Free-standing one-dimensional plasmonic nanostructures

The field of plasmonics has become one of the most interesting and active research areas in nanotechnology, enabling numerous fundamental studies and applications. The ability to tailor the size, shape, and environment of metal nanostructures is the key component for controlling the plasmonic properties of individual or aggregated nanostructures. In this feature article, a category of chemically nanofabricated, unique free-standing one-dimensional (1D) plasmonic nanostructures has been summarized. The dispersible plasmonic nanostructures were obtained in high yield with control over gap size and feature size. This ability was exploited to tune the emerging plasmonic properties overcoming the difficulties of other methods to do so, leading to applications in analytical detection, biological sensing, and nanoelectronics.     

Solar energy conversion with tunable plasmonic nanostructures for thermoelectric devices

The photothermal effect in localized surface plasmon resonance (LSPR) should be fully utilized when integrating plasmonics into solar technologies for improved light absorption. In this communication, we demonstrate that the photothermal effect of silver nanostructures can provide a heat source for thermoelectric devices for the first time. The plasmonic band of silver nanostructures can be facilely manoeuvred by tailoring their shapes, enabling them to interact with photons in different spectral ranges for the efficient utilization of solar light. It is anticipated that this concept can be extended to design a photovoltaic-thermoelectric tandem cell structure with plasmonics as mediation for light harvesting.     

A surface-enhanced Raman spectroscopy study of thin graphene sheets functionalized with gold and silver nanostructures by seed-mediated growth

We describe a simple method for decorating graphene (1–5 layers) with Au and Ag nanostructures (nanoparticles, nanorods, and nanoplates). We deposit graphene electrostatically from highly-oriented pyrolytic graphite onto Si/SiO₂ surfaces functionalized with (aminopropyl)trimethoxysilane and grow the metal nanostructures by a seed-mediated growth method from hexanethiolate-coated Au monolayer-protected cluster “seeds” that are attached to graphene by hydrophobic interactions. Scanning electron microscopy reveals the selective growth of Au or Ag nanostructures on the graphene surface. In the case of Au, the low pH 2.8 growth solution causes etching of the graphene and formation of scroll-like structures. For Ag, the high pH 9.3 solution does not seem to affect the graphene. Raman spectroscopy is consistent with the graphene morphology and reveals that the presence of Au and Ag nanostructures increases the Raman scattering from the graphene by a factor of about 45 and 150, respectively. This work demonstrates a simple method for decorating graphene with noble metal nanostructures that may have interesting optical, electronic, and chemical properties for applications in nanoelectronics, sensing, and catalysis.     

Polytetrafluorethylene-Au as a substrate for surface-enhanced Raman spectroscopy

This study deals with preparation of substrates suitable for surface-enhanced Raman spectroscopy (SERS) applications by sputtering deposition of gold layer on the polytetrafluorethylene (PTFE) foil. Time of sputtering was investigated with respect to the surface properties. The ability of PTFE-Au substrates to enhance Raman signals was investigated by immobilization of biphenyl-4,4'-dithiol (BFD) from the solutions with various concentrations. BFD was also used for preparation of sandwich structures with Au or Ag nanoparticles by two different procedures. Results showed that PTFE can be used for fabrication of SERS active substrate with easy handle properties at low cost. This substrate was sufficient for the measurement of SERS spectrum of BFD even at 10-8 mol/l concentration.     

Rutile TiO2(101) based plasmonic nanostructures

TiO₂/Ag and TiO₂/Au nanocrystalline multilayer thin films were deposited using pulsed laser deposition technique. Investigations have been made to understand the influence of different phases of TiO₂ on the surface plasmon characteristics of the thin films. Rutile phase of TiO₂ is found to be a good host matrix for both Ag and Au nanoparticles. Compared to silver, gold nanoparticles are found to enhance the photocatalytic activity of the films by exhibiting a broad and intense absorption with a significant shift to longer wavelength region.     

Optical sensing nanostructures for porous silicon rugate filters

Porous silicon rugate filters [PSRFs] and combination PSRFs [C-PSRFs] are emerging as interesting sensing materials due to their specific nanostructures and superior optical properties. In this work, we present a systematic study of the PSRF fabrication and its nanostructure/optical characterization. Various PSRF chips were produced with resonance peaks that are adjustable from visible region to near-infrared region by simply increasing the periods of sine currents in a programmed electrochemical etching method. A regression analysis revealed a perfect linear correlation between the resonant peak wavelength and the period of etching current. By coupling the sine currents with several different periods, C-PSRFs were produced with defined multiple resonance peaks located at desired positions. A scanning electron microscope and a microfiber spectrophotometer were employed to analyze their physical structure and feature spectra, respectively. The sensing properties of C-PSRFs were investigated in an ethanol vapor, where the red shifts of the C-PSRF peaks had a good linear relationship with a certain concentration of ethanol vapor. As the concentration increased, the slope of the regression line also increased. The C-PSRF sensors indicated the high sensitivity, quick response, perfect durability, reproducibility, and versatility in other organic gas sensing.     

