Koh Yee Kan
Degree: PhD
Position: Associate Professor
Affiliation: NUS - Dept of Mechanical Engineering
Research Type: Experiment
Office: E2-02-29
Email: mpekyk@nus.edu.sg
Contact: (65) 6516 7601
Research Interests:
Ultrafast spectroscopy
Heat transport (or energy transport)
Transport properties
Thermoelectric properties
Heterostructures of 2D materials
CA2DM Publications:
2023 |
Li, Hongkun; Pandey, Tribhuwan; Jiang, Yi; Gu, Xiaokun; Lindsay, Lucas; Koh, Yee Kan Origins of heat transport anisotropy in MoTe2 and other bulk van der Waals materials Journal Article MATERIALS TODAY PHYSICS, 37 , 2023, ISSN: 2542-5293. @article{ISI:001093005700001, title = {Origins of heat transport anisotropy in MoTe_{2} and other bulk van der Waals materials}, author = {Hongkun Li and Tribhuwan Pandey and Yi Jiang and Xiaokun Gu and Lucas Lindsay and Yee Kan Koh}, doi = {10.1016/j.mtphys.2023.101196}, times_cited = {0}, issn = {2542-5293}, year = {2023}, date = {2023-08-29}, journal = {MATERIALS TODAY PHYSICS}, volume = {37}, publisher = {ELSEVIER}, address = {RADARWEG 29, 1043 NX AMSTERDAM, NETHERLANDS}, abstract = {Knowledge of how heat flows anisotropically in van der Waals (vdW) materials is crucial for thermal management of emerging 2D materials devices and design of novel anisotropic thermoelectric materials. Despite the importance, anisotropic heat transport in vdW materials is yet to be systematically studied and is often presumably attributed to anisotropic speeds of sound in vdW materials due to soft interlayer bonding relative to covalent in-plane networks of atoms. In this work, we investigate the origins of the anisotropic heat transport in vdW materials, through time-domain thermoreflectance (TDTR) measurements and first-principles calculations of anisotropic thermal conductivity of three different phases of MoTe2. MoTe2 is ideal for the study due to its weak anisotropy in the speeds of sound. We find that even when the speeds of sound are roughly isotropic, the measured thermal conductivity of MoTe2 along the c-axis is 5-8 times lower than that along the in-plane axes. We derive meaningful characteristic heat capacity, phonon group velocity, and relaxation times from our first principles calculations for selected vdW materials (MoTe2, BP, h-BN, and MoS2), to assess the contributions of these factors to the anisotropic heat transport. Interestingly, we find that the main contributor to the heat transport anisotropy in vdW materials is anisotropy in heat capacity of the dominant heat-carrying phonon modes in different directions, which originates from anisotropic optical phonon dispersion and disparity in the frequency of heat-carrying phonons in different directions. The discrepancy in frequency of the heat-carrying phonons also leads to similar to 2 times larger average relaxation times in the cross-plane direction, and partially explains the apparent dependence of the anisotropic heat transport on the anisotropic speeds of sound. This work provides insight into understanding of the anisotropic heat transport in vdW materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Knowledge of how heat flows anisotropically in van der Waals (vdW) materials is crucial for thermal management of emerging 2D materials devices and design of novel anisotropic thermoelectric materials. Despite the importance, anisotropic heat transport in vdW materials is yet to be systematically studied and is often presumably attributed to anisotropic speeds of sound in vdW materials due to soft interlayer bonding relative to covalent in-plane networks of atoms. In this work, we investigate the origins of the anisotropic heat transport in vdW materials, through time-domain thermoreflectance (TDTR) measurements and first-principles calculations of anisotropic thermal conductivity of three different phases of MoTe2. MoTe2 is ideal for the study due to its weak anisotropy in the speeds of sound. We find that even when the speeds of sound are roughly isotropic, the measured thermal conductivity of MoTe2 along the c-axis is 5-8 times lower than that along the in-plane axes. We derive meaningful characteristic heat capacity, phonon group velocity, and relaxation times from our first principles calculations for selected vdW materials (MoTe2, BP, h-BN, and MoS2), to assess the contributions of these factors to the anisotropic heat transport. Interestingly, we find that the main contributor to the heat transport anisotropy in vdW materials is anisotropy in heat capacity of the dominant heat-carrying phonon modes in different directions, which originates from anisotropic optical phonon dispersion and disparity in the frequency of heat-carrying phonons in different directions. The discrepancy in frequency of the heat-carrying phonons also leads to similar to 2 times larger average relaxation times in the cross-plane direction, and partially explains the apparent dependence of the anisotropic heat transport on the anisotropic speeds of sound. This work provides insight into understanding of the anisotropic heat transport in vdW materials. |
2022 |
