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Using methods of pulsed laser ablation from a silicon target in helium (He)-nitrogen (N2) gas mixtures maintained at reduced pressures (0.5–5 Torr), we fabricated substrate-supported silicon (Si) nanocrystal-based films exhibiting a strong photoluminescence (PL) emission, which depended on the He/N2 ratio. We show that, in the case of ablation in pure He gas, Si nanocrystals exhibit PL bands centered in the “red - near infrared” (maximum at 760 nm) and “green” (centered at 550 nm) spectral regions, which can be attributed to quantum-confined excitonic states in small Si nanocrystals and to local electronic states in amorphous silicon suboxide (a-SiOx) coating, respectively, while the addition of N2 leads to the generation of an intense “green-yellow” PL band centered at 580 nm. The origin of the latter band is attributed to a radiative recombination in amorphous oxynitride (a-SiNxOy) coating of Si nanocrystals. PL transients of Si nanocrystals with SiOx and a-SiNxOy coatings demonstrate nonexponential decays in the micro- and submicrosecond time scales with rates depending on nitrogen content in the mixture. After milling by ultrasound and dispersing in water, Si nanocrystals can be used as efficient non-toxic 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.

Due to particular physico-chemical characteristics and prominent optical properties, nanostructured germanium (Ge) appears as a promising material for biomedical applications, but its use in biological systems has been limited so far due to the difficulty of preparation of Ge nanostructures in a pure, uncontaminated state. Here, we explored the fabrication of Ge nanoparticles (NPs) using methods of pulsed laser ablation in ambient gas (He or He-N2 mixtures) maintained at low residual pressures (1–5 Torr). We show that the ablated material can be deposited on a substrate (silicon wafer in our case) to form a nanostructured thin film, which can then be ground in ethanol by ultrasound to form a stable suspension of Ge NPs. It was found that these formed NPs have a wide size dispersion, with sizes between a few nm and hundreds of nm, while a subsequent centrifugation step renders possible the selection of one or another NP size fraction. Structural characterization of NPs showed that they are composed of aggregations of Ge crystals, covered by an oxide shell. Solutions of the prepared NPs exhibited largely dominating photoluminescence (PL) around 450 nm, attributed to defects in the germanium oxide shell, while a separated fraction of relatively small (5–10 nm) NPs exhibited a red-shifted PL band around 725 nm under 633 nm excitation, which could be attributed to quantum confinement effects. It was also found that the formed NPs exhibit high absorption in the visible and near-IR spectral ranges and can be strongly heated under photoexcitation in the region of relative tissue transparency, which opens access to phototherapy functionality. Combining imaging and therapy functionalities in the biological transparency window, laser-synthesized Ge NPs present a novel promising object for cancer theranostics.

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.