Plasmonic Device

Abstract

A plasmonic substrate fabrication method is formed using sacrificial nanostructures or microstructures of removable materials printed onto a substrate, and subsequent deposition or growth of a material such as metal or graphene. After the sacrificial structures are removed, plasmonic hotspots of nanoscale or microscale dimension and geometry are obtained on the substrate, enabling various sensing and detection of analytes based on plasmonic techniques.

Claims

1. A method of fabricating a plasmonic device comprising: fabricating structures that are nanostructures or microstructures having a feature size of from 1 to 1000 nm on a substrate; forming a layer of a plasmonic material on the substrate; and removing the structures to form a pattern having first and second plasmonic material portions spaced apart from each other and having a gap therebetween of less than 100 nm, so as to produce a plasmonic device having a plasmonic hotspot at said gap.

2. The method of fabricating a plasmonic device of claim 1, wherein the fabricating of the structures comprises printing the structures on the substrate.

3. The method of fabricating a plasmonic device of claim 2, wherein the printing of the structures comprises formation of a three dimensional object layer by layer using a digital file.

4. The method of fabricating a plasmonic device of claim 2, wherein after printing the structures on the substrate, wherein the layer of plasmonic material is substantially all metal that forms a metal layer on the substrate and on the structures, followed by removal of the printed structures.

5. The method of fabricating a plasmonic device of claim 4, wherein the metal and gap for a plasmonic hotspot are capable of exhibiting a plasmonic effect when irradiated with electromagnetic radiation.

6. The method of fabricating a plasmonic device of claim 5, wherein the metal comprises Au, Ag and/or Cu.

7. The method of fabricating a plasmonic device of claim 1, wherein a supporting layer is deposited on the substrate before and/or after fabricating the structures.

8. The method of fabricating a plasmonic device of claim 1, wherein the structures are removed by heat, chemical treatment and/or physical treatment.

9. The method of fabricating a plasmonic device of claim 8, wherein the structures are formed of an organic or hybrid organic-inorganic photoresist material, and the structures are removed with a solvent capable of dissolving the photoresist material.

10. The method of fabricating a plasmonic device of claim 1, wherein the structures are formed of PMMA and are removed with acetone.

11. The method of fabricating a plasmonic device of claim 1, wherein the fabricating of the structures is via 3D printing, nano-imprinting, dip-pen lithography, or laser writing, and forms the structures with an aspect ratio (height to width ratio) of at least 2.

12. The method of fabricating a plasmonic device of claim 11, wherein the aspect ratio is at least 7.

13. The method of fabricating a plasmonic device of claim 12, wherein the aspect ratio is at least 15.

14. The method of fabricating a plasmonic device of claim 4, wherein a thickness of the metal layer deposited is less than 80% of the height of the structures.

15. The method of fabricating a plasmonic device of claim 14, wherein the thickness of the metal layer is less than 60% of the height of the structures.

16. The method of fabricating a plasmonic device of claim 1, wherein the forming of the layer of a plasmonic material on the substrate comprises growing a graphene layer on the substrate.

17. The method of fabricating a plasmonic device of claim 16, wherein the growing of graphene comprises depositing on the substrate a layer capable of facilitating or catalyzing graphene growth, followed by growing graphene thereon.

18. The method of fabricating a plasmonic device of claim 1, wherein the plasmonic material is a doped metal oxide selected from doped tin oxide, doped zinc oxide, doped cadmium oxide and doped titanium oxide.

19. The method of fabricating a plasmonic device of claim 1, wherein the plasmonic material is a copper deficient chalcogenide or an oxygen deficient transition metal oxide.

20. The method of fabricating a plasmonic device of claim 16, wherein the substrate is a metal capable of facilitating and/or catalyzing graphene, or a metal layer is formed on the substrate which metal layer is capable of facilitating and/or catalyzing graphene.

21. The method of fabricating a plasmonic device of claim 1, wherein a supporting layer is formed on the substrate prior to forming the structures, followed by removal of the structures and removal of the substrate.