Li, Qinshu; Liu, Fang; Hu, Song; Song, Houfu; Yang, Susu; Jiang, Hailing; Wang, Tao; Koh, Yee Kan; Zhao, Changying; Kang, Feiyu; Wu, Junqiao; Gu, Xiaokun; Sun, Bo; Wang, Xinqiang Inelastic phonon transport across atomically sharp metal/semiconductor interfaces Journal Article 22 NATURE COMMUNICATIONS, 13 (1), 2022. @article{ISI:000843206100006, title = {Inelastic phonon transport across atomically sharp metal/semiconductor interfaces}, author = {Qinshu Li and Fang Liu and Song Hu and Houfu Song and Susu Yang and Hailing Jiang and Tao Wang and Yee Kan Koh and Changying Zhao and Feiyu Kang and Junqiao Wu and Xiaokun Gu and Bo Sun and Xinqiang Wang}, doi = {10.1038/s41467-022-32600-w}, times_cited = {22}, year = {2022}, date = {2022-08-20}, journal = {NATURE COMMUNICATIONS}, volume = {13}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Understanding thermal transport across metal/semiconductor interfaces is crucial for the heat dissipation of electronics. The dominant heat carriers in non-metals, phonons, are thought to transport elastically across most interfaces, except for a few extreme cases where the two materials that formed the interface are highly dissimilar with a large difference in Debye temperature. In this work, we show that even for two materials with similar Debye temperatures (Al/Si, Al/GaN), a substantial portion of phonons will transport inelastically across their interfaces at high temperatures, significantly enhancing interface thermal conductance. Moreover, we find that interface sharpness strongly affects phonon transport process. For atomically sharp interfaces, phonons are allowed to transport inelastically and interface thermal conductance linearly increases at high temperatures. With a diffuse interface, inelastic phonon transport diminishes. Our results provide new insights on phonon transport across interfaces and open up opportunities for engineering interface thermal conductance specifically for materials of relevance to microelectronics.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Understanding thermal transport across metal/semiconductor interfaces is crucial for the heat dissipation of electronics. The dominant heat carriers in non-metals, phonons, are thought to transport elastically across most interfaces, except for a few extreme cases where the two materials that formed the interface are highly dissimilar with a large difference in Debye temperature. In this work, we show that even for two materials with similar Debye temperatures (Al/Si, Al/GaN), a substantial portion of phonons will transport inelastically across their interfaces at high temperatures, significantly enhancing interface thermal conductance. Moreover, we find that interface sharpness strongly affects phonon transport process. For atomically sharp interfaces, phonons are allowed to transport inelastically and interface thermal conductance linearly increases at high temperatures. With a diffuse interface, inelastic phonon transport diminishes. Our results provide new insights on phonon transport across interfaces and open up opportunities for engineering interface thermal conductance specifically for materials of relevance to microelectronics. |
Zheng, Weidong; McClellan, Connor J; Pop, Eric; Koh, Yee Kan Nonequilibrium Phonon Thermal Resistance at MoS2/Oxide and Graphene/Oxide Interfaces Journal Article ACS APPLIED MATERIALS & INTERFACES, 14 (19), pp. 22372-22380, 2022, ISSN: 1944-8244. @article{ISI:000812941600001, title = {Nonequilibrium Phonon Thermal Resistance at MoS2/Oxide and Graphene/Oxide Interfaces}, author = {Weidong Zheng and Connor J McClellan and Eric Pop and Yee Kan Koh}, doi = {10.1021/acsami.2c02062}, times_cited = {0}, issn = {1944-8244}, year = {2022}, date = {2022-05-18}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {14}, number = {19}, pages = {22372-22380}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Accurate measurements and physical understanding of thermal boundary resistance (R) of two-dimensional (2D) materials are imperative for effective thermal management of 2D electronics and photonics. In previous studies, heat dissipation from 2D material devices was presumed to be dominated by phonon transport across the interfaces. In this study, we find that, in addition to phonon transport, thermal resistance between nonequilibrium phonons in the 2D materials could play a critical role too when the 2D material devices are internally self-heated, either optically or electrically. We accurately measure the R of oxide/MoS2/oxide and oxide/graphene/oxide interfaces for three oxides (SiO2, HfO2, and Al2O3) by differential time-domain thermoreflectance (TDTR). Our measurements of R across these interfaces with external heating are 2-4 times lower than the previously reported R of the similar interfaces measured by Raman thermometry with internal self-heating. Using a simple model, we show that the observed discrepancy can be explained by an additional internal thermal resistance (Rint) between nonequilibrium phonons present during Raman measurements. We subsequently estimate that, for MoS2 and graphene, Rint approximate to 31 and 22 m2 K GW-1, respectively. The values are comparable to the thermal resistance due to finite phonon transmission across interfaces of 2D materials and thus cannot be ignored in the design of 2D material devices. Moreover, the nonequilibrium phonons also lead to a different temperature dependence than that by phonon transport. As such, our work provides important insights into physical understanding of heat dissipation in 2D material devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Accurate measurements and physical understanding of thermal boundary resistance (R) of two-dimensional (2D) materials are imperative for effective thermal management of 2D electronics and photonics. In previous studies, heat dissipation from 2D material devices was presumed to be dominated by phonon transport across the interfaces. In this study, we find that, in addition to phonon transport, thermal resistance between nonequilibrium phonons in the 2D materials could play a critical role too when the 2D material devices are internally self-heated, either optically or electrically. We accurately measure the R of oxide/MoS2/oxide and oxide/graphene/oxide interfaces for three oxides (SiO2, HfO2, and Al2O3) by differential time-domain thermoreflectance (TDTR). Our measurements of R across these interfaces with external heating are 2-4 times lower than the previously reported R of the similar interfaces measured by Raman thermometry with internal self-heating. Using a simple model, we show that the observed discrepancy can be explained by an additional internal thermal resistance (Rint) between nonequilibrium phonons present during Raman measurements. We subsequently estimate that, for MoS2 and graphene, Rint approximate to 31 and 22 m2 K GW-1, respectively. The values are comparable to the thermal resistance due to finite phonon transmission across interfaces of 2D materials and thus cannot be ignored in the design of 2D material devices. Moreover, the nonequilibrium phonons also lead to a different temperature dependence than that by phonon transport. As such, our work provides important insights into physical understanding of heat dissipation in 2D material devices. |
2020 |
Li, H; Hanus, R; Polanco, C A; Zeidler, A; Koblmueller, G; Koh, Y K; Lindsay, L GaN thermal transport limited by the interplay of dislocations and size effects Journal Article PHYSICAL REVIEW B, 102 (1), 2020, ISSN: 2469-9950. @article{ISI:000553246200001, title = {GaN thermal transport limited by the interplay of dislocations and size effects}, author = {H Li and R Hanus and C A Polanco and A Zeidler and G Koblmueller and Y K Koh and L Lindsay}, doi = {10.1103/PhysRevB.102.014313}, times_cited = {0}, issn = {2469-9950}, year = {2020}, date = {2020-07-29}, journal = {PHYSICAL REVIEW B}, volume = {102}, number = {1}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Measurements and first-principles calculations probe the temperature-dependent thermal conductivity (k) of GaN films with large densities of highly oriented dislocations. We demonstrate that phonon-dislocation scattering is weaker than suggested by previous measurements, likely due to sample and experiment size effects. Nonetheless, dislocation-limited k is observed in samples with large dislocation densities and at lower temperatures where k anisotropy is also observed. Combination of experiment and theory give insights into the interplay of thermal resistance mechanisms limiting GaN functionalities and suggest pathways for tuning k via defect engineering.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Measurements and first-principles calculations probe the temperature-dependent thermal conductivity (k) of GaN films with large densities of highly oriented dislocations. We demonstrate that phonon-dislocation scattering is weaker than suggested by previous measurements, likely due to sample and experiment size effects. Nonetheless, dislocation-limited k is observed in samples with large dislocation densities and at lower temperatures where k anisotropy is also observed. Combination of experiment and theory give insights into the interplay of thermal resistance mechanisms limiting GaN functionalities and suggest pathways for tuning k via defect engineering. |
Zheng, Weidong; Huang, Bin; Koh, Yee Kan Ultralow Thermal Conductivity and Thermal Diffusivity of Graphene/Metal Heterostructures through Scarcity of Low-Energy Modes in Graphene Journal Article ACS APPLIED MATERIALS & INTERFACES, 12 (8), pp. 9572-9579, 2020, ISSN: 1944-8244. @article{ISI:000517360000064, title = {Ultralow Thermal Conductivity and Thermal Diffusivity of Graphene/Metal Heterostructures through Scarcity of Low-Energy Modes in Graphene}, author = {Weidong Zheng and Bin Huang and Yee Kan Koh}, doi = {10.1021/acsami.9b18290}, times_cited = {0}, issn = {1944-8244}, year = {2020}, date = {2020-02-26}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {12}, number = {8}, pages = {9572-9579}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {In many ultralow thermal conductivity materials, interfaces of dissimilar materials are employed to impede heat flow perpendicular to the interfaces. However, when packed within a distance comparable to the phonon wavelengths, these interfaces are coupled and thus ineffective to scatter low-energy phonons, due to either coherent phonon transmission across the closely packed interfaces or weak coupling of the low-energy phonons and the interfaces. Here, we propose to block the propagation of these low-energy phonons by periodically distributed scarcity of available low-energy phonon modes using graphene/metal heterostructures of transferred graphene and ultrathin metal films. We demonstrate the effectiveness of graphene in blocking propagation of low-energy phonons by comparing the effective transmission probabilities of phonons in a wide range of multilayered structures; we find that interfaces in our graphene/metal heterostructures remain decoupled even when the spacing between interfaces is <2 nm. With the proposed strategy, we successfully achieve an ultralow thermal conductivity of Lambda = 0.06 W m(-1) K-1 and a world-record lowest thermal diffusivity of alpha = 2.6 x 10(-4) cm(2) s(-1) suitable for thermal insulation. Moreover, we demonstrate the capability to tune the electronic heat transport across the new materials by creating atomic-scale pinholes on graphene through magnetron sputtering, with electrons carrying approximate to 50% of heat when Lambda is approximate to 0.15 W m(-1) K-1. With the ultralow A and substantial electronic transport, the new graphene/metal heterostructures could be explored for thermoelectric applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In many ultralow thermal conductivity materials, interfaces of dissimilar materials are employed to impede heat flow perpendicular to the interfaces. However, when packed