Prathamesh Pavaskar

Plasmon Resonant Enhancement of Photocatalytic Water Splitting Under Visible Illumination

We demonstrate plasmonic enhancement of photocatalytic water splitting under visible illumination by integrating strongly plasmonic Au nanoparticles with strongly catalytic TiO2. Under visible illumination, we observe enhancements of up to 66× in the photocatalytic splitting of water in TiO2 with the addition of Au nanoparticles. Above the plasmon resonance, under ultraviolet radiation we observe a 4-fold reduction in the photocatalytic activity. Electromagnetic simulations indicate that the improvement of photocatalytic activity in the visible range is caused by the local electric field enhancement near the TiO2 surface, rather than by the direct transfer of charge between the two materials. Here, the near-field optical enhancement increases the electron-hole pair generation rate at the surface of the TiO2, thus increasing the amount of photogenerated charge contributing to catalysis. This mechanism of enhancement is particularly effective because of the relatively short exciton diffusion length (or minority carrier diffusion length), which otherwise limits the photocatalytic performance. Our results suggest that enhancement factors many times larger than this are possible if this mechanism can be optimized.

Plasmonic nanoparticle arrays with nanometer separation for high-performance SERS substrates.

We demonstrate a method for fabricating arrays of plasmonic nanoparticles with separations on the order of 1 nm using an angle evaporation technique. Samples fabricated on thin SiN membranes are imaged with high-resolution transmission electron microscopy (HRTEM) to resolve the small separations achieved between nanoparticles. When irradiated with laser light, these nearly touching metal nanoparticles produce extremely high electric field intensities, which result in surface-enhanced Raman spectroscopy (SERS) signals. We quantify these enhancements by depositing a p-aminothiophenol dye molecule on the nanoparticle arrays and spatially mapping their Raman intensities using confocal micro-Raman spectroscopy. Our results show significant enhancement when the incident laser is polarized parallel to the axis of the nanoparticle pairs, whereas no enhancement is observed for the perpendicular polarization. These results demonstrate proof-of-principle of this fabrication technique. Finite difference time domain simulations based on HRTEM images predict an electric field intensity enhancement of 82400 at the center of the nanoparticle pair and an electromagnetic SERS enhancement factor of 10(9)-10(10).

Plasmon resonant enhancement of dye sensitized solar cells

We report an improvement in the efficiency of dye sensitized solar cells (DSSCs) by exploiting the plasmonic resonance of Au nanoparticles. By comparing the performance of DSSCs with and without Au nanoparticles, we demonstrate a 2.4-fold enhancement in the photoconversion efficiency. Enhancement in the photocurrent extends over the wavelength range from 460 nm to 730 nm. The underlying mechanism of enhancement is investigated by comparing samples with different geometries, including nanoparticles deposited on top of and embedded in the TiO2 electrode, as well as samples with the light absorbing dye molecule deposited on top of and underneath the Au nanoparticles. The mechanism of enhancement is attributed to the local electromagnetic response of the plasmonic nanoparticles, which couples light very effectively from the far field to the near field at the absorbing dye molecule monolayer, thereby increasing the local electron–hole pair (or exciton) generation rate significantly. The UV-vis absorption spectra and photocurrent spectra provide further information regarding the energy transfer between the plasmonic nanoparticles and the light absorbing dye molecules. Based on scanning electron microscope images, we perform electromagnetic simulations of these different Au nanoparticle/dye/TiO2 configurations, which corroborate the enhancement observed experimentally.

A microscopic study of strongly plasmonic Au and Ag island thin films

Thin Au and Ag evaporated films (∼5 nm) are known to form island-like growth, which exhibit a strong plasmonic response under visible illumination. In this work, evaporated thin films are imaged with high resolution transmission electron microscopy, to reveal the structure of the semicontinuous metal island film with sub-nm resolution. The electric field distributions and the absorption spectra of these semicontinuous island film geometries are then simulated numerically using the finite difference time domain method and compared with the experimentally measured absorption spectra. We find surface enhanced Raman scattering (SERS) enhancement factors as high as 108 in the regions of small gaps (≤2 nm), which dominate the electromagnetic response of these films. The small gap enhancement is further substantiated by a statistical analysis of the electric field intensity as a function of the nanogap size. Areal SERS enhancement factors of 4.2 × 104 are obtained for these films. These plasmonic films can also ...

