ALLOYS OF NOBLE METALS WITH AUGMENTED QUALITY FACTORS OF SURFACE PLASMONS

Abstract

A set of improved substrates for plasmonic applications characterized by specific characteristics of surface plasmons, such as resonance frequencies, manifesting, notably, as a change of perceived color of said alloys, as well as alteration of surface plasmon energies and generation efficiency compared to the constituent elements. The disclosed compositions include alloys of gold, platinum, and palladium with other chemical elements, especially alkali metals (such as potassium, and rubidium), alkali earth metals (such as magnesium and barium) as well as transition metals (such as tin and zirconium).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 62/624,097 filed on Jan. 30, 2018 to which priority is claimed under 35 U.S.C. 119 and the contents of which are hereby expressly incorporated by reference herein.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention relates to alloys of precious metals such as gold, platinum, and palladium with other chemical elements.

BACKGROUND

[0003] Phenomena associated with energy and information transfer as well as transient or permanent chemical transformations that occur at the interface between materials with different electromagnetic properties are commonly referred to as plasmonic phenomena. These are produced as a result of generation of coherent delocalized electron oscillations generally known as surface plasmons. Such surface plasmons may be further differentiated as surface plasmon polaritons, and localized surface plasmons; however, for the purposes of the disclosed invention the difference is insubstantial and both surface plasmon polaritons and localized surface plasmons are further referred to generically as surface plasmons.

[0004] Surface plasmon-related phenomena are utilized in a number of industrial and research applications as diverse as high speed optical switching, formation of optical circuits, optical cloaking, superlensing, surface-enhanced spectroscopy, solar light capture, direct transformation of solar light energy into electric energy, photocatalytic chemical processes, and plasmon-based electronics.

[0005] Performance of plasmonic systems depends on the fundamental properties of the underlying material (hereinafter termed `substrate`), as both the electromagnetic surface effects and chemical surface effects are affected by some or all of the following factors:

[0006] composition of the substrate (e.g., pure chemical elements or alloys/mixtures, and if such, of which constituent chemical elements; chemical compounds and, if such, of which constituent chemical elements and in what ratios, structural arrangements and types of chemical bond);

[0007] fine structure or particle size of the substrate (e.g., solid solution (alloy), solid solutions containing crystalline inclusions (grain), and fully crystalline, as well as amorphous, glass-like etc., while microscopic particles and especially nanoparticles presenting with very specific and often desirable properties);

[0008] condition of the surface of the substrate (e.g., polished, roughened, specifically formed and in what shape, for instance, grooved gratings made by deposition of alloying elements, microscopic spheres, and, in specific cases, highly localized conditions on the surfaces of microscopic particles and nanoparticles etc.);

[0009] chemical reactivity of the substrate with its environment and, in case of analytical or preparatory surface phenomena (such as chemical catalysis, photonics, energy capture etc.) with the molecules of the solvent, analyte, precursors, intermediate compounds and products of chemical reactions, as well as desirable and undesirable trace compounds (such as atmospheric gases and water vapor) and contaminants;

[0010] stability of the substrate in regard of changes induced, inter alii, by the incident illumination, especially with energetic photons, such as ultraviolet and, in general, shorter wavelength electromagnetic radiation, as well as aging, formation of fractures, dislocations and other defects;

[0011] other factors arising due to endogenic and exogenic factors, including, in one example, the size of the grain (crystalline inclusions in the alloy), as greater energy loss is known to occur on the grain boundary.

[0012] Of note is the fact that chemical reactivity of the substrate may be both beneficial and detrimental depending on the specifics of the application. Also, high chemical reactivity of a specific substrate in one environment (e.g., in air or water) may change to low chemical reactivity in a different environment (e.g, in a non-polar organic solvent). A substrate that presents with high reactivity in water may suffer shortened usable time due to corrosion, solubilization and degradation; however, such substrate may also present with advantages that eclipse its enhanced wear and degradation.

[0013] Since the 1970s, following the initial discovery of the surface enhancement phenomenon, multiple scientific studies had been undertaken and published that aimed to evaluate suitability and usability of specific chemical elements, their alloys and mixtures for plasmonic technologies (Arnold and Blaber, 2009).

[0014] The consensus is that development and employment of alternative materials, rather then the commonly used coinage metals (copper, silver, and gold) or the sometimes used platinum group metals (platinum and palladium, sometimes also ruthenium, rhodium, iridium, and osmium) are necessary to achieve the desired enhancement of the efficacy of plasmonic devices and technologies.

