Li Shi - US grants

The proposal was submitted in response to the FY2002 Chemical and Transport Systems equipment solicitation, described in NSF Announcement. NSF 01-93. The PIs propose to acquire two pieces of equipment that will establish a significant experimental capability in micro/nanoscale transport phenomena at the University of Texas at Austin. Research activities enabled by this equipment include thermal property measurements of nanostructures and in low conductivity dielectric films, investigation of heat dissipation mechanisms in carbon nanotubes, and near-field laser manufacturing and nanoscale fluorescence imaging. Application of these nanostructures, including carbon nanotubes and semiconductor nanowires, in the areas o nanoelectronics, optoelectronics, and thermoelectric cooling has great technological potoential. The equipment will also be utilized in new graduate courses at U.T.-Austin. Funding is from the Thermal Transport and Thermal Processing of the Chemical and Transport Systems Division.

Abstract

Proposal Number: CTS-0239179
Principal Investigator: Li Shi
Affiliation: University of Texas
Proposal Title: CAREER: Thermal transport and thermoelectric measurements of nanotransistors, nanowires,and superlattices

This career-development program proposes to investigate thermal transport of nanotransistors that are influenced by ballistic charge transport, and to characterize thermoelectric properties of nanowires and superlattices with potentially superior thermoelectric figure of merit. Techniques for nanoscale quantitative mapping of temperature and electric fields will be developed for studying heat dissipation and local hot spots in sub-100 nm silicon-on-insulator (SOI) and SiGe metal-oxide-semiconductor field effect transistors (MOSFETs), and in single wall carbon nanotube (SWCN) devices. A suspended micro- device will be used to characterize thermoelectric properties of nanowires and to investigate quantum thermal transport in SWCNs. The PI will further develop a technique for nanoscale mapping of Seebeck coefficient across individual quantum well and barrier layers of Si/Ge and GaAs/AlAs superlattice coolers.
The PI will adopt effective models for micro-nano scale engineering instruction. He will integrate micro-nano scale thermo-fluids teaching with existing undergraduate courses via a curriculum reform effort focused on Project-Centered Education at his home department. Further, the PI will develop a new graduate course on micro-nano scale thermal/fluid science and technology, and an introductory laboratory section for use in regular summer workshops organized by the PI's institution for K-12 teachers, women and minority students.
The award is supported by the Thermal Transport and Thermal Processing Program of the Chemical and Transport Systems Division.

ABSTRACT

National Science Foundation

Proposal Number CTS-0553649
Principal Investigator Shi, Li
Affiliation University of Texas at Austin
Proposal Title Thermal Transport at Nanoscale Point and Line Constrictions and Interfaces

Thermal transport at nanometer scale point and line constrictions and interfaces is a fundamental problem that is important for a number of technologies, such as scanning probe microscopy, novel thermal interface materials, and nanostructural electronic and thermoelectric devices. As of today, few measurement results of thermal resistances at nanoscale constrictions and interfaces are available. Moreover, although there have been extensive theoretical studies of contact thermal resistance between two solids, most of the existing analytical models have been developed for macro to micro scale contacts.
The research objective of this program is to measure and model thermal transport at nanoscale point and line constrictions and interfaces. Ultrahigh vacuum atomic force microscopy and nanofabricated structures will be employed to measure the thermal resistance of nanometer size point contacts, line interfaces, and Si constrictions. A molecular dynamics (MD) simulation method will be used to calculate the thermal resistance and temperature distribution at these nanoscale constrictions and interfaces. In addition to heat conduction, the calculation will investigate the influences of near- and far- field radiation transfer on the temperature distribution and thermal resistance. The results from the measurements and calculations will be correlated and used to verify and improve analytic models.
Intellectual Merit. The proposed research will obtain measurement data of thermal resistance at nanoscale constrictions and interfaces. The results from the measurements and simulations will fill in a knowledge gap and provide timely support for thermal design and thermal management of electronic and thermoelectric devices as well as new scanning probe microscopy and data storage methods.
Broader Impacts. The research will provide training opportunities for two graduate students and an undergraduate student participant in the NSF Research Experience for Undergraduates (REU) program. The research results will be used as case studies in two graduate courses and one new undergraduate technical elective course. The two investigators will give short lectures in the seminar series organized by local ASME student organization, and will actively participate in K-12 outreach activities sponsored by the university. The close collaboration with an industrial researcher will have mutual benefits for the education of the participating students and the transfer of knowledge gained from this study for industrial applications.

