Photothermoelectric effects and large photovoltages in plasmonic Au nanowires with nanogaps

Nanostructured metals subject to local optical interrogation can generate open-circuit photovoltages potentially useful for energy conversion and photodetection. We report a study of the photovoltage as a function of illumination position in single metal Au nanowires and nanowires with nanogaps formed by electromigration. We use a laser scanning microscope to locally heat the metal nanostructures via excitation of a local plasmon resonance and direct absorption. In nanowires without nanogaps, where charge transport is diffusive, we observe voltage distributions consistent with thermoelectricity, with the local Seebeck coefficient depending on the width of the nanowire. In the nanowires with nanogaps, where charge transport is by tunneling, we observe large photovoltages up to tens of mV, with magnitude, polarization dependence, and spatial localization that follow the plasmon resonance in the nanogap. This is consistent with a model of photocurrent across the nanogap carried by the nonequilibrium, "hot" carriers generated upon the plasmon excitation.Comment: 17 pages, 4 figures + 17 pages/10 figs of supporting informatio

Plasmonic heating in Au nanowires at low Temperatures: The role of thermal boundary resistance

Inelastic electron tunneling and surface-enhanced optical spectroscopies at the molecular scale require cryogenic local temperatures even under illumination - conditions that are challenging to achieve with plasmonically resonant metallic nanostructures. We report a detailed study of the laser heating of plasmonically active nanowires at substrate temperatures from 5 to 60 K. The increase of the local temperature of the nanowire is quantified by a bolometric approach and could be as large as 100 K for a substrate temperature of 5 K and typical values of laser intensity. We also demonstrate that a $\sim 3\times$ reduction of the local temperature increase is possible by switching to a sapphire or quartz substrate. Finite element modeling of the heat dissipation reveals that the local temperature increase of the nanowire at temperatures below $\sim$50 K is determined largely by the thermal boundary resistance of the metal-substrate interface. The model reproduces the striking experimental trend that in this regime the temperature of the nanowire varies nonlinearly with the incident optical power. The thermal boundary resistance is demonstrated to be a major constraint on reaching low temperatures necessary to perform simultaneous inelastic electron tunneling and surface enhanced Raman spectroscopies.Comment: 20 pages, 5 figures + 17 pages supporting materia

Superconductivity in Films of Pb/PbSe Core/Shell Nanocrystals

Superconductivity in films of electronically coupled colloidal lead nanocrystals is reported. The coupling between particles is <i>in situ</i> controlled through the conversion of the oxides present on the surface of the nanoparticles to chalcogenides. This transformation allows for a 10⁹-fold increase in the conductivity. The temperature of the onset of the superconductivity was found to depend upon the degree of coupling of the nanoparticles in the vicinity of the insulator–superconductor transition. The critical current density of the best sample of Pb/PbSe nanocrystals at zero magnetic field was determined to be 4 × 10³ A/cm². In turn, the critical field of the sample shows 50-fold enhancement compared to bulk Pb