Monolayer Of Metal Nanoparticles Research Articles

Spectral Characteristics of a Photonic Crystal Structure with a Monolayer of Metal Nanoparticles

Spectral Characteristics of a Photonic Crystal Structure with a Monolayer of Metal Nanoparticles

Precise regulation and control of hotspots in nanoparticle multilayer SERS substrates

Precise regulation and control of the SERS active hotspot is a significant challenge in SERS technology. This study proposes the precise control of SERS active hotspots in multilayer substrates by combining metal nanoparticles with different dielectric constants. The hotspots of the multilayer SERS substrate composed of different nanoparticle monolayers (Ag, Au, Cu) were investigated by finite element simulation. It was found that the local field hotspots could be transferred controllably between different layers by using heterogeneous nanoparticle multilayer structures. Six multilayer SERS substrates composed of Ag nanoparticle (AgNP) and Au nanoparticle (AuNP) monolayer were fabricated via a layer-by-layer deposition method. The hotspots were precisely probed by the SERS signal of the reporter molecules adsorbed on a specific position. SERS test results showed that the hotspots could be transferred efficiently between layers by combining different metal nanoparticle monolayers, consistent with the simulation results. The results presented in this manuscript strengthen the understanding of surface plasmon on nanoparticle multilayer systems and have profound importance in surface analysis.

IR nanospectroscopy to decipher drug/metal nanoparticle interactions: Towards a better understanding of the spectral signal enhancement and its distribution

In this study, detailed considerations of the interactions appearing at drug/metal nanoparticle interfaces in micro- and nanoscale were performed. Application of the well-known surface-enhanced Raman spectroscopy (SERS) reveals information of how gefitinib, a drug approved in non-small cell lung cancer therapy, connects to the silver and gold nanoparticles (AgNPs and AuNPs, respectively). Such knowledge is important in the context of using the resulting conjugates to increase the effectiveness of cancer therapy. The received data were extended by the nanoscale analysis using atomic force microscopy in combination with the surface-enhanced infrared spectroscopy (AFM-SEIRA) in tapping mode. This allows to better visualize the discussed interaction with ultra-high spatial resolution even reaching the size of the particular nanoparticle (∼15 nm). It should be noted, that the metal nanoparticle mono-layer was enough to enhance the obtained AFM-SEIRA signal, which for the conventional surface-enhanced vibrational techniques would be unattainable. Moreover, since the surface enhancement phenomenon observed in the AFM-SEIRA technique is not fully understood the comparison between this effect and the signal reinforcement in SERS was performed. The drawn conclusions suggest the complementarity of both applied techniques and try to prove the statement that the intensity AFM-SEIRA maps reveal the nature of the drug/metal nanostructure interaction.

Wearable SERS Sensor Based on Omnidirectional Plasmonic Nanovoids Array with Ultra-High Sensitivity and Stability.

Surface-enhanced Raman spectroscopy (SERS) is a promising technology for wearable sensors due to its fingerprint spectrum and high detection sensitivity. However, since SERS-activity is sensitive to both the distribution of "hotspots" and excitation angle, it is profoundly challenging to develop a wearable SERS sensor with high stability under various deformations during movements. Herein, inspired by omnidirectional light-harvesting of the compound eye of Xenos Peckii, a wearable SERS sensor is developed using omnidirectional plasmonic nanovoids array (OPNA), which is prepared by assembling a monolayer of metal nanoparticles into the artificial plasmonic compound-eye (APC). Specifically, APC is an interconnected frame containing omnidirectional "pockets" and acts as an "armour", not only rendering a broadband and omnidirectional enhancement of "hotspots" in the delicate nanoparticles array, but also maintaining an integrity of the "hotspots" against external mechanical deformations. Furthermore, an asymmetry super-hydrophilic pattern is fabricated on the surface of OPNA, endowing the hydrophobic OPNA with the ability to spontaneously extract and concentrate the analytes from sweat. Such an armored SERS sensor can enable the wearable and in situ analysis with high sensitivity and stability, exhibiting great potential in point-of-care analysis.

