Nanostructure having metal nanoparticles and method of assembly thereof

Abstract

A nanostructure and method for assembly thereof are disclosed. An exemplary nanostructure includes a photocatalytic nanoparticle; a first tier of metal nanoparticles, each metal nanoparticle of the first tier being linked about the photocatalytic nanoparticle; and a second tier of metal nanoparticles, each metal nanoparticle of the second tier being linked to one of the metal nanoparticles of the first tier and located a distance from the photocatalytic nanoparticle greater than a distance between a metal nanoparticle of the first tier and the photocatalytic nanoparticle.

Description

FIELD OF THE INVENTION

A nanostructure including metal nanoparticles, and a method of assembling a nanostructure including metal nanoparticles, are disclosed.

BACKGROUND

Gold metal nanoparticles have been used as a pigment to, for example, stain glass. More recently, there has been research into developing metal nanostructure assemblies, including structures made from noble metals such as gold and silver. For example, it is known to use an electromagnetic wave to excite a strong resonance condition in metal nanoparticles, which can lead to enhanced, localized electromagnetic fields.

SUMMARY

A nanostructure is disclosed which includes a photocatalytic nanoparticle, a first tier of metal nanoparticles, and a second tier of metal nanoparticles. Each metal nanoparticle of the first tier is linked about the photocatalytic nanoparticle, and each metal nanoparticle of the second tier is linked to one of the metal nanoparticles of the first tier and is located a distance from the photocatalytic nanoparticle that is greater than a distance between a metal nanoparticle of the first tier and the photocatalytic nanoparticle.

An exemplary method is also disclosed for assembling an exemplary nanostructure. The method includes attaching a first linker about a photocatalytic nanoparticle, attaching a second linker to a first metal nanoparticle, and attaching the first metal nanoparticle about the photocatalytic nanoparticle by connecting the first linker and the second linker. A second metal nanoparticle is attached to the first metal nanoparticle by attaching a third linker to the first metal nanoparticle, attaching a fourth linker to the second metal nanoparticle and connecting the third linker and the fourth linker.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention that together with the description serve to explain the principles of the invention. In the drawings:

FIG. 1 is a diagram of an exemplary nanostructure having two tiers of metal nanoparticles.

FIG. 2 shows graphs depicting exemplary intensity enhancement and metal non-radiative lifetimes in relation to wavelength for the differences in enhancement between single tier and multi-tier nanostructures.

FIG. 3 shows graphs depicting exemplary effects of different types of dielectrics on a resonance wavelength of a nanostructure.

FIGS. 4A and 4B are high level diagrams showing an exemplary method of connecting a photocatalytic nanoparticle and a metal nanoparticle.

DETAILED DESCRIPTION

FIG. 1 is a diagram of an exemplary nanostructure 100, including a photocatalytic nanoparticle 110, first tier 120 of metal nanoparticle, and a second tier 130 of metal nanoparticles. Each metal nanoparticle 120a to 120f of the first tier is linked about the photocatalytic nanoparticle 110 (e.g., attached to the photocatalytic nanoparticle or about a void created to represent the photocatalytic nanoparticle), and each metal nanoparticle 130a to 130f of the second tier is linked to one of the metal nanoparticles 120a to 120f of the first tier, respectively. The metal nanoparticles of the second tier 130 are located a further distance from the photocatalytic nanoparticle 110 than a distance between the metal nanoparticles of the first tier 120 and the photocatalytic nanoparticle 110.

Embodiments of a nanostructure including first and second metal nanoparticle tiers can confine light approaching the nanostructure in almost any direction and excite surface plasmons. These surface plasmons, in turn, can produce a focused electric field in a resonance cavity comprising the first tier 120 and second tier 130.

The two tiers of metal nanoparticles 120, 130 shown in FIG. 1 can act as a three-dimensional feedback structure or resonance cavity that amplifies the electric field within the resonance cavity via electromagnetic enhancement. The enhanced field can be highly localized at a place serving as a “hot spot” for the field enhancement and a site for locating the photocatalytic nanoparticle 110. These metal nanoparticle assemblies can be, for example, any kind of metal nanoparticle structure assembly that has a dielectric constant having a negative real part. Examples of metals for the metal nanoparticles include gold, silver, aluminum, copper, titanium, chromium, and other metals capable of supporting surface plasmons.

