Preparation method and application of graphene-multi-hotspot gold nanostructure composite system

Abstract

The application discloses a preparation method of a graphene-multi-hot-spot gold nanostructure composite system and application thereof. The composite system is obtained by directly thermal evaporation of metal on heated graphene by using a graphene lattice modulation effect; wherein the graphene has a single-atom layer thickness, and the metal nanostructure is a geometric structure with sharp corners. The composite system can be used as an excellent substrate for surface enhanced Raman characterization. The preparation method is economical and efficient, and is of great significance for designing and constructing a structure and a stable SERS substrate, and even an enhanced photoelectric device.

Description

Technical Field

[0001] This invention belongs to the field of spectroscopic measurement technology, specifically relating to a method for preparing a graphene-multi-hotspot gold nanostructure composite system and its application. Background Technology

[0002] Surface-enhanced Raman scattering (SERS) can significantly improve Raman signal intensity, even enabling single-molecule detection, making it an excellent surface analysis method in physics, chemistry, biology, and medicine. The SERS effect stems from two mechanisms: electromagnetic enhancement (EM) and chemical enhancement (CM). Rough surfaces of metals such as gold, silver, and copper are commonly used traditional SERS substrates. Their enhancement primarily originates from EM: incident light irradiates the plasmonic metal, generating plasmons. At resonance, the electric field on the metal surface is greatly enhanced, ultimately significantly increasing the Raman scattering cross-section of monolayers and sub-monolayers adsorbed on the metal surface, providing structural information about the surface molecules. The electric field enhancement factor of localized surface plasmons formed on metal nanoparticles is related to many factors, including size, shape, density, and the orientation and configuration of the analyte molecules near the gold. High-density gold particle arrangement allows for electromagnetic coupling between adjacent gold particles, further increasing the electromagnetic field. The gaps between gold particles are therefore called "hot spots." For metallic nanostructures with sharp shapes (such as triangles), larger electromagnetic field enhancements can exist at sharp tips or edges, thus the geometrized structure itself contains many "hot spots" generated by its morphological features. Chemical enhancement mechanisms can reach up to 10... 13 In the total SERS enhancement intensity, the enhancement magnitude is generally in the range of 10-10. 2 Therefore, it is considered a secondary, auxiliary factor. It is generally believed that the primary cause of chemical enhancement is charge transfer between the metal and the adsorbed molecules. Due to charge transfer, positive and negative charges separate within the molecule, increasing molecular polarizability and thus improving the Raman scattering cross section.

[0003] Graphene possesses a continuous electronic band structure, enabling it to accept any excitation energy and thus suppress electronic and vibrational excitations. Studies have shown that graphene can effectively quench the fluorescence of detected molecules, potentially enabling chemically enhanced SERS detection (Nano Lett., DOI: 10.1021 / nl903414x; Small, DOI: 10.1002 / smll.201203097). Loading noble metal nanoparticles onto graphene can combine both chemical and electromagnetic enhancement effects, resulting in a higher Raman enhancement factor. Conversely, while pure gold substrates effectively enhance the Raman signal of molecules, they also amplify the fluorescence signal. The application of monolayer graphene, however, suppresses the fluorescence signal. Wang et al. deposited gold films of varying thicknesses onto monolayer graphene and used them as SERS substrates for R6G. The results showed that the strongest molecular SERS signal and the weakest fluorescence background were obtained when the gold film thickness was 7 nm (Appl. Phys. Lett., DOI:10.1063 / 1.3505335). Therefore, the graphene-gold film combination system is an ideal choice for SERS substrates.