Hybrid FIB milling strategy for the fabrication of plasmonic nanostructures on semiconductor substrates

The optical properties of plasmonic semiconductor devices fabricated by focused ion beam (FIB) milling deteriorate because of the amorphisation of the semiconductor substrate. This study explores the effects of combining traditional 30 kV FIB milling with 5 kV FIB patterning to minimise the semiconductor damage and at the same time maintain high spatial resolution. The use of reduced acceleration voltages is shown to reduce the damage from higher energy ions on the example of fabrication of plasmonic crystals on semiconductor substrates leading to 7-fold increase in transmission. This effect is important for focused-ion beam fabrication of plasmonic structures integrated with photodetectors, light-emitting diodes and semiconductor lasers.     

ZnO thin film nanostructures for hydrogen gas sensing applications

A ZnO thin film-based gas sensor was fabricated using a SiO₂/Si substrate with a platinum comb-like integrated electrode and heating element. The structural characteristics, morphology, and surface roughness of the as-grown ZnO nanostructure were investigated. The film revealed the presence of a c-axis oriented (002) phase with a grain size of 20.8 nm. The sensor response was tested for hydrogen concentrations of 50, 70, 100, 200, 400, and 500 ppm at the optimum operating temperature of 350 °C. The sensitivities towards 50 and 200 ppm of hydrogen gas at 350 °C were approximately 78% and 98%, respectively. A linear response was observed for hydrogen concentrations within the range of 50 ppm–200 ppm. These results demonstrated the potential application of the ZnO nanostructure for the fabrication of cost-effective and high-performance gas sensors.     

Silver nanoplates prepared by modified galvanic displacement for surface-enhanced Raman spectroscopy

Silver nanoplates were prepared by modified galvanic displacement on commercial copper foil. SEM, TEM, UV-vis and XPS were employed to analyze those closely packed silver nanoplates. This type of surface-enhanced Raman spectroscopy substrates showed strong surface plasmon absorption and reliable surface-enhanced Raman activity.     

Controlled electrodeposition of ZnO nanostructures for enhanced light scattering properties

This paper outlined how to control density and shape of electrodeposited ZnO nanorods to achieve high scattering properties. Light scattering at nanostructured metal–semiconductor interfaces is a proven method to improve absorption in photovoltaic devices. Adjustment of nanostructure shape and mean distance is critical to achieve efficient light scattering. A simple model is introduced that predicts maximal suppression of the specular transmitted light, resulting in maximal light scattering. This model predicts in an ideal case, 50 % nanostructure coverage of the electrode. Furthermore, an optimal nanostructure height has been determined depending on the incident wavelength and the refractive indices. Experimentally, the crystal density on ITO substrates was adjusted by pulsing the deposition potential, thus, removing the requirement for an additional seeding layer. The solution of the diffusion equation indicated a break-to-pulse ratio of at least 2.4 for an efficient control of the crystal density during pulsed electrodeposition. In addition, the structure height was set by varying the number of pulse cycles. Such tailored ZnO nanostructures showed a suppression of the specular transmitted light beam of 83.5 % and a diffusive forward scattering efficiency of 39 % at a wavelength of 406 nm. Thus, the optical absorption of e.g. an 80-nm thick polymeric active layer of P3HT could be increased by 47 % by applying such tailored ZnO nanostructures.     

Fabrication and evolution of multilayer silver nanofilms for surface-enhanced Raman scattering sensing of arsenate

Surface-enhanced Raman scattering (SERS) has recently been investigated extensively for chemical and biomolecular sensing. Multilayer silver (Ag) nanofilms deposited on glass slides by a simple electroless deposition process have been fabricated as active substrates (Ag/GL substrates) for arsenate SERS sensing. The nanostructures and layer characteristics of the multilayer Ag films could be tuned by varying the concentrations of reactants (AgNO₃/BuNH₂) and reaction time. A Ag nanoparticles (AgNPs) double-layer was formed by directly reducing Ag⁺ ions on the glass surfaces, while a top layer (3rd-layer) of Ag dendrites was deposited on the double-layer by self-assembling AgNPs or AgNPs aggregates which had already formed in the suspension. The SERS spectra of arsenate showed that characteristic SERS bands of arsenate appear at approximately 780 and 420 cm^-1, and the former possesses higher SERS intensity. By comparing the peak heights of the approximately 780 cm^-1 band of the SERS spectra, the optimal Ag/GL substrate has been obtained for the most sensitive SERS sensing of arsenate. Using this optimal substrate, the limit of detection (LOD) of arsenate was determined to be approximately 5 μg·l^-1.     