22. The method of fabricating a plasmonic device of claim 21, wherein the structures and substrate are removed at the same time with the same chemical, physical or thermal removal process.

23-36. (canceled)

37. The method of fabricating a plasmonic device of claim 1, wherein the substrate comprises a light transmissive material such that the plasmonic device formed is capable of transmitting at least 90% of the light incident thereon in the infrared, visible or UV spectrum.

38-40. (canceled)

41. A plasmonic device made from the method of claim 1.

42. A device for testing toxins, comprising: the plasmonic device of claim 41, a light source; an optical detector for detecting light transmitted or reflected or refracted through the plasmonic device; and an optical analyzer for collecting the transmitted or reflected light through the plasmonic device.

43-49. (canceled)

50. A method for testing a sample for the presence of toxins, comprising: providing the plasmonic device of claim 41, providing a sample or analyte of interest on the plasmonic device; directing a light beam onto the plasmonic device; detecting the light beam after passing through the plasmonic device; analyzing the light beam for changes; and determining whether a toxin is present in the sample based on any changes detected.

51-53. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The descriptions that follow will be further understood when read with the appended drawings. There are shown in the drawings exemplary embodiments of the disclosed technology for illustration purpose. The invention is not limited to the specific methods, compositions, and devices disclosed. Further, the drawings are not necessarily drawn to scale or proportion.

[0019] FIGS. 1A, 1B, 1C, and 1D are schematic drawings illustrating the fabrication process of a metal-based or semiconductor-based plasmonic device.

[0020] FIG. 2 is a flow chart showing a fabrication process of plasmonic metal-based or semiconductor-based plasmonic device.

[0021] FIG. 3A is a schematic drawing illustrating a top view of the metal-based or semiconductor-based plasmonic device before removal of removable nanostructures, FIG. 3B is a schematic drawing illustrating a top view of the metal-based, or semiconductor-based plasmonic device before removal of removable nanostructures, FIG. 3C is a schematic drawing illustrating one of the hotspot features created, and FIG. 3D is a schematic drawing illustrating the magnified top view of one of the hotspot with a gap distance w.

[0022] FIG. 4A illustrates a metal-based or semiconductor-based plasmonic device before removal of removable nanostructures, FIG. 4B illustrates a top view of a part of device including one double-cylindrical cavity after removal of the nanostructures, and FIGS. 4C, 4D, and 4E illustrate cross-sectional views at different positions across the double-cylindrical nanostructure after each step of fabrication.

[0023] FIGS. 5A-5F illustrate examples of removable nanostructures (top view) which can lead to plasmonic hotspots (denoted with *), where FIG. 5A illustrates one hotspot from a bi-circular pattern, FIG. 5B illustrates three hotspots from a tri-circular pattern, FIG. 5C illustrates five hotspots from a tetra-circular pattern, FIG. 5D illustrates one hotspot from two opposite triangles, FIG. 5E illustrates one hotspot from two squares, and FIG. 5F illustrates four hotspots from a cluster of four squares.

[0024] FIGS. 6A, 6B, 6C, and 6D are schematic drawings showing the fabrication process of a graphene-based plasmonic device.

[0025] FIG. 7 is a flow chart showing the entire fabrication process of plasmonic graphene-based plasmonic device.

[0026] FIG. 8A is a schematic drawing illustrating a top view on the graphene-based plasmonic substrate after removal of removable nanostructures, FIG. 8B is a schematic drawing illustrating one of the hotspot features created, and FIG. 8C is a schematic drawing illustrating the magnified top view of one of the hotspots with a gap distance w.

[0027] FIG. 9A illustrates a graphene-based plasmonic device before removal of removable nanostructures, FIG. 9B illustrates a top view of a part of device including one double-cylindrical cavity after removal of nanostructure, and FIGS. 9C, 9D, and 9E illustrate cross-sectional views at different positions across the double-cylindrical nanostructure after each step of fabrication.

[0028] FIGS. 10A, 10B, 10C, and 10D are schematic drawings illustrating the fabrication process of a plasmonic test filter for collection and detection of analyte in air or solution.

