Plasmonic Metal Nanoparticles Research Articles

Entropy Current through a Strongly Correlated Plexcitonic Nanojunction

We study the entropy current through an excitonic Coulomb blockaded two-level quantum emitter attached to plasmonic metal nanoparticles subjected to a temperature gradient by invoking the noncrossing approximation. We find that boosting the plasmon–exciton coupling enhances the entropy current at any temperature for an infinitesimal bias and temperature gradient, whereas increasing the ambient temperature rapidly suppresses it. Introducing finite temperature gradient serves to increase the entropy current. On the other hand, increasing the bias voltage causes the entropy current to increase slightly around the Kondo temperature for low plasmon–exciton coupling values. Finally, we determine that the entropy current always stays finite at elevated voltage bias values for any nonvanishing plasmon–exciton coupling. We elucidate on the microscopic origin of these intriguing results.

Multifunctional Photonic Nanomaterials for Diagnostic, Therapeutic, and Theranostic Applications.

The last decade has seen dramatic progress in the principle, design, and fabrication of photonic nanomaterials with various optical properties and functionalities. Light-emitting and light-responsive nanomaterials, such as semiconductor quantum dots, plasmonic metal nanoparticles, organic carbon, and polymeric nanomaterials, offer promising approaches to low-cost and effective diagnostic, therapeutic, and theranostic applications. Reasonable endeavors have begun to translate some of the promising photonic nanomaterials to the clinic. Here, current research on the state-of-the-art and emerging photonic nanomaterials for diverse biomedical applications is reviewed, and the remaining challenges and future perspectives are discussed.

Gold nanoparticle plasmon resonance in near-field coupled Au NPs layer/Al film nanostructure: Dependence on metal film thickness

We study the effects of coupling between plasmonic metal nanoparticles and a thin metal film by using light extinction spectroscopy. A planar monolayer of gold nanoparticles located near an aluminum thin film (thicknesses within the range of 0–62 nm) was used to analyze the coupling between the monolayer and the thin metal film. SPR peak area increase for polymer coated Au NPs, non-monotonical behavior of the peak area for bare Au NPs, as well as red shift and broadening of SPR at the increase of the Al film thickness have been observed. These effects are rationalized as a result of coupling of the layer of Au NPs with Al film through the field of localized surface plasmons in Au NPs that causes the excitation of collective plasmonic gap mode in the nanostructure. An additional mechanism for bare Au NPs is the non-radiative damping of SPR that is caused by the electrical contact between metal NPs and film.

The photocatalytic activity of rutile and anatase TiO2 electrodes modified with plasmonic metal nanoparticles followed by photoelectrochemical measurements

In the present work, we investigated the behavior of titanium dioxide photocatalysts decorated with plasmonic metal – gold and silver – nanoparicles (NPs) for the photooxidation of small organic molecules. The TiO2 samples were large surface area thin film electrodes formed either by thermal oxidation of the Ti metal (for the rutile films) or deposited on the same substrate by a sol-gel method (for the anatase films). The TiO2 films were subsequently decorated with Au nanostructures using either photodeposition or vacuum sputtering. The Ag nanostructures were formed on TiO2 by electrodeposition. Although both the Au-TiO2 and the Ag-TiO2 films yielded photoanodic currents under illumination with visible wavelengths, consistent with the resonant frequencies of plasmonic metal NPs, prolonged polarization resulted in the drastic drop of photoactivity. Such deactivation, that did not affect the range of near-UV wavelengths absorbed by the pristine TiO2, has been assigned to the surface oxidation of the plasmonic NPs progressively blocking the charge regeneration within the metal nanoparticles. These results raise questions in regard to the use of Au-TiO2 and Ag-TiO2 photocatalysts for sunlight-driven photodegradation of organic contaminants in water.

