Welcome!

The H-Lab is a research group in the School of Engineering and Applied Science at UCLA, led by Professor Yongjie Hu. Motivated by technological challenges, our lab's research focus is on understanding and engineering nanoscale transport phenomena and nanomaterials for wide applications including energy conversion, storage, and thermal management. We use a variety of experimental and theoretical techniques to investigate nanoscale transport processes, with a particular emphasis on design and chemical synthesis of advanced materials, ultrafast optical spectroscopy, pulsed electronics, and thermal spectral mapping techniques. We invite you to look at the website for more details about our work and facilities.



News

Our group developed a new metrology for anisotropic thermal conductivity measurement. Man and Joon's new technique using an elliptical laser beam, i.e., AB-TDTR was published in Rev. Sci. Instruments. Aug 2, 2018

Our group's study on developing high thermal conductivity boron arsenide was published in Science magzine! This is a highly interdisciplinary project that requires precise materials synthesis, comprehensive structural characterizations, accurate thermal transport measurement, and atomistic ab-initio calculations: Congratulations and thank H-Lab students for the hard work to get it done! See UCLA news here. July 05, 2018

Yongjie received 2018 UCLA Faculty Career Development Award. June 21, 2018

Congrationlations to H-Lab student, Joon Sang Kang, for receiving UCLA Northrop Grumman Corporation Scholarship, for his outstanding graduate research! April 16, 2018

Yongjie received NSF CAREER Award! Many thanks for the support from the National Science Foundation!! Feb 21, 2018

Congratulations to H-Lab students, Joon and Huan for passing their PhD Qualifying Exams! Dec 18, 2017

Congratulations on Joon and Huan's paper published in Nano Letters: Thermal and phonon spectral characterization for energy-efficient thermal management! Nov 8, 2017

Congratulations to H-Lab students: Huan, Huu, Man, and Nicolas for all passing their PhD Candidacy Exam! July 18, 2017

Yongjie received the Doctoral New Investigator Award from the American Chemical Society (ACS)! Many thanks for the support from ACS PRF!! June 23, 2017

New paper on high-performance flexible thin film thermoelectric devices published in Advanced Materials. March 29, 2017

Congratulations on Joon and Ming's paper published in Nano Letters! This is the first time demonstration of in situ thermal-electrochemical characterization of a 2D van der Waals material based lithium ion battery. Feb 23, 2017

Our team is awarded a competitive research grant as part of UCLA Sustainable LA Grand Challenge! Many thanks to the Sustainable LA Grand Challenge and the Anthony and Jeanne Pritzker Family Foundation!! Dec 21, 2016

Yongjie has received the Air Force Young Investigator Award! Many thanks for the support from AFOSR!! Oct 12, 2016

Welcome new graduate students joining our lab: Nicolas, Man, Julia, Junyu, Huu, Huan! Sept 15, 2016

Electrical-gate tuning of thermoelectric transport in single heterostructure nanowires is published on J. Appl. Phys. . June 15, 2016

We have received a grant support from the US Department of Energy, together with Professors Pilon, Dunn, and Tolbert. We very much appreciate the generous support from DOE! May 20, 2016

Developing a refined technique to measure phonon mean free path distributions is published on Scientific Reports. Nov 27, 2015

Manuscript posted on arXiv: No energy transport without discord: Quantum correlations are mandatory for any energy transport. Oct 16, 2015

Congratulations for the new paper published on Nature Nanotechnology: spectral mapping of thermal conductivity through nanoscale ballistic heat transfer! June 10th, 2015

Congratulations to Joon for passing his PhD Candidacy Exam! May 15, 2015

Developing ultra-high thermal condutivity material - Boron Arsenide, is published on Appl. Phys. Lett.: the first experimental measurement on its thermal conductivity. Feb 20, 2015

Spotlight

imageExploring the theoretically predicted conductivity limit in defect-free crystals: Experimental observation of high thermal conductivity in boron arsenide

Improving thermal management of small scale devices requires developing materials with high thermal conductivities. The semiconductor boron arsenide (BAs) is an attractive target due to ab initio calculation indicating single crystals have an ultrahigh thermal conductivity. We synthesized BAs single crystals with undetectable defects, and measured a room temperature thermal conductivity of 1300 W/mK. Our spectroscopy study in conjunction with atomistic theory reveals that the unique band structure of BAs allows for very long phonon mean free paths and strong high-order anharmonicity through the four-phonon process. The single-crystal BAs has better thermal properties than other metals and semiconductors. Our study establishes BAs as a benchmark material for thermal management applications, and exemplifies the power of combining experiments and ab initio theory in new materials discovery. July 5, 2018

imageThermal management application using emerging high thermal conductivity materials: Thermal and phonon spectral characterization of synthetic boron phosphide

We demonstrated a systematic synthesis-experimental-modeling approach to investigate high thermal conductivity material for efficient heat dissipation: we have chemically synthesized high-quality boron phosphide single crystals and measured their thermal conductivity as a record-high 460 W/mK at room temperature. We have, for the first time, experimentally measured the phonon mean free path spectra of boron phosphide and analyzed experimental results by solving three-dimensional and spectral-dependent phonon Boltzmann transport equation using the variance-reduced Monte Carlo method. The experimental results are in good agreement with that predicted by multiscale simulations and density functional theory, which together quantify the heat conduction through the phonon mode dependent scattering process. Our finding underscores the promise of boron phosphide as a high thermal conductivity material for thermal management and provides a microscopic-level understanding of the phonon spectra and thermal transport mechanisms of boron phosphide, to enable a rational design of high thermal conductivity materials and nano- to multiscale devices. Nov 8, 2017

imageHigh-performance flexible thermoelectric devices

A solid-state thermoelectric device is attractive for diverse technological areas such as cooling, power generation and waste heat recovery with unique advantages of quiet operation, zero hazardous emissions, and long lifetime. With the rapid growth of flexible electronics and miniature sensors, the low-cost flexible thermoelectric energy harvester is highly desired as a potential power supply. Here, we demonstrated a flexible thermoelectric copper selenide (Cu2Se) thin film through a low-cost and scalable spin coating process for waste heat harvesting applicatons. March 29, 2017

imageFirst in situ thermal-electrochemical characterization of 2D van der Waals materials in a lithium ion battery

This work presents the first time demonstration of in situ thermal-electrochemical characterization of a 2D van der Waals material in a lithium ion battery. A novel approach to integrate ultrafast optical spectroscopy and electrochemical control has been developed to investigate the thermal transport in the 2D material electrode (black phosphorus) during the battery's normal operation process. The study reveals intriguing anisotropic ion-phonon interactions and highly reversible electrochemical control over the thermal properties. Feb 23, 2017

imagePaper published in Nature Nanotechnology

This work presents a table-top pump-probe spectroscopy approach to measure phonon spectral contribution to heat transfer. Classical diffusion theory fails to describe small scale energy transport, such as to gauge the temperature rise in modern electronics and thermal solar energy harvesting devices, but the new work provides detailed quatification of such non-equilibrium energy transport down to its spetral contributions. Experimental measurement is compared with modeling resutls based on transient frequency-dependent Boltzmann transport equation and multi-scale Monte Carlo methods, which enables us to better understand heat transfer and design energy materials. June 10, 2015

imageExperimental study of the proposed super-thermal-conductor: BAs in Applied Physics Letters

This work reports the first thermal measurement of Boron Asenide, the new candidate material of ultra-high thermal conductivity for thermal management based on first-principles prediction. Despite high density of vacancy impurities, our measurement shows a reasonable high thermal conductivity, as an important first step towards developing high-conducting materails. Feb 20, 2015