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7 pags., 4 figs., Entanglement of excitons holds great promise for the future of quantum computing, which would use individual molecular dyes as building blocks of their circuitry. Studying entangled excitonic eigenstates emerging in coupled molecular assemblies in the near-field with submolecular resolution has the potential to bring insight into the photophysics of these fascinating quantum phenomena. In contrast to far-field spectroscopies, near-field spectroscopic mapping permits direct identification of the individual eigenmodes, type of exciton coupling, including excited states otherwise inaccessible in the far field (dark states). Here we combine tip-enhanced spectromicroscopy with atomic force microscopy to inspect delocalized single-exciton states of charged molecular assemblies engineered from individual perylenetetracarboxylic dianhydride (PTCDA) molecules. Hyperspectral mapping of the eigenstates and comparison with calculated many-body optical transitions reveals a second low-lying excited state of the anion monomers and its role in the exciton entanglement within the assemblies. We demonstrate control over the exciton coupling by switching the assembly charge states. Our results reveal the possibility of tailoring excitonic properties of organic dye aggregates for advanced functionalities and establish the methodology to address them individually at the nanoscale., S.C., R.C.C.F., M.Š ., and J.D. acknowledge the Czech grant agency funding no. 20-18741S and the Charles University Grant Agency project no. 910120. P.M. thanks the ERC Synergy Program (grant no. ERC-2013- SYG-610256, Nanocosmos) for financial support and the “Comunidad de Madrid” for its support to the FotoArt-CM Project (S2018/NMT-4367) through the Program of R&D activities between research groups in Technologies 2013, cofinanced by European Structural Funds.

Interplay between motion of nuclei and excitations has an important role in molecular photophysics of natural and artificial structures. Here we provide a detailed analysis of coupling between quantized librational modes (librons) and charged excited states (trions) on single phthalocyanine dyes adsorbed on a surface. By means of tip-induced electroluminescence performed with a scanning probe microscope, we identify libronic signatures in spectra of chirally adsorbed phthalocyanines and find that these signatures are absent from spectra of symmetrically adsorbed species. We create a model of the libronic coupling based on the Franck-Condon principle to simulate the spectral features. Experimentally measured librational spectra match very well the theoretically calculated librational eigenenergies and peak intensities (Franck-Condon factors). Moreover, the comparison reveals an unexpected depopulation channel for the zero libron of the excited state that can be effectively controlled by tuning the size of the nanocavity. Our results showcase the possibility of characterizing the dynamics of molecules by their low-energy molecular modes using µeV-resolved tip-enhanced spectroscopy., S.C., R.C.C.F., M.Š. and J.D. acknowledge the Czech grant agency funding no. 20-18741 S and the Charles University Grant Agency project no. 910120. P.M. acknowledges grants EUR2021-122006, RYC2020-029800-I and PID2021-125309OA-I00 funded by MCIN/AEI/ 10.13039/501100011033 and European Union NextGenerationEU/PRTR.

Terminating the tip of an atomic force microscope with a CO molecule allows data to be acquired with a well-known and inert apex. Previous studies have shown conflicting results regarding the electrostatic interaction, indicating in some cases that the negative charge at the apex of the CO dominates, whereas in other cases the positive charge at the end of the metal tip dominates. To clarify this, we investigated $$\hbox {CaF}_{2}$$ CaF 2 (111). $$\hbox {CaF}_{2}$$ CaF 2 is an ionic crystal and the (111) surface does not possess charge inversion symmetry. Far from the surface, the interaction is dominated by electrostatics via the negative charge at the apex. Closer to the surface, Pauli repulsion and CO bending dominate, which leads to an unexpected appearance of the complex 3-atom unit cell. We compare simulated data in which the electrostatics are modeled by point particles versus a charge density calculated by DFT. We also compare modeling Pauli repulsion via individual Lennard–Jones potentials versus a total charge density overlap. In doing so, we determine forcefield parameters useful for future investigations of biochemical processes.

