Hybrid two-dimensional heterostructures for optoelectronics – H2DH

The rise of a vast family of two-dimensional (2D) crystals, with unique electronic and optical properties, has opened exciting perspectives for “van der Waals heterostructures”. The latter are only a few atoms thick and exhibit new properties and functionalities that cannot be achieved using bulk crystals. Indeed, 2D crystals feature exposed electron gases, which properties are dramatically influenced by non-covalent coupling to low-dimensional adsorbates. So far, most endeavors have focused on heterostructures based on graphene, boron nitride, and transition metal dichalcogenides (MX2, with M=Mo, W and X= S, Se, Te). In particular, graphene, as a 2D semimetal with extremely high carrier mobility but no bandgap and “monolayer” MX2, as direct bandgap semiconductors with good carrier mobility, are highly promising building blocks for optoelectronic devices (OEDs). Besides, 2D crystals are naturally suited for lateral geometries and can be more easily integrated in OEDs than 0D and 1D nanostructures. Yet, the fabrication of high-quality heterostructures based on graphene and MX2 relies on sophisticated processes, which offer limited scalability and device engineering possibilities.

At the same time, a breakthrough has been achieved in the controlled colloidal synthesis of layered semiconductors, such as core only, core-shell and core-crown 2D nanoplatelets (NPL, or quantum wells) based on metal chalcogenides (CdSe, CdS, CdTe,…). NPL are excellent light-absorbers and emitters. Their thickness, which directly defines their electronic structure and peak emission energy, is controlled at the monolayer level, while their lateral dimensions can attain the µm range. In addition, NPL surface chemistry can be efficiently tailored. Importantly, highly homogeneous ensembles of NPL, with high structural quality can be synthesized, processed and integrated into OEDs. Nevertheless, electron transport in NPL films remains driven by hopping processes, leading to limited carrier mobility.

It therefore seems natural to combine i) graphene and MX2, as semimetallic or semiconducting channels with good transport properties and ii) NPL, as a tunable active materials, into novel hybrid 2-dimensionnal heterostructures (H2DH) and OEDs. The performance of such devices is governed and often limited by band alignment, interactions with the underlying substrate, and crucially, short range phenomena such as charge separation, charge transfer and Förster resonant energy transfer (FRET). FRET is a “dipole-dipole”, non-radiative coupling phenomenon, involving a photoexcited donor and an acceptor, which absorption spectrum overlaps with the emission spectrum of the donor. FRET between a photoexcited NP and a graphene or MX2 layer may bypass direct charge transfer processes, which could lead to an electrical current, useful for optoelectronic applications. FRET may be seen as an efficient way to harvest and funnel energy from photoexcited donors, which is of major interest for photodetection. However, in the absence of a charge separation mechanism, the energy transferred as electronic excitations will be rapidly dissipated into heat. H2DH offer a natural and elegant platform to address these issues and to uncover new regimes of electron transport, charge separation, photoconductivity, photodetection and electrically-controlled luminescence.

For our project, we will grow high quality 2D materials (graphene, MX2, and NPL) that will be assembled intro electrically contacted H2DH, using original fabrication methods based on resist-free processing and electrochemical gating. We will investigate the fundamentals of charge and energy transfer in NPL-graphene and NPL-MX2 H2DH using a complementary set of optical and electron spectroscopy studies, as well as optoelectronic measurements. This fundamental work will guide a more applied, yet equally important effort towards the development and extensive study of a new class of phototransistors based on H2DH.