[0015] In search of such alternative materials, one needs to consider the fact that elements noted for their strong enhancement efficacy (copper, silver, and gold) are known to be in violation of the Madelung's rule of electron shell configuration--all three of these elements possess a single s-electron: copper has the electron shell configuration of [Ar] 3d10 4s1; silver--[Kr] 4d10 5s1, and gold--[Xe] 4f14 5d10 6s1, which may explain the high degree of plasmonic performance of these metals. Other elements of interest may, therefore, include the alkali metals (lithium, sodium, potassium, rubidium and cesium) that also have an unpaired s-electron in their valence shells, especially those comparable in atomic size to the most efficacious element (silver) such as potassium with the electron configuration of [Ar] 4s1, rubidium--[Kr] 5s1, and cesium with the electron configuration of [Xe]6s1, although with the heavier elements relativistic effects may cause reduction of efficacy of plasmonic performance.

[0016] Classical (Maxwellian) electrodynamics provides the following formula for the macroscopic electronic response of a substrate:

.epsilon.(.omega.)=.epsilon.'+i.epsilon.''

wherein .epsilon. is the permittivity of the substrate, the measure of resistance that is encountered when forming an electric field in the substrate; .omega. is the angular frequency (pulsatance, a measure of the rate of change of the phase of a sinusoidal waveform, such as electromagnetic wave); while .epsilon.' is the real part of the permittivity, signifying the storage of energy, in this case, on the surface of the substrate, and .epsilon.'' is the complex part of the permittivity, signifying the rate of loss of energy due to dissipation. However, even though the formula appears simple and straightforward, in reality, especially dealing with a surface of a substrate that is immersed in another medium (air, water, solvents, etc.) and may be also coated with layers of oxides, nitrides, other opportunistic compounds or intentionally reacted with molecules of analyte substance or several such substances, the resulting superimposition of permittivities renders calculations extraordinarily complicated.

[0017] Classical electrodynamics also provides the means of assessment of the quality factors that determine the efficacy of plasmonic effects for a specific substrate when it is illuminated with photons of a narrow wavelength range, such as produced by lasers and laser diodes. The measure of such efficacy is dependent on the complex part of the permittivity i.epsilon.'', and is commonly assessed as the `Quality factor` (QL), which has been calculated (for the narrower case of localized surface plasmons generated on the surface of nanoparticles, yet relatable to all types of plasmons) and published for the majority of chemical elements but not their alloys. (Blaber et al., 2010).

[0018] As expected, quality factors are high for the coinage metals (copper=10.09, silver=97.43, and gold=33.99) and for the alkali metals (lithium=28.82, sodium=35.09, potassium=40.68, rubidium=21.90 and cesium=11.20). These elements may, therefore, be suitable substrates for plasmonic technologies. The only other elements for which the calculated QL factor appears to be appreciable are magnesium (calculated QL of 9.94), palladium with QL of 6.52, and aluminum for which conflicting data exist suggesting the QL of 13.58 or less. These data suggest that alloys of said metals may present with desirable characteristics and high efficacy of their plasmonic response.

[0019] The second consideration of practical value is related to the specifics of incident illumination, which, in theory, may be of any wavelength; however, in practice, there are limitations on the range of useful wavelengths. Photolytic effects are especially likely to occur with shorter wavelengths such as ultraviolet spectrum, destroying or significantly altering the substrate and, if present, molecules adsorbed on the surface of the substrate. At the same time, less energetic longer wavelength photons of the infrared and microwave spectra may be not energetic enough to cause and sustain the development of coherent delocalized electron oscillations. Expense and complexity are likely to produce additional challenges in both shorter than visible and longer than visible wavelengths.

[0020] The most economical and best studied, as well as most commonly implemented, are the techniques that operate with wavelengths of the visible and near-infrared spectrum, between approximately 380 nm and approximately 780 nm (visible) and up to 2500 nm (near-infrared). These implementations work well with standard glass-based optical elements, detectors and sources, which are common and inexpensive. Visible light has the additional potential advantage of allowing visual monitoring of the substrate as well as classification of exemplars, and detection of defects by the human eye.