The photocatalytic nanoparticle 110 is a material having the capability of producing excitons, such as a semiconductor material, or any other desired particle to be exposed to an enhanced field. For example, photocatalytic nanoparticle 110 may be a light-emitting semiconductor material, such as II-VI or III-V semiconductor material, for example CdSe, GaAs, InSb, or LiNbO₃, or other types of semiconductor materials. In other embodiments, the nanoparticle to which the first tier 120 of metal nanoparticles 120a to 120f is attached can be a magnetic particle, or a place-holder nanoparticle that does not need to interact with the surrounding feedback structure and which can be removed if desired. This placeholder nanoparticle can be on the order of 1 nm or smaller, for example although the placeholder nanoparticle also may be larger than 1 nm.

The photocatalytic nanoparticle 110 can also be a core semiconductor nanoparticle covered with a shell layer of another wider bandgap semiconductor material to form a core-shell heterostructure. For example, a CdSe semiconductor nanoparticle can serve as the core and a ZnS epitaxial layer can function as the shell in an exemplary heterostructure embodiment, although other semiconductor materials may be used to form the heterostructure. A heterostructure can minimize the number of surface traps, yield greater charge recombination (quantum yield), reduce leakage current, and improve injection characteristics.

By varying the number of tiers of metal nanoparticles, the relative sizes of the metal nanoparticles, and/or the distances between the metal nanoparticles, parameter values can be determined that allow for extraction of a maximum enhancement from a given nanoparticle configuration. For example, an exemplary nanostructure embodiment may include a metal nanoparticle and/or photocatalytic nanoparticle shape chosen to tune the structure to a particular resonant wavelength. Although the metal nanoparticles shown in FIG. 1 are depicted as spheres, the shape of a metal nanoparticle may be a rod, triangle, plate, pentagon, ellipsoid, or any other desired shape. Other parameters that may be controlled to increase electromagnetic field enhancement include varying the size of a metal nanoparticle and varying the size of metal nanoparticles from one tier group relative to another.

Keeping the distances between the nanoparticles proportional while increasing only the sizes of the nanoparticles can yield a high result gain increase that follows a power law with the sizes of the nanoparticles. The non-radiative decay rate does not necessarily increase correspondingly. By bringing the nanoparticles closer together for a given nanoparticle diameter set, the enhancement and non-radiative lifetime can, for example, increase exponentially.

The associated wavelength of a photocatalytic nanoparticle, for example, a semiconductor nanoparticle, may be adjusted by, for example, controlling a size (e.g., diameter) or shape of the nanoparticle to tune it to a desired emission or lasing wavelength. For example, a range of wavelengths from 490-620 nm, or lesser or greater, can be achieved for CdSe semiconductor nanocrystals by appropriately varying the diameter of the CdSe nanoparticle. Wider or narrower ranges of wavelengths can be achieved by using other semiconductor material compositions.

Referring to FIG. 1, the metal nanoparticles 120a to 120f and 130a to 130f, and the photocatalytic nanoparticle 110 can be linked using organic linkers. A first linker 111, a second linker 112, and a connection point 113 between the first and second linkers, is provided between the photocatalytic nanoparticle 110 and each metal nanoparticle of tier 120. Third linker 121 and fourth linker 122 with an intermediate connection point 123 are provided between each metal nanoparticle of the first tier 120 and each metal nanoparticle of the second tier 130. The linkers connecting the photocatalytic nanoparticle 110 and the tiers of metal nanoparticles 120, 130 can be any organic ligand type, such as alkane linkers or polyethylene glycol (PEG) linkers, or another type of linker composed of organic material.

While FIG. 1 shows nanostructure 100 as having metal nanoparticles 120a to 120f attached to six “sides” of a photocatalytic nanoparticle 110, with three orthogonal axes, other embodiments may be arranged on fewer or more axes and utilize a fewer or greater number of first and second tier nanoparticles per nanostructure to focus the field. The pattern of the overall nanostructures may be self-similar or any other type of pattern. Furthermore, some embodiments of nanostructure geometries may include more than two tiers to provide large localized field enhancements to amplify (e.g., increase or decrease (i.e., attenuate)) the electric field within the cavity. A distance d1 between a metal nanoparticle in tier 120 and the photocatalytic nanoparticle 110, and a distance d2 between a metal nanoparticle in tier 120 and a metal nanoparticle of tier 130 may be set to particular values to control field enhancement. Because the nanostructure 100 can be assembled smaller than the wavelength of light, for example, in sizes on the order of about 100-200 nm, it can have a very high packing density that permits its use in a variety of optical and/or other applications.

