Reduction-induced switching of single-molecule conductance of fullerene derivatives

The effect of electrochemical reduction on the single-molecule conductance of fullerene C60 derivatives was studied by scanning tunneling microscopy. Three types of C60 derivatives were synthesized, a monoadduct having an amino-terminated linker and two bisadducts having two linkers at different positions (trans2 and trans3). Each C60 derivative was immobilized on a gold surface by an amino-gold linkage, confirmed by infrared reflection-absorption spectroscopy. The immobilized C60 derivatives showed reversible and multiple reduction peaks in the cyclic voltammogram in dimethylformamide (DMF) at almost the same potentials as those in solution, showing the redox properties of the molecules are intact on gold. Single-molecule conductances of the bisadducts, which can span between a scanning tunneling microscopy (STM) tip made of gold and substrate with the two linkers, were determined by the STM break-junction measurements in water and DMF. The conductances were 6.1+/-4.5 nS in water and 4.9+/-1.7 nS in DMF for the trans2 bisadduct and 8.4+/-3.4 nS in water and 7.9 nS+/-2.8 in DMF for the trans3 bisadduct. By using a potential-controlled STM setup, the tunneling current through a single molecule was recorded with sweeping the potentials of the tip and substrate. The trans2 bisadduct showed significant changes in the current when the reductions of the C60 moiety occur. Some current curves showed multiple peaks, and the other curves showed stepwise increase and decrease at the C60 reduction and subsequent reoxidation. Statistical analysis afforded stepwise switching of the conductance as the average behavior and suggested that the electron tunneling through the C60 derivative is enhanced as it accepts electrons.     

Current-voltage characteristics of a homologous series of polycyclic aromatic hydrocarbons

A novel alkyl-substituted polycyclic aromatic hydrocarbon (PAH) with D(2h) symmetry and 78 carbon atoms in the aromatic core (C78) was synthesized, thereby completing a homologous series of soluble PAH compounds with increasing size of the aromatic pi system (42, 60, and 78 carbon atoms). The optical band gaps were determined by UV/Vis and fluorescence spectroscopy in solution. Scanning tunneling microscopy (STM) and spectroscopy (STS) revealed diode-like current versus voltage (I-V) characteristics through individual aromatic cores in monolayers at the interface between the solution and the basal plane of graphite. The asymmetry of the current-voltage (I-V) characteristics increases with the increasing size of the aromatic core, and the concomitantly decreasing HOMO-LUMO gap. This is attributed to resonant tunneling through the HOMO of the adsorbed molecule, and an asymmetric position of the molecular species in the tunnel junction. Consistently, submolecularly resolved STM images at negative substrate bias are in good agreement with the calculated pattern for the electron densities of the HOMOs. The analysis provides the basis for tailoring rectification with a single molecule in an STM junction.     

Vibronic states in single molecules: C60 and C70 on ultrathin Al2O3 films

Vibronic states are observed in single C(60) and C(70) molecules by scanning tunneling microscopy. When single fullerene molecules are adsorbed on a thin layer of Al(2)O(3) grown on a NiAl(110) substrate, equally spaced features are observed in the differential conductance (dI/dV), which are clearly resolved in d(2)I/dV(2) spectra. These features are attributed to the vibronic states of the molecule. The vibronic progressions are sensitive to the molecular orientations and can have different spacings in different electronic bands of the same molecule. For C(60,) these vibronic states are associated with the intramolecular A(g) and H(g) vibrational modes. Vibronic states are not resolved in molecules adsorbed on the metal surface. However, inelastic electron tunneling spectroscopy exhibits a vibrational mode at 64 meV for C(60) and 61 meV for C(70) adsorbed on NiAl(110).     

Single-molecule chemistry and analysis: mode-specific dehydrogenation of adsorbed propene by inelastic electron tunneling

A single propene molecule, located in the junction between the tip of a scanning tunneling microscope (STM) and a Cu(211) surface can be dehydrogenated by inelastic electron tunneling. This reaction requires excitation of the asymmetric C-H stretching vibration of the ═CH(2) group. The product is then identified by inelastic electron tunneling action spectroscopy (IETAS).     

Vibrational spectroscopy of individual doping centers in a monolayer organic crystal

A scanning tunneling microscope (STM) is used to study individual Ag doping centers in a monolayer of C60 molecules supported on a thin Al2O3 film grown on the NiAl(110) surface. Vibronic states of the doping centers are observed with differential conductance (dIdV) spectroscopy. The double-barrier nature of the junction results in bipolar transport: same states participate in charge transport at both bias voltage polarities. Identification of the dIdV features corresponding to bipolar conduction enables a new mode of vibrational spectroscopy with STM.     