[0021] Indeed, the most common metals used in plasmonics, such as silver and gold are best for experiments utilizing visible and near-infrared spectra because their plasmon resonance frequencies fall within that range of frequencies. Gold, with its intrinsic yellow color, reflects well in the 550-700 nm range (84% to 94%, a smooth curve), but absorbs fairly strongly in the shorter visible wavelengths, with the absolute minimum at about 450 nm (35% reflectance). This makes gold a potentially desirable substrate for plasmonic applications with the incident illumination in the blue-green spectrum. Similarly, the absorption spectrum of copper also falls within the roughly similar range (Creighton and Eadon, 1991), whereas silver, platinum, and palladium reflect all visible light relatively evenly, as signified by the silvery-gray appearance of their surfaces and think films; however, platinum and palladium nanoscale structures, including nanoparticles and possibly including nanoscale grooves on the surface of a solid object made of these metals, may also display plasmon resonance in the visible and near-infrared wavelengths (Langhammer et al., 2006).

[0022] The wavelength that elicits the most efficacious response from the substrate differs depending on the chemical identity of the element. Once again, the wavelengths corresponding to maximum quality factor QL had been approximated (Blaber M G et al., 2010) and listed below:

Barium=649 nm (visible, red) Copper=709 nm (visible, red) Gold=886 nm (near infrared) Magnesium=310 nm (near ultraviolet) Palladium=approximately 12400 nm (infrared) Platinum=3543 nm (infrared) Potassium=1182 nm (near infrared) Rubidium=1532 nm (near infrared) Silver=1088 nm (near infrared) Tin=551 nm (visible, green-yellow) Zirconium=413 nm (visible, purple) However, while barium, tin, and zirconium may present with desirable plasmonic phenomena when illuminated with visible wavelengths, their enhancement quality factors are low, in the range of 1 to 4 units, which renders these elements in their pure, unalloyed state impractical for technological applications.

[0023] The majority of the high quality factor elements, with the exception of silver, gold and platinum group metals suffer with high reactivity, especially the alkali metals, which are highly reactive under standard conditions and react with air, water and many organic molecules rather violently. Same is true for barium, while zirconium, magnesium and, likely, tin are only passive due to the presence of a dielectric layer of a less reactive oxide, the very essential nature of which, being an insulator, hinders plasmonics.

[0024] The search for a desirable substrate that is also sufficiently excited by the most convenient and inexpensive type of electromagnetic radiation, namely, visible light cannot, therefore, be restricted to single chemical elements.

[0025] However, alloys of two or several elements may possess the desirable characteristics: high quality factors and best or at least high efficiency of excitation in the visible spectrum. Such alloys may be in the form of solid solutions or solid solutions with inclusions of crystals (grains) of intermetallic compounds.

[0026] It is empirically known that some alloys and inter-metallic compounds present with visible color that is different from the color of the constituent elements. The most ancient example is electrum, an alloy of gold and silver that possesses a fairly saturated green color. This phenomenon is explained by the specific alloy preferentially reflecting photons of certain wavelengths of the visible spectrum and absorbing photons of other wavelengths. Such absorption of photons of the visible spectrum is a desirable characteristic, as the energy of the absorbed photons can be used to excite surface plasmons and contribute to plasmonic phenomena (Campion and Kambhampati (1998).

SUMMARY OF INVENTION

[0027] The disclosed invention describes the compositions of improved substrates for surface enhancement technologies and surface event technologies. Said improved substrates are disclosed as alloys of gold, platinum and palladium with other chemical elements, especially of the alkali, such as potassium and rubidium, alkali earths, such as magnesium and barium, as well as certain transition metals such as tin and zirconium taken in specific disclosed combinations and specific disclosed ratios.

[0028] One desirable characteristic of substrates disclosed herein affects the electromagnetic factor of the surface-associated phenomena, such as plasmonics. Surfaces of solid objects, thin films, and particles manufactured from said alloys possess different visible colors, that is, they present with desirable reflectance of specific wavelengths of the visible light, different from the characteristic reflectances of the pure substrate metals (gold, platinum and palldaium). The change in reflectance characteristics of the alloy compared to the pure substrate metal is due to the changes in the energy of surface plasmons, as well as the surface plasma frequencies and the different interband transitions in the alloy compared to the pure substrate metal. These changes are not random but rather fully distinct and characteristic and are due to the influence of the alloying chemical elements and formation of specific phases and crystal structures in the alloy. The use of said substrates, therefore, allows for optimization of resonance at a particular wavelength of the incident light, a highly sought after characteristic in all varieties of plasmonics applications.

