Kaoru Kimura

Catalyst-free fabrication of single crystalline boron nanobelts by laser ablation

We successfully fabricated single crystalline tetragonal boron nanobelts by a laser ablation method in a furnace without using a catalyst or hazardous boron vapor source. The boron nanobelts obtained are rectangular in crosssectional shape with a width-to-thickness ratio of about 5, several tens of nanometers to about 150 nm in width, and several micrometers to the order of millimeters in length. The interior is well crystallized in tetragonal structure and an amorphous structure 2–4 nm thick can be seen along the surface of the nanobelt. Only boron EELS peak was detected from single nanobelts. 2002 Elsevier Science B.V. All rights reserved.

Temperature dependence of electrical conductance in single-crystalline boron nanobelts

We studied electrical transport in single-crystalline boron nanobelts with α-tetragonal crystalline structure. We obtained ohmic contacts to the boron nanobelts by metal electrodes of Ni∕Au bilayer. From the temperature dependence of electrical conductance, we found that the boron nanobelt is a semiconductor. The electrical conductivity was of the order of 10−3(Ωcm)−1 at 295 K. Fitting the results to variable-range-hopping conduction revealed a high density of localized states at the Fermi level compared with bulk β-rhombohedral boron.

Electron density distributions in derivative crystals of α-rhombohedral boron

Boron carbide (B 12 C 3 ) and boron phosphide (B 12 P 2 ) have the similar structures to α-rhombohedral boron (α-B 12 ) and are considered to be derivative crystals of it. The peculiar bonds in these two derivative crystals are visualized by MEM/Rietveld analysis, which involves a combination of the maximum entropy method (MEM) and Rietveld refinement for powder X-ray diffraction (XRD) data. The distinctive bending of bonds observed in α-B 12 is reversed by an insertion of other atoms. This bending arises from the conflict between the crystal structure of the rhombohedral lattice and the icosahedral structure of the B 12 cluster. Bond strength and interatomic distances are also varied by the insertion. In order to estimate the bond strength, we obtained the electron density height at each bond critical point, and compared it with the force constant, which is derived from the Raman shift.

Dependence of photocurrent in single-crystalline boron nanobelts on atmosphere

This letter describes the dependence of photocurrent of single-crystalline boron nanobelts on the atmosphere. In ambient air, slow photoresponse under blue light illumination was observed. Rise and decay times exceeded three days. The magnitude of photoresponse in ambient air and oxygen was greater than that in hydrogen and argon atmospheres. In vacuum, a photoresistivity effect consisting of the continuous decrease of conductance under blue light illumination was observed. Variation of band bending of the nanobelt surface by adsorption or desorption of oxygen and water molecules appeared to switch the photoconduction on and off by the respective trapping and recombination of photoexcited carriers at the nanobelt core and surface.This letter describes the dependence of photocurrent of single-crystalline boron nanobelts on the atmosphere. In ambient air, slow photoresponse under blue light illumination was observed. Rise and decay times exceeded three days. The magnitude of photoresponse in ambient air and oxygen was greater than that in hydrogen and argon atmospheres. In vacuum, a photoresistivity effect consisting of the continuous decrease of conductance under blue light illumination was observed. Variation of band bending of the nanobelt surface by adsorption or desorption of oxygen and water molecules appeared to switch the photoconduction on and off by the respective trapping and recombination of photoexcited carriers at the nanobelt core and surface.

Electrical transport of tetragonal boron nanobelts

We measured electrical conductances in single crystalline boron nanobelts having α-tetragonal crystalline structure. The doping experiment of Mg was carried out by vapor diffusion method. We obtained ohmic contacts to both pure and Mg-doped nanobelt by metal electrodes of Ni∕Au bilayer. From the temperature dependence of electrical conductance, we found that the pure boron nanobelt is a semiconductor. The electrical conductivity of pure boron nanobelt was estimated to be on the order of 10−3(Ωcm)−1 at room temperature. After Mg vapor diffusion, the electrical conductance increased by a factor of 100 but a transition to metal was not observed.