Anisotropic thermal conductivity of graphene wrinkles

Thermal conductivity of wrinkled graphene ring with defects

Graphene rings have great prospects in the fields of biological modulators, electrochemical biosensors, and resonators, but are prone to wrinkling which can affect their physical properties. This work establishes a theoretical model predicting the torsional wrinkling behavior of defective monolayer graphene rings, which provides direct understanding and reliable accuracy of the wrinkle levels. Then the thermal conductivity of wrinkled graphene rings is studied considering different wrinkle levels, defect concentrations and radii. It is found that with increased radius, defect concentration and torsional angle, the ratio of wrinkle amplitude to wavelength increases gradually. Vacancy defects and radii have more significant influences on the thermal conductivity than torsional wrinkles. The main influence mechanism of wrinkles and defects on thermal conductivity is revealed by phonon density of state. This work provides theoretical guidance for thermal manipulation based on the wrinkle-tuning approach.

Efficient modulation of thermal transport in two-dimensional materials for thermal management in device applications

With the development of chip technology, the density of transistors on integrated circuits is increasing and the size is gradually shrinking to the micro-/nanoscale, with the consequent problem of heat dissipation on chips becoming increasingly serious. For device applications, efficient heat dissipation and thermal management play a key role in ensuring device operation reliability. In this review, we summarize the thermal management applications based on 2D materials from both theoretical and experimental perspectives. The regulation approaches of thermal transport can be divided into two main types: intrinsic structure engineering (acting on the intrinsic structure) and non-structure engineering (applying external fields). On one hand, the thermal transport properties of 2D materials can be modulated by defects and disorders, size effect (including length, width, and the number of layers), heterostructures, structure regulation, doping, alloy, functionalizing, and isotope purity. On the other hand, strain engineering, electric field, and substrate can also modulate thermal transport efficiently without changing the intrinsic structure of the materials. Furthermore, we propose a perspective on the topic of using magnetism and light field to modulate the thermal transport properties of 2D materials. In short, we comprehensively review the existing thermal management modulation applications as well as the latest research progress, and conclude with a discussion and perspective on the applications of 2D materials in thermal management, which will be of great significance to the development of next-generation nanoelectronic devices.

Thermal Transport of AlN/Graphene/3C-SiC Typical Heterostructures with Different Crystallinities of Graphene

It is proven that introduction of graphene into typical heterostructures can effectively reduce the high interfacial thermal resistance in semiconductor chips. The crystallinity of graphene varies greatly; thus, we have investigated the effects of single-crystal and polycrystalline graphene on the thermal transport of AlN/graphene/3C-SiC heterostructures by molecular dynamics. The results show that polycrystalline graphene contributes more to the interfacial thermal conductance (ITC) inside the chip with a maximum increase of 75.09%, which is further confirmed by the energy transport and thermal relaxation time. Multiple analyses indicate that grain boundaries lead to the increase in C-Si covalent bonds, and thus, strong interactions improve the ITC. However, covalent bonding further causes local tensile strain and wrinkles in graphene. The former decreases the ITC, and the latter leads to the fluctuation of the van der Waals interaction at the interface. The combined effect of various influential factors results in the increase in the ITC, which are confirmed by phonon transmission with 0-18 THz. In addition, wrinkles and covalent bonding lead to increased stress concentration in polycrystalline graphene. This leads to a maximum reduction of 19.23% in the in-plane thermal conductivity, which is not conducive to the lateral diffusion of hot spots within the chip. The research results would provide important guidance in designing for high thermal transport performance high-power chips.

Anisotropic Thermal Transport in Twisted Bilayer Graphene

Recently, twisted bilayer graphene (TBLG) has attracted significant attentions due to its peculiar electronic properties. In this work, we investigate the anisotropic thermal conductivity of TBLG and report that twisting...

