Luminescence and energy transfer of 432 nm blue LED radiation-converting phosphor Ca₄Y₆O(SiO₄)₆:Eu²⁺, Mn²⁺ for warm white LEDs

Abstract

A series of Ca₄Y₆O(SiO₄)₆:Eu²⁺, Mn²⁺ phosphors are synthesized by a solid state method. Ca₄Y₆O(SiO₄)₆:Eu²⁺, Mn²⁺ can be excited at wavelength ranging from 250 to 500 nm, which is well matched with ultraviolet-visible light emitting diode. Under 432 nm excitation, Ca₄Y₆O(SiO₄)₆:Eu²⁺, Mn²⁺ can create warm white emission by energy transfer from Eu²⁺ to Mn²⁺. A warm white light emitting diode is fabricated by combining a 432 nm blue LED with a single phase warm white emitting phosphor Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺, which has CIE chromaticity coordinates (0.336, 0.319), correlated color temperature (CCT) 4326 K and color rendering index (R_a) 86, respectively. The results indicate Ca₄Y₆O(SiO₄)₆:Eu²⁺, Mn²⁺ may serve as potential warm white emitting phosphor for blue LED based white LEDs.

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1. Introduction

White light emitting diodes (LEDs) are widely used in solid-state lighting because of their advantages, such as energy savings, no mercury pollution, long life, short response time, small size, good hardness and high energy efficiency.^1–3 White LEDs can be fabricated by blue chip-pumped yellow YAG:Ce³⁺, but the color rendering index (CRI) of white light made by complementary blue and yellow emission is deficient because of the lack of contribution from red light.⁴ Therefore, to offset the deficiency, several red emitting phosphors have been developed to add into the YAG:Ce³⁺.⁵ Unfortunately, the extreme difference in degradation between the different host phosphors can produce color aberrations. Design of single-phase phosphor pumped by ultraviolet (UV) or near-UV chips is of significance for white LEDs because single-phase phosphor has excellent color rendering indexes and electro-optical design makes it simple to control different colors in comparison with mixed phosphors.^6–13 However, the efficiency of UV or near-UV chip chips is still lower than that of blue chip. Accordingly, exploration of single phase phosphor with green-to-red emission bands for blue LEDs would be useful.¹⁴ A phosphor could emit radiation couple co-doped activators with f–f or d–d electron configurations, such as Eu²⁺/Mn²⁺, Ce³⁺/Mn²⁺ and Ce³⁺/Eu²⁺, and then energy transfer would occur between activator/co-activator couples by effective resonant type via a multipolar interaction, and the energy transfer to Mn²⁺ would be an exchange interaction.^{6–13,15–17} However, as far as we know, co-doped single-phase phosphors with blue absorption or for blue LEDs rarely have been investigated.¹⁴ Moreover, for indoor illumination, warm white light, similar to most luminescence lamps, CCT (CCT ≤ 4500 K or even CCT ≤ 3500 K), is more favorable for human sight and better recommended.^18–20 Therefore, there is an urgent need for novel and highly efficient warm white emitting phosphor that can be excited by blue LEDs.

Ternary rare-earth-metal silicate Ca₄Y₆O(SiO₄)₆ is an efficient host lattice for luminescence of various rare earth ions and mercury-like ions.^21,22 The oxy-apatite host lattice consists of two cationic sites, that is, the 9-fold coordinated 4f site with C₃ point symmetry and 7-fold coordinated 6h site with C_s point symmetry. Both sites are suitable and easily accommodate a great variety of rare earth and transitional-metal ions.²³ Luminescent properties of Sb³⁺, Pb²⁺, Eu³⁺, Tb³⁺ and Dy³⁺ in Ca₄Y₆O(SiO₄)₆ have been reported.^21,22,24,25 Moreover, Lin et al. explored white emission properties of Ca₄Y₆O(SiO₄)₆:Ce³⁺/Tb³⁺/Mn²⁺, which can be excited effectively by 284 and 358 nm UV radiation.²³ Nonetheless, Ca₄Y₆O(SiO₄)₆:Ce³⁺/Tb³⁺/Mn²⁺ has a low absorption in the blue region. Therefore, in the present work, a warm white emitting phosphor Ca₄Y₆O(SiO₄)₆:Eu²⁺, Mn²⁺ which can be excited by blue light is explored, and the energy transfer property from Eu²⁺ to Mn²⁺ is studied.

