Select Papers

Quantifying Charge Carrier Localization in Chemically Doped Semiconducting Polymers

Shawn A. Gregory, Riley Hanus*, Amalie Atassi, Joshua M. Rinehart, Jamie P. Wooding, Akanksha K. Menon, Mark D. Losego, G. Jeffery Snyder, Shannon K. Yee* Nature Materials (2021) *cooresponding

Charge transport in semiconducting polymers ranges from localized (hopping-like) to delocalized (metal-like), yet no quantitative model exists to fully capture this transport spectrum and its dependency on charge carrier density. In this study, using an archetypal polymer-dopant system, we measure the temperature-dependent electrical conductivity, Seebeck coefficient, and extent of oxidation. We then use these measurements to develop a semi-localized transport (SLoT) model, which captures both localized and delocalized transport contributions. By applying the SLoT model to published data, we demonstrate its broad utility. We are able to determine system dependent parameters such as the maximum localization energy of the system, how this localization energy changes with doping, the amount of dopant required to achieve metal-like conductivity, and the conductivity a system could have in the absence of localization effects. This proposed SLoT model improves our ability to predict and tailor electronic properties of doped semiconducting polymers.

Uncovering design principles for amorphous-like heat conduction using two-channel lattice dynamics

Riley Hanus*, Janine George, Max Wood, Alexander Bonkowski, Yongqiang Cheng,Douglas L. Abernathy, Michael E. Manley, Geoffroy Hautier, G. Jeffrey Snyder, and Raphael P. Hermann*. Materials Today Physics (2021) *cooresponding

Recently density functional theory (DFT) based two-channel lattice dynamics (LD) has emerged as a powerful tool to predict the thermal conductivity of materials ranging from crystalline to disordered. Within this framework, heat conduction can be thought of transporting through (i) the common phonon-gas channel (diagonal) and (ii) the diffuson channel (off-diagonal), which combine additively. We show how two channel LD predicts that the diffuson channel dominates in thermoelectric Yb14(Mg,Mn)Sb11 above room temperature. More importantly we demonstrate how this tool can provide rational design principles for the diffuson channel and therefore provides a clear avenue to engineer the thermal conductivity of disordered and amorphous materials.

Thermal resistance at a twist boundary and a semicoherent heterointerface

Ramya Gurunathan, Riley Hanus*, Anupam Garg, G. Jeff Snyder. Physical Review B (2021) *cooresponding

Traditional models of interfacial phonon scattering, including the acoustic mismatch model and diffuse mismatch model, take into account the bulk properties of the material surrounding the interface, but not the atomic structure and properties of the interface itself. Here, we derive a theoretical formalism for the phonon scattering at a dislocation grid, or two interpenetrating orthogonal arrays of dislocations, as this is the most stable structure of both the symmetric twist boundary and semicoherent heterointerface. With this approach, we are able to separately examine the contribution to thermal resistance due to the step-function change in acoustic properties and due to interfacial dislocation strain fields, which induces diffractive scattering. This work indicates that scattering from misfit dislocation strain fields doubles the thermal boundary resistance of Si-Ge heterointerfaces compared to scattering due to acoustic mismatch alone. This physical treatment can guide the thermal design of devices by quantifying the relative importance of interfacial strain fields, which can be engineered via fabrication and processing methods, versus acoustic mismatch, which is fixed for a given interface.

Analytical Models of Phonon–Point-Defect Scattering

Ramya Gurunathan, Riley Hanus, Maxwell Dylla, Ankita Katre, and G. Jeffrey Snyder. Physical Review Applied (2020)

Point defects exist widely in engineering materials and are known to scatter vibrational modes, resulting in reduction in thermal conductivity. This work reviews the essential physics of the model and compares its predictions with first-principles results for isotope and alloy scattering, demonstrating the model to be a useful metric of material design. A treatment of the scattering parameter for a multiatomic lattice is recommended and compared with other treatments presented in the literature, which have been at times misused to yield incomplete conclusions about the system’s scattering mechanisms. Additionally, we demonstrate a reduced sensitivity of the model to the full phonon dispersion and discuss its origin. Finally, a simplified treatment of scattering in alloy systems with vacancies and interstitial defects is demonstrated to suitably describe the potent scattering strength of these off-stoichiometric defects.

Lattice Softening Significantly Reduces Thermal Conductivity and Leads to High Thermoelectric Efficiency

Riley Hanus, Matthias T. Agne, Alexander J. E. Rettie, Zhiwei Chen, Gangjian Tan, Duck Young Chung, Mercouri G. Kanatzidis, Yanzhong Pei, Peter W. Voorhees, and G. Jeffrey Snyder. Advance Materials (2019)

Two fundamentally different avenues for controlling thermal conductivity are phonon scattering and lattice softening, or the reduction of phonon speed. The latter mechanism is particularly attractive when phonon-phonon scattering is inherently very strong, such as in thermoelectric materials and/or at high temperatures. In this work we show the importance of lattice softening in two specific cases, PbTe and nanocrystalline Si. We have affectionately named this project 'The Bird' since when a phonon dispersion softens it looks like a bird flapping its wings.

