Article pubs.acs.org/JPCC
Third-Order Nonlinear Optical Properties of Infrared Emitting PbS and PbSe Quantum Dots Dominika Wawrzynczyk, Janusz Szeremeta, Marek Samoc, and Marcin Nyk* Advanced Materials Engineering and Modelling Group, Faculty of Chemistry, Wroclaw University of Science and Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland S Supporting Information *
ABSTRACT: The optical properties of small band gap, colloidal quantum dots are presented, with the special emphasis put on the measurements of their nonlinear optical properties in the infrared region of spectra. In particular, two types of colloidal quantum dots (PbS and PbSe), with the first exciton absorption band maxima in the near-infrared region of spectra, were investigated using a tunable femtosecond laser system and the Z-scan technique. The measurements of closed- and open-aperture Z-scan traces allowed for the calculation of real and imaginary parts of cubic nonlinearity, which were presented as appropriate cross sections used to characterize the nonlinear refractive and absorptive properties of the studied quantum dots. The maximum two-photon absorption cross section values taken for a single quantum dot were found to be ∼2400 GM (Goeppert-Mayer units) at 1300 nm and ∼15 500 GM at 1700 nm, for PbS and PbSe QDs, respectively. PbS quantum dots showed two-photon induced emission upon infrared excitation. The obtained results demonstrate the potential of IV−VI group colloidal quantum dots for low-cost photonic devices and two-photon absorbers in the near-infrared and infrared spectral ranges.
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INTRODUCTION The nonlinear optical (NLO) properties of various types of colloidal quantum dots (QDs) have been up to now thoroughly investigated with the use of different experimental techniques. Open-aperture Z-scan,1−5 two-photon excited emission,3,5,6 and four-wave mixing7 measurements reported two-photon absorption cross sections (σ2) values for QDs as high as 1.6 × 105 GM (Goeppert-Mayer units) for CdSe quantum rods,8 and 5 × 107 GM for CdSe noplatelets9the values of more than an order of magnitude higher than those for very good two-photon absorbing organic chromophores,10−12 although comparisons of absolute values of the NLO absorption parameters, without normalizing, e.g., to the mass or volume of a nanoparticle, are skewed.12 Due to the quantum confinement effect, large oneand two-photon absorption cross section, high quantum yield and good chemical and optical stability, the colloidal QDs are considered to be among the best multiphoton absorbing probes for various types of applications.13−17 The peaks of maximum two-photon absorption for most often investigated type II−VI QDs (CdSe, CdS, or CdTe), usually fall in the near-infrared (NIR) region of light, in the wavelength range between 800 and 1300 nm. Above this wavelength range NLO processes of higher orders are often observed, but the absorption cross sections involved, in combination with the typically used light intensities, usually lead to decreased multiphoton excited emission intensities compared to those achievable through two-photon absorption. The search for stable, inorganic two© 2016 American Chemical Society
photon absorbers in the infrared region of spectra (1.5−2.5 μm) has been mostly restricted to red-shifting of the first exciton absorption band by synthesis of larger QDs. However, as the size of QDs exceeds the Bohr exciton radius, characteristic for a particular material, the quantum confinement effect is no longer observed.18 The alternative is to use QDs that are one-photon absorbing in NIR, e.g., PbS or PbSe, which have an additional advantage of large Bohr exciton radii and thus broad size tunability. The wet chemistry synthesis techniques allow one to obtain colloidal solutions of monodisperse PbS19,20 and PbSe21−23 QDs, with the lowest exciton absorption maxima tuned between 800 and 4000 nm, and thus two-photon absorption is expected to be present in the wavelength range between 1.6 and 8.0 μm. However, it should be kept in mind that the actual peaks of two-photon absorption cross section values for QDs do not necessarily need to correspond to the position of their first excitonic transition.24 The NLO properties of PbSe25 and PbS19,26,27 QDs with various sizes, including measurements of nonlinear refractive index (n2) and nonlinear absorption coefficient (β), have been already investigated; however, the reported experiments were mostly performed for single wavelengths. Nanosecond25,26 and picosecond28 532 nm laser excitation and the Z-scan technique Received: June 13, 2016 Revised: August 30, 2016 Published: August 31, 2016 21939
