CA2329436C - Metal nanoshells - Google Patents

Abstract

The present invention is for particulate compositions and methods for producing them that can absorb or scatter electromagnetic radiation. The particles are homogeneous in size and are comprised of a nonconducting inner layer that is surrounded by an electrically conducting material. The ratio of the thickness of the nonconducting layer to the thickness of the outer conducting shell is determinative of the wavelength of maximum absorbance or scattering of the particle. Unique solution phase methods for synthesizing the particles involve linking clusters of the conducting atoms, ions, or molecules to the nonconducting inner layer by linear molecules. This step can be followed by growth of the metal onto the clusters to form a coherent conducting shell that encapsulates the core.

Description

MF_,TAL, NANOSHELLS
BACKGROUND OF THE INVENTION
It is known that solid metal nanoparticles (i.e. solid, single metal spheres of unifon-ri composition and nanonleter diTnensions) possess umique optical properties. In particular, metal nanoparticles (especially the coinage metals) display a pronounced optical resonance.
This so-called plasmon resonarice is due to the collective coupling of the conduction electrons in the metal sphere to the incident electromagnetic field. This resonance can be dominated by absorption or scattering depending on the radius of the nanoparticle with respect to the wavelength of the incident electromagnetic radiation.
Associated with this plasmon resoliance is a strong local field enhancement in the interior of the metal nanoparticle. A variety of potentiall.y usefiil. devices can be fabricated to take advantage of these specific optical properties. For example, optical filters or chemical sensors based on surface enlianced Raman scattering (SERS) have been fabricated.
A serious practical limitation to realizing niany applications of solid metal nanoparticles is the inability to position the plasmon resonance at techilologically impoi-tant wavelengths. For example, solid gold nanoparticles of 10 nrn in diameter have a plasmon resonance centered at 520 nm. This plasmon resonance cannot be controllably shi f:ted by more than approximately 30 iianometers by varying the particle diameter or the specific enibedding medium.
Metal colloids have a variety of useftil optical properties including a strong optical absorption and an extremely large and fast third-order nonlinear optical (NLO) polarizability.
These optical properties are attributed to the phasic response of electrons in the metallic particles to electronlagnetic fields. This collective electron excitation is known as plasmon resonance.
At resonance, dilute metal colloid solutions have the largest electronic NLO
susceptibility of known substances. However, the utility of these solutions is linlited because their plasmon resonance is confined to relatively nar-row wavelength ranges and cannot readily be shifted. For example, silver particles 10 nm in dianieter absorb light niaximally at approximately 355 nm, while similar sized gold particles absorb maximally at about 520 nm.
These absorbance inaximums are insensitive to changes in particle size and various dielectric coatings on the particles.

One method of overcoming this problem is to coat small nonconducting particles with these metals. For exaniple, the reduction of Au on Au2S (reduction of chloroauric acid with.
sodium sulfide) particles has been shown to red shift the gold colloid absorption niaximum froln 520 nm to between approximately 600 nm and 900 nm, depending on the anlount of gold deposited on the Au2S core and the size of the core, Zhou, et al. (1994).
The ratio of the core radius to sl:iell thickness can be controlled by changing the reactant concentrations or by stopping the reaction. In this case, the diameter of the particle core is directly proportional to the red shift in the wavelength of light that induces gold plasmon resonance.
However, gold-sulfide particle diameters are hmited to sizes of approximately 40-45 nm with a thin gold shell (less than 5 nm). -I'he limited size of the gold-sulfide pai-ticles of Zhou et al. limits the absorbance maximum to wavelengths no larger than 900 mn. See, also Averitt et al. (1997).
An additional limitation of such particles as defined by Zhou et al. is that both the core atld the sllell are grown as a result of a single chemical reaction, thus limiting the clloice of the core material and the shell material to Au?S and Au respectively. In addition, only the ratio of the core radius to shell. thickness inay be controlled; independent control of the core radius and the shell thickness is not possible.

Neideljkovic and Patel (1991) disclosed silver-coated silvei- bromide particles that are produced by intense UV irradiation of a mixture of silver bromide, silver, sodium dodecylsulfate (SDS) and ethylenedianline tetraacetic acid (EDTA). The Neideljkovic particles range in size fronl approximately 10 to 40 nm and are irregularly-shaped, as deterrnined by transmission electron niicrography. Predictably, the spectra obtained froni these particle preparations are extremely broad.
In U.S. Patent No. 5,023,139, Birnboir.n et al. disclosed theoretical calculations indicating that metal-coated, semi-conducting, nanometer-sized particles should exhibit third-order nonlinear optical susceptibility relative to uncoated dielectric nanoparticles (due to local field enhancement). Their static calculations were based on hypothetical coinpositions.
The preferred embodiments disclosed by Birnboim et al. are, in fact, not particles with metallic shells on their surfaces. In those embodiments theoretically proposed by Birnboim et al. that do in fact propose ainetal outer shell, there is an additional requirement as to the specific medium in which they must be used in order to properly function.
However, Bimboim does not disclose nlethods for preparing the disclosed hypothetical compositions. Furthermore, Birnboim's calculations do not take into account surface electron scattering. Surface electron scattering strongly modifies the optical response of all metallic structures that possess at least one dimension smaller than the bulk electron mean free path (e.g. in Au at room temperature the bulk electron mean free path is about 40 nm). This effect reduces the local field enhancement factor that in tunl reduces the resonant third order nonlinear optical susceptibility associated with the nanoshell geometry. See, Averitt et al. (1997). Since typical shell thicknesses for these compositions fall below 40 nrn, Birnboim et al.'s theoretical calculations fail to account for this effect which is an important aspect of the optical response for functional metal nanoshells. Finally, it is important to realize that the hypothetical metal nanoshells of Birnboini pertains specifically to the enllancement of the third order nonlinear optical susceptibility.
Moreover, Birnboim-type particles are by def:in.ition particles mu.ch smaller than a wavelength of light (less than 0.10 times a given wavelength of light), and are particles in which the dielectric property of'th.e nanoshell (in those instances wliere it is in fact a metal shell that is used in Birnboim et al.) are defined as the bulk dielectric property of the metal selected. In practice, this requires these smaller-than-a-wavelength particles to have metal shell layer thicknesses of many nanometers (e.g., for Au, such minute pat-ticles meeting the theoretical requirements o:f the Birnboim calculations and the bulk dielectric properties of Au required thereby, would necessarily have shells at least 40 nm in thickness).
The physical lirnitations placed on the construction of such particles is therefore considerable.

Methods and materials are needed that can be used to shift the wavelength of maxin:ium absorption of metal colloids. Methods for producing materials having defined wavelength absorbance maxima across the visible and infrared range of the electromagnetic spectrum are needed. Particularly, such metal nanoshell composites should be constructed in a manner to allow a choice of core t:naterial, core dimensions, and core geoinetry in.dependent of those criteria for the shell material. Compositions produced by these methods should have i-elatively homogeneous structures and should not have to rely on suspen.sion in a particular medium in order to exhibit their desired absoiption characteristics. Moreover, materials and nlethods are needed that are not limited in the radial di.m.ensions of the shell layer by the bulk dielectric properties of the metal selected, and are not limited in size to much smaller than a wavelength of light. Materials impregnated with these compositions could be used :in, such diverse applications as optical switching devices, optical communication systems, infrared detectors, infrared cloaking devices, passive solar radiation collection or deflecting devices and the like.

BRIEF SUMMARY OF TH.E INVENTION

The present invention relates to compositions and methods for synthesizing unique composite particles having homogeneous structures and defined wavelengtll absorbance maxima. The present compositions consist o E' a nonconducting inner layer that is surrounded by a layer made of a conductii-ig material. Also contemplated are unique m.ethods for niaking the present compositions such that the resulting compositions can be tuned to absorb electromagnetic radiation maxir.nally at wavelengths in the visible or infrared regions of the electromagnetic spectrum.