Light-tuned selective photosynthesis of azo- and azoxy-aromatics using graphitic C3N4

Solar-driven photocatalysis has attracted significant attention in water splitting, CO2 reduction and organic synthesis. The syntheses of valuable azo- and azoxyaromatic dyes via selective photoreduction of nitroaromatic compounds have been realised using supported plasmonic metal nanoparticles at elevated temperatures (≥90 °C); however, the high cost, low efficiency and poor selectivity of such catalyst systems at room temperature limit their application. Here we demonstrate that the inexpensive graphitic C3N4 is an efficient photocatalyst for selective syntheses of a series of azo- and azoxy-aromatic compounds from their corresponding nitroaromatics under either purple (410 nm) or blue light (450 nm) excitation. The high efficiency and high selectivity towards azo- and azoxy-aromatic compounds can be attributed to the weakly bound photogenerated surface adsorbed H-atoms and a favourable N-N coupling reaction. The results reveal financial and environmental potential of photocatalysis for mass production of valuable chemicals.

Fermi level equilibration of Ag and Au plasmonic metal nanoparticles supported on graphene oxide.

For the first time, the process of Fermi level equilibration has been studied and compared for plasmonic metal nanoparticles (PMNPs) supported on conducting substrates i.e. graphene oxide (GO) sheets. The extent of Fermi level equilibration has been monitored by recording the changes in the position and intensity of the surface plasmon resonance (SPR) band of Ag and Au PMNPs supported on reduced graphene oxide (rGO). Ag PMNPs supported on rGO show larger variation in the SPR band position and intensity as compared to rGO supported Au PMNPs. The average shift in the chemical potential has been determined through the changes in the SPR band position for Ag, Ag@rGO, Au, and Au@rGO, which are approximately -1812 ± 70 mV, -171 ± 20 mV, -96 ± 8 mV and -29 ± 4 mV, respectively. The calculated values of the shift in chemical potential suggest that Ag and its rGO composite are more prone to Fermi level equilibration as compared to the Au and Au@rGO composite. The electrochemical (galvanostatic) charging/discharging (GCD) measurements also brace the observations from the chemical charging/discharging method with minor variations due to the measurements under two different conditions; particulate films in the case of the former versus the dispersed phase in the case of the latter. Moreover, the average capacitance associated with single nanoparticles (Ag and Au) is estimated using the capacitance values determined from GCD curves and the approximate number of nanoparticles determined from the quantity of PMNPs used in the deposited films for GCD measurements. These values are in close agreement with the quantized double layer capacitance values of monolayer protected clusters reported in the literature. A similar inference is also drawn from the enzyme-less glucose sensing activity of these nanostructures, where Ag and Ag@rGO show better activity in terms of lower values of the limit of detection (LOD) and the limit of quantification (LOQ).

Long-distance transmission of broadband near-infrared light guided by a semi-disordered 2D array of metal nanoparticles.

Near-infrared (NIR) waveguides are a key component of planar photonic devices such as optical communication couplers, image sensors, and spectroscopes for chemical or biological molecules. Conventional NIR waveguides used for signal transmission include silicon-on-insulator (SOI) waveguides and channel/ridge-type metal micro-strips. However, these waveguides usually have limitations of either signal delay or signal loss in optically integrated devices. In this study, a novel NIR waveguide composed of a semi-disordered array of metal nanoparticles (sDAMNPs) on Si substrate was proposed, fabricated, and tested. The disordered metallic nanoparticles array is geometrically localized in the form of 1D metal strips, thus replacing sDAMNPs with less lossy micro strip channel waveguides. From the measurements supported by various computational models, the fabricated waveguides operate effectively in the broadband NIR region (1100 to 1700 nm). The waveguide does not support signal transmission in the ultra violet-visible spectrum due to strong signal absorption, scattering, and localization effects inside the metal nanoparticles. Instead, it is capable of transmitting NIR over a distance longer than 100 μm (signal loss ∼3.85 dB per 100 μm for NIR from 1200 to 1600 nm), which is also sufficiently longer than the conventional surface plasmon polariton propagation distance at the metal-Si interface. Compared to a waveguide-free reference, the waveguide exhibited greatly improved signal transmission efficiency up to a factor of 7.42 × 104 at 1367 nm. It also exhibits a high deflection angle sensitivity of 1.89 dB per 0.01 rad, thus efficiently and straightly guiding the broadband NIR signal over a long distance.

