Single-crystal GaN layer converted from β-Ga2O3 films and its application for free-standing GaN

Yuewen Li , Xiangqian Xiu *, Zening Xiong , Xuemei Hua , Zili Xie *, Peng Chen , Bin Liu , Tao Tao , Rong Zhang * and Youdou Zheng
Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing, 210023, Jiangsu, P. R. China. E-mail:;;

Received 9th August 2018 , Accepted 27th November 2018

First published on 28th November 2018

We have successfully obtained a hexagonal single crystal GaN layer with (002) orientation by nitridating β-Ga2O3 film under NH3 flow despite structural mismatch between β-Ga2O3 and hexagonal GaN. The conversion process of β-Ga2O3 to GaN has also been systematically investigated. The nitridated GaN layer shows a network structure without significant stress, which makes it very suitable to be used as a template for the epitaxial growth of high-quality GaN films. The GaN/Ga2O3 heterostructure can also be used to obtain free-standing GaN (FS-GaN) films by self-separation or chemical lift-off (CLO) process due to the selective etching of β-Ga2O3. Further investigation demonstrates the feasibility of the in situ growth of low-stress FS-GaN substrates by halide vapor phase epitaxy (HVPE).

1. Introduction

GaN and related compounds have attracted considerable attention due to their extensive applications in solid-state lighting and high-power electronic devices. However, these devices are negatively affected by the threading dislocations and strain in GaN epitaxial films due to the large lattice mismatch and thermal expansion coefficient mismatch between GaN and foreign substrates such as sapphire, SiC, or Si.1–3 After years of development, high threading dislocation density and large residual strain are still the main factors limiting the performance of nitride devices. As a result, the homoepitaxial growth of GaN layer on GaN template/substrates is the most ideal technology to obtain high-quality GaN materials. Previous investigations show that hydride vapor phase epitaxy could be used to obtain free-standing GaN (FS-GaN) substrates.4–6 However, the separation of thick GaN films from the sapphire substrate is usually difficult and of low yield due to the large residual strain in hetero-epitaxial GaN. Therefore, novel separation techniques are highly desirable.

Recently, bulk β-Ga2O3 single crystals have also been used as substrates for the epitaxial growth of GaN epilayers and light emitting diodes (LEDs).7,8,10 Some studies have reported the epitaxial relation of GaN grown on β-Ga2O3.7,9,10 In particular, the minimum lattice mismatch between Ga2O3 and GaN is about 2.6%.11 Even though β-Ga2O3 does not have a hexagonal structure that matches GaN, surface reconstruction occurs under NH3 atmosphere,12 which enables the epitaxial growth of GaN on β-Ga2O3 by a low-temperature nitride buffer layer or direct surface nitridation.10,11,13 Besides, β-Ga2O3 films can be selectively corroded by hydrofluoric acid (HF), through which the sacrificial β-Ga2O3 buffer layer can be easily etched by the chemical lift-off (CLO) process.14

The nitridation processes of Ga2O3 are not fully understood.12,15,16 In this study, we have successfully grown a single crystal GaN layer through the high-temperature thermal nitridation of β-Ga2O3 films grown by HVPE. Moreover, HVPE is a simple and cost-effective technique to grow large-sized Ga2O3 films compared with bulk Ga2O3 crystals, which are beneficial in obtaining large-sized FS-GaN after the nitridation in this research. We have systematically investigated the surface morphology, crystal structure, and optical properties of the GaN/Ga2O3 films. Our results indicate that the GaN/Ga2O3 films are highly compatible as growth templates for the growth of group-III nitride layers and the preparation of FS-GaN substrates.

2. Experimental

The β-Ga2O3 films were grown in a home-made HVPE reactor.17 The thickness of the as-grown β-Ga2O3 is about 0.9 μm. The samples were first cleaned with acetone, ethanol, and deionized water in sequence, each for 10 min. The samples were then put in a quartz tube and annealed in NH3 ambient for 60 min at 850 °C, 900 °C, 950 °C, 1000 °C and 1050 °C, respectively. The NH3 flow rate was 200 sccm.