Intermediate-state imaging of electrical switching and quantum coupling of molybdenum disulfide monolayer

SignificanceThin transparent semiconductors of two-dimensional materials are attractive for the practical applications in next-generation nanoelectronic and optoelectronic devices. Probing the electron states and electrical switching mechanisms of a molybdenum disulphide monolayer with atomic-scale thickness (6.5 Å) allows us to unlock the full technological potential of this nanomaterial. We introduced a plasmonic phase imaging method to uncover the underlying mechanism and detailed switching dynamics of an electrical-state switching event. This dramatic phase change can be attributed to the reversible switching of classical electromagnetic coupling and quantum coupling effects interplaying between a single metal nanoparticle and molybdenum disulphide monolayer, and the transient intermediate states during the switching event can be directly imaged by a plasmonic technique.

Absorption of light by ultra-thin silicon films covered with a monolayer of elliptical metal nanoparticles

The absorption of Si films covered with an array of elliptical metal nanoparticles (NPs) of identical orientation and shape is investigated theoretically. We consider silicon films with thickness in the range of 80–200 nm. The symmetry axis of the NPs is taken perpendicular to the surface of silicon surface and their short axis has a radius of 10 nm in the plane of the Si film. A 10-nm-thick SiO2 layer separates the submonolayer NP coverage from the silicon film. The spectral properties of the considered hybrid system are studied in terms of its effective susceptibility. Specifically, the dependence of light absorption by the NPs covering the ultrathin Si film on their morphology and material composition is established. For chromium and silver NPs, the appearance of collective absorption modes in the hybrid structures is predicted.

Nanoscale image of the drug/metal mono-layer interaction: Tapping AFM-IR investigations

The application of metal nanoparticles as an efficient drug delivery system is one of the directions of cancer therapy development. However, this strategy requires precise information about how the drug interacts with the applied nanocarrier. In this study, atomic force microscopy combined with infrared spectroscopy (AFM-IR) was used for the first time to investigate the erlotinib adsorption structure on two different types of 15 nm metal nanoparticle mono-layers, namely, silver nanoparticle (AgNP) and gold nanoparticle (AuNP) mono-layers. Because the metal nanoparticles are loosely bound samples, only the tapping AFM-IR mode is suitable for the collection of IR maps and spectra for such a system. The obtained results indicated the relevance of the AFM-IR technique for characterizing drug interactions with a metal mono-layer surface. The investigated drug interacts with the AgNPs mainly through phenyl rings and methoxy moieties, while quinazoline, amino, and ethoxy moieties appear to be farther from the surface. For the AuNPs, the interaction occurs through both the phenyl ring and the quinazoline moiety. Additionally, the aliphatic groups of erlotinib directly participate in this interaction. The novelty of the present work is also related to the use of the tapping AFM-IR mode to study metal NP mono-layers with a drug adsorbed on them. The collected IR maps for the most enhanced erlotinib bands show specific areas with very high signal intensity. The connection between these areas and the “hot spots” typical for the surface plasmon resonance phenomenon of metals is considered.

Characteristics of fiber-optic photoacoustic transducers with monolayer of metal nanoparticles for systems of technical diagnostics

The work is devoted to the experimental study of the microstructural and morphological properties of nanostructures as part of a prototype of fiber-optic photoacoustic transducer. The transducer has been created to confirm the theoretical investigations previously obtained by the authors during the study the conditions of the most effective photoacoustic gene ration. To solve the main problem that arises when creating photoacoustic transducers, namely reducing the thickness of the absorbing layer, we used a nanostructure based on a monolayer of silver nanoparticles with size gamma-distribution, the average diameter of 35 nm with RMS-size of 12 nm. The method of simultaneous measuring both efficiency of photoacoustic conversion and frequency response of a photoacoustic transducer is proposed for the first time. The method allows experimental investigation of transduces output parameters versus the modulation mode of the optical signal. The proposed method is based on the usage of the main measurement channel for irradiating the photoacoustic transducer and a reference channel based on fiber optical coupler and photodiode. The experiment shows the reliable generation of ultrasound at frequencies of 10 –18 MHz with a prototype of photoacoustic transducer. During one hour irradiation, degradation of two-dimensional surface nanostructures has not been observed. This allows such type of photoacoustic transducer to be used as part of a new generation of technical diagnostics systems.