Particularly, the metal nanoshells of the present invention are not restricted to a single core or single shell material; permutations of materials are made possible by the novel nzethodology disclosed here for the first time to make such nietal nanoshells.
There is no requirement to use the metal nanoshells of the present invention in any given medium in order for them to exhibit their absorptive qualities; in.1'act, it is anticipated that such metal nanoshells niay find particular utility as surface treatments and coatings totally absent any surrounding medium. Because, the core and sllell material is selected independently, any nuniber of such permutations is made possible. The particles of the invention are also relatively uni form in size and shape by virtue of the nlethods of the invention used to construct them. Most importantly, while the metal nanoshells of the present invention may be much smaller than a wavelength of light, they are not limited in the thickness of their metal shells to account for the bulk dielectric properties of the metal comprising the shell. In fact, due to the one-atom-or-molecule-at-a-time approach to building the metal shell disclosed by the present inventors, the thickness of the inetal shell may be controlled from as low as atomic thicknesses.

The spectral location of the maxinium of the plasm.on resonance peak for this geoinetry depends sensitively upon the ratio of the core radius to shell thickness, as well as the dielectric functions of the core and shell. The presence of a dielectric core shifts the plasnlon resonance to longer wavelengths relative to a solid nanoparti.cle made exclusively of the metallic shell material. For a given core radius, a thin shell wil.l have a plasm.on peak that is shiited to longer wavelengths relative to a thicker shell. It is to be emphasized that metal nanoshells possess all of the same technologically viable optical properties as solid nzetal nanoparticles in addition to this extremely important aspect of resonance tunability.

This invention relates in certain regards to a general method for the production of nanoshell composites. In particular, the choice of the core material and geometry can. be determined independently of the shell material. Similarly, the choice of the shell material and shell thickness is independent of the desired core material. It is also important to note that the coating methods and materials described herein will allow for the fabrication of other unique geometries with potentially unique properties; the utility of this method extends far beyond the fabrication of spherical nanoshells. For exainple, coated cubes or pyramids or cyliuiders, planar surfaces, or struc.tures patterned onto or etched into a planar surface, to name a few, can be easily fabricated using the same methods detailed herein.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1: Calculated optical resonances of metal nanoshells (silica core with (yold shell in water) over a range of core radius/shell thickness ratios.

Figure 2: Calculation of optical resonance wavelength versus core radius/shell thickness ratio for metal nanoshells (silica core with gold shell in water).

Figure 3: Growtli of gold shell on 120 nm diameter silica core particles. The lower spectral curves follow the evolution of the optical absorption as coalescence of the gold layer progresses. Corresponding theoretical peaks are plotted with dotted lines. From. right to left these spectra correspond to theoretical sliell thicknesses of 14, 17, 24 and 30 nnl.
Experimental shell thicknesses determined by TEM were 20, 26, 29, 33 nm +/-4 nm.

Figure 4: Growth of gold shell on 340 nm dianieter silica core particles. The dotted lines from right to left correspond to calculations with shell thicknesses of 17, 35, and 48 nm. TEM detennined shell thicknesses were 18, 39, and 53 nm +/-12.

Figure 5: Absorption spectrum of mixture of particles that would absorb or scatter all wavelengths of solar radiation available at the earth's surface.

Figure 6: Absorption spectrum of mixture of particles that would absorb or scatter only the infrared wavelengths of solar radiation available at the earth's surface.
DETAILED DESCRIPTION OF EMBODIMENTS OF INVENTION
The present embodiments have wavelength absorbance maxima in the range of approximately 400 nrn to 20 m. The low wavelength end of th.e range is defined by the natural plasmon resonance of the metal-like conductor in a shell layer. For any given particle, the maximtim absorbance depends upon the ratio of the thickness of the nonconducting layer to the conducting shell layer. Shown in Figure 1 are absoi-ption maxima for particles having core to shell ratios of 60:20, 60:10, 60:7, and 60:5. As shown, these particles have absorbance peaks of approximately 740, 830, 910, and 1010, nanometers respectively.
The specially tailored particles or particle mixtures of tlie invention can be added to polymers during their preparation by methods well known in the art. Suitable polymers include polyethylene, polyvinyl alcohol (PVA), latex, nylon, teflon, acrylic, kevlar, epoxy, glasses and the like. Solubility of nanoparticles into polymers can be facilitated by functionalization of the nanoparticle surfaces with suitable molecules known to those of skill in the art. The resulting coatings and materials can absorb radiation over the wavelength region of the incorporated particles. Embodiments containing these materials can be used in theii-nal management to produce more energy efficient buildings, automobiles and storage chambers creating savings in air conditioning and heating costs. Fullerene and/or polynler thin film chemistry could be used to incorporate the present materials into photovoltaic devices by methods known in that art. This approach extends the spectral response oCsolar cells across the infrared region of the solar emission spectrum, providing more efficient solar cells. Similarly, solar cells or similar devices operated in a photoconductive rather than photovoltaic mode could be used to provide new low-cost, conipact infrared detectors useful for a range of applications, including but iwt limited to environmental emissions testing, medical imaging or night vision surveillance.
The compositions of the present invention are particles that have at least two layers.
At least one layer is immediately adjacent to and surrouilds another layer.
The innermost layer is said to be a core. A layer that surrounds the core is said to be a shell layer. The shell layer is metal-like in that it can conduct electricity and is made of a metal or metal-like niaterial. It is preferred that at least one shell layer readily conduct electricity, however, the invention only requires that one shell layer have a lower dielectric constant than the adjacent inner layer. In sonie embodiments, this metal or metal-like shell layer is the outermost layer.
In other embodiments, the shell layer immediately adjacent the core is not the outer most shell layer. Additional layers, such as a non-conducting layer, a conducting layer, or a sequence of such layers, such as an alternating sequence of non-conducting and conducting layers, may be bound to this sliell layer using the methods described lzerein and using materials and methods known well to those of skill in the relevant art. Thus, for the purposes of this invention the teiin conductor is defined by reference to the adjacent inner layer and includes any material having a lower dielectric constant than its immediately adjacent inner layer.

It is also preferred that the adjacent inner layer to the shell layer be nonconducting.
Specifically contemplated are nonconducting layers inade of dielectric materials and semiconductors. Suitable dielectric materials include but are not limited to silicon dioxide, titanium dioxide, polyinethyl inethacrylate (PMMA), polystyrene, gold sulf:ide and macromolecules such as dendrimers. In certain embodiments of this invention, the nonconducting layer is comprised of a senliconductor material. For example, core particles may be made of CdSe, CdS or GaAs. The material of the nonconducting layer influences the properties of the particle. For example, if the dielectric constant of the shell layer is larger relative to a particle having a core with a given dielectric constant, the absorbance maximunl of the particle will be blue-shifted relative to a particle having a core with a lower dielectric constant. The core may also be a combination or a layered combination of dielectric niatet-ials sucli as those listed above.

One layer of a particle is its core as noted above. In a two layer particle, the core is a nonconducting layer. The preferred core is a monodisperse, spherical particle that is easily synthesized in a wide range of sizes, and has a surface that can be chemically derivatized. It is also preferred that cores be made of dielectric niaterials or semiconductors.

Although in preferred embodiments the core is spherical in shape, the core niay have other shapes such as cubical, cylindrical or hemispherical. Regardless of the geometry of the core, it is preferred that the particles be homogenous in size and shape in preferred embodiments. In other embodiments, mixtures are purposefully constructed wherein there is a controlled size and shape distribution. In sphei-ica1 embodiments, particles have a homogeneous radius that can range froni approximately 1 to 10 nanometers to several n-iicrons depending upon the desired absorbance maximum of the enlbodiment.
For the purposes of this invention, homogeneity exists when over about 99% of the particles do not vary in diameter by more than 100%. Under this definition a particle preparation wherein 991/o of the particles have diameters between about 50 nm to 100 nm would be said to be homogeneous. Specif:ic applications, however, as discussed in the examples, may rely on mixtures of metal nanoshells with different core and shell sizes.

Monodisperse colloidal silica is the preferred nonconducting layer or core material.
These particles can be produced by the base catalyzed reaction of tetraalkoxysilailes, by techniques known well to those of skill in the art. Nearly spherical silica cores having sizes ranging from 10 nm to greater than 4 m with a variation in particle diameter of only a few percent are preferred.

