Abstract:
Disclosed herein describes an SERS sensing substrate comprising upright metal nanostructures made by using oblique angle deposition (OAD) collocating with self-rotation substrate, wherein said upright nanostructures include individual upright nanopillars and metal/dielectric multilayered upright nanopillar stacks. The SERS sensing substrate exhibits higher and enhanced adsorption spectra for unpolarized incident rays in the visible and infrared wavelength regimes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This utility application claims priority to Taiwan application serial number 101101168, filed on Jan. 12, 2012, that is incorporated herein by reference. 
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The invention relates to a surface enhanced Raman spectroscopy (SERS) sensing substrate and the manufacturing method thereof for enhancing Raman signals of specimen to be sensed. 
     2. Brief Description of the Related Art 
     Raman in 1928 invented the Raman scattering method that utilizes scattering property of light for detecting the vibrational spectroscopy signals (Raman spectroscopy) of molecule. However, the relatively minuteness of cross section of molecule beam making the detection of Raman signals difficult causes replacement of this technology by near-infrared spectroscopy (NIRS). 
     After the lasers were invented in the 1960, the lasers have been used as excitation source in Raman spectroscopy since 1964 for magnifying the signals. But the Raman spectroscope is still more expensive than infra-red spectroscopy limiting its application. In 1974, M. Fleischmann et al. found out that the rough metal surface can enhance significantly Raman spectroscopy signals (M. Fleischmann, P. J. Hendra, and A. J. McQulillan, “Raman spectra of pyridine adsorbed at a silver electrode” Chem. Phys. Lett. 26, 123 (1974)) and developed the surface enhanced Raman spectroscopy (SERS). This research has brought a large amount of potential applications. The surface enhanced Raman spectroscopy has improved the differentiation capability of molecule vibrational identification in the chemistry and biological system. Recent research has indicated that introducing of single molecule Raman scattering further enhances the Raman signal sensitivity. This finding has therefore extended the areas of sensor application involving surface enhanced Raman spectroscopy. 
     The state-of-art Raman signal instrumentation includes (a) radiation source; (b) Raman sensor; and (c) detector, wherein the Raman sensor includes surface enhanced Raman spectroscopy sensing substrate. The substrate is exposed to the radiation source and generates the localized plasmonic field, and after the localized plasmonic field is coupled to the specimen molecule being analyze, Raman photons are produced and detected by the Raman photons. Raman spectroscopy, which concerns with inelastic scattering of photons of chemical component, has been a tool for analyzing chemical substances (e.g., biological molecule) other than a tool for analyzing the molecules adsorbed by a surface. 
     The wide application range of surface enhanced Raman spectroscopy includes at least fast medical detection, protein research, pharmaceuticals, scientific discernment, development of biotech medicine, medical detection, health monitoring, single molecule detection, water pollution detection, agricultural products inspection, organic substance detection, environment detection, verification of carbon nano-tube. In the current measuring methods, SERS is also expected to replace the Gas chromatography and High performance liquid chromatography (HPLC). 
     Large area, simple vapor deposition process has attained engraving technique, such as silver zig-zag structures (Yi-Jun Jen, Ching-Wei Yu, Yu-Hsiung Wang, and Jheng-Jie Jhou, “Shape effect on the real parts of equivalent permeability of chevron thin films of silver”, J. Nanophoton. 5, (2011)) and silver aligned nanorod arrays (Yi-Jun Jen, A. Lakhtakia, Ching-Wei Yu, and Chin-Te Lin, “Vapor-deposited thin films with negative real refractive index in the visible regime,” Opt. Express 17, 7784 (2009)). 
     Since the metallic nanopillar structure&#39;s localized electric field strength can effectively enhance the Raman signals, it is proved that the silver aligned nanorod arrays (U.S. Pat. No. 7,658,991 B2) and double layers (silver nanorod/zig-zag dielectric structure) (U.S. Pat. No. 7,956,995 B2) can be applied to SERS. 
    
    
     
       SUMMARY OF THE DISCLOSURE 
         FIG. 1  illustrates how the upright nanopillars structure of SERS sensing substrate of this invention is produced. 
         FIG. 2(   a ),  FIG. 2(   b ),  FIG. 2(   c ),  FIG. 2(   d ),  FIG. 2(   e ),  FIG. 2(   f ),  FIG. 2(   g ),  FIG. 2(   h ) respectively depicts the top view of silver nanopillars and the distribution of diameters thereof based on same deposition speed and different substrate rotation speeds. 
