Phonon thermal properties of graphene from molecular dynamics using different potentials

Thermal transport in a defective pillared graphene network: insights from equilibrium molecular dynamics simulation

Graphene-based hybrid nanostructures have great potential to be ideal candidates for developing tailored thermal transport materials. In this study, we perform equilibrium molecular dynamics simulations employing the Green-Kubo method to investigate the influence of topological defects in three-dimensional pillared graphene networks. Similar to single-layer graphene and carbon nanotubes, the thermal conductivity (k) of pillared graphene systems exhibits a strong correlation with the system size (L), following a power-law relation k ∼ Lα, where α ranges from 0.12 to 0.15. Our results indicate that the vacancy defects significantly reduce thermal conductivity in pillared graphene systems compared to Stone-Wales defects. We observe that, beyond defect concentration, the location of the defects also plays a crucial role in determining thermal conductivity. We further analyze the phonon vibrational spectrum and the phonon participation ratio to obtain more insight into the thermal transport in the defective pillared graphene network. In most scenarios, longitudinal and flexural acoustic phonons experience significant localization within the 15-45 THz frequency range in the defective pillared graphene system.

The carrier mobility and superconducting properties of monolayer oxygen-terminated functionalized MXene Ti2CO2

In this study, the carrier mobility of monolayer Ti2CO2 was evaluated by employing the Boltzmann transport equation and superconducting transition temperature (Tc) of Ti2CO2 was determined by utilizing the Migdal and Eliashberg formalism in the first-principles framework. In contrast to previous studies, the results reveal that optical phonons in monolayer Ti2CO2 have dominant roles in scattering processes, which significantly reduce the mobility of carriers. Alongside the rigid band model, the jellium model is implemented to investigate the screening effects on electron-phonon interactions. Based on the jellium model and full-band electron-phonon calculations, the predicted maximum electron mobility at room temperature is 38 cm2 V-1 s-1 in which 80% of the total scattering rate originates from the intra-valley transitions within the M-valleys, indicating the crucial role of the long wavelength phonon wavevectors in scattering processes. On the other hand, for the p-type material, a maximum room temperature mobility of about 285 cm2 V-1 s-1 is calculated, which can be explained by a relatively small effective mass and tiny scattering phase space. Moreover, a maximum Tc of 39 (10) K is obtained for the n-type monolayer Ti2CO2 based on the rigid (jellium) model. Outcomes indicate that the important peaks of α2F(ω) are mainly caused by the optical phonons. The remarkable couplings between the electron states and phonons are related to the non-zero slope of (near the Brillouin zone center) the longitudinal optical branch denoted by Eu caused by the displacements of oxygen and carbon atoms at intermediate and high energy ranges of phonon dispersion, respectively.

Enhanced Thermal Transport Properties of Graphene/SiC Heterostructures on Nuclear Reactor Cladding Material: A Molecular Dynamics Insight

Owing to the excellent thermal properties of graphene, silicon carbide (SiC) combined with graphene is expected to obtain more outstanding thermal performance and structural stability at high temperatures. Herein, the thermal conductivity of graphene/SiC heterostructures (GS-Hs) with different structures and atomic orientations was calculated through non-equilibrium molecular dynamics (NEMD) simulations. The temperature dependence and size effect on the thermal transport properties of GS-Hs were systematically investigated and discussed. The continuous addition of graphene layers did not always have a positive effect. The thermal transport performance of GS-Hs approached the intrinsic thermal conductivity of SiC when the interaction gradually decreased with the distance between SiC and graphene. Studies on temperature and size dependence show opposite trends. The enhancement effect of graphene was limited at small distances. The thermal conductivity of GS-Hs had a negative correlation with temperature and increased with the system size. Meanwhile, the thermal conductivity of GS-Hs was predicted to be 156.25 (W·m⁻¹·K⁻¹) at the macroscopic scale via extrapolation. The model established in this paper is also applicable to other material simulation processes, as long as the corresponding parameters and potential functions are available. This study will provide inspiration for the optimized design and preparation of highly efficient cladding materials in nuclear reactors.

