Correlated room temperature ferromagnetism and photoluminescence in Ni-doped SnO flower-like architecture synthesized via hydrothermal method

Materials Chemistry and Physics 199 (2017) 216e224

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Correlated room temperature ferromagnetism and photoluminescence in Ni-doped SnO flower-like architecture synthesized via hydrothermal method Ying Li, Wei Zhou, Jianchun Wang, Yuzhe Yang, Ping Wu* Department of Applied Physics, Institute of Advanced Materials Physics, Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Faculty of Science, Tianjin University, Tianjin 300072, People's Republic of China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Novel Ni-doped SnO microflowers have been synthesized by hydrothermal growth method.
The low doping narrows the Eg while the high doping has the opposite effect.
The analysis of PL spectra reveals that surface defects are present in all samples.
The ferromagnetism originated from tin vacancies is discovered firstly in SnO.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 April 2016 Received in revised form 14 May 2017 Accepted 2 July 2017 Available online 3 July 2017

Sn1-xNixO microflowers self-assembled with nanopetals have been synthesized successfully with template-free hydrothermal growth method. Field-emission scanning electron microscopy results exhibit the flower-like architecture consist of nanopetals, which have lateral dimensions of 1e2 mm with a thickness of ~100 nm. X-ray diffraction results show that all the samples possess typical tetragonal structure and Ni would occupy different positions (NiSn and Nii) with various concentrations. The bandgap of SnO tends to shrink firstly then widen after Ni-doping, which is caused by the sp-d exchange interactions and the Burstein-Moss effect. Meanwhile, PL and XPS measurements illustrate that tin vacancies (VSn) and oxygen vacancies (VO) were generated during the process of preparation and the VSn as the origin of the ferromagnetism in pure SnO was verified by air-anneal experiment. In addition, Nidoping can improve the ferromagnetism via enhancing the content of VSn. This literature studies the ferromagnetism of novel SnO flower-like structure firstly and reasonably reveals the desired ferromagnetism originated from the VSn. © 2017 Elsevier B.V. All rights reserved.

Keywords: SnO microflowers Optical bandgap Tin vacancy Photoluminescence Room temperature ferromagnetism

1. Introduction SnO has been widely used in electro-optical devices. High

* Corresponding author. E-mail address: [email protected] (P. Wu). http://dx.doi.org/10.1016/j.matchemphys.2017.07.008 0254-0584/© 2017 Elsevier B.V. All rights reserved.

theoretical specific capacity, wide bandgap, low resistivity and excellent environmental performance produce many applications including lithium rechargeable batteries, storage of solar energy and catalysts for organic synthesis [1e5]. Since the properties and performance of metal oxide semiconductors commonly are influenced by the nanostructures as well as by the crystallinity, various morphologies of SnO have also been synthesized such as diskettes

Y. Li et al. / Materials Chemistry and Physics 199 (2017) 216e224

[6], flowers [7], meshes [8], sheets [9], and cross-like [10]. To explore potential applications of SnO, researchers have tried to introduce dopant ions into them to get certain required properties like bipolar conductivity and better optical properties [11e13]. Spintronics is a novel field in physics, which depends upon on spin-dependent phenomena required for modern electronic devices. Utilizing semiconducting and magnetic properties simultaneously is regarded as an essential combination for spintronics [14]. On account of the special configuration of the bandgap, SnO is considered as an important intrinsic p-type semiconductor [1]. Recently, the wide bandgap semiconductors doped with transition metals (Fe, Mn, Ni, Co, Cr) have been studied broadly due to their potential applications in many fields [15]. In order to make the best of the magnetism in spintronics, Tan et al. [16] proposed that pure SnO is not a spontaneous magnetic semiconductor but Co-doping could induce ferromagnetism due to the double exchange interaction theory. Lately, Farooq et al. [17] experimentally proved that Mn-doped SnO is ferromagnetic while the origin is uncertain. From the above, the ferromagnetic mechanism of SnO is still need to explore further. It is known that the Ni2þ ions as dopant can induce new impurity states at different positions within the host bandgap and strongly interfere with the existing carrier recombination process [18], thus Ni2þ ions are selected as the doping element to explore the ferromagnetic mechanism of SnO. In the present work, hydrothermal method was used to synthesize samples. Through controlling the reaction time, temperature, and the concentration of precursor, novel flower-like pure and Ni-doped SnO have been prepared. The structural and magnetic properties of Sn1-xNixO were characterized and the origin of ferromagnetism was analyzed in detail. Additionally, the performance of optical property is vital to the application for spintronics, therefore, it is also essential to study how the optical property changes after introducing the dopant.

