Ae Ran Lim1,2. 1. Department of Carbon Convergence Engineering, Department of Science Education, Jeonju University Jeonju 55069 Korea aeranlim@hanmail.net arlim@jj.ac.kr. 2. Analytical Laboratory of Advanced Ferroelectric Crystals, Jeonju University Jeonju 55069 Korea.
Organic–inorganic compounds based on two-dimensional (2D) hybrid perovskites, particularly [(CH2NH3)]2BX4 (n = 1, 2, 3,…; B = Mn, Co, Cu, Zn, Cd; X = Cl, Br) and [NH3(CH2)NH3]BX4 (n = 2, 3,…), have attracted considerable attention in recent years. The monoammonium [(CH2NH3)]2BX4 (ref. 1–7) and diammonium [NH3(CH2)NH3]BX4 series have been extensively studied owing to their relative stability and potential applications.[8-13] The properties and structural phase transitions of organic–inorganic hybrid perovskites are related to their structures and the interaction of cationic units with complex anionic sublattices.[14] The phase transitions for diammonium [NH3(CH2)NH3]BX4 compounds have their origin in the dynamics of the cations, in particular from the dynamics of the NH3 groups forming hydrogen bonds with the halogen atoms of the anion layers and hindered rotational motions of the entire alkyl group around the long molecular axis.[10] For B = Mn, Cu, or Cd, the structure consists of the corner shared octahedral (BX6)2− alternated with organic layers. In contrast, for B = Co or Zn, isolated tetrahedral structures are formed in the inorganic layer (BX4)2− sandwiched between layers of organic cations.[15-23] These compounds have attracted attention owing to the multiplicity of their crystal structures, which is correlated to the structural dynamics of cations and anions. Ferroelasticity is commonly observed in materials with a perovskite crystal structure. Recently, the ferroelastic twin domain observed in organic–inorganic hybrid perovskite likewise garnered significant attention.[24-27] 2D hybrid perovskites are promising for a variety of applications, including photovoltaics, photocatalysis, batteries, and energy storage.[28,29]The [NH3(CH2)3NH3]CdBr4 (1,3-propanediammonium tetrabromocadmate) crystal (n = 3; B = Cd; X = Br), a member of the diammonium [NH3(CH2)NH3]BX4 series, belongs to the orthorhombic structure at room temperature. This crystal with Cd is a very special case; it exhibits an unusual phase sequence, i.e., its stable phase at the highest temperature is the one with the lowest symmetry.[30] This crystal undergoes two phase transitions, at temperatures of 326 K (=TC2) and 368 K (=TC1).[10,30] The room temperature phase III was determined in the space group Pnma (point group mmm) of the orthorhombic structure. Its lattice constants have been reported as a = 7.721 Å, b = 19.054 Å, c = 7.898 Å, and Z = 4.[30] In this phase, Cd atoms are surrounded by six bromine atoms forming a nearly regular octahedron CdBr6. Of these six bromine atoms, four Br atoms are bridging atoms shared with the neighboring octahedral, and two Br atoms are terminal atoms resulting in bidimensional anion planes. These formations are connected by hydrogen bonds N–H⋯Br and the cation. The phase II above TC2 has the space group Ima2 (point group mmm) and the same orthorhombic structure. In the highest phase I, this crystal is in the monoclinic structure with the space group P21/m (point group 2/m). The lattice constants a and b increase continuously with rising temperature, but the value of c decreases slightly at TC2 and shortens rapidly at TC1. The monoclinic angle β abruptly increases with increasing temperature, reaching approximately 95.5° in this phase, whereas in phases II and III, its value was constant at β = 90°.[30]For [NH3(CH2)3NH3]CdBr4 crystals, a temperature dependence experiment addressing the 79,81Br nuclear quadrupole resonance (NQR) near the phase transition temperatures was studied by Ishihara et al.[31,32] The X-ray structure analysis at room temperature was likewise reported.[32] Further, the spectroscopic properties of this crystal with its phase sequence were investigated via various experimental methods: differential scanning calorimetry (DSC), infrared (IR), far infrared (FIR), and Raman spectroscopy measurements.[10] Recently, optical and dilatometric studies presented the multidomain states obtained by optical polarizing microscopic observation.[14] Although [NH3(CH2)3NH3]CdBr4 has numerous applications, the physicochemical properties and molecular dynamics of its crystals have not been studied to date.In this study, the structure and phase transition temperatures of [NH3(CH2)3NH3]CdBr4 crystals are investigated via X-ray diffraction and DSC. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) experiments were performed to obtain a better understanding of the thermal properties. The chemical shifts and molecular dynamics were probed by 1H magic-angle spinning nuclear magnetic resonance (MAS NMR) and 13C MAS NMR as a function of temperature to elucidate the role of the [NH3(CH2)3NH3] cation. Furthermore, the chemical shifts for 14N and 113Cd of the CdBr6 anion were recorded by static NMR spectra as a function of the temperature. The spin–lattice relaxation times (T1ρ) in the rotating frame were discussed in terms of the change of temperature. Based on these results, the structural dynamics of the [NH3(CH2)3NH3] cation and CdBr4 anion during the III–II–I phase transitions were discussed. The ferroelastic twin domain walls in all phases were observed as well. Finally, we compare the physicochemical properties of [NH3(CH2)3NH3]CdBr4 with those of [NH3(CH2)3NH3]CdCl4 previously reported. Moreover, the physicochemical properties revealed important information regarding the basic mechanisms that enable their widespread applicability.
