Surface Of Plasmonic Nanostructures Research Articles

Cold Plasma Jet Coupled Nanosecond Laser Ablation Scheme For Plasmonic Nanostructured Surfaces

AbstractThis paper describes a study where an argon cold plasma jet, generated by a dielectric‐barrier discharge (DBD), is combined with nanosecond laser ablation (248 nm, 25 ns, 10 Hz) to deposit silver particle aerosols onto the substrate at atmospheric pressure. The deposition of the particle is examined using various microscopy techniques and absorption spectroscopy for the plasma jet produced by operating DBD in the normal and reversed mode. Plasma facilitated the deposition process by delivering the particle to the substrate and significantly influenced its morphology depending on the jet interaction, length, and substrate position. In both cases, the particles are clustered; however, there is less deposit for the plasma ignited in the reverse mode. The theoretical analysis of the deposition process is performed using ANSYS software and evaluated in terms of plasma‐induced flow velocity. This study infers that the hybrid plasma‐laser deposition scheme considered is attractive for material processing and deposition, especially overextended substrate distances, and for altering the properties of the deposited particles for practical utilization in surface‐enhanced Raman spectroscopy, solar cells, and catalysis.

Diode model of nonuniform irradiation treatment to predict multiscale solar-electrical conversion for the concentrating plasmonic photovoltaic system

Optical concentrators and plasmonic nanostructured surfaces are promising methods to increase the solar irradiation density and enhance light absorption. The combined application of these two methods for solar cells faces challenges of multiscale solar-electrical conversions, especially the treatment of nonuniform irradiation. A multiscale multiphysics method is proposed to predict the electrical outputs of a concentrating plasmonic photovoltaic (CPV) system. In the model, a series-connected diode global model for concentrating-plasmonic-electrical conversion combined with a local one-unit diode for fully coupled optical-thermal-electrical conversion is employed to treat nonuniform concentrating irradiation. The one-unit diode model is calibrated by J-V curve fitting with the local fully coupled optical-thermal-electrical model to characterize the wavelength-dependent light absorption and charge carrier behaviors. An explicit formula is proposed to consider the influences of concentrating irradiation and plasmonic nanostructures. The feasibility of the calibrated one-unit diode under concentration ratios is validated with the maximum power compared to an experimental GaAs-based solar cell under concentrated sunlight. The electrical outputs of the plasmonic solar cell under nonuniform irradiation is obtained by the equivalent circuit with series-connected diodes. Each of the diodes yields a photogenerated current in its individual state, whereas the equivalent series-connected circuit has a reformulated homogeneous current. The cross current of the series-connected circuit is mainly determined by the solar cell zone with the lowest concentration ratio. The method with a uniform one-sun irradiation assumption in the entire solar cell surface can overestimate its working current before the declining point and can underestimate the open-circuit voltage. The concentration enhances the photogenerated current but induces auxiliary series resistance. The maximum power is a trade-off of the improved photogenerated current and unfavorable series resistance. This work provides theoretical guidance for large-scale engineering applications of CPV systems with nanostructured solar cells.

Plasmonic Nanocrystals with Complex Shapes for Photocatalysis and Growth: Contrasting Anisotropic Hot‐Electron Generation with the Photothermal Effect

AbstractIn plasmonics, and particularly in plasmonic photochemistry, the effect of hot‐electron generation is an exciting phenomenon driving new fundamental and applied research. However, obtaining a microscopic description of the hot‐electron states represents a challenging problem, limiting the capability to design efficient nanoantennas exploiting these excited carriers. This paper addresses this limitation and studies the spatial distributions of the photophysical dynamic parameters controlling the local surface photochemistry on a plasmonic nanocrystal. It is found that the generation of energetic electrons and holes in small plasmonic nanocrystals with complex shapes is strongly position‐dependent and anisotropic, whereas the phototemperature across the nanocrystal surface is nearly uniform. The formalism includes three mechanisms for the generation of excited carriers: the Drude process, the surface‐assisted generation of hot‐electrons in the sp‐band, and the excitation of interband d‐holes. The computations show that the hot‐carrier generation originating from these mechanisms reflects the internal structure of hot spots in nanocrystals with complex shapes. The injection of energetic carriers and increased surface phototemperature are driving forces for photocatalytic and photo‐growth processes on the surface of plasmonic nanostructures. Therefore, developing a consistent microscopic theory of such processes is necessary for designing efficient nanoantennas for photocatalytic applications.

