Self-Organized SERS Substrates with Efficient Analyte Enrichment in the Hot Spots.

One of the requirements of an efficient surface-enhanced Raman spectroscopy (SERS) substrate is a developed surface morphology with a high density of "hot spots", nm-scale spacings between plasmonic nanoparticles. Of particular interest are plasmonic architectures that could enable self-localization (enrichment) of the analyte in the hot spots. We report a straightforward method of fabrication of efficient SERS substrates that comply with these requirements. The basis of the substrate is a large-area film of tightly packed SiO2 spheres formed by their quick self-assembling upon drop casting from the solution. Thermally evaporated thin Ag layer is converted by quick thermal annealing into nanoparticles (NPs) self-assembled in the trenches between the silica spheres, i.e., in the places where the analyte molecules get localized upon deposition from solution and drying. Therefore, the obtained substrate morphology enables an efficient enrichment of the analyte in the hot spots formed by the densely arranged plasmonic NPs. The high efficiency of the developed SERS substrates is demonstrated by the detection of Rhodamine 6G down to 10-13 mol/L with an enhancement factor of ∼108, as well as the detection of low concentrations of various nonresonant analytes, both small dye molecules and large biomolecules. The developed approach to SERS substrates is very straightforward for implementation and can be further extended to using gold or other plasmonic NPs.

SERS-substrates based on ZnO nanoflowers prepared by green synthesis.

ZnO nanoparticles (NPs) with a flower-like morphology, synthesized by an affordable colloidal route using an aqueous fungi extract of Ganoderma lucidum as a reducing agent and stabilizer, are investigated as SERS-substrate. Each "flower" has large effective surface that is preserved at packing particles into a dense film and thus exhibits an advantageous property for SERS and similar sensing applications. The mycoextract used in our low-cost and green synthesis as surface stabilizer allows subsequent deposition of metal NPs or layers. One type of SERS substrates studied here was ZnO NPs decorated in situ in the solution by Ag NPs, another type was prepared by thermally evaporating Ag layer on the ZnO NP film on a substrate. A huge difference in the enhancement of the same analyte in the solution and in the dried form is found and discussed. Detection down to 10-7 M of standard dye analytes such as rhodamine 6G and methylene blue was achieved without additional optimization of the SERS substrates. The observed SERS-activity demonstrate the potential of both the free-standing flower-like ZnO NPs and thereof made dense films also for other applications where large surface area accessible for the external agent is crucial, such as catalysis or sensing.

Au Gratings Fabricated by Interference Lithography for Experimental Study of Localized and Propagating Surface Plasmons.

Optical properties of high-frequency Au gratings with a fixed period (296.6 ± 0.5 nm) and a variable modulation depth are studied using measurements of spectral and angular dependence of transmission and reflection of polarized light in order to build the dispersion curves of excited optical modes and to identify their types. It was shown that in gratings with small modulation depth only propagating surface plasmon-polaritons (SPP) modes were observed. With increasing of modulation depth, the intensity of SPP decreases and localized plasmon (LP) resonance appears, which is more intense at small incident angles, and overlaps with the SPP modes. For grating with isolated grooves (nanowires), mostly LP resonance is observed. After additional deposition of gold onto grating with isolated grooves, the intensity of the SPP mode increases again, and the LP band maximum shifts to longer wavelengths.

Enhancing Surface Plasmon Resonance Detection Using Nanostructured Au Chips.

The increase of the sensitivity of surface plasmon resonance (SPR) refractometers was studied experimentally by forming a periodic relief in the form of a grating with submicron period on the surface of the Au-coated chip. Periodic reliefs of different depths and spatial frequency were formed on the Au film surface using interference lithography and vacuum chalcogenide photoresists. Spatial frequencies of the grating were selected close to the conditions of Bragg reflection of plasmons for the working wavelength of the SPR refractometer and the used environment (solution of glycerol in water). It was found that the degree of refractometer sensitivity enhancement and the value of the interval of environment refractive index variation, Δn, in which this enhancement is observed, depend on the depth of the grating relief. By increasing the depth of relief from 13.5 ± 2 nm to 21.0 ± 2 nm, Δn decreased from 0.009 to 0.0031, whereas sensitivity increased from 110 deg./RIU (refractive index unit) for a standard chip up to 264 and 484 deg./RIU for the nanostructured chips, respectively. Finally, it was shown that the working range of the sensor can be adjusted to the refractive index of the studied environment by changing the spatial frequency of the grating, by modification of the chip surface or by rotation of the chip.

