Research Themes

(Selected)

 

1. Advanced Technology Development for Micro/Nanoscale Thermal Probing and Characterization

The objective is to develop advanced platforms to study thermal transport at the micro/nanoscale that is involved in nanomanufacturing, material design, renewable energy application, and material synthesis. Thermal diffusivity, thermal conductivity, and interface thermal contact resistance are three of the most critical properties used for thermal design, material/device performance evaluation, and energy conversion efficiency evaluation (thermoelectric, pyrolysis, and thermogalvanic). When the material size goes down to micro and nanoscale, precise measurement of these properties become extremely challenging. We have been working to develop various advanced techniques to overcome this challenge and conducted unprecedented study of thermal transport down to sub-nm level.  [Details]  

 

2. Thermal Transport down to Sub-nm Scale : Effect of Material Structure

This is a very broad topic, evidenced by a significant number of scholars working in this area world-wide. Our work has been focused on those related to novel material design and structure tailoring, thermoelectric energy conversion, nanoelectronics performance and design. Both experiment and atomistic modeling are being carried out in our lab. Some of our findings/discoveries include the pioneering exploration of the coherent lattice interface in thermoelectric materials, electron tunneling and hopping by organic substrates, and extremely confined electron scattering down to 3-atom thickness. The research is conducted toward nanomanufacturing with optimal quality control, well-defined material property tailoring, new material discovery, mechanisms understanding in energy conversion, and significant efficiency improvement in energy applications. [Details]

 

3. Sub-wavelength to Nanoscale Thermal, Mechanical, and Optical Fields under Optical Near-field Effect

Optical near-field effect has been studied and used widely for surface nanostructuring (nanopatterning, nanomanufacturing), and for surface structure imaging to break-down the diffraction limit. Near-field focusing could concentrate the light to extremely small domains (~ nm). Our work has been focused on modeling, probing, and mapping of the near-field induced temperature, stress, and optical fields. This knowledge is of great importance for system design to achieve scalable nanomanufacturing, and for geometry-design to tailor light-structure interaction. [Details]

 

4. 2D Atomic Layer Interface Energy Coupling

Ripples and corrugations are intrinsic feature of 2D atomic-layer materials, like graphene sheets. 2D complex structure is promising for a broad array of applications including extremely high resolution sensors and high-performance field-effect transistors. Although graphene has a very high in-plane thermal conductivity, its extreme thinness limits its in-plane heat dissipation during device operation. The out-of-plane thermal transport to the adjacent materials plays an important role in heat dissipation for 2D electronics. Extensive computation and novel experimental work are being conducted in our lab to investigate interface energy coupling, and explore the effect of local structure and atomic bonds. [Details]

 

5. Shockwaves in Laser-matter Interaction

In pulsed laser-matter interaction, especially in laser-assisted manufacturing, shockwave could arise when the process is in open-air (non-vacuum). The shockwave will significantly affect the phase explosion, material ablation, plume re-deposition, and solidification. For years, we have been conducting active research to study the formation, inside structure, propagation, and dissipation of shockwaves in pulsed-laser matter interaction. The acquired knowledge is expected to provide the critical information for quality-control and process design in laser-assisted manufacturing. [Details]

 

6. General Physics in laser-matter Interaction

This is a quite long tradition in our lab for studying various physical behavior of target under pulsed laser irradiation. Emphasis has been placed on phase explosion, crystallization, defect formation, stress wave formation and propagation. Atomistic and atomistic-macroscale hybrid modelings have been designed and conducted. [Details]

 


 

 

 

 

A Few Picks

1. Time-domain Differential Raman (TD-Raman)

Recently, we have developed a novel transient thermal characterization technology based on the principles of transient optical heating and Raman probing: time-domain differential Raman. It employs a square-wave modulated laser of varying duty cycle to realize controlled heating and transient thermal probing. Very well defined extension of the heating time in each measurement changes the temperature evolution profile and the probed temperature field at ms resolution. Using this new technique, the transient thermal response of a tipless Si cantilever is investigated along the length direction. A physical model is developed to reconstruct the Raman spectrum considering the temperature evolution, while taking into account the temperature dependence of the Raman emission. By fitting the variation of the normalized Raman peak intensity, wavenumber, and peak area against the heating time, the thermal diffusivity is determined as 9.17x10-5, 8.14x10-5, and 9.51x10-5 m2/s. These results agree well with the reference value of 8.66x10-5 m2/s considering the 10% fitting uncertainty. The time-domain differential Raman provides a novel way to introduce transient thermal excitation of materials, probe the thermal response, and measure the thermal diffusivity, all with high accuracy.

More details can be found in our recent publication: Optics Express, Vol. 23, No. 8 | DOI:10.1364/OE.23.010040, 10040.

2. Frequency-resolved Raman (FR-Raman)

We have developed a new technique for probing the thermal transport in materials using a frequency domain technique based on Raman spectroscopy. In this FR-Raman technique, instead of using a CW laser for heating and simultaneous Raman thermal probing, the laser beam is modulated across a very wide frequency range (up to 1 MHz for our current setup). The thermal response resolution has been pushed to a level of 0.5 us. A theoretical model has been developed for Raman spectrum reconstruction and fitting the Raman property-frequency profile to determine the material's thermal diffusivity/conductivity.

The FR-Raman shares similar concept like the frequency-domain photothermal, photoacoustic, and surface thermal reflectance techniques, but it has very unique advantages such as high selectivity of temperature measurement, least surface preparation, and applicable to nanometer thin materials (2D materials). It significantly extends the scope and capacity of Raman spectroscopy.

More details about this work can be found in our recent publication: Optics Letters, Vol. 41, 80-83, DOI: 10.1364/OL.41.000080.

 

 

Research Support

 

Iowa Energy Center

Department of Enegy

Honeywell Federal Manufacturing & Technologies

Army Research Office

National Science Foundation

California Energy Commission

Office of Naval Research

Air Force Office of Scientific Research

Nebraska Research Initiative

Layman Foundation

Sumitomo Foundation

Nanoconduction, Inc.

University of Nebraska-Lincoln

Iowa State University

 

Position Opening

Two Ph.D. student positions are available in the lab. Highly motivated students with very strong academic backgrounds are encouraged to apply. The starting date can be Spring 2020 or Fall 2020.

 

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