[0004] Methods for stacking metal nanoparticles on the surface of two-dimensional materials such as graphene include physical and chemical methods. However, chemical methods inevitably introduce contaminants such as solvents. Physical microfabrication methods require complex and expensive processing techniques, such as electron beam lithography and focused ion beam etching, which greatly increases processing time and cost and makes large-scale fabrication impossible. Therefore, physical vapor deposition (PVD) is the preferred method for depositing metals on the graphene surface to form a graphene-metal composite SERS substrate. Furthermore, the graphite lattice structure modulates the thermodynamic and kinetic behaviors of metal nanoparticles on its surface, including nucleation, growth, and structural evolution. In 1975, scientists first studied the relationship between the dispersion of gold deposited by physical vapor deposition on highly oriented pyrolytic graphite (HOPG) and substrate temperature and evaporation rate (J. Cryst. Growth, DOI: 10.1016 / 0022-0248(75)90025-1; 10.1016 / 0022-0248(75)90026-3). Since then, many studies have investigated the morphology of gold particles deposited on bulk graphite. For example, Nishitani et al. studied the morphology of gold on HOPG at substrate temperatures of 293–383 K (J. Vac. Sci. Technol., DOI: 10.1116 / 1.585516). At lower temperatures, compact gold islands were obtained, while at higher temperatures, dendritic gold was obtained. Gladfelter et al. observed that gold can form dendritic structures on bulk graphite at room temperature (Langmuir, DOI: 10.1021 / la00051a003). This work shows that the morphology of gold particles deposited on a bulk graphite substrate can be altered by changing the temperature of the substrate. However, research on how the specific surface structure and nanoscale thickness of two-dimensional graphene modulate the morphology of surface gold nanoparticles remains lacking. Graphene-multi-hotspot gold nanostructure composite systems obtained by modulating the morphology of metal nanoparticles with graphene are of great significance for designing and constructing structurally and performance-stable SERS substrates, and even for enhancing optoelectronic devices. Summary of the Invention

[0005] One objective of this invention is to provide a method for preparing a graphene-multi-hotspot metal nanostructure composite system, comprising the following steps:

[0006] Step 1, Micromechanical preparation of graphene film: Using natural highly oriented pyrolytic graphite HOPG as raw material, graphene sheets are prepared by micromechanical exfoliation and placed on the surface of Si / SiO2 substrate to form graphene film.

[0007] In one embodiment of the present invention, the thickness of graphene is initially identified using an optical microscope to find thin layers; the number of layers and quality of the prepared graphene sample are identified by testing its Raman spectrum.

[0008] Step 2: A first layer of plasma metal is thermally vapor-deposited onto the heated graphene surface using a vacuum coating apparatus to form a geometrically shaped metal nanostructure. A metal wire is placed inside a ceramic crucible, and a Si / SiO2 substrate with an attached graphene film is tightly connected to a power thin-film resistor. This resistor is then placed parallel to the ceramic crucible in the vacuum coating apparatus, with a metal baffle placed between them. The process is carried out at a pressure of 10... -4 Under the conditions of Pa, the graphene film was first deposited by applying current to the thin film resistor to maintain the temperature of the Si / SiO2 substrate with the graphene film attached at 80 ℃~120 ℃. At the same time, the ceramic crucible was heated to make the evaporation rate 1.0 Å / s. Then, the metal baffle was removed to start depositing metal atoms. When the metal film thickness reached 4 nm~6 nm, the baffle was opened, the heating of the ceramic crucible was turned off, and the sample was cooled under vacuum to obtain a graphene Si / SiO2 substrate with a metal nanostructure with geometric morphology.

[0009] In one embodiment of the present invention, the magnitude of the current applied to the thin-film resistor is set according to the current-temperature parameter of the thin-film resistor; the thickness of the metal film is measured by a film thickness monitor in the evaporation chamber via a crystal oscillator.

[0010] Step 3: A second layer of plasma metal is deposited by conventional thermal evaporation using a vacuum coating machine to form dense metal particles: The substrate obtained in step 2 is kept at room temperature and another layer of dense metal nanoparticle structure is deposited by conventional evaporation with a film thickness of 4 nm to 6 nm. The gaps between these densely packed metal particles further generate a large number of "hot spots", and finally a graphene-multi-hot spot gold nanostructure composite system is obtained.

[0011] Furthermore, the metal wire is a gold wire or a silver wire.

