stream is pyrolyzed by an
intense laser beam. The intense heat resulting from the absorption
of the laser radiation induces the reaction of the metal compound
precursor in the oxygen or sulfur environment. As the molecular
stream leaves the laser beam, the particles are rapidly
quenched.
A. Particle Production
[0022] Laser pyrolysis has been discovered to be a valuable tool
for the production of nanoscale metal oxide and sulfide particles
of interest. In addition, the metal oxide and sulfide particles
produced by laser pyrolysis are a convenient material for further
processing to expand the pathways for the production of desirable
metal compound particles. Thus, using laser pyrolysis alone or in
combination with additional processes, a wide variety of metal
oxide and sulfide particles can be produced. In some cases,
alternative production pathways can be followed to produce
comparable particles.
[0023] The reaction conditions determine the qualities of the
particles produced by laser pyrolysis. The reaction conditions for
laser pyrolysis can be controlled relatively precisely in order to
produce particles with desired properties. The appropriate reaction
conditions to produce a certain type of particles generally depend
on the design of the particular apparatus. Nevertheless, some
general observations on the relationship between reaction
conditions and the resulting particles can be made.
[0024] Reactant gas flow rate and velocity of the reactant gas
stream are inversely related to particle size so that increasing
the reactant gas flow rate or velocity tends to result in smaller
particle size. Also, the growth dynamics of the particles have a
significant influence on the size of the resulting particles. In
other words, different crystal forms of a metal compound have a
tendency to form different size particles from other crystal forms
under relatively similar conditions. Laser power also influences
particle size with increased laser power favoring larger particle
formation for lower melting materials and smaller particle
formation for higher melting materials.
[0025] Appropriate metal precursor compounds generally include
metal compounds with suitable vapor pressures, i.e., vapor
pressures sufficient to get desired amounts of precursor vapor in
the reactant stream. The vessel holding the precursor compounds can
be heated to increase the vapor pressure of the metal compound
precursor, if desired. Preferred titanium precursors include, for
example, TiCl.sub.4 and Ti[OCH(CH.sub.3).sub.2].sub.4 (titanium
tetra-I-propoxide). Preferred yttrium precursors include
Y.sub.5O(OC.sub.3H.sub.7).sub.13 (yttrium oxide isopropoxide).
Preferred zinc precursors include, for example, ZnCl.sub.2.
ZnCl.sub.2 vapor can be generated by heating and, optionally,
melting ZnCl.sub.2 solids. For example, ZnCl.sub.2 has a vapor
pressure of about 5 mm Hg at a temperature of about 500.degree. C.
When using ZnCl.sub.2 precursor, the chamber and nozzle preferably
are heated to avoid getting condensation of the precursor.
[0026] Preferred reactants suitable as oxygen sources include, for
example, O.sub.2, CO, CO.sub.2, O.sub.3 and mixtures thereof.
Preferred reactants suitable as sulfur sources include, for
example, H.sub.2S. The reactant compound serving as the oxygen of
sulfur source should not react significantly with the metal
precursor compound prior to entering the reaction zone since this
generally would result in the formation of large particles.
[0027] Laser pyrolysis can be performed with a variety of optical
laser frequencies. Preferred lasers operate in the infrared portion
of the electromagnetic spectrum. CO.sub.2 lasers are particularly
preferred sources of laser light. Infrared absorbers for inclusion
in the molecular stream include, for example, C.sub.2H.sub.4,
NH.sub.3, SF.sub.6, SiH.sub.4 and O.sub.3. O.sub.3 can act as both
an infrared absorber and as an oxygen source. The radiation
absorber, such as the infrared absorber, absorbs energy from the
radiation beam and distributes the energy as heat to the other
reactants to drive the pyrolysis.
[0028] Preferably, the energy absorbed from the radiation beam
increases the temperature at a tremendous rate, many times the rate
that energy generally would be produced even by strongly exothermic
reactions under controlled condition. While the process generally
involves nonequilibrium conditions, the temperature can be
described approximately based on the energy in the absorbing
region. The laser pyrolysis process is qualitatively different from
the process in a combustion reactor where an energy source
initiates a reaction, but the reaction is driven by energy given
off by an exothermic reaction.
[0029] An inert shielding gas can be used to reduce the amount of
reactant and product molecules contacting the reactant chamber
components. Appropriate shielding gases include, for example, Ar,
He and N.sub.2.
