Periodic crescent-shaped nano-gap array and preparation method thereof
Technical Field
The invention belongs to the technical field of surface enhanced Raman scattering, and particularly relates to a periodic crescent-shaped nano-gap array with greatly improved surface enhanced Raman scattering performance and a preparation method thereof.
Background
The Surface Enhanced Raman Scattering (SERS) structured film on a substrate is attractive because it is in the plasma[1,2]Non-linear optics[3]Single molecule detection[4,5]Optoelectronics[6]And meta-materials[7]And the like. A large number of adjacent nano-micro particles[8]Nanowires, and a method for producing the same[9]And some more complex nano-microstructures[4-6]The gaps between the nano-microstructures or the nano-microstructures with the tips show good surface enhanced Raman scattering performance.
However, the noble metal nano-microstructure with the gap and the tip is very few, and electron beam etching, focused ion beam etching and the like are generally adopted, and the complicated processes require expensive instruments, are long in time consumption and have low flux, so that the further application of the noble metal nano-microstructure is limited. Therefore, how to select a universal and controllable preparation method becomes the current exploration hotspot.
[1]Ward,D.R.;Hueser,F.;Pauly,F.;Carlos Cuevas,J.;Natelson,D.Nature Nanotechnology 2010,5,732-736.
[2]Kim,S.;Jin,J.;Kim,Y.J.;Park,I.Y.;Kim,Y.;Kim,S.W.Nature 2008,453,757-760.
[3]Danckwerts,M.;Novotny,L.Physical review letters 2007,98,026104.
[4]Chirumamilla,M.;Toma,A.;Gopalakrishnan,A.;Das,G.;Zaccaria,R.P.;Krahne,R.;Rondanina,E.;Leoncini,M.;Liberale,C.;De Angelis,F.;Di Fabrizio,E.Adv.Mater.2014,26,2353-2358.
[5]Shi,X.;Verschueren,D.;Pud,S.;Dekker,C.Small 2017,1703307.
[6]Dong,Z.;Asbahi,M.;Lin,J.;Zhu,D.;Wang,Y.M.;Hippalgaonkar,K.;Chu,H.S.;Goh,W.P.;Wang,F.;Huang,Z.;Yang,J.K.W.Nano Lett.2015,15,5976-5981.
[7]S.P.Burgos,R.de Waele,A.Polman,H.A.Atwater,Nat.Mater.2010,9,407.
[8]Z.Dong,M.Asbahi,J.Lin,D.Zhu,Y.M.Wang,K.Hippalgaonkar,H.S.Chu,W.P.Goh,F.Wang,Z.Huang,J.K.Yang,Nano Lett.2015,15,5976.
[9]Z.Zhou,Z.Zhao,Y.Yu,B.Ai,H.
R.C.Chiechi,J.K.W.Yang,G.Zhang,Adv.Mater.2016,28,2956。
Disclosure of Invention
The invention aims to provide a crescent-shaped nano gap array with greatly improved surface enhanced Raman scattering performance and a preparation method thereof.
The invention uses colloid etching method, metal auxiliary wet etching, physical vapor deposition method, nanometer cutting method, etc., the whole process is simple and easy to operate, and the controllability is high. Crescent nanometer gap arrays with different periods can be prepared by controlling the etching time; by controlling the thickness of the middle layer, crescent nanometer gap arrays with different gap widths can be prepared, the coupling of multiple hot spots is realized, and the surface enhanced Raman scattering performance is greatly improved.
