Water splitting is a promising pathway for hydrogen production, providing an environmentally friendly fuel source. More recently, great attention has been given to transition metal dichalcogenides (TMDCs) because of their interesting chemical and physical properties. In particular, tungsten disulfide (WS2) has garnered significant attention as a catalyst for this application due to its unique layered 2D structure. In this study, few-layered WS2 and phosphorus-doped WS2 (WS2/P) nanoflakes are synthesized on SiO2/Si substrates as electrocatalysts for hydrogen evolution reactions (HER) in acidic conditions. Analyses of the synthesized WS2 and WS2/P films reveal that the few-layered WS2 is of high quality, exhibiting continuity and uniformity. The presence of a strong peak in the photoluminescence spectrum confirms the mono/few layer nature of the synthesized samples. In additionally, scanning force microscopy in quantitative imaging mode reveals that the thinnest layers observed on the substrate have a height of 1.35 nm, indicating the presence of double-layer WS2. The WS2/P electrocatalyst demonstrates superior HER performance compared to pristine WS2, showing a low overpotential of 245 mV at 10 mA.cm−2 and a small Tafel slope of 123 mV.dec−1. Furthermore, WS2/P exhibits a greater electrochemical surface area and excellent catalytic stability under acidic conditions. Consequently, few layer phosphorus-doped WS2 proves to be a highly suitable electrocatalyst for hydrogen production compared to the WS2.

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.

This study explores how laser light affects the morphology of tetracene films, and it presents novel strategies for improving the creation of thin films used in (opto-)electronic devices. We demonstrate that laser light (532 nm, 1.1 W mm-2), applied during tetracene deposition, not only increases grain size but also induces photoalignment. The observed effects arise from enhanced molecular diffusion, resulting from energy transferred by light to the molecules after adsorption, but not from heating the substrate surface underneath. We observe that linearly polarized light promotes photoalignment, while increased crystallite sizes occurs with both linear and circular polarizations. We propose an Ostwald ripening process facilitated by laser illumination, where smaller crystallites get optically heated and dissolve, allowing molecules to surmount step-edge barriers and assemble into larger crystallites. Importantly, the crystallite sizes achieved with laser illumination surpass those attainable by substrate heating alone. The study demonstrates that laser illumination acts as a promising new parameter for controlling thin film properties and is distinct from growth control via substrate temperature and growth rate. Light control also includes the ability for lateral patterning, with implications for the future of molecular materials and their manufacturing technologies. Laser illumination acts a novel growth parameter for thin films, leading to larger crystallite size and molecular alignment in tetracene growth. Laser control is distinct from traditional thermal growth and paves the way for novel materials.

Abstract In topological band theory, phonon boundary modes consequence of a topologically non-trivial band structure feature desirable properties for atomically-precise technologies, such as robustness against defects, waveguiding, and one-way transport. These topological phonon boundary modes remain to be studied both theoretically and experimentally in synthetic materials, such as polymers and supramolecular assemblies at the atomistic level under thermal fluctuations. Here we show by means of molecular simulations, that surface-confined Su-Schrieffer-Heeger (SSH) phonon analogue models express robust topological phonon boundary modes at heavy boundaries and under thermal fluctuations. The resulting bulk-heavy boundary correspondence enables patterning of boundary modes in polymer chains and weakly-interacting supramolecular lattices. Moreover, we show that upon excitation of a single molecule, propagation along heavy-boundary modes differs from free boundary modes. Our work is an entry to topological vibrations in supramolecular systems, and may find applications in the patterning of phonon circuits and realization of Hall effect phonon analogues at the molecular scale.

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 MoS 2 and WS 2 exfoliated on dry mica and exposed to the vapors of water, ethanol, and tetrahydrofuran (THF) is investigated. Photoluminescence and differential reflectance (Δ R / 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 MoS 2 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.

The synergetic combination of transition metal dichalcogenides (TMDCs) with organic dye molecules in functional heterostructures is promising for various optoelectronic applications. Here resonance energy transfer (RET) from a red-emitting WS2 monolayer (1L-WS2) to a layer of near-infrared (NIR) emitting organic dye molecules is demonstrated. It is found that the total photoluminescence (PL) yield of the heterostructures is up to a factor of eight higher as compared to the PL yield of pristine 1L-WS2. This is attributed to the efficient conversion of the mostly non-radiative excitons in 1L-WS2 into radiative excitons in the dye layer. A type-I energy level alignment of the 1L-WS2/dye interface assures the emission of bright PL. From excitation density-dependent PL experiments, it is concluded that RET prevails against defect-assisted non-radiative recombination as well as Auger-type exciton-exciton annihilation in 1L-WS2. The work paves the way for employing organic dye molecules in heterostructures with TMDCs in nanoscale light-emitting devices with improved efficiency and tunable color.