DOI: 10.1021/acs.jpcc.6b05956 J. Phys. Chem. C 2016, 120, 21939−21945
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λem = 1000 nm, 10 mg/mL in toluene) were purchased from Sigma-Aldrich. For the size and shape characterization of the studied PbS and PbSe QDs we used a high-resolution FEI Tecnai G2 20 X-TWIN transmission electron microscope (TEM), with 200 kV accelerating voltage. The absorption spectra of PbS and PbSe QDs toluene solutions were measured in spectral range between 600 and 2000 nm with a JASCO V670 spectrophotometer, while for the emission spectra measurements we have changed the solvent to tetrachloroethylene, which has better NIR transparency, and used an Ocean Optics NIR Quest 512-2.2 fiber coupled spectrophotometer. The measurements of two-photon excited emission of PbS and PbSe QDs and their NLO parameters, i.e., nonlinear refractive index (n2), nonlinear absorption coefficient (β), twophoton absorption cross section (σ2), and nonlinear refraction cross section (σR),31,33 were performed with the excitation from a femtosecond laser system, operated at the repetition rate of 1 kHz, consisting of a Quantronix Integra-C Ti:sapphire regenerative amplifier and a Quantronix Palitra-FS optical parametric amplifier, which allowed for tuning the wavelength from 500 up to 2000 nm. We used the standard open-aperture (OA) and closed-aperture (CA) Z-scan technique introduced by Sheik-Bahae et al.34 for the determination of NLO parameters; the detailed description of the experimental setup can be found in our previous papers.1,5 In order to exclude the contributions of solvent and cuvette cell to the values of NLO parameters, we have used a calculation method, in which the real and imaginary parts of the cubic hyperpolarizability, γ, or the corresponding cross sections, σR and σ2, are derived from the difference between the Z-scans traces obtained for the cell filled with pure solvent and the cell with the solution of the investigated PbS or PbSe QDs35 (see the Supporting Information for the detailed information).
were used to evaluate the NLO coefficients values for PbS and PbSe QDs in the wavelength range of one-photon absorption. Similar measurements conditions, with picosecond 106428 and 1550 nm,29 and femtosecond 780 nm19 excitation, allowed for the n2 and β parameters calculations for out-of-resonance wavelengths. Padilha et al.27 and Nootz et al.30 performed a comprehensive study, both theoretical and experimental, of two-photon absorption in lead-salt QDs, i.e., PbS and PbSe, with the use of Z-scan and two-photon excited emission techniques, showing the increase of two-photon absorption cross section values with the increase of QDs size. Those measurements were performed in a wide wavelength range, for the laser wavelengths from ∼950 up to ∼2500 nm (1.3−0.5 eV). However, no data on the nonlinear refractive index were provided for that range. We believe an independent determination of the NLO parameters, including both nonlinear absorption and nonlinear refraction, would contribute to facilitating various photonic applications of PbS and PbSe QDs, including scattering-free two-photon fluorescence imaging, data storage, or microfabrication. With this in mind, we have investigated the infrared NLO properties of synthesized by us PbSe QDs as well as commercially available PbS QDs in the wavelength range between 1.3 and 2.0 μm, using a tunable femtosecond laser system and the Z-scan technique. Both the sizes and types of the investigated QDs were chosen to ensure strong one-photon absorption and single-photon excited emission in the NIR region of spectra. As suspected, the PbS and PbSe QDs showed two-photon absorption in the red-shifted wavelength range, with the maximum values of σ2 reaching ∼15 500 GM at 1700 nm for 7.3 nm PbSe QDs. Following Balu et al.31 we have presented also the nonlinear refractive properties of the investigated QDs as concentration-independent cross sections (σR), with analogous to two-photon absorption cross section “refractive” Göppert-Mayer units (RGM = 10−50 cm4 s). The determined values of NLO parameters (i.e., σ2, σR, β, and n2) are further confronted with the literature reports for similar QDs systems, and the arising issues resulting from the chosen way of presenting NLO data are discussed. Additionally, it was verified if both PbS and PbSe QDs showed their characteristic emission upon infrared excitation in the two-photon absorption range of 1.7−1.9 μm.
