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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.

The enduring demand for ever-increasing storage capacities inspires the development of new few nanometer-sized, high-performance memory devices. In this work, tri-n-octylphosphine oxide (TOPO)-stabilized sub-10 nm monoclinic HfO2 nanocrystals (NC) with a rod-like and spherical shape are synthesized and used to build up microscale and nanoscale test devices. The electrical characterization of these devices studied by cyclic current–voltage measurements reveals a redox-like behavior in ambient atmosphere and volatile threshold switching in vacuum. By employing a thorough spectroscopic and surface analysis (FT-IR and NMR spectroscopy and XPS), the origin of this behavior was elucidated. While the redox behavior is enabled by residual moisture present during clean-up of the NC and thin film preparation, which leads to a partial desorption of TOPO from the NC surface, threshold switching is obtained for dry TOPO-stabilized HfO2 NC in microchannel as well as in nanoelectrode devices addressing only a few sub-10 nm TOPO-stabilized HfO2 NC. The results show that integration of sub-10 nm HfO2 NC in nanoscale devices is feasible to build up switching elements.

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

Terpyridine derivatives reveal rich coordination chemistry and are frequently used to construct reliable metallo-supramolecular wires, which are promising candidates for optoelectronic or nanoelect...

Naphthalene diimide (NDI) derivatives are extensively studied in the field of supramolecular and materials chemistry due to their versatility in coordination, electron deficiency, and planar aromat...

Self-assembled monolayers (SAMs) of ruthenium-based molecular wires on solid surfaces are of great interest for optoelectronic and nanoelectronic applications. Here, we present a novel reactive Ru precursor, which enabled us to grow SAMs of Ru complex wires on Au surfaces even at room temperature. Thus, the Ru complex wire growth can be performed easily by sequential reaction of the reactive Ru precursor with terpyridine ligands without the harsh reaction conditions needed otherwise. Subsequently, we monitored the stepwise growth using infrared reflection absorption spectroscopy (IRRAS) and surface-enhanced Raman spectroscopy (SERS). A comparison of IRRAS and SERS data with theoretical spectra, derived from density functional theory calculations, enabled us to verify the formation of each individual growth step. Furthermore, we used these data to determine the orientation of the Ru-based molecular wires with respect to the Au surface. Growth step-dependent layer thicknesses obtained from variable angle sp...

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

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.

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.

Close-packed monolayers of fullerenes on metallic substrates are very rich systems with respect to their rotational degrees of freedom and possible interactions with different adsorption sites or next neighbours. In this connection, we report in detail on the (2√3 × 2√3)R30°-superstructure of C 60 with respect to the Au(111)-surface. We use molecular orbital imaging in systematic UHV-STM studies to reveal the delicate balance of interfacial and intermolecular interactions in this system. Thus, bright C 60 -molecules in 5:6-top and 6:6-top geometries are observed depending on the respective next neighbours. Moreover, tiny changes in the appearance of the unoccupied molecular orbitals of dim C 60 -molecules in hex-vac positions are identified which are caused by the respective interaction with the facets surrounding the Au-vacancy.

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.

Forming reliable and reproducible molecule–nanoelectrode contacts is one of the key issues for the implementation of nanoparticles as functional units into nanoscale devices. Utilizing heterometallic electrodes and Janus-type nanoparticles equipped with molecules allowing selective binding to a distinct electrode material represents a promising approach to achieve this goal. Here, the directed immobilization of individual Janus-type gold nanoparticles (AuNP) between heterometallic electrodes leading to the formation of asymmetric contacts in a highly controllable way is presented. The Janus-AuNP are stabilized by two types of ligands with different terminal groups on opposite hemispheres. The heterometallic nanoelectrode gaps are formed by electron beam lithography in combination with a self-alignment procedure and are adjusted to the size of the Janus-AuNP. Thus, by choosing adequate molecular end group/metal combinations, the immobilization direction of the Janus-AuNP is highly controllable. These resul...

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.