Abstract:

NER: Nanoimprint fabrication of stimuli-responsive drug delivery carriers
This proposal was received in response to Nanoscale Science and Engineering initiative, NSF 05-610, category NER. The objective of this research is to fabricate functional three dimensional nanostructures for tissue targeted drug delivery. The approach is to use top-down nanofabrication technology, specifically step and flash nanoimprinting and thermal nanoimprint lithography, coupled with rational polymer chemistry to develop monodisperse, injectable nanocarriers with precise size and shape that can release drug in response to specific disease-associated signals.
The results could not only provide new directions in fabricating drug delivery vehicles with disease-responsive properties, but would also explore the fundamental limitations and practical capabilities of generating complex nanostructures with imprint techniques. If successful, this would eventually lead to the next generation of disease-specific and highly effective therapeutics and also provide novel biomedical applications for nanoimprint lithography. The project is inherently interdisciplinary involving a biomedical engineer and a mechanical/nano-manufacturing engineer. This provides a unique and rewarding educational environment for the graduate students involved. In addition, the results and concepts developed here would directly benefit several graduate courses and would be disseminated into undergraduate and high school education by correlating nanoscale science and engineering with real-life biomedical applications.

CBET-0813986
Shi

Partial support is provided for the 3rd Energy Nanotechnology International Conference, which will be held in Jacksonville, Florida, on August 10 - 14, 2008. The purpose of this meeting is to engage a broad spectrum of the nanotechnology research community to address leading issues related to energy.

With respect to the intellectual merit of the conference, a significant component of the conference is the Workshop on Nanotechnologies for Solar and Thermal Energy Conversion and Storage. The invited participants to this workshop will address current barriers to solar and thermal energy conversion and storage, which are emerging as the most significant barriers to widespread application of renewable energy. The output of this workshop will be a roadmap describing future research directions in this vital area. The workshop report will be published as a journal article, allowing for broad dissemination of the findings.

The broader impacts of the conference include support for women and underrepresented minority participants. The conference, including the workshop discussed above, addresses crucial research needs inhibiting sustainable energy conversion and usage, and thus may lead to reduced consumption of fossil fuel resources.

This project is jointly funded by the Thermal Transport Processes (TTP) Program, of the Chemical, Bioengineering, Environmental, and Transport Systems (CBET) Division, by the Energy for Sustainability Program, also of CBET, and by the Nanoscale Science & Engineering Program, all within the Directorate for Engineering (ENG).

0933454
Shi

The Wiedemann-Franz (W-F) law describing thermal transport by electrons is a hallmark of success of the Sommerfeld theory. For electrons confined in one- or two-dimensional nanostructures, however, several intriguing phenomena can lead to the breakdown of the W-F law. These phenomena include strong correlation effects associated with increased electron-electron interaction, as well as electron localization.
Intellectual Merit. This experimental study addresses a fundamental question regarding whether the Wiedemann-Franz Law is valid in low dimensional conductors. The scientific question being addressed is of a very fundamental nature, and one that will impact multiple disciplines. The experimental study examines electronic thermal transport in copper nanowires, quasi one-dimensional polyacetylene nanofibers, and two-dimensional graphene nanoribbons. The thermal conductivity, electrical conductivity, and Seebeck coefficient of these nanostructures are measured using nanofabricated suspended measurement devices at various temperatures. Chemical doping, electrical field effects, and the thermal Hall effect are employed to tune and determine the electronic thermal conductivity and isolate the lattice contribution. The validity of the W-F law is examined by analyzing the measured Lorenz number and its temperature dependence, and comparing the measurement results with the predictions of different transport theories.
Broader Impacts. The nanostructures of interest are being considered for nanoelectronics, sensors, energy conversion, and thermal management applications, all of which will benefit from a better understanding of electronic thermal transport in these nanostructures. The research provides interdisciplinary education opportunities for graduate and undergraduate students. New example materials for a graduate and an undergraduate course in thermal science are being produced, as are new materials for three K-12 outreach activities designed to attract young girls to engineering professions, and to expose public school students and teachers in Texas to engineering principles.

