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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.

We report a simple and facile procedure to control molecular orientation in a single-molecule junction and relevant rectification properties. Rectification properties of single-molecule junctions consisting of tripodal triptycene and C60 moieties at the termini that couple to the Au electrodes were determined using the break junction method. Because of the strong affinity of the tripodal triptycene moiety to the Au electrode, the molecular orientation and the related rectification direction are uniquely determined. Statistical analysis of the I–V curves of the single-molecule diode reveals the robustness of the rectification behavior with a statistically significant rectification ratio of 1.7.

We report on an experimental analysis of the charge transport properties of single mesitylene (1,3,5-trimethylbenzene) molecular junctions. The electronic conductance and the current–voltage characteristics of mesitylene molecules wired into Au electrodes were measured by a scanning tunnelling microscopy-based break-junction method at room temperature in a liquid environment. We found the molecular junctions exhibited two distinct conductance states with high conductance values of ca. 10−1G0 and of more than 10−3G0 (G0 = 2e2/h) in the electronic conductance measurements. We further performed a statistical analysis of the current–voltage characteristics of the molecular junctions in the two states. Within a single channel resonant tunnelling model, we obtained electronic couplings in the molecular junctions by fitting the current–voltage characteristics to the single channel model. The origin of the high conductance was attributed to experimentally obtained large electronic couplings of the direct π-bonded molecular junctions (ca. 0.15 eV). Based on analysis of the stretch length of the molecular junctions and the large electronic couplings obtained from the I–V analysis, we proposed two structural models, in which (i) mesitylene binds to the Au electrode perpendicular to the charge transport direction and (ii) mesitylene has tilted from the perpendicular orientation.

The length dependence of tunneling current through single phenylene oligomers, i.e., benzenemethanethiol, 4-biphenylmethanethiol and [1,1':4',1''-terphenyl]-4-methanethiol, was determined experimentally by scanning tunneling microscopy (STM). The single phenylene oligomers were isolated in a self-assembled monolayer (SAM) matrix of a disulfide with two spherical bicyclo[2.2.2]octane moieties on Au(111). The STM measurement was conducted at a low temperature to prevent the thermal motions of the isolated molecules that occur at room temperature. The isolated single molecules inserted in the SAM matrix were observed as protrusions in an STM topography using a constant-current mode owing to their higher tunneling ability. The decay constant, β, of the tunneling current through single phenylene oligomers was estimated from the STM heights of the protrusions corresponding to the single phenylene oligomers using a bilayer tunnel junction model. The value of β for tunneling current through single phenylene oligomers was 5.5 ±0.2 nm-1. In a constant-height mode, we measured the conductance of the isolated single molecules. We estimated an averaged single-molecular conductance of the isolated molecules of a biphenyl derivative to be ca. 2 nS. Here, one side of each isolated molecule was bound via an Au–S bond and the other phenyl end was physically in contact with an Au STM tip.

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.

We measured currents through the single molecules of 1,6-hexanedithiol and 1,8-octanedithiol using a scanning tunneling microscope (STM) under ultrahigh vacuum (UHV). Self-assembled monolayers (SAMs) of alkanedithiols were formed on Au(111) surfaces of gold films deposited on mica by dipping them in each 1 mM ethanol solution of alkanedithiols for 1 min. The sample with each SAM was introduced into the UHV-STM chamber. The current was recorded as a bare Au STM tip was repeatedly pushed against and drawn away from the sample surface. In this way, the single molecules of each alkanedithiol compound bridged a gap between the Au(111) surface and the Au STM tip apex by forming S–Au bonds at both ends of the molecules. The single-molecular conductivities Gm6 and Gm8 of alkanedithiols (N=6, 8) in this break junction were found to be approximately 9.4×10-5G0 and 1.3×10-5G0 (G0: quantum conductance), respectively.

The cleaved (111) surfaces on CaF2 were imaged by frequency modulation atomic force microscopy (FM-AFM) in ultrahigh vacuum at room temperature. High-resolution imaging showed the perfect atomic periodicity of a F- sublattice in the outermost layer or a Ca2+ sublattice in the second layer. Gentle contact between a modified Si tip with a CaF2 nanocluster and the CaF2(111) surface induced formation of atomic size defects on the perfect atomic periodicity. We observed two types of atomic contrast on the point defect during successive imaging. In a constant-height FM-AFM image, one appeared as a dark spot corresponding to a region of weaker attraction between the tip and the surface, while the other appeared as a bright spot. The change in the atomic contrast on the point defect can be explained by the change in the sign of a tip-terminating ion.

In recent years, π-conjugated organic compounds have become a widely investigated class of advanced materials as they exhibit a variety of interesting electronic properties. One of the most attractive properties is their small highest occupied molecular orbital-lowest unoccupied molecular orbital gap, which results in the very high charge transport efficiency of the π-conjugated compounds as compared with that of saturated compounds. A new π-conjugated compound was synthesized by a recently developed method. We investigated electronic conduction through an isolated single molecule of the π-conjugated compound by scanning tunneling microscopy (STM) and self-assembly techniques.

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.

Molecular diodes are an interesting topic in the field of single-molecule electronics. Rectification of molecules such as 1,4-aminobenzenethiol (ABT) having different contact areas was reported. However, a more statistical approach is necessary to clarify the rectification of the ABT single-molecule junctions. In this research, we statistically measured the single molecular conductance and I–V characteristics of ABT single-molecule junctions using the scanning tunneling microscope break junction (STM-BJ) method. Two single molecular conductances caused to difference of bridging geometries were observed in the conductance measurements. Statistically significant rectification was not observed for ABT junctions. We concluded rectification does not appear only due to the difference of two anchoring groups in case of a small conjugate molecule such as ABT.