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Heterogrenzflächen von 2D Materialien und organischen Halbleitern

Fachliche Zuordnung Experimentelle Physik der kondensierten Materie
Förderung Förderung von 2018 bis 2023
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 397373225
 
Erstellungsjahr 2022

Zusammenfassung der Projektergebnisse

In our project we were working on the integration of nanolayers (5‐10 nm) of organic semiconductors (OSC) with inorganic 2D materials to form their van der Waals heterostructures (vdWh), which could contribute significantly to technological advancement by exploiting the properties of both materials. For practical application of vdWh, large scale preparation of heterostructures without compromising their crystallinity and interface quality is required. In the first phase of the project, we have successfully optimized reproducible synthesis of large area high‐quality 2D transition metal dichalcogenides (MoS2, WS2, WSe2 and MoSe2) and organic semiconductors (pentacene and DNTT). We have recently shown that the chemical vapor deposition (CVD) based synthesis of TMD monolayers can be significantly improved resulting in a superior structural quality of the samples, which is comparable to the exfoliated ones. This improvement is achieved by controlling the effusion rates of the precursors using Knudsen cells and confining the reactants in a small reaction volume enabling their optimal chemical reaction conditions during the growth. Furthermore, we have analyzed the structural quality of CVD grown TMDs down to the atomic scale by high resolution transmission electron microscopy (HRTEM) and evaluated their defect density, which is found to be in the range of ~10^13/cm2 comparable to the best quality exfoliated monolayers of TMDs. In additions to this, we have shown that by encapsulating the CVD grown TMDs in hexagonal boron nitride (hBN) layers, their intrinsic high optical quality can be accessed, which is demonstrated by low temperature optical spectroscopy. The OSC transfer requires a sacrificial, water‐soluble interlayer for OSC release. We have tested several water swelling materials as polymer thin films in view of their OSC growth properties. We find that it is possible to grow homogeneous OSC films on such suited water‐soluble interlayers. We optimized the deposition technique of the OSC molecules on these soluble interlayers by variation of Knudsen cell distance, rate, and substrate tilt angle. For pentacene and DNTT, two of the most commonly used highly ordered p‐conductors, we obtain huge crystalline grains of several micron, i.e. large enough for optical inspection in a microscope. We have tested different release methods in order to maximize the flake size for transfer. The initial transfer technique was based on full immersion and peel‐off of OSC film. This resulted in mm sized flakes, which however were subject to folds and wrinkles. We further improved the technique by developing a meniscus‐ based transfer method, in which we were able to achieve wafer‐scale transfer of OSC nanosheets. We integrated the OSC nanosheets into various device configurations, including large area OSC field‐effect transistor (FET) device arrays and OSC‐TMD van der Waals heterostructures (vdWh). The high quality of the transferable OSC nanosheets resulted in better performing OSC‐FETs. Furthermore, we have tested the ambipolar and anti‐ambipolar FET characteristics of the OSC‐TMD vdWh devices. Currently, we are investigating the optical response of the OSC‐TMD vdWh by performing photocurrent measurements as well as transient absorption spectroscopy. Combining the electronic properties of graphene and MoS2 in hybrid heterostructures offers the possibility to create high‐performance devices with various functionalities. We have realized all‐CVD graphene‐MoS2 heterostructures based field‐effect transistors with as highly sensitive gate‐tunable Schottky barriers at the contacts. Furthermore, we investigated spin transport and dynamics in the heterostructure channel showing an increased spin‐orbit coupling strength in the proximitized graphene in the heterostructures. These studies open a scalable platform for further improving the fabrication method and its device figure of merits of all‐CVD 2D heterostructure devices. The Schottky barrier at material interfaces can limit the performance of future electronic devices made with 2D materials and OSC. To minimize interface contact resistances, we have explored schemes of work function matching between materials. We found that it is possible to achieve controllable contact resistances to MoS2 with low work function metals (e.g., Gd) and graphene (with tunable work function). As an alternative route to produce atomically clean interfaces, we explore direct van der Waals epitaxy of MoS2 on epitaxial graphene on SiC, which will serve as substrate for future integration with OSC. In addition to this, spatially resolved measurements of the work function of monolayer and few‐layer MoS2, WSe2 and MoSe2 on different metal substrates show that both Fermi‐level pinning as well as Schottky‐Mott limit behaviour can be obtained. The latter is especially promising for tuning energy barriers in TMD/organic heterostructures via the substrate work function. We further continue collaborating on the project, we are progressing in the direction of investigating the optoelectronic response of the OSC‐TMD vdWh. We are exploring various ultrathin optoelectronic and photovoltaic device configurations. Furthermore, we study the nature of band alignment between the OSC‐TMD heterostructures using photoelectron spectroscopy techniques. We expect that our project will further contribute to the development of OSC device technology by integrating 2D TMDs, which enables the realization of high‐performance consumer electronic products.

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