The invention relates to a preparation method of a crescent-shaped nanometer gap array with greatly improved surface enhanced Raman scattering performance, which comprises the following steps:
1) putting a silicon substrate into a mixed solution of 98% by mass of concentrated sulfuric acid and 30% by mass of hydrogen peroxide (the volume ratio of the concentrated sulfuric acid to the hydrogen peroxide is 7:3) heating to 70-90 ℃, and keeping for 3-8 h; placing the silicon substrate to room temperature, taking out the silicon substrate, washing the silicon substrate with a large amount of deionized water and ethanol, drying the silicon substrate with nitrogen, and cleaning the surface of the silicon substrate with oxygen plasma for 2-5 min to obtain a hydrophilic and uniform surface;
2) adding 2-5 mL of deionized water into 1-5 mL of deionized water dispersion liquid of polystyrene microspheres with the concentration of 1-20 wt% and the diameter of 500-900 nm, centrifuging at the rotating speed of 6000-10000 rpm for 8-15 min, adding 2-5 mL of deionized water into a solid obtained after centrifugation, and centrifuging at the rotating speed of 6000-10000 rpm for 8-15 min; repeatedly adding deionized water and centrifuging for 3-5 times; adding 2-5 mL of solid obtained by centrifugation into the solid, wherein the volume ratio of the solid is 1: 1, centrifuging for 8-15 min at the rotating speed of 6000-10000 rpm, adding 2-5 mL of mixed solution of ethanol and deionized water into the solid obtained after centrifugation, and centrifuging for 8-15 min at the rotating speed of 6000-10000 rpm; repeatedly adding a mixed solution of ethanol and deionized water and centrifuging for 6-10 times; adding 2-5 mL of mixed solution of ethanol and deionized water into the finally obtained solid to obtain 1-10% by mass of ethanol and deionized water dispersion liquid of the hydrophobic polystyrene microspheres; sucking 0.1-0.5 mL of ethanol and deionized water dispersion liquid of hydrophobic polystyrene microspheres by using a disposable syringe, dropwise adding the dispersion liquid into a culture dish containing deionized water, arranging the hydrophobic polystyrene microspheres into a single layer at an air-deionized water gas-liquid interface, adding 50-120 mu L of aqueous solution of sodium dodecyl sulfate surfactant with the mass concentration of 1-10 wt% to ensure that the hydrophobic polystyrene microspheres are tightly arranged, slightly supporting the single-layer tightly arranged hydrophobic polystyrene microspheres by using the hydrophilic-treated silicon substrate obtained in the step 1), and naturally drying the silicon substrate on filter paper inclined at 40-60 degrees, thereby obtaining a two-dimensional ordered single-layer tightly arranged hydrophobic polystyrene microsphere array with the diameter of 500-900 nm on the silicon substrate;
3) placing the two-dimensional ordered hydrophobic polystyrene microsphere array substrate which is prepared in the step 2) and is densely arranged in a single layer with the diameter of 500-900 nm in a reactive plasma etching machine, etching for 90-120 s under the conditions that the etching pressure is 5-10 mTorr, the etching temperature is 10-25 ℃, the oxygen flow rate is 10-50 sccm, the Radio Frequency (RF) power is 20-100W and the Inductively Coupled Plasma (ICP) power is 200-400W, and the polystyrene microspheres are etched and gradually reduced in diameter in the etching process;
4) placing the polystyrene microspheres etched in the step 3) and the silicon substrate on a sample table of vacuum thermal evaporation coating equipment, wherein the included angle between the normal of the substrate and the deposition direction is 0 DEG, and the deposition speed is
The thickness of the deposited silver film is 20-40 nm; putting the substrate deposited with the silver film into toluene for ultrasonic treatment for 2-5 min, wherein the ultrasonic power is 30-50W, and removing the polystyrene microspheres and the silver film covered on the polystyrene microspheres to obtain a silver nanopore film array substrate with the period of 500-900 nm and the pore diameter of 200-500 nm;
5) soaking the silver nanopore membrane array substrate prepared in the step 4) into 10-15 mL of etching solution (the volume ratio of hydrofluoric acid to hydrogen peroxide to water is 1: 1: 2) carrying out metal-assisted wet etching for 5-10min, fishing out the etched substrate, washing with ethanol, drying with nitrogen, and obtaining a large-area silicon substrate with the length-diameter ratio of (5-10): 1 (wet etching is to remain the silicon substrate under the silver film, so that the part of the silicon substrate still covered by the silver film is etched and collapsed, i.e. a silicon column array is formed);
