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To optimize the performance of catalytic materials, it is paramount to elucidate the dependence of the chemical reactivity on the atomic arrangement of the catalyst surface.

The synthesis of two-dimensionally extended polycyclic heteroatomic molecules keeps attracting considerable attention. In particular, frameworks bearing planar cyclooctatetraenes (COT) moieties can display intriguing properties, including antiaromaticity. Here, we present an on-surface chemistry route to square-type porphyrin tetramers with a central COT ring, coexisting with other oligomers. This approach employing temperature-induced dehydrogenative porphyrin homocoupling in an ultrahigh vacuum environment provides access to surface-supported, unsubstituted porphyrin tetramers that are not easily achievable by conventional synthesis means. Specifically, monomeric free-base (2H-P) and Zn-metalated (Zn-P) porphines (P) were employed to form square-type free-base and Zn-functionalized tetramers on Ag(100). An atomic-level characterization by bond-resolved atomic force microscopy and scanning tunneling microscopy and spectroscopy is provided, identifying the molecular structures. Complemented by density functional theory modeling, the electronic structure is elucidated, indeed revealing antiaromaticity induced by the COT moiety. The present study thus gives access, and insights, to a porphyrin oligomer, representing both a model system for directly fused porphyrins and a potential building block for conjugated, extended two-dimensional porphyrin sheets.

The spatial arrangement of adsorbates deposited onto a clean surface in vacuum typically cannot be reversibly tuned. Here we use scanning tunneling microscopy to demonstrate that molecules deposited onto graphene field-effect transistors exhibit reversible, electrically-tunable surface concentration. Continuous gate-tunable control over the surface concentration of charged F4TCNQ molecules was achieved on a graphene FET at T = 4.5K. This capability enables precisely controlled impurity doping of graphene devices and also provides a new method for determining molecular energy level alignment based on the gate-dependence of molecular concentration. The gate-tunable molecular concentration can be explained by a dynamical molecular rearrangement process that reduces total electronic energy by maintaining Fermi level pinning in the device substrate. Molecular surface concentration in this case is fully determined by the device back-gate voltage, its geometric capacitance, and the energy difference between the graphene Dirac point and the molecular LUMO level.

Porphyrin-based oligomers were synthesized from the condensation of adsorbed 4-benzaldehyde-substituted porphyrins through the formation of CC linkages, following a McMurry-type coupling scheme. Scanning tunneling microscopy, non-contact atomic force microscopy, and X-ray photoelectron spectroscopy data evidence both the dissociation of aldehyde groups and the formation of CC linkages. Our approach provides a path for the on-surface synthesis of porphyrin-based oligomers coupled by CC bridges - as a means to create functional conjugated nanostructures.

The ability to tune the electronic properties of molecular arrays is an important step in the development of molecule-scale electronic devices. However, control over internal device charge distributions by tuning interactions between molecules has proved challenging. Here, we show that gate-tunable charge patterning can occur in one-dimensional molecular arrays on graphene field-effect transistors. One-dimensional molecular arrays are fabricated using an edge-templated self-assembly process that allows organic molecules (F4TCNQ) to be precisely positioned on graphene devices. The charge configurations of the molecular arrays can be reversibly switched between different collective charge states by tuning the graphene Fermi level via a back-gate electrode. Charge pinning at the ends of the molecular arrays allows the charge state of the entire array to be controlled by adding or removing an edge molecule and changing the total number of molecules in an array between odd and even integers. Charge patterns altered in this way propagate down the array in a cascade effect, allowing the array to function as a charge-based molecular shift register. An extended multi-site Anderson impurity model is used to quantitatively explain this behaviour. One-dimensional molecular arrays on graphene field-effect transistors can be reversibly switched between different periodic charge states by tuning the graphene Fermi level via a back-gate electrode and by manipulating individual molecules, allowing them to function as a nanoscale shift register.

