WO1991006894A1

WO1991006894A1 - Plasmon enhanced photo processes

Info

Publication number: WO1991006894A1
Application number: PCT/US1990/006013
Authority: WO
Inventors: Steven C. Hill; Ramesh C. Patel; Delmar L. Barker; Kim A. Baker; Joseph G. Stumpf; Jeffery E. Jacob; John E. Creange
Original assignee: Research Corporation Technologies, Inc.
Priority date: 1989-10-18
Filing date: 1990-10-18
Publication date: 1991-05-16

Abstract

Methods and apparatus provided with selected nanoparticles that exhibit the plasmon resonance effect to enhance photoprocesses. In a first embodiment, such nanoparticles are used in a method of forming an integrated circuit; and in additional embodiments, the nanoparticles are used to increase the catalytic effect of a metal catalyst to increase the rate at which molecules absorb light, to accelerate the reduction of carbon dioxide to formic acid, to accelerate the decomposition of pollutants, and to accelerate the reaction of two reactants to produce a fuel. Additional embodiments utilize the plasmon resonance effect to enhance the effect of solar energy on a solar responsive electrode of a photochemical battery, and to inhibit the passage of ultraviolet light through a sunscreen. Also, in accordance with the present invention, nanoparticles that exhibit the plasmon resonance effect are used to improve a test for a solution for a bio-agent, and to develop fingerprints on a surface.

Description

PLASMON ENHANCED PHOTO PROCESSES

BACKGROUND OF THE INVENTION

This invention generally relates to the use of the plasmon resonance effect to enhance photoprocesses.

The plasmon resonance effect is shown by certain small particles when dispersed in selected media, and this can result in increased intensity of electromagnetic fields

10 in and around those particles. The extent to which a particle exhibits the plasmon resonance effect depends on a number of factors, including the size and shape of the particle, the material or materials from which the particle is made, and, in a particle made of a plurality of materials, -_je the order, number, shape and dimensions of the materials from which the particle is made. For example, the plasmon resonance effect may be enhanced in particles comprised of shells and cores of metals and dielectrics and that have sizes on the order of magnitude of nanometers. Such

20 particles referred to as composite nanoparticles, are of special interest because they can be made to exhibit an enhanced plasmon resonance effect at a selected electromagnetic frequency.

It has been recognized for many years that the 5 plasmon resonance effect in small metal particles can be responsible for absorption and scattering phenomena of electromagnetic radiation.

Recently, Ker er et al. in Phy. Rev. B. , 26, 4052-4062 (1982) have recognized that by designing composite 0 nanoparticles comprised of metal and dielectric layers, the plasmon resonance can be greatly enhanced. This now makes it possible to use the effect in many processes from nonlinear optics to photochemical catalysis. Consider, for example, a spherically shaped nanoparticle suspended in a medium, and 5 consisting of a spherical core made of a dielectric material surrounded by a shell made of a metal. When electromagnetic radiation of a wavelength much longer than the size of the particle is incident on that particle, the radiation scattering coefficient, a,, of the particle, including the effect thereon of the plasmon resonance effect, is given by the equation:

a₁3T2 3 ι« (e₂-e₁) (e₁-2e₂) + g^*3(2β₂+e-.)(e₁-e₂) 0 (e₂+2e₃)(e₁+2e₂) - g³ (2e₂-2e₃) (e_±-e₂)

-_jc where, q= a/b

=2irb/\

i is the imaginary number, V-ΪT

20 a is the radius of the core of the particle, b is the radius of the particle, is the wavelength of the incident electromagnetic radiation, e₁ is the dielectric constant of the core,

25 e₂ is the dielectric constant of the shell, and e₃ is the dielectric constant of the surrounding medium.

Heretofore, few devices or methods have been _Q specifically designed, constructed, or operated so as to utilize the plasmon resonance effect. Pursuant to the present invention, by providing various devices and methods with selected nanoparticles, the plasmon resonance effect can

35 be effectively employed to enhance dramatically the photoprocesses that occur in those devices and methods.

SUMMARY OF THE INVENTION

An object of the present invention is to use the plasmon resonance effect to enhance photo processes.

Another object of the present invention is to provide various devices in which features of the plasmon resonance effect associated with nanoparticles are used either to make the device function or to function better.

Another object of this invention is to provide various methods and devices with nanoparticles that exhibit the plasmon resonance effect to enhance photo processes that occur in those methods and devices.

A further object of the present invention is to provide a substrate material with nanoparticles that exhibit the plasmon resonance effect to facilitate forming an integrated circuit in that substrate.

Another object of this invention is to provide a metal catalyst in the form of metal coated nanoparticles where those particles exhibit the plasmon resonance effect to increase the catalytic activity of the metal.

Another object of the present invention is to provide a photochemical reaction, of the type where molecules absorb light to reach an excited state, with metal coated nanoparticles that exhibit the plasmon resonance effect to increase the rate at which the molecules absorb light.

Another object of the present invention is to use nanoparticles, designed to take advantage of the plasmon resonance effect, to enhance the electromagnetic fields in and around the nanoparticles in such a way that the rates of photochemical reactions, such as those used for solar energy conversion or photochemical decomposition of pollutants, are increased in the media surrounding the particles and/or in media that has moved into the particles.

Another object of the present invention is to enhance the effectiveness of photoactivated pharmaceutical agents, such as those used in photodynamic therapy, by binding such agents to nanoparticles that were designed to take advantage of the plasmon resonance effect and to enhance the sensitivity of the pharmaceuticals to light.

10 Another object of the present invention is to enhance the effectiveness of photoactivated pharmaceutical agents by binding such agents as well as antibodies (e.g., tumor specific antibodies) to nanoparticles that were designed to take advantage of the plasmon resonance effect -, r- and to enhance the sensitivity of the pharmaceuticals to light.

Another object of the present invention is to use nanoparticles, designed to take advantage of the plasmon resonance effect, to enhance the absorption of 2_Q electromagnetic energy and hence to enhance the photoactivation of the materials comprising the particles, and to use such photocatalytic particles to catalyze such reactions as the decomposition of pollutants, or the deposition of metals in integrated circuits, c Another object of this invention is to decompose pollutants by a photocatalytic process in which nanoparticles that exhibit the plasmon resonance effect are used to accelerate the decomposition of the pollutants.

A further object of the present invention is to

30 produce a fuel by a photo-oxidation reduction process in which nanoparticles that exhibit the plasmon resonance effect are used to accelerate the reaction of two reactants that produce the fuel.

35 Another object of the present invention is to use nanoparticles, designed to take advantage of the plasmon resonance effect, to enhance the absorption of electromagnetic energy in a solar heating device. _t- Another object of this invention is to provide a solar heater device with a liquid including nanoparticles that exhibit the plasmon resonance effect to enhance the absorption of solar energy by liquid.

A further object of the present invention is to apply nanoparticles that exhibit the plasmon resonance effect, to a terminal of a photochemical battery to enhance the conversion of solar energy to electrical energy.

Another object of the present invention is to use coated nanoparticles designed to take advantage of the _TC plasmon resonance effect, to enhance the absorption of electromagnetic energy, and hence to block certain wavelengths of optical energy, and to use such particles in sun screens.

A further object of this invention is to improve 2o the sensitivity of a test of a solution for a bio-agent by dispersing in the solution nanoparticles that, or that can, exhibit the plasmon resonance effect.

Another object of the present invention is to use coated nanoparticles exhibiting the plasmon resonance effect 25 to enhance the fluorescence emission or Raman scattering from molecules near or on the particle, and to use such particles in the detection of chemical or biochemical species.

Another object of the present invention is to use coated nanoparticles exhibiting the plasmon resonance effect

30 to enhance the fluorescence emission or Raman scattering from molecules near or on the particle, and to combine such particles with antibodies and to use such particles in the detection of chemical or biochemical agents.

35 Another object of the present invention is to coat a photo activated pharmaceutical onto nanoparticles that exhibit the plasmon resonance effect to increase the sensitivity of the pharmaceutical to light.

Another object of this invention is to use nanometer sized silica particles as nucleation centers for the particles used in solid propellants to control more accurately the size distribution of those particles.

Another object of the present invention is to use nanoparticles that exhibit the plasmon resonance effect to improve the sensitivity of a method for developing fingerprints on surfaces.

Another object of the present invention is to use coated nanoparticles exhibiting the plasmon resonance effect to enhance the fluorescence emission from molecules near or on the particle, and to use such particles to make fingerprints more visible.

Another object of the present invention is to use nanoparticles to enhance, via a plasmon resonance, the electromagnetic fields in and around the nanoparticles in such a way that plasma formation is initiated with substantially lower incident optical densities than is required when no particles are present.