Energy-resolved plasmonic chemistry in individual nanoreactors.

Plasmonic resonances can concentrate light into exceptionally small volumes, which approach the molecular scale. The extreme light confinement provides an advantageous pathway to probe molecules at the surface of plasmonic nanostructures with highly sensitive spectroscopies, such as surface-enhanced Raman scattering. Unavoidable energy losses associated with metals, which are usually seen as a nuisance, carry invaluable information on energy transfer to the adsorbed molecules through the resonance linewidth. We measured a thousand single nanocavities with sharp gap plasmon resonances spanning the red to near-infrared spectral range and used changes in their linewidth, peak energy and surface-enhanced Raman scattering spectra to monitor energy transfer and plasmon-driven chemical reactions at their surface. Using methylene blue as a model system, we measured shifts in the absorption spectrum of molecules following surface adsorption and revealed a rich plasmon-driven reactivity landscape that consists of distinct reaction pathways that occur in separate resonance energy windows.

SERS Amplification in Au/Si Asymmetric Dimer Array Coupled to Efficient Adsorption of Thiophenol Molecules.

Maximizing the surface-enhanced Raman scattering (SERS) is a significant effort focused on the substrate design. In this paper, we are reporting on an important enhancement in the SERS signal that has been reached with a hybrid asymmetric dimer array on gold film coupled to the efficient adsorption of thiophenol molecules on this array. Indeed, the key factor for the SERS effect is the adsorption efficiency of chemical molecules on the surface of plasmonic nanostructures, which is measured by the value of the adsorption constant usually named K. In addition, this approach can be applied to several SERS substrates allowing a prescriptive estimate of their relative performance as sensor and to probe the affinity of substrates for a target analyte. Moreover, this prescriptive estimate leads to higher predictability of SERS activity of molecules, which is also a key point for the development of sensors for a broad spectrum of analytes. We experimentally investigated the sensitivity of the Au/Si asymmetric dimer array on the gold film for SERS sensing of thiophenol molecules, which are well-known for their excellent adsorption on noble metals and serving as a proof-of-concept in our study. For this sensing, a detection limit of 10 pM was achieved as well as an adsorption constant K of 6 × 10 M. The enhancement factor of 5.2 × 10 was found at the detection limit of 10 pM for thiophenol molecules.

Manipulation and Applications of Hotspots in Nanostructured Surfaces and Thin Films.

The synthesis of nanostructured surfaces and thin films has potential applications in the field of plasmonics, including plasmon sensors, plasmon-enhanced molecular spectroscopy (PEMS), plasmon-mediated chemical reactions (PMCRs), and so on. In this article, we review various nanostructured surfaces and thin films obtained by the combination of nanosphere lithography (NSL) and physical vapor deposition. Plasmonic nanostructured surfaces and thin films can be fabricated by controlling the deposition process, etching time, transfer, fabrication routes, and their combination steps, which manipulate the formation, distribution, and evolution of hotspots. Based on these hotspots, PEMS and PMCRs can be achieved. This is especially significant for the early diagnosis of hepatocellular carcinoma (HCC) based on surface-enhanced Raman scattering (SERS) and controlling the growth locations of Ag nanoparticles (AgNPs) in nanostructured surfaces and thin films, which is expected to enhance the optical and sensing performance.

Comparison of Free-Space and Waveguide-Based SERS Platforms.

