Noriaki Sugimoto

Template synthesis of nanoparticle arrays of gold, platinum and palladium in mesoporous silica films and powders

Mesoporous silica films were prepared on glass plates by a dip coating method, and the film/plate samples were used as a support for Au, Pt and Pd nanoparticles. By H2- or photo-reduction of metal precursors impregnated on the films, metal nanoparticles are formed with an ordered array structure in the mesopores. The nanoparticles are isolated by dissolving the silica film with a dilute HF solution, and the particles are stabilized with ligands such as 1-dodecanethiol and triphenylphosphine. The diameter of the separated nanoparticles is 2.5 nm with narrow distributions, showing that uniform particles are formed in the mesopores. Selective formation of metal nanoparticles rather than nanowires suggests that a 3D-hexagonal structure is involved in the 2D-hexagonal mesoporous silica films. To study the formation of nanoparticles in a 3D-hexagonal structure, HMM-2 powder was used as a support, and in fact Au and Pt nanoparticles are formed in the HMM-2 powders. The XRD study suggests that the Au nanoparticles in the film/plate have an anisotropic orientation in the mesoporous channels. The Au/film/plate sample gives a weak plasmon peak at 570–580 nm using diffuse-reflectance UV-visible spectroscopy, which is shifted to high wavelength by 50–60 nm compared with larger Au particles.

Novel templating fabrication of nano-structured Pt clusters and wires in the ordered cylindrical mesopores of FSM-16 and their unique properties in catalysis and magnetism

Abstract The robust trigonal prismatic cluster anions such as [Pt3(CO)6]52− and [Pt3(CO)6]62− were selectively synthesized in the ordered hexagonal channels (2.8 and 4.7 nm diameter) of FSM-16 and ZrO2-modified FSM-16 by a ‘ship-inbottle’ technique in the reductive carbonylation of H2PtCl6 with CO + H2O at 323 K. The cluster anions were characterized by FTIR, UV-visible, TEM and EXAFS spectroscopy. The Zr oxide-modified FSM-16 was prepared by CVD method using Zr(2-propoxyl)4 impregnated with FSM-16, followed by calcination at 723 K. The controlled removal of CO from [Pt3(CO)6]52− and [Pt3(CO)6]62− in FSM-16 by thermal evacuation at 300–423 K yields nanostructured Pt clusters of 1.5–2 nm diameter highly dispersed in the channels of FSM-16. By contrast, Pt nano-wires were prepared by a templating reduction of Pt salts impregnated in FSM-16 (2.8 and 4.7 nm) under photo- and γ-ray irradiation (see Scheme 1). They exhibited unique properties in CO chemisorption and magnetism due to their anisotropic morphology of Pt nano-wires aligned in the ordered mesoporous channels of FSM-16, which were quite different from those of Pt nano-particles.

Synthesis of borosilicate zeolites by the dry gel conversion method and their characterization

Abstract Borosilicate zeolites, [B]-BEA, [B]-MFI and [B]-MTW, were synthesized by the dry gel conversion technique. Phase selection of the crystallization product depended on the gel composition, mainly upon the SiO 2 /B 2 O 3 and TEAOH/SiO 2 ratio. Nearly complete conversion of gel to zeolites was achieved by this synthesis method, and there was no phase change during the crystallization. It was observed from the 13 C CP MAS NMR of the as-synthesized samples that TEA + was fully intact in [B]-BEA, whereas partial decomposition of the template occurred in [B]-MFI and [B]-MTW. FTIR and 11 B MAS NMR analyses of the samples indicated that boron was located mainly in the tetrahedral framework position. Most of the tetrahedral boron could be removed and substituted by repeatedly refluxing the calcined sample with aluminum nitrate solution.

Nonresonant third‐order nonlinear optical susceptibility of CdS clusters encapsulated in zeolite A and X

Nonresonant third‐order harmonic generation from CdS clusters encapsulated in zeolite A and X was observed at a fundamental wavelength of 1900 nm. To avoid scattering from the surfaces of the small zeolite crystals, the powder samples were dispersed in a liquid with nearly the same refractive index as that of the samples. The third‐order optical susceptibilities of CdS‐encapsulated zeolite A and X estimated from the intensity of their Maker fringe patterns were 4.1×10−12 and 1.1×10−11 esu, respectively. These values were slightly smaller than those reported for the 1.5 nm surface‐capped CdS cluster. The hyperpolarizabilities of CdS clusters encapsulated in zeolite A and X were estimated by assuming the Lorentz local field to be in the range of 380–480×10−36 and 270–390×10−36 esu, respectively.

Temperature Dependence of Cu2ZnSnS4 Photovoltaic Cell Properties

We studied the temperature dependence of current–voltage characteristics of Cu2ZnSnS4 (CZTS) photovoltaic cells to determine their fundamental properties. The open circuit voltage (Voc) in CZTS cells increased from 0.61 to 0.85 V and showed a linear relationship with decreasing temperature in the range from 150 to 350 K. The Voc at 0 K was extrapolated to about 1.2 V, which is lower than the bandgap energy of CZTS. These results suggest that the reason for the lower Voc in CZTS cells is recombination at the interface between the CZTS and buffer layers.