In the present embodinlents, at least one nonconducting layer is surrounded by a layer that is made of a conducting material. Generally, the conducting layer is metallic but it may also be an organic conducting material such as polyacetylene, doped polyanaline and the like.
Suitable metals include the noble and coinage metals bu.t any metal that can conduct electricity is suitable. Metals that are particularly well suited for use in shells include but are not limited to gold, silver, copper, platinum, palladium, lead., iron or the like. Gold and silver are preferred. Alloys or non-homogenous niixtures of such metals may also be used.

The conducting shell layers of the present invention have thicknesses that range from approximately 1 to 100 nm. They inay coat the adjacent inner layer fully and uniforrnly or may partially coat that layer with atomic or molecular clusters. In either embodiment, at least approximately 30% of the adjacent inner layer is coated by the conducting layer.

In certal'n embodiments, the shell layer is liiiked to the dielectric core layer through a linker molecule. Suitable linker molecules include any molecule that is capable of binding both the core and atoms, ions or molecules of the shell. Preferably, linker binding is covalent to both the shell and the inner layer but binding may also be through ionic bonds, lone-pair interactions, hydrogen bonds, Van der Waals interaction or the like. In certain embodiments, the liriher binds existing metallic clusters to the surface of a non-conducting layer. In other embodinients, the linker binds atoms, ions or molecules directly to the surface of a non-conducting layer. Thus, in embodiments that have a core made of CdSe, a suitable linker would be able to bind the CdSe core and molecules in the shell. In preferred embodiments, the silicone dioxide core and gold metallic shell, are linked by aminopropyltriethoxy silane ("APTES").

The present invention also contemplates unique chemical methods for producing the disclosed cornpositions in solut.ion. Generally, assenlbly occurs by way of the following steps. First, core particles are grown or other-wise obtained. Next, a linker molecule is bound to the core. Then, clusters of molecules that comprise the conducting shell layer are reacted with a I:ree reactive end on the linker molecules. These clusters nlay complete the shell layer or forni nucleation sites for the growth of a conlplete shell layer around the core.

The conditions under which each of the synthetic reactions is carried out determines the overall size and makeup of the particle. For example, in the synthesis of metal shells, reactants include certain concentrations of metal and reducing equivalents that can be altered along with reaction times to vary the shell thiclcness and morphology. With certain shell materials, the progress of this reaction can be followed spectrophotometrically due to the distinct absorption peaks of the particles in the visible and infrared regions of the electromagnetic spectrum.

One method for obtaining core particles is by the synthetic method described in Example 11, which can be used to synthesize particles of silicon dioxide.
Alternatively, in some cases suitable core particles may be purchased. For example, silicone dioxide core particles such as LUDOXT'" TM-50 colloidal silica are available from Aldrich Chemical Co., Milwaukee, Wisconsin.

One uniyue aspect of the present niethod is the attachment of conducting materials of the shell to the nonconducting inner layer. In the methods of the invention, this step is carried out in. solution. In this method, linker molecules that are capable of chemically linking the conducting layer to the core are first bound to the core. One method of attachment, described in Exaniple 111, is for the reaction of APTES with silicon dioxide particles.
Other suitable linker molecules include but are not limited to mercaptopropyltrimethoxy silane, 4-amin.obutyldimethoxysilane, and the like. One of skill in the art will readily appreciate that the suitability of a linker nlolecule depends upon the particular embodinieiit including the composition of the core and of the conducting shell that will eventually sun=ound the core.
With this knowledge, one of slcill can identify suitable linkers and bind them to core particles or nonconducting inner layers and then react suitable conducting molecular clusters, ions, or atoms of a suitable conducting material to tliem.

As one of skill in the art can readily appreciate, suitable solvents for linker molecule attachment depend upon the reactants and a variety of solvents niay work under a given set of conditions. In Example III, the solvent of choice for the attachment of APTES
to silicon dioxide is anhydrous ethanol. Generally, where linker-s are attached in condensation reactions, the preferred solvents are anhydrous because such solvents tend to drive the reactions to produce more of the desired final. reacted product. One of skill in the art would be able to select a suitable solvent based on chemical methodologies well known in the chemical arts.

Once the linker 7nolecules are bound to the core, a free reactive moiety on the linker is reacted with clusters of molecules, ions or atoms to produce all or part of a conducting shell.
In certain embodiments, the clusters are metal atoms. When the clusters are metallic, one suitable attachment method is disclosed in Example IV. Metal clusters, ions or atoms that are linked to the core particle through a linker nlolecule are said to be "tethered." In certain embodiments the tethered metal atoms or clusters serve as nucleation sites for the deposition of additional metal from solution. hl other embodiments, the attaclunent of metal clusters completes the synthesis. Methods for the growth of a complete inetallic shell on tethered metal clusters are disclosed in. Exanlple V. Generally, metal is deposited onto the tethered clusters and enlarges the clusters until a coherent metal shell o:fthe desired thickness is formed.
In the method of Example V, the metal can be deposited tlirough reduction process of solution metal onto the tethered clusters. Alternatively, meta] can be deposited on the tethered metal clusters by a "colloid-based" deposition process. The deposition can also be initiated or driven photochemically. The technique of'depositing metal onto metal nucleation sites tethered to nonconducting core materials in solution is one of the novel features of the present niethods.

In certain preferred embodiments, the metallic shell is the terminal layer.
However, attachment of molecules or additional layers can change the physical properties of the particle. A chemical or charge-transfer interaction between the metallic shell and an additional layer, or just the local embedding medium, influences the optical absorption of the particles, as discussed by Kreibig et al.

In addition, the near field of the metallic shel_l can aff:ect the properties of molecules adsorbed on the surface of the nanoparticles. This could be of use in chemical sensing applications. In other embodiments, a non-conducting layer suiz-ounding the metallic layer can provide a steric bar-rier that is useful when processing or organizing the particles into a particular arrangement. Chemical functionalization of the metal surf:ace is also useful for transferring the metal nanoshel.ls between different solvents, as discussed by Sarathy et al., incorporated herein by reference to the extent it provides such methods.
Chemical functionalization may also assist or enable the fonnation of arrays oa=
crystals of these particles, which will possess additional unique optical properties relating to the periodicity of the ar7-ay or crystal structure, in similarity with photonic band gap crystals and arrays.

By varying the conditions of the metal deposition reaction, the ratio of the thickness of the metal. shell to the nonconducting inner layer can be varied in a predictable and controlled way. Particles can be constructed with metallic sheli layer to core layer radius with ratios from 10 to 10-3. This large ratio range coupled with control over the core size results in a particle that has a large, frequency-agile absorbance over most of the visible and infrared regions of the spectrum. A theoretical calculation of tlie plasmon resonance as a function of core/shell ratio is shown in Figure 2.

There are many possible applications of nietal-coated nanoparticles that could utilize the tunability of the plasnlon resonance. Nanoshells could be nlade to absorb or scatter light at specific wavelengths in the visible or infrared range. Such conipositions would be ideal for use in a wide range of niaterials including energy efficient paints, windows, coatings, or fabrics that could be used on. or in vehicles and building structures. These compositions could be suspended as an active agent in inks, for cryptographic marking purposes.
These materials would also be particularly well suited for use in air heating units or in solar collector materials. Such a solar absorber could also be used as a shield or screen that absorbs or scatters incident solar radiation, keeping the structure cooler than i f' it were directly exposed to the solar radiation.

Such materials could be useful in niany other applications to efficiently "manage" the radiation f:rom any thermal source. For exanlple, these compositions could be adsorbed onto or embedded into materials, thin filnis, coatings, or fabrics that convert radiation directly into heat (passive solar energy harvesting), or into devices or device components, that convert radiation into electricity via photovoltaic or photoconductive effects, or that convert radiation into chemical energy (fuel cells). Mixtures of'these compositions could be made to absorb or scatter solar energy across the entire solar radiation spectrum.