6) placing the silicon column array substrate obtained in the step 5) into a vacuum dryer, dropwise adding trichloro- (1H, 1H, 2H, 2H-perfluorooctyl) silane into the dryer, and placing the dryer into an oven for modification at 50-80 ℃ for 2-5H; putting the modified substrate into a culture dish, pouring a prepared Polydimethylsiloxane (PDMS) prepolymer (the mass ratio of polydimethylsiloxane resin to a curing agent is 10:1) into the culture dish, putting the culture dish into an oven to be dried for 2-5 h at 50-80 ℃, and stripping the silicon column array substrate from the flexible polydimethylsiloxane substrate; pouring epoxy resin prepolymer (the volume ratio of epoxy resin to curing agent is 15:2) on the obtained structured flexible polydimethylsiloxane prepolymer substrate to enable the epoxy resin prepolymer to completely cover the structured Polydimethylsiloxane (PDMS) prepolymer; then placing the substrate into a drying oven to be dried for 2-5 h at the temperature of 50-80 ℃, and slightly stripping the obtained structured epoxy resin substrate;
7) placing the structured epoxy resin substrate obtained in the step 6) on a sample table of vacuum evaporation coating equipment, wherein the included angle between the normal of the substrate and the deposition direction is 25-40 degrees, and the deposition speed is
Continuously depositing 3 layers of metal, wherein the thicknesses of the 3 layers of metal are 80-120 nm of a gold film, 1-8 nm of an aluminum film and 80-120 nm of a gold film respectively;
8) placing the substrate obtained in the step 7) into a mold, and pouring epoxy resin prepolymer; placing the obtained substrate into a fixing device of an ultrathin slicer (Leica EM UC 7), slicing the substrate in a direction parallel to the substrate by using an ultrathin diamond knife (the slicing speed is set to be 0.6-1.0 mm/s, the slicing thickness is 80-150 nm), collecting and slicing the silicon substrate deposited with a 60-100 nm gold film in a water phase, drying the silicon substrate, and then soaking the silicon substrate and the slices on the silicon substrate into 2M dilute hydrochloric acid solution for 1-3 hours to remove the aluminum film in the middle of 3 layers of metal; then placing the substrate in a reactive plasma etching machine, etching for 120-200 s under the conditions that the etching pressure is 5-10 mTorr, the etching temperature is 10-25 ℃, the oxygen flow rate is 10-50 sccm, the Radio Frequency (RF) power is 60-100W and the power of Inductively Coupled Plasma (ICP) is 200-300W, and removing all redundant epoxy resin, thereby obtaining the periodic crescent nano-gap array with the surface enhanced Raman scattering performance on the gold film substrate;
placing the structured epoxy resin substrate obtained in the step 6) on a sample table of vacuum evaporation coating equipment, wherein an included angle between the normal of the substrate and the deposition direction is 25-40 degrees, carrying out thermal evaporation deposition on a layer of 80-120 nm gold film, placing the obtained sample in a mold, and pouring epoxy resin prepolymer; then placing the silicon substrate into an ultrathin slicer to slice along the direction parallel to the substrate (the slicing speed is set to be 0.6-1.0 mm/s, the slicing thickness is 80-150 nm), collecting the slices in a water phase by using a silicon substrate deposited with a 60-100 nm gold film, and drying the slices; then placing the gold nanowire array in a reactive plasma etching machine, etching for 120-200 s under the conditions that the etching pressure is 5-10 mTorr, the etching temperature is 10-25 ℃, the oxygen flow rate is 10-50 sccm, the Radio Frequency (RF) power is 60-100W and the power of Inductively Coupled Plasma (ICP) is 200-300W, removing all redundant epoxy resin, and obtaining a crescent gold nanowire array as a reference sample;
respectively placing the crescent-shaped nano gap array and the crescent-shaped nano wire on the gold film substrate prepared in the step 8) on a sample table of a high-resolution excitation Raman spectrometer, testing the Raman spectrum of the sample table, and analyzing the surface enhanced Raman scattering performance of the sample table.
