Multiple PhD/postdoc positions open for 2021

We have multiple PhD/postdoc position openings for Spring/Fall 2021. Please check the Openings page of on our website for details.


1 Funded Projects (NSF, ACS PRF, NRC, and NASA)

1.1 Phonon transport in metamaterials: thermal management, thermoelectrics, etc.


NSF Thermal Transport Processes program (07/2021-06/2026).


Both thermoelectric and thermal barrier materials must have low thermal conductivity, while a high thermal conductivity is desired for many thermal management applications. The increase in the thermoelectric figure of merit ZT based on nanostructuring has reached a plateau, largely because of the strongly coupled electrical conductivity, Seebeck coefficient, and the thermal conductivity of materials. Metamaterials provides a promising route to decouple electron transport and phonon transport. Notably, it provides strong phonon scatterings and possibly induce strong phonon localization. Our group work extensively on designing novel structures using molecular dynamics, lattice dynamics, and Boltzmann equations aided by first-principles methods, and, machine learning (of course!). We aim to develop nanostructured (meta)materials with strong localization of phonons but preserving low scattering of electrons, which will eventually lead to advanced thermoelectric materials with high ZT that could replace traditional power generators and coolers.


NSF CAREER Award recipient Yan Wang investigates phonon waves to develop effective thermal engineering strategies

Publications to date (more updated publication list at our Publications page)

[5] Tengfei Ma, Cheng-Te Lin, and Yan Wang*, "The Dimensionality Effect on Phonon Localization in Graphene/Hexagonal Boron Nitride Superlattices," 2020 2D Mater. 7 035029.

  • Highlights: How does the 3rd dimension affect coherent phonon transport and localization? Is the lower thermal conductivity of aperiodic superlattice than its periodic counterpart caused by localization or scattering? How does each phonon mode behave in an aperiodic superlattice? We have provided some preliminary answers in this paper (and there are many more to explore in the future). 

[4] Pranay Chakraborty, Yida Liu, Tengfei Ma, Xixi Guo, Lei Cao, Run Hu*, and Yan Wang*, "Quenching Thermal Transport in Aperiodic Superlattices: a Molecular Dynamics and Machine Learning Study," ACS Appl. Mater. Interfaces 2020, 12, 7, 8795-8804.

  • Thanks to the recent advancements in machine learning, we were able to use this technique to study the aperiodic superlattice structure, which has a tremendously large design space. Highlights: We demonstrated the effectiveness of the neural network method for predicting the thermal conductivity of aperiodic superlattices. We also proposed a few effective parameters to quantify the randomness of the configuration.

[3] Pranay Chakraborty, Lei Cao, and Yan Wang*, "Ultralow lattice thermal conductivity of the random multilayer structure with lattice imperfection," Sci. Rep. 7 (1), 8134 (2017).

  • Highlights: Randomizing the layer thicknesses in superlattices can localize (a fancier word used nowadays, "quench") the coherent phonons, how to deal with the remaining incoherent phonons? We proposed to use point defects to scatter rather shorter-wavelength incoherent phonons and reduced the thermal conductivity even further. 

[2] Yan Wang, Chongjie Gu, and Xiulin Ruan, "Optimization of the random multilayer structure to break the random-alloy limit of thermal conductivity," Appl. Phys. Lett., 106, 073104, 2015.

  • Highlights: We systematically investigated and proposed physics-informed strategies to optimize aperiodic superlattices, also called random multilayers, toward lower lattice thermal conductivity.

[1] Yan Wang, Haoxiang Huang, and Xiulin Ruan, "Decomposition of coherent and incoherent phonon conduction in superlattices and random multilayers," Phys. Rev. B 90, 165406, 2014.  

  • Highlights: We found greatly reduced thermal conductivity in aperiodic superlattices than their periodic counterparts; we established a rigorous two-phonon model to quantify the contribute of coherent phonons, which can be localized in aperiodic superlattices, and incoherent phonons, which are scattered at the interfaces, to the overall thermal transport.


1.2 Thermal transport in materials under ultrahigh-repetition-rate laser processing


NSF Thermal Transport Processes program (07/2020-06/2023).

Project title: CDS&E: Nanoconfined Heating via Ultrahigh-repetition-rate Lasers for Enhanced Surface Processing

The overarching goals of this project are to predict and control the depth of the heat-affected zone during ultrahigh-repetition-rate laser processing, to model the unique microstructure behaviors of laser-material interactions under extreme conditions, and to develop and apply advanced thermomechanical models to predict the material responses to laser processing. Specifically, the research team will develop, validate, and share advanced computational models for predicting thermal transport behaviors for a broad range of materials under pulsed laser heating at repetition rates up to the terahertz regime. 


