Progress on 2D-MoS2: development of a scalable fabrication method and demonstration of an X-ray detector

Two-dimensional transition metal dichalcogenides (TMDCs) aroused significant interest in the last years as semiconductor materials for application in the field of electronics, due to their tunable bandgap, good carrier mobility, and strong light absorption. Among TMDCs, two-dimensional molybdenum disulfide (2D-MoS2) has been the most investigated for electronic and optoelectronic applications, like transistors and photodetectors. 2D-MoS2 can particularly benefit from the excellent light matter interaction properties in the UV-VIS spectrum combined with good charge carrier transport properties. The literature reports photodetectors based on 2D-MoS2 fabricated with different techniques, including exfoliation, chemical vapor deposition (CVD) and wet chemical synthesis. However, it is still challenging to scale the proposed devices to the industrial level, due to the lack of a versatile fabrication process that ensures both reproducibility and scalability. A possible solution to this could rise from wet chemical synthesis. In the first part of this work, I discuss the development and optimization of a fabrication method for MoS2 thin films based on a sol-gel process which allows for scalable productions. This route allowed the fabrication of large area (~cm2) MoS2 thin films of 200 nm thickness on technological relevant substrates (i.e., glass, gold, silicon). The films displayed good uniformity, although the crystallinity was affected by residual impurities. The films produced with this technique were employed for the fabrication of photodetectors, displaying responsivity of few mA/W in the NUV-VIS-NIR spectrum. However, the performance of the device was affected by a still limited quality of the MoS2 films obtained with the current method that require further optimization. Further studies will overcome the current limitations and solutions to be investigated in future works are proposed. The second part of this work focuses on expanding the detection capability of 2D-MoS2 (currently limited to the UV-VIS-NIR spectrum), by exploring for the first time X-rays sensing, taking advantage of the X-ray cross section of MoS2 associated with the high atomic number Z of Mo. A detector based on an exfoliated MoS2 monolayer (1L-MoS2) was fabricated and characterized for the purpose. The detector showed direct detection of ~10^2 keV X-rays down to dose rates of 0.08 mGy/s, with X-ray sensitivity is in the range 10^8-10^9 µC ⋅Gy-1·cm-3, outperforming most of the reported organic and inorganic materials. A strategy to improve the device response was also studied by adding a scintillator film, which resulted in a three-fold increase of the signal. These results suggest to consider 2D-MoS2 for in-vivo dosimetry applications.