These nanoparticles could be used to sensitize existing photovoltaic, photoconductive, or bolometric devices for enhanced photoresponse and efficiency, and could be used as the functional basis for new device desigils. The strong infrared photoresponse of these compositions may be useful for sensitizat.ion of many different types of semiconductor or polymer surfaces or films for other applications.

For example, the selective infrared absorption inay be useful for laser eye protection, or eye protection from other potentially daniaging soarces of infrared radiation. The enhanced optical. field in the vicinity (1-20 nrn) of a nanoparticle may facilitate photochemistry or photoelectrocliemistry, either on a nanoparticle surface, on a substrate upon whicli a nanoparticle is attaclied, or an electrode upon which the nanoparticle is attached or embedded. Structures containing such compositions could be used in photoconductive applications such as in infrared detectors. Infrared detectors utilizing the properties of these compositions could be used in a wide range of applications such as detecting emissions in environmental monitoring, optical telecommunications networks, wavelength selective, mid-infrared detectors for medical imaging, night vision surveillance equipment or infrared telescopes.

Compositions constructed with different resonant frequencies could be selectively manipulated, levitated, or "sifted" using the wavelength dependent dipole force of a laser beam or beanls. Additionally, nletal nanosliells can be made that possess unique electronic properties that could be useful in specific electronic device applications.
The fabi-ication of homogeneous niet.al shells comprised of sevei-a1 hundred or a few tliousand atoms covering dielectric cores as small as I nm would have well-defined electronic energy levels, similar to niolecules, whose energy level spacings are controllably defined by the nanoshell geometry as described by Puska and Neiminen.

In other words, the energy eigenstates of very small dianleter metal nanoshelis are defined not only by the shell thickness, but by the diameter of the inner core as well. For small core diameters, both the optical and electronic properties are unique to the ultra small core/shell structure. Such metal nanoshells might find application in nanoscale devices, such as single electron transistors or coulomb blockade devices that rely on having well defined electronic energy level spacings. They may also provide useful electronic or electrical properties as components of larger devices. In addition, there could be higher energy optical resonances of nietal nanoshells that lie in the vacuum ultraviolet or x-ray region of the electromagnetic spectrum, a property that could be applied. to the f:abrication of x-ray absorbers or detectors.

The enhanced polarizability at the plasmon resonance of these compositions could be used in chemical sensing or cheniical analysis applications, where in_fornlation concerning the properties of molecules adsorbed onto the nanoparticle surface is obtained.
Such compositions may pennit the use of surface enhanced ranlan scattering (S.ERS) to be performed upon adsorbate or adjacent molecules using laser wavelengths in the near-inf:rared or infrared region. of the spectrum. For conipositions prepared where the shell is incomplete, second-order nonlinear optical effects may be enhanced when such oriented compositions are adsorbed onto a surface or embedded into an appropriate medium.

The following examples are offered by way of illustration and are not intended to lilnit the invention in any manner. In the examples all temperatures are in degrees Celsius and all percentages are by wei~ht for solids and volume if for liquids unless otherwise noted.
EXAMPLE 1. General Method For Metal Nanoshell Colloid Based Synthesis Versatile methods for the synthesis of nanoparticle cores and metal shells of defined sizes have been developed and are described below. Generally, the nlethod comprised the following steps:

(1) first a dielectric or semiconductor nanoparticle core was obtained and dispersed in solution;

(2) then 1-2 nm metal-like "seed" colloids were attached to the surface of the nanoparticle core via molecular linkages, covering the core surface with a discontinuous metal colloid layer;

(3) finally, additional meta.l was deposited onto the nietal-like adsorbates by a soh.ition phase chemical reduction reaction.

This nanoparticle assembly method was carried out with silica nanopai-ticles and gold colloid. Both conimercially available silica nanoparticles and silica nanoparticles grown in .sztu were used successfiilly. The organosilane linkage molecule 4-aminopropyltriethoxy silane was absorbed onto the nanoparticle core. Gold colloid was then introduced into a soltition containinb the core particles. The gold colloidal nanoparticles bound to the organosilane linker molecules and covered the silica cores with a discontinuous layer of metal clusters. Subseduently, gold metal atoms were deposited onto the tetllered nietal clusters by reduction from solution.

EXAMPLE II. Core Particle Synthesis Initially a core material for the nanoparticle was prepared. This material had a spherical shape, and was approximately uniforni in size. The silica particles produced in the following procedure had a standard deviation of less than 10% (4% is routinely achievable).
The niethod of Stobei- (1968), was used to produce monodisperse silicon dioxide particle cores. Tetraethyl orthosilicate (TEOS) 99.999% was obtained from Aldrich Chemical Co., sodium hydroxide was from Fluka Chemical Co. and highly purified water was obtained from a Millipore "TOTALQ"TM system that included "1VIILLIQiJV"TM
and "MILLIQ""T"' filters. All glassware was cleaned with chromic acid solution and thoroughly rinsed with "TOTALQ"TM water.

Variations in water, base concentration, and TEOS concentration were used to produce monodisperse silica spheres of various sizes. Temperature and electrolyte concentration also affected the final diameter of the particles. Generally, the following concentration ranges were used: 0.1 to 0.5 M TEOS, 0.5 to 17 M H20, and 0.5 to 3.0 M
ammonia. In addition, a variety of alcohols were used as solvents, however, ethanol was preferred. Higher aimnonia coticentrations provide larger particles.

Uniforni particles having a diameter of 120 nm as measured by a transmission electron microscope (TEM) were prepared by the following method. Approximately inill.iliters (ml) of dry (100%) ethanol and 4 nil of NH4OH (25% NH3 in water), were stirred in a glass beaker. To this solution, 2.2 ml. of tetraethyl orthosilicate liaving a purity of at least 99.999% was added and allowed to stir for at least 8 hours. By varying the concentrations of NH4OH, water and silicate among other factors, the size of the silica particle was varied fi-orn approximately 20 nm to 500 nm diameter. Larger core particles were grown using a seeded growth tecllnique where additional TEOS and water were added to already formed silica particles. Multiple additions ol'sniall amoimts of additional reactants allowed m.onodisperse core particles to be grown as large as 4 microns.

EXAMPLE 111. Linker Molecule Attachment To asseinble a metallic shell around an inner layer frequently required the use of linker molecules. These nlolecules were chemically linked to the inner layer and served to bind atoms, ions, atomic or molecular clusters of the conducting shell to the inner layer. The conducting shell atonis that bound to the linkers were used as nucleation sites for reduction of the additional atoms or molecules to complete the shell. One method used to attach gold particles to silicon dioxide was to treat the particles with aminopropyltriethoxy silane (APTES). The silanol end groups of the A.PTES molecules attach covalently to the silica core extending their amine groups outward as a new termination of the particle surface.

In this nlethod, 1.0 ml. of a silica particle suspension such as prepared in Example 11, was added to a 50 ml glass beaker. Next, pure aminopropyltriethoxy silane (APTES) was added to the solution. Based on estimates, enough silane was added to coat the particles with multiple layers of silane. For example, 40 microliters of uildiluted APTES was used for particles having diameters of 120 nni. The solution was stirred for 2 hours, diluted to 200 mis anci then heated to a boil for four hours. The heating step promotes the reaction of silanol groups into Si-O-Si bonds and strengthens the attachment of the silane to the silica. This mixture was centriftiged at 2000 x g for 30 minutes. The supernatant was decanted off and the pellet was redispersed ultrasonically. This washing procedure was repeated five times.
Many linker molecules other than aminopropyl triethoxy silane are suitable for use in this procedure. Foi- example, anlinopropyl triinethoxy silane, dianiinopropyl diethoxy silane, or 4-aminobutyl dinlethyln-iethoxysilane and the like can be used. In addition, the surface can be tenninated with a linker that allows for the direct reduction of metal atoms on the surface rather than through a metallic cluster intermediary. In other einbodiments, reaction of tetrahydrothiophene (AuCI) with a silica core coated with diphenyltriethoxy silane leaves a surface terminated with gold chloride ions which can pi-ovide sites for additional gold reduction. In other embodiments, a thin shell of another nonmetallic material, such as CdS or CdSe grown on the exterior of a silica particle allows for a metallic shell to be reduced directly onto the nanoparticle's surface. In other embodiments, functionalized oligomers of conducting polymers can be attached in solution to the functionalized or nonfunctionalized surface of the core nanoparticle and subsequently cross-linked by theimal or photo-induced chemical methods.