These and other objectives are attained by providing various devices and methods with selected nanoparticles that exhibit the plasmon resonance effect and thereby enhance photoprocesses occurring in those devices and methods. A first embodiment of this invention is a method of forming an integrated circuit comprising the step of forming a film by forming a first layer of silver coated nanoparticles, forming a second layer containing TiO- partides over the first layer, and forming a third, protective polymer layer over the second layer. This method further comprises the steps of illuminating the film through a mask to produce Ag centers in the film, and developing the film to change these Ag centers to metallic lines.

Another embodiment of this invention is an improved method in which a metal catalyst is used to accelerate a reaction between first and second chemicals. The metal catalyst is provided in the form of metal coated nanoparticles, and those particles are illuminated so that they exhibit the plasmon resonance effect and thereby

10 increase the catalytic activity of the metal. A further embodiment of the invention is an improved photochemical reaction of the type wherein light absorption causes molecules to reach excited singlet states and then decay to triplet states; and more specifically, the improvement is to

- z illuminate those particles in the presence of metal coated nanoparticles that exhibit the plasmon resonance effect to thereby increase the rate at which the molecules absorb light.

Also, pursuant to the present invention, a method

2o is provided to decompose pollutants using photocatalysis. This method comprises the steps of providing a solution containing the pollutants, locating metal coated nanoparticles in the solution, and exposing the solution to light to decompose the pollutants, wherein the metal coated nanoparticles exhibit the plasmon resonance effect to accelerate the decomposition of the pollutants. A further embodiment of this invention is a method for the photoredox of first and second reactants to produce a fuel, comprising the steps of providing a solution containing those reactants, _Q locating metal coated nanoparticles in the solution, and exposing the solution to light to react the first and second reactants together to produce the fuel, wherein the metal

5 -, coated nanoparticles exhibit the plasmon resonance effect to accelerate the reaction of the two reactants.

Another embodiment of this invention is a solar heater device comprising a liquid including nanoparticles 5 that exhibit the plasmon resonance effect, thereby increasing absorption and heating, and means for holding the liquid while it is exposed to solar energy, wherein the nanoparticles enhance the effect of the solar energy on the liquid. A further embodiment of the present invention is a

-_j^O photochemical battery comprising first and second electrodes, at least one of which is solar responsive such that the electric potential of that electrode changes when it is exposed to light. This battery also comprises a multitude of nanoparticles applied onto that solar responsive electrode, 5 and these particles exhibit the plasmon resonance effect and thus enhance the effect of light on that electrode. With a still additional embodiment of this invention, a sunscreen base is provided with nanoparticles that exhibit the plasmon resonance effect to inhibit the passage of ultraviolet light

2o through the base.

A further embodiment of the present invention is a method for testing a solution for a given bio-agent. The method includes the steps of dispersing in the solution a fluorescent agent that is capable of attaching to the given

25 bio-agent; and also dispersing in the solution, nanoparticles that, or that can, exhibit the plasmon resonance effect to alter the fluorescence of the fluorescent agent. The method further includes the step of testing the solution for fluorescence to determine whether the bio-agent is present in

30 the solution. Also, pursuant to this invention, the sensitivity of a photoactivated pharmaceutical is increased by coating the pharmaceutical onto a nanoparticle that exhibits the plasmon resonance effect.

35 A still additional embodiment of this invention use coated nanoparticles in solid propellants. More specifically, particles including a nanometer sized core of a first material, and at least one layer of a second material formed on that core, are used to form the solid particulates of a solid propellant.

The present invention also uses the plasmon resonance effect in methods for developing fingerprints on a surface. A first such method comprises the steps of applying to the surface a fluorescent dye to attach that dye to the profile of the fingerprint; and also applying to the surface, nanoparticles that exhibit the plasmon resonance effect to attach those particles to the surface outside the profile of the fingerprints. This method further comprises the steps of illuminating the surface to activate the fluorescent dye to generate fluorescent light, wherein the nanoparticles attenuate activity of any fluorescent dye that has become attached to the surface outside the profile of the fingerprints, and sensing the fluorescent light emitted from the surface to detect the profile of the fingerprint. A second method comprises the steps of applying a fluorophore to the surfaces of nanoparticles that exhibit the plasmon resonance effect, and attaching those particles to the profile of the fingerprint. This second method also comprises the steps of illuminating the surface to activate the fluorophore to generate fluorescent light, wherein the nanoparticles enhance the fluorescent light generated by the fluorophore, and sensing the fluorescent light emitted from the surface to detect the profile of the fingerprint.

Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a base or substrate from which an integrated circuit can be made, and which includes TiO-. nanoparticles.

Figure 2 generally illustrates how a nanoparticle can be used to improve detection of a specific bio-agent.

Figure 3 shows an alternate combination of materials also employing nanoparticles to improve detection of a bio-agent.

Figure 4 illustrates a nanoparticle structure that may be used in a process for detecting bio-agents.

Figure 5 depicts the nanoparticle structure of Figure 4, as bio-agents are about to be attached to the structure.

Figures 6-13, which are not drawn to scale, show various particles that may be used in or in conjunction with the present invention.

Figures 14-22 outline several process that may be used to make the particles shown in Figures 6-13.

Figure 23 is a transmission electron micrograph of silver-coated silver bromide nanoparticles.

Figure 24 is a transmission electron micrograph of silver coated silver bromide nanoparticle treated with ammonia.

Figure 25 shows various optical extinction spectra of silver coated silver bromide nanoparticles. (a) to (d) are spectra of various illuminated solutions of Ag, Br and EDTA. In going from (a) to (d) , the illumination time increases. (e) Typical spectrum observed after addition of ammonia to any of the above solutions.

Figure 26 shows computed extinction efficiencies for silver-coated silver bromide particles in water. The diameter of the core particle is 20 nm and the thicknesses of the silver coats are indicated in nm. The spectrum marked solid is that of a homogeneous 20 nm diameter silver sphere.

Figure 27 is an optical extinction spectra of a measured silver coated silver bromide nanoparticle and two computed extinction spectra. The measured spectrum lies between the two computed spectra. In the upper curve all the silver in the coat is assumed to come from the solution. In the lower curve all of the silver is assumed to come from the reduction of AgBr at the particle surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The fabrication of very large scale integrated circuits and the corresponding high density packaging resulting from miniaturization requires extremely high resolution photolithographic methods. The use of excimer lasers to achieve submicron features have been described in recent publications (e.g., M. Rothschild and D.J. Ehrlich, J. Vac. Science and Technology, B6, 1 (1988), and Fujitsu commercially produces IM Byte RAMS using excimer laser based methods.

Particles exhibiting the plasmon resonance effect can be incorporated into, and employed in, such photolithographic methods in various ways.

(1) Two characteristics of metal (e.g., silver or aluminum) coated nanospheres would be exploited: the use of particles with small diameters (-0.1 micron and less), and the high "resonant" light absorption. With reference to Figure 1, a very thin film 10 ( 1 micron) of such particles 12 on a suitable substrate 14 can be formed by using a suspension of the particles in a mixture containing a monomer 5 and an initiator, and spin coating the substrate. A number of suitable polymer systems can be chosen to produce a thin polymer film containing the particles. On top of this layer 10, another film 16 containing Ti0₂ particles of small diameter can be formed in a similar manner, or by allowing a o titanium (IV) organometallic polymer film to hydrolyze directly to yield the Ti0₂ particle film. A third protective polymer film 20, which may contain a reducing agent (aldeyde, benzoqunone) is finally laid. The total film thickness is kept low to achieve high resolution. Illumination of this 5 film by UV light through a suitable mask 22 produces Ag centers which can be developed to produce metallic lines of copper and/or nickel using an appropriate developer.

(2) A variation of the photochemical method, discussed in detail below, which is used to produce metal o (e.g., silver or aluminum) coated particles can be used. In this case, a thin polymer film containing dielectric particles with absorbed metal (e.g., silver or aluminum) ions, and photoactivated reducing agents (e.g., a combination of organic compounds containing keto- and isopropyl groups), can be produced on a suitable substrate. Exposure to UV light will produce metallic silver which will act as development centers.

(3) Instead of using conventional UV light as the source of illumination, an excimer laser (e.g., 248 nm 0 emission) can be used. In this case, latent images with much higher resolution can be produced, and developed using conventional methods.

5 The growing demand for catalysts, in particular noble metal catalysts, in the production of a variety of chemicals (e.g., Ag in the conversion of methanol to formaldehyde) has been highlighted in a recent article (Bruce F. Greek, C&E News, May 29, 1989, p. 29). The method discussed below in detail, for coating silver particles can be adapted for a wide variety of other metals as defined hereinbelow. The capability of accurately controlling the thickness of the coating of a given metal is a basic feature of this method. Consequently, for a given mass of metal, the optimum sμrface area, which is the critical parameter for catalysis, can be readily achieved.

