| Plasmonic nanostructure arrays are demonstrated to be the most powerful platform for realizing controllable light-matter interactions and have found widespread applications in the field of surface-enhancement spectroscopy due to their fascinating and tunable optical properties.Among them,surface-enhanced Raman scattering(SERS)technology can amplify the fingerprint Raman signal of molecules by 6-9 orders of magnitude,enabling for ultra-sensitive detection of trace amounts or even single molecules.It has rapidly evolved into a powerful trace analysis technology with broad application prospects in many fields.When the analytes are located in the nanogaps(i.e.,the hotspots),their Raman signals will be significantly.amplified,and the signal intensity is approximately proportional to the fourth power of the electric field enhancement(i.e.,|E/E0|4).Therefore,designing and fabricating metallic nanogap arrays with small size,high density,and uniform distribution is an important way to construct SERS substrates with high sensitivity and excellent signal reproducibility,as well as the key to the practical application of SERS technology.In addition,with the gap size further reducing down to the nanoscale,it becomes increasingly challenging to deliver large-sized analytes into these gaps,particularly when the gap size is comparable to typical sizes of analytes.This issue fundamentally limits the further performance improvement of SERS sensors.In this thesis,based on highly ordered imprinted anodic aluminum oxide(AAO)template,we designed and constructed a series of high-performance SERS substrates,which were composed of ordered nanostructure arrays with precisely controllable gap size,and the nanogaps are distributed over the entire surface of the substrate in a dense and uniform manner,enabling rapid trace identification of analyte molecules of varying sizes.Furthermore,this SERS substrate fabrication method overcomes challenges such as low signal sensitivity,poor reproducibility,and poor universality caused by an insufficient number and uneven distribution of hotspots in traditional SERS substrates,while also providing advantages such as a simple process,controllable structure,and ease of large area and batch fabrication.The main research content and innovation of this thesis are as follows:1.By easily combining the unique advantages of AAO template technique and atomic layer deposition(ALD)technique,a controllable method for fabricating high density annular hotspots has been developed.Conformal TiO2 and Al2O3 layers were alternately deposited in the nanochannels of an AAO template by ALD,and then combined with electrochemical deposition(or not),chemical etching,and magnetron sputtering,a series of coaxial annular nanoslit arrays were obtained.This new nanofabrication approach can tailor the geometry and dimensions of inner-pillar/outerring or inner-ring/outer-ring nano-arrays at the nanoscale with high reproducibility,and has resulted in a large-area(cm2 scale)ordered,high-density coaxial circular nano-slit array patterns,in which the slit width between the outer ring and the inner pillar/ring can be tailored down to 2 nm.Such narrow metallic nano-slits can generate strong hotspots,leading to a SERS enhancement factor of up to 4×108.This enables detection of rhodamine 6G at concentrations as low as 10 fM,and chlorpyrifos and thiram in lake water at limit of detection of 0.25 ppm and 1.8 ppb,respectively.In comparison to the existing nanogap fabrication techniques,our method allows for more ingenious structural design and manipulation,bringing a new impetus to the field of narrow-gap patterning.Besides the application of SERS sensing,the coaxial structure arrays and their fabrication strategy are expected to have a great potential in other fields like photonics,electronics,and catalysis,etc.2.By introducing a unique anisotropic wet-chemical etching process,we constructed a high-quality imprinting mold-SAIMST,which has abundant sharp tips distributed on its surface,and a cost-effective and universal ’hotspots printing’ strategy is proposed,enabling the rapid,batch and low-cost fabrication of SERS substrates with high-density hotspots.Through a simple wet etching process,an AAO template with abundant sharp tips on the surface is obtained by utilizing the differences in the chemical composition of the inner and outer walls of the AAO template and the hexagonal anisotropic distribution of compressive stress within the barrier layer.Then,it is used as an imprinting mold to directly pattern various materials at room temperature.By imprinting the substrate a different number of times,plasmonic nanogap devices with different hotspot densities can be obtained.After three consecutive shallow imprints,hotspots with a gap width of 5 nm are densely distributed on the silver substrate,which enables the detection of 0.1 pM rhodamine 6G and trace parathionmethyl,with excellent signal uniformity and reproducibility.This research presents an effective strategy for the design of advanced imprinting molds,as well as a paradigm for fabricating various high-performance functional devices by using the mold.In comparison to previous fabrication methods of nanogap devices,this ’hotspots printing’strategy enables the rapid construction of high-density and uniformly distributed nanogaps without the use of special instruments,which has great potential in SERS applications.3.By utilizing the highly ordered conical-pore imprinted AAO template prepared by multi-step anodization and wet-chemical etching process,we realized a universal SERS platform based on a metallic ’slot-under-groove’ nanostructure array that enables strongly coupled field enhancement with a large spatial mode profile through the hybridization of 2D gap-surface plasmons in the upper groove and localized surface plasmon resonance in the lower slot.Large-sized analytes,such as proteins and viruses,usually cannot enter into the tiny nanogaps of traditional SERS substrates,limiting practical SERS-based applications.Here we demonstrate a universal SERS platform for reliable and sensitive identification of a wide size range of analytes,which can simultaneously deliver superior sensitivity for small-sized analytes and identification of large-sized analytes with an unprecedently large Raman gain.A wide size range of analytes,from a few nanometer-size biomarkers to more than one hundred nanometersize SARS-CoV-2 pseudovirus,could be efficiently trapped and label-free identified by our SERS platform.As a result,our results provide a powerful impetus for designing and manufacturing universal SERS platforms for a wide size range of analytes in practical applications.4.We realized a series of highly-ordered metallic plasmonic nano-gap and nanotip arrays with well-defined structural features,accurate dimensions,and scalable fabrication by combining the imprinted AAO template with multiple advanced manufacturing techniques such as magnetron sputtering,evaporation deposition,wet etching,and so on.Meanwhile,these methods can be easily scaled up to large-scale fabrication.The fabricated plasmonic nano-gap and nano-tip arrays are expected to be applied to SERS,nonlinear optics,chemical/biosensors,and other fields.And the purpose of this work is to present a promising and designable AAO template-based nano-gap and nano-tip patterning technique,and discuss the physical principles and manufacturing steps behind each method.The ultimate goal is to investigate novel SERS substrate construction methods and inspire researchers for their cutting-edge study. |
