Method for enhancing NV color center fluorescence based on localized surface plasma resonance effect of precious metal nanoparticles

Abstract

The invention discloses a method for enhancing NV color center fluorescence based on a localized surface plasmon resonance effect of noble metal nanoparticles, and belongs to the technical field of quantum sensing and nano photonics. The invention aims to solve the problems that an existing optical microcavity enhancement method for improving the diamond NV color center fluorescence intensity is extremely high in NV color center positioning precision requirement and complex in process, surface plasmon enhancement depends on a propagation mode of a metal film, the mode size is large, resonance wavelengths are not matched, ohmic loss is serious, and the cost is low. And although the random metal nanoparticles are simple to prepare, the problems of unstable enhancement effect and low signal-to-noise ratio caused by uncontrollable size and arrangement are solved. According to the method, the metal nanoparticle array is prepared on the surface of the diamond, the resonance enhancement requirement of the NV color center zero phonon line (637nm) is met by regulating and controlling the geometric parameters and spatial arrangement of the metal nanoparticles and the coupling relation between the metal nanoparticles and the NV color center, and the fluorescence intensity of the NV color center of the diamond is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of quantum sensing and nanophotonics, and specifically relates to a method for enhancing the fluorescence intensity of diamond nitrogen vacancies by utilizing the localized surface plasmon resonance effect of noble metal nanoparticles. Background Art

[0002] A nitrogen-vacancy center (NV) is a point defect structure in diamond crystals, consisting of a nitrogen atom replacing a carbon atom and a neighboring lattice vacancy. It exhibits a stable spin energy level structure and observable fluorescence at room temperature. The NV center's ground state is a spin triplet state. Under 532nm laser excitation, it transitions to an excited state, subsequently emitting fluorescence in the wavelength range of 637-800nm via radiative transitions. The zero-phonon line (ZPL) at 637nm is its most prominent characteristic peak. Due to this unique energy level structure and excellent optical properties, NV centers have become an important research topic in quantum information science and precision measurement, showing great potential in single-photon source fabrication, quantum bit implementation, and magnetic, electric, and temperature sensing. The high fluorescence intensity of NV centers can significantly improve the signal-to-noise ratio of spin-state readout. Furthermore, as a stable single-photon source at room temperature, the fluorescence intensity of NV centers directly determines the photon emission rate and brightness. However, the fluorescence intensity of NV color center diamonds prepared by conventional methods is generally weak, making it difficult to effectively measure. Therefore, it is very important to improve the fluorescence radiation intensity of NV color centers.

[0003] Currently, methods for enhancing the fluorescence intensity of diamond NV centers primarily include optical microcavity enhancement, surface plasmon (SPP) enhancement, and random metal nanoparticle scattering enhancement. Optical microcavities utilize photonic crystals or resonant cavities to enhance spontaneous emission via the Purcell effect. While effective, these methods require extremely high precision in NV center positioning and are complex to manufacture. Surface plasmon enhancement relies on the propagation mode of metal films, resulting in large mode volumes, resonant wavelength mismatch (typically deviating from the 637nm ZPL), and significant ohmic losses. While random metal nanoparticles are simple to prepare, their uncontrollable size and arrangement lead to unstable enhancement and a low signal-to-noise ratio. Summary of the Invention

[0004] The purpose of the present invention is to solve the problems of existing optical microcavity enhancement methods for improving the fluorescence intensity of diamond NV center, such as extremely high requirements for NV center positioning accuracy and complex processes, surface plasmon enhancement relying on the propagation mode of the metal film, large mode volume, resonant wavelength mismatch and severe ohmic loss, and random metal nanoparticles, although simple to prepare, but due to the uncontrollable size and arrangement, resulting in unstable enhancement effect and low signal-to-noise ratio. The present invention provides a method for enhancing NV center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles.

[0005] The present invention proposes a method for enhancing the fluorescence of NV color centers based on the localized surface plasmon resonance (LSPR) effect of noble metal nanoparticles. A metal nanoparticle array is prepared on the diamond surface. By regulating the geometric parameters, spatial arrangement and coupling relationship between the metal nanoparticles and the NV color centers, the requirements for resonance enhancement of the NV color center zero phonon line (637nm) are met, thereby improving the fluorescence intensity of the diamond NV color center.