[0029] FIG. 11 is a scanning electron micrograph (SEM) image showing the photoresist nanopillars prepared by 3D printing.

DETAILED DESCRIPTION

[0030] A method for fabricating a plasmonic device with controllable nanostructured features is disclosed, enabling effective plasmonic effects for detectors and sensors such as chemo- and/or biosensing devices. The following detailed description with reference to the drawings illustrates the spirit and essence of the disclosed technique. The illustrative embodiments and examples in the description, drawing, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without department from the spirit of the subject matter presented here.

[0031] The term substrate may denote a substrate itself, or a stack structure including a substrate and predetermined layers or films formed on a surface of the substrate. In addition, the term surface of a substrate may denote an exposed surface of the substrate itself, or an external surface of a predetermined layer or a film formed on the substrate.

[0032] It will be further understood that the terms comprises and/or comprising, or includes and/or including when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.

[0033] As used herein, the singular forms a, an and the are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term and/or includes any and all combinations of one or more of the associated listed items and may be abbreviated as /.

[0034] It will be understood that when an element is referred to as being connected or coupled to or on another element, it can be directly connected or coupled to or on the other element or intervening elements may be present. In contrast, when an element is referred to as being directly connected or directly coupled to another element, or as contacting or in contact with another element (or using any form of the word contact), there are no intervening elements present at the point of contact.

[0035] Embodiments described herein will be described referring to plan views and/or cross-sectional views by way of ideal schematic views. Accordingly, the exemplary views may be modified depending on manufacturing technologies and/or tolerances. Therefore, the disclosed embodiments are not limited to those shown in the views, but include modifications in configuration formed on the basis of manufacturing processes. Therefore, regions exemplified in figures may have schematic properties, and shapes of regions shown in figures may exemplify specific shapes of regions of elements to which aspects of the invention are not limited.

[0036] Spatially relative terms, such as beneath, below, lower, above, upper, top, bottom, and the like, may be used herein for ease of description to describe one element's or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as below or beneath other elements or features would then be oriented above the other elements or features. Thus, the term below can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. Items described in the singular herein may be provided in plural, as can be seen, for example, in the drawings. Thus, the description of a single item that is provided in plural should be understood to be applicable to the remaining plurality of items unless context indicates otherwise.

[0037] When a layer or area is referred to as being on (or formed on or deposited on etc.) another layer or area, it may be directly on the other layer or area, or intervening layers or areas may be present therebetween. Conversely, when a layer or area is referred to as being directly on (or formed directly on or deposited directly on etc.) another layer or area, intervening layers or areas are absent therebetween.

[0038] In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Like numbers refer to like elements throughout. Though the different figures show variations of exemplary embodiments, these figures are not necessarily intended to be mutually exclusive from each other. Rather, as will be seen from the context of the detailed description below, certain features depicted and described in different figures can be combined with other features from other figures to result in various embodiments, when taking the figures and their description as a whole into consideration.

[0039] In an example, a metal-based plasmonic device fabrication method and related applications are disclosed. Sacrificial nanostructures of removable materials are formed on a rigid or flexible substrate. In case of a metal-based plasmonic device, a layer of supporting material is deposited, which can be a rigid (e.g., metal, oxide, nitride), flexible (e.g., polydimethyl siloxane), or elastic material (e.g., styrene-butadiene copolymer) depending on whether a rigid, flexible, or elastic plasmonic device is desired. Then, a layer of plasmonic metal (e.g., Au, Ag) is then deposited. This overlayer can be deposited by chemical vapor deposition (CVD), atomic layer deposition (ALD) and physical vapor deposition (PVD).

