Dual-channel response-enhanced metasurface structure Ag-graphene photocathode and preparation method thereof

Abstract

The application discloses a kind of double-channel response enhanced super surface structure Ag-graphene photocathode and preparation method thereof.The photocathode includes P-type Si substrate, Cr adhesion layer, Ag base, Cr adhesion layer, Ag nano-grating structure, graphene cover layer, Cs / O activation layer arranged from bottom to top, wherein Cr adhesion layer, Ag base and nano-grating structure Ag are formed by electron beam evaporation deposition, graphene is transferred to the surface of Ag nano-grating structure after being grown by chemical vapor deposition method.The application designs a kind of Ag nano-grating structure in 405nm and 532nm double-channel absorption enhancement by FDTD optical simulation software, improves the photoemission characteristics of photocathode.On the other hand, by covering graphene on the surface of cathode, the half width at half maximum (FWHM) of absorption peak can be extended, and low work function surface can be obtained by generating Cs2O on the surface of graphene, and the service life of metal photocathode can also be extended.

Description

Technical Field

[0001] This invention belongs to the field of photocathode material technology, specifically a dual-channel response-enhanced metasurface structure Ag-graphene photocathode and its preparation method. Background Technology

[0002] Laser-driven photocathodes are core components of high-end electron accelerators such as free-electron lasers, energy-harvesting linear accelerators, and inverse Compton scattering sources. High-performance photocathodes typically require characteristics such as high quantum efficiency, good operational stability, and long lifespan to stably provide high-brightness electron beams under complex operating conditions.

[0003] Semiconductor photocathodes, while possessing high quantum efficiency, are extremely sensitive to vacuum environments and susceptible to performance degradation due to residual gas contamination, making them unsuitable for long-life applications. Metal photocathodes, despite their lower quantum efficiency, typically requiring ultraviolet excitation and limited to low average current scenarios, offer excellent vacuum compatibility, long operating life, and femtosecond-level response speeds, providing irreplaceable advantages in experiments demanding extremely high time resolution. To overcome the limitations of low quantum efficiency in metal cathodes, researchers have recently explored the introduction of surface plasmon resonance (SPR) technology to enhance light absorption at specific wavelengths. By introducing gratings or nanostructures onto the metal surface, SPR resonances can be excited under phase-matching conditions, significantly improving the coupling efficiency between light and metal. Currently, the United States and France are conducting research on photoemission using metasurface structures on metals such as gold, silver, and copper. Experimental results show that significant improvements in quantum efficiency can be achieved at specific wavelengths. In France, for example, by depositing a silver film on a commercial grating, Cs activation resulted in a quantum efficiency greater than 1% at a wavelength of 355 nm.

[0004] In terms of metal selection, silver has advantages such as a low work function and better stability in air than copper. It also exhibits a strong two-photon photoelectric emission process in the visible light band, possessing higher excitation efficiency compared to multi-photon process materials such as gold and copper. However, silver still faces the problems of high reflectivity and low quantum efficiency, which limit its application in practical cathodes. Summary of the Invention

[0005] The purpose of this invention is to propose a dual-channel response-enhanced metasurface structure Ag-graphene photocathode and its preparation method.

[0006] The technical solution to achieve the purpose of this invention is: a dual-channel response-enhanced metasurface structure Ag-graphene photocathode, comprising, from bottom to top, a P-type Si substrate, a Cr adhesion layer, an Ag substrate, a Cr adhesion layer, an Ag nanograting structure, a graphene capping layer, and a Cs / O activation layer.

[0007] Preferably, the Ag nanograting structure has a height of 29~33nm, a spacing linewidth of 28~44nm, and a period of 395~405nm.

[0008] Preferably, the graphene coating has 5 graphene layers.

[0009] Preferably, the graphene is grown by chemical vapor deposition and then transferred to the surface of the Ag nanograting structure.

[0010] Preferably, the thickness of the Cr adhesion layer deposited by electron beam thermal evaporation is 5 nm.

