COMMUNICATION 1700334 (1 of 7)
Graphene has served as the model 2D system for over a decade, and the effects of grain boundaries (GBs) on its electrical and mechanical properties are very well investigated. However, no direct measurement of the correlation between thermal transport and graphene GBs has been reported. Here, we report a simultaneous comparison of thermal transport in supported single crystalline graphene to thermal transport across an individual graphene GB. Our experiments show that thermal conductance (per unit area) through an isolated GB can be up to an order of magnitude lower than the theoretically anticipated values. Our measurements are supported by Boltzmann transport modeling which uncovers a new bimodal phonon scattering phenomenon initiated by the GB structure. In this novel scattering mechanism, boundary roughness scattering dominates the phonon transport in low-mismatch GBs, while for higher mismatch angles there is an additional resistance caused by the formation of a disordered region at the GB. Nonequilibrium molecular dynamics simulations verify that the amount of disorder in the GB region is the determining factor in impeding thermal transport across GBs.
Reliable fabrication of lateral interfaces between conducting and semiconducting 2D materials is considered a major technological advancement for the next generation of highly packed all-2D electronic circuitry. This study employs seed-free consecutive chemical vapor deposition processes to synthesize high-quality lateral MoS -graphene heterostructures and comprehensively investigated their electronic properties through a combination of various experimental techniques and theoretical modeling. These results show that the MoS -graphene devices exhibit an order of magnitude higher mobility and lower noise metrics compared to conventional MoS -metal devices as a result of energy band rearrangement and smaller Schottky barrier height at the contacts. These findings suggest that MoS -graphene in-plane heterostructures are promising materials for the scale-up of all-2D circuitry with superlative electrical performance.
The ongoing shrinkage in the size of two-dimensional (2D) electronic circuitry results in high power densities during device operation, which could cause a significant temperature rise within 2D channels. One challenge in Raman thermometry of 2D materials is that the commonly used high-frequency modes do not precisely represent the temperature rise in some 2D materials because of peak broadening and intensity weakening at elevated temperatures. In this work, we show that a low-frequency E shear mode can be used to accurately extract temperature and measure thermal boundary conductance (TBC) in back-gated tungsten diselenide (WSe) field-effect transistors, whereas the high-frequency peaks (E and A) fail to provide reliable thermal information. Our calculations indicate that the broadening of high-frequency Raman-active modes is primarily driven by anharmonic decay into pairs of longitudinal acoustic phonons, resulting in a weak coupling with out-of-plane flexural acoustic phonons that are responsible for the heat transfer to the substrate. We found that the TBC at the interface of WSe and Si/SiO substrate is ∼16 MW/m K, depends on the number of WSe layers, and peaks for 3-4 layer stacks. Furthermore, the TBC to the substrate is the highest from the layers closest to it, with each additional layer adding thermal resistance. We conclude that the location where heat dissipated in a multilayer stack is as important to device reliability as the total TBC.
Thermal properties of graphene have attracted much attention, culminating in a recent measurement of its length dependence in ribbons up to 9 µm long. In this paper, we use the improved Callaway model to solve the phonon Boltzmann transport equation while capturing both the resistive (umklapp, isotope, and edge roughness) and non-resistive (normal) contributions. We show that for lengths smaller than 100 µm, scaling the ribbon length while keeping the width constant leads to a logarithmic divergence of thermal conductivity. The length dependence is driven primarily by a ballistic-to-diffusive transition in the in-plane (LA and TA) branches, while in the hydrodynamic regime when 10 µm
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