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The controlled adsorption of polynuclear coordination compounds with specific structural and electronic characteristics on surfaces is crucial for the prospective implementation of molecule-surface interfaces into practical electronic devices. From this perspective, a neutral 3d,4f-coordination cluster [MnII3MnIVYb3O3(OH)(L·SMe)3(OOCMe)9]·2MeCN·3EtOH (1·2MeCN·3EtOH), where L·SMe- is a Schiff base, has been synthesized and fully characterized and its adsorption on two different solid substrates, gold and graphite, has been studied. The mixed-valence compound with a bilayered metal core structure and the structurally exposed thioether groups exhibits a substantially different surface bonding to metallic gold and semimetallic graphite substrates. While on graphite the adsorption takes place only on distinguished attraction points with a locally increased number of potential bonding sites such as terrace edges and other surface defects, on gold the molecules were found to adsorb rather weakly on randomly distributed adsorption sites of the surface terraces. This entirely different behavior provides important information for the development of advanced surface materials that may enable well-distributed ordered molecular assemblies.

Advanced materials interfaces 9(21), 2200461 (2022). doi:10.1002/admi.202200461, Published by Wiley-VCH, Weinheim

This Account highlights recent experimental and theoretical work focusing on the development of polyoxometalates (POMs) as possible active switching units in what may be called "molecule-based memory cells". Herein, we critically discuss how multiply charged vanadium-containing POMs, which exhibit stable metal-oxo bonds and are characterized by the excellent ability to change their redox states without significant structural distortions of the central polyoxoanion core, can be immobilized best and how they may work optimally at appropriate surfaces. Furthermore, we critically discuss important issues and challenges on the long way toward POM-based nanoelectronics. This Account is divided into four sections shedding light on POM interplay in solution and on surfaces, ion soft-landing of mass-selected POMs on surfaces, electronic modification of POMs on surfaces, and computational modeling of POMs on surfaces. The sections showcase the complex nature of far-reaching POM interactions with the chemical surroundings in solution and the properties of POMs in the macroscopic environment of electrode surfaces. Section 2 describes complex relationships of POMs with their counter-cations, solvent molecules, and water impurities, which have been shown to exhibit a direct impact on the resulting surface morphology, where a concentration-dependent formation of micellar structures can be potentially observed. Section 3 gives insights into the ion soft-landing deposition of mass-selected POMs on electrode surfaces, which emerges as an appealing method because the simultaneous deposition of agglomeration-stimulating counter-cations can be avoided. Section 4 provides details of electronic properties of POMs and their modification by external electronic stimuli toward the development of multiple-state resistive (memristive) switches. Section 5 sheds light on issues of the determination of the electronic structure properties of POMs across their interfaces, which is difficult to address by experiment. The studies summarized in these four sections have employed various X-ray-scattering, microscopy, spectroscopy, and computational techniques for imaging of POM interfaces in solution and on surfaces to determine the adsorption type, agglomeration tendency, distribution, and oxidation state of deposited molecules. The presented research findings and conceptual ideas may assist experimentalists and theoreticians to advance the exploration of POM electrical conductivity as a function of metal redox and spin states and to pave the way for a realization of ("brain-inspired") POM-based memory devices, memristive POM-surface device engineering, and energy efficient nonvolatile data storage and processing technologies.

The controlled adsorption of polynuclear coordination compounds with specific structural and electronic characteristics on surfaces is crucial for the prospective implementation of molecule-surface interfaces into practical electronic devices. From this perspective, a neutral 3d,4f-coordination cluster [Mn

Nanoscale advances (2021). doi:10.1039/d1na00190f, Published by Royal Society of Chemistry, Cambridge

The far-reaching interplay between the speciation of polyoxometalates (POMs) in the liquid phase and the POM adsorption characteristics on substrate surfaces yet remains to be understood. The significance of this interplay is however paramount because it indicates the degree of technical applicability of solvent-processable POM molecules. Herein, we target this fundamentally important issue, shedding light on the "POM-counterion-solvent" and "POM-counterion-solvent-substrate" processes. We effectively combine the results from small-angle X-ray scattering in solution with surface sensitive scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements and present on this basis a semi-quantitative analysis which provides an excellent correlation between both approaches. The MeCN-solution speciation of a tris(alkoxo)-ligated Wells-Dawson-type polyoxoanion - explored as a representative of commonly negatively charged POM-based inorganic-organic nanostructures - is strikingly connected with the growth of porous two-dimensional molecular layers on highly oriented pyrolytic graphite (HOPG). Low water amounts dramatically transform intermolecular relationships toward hierarchical agglomeration that inhibits the layer formation on HOPG. The obtained findings lay the groundwork for a mechanistic study of controlled nucleation and growth of POM nanostructures on weakly interacting surfaces.