A class of important semiconductors, such as Si, Ge, or C, has an indirect band gap, which critically limits their optical properties. Lack of efficient emission is especially unfortunate for silicon, where Si light sources could enable realization of the long-awaited on-chip-integrated Si laser for an integrated optical computing CPU architecture. Hence, methods toward the improvement of optical properties of Si-based materials are in high demand. Unlike most of the applied light-emitting semiconductor nanocrystals (NCs) with a direct band gap, the radiative rate in covalent silicon NCs (SiNCs) is size-dependent but remains low even for the smallest SiNCs. Additionally, the radiative rate is also ligand-sensitive, and the covalent bond with ligands is very rigid and static and could be, in principle, used for straining via steric hindrance, further influencing the radiative rates. In this work, we use the self-consistent density functional theory (DFT) simulation together with a "fuzzy"band-structure concept to show the effect of covalently bonded ligands on the electronic structure of NCs and their k - -space projection. For instance, in 2 nm large SiNCs with C-linked organic ligands, we demonstrate that radiative rates can be manipulated by ligands to a considerable extent through an intricate interplay between charge transfer from the core to the ligand, orbital delocalization, and strain by steric hindrance. We propose that the tunability of electronic properties achieved via ligands in covalent systems offers a possible direction toward the design of an ideal Si light-emitting system.

Ion hydration and transport at interfaces are relevant to a wide range of applied fields and natural processes1–5. Interfacial effects are particularly profound in confined geometries such as nanometre-sized channels6–8, where the mechanisms of ion transport in bulk solutions may not apply9,10. To correlate atomic structure with the transport properties of hydrated ions, both the interfacial inhomogeneity and the complex competing interactions among ions, water and surfaces require detailed molecular-level characterization. Here we constructed individual sodium ion (Na+) hydrates on a NaCl(001) surface by progressively attaching single water molecules (one to five) to the Na+ ion using a combined scanning tunnelling microscopy and noncontact atomic force microscopy system. We found that the Na+ ion hydrated with three water molecules diffuses orders of magnitude more quickly than other ion hydrates. Ab initio calculations revealed that such high ion mobility arises from the existence of a metastable state, in which the three water molecules around the Na+ ion can rotate collectively with a rather small energy barrier. This scenario would apply even at room temperature according to our classical molecular dynamics simulations. Our work suggests that anomalously high diffusion rates for specific hydration numbers of ions are generally determined by the degree of symmetry match between the hydrates and the surface lattice. A sodium ion hydrated with three (rather than one, two, four or five) water molecules diffuses orders of magnitude more quickly than the other ion hydrates owing to the interfacial symmetry mismatch.

We develop a deep learning method that predicts atomic structure directly from experimental atomic force microscopy images., Atomic force microscopy (AFM) with molecule-functionalized tips has emerged as the primary experimental technique for probing the atomic structure of organic molecules on surfaces. Most experiments have been limited to nearly planar aromatic molecules due to difficulties with interpretation of highly distorted AFM images originating from nonplanar molecules. Here, we develop a deep learning infrastructure that matches a set of AFM images with a unique descriptor characterizing the molecular configuration, allowing us to predict the molecular structure directly. We apply this methodology to resolve several distinct adsorption configurations of 1S-camphor on Cu(111) based on low-temperature AFM measurements. This approach will open the door to applying high-resolution AFM to a large variety of systems, for which routine atomic and chemical structural resolution on the level of individual objects/molecules would be a major breakthrough.

Ferrocene-based molecules are extremely appealing as they offer a prospect of having built-in spin or charge functionality. However, there are only limited number of studies of structural and electronic properties on surfaces so far. We investigated the self-assembly processes of 1,1′-ferrocene dicarboxylic acid molecules (C12H10FeO4) on both metallic (Ag(111), Au(111), and Cu(110)) and insulating (Cu3N/Cu(110)) surfaces with high-resolution ncAFM/STM, XPS, and NEXAFS. The experimental evidence is corroborated with total energy DFT calculations and ncAFM simulations. The combined experimental and theoretical analysis allows detailed understanding of the unique arrangement and adsorption geometries of the molecules on different substrates, as well as the different chemical stability of the carboxylic (COOH) groups. The molecules on noble (Ag, Au) surfaces show only a weak interaction with the substrate forming a complex self-assembled pattern, driven by weak intermolecular interactions. In contrast, the an...

