Reversible Redox‐Driven Crystallization in a Paracyclophane Monolayer at a Solid–Liquid Interface

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

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

2021 roadmap for sodium-ion batteries

Abstract: Increasing concerns regarding the sustainability of lithium sources, due to their limited availability and consequent expected price increase, have raised awareness of the importance of developing alternative energy-storage candidates that can sustain the ever-growing energy demand. Furthermore, limitations on the availability of the transition metals used in the manufacturing of cathode materials, together with questionable mining practices, are driving development towards more sustainable elements. Given the uniformly high abundance and cost-effectiveness of sodium, as well as its very suitable redox potential (close to that of lithium), sodium-ion battery technology offers tremendous potential to be a counterpart to lithium-ion batteries (LIBs) in different application scenarios, such as stationary energy storage and low-cost vehicles. This potential is reflected by the major investments that are being made by industry in a wide variety of markets and in diverse material combinations. Despite the associated advantages of being a drop-in replacement for LIBs, there are remarkable differences in the physicochemical properties between sodium and lithium that give rise to different behaviours, for example, different coordination preferences in compounds, desolvation energies, or solubility of the solid–electrolyte interphase inorganic salt components. This demands a more detailed study of the underlying physical and chemical processes occurring in sodium-ion batteries and allows great scope for groundbreaking advances in the field, from lab-scale to scale-up. This roadmap provides an extensive review by experts in academia and industry of the current state of the art in 2021 and the different research directions and strategies currently underway to improve the performance of sodium-ion batteries. The aim is to provide an opinion with respect to the current challenges and opportunities, from the fundamental properties to the practical applications of this technology

Switchable White Graphene:Electrochemistry of the Boron Nitride Nanomesh

On Rh(111), a monolayer of hexagonal boron nitride (h-BN, isoelectronic with graphene) forms a so-called nanomesh superstructure [1], characterized by a 3.2-nm lattice constant and strong electronic corrugation, which can be used for trapping atoms and molecules [2,3]. Here, we show that hydrogen underpotential deposition (H upd) is easier on h-BN/Rh(111) than on the naked substrate [4], and leads to submonolayer quantities of hydrogen intercalated between the h-BN overlayer and Rh(111), as demonstrated by electrolyte-to-vacuum transfer experiments and thermal desorption spectroscopy. In situ STM measurements reveal that the intercalation lifts the corrugation of the nanomesh, and that this process is fully reversible under potential control. Copper upd is used to quantify the defect density in the nanomesh, and to increase the understanding of the electrochemical window where the nanomesh is stable. By measuring dynamic contact angles of an electrolyte drop (see Figure), we show that the microscopic change within the 2-dimensional material leads to a macroscopic effect related to a 10% change in adsorption energy [4]. The static friction on the other hand, which can be extracted by extending the Young equation for non-equilibrium effects, remains unchanged for the surface in the two states

Metal Underpotential Deposition to Quantify Defects in 2D Materials

We demonstrate how metal UPD may find use as a general tool to determine the collective defect area in hybrids between 2D materials (graphene, hexagonal boron nitride, etc.) and various substrate metals. By investigating copper UPD on a monolayer of hexagonal boron nitride (h-BN) on Rh(111), we explore how this process can be used to quantify the defects in the h-BN monolayer which form during its chemical vapor deposition. In addition, the UPD signature allows assessing the potential window of the h-BN/metal hybrid, which is important to explore its functionality under ambient and electrochemical conditions. Importantly, UPD itself does not alter the defect area on repeated cycling. Overpotential deposition, on the other hand, is shown to have significant consequences on the defect area. We show that this non-innocent Cu electrodeposition involves intercalation originating at initial defects, causing irreversible delamination of the h-BN layer; this effect therefore may be used for 2D material nanoengineering

Fluoride-free wet-chemical preparation of oxide single crystal surfaces:66th Annual Meeting of the Austrian Physical Society

The ultimate goal to perform surface science studies under technologically relevant conditions includes wet-chemical methods to prepare well-defined oxide surfaces [1]. The most widely practised approach is hydrofluoric acid etching, even though this chemical poses serious health risks and may inadvertently dope the surface with fluorine, an efficient electron donor [2]. Here, we present a rational yet versatile wet-chemical alternative to lengthy sputtering–annealing cycles in ultrahigh vacuum for preparing single crystal oxide samples for surface science investigations. The method does not require hydrofluoric acid, is environmentally benign and is demonstrated on rutile TiO2 (110), rutile TiO2 (011) and SrTiO3 (100), but may have much wider application potential, also for surfaces that are quickly destroyed by acids. The procedure consists of (i) ultrasonication in the presence of a dispersing agent to remove polishing debris; (ii) thermal annealing to produce equilibrium-shaped steps and terraces determined by the crystal miscut; and (iii) oxidative cleaning in an alkaline mixture to remove adsorbed organic contaminants from the surface. Each of the steps is optimised based on AFM and characterisation in ultrahigh vacuum, including by LEED and XPS. Following this wet-chemical preparation, we demonstrate atomically resolved electrochemical scanning tunnelling microscopy on TiO2 (110), on a sample that was never treated by sputtering–annealing

Monodisperse tungsten oxide cluster deposition from solution:66th Annual Meeting of the Austrian Physical Society

Perfectly monodisperse clusters of oxides are critically important model systems for catalysis studies because they allow the rigorous analysis of reaction mechanisms, and variations at the single-atom level can already be reflected in their reactivity. The generation and intact immobilisation on a suitable substrate of such clusters is quite challenging, and usually requires mass spectrometric size selection and sophisticated soft landing protocols to make such studies successful. Tungsten (VI) oxide in particular holds promise as a visible-light photocatalyst, but is quite reactive and can be challenging to immobilise in a well-defined manner in vacuum [1]. Here, we present a solution-based protocol for the preparation of monodisperse cyclic tris (tungsten (VI) trioxide) clusters, (WO3) 3. The clusters can be harvested efficiently on the boron nitride nanomesh [2], an atomically thin layer of hexagonal boron nitride on Rh (111) with strong corrugation, and a promising platform for self-assembly [3] and electrochemical functionality [4]. The triangular (WO3) 3 clusters adsorb in the ‚pores‘ of the nanomesh, where they were imaged with submolecular resolution using electrochemical scanning tunnelling microscopy. The decorated surface was transferred to vacuum where the chemical identity of the clusters was confirmed with XPS. To our knowledge, this is the first successful example of self-assembly on the nanomesh from solution. We expect that proper control over deposition conditions will allow tuning of the number of clusters per pore, making this a promising model system for on-surface catalysis studies. We contrast this finding with deposition of the same source material on rutile TiO2 (110) in liquid, on which the clusters appear to react and form chains, akin to some observations of sublimated WO3 in vacuum [5]. Even though the clusters are likely hydroxylated in aqueous solution, this behaviour indicates surprising parallels with UHV and suggests that, in many cases, solution-based procedures complement vacuum methods