Pulsed laser deposition in He-N2 gaseous mixtures for the synthesis of photoluminescent Si and Ge nanoparticles for bioimaging

Abstract

Owing to particular physico-chemical properties and high biocompatibility, nanostructured silicon (Si) and germanium (Ge) present very promising materials for biomedical applications, but the fabrication of luminescent Si and Ge nanoparticles (NPs) in pure, uncontaminated, water-dispersible state is almost impossible by using conventional methods of wet-chemical synthesis. We recently showed that such a task can be solved by the elaboration of a technique of pulsed laser deposition (PLD) in gaseous medium under reduced gas pressures (0.5-10 Torr). In particular, PLD-prepared Si-based nanocrystalline layers and NPs could exhibit a photoluminescence (PL) band centered in the red- near infrared (maximum at 760 nm) spectral region (when ablated in pure He) or an intense “green-yellow” PL band centered at 580 nm (when ablated in He and N₂ mixture), which were attributed to quantum-confined excitonic states in small Si nanocrystals and a radiative recombination in amorphous oxynitride (a-SiN_xO_y) coating of Si nanocrystals, respectively. While as-prepared Ge nanocrystals exhibited a dominating photoluminescence (PL) band around 450 nm, which was attributed to defects in germanium oxide shell, a size-selected portion of relatively small (5-10 nm) Ge NPs exhibited a red-shifted PL band around 725 nm under 633 nm excitation, which could be attributed to the quantum confinement effect in small Ge nanocrystals. After milling by ultrasound and dispersing in water, all such nanocrystals and NPs can be used as efficient non-toxic markers for bioimaging. Here, we give a comparative analysis of the structural and optical properties of Si and Ge nanostructures produced by methods of PLD in He-N₂ gaseous mixtures and discuss their potential applications in bioimaging.