2. Experimental details Flower-like Sn1-xNixO (x ¼ 0.00, 0.01, 0.02, 0.06, 0.08) were prepared by hydrothermal method using the starting materials SnCl2$2H2O and NiCl2$6H2O. All reagents were of analytical-grate therefore used without any purification. In a typical hydrothermal synthesis 17.6 mmol of SnCl2$2H2O and corresponding amount of NiCl2$6H2O (0, 0.176, 0.352, 1.056, 1.408 mmol) were dissolved in 40 mL de-ionized water to form a homogenous solution. Following

217

that, 57.9 mmol of KOH added in the obtained solution slowly in three steps, mixed by ultrasonically by using the magnetic stirrer for 30 min and transferred into 50 mL Teflon-lined stainless autoclave. These autoclaves were kept inside the electric oven at the maintained temperature of 100  C for 15 h. After the hydrothermal treatment, light blackish fresh precipitates the precipitation were collected, washed several times with de-ionized water and absolute ethyl alcohol to remove salt impurities, dried under vacuum oven at 55  C for 15 h, and collected for characterization. The phase structure and purity of prepared samples were characterized by X-ray diffraction (XRD) (Rigaku D/Max-RA) using Cu Ka radiation at room temperature. Surface morphology and internal structure were examined by field emission scanning electron microscopy (FESEM, JSM-7800 F) and transmission electron microscopy (TEM, Tecnai G2 f20), respectively. The photoluminescence (PL) measurement was carried out by means of a Xe excitation with a wavelength of 300 nm (Jobin Yvon Fluorolog 3e21). Meanwhile, the bonding states of Sn, Ni and O elements were determined by X-ray photoelectron spectroscopy (XPS). The absorption spectra of samples was recorded by a JASCO-670 spectrophotometer in ultravioletevisibleeinfrared (UVevis-IR) range and magnetic properties were measured using a superconducting quantum interference device (SQUID, Quantum Design, MPMSXL5) system from 5 to 300 K.

3. Result and discussion The structure properties of undoped and Ni-doped SnO microflowers were investigated by XRD and the results are shown in Fig. 1a. All the diffraction peaks in the XRD spectra can be indexed to tetragonal SnO (JCPDS card no. 06e0395, lattice parameters a ¼ b ¼ 3.802 Å, and c ¼ 4.836 Å) and no diffraction peak from other crystalline phases such as Sn2O3, Sn3O4, SnO2 and NiO is detected, indicating the formation of tetragonal SnO is high purity. It is seen that both undoped and Ni-doped SnO samples show the highest intensity with the (101) peak and the corresponding intensity increases as enlarging the Ni-doping concentration. In addition, careful examination (as shown in the inset of Fig. 1a) observes that the SnO (101) peak firstly shifts to high angle with the increase of Ni-doping concentration. After reaching at 2 at% Ni doping, it shifts to low angle. The shift of (101) peak reveals the corresponding variation in lattice constant along a-axis and c-axis (see Fig. 1b). With Ni-doping concentration fewer than 2 at%, the lattice constant a and c shrink clearly as relative to the pure SnO. This decrease

Fig. 1. a XRD patterns for Sn1-xNixO particles. The inset shows the magnified part of the (101) peaks; b The variation of the lattice parameters a and c as a function of Ni concentration.

