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Cyclobuta[1,2-b:3,4-b′]ditetracene – an analogue of nonacene with a cyclobutadiene unit embedded in the central part has been synthesized by the combination of solution and on-surface chemistry.

On-surface synthesis has recently been regarded as a promising approach for the generation of new molecular structures. It has been particularly successful in the synthesis of graphene nanoribbons, nanographenes and intrinsically reactive and instable, yet attractive species. It is based on the combination of solution chemistry aimed at preparation of appropriate molecular precursors for further ultra-high vacuum surface assisted transformations. This approach also owes its success to an incredible development of characterization techniques, such as scanning tunneling/atomic force microscopy and related methods, which allow detailed, local characterization at atomic scale. While the surface-assisted synthesis can provide molecular nanostructures with outstanding precision, down to single atoms, it suffers from basing on metallic surfaces and often limited yield. Therefore, the extension of the approach away from metals and the struggle to increase productivity seem to be significant challenges toward wider applications. Herein, we demonstrate the on-surface synthesis approach for generation of non-planar nanographenes, which are synthesized through a combination of solution chemistry and sequential surface-assisted processes, together with the detailed characterization by scanning probe microscopy methods.

The formation of s-indaceno[1,2-b:5,6-b']ditetracene and as-indaceno[2,3-b:6,7-b']ditetracene containing indenofluorene cores from a common precursor has been achieved by a dehydrogenative surface-assisted cyclization on Au(111) and confirmed by bond-resolved non-contact atomic force microscopy. On-surface generated as-indaceno[2,3-b:6,7-b']ditetracenes undergo fusion, which leads to T-shaped adducts by an intermolecular cycloaddition. The same type of cycloaddition, which has no parallel in solution chemistry, has been observed between as-indaceno[2,3-b:6,7-b']ditetracene and pentacene or octacene. These examples of surface-assisted cycloaddition provide perspectives for the rational design and synthesis of molecular nanostructures.

International audience; Surface-assisted synthesis has become a powerful approach for generation of molecular nanostructures, which could not be obtained via traditional solution chemistry. Nowadays there is an intensive search for reactions that could proceed on flat surfaces in order to boost the versatility and applicability of synthesized nano-objects. Here we propose application of atomic hydrogen combined with on-surface synthesis in order to tune the reaction pathways. We demonstrate that atomic hydrogen could be widely applied: (1) as a cleaning tool, which allows removal of halogen residues from the surface after Ullmann couplings/polymerization, (2) by reaction with surface organometallics to provide stable hydrogenated species, and (3) as a reagent for debromination or desulfurization of adsorbed species.

Nanographenes on oxides The growth of nanographene islands and ribbons on metal surfaces can be accomplished on single-crystal metal surfaces through carbon-carbon coupling reactions, but the surfaces of oxides do not assist these reactions. Kolmer et al. show that fluorinated aryl groups can be coupled to form nanographenes on the rutile surface of titanium oxide. The fluorine substitution of the aryl groups was selected so that as the carbon-fluorine bonds were thermally activated, a stepwise process sequentially added aromatic rings around a central aryl group until it was completely substituted. Science , this issue p. 57

A unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on‐surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on‐surface annealing. The structure of the generated acenes has been visualized by high‐resolution non‐contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements.

A nanographene formed by the fusion of 22 benzene rings has been prepared by combining an in-solution Pd-catalyzed cycloaddition reaction and on-surface Au-promoted cyclodehydrogenation. The structure and electronic properties of the resulting three-fold symmetric CH molecule have been characterized by scanning probe microscopy with atomic resolution and corroborated by theoretical modelling., This research was supported by the National Science Center, Poland (2017/26/E/ST3/00855), the EU project PAMS (610446), the Agencia Estatal de Investigacion (MAT2016-78293-C6-3-R, MAT2016-78293-C6-4-R, FIS2017-83780-P and CTQ2016-78157-R), the Xunta de Galicia (Centro singular de investigacion de Galicia, acreditacion 2016-2019, ED431G/09) and the European Regional Development Fund (ERDF). R. Z. acknowledges support received from the National Science Center, Poland (2017/24/T/ST5/00262).

