Surface enhanced infrared absorption stage

A surface enhanced infrared absorption stage may include a substrate, a static island extending from the substrate and a movable nano finger extending from the substrate. The static island may have a plasmonically active island cap and a dimension parallel to the substrate of at least one micrometer. The movable nano finger may be aligned with the dimension. The movable nano finger may have a plasmonically active finger cap closable to less than or equal to 5 nm of the island cap.

BACKGROUND

Surface enhanced infrared absorption (SEIRA) is sometimes used for analyzing the structure of an analyte such as inorganic materials and complex organic molecules. SEIRA focuses infrared radiation or light onto the analyte, wherein the interaction between the light and the analyte is detected for analysis.

DETAILED DESCRIPTION OF EXAMPLES

SEIRA utilizes energy localization gaps between neighboring electrically conductive or metallic surfaces. Reducing the size of the gaps may enhance performance. Disclosed herein are examples of SEIRA stages that offer small, controlled self-limited single digit nanometer gaps for strong energy localization/strong signal strength while at the same time being more economical to fabricate.

The example SEIRA stages comprise flexible nano fingers and wider protruding islands. The islands exhibit a footprint that facilitates operation at infrared wavelengths. The nano fingers are flexible so as to lean towards the islands to create self-limiting nano gaps of less than or equal to 5 nm and as little as 1 to 2 nm, creating hot spots of high optical energy concentration resulting in strong spectroscopic signal response. Because the nano gaps are formed by closure of the nano fingers and the islands, the size of the nano gaps therebetween is not wholly dependent on fabrication density or resolution, facilitating lower cost fabrication techniques.

Disclosed herein is an example SEIRA stage that may comprise a substrate, a static island extending from the substrate and a movable nano finger extending from the substrate. The static island may have a plasmonically active island cap and a dimension parallel to the substrate of at least one micrometer. The movable nano finger may be aligned with the dimension. The movable nano finger may have a plasmonically active finger cap closable to less than or equal to 5 nm of the island cap.

Disclosed herein is an example method for analyzing and analyte using SEIRA. The method may comprise applying an analyte to a SEIRA stage. SEIRA stage may comprise a substrate, a static island extending from the substrate and a movable nano finger extending from the substrate. The static island may have a plasmonically active island cap and a dimension parallel to the substrate of at least one micrometer. The movable nano finger may be aligned with the dimension. The movable nano finger may have a plasmonically active finger cap closable to less than or equal to 5 nm of the island cap. The method may further include closing the finger cap towards the island cap, irradiating the finger cap and the island cap and sensing infrared absorption to analyze the analyte.

Disclosed herein is an example method for forming a SEIRA stage. The method may comprise forming static islands extending from a substrate, each of the static islands having a dimension parallel to the substrate of at least one micrometer, wherein each static island has a plasmonically active island cap. The method may further comprise forming movable nano fingers extending from the substrate, wherein each of the nano fingers is aligned with the dimension of an adjacent static island. Each movable nano finger may have a plasmonically active finger cap closable to less than or equal to 5 nm of the island cap the adjacent static island.

FIG. 1is a side view of a portion of an example SEIRA stage20for use in an SEIRA sensing system, Stage20serves as a surface for supporting an analyte and for enhancing interactions between impinging infrared light and the analyte for enhanced sensor sensitivity. Stage20utilizes an architecture that may be reliably and cost-effectively fabricated, Stage20offers small sized gaps for strong energy localization for enhanced performance. Stage20comprises substrate24, island28and nano finger32.

Substrate24comprises a base, foundation or floor for supporting island28and nano finger32. In one implementation, substrate24comprises a layer of silicon, quartz, ceramic, glass or a polymeric film such as polyethylene teraphalate (PET). In some implementations, substrate24may additionally comprise and interlayer the dielectric material between the silicon substrate and island28as well as nano finger32. Such an interlayer dielectric may form from a material such as SiO2TEOS, a passivation layer of SiC, silicon nitride, etc. In other implementations, other interlayer dielectric materials may be utilized. In still other implementations, substrate24may be formed from other materials.