Surface-Enhanced Raman Spectroscopy (SERS) allows for the highly specific detection of molecules by enhancing the inherently weak Raman signals near the surface of plasmonic nanostructures. A variety of plasmonic nanostructures have been developed for SERS signal excitation and collection in a conventional free-space microscope, among which the gold nanodomes offer one of the highest SERS enhancements. Nanophotonic waveguides have recently emerged as an alternative to the conventional Raman microscope as they can be used to efficiently excite and collect Raman signals. Integration of plasmonic structures on nanophotonic waveguides enables reproducible waveguide-based excitation and collection of SERS spectra, such as in nanoplasmonic slot waveguides. In this paper, we compare the SERS performance of gold nanodomes, in which the signal is excited and collected in free space, and waveguide-based nanoplasmonic slot waveguide. We evaluate the SERS signal enhancement and the SERS background of the different SERS platforms using a monolayer of nitrothiophenol. We show that the nanoplasmonic slot waveguide approaches the gold nanodomes in terms of the signal-to-background ratio. We additionally demonstrate the first-time detection of a peptide monolayer on a waveguide-based SERS platform, paving the way towards the SERS monitoring of biologically relevant molecules on an integrated lab-on-a-chip platform.

Impact of Plasmon-Induced Optically Rectified Electric Fields on Second Harmonic Generation

Plasmon associated optically rectified fields arising from electron tunneling between metal nanojunctions have been shown to impact photocatalytic reactions on the surface of plasmonic nanostructures. However, temporal differences have been shown between quantum regime electron tunneling events and optically rectified surface charge buildup. Computational work shows that electron tunneling across a nanometer-sized gap between two plasmonic entities can be induced from a single ultra-fast laser pulse and occurs on a femtosecond timescale. Our group has shown that charge buildup from optically rectified fields occurs over 10s to 100s of seconds through light-induced redox events in cyclic voltammetry experiments using a CuSO4 solution. Utilizing a femtosecond laser to generate second harmonic signals along with widefield excitation from a continuous light-emitting diode (LED) source, it is shown that surface potentials that average −400 mV are only generated upon continuous illumination of a gold nanoisland plasmonic surface with a widefield LED source. Negligible changes in second harmonic generation (SHG) signals are seen from surface excitation with the femtosecond laser alone, showing the importance of continuous plasmon excitation for sustained surface potentials. It is also shown, through Stark spectroscopy and spectroelectrochemical measurements, that optically rectified fields impact SHG signals in a manner similar to an externally applied bias on pristine plasmonic surfaces. This result shows the potential for utilizing SHG signals to better understand the impact surface heterogeneity has on the optically rectified direct current field and enables the optimization of plasmonic surfaces for future use as photocatalysts.

Expedite SERS Fingerprinting of Portuguese White Wines Using Plasmonic Silver Nanostars.

Surface-enhanced Raman Spectrosocopy (SERS) is a highly sensitive form of Raman spectroscopy, with strong selectivity for Raman-active molecules adsorbed to plasmonic nanostructured surfaces. Extremely intense Raman signals derive from “hotspots”, generally created by the aggregation of a silver nanospheres colloid. An alternative and cleaner approach is the use of anisotropic silver nanoparticles, with intrinsic “hotspots”, allowing a more controlled enhancement effect as it is not dependent on disordered nanoparticle aggregation. Here, a simple SERS-based test is proposed for Portuguese white wines fingerprinting. The test is done by mixing microliter volumes of a silver nanostars colloid and the white wine sample. SERS spectra obtained directly from these mixtures, with no further treatments, are analyzed by principal component analysis (PCA), using a dedicated software. Depending on the duration of the incubation period, different discrimination can be obtained for the fingerprinting. A “mix-and-read” approach, with practically no incubation, allows for a simple discrimination between the three white wines tested. An overnight incubation allows for full discrimination between varieties of wine (Verde or Maduro), as well as between wines from different Maduro wine regions. This use of SERS in a straightforward, fast and inexpensive test for wine fingerprinting, avoiding the need for prior sample treatment, paves the way for the development of a simple and inexpensive authenticity assay for wines from specific appellations.