         FIG. 3(   a ),  FIG. 3(   b ),  FIG. 3(   c ),  FIG. 3(   d ),  FIG. 3(   e ),  FIG. 3(   f ) respectively exemplifies single layer of nanopillar and multiple layers of nanopillar structure configuration. 
         FIG. 4(   a ),  FIG. 4(   b ) is SEM top view showing distribution of diameters of silver nanopillar with thickness (height) of 230 nm. 
         FIG. 5  shows transmisivity, reflectivity and adsorptance spectroscopy for structure in  FIG. 4 , wherein the incidence light has X polarized direction. 
         FIG. 6  shows transmisivity, reflectivity and adsorptance spectroscopy for structure in  FIG. 4 , wherein the incidence light has Y polarized direction. 
         FIG. 7  shows Raman spectroscopy when using SERS substrate of single layer of silver nanopillar to sense R6G of 10 −4  M concentration. 
         FIG. 8(   a ),  FIG. 8(   b ) is SEM top view showing distribution of diameters of silver nanopillar/SiO 2  nanopillar/silver nanopillar multiple layers with thickness (height) of 250 nm for nanopillars. 
         FIG. 9  shows transmisivity, reflectivity and adsorptance spectroscopy for structure in  FIG. 8(   a ), wherein the incidence light has X polarized direction. 
         FIG. 10  shows transmisivity, reflectivity and adsorptance spectroscopy for structure in  FIG. 8(   a ), wherein the incidence light has Y polarized direction. 
         FIG. 11  shows Raman spectroscopy when using SERS substrate of silver nanopillar/SiO 2  nanopillar/silver nanopillar multiple layers to sense R6G of 10 −4  M concentration. 
     
    
    
     While preferred embodiments are depicted in the drawings, those embodiments are illustrative and are not exhaustive, and many other equivalent embodiments may be envisioned and practiced based on the present disclosure by persons skilled in the arts. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention now will be described more fully herein with reference to the accompanied figures, in which embodiments of the invention are shown. This invention may, however, be embodied in many alternate forms and should not be construed as limited to the embodiments set forth herein. 
     Accordingly, while the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims. Like numbers refer to like elements throughout the description of the figures. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. 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. It will be further understood that the terms “comprises” and/or “comprising” used in this specification do not preclude the presence or addition of one or more other selectivity features, steps, operations, elements, components, and/or groups thereof. And the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms defined in commonly used dictionaries will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     To attain the objectives of the invention, the invention provides method for producing surface enhanced Raman spectroscopy (SERS) substrate having at least one metal nanopillar structure. The method first involves on the rotating substrate making metal nanopillar structure and/or dielectric nanopillar structure by oblique angle deposition technique collocating with rotating substrate. The nanopillar structure has a configuration in which the nanopillar structure is about parallel to the substrate&#39;s normal line. 
     In the electron gun evaporation system, as shown in  FIG. 1 , the invention utilizes OAD (oblique angle deposition) collocating with rotating substrate, forming multiple silver nanopillars structure  106  of structure diameter D and height (or thickness) L on silicon wafer substrate  101  (or  107 ) or glass substrate  101  (or  107 ). 
     Oblique angle deposition technique is a physical vaporization deposition by which, during the formation of membrane deposition, the substrate  101  in the electron gun evaporation system is tilted by a deposition angle θ v  of substrate normal line with respect to the incoming vapor flux. In the initial deposition, the random nucleation centers are formed on the substrate and the later deposited flux causes preferential growth of nanopillars towards the direction of deposition due to the influence of shadowing effect. Incoming evaporation deposition source  102  (flux) will impinges obliquely through various different directions making nano structure, during the growing process, orients to the substrate normal line Z. And the nanopillars  106  (D, L) are then formed. 
     In other words, while oblique angle deposition, the substrate  101  rotates along the rotation axis  100 , and the rotation direction is denoted as  103 . In the meanwhile, the evaporation flux comes in along the incoming direction  105 , wherein the deposition angle is θv. 
     In a preferred embodiment, the manufacturing parameters are (a) deposition angle is between 0˜90 degree, and/or (b) deposition speed is between 0.01 nm/s˜100 nm/s, and/or (c) substrate rotation speed is between 0.01 rpm˜1000 rpm. 
