Surface-enhanced Raman spectroscopy device and a mold for creating and a method for making the same

A surface-enhanced Raman spectroscopy device includes a substrate, and an ultraviolet cured resist disposed on the substrate. The ultraviolet cured resist has a pattern of cone-shaped protrusions, where each cone-shaped protrusion has a tip with a radius of curvature equal to or less than 10 nm. The ultraviolet cured resist is formed of a predetermined ratio of a photoinitiator, a cross-linking agent, and a siloxane based backbone chain. A Raman signal-enhancing material is disposed on each of the cone-shaped protrusions.

BACKGROUND

The present disclosure relates generally to surface-enhanced Raman spectroscopy devices, and a mold for creating the same and a method for making the same.

Raman spectroscopy is used to study the transitions between molecular energy states when photons interact with molecules, which results in the energy of the scattered photons being shifted. The Raman scattering of a molecule can be seen as two processes. The molecule, which is at a certain energy state, is first excited into another (either virtual or real) energy state by the incident photons, which is ordinarily in the optical frequency domain. The excited molecule then radiates as a dipole source under the influence of the environment in which it sits at a frequency that may be relatively low (i.e., Stokes scattering), or that may be relatively high (i.e., anti-Stokes scattering) compared to the excitation photons. The Raman spectrum of different molecules or matters has characteristic peaks that can be used to identify the species. As such, Raman spectroscopy is a useful technique for a variety of chemical or biological sensing applications. However, the intrinsic Raman scattering process is very inefficient, and rough metal surfaces, various types of nano-antennas, as well as waveguiding structures have been used to enhance the Raman scattering processes (i.e., the excitation and/or radiation process described above). This field is generally known as surface enhanced Raman spectroscopy (SERS).

DETAILED DESCRIPTION

Embodiments of the surface-enhanced Raman spectroscopy devices disclosed herein may be fabricated on flexible substrates. The devices are made from a master mold, which can be placed onto a roller that is part of a roller/imprint machine, thus enabling rolls of such devices to be fabricated. As such, the method disclosed herein is scalable so that mass fabrication of the substrates may be achieved. In some instances, the periodicity of the cone-shaped protrusions of the devices may be optimized for sensing within a particular wavelength range and/or for detection of a particular chemical species. In other instances, the cone-shaped protrusions may be formed in non-periodic patterns. The master mold used to form the SERS devices is made from a template having cone-shaped features with sub-10 nm radii of curvature. The material used to make the mold and the final device is a rigid ultraviolet curable resist that is capable of duplicating the ultra-fine details of such features.

As used herein, the terms “cone-shaped” or “cone shape” describe a protrusion, or the negative replica of such protrusion, having a three-dimensional geometric shape that tapers from a round perimeter base to a sharp tip (e.g., an apex or vertex). The sharp tip has a radius of curvature that is equal to or less than 10 nm. The height of such protrusions may be up to 2 μm, and the round perimeter base may have a diameter up to 500 nm.

Referring now toFIG. 1, a template10used for forming an embodiment of a master mold (shown inFIG. 2H) is depicted. SEM images of an example of the template10are also shown inFIGS. 8A and 8B, and will be discussed herein in reference to the Example.

The template10includes a substrate12. Non-limiting examples of suitable substrate12materials include single crystalline silicon, polymeric materials (acrylics, polycarbonates, polydimethylsiloxane (PDMS), polyimide, etc.), metals (aluminum, copper, stainless steel, nickel, alloys, etc.), quartz, ceramic, sapphire, silicon nitride, or glass. In some instances, after protrusions12′ are formed on the substrate12, the incoming light may become trapped by the protrusions12′ by mechanisms such as multiple forward scattering or through continuous variation of the index of refraction. The trapped light renders the appearance of the substrate12dark or black. As such, a silicon substrate12having the protrusions12′ thereon may be referred to herein as “black silicon”. The dimensions of the substrate12may vary, depending, at least in part, upon the desirable size of the resulting template10and upon the number and depth of the protrusions12′ to be formed.

