qing hao | University of Arizona (original) (raw)

Papers by qing hao

Thermoelectric materials have attractive applications in electric power generation and solid-stat... more Thermoelectric materials have attractive applications in electric power generation and solid-state cooling. The performance of a thermoelectric device depends on the dimensionless figure of merit (ZT) of the material, defined as ZT = S 2 o-T / k, where S is the Seebeck coefficient, o is the electrical conductivity, k is the thermal conductivity, and T is the absolute temperature. In recent years, the idea of using nanotechnology to further improve the figure of merit of conventional thermoelectric materials has triggered active research and led to many exciting results. Most of the reported ZT enhancements are based on thin films and nanowires in which the thermal conductivity reduction plays a central role. We pursue the nanocomposite approach as an alternative to superlattices in the quest for high ZT materials. These nanocomposites are essentially nano-grained bulk materials that are synthesized by hot pressing nanoparticles into a bulk form. The interfaces inside a nanocomposite strongly scatter phonons but only slightly affect the charge carrier transport. Therefore, we can significantly reduce the lattice thermal conductivity and even somewhat increase the power factor S 2 U, resulting in higher ZT than for bulk materials. Compared with expensive thin-film superlattices, nanocomposites will have significant advantages in mass production, device construction and operation. This thesis covers my studies on bismuth antimony telluride nanocomposites and some recent work on Co 4 Sb 12-based nanocomposites. In bismuth antimony telluride nanocomposites, we have achieved a peak ZT of 1.4 at 100 'C, a 40% increase in ZT over the bulk material. This is the first significant ZT increase in this material system in fifty years. The same approach has also yielded a peak ZT around 1.2 in Yb filled Co 4 Sbi 2 nanocomposites. During the process, great efforts were dedicated to assuring accurate and dependable property measurements of thermoelectric nanocomposites. In addition to comparing measurement results between the commercial setups and a homebuilt measurement system, the high ZT obtained in bismuth antimony telluride nanocomposites was further confirmed by a device cooling test. To better understand the measured thermoelectric properties of nanocomposites, theoretical analysis based on the Boltzmann transport equation was performed. Furthermore, frequency-dependent Monte Carlo simulations of the phonon transport were conducted on 2D periodic porous silicon and 3D silicon nanocomposites. In the thermoelectrics field, the latter one provided the first accurate prediction for phonon size effects in a given nanocomposite. For charge carriers in thermoelectric nanocomposites, The process of writing this thesis provided me an opportunity to revisit many of the exciting moments of my Ph.D. studies. During my stay at MIT, I feel deeply grateful for many people who have taught and inspired me in their own ways. Among these, I would first thank my advisor, Prof. Gang Chen, who not only set an example of conducting first-class research but also helped me to improve myself in many aspects. In addition, I highly appreciate the enthusiastic help and support of my other Ph.D. committee members, including Prof. Mildred S. Dresselhaus, Prof. Zhifeng Ren, and Prof. Borivoje B. Mikic. I benefited a lot from their constructive criticisms of my research and encouragement along the way. With this chance, I would acknowledge the support and guidance of my Master's advisor, Prof. Li Shi at the University of Texas at Austin, who led me into the nanotechnology field from a traditional thermal engineering background. I am also indebted to Prof. Yinping Zhang at Tsinghua University for his advice over the years. Over the past six years, I was fortunate to work with many brilliant and diligent students and visiting scholars in both Boston College and MIT. Among many ex-and current MIT/BC members, I would especially thank the follows for their friendship and many useful discussions:

ES Materials & Manufacturing, 2021

ES Materials & Manufacturing, 2020

ES Materials & Manufacturing, 2020

Machine learning approaches are explored to predict the bandgaps of inorganic compounds using kno... more Machine learning approaches are explored to predict the bandgaps of inorganic compounds using known compositional features, based on a dataset of 3896 compounds with experimentally measured bandgaps. In particular, among various existing methods, we propose a new method, random forest with Gaussian process model as leaf nodes (RF-GP), and show its advantages. We have also investigated ensemble learning methods, which produce superior results over other traditional machine learning methods, but at the cost of extra computational load and further reduced interpretability.