Thermo-plasmonic gold nanofilms for simple and mass-producible photothermal neural interfaces.

In recent years, photothermal stimulation methods using plasmonic metal nanoparticles have emerged as non-genetic optical techniques in neuromodulation. Although nanoparticle-based photothermal stimulation shows great potential in the excitation and the inhibition of neural activity, the complex synthesis processes of the nanoparticles and the lack of large-area deposition methods can be limiting factors for the development of photothermal neural devices. In this paper, we propose a plasmonic gold nanofilm, fabricated by a standard thermal evaporation process, as a simple and mass-producible photothermal neural interface layer for microelectrode array (MEA) chips. The absorption of the gold nanofilm at near infrared wavelengths is optimized to maximize the photothermal effect by varying the thickness and microstructure of the gold nanofilm. With the optimized conditions, a significantly strong photothermal effect is applied on MEAs without affecting the neural signal recording capability. Finally, primary rat hippocampal neuronal cultures are used to show that the photothermal neural inhibition using the gold nanofilm is as effective as that using the plasmonic nanoparticles. Due to the greater simplicity and versatility of the fabrication process, the plasmonic gold nanofilm can provide a promising solution for the mass production of photothermal platforms.

Plasmonic behaviour and plasmon-induced charge separation of nanostructured MoO3-x under near infrared irradiation.

Plasmon-induced charge separation (PICS) allows direct conversion of localized surface plasmon resonance (LSPR) to electron flows and photoelectrochemical reactions. However, PICS has only been achieved using plasmonic noble metal nanoparticles, not with compound nanoparticles. In order to achieve compound PICS, MoO3-x nanostructures were prepared that exhibit LSPR in the near infrared region by using metal oxides or metal nanoparticles as templates. Solid-state cells based on the MoO3-x nanostructure were developed. Their photoresponse to 700-1400 nm infrared light was investigated and analyzed on the basis of their PICS mechanisms.

Optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals.

Optically-active nanocrystals (such as quantum dots and plasmonic noble metal nanoparticles) have received great attention due to their size-tunable optical properties. The indicator displacement assay (IDA) with optically-active nanocrystals has become a common practice for optical sensor development, since no sophisticated surface functionalization of nanoparticles is required. Among the IDA-based optical sensors, the use of metal ions as receptors seems to be attractive. Therefore, in this review, the research progress of optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals has been summarized. In particular, metal ion-mediated selective recognition has been summarized here based on the classical Hard-Soft-Acid-Base (HSAB) principle, which has been seldom mentioned before. Most of the references were therefore categorized according to their located place based on the HSAB theory. Besides, several metal ion modulation strategies that were not related to the HSAB theory (e.g., redox modulation) were also included. Finally, due to the cross-talk of metal ions in selective recognition, we have also summarized sensor array development based on multiple metal ion receptors in IDA sensing with optically-active nanocrystals. Several interesting applications of the IDA sensing with metal ions as receptors and optically-active nanocrystals as indicators are presented, with specific emphasis on the design principles and photophysical mechanisms of these probes.

Modeling of the surface plasmon resonance tunability of silver/gold core-shell nanostructures.

