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-SiNxOy) 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.
Nanocrystalline silicon (Si) films were synthesized by nanosecond laser ablation of crystalline Si targets in low-pressure helium (He) and nitrogen (N2) gas mixtures. Photoluminescence (PL) spectra of the prepared samples were found to depend on the He/N2 ratio in the gas mixture. The ablation pure He atmosphere allowed us to prepare Si nanocrystals (NCs) exhibiting a PL band in red-near-IR range, while samples prepared in the presence of N2 exhibited a strong PL band with maximum in the green-yellow region. Such a modification of PL properties can be explained by the presence of amorphous Si oxynitride (a-SiNxOy) on the surface of Si-NCs. Structural studies of the prepared samples by means of the scanning electron microscopy revealed different morphology for Si-NCs produced under different gas mixtures. After treating of the films by ultrasound and dispersing in water, Si-NCs can be used as novel biodegradable markers for bioimaging, while the observed spectral tailoring effect makes possible an adjustment of the PL emission of such markers to a concrete bioimaging task.
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