Next generation SERS- atomic scale platform for molecular level detection

The research with ultra-sensitive detection by Raman enhancement has traditionally focused on Surface Enhanced Raman Scattering (SERS) or Tip Enhanced Raman Scattering (TERS) from predominantly Raman active materials (monolayers of metallic nanoparticles of gold and silver). However, these traditional SERS & TERS methods have demonstrated limited applicability due to many inherent disadvantages. In this study, we have demonstrated the next generation of Enhanced Raman Sensing by adding a third dimension, by introducing a concept of employing atomic scale features (voids in nanostructures) for molecular level detection. Unlike traditional SERS/TERS, the atomic scale platform also exhibited immunity to the excitation wavelength showing potential for universal sensor. Additionally, this portable platform demonstrated a wide spectrum of applications from trace level detection of environmental pollutants, Raman reporter molecule sensing, detection of biomarkers associated with cancer and detection of cancer from the DNA, opening new doors for many non-invasive, ultrasensitive and remote sensing applications. The atomic scale platform was synthesized with two features- multi-tip probes encapsulated with multiple layers of atomic scale voids. Multi-photon ionization mechanism was used for the synthesis. We have witnessed exponentially increased and highly reproducible Raman enhancement from a traditionally Raman inactive material (semiconductor based ZnO) demonstrating molecular level detection. Utilization of atomic-scale features (voids in nanostructures) for activation of a typically Raman inactive material for molecular level detection will create a paradigm for future studies in the field of Raman enhancement for Raman active as well as Raman inactive materials. This mechanism will initiate avenues for many new materials to be candidates for Raman sensing.

Optical Detection of Small Metabolites for Biological Gas Conversion by using Metal Nanoparticle Monolayers Produced by Capillary-Assisted Transfer.

Detection of small metabolites is essential for monitoring and optimizing biological gas conversion. Currently, such detection is typically done by liquid chromatography with offline sampling. However, this method often requires large equipment with multiple separation columns and is at risk of serious microbial contamination during sampling. Here we propose real-time optical detection of small metabolites using uniform plasmonic nanoparticles monolayers produced by capillary-assisted transfer. We reproducibly fabricate metal nanoparticles monolayers with a diameter of ∼1 mm for the detection of acetate, butyrate, and glucose by a glass capillary tube. Metal nanoparticles monolayers are not only uniform in terms of average interparticle distance but also structurally stable under dynamic fluidic conditions. The monolayers resistant to fluid shear stress with surface-enhanced Raman scattering are able to reversibly monitor the concentration of acetate and sensitively detect acetate and glucose at levels as low as 10 μM, which is more than 2 orders of magnitude lower than the concentration range of typical biological gas conversion. In addition, structurally similar metabolites such as acetate and butyrate, when mixed, become distinguishable by our method.

Predicting the effect of chain-length mismatch on phase separation in noble metal nanoparticle monolayers with chemically mismatched ligands.