As shown inFIG. 1, the substrate12has cone shaped protrusions12′ integrally formed therewith. Such protrusions12′ may also be referred to as nano-grass or surface roughness. Each protrusion12′ has a radius of curvature (r) that is very small, ranging from about 0.1 nm to about 10 nm. The protrusions12′ are formed such that a valley14is formed at substantially flat areas of the substrate12where protrusions12′ are not formed, and crevices (not shown) may be formed in the region proximate two adjacent protrusions12′. A plurality of crevices may resemble an ensemble of pits, each of which has a sharp point or angle (as opposed to the substantially flat areas shown inFIG. 1).

In an embodiment, the protrusions12′ may be formed by deep reactive ion etching and passivation. More specifically, the Bosch process may be used, and this process involves a series of alternating cycles of etching (e.g., using SF6and O2plasmas) and passivation (e.g., using a C4F8plasma). The morphology of the resulting protrusions12′ may be controlled by controlling the conditions (e.g., vacuum pressure, RF power, total processing time, individual etching cycle time, individual passivation cycle time, and gas flow rates) of the process. In one non-limiting example, the etcher is operated at a pressure of 15 mTorr, the coil and platen powers of the etcher are 800 W and 10 W, respectively, each etching cycle (with SF6and O2) is 6 seconds, each passivation cycle (with C4F8) is 5 seconds, and the flow rates for SF6, O2, and C4F8are 100 sccm, 13 sccm, and 100 sccm, respectively. More generally, the flow rate may be any rate up to about 100 sccm.

Regular or non-regular arrays of the protrusions12′ may be formed. The etching and passivation process previously described often results in a non-regular array. It is to be understood that in order to generate a regular array, a fabrication method, such as focused ion-beam, e-beam lithography, or optical lithography. It is believed that the cone-shaped protrusions12′ may be designed in a predetermined manner to enable the resulting device (shown inFIGS. 3D, and4-6) to be sensitive to a targeted range on the Raman spectrum (e.g., capable of producing stronger signals in a particular wavelength).

FIGS. 2A through 2Itogether illustrate the formation of the master mold100, shown inFIG. 2I, using the template10(shown inFIG. 2D).FIGS. 2A through 2Cillustrate the formation of a first portion P1of the mold100,FIGS. 2D and 2Eillustrate the formation of a second portion P2of the mold100, andFIGS. 2F through 2Iillustrate the combining of the first and second portions P1, P2to form the master mold100.

At the outset of the method for creating the mold100, as shown inFIGS. 2A and 2B, a substantially flat film16is formed on a removable substrate18. Any substrate18(e.g., any wafer) of any suitable dimensions and thickness may be selected, as long as the substrate18is removable from the material selected to form the film16, and has a planar surface, so that the resulting substantially flat film16is also planar (i.e., does not have a pattern formed therein). A non-limiting example of a suitable substrate18is silicon. Other suitable examples include those listed for the substrate12discussed hereinabove.

The substantially flat film16is formed of a material that is transparent to ultraviolet radiation (i.e., wavelengths ranging from 320 to 380 nanometers). One non-limiting example of such a material includes polydimethylsiloxane (PDMS). Furthermore, any UV transparent and flexible polymer (i.e., capable of being flexed or bent without breaking) in the silicone family (e.g., PVC) may be used to form the substantially flat film16. The transparent UV material is generally in the form of a liquid and can be deposited on the substrate18via pouring, spray coating, casting, or the like. In one embodiment, the thickness of the material deposited to form the film16ranges from about 5 μm to about 50 mm. Once deposited, the transparent UV material is allowed to harden, for example, in air or under heat (e.g., at 75° C. for about 2 hours, or at 120° C. for about 20 minutes), to form the film16.

Since the substrate18has a planar surface, the resulting film16will also be planar. It is to be understood, however, that in some rare instances the substantially flat film16may have minor and sporadic irregularities on the surface S which transfer from the substrate18during formation of the film16.

The film16may then be removed from the substrate18. Since the film18does not stick to the substrate18, the film16may be peeled off of the substrate18.