Island28comprises an upstanding structure and projecting from substrate24that has a dimension D parallel to substrate24of at least one micrometer. Due to its dimensions of at least one micrometer, island28may serve as an antenna that operates at infrared wavelengths. Island28is substantially static or non-movable relative to nano finger32.

In the example illustrated, island28comprises a base pillar34and a plasmonically active island cap36. Base pillar34serves as a stem, supporting cap36. In one implementation a base pillar34is formed from a polymer. Examples of polymer materials from which each pillar34may be formed include, but are not limited to, photo resist, PDMS, or a material selected from the group, which includes both dielectric and non-dielectric materials, consisting of a highly cross-linked uv-curable or thermal-curable polymer, a highly cross-linked uv-curable or thermal-curable plastic, a polysiloxane compound, silicon, silicon dioxide, spin-on glass, a solgel material, silicon nitride, diamond, diamond-like carbon, aluminum oxide, sapphire, zinc oxide, and titanium dioxide.

Cap36comprises a plasmonically active or electrically conductive structure formed on top of pillar34. A plasmonically active structure material is a material that converts radiation, such as light or photons, into plasmons, a density wave in an electron gas. In one implementation, cap36comprises a metal material that enhances the intensity of electromagnetic radiation interacting with the analyte in the gap. In one implementation, cap36comprises silver, gold, copper, platinum, aluminum, or combinations of these metals in the form of alloys or multilayer systems. In one implementation, cap36may comprise a material such as indium tin oxide, aluminum zinc oxide, fluorine doped tin oxide, doped zinc oxides, titanium nitride, carbon nanotubes networks and graphene. In another implementation, cap36may comprise other materials that provide such intensity enhancement.

Nano finger32comprises an elongated nanometer scale flexible columnar support such as a needle, rod, finger or wire that rises up from substrate24proximate to island36. In one implementation, nano finger32has an aspect ratio of and at least 10:1 (a height of at least 10 times the thickness or diameter). In one implementation, such nano finger32has a thickness or diameter between 50 nm and 100 nm, while, at the same time, having a height of at least 500 nm and, in one implementation, at least 700 nm. In the example illustrated, nano finger32is movable, wherein such columnar structures bend or flex towards island28in response to micro-capillary forces or van der Waals forces, wherein such bending facilitates close spacing between the nano finger32and island28for a smaller gap with enhanced energy localization. In one implementation, pillar44rises up from substrate24at a location sufficiently close to island28such that pillar44is bendable or closable to a position in which the tip of nano finger32is within 1 nm of island36.

As schematically shown byFIG. 1, nano finger32comprises a pillar44supporting a plasmonically active tip or cap46. In one implementation, pillar44comprises an elongate column formed from a polymer material, Pillar44serves as a stem supporting cap46. The polymer material facilitates the use of molding, imprinting or other fabrication techniques to form pillar44. The polymer material further facilitates bending and flexing of pillar44and subsequently closing during use of stage20. In one implementation, pillar44has a diameter of less than a micron. In one implementation of pillar44has a diameter of less than or equal to 500 nm. Examples of polymer materials from which each pillar44may be formed include, but are not limited to, photo resist, PDMS, or a flexible material selected from the group, which includes both dielectric and non-dielectric materials, consisting of a highly cross-linked uv-curable or thermal-curable polymer, a highly cross-linked uv-curable or thermal-curable plastic, a polysiloxane compound, silicon, silicon dioxide, spin-on glass, a solgel material, silicon nitride, diamond, diamond-like carbon, aluminum oxide, sapphire, zinc oxide, and titanium dioxide.