[0040] In another example, a graphene-based plasmonic device fabrication method and related applications are disclosed. Sacrificial nanostructures of removable materials are formed on a rigid or flexible substrate that can facilitate or catalyze graphene synthesis, for instance, Ni or Cu. If another substrate is used, a layer of such material that can facilitate or catalyze graphene synthesis is first deposited after printing the sacrificial nanostructures. Graphene is then synthesized on the substrate surface through CVD methods. Depending on the nature of the substrate and processing condition, a monolayer and/or few-layer graphene can be prepared. Then, a layer of supporting material is deposited onto the graphene, which can be a rigid substrate (e.g., oxide, nitride), a flexible substrate (e.g., polydimethyl siloxane), or substrate made from an elastic material (e.g., styrene-butadiene copolymer) depending on whether a rigid, flexible, or elastic plasmonic device is desired.

[0041] The rigid substrate for either metal-based or graphene-based plasmonic device can be a transmissive substrate (e.g., glass, quartz, sapphire) or an opaque substrate (e.g., metal, polymer, ceramic), depending on considerations such as whether light illumination or light transmission is to be used for the specific application or device configuration.

[0042] Further, the rigid substrate for either metal-based or graphene-based plasmonic device can be optionally removed, depending on whether fluid or gas is required to pass through the plasmonic device in the specific application or device configuration.

[0043] The supporting layer for either a metal-based or graphene-based plasmonic device can be metal, non-metal, organic, inorganic and biomaterial, or a combination of these materials.

[0044] In one configuration, the material forming the nanostructures is a removable material, which may be removed by a solvent, by heat, chemical treatment, physical treatment, or a combination of these techniques. The shape and morphology of the nanostructures are such that plasmonic hotspots can form after removal of the nanostructures when electromagnetic energy is incident in that area during operation of the device. A plasmonic hotspot can be a region, such as a small gap, where an intense electric field and thus plasmonic effect can be produced under light illumination.

[0045] FIGS. 5A-5F illustrate examples of removable nanostructures (top view) which can lead to plasmonic hotspots (denoted with *), where FIG. 5A illustrates one hotspot from a bi-circular pattern, FIG. 5B illustrates three hotspots from a tri-circular pattern, FIG. 5C illustrates five hotspots from a tetra-circular pattern, FIG. 5D illustrates one hotspot from two opposite triangles, FIG. 5E illustrates one hotspot from two squares, and FIG. 5F illustrates four hotspots from a cluster of four squares.

[0046] The removable nanostructures can be deposited such as by printing, e.g. using 3D printing, nano-imprint, dip-pen lithography, laser writing or other suitable printing technique. The printing technique used can be any technique capable of printing high-aspect ratio nanostructures and may have high aspect-ratio up to a height-to-width ratio of 20.

Fabrication Process of Metal-Based and Semiconductor-Based Plasmonic Device

[0047] FIGS. 1A-1D are schematic drawings illustrating a basic fabrication process of a metal-based plasmonic device on a starting substrate 100. FIG. 2 is a flow chart showing a fabrication process of plasmonic metal-based or semiconductor-based plasmonic device. As can be seen in FIG. 1A, a plurality of nanostructures 101 are formed on substrate 100. Then, a supporting layer 102 is deposited (FIG. 1B). Nanoscale structure 101 can be 3D-printed or fabricated using lithographic techniques or other nanofabrication techniques. If printing such as 3D-printing is used, a process to form the nanostructures 101 can be a printing process that leads to formation of a three-dimensional object layer-by-layer using a digital file (e.g. a 3D computer aided design (CAD) file). For example, the 3D-printing can be an additive process whereby layers of material are built up to create a 3D structure.

[0048] The double cylindrical nanostructure 101 is for exemplary purposes only, and other shapes, layouts, dimensions etc. can be used. Compared with lithographic techniques, 3D-printing may have advantages in the disclosed technique because of its ease and lower cost of operation. It also offers a higher degree of freedom and flexibility in designing the nanoscale features. Advantageously, 3D-printing achieves high-aspect ratio nanoscale structures, which is difficult with conventional lithographic techniques. However, other deposition methods can be utilized as mentioned previously.

[0049] Advantageously, nanostructures 101 may be made of dissolvable material that can removed by solvent, by heat, or other appropriate treatment. For example, the material can be poly methyl methacrylate (PMMA) that can be removed by acetone. For some materials used for layer 101, heat or UV curing may be desired to obtain nanostructures that can better survive subsequent processes.