Graphene nanoribbons (GNRs) and their derivatives attract growing attention due to their excellent electronic and magnetic properties as well as the fine-tuning of such properties that can be obtained by heteroatom substitution and/or edge morphology modification. Here, we introduce graphene nanoribbon derivatives—organometallic hybrids with gold atoms incorporated between the carbon skeleton and side Cl atoms. We show that narrow chlorinated 5-AGNROHs (armchair graphene nanoribbon organometallic hybrids) can be fabricated by on-surface polymerization with omission of the cyclodehydrogenation reaction by a proper choice of tailored molecular precursors. Finally, we describe a route to exchange chlorine atoms connected through gold atoms to the carbon skeleton by hydrogen atom treatment. This is achieved directly on the surface, resulting in perfect unsubstituted hydrogen-terminated GNRs. This will be beneficial in the molecule on-surface processing when the preparation of final unsubstituted hydrocarbon structure is desired., This work was supported by the National Science Center, Poland (2019/35/B/ST5/02666), and the EU Project PAMS (610446). A.G.L., D.S.P., and P.B. acknowledge the Spanish Agencia Estatal de Investigación (Grants MAT2016-78293-C6-4-R, PID2019-107338RB-C66, and FIS2017-83780-P), Dep. Educación of the Basque Government and UPV/EHU (Grant IT-756-13), and the European Union (EU) through Horizon 2020 (FET-Open project SPRING Grant 863098) for support.

Miniaturization of electronic circuits into the single-atom level requires novel approaches to characterize transport properties. Due to its unrivaled precision, scanning probe microscopy is regarded as the method of choice for local characterization of atoms and single molecules supported on surfaces. Here we investigate electronic transport along the anisotropic germanium (001) surface with the use of two-probe scanning tunneling spectroscopy and first-principles transport calculations. We introduce a method for the determination of the transconductance in our two-probe experimental setup and demonstrate how it captures energy-resolved information about electronic transport through the unoccupied surface states. The sequential opening of two transport channels within the quasi-one-dimensional Ge dimer rows in the surface gives rise to two distinct resonances in the transconductance spectroscopic signal, consistent with phase-coherence lengths of up to 50 nm and anisotropic electron propagation. Our work paves the way for the electronic transport characterization of quantum circuits engineered on surfaces., This work was supported by the FP7 FET-ICT “Planar Atomic and Molecular Scale devices” (PAMS) project (funded by the European Commission under contract number 610446), the Polish Ministry for Science and Higher Education from financial resources for science in 2013–2017 granted for an international co-financed project (contract number 2913/7.PR/2013/2) and the Spanish Ministerio de Economía y Competitividad (MINECO) (Grant Numbers MAT2016-78293-C6-4-R and FIS2017-83780-P). M.K. acknowledges financial support received from the Polish Ministry of Science and Higher Education, contract number 0341/IP3/2016/74.

Synthesis of a triporous nanographene with 102 sp2 carbon atoms by combining solution and surface chemistry., The synthesis of porous nanographenes is a challenging task for solution chemistry, and thus, on-surface synthesis provides an alternative approach. Here, we report the synthesis of a triporous nanographene with 102 sp2 carbon atoms by combining solution and surface chemistry. The carbon skeleton was obtained by Pd-catalyzed cyclotrimerization of arynes in solution, while planarization of the molecule was achieved through two hierarchically organized on-surface cyclodehydrogenation reactions, intra- and inter-blade. Remarkably, the three non-planar [14]annulene pores of this nanographene further evolved at higher temperatures showing interesting intra-porous on-surface reactivity.

Understanding the mechanisms involved in the covalent attachment of organic molecules to surfaces is a major challenge for nanotechnology and surface science. On the basis of classical organic chemistry mechanistic considerations, key issues such as selectivity and reactivity of the organic adsorbates could be rationalized and exploited for the design of molecular-scale circuits and devices. Here we use tris(benzocyclobutadieno)triphenylene, a singular Y-shaped hydrocarbon containing antiaromatic cyclobutadienoid rings, as a molecular probe to study the reaction of polycyclic conjugated molecules with atomic scale moieties, dangling-bond (DB) dimers on a hydrogen-passivated Ge(001):H surface. By combining molecular design, synthesis, scanning tunneling microscopy and spectroscopy (STM/STS) and computational modeling, we show that the attachment involves a concerted [4+2] cycloaddition reaction that is completely site-selective and fully reversible. This selectivity, governed by the bond alternation induced by the presence of the cyclobutadienoid rings, allows for the control of the orientation of the molecules with respect to the surface DB-patterning. We also demonstrate that by judicious modification of the electronic levels of the polycyclic benzenoid through substituents, the reaction barrier height can be modified. Finally, we show that after deliberate tip-induced covalent bond cleavage, adsorbed molecules can be used to fine tune the electronic states of the DB dimer. This power to engineer deliberately the bonding configuration and electronic properties opens new perspectives for creating prototypical nanoscale circuitry., This work was supported by the FP7 FET-ICT “Planar Atomic and Molecular Scale devices” (PAMS) project (funded by the European Commission under contract no. 610446), by the Polish Ministry for Science and Higher Education from financial resources for science in 2013–2017 granted for an international co-financed project (contract no. 2913/7.PR/2013/2) and by projects CTQ2016-78157-R, MAT2016-78293-C6-3R and MAT2016-78293-C6-4R (AEI/FEDER, UE). EG, DP and DP are grateful for financial support from the Xunta de Galicia (Centro singular de investigación de Galicia accreditation 2016–2019, ED431G/09) and the European Union (European Regional Development Fund-ERDF).

