PSi films were prepared by electrochemical etching of boron-doped (100) c-Si wafers (specific resistivity of 1…10 Ohm*cm) in a mixture of HF (48%):C2
OH (1:1) under etching current density 60
min. The etching was done in a Teflon cell with a platinum counter electrode at room temperature.
In order to obtain free-standing PSi films, a short pulse of the etching current approximately 600
was applied. The free-standing films were rinsed in deionized water and dried in air.
The porosity of the films was measured about 60%
5% by using the gravimetric analysis and low temperature nitrogen adsorption Brunauer-Emmet-Teller (BET) method. The BET method allowed us to estimate the mean diameter of pores equal to 4
The dried films were hand-milled in agate mallet for 15
min to get powder. The prepared powder consisted of small individual PSiNPs (sizes of 10 to 200
nm) and larger particles (sizes above 200
nm) detected by using transmission electron microscopy (not shown).
The prepared powder was covered by PLGA/PVA in the following way. At the first step, the powder was mixed with dimethyl sulfoxide (DMSO) to obtain a suspension with particle concentration approximately 1
mg/ml. Then the suspension was centrifuged for 3
min at 2,000
rpm (rotor's radius approximately
cm) in order to remove largest particles. The supernatant was ultrasonicated for 4
h in order to form the stable suspension. At the second step, 1
ml of the suspension was mixed with 40
mg of PLGA, and the mixture was stirred for 1
-lactide-co-glycolide) (PLGA, MW:5, 000
Da) was purchased from Boehringer Ingelheim Inc (Ridgefield, CT, USA). Polyvinyl alcohol (PVA, MW:20,000
Da) and DMSO were obtained from Sigma-Aldrich Corporation (St. Louis, MO, USA).
In the third step, the suspension of PLGA-coated PSiNPs (1
ml) was mixed with 9
ml of aqueous solution of PVA (45
mg/ml). Then the suspension of PLGA/PVA-coated PSiNPs was stirred for 20
h in order to create a hydrophilic coverage of PSiNPs and to prevent their agglomeration. At the final step, the nanoparticles were triply precipitated by centrifugation (3,000
min) followed by washing and stirring in distilled water to remove excessive DMSO and to form aqueous suspensions of PLGA/PVA-coated PSiNPs.
For comparison we have prepared and studied a suspension of uncoated PSi particles by using the powder of as-prepared PSi films mixed with water. Prior to the investigation, the formed suspensions were subjected to ultrasonication for 15
Some parts of the aqueous suspension were used to deposit the nanoparticles on flat surface of c-Si wafer. The dried samples were investigated by means of scanning electron microscopy (FE-SEM, Sirion, FEI Company, Hillsboro, OR, USA) at an acceleration voltage of 10
kV. Additionally, the samples were studied using a Fourier-transform infrared (FTIR) spectrometer (Alpha-P, Bruker Corporation, Billerica, MA, USA) with attenuated total reflectance mode.
The PL spectra of the aqueous suspensions of PSiNPs were measured using a spectrophotometer Perkin Elmer LS-55 (PerkinElmer Inc., Waltham, MA, USA) under continuous wave excitation by a Xe lamp (with excitation wavelength of 350
nm and spectral width of 10
The PL relaxation transients were detected by R928 photomultiplier tube (Hamamatsu Photonics, Hamamatsu, Shizuoka, Japan) under pulsed laser irradiation by a nitrogen laser (excitation wavelength of 337
nm and pulse duration of 10
ns). The time response of the detection system was better than 1
The PL quantum yield was measured by comparing the PL intensity and absorption of the samples and a solution of Rhodamine 6
G with the PL quantum yield of about 100%.