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Thermal transport properties of patterned binary self-assembled monolayers (SAMs) on Au(111) were examined using scanning thermal microscopy (SThM) with both contact and noncontact methods. We fabricated two-dimensional (2D) patterns with two separate domains of n-hexadecanethiol/benzenethiol, benzenethiol/n-butanethiol, or n-hexadecanethiol/n-butanethiol. In the experimental setup, the efficiency of thermal transport from a SThM tip to the SAM surface can be evaluated in terms of the temperature change at the SThM tip. In the contact regime, where a SThM tip physically contacts the SAM surface, direct thermal transport through the SAM and radiation-based thermal transport through the space where SAMs exist may contribute to a drop in temperature at the tip. In the noncontact regime, thermal transport relies on radiation-based heat dissipation from the heated tip to the SAMs. 2D mapping of the spatial temperature distribution on SAMs reflects the difference in thermal transport properties of the two SAM domains. We found that the contact method is effective for visualizing the temperature contrast, which reflects the thermal transport properties of the constituent molecules when the domains of the SAMs have a similar height, while the noncontact method allows visualization of the temperature distribution, which is related to the height of each domain of the SAMs, rather than the chemical structures of the constituent molecules. Combination of contact and noncontact SThM enables 2D imaging of thermal transport properties and topographic imaging simultaneously and represents a new technique for investigating the thermal properties of materials surfaces, which is essential for nanoscale thermal management.

Electric switches are one of the most fundamental electric circuitries. Recent developments in single-molecule techniques allow us to study various electric-switching phenomena on the single-molecule scale. In this study, the switch of the current through a single molecule of a biphenothiazine derivative was investigated using the break junction technique. The biphenothiazine derivative undergoes acid-base reaction-induced electronic modulation due to the reversible structural transformation between a closed shell and an open shell, resulting in a change in the electronic gap and effective tunneling barrier height. We succeeded in controlling the single-molecule electric conductivity of the biphenothiazine derivative wired to two Au electrodes by allowing the reaction to proceed reversibly on the electrode.

A triphosphasumanene trisulfide was designed and synthesized as an out-of-plane anisotropic π-conjugated molecule. Incorporating three anisotropic phosphine sulfide moieties into a sumanene skeleton induced a cumulative anisotropy with a large dipole moment (12.0 D), which is aligned in perpendicular direction with respect to the π-framework and more than twice as large as those of conventional out-of-plane anisotropic molecules. In the crystal, the molecules align to form columnar structures, in which electron-rich and electron-deficient sides of the π-framework face each other. The interactions between the electron-rich surfaces, which contain three sulfur atoms, and Au(111) were examined by X-ray photoelectron spectroscopy.

The electronic structure of molecular junctions has a significant impact on their transport properties. Despite the decisive role of the electronic structure, a complete characterization of the electronic structure remains a challenge. This is because there is no straightforward way of measuring electron spectroscopy for an individual molecule trapped in a nanoscale gap between two metal electrodes. Herein, a comprehensive approach to obtain a detailed description of the electronic structure in single-molecule junctions based on the analysis of current–voltage (I–V) and thermoelectric characteristics is described. It is shown that the electronic structure of the prototypical C60 single-molecule junction can be resolved by analyzing complementary results of the I–V and thermoelectric measurement. This combined approach confirmed that the C60 single-molecule junction was highly conductive with molecular electronic conductances of 0.033 and 0.003 G0 and a molecular Seebeck coefficient of −12 μV K−1. In addition, we revealed that charge transport was mediated by a LUMO whose energy level was located 0.5≈0.6 eV above the Fermi level of the Au electrode.

To gain a fundamental understanding of atomic- and molecular-scale electronic transport phenomena, as well as the practical application of a single-molecule junction for molecular electronics, it is necessary to establish a reliable electronic and structural characterization method for single-molecule junctions. Recent advances in characterization methods make it possible to elucidate the unique electronic functionality of a quasi-one-dimensional conductor for a single-molecule junction. This review presents some of the current methods used to address the electronic and structural details of a single-molecule junction. We discuss the potential applications of single-molecule junctions as molecular switches, diodes, and transistors in single-molecule electronic devices.

Single-molecule junctions, in which a single molecule bridges a gap between metal electrodes, have attracted significant attention due to their potential applications in ultra-small electronic devices and their unique structure. Single-molecule junctions are one-dimensional nanomaterials having two metal–molecule interfaces. Thus, unconventional properties and functionalities that would not be observed in other phases (e.g., isolated molecules and bulk crystals) are expected to appear in these nanomaterials. Despite interest in these expected unconventional properties, several issues have been noted with the investigation and practical application of the unique properties of single-molecule junctions. To explore new functionality, we have investigated single-molecule junctions using a combined approach comprising fabrication, characterization, and measurement. First, we have explored a new generation of the metal–molecule interfaces formed by direct π-bonding. The interfaces made by the direct π-bonding h...

