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Formation of PbMnI2 alloys: Structural, photoluminescence and nuclear quadrupole resonance studies
Journal Pre-proof Formation of PbMnI2 alloys: Structural, photoluminescence and nuclear quadrupole resonance studies A.P. Bukivskii, I.G. Vertegel, E.D. Chesnokov, P.M. Bukivskij, O.I. Ovcharenko, R.V. Gamernyk, Z.D. Kovalyuk, V.M. Tkach, Yu.P. Gnatenko PII:
S0925-8388(20)30348-0
DOI:
https://doi.org/10.1016/j.jallcom.2020.153985
Reference:
JALCOM 153985
To appear in:
Journal of Alloys and Compounds
Received Date: 27 September 2019 Revised Date:
18 December 2019
Accepted Date: 21 January 2020
Please cite this article as: A.P. Bukivskii, I.G. Vertegel, E.D. Chesnokov, P.M. Bukivskij, O.I. Ovcharenko, R.V. Gamernyk, Z.D. Kovalyuk, V.M. Tkach, Y.P. Gnatenko, Formation of PbMnI2 alloys: Structural, photoluminescence and nuclear quadrupole resonance studies, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153985. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
A.P. Bukivskii: Investigation, Software, Data Curation, Writing - original draft I.G. Vertegel: Investigation, Formal analysis, Validation E.D. Chesnokov: Investigation, Formal analysis P.M. Bukivskij: Investigation, Validation, Visualization O.I. Ovcharenko: Investigation, Methodology R.V. Gamernyk: Investigation, Formal analysis, Methodology Z.D. Kovalyuk: Formal analysis, Visualization, Resources V.M. Tkach: Investigation, Methodology Yu.P. Gnatenko: Conceptualization, Writing - review & editing, Supervision
1
Formation of PbMnI2 alloys: structural, photoluminescence and nuclear quadrupole resonance studies A.P. Bukivskii1, I.G. Vertegel1, E.D. Chesnokov1, P.M. Bukivskij1, O.I. Ovcharenko1, R.V. Gamernyk2, Z.D. Kovalyuk3, V.M. Tkach4 and Yu.P. Gnatenko1a
1
Institute of Physics of NASU, 03028 Kyiv, Nauky ave. 46, Ukraine Lviv National University, 29005 Lviv, Kyrylo and Mefodiy str. 8, Ukraine 3 Chernivtsi department of Institute of Problem of Material Sciences of NASU, 58001 Chernivtsi, Iryny Vilde ave. 5, Ukraine 4 Institute for Superhard Materials of NASU, 2 Avtozavodska Str., Kyiv, 04074, Ukraine 2
a
Corresponding author: Yuriy P. Gnatenko: [email protected]
Abstract The structural, optical and nuclear quadrupole resonance (NQR) measurements of PbMnI2 alloys were carried out. This allows us to study the peculiarities of the formation of such alloys. It was found that pure PbI2 and Pb1-XMnXI2 crystals with X = (0.03-0.05) have the phase of 2H-polytype. It was shown that the investigated alloys can be presented as the crystal regions of Pb1-XMnXI2 solid solutions, embedded in PbI2 crystal matrix. These formations can be considered as platelet-shaped nanoparticles with a surface of several microns and a thickness of several tens of nm. Thus, PbMnI2 alloys are heterogeneous PbI2-PbMnI2 nanocomposites. The results of NQR spectra measurements also show that Mn2+ ions preferably replace Pb2+ ions in the crystalline layers. Here, the local deformations occur because the radii of Pb2+ and Mn2+ ions differ significantly. The broadening of optical and NQR lines is due to the presence of both these deformations and the crystal field fluctuations, which are characteristic of semiconductor solid solutions.
Keywords: semiconductors; nanostructured materials.
disordered
systems;
photoluminescence;
crystal
structure;
• Structural, optical and NQR measurements of PbMnI2 alloys were carried out. • The investigated crystals have a 2H-polytype phase. • The crystal regions of Pb1-XMnXI2 solid solutions are embedded in PbI2 matrix. • These crystal regions can be considered as platelet-shaped nanoparticles. • PbMnI2 alloys are heterogeneous PbI2-PbMnI2 nanocomposites.
1. Introduction The semiconductor PbI2 crystals present considerable interest since they are used for elaboration of high-sensitive non-cooled radiation detector materials for x- and γ-rays [1-3]. PbI2 has a type of layered structure with a repeating unit of a hexagonal close packed layer of Pb2+ sandwiched between two layers of I− in the crystal [4]. It should be noted that at present, the PbI2
2 semiconductor is considered as ideal scintillator, since this material can provide a combination of high light output and short decay time [5,6]. At the same time, in order to realize the great potential of these crystals as scintillation materials, it is necessary to improve their optical and electronic properties. It is well known that an effective method of improving the physical properties of crystals is their doping with different impurities, which leads to the formation of nano- or submicron-sized clusters in the crystal matrix. Recently, it was shown that the growth of Pb1-XCdXI2 alloys is accompanied by the formation of nanostructured materials, where nanoclusters of PbI2 are embedded in the CdI2 crystal matrix. This is due to the fact that Pb1-XCdXI2 alloy is a nonisoelectronic system because the valence electrons of Pb and Cd atoms belong to different electronic configurations, namely (5d106s26p2) and (4d105s2), respectively [7,8]. It was found that this substitution peculiarity of cation sites is a reason for the formation of bulk nanostructured Pb0.30Cd0.70I2 alloys and may be caused by spinodal decomposition of these alloys [9]. It was found that these materials exhibit intense photoluminescence (PL) and luminescence excited by X-rays (XRL) at room temperature [10-14]. Photoluminescence lifetime studies of PbI2 nanoclusters and microcrystallites in Pb0.30Cd0.70I2 alloys have been performed in [11,14,15]. The formation of various cluster structures at the micro/nano scale has also been observed for other layered semiconductors, namely, CdI2 crystals having the same crystal structure as PbI2 crystals [16,17]. It was shown that for CdI2-BiI3 alloys, the clusters consisting of two BiI3 molecules and a cadmium vacancy are formed since cadmium and bismuth ions are non-isovalent. Transfer of Bi3+ ions to the dislocation borders and the interaction between Bi3+ ions, vacancies and anion sublattice may lead to the appearance of BiI3 microphase containing large clusters. Besides, the formation of the cadmium clusters was observed for non-stoichiometric pure crystals CdI2 as a result of long-term effects of aging [18-20]. It is evident that the layered solid solutions, based on the PbI2 or CdI2 crystals, may exhibit features related to the formation of nanostructured materials. Thus, it is very important to study the
3 peculiarities of the formation of Pb1-XMnXI2 alloys containing Mn magnetic atoms. The use of Pb1XMnXI2
alloys can expand the luminescence spectral range of such materials and thus improve their
scintillator characteristics [21]. Besides, PbI2 crystals doped with magnetic atoms are of interest for the development of novel materials – layered diluted magnetic semiconductors. Earlier, the studies of magnetic and magneto-optic properties of Pb1-XMnXI2 alloys were performed which showed that they have very interesting magnetic properties [22-26]. In particular, strong spin exchange ferromagnetic interactions due to the presence of magnetic ions in the PbMnI2 crystals were found [23]. The antiferromagnetic character of the exchange interaction between Mn ions and holes of the valence band was observed in [25]. However, the crystalline structure of PbMnI2 alloys and its effect on the photoluminescent properties of such materials have not yet been sufficiently studied. As for the NQR spectra, they were previously studied for PbCdI2 and PbI2-BiI3 alloys [4,27-31]. In this paper, we report the results of the comprehensive structural (XRD, SEM, EDAX, SAXS), optical (absorption, photoluminescence and reflection spectra) and nuclear quadrupole resonance measurements of PbMnI2 alloys. This allowed us for the first time to study the peculiarities of the formation of such alloys. It was shown that the investigated alloys present the crystal regions of Pb1-XMnXI2 solid solutions with micron-sized surfaces and thickness of several tens of nm distributed heterogeneously in the PbI2 crystal. Thus, PbMnI2 alloys are heterogeneous nanocomposites. The spectral range of photoluminescence of such alloys extends to the shortwavelength region due to the emission of Pb1-XMnXI2 nanoparticles.
2. Experimental details The PbI2 and Pb1-XMnXI2 crystals were grown by the Bridgman technique. The crystals were synthesized by direct alloying of the constituents in sealed quartz ampoules under vacuum. The impurity Mn atoms were introduced by adding them to the melt and the Mn concentration in the studied crystals corresponds to 0 ≤ X ≤ 0.05. The elements were purified by vacuum sublimation and were carefully weighed to obtain the desired impurity concentration.
4 The crystal quality and phases of PbMnI2 alloys were verified by X-ray diffraction experiments (XRD) using a Shimadzu LabX XRD-6000 diffractometer with Kα radiation of Cu (λ = 0.154178 nm). The scanning was performed with scattering angle (2θ) from 10° up to 90° with a step of 0.02°. Qualitative phase analysis of the diffractogram was carried out using the ICDD database, PDF-2 Release 2012. A high-brilliance synchrotron P12 beamline of the EMBL located at the PETRA III storage ring (DESY, Hamburg) was used for small-angle X-ray scattering (SAXS) [32,33]. This method enables one to investigate the size distribution function of nanoparticles, which can be randomly embedded in a nanocomposite system. The investigation of the surface structure of the Pb1-XMnXI2 crystals was performed using a Zeiss EVO 50 XVP scanning electron microscope with an X-Ray dispersion analysis module in phase-contrast regime. This allowed us to obtain not only a visualization of the crystal structure morphology, but also to determine the chemical composition of such areas. Thus, this made it possible to observe precisely the areas where Pb1-XMnXI2 solid solutions are formed in the PbI2 matrix. An SDL-1 grating spectrometer was used to measure the PL spectra [34-37]. The excitation intensity of an LGN-404 argon laser with 488.8 nm emission line was kept below 100 mW/cm2 to avoid heating the sample. The samples were mounted on the cold finger of a variable temperature liquid-helium cryostat. The spectral resolution of the system was of the order of (0.1 – 0.2) meV. The nuclear quadrupole resonance (NQR) spectra of I127 for the investigated crystals were measured at 77 K in the frequency range 5-300 MHz using a pulsed quasi-coherent NQR spin-echo spectrometer. The accuracy of the NQR frequency measurements was determined by the halfwidths of the NQR lines, and for the investigated crystals this value was no worse than ± 25 kHz.
