2D materials beyond graphene

Two-dimensional (2D) materials are emerging as a novel class of functional materials for the miniaturization of electronic and electrochemical devices. Devices based on 2D materials can take advantage of the materials inherent atomic layer thinness to improve performance in terms of energy efficiency and energy density. In particular, they pave the way for electrochemical capacitors with energy densities approaching that of Li-ion batteries with significantly higher power density for powerful and fast-charging battery technology. 2D materials can also be used in novel transistor architectures. The project has focused on fundamental understanding of the defects and interfaces present in these materials, which also govern their functional properties. The defects of interest comprised charged point defects as well as clusters in the material and at its highly influential interfaces (surface, edge, interlayer and substrate). Molybdenum sulfide and oxide (MoS2 and MoOx) 2D films were fabricated by a novel route based on sputter deposition of molybdenum on silicon substrates and post treatment in hydrogen disulfide of oxygen containing atmospheres (H2S or O2/H2O). The fabrication procedure was optimised based on several structural and spectroscopic characterization methods. The MoS2 films, however, displayed vertical growth which is not suited for the planned electrical characterization. In another study, quantitative Atomic Force Microscopy was used to directly measure the adhesion between 2D materials by using a graphene oxide coated AFM-tip. Transmission electron microscopy confirmed that the tip was covered with 6-7 monolayers. Adhesion energies were measured as a function of temperature, and the effect of thermally induced ripples in the 2D materials on the adhesion characteristics was shown for the first time. Films deposited as free-standing membranes have been used to further understand the influence of the substrate. Atomistic modelling has focused on implementing methodology for treating charged point defects in periodic single-layer MoS2 slabs, as well as indium and gallium sulfides and selenides (InSe, GaSe, and GaS). The structure and formation energy of various defects have been considered, including sulfur vacancies and protons associated with sulfur in MoS2. Moreover, the interfacial structure between graphene oxide and iron oxide (Fe2O3) was modelled with respect to chemical bonding, electronic structure and dissociative chemisorption of water as hydroxide and protonic species. The project was managed by SINTEF with partners at University of Oslo (PhD, University of Luxembourg (postdoc) and EPFL (masterstudent).

Prosjektet har gitt betydelig kompetanseutvikling for de norske miljøene og etablert internasjonalt samarbeid. Som et Unge forskertalenter prosjekt, har prosjektet bidratt til betydelig karriereutvikling for prosjektleder (J.M. Polfus), som ila prosjektperioden har gjennomført et forskningsopphold ved Massachusetts Institute of Technology (MIT, USA) og blitt ansatt i fast stilling som førsteamanuensis ved Universitetet i Oslo. Den vitenskapelige tematikken i prosjektet er fundamental og nytteverdiene av resultatene vil hovedsakelig knyttes til økt kunnskap og publisering i faglige tidsskrift. Prosjektet har videre bidratt til utdanning av en PhD kandidat og en MSc student innen kjemi og materialvitenskap. Resultatene og arbeidet i prosjektet har også ført til nye idéer som følges gjennom andre prosjekter og MSc oppgaver.

Two-dimensional (2D) materials are emerging as a novel class of functional materials for the miniaturization of electronic and electrochemical devices. They pave the way for electrochemical capacitors with energy densities approaching that of Li-ion batteries due to stacking efficiency, photovoltaic devices with strong light matter interactions and novel transistor architectures - technology that would represent a significant breakthrough for renewable energy storage and conversion. The project seeks fundamental understanding of the defect chemistry and functional properties of 2D materials, encompassing charged point defects as well as clusters in the material and at its highly influential interfaces (surface, edge, inter-layer and substrate). The project builds upon a fabrication procedure recently demonstrated by the applicants for 2D MoS2, involving PVD techniques and post treatment in H2S at elevated temperature. MoS2 and Ga2S2 will serve as model systems, and other 2D materials and ultra thin films will be pursued, e.g., WSe2, MnO2, TiOxNy. First-principles calculations will be combined with advanced characterization of the prepared 2D films to elucidate the defect chemistry of the materials, including charge compensation mechanisms for intrinsic and extrinsic defects in the material and interfaces. An electrical characterization setup for 2D films under controlled temperature and atmosphere will be developed. 2D heterostucture devices will be constructed to demonstrate application properties. The project will be managed by SINTEF with partners at University of Oslo and University of Luxembourg (transnational funding with FNR) and educate 2 PhD candidates in Norway and Luxembourg. International collaboration plays an important role in the project with participation of 2D research groups at The University of Illinois (US) and EPFL (CH), as well as collaborative and advisory roles from The University of Manchester (UK) and planarTECH LLC (US).