     As shown in  FIG. 3(   a ),  FIG. 3(   b ),  FIG. 3(   c ),  FIG. 3(   d ),  FIG. 3(   e ),  FIG. 3(   f ) respectively, the nano-pillars structure configuration may be (a) metal nanopillar  301 /substrate  302 , or (b) metal nanopillar  303 /dielectric nanopillar  304 /substrate  305 , or (c) metal nanopillar  308 /dielectric nanopillar  307 /metal nanopillar  306 /substrate  309 , or (d) a first metal material nanopillar  310 /a second metal material nanopillar  311 /substrate  312 , or (e) metal nanopillar  313 /dielectric nanopillar  314 /metal nanopillar  315 /dielectric nanopillar  316 /substrate  317 , or (f) metal nanopillar  318 /metal nanopillar  319 /periodical structures of seed layer  320 /substrate  321 . Or, it also can be (g) metal nanopillar/dielectric nanopillar/periodical structures of seed layer/substrate (not shown). The metal nanopillar  318 , or  319  may be nanopillars of identical or distinct metal material. 
     The metal material is selected from a group consisting of group of material capable of enhancing Raman signals including gold (Au), silver (Ag), copper (Cu), aluminum (Al), lithium (Li), palladium (Pd), and platinum (Pt). 
     The nanopillars structure is made on substrate of periodical or nonperiodical structures, or periodical structures of seed layer/substrate. Therefore the substrate  101 ,  107 ,  302 ,  305 ,  309 ,  312 ,  317 ,  321  might be substrate which is flat or has periodical patterns. 
     The seed layer  320  is selected from a group consisting of dielectric or photo resist material. The substrate  101 ,  107 ,  302 ,  305 ,  309 ,  312 ,  317 ,  321  might be selected from a group of good adhesive property materials to metal/dielectric consisting of silicon wafer, acrylic (PMMA), glass, PET, Al2O3, PC etc. 
     The metal or dielectric nanopillar structure has following properties: (a) nanopillar diameter (D): 10 nm˜1.5 μm, (b) distance of center to center of two adjacent nanopillars: 10 nm˜1.5 μm, (c) total height of pillar: 10 nm˜5 μm. 
     From the above recitations, it is known that a surface enhanced Raman spectroscopy (SERS) sensing substrate produced includes a substrate; at least one single layer of metal nanopillar structure formed on the substrate, the nanopillar structure has a configuration in which the nanopillar structure is about parallel to the substrate&#39;s normal line. The configurations are illustrated in  FIG. 3(   a ),  FIG. 3(   b ),  FIG. 3(   c ),  FIG. 3(   d ),  FIG. 3(   e ), or  FIG. 3(   f ). 
     The Raman instrumentation embodied by above methodology includes (a) irradiation source; (b) Raman sensor including the above mentioned sensing substrate as shown in  FIG. 3(   a ),  FIG. 3(   b ),  FIG. 3(   c ),  FIG. 3(   d ),  FIG. 3(   e ),  FIG. 3(   f ); and (c) detector. Wherein the Raman sensor generates localized plasmonic field after being exposed by the irradiation source, and the plasmonic field enhances Raman signals of the specimen to be sensed. 
     The above irradiation source might be laser devices capable of producing single mode wavelength, which includes solid state-laser, semiconductor (diode) lasers, He—Ne laser, and gas laser. 
     Preferred Embodiment 
     The  FIG. 3(   a ) is used again for following reiterations. In an exemplified manufacturing process, the silver particles of 3 mm diameter of purity 99.999% are adopted as evaporation source  102  and the electron beam gun evaporation system is maintained at base pressure of 4×10 −6  Pa. During deposition, the substrate is rotated about the axis of substrate normal line at a constant speed. The lengths of the pillars are controlled by using a quartz thickness monitor placed next to the substrate. The deposition rate is maintained and the deposition angle is kept at θ v . 
       FIG. 2(   a ), ( c ), ( e ), ( g ) are respectively the top view diagrams taken by scanning electron microscope (SEM) for deposition speed=1.2 nm/s, θ v =89 degrees, collocating with four types of substrate rotation speed of 20 rpm, 10 rpm, 1 rpm and 0.1 rpm, wherein in  FIG. 2(   a ), ( c ), ( e ) nanopillar L=120 nm, and in  FIG. 2(   g ) the nanopillar is L=250 nm. 
       FIG. 2(   b ), ( d ), ( f ), ( h ) respectively shows, at deposition speed=1.2 nm/s, θ v =89 degrees collocating with four rotation speeds of substrate 20 rpm, 10 rpm, 1 rpm and 0.1 rpm, the corresponding distribution of nanopillar diameter D. From  FIG. 2(   b ), ( d ), ( f ), it is found that while the deposition speed is unchanged, lowering of substrate rotational speed will result in the distribution of diameter D of nanopillars moving to direction of larger dimension. In addition, increase of nanopillar&#39;s height L ( FIG. 2(   g )) also results in larger of diameter D of nanopillar. 