3. Results and discussion 3.1 Crystal characterization of Pb1-XMnXI2 alloys
5 The XRD patterns of both pure PbI2 and Pb1-XMnXI2 crystals are presented in Fig. 1. The analysis of the patterns shows that the Pb1-XMnXI2 crystal with X=0.03 as well as the pure PbI2 crystal have a single phase of 2H-polytype. A small decrease in the crystal lattice parameters for Pb1-XMnXI2 is observed, which may be caused by the replacement of Pb2+ ions by Mn2+ ions with ionic radii of 1.26 Ǻ and 0.91 Ǻ, respectively. The lattice “a” and “c” parameters for pure PbI2 crystals correspond to 4.561 Ǻ and 6.989 Ǻ, respectively. At the same time, these values for Pb1XMnXI2
crystals are equal to 4.557 Ǻ and 6.981 Ǻ. Thus, the formation of Pb1-XMnXI2 solid
solutions occurs.
Fig. 1 XRD patterns of PbI2 (A) and Pb1-XMnXI2 (B) crystals (X=0.03).
6 This conclusion is supported by the results of a surface morphology study of Pb1-XMnXI2 crystals performed using a Zeiss EVO 50 XVP scanning electron microscope with an X-Ray dispersion analysis module at room temperature shown in Fig. 2A. The surface morphology of the studied crystals was obtained in the phase contrast mode. In this case, there are areas with different brightness. Therefore, it is very important to determine the local chemical composition of the crystal for different surface spots. These results are shown in Fig. 2 and Table 1.
Fig. 2 SEM images (A) and EDS spectrum (B) of Pb1-XMnXI2 crystals (X=0.03). The spots 1-6 in SEM images indicate the areas of the crystal surface, where its chemical composition was determined. The EDS spectrum corresponds to spot 4. Table 1. The chemical composition of Pb1-XMnXI2 crystals with (X=0.03) and X=0.05 shown in Fig. 2 A and Fig. 3, respectively.
Spot № 1 2 3 4 5 6
Pb1-XMnXI2 Х = 0.03 Mn, at% I, at% 0.97 0.90 0 8.53 1.66 0.99
67.61 70.46 68.12 64.99 66.51 68.04
Pb, at% 31.42 28.64 31.88 26.48 31.83 30.97
Spot № 1 2 3 4 5
Pb1-XMnXI2 Х = 0.05 Mn, at% I, at% Pb, at% 1.08 13.28 4.55 1.62 5.06
67.88 62.41 71.97 73.34 64.79
31.04 24.31 23.48 25.04 30.15
7 As can be seen from Table 1, the chemical analysis of Pb1-XMnXI2 crystals (X=0.03) for spots 1,2 and 5, which are the bright areas, shows that they contain a small concentration of Mn element (smaller than two percent). It should be noted that visually the brightness of these areas is approximately the same. However, their chemical composition for spots 1 (or 2) and 5 is different. It should be noted that the depth of X-ray photons penetration is about 1.0 micron. Thus, we assume that different Mn concentrations for different spots can be caused by the inhomogeneous composition of the crystal both at the surface and in depth, i.e. can be a result of the existence of crystal layers of various composition, located parallel to the substrate surface. Apparently, these areas are associated with the formation of the Pb1-XMnXI2 solid solutions. In this case, there exists an inhomogeneous PbI2 crystal, where the areas of the solid solutions of micron sizes are embedded in the PbI2 crystal, i.e. such crystals are a PbI2-PbMnI2 composite. It should be noted that the presented SEM images are similar for the different regions of the same crystal. Thus, this makes it possible to extrapolate information based on the presented SEM images to the whole crystal. The brightest area marked as spot 3 corresponds to pure PbI2 crystal. The size of this area is considerable. There are also other areas that are typical of this crystal dark gray color, where the concentration of Mn atoms corresponds to about ten percent. Such an area is shown in Fig. 2 A as spot 4. They are rarely found in the crystals. It is expected that the
Fig. 3 SEM images of Pb1-XMnXI2 crystals (X=0.05). The spots 1-5 in SEM images indicate the areas of the crystal surface where its chemical composition has been determined.
8 formation of both Pb1-XMnXI2 solid solutions and, possibly, MnI2 compound can be characteristic of these areas. It should be noted that for such crystal areas the concentration of Pb element decreases. Fig. 3 shows SEM images of the Pb1-XMnXI2 crystal surface (X=0.05). In comparison with the Pb1-XMnXI2 crystal with X=0.03 there are many areas of dark gray color here, where the concentration of the Mn element corresponds to about 5 percent. This means that for this crystal the area size of Pb1-XMnXI2 solid solution is larger than for the crystal with X=0.03. In order to obtain information on the structure of inhomogeneities (their shape and sizes) of the investigated materials we carried out small-angle X-ray scattering (SAXS) measurements [37]. In this case, the study of the dependence of X-ray intensity on the wave vector q, namely I(q), where q = 4πsinθ / λ (θ is half the scattering angle, λ is the x-ray wavelength) was carried out. It should be noted that such a dependence can be described by the following equation: I(q) = q-D [38]. The values of D = 1, 2 and 3 indicate that the scattering objects are particles of rod, disk and spherical shape, respectively. It should be noted that for the surface fractals their dimension varies within 2 < Ds <3 and is calculated by the formula Ds= 6 – D [39].
Fig. 4 The dependence I(q)= q-D shown on a double logarithmic scale.
In Fig. 4 the dependence I(q) is shown on a double logarithmic scale to emphasize the
power law behavior. This allowed us to determine the value of Ds, which for alloys with X = 0.03 corresponds to 2.1. This means that the structural inhomogeneities (in our case crystal regions, where Pb1-XMnXI2 solid solutions are formed) have the platelet-like shape. For the crystals with X=0.05 the value Ds equals to 2.35, which indicates the fractal geometry of the objects. It should be
9 noted that the analysis of the dependence I(q) also allows us to obtain information on the size distribution
Fig. 5 The distance distribution function p(d) of nanoparticles for Pb1-XMnXI2 (X=0.03) alloys.
of the particles, namely to determine the distance distribution function f(d), where d is the diameter of the particles of spherical shape or the thickness of the platelet-like particles. In Fig. 5 the distance distribution function f(d) is presented for the alloys with X=0.03. As can be seen from Fig. 5, this distribution contains maximums at d equal to 14 nm, 36 nm and 69 nm, which can be associated with the average sizes of the crystal regions where Pb1-XMnXI2 solid solutions are formed. Taking into account the results of SEM image studies we can assume that such crystal regions present the formations of platelet-like or fractal shape with the sizes of several microns in the plane and the average thickness of several tens of nm, i.e. are nanoparticles. Therefore, the studied alloys Pb1XMnXI2
can be considered as PbI2-PbMnI2 nanocomposites.
3.2 Photoluminescence spectra of Pb1-XMnXI2 alloys Fig. 6 (curve 1) shows a PL spectrum of pure PbI2 crystals at T=4.5 K which have mainly 2H-polytype [40]. In this case the exciton PL spectrum contains several lines at 2.497 eV, 2.492 eV, 2.484 eV, 2.472 eV and 2.459 eV. These lines correspond to the bound excitons and their phonon replicas with the participation of low-frequency optical phonons [40]. In particular, the intense line
10
Fig. 6 Photoluminescence (curve 1) and exciton reflection (curve 2) spectra of PbI2 crystals.
at 2.492 eV corresponds to the emission of excitons bound with the neutral donors, i.e., (DoX)-line for PbI2 crystal of 2H-modification. The line at 2.497 eV corresponds to the emission of other bound excitons in PbI2 crystal of 2H-polytype. The weak line at 2.503 eV may be associated with the exciton line in PbI2 crystal of 4H-polytype or correspond to the emission of free excitons from the lower polaritons. The latter assumption is supported by the observation of free excitons for this crystal in the exciton reflection spectrum at 2.503 eV, which is shown in Fig. 6 (curve 2). In addition, two more intense broad lines at 2.435 eV and 2.381 eV are observed, which are associated with the emission of donor-acceptor pairs with the participation of two different acceptor levels. In this case, the phonon replicas of these zero-phonon lines are also observed.
11 Fig. 7 Photoluminescence (curve 1) and exciton reflection (curve 2) spectra of Pb1-XMnXI2 alloys for X=0.03. The analysis of the PL spectrum of Pb1-XMnXI2 crystals for X=0.03, shown in Fig. 7 (curve 1), reveals free and bound exciton lines characteristic of PbI2 crystal of 2H-polytype. The formation of free excitons in the investigated crystal is also observed in the exciton reflection spectrum in Fig. 7 (curve 2). The most intense PL line at 2.436 eV is due to the emission of donor-acceptor pairs. In addition, a new line at 2.546 eV was found in the short-wavelength region. Since the energy position of this line is larger than the band gap of PbI2 crystal, it may be caused by the exciton emission in Pb1-XMnXI2 solid solutions, which, accordingly to the SEM images and EDX measurements, present the crystal regions with micron-sized surfaces distributed heterogeneously in PbI2 crystal.
Fig. 8. Photoluminescence (curve 1) and exciton reflection (curve 2) spectra of Pb1-XMnXI2 alloys with X=0.05 at T=4.5 K. As can be seen from Fig. 8, the PL spectrum (curve 1) of Pb1-XMnXI2 crystals with X=0.05 contains two intense exciton lines at 2.500 eV and 2.556 eV, as well as a very intense line at 2.436 eV, which corresponds to the radiative recombination of donor-acceptor pairs. A comparison of this PL spectrum with the spectrum of a pure PbI2 crystal (see Fig. 6) shows that there is no PL band at 2.381 eV due to the emission of donor-acceptor pairs with the participation of a deeper acceptor center. It is possible that such acceptor center is associated with lead vacancy. Thus, due to the doping of the PbI2 crystal with Mn atoms, the number of lead vacancies decreases.
