No abstract

The impact of colloidal silicon quantum dots (SiQDs) on next-generation light sources is promising. However, factors determining the efficiency of SiQDs, such as the photoluminescence (PL) wavelength, PL quantum yield (PLQY), and the SiQD LED performance based on the type of ligand, ligand coverage, stress, and dangling bonds, have not been quantified. Characterizing these variables would accelerate the design and implementation of SiQDs. Herein, colloidal SiQDs were synthesized by pyrolyzing hydrogen silsesquioxane and their surfaces were terminated with 1-decene by either thermal hydrosilylation (HT-SiQDs) or room-temperature hydrosilylation using PCl 5 (RT-SiQD). As a result, PL, PL-excitation, and ultraviolet−visible absorption spectra were similar, but their PLQYs were significantly different: 54% (RT-SiQDs) vs 19% (HT-SiQDs). To understand their similarities and differences, surface coverages (dangling bonds, Si−H (Si−H 1 , Si−H 2 , and −Si−H 3 ), Si−O−Si, Si−C, Si−Cl) were determined. A core stress analysis established that a single ligand terminated to a SiQD bond site stretched the Si−Si bond length by 0.3%. From the two well-defined SiQDs, the PLQY and SiQD LED efficiency were attributed to four factors: low coverage of insulator ligands, the Cl ligand effect on radiative and nonradiative rates, negligible dangling bonds, and a SiQD core with low tensile stress. The PLQY of the RT-SiQDs in toluene was 80%. In addition, the 20× electroluminescence intensity difference of the LEDs originated from a 10× difference in current density and a 2× difference in Auger recombination. The concepts demonstrated here can be applied to further improve the PLQY and LED efficiencies of SiQDs with other ligands.

Nanosecond pulsed laser ablation of gold with an excitation wavelength of 532 nm was conducted in supercritical CO2 to generate gold nanoparticles, which were then investigated by scanning electron microscopy and small-angle X-ray scattering, and their extinction spectra and simulated extinction spectra were studied. Both the morphology and amount of gold nanoparticles changed significantly with changes in the density of supercritical CO2 during laser ablation. In a gaslike density, a network structure consisting of nanonecklaces was the major product, whereas in a liquidlike density, large nanospheres with an average diameter (⟨D⟩) of 500 nm were produced. After absorption of multiphoton of excitation light, the gold nanonecklaces and large nanospheres were generated by the fragmentation and solidification, respectively, of liquid gold droplets with ⟨D⟩ = 500 nm. The amount of both products changed according to the branching ratio, which determined whether the liquid gold droplets followed the fragmentation or solidification channel. The local structure of supercritical CO2 in the vicinity of the gold nanoparticles determined the preferred reaction channel. A significant change in the branching ratio occurred near the density ρr = 0.7, where both the enhancement of the local density of supercritical CO2 and the degree of solvation of fluid molecules around the gold nanoparticles reached a maximum. To the best of our knowledge, this is the first study to observe the density dependence of morphological changes in gold nanoparticles fabricated by laser ablation in a supercritical fluid and the local structure of the supercritical fluid that determines the morphology and amount of nanoparticles.

Silicon (Si) nanomaterials with bright luminescence in the visible region are promising materials for use as the nextgeneration light source in displays, lighting, and biomedical imaging. A scalable and cost-effective method for the synthesis of Si quantum dots (SiQDs) is essential for research and development in the field of quantum dots. Herein, we show a facile and costeffective method for controlling the structure and properties of SiQDs, obtained using the pyrolysis of hydrogen silsesquioxane (HSQ) polymer precursors synthesized using methanol. The amount of methanol added to trichlorosilane prior to the addition of water is a key factor that determines the structure and crosslinking density of the HSQ polymer used as the precursor. In turn, these features control the SiQD size, crystallinity, and luminescence efficiency. Dodecyl-passivated SiQDs of size 3−4 nm are obtained as a final product and show red photoluminescence (PL) at approximately 700−800 nm with the peak wavelength depending on the size of SiQDs. The PL quantum yield ranged from 10 to 25% with the highest value obtained for the smaller SiQDs with higher crystallinity. The present study provides new insight into the SiQD synthesis procedure and the understanding of the reaction mechanism. Furthermore, it was found that only methanol is the crucial reagent and the facile and cost-effective synthesis method can be controlled merely by changing the amount of methanol.

