In situ wiring of single molecules into an electrical circuit via electrochemical distance tunneling spectroscopy

The electrical conductance of single n-alkanethiol and alpha,omega-alkanedithiol molecules was measured via in situ distance tunneling spectroscopy in aqueous 0.1 M KOH solution. The statistical analysis of the conductance values show that the alpha,omega-alkanedithiol molecule trapped in the STM break junction can adopt two distinct geometries that result in "lower" and "higher" conductivity values. In contrast, n-alkanethiol molecules trapped in the junction show only a single conductivity value characteristic for a particular molecule. Furthermore, the "lower" conductivity value determined for alpha,omega-alkanedithiol is virtually identical to the electrical conductivity of the n-alkanethiol containing the same number of atoms in the backbone. Moreover when the STM tip is polarized to electrochemical potential preventing a chemical reaction between the terminal -SH group and Au, only "lower" conductivity values are observed for alpha,omega-alkaneditiols.     

Observation of single dinuclear metal-complex molecules using scanning tunneling microscopy

We report a scanning tunneling microscopy (STM) investigation of a dinuclear organometallic molecule, trans-[Cl(dppe)2Ru(C[triple bond]C)6Ru(dppe)2Cl] (Ru2), absorbed on a Au(111) surface; this molecule is a potential candidate for use in molecular quantum-dot cellular automata (QCA) devices. Isolated Ru2 molecules were observed under ultra-high-vacuum conditions. Submolecular structure was clearly discernible in the STM images, with a bright feature corresponding to each of the two Ru-ligand complexes within the Ru2 molecule. Rotation and translation of the Ru2 molecules were observed to be induced by the STM tip under some tunneling conditions.     

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.     

Conductivity in alkylamine/gold and alkanethiol/gold molecular junctions measured in molecule/nanoparticle/molecule bridges and conducting probe structures

Charge transport through alkane monolayers on gold is measured as a function of molecule length in a controlled ambient using a metal/molecule/nanoparticle bridge structure and compared for both thiol and amine molecular end groups. The current through molecules with an amine/gold junction is observed to be more than a factor of 10 larger than that measured in similar molecules with thiol/gold linkages. Conducting probe atomic force microscopy is also used to characterize the same monolayer systems, and the results are quantitatively consistent with those found in the nanoparticle bridge geometry. Scaling of the current with contact area is used to estimate that approximately 100 molecules are probed in the nanoparticle bridge geometry. For both molecular end groups, the room-temperature conductivity at low bias as a function of molecule length shows a reasonable fit to models of coherent nonresonant charge tunneling. The different conductivity is ascribed to differences in charge transfer and wave function mixing at the metal/molecule contact, including possible effects of amine group oxidation and molecular conformation. For the amine/Au contact, the nitrogen lone pair interaction with the gold results in a hybrid wave function directed along the molecule bond axis, whereas the thiol/Au contact leads to a more localized wave function.     

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.     

Optical and electrical properties of three-dimensional interlinked gold nanoparticle assemblies