Single-molecule junctions are elementary building blocks in novel electronics devices, and therefore, it is essential to understand the charge transport mechanisms at the single-molecular level. According to recent studies, the linker atoms connecting the organic molecule with the electrodes play a crucial role in optimizing the transport properties as well as thermodynamical stability of a molecular junction. We address this issue by considering N, O, S, and Se atoms as prospective linkers anchoring the benzene molecule to the Au(1 0 0) electrodes. Calculations based on non-equilibrium Green's function approach are performed within the framework of the density functional theory. Electron transport is studied in detail by analyzing the transmission spectra, density-of-states distributions, and current–voltage characteristics. Results show that the choice of linkers strongly affects the conductance of the junctions under study: at low bias regime, the current through N-linked molecules is remarkably higher as compared to the case of S and Se linkers, whereas the thermodynamical stability is similar. This offers an additional means of modifying the current–voltage characteristics of benzene-based molecular junctions by an appropriate selection of linkers.

In this Letter, the links to Supplementary Videos 5, 7, 9 and 10 were incorrect, and there were some formatting errors in the Supplementary Video legends. These errors have been corrected online.

Nitrogen doping of graphene significantly affects its chemical properties, which is particularly important in molecular sensing and electrocatalysis applications. However, detailed insight into interaction between N-dopant and molecules at the atomic scale is currently lacking. Here we demonstrate control over the spin state of a single iron(II) phthalocyanine molecule by its positioning on N-doped graphene. The spin transition was driven by weak intermixing between orbitals with z-component of N-dopant (pz of N-dopant) and molecule (dxz, dyz, dz2) with subsequent reordering of the Fe d-orbitals. The transition was accompanied by an electron density redistribution within the molecule, sensed by atomic force microscopy with CO-functionalized tip. This demonstrates the unique capability of the high-resolution imaging technique to discriminate between different spin states of single molecules. Moreover, we present a method for triggering spin state transitions and tuning the electronic properties of molecules through weak non-covalent interaction with suitably functionalized graphene., Molecules can change their electronic properties when they are adsorbed on substrates, which can be useful for sensing and catalysis. Here, the authors use atomic force microscopy to show that the spin state of an iron complex can be changed upon displacing the molecule to different sites of a nitrogen-doped graphene

Chemical doping is one of the most suitable ways of tuning the electronic properties of graphene and a promising candidate for a band gap opening. In this work we report a reliable and tunable method for preparation of high-quality boron and nitrogen co-doped graphene on silicon carbide substrate. We combine experimental (dAFM, STM, XPS, NEXAFS) and theoretical (total energy DFT and simulated STM) studies to analyze the structural, chemical, and electronic properties of the single-atom substitutional dopants in graphene. We show that chemical identification of boron and nitrogen substitutional defects can be achieved in the STM channel due to the quantum interference effect, arising due to the specific electronic structure of nitrogen dopant sites. Chemical reactivity of single boron and nitrogen dopants is analyzed using force-distance spectroscopy by means of dAFM.

Charge distribution on surfaces and molecules plays an important role in many physical and chemical processes including catalytic reactions, adsorption, adhesion and charge transport. However, detailed mapping of the spatial charge distribution on atomic scale still remains a challenge, despite recent advances in the field of scanning probe microscopy. Consequently, we are witnessing extensive activity in search for a reliable and robust protocol to image the charge distribution. In this chapter, we will discuss different methods which provide information about charge distribution at atomic or submolecular scale, including Kelvin probe force microscopy, scanning quantum dot microscopy or high-resolution imaging with functionalized tips. We will briefly discuss the mechanism of each of the different methods as well as their advantages and drawbacks. Finally, we will provide a brief outlook and perspectives for further improvement of charge distribution mapping on the submolecular level.

