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. 2020 Jan 21;25(3):440.
doi: 10.3390/molecules25030440.

Tailoring Photoluminescence From Si-Based Nanocrystals Prepared by Pulsed Laser Ablation in He-N 2 Gas Mixtures

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Free PMC article

Tailoring Photoluminescence From Si-Based Nanocrystals Prepared by Pulsed Laser Ablation in He-N 2 Gas Mixtures

Anastasiya A Fronya et al. Molecules. .
Free PMC article

Abstract

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.

Keywords: bioimaging; photoluminescence; pulsed laser ablation in gases; pulsed laser deposition; quantum confinement; silicon nanoparticles; silicon oxynitride; silicon quantum dots.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Scheme of pulsed laser ablation to deposit Si-based nanostructured films. A beam from UV excimer laser is directed at the angle of 45 deg. onto a rotated Si target. The ablated material is deposited on a substrate located 2–3 cm above the target surface. The target–substrate distance is selected in such a way that visible plasma plume area contacts the substrate surface.
Figure 2
Figure 2
Top-view SEM images of laser-ablated samples obtained at different ratio R between partial pressures of helium and nitrogen: R = 0 (a); R = 0.1 (b); R = 1 (c).
Figure 3
Figure 3
(a) Raman spectra of nc-Si sample obtained at different ratio of helium to nitrogen pressures, as well as one for c-Si substrate; (b) Raman and PL spectra measured in a wide range. Vertical dashed lines indicate the Raman line position (520.5 cm−1) for c-Si.
Figure 4
Figure 4
Fourier transform infrared (FTIR) absorbtion spectra of laser-ablated samples obtained at different ratio of helium to nitrogen pressures. Vertical red and black arrows indicate the spectral position of the Si-O and Si-N bonds, respectively.
Figure 5
Figure 5
Photoluminescence spectra of laser-ablated Si films prepared under different gas mixtures: 5 Torr He (red); 4.5 Torr He + 0.5 Torr N2 (green); 1 Torr He + 1 Torr N2 (black).
Figure 6
Figure 6
PL transients from samples Si-1 and Si-3 obtained for two different ratios between helium and nitrogen pressures (R = 0 and R = 1, respectively). Dashed line indicates time response of the detection system, while solid black and red lines are power law approximations with exponents of 0.5 and 1.25, respectively. Left and right insets show digital images of the PL spots (2 × 3 mm2) on the top of samples Si-1 and Si-3, respectively. PL was excited by 20 ns laser pulses at 351 nm.

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