The microstructural aspects of the β-Ga2O3 films and the nitridated samples were analyzed by means of high-resolution X-ray diffraction (XRD, BRUKER-D8), Raman spectroscopy (Raman, OLYMPUS-BX41), photoluminescence (PL) and transmission electron microscopy (TEM, FET-TF20). The surface morphologies were studied by scanning electron microscopy (SEM, JSM-700F), and energy dispersive spectroscopy (EDS, FET-TF20) was performed by line scanning to determine the chemical compositions.

3. Results and discussions

3.1 The nitridation of β-Ga2O3 films

The β-Ga2O3 samples are highly transparent with a flake-like morphology, and the nitridated samples were yellowish and translucent. The yellow color of the nitridated samples is considered to be linked to the microstructural defects.18,19Fig. 1 shows the SEM micrographs of the β-Ga2O3 samples and the as-nitridated films at 850 °C, 900 °C, 950 °C, 1000 °C and 1050 °C. The SEM images confirm a surface morphology change with the increase in the nitridation temperatures. After nitridation, the surface becomes rough, with a considerable amount of hillock pits formed on the surface. Atomic force microscopy reveals that the root mean square (RMS) roughness of the β-Ga2O3 film was about 4 nm over an area of 5 × 5 μm2 and that the nitridation process results in a larger roughness from 5 nm (850 °C) to 17 nm (1050 °C), as shown in the insert images of Fig. 1. The density and diameter of the hillock pits increased with increasing temperature. When the temperature reached 1050 °C, a porous network structure formed on the surface.
image file: c8ce01336e-f1.tif
Fig. 1 Top-view SEM micrographs of as-grown β-Ga2O3 (a) and as-nitridated films at different temperatures (b–f).

It has been reported that a very small amount of H2 in flowing gas can cause low-temperature (350 °C) decomposition of Ga2O3 with a decomposition rate that increases with the increase in temperature.20 Some studies indicated that Ga2O3 can dissociate into Ga2O or Ga in the presence of H2, NH or NH2 species due to the decomposition of NH3 at high temperatures, which may eventually result in the conversion of Ga2O3 to GaN in ambient NH3.21–23 The roughened porous network morphology was a result of the hydrogen etching of the β-Ga2O3 film. This structure is beneficial for strain release during the growth of the GaN layer, and it also makes the CLO process function much more easily. The porous network-structure surface can also be used for the laterally epitaxial overgrowth of GaN to improve crystal quality.24–27 Our results show that the thickness of the samples decreased by about 0.1–0.2 microns after nitridation at 1050 °C for 60 min, which can be attributed to the formation of volatile gallium suboxides during the nitridation process.

Raman spectroscopy was carried out to investigate the phase transformation from β-Ga2O3 to GaN. Fig. 2 shows the Raman spectra of the β-Ga2O3 film and the as-nitridated films at different temperatures. For β-Ga2O3, eight Raman peaks (marked by the symbol “*” in Fig. 2) were observed at 144, 169, 200, 346, 416, 576, 657 and 751 cm−1 with no blue shift as compared to the bulk Raman peaks.28 The strong low-frequency peak located at 200 cm−1 is the characteristic translation/vibration mode of the Ga–O chains.29,30 The intensities of the Raman peaks corresponding to Ga2O3 gradually reduce with increasing temperature, and the peaks completely disappear after nitridation at 1000 °C. At the same time, the Raman peak at 568 cm−1 for the E2 (high) phonon modes of GaN appears at 950 °C and becomes dominant with increasing temperature. The above results indicate that the surface of β-Ga2O3 has converted to GaN at these nitridation temperatures. The E2 (high) and A1 (LO) phonon modes of GaN are consistent with those of the bulk GaN crystal,31,32 which reveals high-quality porosity of the GaN network layer without significant compressive or tensile stress of the as-transformed. Some peaks related to GaN at 142, 537, 733 cm−1 from A1 (LO) can also be observed. The peak located at 421 cm−1 is generally related to the zone-boundary phonon or to the N-rich GaN configuration.33,34

image file: c8ce01336e-f2.tif
Fig. 2 Raman spectra of β-Ga2O3 and the nitridated films at different temperatures.