Scanning probe microscopy has been extensively applied to probe interfacial water in many interdisciplinary fields but the disturbance of the probes on the hydrogen-bonding structure of water has remained an intractable problem. Here, we report submolecular-resolution imaging of the water clusters on a NaCl(001) surface within the nearly noninvasive region by a qPlus-based noncontact atomic force microscopy. Comparison with theoretical simulations reveals that the key lies in probing the weak high-order electrostatic force between the quadrupole-like CO-terminated tip and the polar water molecules at large tip–water distances. This interaction allows the imaging and structural determination of the weakly bonded water clusters and even of their metastable states with negligible disturbance. This work may open an avenue for studying the intrinsic structure and dynamics of ice or water on surfaces, ion hydration, and biological water with atomic precision., Scanning probe microscopy has been extensively applied to probe interfacial water but the probes tend to disturb the structure of water easily. Here, the authors report submolecular-resolution imaging of water clusters within the nearly non-invasive region by qPlus noncontact atomic force microscopy.

Here we show scanning tunneling microscopy (STM), noncontact atomic force microscopy (AFM), and inelastic electron tunneling spectroscopy (IETS) measurements on an organic molecule with a CO-terminated tip at 5 K. The high-resolution contrast observed simultaneously in all channels unambiguously demonstrates the common imaging mechanism in STM/AFM/IETS, related to the lateral bending of the CO-functionalized tip. The IETS spectroscopy reveals that the submolecular contrast at 5 K consists of both renormalization of vibrational frequency and variation of the amplitude of the IETS signal. This finding is also corroborated by first principles simulations. We extend accordingly the probe-particle AFM/STM/IETS model to include these two main ingredients necessary to reproduce the high-resolution IETS contrast. We also employ the first principles simulations to get more insight into a different response of frustrated translation and rotational modes of the CO tip during imaging.

We report a straightforward method to produce high-quality nitrogen-doped graphene on SiC(0001) using direct nitrogen ion implantation and subsequent stabilization at temperatures above 1300 K. We demonstrate that double defects, which comprise two nitrogen defects in a second-nearest-neighbor (meta) configuration, can be formed in a controlled way by adjusting the duration of bombardment. Two types of atomic contrast of single N defects are identified in scanning tunneling microscopy. We attribute the origin of these two contrasts to different tip structures by means of STM simulations. The characteristic dip observed over N defects is explained in terms of the destructive quantum interference.

We present a robust but still efficient simulation approach for high-resolution scanning tunneling microscopy with a flexible tip apex showing sharp submolecular features. The approach takes into account the electronic structure of sample and tip and relaxation of the tip apex. We validate our model by achieving good agreement with various experimental images which allows us to explain the origin of several observed features. Namely, we have found that high-resolution STM mechanism consists of the standard STM imaging, convolving electronic states of the sample and the tip apex orbital structure, with the contrast heavily distorted by the relaxation of the flexible apex caused by interaction with the substrate., 6 pages, 3 figures

Electron donor–acceptor molecules are of outstanding interest in molecular electronics and organic solar cells for their intramolecular charge transfer controlled via electrical or optical excitation. The preservation of their electronic character in the ground state upon adsorption on a surface is cardinal for their implementation in such single-molecule devices. Here, we investigate by atomic force microscopy and scanning tunneling microscopy a prototypical system consisting of a π-conjugated tetrathiafulvalene-fused dipyridophenazine molecule adsorbed on thin NaCl films on Cu(111). Depending on the adsorption site, the molecule is found either in a nearly undisturbed free state or in a bound state. In the latter case, the molecule adopts a specific adsorption site, leading to the formation of a chelate complex with a single Na+ alkali cation pulled out from the insulating film. Although expected to be electronically decoupled, the charge distribution of the complex is drastically modified, leading to t...

