Laser assisted synthesis of blue-luminescent Si colloidal quantum dots

Full text: The present work reports on the synthesis and characterization of Si nanoparticle (SiNP) colloids via laser ablation in water. Due to quantum confinement effects, semiconductor materials' nanoparticles present size-based tunable luminescence under optical excitation; and, thus, can be referred as quantum dots (QD). The success in synthesizing SiNP by laser ablation may open a low-cost, no-chemical route for obtaining semiconductor nanostructures that are suitable for many applications in photonics. Si QD produced in water are particularly important for biophotonics applications. In this sense, nanoparticle size-control constitutes a challenging task in the realm of laser ablation routes. In our experiments, Si QD with sizes around 1 nm were obtained under a three-step process. First, SiNP were produced by irradiating a water immersed Si target with the first harmonic (1064nm) of a Nd:YAG laser operating at 1.5 kHz and delivering 200 ns pulses (3 mJ per pulse). As a second step, colloids were centrifuged so to obtain a narrower size distribution; and in the third-step, these solutions were irradiated further with the fourth harmonic (266 nm) of a 10 Hz, 5 ns, high pulse energy (45 mJ) Nd:YAG laser aiming at NP photofragmentation and size reduction. Nanoparticles were characterized by Dynamic Light Scattering, UV-Vis spectroscopy, Transmission Electron Microscopy measurements, X-ray diffraction and photoluminescence. Our experimental data indicate the production of ultra-small NP (with size around 1 nm) that present quantum confinement features, particularly a blue luminescence centered at 445 nm. This blue luminescence is characteristic of Si/SiO₂ QD. In addition, Si QD were also synthesized in the presence of gold NP resulting in core-shell like structures with modified luminescence properties. Our major contribution consists of establishing a novel route to obtain Si NP which is based on laser ablation / photofragmentation and yields highly pure Si QD in water. (author)