Construction method of plasmon nanocavity for enhancing two-photon fluorescence of quantum dots through double resonance

Abstract

The invention provides a construction method of a plasmon nanocavity for enhancing two-photon fluorescence of quantum dots through double resonance. The construction method comprises the following steps: S1, preparing an ultra-smooth metal film; s2, preparing a dielectric isolation layer on the ultra-smooth metal film; s3, spin-coating a quantum dot solution on the surface of the dielectric isolation layer to form a uniformly distributed quantum dot film; s4, metal nanoparticles are dropped on the quantum dot film through a liquid dropping method, and the plasmon nanocavity is obtained. And S5, testing two-photon fluorescence enhancement. According to excitation wavelength selection, the isolation layer is evaporated between the metal nanoparticles and the metal film, and the thickness of the isolation layer is adjusted, so that a high-order mode (emission resonance) and a low-order mode (excitation resonance) are synergistically enhanced, and double-resonance enhancement is formed. A dielectric isolation layer does not need to be additionally deposited on the quantum dot film, and exciting light can be directly matched with magnetic mode resonance.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanophotonics, and specifically relates to a method for double-resonance enhanced two-photon fluorescence of quantum dots based on plasmonic nanocavities, which is applicable to fields such as bioimaging, optoelectronic devices, and ultrasensitive sensing. Background Art

[0002] Quantum dots have unique optical properties and have broad application prospects in bioimaging, optoelectronic devices, quantum information processing, etc. Two-photon fluorescence technology has been widely used in bioimaging due to its advantages such as deep penetration and low light damage. In the field of optoelectronic devices, enhancing the two-photon fluorescence of quantum dots can be used to improve the sensitivity and response speed of photodetectors, and can also be used to develop optoelectronic devices such as new light-emitting diodes to achieve higher luminous efficiency and narrower emission spectra. However, the low two-photon absorption cross-section and fluorescence efficiency limit its application in high-sensitivity detection and deep tissue imaging. At present, several methods for enhancing the two-photon fluorescence of quantum dots have been proposed. A common method is to use an optical resonator to increase the local photon state density of the environment where the quantum dots are located. But they often have problems related to mode matching. It is challenging to precisely align the emission spectrum of the quantum dots with the microcavity mode, which will lead to low coupling efficiency and limited enhancement effect. Another method is to use a high-intensity laser light source to increase the two-photon absorption intensity. However, this method may cause a decrease in the stability and performance of the quantum dots. In addition, plasmonic nanostructures have also been used to explore the research on enhancing two-photon fluorescence. In the prior art, the fluorescence efficiency can be improved by local field enhancement in plasmonic nanocavities, but the traditional structure has the following problems: it is difficult for the single resonance mode to simultaneously match the excitation and emission wavelengths; the scattering loss at the metal-quantum dot interface is large, resulting in a limited enhancement factor; the in-plane plasmonic structure has high requirements for micro-nano processing technology; the accuracy of nano-gap regulation is insufficient, and it is difficult to achieve double resonance; therefore, there is an urgent need for a method for constructing a plasmonic nanocavity that can precisely control the double resonance mode. Summary of the Invention

[0003] The invention object of this project is to improve the luminescence efficiency of two-photon fluorescence, provide a diversified plasmonic structure, avoid the problems in the preparation process of the current in-plane plasmonic structure such as complex process, high requirement for instrument precision, long preparation period, and high cost, adopt the out-of-plane metal particle-quantum dot-metal film structure to achieve precise control of the plasmonic structure, and design a double resonance mode to enhance two-photon fluorescence by using the existence of multiple modes.

[0004] The present invention proposes a method for constructing a plasmonic nanocavity for double-resonance enhanced two-photon fluorescence of quantum dots, and the steps are as follows:

[0005] S1: Prepare a super-smooth metal film;

[0006] S2: Deposit a dielectric isolation layer of a certain thickness on the ultrasmooth metal film at a growth rate of 1 - 5 Å / cyc by atomic layer deposition technology to isolate the quantum dots from the metal substrate and prevent electron tunneling.

[0007] S3: Spin - coat the quantum dot solution on the surface of the dielectric isolation layer to form a quantum dot thin film with uniform distribution.