AbstractDie Inkorporation von Nanoporen in Graphen‐Nanostrukturen wurde als effiziente Methode zur Anpassung der Bandlücke und elektronischen Struktur dieser Materialien demonstriert. Allerdings ist die atomgenaue Einbettung uniformer Nanoporen in Graphen‐Nanostreifen (GNS, engl. GNR) auf atomarer Ebene durch Mangel an effizienten Synthesestrategien bislang unterentwickelt. Wir berichten hier vom ersten lösungschemisch dargestellten, vollständig durch Scholl‐Reaktion konjugierten porösen GNS (pGNS), welchen wir durch das atomgenau dargestellte Polyphenylen P1 mit vorinstallierten hexagonalen Makrozyklen zugänglich machen konnten. pGNS enthält periodische Poren im sub‐Nanometerbereich (0.6 nm) mit einem Abstand von 1.7 nm zueinander. Um unsere Synthesestrategie zu untermauern, wurden zwei poröse Modellverbindungen (1 a, 1 b) mit einer zu pGNS identischen Pore erfolgreich dargestellt. Die chemische Struktur und photophysikalischen Eigenschaften von pGNS wurden durch verschiedene spektroskopische Methoden untersucht. Die eingebetteten periodischen Nanoporen vermindern die π‐Konjugation und damit die Interaktion zwischen den Nanostreifen, verglichen mit nicht porösen GNS von ähnlicher Breite. pGNS zeigt demzufolge eine erhöhte Bandlücke und eine verbesserte nasschemische Prozessierbarkeit.

Conjugated single-layered two-dimensional covalent organic frameworks are flat and extended polymer networks with a unique combination of material properties, giving rise to potential applications in sensing, optoelectronics, and photonics. Despite their great potential, thus far only a few reactions to access such extended conjugated 2D polymers have been reported. Here, the on-surface polymerization of the first vinylene-linked single layered two-dimensional covalent organic framework using reversible Knoevenagel polycondensation under solvothermal conditions is described. Self-assembly of the two monomer building blocks at the solid-liquid interface led to the formation of extended covalent networks at room temperature without the need of additional catalysts or reagents. The described approach grants access to extended conjugated 2D polymers under unprecedentedly mild conditions and paves the way to new hybrid material systems.

Collective excited states form in organic two-dimensional layers through Coulomb coupling of the molecular transition dipole moments. They manifest as characteristic strong and narrow peaks in the excitation and emission spectra that are shifted to lower energies compared with the monomer transition. We study experimentally and theoretically how robust the collective states are against homogeneous and inhomogeneous broadening, as well as spatial disorder that occurs in real molecular monolayers. Using a microscopic model for a two-dimensional dipole lattice in real space, we calculate the properties of collective states and their extinction spectra. We find that the collective states persist even for 1-10% random variation in the molecular position and in the transition frequency, with a peak position and integrated intensity similar to those for the perfectly ordered system. We measured the optical response of a monolayer of the perylene derivative MePTCDI on two-dimensional materials. On the wide-band-gap insulator hexagonal boron nitride, it shows strong emission from the collective state with a line width that is dominated by the inhomogeneous broadening of the molecular state. When the semimetal graphene is used as a substrate, however, the luminescence is completely quenched. By combining optical absorption, luminescence, and multiwavelength Raman scattering, we verify that the MePTCDI molecules form very similar collective monolayer states on hexagonal boron nitride and graphene substrates, but on graphene the line width is dominated by nonradiative excitation transfer from the molecules to the substrate. Our study highlights the transition from the localized molecular state of the monomer to a delocalized collective state in the two-dimensional molecular lattice that is entirely based on Coulomb coupling between optically active excitations of the electrons and molecular vibrations. The excellent properties of organic monolayers make them promising candidates for components of soft-matter optoelectronic devices.

