Abstract
The present invention relates to nanograting sensor devices and fabrication methods thereof. The nanograting sensor device includes a light transmissive optical component comprising a plasmonic thin film with nanostructure patterns. The nanostructure has a smooth shape profile which can enhance the efficiency of plasmonic coupling and light transmission and increase the sensing ability. Methods of the present invention provide a means of fabricating such plasmonic thin film structures. The sensor described in the present invention utilizes the changes of the plasmonic resonances to detect analytes and/or determine the concentration of analytes at the plasmonic thin film surface or in the fluid near the plasmonic thin film surface.
Claims
1. A nanograting sensor device, comprising: a substrate, wherein a plurality of nanostructures are formed with smooth profiles; a metallic thin film layer coated on the substrate.
2. The nanograting sensor device of claim 1, wherein the nanostructures have a periodicity p, or a certain symmetry.
3. The nanograting sensor device of claim 1, wherein the preferred thickness of the metallic thin film layer is 10-60 nm.
4. The nanograting sensor device of claim 1, wherein the metallic thin film layer is an electrically conductive material.
5. A method of making a nanograting sensor device, the method comprising: providing a substrate; generating a plurality of nanostructures on the substrate; forming a smooth profile of the nanostructures; coating a metallic thin film layer.
6. The method of claim 5, wherein the nanostructures are formed by electron beam lithography, focus ion beam, interference lithography, stamping or molding.
7. The method of claim 5, wherein the smooth profile is formed by coating the nanostructure patterned substrate with a polymer layer, a copolymer layer or a combination layer, and the preferred thickness of the layer is approximately 10-20 nm
8. The method of claim 5, wherein the smooth profile is formed by depositing an organic film or an inorganic film via chemical vapor deposition or physical vapor deposition, and the preferred thickness of the film is approximately 10-20 nm
9. The method of claim 5, wherein generating the nanostructures and forming the smooth profile are made in one process, and a stamp or mold comprising a plurality of nanostructures with a smooth profile is brought into contact with a substrate to form a plurality of nanostructures
10. The method of claim 9, wherein the substrate is coated with a polymer layer, a co-polymer layer or a combination of a polymer and copolymer layer
11. The method of claim 5, wherein generating the nanostructures and forming the smooth profile are made in one process, and a substrate material in liquid form can be poured onto a stamp or mold comprising a plurality of nanostructures with a smooth profile and then solidifies to form a plurality of nanostructures.
12. The method of claim 11, wherein the substrate material can be a polymer, a co-polymer, a combination of a polymer and copolymer, or glass.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1a and 1b show schematics of a nanograting sensor structure. FIG. 1a is a perspective view illustrating a nanograting sensor structure. FIG. 2b shows a schematic cross-section of a nanograting sensor structure.
[0013] FIGS. 2a and 2b illustrate a sensing method of detecting transmitted light. FIG. 2a is a schematic sensing method of light from the substrate side and transmitted light detected from the sensing side. FIG. 2b is a schematic sensing method of light from the sensing side and transmitted light detected from the substrate side. Reflected light also can be used for detection.
[0014] FIGS. 3a and 3b are graphs of experimental data obtained by a spectrophotometer. FIG. 3a shows the transmission spectra of the nanograting devices with the grating period of 500, 550, 600, 650 and 700 nm, corresponding to resonance peaks from left to right. FIG. 3b shows the intensity/wavelength changes of the transmitted light when different analytes or analytes with different concentration are disposed on the sensor surface.
[0015] FIG. 4 illustrates a nanograting sensor fabrication method with a dielectric coating process to form a smooth profile in a nanograting structure
[0016] FIG. 5 illustrates a nanograting sensor fabrication method with a thermal process to form a smooth profile in a nanograting structure
[0017] FIG. 6 illustrates a nanograting sensor fabrication method with a thermal or transferring process to form a smooth profile in a nanograting structure
[0018] FIG. 7 illustrates a transferring process to form a smooth profile in a nanograting structure
[0019] FIGS. 8a and 8b show scanning electron microscopy (SEM) cross-section images of the sensor devices at the periodicity of 600 nm.