Tunable plasmonic noble metal nanoparticles are indispensable for chemical sensors and optical near field enhancement applications. Laser wavelengths within the absorption spectrum of the nanoparticle and Localized Surface Plasmon Resonances (LSPR) in the visible and near infrared range are the key points to be met for the successful utilization in the field of aforementioned high sensitivity sensors. This way, Surface Enhanced Raman Spectroscopy (SERS) has been pushed to the sensitivity level of single molecule. The tunability, i.e. the modulation of the surface plasmon resonance wavelength as a function of the ambient refractive index is one of the important criteria to be understood clearly. Among various noble metals, gold and silver nanoparticles have the strongest surface enhancement factors for the Raman signal and their tunability for many practical applications has been experimentally demonstrated. We present a comprehensive numerical investigation by means of a finite element analysis on Ag/Au core–shell nanospheres including agglomerated and non-agglomerated dimers. Tunability as a function of shell thickness, total nanosphere radius and fraction of overlap between the dimer is discussed. Our studies show that tunability is considerably affected by the nanosphere radius rather than the shell thickness. These findings may be helpful in the synthesis of nanoplasmonic structures, especially related to an optimized use of gold as the shell material for the targeted application.

Numerically investigating the optical properties of plasmonic metallic nanoparticles for effective solar absorption and heating

Solar heating with plasmonic nanoparticles (NPs) has great potential for application in optical storage, solar thermal collectors, and thermo-photovoltaic technologies. The performance of solar thermal conversion applications depends on the NP parameters. Herein, we present a comparative analysis of the solar absorption properties of noble metallic NPs (Au, Ag, Cu, and Al), to determine suitable parameters for effective solar heating by using the finite-difference time-domain method and the finite-element method. Results show that light absorption plays a major role in the interaction of light with the small NP size, small sphericity, or small dielectric constant of the surrounding environment. Au and Cu NPs have higher solar absorption power and absorption ratio. The NP size has little effect on the peak absorption wavelength. A Au sphere smaller than 30 nm has greater solar absorption ability for the solar heating process when considering the absorption power per unit volume. The solar absorption power first increases rapidly and subsequently decreases slightly with increasing dielectric constant, and can become as high as 0.0045 nW when the dielectric constant is 1.33–2.5 in the calculation samples. The solar scattering power and solar absorption power decrease with increasing sphericity, i.e., ranging from cubic to cylindrical and spherical. Finally, simulation results of NP solar heating show that the Au cube obtains a higher maximum temperature than the Au sphere and Au cylinder, which further verifies that Au NPs with low sphericity can significantly enhance solar heating ability in the simulation cases.

Plasmonically Induced Transparency in Graphene Oxide Quantum Dots with Dressed Phonon States

The absorption in quantum dots (QDs) embedded within a semiconductor matrix can be manipulated by resonant optical excitation of phonons. The far-field interaction of the light leading to dressed phonon states within reduced graphene oxide quantum dots has been coherently modified by a near-field optical driving field at the nanoscale limit. The near-field optical excitation was introduced by resonant excitation of localized plasmons in metal nanoparticles coupled to QDs for ultrafast light manipulation. The coherent interaction of photons due to localized plasmon induced a change in the transient absorption of graphene oxide quantum dots conjugated to silver nanoparticles. Resonant pumping of plasmons and phonons with 400 nm pump photons induces a coherent change in excitonic absorption within QDs which results in phonon-assisted plasmon induced transparency at room temperature. This novel effect can be related to the appearance of the coherent effects such as forming dark states and coherent population ...

Graphene nano-mesh-Ag-ZnO hybrid paper for sensitive SERS sensing and self-cleaning of organic pollutants

