Germanium Nanoparticles Prepared by Laser Ablation in Low Pressure Helium and Nitrogen Atmosphere for Biophotonic Applications

Abstract

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.

Keywords: Ge nanocrystals; laser ablation; nanoparticles; nanostructured germanium (Ge); photoluminescence; phototherapy; pulsed laser deposition; theranostics.

Figure 1

Experimental setup. A rotated Ge target is irradiated by a focused beam from UV excimer laser at the angle of 45° to initiate ablation of Ge nanoclusters in residual gases (He or He-N₂ mixtures). The nanoclusters are deposited on a substrate located 2 cm from the target surface.

Figure 2

(a) Typical SEM image and corresponding size distribution of Ge NPs obtained by grinding of laser-ablated Ge nanostructured films (sample Ge2.5/2.5 NPs) and dispersed in ethanol after the centrifugation. The size distribution was calculated using a neural network analysis [47]. Typical TEM images of Ge NPs obtained by grinding of Ge nanostructured films and dispersed in ethanol: sample Ge 5.0 NPs (b,c) Ge2.5/2.5 NPs (d,e). Yellow dashed circles in panel (e) highlight locations of Ge nanocrystals.

Figure 3

(a) XRD spectra in the vicinity of the (111) crystalline lattice of c-Ge from dried suspensions of Ge2.5/2.5 NPs (open and dark circles) after applying Gaussian function fits (red lines) and that of a c-Ge powder (grey line); the middle and upper spectra correspond to Ge NPs from the integral (whole) ensemble and the small size fraction (upper fraction), respectively. (b) Size distribution of Ge NPs from the DLS data. Inset shows the photographic image of a vessel with Ge NP suspension in ethanol.

Figure 4

Raman spectra from Ge NPs obtained by grinding of laser-ablated Ge-based nanostructured films. Red curve (upper spectrum) and blue curve (lower spectrum) correspond to samples Ge5.0 and Ge2.5/2.5, respectively. Inset shows a selected region of the Raman spectrum of the dried small size fraction of Ge2.5/2.5 NPs.

Figure 5

Photoluminescence spectrum from small size fraction of Ge NPs from Ge2.5/2.5 sample under 633 nm laser excitation. RS signature shows the position of Raman scattering peak at 298–299 cm⁻¹ associated with Ge nanocrystals.

Figure 6

(a) Extinction spectra from a suspension of laser-synthesized Ge NPs (Ge2.5/2.5 sample) at concentration of 0.1 mg/mL (black curve) and that of laser-synthesized Si NPs at the same concentration (red dashed curve) from Ref. [46]; (b) temperature growth of the suspension of Ge NPs (sample Ge2.5/2.5) under photoexcitation with the wavelengths of 650 (red circles) and 810 (black squares) nm.