12 It should be noted that the energy position of the exciton line at 2.500 eV is close to that of bound excitons (DoX-line) for PbI2 crystals of 2H-polytype. The long-wavelength edge of this line shows a low intensity line at 2.492 eV, which may be associated with the phonon replica of DoXline. The short-wavelength edge of DoX-line also contains a line of low intensity at 2.507 eV, which may be caused by the emission of bound excitons characteristic of PbI2 crystals of 4H-polytype. On the other hand, this line can be associated with free exciton emission of PbI2 crystals of 2H-polytype from the lower (transverse or XT) polariton branch [41]. This assumption follows from a comparison of the energy positions of PL and exciton reflection bands. It should be noted that the exciton reflection spectrum, presented in Fig.8 (curve 2), shows two bands at 2.515 eV and 2.573 eV, marked as FER-1 and FER-2 bands, respectively, which are usually associated with the formation of free excitons. As can be seen from Fig. 8, the energy position of PL band at 2.507 eV coincides with the maximum of the first exciton reflection band, i.e the energy position of free exciton (FER-1 band). The FER-1 band at 2.515 eV is slightly (12 meV) shifted to the short-wavelength region in comparison with the pure PbI2 crystal. It is evident that this is due to the presence of Mn atoms in the investigated crystals. Thus, we assume that for the investigated Pb1-XMnXI2 alloys with X=0.05 the crystal matrix presents PbI2 crystal with a small concentration of Mn atoms. This concentration can be determined using the following equation: Eex(X) = (2.5 + 1.3) eV, which was proposed for the energy position of the free exciton for Pb1-XMnXI2 crystals at T=4.2 K [42]. This allowed us to determine the concentration of Mn atoms in the PbI2 matrix which is approximately 1%. It should be noted that the exciton reflection FER-1 band is broadened, which may be produced by overlapping exciton reflection bands associated with the formation of free excitons from both crystal regions of pure PbI2 and crystal regions where a Pb1-XMnXI2 solid solution with a low concentration of manganese atoms is formed (about 1%). The other PL line at 2.556 eV is shifted to the short-wavelength region in comparison with the pure PbI2 crystal. The energy shift of this line corresponds to 64 meV in comparison with that for pure PbI2 crystals. This means that there are the crystalline regions of Pb1-XMnXI2 solid solutions
13 with a higher concentration of Mn atoms which are embedded in PbI2 crystal matrix doped with small concentrations (about 1%) of Mn atoms. According to the results of the structural studies, presented in this work, such crystal regions present the nanoparticles of platelet-like shape with the sizes of several microns in the plane and the average thickness of several tens of nm. Thus, the results of PL measurements also indicate that the investigated Pb1-XMnXI2 alloys can be considered as PbI2-PbMnI2 nanocomposites. The other exciton reflection band at 2.573 eV (FER-2 band) is evidently due to the formation of free excitons in Pb1-XMnXI2 nanoparticles with a larger concentration of Mn atoms. Taking into account the energy shift of free excitons (64 meV) for Pb1-XMnXI2 crystals with X=0.05 (FER-2 band) in comparison with that for PbI2 crystals, it is possible to estimate the concentration of Mn atoms. According to the above mentioned equation, which determines the energy position of free excitons in Pb1-XMnXI2 crystals, the concentration of Mn atoms is equal to about 5.3 at. %. This value is close to that characteristic of typical crystal areas of the Pb1-XMnXI2 crystal with X=0.05 obtained as a result of EDS measurements.
14 Fig. 9 Temperature dependences of PL bands at 2.556 eV (A) and 2.500 eV (B). With the aim to determine the nature of PL line at 2.556 eV we also carried out the comparison of the energy positions of this line and the FER-2 band at 2.573 eV. It was found that the energy position of PL line at 2.556 eV coincides with the maximum of the exciton reflection FER-2 band. This situation is a characteristic feature of any semiconductor solid solution [43]. This means that in this case the emission line is evidently due to the emission of localized excitons in Pb1-XMnXI2 nanoparticles with a larger concentration of Mn atoms. The formation of the localized excitons in Pb1-XMnXI2 is due to the crystal field fluctuations. For a more detailed study of the nature of the observed exciton PL lines, their temperature dependences were measured. Fig. 9A shows the temperature dependence of the PL exciton line at 2.556 eV, which was previously identified by us as the line caused by the emission of localized excitons. As can be seen from Fig. 9A, the long-wavelength edge of this line contains another PL line at 2.54 eV indicated by an arrow. An increase in temperature leads to a decrease in the intensity of this line relative to the main line at 2.556 eV, which becomes almost symmetric at temperatures above 30 K. Such a temperature dependence of the intensities of the exciton PL lines indicates that the line at 2.54 eV is associated with emission of bound excitons in PbMnI2 nanoparticles. It should be noted that a strong decrease in the intensities of PL lines is characteristic of bound excitons in any semiconductors, where their binding energy is several meV [41]. At the same time, the main PL line at 2.556 eV is obviously caused by the emission of localized excitons in PbMnI2 nanoparticles. Such emission still remains at temperatures above 50 K. An additional confirmation of the nature of this line is that the energy position of its maximum exhibits a short-wavelength shift of 3.7 meV with increasing temperature from 4.5 to 50 K. It should be pointed out that the energy position of free excitons, obtained from the exciton reflection spectra, practically does not change in this temperature region. This indicates that the short-wavelength shift of localized excitons is due to their thermal excitation from lower to higher excited states due to a temperature increase.
15 The temperature dependence of PL line at 2.500 eV is presented in Fig. 9B. The line shows the structure. In this case, there are two components at 2.500 and 2.502 eV, indicated by the arrows for the PL spectrum at T = 25 K. They have different temperature dependencies of their intensity. As can be seen, the intensity of latter component becomes determinative at T = 30 K. It should be noted that the long-wavelength edge of this line also contains the structure caused by the presence of other lines associated with low intensity emission of other bound excitons. A further increase in temperature leads to a decrease in the PL intensity of bound excitons, which is practically not manifested at T = 50 K, which is due to their thermal dissociation and additional generation of free excitons. Thus, the PL line at 2.508 eV associated with the emission of free excitons becomes determining in the exciton spectrum in this spectral region at higher temperatures (above 40 K).
It is a very interesting task to compare the photoluminescence and reflection exciton spectra with the absorption spectrum for both pure PbI2 crystals and PbMnI2 alloys. This makes it possible to obtain additional information on the distribution of Mn atoms in the depth of the alloys, as well as on the possible reabsorption of the emission observed for the materials under study. Fig. 10A shows the absorption (curve 1), exciton reflection (curve 2) and photoluminescence (curve 3) spectra of PbI2 crystals at T=4.5 K. In the absorption spectrum an exciton line is observed. It should be noted that the energy position of this line practically coincides with that for the exciton reflection band. However, in this case, the intensity of the exciton absorption band is lower than it was expected, since it was impossible to obtain a crystal sample with a thickness of less than 1 µm for this measurement. As can be seen from Fig. 10A, the energy position of bound excitons in the PL spectrum is located at the long-wavelength edge of the absorption of free excitons. The difference between the energy positions of the exciton absorption line and that for bound excitons determines the binding energy of bound excitons. This value for the bound excitons associated with the PL band at 2.492 eV (the most intense PL exciton line) corresponds to 11 meV. The photoluminescence, exciton reflection and absorption spectra for Pb1-XMnXI2 (X=0.05) alloys are presented in Fig. 10B. In this case, we were also unable to measure the absorption
16 spectrum, including the maximum of the exciton line associated with the formation of free excitons. This is
Fig. 10 Absorption (1), exciton reflection (2) and photoluminescence (3) spectra of pure PbI2 (A) and Pb1-XMnXI2 (X=0.05) alloys (B) at T=4.5 K. due to the use of a relatively thick sample of the crystal (about 12 microns), as well as the presence of heterogeneities in the crystal structure of the studied PbMnI2 alloys. So, we have a strongly heterogeneous crystalline system where separate crystal regions of Pb1-XMnXI2 solid solutions are embedded in the PbI2 crystal matrix. We observe exciton emission from relatively homogeneous crystalline regions (nanoparticles), since the widths of exciton reflection bands (the distance between the maximum and minimum of the bands) are close to those for PbI2 crystals and crystal regions, where Pb1-XMnXI2 solid solutions are formed (19 meV and 32 meV, respectively). The larger bandwidth for Pb1-XMnXI2 (X=0.05) is due to the presence of the crystal field fluctuations for the solid solutions as a result of the inhomogeneous distribution of Mn atoms in the Pb1-XMnXI2 nanoparticles. It should be noted that for the crystals with X=0.03 the width of the exciton reflection band corresponds to 6 meV. Therefore, the homogeneity of such crystals is considerably better than for the crystals with X=0.05 due to the smaller concentration of Mn atoms.