[0012] A second objective of this invention is to use the graphene-multi-hotspot gold nanostructure composite system prepared by the above method as a substrate for surface-enhanced Raman scattering (SERS) characterization. This graphene-multi-hotspot metal nanostructure composite system, as a SERS substrate, can enhance the Raman signal of adsorbed molecules.

[0013] Compared with existing technologies, the graphene-multi-hotspot metal nanostructure composite system provided by this invention, which can be used as a SERS substrate, has the following advantages:

[0014] (1) By utilizing the effect of substrate temperature increase on the nucleation and growth of deposited metal, multi-hotspot metal nanostructures can be obtained simply, economically and efficiently. When the temperature increases, the kineticly controlled island growth intensifies, and the merging and growth events of particles become more significant, resulting in the three-dimensionalization and geometric shaping of metal islands.

[0015] (2) Achieving a large enhancement factor SERS effect at low cost can be further applied to various trace molecule detection.

[0016] (3) The application of this graphene-multi-hotspot metal nanostructure composite system can be expanded, for example, to form optoelectronic devices based on two-dimensional materials, providing a new way for device structure optimization and performance improvement. Attached Figure Description

[0017] Figure 1 This is a schematic diagram illustrating the preparation process of the graphene-multi-hotspot metal nanostructure composite system of the present invention. Wherein: 1 represents a silicon substrate, 2 represents graphene, 3 represents the first layer of geometrically shaped metal nanostructures thermally evaporated under heated substrate conditions, and 4 represents the second layer of metal nanoparticles covered by a conventional thermal evaporation method.

[0018] Figure 2 The images show SEM images of two different geometric morphologies of metallic nanostructures formed by thermally depositing a first layer of gold on heated graphene surfaces with different numbers of layers using a vacuum deposition apparatus in Example 1. The scale bar in the images is 500 nm.

[0019] Figure 3 The first layer of geometrically shaped metal nanostructures deposited by thermal evaporation in Example 1 enhances the Raman signal of graphene itself.

[0020] Figure 4 This is a SEM image of the graphene-multi-hotspot gold nanostructure composite system obtained after evaporating the second layer of gold nanoparticle film in Example 1. The scale bar in the image is 500 nm.

[0021] Figure 5 The graphene-multi-hot spot gold nanostructure composite system obtained after thermally depositing two layers of gold film in Example 1, and the SERS signal of gentian violet molecules. Implementation

[0022] The preferred embodiments of the present invention will now be described in detail with reference to specific examples. It should be understood that the following examples are given for illustrative purposes only and are not intended to limit the scope of the invention. Those skilled in the art can make various modifications and substitutions to the present invention without departing from its spirit and essence.

[0023] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0024] Unless otherwise specified, all materials and reagents used in the following examples are commercially available. Example 1

[0025] like Figure 1 In this embodiment, a graphene-multi-hotspot gold nanostructure composite system was obtained by thermally depositing 4 nm polygonal gold nanostructure films and 4 nm gold particle films on graphene with different numbers of layers using a vacuum coating instrument (SBC-2). The Raman enhancement effect of the gold nanostructure on the graphene itself was verified by testing. Furthermore, the composite system was used as a SERS substrate to achieve trace detection of gentian violet molecules.

[0026] First, a small layer of graphite is peeled off from a block of highly oriented pyrolytic graphite (Alfa Aesar Graphite flake, natural) using transparent tape. The tape is then repeatedly applied between layers, gradually thinning the graphite sheets. Once the graphite adhered to the tape is thin and appears transparent, the tape is then applied to a Si / SiO2 substrate. After gentle rubbing and peeling off the tape, single-layer and thin-layer graphene can be found within the graphite sheets deposited on the substrate. These single-layer and thin-layer graphite on the Si / SiO2 substrate can be observed under an optical microscope, and the number of layers is determined by Raman peak positions obtained through Renishaw in Via plus.

