Enhanced photoelectrical performance of chemically processed SnS₂ nanoplates

Abstract

In this work, tin disulfide (SnS₂) nanoplates have been synthesized through a facile hydrothermal method. The structural and morphological properties of SnS₂ were investigated via scanning electron microscopy (SEM) which actually revealed the nanoplates like morphology of the obtained samples. Heterojunction diodes comprising SnS₂ nanoplates and p-type silicon (Si) were fabricated and been demonstrated. Their electrical properties were studied using current–voltage characteristics and impedance spectroscopy. The diodes were found to exhibit excellent rectifying behavior with significant increase in reverse current under illumination. Impedance results identified the resistance of the device to reduce considerably under light irradiation. The enhanced photoelectrical properties of the heterojunctions were actually promoted by the electric field at the heterojunction interface, which further results with the effective separation of photogenerated electron hole pairs. The obtained results also suggest the potential of chemically processed SnS₂ nanoplates for applications in photodetection and sensors applications.

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Introduction

Recently, semiconductor based metal sulfides are extensively studied for producing nanoscale optical and electronic devices in new forms and functions, enhanced performance, and potential for low-cost mass production. Among the metal sulfides, tin disulfide (SnS₂) is believed to be a pioneer material due to its properties such as n-type electrical conductivity, remarkable optical properties and strong photo-conducting behaviors.¹ It possess a CdI layered structure, composed of tin atoms located at octahedral sites between two close-packed sulfur atoms and the adjacent sulfur layers connected by weak van der Waals (VDW) interactions.^2,3 Additionally, its constituent elements are also nontoxic, eco-friendly, and abundant in nature. With a band gap of around 2.1–2.4 eV, SnS₂ has attracted attention for many different applications such as photovoltaics, photodetectors, phototransistors, sensors, photcatalysis and lithium ion batteries.^4–11 Different morphologies like nanorods, nanowires, nanobelts, nanotubes, nanosheets and nanoplates of tin sulfides have been reported by many groups in this regard.^12–23

Two dimensional (2D) nanostructured forms like nanoplates and nanosheets in particular could be easily integrated in micro/nanoscale structures for development of new electronic and optoelectronics applications.^24,25 A variety of methods such as chemical vapor deposition, solvothermal and hydrothermal methods have been employed for the synthesis of SnS₂ nanoplates and nanosheets.^26,27 Among them hydrothermal methods is highly suitable for large scale production of nanoplates. The integration of this kind of solution processed nanoplates on Si substrate are attractive for photodiode functions based on Si devices. This can actually be achieved by employing a dispersion of nanoplates, which are further processed through low-cost techniques such as drop-casting, spin-coating, spray-deposition, etc.

In this work we demonstrate a simple hydrothermal synthesis of SnS₂ hexagonal nanoplates that are processed into thin films by adopting a simple spin coating method onto Si substrates for the fabrication of p–n photodiode. The electrical properties of the heterojunction diode revealed excellent rectification. Additionally the photoresponse properties of the heterojunction diodes were found to exhibit excellent current rectification and sensitivity under light irradiation. The enhancement of the photoelectrical properties were further studied through impedance spectroscopy. A physical mechanism for the enhancement of the photocurrent is also explained in detail using a band diagram.

Experiment

Materials

All the chemicals used in the experiments were of analytical grade and used thereafter. Tin(IV) chloride pentahydrate, thioacetamide (TAA) and ethanol were procured from Sigma-Aldrich. The p-Si substrate used in the fabrication of heterojunction were pre-cleaned in acetone, ethanol and deionized water. Electrical contacts were made using conducting silver paste (Silbest).

Synthesis of SnS₂ nanoplates

In a typical process, tin(IV) chloride pentahydrate (SnCl₄·5H₂O) and thioacetamide (TAA) (1 : 4) were dissolved in 60 mL of deionized water. The solution was stirred for about an hour to form a transparent solution. The resultant solution was then transferred to a Teflon-lined stainless steel autoclave of 75 mL capacity and heated at 160 °C for 12 h in an electric oven and finally cooled to room temperature. After the hydrothermal synthesis, the resultant product was centrifuged at 8000 rpm for 20 min and washed several times with ethanol and deionized water. Finally, the SnS₂ product were obtained after drying at 60 °C for 5 h.