EXAMPLE IV. Attachment Of Metal Clusters Metal clusters were attached to the linker molecules on the core by im:mersirzg the derivatized core particles in a metal colloid bath. Any metal that can be made in colloidal form could be attached as a metal cluster. For exatnple, silver, platinum, palladium, lead and the like could be used. In addition, metal-like organic niolecules are suitable. Such compounds include polyacetylene and pol.yaniline. Gold clusters having a diameter of 1-3 nm were grown using the reduction reaction as described by Duff. A solution of 45 ml of water, 300 microliters of I M NaOH and 1 mL of a freshly diluted 1% aqueous solution of tetrakis (hydroxyznethyl) phosphonium chloride (THPC) was stirred in a 100 ml flat bottom beaker with a pyrex coated magnetic stir bar. After 2.minutes, 2 ml of chloroauric acid (25 mM darlc-aged stock solution, hydrogen tetrachloroaurate (III) trihydrate 99.999% from Aldrich) was added. This i-eaction mix was used to form gold pai-ticles in solution with an. average particle diameter of 1-2 nm. To increase the size of the particles higher concentrations of gold chloride could be used. Particles prepared in this fashion were refen-ed to as ultra small gold particles or (UG).

Generally, the UG solution was mixed with silica particles in an amount that would theoretically cover the core particle surface five to ten times. The solution was allowed to react for 3 hours under gentle stirring. In the preferred embodiment the gold was used 5-30 days after it was nlade.

Typically, after tht-ee hours, unreacted gold colloid was separated from the gold-decorated silica particles by centril'ugation at 1000 RCF. The minimum amount of centritugal force required to effect separation was used to avoid coalescence of the particles. Particles were washed twice by resuspension and centri fugation..

The inventors made the surprising discovery that the gold decorated particles did not aggregate after being centi-ifuged and redispersed in the absence of additional stabilizing compounds. This discovery allowed the convenient separation of the decorated silica from colloidal gold, leaving the gold attached to silica in a chemically reactive state. Various protectants could be added before centrifugation to facilitate later resuspension of the particles. These protectants include polyvinyl alcohol, polyethylene glycol or phosphine ligands, and thiol-terininated carboxylic acid linkages. Resuspension was easily acconlplished when a minimum anlount of force was used in the centrifugation step and any aggregates of particles could be redispersed by treatment with sonification. A
dynamic light scattering instniment was used according to standard and well known methods to veri f:y that the particles were dispersed. The dispersed particles were diluted to 10 mis and used as a stock solution for the growth of the complete metal shell.
EXAMPLF, V. Growth of the Shell The metal clusters were enlarged by deposition of gold using a variety of reductants such. as hydroxylamine hydroclioloride, sodium borohydride, and formaldehyde.
Formaldehyde was preferred. A solution of 25 nlg anhydrous potassium carbonate was added to 100 ml of water containing 1.5 nil of 25 mM chloroauric acid solution (PCG). This solution was allowed to age in the dark for one day. Approximately 10 ml +/-5 inl of PCG
was rapidly stirred with 2-5 mis of the gold clust.ered silica solution. A 100 microliter aliquot of freshly prepared formaldehyde solution (2% by volume in water) was slowly added.
Before enlargement of the metal clusters, the metal clusters attached to the particles had the same UV-visible absorption spectrum as their natural colloidal fornn.
As additional metal was deposited onto the chisters, the absoi-bance maximum of the pai-ticle shifted to longer wavelengths, as shown in the lower curves of Figures 3 and 4. When the gold shell was complete. the particles' absorbance nlaxinlunl was related to its geometry, specifically, to the ratio of the thickness of the inner nonconducting layer to the thickness of the outer conducting layer. As the conducting layer grew thicker, the absorbance maximurn of the particle shifted to shorter wavelengths, as shown in the upper curves of Figures 3 and 4. The progress of this reaction was followed spectrophotometricall y and terminated when the desired wavelength for the absorbance maximum was obtained. Typically a color change occuiTed within 10 minutes. For 110 nm diameter core particles, typically a visible color change is apparent, from faint brown to purple, blue, green, or yellow. Some of the other factors that influenced the optical absorptioii of the spectrum are the size of the core, the roughness of the shell, the shape of the core, additional reactants in solution that may be incorporated into the core during the reduction, the continuity of the shell, and the degree of aggregation of the particles.

Many different methods can be used to complete the metal shell once the nucleation sites are in place. One of skill in the art will realize that any method that can be used to develop a metal colloid into a larger metal colloid should be successful for the shell growth.
For example, silver solutions such as the commercially available LI silver from Nanoprobes, Inc. may work. In addition, it is not necessary that the tethered seed particle be of the same material as the shell material. l:n one embodiment silver nitrate is reduced onto silica coated with UG. This is done in a basic solution with formaldehyde as a reductant and results in a silver shell. Photo-induced deposition of the rnetal shell onto the prepared nanoparticle surface is also possible.

Direct redliction of silver onto a non-conducting core can be accomplished with the reduction of silver directly onto a CdS semiconductor layer. In order to construct a CdS with a diameter greater than 20 nm it was necessary to first grow a CdS layer onto a silica core.
This can be acconlplished using water in oil microemulsions, f:or example. In one embodiment silver was reduced onto a silica/CdS particle by adding the particles to a solution of AgNO3 and NH4 and then slowly adding a NH3OHCI solution to develop the shell.

EXAMPLE VI. Nanoengineered Thermal Management Materials and Coatings The present application takes adva.ntage of the fact that the sun's maximum radiant power that reaches the Ear=th's surface is distributed broadly across the visible and infi-ared regions of the electromagnetic spectrum and a mixture of nanoparticles can be developed to either absorb or scatter energy througllout that entire spectruin. The present technology is the only method known for systeniatic control of absorption or scattering of radiation across the entire range of the solar etnission spectrum. In Figure 5 is shown an example of a mixture of the present embodiments that can absorb the entire solar spectrum. In this Figure is shown the individual spectra of eight particles having varied core to shell ratios and that absor=b distinct segments of the solar radiation spectrum. A mixture of these partictes is capable of absorbing radiation across the entire solar spectrum.

Such a mixture can be incorporated into polymers, glasses, paints, epoxies, or other coating matrices by standard nlethods well known in the art. The thermal properties of these materials can then be used in appropriate applications that rely ori absorption and scattering of solar energy or any source of electromagnetic radiation across the wavelength range of tlle nlixture.