This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence., Dangling bond (DB) arrays on Si(001):H and Ge(001):H surfaces can be patterned with atomic precision and they exhibit complex and rich physics making them interesting from both technological and fundamental perspectives. But their complex behavior often makes scanning tunneling microscopy (STM) images difficult to interpret and simulate. Recently it was shown that low-temperature imaging of unoccupied states of an unpassivated dimer on Ge(001):H results in a symmetric butterfly-like STM pattern, despite that the equilibrium dimer configuration is expected to be a bistable, buckled geometry. Here, based on a thorough characterization of the low-bias switching events, we propose a new imaging model featuring a dynamical two-state rate equation. This model allows us to reproduce the features of the observed symmetric empty-state images which strongly corroborates the idea that the patterns arise due to fast switching events and provides insight into the relation between the tunneling current and switching rates. Our new imaging model is general and can be applied to other systems that exhibit rapid fluctuations during STM experiments., The work is funded by the European Commission under the FP7 FET-ICT “Planar Atomic and Molecular Scale devices” (PAMS) project (Contract No. 610446) and National Science Centre, Poland (2014/15/D/ST3/02975). ME, TF, and DSP also acknowledge support from the Spanish Ministerio de Economía y Competitividad (MINECO) (Grant No. MAT2013-46593-C6-2-P) and the Basque Dep. de Educación and the UPV/EHU (Grant No. IT-756-13). MK acknowledges financial support received from the Foundation for Polish Science (FNP). RZ acknowledges support received from KNOW (scholarship KNOW/44/SS/RZ/2015)., We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).

Transformations of molecular structures formed by perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules on a rutile TiO2(110) surface are studied with low-temperature scanning tunnelling microscopy. We demonstrate that metastable molecular assemblies transform into differently ordered structures either due to additional energy provided by thermal annealing or when the influence of intermolecular forces is increased by the enlarged amount of deposited molecules. Proper adjustment of molecular coverage and substrate temperature during deposition allows for fabrication of desired assemblies. Differences between PTCDA/TiO2(110) and PTCDA/TiO2(011) systems obtained through identical experimental procedures are discussed.

The on-surface synthesis of nonacene has been accomplished by dehydrogenation of an air-stable partially saturated precursor, which could be aromatized by using a combined scanning tunnelling and atomic force microscope (STM/AFM) tip as well as by on-surface annealing. This transformation allowed the in-detail analysis of the electronic properties of nonacene molecules physisorbed on Au(111) by scanning tunnelling spectroscopy (STS) measurements, which were corroborated by density theory functional (DFT) calculations thus confirming the spatial mapping of its molecular orbitals. Furthermore, the thermally-induced dehydrogenation uncovered the isomerisation of intermediate dihydrononacene species, which allowed for their in-depth structural and electronic characterization.

cited By 18; International audience; Fabrication of single-molecule logic devices requires controlled manipulation of molecular states with atomic-scale precision. Tuning molecule–substrate coupling is achieved here by the reversible attachment of a prototypical planar conjugated organic molecule to dangling bonds on the surface of a hydrogenated semiconductor. We show that the ground electronic state resonance of a Y-shaped polyaromatic molecule physisorbed on a defect-free area of a fully hydrogenated surface cannot be observed by scanning tunneling microscopy (STM) measurements because it is decoupled from the Ge bulk states by the hydrogen-passivated surface. The state can be accessed by STM only if the molecule is contacted with the substrate by a dangling bond dimer. The reversibility of the attachment processes will be advantageous in the construction of surface atomic-scale circuits composed of single-molecule devices interconnected by the surface dangling bond wires.

Graphical abstractDisplay Omitted HighlightsWe prepare and characterize fully passivated Ge(001)-(2i?1):H surface.We describe a protocol for dimer-by-dimer STM tip-induced hydrogen desorption.Atomically precise dangling bond (DB) logic gate is successfully constructed.Feasibility of DB logic gate implementation on hydrogenated Ge is demonstrated. We describe a complete protocol for atomically precise dangling bond (DB) logic gate construction on a hydrogenated Ge(001):H surface. Starting from the preparation of the reconstructed Ge(001) surface followed by its passivation with hydrogen atoms we end up with the platform for scanning tunneling microscopy (STM) atomic-scale lithography. Finally with the use of dimer-by-dimer STM tip-induced hydrogen desorption from the Ge(001)-(2i?1):H surface the DB nanostructures of pre-designed form are fabricated. Furthermore, the STM tip manipulation provides the control over the buckling phase of a single DB dimer incorporated into the DB logic gate structure, which is of crucial importance for the final electronic properties of the system. Our results prove feasibility of DB atomic scale logic gate implementation on the passivated semiconductor surfaces.