2. Experimental section

2.1. Sample preparation

A series of Ca_4−x−yY₆O(SiO₄)₆:xEu²⁺, yMn²⁺ (x, y: mole concentration) samples were synthesized by a high temperature solid state method. Initial materials CaCO₃ (A.R.), Y₂O₃ (A.R.), SiO₂ (A.R.), MnCO₃ (99.99%) and Eu₂O₃ (99.99%), weighted in stoichiometric proportion, were thoroughly mixed and ground by an agate mortar and pestle for more than 30 min till they were uniformly distributed. The obtained mixtures were put into crucibles and heated at 900 °C for 2 h in an atmosphere. After that, the samples were thoroughly ground and sintered at 1350 °C for 4 h in a reducing atmosphere (5% H₂/95% N₂), then slowly cooled down to room temperature. The samples were then ground into powder for taking measurements.

2.2. Materials characterization

Phase formation was determined by X-ray diffraction (XRD) in a Bruker AXS D8 advanced automatic diffractometer (Bruker Co., Germany) with Ni-filtered Cu Kα1 radiation (λ = 0.15406 nm), and a scan rate of 0.02° s⁻¹ was applied to record the patterns in the 2θ range from 10° to 60°. Steady time resolved photoluminescence spectra were detected by a FLS920 fluorescence spectrometer, with the excitation source being a 450 W Xe lamp. Curve fittings are performed on the luminescence decay curves to confirm the decay time. Commission International de I'Eclairage (CIE) chromaticity coordinates were measured by a PMS-80 spectra analysis system. Quantum efficiency was analyzed with a photoluminescence quantum efficiency measurement system (C9920-02, Hamamatsu Photonics, Shizuoka) containing a 150 W xenon lamp. All measurements were carried out at room temperature. High-temperature photoluminescence spectra were detected by a fluorescence spectrophotometer (Hitachi F-4600) with a TAP-02 high temperature control system, scanning wavelength range from 400 to 700 nm, spectral resolution 0.2 nm, and excitation source a 450 W Xe lamp.

3. Results and discussion

3.1. Phase formation

XRD patterns of Ca_4−x−yY₆O(SiO₄)₆:xEu²⁺, yMn²⁺ were recorded and similar diffraction patterns were observed for each sample. As a representative, the XRD patterns of Ca_3.95Y₆O(SiO₄)₆:0.05Eu²⁺, Ca_3.95Y₆O(SiO₄)₆:0.05Mn²⁺ and Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺ are shown in Fig. 1. When the diffraction data were compared with the standard JCPDS card (no. 27-0093), there was no difference between the doped impurity Ca₄Y₆O(SiO₄)₆ and Ca₄Y₆O(SiO₄)₆. The uniform diffraction patterns indicate that the phase formation of Ca₄Y₆O(SiO₄)₆ is not influenced by small amounts of Eu²⁺, Mn²⁺ and Eu²⁺/Mn²⁺. Ca₄Y₆O(SiO₄)₆ has a hexagonal crystal structure, with a space group P6₃/m (176), and cell parameters a = b = 0.9356 nm, c = 0.6793 nm, α = β = 90°, and γ = 120°. However, as shown in Fig. S1,† it is noted that the diffraction peaks (2θ) shift to a small (Eu²⁺) or large (Mn²⁺) degree compared with those of the standard PDF card, which implies diversification of lattice parameters. According to the equation 1/d² = (h² + k² + l²)/a², the lattice constant a can be achieved. For Ca_3.95Y₆O(SiO₄)₆:0.05Mn²⁺ and Ca_3.95Y₆O(SiO₄)₆:0.05Eu²⁺, the a values are calculated to be 0.9317 nm and 0.9396 nm, respectively. The values are a little smaller (or bigger) than that of blank hexagonal Ca₄Y₆O(SiO₄)₆ phase (a = 0.9356 nm). The contraction may be attributed to the ionic radius of replacement ions being smaller than that of host ions, whereas the expansion may because the ionic radius of replacement ions is larger than that of host ions. Therefore, in view of the similar ion radius and valence, Eu²⁺ (0.130 nm) ions are expected to substitute Ca²⁺ (0.118 nm) sites in the compound Ca₄Y₆O(SiO₄)₆.²⁶ According to the same principle and ref. 23, Ca²⁺ sites are expected to be replaced by Mn²⁺ (0.090 nm) ions in the same crystal structure.