Nanoparticles (NPs) protected with a ligand monolayer hold promise for a wide variety of applications, from photonics and catalysis to drug delivery and biosensing. Monolayers that include a mixture of ligand types can have multiple chemical functionalities and may also self-assemble into advantageous patterns. Previous work has shown that both chemical and length mismatches among these surface ligands influence phase separation. In this work, we examine the interplay between these driving forces, first by using our previously-developed configurationally-biased Monte Carlo (CBMC) algorithm to predict, then by using our matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) technique to experimentally probe, the surface morphologies of a series of two-ligand mixtures on the surfaces of ultrasmall silver NPs. Specifically, we examine three such mixtures, each of which has the same chemical mismatch (consisting of a hydrophobic alkanethiol and a hydrophilic mercapto-alcohol), but varying degrees of chain-length mismatch. This delicate balance between chemical and length mismatches provides a challenging test for our CBMC prediction algorithm. Even so, the simulations are able to quantitatively predict the MALDI-MS results for all three ligand mixtures, while also providing atomic-scale details from the equilibrated ligand structures, such as patch sizes and co-crystallization patterns. The resulting monolayer morphologies range from randomly-mixed to Janus-like, demonstrating that chain-length modifications are an effective way to tune monolayer morphology without needing to alter chemical functionalities.

Photoacoustic Generation with Surface Noble Metal Nanostructures

Thin layers of noble metal (Ag, Au) nanoparticles on the surface of optical fiber edge are studied from the viewpoint of photoacoustic fiber-optic generation. In this paper we present theoretical investigation of laser radiation absorbtion by the monolayer of noble metal nanoparticles deposited on the surface of the optical fiber edge. Different nanostructures on the edge of the optical fiber are proposed for effective photoacoustic generation based on lasers with wavelength of 405, 445, 450, 510–530 nm.

Nonlinear Viscoelasticity of Highly Ordered, Two-Dimensional Assemblies of Metal Nanoparticles Confined at the Air/Water Interface.

In this study, we investigated the viscoelastic properties of metal nanoparticle monolayers at the air/water interface by dilational rheology under periodic oscillation of surface area. Au nanoparticles capped with oleylamine form a stable, dense monolayer on a Langmuir film balance. The stress response function of a nanoparticle monolayer was first analyzed using the classical Kelvin-Voigt model, yielding the spring constant and viscosity. The obtained results suggest that the monolayer of nanoparticles is predominantly elastic, forming a two-dimensional physical gel. As the global shape of the signal exhibited a clear nonlinearity, we further analyzed the data with the higher modes in the Fourier series expansion. The imaginary part of the higher mode signal was stronger than the real part, suggesting that the dissipative term mainly causes the nonlinearity. Intriguingly, the response function measured at larger strain amplitude became asymmetric, accompanied by the emergence of even modes. The significance of interactions between nanoparticles was quantitatively assessed by calculating the potential of mean force, indicating that the lateral correlation could reach up to the distance much larger than the particle diameter. The influence of surface chemical functions and core metal has also been examined by using Au nanoparticles capped with partially fluorinated alkanethiolate and Ag nanoparticles capped with myristic acid. The combination of dilational rheology and correlation analyses can help us precisely control two-dimensional colloidal assembly of metal nanoparticles with fine-adjustable localized surface plasmon resonance.

Optical generation in an amplifying photonic crystal with metal nanoparticles

We show the possibility of selective polarization generation in photonic crystal structure with the monolayer of metal nanoparticles. Optical properties of the monolayer are defined by the surface concentration and shape factor of the nanoparticles. Wavelength dependence of the monolayer reflectance and transmittance possesses a resonance behavior resulting from plasmon resonance in nanoparticles. We show that this allows, depending on the relative orientation of the monolayer anisotropy axis and the polarization direction of the light, to achieve the mode discrimination of the photonic structure.