[0050] The supporting layer 102 deposited onto the structure in FIG. 1B can be made of a rigid (e.g., oxide, nitride), flexible (e.g., polydimethyl siloxane or other siloxane or silicone based material), or elastic material (e.g., styrene-butadiene copolymer) depending on whether a rigid, flexible, or elastic plasmonic device is desired.

[0051] A sacrificial interlayer also made of a dissolvable material can be added between layers 100 and 102 to assist in the removal of layer 102 from substrate 100. This sacrificial interlayer can be of the same material as for 101, or another removable material that can be easily removed by solvent, by heat, or other appropriate treatment.

[0052] A plasmonic material layer 103 is then deposited onto the supporting layer 102 as illustrated in FIG. 1C. Examples of plasmonic metal are Au, Ag, Cu, Al, Mg, Pt, and Rh, which have been widely explored both experimentally and theoretically. Non-metal plasmonic materials include extrinsically doped oxides and semiconductors such as Sn-doped In.sub.2O.sub.3 (ITO), Al-doped ZnO (AZO), Sb-doped SnO.sub.2, In-doped CdO, and Ne-doped TiO.sub.2; and self-doped oxides and semiconductors such as copper-deficient chalcogenides (Cu.sub.2-xE, E=S, Te, Se), MoS.sub.2-x and oxygen-deficient transition metal oxides, including TiO.sub.2-x, WO.sub.3-x, MoO.sub.3-x, and ZnO.

[0053] Nanostructures 101 are then dissolved or removed by one or more of a solvent, heat, chemical treatment, and physical treatment.

[0054] In another example, the substrate 100 is not separated from supporting layer 102 (and no sacrificial layer is deposited between substrate 100 and supporting layer 102). In such a case, after dissolving nanostructures 101, a metal-based plasmonic device on a substrate 100 is obtained with tailor-designed hotspots exhibiting plasmonic effects for various applications.

[0055] If the example with a sacrificial layer between layers 100 and 102, the sacrificial layer will dissolve together with the removal of nanostructures 101, so as to remove substrate 100, such that a free-standing metal-based plasmonic device in membrane form with thru holes or cavities is obtained as it is released from the substrate 100.

[0056] Regardless of the presence of a sacrificial layer between 100 and 102, if the substrate 100 is made of removable material and can be removed together with the nanostructures 101 (and the sacrificial layer between 100 and 102), a free-standing metal-based plasmonic device in membrane form with thru holes or cavities can also be obtained.

[0057] Specific to the double-cylindrical nanostructures used in this description, the plasmonic hotspots formed are the nanoscale metal gaps created after removal of nanostructures 101 (FIGS. 3C and 3D), where w is the gap distance that is critical to the strength of the plasmonic effect exhibited upon light illumination, with w ranges from sub-nanometer to 100 nm.

[0058] To facilitate better visualization of the formation of gap w, FIG. 4A shows the metal-based plasmonic device before removal of substrate 100 and nanostructures 101. FIG. 4B shows the device after their removal.

[0059] FIGS. 4C, 4D and 4E shows the cross-sectional views along Plane X, Plane Y and Plane Z as denoted in FIG. 4B. For example, w can be selected to be within the range of from 10 to 85 nm, or w can be selected to be less than 10 nm (e.g. from 1 to 10 nm). Alternatively, w can be less than 1 nm, e.g. from 0.2 to 0.9 nm. As such, the gap distance w of metal-based plasmonic hotspot is tunable and controllable by adjusting the shape, morphology, and arrangement of the nanostructures 101 printed or deposited. In addition, the density and position of hotspots can be accurately and precisely controlled and arranged.