17 The presence of heterogeneities in the crystal structure of Pb1-XMnXI2 (X=0.05) alloys causes a strong inhomogeneous broadening of the exciton line in the absorption spectrum. Unlike for the pure PbI2 crystal, the absorption minimum observed at the short-wavelength edge of the exciton absorption band is not observed for the PbMnI2 alloys with X = 0.05. So, we observe only the long-wavelength exciton absorption edge for the PbI2 crystal matrix. It should be noted that the energy position of exciton reflection band is shifted to the long-wavelength spectral region relative to the expected position of free excitons in the reflection spectrum. The magnitude of this shift can be estimated based on the energy difference at a fixed absorption value for a pure PbI2 crystal and PbMnI2 alloys. This value is 25 meV. At the same time, as shown above, the energy position of the exciton reflection band for the PbI2 crystal matrix (FER-1 band, see Fig. 8) is 13 meV higher than for pure PbI2. This means that the additional shift of the expected energy position for free excitons in the absorption spectra of Pb1-XMnXI2 nanoparticles relative to the energy position of FER-2 band is approximately 12 meV. In our opinion, this energy difference may be caused by the inhomogeneous distribution of Mn atoms in the depth of the crystal sample. According to the energy position of the FER-1 exciton reflection band, the concentration of Mn atoms on the surface of the investigated crystal sample is approximately 1%. However, the average concentration of Mn atoms in the depth of the investigated crystal sample is about 2%. Thus, for the investigated PbMnI2 alloys there is a certain dispersion in the concentration of Mn atoms both at the surface of the crystal samples and inside them. This conclusion is in good agreement with the results of the structural studies presented in this paper. One of the important issues in the elaboration of materials for modern optoelectronics, in particular, for the development of scintillation detectors or photodiodes, is the analysis of the possibility of reabsorption of emission in such materials. Recently, using photoluminescence and depth-resolved cathodoluminescence methods the emission was probed for organic-inorganic leadbased perovskites from surface and near-surface regions at a depth of several microns [44]. Here, the reabsorption effect for the CH3NH3PbBr3 single crystal is due to the difference in the optical bandgap between the surface and the bulk of the single crystals. It was shown that surface
18 photoluminescence of the crystal at a depth of several tens of nm, does not manifest reabsorption of the emission near the absorption edge [44]. In this case, the emission is symmetric and centered close to the absorption band edge. At the same time, in the case of the photoluminescence transmitted through the thicknesses of the crystal samples between 50 µm and 600 µm, the reabsorption of the emission was observed. It was found that the PL band becomes asymmetric with a sharp edge on the high-energy side and the full width at half maximum (fwhm) decreases with increasing voltage (for cathodoluminescence). Additionally, a red shift of the PL band occurs. It should be noted that the studies of PbMnI2 alloys, presented in this work, were carried out using the surface photoluminescence method, i.e. their emission was measured from the front surface of the crystal. The depth of excitation was several tens of nm. An analysis of the shape and energy position of the exciton PL lines of PbMnI2 alloys, as well as their temperature dependences, as expected, showed that the above-described signs of reabsorption observed for perovskite crystals were not observed. It is expected that the reabsorption effect in the investigated crystals can occur in the case of photoluminescence transmitted through the thickness of the crystal sample. First of all, this concerns the exciton PL line at 2.556 eV, associated with the emission of localized excitons in PbMnI2 nanoparticles, since its energy position is overlapped with the band-to-band optical transitions of PbI2 crystal matrix. In this case, it is expected that the energy will be transferred from the localized excitons to the crystal matrix. It was shown that the reabsorption process causes an extension of the decay of the transmitted PL and the presence of a long rise time [44]. It should be noted that we also studied the kinetics of photoluminescence of exciton lines in PbMnI2 crystals. Here, we used the novel analysis of kinetic dependence proposed by us in [14,15]. This allows us to obtain the distribution of lifetimes associated with the different recombination processes. It was established that the lifetimes, associated with different exciton recombination processes that cause the emission in the spectral region of 2.48 - 2.54 eV correspond to the values from 25 ps (free excitons) to several ns (bound and localized excitons). Thus, the results of such studies are an
19 additional argument for the absence of reabsorption in the exciton photoluminescence spectra of the crystals under study.
3.3 Nuclear quadrupole resonance spectra of Pb1-XMnXI2 alloys It is well known that NQR technique is very sensitive when studying various crystal deformations induced by the presence of impurity atoms in the crystal lattice. In particular, the frequency of NQR line and its width are highly dependent on such deformations. It was previously shown that this method is effective for studying the crystal structure of Pb1-XCdXI2 solid solutions in which the formation of crystalline clusters takes place [4]. In order to obtain additional information on the features of Pb1-хMnxI2 alloy formation, the I127 NQR spectra of the investigated crystals were measured at T=77 K for different Mn concentrations. The quadrupole interaction constant (QIC) and the asymmetry parameter of the electric field gradient (APEFG), namely η(х) = (qxx - qyy)/qzz, where qxx, qyy and qzz are the axes of electric field gradient tensor at I127 nuclei, were determined. In this case, we used the results of measuring the frequencies ν1 and ν2 of
127
I NQR spectrum, corresponding to the transitions ±1/2↔±3/2 and
±3/2↔±5/2, respectively. First of all, we studied pure PbI2 crystals, which have mainly a 2Hpolytype and contain a single-layer packet (3 atoms) in an elementary cell. As can be seen from Fig.9 (curve 1), in this case the NQR spectrum of 127I (± 3/2 ↔ ± 5/2 transition) contains only one line with a frequency of ν2= 8.930 MHz at T = 77K. At the same time, PbI2 crystals of 4H-polytype contain two-layer packets (6 atoms) in the elementary cell. For such crystals the NQR spectrum of 127
I (± 3/2 ↔ ± 5/2) consists of two lines with frequencies of 9.8 MHz and 10.3 MHz, which are
due to the presence of two nonequivalent positions of the 127I nucleus in the elementary cell.
20
Fig. 11 Dependence of the intensity of I127 NQR band for (± 3/2 ↔ ± 5/2) transition on the NQR frequency. It was found that the I127 NQR spectrum of PbI2 crystal contains two bands at the frequencies of ν1 (±1/2 ↔ ± 3/2) = 4.465 MHz and ν2 (± 3/2 ↔ ± 5/2) = 8.935 MHz. Using these frequency values, the QIC and the asymmetry parameter of the electric field gradient tensor were determined. It was shown that these parameters correspond to 29.778 MHz and η = 0, respectively. Therefore, we assume that the major qxx and qyy axes of electric field gradient at I127 nuclei are in the plane layer and their values do not depend on the orientation in the plane of this layer [27]. The obtained data are consistent with the results of the XRD and optical measurements and indicate that the investigated crystals have a 2H polytype structure that is most stable at room temperature. Table 2. Parameters of the 127I NQR spectra
Pb1-хMnxI2 alloys
Width of NQR line (∆υ), MHz
Frequency υ2 (±3/2 ↔ ±5/2), MHz
х = 0.0 х = 0.03 х = 0.05
0.190 0.218 0.468
8.935 8.930 8.932
Frequency υ2 (±1/2 ↔ ±3/2), MHz 4.465 4.460 4.468
QIC, MHz
APEFG (η)
29.778 29.778 29.778
0 0 0
Similar results were obtained for the Pb1-хMnxI2 alloys. They are shown in Fig. 9 for X=0.03 and X=0.05 (curves 2 and 3, respectively). As can be seen from Table 2, such parameters of the I127 NQR spectrum as frequency, QIC and η do not change in comparison with a pure PbI2 crystal. This
21 indicates that these crystals also belong to 2H-polytype. It should be noted that for the alloys the line is strongly broadened. In this case, the width of the NQR line is almost doubled. This may be due to the fact that the intralayer anisotropy breaks, since the radii of Pb2+ and Mn2+ ions differ significantly. In addition, it is evident that the broadening of the NQR line is also due to the deformations as a result of the presence of the crystal field fluctuations. Such fluctuations are caused by the formation of Pb1-хMnxI2 solid solutions. In this case, the intralayer mechanical stresses of the crystalline lattice appear, which induces the broadening of the I127 NQR line. Since the frequency of the I127 NQR line (± 3/2 ↔ ± 5/2) does not change, this means that the crystallographic position of the Mn atoms remains unchanged. In this case, Mn2+ ions preferably replace Pb2+ ions in the crystalline layers.
4. Conclusions The complex structural and optical studies of PbMnI2 alloys were carried out. It was found that the alloys mostly have a phase of 2H-polytype, where the crystal areas of Pb1-XMnXI2 solid solutions are formed. Thus, in this case, inhomogeneous PbI2 crystals exist, where the crystal regions of Pb1-XMnXI2 nanosized solid solutions are embedded in the PbI2 crystal, i.e., such crystals represent the PbI2-PbMnI2 nanocomposite. These results are supported by the PL measurements for the alloys with X=0.05, where two bands are observed in the exciton PL and reflection spectra. Thus, the spectral range of photoluminescence of such crystals extends to the short-wavelength region due to the emission of Pb1-XMnXI2 solid solutions. The bands in the exciton reflection spectrum are associated with the formation of free excitons in PbI2 crystals of 2H-polytype and in Pb1-XMnXI2 solid solutions. This conclusion coincides with the results of NQR spectra measurements, which show that Mn2+ ions preferably replace Pb2+ ions in the crystalline layers. In this case, the intralayer anisotropy is broken, since the radii of Pb2+ and Mn2+ ions differ significantly and the crystal deformation occurs. The broadening of the optical and NQR lines is due to both these deformations and the presence of the crystal field fluctuations. Such fluctuations
22 are caused by the formation of Pb1-хMnxI2 solid solutions. In this case, intralayered mechanical stresses of the crystalline lattice arise, which induces the broadening of the I127 NQR and optical lines.
Acknowledgement This work was supported by the National Academy of Sciences of Ukraine (Grant No B94). The authors are grateful to Dr. Pavlo Demchenko for the support in performing XRD measurements.
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26 Figure Captions
Fig. 1 XRD patterns of PbI2 (A) and Pb1-XMnXI2 (B) crystals (X=0.03). Fig. 2 SEM images (A) and EDS spectrum (B) of Pb1-XMnXI2 crystals (X=0.03). The spots 1-6 in SEM images indicate the areas of the crystal surface where its chemical composition has been determined. EDS spectrum corresponds to spot 4. Fig. 3 SEM images of Pb1-XMnXI2 crystals (X=0.05). The spots 1-5 in SEM images indicate the areas of the crystal surface where its chemical composition has been determined. Fig. 4 The dependence I(q) is shown in double logarithmic scale. Fig. 5 The distance distribution function p(d) of nanoparticles for Pb1-XMnXI2 (X=0.03) alloys. Fig. 6 Photoluminescence (curve 1) and exciton reflection (curve 2) spectra of PbI2 crystals. Fig. 7 Photoluminescence (curve 1) and exciton reflection (curve 2) spectra of Pb1-XMnXI2 alloys for X=0.03. Fig. 8. Photoluminescence (curve 1) and exciton reflection (curve 2) spectra of Pb1-XMnXI2 alloys with X=0.05 at T=4.5 K. Fig. 9 Temperature dependences of PL bands at 2.556 eV (A) and 2.500 eV (B). Fig. 10 Absorption (1), exciton reflection (2) and photoluminescence (3) spectra of pure PbI2 (A) and Pb1-XMnXI2 (X=0.05) alloys (B) at T=4.5 K. Fig. 11 Dependence of the intensity of I127 NQR band for (± 3/2 ↔ ± 5/2) transition on the NQR frequency.