within a distance comparable to the phonon wavelengths, these interfaces are coupled and thus ineffective to scatter low-energy phonons, due to either coherent phonon transmission across the closely packed interfaces or weak coupling of the low-energy phonons and the interfaces. Here, we propose to block the propagation of these low-energy phonons by periodically distributed scarcity of available low-energy phonon modes using graphene/metal heterostructures of transferred graphene and ultrathin metal films. We demonstrate the effectiveness of graphene in blocking propagation of low-energy phonons by comparing the effective transmission probabilities of phonons in a wide range of multilayered structures; we find that interfaces in our graphene/metal heterostructures remain decoupled even when the spacing between interfaces is <2 nm. With the proposed strategy, we successfully achieve an ultralow thermal conductivity of Lambda = 0.06 W m(-1) K-1 and a world-record lowest thermal diffusivity of alpha = 2.6 x 10(-4) cm(2) s(-1) suitable for thermal insulation. Moreover, we demonstrate the capability to tune the electronic heat transport across the new materials by creating atomic-scale pinholes on graphene through magnetron sputtering, with electrons carrying approximate to 50% of heat when Lambda is approximate to 0.15 W m(-1) K-1. With the ultralow A and substantial electronic transport, the new graphene/metal heterostructures could be explored for thermoelectric applications. |
2019 |
Zeng, Qingsheng; Sun, Bo; Du, Kezhao; Zhao, Weiyun; Yu, Peng; Zhu, Chao; Xia, Juan; Chen, Yu; Cao, Xun; Yan, Qingyu; Shen, Zexiang; Yu, Ting; Long, Yi; Koh, Yee Kan; Liu, Zheng Highly anisotropic thermoelectric properties of black phosphorus crystals Journal Article 2D MATERIALS, 6 (4), 2019, ISSN: 2053-1583. @article{ISI:000474688300004, title = {Highly anisotropic thermoelectric properties of black phosphorus crystals}, author = {Qingsheng Zeng and Bo Sun and Kezhao Du and Weiyun Zhao and Peng Yu and Chao Zhu and Juan Xia and Yu Chen and Xun Cao and Qingyu Yan and Zexiang Shen and Ting Yu and Yi Long and Yee Kan Koh and Zheng Liu}, doi = {10.1088/2053-1583/ab2816}, times_cited = {3}, issn = {2053-1583}, year = {2019}, date = {2019-10-01}, journal = {2D MATERIALS}, volume = {6}, number = {4}, publisher = {IOP PUBLISHING LTD}, address = {TEMPLE CIRCUS, TEMPLE WAY, BRISTOL BS1 6BE, ENGLAND}, abstract = {Black phosphorus captures enormous research attention on the anisotropic properties due to its orthorhombic crystal structure. Here the in-plane anisotropic thermoelectric behaviors of bulk black phosphorus crystals in the temperature range from 300 K to 600 K are reported for the first time. Based on the home-grown big size and high-quality black phosphorus crystals, the electrical conductivity and Seebeck coefficient are simultaneously measured by a ZEM-3 instrument system, and the thermal conductivity is measured by time-domain thermoreflectance (TDTR). For each individual parameter, the values along the zigzag and armchair directions show the same temperature-dependent trend. However, the electrical conductivity along the armchair direction is similar to two times of that along the zigzag direction, while the thermal conductivity along the armchair direction is only similar to one third of that along the zigzag direction. Furthermore, the Seebeck coefficients show almost isotropic behavior. As a result, the figure of merit ZT along the armchair direction is as large as similar to 5.5 times of that along the zigzag direction, exhibiting high anisotropy. The results of intrinsic, orientation-dependent thermoelectric behaviors not only shed light on the fundamental physical properties of black phosphorus but also provide guidelines in the device design for potential thermoelectric applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Black phosphorus captures enormous research attention on the anisotropic properties due to its orthorhombic crystal structure. Here the in-plane anisotropic thermoelectric behaviors of bulk black phosphorus crystals in the temperature range from 300 K to 600 K are reported for the first time. Based on the home-grown big size and high-quality black phosphorus crystals, the electrical conductivity and Seebeck coefficient are simultaneously measured by a ZEM-3 instrument system, and the thermal conductivity is measured by time-domain thermoreflectance (TDTR). For each individual parameter, the values along the zigzag and armchair directions show the same temperature-dependent trend. However, the electrical conductivity along the armchair direction is similar to two times of that along the zigzag direction, while the thermal conductivity along the armchair direction is only similar to one third of that along the zigzag direction. Furthermore, the Seebeck coefficients show almost isotropic behavior. As a result, the figure of merit ZT along the armchair direction is as large as similar to 5.5 times of that along the zigzag direction, exhibiting high anisotropy. The results of intrinsic, orientation-dependent thermoelectric behaviors not only shed light on the fundamental physical properties of black phosphorus but also provide guidelines in the device design for potential thermoelectric applications. |