Fabrication Process of Graphene-Based Plasmonic Device

[0060] As an alternative to the metal-based and semiconductor-based materials mentioned above, 2D materials can also exhibit plasmonic effects, for instance, graphene, graphene oxides, hexagonal boron nitride, pnictogens, and MXenes (e.g. 2D plasmonic materials that include nitrides, carbides or carbonitrides of early transition metals, including MXenes with a general formula M.sub.n+1X.sub.nT.sub.x, where M refers to an early transition metal element, X is nitrogen or carbon; and T corresponds to e.g. O, F, or OH surface termination).

[0061] FIGS. 6A, 6B, 6C, and 6D are schematic drawings illustrating a basic fabrication process of a graphene-based plasmonic device. FIG. 7 is a flow chart showing the entire fabrication process of plasmonic graphene-based plasmonic device. As can be seen in FIG. 6A, on a starting substrate 200 nanoscale structures 201 can be 3D-printed or fabricated using lithographic techniques or other nanofabrication techniques. Advantageously, nanostructures 201 may be made of a dissolvable material that can removed by solvent, by heat, or other appropriate treatment. For example, poly methyl methacrylate (PMMA) can be used that can be removed by acetone. For some materials used for nanoscale structures 201, heat or UV curing may be desirable to obtain nanostructures that can better withstand subsequent processes.

[0062] The double cylindrical nanostructure 201 is for exemplary purposes only, and other shapes, layouts, dimensions etc. can be used. Compared with lithographic techniques, 3D-printing may have advantages in the disclosed technique because of its ease and lower cost of operation. It also offers a higher degree of freedom and flexibility in designing the nanoscale features. Advantageously, 3D-printing achieves high-aspect ratio nanoscale structures, which is difficult with conventional lithographic techniques. Other deposition methods can be utilized as mentioned previously.

[0063] The starting substrate 200 serves as a support for subsequent preparation of nanostructures 201, and to facilitate or catalyze graphene synthesis. Preferably, it is made of Ni, Cu or a transition metal or other material that can facilitate or catalyze graphene synthesis, which can be amorphous, polycrystalline, or single-crystalline form.

[0064] If a starting substrate other than a material capable of facilitating or catalyzing graphene is used, a layer of such a material that can facilitate or catalyze graphene synthesis is first deposited before subsequent graphene synthesis.

[0065] Graphene layer 202 is then synthesized on the substrate surface through CVD methods (FIG. 6B). Depending on the nature of the substrate and processing condition, monolayer and/or few-layer graphene can be prepared so that different CVD methods can used.

[0066] Another overlayer 203, as a supporting layer, is further deposited onto the graphene layer 202 as illustrated in FIG. 6C, which can be made of a rigid (e.g., oxide, nitride), flexible (e.g., polydimethyl siloxane or other siloxane or silicone based material), or elastic material (e.g., styrene-butadiene copolymer) depending on whether a rigid, flexible, or elastic plasmonic device is desired, and which can be also made of metal, non-metal, organic, inorganic and biomaterial, or a combination of these materials.

[0067] Nanostructures 201 are then dissolved or removed by one or more of a solvent, heat, chemical treatment, and physical treatment.

[0068] After removal of nanostructures 201, the graphene containing multilayer (with graphene sandwiched between overlayer 203 above and starting substrate 200 beneath) obtained is then subjected to metal etching treatment to completely remove substrate 200, or the overlayer deposited before graphene synthesis, if any. For example, if the substrate 200 is Cu that is a common substrate for CVD graphene, the Cu substrate can be removed with 0.05 g/mL aqueous solution of Fe(NO.sub.3).sub.3, followed by copious rinsing with deionized (DI) water.

[0069] After removing nanostructures 201 and substrate 200, the released nanoporous membrane is flipped over, such that a graphene-based plasmonic device can be used and having tailor-designed hotspots exhibiting plasmonic effects for various application (FIG. 8A).

[0070] Specific to the double-cylindrical nanostructures used in this description, the plasmonic hotspots formed are the nanoscale gaps within the graphene layer (FIGS. 8B and 8C), where w is the gap distance that is critical to the strength of the plasmonic effect exhibited upon light illumination.