The optical and electrical properties of 11-20 nm thick films composed of approximately 4 nm gold nanoparticles (Au-NPs) interlinked by six organic dithiol or bis-dithiocarbamate derivatives were compared to investigate how these properties depend on the core of the linker molecule (benzene or cyclohexane) and its metal-binding substituents (thiol or dithiocarbamate). Films prepared with the thiol-terminated linker molecules, (1,4-bis(mercaptomethyl)benzene, 1,4-bis(mercaptomethyl)cyclohexane, 1,4-bis(mercaptoacetamido)benzene, and 1,4-bis(mercaptoacetamido)cyclohexane), exhibit thermally activated charge transport. The activation energies lie between 59 and 71 meV. These films show distinct plasmon absorption bands with maxima between 554 and 589 nm. In contrast, the film prepared with 1,4-cyclohexane-bis(dithiocarbamate) has a significantly red-shifted plasmon band ( approximately 626 nm) and a pronounced absorbance in the near infrared. The activation energy for charge transport is only 14 meV. These differences are explained in terms of the formation of a resonant state at the interface due to overlap of the molecular orbital and metal wave function, leading to an apparent increase in NP diameter. The film prepared with 1,4-phenylene-bis(dithiocarbamate) exhibits metallic properties, indicating the full extension of the electron wave function between interlinked NPs. In all cases, the replacement of the benzene ring with a cyclohexane ring in the center of the linker molecule leads to a 1 order of magnitude decrease in conductivity. A linear relationship is obtained when the logarithm of conductivity is plotted as a function of the number of nonconjugated bonds in the linker molecules. This suggests that nonresonant tunneling along the nonconjugated parts of the molecule governs the electron tunneling decay constant (beta(N)(-)(CON)), while the contribution from the conjugated parts of the molecule is weak (corresponding to resonant tunneling). The obtained value for beta(N)(-)(CON) is approximately 1.0 (per non-conjugated bond) and independent of the nanoparticle-binding group. Hence, the molecules can be viewed as consisting of serial connections of electrically insulating (nonconjugated) and conductive (conjugated) parts.     

Electron Transport through Single π‐Conjugated Molecules Bridging between Metal Electrodes

Understanding electron transport through a single molecule bridging between metal electrodes is a central issue in the field of molecular electronics. This review covers the fabrication and electron‐transport properties of single π‐conjugated molecule junctions, which include benzene, fullerene, and π‐stacked molecules. The metal/molecule interface plays a decisive role in determining the stability and conductivity of single‐molecule junctions. The effect of the metal–molecule contact on the conductance of the single π‐conjugated molecule junction is reviewed. The characterization of the single benzene molecule junction is also discussed using inelastic electron tunneling spectroscopy and shot noise. Finally, electron transport through the π‐stacked system using π‐stacked aromatic molecules enclosed within self‐assembled coordination cages is reviewed. The electron transport in the π‐stacked systems is found to be efficient at the single‐molecule level, thus providing insight into the design of conductive materials.     

Chemical imaging of single 4,7,12,15-tetrakis[2.2]paracyclophane by spatially resolved vibrational spectroscopy

Single 4,7,12,15-tetrakis[2.2]paracyclophane were deposited on NiAl(110) surface at 11 K. Two adsorbed species with large and small conductivities were detected by the scanning tunneling microscope (STM). Their vibrational properties were investigated by inelastic electron tunneling spectroscopy (IETS) with the STM. Five vibrational modes were observed for the species with the larger conductivity. The spatially resolved vibrational images for the modes show striking differences, depending on the coupling of the vibrations localized on different functional groups within the molecule to the electronic states of the molecule. The vibrational modes are assigned on the basis of ab initio calculations. No IETS signal is resolved from the species with the small conductivity.     

Self-assembly of small polycyclic aromatic hydrocarbons on graphite: a combined scanning tunneling microscopy and theoretical approach

Self-assembled monolayers of chrysene and indene on graphite have been observed and characterized individually with scanning tunneling microscopy (STM) at 80 K under low-temperature, ultrahigh vacuum conditions. These molecules are small, polycyclic aromatic hydrocarbons (PAHs) containing no alkyl chains or functional groups that are known to promote two-dimensional self-assembly. Energy minimization and molecular dynamics simulations performed for small groups of the molecules physisorbed on graphite provide insight into the monolayer structure and forces that drive the self-assembly. The adsorption energy for a single chrysene molecule on a model graphite substrate is calculated to be 32 kcal/mol, while that for indene is 17 kcal/mol. Two distinct monolayer structures have been observed for chrysene, corresponding to high- and low-density assemblies. High-resolution STM images taken of chrysene with different bias polarities reveal distinct nodal structure that is characteristic of the molecular electronic state(s) mediating the tunneling process. Density functional theory calculations are utilized in the assignment of the observed electronic states and possible tunneling mechanism. These results are discussed within the context of PAH and soot particle formation, because both chrysene and indene are known reaction products from the combustion of small hydrocarbons. They are also of fundamental interest in the fields of nanotechnology and molecular electronics.     

