JP7296681B1 - Optical device and its manufacturing method - Google Patents

Abstract

An object of the present invention is to provide an optical device with a novel structure that can be applied to a living body and is resistant to deterioration over time. At least a laminated film in which a support substrate, a noble metal crystal layer, and a light transmissive flat layer are laminated in this order, the noble metal crystal layer has a nanohole structure, and the light transmissive flat layer has an atomic level An optical device having flatness and heat resistance, in which excitation light is incident on the nanohole structure from the light-transmitting flat layer side to excite surface plasmons.

Description

The present invention relates to an optical device using surface-enhanced Raman scattering and a method of manufacturing the same.

In surface-enhanced Raman scattering (SERS), the Raman scattering intensity of molecules adsorbed on the surface of precious metal particles such as silver and gold is 10 ⁸ times or more due to localized surface plasmon resonance (LSPR). In recent years, technology using this phenomenon has attracted attention as a technology that can diagnose infectious diseases, cancers, etc., and can quickly and easily test food and water quality.
Noble metals, such as silver and gold, are used as materials for expressing this surface-enhanced Raman scattering (SERS), and in order to obtain high sensitivity, devices are known in which silver and gold deposited on glass or _SiO2 substrates are microfabricated. (See, for example, Patent Document 1).

Japanese Patent Application Publication No. 2012-508881

However, the technique described in Patent Document 1 uses metal as it is as a SERS activator for enhancing Raman scattering, but metal is difficult to apply to living organisms because of its high antibacterial action and thermal conductivity. Since the surface changes due to chemical reactions, it has the disadvantage of being prone to deterioration. In addition, the technology described in Patent Document 1 is composed of complex convex nanostructures, the manufacturing process is complicated and expensive, and there are problems with uniformity and reproducibility.
Accordingly, it is an object of the present invention to provide an optical device with a novel configuration for overcoming these drawbacks and problems.

In order to solve the aforementioned problems, an optical device according to the present invention comprises at least a support substrate, a noble metal single crystal layer, and a light transmissive flat layer for transmitting incident light that excites surface plasmons , laminated in this order. The noble metal single crystal layer has a nanohole structure, and the light transmissive flat layer has atomic level flatness and heat resistance to form the noble metal single crystal layer by epitaxial growth .

A silver single crystal thin film can be used as the noble metal single crystal layer, and sodium chloride or lithium fluoride having a NaCl structure can be used as well as mica as the light-transmitting flat layer. It may be laminated. In addition, the optical device can be used as a biosensor by fixing an antigen sandwiched between antibodies .

1 is a cross-sectional view of an optical device (Example 1) according to an embodiment of the present invention; FIG. 4 is a graph showing surface plasma polariton propagation lengths of silver thin films. 1 is a graph showing an X-ray diffraction spectrum of a silver thin film formed on a mica substrate; 4 is a graph showing surface-enhanced Raman scattering signals from the silver surface side. 4 is a graph showing surface-enhanced Raman scattering signals from the mica substrate side. 1 is an SEM photograph of silver nanodots formed on a mica substrate. 4 is a SEM photograph of silver nanoholes formed on a mica substrate. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the manufacturing method of a general nanohole structure, (a) has shown the 1st process, (b) has shown the 2nd process, and (c) has shown the 3rd process. It is a top view in the state of Fig.8 (a). It is a top view in the state of FIG.8(b). BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the manufacturing method of the optical device (Example 1) which concerns on embodiment of this invention, (a) is a 1st process, (b) is a 2nd process, (c) is a 3rd process. , (d) shows the completed optical device in action. 1 is a cross-sectional view of an optical device (Example 2) according to an embodiment of the present invention; FIG. 4 is a graph showing surface-enhanced Raman scattering signals from the graphene substrate side; 4 is a graph showing surface-enhanced Raman scattering signals from the side of a substrate on which graphene and silver are laminated; FIG. 3 is a cross-sectional view of an optical device (Example 3) according to an embodiment of the present invention; FIG. 3 is a diagram illustrating a method for manufacturing an optical device (Example 3) according to an embodiment of the present invention, in which (a) is the first step, (b) is the second step, and (c) is the third step. , (d) shows the completed optical device in action.