The understanding of the molecular state of vanadium-oxo clusters (polyoxovanadates, POVs) in solution and on surface is a key to their target application in catalysis as well as molecular electronics and spintronics. We here report the results of a combined experimental and computational study of the behavior of nucleophilic polyoxoanions [VIV10VV8O42(I)]5– charged balanced by Et4N+ in water, in a one-phase organic solution of N,N-dimethylformamid (DMF) or acetonitrile (MeCN), in a mixed solution of MeCN–water, and at the hybrid liquid–surface interface. The molecular characteristics of the compound (NEt4)5[V18O42(I)] (1) in the given environments were studied by microspectroscopic, electrochemical, scattering, and molecular mechanics methods. Contrary to the situation in pure water, where we observe great agglomeration with a number of intercalated H2O molecules between POVs that are surrounded by the Et4N+ ions, no or only minor agglomeration of redox-active POVs in an unprecedented cation-mediated fas...

The performance of molecule based electronic devices can be improved by use of transition metal oxides (TMO) as charge injection buffer layers between electrodes and organic semiconductors. It is known that the appropriate energy level alignment at the molecule/TMO interface determines the efficiency of the respective TMO. Herein, scanning tunneling microscopy is employed to characterize the interface formed by fullerenes (C60) deposited on a titanium oxide (TiO) ultrathin film, which is created by oxidation of a Pt3Ti(111) surface. Individual C60 are identified with orbital resolution, and the interfacial and intermolecular interactions are characterized in detail. Furthermore, the energy level alignment at the C60/TiO ultrathin film interface is deduced based on scanning tunneling spectroscopy data. The results demonstrate that the C60/TiO interface corresponds to a type-I heterojunction and, thus, is useful to decouple C60 molecules from the metallic alloy surface.

The sustainable development of IT-systems requires a quest for novel concepts to address further miniaturization, performance improvement, and energy efficiency of devices. The realization of these goals cannot be achieved without an appropriate functional material. Herein, we target the technologically important electron modification using single polyoxometalate (POM) molecules envisaged as smart successors of materials that are implemented in today's complementary metal-oxide-semiconductor (CMOS) technology. Lindqvist-type POMs were physisorbed on the Au(111) surface, preserving their structural and electronic characteristics. By applying an external voltage at room temperature, the valence state of the single POM molecule could be changed multiple times through the injection of up to 4 electrons. The molecular electrical conductivity is dependent on the number of vanadium 3d electrons, resulting in several discrete conduction states with increasing conductivity. This fundamentally important finding illustrates the far-reaching opportunities for POM molecules in the area of multiple-state resistive (memristive) switching.

The local electronic properties of tantalum oxide (TaOx, 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO3) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaOx films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO3, we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaOx and SrRuO3 differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides.

Increasing the efficiency and stability of bimetallic electro catalysts is particularly important for future clean energy technologies. However, the relationship between the surface termination of these alloys and their catalytic activity is poorly understood. Therefore, we report on fundamental UHV-SPM, LEED, and DFT calculations of the Pt3Ti(111) single crystal surface. Using voltage dependent imaging the surface termination of Pt3Ti(111) was studied with atomic resolution. Combining these images with simulated STM maps based on ab initio DFT calculations allowed us to identify the three upper layers of the Pt3Ti(111) single crystal and their influence upon the surface electronic structure. Our results show that small changes in the composition of the second and third atomic layer are of significant influence upon the surface electronic structure of the Pt3Ti electro catalyst. Furthermore, we provide relevant insights into the dependence of the surface termination on the preparation conditions.