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).

The objective of this research is to develop nanomanufacturing methods for fabrication of shape-specific ?smart? nanoparticles capable of delivering drugs or imaging agents to targeted tissues in response to disease-specific or physiological signals. Specifically, high throughput, bio-compatible nanoimprint manufacturing processes are developed in this research to fabricate highly monodisperse, enzymatically-triggered nanoscale carriers of drug and imaging agents. The sizes and shapes of the nanoparticles are controlled during the top-down nano-imprint process. Experiments are carried out to evaluate nanoparticle loading, the release of drug and imaging agents, and to characterize the effects of nano-carrier size and shape on carrier transport and cellular uptake in cell cultures and microfluidic environments.

This research can transform the manufacturing of nanoparticles for drug and imaging agent delivery as well as address fundamental questions regarding the optimal size and shape of nano-carriers. The obtained nano-carriers can significantly improve therapeutic care of complex diseases such as cancer or cardiovascular diseases. Moreover, the results from this research would not only provide new directions in fabricating drug delivery vehicles with disease-responsive properties, but would also explore the fundamental limitations and practical capabilities of generating three-dimensional, complex structures with nanomanufacturing techniques. If successful, this would eventually lead to the next generation of disease-specific and highly effective therapeutics and also provide novel biomedical applications for nanoimprint lithography. The project is inherently interdisciplinary and involves principles from manufacturing, mechanical, and biomedical engineering. This provides a unique and rewarding educational environment for the students involved including students from underrepresented groups in engineering professions. The results and concepts developed here directly benefit several graduate and undergraduate courses and are disseminated into industry and public by the active participations of the investigators in short courses and seminars for industry and K-12 teachers, students, and parents.

1048767

Shi

This project seeks to develop novel thermoelectric materials for use in prototype thermoelectric modules to promote the cost-effective conversion of waste heat in vehicle exhaust systems.

Intellectual Merit: This proposal addresses four elements that are critical for successful implementation of thermoelectric devices for waste heat recovery from vehicle exhaust. These include development of new thermoelectric materials, system-level modeling, heat sink development, and reduction of thermal and electric resistances at material-material interfaces. The thermoelectric materials of interest are silicides, in which the lattice thermal conductivity of both the p- and n-type material will be reduced through nanostructuring, hence increasing the efficacy with which the material will perform in waste heat recovery scenarios. Once developed, the new material will be incorporated into thermoelectric modules, and the modules will be installed on a 6.7 liter diesel engine to measure system performance under realistic operating conditions. A system-level model will be developed and utilized to identify opportunities to further increase and optimize the overall design. Material properties will be measured at the PIs institutions as well as at Oak Ridge National Laboratory.

Broader Impact: The successful development and implementation of new thermoelectric materials and module designs will improve fuel economy and reduce emissions. Graduate students will be involved in the research. Video course modules specific to thermoelectric waste heat recovery will be developed and disseminated via the Internet for K-12, undergraduate, and graduate students. Outreach to a broad segment of the local population will be conducted.

1125957
Shi

The workshop will provide a critical review of the current state of the art in nanoscale transport and provide directions for future activities. A broad range of application areas are targeted including thermoelectric, optoelectronic, photovoltaic and nano/micro fluidic systems.