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RESULTS AND DISCUSSION The investigated PbS and PbSe QDs showed good colloidal stability, and no visible precipitation of the particles in the toluene solution was observed. The absence of QDs aggregation was also confirmed by TEM images (Figure 1, parts a and c), where well-separated spherical particles could be seen. On the basis of the TEM images, by measuring the sizes of over 100 particles, we have calculated the mean diameter of the studied QDs to be 3.22 ±0.03 and 7.28 ±0.07 nm for PbS and PbSe QDs, respectively. For the larger PbSe QDs we were able to observe lattice fringes (inset in Figure 1c), which indicated high crystallinity of the synthesized QDs. Parts b and d of Figure 1 present size distribution histograms showing monodispersity of the QDs morphology. The small sizes of the studied QDs, below the exciton Bohr radius characteristic for those materials, ensure the high quantum confinement regime. The sizes of PbS and PbSe QDs estimated from TEM images were further correlated with the band gap energy (E0) calculated based on absorption spectra (Figure 2a). The inset in Figure 2a shows that the experimental data (dots) correspond well with literature sizing curves obtained based on the following equations for PbS36 and PbSe37 QDs, respectively: PbS QDs:
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MATERIALS AND METHODS The synthesis of PbSe was carried out as described previously, with some modifications.32 In a typical synthesis, first 3 mmol of Se nanopowder (95%, Sigma-Aldrich) was dissolved in 3 mL of TOP (trioctylphosphine, Sigma-Aldrich) under inert atmosphere to obtain the selenium precursor. Next, Pb3O4 (1 mmol), oleic acid (3 mL), and benzyl ether (4 mL) were mixed in a reaction flask. To remove water and oxygen, the flask was evacuated and purged several times with dry nitrogen at 5 min intervals at 110 °C. The transparent solution was then heated to 200 °C under nitrogen atmosphere and vigorous stirring. At this temperature the selenium stock solution was swiftly injected into the reaction vessel in a single step. Upon injection, the temperature of the solution dropped ∼30 °C below the injection temperature. The reaction was stopped 1 min after the injection by cooling the flask with cold air. The mixture was precipitated with methanol in an ultrasonic bath and collected by centrifugation at 10 000 rpm for 30 min. The precipitate was dried in vacuum and dispersed in anhydrous toluene. The PbS quantum dots (oleic acid coated, fluorescence
E0 = 0.41 + 21940
1 0.0252d 2 + 0.283d DOI: 10.1021/acs.jpcc.6b05956 J. Phys. Chem. C 2016, 120, 21939−21945
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Obviously, presenting the nonlinear absorption and refraction for nanoparticles dispersed in solvents in terms of macroscopic parameters is not a good idea since the parameters are concentration-dependent. Two alternate but equivalent ways can be chosen here, either presenting the data as the complex hyperpolarizability of the nanoparticles (as in our former papers1,24,38) or, following refs 8, 31, and 39, as cross sections, σ2 and σR, for the two-photon absorption and nonlinear refraction, respectively. Such a single-nanoparticle-oriented presentation of experimental results allows for comparison of NLO parameters between different types of nanoparticles. The wavelength dispersions of both nonlinear refraction and absorption can then be conveniently presented since those two effects should be related through Kramers−Kronig transforms, and thus, e.g., changes of the nonlinear refraction sign are often observed in the peak region of nonlinear absorption.31 However, for comparing nanoparticles of different sizes, both for σ2 and σR one may be also interested in the merit factors that arise when the cross sections are normalized to the molecular weight or volume of the investigated species. On the basis of the known sizes of the studied QDs and the density of their bulk counterparts, we have calculated the molecular weight of single PbS and PbSe QDs to be 64 600 and 1 077 000 g/mol, respectively. The gravimetric analysis, after evaporation of certain amounts of solvent, allowed us to estimate the concentration of QDs in toluene solutions to be approximately 2% by weight. Parts a and b of Figure 3 show the dispersion of
Figure 1. TEM images showing the morphology of PbS (a) and PbSe (c) QDs, together with corresponding size distribution histograms (b and d) obtained based on measuring the size of over 100 dots. Inset in part c shows a high-resolution TEM image of PbSe QDs.