Synthetic investigation of CIT-5 catalyst

Abstract Syntheses of the all-silica and the aluminosilicate versions of CIT-5 (International Zeolite Association Code: CFI) were studied. As structure-directing agent (SDA), N(16)-methylsparteinium (MeSPA+) cation was used. Under static conditions, combination of this SDA and lithium cations was considered to be a key factor for efficient synthesis of the zeolite. The synthesis vessel has been observed to play a significant role during the synthesis. Synthesis in a sealed quartz tube succeeded whereas that in a Teflon-lined stainless steel autoclave failed. CIT-5 with high purity was obtained inside the autoclave in the presence of hollow quartz tubes. When rotating the Teflon-lined autoclave, CIT-5 was obtained in the absence of the quartz tubes. With or without rotation, very high silica [Al]-CIT-5 was synthesized using fumed silica (Cab-O-Sil) and aluminum nitrate as silica source and aluminum source, respectively. With rotation, [Al]-CIT-5 with relatively lower Si/Al ratio was obtained by using highly dealuminated Y zeolite (HDY) as silica source and aluminum nitrate as aluminum source. In each aluminosilicate sample, only tetrahedral aluminum was observed. The particle size of the product was highly dependent on the synthesis method.

Control of shape of silicon needles fabricated by highly selective anisotropic dry etching

The process to fabricate a needle-shaped silicon crystal (silicon needle) has been developed. These silicon needles are fabricated by highly selective anisotropic dry etching, where the etching mask is oxygen precipitation in the silicon substrate. In order to apply these silicon needles to field emitters, the shape of the silicon needle should be controlled. This is because a high aspect ratio can lead to electric field enhancement around tips and a flared base of the silicon needle can lead to mechanical and thermal stability. In this article we studied how to control the shape of the silicon needles. We found that the silicon needles with a high aspect ratio could be obtained by either lowering the deposition rate of sidewall passivation film or increasing etching depth, unlike conventional silicon cones fabricated by isotropic dry etching. Furthermore, a high aspect ratio could induce a flared base of the silicon needle, since incident ions were reduced in the vicinity of the base due to a shadowing effect by the silicon needle with a high aspect ratio.The process to fabricate a needle-shaped silicon crystal (silicon needle) has been developed. These silicon needles are fabricated by highly selective anisotropic dry etching, where the etching mask is oxygen precipitation in the silicon substrate. In order to apply these silicon needles to field emitters, the shape of the silicon needle should be controlled. This is because a high aspect ratio can lead to electric field enhancement around tips and a flared base of the silicon needle can lead to mechanical and thermal stability. In this article we studied how to control the shape of the silicon needles. We found that the silicon needles with a high aspect ratio could be obtained by either lowering the deposition rate of sidewall passivation film or increasing etching depth, unlike conventional silicon cones fabricated by isotropic dry etching. Furthermore, a high aspect ratio could induce a flared base of the silicon needle, since incident ions were reduced in the vicinity of the base due to a shadowing e...

Study on a condition for forming the high density of silicon needles with high aspect ratio

We have developed a process to fabricate needle-shaped silicon crystals (silicon needles) by highly selective anisotropic dry etching. The etching masks are fine SiOx clusters in the silicon substrate. Under high density of these clusters, side etching easily occurs at the silicon needles in the process of dry etching. This mechanism is as follows. As the dry etching proceeds, the etching area becomes smaller and smaller due to existing high density of silicon needles. This leads to reduction in the deposition rate of sidewall passivation film, because gas for this film comes from the etched silicon substrate. Accordingly, for forming high density of the silicon needles, the deposition rate must be kept higher than the etching rate of the sidewall passivation film during the dry etching. By adjusting the deposition rate, we confirmed this mechanism and obtained high density 3×108 cm−2 of the silicon needles with an aspect ratio of 5 without side etching. Such silicon needles can be expected to lead to hig...

Resonant tunneling diodes as energy-selective contacts used in hot-carrier solar cells

Among the four features unique to hot-carrier solar cells (HC-SCs): (i) carrier thermalization time and (ii) carrier equilibration time in the absorber, (iii) energy-selection width and (iv) conductance of the energy-selective contacts (ESCs), requisites of (i)-(iii) for high conversion efficiency have been clarified. We have tackled the remaining issues related to (iv) in the present study. The detailed balance model of HC-SC operation has been improved to involve a finite value of the ESC conductance to find the required values, which in turn has been revealed to be feasible using resonant tunneling diodes (RTDs) consisting of semiconductor quantum dots (QDs) and quantum wells (QWs) by means of a formulation to calculate the conductance of the QD- and QW-RTDs derived using the rigorous solutions of the effective-mass Hamiltonians. Thus, all of the four requisites unique to HC-SCs to achieve high conversion efficiency have been elucidated, and the two requisites related to the ESCs can be fulfilled using the QD- and QW-RTDs.

Field-emission characteristics of a silicon tip defined by oxygen precipitate

We have developed a silicon tip, fabricated by highly selective anisotropic dry etching using oxygen precipitates (SiOx,x∼2) in a silicon substrate as the etching mask. The gated field-emission device with the single silicon tip was characterized, under a pressure of 10−8-Torr range after an aging process. Although the turn-on voltage of as low as 20 V was obtained, it fluctuated in a range of 20–30 V. In order to reveal the origin of this fluctuation, we investigated the fluctuations of the emitting area (α) and the tip radius (r), using the Fowler-Nordheim theory. As a result, these two were found to fluctuate with a good relation of α∝r2e1.1r, assuming that the work function is constant. This demands that the fluctuation of the turn-on voltage was caused by the variations in the emitting area and the tip radius, not in the work function.