Fig. 1 XRD patterns of Ca_3.95Y₆O(SiO₄)₆:0.05Eu²⁺, Ca_3.95Y₆O(SiO₄)₆:0.05Mn²⁺ and Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺ with the standard data of Ca₄Y₆O(SiO₄)₆ (JCPDS no. 27-0093).

3.2. Photoluminescence properties

Fig. 2(a) shows that Ca_3.95Y₆O(SiO₄)₆:0.05Eu²⁺ has green emission under 380 nm and 432 nm excitation. Monitored at 527 nm, the excitation spectrum of Ca₄Y₆O(SiO₄)₆:Eu²⁺ has two obvious excitation bands, which are mainly caused by the transitions of Eu²⁺ from the 4f⁷ ground state to the 4f⁶5d¹ excited state.^7,8 Eu²⁺ ions have many excited states, and consequently, they have an unresolved broad excitation spectrum. The excitation spectrum shows that excitation wavelength can extend from 250 to 500 nm, although the peak is at 380 nm; however, there is an obvious excitation intensity in the blue region (such as at 432 nm). As shown in Fig. 2(a), Ca₄Y₆O(SiO₄)₆:Eu²⁺ can be excited effectively by 432 nm blue light, and produces green emission. The inset of Fig. 2(a) shows that emission intensities of Ca₄Y₆O(SiO₄)₆:Eu²⁺ enhance with increasing Eu²⁺ concentration to a point, then decrease with further increasing Eu²⁺ concentration, The optimal Eu²⁺ concentration is x = 0.05.

Fig. 2 (a) Emission and excitation spectra of Ca_3.95Y₆O(SiO₄)₆:0.05Eu²⁺ (λ_ex = 380 nm, 432 nm; λ_em = 527 nm); (b) emission and excitation spectra of Ca_3.95Y₆O(SiO₄)₆:0.05Mn²⁺ (λ_ex = 405 nm; λ_em = 615 nm); (c) emission spectra of Ca_3.95−yY₆O(SiO₄)₆:0.05Eu²⁺, yMn²⁺ (λ_ex = 432 nm); (d) decay curves of Ca_3.95−yY₆O(SiO₄)₆:0.05Eu²⁺, yMn²⁺ (λ_ex = 432 nm). Inset: emission intensities ((a) Eu²⁺ and (b) Mn²⁺) and (c) η_T as a function of impurity concentration.

Fig. 2(b) shows typical emission and excitation spectra for Ca_3.95Y₆O(SiO₄)₆:0.05Mn²⁺ under 405 nm excitation. As is well known, d–d transitions of Mn²⁺ are spin- and parity-forbidden, thus excitation transition of Mn²⁺ doped Ca₄Y₆O(SiO₄)₆ is difficult to pump and the emission intensity is very weak. A broad emission band peak at 615 nm is attributed to the spin-forbidden ⁴T₁(4G)–⁶A₁(6S) transition. Its excitation spectrum consists of several bands peaking at 365, 405, and 441 nm corresponding to the transitions from ⁶A₁(6S) to ⁴T₂(⁴D), [⁴A₁(⁴G), ⁴E(⁴G)] and ⁴T₂(⁴G), respectively.^7–10 The inset of Fig. 2(b) shows that the Mn²⁺ optimal concentration is y = 0.05.