Cap46is similar to cap36of island28. Cap46comprises a plasmonically active or electrically conductive structure formed on top of pillar44. A plasmonically active structure material is a material that converts radiation, such as light or photons, into plasmons, a density wave in an electron gas. In one implementation, cap46comprises a metal material that enhances the intensity of electromagnetic radiation interacting with the analyte in the gap. In one implementation, cap46comprises silver, gold, copper, platinum, aluminum, or combinations of these metals in the form of alloys or multilayer systems. In one implementation,36may comprise a material such as indium tin oxide, aluminum zinc oxide, fluorine doped tin oxide, doped zinc oxides, titanium nitride, carbon nanotubes networks and graphene. In another implementation, nano finger cap46may comprise other materials that provide such intensity enhancement. In the example illustrated, pillar44rises up from substrate24at a location sufficiently close to island28such that pillar44is bendable or closable to a position in which the nano finger cap46less than or equal to 5 nm of island cap36. In some implementations, pillar44is bendable or closable to a position in which the nano finger cap46is spaced less than or equal to 2 nm from island cap36.

FIG. 2is a flow diagram of an example method100that may be used to form a SEIRA stage having small gaps for strong energy localization and enhanced performance. Although method100is described with respect to forming a SEIRA state such as stage20, method100may be utilized to form any of the SEIRA stages disclosed herein or similar SEIRA stages. As indicated by block110, static islands extending from a substrate are formed. The static islands each have a dimension D parallel to the substrate of at least one micrometer. Each static island has a plasmonically active or electrically conductive island cap36.

In one implementation, each static island28has an elongated cross sectional shape such as an elongated oval or a rectangle, the major dimension or length of the elongated oval or rectangle being at least one micrometer. In one implementation, each static island28is triangular, having a length or height of at least one micrometer. In yet another implementation, the static island28is circular, having a diameter of at least one micrometer. In yet other implementations, the static island28may have other cross-sectional shapes having a dimension of at least one micrometer.

As indicated by block114, movable nano fingers are formed on the substrate in alignment with the dimension of an adjacent static island. The phrase “in alignment with the dimension of an adjacent static island” means that an imaginary line extending along the dimension of the island that is at least one micrometer intersects the nano finger. Each of the formed nano fingers has a plasmonically active finger cap that is closable to lessen or equal to 5 nm of the island cap36of the adjacent static island28. In one implementation, each of the formed nano fingers has a plasmonically active finger cap that is closable to less than or equal to 2 nm of the island cap36of the adjacent static island28. In other words, the nano fingers are formed from a material and dimensioned so as to be bendable or flexible such that the cap of the nano finger may move towards the island to provide a gap of less than or equal to 5 nm or less than or equal to 2 nm between the tips or caps of the islands28and the nano fingers32.

In one implementation, islands28and nano fingers32are formed on substrate24using nano imprint lithography. With nano imprint lithography, a layer of moldable material, such as a UV resist, on substrate24is imprinted or embossed with an imprint master, such as a quartz master, wherein ultraviolet light is applied to solidify or cure the patterned UV resist. Sacrifice or withdraw of the imprint master leaves the remaining substrate and formed pillars34,44up standing from the substrate24. The pillars are then topped with the caps36,46.

FIG. 3is a schematic diagram of a portion of an example SEIRA sensing system200comprising stage20. In addition to stage20, sensing system200comprises infrared emitter202and infrared sensor204. Emitter202and sensor204interrogate analyte molecules at a frequency resonant to those of molecular vibrations. Emitter202comprises a device that emits and directs infrared (wavelength 3-30 microns) or near infrared (wavelength 0.7-3 microns) radiation towards stage20, towards island28and nano finger32. Infrared sources may include, but are not limited to, thermal sources such as Globar filaments, infrared LEDs and infrared lasers sources. Infrared sensor204comprise a device that senses the infrared radiation absorbed by the analyte molecules such as by sensing the quantity of the emitted infrared radiation that is transmitted or reflected back towards sensor204. Infrared sensors may include Mercurium-Cadmium-Telluride sensors, microbolometers, Indium gallium arsenide, indium antimonide other pyroelectric and imaging arrays of such sensing elements. The amount of energy absorbed by a molecule may serve as a fingerprint facilitating identification of the analyte molecules or to identify characteristics of the analyte molecules.

