Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Charge Transfer-Induced Photobrightening of Silicon Quantum Dots in Water Containing a Molecular Reductant Kosuke Inoue, Takuya Kojima, Hiroshi Sugimoto,* and Minoru Fujii* Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan
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S Supporting Information *
ABSTRACT: The effect of molecular reductants on the charge transferinduced brightening of near-infrared photoluminescence (PL) from silicon (Si) quantum dots (QDs) was studied. Without a molecular reductant, a temporal decrease of the PL quantum yield (QY) during light irradiation in water was observed. The temporal photodarkening was reversed when a molecular reductant [sodium sulfites (Na2SO3)] was added in water. In Na2SO3-dissolved water, the PL QY increased gradually during light irradiation. The photobrightening behavior depended strongly on the amount of reductant molecules, excitation power, and the size of Si QDs. The observed phenomena suggest that an excess hole generated by trapping a photoexcited electron to a trap level is effectively removed by a reductant molecule. The observed charge transfer-induced photobrightening paves way to realize high efficiency and stable Si QDs-based phosphors usable in aqueous media.
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INTRODUCTION Charge transfer interactions between semiconductor quantum dots (QDs) and molecules provide rich phenomena such as chemical doping,1−4 photodoping (photochemical electronic doping),5−8 photodarkening,9,10 photobrightening,9,10 and electrobrightening11−13 of a QD. In particular, photoassisted charge transfer is an efficient route for the control of charge carriers in a QD. Rinehart et al.6 demonstrated n-type photodoping in cadmium selenide (CdSe) QDs in the presence of a borohydride (Li[Et3BH]) hole quencher. They monitored the doping by the bleaching of the band edge absorption, the appearance of a new infrared (IR) absorption band, and the quenching of the excitonic photoluminescence (PL). Recently, Araujo et al.3 succeeded in achieving photodoping of lead selenide (PbSe) QDs to the level of ∼1020 cm−3. The heavy doping resulted in complete filling of the 1Se orbitals of the conduction band. Charge transfer-mediated darkening and brightening have also been widely studied. Krivenkov et al.9 observed both phenomena in ligand-stabilized CdSe/zinc sulfide (ZnS) QDs and demonstrated that negative or positive charging of a QD by charge transfer from/to the ligands results in PL darkening, while neutralization of a QD by charge transfer results in the brightening. Darkening and brightening of QD PL by charging can also be controlled by the electrochemical potential. Weaver and Gamelin11 showed in zinc selenide (ZnSe) QDs that reductive passivation of midgap surface electron traps by electron transfer upon applications of reducing potential largely improves the luminescence efficiency. Similarly, Brovelli et al.13 demonstrated both the brightening and darkening in copper (Cu)-doped ZnSe/CdSe QDs by controlling the © XXXX American Chemical Society
potential. Passivation of midgap states by chemical reductants also improves the luminescence efficiency. Rinehart et al.14 demonstrated brightening of manganese (Mn)-doped ZnSe QDs by adding a molecular reductant (NaK2) to the solution. In contrast to rich photophysical phenomena studied in II− IV and IV−VI QDs, those on silicon (Si) QDs have emerged only recently despite the importance in optoelectronics and biophotonics because of the high environmental friendliness and the high biocompatibility.15−18 The quality of Si QDs, especially the colloidal solution, has been improved rapidly over the past decade,19−21 and now very high quality colloidal solutions with the luminescence quantum yield (QY) exceeding 70% are available.19 In accordance with the development of high-quality materials, reports on charge transfer-induced phenomena in Si QDs have been emerging. In amphiphilic polymer-coated Si QDs, a high photocatalytic efficiency and the clear size dependence were observed for a model organic contaminant (methanol) in water.22 Charge transfer from Si QDs to molecules was used as a tool to determine absolute energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels by monitoring the charge transfer-induced quenching of the PL for molecules with different redox potentials.23 Our group succeeded in observing clear size dependence on charge transfer-induced PL brightening24 and photocatalytic activities25 of Si QDs. Kislitsyn et al.26 succeeded in controlling and visualizing trapping and detrapping of an electron in midgap Received: November 23, 2018 Revised: December 28, 2018 Published: December 28, 2018 A