Anisotropic thermal conductivity and corrugated patterns in single-layer black phosphorus nanoribbon subjected to shear loading: a molecular dynamics study

Single-layer black phosphorus (SLBP) also known as phosphorene is a recently introduced two-dimensional material with unique structure and promising physical properties that has drawn considerable attention in the field of nanodevices. This structure demonstrates a high anisotropy in mechanical and thermal behavior along zigzag (ZZ) and armchair (AC) principal in-plane directions. Here in this study, it is shown that implementing shear strain on 10 nm × 50 nm SLBP nanoribbons (SLBPNRs) along ZZ and AC directions, the anisotropy leads to different corrugated patterns on the pristine structure. Applying non-equilibrium molecular dynamics under a parameterized Stillinger-Weber potential for modelling SLBP, thermal conductivity (TC) behavior of the sheared SLBPNRs with corrugated patterns are examined. The results show a higher amplitude and wavelength of the corregations on the ZZ-aligned SLBPNRs, which is around two times higher than that of AC-aligned counterparts. Although, it is also shown that unlike some other 2D materials, such as graphene, the wrinkling does not have such a significant effect on TC of SLBP. The phonon density of states results obtained in this work as well as phonon dispersion curves by first-principle calculations in other works concrete this finding. The results show small frequency shifts in both high- and low-frequency phonons, which are not strong enough to affect TC in SLBPNRs. This interesting thermal property of SLBP under shear strain suggests the great potential application of these corrugated structures in nanodevices without any loss of TC abilities.

Phonon transport in graphene based materials

Graphene, due to its atomic layer structure, has the highest room temperature thermal conductivity k for all known materials. Thus, it is expected that graphene based materials are the best candidates for thermal management in next generation electronic devices. In this perspective, we first review the in-plane k of monolayer graphene and multilayer graphene obtained using experimental measurements, theoretical calculations and molecular dynamics (MD) simulations. Considering the importance of four-phonon scattering in graphene, we also compare the effects of three-phonon and four-phonon scattering on phonon transport in graphene. Then, we review phonon transport along the cross-plane direction of multilayer graphene and highlight that the cross-plane phonon mean free path is several hundreds of nanometers instead of a few nanometers as predicted using classical kinetic theory. Recently, hydrodynamic phonon transport has been observed experimentally in graphitic materials. The criteria for distinguishing the hydrodynamic from ballistic and diffusive regimes are discussed, from which we conclude that graphene based materials with a high Debye temperature and high anharmonicity (due to ZA modes) are excellent candidates to observe the hydrodynamic phonon transport. In the fourth part, we review how to actively control phonon transport in graphene. Graphene and graphite are often adopted as additives in thermal management materials such as polymer nanocomposites and thermal interface materials due to their high k. However, the enhancement of the composite's k is not so high as expected because of the large thermal resistance between graphene sheets as well as between the graphene sheet and matrix. In the fifth part, we discuss the interfacial thermal resistance and analyze its effect on the thermal conductivity of graphene based materials. In the sixth part, we give a brief introduction to the applications of graphene based materials in thermal management. Finally, we conclude our review with some perspectives for future research.

Mechanical and Viscoelastic Properties of Wrinkled Graphene Reinforced Polymer Nanocomposites - Effect of Interlayer Sliding within Graphene Sheets

Multilayer graphene sheets (MLGSs) are promising nano-reinforcements that can effectively enhance the properties of polymer matrices. Despite many studies on MLGSs-reinforced polymer nanocomposites, the effect of wrinkles formed in MLGSs on the reinforcement effect and the viscoelastic properties of polymer nanocomposites has remained unknown. In this study, building upon previously developed coarse-grained models of MLGSs and poly(methyl methacrylate) coupled with molecular dynamics simulations, we have systematically investigated nanocomposites with different numbers of graphene layers and various wrinkle configurations. We find that with decreasing degree of waviness and increasing numbers of layers, the elastic modulus of the nanocomposites increases. Interestingly, we observe a sudden stress drop during shear deformation of certain wrinkled MLGSs-reinforced nanocomposites. We further conduct small amplitude oscillatory shear simulations on these nanocomposites and find that the nanocomposites with these specific wrinkle configurations also show peculiarly large loss tangents, indicating an increasing capability of energy dissipation. These behaviors are attributed to the activation of the interlayer sliding among these wrinkled MLGSs, as their interlayer shear strengths are indeed lower than flat MLGSs measured by steered molecular dynamics technique. Our study demonstrates that the viscoelastic properties and deformation mechanisms of polymer nanocomposites can be tuned through MLGS wrinkle engineering.

