Marcus Theory and Long-Range Activationless Transport in Molecular Junctions.

Intrachain transport in molecular junctions (MJs) longer than 5 nm has been modeled within the theoretical framework of Marcus theory. We show that in oligo(bisthienylbenzene)-based MJs, electronic transport involves polarons, localized on three monomers that are close to 4 nm in length. They hop and tunnel between adjacent localized sites with reorganization energies λ close to 400-600 meV and electronic coupling parameters Hab close to λ/2. As a consequence, the activation energy for intrachain transport, given by the equation ΔG* = (λ/4)(1 - 2Hab/λ)2, is close to zero, and transport along the chain is activationless, in agreement with experimental observation. On the contrary, similar calculations on conjugated oligonaphthalenefluoreneimine wires show that Hab is much less than λ/2 and predict that the activation energies for intrachain hopping between adjacent sites, close to λ/4, are ∼115 meV. This work proposes a new perspective for understanding long-range activationless transport in MJs beyond the tunneling regime.

Molecular Diodes Induced by a Schottky Barrier with a Gold-Silicon Doped Electrode.

To create complementary metal oxide semiconductor compatible molecular devices, more insights into the electrode property regarding its metal/semiconductor doping level and creating a functional molecular device are required. In this work, we constructed an EGaIn/alkanethiol/Au-Si molecular diode (with a rectification ratio R of 50.70) induced by Schottky barriers within a gold-silicon doped electrode instead of the functional property of molecules. The relationship between the rectification ratio and the number of methylene units in alkanethiol was analyzed, revealing a gradual increase in the ratio from 3.33 for C6H14S to 50.70 for C16H34S. The rectification ratio of the junction is well modulated by the temperature due to the change in the Schottky barrier. Such a mechanism is explained by the energy band diagrams of the surface space charge region and a combination of density functional theory and Keldysh-Green formalism calculations.

Spatially Extended Charge Density Wave Switching by Nanoscale Local Manipulation in a VTe2 Monolayer.

Monolayer transition metal dichalcogenide VTe2 exhibits multiple charge density wave (CDW) phases, mainly (4 × 4) and (4 × 1). Here we report facile dynamic and tens-of-nanometer scale switching between these CDW phases with gentle bias pulses in scanning tunneling microscopy. Bias pulses purposely stimulate a reversible random CDW symmetry change between the isotropic (4 × 4) and anisotropic (4 × 1) CDWs, as well as CDW phase slips and rotation. The switching threshold of ∼1.0 V is independent of bias polarity, and the switching rate varies linearly with the tunneling current. Density functional theory calculations indicate that a coherent CDW phase switching incurs an energy barrier of ∼2.0-3.0 eV per (4 × 4) unit cell. While there is a challenge in understanding the observed large-area CDW random fluttering, we provide some possible explanations. The ability to manipulate electronic CDW phases sheds new light on tailoring CDW properties on demand.

Destructive quantum interference in meta-oligo(phenyleneethynylene) molecular wires with gold-graphene heterojunctions.

Quantum interference (QI) is well recognised as a significant contributing factor to the magnitude of molecular conductance values in both single-molecule and large area junctions. Numerous structure-property relationship studies have shown that para-connected oligo(phenyleneethynylene) (OPE) based molecular wires exemplify the impact of constructive quantum interference (CQI), whilst destructive quantum interference (DQI) effects are responsible for the orders of magnitude lower conductance of analogous meta-contacted OPE derivatives, despite the somewhat shorter effective tunnelling distance. Since molecular conductance is related to the value of the transmission function, evaluated at the electrode Fermi energy, T(EF), which in turn is influenced by the presence and relative energy of (anti)resonances, it follows that the relative single-molecule conductance of para- and meta-contacted OPE-type molecules is tuned both by the anchor group and the nature of the electrode materials used in the construction of molecular junctions (gold|molecule|gold vs. gold|molecule|graphene). It is shown here that whilst amine-contacted junctions show little influence of the electrode material on molecular conductance due to the similar electrode-molecule coupling through this anchor group to both types of electrodes, the weaker coupling between thiomethyl and ethynyl anchors and the graphene substrate electrode results in a relative enhancement of the DQI effect. This work highlights an additional parameter space to explore QI effects and establishes a new working model based on the electrode materials and anchor groups in modulating QI effects beyond the chemical structure of the molecular backbone.

