CN113200512A - Small-gap metal nano cavity structure, preparation method and equipment - Google Patents
Small-gap metal nano cavity structure, preparation method and equipment Download PDFInfo
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Abstract
The invention discloses a method for manufacturing a small-gap metal nano cavity structure, which comprises the following steps: and carrying out laser irradiation on the metal nano-structure pair adhered to the substrate to melt the metal nano-structure to form a spherical or hemispherical structure pair, and then solidifying to finally obtain the small-gap metal nano-cavity structure. The invention also discloses a small-gap metal nano-cavity structure obtained by the method. An apparatus capable of implementing the above method is also disclosed. The invention utilizes nanosecond pulse laser to heat the metal nano structure to a melting point, so that the metal nano structure is melted at high temperature. After the laser pulse, the re-solidified gold nano-material gradually evolves from a cuboid or an elliptic cylinder to a stable sphere or a hemisphere structure, so that gaps between adjacent metal nano-structures are compressed.
Description
Technical Field
The invention belongs to the field of advanced 3D micro-nano manufacturing, and particularly relates to a small-gap metal nano-cavity structure, a preparation method and equipment.
Background
Thanks to the surface plasmon mode coupling and the space local effect, the nano cavity formed in the dielectric gap between a pair of adjacent gold nanoparticles not only has extremely large electromagnetic field local enhancement, but also has extremely small electromagnetic field mode volume. These excellent characteristics make the paired gold nanoparticles have important significance and practical value in the field of modern nanophotonics: for example, as a high-sensitivity sensing device, a small change in refractive index in a gold nano cavity can cause a local surface plasmon resonance wavelength of a metal nano structure to significantly shift, thereby realizing ultra-sensitive chemical and biological sensing, and even reaching the detection level of a Single Molecule (a. kinkhabwala et al, "Large Single-Molecule Fluorescence enhanced by a bouw Nanoantenna", Nature Photonics 3,654 (2009); c. cautech, et al, "ultrasensive sensitive sensing in air using optical fiber spectra", Nature Communications 7, 1-8 (2016)). For another example, the enhancement and regulation of spontaneous emission of fluorescent materials and quantum dots have important application in the fields of ultrafast photoelectric devices such as modulators, ultrafast photodiodes, on-chip ultrafast quantum information processing based on single photon sources, and the like. For example, nano-optical tweezers capable of capturing Single atoms, surface enhanced Single molecule Raman spectroscopy, strong coupling of Single molecules to the nanocavities, coherent high-energy electron pulsed radiation, ultrafast optical field driven electrical switches, Single atom level quantum dot touch switches, coherent nano-optical comb light sources, quantum electrodynamics, etc. (A.V.Akimov., et al, "" Generation of Single optical plasma in metallic nano-wires coupled to quantum dots ", Nature 450, 402. quadrature 406 (2007); S.Y., et al." "Nano-substrate plasma-enhanced Raman spectroscopy for surface analysis of Materials", Nature review Materials 1,1-16(2016), "R.K." fluorescence, molecular-spectroscopy, chemical analysis of Materials ", Nature reflection Materials 127, 2016. sub.2016. sub.10, "Nano Letters 13,674-678(2013)," Ultrafast string-field photoemission from plasma nanoparticles "; gen, t.t., et al, "Frequency comb transferred by surface plasma response," Nature Communications 7,1 (2016)).
The continuous reduction of the size of gold nanocavities is a constantly sought goal because, in a certain size range, the electric field enhancement coefficient within metal nanocavities increases exponentially with decreasing slot width, and some quantum optical effects can only be observed in very small nanocavities (r. estban, et al., "bright quantum and structural plastics with a quantum-corrected model." Nature Communications 3,825 (2012); j. baumberg, et al., "Extreme nanophotonics from metallic alloys." Nature Materials 18,668-678 (2019)). For example, when the gap width of the metal nanostructure is close to the de broglie wavelength of electrons, the electric field enhancement coefficient thereof reaches the limit of the classical electrodynamics, and the quantum tunneling effect significantly suppresses the electromagnetic field enhancement in the gap, which is called "quantum kiss" (k.j. savage, et al, "reforming the quantum region in the tuning plasma," Nature 491,574-577 (2012)). For these quantum optical effects, people have conducted extensive and intensive theoretical studies, but limited by the limitations of micro-nano processing technology, it is extremely difficult to observe these phenomena in experiments. The method for manufacturing the metal nano structure with the nano cavity structure generally comprises two steps: chemical synthesis and/or nano particle self-assembly in a bottom-up approach, ion beam etching or electron beam exposure in a top-down approach and the like.
