Thermal spin injection and interface insensitivity in permalloy/aluminum metallic non-local spin valves

We present measurements of thermal and electrical spin injection in nanoscale metallic non-local spin valve (NLSV) structures. Informed by measurements of the Seebeck coefficient and thermal conductivity of representative films made using a micromachined Si-N thermal isolation platform, we use simple analytical and finite element thermal models to determine limits on the thermal gradient driving thermal spin injection and calculate the spin dependent Seebeck coefficient to be $-0.5\ \mu\mathrm{V}/\mathrm{K}< S_{s}<-1.3\ \mu\mathrm{V}/\mathrm{K}$. This is comparable in terms of the fraction of the absolute Seebeck coefficient to previous results, despite dramatically smaller electrical spin injection signals. Since the small electrical spin signals are likely caused by interfacial effects, we conclude that thermal spin injection is less sensitive to the FM/NM interface, and possibly benefits from a layer of oxidized ferromagnet, which further stimulates interest in thermal spin injection for applications in sensors and pure spin current sources

Heat, Charge and Spin Transport of Thin Film Nanostructures

Understanding of fundamental physics of transport properties in thin film nanostructures is crucial for application in spintronic, spin caloritronics and thermoelectric applications. Much of the difficulty in the understanding stems from the measurement itself. In this dissertation I present our thermal isolation platform that is primarily used for detection of thermally induced effects in a wide variety of materials. We can accurately and precisely produce in-plane thermal gradients in these membranes, allowing for thin film measurements on 2-D structures. First, we look at thermoelectric enhancements of doped semiconducting single-walled carbon nanotube thin films. We use the Wiedemann-Franz law to calculate contributions to thermal conductivity and find interesting underlying physics as we dope the films, thus changing the Fermi level. Adapting the tube diameter leads to structural differences, which greatly affects both phonon and electron contributions to thermal conductivity. These unique films can be designed as thermoelectric materials that are easy to manufacture and can be utilized in a variety of situations. Second, we look at work measuring enhanced contributions to thermopower and thermal conductivity of unique ferromagnetic metals. We observe improved thermopower due to the ultra-low damping of the magnon system. For spintronic and spin caloritronic applications, having a low damping is important for device engineering and allows for long spin lifetimes. Third, we present on spin transport through disordered magnetic insulators. We observe spin Hall effect driven magnon transport through materials with no long-range order but with local antiferromagnetic exchange interactions. We are the first to observe this type of transport, which may lead spintronic investigations in a new and profound direction. Finally, we look at transverse effects in a thin ferromagnetic metal. Our observation of the planer Nernst effect and planar Hall effect across long length scales shows that effects in this range are dominated by traditional magneto-thermoelectric effects without any evidence of spin transport. A careful understanding of thermal and electric gradients is needed to aid in understanding of transport properties of thin films

Transport and Spectroscopic Studies of the Effects of Fullerene Structure on the Efficiency and Lifetime of Polythiophene-based Solar Cells

Time-dependent measurements of both power conversion efficiency and ultraviolet-visible absorption spectroscopy have been observed for solar cell blends containing the polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) with two different functionalized C60 electron acceptor molecules: commercially available [6,6]-phenyl C61 butyric acid methyl ester (PCBM) or [6,6]-phenyl C61 butyric acid octadecyl ester (PCBOD) produced in this laboratory. Efficiency was found to decay with an exponential time dependence, while spectroscopic features show saturating exponential behavior. Time constants extracted from both types of measurements showed reasonable agreement for samples produced from the same blend. In comparison to the PCBM samples, the stability of the PCBOD blends was significantly enhanced, while both absorption and power conversion efficiency were decreased.Comment: manuscript submitted to Solar Energy Materials and Solar Cell

Large n- and p-type thermoelectric power factors from doped semiconducting single-walled carbon nanotube thin films

Lightweight, robust, and flexible single-walled carbon nanotube (SWCNT) materials can be processed inexpensively using solution-based techniques, similar to other organic semiconductors. In contrast to many semiconducting polymers, semiconducting SWCNTs (s-SWCNTs) represent unique one-dimensional organic semiconductors with chemical and physical properties that facilitate equivalent transport of electrons and holes. These factors have driven increasing attention to employing s-SWCNTs for electronic and energy harvesting applications, including thermoelectric (TE) generators. Here we demonstrate a combination of ink chemistry, solid-state polymer removal, and charge-transfer doping strategies that enable unprecedented n-type and p-type TE power factors, in the range of 700 μW m−1 K−2 at 298 K for the same solution-processed highly enriched thin films containing 100% s-SWCNTs. We also demonstrate that the thermal conductivity appears to decrease with decreasing s-SWCNT diameter, leading to a peak material zT ≈ 0.12 for s-SWCNTs with diameters in the range of 1.0 nm. Our results indicate that the TE performance of s-SWCNT-only material systems is approaching that of traditional inorganic semiconductors, paving the way for these materials to be used as the primary components for efficient, all-organic TE generators