[0029] The second desirable characteristic of disclosed substrates affects the chemical factor of surface phenomena. While pure substrate metals, especially gold and platinum, and, to the lesser extent, palladium and silver, are fairly inert (`noble`) and have lesser affinities for many molecules and functional groups, the alloys as disclosed are potentially more reactive, allowing to stronger coupling and different types of coupling, thus causing a change in the adsorbed molecule, its shape, conformation, density of its electron cloud, position and shape of functional groups and other characteristics. Therefore, the chemical enhancement variation due to the presence of alloying elements with intrinsically different electrochemical properties, different reactivity and affinity for various functional groups of the adsorbed molecules is useful in a variety of plasmonics technologies.

[0030] Embodiments of the present invention include alloys of gold, platinum and palladium presenting with distinct and uniform values of surface plasmon generation as well as surface reactivity that are different from the constituent elements.

[0031] The disclosed substrates may be used as bulk and macroscopic objects, thin films, and particles on the micrometer and nanometer scales.

[0032] The present invention posits the use of specifically formulated alloys of said metals to facilitate the following:

[0033] changes in resonance frequencies of the surface plasmons, allowing excitation with a wider range of photon wavelengths or, conversely, allowing excitation only with specific ranges of wavelengths, thus, potentially reducing the interfering effects of fluorescence;

[0034] formation of chemical bonds between the adsorbed molecule and the components of the alloy, such as separate atoms and crystalline inclusions (grains);

[0035] utility in photovoltaic, that is, conversion of photic energy to electric potentials or currents; photocatalytic, that is facilitation of chemical reactions utilizing the energy of electromagnetic radiation; plasmon-based electronics, that is, utilizing different alloys as disclosed to form dual- and triple-layered structures that can function as diodes and transistors.

BRIEF DESCRIPTION OF DRAWINGS

[0036] FIG. 1 presents the graphical representation of measurement of reflectance of three experimental alloys (2Au:1K, 2Au:1Rb, and 5Au:1Rb) as well as pure gold (Au).

[0037] FIG. 2 presents the graphical representation of measurement of reflectance of two experimental alloys (1Mg:1Pt:1Sn and 1Mg:1Pd:1Sn) as well as pure gold (Au).

DESCRIPTION OF EMBODIMENTS

[0038] The inventors, a professional jeweler and a researcher in the field of non-linear optics, embarked upon creation of alloys of gold, platinum and platinum group metals suitable for plasmonic applications with the understanding of the following criteria:

criterion A. The alloy needs to be visibly colored, preferentially absorbing and reflecting certain parts of the visible spectrum; criterion B. The alloy needs to be prepared in a facile fashion.

[0039] In the experiments carried out and directed solely by the inventors, series of alloys of various composition were prepared both by conventional techniques (alloying under inert gas) and by alternative techniques (vapor deposition, sintering, electrochemical deposition from polar and organic electrolytes). Reflectance of obtained alloys was measured under standard conditions with a scanning reflectometer. in some of the cases, such as alloys with high reactivity and susceptibility to tarnishing in air, reflectances were measured in the same quartz ampoule in which alloys were prepared.

[0040] The alloys were prepared from chemically pure (no less than 99.9% purity, under vacuum) elements obtained from Smart Elements GmbH (Austria) and from a Russian commercial supplier. Gold, palladium and platinum were processed by a bullion supplier (Produits Artistiques Metaux Precieux, Ticino, Switzerland), all sealed in original cards and assayed with 99.99% or better marks of purity. Due to the high gold content of the alloys and the need for multiple samples, 99% pure gold was also prepared from jewelry scrap by a modified electrolytic dissolution method and alloys prepared from such lesser purity gold were used in preliminary experiments.

[0041] Preparation of alloys presented with some challenges, especially alloys made from metals with vastly different melting points (e.g., gold and rubidium). In those cases, the more difficult to melt metal was first rolled into thin foil (.about.0.05 mm thick for gold, .about.0.08 mm thick for platinum and palladium), weighted, acid washed and ultrasonically cleaned, dried, re-weighted, and introduced into quartz ampoule with the other metal already present under protective atmosphere of argon gas. Metals supplied under protective mineral oil (such as alkali metals) were washed in a great excess of n-hexane under argon atmosphere and the residual n-hexane was evaporated with a stream of argon gas at room temperature. The ampoule was introduced into an electric furnace filled with argon gas and heated with periodic agitation. Temperatures for the alloy formation were taken from multiple references and corresponded to binary alloy phase diagrams. In all cases the samples were overheated by .about.50 degrees C. to allow for liquidus formation. Cooling of the prepared samples was at the rate of 10 degrees C. per hour around the expected solidus formation temperature, and subsequently at 100 degree C. per hour, allowing for formation of a suitably looking sample. The preparation of the gold-zirconium alloy was carried under similar procedure, except zirconium foil was introduced into a quantity of molten gold and incubated for 24 hours at 1280 degrees C.