FIG. 2 is a graph showing increased enhancement provided by a multi-tiered structure. As shown in FIG. 2, the graph 210 represents enhancement provided by a nanostructure having a first tier including two metal nanoparticles around a centrally located nanoparticle, and the graph 220 represents enhancement provided by adding a second tier of metal nanoparticles. The second tier provides two additional metal nanoparticles, resulting in a nanostructure with a total of four metal nanoparticles around a centrally located nanoparticle. As can be seen in FIG. 2, a nanostructure having only a single metal nanoparticle tier can provide a slight enhancement to the field. However, providing additional metal nanoparticle tiers can substantially increase enhancement. For example, providing a second metal nanoparticle tier located a distance further from the first metal nanoparticle tier can increase enhancement by up to an order of magnitude, or even greater.

Graph 230 of FIG. 2 represents a metal non-radiative lifetime (i.e., a rate at which the photocatalytic nanoparticle couples to non-radiative modes of the metal nanoparticles), resulting from both the one-tier structure corresponding to graph 210 and the two-tier structure corresponding to graph 220. A negligible increase in the metal non-radiative lifetime is incurred when a second tier is provided to the nanostructure, and the graph of the metal non-radiative lifetime for the one-tier structure coincides (overlaps) with the graph of the metal non-radiative lifetime for the two-tier structure, as shown in graph 230. As can be seen from FIG. 2, enhancement can be an order of magnitude greater when nanoparticles are provided further away from the photocatalytic nanoparticle with no substantial increase of coupling to the non-radiative lifetime. A significant increase in enhancement can be obtained almost independently of non-radiative loss, by optimizing the geometry of the system.

The plasmonic resonance condition can change with the dielectric constant of the environment surrounding the nanostructures. For instance, it is possible to tune the enhancement and/or the resonance wavelength of the nanostructure by changing the surrounding coating material and its corresponding dielectric. For example, an enhancement control variable may be the inclusion of a coating on the metal nanoparticles that has a different dielectric constant than the metal, for example, silicon dioxide. The nanostructures may be embedded in a polymer or other type of material to provide support to the nanostructure. This can permit another aspect of dielectric tuning. The frequency and/or intensity of the plasmonic resonance are known to be sensitive to the dielectric properties of the surrounding medium. For example, the plasmonic resonance can be sensitive to the refractive index of matter close to the nanostructure surface.

In FIG. 3, exemplary effects of a dielectric constant on the field enhancement of the nanostructure are shown, as well as the corresponding metal non-radiative lifetime. Graphs 310, 320, and 330 show intensity enhancements in the structure for the surrounding environments of air, glass, and water, respectively. As shown, when the dielectric constant of a material surrounding the nanostructure increases, the field enhancement correspondingly increases. The wavelength is also red-shifted as the dielectric constant of the surrounding material increases.

Additionally graphs 311, 321, and 331 of FIG. 3 represent the metal non-radiative lifetime corresponding to the non-radiative recombination rate for air, glass, and water, respectively. The non-radiative lifetime is shown as decreasing as the dielectric constant of the surrounding material increases. FIG. 3 shows that as the enhancement increases by an order of magnitude between air and glass, the non-radiative decay rate decreases only by a factor of 2. As enhancement grows, the photocatalytic nanoparticle non-radiative recombination rate into the metal grows as well, but much less than the actual enhancement, making dielectric shifting an efficient enhancement tuning method. Not only does the surrounding material provide a means for fine-tuning the enhancement strength and peak wave length, it can also provide support for the nanostructure.