FIG. 4is a flow diagram of an example method300for analyzing an analyte using SEIRA, Method300is described as carried out using system200and is illustrated inFIG. 3. It should be appreciated that method300may be carried out using other SEIRA sensing systems or other SEIRA stages similar to stage20.

As indicated by block310and illustrated inFIG. 3, an analyte50is applied to an SEIRA stage, such as stage20. As described above, the stage comprises a substrate24, a static island28and a movable nano finger32. The static island28extends from the substrate24and has a dimension D parallel to the substrate of at least one micrometer and a plasmonically active/electrically conductive island cap36, The movable nano finger32extends from the substrate and is aligned with the dimension D. The movable nano finger32has a plasmonically active or electrically conductive finger cap46and is closable to less than or equal to 5 nm of the island cap36. In one implementation, cap46is bendable or closable to less than or equal to 2 nm of the island cap36.

As indicated by block314and further shown inFIG. 3, the finger cap46is closed with respect to or towards the island cap36. In one implementation, the analyte50is applied as part of a liquid, wherein liquid is evaporated, leaving the analyte deposited upon island cap36and finger cap46, captured between or within the gap separating island cap36and finger cap46. The evaporation of the liquid creates capillary forces sufficient to draw and bend nano finger32towards island28. In one implementation, the evaporation of the liquid is accelerated through the application of heat to stage20.

In other implementations, the closing of the island cap36and the finger cap46may be facilitated in other fashions. For example, in other implementations, nano finger32may be heated to a temperature approaching or above its glass transition temperature, causing nano finger32to collapse and bend towards island28. The larger size and mass of island28may slow the rate at which the temperature of island28rises such that nano finger32collapses towards island28.

As indicated by block318and illustrated inFIG. 3, IR emitter202irradiates stage20, impinging the analyte50, island cap36and finger cap46with infrared or near infrared radiation54. As indicated by block320, sensor204senses infrared radiation absorption by the molecules50. In one implementation, sensor204determines the amount of infrared radiation that has been absorbed based upon the quantity of infrared radiation directed at stage20and by sensing the quantity of infrared radiation not absorbed, transmitted or reflected back towards sensor204. The quantity of infrared radiation absorbed by analyte50(or not absorbed by analyte50) may be utilized to analyze the analyte50, indicating an identity of the analyte50or indicating characteristics of the analyte50.

FIGS. 5 and 6illustrate SEIRA stage420, an example implementation of SEIRA stage20. SEIRA stage420may be utilized as part of sensing system200in place of stage20. Stage420is similar to stage20described above except that stage420is specifically illustrated as comprising a static island428in view of island28. Those remaining components of stage420which correspond to components of stage20are numbered similarly.

Static island428is similar to island28except that static island428is specifically illustrated as having an elongate rectangular or rod cross sectional shape having a major dimension D of at least 1 μm. Similar to island28, static island428comprise a stem or base pillar34and a static island cap36. The major dimension C of static island428is aligned with nano finger28. In other words, an imaginary linear line451extending along dimension D, through or upon the top of cap36, intersects cap46of nano finger32.

As with static island28, because the dimension D that is aligned with nano finger28is at least 1 μm, static island428serves as an antenna that operates at infrared wavelengths. As with stage20, nano finger32has an elongated aspect ratio facilitating bending of nano finger32towards island36, namely towards the end of the elongated rectangle, so as to close caps36and46to less than or equal to 5 nm, and in one implementation less than or equal to 2 nm, of one another. This single-digit nanometer scale gap G, after closure or bending, provides a hotspot of high optical energy concentration which results in strong spectroscopic signal response.