STM study of morphology and electron transport features in cytochrome c and nanocluster molecule monolayers.

The morphology and electron tunneling through single cytochrome c and nanocluster Pt(5)(CO)(7)[P(C(6)H(5))](4) molecules organized as monolayer Langmuir-Blodgett (LB) films on graphite substrate have been studied experimentally using scanning tunneling microscopy (STM) and spectroscopy techniques with sub-nanometer spatial resolution in a double barrier tunnel junction configuration STM tip-monomolecular film-conducting substrate at ambient conditions. STM images of the films revealed globular structures with characteristic diameters (approximately 3.5 nm for the protein molecule and approximately 1.2 nm for the nanocluster). The spectroscopic study by recording the tunneling current-bias voltage (I-V) curves revealed tunneling I-V characteristics with features as steps of different width and heights that are dependent on the STM tip position over the molecule in the monolayer, giving evidence for sequential discrete electron-tunneling effects with the combination of the single electron Coulomb-charging energy and the electronic energy level separation (molecular spectrum) in such immobilized metalloprotein and nanocluster structures that can be of interest for the development of bioelectronic and hybrid functional nanosystems.     

隧道结电场辅助下叔丁胺分子在Cu(111)表面跳跃(英文)

采用扫描隧道显微镜(STM)于78 K研究了单个叔丁胺分子在Cu(111)表面的横向跃迁现象.研究发现叔丁胺分子的跳跃几率随隧道电流的增加而线性增加,这表明该过程是单电子激发过程;在不同极性的隧道结电场作用下,叔丁基胺分子跳跃行为发生的几率不同,这种现象可以用电场辅助的扩散过程解释.在不同极性电场作用下叔丁胺分子在Cu(111)表面的吸附能和扩散势垒不同,从而表现出不同的跳跃几率.     

SERS and tunneling spectroscopy investigation of iron-protoporphyrin IX adsorbed on a silver tip

Iron-protoporphyrin IX adsorbed on a scanning tunneling microscopy silver tip is investigated by combining surface enhancement Raman (SERS) and tunneling spectroscopies down to single molecule regime. Both the Raman signals and the tunneling current intensity reveal fast switching between the iron oxidation states and present analogous, significant fluctuations in time. The results point out the occurrence of a strong electronic coupling between the molecule and the metal, and may contribute to elucidate the dynamical phenomena at the molecule-metal interface, deserving some nanotechnological interest.     

Schemes for Single Electron Transistor Based on Double Quantum Dot Islands Utilizing a Graphene Nanoscroll,Carbon Nanotube and Fullerene

The single electron transistor (SET) is a nanoscale switching device with a simple equivalent circuit. It can work very fast as it is based on the tunneling of single electrons. Its nanostructure contains a quantum dot island whose material impacts on the device operation. Carbon allotropes such as fullerene (C₆₀), carbon nanotubes (CNTs) and graphene nanoscrolls (GNSs) can be utilized as the quantum dot island in SETs. In this study, multiple quantum dot islands such as GNS-CNT and GNS-C₆₀ are utilized in SET devices. The currents of two counterpart devices are modeled and analyzed. The impacts of important parameters such as temperature and applied gate voltage on the current of two SETs are investigated using proposed mathematical models. Moreover, the impacts of CNT length, fullerene diameter, GNS length, and GNS spiral length and number of turns on the SET’s current are explored. Additionally, the Coulomb blockade ranges (CB) of the two SETs are compared. The results reveal that the GNS-CNT SET has a lower Coulomb blockade range and a higher current than the GNS-C₆₀ SET. Their charge stability diagrams indicate that the GNS-CNT SET has smaller Coulomb diamond areas, zero-current regions, and zero-conductance regions than the GNS-C₆₀ SET.     

Electron transfer processes in scanning tunneling spectroscopy through small supported particles

Mechanism of the staircase like I–V curve observed recently in scanning tunneling spectroscopy (STS) of a metal fine particle supported on an oxide covered substrate is clarified based on theoretical simulations. It is discussed how the step structures are influenced by the coupling of the fine particle charge to the remaining degrees of freedom, such as induced charge in the surrounding medium. Relation between the characteristic features of single electron tunneling (SET) and the STS of micro-clusters is discussed     

Long-range electron transfer through monolayers and bilayers of alkanethiols in electrochemically controlled Hg[bond]neling junctions