[0042] The technical difficulty of preparation of alloys of gold with reactive metals, such as alkali metals, prompted the development of a principally novel method of manufacturing of alloys, especially suitable for preparation of alloys of metals with vastly different reactivity, such as alloys of gold and alkali metals.

This method consisted of the following steps:

[0043] foil of the main element of the alloy such as gold, platinum or palladium was produced by conventional metallurgy methods such as cold rolling, resulting in a foil with minimal surface roughness as evident by a mirror-like sheen;

[0044] said foil was thoroughly cleaned and immersed into water-free non-polar organic electrolyte comprised of one or several of the following chemical compounds: propylene carbonate (CH3C2H3O2CO), ethylene carbonate ((CH2O)2CO), dimethyl carbonate (OC(OCH3)2), poly(oxyethylene) (a polymer with the general formula HO--(CH2CH2O)n-H), diethyl carbonate (OC(OCH2CH3)2), and similar organic electrolytes, or, in some of the experiments, into ionic liquids such as ethylammonium nitrate also known as ethylamine nitrate (C2H8N2O3) and similar compounds in which was dissolved a salt of the alloying element, such as potassium chloride (KCl), rubidium chloride (RbCl) or cesium chloride (CsCl) with optional additive in the form of aluminum chloride (AlCl3);

electric current was introduced with the foil serving as cathode and a graphite rod serving as the anode;

[0045] electrochemical deposition of the alloying element such as potassium, rubidium, cesium on the surface of the foil made of the main element of the alloy occurred and the amount of the deposited alloying element was quantified by determination of voltage and current;

[0046] the resulting foil with electrochemically deposited alloying metal was taken out of the electrolyte, washed with n-hexane and dried under a stream of argon;

[0047] subsequent treatment included heating under protective argon atmosphere for 24 hours at 500 degrees C., facilitating diffusion of atoms and formation of the alloy, or, optionally, heated to higher or lower temperature depending on the identity of the alloying element (not to exceed the boiling point of potassium at 759 degrees C., boiling point of rubidium at 688 degrees C., or boiling point of cesium at 671 degrees C., correspondingly);

[0048] mechanical treatment of the resulting alloy followed and included smoothing, rolling or burnishing with an agate rod under the protective atmosphere of argon gas.

[0049] Upon examination, surfaces of thus manufactured alloys were found to present with specific colors, such as the dark green color when the amount of electrochemically deposited rubidium was 17.82 weight percent to 82.17 weight percent of gold.

[0050] A variation of the aforementioned method comprised a preparatory step, in which the goal was to produce a roughened surface of the foil of the main element of the alloy, such as gold prior to the electrochemical deposition of the alloying element. The usefulness of such roughened surface is well known in plasmonics and the resulting nano-scale (<100 nm) striated and sponge-like surface features upon electrochemical deposition of the alloying element will form a striated or sponge-like surface of the alloy.

[0051] To achieve the roughening of the surface of the foil made from the main element of the alloy and formation of nanoscale surface features, an intermediary alloy of the main element was created. In one approach, the intermediary alloy was comprised of less than 25 weight percent of gold with the balance being silver. Specifically, the intermediary alloy was prepared by co-melting under argon atmosphere of 22 weight percent pure gold no less than 99.99 purity and 78 weight percent pure silver of no less than 99.9 purity. The resulting alloy was cold rolled to 1 mm thickness and subjected to acid etching with aqueous nitric acid (specific gravity of 1.25 g/ml) for several hours at .about.95 degrees C. (the hot acid etching was chosen as cold acid etching resulted in generation of fine particles) with subsequent washes in boiling water, followed with a prolonged wash in 1 percent solution of urea in water to neutralize remaining residues of nitric acid and final washes with boiling and cold water.