The method has the advantages that the operation of each step is simple, the controllability is strong, in the preparation of the crescent-shaped nanometer gap array, the characteristic peak intensity of the Raman spectrum is gradually increased along with the reduction of the gap width, the Raman intensity is maximized when the gap width is 1nm, the periodic crescent-shaped nanometer gap array with the improved surface enhanced Raman scattering performance is obtained, and the method can be applied to practical applications such as photoelectric devices.
Drawings
FIG. 1 is a flow chart of the preparation of an epoxy column array template, in which various materials utilized and major operating steps are identified; wherein, the polystyrene microsphere is 1nm or 700 nm; a substrate 2, a silicon substrate; 3. a silver nanoporous film; 4. polydimethylsiloxane; 5. an array of epoxy columns;
FIG. 2A is a Scanning Electron Microscope (SEM) photograph of the etched polystyrene microsphere array obtained in step 3); FIG. 2B is a Scanning Electron Microscope (SEM) photograph (top view) of the silicon pillar array obtained by wet etching in step 5); fig. 2C is a Scanning Electron Microscope (SEM) photograph (30 ° side view) of the silicon pillar array obtained in step 5); fig. 2D is a Scanning Electron Microscope (SEM) photograph (cross-sectional view) of the silicon pillar array obtained in step 5); the scales are all 1 μm;
FIG. 3 is a flow chart for preparing a crescent-shaped nanogap array; corresponding to steps 7 and 8; the various materials utilized and the major operating steps are labeled in the figures; wherein 5 is an epoxy resin column array;
FIG. 4A is a schematic diagram of structural parameters of a crescent-shaped nanogap array (H represents the height (100nm) of a crescent-shaped nanogap, W represents the thickness (80nm) of a gold film, G represents the width (1-8 nm, the thickness of an aluminum film) of a nanogap, and P represents the period (i.e., the size of an original polystyrene microsphere is 700nm) of the crescent-shaped nanogap; FIGS. B-F are Scanning Electron Microscope (SEM) photographs of crescent-shaped nanogap arrays having different gap widths; b is a top view of the crescent-shaped nano-gap array with the gap width of 1 nm; c is a top view of the crescent-shaped nano-gap array with the gap width of 2 nm; d is a top view of the crescent-shaped nano-gap array with the gap width of 3 nm; e is a top view of the crescent-shaped nano-gap array with the gap width of 5 nm; f is a top view of the crescent-shaped nano-gap array with the gap width of 8 nm;
fig. 5 is a raman spectrum measured by a high-resolution laser raman spectrometer.
FIG. 5A is a Raman spectrum of a crescent-shaped nanogap array with different gap widths, which is sequentially a Raman spectrum with a gap width of 1nm, a Raman spectrum with a gap width of 2nm, a Raman spectrum with a gap width of 3nm, a Raman spectrum with a gap width of 5nm, and a Raman spectrum with a gap width of 8nm from top to bottom, and illustrates that the Raman intensity is greatly improved and the surface enhanced Raman scattering performance is good as the gap width is reduced;
FIG. 5B shows a 1077cm crescent-shaped nanogap array with different gap widths-1The Raman intensity of the Raman scattering film is in a monotone increasing trend along with the reduction of the nano-gap, which shows that the crescent nano-gap array has good performanceGood surface enhanced Raman scattering performance;
FIG. 5C is a Raman spectrum of the crescent gold nanowires (corresponding to step 8), wherein the characteristic peak intensity of the crescent gold nanowires is significantly lower than that of the crescent nanowire array with gaps in the graph A, which indicates that the gap-enhanced surface-enhanced Raman scattering plays an important role;
fig. 5D is a raman spectrogram obtained by randomly selecting 16 positions on a crescent-shaped nanogap array having a gap width of 1nm and measuring with a high-resolution laser raman spectrometer, wherein the profiles of the raman spectrogram are substantially the same, which indicates that the obtained structure has good uniformity.