Quite apart from the advantages gained from this surface area optimization, it has already been shown that small metal particles of silver, gold and copper possess unusual catalytic properties which are dramatically different from that of bulk material (A. Henglein, Ber. Bunsneger. Phys, Chem. , 81, 556 (1977); 82, 1335 (1978)). Consequently, metal coated particles in which the coating layer thickness is very small (1-10 nm) , should possess very high catalytic activity.

In a conventional photochemical reaction, light absorption causes a molecule to reach an excited singlet state, and then decay to a triplet state by intersystem crossing, usually with rather low efficiency. Reactive intermediates with the longer lived triplet lifetimes can undergo reactions much more efficiently. It is well known that heavy atoms (e.g., Ag, Hg, etc.) promote the singlet to triplet conversion. Silver coated silica particles can be made so that the reaction solution is transparent, with negligible turbidity, so that the process of light absorption is very efficient. Two possibilities for the enhancement of a photochemical reaction can be realized:

(1) For a normal catalytic reaction as described above under catalysis (e.g., methanol to formaldehyde conversion) , illumination of the silver coated particles at the peak absorption will cause increased catalytic activity.

(2) In a conventional photochemical reaction in the presence of silver coated particles, light absorption by the reacting molecules will be increased as a result of energy transfer, in addition to increased yield of triplet formation. Both effects will contribute to accelerating the reaction.

The number of chemical systems which will find a useful application from this new class of metal coated (possibly multi-layered) particles cannot be enumerated in a brief summary. All known systems which require metal, metal-semiconductor, and metal oxide catalysts will find greatly improved performance with the described particles. The importance of combined noble metal-semiconductor photocatalysts in the two-electron-transfer reduction of aldehydes and related derivatives has been recently described (Shozo Yanagida et al., J. Phys. Chem. j?3_, 2576-2582 (1989)). It is clear from this and related articles that the availability of metal coated nanoparticles will lead to greatly improved yields.

In accordance with another embodiment of this invention, metal-coated nanoparticles are used to enhance the rates and/or make more efficient a variety of photochemical reactions. The development is based on the fact that optical fields are resonantly enhanced near the surface of a metal-coated particle having appropriate thickness of cores and coats, and on the fact that the metal-coated particles can be particularly good absorbers of light. There are many reactions in which light either acts to catalyze a reaction or provides the energy to drive a reaction. For example, concentrated sunlight can increase the speed of reactions in which toxic pollutants are decomposed. When such reactions go to completion, toxins such as pesticides or PCB's are decomposed into CO- , water and simple acids. The reactions go faster when the light is more intense. Or, for example, the energy from sunlight can be used to generate electricity in a storage battery, a photovoltaic cell or a photoelectrochemical cell. Or, the energy from sunlight can be used to drive reactions such as photoredox reactions that produce fuels such as formaldehyde or methanol from water and carbon dioxide or ammonia from nitrogen and water. 1. Photocatalytic Decomposition of Pollutants

The preferred embodiment is a metal-coated dielectric particle that is coated with Ti0₂- The thickness of the core, the metal coat, and the Ti0₂ coat are chosen so as to maximize the intensity of optical fields in and outside ^{tne τ}i°2 l^aY^er* ^τ^^e particles may be made by processes described below.

In one embodiment, the particles are dispersed in the liquid that contains the pollutants.

In another embodiment, the particles are bonded to a surface and only part of the particles is in direct contact with the liquid.

In another embodiment, the particles are trapped in an agarose or sepharose gel having pores small enough that the particles remain in the gel.

In another embodiment, the particles are chemically bonded to gel. 2. Photoredox Reactions that Produce Fuels The preferred embodiments are the same as those described above.

In particular, the preferred embodiment is a metal-coated dielectric particle that is coated with Ti0₂. The thickness of the core, the metal coat, and the Ti0₂ coat are chosen so as to maximize the intensity of optical fields in and outside the i0₂ layer. The particles may be made by processes described below.

In one embodiment, the particles are dispersed in the liquid that contains the reactants.

In another embodiment, the particles are bonded to a surface and only part of the particles is in direct contact with the liquid. 5 In another embodiment, the particles are trapped in an agarose or sepharose gel having pores small enough that the particles remain in the gel.

In another embodiment, the particles are chemically bonded to gel. o In accordance with another embodiment of the present invention, the plasmon resonance effect can be used with solar power technology.

For example, highly absorbing beads could be placed in water or other liquid to heat up the fluid in solar heater 5 devices.

Also, photochemical batteries such as the AgCl photo voltaic battery could be enhanced by the use of small beads on the AgCl electrode. Work already exists that shows by roughening the electrode, the photo current increases 0 markedly as well as SERS effects at the surface of the electrode.

Beads can be used as a UV-blocking sun screen. The

5 implementation only requires a transparent base to support the beads.

Pursuant to a further embodiment, a sphere is designed to improve fluorescence detection of a specific bio-agent. With reference to Figure 2, in general, one must attach a fluorescent agent (F) to an antibody or antigen 56 with a plasmon resonating sphere 60 in the neighborhood such that the fluorescence of agent F is increased or decreased, depending on whether the sphere goes off or on resonance.

The fluorescence of agent F should be enhanced and, therefore, make the above process more easily detectable. There are many other combinations that include flat surfaces such as surface 62 shown in Figure 3.

Another point to consider is that if the sphere 64 shown in Figure 4 is built to be on resonance (highly absorbing at a particular wavelength) , then as an antigen 66 is attached, as illustrated in Figure 5, the sphere 64 should go off resonance and the colloidal suspension should suddenly begin transmitting light. Alternately, a nonlinear core can be used such that a second or third harmonic signal is produced. This would be at a wavelength considerably different than the pump beam.

Any photo activated pharmaceutical could be coated onto an active sphere which would then become more sensitive to any light shined on it. That would mean shorter treatment time or less intense light to activate the light sensitive drug.

In addition to metals such as silver and gold discussed herein, any material having a negative real part of the dielectric constant (NRDC) can also be used in sunscreens, in florescence detection, and in photo activated pharmaceuticals. The NRDC materials includes super conductors, conducting polymers and materials with an anomalous dispersion of carrier electrons and heavily doped semiconductors where free carrier motion dominates the dielectric function, and the like.

In accordance with another embodiment of this i- invention, nanometer-sized dielectric particles are used as nucleation centers for the particles used in solid propellants. By growing particles on well defined nucleation centers, the particle sizes can be more accurately controlled, and smaller sizes can be obtained. ^• Consequently, o the properties of the propellant can be more accurately controlled. The size distributions of the particles in solid propellants is important in determining the burn rate and other properties.

Well-controlled, uniformly-sized particles can be generated by using very-small dielectric particles as nucleation centers for some salts. These methods can also be applied to other salts such as ammonium nitrate, ammonium perchlorate or other solid phase components of solid propellants. Similar methods can also be used to generate o accurately-sized metallic particles such as aluminum or magnesium which are used in some solid propellants.

The major advantages of using accurately sized particles grown on dielectric are that the size distributions can be very well controlled and the particles generated can be much smaller than those presently used. The size distributions now obtained using conventional crystal-growth processes have relatively large standard deviations. The size distributions now obtained using liquid-liquid mixing processes are determined primarily by the shear forces

30 developed during the mixing. Another possible advantage is that the equipment required for making and mixing these particles may be less expensive than presently used equipment. For example, dielectric particles such as silica,

35 particles of various sizes can be inexpensive and can be purchased in large volumes.

A further embodiment of the present invention relates to using metal coated nanoparticles to help develop fingerprints from paper and similar surfaces. This is currently a very active area, and a current method used is to fluorescent label the fingerprint profile by means of a suitable dye, and scan the fluorescence emission to achieve a spatial resolution by means of a gated diode array detector. One problem with this method is the relatively high background fluorescence which originates from the substrate. To circumvent this, the time resolved, longer lived (but generally very weak) triplet emission is detected by the diode array spectrometer operated in the gated mode. The properties of the metals, (e.g. silver or gold) coated nanoparticles can be exploited in two ways:

(1) The particles can be prepared with a suitable crosslinking agent (amino acids or proteins) attached to the surface, enabling them to be covalently bonded to the fingerprint profile. This may require photo-initiation, similar to the well known photo-affinity labelling used in protein chemistry. If this region is now scanned by a laser (e.g. He-Ne), the reflected beam will be significantly attenuated in those regions which contain particles. A spatial resolution of the reflectance spectrum can be achieved by directly digitizing the optical signal. This aspect of signal processing is very similar to the reading of bar codes in super markets, and should make it relatively simple to mass produce. At the same time, the method would be extremely sensitive, free from interference, and inexpensive.

