Thermoelectric Nanowire Junction Photoresponse
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Thermoelectric Nanowire Junction Photoresponse Tito E. Huber1, Scott D. Johnson2 , Tina Brower1, Quinton Barclift2, Benjamin Panga2 , and Gary Harris.1 1
Howard Nanoscale Science and Engineering Facility, Howard University, Washington DC 20059, USA 2
Engineering, Prince George’s Community College, Largo MD 20774, USA.
ABSTRACT
Recently there have been reports of hot carrier thermoelectric response in nanostructured materials like graphene and MoS. We report observing that thermoelectric nanowire junctions detect light. In these experiments we employed devices composed of bismuth nanowire arrays which are capped with a transparent indium tin oxide electrode. The incident surface features very low optical reflectivity and enhanced light trapping. The unique attributes of the thermoelectric arrays are the combination of strong temporal and optical wavelength dependences of the photocurrent. Under infrared illumination, the signal can be completely described by "quasi-equilibrium" thermoelectric effects considering cooling rates given by heat diffusion through the array. The thermal diffusivity is found to be less (by a factor of 3.5) than in the bulk, a result that we discuss in terms of phonon confinement effects. In addition to a thermoelectric response, under visible illumination, we observe a photovoltaic response.
INTRODUCTION Optoelectronics are devices that convert optical energy into electrical energy and vice versa. For semiconductor optoelectronic devices such as silicon solar cells, photon absorption leads to the transfer of charge between different electronic bands, resulting in a photocurrent (PC). It recently has been shown that the archetypical two dimensional material graphene exhibit hot carrier thermoelectric (TE) photoresponse, an effect where electrical power is generated by the heating associated with the absorption of light at the nanoscale.[1,2] There have been no reports of this effect in nanoscale systems based on conventional bulk thermoelectrics. Here we show that nanowire arrays (NWAs) of bismuth feature a TE photoresponse and we study the nanoscale optical and TE properties that give rise to the effect. TE effects are caused by the difference in the broadening of the energy distribution in the electronic bands owing to a temperature gradient. Traditional TE, including bismuth, have large thermopower values and a large electrical conductivity tied with poor thermal conductivity leading to exceptionally large efficiency.[3] Bulk TE crystals are not good candidates in the search for TE photoresponse effects because energy dissipates quickly in these systems. Also, the high optical reflectivity of the front surface of bulk thermoelectrics is not conducive for conversion. In their investigation of PC’s in solar cells, researchers have discovered that the
nanostructuring of bulk materials into nanowires or sharp points aligned along the optical incident direction , like ours, results in reduced optical reflection and induced light trapping. [4] EXPERIMENT We fabricated devices composed of
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