Table Captions Table 1. The chemical composition of Pb1-XMnXI2 crystals with (X=0.03) and X=0.05 shown in Fig. 2 A and Fig. 3, respectively. Table 2. Parameters of the 127I NQR spectra
• • • • •
Structural, optical and NQR measurements of PbMnI2 alloys were carried out. The investigated crystals have a 2H-polytype phase. The crystal regions of Pb1-XMnXI2 solid solutions are embedded in PbI2 matrix. These crystal regions can be considered as platelet-shaped nanoparticles. PbMnI2 alloys are heterogeneous PbI2-PbMnI2 nanocomposites.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30348-0
DOI:
https://doi.org/10.1016/j.jallcom.2020.153985
Reference:
JALCOM 153985
To appear in:
Journal of Alloys and Compounds
Received Date: 27 September 2019 Revised Date:
18 December 2019
Accepted Date: 21 January 2020
Please cite this article as: A.P. Bukivskii, I.G. Vertegel, E.D. Chesnokov, P.M. Bukivskij, O.I. Ovcharenko, R.V. Gamernyk, Z.D. Kovalyuk, V.M. Tkach, Y.P. Gnatenko, Formation of PbMnI2 alloys: Structural, photoluminescence and nuclear quadrupole resonance studies, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153985. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
A.P. Bukivskii: Investigation, Software, Data Curation, Writing - original draft I.G. Vertegel: Investigation, Formal analysis, Validation E.D. Chesnokov: Investigation, Formal analysis P.M. Bukivskij: Investigation, Validation, Visualization O.I. Ovcharenko: Investigation, Methodology R.V. Gamernyk: Investigation, Formal analysis, Methodology Z.D. Kovalyuk: Formal analysis, Visualization, Resources V.M. Tkach: Investigation, Methodology Yu.P. Gnatenko: Conceptualization, Writing - review & editing, Supervision
1
Formation of PbMnI2 alloys: structural, photoluminescence and nuclear quadrupole resonance studies A.P. Bukivskii1, I.G. Vertegel1, E.D. Chesnokov1, P.M. Bukivskij1, O.I. Ovcharenko1, R.V. Gamernyk2, Z.D. Kovalyuk3, V.M. Tkach4 and Yu.P. Gnatenko1a
1
Institute of Physics of NASU, 03028 Kyiv, Nauky ave. 46, Ukraine Lviv National University, 29005 Lviv, Kyrylo and Mefodiy str. 8, Ukraine 3 Chernivtsi department of Institute of Problem of Material Sciences of NASU, 58001 Chernivtsi, Iryny Vilde ave. 5, Ukraine 4 Institute for Superhard Materials of NASU, 2 Avtozavodska Str., Kyiv, 04074, Ukraine 2
a
Corresponding author: Yuriy P. Gnatenko: [email protected]
Abstract The structural, optical and nuclear quadrupole resonance (NQR) measurements of PbMnI2 alloys were carried out. This allows us to study the peculiarities of the formation of such alloys. It was found that pure PbI2 and Pb1-XMnXI2 crystals with X = (0.03-0.05) have the phase of 2H-polytype. It was shown that the investigated alloys can be presented as the crystal regions of Pb1-XMnXI2 solid solutions, embedded in PbI2 crystal matrix. These formations can be considered as platelet-shaped nanoparticles with a surface of several microns and a thickness of several tens of nm. Thus, PbMnI2 alloys are heterogeneous PbI2-PbMnI2 nanocomposites. The results of NQR spectra measurements also show that Mn2+ ions preferably replace Pb2+ ions in the crystalline layers. Here, the local deformations occur because the radii of Pb2+ and Mn2+ ions differ significantly. The broadening of optical and NQR lines is due to the presence of both these deformations and the crystal field fluctuations, which are characteristic of semiconductor solid solutions.
Keywords: semiconductors; nanostructured materials.
disordered
systems;
photoluminescence;
crystal
structure;
• Structural, optical and NQR measurements of PbMnI2 alloys were carried out. • The investigated crystals have a 2H-polytype phase. • The crystal regions of Pb1-XMnXI2 solid solutions are embedded in PbI2 matrix. • These crystal regions can be considered as platelet-shaped nanoparticles. • PbMnI2 alloys are heterogeneous PbI2-PbMnI2 nanocomposites.
1. Introduction The semiconductor PbI2 crystals present considerable interest since they are used for elaboration of high-sensitive non-cooled radiation detector materials for x- and γ-rays [1-3]. PbI2 has a type of layered structure with a repeating unit of a hexagonal close packed layer of Pb2+ sandwiched between two layers of I− in the crystal [4]. It should be noted that at present, the PbI2
2 semiconductor is considered as ideal scintillator, since this material can provide a combination of high light output and short decay time [5,6]. At the same time, in order to realize the great potential of these crystals as scintillation materials, it is necessary to improve their optical and electronic properties. It is well known that an effective method of improving the physical properties of crystals is their doping with different impurities, which leads to the formation of nano- or submicron-sized clusters in the crystal matrix. Recently, it was shown that the growth of Pb1-XCdXI2 alloys is accompanied by the formation of nanostructured materials, where nanoclusters of PbI2 are embedded in the CdI2 crystal matrix. This is due to the fact that Pb1-XCdXI2 alloy is a nonisoelectronic system because the valence electrons of Pb and Cd atoms belong to different electronic configurations, namely (5d106s26p2) and (4d105s2), respectively [7,8]. It was found that this substitution peculiarity of cation sites is a reason for the formation of bulk nanostructured Pb0.30Cd0.70I2 alloys and may be caused by spinodal decomposition of these alloys [9]. It was found that these materials exhibit intense photoluminescence (PL) and luminescence excited by X-rays (XRL) at room temperature [10-14]. Photoluminescence lifetime studies of PbI2 nanoclusters and microcrystallites in Pb0.30Cd0.70I2 alloys have been performed in [11,14,15]. The formation of various cluster structures at the micro/nano scale has also been observed for other layered semiconductors, namely, CdI2 crystals having the same crystal structure as PbI2 crystals [16,17]. It was shown that for CdI2-BiI3 alloys, the clusters consisting of two BiI3 molecules and a cadmium vacancy are formed since cadmium and bismuth ions are non-isovalent. Transfer of Bi3+ ions to the dislocation borders and the interaction between Bi3+ ions, vacancies and anion sublattice may lead to the appearance of BiI3 microphase containing large clusters. Besides, the formation of the cadmium clusters was observed for non-stoichiometric pure crystals CdI2 as a result of long-term effects of aging [18-20]. It is evident that the layered solid solutions, based on the PbI2 or CdI2 crystals, may exhibit features related to the formation of nanostructured materials. Thus, it is very important to study the
3 peculiarities of the formation of Pb1-XMnXI2 alloys containing Mn magnetic atoms. The use of Pb1XMnXI2
alloys can expand the luminescence spectral range of such materials and thus improve their
scintillator characteristics [21]. Besides, PbI2 crystals doped with magnetic atoms are of interest for the development of novel materials – layered diluted magnetic semiconductors. Earlier, the studies of magnetic and magneto-optic properties of Pb1-XMnXI2 alloys were performed which showed that they have very interesting magnetic properties [22-26]. In particular, strong spin exchange ferromagnetic interactions due to the presence of magnetic ions in the PbMnI2 crystals were found [23]. The antiferromagnetic character of the exchange interaction between Mn ions and holes of the valence band was observed in [25]. However, the crystalline structure of PbMnI2 alloys and its effect on the photoluminescent properties of such materials have not yet been sufficiently studied. As for the NQR spectra, they were previously studied for PbCdI2 and PbI2-BiI3 alloys [4,27-31]. In this paper, we report the results of the comprehensive structural (XRD, SEM, EDAX, SAXS), optical (absorption, photoluminescence and reflection spectra) and nuclear quadrupole resonance measurements of PbMnI2 alloys. This allowed us for the first time to study the peculiarities of the formation of such alloys. It was shown that the investigated alloys present the crystal regions of Pb1-XMnXI2 solid solutions with micron-sized surfaces and thickness of several tens of nm distributed heterogeneously in the PbI2 crystal. Thus, PbMnI2 alloys are heterogeneous nanocomposites. The spectral range of photoluminescence of such alloys extends to the shortwavelength region due to the emission of Pb1-XMnXI2 nanoparticles.
2. Experimental details The PbI2 and Pb1-XMnXI2 crystals were grown by the Bridgman technique. The crystals were synthesized by direct alloying of the constituents in sealed quartz ampoules under vacuum. The impurity Mn atoms were introduced by adding them to the melt and the Mn concentration in the studied crystals corresponds to 0 ≤ X ≤ 0.05. The elements were purified by vacuum sublimation and were carefully weighed to obtain the desired impurity concentration.
4 The crystal quality and phases of PbMnI2 alloys were verified by X-ray diffraction experiments (XRD) using a Shimadzu LabX XRD-6000 diffractometer with Kα radiation of Cu (λ = 0.154178 nm). The scanning was performed with scattering angle (2θ) from 10° up to 90° with a step of 0.02°. Qualitative phase analysis of the diffractogram was carried out using the ICDD database, PDF-2 Release 2012. A high-brilliance synchrotron P12 beamline of the EMBL located at the PETRA III storage ring (DESY, Hamburg) was used for small-angle X-ray scattering (SAXS) [32,33]. This method enables one to investigate the size distribution function of nanoparticles, which can be randomly embedded in a nanocomposite system. The investigation of the surface structure of the Pb1-XMnXI2 crystals was performed using a Zeiss EVO 50 XVP scanning electron microscope with an X-Ray dispersion analysis module in phase-contrast regime. This allowed us to obtain not only a visualization of the crystal structure morphology, but also to determine the chemical composition of such areas. Thus, this made it possible to observe precisely the areas where Pb1-XMnXI2 solid solutions are formed in the PbI2 matrix. An SDL-1 grating spectrometer was used to measure the PL spectra [34-37]. The excitation intensity of an LGN-404 argon laser with 488.8 nm emission line was kept below 100 mW/cm2 to avoid heating the sample. The samples were mounted on the cold finger of a variable temperature liquid-helium cryostat. The spectral resolution of the system was of the order of (0.1 – 0.2) meV. The nuclear quadrupole resonance (NQR) spectra of I127 for the investigated crystals were measured at 77 K in the frequency range 5-300 MHz using a pulsed quasi-coherent NQR spin-echo spectrometer. The accuracy of the NQR frequency measurements was determined by the halfwidths of the NQR lines, and for the investigated crystals this value was no worse than ± 25 kHz.