EXAMPLE VII. Nanoshell Windows In Figure 6 is shown ari example of ainixture of the present enlbodiments that can absorb the entire infrared region of tlie solar spectrum while allowing visible light from the sun to pass through. In this Figure is shown the individual and combined spectra of three distinct particles having varied core to shell ratios in a particle mixture and that absorb distinct segnlents of the solar radiation spectruni. These particles may be a subset of the mixture of Example VI.
Such a mixture can be incorporated into polymers, glasses, paints, epoxies, or other coating matrices by standard nlethods well known in the art. The thernial properties of these materials can then be used in appropriate applications that rely on trapping solar energy while at the sanle time allowing visible wavelengths to pass. Such materials could be incorporated into sunroofs and windows that would trap sonle fraction of incident radiation from a solar or nonsolar source while allowing a clear field of vision. Similarly incorporation of these particles into heat i-esistant paint would allow for the production of colored paints that have the ability to absorb or to scatter most of the sun's heat energy.
Siniilarly, it is possible to engineer "transmission windows" through. which selected wavelengths of electromagnetic radiation can pass and incorporate those particles into similar materials. It is also possible to engiiieer nzaterials that will strongly absorb oi- scatter ovei- a specific wavelengtll range corresponding to the operating range of a laser or lasers, and be either translucent or transparent below and above the specific wavelength range. This type of material could be used for eye protection in the presence of a laser source or sources.
EXAMPLE VIII. Nanoengineered Solar Cells Current solar energy cells only convert a small fraction of the sun's radiant energy into electrical. energy. Clearly, converting absorbed radiation from across the entire solar specti-um into electricity would achieve higher solar cell efficiencies than is currently available. The present embodiment relies on the electron aecepting capability of thin solid films of buckniinisterfullerene or C60, a spherical carbon cage molecule. The photovoltaic effect of C60 based thin film, which is enhanced by forming a heterojunction with a conducting polymer film, such as PPV, is sensitized to an enhanced photoresponse at longer wavelengths by introducing a layer of suitably designed metal nanoshells between the C60 and the PPV. A
dilute or dense layer of metal nanoshells is embedded at the interface between the conducting polyiner and the C60. The electrostatic field created by the heterojunction establishes an assymetric potential across the metal nanoshell layer. The strong electron donor-acceptor interaction between the metal-like conductor of the shell layer and the C60 will cause the metal-like conductor of ttle shells to inject electrons into the C60 layer when the shell layers are illuminated with radiation corresponding to their absorption resonance.
The released electrons are detected as either a photocurrent or photovoltage by methods known in the art.
Such a device could be made using spun polyiner films, vacuum deposition, or other rnethods known to one skilled in the art. Any pair of electron donor-acceptor materials that could be sequentially deposited into such a layer stnicture could be used with metal nanoshells as photosensitive devices. This device may also consist of more than one layer of electron donor/tnetal nanoshell/electron acceptor layers.

It will be readily apparent to one skilled in the art that various substitutions and inodif'lcations may be made to the invention disclosed herein without departing from the scope and spirit of the invention. For example, the nanopal-ticle core used not be spherical and nlay retain other shapes such as planar sheets. Similarly, the invention may be employed to place metal nanoshells on planar sheets and other surfaces.

EXAMPLE IX. Optically Active Materials with Metal Nanoshell Dopants The adjustable plasmon resonance of metal nanoshells can be used as dopants or additives to influence and modify the optical properties of optically active host materials, such as, but not limited to, coriducting polymers or organic semiconductors.
The nanoshell resonance can be designed to overlap with either the singlet or the triplet exciton resonances of the host material. Since metal nanoparticle resonances have excitation lifetimes of only a few picoseconds, any donor-acceptor interaction between the coinparatively long-lived excitons of the conducting polymer and the nanoparticles will result in strong quenching of the specific excitation or excitations of the host nlaterial to which the nanoshell resonance has been tuned. This provides a method for strongly modifying the optical properties of a nzaterial with little rnodification of its cllemical properties.

EXAMPLE X. Optically active species inside Metal Nanoshell nanocavities A straightf:orward variation of the fabrication niethod outlined above enables the insertion of optical absorbers and 1uminescent species into the dielectric cores of metal nanoshells. Procedures for bonding functionalized dye molecules into silica nanoparticles grown by the Stober method have been demonstrated by van Blaaderen and Vrij, incorporated herein by reference to the extent it discloses such methods.
Introducing an optically absorbing species into the core will strongly influence the plasnion resonance sliift and width. Conversely, the optical field enhancement within the metal "nanocavity" niodifies the absorption aild fluorescent properties of the chromophores incorporated within this nanostructure.
EXAMPLE XI. Metal Nanoshell Structures via planar fabrication methods There also exists the possibility of' fabricating spherical, henlisplierical, or other types of geometries directly froni planar semiconductor or dielectric surfaces. For example, on silicon substrates specific chemical etching techniques result in well-defined nanoscale structures extending up from the surface of the substrate. Using the unique methods described herein, such nanostructures can be metal-coated, resulting in devices or regions of devices with unique properties. For example, an array of nanometer-sized hemispheres etched onto a silicon substrate could be coated with a thin layer of gold. Such a substrate would have the same optical tunability as nanoshells, that is, the optical absoiption of the substrate would have a resonance defined by the size of the hemisphei-es and the thickness of the metal layer.
Such substrates would be useful as SERS-based chemical sensors. Similarly, nanoshells attached or directly fonned onto small cantilevers, such as those used in scanning force microscopy, could be used as small localized probes for the wavelength-specific detection of radiation: in this embodiment, the deflection or the vibrational frequency of the cantilever may be monitored with either response being proportional to the absorbed energy content of the nanoshell or nanoshells attached or fabricated onto the cantilever structure.
Such microbolometers would find applications in a variety of diagnostic environments requiring localized. optical detection, or optical detection by a very small (micro-sized) structure.
Further to the description set out in the foregoing examples, various descriptions of the preferred embodiments are as follows: a nanoparticle comprising at least one non-conducting core layer and at least one conducting shell layer, the shell layer being immediately adjacent to and independently layered upon the core layer, wherein radial thickness of the shell layer is less than that whose properties are described by a bulk dielectric function of the material comprising the conducting shell layer. The conducting shell layer preferably comprises a metal. The metal is preferably selected from the group consisting of the coinage metals, noble metals, transition metals, and synthetic metals such as polyacetylene and polyanaline. The conducting shell layer preferably comprises a metal-like material. The conducting shell layer preferably comprises a metal alloy. There is preferably an innermost conducting shell layer immediately adjacent to the non-conducting core layer, and at least one additional conducting shell layer. The non-conducting core layer preferably comprises a dielectric material, which is selected from the group consisting of silicon dioxide, titanium dioxide, PMMA, polystyrene, and dendrimers. The non-conducting core layer preferably comprises a semi-conducting material. The non-conducting core layer more preferably comprises an organic molecule or an organic supermolecular structure. The non-conducting core layer can alternatively comprise a mixture of non-conducting materials, an optically absorbing material, or a fluorescent material. There can be an outermost non-conducting core layer bound to the conducting shell layer, and at least one additional non-conducting core layer. There can be a non-conducting layer sandwiched between two conducting layers. The nanoparticle can also comprise a conducting layer sandwiched between two non-conducting layers. The diameter of the particles preferably varies by less than 100% of the diameter of the smallest particles. The conducting shell layer coats at least 10% of the non-conducting core layer. The conducting shell layer preferably coats at least 10% but less than 100% of the non-conducting core layer and is a component in a second-order, nonlinear optical material. The conducting shell layer can alternatively completely surround the non-conducting core layer. The conducting shell layer is preferably linked to the non-conducting core layer by a linker molecule. The particle is approximately preferably spherical, but can be non-spherical.
The present nanoparticles can be used as or in: x-ray absorbing devices, light detecting devices, chemical sensing device, bolometers, microbolometers, sensing cantilever devices, dopants, polymers, photovoltaic solar cells, and infrared detectors.
The present nanoparticles can also be described as particles each comprising a core that is between 1 nm and 2 m in diameter, the core further comprising silicon dioxide, the core being encapsulated by a layer of a metal that is less than approximately 40 nm thick, the metal comprising gold, wherein the metal is linked to the core through a linker molecule, the particle having a wavelength absorbance maximum between 300 nm and 20 m. The present particles absorb electromagnetic radiation and preferably comprise at least a first and a second layer, the second layer surrounding the first layer, the particle having a diameter of greater than approximately 1 nm, wherein the first layer comprises a non-conducting material and the second layer comprises a conducting material. The present nanoparticles can be used to form a mixture that is capable of absorbing electromagnetic radiation, the mixture further comprising particles between approximately 5 run and 2 m thick.
The present invention also includes a method of making a nanoparticle comprising binding a linker molecule to a non-conducting core layer, binding conducting molecules to the linker molecule, and forming a metal conducting shell layer around the core by reducing additional conducting molecules onto the conducting molecules bound to the linker, wherein each of the steps is carried out in solution. A method for metallizing a surface comprises layering the present nanoparticles onto the surface. Another method of metallizing a surface comprises binding linker molecules to a non-conducting core layer, binding clusters of conducting atoms, ions, or molecules to the linker molecules, and reducing additional conducting atoms, ions or molecules onto the clusters.