As shown in Fig. 2(a) and (b), there is a spectral overlap between the emission band of Ca₄Y₆O(SiO₄)₆:Eu²⁺ and the excitation band of Ca₄Y₆O(SiO₄)₆:Mn²⁺. Accordingly, an energy transfer from Eu²⁺ to Mn²⁺ is expected in Ca₄Y₆O(SiO₄)₆. Under 432 nm excitation, emission spectra of Ca_3.95−yY₆O(SiO₄)₆:0.05Eu²⁺, yMn²⁺ with increasing Mn²⁺ concentration were measured, and the variation in emission spectra and emission intensity of samples are shown in Fig. 2(c). The results show that the emission intensity of Mn²⁺ increases systematically from y = 0.01 to y = 0.05, and reaches saturation at y ≥ 0.05. In other words, there is an energy transfer from Eu²⁺ to Mn²⁺ in Ca₄Y₆O(SiO₄)₆. To aid understanding of the energy transfer process, energy transfer efficiency (η_T) from Eu²⁺ to Mn²⁺ of samples was calculated. The η_T from Eu²⁺ to Mn²⁺ can be expressed according to Paulose et al.²⁷

η_T = 1 − (I_S/I_S0) (1)

where I_S0 and I_S are luminescent intensities of sensitizer Eu²⁺ in the absence and presence of activator Mn²⁺. The η_T values from Eu²⁺ to Mn²⁺ for Ca_3.95−yY₆O(SiO₄)₆:0.05Eu²⁺, yMn²⁺ (0 ≤ y ≤ 0.1) were calculated and are shown in the inset of Fig. 2(c). The η_T is found to increase gradually with increasing Mn²⁺ concentration. The results imply that energy transfer from Eu²⁺ to Mn²⁺ exists in Ca₄Y₆O(SiO₄)₆:Eu²⁺, Mn²⁺.

It is known that if energy is transferred from a donor to an acceptor, the temporal decay of donor decreases. Monitored at 527 nm emission, the temporal decay of Eu²⁺ as a function of Mn²⁺ concentration was investigated, and the results are shown in Fig. 2(d). For Eu²⁺ and Mn²⁺ co-doped samples, if Eu²⁺ and Mn²⁺ work independently, and there is no energy transfer between them, the photoluminescence lifetime of both activators will be as same as in the single-doped samples. If energy transfer from Eu²⁺ to Mn²⁺ exists in Ca₄Y₆O(SiO₄)₆, the decay of Eu²⁺ excitation state will be accelerated by this energy transfer channel, and consequently the lifetime of Eu²⁺ will be shortened. As shown in Fig. 2(d), shortening the lifetime of Eu²⁺ confirmed the energy transfer between Eu²⁺ and Mn²⁺ ions. In other words, the energy transfer Eu²⁺ → Mn²⁺ exists in Ca₄Y₆O(SiO₄)₆.

Generally, there are two aspects to the resonant energy-transfer mechanism: one is exchange interaction and the other is multipolar interaction. It is well known that if energy transfer results from the exchange interaction, the critical distance between sensitizer and activator should be shorter than 5 Å. In many cases, concentration quenching results from energy transfer from one activator to another until an energy sink is reached in the lattice. The critical distance (R_c) for energy transfer from Eu²⁺ to Mn²⁺ ions can be calculated using the concentration quenching method, and the critical distance R_Eu–Mn between Eu²⁺ and Mn²⁺ can be estimated by²⁸