Device fabrication

Initially, 1 mg of SnS₂ nanoplates were dispersed in 10 mL ethanol solution and sonicated for about 1 h to make the colloidal suspension of SnS₂ nanoplates. Then the suspension was spin coated on top of the precleaned p-Si. After the spin coating process, the film is heated at 100 °C for 5 min. This process was repeated for several cycles to obtain a continuous film. Finally, Ag was coated on top of SnS₂ and p-Si to complete the diode structure.

Characterization

The morphological evolution of the sample was examined using high resolution scanning electron microscopy (HRSEM, Philips). The phase purity and crystal structure of SnS₂ nanoplates was inferred through X-ray RINT 2500 diffractometer with Cu Kα radiation (k = 0.154 nm) and Micro Raman spectrometer. The absorbance spectrum was recorded using a Cary UV/VIS/NIR spectrophotometer, while the room temperature emission properties were studied using photoluminescence spectrum. The room-temperature current–voltage (I–V) characteristics were studied using a Keithley 617 semiconductor parameter analyzer. A solar simulator (Newport, AM 1.5) was employed to study photoresponse of the device under different light intensity. Electrochemical impedance studies was performed using a ZIVE impedance analyzer.

Results and discussion

Fig. 1a shows the surface morphology of the as prepared SnS₂ with the inset showing their size distribution. As seen from the SEM images the observed morphology appears to be of hexagonal like nanoplates with diameter ranging from 30 to 60 nm. Transmission electron microscopy (TEM) images in Fig. 1b clearly shows that the SnS₂ nanocrystals obtained are in form of hexagonal nanoplates and agrees with SEM result. X-ray diffraction (XRD) experiments were additionally carried out to elucidate the structural properties of SnS₂ nanoplates. The diffraction patterns of the as synthesized SnS₂ nanoplates are shown in Fig. 1c, where all the observed peaks could be assigned to the hexagonal phase of SnS₂. Interestingly, a sharp diffraction peak was observed for (001) plane, indicating the preferential c-axis orientation of SnS₂.²⁸ Furthermore, the absence of impurities peaks other than SnS₂ implies the high quality of the obtained product. The structural composition of the obtained SnS₂ nanoplates was further studied by Raman analysis. The Raman spectrum of SnS₂ nanoplates shown in Fig. S1,† displays two peak at 314 and 204 cm⁻¹ respectively. The peak at 314 cm⁻¹ can be assigned to A_1g phonon modes whereas later one corresponds to the E_g mode of the SnS₂.^29–31

Fig. 1 (a) SEM image of the as-prepared SnS₂ nanoplates and inset shows size distribution of SnS₂ nanoplates. (b) TEM image of the SnS₂ nanoplates. (c) XRD pattern and (d) UV absorption spectrum of the SnS₂ nanoplates with the inset that shows the Tauc's plot.

UV absorption measurements were carried out to evaluate the optical properties of the SnS₂ nanoplates. Fig. 1d shows the absorption spectra of the synthesized SnS₂ nanoplates measured over a wavelength range of 200–900 nm. Here, the band gap of the synthesized SnS₂ nanoplates was determined by using Tauc's plot. Inset of Fig. 1d shows the (αhν)² against hν plot. By extrapolating the linear portion of the plot, the optical band gap of SnS₂ nanoplates was found to be 2.2 eV. This indicates the light absorption region of SnS₂ to spread across the visible region. The value of the band gap is also in agreement with the reported ones.³²

The electrical properties of the SnS₂ nanoplate made films (refer experiment procedure) were investigated using Mott–Schottky (MS) plots to determine the nature of carrier type, apparent donor density (N_D) and the flat band potential (V_FB).^33,34 Fig. S2† shows the MS plot of the SnS₂ nanoplates. The positive slope obtained along the linear part of the plot actually suggests the n-type characteristic behavior of SnS₂ nanoplates. The corresponding N_D determined from the plot was found to be around 1.46 × 10¹⁹ cm⁻³. To explore the photoconducting property of the hexagonal SnS₂ nanoplates, I–V curve for the SnS₂ nanoplates was measured under dark and illumination condition (Fig. S3†). Here, we found the I–V characteristics to reveal asymmetric behavior and display a non ohmic contact between Ag electrodes under dark condition. Additionally, the current was noted to increase under illumination conditions, due to increase of photogenerated carriers.^35,36 This actually indicates the photoconductive behavior of SnS₂ nanoplates.