cited By 0; International audience; Direct characterization of planar atomic or molecular scale devices and circuits on a supporting surface by multi-probe measurements requires unprecedented stability of single atom contacts and manipulation of scanning probes over large, nanometer scale area with atomic precision. In this work, we describe the full methodology behind atomically defined two-probe scanning tunneling microscopy (STM) experiments performed on a model system: dangling bond dimer wire supported on a hydrogenated germanium (0 0 1) surface. We show that 70 nm long atomic wire can be simultaneously approached by two independent STM scanners with exact probe to probe distance reaching down to 30 nm. This allows direct wire characterization by two-probe I–V characteristics at distances below 50 nm. Our technical results presented in this work open a new area for multi-probe research, which can be now performed with precision so far accessible only by single-probe scanning probe microscopy (SPM) experiments.

Titanium dioxide is one of the most frequently studied metal oxides, and its (110) rutile surface serves as a prototypical model for the surface science of such materials. Recent studies have also shown that the (011) surface is relatively easy for preparation in ultra-high vacuum (UHV) and that both the (110) and (011) surfaces could be precisely characterized using scanning tunneling microscopy (STM). The supramolecular self-assembly of organic molecules on the surfaces of titanium dioxide plays an important role in nanofabrication, and it can control the formation and properties of nanostructures, leading to wide range of applications covering the fields of catalysis, coatings and fabrication of sensors and extends to the optoelectronic industry and medical usage. Although the majority of experiments and theoretical calculations are focused on the adsorption of relatively small organic species, in recent years, there has been increasing interest in the properties of larger molecules that have several aromatic rings in which functional units could also be observed. The purpose of this review is to summarize the achievements in the study of single polycyclic molecules and thin layers adsorbed onto the surfaces of single crystalline titanium dioxide over the past decade.

Present-day electronic technology, based to a large extent on silicon fabricated devices, is surely approaching size limitations arising from quantum effects. The effort to achieve rapid development of electronic devices requires implementation of entirely new ideas that will allow existing technological constraints to be overcome. Among a wide range of concepts, utilization of single organic molecules, acting as active blocks performing logic operations, appears as one of the most appealing and is based on the state-of-the-art use of modern nanotechnology. In this short review, we offer a selection of recent experiments addressing milestones of single-molecule computing devices technology. Along with discussion of the latest achievements in atomic- and molecular-scale technologies, we discuss their future perspectives, challenges, and still unsolved issues standing in the way of practical implementation of single-molecule devices. Concept of a single-molecule electronic device. The molecule is equipped with various functional groups responsible for logic operations and bonding to an underlying surface and metallic nanoelectrodes. The monomolecular nanodevice is deposited on a thin buffer layer assuring the decoupling from a (semi-)conducting substrate.

The adsorption of individual Violet Lander molecules self-assembled on the c(8x2) reconstructed InSb(001) surface in its native form and on the surface passivated with one to three monolayers of KBr is investigated by means of low-temperature scanning tunneling microscopy (STM). Preferred adsorption sites of the molecules are found on flat terraces as well as at atomic step edges. For molecules immobilized on flat terraces, several different conformations are identified from STM images acquired with submolecular resolution and are explained by the rotation of the 3,5-di-tert-butylphenyl groups around sigma bonds, which allows adjustment of the molecular geometry to the anisotropic substrate structure. Formation of ordered molecular chains is found at steps running along substrate reconstruction rows, whereas at the steps oriented perpendicularly no intermolecular ordering is recorded. It is also shown that the molecules deposited at two or more monolayers of the epitaxial KBr spacer do not have any stable adsorption sites recorded with STM. Prospects for the manipulation of single molecules by using the STM tip on highly anisotropic substrates are also explored, and demonstrate the feasibility of controlled lateral displacement in all directions.

The structure of terephthalic acid (TPA) molecules adsorbed on rutile TiO2(110)-(1 × 1) has been investigated by scanning tunneling microscopy (STM). Molecularly resolved STM images show formation of monolayer with the tendency of TPA molecules to form dimer rows along the [001] substrate direction. The ability to image functional groups by the STM tip, and single molecule diffusion are demonstrated. The quality and stability of the monolayer are tested, including the resistance against air exposure. On the basis of results obtained, the model of TPA adsorption on the rutile TiO2(110)-(1 × 1) is proposed.