Hereinafter, the configuration of an optical device according to embodiments of the present invention will be described with reference to the drawings. In the following description, the same reference numerals in different figures denote portions having the same function, and duplication of description in each figure will be omitted as appropriate.

[Example 1]
FIG. 1 is a cross-sectional view of an optical device (Example 1) according to an embodiment of the present invention, showing an example of application as a biosensor. The optical device is a surface-enhanced Raman scattering element, and comprises a supporting substrate 11 made of a glass substrate or the like, a silver single crystal layer 12, and a mica layer 13 as a light-transmitting flat layer, which are laminated in this order. membrane. However, as a manufacturing process, the support substrate 11, the silver single crystal layer 12, and the mica 13 are not laminated in this order. This will be described later in the description of the manufacturing method.

In the optical device according to the embodiment of the present invention (Example 1), the silver single crystal layer 12 has holes and grooves (hereinafter referred to as “nanoholes”) having a microstructure for activating surface-enhanced Raman scattering. formed by self-organization. In addition, although the film thickness of the silver single crystal layer 12 of the optical device (Example 1) according to the embodiment of the present invention is 70 nm, nanoholes are self-organized when the film thickness is 50 to 100 nm. The film thickness of the mica 13 is desirably 50 μm or less, and is 20 μm in the optical device (Example 1) according to the embodiment of the present invention.

As FIG. 1 shows, the surface of the optical device according to the embodiment of the invention is mica 13 . Since mica (mica) has atomic level flatness and hydrophilicity, it is possible to stably fix a hydrophilic sample S thereon. FIG. 1 shows how the Raman scattering emitted by the excitation light Lex incident on the sample S is amplified by the optical device according to the embodiment of the present invention and emitted as surface-enhanced Raman spectroscopy Ls. there is However, this is a diagram for explanation, and the incident angle is not necessarily set as described above, and may be vertically incident. Further, when mounted as an actual device, other light such as Rayleigh scattered light is removed by using an appropriate filter or the like, and only Raman scattered light is taken out.

As is clear from FIG. 1, the silver single crystal layer 12, which is the fine structure, is sealed with mica 13. As shown in FIG. Therefore, antibacterial action is prevented and chemical stability is maintained. Normally, when producing a convex nanostructure, for example, in the case of a silver thin film, a nanostructure is formed by microfabrication of a laminated film formed by evaporating silver on a glass substrate. The film becomes polycrystalline. However, in the present invention, the laminated film deposited by self-organization on the mica substrate after setting conditions such as temperature becomes a single crystal. A single-crystal silver thin film has an improved index (called a Q value) indicating the degree of loss of scattered light when light is scattered, compared to a polycrystalline one, and has higher sensitivity. FIG. 2 is a graph showing the surface plasma polariton propagation length of a silver thin film, and it is understood that silver single crystals have higher sensitivity than silver polycrystals. From this result, it can be seen that the molecules to be detected are affected by plasmon even if they are separated from the silver single crystal thin film by about 50 μm. FIG. 3 is a graph showing the X-ray diffraction spectrum of the silver thin film formed on the mica substrate, which indicates that it is a single crystal oriented in the [111] direction.

In the laminate of mica and the silver single crystal layer, since mica is light transmissive, a surface-enhanced Raman signal should be obtained even when the excitation light is incident from the back side of the silver single crystal layer. be. The original aim of the present invention is to make use of this fact to seal a silver single crystal layer with mica so that it can be applied to a living body. In addition to this, the present invention was able to confirm an advantageous effect in terms of sensitivity. This is illustrated in FIGS. 4 and 5, which are graphs evaluating the surface-enhanced Raman scattering of a sample in which the dye rhodamine 6G aqueous solution (10@ ^-6 M ) was dropped on both sides of the sample . The peak indicated by "*" in the graph is the peak due to rhodamine 6G. FIG. 4 shows surface-enhanced Raman scattering signals from the silver surface side, and FIG. 5 shows surface-enhanced Raman scattering signals from the mica substrate side. In FIG. 5, the second peak from the smaller wavenumber side is not seen in FIG. 4, and this peak is caused by mica. Since the peak from the mica substrate side is more enhanced than the peak from the silver surface side, it can be understood that the flat surface exhibits the function of improving the sensitivity of the optical device. Yo.