(2) In a variation of (1) and the existing fluorescence method, a suitable fluorophore can be attached to the surface of the particles, which would mark the fingerprint profile as in (1). It is likely that the triplet emission will be enhanced due to the heavy atom effect, and the fluorescence emission increased due to the presence of silver coated particles. The net result will be much higher sensitivity, and reduction from background fluorescence.

Laser induced plasmas are formed when the optical intensities in materials are sufficiently large [L.J. Radziemski and D.A. Gremers, eds., Laser Induced Plasmas, (Marcel Decker: New York), 1989]. Once the plasma is initiated it more readily absorbs the incident radiation and grows. The plasma may then protect the underlying surface from the laser beam. The plasma may also be used as a broadband light source, or may be useful for materials processing or other applications. Also pursuant to this invention, a material is provided that provides for plasmas to be generated with lower laser intensities than are now required. In one embodiment of this invention, metal coated nanospheres, designed to enhance the fields at the wavelength of an incident laser beam, are used to decrease the intensity of the incident beam that is required for plasma formation. The nanospheres provide a means of generating plasmas with lower intensity lasers than have been used previously. One very small (20 to 50 nm) nanosphere can enhance the intensity in a small region near the sphere by a factor that is in the hundreds. Since only a very small region of high intensity is required for the plasma formation, a low concentration of very small spheres can be used.

As will be appreciated by those of ordinary skill in the art, numerous specific types and shapes of particles may be used in the practice of the present invention. Generally, each of these particles includes a core surrounded by a shell, and at least one of the core and shell consists essentially of a metal.

As defined herein, the metals include the transition metals, the lanthanides and the Group IIIA metals, and the like. The especially preferred metals include the Group VIII and IB metals, especially copper, silver, gold, iron, nickel, palladium, platinum, cobalt, rhodium, iridium, ruthenium, aluminum and the like. Especially preferred metals include copper, silver, gold, nickel, palladium, platinum and aluminum.

With respect to the photo processes described herein, these metals will exhibit the desired plasmon resonance effect in wavelength regions where the real part of the dielectric is a negative constant. The other of the core and shell, for example, may consist essentially of a dielectric material. The term "dielectric material or core as used herein refers to a material which is a non-conductor or a semi conductor. The conductivity of the material may range from 0, but preferable as low as 10^~ to 10 mhos. In a preferred embodiment the conducti .vi.ty ranges from 10-40 to 105 mhos. In a most preferred embodiment, the conductivity ranges from 10 -30 to

4

10 mhos. Examples of dielectric material include glass, silica, cadmium sulfide, gallium arsenide, polydiacetylene, lead sulfide, titanium dioxide, polymethylacrylate (PMMA), silver bromide, carbon fibers, copper sulfide, silver sulfide and the like.

Figures 6-9 show four types of particles, referenced at 70, 72, 74, and 76, respectively, that may be employed in the methods and apparatuses of this invention.

Particle 70 consists of core 70a and shell 70b, the core consists essentially of a dielectric material such as silica, and the shell consists essentially of a metal, such as silver, and is disposed immediately over and substantially completely covers core 70a. Particle 72 consists of core 72a, first shell 72b and second shell 72c. Core 72a consists essentially of a dielectric material such as silica, shell 72b consists essentially of a first metal and is disposed immediately over and substantially completely covers core 72a, and shell 72c consists essentially of a second metal and is disposed immediately over and substantially completely covers shell 72b.

If it is desired to provide two metal layers over the dielectric core, it may be preferred, as is done in particle 74, to separate or space the metal layers from each other to prevent those two metals from chemically reacting with each other. In particular, particle 74 consists of dielectric core 74a, a layer of a first metal 74b disposed immediately over and substantially completely covering layer 74a, a layer of dielectric material 74c such as a polymer, disposed immediately over and substantially completely covering layer 74b, and a layer of a second metal 74d disposed immediately over and substantially completely covering layer 74c.

It is not necessary that, in the nanoparticles that are used in the practice of the present invention, the metal form a shell or coating, and Figure 9 shows a fourth particle 76 that may be used in the practice of this invention and which comprises core 76a comprised of a metal and shell 76b comprised of a dielectric material. Of course, particle 76 may be provided with additional layers over shell 76b.

Figures 10-13 show four additional particles, referenced at 80, 82, 84 and 86, respectively, that may be used in, or in conjunction with, the present invention. Each of these particles includes at least a core surrounded by a shell; and in each of these particles, one of the core and shell includes silver halide, and the other of the core and shell includes a dielectric material. For example, particle 80 consists of core 80a and shell 80b, the core consists essentially of a dielectric material such as silica, and the shell consists essentially of silver halide. Further, with this particle, shell 80b is disposed immediately over and substantially completely covers core 80a. This particle does not itself include any metal and thus does not exhibit the plasmon resonance effect. However, the silver halide in the particle may be changed to metal silver, either to form a layer of metal silver on the particle or to help form a layer of another metal thereon, and to thereby form a particle that does exhibit the plasmon resonance effect.

In particle 82, a metal coating such as silver, copper, aluminum, gold or palladium is disposed between the dielectric core and the silver halide shell to increase the sensitivity of the silver halide to light. This increased sensitivity is caused by the plasmon resonance effect produced by the metal coating. More specifically, particle 82 consists of dielectric core 82a, metal coating 82b disposed immediately over and covering that core, and a layer of silver halide 82c disposed immediately over and covering layer 82b.

If it is desired to use a layer of metal between the dielectric core and the silver halide shell, it may be preferred, as is done in particle 84, to separate or space the metal from the silver halide to prevent the silver halide from chemically reacting with the metal. In particular, particle 84 consists of dielectric core 84a, a layer of silver 84b disposed immediately over and covering core 84a, a layer of dielectric material 84c such as a polymer, disposed immediately over and substantially covering the silver layer, and shell 84d formed of silver halide disposed immediately over and substantially completely covering layer 84c.

In a silver halide nanoparticle used in or in conjunction with the present invention and that includes both r- silver halide and a dielectric material, it is not necessary that the dielectric material and the silver halide form the core and shell of the particle, respectively; and Figure 13 shows silver halide particle 86 that may be used in or with the present invention and which comprises core 86a comprised 0 of silver halide and shell 86b comprised of a dielectric material. With the particle 86 shown in Figure 13, it may be desirable to provide the particle with a layer of metal (not shown) to enhance the sensitivity of the silver halide to light; and if this is done, to further provide the particle with a still further coating of a dielectric material (also not shown) between that metal layer and the silver halide core of the particle to prevent the metal and the silver halide from chemically reacting with each other. Figures 6-13 are only representative of 0 nanoparticles that may be used in the present invention, and in particular, only illustrate the general relationship between the cores and the shells of the shown particles. In any nanoparticle used in this invention, the particle and the core thereof may have any suitable shapes, and specifically, the particles and the cores may have shapes other than spherical. For instance, the particles and the cores may be cylindrical or ellipsoidal, have a thread-like shape, or be crystalline shaped. The actual crystal form of the core may be any suitable form; and, for example, these cores may be: _Q Tetragonal crystal forms,

Orthorhombic crystal forms, Monoclinic crystal forms, Triclinic crystal forms,

5 Isometric crystal forms, and Hexagonal crystal forms. Also, the shapes of the nanoparticles may change as they are made. Further, in any particle having a dielectric material, any suitable dielectric material may be used, and in particular, the dielectric material may be linear or non-linear. In addition, as the term is used herein, "metal" includes any material having a negative dielectric constant, and so can include superconductors, conducting polymers, materials with an anomalous dispersion of carrier electrons, and heavily doped semiconductors where free carrier electron motion dominates the dielectric function.

Any suitable procedure may be used to prepare the coated particles used in or with the present invention. In accordance with the present invention, a metal-halide coated nanoparticle can be prepared by providing a source of metal ions and a source of halide ions in a liquid carrier having dispersed therein charged colloidal dielectirc particles and reacting the halide ions with the metal ions in the presence of the dielectric particles to form coatings of metal halide over individual dielectric particles. For example, with reference to Figure 14, silver halide coated dielectric particles, such as particle 70 of Figure 6, may be made by a process generally comprising the steps of providing an aqueous solution including negatively charged colloidal dielectric particles, positively charged silver ions, and a halide, and reacting the halide with the silver ions to bond, or grow, coatings of silver halide completely covering individual dielectric particles. Preferably, the concentrations of dielectric particles, silver ions and halide in the solution, and the length of time over which the coatings are allowed to grow on the dielectric particles, are selected so that coatings of a uniform preselected thickness are grown on those particles. The specific order in which the dielectric particles, the silver ions and the halide are added to the aqueous solution is not critical; and, for example, the dielectric particles may be dispersed in the solution, then the silver ions may be added, and then the halide may be added.