Detailed Description
Example 1: preparation of hydrophilic silicon substrate
Cutting the silicon wafer into 2 × 2cm pieces with a glass cutter2And (2) putting the silicon wafer into a mixed solution (volume ratio of concentrated sulfuric acid to hydrogen peroxide is 7:3) of concentrated sulfuric acid (mass fraction is 98%) and hydrogen peroxide (30%), heating to 80 ℃, keeping the temperature for 5 hours, putting the silicon wafer to room temperature, washing the silicon wafer with a large amount of deionized water to obtain a hydrophilic silicon wafer, washing the silicon wafer with ethanol, drying the silicon wafer with nitrogen, and cleaning the surface for 3 minutes by using an oxygen plasma machine to obtain a hydrophilic and uniform surface.
Example 2: preparation of hydrophobic polystyrene microspheres
Adding 3mL of deionized water into 1mL of polystyrene microsphere aqueous dispersion with the diameter of 700nm at room temperature for 10min by ultrasonic treatment, centrifuging at 8900rpm for 10min, sucking supernatant, adding 3mL of deionized water into the remaining solid substance, ultrasonic treatment and centrifuging again, and repeating the process for 4 times. After the supernatant liquid is absorbed for the last time, 3mL of mixed liquid of ethanol and deionized water (the volume ratio of the ethanol to the deionized water is 1: 1) is added into the solid matter, the mixture is centrifuged at 8900rpm for 10min, the supernatant liquid is absorbed, then 3mL of mixed liquid of the ethanol and the deionized water is added into the remaining solid matter, the mixture is subjected to ultrasound and centrifugation again, the process is repeated for 8 times, after the supernatant liquid is absorbed for the last time, the remaining solid matter is dried in an oven at 30 ℃ for 8h, and the mixed liquid of the ethanol and the deionized water is added to prepare 5 wt% of dispersion liquid for later use.
Example 3: preparation of hexagonal close-packed monolayer polystyrene colloidal crystal
0.2mL of the ethanol and deionized water dispersion of the hydrophobic polystyrene microspheres with the diameter of 700nm prepared in example 2 was sucked by a disposable syringe, and was slowly dropped onto the air-deionized water interface of a petri dish by an instrument, 80. mu.L of an aqueous solution of sodium dodecyl sulfate with a mass concentration of 10 wt% was added along one side of the petri dish, and after standing for a while, the polystyrene microspheres formed a monolayer hexagonal close-packed morphology on the air-deionized water interface. Taking the hydrophilic silicon substrate obtained in the example 1 as an example, the hydrophilic silicon substrate is deeply inserted into the water surface, is slowly lifted upwards from the position below the compact single-layer polystyrene microsphere, and is placed on filter paper with an inclined plane of 60 degrees for natural drying, so that the single-layer hexagonal close-packed polystyrene colloidal crystal with the diameter of 700nm is formed on the silicon substrate.
Example 4: preparation of silver nanopore membrane array
Placing the silicon substrate of the 700 nm-diameter two-dimensional ordered single-layer hexagonal close-packed polystyrene colloidal crystal array prepared in the example 3 in a chamber of a reactive plasma etching machine, and etching for 120s (as shown in fig. 2A) under the conditions that the etching pressure is 10mTorr, the etching temperature is 20 ℃, the oxygen flow rate is 50sccm, the Radio Frequency (RF) power is 30W and the Inductively Coupled Plasma (ICP) power is 300W; placing the etched polystyrene microsphere array with the reduced diameter on a sample table of vacuum coating equipment, wherein the included angle between the normal line of the sample and the deposition direction is 0 DEG, and the thermal evaporation deposition speed is

The thickness of the deposited silver film is 20 nm; immersing the substrate deposited with the silver film into toluene for ultrasonic treatment for 4min, wherein the power is 40w, and removing the microspheres to obtain a silver nanopore film array substrate with the period of 700nm and the aperture of 350 nm;
example 5: preparation of large-area high-length-diameter ratio silicon column array