[0071] To facilitate better visualization of the formation of gap w, FIG. 9A shows the graphene-based plasmonic device before removal of substrate 200 and nanostructures 201. FIG. 9B shows the device after their removal.

[0072] FIGS. 9C, 9D and 9E are cross-sectional views (taken along Plane X, Plane Y and Plane Z, respectively, as denoted in FIG. 9B) of the method of forming the plasmonic device.

[0073] The gap distance w of the graphene-based plasmonic hotspot is tunable and controllable by adjusting the shape, morphology, and/or arrangement of the nanostructures 201 printed or deposited, with w ranging, as in the metal and semiconductor based examples above, from sub-nanometer to 100 nm. In addition, the density and position of hotspots can also be accurately and precisely controlled and arranged.

A. Plasmonic Test Device for Food Toxins Detection

[0074] Plasmonics is a unique field in nanophotonics, in which light energy is confined to a nm-scale oscillating field of free electrons, known as a surface plasmon, with the use of nanostructures or nanoarchitectures offering nanosized region (such as nanogaps, nanoparticular cluster, nanostructure array) for strong coupling of light and the collective oscillation of free electrons at the metal-dielectric interface. Such coupling enables significant enhancement and localization of resonance field and amplification of optical response.

[0075] For example, the Raman scattering signals from molecules within the nanogap can be much enhanced, resulting in surface-enhanced Raman scattering (SERS). Raman scattering is the inelastic scattering of photons by matter, leading to both an exchange of energy and a change in the direction of light. While different molecules have their characteristic scattering modes and patterns, giving their unique fingerprint Raman spectra, Raman scattering can be used for molecular identification. When a molecule falls within a hotspot, for example a nanogap of plasmonic material, plasmonic effect exhibited due to light confinement within the nanoscale space can enhance the Raman scattering signals (i.e., SERS), which can significantly facilitate chemical analysis and identification in molecular level.

[0076] Another example is the enhanced (or extraordinary) optical transmission brought by surface plasmons (SP-EOT) from the coupling between light and a nanopore array. The periodic array of nanopores acts as a diffraction grating and convert incident light into surface plasmonic waves at resonance wavelengths, thereby creating a series of intense peaks in the optical transmission spectra. Attachment or adsorption of analyte molecule or species on the nanopore array can change the optical transmission characteristics.

[0077] The size, shape, arrangement, and composition of the nanostructures can also be designed and engineered to control the properties of the plasmons, e.g., resonance frequency and localization, thus enabling different applications in various field.

[0078] A plasmonic device such as disclosed above can be used, for example, to wipe a food sample surface (e.g., fruit, vegetable) to collect chemicals and substances on the surface, which would be transferred and attached onto the plasmonic device with hotspots after wiping. The device is then subjected to analysis based on plasmonic effects, such as SERS or SP-EOT for rapid identification of target toxin molecules, and even quantitative analysis if appropriate calibration is performed.

[0079] Food toxins detection can also be achieved by extracting sample fluid from the food sample by pressing, grinding, squeezing, or any other suitable method. The collected sample fluid is then dropped onto the plasmonic device for subsequent analysis based on plasmonic effects, such as SERS or SP-EOT for rapid identification of target toxin molecules, and even quantitative analysis if appropriate calibration is performed.

B. Plasmonic Test Filter for Collection and Detection of Analyte in Air or Solution

[0080] A plasmonic device as disclosed above can also be provided in the form of a nanopore array membrane, it can be used to collect analyte by allowing a gas or liquid to pass through the plasmonic nanopore array membrane. The analyte molecules trapped in the cavities, particularly those within the hotspots, can be analyzed by analysis based on plasmonic effects, such as SERS or SP-EOT for rapid identification of target analytes.

[0081] FIGS. 10A-10D are schematic drawings illustrating a basic fabrication process of a metal-based plasmonic device. As seen in FIG. 10C, on a starting substrate 300, nanopillars 301 can be 3D-printed or fabricated using lithographic techniques or other nanofabrication techniques. 3D-printing has the benefit of ease and lower cost of operation, as well as a high degree of freedom and flexibility in designing the nanoscale features. Advantageously, 3D-printing achieves high-aspect ratio nanoscale structures, which is difficult with conventional lithographic techniques. However, other deposition methods can be utilized as mentioned previously.

