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ACS applied nano materials (2022). doi:10.1021/acsanm.2c04165, Published by American Chemical Society Publications, Washington, DC

Transition metal complexes are electrofunctional molecules due to their high conductivity and their intrinsic switching ability involving a metal-to-ligand charge transfer. Here, a method is presented to contact reliably a few to single redox-active Ru-terpyridine complexes in a CMOS compatible nanodevice and preserve their electrical functionality. Using hybrid materials from 14 nm gold nanoparticles (AuNP) and bis-{4′-[4-(mercaptophenyl)-2,2′:6′,2″-terpyridine]}-ruthenium(II) complexes a device size of 302 nm2 inclusive nanoelectrodes is achieved. Moreover, this method bears the opportunity for further downscaling. The Ru-complex AuNP devices show symmetric and asymmetric current versus voltage curves with a hysteretic characteristic in two well separated conductance ranges. By theoretical approximations based on the single-channel Landauer model, the charge transport through the formed double-barrier tunnel junction is thoroughly analyzed and its sensibility to the molecule/metal contact is revealed. It can be verified that tunneling transport through the HOMO is the main transport mechanism while decoherent hopping transport is present to a minor extent.

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

Local potential differences between the 36 and 32.4.3.4 vertex configurations are identified within a two-dimensional dodecagonal fullerene monolayer. In a local area of the 8/3 approximant, rotational switching fullerenes on 36 vertex sites are revealed by scanning tunneling microscopy., One of the essential components of molecular electronic circuits are switching elements that are stable in two different states and can ideally be switched on and off many times. Here, distinct buckminsterfullerenes within a self-assembled monolayer, forming a two-dimensional dodecagonal quasicrystal on a Pt-terminated Pt3Ti(111) surface, are identified to form well separated molecular rotational switching elements. Employing scanning tunneling microscopy, the molecular-orbital appearance of the fullerenes in the quasicrystalline monolayer is resolved. Thus, fullerenes adsorbed on the 36 vertex configuration are identified to exhibit a distinctly increased mobility. In addition, this finding is verified by differential conductance measurements. The rotation of these mobile fullerenes can be triggered frequently by applied voltage pulses, while keeping the neighboring molecules immobile. An extensive analysis reveals that crystallographic and energetic constraints at the molecule/metal interface induce an inequality of the local potentials for the 36 and 32.4.3.4 vertex sites and this accounts for the switching ability of fullerenes on the 36 vertex sites. Consequently, a local area of the 8/3 approximant in the two-dimensional fullerene quasicrystal consists of single rotational switching fullerenes embedded in a matrix of inert molecules. Furthermore, it is deduced that optimization of the intermolecular interactions between neighboring fullerenes hinders the realization of translational periodicity in the fullerene monolayer on the Pt-terminated Pt3Ti(111) surface.

Nanoscale advances 3(2), 538-549 (2021). doi:10.1039/D0NA00925C, Published by Royal Society of Chemistry, Cambridge

European journal of inorganic chemistry 2020(40), 3798-3810 (2020). doi:10.1002/ejic.202000629 special issue: "ADUC Prize Winners", Published by Wiley-VCH, Weinheim

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.

Functionalized naphthalene diimides (NDIs) are attractive candidates as small molecule acceptors for various molecular electronic applications due to their reversible two-step one-electron reductions at relatively low potentials. Here, we utilize low-temperature scanning tunneling microscopy (STM) to study the spatial extent and the electronic structure of 2,7-dibenzyl 1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI) adsorbed on the Pt(111) surface. We succeeded to map in real space the electronic structure of this three-dimensional (3D) molecule with orbital resolution, and thus were able to image an in-plane π-nodal plane located at the benzyl side arms. Furthermore, on the basis of the comparison of voltage dependent STM images and ab initio density functional theory simulations, we are able to explain the STM features of BNTCDI in terms of a convolution between its 3D shape and electronic structure. Importantly, for this weakly coupled molecule on the Pt(111) substrate, the intramolecular N···H–C hydrogen bonds (i) stabilize the protruding π-systems of the benzyl groups perpendicular to the flat NDI core and (ii) open an effective transport path around Fermi energy.