Controlling the strength of the coupling between organic molecules and single atoms provides a powerful tool for tuning electronic properties of single-molecule devices. Here, using scanning tunneling microscopy and spectroscopy (STM/STS) supported by theoretical modeling, we study the interaction of a planar organic molecule (trinaphthylene) with a hydrogen-passivated Ge(001):H substrate and a single dangling bond quantum dot on that surface. The electronic structure of the molecule adsorbed on the hydrogen-passivated surface is similar to the gas phase structure and the measurements show that HOMO and LUMO states contribute to the STM filled and empty state images, respectively. Furthermore, we show that the electronic properties are not significantly affected when the molecule is attached to the single dangling bond, which is in contrast with the strong interaction of the molecule with a dangling bond dimer. Our results show that the dangling bond quantum dots could stabilize organic molecules on a hydrogenated semiconductor without affecting their originally designed gas phase electronic properties. Together with the ability to laterally manipulate the molecules on the surface, this will be advantageous in the construction of single-molecule devices, where the coupling and positioning of the molecules on the substrate could be tuned by a proper design of the surface quantum dot arrays, comprising both single and dimerized dangling bonds., This work was supported by the Polish Ministry for Science and Higher Education, contract no. 0322/IP3/2013/72. The experiment was carried out using equipment purchased with financial support from the European Regional Development Fund within the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08). ME, AGL and DSP acknowledge funding by the FP7 FET-ICT “Planar Atomic and Molecular Scale devices” (PAMS) project (funded by the European Commission under contract No. 610446), the Spanish Ministerio de Economia y Competitividad (MINECO) (Grant No. MAT2013-46593-C6-2-P) and the Basque Department of Education and the UPV/EHU (Grant No. IT-756-13). MK acknowledges financial support received from the Foundation for Polish Science (FNP). SG acknowledges the support from the National Science Centre, Poland (2014/15/D/ST3/02975). AME acknowledges the MINECO (Grant No. CTQ2013-42106-P). HK acknowledges the A*STAR Computational Resource Centre (A*CRC) for the computational resources and support. RZ acknowledges support received from KNOW (scholarship KNOW/44/SS/RZ/2015).

The ordering of zinc containing porphyrin molecules on surface of rutile $TiO_{2}$(110)-(1 $\times$ 1) has been investigated using scanning tunneling microscopy (STM) in ultra-high vacuum at room temperature. It is demonstrated that a carboxylic group (COOH) has a profound impact on the immobilization of the molecules. At coverages below 0.1 monolayer only molecules equipped with the group COOH could be anchored to the surface and imaged with STM. At higher coverage both species, with and without the carboxyl substituent, assemble into ordered structures, forming complete monolayers. It is found, however, that the rhomboid unit cells of these structures exhibit differences in size.

cited By 6; International audience; One of the key challenges in the construction of atomic-scale circuits and molecular machines is to design molecular rotors and switches by controlling the linear or rotational movement of a molecule while preserving its intrinsic electronic properties. Here, we demonstrate both the continuous rotational switching and the controlled step-by-step single switching of a trinaphthylene molecule adsorbed on a dangling bond dimer created on a hydrogen-passivated Ge(001):H surface. The molecular switch is on-surface assembled when the covalent bonds between the molecule and the dangling bond dimer are controllably broken, and the molecule is attached to the dimer by long-range van der Waals interactions. In this configuration, the molecule retains its intrinsic electronic properties, as confirmed by combined scanning tunneling microscopy/spectroscopy (STM/STS) measurements, density functional theory calculations, and advanced STM image calculations. Continuous switching of the molecule is initiated by vibronic excitations when the electrons are tunneling through the lowest unoccupied molecular orbital state of the molecule. The switching path is a combination of a sliding and rotation motion over the dangling bond dimer pivot. By carefully selecting the STM conditions, control over discrete single switching events is also achieved. Combined with the ability to create dangling bond dimers with atomic precision, the controlled rotational molecular switch is expected to be a crucial building block for more complex surface atomic-scale devices.

Construction of single-molecule electronic devices requires the controlled manipulation of organic molecules and their properties. This could be achieved by tuning the interaction between the molecule and individual atoms by local “on-surface” chemistry, i.e., the controlled formation of chemical bonds between the species. We demonstrate here the reversible attachment of a planar conjugated polyaromatic molecule to a pair of unpassivated dangling bonds on a hydrogenated Ge(001):H surface via a Diels–Alder [4+2] addition using the tip of a scanning tunneling microscope (STM). Due to the small stability difference between the covalently bonded and a nearly undistorted structure attached to the dangling bond dimer by long-range dispersive forces, we show that at cryogenic temperatures the molecule can be switched between both configurations. The reversibility of this covalent bond forming reaction may be applied in the construction of complex circuits containing organic molecules with tunable properties., This work was supported by the National Science Centre, Poland (2014/15/D/ST3/02975). The experiment was carried out using equipment purchased with financial support from the European Regional Development Fund within the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08). ME, AGL and DSP acknowledge funding by the FP7 FET-ICT “Planar Atomic and Molecular Scale devices” (PAMS) project (funded by the European Commission under contract no. 610446), the Spanish Ministerio de Economia y Competitividad (MINECO) (Grant No. MAT2013-46593-C6-2-P) and the Basque Department of Education and the UPV/EHU (Grant No. IT-756-13). MK acknowledges financial support received from the Foundation for Polish Science (FNP). SG acknowledges the support from the Polish Ministry for Science and Higher Education, contract no. 0322/IP3/2013/72. AME acknowledges the MINECO (Grant No. CTQ2013-42106-P). HK acknowledges the A*STAR Computational Resource Centre (A*CRC) for the computational resources and support. CJ acknowledges the MANA NEXT financial support during this work. RZ acknowledges support received from KNOW (scholarship KNOW/44/SS/RZ/2015).

