Plasmonic Nanostructures Based On Nanoskiving And Optical Properties

Noble metal nanostructures have been able to interact with incident light to produce surface plasmon resonance(SPR),and cause significantly enhancements of electro-magnetic(EM)fields,making them attractive in sensing,imaging,non-linear optics and metamaterials applications.However,in order to obtain metal nanostructures with huge enhanced EM fields,we need to construct metals at nano/micro-scale.In addition,in practical application,noble metal nanostructures are required not only to have remarkable and controllable plasmonic resonance performance to build an efficient and stable plasmonic sensing platform,but also to have simple,easy operation,low cost,high yield and good integration fabrication techniques.In recent years,nanostructures with nanogaps and sharp tips have attracted much attention due to their large EM field enhancements.On the one hand,when the light irradiates the nanogaps structures,the nanometric gaps between noble metals squeeze light through them can generate extreme subwavelength confinement of EM energy,thereby enhance the EM field intensity in the nanogap.The classical electromagnetic theory model gives the EM field distribution in the nanogaps by the solution of Maxwell?s equations.This approach predicts monotonically increasing electric field enhancements with decreasing nanogap size,prompting the development of nanotechnology for producing plasmonic structures with ultranarrow gaps.In particular,when the gap size is less than 10 nm,the strong EM field excited in the gap can be applied to single molecule detection.On the other hand,as an alternative way to generate optical hot spots,nanotips can readily confine optical energy and lead to local EM field enhancement via plasmonic nanofocusing.Therefore,the preparation of nanostructures consisting of nanogaps and sharp tips with strong EM field enhancement is the primary task of contemporary scientists.However,the traditional fabrication technology has certain limitations in prepare nanostructure.Especially,it is difficult to fabricate vertically oriented nanostructures with both nanogaps and sharp tips.In order to prepare large area,low cost,highly integrated and complex plasmonic nanostructure,a relatively novel fabrication technology called nanoskiving has emerged,and the prepared nanostructure can be transferred to any substrate,which is more convenient in construct nanostructure in three-dimensional.In this paper,we use nanoskiving technology to obtain vertically oriented plasmonic nanostructures,and introduce nanogap,sharp tips and heterogeneous materials to enhance the plasmonic resonance properties.We also study their applications in sensing,surface enhancement spectroscopy,nonlinear optics.In the second chapter,we combine anisotropic wet etching and nanoskiving to create a novel three-dimensional(3D)nano-antenna for plasmonic nanofocusing,vertically aligned zig-zag nanogaps,constituted of nanogaps with defined angles.Instead of conventional lithography,we used the thickness of a self-assembled monolayer(SAM)to define nanogaps with high throughput,and anisotropic etching of Si V-grooves to naturally define ultra-sharp tips.Both nanogaps and sharp tips can synergistically squeeze the electro-magnetic(EM)field and excite 3D nanofocusing,enabling great potential applications in chemical sensing and plasmonic devices.The dependence of the EM field enhancement on structural features is systematically investigated and optimized.We found that the field enhancement and confinement are stronger at the tipped-nanogap compared to what standalone tips or nanogaps produce.The intensity of surface-enhanced Raman spectroscopy(SERS)recorded on the 70.5° tipped-nanogaps is 45 times higher than that recorded with linear nanogaps and 5 times higher than that recorded with tip-only nanowires,which is attributed to the integration of the tip and gap in plasmonic nanostructures.This proposed nanofabrication technique and the resulting structures equipped with a strongly enhanced EM field will promote broad applications for nanophotonics and surface-enhanced spectroscopy.In the third chapter,a novel fabrication route is reported for the generation of free-standing asymmetric metal nanostructures.We combine a sacrificial nanosphere template and conventional thin film deposition method and nanoskiving for subsequent sectioning the patterned polymeric sample.The technique is demonstrated for the fabrication of crescent-shaped and opposing crescent-shaped gold nanowires with or without nanogaps.Optical properties of localized surface plasmon resonances induced in such structures are studied by surface-enhanced Raman spectroscopy(SERS)and finite-difference time domain(FDTD)simulations.Simulations reveal optical near field enhancements is dependent on the polarization of incident light.The strong electric-field enhancements in the opposing crescent nanogaps structures have great potential applications in nonlinear optics,optical trapping,and surface-enhanced spectroscopy.In the fourth chapter,we developed a simple and systematic method to fabricate optically tunable and thermally and chemically stable Au-Ag nanowire-based plasmonic metamaterials.The Au-Ag bimetallic heterogeneous nanogaps with desirable optical properties were fabricated by a novel(simple,ultra-rapid and robust)nanoskiving technique.Its optical properties were controlled by gap width between the Au-Ag nanowires.Also,as these sub-10 nm Au-Ag heterogeneous nanogaps can be fabricated thousands of one time,these results required for various applications in spectroscopy and nanophotonics.Compared to the monmetallic linear Ag-Ag or Au-Au nanogaps,the Au-Ag bimetallic heterogeneous nanogaps exhibited significant SERS enhancement,which was mainly attributed to two important factors,the nanogaps between the adjacent Au/Ag nanowires and the composite of the Ag/Au bimetallic film.Notably,the novel 3D bimetallic heterogeneous nanogaps were successively built by a simple stacking proceduce,it produces the even larger local fields than the 1D linear nanogaps.This makes it have more potential applications in surface enhanced spectroscopy.