[0030] An appropriate laser pyrolysis apparatus generally includes
a reaction chamber isolated from the ambient environment. A
reactant inlet connected to a reactant supply system produces a
molecular stream through the reaction chamber. A laser beam path
intersects the molecular stream at a reaction zone. The molecular
stream continues after the reaction zone to an outlet, where the
molecular stream exits the reaction chamber and passes into a
collection system. Generally, the laser is located external to the
reaction chamber, and the laser beam enters the reaction chamber
through an appropriate window.
[0031] Referring to FIG. 1, a particular embodiment 100 of a
pyrolysis apparatus involves a reactant supply system 102, reaction
chamber 104, collection system 106 and laser 108. Reactant supply
system 102 includes a source 120 of metal compound precursor. For
liquid precursors, a carrier gas from carrier gas source 122 can be
introduced into precursor source 120, containing liquid precursor
to facilitate delivery of the precursor. The carrier gas from
source 122 preferably is either an infrared absorber or an inert
gas and is preferably bubbled through the liquid, metal compound
precursor. The quantity of precursor vapor in the reaction zone is
roughly proportional to the flow rate of the carrier gas.
[0032] Alternatively, carrier gas can be supplied directly from
infrared absorber source 124 or inert gas source 126, as
appropriate. The reactant serving as the oxygen or sulfur source is
supplied from reactant source 128, which can be a gas cylinder or
other appropriate container. The gases from the metal compound
precursor source 120 are mixed with gases from reactant source 128,
infrared absorber source 124 and inert gas source 126 by combining
the gases in a single portion of tubing 130. The gases are combined
a sufficient distance from reaction chamber 104 such that the gases
become well mixed prior to their entrance into reaction chamber
104. The combined gas in tube 130 passes through a duct 132 into
rectangular channel 134, which forms part of an injection nozzle
for directing reactants into the reaction chamber.
[0033] Flow from sources 122, 124, 126 and 128 are preferably
independently controlled by mass flow controllers 136. Mass flow
controllers 136 preferably provide a controlled flow rate from each
respective source. Suitable mass flow controllers include, for
example, Edwards Mass Flow Controller, Model 825 series, from
Edwards High Vacuum International, Wilmington, Mass.
[0034] Inert gas source 138 is connected to an inert gas duct 140,
which flows into annular channel 142. A mass flow controller 144
regulates the flow of inert gas into inert gas duct 140. Inert gas
source 126 can also function as the inert gas source for duct 140,
if desired.
[0035] The reaction chamber 104 includes a main chamber 200.
Reactant supply system 102 connects to the main chamber 200 at
injection nozzle 202. The end of injection nozzle 202 has an
annular opening 204 for the passage of inert shielding gas, and a
rectangular slit 206 for the passage of reactant gases to form a
molecular stream in the reaction chamber. Annular opening 204 has,
for example, a diameter of about 1.5 inches and a width along the
radial direction of about 1/16 in. The flow of shielding gas
through annular opening 204 helps to prevent the spread of the
reactant gases and product particles throughout reaction chamber
104.
[0036] Tubular sections 208, 210 are located on either side of
injection nozzle 202. Tubular sections 208, 210 include ZnSe
windows 212, 214, respectively. Windows 212, 214 are about 1 inch
in diameter. Windows 212, 214 are preferably plane-focusing lenses
with a focal length equal to the distance between the center of the
chamber to the surface of the lens to focus the beam to a point
just below the center of the nozzle opening. Windows 212, 214
preferably have an antireflective coating. Appropriate ZnSe lenses
are available from Janos Technology, Townshend, Vt. Tubular
sections 208, 210 provide for the displacement of windows 212, 214
away from main chamber 200 such that windows 212, 214 are less
likely to be contaminated by reactants or products. Window 212, 214
are displaced, for example, about 3 cm from the edge of the main
chamber 200.
[0037] Windows 212, 214 are sealed with a rubber o-ring to tubular
sections 208, 210 to prevent the flow of ambient air into reaction
chamber 104. Tubular inlets 216, 218 provide for the flow of
shielding gas into tubular sections 208, 210 to reduce the
contamination of windows 212, 214. Tubular inlets 216, 218 are
connected to inert gas source 138 or to a separate inert gas
source. In either case, flow to inlets 216, 218 preferably is
controlled by a mass flow controller 220.