[0008] S4: According to the fluorescence emission peak of the quantum dot thin film, select metal nanoparticles with appropriate sizes to resonate the high - order surface plasmon mode or other lateral modes that are not affected or less affected by the gap with the fluorescence peak. Then, use the drop - casting method to drop the metal nanoparticles on the quantum dot thin film, and after a certain time, dry it with nitrogen to obtain the plasmonic nanocavity.

[0009] S5: Test the two - photon fluorescence enhancement.

[0010] Furthermore, step S1 is specifically as follows:

[0011] S11: Select a flat substrate, and deposit an ultrasmooth metal film on the substrate by thermal evaporation technology. The film thickness should be greater than the skin depth of light in the metal.

[0012] S12: Drop the ultraviolet - curable glue on the transparent and smooth transparent substrate to ensure that there are no bubbles in the liquid drop.

[0013] S13: Invert the substrate with the metal film on the substrate with the dropped liquid drop, so that the substrate and the substrate form a sandwich structure.

[0014] S14: Place the sandwich structure under an ultraviolet lamp for 2 hours to cure the ultraviolet glue, with the transparent substrate facing up, ensuring that the ultraviolet - curable glue can receive a certain intensity of ultraviolet light.

[0015] S15: Put the structure into an oven and bake it for 4 hours to cure the residual glue.

[0016] S16: Use a blade to peel the metal film off the flat substrate to obtain an ultrasmooth metal film. The roughness of the ultrasmooth metal film should have good flatness, RMS < 0.5 nm.

[0017] Furthermore, the flat substrate can be selected from single - side polished quartz, silicon wafers, and mica sheets; the metal target materials can be gold, silver, and aluminum.

[0018] Furthermore, in S2, the dielectric isolation layer can be selected from silicon oxide, titanium oxide, and aluminum oxide; the thickness of the dielectric isolation layer is 1 - 2 nm; the growth temperature is above 200 degrees Celsius.

[0019] Furthermore, in step S3, the concentration of the quantum dot solution is 0.1-0.5 mg / mL, the speed of the coater is 5000-6000 rpm, the spin coating time is 0.5-1 min, and the thickness of the quantum dot film is 5-10 nm; the thickness of the quantum dot film can be precisely controlled by the concentration of the quantum dot solution, the speed of the coater, and the spin coating time to ensure uniform distribution of the quantum dots.

[0020] Furthermore, the method for confirming the plasmon mode or the transverse mode in step S4 needs to rely on dark field scattering or absorption spectroscopy.

[0021] Furthermore, step S5 is specifically as follows:

[0022] S51: First test the excitation power threshold of system damage, and then test at a power lower than the threshold power.

[0023] S52: Using the two-photon excitation method, the excitation light is 800nm, and a filter is added to filter out the excitation light, retaining only the two-photon fluorescence signal, and measure the changes in the two-photon fluorescence intensity of quantum dots without structure and with structure. Then measure the spot area and compare it with the metal particle size to obtain the real two-photon fluorescence enhancement.

[0024] Compared with the prior art, this patent application has the following advantages:

[0025] 1. According to the excitation wavelength selection, an isolation layer is evaporated between the metal nanoparticles and the metal film, and the thickness of the isolation layer is adjusted to enhance the high-order mode (emission resonance) and the low-order mode (excitation resonance) synergistically to form a double resonance enhancement.

[0026] 2. There is no need to deposit an additional dielectric isolation layer on the quantum dot film, and the excitation light can be directly matched with the magnetic mode resonance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of a plasmon nanocavity;

[0028] Figure 2 It is a uniformly spin-coated quantum dot film;

[0029] Figure 3 Dark field scattering of plasmon structures and selection of metal particle size and excitation wavelength;

[0030] Figure 4 It is the two-photon fluorescence enhancement effect. DETAILED DESCRIPTION

[0031] The implementation of the present invention is described in detail below. Examples of the implementation are shown in the accompanying drawings, which are intended to be used to explain the present invention but should not be understood as limiting the present invention.

[0032] like Figure 1As shown, a method for constructing a plasmonic nanocavity for double-resonance enhanced two-photon fluorescence of quantum dots is as follows:

[0033] S1: Prepare a super-smooth metal film

[0034] S11: Select a flat silicon wafer as the substrate, and deposit a super-smooth gold film 2 on the substrate through thermal evaporation technology. The film thickness should be greater than the skin depth of light in the metal.

