Session S62: Thermal Transport in Nanostructures II

Thermoelectric Transport in Topological Crystalline Insulator and Topological Semimetal Nanowires

Many of the topological materials (e.g. Bi₂Te₃ and Sb₂Te₃) are also well-known thermoelectric materials with high figures of merit ZTs. This ‘coincidence’ is not accidental but is because the two groups of materials share some similar properties, i.e. they are composed of heavy elements and have small bulk band gaps. Theoretical calculations have suggested the possibility of enhancing thermoelectric properties by utilizing topological non-trivial states. In this talk, I will present our recent experimental studies of thermoelectric transport in some representative topological materials, including SnTe-based topological crystalline insulators (TCIs) and iridium oxide-based topological semimetals (TSMs). Grown by chemical vapor deposition, the TCIs and TSMs are (quasi-)one-dimensional nanowires with large surface-area-to-volume ratios and well-defined facets. Measurements of the Seebeck coefficient, and electrical and thermal conductivities were performed on the same individual nanowires as a function of temperature, allowing for accurate determination of their ZTs. I will discuss the enhancement of Seebeck coefficient and suppression of thermal conductivity in connection with the topological states, doping, alloying, and nanostructuring.   

Tunable Dimensionality Effects on the Thermoelectric Performance of Solution-Processable Hybrid Tellurium Nanowire Composites

Hybrid composite materials have demonstrated a high degree of thermoelectric (TE) performance and manage to leverage positive aspects of both their organic and inorganic counter parts such as ease of solution processability, cost effectiveness, conformal geometries and device printability. To date, there is limited understanding of the physical factors underlying high performance in this class of materials. Here, we investigate the impact of ligand chain length on the physical and TE properties of tellurium nanowires (TeNWs) grown in a simple one-pot synthesis. This work reveals the importance of TeNW dimensionality and polydispersity on TE transport in TeNW systems. Using an aqueous chemical modification technique, conductive polymer ligands are affixed to the surface of the top performing TeNWs, resulting in a p-type hybrid composite TE ink system with an optimized power factor of 130 uW/mK². This study provides insight into how synthetic chemistry can provide previously unused tools for optimizing performance and understanding transport in such complex hybrid systems.   

Thermal transport in spin ladder compound Sr14Cu24O41 microstructures

Recently, large magnetic contribution to thermal conductivity has been observed in some cuprates with strong antiferromagnetic coupling. One prominent example is the spin ladder compound Sr₁₄Cu₂₄O₄₁ with an incommensurate layered structure. Steady-state thermal transport measurements of single crystals revealed a large anisotropic ratio of thermal conductivity due to the magnons propagating along spin ladders. Previous studies have been devoted to understand the coupling of magnons with phonons and electrons in Sr₁₄Cu₂₄O₄₁ single crystals; however, the effects of boundaries and defects on anisotropic magnon transport have been under-examined. Here, we investigate the thermal transport in Sr₁₄Cu₂₄O₄₁ microrods, which are prepared by co-precipitation synthesis. TEM studies indicate that these microrods are single crystals grown preferentially along the spin ladder axis. The thermal conductivity of Sr₁₄Cu₂₄O₄₁ microrods prepared at different temperatures is measured using a four-probe thermal transport measurement method. The thermal conductivity contribution and mean free paths of magnons are evaluated using a kinetic model of 1D magnon transport. Our results show pronounced effects of defects and boundaries on anisotropic magnon thermal transport in the magnetic microstructures.   

Probing thermal transport in single-molecule junctions

Charge and energy transport in single-molecule junctions is of great interest to reveal exotic quantum transport phenomena at the fundamental size limit, and with potential applications in nanoelectronics and nanophotonics. A key quantity—the thermal conductance of single-molecule junctions—has eluded experimental determination due to challenges in detecting extremely small picowatt-level heat currents that occur in such systems. Here, by employing custom-developed scanning thermal microscopy (SThM) probes that enable picowatt-resolution, we report the quantitative measurement of the thermal conductance of single-molecule junctions and investigate the dependence of thermal transport on molecular length. Our experiments were performed on prototypical Alkanedithiol molecules, revealing that the thermal conductance is approximately independent of the molecular length and is consistent with ab initio simulations. The techniques presented here represent a breakthrough that will enable further studies of thermal transport in many other 1D organic systems, short molecules and polymers, for which interesting thermal transport properties have been theoretically predicted but remain experimentally inaccessible.
Reference: L. Cui et al., Nature 572, 628–633 (2019).   