[0027] Secondly, the combined use of mechanical and molecular pumps to create a vacuum results in an internal pressure of ~10. -4 The gold wire was placed in a parallel ceramic crucible and melted and deposited using resistance heating. The evaporation rate was controlled at 1.0 Å / s by adjusting the current applied to the ceramic crucible. The silicon substrate with the graphene sample attached was tightly bonded to the surface of a power thin-film resistor (Caddock MP9100) and kept parallel to the surface of the ceramic crucible. While the cavity was evacuated, a current of 1A was applied to the power thin-film resistor and stabilized for about half an hour to maintain the temperature of the silicon substrate at 80 °C. A metal baffle was placed between the graphene sample and the crucible to shield the graphene sample from the thermal radiation emitted by the gold wire heating the crucible. Before evaporation, the baffle was installed to stabilize the current in the crucible and the evaporation rate. Then the baffle was removed, allowing the graphene sample to face the evaporation source, and the deposition of gold atoms began. The thickness of the gold film was measured by a film thickness monitor (FTM-V) in the evaporation cavity using a crystal oscillator.

[0028] like Figure 2As shown, two geometries of gold nanostructures deposited on graphene were observed using a scanning electron microscope (Hitachi S-4800). It can be seen that the geometric gold islands deposited on the graphene surface exhibit numerous edges and small crystal faces. The metal film of morphology one was deposited by vapor deposition while the substrate was kept heated; the metal film of morphology two was deposited by vapor deposition during the cooling process while the power thin-film resistor was turned off. As the substrate temperature increases, the metal particles bond together to form three-dimensional and geometric islands. Furthermore, since the metal atoms are epitaxially distributed on the graphite surface, many edges of the particles should be modulated and grow along the crystal orientation of the graphene.

[0029] Specifically, morphology one ( Figure 2 Characteristic a) is that the gold islands on the surface have a clear tendency to align in a specific direction, forming gold structures with characteristic side lengths of 10-100 nm, and many geometric shapes such as triangles, quadrilaterals, hexagons, octagons, and multiple twins; Morphology two ( Figure 2 (b) The characteristic is that the multiple twin structure exhibits a certain regularity of dendritic structure pattern. The tips and close-packed junctions of these geometric structures are "hot spots" that can generate a large electromagnetic field to enhance SERS.

[0030] Step 3: The deposition of the second layer of gold granular film is performed without turning on the thin-film resistor. In this embodiment, the thickness of both the first and second gold films is 4 nm. When the film thickness monitor shows that the predetermined film thickness of 4 nm has been reached, the baffle is immediately applied and the current to the heating crucible is turned off. The sample is allowed to cool by maintaining a high vacuum for a period of time, thus completing the deposition process.

[0031] like Figure 3 As shown, Raman spectroscopy was performed on single / double-layer graphene samples with two gold film morphologies, each with a thickness of 4 nm, obtained by the above-mentioned heated substrate evaporation method. Typical SERS spectra were obtained. The blue line at the bottom is the ordinary Raman spectrum of the un-gold-coated graphene sample; the spectral intensity was magnified tenfold for easy comparison. It can be seen from the figure that the Raman signal of both single and double-layer graphene samples was significantly enhanced after gold coating. Both morphologies of multi-hotspot geometric gold films significantly enhanced the Raman signal of the graphene material itself. The Raman enhancement factor EF is defined as the ratio between the integral areas of the Raman peaks before and after gold coating. Among them, the polygonal geometric gold nanostructure of morphology one has a larger EF value for the G peak of single-layer graphene, reaching 272, and an EF value for the 2D peak, reaching 147, both of which are historical highs. Figure 3 (a) Additionally, such as Figure 3As shown in Figure b, the SERS enhancement factor EF of gold nanostructures on bilayer graphene is reduced for both the G and 2D peaks. The decrease in EF values ​​with increasing graphene layer number is consistent with previous findings by Lee et al. on the enhancement of 1-3 layer graphene. They suggest this is because monolayer graphene exhibits the strongest interaction with metals, thus possessing the largest enhancement factor. For bilayer and thicker graphene layers, the electronic structure properties differ significantly from monolayer graphene due to the van der Waals interactions between layers. Consequently, interactions such as charge transfer with metals are expected to decrease considerably.