Substrate Engineering for CVD Growth of Single Crystal Graphene

Single crystal graphene (SCG) has attracted enormous attention for its unique potential for next-generation high-performance optoelectronics. In the absence of grain boundaries, the exceptional intrinsic properties of graphene are preserved by SCG. Currently, chemical vapor deposition (CVD) has been recognized as an effective method for the large-scale synthesis of graphene films. However, polycrystalline films are usually obtained and the present grain boundaries compromise the carrier mobility, thermal conductivity, optical properties, and mechanical properties. The scalable and controllable synthesis of SCG is challenging. Recently, much attention has been attracted by the engineering of large-size single-crystal substrates for the epitaxial CVD growth of large-area and high-quality SCG films. In this article, a comprehensive and comparative review is provided on the selection and preparation of various single-crystal substrates for CVD growth of SCG under different conditions. The growth mechanisms, current challenges, and future development and perspectives are discussed.

Thermal transport across wrinkles in few-layer graphene stacks

Wrinkles significantly influence the physical properties of layered 2D materials, including graphene. In this work, we examined thermal transport across wrinkles in vertical assemblies of few-layer graphene crystallites using the Raman optothermal technique supported by finite-element analysis simulations. A high density of randomly oriented uniaxial wrinkles were frequently observed in the few-layer graphene stacks which were grown by chemical vapor deposition and transferred on Si/SiO₂ substrates. The thermal conductivity of unwrinkled regions was measured to be, κ ∼ 165 W m^-1 K^-1. Measurements at the wrinkle sites revealed local enhancement of thermal conductivity, with κ ∼ 225 W m^-1 K^-1. Furthermore, the total interface conductance of wrinkled regions decreased by more than an order of magnitude compared to that of the unwrinkled regions. The physical origin of these observations is discussed based on wrinkle mediated decoupling of the stacked crystallites and partial suspension of the film. Wrinkles are ubiquitous in layered 2D materials, and our work demonstrates their strong influence on thermal transport.

Thermo-mechanical correlation in two-dimensional materials

Two-dimensional (2D) materials have received tremendous attention from the research community in the past decades, because of their numerous striking physical, chemical and mechanical properties and promising potential in a wide range of applications. This field is strongly interdisciplinary, requiring efficient integration of knowledge with different insights. In this review, we summarize the up-to-date research on the thermal and mechanical properties and thermo-mechanical correlation in 2D materials, including both theoretical and experimental insight. Firstly, the mechanical properties of 2D nanomaterials are discussed, in which the underlying physics is summarized. Then, we discuss the impacts of thermal fluctuation on the mechanical properties. Next, from experimental points of view, we present the methods to introduce strain in 2D materials experimentally and the experimental tools to measure the degree of strain. Finally, we discuss the fundamental phonon and thermal properties of 2D materials, including the strain effects on phonon dispersion, phonon hydrodynamic behavior, phonon topological feature, ballistic thermal conductance and diffusive thermal conductivity. This article presents an advanced understanding of the mechanical and thermal properties of 2D materials, which provides new opportunities for promoting their applications in nanoscale electronic, optoelectronic, and thermal functional devices.

Temperature Effects on the Fracture Dynamics and Elastic Properties of Popgraphene Membranes

Popgraphene (PopG) is a new 2D planar carbon allotrope which is composed of 5-8-5 carbon rings. PopG is intrinsically metallic and possesses excellent thermal and mechanical stability. In this work, we report a detailed study of the thermal effects on the mechanical properties of PopG membranes using fully-atomistic reactive (ReaxFF) molecular dynamics simulations. Our results showed that PopG presents very distinct fracture mechanisms depending on the temperature and direction of the applied stretching. The main fracture dynamics trends are temperature independent and exhibit an abrupt rupture followed by fast crack propagation. The reason for this anisotropy is due to the fact that y-direction stretching leads to a deformation in the shape of the rings that cause the breaking of bonds in the pentagon-octagon and pentagon-pentagon ring connections, which is not observed for the x-direction. PopG is less stiff than graphene membranes, but the Young's modulus value is only 15 % smaller.