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Fig. 2. a and b are the typical EDS patterns of samples corresponding to SnO and Sn0.98Ni0.02O, respectively.

would be expected when Sn2þ ions are replaced by Ni2þ ions, since the relatively small radius of Ni2þ (0.55 Å) [19] compared with that of Sn2þ (0.93 Å). For the 6 and 8 at% Ni-doping samples, lattice constant a and c start to increase, Ni2þ ions seem to incorporate at interstitial sites mainly because of the limited solubility of NiSn and consequently cause lattice distortion [20,21]. Moreover, for confirming Ni2þ ions exist in the prepared samples, the EDS analysis spectra of undoped and doped SnO samples are shown in Fig. 2a and b respectively. Fig. 2a indicates that the only compositions are of Sn and O. Fig. 2b shows the EDS spectrum of Sn0.98Ni0.02O, it states that the product contains Sn, Ni and O elements. Structural morphologies of synthesized SnO microflowers were examined by using FESEM. Fig. 3a illustrates a representative overview of the flower-like architecture at low magnification. The high magnification FESEM image in Fig. 3b shows the morphology of an individual three-dimensional (3-D) flower-shaped

architecture and the average size of the flower-like structure is about 10 mm. It is prone to form micro-flower due to the structure feature of SneOeSn layers, which is stacked along the c direction with Vander Waals interactions between the two layers [22]. It can be seen a lot of 3D microflowers which assembled by high-density 2D nanopetals in the figure. The straight and isolated nanopetals are aligned to the surface of the substrate. The higher magnification FESEM image shown in Fig. 3c indicates that these nanopetals have lateral dimensions of 1e2 mm with a thickness  100 nm (see the inset of Fig. 3c). The results show that SnO nanopetals can be obtained and self-assembling into 3D hierarchical microflowers architectures by the facile method. Fig. 3d shows the morphology of Sn0.98Ni0.02O, it has the same size as the undoped SnO. However, some degradation has occurred at the particle surface, which more easily leads to generate more defects. Fig. 4 shows TEM images of individual nanopetals. Fig. 4a shows

Fig. 3. a-c The typical FESEM images of the as-synthesized pure SnO particles at different magnifications; d The FESEM images of Sn0.98Ni0.02O sample.

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Fig. 4. a-c the TEM images of pure SnO samples; a TEM images of individual nanopetals; b HRTEM image with corresponding magnified grain boundary pattern (inset) from the square area of pure SnO sample; c SAED pattern of certain area; d and e TEM image and HRTEM image of Sn0.98Ni0.02O.

their typical rectangular morphology for pure SnO particles. The average size of the petal edge is in the rage of 0.5e1 mm, which is consistent with the FESEM results. To ascertain the lattice structure, the high-resolution TEM images were recorded from the nanopetal as shown in Fig. 4b. The clear lattice fringes confirm the wellcrystalline nature of the as-prepared SnO and the red arrows indicate the lattice lanes of SnO. From the inset of Fig. 4c, the distances (0.27 nm and 0.304 nm) between the adjacent lattice fringes agree well with the d110 and d101 spacing of the literature values (0.2688 nm and 0.2989 nm) (JCPDS card no. 06e0395). The selected area electron diffraction (SAED) pattern from an individual part (in Fig. 4c) shows very sharp diffraction spots, indicating the nanopetals consist of a single-crystalline tetragonal structure [23]. Fig. 4e and d shows the TEM images of Sn0.98Ni0.02O, the interplanar spacing of (101) places shown in Fig. 4e is about 0.29 nm, which is smaller than that of pure SnO (about 0.30 nm). It means the Ni doping can affect the lattice structure [21], which is consistent with the XRD analysis. In order to investigate the effect of dopant on optical property,

the absorption spectra of Sn1-xNixO were measured as shown in Fig. 5a. All Sn1-xNixO samples show a sharp absorption edge at around 300e325 nm which can be attributed to the photoexcitation of electrons directly from valence band to conduction band. The absorption edge of different samples slightly varies as fluctuating the concentration of Ni in SnO particles. It is known that the fundamental band gap of SnO is of an indirect transition type [24], however, it is difficult to determine the indirect bandgap because the absorption coefficient is too small [1]. Here, we discuss the change in the bandgap determined by the Tauc relation [25] with respect to Ni doping (see Fig. 5b),

ahn ¼ A hn  Eg


n

(1)

where a is the absorption coefficient, A is a constant and n ¼ 1/2 for the direct-allowed transition. An extrapolation of the linear region of a plot of (ahn)2 vs. hn gives the value of the optical bandgap Eg. Clearly, for the undoped sample, the bandgap is 3.39 eV, which is in good agreement with the reported values (2.7e3.4 eV) [1]. After Ni