Rationally designed self-assemble, functional 3D architecture of graphene nano-meshes with unique plasmonic resonance metal nanoparticles can be excellent nanohybrid Raman scattering (SERS) substrate in spotting the low levels of environmental organic pollutants. In this study, graphene nano-mesh-Ag-ZnO metal (GNMM) nanohybrid was prepared by self-assembled hydrothermal reduction of graphene oxide (GO) followed by thermal annealing of site-localized metal-salts as a highly sensitive surface-enhanced Raman scattering (SERS) substrate. The crumpled graphene nano-meshes decorated with Ag-ZnO nanoparticles created SERS hot spots (holes) that manifested high SERS sensitivity to model organic pollutants such as methyl orange (MO), rhodamine 6G (Rh-6G) and paraquat (PQ) with detection limit as low as 10−11, 10−13 and 10−14 M, respectively. A maximum enhancement factor (EF) of 1.97 × 1011 was obtained with PQ molecules on synthesized GNMM substrate. The enhancement in signal can be supported through charge transfer mechanism that induced strong enhancement of the local electric field on free available graphene nano-mesh edges abundant with hotspots. This novel nanohybrid SERS substrate realized self-cleaning by photocatalytic degradation of model organic Rh-6G molecules adsorbed on the nanohybrid paper and shows reusability during the detection process. Meanwhile, GNMM nanohybrid also possesses highly effective antimicrobial activities for Escherichia coli. The detection of organic pollutants by GNMM nanohybrid SERS substrate has several advantages such as low cost, easy and simple preparation method, sensitive detection and reusability that can be potentially applied for detection of variety of environmental pollutants and antimicrobial agent applications.

Enhanced efficiency of thin film GaAs solar cells with plasmonic metal nanoparticles

ABSTRACTIn the present work, a thin film solar cell structure consisting of thin GaAs layer, metallic nanoparticle array and aluminum back reflector is proposed and studied to calculate optical absorption and current density using finite difference time domain (FDTD) numerical simulation method. Simulation results show that in comparison to other metallic nanoparticles, aluminum nanoparticles have strong plasmonic scattering effect at optimized period of 100nm and radius 40nm to improve the efficiency of thin film GaAs solar cells through enhancement of current density integrated over solar spectrum by 31% compared to bare thin film GaAs solar cells.

Analysis of Localized Surface Plasmon Resonances in Spherical Jellium Clusters and Their Assemblies

Due to multiple possible applications of physico-chemical properties of plasmonic metal nanoparticles and particle systems, there is high interest to understand the mechanisms that underlie the birth of localized surface plasmon resonance (LSPR). Here we studied the birth of the LSPR in spherical jellium clusters with the density of sodium and with 8, 20, 34, 40, 58, 92, 138, and 186 electrons, by using the linear response time-dependent density functional theory (lr-TDDFT). The coupling of the individual plasmon resonances in dimer, trimer, tetramer, and hexamer cluster assemblies consisting of the 8-electron cluster was also studied. The Kohn-Sham electron-hole transitions contributing to the absorption peaks were analyzed using time dependent density functional perturbation theory (TD-DFPT) and visualized using the transition contribution map (TCM) analysis. The plasmonicity of an absorption peak was analyzed by examining the number of the e-h transitions contributing to it, the relative strengths of t...

Minimizing structural deformation of gold nanorods in plasmon-enhanced dye-sensitized solar cells

Plasmonic metal nanoparticles have shown great promise in enhancing the light absorption of organic dyes and thus improving the performance of dye-sensitized solar cells (DSSCs). However, as the plasmon resonance of spherical nanoparticles is limited to a single wavelength maximum (e.g., ~ 520 nm for Au nanoparticles), we have here utilized silica-coated gold nanorods (Au@SiO2 NRs) to improve the performance at higher wavelengths as well. By adjusting the aspect ratio of the Au@SiO2 NRs, we can shift their absorption maxima to better match the absorption spectrum of the utilized dye (here we targeted the 600–800 nm range). The main challenge in utilizing anisotropic nanoparticles in DSSCs is their deformation during the heating step required to sinter the mesoporous TiO2 photoanode and we show that the Au@SiO2 NRs start to deform already at temperatures as low as 200 °C. In order to circumvent this problem, we incorporated the Au@SiO2 NRs in a TiO2 nanoparticle suspension that does not need high sintering temperatures to produce a functional photoanode. With various characterization methods, we observed that adding the plasmonic particles also affected the structure of the produced films. Nonetheless, utilizing this low-temperature processing protocol, we were able to minimize the structural deformation of the gold nanorods and preserve their characteristic plasmon peaks. This allowed us to see a clear redshift of the maximum in the incident photon-to-current efficiency spectra of the plasmonic devices (Δλ ~ 14 nm), which further proves the great potential of utilizing Au@SiO2 NRs in DSSCs.

