Kenji Kisoda

Spectroscopic observation of oxidation process in InN

Spectroscopic observations of high-quality wurtzite InN have shown that oxygen is easily incorporated in the crystal by thermal treatments in the air. Incorporation of oxygen may play a key role in determining the apparent properties of InN including the bandgap and the lattice constant. It is shown that Raman scattering is a sensitive tool to probe the oxygen incorporation process and associated deterioration in crystallinity.

Chiral sum frequency spectroscopy of thin films of porphyrin J-aggregates.

Thin films of chiral porphyrin J-aggregates have been studied by vibrationally and electronically doubly resonant sum frequency generation (SFG) spectroscopy. It was revealed that the chiral supramolecular structures of porphyrin aggregates in solutions were retained in the thin film samples, and their chirality was determined by using chiral vibrational SFG spectroscopy. Electronic resonance profiles of some vibrational bands in achiral and chiral SFG were different from each other, and both were distinct from electronic absorption spectra. To account for these peculiar profiles, we have proposed interference effects of Raman tensor components in achiral and chiral SFG susceptibilities, which is analogous to that of resonance Raman scattering.

Influence of the growth morphology of single-walled carbon nanotubes on gas sensing performance

We investigated the impact of the growth morphology of single-walled carbon nanotubes (SWNTs) on gas sensing performance. An SWNT film was directly synthesized on alumina substrate by thermal chemical vapour deposition. Different morphologies of the SWNTs in terms of density, diameter distribution and orientation were obtained by varying the growth temperature. Vertically aligned SWNTs with a high density were grown at 750 °C, while horizontally lying SWNT networks with a low density were grown in the temperature range 800–950 °C. The sensor response of the resultant SWNTs to NO2 was characterized at room temperature. It was found that the density of SWNTs strongly dominates sensor performance; the SWNT networks with the lowest density exhibited the highest sensor sensitivity. This was evidenced by characterization of density-controlled SWNTs synthesized using different thicknesses of an Fe/Al multilayer catalyst. The high sensor sensitivity for low-density SWNT networks is likely to be attributed to suppression of the formation of SWNT bundles and reduction of narrow-band-gap conduction paths, resulting in the enhancement of the adsorption probability and chemical gating efficiency of gas molecules on SWNTs.

Raman determination of structures of long‐period SiC polytypes

Raman spectroscopy has been applied to identification of longer‐period SiC polytypes. The stacking structures of 51R and 132R polytypes have been examined, the result of which is consistent with electron‐ and x‐ray‐diffraction analyses. The possibility is discussed for Raman determination of the period and stacking structure of longer period polytypes. It is demonstrated that Raman scattering is useful for the determination of the structure of SiC polytypes.

Catalytic mechanism of a Fe–Co bimetallic system for efficient growth of single-walled carbon nanotubes on Si∕SiO2 substrates

Single-walled carbon nanotubes (SWCNTs)were grown on Si∕SiO2 substrates by catalytic chemical vapor deposition using ethanol as a carbon source. We found that a bimetallic catalyst, Fe–Co, worked efficiently in growing SWCNTs in contrast to Fe or Co catalyst only. The underlying mechanism was carefully studied by observing the catalyst-substrate interactions by transmission electron microscopy. It is shown that preferential diffusion of Fe into a SiO2 layer occurs in the bimetallic case, which suppresses aggregation of nanometer- sized Co particles that play key roles in growing SWCNTs.

Direct Growth of Single-Walled Carbon Nanotube Networks on Alumina Substrate: A Novel Route to Ultrasensitive Gas Sensor Fabrication

We explored structural and electrical properties of single-walled carbon nanotube (SWNT) networks directly grown on alumina substrates. The netlike SWNTs were uniformly grown on the substrate at a high quality. From the Raman spectroscopy analysis it was found that the as-grown SWNT networks were a mixture of metallic and semiconducting SWNTs, while the SWNT networks after their electrical breakdown exhibited a predominance of the semiconducting property. The direct growth method and subsequent electrical breakdown facilitated high-throughput production of practical ultrasensitive sensors for pollutant gases with a high sensitivity, which was demonstrated by NO2 detection.

Growth of Single-Walled Carbon Nanotubes Rooted from Fe/Al Nanoparticle Array

We explored the growth of single-walled carbon nanotubes (SWNTs) from nanoparticle array made of an Fe/Al multilayer catalyst by thermal chemical vapor deposition. Fe nanoparticles with a high number density and a narrow size distribution (1–5 nm) were efficiently formed by annealing the Fe/Al thin layer, resulting in the high-yield growth of SWNTs. Moreover, it was found that isolated SWNTs are rooted from patterned Fe/Al islands. The SWNTs bridged between electrodes exhibited semiconducting and metallic properties.

Peculiar noise properties of phonons generated by femtosecond laser pulses in antimony

In femtosecond pump-probe experiments on antimony, we have detected coherent oscillations that exhibit phase-dependent noise. The Fourier transforms of the coherent amplitude and its variance show that the two fluctuate at different frequencies, suggesting that the phonons created in the pump-probe experiment are of a squeezed nature.

Few-layer epitaxial graphene grown on vicinal 6H–SiC studied by deep ultraviolet Raman spectroscopy

Few layer epitaxial graphenes (1.8–3.0 layers) grown on vicinal 6H–SiC (0001) were characterized by deep ultraviolet Raman spectroscopy. Shallow penetration depth of the probe laser enabled us to observe G-peak of graphene without subtraction of the SiC substrate signal from observed spectra. The G-peak was greatly shifted to higher frequency compared to that of graphite due to in-plane compressive stress deriving from the substrate. The frequency shift decreased with the number of graphene layers because of stress relaxation from layer to layer. Our experiment suggests that the stress is completely relaxed within five to six graphene layers.

Synthesis and Characterization of Mn-Doped BiFeO_3 Nanoparticles

BiFeO3 is a multiferroic material showing antiferromagnetic ordering and ferroelectric behavior simultaneously. Here, Mn-doped BiFeO3 nanoparticles were synthesized up to 10% of Mn composition by a sol-gel process. The samples showed high crystallinity with no secondary phase up to 2% of Mn doping. A phonon peak at 1250 cm−1 in undoped BiFeO3 showed anomalous intensity enhancement in the magnetically ordered phase below TN = 643 K due to a spin-phonon coupling. This behavior was less pronounced in the Mn-doped samples, suggesting a suppression of magnetic ordering between Fe spins by Mn doping.