Layering Boosts Heat Flow in New Material

Researchers have identified stacking order as a key control mechanism for heat dissipation in two-dimensional materials, a critical challenge for advancing van der Waals electronics. Yongjian Zhou from the Walker Department of Mechanical Engineering at The University of Texas at Austin, Haoran Cui from the Department of Mechanical Engineering at the University of Nevada, Reno, and Zefang Ye, working with colleagues including Jung-Fu Lin from the Department of Geological Sciences, Jackson School of Geosciences at The University of Texas at Austin, and Yan Wang and Yaguo Wang from the Department of Mechanical Engineering, University of Nevada, Reno, and the Walker Department of Mechanical Engineering at The University of Texas at Austin, demonstrate that manipulating the arrangement of Rhenium Disulfide (ReS2) layers significantly alters thermal conductivity. Their findings reveal unexpectedly long phonon mean free paths and a transition to ballistic heat transport, with AA stacking exhibiting substantially higher thermal conductivity than AB stacking due to enhanced phonon lifetimes. This research establishes ReS2 as a promising material for engineering thermal properties across different regimes, offering a novel pathway for effective thermal management in future nanoscale devices.

Efficient heat dissipation is a persistent challenge in miniaturising electronic devices. New work with the material rhenium disulfide reveals a surprising degree of control over how heat flows through layered structures. By carefully stacking atomic sheets, scientists can dramatically alter thermal conductivity, paving the way for better thermal management in future electronics.

Scientists have uncovered a new level of control over heat dissipation in two-dimensional materials, potentially revolutionising thermal management in future electronics. Research focused on multilayer Rhenium Disulfide (ReS2) demonstrates that the way these layers are stacked dramatically influences how efficiently heat travels through the material.

Specifically, the stacking order, whether layers align directly (AA stacking) or with a slight offset (AB stacking), acts as a precise “knob” to tune thermal conductivity. Thermal conductivity measurements reveal remarkably long cross-plane phonon mean free paths (MFPs) exceeding 200-300 nanometres, indicating that heat can travel surprisingly far before being scattered within the material.

This work provides the first direct observation of a transition from quasi-ballistic to ballistic transport, a regime where phonons, the primary carriers of heat, travel almost without resistance. AA stacking exhibits nearly double the cross-plane thermal conductivity compared to AB stacking, a result attributed to more “coherent” interlayer connections that allow acoustic phonons to travel further without losing energy.

Integrated molecular dynamics simulations, enhanced by a deep neural network, reveal that ReS2 acts as a frequency-selective filter for phonons. Weak interlayer interactions block high-frequency phonons, while stronger coupling broadens the range of frequencies that can transmit heat. These findings establish ReS2 as a model material for engineering thermal transport across different regimes, from diffusive (where heat spreads randomly) to quasi-ballistic and fully ballistic (where heat travels directionally with minimal scattering).

This capability offers a new framework for designing advanced thermal management systems in two-dimensional electronics, potentially leading to more efficient and reliable devices. The ability to manipulate heat flow at this level could be crucial for overcoming a key bottleneck in the development of next-generation electronic components.

Determining cross-plane thermal conductivity in ReS2 flakes via 3ω method and stacking configuration analysis

Thermal conductivity measurements were performed on multilayer Rhenium Disulfide (ReS2) flakes to investigate cross-plane heat transport, a key limitation in two-dimensional electronic devices. ReS2 samples of varying thicknesses were exfoliated onto silicon dioxide substrates and patterned using electron beam lithography to define micro-scale devices.

Cross-plane thermal conductivity was then determined using a 3ω method, where a metallic heater acts as both heat source and thermometer, allowing precise temperature rise measurements as a function of frequency. This technique is particularly sensitive to the thermal properties of the underlying material and minimizes the influence of contact thermal resistance.

To establish the relationship between stacking order and thermal transport, ReS2 flakes were intentionally stacked in both AA and AB configurations, leveraging van der Waals heterostructure assembly techniques. Atomic force microscopy and Raman spectroscopy were employed to confirm the stacking arrangement and material quality of each sample. The resulting devices were subjected to the same 3ω measurements, enabling a direct comparison of cross-plane thermal conductivity between the two stacking orders.

Further insight into the phonon behaviour within ReS2 was gained through deep molecular dynamics simulations. These simulations employed a Nosé thermostat to maintain a constant temperature, accurately modelling the atomic vibrations and energy transfer processes. The simulations incorporated a spectral energy density approach to predict phonon dispersion relations and lifetimes, providing a detailed understanding of how interlayer coupling filters phonon frequencies. This computational methodology allowed researchers to explore the fundamental mechanisms governing coherent and incoherent phonon transport, complementing the experimental findings and revealing the frequency-selective nature of heat conduction in ReS2.

Stacking-dependent ballistic heat transfer and extended phonon mean free paths in multilayer Rhenium Disulfide

Cross-plane thermal conductivity measurements reveal remarkably long phonon mean free paths exceeding 200-300nm in multilayer Rhenium Disulfide (ReS2), directly observing a transition from quasi-ballistic to a thickness-independent ballistic limit of heat transfer. Specifically, the research demonstrates a substantial increase in cross-plane thermal conductivity as ReS2 thickness increases, with AA stacking exhibiting values nearly double those of AB stacking.

This difference arises from more coherent interlayer registry in the AA configuration, leading to extended acoustic phonon lifetimes. Ultrathin ReS2 films, below 100nm in thickness, display similar thermal conductivities for both stacking orders, consistent with boundary scattering dominating heat transfer at these dimensions. However, as thickness surpasses 100nm, a clear divergence emerges, with AA-stacked samples reaching a saturation plateau of approximately 4.0W m-1 K-1.

AB stacking, conversely, shows a slower increase in thermal conductivity and does not achieve the same plateau, indicating a less efficient pathway for heat dissipation. The observed difference in thermal conductivity is directly linked to the interlayer coupling and phonon lifetimes within each stacking configuration. Integrated deep neural-network molecular dynamics simulations reveal that weak van der Waals coupling acts as a low-pass filter for phonons, while stronger coupling broadens the transmission passband.

This frequency-selective phonon filtering explains how ReS2 can transition between diffusive, quasi-ballistic, and ballistic regimes of heat conduction. The study further establishes that the difference between AA and AB stacking is measurable using Raman spectroscopy, identifying a peak separation of approximately 13cm-1 for AA stacking and 20cm-1 for AB stacking, allowing for reliable characterisation of sample configurations. These findings position ReS2 as a model material for engineering thermal management in two-dimensional electronics through precise control of stacking order and interlayer interactions.

Layered rhenium disulphide exhibits unexpectedly efficient nanoscale heat conduction through stacking manipulation

Scientists have long recognised that managing heat dissipation is paramount in ever-shrinking electronic devices. However, understanding how heat travels at the nanoscale, particularly across layers in 2D materials, has proven remarkably difficult. Traditional views of heat transfer often assume a chaotic, diffusive process where phonons, the quantum units of heat, scatter frequently, limiting how far they can travel before losing energy.

This work on rhenium disulphide (ReS2) challenges that assumption, revealing conditions where heat can flow almost unimpeded, akin to a ‘ballistic’ trajectory. The significance lies in demonstrating a degree of control over this thermal transport. By manipulating the stacking order of ReS2 layers, researchers have shown they can nearly double thermal conductivity.

This isn’t merely an incremental improvement; it suggests a pathway towards engineering materials where heat flow is dictated by design, not just material properties. The discovery that interlayer coupling acts as a frequency-selective filter for phonons is particularly insightful, offering a new parameter for thermal tuning. While the experiments and simulations point to ballistic-like behaviour over considerable distances, maintaining this coherence in complex, real-world devices will be a substantial hurdle.

The precise impact of defects and imperfections, inevitable in manufactured materials, needs further investigation. Nevertheless, this work opens exciting possibilities. Beyond ReS2, the principles established here could guide the development of entirely new classes of 2D materials and heterostructures tailored for efficient thermal management, potentially paving the way for smaller, faster, and more reliable electronics.