Electrical and structural characterisation of Transition Metal Dichalcogenides

Abstract

This thesis explores modification of transition metal dichalcogenides (TMDs) through doping and synthesis techniques to tailor their electronic properties, including sensor technology. The work is divided into three main studies: Rhenium (Re) doping in Molybdenum Disulfide (MoS2) to achieve n-type characteristics, Niobium (Nb) doping in Tungsten Disulfide (WS2) to induce p-type behavior, and the synthesis and application of PtSe2 in strain sensors. The first study investigates Re doping in Thermally Assisted Conversion (TAC) MoS2 films, with three compositions: pristine, 2.63 at.% Re, and 10.48 at.% Re. Based on Hall effect measurements, both 2.63 at.% and 10.48 at.% Re-doped MoS2 showed n-type behavior with reliable signal-to-noise ratio. Optimal 2.63 at.% Re doping enhanced conductivity and reduced activation energy, as confirmed by Arrhenius plots. Hall measurements showed the highest mobility at 2.63 at.% Re doping (0.51 cm2/V·s) and a resistivity of 2.5 Ω·cm. TEM and EELS analyses indicated Re segregation and integration into the MoS2 matrix, providing insights into polycrystalline MoS2 film doping. The second study addresses achieving weak p-type conductivity in TAC grown WS2 through Nb doping, with two compositions: pristine and 1.76 at.% Nb. Introducing Nb at 1.76 at.% and converting at 900◦C for 30 minutes resulted in highly textured structures in both pristine and Nb-doped WS2 samples. Field effect measurements showed pristine WS2 had n-type behavior with a mobility of approximately 0.1 cm2/V·s and an on/off ratio of 8, while Nb-doped WS2 exhibited weak p-type iii conduction, attributed to incomplete conversion. This study underscores the need for further optimization of the doping process and provides a basis for improving doped WS2 film quality. The third study focuses on PtSe2 synthesis and its application in sensors, moti- vated by an energy-efficient electrochemical exfoliation (EE) process, large PtSe2 flakes with aspect ratios exceeding 1000 were produced and deposited using the Langmuir-Schaefer technique to achieve highly aligned networks. The semitrans- parent sensors, with ≈ 60% transparency (light passes through) at 1000 nm, showed potential applications in textile electronics. The sensors exhibited a consistent response with 0.5% strain with a negative gauge factor of −5.45, attributed to changes in the band structure upon deformation, and remained robust for 1000 cycles. This thesis provides insight into semiconducting material synthesis and characterization, and suggests future pathways to optimize these materials.