In a preferred process, after the dielectric particles are added to the solution, the pH of that solution is adjusted to and thereafter maintained at a level slightly above 2, and even more preferably, between about 3 and 5. With this procedure, the dielectric particles do not have to be negatively charged when they are added to the solution, and, instead, the acidity of the aqueous solution causes the dielectric particles to become negatively charged once the particles are in the solution. Further, with the preferred process, the initial concentration of the silver ions in the solution is relatively low, less than 10~ M; the initial concentration of the halide in the solution is slightly greater than, such as about 10% greater than, the concentration of the silver ions in the solution; and also, the solution is constantly stirred while the halide is being added to it.

The silver ions may be added to the solution in any suitable form, and for instance, these ions may be added in the form of a silver salt soluble in water, e.g., silver nitrate. Likewise, the halide that is added to the solution may be any suitable halide, such as alkali halide, e.g., sodium bromide, potassium bromide, sodium chloride, or potassium chloride, and the like. In addition, any suitable dielectric may be used in the above-discussed process, and the dielectric may be linear or non-linear and may have any suitable shape and size. For example, the dielectric particles may be spherically shaped silica particles. When, ^ first, the dielectric particles are these silica particles, second, the silver ions are added to the solution in the form of silver nitrate, and third, the halide is sodium bromide, then the silver from the silver nitrate reacts with the r- bromide from the sodium bromide to form silver bromide, which bonds to and forms layers over the silica particles.

Figure 15 generally outlines a process for making a metal coating on a dielectric particle, such as coating 72b of particle 72, or coating 74b of particle 74. This process 0 generally comprises the steps of providing an aqueous solution including negatively charged colloidal dielectric particles, metal ions, a secondary alcohol, preferably a lower secondary alcohol containing 3-6 carbon atoms (e.g., isopropanol) and a ketone, preferably containing 3-6 carbon atoms, such as acetone; removing oxygen from the solution; and exposing the solution to ultraviolet light to cause the metal ions to attach to the dielectric particles and form metal coatings completely covering individual dielectric particles. Preferably, the concentrations of the dielectric particles, the metal ions, the isopropanol and the acetone, and the length of time the solution is exposed to the ultraviolet light are selected so that coatings of a uniform, preselected thickness are formed on the dielectric particles.

As used herein, the term lower alkyl, when used alone or in combination, contains 1-7 carbon atoms. These alkyl groups may be straight chained or branched and include such groups as methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, isobutyl, t-butyl, pentyl, amyl, hexyl, heptyl and the like.

As used herein, a secondary alkanol refers to a lower alkyl alcohol in which the hydroxy group is attached to a secondary carbon. Such groups include isopropanol, sec-butanol and the like. The preferred ketone is acetone.

In the above-discussed procedure, without wishing to be bound, it is believed that the acetone absorbs energy from the ultraviolet light and then reacts with isopropanol to form isopropyl radicals. These radicals are powerful reducing agents and cause metal ions that have become attached to the dielectric particles to form metal molecules. The particular order in which the dielectric particles, the metal ions, the isopropanol and the acetone are added to the aqueous solution is not critical; and, for instance, the isopropanol and acetone may be added to the solution, the dielectric particles may then be dispersed in the solution, and then the metal ions may be added.

With a preferred process, as with the process outlined in Figure 14, after the dielectric particles are added to the solution, the pH of the solution is adjusted to and thereafter maintained at a level slightly above 2, and even more preferably, between about 3 and 5. In this way, the dielectric particles do not have to be negatively charged when they are added to the solution and the acidity of the aqueous solution causes the dielectric particles to become negatively charged. In addition, the initial concentration of the metal ions in the solution is relatively low, such as 2 x 10 -4M; and the initial concentration of the acetone and isopropanol in th solution are about equal to each other and much greater than, such as about 400 times greater than, the initial concentration of the metal ion in the solution. In addition, preferably the solution is stirred while exposed to the ultraviolet light.

Numerous specific types of metal coatings may be made using a procedure as described above, and for example, the process may be used to form silver coated dielectric particles, gold coated particles or palladium coated particles. In addition, the metal ions may be provided in the solution in any suitable manner; and, for example, these ions may be provided by adding a water soluble metal salt such as silver nitrate, to the solution.

Moreover, any suitable dielectric may be used in the above-discussed process, and the dielectric may be linear or non-linear and may have any suitable shape and size. For instance, the dielectric particles may be spherically shaped silica particles. When such dielectric particles are used, and the metal ions are added to the solution in form of silver nitrate, then the ultraviolet light, in combination with the acetone and the isopropanol, causes the silver ions to bond to and form metal coatings over the silica particles. The following example illustrates this process for forming metal coated dielectric particles.

EXAMPLE 1

An aqueous solution is prepared by mixing the following solutions in a 50 ml beaker:

(1) 0.5 ml of 0.01 M g 0₃, 5 (2) 0.5 ml of 0.50 M of low porosity SiO-, particles. The particle diameter was chosen to be between 5 to 20 nanometers, although other sizes can be readily substituted,

(3) 1.5 ml of pure isopropanol,

(4) 1.5 ml of pure acetone. o All chemicals used are of reagent grade quality, unless otherwise specified. The above mixture is diluted with 16 ml of distilled water, and the pH adjusted to be between 4 to 5 by dropwise addition of a 0.01 M nitric acid solution. In this pH range, the silica particles are 5 negatively charged, causing the positively charged silver ions to be bound to the surface. After thorough mixing by stirring for one minute using a magnetic stirrer, the sample is transferred to a UV photolysis vessel, equipped with a quartz window and provision for careful deoxygenation by o bubbling nitrogen gas for one hour. It is important that no oxygen be present in the solution. The sample is irradiated by a 450 Watt Hg-Xe lamp for one hour, with gentle stirring continued by means of a magnetic stirrer. The solution color, and consequently the thickness of the coat, can be 5 controlled by adjusting the period of illumination by UV light. This forms the basis for the preparation of the silver coated silica particles in the present example.

0

5 Silver coated dielectric particles may also be made by a process employing photoreduction of silver halide, and one such process is outlined in Figure 16. In this process, silver halide coated dielectric particles are made, for example, by the process discussed above in connection with Figure 14, and then the coated particles are exposed to light to change the silver halide coatings over the individual particles to metal silver coatings.

Preferably, though, a more integrated process, generally outlined in Figure 17, is used to form silver coated dielectric particles. In accordance with this process, dielectric particles are dispersed in a solution including silver ions, a halide and an electron hole scavanger, and the metal ions react with the halide to form silver halide coatings completely covering the dielectric particles. The solution is then exposed to ultraviolet light, and this light changes the silver halide coatings to silver coatings. Preferably, the concentrations of the dielectric particles, the silver ions, the halide and the electron hole scavenger in the solution, and the length of time the solution is exposed to the ultraviolet light are selected so that coatings of a uniform, preselected thickness are formed on the dielectric particles.

Preferably, with this process, the initial concentration of silver ions in the solution is greater than the initial concentration of the halide in the solution; and for instance, the former concentration may be about 5 times the latter concentration. The silver ions may be in the solution in any suitable form. For instance, the silver ion may be added to the solution in the form of silver nitrate. Similarly, the halide that is added to the solution may be any suitable halide such as alkali halide, e.g., sodium bromide, potassium bromide, sodium chloride, potassium chloride and the like. Further, any suitable dielectric may be used in this process, and the dielectric be linear or non-linear and have any suitable shape and size. For example, the dielectric particles may be spherically shaped silica particles. When (i) the dielectric particles are the silica particles, (ii) the silver ions are added to the solution in the form of silver nitrate, and (iii) the halide is sodium bromide, then the silver from the silver nitrate reacts with the bromide from the sodium bromide to form silver bromide; and the ultraviolet light, in the presence of EDTA, then reduces the silver bromide coatings to metallic silver.

In the above procedure, it is preferred that the light source used contain ultraviolet light. It is preferred that the light source contain wavelengths of 150 - 550 nm. The preferred wavelengths range from 200-400 nm.

Furthermore, it is preferred that the intensity of light used ranges from 50 watts to 1.5 kilowatts, with the preferred intensity ranging from 250-1000 watts. Especially preferred intensity ranges from 350-550 watts, with an intensity of about 450 watts being the most preferred.

The following example illustrates this process for forming silver coated dielectric particles.