[0035] S12: Drop ultraviolet curable glue on the transparent and smooth transparent substrate 1, ensuring that there are no bubbles in the liquid droplet;

[0036] S13: Invert the substrate with the gold film 2 on the substrate 1 with the liquid droplet dropped, so that the substrate and the substrate 1 form a sandwich structure;

[0037] S14: Place the sandwich structure under an ultraviolet lamp for 2 hours to cure the ultraviolet glue, with the transparent substrate facing up, ensuring that the ultraviolet curable glue can receive a certain intensity of ultraviolet light;

[0038] S15: Put the structure in an oven and bake for 4 hours to cure the residual glue;

[0039] S16: Use a blade to peel off the gold film 2 from the flat substrate to obtain a super-smooth metal film. The roughness of the super-smooth metal film should have good flatness, RMS < 0.5nm;

[0040] The roughness of the super-smooth gold film is determined by the flatness of the flat substrate surface. The smooth gold film can reduce defect scattering and improve the quality factor of the plasmonic nanocavity;

[0041] S2: Through atomic layer deposition technology, grow a certain thickness of alumina as the dielectric isolation layer 3 on the super-smooth gold film 2 at a growth rate of 1 - 5 Å / cyc to isolate the quantum dots from the metal substrate and prevent electron tunneling. Among them, the thickness of the dielectric isolation layer 3 is 2nm; the growth temperature is above 200 degrees.

[0042] S3: Spin-coat the quantum dot solution on the surface of the dielectric isolation layer 3 to form a uniformly distributed quantum dot thin film 4;

[0043] Among them, the concentration of the quantum dot solution is 0.1mg / mL, the rotation speed of the spin coater is 5000rpm, the spin coating time is 0.5min, and the thickness of the quantum dot thin film 4 is 5nm; the thickness of the quantum dot thin film 4 can be precisely controlled by the concentration of the quantum dot solution, the rotation speed of the spin coater, and the spin coating time to ensure uniform distribution of the quantum dots.

[0044] S4: According to the fluorescence emission peak of the quantum dot film 4, select metal nanoparticles 5 with appropriate sizes to make the high-order plasmon mode or other lateral modes that are not affected by the gap or are less affected resonate with the fluorescence peak. Then, use the drop-casting method to drop the metal nanoparticles 5 on the quantum dot film 4, and after a certain period of time, dry it with nitrogen to obtain the plasmonic nanocavity. The method for confirming the plasmon mode or the lateral mode needs to rely on dark-field scattering or absorption spectroscopy.

[0045] S5: Test the two-photon fluorescence enhancement.

[0046] S51: First, test the excitation power threshold at which the system is damaged, and test it at a power lower than the threshold power.

[0047] S52: Using the two-photon excitation method, the excitation light is 800 nm, and a filter is added to filter out the excitation light, leaving only the two-photon fluorescence signal. Measure the changes in the two-photon fluorescence intensity of the quantum dots at the unstructured and structured locations. The two-photon fluorescence test shows that the fluorescence intensity on the spectrum has a 10 4 -fold increase; measure the spot area, compare it with the size of the metal particles, and obtain the true two-photon fluorescence enhancement. The results show that the enhancement multiple of the two-photon fluorescence intensity is greater than 10 5 -fold.

[0048] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for constructing a plasmonic nanocavity for double-resonance enhanced two-photon fluorescence of quantum dots, characterized in that, The steps are as follows: S1: Prepare a super-smooth metal film; S2: Through atomic layer deposition technology, grow a dielectric isolation layer with a certain thickness on the super-smooth metal film at a growth rate of 1-5 Å / cyc, which is used to isolate the quantum dots from the metal substrate and prevent electron tunneling; S3: Spin-coat the quantum dot solution on the surface of the dielectric isolation layer to form a uniformly distributed quantum dot thin film; S4: According to the fluorescence emission peak of the quantum dot thin film, select metal nanoparticles with appropriate sizes to make the higher-order surface plasmon modes or other transverse modes that are not affected or less affected by the gap resonate with the fluorescence peak, and then use the drop-casting method to drop the metal nanoparticles on the quantum dot thin film, and after a certain time, dry it with nitrogen to obtain a plasmonic nanocavity; S5: Test the two-photon fluorescence enhancement.

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