Polarization memory effect in the photoluminescence of nc-Si-SiOx light-emitting structures.

The polarization memory (PM) effect in the photoluminescence (PL) of the porous nc-Si-SiOx light-emitting structures, containing nanoparticles of silicon (nc-Si) in the oxide matrix and passivated in a solution of hydrofluoric acid (HF), has been investigated. The studied nc-Si-SiOx structures were produced by evaporation of Si monoxide (SiO) powder in vacuum and oblique deposition on Si wafer, and then the deposited silicon oxide (SiOx) films were annealed in the vacuum at 975 °C to grow nc-Si. It was found that the PM effect in the PL is observed only after passivation of nanostructures: during etching in HF solution, the initial symmetric nc-Si becomes asymmetric elongated. It was also found that in investigated nanostructures, there is a defined orientational dependence of the PL polarization degree (ρ) in the sample plane which correlates with the orientation of SiOx nanocolumns, forming the structure of the porous layer. The increase of the ρ values in the long-wavelength spectral range with time of HF treatment can be associated with increasing of the anisotropy of large Si nanoparticles. The PM effect for this spectral interval can be described by the dielectric model. In the short-wavelength spectral range, the dependence of the ρ values agrees qualitatively with the quantum confinement effect.

Fabrication of Periodic Plasmonic Structures Using Interference Lithography and Chalcogenide Photoresist.

This study reports on the employment of the interference lithography (IL) technique, using photoresist based on the chalcogenide glass (ChG) films, for fabrication of one-dimensional (gratings) and two-dimensional (arrays) periodic plasmonic structures on the surface of glass plates. The IL technique was optimized for patterning of the Au and Al layers and formation of gratings and arrays with a spatial frequency of 2000 mm(-1). Optical properties of obtained structures were studied using measurements of spectral and angular dependence of transmission and reflection of polarized light. It was shown that the spectral and angular position of the surface plasmon polariton and local surface plasmon resonance, which are observed in these samples, can be adjusted over a wide range by selecting the geometric parameters of structures and technological modes of their manufacturing.

The nanostructuring of surfaces and films using interference lithography and chalcogenide photoresist.

The reversible and transient photostimulated structural changes in annealed chalcogenide glass (ChG) layers were used to form interference periodic structures on semiconductor surfaces and metal films. It was shown that negative-action etchants based on amines dissolve illuminated parts of a chalcogenide film, i.e., act as positive etchants. The diffraction gratings and 2-D interference structures on germanium ChGs - more environmentally acceptable compounds than traditionally used arsenic chalcogenides - were recorded, and their characteristics were studied.

Photoluminescence decay rate of silicon nanoparticles modified with gold nanoislands.

We investigated plasmon-assisted enhancement of emission from silicon nanoparticles (ncs-Si) embedded into porous SiOx matrix in the 500- to 820-nm wavelength range. In the presence in the near-surface region of gold nanoisland film, ncs-Si exhibited up to twofold luminescence enhancement at emission frequencies that correspond to the plasmon resonance frequency of Au nanoparticles. Enhancement of the photoluminescence (PL) intensity was attributed to coupling with the localized surface plasmons (LSPs) excited in Au nanoparticles and to increase in the radiative decay rate of ncs-Si. It has been shown that spontaneous emission decay rate of ncs-Si modified by thin Au film over the wide emission spectral range was accelerated. The emission decay rate distribution was determined by fitting the experimental decay curves to the stretched exponential model. The observed increase of the PL decay rate distribution width for the Au-coated nc-Si-SiOx sample in comparison with the uncoated one was explained by fluctuations in the surface-plasmon excitation rate. PACS: 78. 67. Bf; 78.55.-m.