By the way, in a sample having a mica substrate with a film thickness of 145 μm, a signal of surface-enhanced Raman scattering could not be observed from the mica side. However, when the film thickness of the mica substrate was reduced to about 50 μm, it was confirmed that signals of surface-enhanced Raman scattering were observed even from the mica side. 4 and 5 are the observation results when the film thickness of the mica substrate is 20 μm.

In fact, it is possible to observe surface-enhanced Raman scattering signals even when a nanodot structure is formed on mica. The nanodot structure can be produced by subjecting a silver thin film with a thickness of 10 nm to a high temperature of about 400.degree. However, the substrate with the nanohole structure was more sensitive than the substrate with the nanodot structure. The reason for this is that in the nanodot substrate, surface-enhanced Raman scattering occurs only by localized surface plasmon resonance (LSPR), whereas in the nanohole structure, in addition to LSPR, surface plasmon polariton (SPP: surface Plasmon Polariton) is also involved.

For reference, the sample used when observing the surface-enhanced Raman scattering signal is shown. FIG. 6 is an SEM photograph of silver nanodots formed on the mica substrate, and FIG. 7 is an SEM photograph of silver nanoholes formed on the mica substrate. Since nanoholes are formed by self-organization, their sizes vary.

[Manufacturing method of Example 1]
Before describing the method for manufacturing an optical device (Example 1) according to an embodiment of the present invention, a general method for manufacturing a nanohole structure will be described with reference to FIGS. As a first step, as shown in FIG. 8A, silica fine particles 22 are dispersed and arranged on a supporting substrate 11 such as glass. FIG. 9 is a top view in this state. As a second step, as shown in FIG. 8B, a silver thin film 23 is formed by vapor deposition. As a third step, the silica fine particles are removed by an appropriate etching treatment to form the nanohole structure shown in FIG. 8(c). FIG. 10 is a top view in this state. Since the steps described above include the step of arranging and removing silica fine particles, the production is complicated.

Next, a method for manufacturing an optical device (Example 1) according to an embodiment of the present invention will be described with reference to FIGS. As a first step, as shown in FIG. 11(a), a silver single crystal layer 12 is formed on mica 13. Next, as shown in FIG. Specifically, it is only necessary to deposit a silver single crystal thin film on the substrate of mica 13 at a high temperature using an ultra-high vacuum sputtering apparatus. Since the mica substrate has a laminated structure and the cleaved surface has atomic-level flatness, a single crystal can be formed by van der Waals epitaxy by depositing a silver thin film at a high temperature of about 300°C. In this way, nanoholes with a diameter of about 120 nm can be self-organized. As a second step, as shown in FIG. 11B, the laminate of mica 13 and silver single crystal layer 12 is turned upside down above a supporting substrate 11 made of a glass substrate or the like. FIG. 11(c) shows, as the third step, fixing the laminated film in which the silver single crystal layer 12 and the mica 13 are laminated in this order to the supporting substrate 11. FIG. FIG. 11(d) shows the completed optical device in action.

Further, if necessary, the mica substrate is subjected to hydrophobic treatment such as HDMS treatment. Since the antibody binds to the substrate surface through hydrophobic interaction, it is necessary to hydrophobilize the hydrophilic mica substrate surface. The HDMS treatment is a hydrophobization treatment using hexamethyldisilazane, which forms a chemical bond with the silanol groups on the mica surface for hydrophobization, so that a highly homogeneous hydrophobization treatment can be performed.

Since the steps described above utilize a self-organized nanohole structure, the production is inexpensive and simple. Moreover, since the silver thin film formed is a single crystal, the Q value is advantageous.

The following effects can be mentioned as advantageous points of the first embodiment described so far.
(1) Since the minute parts of silver are sealed, it is advantageous in terms of biocompatibility and aged deterioration.
(2) A silver thin film formed as a single crystal exhibits a high Q value.
(3) Since the nanohole structure of silver is self-organized, it is cheap and easy to manufacture.
(4) Since the flat mica surface is used, the Raman scattering intensity is high and the uniformity is excellent.
The above effect (1) is the effect that was obtained in order to solve the problem, but (2) and (3) were discovered in the process and were obtained accidentally. and (4) is an unexpected effect obtained unexpectedly.

