Extrinsic Effects on Heat and Electron Transport In Two-Dimensional Van-Der Waals Materials- A Boltzmann Transport Study
Creators: name: Majee, Arnab K
Publisher: name: University of Massachusetts Amherst
Year: 2016

description: Two-dimensional van der Waals materials have been a subject of intense research interest in recent years. High thermal conductivity of graphene can be utilized for many thermal management applications. In spite of possessing very high electron mobility, graphene can’t be used as transistors because of the absence of band gap; however transition metal dichalcogenides are another class of two-dimensional van der Waals materials with inherent band gap and show a great promise for future nanoelectronic applications. But in order to tailor these properties for commercial applications, we should develop a better understanding of the effect of extrinsic factors like size, rough edges, grain boundaries, mass-impurities, interaction with substrate etc. on thermal and electrical transport.
Most materials exhibit a smooth ballistic-to-diffusive type of thermal transport in which when the sample size is small as compared to mean-free-path of phonons the transport is ballistic, whereas, when the sample size is large as compared to phonon mean-free-path, phonons undergo multiple scattering events and the thermal transport becomes diffusive in nature. However, graphene exhibits an atypical thermal transport behavior where thermal conductivity shows an increasing logarithmic trend even for samples far greater than the mean-free-path of phonons. We show that this anomalous behavior can be attributed to the significant contribution coming from momentum-conserving normal phonon-phonon scattering. Secondly, graphene grain boundaries have been found to significantly reduce thermal conductivity even in the presence of substrates. In spite of numerous studies on the effect of grain boundaries (GBs) on thermal conductivity in graphene, there lacks a complete correlation between GB resistance and misorientation angle across graphene GBs. We show a direct correlation between thermal GB resistance and mismatch angles with low angle mismatch can be captured only by GB roughness, whereas, large mismatch angles will lead to the formation of a disordered patch at the interface and it could significantly deteriorate the overall thermal conductivity even in the presence of substrates.
GBs are found to affect electrical transport in two-dimensional systems as well. Owing to the excellent electronic properties and compactness of these two-dimensional materials, high quality 2D heterojunctions are the subject of intense research interest in recent years. Graphene-MoS<sub>2</sub> heterojuctions are found to form ohmic contacts and show great potential for future nanoelectronic applications. We show that the interface resistance in Gr-MoS<sub>2</sub> heterojuctions can affect the overall resistance of the device if the channel (MoS<sub>2</sub>) length is small at low carrier densities, whereas, at high carrier densities interface resistance do not play much role in determining the resistance of the entire device. However, if graphene and MoS<sub>2</sub> grains are misorientated then interface resistance can play a crucial role in determining the overall resistance of the device. We also show a weak dependence of misorientation angles on GB resistance across MoS<sub>2</sub> grain boundaries.