Electrical and Computer Engineering Professor Zlatan Aksamija and his graduate students Arnab K. Majee and Cameron J. Foss are publishing a paper on the electronic properties of graphene and MoS grain boundaries, coming out on November 29th in Scientific Reports, published by the Nature Publishing group. The title of their paper is “Impact of Mismatch Angle on Electronic Transport Across Grain Boundaries and Interfaces in 2D Materials.”
As Aksamija says about the research in the Scientific Reports paper, “We study atomic monolayer materials, such as graphene, because of their potential applications in future ultra-nanoscale devices. Graphene is a material of superlatives, including very low electrical resistivity, but it’s missing a key property that other semiconductors have: an energy bandgap. The lack of a bandgap means graphene devices cannot be completely turned off.”
Molybdenum disulphide (or MoS2 for short) is similar to graphene in that it forms atomically-thin layers, but it possesses a bandgap so it can be made into energy-efficient and ultra-small transistors.
“Graphene and MoS2 can also be combined together,” explains Aksamija, “with graphene playing the role of contact and MoS2 the role of a switch. In that application, an interface forms between the graphene and the MoS2, adding resistance due to mismatch of materials properties and partial reflection of electrons across the interface. Our work shows that the interface resistance depends on how well the atoms at the interface are aligned.”
Aksamija, along with graduate students Majee and Foss, is part of the Nanoelectronics Theory and Simulation Lab, which focuses its research activities in theoretical and computational nanosciences with engineering applications related to emerging semiconductor nanostructures, post-CMOS nanoelectronic devices and computing paradigms, and nanoenergy materials.
As the three authors describe the research in their Scientific Reports paper, “We study the impact of grain boundaries (GB) and misorientation angles between grains on electronic transport in 2-dimensional materials. Here we have developed a numerical model based on the first-principles electronic bandstructure calculations in conjunction with a method which computes electron transmission coefficients from simultaneous conservation of energy and momentum at the interface to essentially evaluate GB/interface resistance in a Landauer formalism.”
Large-area 2-dimensional materials are grown by chemical vapor deposition, a process that does not produce perfectly ordered crystalline samples. Instead, they grow as a patchwork of microscale grains, separated by grain boundaries and containing crystalline regions at various angles. One of the primary features of graphene that makes it less resistive than the other two-dimensional materials like MoS2 is its steep electron bands, often referred to as the “Dirac cone”. Ironically, this also turns out to be the reason why misorientation between adjacent grains deteriorates conductance across a graphene grain boundary, far more than that in MoS2.
In their Scientific Reports abstract, the researchers say that they find the resistance across graphene GBs varies over a wide range depending on misorientation angles and type of GBs, starting from 53 Ω μm for low-mismatch angles in twin (symmetric) GBs to about 1020 Ω μm for 21° mismatch in tilt (asymmetric) GBs.
“On the other hand,” as the researchers explain, “misorientation angles have weak influence on the resistance across MoS2 GBs, ranging from about 130 Ω μm for low mismatch angles to about 6000 Ω μm for 21°. The interface resistance across graphene-MoS2 heterojunctions also exhibits a strong dependence on misorientation angles with resistance values ranging from about 100 Ω μm for low-mismatch angles in Class-I (symmetric) interfaces to 1015 Ω μm for 14° mismatch in Class-II (asymmetric) interfaces. Overall, symmetric homo/heterojunctions exhibit a weak dependence on misorientation angles, while in MoS2 both symmetric and asymmetric GBs show a gradual dependence on mismatch angles.”
The authors conclude that MoS2 is relatively insensitive to grain mismatch, ensuring that wafer-scale MoS2 retains its desirable electronic properties.
Aksamija and Majee conducted related research in 2015, examining how heat transfer works in graphene, which was first published in the peer-reviewed journal Nano Letters ("Bimodal Phonon Scattering in Graphene Grain Boundaries") and was subsequently covered in many scientific news outlets, including Science Daily, Nanowerk News, Chemeurope.com, Nanotechnology Now, Science Newsline, Lab Manager magazine, Controlled Environments, and Iconnect007.com. (November 2017)
IMAGE - (a) The side- and top-views of a perfectly-aligned, theoretical 2-dimensional grain boundary between graphene (left) and MoS2 (right) grains. (b) A schematic of the misorientation between two grains. The honeycomb (hexagonal) structures in (a) are synonymous with the hexagons in (b). The interface resistance as a function of misorientation for graphene-graphene and MoS2-MoS2 grain boundaries are shown in (c) and (d), respectively.