Aromaticity is an established and widely used concept for the prediction of the reactivity of organic molecules. However, in on-surface chemistry – where the interaction with the substrate can significantly alter the electronic and geometric structure of the adsorbates – the role of aromaticity is not understood. Here we investigate how aromaticity affects the reactivity of alkyne-substituted porphyrin molecules in cyclization and coupling reactions on a Au(111) surface. We examine and quantify the regioselectivity of the reactions by scanning tunneling microscopy and bond-resolved atomic force microscopy at the single-molecule level. Our experiments show a substantially lower reactivity of carbon atoms that are stabilized by the aromatic diaza[18]annulene pathway of free-base porphyrins. The results are corroborated by density functional theory calculations, revealing a direct correlation between aromaticity and thermodynamic stability of the reaction products. These insights will help to design and understand reactions in on-surface chemistry and heterogeneous catalysis.

Discovered exactly 200 years ago in 1821, thermoelectricity is nowadays of global interest as it allows to directly interconvert thermal and electrical energy via the Seebeck/Peltier effect, which could be exploited to enhance energy efficiency. In their seminal work, Mahan and Sofo mathematically derived the conditions for ’the best thermoelectric’ − a delta-distribution-shaped electronic transport function, where charge carriers contribute to transport only in an infinitely narrow energy interval. So far, however, only approximations to this concept were expected to really exist in nature. Here, we propose as a physical realisation of this scenario the Anderson transition in an impurity band, i.e. the transition from Anderson-localised to extended quantum states. We obtained a significant enhancement and dramatic change of the thermoelectric properties from p-type to n-type in the stoichiometric Heusler compound Fe2VAl, which we assign to a narrow region of delocalised electrons in the energy spectrum near the Fermi energy. We achieved this through an innovative approach of driving the Anderson transition via continuous disorder tuning: variable amounts of atomic defects are induced in a controlled fashion by thermal quenching from high temperatures (950 − 1380 °C). Based on our experimental electronic transport and magnetisation results, supported by Monte-Carlo and density functional theory calculations, we demonstrate a universal enhancement strategy towards colossal thermoelectric performance that is applicable to diverse material classes.

The breakdown of macrocyclic compounds is of utmost importance in manifold biological and chemical processes, usually proceeding via oxygenation-induced ring-opening reactions. Here, we introduce a surface chemical route to selectively break a prototypical porphyrin species, cleaving off one pyrrole unit and affording a tripyrrin derivative. This pathway, operational in an ultrahigh vacuum environment at moderate temperature is enabled by a distinct molecular conformation achieved via the specific interaction between the porphyrin and its copper support. We provide an atomic-level characterization of the surface-anchored tripyrrin, its reaction intermediates, and byproducts by bond-resolved atomic force microscopy, unequivocally identifying the molecular skeletons. The ring-opening is rationalized by the distortion reducing the macrocycle's stability. Our findings open a route to steer ring-opening reactions by conformational design and to study intriguing tetrapyrrole catabolite analogues on surfaces.

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., Let′s tessellate: The complex evolution of structure as a function of the chemical state is visualized by scanning probe microscopy and X‐ray photoelectron spectroscopy. With progressive deprotonation of an adsorbate on Ag(111), complex networks featuring chiral pores are revealed. Their functionality is exemplified in a polyporous network where chiral supramolecular guests form enantioselectively.

The design of organometallic complexes is at the heart of modern organic chemistry and catalysis. Recently, on-surface synthesis has emerged as a disruptive paradigm to design previously precluded compounds and nanomaterials. Despite these advances, the field of organometallic chemistry on surfaces is still at its infancy. Here, we introduce a protocol to activate the inner diacetylene moieties of a molecular precursor by copper surface adatoms affording the formation of unprecedented organocopper metallacycles on Cu(111). The chemical structure of the resulting complexes is characterized by scanning probe microscopy and X-ray photoelectron spectroscopy, being complemented by density functional theory calculations and scanning probe microscopy simulations. Our results pave avenues to the engineering of organometallic compounds and steer the development of polyyne chemistry on surfaces., The diacetylene skeletons of DNBD precursors are attacked on Cu(111) by copper adatoms resulting in the synthesis of organocopper metallacycles.