[0038] Laser 108 is aligned to generate a laser beam 222 that
enters window 212 and exits window 214. Windows 212, 214 define a
laser light path through main chamber 200 intersecting the flow of
reactants at reaction zone 224. After exiting window 214, laser
beam 222 strikes power meter 226, which also acts as a beam dump.
An appropriate power meter is available from Coherent Inc., Santa
Clara, Calif. Laser 108 can be replaced with an intense
conventional light source such as an arc lamp. Preferably, laser
108 is an infrared laser, especially a CW CO.sub.2 laser such as an
1800 watt maximum power output laser available from PRC Corp.,
Landing, N.J.
[0039] Reactants passing through slit 206 in injection nozzle 202
initiate a molecular stream. The molecular stream passes through
reaction zone 224, where reaction involving the metal precursor
compound takes place. Heating of the gases in reaction zone 224 is
extremely rapid, roughly on the order of 10.sup.5.degree. C./sec
depending on the specific conditions. The reaction is rapidly
quenched upon leaving reaction zone 224, and particles 228 are
formed in the molecular stream. The nonequilibrium nature of the
process allows for the production of particles with a highly
uniform size distribution and structural homogeneity.
[0040] The path of the molecular stream continues to collection
nozzle 230. Collection nozzle 230 is spaced about 2 cm from
injection nozzle 202. The small spacing between injection nozzle
202 and collection nozzle 230 helps reduce the contamination of
reaction chamber 104 with reactants and products. Collection nozzle
230 has a circular opening 232. Circular opening 232 feeds into
collection system 106.
[0041] The chamber pressure is monitored with a pressure gauge
attached to the main chamber. The chamber pressure generally ranges
from about 5 Torr to about 1000 Torr.
[0042] Reaction chamber 104 has two additional tubular sections not
shown. One of the additional tubular sections projects into the
plane of the sectional view in FIG. 1, and the second additional
tubular section projects out of the plane of the sectional view in
FIG. 1. When viewed from above, the four tubular sections are
distributed roughly, symmetrically around the center of the
chamber. These additional tubular sections have windows for
observing the inside of the chamber. In this configuration of the
apparatus, the two additional tubular sections are not used to
facilitate production of particles.
[0043] Collection system 106 can include a curved channel 250
leading from collection nozzle 230. Because of the small size of
the particles, the product particles follow the flow of the gas
around curves. Collection system 106 includes a filter 252 within
the gas flow to collect the product particles. A variety of
materials such as teflon, glass fibers and the like can be used for
the filter as long as the material is inert and has a fine enough
mesh to trap the particles. Preferred materials for the filter
include, for example, a glass fiber filter from ACE Glass Inc.,
Vineland, N.J.
[0044] Pump 254 is used to maintain collection system 106 at a
selected pressure. A variety of different pumps can be used.
Appropriate pumps for use as pump 254 include, for example, Busch
Model B0024 pump from Busch, Inc., Virginia Beach, Va. with a
pumping capacity of about 25 cubic feet per minute (cfm) and
Leybold Model SV300 pump from Leybold Vacuum Products, Export, Pa.
with a pumping capacity of about 195 cfm. It may be desirable to
flow the exhaust of the pump through a scrubber 256 to remove any
remaining reactive chemicals before venting into the atmosphere.
The entire apparatus 100 can be placed in a fume hood for
ventilation purposes and for safety considerations. Generally, the
laser remains outside of the fume hood because of its large
size.
[0045] The apparatus is controlled by a computer. Generally, the
computer controls the laser and monitors the pressure in the
reaction chamber. The computer can be used to control the flow of
reactants and/or the shielding gas. The pumping rate is controlled
by either a manual needle valve or an automatic throttle valve
inserted between pump 254 and filter 252. As the chamber pressure
increases due to the accumulation of particles on filter 252, the
manual valve or the throttle valve can be adjusted to maintain the
pumping rate and the corresponding chamber pressure.
[0046] The reaction can be continued until sufficient particles are
collected on filter 252 such that the pump can no longer maintain
the desired pressure in the reaction chamber 104 against the
resistance through filter 252. When the pressure in reaction
chamber 104 can no longer be maintained at the desired value, the
reaction is stopped, and the filter 252 is removed. With this
embodiment, about 3-75 grams of particles can be collected in a
single run before the chamber pressure can no longer be maintained.