The rates of electron tunneling through monolayers and bilayers of alkanethiols self-assembled in a potentiostatically controlled Hg-Hg junction are reported. An alkanethiolate monolayer is formed in situ on one or both Hg drops via oxidative adsorption at the controlled potential. Subsequently, the Hg drops are brought into contact using micromanipulators. The junction formation is instantly followed by the flow of a steady-state tunneling current between the two electrodes. A plot of the logarithm of the tunneling current density vs the total number of carbon atoms in each junction yields identical tunneling coefficients, beta = 1.06 +/- 0.04/-CH(2)- and beta = 1.02 +/- 0.07/-CH(2)-, for monolayers and bilayers of alkanethiols, respectively. Careful examination of the tunneling data indicates that the solvent and ions are ejected from the junction area. The tunneling current recorded for a bilayer of 1-octanethiol or 1-nonanethiol is ca. 2-fold larger than a corresponding tunneling current recorded for monolayers of 1-hexadecanethiol or 1-octadecanethiol, respectively. This result is explained in terms of weak electronic coupling across the noncovalent molecule/electrode interface.     

Adsorbed states and scanning tunneling microscopy induced migration of acetylene on Cu(110)

The authors have studied adsorption of acetylene on Cu(110) by means of low-temperature scanning tunneling microscopy. Adsorbed molecules preferentially aggregate at 40 K to yield dimer, trimer, and larger islands on the surface. Isolated species (monomer) adsorbs on the fourfold hollow site with approximately sp3 rehybridization as characterized by inelastic electron tunneling spectroscopy. Tunneling electron induces an acetylene molecule to migrate along the trough of Cu(110). The migration proceeds in two steps: the molecule first hops to the adjacent long-bridge site and then to the next fourfold site. The voltage and current dependencies of the hopping probability show that the migration is induced by inelastic electron tunneling that causes vibrational excitation of mainly C-H stretch mode.     

Spin-dependent transport through a magnetic carbon nanotube-molecule junction

The electronic structure and spin-dependent conductance of a magnetic junction consisting of two Fe-doped carbon nanotubes and a C60 molecule are investigated using a first-principles approach that combines the density functional theory with the nonequilibrium Greens function technique. The tunneling magnetoresistance ratio is found to be 11%. The density of states and transmission coefficient through the molecular junction are analyzed and compared to layered magnetic tunneling junctions. Our findings suggest new possibilities for experiments and for future technology.     

Intermolecular dynamics in crystalline iron octaethylporphyrin (FeOEP)

The new technique of nuclear resonance vibrational spectroscopy (NRVS) has increased the range and quality of dynamical data from Fe-containing molecules that when combined with Raman and infrared spectroscopies impose stricter constraints on normal mode simulations, especially at lower frequencies. Going beyond the usual single molecule approximation, a classical normal-mode analysis that includes intermolecular coupling and the full crystalline symmetry is found to produce a better fit with fewer free parameters for the heme compound iron octaethylporphyrin (FeOEP), using NRVS data from polycrystalline material. Off-diagonal force constants were completely unnecessary, indicating that their role in previous single molecule fits was just to emulate intermolecular coupling. Sound velocities deduced from the calculated phonon dispersion curves are compared to NRVS measurements to further constrain the intermolecular force constants. The NRVS data by themselves are insufficient to rigorously determine all unknown force constants for molecules of this size, but the improved crystal model fit indicates the necessity of including intermolecular interactions for normal-mode analyses.     

Surface reactions of 3-butenenitrile on the Si(001)-2 x 1 surface at room temperature

Using a combination of local -- scanning tunneling microscopy -- and spatially integrated, but chemically sensitive probes -- X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure spectroscopy -- we have examined how 3-butenenitrile reacts with the Si(001)-2 x 1 surface at room temperature. Electron spectroscopies indicate three different nitrogen chemical bonds: a Si-C=N-Si bond, a C=C=N cumulative double bond, and a CN moiety datively bonded to a silicon atom. All molecular imprints detected by scanning tunneling microscopy (STM) involve two adjacent silicon dimers in the same row. The three geometries we propose -- a double di-sigma bonding via the CN and the C=C, a cumulative double bond formation associated with alphaC-H bond dissociation, and a di-sigma vinyl bonding plus a CN datively bonded to a silicon atom -- are all compatible with electron spectroscopies and data. Real-time Auger yield kinetic measurements show that the double di-sigma bonding geometry is unstable when exposed to a continuous flux of 3-butenenitrile molecules, as the Si-C=N-Si unit transforms into a CN moiety. A model is proposed to explain this observation.     