To explore the origin of the Fermi level pinning in germanium we investigate the Ge(001) and Ge(001):H surfaces. The absence of relevant surface states in the case of Ge(001):H should unpin the surface Fermi level. This is not observed. For samples with donors as majority dopants the surface Fermi level appears close to the top of the valence band regardless of the surface structure. Surprisingly, for the passivated surface it is located below the top of the valence band allowing scanning tunneling microscopy imaging within the band gap. We argue that the well known electronic mechanism behind band bending does not apply and a more complicated scenario involving ionic degrees of freedom is therefore necessary. Experimental techniques involve four point probe electric current measurements, scanning tunneling microscopy and spectroscopy., Comment: 5 pages, 4 figures

This work is licensed under a Creative Commons Attribution 4.0 International License., We present a combined experimental and theoretical study of the electronic properties of close-spaced dangling-bond (DB) pairs in a hydrogen-passivated Si(001):H p-doped surface. Two types of DB pairs are considered, called >cross> and >line> structures. Our scanning tunneling spectroscopy (STS) data show that, although the spectra taken over different DBs in each pair exhibit a remarkable resemblance, they appear shifted by a constant energy that depends on the DB-pair type. This spontaneous asymmetry persists after repeated STS measurements. By comparison with density functional theory (DFT) calculations, we demonstrate that the magnitude of this shift and the relative position of the STS peaks can be explained by distinct charge states for each DB in the pair. We also explain how the charge state is modified by the presence of the scanning tunneling microscopy (STM) tip and the applied bias. Our results indicate that, using the STM tip, it is possible to control the charge state of individual DBs in complex structures, even if they are in close proximity. This observation might have important consequences for the design of electronic circuits and logic gates based on DBs in passivated silicon surfaces., This work is funded by the FP7 FET-ICT “Planar Atomic and Molecular Scale devices” (PAMS) project (funded by the European Commission under contract No. 610446). ME, TF, AGL and DSP also acknowledge support from the Spanish Ministerio de Economía y Competitividad (MINECO) (Grant No. MAT2013-46593-C6-2-P) and the Basque Dep. de Educación and the UPV/EHU (Grant No. IT-756-13). MK acknowledges financial support received from the Polish National Science Centre for preparation of his PhD dissertation (decision number: DEC-2013/08/T/ST3/00047) and from the Foundation for Polish Science (FNP).

We show that a dangling bond (DB) dimer on Ge(001):H exhibits a dynamical behavior when the empty states are imaged with scanning tunneling microscopy (STM) at liquid helium temperature. Large amplitude Ge atom vibrations are decisive in facilitating a specific appearance of the structure in the STM images. The underlying mechanism is unraveled using a theoretical model and calculations within the density functional theory framework. Furthermore, we demonstrate the ability to induce controlled switching of the DB dimer with noncontact atomic force microscope and the stabilizing role of the dimer-dimer interaction.

Based on scanning tunneling microscopy experiments, we show that the covalent coupling of aryl halide monomers on the rutile TiO2(011)-(2 \times 1) surface is controlled by the density of surface hydroxyl groups. The efficiency of the polymerization reaction depends on the level of surface hydroxylation, but the presence of hydroxyl groups is also essential for the reaction to occur.

The structure of a hydrogenated Ge(001) surface is examined by means of non-contact atomic force microscopy (NC-AFM) at 5 K. Three-dimensional force spectroscopy of a bond defect surrounded by a passivated area shows a qualitatively distinct spatial dependence of the interaction. The force curve over a defect has a range where the short-range attractive force is detected, whereas over the hydrogen-terminated bonds, it is missing: atomic-scale imaging of hydrogens is possible only in the repulsive range. Thus, NC-AFM allows for clear determination of the different chemical activities of the defect compared to the surrounding passivated area.

cited By 17; International audience; The design and construction of the first prototypical QHC (Quantum Hamiltonian Computing) atomic scale Boolean logic gate is reported using scanning tunnelling microscope (STM) tip-induced atom manipulation on an Si(001):H surface. The NOR/OR gate truth table was confirmed by dI/dU STS (Scanning Tunnelling Spectroscopy) tracking how the surface states of the QHC quantum circuit on the Si(001):H surface are shifted according to the input logical status.

