Stijn F. L. Mertens

In this talk, I will discuss mechanisms for controlled switching of the structure and properties of 2D materials at solid–liquid interfaces, from the collective scale down to single-molecule manipulation, at ambient temperatures and pressures. The 2D materials are self-assembled structures of tailored organic molecules, and the switching events depend on tipping the balance between intermolecular and adsorbate–substrate interactions [1,2] through external stimuli: the interfacial potential at electrochemical solid–liquid interfaces [3-5], the chemical potential of ionic species in solution [6], or the electric field between an STM tip and the substrate [7]. References S.F.L. Mertens, Adsorption and self-organization of organic molecules under electrochemical control, Encyclopedia of Interfacial Chemistry (ed. K. Wandelt), Vol. 4, Elsevier, 13 (2018). K. Cui, I. Dorner, S.F.L. Mertens, Interfacial supramolecular electrochemistry, Curr. Opin. Electrochem. 8, 156 (2018). K. Cui, K.S. Mali, O. Ivasenko, D. Wu, X. Feng, M. Walter, K. Müllen, S. De Feyter, S.F.L. Mertens, Squeezing, then stacking: from breathing pores to 3-dimensional ionic self-assembly under electrochemical control, Angew. Chem. Int. Ed. 53, 12951 (2014). K. Cui, O. Ivasenko, K.S. Mali, D. Wu, X. Feng, K. Müllen, S. De Feyter, S.F.L. Mertens, Potential-driven molecular tiling of a charged polycyclic aromatic compound,Chem. Commun. 50, 10376 (2014). Z. Li, K. Mali, P. Hapiot, S. De Feyter, A. J. Attias, S. F. L. Mertens, Reversible Redox-Driven Crystallization in a Paracyclophane Monolayer at a Solid–Liquid Interface, Adv. Funct. Mater. 2315861 (2024). K. Cui, K.S. Mali, X. Feng, K. Müllen, M. Walter, S. De Feyter, S.F.L. Mertens, Reversible anion-driven switching of an organic 2D crystal at a solid–liquid interface, Small 13, 1702379 (2017). K. Cui, K.S. Mali, D. Wu, X. Feng, K. Müllen, M. Walter, S. De Feyter, S.F.L. Mertens, Ambient bistable single dipole switching in a molecular monolayer, Angew. Chem. Int. Ed. 59, 14049 (2020). Figure 1

Graphene, a one-atom-thick sp²-hybridized carbon sheet, has enormous potential for fabricating flexible transparent electronics due to its unique electronic and mechanical properties. However, the intrinsic lack of a band gap, the low reactivity, and the poor solubility of pristine graphene have largely hindered wide-ranging applications so far. One of the most attractive ways to resolve these issues is to modify the graphene surface through molecular physisorption or chemisorption. In this review, we summarize the recent progress in fabricating graphene-based molecular electronics through manipulating small functional molecules on the graphene surface towards chemical reactivity adjustment, molecular doping, and band gap opening via non-covalent and covalent interactions, and draw attention to challenges and opportunities. We also suggest future research directions for graphene-based molecular electronics.

The development and integration of cyclophanes into future functional materials require a detailed understanding of the physicochemical principles that underlie their properties, phase behavior, and in particular the relationship between structure and function. Here, electrochemically switchable crystallization of a ferrocene‐bearing 3D Janus tecton (M‐Fc) at the interface between highly oriented pyrolytic graphite (HOPG) and an electrolyte solution is demonstrated. The M‐Fc adlayer is successfully visualized under both ambient and electrochemical conditions using scanning tunneling microscopy. Voltammetric measurements show a surface‐confined redox process for the M‐Fc modified surface that drives the phase transition between a visible 2D ordered linear phase (M‐Fc⁰, with ferrocene in the neutral state) and an invisible gas‐like adsorption layer with high mobility when ferrocene is oxidized, M‐Fc⁺, and a “square scheme” mechanism explains the data. Analogous experiments in a ferrocene‐free tecton adlayer show no phase transition and confirm that the dynamics in M‐Fc are redox‐driven. On‐surface 3D nanoarchitectures are also demonstrated by forming inclusion complexes between M‐Fc and β‐cyclodextrin and device behavior through electrochemical scanning tunneling spectroscopy (STS). These results showcase the functional potential of this class of cyclophanes, which can find use in actuators, optical crystals, and other smart materials.

Physisorption on hexagonal boron nitride (hBN) gained interest over the years thanks to its properties (chemically and thermally stable, insulating properties, etc.) and similarities to the well-known graphene. A recent study showed flat-on adsorption of several cationic thiacarbocyanine dyes on hBN with a tendency to form weakly coupled H- or I-type aggregates, while a zwitterionic thiacarbocyanine dye rather led to a tilted adsorption. With this in-depth time-resolved study using the TC-SPC technique, we confirm the results proven by adsorption isotherms, atomic force microscopy, and stationary state spectroscopy combined with molecular mechanics simulations and estimation of the corresponding exciton interaction. The absence of a systematic trend for the dependence of the decay times, normalized amplitudes of the decay components, and contribution of different components to the stationary emission spectra upon the emission wavelength observed for all studied dyes and coverages suggests the occurrence of a single emitting species. At low coverage levels, the non-mono-exponential character of the decays was attributed to adsorption on different sites characterized by different intramolecular rotational freedom or energy transfer to nonfluorescent traps or a combination of both. The difference between the decay rates of the four dyes reflects a different density of the nonfluorescent traps. Although the decay time of the unquenched dyes was in the order of magnitude of that of dye monomers in a rigid environment, it is also compatible with weakly coupled aggregates such as proposed earlier based on the stationary spectra. Hence, the adsorption leads to a rigid environment of the dyes, blocking internal conversion. Increasing the concentration of the dye solution from which the adsorption on hBN occurs increases not only the coverage of the hBN surface but also the extent of energy transfer to nonfluorescent traps. For TDC (5,5-dichloro-3-3'-diethyl-9-ethyl-thiacarbocyanine) and TD2 (3-3'-diethyl-9-ethyl-thiacarbocyanine), besides direct energy transfer to traps, exciton hopping between dye dimers followed by energy transfer to these traps occurs, which resulted in a decreasing decay time of the longest decaying component. For all dyes, it was also possible to analyze the fluorescence decays as a stretched exponential as would be expected for energy transfer to randomly distributed traps in a two-dimensional (2D) geometry. This analysis yielded a fluorescence decay time of the unquenched dyes similar to the longest decay time obtained by analysis of the fluorescence decays as a sum of three of four exponentials.