[0006] A method for enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles is specifically completed in the following steps:

[0007] 1. Preprocessing:

[0008] ①, ultrasonically clean the diamond substrate to remove organic pollutants on its surface, and obtain an ultrasonically cleaned diamond substrate;

[0009] ② Immerse the ultrasonically cleaned diamond substrate in a mixture of sulfuric acid and nitric acid, then heat it to a high temperature and pickle it for a period of time to remove metal residues on its surface, thereby obtaining an acid-washed diamond substrate;

[0010] ③. Cleaning the acid-washed diamond substrate with oxygen plasma to activate its surface and obtain a pre-treated diamond substrate;

[0011] 2. Attaching a porous anodic aluminum oxide film (AAO) mask:

[0012] ① Cut the porous anodic aluminum oxide film mask and immerse it in acetone for a period of time to completely dissolve the polymethyl methacrylate (PMMA) supporting structure of the mask to obtain a porous anodic aluminum oxide film;

[0013] ② Use the pre-treated diamond substrate to pick up the porous anodic aluminum oxide film and make it adhere evenly to the surface of the diamond substrate and completely cover the diamond substrate to obtain a diamond substrate with a porous anodic aluminum oxide film;

[0014] ③ Immerse the diamond substrate with the porous anodic aluminum oxide film in acetone for a period of time, take it out and dry it;

[0015] ④. Repeat steps 2 and 3 two to three times to obtain a diamond substrate with a porous anodic aluminum oxide film attached;

[0016] 3. Preparation of Metal Nanoparticles:

[0017] ①. Fill the evaporation boat with precious metal materials;

[0018] ② Fix the diamond substrate with porous anodic aluminum oxide film on the substrate holder in the coating equipment, install the evaporation boat, close the chamber, and pump to high vacuum;

[0019] ③. Increase the current and heat until the evaporation boat glows and the precious metal material melts. At this time, the current is 30A to 40A. Keep the molten state for a period of time to remove impurities.

[0020] ④. Increase the current, adjust the deposition rate, open the baffle, evaporate the precious metal material, and immediately close the baffle and stop heating when the film thickness reaches the expected thickness;

[0021] ⑤. Keep the vacuum environment cool for a while, then let air in to normal pressure, open the hatch, take out the diamond sample, and use brown high-temperature tape to remove the porous anodic aluminum oxide film to obtain a precious metal nanoparticle / diamond composite structure, completing a method for enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of precious metal nanoparticles.

[0022] Principle of the present invention:

[0023] The present invention adopts the localized surface plasmon resonance (LSPR) technology of noble metal nanoparticles. By precisely controlling the geometric parameters of the nanoparticles so that their resonance peak matches the ZPL wavelength, combined with an ordered array preparation process, it achieves high local field enhancement while having the advantages of process compatibility and position tolerance, providing an efficient and controllable solution for NV color center fluorescence enhancement.

[0024] Beneficial effects of the present invention:

[0025] 1. This invention utilizes the localized surface plasmon resonance effect of noble metal nanoparticles to provide a new technical approach for enhancing the fluorescence performance of NV color centers, breaking through the diffraction limit and achieving an enhanced effect of NV color center luminescence;

[0026] Second, the present invention provides a simple, low-cost solution for enhancing NV color center fluorescence. Compared to methods such as photonic crystals or optical resonant cavities, which require complex micro-nanofabrication, the present invention uses a thermal evaporation coating process, which can achieve the preparation of high-performance enhanced structures by optimizing process parameters.

[0027] 3. The implementation of this invention has laid an important foundation for promoting the practical application of NV color centers in fields such as quantum computing, quantum communication and precision measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic diagram of the Au nanoparticle / diamond composite structure prepared by the mask method in Example 1;

[0029] Figure 2 This is a scanning electron microscope image of the gold nanoparticles on the Au nanoparticle / diamond composite structure prepared in Example 1;

[0030] Figure 3 Comparison of the color center fluorescence intensity of the diamond substrate and the Au nanoparticle / diamond composite structure prepared in Example 1. DETAILED DESCRIPTION

[0031] Specific embodiment 1: This embodiment is a method for enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles, which is specifically completed by the following steps:

[0032] 1. Preprocessing:

[0033] ①, ultrasonically clean the diamond substrate to remove organic pollutants on its surface, and obtain an ultrasonically cleaned diamond substrate;

[0034] ② Immerse the ultrasonically cleaned diamond substrate in a mixture of sulfuric acid and nitric acid, then heat it to a high temperature and pickle it for a period of time to remove metal residues on its surface, thereby obtaining an acid-washed diamond substrate;

[0035] ③. Cleaning the acid-washed diamond substrate with oxygen plasma to activate its surface and obtain a pre-treated diamond substrate;

[0036] 2. Attaching the porous anodized aluminum film mask:

