Jun Liu

Our team aims to solve the paradoxical diversity of the CISS effect by elucidating its fundamental physical origins and structure-property-relations in conjugated helical polymers possessing tunable chirality and electrical/thermal conductivities. We envision that the biocompatibility of the helical polymers and their chiral assemblies will provide an ideal platform for the development of bio-inspired spintronic applications and biosensors.

The research goal is to understand how thermal conductivity of ferroelectric materials can be switched with external electric fields and how this switching behavior can be engineered through the reconfigurable ferroelectric domains. This is motivated by the possible new paradigm of energy regulation for harvesting, saving, and management if dynamic control of thermal energy transport can be achieved. Seeking solutions for thermal control in materials are limited by the traditional toolkit of linear, static, and passive thermal components, such as thermal resistors and thermal capacitors. This paucity of thermal options pales in comparison to the rich selection of highly nonlinear, switchable, and active components in the electrical domain. Since switchable and nonlinear thermal components are not nearly as mature as their electrical counterparts, the pursuit of these thermal components remains a motivating interdisciplinary challenge to researchers for years. In this project, we will measure the thermal conductivity- domain structure correlation to reveal the effects of domain walls and domain engineering. The studies on the manipulation of domain walls using domain engineering has translational implications for a range of applications, such as infrared imaging, solid-state cooling, and waste-heat energy conversion due to pyroelectric and electrocaloric effects, and sensors based piezoelectric effects.

The research goal is to understand the microscopic mechanisms of the vibrational energy transport and push the lower limit of thermal conductivity in layered materials. As a general scientific interest, a large and ongoing effort has been set forth to expand the limits of heat conduction. On one end of the spectrum, materials with low thermal conductivity have many applications such as thermal insulation and thermoelectric. Even though the amorphous phases of materials usually exhibit low thermal conductivity, the record-low thermal conductivity in fully dense solid, ~0.05 W m-1 K-1, has been found in two crystalline materials: disordered layered WSe2 crystals and fullerene derivative. Can we further push the lower limit to achieve exceptionally-low thermal conductivity? We will focus on layered materials, such as MoS2, to specifically test two hypotheses : (1) Expanding the interlayer spacing would further decrease the thermal conductivity due to the weakened interlayer coupling and enhanced bonding anisotropy; (2) inhomogeneous variation of interlayer spacing can create a much larger reduction in thermal conductivity than the homogenous one. If successful, the proposed strategy combining bonding anisotropy and inhomogeneous layer expansion to achieve record-low thermal conductivity could be potentially applied for thermal insulation or thermoelectrics. Moreover, they also provide transformative opportunities to develop thermal functional materials with thermal conductivities widely tuned by external stimuli, due to the reversibility of electrochemistry process.

Understanding and controlling surface phenomena at solid-liquid interfaces is of primary importance to petroleum engineering. One of the remaining challenges is to design durable and scalable engineered surfaces with a strong repellency to the broadest range of liquids. Recent experimental results demonstrated that surfaces grafted with covalently attached polymer brushes will sustainably remove liquids without air or liquid lubrication. However, the molecular-level understanding of the underlying mechanisms and a clear design guideline is missing. The goal of this proposal is to understand, at the molecular level, how surfaces grafted with polymer brushes will passively remove liquids, regardless of their intrinsic wettability. To provide a comprehensive molecular-level picture of interactions between polymer brushes and liquid molecules, molecular dynamics simulations will be conducted to control the properties of polymer brushes, such as inter-tether distance, chain-backbone flexibility, and terminal functional group, and then correlate the static and dynamic contact angles with these properties. By elucidating the roles of properties of polymer brushes, the proposed research will provide fundamental guidance on how to achieve a non-textured liquid-like grafted surface with tunable surface functionality.

Part of the mission of the NSF, and in particular the Engineering Directorate, is to attract young talented researchers and to mentor them for a career in science and engineering. In the spirit of this mission, this proposal seeks NSF travel funding for 43 students and postdocs to attend the 2019 ASME International Mechanical Engineering Congress and Exposition (IMECE) in Salt Lake City, UT so they can present their research at the society-wide micro-nano poster forum. The ASME Society-Wide Micro and Nano Technology Forum (Topic 17-15) focuses on new developments in the field of micro and nanoengineering and sciences. As co-chairs of this productive gathering, we value the chance to support these upcoming researchers as they showcase their scientific accomplishments, interact with their peers and extend their network within the broader academic community. This support will create future career opportunities for students and postdoctoral researchers such as faculty and researcher appointments and positions within industry. Priority will be given to student participants who are women or who come from underrepresented groups, and to undergraduate students, so as to promote diverse participation at the ASME conference, in the short term, and in STEM fields, more long term.

The space exploration capability, especially for imaging missions, greatly depends on the performance of onboard memory system. The next-generation memory chip for space missions requires larger data capacity and higher data density with lower power consumption and shorter access time. One promising candidate is the spin-transfer-torque (STT) based spintronic memory device. The objective of this program is to: (1) experimentally study the mechanism of the novel thermal-induced STT and demonstrate the ability to locally manipulate a memory bit; (2) train undergraduate and graduate students with the fundamental knowledge and technical skills in this area; and (3) collect preliminary experimental data for NASA fellowship and grants.