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Superlattices and proximity effects in 2D materials/molecules hybrid van der Waals heterostructures

Periodic Reporting for period 1 - SUPER2D (Superlattices and proximity effects in 2D materials/molecules hybrid van der Waals heterostructures)

Reporting period: 2017-11-01 to 2019-10-31

The fabrication of materials with on-demand electronic, optical and magnetic properties lies at the heart of Materials Science. Efforts in the quest for materials by design have produced exquisite results in basic research and technology, which have impacted a number of research fields directly linked to applications (communications, information, mobility), ultimately revolutionizing everyday life. A strong connection exists between basic research in materials science and electronics, since the design of ultimately all electronic components relies on peculiar material capabilities. In this regard, the demonstration of new materials with tailored properties could improve current technology and inspire novel device concepts beyond conventional micro-electronics.
In this search for materials by design, graphene and other two-dimensional materials (2DMs) offer a unique opportunity, since their reduced dimensionality makes it possible to controllably tune their intrinsic physical properties through ad-hoc modifications of their surface. Interfacing 2DMs with molecular thin films holds immense potential, since specific functional groups can be integrated in the molecular layer to provide programmable capabilities to 2DMs.
SUPER2D aimed at combining 2D materials with molecular self-assembled monolayer, to obtain hybrid heterostructures with tunable properties. The project has exploited different, ad-hoc chosen molecular monolayers to introduce deterministic modifications in the intrinsic physical properties of 2DMs, with the ultimate goal of generating novel materials for next generation electronics. Special focused was dedicated to (i) silane-based functional molecules, with the goal of improving the performances of 2DM-based field effect transistors and manipulating the superconductive transition in superconductive 2DMs; (ii) organometallic molecules, with the goal of altering the magnetic properties of 2D materials.
The ultimate objective of SUPER2D was the demonstration that 2DM/molecule interfaces represent an ideal experimental platform to design technologically relevant materials with programmable properties, fulfilling the central challenge of materials science.
In the course of SUPER2D, the interface between different 2D Materials (2DMs) and highly ordered molecular monolayers were investigated. Very different 2DMs were explored: a semiconductor, a superconductor, a layered ferromagnetic material and two compounds which could potentially display intrinsic magnetic ordering. These materials were interfaced to different ultrathin molecular films. A special focus was dedicated to molecules composed of a metal atom coordinated by organic ligands, which possess predictable spin configuration, and generate a net of metallic atoms orderly arranged on the 2DM surface. Additionally, self-assembled monolayers (SAMs) composed of silane-based molecules were employed, which induce predictable doping effects.
Different strategies were explored to obtain high-quality 2DMs. Most 2DMs were grown by molecular beam epitaxy. To confirm the atomic thickness and the desired stoichiometry, the so-obtained single layers were characterized through scanning probe techniques and spectroscopic techniques. Additionally, in some cases 2DMs were obtained by micromechanical exfoliation.
The growth of molecular overlayers was also optimized by following different strategies. Silane-based self-assembled monolayers were grown through a vapor phase deposition. Ultrathin films of organometallic molecules were grown by sublimation in ultra-high vacuum.
Through spectroscopic techniques, it was found that SAMs introduce electric field effects similar to those of a gate terminal, thereby introducing doping effects in the 2DMs. Importantly, the direction of the electric field could be inversed by using different molecules. This effect was exploited in two different studies, leading to two main results:
1. We engineered the charge transport in semiconducting 2DMs, demonstrating high performance p-type or n-type field effect transistors.
2. We showed that the superconducting transition in a single-layer superconductor could be manipulated via the formation of self-assembled monolayers.
Finally, we characterized how the magnetic properties of 2DMs are affected by the presence of a molecular overlayer through X-Ray circular dichroism, which provides information on the magnetic properties with elemental resolution. It was found that (i) VSe2 is not intrinsically ferromagnetic at the single layer limit, in contrast to a previous report reporting ferromagnetism; (ii) the metallic center in organometallic molecules does not couple magnetically to ferromagnetic van der Waals materials.
These results obtained in SUPER2D were presented to the scientific community in four conferences and a workshop, in three occasions as invited contributions. Moreover, to spark interest of the scientific community in the topic of SUPER2D, a perspective and a review paper were published discussing different aspects of 2D materials/molecules interfaces. The results which emerged from the experimental work performed during SUPER2D are described so far in two published works, one focusing on the engineering of charge transport in semiconducting WSe2, and one on the absence of ferromagnetism in VSe2. Other two articles are in preparation.
Moreover, various activities were performed with the purpose of bringing science closer to students/general public, such as Lab visits and demonstrations in front of high-school students and participation to the Science Week.
This project focused on the possibility to exploit 2D Material/molecule interfaces to improve the performances of 2D-Material-based devices and to manipulate their fundamental physical properties, moving at the crossroad between physics, chemistry and engineering in the interdisciplinary field of materials science.
Several results achieved in this project moved beyond the state of art. It was found that self-assembled monolayers introduce an electric field effect which could be used to introduce a predictable doping in 2DMs. While several studies had previously reported how specific molecules produce changes in the charge carrier density of 2DMs (doping), in this project we took a step further, exploiting the molecule-induced doping in two very different ways. First, we engineered the energy levels of a prototypical single layer semiconductor with the purpose of demonstrating high performance p- and n- field effect transistors. Second, we were able to demonstrate that an intrinsic physical property of a 2DMs, such as its superconducting transition, can be manipulated determinstically through molecular engineering.
Moreover, our search for proximity effect at 2DM/molecule interfaces has provided significant insight on the intrinsic magnetic properties of 2D Materials and on the magnetic coupling across van der Waals interfaces.
The project has a societal impact in view of the potential relevance of the materials and devices studied in this action for next-generation technology. Additionally, the fellow was attended not only specialized events with the purpose of promoting the results of the fellowship to a broad audience of researchers, but also public activities to bring science closer to the public.
SUPER2D focused on the investigation of 2D Material/molecule interfaces.