By performing in situ growth studies during pulsed laser deposition, we observed a strong reduction of the surface diffusion coefficients for slightly non-stoichiometric SrTiO 3 . Both, stoichiometric and non-stoichiometric thin films exhibit 2D layer by layer growth. However, in the non-stoichiometric case the 2D island coalescence is significantly delayed, which goes along with a shift of the reflection high electron energy diffraction (RHEED) minimum. We could explain this shift of the RHEED minimum by developing a model for the step density evolution taking into account finite surface diffusion.

Low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and density functional theory (DFT) calculations have been used to investigate the atomic and electronic structure of gold deposited (between 0.8 and 1.0 monolayer) on the Pt(111) face in ultrahigh vacuum at room temperature. The analysis of LEED and STM measurements indicates two-dimensional growth of the first Au monolayer. Change of the measured surface lattice constant equal to 2.80 A after Au adsorption was not observed. Based on DFT, the distance between the nearest atoms in the case of bare Pt(111) and Au/Pt(111) surface is equal to 2.83 A, which gives 1% difference in comparison with STM values. The first and second interlayer spacing of the clean Pt(111) surface are expanded by +0.87% and contracted by −0.43%, respectively. The adsorption energy of the Au atom on the Pt(111) surface is dependent on the adsorption position, and there is a preference for a hollow fcc site. For the Au/Pt(111) surface, the top interlayer spacing is expanded by +2.16% with respect to the ideal bulk value. Changes in the electronic properties of the Au/Pt(111) system below the Fermi level connected to the interaction of Au atoms with Pt(111) surface are observed.

The Pt 3 Ti(111) surface is a typical example of an ordered binary intermetallic alloy system consisting of a reactive transition metal and a noble metal. Atomically resolved scanning tunneling microscopy images reveal an either one- or two-layer-thick Pt termination of the reconstructed alloy surface depending on the preparation conditions. As a consequence of the resulting surface states, the system exhibits unique adsorption properties, which is demonstrated by the 2D quasicrystalline growth of C 60 molecules. The lowered reactivity of the alloy compared to pure titanium allows the controlled growth of several atomically thin and highly ordered titanium oxide phases by simple oxygen exposure at elevated temperature. These oxide films can be utilized, for example, as surface template or charge injection buffer layer for organic molecules.

Ordered ultrathin titanium oxide films have been produced under ultrahigh vacuum (UHV) conditions by oxidation of a Pt3Ti(111) single crystal at elevated temperatures. Depending on substrate temperature, oxygen dosage, and partial pressure, four different titania phases have been observed. All phases have been investigated by low- energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Two commensurate phases with a rect-(6 × 3√3) and a hex-(7 × 7)R21.8° superstructure, respectively, are obtained at low oxygen pressures. Both structures form homogeneous films, which wet the complete surface and are stable against further annealing. At high oxygen partial pressures two incommensurate structures can be prepared. The first one contains several holes to release stress arising from the lattice mismatch between the oxide film and substrate. The second one shows a very rough surface morphology and is, like the other incommensurate phase, unstable against thermal treatment. The structures of the different phases are very similar to the structures found for the systems TiOx/Pt(111), VOx/ Rh(111), and VOx/Pd(111), which have been described extensively in the literature.

Breakthroughs in future information technology mandate new materials. For the most pressing challenges in current information technology—i.e., a drastic decrease in energy dissipation and augmented device functionality beyond classical von Neumann computation architectures—several pathways have been proposed. A particularly promising direction is that of spintronics, where electronic spin state control complements electronic charge-state control (1) . More recently, neuromorphic computing gained significant interest, characterized by a focus on synaptic functionality to realize neural networks and associative memory concepts. In the context of these developments, polyoxometalates, characterized by their unique structural versatility, their potential for functionalization and a highly versatile redox chemistry, provide several key advantages that motivate their exploration and use as single-molecule components of novel electronic and spintronic devices. As will be exemplified in this chapter, polyoxometalates provide near-perfect model systems to study the basic phenomena associated with single-molecule electronics and spintronics. Given the infancy of this field, this chapter thus is intended not a classical review, but as a map of potential departure points for avenues to polyoxometalate-based single-molecule devices.

The adsorption of metal-free phthalocyanine molecules on an anisotropic Au(1 1 0)(1 × 2) surface has been studied with ultraviolet (UV) photoemission, low-energy electron diffraction and low-temperature scanning tunneling microscopy. In all cases, the molecules form rows in the [1 [Formula: see text] 0] direction, i.e. along the troughs of the reconstructed substrates. However, depending on the exposure and adsorption temperature, the substrate maintains (1 × 2)- or transforms into a (1 × 3)-reconstruction, and the molecular separation along the rows shrink from six to five times the Au-Au interatomic distance. The results are in agreement with previous density functional theory (DFT) calculations.