Nanosecond pulsed laser ablation of bulk silicon crystal upon the excitation of 532 nm was conducted in supercritical CO 2 to generate silicon nanocrystals, whose properties were studied by seven experimental methods. According to the photoluminescence spectra and fluorescence microscope images, emissions of near-ultraviolet, violet, blue, green, and red were observed in air, at room temperature, and without cooling in liquid nitrogen or a helium cryogenic system. A preferable emission channel of carriers, generated by photoexcitation of Si/SiO 2 of core/shell structure, was responsible for interface states with defect sites. This luminescence process caused color changes and intensity increase, enhanced by a factor of 100, where thermal properties of supercritical CO 2 were maximized, due to critical anomaly. It was found that colors and intensities of photoluminescence of silicon nanocrystals are controlled by a cooling rate during ablation, whose quantity is manipulated by the supercritical fluid pressure.

Small-angle x-ray scattering (SAXS) experiments using synchrotron radiation were carried out for supercritical water along isotherms at the temperatures T=660.0, 661.5, 663.0, 677.0, and 687.5 K, from a gaslike density region to a liquidlike one, including an intermediate density region. The high-temperature and high-pressure sample holder for SAXS measurements suitable for supercritical water was redesigned for more precise measurements. The curves illustrating the density dependence of density fluctuations and correlation lengths show a slight shift of the maximum from critical isochore. The deviations become larger with increasing temperature. The results for the density fluctuations and correlation lengths for supercritical water are compared with those for supercritical CO2 and CF3H at T/Tc=1.02 and 1.06. The comparison allows us to draw the conclusion that the behavior in the long-range inhomogeneity of water in the supercritical state is in discord with the ordinary behaviors for other molecular substances. Density fluctuations in water are also compared with those of Ar and Hg calculated thermodynamically by use of the equations of state. The correlation of the symmetry between the contour of density fluctuations and the vapor–liquid coexistence curve is discussed.

We study the mixing schemes or the molecular processes occurring in aqueous acetonitrile (ACN) and acetone (ACT) by near-infrared spectroscopy (NIR). Both solutions (any other aqueous solutions) are not free from strong and complex intermolecular interactions. To tackle such a many-body problem, we first use the concept of the excess molar absorptivity, epsilonE, which is a function of solute mole fraction in addition to that of wavenumber, nu. The plots of epsilonE calculated from NIR spectra for both aqueous solutions against nu showed two clearly separated bands at 5020 and 5230 cm(-1); the former showed negative and the latter positive peaks. At zero and unity mole fractions of solute, epsilonE is identically zero independent of nu. Similar to the thermodynamic excess functions, both negative and positive bands grow in size from zero to the minimum (or the maximum) and back to zero, as the mole fraction varies from 0 to 1. Since the negative band's nu-locus coincides with the NIR spectrum of ice, and the positive with that of liquid H(2)O, we suggest that on addition of solute the "ice-likeness" decreases and the "liquid-likeness" increases, reminiscent of the two-mixture model for liquid H(2)O. The modes of these variations, however, are qualitatively different between ACN-H(2)O and ACT-H(2)O. The former ACN is known to act as a hydrophobe and ACT as a hydrophile from our previous thermodynamic studies. To see the difference more clearly, we introduced and calculated the excess partial molar absorptivity of ACN and ACT, epsilon(E)(N) and epsilon(E)(T), respectively. The mole fraction dependences of epsilon(E)(N) and epsilon(E)(T) show qualitatively different behavior and are consistent with the detailed mixing schemes elucidated by our earlier differential thermodynamic studies. Furthermore, we found in the H(2)O-rich region that the effect of hydrophobic ACN is acted on the negative band at 5020 cm(-1), while that of hydrophilic ACT is on the positive high-energy band. Thus, the present method of analysis adds more detailed insight into the difference between a hydrophobe and a hydrophile in their effects on H(2)O.

Field enhancement is investigated by spectroscopy, microscopy, and calculations at the same position. The enhancement factor and mechanism change with the thickness.