Browsing by Author "Abbasi, Mahdiyeh"

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    Photothermoelectric effect study in gold nanowires
    (2019-08-09) Abbasi, Mahdiyeh; Natelson, Douglas
    The thermoelectric effect is the conversion of electrical to thermal energy and vice versa, and it is of great interest in energy harvesting, cooling mechanisms based on the Peltier effect, and photodetectors based on the Seebeck effect. In the photothermoelectric effect or PTE, photons are used to produce thermal energy and a temperature gradient, and then electrical energy. Previous studies show the unexpected localized variation in the Seebeck coefficient, and further efforts in the Natelson Lab have been directed to understanding these behaviors. In the first part of this thesis, studies of thermocouples with gold single crystals are introduced, to help us figure out the thermoelectric behavior of individual grains, to push us one step closer to solving the localized variation of Seebeck coefficient puzzle. Experimental and simulation results are being introduced. Photodetectors based on photothermoelectric effect can be made, and this is the focus of the second part of this thesis. Asymmetric gold nanostructures can make localized optical absorption in the structure; combine that with Seebeck coefficient changes and the result will be an electrical signal when the whole geometry is illuminated. Geometries that can have plasmon resonances at desired wavelengths can be used to increase the absorption for that wavelength and so make the photodetector spectrally sensitive. Simulations and experiments are been discussed.
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    Photothermoelectric response and hot carrier tunneling in gold nanowires
    (2021-10-27) Abbasi, Mahdiyeh; Natelson, Douglas
    The thermoelectric effect is the conversion of electrical to thermal energy and vice versa. In the photothermoelectric effect (PTE), photons are used as a heat source to apply a temperature distribution. Photodetectors based on PTE can be made. If we wisely choose noble metals in our PTE-based photodetectors, we can use the plasmonic characteristic of the metals to our benefit. Plasmons are the incompressible oscillation of electrons that can be excited by (coupled to) light. These oscillations have different energy levels and they depend on the geometry as well as the permittivity of the metal. All plasmon modes can decay nonradiatively and produce heat. Dipolar plasmon modes can couple to the far field. In the first section of this thesis, the design, simulation, and experimental results of single metal (gold) photodetectors that operate based on dipolar plasmon modes of gold nanowires are discussed. Later we discuss how PTE voltages in single-crystal nanowires are sensitive to lattice distortions and trace impurities. As a result, PTE measurement can be used to detect internal properties of gold nanowire. Using COMSOL simulations we can characterize the internal strain as well as platinum impurity concentration in gold single crystalline nanowires. In the second part of the thesis, we consider the effects on plasmon modes in nanogaps, both in the generation of light through inelastic tunneling, and in their photovoltage response that can be used for photodetection. A nanowire may be broken to form a nanogap. That nanogap, thanks to broken geometrical symmetry, can host localized plasmon modes of a variety of energies. These modes can be excited either by applied light or by inelastic tunneling of electrons under an applied voltage. When these plasmon modes decay, they produce electron-hole pairs. Radiative recombination of these pairs can cause light emission out of the gap. This light emission is shaped by the plasmonic characteristics of the metallic nanostructure. In the presence of high current densities, so that the energy of more than one inelastically excited plasmon can be present at a time in the junction region, the emitted light can be above threshold (photon energy greater than the applied voltage). Here, the simulations of characterizing the plasmonic modes of these nanogaps are discussed. The localized plasmon modes can be excited by light and can be detected by measuring the open circuit voltage thanks to two different mechanisms. In an unbroken nanowire, an optically generated temperature gradient leads to a PTE voltage. In nanogap structures, hot carrier tunneling can also lead to an open circuit photovoltage. The open circuit voltage after forming the gap is up to 1000 stronger than the PTE voltage in an unbroken nanowire. When the gap is formed, local plasmon modes can be excited. These modes decay and produce hot carriers, these hot carriers tunnel across the gap from one electrode to the other. An open circuit voltage develops to counterbalance this hot electron current. When both electrodes are made from same metal (same electrical permittivity) and same geometry, the hot carriers don’t have a preferred tunneling direction. So, the polarity of the net open circuit voltage is completely random from device to device and depends on the small changes in gap geometry. If the direction of hot carrier tunneling in these devices can be controlled, we can make photodetectors that ×100 times faster with ×1000 times higher responsivity. Here we show the experimental results of preferred hot carrier tunneling direction in gold, platinum MIM structure.
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    Tuning Light Emission Crossovers in Atomic-Scale Aluminum Plasmonic Tunnel Junctions
    (American Chemical Society, 2022) Zhu, Yunxuan; Cui, Longji; Abbasi, Mahdiyeh; Natelson, Douglas
    Atomic-sized plasmonic tunnel junctions are of fundamental interest, with great promise as the smallest on-chip light sources in various optoelectronic applications. Several mechanisms of light emission in electrically driven plasmonic tunnel junctions have been proposed, from single-electron or higher-order multielectron inelastic tunneling to recombination from a steady-state population of hot carriers. By progressively altering the tunneling conductance of an aluminum junction, we tune the dominant light emission mechanism through these possibilities for the first time, finding quantitative agreement with theory in each regime. Improved plasmonic resonances in the energy range of interest increase photon yields by 2 orders of magnitude. These results demonstrate that the dominant emission mechanism is set by a combination of tunneling rate, hot carrier relaxation time scales, and junction plasmonic properties.