Nonuniform STM Contrast of Self-Assembled Tri-n-octyl-triazatriangulenium Tetrafluoroborate on HOPG.

We have assembled 4,8,12-tri-n-octyl-4,8,12-triazatrianguleniumtetrafluoroborate (TATA-BF4) on highly oriented pyrolytic graphite (HOPG) and have studied the structure and tunneling properties of this self-assembled monolayer (SAM) using scanning tunneling microscopy (STM) under ambient conditions. We show that the triazatriangulenium cations TATA+ form hexagonally packed structures driven by the interaction between the aromatic core and the HOPG lattice, as evidenced by density functional theory (DFT) modeling. According to the DFT results, the three alkyl chains of the platform tend to follow the main crystallographic directions of HOPG, leading to a different STM appearance. The STM contrast of the SAM shows that the monolayer is formed by two types of species, namely, TATA+ with BF4- counterions on top and without them. The cationic TATA+ platform gives rise to a seemingly higher appearance than neutral TATA-BF4, in contrast to observations made on metallic substrates. The variation of the STM tunneling parameters does not change the relative difference of contrast, revealing the stability of both species on HOPG. DFT calculations show that TATA-BF4 on HOPG has sufficient binding energy to resist dissociation into TATA+ and BF4-, which might occur under the action of the electric field in the tunneling gap during STM scanning.

Quantum Transport in Large-Scale Patterned Nitrogen-Doped Graphene.

It has recently been demonstrated how the nitrogen dopant concentration in graphene can be controlled spatially on the nano-meter scale using a molecular mask. This technique may be used to create ballistic electron optics-like structures of high/low doping regions; for example, to focus electron beams, harnessing the quantum wave nature of the electronic propagation. Here, we employ large-scale Greens function transport calculations based on a tight-binding approach. We first benchmark different tight-binding models of nitrogen in graphene with parameters based on density functional theory (DFT) and the virtual crystal approximation (VCA). Then, we study theoretically how the random distribution within the masked regions and the discreteness of the nitrogen scattering centers impact the transport behavior of sharp n-p and n-n' interfaces formed by different, realistic nitrogen concentrations. We investigate how constrictions for the current can be realized by patterned high/low doping regions with experimentally feasible nitrogen concentrations. The constrictions can guide the electronic current, while the quantized conductance is significantly washed out due to the nitrogen scattering. The implications for device design is that a p-n junction with nitrogen corrugation should still be viable for current focusing. Furthermore, a guiding channel with less nitrogen in the conducting canal preserves more features of quantized conductance and, therefore, its low-noise regime.

Enhancement of the Solar Water Splitting Efficiency Mediated by Surface Segregation in Ti-Doped Hematite Nanorods.

Band engineering is employed thoroughly and targets technologically scalable photoanodes for solar water splitting applications. Complex and costly recipes are necessary, often for average performances. Here, we report simple photoanode growth and thermal annealing with effective band engineering results. By comparing Ti-doped hematite photoanodes annealed under nitrogen to photoanodes annealed in air, we found a strongly enhanced photocurrent of more than 200% in the first case. Using electrochemical impedance spectroscopy and synchrotron X-ray spectromicroscopy, we demonstrate that oxidized surface states and increased density of charge carriers are responsible for the enhanced photoelectrochemical (PEC) activity. Surface states are found to be related to the formation of pseudo-brookite clusters by surface Ti segregation. Spectro-ptychography is used for the first time at the Ti L3 absorption edge to isolate Ti chemical coordination arising from pseudo-brookite cluster contribution. Correlated with electron microscopy investigation and density functional theory calculations, the synchrotron spectromicroscopy data unambiguously prove the origin of enhanced PEC 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 PEC activity for the hematite-based photoanodes.