Immersing the silver nanopore membrane array substrate obtained in the example 4 into etching liquid (the volume ratio of hydrofluoric acid to hydrogen peroxide to deionized water is 1: 1: 2), etching for 10min, gradually etching the silicon substrate under the silver membrane, washing the etched wafer with ethanol, and drying the wafer with nitrogen to obtain the silver nanopore membrane array substrate with the cycle of 700nm and the diameter ratio of 350nm of 6: 1, corresponding to fig. 2B, 2C, 2D;
example 6: replicated epoxy column array
Placing the silicon column array substrate obtained in the example 5 into a vacuum dryer, dropwise adding 2 drops of trichloro- (1H, 1H, 2H, 2H-perfluorooctyl) silane into the dryer, and placing the dryer into an oven for modification at 60 ℃ for 3 hours; (ii) a Placing the modified substrate into a culture dish, pouring the prepared polydimethylsiloxane prepolymer into the culture dish, placing the culture dish into an oven to dry for 3 hours at the temperature of 60 ℃, and stripping the silicon column array substrate from the flexible polydimethylsiloxane substrate; pouring epoxy resin prepolymer on the structured flexible substrate, putting the substrate into an oven to be dried for 3 hours at 60 ℃, slightly stripping the structured epoxy resin substrate, and standing the substrate for later use;
example 7: three-layer metal gold/aluminum/gold continuous deposition
The replicated epoxy resin substrate obtained in example 6 was placed on a vacuum evaporation-coated sample stage, and the angle (incident angle) between the normal to the sample and the deposition direction was 35 °, with the deposition rate being set as
Performing thermal evaporation to deposit 3 layers of metal gold/aluminum/gold, wherein five samples with the deposition thicknesses of 80nm/1, 2, 3, 5 and 8nm/80nm are respectively deposited;
example 8: preparation of crescent-shaped nano gap array
The sample obtained in example 7 was cut into 2X 2mm2Placing the block into a mold and pouring more epoxy resin prepolymer; placing the obtained sample in a fixture of a microtome (Leica EM UC 7), slicing in a direction parallel to the substrate by using a diamond knife (the slicing speed is set to be 0.8mm/s, and the thickness of the slice is 100nm), collecting continuous slices in an aqueous phase by using a silicon substrate deposited with an 80nm thick gold film, and drying; the collected slices were immersed in 2M dilute hydrochloric acid solution for two hours for removalDrying the interlayer aluminum layer with ethanol, and drying at room temperature; then placing the substrate in a chamber of a reactive plasma etcher, etching for 180s under the conditions that the etching pressure is 10mTorr, the etching temperature is 20 ℃, the oxygen flow rate is 50sccm, the Radio Frequency (RF) power is 60W and the Inductively Coupled Plasma (ICP) power is 200W to remove the epoxy resin, and obtaining a crescent-shaped nano gap array on the gold film substrate, which corresponds to the graph 4;
example 9: method for detecting SERS enhanced performance of crescent-shaped nanogap array
Placing the crescent-shaped nano-gap array obtained in the embodiment 8 on a sample table of a high-resolution excitation Raman spectrometer, and testing the Raman spectrum of the crescent-shaped nano-gap array; the laser wavelength is 663nm at 800-1800 cm-1The reflection spectrum of the crescent-shaped nano-gap array is tested in the range of (1), which proves that the crescent-shaped nano-gap array has stronger Raman enhancement performance and corresponds to the graph in FIG. 5A; and as the gap width is reduced, the raman intensity is monotonically increased (fig. 5B), and by comparison with the crescent-shaped gold nanowires (fig. 5C), the gap plays a decisive role in raman enhancement. The 16 positions are randomly selected on the crescent-shaped nano-gap array with the gap width of 1nm, and the Raman spectrum profiles of the Raman spectrum images measured by the high-resolution laser Raman spectrometer are basically the same, which shows that the obtained structure has good uniformity and corresponds to FIG. 5D.
The above description is only a preferred embodiment of the present invention, and is not intended to limit the method scheme of the present invention in any way. Any simple modification, equivalent changes and modifications of the above embodiments according to the method substance of the present invention fall within the scope of protection of the present invention.