The aforementioned photoconductivity of the as-synthesized SnS₂ makes the material attractive for fabrication of heterojunction diodes for optoelectronic functions. The heterojunction comprising of SnS₂ nanoplates and Si were then established at room temperature without any strenuous treatment. The detailed procedure for the fabrication of heterojunction diode is explained in the Experimental section. Fig. 2a shows the schematic diagram of SnS₂/p-Si device. I–V characteristic of the SnS₂/p-Si diodes measured in dark condition is shown in Fig. 2b. Here, it is evident that the device exhibits good rectifying characteristics with a current rectification ratio of about 100 at ±2.0 V. The excellent rectification could be attributed to the interface between p-type Si and n-type SnS₂. The turn-on voltage and reverse leakage current values were found to be 0.5 V and 5.5 × 10⁻⁵ A, respectively. Generally, the relation between I–V characteristics for a heterojunction diode is fitted by the equation given below

I = I_s exp[e(V − IR_s)/nk_BT − 1] (1)

where I_s, n, R_s, and k_B are reverse saturation current, ideality factor, sheet resistance and Boltzmann constant respectively. The value of ideality factor and reverse saturation current were estimated from the fitted curve (red line) Fig. 2b, to be 3.8 and 8.15 × 10⁻⁵ A cm⁻². Since the n value is greater than 2, it signifies that the diode deviates from the ideal thermionic behavior. And this may be attributed to existence of interface states and series resistance along the heterojunction interface.³⁷ Additionally, we would like to emphasize that the heterojunction processed through chemical synthesis may also be accompanied by some other chemical impurities at the surface/interface region (which might have resulted in higher n value). The low turn-on voltage and excellent rectification ratio also demonstrates the high quality of the heterojunction for its applications in optoelectronic devices.

Fig. 2 (a) Schematic representation of n-SnS₂/p-Si heterojunctions device. (b) I–V characteristics of vertical n-SnS₂/p-Si heterojunctions under dark condition (dotted line) and the red line shows fitted data. (c) I–V characteristics of vertical n-SnS₂/p-Si heterojunction under dark and light illumination. (d) Schematic representation of band diagram of the n-SnS₂/p-Si heterojunction under reverse bias.

The photoresponse properties of the heterojunction device was evaluated by measuring the I–V characteristics under white light illumination. Fig. 2c shows the I–V characteristic of the SnS₂/p-Si heterojunction diode measured under white light illumination. Here, no differences were observed between the dark and photocurrent under forward bias. However, under reverse bias the photocurrent was noted to exhibit a marked difference in the dark current. The high ratio of photocurrent to dark current in the reverse bias could be attributed to the high separation of photogenerated charge carriers. This also explains that the heterojunction works under reverse bias than the forward bias, demonstrating its potential photodetection performance. The enhancement of photocurrent in reverse bias could be explained on the basis of the energy band diagram shown in Fig. 2d. In the present case, the large surface-to-volume ratio and high specific surface area of the hexagonal nanoplates plays a crucial role in enhancing the light absorption. Also, the nanoplates exhibit few dangling bonds and represent interconnected networks for the assembly of highly uniform continuous thin films with greatly reduced grain boundaries with clean interface. This can therefore promise efficient charge transport across neighboring nanoplates and throughout the entire thin film to enable excellent electronic performance. When n-SnS₂ and p-Si side are made in contact, a depletion region is formed across the interface. Here, the electrons from SnS₂ will flow into p-Si until an electronic equilibrium is reached. Under illumination, the electron–hole pairs are generated in the depletion region due to absorption of photons with higher photo-energy than the band gap of the semiconductor materials. Due to this built in electric field, the photogenerated electron and hole in the depletion region will be separated and are carried away to the n-SnS₂ and p-Si respectively. Such processes in turn contribute towards the enhancement of photocurrent along reverse bias direction.