ES Materials & Manufacturing, 2019

In this work, high-temperature annealing above 1073 K has been carried out to study the possible ... more In this work, high-temperature annealing above 1073 K has been carried out to study the possible shape change of nanoporous Si thin films. Under 1273 K, it is found that the pore size can still be largely maintained when the nanoporous Si thin film is on a SiO /Si substrate. 2 However, the pore size can significantly shrink when the film is suspended. The contrast suggests that the SiO /Si substrate can play an 2 important role in maintaining the nanoporous patterns at a high temperature. This finding can be important for the high-temperature applications of these porous thin films, such as thermoelectric power generation.

Carbon, 2018

Charge carrier scattering is critical to the electrical properties of two-dimensional materials s... more Charge carrier scattering is critical to the electrical properties of two-dimensional materials such as graphene, transition metal dichalcogenide monolayers, black phosphorene, and tellurene. Beyond pristine two-dimensional materials, further tailored properties can be achieved by nanoporous patterns such as nano-or atomic-scale pores (antidots) across the material. As one example, structure-dependent electrical/optical properties for graphene antidot lattices (GALs) have been studied in recent years. However, detailed charge carrier scattering mechanism is still not fully understood, which hinders the future improvement and potential applications of such metamaterials. In this paper, the energy sensitivity of charge-carrier scattering and thus the dominant scattering mechanisms are revealed for GALs by analyzing the maximum Seebeck coefficient with a tuned gate voltage and thus shifted Fermi levels. It shows that the scattering from pore-edge-trapped charges is dominant, especially at elevated temperatures. For thermoelectric interests, the gate-voltage-dependent power factor of different GAL samples are measured as high as 554 μW/cm·K at 400 K for a GAL with the square

Journal of Applied Physics, 2018

In recent years, hierarchical structures have been intensively studied as an effective approach t... more In recent years, hierarchical structures have been intensively studied as an effective approach to tailor the electron and phonon transport inside a bulk material for thermoelectric applications. With atomic defects and nano- to micro-scale structures in a bulk material, the lattice thermal conductivity can be effectively suppressed across the whole phonon spectrum, while maintaining or somewhat enhancing the electrical properties. For general materials with superior electrical properties, high thermoelectric performance can be achieved using hierarchical structures to minimize the lattice thermal conductivity. Despite many encouraging experimental results, accurate lattice thermal conductivity predictions are still challenging for a bulk material with hierarchical structures. In this work, an effective medium formulation is developed for nanograined bulk materials with embedded nanostructures for frequency-dependent phonon transport analysis. This new formulation is validated with ...

Review of Scientific Instruments, 2009

The maximum efficiency of a thermoelectric generator is determined by the material’s dimensionles... more The maximum efficiency of a thermoelectric generator is determined by the material’s dimensionless figure of merit ZT. Real thermoelectric material properties are highly temperature dependent and are often measured individually using multiple measurement tools on different samples. As a result, reported ZT values have large uncertainties. In this work we present an experimental technique that eliminates some of these uncertainties. We measure the Seebeck coefficient, electrical conductivity, and thermal conductivity of a single element or leg, as well as the conversion efficiency, under a large temperature difference of 2–160 °C. The advantages of this technique include (1) the thermoelectric leg is mounted only once and all measurements are in the same direction and (2) the measured properties are corroborated by efficiency measurements. The directly measured power and efficiency are compared to the values calculated from the measured properties and agree within 0.4% and 2%, respec...

Journal of Nanoscience and Nanotechnology, 2008

We describe a one-step, one-pot non-aqueous route for the synthesis of Sb2Te3 nanocrystals with h... more We describe a one-step, one-pot non-aqueous route for the synthesis of Sb2Te3 nanocrystals with hexagonal shape and highly anisotropic nanostructures. The as-prepared nanostructures were characterized by XRD, TEM and HRTEM. The effect of the stabilizers on the nanocrystal morphology has been discussed in detail. We have studied the thermal conductivity of the compacted bulk from the Sb2 Te3 nanostructures. The results indicated that a very low thermal conductivity of about 1 W/mK at 300 K, comparing to 4.7 W/mK of the polycrystalline bulk, was achieved. The results indicated that nanostructured Sb2 Te3 is potentially a good candidate for engineered nanocomposites that can lead to high thermoelectric figure-of-merit.