Integration of molecule-capped gold nanoparticles (AuNP) into nanoelectronic devices requires detailed knowledge about the AuNP-electrode interface. Here, we report the pH-dependent adsorption of amine or carboxylic acid-terminated gold nanoparticles on platinum or gold/palladium (30% Pd) alloy, respectively. We synthesized amine-terminated AuNP, applying a new solid phase supported approach, as well as AuNP exhibiting carboxylic acid as terminal groups. The pH-induced agglomeration of the synthesized AuNP was investigated by UV-vis, DLS, and ζ-potential measurements. Depending on the pH and the ionic strength of the AuNP solution a preferential adsorption on the different metals occurred. Thereby, we demonstrate that by choosing the appropriate functional group and adjusting the pH as well as the ionic strength a directed binding can be achieved, which is an essential prerequisite for applications of these particles in nanoelectronics. These findings will pave the way for a controlled designing of the interface between molecule-capped AuNP and metallic electrodes for applications in nanoelectronics.

One concept to build up hybrid electronic devices based on molecules or nanoparticles with rectifying properties is based on nanoscale objects that are immobilized between two electrodes composed of different metals forming asymmetric contacts. Following this concept, we introduce an optimized procedure to fabricate heterometallic nanoelectrodes with a separation of only 5 nm. Gold nanoparticles (AuNPs) with a diameter of 15 nm, stabilized with 4-mercaptophenylamine, were used to form electrode1-molecule/AuNP/molecule-electrode2 devices comprising at most a small number of AuNPs. Immobilization was performed by dielectrophoretic trapping. The molecular properties of 4-mercaptophenylamine are reflected in transition voltage spectroscopy features of the device. Cyclic current–voltage measurements on 20 functional devices revealed distinct differences in conductivities based on minor differences in device geometry. Analysis of the electron transport characteristics discloses that under these experimental con...

In this article, we briefly overview our experimental and theoretical studies aimed to investigate the electronic structure and the transport properties of several carboxylates chemisorbed on the Cu(110) surface. In particular, we have theoretically explored the possibility to chemically functionalize a carboxylate–Cu(110) system to specifically tune its electronic structure. Experimentally, it could be proven that the transport properties of the carboxylate–Cu(110) interface indeed revealed specific features in the d spectra corresponding to the used functionalization process. Moreover, the d spectra can be used to identify specific adsorption geometries or conformations of carboxylates on Cu(110). Furthermore, transport calculations for molecules with different anchoring groups between two Cu(110) electrodes are also discussed. Schematic view of a STM tip probing a carboxylate molecule chemisorbed on the Cu(110) surface with the corresponding energy level alignment in the case of a zero bias voltage.

Besides current–voltage spectroscopy, also transition voltage spectroscopy (TVS) becomes an interesting tool to investigate the energetic position of the molecular orbitals involved in the tunneling process. We used scanning tunneling spectroscopy to perform both spectroscopy techniques as a function of the tip–substrate distance. Employing our model system, benzoic acid on a Cu(110) surface, we could observe a step in the transition voltage using distance-dependent TVS. Combining the spectroscopic results with density functional theory based calculations, it was possible to identify the molecular orbitals responsible for charge transport. Moreover, it was found that different molecular orbitals are responsible for charge transport if varying STM tip–substrate distances are examined.

Systematic studies of domain boundaries of self-assembled monolayers of octanethiols on (1 1 1) oriented gold surfaces have been performed by ultrahigh vacuum scanning tunnelling microscopy (STM). The monolayers consist of domains that exhibit the c ( 4 × 2 ) superstructure of the hexagonal ( 3 × 3 ) R 30 ∘ structure of alkanethiols on gold. By high-resolution images domain boundaries are displayed with molecular resolution. These are used to deduce the exact relation between the adsorption sites of the molecules belonging to different domains on the gold substrate. A translational transition is shown here in detail and it is demonstrated that hexagonal close-packed (hcp), and face-centred cubic (fcc) triple hollow sites are occupied similarly by octanethiols.

The self-assembly of ω-ferrocenylalkanethiols (FcCnSH) with different alkyl-spacer lengths on Au(1 1 1) substrates has been studied by scanning tunneling microscopy (STM). Upon deposition at room temperature FcCnSH molecules tend to form multilayers, while by thermal treatment monolayer formation, a rearrangement of the molecules and the formation of ordered domains is achieved. The surface structure of the resulting full coverage self-assembled monolayers is resolved with molecular resolution by STM. The ordered monolayer structure of ω-ferrocenylpropanethiol is discussed in comparison with its bulk crystal structure, derived from single crystal X-ray analysis. Based on these results a monolayer structure of ω-ferrocenylalkanethiols with longer alkyl chains closely related to the bulk crystal structure of the shorter alkyl-spacer derivates is suggested. Our results provide detailed insight into the self-assembly of FcCnSH on gold substrates.