The photoresponse of the heterojunction was additionally studied as a function of illumination intensities. Fig. 3a shows the I–V curves of SnS₂/p-Si heterojunction at different light intensities. As seen from the Fig. 3a, with increasing intensities of light the reverse current increases linearly. This shows that the heterojunction is sensitive to light and exhibits photoconducting behavior. A photoresponsivity value of about 22.8 A W⁻¹ under 1.4 mW cm⁻² was obtained from this heterojunction. The variation of photocurrent with different light intensities is also shown in Fig. 3b. The curve nearly exhibits a linear relationship, suggesting that the charge carrier photogeneration efficiency is approximately proportional to the number of photons absorbed by the SnS₂ nanoplates. The photoresponse stability of the heterojunction device was performed by switching on and off the light periodically for number of cycles. This time dependent photocurrent response of the heterojunction is shown in Fig. 3c. The photocurrent of the device rises under light illumination, and then drops quickly when the illumination is turned off. The rise time and fall time of the device was estimated to be around 2 s and 5 s under 1.4 mW cm⁻². The observed photocurrent shows no fluctuation under constant illumination and the results are also stable for a number of cycles. The reproducible response of the photocurrent actually demonstrates the excellent stability of the device.

Fig. 3 (a) Variation of the photo-response characteristics of the n-SnS₂/p-Si heterojunction under different intensities of light illumination. (b) Photocurrent plot under different light intensities. (c) Time dependent photocurrent response of the n-SnS₂/p-Si heterojunction under light illumination.

To better understand the electrical variation of the heterojunction, we carried out impedance measurements over a frequency range to study their interfacial structure.^38,39 This electrochemical impedance spectroscopy (EIS) technique involves the measurement of electrical impedance Z as a function of the frequency of input signal. Fig. 4a shows the Bode plots representing the total impedance over a wide frequency range under dark and illumination. Here, the Z′ curves appears to be frequency dependent with a constant slope, demonstrating the occurrence of a single peak. This features suggests the active role of single time constant in the impedance spectra. However, the reduced Z′ curves on exposure to light illumination further demonstrates the improved conductivity of the diodes. And this behavior could be inferred to the improvement of charge transfer process along the heterojunction interface. For a better clarity, the EIS spectra obtained by plotting the real vs. imaginary parts of the impedance was also evaluated. From this, the equivalent circuit parameters which are representative of the physical processes taking place in the sample was determined.

Fig. 4 (a) Bode plot and (b) Nyquist plots of the SnS₂/p-Si heterojunction diode under dark and light illumination. The box represent the experimental data and the dot represents the fitted model. Inset shows the equivalent circuit model.

Fig. 4b shows Nyquist plot of the heterostructure measured under dark and light. It is clear from the figure that the plot appears to be in nearly semicircular form, indicating the heterojunction to be composed of a single time constant, which is in agreement to the Bode plots shown in Fig. 4a. The fitting of the obtained Nyquist plot reveals the heterojunction to be modelled with resistance and capacitance connected in parallel configuration. Inset of Fig. 4b shows the equivalent circuit of the heterojunction, where the parameters R_s represents series resistance, R_p represents the interface resistance between SnS₂ and p-Si. The box represent the experimental data, whereas the dot represents the proposed model. The value of resistance obtained from the fitted data was found to be higher for dark conditions, which in turns gets strongly influenced by the incident light irradiation. Here, the radius of semicircle shortens, indicating a significant decrease in resistance and increase in capacitance (C_p) values of the device. This observation can be attributed to the generation of charge carriers in the depletion region established between SnS₂ and p-Si.⁴⁰ The resistance value was actually estimated to get lowered to 55.8 kΩ from 116 kΩ on light illumination. The reduced resistance for the heterojunction under light displays the enhanced charge transport and effective separation of photogenerated electron–hole pairs along the heterojunction.

Next to understand the charge transport process along the diode configuration, Nyquist plot were recorded under various bias voltages. The spectrum shown in Fig. 5a displays a single semicircular arc for all bias voltage. Interestingly, the diameter of semicircles were noted to shorten on increasing the applied bias voltages. This kind of shortening behavior indicates the presence of a depletion layer along the samples. Based on the impedance spectra, the fitting values of C_p and R_p were additionally estimated from the equivalent circuit model. The variation of R_p and C_p as a function of bias potential is shown in Fig. 5b. Here, the value of resistance decreases, while a significant improvement in the capacitance was noted on increasing the bias potential. This behavior could be reasoned with the improved electron transfer kinetics that facilitates significant improvement in the charge transport process. So, from the above-mentioned results, SnS₂/Si heterojunction displays an enhanced rectifying effect and obvious photo-response, which enables the application of SnS₂ heterojunction in the field of photoelectronics.