The concise synthesis of C3-symmetrical [12]CPP-hexacarboxylate has been achieved through macrocyclization by the rhodium-catalyzed intermolecular cross-cyclotrimerization and subsequent reductive aromatization. C3-Symmetrical functionalization of CPP with highly polar alkoxycarbonyl groups enabled the structurally uniform nanotube assembly in the crystalline state through multiple hydrogen-bonding interactions giving a dimer followed by one-dimensional stacking.

By use of the reversible trimerization of boronic acids, the series of boroxine cages 3-mer, 6-mer, and 12-mer were constructed from rationally designed diboronic acids whose bond angles between two C-B bonds are 60°, 84°, and 117°, respectively. Boroxine cages 6-mer and 12-mer have 1.5 and 2.5 nm sized cavities, respectively.

Aromatic stacks formed through self-assembly are promising building blocks for the construction of molecular electronic devices with adjustable electronic functions, in which noncovalently bound π-stacks act as replaceable modular components. Here we describe the electron-transport properties of single-molecule aromatic stacks aligned in a self-assembled cage, using scanning probe microscopic and break junction methods. Same and different modular aromatic pairs are noncovalently bound and stacked within the molecular cage holder, which leads to diverse electronic functions. The insertion of same pairs induces high electronic conductivity (10(-3)-10(-2) G0, G0 = 2e(2)/h), while different pairs develop additional electronic rectification properties. The rectification ratio was, respectively, estimated to be 1.4-2 and10 in current-voltage characteristics and molecular orientation-dependent conductance measurements at a fixed bias voltage. Theoretical calculations demonstrate that this rectification behavior originates from the distinct stacking order of the internal aromatic components against the electron-transport direction and the corresponding lowest unoccupied molecular orbital conduction channels localized on one side of the molecular junctions.

We have studied the electrical conductance of the single 1,2-ethanedithiol molecular junction prepared in ultrahigh vacuum (UHV). The single molecular junction was fabricated by breaking the Au contacts with mechanically controllable break junction technique in the presence of 1,2-ethanedithiol. The conductance of the single 1,2-ethanedithiol molecular junction was 0.13 G0 (2e2/h), larger than the value expected for the single molecular junction fabricated in the molecular solution. The present study revealed that the molecular junctions created in UHV exhibited lower contact resistances due to the clean vacuum environment.

Atomic-scale charge-transport properties of Au–Ag ion clusters aligned as wires in host molecular cages were measured by a scanning tunneling microscopy-based break junction method. The Au–Ag ion wires were prepared by doping guest Ag ions into Au ion wires through a self-assembly process. We demonstrated that the electronic conductance of the Au ion wires can be modulated by 10 times by doping the guest Ag ion based on statistical analysis of the electronic investigations.

A single molecular resistive (conductance) switch via control of anchoring positions was examined by using a molecule consisting of more than two same anchors. For this purpose, we adopted the covered quaterthiophene (QT)-based molecular wire junction. The QT-based wire consisted of two thiophene ring anchors on each side; thus, shift of anchors was potentially possible without a change in the binding modes and distortion of the intramolecular structure. We observed three distinct conductance states by using scanning tunneling microscope-based break junction technique. A detailed analysis of the experimental data and first-principles calculations revealed that the mechanism of the resistive switch could be explained by standard length dependence (exponential decay) of conductance. Here, the length is the distance between the anchoring points, i.e., length of the bridged π-conjugated backbone. Most importantly, this effective tunneling length was variable via only controlling the anchoring positions in the same molecule. Furthermore, we experimentally showed the possibility of a dynamic switch of anchoring positions by mechanical control. The results suggested a distinct strategy to design functional devices via contact engineering.