Control of the Organization of 4,4'-bis(carbazole)-1,1'-biphenyl (CBP) Molecular Materials through Siloxane Functionalization.

We show that through the introduction of short dimethylsiloxane chains, it was possible to suppress the crystalline state of CBP in favor of various types of organization, transitioning from a soft crystal to a fluid liquid crystal mesophase, then to a liquid state. Characterized by X-ray scattering, all organizations reveal a similar layered configuration in which layers of edge-on lying CBP cores alternate with siloxane. The difference between all CBP organizations essentially lay on the regularity of the molecular packing that modulates the interactions of neighboring conjugated cores. As a result, the materials show quite different thin film absorption and emission properties, which could be correlated to the features of the chemical architectures and the molecular organizations.

Hybrid molecular graphene transistor as an operando and optoelectronic platform.

Lack of reproducibility hampers molecular devices integration into large-scale circuits. Thus, incorporating operando characterization can facilitate the understanding of multiple features producing disparities in different devices. In this work, we report the realization of hybrid molecular graphene field effect transistors (m-GFETs) based on 11-(Ferrocenyl)undecanethiol (FcC11SH) micro self-assembled monolayers (µSAMs) and high-quality graphene (Gr) in a back-gated configuration. On the one hand, Gr enables redox electron transfer, avoids molecular degradation and permits operando spectroscopy. On the other hand, molecular electrode decoration shifts the Gr Dirac point (VDP) to neutrality and generates a photocurrent in the Gr electron conduction regime. Benefitting from this heterogeneous response, the m-GFETs can implement optoelectronic AND/OR logic functions. Our approach represents a step forward in the field of molecular scale electronics with implications in sensing and computing based on sustainable chemicals.

Structural properties of Bi/Au(110).

Atomically thin bismuth films (2D Bi) are becoming a promising research area due to their unique properties and their wide variety of applications in spintronics, electronic and optoelectronic devices. We report on the structural properties of Bi on Au(110), explored by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. At a Bi coverage lower than one monolayer (1 ML) various reconstructions are observed, we focus on Bi/Au(110)-c(2 × 2) reconstruction (at 0.5 ML) and Bi/Au(110)-(3 × 3) structure (at 0.66 ML). We propose models for both structures based on STM measurements and further confirm by DFT calculations.

Negative Differential Resistance in Spin-Crossover Molecular Devices.

We demonstrate, based on low-temperature scanning tunneling microscopy (STM) and spectroscopy, a pronounced negative differential resistance (NDR) in spin-crossover (SCO) molecular devices, where a FeII SCO molecule is deposited on surfaces. The STM measurements reveal that the NDR is robust with respect to substrate materials, temperature, and the number of SCO layers. This indicates that the NDR is intrinsically related to the electronic structure of the SCO molecule. Experimental results are supported by density functional theory (DFT) with nonequilibrium Green's function (NEGF) calculations and a generic theoretical model. While the DFT+NEGF calculations reproduce NDR for a special atomically sharp STM tip, the effect is attributed to the energy-dependent tip density of states rather than the molecule itself. We, therefore, propose a Coulomb blockade model involving three molecular orbitals with very different spatial localization as suggested by the molecular electronic structure.

Phthalocyanine reactivity and interaction on the 6H-SiC(0001)-(3 × 3) surface investigated by core-level experiments and simulations.

The adsorption of phthalocyanine (H2Pc) on the 6H-SiC(0001)-(3 × 3) surface is investigated using X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS), and density functional theory (DFT) calculations. Spectral features are tracked from the submonolayer to the multilayer growth regime, observing a significant modification of spectroscopic signals at low coverage with respect to the multilayer films, where molecules are weakly interacting. Molecules stay nearly flat on the surface at the mono and submonolayers. Previously proposed adsorption models, where the molecule binds by two N atoms to corresponding Si adatoms, do not reproduce the experimental spectra at the submonolayer coverage. We find instead that another adsorption model where the molecule replaces the two central H atoms by a Si adatom, effectively forming Si-phthalocyanine (SiPc), is both energetically more stable and yields in combination a better agreement between the experimental and simulated spectra. This suggests that the 6H-SiC(0001)-(3 × 3) surface may be a candidate substrate for the on-surface synthesis of SiPc molecules.