[0052] The resulting foil of pure gold was observed under the microscope to have sponge-like surface and was used in the aforementioned electrochemical deposition process resulting in a formation of a sponge-like structure of the alloy of the alkali metal with gold.

[0053] Alternatively, to generate a surface characterized by nanoscale striations, a quantity of pure gold foil was pressed with a quantity of pure copper foil pre-washed with boiling 10% solution of citric acid with subsequent boiling water washes under protective atmosphere to remove oxidation layers from the copper foil. With application of constant pressure at 100 kg/cm2 the bilayer foil was incubated at 400 degrees C., facilitating the formation of Au--Cu intermetallics. The fused bilayer foil was then subjected to boiling in nitric acid until full dissolution of the copper component. The surface of the remaining gold component was found to be striated with the average size of the striation less than 100 nm. This gold foil with striated surface was subsequently subjected to electrochemical deposition of the alkali metal as described above.

[0054] Other intermediary alloys, such as a ternary alloy of gold-silver-copper and quartenary alloy of gold-silver-copper-zinc were also successfully used to generate other types of roughened surfaces.

[0055] Other etching solutions were used, including various concentrations of nitric acid-water mixtures, other inorganic acids, including selenic acid (H2SeO4), water-based solutions that generated halogens in situ, such as the mixture of sodium hypochlorite (NaClO) with hydrogen peroxide (H2O2), solutions of free halogens, such as iodine-iodide solution (I2 in KI), as well as salts of the thiosulfate anion (Na2S2O3) under acidic pH and solutions of thiourea under various pH. All these solutions were utilized upon the primary dealloying of the intermediate alloy with the goal of etching the gold component with either smoothing of the surface or, alternately, increased roughening of the surface.

[0056] The aforementioned methods were utilized to produce multiple samples of gold-based, as well as platinum- and palladium-based alloys with desirable characteristics, such as desirable surface color and desirable reflectance curves. Other methods for alloy preparation are well known to persons familiar with the metallurgic science and are not a part of the disclosed invention.

[0057] Multiple samples of the gold alloys were prepared, with alloying elements added in weight percentages that fell into the range indicated in Table 1, for example, for the Gold-Potassium alloy 1 four samples were prepared: with potassium at 2.5, 3.0, 3.5, 3.82 (calculated ideal ratio corresponding to the 1/6 atomic parts of K to 5/6 atomic parts of Au) and 4.0 weight percent. The samples were evaluated for desirable characteristics, such as change in reflectivity spectrum compared to the pure metals (gold, platinum and palladium) and reactivity with air, water and common solvents including anhydrous ethanol, acetone and n-hexane.

[0058] Composition of alloys, especially the atomic percentages of the comprising chemical elements, was confirmed with X-ray fluorescence utilizing the Bruker S1 Titan XRF analyzer.

[0059] Of the large number of prepared alloys, only a small portion was found to exhibit the desired characteristics as discussed above and presented below:

TABLE-US-00001 Atomic Weight Formation Color as Alloyed elements Label ratios percent at .degree. C. observed Gold, Potassium 5Au: Au: 5/6; Au 96.18%, 800-900 dark green 1K K: 1/6 K 3.82% Gold, Potassium 2Au: Au: 2/3; Au 90.97%, 650 reddish- 1K K: 1/3 K 9.03% purple Gold, Rubidium 5Au: Au: 5/6; Au 92.2%, 730 dark green- 1Rb Rb: 1/6 Rb 7.98% yellow Gold, Rubidium 2Au: Au: 2/3; Au 82.17%, 580 dark green 1Rb Rb: 1/3 Rb 17.82% Gold, Barium 5Au: Au: 5/6; Au 87.76%, 800-900 gray-blue, 1Ba Ba: 1/6 Ba 12.24% shimmering Gold, Zirconium 3Au: Au: 3/4; Au 86.62%, 1280 gray-blue, 1Zr Zr: 1/4 Zr 13.37% shimmering Gold, Magnesium, 1Mg: Au: 1/3; Au 57.93% 780 reddish- Tin 1Au: Mg: 1/3; Mg 7.15%, purple 1Sn Sn: 1/3 Sn 34.92% Platinum, 1Mg: Pt: 1/3: Pt 57.70% 920 dark Magnesium, Tin 1Pt: Mg: 1/3; Mg 7.19%, reddish- 1Sn Sn: 1/3 Sn 35.11% orange Palladium, 1Mg: Pd: 1/3; Pd 42.55% 880 brass Magnesium, Tin 1Pd: Mg: 1/3; Mg 9.75%, yellow 1Sn Sn: 1/3 Sn 47.59%

[0060] Six of the listed alloys exhibiting color when viewed with the naked eye were subjected to measurement of reflectance of the visible light. The results of these measurements are presented in FIG. 1 and FIG. 2.

