Zuwei Liu

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.

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.

Thermal emission spectra from individual suspended carbon nanotubes.

We study the thermal emission spectra of individual suspended carbon nanotubes induced by electrical heating. Semiconducting and metallic devices exhibit different spectra, based on their distinctive band structures. These spectra are compared with the ideal blackbody emission spectrum. In the visible wavelength range, the thermal emission spectra of semiconducting devices agree well with Plancks law, while the spectra of metallic devices show an additional peak between 1.5 and 1.9 eV. In the near-infrared wavelength range, the semiconducting nanotubes exhibit a peak around 1 eV. These additional peaks are attributed to the E11M and E22SC transitions that are thermally driven under these high applied bias voltages. These peaks show a strong polarization dependence, while the blackbody tail is unpolarized, which provides further evidence for electron-hole recombination in thermal emission. For semiconducting devices, the temperature of the nanotube is fit to Plancks law and compared with the temperatures obtained from the G band and 2D band Raman downshifts, as well as the anti-Stokes/Stokes intensity ratio. For devices showing thermal non-equilibrium, the electron temperature agrees well with G+ downshift but deviates from G_ downshift.

Optical manipulation of plasmonic nanoparticles, bubble formation and patterning of SERS aggregates

We present an optical method for patterning SERS (surface-enhanced Raman spectroscopy)--enhancing aggregates of gold nanoparticles, using a focused laser beam to optically trap the nanoparticles in suspension. At high laser powers, heat generated from the plasmonic excitation causes boiling of the aqueous suspension and the formation of gaseous bubbles of water vapor. By measuring the Raman peak of the hydroxyl bond of water, the temperature in the laser spot during the aggregation can be determined in situ. The hydrophilic nanoparticles are found to aggregate at the liquid-vapor interface. By allowing the suspension to dry, a ring of gold nanoparticles is deposited on the substrate, producing a highly SERS-active region. These aggregates are studied using optical microscopy, scanning electron microscopy and micro-Raman spectroscopy.

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.

Electromechanical resonance behavior of suspended single-walled carbon nanotubes under high bias voltages

We characterize the nanoelectromechanical response of suspended individual carbon nanotubes under high voltage biases. An abrupt upshift in the mechanical resonance frequency of approximately 3 MHz is observed at high bias. While several possible mechanisms are discussed, this upshift is attributed to the onset of optical phonon emission, which results in a sudden contraction of the nanotube due to its negative thermal expansion coefficient. This, in turn, causes an increase in the tension in the suspended nanotube, which upshifts its mechanical resonance frequency. This upshift is consistent with Raman spectral measurements, which show a sudden downshift of the optical phonon modes at high bias voltages. Using a simple model for oscillations on a string, we estimate the effective change in the length of the nanotube to be ΔL/L ≈ −2 × 10−5 at a bias voltage of 1 V.