GALACTIC · Graphene bAsed switchabLe mAterials - towards responsive eleCTronICs : An Intra-European Fellowship for career development

Graphene bAsed switchabLe mAterials - towards responsive eleCTronICs : An Intra-European Fellowship for career development

After the Nobel Prize in 2010 on graphene [https://www.nobelprize.org/nobel_prizes/physics/laureates/2010/press.pdf], the interest of the scientific community shifted from bulk van der Waals (vdW) heterostructures to two-dimensional (2D) materials such as graphene, MoS2, WSe2, etc. that show physical properties totally different from the original 3D materials due to the quantum confinement of electrons. In particular, being composed by 2D surface without any bulk, they are extremely sensitive to surface modification. Several examples of artificial materials have been indeed reported in which magnetic [Wang et al. Phys. Rev. Lett. (2015)], electrical [Wang et al. Science (2015); Ponomarenko et al. Nature (2013)] and optical [Buscema et al. Nano Research (2014)] properties of 2D materials have been modified by the interaction with different substrates. More recently, the possibility to obtain vdW heterostructures by mechanically superimposing different 2D materials is opening new perspectives in this field, since these new vdW heterostructures have properties different from the starting materials [Novoselov et al. Science (2016)]. In this field, chemistry plays an important role since libraries of molecules with specific doping moieties can be designed for fine-tuning the properties of such 2D materials. In these last years, indeed, numerous different molecules such as TCNQ, TTF, etc. have been used as electron-donating or -withdrawing units able to induce, respectively, a p- or n-type doping effect on 2D materials [among others: Balakrishnan et al. Nature Physics (2013); Mouri et al. Nano Lett. (2013); Anami et al. Science (2015)]. By combining bottom-up and top-down approaches, the GALACTIC project was targeted at forming highly ordered and phototunable self-assembled monolayer interfaces suitable for the fabrication of graphene-based light-responsive nanoelectronic device. Compared to the state of the art, the use of photochromic molecules as doping units has guaranteed the possibility to tune in situ the electrical/optical properties of the so-obtained hybrid organic/inorganic structures by taking advantage of the different isomer properties (e.g. different dipole moment of spiropyran/merocyanine isomers). Moreover, the GALACTIC research has developed an easy strategy to ensure a big change in the properties of 2D flakes by self-assembling a highly ordered molecular monolayer on top of them. A supramolecular approach has been used to reach this goal. In particular, photochromic molecules with long alkyl chains have been synthetized in collaboration with Prof. Hecht group in Berlin to exploit the well-known vdW interactions among the alkyl units and, at the same time, between the organic self-assembled monolayer and the inorganic 2D flakes. Interestingly, the GALACTIC approach allowed to easily tune i) the doping unit by a suitable light input and potentially ii) the order and iii) density of the self-assembled monolayer by changing, for instance, the length of the alkyl chain. A solid understanding of the morphology of these phototunable self-assembled monolayers at the nanoscale was crucial for the GALACTIC project. Scanning tunneling microscopy (STM) was used – among others – to investigate the structure of the molecular assembly. Moreover, X-ray photoelectron spectroscopy (XPS) was used – in parallel with UV-Vis characterization –to monitor the in situ optical switch of the photochromic head units in the assembly. In order to understand the correlation between the organic architecture and the electrical properties of graphene (2D material), graphene-based field effect transistors (FETs) has been developed and a light-tunable electrical response of the 2D material was observed and correlated to the nature of the photochromic isomer on top of it. Impact: The knowledge acquired in GALACTIC is central both to the optimization of organic self-assembled monolayers physisorbed on different inorganic 2D materials, and to the development of new hybrid organic/inorganic vdW heterostructures. In particular, phototunable hybrid heterostructures possess a great potential to create multifunctional materials with properties on demand. The fellow has received multidisciplinary training in a range of experimental techniques, built new academic and industrial collaborations and has worked in world-class laboratories. Moreover, GALACTIC gave the fellow the possibility to strengthen her background in the interdisciplinary field of 2D-material-based electronics and to further develop her research career in Europe by performing cutting-edge science.

Data: CORDIS, © European Union