Experimental method
An aqueous solution containing NH2(CH2)3NH2·2HBr and CdBr2 was slowly evaporated at a constant temperature of 300 K to yield single crystals of [NH3(CH2)3NH3]CdBr4. The [NH3(CH2)3NH3]CdBr4 single crystal grown here has a colorless and transparent square shape.The structure of the [NH3(CH2)3NH3]CdBr4 crystal at 298 K was analyzed using an X-ray diffraction system equipped with a Cu-Kα radiation source. The lattice parameters were determined by single crystal X-ray diffraction at the Western Seoul Center of Korea Basic Science Institute (KBSI). The crystals were mounted on a Bruker D8 Venture equipped with an I μS micro-focus sealed tube Mo-Kα and a PHOTON III M14 detector.DSC (TA, DSC 25) experiments were conducted at a heating rate of 10 K min−1 in a temperature range of 200–600 K in nitrogen atmosphere. TGA and DTA experiments were performed on a thermogravimetric analyzer (TA Instrument) in the temperature range of 300–870 K with the same heating rate. The type of fan used in experiment was Al.The NMR spectra of [NH3(CH2)3NH3]CdBr4 crystals were measured on a 400 MHz Avance II+ Bruker solid-state NMR spectrometer at the Western Seoul Center of KBSI. The MAS 1H and 13C NMR experiments were conducted at the Larmor frequencies of 400.13 and 100.61 MHz, respectively. To minimize the spinning sideband, a MAS rate of 10 kHz was employed. Tetramethylsilane (TMS) was used as the standard to record the NMR spectra. T1ρ values were measured using the π/2 − τ sequence method by varying spin-locking pulses. The width of the π/2 pulse for 1H and 13C was 3.56–3.72 μs. Further, static 14N NMR and 113Cd NMR spectra of a [NH3(CH2)3NH3]CdBr4 single crystal were measured at Larmor frequencies of 28.90 and 88.75 MHz, respectively, and the chemical shift was referenced with respect to NH3NO3 and CdCl2O8·6H2O as standard samples, respectively. The 14N NMR experiments were performed using a solid-state echo sequence. An almost constant temperature within error range ±0.5 K was maintained, even when the rate of flow of nitrogen gas and the heater current were adjusted.The ferroelastic domain pattern in the (001) plane was studied using an optical polarizing microscope. A hot stage (Linkam, THMS 600) and temperature controller maintained the temperature of the crystal.
Results and discussion
The X-ray powder diffraction pattern of the [NH3(CH2)3NH3]CdBr4 at 298 K are displayed in Fig. 1. And, the lattice constants for [NH3(CH2)3NH3]CdBr4 crystal are determined to be a = 7.711 ± 0.003 Å, b = 19.148 ± 0.008 Å, and c = 7.856 ± 0.004 Å. This result is consistent with that reported previously.[30]
Fig. 1
X-ray diffraction pattern of the [NH3(CH2)3NH3]CdBr4 crystal at 298 K.