A single run generally can last from about 10 minutes to about 3
hours depending on the type of particle being produced and the
particular filter. Therefore, it is straightforward to produce a
macroscopic quantity of particles, i.e., a quantity visible with
the naked eye.
[0047] The reaction conditions can be controlled relatively
precisely. The mass flow controllers are quite accurate. The laser
generally has about 0.5 percent power stability. With either a
manual control or a throttle valve, the chamber pressure can be
controlled to within about 1 percent.
[0048] The configuration of the reactant supply system 102 and the
collection system 106 can be reversed. In this alternative
configuration, the reactants are supplied from the bottom of the
reaction chamber, and the product particles are collected from the
top of the chamber. This alternative configuration tends to result
in a slightly higher collection of product for particles that tend
to be buoyant in the surrounding gases. In this configuration, it
is preferable to include a curved section in the collection system
so that the collection filter is not mounted directly above the
reaction chamber.
[0049] An alternative design of a laser pyrolysis apparatus has
been described. See, commonly assigned U.S. patent application Ser.
No. 08/808,850, now U.S. Pat. No. 5,958,348, entitled "Efficient
Production of Particles by Chemical Reaction," incorporated herein
by reference. This alternative design is intended to facilitate
production of commercial quantities of particles by laser
pyrolysis. A variety of configurations are described for injecting
the reactant materials into the reaction chamber.
[0050] The alternative apparatus includes a reaction chamber
designed to minimize contamination of the walls of the chamber with
particles, to increase the production capacity and to make
efficient use of resources. To accomplish these objectives, the
reaction chamber conforms generally to the shape of an elongated
reactant inlet, decreasing the dead volume outside of the molecular
stream. Gases can accumulate in the dead volume, increasing the
amount of wasted radiation through scattering or absorption by
nonreacting molecules. Also, due to reduced gas flow in the dead
volume, particles can accumulate in the dead volume causing chamber
contamination.
[0051] The design of the improved reaction chamber 300 is
schematically shown in FIGS. 2 and 3. A reactant gas channel 302 is
located within block 304. Facets 306 of block 304 form a portion of
conduits 308. Another portion of conduits 308 join at edge 310 with
an inner surface of main chamber 312. Conduits 308 terminate at
shielding gas inlets 314. Block 304 can be repositioned or
replaced, depending on the reaction and desired conditions, to vary
the relationship between the elongated reactant inlet 316 and
shielding gas inlets 314. The shielding gases from shielding gas
inlets 314 form blankets around the molecular stream originating
from reactant inlet 316.
[0052] The dimensions of elongated reactant inlet 316 preferably
are designed for high efficiency particle production. Reasonable
dimensions for the reactant inlet for the production of metal oxide
or metal sulfide particles, when used with a 1800 watt CO.sub.2
laser, are from about 5 mm to about 1 meter.
[0053] Main chamber 312 conforms generally to the shape of
elongated reactant inlet 316. Main chamber 312 includes an outlet
318 along the molecular stream for removal of particulate products,
any unreacted gases and inert gases. Tubular sections 320, 322
extend from the main chamber 312. Tubular sections 320, 322 hold
windows 324, 326 to define a laser beam path 328 through the
reaction chamber 300. Tubular sections 320, 322 can include
shielding gas inlets 330, 332 for the introduction of shielding gas
into tubular sections 320, 322.
[0054] The improved apparatus includes a collection system to
remove the particles from the molecular stream. The collection
system can be designed to collect a large quantity of particles
without terminating production or, preferably, to run in continuous
production by switching between different particle collectors
within the collection system. The collection system can include
curved components within the flow path similar to curved portion of
the collection system shown in FIG. 1. The configuration of the
reactant injection components and the collection system can be
reversed such that the particles are collected at the top of the
apparatus.
[0055] As noted above, properties of the metal compound particles
can be modified by further processing. For example, oxide nanoscale
particles can be heated in an oven in an oxidizing environment or
an inert environment to alter the oxygen content and/or crystal
structure of the metal oxide. The processing of nanoscale metal
oxides in an oven is further discussed in commonly assigned and
copending, U.S. patent application Ser. No. 08/897,903, now U.S.
Pat. No. 5,989,514, entitled "Processing of Vanadium Oxide
Particles With Heat," incorporated herein by reference.
[0056] In addition, the heating process can be used possibly to
remove adsorbed compounds on the particles to increase the quality
of the particles. It has been discovered that use of mild
conditions, i.e., temperatures well below the melting point of the
particles, can result in modification of the stoichiometry or
crystal structure of metal oxides without significantly sintering
the particles into larger particles.
