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 fundamental transport phenomena and new materials for wide applications including energy, electronics, thermal management, and micro/nano sensors. We use a variety of experimental and theoretical techniques to investigate nanoscale transport processes and develop device applications, 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.


Thanks for the nice article in Scientific American that highlights our research. Jan 8, 2024

See UCLA news highlight on our recent study. Nov 9, 2023

Our study on dynamic tunning over thermal transport is published in Science! Congratulations to Man, Huan, Zihao, Huu, and wonderful collaboration team! Nov 3, 2023

Congrats to Huu Nguyen for passing the PhD exam! Nov 2, 2023

Congrats to our recent PhD graduate Ming Ke for starting her career at NASA Goddard Center! October 1, 2023

Welcome new PhD students: Chuanjin and Zhihan joined the group from Tsinghua and Shanghai Jiaotong. Sept 1, 2023

Our research has been highlighted by Scientific American, National Science Foundation, and National Academy of Engineering. April 28, 2023

See UCLA news highlight on our recent study. Nov 28, 2022

Our study on thermal energy transport under extreme conditions is published in Nature! Congratulations to Suixuan, Zihao, Huan, Man! Nov 30, 2022

Man and Lingyun published on machine Learning for harnessing thermal energy. Oct 1, 2022

Congratulations to Joon for starting as an assistant professor in KAIST. Best of luck! September 1, 2022

Zihao, Man, and Jessica's paper published on thermal management materials for energy-efficient and sustainable future buildings. Oct 19, 2021

Our research published in Nature Electronics: Our work developed heterogenous integration of BAs with wide bandgap semiconductors for device cooling of AlGaN/GaN HEMTs. Congratulations to Joon, Man, Huan, and Huu! June 18, 2021

Our research published in Nature Communications: Ying's paper on developing flexible thermal management for electronics and soft robotics. February 28, 2021

Huan and Hang's paper on ultrahigh thermal conductivity was accepted for publication. January 1, 2021

Congrats to H-Lab students, Ying, Zihao, and Suixuan for all passing the PhD Qualifying Exam! Dec 21, 2020

UCLA Samueli School of Engineering recognized Yongjie with the Vernroy Makoto Watanabe Excellence in Research Award for outstanding accomplishments in research. Feb 22, 2020

Our work on demonstrating complementary doping of van der Waals materials for monolithically integrated electronic circuits was published. Jan 2020

Ying and Man's paper on thermal inferface materials for nanoelectronics was published. Jan 2020

Yongjie received the 2019 Bergles-Rohsenow Young Investigator Award in Heat transfer. Thanks for the recognition by ASME! Nov 12, 2019

We developed an experimental database that reports systematically the optical, mechanical, and thermal properties of cubic boron arsenide. The work was published and featured in Appl. Phys. Lett. Also see the APL Scilight. Sept 16, 2019

Hang, Huan, and Lucas' theory work on "Ab initio investigation of single-layer high thermal conductivity boron compounds" was published in Phys. Rev. B. July 24, 2019

Joon and Huan's work on investigating the intrinsic ultralow thermal conductivity and phonon renormalization due to strong anharmonicity of tin selenide is published in Nano Letters. July 2, 2019

Man's work on studying interface energy transport and anisotropic thermal boundary resistance for near-junction thermal management was published in Advanced Materials. June 24, 2019

Yongjie is selected to participate in the National Academy of Engineering's 2019 U.S. Frontiers of Engineering. June 1, 2019

Our work was published on developing a facile ambient process for manufacturing ultralight, flexible, and superhydrophobic thermal insulator. May 27, 2019

H-Lab student Joon Sang Kang has been awarded the Gold medal for this year's MRS Graduate Student Award for his outstanding PhD research. Congratulations! April 30, 2019

Yongjie is selected as 2019 Alfred P. Sloan Research Fellow! Many thanks for the honor and support from Sloan Foundation! Feb 19, 2019

Huu's work on developing nanoscale sensors and systems beyond solid state electronics was published online: high-performance emitter for nanoscale vacuum channel transistors. Jan 22, 2019

Nicolas's work was accepted for publication, demonstrating flexible thin film devices to recover lower-grade waste heat and power electronics and sensors. Jan 21, 2019

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

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

Our new 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

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


imageEllectrically Gated Thermal Switch

Controlling heat flow is a key challenge for electronics, sustainable energy, industrial processing, and thermal therapy. We demonstrate a first-of-its-kind electronically gated three-terminal device, allowing continuously and reversibly regulate heat flow by an electric field through carefully controlled chemical bonding and charge distributions within the molecular interface. The devices have ultrahigh switching speeds above 1 megahertz, have on/off ratios in thermal conductance greater than 1300%, and can be switched more than 1 million times. We anticipate that these advances will generate opportunities in molecular engineering for thermal management systems and thermal circuit design. November 3, 2023

imageThermal Energy Transport Under Extreme Conditions

We observed a new physics principle governing how thermal energy transport under extreme conditions. We performed in situ ultrafast optics, Raman spectroscopy, and inelastic X-ray scattering measurements to examine the phonon bandstructure evolution and confirmed the anomalous high-pressure behaviour from competitive heat conduction channels from interactive high-order anharmonicity physics. Our spectroscopy study combining atomistic theory represents as a powerful approach to probe complex phonon physics and provide fundamental insights for understanding microscopic energy transport in materials of extreme properties. December 8, 2022

imageIntegration of Boron Arsenide Cooling Substrates Into Gallium Nitride Devices for High Power Electronics

