(Invited) Solution Processing and Structural Control of 2D Materials of Bi2Te3, MoS2 and V2O5 and Their Applications
Authors: Carroll Elaine , Buckley Darragh , McNulty David , Mogili Vishnu , Moreno Sergio , Glynn Colm , Collins Gillian , Holmes Justin D , Razeeb Kafil M. , O'Dwyer Colm 
Published: February 2020

Embedding 2D layered materials into polymers can form a paint with many useful applications. In bulk form the thermoelectric material bismuth telluride (Bi2Te3) has a high figure of merit (ZT) at room temperature. When bulk Bi2Te3 is exfoliated to smaller dimensions, such as its 2D form (nanosheets), its thermoelectric effects are improved by reduction in thermal conductivity due to phonon confinement and scattering, thus increasing the ZT. The difficulty involved in incorporating nanosheets into energy harvesting systems motivated the search for a form of Bi2Te3 that retains the high ZT apparent at the nanoscale with the capability of being deposited or painted onto device infrastructures.

This work highlights a method whereby solvent exfoliation of Bi2Te3 into solution-dispersible 2D nanosheets can form a practical thin film that can be distributed across a surface. Optical transmission measurements quantify the relationship between efficient and stable exfoliation and the reduction in optical density.

Optimized exfoliated suspension are also shown to form smooth, uniform blends when mixed with poly ethylene glycol and other polymers to produce a paintable Bi2Te3film that can be applied to surfaces using an innovative painting technique.

Atomic force microscopy, transmission electron spectroscopy, Raman spectroscopy and scanning electron spectroscopy are used to examine the structure of the 2D nanosheets and the highly reproducible Bi2Te3thin films. Electrical transport studies show that the films have conductive pathways over a range of surfaces and various structural formations, linking the conductivity to the percolating conduction through the nanosheet ensemble. The combination of the facile preparation method and the scope for diverse surface coating as a cohesive and conductive thin film offers methods for integration with heat producing devices for energy harvesting applications. Direct Seebeck coefficient and thermal diffusivity measurements examine the role of the pain conductivity on its thermoelectric response.

Separately, MoS2 nanosheets are also demonstrated by solvent exfoliation of uniquely formed hexagonal single crystals. These crystals are formed under high pressure from layered MoS2 and this talk demonstrates how multi-micron scale dimension flakes of 2D and few-layer hexagonal MoS2can be formed from compressed soluble solutions of the 2D and layered material.

Finally, we demonstrate a method whereby 2D layers of V2O5 can be turbostratically assembled using organic-inorganic interactions with alkylamines under hydrothermal conditions, to scroll into very high quality V2O5 nanotubes. V2O5 layered materials intercalated with 1D Pt-Pt metal complexes exhibit high directional conductivity. We also show that the method can be used to make nanotubes of 2D and layered materials, and how MoS2 sheets can also be rolled into nanotubular form.