Electrochemical gating for single-molecule electronics with hybrid Au|graphene contacts.

The single-molecular conductance of a redox active viologen molecular bridge between Au|graphene electrodes has been studied in an electrochemical gating configuration in an ionic liquid medium. A clear "off-on-off" conductance switching behaviour has been achieved through gating of the redox state when the electrochemical potential is swept. The Au|viologen|graphene junctions show single-molecule conductance maxima centred close to the equilibrium redox potentials for both reduction steps. The peak conductance of Au|viologen|graphene junctions during the first reduction is significantly higher than that of previously measured Au|viologen|Au junctions. This shows that even though the central viologen moiety is not directly linked to the enclosing electrodes, substituting one gold contact for a graphene one nevertheless has a significant impact on junction conductance values. The experimental data was compared against two theoretical models, namely a phase coherent tunnelling and an incoherent "hopping" model. The former is a simple gating monoelectronic model within density functional theory (DFT) which discloses the charge state evolution of the molecule with electrode potential. The latter model is the collective Kuznetsov Ulstrup model for 2-step sequential charge transport through the redox centre in the adiabatic limit. The comparison of both models to the experimental data is discussed for the first time. This work opens perspectives for graphene-based molecular transistors with more effective gating and fundamental understanding of electrochemical electron transfer at the single molecular level.

Electron beam analysis induces Cl vacancy defects in a NaCl thin film.

This work reports on the electron-induced modification of NaCl thin film grown on Ag(110). We show using low energy electron diffraction that electron beam bombardment leads to desorption and formation of Cl vacancy defects on NaCl surface. The topographic structure of these defects is studied using scanning tunneling microscopy (STM) showing the Cl defects as depressions on the NaCl surface. Most of the observed defects are mono-atomic vacancies and are located on flat NaCl terraces. Auger electron spectroscopy confirms the effect of electron exposure on NaCl thin films showing Cl atoms desorption from the surface. Using density functional theory taken into account the van der Waals dispersion interactions, we confirm the observed experimental STM measurements with STM simulation. Furthermore, comparing the adsorption of defect free NaCl and defective NaCl monolayer on Ag(110) surfaces, we found an increase of the adhesion energy and the charge transfer between the NaCl film and the substrate due to the Cl vacancy. In details, the adhesion energy increases between the NaCl film and the metallic Ag substrate from 30.4 meV Å-2for the NaCl film without Cl vacancy and from 39.5 meV Å-2for NaCl film with a single Cl vacancy. The charge transfer from the NaCl film to the Ag substrate is enhanced when the vacancy is created, from 0.63e-to 1.25e-.

Oligothiophene molecular wires at graphene-based molecular junctions.

The use of graphene as a new type of electrode at molecular junctions has led to a renewal of molecular electronics. Indeed, the symmetry breaking induced by the graphene electrode yields different electronic behaviors at the molecular junction and in particular enhanced conductance for longer molecules. In this respect, several studies involving different molecular backbones and anchoring groups have been performed. Here in the same line, we consider oligopthiophene based hybrid gold-graphene junctions and we measure their electrical properties using the STM-I(s) method in order to determine their attenuation factor and the effect of specific anchoring groups. The results are supported by density functional theory (DFT) calculations, and exhibit a similar behavior to what is observed at alkane-based junctions.

Flat epitaxial quasi-1D phosphorene chains.

The emergence of peculiar phenomena in 1D phosphorene chains (P chains) has been proposed in theoretical studies, notably the Stark and Seebeck effects, room temperature magnetism, and topological phase transitions. Attempts so far to fabricate P chains, using the top-down approach starting from a few layers of bulk black phosphorus, have failed to produce reliably precise control of P chains. We show that molecular beam epitaxy gives a controllable bottom-up approach to grow atomically thin, crystalline 1D flat P chains on a Ag(111) substrate. Scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and density functional theory calculations reveal that the armchair-shaped chains are semiconducting with an intrinsic 1.80 ± 0.20 eV band gap. This could make these P chains an ideal material for opto-electronic devices.