Heterogeneous deformation of two-dimensional materials for emerging functionalities

Atomically thin 2D materials exhibit strong intralayer covalent bonding and weak interlayer van der Waals interactions, offering unique high in-plane strength and out-of-plane flexibility. While atom-thick nature of 2D materials may cause uncontrolled intrinsic/extrinsic deformation in multiple length scales, it also provides new opportunities for exploring coupling between heterogeneous deformations and emerging functionalities in controllable and scalable ways for electronic, optical, and optoelectronic applications. In this review, we discuss (i) the mechanical characteristics of 2D materials, (ii) uncontrolled inherent deformation and extrinsic heterogeneity present in 2D materials, (iii) experimental strategies for controlled heterogeneous deformation of 2D materials, (iv) 3D structure-induced novel functionalities via crumple/wrinkle structure or kirigami structures, and (v) heterogeneous strain-induced emerging functionalities in exciton and phase engineering. Overall, heterogeneous deformation offers unique advantages for 2D materials research by enabling spatial tunability of 2D materials' interactions with photons, electrons, and molecules in a programmable and controlled manner.

The MoSeS dynamic omnigami paradigm for smart shape and composition programmable 2D materials

The properties of 2D materials can be broadly tuned through alloying and phase and strain engineering. Shape programmable materials offer tremendous functionality, but sub-micron objects are typically unachievable with conventional thin films. Here we propose a new approach, combining phase/strain engineering with shape programming, to form 3D objects by patterned alloying of 2D transition metal dichalcogenide (TMD) monolayers. Conjugately, monolayers can be compositionally patterned using non-flat substrates. For concreteness, we focus on the TMD alloy MoSe[Formula: see text]S[Formula: see text]; i.e., MoSeS. These 2D materials down-scale shape/composition programming to nanoscale objects/patterns, provide control of both bending and stretching deformations, are reversibly actuatable with electric fields, and possess the extraordinary and diverse properties of TMDs. Utilizing a first principles-informed continuum model, we demonstrate how a variety of shapes/composition patterns can be programmed and reversibly modulated across length scales. The vast space of possible designs and scales enables novel material properties and thus new applications spanning flexible electronics/optics, catalysis, responsive coatings, and soft robotics.

Thermal management applied laminar composites with SiC nanowires enhanced interface bonding strength and thermal conductivity

Design and fabrication of oriented thermal management materials has great significance in meeting the requirements of high-power heat dissipation device applications. To synchronously improve the structure stability and thermal management performance, in this study, large-scale silicon carbide (SiC) nanowires were deposited on the graphite film (GF) surface to reinforce the aluminum-based laminar composites. Highly thermally conductive SiCnws-GF multiscale architecture reinforced Al laminar composites with enhanced interlayer bonding strength were achieved by an innovative pressure infiltration strategy. The embedding of the silicon carbide nanowires not only improved the thermal conductivity of the laminar composites but also enhanced the interface bonding strength between the Al matrix and the SiCnws-GF multiscale structure robustly. The interlaminar shear strength of the SiCnws-GF reinforced Al laminar composites was 134.1 MPa, which was 2.4 times the value of GF reinforced Al composites. The in-plane thermal conductivity of the best-performing SiCnws-GF reinforced Al laminar composites was 868.9 W (m K)-1, which was 16.9% higher than the value of the GF reinforced Al laminar composites. The outstanding interlaminar shear strength and superior thermal conductivity of the SiCnws-GF reinforced Al laminar composites revealed that a potential and competitive thermal management material was obtained.

Wrinkling of two-dimensional materials: methods, properties and applications

Recently, two-dimensional (2D) materials, including graphene, its derivatives, metal films, MXenes and transition metal dichalcogenides (TMDs), have been widely studied because of their tunable electronic structures and special electrical and optical properties. However, during the fabrication of these 2D materials with atomic thickness, formation of wrinkles or folds is unavoidable to enable their stable existence. Meaningfully, it is found that wrinkled structures simultaneously impose positive changes on the 2D materials. Specifically, the architecture of wrinkled structures in 2D materials additionally induces excellent properties, which are of great importance for their practical applications. In this review, we provide an overview of categories of 2D materials, which contains formation and fabrication methods of wrinkled patterns and relevant mechanisms, as well as the induced mechanical, electrical, thermal and optical properties. Furthermore, these properties are modifiable by controlling the surface topography or even by dynamically stretching the 2D materials. Wrinkling offers a platform for 2D materials to be applied in some promising fields such as field emitters, energy containers and suppliers, field effect transistors, hydrophobic surfaces, sensors for flexible electronics and artificial intelligence. Finally, the opportunities and challenges of wrinkled 2D materials in the near future are discussed.