The incorporation of nanopores into graphene nanostructures has been demonstrated as an efficient tool in tuning their band gaps and electronic structures. However, precisely embedding the uniform nanopores into graphene nanoribbons (GNRs) at the atomic level remains underdeveloped especially for in-solution synthesis due to the lack of efficient synthetic strategies. Herein we report the first case of solution-synthesized porous GNR (pGNR) with a fully conjugated backbone via the efficient Scholl reaction of tailor-made polyphenylene precursor (P1) bearing pre-installed hexagonal nanopores. The resultant pGNR features periodic subnanometer pores with a uniform diameter of 0.6 nm and an adjacent-pores-distance of 1.7 nm. To solidify our design strategy, two porous model compounds (1 a, 1 b) containing the same pore size as the shortcuts of pGNR, are successfully synthesized. The chemical structure and photophysical properties of pGNR are investigated by various spectroscopic analyses. Notably, the embedded periodic nanopores largely reduce the π-conjugation degree and alleviate the inter-ribbon π-π interactions, compared to the nonporous GNRs with similar widths, affording pGNR with a notably enlarged band gap and enhanced liquid-phase processability.

Manipulation of the structure of covalent organic frameworks at the molecular level is an efficient strategy to shift their biological, physicochemical, optical, and electrical properties in the desired windows. In this work, we report on a new method to construct chiral triazine frameworks using metal-driven polymerization for enantiodiscrimination. The nucleophilic substitution reaction between melamine and cyanuric chloride was performed in the presence of PdCl2, ZnCl2, and CuCl2 as chirality-directing agents. Palladium, with the ability of planar complex formation, was able to assemble monomers in two-dimensions and drive the reaction in two directions, leading to a two-dimensional triazine network with several micrometers lateral size. Nonplanar arrangements of monomers in the presence of ZnCl2 and CuCl2, however, resulted in calix and bouquet structures, respectively. While 2D and bouquet structures showed strong negative and positive bands in the CD spectra, respectively, their calix counterparts displayed long-range weak negative bands. In spite of the ability of both calix and bouquet networks to load l-histidine 35 and 50% more than d-histidine from pure enantiomers, respectively, only calix counterparts were able to take up this enantiomer (78%) from the racemic mixture. The two-dimensional polytriazine network did not show any specific interactions with pure enantiomers or their racemic mixtures.

Atomic surface structure imaging is instrumental for the understanding of surface-related phenomena. Here, we show that conventional tapping mode atomic force microscopy with high cantilever eigenmodes and subnanometer amplitudes allow routine atomic imaging at atmospheric pressures. We identify the reasons for failure of atomic resolution imaging employing low eigenmodes. Strong tip-surface interactions cause significant differences between the oscillatory behaviors of the inclination of the cantilever as detected by conventional instruments and of the vertical position of the tip, which prevents correct functioning of instrumental feedback control loops. However, high effective spring constants of high eigenmodes make it possible to overcome the problem. Furthermore, the combination of high effective elastic constants of high cantilever eigenmodes with the high flexibility of the cantilever substantially enhances the imaging stability, thereby universally allowing atomic imaging of solid surfaces in gaseous environments and at elevated temperatures. Demonstrated imaging examples include single sulfur vacancies at the surface of MoS2 crystals imaged at temperatures ranging from room temperature to 250 degrees C and potassium ions on hydrophilic and highly adhesive muscovite mica surfaces. Moreover, the high imaging stability allows knocking atoms off the MoS2 surface by hard tapping, indicating the potential for ultrahigh resolution lithography.

We demonstrate a new localized excitonic state in patterned monolayer 2D semiconductors. This state is not associated with lattice disorder but is extrinsic, i.e. results from external molecules on the material surface. The signature of an exciton associated with that state is observed in the photoluminescence spectrum after electron beam exposure of several 2D semiconductors. The localized state, which is distinguished by non-linear power dependence, survives up to room temperature and is patternable down to 20 nm resolution. We probe the response of the new exciton to the changes of electron energy, nanomechanical cleaning, and encapsulation via multiple microscopic, spectroscopic, and computational techniques. All these approaches suggest that the state does not originate from irradiation-induced structural defects or spatially non-uniform strain, as commonly assumed. Instead, we show that it is extrinsic, likely a charge transfer exciton associated with the organic substance deposited onto the 2D semiconductor. By demonstrating that structural defects are not required for the formation of localized excitons, our work opens new possibilities for further understanding of these states and using them for example in chemical sensing and quantum technologies.

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.

Abstract Macrocyclic oligothiophenes and their π‐expanded derivatives constitute versatile building blocks for the design of (supra)molecularly engineered active interfaces, owing to their structural, chemical, and optoelectronic properties. Here, it is demonstrated how resonant tunneling effect induces single molecular isomerization in a 2D crystal, self‐assembled at solid–liquid interfaces under ambient conditions. Monolayers of a series of four π‐expanded oligothiophene macrocycles are investigated by means of scanning tunneling microscopy and scanning tunneling spectroscopy (STS) at the interface between their octanoic acid solutions and the basal plane of highly oriented pyrolytic graphite. Current–voltage characteristics confirm the donor‐type character of the macrocycles, with the highest occupied molecular orbital and the lowest unoccupied molecular orbital (LUMO) positions consistent with time‐dependent density functional theory calculations. Cyclic STS measurements show the redox isomerization from Z,Z‐8T6A to its isomer E,E‐8T6A occurring in the 2D crystal, due to the formation of a negatively charged species when the tunneling current is in resonance with the LUMO of the macrocycle.