CO adsorption on clean and oxidized Pt3Ti(111) surfaces has been investigated by means of Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and High Resolution Electron Energy Loss Spectroscopy (HREELS). On clean Pt3Ti(111) the LEED patterns after CO adsorption exhibit either a diffuse or a sharp c(4 × 2) structure (stable up to 300 K) depending on the adsorption temperature. Remarkably, the adsorption/desorption behavior of CO on clean Pt3Ti(111) is similar to that on Pt(111) except that partial CO decomposition on Ti sites and partial CO oxidation have also been evidenced. Therefore, the clean surface cannot be terminated by a pure Pt plane. Partially oxidized Pt3Ti(111) surfaces ( 220 L O2 exposure at 1000 K) allow no CO adsorption indicating that the titanium oxide film prepared under these conditions is completely closed.

Structural and electronic properties of ultrathin Au films deposited on Pt(111) and annealed at different temperatures have been studied by ultraviolet photoelectron spectroscopy (UPS), photoemission of adsorbed xenon (PAX) and low energy electron diffraction (LEED). The LEED measurements indicate an initial pseudomorphic growth of the Au films. The UPS and PAX experiments show a strong temperature dependence of the surface morphology. The surface covered with Au at 150 K is quite rough but smoothens significantly above room temperature. At a temperature of 750 K intermixing and the formation of an Au–Pt surface alloy start at the interface. The electronic properties of this surface alloy seem to be nearly independent from the originally deposited Au amount in the investigated range of 1–10 monolayers. The removal of Au from the surface regions has also been verified by scanning tunneling microscopy. Adsorption experiments with CO as a titration agent show a significantly lower affinity of the Au–Pt surface alloy in comparison with the clean Pt surface.

Low temperature scanning tunneling microscopy (LT-STM) and scanning tunneling spectroscopy (STS) have been used to investigate adsorbed copper phthalocyanine (C32H16N8Cu) molecules on an ordered ultrathin Al2O3 film on the Ni3Al(111) surface as a function of coverage and annealing temperature. For sub-monolayer coverage and a deposition temperature of 140 K two different planar molecular adsorption configurations rotated by 30° with respect to each other were observed with submolecular resolution in the STM images. The template effect of the underlying oxide film on the CuPc orientation, however, is only weak and negligible at higher coverages. For θ CuPc ≈ 1 ML, before completion of the first layer, the growth of a second layer was already observed. The measured spacing of 3.5 Å between first and second layer corresponds to the distance between the layers in the greek small letter alpha-modification of crystalline CuPc. The molecules deposited at 140 K are thermally stable upon prolonged annealing to temperatures up to 250 K. By the use of STS the lowest unoccupied molecular orbital (LUMO) of the adsorbed copper phthalocyanine molecules has been identified at an energy of 1.2 eV above EF. The lateral distribution of the electronic states of the CuPc has been analyzed and mapped by STS.

A detailed understanding of the resistive switching mechanisms that operate in redox-based resistive random-access memories (ReRAM) is key to controlling these memristive devices and formulating appropriate design rules. Based on distinct fundamental switching mechanisms, two types of ReRAM have emerged: electrochemical metallization memories, in which the mobile species is thought to be metal cations, and valence change memories, in which the mobile species is thought to be oxygen anions (or positively charged oxygen vacancies). Here we show, using scanning tunnelling microscopy and supported by potentiodynamic current-voltage measurements, that in three typical valence change memory materials (TaO(x), HfO(x) and TiO(x)) the host metal cations are mobile in films of 2 nm thickness. The cations can form metallic filaments and participate in the resistive switching process, illustrating that there is a bridge between the electrochemical metallization mechanism and the valence change mechanism. Reset/Set operations are, we suggest, driven by oxidation (passivation) and reduction reactions. For the Ta/Ta2O5 system, a rutile-type TaO2 film is believed to mediate switching, and we show that devices can be switched from a valence change mode to an electrochemical metallization mode by introducing an intermediate layer of amorphous carbon.