Suppression of wrinkle formation in graphene on Ir(111) by high-temperature, low-energy ion irradiation

Graphene on Ir(111) is irradiated with small fluences of 500 eV He ions at temperatures close to its chemical vapor deposition growth temperature. The ion irradiation experiments explore whether it is possible to suppress the formation of wrinkles in Gr during growth. It is found that the release of thermal mismatch strain by wrinkle formation can be entirely suppressed for an irradiation temperature of 880 °C. A model for the ion beam induced suppression of wrinkle formation in supported Gr is presented, and underpinned by experiments varying the irradiation temperature or involving intercalation subsequent to irradiation.

Phase transformation in two-dimensional covalent organic frameworks under compressive loading

As a new class of two-dimensional (2D) materials, 2D covalent organic frameworks (COFs) are proven to possess remarkable electronic and magnetic properties. However, their mechanical behaviours remain almost unexplored. In this work, taking the recently synthesised dimethylmethylene-bridged triphenylamine (DTPA) sheet as an example, we investigate the mechanical behaviours of 2D COFs based on molecular dynamics simulations together with density functional theory calculations. A novel phase transformation is observed in DTPA sheets when a relatively large in-plane compressive strain is applied to them. Specifically, the crystal structures of the transformed phases are topographically different when the compressive loading is applied in different directions. The compression-induced phase transformation in DTPA sheets is attributed to the buckling of their kagome lattice structures and is found to have significant impacts on their material properties. After the phase transformation, Young's modulus, band gap and thermal conductivity of DTPA sheets are greatly reduced and become strongly anisotropic. Moreover, a large in-plane negative Poisson's ratio is found in the transformed phases of DTPA sheets. It is expected that the results of the compression-induced phase transformation and its influence on the material properties observed in the present DTPA sheets can be further extended to other 2D COFs, since most 2D COFs are found to possess a similar kagome lattice structure.

Bridging the Gap between Reality and Ideal in Chemical Vapor Deposition Growth of Graphene

Graphene, in its ideal form, is a two-dimensional (2D) material consisting of a single layer of carbon atoms arranged in a hexagonal lattice. The richness in morphological, physical, mechanical, and optical properties of ideal graphene has stimulated enormous scientific and industrial interest, since its first exfoliation in 2004. In turn, the production of graphene in a reliable, controllable, and scalable manner has become significantly important to bring us closer to practical applications of graphene. To this end, chemical vapor deposition (CVD) offers tantalizing opportunities for the synthesis of large-area, uniform, and high-quality graphene films. However, quite different from the ideal 2D structure of graphene, in reality, the currently available CVD-grown graphene films are still suffering from intrinsic defective grain boundaries, surface contaminations, and wrinkles, together with low growth rate and the requirement of inevitable transfer. Clearly, a gap still exits between the reality of CVD-derived graphene, especially in industrial production, and ideal graphene with outstanding properties. This Review will emphasize the recent advances and strategies in CVD production of graphene for settling these issues to bridge the giant gap. We begin with brief background information about the synthesis of nanoscale carbon allotropes, followed by the discussion of fundamental growth mechanism and kinetics of CVD growth of graphene. We then discuss the strategies for perfecting the quality of CVD-derived graphene with regard to domain size, cleanness, flatness, growth rate, scalability, and direct growth of graphene on functional substrate. Finally, a perspective on future development in the research relevant to scalable growth of high-quality graphene is presented.

Shear deformation-induced anisotropic thermal conductivity of graphene

Investigation of anisotropic thermal transport in graphene wrinkles considering the effect of both shear strain and strain-induced wrinkling configurations.