Journal of Nanoscience and Nanotechnology, 2008

Nanostructured Ni-doped skutterudites Co1−xNixSb3 (with x ranging from 0.01 to 0.09) were prepare... more Nanostructured Ni-doped skutterudites Co1−xNixSb3 (with x ranging from 0.01 to 0.09) were prepared by ball milling and direct-current induced hot press. It was found that the thermal conductivity was reduced due to strong electron–phonon scattering from Ni-doping as well as phonon scattering from the increased grain boundary of the nanostructures. A maximum dimensionless figure-of-merit of 0.7 was obtained in Co0.91Ni0.09Sb3 at 525 °C.

Chemistry of Materials, 2013

ES Materials & Manufacturing, 2019

ES Materials & Manufacturing, 2018

Applied Physics Letters

Heat exchange between a solid material and the gas environment is critical for the heat dissipati... more Heat exchange between a solid material and the gas environment is critical for the heat dissipation of miniature electronic devices. In this aspect, existing experimental studies focus on non-porous structures such as solid thin films, nanotubes, and wires. In this work, the proposed two-layer model for the heat transfer coefficient (HTC) between a solid sample and the surrounding air is extended to 70-nm-thick nanoporous Si thin films that are patterned with periodic rectangular nanopores having feature sizes of 100–400 nm. The HTC values are extracted using the 3[Formula: see text] method based on AC self-heating of a suspended sample with better accuracy than steady-state measurements in some studies. The dominance of air conduction in the measured HTCs is confirmed by comparing measurements with varied sample orientations. The two-layer model, developed for nanotubes, is still found to be accurate when the nanoporous film is simply treated as a solid film in the HTC evaluation a...

Engineered Science, 2021

Nanograined bulk alloys based on bismuth telluride (Bi2Te3) are the dominant materials for roomte... more Nanograined bulk alloys based on bismuth telluride (Bi2Te3) are the dominant materials for roomtemperature thermoelectric applications. In numerous studies, existing bulk phonon mean free path (MFP) spectra predicted by atomistic simulations suggest sub-100 nm grain sizes are necessary to reduce the lattice thermal conductivity by decreasing phonon MFPs. This is in contrast with available experimental data, where a remarkable thermal conductivity reduction is observed even for micro-grained Bi2Te3 samples. In this work, first-principles phonon MFPs along both the inplane and cross-plane directions are re-computed for bulk Bi2Te3. These phonon MFPs can explain new and existing experimental data on flake-like Bi2Te3 nanostructures with various thicknesses. For polycrystalline Bi2Te3-based materials, a better explanation of the experimental data requires

ES Materials & Manufacturing, 2021

As one fundamental problem in materials science research, thermal transport across grain boundari... more As one fundamental problem in materials science research, thermal transport across grain boundaries is critical to many energy-related applications. Due to the complexity of grain boundaries, the current understanding on how a grain boundary interacts with heat carriers is still in its infancy. This review summarizes the current progresses of this important topic, with its further extension to general interfaces. One focus is on major modeling and simulation techniques to predict the thermal resistance of a grain boundary. The corresponding thermal measurements of individual grain boundaries and grain-boundary thermal engineering are also discussed. A better understanding of the grain-boundary phonon transport is critical to many energy-related applications, where the concerned thermal transport within a polycrystalline material can be largely suppressed by grain boundaries.

16 The rapid development in the synthesis and device fabrication of 2D materials provides new 17 ... more 16 The rapid development in the synthesis and device fabrication of 2D materials provides new 17 opportunities for their wide applications in fields including thermoelectric energy conversion. 18 As one important research direction, the possibly poor thermoelectric performance of the 19 pristine 2D materials can be dramatically improved with patterned nanoporous structures and/or 20 heterostructures. This article reviews the recent advancement along this direction, with 21 emphasis on both fundamental understanding and practical problems. 22

Physical Review Applied, 2020

For thermoelectric applications, thermoelectric Si thin films with periodic circular pores have b... more For thermoelectric applications, thermoelectric Si thin films with periodic circular pores have been intensively studied because of the low price and earth abundance of Si. In this work, a different periodic nanoporous pattern is investigated for its potential thermoelectric benefit, i.e., a Si thin film with periodic nanoslots. Inside such structures, the neck between adjacent nanoslots functions as the nanorestriction to suppress the phonon transport, leading to a dramatically reduced lattice thermal conductivity. When the neck width is still longer than the mean free paths of majority charge carriers, bulklike electron transport can be maintained so that the thermoelectric ZT can be enhanced. For the thermal designs of these porous thin films, a simple but accurate analytical model based on the mean-free-path modification with a characteristic length is derived and is used to predict their thermoelectric properties. For heavily doped Si films with the neck width reduced to 5 nm, the computed ZT can reach 0.58 at 1100 K. The proposed nanoslot pattern can be extended to general thin films and atomic thick materials to tune their transport properties.

