Research

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 down to the scale of single atoms and molecules, the fundamental building blocks of any material. These developments lend us unique opportunities to address open questions towards understanding of properties and performance of nanoscale thermal, electrical, and optical devices, as well as tackle practical challenges ranging from heat management, renewable energy, to conductive yet lightweight thermal materials.

1. Scanning Probe Microscopy and Ultrasensitive Calorimetry/Thermometry

 

2. Atomic Electronic and Heat Transport

3. Molecular and Quantum Thermoelectrics

4. Near-field Radiative Heat Transfer and Energy Conversion

5. Nano-optics and Surface-Enhanced Raman Spectroscopy

Scanning Probe Microscopy and Ultrasensitive Calorimetry/Thermometry

 

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 their fundamental limits.

Examples of our custom fabricated micro- and nano-scale devices, sensors and scanning probes

Atomic Electronic and Heat Transport

 

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, switches) 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, heat transfer and thermal dissipation processes at the atomic and molecular scale follow different rules, strongly depending on the transport and interactions of elementary thermal and electrical excitations in materials at the spatial-temporal scales difficult to access experimentally. We develop experimental techniques to study quantum electron and heat transport at the space-time limit. Our research aims to understand the thermal and energy dissipation properties at the size limit of nanoelectronics and provide guidelines and design tools for molecular and atomic-scale precision manufacturing/synthesis of next generation functional materials. 

Recent publications:

  • L. Cui, S. Hur, Z. A. Akbar, J. C. Klöckner, W. Jeong, F. Pauly, S.-Y. Jang, P. Reddy, E. Meyhofer, "Thermal conductance of single-molecule junctions", Nature (2019). 

 

  • 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 and Quantum Thermoelectrics 

 

The ​inevitable irreversibility of energy processes involving heat transfer and conversion, ruled by the second law of thermodynamics, sets the theoretical 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, capable of generating clean energy, and achieving solid-state heating and cooling, constitutes one potential future of recycling waste heat for sustainability. We study in the field of molecular and quantum thermoelectrics, exploring the possibility of discovering novel thermoelectric materials and energy conversion mechanisms with high efficiency and power output beyond the existing practices which are still far from the theoretical limits. Our work leverages the huge chemical design space of molecules and their assembly, as well as quantum engineering of electrical and thermal transport properties, with the potential to manufacturing molecular and quantum thermoelectric devices at scale.

 

Recent publications:

  • 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 and Energy Conversion

 

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.

Recent publications:

  • 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). 

  • L. Cui, Y. Huang, J. Wang, K. Y. Zhu, "Ultrafast modulation of near-field heat transfer with tunable metamaterials", Applied Physics Letters, 102, 053106 (2013). 

  • L. Cui, Y. Huang, J. Wang, "Near-field radiative heat transfer between chiral metamaterials", Journal of Applied Physics, 112, 084309 (2012). 

(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

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 and hot-carrier dynamics.

 

Recent publications:

  • L. Cui, Y. Zhu, M. Abbasi, A. Ahmadivand, B. Gerislioglu, P. Nordlander, D. Natelson, "Electrically driven hot-carrier generation and above-threshold light emission in plasmonic tunnel junctions", Nano Letters (2020).

(Left) Schematics of surface plasmon-induced above-threshold (ħω > eV) light emission from an electrically driven plasmonic tunnel junction due to the recombination of hot electrons and hot holes generated by non-radiatively decay of localized plasmons; (Right) Effective temperatures of hot carriers linearly are found to increase linearly with V, rather than IV (the Joule heating picture).