The present invention relates generally to methods for fabricating crystalline metal structures.

Wet chemistry techniques are conventionally used to grow silver crystals in a variety of shapes. Such techniques can be employed to form high quality crystals shaped, for example, as pyramids, bipyramids, cubes, triangular prisms, hexagonal prisms and nanowires. These techniques, however, have significant drawbacks. For example, they cannot be effectively utilized to control the location of crystal growth. Further, such techniques generally provide a slow rate of crystal growth. Traditional wet chemistry methods typically require between 0.5 and 30 hours at a typical refluxing temperature of 120 to 160°C to grow silver crystals between 80 nm and 280 nm in size.

Femtosecond laser direct-writing methods have also been used to grow silver in engineered patterns in two and three dimensions. However, such methods generally lead to silver structures that are composed of nanoparticle agglomerations, rather than single crystal domains. For example, such silver structures can consist of silver nanoparticle clusters with void, polymer, or multiple domain inclusions.

Hence, there is a need for improved methods for generating crystalline metal structures.

Summary

A method for generating crystalline metal structures is disclosed, which comprises generating a mixture of a metal precursor, a solvent and a polymer, applying said mixture to a surface of a substrate, curing said mixture to generate a cured mixture, and irradiating at least one location of said cured mixture so as to form a crystalline metal structure within at least a portion of the irradiated region. The cured mixture can include a plurality of metal ions associated with the metal precursor distributed therein. The generated crystalline metal structure(s) can exhibit a plasmonic resonance. Some examples of suitable polymers include, without limitation, polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), polyvinylcarbazole (PVK),

polymethylmethacrylate (PMMA), polystyrene (PS). In some embodiments, the solvent can be an organic solvent or a mixture of an organic solvent with water. In some embodiments, the solvent can be a mixture of two or more organic solvents. Some examples of suitable solvents include, without limitation, an alcohol, such as ethanol, ethylene glycol, or mixture thereof with water. In some embodiments, additives, such as, 5- 20 nm silver nanoparticles, citric acid, maleic acid, captopril, hydrogen peroxide, sodium borohydride (NaBH4), iron (II) sulfate heptahydrate (FeS0 ₄ .7H ₂ 0), KC1, KOH, and sulfuric acid, can be added to the mixture.

The step of irradiating can comprise applying one or more radiation pulses to at least one location of the cured mixture. The radiation pulses can have a pulse width in a range of about 10 femtoseconds to about 500 picoseconds, e.g., in a range of about 50 femtoseconds to about 300 femtoseconds. The radiation pulses can have a central wavelength in a range of about 500 nm to about 1550 nm and a pulse energy in a range of about 0.5 nJ to about 40 nJ, e.g., in a range of about 3 nJ to about 6 nJ. In some embodiments, the total energy deposition in each irradiated location can take place over a timescale in a range of about 100 ns to about 100 ms. In some embodiments, the total optical energy irradiated onto each location (e.g., the focal volume) can be, e.g., in a range of about 9 μΐ to about 26 μί. Further, the pulses can be applied to the cured mixture at a repetition rate in a range of about 1 MHz to about 90 MHz, e.g., 11 MHz.

The irradiating step can cause chemical reduction of at least a portion of the metal ions so as to form the crystalline metal structure(s). In many embodiments, the irradiating step comprises focusing radiation onto said at least one location of the cured mixture such that said focused radiation has a sufficiently high intensity at said location so as to undergo non-linear absorption by at least one radiation-absorbing constituent of said cured mixture, thereby mediating the chemical reduction of the metal ions. By way of example, the radiation can be focused onto one or more locations of the cured mixture at a numerical aperture in a range of about 0.4 to about 1.45. The generated crystalline metal structures can exhibit a variety of shapes, such as pyramid, bipyramid, cube, triangular prism, hexagonal prism or a nanowire. In some embodiments, the crystalline metal structure(s) comprise a single crystal domain. In some embodiments, the crystalline metal structure has a size in at least one dimension, and in some cases in two or three dimensions (e.g., X-, Y- and Z- dimensions) of at least about 40 nm, or at least about 100 nm, or at least about 200 nm, or at least about 300 nm, or at least about 400 nm, or at least about 500 nm, or at least about 1 micrometer. In some embodiments, the generated crystalline metal structures have a size in at least one dimension, and in some cases in two or three dimensions (e.g., X-, Y- and Z-) that is less than 40 nm, for example, less than about 20 nm, or less than about 10 nm, e.g., in a range of about 5 nm to about 20 nm.