Three endothermic peaks at 328 K, 363 K, and 557 K were observed in the DSC curves of [NH3(CH2)3NH3]CdBr4, as shown in Fig. 2. Two endothermic peaks at 328 K and 363 K are consistent with those previously reported by Staskiewicz et al.[30] In order to understand the peak of 557 K, we performed the TGA and DTA experiments and the results are presented in Fig. 3. On the DTA curve, two small endothermic peaks at 328 and 363 K are assigned to the structural phase transitions detected in the DSC experiment. A large endothermic peak at 557 K is assigned to the onset of thermal decomposition temperature (=Td) by the DTA and a polarizing microscope experiments. This was characterized by a loss in the weight of the compound. It was observed that [NH3(CH2)3NH3]CdBr4 (Mw = 508.16 mg) crystals begin to lose weight as the temperature rises. The amount remaining as solid residue is calculated from the molecular weights and balanced chemical reactions. [NH3(CH2)3NH3]CdBr4 lost 16 and 32% of its weight at temperatures of approximately 614 and 638 K, respectively. The weight loss can be attributed to the decomposition of HBr and 2HBr moieties, respectively, as shown in Fig. 3. At approximately 850 K, the 98% of the total weight of this crystal is lost.
Fig. 2
Differential scanning calorimetry (DSC) thermogram of [NH3(CH2)3NH3]CdBr4.
Fig. 3
Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) curves for [NH3(CH2)3NH3]CdBr4 crystal.
The temperature-dependent 1H NMR chemical shifts for [NH3(CH2)3NH3]CdBr4 crystals were recorded by MAS NMR, as shown in Fig. 4. Only one resonance signal was observed at low temperature. The observed resonance signal exhibits asymmetric shapes due to overlapping lines of the 1H for two types of NH3 and CH2 in [NH3(CH2)3NH3] cations. The spinning sidebands were marked with open circles. At 180 K, a single resonance line is present at 7.35 ppm, which subsequently splits into two resonance lines above 300 K (inset Fig. 4).
Fig. 4
In situ
1H MAS NMR spectra for CH2 and NH3 of [NH3(CH2)3NH3]CdBr4 at several temperatures. The spinning sidebands are marked with open circles (inset: expansion of 1H NMR signals above 300 K).
At 330 K, which is higher than TC2, the NMR spectrum is separated by two resonance lines showing chemical shifts of 7.24 and 3.88 ppm for NH3 and CH2, respectively. The 1H chemical shifts of NH3 indicated by dotted lines in Fig. 4 are almost independent of the temperature, while those of CH2 slightly shift toward the lower side as the temperature increases. From these results, the surrounding environment of H of NH3 does not change depending on the temperature, and that of H of CH2 changes slightly according to the temperature.The 1H MAS NMR spectrum was measured with respect to several delay times at each temperature. The relationship between the intensities of the NMR signals and the delay times is as follows:[33-35]where I(t) and I(0) are the signal intensities at times t and t = 0, respectively, and T1ρ denotes the spin–lattice relaxation time in the rotating frame. 1H NMR signals for [NH3(CH2)3NH3]CdBr4 at 300 K were recorded by varying the delay times of 0.2–160 ms, as shown in the inset of Fig. 5. The decay curves were fit to a single exponential function shown in eqn (1). The T1ρ values of the protons in [NH3(CH2)3NH3]CdBr4 were obtained as a function of the inverse temperature, and their results are represented in Fig. 5. The T1ρ values initially increased and then abruptly decreased when the temperature was increased. At 328 K (=TC2), the change in T1ρ values is more or less continuous; however, at approximately 363 K (=TC1), there is a slight inflection point, marking a discontinuity. This experimental result is considered to be consistent with previously reported X-ray results,[30] stating that the c-value of the lattice constant is slightly shorter in the vicinity of TC2, but the c value is rapidly shortened in the vicinity of TC1. The T1ρ values of the protons in the [NH3(CH2)3NH3] cation are 10–300 ms over the measured temperature range. At 180 K, T1ρ is approximately 72 ms, and at 290 K, it has a very long duration of 303 ms, and at 430 K, it exhibits a very short duration of 11 ms. The 1H T1ρ values for NH3 and CH2 separated from temperatures above 320 K have highly similar values. The T1ρ values experience fast motion below TC2 and slow motion above TC2. The general behavior of the T1ρ for random motions of the Arrhenius type with a correlation time τC is described in three regimes, including both fast and slow motion regimes. The fast motion regime is described as ω1τC ≪ 1, T1ρ−1 ∝ exp(Ea/kBT), and the slow motion regime as ω1τC ≫ 1, T1ρ ∝ ω1−2 exp(−Ea/kBT), where ω1 denotes the radiofrequency power of the spin lock pulse, and Ea represents the activation energy. Different limits are satisfied for ω1τC in each of the three temperature ranges, separated by TC1 = 363 K and TC2 = 328 K. Specifically, the limit ω1τC ≫ 1 applies for both T > TC1 and TC2 < T < TC1, and the limit ω1τC ≪ 1 for T < TC2. As indicated by the solid lines in Fig. 5, Ea = 5.21 ± 0.38 kJ mol−1 at temperature below TC2, while above TC1, Ea = 35.98 ± 0.68 kJ mol−1. The decrease in T1ρ values with temperature indicates an increase in proton mobility at higher temperatures.