[0037] ① Cut the porous anodic aluminum oxide film mask and immerse it in acetone for a period of time to completely dissolve the PMMA supporting structure of the mask to obtain a porous anodic aluminum oxide film;

[0038] ② Use the pre-treated diamond substrate to pick up the porous anodic aluminum oxide film and make it adhere evenly to the surface of the diamond substrate and completely cover the diamond substrate to obtain a diamond substrate with a porous anodic aluminum oxide film;

[0039] ③ Immerse the diamond substrate with the porous anodic aluminum oxide film in acetone for a period of time, take it out and dry it;

[0040] ④. Repeat steps 2 and 3 two to three times to obtain a diamond substrate with a porous anodic aluminum oxide film attached;

[0041] 3. Preparation of Metal Nanoparticles:

[0042] ①. Fill the evaporation boat with precious metal materials;

[0043] ② Fix the diamond substrate with porous anodic aluminum oxide film on the substrate holder in the coating equipment, install the evaporation boat, close the chamber, and pump to high vacuum;

[0044] ③. Increase the current and heat until the evaporation boat glows and the precious metal material melts. At this time, the current is 30A to 40A. Keep the molten state for a period of time to remove impurities.

[0045] ④. Increase the current, adjust the deposition rate, open the baffle, evaporate the precious metal material, and immediately close the baffle and stop heating when the film thickness reaches the expected thickness;

[0046] ⑤. Keep the vacuum environment cool for a while, then let air in to normal pressure, open the hatch, take out the diamond sample, and use brown high-temperature tape to remove the porous anodic aluminum oxide film to obtain a precious metal nanoparticle / diamond composite structure, completing a method for enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of precious metal nanoparticles.

[0047] Specific Embodiment 2: This embodiment differs from Specific Embodiment 1 in that the diamond substrate described in step 1 (1) is a CVD single-crystal diamond. In step 1 (1), the diamond substrate is ultrasonically cleaned using acetone, anhydrous ethanol, and deionized water, sequentially for 10 to 15 minutes each, to remove organic contaminants from its surface, thereby obtaining an ultrasonically cleaned diamond substrate. The remaining steps are the same as those in Specific Embodiment 1.

[0048] Specific embodiment 3: This embodiment differs from either specific embodiment 1 or 2 in that the volume ratio of sulfuric acid to nitric acid in the sulfuric acid and nitric acid mixture described in step 1 (2) is 3:1, the mass fraction of sulfuric acid is 98%, and the mass fraction of nitric acid is 68%. The other steps are the same as those in specific embodiments 1 or 2.

[0049] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that the high-temperature pickling temperature in step 1 ② is 250°C to 300°C and the time is 2h to 3h. The other steps are the same as specific embodiments 1 to 3.

[0050] Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that the oxygen plasma cleaning power described in step 1 (3) is 100W to 150W, the oxygen flow rate is 10sccm to 15sccm, and the cleaning time is 5 to 10 minutes. The other steps are the same as specific embodiments 1 to 4.

[0051] Specific embodiment 6: This embodiment differs from specific embodiments 1 to 5 in that: in step 2①, the porous anodic aluminum oxide film mask is cut and then immersed in acetone for 10 to 20 minutes. The other steps are the same as specific embodiments 1 to 5.

[0052] Specific embodiment 7: This embodiment differs from specific embodiments 1 to 6 in that in step 2 (3), the diamond substrate with the porous anodic aluminum oxide film is immersed in acetone for 10 to 20 minutes. The other steps are the same as those of specific embodiments 1 to 6.

[0053] Specific Embodiment 8: This embodiment differs from Specific Embodiments 1 to 7 in that the precious metal material in step 3 (1) is Au or Ag; in step 3 (2), the substrate holder is 20 to 25 cm away from the evaporation boat; and the pre-evaporation time in step 3 (3) is 1 to 2 minutes. The other steps are the same as in Specific Embodiments 1 to 7.

[0054] Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that the current in step 3 (4) is 55A to 61A; the deposition rate in step 3 (4) is 0.1 Å / s to 0.5 Å / s; and the film thickness in step 3 (4) is 20nm to 50nm. The other steps are the same as specific embodiments 1 to 8.

[0055] Specific embodiment 10: This embodiment differs from specific embodiments 1 to 9 in that: in step 3 (5), the vacuum cooling environment is maintained for 10 to 20 minutes; and the high-temperature tape in step 3 (5) has a heat resistance of 200°C to 300°C. The other steps are the same as specific embodiments 1 to 9.