Phonon diffraction and dimensionality crossover in phonon-interface scattering

Riley Hanus, Anupam Garg, G. Jeff Snyder. Comms. Phys. (2018)

Thermal resistance across interfaces and grain boundaries is an inherently complex topic. It is widely accepted that the structure of the interface at the nanoscale is critically important when describing phonon transmitance and reflectance. Many boundaries in cyrstalline materials can be described as arrays of linear defects. This paper presents several phyiscal mechanisms that arrise when considering such a structure. An analytical theory is presented to describe phonon grain boundary scattering/transmisivity which contains information about the nanoscale structure of the interfaces.

Understanding of the Band Convergence in Thermoelectric CoSb3 Skutterudites: Influence of Electron Population, Local Thermal Expansion, and Bonding Interactions

Riley Hanus, Xingyu Guo, Yinglu Tang, Goudong Li, G. Jeff Snyder, Wolfgang G. Zeier. Chem. Mat. (2017)

As was identified by Yinglu Tang et el. the electronic origin of the superior thermoelectric properties is attributed to a secondary conduction band with 12 carrier pockets. Using a combined computational and experimental approach the chemical nature of critical features in the band structure are highlighted. Synchrotron diffraction and refinement studies reveal interesting structural behavior upon doping and upon heating. The work of Korotaev and Yanilkin supplements this study nicely, and identifies that electron-phonon interactions are indeed required to explain the experimentally observed band convergence with temperature.

Skutterudite with graphene-modified grain-boundary complexion enhances zT enabling high-efficiency thermoelectric device

Peng-an Zong*, Riley Hanus*, Maxwell Dylla, Yunshan Tang, Jingcheng Liao, Qihao Zhang, G. Jeff Snyder and Lidong Chen. Energy Environ. Sci. (2016) *contributed equally

Incorporating 2D materials into the grain-boundary complexion can modify transport across the grain boundary (GB) in fundamentally different ways than traditional GB phases. We find that wrapping the grains of skudderudite with several layers of graphene dramatically increases the thermal boundary resistance while hardly effecting the electrical properties. This effect was observed in both n and p-type skutterudite and the resulting 16 TE module exhibited a 24% increase in conversion efficiency due to this GB engineering.

Minimum thermal conductivity in the context of diffuson-mediated thermal transport

Matthias T. Agne, Riley Hanus, G. Jeff Snyder. Energy Environ. Sci. (2018)

Estimating how low the thermal conductivity can be engineered can set practical limits for a variety of applications. A model for the thermal conductivity of bulk solids is proposed in the limit of diffusive transport mediated by diffusons as opposed to phonons is presented.

High Thermoelectric Performance in SnTe−AgSbTe2 Alloys from Lattice Softening, Giant Phonon−Vacancy Scattering, and Valence Band Convergence

Gangjian Tan, Shiqiang Hao, Riley Hanus, et al. ACS Energy Let. (2018)

In this collaboration between the Kanatzidis, Wolverton, Snyder and Dravid groups, we report on the underlying mechanisms that enable the SnTe−AgSbTe2 system to exhibit superior thermoelectric figure of merit (zT ) compared to its parent compound SnTe. Specifically, alloying with Ag and Sb on the Sn site promotes the formation of cation vacancies which softens the materials lattice slowing the propagation of phonons while also inducing phonon scattering.


Nanoscale thermal transport lecture series (Georgia Tech, Mech. E./Mat. Sci. & E.)

The goal of this five part lecture series is to establish a working knowledge of thermal transport theory at the nano-scale. After this course the participants will have the requisite knowledge to interpret current research in the computational, theoretical, and experimental thermal sciences. Additionally, they will have the foundation required to begin building thermal transport models which are useful in guiding independent research and interpreting results.

Time: 1:30pm to 2:30pm

Dates and locations: (1) Feb. 17th MRDC 3515 (2) Feb. 24th MRDC 3515 (3) Mar. 2nd Student Center 332 (4) Mar. 9th MRDC 3515 (5) Mar. 16th MRDC 3515

Lecture 1: [PDF] [PowerPoint]

Lecture 2: [PDF] [PowerPoint]

Lecture 3: [PDF] [PowerPoint]

Lecture 4: [PDF] [PowerPoint]

Lecture 5: [PDF] [PowerPoint]


March 2021
DOE research highlight: Glass-like quantum energy hopping limits thermal insulation in crystals

Work on amorphous-like heat conduction of Yb14(Mn,Mg)Sb11 is highlight by the Department of Energy, Physical Sciences Directorate. Highlight article can be found here .

April 2020
Why Science? - Riley Hanus

Riley is highlighted as a "talented young researcher", and explains why he chose science in Oak Ridge National Laboratory Review Vol. 53 No.1. The full article can be found here .

May 2019
Science Graduate Student Research (ScGSR) Award

Appointment begins at Oak Ridge National Lab. List of ScGSR awardees can be found here.

Photo: Riley (left) and Raphael Hermann perform an experiment on the ARCS beamline at ORNL's Spallation Neutron Source (SNS).

March 2019
Julia and Hans Weertman: A legacy of scholarship, mentorship, and lives well-lived

Sonal Rangnekar, Riley Hanus*. MRS bulletin (March 2019). * 2018 Weertman Fellowship recipient.

Article published recounting the Symposium in Honor and Remembrance of Johannes (Hans) and Julia Weertman, held November 16 in Evanston, Ill.