Contamination control and optimisation of hot pick-up van der Waals heterostructures assembly method for automatic systems in inert conditions

Since the discovery of graphene in 2004, two-dimensional materials have been extensively studied. A wide range of these materials can be grown or thinned down to monolayers by variety of methods and then re-assembled layer by layer into three-dimensional van der Waals heterostructures. Arbitrary order of layers makes the number of potential combinations, as well as applications close to infinity.

Further and further rise the demand for high quality heterostructures with atomically clean interlayer interfaces. Many methods have been developed to fabricate such structures; however, quality of fabricated structures varies from lab to lab from one scientist to another due to complexity of the processes. This complexity comes from fragile and sensitive nature of two-dimensional materials and manual component of fabrication.

One of the way to improve quality of the heterostructures and incorporate new water-sensitive materials is performing fabrication in inert conditions as well as process automation to eliminate human factor. However, such goals requires stable and reliable fabrication processes as well as simple and fast characterization techniques for quality control.

In this work, the hot pick up technique has been deeply analysed to reveal factors critical for process stability and reveal contamination sources. Multiple technical details were clarified and optimized, such as optimal concentration of polypropylene carbonate, samples cleaning procedures, significant improvement of the stacking setup. New application of terahertz time domain spectroscopy using scattering scanning nearfield optical microscopy as contamination and properties control method of heterostructures is presented.