DOI: 10.1021/acs.jpcc.8b11359 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C states of single Si QDs by scanning tunneling spectroscopy. Wheeler et al.27 performed comprehensive studies on the interaction between Si QDs and various donor molecules. This work is an extension of these recent activities on charge transfer-induced photophysical phenomena in Si QDs. In this paper, we report for the first time photoinduced brightening of Si QDs in water containing molecular reductants. For this study, we employ all-inorganic boron (B)- and phosphorus (P)-codoped Si QDs developed in our group.28 The QD is dispersible in polar solvents such as water and alcohol without organic ligands because of the negative surface potential (ζ potential: −35 mV) induced in the heavily B- and P-codoped shell.29,30 Because of the ligand-free surface, molecules can access the QD surface easily and thus efficient charge transfer is expected.24,25 The codoped Si QDs exhibit efficient sizetunable PL in water in the near IR (NIR) range because of the transition from the donor to acceptor states.31,32 Although the PL is much more stable than water-soluble organic molecules emitting in the same wavelength range, for example, fluorescein isothiocyanate,33 slight temporal photodarkening, that is, darkening which recovers during storage in dark, occurs under continuous excitation in water.29 The most plausible mechanism of the temporal darkening is trapping of a photoexcited electron (hole) to a surface-related trap state, and hole (electron) accumulation in a Si QD, which promotes Auger recombination of photoexcited carriers.14 A promising strategy to overcome the problem is introducing a molecular reductant (oxidant) and promptly removing an extra carrier from a QD to bring it back to the luminescing state.34 In this work, we study charge transfer interactions between Si QDs and a molecular reductant in water under light irradiation. As a molecular reductant, we employ sodium sulfite (Na2SO3), which is a typical food additive to preserve the freshness and thus is harmless for biological substances unless the concentration is too high.35 We show that Na2SO3 molecules can not only prevent the photodarkening, but also reverse the effect. In Na2SO3-dissolved water, PL of Si QDs continuously increases during irradiation. The photobrightening behavior depends on the amount of the molecular reductant, the excitation power, and the size of Si QDs. We will propose a model in which the synergy of photoexcitation and charge transfer interaction enhances the PL.
Figure 1. (a) Photograph of water solution of codoped Si QDs. (b) TEM image of a codoped Si QD grown at 1100 °C. The lattice fringe corresponds to {111} planes of the Si crystal.
shows an example of the TEM (JEM-2100F, JEOL) image of a Si QD grown at 1100 °C. The lattice fringe corresponds to {111} planes of the Si crystal. The diameter of the QD is around 3.9 nm. In most part of this work, we fixed the growth temperature of Si QDs to 1100 °C and the concentration of Si QDs in water to 1 μM (0.05 mg/mL), unless otherwise stated. In a water solution of codoped Si QDs (1 μM, 0.05 mg/mL), Na2SO3 was added as a reductant. The concentration was changed from 10 to 500 mM. PL spectra of a water solution of Si QDs were measured by using a single spectrometer equipped with a liquid N2-cooled InGaAs diode array (OMA-V-SE, Roper Scientific) and a charge-coupled device (CCD) (Roper Scientific). The excitation wavelength, the excitation power, and the PL accumulation time were 405 nm, 0.8 mW (1 mm in diameter), and 1 s, respectively, unless otherwise stated. For the measurements of PL decay curves, a NIR photomultiplier (R5509-72, Hamamatsu Photonics) was used as a detector.
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RESULTS AND DISCUSSION Figure 2a shows PL images of a water solution in which Si QDs are dispersed. Na2SO3 is not dissolved. The excitation light (405 nm) is irradiated from the left, and the emission image is taken from the front surface. We can see dimming of the PL after 10 s continuous excitation (right) compared to that just after excitation (left) (see Movie S2 in the Supporting Information). Figure 2b shows the evolution of the PL spectra during continuous irradiation (0−60 s). The PL intensity decreases continuously during the irradiation, while the spectral shape is not strongly modified. The photodarkening effect is reversed when a molecular reductant (Na2SO3) is dissolved in water. In Figure 2c (Na2SO3 concentration: 100 mM), we can see significant brightening of the PL after 10 s irradiation. The brightening phenomenon can be seen more clearly in the movie (Movie S3 in the Supporting Information). Figure 2d shows the PL spectra of Si QDs in Na2SO3-dissolved water. The PL intensity increases significantly during the irradiation. It is important to note that the enhancement of the PL intensity is predominantly due to that of the QY because the absorption spectrum of a Si QD solution is almost not affected by the addition of Na2SO3 within the concentration range studied (Figure S1 in the Supporting Information). By comparing Figure 2b,d, we notice that the spectral shape is slightly modified by the addition of Na2SO3. The PL peak shifts to shorter wavelength and the spectrum is broadened. At present, the mechanism of
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EXPERIMENTAL PROCEDURE Colloidal solutions of B- and P-codoped Si QDs were fabricated by a cosputtering method. Details of the preparation procedure are shown in our previous papers.28,31 Briefly, Si, SiO2, B2O3, and P2O5 were simultaneously sputtered, and a Sirich borophosphosilicate glass (BPSG) film was deposited on a stainless steel plate. The film was peeled off from the plate and annealed in a N2 gas atmosphere (1050−1200 °C) for 30 min to grow codoped Si QDs in a BPSG matrix. Si QDs were then extracted from a matrix by hydrofluoric acid (HF) etching. Si QDs in a HF solution were transferred to methanol, and then the solvent was exchanged with distilled water (methanol concentration
