Effect of van der Waals forces on the stacking of coronenes encapsulated in a single-wall carbon nanotube and many-body excitation spectrum

We investigate the geometry, stability, electronic structure and optical properties of C24H12 coronenes encapsulated in a single-wall (19,0) carbon nanotube. By an adequate combination of advanced electronic-structure techniques, involving weak and van derWaals interaction, as well as many-body effects for establishing electronic properties and excitations, we have accurately characterized this hybrid carbon nanostructure, which arises as a promising candidate for opto-electronic nanodevices. In particular, we show that the structure of the stacked coronenes inside the nanotube is characterized by a rotation of every coronene with respect to its neighbors through van derWaals interaction, which is of paramount importance in these systems. We also suggest a tentative modification of the system in order this particular rotation to be observed experimentally. A comparison between the calculated many-body excitation spectrum of the systems involved reveals a pronounced optical red-shift with respect to the coronene-stacking gas-phase. The origin of this red-shift is explained in terms of the confinement of the coronene molecules inside the nanotube, showing an excellent agreement with the available experimental evidence

Electroluminescence from a polythiophene molecular wire suspended in a plasmonic scanning tunneling microscope junction

The electroluminescence of a polythiophene wire suspended between two metallic electrodes is probed using a scanning tunneling microscope. Under positive sample voltage, the spectral and voltage dependencies of the emitted light are consistent with the fluorescence of the wire junction mediated by localized plasmons. This emission is strongly attenuated for the opposite polarity. Both emission mechanism and polarity dependence are similar to what occurs in organic light emitting diodes (OLED) but at the level of a single molecular wire.Comment: to be published in Physical Review Letter

Adsorption and STM imaging of polycyclic aromatic hydrocarbons on graphene

International audienceThe structural characterization of polycyclic aromatic hydrocarbon molecules adsorbed on graphene is of fundamental importance in view of the use of graphene or graphene nanoribbons for electronic applications. Before reaching this point, one has to determine the structure of the adsorbed molecules. Here, we study the case of benzene, coronene, and hexabenzocoronene on a graphene layer. First, the adsorption properties of single molecules are calculated using first-principles calculations at the level of density functional theory. We benefit from a recent scheme, particularly adapted for weakly adsorbed molecules, allowing us to precisely calculate the van der Waals contribution. Then, scanning tunneling microscopy (STM) is used to produce images of self-assembled molecules comparing different theoretical approaches to experimental observations. Finally, we consider the imaging of isolated molecules, and we show how the STM tip influences the molecule position by soft mechanical interaction during the scanning process

Pulling and Stretching a Molecular Wire to Tune its Conductance

A scanning tunnelling microscope is used to pull a polythiophene wire from a Au(111) surface while measuring the current traversing the junction. Abrupt current increases measured during the lifting procedure are associated to the detachment of molecular sub-units, in apparent contradiction with the expected exponential decrease of the conductance with wire length. \textit{Ab initio} simulations reproduce the experimental data and demonstrate that this unexpected behavior is due to release of mechanical stress in the wire, paving the way to mechanically gated single-molecule electronic devices

Intrinsic photoanode band engineering: enhanced solar water splitting efficiency mediated by surface segregation in Ti-doped hematite nanorods

Band engineering is thoroughly employed nowadays targeting technologically scalable photoanodes for solar water splitting applications. Most often complex and costly recipes are necessary, for average performances. Here we report very simple photoanode growth and thermal annealing, with effective band engineering results. Strongly enhanced photocurrent, of more than 200 %, is measured for Ti-doped hematite nanorods grown from aqueous solutions and annealed under Nitrogen atmosphere, compared to air annealed ones. Oxidized surface states and increased density of charge carriers are found responsible for the enhanced photoelectrochemical activity, as shown by electrochemical impedance spectroscopy and synchrotron X-rays spectromicroscopies. They are found related to oxygen vacancies, acting as n-dopants, and the formation of pseudo- brookite clusters by surface Ti segregation. Spectro-ptychography is used for the first time at Ti L3 absorption edge to isolate Ti chemical coordination arising from pseudo-brookite clusters contribution. Correlated with electron microscopy investigation and Density Functional Theory (DFT) calculations, our data unambiguously prove the origin of the enhanced photoelectrochemical activity of N2-annealed Ti-doped hematite nanorods. Finally, we present here a handy and cheap surface engineering method beyond the known oxygen vacancy doping, allowing a net gain in the photoelectrochemical activity for the hematite-based photoanodes.Comment: 2 parts: first main manuscript with 39 pages, second supplementary informations with 14 page

Graphene as a Promising Electrode for Low-Current Attenuation in Nonsymmetric Molecular Junctions

International audienceWe have measured the single-molecule conductance of 1,$\it n$-alkanedithiol molecular bridges ($\it n$ = 4, 6, 8, 10, 12) on a graphene substrate using scanning tunneling microscopy (STM)-formed electrical junctions. The conductance values of this homologous series ranged from 2.3 nS ($\it n$= 12) to 53 nS ($\it n$= 4), with a decay constant β$_n$ of 0.40 per methylene (−CH$_2$) group. This result is explained by a combination of density functional theory (DFT) and Keldysh− Green function calculations. The obtained decay, which is much lower than the one obtained for symmetric gold junctions, is related to the weak coupling at the molecule−graphene interface and the electronic structure of graphene. As a consequence, we show that using graphene nonsymmetric junctions and appropriate anchoring groups may lead to a much-lower decay constant and more-conductive molecular junctions at longer lengths

Fragmentation and Distortion of Terpyridine-Based Spin-Crossover Complexes on Au(111)

Spin-crossover complexes are attractive for their spin-switching functionality. However, only a few compounds have been found to remain intact in direct contact to metal surfaces. For the design of new spin-crossover complexes, it is important to understand the mechanisms leading to fragmentation. Here, we investigate, using low-temperature scanning tunneling microscopy along with density functional theory calculations, two Fe(terpyridine)2 complexes deposited on Au(111) by electrospray ionization with in-line mass selection. Only fragments of the first compound are observed on the surface, while the second compound is strongly flattened. On the basis of a detailed analysis of the adsorbates on the surface, possible mechanisms for the fragmentation and molecular distortion are proposed

Effect of Asymmetric Anchoring Groups on Electronic Transport in Hybrid Metal/Molecule/Graphene Single Molecule Junctions.

A combined experimental and theoretical study on molecular junctions with asymmetry in both the electrode type and in the anchoring group type is presented. A scanning tunnelling microscope is used to create the "asymmetric" Au-S-(CH2 )n-COOH-graphene molecular junctions and determine their conductance. The measurements are combined with electron transport calculations based on density functional theory (DFT) to analyze the electrical conductance and its length attenuation factor from a series of junctions of different molecular length (n). These results show an unexpected trend with a rather high conductance and a smaller attenuation factor for the Au-S-(CH2 )n -COOH-graphene configuration compared to the equivalent junction with the "symmetrical" COOH contacting using the HOOC-(CH2 )n -COOH series. Owing to the effect of the graphene electrode, the attenuation factor is also smaller than the one obtained for Au/Au electrodes. These results are interpreted through the relative molecule/electrode couplings and molecular level alignments as determined with DFT calculations. In an asymmetric junction, the electrical current flows through the less resistive conductance channel, similarly to what is observed in the macroscopic regime