Engineered Science, 2020

The rapid development in the synthesis and device fabrication of two-dimensional (2D) materials p... more The rapid development in the synthesis and device fabrication of two-dimensional (2D) materials provides new opportunities for their wide applications in a variety of fields including thermoelectric energy conversion, thermal management, and thermal logics. As one important research direction, the possibly poor thermoelectric performance of the pristine 2D materials can be dramatically improved with patterned nanostructures, and stacking of different 2Ds to form layered heterostructures, as well as electrostatic gating, which allow the fine-tuning of both the quasi Fermi level and phonon transport. This article reviews the recent advancement in this direction, with an emphasis on both fundamental understanding and practical problems.

International Journal of Heat and Mass Transfer, 2020

As one simple metamaterial, nanopatterns are often fabricated across a thin film so that the ther... more As one simple metamaterial, nanopatterns are often fabricated across a thin film so that the thermal transport can be manipulated. The etched sidewalls for these nanostructures are usually rough due to surface defects introduced during the nanofabrication, whereas the top and bottom film surfaces are smoother. In existing analytical models, the contrast between these surfaces has not been addressed and all boundaries are assumed to be diffusive for phonon reflection. In this paper, a new two-step approach to address this issue is proposed for phonon transport modeling of general thin-film-based structures. In this approach, the effective in-plane phonon mean free paths (Λ) are first modified from the bulk phonon MFPs to account for the influence of the top/bottom film surfaces, with possibly enhanced probability of specular phonon reflection at cryogenic temperatures. This Λ is further modified to include the scattering by etched sidewalls with almost completely diffusive phonon scattering. Such a two-step phonon mean free path modification yields almost identical results as frequency-dependent phonon Monte Carlo simulations for etched nanowires and representative nanoporous thin films. This simple yet

Thermoelectric materials have attractive applications in electric power generation and solid-stat... more Thermoelectric materials have attractive applications in electric power generation and solid-state cooling. The performance of a thermoelectric device depends on the dimensionless figure of merit (ZT) of the material, defined as ZT = S 2 o-T / k, where S is the Seebeck coefficient, o is the electrical conductivity, k is the thermal conductivity, and T is the absolute temperature. In recent years, the idea of using nanotechnology to further improve the figure of merit of conventional thermoelectric materials has triggered active research and led to many exciting results. Most of the reported ZT enhancements are based on thin films and nanowires in which the thermal conductivity reduction plays a central role. We pursue the nanocomposite approach as an alternative to superlattices in the quest for high ZT materials. These nanocomposites are essentially nano-grained bulk materials that are synthesized by hot pressing nanoparticles into a bulk form. The interfaces inside a nanocomposite strongly scatter phonons but only slightly affect the charge carrier transport. Therefore, we can significantly reduce the lattice thermal conductivity and even somewhat increase the power factor S 2 U, resulting in higher ZT than for bulk materials. Compared with expensive thin-film superlattices, nanocomposites will have significant advantages in mass production, device construction and operation. This thesis covers my studies on bismuth antimony telluride nanocomposites and some recent work on Co 4 Sb 12-based nanocomposites. In bismuth antimony telluride nanocomposites, we have achieved a peak ZT of 1.4 at 100 'C, a 40% increase in ZT over the bulk material. This is the first significant ZT increase in this material system in fifty years. The same approach has also yielded a peak ZT around 1.2 in Yb filled Co 4 Sbi 2 nanocomposites. During the process, great efforts were dedicated to assuring accurate and dependable property measurements of thermoelectric nanocomposites. In addition to comparing measurement results between the commercial setups and a homebuilt measurement system, the high ZT obtained in bismuth antimony telluride nanocomposites was further confirmed by a device cooling test. To better understand the measured thermoelectric properties of nanocomposites, theoretical analysis based on the Boltzmann transport equation was performed. Furthermore, frequency-dependent Monte Carlo simulations of the phonon transport were conducted on 2D periodic porous silicon and 3D silicon nanocomposites. In the thermoelectrics field, the latter one provided the first accurate prediction for phonon size effects in a given nanocomposite. For charge carriers in thermoelectric nanocomposites, The process of writing this thesis provided me an opportunity to revisit many of the exciting moments of my Ph.D. studies. During my stay at MIT, I feel deeply grateful for many people who have taught and inspired me in their own ways. Among these, I would first thank my advisor, Prof. Gang Chen, who not only set an example of conducting first-class research but also helped me to improve myself in many aspects. In addition, I highly appreciate the enthusiastic help and support of my other Ph.D. committee members, including Prof. Mildred S. Dresselhaus, Prof. Zhifeng Ren, and Prof. Borivoje B. Mikic. I benefited a lot from their constructive criticisms of my research and encouragement along the way. With this chance, I would acknowledge the support and guidance of my Master's advisor, Prof. Li Shi at the University of Texas at Austin, who led me into the nanotechnology field from a traditional thermal engineering background. I am also indebted to Prof. Yinping Zhang at Tsinghua University for his advice over the years. Over the past six years, I was fortunate to work with many brilliant and diligent students and visiting scholars in both Boston College and MIT. Among many ex-and current MIT/BC members, I would especially thank the follows for their friendship and many useful discussions:

