Ultra-high Resolution Scanning Thermal Microscopy
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 probe energy transport, conversion, and dissipation processes down to the scale of single atoms and molecules, the building blocks of any material. These developments lend us unique opportunities to address open questions towards fundamental understanding of nanoscale thermal, electrical, and optical devices, as well as tackle real-world challenges ranging from heat management, renewable energy, to high performance materials.
Quantum 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.
(Left) heat flow across single atoms of Au showing quantized thermal conductance;
(Right) phonon coherently transport through a single molecule chain of alkane-dithiol
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.
An electrically-biased Atomic Force Microscope tip on contact with a self-assembled monolayer of molecules to generate thermoelectric cooling
Nanoscale Thermal Radiation and Thermophotovoltaics
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.
Hot Carrier Enhanced Energy Conversion and Plasmonics
Leveraging nano-optical effects to enlarge the unique vibrational fingerprints of materials by many orders of magnitude, surface plasmonics and associated plasmon-enhanced spectroscopy lend ultrahigh 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. 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 fabricate and study plasmonically active nanostructures and applications based on plasmon-induced hot-carrier dynamics such as light sources and highly responsive photodetector.