[0011] Preferably, the thickness of the Ag substrate deposited by electron beam thermal evaporation is 150~250 nm.

[0012] Preferably, the thickness of the p-type Si substrate is 0.5 mm, and the doping concentration is N. A 1×10 13 ~1×10 16 cm -3 .

[0013] This invention also provides a method for preparing a dual-channel response-enhanced metasurface structure Ag-graphene photocathode, the specific steps of which are as follows:

[0014] Step 1: On the substrate, deposit a Cr adhesion layer and an Ag substrate sequentially;

[0015] Step 2: Spin-coat positive photoresist onto the substrate, and pattern it after electron beam exposure;

[0016] Step 3: Secondary deposition of Cr adhesion layer and Ag film, the thickness of Ag film being the height of the nanograting structure;

[0017] Step 4: The patterned photoresist and excess Ag film are removed by the lift-off process to obtain the nanograting structure;

[0018] Step 5: Transfer the 5-layer graphene grown by chemical vapor deposition to the surface of the Ag nanograting structure using a PMMA-assisted wet or dry method.

[0019] Step 6: Using a high-vacuum activation process, Cs / O activation is performed on the surface of the graphene coating to cover the graphene surface with a Cs / O activation layer, thereby reducing the work function of the photocathode.

[0020] Preferably, the high vacuum activation process in step 6 specifically refers to the Cs / O activation process in a high vacuum environment, with a Cs / O activation layer thickness of 0.5–1.5 nm.

[0021] Compared with the prior art, the significant advantages of this invention are as follows: 1) This invention designs a single-period structure, dual-absorption channel Ag photocathode based on SPP resonance, using a nano-grating array to form a surface impedance matching structure with an aspect ratio of 1, which is easy to process and has high redundancy for linewidth errors; 2) The two absorption peaks of the structure of this invention are located at the wavelengths of two commonly used visible light lasers, 405 nm and 532 nm, and are dominated by two different absorption modes, respectively. The positions of the absorption peaks can be adjusted by changing the structural parameters; 3) Based on the Ag nano-grating cathode, this invention extends the half-width at half-maximum (WHM) of the surface plasmon resonance absorption peak by covering the cathode surface with graphene, and obtains a low work function surface by generating Cs2O on the graphene surface, while also extending the working life of the metal photocathode. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of a dual-channel response-enhanced metasurface structure Ag-graphene photocathode in one of the embodiments.

[0023] Figure 2 This is a top view of the Ag nanograting structure in the embodiment.

[0024] Figure 3 This is a top view of the two-dimensional Ag nanograting structure in the embodiment.

[0025] Figure 4 This is the absorption spectrum of an Ag-graphene photocathode with a dual-channel response-enhanced metasurface structure, as described in this embodiment. Detailed Implementation

[0026] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. The drawings are for illustrative purposes only and are not drawn to scale. It should be noted that these embodiments are only used to explain the present invention and do not constitute any limitation on the scope of protection of the present invention. Any parameter adjustments, equivalent material substitutions, or simple modifications to process steps made based on the concept of the present invention and using conventional means skilled in the art should be included within the scope of protection of the present invention.

[0027] Example 1

[0028] Fabrication and Structure of Ag-Graphene Photocathode with Enhanced Dual-Channel Response

[0029] This embodiment provides a dual-channel response-enhanced metasurface structure Ag-graphene photocathode, whose layered structure is as follows: Figure 1 As shown, the photocathode comprises, from bottom to top, a P-type Si substrate, a first Cr adhesion layer, an Ag substrate, a second Cr adhesion layer, a patterned Ag nanograting structure, and an uppermost graphene capping layer.

[0030] Specifically, the P-type Si substrate is a double-sided polished single-crystal silicon wafer with a thickness of 0.5 mm, and its doping concentration NA is preferably 1×10¹. 5 cm⁻³, to ensure good conductivity for collecting photogenerated electrons.

