Fleksibilni uređaji bazirani na slojevitim 2D materijalima

Abstract

Present-day flexible and stretchable electronic devices, such as bendable phones or displays, are mostly built from rigid microscopic components (sensors and transistors), where rigidity ultimately sets limitations to the devices’ mechanics and performances. Due to their atomically thin nature, 2D materials (2DMs) exhibit impressive mechanical attributes which offer, in conjunction with the tunability of their other properties, significant advancements of the next generation of flexible devices. Although there is a massive interest in 2DMs and their applications, many steps are still needed to fully exploit 2DMs for flexible and stretchable devices. Within this thesis, we have investigated MoS_2, borophene, and graphene monolayers as well as MoS_2/graphene heterostructures in view of flexible devices. Different segments which are required for the understanding of physical properties and device implementation of these materials have been investigated. For borophene and heterostructures of MoS2/graphene, the focus was on mechanical manipulation. Graphene monolayers were mainly used for testing of the transfer and transport methods. MoS_2 monolayers and respective devices, as the most advanced systems we constructed, constitute the basis of strain-dependent measurements in our work. More specifically, the first part of our research was dedicated to different device fabrication procedures in order to determine the optimum type of 2DM growth, sample morphology, and method of transfer. In this effort, several experimental techniques have been used: AFM, SEM, Raman and PL spectroscopy, and transport measurements. Large-scale monolayers (>100 μm) proved to be advantageous over smaller isolated flakes. Mechanical stamping method and electrochemical delamination yielded good results for the transfer of 2DMs from the growth substrates to arbitrary ones, with minimum contaminations or damage. E-beam and stencil lithography were used on both rigid and flexible substrates for the fabrication of electrodes of the 2DM-based devices. In the second part of the thesis, strain-dependent measurements with uniaxial bending were conducted, thus enabling the investigation of tensile strain on devices’ performances. For example, our devices did not show any degradation in terms of electrical resistance even after 40 bending cycles at strain values as high as 1%. Also, the photoresponse of MoS_2 can be significantly enhanced by the application of strain, where the increase of photocurrent and broadening of spectral sensitivity have been found. Additionally, we have shown that on encapsulated devices strain has uniformly propagated over the whole surface of 2DMs, while with a transfer of 2DMs on pre-fabricated electrodes, the electrodes are a source of inhomogeneous strain distribution within the 2DM. Overall, the research presented in this thesis provides a new insight into the physical phenomena behind the synthesis and manipulation of 2DMs, and can serve as a guideline for the fabrication of novel devices based on 2DMs. In line with this, the conducted straindependent characterisation brings unique properties of 2DMs closer to real-life applications.Istraživanje provedeno u sklopu ove teze predstavlja nove uvide u fizikalne fenomene u pozadini sinteze i manipulacije 2DM-a, koji mogu služiti kao smjernice za izradu novih uređaja na bazi 2DM-a. Konkretno, pokazali smo kako elektrokemijski transfer može biti korišten za mehaničku manipulaciju osjetljivih materijala poput Bo te kompleksnih materijala poput MoS_2/Gr heterostrukture. Nakon toga detaljno smo istražili svojstva MoS_2 te demonstrirali kako se PDMS transfer te stencil i e-beam litografija mogu primijeniti za konstrukciju uređaja na bazi 2DM-a. Dodatno smo pokazali kako se sa raznim dopiranjima može utjecati na energije kvazičestica te da se MoS_2 može iskoristiti za FET-ove na Si waferima. Poseban naglasak u istraživanju stavljen je na karakterizaciju izrađenih fleksibilnih uređaja MoS_2-a u ovisnosti o naprezanju. Utvrdili smo da granice zrna ne sputavaju prijenos naprezanja unutar large-scale 2DM-a, zatim kako se MoS_2 može iskoristiti za fleksibilne senzore naprezanja i pritiska te u konačnici da se primjenom naprezanja mogu poboljšati performanse fleksibilnih fotodetektora na bazi MoS_2. Ostvareni rezultati značajno približavaju iskorištavanje jedinstvenih svojstava 2DM-a za svakodnevne uređaje otporne na drastične mehaničke deformacije poput rastezanja i savijanja