Atsushi Taninaka

Pentagonal‐Dodecahedral La2 Charge Density in [80‐Ih]Fullerene: La2@C80

A double shell of Platonic solids: A trajectory that connects the six-membered rings of Ih -C80 is followed by the two La atoms of La2 @C80 according to maximum entropy/Rietveld analysis of the synchrotron powder-diffraction data. The result is a pentagonal-dodecahedral charge-density distribution for La2 (brown in the picture) inside the icosahedral C80 cage (green).

Production and EPR characterization of exohedrally perfluoroalkylated paramagnetic lanthanum metallofullerenes: (La@C82)–(C8F17)2

Abstract A strategy to chemically derivatize the outer sphere of endohedral metallofullerenes by using a fluorous synthesis-partitioning approach has been developed. The newly synthesized materials were found to be paramagnetic species and were characterized by electron paramagnetic resonance spectroscopy (EPR), laser desorption time-of-flight spectrometry (LD-TOF) and electronic absorption spectroscopy (UV–VIS–NIR). The fluorous-phase partitioning method (or liquid–liquid extraction), successfully applied for the first time in fullerene chemistry, aided by multi-stage recycling high performance liquid chromatography (HPLC) resulted in their isolation in isomer-free form. The present study opens the way to organic transformations of fullerene-based materials as a powerful separation technique.

Structural and electronic properties of ordered La@C82 films on Si(111)

The growth of endohedral fullerene La@C82 films on Si(1 1 1) has been explored by scanning tunneling microscopy (STM). The STM images reveal the molecules are hexagonally close-packed in multilayer films with the inter-molecular separation similar to that of the La@C82 crystal. Electronic properties of the films have been investigated by ultraviolet photoelectron spectroscopy (UPS) and tunneling spectroscopy, which provide comparable results for the electronic structure of the occupied electronic states. Tunneling spectroscopy also gives an insight into the unoccupied states and shows that the edges of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) both approach the Fermi level, indicating that the film is metallic or semi-metallic. An electronic structure of the La@C82 film is proposed from the spectroscopic results.

Reconsideration of Dynamic Force Spectroscopy Analysis of Streptavidin-Biotin Interactions

To understand and design molecular functions on the basis of molecular recognition processes, the microscopic probing of the energy landscapes of individual interactions in a molecular complex and their dependence on the surrounding conditions is of great importance. Dynamic force spectroscopy (DFS) is a technique that enables us to study the interaction between molecules at the single-molecule level. However, the obtained results differ among previous studies, which is considered to be caused by the differences in the measurement conditions. We have developed an atomic force microscopy technique that enables the precise analysis of molecular interactions on the basis of DFS. After verifying the performance of this technique, we carried out measurements to determine the landscapes of streptavidin-biotin interactions. The obtained results showed good agreement with theoretical predictions. Lifetimes were also well analyzed. Using a combination of cross-linkers and the atomic force microscope that we developed, site-selective measurement was carried out, and the steps involved in bonding due to microscopic interactions are discussed using the results obtained by site-selective analysis.

Dynamic-force spectroscopy measurement with precise force control using atomic-force microscopy probe

The accuracy of dynamic-force spectroscopy (DFS), a promising technique of analyzing the energy landscape of noncovalent molecular bonds, was reconsidered in order to justify the use of an atomic-force microscopy (AFM) cantilever as a DFS force probe. The advantages and disadvantages caused, for example, by the force-probe hardness were clarified, revealing the pivotal role of the molecular linkage between the force probe and the molecular bonds. It was shown that the feedback control of the loading rate of tensile force enables us a precise DFS measurement using an AFM cantilever as the force probe.

Ultrafast photoinduced carrier dynamics in GaNAs probed using femtosecond time-resolved scanning tunnelling microscopy

The combination of scanning tunnelling microscopy (STM) with optical excitation using ultrashort laser pulses enables us, in principle, to simultaneously obtain ultimate spatial and temporal resolutions. We have developed the shaken-pulse-pair-excited STM (SPPX-STM) and succeeded in detecting a weak time-resolved tunnelling current signal from a low-temperature-grown GaNAs sample. To clarify the underlying physics in SPPX-STM measurements, we performed optical pump‐probe reflectivity measurements with a wavelength-changeable ultrashort-pulse laser. By comparing the results obtained from the two methods with an analysis based on the nonlinear relationship between the photocarrier density and tunnelling current, we obtained a comprehensive explanation that the photocarrier dynamics is reflected in the SPPX-STM signal through the surface photovoltage effect. (Some figures in this article are in colour only in the electronic version)