Molecular assembly of rubrene on a metal/metal oxide nanotemplate

We investigated the adsorption properties and self-assembly of rubrene molecules on the copper oxide nanotemplate formed by high-temperature exposure of Cu(110) to molecular oxygen. Using high-resolution scanning tunneling microscopy under ultrahigh-vacuum conditions, we observed a complex variety of self-assembled motifs, driven by competing effects such as the chemical affinity between the organic molecule and the surface, surface coverage, and spatial confinement of the rubrene molecules within the rows of the template.     

Single‐Molecule Sensing of Environmental pH—an STM Break Junction and NEGF‐DFT Approach

Sensors play a significant role in the detection of toxic species and explosives, and in the remote control of chemical processes. In this work, we report a single‐molecule‐based pH switch/sensor that exploits the sensitivity of dye molecules to environmental pH to build metal–molecule–metal (m‐M‐m) devices using the scanning tunneling microscopy (STM) break junction technique. Dyes undergo pH‐induced electronic modulation due to reversible structural transformation between a conjugated and a nonconjugated form, resulting in a change in the HOMO–LUMO gap. The dye‐mediated m‐M‐m devices react to environmental pH with a high on/off ratio (≈100:1) of device conductivity. Density functional theory (DFT) calculations, carried out under the non‐equilibrium Green’s function (NEGF) framework, model charge transport through these molecules in the two possible forms and confirm that the HOMO–LUMO gap of dyes is nearly twice as large in the nonconjugated form as in the conjugated form.     

Imaging water on Ag(111): field induced reorientation and contrast inversion

Water adsorbed on Ag(111) at 70 K forms circular clusters that consist of six molecules. In scanning tunneling microscopy, this cyclic hexamer is imaged as a protrusion for voltages below V(SS)=-93 meV and as a depression for voltages above V(SS). The electronic density of states, however, increases around V(SS). We explain this counterintuitive result with the aid of calculated images by a change from constructive to destructive interference between different tunneling channels due to a field induced reorientation of the molecule under the tunneling tip.     

Diffusion of Formaldehyde on Rutile TiO2(110) Assisted by Surface Hydroxyl Groups

As the photo-dissociation product of methanol on the TiO₂(110) surface,the diffusion and desorption processes of formaldehyde (HCHO) were investigated by using scanning tunneling microscope (STM) and density functional theory (DFT).The molecular-level images revealed the HCHO molecules could diffuse and desorb on the surface at 80 K under UV laser irradiation.The diffusion was found to be mediated by hydrogen adatoms nearby,which were produced from photodissociation of methanol.Diffusion of HCHO was significantly decreased when there was only one H adatom near the HCHO molecule.Furthermore,single HCHO molecule adsorbed on the bare TiO₂(110) surface was quite stable,little photo-desorption was observed during laser irradiation.The mechanism of hydroxyl groups assisted diffusion of formaldehyde was also investigated using theoretical calculations.     

Chemisorption and dissociation of single oxygen molecules on Ag110

The chemisorption of single oxygen molecules on Ag110 and the dissociation of the adsorbed molecules induced by tunneling electrons were studied at 13 K using a variable-low-temperature scanning tunneling microscope. Two predominant types of chemisorbed O2 molecules were identified, one with the O2 molecular axis aligned along the [001] direction of the substrate [O2(001)], and the other with the molecular axis aligned along the [110] direction [O2(110)]. Tunneling of electrons between the scanning tunneling microscope tip and O2(001) caused the molecule either to rotate or dissociate, depending on the direction of electron tunneling. In contrast, electron tunneling caused O2(110) to dissociate regardless of tunneling direction. In addition to O2(001) and O2(110), several other oxygen species and their dynamical behaviors were observed.     