[0061] The measurements were performed at room temperature and normal barometric pressure in the visual spectrum extending from 400 nm (violet) to 700 nm (deep red) utilizing a custom made spectroscopic reflectometer set at 20 degree incident angle reporting total reflectance at 7 bands set at 400, 450, 500, 550, 600, 650 and 700 nm which were supplied by a tunable source of near-monochromatic light (Fastie-Ebert in-line monochromator, effective aperture of f/4 and focal length of 75 mm with 1800 lines/mm diffraction grating and 0.5 mm slit width, with wavelength precision of approximately 7.5 nm). Reflectance (R) was measured as:

R=.PHI.r/.PHI.i

[0062] Where .PHI.r is the radiance reflected by the surface and .PHI.i is the radiance received by the surface.

[0063] FIG. 1 presents a plot of experimental data obtained by measuring reflectance of four samples: 99.99% pure gold (reference sign 10); alloy 2Au:1K (reference sign 21), appearing reddish-purple to the eye; alloy 2Au:1Rb (reference sign 22), appearing dark green to the eye; alloy 5Au:1Rb (reference sign 23) appearing dark greenish-yellow to the eye.

[0064] It is evident from FIG. 1 that all three alloys of gold with alkali metals possessed significantly different reflectance characteristics, representative of desirable changes in the absorption and re-emittance of incident visible light that correspond to desirable parameters of surface plasmon polaritons.

[0065] FIG. 2 presents a plot of experimental data obtained by measuring reflectance of two experimental samples overlaid with the data from FIG. 1 for pure gold (reference sign 10) for comparison. The two experimental alloys were 1Mg:1Pt:1Sn (reference sign 31), appearing dark reddish-orange to the eye and 1Mg:1Pd:1Sn (reference sign 32) appearing brass-yellow to the eye.

[0066] Measurements were performed at same wavelengths as for FIG. 1 and calculation of reflectance was also performed as for FIG. 1. From the measured data it is evident that both alloys, of magnesium and tin with platinum or palladium also possessed significantly different reflectance characteristics, representative of desirable changes in the absorption and re-emittance of incident visible light that correspond to desirable parameters of surface plasmons.

[0067] Of these, as expected, the Gold-Potassium, Gold-Rubidium and Gold-Barium alloys, despite the very high weight percent of gold in the composition, exhibited higher reactivity and tarnished under humid air, necessitating measurement of reflection while the sample was confined in a quartz ampoule and its exposed surfaces were coated with mineral oil. The Gold-Zirconium, Gold-Magnesium-Tin, Platinum-Magnesium-Tin and Palladium-Magnesium-Tin alloys were less reactive, however, in some instances they were also measured while confined to a quartz ampoule and their exposed surfaces coated with mineral oil of same composition as the alkali-gold alloys.

[0068] Alloys presented in Table 1 were exhibiting significant visible color that was distinct from the colors of the constituent metals and were, therefore, considered especially desirable, as they met the above mentioned Criterion A. The purple-reddish colored alloys (such as Gold-Potassium 2Au:1K and Gold-Magnesium-Tin alloy) presented with very strong interband electronic transitions around 3 eV, corresponding to photons of wavelengths around 410-420 nm, explaining their visible color and the reflectance curve; while the electronic transitions of the green-colored alloys are likely to correspond to the energies of 2.0-2.75 eV (wavelengths of approximately 450 nm and 600 nm). Since these wavelengths are in the visible spectrum, the Criterion A is met undeniably; while the electrolytic deposition method of manufacturing allows these alloys to meet Criterion B.

[0069] Of note is the opportunity to produce gold and platinum group metal-based alloys of alkali or alkali-earth metals formed into objects or particles which are then partially dealloyed with water or water-organic solvent mixtures. Roughness of the surface is achieved by reaction with water in large or trace amounts and surface structures that are cubic-derived and hexagonal derived are formed, providing an entirely different type of roughening of the surface compared with the conventional acid etching of pure gold and silver. If the dealloying is stopped prior to complete removal of the reactive atoms, the remaining atoms of the alloying element (such as rubidium in gold-rubidium alloys and magnesium in palladium-magnesium-tin alloys) may serve as partial enhancers, especially of the chemical enhancement. The palladium-magnesium-tin alloy undergoes partial deallying upon etching with weak organic acids and may serve as a replacement of the more expensive solid palladium in a multitude of applications. The dealloyed grainy structure rolled or beaten with a cover gold leaf as thin as to allow green-blue pass through as well as formation of evanescent waves may also be used.