R_Eu–Mn = 2[3V/(4πx_cN)]^1/3 (2)

where x_c is the total concentration of Eu²⁺ and Mn²⁺, N is the number of Z ions in the unit cell (for Ca₄Y₆O(SiO₄)₆, N = 10), and V is the volume of the unit cell (for Ca₄Y₆O(SiO₄)₆, V = 514.96 ų). The estimated distances (R_Eu–Mn) for Ca_3.95−yY₆O(SiO₄)₆:0.05Eu²⁺, yMn²⁺ phosphors (x_c = 0.05, 0.06, 0.08, 0.10, 0.12 and 0.15) are 12.53, 11.79, 10.71, 9.94, 9.36 and 8.69 Å, respectively. The distances between Eu²⁺ and Mn²⁺ become shorter with increasing Mn²⁺ concentration. x is the critical concentration at which the emission intensity of donor (Eu²⁺) in the presence of acceptor (Mn²⁺) is half that in the absence of the acceptor (Mn²⁺). Therefore, the critical distance (R_c) of energy transfer is calculated to be about 10.71 Å for Ca_3.95−yY₆O(SiO₄)₆:0.05Eu²⁺, yMn²⁺. R_Eu–Mn for various Eu²⁺ content levels is much larger than the typical critical distance for exchange interaction (5 Å).²⁹ The results indicate that there is little possibility of energy transfer via the exchange interaction mechanism. Thus, the electric multipolar interaction may take place for energy transfer between the Eu²⁺ and Mn²⁺ ions.

On the basis of Dexter's energy transfer formula for exchange and multipolar interactions, the following relation can be obtained^30–35

ln(η₀/η) ∝ C (3)

(η₀/η) ∝ C^α/3 (4)

where η₀ and η are luminescence quantum efficiency of Eu²⁺ in the absence and presence of Mn²⁺, respectively; C is the total concentration of Eu²⁺ and Mn²⁺. Eqn (3) corresponds to exchange interaction, and eqn (4) with α = 6, 8, 10 is ascribed to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively.

The relationships of ln(I_S0/I_S) ∝ C and (I_S0/I_S) ∝ C^α/3 are illustrated in Fig. 3. By consulting the fitting factor R, the relation (I_S0/I_S) ∝ C^8/3 has the best fitting, implying that the dipole–quadrupole interaction is applied for energy transfer from Eu²⁺ to Mn²⁺. The results also prove the conclusion using the critical distance (R_c) of energy transfer.

Fig. 3 Dependence of ln(I_S0/I_S) of Eu²⁺ on (a) C and I_S0/I_S of Eu²⁺ (b) C^6/3, (c) C^8/3, and (d) C^10/3.

For white LED application, quantum efficiency (QE) is an important parameter for phosphors. Tunable white light emission with suitable QE can be obtained in Ca_3.95−yY₆O(SiO₄)₆:0.05Eu²⁺, yMn²⁺ by energy transfer from Eu²⁺ to Mn²⁺ ions, as shown in Table 1. For warm white light emitting phosphor Ca_3.95−yY₆O(SiO₄)₆:0.05Eu²⁺, yMn²⁺, the QE can reach 39.6%. The data indicate that Ca_3.95−yY₆O(SiO₄)₆:0.05Eu²⁺, yMn²⁺ has relatively appropriate QE and warm white light emission, therefore it could be used as phosphor for white LEDs.

Table 1 CIE chromaticity coordinates (X, Y), CCT (K) and quantum efficiency (QE) for Ca_3.95−yY₆O(SiO₄)₆:0.05Eu²⁺, yMn²⁺ (λ_ex = 432 nm)

Samples	CIE coordinates (X, Y)	CCT (K)	Quantum efficiency (QE)
y = 0	(0.306, 0.502)	7119	37.1%
y = 0.01	(0.319, 0.436)	5619	38.3%
y = 0.03	(0.336, 0.319)	4326	39.6%
y = 0.05	(0.356, 0.313)	3601	39.1%
y = 0.07	(0.438, 0.335)	3209	38.3%
y = 0.1	(0.581, 0.366)	2653	38.1%

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Color coordinates an important factor for evaluating the performance of phosphors. Table 1 includes the measured Commission Internationale de I'Eclairage (CIE 1931) chromaticity coordinates and CCT of Ca_3.95−yY₆O(SiO₄)₆:0.05Eu²⁺, yMn²⁺. Eu²⁺ concentration is fixed at 0.05 as the concentration of Mn²⁺ increases from 0 to 0.1, with the corresponding color of phosphor shifting from green to white light, then to red. In particular, CCT of white light can also be tuned by appropriately tuning Mn²⁺ concentration. It is clear that warm white light can be created for different practical applications by varying Mn²⁺ concentration in Ca_3.95−yY₆O(SiO₄)₆:0.05Eu²⁺, yMn²⁺.