A first portion A of an ultraviolet curable resist20is then deposited on the film16. Since the ultraviolet curable resist20is ultimately used to generate the negative replica of the desirable cone-shaped pattern in the mold100, the resist20is selected to have a sufficient rigidity to be able to conform to, and to duplicate/replicate with precision, the cone-shaped protrusions12′ of the template10. As illustrated in the Example provided herein, any resist may not be selected, as not all resist can replicate the ultra-fine features of the cone-shaped protrusions12′ disclosed herein. Suitable ultraviolet curable resists20for the embodiments disclosed herein include a photoinitiator (i.e., a compound that generates a radical in response to UV radiation exposure), a cross-linking agent, and a siloxane based backbone chain. Non-limiting examples of suitable photoinitiators include azobisisobutyronitrile (AIBN), IRGACURE® 184 and IRGACURE® 810 (commercially available from BASF Corp., Florham Park, N.J.), and non-limiting examples of the cross-linking agent includes various species having more than one double or triple bond that opens up and polymerizes upon curing. In one embodiment, additional solvents are not included such UV curable resists20, at least in part because of the presence of the siloxane based backbone. The siloxane based backbone may include double bonded terminal functional groups, such as acryls.

The components of the UV resist20are included in a predetermined ratio of photoinitiator to cross-linking agent to siloxane backbone. Each component can be present in a range of 0.05% to 99.9% of the total weight of the resist20. In one embodiment, the UV resist20includes from about 0.5 wt % to about 2 wt % of the radial initiator, from about 88 wt % to about 92 wt % of the UV curable monomer species (i.e., the siloxane based backbone chain), and from about 7 wt % to about 11 wt % of the cross-linking agent. In one non-limiting example, the UV resist20includes 1 wt % of the radial initiator, 90 wt % of the UV curable monomer species (i.e., the siloxane based backbone chain), and 9 wt % of the cross-linking agent. Commercially available resists that may be used for the curable resist20include NXR-2010 (Nanonex Corp., Monmouth Junction, N.J.), and AR-UV-01 (Nanolithosolution, Inc., San Marcos, Calif.).

The first portion A of the UV curable resist20may be deposited on the film16via any suitable technique, such as spin coating, drop coating, dip-coating, or the like. The thickness of the first portion A ranges from about 20 nm to about 10 μm. Furthermore, as shown inFIG. 2C, the deposited first portion A of the UV curable resist20has a substantially planar surface22.

The formation of the second portion P2is shown inFIGS. 2D and 2E. As illustrated inFIG. 2D, the template10is utilized in the formation of the second portion P2. A second portion B of the UV curable resist20is deposited onto the template10such that the second portion B conforms to the shape of, and covers, each of the cone-shaped protrusions12′. As such, a negative replica NR of the pattern of the cone-shaped protrusions12′ is formed in the second portion B of the curable resist20. The second portion B is also deposited to extend above the tips of each of the protrusions12′ so that a substantially planar surface24is formed. As such, the thickness of the second portion B of the UV resist will depend, at least in part, on the height of the protrusions12′. The second portion B may be deposited using the same techniques that are suitable for depositing the first portion A of the UV curable resist20.

Once the first and second portions P1, P2are formed, the steps for forming the master mold100are performed. These steps are illustrated inFIGS. 2F through 2I. As illustrated inFIG. 2F, the first portion P1is aligned with the second P2so that the surfaces22,24will be in contact when the first portion P1is placed on the second portion P2. In some instances, the first and second portions P1, P2will have the same dimensions so that all of the surface22is in contact with all of the surface24. In other instances, the dimensions of one of the portions P1, P2may be smaller than the dimensions of the other portion P2, P1so that the entire surface22,24of the one portion P1, P2contacts a portion of the surface24,22of the other portion P2, P1.

When aligned and placed into contact, the first and second portions A, B of the UV curable resist20are able to intermingle at the interface of the two surfaces22,24. The same UV curable resist20is selected for both the first and second portions A, B, so that the materials join together when in contact, and so that curing results in the formation of a single cured resist20′ (seeFIG. 2H). While the surfaces22,24are in contact, ultraviolet radiation (in the form of light) is directed toward the portions A, B of the curable resist20at least through the film16. Since the film16is formed of a UV transparent material, the ultraviolet radiation passes through the film16and cures the curable resist20, thereby forming a single UV cured resist20′. It is to be understood that the UV radiation may also be directed toward the resist20at any other desirable angle. Any suitable source of UV radiation may be used to initiate curing, such as, for example, UV lamps or plasma torches or lasers operating in the UV range. The actual wavelength (within the UV range of 320 nm to 380 nm) and intensity of the ultraviolet radiation used may vary, depending at least in part, upon the UV curable resist20selected.

