@article{kim_vetter_yan_yang_wang_sun_yang_comstock_li_zhou_et al._2023, title={Chiral-phonon-activated spin Seebeck effect}, volume={2}, ISSN={["1476-4660"]}, url={http://dx.doi.org/10.1038/s41563-023-01473-9}, DOI={10.1038/s41563-023-01473-9}, abstractNote={Utilization of the interaction between spin and heat currents is the central focus of the field of spin caloritronics. Chiral phonons possessing angular momentum arising from the broken symmetry of a non-magnetic material create the potential for generating spin currents at room temperature in response to a thermal gradient, precluding the need for a ferromagnetic contact. Here we show the observation of spin currents generated by chiral phonons in a two-dimensional layered hybrid organic-inorganic perovskite implanted with chiral cations when subjected to a thermal gradient. The generated spin current shows a strong dependence on the chirality of the film and external magnetic fields, of which the coefficient is orders of magnitude larger than that produced by the reported spin Seebeck effect. Our findings indicate the potential of chiral phonons for spin caloritronic applications and offer a new route towards spin generation in the absence of magnetic materials.}, journal={NATURE MATERIALS}, publisher={Springer Science and Business Media LLC}, author={Kim, Kyunghoon and Vetter, Eric and Yan, Liang and Yang, Cong and Wang, Ziqi and Sun, Rui and Yang, Yu and Comstock, Andrew H. and Li, Xiao and Zhou, Jun and et al.}, year={2023}, month={Feb} }
 @article{kim_kim_garcia_fang_jiang_2021, title={Liquid metallic laser ultrasound transducer for high-temperature applications}, volume={118}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0046052}, DOI={10.1063/5.0046052}, abstractNote={This study aims to investigate a laser ultrasound (LUS) transducer for high-temperature (>100 °C) applications. For decades, many researchers have investigated efficient LUS transducers, yet studies on laser ultrasound transducers capable of performing at the high-temperature condition are rarely found in the literature. Most current LUS transducers still utilize a polymer-based composite material, that is, not stable at varying temperature conditions. This study introduces a liquid metallic (LM) LUS transducer that utilizes field's metal, which has a high thermal expansion (∼3 × 10−4 K−1). We hypothesized that such a high thermal expansion of the liquid metal can effectively produce laser-generated ultrasound waves, substituting for conventional polymer-based transducers. A numerical simulation predicted that the LM LUS transducer would produce higher LUS intensity (∼22 dB) than that without the LUS transducer. Experiment results confirmed that the LM transducer effectively intensifies the ultrasound wave signals, obtaining a signal-to-noise gain over 30 dB. Moreover, the transducer was found capable of transmitting detectable wave packets in relatively high-temperature conditions (∼400 °C), while conventional candle soot nanoparticle-polydimethylsiloxane could not perform stably at these elevated temperatures. The investigations introduced in this article are scientifically significant since we demonstrated the engineering feasibility of liquid metallic materials for LUS transducers.}, number={18}, journal={APPLIED PHYSICS LETTERS}, author={Kim, Howuk and Kim, Kyunghoon and Garcia, Nicholas and Fang, Tiegang and Jiang, Xiaoning}, year={2021}, month={May} }
 @article{subramanyan_zhang_he_kim_li_liu_2019, title={Role of angular bending freedom in regulating thermal transport in polymers}, volume={125}, ISSN={["1089-7550"]}, url={http://dx.doi.org/10.1063/1.5086176}, DOI={10.1063/1.5086176}, abstractNote={Polymers, despite their desirable structural properties, suffer from low thermal conductivity, which restricts their use. Previous studies have indicated that the strong bond-stretching and angular-bending interactions along the chain are believed to have saturated the maximum achievable thermal conductivity in the along-the-chain direction. Contrary to this belief, our results show an improvement in thermal conductivity. By increasing the bond and angle potential, we studied the effect on the thermal conductivity of polyethylene using non-equilibrium molecular dynamics simulations. In comparison to restricting the bond stretching, we found that restricting angular bending freedom plays a crucial role in improving the thermal transport along the chain. We observed significant changes in the morphology of the polyethylene chains when the angle potential was increased. We also found a remarkable increase in the phonon group velocity accompanied by large shifts in the longitudinal acoustic branch of the dispersion curve. These results when coupled with the structural changes strongly support the argument that thermal conductivity can be controlled by restricting the angular bending freedom.}, number={9}, journal={JOURNAL OF APPLIED PHYSICS}, author={Subramanyan, Harish and Zhang, Weiye and He, Jixiong and Kim, Kyunghoon and Li, Xiaobo and Liu, Jun}, year={2019}, month={Mar} }
