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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).