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1. Significant ballistic thermal transport across graphene layers: Effect of nanoholes and lithium intercalation

   https://doi.org/10.1063/5.0275738  

   Crosby, John; Cui, Haoran; Wang, Yan (August 2025, Applied Physics Letters)

   Porous graphene and graphite are increasingly utilized in electrochemical energy storage and solar-thermal applications due to their unique structural and thermal properties. In this study, we conduct a comprehensive analysis of the lattice thermal transport and spectral phonon characteristics of holey graphite and multilayer graphene. Our results reveal that phonon modes propagating obliquely with respect to the graphene basal planes are the primary contributors to cross-plane thermal transport. These modes exhibit a predominantly ballistic nature, resulting in an almost linear increase in cross-plane thermal conductivity with the number of layers. The presence of nanoholes in graphene induces a broadband suppression of cross-plane phonon transport, whereas lithium-ion intercalation shows potential to enhance it. These findings provide critical insights into the mechanisms governing cross-plane heat conduction in key graphene-based structures, offering valuable guidance for thermal management and engineering of van der Waals materials. 

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2. Thermal conductivity of hexagonal BC ₂ P – a first-principles study

   https://doi.org/10.1039/d0ra08444a  

   Muthaiah, Rajmohan; Tarannum, Fatema; Annam, Roshan Sameer; Nayal, Avinash Singh; Danayat, Swapneel; Garg, Jivtesh (November 2020, RSC Advances)

   null (Ed.)

   In this work, we report a high thermal conductivity ( k ) of 162 W m −1 K −1 and 52 W m −1 K −1 at room temperature, along the directions perpendicular and parallel to the c -axis, respectively, of bulk hexagonal BC 2 P (h-BC 2 P), using first-principles calculations. We systematically investigate elastic constants, phonon group velocities, phonon linewidths and mode thermal conductivity contributions of transverse acoustic (TA), longitudinal acoustic (LA) and optical phonons. Interestingly, optical phonons are found to make a large contribution of 30.1% to the overall k along a direction perpendicular to the c -axis at 300 K. BC 2 P is also found to exhibit high thermal conductivity at nanometer length scales. At 300 K, a high k value of ∼47 W m −1 K −1 is computed for h-BC 2 P at a nanometer length scale of 50 nm, providing avenues for achieving efficient nanoscale heat transfer. 

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3. Highly tailorable thermomechanical properties of nanograined silicon: Importance of grain size and grain anisotropy

   https://doi.org/10.1063/5.0185911  

   Cao, Jiahui; Wang, Han; Ferrer-Argemi, Laia; Cao, Penghui; Lee, Jaeho (February 2024, Applied Physics Letters)

   Nanocrystalline silicon can have unique thermal transport and mechanical properties governed by its constituent grain microstructure. Here, we use phonon ray-tracing and molecular dynamics simulations to demonstrate the largely tunable thermomechanical behaviors with varying grain sizes (a0) and aspect ratios (ξ). Our work shows that, by selectively increasing the grain size along the heat transfer direction while keeping the grain area constant, the in-plane lattice thermal conductivity (kx) increases more significantly than the cross-plane lattice thermal conductivity (ky) due to anisotropic phonon–grain boundary scattering. While kx generally increases with increasing ξ, a critical value exists for ξ at which kx reaches its maximum. Beyond this transition point, further increases in ξ result in a decrease in kx due to substantial scattering of low-frequency phonons with anisotropic grain boundaries. Moreover, we observe reductions in the elastic and shear modulus with decreasing grain size, and this lattice softening leads to significant reductions in phonon group velocity and thermal conductivity. By considering both thermal and mechanical size effects, we identify two distinct regimes of thermal transport, in which anisotropic phonon–grain boundary scattering becomes more appreciable at low temperatures and lattice softening becomes more pronounced at high temperatures. Through phonon spectral analysis, we attribute the significant thermal conductivity anisotropy in nanograined silicon to grain boundary scattering of low-frequency phonons and the softening-driven thermal conductivity reduction to Umklapp scattering of high-frequency phonons. These findings offer insights into the manipulation of thermomechanical properties of nanocrystalline silicon via microstructure engineering, carrying profound implications for the development of future nanomaterials. 

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4. Two stage decoherence of optical phonons in long oligomers

   https://doi.org/10.1063/5.0222580  

   Burin, Alexander L; Rubtsov, Igor V (September 2024, The Journal of Chemical Physics)

   Molecular vibrations are generally responsible for chemical energy transport and dissipation in molecular systems. This transport is fast and efficient if energy is transferred by optical phonons in periodic oligomers, but its efficiency is limited by decoherence emerging due to anharmonic interactions with acoustic phonons. Using a general theoretical model, we show that in the most common case of the optical phonon band being narrower than the acoustic bands, decoherence takes place in two stages. The faster stage involves optical phonon multiple forward scattering due to absorption and emission of transverse acoustic phonons, i.e., collective bending modes with a quadratic spectrum; the transport remains ballistic and the speed can be altered. The subsequent slower stage involves phonon backscattering in multiphonon processes involving two or more acoustic phonons resulting in a switch to diffusive transport. If the initially excited optical phonon possesses a relatively small group velocity, then it is accelerated in the first stage due to its transitions to states propagating faster. This theoretical expectation is consistent with the recent measurements of optical phonon transport velocity in alkane chains, increasing with increasing the chain length. 

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5. Moiré Pattern Controlled Phonon Polarizer Based on Twisted Graphene

   https://doi.org/10.1002/adma.202312176  

   Qin, Zihao; Dai, Lingyun; Li, Man; Li, Suixuan; Wu, Huan; White, Katherine E; Gani, Gilad; Weiss, Paul S; Hu, Yongjie (June 2024, Advanced Materials)

   Twisted van der Waals materials featuring Moiré patterns present new design possibilities and demonstrate unconventional behaviors in electrical, optical, spintronic, and superconducting properties. However, experimental exploration of thermal transport across Moiré patterns has not been as extensive, despite its critical role in nanoelectronics, thermal management, and energy technologies. Here, the first experimental study is conducted on thermal transport across twisted graphene, demonstrating a phonon polarizer concept from the rotational misalignment between stacked layers. The direct thermal and acoustic measurements, structural characterizations, and atomistic modeling, reveal a modulation up to 631% in thermal conductance with various Moiré angles, while maintaining a high acoustic transmission. By comparing experiments with density functional theory and molecular dynamics simulations, mode‐dependent phonon transmissions are quantified based on the angle alignment of graphene band structures and attributed to the coupling among flexural phonon modes. The agreement confirms the dominant tuning mechanisms in adjusting phonon transmission from high‐frequency thermal modes while having negligible effects on low‐frequency acoustic modes near Brillouin zone center. This study offers crucial insights into the fundamental thermal transport in Moiré structures, opening avenues for the invention of quantum thermal devices and new design methodologies based on manipulations of vibrational band structures and phonon spectra. 

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