RESUMO
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 PCl5 (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-H1, âSi-H2, and -Si-H3), 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.
RESUMO
In the present study, we investigated auditory event-related potentials in adults with Asperger disorder and normal controls using an auditory oddball task and a novelty oddball task. Task performance and the latencies of P300 evoked by both target and novel stimuli in the two tasks did not differ between the two groups. Analysis of variance revealed that there was a significant interaction effect between group and electrode site on the mean amplitude of the P300 evoked by novel stimuli, which indicated that there was an altered distribution of the P300 in persons with Asperger disorder. In contrast, there was no significant interaction effect on the mean P300 amplitude elicited by target stimuli. Considering that P300 comprises two main subcomponents, frontal-central-dominant P3a and parietal-dominant P3b, our results suggested that persons with Asperger disorder have enhanced amplitude of P3a, which indicated activated prefrontal function in this task.