Thermal transport in monolayer zinc-sulfide: effects of length, temperature and vacancy defects

Of late, atomically thin two-dimensional zinc-sulfide (2D-ZnS) shows great potential for advanced nanodevices and as a substitute to graphene and transition metal di-chalcogenides owing to its exceptional optical and electronic properties. However, the functional performance of nanodevices significantly depends on the effective heat management of the system. In this paper, we explored the thermal transport properties of 2D-ZnS through molecular dynamics simulations. The impact of length, temperature, and vacancy defects on the thermal properties of 2D-ZnS are systematically investigated. We found that the thermal conductivity (TC) rises monotonically with increasing sheet length, and the bulk TC of ∼30.67 W mK^-1is explored for an infinite length ZnS. Beyond room temperature (300 K), the TC differs from the usual 1/Trule and displays an abnormal, slowly declining behavior. The point vacancy (PV) shows the largest decrease in TC compared to the bi vacancy (BV) defects. We calculated phonon modes for various lengths, temperatures, and vacancies to elucidate the TC variation. Conversely, quantum corrections are used to avoid phonon modes' icing effects on the TC at low temperatures. The obtained phonon density of states (PDOS) shows a softening and shrinking nature with increasing temperature, which is responsible for the anomaly in the TC at high temperatures. Owing to the increase of vacancy concentration, the PDOS peaks exhibit a decrease for both types of defects. Moreover, the variation of the specific heat capacity and entropy with BV and PV signify our findings of 2D-ZnS TC at diverse concentrations along with the different forms of vacancies. The results elucidated in this study will be a guide for efficient heat management of ZnS-based optoelectronic and nano-electronic devices.

Thermal conductivity of graphene under biaxial strain: an analysis of spectral phonon properties

Thermal transport in graphene is strongly influenced by strain. We investigate the influence of biaxial tensile strain on the thermal conductivity of zigzag and armchair graphene (AG and ZG) using non-equilibrium molecular dynamics simulations (NEMD). We observe that the thermal conductivity is significantly reduced under strain with a maximum reduction obtained at equi-biaxial strain. It is interesting to note that the high lateral to longitudinal strain ratios reduce the negative impact of strain on the thermal conductivity of AG and ZG. The in-plane acoustic modes are found to be the major heat carriers in unstrained graphene but are severely softened due to strain, and hence, their contribution to the conductivity drops down significantly. Strain alleviates the out-of-plane fluctuations in graphene and the group velocity of the out-of-plane acoustic mode (ZA) increases due to the linearisation of its dispersion relation. These factors result in the dominance of ZA mode in the thermal transport of strained graphene. Significant increase in the size dependence of the thermal conductivity of strained graphene is observed, which is attributed to the long-wavelength ZA phonons. The discrepancies between the results of BTE studies and NEMD are also discussed. This study suggests that biaxial strain can be an effective method to tune the thermal transport in graphene. Our findings can lead to better phonon engineering of graphene for various nanoscale applications.

Thermal transport of carbon nanomaterials

The diversity of thermal transport properties in carbon nanomaterials enables them to be used in different thermal fields such as heat dissipation, thermal management, and thermoelectric conversion. In the past two decades, much effort has been devoted to study the thermal conductivities of different carbon nanomaterials. In this review, different theoretical methods and experimental techniques for investigating thermal transport in nanosystems are first summarized. Then, the thermal transport properties of various pure carbon nanomaterials including 1D carbon nanotubes, 2D graphene, 3D carbon foam, are reviewed in details and the associated underlying physical mechanisms are presented. Meanwhile, we discuss several important influences on the thermal conductivities of carbon nanomaterials, including size, structural defects, chemisorption and strain. Moreover, we introduce different nanostructuring pathways to manipulate the thermal conductivities of carbon-based nanocomposites and focus on the wave nature of phonons for controlling thermal transport. At last, we briefly review the potential applications of carbon nanomaterials in the fields of thermal devices and thermoelectric conversion.

Phonon transport and thermal conductivity of diamond superlattice nanowires: a comparative study with SiGe superlattice nanowires

Due to the coupling of a superlattice's longitudinal periodicity to a nanowire's radial confinement, the phonon transport properties of superlattice nanowires (SLNWs) are expected to be radically different from those of pristine nanowires. In this work, we present the comparative investigation of phonon transport and thermal conductivity between diamond SLNWs and SiGe SLNWs by using molecular dynamics simulations. In the case of period length ∼ 25 Å, the thermal conductivities of diamond SLNWs and SiGe SLNWs both increase linearly with increasing the period number, which implies the wave-like coherent phonons dominate the heat transport of SLNWs. In the case of period length ∼ 103 Å, the thermal conductivity of SiGe SLNWs is length-independent with increasing the period number, indicating that the particle-like incoherent phonons in SiGe SLNWs control the heat transport, because the phonon–phonon scattering causes phonons to not retain their phases and the coherence is destroyed before the reflection at interfaces. However in diamond SLNWs the coherent phonons still dominate heat conduction and the thermal conductivity is length-dependent, because the mean free path of phonon–phonon scattering in diamond SLNWs is much longer. The spatial distribution of phonon localized modes further supports these opinions. These results are helpful not only to understand the coherent and incoherent phonon transport, but also to modulate the thermal conductivity of SLNWs.

We present the comparative investigation of phonon transport and thermal conductivity between diamond SLNWs and SiGe SLNWs by molecular dynamics simulations.