Embodiments can include a nanoparticle having non-linear optical properties placed in a location of the focused high field (i.e., an enhanced field part of a nanostructure). Such nanoparticle materials may exhibit second or third order non-linearity. The optical properties of these non-linear nanoparticle materials can change as a function of a field in which the nanoparticle is placed. For example, use of non-linear materials for the nanoparticles in these structures would permit optic behavior that would normally occur at a high intensity to occur at lower (e.g., order of magnitude lower) intensity. This can substantially increase the potential applications for such a structure, such as more efficient optical switching

A self-assembly method for creating this nanostructure can permit control to be retained over the number of layers, particle material composition, size, shape and/or overall development of the structure. This method can permit the structure to have a self-guided assembly that yields a high volume of product due to the selective nature of self-assembly chemistry (i.e. because each particle can selectively bind to another kind of particle, there can be a high yield of the desired product). This can be carried out by using organic linkers that can attach to another kind of linkers but not to themselves.

The first linkers that are attached about and/or to the photocatalytic nanoparticle are linkers that can favorably bond to a photocatalytic particle at one end and have a terminal functional group at the other end that favorably bonds to a second kind of linkers. The second kind of linkers can be selected to favorably bond to the metal nanoparticles at one end and include a functional group at the other end that will favorably bond to the first kind of linkers. These linkers can act analogously to magnets. Just as magnets have two distinct poles which only connect north to south, these linkers attach only at certain ends, and not others, and use these attachment points to connect together in a manner similar to stacking blocks.

In general, exemplary linkers will be of the form H₂N—R—COOH, where R is an organic linker of any desired length. The H₂N (amine functional group) and the COOH (carboxylic acid functional group) are two exemplary terminal groups for the different types of linkers. This can permit the linkers to connect by peptide bonding (i.e., the bonding mechanism of amino acids), where the amine functional groups can, for example, bond to the carboxylic acid functional groups. In an exemplary embodiment, to form the first tier of metal nanoparticles around a photocatalytic nanoparticle, the photocatalytic particle can be coated with a plurality of first linkers having one of the two exemplary terminal groups. A plurality of first metal nanoparticles can be coated with a plurality of second linkers having the other of the two exemplary terminal groups. When these two types of nanoparticles are combined, the amine end of each of the linkers can combine with the carboxylic acid end of each of the linkers. This permits the nanoparticles to be linked in a way that can prevent the first tier metal nanoparticles from attaching to each other. To add an additional tier of metal nanoparticles, the first tier of metal nanoparticles is coated with a plurality of third linkers similar to or the same as the first linkers and a second tier of metal nanoparticles is coated with a plurality of fourth linkers similar to or the same as the second linkers. These linkers can be connected in the manner described above. In some embodiments, the nanostructure possesses two tiers; however, additional tiers of metal nanoparticles are possible and can be added in a manner similar to the first tier or in some other manner.

FIGS. 4A and 4B are diagrams showing an exemplary method for assembling (i.e., fabricating) a nanostructure. In FIG. 4A, photocatalytic nanoparticle 410 and metal nanoparticle 420 are shown. The photocatalytic nanoparticle has an organic linker 401 of any desired length attached to the nanoparticle, and the metal nanoparticle has a similar organic linker 402 of any desired length attached to the nanoparticle. Each of these linkers has a preferred terminal group attached to the other end.

For example, in FIG. 4A, a carboxylic acid functional group 403 is attached to the terminating end of the linker 401 connected to the photocatalytic nanoparticle and an amine functional group 404 is attached to the terminating end of the linker 402 connected to the metal nanoparticle.

In FIG. 4B, the amine and carboxylic acid functional groups of FIG. 4A have reformed to arrange the nitrogen of the amine group and the doubly bonded oxygen of the carboxylic acid group into group 405, effectively joining photocatalytic nanoparticle 410 and metal nanoparticle 420. In exemplary embodiments, the functional groups used for linking nanoparticles are amines and carboxylic acids. However, other suitable functional groups or chemical moieties can be used for connected different kinds of nanoparticles.

Because of the unique optical properties that can result from the interactions between the photocatalytic nanoparticle and the feedback structure, and the three-dimensional confinement these nanostructure assemblies are able to achieve, there is great potential for novel applications for these structures. Unlike a two-dimensional surface where a field can involve a certain polarization, where light entry can be limited to a certain direction, and where the amount of field inside can depend strongly on the direction (e.g., nanorods and nanowires that confine light only in two dimensions), the three-dimensional (3-D) aspect disclosed herein is substantially directionally independent. The 3-D structures described herein can confine light propagating from almost any direction, resulting in a capacity for a greatly enhanced localized electric field.