Our study demosntrate heterogenous integeration of BAs and BP cooling substrates with wide-bandgap gallium nitride high-electron-mobility transistor devices. GaN-on-BAs structures exhibit a high thermal boundary conductance of 250MW/m2K, and comparison of device-level hot-spot temperatures with length-dependent scaling (from 100 um to 100 nm) shows that the power cooling performance of BAs exceeds that of reported diamond devices. Furthermore, operating AlGaN/GaN high-electron-mobility transistors with BAs cooling substrates exhibit substantially lower hot-spot temperatures than diamond and silicon carbide at the same transistor power density, illustrating their potential for use in the thermal management of radiofrequency electronics. June 17, 2021

imageSelf-Assembled Manufacturing for Thermal Management of Wearable Electronics and Robotics

As an emerging field, thermal management for wearable electronics and robots requires advanced building blocks to provide not only efficient heat dissipation but also high softness and flexibility. Using new materials, modeling design, and self-assembled manufacturing, we report high performance thermal management with excellent elastic compliance similar to that of soft biological tissues down to 100 kPa, high flexibility, and preserves over at least 500 bending cycles a high thermal conductivity over about 100 times improvement of a typical polymer. The superior cooling performance has been verified by device integration with power LEDs to show a 45 'C reduction in the hot spot temperature. Together, this study demonstrates scalable manufacturing of a new generation of energy-efficient and flexible thermal materials that holds great promise for advanced thermal energy technologies. March 12, 2021

imageUltrahigh Thermal Conductivity Identified in Ternary Compounds

Building on our previous efforts, this work investigates the fundamental lattice vibrational spectra and thermal conductivity of ternary compounds involving four-phonon process. We find an ultrahigh room-temperature thermal conductivity through strong carbon-carbon bonding up to 2100 W/mK beyond most common materials and the recently discovered boron arsenide. This study provides fundamental insight into the atomistic design of thermal conductivity and opens up opportunities in new materials searching towards complicated compound structures. March 12, 2021

imageIntrinsic low thermal conductivity and phonon renormalization due to strong anharmonicity of single-crystal tin selenide

This study reports the experimental measurements and theoretical calculations of phonon transport in the record-high thermoelectric material, i.e. SnSe but focuses on its undoped and fully dense single crystals. To investigate the phonon anharmonicty, we developed experimental approach combining picosecond ultrasonics and X-ray diffraction, to enable direct measurement of temperature-dependent sound velocity, thermal expansion coefficient, and Gruneisen parameter. The measured Gruneisen parameter suggests an abnormally large temperature effect on phonon dispersion that contributes to over 90% of phonon frequency shifts. Furthermore, we performed ab initio calculations using different methods: in comparison with self-consistent phonon theory, the harmonic and quasi-harmonic models that have been widely used in current phonon calculations, fail to accurately predict these important thermophysical properties. Our study reveals an extremely strong intrinsic anharmonicity in SnSe that introduces phonon renormalization near room temperature. This study represents an important research benchmark in characterizing high-performance thermal energy materials, and provides fundamental insight into advancing modern calculation methods for phonon transport theory. July 4, 2019

imageObservation of anisotropic thermal boundary resistance for near-junction thermal management: experiment and atomistic modeling of interfacial energy transport

Interfacial thermal boundary resistance (TBR) plays a critical role in near-junction thermal management of modern electronics. In particular, TBR can dominate heat dissipation and has become increasingly important due to the continuous emergence of novel nanomaterials with promising electronic and thermal applications. A highly anisotropic TBR across a prototype 2D material, i.e., black phosphorus, is reported through a crystal orientation-dependent interfacial transport study. The measurements show that the metal-semiconductor TBR of the cross-plane interfaces is 241% and 327% as high as that of the armchair and zigzag direction-oriented interfaces, respectively. Atomistic ab initio calculations are conducted to analyze the anisotropic and temperature-dependent TBR using density functional theory (DFT)-derived full phonon dispersion relation and molecular dynamics simulation. The measurement and modeling work reveals that such a highly anisotropic TBR can be attributed to the intrinsic band structure and phonon spectral transmission. Furthermore, it is shown that phonon hopping between different branches is important to modulate the interfacial transport process but with directional preferences. A critical fundamental understanding of interfacial thermal transport and TBR-structure relationships is provided, which may open up new opportunities in developing advanced thermal management technology through the rational control over nanostructures and interfaces. June 24, 2019

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