Fig. 3a shows the XRD pattern (θ–2θ scan) of β-Ga2O3 and nitridated films. The XRD peaks at 18°, 38° and 59° correspond to diffractions from the (−201), (−402) and (−603) planes of β-Ga2O3 (marked by * in Fig. 3a), respectively. After nitridation, the peak for the wurtzite GaN (002) reflection was observed at 34.45°, with the diffraction intensity increasing with increasing nitridation temperature. No other diffraction peaks were detected, which suggests that the GaN layer had a (002) preferred orientation and that the (002) plane of GaN is parallel to the (−201) β-Ga2O3 film as described by GaN (002)||β-Ga2O3 (−201).

image file: c8ce01336e-f3.tif
Fig. 3 XRD patterns of β-Ga2O3 and nitridated films (a), XRC patterns of the GaN (002) and (102) planes (b).

In order to characterize the crystal quality, the X-ray rocking curves (XRC) of the GaN (002) and (102) planes were obtained from nitridated samples above 850 °C, in which the full width at half maximum (FWHM) of the diffraction peaks can show the magnitude of the screw dislocation and edge dislocation, respectively. As can be seen in Fig. 3b, the FWHM of the GaN (002) plane decreases with the increase in temperature until 1000 °C. The lowest FWHM value is measured as 1.72°, corresponding to the sample nitridated at 1000 °C, which means this sample has a relative crystallinity. For the nitridated sample at 1050 °C, the FWHM value slightly increases and the GaN (002) peak intensity (Fig. 3a) decreases, which can be attributed to the denser network structure on the sample surface. The FWHM of the GaN (102) peak increases first and then decreases upon increasing the nitridation temperature, indicating that the quality of the nitride layer is improved.

Fig. 4 shows the PL spectra of the samples nitridated at different temperatures. There are three peaks located at 2.58 eV, 2.78 eV, and 3.06 eV for each sample. The peak at 2.58 eV is consistent with VGa defects in GaN.35 The peak at 2.78 eV can be attributed to defects related to vacancies, antisites and interstitials.36 The peak at 3.06 eV is probably associated with PL emission from GaNxOy or the recombination of self-trapped excitons.37,38 As the nitridation temperature increases, the near band edge (NBE) emission of GaN at ∼3.39 eV starts to appear and becomes significant when the nitridation temperatures reach 1000 °C and 1050 °C.

image file: c8ce01336e-f4.tif
Fig. 4 PL spectra of the samples nitridated at different temperatures.

Fig. 5a shows a typical cross-sectional TEM micrograph of the sample nitridated at 1050 °C with a thickness of 0.7 μm. Three regions are marked: (A) the interface between the sapphire and β-Ga2O3, (B) the β-Ga2O3 layer in the middle of the sample, and (C) the nitridated layer. The magnified cross-sectional TEMs are shown in Fig. 5b–d. Region A exhibits a high density of dislocations and stacking faults as indicated by arrows in Fig. 5b. The selected area electron diffraction (SAED) pattern shows two sets of diffraction patterns corresponding to Al2O3 and β-Ga2O3. In region B, the atomically resolved TEM image of β-Ga2O3 can be (Fig. 5c) suggestive of the high crystallinity of β-Ga2O3 layer. The pattern shows lattice planes aligned obliquely upward to the right at 103°. The lattice planes with a slope of 103° corresponded to the (−200) and (001) planes of β-Ga2O3.39 In region C (Fig. 5d), the SAED pattern indicates that the GaN layer was converted from β-Ga2O3, with the angle of the lattice planes changing from 62° near β-Ga2O3 layer to 60° of the top GaN layer.

image file: c8ce01336e-f5.tif
Fig. 5 The cross-sectional TEM image (a), HRTEM and SAED images of the nitridated sample: the interface between sapphire/β-Ga2O3 (b), β-Ga2O3 layer (c), and the nitridated layer (d).

The schematic diagrams of atomic structures of β-Ga2O3 and GaN are shown in Fig. 6. During the nitridation process, the N atoms decomposing from NH3 would sit at the O sites of β-Ga2O3 and the Ga atoms would rearrange to form GaN.40 Therefore, we can observe the angle of Ga–Ga changing from 65° to 60° from the HRTEM images in Fig. 5d. As can be seen, the Ga atoms exhibit a similar arrangement in the (−201) plane of β-Ga2O3 and the (002) plane of GaN, which further verifies the GaN (002)||β-Ga2O3 (−201) from XRD analysis.

image file: c8ce01336e-f6.tif
Fig. 6 Atomic structures of the interface between β-Ga2O3 (−201) and GaN (002).