[0070] Other technical advantages and applications of the disclosed invention may become readily apparent to one of ordinary skill in the art upon familiarization with the disclosed figures and description. The scope of the invention is defined by the scope of the claims.

Claims

1. A composition of matter such as an alloy with the properties of: (i) being comprised of at least one metal selected from gold (Au), platinum (Pt), and palladium (Pd) as the main element of the alloy; (ii) being comprised of at least one alloying component promoting change in the energy of surface plasmons of the resulting alloy preferably chosen from alkali metals such as potassium (K), and rubidium (Rb), as well as alkali earth metals such as magnesium (Mg), and barium (Ba), as well as transition metals inclusive of tin (Sn), and zirconium (Zr); (iii) the resulting alloy necessarily and essentially presenting with the property of substantially different energy characteristics of the surface plasmons as present on the surface of objects made from the alloy, including solid items, thin layers and nanoparticles, as compared to any or all of the constituent elements and assessed by one or all of the following manifestations: a change of surface color as perceived by the human eye as a function of reflectance of specific wavelengths of incident white light; a change in specific absorbance and reflectance characteristics when illuminated with electromagnetic radiation including ultraviolet, visible, and infrared light as polychromatic mixture of wavelengths, monochromatic illumination or illumination with multiple narrow wavelengths; a change in efficacy of generation, surface plasmon resonance frequency, specific energies of surface plasmons generated upon illumination of the surface of the alloy with electromagnetic radiation, including ultraviolet, visible and infrared light as polychromatic mixture of photons of multiple wavelengths, monochromatic illumination or illumination with photons of multiple narrow ranges of wavelengths.

2. A composition of matter comprising: an alloy consisting primarily of gold and potassium at atomic ratios of two parts of gold to one part of potassium, with deviation from said ratios not exceeding five atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as reddish-purple as perceived by the human eye, with said color development being of essence.

3. A composition of matter comprising: an alloy consisting primarily of gold and potassium at atomic ratios of five parts of gold to one part of potassium, with deviation from said ratios not exceeding five atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as dark green as perceived by the human eye, with said color development being of essence.

4. A composition of matter comprising: an alloy consisting primarily of gold and rubidium at atomic ratios of five parts of gold to one part of rubidium, with deviation from said ratios not exceeding five atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as dark green-yellow as perceived by eye, with said color development being of essence.

5. A composition of matter comprising: an alloy consisting primarily of gold and rubidium at atomic ratios of two parts of gold to one part of rubidium, with deviation from said ratios not exceeding five atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as dark green as perceived by the human eye, with said color development being of essence.

6. A composition of matter comprising: an alloy consisting primarily of gold and barium at atomic ratios of five parts of gold to one part of barium, with deviation from said ratios not exceeding five atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as gray-blue and shimmering as perceived by the human eye, with said color development being of essence.

7. A composition of matter comprising: an alloy consisting primarily of gold and zirconium at atomic ratios of three parts of gold to one part of zirconium, with deviation from said ratios not exceeding five atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as gray-blue and shimmering as perceived by the human eye, with said color development being of essence.

8. A composition of matter comprising: an alloy consisting primarily of gold, magnesium and tin at atomic ratios of one part of gold to one part of magnesium and one part of tin, with deviation from said ratios not exceeding two atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as reddish-purple as perceived by the human eye, with said color development being of essence.

9. A composition of matter comprising: an alloy consisting primarily of platinum, magnesium and tin at atomic ratios of one part of platinum to one part of magnesium and one part of tin, with deviation from said ratios not exceeding two atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as dark reddish-orange as perceived by the human eye, with said color development being of essence.

10. A composition of matter comprising: an alloy consisting primarily of palladium, magnesium and tin at atomic ratios of one part of palladium with one part of magnesium and one part of tin, with deviation from said ratios not exceeding two atomic percent, which develops, upon preparation of the alloy, the desirable surface color described as brass-like yellow as perceived by the human eye, with said color development being of essence.