Tailoring of size, emission and surface chemistry of germanium nanoparticles via liquid-phase picosecond laser ablation

Germanium nanoparticles (Ge-NPs) have been studied as optical probes, and label-free and therapeutic agents for biomedical applications over the past few years. However, little progress in their synthesis has been made due to limited emission tunability and poor water stability of Ge-NPs. In this work, Ge-NPs suspensions with different emission colors, blue (approximate to 450 nm), green (approximate to 510 nm) and red (approximate to 650 nm), were prepared at room temperature via 60 ps pulsed laser ablation of the Ge target in liquid. The emission color of the Ge-NPs is tuned by adjusting the laser wavelengths (lambda = 355 nm, lambda = 532 nm and lambda = 1064 nm), and is explained in terms of an in situ ablation/photo-fragmentation process. Results also showed that laser wavelengths control the average size of Ge-NPs (from approximate to 3 to 18 nm). The effects of variables such as laser wavelengths, dispersing medium and aging time were studied. Optical (PL, FTIR, Raman) and structural (XRD, TEM, electron diffraction) investigations show that Ge-NPs prepared in organic media are more resistant to oxidation, resulting from the passivation of the Ge-NP surface, i.e., the Ge-C bond at a frequency of approximate to 700 cm(-1). We found out that the laser wavelengths act on the surface passivation degree, showing a higher Ge-C/Ge-O ratio at 532 nm laser wavelength.