In some embodiments, a method for generating a crystalline metal structure is disclosed, which comprises irradiating at least one location of a polymeric matrix in which a plurality of metal ions are distributed with one or more short laser pulses so as to cause chemical reduction of at least a portion of the ions at the irradiated location(s) to form crystalline metal structures.

In some embodiments, a method for generating a crystalline metal structure is disclosed, which comprises generating a mixture of a metal precursor, a solvent and a polymer, where the mixture comprises a plurality of metal ions associated with said precursor. The method further calls for applying said mixture to a surface of a substrate, and curing the mixture, e.g., via heat treatment, to generate a cured mixture. One or more radiation pulses can then be focused onto at least one location of the cured mixture so as to cause the chemical reduction of at least a portion of the metal ions in the irradiated location(s), where at least one parameter of the focused radiation is selected such that the reduced atoms, or at least a portion thereof, form a crystalline metal structure. The parameter of the radiation can be any of, or a combination of, pulse duration, central wavelength, pulse energy of the radiation pulses.

In some embodiments, single crystal prism, e.g., hexagonal silver prism, growth is achieved through irradiation with focused femtosecond laser pulses in targeted locations of a substrate, e.g., a polymer film, in which silver ions are distributed. In some cases, such single crystal prisms are in the form of nanostructures, which by virtue of being single crystals, can exhibit more desirable material properties compared to silver nanostructures grown in previous femtosecond laser direct writing techniques.

Further understanding of various aspects of the invention can be obtained by reference to the following detailed description in conjunction with the associated drawings, which are described briefly below.

Brief Description of the Drawings FIG. 1 is a flow chart depicting various steps in a method according to an embodiment of the teachings of the invention for generating crystalline metal structures,

FIG. 2A schematically depicts a substrate with a polymeric mixture according to the present teachings disposed on a surface thereof,

FIG. 2B schematically depicts the substrate and the polymeric mixture of FIG. 2A after curing of the mixture, e.g., via a heat treatment, to form a polymeric layer,

FIG. 2C schematically depicts application of radiation pulses to selected locations of the polymeric layer shown in FIG. 2B,

FIG. 3 schematically depicts a system suitable for performing the teachings of the invention for generating crystalline metal structures, FIG. 4A shows a scanning electron microscopy (SEM) image of a 900-nm crystal grown in accordance with an embodiment of the invention,

FIG. 4B shows an SEM image of a 500-nm crystal grown in accordance with an embodiment of the invention,

FIG. 5A shows an SEM image of a crystal grown in accordance with an

embodiment of the invention, FIG. 5B shows an energy-dispersive x-ray spectroscopy (EDS) elemental map of silver in the grown crystal, FIG. 5C shows an EDS elemental map of carbon in the probed volume,

FIG. 5D shows an EDS spectrum of the crystal shown in FIG. 5 A,

FIG. 6A shows an SEM image of a crystal grown in accordance with an

embodiment of the invention,

FIG. 6B shows an inverse poll figure from electron backscatter diffraction (EDSD) analysis, FIG. 6C shows unit cell orientation from EBSD analysis,

FIG. 6D shows a poll figure from EBSD analysis,

FIGs. 7A-7D show some examples of crystal shapes that can be obtained via the methods according to the present teachings.

Detailed Description

The present invention relates generally to methods and systems for generating crystalline metal structures, e.g., in a polymeric substrate, and to such substrates having a plurality of crystalline metal structures therein and the resultant crystalline metal structures.