Hidden variety of biotin–streptavidin/avidin local interactions revealed by site-selective dynamic force spectroscopy

By site-selective dynamic force spectroscopy realized with the combination of cross-linkers and anatomic force microscope with a force feedback system, we have revealed, for the first time, that the slight difference between the local structures of amino acid residues at the middle sites, SER45 and THR35 for streptavidin and avidin, respectively, strongly affects the microscopic reaction processes, i.e., the variation governs the type of bond as well as the fine structure of the potential landscape. For streptavidin, a bridged or direct hydrogen bond is induced depending on the molecular structure in the buffer solution. For avidin, in contrast, only a direct hydrogen bond is observed for all the buffer solutions used in the experiment. Since final functions in a system are realized through the assembly of local effects, the obtained results indicate the importance of analyzing the reaction processes with respect to the local structures of molecules, for further development of nanoscale functional devices.

Investigations into the fragmentation and ionization of highly excited La@C82

We report the first measurements of delayed ionization of the metallofullerene La@C82. When highly excited in laser desorption, La@C82 was found to ionize over several microseconds. The rate of delayed ionization of La@C82 was compared to C60 under the same extraction conditions, and found to be significantly different. A theoretical model was used for the fitting of the ion signal of the two fullerenes. The metallofullerene results can be fitted well over a time window <30 μs and are consistent with a low ionization potential (6.3 eV) and a high radiative cooling rate. The C60 data cannot be fitted over the same time window, indicative of an additional time-dependent competing decay channel.

Mechanically activated switching of Si-based single-molecule junction as imaged with three-dimensional dynamic probe

Understanding and extracting the full functions of single-molecule characteristics are key factors in the development of future device technologies, as well as in basic research on molecular electronics. Here we report a new methodology for realizing a three-dimensional (3D) dynamic probe of single-molecule conductance, which enables the elaborate 3D analysis of the conformational effect on molecular electronics, by the formation of a Si/single molecule/Si structure using scanning tunnelling microscopy (STM). The formation of robust covalent bonds between a molecule and Si electrodes, together with STM-related techniques, enables the stable and repeated control of the conformational modulation of the molecule. By 3D imaging of the conformational effect on a 1,4-diethynylbenzene molecule, a binary change in conductance with hysteresis is observed for the first time, which is considered to originate from a mechanically activated conformational change.

Formation of small lanthanum–carbide ions from laser induced fragmentation of La@C82

The fragmentation behavior of the endohedral metallofullerene La@C82 was studied using gas phase time-of-flight mass spectrometry. The results were compared with the fragmentation of C60. When the metallofullerene was excited by a 337 nm ns laser, small lanthanum–carbide fragments LaCn+, n=0–6, were detected. A simple statistical maximum entropy model was used to simulate the excitation energy dependence of the loss of the LaCn+ molecules as well as C2 evaporation to form smaller metallofullerenes. By comparing experimental ion intensities with the model, the appearance energies of LaCn+, n=0–6, were found to lie above 65 eV. The lanthanum–carbide fragments ejected from La@C82 decrease in size with increasing internal energy. The ejection of La+ and LaC2+ is preferred at the expense of larger fragments such as LaC4+ and metallofullerenes at the higher excitation.The fragmentation behavior of the endohedral metallofullerene La@C82 was studied using gas phase time-of-flight mass spectrometry. The results were compared with the fragmentation of C60. When the metallofullerene was excited by a 337 nm ns laser, small lanthanum–carbide fragments LaCn+, n=0–6, were detected. A simple statistical maximum entropy model was used to simulate the excitation energy dependence of the loss of the LaCn+ molecules as well as C2 evaporation to form smaller metallofullerenes. By comparing experimental ion intensities with the model, the appearance energies of LaCn+, n=0–6, were found to lie above 65 eV. The lanthanum–carbide fragments ejected from La@C82 decrease in size with increasing internal energy. The ejection of La+ and LaC2+ is preferred at the expense of larger fragments such as LaC4+ and metallofullerenes at the higher excitation.