Mercury-platinum tunneling junctions incorporating supramolecular host-guest assemblies

Molecular junctions incorporating monolayers of surface-active cyclodextrins have been formed by bringing macroscopic platinum and mercury electrodes together. Monolayers of the host di-6A,6B-deoxy-6-(4-pyridylmethyl)amino-gamma-cyclodextrin (CD) have been formed on platinum electrodes and backfilled with 1-nonanethiol. Significantly, the tunneling current for bilayer junctions in which the mercury surface is modified with a C16 alkane thiol depends markedly on the nature of the CD guest. Junctions where nonconjugated guests, such as adamantane, are included in the CD show an order of magnitude lower current than junctions incorporating the conjugated guest C60. Moreover, monolayer junctions of CD backfilled with 1-nonanethiol exhibit potential-dependent currents in the presence of CD guest molecule [Co(biptpy)2](2+) but not for [Co(tpy)2](2+), which is structurally analogous but cannot associate with CD. Significantly, unlike bilayer systems, when only one electrode is modified, mechanical pressure is required to form a stable tunneling junction and these junctions cannot be formed and separated reversibly.     

Noncontact to contact tunneling microscopy in self-assembled monolayers of alkylthiols on gold

The mechanical interaction between a scanning tunneling microscopy (STM) probe and hexadecane (C16) alkylthiol molecules in a self-assembled monolayer was investigated by sensing the force during constant current mode STM imaging. The force regime changed from attractive to repulsive over the insulating molecule islands under feedback control of the current. The repulsive force on the molecule was strongly dependent on the setpoint value of the current during STM operation. In our experiments, the threshold for contact was found at a tunneling current of 1 pA when the sample bias is 2 V. At higher current, the apparent height of molecular islands changed logarithmically with current. In addition, the current as a function of applied load revealed a stepwise increase, indicative of discrete molecular tilting events. A tunneling decay constant beta of =0.53+/-0.02 A(-1) was obtained based on the measurement of the height of molecules and the tunneling current.     

Infrared absorption and electron paramagnetic resonance studies of vinyl radical in noble-gas matrices

Vinyl radicals produced by annealing-induced reaction of mobilized hydrogen atoms with acetylene molecules in solid noble-gas matrices (Ar, Kr, and Xe) were characterized by Fourier transform infrared and electron paramagnetic resonance (EPR) spectroscopies. The hydrogen atoms were generated from acetylene by UV photolysis or fast electron irradiation. Two vibrational modes of the vinyl radical (nu7 and nu5) were assigned in IR absorption studies. The assignment is based on data for various isotopic substitutions (D and 13C) and confirmed by comparison with the EPR measurements and density-functional theory calculations. The data on the nu7 mode is in agreement with previous experimental and theoretical results whereas the nu5 frequency agrees well with the computational data but conflicts with the gas-phase IR emission results.     

Propensity rules for inelastic electron tunneling spectroscopy of single-molecule transport junctions

Using a perturbative approach to simple model systems, we derive useful propensity rules for inelastic electron tunneling spectroscopy (IETS) of molecular wire junctions. We examine the circumstances under which this spectroscopy (that has no rigorous selection rules) obeys well defined propensity rules based on the molecular symmetry and on the topology of the molecule in the junction. Focusing on conjugated molecules of C(2h) symmetry, semiquantitative arguments suggest that the IETS is dominated by a(g) vibrations in the high energy region and by out of plane modes (a(u) and b(g)) in the low energy region. Realistic computations verify that the proposed propensity rules are strictly obeyed by medium to large-sized conjugated molecules but are subject to some exceptions when small molecules are considered. The propensity rules facilitate the use of IETS to help characterize the molecular geometry within the junction.     

Polarized near-edge x-ray-absorption fine structure spectroscopy of C60-functionalized 11-amino-1-undecane thiol self-assembled monolayer: molecular orientation and evidence for C60 aggregation

Near-edge x-ray-absorption fine structure (NEXAFS) spectroscopy was adopted to probe the unoccupied electronic states of C60 anchored onto an organized assembly of 11-amino-1-undecane thiol on Au(111). The polarization dependence of the intensity of pi* resonance associated with C60 pi network revealed the self-assembled monolayer (SAM) system to be oriented with an average molecular tilt angle of 57 degrees with respect to the surface normal. Invoking the absence of solid-state band dispersion effects and in comparison to solid C60 and /or 1-ML C60/Au(111), the electronic structure of the resulting assembly was found dominated by spectral position shift and linewidth and intensity changes of the lowest unoccupied molecular orbital (LUMO), LUMO+1, and LUMO+2 orbitals. The latter implied hybridization between N Pz of -NH2 group of thiolate SAM and pi levels of C60, resulting in a nucleophilic addition with a change in the symmetry of C60 from Ih to C1 in the SAM. Occurrence of a new feature at 285.3 eV in the NEXAFS spectrum, assigned previously to pi* graphitic LUMO, signified the formation of aggregated clusters, (C60)n of C60 monomer. Low tunneling current scanning tunneling microscopy confirmed them to be spherical and stable aggregates with n approximately 5.