There is also according to the invention a method of producing particles that have a wavelength absorbance maximum in the range of approximately 300 nm to 20 m comprising the steps of: forming a nonconducting core layer and binding conducting molecules, ions or atoms to the core particle in solution. The step of binding conducting molecules to the core layer can further include the steps of: binding linker molecules to the core layer and binding clusters of conducting atoms, ions, or molecules to the linker molecules wherein each of the steps is carried out in solution. The method can further include the step of reducing conducting atoms, ions or molecules onto the clusters.

The invention also includes a method of optically tuning a mixture of nanoparticles comprising determining the wavelengths of light to be absorbed or to be scattered by the mixture, making at least two of the nanoparticles differing in their ability to absorb or to scatter light, and mixing the nanoparticles to achieve the desired degree of absorption or scattering.

The invention also includes a nanoparticle that absorbs electromagnetic radiation, the particle having a diameter greater than approximately 1 nm and comprising: at least a first and a second layer; the second layer surrounding the first layer; and the first layer comprising a non-conducting material and the second layer comprising a conducting material and having a thickness that is independent of the thickness of the first layer; wherein the plasrnon resonance of the nanoparticle can be varied by varying the thickness of the shell layer.
Another embodiment comprises at least one non-conducting core layer and at least one conducting shell layer, characterized in that: the shell layer is independently layered upon the core layer, such that the thickness of the shell layer is independent of the radius of the core layer; the thickness of the sllell layer is less than that of a shell layer whose dielectric properties are described by a bulk dielectric function of the material comprising the conducting shell layer; and the nanoparticle has a wavelength extinction maximum between 300 nm and 20 m.
Another method of making a nanoparticle comprises: providing at least one non-conducting core layer; independently layering on the core layer at least one conducting shell layer such that the thickness of the shell layer is independent of the radius of the core layer;
wherein the thickness of the shell layer is less than that of a shell layer whose dielectric properties are described by a bulk dielectric function of a material comprising the conducting shell layer. Another embodiment of the present particles comprises: a core that is between 1 nm and 2 m in diameter, the core comprising silicon dioxide; and a layer of a metal encapsulating the core, the layer being less than approximately 40 rim thick, the metal comprising gold, wherein the metal is linked to the core through a linker molecule, the particle having a wavelength absorbance maximum between 300 nm and 20 m.

Another method of making a composite particle comprises: providing a electrically non-conducting core; attaching a plurality of clusters of an electrically conducting material to the core; and converting the plurality of precursors to a shell layer around the core; wherein the shell layer has a controllable thickness. Still another method comprises:
(a) providing a electrically non-conducting core; (b) binding a plurality of linker molecules to the non-conducting core; (c) binding a electrically conducting material to the linker molecules; and (d) forming a shell layer around the core by reducing additional electrically conducting material onto the electrically conducting material already bound to the linker; wherein each of the steps (a)-(d) is carried out in solution and the plasmon resonance of the nanoparticle can be varied by varying the thickness of the shell layer.
A method of metallizing a surface, comprises: (a) binding linker molecules to the surface; (b) binding clusters of conducting atoms, ions, or molecules to the linker molecules;
and (c) reducing additional conducting atoms, ions or molecules onto the clusters to form a shell layer; wherein steps (a), (b) and (c) are carried out in solution and the plasmon resonance of the metallized surface can be varied by varying the thickness of the shell layer.
A method of producing a particle, comprises the steps of (a) forming a non-conducting core layer; and (b) binding conducting molecules, ions or atoms to the core particle in solution; and (c) wherein the thickness of the shell layer is independent of the radius of the core layer; wherein the resulting particle has a wavelength absorbance maximum in the range of approximately 300 nm to 20 m and the wavelength absorbance maximum can be varied by varying the thickness of the shell layer. The step (b) preferably includes the steps of: binding linker molecules to the core layer; and binding clusters of conducting atoms, ions, or molecules to the linker molecules wherein each of the steps is carried out in solution. The step (d) preferably comprises reducing conducting atoms, ions or molecules onto the clusters.

A method of optically tuning a mixture of nanoparticles comprises determining the wavelengths of light to be absorbed or to be scattered by the mixture, making at least two of the nanoparticles made as in claim 1 differing in their ability to absorb or to scatter light, and mixing the nanoparticles to achieve the desired degree of absorption or scattering.
Another embodiment of the present nanoparticle comprises at least one non-conducting core layer; and at least one conducting shell layer, the shell layer independently layered upon the core layer such that the thickness of the shell layer is independent of the radius of the core layer; wherein the thickness of the shell layer is less than that for which the nanoparticle has a plasmon resonance peak width described by a bulk dielectric function of the material comprising the shell layer and the plasmon resonance of the nanoparticle can be varied by varying the thickness of the shell layer.

References Cited:
Meyer H. Bimboim and Arthur K. Noeves, U. S. Patent No. 5,023,139 issued June 11,1991;
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Daniel G. Duff, Alfons Baiker. Ian Gameson, and Peter P. Edwards (1993) Langmuir 9, 2310-2317. A New Hydrosol oCGold Clusters. 2. A Comparison of Some Different Measurement Techniques.

Jovan Nedcljkovic and Ramesh C. Patel (1991) Appl. Phys. Lett. 58,2461-2463.
Observation. of Plasmon-Enlianced Optical Extinction in Siiver-Coated Silver Bromide Nanoparticles.

M. J. Puska and R. M. Nieminen, (1993) Physical Review A 47,1181-1186.
Photoabsorption of atoms inside C60.

Werner Stober, Artlnir Fink, and Ernst Bohn, (1968) J. Colloid. and Interface Science 26,62-69. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range.

H. . S. Zhou, I. Honma, and H. Konliyania, and J. W. Haus, (1994) Phys. Rev., 50,12,052-12,056. Controlled Synthesis and Quantum-Size Effect in Gold-Coated Nanoparticles.

R. D. Averitt, D. Sarkar, and N. J. Halas, (1997) Phys. Rev. Lett. 78,4217-4220. Plasnion Resonance Shifts of Au-Coated Au2S Nanoshells: Insight into Multico.mpon.ent Nanoparticle Growth.

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Claims (63)

What is claimed is:

1. A method of making a composite particle, the method comprising:
(a) providing an electrically non-conducting core;
(b) attaching a plurality of clusters of an electrically conducting material to the core; and (c) converting the plurality of clusters to at least one shell layer around the core; wherein the shell layer has a controllable thickness independent of the radius of the core.

2. The method of claim 1, wherein (b) comprises binding a plurality of linker molecules to the non-conducting core, and binding the electronically conducting material to the linking molecules, wherein the method further comprises reducing additional electrically conducting material onto the electrically conducting material already bound to the linker molecules, wherein the composite particle comprises a nanoparticle having a plasmon resonance, wherein each of (a)-(c) is carried out in solution and the plasmon resonance of the nanoparticle can be varied by varying the thickness of the shell layer.

3. A particle comprising:

at least a first and a second layer;
said second layer surrounding said first layer; and said first layer comprising a non-conducting material and said second layer comprising a conducting material and having a thickness that is independent of the thickness of said first layer;

wherein the plasmon resonance of the particle can be varied by varying the thickness of said second layer independently of the thickness of said first layer.

4. The particle of claim 3, wherein said particle is a nanoparticle, wherein said first layer is a non-conducting core layer; and wherein said second layer comprises at least one conducting shell layer, said shell layer independently layered upon said core layer such that the thickness of said shell layer is independent of the radius of said core layer;

wherein the thickness of said shell layer is less than that for which the nanoparticle has a plasmon resonance peak width described by a bulk dielectric function of the material comprising the shell layer.

5. The nanoparticle of claim 4 wherein the nanoparticle has a wavelength extinction maximum between 300 rim and 20 µm.

6. The method of claim 1 wherein the composite particle comprises an optically tunable particle; the electrically non-conducting core comprises at least one non-conducting core layer; and wherein the thickness of said shell layer is less than that of a shell layer whose dielectric properties are described by a bulk dielectric function of a material comprising said conducting shell layer.

