Imaging Reconfigurable Molecular Concentration on a Graphene Field-Effect Transistor

The spatial arrangement of adsorbates deposited onto a clean surface under vacuum typically cannot be reversibly tuned. Here we use scanning tunneling microscopy to demonstrate that molecules deposited onto graphene field-effect transistors (FETs) exhibit reversible, electrically tunable surface concentration. Continuous gate-tunable control over the surface concentration of charged F₄TCNQ molecules was achieved on a graphene FET at T = 4.5K. This capability enables the precisely controlled impurity doping of graphene devices and also provides a new method for determining molecular energy level alignment based on the gate-dependence of molecular concentration. Gate-tunable molecular concentration is explained by a dynamical molecular rearrangement process that reduces total electronic energy by maintaining Fermi level pinning in the device substrate. The molecular surface concentration is fully determined by the device back-gate voltage, its geometric capacitance, and the energy difference between the graphene Dirac point and the molecular LUMO level.

Alkylated Benzodithienoquinolizinium Salts as Possible Non-Fullerene Organic N-Type Semiconductors: An Experimental and Theoretical Study

Three photobicyclized benzodithienoquinolizinium tetrafluoroborates (BPDTQBF4) were prepared and evaluated by UV-Vis and fluorescence spectral, electrochemical analysis, and by theoretical calculations as possible organic n-type semiconductors. Evaluation and comparison of their LUMO levels, HOMO-LUMO energy gaps as monomeric and π-stacked dimers with those of other materials, suggest their potential as organic n-type semiconductors. Calculations of their relative charge carrier mobilities confirmed this potential for one derivative with a long (C-14) alkyl chain appended to the polycyclic planar π-system.

Reliable Computational Prediction of the Supramolecular Ordering of Complex Molecules under Electrochemical Conditions

We propose a computationally lean, two-stage approach that reliably predicts self-assembly behavior of complex charged molecules on metallic surfaces under electrochemical conditions. Stage one uses ab initio simulations to provide reference data for the energies (evaluated for archetypical configurations) to fit the parameters of a conceptually much simpler and computationally less expensive force field of the molecules: classical, spherical particles, representing the respective atomic entities; a flat and perfectly conducting wall represents the metallic surface. Stage two feeds the energies that emerge from this force field into highly efficient and reliable optimization techniques to identify via energy minimization the ordered ground-state configurations of the molecules. We demonstrate the power of our approach by successfully reproducing, on a semiquantitative level, the intricate supramolecular ordering observed experimentally for PQP+ and ClO4- molecules at an Au(111)-electrolyte interface, including the formation of open-porous, self-host-guest, and stratified bilayer phases as a function of the electric field at the solid-liquid interface. We also discuss the role of the perchlorate ions in the self-assembly process, whose positions could not be identified in the related experimental investigations.

Ambient Bistable Single Dipole Switching in a Molecular Monolayer

Reported here is a molecular dipole that self-assembles into highly ordered patterns at the liquid-solid interface, and it can be switched at room temperature between a bright and a dark state at the single-molecule level. Using a scanning tunneling microscope (STM) under suitable bias conditions, binary information can be written at a density of up to 41 Tb cm^-2 (256 Tb/in² ). The written information is stable during reading at room temperature, but it can also be erased at will, instantly, by proper choice of tunneling conditions. DFT calculations indicate that the contrast and switching mechanism originate from the stacking sequence of the molecular dipole, which is reoriented by the electric field between the tip and substrate.

Reversible Anion-Driven Switching of an Organic 2D Crystal at a Solid-Liquid Interface

Ionic self-assembly of charged molecular building blocks relies on the interplay between long-range electrostatic forces and short-range, often cooperative, supramolecular interactions, yet has been seldom studied in two dimensions at the solid-liquid interface. Here, we demonstrate anion-driven switching of two-dimensional (2D) crystal structure at the Au(111)/octanoic acid interface. Using scanning tunneling microscopy (STM), three organic salts with identical polyaromatic cation (PQPC₆⁺ ) but different anions (perchlorate, anthraquinonedisulfonate, benzenesulfonate) are shown to form distinct, highly ordered self-assembled structures. Reversible switching of the supramolecular arrangement is demonstrated by in situ exchange of the anion on the pre-formed adlayer, by changing the concentration ratio between the incoming and outgoing anion. Density functional theory (DFT) calculations reveal that perchlorate is highly mobile in the adlayer, and corroborate why this anion is only resolved transiently in STM. Surprisingly, the templating effect of the anion persists even where it does not become part of the adlayer 2D fabric, which we ascribe to differences in stabilization of cation conformations by the anion. Our results provide important insight into the structuring of mixed anion-cation adlayers. This is essential in the design of tectons for ionic self-assembled superstructures and biomimetic adaptive materials and valuable also to understand adsorbate-adsorbate interactions in heterogeneous catalysis.