[0057] A variety of apparatuses can be used to perform the heat
processing. An example of an apparatus 400 to perform this heat
processing is displayed in FIG. 4. Apparatus 400 includes a tube
402 into which the particles are placed. Tube 402 is connected to a
reactive gas source 404 and inert gas source 406. Reactant gas,
inert gas or a combination thereof to produce the desired
atmosphere are placed within tube 402.
[0058] Preferably, the desired gases are flowed through tube 402.
Appropriate reactant gases to produce an oxidizing environment
include, for example, O.sub.2, O.sub.3, CO, CO.sub.2, and
combinations thereof. The reactant gases can be diluted with inert
gases such as Ar, He and N.sub.2. The gases in tube 402 can be
exclusively inert gases, if desired. The reactant gases may not
result in changes to the stoichiometry of the particles being
heated.
[0059] Tube 402 is located within oven or furnace 408. Oven 408
maintains the relevant portions of the tube at a relatively
constant temperature, although the temperature can be varied
systematically through the processing step, if desired. Temperature
in oven 408 generally is measured with a thermocouple 410. The
particles can be placed in tube 402 within a vial 412. Vial 412
prevents loss of the particles due to gas flow. Vial 412 generally
is oriented with the open end directed toward the direction of the
source of the gas flow.
[0060] The precise conditions including type of active gas (if
any), concentration of active gas, pressure or flow rate of gas,
temperature and processing time can be selected to produce the
desired type of product material. The temperatures generally are
mild, i.e., significantly below the melting point of the material.
The use of mild conditions avoids interparticle sintering resulting
in larger particle sizes. Some controlled sintering of the metal
oxide particles can be performed in oven 408 at somewhat higher
temperatures to produce slightly larger average particle
diameters.
[0061] For the processing of titanium oxides and zinc oxides, the
temperatures preferably range from about 50.degree. C. to about
1000.degree. C. and more preferably from about 80.degree. C. to
about 500.degree. C. The particles preferably are heated for about
1 hour to about 100 hours. Some empirical adjustment may be
required to produce the conditions appropriate for yielding a
desired material.
B. Particle Properties
[0062] A collection of preferred particles has an average diameter
of less than a micron, preferably from about 5 nm to about 500 nm
and more preferably from about 5 nm to about 100 nm, and even more
preferably from about 5 nm to about 50 nm. The particles generally
have a roughly spherical gross appearance. Upon closer examination,
the particles generally have facets corresponding to the underlying
crystal lattice. Nevertheless, the particles tend to exhibit growth
that is roughly equal in the three physical dimensions to give a
gross spherical appearance. Diameter measurements on particles with
asymmetries are based on an average of length measurements along
the principle axes of the particle. The measurements along the
principle axes preferably are each less than about 1 micron for at
least about 95 percent of the particles, and more preferably for at
least about 98 percent of the particles.
[0063] Because of their small size, the particles tend to form
loose agglomerates due to van der Waals forces between nearby
particles. Nevertheless, the nanometer scale of the particles
(i.e., primary particles) is clearly observable in transmission
electron micrographs of the particles. For crystalline particles,
the particle size generally corresponds to the crystal size. The
particles generally have a surface area corresponding to particles
on a nanometer scale as observed in the micrographs.
[0064] Furthermore, the particles manifest unique properties due to
their small size and large surface area per weight of material. Of
particular relevance, the particles have an altered band structure,
as described further below. The high surface area of the particles
generally leads to high luminosity of the particles.
[0065] As produced, the particles preferably have a high degree of
uniformity in size. As determined from examination of transmission
electron micrographs, the particles generally have a distribution
in sizes such that at least about 95 percent of the particles have
a diameter greater than about 40 percent of the average diameter
and less than about 160 percent of the average diameter.
Preferably, the particles have a distribution of diameters such
that at least about 95 percent of the particles have a diameter
greater than about 60 percent of the average diameter and less than
about 140 percent of the average diameter. The narrow size
distributions can be exploited in a variety of applications, as
described below. For some of the applications, it may be desirable
to mix several collections of particles, each having a narrow
diameter distribution, to produce a desired distribution of
particle diameters and compositions.