Fig. 5
1H NMR spin–lattice relaxation times T1ρ of [NH3(CH2)3NH3]CdBr4 as a function of inverse temperature (inset: recovery curves for delay times of 1H MAS NMR spectrum in [NH3(CH2)3NH3]CdBr4 at 300 K).
The 13C MAS NMR chemical shifts in [NH3(CH2)3NH3]CdBr4 were measured with respect to the change in temperature, as shown in Fig. 6. The 13C MAS NMR spectrum for TMS was obtained at 38.3 ppm at 300 K. This peak at 38.3 ppm was taken as the standard and calibrated as a peak at zero ppm. Here, the CH2 sandwiched between two other CH2 is labeled as CH2-1, and the CH2 close to NH3 is labeled CH2-2. At 300 K, the carbon signals corresponding to the CH2-1 and CH2-2 in [NH3(CH2)3NH3]CdBr4 appear at 25.55 and 39.96 ppm, respectively. Below TC2, 13C resonance signals show two resonance lines for CH2-1 and CH2-2. Between TC2 and TC1, 13C resonance signal was separated into three or four resonance lines. Their resonance lines at temperatures above TC1 were again reduced to two. Here, below TC2, the chemical shifts of CH2-1 and CH2-2 are shifted slightly upward as the temperature increases, but at temperatures above TC1, the chemical shifts of the two resonance lines are almost independent of the temperature.
Fig. 6
In situ
13C MAS NMR spectra for CH2-1 and CH2-2 of [NH3(CH2)3NH3] CdBr4 as a function of temperature.
The change in the full width at half maximum (FWHM) for 13C NMR spectra with respect to the temperature is shown in Fig. 7. The 13C NMR line widths for CH2-1 and CH2-2 decreased with an increase in temperature. The line width of the resonance line is approximately 2.8 and 4.5 ppm, respectively, for CH2-1 and CH2-2 at low temperature. However, as the temperature increases, the line width changes from a Gaussian to a Lorentzian shape, and it decreases rapidly. In particular, it is markedly reduced near TC2 and TC1. The line width decreases with increasing temperature due to internal molecular motion, and that of CH2-2 is broader than that of CH2-1.
Fig. 7
Line widths of 13C MAS NMR signal for CH2-1 and CH2-2 in [NH3(CH2)3NH3]CdBr4 as a function of temperature.
