Tetrahedral nanopyramids made of silver and gold over ITO/glass surfaces are fabricated. Our protocol is based on nanosphere lithography (NSL) with the deposition of thicker metal layers. After removing the microspheres used in the NSL process, an array of metallic tetrahedral nanostructures of ∼350–400 nm height is formed. The reported procedure avoids the use of any stabilizing surfactant molecules that are generally necessary to segregate the individual particles onto surfaces. We focus here on the optical and the physical properties of these plasmonic surfaces using near-field spectroscopy in conjunction with finite difference time domain (FDTD) modeling of the electric field. Remarkably, FDTD shows that the localized surface plasmon resonance is confined in the plane formed by the edges of two facing pyramids that is parallel to the polarization of the impinging excitation laser. The variable gap between the edges of two adjacent pyramids shows a broader localized surface plasmon and a larger specific surface as opposed to the usual nanotriangle array. Localized enhancement of the electric field is experimentally investigated by coating the plasmonic surface with a thin film of photosensitive azopolymer onto the surface of the nanopyramids. Upon irradiation, the deformation of the surface topography is visualized by atomic force microscopy and suggests the potentiality of these 3D nanopyramids for near-field enhancement. This last feature is clearly confirmed by surface-enhanced Raman scattering measurement with 4-nitrothiophenol molecules deposited on the pyramid platforms. The potentiality of such 3D nanostructures in plasmonics and surface spectroscopy is thus clearly demonstrated.
Finite-Difference Time-Domain (FDTD) calculations are used to characterize the electric field in the vicinity of a sharp silver or gold cone with an apex diameter of 10 nm. The simulations are utilized to predict the intensity and the distribution of the locally enhanced electric field in tip-enhanced Raman spectroscopy (TERS). A side-by-side comparison of the enhanced electric field induced by a radially and a linearly polarized light in both gap-mode and conventional TERS setup is performed. For this purpose, a radially polarized source is introduced and integrated into the FDTD modeling. Additionally, the optical effect of a thin protective layer of alumina on the enhancement of the electric field is investigated.
In the ectopic biomineralization of calcium oxalate kidney stones, the competition between calcium oxalate monohydrate (COM) formation and its inhibition by the phosphoprotein osteopontin (OPN) plays a key role in COM stone-forming processes. To get more insights into these processes, tip-enhanced Raman spectroscopy (TERS) was used to provide surface-specific information about the adsorption of OPN to faces of COM crystals. In TERS, the surface plasmon resonance of a metallic AFM tip is locally excited when the tip is placed in the optical near-field of a laser focused on the crystal surface. Excitation of this localized surface plasmon resonance allows the enhancement of the Raman signal as well as the improvement of the spatial resolution beyond the diffraction limit of the light. As TERS works label free and noninvasively, it is an excellent technique to study the distribution of adsorbed proteins on crystal faces at the submicrometer scale. In the present work, we generated Raman intensity maps indicating high spatial resolution and a distinct variation in relative peak intensities. The collected TERS spectra show that the OPN preferentially adsorbs to edges and faces at the ends of COM crystals (order: {100}/{121} edge > {100} face > {100}/{010} edge ≈ {121}/{010} edge > {010} face) providing also relevant information on the inhibition of crystal growth. This study demonstrates that TERS is an excellent technique for detailed investigations of biomolecules adsorbed, layered, or assembled to a large variety of surfaces and interfaces.
as plasmonic hot-spots. [2] The resulting spectroscopic technique, known as surface-enhanced Raman spectroscopy (SERS), [3] surpasses the inherently low sensitivity of Raman by counterbalancing its low scattering efficiency [4] through spatial localization of the sample molecules proximal to the hot-spots on SERS substrates. So far colloidal nanoparticles of various shapes and sizes have been extensively used in SERS; [5][6][7][8][9] however, the lack of control over their relative orientation and separation limits efficient plasmonic coupling therebetween. [10] While this issue is addressed in SERS substrates made of immobilized nanoparticles, [11][12][13][14] interparticle gaps in these substrates are typically optimized to generate maximum field confinement for single wavelengths. Broadband SERS substrates can be made by immobilizing a mixture of nanoparticles resonant over a range of laser wavelengths on surface plasmon polariton (SPP)-supporting thin films. [15,16] However, the magnitude of SERS signal enhancement and the broadband nature of such substrates remain highly dependent on the nanoparticle-film separation as well as the interparticle distances which are difficult to control in practice. More robust broadband SERS substrates have been recently fabricated by sputtering randomly sized silver nanoparticles on glass-silver-glass multilayered substrates, [17] yielding plasmonic resonances over the 400-1100 nm
Tip-enhanced Raman spectroscopy (TERS) is used to investigate the influence of strains in isolated and overlapping silicon nanowires prepared by chemical etching of a (100) silicon wafer. An atomic force microscopy tip made of nanocrystalline diamond coated with a thin layer of silver is used in conjunction with an excitation wavelength of 532 nm in order to probe the first order optical phonon mode of the [100] silicon nanowires. The frequency shift and the broadening of the silicon first order phonon are analyzed and compared to the topographical measurements for distinct configuration of nanowires that are disposed in straight, bent or overlapping configuration over a microscope coverslip. The TERS spatial resolution is close to the topography provided by the nanocrystalline diamond tip and subtle spectral changes are observed for different nanowire configurations.
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