cited By 21; International audience; Specific surfaces allowing the ultra-high vacuum (UHV) creation of electronic interconnects and atomic nanostructures are required for the successful development of novel nanoscale electronic devices. Atomically flat and reconstructed Si(0 0 1):H surfaces are serious candidates for that role. In this work such Si:H surfaces were prepared in a cleanroom environment on 200 mm silicon wafers with a hydrogen bake and were subsequently bonded together to ensure the surface protection, and allow their transportation and storage for several months in air. Given the nature of the bonding, which was hydrophobic with weak van der Waals forces, we were then able to de-bond them in UHV. We show that the quality of the de-bonded Si:H surface enables the “at will” construction of sophisticated and complex dangling bond (DB) nanostructures by atomically precise scanning tunneling microscope (STM) tip induced desorption of hydrogen atoms. The DB structures created on slightly doped Si:H samples were characterized by scanning tunneling microscopy and spectroscopy (STM/STS) performed at 4 K. Our results demonstrate that DB nanostructures fabricated on UHV de-bonded Si(0 0 1):H wafers could be directly incorporated in future electronics as interconnects and parts of nanoscale logic circuits.

On-surface polymerization is feasible on semiconducting surfaces. Thermally triggered covalent coupling of 10,10′-dibromo-9,9′-bianthryl molecules on the TiO2(011)-(2×1) surface is demonstrated. The result paves the way for application of the thermally driven on-surface polymerization on semiconducting surfaces and indicates that methods based on such a reaction are more universal than previously thought.

We report on studies concerning individual trinaphthylene molecules (Y molecules) deposited and anchored on the hydrogenated Ge(001):H surface. The characterization of single Y molecules has been performed by means of cryogenic temperature STM imaging using conventional STM tungsten tips and tuning fork-based sensors. In the latter case, a qPlus sensor facilitated simultaneous STM and NC-AFM measurements and thus molecular states were probed by both tunneling current and atomic forces concurrently. We show that the molecules are physisorbed, thus weakly interacting with the substrate. Contrary to the measurements on hydrogenated silicon, for planar aromatic molecules on the hydrogenated germanium, both empty and filled molecular states could be probed by STM.

Atomically precise dangling-bond (DB) lines are constructed dimer-by-dimer on a hydrogen-passivated Ge(001)-(2$\ifmmode\times\else\texttimes\fi{}$1):H surface by an efficient scanning tunneling microscope (STM) tip-induced desorption protocol. Due to the smaller surface band gap of the undoped Ge(001) substrate compared to Si(001), states associated with individually created DBs can be characterized spectroscopically by scanning tunneling spectroscopy (STS). Corresponding $dI/dV$ spectra corroborated by first-principle modeling demonstrate that DB dimers introduce states below the Ge(001):H surface conduction band edge. For a DB line parallel to the surface reconstruction rows, the DB-derived states near the conduction band edge shift to lower energies with increasing number of DBs. The coupling between the DB states results in a dispersive band spanning 0.7 eV for an infinite DB line. For a long DB line perpendicular to the surface reconstruction rows, a similar band is not formed since the interdimer coupling is weak. However, for a short DB line (2--3 DBs) perpendicular to the reconstruction rows a significant shift is still observed due to the more flexible dimer buckling.

Adsorption and self-assembly of large π-conjugated 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) molecules on rutile TiO(2)(110) surface have been investigated using a combination of high-resolution scanning tunneling microscopy (STM), low-energy electron diffraction, and density functional theory calculations with inclusion of Grimme treatment of the dispersion forces (DFT-D). Evolution of the STM images as a function of PTCDA coverage is caused by transition of the adsorption mode from physisorbed single adspecies and meandering stripes into spontaneously ordered chemisorbed molecular assemblies. This change in the adsorption fashion is accompanied by significant bending of the intrinsically flat, yet elastic, PTCDA molecule, which allows for strong electronic coupling of the dye adspecies with the TiO(2) substrate. Extensive DFT-D modeling has revealed that adsorption is controlled by interfacial and intermolecular dispersion forces playing a dominant role in the adsorption of single PTCDA species, their self-organization into the meandering stripes, and at the monolayer coverage acting collectively to surmount the chemisorption energy barrier associated with the molecule bending. Analysis of the resulting density of states has revealed that alignment of the energy levels and strong electronic coupling at the PTCDA/TiO(2) interface are beneficial for dye sensitization purposes.