For application of high power LEDs, the thermal stability of a sample is an important issue for consideration. Fig. 4 shows temperature dependence of emission spectra for Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺, under 432 nm excitation. The inset shows the temperature quenching characteristics of commercial YAG:Ce and Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺ in the temperature range from 25 °C to 250 °C. Compared with YAG:Ce, the emission intensity of Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺ is very close to that of YAG:Ce when the temperature is raised up to 200 °C. The intensity of the sample drops to about 81% when the temperature is 200 °C, while that of YAG:Ce decreases to 83% of the initial value. The results indicate that the sample has good thermal quenching properties.

Fig. 4 Temperature dependent emission spectra of Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺ (λ_ex = 432 nm). Inset: normalized intensity of Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺ and YAG:Ce as a function of temperature.

To further understand the temperature dependence, Arrhenius fitting was conducted following the below equation³⁶

I_T = I₀/[1 + c exp(−ΔE/kT)] (5)

where I₀ is the initial emission intensity of phosphor at room temperature, I_T is the emission intensity at T K, c is a constant, ΔE is the activation energy for thermal quenching, and k is the Boltzmann constant (8.617 × 10⁻⁵ eV). The fitting result is shown in Fig. 5, with activation energy ΔE achieved at 0.162 eV. The relatively high activation energy indicates that Ca₄Y₆O(SiO₄)₆:Eu²⁺, Mn²⁺ has good thermal stability, and may be applied for high-powered LED application.

Fig. 5 Arrhenius fitting of emission intensity of Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺ and the calculated activation energy (ΔE) for thermal quenching.

3.3. Application of Ca₄Y₆O(SiO₄)₆:Eu²⁺, Mn²⁺

To demonstrate application of Ca₄Y₆O(SiO₄)₆:Eu²⁺, Mn²⁺, a white LED was fabricated by coating Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺ phosphor on a 432 nm UV-chip driven by a 20 mA current. Fig. 6 shows the emission spectrum of a white LED, with CIE chromaticity coordinates (0.336, 0.319) and correlated color temperature (CCT) 4326 K, and color rendering index (R_a) 86. Performance of white LEDs based on Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺ is superior to that of white LEDs fabricated by coating YAG:Ce yellow phosphor on blue chips [(0.292, 0.325), CCT = 7756 K] because the former shows higher color rendering index and lower CCT value.⁴ Moreover, Table S1† depicts the CIE chromaticity coordinates and correlated color temperature of Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺ samples in a white LED device with current 20–200 mA. The results show that the white LEDs have a relatively appropriate color stability.

Fig. 6 Emission spectrum of a pc-LED lamp fabricated with a 432 nm LED chip and warm white emitting phosphor Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺. Inset shows appearance of a well-packaged trichromatic LED lamp in operation.

4. Conclusion

In summary, Ca₄Y₆O(SiO₄)₆:Eu²⁺, Mn²⁺ can be effectively excited by 432 nm blue LED, and produce warm white emission by energy transfer from Eu²⁺ to Mn²⁺, with QE about 39.6%. A warm white light emitting diode is fabricated by combining a 432 nm blue LED with the warm white emitting phosphor Ca_3.92Y₆O(SiO₄)₆:0.05Eu²⁺, 0.03Mn²⁺. The white LEDs have CIE coordinates (0.336, 0.319), correlated color temperature (CCT) 4326 K and color rendering index (R_a) 86. The results indicate that Ca₄Y₆O(SiO₄)₆:Eu²⁺, Mn²⁺ may be a potential single-phase white emitting phosphor for blue based LEDs.

Acknowledgements

The work is supported by the National Natural Science Foundation of China (no. 50902042), the Natural Science Foundation of Hebei Province, China (no. A2014201035; E2014201037) and the Education Office Research Foundation of Hebei Province, China (no. ZD2014036; QN2014085).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08283d

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