EXAMPLE 2 Metallic silver on Si0₂ particles can be obtained by photoreduction of silver halides, which are typically prepared in the presence of excess Ag ions. A hole (h ) scavenger, EDTA, is added to the solution. One ml of a 0.002 M NaBr solution is added to 19 ml of a solution which was prepared in a 50 ml beaker mixing the following:

(1) 1 ml of 0.01 M AgN0₃,

(2) 0.5 ml of 0.50 M of low porosity Si0₂ particles. The particle diameter was 12 nanometers, although other sizes can be readily substituted,

(3) 1 ml of 0.02 M EDTA,

(4) 16 ml of distilled water.

After thorough mixing, the solution is transferred to a 1 cm UV quartz cuvette and exposed to a 375 Watt tungsten halogen light source. Under these conditions, very little light is actually absorbed since the colloidal AgBr has a very low absorbance above 350 nm. A possible mechanism for the reduction process is given by:

AgBr -> AgBr(e~ + h⁺)

AgBr + e~ -> Ag° + Br^"

EDTA + h > product

Br^~ + Ag (excess) > AgBr

The duration of illumination, which is in the order of minutes, determines the color of the silver coated silica particles. This color is a result of the thickness of the silver layer, and can range from yellow to a purplish gray.

Once the silver coated silica spheres are prepared, they are purified by dialysis and then placed in a sodium dodecyl sulfate micellar solution, or a micro emulsion.

A variation of the process described above may be employed to form metal coatings other than silver on nanoparticles, and this variation utilizes the fact that metallic silver on the dielectric particles will act as a catalyst to help grow metal coatings on those particles from other metal ions in the solution. In accordance with this variation, which is outlined in Figure 18, a solution is provided including dielectric particles, silver halide is formed on those particles, the solution is exposed to light to change at least a portion of the silver halide to metallic silver, and ions of a metal are added to the solution to form coatings of that metal completely covering individual dielectric particles, with the metallic silver on those particles acting as a catalyst to accelerate the formation of the metal coatings. These metal ions may be added to the solution in any suitable manner, and for instance, conventional photographic developing solutions may be added to the solution to add the metal ions.

Only minute amounts of metallic silver are needed on the dielectric particles to help grow the metal coatings thereon; and hence, in the above-described process, it is only necessary to form minute amounts of silver halide on the dielectric particles. Alternatively, complete coatings of silver halide may be formed on the dielectric particles, with only minute amounts of the silver halide on individual particles being changed to metallic silver. With another variation, silver halide coatings may be made completely covering dielectric particles, only minute amounts of the silver halide may be changed to metallic silver on individual particles, and then these minute amounts of metallic silver may be used to help form metal coatings completely covering the silver halide that remains on the dielectric particles. The resulting product comprises a dielectric core, a first coating of silver that substantially completely covers the dielectric core, and a second coating of a metal that completely covers the layer of silver halide.

The following example illustrates the coating of silver on an dielectric core of silver bromide. The silver bromide nanoparticles exposed briefly the intense UV light in the presence of EDTA have optical extinction spectra similar to those computed for distribution of silver coated silver bromide nanoparticles. By intense, it is meant that the intensity of the light ranges from 50 watts to 1.5 kilowatts, with the preferred range being 250-550 watts, and the most preferred having a range of 350-550 watts.

As clearly shown by the following discussion, with shorter exposure time, the plasmon resonance maximum is shifted to lower wavelengths, a result consistent with theory so long as the coat thickness increases with exposure to light. The resonance maximum of the distributions of coated particles can be controllably shifted to 600 to 700 nm.

EXAMPLE 3 Silver bromide colloids were prepared by rapidly mixing equal volumes of AgN0₃ and NaBr solutions. A growth stabilizer (SDS) and an electron donor (EDTA) were added immediately after precipitation. Typically the final concentrations were 1 x 10 -4 M Br-, 4 x 10-4 M Ag+, 5 x 10-4

M SDS, and 5 x 10 M EDTA. The concentration of SDS was far

_2 below the critical micellization concentration (10 M) .

Freshly prepared solutions were exposed to light from a 450

Watt Hg-Xe lamp for a few seconds. With the shortest exposures the spectra appeared blue. With longer exposures the solutions appeared orange. When ammonia, which dissolves

AgBr by forming complexes with Ag , was added to any of the illuminated solutions the color changed to a yellow color characteristic of small metallic silver colloids.

The particle size distributions were characterized with transmission electron microscopy (JEOL 1200EX) . A typical micrograph is shown in Fig 23. A size distribution consistent with the limited micrograph data is the log normal distribution.

N(r) = N₀exp(-((ln(r) - ln(r_m) )/ln(s) )²) , with rm equal to 1 nm or less and s in the renge of 4 to 4.5 nm. The size distributions as determined by TEM did not appear to change markedly with exposure to light.

After the addition of ammonia to any of the illuminated samples only small particles having diameters 5 nm or less were observed in the TEM (Fig 24) . The most likely interpretation is that only part of the AgBr was reduced to Ag during the illumination and that the larger particles are AgBr/Ag composites.

Example optical extenction spectra measured shortly after exposure are shown in Figs. 25a) to d) . The exposure time and/or EDTA concentration, and hence the reduction of Ag⁺, increases in going from a) to d) . The peak extinction shifts to shorter wavelengths as the illumination time is increased. This result is consistent with theory so long as the coat thickness increases with exposure. A spectrum of the ammonia treated solution, shown in Fig 25e) , is typical of homogeneous silver nanoparticles. The general shapes of the above spectra are readily reproducible. At comparable illumination times, in the absence of Br , the appearance of color in a given sample is negligible.

Theoretical optical extinction spectra of individual silver coated spheres are shown in Fig. 26. The peak of the theoretical extinction shifts from red to blue as the ratio of coat thickness to core radius increases. This data is consistent with the measured spectra where the absorption maxima shift toward the blue as the time of exposure increases, since the coat thickness should increase with exposure time. The computed spectra are very sensitive to the coat thickness. The measured spectra are much more broad than the spectra shown in Fig. 26 because of the distributions of core diameters and coat thicknesses.

The magnitudes of the extinction spectra are also characteristic of silver coated particles. For example, at a wavelength of 700 nm the extinction cross section per unit volume of silver is 100's of times larger in a silver coated nanoparticle having the appropriate ratio to core radius to coat thickness that it is in a solid silver sphere. The fact that the theoretical extinction is so large can be used to help verify that the particles are coated with silver. However, since there is a broad distribution of sizes, care must be taken in making the comparision.

Here we started with the size distribution of core particles described by the above equation, then used trial and error to determine the distributions of coat thicknesses required to match the measured spectra, and then found that the magnitudes of the spectra were within the range of values expected from the initial concentrations of Ag and Br~.

The assumptions made in computing the spectra are as follows:

1. The reduced silver is in the form of a smooth coat on the surface of a spherical AgBr particle. The extinction efficiencies were computed using the separation of variables solution for concentric spheres based on algorithms.

2. The size distribution of the core particles is described by the log-normal distribution of the above equation. The values of N were determined by setting the total volume of all the particles prior to illumination in the distribution equal to the volume of AgBr. The initial total volume of AgBr was determined by solving the ionic equilibria equations including the Ag -EDTA complex.

3. The size distribution of the coat thickness is a Gaussian, typically with a standard deviation of 2 to 8 nm.

4. The silver coat may be formed either from the reduction of the silver halide of the initial particle, or from the reduction of Ag from solution. Computations have been done for each of the two limiting cases.

5. The total extinction is computed by numerically integrating over distributions of core radii and coat thicknesses. b_e(\) = jN_n(r_c)N_g(t)Q(r_c,t,m_c,m_t,-χ)TTr²dr_cdt (2) where Nn is the size distribution of the cores,

N is the size disbribution of the coats, Q is y the extinction efficiency, m is the refractive index of the core, and m. is the refractive index of the coat. Typically the integrations over cores were from r=2 to r=18.

6. The refractive index of the silver was computed from the data of Hagemann et al. in J. Ojg. Soc. Am., 65, 742-744 (1975) and Kerker, in J. Op. Soc. Am. B., 1327-1329 (1985) either by itself, or combined with a Drude model in which the increased electron scattering at the surfaces of the very thin coat was taken into account. The refractive index data of Johnson and Christy in Phy_. Rev. B, S , 4370-4379 (1972) was also used for some computations not shown. Linear interpolation was used to obtain the values of refractive index at points not in the data.

7. The refractive index os AgBr was obtained by combining the data from White, J. Opt. Soc. Am, 62, 212 (1972) and James, "Theory of the Photographic Process," McMillan (1977) p 216.