FIG. 7is a top view of an example SEIRA stage520fabricated using nano imprint lithography. In other implementations, stage520may be formed using other techniques. Stage520comprises a grid or array of island-nano finger pairs522, each pair522comprising a static island428and an associated nano finger32as described above. Each pair522provides a self-limited single-digit nanometer sized gap, upon closure of the caps36,46, to provide multiple hotspots of high optical energy concentration which result in strong spectroscopic signal response.

FIG. 8is a top view of a portion of another example SEIRA stage620that may be fabricated using nano imprint lithography. In other implementations, stage620may be formed using other techniques. Stage620may be utilized as part of SEIRA sensing system200described above in place of stage20. Stage620is similar to stage520described above except that stage620comprises a grid or array of island-nano finger pairs622in place of pairs522. Pairs622are similar to pairs522except that each pair622additionally comprises a second movable nano finger634on an opposite and of island428as nano finger34. Nano finger634is similar to nano finger34in that nano finger34comprises a pillar44(shown inFIG. 5) that supports a cap46. As with nano finger34, the major dimension D of static island428is aligned with nano finger634.

Like nano finger34, nano finger634is bendable or closable with respect to island428. The cap46of nano finger634is closable to within 1 nm of cap36of island428. Unlike pairs522, pairs622of stage620each provide two self-limited single-digit nanometer sized gaps, upon closure of the caps36,46, to provide multiple hotspots of high optical energy concentration which result in strong spectroscopic signal response.

FIG. 9is a top view of a portion of another example SEIRA stage720that may be fabricated using nano imprint lithography. In other implementations; stage720may be formed using other techniques. Stage720is similar to stage520described above except that stage720comprises a grid or array of island-nano finger pairs722in place of pairs522. Pairs722are similar to pairs522except that each of pairs722comprises a triangular shaped island728paired with a movable nano finger32(described above).

Island728each have an underlying base pillar that supports a plasmonically active cap736. As shown byFIG. 9, the underlying base pillar and the cap736each have a triangular cross sectional shape. The underlying base pillar has a size and shape corresponding to the size and shape of the cap736seen in the top view shown inFIG. 9. Each of the triangular islands has a height dimension H (the dimension from the base to the apex of the triangle opposite nano finger32) of at least one micrometer. The triangular shape of island728serves as an infrared antenna which is more broadband, facilitating use of a wider range of wavelengths of radiation in the infrared spectrum when performing SEIRA analysis.

The apex of the triangular shape of each island728points to and extends opposite to the paired movable nano finger32. As in the above described stages, each pair722provides a self-limited single-digit nanometer sized gap, upon closure of the caps736,46, to provide a hotspot of high optical energy concentration which results in a strong spectroscopic signal response. Although each of the pairs722are illustrated as having the same orientation, in other implementations, such pairs722may have opposite orientations or may have a variety of different orientations on substrate24.

FIG. 10is a top view of a portion of another example SEIRA stage820that may be fabricated using nano imprint lithography. In other implementations, stage820may be formed using other techniques. Stage820is similar to stage520described above except that stage820comprises a grid or array of island-nano finger clusters822in place of pairs522. Clusters822each comprise a static cylindrical center island828surrounded by or encircled by multiple spaced movable nano fingers32(described above).

Each of islands828has a cylindrical underlying base pillar that supports a plasmonically active cap836. As shown byFIG. 10, the underlying base pillar and the cap736each have a circular cross sectional shape. The underlying base pillar has a size and shape corresponding to the size and shape of the cap836seen in the top view shown inFIG. 10. Each of the cylindrical islands has a cap836with a diameter at least one micrometer.

During closure and movement of the nano fingers32, nano fingers32bend or flex inwardly towards the center of their associated center island828. During such closure, caps46of nano fingers32close to within 1 nm of the cap836of island828. Each cluster822provides a multitude of self-limited single-digit nanometer sized gaps corresponding to the number of nano fingers32surrounding island828, upon closure of the caps836,46, to provide a multitude of hotspots of high optical energy concentration which results in a strong spectroscopic signal response. Although each of the clusters822are illustrated as comprising a same number of nano fingers32and as specifically comprising 12 nano fingers32about each island828, in other implementations, clusters822may have a different total number of nano fingers32about each island828, Moreover, in some implementations, different clusters may have different numbers of nano fingers832about the respective islands828.