[0082] Advantageously, nanopillars 301 may be made of dissolvable material that can removed by solvent, heat, or other appropriate treatment. For example, if poly methyl methacrylate (PMMA) is used, it can be removed e.g. by acetone. For some materials used for layer 301, heat or UV curing may be desirable to obtain nanopillars that can survive subsequent processes.

[0083] A supporting layer 302 is deposited (FIG. 10B), which can be made of a rigid (e.g., metal, oxide, nitride), flexible (e.g., polydimethyl siloxane), or elastic material (e.g., styrene-butadiene copolymer) depending on whether a rigid, flexible, or elastic plasmonic device is desired.

[0084] A sacrificial interlayer made of the same dissolvable material is added between layers 300 and 302 if supporting layer 302 cannot be easily detached from starting substrate 300. This sacrificial interlayer can be of the same material as for 301, or other removable material that can be easily removed by solvent, by heat, or other appropriate treatment.

[0085] A plasmonic material layer 303 is then deposited onto the supporting layer 302 (FIG. 10B). Examples of plasmonic metals are Au, Ag, Cu, Al, Mg, Pt, and Rh, and examples of non-metal plasmonic materials include extrinsically doped oxides and semiconductors such as Sn-doped In.sub.2O.sub.3 (ITO), Al-doped ZnO (AZO), Sb-doped SnO.sub.2, In-doped CdO, and Ne-doped TiO.sub.2; and self-doped oxides and semiconductors such as copper-deficient chalcogenides (Cu.sub.2-xE, E=S, Te, Se), MoS.sub.2-x and oxygen-deficient transition metal oxides, including TiO.sub.2-x, WO.sub.3-x, MoO.sub.3-x, and ZnO.

[0086] Finally, nanopillars 301 (and the sacrificial layer between layers 300 and 302, if present) are then dissolved or removed by one or more of a solvent, heat, chemical treatment, and physical treatment. After this final treatment, a free-standing plasmonic nanopore array membrane is obtained. The dimension of the 3D-printed or deposited nanopillars would dictate the dimension and morphology of the nanocavity to be constructed. For example, if 500 nm wide circular nanopillars are 3D-printed, cylindrical nanochannels with a diameter of around 500 nm are formed in the plasmonic nanoporous membrane. As such, there is extremely high manufacturing flexibility and control of the dimension of the nanocavity required in the plasmonic nanoporous membrane. For example, an advanced commercial 3D printer could be used that can achieve a linewidth of 50 nm.

[0087] 3D printing objects with a feature size of approximately 100 nm can be achieved such as by two-photon absorption, which involves using femtosecond lasers to achieve precision 3D photon absorption that polymerizes and solidifies a liquid resin that is sensitive to light. Alternatively, two-step absorption can be used where a special photoinitiator called benzil together with a single light source is used to create polymerization and solidification.

[0088] FIG. 11 is a scanning electron micrograph (SEM) image showing photoresist nanopillars prepared by 3D printing. The SEM shows an example of a 33 array of photoresist nanopillars 3D printed on a silicon substrate. The array has three rows of nanopillars of three diameters: 1125 nm, 750 nm, and 500 nm. All of them are 2 m tall.

[0089] The descriptions and examples herein are intended as non-limiting examples to serve to demonstrate the disclosed technology and can be modified by one having ordinary skill in the art to which the claimed invention pertains within the scope of the subject matter of the claimed invention. On the other hand, the present invention is not limited by the examples disclosed in the specification of the subject application, and the scope of the present invention should be interpreted based on the claims, and to include all techniques that are within the equivalent scope.

[0090] From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration. The present invention is not limited necessarily to the embodiments specifically disclosed, but that substitutions, modifications, and variations may be made to the present invention and its uses without departing from the spirit and scope of the invention. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope and spirit being indicated by the following claims.