Photothermoelectric detection of strain variation in gold nanostructure

Controlling morphology and composition via nanoscale structuring gives opportunities to improve the thermoelectric properties of materials for energy conversion and photodetection. In this study, we report the detection of the strain variation via open circuit photothermoelectric voltage detection on thin-film Au nanowire devices as a function of the position of an optical heat source. A focused laser beam is used to locally heat the metal nanostructure. The first mapping scan of the photothermoelectric voltage shows a relative high value compare with the second scan on the same device after 12 hours annealing. We also conduct a control experiment to shift the work function of the gold nanowire via self-assembled monolayer. Combining the kelvin probe measurement with the photothermoelectric voltage shows the work function variation of the nanowire doesn’t play a key role on the photothermoelectric voltage response. These experiments argue that the strain distribution within the gold nanowire is likely the dominant effect causing the variation of the photothermoelectric voltage. Photothermoelectric voltage measurement provides a sensitive new method to strain distributions within a nanostructured metal.   

Energy Transport in Extreme Thermal Materials

Controlling thermal transport is critical for many applications including electronics and energy technologies. Discovering new materials and nanostructures with extreme conductivity that can efficiently insulate or dissipate heat are in urgent need. In this talk, I will first describe our recent effort in developing emerging high thermal conductivity semiconductors, including boron arsenide (BAs) and boron phosphide (BP) with a room temperature thermal conductivity of 1300 W/mK (Science 361, 575, 2018) and 500 W/mK (Nano Letters 17, 7507, 2017) respectively, beyond most common semiconductors and metals. Ultrafast spectroscopy study in conjunction with atomistic theory reveals that the unique band structure of BAs allows for very long phonon mean free paths and strong high-order anharmonicity through the four-phonon process. Our study establishes BAs and BP as benchmark materials for thermal management, and exemplifies the power of combining experiments and ab initio theory in new materials discovery. On the other hand, we investigated ultralow thermal conductivity materials with record-high thermoelectric performance. Examining the intrinsic crystals of tin selenide (Nano Letters 19, 4941, 2019), we observed abnormally strong phonon renormalization at room temperature due to high-order anharmonicity, as well as the failure of widely used quasi-harmonic model in predicting thermophysical properties. In addition, I will briefly describe our effort in developing in-situ techniques to characterize electrochemical materials (Nano Letters 17, 1431, 2017) and quantifying interface energy transport (Advanced Materials 31, 1901021, 2019) that enable better fundamental understanding of phonon spectra and defect scattering for electronics, sensors, batteries, and quantum information.   

Super Compliant and Soft (CH3NH3)3Bi2I9 Crystals with Ultralow Thermal Conductivity

In this work, we show the phonon dispersion of (CH₃NH₃)₃Bi₂I₉ single crystals at 300 K measured by inelastic x-ray scattering. The frequencies of acoustic phonons are among the lowest of crystals. Nanoindentation measurements verified that these crystals are very compliant and considerably soft. The frequency overlap between acoustic and optical phonons results in strong acoustic-optical scattering. All these features lead to an ultralow thermal conductivity, as validated by the laser flash method. The fundamental knowledge obtained from this study will accelerate the design of novel hybrid materials for energy applications. This work has been published in Phys. Rev. Lett. 123, 155901 (2019).   