[0032] like Figure 4 As shown in the SEM images, in this embodiment, after depositing a 4 nm thick layer of a two-dendritic metal nanostructure on graphene, a second 4 nm thick layer of dense metal nanoparticles was conventionally deposited (with the substrate maintained at room temperature), resulting in a typical graphene-multi-hotspot gold nanostructure composite system. This system was used as a SERS substrate and immersed in 2×10⁻⁶ layers. -6 Molecular deposition was performed in a gentian violet solution of concentration M. To ensure that molecular adsorption and deposition on the sample reached equilibrium, the deposition time was allowed to stand for more than 2 hours. Next, the soaked substrate was removed and gently rinsed in deionized water to remove molecules floating on the surface. Finally, the sample was very gently dried with a small stream of nitrogen and left at room temperature for at least 2 hours to allow it to dry completely.

[0033] like Figure 5 As shown, the Raman spectra in the figure present the gentian violet molecular signals (solid lines) on graphene-multi-hotspot gold nanostructure composite systems composed of graphene with different numbers of graphene layers and on silicon substrates with only dense gold particles. The signals are compared with those on unplated graphene substrates with different numbers of graphene layers and on unplated silicon substrates (dashed lines). The Raman spectra in the figure were obtained using an excitation wavelength of 514 nm. The concentration of the gentian violet molecular solution was 2 × 10⁻⁶. -6 M. The results show that the highest SERS enhancement is obtained by combining the EM enhancement brought about by the multi-hotspot gold nanostructure with the CM enhancement of graphene, and the largest signal is obtained at low concentrations of gentian violet. In addition, the Raman enhancement efficiency still shows a correlation with the number of layers: the thicker the graphene layer, the weaker the Raman enhancement.

[0034] The above are merely exemplary embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing a graphene-multi-hotspot metal nanostructure composite system, characterized in that, It comprises the following steps: Step 1, preparing graphene film by micro-mechanical method: graphene sheet is prepared by micro-mechanical exfoliation method with natural highly oriented pyrolytic graphite (HOPG) as raw material, and is placed on the surface of Si / SiO2 substrate to form graphene film; Step 2, forming geometrically patterned metal nanostructures by thermal evaporation of the first layer of plasmonic metal on the heated graphene surface through a vacuum coater: placing a metal wire in a ceramic crucible, tightly connecting the Si / SiO2 substrate with the graphene film to a power thin film resistor, then placing the two in parallel in the vacuum coater with a metal baffle between them, and evaporating under a pressure of 10 -4 Pa, first applying a current to the thin film resistor to keep the temperature of the Si / SiO2 substrate with the graphene film at 80 ℃-120 ℃, while heating the ceramic crucible to make the evaporation rate 1.0 Å / s, then removing the metal baffle to start depositing metal atoms, opening the baffle when the metal film thickness reaches 4 nm-6 nm, turning off the ceramic crucible heating, and keeping the vacuum state to cool the sample, obtaining a graphene Si / SiO2 substrate with geometrically patterned metal nanostructures attached thereto; The geometricized morphological features include triangular, quadrilateral, hexagonal, octagonal and multiple twin nanometer structures; Step 3, forming dense metal particle by conventional thermal evaporation of vacuum coating instrument: the substrate obtained in step 2 is kept at room temperature and is then coated with a layer of dense metal nanoparticle structure by conventional thermal evaporation, with a film thickness of 4-6 nm, to obtain graphene-multi-hot-spot gold nanostructure composite system.

2. The production method according to claim 1, characterized by, The metal wire is gold wire or silver wire.

3. Application of graphene-multi-hot-spot gold nanostructure composite system prepared by the method of claim 1 in surface enhanced Raman scattering characterization.

4. Use according to claim 3, characterized in that, The graphene-multi-hot-spot gold nanostructure composite system is used as a substrate in surface enhanced Raman scattering characterization.

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