Energy dispersive spectroscopy (EDS) was performed on the samples by line-scanning to determine the chemical composition in different regions of the sample. Fig. 7b shows the EDS spectra for the sample nitridated at 1050 °C, and the white solid line in Fig. 7a shows the line scanning range from the interface of Al2O3/β-Ga2O3 to the surface along the cross-section HRTEM. It is verified that the sample thickness is about 0.7 μm. Gallium is uniformly distributed across the entire line scanning area with a distance of 0.7 μm from the Al2O3/Ga2O3 interface to the top surface; oxygen is dominated at a thickness of 0.3 μm from the sapphire substrate. At a distance of 0.3 μm, a significant decrease in oxygen and an increase in nitrogen can be observed. The top layer with a thickness of 0.4 μm was entirely nitridated under 1050 °C. The valley of elemental Ga and N is due to the line-scanning through a void.

image file: c8ce01336e-f7.tif
Fig. 7 The cross-sectional TEM micrograph of the sample nitridated at 1050 °C (a); EDS spectra by a line-scanning for the nitridated sample (b).

3.2 Application of the as-nitridated GaN layer//β-Ga2O3: the homoepitaxial growth and the separation of GaN from the sapphire substrate

As mentioned above, the nitridated GaN/Ga2O3 films can be used to fabricate FS-GaN as a low-stress template or sacrificial layer. We carried out epitaxial-growth of GaN thick film on nitridated GaN/β-Ga2O3/sapphire substrate by HVPE. We then tried to separate the HVPE-GaN thick films by self-separation or selectively etching off the Ga2O3 layer.

Fig. 8 shows the resultant XRD pattern and Raman spectra. The XRD pattern shows a sharp GaN (002) diffraction peak, which indicates preferred orientation and the high quality of HVPE-GaN. The Raman spectra (Fig. 8b) shows only Raman peaks of GaN, and the E2 (high) peak shows no offset from that of bulk GaN, which means that the HVPE-GaN is free of stress. It can also be attributed to the porous GaN layer formed after nitridation.24,25

image file: c8ce01336e-f8.tif
Fig. 8 XRD pattern (a) and Raman spectrum (b) of the nitridated GaN layer and HVPE-GaN.

Fig. 9 shows the omega XRD rocking curves of the (002) and (102) peaks for the nitridated GaN layer and HVPE-GaN, respectively. The FWHM of the (002) and (102) peaks of HVPE-GaN are 0.21° and 1.15°, respectively, which are significantly smaller than those of the nitridated GaN layer (2.33° and 2.58°). The crystal quality of the HVPE-GaN film grown on nitridated β-Ga2O3 can be better than that of GaN on bulk β-Ga2O3 crystal by only slight nitridation treatment.41 The further optimization of the growth conditions is in progress to improve the quality of re-epitaxial GaN.

image file: c8ce01336e-f9.tif
Fig. 9 XRC patterns of the GaN (002) and (102) plane.

The stress is released for two reasons. One is related to the low-stress nitridated layer, and the other is self-separation from the HVPE-GaN to Ga2O3/sapphire. The self-separation can be observed from the cross-sectional SEM image (Fig. 10a). This result means that FS-GaN substrates would be obtained by self-separation using the nitridated Ga2O3 as the template.

image file: c8ce01336e-f10.tif
Fig. 10 The cross-sectional SEM image of HVPE-GaN (a) and the rear view of the sample etched by hydrofluoric acid (b).

Fig. 10b shows the rear view of the sample after HF etching. It is evident that the separation occurs between the HVPE-GaN and the sapphire as expected, which demonstrates that HF can effectively etch off the β-Ga2O3 layer. Also, the porous GaN layer is also beneficial for the separation of HVPE-GaN.26,27 Although the sample used in this experiment is small, our tests verified the idea for fabricating high-quality FS-GaN substrates on nitridated β-Ga2O3 films as a template by the CLO process. Further experiments would be performed to obtain large-sized FS-GaN substrates.

It should be noted that the growth and nitridation of β-Ga2O3 and the re-growth of GaN can all be completed in situ in the HVPE system, which is same and uncomplicated compared to the usual growth process for GaN. Due to the stress-free situation, the re-grown GaN thick films on the sapphire would remain intact until the cooling process in HVPE, which will greatly improve the production rate of FS-GaN.