[0031] On a Si substrate, a first Cr adhesion layer is deposited using electron beam thermal evaporation, with a thickness controlled at 5 nm. This thin Cr layer, due to its excellent adhesion, effectively prevents the upper Ag film from peeling off under subsequent processes or thermal stress.

[0032] Subsequently, an Ag substrate is deposited on the first Cr adhesion layer. In this embodiment, the thickness of the Ag substrate is set to 200 nm. This thickness of Ag layer can completely cover the substrate, forming a smooth, low-loss conductive layer, and providing the necessary boundary conditions for plasmon resonance of the upper nanograting structure.

[0033] On the Ag substrate, a second Cr adhesion layer, also 5 nm thick, is deposited using electron beam thermal evaporation. This layer serves as an adhesion medium between the subsequent Ag nanograting structure and the underlying Ag substrate, and can also suppress interdiffusion between the upper and lower Ag layers to a certain extent, ensuring the structural integrity of the nanograting.

[0034] The core of this invention lies in a meticulously designed Ag nanograting structure. This structure is located on top of a second Cr adhesion layer. Optical simulation optimization was performed using the finite-difference time-domain (FDTD) method, ultimately determining the grating geometry parameters that enable dual-channel absorption enhancement at 405 nm and 532 nm. Figure 2 As shown, the Ag nanograting structure consists of a series of periodically arranged one-dimensional gratings. In this embodiment, the grating height is 30 nm, the spacing linewidth is 36 nm, and the grating period is 400 nm. This structure, with an aspect ratio (height / linewidth) close to 1:1, can not only effectively excite different types of surface plasmon resonance modes at specific wavelengths to enhance light absorption (exciting grating plasmon resonance near 405 nm and Fabry-Perot resonance near 532 nm), but also has a high tolerance for linewidth errors in micro-nano fabrication processes such as electron beam lithography, which is beneficial for improving the fabrication yield.

[0035] Furthermore, if an identical periodic grating structure is added in a direction orthogonal to the existing grating groove, a mesh-like two-dimensional nanograting is formed, such as Figure 3 As shown, its absorption characteristics are basically the same as those of a one-dimensional grating structure, but it eliminates the polarization dependence of the one-dimensional grating structure.

[0036] A graphene capping layer is applied over the Ag nanograting structure. In this embodiment, the graphene consists of five layers, independently grown on copper foil using chemical vapor deposition (CVD), and then completely and non-destructively bonded to the surface of the Ag nanograting structure using a polymethyl methacrylate (PMMA)-assisted wet transfer technique. The graphene capping not only utilizes its excellent chemical inertness to provide a physical barrier for the easily oxidized Ag surface, extending the device's lifespan, but also leverages the broadband light absorption characteristics of graphene and its coupling with the near-field of plasmon resonances on the Ag surface to effectively broaden the full width at half maximum (FWHM) of the original plasmon resonance absorption peak, thereby enhancing the robustness to absorption of non-strictly monochromatic light or wavelength-drift lasers.

[0037] Finally, to reduce the cathode work function and improve photoelectric emission efficiency, the above samples were placed in a high-vacuum activation system for Cs / O activation. Under high vacuum, a Cs / O activation layer with a thickness of approximately 1.5 nm was deposited on the surface of the graphene capping layer using an alternating Cs / O method. This activation layer effectively reduces the surface work function of the material, making it easier for photogenerated electrons to escape into the vacuum, thereby improving quantum efficiency.

[0038] Example 2

[0039] A complete fabrication process for a dual-channel response-enhanced metasurface structure Ag-graphene photocathode

[0040] This embodiment describes in detail the specific process steps for preparing the photocathode of the present invention based on the method described in claim 8. This process corresponds to the structure described in embodiment 1.