Scanning tunneling microscopy and spectroscopy of supported nanostructures

Recent advances in low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) have provided new opportunities for the investigation of the local geometric, electronic, magnetic, and optical properties of nanostructures. This review focuses on the presentation and discussion of single molecules, supramolecular assemblies, and other nanostructures; all research results obtained in our laboratory. The emphasis is directed to the observation of new effects, where the properties of matter at the nanoscale differ from those at the mesoscopic or macroscopic scale: small is different. This fact is illustrated for the conservation of chirality in a hierarchical supramolecular assembly of organic molecules and for local light emission from supported molecules. The latter indicates a possible route towards an optical spectroscopic analysis on the scale of single molecules.     

Solid-state thermal dewetted silver nanoparticles onto electrochemically grown self-standing vertically aligned ZnO nanorods for three-dimensional plasmonic nanostructures

Three dimensional (3D) plasmonic nanostructures composed of silver nanoparticles decorated ZnO NRs arrays, have been fabricated by a process combining the electrochemical growth of ZnO NRs and further formation of Ag nanoparticles by the solid-state thermal dewetting (SSD) process. The effect of SSD parameters on the morphological, structural and optical properties of the Ag NPs decorated ZnO NRs arrays has been investigated. It is possible to tune the bandgap of the Ag NPs@ZnO nanorods array 3D plasmonic nanostructure by tailoring the Ag nanoparticle sizes, allowing light manipulation at the nanoscale. The silver nanoparticles attached to the ZnO NRs arrays experienced surface plasmonic coupling effect, causing enhancement in the room temperature photoluminescence (PL) UV emission and quenching the corresponding visible light one. An enhancement in the near band edge emission PL intensity of ZnO to the deep level emission PL intensity ratio after Ag NPs decoration of the ZnO nanostructures corresponding to ca. 11 folds has been observed, indicating that the defect emission is obviously suppressed.     

Multiple depositions of Ag nanoparticles on chemically modified agarose films for surface-enhanced Raman spectroscopy

A facile and cost-effective approach for the preparation of a surface-enhanced Raman spectroscopy (SERS) substrate through constructing silver nanoparticle/3-aminopropyltriethoxysilane/agarose films (Ag NPs/APTES/Agar film) on various solid supports is described. The SERS performance of the substrate was systematically investigated, revealing a maximum SERS intensity with four layers of the Ag NP deposition. The enhancement factor of the developed substrate was calculated as 1.5 × 10(7) using rhodamine 6G (R6G) as the probe molecule, and the reproducibility of the SERS signals was established. A high throughput screening platform was designed, manufactured and implemented which utilised the ability to cast agarose to assemble arrays. Quantitative analysis of 4-aminobenzoic acid (4-ABA) and 4-aminothiophenol (4-ATP) was achieved over a ～0.5 nM-0.1 μM range.     

Ultrahigh refractive index sensing performance of plasmonic quadrupole resonances in gold nanoparticles

The refractive index sensing properties of plasmonic resonances in gold nanoparticles (nanorods and nanobipyramids) are investigated through numerical simulations. We find that the quadruple resonance in both nanoparticles shows much higher sensing figure of merit (FOM) than its dipolar counterpart, which is attributed mainly to the reduction in resonance linewidth. More importantly, our results predict that at the same sensing wavelength, the sensing FOM of the quadrupole mode can be significantly boosted from 3.9 for gold nanorods to 7.4 for gold nanobipyramids due to the geometry-dependent resonance linewidth, revealing a useful strategy for optimizing the sensing performance of metal nanoparticles.     

Electrospray surface-enhanced Raman spectroscopy (ES-SERS) for studying organic coatings of atmospheric aerosol particles

Heterogeneous reactions between atmospheric aerosol particles and gaseous pollutants, such as those forming brown carbon (BrC), represent an important mechanism. These reactions alter the particle chemical compositions and aerosol-climate interactions. While most studies assume homogeneous particle compositions, organic coatings can be formed on solid or highly viscous particles due to heterogeneous reactions but the underlying mechanism is relatively less examined. We used electrospray surface-enhanced Raman spectroscopy (ES-SERS) to directly probe the formation of BrC coatings on methylaminium sulfate, nitrate, and chloride particles from heterogeneous reactions with gas-phase glyoxal. To create BrC coatings on particle surfaces, heterogeneous reactions were performed under low relative humidity (RH) conditions (i.e., 10 or 30% RH). The reacted particles fluoresced when irradiated at 532?nm in normal Raman analysis, indirectly suggesting the presence of light-absorbing species in them. Further ES-SERS analyses showed Raman bands of 1,3-dimethylimidazole, one of the major known products of reactions of glyoxal with methylaminium, from all the reacted particles at 30% RH. However, only methylaminium sulfate particles showed the formation of BrC coatings at 10% RH. We speculate that methylaminium sulfate particles may have more surface adsorbed water (SAW) than the other particle samples to initiate the formation of BrC coatings detectable by ES-SERS. The present study highlights the surface sensitivity of ES-SERS as well as the potential importance of SAW in heterogeneous reactions of atmospheric particles with gaseous pollutants.

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