Fig. 5 (a) Nyquist plots of the SnS₂/p-Si heterojunction diode under various bias voltages and the corresponding fitting results. Inset shows the equivalent circuit model. (b) Variation of resistance (R_p) and the capacitance (C_p) for SnS₂/p-Si heterojunction diode under various applied bias voltages.

Conclusion

In conclusion, SnS₂ nanoplates were synthesized through a simple hydrothermal method. X-ray diffraction and Raman measurements revealed the samples to be well crystallized. SEM measurements revealed the morphology of obtained samples to be plates-like. p–n heterojunction were fabricated by using the hydrothermally synthesized SnS₂ on p-Si. The electrical characteristics of the heterojunction exhibited rectification ratio of 10² and low turn on voltage of 0.5 V. The resistance of the device was noted to decrease significantly on light illumination. Enhanced photoelectrical performance of the heterojunction was reasoned with the charge transfer process and effective charge separation along the heterojunction interface. The obtained results appear to be promising and suggests the heterojunction diode for potential application in photodetection and sensors.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015-066177).

References

1. X.-L. Gou, J. Chen and P.-W. Shen, Mater. Chem. Phys., 2005, 93, 557–566.
2. D. L. Greenaway and R. Nitsche, J. Phys. Chem. Solids, 1965, 26, 1445–1458.
3. C. Julien, M. Eddrief, I. Samaras and M. Balkanski, Mater. Sci. Eng., B, 1992, 15, 70–72.
4. F. Tan, S. Qu, J. Wu, K. Liu, S. Zhou and Z. Wang, Nanoscale Res. Lett., 2011, 6, 298.
5. J. Xia, D. Zhu, L. Wang, B. Huang, X. Huang and X.-M. Meng, Adv. Funct. Mater., 2015, 25, 4255–4261.
6. Y. Tao, X. Wu, W. Wang and J. Wang, J. Mater. Chem. C, 2015, 3, 1347–1353.
7. Y. Huang, H.-X. Deng, K. Xu, Z.-X. Wang, Q.-S. Wang, F.-M. Wang, F. Wang, X.-Y. Zhan, S.-S. Li, J.-W. Luo and J. He, Nanoscale, 2015, 7, 14093–14099.
8. J. Zhen Ou, W. Ge, B. Carey, T. Daeneke, A. Rotbart, W. Shan, Y. Wang, Z. Fu, A. Chrimes, W. Wlodarski, S. Peter Russo, Y. Xiang Li and K. Kalantar-Zadeh, ACS Nano, 2015, 9, 10313–10323.
9. G. Su, V. G. Hadjiev, P. E. Loya, J. Zhang, S. Lei, S. Maharjan, P. Dong, P. M. Ajayan, J. Lou and H. Peng, Nano Lett., 2015, 15, 506–513.
10. J. Seo, J. Jang, S. Park, C. Kim, B. Park and J. Cheon, Adv. Mater., 2008, 20, 4269–4273.
11. C. Zhai, N. Du and H. Z. D. Yang, Chem. Commun., 2011, 47, 1270–1272.
12. X. Zhou, Q. Zhang, L. Gan, H. Li and T. Zhai, Adv. Funct. Mater., 2016, 26, 4405–4413.
13. J. Zai, X. Qian, K. Wang, C. Yu, L. Tao, Y. Xiao and J. Chen, CrystEngComm, 2012, 14, 1364–1375.
14. J. Ma, D. Lei, L. Mei, X. Duan, Q. Li, T. Wang and W. Zheng, CrystEngComm, 2012, 14, 832–836.
15. P. Wu, N. Du, H. Zhang, J. Liu, L. Chang, L. Wang, D. Yang and J. Z. Jiang, Nanoscale, 2012, 4, 4002–4006.