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Fig. 5. a UVeVisible absorption spectra of Ni-doped SnO particles; b Typical plot of (ahn)2 vs hn of Sn1-xNixO, the top-inset shows the partial enlarge detail and the button-inset is the corresponding bandgaps with different Ni concentration for Sn1-xNixO.

being doped, the bandgap is narrowed when x  0:02, which can be attributed to the sp-d exchange interactions between the band electrons and the localized d electrons of the Ni substituting Sn. It is known that in the band structure of SnO, the contributions of Sn 5s and O 2p are predominant near the valence-band maximum (VBM). In the conduction band, the O 2p component is relatively small and the states near the conduction-band minimum (CBM) are mainly formed by Sn 5p [24,26]. The s-d and p-d exchange interactions give rise to a positive and negative correction to the valance-band and conduction-band edges respectively, leading to a band narrowing [27]. With further Ni doping (x > 0:02), the bandgap widens. The observed blue shift can be explained on the basis of the BursteinMoss effect [28,29]. When Fermi level shifts close to the conduction band due to the increase in the carrier concentration, the low energy transitions are blocked and the value of bandgap increases. As Ni2þ ions locating at interstitial sites, carrier concentration will increase due to introduction of additional electrons. Therefore, the observed red or blue shift indicates slight modification in the band structure in Ni doped SnO nanostructures due to the incorporation of Ni2þ ions. Additionally, Fig. 5a obviously shows that another “relatively weak peaks” are present at around 720 nm and these extra peaks can be ascribed to the indirect-allowed transitions between the bandgap [1]. In general, the influence of d states due to

Ni-doping on the optical bandgap is subtle and the intrinsic characteristic of wide bandgap is stable. PL is an effective technique to determine defects in wide bandgap oxides. At the following stage, PL spectra of Sn1-xNixO were measured in the range of 310e650 nm at room temperature using a Xe lamp with an excitation wavelength of 300 nm. The results are provided in Fig. 6a and all curves have been normalized. The spectra are broad and asymmetric with a center at approximately 400 nm, so it should consist of more than one component. In order to investigate the reason behind the origin of the different emission peaks, the well-fitted PL spectra for pure SnO with the multiple Gaussian peaks are shown in Fig. 6b. The first peak ( 383.93 nm) is widely believed to be corresponded to the violet luminescence and attributed to the direct band to band transition by recombination of electron from conduction band to the hole in valance band [30e32]. This peak slightly shifts to longer or shorter wavelength as Ni2þ ions being introduced into host crystal (see Supporting Information, Fig. S1). The shift is mainly due to the lattice distortion caused by Ni-doping [20]. According to the XRD analysis, substitutional Ni2þ ions result in the decrease of lattice constant, generating compressive stress. For the case of interstitial doping, tensile stress is produced. Thus, it is reasonable to deduce the red- and blue-shifts are associated with the

Fig. 6. a PL spectra of as-grown Sn1-xNixO; b Gaussian fit of PL spectrum for pure SnO.

Y. Li et al. / Materials Chemistry and Physics 199 (2017) 216e224 Table 1 The distribution of blue and green emission peaks for as-grown samples.

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Table 2 The distribution of blue and green emission peaks for air-annealed samples.

As-grown

0%

1%

2%

6%

10%

Air-annealed

0%

1%

2%

6%

10%

Blue peak area (a.u.) Green peak area (a.u.)

48.66 29.44

50.06 37.83

50.82 48.28

48.47 37.43

49.21 35.86

Blue peak area (a.u.) Green peak area (a.u.)