Interplay of hot electrons from localized and propagating plasmons

Plasmon-induced hot-electron generation has recently received considerable interest and has been studied to develop novel applications in optoelectronics, photovoltaics and green chemistry. Such hot electrons are typically generated from either localized plasmons in metal nanoparticles or propagating plasmons in patterned metal nanostructures. Here we simultaneously generate these heterogeneous plasmon-induced hot electrons and exploit their cooperative interplay in a single metal-semiconductor device to demonstrate, as an example, wavelength-controlled polarity-switchable photoconductivity. Specifically, the dual-plasmon device produces a net photocurrent whose polarity is determined by the balance in population and directionality between the hot electrons from localized and propagating plasmons. The current responsivity and polarity-switching wavelength of the device can be varied over the entire visible spectrum by tailoring the hot-electron interplay in various ways. This phenomenon may provide flexibility to manipulate the electrical output from light-matter interaction and offer opportunities for biosensors, long-distance communications, and photoconversion applications.

On quantum approach to modeling of plasmon photovoltaic effect

Surface plasmons in metallic nanostructures including metallically nanomodified solar cells are conventionally studied and modeled by application of the Mie approach to plasmons or by the finite element solution of differential Maxwell equations with imposed boundary and material constraints (e.g., upon commercial COMSOL software system). Both approaches are essentially classical ones and neglect quantum particularities related to plasmon excitations in metallic components. We demonstrate that these quantum plasmon effects are of crucial importance especially in theoretical simulations of plasmon-aided photovoltaic phenomena. Quantum corrections considerably improve both the Mie and COMSOL approaches in this case. We present the semiclassical random phase approximation description of plasmons in metallic nanoparticles and apply the quantum Fermi golden rule scheme to assess the sunlight energy transfer to the semiconductor solar cell mediated by surface plasmons in metallic nanoparticles deposited on the top of the battery. In addition, short-ranged electron–electron interaction in metals is discussed in the framework of the semiclassical hydrodynamic model. The significance of the related quantum corrections is illustrated by the quantumly improved COMSOL simulations.

Effects of Nanoscale Interfacial Design on Photocatalytic Hydrogen Generation Activity at Plasmonic Au–TiO2 and Au–TiO2/Pt Aerogels

We demonstrate that composite catalytic aerogels represent a superior materials design motif for the creation of solar fuels photocatalysts. We couple surface plasmon resonant (SPR) guests to the inherent compositional and interfacial design flexibility of catalytic aerogels to photogenerate molecular hydrogen (H2). We investigate the effects of synthetically modifying the TiO2 aerogel network and the nanoparticulate Au||TiO2 interfaces in plasmonic Au–TiO2 aerogels on H2 evolution under both broadband (i.e., UV + visible light) and visible excitation. We also introduce non-plasmonic Pt co-catalyst nanoparticles into our composite aerogels, creating Au–TiO2/Pt aerogels that perform visible light SPR-driven photocatalytic reduction of water to generate H2. The fuels production achieved with this multicomponent photocatalytic nanoreactor demonstrates that the nanostructured high-surface-area network in the aerogel can spatially and effectively separate charge while electrochemically connecting plasmonic nanoparticle sensitizers and metal nanoparticle. In doing so, we prove several crucial concepts: (1) integration of a plasmonic sensitizer with a separate water reduction co-catalyst within the ultraporous aerogel nanoarchitecture; (2) wiring the electron–hole pairs generated under visible light at the plasmonic Au||TiO2 interface to the co-catalyst via the nanoscale TiO2 network; and (3) combining both the photocatalytic oxidation and reduction reactions critical to solar fuels photocatalysis into one composite material at length scales compatible with the reaction kinetics. This work is supported by the Office of Naval Research.