Linking the fundamental physics of band structure and scattering theory with macroscopic features such as measured temperature dependencies of electronic and thermal transport is indispensable to a thorough understanding of thermoelectric phenomena and ensures more targeted and efficient experimental research. Regarding ${\mathrm{Fe}}_{2}\mathrm{VAl}$-based compounds, experimental work has seen mostly qualitative and often speculative interpretations, preventing this class of materials from tapping their full potential when it comes to applications. In this paper, the temperature-dependent Seebeck coefficient and electrical resistivity of a set of $p$-type and $n$-type samples with the composition ${\mathrm{Fe}}_{2}{\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Ta}}_{x}{\mathrm{Al}}_{1\ensuremath{-}y}{\mathrm{Si}}_{y}$ are presented from 4 K up to 800 K as well as the Hall mobility and carrier concentration from 4 K to 520 K. We attempt a quantitative analysis of our data using a parabolic two- and three-band model and compare the model results with those from density functional theory calculations. Our findings show an increase of the band gap ${E}_{\text{g}}$ from almost zero in undoped ${\mathrm{Fe}}_{2}\mathrm{VAl}$ toward ${E}_{\text{g}}\ensuremath{\approx}0.1\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ with increasing Ta substitution, consistent with results from first-principles calculations. Due to the resulting enhancement of the Seebeck coefficient, the maximum power factor is boosted up to 10.3 mW/${\mathrm{mK}}^{2}$, which is, to the best of our knowledge, the highest value among $n$-type bulk semiconductor systems reported near room temperature up until now. We further show that for the $p$-type ${\mathrm{Fe}}_{2}{\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Ta}}_{x}\mathrm{Al}$ compounds, the dominant scattering mechanism of electrons is intrinsically different compared to the $n$-type samples, for which acoustic phonon scattering can well describe the temperature-dependent Hall mobility in a broad range of temperatures.

Full-Heusler compounds with the composition Fe2V1-XTaxAl1-YSiy have recently shown to exhibit some of thehighest power factors reported so far for bulk materials due to the band convergence and band gap opening causedby the V/Ta substitution. Therefore, the solubility limit of Ta and Si regarding the stability of the L21 phase isinvestigated in this study. The crystal structure and microstructure of a large number of samples is probed by X-raydiffraction as well as scanning electron microscopy and energy dispersive X-ray analysis. The results show that the Al/Si substitution significantly hampers the solubility of Ta within the Heusler structure. Furthermore, Fe2V0.9Ta0.1Al and Fe2V0.95Ta0.05Al0.9Si0.1 reveal nanoscale impurity precipitates in the microstructure, together with diffuse contrasts that indicate a non-equilibrium metastable state. For that reason, different annealing conditions, varying the temperature and time, have been applied to the latter and the effect on the microstructure and thermoelectric properties is investigated. It is found that additional annealing leads to further phase segregation and grain growth of the impurity precipitates, which due to their metallic-like nature have a detrimental effect on the Seebeck coefficient. They can however effectively reduce the lattice thermal conductivity if the average grain size remains below the phonon mean free path. The thermoelectric efficiency in terms of the dimensionless gure of merit ZT is increased up to ZT = 0.3 - 0.34 at 300K which is beyond the values previously reported for Fe2VAl-based bulk materials.

The use of molecular materials in solar cells and nano-electronics demands a fundamental understanding and control of their electronic properties. Particularly relevant is the molecular response to the environment, that is, the interaction with the support and adjacent molecules, as well as the influence of electrostatic gating. Here, the control of molecular level alignment and charge states of fluorinated cobalt phthalocyanines (F16CoPc) on atomically thin hexagonal boron nitride (h-BN) sheets on Cu(111) is reported using scanning tunneling microscopy (STM) and spectroscopy (STS), as well as atomic force microscopy (AFM) and complementary density functional theory (DFT) calculations. Three parameters that govern the electronic level alignment of F16CoPc orbitals are investigated: i) template-induced gating by the work function variation of the h-BN/Cu(111) substrate, ii) gating by the STM tip, and iii) screening by neighboring molecules. The interplay of these parameters influences the charge distribution in the studied molecular arrangements and thus provides the possibility to tune their physicochemical behavior, for instance, the response toward electronic or optical excitation, charge transport, or binding of axial adducts.