The 'bottom-up' method can theoretically realize the size characteristics of a few nanometers and even a sub-nanometer structure, but the prepared metal nanometer structure has larger randomness, is difficult to realize a nanometer antenna array with a specified structure, and faces the problems of how to implant probe molecules or luminescent materials into a nanometer cavity and the like.
The method of ion beam etching or electron beam exposure and the like in the 'top-down' way can well design and process the appointed structure such as the metal nano antenna with the larger electromagnetic field enhancement effect. Generally, gold nanocavities with widths of about 10-20 nm can be easily realized by electron beam exposure. Using gallium ions (Ga)+) The beam etching method can realize gold nanometer cavities of about 10 nanometers, but Ga atoms are remained on the sample surface. Using focused helium ions (He)+) The beam can theoretically realize a gold nanometer cavity with the width of 5nm, and the surface of the structure is smooth and clean. But the bottleneck of 5nm characteristic parameters is difficult to break through by directly utilizing the methods of electron beam exposure and ion beam etching. The process of directly preparing a small-gap metal nano-cavity by using the processes of focused ion beam etching and electron beam exposure is very complicated and expensive. The method for enhancing femtosecond laser near-field ablation by using electron beam exposure (sum) or ion beam etching combined with surface plasmon is expected to realize gap<Relatively low cost fabrication of 5nm metal nanostructures, but are prone to leave photo-induced contaminant residues on the sample surface after femtosecond laser irradiation (L. P. Shi et al, "Generation ultra wideband Deep-UV Radiation and Sub-10nm Gap by Hybrid-Morphology Gold anchors." Nano Letters 19, 4779-.
In general, it remains a challenge to produce high quality, reproducible, large scale, integrated, paired metal nanostructures with gaps less than 10 nm. The problem can be solved, and important breakthrough can be brought to a plurality of advanced fields such as atomic resolution spectral imaging, extreme nonlinear optics, ultra-sensitive optical sensing, cavity electrodynamics, quantum optics, ultra-fast electronics and the like.
Disclosure of Invention
The invention relates to a gold nano structure with local surface plasmon resonance and huge local electric field enhancement effect in a visible near-infrared short wave band (600-1500 nm), in particular to a preparation method of a small-gap metal nano cavity structure.
A method for manufacturing a small-gap metal nano-cavity structure comprises the following steps: and carrying out laser irradiation on the metal nano structure pair adhered to the substrate, so that the metal nano structure is melted to form a sphere structure pair or a hemisphere structure pair, the gap between the two metal nano structure pairs is reduced, then, the metal nano structure is solidified, the small-gap metal nano cavity is formed between the two sphere structure pairs or hemisphere structure pairs, and finally, the small-gap metal nano cavity structure is obtained.
In the present invention, the pair of metal nanostructures consists of two metal nanostructures, and one or more metal nanostructure pairs may be disposed on the substrate. The metal nano-structure pair can be obtained by adopting the existing method, and the size of the gap between the two metal nano-structures is generally controllable before laser irradiation. After laser irradiation, the two metal nano structures are converted into approximate hemispherical structures, and the gap between the two metal nano structures in the center direction is reduced to obtain the small-gap metal nano cavity.
The "spherical structure" and "hemispherical structure" include not only a spherical structure or hemispherical structure in geometric sense, but also an approximate spherical structure or an approximate hemispherical structure or convex spherical structure, etc.
