Atomistic investigation of surface characteristics and electronic features at high-purity FeSi(110) presenting interfacial metallicity.

Iron silicide (FeSi) is a fascinating material that has attracted extensive research efforts for decades, notably revealing unusual temperature-dependent electronic and magnetic characteristics, as well as a close resemblance to the Kondo insulators whereby a coherent picture of intrinsic properties and underlying physics remains to be fully developed. For a better understanding of this narrow-gap semiconductor, we prepared and examined FeSi(110) single-crystal surfaces of high quality. Combined insights from low-temperature scanning tunneling microscopy and density functional theory calculations (DFT) indicate an unreconstructed surface termination presenting rows of Fe-Si pairs. Using high-resolution tunneling spectroscopy (STS), we identify a distinct asymmetric electronic gap in the sub-10 K regime on defect-free terraces. Moreover, the STS data reveal a residual density of states in the gap regime whereby two in-gap states are recognized. The principal origin of these features is rationalized with the help of the DFT-calculated band structure. The computational modeling of a (110)-oriented slab notably evidences the existence of interfacial intragap bands accounting for a markedly increased density of states around the Fermi level. These findings support and provide further insight into the emergence of surface metallicity in the low-temperature regime.

On-Surface Isomerization of Indigo within 1D Coordination Polymers.

Natural products are attractive components to tailor environmentally friendly advanced new materials. We present surface-confined metallosupramolecular engineering of coordination polymers using natural dyes as molecular building blocks: indigo and the related Tyrian purple. Both building blocks yield identical, well-defined coordination polymers composed of (1 dehydroindigo : 1 Fe) repeat units on two different silver single crystal surfaces. These polymers are characterized atomically by submolecular resolution scanning tunnelling microscopy, bond-resolving atomic force microscopy and X-ray photoelectron spectroscopy. On Ag(100) and on Ag(111), the trans configuration of dehydroindigo results in N,O-chelation in the polymer chains. On the more inert Ag(111) surface, the molecules additionally undergo thermally induced isomerization from the trans to the cis configuration and afford N,N- plus O,O-chelation. Density functional theory calculations confirm that the coordination polymers of the cis-isomers on Ag(111) and of the trans-isomers on Ag(100) are energetically favoured. Our results demonstrate post-synthetic linker isomerization in interfacial metal-organic nanosystems.

Depositing Molecular Graphene Nanoribbons on Ag(111) by Electrospray Controlled Ion Beam Deposition: Self-Assembly and On-Surface Transformations.

The chemical processing of low-dimensional carbon nanostructures is crucial for their integration in future devices. Here we apply a new methodology in atomically precise engineering by combining multistep solution synthesis of N-doped molecular graphene nanoribbons (GNRs) with mass-selected ultra-high vacuum electrospray controlled ion beam deposition on surfaces and real-space visualisation by scanning tunnelling microscopy. We demonstrate how this method yields solely a controllable amount of single, otherwise unsublimable, GNRs of 2.9 nm length on a planar Ag(111) surface. This methodology allows for further processing by employing on-surface synthesis protocols and exploiting the reactivity of the substrate. Following multiple chemical transformations, the GNRs provide reactive building blocks to form extended, metal-organic coordination polymers.

N-Heterocyclic Carbenes: Molecular Porters of Surface Mounted Ru-Porphyrins.

Ru-porphyrins act as convenient pedestals for the assembly of N-heterocyclic carbenes (NHCs) on solid surfaces. Upon deposition of a simple NHC ligand on a close packed Ru-porphyrin monolayer, an extraordinary phenomenon can be observed: Ru-porphyrin molecules are transferred from the silver surface to the next molecular layer. We have investigated the structural features and dynamics of this portering process and analysed the associated binding strengths and work function changes. A rearrangement of the molecular layer is induced by the NHC uptake: the NHC selective binding to the Ru causes the ejection of whole porphyrin molecules from the molecular layer on silver to the layer on top. This reorganisation can be reversed by thermally induced desorption of the NHC ligand. We anticipate that the understanding of such mass transport processes will have crucial implications for the functionalisation of surfaces with carbenes.

Assembly and Manipulation of a Prototypical N-Heterocyclic Carbene with a Metalloporphyrin Pedestal on a Solid Surface.