A surface enhanced Raman scattering based colloid nanosensor for developing therapeutic drug monitoring

Competitive reactions, on the surface of plasmonic nanostructures, allow exploiting SERS signals for quantitative Therapeutic Drug Monitoring. As an example, the concentration of Erlotinib, an anti-EGFR small molecule, used for the treatment of non-small cell lung and pancreatic cancer, is determined. The numerous side effects and the variability of patient responses make Erlotinib a good candidate for monitoring. The new SERS based sensor can estimate Erlotinib down to nanomolar concentration and is based on the chemical reaction of the drug and of a competitor SERS reporter on the surface of gold nanostructures. Colloid solutions of naked gold nanoparticles obtained by laser ablation in solution were used for obtaining nanostructures with very efficient hot spots for SERS and with a clean surface for chemistry. Detection of the drug in the nanomolar concentration range is shown to be possible also in spiked plasma samples.

Engineering plasmonic nanostructured surfaces by pulsed laser deposition

The synthesis and the optical response of gold nanoparticles (NPs) and thin nanostructured films grown by pulsed laser deposition (PLD) are here studied. Different PLD process parameters – including background gas pressure and the number of laser shots as well as post-deposition annealing treatments – have been varied to control the growth of Au NPs and films, thus tuning the surface plasmon characteristics. The mechanisms of NPs and film growth have been explored performing a morphological characterization by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM), and the correlation with the optical behavior is investigated. We show that the size distribution and the morphology of the as deposited Au NPs depend on growth mechanisms which are controlled by tuning the deposition process, while the optical behavior is strongly affected by the average size and surface density of NPs or by the length of percolated Au domains. Furthermore, nucleation in gas phase has been reported at high (1000 Pa Ar) background pressures, enabling independent control of NP size and coverage, contrary to surface driven NP growth by diffusion and aggregation on substrate.

Surface-Enhanced Raman Spectroscopy and Density Functional Theory Calculations of a Rationally Designed Rhodamine with Thiol Groups at the Xanthene Ring

Rhodamines are widely used dyes in fluorescence and surface-enhanced Raman spectroscopy (SERS). The latter requires adsorption of the dye onto the surface of plasmonic nanostructures, a process which requires attractive molecule–surface interactions. Here, we report an experimental SERS and computational density functional theory (DFT) study investigating the role of thiol functionalization at the xanthene ring of the rhodamine in the adsorption onto gold nanoparticles. For this purpose, a new bisthiolated rhodamine derivative was rationally designed and synthesized via a PPh3/I2 reduction route. The introduction of two thiol moieties directly at the xanthene ring provides the shortest possible distance between the molecular π-system and the metal surface for maximum SERS enhancement combined with the strong Au–S interaction for chemisorption. The comparison of experimental SERS spectra obtained from gold nanostars and a film of gold nanoparticles with results from DFT calculations (molecular electrostati...

SERS-Modeling in Molecular Sensing

Surface enhanced Raman spectroscopy (SERS) is an ultrasensitive vibrational spectroscopic technique that useful tools in detecting biomolecules at near or on the surface of plasmonic nanostructures. Unique physicochemical and optical properties of noble metal nanostructures allow the assimilation of biomolecular probes and exhibit distinctive spectra, prompting the development of a plethora of biosensing platforms in molecular diagnostics. In SERS biosensor, signal to noise ration such as recognition and transducer elements that provide fingerprint spectrum at the lower limit of detection with specific binding or hybridized event, increasing reliability and sensitivity. Since the localized surface plasmon resonance (LSPR) of nanoparticle lies at the heart of SERS. It is essential to control all of the LSPR influencing factors in highly sensitivity signal strength that ensures reproducibility of SERS signals. SERS active substrates, transducer elements, metal surfaces modification, interparticle spacing, dielectric environment and selection of biorecognition molecules contribute in SERS signal strength. Modified metal structure with bioprobe and Raman reporter molecules provides a strong signature fingerprints that surely contribute to noble biosensor structural designing. We reviewed here ideal fabrication of nanostructure for SERS application in molecular sensing research fields.