Photothermoelectric detection of local strain variations in gold single-crystal and bicrystal stripes

Nanoscale manipulation of the thermoelectric effect has recently been used in photodetection and energy conversion applications. In metals, the electronic Seebeck coefficient depends on the energy-dependent electrical conductivity. At the nanoscale, single metal thermocouples are created by modifying the conductivity via nanostructuring, changing the energy-dependent mean free path of the charge carriers. Here, we present scanning photothermoelectric measurements of gold single crystal devices and compare the photovoltages with electron back-scatter diffraction measurements, which provide insight on the local strain variation in the device. The good correlation between these measurements show that photovoltage measurements are sensitive to local variation in strain. Finite-element modeling suggests that these results are consistent with long-scale gradients in the Seebeck coefficient, which yield reasonable estimates of local strain. Extending these measurements to devices with individual grain boundaries demonstrate that local strain has a larger effect than simple changes in crystallographic orientation. These measurements show that residual strain gradients in nanostructures can dominate thermoelectric response.   

Modeling quasiballistic phonon transport from a cylindrical electron beam heat source

Electron microscopy experiments use focused electron beams as nanoscale heat sources or thermometers. However, when the electron beam radius is smaller than the heat carrier mean free path, Fourier’s law will underpredict the electron-beam induced heating. Here, beam heating in nonmetallic samples is modeled by applying a general solution of the Boltzmann Transport Equation (BTE) under the relaxation time approximation [Hua and Minnich, PRB 90.21 (2014): 214306]. BTE results show that ballistic phonon effects in this radial heat spreading scenario are conveniently represented using a ballistic thermal resistance. Calculations of this ballistic resistance for Si, GaAs, and 3C-SiC show that ballistic effects dominate the total thermal resistance for typical beam radii (<10 nm), indicating that the ballistic resistance could be measured using electron beam heating experiments. However, the magnitude of the temperature rise remains small (<1 K), even when considering these ballistic effects. These BTE modeling results can be used to quantify electron-beam induced heating or to design experiments probing ballistic phonon transport using electron beam heat sources.

Ref.: Wehmeyer, Journal of Applied Physics 126, 124306 (2019).   

Nondestructive probing of the transport and elastic properties of nanostructured metalattices using coherent EUV beams

Nanoscale metamaterials exhibit engineered thermal, magnetic, and electronic properties, which are essential for nanoelectronics, thermoelectrics, and ultralight media. Nanostructured metalattices are artificial 3D solids, periodic on length scales 1-100nm, that enable the functional properties of materials to be tuned. However, characterizing such materials is extremely challenging. Here we overcome this challenge by using tabletop high harmonic extreme ultraviolet (EUV) beams, with wavelengths (≈30nm) and pulse durations (≈10fs) that are well-matched for probing nanoscale structure and function. We demonstrate nondestructive measurements of the mechanical, structural, and transport properties of complex 3D nanostructured media—specifically in metalattices made from 14-30nm silica nanosphere templates infiltrated with silicon. We extract the mechanical and structural properties from the measured acoustic dispersion, which agrees with continuum models. In contrast, the heat flow through the metalattices deviates from macroscopic models due to the nanoscale structure. These samples have not only a lower thermal conductivity than expected, but also exhibit transport properties that depend on the heat source geometry. We support these findings with advanced atomistic models.   

Designing Autoregulatory Nanoparticle Array and Polymer Composite Materials

The composite material: a gold nanoparticle array embedded in a hydrogel polymer with a fluorinated surface layer; combines length scales from atomistic to microscale. When a laser excites the array local heating occurs, causing the polymer to shrink and the surface to wrinkle. The wrinkles deflect lasing and allow the system to relax. Using FTDT, the array geometry has been designed to maximize temperate change. The alteration of the mechanical properties of the hydrogel polymer due to the temperature change is essential to reproduce the wrinkling. When heated, the concentration of water in the polymer layer changes, altering the thermal and mechanical properties explored via molecular dynamics. Thermal transport is modeled using Reverse Non-Equilibrium Molecular Dynamics.¹ A pure water/polymer system is compared to a system with different salts.This atomistic information feeds into finite element mechanics calculations, where previous studies have displayed wrinkling in similar systems.² All these pieces help to create an autoregulatory humidity sensor.

1.S. Kuang, J.D. Gezelter. Molecular Physics 2012 110(9-10), 691-701
2.C.T. Chapman, J.T. Paci, W-K. Lee, C.J. Engel, T.W. Odom, G.C. Schatz. ACS Applied Materials & Interfaces 2016 8 (37), 24339-24344