DETAILED DESCRIPTION OF THE INVENTION
[0020] Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.
[0021] FIGS. 1a and 1b illustrate schematics of a nanograting sensor device, comprising a transparent substrate or a supporting layer with nanograting structures having smooth profiles 12 and a coated metallic thin film layer 10. The patterned substrate 12 that supports the metallic thin film layer 10 can be a single substrate or a layer of materials. The metallic thin film layer 10 may be composed of gold, silver, or any metallic materials such as highly-doped zinc oxide (ZnO) and so on that can excite surface plasmon resonances. In certain embodiments, a sensor device includes a metallic thin film with nanograting structures having smooth profiles. Varying the periodicity 14 in the nanograting structures one can tune transmission peak position or resonance wavelength. The resonance wavelength can be designed to match the periodicity of the nanograting structure. Although the nanograting structure has been shown in the embodiments, other nanostructures or a plurality of nanostructure arrays with certain symmetries may be used, such as bullseye and nanodot structures.
[0022] FIGS. 2a and 2b schematically illustrate a sensing method using the present invention. Polarized or non-polarized broadband or polychromatic light illuminated from the side of the substrate or metallic thin film is incident to the present sensor device 20. FIG. 2a is a schematic sensing method of light from the substrate side and transmitted light detected from the sensing side. FIG. 2b is a schematic sensing method for light illuminated from the sensing side and transmitted light detected from the substrate side. Reflected light also can be used for detection. Analytes sought to be detected are disposed in contact with or in the vicinity of the metallic thin film surface. These analytes change the local refractive index around the nanostructures, which in turn affect the constructive or destructive interferences of the surface plasmon and evanescent electromagnetic waves. The detection is based on a change or difference of the light before and after the contacting of the analytes with a nanograting sensor device. The collected light signal comprises light from a transmission mode, a reflection mode or a combination of both. The incident and detected light can be set perpendicular or with a certain angle to the surface of the nanograting sensor device.
[0023] FIGS. 3a and 3b show graphs of experimental data obtained by a spectrophotometer using the method illustrated in FIG. 2a. The transmission spectra were obtained from the nanograting devices with the grating periods of 500, 550, 600, 650 and 700 nm which correspond to the resonance peaks from left to right in FIG. 3a. Using the present nanostructures, sharp Fano resonances were obtained with the full width of half maximums (FWHMs) around 10 nm under transverse magnetic (TM) light in zero order transmission. The transmission efficiency surpasses that of a metal thin film with the same area and thickness at the resonance maxima. The resonance coupling and transmission efficiency is enhanced by shaping plasmonic nanostructures with a smooth profile. FIG. 3b shows the detection of spectral shifts in NaCl solutions at different concentrations (5%, 10%, 15%, 20%) in deionized (DI) water. Refractive index sensitivity up to 570 nm/RIU (S: nm per refractive index unit) was achieved. Using the perturbation theory, the refractive index sensitivity can be theoretically obtained as: /=/n/n. In fact, it has an upper bound on the spectral sensitivity (S/n) for the normal transmission. Since the period of 600 nm nanograting sensor device is used for the detection, the upper bound of this device is 600 nm/RIU (the refractive index of air n1). The sensitivity of the nanograting sensor device is very close to the upper limit. Sensitivity over this limit can be achieved by adjusting incident or detection angles.
[0024] Referring to FIG. 4, embodiments of the invention provide a fabrication method for a nanograting sensor device, comprising: a substrate 40, a plurality of nanostructures 42, a coating layer 46 and a metallic thin film layer 48. The substrate 40 may be a substrate or a layer of material. A plurality of nanostructures 42 can be formed in predetermined patterns on the substrate 40 by electron beam lithography, focused ion beam etching, nanoimprint or any other appropriate method known in the art. Then a thin layer 44 can be coated wherein by spin coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), evaporation, or any other appropriate method. The smooth profile of the nanostructures 46 can be obtained during the process or formed by a heat or etching process. For example, the smooth profile can be formed by depositing 10 nm zirconium oxide ZrO.sub.2 using atomic layer deposition (ALD) or by 170 C. baking process after coating 10 nm PMMA on the nanostructure patterned substrate. After forming the smooth profile, a metallic layer 48 is deposited.