Intraconfigurational Transition due to Surface-Induced Symmetry Breaking in Noncovalently Bonded Molecules.

The interaction of molecules with surfaces plays a crucial role in the electronic and chemical properties of supported molecules and needs a comprehensive description of interfacial effects. Here, we unveil the effect of the substrate on the electronic configuration of iron porphyrin molecules on Au(111) and graphene, and we provide a physical picture of the molecule-surface interaction. We show that the frontier orbitals derive from different electronic states depending on the substrate. The origin of this difference comes from molecule-substrate orbital selective coupling caused by reduced symmetry and interaction with the substrate. The weak interaction on graphene keeps a ground state configuration close to the gas phase, while the stronger interaction on gold stabilizes another electronic solution. Our findings reveal the origin of the energy redistribution of molecular states for noncovalently bonded molecules on surfaces.

Blue phosphorene reactivity on the Au(111) surface.

The synthesis of blue phosphorene by molecular beam epitaxy (MBE) has recently come under the spotlight due to its potential applications in electronic and optoelectronic devices. However, this synthesis remains a significant challenge. The surface reactivity between the P atoms and the Au atoms should be considered for the P/Au(111) system. In the MBE process, the temperature of the substrate is a key parameter for the growth of blue phosphorene. During the initial growth stage, irregularly shaped Phosphorus clusters grow on top of Au(111) surface at room temperature. When the substrate temperature is increased, these clusters transform into a phosphorene-like structure with a honeycomb lattice. An atom exchange reaction is observed between the P and first layer Au atoms under thermal activation at higher temperature, where the P atoms replace Au atoms to form a blue phosphorene structure within the top Au layer and at the step edges.

Direct Observation of the Reduction of a Molecule on Nitrogen Pairs in Doped Graphene.

Incorporating functional atomic sites in graphene is essential for realizing advanced two-dimensional materials. Doping graphene with nitrogen offers the opportunity to tune its chemical activity with significant charge redistribution occurring between molecules and substrate. The necessary atomic scale understanding of how this depends on the spatial distribution of dopants, as well as their positions relative to the molecule, can be provided by scanning tunneling microscopy. Here we show that a noncovalently bonded molecule such as CoPc undergoes a variable charge transfer when placed on N-doped graphene; on a nitrogen pair, it undergoes a redox reaction with an integral charge transfer whereas a lower fractional charge transfer occurs over a single nitrogen. Thus, the charge state of molecules can be tuned by suitably tailoring the conformation of dopant atoms.

Charge transport in hybrid platinum/molecule/graphene single molecule junctions.

The single molecule conductance of hybrid platinum/alkanedithiol/graphene junctions has been investigated with a focus on understanding the influence of employing two very different contact types. We call this an "anti-symmetric" configuration, with the two different contacts here being platinum and graphene, which respectively provide very different electronic coupling to the alkanedithiol bridge. The conductance of these junctions is experimentally investigated by using a non-contact scanning tunneling microscopy (STM) based method called the I(s) technique. These experimental determinations are supported by density functional theory (DFT) calculations. These alkanedithiol bridging molecules conduct electric current through the highest occupied molecular orbital (HOMO), and junctions formed with Pt/graphene electrode pairs are slightly more conductive than those formed with Au/graphene electrodes which we previously investigated. This is consistent with the lower work function of gold than that of platinum. The measured conductance decays exponentially with the length of the molecular bridge with a low tunneling decay constant, which has a similar value for Pt/graphene and Au/graphene electrode pairs, respectively. These new results underline the importance of the coupling asymmetry between the two electrodes, more than the type of the metal electrode itself. Importantly, the tunneling decay constant is much lower than that of alkanedithiols with the symmetrical equivalent, i.e. identical metal electrodes. We attribute this difference to the relatively weak van der Waals coupling at the graphene interface and the strong bond dipole at the Pt-S interface, resulting in a decrease in the potential barrier at the interface.