Objective (1) was accomplished by leveraging on the expertise of ISIS in covalent and non-covalent functionalization strategies for tailoring the properties of nanomaterials. Such chemical methods were specifically developed for TMD-nanosheet/switchable-molecule heterostructures. The research fellow – holding a PhD in physics and previous experience in device engineering and microfabrication – had the opportunity to acquire all the fundamental knowledge and practical methodologies necessary for chemically functionalizing TMDs. The chemical approaches utilized during MULTI2DSWITCH were systematically presented and critically analyzed in a review paper (Bertolazzi et al., Chem. Soc. Rev. 2018, 47, 6845). In this context, a strategy to engineer chemically-active defects (e.g. sulfur vacancies) in 2D semiconductors has been developed to enable chemical functionalization of monolayer MoS2 with molecules carrying thiol functional groups (Figure 1). The switchable properties of the newly-synthesized hybrid materials have been exploited for developing multifunctional devices, such as photoswitchable field-effect transistors (FETs) capable of sensing the occurrence of molecular photoisomerization processes (e.g. Figure 2).
Objective (2) was achieved thanks to the background and skills of the research fellow in solid-state physics, device engineering, as well as material characterization and clean-room techniques. Throughout the development of MULTI2DSWITCH’s research plan, several hybrid 2D sheets have been integrated within FET device architectures and their properties have been characterized through a variety of optical and electrical measurements (e.g. photoluminescence (PL) and Raman spectroscopy, time-dependent photocurrent monitoring, acquisition of I–V curves and extraction of charge-carrier mobilities, etc.).
As far as objective (3), light-responsive field-effect transistors (FETs) – based on different combinations of photochromic molecules and 2D crystals – were optimized for efficient photoswitching. Photochromic molecules (azobenzene, spiropyran) could be switched between different isomerization states upon illumination with light at different wavelengths (e.g. UV and visible). Such states exhibit diverse physical properties, such as dipole moments, energy levels, conformations, etc. The on-surface switching of the photochromic molecules on 2D TMDs enabled to light-modulate the electrical properties of the latter component (e.g. Figure 2).
The results concerning the defect-engineering approach for the covalent functionalization of TMDs have been published in Advanced Materials (2017, 29, 1606760) and the chemical approaches for tuning the properties of TMDs were discussed in a review article published in the journal Chemical Society Reviews (2018, 47, 6845) that is read by a widest scientific community. Such results can be exploited to develop air-stable heterostructures by leveraging on the chemical reactivity of sulfur vacancies with thiol functional groups.
The newly developed molecules and hybrid 2D materials will be further explored for applications in molecular switches and nanoelectronic devices. These materials/molecules will be also exploited for future research projects and collaborations.
In the framework of MULTI2DSWITCH, a major technical problem in 2D semiconductor optoelectronics has been addressed, namely the occurrence of strong persistent photocurrent (PPC) effects that degrade the devices’ optical performances. The solutions provided to this problem can be exploited to disentangle the contributions of PPC and molecular switching, which is necessary for a deep understanding of molecular collective/cooperative phenomena at interfaces, as well as for boosting the FoM of multifunctional optoelectronic devices.