Self-Limiting Adsorption of WO3 Oligomers on Oxide Substrates in Solution

Electrochemical surface science of oxides is an emerging field with expected high impact in developing, for instance, rationally designed catalysts. The aim in such catalysts is to replace noble metals by earth-abundant elements, yet without sacrificing activity. Gaining an atomic-level understanding of such systems hinges on the use of experimental surface characterization techniques such as scanning tunneling microscopy (STM), in which tungsten tips have been the most widely used probes, both in vacuum and under electrochemical conditions. Here, we present an in situ STM study with atomic resolution that shows how tungsten(VI) oxide, spontaneously generated at a W STM tip, forms 1D adsorbates on oxide substrates. By comparing the behavior of rutile TiO₂(110) and magnetite Fe₃O₄(001) in aqueous solution, we hypothesize that, below the point of zero charge of the oxide substrate, electrostatics causes water-soluble WO₃ to efficiently adsorb and form linear chains in a self-limiting manner up to submonolayer coverage. The 1D oligomers can be manipulated and nanopatterned in situ with a scanning probe tip. As WO₃ spontaneously forms under all conditions of potential and pH at the tungsten-aqueous solution interface, this phenomenon also identifies an important caveat regarding the usability of tungsten tips in electrochemical surface science of oxides and other highly adsorptive materials.

Electric-field-controlled phase transition in a 2D molecular layer

Self-assembly of organic molecules is a mechanism crucial for design of molecular nanodevices. We demonstrate unprecedented control over the self-assembly, which could allow switching and patterning at scales accessible by lithography techniques. We use the scanning tunneling microscope (STM) to induce a reversible 2D-gas-solid phase transition of copper phthalocyanine molecules on technologically important silicon surface functionalized by a metal monolayer. By means of ab-initio calculations we show that the charge transfer in the system results in a dipole moment carried by the molecules. The dipole moment interacts with a non-uniform electric field of the STM tip and the interaction changes the local density of molecules. To model the transition, we perform kinetic Monte Carlo simulations which reveal that the ordered molecular structures can form even without any attractive intermolecular interaction.

Aggregation phenomena in photobicyclised pyridinium salts

Absorption and emission spectra of PQPBF₄ and BPDTQClO₄ in water provide evidence, supported by theoretical calculations, of H-aggregation in this solvent.

Equilibrium structures of anisometric, quadrupolar particles confined to a monolayer

We investigate the structural properties of a two-dimensional system of ellipsoidal particles carrying a linear quadrupole moment in their center. These particles represent a simple model for a variety of uncharged, non-polar conjugated organic molecules. Using optimization tools based on ideas of evolutionary algorithms, we first examine the ground state structures as we vary the aspect ratio of the particles and the pressure. Interestingly, we find, besides the intuitively expected T-like configurations, a variety of complex structures, characterized with up to three different particle orientations. In an effort to explore the impact of thermal fluctuations, we perform constant-pressure molecular dynamics simulations within a range of rather low temperatures. We observe that ground state structures formed by particles with a large aspect ratio are in particular suited to withstand fluctuations up to rather high temperatures. Our comprehensive investigations allow for a deeper understanding of molecular or colloidal monolayer arrangements under the influence of a typical electrostatic interaction on a coarse-grained level.

Living on the edge: Tuning supramolecular interactions to design two-dimensional organic crystals near the boundary of two stable structural phases

One of the benefits of supramolecular assemblies that form at dynamic interfaces is the opportunity to develop condensed phase systems that respond to environmental stimuli. A prerequisite of this responsive behavior is that the supramolecular system be designed to sit very near the stability of two or more crystal structures. We have created such a bi-phasic system with aryl-triazole oligomers by investigating how phase morphology is controlled by the interplay between interactions that involve the oligomer's dipolar cores (Δμ = 3.5 debye), van der Waals contacts of their pendant alkyl chains (C4-C18), and close-contact hydrogen bonding. Scanning tunneling microscopy experiments conducted at the solution-graphite interface allow sub-molecular resolution of the ordered monolayers to unambiguously determine the packing and structure of two principle phases, α and β. The system is balanced very near the edge of phase stability, evidenced by co-existent phases present over short time frames and by the changes in preference between the two 2D supramolecular assemblies that occur with small modifications to the molecular structure. We demonstrate that the bi-phasic behavior can be understood as a balance between electrostatic interactions and van der Waals contacts, two variables within a larger parameter space, allowing synthetic design to move this solution-surface system across the stability boundary of different condensed-phase structures. These findings are a foundation for the development of environmentally responsive 2D supramolecular arrays.

Selective anion-induced crystal switching and binding in surface monolayers modulated by electric fields from scanning probes

Anion-selective (Br(-) and I(-)) and voltage-driven crystal switching between two differently packed phases (α ⇆ β) was observed in 2D crystalline monolayers of aryl-triazole receptors ordered at solution-graphite interfaces. Addition of Br(-) and I(-) was found to stimulate the α → β phase transformation and to produce ion binding to the β phase assembly, while Cl(-) and BF4(-) addition retained the α phase. Unlike all other surface assemblies of either charged molecules or ion-templated 2D crystallization of metal-ligand or receptor-based adsorbates, the polarity of the electric field between the localized scanning tip and the graphite substrate was found to correlate with phase switching: β → α is driven at -1.5 V, while α → β occurs at +1.1 V. Ion-pairing between the countercations and the guest anions was also observed. These observations are supported by control studies including variation of anion species, relative anion concentration, surface temperature, tip voltage, and scanning time.