Using UHV-STM investigations and density-functional theory calculations we prove the contribution of Cu-adatoms to the stabilization of a new high-density phase of benzoate molecules on a Cu(110) substrate. We show that two different chemical species, benzoate and benzoate Cu-adatoms molecules, build the new close-packed structure. Although both species bind strongly to the copper surface, we identify the benzoate Cu-adatoms molecules as the more mobile species on the surface due to their reduced dipole moment and their lower binding energy compared to benzoate molecules. Therefore, the self-assembly process is supposed to be mediated by benzoate Cu-adatom species, which is analogous to the gold-thiolate species on Au(111) surfaces.

The molecular structure of submonolayer-coverage phases of 3-(thioacetyl)-propanoylferrocene and 5-ferrocenylpentanethiol in mixed layers with alkanethiols on Au(111) was resolved by scanning tunneling microscopy. The ferrocenes formed a striped surface phase, similar to the lying-down structures of alkanethiols, resulting in equally spaced rows of the ferrocene moieties. The obtained nanoscale lattice of functional groups on the surface offers an interesting potential for the patterning of small, periodic structures with precise distance control via a hydrocarbon spacer.

The electron-transfer properties of a full-coverage self-assembled monolayer of biphenylethanethiol (CH3-C6H4-C6H4-(CH2)2-SH, BP2) is studied by current versus distance spectroscopy. With this, the electrical properties of different functional groups in one molecule are vertically resolved. The current versus distance plot is shown to consist of three different parts, whose logarithmic slope is interpreted as the decay length of the vacuum, the biphenyl group, and the alkane chain. This finding is described in a trilayer tunnel junction model and the decay constant, βPh, of the tunneling current through the biphenyl group is estimated. Besides characterizing the biphenylalkanethiol monolayers electrically, the structure of these layers is determined by UHV-STM. Two new structures with rectangular (4 × 6√3) lattices are identified for 4‘-methyl-1,1‘-biphenyl-4-ethanethiol self-assembled monolayers on (111)-oriented gold surfaces.

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 bottom-up fabrication route for PbTiO 3 nanograins grown on predefined TiO 2 nanostructures used as seeds is presented. The structuring of the TiO 2 seeds is performed using a self-organized template constructed from a gold-loaded micellar mono-film. With this fabrication process, TiO 2 seeds and PbTiO 3 grains with diameters of 12 and 30 nm, respectively, are prepared without the need for electron-beam lithography. The dimensions of the structure imposed by the micellar template are transferred through all the processing steps to the final PbTiO 3 grains. Furthermore, it is shown that the intermicelle distance and the degree of order in the dried nionofilm is mainly determined by the preparation conditions, such as the pulling velocity in the dipping process and the strength of the surface-micelle interaction, and not necessarily by the architectural properties (block length and ratio) of the diblock copolymers that build the micelles. The intermicelle spacing in the dried film is much smaller than the micelle dimensions in solution, and approaches the dimensions of a fully collapsed micelle when the dipping process is performed slowly enough.

[11-(Acetylthio)-l-oxoundecyl]-ferrocene and 1,1"-Dithiobis-ferrocene are inserted into alkanethiolate self-assembled monolayers from solution. Upon annealing, the inserted molecules form separated domains, which protrude as islands from the surrounding monolayer. The resulting thin-films are studied by UHV-STM. For both compounds the density of the molecules in close packed SAMs is determined. Furthermore for ferroceneundecanethiols the molecular structure and orientation of the endgroups is resolved.

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 two-step deposition process for the formation of atomically flat gold films on mica is developed. The process starts with a high deposition rate followed by a 1:100 reduced rate. Using this combination a completely wetting of mica by gold resulting in a two-dimensional growth mode and finally extremely flat gold films with large terraces are achieved. Additionally hexagonal faceting of the gold films on mica is observed at moderate temperatures which can be related to the relaxation of stress caused by different thermal coefficients of expansion of mica and gold. The stress release leads to the generation of misfit dislocations that glide along the (1 0 0) planes producing facets on the surface. Annealing experiments in a UHV-STM and thermogravimetry point to the inset of intensified hexagonal faceting due to the starting decomposition of mica at elevated temperatures.