The photon-like behavior of electrons in graphene causes unusual confinement properties that depend strongly on the geometry and strength of the surrounding potential. We report bottom-up synthesis of atomically-precise one-dimensional (1D) arrays of point charges on graphene that allow exploration of a new type of supercritical confinement of graphene carriers. The arrays were synthesized by arranging F4TCNQ molecules into a 1D lattice on back-gated graphene, allowing precise tuning of both the molecular charge and the array periodicity. While dilute arrays of ionized F4TCNQ molecules are seen to behave like isolated subcritical charges, dense arrays show emergent supercriticality. In contrast to compact supercritical clusters, these extended arrays display both supercritical and subcritical characteristics and belong to a new physical regime termed “frustrated supercritical collapse”. Here carriers in the far-field are attracted by a supercritical charge distribution, but their fall to the center is frustrated by subcritical potentials in the near-field, similar to trapping of light by a dense cluster of stars in general relativity., Placement of charge centres with atomic precision on graphene allows exploration of new types of confinement of charge carriers. Here, the authors fabricate atomically precise arrays of point charges on graphene and observe the onset of a frustrated supercritical regime.

The vast potential of organic materials for electronic, optoelectronic and spintronic devices entails substantial interest in the fabrication of π-conjugated systems with tailored functionality directly at insulating interfaces. On-surface fabrication of such materials on non-metal surfaces remains to be demonstrated with high yield and selectivity. Here we present the synthesis of polyaromatic chains on metallic substrates, insulating layers, and in the solid state. Scanning probe microscopy shows the formation of azaullazine repeating units on Au(111), Ag(111), and h-BN/Cu(111), stemming from intermolecular homo-coupling via cycloaddition reactions of CN-substituted polycyclic aromatic azomethine ylide (PAMY) intermediates followed by subsequent dehydrogenation. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry demonstrates that the reaction also takes place in the solid state in the absence of any catalyst. Such intermolecular cycloaddition reactions are promising methods for direct synthesis of regioregular polyaromatic polymers on arbitrary insulating surfaces., This work was financially supported by the European Research Council Consolidator Grant NanoSurfs (no. 615233), the Advanced Grant (no. 694097), the Horizon 2020 research and innovation program 2D ink (no. 664878) and the National Science Foundation of China (no. 11974403 and Sino-German Project no. 51761135130). W.A. acknowledges funding by the DFG via a Heisenberg professorship. M.R., R.B., and X.F. thank the German Research Foundation (DFG) within the Cluster of Excellence “Center for Advancing Electronics Dresden (cfaed)” and EnhanceNano (No. 391979941). A.P.P. and A.Ru. thank the Cluster of Excellence "Advanced Imaging of Matter (AIM)" and Grupos Consolidados (IT1249-19). M.G. acknowledges funding by the H2020-MSCA-IF−2014 program under GA no. 658070 (2DNano).

The templated synthesis of porphyrin-based oligomers and heterosystems is of considerable interest for materials with tunable electronic gaps, photovoltaics, or sensing device elements. In this work, temperature-induced dehydrogenative coupling between unsubstituted free-base porphine units and their attachment to graphene nanoribbons on a well-defined Ag(111) support are scrutinized by bond-resolved scanning probe microscopy techniques. The detailed inspection of covalently fused porphine dimers obtained by in vacuo on-surface synthesis clearly reveals atomistic details of coupling motifs, whereby also putative reaction intermediates are identified. Moreover, the covalent attachment of porphines at preferred locations of atomically precise armchair-type graphene nanoribbons is demonstrated.