[0056] The following examples are used to verify the beneficial effects of the present invention:

[0057] Example 1: A method for enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles is specifically accomplished by the following steps:

[0058] 1. Preprocessing:

[0059] ① Ultrasonic cleaning of the diamond substrate was performed using acetone, anhydrous ethanol, and deionized water in sequence, each for 15 minutes, to remove organic pollutants on the surface, thereby obtaining an ultrasonically cleaned diamond substrate;

[0060] The diamond substrate described in step 1① is a CVD single crystal diamond with a size of 3mm×3mm×0.5mm;

[0061] ② Immerse the ultrasonically cleaned diamond substrate in a mixture of sulfuric acid and nitric acid, then heat it to 250°C and pickle it at 250°C for 2 hours to remove metal residues on its surface, thereby obtaining an acid-washed diamond substrate;

[0062] The volume ratio of sulfuric acid to nitric acid in the mixed solution of sulfuric acid and nitric acid described in step 1② is 3:1, wherein the mass fraction of sulfuric acid is 98% and the mass fraction of nitric acid is 65%;

[0063] ③. Place the acid-washed diamond substrate into a microwave plasma cleaning machine and perform oxygen plasma cleaning on the acid-washed diamond substrate to activate its surface and improve the adhesion of the nanoparticles to obtain a pre-treated diamond substrate;

[0064] The power of the oxygen plasma cleaning described in step 1 (3) is 100 W, the oxygen flow rate is 10 sccm, and the cleaning time is 5 min;

[0065] 2. Attaching the mask:

[0066] ① Cut the porous anodic aluminum oxide film mask to obtain a mask with a size of 5mm×5mm. Immerse the cut mask in acetone for 10 minutes to completely dissolve the polymethyl methacrylate (PMMA) supporting structure of the mask to obtain a porous anodic aluminum oxide film (AAO);

[0067] The porous anodic aluminum oxide film mask described in step 2① is model UT 100-65-200, purchased from Shenzhen Topology Fine Film Technology Co., Ltd.;

[0068] ② Place the pretreated diamond substrate in acetone, lift up the porous anodic aluminum oxide film (AAO), and make it adhere evenly to the surface of the diamond substrate and completely cover the diamond substrate to obtain a diamond substrate with a porous anodic aluminum oxide film;

[0069] ③ Immerse the diamond substrate with the porous anodic aluminum oxide film in acetone for 10 minutes, take it out and dry it;

[0070] ④ Repeat steps 2 and 3 twice to make the AAO adhere tightly to the diamond substrate, and obtain a diamond substrate with a porous anodic aluminum oxide film attached;

[0071] 3. Preparation of Metal Nanoparticles:

[0072] ①. Fill the evaporation boat with precious metal materials;

[0073] The precious metal material described in step 3① is Au;

[0074] ② Fix the diamond substrate with porous anodic aluminum oxide film on the substrate holder in the coating equipment, install the evaporation boat, close the chamber, start the mechanical pump to low vacuum (~10 -1 Pa), switch to the molecular pump, and pump to high vacuum (≤5×10 -5 Pa), to ensure that no gas residue affects the coating quality;

[0075] Step 3② The substrate holder is 20 cm away from the evaporation boat;

[0076] ③. Increase the current and heat until the evaporation boat glows and the precious metal material melts. At this time, the current is 40A. Keep the molten state and pre-steam for 2 minutes to remove impurities.

[0077] ④. Increase the current to 61A, adjust the deposition rate, open the baffle, start evaporating gold, and immediately close the baffle when the film thickness reaches 20nm and stop heating.

[0078] The evaporation rate described in step 3 (4) is 0.1 angstroms per second;

[0079] ⑤. Maintain the vacuum environment and cool for 15 minutes, then ventilate the chamber to atmospheric pressure, open the hatch, remove the diamond sample, and use brown high-temperature tape to remove the porous anodic aluminum oxide film to obtain an Au nanoparticle / diamond composite structure, completing a method for enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles;

[0080] The heat resistance of the high temperature tape described in step 3⑤ is between 200°C and 300°C.

[0081] The Au nanoparticle / diamond composite structure prepared in Example 1 was characterized by scanning electron microscopy (SEM) and photoluminescence (PL) testing;

[0082] Figure 2 This is a scanning electron microscope image of the gold nanoparticles on the Au nanoparticle / diamond composite structure prepared in Example 1;

[0083] from Figure 2 It can be seen that Au nanoparticles are evenly and orderly distributed on the surface of the diamond sample without obvious clustering phenomenon.

[0084] Figure 3 Comparison of the fluorescence intensity of the color centers of the diamond substrate and the Au nanoparticle / diamond composite structure prepared in Example 1;

[0085] Figure 3 It shows that the method of Example 1 makes diamond NV - The color center fluorescence has been significantly improved, and NV 0 The fluorescence of the color center is weakened.