Charge transfer at solid interfaces and ensuinginterfacial electricfields on the order of 109V/m are ubiquitous innanostructures and hybrid materials. Here, we address how intrinsicinterfacial electricfields alter the structural properties of intercalatedmolecules considering the optically transparent and atomicallyflatgraphene-mica interface and confined rhodamine 6G dyes as amodel system. Using a combination of Raman spectroscopy andatomistic simulations based on density-functional theory and classicalmolecular dynamics, we show that the observed softening of Raman-active modes of the confined molecules is due to mechanicaldeformations within the latter and to the action of interfacial electricfields exceeding 109V/m. Ourfindings contribute to the generalunderstanding of the role of interfacial electricfields in molecule/solid interfaces, thereby opening new perspectives for controllingcatalytic activities of such complex systems.

Atomic force microscopy (AFM) imaging was used to study swelling of individual bilayer graphene oxide (GO) flakes in water and ethanol vapors. We found that within 5 days after sample deposition the swelling of bilayered Hummers GO (HGO) in ethanol vapors disappears nearly completely, whereas the swelling in water remains not affected. Swelling can therefore be used as a sensitive indicator of rapid aging of bilayered HGO, which occurs both in air and under inert gas. The surprising ability of 5-days-aged bilayered HGO to swell in water but not in ethanol fits to the effects observed in mu m-thick GO membranes after several years of aging. Remarkably, bilayered Brodie GO (BGO) maintains swelling in ethanol after storing it under the same conditions as HGO. Moreover, we demonstrate that AFM can be used to detect hole defects in individual GO sheets. The BGO bilayer swelling in ethanol vapors starts either on the flake edges or in very few points, forming similar to 2 angstrom height fronts, which propagate slowly within interlayer spaces in all directions. The increase of the average distance between HGO sheets occurs simultaneously all over the flake, demonstrating a high abundance of hole defects. Our results imply that the permeation path length in HGO membranes must be significantly shorter compared to BGO, which is important to take into account in the modeling of membrane permeation.

On 12 December 2019, our colleague and friend Professor Dr. Gerhard H. Findenegg (cf. Figure 1) passed away after just having completed the eighty-first year of his life. He leaves behind his wife ...

We demonstrate a simple approach to pattern a novel localized excitonic state in monolayer MoS2. We raster an electron beam over pristine monolayer MoS2 with a resolution down to 20 nm. In the exposed areas we observe new narrow (1 meV) peaks near 1.72 eV in the photoluminescence (PL) spectrum. We investigate the nature of the state responsible for the peaks using AFM and STEM microscopies, PL spectroscopy, and by examining the response of the state to AFM mechanical cleaning. All these techniques indicate that the state is not associated with structural defects in MoS2, as commonly assumed for irradiation-induced states, and results neither from strain nor from local dielectric screening. Instead, we suggest that this state is a charge transfer exciton associated with the organic substance deposited onto the MoS2 during the e-beam exposure. We show that this new state is well-resolved, easy-to-generate via conventional EBL, and its emission is visible at room temperature. It can be used to study localized excitons, to enable chemical sensing, and to generate new many-body excitonic states.

In the 1960s, optical magnification and the concept of microsurgery entered the operating room and revolutionized the effectiveness and safety of neurosurgical procedures. By utilizing natural anatomic pathways within the brain, the new conceptual foundation, combined with the technological advances in surgical microscopy, allowed manually invading the brain with fewer harmful effects. In recent years, the rapidly growing field of computational neuroscience has opened up new perspectives for exploring brain function at the micro and macro scales. Neural network simulations approximate the mechanistic prerequisites of even the highest cognitive functions by modeling their material basis in terms of neural systems.1 Those systems are characterized by interactive, distributed, dynamic, and adaptable processing within neuron populations interlinked by local and long-distance connectivity within and across functional domains.2 These models can explain some degree of resilience of large parts of the brain to injury or degeneration and may offer novel perspectives on predicting variable effects of local injuries.3 We believe that the current decade will witness another paradigm shift: the application of network thinking in clinical neurosurgery through the development of appropriate models and technological means to perform network-based neurosurgery.