[0066] At small crystalline diameters the band properties of the
particles are altered. The increase in band gap is approximately in
proportion to 1/(particle size).sup.2. For especially small
particle sizes, the density of states may become low enough that
the band description may become incomplete as individual molecular
orbitals play a more prominent role. The qualitative trends should
hold regardless of the need to account for a molecular orbital
description of the electronic properties.
[0067] In addition, with a uniform distribution of small particles,
the emission spectrum narrows because of the reduction of
inhomogeneous broadening. The result is a sharper emission spectrum
with an emission maximum that depends on the average particle
diameter. Thus, the use of very small particle diameters may allow
for adjustment of emission characteristics without the need to
activate the particles with a second metal.
[0068] Furthermore, the small size of the particles allows for the
formation of very thin layers. This is advantageous for use with
low velocity electrons since the electrons may not penetrate deeply
within a layer. The small size of the particles is also conducive
to the formation of small patterns, for example using
photolithography, with sharp edges between the elements of the
pattern. The production of small, sharply separated elements is
important for the formation of high resolution displays.
[0069] In addition, the particles produced as described above
generally have a very high purity level. Metal oxide and sulfide
particles produced by the above methods are expected to have a
purity greater than the reactant gases because the crystal
formation process tends to exclude contaminants from the lattice.
Furthermore, metal oxide and sulfide particles produced by laser
pyrolysis generally have a high degree of crystallinity and few
surface distortions.
[0070] Although under certain conditions mixed phase material may
be formed, laser pyrolysis generally can be effectively used to
produce single phase crystalline particles. Primary particles
generally consist of single crystals of the material. The single
phase, single crystal properties of the particles can be used
advantageously along with the uniformity and narrow size
distribution. Under certain conditions, amorphous particles may be
formed by laser pyrolysis. Some amorphous particles can be heated
under mild conditions to form crystalline particles.
[0071] Zinc oxides can have a stoichiometry of, at least, ZnO
(hexagonal crystal, Wurtzite structure) or ZnO.sub.2. The
production parameters can be varied to select for a particular
stoichiometry of zinc oxide. Zinc sulfide has a cubic crystal
lattice generally with a zincblend structure. Y.sub.2O.sub.3 has a
cubic crystal lattice.
[0072] Titanium dioxide is known to exist in three crystalline
phases, anatase, rutile and brookite, as well as an amorphous
phase. The anatase and rutile phases have a tetrahedral crystal
lattice, and the brookite phase has an orthorhombic crystal
structure. The conditions of the laser pyrolysis can be varied to
favor the formation of a single, selected phase of TiO.sub.2. In
addition, heating of small metal oxide particles under mild
conditions may be useful to alter the phase or composition of the
materials.
C. Phosphors and Displays
[0073] The particles described in this application can be used as
phosphors. The phosphors emit light, preferably visible light,
following excitation. A variety of ways can be used to excite the
phosphors, and particular phosphors may be responsive to one or
more of the excitation approaches. Particular types of luminescence
include cathodoluminescence, photoluminescence and
electroluminescence which, respectively, involve excitation by
electrons, light and electric fields. Many materials that are
suitable as chathodoluminescence phosphors are also suitable as
electroluminescence phosphors.
[0074] In particular, the particles preferably are suitable for
low-velocity electron excitation, with electrons accelerated with
potentials below 1 KV, and more preferably below 100 V. The small
size of the particles makes them suitable for low velocity electron
excitation. Furthermore, the particle produce high luminescence
with low electron velocity excitation. The phosphor particles can
be used to produce any of a variety of display devices based on low
velocity electrons, high velocity electrons, or electric
fields.
[0075] Referring to FIG. 5, a display device 500 includes an anode
502 with a phosphor layer 504 on one side. The phosphor layer faces
an appropriately shaped cathode 506, which is the source of
electrons used to excite the phosphor. A grid cathode 508 can be
placed between the anode 502 and the cathode 506 to control the
flow of electrons from the cathode 506 to the anode 502.
[0076] Cathode ray tubes (CRTs) have been used for a long time for
producing images. CRTs generally use relatively higher electron
velocities. Phosphor particles, as described above, can still be
used advantageously as a convenient way of supplying particles of
different colors, reducing the phosphor layer thickness and
decreasing the quantity of phosphor for a given luminosity. CRTs
have the general structure as shown in FIG. 5, except that the
anode and cathode are separated by a relatively larger distance and
steering electrodes rather than a grid electrode generally are used
to guide the electrons from the cathode to the anode.