Intellectual Merit: The proposed workshop will examine the fundamentals of nanoscale transport including phonon, electron and photon interactions and examine how this understanding can be used to improve a wide array of nanoscale devices. The workshop will attract a number of key researchers in this field, including some who have not been active participants in this series of workshops. The PIs will provide a summative assessemnt of the state of the art, progress and challenges and future research directions that will be submitted for consideration for a journal publication and NSF report. The PIs are encouraged to look beyond simplistic modeling at the MD level in idealized geometries and consider how the advances made in the nanoscience field in the last two decades can now begin to be applied to nanodevices with increasing complexity and functionality.

Broader Impacts: The workshop will bring together a diverse group of people in an international setting and will therefore foster broad dissemination of the ongoing work and active interchange of ideas. The requested support is significantly biased toward supporting junior faculty.

This award to the University of Texas at Austin is for the acquisition of a Spark Plasma Sintering (SPS) system. Spark plasma sintering is an innovative technique that emerged in recent years for material syntheses and consolidation. In SPS both external pressure and pulsed current are applied simultaneously to enhance the consolidation of a wide range of ceramic, metallic, and composite powders. Local heating occurs primarily at gaps between particles where the applied electric field induces sparks and the formation of a high-energy plasma. As a result the consolidation can be completed within a shorter time, which allows grain growth and ion diffusion to be efficiently controlled and prevented. These unique features makes SPS suitable to fabricate more complex materials such as heterogeneous materials with specifically defined interface structure, nanostructured materials, and composite materials. The SPS system will be used to develop novel materials for energy applications such as thermoelectrics to recover waste heat, high-temperature solid oxide fuel cells to efficiently convert chemical energy directly into electricity, solid-state electrolytes for high-voltage lithium ion batteries, and low temperature polymer-electrolyte fuel cell plates with high corrosion resistance and low electrical contact resistance.

The SPS system will be one of the major facilities for materials fabrication at the University of Texas at Austin. The system will be managed through the Materials Science and Engineering Program and will be accessible to researchers across campus. The SPS technique adds a new capability to explore a much broader range of materials that cannot be made using conventional sintering. Research activities with the SPS technique will be integrated into the graduate curriculum in the Texas Materials Institute. In addition to lectures on the SPS technique, the availability of this new equipment will offer students in the materials science program an excellent hands-on experience with an advanced materials synthesis techniques. The PIs and their students will give lectures and/or demonstrations of clean energy experiments to students at middle and high schools as well as to the general public and K-12 students at Explore UT, a campus-wide open house.

CBET-1336968
PI: Li Shi (U Texas, Austin)

Different electron and phonon populations can be driven out of local thermal equilibrium in nanoelectronic devices, laser materials processing, and thermal transport measurements. A better understanding of the highly non-equilibrium transport phenomena is necessary for the design of next-generation devices and material structures with enhanced performance and reliability. However, current experimental capabilities are inadequate for probing local temperatures of these different energy excitations. Despite the recent progresses in scanning thermal microscopy (SThM), infrared spectroscopy, and micro-Raman spectroscopy, there is a lack of experimental methods for resolving the local temperature of the acoustic phonons that dominate heat conduction, as well as low-frequency acoustic phonons that may be in the ballistic transport regime in nanostructures. The objective of this research is to investigate new experimental methods for probing the local temperature of acoustic phonons during highly non-equilibrium transport processes in nanostructures and devices. The techniques to be investigated include a new method based on micro-Brillouin light scattering (BLS) for probing the local temperature of low-frequency (0.5 GHz to 100?s GHz) acoustic phonons with sub-micron spatial resolution. Based on preliminary BLS measurements of local acoustic phonon temperatures in glass, this technique will be investigated further for probing the local acoustic phonon temperature in silicon nanostructures that are either electrically biased or optically excited. The obtained acoustic phonon temperature will be correlated with those measured by micro-Raman spectroscopy, infrared spectroscopy, and SThM to quantify local non-equilibrium between electrons, acoustic and optical phonons.