As illustrated inFIG. 2H, the mold100includes the film16attached to the cured resist20′. After curing, the mold100is formed and is removed from the template10. The materials of the template10and the UV cured resist20′ are not adhered to one another after curing, and thus removal may be accomplished by peeling the mold100off of the template10.

The cured resist20′ has the negative replica NR of the pattern of the cone-shaped protrusions12′ of the template10. As previously mentioned, the resist20,20′ has sufficient rigidity to replicate the sub-10 nm radius of curvature as well as the larger features (e.g., the base diameter) of the cone-shaped protrusions12′ in the form of a negative replica NR.

As shown inFIG. 2I, the mold100(shown as100′ in this Figure) also includes a release layer26. This layer26is used to ensure that adhesion to subsequent molded materials does not result. It is to be understood that the adhesion layer26may be used even when the mold100,100′ is to be subsequently used to pattern materials that will not likely adhere to the UV cured resist20′. The thin release layer26is coated on the side of the cured resist20′ having the negative replica NR of the pattern of cone-shaped protrusions formed therein. The release layer26is conformally deposited on the negative replica NR of the cone-shaped pattern such that the sub-10 nm tip radius of curvature is not lost for any of the negatively replicated cones. It is to be understood that the height and diameter of the negatively replicated cones may be slightly reduced by the addition of the release layer26. Such a reduction will depend upon the thickness of the release layer26. However, in an embodiment, the thickness of the release layer26is on the order of one molecule thick (i.e., about 2 nm), and thus will not deleteriously affect the thickness of the features of the negative pattern (or the resulting cones formed therefrom). The release layer26is a self-assembled monolayer (SAM) generated via suitable SAM-forming techniques. In one embodiment, the release layer26is (1H,1H,2H,2H-perfluorooctyl)trichlorosilane (FOTS). The release layer26essentially adds a non-stick coating to the mold100′ so that the mold100′ does not subsequently adhere to materials being patterned therewith. It is to be understood that the release layer26is added when the material(s) to be patterned with the mold100′ will adhere to the UV cured resist20′ and/or when it is desirable to ensure that the mold100′ may be used for a variety of materials (even those that are unlikely to stick, as previously mentioned).

The mold100or100′ may then be used make a surface-enhanced Raman spectroscopy device1000(shown inFIG. 3D).FIGS. 3A through 3Dillustrate an embodiment of the method for making such a device1000.

As shown inFIG. 3A, an ultraviolet curable resist28that is to subsequently be patterned is established on a substrate30. Non-limiting examples of suitable substrate30materials include single crystalline silicon, polymeric materials (acrylics, polycarbonates, polydimethylsiloxane (PDMS), polyimides, poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, polyanilines, poly(p-phenylene sulfide), and poly(para-phenylene vinylene)s (PPV), etc.), metals (aluminum, copper, stainless steel, alloys, etc.), quartz, ceramic, sapphire, silicon nitride, glass, silicon-on-insulators (SOI), or diamond like carbon films. Flexible materials may also be desirable for the substrate30, so that the resulting device1000is flexible and can be rolled up. Non-limiting examples of flexible materials include thermoplastic polyolefin substrates, such as polyethylene or polypropylene, or a polyimide substrate.

The substrate30may have any desirable dimensions. In particular, the substrate30may be small enough to fabricate a single device1000or may be large enough to fabricate a plurality of devices1000thereon.

The resist28that is selected to be deposited on the substrate30is an ultraviolet curable resist that is capable of replicating the ultra-fine features of the negative replica NR of the mold100or100′. As such, in one embodiment, the resist28is the same ultraviolet curable resist20used to form the mold100,100′. The resist28may be deposited using a roller coating process, or any of the techniques previously described for depositing the resist20. The deposited resist28has a thickness such that the mold100,100′ may be pressed therein enough to transfer the pattern of the mold100,100′ (e.g., negative replica NR) to the resist28. As such, in one embodiment, the thickness of the resist28corresponds with the height of the cones in the negative replica NR, and thus is up to 2 μm thick. It is to be understood that the thickness of the resist28may be thicker if the height of the cones in the negative replica NR of the cone-shaped pattern is greater, and/or if it is desirable that a portion of the resist28remain unpatterned (see, e.g.,FIG. 3Dwhere the pattern does not extend through the resist28to the underlying substrate30).