3. Results and discussion 3.1 Crystal characterization of Pb1-XMnXI2 alloys
5 The XRD patterns of both pure PbI2 and Pb1-XMnXI2 crystals are presented in Fig. 1. The analysis of the patterns shows that the Pb1-XMnXI2 crystal with X=0.03 as well as the pure PbI2 crystal have a single phase of 2H-polytype. A small decrease in the crystal lattice parameters for Pb1-XMnXI2 is observed, which may be caused by the replacement of Pb2+ ions by Mn2+ ions with ionic radii of 1.26 Ǻ and 0.91 Ǻ, respectively. The lattice “a” and “c” parameters for pure PbI2 crystals correspond to 4.561 Ǻ and 6.989 Ǻ, respectively. At the same time, these values for Pb1XMnXI2
crystals are equal to 4.557 Ǻ and 6.981 Ǻ. Thus, the formation of Pb1-XMnXI2 solid
solutions occurs.
Fig. 1 XRD patterns of PbI2 (A) and Pb1-XMnXI2 (B) crystals (X=0.03).
6 This conclusion is supported by the results of a surface morphology study of Pb1-XMnXI2 crystals performed using a Zeiss EVO 50 XVP scanning electron microscope with an X-Ray dispersion analysis module at room temperature shown in Fig. 2A. The surface morphology of the studied crystals was obtained in the phase contrast mode. In this case, there are areas with different brightness. Therefore, it is very important to determine the local chemical composition of the crystal for different surface spots. These results are shown in Fig. 2 and Table 1.
Fig. 2 SEM images (A) and EDS spectrum (B) of Pb1-XMnXI2 crystals (X=0.03). The spots 1-6 in SEM images indicate the areas of the crystal surface, where its chemical composition was determined. The EDS spectrum corresponds to spot 4. Table 1. The chemical composition of Pb1-XMnXI2 crystals with (X=0.03) and X=0.05 shown in Fig. 2 A and Fig. 3, respectively.
Spot № 1 2 3 4 5 6
Pb1-XMnXI2 Х = 0.03 Mn, at% I, at% 0.97 0.90 0 8.53 1.66 0.99
67.61 70.46 68.12 64.99 66.51 68.04
Pb, at% 31.42 28.64 31.88 26.48 31.83 30.97
Spot № 1 2 3 4 5
Pb1-XMnXI2 Х = 0.05 Mn, at% I, at% Pb, at% 1.08 13.28 4.55 1.62 5.06
67.88 62.41 71.97 73.34 64.79
31.04 24.31 23.48 25.04 30.15
7 As can be seen from Table 1, the chemical analysis of Pb1-XMnXI2 crystals (X=0.03) for spots 1,2 and 5, which are the bright areas, shows that they contain a small concentration of Mn element (smaller than two percent). It should be noted that visually the brightness of these areas is approximately the same. However, their chemical composition for spots 1 (or 2) and 5 is different. It should be noted that the depth of X-ray photons penetration is about 1.0 micron. Thus, we assume that different Mn concentrations for different spots can be caused by the inhomogeneous composition of the crystal both at the surface and in depth, i.e. can be a result of the existence of crystal layers of various composition, located parallel to the substrate surface. Apparently, these areas are associated with the formation of the Pb1-XMnXI2 solid solutions. In this case, there exists an inhomogeneous PbI2 crystal, where the areas of the solid solutions of micron sizes are embedded in the PbI2 crystal, i.e. such crystals are a PbI2-PbMnI2 composite. It should be noted that the presented SEM images are similar for the different regions of the same crystal. Thus, this makes it possible to extrapolate information based on the presented SEM images to the whole crystal. The brightest area marked as spot 3 corresponds to pure PbI2 crystal. The size of this area is considerable. There are also other areas that are typical of this crystal dark gray color, where the concentration of Mn atoms corresponds to about ten percent. Such an area is shown in Fig. 2 A as spot 4. They are rarely found in the crystals. It is expected that the
Fig. 3 SEM images of Pb1-XMnXI2 crystals (X=0.05). The spots 1-5 in SEM images indicate the areas of the crystal surface where its chemical composition has been determined.
8 formation of both Pb1-XMnXI2 solid solutions and, possibly, MnI2 compound can be characteristic of these areas. It should be noted that for such crystal areas the concentration of Pb element decreases. Fig. 3 shows SEM images of the Pb1-XMnXI2 crystal surface (X=0.05). In comparison with the Pb1-XMnXI2 crystal with X=0.03 there are many areas of dark gray color here, where the concentration of the Mn element corresponds to about 5 percent. This means that for this crystal the area size of Pb1-XMnXI2 solid solution is larger than for the crystal with X=0.03. In order to obtain information on the structure of inhomogeneities (their shape and sizes) of the investigated materials we carried out small-angle X-ray scattering (SAXS) measurements [37]. In this case, the study of the dependence of X-ray intensity on the wave vector q, namely I(q), where q = 4πsinθ / λ (θ is half the scattering angle, λ is the x-ray wavelength) was carried out. It should be noted that such a dependence can be described by the following equation: I(q) = q-D [38]. The values of D = 1, 2 and 3 indicate that the scattering objects are particles of rod, disk and spherical shape, respectively. It should be noted that for the surface fractals their dimension varies within 2 < Ds <3 and is calculated by the formula Ds= 6 – D [39].
Fig. 4 The dependence I(q)= q-D shown on a double logarithmic scale.
In Fig. 4 the dependence I(q) is shown on a double logarithmic scale to emphasize the
power law behavior. This allowed us to determine the value of Ds, which for alloys with X = 0.03 corresponds to 2.1. This means that the structural inhomogeneities (in our case crystal regions, where Pb1-XMnXI2 solid solutions are formed) have the platelet-like shape. For the crystals with X=0.05 the value Ds equals to 2.35, which indicates the fractal geometry of the objects. It should be
9 noted that the analysis of the dependence I(q) also allows us to obtain information on the size distribution
Fig. 5 The distance distribution function p(d) of nanoparticles for Pb1-XMnXI2 (X=0.03) alloys.
of the particles, namely to determine the distance distribution function f(d), where d is the diameter of the particles of spherical shape or the thickness of the platelet-like particles. In Fig. 5 the distance distribution function f(d) is presented for the alloys with X=0.03. As can be seen from Fig. 5, this distribution contains maximums at d equal to 14 nm, 36 nm and 69 nm, which can be associated with the average sizes of the crystal regions where Pb1-XMnXI2 solid solutions are formed. Taking into account the results of SEM image studies we can assume that such crystal regions present the formations of platelet-like or fractal shape with the sizes of several microns in the plane and the average thickness of several tens of nm, i.e. are nanoparticles. Therefore, the studied alloys Pb1XMnXI2
can be considered as PbI2-PbMnI2 nanocomposites.
3.2 Photoluminescence spectra of Pb1-XMnXI2 alloys Fig. 6 (curve 1) shows a PL spectrum of pure PbI2 crystals at T=4.5 K which have mainly 2H-polytype [40]. In this case the exciton PL spectrum contains several lines at 2.497 eV, 2.492 eV, 2.484 eV, 2.472 eV and 2.459 eV. These lines correspond to the bound excitons and their phonon replicas with the participation of low-frequency optical phonons [40]. In particular, the intense line
10
Fig. 6 Photoluminescence (curve 1) and exciton reflection (curve 2) spectra of PbI2 crystals.
at 2.492 eV corresponds to the emission of excitons bound with the neutral donors, i.e., (DoX)-line for PbI2 crystal of 2H-modification. The line at 2.497 eV corresponds to the emission of other bound excitons in PbI2 crystal of 2H-polytype. The weak line at 2.503 eV may be associated with the exciton line in PbI2 crystal of 4H-polytype or correspond to the emission of free excitons from the lower polaritons. The latter assumption is supported by the observation of free excitons for this crystal in the exciton reflection spectrum at 2.503 eV, which is shown in Fig. 6 (curve 2). In addition, two more intense broad lines at 2.435 eV and 2.381 eV are observed, which are associated with the emission of donor-acceptor pairs with the participation of two different acceptor levels. In this case, the phonon replicas of these zero-phonon lines are also observed.
11 Fig. 7 Photoluminescence (curve 1) and exciton reflection (curve 2) spectra of Pb1-XMnXI2 alloys for X=0.03. The analysis of the PL spectrum of Pb1-XMnXI2 crystals for X=0.03, shown in Fig. 7 (curve 1), reveals free and bound exciton lines characteristic of PbI2 crystal of 2H-polytype. The formation of free excitons in the investigated crystal is also observed in the exciton reflection spectrum in Fig. 7 (curve 2). The most intense PL line at 2.436 eV is due to the emission of donor-acceptor pairs. In addition, a new line at 2.546 eV was found in the short-wavelength region. Since the energy position of this line is larger than the band gap of PbI2 crystal, it may be caused by the exciton emission in Pb1-XMnXI2 solid solutions, which, accordingly to the SEM images and EDX measurements, present the crystal regions with micron-sized surfaces distributed heterogeneously in PbI2 crystal.
Fig. 8. Photoluminescence (curve 1) and exciton reflection (curve 2) spectra of Pb1-XMnXI2 alloys with X=0.05 at T=4.5 K. As can be seen from Fig. 8, the PL spectrum (curve 1) of Pb1-XMnXI2 crystals with X=0.05 contains two intense exciton lines at 2.500 eV and 2.556 eV, as well as a very intense line at 2.436 eV, which corresponds to the radiative recombination of donor-acceptor pairs. A comparison of this PL spectrum with the spectrum of a pure PbI2 crystal (see Fig. 6) shows that there is no PL band at 2.381 eV due to the emission of donor-acceptor pairs with the participation of a deeper acceptor center. It is possible that such acceptor center is associated with lead vacancy. Thus, due to the doping of the PbI2 crystal with Mn atoms, the number of lead vacancies decreases.