Fig. 27 shows a measured spectrum and two computed spectra. In the topmost curve the Ag in the coat is assumed to come only from the solution, i.e., the AgBr cores are not reduced in size as the coat grows. In the bottom curve the Ag in the coat is assumed to come only from the reduction of AgBr at the surface of the particle and so the core shrinks as the coat grows. Since the measured curve lies between the two computed spectra, the magnitudes of the plasmon enhanced extinction is in the range of values computed. The main parameters that can be adjusted in fitting the distributions to the spectra are: 1) the thickness and standard deviation of the coats and the limits of the numerical integration for the coats. 2) the size distribution and the limits of integration for the cores. 3) the date for the refractive index of silver. 4) the fraction of the reduced silver that came from solution. The computed spectra are very sensitive to the distributions of cores and coats chosen and to the limits of integration, which also define the size distributions. The computed spectra depend on the refractive index of silver used. However, by varying the size distributions, similar spectra can be obtained with the different models for silver. The effect of the different assumptions about the source of the Ag for the coat can be seen in Fig. 27. In a preliminary experiment without excess silver a spectrum similar to that shown in Fig. 25d) was generated.

Without wishing to be bound, it is believed that the silver coat is formed by the coalescing of many small silver particles. The coat may also contain some AgBr or voids, but it is homogeneous enough to have a refractive index similar to that of bulk islver. The bonds between the particles may be relatively weak because the coat breaks into many small particles when the solution is treated with ammonia.

It might have been thought that the spectra could be accounted for by nonspherical silver particles. The fact that ammonia, which dissolves AgBr but not Ag, reduces the spectrum to that of small solid silver particles, and the fact that the particles in the TEM do not have large eccentricites, argue against this hypotheses. Also, the particle shapes do not seem to be related to the colors of the solutions. In summary, the predicted tunability of the surface-plasmon resonace frequency and enhanced extinction at longer wavelengths was experimentaly confirmed with Ag-AgBr colloidal composites. The particles scatter as if the Ag is smoothly coated on the AgBr.

Silver coated dielectric particles may also be formed by a process utilizing chemical reduction of silver ions by hydroquinone at elevated temperatures. The following example, generally outlined in Figure 19, illustrates this process.

EXAMPLE 4

100 ml of a silica solution (particle diameter 7 nm) which had been purified by overnight dialysis was transferred to a 250 ml beaker, and the pH adjusted to 4.0 by dropwise addition of 0.01 M nitric acid. This was heated to 90°C, and 0.01 M AgN0₃ solution added dropwise under gentle stirring to achieve the final concentration shown in the table below. After about 2 minutes, sufficient quantity of 0.01 M hydroquinone was added in a similar manner. The reduction to metallic silver takes place gradually over a time period of about five minutes, accompanied by a color change from pale yellow to dark brown. The rate of silver deposition by this method can be controlled by varying the temperature between 85 to 95°C. A transparent solution is obtained in every case, and is allowed to cool and then purified by dialysis.

The following table summarizes the experimental conditions, including final concentrations, which were used in four different sets:

The amount of silver deposited increases from I to IV, and is evident from the color of the solutions (light yellow to dark brown) . Electron microscopy also provided evidential support. The optical absorption spectra show the presence of a single peak maximum at about 400 nm. ELECTRON MICROSCOPIC RESULTS:

Solution I consists of particles which are smaller and better defined, appear darker, and were in the size range of 10 to 30 nm. In solutions II, III and IV, the particle size range was found to be between 40 to 100 nm, the particles were similarly dark, but contained elongated as well as spherical shapes. The final size distribution may be due in part to the non uniform size of the silica core particles, found to be between 7 to 11 nm by electron microscopy.

With all of the processes described above, after the coated particles are prepared, they may be removed from the solution in which they were prepared by dialysis, and then placed in a sodium dodecyl sulfate micellar solution or a micro emulsion. Additional coatings of either silver halide, a metal or a polymer, may then be added until the desired final configuration is reached. Polymer coating of any of these particles may be readily achieved in a solution by the well known emulsion polymerization method, in which a suitable amount of monomer and initiator have been added.

For instance, the following process, outlined in Figure 20, shows how a polymer coating may be made on a silver coated particle.

The following aqueous stock solutions were prepared:

(I) 0.1 M KH₂ P0₄,

(II) 0.1 M NaOH,

(III) 2 % solution of sodium salt of styrene sulfonic acid, NaSS (co-monomer), (I ) 3 % solution of K₂S₂O_g.

All solutions were prepared in doubly distilled water, and all chemicals were reagent grade. 131.6 ml of 1 % solution of the silver coated silica particles were transferred to a three necked flask. 8 ml of solution IV, followed by 6.4 ml of solution II, were added with constant stirring using a magnetic stirrer. The flask was equipped with a condenser, and a platinum thermometer, which, in combination with a thermoregulator and a heating mantle, allowed regulation of the temperature of the flask to 65 ± 1°C. At this temperature, nitrogen gas was bubbled through the mixture continuously, and 30 ml of styrene added. After 15 minutes, 10 ml of solution III were added, and after another 20 minutes 4 ml of the initiator, solution IV, were added. Depending upon the thickness of the polymer film desired, the reaction can be terminated by addition of 25 ml of a 1 % solution of hydroquinone, and cooling the reaction mixture to room temperature. The particles are filtered, washed several times with double distilled water, resuspended in water, and further purified by dialysis.

EXAMPLE 5 Coating of carbon fibres with copper was carried out by photochemical reduction of Cu using highly reductive short lived 1-hydroxy-l-methylethyl radicals. These radicals were produced in situ by illuminating a mixuture of 1 M acetone and 1 M propanol-2 with a UV source of Hg-Xe lamp operated at 450 watt. The reaction can be presented by

(CH₃)₂C0 —> (CH₃)₂C0*

(CH₃)₂CO* + (CH₃)₂CH0H --^■=> 2(CH-_)₂C0H

2(CH₃)₂COH + Cu⁺² -- 2(CH₃)₂CO + Cu + 2H⁺

nCu —> Cu_n

Two different solutions of Cu ++ (1 x 10-2 M and 1 x 10 -3 M) were used to achieve two different coating thicknesses. Both solutions contained 1 M acetone, 1 M propanol-2, and carbon fibers. The illumination time was two hours.

These coated fibres, washed with distilled water and observed under an optical microscope, show a very fine and smooth coating and visibly exhibit a metallic lustre of copper. The amount of copper on these fibres was detected using atomic absorption spectroscopy after removing the coat with 1 M nitric acid. The presence of copper on these fibres was also confirmed using Energy Dispersive Spectroscopy

(EDS) , which shows a peak for copper. The thickness of the coat can be controlled by the copper concentration in solution and the duration of illumination. It can be readily varied in the range of tens of nanometers to microns. The processes discussed above may be used in various combinations to form particles of a desired configur tion. For example. Figure 21 generally outlines a procedure to make particle 72 of Figure 7. First, metal coating 72b is formed over dielectric core 72a, for example using the method of illustrated in Figure 17; and then silver halide coating 72c is made over metal layer 72b, for instance by generally following the method shown in Figure 14. Similarly, Figure 22 generally illustrates a procedure to make particle 74 of Figure 8. In this procedure, first, metal coating 74b is formed over dielectric core 74a, for example by the process described above in connection with Figure 15, then polymer coating 74c is applied over coating 74b, and then silver halide layer 74d is formed over coating 74c, for example by generally following the procedure discussed above in connection with Figure 14.

Although the text hereinabove refers to halides, the halides may be replaced with organic anions to form other metal complexes. A property of these organic anions is that they must be capable of forming stable complexes. These organic ions include such anions as acetate, formate, citrate, EDTA, malonate, and polypeptides prepared from the natural amino acids, such as poly GLU, poly ASP, and the like. While it is apparent that the invention herein disclosed is well calculated to fulfill the objects

previously stated, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.

Claims

WHAT IS CLAIMED IS:

1. A method of forming an integrated circuit, comprising the steps of: forming a film, including the steps of

(i) forming a first layer of silver coated nano¬ particles on a substrate,

(ii) forming a second layer containing i0₂ particles over the first layer, and

(ii) forming a third, protective polymer layer 0 over the second layer; illuminating the film through a mask to produce Ag centers in the film; and developing the film to change the Ag centers to metallic lines. 5

2. A method according to Claim 1, wherein the step of forming the first layer includes the steps of: suspending the silver coated nanoparticles in a mixture including a monomer and an initiator; and spin coating the substrate. o

3. A method according to Claim 1, wherein the step of forming the first layer includes the steps of: providing a thin polymer film containing dielectric nanoparticles with absorbed silver ions and a photo activated reducing agent; and 5 exposing said dielectric nanoparticles to ultraviolet light to produce metallic silver on said nanoparticles.