The demonstration of the micro-BLS technique as a thermal microscopy tool for low-frequency phonons will be of value for the experimental thermal transport research community. Meanwhile, the measurement data can be used by theoretical and computational thermal transport researchers to establish a better understanding of several intriguing and important non-equilibrium transport phenomena. Such understanding can impact further advances in nanoelectronic devices, laser materials processing, and thermal measurement techniques. In addition, this research will provide student training opportunities in state-of-the-art experimental techniques, and result in new example materials for undergraduate and graduate courses. It will also generate new demonstration materials to be used in outreach activities for attracting students from underrepresented groups to engineering and science professions, and for exposing university research to K-12 students, parents, and teachers in Texas.

Three-dimensional (3D) ultrathin foams of two-dimensional (2D) materials hold high potential for a variety of applications, including structure reinforcement, thermal management, and energy storage. However, their industrial adoption has been limited due to the substantial difficulties in controlling the pore size and strut wall thickness of the foams to desired dimensions with existing materials processing technologies. This award supports research to address this challenge, by investigating an original and novel process that can lead to highly controllable growth of 3D foams with tunable pore sizes. This research will accelerate the processing technology of 3D porous materials with substantial impact on an array of applications including energy storage, thermal management, and flexible electronics. It will also provide opportunities for educating the next generation workforce to enhance the competitiveness of the US in materials processing and manufacturing technologies.

The goal of this research is to investigate an innovative process for manufacturing 3D foam architectures of 2D ultrathin graphite (UG) as well as similar foams of dielectric 2D hexagonal boron nitride (h-BN) with controllable pore sizes between 1 micon and 100 microns, and tunable strut wall thickness from 1 to 1000 nm. The materials processing method consists of a new approach for fabricating Ni foams as catalytic templates with reduced pore sizes or multiple level porosity, an energy effective and rapid RF induction heating method for growth of large-scale UG and h-BN materials, and an efficient Ni etching process. This work will also establish a fundamental understanding of the effects of pore size, wall thickness, grain boundaries and structure on the thermal properties of UG and h-BN foams and composites. New knowledge in materials processing will be obtained for manufacturing 3D multilevel porous UG and h-BN materials with enhanced thermal management properties. The understanding and knowledge resulting from this work will also potentially lead to a general approach for manufacturing a variety of other 2D materials into 3D foam structures.

Graphitic materials exhibit some of the highest thermal conductivity values found in solids. This feature has resulted in existing and emerging applications of these materials for transferring heat and keeping operating devices cool. However, the actual mechanisms behind their high thermal conductivity are not well understood. Recent theoretical studies have suggested that frequent scattering among a peculiar group of high-population phonons in graphitic materials does not either cause resistance directly or limit their thermal conductivity contribution. This unusual phonon transport behavior has been referred as hydrodynamic phonon transport. Observation of hydrodynamic phonon transport has never been experimentally observed in any materials at a sufficiently high temperature that is relevant to technological applications. Hydrodynamic phonon transport can have practical implications in the design and thermal modeling of graphitic materials. Thus, a set of theoretically guided advanced experiments are conducted in this research to verify the existence of the unusual thermal transport behavior in graphitic materials. Also, the research findings and methods are integrated into three undergraduate and graduate courses and two demonstration modules for general public.

The objective of this research is to elucidate the influence of hydrodynamic phonon transport on the thermal transport properties of graphitic materials. This will be accomplished through theoretical and experimental efforts, including first principles based theoretical studies of phonon transport, nanoscale thermal transport measurements of the intrinsic thermal conductance, and ultrafast opto-thermal measurements of second sound in isotopically purified nanotube, graphene, and thin graphite samples. These theoretical and experimental efforts can further lead to a better understanding of the quantum theory of energy transport by lattice vibrations. If successful, the research can lead to a new approach to simulate phonon transport. In addition, it can advance the frontier of nanoscale thermal transport measurements by overcoming a critical challenge in probing the intrinsic thermal transport properties of nanostructures. Moreover, it can result in a cutting edge ultrafast thermal transport method for probing phonon transport.