[0041] Step 1, Substrate cleaning and Ag substrate deposition:

[0042] A sample with a thickness of 0.5 mm and a doping concentration of 1 × 10¹ was used. 5 A p-type Si(100) substrate with a diameter of cm⁻³ was sequentially ultrasonically cleaned for 5 minutes each with acetone, anhydrous ethanol, and deionized water to remove surface organic contaminants and particles, followed by drying with high-purity nitrogen. The cleaned substrate was immediately placed into the chamber of an electron beam evaporation coating machine. The vacuum level was then evaporated to a value better than 5 × 10⁻⁻⁻⁶ cm⁻³. 4 At Pa, deposition begins. First, a 5 nm thick Cr film is deposited at a rate of 0.1 Å / s to form the first Cr adhesion layer. Then, without breaking the vacuum, a 200 nm thick Ag film is deposited at a rate of 0.5 Å / s to form the Ag substrate.

[0043] Step 2: Photoresist spin coating and electron beam patterning:

[0044] On the sample surface with deposited Ag substrate, ZEP520A positive electron beam photoresist was spin-coated at 4000 rpm for 60 seconds, followed by baking on a hot plate at 180°C for 3 minutes to obtain a photoresist layer with a thickness of approximately 300 nm. Using an electron beam exposure system, the photoresist was exposed according to a pre-designed grating pattern with a period of 400 nm and a linewidth of 36 nm. After exposure, the sample was immersed in ZED-N50 developer for 60 seconds, fixed with isopropanol for 30 seconds, and dried with high-purity nitrogen gas, thus obtaining the desired grating pattern openings on the photoresist layer.

[0045] Step 3: Secondary deposition of Cr adhesion layer and Ag film:

[0046] The sample with patterned photoresist was placed back into the electron beam evaporation coating machine. Under high vacuum conditions, a 5 nm thick Cr film was first deposited to form the second Cr adhesion layer. Immediately afterwards, a 30 nm thick Ag film was deposited; this thickness represents the final height of the nanograting structure. The deposition rate had to be strictly controlled during the deposition process to ensure film uniformity and the coverage quality of the grating sidewalls.

[0047] Step 4, Lift-off stripping process:

[0048] The sample with the deposited metal film was immersed in ZDMAC stripping solution, heated to 60°C, and gently stirred. The photoresist and the excess Ag / Cr composite film layer covering it gradually dissolved and peeled off. After complete stripping, the sample was removed and repeatedly washed with acetone and isopropanol, ultimately leaving an Ag nanograting structure with precise period and linewidth on the substrate, consisting of a second Cr adhesion layer and an Ag film.

[0049] Step 5, Graphene Transfer:

[0050] A PMMA-assisted wet transfer technique was employed. First, PMMA was spin-coated onto a 5-layer graphene layer grown on a copper foil surface using CVD, and after curing, it served as a support layer. The copper foil was removed by chemical etching, yielding a PMMA / graphene composite film floating on a deionized water surface. Using a sample with an Ag nanograting structure as the target substrate, the PMMA / graphene film was retrieved and tightly adhered to the grating structure surface. After natural drying at room temperature, the PMMA layer was removed by acetone vapor, resulting in a clean and complete 5-layer graphene coating on the Ag nanograting structure surface.

[0051] Step 6, Cs / O high vacuum activation:

[0052] The sample, after completing the above steps, was transferred to a dedicated activation chamber for Cs / O activation. The Cs and O sources were alternately turned on, and Cs / O deposition was performed by monitoring changes in photocurrent until the optimal activation state was reached. Finally, a Cs / O activation layer with a thickness of approximately 1.5 nm was formed on the surface of the graphene capping layer.

[0053] Example 3

[0054] A dual-channel response-enhanced metasurface structure Ag-graphene photocathode fabricated using dry transfer technology

[0055] This embodiment, based on Embodiment 2, focuses on illustrating the process route for coating multilayer graphene onto the surface of Ag nanogratings using dry transfer technology and the resulting structural optimization effects. This embodiment aims to demonstrate the diversity and process compatibility of the present invention, particularly by avoiding contact with chemical reagents in wet transfer, thereby further improving the cleanliness and coupling efficiency of the graphene-Ag interface.