Preferably, a bonding layer is arranged between the metal nanostructure and the substrate. More preferably, the bonding layer is a titanium or/and chromium thin film. The titanium or/and chromium film is arranged between the metal nano structure and the substrate, so that one side of the metal nano structure is adhered to the surface of the glass or sapphire substrate, and the metal nano structure shaped by photo-thermal shaping is of a hemispherical structure by the adhesion of the metal nano structure and the substrate material. The planar side of the metal nanostructure is affixed to the substrate, and the spherical side of the metal nanostructure is exposed to air or immersed in a liquid environment. Preferably, the titanium or/and chromium thin film has a thickness of 3 to 5 nm. Further preferably a titanium (Ti) or chromium (Cr) film of 3 to 5nm thickness.
According to the invention, when no bonding layer is additionally arranged, under the action of laser, the two metal nano structures are melted to form a spherical structure; when the bonding layer is arranged, the bonding layer provides a certain positioning effect for the metal nano structure, and at the interface of the substrate and the hemispherical gold nano structure, the gap between two adjacent gold nano structures realizes self-compression through the photo-thermal effect to form the hemispherical structure.
For convenience of processing, before laser irradiation, each metal nano structure is a cuboid or elliptic cylinder nano structure, or any intermediate state nano structure in the process of converting the cuboid or elliptic cylinder nano structure into a spherical structure or a hemispheroid structure; the long edges (cuboids) or long axes (elliptic cylinders) of the two metal nano structures are parallel to each other, and the central connecting line is perpendicular to the long edges or long axes. In the present invention, the elliptic cylinder may be a cylinder obtained by pulling an ellipse in a direction perpendicular to a plane in which the ellipse is located, that is, a right cylinder having an elliptic cross section.
Preferably, the pair of metal nanostructures is a pair of rectangular parallelepiped or elliptic cylinder nanostructures with the same structure.
Preferably, the direction of the long edge or the long axis of the cuboid or the elliptic cylinder nano structure is taken as the x direction, the direction of the central connecting line of the two cuboid nano structures or the elliptic cylinder nano structures is taken as the y direction, the thickness direction is taken as the z direction, and the size of the metal nano structure in the y direction is smaller than the diameter size of an equivalent sphere or a hemisphere. After laser irradiation, the two metal nano structures are converted into a spherical structure or a hemispherical structure, the radial size in the y direction is increased, and meanwhile, due to the existence of adhesion, the central positions of the two metal nano structures are almost unchanged, so that the minimum gap is reduced, and the small-gap metal nano cavity is finally obtained.
Preferably, the pair of spherical structures or the pair of hemispherical structures (hemispherical metal nanostructures) is obtained by photo-thermal shaping of a metal nanostructure of a rectangular parallelepiped or an elliptic cylinder. Firstly, a pair of cuboid or elliptic cylinder single metal nano structures or arrays are processed on a glass or sapphire substrate by utilizing a focused ion beam etching or electron beam exposure process. The array period and size are not limited. Taking the cuboid metal nano-structure as an example, the geometrical arrangement mode of the paired cuboid nano-structures is as follows: in the top view, the central connecting line of the two is along the short side direction of the rectangle. The size of the cuboid metal nano-structure and the distance between the two structures need to be designed according to the specification of the actual requirement.
In the invention, the cuboid metal nano structure is continuously irradiated by a beam of focused nanosecond pulse laser, and the thermal shaping of the cuboid metal nano structure is realized by using a photo-thermal effect.
In the invention, the material of the metal nano structure is one or any two or more of gold, platinum, silver and aluminum; further preferably a gold nanostructure.
Preferably, the laser used is a nanosecond pulsed laser; laser energy is distributed into flat-top light spots; the focused power density is higher than the melting threshold of the metal nanostructure and lower than the ablation threshold of the metal nanostructure.
In the invention, the nanosecond pulse laser has unlimited repetition frequency and central wavelength, and the light spot mode is in flat-top distribution. The laser is focused onto the sample surface and heats the metal nanostructures. The focal point of the laser is located at the interface of the metal nanostructure and the substrate. And a half-wave plate is matched with the analyzer to gradually increase the incident energy of the pulse laser.