[0025] Referring to FIG. 5, embodiments of the invention provide an alternative method for a nanograting sensor device, comprising: a substrate 40, a plurality of nanostructures 52, and a metallic thin film layer 48. In this method, a nanostructure array 50 can be formed on a substrate 40 by electron beam lithography, focused ion beam etching, nanoimprint or any other appropriate method known in the art. Then a heating or etching process is used to round the corner and smoothen the surface to form a smooth shape profile 52. Then a metallic layer can be deposited wherein to form the present sensor device. Furthermore, the desired smooth profile can also be obtained by molding or printing at approximate temperature.
[0026] Referring to FIG. 6, embodiments of the invention further provide a fabrication method for a nanograting sensor device, comprising: a substrate or supporting layer with nanograting structures 62, and a metallic thin film layer 48. In this method, a substrate with nanograting structures 60 are pre-formed by electron beam lithography, focused ion beam etching, nanoimprint, molding, printing or any other appropriate method known in the art. Baking or stamp transferring process can be used to alleviate sharp edges or corners to form the nanograting structures 62. Then a metallic layer can be deposited wherein to form the present sensor device.
[0027] Referring to FIG. 7, the invention further demonstrates a method to fabricate the nanostructures with a smooth profile as shown in FIG. 6. In this method, a substrate with nanostructures are pre-formed by electron beam lithography, focused ion beam etching, nanoimprint, molding, printing or any other appropriate method known in the art. A stamp transferring process can be used to make a stamp or mold 72 and transfer nanostructures with a smooth profile in a substrate 60 to another substrate or a layer 62. Then a metallic layer can be deposited wherein to form the present sensor device. The material of the stamp or mold 70 can be a polymer, a co-polymer, a combination of a polymer and copolymer, or glass. The material of the device substrate 74 can be a polymer, a co-polymer, a combination of a polymer and copolymer, or glass. For example, nanostructure patterns can be generated on a silicon substrate by well-developed nanofabrication techniques, such as electron beam lithography, focus ion beam, and interference lithography. Then, a thin polymethyl methacrylate (PMMA) layer is spin-coated on the patterned silicon substrate and baked at 170 C. to create a smooth shape profile. Next, an elastomer (polydimethylsiloxane, PDMS) is cast onto the pattern silicon substrate to duplicate the nanoscale features that create a PDMS stamp with nanostructures. The stamp is brought into contact with SU-8 coated glass slides under a weight pressing for 2 min. The stamped glass slides are then exposed by a broadband mask aligner to harden or cure the SU-8 photoresist.
[0028] FIGS. 8a and 8b show scanning electron microscopy (SEM) cross-section images of the sensor devices at the periodicity of 600 nm. FIG. 8a shows a SEM cross-section image of the sensor device using the fabrication method involving a polymer coating process. In this structure profile, a substrate with nanograting structures are pre-formed by electron beam lithography, focused ion beam etching, nanoimprint, molding, printing or any other appropriate method known in the art. Next a thin polymethyl methacrylate (PMMA) layer is spin-coated on the patterned substrate. Baking or stamp transferring process can be used to alleviate sharp edges or corners to form the smooth profile as shown in FIG. 8a. FIG. 8b shows a SEM cross-section image of the sensor device using the fabrication method involving a CVD process. The smooth profile of the nanostructures in FIG. 8b can be obtained by a CVD process. For example, the smooth profile can be formed by depositing 10 nm silicon oxide SiO.sub.2 using atomic layer deposition (ALD) on the nanostructure patterned substrate. After forming the smooth profile, a metallic layer can be deposited to form the sensor device.
[0029] In the fabrication process, it is important to alleviate or eliminate sharp edges or corner in the nanostructures. After forming the smooth shape profiles, the height of the single nanostructure is 10 nm or above, the width of the nanostructure is in the subwavelength range. The preferred height of the single nanostructure is 20-100 nm, and the preferred width is 20-200 nm. The preferred thickness of the metallic layer is 10-60 nm.
REFERENCES
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