Anomalous temperature dependent thermal conductivity of two-dimensional silicon carbide

Recently, two-dimensional silicon carbide (2D-SiC) has attracted considerable interest due to its exotic electronic and optical properties. Here, we explore the thermal properties of 2D-SiC using reverse non-equilibrium molecular dynamics simulation. At room temperature, a thermal conductivity of ∼313 W mK^-1 is obtained for 2D-SiC which is one order higher than that of silicene. Above room temperature, the thermal conductivity deviates the normal 1/T law and shows an anomalous slowly decreasing behavior. To elucidate the variation of thermal conductivity, the phonon modes at different length and temperature are quantified using Fourier transform of the velocity auto-correlation of atoms. The calculated phonon density of states at high temperature shows a shrinking and softening of the peaks, which induces the anomaly in the thermal conductivity. On the other hand, quantum corrections are applied to avoid the freezing effects of phonon modes on the thermal conductivity at low temperature. In addition, the effect of potential on the thermal conductivity calculation is also studied by employing original and optimized Tersoff potentials. These findings provide a means for better understating as well as designing the efficient thermal management of 2D-SiC based electronics and optoelectronics in near future.

Effect of phonon scattering by substitutional and structural defects on thermal conductivity of 2D graphene

The ability to tailor the thermal conductivity of graphene by introducing crystalline defects has attracted considerable research attention. In this study, nonequilibrium molecular dynamics calculation is used to investigate the effect of crystalline defects on the thermal conductivity of 2D graphene. The defects considered include substitutional nitrogen and silicon, pure structural single vacancy and Stone-Wales defects, and structurally different pyridinic nitrogen. In particular, this study focuses on the unique phonon scattering behaviors arising from the low dimensionality of graphene. The results reveal that the low dimensionality of graphene has a negligible effect on phonon scattering in substitutionally defected graphene, for which the Klemens scattering model is accurate without the need for any corrections. The substitutional silicon defect leads to more effective reduction of the thermal conductivity than the structural defects because of the effect of change in the hybridization and the mass on the scattering. Almost equal reductions are observed for the two structural defects, the scattering strengths of which are significantly weakened by the two dimensionality of graphene. Callaway analysis of the vacancy scattering reveals that even with the perturbation of the vacancy, the 2D honeycomb structure preserves considerable phonon stability compared with a 3D material. In addition, the absence of mass deficiency for the Stone-Wales defect suggests that the contribution of mass deficiency is minimized for structural defects of graphene. Finally, opposite to the findings for the substitutional nitrogen defect, the introduction of pyridinic nitrogen leads to further reduction of the thermal conductivity compared with that for a single vacancy defect.

Decomposition of the Thermal Boundary Resistance across Carbon Nanotube-Graphene Junctions to Different Mechanisms

Three different mechanisms are identified to contribute to thermal resistances across a carbon nanotube-graphene junction: material mismatch, nonplanar junction, and defects. To isolate the contributions of each mechanism, we have designed five types of junctions and performed nonequilibrium molecular dynamics simulations. The results show that the contributions from the three mechanisms are similar, each at around 2.5 × 10^-11 m² K/W. The relations between thermal boundary resistance and both defect number and turning angle at the interface are also studied.

Anisotropic thermal conductivity in carbon honeycomb

Carbon honeycomb, a new kind of 3D carbon allotrope experimentally synthesized recently, has received much attention for its fascinating applications in electronic device and energy storage. In the present work, we perform equilibrium molecular dynamics (EMD) to study the thermal transport properties of carbon honeycombs with different chirality. It is found that the thermal conductivity along the honeycomb axis ([Formula: see text]) is three times larger than that normal to the axis ([Formula: see text]), which shows strong anisotropy reflecting their geometric anisotropy. Lattice dynamics calculations reveal that this anisotropy stems from the orientation-dependent phonon group velocities. Moreover, when ambient temperature ([Formula: see text]) increases from 200 K to 800 K, the [Formula: see text] dependence of [Formula: see text] is observed due to the enhanced Umklapp scattering. The detailed phonon spectra analyses indicate phonon group velocities are insensitive to the variation of ambient temperature, and the temperature dependence of the relaxation times of low-frequency phonons (<20 THz) follows [Formula: see text] behavior. Our results have a certain guiding significance to develop carbon honeycomb for effective thermal channeling devices.

Path-integral simulation of graphene monolayers under tensile stress

Finite-temperature properties of graphene monolayers under tensile stress have been studied by path-integral molecular dynamics (PIMD) simulations.

Thermal wave: from nonlocal continuum to molecular dynamics

To accommodate effects of thermomass and size-dependency of thermophysical properties on heat transport and to remove the theoretical singularity of temperature gradients across the thermal wavefront NL FTPL heat conduction, corroborated with MD simulation, is introduced.