12 It should be noted that the energy position of the exciton line at 2.500 eV is close to that of bound excitons (DoX-line) for PbI2 crystals of 2H-polytype. The long-wavelength edge of this line shows a low intensity line at 2.492 eV, which may be associated with the phonon replica of DoXline. The short-wavelength edge of DoX-line also contains a line of low intensity at 2.507 eV, which may be caused by the emission of bound excitons characteristic of PbI2 crystals of 4H-polytype. On the other hand, this line can be associated with free exciton emission of PbI2 crystals of 2H-polytype from the lower (transverse or XT) polariton branch [41]. This assumption follows from a comparison of the energy positions of PL and exciton reflection bands. It should be noted that the exciton reflection spectrum, presented in Fig.8 (curve 2), shows two bands at 2.515 eV and 2.573 eV, marked as FER-1 and FER-2 bands, respectively, which are usually associated with the formation of free excitons. As can be seen from Fig. 8, the energy position of PL band at 2.507 eV coincides with the maximum of the first exciton reflection band, i.e the energy position of free exciton (FER-1 band). The FER-1 band at 2.515 eV is slightly (12 meV) shifted to the short-wavelength region in comparison with the pure PbI2 crystal. It is evident that this is due to the presence of Mn atoms in the investigated crystals. Thus, we assume that for the investigated Pb1-XMnXI2 alloys with X=0.05 the crystal matrix presents PbI2 crystal with a small concentration of Mn atoms. This concentration can be determined using the following equation: Eex(X) = (2.5 + 1.3) eV, which was proposed for the energy position of the free exciton for Pb1-XMnXI2 crystals at T=4.2 K [42]. This allowed us to determine the concentration of Mn atoms in the PbI2 matrix which is approximately 1%. It should be noted that the exciton reflection FER-1 band is broadened, which may be produced by overlapping exciton reflection bands associated with the formation of free excitons from both crystal regions of pure PbI2 and crystal regions where a Pb1-XMnXI2 solid solution with a low concentration of manganese atoms is formed (about 1%). The other PL line at 2.556 eV is shifted to the short-wavelength region in comparison with the pure PbI2 crystal. The energy shift of this line corresponds to 64 meV in comparison with that for pure PbI2 crystals. This means that there are the crystalline regions of Pb1-XMnXI2 solid solutions
13 with a higher concentration of Mn atoms which are embedded in PbI2 crystal matrix doped with small concentrations (about 1%) of Mn atoms. According to the results of the structural studies, presented in this work, such crystal regions present the nanoparticles of platelet-like shape with the sizes of several microns in the plane and the average thickness of several tens of nm. Thus, the results of PL measurements also indicate that the investigated Pb1-XMnXI2 alloys can be considered as PbI2-PbMnI2 nanocomposites. The other exciton reflection band at 2.573 eV (FER-2 band) is evidently due to the formation of free excitons in Pb1-XMnXI2 nanoparticles with a larger concentration of Mn atoms. Taking into account the energy shift of free excitons (64 meV) for Pb1-XMnXI2 crystals with X=0.05 (FER-2 band) in comparison with that for PbI2 crystals, it is possible to estimate the concentration of Mn atoms. According to the above mentioned equation, which determines the energy position of free excitons in Pb1-XMnXI2 crystals, the concentration of Mn atoms is equal to about 5.3 at. %. This value is close to that characteristic of typical crystal areas of the Pb1-XMnXI2 crystal with X=0.05 obtained as a result of EDS measurements.
14 Fig. 9 Temperature dependences of PL bands at 2.556 eV (A) and 2.500 eV (B). With the aim to determine the nature of PL line at 2.556 eV we also carried out the comparison of the energy positions of this line and the FER-2 band at 2.573 eV. It was found that the energy position of PL line at 2.556 eV coincides with the maximum of the exciton reflection FER-2 band. This situation is a characteristic feature of any semiconductor solid solution [43]. This means that in this case the emission line is evidently due to the emission of localized excitons in Pb1-XMnXI2 nanoparticles with a larger concentration of Mn atoms. The formation of the localized excitons in Pb1-XMnXI2 is due to the crystal field fluctuations. For a more detailed study of the nature of the observed exciton PL lines, their temperature dependences were measured. Fig. 9A shows the temperature dependence of the PL exciton line at 2.556 eV, which was previously identified by us as the line caused by the emission of localized excitons. As can be seen from Fig. 9A, the long-wavelength edge of this line contains another PL line at 2.54 eV indicated by an arrow. An increase in temperature leads to a decrease in the intensity of this line relative to the main line at 2.556 eV, which becomes almost symmetric at temperatures above 30 K. Such a temperature dependence of the intensities of the exciton PL lines indicates that the line at 2.54 eV is associated with emission of bound excitons in PbMnI2 nanoparticles. It should be noted that a strong decrease in the intensities of PL lines is characteristic of bound excitons in any semiconductors, where their binding energy is several meV [41]. At the same time, the main PL line at 2.556 eV is obviously caused by the emission of localized excitons in PbMnI2 nanoparticles. Such emission still remains at temperatures above 50 K. An additional confirmation of the nature of this line is that the energy position of its maximum exhibits a short-wavelength shift of 3.7 meV with increasing temperature from 4.5 to 50 K. It should be pointed out that the energy position of free excitons, obtained from the exciton reflection spectra, practically does not change in this temperature region. This indicates that the short-wavelength shift of localized excitons is due to their thermal excitation from lower to higher excited states due to a temperature increase.
15 The temperature dependence of PL line at 2.500 eV is presented in Fig. 9B. The line shows the structure. In this case, there are two components at 2.500 and 2.502 eV, indicated by the arrows for the PL spectrum at T = 25 K. They have different temperature dependencies of their intensity. As can be seen, the intensity of latter component becomes determinative at T = 30 K. It should be noted that the long-wavelength edge of this line also contains the structure caused by the presence of other lines associated with low intensity emission of other bound excitons. A further increase in temperature leads to a decrease in the PL intensity of bound excitons, which is practically not manifested at T = 50 K, which is due to their thermal dissociation and additional generation of free excitons. Thus, the PL line at 2.508 eV associated with the emission of free excitons becomes determining in the exciton spectrum in this spectral region at higher temperatures (above 40 K).
It is a very interesting task to compare the photoluminescence and reflection exciton spectra with the absorption spectrum for both pure PbI2 crystals and PbMnI2 alloys. This makes it possible to obtain additional information on the distribution of Mn atoms in the depth of the alloys, as well as on the possible reabsorption of the emission observed for the materials under study. Fig. 10A shows the absorption (curve 1), exciton reflection (curve 2) and photoluminescence (curve 3) spectra of PbI2 crystals at T=4.5 K. In the absorption spectrum an exciton line is observed. It should be noted that the energy position of this line practically coincides with that for the exciton reflection band. However, in this case, the intensity of the exciton absorption band is lower than it was expected, since it was impossible to obtain a crystal sample with a thickness of less than 1 µm for this measurement. As can be seen from Fig. 10A, the energy position of bound excitons in the PL spectrum is located at the long-wavelength edge of the absorption of free excitons. The difference between the energy positions of the exciton absorption line and that for bound excitons determines the binding energy of bound excitons. This value for the bound excitons associated with the PL band at 2.492 eV (the most intense PL exciton line) corresponds to 11 meV. The photoluminescence, exciton reflection and absorption spectra for Pb1-XMnXI2 (X=0.05) alloys are presented in Fig. 10B. In this case, we were also unable to measure the absorption
16 spectrum, including the maximum of the exciton line associated with the formation of free excitons. This is
Fig. 10 Absorption (1), exciton reflection (2) and photoluminescence (3) spectra of pure PbI2 (A) and Pb1-XMnXI2 (X=0.05) alloys (B) at T=4.5 K. due to the use of a relatively thick sample of the crystal (about 12 microns), as well as the presence of heterogeneities in the crystal structure of the studied PbMnI2 alloys. So, we have a strongly heterogeneous crystalline system where separate crystal regions of Pb1-XMnXI2 solid solutions are embedded in the PbI2 crystal matrix. We observe exciton emission from relatively homogeneous crystalline regions (nanoparticles), since the widths of exciton reflection bands (the distance between the maximum and minimum of the bands) are close to those for PbI2 crystals and crystal regions, where Pb1-XMnXI2 solid solutions are formed (19 meV and 32 meV, respectively). The larger bandwidth for Pb1-XMnXI2 (X=0.05) is due to the presence of the crystal field fluctuations for the solid solutions as a result of the inhomogeneous distribution of Mn atoms in the Pb1-XMnXI2 nanoparticles. It should be noted that for the crystals with X=0.03 the width of the exciton reflection band corresponds to 6 meV. Therefore, the homogeneity of such crystals is considerably better than for the crystals with X=0.05 due to the smaller concentration of Mn atoms.