Measurement of specific heat and thermal conductivity of supported and suspended graphene by a comprehensive Raman optothermal method

The last decade has seen the rapid growth of research on two-dimensional (2D) materials, represented by graphene, but research on their thermophysical properties is still far from sufficient owing to the experimental challenges. Herein, we report the first measurement of the specific heat of multilayer and monolayer graphene in both supported and suspended geometries. Their thermal conductivities were also simultaneously measured using a comprehensive Raman optothermal method without needing to know the laser absorption. Both continuous-wave (CW) and pulsed lasers were used to heat the samples, based on consideration of the variable laser spot radius and pulse duration as well as the heat conduction within the substrate. The error from the laser absorption was eliminated by comparing the Raman-measured temperature rises for different spot radii and pulse durations. The thermal conductivity and specific heat were extracted by analytically fitting the temperature rise ratios as a function of spot size and pulse duration, respectively. The measured specific heat was about 700 J (kg K)^-1 at room temperature, which is in accordance with theoretical predictions, and the measured thermal conductivities were in the range of 0.84-1.5 × 10³ W (m K)^-1. The measurement method demonstrated here can be used to investigate in situ and comprehensively the thermophysical properties of many other emerging 2D materials.

Hexagonal boron nitride: a promising substrate for graphene with high heat dissipation

Supported graphene on a standard SiO₂ substrate exhibits unsatisfactory heat dissipation performance that is far inferior to the intrinsic ultrahigh thermal conductivity of a suspended sample. A suitable substrate for enhancing thermal transport in supported graphene is highly desirable for the development of graphene devices for thermal management. By using molecular dynamics simulations, here we demonstrate that bulk hexagonal boron nitride (h-BN) is a more appealing substrate to achieve high performance heat dissipation in supported graphene. Notable length dependence and high thermal conductivity are observed in h-BN-supported single-layer graphene (SLG), suggesting that the thermal transport characteristics are close to that of suspended SLG. At room temperature, the thermal conductivity of h-BN-supported SLG is as high as 1347.3 ± 20.5 Wm^-1 K^-1, which is about 77% of that for the suspended case, and is more than twice that of the SiO₂-supported SLG. Furthermore, we find that the smooth and atomically flat h-BN substrate gives rise to a regular and weak stress distribution in graphene, resulting in a less affected phonon relaxation time and dominant phonon mean free path. We also find that stacking and rotation significantly impacts the thermal transport in h-BN-supported graphene. Our study provides valuable insights towards the design of graphene devices on realistic substrate for high performance heat dissipation applications.

Coherent and incoherent phonon transport in a graphene and nitrogenated holey graphene superlattice

Coherent and incoherent phonon transport in a graphene and nitrogenated holey graphene superlattice are investigated comprehensively for the first time.

Ab Initio-Based Bond Order Potential to Investigate Low Thermal Conductivity of Stanene Nanostructures

We introduce a bond order potential (BOP) for stanene based on an ab initio derived training data set. The potential is optimized to accurately describe the energetics, as well as thermal and mechanical properties of a free-standing sheet, and used to study diverse nanostructures of stanene, including tubes and ribbons. As a representative case study, using the potential, we perform molecular dynamics simulations to study stanene's structure and temperature-dependent thermal conductivity. We find that the structure of stanene is highly rippled, far in excess of other 2-D materials (e.g., graphene), owing to its low in-plane stiffness (stanene: ∼ 25 N/m; graphene: ∼ 480 N/m). The extent of stanene's rippling also shows stronger temperature dependence compared to that in graphene. Furthermore, we find that stanene based nanostructures have significantly lower thermal conductivity compared to graphene based structures owing to their softness (i.e., low phonon group velocities) and high anharmonic response. Our newly developed BOP will facilitate the exploration of stanene based low dimensional heterostructures for thermoelectric and thermal management applications.

Graphene oxide coated quartz sand as a high performance adsorption material in the application of water treatment

GO firmly planted on the surface of quartz sand, will not fall off and cause secondary pollution. A series of experiments show that the GO coated sand (GOS) granules have a strong adsorption performance for organic matter and heavy metal ions.

A comprehensive review on the molecular dynamics simulation of the novel thermal properties of graphene

This review summarizes state-of-the-art progress in the molecular dynamics simulation of the novel thermal properties of graphene.