We investigate the thermally induced cyclization of 1,2-bis(2-phenylethynyl)benzene on Au(111) using scanning tunneling microscopy and computer simulations. Cyclization of sterically hindered enediynes is known to proceed via two competing mechanisms in solution: a classic C1–C6 (Bergman) or a C1–C5 cyclization pathway. On Au(111), we find that the C1–C5 cyclization is suppressed and that the C1–C6 cyclization yields a highly strained bicyclic olefin whose surface chemistry was hitherto unknown. The C1–C6 product self-assembles into discrete noncovalently bound dimers on the surface. The reaction mechanism and driving forces behind noncovalent association are discussed in light of density functional theory calculations., Research was supported by the U.S. Department of Energy Office of Basic Energy Sciences Nanomachine Program under Contract No. DE-AC02-05CH11231 (STM imaging), by the Office of Naval Research BRC Program (molecular synthesis), by the European Research Council Grants ERC-2010-AdG267374-DYNamo and ERC-2014-STG-635919-SURFINK (computational resources and surface analysis, respectively), by Spanish Grant No. FIS2013-46159-C3-1-P (simulated reaction landscape), and by Grupos Consolidados UPV/EHU del Gobierno Vasco No. IT-578-13 (simulated dimer binding energy). A.P.P. acknowledges postdoctoral fellowship support rom “Ayuda para la Especializacion de Personal Investigador ́ del Vicerrectorado de Investigacion de la UPV/EHU-2013 ́ ” and from the Spanish “Juan de la Cierva-incorporacioń ” program (IJCI-2014-20147).

Full-Heusler compounds with the composition Fe 2 V 1 − x Ta x Al 1 − y Si y have recently shown to exhibit some of the highest thermoelectric power factors reported so far among bulk materials due to the band convergence and band gap opening caused by the V/Ta substitution. Therefore, the solubility limit of Ta and Si regarding the stability of the L2 1 phase is investigated in this study. The crystal structure and microstructure of a large number of samples is probed by X-ray diffraction as well as scanning electron microscopy and energy dispersive X-ray analysis. The results show that the Al/Si substitution significantly hampers the solubility of Ta within the Heusler structure. Furthermore, Fe 2 V 0.9 Ta 0.1 Al and Fe 2 V 0.95 Ta 0.05 Al 0.9 Si 0.1 reveal nanoscale impurity precipitates in the microstructure, together with diffuse contrasts that indicate a non-equilibrium metastable state. For that reason, different annealing conditions, varying temperature and time, have been applied to the latter and the effect on the microstructure and thermoelectric properties is investigated. It is found that additional annealing leads to further phase segregation and grain growth of the impurity precipitates, which have a detrimental effect on the Seebeck coefficient due to their metallic-like nature. They can, however, effectively reduce the lattice thermal conductivity if their average size remains below the phonon mean free path. The thermoelectric efficiency in terms of the dimensionless figure of merit Z T is increased up to Z T = 0.3 − 0.34 at 300 K which is beyond the values previously reported for Fe 2 VAl -based bulk materials.

Despite the markedly low chemical reactivity of the nonpolar (110) surfaces of III–V semiconductors, the covalent functionalization of GaP(110) surfaces with perfluorophenyl azide (PFPA) molecules by a Staudinger-type reaction occurs only slightly above room temperature (325 K). Scanning tunneling microscopy observations, combined with density functional theory calculations, support the formation of stable, covalent perfluorophenyl nitride (PFPN) molecule–surface bonds, which can be described as Lewis acidic Ga-stabilized phosphine imides. π–π stacking between aromatic, electron-deficient PFPN units results in compact, commensurate 2D molecular assembly at the surface. PFPA deposition on GaP(110) at room temperature with no additional annealing leads to an intermediate phase consistent with an alternating 1D array of physisorbed and chemisorbed molecular units. This work provides a new route for covalently bonding molecular linkages to the (110) surfaces of III–V semiconductors.

Its excellent spatial resolution makes scanning probe microscopy a capable method to investigate chemical reactions at the single-molecule level and obtain fascinating and unprecedented insights into the mechanisms of chemical transformations. Particularly exciting are recent advances in atomic force microscopy that allow bond-resolved imaging and thus make the chemical identification of organic molecular reaction intermediates and products possible. In this chapter we will give an overview about recent fundamental research on reaction mechanisms and kinetics of surface-supported reactions by scanning probe microscopy. Particular emphasis will be placed on the stabilization and statistical analysis of intermediates, which provides fundamental understanding of the microscopic driving forces of complex chemical transformations of organic molecules.

