The research of our group is focused on developing nano-engineering and nano-structure based techniques and tools capable of resolving energy transfer, conversion, and dissipation processes down to the scale of single atoms and single molecules, the fundamental building blocks of any material. These developments lend us unique opportunities to address open questions towards fundamental understanding of the properties and performance of nanoscale thermal, electrical, and optical devices, as well as tackle practical challenges ranging from thermal management of high power electronics, cheap renewable energy with high efficiency, to conductive yet lightweight thermal materials for aerospace and wearable applications.
Research topics of our interest include:
Scanning Thermal Probe Microscopy and Picowatt-resolution Calorimetry
Scanning probe microscopies such as Scanning Tunneling Microscope (STM) and Atomic Force Microscope (AFM), since their invention, have been the primary lens through which the atomic and molecular world is directly viewed and rich physics and chemistry are unraveled. Our current efforts have been directed to the development of scanning thermal probe microscopy (SThM) integrated with ultrahigh resolution calorimetry. The capability of simultaneously achieved sub-nanometer spatial resolution and picowatt (one trillionth of a watt) energy resolution makes it possible to observe various energy processes at the atomic and single-molecule scale to explore their fundamental limits.
Examples of our custom fabricated micro- and nano-scale devices, sensors and scanning probes
Atomic and Molecular Heat Tranport
The demand for faster and more affordable computing and storage has advanced the modern electronics industry over the past half-century to continuously miniaturize individual functional units (e.g., transistors) to approach the atomic and molecular scale, the endgame of Moore's law. Whereas macroscopic devices and materials can be effectively approximated by classical laws such as Fourier's law, Atomic and molecular scale heat transfer follow different rules, depending on the transport and interactions of the elementary heat carriers such as phonons and electrons. We study the quantum energy transport of atomic-scale structures and heat transfer in single molecules, to understand the thermal properties at the size limit of nanoelectronics and provide guidelines for molecular design of polymer thermal materials.
L. Cui, W. Jeong, S. Hur, M. Matt, J. C. Klöckner, F. Pauly, P. Nielaba, J. C. Cuevas, E. Meyhofer, P. Reddy, "Quantized thermal transport in single-atom junctions", Science 355, 1192 (2017).
(Left) heat flow across single atoms of Au showing quantized thermal conductance;
(Right) phonon coherently transport through a single molecule chain of alkane-dithiol
Molecular Thermoelectric Energy and Refrigeration
The inevitable irreversibility of energy processes involving thermal transfer and conversion, ruled by the second law of thermodynamics, sets the limits of any heat engine and renders heat the most ubiquitous energy form. More than 65% of the U.S. energy consumption ends up as wasted heat. Thermoelectric energy conversion in terms of generating clean energy and cooling constitutes one potential future of recycling waste heat for sustainability. We study in the field of molecular thermoelectrics, exploring the possibility of discovering thermoelectric materials with high efficiency and power output. Our work leverages the huge chemical design space of molecules and their assembly, as well as quantum engineering of molecular transport properties.
L. Cui, R. Miao, K. Wang, D. Thompson, L. A. Zotti, J. C. Cuevas, E. Meyhofer, P. Reddy, "Peltier cooling in molecular junctions", Nature Nanotechnology 13, 122-127 (2018).
R. Miao, H. Xu, M. Skripnik, L. Cui, K. Wang, K. G. L. Pedersen, M. Leijnse, F. Pauly, K. Wärnmark, E. Meyhofer, P. Reddy, H. Linke, "Influence of quantum interference on the thermoelectric properties of molecular junctions", Nano Letters 18, 9, 5666-5672 (2018).
L. Cui, R. Miao, C. Jiang, E. Meyhofer, P. Reddy, "Perspective: Thermal and thermoelectric transport in molecular junctions", Journal of Chemical Physics, 146, 092201 (2017).
An electrically-biased Atomic Force Microscope tip on contact with a self-assembled monolayer of molecules to generate thermoelectric cooling
Near-field Radiative Heat Transfer
Thermal radiation at the macroscopic scale is strictly bounded by Planck's law for blackbody radiation. In contrast, when objects are brought into close proximity (with spacing less than thermal wavelength, ~ 10 microns at room temperature), heat transfer between two surfaces is strongly modulated by near-field electromagnetic effects such as surface plasmon and phonon polaritons, resulting in heat flux exceeding the blackbody limit by several orders of magnitude. Understanding this phenomenon holds great promise to improve a series of technologies including thermophotovoltaics (TPV), non-contact thermal imaging, heat-assisted magnetic recording (HAMR), and nanolithography. We are particularly interested in probing the giant heat transfer enhancement to identify the limitation of existing theoretical frameworks and leveraging the nanoscale thermal radiation effect for energy conversion applications.
L. Cui, W. Jeong, V. Fernández-Hurtado, J. Feist, F. J. García-Vidal, J. C. Cuevas, E. Meyhofer, P. Reddy, "Study of radiative heat transfer in Angstrom- and nanometer-sized gaps", Nature Communications 8, 14479 (2017).
H. Wu, Y. Huang, L. Cui, K. Zhu, "Active magneto-optical control of near-field radiative heat transfer between graphene sheets", Physical Review Applied 11(05), 054010 (2019).
K. Kim, B. Song, V. Fernández-Hurtado, W. Lee, W. Jeong, L. Cui, D. Thompson, J. Feist, MT Homer Reid, F. J García-Vidal, J. C. Cuevas, E. Meyhofer, P. Reddy, "Radiative heat transfer in the extreme near field", Nature 528, 387 (2015).
(Left) Schematic and SEM image of the scanning thermal probe, featuring an embeded nanoscale thermocouple as the sensitive thermal sensor; (Right) Calculated surface contour plot showing the spatial distribution of Poynting flux pattern due to near-field thermal radiation effect.
Nano-optics and Surface-Enhanced Raman Spectroscopy (SERS)
Surface-enhanced Raman spectroscopy, by leveraging nano-optical (surface plasmonics) and chemical enhancement effects to enlarge the unique vibrational fingerprints of materials by many orders of magnitude, allows highly sensitive detection at the single-molecule level. The presence of low-abundance molecular markers in various applications ranging from biological diagnosis, chemical identification, to environmental analysis can be recognized and imaged using SERS. The efforts in our group include the development of single-molecule Raman spectroscopy focusing on probing vibrational and electronic coupling in single molecules and nanostructures, which is crucial to understand heat dissipation and chemical reaction in various systems. We are also active in the study of electro-luminescence and photo-luminescence of atomic-scale plasmonic structures.