As discussed in more detail below, such crystalline metal structures can be formed by exposing selected locations of a polymeric substrate (e.g., a polymeric matrix) through which metal ions are distributed to short laser pulses. For example, a plurality of laser pulses can be focused onto the polymeric substrate so as to cause the chemical reduction of at least a portion of ions in the focal volume of the pulses so as to form crystalline metal structures. The term "polymer" is used herein consistent with its common meaning in the art to refer to a macromolecule formed by the chemical union of five or more repeating chemical units, e.g., by repeating monomers.

The terms "reduction" and "chemical reduction" are used herein consistent with the use of these terms in the art to refer to a chemical reaction in which a chemical species decreases its oxidation number, typically by gaining one or more electrons. The term "photoreduction" as used herein refers to a chemical reduction that is mediated by photons.

The term "substantially transparent," as used herein for describing a material, is intended to mean that the linear absorption coefficient of the material for a radiation wavelength is less than about 25%, and preferably less than about 5%. In other words, radiation having that wavelength can penetrate into the material without much absorption by that material.

The term "short radiation pulses," as used herein, refers to pulses of electromagnetic radiation having a temporal duration in a range of about 10 femtoseconds to about 500 picoseconds.

The term "focal volume" is used herein consistent with its common meaning in the art to refer to a volume extended axially about a focal plane, a plane at which a focused radiation beam exhibits a minimum beam waist and a maximum intensity, up to a plane at which the beam exhibits a beam waist that is larger than the minimum beam waist by a factor of about V2.

The term "nanowire" is used herein to refer to a material structure having a cross- sectional dimension (e.g., a diameter) that is equal to or less than about 1 micrometer, e.g., equal to or less than about 500 nm, or equal to or less than about 200 nm, or equal to or less than about 100 nm, e.g., in a range of about 2 nm to about 20 nm, and having an unconstrained length.

The term "crystalline metal structure" is used consistent with its ordinary meaning in the art to refer to a solid formed by metal atoms, ions, which are arranged in a repeating, three dimensional pattern to provide long-range order. It should be understood that a crystalline metal structure can, nonetheless, include defects, e.g., impurities, dislocations, etc. Some examples of crystal structures include, without limitation, face-centered cubic, body-centered cubic, among others. With reference to Figures 1, 2A, 2B, and 2C, in an exemplary method of an embodiment according to the teachings of the invention, a mixture of a polymer, a metal precursor, and a solvent is formed (step A) and applied to a surface 12 of a substrate 14 (step B), e.g., to form a thin layer of the mixture over the surface. The mixture can be in the form of a solution or a colloid. The mixture can include a plurality of ions associated with metal of the metal precursor. By way of example, the mixture can be spin-coated onto the substrate surface, though other techniques can be used as well. In some cases, the substrate surface is treated, e.g., via plasma treatment and/or salinization, prior to application of the mixture thereto. For example, in some embodiments, the substrate surface can be treated with a variety of silanes, such as Acryloxy Propyl Methoxy Silane (APMS) or Mercapto Propyl Trimethoxy Silane.

A variety of substrates can be employed. Some examples of suitable substrates include, without limitation, glass, polymer or other organics, and semiconductor substrates (e.g., silicon). Some examples of suitable of metal precursors include, without limitation,

Ag 0 ₃ , AgC10 ₄ , AgBF ₄ , HAuCl ₄ , among others. Some examples of suitable polymers include, without limitation, polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), polyvinylcarbazole (PVK), polymethylmethacrylate (PMMA), polystyrene (PS).

A variety of solvents can be utilized. In some embodiments, the solvent can include at least one component that can cause the reduction of the metal ions even in absence of the applied radiation, though in many cases the applied radiation can enhance the reducing effect (the rate of reduction) of that solvent component. For example, in many

embodiments, the solvent can include an alcohol, e.g., ethanol. Some examples of suitable solvents include, without limitation, ethanol, ethylene glycol, or a mixture thereof. In some embodiments, the solvent can be an aqueous solution of an organic solvent, such as one or more of the above solvents, with water.