PbSe QDs: E0 = 0.278 +
1 0.016d + 0.209d + 0.45 2
Upon 900 nm femtosecond laser excitation both types of the studied QDs showed emission in the NIR region of spectra (Figure 2b). The emission band maximum of PbS QDs was situated at ∼1000 nm, with a 100 nm (130 meV) Stokes shift with respect to the absorption exciton band position. For the PbSe QDs we have observed broad-band emission spectra peaking at approximately 1650 nm (Figure 2b), close to the first exciton maximum in the absorption spectra. Having well-characterized materials, with one-photon absorption spectra peaking in the NIR region of light, we performed OA and CA Z-scan measurements, and calculated the relevant NLO parameters in the red-shifted region of excitation. Literature data on NLO parameters of various nanomaterials suffer from poor reproducibility, partly because of the lack of standards on how the data should be presented.
Figure 3. Dispersion of two-photon absorption (σ2, black squares) and nonlinear refraction (σR, red circles) cross sections for PbS (a) and PbSe (b) QDs.
Figure 2. Absorbance (a) and emission spectra under 900 nm excitation (b) of PbS (black line) and PbSe (red) QDs solutions. 21941
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which was equal to ∼0.04 and ∼0.01 (in GM·mol/g) for PbS QDs at 1300 nm and PbSe QDs at 1700 nm, respectively. Those values are similar to corresponding ones obtained in our laboratory for other types of colloidal QDs1,4,5,24 and lanthanide-doped nanoparticles.38,42,43 Another, way of presenting the values of relevant NLO parameters, convenient for comparison between different types of materials, is volume normalization, which in our case leads to the corresponding σ2/ V values of ∼140 and ∼76 (in GM/nm3) for PbS QDs at 1300 nm and PbSe QDs at 1700 nm, respectively. A wide wavelength range, between 950 and 2500 nm, investigation of two-photon absorption cross section for PbS QDs of different sizes was previously performed by Padilha et al.27 Additionally, Nootz et al.30 investigated, both theoretically and experimentally, the positions of two-photon absorption maxima in PbS and PbSe QDs of different sizes. Those authors have reported an increase of σ2 values with the decrease of quantum confinement effect and increase of the size of the QDs. For the PbS QDs dispersed in toluene, with similar size to ones investigated by us (3.7 vs 3.2 nm), they have reported much larger two-photon absorption cross sections reaching 50 000 GM at 1460 nm, which corresponds to the σ2/M parameter equal to ∼0.4. However, taking into account the local field penetration factor f,27 which accounts for the dielectric constants of both the solvent and the QDs material, the results obtained by us are comparable to the literature reports (see Table 1). Presenting the values of the obtained NLO parameters in terms of the microscopic properties of materials (real and imaginary parts of cubic hyperpolarizability, γ, or the corresponding cross sections) gives the possibility for comparison between different types of molecules or nanoparticles; however, as mentioned, the literature usually reports the corresponding macroscopic parameters: the nonlinear refractive index (n2) and nonlinear absorption coefficient (β). Such results must be taken with care since the macroscopic parameters obviously depend on the concentration of the active species in the medium that is being investigated (which could be neat material, solution, or solid solution) and often the contribution of the matrix cannot be excluded. For the precise conversion between microscopic and macroscopic NLO parameters the exact values of materials concentration and type of solvent used are necessary; such information is, however, often missing from the published results. In order to compare our results with the corresponding values obtained by other groups we have calculated the macroscopic parameters values for the studied PbS and PbSe QDs (Table 1). Simultaneously, if all the necessary information (QDs concentration and solvent type) was provided, we have calculated corresponding cross sections for the literature data (Table 1). It should be emphasized that the values of NLO parameters, for any types of materials, are highly wavelengthdependent. The measurements of NLO absorption, with the use of the Z-scan technique, should be performed using low repetition rate, short laser pulses excitation (most preferable are kilohertz, femtosecond excitation sources) in order to minimize the unwanted contribution from thermal effects or excited state absorption.44 While performing the measurements in the wavelength region of non-negligible linear absorption, the effect of saturable absorption should also be taken into account. On the other hand, the calculation of n2 values should account for the contributions from solvent and cuvette cells to obtain the values characterizing the pure material.44 The observed scatter of the calculated n2 and the corresponding σR values
