Tunable work function and optical nonlinearity of nanocomposites

In this paper, through a vector-spherical harmonics approach, we investigate the optical spectra of plasmonic Au nanoparticles excited by two special accelerating beams: a non-paraxial Airy beam and a Bessel beam. We systematically analyze the impacts of the beam profile, phase, and helical wave front of the electromagnetic fields on the optical spectrum and the excitation of the surface plasmon resonance (SPR). We find that the high-order phase in the Airy beam would result in strong plasmonic oscillations in the optical spectra, while the cone angle and orbital angular momentum carried by the Bessel beam could be employed to engineer the plasmon modes excited in Au nanoparticles. Furthermore, the optical spectrum excited by a combined Airy–Bessel–Gauss beam is discussed. The study could help to deeply explore new ways to manipulate SPR in metal nanoparticles via the wave front engineering of optical beams for enhancing light–matter interaction and optical sensing performance.

Silver (Ag) and gold (Au) nanoparticles (NPs) has unique properties associated with their Nano sized particulate composition, shape, and size. Ag and Au NPs were approved as a biocompatible and biomedical activity. Ag and Au colloidal NPs were chemically synthesized by chemical reduction of AgNO3 and HAuCl4 respectively at different concentrations. The surface Plasmon resonance absorption peaks were detected at (415 and 514) nm for Ag and Au colloidal NPs respectively, the absorbance peaks remains at the same wavelength with most possible for different concentrations. SEM micrographs of the synthesized NPs revealed spherical, relatively homogenous nanoparticles. Chemogenic Ag and Au NPs were examined for its antimicrobial activity using three different concentrations against Pseudomonas aeruginosa isolated from clinical samples and identified by postgraduate students. The results revealed that both types of NPs expressed antimicrobial activity against Pseudomonas aeruginosa, with higher antimicrobial activity for Ag than Au NPs at the same concentrations.

Organic solar cells (OSCs) are promising candidates for solar light harvesting due to their standout advantages both in material properties and manufacturing process. During past decades, remarkable progress has been achieved. Efficiency for single-junction cells over 9% and tandem cells over 10% has been reported. For high performance OSCs towards commercialization, sufficient light absorption and high quality buffer layers are still two challenges, which are addressed in this thesis by investigating the plasmonic effects on OSCs and interface engineering.\n\n\n\nHere, the mechanisms of plasmonic effects on OSC are explored by incorporating metallic Au nanoparticles (NPs) in individual anode buffer layer and active layer, respectively, and finally in both layers simultaneously. When Au NPs are incorporated into the buffer layer, surface plasmonic resonance (SPR) induced absorption enhancement due to incorporation of Au NPs is evidenced theoretically and experimentally to be only minor contributor to the performance improvement. The increased interfacial contact area between the buffer layer and active layer, together with the reduced resistance of the buffer layer due to the embedded Au NPs, are revealed to benefit hole collection and thus are main contributors to the performance improvement. When Au NPs are embedded in the active layer, Au NPs induced SPR indeed contributes to enhanced light absorption. However, when large amount of Au NPs are incorporated, the negative effects of NPs on the electrical properties of OSCs can counter-diminish the optical enhancement from SPR, which limits the overall performance improvement. When Au NPs are embedded into both layers, both advantages of incorporating NPs in individual layers can be utilized together to achieve more pronounced improvement in photovoltaic performance; as a result, accumulated enhancements in device performance can be achieved. The results herein are applicable to other metallic NPs such as Ag NPs, Pt NPs, etc. The study herein has clarified the degree of contribution of SPR effects on OSCs and revealed the mechanisms behind. It has also highlighted the importance of considering both optical and electrical effects when employing metallic NPs as strategies to enhance the photovoltaic performance of OSCs. Consequently, the study contributes both physical understanding and technological development of applying metallic NPs on OSCs.\n\n\n\nRegarding interface engineering, we first propose a simple method to modify the substrate work function for efficient hole collection by using an ultra-thin ultraviolet-ozone treated Au. The method can be used in other situations such as modifying the work function of multilayer graphene as transparent electrode. Then we propose a general method to synthesize solution-processed transition metal oxides (TMOs). Besides high material quality, desirable electrical properties, and good stability, our method stands out particular in that the synthesized TMOs can be dispersed in water-free solvents and the TMO films require only low temperature treatment, which is very compatible with the organic electronics. Our method can also be used to synthesize other TMOs other than the demonstrated molybdenum oxide and vanadium oxide. The proposed method herein is applicable in semiconductor industry.