Topography Structure and Scanning Tunneling Spectrum of Nickel（Ⅱ）-tetraphenylporphyrin Molecules on Au（111）

1 INTRODUCTION Metalloporphyrins are intensively studied for many reasons. They have been comprehensively used in biochemistry, analytical chemistry and so on. They play an important role in biological processes such as oxygen transport photosynthesis and enzyme catalysis. They can act as catalysts[1], and can undergo reversible redox reactions in which the site of electron transfer may be localized on the por- phyrin ring or on the central metal ion. Both reaction types are important in…     

Topography Structure and Scanning Tunneling Spectrum of Nickel(Ⅱ)-tetraphenylporphyrin Molecules on Au(111)

Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) were performed on monolayer film of NiTPP supported on Au(111) under ultrahigh vacuum (UHV) conditions. The constant current STM images show remarkable bias dependence. High resolution STM data clearly show the individual NiTPP molecules and allow easy differentiation between NiTPP and CoTPP reported before. Scanning tunneling spectra, as a function of molecule-tip separation, were acquired over a range of tip motion of 0.42 nm. Spectra do not show the variation in band splitting with tip distance. It appears for molecules such as NiTPP that the average potential at the molecule is essentially the same at the same metal substrate. For molecules of the height of NiTPP, the scanning tunneling spectra should give reliable occupied and unoccupied orbital energies over a wide range of tip-molecule distances.     

The initiation and characterization of single bimolecular reactions with a scanning tunneling microscope

A scanning tunneling microscope (STM) operating at 9 K in ultrahigh vacuum was used to initiate a bimolecular reaction between isolated hydrogen sulfide and dicarbon molecules on the Cu(001) surface. The reaction products ethynyl (CCH) and sulfhydryl (SH) were identified by inelastic electron tunneling spectroscopy (STM-IETS) and by sequentially removing hydrogen atoms from an H2S molecule using energetic tunneling electrons. For comparison, the thermal diffusion and reaction of H2S and CC at 45 K and H2O and CC at 9 K were also observed.     

Selective effect of guest molecule length and hydrogen bonding on the supramolecular host structure

A host supramolecular structure consisting of bis-(2,2':6',2' '-terpyridine)-4'-oxyhexadecane (BT-O-C16) is shown to respond to guest molecules in dramatic ways, as observed by using scanning tunneling microscopy (STM) on a highly oriented pyrolytic graphite surface under ambient conditions. It is observed that small linear molecules can be encapsulated within the host supramolecular lattice. The characteristics of the host structure were nearly unaffected by the encapsulated guest molecules of terphthalic acid (TPA) dimers, whereas appreciable changes in cavity dimension can be observed with azobenzene-4,4'-dicarboxylic acid. The STM study and density functional theory (DFT) analysis reveal that intermolecular hydrogen bonding interaction plays an essential role in forming the assembling structures. The difference in guest molecule length is considered the important cause for the different guest-host complexes.     

An investigation of the change in the electrical conductivity during photochemical reactions in solutions of safranine T

The reduction of electrical conductivity on illumination has been investigated in greater detail for a pyridine solution of safranine, and has also been observed for some dyestuffs of similar structure. It is suggested that the cause of the conductivity reduction is due to a diminution of the mobility of the cation of the dyestuff due to complex-formation between this and the molecules of the reducing agent due to the phototransfer of an electron. It has been shown experimentally that change in conditions may lead to a change in the sign of the effect, giving an increase in conductivity on illumination, which may be explained by the capacity of the molecule to undergo transition from reduction to oxidation under the changed conditions.     

Hydrophilicity and Microsolvation of an Organic Molecule Resolved on the Sub‐molecular Level by Scanning Tunneling Microscopy

Low‐temperature scanning tunneling microscopy was used to follow the formation of a solvation shell around an adsorbed functionalized azo dye from the attachment of the first water molecule to a fully solvated molecule. Specific functional groups bind initially one water molecule each, which act as anchor points for additional water molecules. Further water attachment occurs in areas close to these functional groups even when the functional groups themselves are already saturated. In contrast, water molecules surround the hydrophobic parts of the molecule only when the two‐dimensional solvation shell closes around them. This study thus traces hydrophilic and hydrophobic properties of an organic molecule down to a sub‐molecular length scale.