In the above scheme, the flat-top light spot is used for irradiating the sample, which has the advantage that once the energy density is adjusted, the sample can be exposed at one time, and the finished product of the whole metal nanostructure array is realized. And the transmitted or reflected spectrum is measured, so that the in-situ monitoring of the metal nano structure shaping process can be realized.
Preferably, the pair of metal nanostructures is one or more pairs. By using the method of the invention, the metal nano structure array with the same size requirement can be processed at one time.
The metal nano structure can be prepared by adopting an evaporation method.
After the nano structure is processed, a nano cavity with the gap smaller than 10 nanometers can be obtained.
In the invention, the metal nano structure after laser irradiation is finally cleaned by using acetone, ultrasonic waves and plasma cleaning methods to remove the pollutant residue on the surface of the sample caused by laser irradiation.
A small-gap metal nano-cavity structure is prepared by any one of the manufacturing methods.
An apparatus for preparing the small-gap metal nano-cavity structure of any one of the above technical solutions, comprising:
the laser emitter is used for providing required laser;
the diffraction optical element converts incident Gaussian laser into flat-top laser;
the light intensity adjusting element is used for adjusting the energy of the input flat-top laser;
the first lens element focuses and emits the laser with adjusted light intensity to the metal nano-structure pair;
a signal guide and collection element for collecting and outputting the laser reflection light to the second lens element;
and the second lens element focuses the collected laser reflection light to the fiber spectrometer for measurement.
The light intensity adjusting element generally comprises an optical half-wave plate and an optical analyzer, and is used for adjusting the light intensity of the laser to obtain the laser with the required energy.
The signal directing and collecting elements may generally employ one or more beam splitting chips or a combination of beam splitting chips and mirrors to direct and collect light by transmitting or reflecting light beams of a particular wavelength.
Preferably, the device further comprises an industrial camera for adjusting the spatial position of the laser spot and the metal nanostructure and simultaneously realizing the image acquisition of the metal nanostructure.
During the fabrication process, laser light is incident on the sample from one side of the metal nanostructure. The laser polarization direction is parallel to the long side of the cuboid or the long axis direction of the elliptic cylinder, namely the x-axis direction, so that the photothermal effect can be more fully utilized. In the laser irradiation process, due to local surface plasmon resonance, the metal nano structure of the cuboid or the elliptic cylinder can effectively enhance the absorption of laser. Due to the photo-thermal effect of surface plasmon enhancement, the metal nano structure of a cuboid or an elliptic cylinder is deformed due to high-temperature melting caused by pulse laser irradiation, and the corresponding scattered light intensity can be gradually changed along with the deformation of the metal nano structure. When the reflected light intensity no longer changes within a certain time (depending on the laser repetition frequency) range, indicating that the metal nanostructure has been integrated into a stable structure, the laser irradiation may be stopped.
The invention firstly determines the initial distance between the sizes of the cuboid or the elliptic cylinder according to the processing requirement of a target, and then utilizes the technique of focused ion beam or electron beam exposure to prepare a pair of adjacent cuboid or elliptic cylinder metal nano structures on a glass or sapphire substrate. And finally, heating the metal nano structure to a melting point by using nanosecond pulse laser to melt the metal nano structure at a high temperature. After the laser pulse, the re-solidified gold nano-material gradually evolves from a cuboid or an elliptic cylinder into a stable sphere or a hemisphere structure, so that gaps between adjacent metal nano-structures are compressed. In the preparation process, the in-situ monitoring of the shaping process of the metal nano structure is realized by utilizing the intensity of reflected laser.
In the present invention, the advantage of heating the metal nanostructure with the nanosecond laser is that its peak electric field intensity is weaker than that of the femtosecond laser with the same energy density, so that it does not cause non-thermal damage to the material including the metal nanostructure (such as gold) and the substrate due to the strong field effect. Meanwhile, the energy density required for inducing the metal nano structure to generate photo-thermal shaping is lower than that of long pulse and continuous laser, so that the precise processing of the metal nano structure in a micro-nano range can be realized, and the thermal damage to the nearby structure, such as a gold nano connecting line required in the application of a photoelectric effect, can not be caused.
