Laboratory for Surface Modification (LSM)

Seminars Archives

December 2017 | January 2018 | February 2018

Thursday, January 18, 2018
Polymer-Dopant Synergies for Thermoelectric Performance in Sustainable Materials
Howard Katz
Department of Materials Science and Engineering
Johns Hopkins University
12:00 NOON Chem 260

Conjugated polymers are considered in thermoelectric applications because of their possible contribution to thermoelectric composites based on sustainable materials and fabrication processes. The power factor (PF) of thermoelectric materials, S2σ, where S is Seebeck coefficient and σ is electrical conductivity, requires high charge density at an energy level ca. 0.1 eV below the transport level, and high mobility of charge carriers in that level. Creating stable charge carriers in a semiconducting polymer structure that maintains facile charge transport is a materials chemistry challenge. This talk will discuss two approaches to this challenge. For hole conductivity, we modified a standard thiophene polymer structure (PQT12) with electron donating sulfur atoms between the dodecyl chains and thiophene rings, and with ethylenedioxy substitution (like in PEDOT-PSS) on half the thiophene rings. Both these modifications are intended to stabilize holes and achieve unusually high nonionic polymer conductivity. For each of the modifications, one particular dopant yielded the highest σ and PF.[1] For electron conductivity, we employed an emerging n-type polymer with enhanced electron accepting properties and air-stable ionic dopants, one an inorganic salt and another a particle made from common elements to achieve the first step toward air stability of electron σ and PF.[2] One notable aspect of both of these investigations is the consistent correlations of S and σ with predictions of recently published models, which indicates high mobility of doped forms of the polymers. A second aspect is the constancy of S over the minutes time scale following imposition of a temperature difference, decreasing the likelihood of a major ionic contribution to S. Spectroscopic measurements were used as alternate means of observing charge carriers, transistor data provided estimations of mobility, and x-ray scattering revealed the effects of doping on polymer chain packing.

Figure 1. Structures of semiconducting polymers used in this study

[1] H. Li, M.E. DeCoster, R.M. Ireland, J.; Song, P.E. Hopkins, H.E. Katz,
J. Am Chem. Soc. 2017, 139, 11149-11157.
[2] X. Zhao, D. Madan, Y. Cheng, J. Zhou, H. Li, S.M. Thon, A.E. Bragg, M.E. DeCoster, P.E.; Hopkins, H.E.; Katz, Adv. Mater. 2017, 29, 1606928; D. Madan; X. Zhao; R.M. Ireland; D. Xiao; H.E. Katz, APL Materials 5, 086106
Thursday, January 25, 2018
Time-resolved magneto-thermal microscopy: dynamic imaging of magnetic metals and magnetic insulators in devices
Gregory David Fuchs
School of Applied and Engineering Physics
Cornell University
12:00 Noon CHEM 260

Research in spintronics would be accelerated by a table-top magnetic imaging technology that possesses the simultaneous temporal resolution and spatial resolution to study magnetization dynamics in emerging magnetic devices – magnetic memory, logic, and oscillators. In addition, many of the most exciting magnetic material systems, including ultrathin magnetic insulators buried beneath heavy metals or topological insulator layers, are difficult to image with any method. To address these challenges, we have developed a spatiotemporal magnetic microscope based on picosecond heat pulses that stroboscopically transduces an in-plane magnetization into a voltage signal. When the device contains a magnetic metal like FeCoB or NiFe, we use the time-resolved anomalous Nernst effect [1,2]. When it contains a magnetic insulator/normal metal bilayer like yttrium iron garnet/platinum, we use the combination of the time-resolved longitudinal spin Seebeck effect and the inverse spin Hall effect [3]. We demonstrate that these imaging modalities have time resolution in the range of 10 100 ps and sensitivities in the range of 0.1 0.3°/√Hz, which enables both static magnetic imaging and phase-sensitive ferromagnetic resonance imaging. One application of our microscope is imaging a magnetic torque vector, which we apply to spin-orbit torques in a spin Hall device [4]. We find an unexpected spatial variation in the orientation of the spin torque vector. These results show that spin torques in magnetic devices can have more complicated dynamics than expected, and that the all-electrical FMR measurements of spin torque vectors can produce a systematic error as large as 30%. Finally, I will describe how time-resolved magnetic imaging can be extended to greatly exceed the optical diffraction limit, both theoretically [5] and experimentally. We demonstrate scanning a sharp gold tip to create near-field thermal transfer from a picosecond laser pulse to a magnetic sample as the basis of a nanoscale spatiotemporal microscope.
[1] J. M. Bartell, D. H. Ngai, Z. Leng, and G. D. Fuchs, Nat. Commun. 6, 8460 (2015).
[2] F. Guo, J. M. Bartell, D. H. Ngai, and G. D. Fuchs, Phys. Rev. Appl. 4, 044004 (2015).
[3] J. M. Bartell, C. L. Jermain, S. V. Aradhya, J. T. Brangham, F. Yang, D. C. Ralph, and G. D. Fuchs, Phys. Rev. Appl. 7, 044004 (2017).
[4] F. Guo, J. M. Bartell, and G. D. Fuchs, Phys. Rev. B 93, 144415 (2016).
[5] J. C. Karsch, J. M. Bartell, and G. D. Fuchs, APL Photonics 2, 086103 (2017).

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