Expanding the range of synthetic 2D materials beyond graphene : production of dilute magnetic semiconductors thinned to the 2-dimensional limit by scalable liquid-phase exfoliation

Two-dimensional (2D) materials beyond graphene have opened a new frontier of research in material science after the discovery of high-quality crystals only one atom thick. Synthetic strategies that are high-yield, high-tunability and cost-effective are highly desirable. Doping and alloying have shown much potential for tailoring the electronic and magnetic properties of materials to meet the increasing demands in both fundamental studies and diverse applications. Dilute magnetic semiconductors are one such candidate, allowing excellent preservation of both semiconducting and magnetic properties, though the experimental production is still challenging. In this study, I have explored the fabrication strategies of layered 2D materials, alloying of structurally similar metal sulfides and oxides, and production of dilute magnetic semiconductors via doping of magnetic atoms into molybdenum disulfide, through a combination of chemical synthetic method and liquid-phase exfoliation. A soft processing method toward layered and 2D materials via single and dual-molecular precursors was designed and studied in this work. Layered transition metal dichalcogenides (TMDCs) (MoS2, WS2, MnS, FeS and CoS), transition metal oxides (MoO3 and WO3), and their alloys (Mo1-xWxS2 and Mo1-xWxO3) have been produced. Transition metal doped MoS2 (M1-xMoxS2, M=Mn, Fe and Co; x=0, 0.01, 0.05, 0.1, 0.25, 0.5, 0.75 and 1) have been synthesised and characterised. Furthermore, the formation of pure 2H-MoS2 and polycrystalline MoO3 from direct thermolysis of precursor Mo(DTC)4 under 450 degree Celsius for one hour has been reported for the first time. In addition, the combination of liquid phase exfoliation has been applied to exfoliate bulk layered materials into 2D thin-layers for nanoscale analysis such as HR-S/TEM, HR-S/TEM-EDX, and AFM, whilst other characterisations have been measured on pristine powder using Raman, PXRD, XPS, SEM/SEM-EDS, EPR, and SQUID. The pure MoS2 thin-layer produced has a mean flake thickness of about 10 nm and the Mo1-xWxS2 alloys were found to show a thickness range of 1-10 nm. Magnetic atoms (Mn, Fe and Co) doped MoS2 show stronger magnetic moments when compared to undoped MoS2 at 5K, and a midway saturation magnetisation is found for all the 10% doping samples, which is between the extremes that were reported previously. The successful doping with magnetic moments that are in reasonable agreement with theoretical studies demonstrates the great potential of this soft processing method for multifunctional purposes. The current work proposes a convenient soft synthetic strategy which does not involve high temperature or long-range solid-state diffusion, and thus is of interest for both scalable production of layered and 2D materials, as well as the expansion of the 2D library via doping and alloying, which shows promise for production of future dilute magnetic semiconductors.