4. Conclusions

Single-crystal GaN layers were formed by nitridating β-Ga2O3 films under NH3 atmosphere. SEM images show that the nitridated surface has a porous network morphology. Raman analysis demonstrates that both the nitridated GaN layer and the re-grown GaN are almost stress-free. The separation between the GaN epilayer and the sapphire substrate can be achieved by etching β-Ga2O3 in hydrofluoric acid. Our results suggest that β-Ga2O3, after nitridation to form GaN, is an ideal substrate for the in situ growth of FS-GaN substrates by HVPE.

Conflicts of interest

There are no conflicts of interest to declare.


This study is supported by the National Key R&D Program of China (2017YFB0404201), the Solid State Lighting and Energy-Saving Electronics Collaborative Innovation Center, PAPD, and the State Grid Shandong Electric Power Company. We also thank Professor Duo LIU from Shandong University for the linguistic assistance during the preparation of this manuscript.


  1. S. Nakamura, M. Senoh and N. Iwasa, Jpn. J. Appl. Phys., 1995, 34(7A), L797 CrossRef CAS.
  2. S. Nakamura, M. Senoh and S. Nagahama, Jpn. J. Appl. Phys., 1996, 35(2B), L217 CrossRef CAS.
  3. T. Hashizume, J. Kotani and H. Hasegawa, Appl. Phys. Lett., 2004, 84(24), 4884–4886 CrossRef CAS.
  4. T. Sochacki, Z. Bryan and M. Amilusik, Appl. Phys. Express, 2013, 6(7), 075504 CrossRef.
  5. T. Wei, J. Yang and Y. Wei, Sci. Rep., 2016, 6, 28620 CrossRef CAS PubMed.
  6. S. F. Chichibu, Y. Ishikawa and M. Tashiro, ECS Trans., 2013, 50(42), 1–8 CrossRef.
  7. K. Shimamura, E. G. Víllora, K. Domen, K. Yui, K. Aoki and N. Ichinose, Jpn. J. Appl. Phys., 2004, 44(1L), L7 Search PubMed.
  8. Z. L. Xie, R. Zhang, C. T. Xia, X. Q. Xiu, P. Han, B. Liu and Y. D. Zheng, Chin. Phys. Lett., 2008, 25, 2185–2186 CrossRef CAS.
  9. T. Y. Tsai, S. L. Ou, M. T. Hung, D. S. Wuu and R. H. Horng, J. Electrochem. Soc., 2011, 158(11), H1172–H1178 CrossRef CAS.
  10. S. Ohira, M. Yoshioka, T. Sugawara, K. Nakajima and T. Shishido, Thin Solid Films, 2006, 496(1), 53–57 CrossRef CAS.
  11. E. G. Víllora, K. Shimamura, K. Kitamura, K. Aoki and T. Ujiie, Appl. Phys. Lett., 2007, 90(23), 234102 CrossRef.
  12. G. V. Encarnación, S. Kiyoshi, A. Kazuo and I. Noboru, J. Cryst. Growth, 2004, 270(3–4), 462–468 Search PubMed.
  13. S. Ohira, N. Suzuki, H. Minami, K. Takahashi, T. Araki and Y. Nanishi, Phys. Status Solidi C, 2007, 4(7), 2306–2309 CrossRef CAS.
  14. T. Y. Tsai, R. H. Horng, D. S. Wuu, S. L. Ou, M. T. Hung and H. H. Hsueh, Electrochem. Solid-State Lett., 2011, 14(11), H434–H437 CrossRef CAS.
  15. X. Li, C. Xia, X. He, G. Pei, J. Zhang and J. Xu, Chin. Opt. Lett., 2008, 6(4), 282–285 CrossRef CAS.
  16. X. J. Cui and L. L. Wang, Mod. Phys. Lett. B, 2017, 31(10), 1750108 CrossRef CAS.
  17. Z. N. Xiong, X. Q. Xiu, Y. W. Li, X. M. Hua, Z. L. Xie, P. Chen, B. Liu, R. Zhang and Y. D. Zheng, Chin. Phys. Lett., 2018, 35(5), 058101 CrossRef.