ES Materials & Manufacturing, 2021

ES Materials & Manufacturing, 2020

ES Materials & Manufacturing, 2020

Machine learning approaches are explored to predict the bandgaps of inorganic compounds using kno... more Machine learning approaches are explored to predict the bandgaps of inorganic compounds using known compositional features, based on a dataset of 3896 compounds with experimentally measured bandgaps. In particular, among various existing methods, we propose a new method, random forest with Gaussian process model as leaf nodes (RF-GP), and show its advantages. We have also investigated ensemble learning methods, which produce superior results over other traditional machine learning methods, but at the cost of extra computational load and further reduced interpretability.

ES Materials & Manufacturing, 2019

In this work, high-temperature annealing above 1073 K has been carried out to study the possible ... more In this work, high-temperature annealing above 1073 K has been carried out to study the possible shape change of nanoporous Si thin films. Under 1273 K, it is found that the pore size can still be largely maintained when the nanoporous Si thin film is on a SiO /Si substrate. 2 However, the pore size can significantly shrink when the film is suspended. The contrast suggests that the SiO /Si substrate can play an 2 important role in maintaining the nanoporous patterns at a high temperature. This finding can be important for the high-temperature applications of these porous thin films, such as thermoelectric power generation.

Carbon, 2018

Charge carrier scattering is critical to the electrical properties of two-dimensional materials s... more Charge carrier scattering is critical to the electrical properties of two-dimensional materials such as graphene, transition metal dichalcogenide monolayers, black phosphorene, and tellurene. Beyond pristine two-dimensional materials, further tailored properties can be achieved by nanoporous patterns such as nano-or atomic-scale pores (antidots) across the material. As one example, structure-dependent electrical/optical properties for graphene antidot lattices (GALs) have been studied in recent years. However, detailed charge carrier scattering mechanism is still not fully understood, which hinders the future improvement and potential applications of such metamaterials. In this paper, the energy sensitivity of charge-carrier scattering and thus the dominant scattering mechanisms are revealed for GALs by analyzing the maximum Seebeck coefficient with a tuned gate voltage and thus shifted Fermi levels. It shows that the scattering from pore-edge-trapped charges is dominant, especially at elevated temperatures. For thermoelectric interests, the gate-voltage-dependent power factor of different GAL samples are measured as high as 554 μW/cm·K at 400 K for a GAL with the square

Journal of Applied Physics, 2018

In recent years, hierarchical structures have been intensively studied as an effective approach t... more In recent years, hierarchical structures have been intensively studied as an effective approach to tailor the electron and phonon transport inside a bulk material for thermoelectric applications. With atomic defects and nano- to micro-scale structures in a bulk material, the lattice thermal conductivity can be effectively suppressed across the whole phonon spectrum, while maintaining or somewhat enhancing the electrical properties. For general materials with superior electrical properties, high thermoelectric performance can be achieved using hierarchical structures to minimize the lattice thermal conductivity. Despite many encouraging experimental results, accurate lattice thermal conductivity predictions are still challenging for a bulk material with hierarchical structures. In this work, an effective medium formulation is developed for nanograined bulk materials with embedded nanostructures for frequency-dependent phonon transport analysis. This new formulation is validated with ...

