Tunable electron and phonon properties of folded single-layer molybdenum disulfide

The single layer molybdenum disulfide is a two dimensional group VI transition metal dichalcogenide semiconductor that has attracted significant interest owing to its technological applications. Researchers have in fact proposed its optoelectronic as well as energy harvesting application reference to its intrinsic direct band gap, large mobility, and flexible structure. A recent investigation on its thermoelectric performance has also demonstrated a possibility of enhancing the figure of merit by phonon engineering.

Recent research works have found that folding could naturally occur in single layer molybdenum disulfide during processing and fabrication. However, photoluminescence measurements of the folded portions have identified a blue shift that can be traced back to the exciton screening initiated by the folded bilayer structure. Another study has shown that changing the layer stacking in the folded flakes of the single-layer molybdenum disulfide, could result in a decrease in interlayer coupling and an improvement in the photoluminescence emission yield.

Unfortunately, there is lacking some information on how folding affects the local ground state electronic as well as thermal properties. In view of previous research works, folding seems not to induce chemical reactivity and results in a well-defined structure, whose attributes are seemingly not influenced by the manner in which it is developed.

Jie Peng and Peter Chung at University of Maryland in collaboration with Madan Dubey and Raju Namburu at US Army Research Laboratory determined the equilibrium structure, ground state electronic properties, and phonon transport of folded arrangements of single-layer molybdenum disulfide implementing a combination of methods such as methods based on vibrational mechanics, density functional theory, and classical potentials. They examined the size dependence of these attributes by analyzing their changes as a function of the length of the overlapping bilayer regions. Their research work is published in Nano Research.

The authors applied a combination of computational methods such as methods based on engineering mechanics, density functional theory, and classical potentials. They first carried out highly precise computations of the reference folded structure in a bid to show that the folded structure was largely insensitive to the wrapping length.

Implementing the different modelling methods, the authors demonstrated that the folding structure as well as dimensional characteristics were independent of the wrapping length. A fold developed by wrapping the armchair axis of the single-layer molybdenum disulfide onto itself generated a loop feature of about 5-5.6 nm in diameter.

The electronic band gap indicated a strong dependence on the wrapping length, which monotonically converged to the band gap value of the bilayer structure. The authors could vary the gap by approximately 50% by changing the wrapping length applied in the folding. On the contrary, thermal attributes being sensitive to the folding, which reduced thermal conductivity by about 60%, were quite insensitive to the wrapping length once the fold was formed.

Their results provide for a route for designing molybdenum disulfide-based devices whose electronic band structure will be tuned via a directed symmetry breaking, without having an impact on the thermal properties.