The controlled arrangement of N-heterocyclic carbenes (NHCs) on solid surfaces is a current challenge of surface functionalization. We introduce a strategy of using Ru porphyrins in order to control both the orientation and lateral arrangement of NHCs on a planar surface. The coupling of the NHC to the Ru porphyrin is a facile process which takes place on the interface: we apply NHCs as functional, robust pillars on well-defined, preassembled Ru porphyrin monolayers on silver and characterize these interfaces with atomic precision via a battery of experimental techniques and theoretical considerations. The NHCs assemble at room temperature modularly and reversibly on the Ru porphyrin arrays. We demonstrate a selective and complete functionalization of the Ru centers. With its binding, the NHC modifies the interaction of the Ru porphyrin with the Ag surface, displacing the Ru atom by 1 Å away from the surface. This arrangement of NHCs allows us to address individual ligands by controlled manipulation with the tip of a scanning tunneling microscope, creating patterned structures on the nanometer scale.

Single Molecules in Strong Optical Fields: A Variable-Temperature Molecular Junction Spectroscopy Setup.

In order to advance the development of molecular electronic devices, it is mandatory to improve the understanding of electron transport and functionalities in single molecules, integrated in a well-defined environment. However, limited information can be obtained by solely analyzing I-V characteristics, whence multiparameter studies are required to obtain more information on such systems including chemical bonds, geometry, and intramolecular strain. Therefore, we developed an analytical method incorporating an optical near-field technique, which allows us to investigate single-molecule junctions at variable temperatures in strong optical fields. An apertureless near-field emitter acts as a counter electrode and a plasmonic waveguide to focus surface plasmon polaritons into the molecular junctions, where a strongly enhanced evanescent field is confined to only a few nanometers around the apex of the tip. The proof of concept, even at low temperatures, is demonstrated by simultaneously investigating electronic and optical features of the molecule p-terphenyl-4,4″-dithiol in dependence of its charge state. This multichannel method can be employed to analyze a variety of nearly unexplored properties in single-molecule junctions such as photoconductance and photocurrent generation and allows a characterization of the molecular junctions by spectroscopic means as well.

Rotation in an Enantiospecific Self-Assembled Array of Molecular Raffle Wheels.

Tailored nano-spaces can control enantioselective adsorption and molecular motion. We report on the spontaneous assembly of a dynamic system-a rigid kagome network with each pore occupied by a guest molecule-employing solely 2,6-bis(1H-pyrazol-1-yl)pyridine-4-carboxylic acid on Ag(111). The network cavity snugly hosts the chemically modified guest, bestows enantiomorphic adsorption and allows selective rotational motions. Temperature-dependent scanning tunnelling microscopy studies revealed distinct anchoring orientations of the guest unit switching with a 0.95 eV thermal barrier. H-bonding between the guest and the host transiently stabilises the rotating guest, as the flapper on a raffle wheel. Density functional theory investigations unravel the detailed molecular pirouette of the guest and how the energy landscape is determined by H-bond formation and breakage. The origin of the guest's enantiodirected, dynamic anchoring lies in the specific interplay of the kagome network and the silver surface.

Conformational Control of Chemical Reactivity for Surface-Confined Ru-Porphyrins.

We assess the crucial role of tetrapyrrole flexibility in the CO ligation to distinct Ru-porphyrins supported on an atomistically well-defined Ag(111) substrate. Our systematic real-space visualisation and manipulation experiments with scanning tunnelling microscopy directly probe the ligation, while bond-resolving atomic force microscopy and X-ray standing-wave measurements characterise the geometry, X-ray and ultraviolet photoelectron spectroscopy the electronic structure, and temperature-programmed desorption the binding strength. Density-functional-theory calculations provide additional insight into the functional interface. We unambiguously demonstrate that the substituents regulate the interfacial conformational adaptability, either promoting or obstructing the uptake of axial CO adducts.

Electron-Phonon Coupling in Current-Driven Single-Molecule Junctions.

Vibrational excitations provoked by coupling effects during charge transport through single molecules are intrinsic energy dissipation phenomena, in close analogy to electron-phonon coupling in solids. One fundamental challenge in molecular electronics is the quantitative determination of charge-vibrational (electron-phonon) coupling for single-molecule junctions. The ability to record electron-phonon coupling phenomena at the single-molecule level is a key prerequisite to fully rationalize and optimize charge-transport efficiencies for specific molecular configurations and currents. Here we exemplarily determine the pertaining coupling characteristics for a current-carrying chemically well-defined molecule by synchronous vibrational and current-voltage spectroscopy. These metal-molecule-metal junction insights are complemented by time-resolved infrared spectroscopy to assess the intramolecular vibrational relaxation dynamics. By measuring and analyzing the steady-state vibrational distribution during transient charge transport in a bis-phenylethynyl-anthracene derivative using anti-Stokes Raman scattering, we find ∼0.5 vibrational excitations per elementary charge passing through the metal-molecule-metal junction, by means of a rate model ansatz and quantum-chemical calculations.