     Other than single layer of metal nanopillar, pillars of different materials may be formed in a stack by same process. As shown in  FIG. 3  and above recitations, the substrate  302  (or  305 ,  309 ,  312 ,  317 ,  321 ) might be substrate having periodical structure patterns, the dimensions of seed layer  320  and substrate of periodical structure are in nano-scale as well. Other than the substrate, this configuration may further extend to a configuration which is formed by further upward periodical stacks. 
       FIG. 4(   a ) shows the top view diagram taken by SEM on the specimen of single layer of silver nanopillars of L=230 nm obtained by process under the deposition speed 1.2 nm/s, θ v =89 degrees collocating with rotational substrate speed of 10 rpm.  FIG. 4(   b ) shows diameter D distribution of single layer of silver nanopillars. 
       FIG. 5  and  FIG. 6  respectively shows the transmissivity (T), reflectivity (R) and adsorbtivity (A) spectroscopy for structure of  FIG. 4  while x polarized incidence light and y polarized incidence light are respectively used. From  FIG. 5  and  FIG. 6 , the spectroscopy property of this single layer of silver nanopillar structure is not correlated with polarization, wherein the adsorbtivity (A) exceeds 78% in range of 400 nm˜850 nm. 
       FIG. 7  shows Raman spectroscopy at different locations (p0, p1, p2, p3) of surface while configuration of  FIG. 3(   a ) is used as SERS substrate. The 532 nm laser of 100 mW is used for the excitation light source, the measurement are made at above locations for Rhodamine (R6G) of 10 −4  M concentration. In particular, the excitation light source laser has light-spot size of 1 μm over the nano structure. Rhodamine can be obtained by diluting of de-ionized water to needed 10 −4  M concentration. The p0 distribution is Raman spectroscopy of reference point, the p1 distribution is Raman spectroscopy for location distant from the reference point p0 by 100 μm, the p2 distribution is Raman spectroscopy for location distant from the reference point p0 by 200 μm, the p3 distribution is Raman spectroscopy for location distant from the reference point p0 by 2000 μm. From  FIG. 7 , compared to Raman spectroscopy of silver aligned nanopillar arrays, it is observed that the Raman signals of the silver nanopillars shown in  FIG. 3(   a ) configuration are stronger than that of the silver aligned arrays. 
     In the followings, the triple layers of metal  306 /dielectric  307 /metal  308 /substrate  309  shown in  FIG. 3  ( c ) is used for explaining the advantage or benefit of the invention. 
       FIG. 8(   a ) shows the top view diagram taken by SEM on the specimen of single layer of silver nanopillars of L=250 nm obtained by process under the deposition speed 1.2 nm/s, θ v =89 degrees collocating with rotational substrate speed of 10 rpm.  FIG. 8(   b ) shows diameter D distribution of single layer of silver nanopillars. 
       FIG. 9  and  FIG. 10  respectively shows the transmissivity (T), reflectivity (R) and adsorbtivity (A) spectroscopy for structure of  FIG. 8(   a ) while X polarized incidence light and Y polarized incidence light are respectively used. From  FIG. 9  and  FIG. 10 , the spectroscopy property of this single layer of silver nanopillar structure is not correlated with polarization, wherein the adsorbtivity (A) exceeds 57% in range of 400 nm˜850 nm. 
       FIG. 11  shows Raman spectroscopy at different locations (s0, s1, s2) of surface while configuration of silver nanopillar/SiO 2  nanopillar/silver nanopillar multiple layers of is used as SERS substrate. The 532 nm laser of 100 mW is used for the excitation light source, the measurement are made at above locations for Rhodamine (R6G) of 10 −4  M concentration. In particular, the excitation light source laser has light-spot size of 1 μm over the nano structure. Rhodamine can be obtained by diluting of de-ionized water to needed 10 −4  M concentration. The p0 distribution is Raman spectroscopy of reference point, the p1 distribution is Raman spectroscopy for location distant from the reference point p0 by 100 μm, the p2 distribution is Raman spectroscopy for location distant from the reference point p0 by 200 μm. From  FIG. 7 , compared to Raman spectroscopy of tilting silver nanopillars array, it is observed that stronger and stable SERS signals are resulted by nanopillars structure shown in  FIG. 3(   c ) configuration. 
     The scope of protection is limited solely by the claims, and such scope is intended and should be interpreted to be as broad as is consistent with the ordinary meaning of the language that is used in the claims when interpreted in light of this specification and the prosecution history that follows, and to encompass all structural and functional equivalents thereof.