In some embodiments, additives can be added to the mixture that can facilitate the formation of the crystalline metal structures. By way of example, such additives can provide nucleation seeds, charge stabilization and/or more facile reduction of the metal ions. Some examples of such additives include silver nanoparticles (e.g., 1-25 nm silver nanoparticles, for example, 5-20 nm silver nanoparticles), citric acid, maleic acid, captopril, hydrogen peroxide, sodium borohydride ( aBH4), iron (II) sulfate heptahydrate

(FeS0 ₄ .7H ₂ 0), KC1, KOH, and sulfuric acid.

The mixture applied to the substrate surface can be cured (step C), e.g., via a heat treatment, to generate a polymeric layer 16 through which metal ions associated with the metal precursor are distributed. For example, the mixture can be exposed to a temperature in a range of about 40 °C to about 150 °C, e.g., in a range of about 50 °C to about 100 °C, for a time duration in a range of about 30 minutes to about 2 hours. The heating can cause the mixture to form a more viscous polymeric layer (e.g., a solid polymeric matrix) through which the metal ions are distributed. For example, the cured polymer can be in the form of a polymeric film having a thickness in a range of about 4 microns to about 20 microns.

Subsequently, a plurality of short radiation pulses at a wavelength to which the polymeric film is substantially transparent can be focused onto a plurality of locations 18 within the polymeric film 16 (step D) to selectively cause chemical reduction of at least some of the metal ions with the focal volume of the pulses. The polymer, the solvent and the parameter of the applied pulses are selected according to the present teachings such that the chemical reduction of the metal ions mediated by the applied laser pulses would lead to formation of crystalline metal structures. By way of example, the central wavelength of the applied pulses can be in a range of about 500 nm to about 1550 nm, e.g., in a range of about 795 nm to about 1064 nm, the pulse widths can be in a range of about 10 fs to about 500 ps, e.g., in a range of about 50 fs to about 500 fs, the pulse energy can be in a range of about 0.4 nJ to about 40 nJ, e.g., in a range of about 3 nJ to about 8 nJ. The pulses can be applied to each selected location of the polymeric film at a repetition rate in a range of about 1 MHz to about 90 MHz, e.g., 11 MHz. In some embodiment, the total optical energy irradiated into each location (e.g., the focal volume) can be, e.g., in a range of about 9 μΐ to about 26 μΐ

Without being limited to any particular theory, in some embodiments, the short laser pulses can be tightly focused onto a focal volume of the cured mixture (e.g., the polymeric film) such that multiple photons converge in time and space to collectively bridge an electronic energy gap of at least one constituent of the cured mixture to cause reduction of at least a portion of the metal ions. For example, the radiation intensity at the focal volume - cl ean be sufficiently high such that non-linear absorption of the radiation by one or more constituents of the polymeric film can occur.

In some embodiments, the generated crystalline metal structures can be in the form of particles having a size in at least one dimension, and in some cases in two or three dimensions, greater than about 40 nm, or greater than about 100 nm, or greater than about 200 nm, or greater than about 300 nm, or greater than about 400 nm, or greater than about 500 nm, and in some cases up to about 1 micrometer, though other sizes are also possible. In some embodiments, the generated crystalline metal structures can have a size in at least one dimension, and in some cases in two or three dimensions, as low as 10 nanometers

(nm).

As noted above, the crystalline metal structures can have a variety of shapes, such as pyramid, bipyramid, cube, triangular prism, hexagonal prism or a nanowire. In some embodiments, the shape of the generated metal (e.g., silver) crystals can be changed by varying the ratio of the polymer (e.g., PVP) relative to the metal precursor (e.g., Ag Os).