The intensities of 13C MAS NMR signals for CH2-1 and CH2-2 in [NH3(CH2)3NH3]CdBr4 were measured by varying the delay times at each temperature. The decay curves for CH2-1 and CH2-2 were fitted to a single exponential equation in eqn (1). 13C T1ρ values were measured by the spin-locking pulse sequence with a locking pulse of 69.44 kHz. From the slope of their recovery traces, the 13C T1ρ values were obtained for CH2-1 and CH2-2 as a function of 1000/temperature, as shown in Fig. 8. The values of T1ρ are somewhat continuous near TC2, and discontinuous near TC1. Similar to the results of 1H T1ρ, the discontinuous change near TC1 is consistent with the result of a rapid shortening of the lattice constant c-value. The T1ρvs. inverse temperature curves shows minima at 32.65 and 29.14 ms for CH2-1 and CH2-2 at 230 K, respectively. This trend indicates that distinct molecular motions exist, where the minimum T1ρ was attributed to the uniaxial rotation of CH2 ions. The T1ρ values were described by the correlation time τC for molecular motion. The T1ρ value for the molecular motion is given by:[34]where C is a coefficient, γC and γH are the gyromagnetic ratios for 13C and 1H, respectively, ℏ is the Planck constant, r is the distance between the proton and the carbon, ωH and ωC are the Larmor frequencies of 1H and 13C, respectively, and ω1 is the spin-lock field. When ω1τC = 1, T1ρ has a minimum value, and the coefficient of eqn (2) is obtained from the relationship between T1ρ and ω1. From this coefficient, the τC values can be obtained relative to the temperature. The local field fluctuation is governed by the thermal motion of CH2-1 and CH2-2, which is activated by thermal energy. The τC is described by Arrhenius behavior: τC = τo exp(−Ea/kBT), where τo, Ea, and kB denote the pre-correlation time, activation energy of the motions, and Boltzmann constant, respectively.[33] As the magnitude of Ea depends on the molecular dynamics, we plotted on a logarithmic scale τCvs. 1000/temperature (inset of Fig. 8). Below TC2, Ea values for CH2-1 and CH2-2 are 13.66 ± 0.58 and 14.59 ± 0.72 kJ mol−1, respectively, and above TC1, Ea values for CH2-1 and CH2-2 are 30.34 ± 3.41 and 35.64 ± 2.16 kJ mol−1, respectively.
Fig. 8
13C NMR spin–lattice relaxation times T1ρ for CH2-1 and CH2-2 of [NH3(CH2)3NH3]CdBr4 as a function of inverse temperature (inset: the correlation times for CH2-1 and CH2-2 as a function of inverse temperature).
Static 14N NMR investigations of [NH3(CH2)3NH3]CdBr4 single crystal were conducted over the temperature range of 180–430 K. The 14N spectra obtained using the solid-state echo method by static NMR is shown in Fig. 9. Two 14N NMR signals were recorded from the quadrupole interactions due to the spin number I = 1. The lines with the same color below TC2 indicate the same pairs for 14N. Near 328 K (=TC2), the number of resonance lines and chemical shifts of the NMR spectrum showed abrupt changes. The changes in the 14N chemical shift as a function of temperature were attributed to variations in the structural geometry. The chemical shift of the 14N signals below TC2 exhibited almost continuous change, and it was difficult to observe the N signal due to the wider line width above the TC2 temperature. Near TC2, the electric field gradient tensors at N sites varied, reflecting changes in the atomic configuration around the nitrogen atom. The environment around 14N in the NH3 groups indicates that the change is large near TC2. Furthermore, at the temperature below TC2, two different 14N spectra were explained as follows. According to previously reported X-ray results,[30] there are no reports related to two different N sites, whereas there are reports of twin domain observations; therefore, the two different N sites are attributed to the twin domain.
Fig. 9
Static 14N chemical shifts of [NH3(CH2)3NH3]CdBr4 single crystal as a function of temperature (inset: 14N chemical shift at 180 K).
Static 113Cd NMR experiments were employed to examine the structural environment in CdBr6 anions of the [NH3(CH2)3NH3]CdBr4 single crystal. The spectrum exhibits only one peak due to the spin of I = 1/2. 113Cd NMR spectra were obtained at several temperatures, as shown in Fig. 10. The NMR chemical shift was recorded using CdCl2O8·6H2O as the standard. At 300 K, the line width is 37.95 ppm. As shown in Fig. 10, the chemical shifts for 113Cd are almost constant for temperatures increasing from 180 to 410 K. This result indicates that the environment of the Cd atom surrounded by six Br atoms does not change with increasing temperature.
Fig. 10
Static 113Cd chemical shifts of [NH3(CH2)3NH3]CdBr4 single crystal as a function of temperature.