Graphene can be referred to as an infinite polycyclic aromatic hydrocarbon (PAH) consisting of an infinite number of benzene rings fused together. However, at the nanoscale, nanographene's properties lie in between those of bulk graphene and large PAH molecules, and its electronic properties depend on the influence of the edges, which disrupt the infinite π-electron system. The resulting modulation of the electronic states depends on whether the nanographene edge is the armchair or zigzag type, corresponding to the two fundamental crystal axes. In this Account, we report the results of fabricating both types of edges in the nanographene system and characterizing their electronic properties using a scanning probe microscope. We first introduce the theoretical background to understand the two types of finite size effects on the electronic states of nanographene (i) the standing wave state and (ii) the edge state which correspond to the armchair and zigzag edges, respectively. Most importantly, characterizing the standing wave and edge states could play a crucial role in understanding the chemical reactivity, thermodynamic stability and magnetism of nanosized graphene--important knowledge in the design and realization of promising functionalized nanocarbon materials. In the second part, we present scanning probe microscopic characterization of both edge types to experimentally characterize the two electronic states. As predicted, we find the armchair-edged nanographene to have an energetically stable electronic pattern. The zigzag-edged nanographene shows a nonbonding (π radical) pattern, which is the source of the material's electronic and magnetic properties and its chemical activity. Precise control of the edge geometry is a practical requirement to control the electronic structure. We show that we can fabricate the energetically unstable zigzag edges using scanning probe manipulation techniques, and we discuss challenges in using these techniques for that purpose.

The link between changes in the material crystal structure and its mechanical, electronic, magnetic, and optical functionalities known as the structure-property relationship is the cornerstone of the contemporary materials science research. The recent advances in scanning transmission electron and scanning probe microscopies (STEM and SPM) have opened an unprecedented path towards examining the materials structure property relationships on the single-impurity and atomic-configuration levels. Lacking, however, are the statistics-based approaches for cross-correlation of structure and property variables obtained in different information channels of the STEM and SPM experiments. Here we have designed an approach based on a combination of sliding window Fast Fourier Transform, Pearson correlation matrix, linear and kernel canonical correlation, to study a relationship between lattice distortions and electron scattering from the SPM data on graphene with defects. Our analysis revealed that the strength of coupling to strain is altered between different scattering channels which can explain coexistence of several quasiparticle interference patterns in the nanoscale regions of interest. In addition, the application of the kernel functions allowed us extracting a non-linear component of the relationship between the lattice strain and scattering intensity in graphene. Lastly, the outlined approach can be further utilized to analyzing correlations in various multi-modalmore » imaging techniques where the information of interest is spatially distributed and has usually a complex multidimensional nature.« less

Bowl-shaped π-conjugated compounds, or buckybowls, are a novel class of sp(2)-hybridized nanocarbon materials. In contrast to tubular carbon nanotubes and ball-shaped fullerenes, the buckybowls feature structural flexibility. Bowl-to-bowl structural inversion is one of the unique properties of the buckybowls in solutions. Bowl inversion on a surface modifies the metal-molecule interactions through bistable switching between bowl-up and bowl-down states on the surface, which makes surface-adsorbed buckybowls a relevant model system for elucidation of the mechano-electronic properties of nanocarbon materials. Here, we report a combination of scanning tunneling microscopy (STM) measurements and ab initio atomistic simulations to identify the adlayer structure of the sumanene buckybowl on Au(111) and reveal its unique bowl inversion behavior. We demonstrate that the bowl inversion can be induced by approaching the STM tip toward the molecule. By tuning the local metal-molecule interaction using the STM tip, the sumanene buckybowl exhibits structural bistability with a switching rate that is two orders of magnitude faster than that of the stochastic inversion process.

Edges of finite size graphene, particularly nanographene, play an important role in giving unconventional electronic structure owing to the massless Dirac fermion features. This emerges as the creation of nonbonding edge state localized in the zigzag-shaped edges and electron wave interference in the armchair-shaped ones. We fabricated zigzag-edged and armchair-edged graphene nanoribbons, and investigated the Raman G-band and STM superlattice which are governed by the electron wave interference at the armchair edges.

We report a simple approach to producing nanosized graphene on the basis of chemical oxidation of a graphene sheet followed by cutting of the sheet using a scanning probe microscopic (SPM) manipulation technique. The structural and electronic properties of the oxidized sheet are characterized by noncontact atomic force microscopic (NC-AFM) imaging and SPM spectroscopy under ultrahigh vacuum conditions. Regularly spaced linear defects with a spacing of 5-10 nm and a length of100 nm were found on the sheet, which can be attributed to the result of linear arrangement of epoxide functional groups. The cutting experiments are performed on sheets in which the linear defects were observed in advance. Cutting is initiated by a point contact between the preoxidized sheet and the AFM probe. The local mechanical stress caused by the point contact leads to rupture of the sheet, which proceeds linearly along the linear defect of the epoxide groups. We propose that the linear defect structures can be used as a template to determine the cutting direction of the sheet. According to recently proposed theoretical predictions, the linear epoxide groups have preferential alignment along a zigzag direction in the graphene lattice, and therefore, the cut edge shape could have well-defined alignment along the zigzag direction. This cutting procedure of the graphene sheet could be a useful method for the production of nanosized graphene with well-defined edges.