[0077] Other preferred applications include the production of flat
panel displays. Flat panel displays can be based on, for example,
liquid crystals or field emission devices. Liquid crystal displays
can be based on any of a variety of light sources. Phosphors can be
useful in the production of lighting for liquid crystal displays.
Referring to FIG. 6, a liquid crystal element 530 includes at least
partially light transparent substrates 532, 534 surrounding a
liquid crystal layer 536. Lighting is provided by a phosphor layer
538 on an anode 540. Cathode 542 provides a source of electrons to
excite the phosphor layer 538. Alternative embodiments are
described, for example, in U.S. Pat. No. 5,504,599, incorporated
herein by reference.
[0078] Liquid crystal displays can also be illuminated with
backlighting from an electroluminescenct display. Referring to FIG.
7, electroluminescent display 550 has a conductive substrate 552
that functions as a first electrode. Conductive substrate 552 can
be made from, for example, aluminum, graphite or the like. A second
electrode 554 is transparent and can be formed from, for example,
indium tin oxide. A dielectric layer 556 may be located between
electrodes 552, 554, adjacent to first electrode 552. Dielectric
layer 556 includes a dielectric binder 558 such as cyanoethyl
cellulose or cyanoethyl starch. Dielectric layer 556 can also
include ferroelectric material 560 such as barium titanate.
Dielectric layer 556 may not be needed for dc-driven (in contrast
with ac-driven) electro-luminescent devices. A phosphor layer 562
is located between transparent electrode 554 and dielectric layer
562. Phosphor layer 562 includes electroluminescent particles 564
in a dielectric binder 566.
[0079] Electroluminescent display 550 also can be used for other
display applications such as automotive dashboard and control
switch illumination. In addition, a combined liquid
crystal/electroluminescent display has been designed. See, Fuh, et
al., Japan J. Applied Phys. 33:L870-L872 (1994), incorporated
herein by reference.
[0080] Referring to FIG. 8, a display 580 based on field emission
devices involves anodes 582 and cathodes 584 spaced a relatively
small distance apart. Each electrode pair form an individually
addressable pixel. A phosphor layer 586 is located between each
anode 582 and cathode 584. The phosphor layer 586 includes
phosphorescent nanoparticles as described above. Phosphorescent
particles with a selected emission frequency can be located at a
particular addressable location. The phosphor layer 586 is excited
by low velocity electrons travelling from the cathode 584 to the
anode 582. Grid electrodes 588 can be used to accelerate and focus
the electron beam as well as act as an on/off switch for electrons
directed at the phosphor layer 586. An electrically insulating
layer is located between anodes 582 and grid electrodes 588. The
elements are generally produced by photolithography or a comparable
techniques such as sputtering and chemical vapor deposition for the
production of integrated circuits. As shown in FIG. 8, the anode
should be at least partially transparent to permit transmission of
light emitted by phosphor 586.
[0081] Alternatively, U.S. Pat. No. 5,651,712, incorporated herein
by reference, discloses a display incorporating field emission
devices having a phosphor layer oriented with an edge (rather than
a face) along the desired direction for light propagation. The
construction displayed in this patent incorporates color filters to
produce a desired color emission rather than using phosphors that
emit at desired frequencies. Based on the particles described
above, selected phosphor particles preferably would be used to
produce the different colors of light, thereby eliminating the need
for color filters.
[0082] The phosphor particles can be adapted for use in a variety
of other devices beyond the representative embodiments specifically
described.
[0083] The nanoparticles can be directly applied to a substrate to
produce the above structures. Alternatively, the nanoparticles can
be mixed with a binder such as a curable polymer for application to
a substrate. The composition involving the curable binder and the
phosphor nanoparticles can be applied to a substrate by
photolithography or other suitable technique for patterning a
substrate such as used in the formation of integrated circuit
boards. Once the composition is deposited at a suitable positions
on the substrate, the material can be exposed to suitable
conditions to cure the polymer. The polymer can be curable by
electron beam radiation, UV radiation or other suitable
techniques.
[0084] The embodiments described above are intended to be
representative and not limiting. Additional embodiments of the
invention are within the claims. As will be understood by those
skilled in the art, many changes in the methods and apparatus
described above may be made by the skilled practitioner without
departing from the spirit and scope of the invention, which should
be limited only as set forward in the claims which follow
* * * * *