[0056] Step 1, Substrate cleaning and Ag substrate deposition:

[0057] This step is exactly the same as step 1 in Example 2. A sample with a thickness of 0.5 mm and a doping concentration of 1 × 10¹ is used. 5 A p-type Si(100) substrate with a diameter of cm⁻³ was sequentially ultrasonically cleaned with acetone, anhydrous ethanol, and deionized water, and then dried with high-purity nitrogen. The cleaned substrate was then placed into the chamber of an electron beam evaporation coating machine, where it was coated under a vacuum of better than 5 × 10⁻⁻⁻⁶ cm⁻³. 4 Under the condition of Pa, a 5 nm thick Cr film was first deposited at a rate of 0.1 Å / s to form the first Cr adhesion layer; then a 200 nm thick Ag film was deposited at a rate of 0.5 Å / s to form the Ag substrate.

[0058] Step 2: Photoresist spin coating and electron beam patterning:

[0059] This step is exactly the same as step 2 in Example 2. ZEP520A positive electron beam photoresist is spin-coated onto the sample surface with the deposited Ag substrate. After baking, an electron beam exposure system is used to expose the sample according to a pre-designed grating pattern with a period of 400 nm and a linewidth of 36 nm. After development, the desired grating pattern openings are obtained on the photoresist layer.

[0060] Step 3: Secondary deposition of Cr adhesion layer and Ag film:

[0061] This step is exactly the same as step 3 in Example 2. The sample with patterned photoresist is placed back into the electron beam evaporation coating machine, where a 5 nm thick Cr film is first deposited to form the second Cr adhesion layer; then, a 30 nm thick Ag film is immediately deposited, which is the height of the final nanograting structure.

[0062] Step 4, Lift-off stripping process:

[0063] This step is exactly the same as step 4 in Example 2. The sample with the deposited metal film is immersed in ZDMAC stripping solution, heated and gently stirred. After the photoresist and the excess Ag / Cr composite film layer covering it are completely stripped off, the sample is taken out and cleaned, leaving an Ag nanograting structure with precise period and linewidth on the substrate, consisting of a second Cr adhesion layer and an Ag film.

[0064] Step 5: Dry transfer of graphene:

[0065] This step is the key difference from Example 2. This example uses a dry transfer technique assisted by heat-release tape, and the specific process is as follows:

[0066] First, five layers of graphene were grown on a copper foil substrate using chemical vapor deposition. A layer of PMMA was then spin-coated onto the graphene surface as a temporary support layer and cured to form a three-layer structure of PMMA / graphene / copper foil.

[0067] Subsequently, the PMMA / graphene / copper foil sample was placed in an oxygen plasma etching machine to perform a short-term light etching on the back of the copper foil to remove any small amount of graphene that might be present on the back of the copper foil. Then, a piece of thermally released adhesive tape was smoothly attached to the back of the copper foil.

[0068] Utilizing the strong adhesion of heat-release tape, the tape, along with the copper foil, is slowly and mechanically peeled off from the PMMA / graphene, ensuring complete removal of the copper foil and leaving a clean graphene film on the PMMA support layer. This process avoids the use of chemical etching solutions, eliminating metal ion residue and chemical doping effects.

[0069] Next, the PMMA / graphene sample (graphene surface down) is aligned and placed above the target substrate where steps 1-4 have been completed, ensuring complete graphene coverage of the Ag nanograting structure region. It is then placed in a thermo-press bonding apparatus. First, slight mechanical pressure is applied to create a tight physical contact between the graphene and the Ag nanograting surface. The temperature is then raised to 120°C and maintained at this temperature for 10 minutes, allowing the heat-release adhesive tape to automatically lose its adhesiveness and detach from the sample. The temperature is maintained at 120°C, and the pressure is applied for another 15 minutes to promote van der Waals interactions between the graphene and Ag surface atoms, achieving a tight bond.

[0070] Finally, the sample was naturally cooled to room temperature and immersed in acetone solution for 12 hours to dissolve the PMMA support layer. It was then washed with isopropanol and dried with nitrogen to obtain the graphene-covered Ag nanograting structure.