H- and Cl-terminal groups of bicyclo[2.2.2]octane (BCO) derivatives in a mixed self-assembled monolayer (SAM) on Au(111) were imaged using a modified Si tip with a CaF(2) nanocluster to differentiate the two terminals, which have different electronegativities. In order to achieve this we fabricated a new sample holder, on which a CaF(2) single crystal and the mixed SAM on Au(111) could be mounted side by side. We transferred the holder with the two samples into a ultrahigh vacuum (UHV) atomic force microscopy (AFM) chamber. Upon cleaving the CaF(2) single crystal under UHV, a fresh and clean CaF(2)(111) surface parallel with the SAM surface appeared within 2 mm of the separation. The modified Si tip was prepared by repeatedly making contact between a Si tip and the CaF(2)(111) surface. The resulting modified tip could image the atomic periodicity of a Ca(2+) and an F(-) sublattice on the CaF(2)(111) surface depending on the sign of the tip-terminating ion, i.e. an F(-) and a Ca(2+) ion, respectively, as reported previously (Foster et al 2002 Phys. Rev. B 66 235417). Using the modified Si tip with the known tip-terminating ion, we observed the Cl-terminal in the surrounding H-terminals in the mixed SAM by noncontact (NC) AFM. Here, the Cl-terminal is negatively charged due to its electronegativity and thus the BCO moiety with the Cl-terminal is terminated by a C(δ+)-Cl(δ-) permanent dipole, while the H-terminal is almost neutral. The Cl-terminal appeared brighter (more attractive) and darker (more repulsive) than the surrounding H-terminals in NC-AFM images depending on the sign of the tip-terminating ion, i.e. a Ca(2+) and an F(-) on the modified tip, respectively, although the relationship between the image contrast and the sign of the tip-terminating ion was not always perfect because of the instability of the tip-terminating ion on the nanocluster. The present method can be used to distinguish terminal groups with different electronegativities.

A self-assembled monolayer (SAM) of a disulfide with a pair of bicyclo[2.2.2]octane (BCO) moieties was prepared on Au(1 1 1). The resulting SAMs were separately imaged by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (NC-AFM) under ultra-high vacuum at room temperature. The images of the BCO SAM by NC-AFM were similar to those by STM, in which adsorbed molecules were hexagonally packed with the spacing of ca. 6.2 nm.

We performed simultaneous measurements of the thermopower, and conductance of Au atomic contacts during the self-breaking process under temperature control. During the whole measurement temperature regime (290-330 K), the thermopower randomly fluctuated from positive to negative in sign, and the average thermopower was negligibly small with respect to the variation of the thermopower of the contact. Meanwhile, the standard deviation of the thermoelectric voltage increased linearly with the temperature difference across the contacts. Above 320 K, we observed a decrease in the standard deviation of thermopower, which suggested a decrease in the density of defects near the contacts. The linear increase in the standard deviation of the thermoelectric voltage, and the decrease in the standard deviation of the thermopower above 320 K, indicate that the standard deviation of thermopower provides insight into the thermopower of an individual Au atomic contact and the atomic structure of Au atomic contacts, such as crystallinity and the distribution of defects near the contacts.

Cut into pieces: The π-electron distribution in nanographene fragments isolated between oxidized graphene areas is investigated using scanning probe microscopy (see picture). The edge-shape-dependent localization and migration of the Clar sextet explains the observed π-state distributions and enables investigation of the electronic properties.

We observed the dynamic behaviour of a single conjugated molecule, 4-biphenylmethanethiol (BPMT), inserted in a self-assembled monolayer (SAM) matrix of a bicyclo[2.2.2]octane derivative (BCO) using scanning tunnelling microscopy at . At room temperature (Wakamatsu et al 2003 Nanotechnology 14 258), stochastic switching of BPNT and lateral movement of the BPMT and the BCO molecules were observed even in the well-ordered BCO SAM matrix on Au(111) terraces. Such movement was, however, rarely observed in the SAM on Au(111) terraces at . Single BPMT molecules were fixed in the well-ordered BCO SAM matrix at , except at step edges and edges of one atom deep pits of the Au(111) substrate. When a disordered BCO SAM matrix was created intentionally by applying voltage pulses, the stochastic switching of the single BPMT molecule and the lateral movement of the BCO and the BPMT molecules were observed more frequently than those at the step edges and the pit edges at .