Helicenes are carbon-rich materials with an ordered structure that possess interesting and remarkable electronic, chiroptical, mechanical and magnetic properties either in bulk or as single molecule devices. We report on the first attempt to anchor and organize {[}11]anthrahelicene ({[}11]AH) molecules onto a metal oxide surface. Molecular structures obtained on the (110) and (011) faces of rutile titania are characterized using scanning probe microscopy. On the (110) surface, {[}11]AH molecules form islands that are comprised of small domains of two types that are mirror reflections of each other and have an ordered quasi-hexagonal lattice. In contrast, molecules form unordered clusters on the (011) surface. Single molecules on both faces are imaged with sub-molecular resolution. (C) 2012 Elsevier B.V. All rights reserved

The demand to discover new, alternative solutions in future electronics has currently focused the attention on the possible use of single atomic wires and conductive mesa pads as components in electronic circuits. For such applications, we need a good knowledge of structural and electronic properties of 1D and 2D conductive nanostructures fabricated on large and/or moderate band gap semiconductor surfaces. This chapter is reporting on case studies invoking preparation of well organized atomic wires and 2D conductive pads by self-assembly of gold atoms on clean, reconstructed Ge(001) surface. Structural characterization of the fabricated nanostructures was performed with atomic resolution on low temperature scanning tunneling microscope (LT-STM) and multi-probe STM designed by Omicron Nanotechnology GmbH. Using the far field, high resolution scanning electron microscope (SEM), the pre-prepared nanostructures were identified after an UHV transfer into a multi-probe station of the system, and their surface conductance was measured with two STM probes. The conductance dependence as a function of the inter-probe distance confirmed 2D (surface) character of the Au rich nanostructures assembled on Ge(001).

The self-assembling of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules deposited on an InSb(001) c(8 $\times$ 2) surface at sub-monolayer quantities has been investigated at low temperature (77 K) using scanning tunnelling microscopy. Sub-molecular resolution was obtained on PTCDA molecules. The results reveal that individual PTCDA molecules are arranged on the substrate in chains parallel to the [110] crystallographic direction, correlated with characteristic features of the low temperature InSb(001) c(8 $\times$ 2) surface electronic structure. A structural model for PTCDA molecules adsorbed on InSb is proposed.

Periodic DFT calculations were used to explore structural properties of terephthalic acid (TPA) deposited on the rutile TiO2(110)-(1 x 1) surface at low and high coverage. Theoretical results were compared with scanning tunneling microscopy imaging data. At low loading the TPA molecules adsorb dissociatively as discrete entities adopting a flat-lying plane-on geometry. The resultant terephthalic anion is attached to the surface by two covalent bonds between the carboxylic oxygens and the 5-fold coordinated Ti atoms with an additional stabilization due to the hydrogen bond formation with the adjacent surface hydroxyls. When the saturation coverage is achieved, a well-ordered monolayer of the vertically oriented molecules is formed. In both cases the TPA admolecules bind to the surface via carboxylic groups as terephthalic anions. Formation of dimers results from the formation of hydrogen bonds between the adjacent TPA molecules. To elucidate the reactivity of the functionalized surface, we deposited zinc formate ions on top of the compact TPA monolayer. Calculations showed that the anchoring properties of the TiO2/TPA system are not perturbed by the dimer formation, auguring well for its prospective application as a promising chemically functionalized surface for on-top growth of metal organic frameworks.

Self-assembly of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules on a c (8 × 2) reconstructed InSb(0 0 1) surface as well as on an asymmetrical (1 × 3) InSb(0 0 1) is investigated by means of high resolution scanning tunneling microscopy. The formation of well-ordered one-dimensional molecular lines is observed at low coverage on a former face, whereas on the latter molecules are found as isolated entities. At room temperature spontaneous hopping between energetically similar sites is observed within the lines. At a monolayer coverage flat molecular surface is obtained for both reconstructions, on the c (8 × 2) the layer is composed of neighboring one-dimensional lines, on the (1 × 3) molecular structure does not reveal any long range order. Application of the special experimental procedure provides direct comparison of molecule behavior on both reconstructions of the same sample.

An extensive scanning tunneling microscopy (STM) study of adsorption of submonolayer coverages of PTCDA molecules on the $c(8\ifmmode\times\else\texttimes\fi{}2)$ reconstructed InSb(001) surface is presented together with ab initio density functional theory (DFT) calculations. Our DFT calculations explain the variety of adsorption sites seen on the experimental STM images. In particular, we prove that the molecules are oriented with their long axes along the [110] direction. Calculated STM images of the molecule agree well with the high-resolution STM images obtained at the temperature of 77 K. We find that molecules form four covalent bonds between edge oxygen atoms of the PTCDA and In atoms of the surface. We also study in detail their diffusion mechanism and explain their ability to form experimentally observed chains along the [110] direction.

Molecular reorientation: Copper (II) phthalocyanines are investigated on the rutile TiO2(011)-(2×1) surface. At first, a quasi-ordered wetting monolayer is formed, after which well-ordered islands composed of flat-lying molecules appear (picture, left). Post-deposition thermal annealing at 200 °C leads to a complete reorientation of the second layer so that the molecules adopt an upright configuration (picture, right).