On-surface Synthesis of a Semiconducting 2D Metal-Organic Framework Cu3 (C6 O6 ) Exhibiting Dispersive Electronic Bands.

A 2D metal-organic framework (2D-MOF) was formed on a Cu(111) substrate using benzenehexol molecules. By means of a combination of scanning tunneling microscopy and spectroscopy, X-ray photoelectron spectroscopy and density-functional theory, the structure of the 2D-MOF is determined to be Cu3 (C6 O6 ), which is stabilized by O-Cu-O bonding motifs. We find that upon adsorption on Cu(111), the 2D-MOF features a semiconductor band structure with a direct band gap of 1.5 eV. The O-Cu-O bonds offer efficient charge delocalization, which gives rise to a highly dispersive conduction band with an effective mass of 0.45 me at the band bottom, implying a high electron mobility in this material.

Optical-field-induced current in dielectrics.

The time it takes to switch on and off electric current determines the rate at which signals can be processed and sampled in modern information technology. Field-effect transistors are able to control currents at frequencies of the order of or higher than 100 gigahertz, but electric interconnects may hamper progress towards reaching the terahertz (10(12) hertz) range. All-optical injection of currents through interfering photoexcitation pathways or photoconductive switching of terahertz transients has made it possible to control electric current on a subpicosecond timescale in semiconductors. Insulators have been deemed unsuitable for both methods, because of the need for either ultraviolet light or strong fields, which induce slow damage or ultrafast breakdown, respectively. Here we report the feasibility of electric signal manipulation in a dielectric. A few-cycle optical waveform reversibly increases--free from breakdown--the a.c. conductivity of amorphous silicon dioxide (fused silica) by more than 18 orders of magnitude within 1 femtosecond, allowing electric currents to be driven, directed and switched by the instantaneous light field. Our work opens the way to extending electronic signal processing and high-speed metrology into the petahertz (10(15) hertz) domain.

Snapshots of Dynamic Adaptation: Two-Dimensional Molecular Architectonics with Linear Bis-Hydroxamic Acid Modules.

Linear modules equipped with two terminal hydroxamic acid groups act as the building block of diverse two-dimensional supramolecular motifs and patterns with room-temperature stability on the close-packed single-crystal surfaces of silver and gold, revealing a complex self-assembly scenario. By combining multiple investigation techniques (scanning tunneling microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations), we analyze the characteristics of the ordered assemblies which range from close-packed structures to polyporous networks featuring an exceptionally extended primitive unit cell with a side length exceeding 7 nm. The polyporous network shows potential for hosting and promoting the formation of chiral supramolecules, whereas a transition from 1D chiral randomness to an ordered racemate is discovered in a different porous phase. We correlate the observed structural changes to the adaptivity of the building block and surface-induced changes in the chemical state of the hydroxamic acid functional group.

Voltage-Driven Conformational Switching with Distinct Raman Signature in a Single-Molecule Junction.

Precisely controlling well-defined, stable single-molecule junctions represents a pillar of single-molecule electronics. Early attempts to establish computing with molecular switching arrays were partly challenged by limitations in the direct chemical characterization of metal-molecule-metal junctions. While cryogenic scanning probe studies have advanced the mechanistic understanding of current- and voltage-induced conformational switching, metal-molecule-metal conformations are still largely inferred from indirect evidence. Hence, the development of robust, chemically sensitive techniques is instrumental for advancement in the field. Here we probe the conformation of a two-state molecular switch with vibrational spectroscopy, while simultaneously operating it by means of the applied voltage. Our study emphasizes measurements of single-molecule Raman spectra in a room-temperature stable single-molecule switch presenting a signal modulation of nearly 2 orders of magnitude.

Dynamics of Spatially Confined Bisphenol A Trimers in a Unimolecular Network on Ag(111).

Bisphenol A (BPA) aggregates on Ag(111) shows a polymorphism between two supramolecular motifs leading to formation of distinct networks depending on thermal energy. With rising temperature a dimeric pairing scheme reversibly converts into a trimeric motif, which forms a hexagonal superstructure with complex dynamic characteristics. The trimeric arrangements notably organize spontaneously into a self-assembled one-component array with supramolecular BPA rotors embedded in a two-dimensional stator sublattice. By varying the temperature, the speed of the rotors can be controlled as monitored by direct visualization. A combination of scanning tunneling microscopy and dispersion-corrected density-functional tight-binding (DFTB-vdW(surf)) based molecular modeling reveals the exact atomistic position of each molecule within the assembly as well as the driving force for the formation of the supramolecular rotors.