Plasmonic colorimetry: visual detection of environmentally relevant species

Colorimetric detection of chemical species based on specially tailored noble metal nanoparticles is garnering increasing interest in the scientific community. The aggregation/dispersion of unmodified or ligandfunctionalised plasmonic nanostructures upon binding to an environmental pollutant results in a colorimetric response, which can be easily discerned with the naked eye. This review offers a snapshot of recent advances in this field of plasmonic colorimetry. The identification and detection of anionic, neutral and cationic chemical species among a wide variety of other environmentally significant pollutants remains a particularly important but difficult challenge. The optical transduction of target-ligand interactions at the surface of plasmonic nanostructures via a distinct colour change is very attractive for the development of environmental sensors. Such a litmus test-type colorimetric transduction sensing scheme offers extreme simplicity and excellent robustness for practical applications.

Full Color Plasmonic Nanostructured Surfaces and Their Sensor Applications

In this Article, we report on plasmonic nanostructured surfaces with almost all colors by nanosphere lithography. The feature sizes of the plasmonic nanostructures can be controlled by O2 reactive ion etching and gold evaporation conditions. The optical properties of the plasmonic nanostructured surfaces can be easily tuned in the visible light region by changing etching time, and the color of plasmonic nanostructured surfaces does not change with the viewing angle up to 45°. More importantly, such surfaces show high bulk sensitivity to the refractive index variation, and they can be used as label-free, real-time biosensors to detect biomacromolecules.

Polyelectrolyte Layer-by-Layer Assembly To Control the Distance between Fluorophores and Plasmonic Nanostructures.

In the past several years we have demonstrated the metal-enhanced fluorescence (MEF) and the significant changes in the photophysical properties of fluorophores in the presence of metallic nanostructures and nanoparticles. MEF is largely dependent on several factors, such as chemical nature, size, shape of the nanostructure, and its distance from the interrogating fluorophore. Herein, we elucidate the potential of layer-by-layer (LbL) assembly to understand the distance dependence nature of MEF from sulforhodamine B (SRB) assembled on the plasmonic nanostructured surfaces [in the form of Silver Islands films (SIFs)]. The varied proximity of fluorophores from the SIF surfaces was controlled by constructing different numbers of alternate layers of poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH). An anionic laser dye SRB could be electrostatically attached to the positively charged PAH layer. Orientation of the SRB probe molecule adsorbed in PSS/PAH-layered assembly was determined by polarized absorption spectroscopy. The observed tilt angle of the probe transition dipole moment with respect to the surface normal was 40°. Our results show that MEF is indeed distance-dependent. Accordingly, we observed a maximum of a ~6-fold increase in the fluorescence intensity from a monolayer of the SRB at a distance of ~9 nm from the metal-nanostructured surface, with the enhancement decreasing down to ~1.5-fold at about a 30 nm separation distance. Consistently, the minimum lifetimes were about 4-fold shorter than those on glass slides without silver, with the lifetimes being about nearly the same for 15 layers of the PSS/PAH assembly. The intensity-time decays were analyzed with a lifetime distribution model to underpin the distance effect on the metal-fluorophore interaction in the nanometric range. The present study provides improved understanding of the interaction between plasmonic nanostructures and fluorophores and, more importantly, their distance dependence nature, where we used a robust, easy, and inexpensive alternate electrostatic LbL assembly as a bottom-up nanofabrication technique to control the probe distance from the surface.