Principles And Mechanisms Of Thermal Properties Of Functionalizied Graphene

As a 2D plate-like single layer nanomaterial gifted with exceptional properties in terms of chemical,electrical,mechanical and optical characteristics,graphene has attracted a great deal of research attention over the last decades.Graphene is not only a good conductor of electricity with zero band gap and remarkable electron mobility compared to silicon,it has also been well known for its outstanding thermal property with a bunch of potential applications.With the development of nanotechnology,a critical challenge in many nanoelectronic systems is the highly efficient heat dissipation requirement for ensuring the performance and lifetime of nanodevices,and Graphene has been acknowledged as a promising thermal material to be used in next-generation nanoelectronic devices.To achieve enhanced functionalities,the thermal conductivity of graphene should also be tunable according to the requirements for the specific applications.With the advancement in the synthesis and assembly of 2D nanomaterials,graphene can be effectively modified via surface modification with other adatoms,functional group and other material components.Surface modification has been widely accepted as an efficient way to manipulate not only the thermal properties but also conductive behavior of graphene.Energy in nature is recognized to be transported by electrons and photons during electrical and thermal conduction,which are two fundamental natural phenomena with equal importance.The electric power transmission has already been tremendously investigated and sophisticatedly manipulated with the invention of electronic diodes,transistor and other relevant devices.However,the counterpart devices for the control of heat conduction are still at the early stage of scientific exploration.The design and fabrication of devices for heat flow control have great application potentials for data storage,photonics circuit and thermal management.In this study,the thermal characteristics of graphene nanoribbons with surface modification are investigated using Non-Equilibrium Molecular Dynamics(NEMD)simulations.The surface hydrogenation and planar heterogeneous hybridization methods have been focused on in the study.For the surface hydrogenation,thermal conductivity(?)of graphene nanoribbons with fully hydrogenated domain(graphane)are studied by calculating the Kapitza conductance across the graphene-graphane interface.The ? of hybrid nanoribbon is revealed to depend on the length as well as charity and initial temperature,and been systematically interpreted from the perspectives of morphology and phonon vibrational spectra.More interestingly,remarkable thermal rectification is noticed for hybrid nanoribbon with graphenegraphane interface.The ? under heat flux from graphane to graphene is higher than that under opposite heat flux.Such thermal rectification decays with the length of nanoribbon and a critical length of 10 nm is identified for single layer nanoribbon beyond which the thermal rectification disappeared.We also studied the thermal properties of graphene nanoribbon with gradient hydrogen arrangement at the surface.Gradient hydrogenation can provide graphene nanoribbon with similar thermal rectification while eliminating the dependency on chirality and length.The proposed gradient hydrogenation can be used for length-insensitive thermal diode with practical application.Moreover,the thermal transfer efficiency of the hybrid grain boundary(GB)in graphene/h-BN heterostructure is revealed to be dependent not only on the mismatch angle of grains but also on the direction of the thermal flux.Thermal transfer efficiency from graphene to h-BN is higher than that from h-BN to graphene,which is interpreted by the difference between thermal properties of graphene and h-BN.Currently,grain boundary(GB)defects are unavoidably present in the graphene fabricated by Chemical Vapor Deposition(CVD)method.And the strain is also avoidable in flexible graphene based devices either at microscale or nanoscale.Moreover,it has been confirmed that GB in polycrystalline graphene(PG)plays an important role in its surface functionalization and mechanical responses.In this study,the ? of PG with surface hydrogenation under varying average grain size are investigated.The principles and mechanisms for the change of ? with in-plane strain and surface hydrogenation are interpreted combining thermal transport theory and phonon density of states(PDOS)analysis.The thermal property of PG under tension is found to be related with the average stress in PG as a result of the suppression of mean free path and the softening of phonon modes.The PG with varying grain sizes show different sensitivity to the tensile strain.The mechanism is also revealed for the size effect on the thermal property of PG under compression.Additionally,the dependency of ? on surface hydrogenation of PG is investigated,and an unexpected two stages evolution of ? with hydrogenation coverage is interpreted preliminarily from the circumference and arrangement of functionalized domains.Furthermore,the coupling effect between hydrogenation and strain on the ? of PG is revealed,and the ? of PG becomes insensitive to the in-plane strain under the higher hydrogenation.These results provide new insights into the role of GB on the thermal manipulation of PG.Moreover,it has been found that the coupling effects between interlayer and intralayer phonon transport play an essential role in the layer-dependent thermal conductivity of multilayer PG.It has been observed that graphene layers are usually stacked to form multilayer structures in practice.However,the reported investigations are all about the in-plane thermal control of two dimensional nanomaterials.Two dimensional nanomaterials with asymmetry out-of-plane ? has not been studied in literatures.In this study,a novel laminated structure composed of multilayer hydrogenated graphene as out-of-plane thermal rectifier is shown.Each layer of graphene has a different hydrogenation ratio to form gradient interlayer hydrogenation.The proposed multilayer graphene structure has thermal rectification along the thickness of graphene regardless of the in-plane size.Such 2D thermal rectifier with out-of-plane thermal control can be used at both microscopic and macroscopic for heat passivation as well as conservation.Moreover,a theoretical model is established to analyze the dependencies of thermal rectification on the environmental temperature and out-of-plane strain.Further,an efficient solder-free approach to weld the independent graphene assemblies and multilayers is reported.The graphene-based materials are covalently bonded together with strong joint strength after rapid high temperature Joule heating process.Such facile high temperature Joule heating strategy will provide potential opportunities to facilitate the bottom-up manufacturing of carbon-based materials with sophisticated 3D configurations.Our work reveals detailed principles and mechanisms in the thermal properties of graphene with surface modifications,and offer theoretical guideline for the design of graphene based flexible devices for thermoelectric and thermal management applications.