Five-vertex Archimedean surface tessellation by lanthanide-directed molecular self-assembly.

The tessellation of the Euclidean plane by regular polygons has been contemplated since ancient times and presents intriguing aspects embracing mathematics, art, and crystallography. Significant efforts were devoted to engineer specific 2D interfacial tessellations at the molecular level, but periodic patterns with distinct five-vertex motifs remained elusive. Here, we report a direct scanning tunneling microscopy investigation on the cerium-directed assembly of linear polyphenyl molecular linkers with terminal carbonitrile groups on a smooth Ag(111) noble-metal surface. We demonstrate the spontaneous formation of fivefold Ce-ligand coordination motifs, which are planar and flexible, such that vertices connecting simultaneously trigonal and square polygons can be expressed. By tuning the concentration and the stoichiometric ratio of rare-earth metal centers to ligands, a hierarchic assembly with dodecameric units and a surface-confined metal-organic coordination network yielding the semiregular Archimedean snub square tiling could be fabricated.

Surface-Guided Formation of an Organocobalt Complex.

Organocobalt complexes represent a versatile tool in organic synthesis as they are important intermediates in Pauson-Khand, Friedel-Crafts, and Nicholas reactions. Herein, a single-molecule-level investigation addressing the formation of an organocobalt complex at a solid-vacuum interface is reported. Deposition of 4,4'-(ethyne-1,2-diyl)dibenzonitrile and Co atoms on the Ag(111) surface followed by annealing resulted in genuine complexes in which single Co atoms laterally coordinated to two carbonitrile groups undergo organometallic bonding with the internal alkyne moiety of adjacent molecules. Alternative complexation scenarios involving fragmentation of the precursor were ruled out by complementary X-ray photoelectron spectroscopy. According to density functional theory analysis, the complexation with the alkyne moiety follows the Dewar-Chatt-Duncanson model for a two-electron-donor ligand where an alkyne-to-Co donation occurs together with a strong metal-to-alkyne back-donation.

Surface-Assisted Cyclodehydrogenation; Break the Symmetry, Enhance the Selectivity.

Selectivity in chemical reactions is a major objective in industrial processes to minimize spurious byproducts and to save scarce resources. In homogeneous catalysis the most important factor which determines selectivity is structural symmetry. However, a transfer of the symmetry concept to heterogeneous catalysis still requires a detailed comprehension of the underlying processes. Here, we investigate a ring-closing reaction in surface-confined meso-substituted porphyrin molecules by scanning tunneling microscopy, temperature-programmed desorption, and computational modeling. The identification of reaction intermediates enables us to analyze the reaction pathway and to conclude that the symmetry of the porphyrin core is of pivotal importance regarding product yields.

Photoinduced C-C reactions on insulators toward photolithography of graphene nanoarchitectures.

On-surface chemistry for atomically precise sp(2) macromolecules requires top-down lithographic methods on insulating surfaces in order to pattern the long-range complex architectures needed by the semiconductor industry. Here, we fabricate sp(2)-carbon nanometer-thin films on insulators and under ultrahigh vacuum (UHV) conditions from photocoupled brominated precursors. We reveal that covalent coupling is initiated by C-Br bond cleavage through photon energies exceeding 4.4 eV, as monitored by laser desorption ionization (LDI) mass spectrometry (MS) and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) gives insight into the mechanisms of C-Br scission and C-C coupling processes. Further, unreacted material can be sublimed and the coupled sp(2)-carbon precursors can be graphitized by e-beam treatment at 500 °C, demonstrating promising applications in photolithography of graphene nanoarchitectures. Our results present UV-induced reactions on insulators for the formation of all sp(2)-carbon architectures, thereby converging top-down lithography and bottom-up on-surface chemistry into technology.

Structural adaptations of electrosprayed aromatic oligoamide foldamers on Ag(111).

Aromatic foldamers are promising for applications such as molecular recognition and molecular machinery. For many of these, defect free, 2D-crystaline monolayers are needed. To this end, submonolayers were prepared in ultra-high vacuum (UHV) on Ag(111) via electrospray controlled ion beam deposition (ES-CIBD). On the surface, the unfolded state is unambiguously identified by real-space single-molecule imaging using scanning tunnelling microscopy (STM) and it is found to assemble in regular structures.