An investigation of the wavelength dependent extinction spectra of coated sphere with different core@shell compositions based on discrete dipole approximation technique has been presented in this paper. We have used combinations of A g, A u, and S i O 2 for this analysis. Specifically, we study the impact of spherical core-shell thickness on its surface plasmon resonance (SPR) peak positions and corresponding spectral widening in distinct regimes of the spectrum. We observe that SPR peak of core-shell nanoparticle(CSNP) can be tuned over the visible to near-infrared spectrum region by manipulating the core/shell ratio and composition of core and shell. Specifically, for dielectric@metal (core@shell) nanoparticle, SPR peak position shifted towards lower wavelength as we increase the shell thickness, which is opposite to the SPR behavior of metal@dielectric. The extinction spectrum shows linear relation between SPR position and thickness of the shell. Further, we observed two resonant peaks for the case of metal@metal CSNP. The SPR peak of Au@Ag (a eff 100 nm with shell thickness 8 nm) reveals two resonant peak corresponding to Au (594 nm) in red domain, while the peak in blue domain corresponds to Ag (402 nm). We also observe that optical resonance of CSNP can be tuned across the near-infrared region by changing the surrounding medium of higher refractive index. Further, near field pattern of core@shell geometry at resonance wavelength is also shown in the present study. We have also compared the numerical and analyticalmethod for smaller size CSNP with varying thickness and the results show good agreement.

Imprinted solution-processible gold nanoparticle (AuNP) grating films were fabricated for use as a grating-coupled surface plasmon resonance (SPR) excitation substrate. The imprinted AuNP grating pattern, which consists of a 1.72 μm grating pitch, was fabricated on flat glass substrates by imprinting AuNP on silicon grating templates. In this study, the SPR measurements were carried out on an SPR device in the grating-coupling configuration with a He–Ne laser ( λ = 632.8 nm) as the excitation source, and multimode surface plasmon excitations were observed upon irradiation with white light. SPR excitation of our substrate was observed at the incident angle of 47.1°. For SPR measurements using white light irradiation, multimode surface plasmon excitation resulting from several diffraction orders was observed in the wavelength region of 500–850 nm. The surface plasmon dispersion branches were calculated to obtain the diffraction order in this region. SPR excitation of the imprinted AuNP grating substrate was further studied by fabrication of a layer-by-layer ultrathin film of iron containing bis(terpyridine) polymer (Fe(II)-BTP) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). A shift of the SPR excitation spectrum was observed when the ultrathin film was deposited on the imprinted AuNP grating surface. Thus, this substrate should be a useful SPR substrate in a variety of applications such as photoelectric conversions and sensors.

Visible and near-infrared photoluminescence enhanced by Ag nanoparticles in Sm3+-doped aluminoborate glass

Clustering of gold particles in Au implanted CrN thin films: The effect on the SPR peak position

Au-Ag alloy films deposited on the glass substrates are used, for the first time, as a wavelength-interrogated near infrared surface plasmon resonance (SPR) sensor. The values of resonance wavelength (λR) of the sensor at different angles of incidence are determined by absorptiometry and its refractive-index (RI) sensitivity is investigated using aqueous glucose solutions as the standard RI samples. As the incident angle increases from 7.5° to 9.5°, the SPR absorption peak shifts from λR = 1215 nm to 767.7 nm, the full width at half magnitude (FWHM) of the peak reduces from 292.8 nm to 131.4 nm, and the RI sensitivity decreases from 35648.3 nm/RIU down to 9363.6 nm/RIU. At the same initial λR, the SPR sensor with the Au-Ag alloy film shows a higher sensitivity than that with the pure Au film (S = 29793.9 nm/RIU at λR=1215 nm with a pure Au film). Adsorption of bovine serum album molecules from the aqueous solution of 1 μmol/L protein results in a redshift of ΔλR = 12.1 nm with the Au-Ag alloy film and ΔλR=9.5 nm with the pure Au film. The experimental data also indicate that the FWHM of the SPR absorption peak with the Au-Ag alloy film is larger than that at the same λR with the pure Au film, leading to a lower spectral resolution than that of the latter.

The properties of gold nanoparticles (GNP) holds promising potential for drug delivery, diagnostics, plasmonic photothermal therapy of diseases, electronics, catalysis, and photovoltaics (Liu et al., 2006; Huang et al., 2003; Thompson, 2007; Atwater and Polman, 2010). This field is growing at a fast pace with the demand for GNPs ever increasing with slow progress on development of synthesis techniques. The objective of this study was to develop a new synthesis technique that produces gold/gold sulfide (GGS) nanoparticles with a high yield. This process can control the equilibrium shift of the surface plasmon resonance (SPR) of the nanoparticles (Patel, 2012). The goal of this research is to increase the total volume of GGS nanoparticles to be synthesized while keeping their tunability for the SPR absorption peak. This research has demonstrated the ability of cellulose membrane to reduce the need for purification steps, which are associated with traditional synthesis techniques for removing small colloidal gold (< 10 nm). It was found that controlling the surface area to volume ratio (SA/Vol) of the cellulose to GGS solution and temperature of the synthesis process, would provide, greater control over the SPR peak. Using these principles allows for larger volume synthesis to be performed while still retaining the ability to tune the SPR peak. It was also discovered that dialyzing out ions during the synthesis process makes bare GGS nanoparticles more stable over time. The time for the reaction to reach equilibrium was observed and showed an increase in temperature and/or SA/Vol reduces the reaction time. In conclusion, this study demonstrated how increasing the temperature and SA/Vol shifts the SPR peak and reduces the time for the reaction to reach equilibrium.