[Example 2]
In Example 1, the HDMS treatment was performed as necessary, but the surface of the mica substrate can be made hydrophobic by another method. It uses graphene, a two-dimensional transparent carbon material with hydrophobic properties. Another embodiment using graphene will be described below. FIG. 12 is a cross-sectional view of an optical device (Example 2) according to another embodiment. The optical device is a surface-enhanced Raman scattering element, and on a support substrate 11 formed of a glass substrate or the like, a silver single crystal layer 12, a mica layer 13 as a light-transmitting flat layer, and a light-transmitting flat layer , the graphene 14 is laminated in this order. However, as a manufacturing process, the support substrate 11, the silver single crystal layer 12, the mica 13, and the graphene 14 are not laminated in this order. After the silver single crystal layer 12 was formed on the mica 13 by the same manufacturing method as in Example 1, the laminate of the mica 13 and the silver single crystal layer 12 was reversed up and down. Graphene 14 is transferred to Then, this laminate is fixed to a support substrate 11 formed of a glass substrate or the like. Since graphene is hydrophobic, it is possible to stably fix a hydrophobic specimen S such as tissue or blood.

In an optical device according to another embodiment (Example 2), nanoholes for activating surface-enhanced Raman scattering are formed in the silver single crystal layer 12 by self-organization. In the optical device according to another embodiment (Example 2), the film thickness of the silver single crystal layer 12 is 70 nm, the film thickness of the mica 13 is 20 μm, and the film thickness of the graphene is one carbon atom. Although it is 1 nm or less, it is not limited to these numerical values, and may be appropriately set within a range in which nanoholes are self-organized and localized surface plasmon resonance is expressed.

In addition to the property of being hydrophobic, graphene also has the property of surface plasmon polariton (SPP). FIG. 13 is a graph showing a surface-enhanced Raman scattering signal from the side of the graphene substrate transferred onto the silicon oxide film, and peaks attributed to graphene are indicated by D, G, and 2D. In addition, FIG. 14 is a graph showing the surface-enhanced Raman scattering signal from the substrate side where graphene and silver are laminated, and in addition to the peaks (G, 2D) due to graphene, the peak due to rhodamine 6G is highly sensitive. Observed. This indicates that in the optical device according to another embodiment (Example 2), graphene not only plays a role of hydrophobization, but also contributes to sensitivity improvement as a sensor.

[Example 3]
A gold particle encapsulation technique can be used when fixing the layered body composed of the silver single crystal layer 12, the mica 13, and the graphene 14 to the support substrate 11 formed of a glass substrate or the like. This will be explained below. FIG. 15 is a cross-sectional view of an optical device (Example 3) according to another embodiment. The optical device is a surface-enhanced Raman scattering element, and on a support substrate 11 formed of a glass substrate or the like, a silver single crystal layer 12, a mica layer 13 as a light-transmitting flat layer, and a light-transmitting flat layer , the graphene 14 is laminated in this order. However, the silver single crystal layer 12 is fixed to the support substrate 11 by sealing with gold particles 20 . Since gold has a plasmon property, it has the advantage that it can be sealed while maintaining sensitivity to surface-enhanced Raman scattering.

[Example 4]
In Example 4 (not shown), instead of the graphene 14 in Example 3, MoS ₂ is laminated. Besides MoS ₂ , two-dimensional materials such as WS ₂ , ZnSe ₂ , MoTe ₂ and h-BN can also be used. Using these two-dimensional materials, they act as biosensors due to charge-transfer enhancing effects that differ from the previously described surface-enhanced Raman scattering (SERS) mechanism. Conventionally, the enhancement effect by this mechanism was thought to be about 10 ³ times. In recent years, however, it is known that the Raman signal is enhanced by a factor of 10 ⁶ or more due to the dipole-dipole interaction that occurs between the two-dimensional material and the sample molecule.