σ2 and σR values calculated for PbS and PbSe QDs based on OA and CA Z-scan traces in the wavelength ranges of 1300− 2000 and 1600−2000 nm, respectively. The maximum σ2 values taken for a single QD for PbS and PbSe QDs were found to be ∼2400 GM at 1300 nm and ∼15 500 GM at 1700 nm, respectively (Figure 3). However, it should be stressed that the measurements for the PbSe QDs were performed close to onephoton transition, but no saturable absorption effect was detected. Simultaneously, for both types of investigated QDs the σR showed negative sign in the whole investigated spectral region, with the most negative values falling in the strong twophoton absorption region (Figure 3). The ratio σR/σ2, which can be treated as a figure of merit for, e.g., optical switching application,31,33 was equal to ∼4 at 1800 nm and ∼5 at 1900 nm for PbS and PbSe QDs, respectively. We also found that the position of the maximum of two-photon absorption cross section corresponds roughly to twice the wavelength of the maximum of the second absorption band as seen in the onephoton absorption spectrum of the PbS QDs. These observations converge with those of Padilha et al.3 as well as with our previous study performed for CdSe and CdS QDs.1,24 Nootz et al.30 predicted theoretically, with the use of isotropic k·p four-band envelope function formalism, and proved experimentally, that for lead-salt PbS and PbSe QDs the additional two-photon transitions are observed at energies where one-photon transitions are expected. This phenomenon is attributed to the band anisotropy, where the breaking of the inversion symmetry of the wave functions has to be included. Figure 4 shows representative raw CA and OA Z-scan traces for PbS QDs at 1350 nm (Figure 4a) and PbSe QDs at 1700
Figure 4. Representative raw open- and closed-aperture Z-scan traces measured at 1350 nm for PbS QDs (a) and at 1700 nm for PbSe QDs (b) together with their theoretical fits.
nm (Figure 4b), together with corresponding fits. Additionally, the unperturbed Z-scan traces for cuvette filled with solvent only are presented in Figure S1. The two-photon character of the observed absorptive processes was assured based on the accuracy of the theoretical fits to the raw OA Z-scan traces (Figure 4). The occurrence of the higher order processes would result in the tell-tale narrowing of the transmittance dips.40,41 As mentioned above, due to the large difference in the studied QDs sizes, and also for better comparison with different types of nonlinear absorbers, it is convenient to present the results by means of the molecular weight scaled NLO parameter, σ2/M, 21942
DOI: 10.1021/acs.jpcc.6b05956 J. Phys. Chem. C 2016, 120, 21939−21945
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Table 1. Nonlinear Absorption Coefficient (β), Nonlinear Refractive Index (n2), Together with Corresponding Cross Sections (σ2 and σR) and Molecular Weight (M) or Volume (V) Scaled Corresponding NLO Parameters (σ2/M, σR/M, σ2/V, and σR/V) for PbS and PbSe QDs Measured with the Open- and Closed-Aperture Femtosecond Z-Scan Technique sample (size)
λ [nm]
β [m/W] −11
σ2 [GM]
PbS QDs (3.2 nm)
1900
7.9 × 10
PbS QDs (3.2 nm)
1700
3.3 × 10−10
470 (3700)
PbS QDs (3.2 nm)
1500
1.3 × 10−9
1100 (8500)
102 (785)
PbS QDs (2.96 nm)
∼1150
3953 (30 500)
PbS QDs (5.2 nm)
∼2150
16 330 (126 000)
PbS QDs (6.1 nm)
1550a
−9 × 10−10
satd abs
PbSe QDs (7.3 nm)
1900
7.0 × 10−10
8130 (62 730)
PbSe QDs (7.3 nm)
2000
6.6 × 10−10
7910 (60 900)
PbSe QDs (5.8 nm)
1640b
σ2/M [GM·mol/g] σ2/V [GM/nm3] 0.0016 (0.012) 6 (46) 0.0073 (0.056) 28 (215) 0.017 (0.13) 65 (500) 0.064 (0.49) 288 (2220) 0.048 (0.37) 218 (1680)
0.0076 (0.058) 42 (320) 0.0074 (0.057) 41 (315)
n2 [m2/W] −3.51 × 10
−17
σR/M [GM·mol/g] σR/V [GM/nm3]
σR [RGM] −2.4 × 10
3
−1.72 × 10−17
−0.9 × 103
−2.38 × 10−17
−1 × 103
0.037 141 0.014 53 0.016 59
ref this work this work this work 27 27
−25 × 10−16 −1.67 × 10−12c −9.18 × 10−17
−14.4 × 109
−5.85 × 10−17
−24.8 × 103
−3.0 × 10−15 −1.8 × 10−11c
−43.1 × 103
−43.7 × 109
−2.6 × −1.2 × 0.040 221 0.023 127 −9.9 × −4.3 ×
104 108
29 this work this work
104 108
45
a
Measurements done with picosecond, 10 MHz laser pulses for sample dispersed in C2Cl4. bMeasurements done with femtosecond, 82 MHz laser pulses for sample dispersed in C2Cl4. cExtrapolated to the pure substance; in parentheses, the same values after correction by the local field penetration factor f (ref 27).