Surface plasmon resonance using noble metal nanoparticles is regarded as an attractive and viable strategy to improve the optical absorption and/or photocurrent in dye-sensitized solar cells (DSSCs). However, no significant improvement in device performance has been observed. The bottleneck is the stability of the noble-metal nanoparticles caused by chemical corrosion. Here, we propose a simple method to synthesize high-performance DSSCs based on polyvinylpyrrolidone-coated Au-TiO2 microspheres that utilize the merits of TiO2 microspheres and promote the coupling of surface plasmons with visible light. When 0.4 wt % Au nanoparticles were embedded into the TiO2 microspheres, the device achieved a power conversion efficiency (PCE) as high as 10.49%, a 7.9% increase compared with pure TiO2 microsphere-based devices. Simulation results theoretically confirmed that the improvement of the PCE is caused by the enhancement of the absorption cross-section of dye molecules and photocurrent.

An investigation of Au/SiO2 composite multilayer films with a large third-order optical nonlinearity is reported in this article. Multilayer films containing Au nanoparticles with narrow size and shape distributions can be obtained by controlling the thickness of the Au layer, as well as the annealing temperature and time duration. It is found that the peak of surface plasmon resonance (SPR) of films with mean nanoparticle size of 30 nm is located at 2.35 eV with a width of 350 meV. The SPR peak of films containing larger or smaller particles red-shifts and broadens due to the particle size and shape effects. The third-order optical nonlinear susceptibility χ(3) of these films has a maximum value of about 5.1×10−6 esu at 532 nm when the mean nanoparticle size is around 30 nm. We attribute the observed size dependence of χ(3) to the competition between the enhancement of the local field factor and the skin-depth effect.

Impact of photoinduced energy transfer and LSPR of Au and Ag nanoparticles on nonlinear optical response of methyl orange

A new scheme of variable thickness ladder-like stepped metal film for simultaneous excitation of surface plasmon resonance (SPR) with different wavelength based on modified three layer Kretschmann configuration has been proposed. Metal thickness has been optimized for four different wavelengths ranging from visible to near infrared. The thickness has also been optimized for different substrates as well as different metal films in order to show the dependency of the substrate and metal layers on the ultimate performance of the plasmonic structure. A new concept of Reflectance Constrained Dynamic Range has been employed and its value has been calculated numerically and graphically for each SPR structure under consideration and for each wavelength considering water as reference sample and using the optimized metal thickness. Electric field enhancement has been evaluated for different structures and the full width half maximum has also been calculated for four working wavelengths in order to study the field confinement and measurement accuracy. Longer wavelength provides larger dynamic range and better measurement precision, which is desirable for biochemical sensing. Moreover, this multi-channel ladder-like stepped metallic structure can also be utilized for simultaneous sensing of different samples.

Surface plasmon resonance (SPR) has found wide applications in sensing down to molecular level due to its extreme sensitivity to change of dielectric properties. An unavoidable effect in SPR is surface deformation (thermal bump) due to local heating by incident laser light used in SPR excitation. In addition, the change in reflection phase from an SPR-supported metal film could also contribute to the SPR signal variation, and thus proper handling of measured SPR signals is very important in order to broaden its potential applications. Here a simple Fabry Perot (FP) interference technique for measuring, simultaneously, the thermal bump as well as the reflection phase shift of the gold film used for SPR excitation is reported. The FP signal change is dominated by the appearance of the thermal bump while it is small for the effect of the reflection phase shift due to the change of dielectric property of the metal. To support the experimental results, model simulations for the SPR system are performed and good agreement with the experiment is obtained. The effect of the thermal bump on the resonance of surface plasmon is also investigated and a polarization-dependent reflection phase shift is observed in SPR. Since both amplitude and phase can be measured, our method could lead to a better characterization of SPR and can also be applied to the study of active metasurfaces under external excitation.

The two-dimensional (2D) periodic Au nanosphere array with silver coating was prepared by using a colloidal monolayer template to obtain a Au nanosphere array and subsequently depositing silver thin coating on it, which could be used as an optical sensor to effectively detect H2S. Such periodic Au nanosphere array with silver coating displayed a surface plasmonic resonance (SPR) peak and an optical diffraction peak. Compared with the SPR peak, the diffraction peak, originated from the periodic arrangements of the obtained array, demonstrated a more sensitive optical change to detect H2S with a significant redshift as the H2S concentration increased. It was attributed to the increase of the refractive index of the environment around the Au nanosphere arrays with silver coating due to the partial formation of Ag2S after detecting H2S. Furthermore, the H2S sensor based on the change of the optical diffraction peak, showed an excellent selectivity and it was very sensitive to detect H2S from 2 to 30 μM. This method was investigated by the analysis in H2S-spiked blood samples, which indicates that the method has the potential to detect H2S in blood samples. The presented work provides a new strategy of utilizing the optical diffraction peak of the periodic array to develop promising sensors.