For example, the enhanced electric field created by a super-structure arranged around some light-emitting nanoparticle, such as a quantum dot or other photocatalytic nanoparticle, can be used to stimulate an increase in emission from that light-emitting nanoparticle. The 3-D super-structure can alternatively be arranged around a non-linear material, a magnetic material or even a molecule of heavy water, for the purpose of confining light and focusing the electromagnetic energy from all directions into localized spot. Additionally, while 3-D confinement is present in certain existing applications such as photonic band gap crystals, these crystals can include many defects, and their growth and resulting form can be difficult to control. To generate enhancement, the crystals are thousands of layers thick. As described herein, a means of generating 3-D confinement can be achieved using several layers of nanoparticles.

It will be apparent to those skilled in the art that various changes and modifications can be made in the method and system for accumulating and presenting device capability information of the present invention without departing from the spirit and scope thereof. Thus, it is intended that the invention cover the modifications of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

1. A nanostructure comprising:

a photocatalytic nanoparticle;

a first tier of metal nanoparticles, each metal nanoparticle of the first tier being linked about the photocatalytic nanoparticle; and

a second tier of metal nanoparticles, each metal nanoparticle of the second tier being linked to one of the metal nanoparticles of the first tier and located a distance from the photocatalytic nanoparticle greater than a distance between a metal nanoparticle of the first tier and the photocatalytic nanoparticle.

2. The nanostructure according to claim 1, comprising:

at least one first linker attached to the photocatalytic nanoparticle;

a second linker attached to each metal nanoparticle of the first tier;

a third linker attached to each metal nanoparticle of the first tier; and

a fourth linker attached to second metal nanoparticles of the second tier, wherein like ones of the first through fourth linkers do not attach to one another.

3. The nanostructure according to claim 1, wherein the photocatalytic nanoparticle is semiconductor material.

4. The nanostructure according to claim 3, wherein the photocatalytic nanoparticle is a quantum dot.

5. The nanostructure according to claim 3, wherein the semiconductor material comprises a heterojunction.

6. The nanostructure according to claim 1, wherein the metal nanoparticles are one of gold, silver, aluminum, copper, titanium, and chromium.

7. The nanostructure according to claim 1, wherein the nanostructure is located in a dielectric medium.

8. The nanostructure according claim 7, wherein the medium shifts the resonant wavelength of the nanostructure.

9. The nanostructure according to claim 7, wherein the dielectric constant of the dielectric medium is adjustable.

10. The nanostructure according to claim 1, wherein metal nanoparticles of the first and second tiers are coated with a material having a dielectric constant different from a dielectric constant of the metal nanoparticles.

11. The nanostructure according to claim 2 where the first through fourth linkers are alkane linkers.

12. The nanostructure according to claim 2 where the first through fourth linkers are polyethylene glycol (PEG) linkers.

13. The nanostructure according to claim 1, wherein the size of metal nanoparticles of the first group is different from the size of the metal nanoparticles of the second tier.

14. The nanostructure according to claim 13, wherein metal nanoparticles of the second tier are larger than metal nanoparticles of the first tier.

15. A method for assembling a nanostructure, comprising:

attaching a first linker to a photocatalytic nanoparticle;

attaching a second linker about a first metal nanoparticle;

attaching the first metal nanoparticle about the photocatalytic nanoparticle by connecting the first linker and the second linker;

attaching a third linker to the first metal nanoparticle;

attaching a fourth linker to a second metal nanoparticle; and

attaching the second metal nanoparticle to the first metal nanoparticle by connecting the third linker and the fourth linker.

16. The method according to claim 15, wherein like ones of the first through fourth linkers do not attach to one another.

17. The method according to claim 15, wherein the metal nanoparticles are one of gold, silver, aluminum, copper, titanium, and chromium.

18. The method according to claim 15, wherein the first, second, third, and fourth linkers are each of a same length.

19. The method according to claim 15, wherein the first and third linkers have one end terminating in a carboxylic acid group.

20. The method according to claim 15, wherein the second and fourth linkers have one end terminating in an amine group.