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1. Perspectives on interfacial thermal resistance of 2D materials: Raman characterization and underlying physics

   https://doi.org/10.1007/s44251-024-00037-6  

   Liu, Jing ; Al Keyyam, Ibrahim ; Xie, Yangsu ; Wang, Xinwei ( February 2024 , Surface Science and Technology)

   Interfacial thermal resistance plays a crucial role in efficient heat dissipation in modern electronic devices. It is critical to understand the interfacial thermal transport from both experiments and underlying physics. This review is focused on the transient opto-thermal Raman-based techniques for measuring the interfacial thermal resistance between 2D materials and substrate. This transient idea eliminates the use of laser absorption and absolute temperature rise data, therefore provides some of the highest level measurement accuracy and physics understanding. Physical concepts and perspectives are given for the time-domain differential Raman (TD-Raman), frequency-resolved Raman (FR-Raman), energy transport state-resolved Raman (ET-Raman), frequency domain ET-Raman (FET-Raman), as well as laser flash Raman and dual-wavelength laser flash Raman techniques. The thermal nonequilibrium between optical and acoustic phonons, as well as hot carrier diffusion must be considered for extremely small domain characterization of interfacial thermal resistance. To have a better understanding of phonon transport across material interfaces, we introduce a new concept termed effective interface energy transmission velocity. It is very striking that many reported interfaces have an almost constant energy transmission velocity over a wide temperature range. This physics consideration is inspired by the thermal reffusivity theory, which is effective for analyzing structure-phonon scattering. We expect the effective interface energy transmission velocity to give an intrinsic picture of the transmission of energy carriers, unaltered by the influence of their capacity to carry heat.

    

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2. Direct Characterization of Thermal Nonequilibrium between Optical and Acoustic Phonons in Graphene Paper under Photon Excitation

   https://doi.org/10.1002/advs.202004712  

   Zobeiri, Hamidreza ; Hunter, Nicholas ; Wang, Ridong ; Wang, Tianyu ; Wang, Xinwei ( May 2021 , Advanced Science)

   Raman spectroscopy has been widely used to measure thermophysical properties of 2D materials. The local intense photon heating induces strong thermal nonequilibrium between optical and acoustic phonons. Both first principle calculations and recent indirect Raman measurements prove this phenomenon. To date, no direct measurement of the thermal nonequilibrium between optical and acoustic phonons has been reported. Here, this physical phenomenon is directly characterized for the first time through a novel approach combining both electrothermal and optothermal techniques. While the optical phonon temperature is determined from Raman wavenumber, the acoustic phonon temperature is precisely determined using high‐precision thermal conductivity and laser power absorption that are measured with negligible nonequilibrium among energy carriers. For graphene paper, the energy coupling factor between in‐plane optical and overall acoustic phonons is found at (1.59–3.10) × 10¹⁵W m⁻³K⁻¹, agreeing well with the quantum mechanical modeling result of 4.1 × 10¹⁵W m⁻³K⁻¹. Under ≈1 µm diameter laser heating, the optical phonon temperature rise is over 80% higher than that of the acoustic phonons. This observation points out the importance of subtracting optical–acoustic phonon thermal nonequilibrium in Raman‐based thermal characterization.

    

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3. Enhanced Thermal Boundary Conductance in Few‐Layer Ti 3 C 2 MXene with Encapsulation

   https://doi.org/10.1002/adma.201801629  

   Yasaei, Poya ; Hemmat, Zahra ; Foss, Cameron J. ; Li, Shixuan Justin ; Hong, Liang ; Behranginia, Amirhossein ; Majidi, Leily ; Klie, Robert F. ; Barsoum, Michel W. ; Aksamija, Zlatan ; et al ( September 2018 , Advanced Materials)

   Van der Waals interactions in 2D materials have enabled the realization of nanoelectronics with high‐density vertical integration. Yet, poor energy transport through such 2D–2D and 2D–3D interfaces can limit a device's performance due to overheating. One long‐standing question in the field is how different encapsulating layers (e.g., contact metals or gate oxides) contribute to the thermal transport at the interface of 2D materials with their 3D substrates. Here, a novel self‐heating/self‐sensing electrical thermometry platform is developed based on atomically thin, metallic Ti₃C₂MXene sheets, which enables experimental investigation of the thermal transport at a Ti₃C₂/SiO₂interface, with and without an aluminum oxide (AlO_x) encapsulating layer. It is found that at room temperature, the thermal boundary conductance (TBC) increases from 10.8 to 19.5 MW m⁻²K⁻¹upon AlO_xencapsulation. Boltzmann transport modeling reveals that the TBC can be understood as a series combination of an external resistance between the MXene and the substrate, due to the coupling of low‐frequency flexural acoustic (ZA) phonons to substrate modes, and an internal resistance between ZA and in‐plane phonon modes. It is revealed that internal resistance is a bottle‐neck to heat removal and that encapsulation speeds up the heat transfer into low‐frequency ZA modes and reduces their depopulation, thus increasing the effective TBC.

    

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4. Distinguishing Optical and Acoustic Phonon Temperatures and Their Energy Coupling Factor under Photon Excitation in nm 2D Materials

   https://doi.org/10.1002/advs.202000097  

   Wang, Ridong ; Zobeiri, Hamidreza ; Xie, Yangsu ; Wang, Xinwei ; Zhang, Xing ; Yue, Yanan ( May 2020 , Advanced Science)

   Under photon excitation, 2D materials experience cascading energy transfer from electrons to optical phonons (OPs) and acoustic phonons (APs). Despite few modeling works, it remains a long‐history open problem to distinguish the OP and AP temperatures, not to mention characterizing their energy coupling factor (G). Here, the temperatures of longitudinal/transverse optical (LO/TO) phonons, flexural optical (ZO) phonons, and APs are distinguished by constructing steady and nanosecond (ns) interphonon branch energy transport states and simultaneously probing them using nanosecond energy transport state‐resolved Raman spectroscopy. ΔT_{OP −AP}is measured to take more than 30% of the Raman‐probed temperature rise. A breakthrough is made on measuring the intrinsic in‐plane thermal conductivity of suspended nm MoS₂and MoSe₂by completely excluding the interphonon cascading energy transfer effect, rewriting the Raman‐based thermal conductivity measurement of 2D materials.G_OP↔APfor MoS₂, MoSe₂, and graphene paper (GP) are characterized. For MoS₂and MoSe₂,G_OP↔APis in the order of 10¹⁵and 10¹⁴W m⁻³K⁻¹andG_ZO↔APis much smaller thanG_LO/TO↔AP. Under ns laser excitation,G_OP↔APis significantly increased, probably due to the reduced phonon scattering time by the significantly increased hot carrier population. For GP,G_LO/TO↔APis 0.549 × 10¹⁶W m⁻³K⁻¹, agreeing well with the value of 0.41 × 10¹⁶W m⁻³K⁻¹by first‐principles modeling.

    

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5. Differential multi-probe thermal transport measurements of multi-walled carbon nanotubes grown by chemical vapor deposition

   https://doi.org/10.1016/j.ijheatmasstransfer.2023.124535  

   Jia, Qianru ; Zhou, Yuanyuan ; Li, Xun ; Lindsay, Lucas ; Shi, Li ( December 2023 , International Journal of Heat and Mass Transfer)

   Carbon nanotubes (CNTs) are quasi-one dimensional nanostructures that display both high thermal conductivity for potential thermal management applications and intriguing low-dimensional phonon transport phenomena. In comparison to the advances made in the theoretical calculation of the lattice thermal conductivity of CNTs, thermal transport measurements of CNTs have been limited by either the poor temperature sensitivity of Raman thermometry technique or the presence of contact thermal resistance errors in sensitive two-probe resistance thermometry measurements. Here we report advances in a multi-probe measurement of the intrinsic thermal conductivity of individual multi-walled CNT samples that are transferred from the growth substrate onto the measurement device. The sample-thermometer thermal interface resistance is directly measured by this multi-probe method and used to model the temperature distribution along the contacted sample segment. The detailed temperature profile helps to eliminate the contact thermal resistance error in the obtained thermal conductivity of the suspended sample segment. A differential electro-thermal bridge measurement method is established to enhance the signal-to-noise ratio and reduce the measurement uncertainty by over 40%. The obtained thermal resistances of multiple suspended segments of the same MWCNT samples increase nearly linearly with increasing length, revealing diffusive phonon transport as a result of phonon-defect scattering in these MWCNT samples. The measured thermal conductivity increases with temperature and reaches up to 390 ± 20 W m-1 K-1 at room temperature for a 9-walled MWCNT. Theoretical analysis of the measurement results suggests submicron phonon mean free paths due to extrinsic phonon scattering by extended defects such as grain boundaries. The obtained thermal conductivity is decreased by a factor of 3 upon electron beam damage and surface contamination of the CNT sample. 

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