Intercalation of molecules into layered materials is actively researched in materials science, chemistry, and nanotechnology, holding promise for the synthesis of van der Waals heterostructures and encapsulated nanoreactors. However, the intercalation of organic molecules that exhibit physical or chemical functionality remains a key challenge to date. In this work, we present the synthesis of heterostructures consisting of porphines sandwiched between a Cu(111) substrate and an insulating hexagonal boron nitride ( h-BN) monolayer. We investigated the energetics of the intercalation, as well as the influence of the capping h-BN layer on the behavior of the intercalated molecules using scanning probe microscopy and density functional theory calculations. While the self-assembly of the molecules is altered upon intercalation, we show that the intrinsic functionalities, such as switching between different porphine tautomers, are preserved. Such insulator/molecule/metal structures provide opportunities to protect organic materials from deleterious effects of atmospheric environment, can be used to control chemical reactions through spatial confinement, and give access to layered materials based on the ample availability of synthesis protocols provided by organic chemistry.

Nanographenes, namely polycyclic aromatic hydrocarbons (PAHs) with nanoscale dimensions (>1 nm), are atomically precise cutouts from graphene. They represent prime models to enhance the scope of chemical and physical properties of graphene through structural modulation and functionalization. Defined nitrogen doping in nanographenes is particularly attractive due to its potential for increasing the number of π-electrons, with the possibility of introducing localized antiaromatic ring elements. Herein we present azomethine ylide homocoupling as a strategy to afford internally nitrogen-doped, non-planar PAH in solution and planar nanographene on surfaces, with central pyrazine rings. Localized antiaromaticity of the central ring is indicated by optical absorption spectroscopy in conjunction with theoretical calculations. Our strategy opens up methods for chemically tailoring graphene and nanographenes, modified by antiaromatic dopants., Polyaromatic hydrocarbons can be precisely manipulated to yield ever more complex and discrete graphene analogs, such as nanographenes. Here, the authors use azomethine ylide homocoupling to insert an antiaromatic pyrazine ring into the core of a nanographene, and characterize the molecule’s unique electronic character.

International audience; Atomically thin hexagonal boron nitride (h-BN) layers on metallic supports represent a promising platform for the selective adsorption of atoms, clusters, and molecular nanostructures. Specifically, scanning tunneling microscopy (STM) studies revealed an electronic corrugation of h-BN/Cu(111), guiding the self-assembly of molecules and their energy level alignment. A detailed characterization of the h-BN/Cu(111) interface including the spacing between the h-BN sheet and its support—elusive to STM measurements—is crucial to rationalize the interfacial interactions within these systems. To this end, we employ complementary techniques including high-resolution noncontact atomic force microscopy, STM, low-energy electron diffraction, X-ray photoelectron spectroscopy, the X-ray standing wave method, and density functional theory. Our multimethod study yields a comprehensive, quantitative structure determination including the adsorption height and the corrugation of the sp2 bonded h-BN layer on Cu(111). Based on the atomic contrast in atomic force microscopy measurements, we derive a measurable–hitherto unrecognized–geometric corrugation of the h-BN monolayer. This experimental approach allows us to spatially resolve minute height variations in low-dimensional nanostructures, thus providing a benchmark for theoretical modeling. Regarding potential applications, e.g., as a template or catalytically active support, the recognition of h-BN on Cu(111) as a weakly bonded and moderately corrugated overlayer is highly relevant.

The ability to understand and control the electronic properties of individual molecules in a device environment is crucial for developing future technologies at the nanometre scale and below. Achieving this, however, requires the creation of three-terminal devices that allow single molecules to be both gated and imaged at the atomic scale. We have accomplished this by integrating a graphene field effect transistor with a scanning tunnelling microscope, thus allowing gate-controlled charging and spectroscopic interrogation of individual tetrafluoro-tetracyanoquinodimethane molecules. We observe a non-rigid shift in the molecule's lowest unoccupied molecular orbital energy (relative to the Dirac point) as a function of gate voltage due to graphene polarization effects. Our results show that electron–electron interactions play an important role in how molecular energy levels align to the graphene Dirac point, and may significantly influence charge transport through individual molecules incorporated in graphene-based nanodevices., The development of single-molecule electronics calls for precise tuning of the electronic properties of individual molecules that go beyond two-terminal control. Here, Wickenburg et al. show gate-tunable switch of charge states of an isolated molecule using a graphene-based field-effect transistor.