[0071] Step 6, Cs / O high vacuum activation:

[0072] This step is exactly the same as step 6 in Example 2. The sample after completing the above steps is transferred to a dedicated activation chamber for Cs / O activation. The Cs source and O source are alternately turned on, and Cs / O alternating deposition is carried out by monitoring the photocurrent changes until the optimal activation state is reached. Finally, a Cs / O activation layer with a thickness of about 1.5 nm is formed on the surface of the graphene capping layer.

[0073] Through the aforementioned dry transfer process, this embodiment achieves clean transfer of graphene onto the surface of an Ag nanograting while avoiding contact with chemical reagents. The residual contaminants at the graphene-Ag interface are significantly reduced, resulting in a tighter and more uniform adhesion. The lattice integrity of the graphene is maintained, which is beneficial for the coupling of graphene with the near-field of plasmon resonances on the Ag surface. Simultaneously, the dense and complete graphene capping layer effectively blocks residual gas molecules from contacting the Ag surface and the Cs / O activation layer, thereby protecting the cathode's operational lifespan.

[0074] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Those skilled in the art can make various improvements and modifications without departing from the spirit and principle of the invention, such as combining wet and dry transfer processes or using other types of two-dimensional materials as a covering layer. However, these improvements and modifications should be considered within the scope of the invention and protected by the claims of the present invention.

Claims

1. A dual-channel response-enhanced metasurface structure Ag-graphene photocathode, characterized in that, It includes, from bottom to top, a P-type Si substrate, a Cr adhesion layer, an Ag substrate, a Cr adhesion layer, an Ag nanograting structure, a graphene capping layer, and a Cs / O activation layer.

2. The dual-channel response-enhanced metasurface structure Ag-graphene photocathode according to claim 1, characterized in that, The Ag nanograting structure has a height of 29~33nm, a spacing linewidth of 28~44nm, and a period of 395~405nm.

3. The dual-channel response-enhanced metasurface structure Ag-graphene photocathode according to claim 1, characterized in that, The graphene coating has 5 graphene layers.

4. The dual-channel response-enhanced metasurface structure Ag-graphene photocathode according to claim 3, characterized in that, Graphene was grown by chemical vapor deposition and then transferred to the surface of an Ag nanograting structure.

5. The dual-channel response-enhanced metasurface structure Ag-graphene photocathode according to claim 1, characterized in that, The Cr adhesion layer was deposited by electron beam thermal evaporation and has a thickness of 5 nm.

6. The dual-channel response-enhanced metasurface structure Ag-graphene photocathode according to claim 1, characterized in that, The Ag substrate was deposited by electron beam thermal evaporation, with a thickness of 150~250 nm.

7. The dual-channel response-enhanced metasurface structure Ag-graphene photocathode according to claim 1, characterized in that, The thickness of the p-type Si substrate is 0.5 mm, and the doping concentration is N. A 1×10 13 ~1×10 16 cm -3 .

8. A method for preparing a photocathode according to any one of claims 1 to 7, characterized in that, The specific steps are as follows: Step 1: On the substrate, deposit a Cr adhesion layer and an Ag substrate sequentially; Step 2: Spin-coat positive photoresist onto the substrate, and pattern it after electron beam exposure; Step 3: Secondary deposition of Cr adhesion layer and Ag film, the thickness of Ag film being the height of the nanograting structure; Step 4: The patterned photoresist and excess Ag film are removed by the lift-off process to obtain the nanograting structure; Step 5: Transfer the 5-layer graphene grown by chemical vapor deposition to the surface of the Ag nanograting structure using a PMMA-assisted wet or dry method. Step 6: Using a high-vacuum activation process, Cs / O activation is performed on the surface of the graphene coating to cover the graphene surface with a Cs / O activation layer, thereby reducing the work function of the photocathode.

9. The method for preparing a photocathode according to claim 8, characterized in that, The high vacuum activation process mentioned in step 6 specifically refers to the Cs / O activation process in a high vacuum environment, with a Cs / O activation layer thickness of 0.5–1.5 nm.

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