Photoinduced Electron-Transfer Quenching of Luminescent Silicon Nanocrystals as a Way To Estimate the Position of the Conduction and Valence Bands by Marcus Theory

Abstract

Photoluminescence of silicon nanocrystals (SiNCs) in the presence of a series of quinone electron acceptors and ferrocene electron donors is quenched by oxidative and reductive electron transfer dynamic processes, respectively. The rate of these processes is investigated as a function of (a) the thermodynamic driving force of the reaction, by changing the reduction potentials of the acceptor or donor molecules, (b) the dimension of SiNCs (diameter = 3.2 or 5.0 nm), (c) the surface capping layer on SiNCs (dodecyl or ethylbenzene groups), and (d) the solvent polarity (toluene vs dichloromethane). The results were interpreted within the classical Marcus theory, enabling us to estimate the position of the valence and conduction bands, as well as the reorganization energy (particularly small, as expected for quantum dots) and electronic transmission coefficients. The last parameter is in the range 10 −5 −10 −6 , demonstrating the nonadiabaticity of the process, and it decreases upon increasing the SiNC dimensions: this result is in line with a larger number of excitons generated in the inner silicon core for larger SiNCs and thus resulting in a lower electronic coupling with the quencher molecules.

Improved photon absorption in dye-functionalized silicon nanocrystals synthesizedviamicrowave-assisted hydrosilylation

“…Locritani et al demonstrated that bonded pyrene molecules can increase brightness of SiNc emission by almost 300% at 378 nm excitation. [29][30][31][32][33] In addition, Fermi et al indicated that a porphyrin chromophore can be also used as antenna for SiNc. However, a significant decrease in PLQY from 40% (for bare SiNc) to 8% (for the conjugates with the porphyrin) was observed upon excitation at 427 nm.…”

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“…In PET, a complex is formed between an electron donor (D) and an acceptor (A); upon excitation, the donor transfers an electron to the acceptor, forming the charge transfer complex [D + A − ]* . For oxidative quenching (where the luminophore is the electron donor), the overall free energy change (Δ G ) for electron transfer is given by:where n is the number of electrons involved in the reaction, F is the Faraday constant, E (NC + /NC) and E (A/A − ) are the reduction potentials of the Si NCs and analyte ions (electron acceptors), E 00 ( NC */ NC ) is the one electron potential corresponding to the optical band gap of the Si NCs, while w accounts for the Coulombic interaction between ion pair after electron transfer. For the results reported above, both E (NC + /NC) and E 00 (NC*/NC) remain constant, so the differences in luminescence quenching observed are due to the reduction potentials of the analyte ions.…”

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“…These results are strongly encouraging for the application of this class of luminescent QDs in LSCs, even if the relatively low QY reported in the paper (46%) is limiting the overall performance of the device. While higher QY values have been reported, the extremely sensitive surface chemistry and the long PL lifetime are responsible for a strong influence of surface trap states and intermolecular excited state quenching phenomena on the QY. In addition, the indirect band gap is responsible for the low absorption coefficient and the limited CRI tunability.…”

“…While higher QY values have been reported, [107] the extremely sensitive surface chemistry and the long PL lifetime are responsible for a strong influence of surface trap states [108] and intermolecular excited state quenching phenomena [109,110] on the QY. While higher QY values have been reported, [107] the extremely sensitive surface chemistry and the long PL lifetime are responsible for a strong influence of surface trap states [108] and intermolecular excited state quenching phenomena [109,110] on the QY.…”