Without being limited to any particular theory, PVP can play an important role in controlling the size and shape of silver structures: the strong affinity of N and O atoms in the amide groups of PVP to surfaces of transition-metal clusters can restrain their growth. Furthermore, PVP interacts preferentially to { 100} planes than those of { 1 11 } of silver crystals and result in faster growth in certain crystal planes. Therefore, different ratios of

PVP and Ag C^ can lead to the growth of a silver particle as twinned, cubic, wire, and spherical crystals. In some embodiments, salts may be added to yield various shapes. Sodium citrate, for example, can be used in some cases to generate triangular or nanobelt structures.

In some embodiments, the crystalline metal structures are composed of a single crystal lattice, e.g., FCC.

In some embodiments, once the crystalline metal structures are formed, the polymer film doped with the crystalline metal structures can be separated from the underlying substrate (e.g., the glass substrate), for example, via washing by a solvent, such as ethanol. In some embodiments, the metal structures can be extracted from the polymer matrix, e.g., via dissolving the polymeric matrix in a solvent. In some embodiments, ethanol can play a role in promoting the crystal growth process. It should, however, be understood that the teachings of the invention are not limited to the use of ethanol as a solvent, but a variety of solvents such as those listed above can be employed. Without being limited to a particular theory, ethanol can play a crucial role as a reducing agent, expediting the initial nucleation process and leading to energetic pathways that promote single crystal growth, rather than agglomerations of smaller nanoparticles. The timescale of the crystal growth indicates novel phenomena not previously reported. Again, without being limited to a particular theory, the ultrafast nature of the energy deposition employed during growth can also contribute to significantly enhanced rate of crystal growth relative to traditional wet chemistry approaches, e.g., as discussed below in some cases an increase of crystal growth by a 10 ⁸ factor was observed. Again without being limited to any particular theory, extremely high temperatures created by the high intensity pulses may aid in increasing the growth rates.

The crystalline metal structures generated according to the present teachings, such as the silver crystals discussed below, can find a variety of applications. Some of these applications include, without limitation, plasmonic applications, biomedical devices, metamaterials, sensing applications, among others.

The following examples are provided to further elucidate various aspects of the invention, and are not intended to convey necessarily the optimal ways of practicing the teachings of the invention and/or the optimal results that can be obtained.

Examples

A mixture of polyvinylpyrrolidone (PVP), silver nitrate (AgNC^), ethanol, and water (H ₂ O) was prepared in the following manner. At room temperature, 2 mL

(milliliters) of deionized water was deposited in a vial. In another vial, 10 mL of ethanol was deposited. 0.25 g of PVP was added to the 10 mL of ethanol at room temperature. The mixture was stirred using a vortex mixer until a clear solution was obtained. In a separate vial, 0.4 g of Ag 0 ₃ was dissolved in the 2mL of deionized H ₂ 0. The mixture was stirred using a vortex mixer until a clear solution was obtained. Once both mixtures were dissolved, the mixtures were mixed together and stirred for 25 minutes at room

temperature. To minimize silver nanoparticle formation, ultraviolet light was filtered from the room lighting. The surface of a glass slide, which was coated with a thin 20 nm layer of indium tin oxide (ITO), was treated with oxygen plasma and then silanized with (3- acryloxypropyl)trimethoxysilane. The slide was then coated with the prepared solution through spin-coating (1000 rpm for 30 seconds). The sample was baked in an oven for 25 minutes at 1 10 °C to form a polymer film doped with silver ions on a glass substrate. The sample was then removed from the oven and let cool for 30 minutes.