Layered titanate nanostructures offer promising photoelectronic properties that are subject to surface chemistry-induced morphology changes. For a systematic evaluation of the bulk and surface contributions to the photoactivity of these structures, we investigated their photoelectronic properties and in particular their dependence on the condition of the gas-solid interface. We comprehensively explored the stability of Na(2)Ti(3)O(7) nanowires and scrolled up H(2)Ti(3)O(7) nanotubes by means of transmission electron microscopy, Raman, and FT-IR spectroscopy and subjected both titanate sheet-based structures to controlled thermal activation treatment under high vacuum conditions. We found that throughout thermal annealing up to T = 870 K the structure and morphology of Na(2)Ti(3)O(7) nanowires are retained. Consistent with the significant photoluminescence emission that is attributed to radiative exciton annihilation in the bulk, UV-induced charge separation is strongly suppressed in these structures. H(2)Ti(3)O(7) nanotubes, however, undergo transformation into elongated anatase nanocrystals during annealing at temperatures TOR= 670 K. Photoexcitation experiments in O(2) atmosphere reveal that these structures efficiently sustain the separation of photogenerated charges. Trends in the abundance of trapped holes and scavenged electrons were characterized quantitatively by tracking the concentration of paramagnetic O(-) and O(2)(-) species with electron paramagnetic resonance spectroscopy EPR, respectively. An incisive analysis of these results in comparison to those obtained on airborne anatase nanocrystals underlines the critical role of surface composition and structure on charge separation and, in consequence, on the chemical utilization of photogenerated charge carriers.

Fur Festkorper mit einer raumlichen Ausdehnung von wenigen Nanometern ergeben sich oft eindeutige Zusammenhange zwischen Festkorpergestalt und bestimmten physikalischen und chemischen Eigenschaften, die hinsichtlich der Verwendung von Nanomaterialien als Funktionswerkstoffe von Bedeutung sein konnen. In diesem Artikel wird exemplarisch aufgezeigt, dass Ensembles von Nanokristallen mit einheitlicher und enger Grosenverteilung wertvolle Modellsysteme zur Aufklarung von Prozessen an der Grenzflache und im Inneren des Festkorpers sein konnen. Auch aus Sicht der Materialsynthese haben Nanokristalle mit ausgezeichneter und einheitlicher Morphologie grose Bedeutung. Dort konnen sie als elementare Bauelemente zum orientierungsabhangigen Wachstum komplexerer Nanostrukturen eingesetzt werden. Nanometer-sized solids of uniform size distribution and well-defined morphology often show a clear correspondence between shape and specific physico-chemical properties that eventually may lead to a particular functionality. This contribution is about the potential of morphologically well-defined nanocrystals to serve as model systems for the detailed exploration of chemical surface and bulk processes. Moreover, also with respect to synthesis and engineering of functional materials they are of great interest. As basic building block elements they can be utilized for the orientation dependent growth of complex nanostructures.

The photoelectronic properties of layered titanate nanostructures can be adjusted by changing the nature and bonding state of ions in the interlayer region. We studied the optical properties of titanate nanowires and nanotubes obtained after soft-chemical treatment of TiO2 anatase powders. A photoluminescence emission process originating from exciton states trapped in [TiO6] units was observed in anatase TiO2 and, with significantly enhanced intensity, in nanowires made of titanate nanosheets. On the basis of a correlation between emission intensity and the concentration of intercalated alkali ions, we conclude that protonation of the [TiO6] octahedra that constitute the titanate sheet structure suppresses radiative deactivation of trapped excitons and can be reversed by ion exchange.