We have investigated the growth of ultrathin titanium oxide (TiO(x)) films on a Pt(3)Ti(111) single crystal surface as a function of oxidation temperature (300-1000 K) and oxygen exposure (up to 4500 l) by means of Auger electron spectroscopy, low-energy electron diffraction, ultraviolet photoelectron spectroscopy and high-resolution electron energy loss spectroscopy (HREELS). Both the surface composition and the surface structure of the resulting TiO(x) films exhibit a strong dependence on the preparation conditions. Loss of the chemical order and Ti segregation are observed at the Pt(3)Ti(111) surface upon oxygen exposures of more than 135 l at 1000 K. Increasing oxygen exposure enhances Ti segregation and oxide growth. At a threshold of ≈220 l (at 1000 K) a transition in the oxide structure occurs, namely from a (6 × 3√3) rectangular structure (a = 16.6 Å, b = 14.4 Å) below 220 l to a (7 × 7)R21.8° hexagonal structure (a = b = 19.3 Å) above 220 l. Two additional incommensurate rectangular metastable structures are observed for the highest oxygen exposures (above 900 l) at intermediate oxidation temperatures (800-900 K). In all cases the changes in the valence band spectra and the work function with respect to the clean Pt(3)Ti(111) surface are independent of the chosen oxidation parameters. Based on their HREELS spectra we identify the (6 × 3√3) and (7 × 7)R21.8° structures grown at 1000 K with a stoichiometric TiO phase, while the other and less stable oxide phases grown at 800-900 K exhibit more complex phonon structures that could not simply be associated with any of the stoichiometric phases TiO, Ti(2)O(3) or TiO(2). Our results are rather similar to those found by Granozzi et al for the deposition of Ti onto a Pt(111) surface in an oxygen atmosphere, except a few interesting deviations as a consequence of the different preparation conditions.

In the present study the thermal behavior of ultrathin Au films deposited on different noble metal single crystal surfaces was studied by means of ultraviolet photoelectron spectroscopy and photoemission of adsorbed xenon. The primary goal of this work was to demonstrate the power of photoemission of adsorbed xenon as an easy to handle and very surface sensitive technique for the characterization of multi-component systems. All investigated systems, Au–Pt(111), Au–Pd(111) and Au–Pd(110) show a particular dependence of their electronic and structural properties on the annealing temperature. Au multilayer films with a thickness of 3 monolayer prepared at 150 K undergo in all cases a smoothening by annealing up to room temperature. While on Pd this effect is mainly detectable on the topmost atomic layer the ordering process on Pt is also affecting the deeper layers. Further annealing of the Au–Pd system to 450 K and the Au–Pt system to 750 K, respectively, results in the onset of the alloying process at the interface between metal film and substrate. After annealing to 650 and 950 K, respectively, the alloying finally extends to the very surface. The complete loss of Au from the surface can be detected at 1050 K on Pd and at 1150 K on Pt. The orientation of the substrate has only a weak influence, showing up e.g. in additional electronic states of the not alloyed Au–Pd(111) and Au–Pd(111) systems. Adsorption of CO as a probe molecule illustrates the formation of different adsorption sites on the bimetallic systems during the Au diffusion process.

Resistive switching in transition metal oxide‐based metal‐insulator‐metal structures relies on the reversible drift of ions under an applied electric field on the nanoscale. In such structures, the formation of conductive filaments is believed to be induced by the electric‐field driven migration of oxygen anions, while the cation sublattice is often considered to be inactive. This simple mechanistic picture of the switching process is incomplete as both oxygen anions and metal cations have been previously identified as mobile species under device operation. Here, spectromicroscopic techniques combined with atomistic simulations to elucidate the diffusion and drift processes that take place in the resistive switching model material SrTiO3 are used. It is demonstrated that the conductive filament in epitaxial SrTiO3 devices is not homogenous but exhibits a complex microstructure. Specifically, the filament consists of a conductive Ti3+‐rich region and insulating Sr‐rich islands. Transmission electron microscopy shows that the Sr‐rich islands emerge above Ruddlesden–Popper type antiphase boundaries. The role of these extended defects is clarified by molecular static and molecular dynamic simulations, which reveal that the Ruddlesden–Popper antiphase boundaries constitute diffusion fast‐paths for Sr cations in the perovskites structure.