Study of the plasmon energy transfer processes in dye sensitized solar cells

We report plasmon enhanced absorption in dye sensitized solar cells (DSSC) over a broad wavelength range. 45% enhancement in the power conversion efficiency is observed with the inclusion of plasmonic gold nanoparticles (NPs). Photocurrent spectra show enhancement over the entire dye absorption range from 450 nm to 700 nm, as well as in the near infrared (NIR) region above 700 nm due to the strong plasmon-induced electric fields produced by the gold NPs. The plasmon-induced electric field distribution of the island-like gold film is also investigated using finite-difference-time-domain (FDTD) calculations. Furthermore, photoluminescence spectra are performed in order to rule out the mechanism of plasmon energy transfer through Forster resonance energy transfer.

Iterative optimization of plasmon resonant nanostructures

We perform finite difference time domain simulations of two-dimensional clusters of metal nanoparticles with incident planewave irradiation. An iterative optimization algorithm is used to determine the configuration of the nanoparticles that gives the maximum electric field intensity at the center of the cluster. The optimum configurations of these clusters have mirror symmetry about the axis of planewave propagation, but are otherwise nonsymmetric and nonintuitive. The maximum field intensity is found to increase monotonically with the number of nanoparticles in the cluster, producing intensities that are five times larger than linear chains of nanoparticles and 2500 times larger than the incident electromagnetic field.

Plasmonic mode mixing in nanoparticle dimers with nm-separations via substrate-mediated coupling

We fabricate arrays of metallic nanoparticle dimers with nanometer separation using electron beam lithography and angle evaporation. These “nanogap” dimers are fabricated on thin silicon nitride membranes to enable high resolution transmission electron microscope imaging of the specific nanoparticle geometries. Plasmonic resonances of the pairs are characterized by dark-field scattering micro-spectroscopy, which enables the optical scattering from individual nanostructures to be measured by using a spatially-filtered light source to illuminate a small area. Scattering spectra from individual dimers are correlated with transmission electron microscope images and finite-difference time-domain simulations of their electromagnetic response, with excellent agreement between simulation and experiment. We observe a strong polarization dependence with two dominant scattering peaks in spectra taken with the polarization aligned along the dimer axis. This response arises from a unique Fano interference, in which the bright hybridized modes of an asymmetric dimer are able to couple to the dark higherorder hybridized modes through substrate-mediated coupling. The presence of this interference is strongly dependent on the nanoparticle geometry that defines the plasmon energy profile but also on the intense localization of charge at the dielectric surface in the nanogap region for separations smaller than 6 nm.

Plasmon Resonant Enhancement of Photocatalytic Solar Fuel Production

We have recently demonstrated plasmonic enhancement of several photochemical processes (water splitting, CH4 formation from CO2, methyl orange decomposition, and CO oxidation) by integrating strongly plasmonic metal nanostructures with strongly catalytic metal oxide semiconductors. Irradiating these catalysts with visible light near the plasmon resonance frequency generates intense electric fields and immense plasmonic charge, which drive these photocatalytic processes at an accelerated rate. Enhancement factors up to 66X have been observed under visible light illumination, while, under ultraviolet radiation, we observe a 4-fold reduction in the photocatalytic activity. Finitedifference time-domain (FDTD) simulations indicate that the enhanced photocatalytic activity in the visible range is due to the local electric field enhancement near the TiO2 surface, rather than the direct transfer of charge between the two materials. These simulation results also indicate that enhancement factors many times larger than this can be achieved if the geometry of the plasmonic nanoparticles can be optimized.

Correction: Plasmon-enhanced water splitting on TiO2-passivated GaP photocatalysts

Correction for ‘Plasmon-enhanced water splitting on TiO2-passivated GaP photocatalysts’ by Jing Qiu et al., Phys. Chem. Chem. Phys., 2014, 16, 3115–3121.

Iteratively optimized nonperiodic plasmon resonant nanostructures

FDTD simulations are performed on two-dimensional clusters of plasmonic metal nanoparticles in response to incident planewave irradiation. Using an iterative optimization algorithm, we determine the spatial configuration of the nanoparticles that gives the maximum electric field intensity at the center of the cluster. The optimum configurations of these clusters have mirror symmetry about the axis of planewave propagation, but are otherwise non-symmetric and non-intuitive. The optimized electric field intensity increases monotonically with the number of nanoparticles in the cluster, producing surface enhanced Raman spectroscopy (SERS) enhancements that are 25 times larger than linear chains of nanoparticles and 6 million times larger than the incident electromagnetic field.