1.3 Optical absorption, chemical reaction, and thermal transport in laser-based atomic-scale drilling process for graphene-based supercapacitors


NSF Manufacturing Machines & Equipment program (09/2018-09/2021).

Project title: Collaborative Research: Photon-Enabled Atomic Drilling of Graphene for Supercapacitors

The goal of this project is to establish a novel photon-enabled atomic drilling process to fabricate porous graphene frameworks toward supercapacitor applications. The photon-enabled atomic drilling approach utilizes photons from a nanosecond pulsed laser to initiate atomic drilling of nanoholes in the graphene basal planes for porous graphene framework fabrication with high efficiency and controllability. The major research objective is to establish a fundamental understanding of the mechanisms involved in the photon-enabled atomic drilling process responsible for the formation of porous graphene frameworks. A computational model capable of predicting photon-enabled bond excitation and dissociation as affected by the laser processing parameters will be developed.


1.4 Thermal transport in clathrate hydrate: phonons, lattice instability, structural disorder, and more


ACS Petroleum Research Foundation Doctoral New Investigator award (09/2019-09/2021) 

Project title: Thermal Transport in Clathrate Hydrate: Role of Lattice Instability and Irregularity

We will investigate the thermal properties of clathrate hydrates under various thermodynamic conditions using atomistic methods and first-principles calculations. Particular attention will be paid to the effect of lattice instability and irregularity on thermal transport in this complex but interesting and important category of materials that are of significant interest to the petroleum industry. 



2 General Topics

2.1 Phonon-phonon and phonon-electron interactions in metals and semiconductors

Phonon (quantum of lattice vibrations) and electron are the two major heat carriers in most metals and semiconductors. We use and develop various approaches, including molecular dynamics (and two-temperature molecular dynamics), Boltzmann transport equation, atomistic Green's function, and first-principles (based on the density-functional theory, or DFT) approaches to investigate the transport of phonons and electrons as well as the interactions between these heat carriers. 

Publications to date (more updated publication list at our Publications page)

[8] (Invited Review) Tengfei Ma, Pranay Chakraborty, Xixi Guo, Lei Cao, and Yan Wang*, “First-principles Modeling of Thermal Transport in Materials: Achievements, Opportunities, and Challenges", International Journal of Thermophysics 41, no. 1 (2020): 1-37.

  • Highlights: A very interesting (I believe) and quite comprehensive review paper by us. 

[7] Xiangyu Li, Wonjun Park, Yan Wang, Yong P. Chen, and Xiulin Ruan*, "Reducing Interfacial Thermal Resistance between Metal and Dielectric Materials by a Metal Interlayer," Journal of Applied Physics 125, 045302 (2019)

  • Highlights: This is an experimental proof of our theoretical prediction in Ref. [2] below (or, J. Appl. Phys. 119, 065103 (2016)). 

[6] Pranay Chakraborty, Guoping Xiong, Lei Cao*, and Yan Wang*, "Lattice thermal transport in superhard hexagonal diamond and wurtzite boron nitride: A comparative study with cubic diamond and cubic boron nitride," Carbon, vol. 139, pp. 85-93, (2018)

  • Highlights: Diamond is in a cubic lattice (fcc primitive unit cell containing two basis atoms) in most cases, while recently hexagonal diamond (hcp primitive unit cell containing four basis atoms) was found. Still, cubic diamond and hexagonal diamond have the same C-C bonds (almost the same strength), should they have the same lattice thermal conductivity?  Our short answer is no. Specifically, the hexagonal diamond has much denser phonon dispersion relations (smaller phonon bandgaps), leading to larger phonon scattering phase space and thus lower thermal conductivity.  

[5] Zexi Lu, Yan Wang, and Xiulin Ruan, "The critical particle size for enhancing thermal conductivity in metal nanoparticle-polymer composites," J. Appl. Phys. 123 (7), 074302 (2018)

  • Highlights: We built analytical models to predict the optimal particle size for maximizing the thermal conductivity of metal nanoparticle-polymer composites. As we demonstrated, electron-phonon nonequilibrium near the metal-polymer interfaces is important. 

[4] Yan Wang, Zexi Lu, and Xiulin Ruan, "First principles calculation of lattice thermal conductivity of metals considering phonon-phonon and phonon-electron scattering," J. Appl. Phys. 119, 225109 (2016)

  • Highlights: Electrons can scatter phonons so significantly in certain metals (Ni, Cr, Ti, Pt, etc.) that the lattice thermal conductivity is much lower than what's expected from first-principles calculations only consider phonon-phonon scatterings. 