47.29 51.03

43.06 74.36

45.10 79.34

44.31 59.83

46.36 54.57

compressive and the tensile stress respectively [20]. The next two extrinsic peaks reveal trap levels exist between conduction band (CB) and valence band (VB). These trap levels may be generated due to defects on the surface of flower-like type morphology [31]. Further analysis showed that the blue peak at  417.63 nm could be assumed to the VO [20,32] and the green peak at  480.74 nm could be attributed to VSn in SnO [26,30,33]. The detailed information of each PL peaks is shown in Table 1. From Table 1, the relative area of the green peak increases firstly and then decreases, which is in line with the various of lattice parameters, indicating that substitutional Ni2þ ions in favour of generating VSn while interstitial Ni2þ ions restrain the growth of VSn. For further confirming this prediction, all samples were annealed in air at 200  C for 2 h and the PL spectra were measured again as shown in Fig. 7. Similarly, the asymmetrical emission peak can be well fitted with the multiple Gaussian peaks. Compared with Tables 1 and 2, the intensity of blue emission peak weakens while the green strengthens after being annealed, respectively (detail information can be seen in Supporting Information, Fig. S1 and Fig. S2). As we know, air anneal favors the formation of VSn and suppresses the formation of VO. Therefore, the conclusion of emission peak around at  410 nm attributed to the VO and the emission peak around at  480 nm assumed to VSn is reliable. The chemical states of the compositional elements in SnO (sample I) and Sn0.98Ni0.02O (sample II) were revealed by XPS, and the corresponding spectra are shown in Fig. 8, where Fig. 8aed are the survey spectra, Ni 2p, O 1s and Sn 3d core-level spectra, respectively. From analysis of the XPS results (Fig. 8a), in addition to C, only Sn, Ni(only in sample II), and O elements can be observed. The peaks located at 852.21 and 872.58 eV are identified with the binding energies of Ni2þ [34], shown in Fig. 8b. The O 1s peaks of two samples are respectively fitted into three peaks by LorentzianGaussian function, centered at 529.98 (Oa), 531.74 (Ob) and 532.80 (Oc) eV, corresponding to the lattice oxygen (OeSn bond), the loss of oxygen (VO), and binding in surface O2, respectively [21,35]. It is obvious that the relative area of Ob increases after Ni being doped

into SnO, which means that VO content would increase with Ni2þ ions being introduced into the host crystal. The double spectral lines of Sn 3d (Fig. 8d) appear at binding energies of 485.79 (Sn 3d5/ 2) and 494.26 eV (Sn 3d3/2), which coincides with the findings for Sn2þ bound to oxygen in the SnO matrix [36]. The Sn 3d5/2 peak shows only one symmetric component without a shoulder peak, indicating the absence of Sn4þ ions. Quantification of Sn 3d and O 1s gives an average [Sn]/[O] atomic ratio. The [Sn]/[O] ratios (identified as RI, RII) were determined to be 0.94 and 0.92 for sample I and II, respectively. Combined with the corresponding core-level spectra, it can be confirmed that the existence of Sn deficiency (VSn) [36] and there are more VSn in sample II (RI > RII). The XPS results further confirm the defect states in the synthesized samples, corresponding to the consequence of PL analysis. Fig. 9a shows the magnetic hysteresis (M-H) curves of Sn1-xNixO samples taken at 300 K. The diamagnetic signals have already been subtracted linearly using the high-filed magnetization. All the samples exhibit hysteresis phenomena, indicating they are ferromagnetic with a Curie temperature (Tc) higher than 300 K. The dependence of the saturation magnetization (Ms) and Ni-doping concentration is described with the black dashed line in Fig. 9d. The pure SnO exhibits weak Ms of ~0.5  103 emu/g, which is inconsistent with other reported results on theory and experiment respectively [16,17]. The unexpected ferromagnetism is likely to be attributed to point defects and will be further investigated later. As Ni being doped, Ms rises firstly, reaching its highest value at 2 at% doping (~1.8  103 emu/g) and then tends to decrease with further doping, which may be resulted from the exotic Ni clusters or be the intrinsic nature of the material. In order to illuminate the ferromagnetic origin, the temperature dependence of magnetization for the as-grown sample of Sn0.98Ni0.02O under the field-cooled (FC) and zero-field-cooled (ZFC) mode is shown in Fig. 8b. The FC and ZFC curves keep separated to 300 K, confirming that the Curie temperature of the sample should be above room temperature [21]. Furthermore, no characteristic peak or hump in the ZFC curve is observed, signifying absence of ferromagnetic contamination (Ni

Fig. 7. a PL spectra of Sn1-xNixO with air annealed; b Gaussian fit of PL spectrum for pure SnO with air annealing.