A crystal is ferroelastic when it has two or more orientation states in the absence of mechanical stress, and is capable to shift from one to another by mechanical stress. Several parallel lines representing ferroelastic twin domain walls are present at room temperature phase III (Fig. 11(a)), and we did not observe any changes to the domain pattern in phase II. The symmetry of phase II remains orthorhombic, and the domain wall is preserved (Fig. 11(b)). Similarly, the same domain pattern was observed in the monoclinic phase I (Fig. 11(c)). The twin boundary exists in the same direction at all temperatures. Here, the [NH3(CH2)3NH3]CdBr4 crystal exists in two crystallographic phases: monoclinic (2/m) above 363 K, orthorhombic (mmm) between 328 and 363 K, and orthorhombic (mmm) below 328 K. For the transition from the mmm of the orthorhombic phase to the 2/m of the monoclinic phase, the domain wall orientations were expressed as x = 0 and z = 0. According to Aizu[36] and Sapriel,[37] the equations of the twin domain walls are reflected the ferroelasticity of mmmF2/m. In this case, this corresponds to “inverse” mmmF2/m suggested by Prezeslawski et al.,[14] unlike the mmmF2/m reported by Sapriel. Hence, our results support the mechanism of ferroelastic twin domains. Further, the two types of inequivalent 14N NMR lines are attributed to the ferroelastic twin domain structure.
Fig. 11
Ferroelastic domain wall patterns of [NH3(CH2)3NH3]CdBr4 crystal at (a) phase III (297 K), (b) phase II (333 K), and (c) phase I (383 K).
Conclusions
We considered the physical properties of organic–inorganic hybrid perovskite [(NH3)(CH2)3(NH3)]CdBr4 crystals. First, the structure and phase transition temperatures (328 K and 363 K) were confirmed by X-ray diffraction and DSC experiments, respectively. We found that the TGA curve exhibits stability until 557 K. Second, the 1H NMR chemical shifts of CH2 for crystallographic environments changed more significantly with temperature than those for NH3. At temperatures below TC2, the 13C chemical shifts change to a slightly upward chemical shift, whereas above TC1, the chemical shift hardly changes. Furthermore, the 14N chemical shift changes with temperature, whereas the 113Cd chemical shift is independent of it. This is because the environments around N change, while the 113Cd chemical shifts were not attributed to the rotation of CdBr6 octahedra. Finally, 1H T1ρ shows fast motion at low temperature and slow motion at high temperature, while 13C T1ρ shows molecular motion at low temperature and slow motion at high temperature. Shorter T1ρ values at high temperature indicate that 1H and 13C in the organic chains are more flexible at these temperatures.We compared the physical properties of the previously reported [(NH3)(CH2)3(NH3)]CdCl4 crystals,[38] and those of [NH3(CH2)3NH3]CdBr4 examined in this study. This is summarized in Table 1. The decomposition temperature of [NH3(CH2)3NH3]CdBr4 is higher than that of [NH3(CH2)3NH3]CdCl4. In the two compounds, the tendency of the 1H and 13C T1ρ values in terms of the temperature change is very similar, while their activation energies are very different, as shown in Table 1; [NH3(CH2)3NH3]CdBr4 has a large Ea at high temperatures, whereas [NH3(CH2)3NH3]CdCl4 has a large Ea at low temperature. Notably, their 113Cd chemical shifts depend on the temperature change, which indicates the difference between the surrounding environments of CdBr6 and CdCl6 octahedra in the two compounds. Although the two crystals contain the same cations, the observed differences in structural dynamics obtained from the chemical shifts and T1ρ values of the two compounds can be attributed to the differences in halogen atoms Br− and Cl− surrounding of Cd. These results provide insights into the molecular dynamics of the [NH3(CH2)3NH3]CdBr4 crystals, and are expected to facilitate their potential applications.
Structure, space group, lattice constant (Å), phase transition temperature TC (K), thermal decomposition temperature Td (K), 1H spin–lattice relaxation time T1ρ (ms), 13C spin–lattice relaxation time T1ρ (ms), and activation energies Ea (kJ mol−1) for 1H and 13C in [NH3(CH2)3NH3]CdBr4 and [NH3(CH2)3NH3]CdCl4 crystals
Authors: Yongtao Liu; Liam Collins; Roger Proksch; Songkil Kim; Brianna R Watson; Benjamin Doughty; Tessa R Calhoun; Mahshid Ahmadi; Anton V Ievlev; Stephen Jesse; Scott T Retterer; Alex Belianinov; Kai Xiao; Jingsong Huang; Bobby G Sumpter; Sergei V Kalinin; Bin Hu; Olga S Ovchinnikova Journal: Nat Mater Date: 2018-08-27 Impact factor: 43.841
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Fig. 11