4. A method according to claim 3 wherein the dielectric is silica, alumina, titania, chromium hydroxide or 0 plastic.

5. A method according to Claim 1, wherein the step of illuminating the film includes the step of illuminating the film with ultraviolet light.

5

6. A method according to Claim 1, wherein the step of illuminating the film includes the step of illuminating the film with an excimer laser.

7. In a method for reacting at least first and second chemicals together, of the type wherein a metal is used as a catalyst to accelerate the reaction of said chemicals, the improvement comprising: providing said metal in the form of metal coated nanoparticles; and illuminating said nanoparticles so that said particles exhibit a plasmon resonance effect to increase the catalytic activity of the metal.

8. The improvement of Claim 7, wherein said illuminating step includes the step of illuminating said particles with light having a frequency equal to a plasmon resonance frequency of the nanoparticles.

9. The improvement of Claim 7, wherein the metal is a noble metal.

10. The improvement of Claim 9, wherein said noble metal is silver.

11. In a photochemical reaction of the type wherein light absorption causes molecules to reach an excited singlet states and then decay to triplet state, the improvement comprising the step of illuminating the molecules in the presence of metal coated nanoparticles that exhibit a plasmon resonance effect to increase the rate at which the molecules absorb light.

12. The improvement of Claim 11, wherein the metal coated nanoparticles are silver coated silica nanoparticles.

13. A method for the photocatalytic decomposition of pollutants, comprising the steps of: providing a solution containing the pollutants; locating metal coated nanoparticles in the solution; and exposing the solution to light to decompose the pollutants, wherein the metal coated nanoparticles exhibit a plasmon resonance effect to accelerate the decomposition of the pollutants.

14. A method according to Claim 13, wherein the locating step includes the step of dispersing the metal coated nanoparticles in the solution.

15. A method according to Claim 13, wherein the locating step includes the steps of: bonding the metal coated nanoparticles to a surface; and placing a portion of the surface in the solution to place a portion of the nanoparticles therein.

16. A method according to Claim 13, wherein the locating step includes the steps of: trapping the nanoparticles in a gel; and. placing the gel in the solution.

17. A method according to Claim 13, wherein the locating step includes the steps of: chemically bonding the nanoparticles to a gel; and placing the gel in the solution.

18. A method according to Claims 13, 14, 15, 16 or 17, wherein the nanoparticles are dielectric particles coated with Ti0₂-

19. A method for the photoredox of first and second reactants to produce a fuel, comprising the steps of: providing a solution containing the first and second reactants; locating metal coated nanoparticles in the solution; and exposing the solution to light to react the first and second reactants together to produce the fuel; wherein the metal coated nanoparticles exhibit a plasmon resonance effect to accelerate the reaction of the first and second reactants.

20. A method according to Claim 19, wherein the locating step includes the step of dispersing the metal coated nanoparticles in the solution.

21. A method according to Claim 19, wherein the locating step includes the steps of: bonding the metal coated nanoparticles to a surface; and placing a portion of the surface in the solution to place a portion of the nanoparticles therein.

22. A method according to Claim 19, wherein the 5 locating step includes the steps of: trapping the nanoparticles in a gel; and placing the gel in the solution.

23. A method according to Claim 19, wherein the locating step includes the steps of: o chemically bonding the nanoparticles to a gel; and placing the gel in the solution.

24. A method according to Claims 19, 20, 21, 22 or 23, wherein the nanoparticles are dielectric particles coated with Ti0₂. 5

25. A solar heater device comprising: a liquid including nanoparticles that exhibit a plasmon resonance effect; and means for holding the liquid while the liquid is exposed to solar energy; 0 wherein the nanoparticles enhance the effect of the solar energy on the liquid.

26. A photochemical battery comprising: first and second electrodes, the first electrode

5 2 being responsive to solar energy so that the electric potential of the first electrode changes when exposed to solar energy; and a multitude of nanoparticles applied on the first electrode, said nanoparticles exhibiting a plasmon resonance effect to enhance the effect of solar energy on the first electrode.

27. A battery according to Claim 26, wherein the first electrode is comprised of AgCl. 0

28. A battery according to claim 26, wherein the nanoparticles comprise dielectric cores, and metal coatings covering said cores.

29. A sunscreen, comprising: a base; and nanoparticles supported by the base, the nanoparticles exhibiting a plasmon resonance effect and inhibiting the passage of ultraviolet light through the base.

30. A sunscreen according to Claim 29, wherein the base is transparent. o

31. A method for testing a solution for a given bio-agent, comprising the steps of: dispersing in the solution a fluorescent agent that is capable of attaching to the given bio-agent; dispersing nanoparticles in the solution, the nanoparticles altering the fluorescence of the fluorescent agent; and testing the solution for fluorescence to determine whether the bio-agent is present in the solution.

32. A method according to Claim 31, wherein the _Q step of dispersing nanoparticles in the solution includes the step of attaching to the nanoparticles a bio-agent that is capable of attaching to the given bio-agent.

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33. A method according to Claim 32, wherein the nanoparticles exhibit a plasmon resonance effect when they are initially dispersed in the solution, and the nanoparticles do not exhibit the plasmon resonance effect if they become attached to the given bio-agent.

34. A method according to Claim 32, wherein the nanoparticles do not exhibit a plasmon resonance effect when they are initially dispersed in the solution, and the nanoparticles do exhibit the plasmon resonance effect if they become attached to the given bio-agent.

35. A method for increasing the sensitivity of a photoactivated pharmaceutical, comprising the step of coating the pharmaceutical onto a nanoparticle that exhibits a plasmon resonance effect.

36. A pharmaceutical comprising: nanoparticles that exhibit a plasmon resonance effect; and a light sensitive drug attached to the nanoparticles, wherein the nanoparticles exhibit the plasmon resonance effect to increase the sensitivity of the drug to light.

37. In a solid propellant of the type comprising solid particulates, the improvement comprising, each of at least a multitude of the particles includes a nanometer sized core of a first material, and at least one layer of a second material formed on said core.

38. A solid propellant according to Claim 37, wherein the second material is an explosive material.

39. A solid propellant according to Claim 38, wherein the second material is selected from the group consisting of ammonium nitrate and ammonium perchlorate.

40. A solid propellant according to Claim 37, wherein the second material is a metal.

41. A solid propellant according to Claim 40, wherein the second material is selected from the group consisting of aluminum and magnesium.

42. A solid propellant according to claim 37, 38, 39, 40 or 41, wherein the first material is silica.

43. A method of developing a fingerprint on a surface, comprising the steps of: applying to the surface a fluorescent dye to attach the dye to a profile of the fingerprint; also applying to the surface nanoparticles that exhibit a plasmon resonance effect to attach said nanoparticles to the surface outside the profile of the fingerprint; illuminating the surface to activate the fluorescent dye to generate fluorescent light, wherein the nanoparticles attenuate activity of any fluorescent dye that has become attached to the surface outside the profile of the fingerprint; and sensing the fluorescent light emitted from the surface to detect the profile of the fingerprint.

44. A method according to Claim 43, wherein the step of also applying the nanoparticles to the surface, includes the steps of: applying a first cross linking agent to the surface outside the profile of the fingerprint; and applying a second cross linking agent to the nanoparticles, the second cross linking agent being adapted to attach to the first cross linking agent when the nanoparticles are applied to the surface.

45. A method for developing a fingerprint on a surface, comprising the steps of: applying a fluorophore to the surfaces of nanoparticles that exhibit a plasmon resonance effect; attaching the nanoparticles to a profile of the fingerprint; illuminating the surface to activate the fluorophore to generate fluorescent light, wherein the nanoparticles enhance the fluorescent light generated by the fluorophore; and sensing the fluorescent light emitted from the surface to detect the profile of the fingerprint.

46. A method according to Claim 45, wherein the step of attaching the nanoparticles to the profile of the fingerprint includes the steps of: applying a first cross linking agent to the profile of the fingerprint; applying a second cross linking agent to the 5 nanoparticles, the second cross linking agent being adapted to attach to the first cross linking agent; and applying the nanoparticles to the surface to attach the second cross linking agent and the nanoparticles to the first cross linking agent on the surface. o

47. A material that generates a plasma when illuminated with a relatively low laser intensity, where the material is comprised of: a material that may be a solid or a liquid; and a multitude of metal coated nanoparticles that are 5 designed to have a plasmon resonant enhancement of the fields at the frequency of the laser, and where the particles are either dispersed in the solid or liquid or are resting on the surface of the particle or liquid.

48. A material according to claim 47, wherein the 0 metal coat is silver, copper, gold, platinum or paladium.

49. A material according to claim 47, wherein the cores of the nanoparticles are silica, polystyrene or polydiacetylene.

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