As shown inFIG. 3B, the mold100,100′ is then pressed into the UV curable resist28so that the resist28conforms to the pattern of the mold100,100′. While the mold100,100′ is pressed into the UV curable resist28, ultraviolet radiation (in the form of light) is directed toward the curable resist28. It is to be understood that the UV radiation may be directed toward the resist28from any desirable angle and from any desirable source (such as those previously discussed), as long as the resist28is exposed to the radiation and curing is accomplished. The wavelength and intensity of the ultraviolet radiation used may vary, depending at least in part, upon the UV curable resist28selected. In one embodiment, the resist28is exposed to UV light for a time ranging from about 10 seconds to about 20 minutes.

Curing sets the negative replica NR pattern of the mold100,100′ into the cured resist28′ such that the pattern transferred to the cured resists28′ resembles the pattern of the template10. As such, the patterned cured resist18′ has cone-shaped protrusions32which have a radius of curvature, r, equal to or less than 10 nm. Referring briefly toFIG. 3E, an enlarged view of the cross-section of the tip of one of the protrusions32is shown. The radius of curvature, r, is also shown. In particular, the radius of curvature, r, is the radius of the approximate circle C that results when points are drawn on part of the curved portion of the32. As previously mentioned, the pattern and dimensions of the cone-shaped protrusions32are transferred from the original template10using the mold100or100′.

As illustrated inFIG. 3C, after curing, the mold100′ is removed from the patterned and cured resist28′ (which remains on substrate30). In this particular non-limiting example, the cured resist28′ is made of the same material as the cured resist20′ of the mold100′, and thus the mold100′ with the release layer26is utilized to ensure that the resists20,28do not adhere together during curing. This release layer26also enables the mold100′ to simply be lifted or peeled off of the patterned cured resist28′.

The mold100(i.e., without the release layer26) may be used when the cured resist20′ of the mold100is not the same as and will not adhere to the material used for the curable resist28during curing. When mold100is used, removal of the mold100may also be accomplished via lifting or peeling, or by etching away or dissolving the mold100. Etching or dissolution may be accomplished using etchants or solvents that eat away at the mold100materials, but will not deleteriously affect the cured resist28′. The etchant or solvent used will depend upon the mold100material. In an embodiment, a buffer oxide etch (BOE) or a potassium hydroxide (KOH) etchant is used.

Once the patterned cured resist28′ is formed, a Raman signal-enhancing material34is coated on the cone-shaped protrusions32. It is to be understood that the phrase “Raman signal-enhancing material” as used herein means a material that, when established on the protrusions32, is capable of increasing the number of Raman scattered photons when an analyte (or other material of interest) is located proximate to that protrusion32, and when the analyte and material are subjected to electromagnetic radiation. Raman signal-enhancing materials include, but are not limited to, silver, gold, and copper.

The Raman signal-enhancing material34may be established by any suitable deposition or other coating technique. A blanket deposition technique may be used so that the material34is established on all of the exposed portions of the cured resist28′ (see, e.g.,FIG. 5). As a non-limiting example, the material34may be deposited via electron-beam (e-beam) evaporation or sputtering. In still another non-limiting example, the Raman signal-enhancing material34can be pre-formed nanoparticles (e.g., of silver, gold, copper, etc.), which are coated onto the cured resist28′ (seeFIGS. 3D,4and6). Such nanoparticles have an average diameter ranging from about 1 nm to about 10 nm. It is believed that the presence of the material34nanoparticles (rather than a continuous coating of material34) at the apex further enhances the electric field during SERS operation. The material34itself may also have a surface roughness that spontaneously forms during the deposition process. Such surface roughness can act as additional optical antennas to increases the SERS-active sites over each protrusion32and/or adjacent each protrusion32.

