ABSTRACT
The emission of silicon nanocrystals (Si-NCs), synthesized by pulsed laser ablation in water, was investigated on varying the pH of the solution. These samples emit µs decaying orange photoluminescence (PL) associated with radiative recombination of quantum-confined excitons. Time-resolved spectra reveal that both the PL intensity and the lifetime increase by a factor of â¼20 when the pH decreases from 10 to 1 thus indicating that the emission quantum efficiency increases by inhibiting nonradiative decay rates. Infrared (IR) absorption and electron paramagnetic resonance (EPR) experiments allow addressing the origin of defects on which the excitons nonradiatively recombine. The linear correlation between the PL and the growth of SiH groups demonstrates that H+ ions passivate the nonradiative defects that are located in the interlayer between the Si-NC core and the amorphous SiO2 shell.
ABSTRACT
A 62-year-old female patient was admitted to our hospital because of a relapse of pericardial effusion complicating a recent aortic root replacement. At admission to our hospital, echocardiogram showed the reappearance of pericardial effusion and the presence of intrapericardial clots. Color Doppler and Pulsed-wave examinations showed a localized systolic flow within the pericardial space. Contrast enhanced computed tomography excluded abnormal communications with the cardiac chambers or with the aorta. The possible causes of this unusual Doppler finding are discussed.
ABSTRACT
ZnO nanoparticles (NPs) synthesized by pulsed laser ablation (PLAL) of a zinc plate in deionized water were investigated by time-resolved photoluminescence (PL) and complementary techniques (TEM, AFM, µRaman). HRTEM images show that PLAL produces crystalline ZnO NPs in wurtzite structure with a slightly distorted lattice parameter a. Consistently, optical spectra show the typical absorption edge of wurtzite ZnO (Eg = 3.38 eV) and the related excitonic PL peaked at 3.32 eV with a subnanosecond lifetime. ZnO NPs display a further PL peaking at 2.2 eV related to defects, which shows a power law decay kinetics. Thermal annealing in O2 and in a He atmosphere produces a reduction of the A1(LO) Raman mode at 565 cm(-1) associated with oxygen vacancies, accompanied by a decrease of defect-related emission at 2.2 eV. Based on our experimental results the emission at 2.2 eV is proposed to originate from a photo-generated hole in the valence band recombining with an electron deeply trapped in a singly ionized oxygen vacancy. This investigation clarifies important aspects of the photophysics of ZnO NPs and indicates that ZnO emission can be controlled by thermal annealing, which is important in view of optoelectronic applications.
