Dipole-driven self-organization of zwitterionic molecules on alkali halide surfaces

Facile surface functionalization of triboelectric layers via electrostatically self-assembled zwitterionic molecules for achieving efficient and stable antibacterial flexible triboelectric nanogenerators

Manipulation of the surface properties of the triboelectric layer has been proven to be one of the key parameters to achieve high-performance and stable triboelectric nanogenerators (TENG). Herein, a pragmatic surface engineering strategy that can substantially boost the performance and stability of flexible TENG is elaborated by incorporating the zwitterionic molecule dimethylethylammoniumpropane sulfonate (NDSB) as the surface modification layer. Given that zwitterionic molecules tend to form aggregated structures, realizing ordered arrangement on the substrate surface remains challenging to date. To address this issue, in this work, a combination of multiple surface treatments and molecular manipulation strategy is proposed. Our results prove that NDSB is effective in modifying the surface properties of the dielectric layer and electrode layer, leading to a remarkable power density and specific power of 2.86 W m-2 and 20.73 mW g-1 for flexible TENG, respectively. In addition, due to the strong interaction between the NDSB/dielectric and NDSB/electrode, a water-resistant long-term stable flexible TENG is realized. More encouragingly, our strategy is compatible with a cost-effective dip-coating technique, and an unprecedented demonstration of batch fabrication of TENG using NDSB to functionalize the surface of the dielectric layer and electrode layer synchronously can be realized, which is advantageous for rapid and up-scalable manufacturing of TENG. We also prove that the TENG based on zwitterionic materials reveals exceptional antibacterial properties against Escherichia coli. This study represents an important step towards the development of long-term stable flexible TENG that possesses a high output performance and excellent antibacterial activity based on a facile and economical strategy, enabling TENG technology to show bright prospects in a wide variety of application domains.

Creating a regular array of metal-complexing molecules on an insulator surface at room temperature

Controlling self-assembled nanostructures on bulk insulators at room temperature is crucial towards the fabrication of future molecular devices, e.g., in the field of nanoelectronics, catalysis and sensor applications. However, at temperatures realistic for operation anchoring individual molecules on electrically insulating support surfaces remains a big challenge. Here, we present the formation of an ordered array of single anchored molecules, dimolybdenum tetraacetate, on the (10.4) plane of calcite (CaCO₃). Based on our combined study of atomic force microscopy measurements and density functional theory calculations, we show that the molecules neither diffuse nor rotate at room temperature. The strong anchoring is explained by electrostatic interaction of an ideally size-matched molecule. Especially at high coverage, a hard-sphere repulsion of the molecules and the confinement at the calcite surface drives the molecules to form locally ordered arrays, which is conceptually different from attractive linkers as used in metal-organic frameworks. Our work demonstrates that tailoring the molecule-surface interaction opens up the possibility for anchoring individual metal-complexing molecules into ordered arrays.

Nanostructuring of an alkali halide surface by low temperature plasma exposure

Templating insulating surfaces at the nanoscale is an interesting prospect for applications that involve the adsorption of molecules or nanoparticles where electronic decoupling of the adsorbed species from the substrate is needed. In this study, we present a method to structure alkali halide surfaces at the nanoscale using a combination of low temperature plasma exposure and annealing, and characterize the surfaces by atomic force microscopy. We find that nanostructurating can be controlled by the duration of the exposure, the atomic mass of the plasma gas and the subsequent step-by-step annealing process. In contrast to previous studies with electron or high energy (few keV) ion irradiation, our approach of employing moderate particle energy (10-15 eV Ar⁺ or He⁺ ions) results in fine nanostructuring at length scales of nanometers and even single atom vacancies.

Imaging prototypical aromatic molecules on insulating surfaces: a review

Insulating substrates allow for in-plane contacted molecular electronics devices where the molecule is in contact with the insulator. For the development of such devices it is important to understand the interaction of molecules with insulating surfaces. As substrates, ionic crystals such as KBr, KCl, NaCl and CaF₂ are discussed. The surface energies of these substrates are small and as a consequence intrinsic properties of the molecules, such as molecule-molecule interaction, become more important relative to interactions with the substrates. As prototypical molecules, three variants of graphene-related molecules are used, pentacene, [Formula: see text] and PTCDA. Pentacene is a good candidate for molecular electronics applications due to its high charge carrier mobility. It shows mainly an upright standing growth mode and the morphology of the islands is strongly influenced by dewetting. A new second flat-lying phase of the molecule has been observed. Studying the local work function using the Kelvin method reveals details such as line defects in the center of islands. The local work function differences between the upright-standing and flat-lying phase can only be explained by charge transfer that is unusual on ionic crystalline surfaces. [Formula: see text] nucleation and growth is explained by loosely bound molecules at kink sites as nucleation sites. The stability of [Formula: see text] islands as a function of magic numbers is investigated. Peculiar island shapes are obtained from unusual dewetting processes already at work during growth, where molecules 'climb' to the second molecular layer. PTCDA is a prototypical semiconducting molecule with strong quadrupole moment. It grows in the form of elongated islands where the top and the facets can be molecularly resolved. In this way the precise molecular arrangement in the islands is revealed.

Surface induced vibrational modes in the fluorescence spectra of PTCDA adsorbed on the KCl(100) and NaCl(100) surfaces

We report a combined experiment-theory study on low energy vibrational modes in fluorescence spectra of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) molecules.

Tuning molecular self-assembly on bulk insulator surfaces by anchoring of the organic building blocks

Molecular self-assembly constitutes a versatile strategy for creating functional structures on surfaces. Tuning the subtle balance between intermolecular and molecule-surface interactions allows structure formation to be tailored at the single-molecule level. While metal surfaces usually exhibit interaction strengths in an energy range that favors molecular self-assembly, dielectric surfaces having low surface energies often lack sufficient interactions with adsorbed molecules. As a consequence, application-relevant, bulk insulating materials pose significant challenges when considering them as supporting substrates for molecular self-assembly. Here, the current status of molecular self-assembly on surfaces of wide-bandgap dielectric crystals, investigated under ultrahigh vacuum conditions at room temperature, is reviewed. To address the major issues currently limiting the applicability of molecular self-assembly principles in the case of dielectric surfaces, a systematic discussion of general strategies is provided for anchoring organic molecules to bulk insulating materials.

Near-field scanning optical microscopy enables direct observation of Moiré effects at the nanometer scale

This work reports probing the Moiré effect directly at the nanometer scale via near-field scanning optical microscopy (NSOM). Periodic metal nanostructures of Au and Cu have been produced sequentially using particle lithography, and the overlapped regions serve as Moiré patterns at nanometer scale. The Moiré effect in these regions can be directly visualized from NSOM images, from which periodicity and structural details are accurately determined. In addition, the near-field Moiré effect was found to be very sensitive to structural changes, such as lateral displacement and/or rotations of the two basic arrays with respect to each other. Further, nanostructures of Cu exhibited higher photon transmission than Au from NSOM images. Collectively, NSOM enables direct visualization of the Moiré effect at nanoscale levels, from optical read out, and without enhancements or modification of the structures. The results demonstrate the feasibility to extend applications of the Moiré effect-based techniques to nanometer levels.