After deposition of the material34, each protrusion32remains substantially unchanged in terms of its tip/apex angle and radius of curvature r, and in terms of any crevice angles, as a relatively uniform coating (often in the form of numerous small nanoparticles on the slopes and/or tips of the protrusions32) of the material34is produced.

It is to be understood that the device1000disclosed herein may, in some embodiments, include additional components, such as, for example, a grating structure36(seeFIG. 4), a reflective layer38(also referred to as a mirrored structure, seeFIG. 5), and/or an adhesive layer40(seeFIG. 6). Such additional components may be used alone (as shown inFIGS. 4 through 6) or in any combination in a single device1000, for example, a grating and a reflective layer may be incorporated into the same device1000(not shown).

Referring now specifically toFIG. 4, the device1000′ includes the grating structure36positioned between the substrate30and the cured resist28′. During formation of such a device1000′, the grating structure36is formed on the substrate30prior to depositing the curable resist28thereon, imprinting the curable resist28via the mold100,100′, and curing the patterned curable resist28. The grating layer36may be a dielectric or metal layer having grating holes or openings37formed therein. Non-limiting examples of such grating structures36and how they are formed are described in U.S. patent application Ser. No. 12/771,753, filed on Apr. 30, 2010, entitled “ENHANCING SIGNALS IN SURFACE ENHANCED RAMAN SPECTROSCOPY (SERS)”, which is incorporated herein by reference in its entirety.

Referring now specifically toFIG. 5, the device1000″ includes the reflective layer38positioned between the substrate30and the cured resist28′. During formation of such a device1000″, the reflective layer38is formed on the substrate30prior to depositing the curable resist28thereon, imprinting the curable resist28via the mold100,100′, and curing the patterned curable resist28. The reflectivity of such a reflective/mirrored layer38is generally above 90%. As such, any metal having this reflectivity may by used, including, but not limited to gold. Such a layer38may be deposited via electron beam (e-beam) evaporation, sputtering, or the like. A suitable thickness for the layer38generally ranges from about 5 nm to about 5 mm. Non-limiting examples of such reflective/mirrored layers38are described in U.S. patent application Ser. No. 12/771,824, filed on Apr. 30, 2010, entitled “APPARATUS FOR PERFORMING SERS”, which is incorporated herein by reference in its entirety. While the reflective/mirrored layer38shown inFIG. 5is substantially planar, a concave reflective/mirrored layer38may be included instead. Such concave reflective/mirrored layers38are described in U.S. patent application Ser. No. 12/771,753 filed on Apr. 30, 2010, entitled “ENHANCING SIGNALS IN SURFACE ENHANCED RAMAN SPECTROSCOPY (SERS)”(already incorporated by reference herein).

Referring now specifically toFIG. 6, the device1000′″ includes the adhesive layer40positioned between the substrate30and the cured resist28′. During formation of such a device1000′″, the adhesive layer40is formed on the substrate30prior to depositing the curable resist28thereon, imprinting the curable resist28via the mold100,100′, and curing the patterned curable resist28. The adhesive layer40may be formed of a metal or 3-acryloxypropyl trichlorosilane. In one embodiment, the adhesive layer40is a self-assembled monolayer (SAM) that is a single molecule thick. Generally, the thickness of the adhesive layer ranges from about 0.5 nm to about 5 nm, and is used to strengthen the adhesion of the substrate30to the cured resist28′. It is to be understood that the adhesive layer40may be used in any embodiment in which it is not desirable to remove the cured resist28′ from the substrate30. The addition of adhesive layer40aids in the adhesion between these two components28′ and30.

FIG. 7illustrates components that are used in conjunction with the device1000(or any other embodiment of the devices1000′,1000″,1000′″ disclosed herein) in order to perform Raman spectroscopy. Such additional components include a stimulation/excitation light source42, and a detector44. In some instances, the additional components also include an optical component (e.g., optical microscope46), which is positioned between the light source42and the device1000. The optical component46focuses the light from the light source42to a desirable area of the device1000, and then again collects the Raman scattered light and passes such scattered light to the detector44. Analyte molecules (not shown) may be introduced across the Raman active structures protrusions32, where they may be exposed to stimulating/excitation wavelengths from the light source42, and the resulting signals may be detected by the Raman detection unit44. It is believed that the total surface area of the cone-shaped protrusions32enables a large number of target molecules to be trapped on the substrate device1000, and thus more Raman photons can contribute to the overall SERS signal.

