ИСТИНА

Silicon (Si) nanostructures attract scientific interest for their possible application in the biomedicine because of the properties of Si itself (biocompatibility, biodegradability and low toxicity), and also due to the possibility of changing the semiconductor properties when switching to nanoscale. For example, small-size nanoparticles (less than 7 nm) demonstrate quantum-size effects that lead to effective photoluminescence (PL) in the visible and near infrared range, corresponding to transparent diagnostic window of the biotissue (700-1300 nm), which makes it possible to use such nanoparticles as photoluminescent markers inside the living organisms for medical diagnosis [1]. For such applications it is important to develop fabrication methods, which allow obtaining Si nanoparticles with small size, desired optical properties and minimal chemical contamination simultaneously. To solve this task, in this work, Si nanoparticles are formed by a hybrid approach that includes sequential application of the electrochemical etching of Si in a hydrofluoric acid solution (HF:H2O:C2H5OH = 2:3:5) and pulsed laser ablation of etched Si wafers in various liquids (distilled water, liquid nitrogen, ethanol). Femtosecond (1250 nm, 180 fs, 10 Hz, 10 J/cm2) and picosecond (1064 nm, 30ps, 10 Hz, 17 J/cm2) laser pulses were used for ablation. As a result of etching, porous Si is formed, consisting of Si nanocrystals and air pores. Such structure demonstrates effective photoluminescence. As a result of laser ablation of the porous silicon, nanoparticles suspensions with narrow distribution in size are formed. Atomic force microscopy and scanning electron microscopy revealed the large fraction of nanoparticles with size less than 20 nm, which shows higher efficiency of the proposed two-stage method compared to single-stage laser ablation or mechanical grinding. Raman spectroscopy revealed red shift and broadening of the crystalline Si TO phonon mode, indicating the presence of a quantum-size effect in the formed nanoparticles. PL spectra of the nanoparticles produced in liquid nitrogen and ethanol demonstrate maximum at 746 and 720 nm correspondingly, which is within biotissue transparency window. On the other hand, nanoparticles produced in distilled water showed absence of PL, which is probably explained by formation of oxygen and hydrogen defects in the nanoparticles during ablation. It was also demonstrated that PL lifetime of the Si nanoparticles ensembles produced in liquid nitrogen and ethanol increases by more than an order of magnitude after laser ablation. Obtained characteristics of the fabricated Si nanoparticles suspensions allow us to conclude that such objects can find application as photoluminescent markers for biotissue objects.