17 The presence of heterogeneities in the crystal structure of Pb1-XMnXI2 (X=0.05) alloys causes a strong inhomogeneous broadening of the exciton line in the absorption spectrum. Unlike for the pure PbI2 crystal, the absorption minimum observed at the short-wavelength edge of the exciton absorption band is not observed for the PbMnI2 alloys with X = 0.05. So, we observe only the long-wavelength exciton absorption edge for the PbI2 crystal matrix. It should be noted that the energy position of exciton reflection band is shifted to the long-wavelength spectral region relative to the expected position of free excitons in the reflection spectrum. The magnitude of this shift can be estimated based on the energy difference at a fixed absorption value for a pure PbI2 crystal and PbMnI2 alloys. This value is 25 meV. At the same time, as shown above, the energy position of the exciton reflection band for the PbI2 crystal matrix (FER-1 band, see Fig. 8) is 13 meV higher than for pure PbI2. This means that the additional shift of the expected energy position for free excitons in the absorption spectra of Pb1-XMnXI2 nanoparticles relative to the energy position of FER-2 band is approximately 12 meV. In our opinion, this energy difference may be caused by the inhomogeneous distribution of Mn atoms in the depth of the crystal sample. According to the energy position of the FER-1 exciton reflection band, the concentration of Mn atoms on the surface of the investigated crystal sample is approximately 1%. However, the average concentration of Mn atoms in the depth of the investigated crystal sample is about 2%. Thus, for the investigated PbMnI2 alloys there is a certain dispersion in the concentration of Mn atoms both at the surface of the crystal samples and inside them. This conclusion is in good agreement with the results of the structural studies presented in this paper. One of the important issues in the elaboration of materials for modern optoelectronics, in particular, for the development of scintillation detectors or photodiodes, is the analysis of the possibility of reabsorption of emission in such materials. Recently, using photoluminescence and depth-resolved cathodoluminescence methods the emission was probed for organic-inorganic leadbased perovskites from surface and near-surface regions at a depth of several microns [44]. Here, the reabsorption effect for the CH3NH3PbBr3 single crystal is due to the difference in the optical bandgap between the surface and the bulk of the single crystals. It was shown that surface
18 photoluminescence of the crystal at a depth of several tens of nm, does not manifest reabsorption of the emission near the absorption edge [44]. In this case, the emission is symmetric and centered close to the absorption band edge. At the same time, in the case of the photoluminescence transmitted through the thicknesses of the crystal samples between 50 µm and 600 µm, the reabsorption of the emission was observed. It was found that the PL band becomes asymmetric with a sharp edge on the high-energy side and the full width at half maximum (fwhm) decreases with increasing voltage (for cathodoluminescence). Additionally, a red shift of the PL band occurs. It should be noted that the studies of PbMnI2 alloys, presented in this work, were carried out using the surface photoluminescence method, i.e. their emission was measured from the front surface of the crystal. The depth of excitation was several tens of nm. An analysis of the shape and energy position of the exciton PL lines of PbMnI2 alloys, as well as their temperature dependences, as expected, showed that the above-described signs of reabsorption observed for perovskite crystals were not observed. It is expected that the reabsorption effect in the investigated crystals can occur in the case of photoluminescence transmitted through the thickness of the crystal sample. First of all, this concerns the exciton PL line at 2.556 eV, associated with the emission of localized excitons in PbMnI2 nanoparticles, since its energy position is overlapped with the band-to-band optical transitions of PbI2 crystal matrix. In this case, it is expected that the energy will be transferred from the localized excitons to the crystal matrix. It was shown that the reabsorption process causes an extension of the decay of the transmitted PL and the presence of a long rise time [44]. It should be noted that we also studied the kinetics of photoluminescence of exciton lines in PbMnI2 crystals. Here, we used the novel analysis of kinetic dependence proposed by us in [14,15]. This allows us to obtain the distribution of lifetimes associated with the different recombination processes. It was established that the lifetimes, associated with different exciton recombination processes that cause the emission in the spectral region of 2.48 - 2.54 eV correspond to the values from 25 ps (free excitons) to several ns (bound and localized excitons). Thus, the results of such studies are an
19 additional argument for the absence of reabsorption in the exciton photoluminescence spectra of the crystals under study.
3.3 Nuclear quadrupole resonance spectra of Pb1-XMnXI2 alloys It is well known that NQR technique is very sensitive when studying various crystal deformations induced by the presence of impurity atoms in the crystal lattice. In particular, the frequency of NQR line and its width are highly dependent on such deformations. It was previously shown that this method is effective for studying the crystal structure of Pb1-XCdXI2 solid solutions in which the formation of crystalline clusters takes place [4]. In order to obtain additional information on the features of Pb1-хMnxI2 alloy formation, the I127 NQR spectra of the investigated crystals were measured at T=77 K for different Mn concentrations. The quadrupole interaction constant (QIC) and the asymmetry parameter of the electric field gradient (APEFG), namely η(х) = (qxx - qyy)/qzz, where qxx, qyy and qzz are the axes of electric field gradient tensor at I127 nuclei, were determined. In this case, we used the results of measuring the frequencies ν1 and ν2 of
127
I NQR spectrum, corresponding to the transitions ±1/2↔±3/2 and
±3/2↔±5/2, respectively. First of all, we studied pure PbI2 crystals, which have mainly a 2Hpolytype and contain a single-layer packet (3 atoms) in an elementary cell. As can be seen from Fig.9 (curve 1), in this case the NQR spectrum of 127I (± 3/2 ↔ ± 5/2 transition) contains only one line with a frequency of ν2= 8.930 MHz at T = 77K. At the same time, PbI2 crystals of 4H-polytype contain two-layer packets (6 atoms) in the elementary cell. For such crystals the NQR spectrum of 127
I (± 3/2 ↔ ± 5/2) consists of two lines with frequencies of 9.8 MHz and 10.3 MHz, which are
due to the presence of two nonequivalent positions of the 127I nucleus in the elementary cell.
20
Fig. 11 Dependence of the intensity of I127 NQR band for (± 3/2 ↔ ± 5/2) transition on the NQR frequency. It was found that the I127 NQR spectrum of PbI2 crystal contains two bands at the frequencies of ν1 (±1/2 ↔ ± 3/2) = 4.465 MHz and ν2 (± 3/2 ↔ ± 5/2) = 8.935 MHz. Using these frequency values, the QIC and the asymmetry parameter of the electric field gradient tensor were determined. It was shown that these parameters correspond to 29.778 MHz and η = 0, respectively. Therefore, we assume that the major qxx and qyy axes of electric field gradient at I127 nuclei are in the plane layer and their values do not depend on the orientation in the plane of this layer [27]. The obtained data are consistent with the results of the XRD and optical measurements and indicate that the investigated crystals have a 2H polytype structure that is most stable at room temperature. Table 2. Parameters of the 127I NQR spectra
Pb1-хMnxI2 alloys
Width of NQR line (∆υ), MHz
Frequency υ2 (±3/2 ↔ ±5/2), MHz
х = 0.0 х = 0.03 х = 0.05
0.190 0.218 0.468
8.935 8.930 8.932
Frequency υ2 (±1/2 ↔ ±3/2), MHz 4.465 4.460 4.468
QIC, MHz
APEFG (η)
29.778 29.778 29.778
0 0 0
Similar results were obtained for the Pb1-хMnxI2 alloys. They are shown in Fig. 9 for X=0.03 and X=0.05 (curves 2 and 3, respectively). As can be seen from Table 2, such parameters of the I127 NQR spectrum as frequency, QIC and η do not change in comparison with a pure PbI2 crystal. This
21 indicates that these crystals also belong to 2H-polytype. It should be noted that for the alloys the line is strongly broadened. In this case, the width of the NQR line is almost doubled. This may be due to the fact that the intralayer anisotropy breaks, since the radii of Pb2+ and Mn2+ ions differ significantly. In addition, it is evident that the broadening of the NQR line is also due to the deformations as a result of the presence of the crystal field fluctuations. Such fluctuations are caused by the formation of Pb1-хMnxI2 solid solutions. In this case, the intralayer mechanical stresses of the crystalline lattice appear, which induces the broadening of the I127 NQR line. Since the frequency of the I127 NQR line (± 3/2 ↔ ± 5/2) does not change, this means that the crystallographic position of the Mn atoms remains unchanged. In this case, Mn2+ ions preferably replace Pb2+ ions in the crystalline layers.
4. Conclusions The complex structural and optical studies of PbMnI2 alloys were carried out. It was found that the alloys mostly have a phase of 2H-polytype, where the crystal areas of Pb1-XMnXI2 solid solutions are formed. Thus, in this case, inhomogeneous PbI2 crystals exist, where the crystal regions of Pb1-XMnXI2 nanosized solid solutions are embedded in the PbI2 crystal, i.e., such crystals represent the PbI2-PbMnI2 nanocomposite. These results are supported by the PL measurements for the alloys with X=0.05, where two bands are observed in the exciton PL and reflection spectra. Thus, the spectral range of photoluminescence of such crystals extends to the short-wavelength region due to the emission of Pb1-XMnXI2 solid solutions. The bands in the exciton reflection spectrum are associated with the formation of free excitons in PbI2 crystals of 2H-polytype and in Pb1-XMnXI2 solid solutions. This conclusion coincides with the results of NQR spectra measurements, which show that Mn2+ ions preferably replace Pb2+ ions in the crystalline layers. In this case, the intralayer anisotropy is broken, since the radii of Pb2+ and Mn2+ ions differ significantly and the crystal deformation occurs. The broadening of the optical and NQR lines is due to both these deformations and the presence of the crystal field fluctuations. Such fluctuations
22 are caused by the formation of Pb1-хMnxI2 solid solutions. In this case, intralayered mechanical stresses of the crystalline lattice arise, which induces the broadening of the I127 NQR and optical lines.
Acknowledgement This work was supported by the National Academy of Sciences of Ukraine (Grant No B94). The authors are grateful to Dr. Pavlo Demchenko for the support in performing XRD measurements.
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26 Figure Captions
Fig. 1 XRD patterns of PbI2 (A) and Pb1-XMnXI2 (B) crystals (X=0.03). Fig. 2 SEM images (A) and EDS spectrum (B) of Pb1-XMnXI2 crystals (X=0.03). The spots 1-6 in SEM images indicate the areas of the crystal surface where its chemical composition has been determined. EDS spectrum corresponds to spot 4. Fig. 3 SEM images of Pb1-XMnXI2 crystals (X=0.05). The spots 1-5 in SEM images indicate the areas of the crystal surface where its chemical composition has been determined. Fig. 4 The dependence I(q) is shown in double logarithmic scale. Fig. 5 The distance distribution function p(d) of nanoparticles for Pb1-XMnXI2 (X=0.03) alloys. Fig. 6 Photoluminescence (curve 1) and exciton reflection (curve 2) spectra of PbI2 crystals. Fig. 7 Photoluminescence (curve 1) and exciton reflection (curve 2) spectra of Pb1-XMnXI2 alloys for X=0.03. Fig. 8. Photoluminescence (curve 1) and exciton reflection (curve 2) spectra of Pb1-XMnXI2 alloys with X=0.05 at T=4.5 K. Fig. 9 Temperature dependences of PL bands at 2.556 eV (A) and 2.500 eV (B). Fig. 10 Absorption (1), exciton reflection (2) and photoluminescence (3) spectra of pure PbI2 (A) and Pb1-XMnXI2 (X=0.05) alloys (B) at T=4.5 K. Fig. 11 Dependence of the intensity of I127 NQR band for (± 3/2 ↔ ± 5/2) transition on the NQR frequency.
Table Captions Table 1. The chemical composition of Pb1-XMnXI2 crystals with (X=0.03) and X=0.05 shown in Fig. 2 A and Fig. 3, respectively. Table 2. Parameters of the 127I NQR spectra
• • • • •
Structural, optical and NQR measurements of PbMnI2 alloys were carried out. The investigated crystals have a 2H-polytype phase. The crystal regions of Pb1-XMnXI2 solid solutions are embedded in PbI2 matrix. These crystal regions can be considered as platelet-shaped nanoparticles. PbMnI2 alloys are heterogeneous PbI2-PbMnI2 nanocomposites.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: