Ultra-Low Thermal Conductivity of Moiré Diamanes

At a Glance

Metadata	Details
Publication Date	2022-09-25
Journal	Membranes
Authors	Suman Chowdhury, V. A. Demin, Л. А. Чернозатонский, Alexander G. Kvashnin
Institutions	Shiv Nadar University, Institute of Biochemical Physics NM Emanuel
Citations	7
Analysis	Full AI Review Included

Executive Summary

The study investigates the lattice thermal conductivity (K_L) of hydrogenated (H-Dnθ) and fluorinated (F-Dnθ) Moiré Diamanes, revealing a strong dependence on the bi-layer twist angle (θ).

Tunable Thermal Conductivity: K_L is highly tunable, decreasing sharply from high values in untwisted structures (θ=0°) to ultra-low values as the twist angle increases toward 30°.
Ultra-Low K_L Achievement: Hydrogenated Moiré diamane (H-Dn27, θ=27.8°) exhibits an ultra-low K_L of 32 W/mK at 300 K, representing a reduction of over 97% compared to the ordered AB-stacked diamane (1360 W/mK).
Mechanism of Reduction: The drastic decrease in K_L is linked to increased structural disorder, strong symmetry breaking, and high anharmonicity (enhanced Umklapp scattering), particularly pronounced in hydrogenated systems at high twist angles.
Passivation Effect: Fluorinated diamanes generally show lower K_L than hydrogenated ones at θ=0° (361 W/mK vs. 1360 W/mK), but the K_L reduction with increasing twist angle is less critical, suggesting heavy fluorine atoms stabilize the structure.
Methodology: Calculations utilized a combination of Density Functional Theory (DFT) and passively trained Machine Learning Interatomic Potentials (Moment Tensor Potentials, MTP) to solve the phonon Boltzmann Transport Equation (BTE).
Prospective Materials: Diamanes offer a unique platform for thermal management, capable of serving as both high-conductivity heat sinks and low-conductivity insulators, depending on the engineered twist angle.

Technical Specifications

The following table summarizes the key thermal and structural parameters derived from the computational analysis of Moiré Diamanes at 300 K.

Parameter	Value	Unit	Context
Highest K_L (H-Dn0)	1360	W/mK	Hydrogenated AB-stacked diamane (θ=0°).
Lowest K_L (H-Dn27)	32	W/mK	Hydrogenated Moiré diamane (θ=27.8°).
K_L (F-Dn0)	361	W/mK	Fluorinated AB-stacked diamane (θ=0°).
K_L (F-Dn27)	90	W/mK	Fluorinated Moiré diamane (θ=27.8°).
Twist Angles (θ) Studied	0, 13.2, 21.8, 27.8	Degrees (°)	Angles corresponding to small unit cells in twisted bi-layer graphene.
H-Dn0 K_L ~ T^α Fit (α)	-1.98	Dimensionless	Temperature dependence fitting parameter for ordered H-diamane.
F-Dn0 K_L ~ T^α Fit (α)	-1.07	Dimensionless	Temperature dependence fitting parameter for ordered F-diamane.
H-Dn27 K_L ~ T^α Fit (α)	-1.36	Dimensionless	Temperature dependence fitting parameter for highly disordered H-diamane.
H-Diamane High-Frequency Modes	~85	THz	Corresponds to surface hydrogen adatom vibrations perpendicular to the film.
C-C Interlayer Bond Length Range	1.5 to 1.8	Angstrom (A)	Non-zero distribution observed in disordered Moiré diamane (Dn13).

Key Methodologies

The lattice thermal conductivity (K_L) was determined using a multi-step computational approach combining first-principles methods with machine learning potentials.

Geometry Optimization (DFT):
- Structures were relaxed using the VASP package, employing the Generalized Gradient Approximation (GGA, Perdew-Burke-Ernzerhof functional) and the Projector Augmented Wave (PAW) method.
- A plane-wave energy cutoff of 500 eV was applied.
- Relaxation continued until the change in total energy was less than 10^-4 eV.
Machine Learning Potential (MTP) Training:
- Moment Tensor Potentials (MTP) were trained to accurately calculate anharmonic force constants, replacing hundreds of expensive DFT calculations.
- Training sets were generated via Ab Initio Molecular Dynamics (AIMD) simulations (2000 time steps, 1 fs time step).
- Two AIMD configurations were used: constant temperature (50 K) and temperature reduction (1000 K down to 200 K).
Phonon Property Calculation:
- MTP-derived force constants were used in the PHONOPY package to calculate phonon dispersion curves.
- Fifth-nearest neighbor interactions were included to ensure accurate K_L calculation.
Lattice Thermal Conductivity (K_L) Determination:
- K_L was calculated by solving the phonon Boltzmann Transport Equation (BTE) using the full iterative solution implemented in the ShengBTE package.
- A correction factor was applied for 2D materials: K_L was multiplied by the vacuum thickness and divided by the effective thickness (distance between the upper and bottom layers).

Commercial Applications

The ability to precisely tune the thermal conductivity of Moiré Diamanes across a wide range makes them highly valuable for advanced thermal management and energy conversion technologies.

Thermoelectric Devices:
- Moiré diamanes with ultra-low K_L (e.g., H-Dn27) are ideal for maximizing the thermoelectric figure of merit (ZT), as low thermal conductivity is essential for efficient heat-to-electricity conversion.
Thermal Insulation and Protection:
- The highly twisted structures (θ > 20°) provide excellent thermal insulation (K_L as low as 32 W/mK), suitable for use as protective and insulating layers in micro- and nano-electronic devices where heat leakage must be minimized.
High-Performance Heat Sinks:
- Untwisted or low-twist diamanes (H-Dn0, K_L = 1360 W/mK) possess high thermal conductivity comparable to bulk diamond, making them prospective materials for heat dissipation and thermal spreading in high-power nanodevices.
Tunable Thermal Switches/Regulators:
- The strong dependence of K_L on the twist angle offers a pathway for engineering materials with spatially varying or dynamically tunable thermal properties for active thermal regulation systems.
Advanced 2D Materials Engineering:
- Diamanes, combining high stiffness with flexibility and wide electronic band gaps, are foundational materials for next-generation quasi-2D films used in electronics and optics.

View Original Abstract

Ultra-thin diamond membranes, diamanes, are one of the most intriguing quasi-2D films, combining unique mechanical, electronic and optical properties. At present, diamanes have been obtained from bi- or few-layer graphene in AA- and AB-stacking by full hydrogenation or fluorination. Here, we study the thermal conductivity of diamanes obtained from bi-layer graphene with twist angle θ between layers forming a Moiré pattern. The combination of DFT calculations and machine learning interatomic potentials makes it possible to perform calculations of the lattice thermal conductivity of such diamanes with twist angles θ of 13.2∘, 21.8∘ and 27.8∘ using the solution of the phonon Boltzmann transport equation. Obtained results show that Moiré diamanes exhibit a wide variety of thermal properties depending on the twist angle, namely a sharp decrease in thermal conductivity from high for “untwisted” diamanes to ultra-low values when the twist angle tends to 30∘, especially for hydrogenated Moiré diamanes. This effect is associated with high anharmonicity and scattering of phonons related to a strong symmetry breaking of the atomic structure of Moiré diamanes compared with untwisted ones.

Tech Support

Original Source

DOI: https://doi.org/10.3390/membranes12100925

References

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