Sun, Bo; Haunschild, Georg; Polanco, Carlos; Ju, James (Zi-Jian); Lindsay, Lucas; Koblmueller, Gregor; Koh, Yee Kan Dislocation-induced thermal transport anisotropy in single-crystal group-III nitride films Journal Article NATURE MATERIALS, 18 (2), pp. 136-+, 2019, ISSN: 1476-1122. @article{ISI:000456325600014, title = {Dislocation-induced thermal transport anisotropy in single-crystal group-III nitride films}, author = {Bo Sun and Georg Haunschild and Carlos Polanco and James (Zi-Jian) Ju and Lucas Lindsay and Gregor Koblmueller and Yee Kan Koh}, doi = {10.1038/s41563-018-0250-y}, times_cited = {0}, issn = {1476-1122}, year = {2019}, date = {2019-02-01}, journal = {NATURE MATERIALS}, volume = {18}, number = {2}, pages = {136-+}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {Dislocations, one-dimensional lattice imperfections, are common to technologically important materials such as III-V semiconductors, and adversely affect heat dissipation in, for example, nitride-based high-power electronic devices. For decades, conventional nonlinear elasticity models have predicted that this thermal resistance is only appreciable when the heat flux is perpendicular to the dislocations. However, this dislocation-induced anisotropic thermal transport has yet to be seen experimentally. Using time-domain thermoreflectance, we measure strong thermal transport anisotropy governed by highly oriented threading dislocation arrays throughout micrometre-thick, single-crystal indium nitride films. We find that the cross-plane thermal conductivity is almost tenfold higher than the in-plane thermal conductivity at 80 K when the dislocation density is similar to 3 x 10(10) cm(-2). This large anisotropy is not predicted by conventional models. With enhanced understanding of dislocation-phonon interactions, our results may allow the tailoring of anisotropic thermal transport with line defects, and could facilitate methods for directed heat dissipation in the thermal management of diverse device applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Dislocations, one-dimensional lattice imperfections, are common to technologically important materials such as III-V semiconductors, and adversely affect heat dissipation in, for example, nitride-based high-power electronic devices. For decades, conventional nonlinear elasticity models have predicted that this thermal resistance is only appreciable when the heat flux is perpendicular to the dislocations. However, this dislocation-induced anisotropic thermal transport has yet to be seen experimentally. Using time-domain thermoreflectance, we measure strong thermal transport anisotropy governed by highly oriented threading dislocation arrays throughout micrometre-thick, single-crystal indium nitride films. We find that the cross-plane thermal conductivity is almost tenfold higher than the in-plane thermal conductivity at 80 K when the dislocation density is similar to 3 x 10(10) cm(-2). This large anisotropy is not predicted by conventional models. With enhanced understanding of dislocation-phonon interactions, our results may allow the tailoring of anisotropic thermal transport with line defects, and could facilitate methods for directed heat dissipation in the thermal management of diverse device applications. |
2018 |
Zheng, Weidong; Huang, Bin; Li, Hongkun; Koh, Yee Kan Achieving Huge Thermal Conductance of Metallic Nitride on Graphene Through Enhanced Elastic and Inelastic Phonon Transmission Journal Article ACS APPLIED MATERIALS & INTERFACES, 10 (41), pp. 35487-35494, 2018, ISSN: 1944-8244. @article{ISI:000447954600073, title = {Achieving Huge Thermal Conductance of Metallic Nitride on Graphene Through Enhanced Elastic and Inelastic Phonon Transmission}, author = {Weidong Zheng and Bin Huang and Hongkun Li and Yee Kan Koh}, doi = {10.1021/acsami.8b12480}, times_cited = {0}, issn = {1944-8244}, year = {2018}, date = {2018-10-17}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {10}, number = {41}, pages = {35487-35494}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Low thermal conductance of metal contacts is one of the main challenges in the thermal management of nanoscale devices of graphene and other two-dimensional (2D) materials. Previous attempts to search for metal contacts with high thermal conductance yielded limited success because of the incomplete understanding of the origins of low thermal conductance. In this paper, we carefully study the intrinsic thermal conductance of metal/graphene/metal interfaces to identify the heat transport mechanisms across graphene interfaces. We find that unlike metal/diamond interfaces, the intrinsic thermal conductance of most graphene interfaces (except Ti and TiNx) is only approximate to 50% of the phonon radiation limit, suggesting that heat is carried across graphene interfaces mainly through the elastic transmission of phonons. We thus propose a convenient approach to substantially enhance the phononic heat transport across metal contacts on graphene, by better matching the energy of phonons in metals and graphene, for example, using metallic nitrides. We test the idea with TiNx with phonon frequencies of up to 1.2 x 10(14) rad/s, 39% of the highest phonon frequencies in graphene of 3.1 x 10(14)/s. Interestingly, we obtain a huge thermal conductance of 270 MW m(-2) K-1 for the TiNx/graphene interface, which is approximate to 140% of the phonon radiation limit. Thus, the huge thermal conductance cannot be fully explained by enhanced elastic phonon transport alone, but may be partially attributed to inelastic phonon transport across the TiNx/graphene interface. Our work provides guidance for the search for good metal contacts on 2D materials and devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Low thermal conductance of metal contacts is one of the main challenges in the thermal management of nanoscale devices of graphene and other two-dimensional (2D) materials. Previous attempts to search for metal contacts with high thermal conductance yielded limited success because of the incomplete understanding of the origins of low thermal