16. Y. T. Lin, J. B. Shi, Y. C. Chen, C. J. Chen and P. F. Wu, Nanoscale Res. Lett., 2009, 4, 694–698.
17. J. Wang, J. Liu, H. Xu, S. Ji, J. Wang, Y. Zhou, P. Hodgson and Y. Li, J. Mater. Chem. A, 2013, 1, 1117–1122.
18. L. Wang, L. Zhuo, Y. Yu and F. Zhao, Electrochim. Acta, 2013, 112, 439–447.
19. L. L. Cheng, M. H. Liu, S. C. Wang, M. X. Wang, G. D. Wang, Q. Y. Zhou and Z. Q. Chen, Semicond. Sci. Technol., 2013, 28, 015020.
20. A. Yella, E. Mugnaioli, M. Panthofer, H. A. Therese, U. Kolb and W. Tremel, Angew. Chem., Int. Ed., 2009, 48, 6426–6430.
21. G. Radovsky, R. Popovitz-Biro, M. Staiger, K. Gartsman, C. Thomsen, T. Lorenz, G. Seifert and R. Tenne, Angew. Chem., Int. Ed., 2011, 50, 12316–12320.
22. J. Xia, G. Li, Y. Mao, Y. Li, P. Shen and L. Chen, CrystEngComm, 2012, 14, 4279–4283.
23. M. X. Wang, G. H. Yue, Y. D. Lin, X. Wen, D. L. Peng and Z. R. Geng, Nano-Micro Lett., 2013, 5, 1–6.
24. Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman and M. S. Strano, Nat. Nanotechnol., 2012, 7, 699–712.
25. K. S. Novoselov, V. I. Fal'ko, L. Colombo, P. R. Gellert, M. G. Schwab and K. Kim, Nature, 2012, 490, 192–200.
26. T. Kim, C. Kim, D. Son, M. Choi and B. Park, J. Power Sources, 2007, 167, 529–535.
27. Y. Lei, S. Song, W. Fan, Y. Xing and H. Zhang, J. Phys. Chem. C, 2009, 113, 1280–1285.
28. P. Chen, Y. Su, H. Liu and Y. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 12073–12082.
29. B. Qu, C. Ma, G. Ji, C. Xu, J. Xu, Y. S. Meng, T. Wang and J. Y. Lee, Adv. Mater., 2014, 26, 3854–3859.
30. Y. Du, Z. Yin, X. Rui, Z. Zeng, X. J. Wu, J. Liu, Y. Zhu, J. Zhu, X. Huang, Q. Yan and H. Zhang, Nanoscale, 2013, 5, 1456–1459.
31. L. P. Price, I. P. Parkin, A. M. E. Hardy, R. J. H. Clark, T. G. Hibbert and K. C. Molloy, Chem. Mater., 1999, 11, 1792–1799.
32. Z. Zhang, C. Shao, X. Li, Y. Sun, M. Zhang, J. Mu, P. Zhang, Z. Guo and Y. Liu, Nanoscale, 2013, 5, 606–618.
33. Y. Sun, H. Cheng, S. Gao, Z. Sun, Q. Liu, Q. Liu, F. Lei, T. Yao, J. He, S. Wei and Y. Xie, Angew. Chem., Int. Ed., 2012, 51, 8727–8731.
34. X. An, J. C. Yu and J. Tang, J. Mater. Chem. A, 2014, 2, 1000–1005.
35. H. Geng, Y. Su, H. Wei, M. Xu, L. Wei, Z. Yang and Y. Zhang, Mater. Lett., 2013, 111, 204–207.
36. H. Zhong, G. Yang, H. Song, Q. Liao, H. Cui, P. Shen and C. X. Wang, J. Phys. Chem. C, 2012, 116, 9319–9326.
37. R. K. Gupta and F. Yakuphanoglu, Sol. Energy, 2012, 86, 1539–1545.
38. K. J. Jiao, X. L. Wang, Y. Wang and Y. F. Chen, J. Mater. Chem. C, 2014, 2, 7715–7721.
39. D. Gross, I. Mora-Sero, T. Dittrich, A. Belaidi, C. Mauser, A. J. Houtepen, E. Da Como, A. L. Rogach and J. Feldmann, J. Am. Chem. Soc., 2010, 132, 5981–5983.
40. S. Ebrahim, Polym. Sci., Ser. A, 2011, 53, 1217–1226.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20491k

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