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Supplementary material for

“Enhanced quantum yield of photoluminescent porous silicon prepared by supercritical drying”

Jinmyoung Joo,^1,2,a) Thomas Defforge,^3,a) , Armando Loni,^4,a) Dokyoung Kim,¹ Z. Y. Li,⁵ Michael J. Sailor,^1,b) Gael Gautier,^3,b) and Leigh T. Canham ^{4,5 b)
1} Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, USA

² Biomedical Engineering Research Center, Asan Institute for Life Sciences, Asan Medical Center, University of Ulsan College of Medicine, Seoul 05505, Rep. of Korea

³ Universite Francois Rabelais de Tours, CNRS CEA, INSA-CVL, GREMAN UMR 7347, 37071 Tours Cedex 2, France

⁴ pSiMedica Ltd., Malvern Hills Science Park, Geraldine Road, Malvern, Worcestershire WR14 3SZ, UK

⁵ Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Experimental

Preparation of pSi: PSi powders were produced from the electrochemical etching of single crystalline (FZ) 30 – 50 Ω.cm p-type Si wafer. The etching parameters are similar to the process developed to produce powders whose surface area exceeds 1000 m²/g and described in a previous publication.¹ The electrolyte was composed of highly concentrated hydrofluoric acid (HF – 30 wt. %) mixed with sulfuric acid (38 wt. %). The porous silicon layer was formed by anodization under galvanostatic control (65 mA/cm²) for 108 seconds using a Keithley 2400 as source. After anodization, the porous powders were first lifted off the parent substrate thanks to a second anodization step (65 mA/cm² for 30 seconds) in dilute HF (3 wt%) in isopropyl alcohol (IPA 10 wt%) solution, the powders were then collected and carefully rinsed in IPA.

Supercritical drying: The drying process reported previously,¹ to obtain extremely high surface area pSi involved transferring the wet pSi sample from storage in IPA, to the drying chamber of a Quorum Technologies Ltd K850 critical point dryer; each sample was dried individually and always remained wet until flushing with liquid CO₂ several times before the final drying step.

Characterization: Aberration corrected scanning transmission electron microscopy (AC-STEM) analysis was conducted using a 200kV JEM 2100F equipped with a CEOS probe corrector. SCD-pSi sample was subjected to particle size reduction via grinding between two highly polished silicon wafers. The resulting particles were then blown off the wafer surface directly onto TEM grids using a nitrogen gun. The nanocrystalline particle size distribution obtained by Feret diameter analysis of the BF-STEM images using ImageJ.² Powder X-ray diffraction (XRD) analysis was carried out using a Bruker D8 Advance diffractometer at 40 kV, 40 mA for Cu Kα. Nitrogen adsorption-desorption isotherms of porous Si microparticles were recorded at 77 K using a Micromeritics TriStar 3000 volumetric apparatus. Prior to the adsorption experiment, the porous Si samples were degassed under vacuum overnight. The surface area of the sample was measured by the BET (Brunnauer-Emmett-Teller) method, which yields the amount of adsorbate corresponding to a molecular monolayer.

Measurement of quantum yield: Adsorption and photoluminescent characteristics of pSi powders were obtained using an absolute measurement setup equipped with an integrating sphere (Labsphere, NH, USA) and a spectrometer (QE Pro, Ocean Optics, CA, USA). The quantum yields were determined by comparing the total number of emitted and absorbed photons. For the PL excitation, a light emitting diode (LED) emitting at 365nm (Ocean Optics) was used. As a base line, the intensity of the excitation was measured with the integrating sphere containing a glass slide only with a dual-side sticky tape on. The pSi powders were then rubbed onto the tape to form a well dispersed fine powder film. The absorption and PL signal, derived from the changes of the base line signal, were determined simultaneously. The quantum yield, defined as the number photons emitted per absorbed photon, was determined according to the method outlined by de Mello.³ In this approach, the quantum yield is given by

where

According to the notation, L_d and L_i are the integrated photoluminescence as a result of direct and indirect excitation, respectively. The latter emission is due to excitation light reflected from the sphere walls hitting the sample, which in turn is not directly in the path of the excitation beam. A is the absorbance of the pSi sample, which is found by measuring the integrated excitation profiles: E_d is the integrated excitation when the pSi sample is directly excited and E_i is the integrated excitation when the excitation light originates from the sphere walls as described above. E₀ is the integrated excitation profile for an empty sphere. Figure 3 in the manuscript shows the PL spectrum of pSi upon excitation at 365 nm. In order to insure the accuracy of the method, measurements were made on several standards: measurements of 9, 10-diphenylanthracene, poly(9, 9-di(ethylhexyl)fluorene) and MEH-PPV were performed using the same setup, and the results agreed with the published values within ± 3% (not shown here).

Measurement of PL decay time: Time-resolved PL spectra of the pSi samples mounted in the integrating sphere were acquitted using an intensified CCD camera (iSTAR 334T, Andor Technology Ltd.) equipped with a monochromator. The excitation source was provided by a light emitting diode (LED, λ_ex = 365 nm). The LED output was modulated by a periodic square-wave pulse. The PL decay at specific emission wavelength was measured with 10 μs steps, and normalized to the initial intensity (Fig. 4 in the manuscript).⁴

References

1. A. Loni, L. T. Canham, T. Defforge and G. Gautier, ECS J. Solid State Sc. 4 (8), P289 (2015).

2. H. G. Merkus, Particle Size Measurements: Fundamentals, Practice, Quality. (Springer Netherlands, 2009).

3. J. C. deMello, H. F. Wittmann and R. H. Friend, Adv. Mater. 9 (3), 230 (1997).

4. J. Joo, X. Y. Liu, V. R. Kotamraju, E. Ruoslahti, Y. Nam and M. J. Sailor, ACS Nano 9 (6), 6233 (2015).

a^ These authors contributed equally to this work.

b^ Author to whom correspondence should be addressed. Electronic mail: [e-mail], [e-mail], [e-mail].