Review of Scientific Instruments, 2009

The maximum efficiency of a thermoelectric generator is determined by the material’s dimensionles... more The maximum efficiency of a thermoelectric generator is determined by the material’s dimensionless figure of merit ZT. Real thermoelectric material properties are highly temperature dependent and are often measured individually using multiple measurement tools on different samples. As a result, reported ZT values have large uncertainties. In this work we present an experimental technique that eliminates some of these uncertainties. We measure the Seebeck coefficient, electrical conductivity, and thermal conductivity of a single element or leg, as well as the conversion efficiency, under a large temperature difference of 2–160 °C. The advantages of this technique include (1) the thermoelectric leg is mounted only once and all measurements are in the same direction and (2) the measured properties are corroborated by efficiency measurements. The directly measured power and efficiency are compared to the values calculated from the measured properties and agree within 0.4% and 2%, respec...

Journal of Nanoscience and Nanotechnology, 2008

We describe a one-step, one-pot non-aqueous route for the synthesis of Sb2Te3 nanocrystals with h... more We describe a one-step, one-pot non-aqueous route for the synthesis of Sb2Te3 nanocrystals with hexagonal shape and highly anisotropic nanostructures. The as-prepared nanostructures were characterized by XRD, TEM and HRTEM. The effect of the stabilizers on the nanocrystal morphology has been discussed in detail. We have studied the thermal conductivity of the compacted bulk from the Sb2 Te3 nanostructures. The results indicated that a very low thermal conductivity of about 1 W/mK at 300 K, comparing to 4.7 W/mK of the polycrystalline bulk, was achieved. The results indicated that nanostructured Sb2 Te3 is potentially a good candidate for engineered nanocomposites that can lead to high thermoelectric figure-of-merit.

Journal of Nanoscience and Nanotechnology, 2008

Nanostructured Ni-doped skutterudites Co1−xNixSb3 (with x ranging from 0.01 to 0.09) were prepare... more Nanostructured Ni-doped skutterudites Co1−xNixSb3 (with x ranging from 0.01 to 0.09) were prepared by ball milling and direct-current induced hot press. It was found that the thermal conductivity was reduced due to strong electron–phonon scattering from Ni-doping as well as phonon scattering from the increased grain boundary of the nanostructures. A maximum dimensionless figure-of-merit of 0.7 was obtained in Co0.91Ni0.09Sb3 at 525 °C.

Chemistry of Materials, 2013

ES Materials & Manufacturing, 2019

ES Materials & Manufacturing, 2018

Applied Physics Letters

Heat exchange between a solid material and the gas environment is critical for the heat dissipati... more Heat exchange between a solid material and the gas environment is critical for the heat dissipation of miniature electronic devices. In this aspect, existing experimental studies focus on non-porous structures such as solid thin films, nanotubes, and wires. In this work, the proposed two-layer model for the heat transfer coefficient (HTC) between a solid sample and the surrounding air is extended to 70-nm-thick nanoporous Si thin films that are patterned with periodic rectangular nanopores having feature sizes of 100–400 nm. The HTC values are extracted using the 3[Formula: see text] method based on AC self-heating of a suspended sample with better accuracy than steady-state measurements in some studies. The dominance of air conduction in the measured HTCs is confirmed by comparing measurements with varied sample orientations. The two-layer model, developed for nanotubes, is still found to be accurate when the nanoporous film is simply treated as a solid film in the HTC evaluation a...

Engineered Science, 2021

Nanograined bulk alloys based on bismuth telluride (Bi2Te3) are the dominant materials for roomte... more Nanograined bulk alloys based on bismuth telluride (Bi2Te3) are the dominant materials for roomtemperature thermoelectric applications. In numerous studies, existing bulk phonon mean free path (MFP) spectra predicted by atomistic simulations suggest sub-100 nm grain sizes are necessary to reduce the lattice thermal conductivity by decreasing phonon MFPs. This is in contrast with available experimental data, where a remarkable thermal conductivity reduction is observed even for micro-grained Bi2Te3 samples. In this work, first-principles phonon MFPs along both the inplane and cross-plane directions are re-computed for bulk Bi2Te3. These phonon MFPs can explain new and existing experimental data on flake-like Bi2Te3 nanostructures with various thicknesses. For polycrystalline Bi2Te3-based materials, a better explanation of the experimental data requires

