Formation of Si/SiO2 Luminescent Quantum Dots From Mesoporous Silicon by Sodium Tetraborate/Citric Acid Oxidation Treatment

Abstract

We propose a rapid, one-pot method to generate photoluminescent (PL) mesoporous silicon nanoparticles (PSiNPs). Typically, mesoporous silicon (meso-PSi) films, obtained by electrochemical etching of monocrystalline silicon substrates, do not display strong PL because the silicon nanocrystals (nc-Si) in the skeleton are generally too large to display quantum confinement effects. Here we describe an improved approach to form photoluminescent PSiNPs from meso-PSi by partial oxidation in aqueous sodium borate (borax) solutions. The borax solution acts to simultaneously oxidize the nc-Si surface and to partially dissolve the oxide product. This results in reduction of the size of the nc-Si core into the quantum confinement regime, and formation of an insulating silicon dioxide (SiO₂) shell. The shell serves to passivate the surface of the silicon nanocrystals more effectively localizing excitons and increasing PL intensity. We show that the oxidation/dissolution process can be terminated by addition of excess citric acid, which changes the pH of the solution from alkaline to acidic. The process is monitored in situ by measurement of the steady-state PL spectrum from the PSiNPs. The measured PL intensity increases by 1.5- to 2-fold upon addition of citric acid, which we attribute to passivation of non-radiative recombination centers in the oxide shell. The measured PL quantum yield of the final product is up to 20%, the PL activation procedure takes <20 min, and the resulting material remains stable in aqueous dispersion for at least 1 day. The proposed phenomenological model explaining the process takes into account both pH changes in the solution and the potential increase in solubility of silicic acid due to interaction with sodium cations.

Keywords: biomedical application; photoluminescence; porous silicon (PS); silicon nanoparticles (SiNPs); theranostics.

Figure 1

Electron microscope study of Perf-PSi. SEM images of Perf-PSi layer: cross-sectional view (A) and plan-view (B); Pore distribution, calculated from SEM image (inset B); TEM image of Perf-PSi piece, that was isolated midway through the ultrasonic fracturing process (C); Electron diffraction pattern corresponding to TEM image (inset C); Dark-field image of the same piece (D). Bright spots correspond to silicon nanocrystals; Diameter distribution of the nanocrystals (inset D).

Figure 2

TEM image of PSiNPs, sedimented from aqueous suspension (A); Raman spectra of initial Perf-PSi (red), oxidized Perf-PSi (green), and reference c-Si (black) (B). Pore diameter distribution of PSiNPs sedimented from the aqueous suspensions by using the low-temperature adsorption technique (C).

Figure 3

Family of PL spectra of porous silicon nanoparticles obtained during aqueous borax oxidation (A); evolution of the PL intensity (measured at the emission maximum) during borax oxidation without (black curve—B) and with termination by citric acid (red curve—B); schematic view of the oxidation process starting from as-prepared PSiNPs through maximal emissive state to complete oxidation in borax (C). Green—Si skeleton, yellow—SiO₂ shell, black circles—silicon nanocrystals (nc-Si) indicating the size of the nanocrystals, some of which fit the quantum confinement criteria for efficient emission, indicated with the red arrows.

Figure 4

Photoluminescence of PSiNPs during oxidation in aqueous borax with addition of citric acid to terminate the reaction (showed by vertical arrows) (A). Initial borate concentrations for each trace are given in the legend. Absorbance of PSiNPs during the same process (B).

Figure 5

Oxidation rate of PSiNPs vs. borax concentration in suspension obtained from photoluminescence (red curve) and absorbance (black curve) measurements. Dashed line traces are included as a guide to the eye.

Figure 6

PL quantum yield of PSiNPs vs. borax concentration (A) before (black curve) and after CA termination (red curve); PL quantum yield of PSiNPs vs. their absorbance at 365 nm (B). Higher absorbance corresponds to both a larger quantity of silicon and larger silicon nanocrystals.

Figure 7

FTIR transmission spectra of as-prepared (black curve) and borax-oxidized (red curve) PSi films.

Figure 8

XPS spectra of “fresh” and “aged” PSiNPs. Dots: experimental data for “fresh” (A,B) and “aged” (C,D) PSiNPS; Solid lines: approximation; Dashed: deconvolution into Gaussians.

Figure 9

Schematic cross-section of a PSiNP subjected to borax-induced oxidation/dissolution. Borax solution (blue) penetrates into the SiO₂ shell (yellow), which covers the silicon core silicon nanocrystallite (green). An exciton confined in the silicon nanocrystallite is shown as ≪ + ≫ and ≪ – ≫ in circles and photoluminescence is indicated with the red arrow. A notional insoluble protective surface layer is shown as a red region.