Mixed monolayers of mercaptoundecylferrocene and undecanethiol were deposited from solution by coadsorption and by a two-step insertion method, using the alkanethiol monolayer as insulating matrix. The resulting layers were characterized by UHV-STM, showing molecular resolution. For insertion-processed samples, a mesh-like surface structure of ferrocenes was observed, due to the preferential incorporation of molecules along domain boundaries and defect sites of the alkanethiol monolayer. For monolayers in the intermediate coverage regime, a crystalline phase was observed.

RSC Advances 5(125), 102981-102992(2015). doi:10.1039/C5RA17545C, Published by RSC Publishing, London

A method is presented for depositing mixed self-assembled monolayers (SAMs) of dodecanethiol (C12) and 4'-methyl-1,1'-biphenyl-4-butane (H3C-C6H4-C6H4-(CH2)4-SH, BP4) by insertion of BP4 into a closely packed SAM of dodecanethiol on Au(111). Insertion takes place at defect sites such as domain boundaries or etch pits in the gold surface that are characteristic of C12 monolayers on gold. With a lower probability, insertion also occurs beside defect sites inside dodecanethiol domains. Insertion at defect sites results in domains of BP4, whereas insertion into C12 domains leads to isolated BP4 molecules. The isolated BP4 molecules are shown not to move at room temperature. By comparing the apparent height of the isolated BP4 molecules and BP4 domains, it is proposed that the isolated molecules have the same conformation as in the full-coverage phase. A simple two-layer model is proposed to characterize the current transport through BP4. The decay constant beta for the phenylene groups is deduced from the apparent STM heights of the inserted BP4 islands compared to the STM heights of the C12 closely packed monolayers.

A rectangular (3 x 2 radical3) surface lattice for long-term-annealed dodecanethiol self-assembled monolayers on Au(111) is observed by ultra-high-vacuum scanning tunneling microscopy. The new lattice has the same density and a unit cell of the same size as the well-known c(4 x 2) reconstruction. In contrast, it does not show hexagonal symmetry but rather a sort of thiol pairing, leading to a shift in the binding position of every second molecule. The described structure is believed to be an intermediate phase close to desorption.

The synthesis, adsorption behavior, surface structure, and the charge transport properties of a persulfurated coronene asterisk with a 3D-polyaromatic system, namely dodecakis(phenylthio) coronene (DPTC), deposited on HOPG(0001) and Au(111) surfaces, are investigated by means of scanning tunneling microscopy (STM) and spectroscopy (STS). DPTC molecules adsorbed on HOPG(0001) show an orbital mediated tunneling through mainly undisturbed frontier molecular states. DPTC molecules self-assemble on Au(111) into a highly ordered p-stacked columnar "edge-on pattern. The columnar stacking is a gold surface mediated process, as ascertained by fluorescence correlation spectroscopy (FCS). DPTC was monomeric in the precursor solution before assembly. The tunneling spectra of ordered DPTC stacks on Au(111) show an energetic splitting of the frontier molecular states, indicating orbital overlap and supramolecular pi pi interactions of adjacent molecules. DPTCs are sufficiently flexible to facilitate dense 1D stacks. The multiple aryl-sulfur bonds play a dominant role in the modulation of the self-assembly properties of the coronenes which in turn affect their electronic properties. Our results encourage further applications in dendrimer chemistry toward molecular electronics, by using the functionalized coronene core and its multidirectional 3D properties.