(Table 1) may partly result from the difficulty to fully separate the contributions from the QDs and the solvent. A dependence of the NLO refractive coefficient of the PbS QDs on the dispersant type has been reported by Cheng et al.,19 but the observed differences between n2 values measured for toluene (−9.005 × 10−20 m2/W), n-hexane (−2.377 × 10−19 m2/W), and tetrachloromethane (−1.285 × 10−19 m2/W) solutions of PbS QDs may partly result from the hard to be excluded contribution from the solvent itself. The above issue was discussed by Ferdinandus et al.,33 who have suggested a new dual-arm Z-scan technique to precisely extract small nonlinearities of the materials from large solvent signals. While the literature reports different strengths of NLO absorption (Table 1) for PbS and PbSe QDs, to make any comparisons the size of the QDs should be taken into account, while also attention should be paid to the wavelength and the duration of the laser pulse.44 We believe the highest reported values of β were equal to 2.16 × 10−9 and 7.0 × 10−10 m/W for PbS nanorods26 and PbSe QDs, respectively. The investigated PbS nanorods had, however, much bigger sizes than other studied QDs systems, and additionally the Z-scan measurements were in that case performed with the green, nanosecond laser excitation. The enhancement of NLO absorption in anisotropic CdSe QDs has been also reported;8 thus, the combination of the mentioned factors could affect the final β value. Finally, we have investigated the possibility of obtaining PbS and PbSe QDs emission upon infrared excitation, and the corresponding spectrum for PbS QDs in tetrachloroethylene is shown in Figure S2 (Supporting Information). QDs showed their characteristic emission after anti-Stokes excitation, which further expands their possible applications beyond being good two-photon absorbers and emitters in the infrared region of spectra. Unfortunately, due to the significant aggregation and precipitation of PbSe QDs in tetrachloroethylene under high-
intensity laser excitation, we were unable to record reliable twophoton excited emission spectra for those QDs in that solvent, while the emission spectra in toluene were distorted through strong solvent absorption in the range of PbSe QDs emission.
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CONCLUSIONS
In conclusion, we evaluated nonlinear optical properties of colloidal PbS and PbSe QDs in the important wavelength range between 1.3 and 2.0 μm. In particular, we have used the Z-scan technique, with tunable femtosecond pulses, to obtain the values of relevant NLO parameters: two-photon absorption and nonlinear refraction cross sections, nonlinear absorption coefficient, and nonlinear refractive index corresponding to pure PbS and PbSe QDs. By the comparison of our data with the literature reports, we have demonstrated that it is important to characterize those NLO factors with the use of short laser pulses, in a wide wavelength range, away from the linear absorption region, and with special emphasis put on the exclusion of solvent and thermal effects. The maximum values of σ2 and σ2/M parameters found for both PbS and PbSe QDs are in the same range as the corresponding values calculated for other types of semiconducting nanoparticles colloidal systems. However, in the case of the studied QDs the peaks of these values are found in the strongly red-shifted region of spectra. Additionally, besides being good two-photon absorbers in the infrared region, the PbS QDs showed two-photon induced photoluminescence under excitation with laser beam at wavelengths corresponding to the maxima of σ2. The possibility to effectively excite those QDs with infrared light, through multiphoton processes, is likely to be of importance for various photonics applications. 21943
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b05956. Extended description of the method used for calculations of relevant nonlinear optical parameters and two-photon induced luminescence of PbS QDs (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +48 71 320 4069. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge support from the National Science Centre under Grant DEC-2012/05/B/ST5/00256 and by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw University of Science and Technology.
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