conductance. In this paper, we carefully study the intrinsic thermal conductance of metal/graphene/metal interfaces to identify the heat transport mechanisms across graphene interfaces. We find that unlike metal/diamond interfaces, the intrinsic thermal conductance of most graphene interfaces (except Ti and TiNx) is only approximate to 50% of the phonon radiation limit, suggesting that heat is carried across graphene interfaces mainly through the elastic transmission of phonons. We thus propose a convenient approach to substantially enhance the phononic heat transport across metal contacts on graphene, by better matching the energy of phonons in metals and graphene, for example, using metallic nitrides. We test the idea with TiNx with phonon frequencies of up to 1.2 x 10(14) rad/s, 39% of the highest phonon frequencies in graphene of 3.1 x 10(14)/s. Interestingly, we obtain a huge thermal conductance of 270 MW m(-2) K-1 for the TiNx/graphene interface, which is approximate to 140% of the phonon radiation limit. Thus, the huge thermal conductance cannot be fully explained by enhanced elastic phonon transport alone, but may be partially attributed to inelastic phonon transport across the TiNx/graphene interface. Our work provides guidance for the search for good metal contacts on 2D materials and devices. |
2017 |
Huang, Bin; Koh, Yee Kan Negligible Electronic Contribution to Heat Transfer across Intrinsic Metal/Graphene Interfaces Journal Article ADVANCED MATERIALS INTERFACES, 4 (20), 2017, ISSN: 2196-7350. @article{ISI:000413572500014, title = {Negligible Electronic Contribution to Heat Transfer across Intrinsic Metal/Graphene Interfaces}, author = {Bin Huang and Yee Kan Koh}, doi = {10.1002/admi.201700559}, times_cited = {1}, issn = {2196-7350}, year = {2017}, date = {2017-10-23}, journal = {ADVANCED MATERIALS INTERFACES}, volume = {4}, number = {20}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {Despite the importance of high thermal conductance (G) of metal/graphene interfaces to thermal management of graphene devices, prior reported G of graphene interfaces is all relatively low. One possible route to improve G of metal/graphene interfaces is through additional heat conduction by electrons, since graphene can be easily doped by metals. In this study, we evaluate the electronic heat conduction across metal/graphene interfaces by measuring G of palladium (Pd)/transferred graphene (trG)/Pd interfaces, prepared by either thermal evaporation or magnetron sputtering. It is found that, for Pd/trG/Pd samples prepared by thermal evaporation, G = 42 MW m(-2) K-1 and G only weakly depends on temperature, suggesting that heat is predominantly carried by phonons. However, for Pd/trG/Pd samples prepared by sputtering, a significant increment of G is observed. G = 300 MW m(-2) K-1, and is roughly proportional to temperature. We attribute the enhancement of G to an additional channel of heat transport by electrons via atomic-scale pinholes generated in the graphene by ion bombardment during magetron sputtering. Thus, it is concluded that electrons play a negligible role in heat conduction across intrinsic metal/graphene interfaces, and the contribution of electrons is only substantial if graphene is damaged.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Despite the importance of high thermal conductance (G) of metal/graphene interfaces to thermal management of graphene devices, prior reported G of graphene interfaces is all relatively low. One possible route to improve G of metal/graphene interfaces is through additional heat conduction by electrons, since graphene can be easily doped by metals. In this study, we evaluate the electronic heat conduction across metal/graphene interfaces by measuring G of palladium (Pd)/transferred graphene (trG)/Pd interfaces, prepared by either thermal evaporation or magnetron sputtering. It is found that, for Pd/trG/Pd samples prepared by thermal evaporation, G = 42 MW m(-2) K-1 and G only weakly depends on temperature, suggesting that heat is predominantly carried by phonons. However, for Pd/trG/Pd samples prepared by sputtering, a significant increment of G is observed. G = 300 MW m(-2) K-1, and is roughly proportional to temperature. We attribute the enhancement of G to an additional channel of heat transport by electrons via atomic-scale pinholes generated in the graphene by ion bombardment during magetron sputtering. Thus, it is concluded that electrons play a negligible role in heat conduction across intrinsic metal/graphene interfaces, and the contribution of electrons is only substantial if graphene is damaged. |
Sun, Bo; Gu, Xiaokun; Zeng, Qingsheng; Huang, Xi; Yan, Yuexiang; Liu, Zheng; Yang, Ronggui; Koh, Yee Kan Temperature Dependence of Anisotropic Thermal-Conductivity Tensor of Bulk Black Phosphorus Journal Article ADVANCED MATERIALS, 29 (3), 2017, ISSN: 0935-9648. @article{ISI:000392729800005, title = {Temperature Dependence of Anisotropic Thermal-Conductivity Tensor of Bulk Black Phosphorus}, author = {Bo Sun and Xiaokun Gu and Qingsheng Zeng and Xi Huang and Yuexiang Yan and Zheng Liu and Ronggui Yang and Yee Kan Koh}, doi = {10.1002/adma.201603297}, times_cited = {0}, issn = {0935-9648}, year = {2017}, date = {2017-01-18}, journal = {ADVANCED MATERIALS}, volume = {29}, number = {3}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {The anisotropic thermal-conductivity tensor of bulk black phosphorus (BP) for 80 <= T <= 300 K is reported. Despite the anisotropy, phonons are predominantly scattered by Umklapp processes in all the crystallographic orientations. It is also found that the phonon mean-free-paths of BP are rather long (up to 1 mu m) in the through-plane direction.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The anisotropic thermal-conductivity tensor of bulk black phosphorus (BP) for 80 <= T <= 300 K is reported. Despite the anisotropy, phonons are predominantly scattered by Umklapp processes in all the crystallographic orientations. It is also found that the phonon mean-free-paths of BP are rather long (up to 1 mu m) in the through-plane direction. |