Semiconductor research and development over the past several decades have enabled widespread use of electronic devices in society. However, the semiconductor industry is facing significant challenges, such as the difficulty in dissipating the large amount of heat generated in silicon electronic chips. The challenges have motivated the investigation of emerging two-dimensional (2D) electronic materials because of their superior electronic and thermal properties. One representative 2D material is graphene that is made of a single layer of carbon atoms. However, compared to the abundant knowledge of silicon electronics, there is only limited understanding of heat dissipation in 2D electronic materials. Therefore, the goal of this project is to establish an in-depth understanding of the distinctly different heat generation and dissipation processes in 2D materials and devices. The obtained knowledge will be used to develop open-source simulation tools, enhance online courses and classroom instruction, and develop hands-on outreach activities to improve the recruitment and training of a diverse population of next-generation workforce in thermal engineering.

The goal of this project is to understand the unique fundamental mechanisms underlying heat generation and conduction processes in emerging 2D electronic and optoelectronic materials, including graphene and transition metal dichalcogenides (TMD). Specifically, three outstanding questions that are relevant to the performance and thermal reliability of 2D electronic and optoelectronic materials will be addressed: (1) the importance of four-phonon scattering processes in graphene and other 2D materials; (2) the length scales for different phonon polarizations and electronic excitations to establish local thermal equilibrium in 2D semiconductors; and (3) the phonon mode conversion and transmission processes across the dimensionally mismatched interfaces between a 2D layer and its 3D support. The questions will be addressed via first principles and atomistic theoretical calculations that resolve the modal scattering and coupling behaviors of phonons and electrons, in conjunction with thermal transport and inelastic light scattering measurements of graphene and TMD. Improved understanding of the three specific questions helps to build the foundation for modeling and controlling heat dissipation processes in 2D materials and devices.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

This award is to support the Workshop on Emerging Opportunities at the Intersection of Quantum and Thermal Sciences to be held virtually on June 28-30, 2021. Advances in quantum science over the past century have laid the foundation for both modern information technologies and solid-state energy-conversion devices that have emerged and altered the course of human history. As these classical devices and technologies approach their theoretical limits, progress is now being made to store information in quantum systems to enable secured quantum communication, quantum sensing, and quantum computation of complex problems that cannot be handled by current classical computers. Among the various grand challenges that need to be overcome for the establishment and deployment of quantum information technologies, innovative thermal science and engineering solutions are required for cooling the quantum hardware to an extremely low temperature. Meanwhile, thermal measurements can provide unique probes of exotic quantum materials and may provide unanticipated new directions in quantum hardware designs. In response to these challenges and opportunities, nineteen leading researchers in relevant fields have been invited to present forward-looking perspectives and participate in panel discussions in the three-day virtual workshop. The workshop is open to the broad research community, including both students and established researchers from different fields.

The goal of this three-day virtual workshop is to identify emerging research opportunities at the intersection of quantum and thermal sciences, especially on thermal challenges and opportunities in quantum computation and hardware, thermal science and engineering of quantum sensors and materials, and coherent phonons and magnons for quantum information technologies. This goal will be pursued via invited talks from leading experts in quantum and thermal science communities in US, Europe, and Israel, as well as panel discussions that will stimulate scientific exchanges and initiate interdisciplinary collaborations between the different communities in quantum theories, thermal science, material science, and advanced manufacturing. These cross-disciplinary interactions will help identify research needs in thermal science and engineering for advancing quantum information science and technologies. The workshop is also expected to promote the applications of quantum sensing and computation for measurements and computation of thermal problems. These reciprocal interactions between quantum and thermal sciences will stimulate future research opportunities in materials science and advanced manufacturing for realizing thermal devices and components in quantum systems and quantum hardware for thermal applications. Besides identifying collaboration opportunities for the research communities, the workshop will contribute to the education of students from diverse backgrounds for the future workforce of the quantum information technology industry.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.