The electron transport through molecules in molecular devices is typically influenced by the nature of the interfaces with the contacting electrodes and by the interactions between neighbouring molecules. It is a major goal of molecular electronics to adjust the electronic function of a molecular device by tailoring the intrinsic molecular properties and the interfacial and intermolecular interactions. Here, we report on the tunability of the electronic properties of coronene derivatives, namely dodecakis(arylthio)coronenes (DATCs), which are found to exhibit a three-dimensional aromatic system. Scanning tunnelling microscopy (STM), spectroscopy (STS) and simulations based on the density functional theory (DFT) are employed to characterize the structural and electronic properties of these molecules deposited on Au(111) surfaces. It is shown that modifications of the peripheral aryl-groups allow us to specifically affect the self-assembly and the charge transport characteristics of the molecules. Molecular assemblies like supramolecular wires with highly delocalized orbitals and single molecules with molecular "quantum dot'' characteristics are obtained in this way.

The molecule-metal interface formed by pyridine-2,5-dicarboxylic acid chemically bonded to the Cu (110) surface is investigated by scanning tunneling microscopy and first-principles calculations. Our current-voltage spectroscopy studies reveal an electronic mapping of molecular orbitals as a function of tip position. By combining experimental and theoretical investigations, individual molecular orbitals are characterized by their energy and spatial distribution. The importance of adsorption geometries and conformational changes on the electron transport properties is highlighted.

Copper nanocontacts and molecular-sized nanogaps were prepared and characterized at electrified solid/liquid interfaces employing lithographic and electrochemical techniques. A dedicated four-electrode potentiostat was developed for controlling the electrochemical fabrication process and for monitoring the electrical characteristics of the nanostructures created. The formation and breaking of Cu nanocontacts exhibits conductance quantization characteristics. The statistical analysis of conductance histograms revealed a preferential stability of nanocontacts with integer values of G0, with a clear preference for 1 G0, 2 G0 and 3 G0. The growth of molecular-sized gaps shows quantized tunneling current, which is attributed to the discrete nature of Cu atoms, water molecules, and specifically adsorbed ions.

The resistive switching behavior of devices consisting of aluminum top electrode, molecular layer (rose bengal), and bottom electrode (zinc oxide and indium tin oxide) is examined. By measuring the current versus voltage dependence of these devices for various frequencies and by systematically varying the composition of the device, we show that the switching is an extrinsic effect that is not primarily dependent on the molecular layer. It is shown that the molecular layer is short circuited by filaments of either zinc oxide or aluminum and that the switching effect is due to a thin layer of aluminum oxide at the zinc oxide/aluminum interface. (c) 2006 American Institute of Physics.

A self-assembled monolayer of dodecanethiol is grown onto (111) oriented gold by vacuum phase deposition and studied by ultrahigh vacuum scanning tunneling microscopy (STM). The films consist of domains that exhibit the c(4 x 2) over-structure of the hexagonal (square root of 3 x square root of 3)R30 of alkanethiols on gold. The domain size is only limited by the terrace size of the underlying gold. By higher resolution scans a new phase of the c(4 x 2) structure consisting of four inequivalent molecules that display different heights in the STM images is discovered.

SrZrO3 thin films grown by pulsed laser deposition on SrRuO3-buffered SrTiO3 substrates have been investigated by means of x-ray diffraction, atomic force microscopy and Rutherford backscattering spectroscopy. The compressive stress originated in the coherent SrZrO3/SrRuO3 interface due to the lattice mismatch (approximate to-4.5%) forces epitaxial SrZrO3 to grow in a three-dimensional habit. So-deposited SrZrO3 is used to fabricate nanoarrays of dielectric wires from a nanopatterned SrRuO3 surface. The influence of the geometric shadowing effect produced by the non-perpendicular incidence of the ablated particles on the SrZrO3 growth habit is discussed.

Thin films of 1,3-diethylbenzimidazol-2-ylidene (BIEt) were fabricated from THF solution on solid gold substrates and characterised by high-resolution X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopy. The surface-analytical data are in accord with the formation of self-assembled monolayers of BIEt molecules exhibiting an approximately vertical orientation on the substrate. The crystal structure of (BIEt)2 was also determined.