Plasmon-excitonic polaritons in superlattices

A theory for propagation of polaritons in superlattices with resonant plasmon-exciton coupling is presented. A periodical superlattice consists of a finite number of cells with closely located a quantum well and a monolayer of metal nanoparticles. Under study is the spectrum of hybrid modes formed of the quasitwo- dimensional excitons of quantum wells and the dipole plasmons of metal particles. The problem of electrodynamics is solved by the method of Green’s functions with taking account of the resonant polarization of quantum wells and nanoparticles in a self-consistent approximation. The effective polarizability of spheroidal particles occupying a square lattice is calculated with taking into consideration the local-field effect of dipole plasmons of the layer and their images caused by the excitonic polarization of nearest quantum well. Optical reflection spectra of superlattices with GaAs/AlGaAs quantum wells and silver particles are numerically analyzed. Special attention is paid to the superradiant regime originated in the Bragg diffraction of polaritons in superlattice. Superradiance is investigated separately for plasmons and excitons, and then for hybrid plasmonexcitonic polaritons. It is demonstrated that the broad spectrum of reflectance associated with plasmons depends on the number of cells in superlattice, and it has a narrow spectral dip in the range of plasmon-excitonic Rabi splitting.

Oxidation of Small Organic Molecules on Noble Metals and Pseudomorphic Layers of Noble Metals

Ethanol, due to its low toxicity and relatively high energy density, is a promising candidate to be used in fuel cells. It is mainly obtained from biomass, thus its oxidation does not influence the total CO2 amount in the biosphere. However incomplete and slow electrooxidation of ethanol impedes the commercialization of low-temperature ethanol fuel cells. Electrocatalytic oxidation of ethanol is a complex, multi-step process with not completely understood mechanism. Factors influencing the catalytic activity of many systems towards ethanol electrooxidation are still unclear, and effective catalyst for ethanol electrooxidation has not been discovered yet. Most systems used for electrocatalytic oxidation of ethanol contain platinum-group metals in the form of nanoparticles, but even for those systems high overpotentials and incomplete oxidation of ethanol is observed. Another factor influencing the catalytic activity towards ethanol oxidation is poisoning of catalysts’ surface by products of incomplete oxidation. In particular for Pt, a quick decrease in surface activity during ethanol electrooxidation has been observed, which is related to surface poisoning by strongly adsorbed CO, a by-product of ethanol electrooxidation reaction. Adsorption of CO inhibits further electrooxidation of ethanol, and as a result high electrode potential is required to oxidize ethanol, as the surface CO must be oxidized first. To overcome the problem of surface poisoning by CO, bi-metallic surfaces, such as Pt-Ru and Pt-Sn were proposed, due to the fact, that the second metal provides adsorbed OH groups, required to oxidize the adsorbed CO, at lower potential than Pt. This so-called “bifunctional mechanism” was firstly described for methanol electrooxidation on Pt-Ru by Watanabe and Motoo in 1975. However in the case of ethanol electrooxidation the presence of Ru or Sn on Pt surface, despite yielding higher current density, leads to even further decrease in CO2 yield, increase the amount of acetic acid produced. This shift in product distribution significantly decrease the maximum amount of energy, which can be obtained from ethanol electrooxidation in conditions relevant to fuel cells on Pt-Sn or Pt-Ru, as it is impossible to oxidize acetic acid in those conditions, making oxidation of ethanol effectively a 4-electron process. It is thus imperative to develop new catalysts more active towards C-C bond scission, which is impossible without good understanding of the factors influencing catalytic activity in general. It is postulated, that electronic properties are in part responsible for catalytic activity of a given material. Theoretical DFT calculations suggest that chemisorption energy, activation barrier and energy of dissociation of small molecules on metal surface can be correlated to the d-band center of gravity of that metal (so called d-band center theory). This holds true for many systems and reactions it is possible that for more complex system other factors, not included in the d-band center theory, can play a significant role. For instance for more complex reactions, such as oxidation of simple alcohols, the relation between electronic and electrocatalytic properties are much more complicated, because those reactions are multi-step processes and the change of electronic properties may influence the elementary reactions to a different degree. One of the model systems allowing for studying the relation between catalytic and electronic properties of transition metals are pseudomorphic layers. Such systems consist of one metal (substrate) on which another metal is deposited forming a thin layer, usually a monolayer. It has been discovered, that in those systems the top atom layer mimics the crystallographic structure and lattice parameter of the substrate. Possible examples of such systems are monocrystal substrate with a monolayer of other metal or core-shell nanoparticles. Pseudomorphic layers allow to introduce and study changes in electronic properties due to modification of lattice parameter of the topmost layer and changes resulting from possible partial charge transfer between topmost layer and substrate material. At the same time, if pseudomorphic layer provides full coverage of the substrate’s surface, non-electronic effects, such as above mentioned bi-functional mechanism, can be eliminated. We focused on correspondence between electronic properties of noble metals (especially Pt and Pd) and their pseudomorphic layers observed using X-ray Photoelectron Spectroscopy (XPS) and UV Photoelectron Spectroscopy (UPS) and catalytic activity towards CO, formic acid and ethanol electrooxidation reactions. Catalytic activity was investigated using electrochemical methods. Oxidation of formic acid was selected as a model electrode reaction while oxidation of CO is a crucial step in electrooxidation of ethanol on noble metals. This project was funded from Polish National Science Centre budget based on decision number DEC-2013/09/B/ST4/00099