The manufacturing method of Example 4 will be described with reference to FIG. As a first step, as shown in FIG. 16(a), a silver single crystal layer 12 is formed on mica 13. Next, as shown in FIG. Specifically, it is only necessary to deposit a silver single crystal thin film on the substrate of mica 13 at a high temperature using an ultra-high vacuum sputtering apparatus. Since the mica substrate has a laminated structure and the cleaved surface has atomic-level flatness, a single crystal can be formed by van der Waals epitaxy by depositing a silver thin film at a high temperature of about 300°C. In this way, nanoholes with a diameter of about 120 nm can be self-organized. As a second step, as shown in FIG. 16B, the laminate of mica 13 and silver single crystal layer 12 is turned upside down above a support substrate 11 made of a glass substrate or the like. Gold particles 20 are arranged at predetermined positions on the support substrate 11 . As a third step, as shown in FIG. 16(c), the silver single crystal layer 12 is fixed to the support substrate 11 by Au bump bonding. As for the method of bonding, it is desirable to use a method with high bump bonding strength, such as using ultrasonic waves. Gold has good adaptability to the surface shape of the bonding interface and is the most suitable bonding material. FIG. 16(d) shows an optical device completed as a laminated film in which a support substrate 11, silver single crystal layer 12, mica 13, and MoS ₂ 15 are laminated in this order. In addition to MoS ₂ , two-dimensional materials such as WS ₂ , ZnSe ₂ , MoTe ₂ and h-BN may be laminated.

[Application example]
As already mentioned, the present invention is advantageous in terms of biocompatibility and deterioration over time because the minute parts of silver are sealed. It can be effectively used as a biosensor in practice. More specifically, as an analytical biochemical assay called an enzyme-linked immunosorbent assay (ELISA assay), it is possible to immobilize an antigen sandwiched between antibodies on the optical device of the present invention. HRP (enzyme) may be used as a reporter.

Although the optics of the embodiments according to the present invention have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments, and is within the scope of the present invention. Even if there is a change in design, etc., it is included in the present invention. For example, it is possible to use gold instead of silver as the noble metal crystal. Also, as the noble metal crystal, a silver single crystal self-organized on mica was used, but it is also possible to form a polycrystalline silver thin film by ordinary silver vapor deposition and use it as an optical device. A single crystal is more advantageous in terms of Q value, but it has technical significance in that it can be applied to a living body by sealing the precious metal crystal layer with mica. not something. In that sense, it is possible to apply sodium chloride or lithium fluoride instead of mica as the light-transmitting flat layer.

In addition, each of the above-described embodiments can be combined by diverting each other's techniques unless there is a particular contradiction or problem in the purpose, configuration, or the like.

11: Support substrate 12: Silver single crystal layer 13: Mica 14: Graphene 15: MoS ₂ (two-dimensional material)
20: gold particles 22: silica fine particles

Claims (9)

At least a supporting substrate, a noble metal single crystal layer, and a light transmissive flat layer for transmitting incident light that excites surface plasmons are laminated in this order,
The noble metal single crystal layer has a nanohole structure,
The optical device according to claim 1, wherein the light-transmitting flat layer has atomic-level flatness and heat resistance that enable formation of the noble metal single crystal layer by epitaxial growth . 2. The optical device according to claim 1, wherein the noble metal single crystal layer is a silver single crystal thin film. 3. The optical device according to claim 1, wherein the light-transmitting flat layer is a single layer selected from sodium chloride and lithium fluoride having a mica or rock salt type structure. 4. The optical device according to any one of claims 1 to 3, wherein a two-dimensional material is further laminated on the light transmissive flat layer. 5. The optical device of claim 4, wherein the two-dimensional material is graphene. 6. The optical device according to claim 4, wherein the antigen can be fixed in a state sandwiched between the antibodies. A noble metal single crystal film deposition step of depositing a noble metal single crystal film on a light transmissive structure having atomic level flatness and heat resistance capable of forming a base metal single crystal by epitaxial growth ;
a support substrate fixing step of fixing the noble metal single crystal film side to the support substrate;
including at least
A method for manufacturing an optical device, wherein a nanohole structure is self-organized in the step of depositing a noble metal single crystal film. 8. The method of manufacturing an optical device according to claim 7, further comprising a lamination step of laminating a two-dimensional material on a surface of the light transmissive structure opposite to the noble metal single crystal film. 9. The method of manufacturing an optical device according to claim 8, wherein the lamination step of laminating the two-dimensional material is a transfer step of transferring graphene.

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