Nikolai Severin

Y6 is a high-performance non-fullerene acceptor widely used in organic solar cells. The assembly behavior of Y6 at interfaces, however, remains mostly unknown and respective in-depth investigations are missing. Here, we use room temperature scanning tunneling microscopy to investigate the self-assembly of Y6 at the solid-liquid interface. We show how coadsorption of polycyclic aromatic hydrocarbon donor molecules leads to the formation of a series of donor-acceptor interfacial cocrystals with tailored molecular structure and order on a graphite surface. The gained understanding of the intricate interplay of the underlying intermolecular interactions should facilitate the engineering of multicomponent interfacial nanostructures and help to potentially improve charge separation and transport in organic (opto)electronic devices.

Supramolecular organization governs the structure and optoelectronic properties of organic thin films. This study shows that films based on the non-fullerene acceptor Y6 can be precisely structured via assembly at the air-water interface. Theoretical cross-sectional areas, Langmuir isotherms, and Brewster angle microscopy reveal that Y6, despite its complex structure, is sufficiently amphiphilic to form well-defined two-dimensional layers. Mechanical annealing through compression-expansion (CE) cycles systematically improves structural uniformity, as evidenced by narrower in-situ detected fluorescence spectra. Repeated CE-cycles also shift the maximum of the compressional modulus towards denser packing. Compared to spin-cast films, Langmuir-Schaefer (LS) layers exhibit a significantly reduced Stokes shift, suggesting less reorganization after photoexcitation and thus a higher supramolecular order. Organic thin-film transistors (OTFTs) fabricated using the LS technique achieve mobilities comparable to those of spin-cast films, despite being substantially thinner (≤ 3 nm, determined by atomic force microscopy), thus requiring considerably less material. Notably, Y6-LS OTFTs outperform previously reported polymer-based LS-OTFTs by one order of magnitude in charge carrier mobility. This work highlights the potential of interfacial assembly for thin film fabrication and underscores the advantages of mechanical annealing and in-situ spectroscopy to enhance the performance of organic optoelectronic devices.

Studying molecular materials at the nanoscale allows us to gain a deeper understanding of supramolecular structure formation and serves as the basis for rationally controlling the resulting interfacial properties. Here, we describe the formation of extended Moire patterns resulting from the assembly of dipolar pi-conjugated molecules on highly oriented pyrolytic graphite at the liquid-solid interface as characterized by scanning tunneling microscopy (STM). By switching the bias of the sample and thus the orientation of the external electric field in the vicinity of the STM junction, structural reorganization of the molecular building blocks and the resulting organic 2D crystal is induced and can conveniently be monitored in situ by the appearance and disappearance of the Moire patterns. Importantly, the formation and loss of the Moire patterns are fully reversible, providing exquisite control over epitaxial molecular crystals. Our approach provides fundamental insights into the supramolecular organization and resulting superstructure formation of incommensurable 2D lattices upon applying an electric field and enables the rational tuning of Moire patterns as a key step toward the potential integration of organic 2D crystals in molecular nanodevices.

Transition metal dichalcogenides (TMDCs) are often mechanically exfoliated on mica and examined under ambient conditions. It is known that above a certain relative humidity, a molecularly thin layer of water intercalates between the mica and the TMDC. Herein, the effect of molecularly thin liquid layers on the optical spectra of MoS₂ and WS₂ exfoliated on dry mica and exposed to the vapors of water, ethanol, and tetrahydrofuran (THF) is investigated. Photoluminescence and differential reflectance (ΔR/R) spectra on the TMDCs on dry mica show dominant trion emission due to n‐doping. Intercalation of water removes charge doping and results in purely neutral exciton emission, while an ethanol layer, which can be reversibly exchanged with water, does not completely suppress charge. Similarly, THF intercalates between TMDC and mica, as shown by atomic force microscopy, but it does not suppress the charging of mica. In MoS₂ bi‐ and trilayers, an intercalated water layer leads to a near doubling of the intensity of the indirect band transition. The described charging/discharging of TMDCs by molecular thin liquid layers can provide important clues to better control the optical properties of TMDCs under environmental conditions.

Interfacial water is a widespread lubricant down to the nanometer scale. We investigate the lubricities of molecularly thin H2O and D2O films confined between mica and graphene, via the relaxation of initially applied strain in graphene employing Raman spectroscopy. Surprisingly, the D2O films are at least 1 order of magnitude more lubricant than H2O films, despite the similar bulk viscosities of the two liquids. We propose a mechanism based on the known selective permeation of protons vs deuterons through graphene. Permeated protons and left behind hydroxides may form ion pairs clamping across the graphene sheet and thereby hindering the graphene from sliding on the water layer. This explains the lower lubricity but also the hindering diffusivity of the water layer, which yields a high effective viscosity in accordance with findings in dewetting experiments. Our work elucidates an unexpected effect and provides clues to the behavior of graphene on hydrous surfaces.

Understanding and controlling charge transfer through molecular nanostructures at interfaces is of paramount importance, particularly for electronic devices but also for contact electrification or in bio-electronics. We investigate the influence of intercalation and exchange of molecularly thin layers of small molecules (water, ethanol, 2-propanol and acetone) on charge transfer at the well-defined interface between an insulator (muscovite mica) and a conductor (graphene). Raman spectroscopy is used to probe the charge carriers in graphene. While a molecular layer of water blocks charge transfer between mica and graphene, a layer of the organic molecules allows for it. The exchange of molecular water layers with ethanol layers switches the charge transfer very efficiently from off to on and back. We propose a charge transfer model between occupied mica trap states and electronic states of graphene, offset by the electrostatic potentials produced by the molecular dipole layers, as supported by molecular dynamics simulations. Our work demonstrates how intercalation of molecules of volatile liquids can reversibly affect charge transfer at interfaces. This implies its strong impact on the function of hybrid inorganic-organic electronic devices in different ambients, and potential applications, including sensors and actuators.

Hydration of interfaces with a layer of water is a ubiquitous phenomenon, which has important implications for numerous natural and technologically important processes. Nevertheless, at the nanoscale the understanding of the wetting process is still limited, since it is experimentally difficult to follow. Here, graphene and monolayers of MoS2 deposited on dry mica are used to investigate wetting of the 2D material-mica interfaces with a molecularly thin layer of water employing scanning force microscopy in different modes. Wetting occurs non-monotonously in time and space for both types of interfaces. It starts at relative humidities (RH) of 10-17 % for graphenes and 8-9 % for MoS2, and it concludes with a homogeneous layer at 25-30 % and 15-20 %, respectively. Investigation of the process at the graphene-mica interface indicates that up to about 25 % RH, initially a highly compliant and unstable layer of water spreads, which subsequently stabilizes by developing labyrinthine nanostructures. Moreover, these nanostructures exhibit distinct mechanical deformability and dissipation, which is ascribed to different densities of the confined water layer. The laterally structured morphology is explained by the interplay of counteracting long-range dipole-dipole repulsion and short-range line tension, with the latter caused at least in part by the mechanical deformation of the 2D material. The proposed origins of the interactions are common for thin layers of polar molecules at interfaces, implying that the lateral structuring of thin wetting layers at sub-monolayer concentrations may also be a quite general phenomenon.