We present a numerical model that allows us to study the response of an oscillating probe in electrostatic force spectroscopy to charge switching in quantum dots at various time scales. The model provides more insight into the behavior of frequency shift and dissipated energy under different scanning conditions when measuring a temporarily charged quantum dot on a surface. Namely, we analyze the dependence of the frequency shift, the dissipated energy, and their fluctuations on the resonance frequency of the tip and on the electron tunneling rates across the tip-quantum dot and quantum dot-sample junctions. We discuss two complementary approaches to simulating the charge dynamics, a stochastic and a deterministic one. In addition, we derive analytic formulas valid for small amplitudes, describing relations between the frequency shift, dissipated energy, and the characteristic rates driving the charging and discharging processes.

We report on our systematic investigation of Au nanoparticles highly dispersed in the mesopores of (s)-(–)-2-pyrrolidinone-5-carboxylic acid (Py)-modified SBA-15 (Au/SBA-15-Py) by using a series of modern techniques. 13C NMR and IR spectroscopies indicate that Py species are successfully grafted on the surface of mesopores in SBA-15; XRD patterns and N2 adsorption isotherms show that the sample mesostructures are well preserved; TEM images clearly confirm the uniform Au nanoparticles in the mesopores; and XPS suggests an interaction between Au nanoparticles with Py species. Interestingly, Au/SBA-15-Py catalysts always exhibit superior catalytic properties in the oxidation of cyclohexene and styrene by molecular oxygen at atmospheric pressure, compared with the pyrrolidone-free SBA-15 supported Au catalyst (Au/SBA-15-N). This phenomenon is reasonably related to the interaction between Au nanoparticles with Py species, which is consistent with results analyzed from our density-functional theory (DFT) calculations.

Atomic force microscopy is capable of resolving the chemical structure of a single molecule on a surface. In previous research, such high resolution has only been obtained at low temperatures. Here we demonstrate that the chemical structure of a single molecule can be clearly revealed even at room temperature. 3,4,9,10-perylene tetracarboxylic dianhydride, which is strongly adsorbed onto a corner-hole site of a Si(111)–(7 × 7) surface in a bridge-like configuration is used for demonstration. Force spectroscopy combined with first-principle calculations clarifies that chemical structures can be resolved independent of tip reactivity. We show that the submolecular contrast over a central part of the molecule is achieved in the repulsive regime due to differences in the attractive van der Waals interaction and the Pauli repulsive interaction between different sites of the molecule., Atomic force microscopy is capable of resolving the chemical structure of a single molecule on a surface, usually at low temperatures. Here, the authors demonstrate that the chemical structure of a single molecule strongly adsorbed onto a silicon surface can be determined at room temperature.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al., The forces between two single molecules brought into contact, and their connection with charge transport through the molecular junction, are studied here using non contact AFM, STM, and density functional theory simulations. A carbon monoxide molecule approaching an acetylene molecule (C2H2) initially feels weak attractive electrostatic forces, partly arising from charge reorganization in the presence of molecular. We find that the molecular contact is chemically passive, and protects the electron tunneling barrier from collapsing, even in the limit of repulsive forces. However, we find subtle conductance and force variations at different contacting sites along the C2H2 molecule attributed to a weak overlap of their respective frontier orbitals., The research was supported by DFG (Grant No. Sfb 658), the Czech Science Foundation (GAČR) Project No. 14-02079S, GAAV Grant No. M100101207, and the Spanish MINECO (Grant No. MAT2013-46593-C6-01). M. C. acknowledges support from the Alexander von Humboldt Foundation.

We discuss the history and recent progress of simultaneous AFM/STM measurements with atomic resolution. We demonstrate, that the technique can provide complex information about chemical and physical processes at atomic scale as well as about material properties of surfaces and nanostructures. We briefly overview one of the most fascinating achievements, high-resolution imaging with functionalized tips. The complexity of this technique calls for a new theoretical approach where relaxation of functionalized probe is considered in both AFM and STM modes. We describe mechanisms responsible for the high-resolution contrast introducing a numerical model, which provides deeper understanding of the AFM/STM measurements.