Electrophoretic Deposition of Metal Nanoparticle Monolayers from Nonpolar Solvents for Hydrogen Sensing

We report on the electrophoretic deposition (EPD) of metal nanoparticles (NPs) prepared in reverse micelles on semiconductor substrates with the aim to fabricate sensitive Schottky-based hydrogen sensors with fast response and high degree of selectivity. We discuss the mechanism of NP monolayer formation and show which parameters are essential for the transition from three-dimensional to two-dimensional growth.

Fabrication and optical properties of assembled gold nanoparticles film on elastomeric substrate

Abstract We report an easy, scalable and low cost interfacial self-assembly approach to fabricate dense metal nanoparticle monolayer films deposited on elastomeric substrate which exhibit tunable plasmonic responses to uniaxial mechanical stretching. The nanoparticle monolayers deposited on silicon substrate by water-in-oil self-assembling were quantitatively transferred to the elastic substrate by a wet lift-up soft lithography approach. By uniaxial stretching of the resulting nanoparticle film the plasmonic resonance peak experience a blue shift proportional to the applied strain. Numerical simulations were performed on the plasmon modes of the gold nanoparticles film under mechanical stress to explain the observed behaviors. Such metal nanoparticle film can be exploited in a wide range of applications in nanophotonics including strain sensors.

Distinguishing plasmonic absorption modes by virtue of inversed architectures with tunable atomic-layer-deposited spacer layer

We demonstrated the distinguishing between plasmonic absorption modes by exploiting an inversed architecture with tunable atomic-layer-deposited dielectric spacer layer. The dielectric spacer layer was manipulated between the bottom metal–nanoparticle monolayer and the upper metal film to inspect the contributions of metal nanoparticles and dielectric film in a step-by-step manner. The experimental and simulated differences between the two peak absorption positions (Δf) and between the corresponding half width at half maxima (Δw) confirmed the evolutions of gap plasmon and interference-enhanced local surface plasmon resonance absorption modes in the plasmonic metamaterial absorbers (PMAs), which were useful for understanding the underlying mechanism of amorphous PMAs.

Hyperbolic metamaterials based on quantum-dot plasmon-resonator nanocomposites.

We theoretically demonstrate that nanocomposites made of colloidal semiconductor quantum dot monolayers placed between metal nanoparticle monolayers can function as multilayer hyperbolic metamaterials. Depending on the thickness of the spacer between the quantum dot and nanoparticle layers, the effective permittivity tensor of the nanocomposite is shown to become indefinite, resulting in increased photonic density of states and strong enhancement of quantum dot luminescence. This explains the results of recent experiments [T. Ozel et al., ACS Nano 5, 1328 (2011)] and confirms that hyperbolic metamaterials are capable of increasing the radiative decay rate of emission centers inside them. The proposed theoretical framework can also be used to design quantum-dot/nanoplasmonic composites with optimized luminescence enhancement.