Fan-Cheng Lin

The influence of shell thickness of Au@TiO2 core–shell nanoparticles on the plasmonic enhancement effect in dye-sensitized solar cells

Plasmonic core-shell nanoparticles (PCSNPs) can function as nanoantennas and improve the efficiency of dye-sensitized solar cells (DSSCs). To achieve maximum enhancement, the morphology of PCSNPs needs to be optimized. Here we precisely control the morphology of Au@TiO2 PCSNPs and systematically study its influence on the plasmonic enhancement effect. The enhancement mechanism was found to vary with the thickness of the TiO2 shell. PCSNPs with a thinner shell mainly enhance the current, whereas particles with a thicker shell improve the voltage. While pronounced plasmonic enhancement was found in the near infrared regime, wavelength-independent enhancement in the visible range was observed and attributed to the plasmonic heating effect. Emission lifetime measurement confirms that N719 molecules neighboring nanoparticles with TiO2 shells exhibit a longer lifetime than those in contact with metal cores. Overall, PCSNPs with a 5 nm shell give the highest efficiency enhancement of 23%. Our work provides a new synthesis route for well-controlled Au@TiO2 core-shell nanoparticles and gains insight into the plasmonic enhancement in DSSCs.

Facet‐Dependent and Light‐Assisted Efficient Hydrogen Evolution from Ammonia Borane Using Gold–Palladium Core–Shell Nanocatalysts

Au-Pd core-shell nanocrystals with tetrahexahedral (THH), cubic, and octahedral shapes and comparable sizes were synthesized. Similar-sized Au and Pd cubes and octahedra were also prepared. These nanocrystals were used for the hydrogen-evolution reaction (HER) from ammonia borane. Light irradiation can enhance the reaction rate for all the catalysts. In particular, Au-Pd THH exposing {730} facets showed the highest turnover frequency for hydrogen evolution under light with 3-fold rate enhancement benefiting from lattice strain, modified surface electronic state, and a broader range of light absorption. Finite-difference time-domain (FDTD) simulations show a stronger electric field enhancement on Au-Pd core-shell THH than those on other Pd-containing nanocrystals. Light-assisted nitro reduction by ammonia borane on Au-Pd THH was also demonstrated. Au-Pd tetrahexahedra supported on activated carbon can act as a superior recyclable plasmonic photocatalyst for hydrogen evolution.

HNO3-Assisted Polyol Synthesis of Ultralarge Single-Crystalline Ag Microplates and Their Far Propagation Length of Surface Plasmon Polariton

We developed a HNO3-assisted polyol reduction method to synthesize ultralarge single-crystalline Ag microplates routinely. The edge length of the synthesized Ag microplates reaches 50 μm, and their top facets are (111). The mechanism for dramatically enlarging single-crystalline Ag structure stems from a series of competitive anisotropic growths, primarily governed by carefully tuning the adsorption of Ag(0) by ethylene glycol and the desorption of Ag(0) by a cyanide ion on Ag(100). Finally, we measured the propagation length of surface plasmon polaritons along the air/Ag interface under 534 nm laser excitation. Our single-crystalline Ag microplate exhibited a propagation length (11.22 μm) considerably greater than that of the conventional E-gun deposited Ag thin film (5.27 μm).

Mode Conversion in High-Definition Plasmonic Optical Nanocircuits

Symmetric and antisymmetric guided modes on a plasmonic two-wire transmission line have distinct properties and are suitable for different circuit functions. Being able to locally convert the guided modes is important for realizing multifunctional optical nanocircuits. Here, we experimentally demonstrate successful local conversion between the symmetric and the antisymmetric modes in a single-crystalline gold plasmonic nanocircuit with an optimally designed mode converter for optical signals at 194.2 THz. Mode conversion may find applications in controlling nanoscale light-matter interaction.

Influence of morphology on the plasmonic enhancement effect of Au@TiO 2 core-shell nanoparticles in dye-sensitized solar cells

Plasmonic core-shell nanoparticles (PCSNPs) can function as nanoantennas and improve the efficiency of dye-sensitized solar cells (DSSCs). To achieve maximum enhancement, the morphology of PCSNPs needs to be optimized. Here we precisely control the morphology of Au@TiO2 PCSNPs and systematically study its influence on the plasmonic enhancement effect. Enhancement mechanism was found to vary with the thickness of TiO2 shell. PCSNPs with thinner shell mainly enhance the current, whereas particles with thicker shell also improve the voltage. While pronounced plasmonic enhancement was found in the near infrared regime, wavelength-independent enhancement in the visible range was observed and attributed to plasmonic heating effect. Emission lifetime measurement confirms that N719 molecules neighboring nanoparticles with TiO2 shell exhibit longer lifetime than those in contact with metal cores. Overall, PCSNPs with 5-nm shell give highest efficiency enhancement of 23%. Our work provides a new synthesis route for well-controlled Au@TiO2 core-shell nanoparticles and gains insight into the plasmonic enhancement in DSSCs.

Photoluminescence-driven Broadband Transmitting Directional Optical Nanoantennas

Optical nanoantennas mediate near and far optical fields. Operating a directional nanoantenna in transmitting mode is challenging because the antenna needs to be driven by a nanosized optical frequency generator, which must work at the antennas resonance frequency and be precisely attached to the antennas feed with correct polarization. Quantum emitters have been used as optical nanogenerators, but their precise positioning relative to the nanoantenna is technically challenging, setting up a barrier to the practical implementation. One unique source to drive nanoantenna is the photoluminescence from the material of the nanoantenna because the high operational frequency of the antenna reaches the regime for the electronic transitions in matter. Here, we exploit plasmon-modulated photoluminescence (PMPL) as an effective optical source to drive directional nanoantennas. We experimentally realize two technically challenging theoretical proposals, namely, an optical nanospectrometer based on Yagi-Uda nanoantennas and tunable broadband directional emission from log-periodic nanoantennas. Using photoluminescence from the nanoantenna as an optical source promotes practical implementation of transmitting optical nanoantennas.

Ultrafast second-harmonic generations in a plasmonic two-wire transmission-line

Nonlinear optical frequency conversion in plasmonics has attracted immense research attention recently. Through various device geometries, harmonic generations of various orders have been reported. However, in these demonstrations, the near-field effect utilized has been limited to localized surface plasmons. It was not until last year that nonlinear frequency conversion utilizing propagating surface plasmon polaritons being reported in a single plasmonic nanowire [1]. A plasmonic two-wire transmission-line (TWTL) is comprised of two metallic nanowires with a nano-gap between these two nanowires [2,3]. Depending on the laser excitation polarization, TE or TM mode could be selectively excited within a plasmonic TWTL. It has been demonstrated the two modes could be freely converted through the geometrical design to the TWTL [3,4]. Here we experimentally demonstrate for the first time, ultrafast second-harmonic generations (SHG) in a plasmonic (TWTL). We also show that regardless of the fundamental harmonic mode excited by the laser beam, the SHG signals are always the TM mode.

Two-photon Photoluminescence Investigation of Transverse Plasmonic Mode of Single-crystalline Gold Nanoantennas

Polarization-dependent spectra of two-photon photoluminescence (TPPL) from three series of single-crystalline gold nanoantennas were studied. The peak position in transverse TPPL spectra was found to shift with the arm width, revealing the depolarization nature of nanoantenna-mediated radiation.