Recently, the family of high-resolution scanning probe imaging techniques using decorated tips has been complimented by a method based on inelastic electron tunneling spectroscopy (IETS). The new technique resolves the inner structure of organic molecules by mapping the vibrational energy of a single carbonmonoxide (CO) molecule positioned at the apex of a scanning tunnelling microscope (STM) tip. Here, we explain high-resolution IETS imaging by extending the model developed earlier for STM and atomic force microscopy (AFM) imaging with decorated tips. In particular, we show that the tip decorated with CO acts as a nanoscale sensor that changes the energy of the CO frustrated translation in response to the change of the local curvature of the surface potential. In addition, we show that high resolution AFM, STM and IETS-STM images can deliver information about intramolecular charge transfer for molecules deposited on a~surface. To demonstrate this, we extended our numerical model by taking into the account the electrostatic force acting between the decorated tip and surface Hartree potential., 5 pages, 4 figures

The computational simulation of photo-induced processes in large molecular systems is a very challenging problem. Firstly, to properly simulate photo-induced reactions the potential energy surfaces corresponding to excited states must be appropriately accessed; secondly, understanding the mechanisms of these processes requires the exploration of complex configurational spaces and the localization of conical intersections; finally, photo-induced reactions are probability events, that require the simulation of hundreds of trajectories to obtain the statistical information for the analysis of the reaction profiles. Here, we present a detailed description of our implementation of a molecular dynamics with electronic transitions algorithm within the local-orbital density functional theory code FIREBALL, suitable for the computational study of these problems. As an example of the application of this approach, we also report results on the [2 + 2] cycloaddition of ethylene with maleic anhydride and on the [2 + 2] photo-induced polymerization reaction of two C60 molecules. We identify different deactivation channels of the initial electron excitation, depending on the time of the electronic transition from LUMO to HOMO, and the character of the HOMO after the transition.

We discuss the possibility of a two-dimensional ordered structure formed upon deposition of Si on metal surfaces. We investigate the atomic and electronic structure of the Si-$(\sqrt{19}\ifmmode\times\else\texttimes\fi{}\sqrt{19})R23.{4}^{\ensuremath{\circ}}$/Pt(111) surface reconstruction by means of a set of experimental surface-science techniques supported by theoretical calculations. The theory achieves very good agreement with the experimental results and is corroborating beyond any doubt that this phase is a surface alloy consisting of ${\mathrm{Si}}_{3}$Pt tetramers that resembles a twisted kagome lattice. These findings render unlikely any formation of silicene or germanene on Pt(111) and other transition-metal surfaces.

High resolution Atomic Force Microscopy (AFM) and Scanning Tunnelling Microscopy (STM) imaging with functionalized tips is well established, but a detailed understanding of the imaging mechanism is still missing. We present a numerical STM/AFM model, which takes into account the relaxation of the probe due to the tip-sample interaction. We demonstrate that the model is able to reproduce very well not only the experimental intra- and intermolecular contrasts, but also their evolution upon tip approach. At close distances, the simulations unveil a significant probe particle relaxation towards local minima of the interaction potential. This effect is responsible for the sharp sub-molecular resolution observed in AFM/STM experiments. In addition, we demonstrate that sharp apparent intermolecular bonds should not be interpreted as true hydrogen bonds, in the sense of representing areas of increased electron density. Instead they represent the ridge between two minima of the potential energy landscape due to neighbouring atoms.