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Fig. 8. The XPS spectra of SnO (I) and Sn0.98Ni0.02O (II) samples. a The survey spectra; b The high-resolution core level XPS spectrum of Ni 2p for sample II; c and d The highresolution core level spectra of O 1s and Sn 3d for sample I, II.

cluster) in the sample, which suggests the observed ferromagnetism of the Sn1-xNixO samples should be nature of the material [21,35]. It is known that Ni is a transition metal with d orbitals. Several mechanisms such as Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, super exchange interaction, double exchange interactions, free-carrier-mediated exchange, and sp-d exchange could not produce long-range magnetic order for the samples with the low doping concentration [37]. Otherwise, in most cases, defect-related ferromagnetic moment reported in the literature is of quite low magnitude (of the order of 106 to 103 emu/g) [38]. Therefore, the ferromagnetism of Sn1-xNixO discovered in our experiment can be ascribed to the surface defects generated in the process preparation. In order to analyze the ferromagnetism origin further, the XRD and magnetic property of annealed samples were measured. In Fig. 9c, the RT M-H curves of air-annealed Sn1-xNixO is presented. For the pure sample, ferromagnetism is enhanced to 1.0  103 emu/g after annealing in air. Air anneal favors the formation of VSn and suppresses the formation of VO, thus the origin of ferromagnetism in pure SnO could be attributed to VSn, which creates holes at neighboring O ligands, with electrons being localized for spin up while partially localized for spin down [39]. Meanwhile, the ferromagnetism of as-grown samples enhances firstly and then reduces with Ni concentration increasing. This also can be associated with the content of VSn, which is determined by the Ni-doping concentration through the PL analysis. Further study in Fig. 9c, all of Ni-doped SnO show a relatively strong ferromagnetism. The XRD-annealed results in the inset of Fig. 8b show that

all samples can be indexed to tetragonal SnO and no impurities is produced, indicating the structure of samples are stable after being annealed. It demonstrates that the ferromagnetism of the asannealed samples is the intrinsic behavior. Compared with asgrown samples, the ferromagnetism of all the samples is enhanced 0.5  103 -0.7  103 emu/g after being annealed in air as shown in Fig. 9d. Combined with the results of PL spectra in Fig. 7 and Tables 1 and 2, it can be furtherly verified that the origin of ferromagnetism in prepared Sn1-xNixO could be VSn. 4. Conclusion We have successfully synthesized Ni-doped SnO microflowers using the template-free hydrothermal growth method. XRD results show that the Ni2þ ions are incorporated into the SnO particles and locating at different positions (NiSn and Nii) as the Ni concentration increasing. UVevisible absorption spectra and the derived optical bandgap results indicate that pure SnO has a wide bandgap of 3.39 eV and Ni-doping can induce subtle change due to the introduction of Ni2þ ions d orbital electron. The analyses of PL and XPS spectra prove that the surface defects exist in the synthesized samples. And annealing treatment further testified that these possible defects are VSn and VO. Combining the M-H curves of asgrown and as-annealed samples, it is clear that the ferromagnetism of Sn1-xNixO is originated from the surface VSn and Ni-doping can affect the ferromagnetism by means of changing the content of VSn. Thus, RTFM is discovered successfully in SnO. In this article, we suggest that the novel morphology, reasonable optical, magnetic

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Fig. 9. a Room-temperature M-H loops of as-grown Sn1-xNixO; b The FC-ZFC plots for the as grown sample of Sn0.98Ni0.02O in a constant field of 500 Oe; c Room-temperature M-H loops of as-annealed Sn1-xNixO. The inset shows the XRD patterns of samples with air anneal; d The Ms as a function of the Ni doping concentration for the samples with and without air anneal.

properties of SnO and the easy preparation process make it suitable for optomagnetic and spintronics devices applications.

[8]

Acknowledgment [9]

This work was supported by the National Natural Science Foundation of China (51572190).

[10]

Appendix A. Supplementary data

[11]

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2017.07.008.

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References

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