Claims

1. A method for enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles, characterized in that The method is specifically completed according to the following steps:

1. Preprocessing: ①, ultrasonically clean the diamond substrate to remove organic pollutants on its surface, and obtain an ultrasonically cleaned diamond substrate; ② Immerse the ultrasonically cleaned diamond substrate in a mixture of sulfuric acid and nitric acid, then heat it to a high temperature and pickle it for a period of time to remove metal residues on its surface, thereby obtaining an acid-washed diamond substrate; ③. Cleaning the acid-washed diamond substrate with oxygen plasma to activate its surface and obtain a pre-treated diamond substrate; 2. Attaching the porous anodized aluminum film mask: ① Cut the porous anodic aluminum oxide film mask and immerse it in acetone for a period of time to completely dissolve the PMMA supporting structure of the mask to obtain a porous anodic aluminum oxide film; ② Use the pre-treated diamond substrate to pick up the porous anodic aluminum oxide film and make it adhere evenly to the surface of the diamond substrate and completely cover the diamond substrate to obtain a diamond substrate with a porous anodic aluminum oxide film; ③ Immerse the diamond substrate with the porous anodic aluminum oxide film in acetone for a period of time, take it out and dry it; ④. Repeat steps 2 and 3 two to three times to obtain a diamond substrate with a porous anodic aluminum oxide film attached; 3. Preparation of Metal Nanoparticles: ①. Fill the evaporation boat with precious metal materials; ② Fix the diamond substrate with porous anodic aluminum oxide film on the substrate holder in the coating equipment, install the evaporation boat, close the chamber, and pump to high vacuum; ③. Increase the current and heat until the evaporation boat glows and the precious metal material melts. At this time, the current is 30A to 40A. Keep the molten state for a period of time to remove impurities. ④. Increase the current, adjust the deposition rate, open the baffle, evaporate the precious metal material, and immediately close the baffle and stop heating when the film thickness reaches the expected thickness; ⑤. Keep the vacuum environment cool for a while, then let air in to normal pressure, open the hatch, take out the diamond sample, and use brown high-temperature tape to remove the porous anodic aluminum oxide film to obtain a precious metal nanoparticle / diamond composite structure, completing a method for enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of precious metal nanoparticles.

2. The method of enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles according to claim 1, characterized in that The diamond substrate described in step 1① is a CVD single crystal diamond; in step 1①, the diamond substrate is ultrasonically cleaned with acetone, anhydrous ethanol and deionized water in sequence, each cleaning for 10 minutes to 15 minutes to remove organic pollutants on its surface, thereby obtaining an ultrasonically cleaned diamond substrate.

3. The method of enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles according to claim 1, characterized in that The volume ratio of sulfuric acid to nitric acid in the mixed solution of sulfuric acid and nitric acid described in step 1② is 3:1, wherein the mass fraction of sulfuric acid is 98%, and the mass fraction of nitric acid is 68%.

4. The method of enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles according to claim 1, characterized in that The temperature of the high temperature pickling described in step 1② is 250℃~300℃, and the time is 2h~3h.

5. The method of enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles according to claim 1, characterized in that The power of the oxygen plasma cleaning described in step 1 (3) is 100W to 150W, the oxygen flow rate is 10sccm to 15sccm, and the cleaning time is 5min to 10min.

6. The method of enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles according to claim 1, characterized in that In step 2①, the porous anodic aluminum oxide film mask is cut and then immersed in acetone for 10 to 20 minutes.

7. The method of enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles according to claim 1, characterized in that In step 2③, the diamond substrate with the porous anodic aluminum oxide film is immersed in acetone for 10 minutes to 20 minutes.

8. The method of enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles according to claim 1, characterized in that The precious metal material described in step 3 ① is Au or Ag; in step 3 ②, the substrate holder is 20 cm to 25 cm away from the evaporation boat; and the pre-evaporation time described in step 3 ③ is 1 min to 2 min.

9. The method of enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles according to claim 1, characterized in that The current described in step 3④ is 55A~61A; The deposition rate in step 3④ is 0.1 Å / s to 0.5 Å / s; the film thickness in step 3④ is 20 nm to 50 nm.

10. The method of enhancing NV color center fluorescence based on the localized surface plasmon resonance effect of noble metal nanoparticles according to claim 1, characterized in that In step 3⑤, the vacuum environment is maintained for cooling for 10 minutes to 20 minutes; the heat resistance of the high-temperature tape described in step 3⑤ is 200℃ to 300℃.

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