Takaaki Orii

Size-dependent crystallization of Si nanoparticles

Crystallization temperature dependency on Si nanoparticles size was studied by using Raman scattering spectroscopy. Si nanoparticles synthesized by pulsed laser ablation were annealed at various temperatures while they were suspended in helium background gas, and then were classified by a differential mobility analyzer. After the size classification, Si nanoparticles showed a narrow size distribution which enabled investigation of the size-dependent crystallization. The temperature threshold for the transition from amorphous to crystalline (Tc) decreased as the particle size decreased: the Tc values of the 10, 8, 6, and 4nm particles were 1273, 1173, 1073 and 773K, respectively.

Tunable, narrow-band light emission from size-selected Si nanoparticles produced by pulsed-laser ablation

We have demonstrated narrow-band visible light emission from size selected silicon nanoparticles (np-Si), with a wavelength controlled by size tuning. The np-Si were synthesized by pulsed-laser ablation of a silicon single-crystal target in high-purity He background gas. A postannealing process improved morphology and crystallinity. Using a differential mobility analyzer, nanoparticles were classified with a diameter tunable from 3 to 6 nm. Monodispersed np-Si deposited on substrate exhibited a sharp photoluminescence band. The energy of this band increased from 1.34 to 1.79 eV with decrease in particle size, and narrowed to approximately 0.22 eV full width at half maximum due to highly resolved size-selection and improvement in crystallinity. The results suggest that tunable, narrow-band light emitting np-Si produced by gas phase synthesis have good possibilities for application as optoelectronic devices.

Fabrication of silicon nanostructured films by deposition of size-selected nanoparticles generated by pulsed laser ablation

Nanostructured films composed of size-controlled silicon nanoparticles were fabricated by pulsed laser ablation in helium background gas. The size distribution of the silicon nanoparticles in the gas phase was controlled and measured by the electrostatic size classification technique using a differential mobility analyzer coupled with an electrometer. The microstructures and crystallinity of the as-deposited and size-classified nanostructured films were analyzed by scanning electron microscopy and Raman spectroscopy, respectively. The effects of the background gas pressure and particle size on the morphology of the films are discussed.

Collisional reaction of water cluster cations (H2O)n+ (n=2 and 3) with D2O

Abstract Collisional reactions of size- and energy-selected unprotonated water clusters (H 2 O) n + ( n =2 and 3) in a single-collision with D 2 O were investigated at very low collision energy ranging from 0.05 to 2.0 eV using guided-ion beam tandem mass spectrometry. First, in the unimolecular and collision-induced dissociations of (H 2 O) 2 + and (H 2 O) 3 + , only the H 2 O + fragment, but not the H 3 O + fragment, was observed. These results suggest that the structure of (H 2 O) 2 + is primarily hemibonded ‘(H 2 O⋯OH 2 ) + ’ rather than proton-transferred ‘(H 3 O + ⋯OH)’. Second, the measured incorporation cross-section of (H 2 O) 2 + was found to be smaller than that of H + (H 2 O) 2 . From the dipole moment obtained by ab initio calculation, we found that the small values of the incorporation cross-section for (H 2 O) 2 + were caused by the small value, 0.0 debye, of the dipole moment of the cluster.

Reaction cross section for incorporation of ND3 into NH4+(NH3)n−1 (n=3–9) at very low energy collision

Cross sections for the collision-induced reactions between protonated ammonia cluster ions, NH4+(NH3)n−1 (n=3–9), with ND3 were measured at a collision energy ranging from 0.02 to 1.4 eV in the center-of-mass frame with an extremely narrow energy distribution of 0.02 eV. Two types of reaction, incorporation and dissociation, were observed at the same collision energy. The incorporation cross section increased drastically with a decrease in the collision energy, especially in the collision energy region below about 0.2 eV. The incorporation cross section at a collision energy of about 0.02 eV was larger than twice the geometrical reaction cross section. It is suggested that the increase of the incorporation cross section corresponds to the increase of the collision cross section between the cluster ion and the neutral molecule at the low collision energies used in this study because of the presence of the electrostatic attractive force. The reaction probability for incorporation also increased with a decre...