  18. G. Li, S. J. Chua, S. J. Xu, W. Wang, P. Li, B. Beaumont and P. Gibart, Appl. Phys. Lett., 1999, 74(19), 2821–2823 CrossRef CAS.
  19. J. Neugebauer and C. G. Walle, Appl. Phys. Lett., 1996, 69(4), 503–505 CrossRef.
  20. R. Togashi, K. Nomura, C. Eguchi, T. Fukizawa, K. Goto and Q. T. Thieu, Jpn. J. Appl. Phys., 2015, 54, 041102 CrossRef.
  21. H. Kiyono, T. Sakai, M. Takahashi and S. Shimada, J. Cryst. Growth, 2010, 312(19), 2823–2827 CrossRef CAS.
  22. C. M. Balkaş and R. F. Davis, J. Am. Ceram. Soc., 1996, 79(9), 2309–2312 CrossRef.
  23. L. Luo, K. Yu, Z. Zhu, Y. Zhang, H. Ma, C. Xue and S. Chen, Mater. Lett., 2004, 58(22–23), 2893–2896 CrossRef CAS.
  24. F. Yun, M. A. Reshchikov and L. He, Appl. Phys. Lett., 2002, 81(22), 4142–4144 CrossRef CAS.
  25. J. K. Jeong, H. J. Kim and H. C. Seo, Electrochem. Solid-State Lett., 2004, 7(4), C43–C45 CrossRef CAS.
  26. Y. Oshima, T. Eri and M. Shibata, Phys. Status Solidi A, 2002, 194(2), 554–558 CrossRef CAS.
  27. T. Yoshida, Y. Oshima and T. Eri, J. Cryst. Growth, 2008, 310(1), 5–7 CrossRef CAS.
  28. R. Rao, A. M. Rao, B. Xu, J. Dong, S. Sharma and M. K. Sunkara, J. Appl. Phys., 2005, 98(9), 094312 CrossRef.
  29. S. Kumar, C. Tessarek, S. Christiansen and R. Singh, J. Alloys Compd., 2014, 587, 812–818 CrossRef CAS.
  30. S. Kumar, V. Kumar, T. Singh, A. Hähnel and R. Singh, J. Nanopart. Res., 2014, 16(1), 2189 CrossRef.
  31. L. Filippidis, H. Siegle, A. Hoffmann, C. Thomsen, K. Karch and F. Bechstedt, Phys. Status Solidi A, 1996, 198(2), 621–627 CrossRef CAS.
  32. P. Perlin, J. Camassel, W. Knap, T. Taliercio, J. C. Chervin, T. Suski and S. Porowski, Appl. Phys. Lett., 1995, 67(17), 2524–2526 CrossRef CAS.
  33. C. C. Chen, C. C. Yeh, C. H. Chen, M. Y. Yu, H. L. Liu, J. J. Wu and Y. F. Chen, J. Am. Chem. Soc., 2001, 123(12), 2791–2798 CrossRef CAS PubMed.
  34. J. Q. Ning, S. J. Xu, D. P. Yu, Y. Y. Shan and S. T. Lee, Appl. Phys. Lett., 2007, 91(10), 103117 CrossRef.
  35. P. Chao, J. S. Jun, Z. Yan, K. S. A. Butcher, T. L. Tansley and J. E. Downes, Chin. Phys. Lett., 2007, 24(7), 2048 CrossRef.
  36. H. Morkoc, Nitride Semiconductors and Devices, Springer, 1999, vol. 32 Search PubMed.
  37. X. Li, C. Xia, X. He, G. Pei, J. Zhang and J. Xu, Chin. Opt. Lett., 2008, 6(4), 282–285 CrossRef CAS.
  38. L. L. Liu, M. K. Li, D. Q. Yu, J. Zhang, H. Zhang, C. Qian and Z. Yang, Appl. Phys. A: Mater. Sci. Process., 2010, 98(4), 831–835 CrossRef CAS.
  39. S. Nakagomi and Y. Kokubun, Phys. Status Solidi B, 2016, 253(6), 1217–1221 CrossRef CAS.
  40. P. Song, Z. Wu, X. Shen, J. Kang, Z. Fang and T. Y. Zhang, CrystEngComm, 2017, 19(4), 625–631 RSC.
  41. K. Kachel, M. Korytov and D. Gogova, CrystEngComm, 2012, 14(24), 8536–8540 RSC.

This journal is © The Royal Society of Chemistry 2019