ES Materials & Manufacturing, 2021

As one fundamental problem in materials science research, thermal transport across grain boundari... more As one fundamental problem in materials science research, thermal transport across grain boundaries is critical to many energy-related applications. Due to the complexity of grain boundaries, the current understanding on how a grain boundary interacts with heat carriers is still in its infancy. This review summarizes the current progresses of this important topic, with its further extension to general interfaces. One focus is on major modeling and simulation techniques to predict the thermal resistance of a grain boundary. The corresponding thermal measurements of individual grain boundaries and grain-boundary thermal engineering are also discussed. A better understanding of the grain-boundary phonon transport is critical to many energy-related applications, where the concerned thermal transport within a polycrystalline material can be largely suppressed by grain boundaries.

16 The rapid development in the synthesis and device fabrication of 2D materials provides new 17 ... more 16 The rapid development in the synthesis and device fabrication of 2D materials provides new 17 opportunities for their wide applications in fields including thermoelectric energy conversion. 18 As one important research direction, the possibly poor thermoelectric performance of the 19 pristine 2D materials can be dramatically improved with patterned nanoporous structures and/or 20 heterostructures. This article reviews the recent advancement along this direction, with 21 emphasis on both fundamental understanding and practical problems. 22

Physical Review Applied, 2020

For thermoelectric applications, thermoelectric Si thin films with periodic circular pores have b... more For thermoelectric applications, thermoelectric Si thin films with periodic circular pores have been intensively studied because of the low price and earth abundance of Si. In this work, a different periodic nanoporous pattern is investigated for its potential thermoelectric benefit, i.e., a Si thin film with periodic nanoslots. Inside such structures, the neck between adjacent nanoslots functions as the nanorestriction to suppress the phonon transport, leading to a dramatically reduced lattice thermal conductivity. When the neck width is still longer than the mean free paths of majority charge carriers, bulklike electron transport can be maintained so that the thermoelectric ZT can be enhanced. For the thermal designs of these porous thin films, a simple but accurate analytical model based on the mean-free-path modification with a characteristic length is derived and is used to predict their thermoelectric properties. For heavily doped Si films with the neck width reduced to 5 nm, the computed ZT can reach 0.58 at 1100 K. The proposed nanoslot pattern can be extended to general thin films and atomic thick materials to tune their transport properties.

Engineered Science, 2020

The rapid development in the synthesis and device fabrication of two-dimensional (2D) materials p... more The rapid development in the synthesis and device fabrication of two-dimensional (2D) materials provides new opportunities for their wide applications in a variety of fields including thermoelectric energy conversion, thermal management, and thermal logics. As one important research direction, the possibly poor thermoelectric performance of the pristine 2D materials can be dramatically improved with patterned nanostructures, and stacking of different 2Ds to form layered heterostructures, as well as electrostatic gating, which allow the fine-tuning of both the quasi Fermi level and phonon transport. This article reviews the recent advancement in this direction, with an emphasis on both fundamental understanding and practical problems.

International Journal of Heat and Mass Transfer, 2020

As one simple metamaterial, nanopatterns are often fabricated across a thin film so that the ther... more As one simple metamaterial, nanopatterns are often fabricated across a thin film so that the thermal transport can be manipulated. The etched sidewalls for these nanostructures are usually rough due to surface defects introduced during the nanofabrication, whereas the top and bottom film surfaces are smoother. In existing analytical models, the contrast between these surfaces has not been addressed and all boundaries are assumed to be diffusive for phonon reflection. In this paper, a new two-step approach to address this issue is proposed for phonon transport modeling of general thin-film-based structures. In this approach, the effective in-plane phonon mean free paths (Λ) are first modified from the bulk phonon MFPs to account for the influence of the top/bottom film surfaces, with possibly enhanced probability of specular phonon reflection at cryogenic temperatures. This Λ is further modified to include the scattering by etched sidewalls with almost completely diffusive phonon scattering. Such a two-step phonon mean free path modification yields almost identical results as frequency-dependent phonon Monte Carlo simulations for etched nanowires and representative nanoporous thin films. This simple yet