[3] Zexi Lu, Yan Wang, and Xiulin Ruan, "Metal/dielectric thermal interfacial transport considering cross-interface electron-phonon coupling: Theory, two-temperature molecular dynamics, and thermal circuit," Phys. Rev. B 93, 064302 (2016)

  • Highlights: Cross-interface electron-phonon coupling can also be important! 

[2] Yan Wang, Zexi Lu, Ajit Roy, and Xiulin Ruan, "Effect of interlayer on interfacial thermal transport and hot electron cooling in metal-dielectric systems: an electron-phonon coupling perspective," J. Appl. Phys. 119, 065103 (2016)

  • Highlights: Electron-phonon nonequilibrium near metal-nonmetal interface imposes a resistance to thermal transport. In this paper, we proposed to use a metallic interlayer with strong electron-phonon coupling between metal-semiconductor interface to enhance the interfacial thermal transport (and the effect is significant!). Also, we build a two-temperature Boltzmann transport equation method to simulate heat transfer in metal-semiconductor systems. We are glad to share the code upon request (we may open-source it on when we have some free time). 

[1] Yan Wang, Xiulin Ruan, and Ajit K. Roy, "Two-temperature nonequilibrium molecular dynamics simulation of thermal transport across metal-nonmetal interfaces," Phys. Rev. B, 85, 205311, 2012

  • Highlights: We developed a two-temperature non-equilibrium molecular dynamics approach to model electron-phonon coupled thermal transport across metal-nonmetal interfaces. Want to try it? Play with our open-sourced online two-temperature molecular dynamics simulator on  


2.2 Light (more generally, electromagnetic waves including laser, sunlight, and microwave)-matter interaction 

We strive to understand how light and microwave interact with materials, e.g., how they are absorbed, how they affect the electron and phonon properties, how they affect the microstructure and mechanical properties of materials, and how they affect the stability of materials using the density-functional theory, molecular dynamics, lattice dynamics, and experimental approaches. These topics and the resulting research findings will be important for laser-based additive manufacturing of metals and semiconductors, solar-thermal energy conversion, and photochemical processing of materials. 

Publications to date (more updated publication list at our Publications page)

[5] Dini Wang, Rui Dai, Xing Zhang, Lei Liu, Houlong Zhuang, Yongfeng Lu, Yan Wang, Yiliang Liao, Qiong Nian*, "Scalable and controlled creation of nanoholes in graphene by microwave-assisted chemical etching for improved electrochemical properties," Carbon, 2020.

  • Highlights: Using microwave to locally facilitate the chemical etching of graphene, through which nanoholes with controllable geometric parameters can be obtained. The resulting porous graphene structure will be useful as supercapacitors.    

[4] Tengfei Ma, Pranay Chakraborty, Xixi Guo, Lei Cao, and Yan Wang*, “First-principles Modeling of Thermal Transport in Materials: Achievements, Opportunities, and Challenges", International Journal of Thermophysics 41, no. 1 (2020): 1-37. PDF

  • Highlights: Our review paper on state-of-the-art modeling methodologies and their applications, including for laser-based additive manufacturing.   

[3] Wu S, Xiong G, Yang H, Tian Y, Gong B, Wan H, Wang Y, Fisher TS, Yan J, Cen K, Bo Z. Scalable Production of Integrated Graphene Nanoarchitectures for Ultrafast Solar-Thermal Conversion and Vapor Generation. Matter. 2019 Oct 2;1(4):1017-32.

  • Highlights: Scalable production of hierarchical structures for water desalination.   

[2] Amir Hassan Zahiri, Pranay Chakraborty, Yan Wang, Lei Cao*, "Strong strain hardening in ultrafast melt-quenched nanocrystalline Cu: The role of fivefold twins," Journal of Applied Physics 126, 075103 (2019)

  • Highlights: We predicted that ultrafast melt-quenching of Cu can produce extensive five-fold twins, leading to strong strain hardening in the material.   

[1] Shenghao Wu, Guoping Xiong, Huachao Yang, Biyao Gong, Yikuan Tian, Chenxuan Xu, Yan Wang, Timothy Fisher, Jianhua Yan, Kefa Cen, Tengfei Luo, Xin Tu, Zheng Bo*, Kostya Ostrikov, " Multifunctional Solar Waterways: Plasma‐Enabled Self‐Cleaning Nanoarchitectures for Energy‐Efficient Desalination," Advanced Energy Materials 2019, 9, 1901286

  • Highlights: A hierarchical structure demonstrating efficient absorption of sunlight and conversion to heat that vaporizes water quickly. The structure could be useful for water desalination.   


More to come... Still under construction.