In the invention, the photo-thermal effect is used for shaping the cuboid or elliptic cylinder metal nano structure, and compared with a processing method directly using electron beam exposure or ion beam etching, the processing method has the outstanding advantages that: utilizing the surface tension of the metal nano structure in a high-temperature melting state to integrate the metal nano structure into a spherical or hemispherical structure; and the geometric characteristics of two adjacent metal nano structures are skillfully utilized to realize the self-compression of the gap. Through photo-thermal shaping, the spatial resolution limit of processing methods such as electron beam exposure or ion beam etching and the like which are directly utilized is broken through, and the processing difficulty and cost are greatly reduced.
The metal nano structure prepared by the invention has important application prospects in the applications of surface-enhanced monomolecular Raman spectroscopy, monomolecular fluorescence spectroscopy, single-photon nonlinear optical effects, terahertz ultrafast photoelectric switches and the like.
Drawings
Fig. 1 (a) - (b) are schematic diagrams of the nano-structure before and after laser irradiation when the small-gap metal nano-cavity structure is prepared according to the present invention.
FIG. 2 is a schematic diagram of the apparatus for realizing self-assembly of the gap by nanosecond laser induced self-assembly of the metal nanostructure according to the present invention.
FIG. 3 is a scanning electron microscope picture before and after laser irradiation for actually preparing a small-gap metal nano-cavity structure according to the present invention.
Detailed Description
The invention will be further described with reference to the accompanying drawings in which:
as shown in fig. 1 (a), a pair of metal nanostructures 2 and 3 in a quasi-rectangular parallelepiped shape is prepared on a glass or sapphire substrate 1 by using a focused ion beam etching or electron beam exposure process, and both the metal nanostructures 2 and 3 are gold nanostructures. The metal nano-structure can also be prepared into an array structure, and the period and the size of the array are not limited and depend on the specific use requirement. The substrate 1 may be a glass or sapphire substrate, and a glass substrate is used in this embodiment. In other embodiments, a 3 nm titanium (Ti) film may also be evaporated between the substrate 1 and the metal nanostructures 2, 3 to enhance the adhesion of gold to the substrate. Thereby enabling the metal nano structure shaped by the final photo-thermal to be close to a hemisphere. In this embodiment, the metal nanostructure is directly formed on the substrate without a bonding layer. As shown in fig. 1 (a) and fig. 3, the length, width and height of the rectangular parallelepiped nanostructure are represented as l, w and t, respectively. The gap between the two is d. The specific dimensions of the above four parameters are also precisely designed and determined according to actual requirements, and in this embodiment, the length l of the metal nanostructure is 280 nanometers, and the width w is 140 nanometers; thickness or height t 100 nm; the gap d is 70 nm.
As shown in fig. 2, the device for realizing self-assembly of the gap by using nanosecond laser to induce self-assembly of the metal nanostructure comprises a diffractive optical element 4, a beam splitting sheet 9, a first lens 5, a reflecting mirror 10, a second lens 12, a fiber spectrometer 13 and an industrial camera 14. The diffractive optical element 4 converts incident gaussian laser light (nanosecond pulse laser light 6) into flat-top laser light. The beam splitting plate 9 guides the incident laser light 6 and collects the return light signal. The laser light is focused to the sample surface via a first lens 5. The detection signal is transmitted to the second lens 12 by the mirror 10 and finally to the fiber spectrometer 13.
A beam of nanosecond pulse laser 6 is focused to the surface of a cuboid metal nanostructure shown in figure 1 by a combination of a diffractive optical element 4 and a first lens 5, and simultaneously, initial Gaussian laser is converted into a flat-top light spot. The energy density of the pulse laser at the focus depends on the size of the cuboid metal nanostructure, and the energy of the incident pulse laser is controlled by an optical half-wave plate and an optical analyzer (which forms the light intensity adjusting element of the invention and is arranged between the diffractive optical element 4 and the beam splitting plate 9, not shown in the figure). The laser is incident on the sample from one side of the metal nanostructure. The laser polarization direction is parallel to the long side of the cuboid, namely the x-axis direction, so that the optothermal effect can be more fully utilized.