In addition to providing a multitude of hotspots equal to the number of nano fingers32about the island828, the dot-flower arrangement of each cluster822further provides polarization insensitivity. In other words, clusters822may provide high degrees of SEIRA sensitivity when unpolarized infrared or near-infrared light is being used to illuminate, interrogate or irradiate the analyte captured and retained on each cluster822. In other instances, the center island may have an oval cross-section instead of circular to allow controlled polarization-selective antenna responses,

FIG. 11is a diagram schematically illustrating another example sensing system900. Sensing system900offers greater versatility in that sensing system900facilitates both surface enhanced SEIRA analysis and surface enhanced Raman spectroscopy analysis of an analyte. In addition to being able to carry out both Raman spectroscopy and SEIRA analysis, system900may apply both types of analysis to a same analyte on a same stage.

Sensing system900comprises stage620, infrared emitter/detector902, Raman emitter/detector906, input908and controller910. Stage620is described above. It should be appreciated that system900may be utilized with any of the stages described above as well as other similar stages having a static island have a plasmonically active cap with a dimension parallel to the substrate of at least one micrometer and a movable nano finger aligned with the dimensions of the cap, wherein the nano finger has a plasmonically active cap closable to a spacing of less than or equal to 5 nm from the island cap.

Infrared emitter/detector902comprises IR emitter202and sensor204described above with respect to system200. Infrared emitter/detector902interrogates analyte molecules at a frequency at which there are molecular vibrations without shifting the frequency of such molecules. Infrared emitter/detector902comprises a device that emits and directs infrared or near infrared radiation towards stage620, towards island428and nano fingers32,632. Infrared emitter/detector902further comprises a device that senses the infrared radiation absorbed by the analyte molecules such as by sensing the quantity of infrared radiation that is reflected back or transmitted towards the infrared emitter/detector902. The amount of energy absorbed by a molecule may serve as a fingerprint facilitating identification of the analyte molecules or to identify characteristics of the analyte molecules.

Raman emitter/detector906comprises a device that directs light, such as a laser beam of light, towards and onto stage620and a device that focuses, gathers and detects and SERS spectra resulting from light scattering by the sample analyte on island428and nano fingers32,632. In one implementation, emitter/detector906comprises an infrared laser to emit a beam having a wavelength of 785 nm onto island428and nano fingers32,632. To direct the beam of light and focus the SERS spectra, resulting from scattering of the light by the island428and nano fingers32,632, onto a sensing panel, Raman emitter/detector906may include one or more optical components such as lenses and mirrors. The received SERS spectra, including shifts in the frequency of light, is compared against previous identified spectrum fingerprints or signatures to identify characteristics of the sample analyte.

Input908comprise a device by which selections or commands may be provided to controller910indicating whether system900is to operate in either an SEIRA detection mode or a Raman spectroscopy detection mode. Input908may comprise a touch screen, a mouse, a keyboard, a touchpad, a microphone with speech recognition and the like. In some implementations, input908may comprise a pushbutton, toggle switch or other manual input device.

Controller910comprises electronic hardware, such as a processing unit, to carry out instructions contained in a non-transitory computer-readable medium or memory. Controller910selectively activates emitter/detector902or emitter/detector906in response to instructions received via the input908. Because stage620(or any of the other stages described above, comprises at least one nano finger32that is bendable or closable towards and associated island20,428,728,828, stage620is well-suited for serving as an analyte supporting surface that facilitate surface enhanced Raman spectroscopy. As a result, through appropriate input to controller910, system900may be used to first identify one or more characteristics of an analyte using SEIRA and then to either confirm the results or to identify additional characteristics of the analyte using surface enhanced Raman spectroscopy (SERS).