The compressive stress originated in the coherent epitaxial-SrRuO3/LaAlO3 interface has been used as a nano-patterning tool for the creation of island and wire arrays in ultrathin (

Memory cells based on the cation migration and filament formation and rupture in a solid electrolyte have attracted much interest due to low switching voltages and a prospective high scalability. In this study we indirectly visualized the growth and dissolution of the conductive filament in Ag–Ge–Se samples with Ag bottom electrodes by surface analysis with Conductive Atomic Force Microscopy (CAFM). By application of a negative voltage to the inert CAFM tip, conductive filaments were grown on the scanned area and they were dissolved under reversed bias. The local conductivity changes directly corresponded to changes in the topography, i.e. to the filament protrusion and dissolution. Topography changes could be circumvented by limiting the maximum current. By placing the CAFM tip on a random spot on the sample, filaments with a diameter as low as 20 nm were grown by local current–voltage measurements. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 reliable fabrication of nanoelectrode pairs with predefined separations in the few nanometer range is an essential prerequisite for future nanoelectronic devices. Here we demonstrate a fine-tuned electron-beam lithographic (EBL) fabrication route which is suitable for defining nanoelectrode pairs with a gap size down to 3 ± 1 nm and with a yield of 55%. This achievement is based on an optimized two-layer resist system in combination with an adopted developer system, as well as on an elaborated nanoelectrode pattern design taking into consideration the EBL inherent proximity effect. Thus, even a structural control in the nanometer scale is achieved in the EBL process.

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.

One of the challenges in molecular electronics is to design molecules which can be used as functional units in electronic devices. The subject of our investigations is an asymmetrical molecule, dihydroxy(4-thiomorpholinomethyl)benzoic acid (TMBA), whose structural and electronic properties are characterized. The self-assembly behavior of TMBA on Au(111) surfaces resulting in highly ordered monolayers is obtained using scanning tunneling microscopy (STM). Furthermore, investigations on the electronic properties of the combined metal/molecule system reveal an orbital mediated tunneling process and tunneling decay constants for the carboxylic and thiomorpholino group. Thus, a diode-like character of TMBA is shown to be caused by intrinsic electronic properties of different molecular moieties.

In this review, possibilities to modify intentionally the electronic transport properties of metal/molecule/metal devices (MMM devices) are discussed. Here especially the influence of the metal work function, the metal–molecule interface, the molecule dipole and different tunneling mechanisms are considered. A route to evaluate the effective surface work function of metal–molecule systems is given and, based on experimental results, an exemplary estimation is performed. The electron transport across different metal–molecule interfaces is characterized by relating transmission coefficients extracted from experimentally derived molecular conductances, decay constants or tunneling barrier heights. Based on the reported results the tunneling decay constant can be assumed to be suitable to characterize intrinsic molecular electron transport properties, while the nature of the metal–molecule contacts is properly described by the transmission coefficient. A clear gradation of transmission efficiencies of metal–anchoring group combinations can be given.

Molecular components with their inherent scalability are expected to be promising supplements for nanoscale electronic devices. Here we report on how to specifically tune the electronic structure of chemisorbed molecules and thus to gain control of molecular transport properties. The electronic structure of our prototype π-conjugated carboxylic acid anchored on the Cu(110) surface is modified systematically by inserting nitrogen atoms in a six-membered aromatic ring, a carboxylic functional group at the aromatic ring or both. Depending on the specific nature of the substituent, the relative position of the occupied or unoccupied electronic states with respect to the Fermi level can be specifically controlled and thus the transport properties of the studied molecular systems are modified intentionally, as proven by our scanning tunneling spectroscopy measurements. On the basis of the insight gained by our systematic experiment and first-principles calculations we are also able to predict the specific molecular character (σ or π) of the orbitals involved in the transport process of a carboxylate–Cu(110) system, depending on the functionalization pattern employed.

The possibility to control the charge transport properties in molecular scale devices strongly depends upon the nature of the molecule―metal interfaces, causing an intense request to engineer at molecular level the interface properties. Here, we report on single electron tunneling effects observed in a STM-tip/single molecule/substrate device at room temperature using a molecule with a three-dimensional aromatic system. The molecule consists of a coronene core per-substituted with arylthio groups which are tailored in such a way that the aromatic system is efficiently decoupled from the metal substrate, and thus a double-barrier tunnel junction is created by means of a built-in insulating spacer. Comparing ab initio calculations with the experimental observations allows us to identify the specific arrangement of the substituents in the most stable conformer of this molecule. The tailored molecular structure in combination with the identified adsorption geometry controls the electron transport behavior and results in single electron transport features.