Cluster size dependence of the H/D exchange reaction rate constant for protonated water clusters H+(H2O)n (n=2–11)

Abstract The collisions of size- and energy-selected H + (H 2 O) n ( n =2–11) with D 2 O have been investigated using guided-ion beam mass spectrometry. The H/D exchange reaction was observed by incorporation of D 2 O into H + (H 2 O) n . The measured rate constants of this reaction are found to depend strongly on the cluster size; the rate constants drastically decrease up to n =6, and then level off for 6⩽ n ⩽11. This size dependence is discussed in terms of the structure of protonated water clusters: the core H 3 O + and H 5 O 2 + ions, so-called Eigen and Zundel cations respectively, are completely surrounded by the first water layer and cause the rate constants for 6⩽ n ⩽11 essentially insensitive to n .

Effect of in situ annealing on structure and optical properties of ZnTe nanoparticles produced by pulsed laser ablation

An improvement in morphology, crystallinity, and optical property of ZnTe nanoparticles produced by pulsed laser ablation (PLA) was achieved by in situ annealing. ZnTe nanoparticles produced in argon gas ambience by PLA were annealed in the gas flow at a temperatures Ta ranging from 300 °C to 800 °C and size-selected by a differential mobility analyzer. The bimodal size distribution of the ZnTe nanoparticles changed to unimodal at Ta = 600 °C. In this condition, the shape of the monodispersed ZnTe nanoparticles, classified into around 20 nm, became uniformly spherical and their crystallinity estimated by x-ray diffraction was extremely improved. These improvements by the in situ annealing were examined for ZnTe nanoparticles produced from off-stoichiometric target. Although the optical property of ZnTe nanoparticles produced from a zinc rich target was improved, those produced from a tellurium rich target could not be improved. It was found that the effect of in situ annealing on optical properties of ZnTe nanoparticles was dependent upon its content.

Theoretical study on ammonia cluster ions: Nature of kinetic magic number

We theoretically investigated collision cross sections due to the attractive forces between NH4+(NH3)n−1 (n=2–8) and ND3. We found that the dependence of the collision cross sections on collision energy and cluster size are comparable to those of measured fusion cross sections. The kinetic magic number, n=5, is related to the structure of the pentamer. Namely, the center ion in the pentamer is surrounded by first-shell ammonia molecules.

Theoretical study on ammonia cluster ions: nature of thermodynamic magic number

Abstract Stable geometries and electronic structures of ammonia cluster ions NH4+(NH3)n−1 (n=1–17) are investigated by the ab initio theory in order to clarify the origin of the observed magic number. Since the ammonium ion NH4+ brings about a large attraction to ammonia monomers, the stable geometries of NH4+(NH3)n−1 (n=1–17) have shell structures around the ion. The calculated binding energy, which well reproduces the experimental ones, decreases monotonically as the cluster size increases. Gibbs free energies are also estimated with the use of the calculated electronic and vibrational energies. The Gibbs free energy curve with respect to the cluster size gives a minimum at n=5 in comparatively wide temperature and pressure region, which corresponds to the experimental magic number. The minimum is found to be due to two competitive factors; that is, the nonlinear aspect of the binding energy and the linear instability of the translational entropy as the cluster size increases.

Incorporation Probability of a Water Molecule into a Water Cluster

Abstract The cross-sections for incorporation of D 2 O into size- and energy-selected water cluster ions H + (H 2 O) n ( n =2–9) were experimentally measured under single-collision conditions using guided-ion beam tandem mass spectrometry. After estimation of the collision cross-section, the reaction probability for incorporation of D 2 O into H + (H 2 O) n was investigated as a function of the collision energy. The reaction probability for incorporation was found to be equal to unity at low collision energy by using the experimental data and theoretical curve derived from our model related to the recoil energy of the incident molecule. We examined, by calculation, the temperature dependence of the probability of the incorporation of H 2 O into neutral water clusters (H 2 O) n .