2016 |
Koh, Yee Kan; Lyons, Austin S; Bae, Myung-Ho; Huang, Bin; Dorgan, Vincent E; Cahill, David G; Pop, Eric Role of Remote Interfacial Phonon (RIP) Scattering in Heat Transport Across Graphene/SiO2 Interfaces Journal Article NANO LETTERS, 16 (10), pp. 6014-6020, 2016, ISSN: 1530-6984. @article{ISI:000385469800007, title = {Role of Remote Interfacial Phonon (RIP) Scattering in Heat Transport Across Graphene/SiO_{2} Interfaces}, author = {Yee Kan Koh and Austin S Lyons and Myung-Ho Bae and Bin Huang and Vincent E Dorgan and David G Cahill and Eric Pop}, doi = {10.1021/acs.nanolett.6b01709}, times_cited = {0}, issn = {1530-6984}, year = {2016}, date = {2016-10-01}, journal = {NANO LETTERS}, volume = {16}, number = {10}, pages = {6014-6020}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {heat transfer across interfaces of graphene and polar dielectrics (e.g.; SiO2) could be Mediated by direct: phonon coupling, as well as electronic coupling with remote interfacial phonons (RIPs). To understand-the relative contribution of each component, we develop a new pump probe technique called voltage-modulated thermorefleetance (VMTR) to accurately measure the change-of interfacial: thermal conductance under an electrostatic field. We employed VMTR on top gates of graphene field-,effect-transistors. and find that the thermal conductance of SiO2/graphene/SiO2 interfaces increases by up to Delta G approximate to 0.8 MW M-2 K-1 under electrostatic fields of <0.2 V nm(-1). We propose two possible explanations for-the small observed Delta G. First, because the applied electrostatic field induces charge carriers in graphene, out VMTR measurements could originate from heat transfer-between the charge carriers in graphene and RIPs in SiO2. Second, the increase in heat conduction could be caused by better conformity of graphene interfaces under electrostatic pressure exerted by the induced charge carriers. Regardless of the origins of the observed Delta G, our VMTR measurements eStablish, an upper limit for heat transfer from unbiased graphene to SiO2 substrates via RIP scattering; for example, only <2% of the interfacial heat transport is facilitated' y RIP scattering even at a carrier concentration of similar to 4 X 10(12) cm(-2).}, keywords = {}, pubstate = {published}, tppubtype = {article} } heat transfer across interfaces of graphene and polar dielectrics (e.g.; SiO2) could be Mediated by direct: phonon coupling, as well as electronic coupling with remote interfacial phonons (RIPs). To understand-the relative contribution of each component, we develop a new pump probe technique called voltage-modulated thermorefleetance (VMTR) to accurately measure the change-of interfacial: thermal conductance under an electrostatic field. We employed VMTR on top gates of graphene field-,effect-transistors. and find that the thermal conductance of SiO2/graphene/SiO2 interfaces increases by up to Delta G approximate to 0.8 MW M-2 K-1 under electrostatic fields of <0.2 V nm(-1). We propose two possible explanations for-the small observed Delta G. First, because the applied electrostatic field induces charge carriers in graphene, out VMTR measurements could originate from heat transfer-between the charge carriers in graphene and RIPs in SiO2. Second, the increase in heat conduction could be caused by better conformity of graphene interfaces under electrostatic pressure exerted by the induced charge carriers. Regardless of the origins of the observed Delta G, our VMTR measurements eStablish, an upper limit for heat transfer from unbiased graphene to SiO2 substrates via RIP scattering; for example, only <2% of the interfacial heat transport is facilitated' y RIP scattering even at a carrier concentration of similar to 4 X 10(12) cm(-2). |
Huang, Bin; Koh, Yee Kan Improved topological conformity enhances heat conduction across metal contacts on transferred graphene Journal Article CARBON, 105 , pp. 268-274, 2016, ISSN: 0008-6223. @article{ISI:000376607200031, title = {Improved topological conformity enhances heat conduction across metal contacts on transferred graphene}, author = {Bin Huang and Yee Kan Koh}, doi = {10.1016/j.carbon.2016.04.005}, times_cited = {0}, issn = {0008-6223}, year = {2016}, date = {2016-08-01}, journal = {CARBON}, volume = {105}, pages = {268-274}, publisher = {PERGAMON-ELSEVIER SCIENCE LTD}, address = {THE BOULEVARD, LANGFORD LANE, KIDLINGTON, OXFORD OX5 1GB, ENGLAND}, abstract = {Thermal conductance of metal contacts on transferred graphene (trG) could be significantly reduced from the intrinsic value due to additional resistance by increased roughness, residues, oxides and voids. In this paper, we compare the thermal conductance (G) of Al/trG/Cu interfaces with that of Al/grG/Cu interfaces. We find that for the Al/grG/Cu interfaces of as-grown CVD graphene, G = 31 MW m(-2) K-1 at room temperature, two orders of magnitude lower than that of Al/Cu interfaces. For most as-transferred graphene on Cu films, G approximate to 20 MW m(-2) K-1, approximate to 35 % lower than that of as-grown CVD graphene. We rule out the contributions of residues, native oxides and interfaces roughness, and attribute the difference to different degrees of conformity of graphene to the Cu substrates. We find that a contact area of approximate to 50 % only reduces the thermal conductance by approximate to 35 %, suggesting that a small amount of heat transfer occurs across "voids" at graphene interfaces. We improve the conformity of the as-transferred graphene by annealing the samples at 300 degrees C, and thus enhancing the thermal conductance to the intrinsic value. From the temperature dependence measurements, we also confirm that phonons are the dominant heat carries across the metal/graphene/metal interfaces despite a substantial carrier concentration of approximate to 3 x 10(12) cm(-2) induced in the graphene. (C) 2016 Elsevier Ltd. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Thermal conductance of metal contacts on transferred graphene (trG) could be significantly reduced from the intrinsic value due to additional resistance by increased roughness, residues, oxides and voids. In this paper, we compare the thermal conductance (G) of Al/trG/Cu interfaces with that of Al/grG/Cu interfaces. We find that for the Al/grG/Cu interfaces of as-grown CVD graphene, G = 31 MW m(-2) K-1 at room temperature, two orders of magnitude lower than that of Al/Cu interfaces. For most as-transferred graphene on Cu films, G approximate to 20 MW m(-2) K-1, approximate to 35 % lower than that of as-grown CVD graphene. We rule out the contributions of residues, native oxides and interfaces roughness, and attribute the difference to different degrees of conformity of graphene to the Cu substrates. We find that a contact area of approximate to 50 % only reduces the thermal conductance by approximate to 35 %, suggesting that a small amount of heat transfer occurs across "voids" at graphene interfaces. We improve the conformity of the as-transferred graphene by annealing the samples at 300 degrees C, and thus enhancing the thermal conductance to the intrinsic value. From the temperature dependence measurements, we also confirm that phonons are the dominant heat carries across the metal/graphene/metal interfaces despite a substantial carrier concentration of approximate to 3 x 10(12) cm(-2) induced in the graphene. (C) 2016 Elsevier Ltd. All rights reserved. |
Sun, Bo; Koh, Yee Kan REVIEW OF SCIENTIFIC INSTRUMENTS, 87 (6), 2016, ISSN: 0034-6748. @article{ISI:000379177000065, title = {Understanding and eliminating artifact signals from diffusely scattered pump beam in measurements of rough samples by time-domain thermoreflectance (TDTR)}, author = {Bo Sun and Yee Kan Koh}, doi = {10.1063/1.4952579}, times_cited = {0}, issn = {0034-6748}, year = {2016}, date = {2016-06-01}, journal = {REVIEW OF SCIENTIFIC INSTRUMENTS}, volume = {87}, number = {6}, publisher = {AMER INST PHYSICS}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {Time-domain thermoreflectance (TDTR) is a pump-probe technique frequently applied to measure the thermal transport properties of bulk materials, nanostructures, and interfaces. One of the limitations of TDTR is that it can only be employed to samples with a fairly smooth surface. For rough samples, artifact signals are collected when the pump beam in TDTR measurements is diffusely scattered by the rough surface into the photodetector, rendering the TDTR measurements invalid. In this paper, we systemically studied the factors affecting the artifact signals due to the pump beam leaked into the photodetector and thus established the origin of the artifact signals. We find that signals from the leaked pump beam are modulated by the probe beam due to the phase rotation induced in the photodetector by the illumination of the probe beam. As a result of the modulation, artifact signals due to the leaked pump beam are registered in TDTR measurements as the out-of-phase signals. We then developed a simple approach to eliminate the artifact signals due to the leaked pump beam. We verify our leak-pump correction approach by measuring the thermal conductivity of a rough InN sample, when the signals from the leaked pump beam are significant. We also discuss the advantages of our new method over the two-tint approach and its limitations. Our new approach enables measurements of the thermal conductivity of rough samples using TDTR. Published by AIP Publishing.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Time-domain thermoreflectance (TDTR) is a pump-probe technique frequently applied to measure the thermal transport properties of bulk materials, nanostructures, and interfaces. One of the limitations of TDTR is that it can only be employed to samples with a fairly smooth surface. For rough samples, artifact signals are collected when the pump beam in TDTR measurements is diffusely scattered by the rough surface into the photodetector, rendering the TDTR measurements invalid. In this paper, we systemically studied the factors affecting the artifact signals due to the pump beam leaked into the photodetector and thus established the origin of the artifact signals. We find that signals from the leaked pump beam are modulated by the probe beam due to the phase rotation induced in the photodetector by the illumination of the probe beam. As a result of the modulation, artifact signals due to the leaked pump beam are registered in TDTR measurements as the out-of-phase signals. We then developed a simple approach to eliminate the artifact signals due to the leaked pump beam. We verify our leak-pump correction approach by measuring the thermal conductivity of a rough InN sample, when the signals from the leaked pump beam are significant. We also discuss the advantages of our new method over the two-tint approach and its limitations. Our new approach enables measurements of the thermal conductivity of rough samples using TDTR. Published by AIP Publishing. |