A mixed self-assembled monolayer (SAM) of two kinds of thiolate with a hydrogen- or a chlorine-terminated bicyclo[2.2.2]octane moiety was formed on Au(111). The terminal groups (X = H and Cl) on the bicyclo[2.2.2]octane moieties were identified with noncontact atomic force microscopy (NC-AFM) using a Si tip with a native oxide layer.

Self-assembled monolayers (SAMs) of thiolates with a cycloalkane or a bicycloalkane moiety were formed on Au(111). The structure of the SAMs was characterized using scanning tunnelling microscopy (STM) and compared with that of linear alkane thiolates. In addition, their electrical properties in terms of dependence of tunnelling currents on the number of the alkane chains in the alicyclic moieties were investigated. For this purpose, we prepared mixed SAMs, in which two of three kinds of thiolate were molecularly mixed. By comparing STM height differences between the two thiolates in the mixed SAMs in a constant-current mode, the charge transport properties were investigated.

An organodisulfide with a pair of adamantane moieties was synthesized, and its self-assembled monolayer (SAM) was formed on Au(111). The adamantane moieties are almost spherical and much bulkier than alkyl chains. The structure was characterized by scanning tunneling microscopy. Two-dimensional crystals of the SAM were found to be four orientationally different hexagonals with almost the same lattice constant with 4 radical 3a/3 and 7a/3 (a = 0.2884 nm, the Au lattice constant). The structure is assigned to four of the high-order commensurate adlayers. The present study of geometry and energetics for self-assembling of such an organosulfur compound with spherical cages provides a new insight into the probable SAM structure of various thiolate derivatives on Au(111).

A chiral allyltitanium compound 2, prepared in situ by the reaction of optically active acrolein 1,2-dicyclohexylethylene acetal (3) with (eta(2)-propene)Ti(O-i-Pr)(2) (1), reacts with a variety of acyclic and cyclic imines 4 in a regiospecific way to afford alpha-addition products 5 as a mixture of the E- and Z-isomers in good combined yield, where the former is predominant in a ratio of 92:8 to95:5. The mixture of (E)- and (Z)-5 and pure (E)-5 which could be isolated in several cases were respectively converted to the corresponding beta-amino ester 6 to confirm the absolute configuration and enantiomeric purity. The ee of the newly formed asymmetric center of 5 is more than 78% for the mixture of (E)- and (Z)-5 and more than 96% for pure (E)-5. By taking advantage of the versatility of the vinyl ether moiety in 5, optically active gamma-amino aldehydes 8, gamma-amino aldehyde acetals 7 and 10, gamma-amino acids 9, beta-amino esters 6, and pyrrolidinoisoquinolines 12 were readily prepared. In the reaction of 2 with optically active alpha-silyloxyimine 4n, remarkable double stereodifferentiation was observed; thus, the reaction of 2 derived from (S,S)- or (R,R)-3 provided syn- and anti-5n in a ratio of 55:45 or 0:100, respectively. Meanwhile, the stereochemistry of the product in the reaction of 2 with beta-silyloxyimine 4o was controlled mainly by 2. Thus, the reaction of beta-silyloxyimine 14 with 2 derived from 1 and (R,R)-3 afforded gamma-silyloxyimine 15 with 92% diastereoselectivity, from which 4-amino-6-hydroxypentadecanal dimethyl acetal (13), a key intermediate for the synthesis of batzelladine D, was synthesized.

We demonstrate here a new sample preparation method satisfying requirements for ideal single-molecule conductance measurements using break junction methods. By diluting a targeted dithiol in an ins...

A series of disulfides containing bicyclo[2.2.2]octane moieties have been synthesised and their self-assembled monolayers (SAMs) on Au(111) have been characterized using scanning tunnelling microscopy (STM).

A point defect on a cleaved CaF2(111) surface is imaged by non-contact atomic force microscopy in ultrahigh vacuum at room temperature to identify the nature of the point defect. First, atomic contrast formation on the point defect is examined using a negatively and a positively charged tip (i.e. F−- and Ca2+-terminated tips), which are prepared by modifying a Si tip with a CaF2 cluster. Analysis of the two types of atomic contrast imaged by the F−- and Ca2+-terminated tips reveals that the point defect has a positive charge and the positive charge is located on a Ca2+ sublattice. Second, the lateral mobility of the point defect is investigated. We repeated imaging on the point defect by decreasing the tip–sample distance. At the smaller tip–sample distance, the point defect moved laterally to one of six neighbouring sites of the defect when the tip scanned the surface across the point defect. Taking into account (i) the atomic contrast on the point defect by the charged tips with a known terminating ion (i.e. F− or Ca2+) and (ii) the lateral mobility of the point defect, we find a probable simple model of the point defect among six possible candidates.