Magnetic field dependence of singlet oxygen generation by nanoporous silicon
© Amonkosolpan et al.; licensee Springer. 2014
Received: 22 April 2014
Accepted: 19 June 2014
Published: 9 July 2014
Energy transfer from photoexcited excitons localized in silicon nanoparticles to adsorbed oxygen molecules excites them to the reactive singlet spin state. This process has been studied experimentally as a function of nanoparticle size and applied external magnetic field as a test of the accepted understanding of this process in terms of the exchange coupling between the nano-Si exciton and the adsorbed O2 molecules.
Since the discovery that photoexcited silicon nanoparticles can act as energy donors to molecular oxygen acceptors and can thereby excite oxygen to a highly reactive singlet state[1–3], there has been much work on the potential exploitation of this process. Applications that have been demonstrated range from photodynamic cancer therapy[4, 5] to optically activated reactors in chemical engineering.
In early work, it was demonstrated that the efficiency of the energy transfer process is sensitive to an externally applied magnetic field (the energy transfer efficiency may be monitored by its quenching of the nano-Si residual photoluminescence), and this provided key evidence for the understanding of the process as a result of exchange coupling between an exciton confined within a silicon nanoparticle and an adsorbed oxygen molecule (the Dexter exchange mechanism). The applied magnetic field B lifts the spin degeneracy of both the exciton and oxygen spin manifolds; both oxygen molecules and silicon excitons will then relax predominantly into their lowest energy spin states at temperatures T for which g μ B B ≥ kT where g = 2.0 is the gyromagnetic ratio and μ B is the Bohr magneton. The energy transfer process between these lowest energy spin states has a low probability due to angular momentum selection rules, so that the effect of the magnetic field at low temperatures is to suppress the energy transfer from the exciton to the molecular oxygen. As a result, the silicon photoluminescence intensity is restored towards the intensity observed when oxygen is not present.
Although earlier investigations proposed this model, the response to a magnetic field has not been investigated or modelled quantitatively in terms of the dynamics of the energy transfer and other excitation and relaxation processes. Furthermore, the dependence of the efficiency of the process on oxygen concentration has never been investigated. Here, we show results of experimental investigations at lower oxygen concentrations than used previously, and we set out a preliminary model which makes some simplifying assumptions but which has the features required to describe our experimental data. This model is a starting point for a full theoretical description of the energy transfer phenomenon and can be expanded to model the energy transfer process as a function of, for example, nanoparticle size. Even at the present level of approximation, the modelling turns out to be a fairly complicated task requiring a large set of input parameters, though many of these are available in the literature; some we use have been estimated as part of the present work.
The samples were produced in the form of porous silicon layers (thickness of approximately 8 μ m) on bulk crystalline substrates by conventional electrochemical etching from wafers consisting typically of p-type boron-doped CZ <100> silicon with resistivities of 1 to 25 Ω cm. Room temperature anodization was performed in a 1:1 solution of 49% aqueous HF and hydrous ethanol; the porosity p was varied by variation of the current (10 to 40 mA/cm2) and was determined by fitting of the Fabry-Pérot interference fringes in a broad-band optical reflectance measurement to be typically p = 63% to 70%. The etched layers were left attached to the substrates for better mechanical strength and were glued to a copper cold finger with heater and thermometer resistors attached. The samples were held either in a continuous-flow cryostat (base temperature of approximately 10 K) or a superconducting magnet in superfluid helium (base temperature of approximately 1.5 K). The magnetic field was varied up to 6 T and was oriented either parallel or perpendicular to the sample normal. The orientation of the field plays no role in the following experiments, in which the optical polarisation of the photoluminescence (PL) emission was not analysed. The effects we discuss here depend only on the magnitude of the induced Zeeman splittings in the exciton and oxygen triplet states (polarisation-dependent studies are under way at present). In both cryostats, the cold finger could be raised to the top of the cryostat to expose the cold sample briefly to oxygen gas and it could be heated whilst in vacuum to desorb oyxgen. PL was excited by a continuous wave solid state diode laser (wavelength approximately 450 nm, power approximately 5 mW at the sample, with a weakly focused laser spot, size a few hundred microns) and detected with an intensified CCD camera and compact single-grating spectrometer.
Results and discussion
There are two notable features: Firstly, the strongest quenching of the PL occurs precisely for NPs having an exciton energy equal to the oxygen 3Σ to 1Σ transition energy of 1.63 eV. Secondly, the spectra show a large number of other sharp downward-pointing peaks or dips which originate from the enhanced energy transfer to oxygen for NPs whose exciton energies differ from 1.63 eV by energies corresponding to one or more momentum- and energy-conserving phonons (located at K and Γ points of the silicon phonon dispersion, respectively). These phonon effects have been discussed elsewhere, where details of the relevant phonon energies are given. Two prominent dips of this type can be seen near 1.9 and 2.0 eV; these are also related to energy transfer to oxygen but will be discussed in future work; here, we shall model only the energy transfer process without phonon participation.Figure2 demonstrates that significant PL is again observed above the threshold for energy transfer to oxygen, even at this higher oxygen concentration. Furthermore, the PL both above and below this threshold shows a much stronger recovery of intensity as the magnetic field is increased, by factor of about 3 times, and unlike the case of Figure1, the recovery of the PL has not saturated up to a magnetic field of 6 T.
Silicon nanoparticles without oxygen
where F is the total fraction of NPs with adsorbed oxygen.
Silicon nanoparticles with oxygen
and this expression can be evaluated as a function of magnetic field; note that n ij , w i and, in principle, u i are all functions of magnetic field through the field dependence of γ ij and β ij .
Comparison to experiment
The fraction F of NPs with adsorbed oxygen was varied from 0.75 (Figures1 and5, blue) to 0.85 (Figures2 and5, red), and 1/t varied from 10-5 to 10-7 s. More work is needed before we would attempt to interpret these parameters directly, but we note that these transfer times are in good agreement with previously measured values, and as is necessary for the evenly matched competition between radiative recombination and energy transfer, they are comparable to the radiative lifetimes 1/r1,1/r0. In the simulations, we also varied the temperature, since the field at which the PL recovery approaches saturation is sensitive to the relationship between g μ B B and kT. As can be seen from Figure5, the simulations agree well with the experimental results taking the nominal experimental temperature of 1.5 K. We will report elsewhere on studies of the excitation intensity dependence of the effect; there, we find we must take into account an increase in temperature for high excitation intensities (here, these were the same for Figures1 and2 and were low).
Using the simple model set out above, the dependence of the photoluminescence spectra of silicon nanoparticles with adsorbed oxygen molecules has been studied and it is shown that a realistic set of parameters can give an adequate description of the recovery of the PL intensity with increasing magnetic field, confirming the proposed spin-dependent exchange-coupled mechanism for the energy transfer process. In particular, one set of parameters can describe the behaviour of the magnetic field dependence for high and low oxygen coverage of the sample by changing only the parameters directly relevant to the energy transfer process. This represents the first detailed and quantitative investigation of magnetic field effects in the photogeneration of singlet oxygen by use of silicon nanoparticles and provides a model which can easily be expanded in order to investigate the dependence of the energy transfer process on nanoparticle size, excitation intensity, and temperature; this work is in progress.
This work was supported by the Engineering and Physical Sciences Research Council (UK) under grant EP/J007552/1.
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