The following article appeared in Journal of Chemical Physics 138.15 (2013): 154106 and may be found at http://scitation.aip.org/content/aip/journal/jcp/138/15/10.1063/1.4801511, Most of today's molecular-dynamics simulations of materials are based on the Born-Oppenheimer approximation. There are many cases, however, in which the coupling of the electrons and nuclei is important and it is necessary to go beyond the Born-Oppenheimer approximation. In these methods, the non-adiabatic coupling vectors are fundamental since they represent the link between the classical atomic motion of the nuclei and the time evolution of the quantum electronic state. In this paper we analyze the calculation of non-adiabatic coupling vectors in a basis set of local orbitals and derive an expression to calculate them in a practical and computationally efficient way. Some examples of the application of this expression using a local-orbital density functional theory approach are presented for a few simple molecules: H3, formaldimine, and azobenzene. These results show that the approach presented here, using the Slater transition-state density, is a very promising way for the practical calculation of non-adiabatic coupling vectors for large systems., This work was partially supported by Spanish Ministerio de Economía y Competitividad (Contract No.FIS2010-16046), the Comunidad de Madrid (Contract No.S2009/MAT-1467), the Office of Science, Basic Energy Sciences in the US Department of Energy (Grant No. DEFG02-10ER16164), the Czech Science Foundation (GAČR)(Project No. 204/10/0952), the Grant of the MŠMT of the Czech Republic (Grant No. ME 09048), and COST-CMTS Action CM1002 (CODECS). J.O. gratefully acknowledges support from the Spanish Ministerio de Ciencia e Innovación (PR2008-0027). E.A. gratefully acknowledges financial support by the Consejería de Educación de la Comunidad de Madrid and Fondo Social Europeo.

In this paper, we discuss the validity of the band structure concept in silicon nanocrystals a few nanometers in size. We introduce a general method which allows reconstruction of a fuzzy electronic band structure of nanocrystals from ordinary real-space electronic structure calculations. A comprehensive study of the fuzzy band structure of a realistic nanocrystal is given including full geometric and electronic relaxation with the surface passivating groups. In particular, we combine this method with large-scale density functional theory calculations to obtain insight into the luminescence properties of silicon nanocrystals up to 3 nm in size depending on the surface passivation and geometric distortion. We conclude that the band-structure concept is applicable to silicon nanocrystals with a diameter larger than $\ensuremath{\approx}$2 nm with certain limitations. We also show how perturbations due to polarized surface groups or geometric distortion can lead to considerable moderation of momentum space selection rules.

The coupling between two atomically sharp nanocontacts provides tunable access to a fundamental underlying interaction: the formation of the bond between two atoms as they are brought into contact. Here we report a detailed experimental and theoretical analysis of the relation between the chemical force and the tunneling current during bond formation in atom-scale metallic junctions and their dependence on distance, junction structure, and material. We found that the short-range force as well as the conductance in two prototypical metal junctions depend exponentially on the distance and that they have essentially the same exponents. In the transition regime between tunneling and point contact, large short-range forces generate structural relaxations which are concomitant with modifications of the surface electronic structure and the collapse of the tunneling barrier.

Silicon, a semiconductor underpinning the vast majority of microelectronics, is an indirect-gap material and consequently is an inefficient light emitter. This hampers the ongoing worldwide effort towards the integration of optoelectronics on silicon wafers. Even though silicon nanocrystals are much better light emitters, they retain the indirect-gap nature. Here, we propose a solution to this long-standing problem: silicon nanocrystals can be transformed into a material with fundamental direct bandgap via a concerted action of quantum confinement and tensile strain. We document this transformation by DFT calculations mapping the E(k) band-structure of Si nanocrystals. The experimental proofs are then given firstly by a 10 000× increase in the photon emission rate of strained silicon nanocrystals together with their altered absorbance spectra, both of which point to direct dipole-allowed transitions, secondly by single nanocrystal spectroscopy, confirming reduced phonon energies and thus the presence of tensile strain, and lastly by photoluminescence studies under external hydrostatic pressure.

Single molecules in a tunnel junction can now be interrogated reliably using chemically functionalized electrodes. Monitoring stochastic bonding fluctuations between a ligand bound to one electrode and its target bound to a second electrode ('tethered molecule-pair' configuration) gives insight into the nature of the intermolecular bonding at a single molecule-pair level, and defines the requirements for reproducible tunneling data. Simulations show that there is an instability in the tunnel gap at large currents, and this results in a multiplicity of contacts with a corresponding spread in the measured currents. At small currents (i.e. large gaps) the gap is stable, and functionalizing a pair of electrodes with recognition reagents (the 'free-analyte' configuration) can generate a distinct tunneling signal when an analyte molecule is trapped in the gap. This opens up a new interface between chemistry and electronics with immediate implications for rapid sequencing of single DNA molecules.