An official website of the United States government
Abstract Spatially resolved vibrational mapping of nanostructures is indispensable to the development and understanding of thermal nanodevices 1 , modulation of thermal transport 2 and novel nanostructured thermoelectric materials 3–5 . Through the engineering of complex structures, such as alloys, nanostructures and superlattice interfaces, one can significantly alter the propagation of phonons and suppress material thermal conductivity while maintaining electrical conductivity 2 . There have been no correlative experiments that spatially track the modulation of phonon properties in and around nanostructures due to spatial resolution limitations of conventional optical phonon detection techniques. Here we demonstrate two-dimensional spatial mapping of phonons in a single silicon–germanium (SiGe) quantum dot (QD) using monochromated electron energy loss spectroscopy in the transmission electron microscope. Tracking the variation of the Si optical mode in and around the QD, we observe the nanoscale modification of the composition-induced red shift. We observe non-equilibrium phonons that only exist near the interface and, furthermore, develop a novel technique to differentially map phonon momenta, providing direct evidence that the interplay between diffuse and specular reflection largely depends on the detailed atomistic structure: a major advancement in the field. Our work unveils the non-equilibrium phonon dynamics at nanoscale interfaces and can be used to study actual nanodevices and aid in the understanding of heat dissipation near nanoscale hotspots, which is crucial for future high-performance nanoelectronics.
more »
« less
Thermoelectric properties of Marcus molecular junctions
Abstract In the present work we theoretically analyze thermoelectric transport in single-molecule junctions (SMJ) characterized by strong interactions between electrons on the molecular linkers and phonons in their nuclear environments where electron hopping between the electrodes and the molecular bridge states predominates in the steady state electron transport. The analysis is based on the modified Marcus theory accounting for the lifetime broadening of the bridge’s energy levels. We show that the reorganization processes in the environment accompanying electron transport may significantly affect SMJ thermoelectric properties both within and beyond linear transport regime. Specifically, we study the effect of environmental phonons on the electron conductance, the thermopower and charge current induced by the temperature gradient applied across the system.
more »
« less
- Award ID(s):
- 2122102
- PAR ID:
- 10495843
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Journal of Physics: Condensed Matter
- Volume:
- 36
- Issue:
- 18
- ISSN:
- 0953-8984
- Page Range / eLocation ID:
- 185301
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Organic semiconductors (OSCs) have garnered significant attention due to their potential use in flexible, lightweight, and cost‐effective electronic devices. Despite their promise, the assembly of organic molecules into the condensed phase promotes a diverse set of lattice dynamics that introduce a detrimental modulation in the intermolecular electronic structure—termed dynamic disorder—that results in charge carrier mobilities that are orders of magnitude lower than inorganic semiconductors. This dynamic disorder is generally associated with low‐frequency phonons, yet whether a small subset of modes or a broad range of phonons drives dynamic disorder remains contested. Resolving this debate is critical for defining how targeted phonon engineering could practically improve OSC performance. In this review, we explore progress toward uncovering the interplay between lattice dynamics and charge transport in OSCs, focusing on the critical role of thermally activated phonons. We describe the powerful insight that mode‐resolved analyses of electron–phonon interactions lends toward the rational design of new materials. We highlight recent efforts to achieve this, showcasing proposed strategies to mitigate dynamic disorder through molecular and crystal design. This work offers an overview of the insight gained toward understanding the fundamental mechanisms governing charge transport in OSCs and outlines pathways for enhancing performance via targeted manipulation of interatomic/intermolecular interactions and resulting phonon modes.more » « less
-
Abstract Topological insulators and semimetals have been shown to possess intriguing thermoelectric properties promising for energy harvesting and cooling applications. However, thermoelectric transport associated with the Fermi arc topological surface states on topological Dirac semimetals remains less explored. This work systematically examines thermoelectric transport in a series of topological Dirac semimetal Cd3As2thin films grown by molecular beam epitaxy. Surprisingly, significantly enhanced Seebeck effect and anomalous Nernst effect are found at cryogenic temperatures when the Cd3As2layer is thin. In particular, a peak Seebeck coefficient of nearly 500 µV K−1and a corresponding thermoelectric power factor over 30 mW K−2 m−1are observed at 5 K in a 25‐nm‐thick sample. Combining angle‐dependent quantum oscillation analysis, magnetothermoelectric measurement, transport modeling, and first‐principles simulation, the contributions from bulk and surface conducting channels are isolated and the unusual thermoelectric properties are attributed to the topological surface states. The analysis showcases the rich thermoelectric transport physics in quantum‐confined topological Dirac semimetal thin films and suggests new routes to achieving high thermoelectric performance at cryogenic temperatures.more » « less
-
Abstract Topological semimetals represent a novel class of quantum materials displaying non‐trivial topological states that host Dirac/Weyl fermions. The intersection of Dirac/Weyl points gives rise to essential properties in a wide range of innovative transport phenomena, including extreme magnetoresistance, high mobilities, weak antilocalization, electron hydrodynamics, and various electro‐optical phenomena. In this study, the electronic, transport, phonon scattering, and interrelationships are explored in single crystals of the topological semimetal HfAs2. It reveals a weak antilocalization effect at low temperatures with high carrier density, which is attributed to perfectly compensated topological bulk and surface states. The angle‐resolved photoemission spectroscopy (ARPES) results show anisotropic Fermi surfaces and surface states indicative of the topological semimetal, further confirmed by first‐principle density functional theory (DFT) calculations. Moreover, the lattice dynamics in HfAs2are investigated both with the Raman scattering and density functional theory. The phonon dispersion, density of states, lattice thermal conductivity, and the phonon lifetimes are computed to support the experimental findings. The softening of phonons, the broadening of Raman modes, and the reduction of phonon lifetimes with temperature suggest the enhancement of phonon anharmonicity in this new topological material, which is crucial for boosting the thermoelectric performance of topological semimetals.more » « less
-
Abstract Heat transport in nanoscale carbon materials such as carbon nanotubes and graphene is normally dominated by phonons. Here, measurements of in‐plane thermal conductivity, electrical conductivity, and thermopower are presented from 77–350 K on two films with thickness <100 nm formed from semiconducting single‐walled carbon nanotubes. These measurements are made with silicon–nitride membrane thermal isolation platforms. The two films, formed from disordered networks of tubes with differing tube and bundle size, have very different thermal conductivity. One film matches a simple model of heat conduction assuming constant phonon velocity and mean free path, and 3D Debye heat capacity with a Debye temperature of 770 K. The second film shows a more complicated temperature dependence, with a dramatic drop in a relatively narrow window near 200 K where phonon contributions to thermal conductivity essentially vanish. This causes a corresponding large increase in thermoelectric figure‐of‐merit at the same temperature. A better understanding of this behavior can allow significant improvement in thermoelectric efficiency of these low‐cost earth‐abundant, organic electronic materials. Heat and charge conductivity near room temperature is also presented as a function of doping, which provides further information on the interaction of dopant molecules and phonon transport in disordered nanotube films.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1088/1361-648X/ad21ac
