Controlling the hot carrier tunneling direction in nanogaps*

Plasmonic structures can be tuned to absorb light resonantly at a particular wavelength. When the width of the nanowire in the middle of a gold bowtie structure is resonant to a wavelength, we observed increased thermal absorption at the nanowire. The Seebeck coefficient of a metal depends on the energy-dependent electrical conductivity, which in turn depends on the energy-dependent electron mean free path. In a single metal bowtie structure, the Seebeck coefficient changes as a function of the width of the device. Combined with a plasmonically resonant width for the nanowire, this leads to an open-circuit photovoltage optimized for a wavelength when the device is illuminated with a laser. Forming a nanometer-scale gap in the gold nanowire can increase the photovoltage signal by up to 1000 times due to hot electron tunneling. Our previous study showed that the polarity of the photothermoelectric signal in the gap in pure Au wires is sensitive to atomic-scale details of the junction. We propose a structure with a nanogap between gold and palladium nanowires such that plasmon-generated hot electrons in the Au have a preferred direction to tunnel. This structure enables the control of hot carrier tunneling direction. We will share preliminary results.