A system 100 schematically shown in FIG. 3 was employed for irradiating the metal-doped polymer film with radiation pulses having a central wavelength of 795 nm, a pulse width of 300 femtoseconds and a repetition rate of 1 1 MHz. Briefly, the system 100 includes a 3-axis translation stage 102 on which a substrate, e.g., a glass slide, can be mounted. It further includes a TkSaphhire laser 104 for generating femtosecond pulses (the pulse width of the pulses generated by the laser 104 was 50 fs in this example). After passage through an isolator 106 and a compressor 108, the pulses are received by an acousto-optic modulator (AOM) 1 10, which functions as a shutter to produce exposure windows in a range of about 100 to about 800 microseconds (μβ) during which the polymer film was irradiated. A neutral density filter 112 was employed to adjust the pulse energies to be in a range of about 3 to about 8 nJ. A dichroic mirror 1 14 reflects the laser pulses onto a microscope objective 1 16, which in turn focuses the radiation pulses onto the polymer film. The polymer film can be observed in-situ using a CCD camera 1 18, e.g., under illumination provided by the microscope illumination source. The pulse width of the pulses after the microscope objective was 300 fs. Further details regarding a system suitable for use in performing the processing steps disclosed herein can be found, e.g., in published PCT application PCT/US2012/022036 entitled "Micro- and Nano-Fabrication of Connected and Disconnected Metallic Structures in Three-Dimensions Using Ultrafast

Laser Pulses," which is herein incorporated by reference in its entirety.

In use, the laser beam was initially blocked before reaching the microscope and the glass slide on which the polymer film was formed was placed on the 3-axis translation stage. The illumination source of the microscope was turned on to observe the sample using the CCD camera. The stage was translated along the z-axis to find an interface between the glass substrate and the polymer film. The laser beam was then unblocked and a motion-controller software was initiated to translate the sample in x-, y- and z- directions at a speed of 100 micrometers/second. Each of a plurality of separated voxels of the metal- doped polymer was irradiated for 10 microseconds. The neighboring irradiated voxels were separated from one another by at least several micrometers for clear in-situ imaging. After irradiation, the polymer film was removed via washing step in ethanol.

Crystal growth in a variety of conditions was observed. Figure 4A shows a scanning electron microscopy (SEM) image of a 900 nm crystal grown with a total radiation exposure time of 100 μ8 at 8.2 nJ per voxel, and Figure 4B shows an SEM image of a 500 nm crystal grown with a radiation exposure of 800 μ8 and a pulse energy of 3 nJ. These exposure times are significantly lower (by 8 orders of magnitude) than the time required to grow these types of crystals through traditional wet chemistry methods.

Figure 5A shows an SEM image of a hexagonal crystal grown at a radiation exposure time of 800 μ8 and its corresponding energy dispersion x-ray spectroscopy (EDS) maps (Figures 5B-5D). The EDS elemental maps, showing data for silver (5B) and carbon (5C), indicate that the hexagonal crystal is composed of silver. The generated crystals were also characterized via electron backscatter diffraction (EBSD) measurements. Figure 6B shows EBSD measurements indicating a single crystal silver lattice structure in the probed volume. Through EDS and EBSD analysis, it was concluded that femtosecond laser direct metal writing can lead to single crystal nanoprism growth under appropriate conditions. Crystal growth was also observed with irradiation from a 1050 nm laser with 270 fs pulses, as well as 800 nm laser with 50 fs pulses.

We have also observed various crystal shapes including nanowires, triangular prisms, emerald shapes, and more exotic crystal shapes as shown in Figures 7A-7D that varies in size from 300 nm to several micrometers.

Fabrication parameters were chosen carefully to avoid damage to the substrate during fabrication due to a drop in effective damage threshold by a factor 20. In absence of a doped polymer film, radiation pulses with a fluence greater than 54 kJ/m ² would damage the ITO substrate. In contrast, with the thin film deposited over the substrate, a fluence of 3 kJ/m ² was sufficient to damage the substrate. Without being limited to a particular theory, a combination of effects can be responsible for the damage. The laser irratidation produces surface plasmon resonances in the growing silver, thus increasing field intensity near the substrate. Additionally, silver increases the absorption of incident pulse energy and the temperature in the focal volume and surrounding material can be raised to the order of 1000 K. Both effects can be combined to reduce the required laser fluence to damage the substrate.

Those having ordinary skill in the art will appreciate that various changes can be made to the above embodiments without departing from the scope of the invention.