The rational construction of desired molecular nanostructures should be required to realize molecular nanodevices, ideally with these structures supported on suitable substrates. We report the formation of surface-supported supramolecular structures whose size and aggregations patterns are rationally controlled by using selective and directional intermolecular interactions between cyanophenyl substituents. Using low-temperature scanning tunneling microscopy and molecular orbital calculations, we have analyzed the conformation and arrangement of adsorbed porphyrin molecules on Au(111) surface. In case of cyanophenyl-substituted porphyrins, monomers, trimers, tetramers, or extended wire-like structures is selectively formed on the Au(111) surface by modifying substituents. These structures should be induced by local dipole-dipole interaction between cyanophenyl substituents. Our findings indicate that the rational design and construction of surface-supported supramolecular architectures will be allowed by introducing non-covalent interactions.
The interest in single molecular electronics has recently grown due to the developments of scanning probe techniques and of precise synthesis of supramolecules. The electronic properties of molecules, which depends on the molecular orbital energy level relative to the Fermi level of the metal, can be modified by doping holes or electrons from a metal substrate. In order to study the electron transfer between the molecule and the metal substrate, we observed spatial distribution of effective tunnel barrier height by using scanning tunneling microscopy (STM). For completely fused porphyrin array, which is expected as a conducting molecular wire, the tunneling barrier height measured above the molecule is smaller than above the metal substrate, meaning that the molecule is positively charged. In case of deoxyribonucleic acid (DNA), whereas the molecular orbital is observed near Fermi level of the metal by photoemission analysis, the electron transfer was not detected by the STM, because deoxyribose-phosphate chains screen base pairs from the metal substrate.
Organic monolayers anchored to silicon surfaces via silicon-carbon covalent bonds were prepared by wet process and 2D-patterning of the surface was done by local oxidation with an atomic force microscope (AFM). The surfaces were anodized with a contact-mode AFM by applying a positive bias voltage to the surface with respect to the platinum-coated cantilever under ambient conditions, which resulted in nanometer-scale oxidation of the surfaces. The anodized areas were etched and terminated with hydrogen atoms by NH4F solution, in which various molecules having C=C bonds could be immobilized. We put allylamine molecules to which organic dyes such as fluorescein and porphyrin were anchored by chemical reactions. The intensity of luminescence varied depending on dopant concentration of the substrates possibly due to energy transfer between the dyes and substrates. The method demonstrated here is one of the promising way to fabricate 3 D-assemblies of molecular scale electronic devices on silicon with a stable interface.
This article describes our recent research results on chemical methodology for bonding one-dimensional silicon backbone polymer, or polysilane, onto a solid surface. The method, which we call end-graft technique, is based on the proper combination of polysilane chemistry and surface chemistry. The synthesis and characterization of end-grafted polysilanes are briefly introduced. Our special focus is on the solid surface-selective technique for grafting polysilanes on SiO2, Si(111) and Au surfaces.
We have formed insulated molecular wires of conducting polymers, polyaniline (PANI), and cyclodextrin (CD) or the molecular nanotubes synthesized from CDs, and observed the structures of the wires by atomic force microscopy (AFM). CD or the molecular nanotube can isolate a single conducting polymer chain by shielding the attractive interaction between polymer chains. In addition, since CD or the nanotube confines the conformation of the polymer chain to a rodlike one owing to the very small cavity, the defects of the π-conjugated system should be suppressed on the polymer chain. Further four-term electrodes with gaps of the order of 100 nm have been made by two different methods using AFM, the tip-induced local oxidation and AFM lithography, in order to measure the conductivity of the insulated molecular wires. With the electrodes fabricated by the tip-induced local oxidation, it was difficult to measure the conductivity because of the native oxide layer on the electrode surface. On the other hand, the AFM lithography provided us with the electrodes appropriate for the conductivity measurements of molecular wires.
Nanometer-scale measurements of the physical properties of materials are needed in nano-electronics. We constructed a new probing system, a dual-probe scanning tunneling microscope (D-STM) with two STM probes. The D-STM permits us to measure the current-voltage curves of a single nanotube ring (20−100 nm in diameter). We found that we could make the nanotube ring behave as a transistor. However, the D-STM we constructed cannot be applied to insulators, since the instrument used a conventional tunneling current response circuit. Then, we developed a triple-probe atomic force microscope (T-AFM) with three electric probes that could be connect to the samples. Using T-AFM, we have measured a three-terminal single molecule DNA device. These results suggest that there are possibilities of developing nanometer-scaled electronics circuits.
Core-level spectra of individual heteroepitaxial nanocrystals were measured with a spectroscopic photoelectron emission and low-energy electron microscope that allows laterally resolved photoemission spectroscopy. The nanocrystals were obtained by depositing nominally 2 and 4 monolayers of InAs on a Se-terminated GaAs(001) surface. Using the high lateral resolution of our microscope in combination with the well-known chemical sensitivity of synchrotron radiation photoelectron spectroscopy, we were able to obtain spectra from individual nanocrystals and to deduce their elemental composition. Consequently, during the growth of InAs, interdiffusion and segregation take place. Furthermore, the differences in work function and valence band edge energy between nanocrystals and the substrate were observed, which indicates that Se diffusion onto the InAs nanocrystals during nanocrystal growth leads to the formation of a Se termination of the InAs.