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John O | May 2017

Rice research describes enhanced voltage in nanogaps in plasmonic gold wires

a team of researcher at rice university (houston, texas) discovered that hot electrons tunneling through a nanogap into neighboring metal can create a photovoltage that is about a thousand times larger than if no gap existed, meaning it could be possible to create nanoscale photodetectors converting light into electricity in sensors and other electronics.


rice university scientists discovered that “hot” electrons can create a photovoltage about a
thousand times larger than ordinary temperature differences in nanoscale gaps
in gold wires. (natelson group/rice university)


according to an article on the rice website, the researchers worked with gold nanowires that had gaps of a few nanometers to see if electrons would jump across the gap under different conditions.


one such condition that the rice team observed was the seebeck (thermoelectric)effect in which heat is converted to electricity at the junction of two wires of different material in the presence of a temperature gradient. using a laser to create a temperature difference across a bowtie-shaped gold nanowire that had been split, a votage was registered.


the article explained, “gold is a plasmonic metal, one of a class of metals that can respond to energy input from a laser or other source by exciting plasmons on their surfaces. plasmon excitations are the back-and-forth sloshing of electrons in the metal, like water in a basin.”


it continued, “in the bowties, laser light absorbed by the plasmons created hot electrons that eventually transferred their energy to the atoms in the metal, vibrating them as well. that energy is dissipated as heat. in continuous, solid wires, the temperature difference caused by the laser also created small voltages. but when nanogaps were present, the hot electrons passed through the void and created much larger voltages before dispersing.”


the scientists can now tune the thermoelectric properties of metals by altering the structure on the nanoscale and can use a laser to map the effects. shining a light on the material created a photovoltage, but that voltage was multiplied by a thousand when nanoscale tunneling gaps were present because it uses the high-energy electrons before the energy is lost to heat.


“the researchers acknowledge several possible reasons for the dramatic effect, but they strongly suspect tunneling by the photo-generated hot carriers is responsible,” the article concluded.


the research was recently published in the journal of physical chemistry letters. the abstract read:


“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 plasmon excitation.”

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