The University of Massachusetts Amherst
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Article by Aksamija and Colleagues Selected for Influential Highlights of Nanotechnology

Zlatan Aksamija

Zlatan Aksamija

An article co-authored by Zlatan Aksamija, an assistant professor in the Electrical and Computer Engineering (ECE) Department and the principal investigator in the Nanoelectronics Theory and Simulation Lab (NET Lab), was included in the 2017 highlights of the scientific journal Nanotechnology. As the journal described its prestigious highlights: “This collection includes outstanding articles and topical reviews published in the journal during 2017. These articles were selected on the basis of a range of criteria including referee endorsements, presentation of outstanding research, and popularity with our online readership.”

“In this contribution,” wrote Aksamija and his fellow researchers about their highlighted article, “we develop a computationally efficient first-principles model for phonon transport across the interface of a monolayer supported on a disordered bulk substrate, and we use our model to study interface thermal conductance of graphene and molybdenum disulfide (MoS2) on silicon dioxide (SiO2).”

The title of the Nanotechnology article was “Interface thermal conductance of van der Waals monolayers on amorphous substrates,” and Aksamija’s co-authors were his graduate student, Cameron Foss, and ECE alumna Gabriela Correa. The article described NET Lab research on the thermal boundary conductance between van der Waals atomic layers and substrates, with impact on device applications on graphene, MoS2, and related materials. The study continued the NET Lab’s work on atomic monolayer materials, such as graphene, because of their potential applications in future ultra-nanoscale devices.

As Aksamija and his collaborators wrote in their Nanotechnology piece, “Heterostructures based on atomic monolayers are emerging as leading materials for future energy efficient and multifunctional electronics.”

The highlighted Nanotechnology article explained that “Graphene has one of the highest known thermal conductivities of any material, making it a promising candidate for removing heat from electronic devices. A material of superlatives, graphene is 97.7% transparent, more flexible than paper, highly conductive to heat and electricity, and the strongest material known to man.”

The Nanotechnology article observed that graphene on SiO2 was initially used in the research model and contrasted against available experimental data; the model was then applied to a MoS2 on SiO2 substrate. As the article noted, “Our findings show the dominant carrier of heat in both graphene and MoS2 in the cross-plane direction is the flexural (ZA) phonon mode, owing to the large overlap between graphene ZA and substrate vibrational density of states.”

The Nanotechnology article continued the research reported by Aksamija and Foss on the electronic properties of graphene and molybdenum disulfide grain boundaries as published on November 29th, 2017, in Scientific Reports.

As Aksamija said then, “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.”

MoS2 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,” explained 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.”

In general, the NET Lab 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.

Improved thermal and thermo-electric simulation tools for devices and materials that are relevant to future advances in nanotechnology enable a detailed understanding of coupling between electron and phonon (charge and thermal) transport, their interdependence, and the impact on the performance of future nanoscale transistors, thermoelectric converters, and sensors. (March 2018)