As shown in fig. 2, in the direction of laser reflection, the reflected laser reflection light 11 is collected by two beam splitting sheets 9, a reflecting mirror 10 and a quartz lens 5, and the reflected light 11 is input into a fiber spectrometer 13 through a quartz lens 12 for measurement. An industrial camera 14 is used on the side of the sample to adjust the laser spot to coincide with the space of the metal nanostructures 2 and 3. Due to localized surface plasmon resonance, the cuboid metal nanostructure can effectively enhance the absorption of laser light. Due to the photo-thermal effect of surface plasmon enhancement, the metal nano structure of the cuboid is deformed due to high-temperature melting caused by pulse laser irradiation, and the corresponding scattered light intensity is gradually changed along with the deformation of the metal nano structure. The intensity of the reflected light is detected by the fiber spectrometer 13, and when the intensity of the reflected light does not change within the time range of 10 seconds, it indicates that the metal nanostructure has been reformed into a stable structure, and at this time, the laser irradiation can be stopped.
As shown in fig. 3, the rectangular metal nanostructure after laser thermal shaping forms a sphere structure 15 and a sphere structure 16 with radius r because the sphere has the smallest surface tension and the most stable structure. As shown in the scanning electron micrograph of the right image in fig. 3. The minimum gap between adjacent metal nanostructures decreases from d 70nm to d1(in this example, d1About 20 nanometers). Soaking the sample irradiated by the nanosecond pulse laser in an acetone solution for 24 hours, cleaning and removing organic matter residues caused by pulse laser irradiation by using ultrasonic waves and plasma, and finally drying by using nitrogen to finish the final sample preparation. Finally, the prepared sample can be characterized by a scanning electron microscope and a linear transmission spectrum.
Claims (10)
1. A method for manufacturing a small-gap metal nano-cavity structure is characterized by comprising the following steps: and carrying out laser irradiation on the metal nano structure pair adhered to the substrate to melt the metal nano structure to form a sphere structure pair or a hemisphere structure pair, and then solidifying to finally obtain the small-gap metal nano cavity structure.
2. The method of claim 1, wherein a bonding layer is disposed between the metal nanostructures and the substrate.
3. The method for manufacturing a small-gap metal nano-cavity structure according to claim 1, wherein before laser irradiation, each metal nano-structure is a cuboid or an elliptic cylindrical nano-structure, or any intermediate state nano-structure in the process of converting the cuboid or the elliptic cylindrical nano-structure into a spherical structure or a hemispherical structure; the long sides or long axes of the two metal nanostructures are parallel to each other, and the central connecting line is perpendicular to the long sides or long axes.
4. The method of claim 3, wherein the pair of metal nanostructures are cuboid or cylindroid nanostructures of the same structure.
5. The method of claim 1, wherein the metal nanostructure is made of one or an alloy of any two or more of gold, platinum, silver, and aluminum.
6. The method of claim 1, wherein the laser used is a nanosecond pulsed laser; laser energy is distributed into flat-top light spots; the focused power density is higher than the melting threshold of the metal nanostructure and lower than the ablation threshold of the metal nanostructure.
7. The method of claim 3, wherein the laser polarization direction is a long side or long axis direction of the metal nanostructure.
8. A small-gap metal nanocavity structure, characterized by being prepared by the manufacturing method of any one of claims 1 to 7.
9. An apparatus for preparing the small-gap metal nanocavity structure of claim 8, comprising:
a laser transmitter;
the diffraction optical element converts incident Gaussian laser into flat-top laser;
the light intensity adjusting element is used for adjusting the energy of the input flat-top laser;
the first lens element focuses and emits the laser with adjusted light intensity to the metal nano-structure pair;
a signal guide and collection element for collecting and outputting the laser reflection light to the second lens element;
and the second lens element focuses the collected laser reflection light to the fiber spectrometer for measurement.
10. The apparatus of claim 9, further comprising an industrial camera for adjusting the spatial position of the laser spot and the metal nanostructure, and for simultaneously achieving image acquisition of the metal nanostructure.
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