 @article{zhou_hao_clark_kim_zhu_liu_cheng_li_2019, title={Sono-Assisted Surface Energy Driven Assembly of 2D Materials on Flexible Polymer Substrates: A Green Assembly Method Using Water}, volume={11}, ISSN={["1944-8252"]}, url={http://dx.doi.org/10.1021/acsami.9b10469}, DOI={10.1021/acsami.9b10469}, abstractNote={The challenges in achieving a green and scalable integration of two dimensional (2D) materials with flexible polymer substrates present a major barrier for the application of 2D materials such as graphene, MoS2, and h-BN for flexible devices. Here, we create a sono-assisted surface energy driven assembly (SASEDA) method that can achieve foot-scale to micrometer-scale assembly of 2D materials, form a conductive network in as short as 10 seconds, and build hierarchical and hybrid flexible devices such as sensors, resistors, and capacitors by using water as the dispersion solvent. SASEDA highlights two counter-intuitive innovations. First, we use an "unfavorable" solvent (i.e. water) for both 2D materials (e.g. graphene, MoS2, h-BN) and polymer substrates (e.g. polydimethylsiloxane) to drive the assembly process. Second, we use a weak sono-field (0.3 W/cm2) generated by a regular sonication bath cleaner to enhance the assembly efficiency and reorganize and unify the assembly network. This method and its principle pave the way towards affordable large-scale 2D materials-based flexible devices.}, number={36}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Zhou, Dong and Hao, Ji and Clark, Andy and Kim, Kyunghoon and Zhu, Long and Liu, Jun and Cheng, Xuemei and Li, Bo}, year={2019}, month={Sep}, pages={33458–33464} }
 @article{he_kim_wang_liu_2018, title={Strain effects on the anisotropic thermal transport in crystalline polyethylene}, volume={112}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.5010986}, DOI={10.1063/1.5010986}, abstractNote={Thermal transport in the axial direction of polymers has been extensively studied, while the strain effect on the thermal conductivity, especially in the radial direction, remains unknown. In this work, we calculated the thermal conductivity in the radial direction of a crystalline polyethylene model and simulated the uniaxial strain effect on the thermal conductivity tensor by molecular dynamics simulations. We found a strong size effect of the thermal transport in the radial direction and estimated that the phonon mean free path can be much larger than the prediction from the classic kinetic theory. We also found that the thermal conductivity in the axial direction increases dramatically with strain, while the thermal conductivity in the radial direction decreases with uniaxial strain. We attribute the reduction of thermal conductivity in the radial direction to the decreases in inter-chain van der Waals forces with strains. The facts that the chains in the crystalline polyethylene became stiffer and more ordered along the chain direction could be the reasons for the increasing thermal conductivity in the axial direction during stretching. Besides, we observed longer phonon lifetime in acoustic branches and higher group velocity in optical branches after uniaxial stretching. Our work provides fundamental understandings on the phonon transport in crystalline polymers, the structure-property relationship in crystalline polymers, and the strain effect in highly anisotropic materials.}, number={5}, journal={APPLIED PHYSICS LETTERS}, author={He, Jixiong and Kim, Kyunghoon and Wang, Yangchao and Liu, Jun}, year={2018}, month={Jan} }
 @article{shourav_kim_kim_kim_2016, title={Wide field-of-view fluorescence imaging with optical-quality curved microfluidic chamber for absolute cell counting}, volume={7}, number={7}, journal={Micromachines}, author={Shourav, M. K. and Kim, K. and Kim, S. and Kim, J. K.}, year={2016} }
 @article{kim_kim_2015, title={Visualization of Biosurfactant Film Flow in a Bacillus subtilis Swarm Colony on an Agar Plate}, volume={16}, ISSN={["1422-0067"]}, DOI={10.3390/ijms160920225}, abstractNote={Collective bacterial dynamics plays a crucial role in colony development. Although many research groups have studied the behavior of fluidic swarm colonies, the detailed mechanics of its motion remains elusive. Here, we developed a visualization method using submicron fluorescent beads for investigating the flow field in a thin layer of fluid that covers a Bacillus subtilis swarm colony growing on an agar plate. The beads were initially embedded in the agar plate and subsequently distributed spontaneously at the upper surface of the expanding colony. We conducted long-term live cell imaging of the B. subtilis colony using the fluorescent tracers, and obtained high-resolution velocity maps of microscale vortices in the swarm colony using particle image velocimetry. A distinct periodic fluctuation in the average speed and vorticity of flow in swarm colony was observed at the inner region of the colony, and correlated with the switch between bacterial swarming and growth phases. At the advancing edge of the colony, both the magnitudes of velocity and vorticity of flow in swarm colony were inversely correlated with the spreading speed of the swarm edge. The advanced imaging tool developed in this study would facilitate further understanding of the effect of micro vortices in swarm colony on the collective dynamics of bacteria.}, number={9}, journal={INTERNATIONAL JOURNAL OF MOLECULAR SCIENCES}, author={Kim, Kyunghoon and Kim, Jung Kyung}, year={2015}, month={Sep}, pages={20225–20238} }