In certain embodiments, the detector44may also be operably coupled to a computer (not shown) which can process, analyze, store and/or transmit data on analytes present in the sample.

While not shown in all embodiments, it is to be understood that a release layer may be used during any of the steps of the method disclosed herein which involve peeling of one material from another. For example, a release layer may be used at the interface of the substrate12and the second portion B of the resist20when forming the second portion P2of the mold100,100′.

To further illustrate embodiment(s) of the present disclosure, an example is given herein. It is to be understood that this example is provided for illustrative purposes and is not to be construed as limiting the scope of the disclosed embodiment(s).

EXAMPLE

Template Formation

Templates were formed using crystalline silicon substrates. The cone-shaped protrusions were fabricated using the Bosch process. In particular, an STS (Surface Technology Systems) etcher was used for the etching of silicon wafers of six inches diameter. No prior cleaning was used for the silicon wafers. The etcher was operated at a pressure of 15 mTorr, and the coil and platent powers were 800 W and 10 W, respectively. Each cycle of etching (with SF6and O2) and passivation (with C4F8) was 6 seconds and 5 seconds, respectively. The flow rates for SF6, O2and C4F8were 100 sccm, 13 sccm and 100 sccm, respectively. SEM images of the template are shown inFIGS. 8A and 8B. The average radius of curvature of the cones was 10 nm or less.

Mold and Comparative Mold Formation

The mold fabricated in accordance with the embodiments disclosed herein included two portions that were adhered together. The first portion was a UV nanoimprint resist (with a thickness between 1 and 2 μm) formulated according to an embodiment disclosed herein and coated on a polydimethylsiloxane (PDMS) planar film, and the second portion was the same UV curable resist (with a thickness between 1 and 2 μm) coated to cover the cone-shaped protrusions of one of the templates. The UV curable resist layers of each of the portions were put in contact with each other and cured by exposing them to a UV lamp for 15 minutes. After curing the mold was removed from the template. A release layer of 1H,1H,2H,2H-perfluorooctyl)trichlorosilane was formed using a self-assembling technique on the negatively patterned portion of the mold.

The comparative mold was fabricated by pouring space grade PDMS elastomer onto another of the templates. The PDMS elastomer was allowed to harden (by exposing it to heat in an oven), and then was removed from the template.

SERS Substrate Formed with Mold

A UV curable resist (i.e., AR-UV-01) was spin coated onto a planar silicon substrate. The mold (fabricated in accordance with the embodiments disclosed herein) was pressed into the AR-UV-01 UV curable resist. While the mold was pressed into the curable resist, the curable resist was exposed to ultraviolet radiation using a UV lamp. After curing, the mold was removed. The pattern of the mold was transferred to the cured resist without losing the ultra-fine features of the mold and original template. The resulting patterned substrate is shown inFIGS. 9A and 9B. As illustrated, the sub-10 nm features from the original template were transferred using the UV resist mold disclosed herein.

Comparative SERS Substrate Formed with Comparative Mold

Norland Optical Adhesive 83H (“NOA83H”) was spin coated on a silicon substrate. The comparative mold was pressed into the NOA83H resist. While the comparative mold was pressed into the NOA83H resist, the NOA83H resist was exposed to ultraviolet radiation using a UV lamp. After curing, the comparative mold was removed. The resulting patterned substrate is shown inFIGS. 10Aand10B. As illustrated (especially when comparingFIGS. 9A and 9BwithFIGS. 10A and 10B), the ultra-fine features of the original template were not duplicated using PDMS as the mold.

It is to be understood that the ranges provided herein include the stated range and any value or sub-range within the stated range. For example, a weight percent (wt %) range of approximately 1 wt % to about 20 wt % should be interpreted to include not only the explicitly recited weight percent limits of 1 wt % to 20 wt %, but also to include individual weight percentages, such as 2 wt %, 3 wt %, 4 wt %, etc., and sub-ranges, such as 5 wt % to 15 wt %, 10 wt % to 20 wt %, etc.