In ULISSES we are actively researching its use for photodetectors. Its main strength is that, in contrast to many other materials, it can be synthesized at a relatively low temperature compatible to common CMOS processing, without strict substrate requirements. We have studied the nanocrystalline nature of the material to better understand how it influences its properties and to provide insights for tailoring the material properties as desired.

The layered van-der-Waals material PtSe₂, a two-dimensional noble-metal-based material from the group of the transition metal dichalcogenides, has gained an increasing amount of attention over the last years. Due to its interesting electronic, optoelectronic, and piezoresistive properties, it has been proposed for various applications in electronics and sensing, including piezoresistive pressure sensors or microphones, highly sensitive photodetectors, or gas sensors with both high sensitivity and specificity. A major advantage is the large-scale growth method called thermally assisted conversion (TAC), which yields high quality thin films of PtSe₂ while at the same time staying within the temperature budget of CMOS back-end-of-line processing technology. This could imply a large step forward to potential integration within existing CMOS technology in industry.

Experiments and measurements with TAC-grown PtSe₂ thin films have, however, often yielded very different results in terms of piezoresistive and (opto-)electronic properties. To investigate this issue, we have examined the nanocrystalline structure of the PtSe₂ films in detail using scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. At the same time, we fabricated devices to measure the electronic and piezoresistive properties of the material.

The study allowed us to show strong correlations between the nanocrystalline structure and the electronic properties of the TAC-grown PtSe₂ layers. A variation of the nano-crystallite size and orientation within the film influences the electronic properties across many orders of magnitude. At the same time, we showed that Raman spectroscopy can be used as a very helpful and fast tool to assess the material quality based on the width of the characteristic peaks of the Raman spectrum of crystalline PtSe₂.

Our experiments show that very high-quality films can be produced using the large-scale synthesis approach, which is a great advantage for the integration of PtSe₂-based devices in CMOS technology. Additionally, the findings suggest that by precisely controlling the synthesis conditions during PtSe₂ fabrication, the material properties can not only be optimised but also be tailored according to the application needs. This makes PtSe₂ a very versatile novel material for several fields of applications.