7. The particle of claim 3, wherein said first layer comprises a core that is between 1 nm and 2 µm in diameter, said core comprising silicon dioxide; and said second layer comprises a layer of a metal, said second layer at least partially encapsulating said core, said second layer being less than approximately 40 nm thick, said metal comprising gold, wherein said metal is linked to said core through a linker molecule, said particle having a wavelength absorbance maximum between 300 nm and 20 µm.

8. The method of claim 1 wherein the electrically conducting material comprises conducting molecules, ions or atoms , wherein (b) and (c) are carried out in solution to form a shell layer, and wherein the resulting particle has a wavelength absorbance maximum that can be varied by varying the thickness of the shell layer.

9. The method of claim 8 wherein the wavelength absorbance maximum is in the range of approximately 300 nm to 20 µm.

10. The method of claim 8 wherein (b) further includes binding linker molecules to the core layer and binding clusters of conducting atoms, ions, or molecules to said linker molecules, wherein each of the steps is carried out in solution.

11. The method of claim 10, further comprising reducing conducting atoms, ions or molecules onto said clusters.

12. The method of claim 1, wherein the core layer comprises a surface, wherein (b) comprises binding a plurality of linker molecules to the surface, and binding clusters of conducting atoms, ions, or molecules to said linker molecules, and wherein the method further comprises reducing additional conducting atoms, ions or molecules onto said clusters to form a shell layer, and wherein (a)-(c) are carried out in solution and the plasmon resonance of the metallized surface can be varied by varying the thickness of the shell layer.

13. A method of optically tuning a mixture of nanoparticles comprising:
determining the wavelengths of light to be absorbed or to be scattered by the mixture, making at least two of said nanoparticles made as in claim 1 differing in their ability to absorb or to scatter light, and mixing the nanoparticles to achieve the desired degree of absorption or scattering.

14. The method of claim 1 wherein (a) comprises providing a mixture of electrically non-conducting cores, wherein the mixture has a distribution of core diameters with a standard deviation of up to about 20%.

15. The method of claim 1 wherein the average diameter of the clusters of electrically conducting material is between about 1 nm and about 10 nm.

16. The method of claim 2 wherein the wavelength of the plasmon resonance of the composite particle is between about 300 nm and 20 µm.

17. The method of claim. 2, further comprising tuning the plasmon resonance of the composite particle by spectrophotometrically following the progress of (c) and terminating (c) when a predetermined wavelength for a maximum of the plasmon resonance is achieved.

18. The method of claim 2 wherein (b) comprises adsorbing the linker molecules onto the non-conducting core layer.

19. The method of claim 2 wherein each of the linker molecules comprises an atomic site having an affinity for the electrically conducting material.

20. The method of claim 2 wherein the electrically conducting material comprises gold and (b) comprises covalently binding the electrically conducting material to the linker molecules.

21. The method of claim 1 wherein the core comprises a material selected from the group consisting of dielectric materials, semi-conductors, and combinations thereof.

22. The method of claim 1 wherein the core additionally comprises: a second shell around the core, and wherein at least one of the core and the second shell comprises a metal and the other of the core and the second shell comprises a material selected from the group consisting of dielectric materials and semi-conductors.

23. The method of claim 2 wherein (c) comprises growing the electrically conducting material into the shell layer.

24. The method of claim 1 wherein the electrically conducting material is selected from the group consisting of gold, silver, palladium, platinum, lead, iron, copper, and combinations thereof.

25. The method of claim 1 wherein the electrically conducting material is gold.

26. The method of claim 1 wherein the electrically conducting material is gold and the non-conducting core comprises an oxide.

27. The method of claim 2 wherein the non-conducting core comprises an oxide compound and the linker molecule comprises a silane.

28. The method of claim 27 wherein the silane is selected from the group consisting of aminopropyltriethoxy silane, aminopropyltrimethoxy silane, diaminopropy-diethoxy silane, 4-aminobutyldimethylmethoxy silane, mercaptopropyltrimethoxy silane, and combinations thereof.

29. The method of claim 1 wherein the non-conducting core comprises a dielectric material selected from the group consisting of silicon dioxide, titanium dioxide, polymethyl methacrylate, polystyrene, gold sulfide, dendrimers, and combinations thereof.

30. The method of claim 2 wherein the non-conducting core comprises an oxide compound, the linker molecule comprises a silane, and the electrically conducting material is selected from the group consisting of gold, silver, palladium, platinum, lead, iron, copper, and combinations thereof.

31. The particle of claim 3 wherein said particle absorbs electromagnetic radiation; and said particle has a diameter greater than approximately 1 nm.

32. The nanoparticle of claim 4 wherein said conducting shell layer comprises a metal, an organic conducting material, a metal alloy, or combinations thereof.

33. The nanoparticle of claim 32 wherein said metal is comprised of a metal selected from the group consisting of the coinage metals, noble metals, transition metals, and synthetic metals.

34. The nanoparticle of claim 4 wherein said nanoparticle comprises at least one additional non-conducting core layers in an alternating sequence with additional conducting shell layers.

35. The nanoparticle of claim 4 wherein said non-conducting core layer comprises a dielectric material, a semi-conducting material, an organic molecule, an organic supermolecular structure, or combinations thereof.

36. The nanoparticle of claim 35 wherein said dielectric material is selected from the group consisting of silicon dioxide, titanium dioxide, PMMA, polystyrene, and dendrimers.

37. The nanoparticle of claim 4 wherein said non-conducting core layer comprises a mixture of non-conducting materials.

38. The nanoparticle of claim 4 wherein said non-conducting core layer comprises an optically absorbing material or a fluorescent material.

39. The nanoparticle of claim 4 further comprising an outermost non-conducting core layer bound to an adjacent conducting shell layer.

40. The nanoparticle of claim 4, further comprising a non-conducting layer sandwiched between two conducting layers.

41. The nanoparticle of claim 4, further comprising a conducting layer sandwiched between two non-conducting layers.

42. The nanoparticle of claim 4 wherein said conducting shell layer partially coats said non-conducting core layer.

43. The nanoparticle of claim 4 wherein said conducting shell layer completely surrounds said non-conducting core layer.

44. The nanoparticle of claim 4 wherein said conducting shell layer is linked to said non-conducting core layer by a linker molecule.

45. The nanoparticle of claim 4 wherein said nanoparticle is substantially spherical.

46. The nanoparticle of claim 4 wherein said nanoparticle is non-spherical.

47. An x-ray absorbing device comprising the nanoparticle of claim 4.

48. A light detecting device comprising the nanoparticle of claim 4.

49. A chemical sensing device comprising the nanoparticle of claim 4.

50. A bolometer comprising the nanoparticle of claim 4.

51. A microbolometer comprising the nanoparticle of claim 4.

52. A sensing cantilever device comprising the nanoparticle of claim 4.

53. A dopant in an optically active material comprising the nanoparticle of claim 4.

54. A polymer comprising the nanoparticle of claim 4.

55. A photovoltaic solar cell comprising the nanoparticle of claim 4.

56. An infrared detector comprising the nanoparticle of claim 4.

57. A mixture of nanoparticles as in claim 4, said mixture capable of absorbing electromagnetic radiation, said mixture further comprising nanoparticles between approximately 5 nm and 2 µm thick.

58. The particle of claim 7, wherein said linker molecule comprises aminopropyltriethoxy silane, aminopropyltrimethoxy silane, diaminopropyl diethoxy silane, 4-aminobutyl dimethylmethoxysilane, diaminopropyl diethoxy silane, diphenyltriethoxy silane reacted with tetrahydrothiophene(AuCl), or combinations thereof.

59. The particle of claim 7, wherein said linker molecule further comprises a linker upon which direct reduction of atoms of said metal may be conducted.

60. The particle of claim 7, wherein said linker molecule further comprises a second non-metallic material.

61. The particle of claim 60, wherein said second non-metallic material further comprises either CdS or CdSe.

62. The particle of any of claim 7, wherein said linker molecule is crosslinked to another linker molecule.

63. The particle of claim 7, wherein said wavelength absorbance maximum is between 900 nm and 5 µm.