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MULTIfunctional optoelectronic devices based on hybrid heterostructures of 2D materials and photochromic SWITCHes

Periodic Reporting for period 1 - MULTI2DSWITCH (MULTIfunctional optoelectronic devices based on hybrid heterostructures of 2D materials and photochromic SWITCHes)

Reporting period: 2016-08-21 to 2018-08-20

Two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides (TMDs), have attracted a great deal of attention due to their outstanding physical properties that can be conveniently exploited in a wide range of micro- and nanotechnologies. Nowadays, a number of physico-chemical approaches are being developed for tailoring various properties of 2D materials in order to improve the performance of existing 2D devices for various technologically applications, which also include the design of multifunctional technologies.

Within this framework, MULTI2DSWITCH aimed at implementing novel multifunctional optoelectronic devices based on heterostructures of switchable molecular systems (i.e. photochromics) and 2D materials, such as semimetallic graphene and semiconducting TMDs. Photochromic molecules – such as azobenzenes, diarylethenes and spiropyrans – are special photosensitive molecules that undergo a reversible photochemical isomerization process upon exposure to UV/visible light, i.e. they switch between two (meta)stable states with markedly different properties, resulting in a modification of the macroscopic properties of the adjacent 2D semiconductors.

Implementing 2D-material/photochromic devices required accomplishing the following three objectives: (1) developing sound chemical approaches for “bridging” 2D crystals and functional molecules, (2) integrating the newly developed hybrid materials into practical devices, and (3) optimizing the device architecture and properties (e.g. dielectric/semiconductor interface, metal contacts) to boost the performance in optoelectronic applications. Such objectives were pursued by making use of a plethora of experimental methodologies belonging to different scientific disciplines, such as (supra)molecular chemistry, solid-state physics and device engineering.

Besides demonstrating new photoswitchable transistors based on molecular systems and 2D semiconductors, MULTI2DSWITCH provided new fundamental knowledge in the key field of 2D materials-based multifunctional electronics.
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.
Besides demonstrating new photoswitchable transistors based on molecular systems and 2D semiconductors (azobenzene/spiropyran and TMDs), MULTI2DSWITCH resulted in state-of-the-art methods for tuning the properties of 2D materials with functional molecular systems and helped solving critical technical challenges in the key field of 2D materials-based multifunctional electronics. Critical knowledge has been produced, particularly on the interaction between molecular switches and 2D semiconductors, as well as on the chemical approaches to functionalize TMDs and impart them a multifunctional/multiresponsive nature. This information will be crucial for developing novel energy-efficient, flexible/wearable smart materials and devices for sustaining the expansion of the IoT/IoE within our hyper-connected society, in line with important industrial megatrends that point towards smart utilities and smart environments (houses, hospitals, fabs and cities), as well as AI at the edge, which could benefit from the unique properties of multifunctional hybrid 2D materials.

The scientific knowledge and technical advancements achieved with MULTI2DSWITCH represent a significant contribution to the numerous efforts of the European community for taking graphene and related 2D materials from the realm of academic laboratories into practical technologies to the benefit of the European society.
Figure 2. Sensing molecular photoswitching at the solid/liquid interface with monolayer MoS2 FETs.
Figure 1. (a) Ion-bombardment experiment. (b) Transfer curves of a monolayer MoS2 FET.