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Formation mechanism of luminescence spectra of carbon nitride films doped by europium chloride CNx: EuCl3
Journal of Luminescence 186 (2017) 247–254
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Formation mechanism of luminescence spectra of carbon nitride films doped by europium chloride CNx: EuCl3 R.Yu. Babkin a, K.V. Lamonova a, S.M. Orel a, A.M. Prudnikov a, Yu.G. Pashkevich a, O.V. Gornostaeva b,c,n, O.G. Viagin d, P.O. Maksimchuk d, Yu.V. Malyukin d a
O. O. Galkin Donetsk Institute for Physics and Engineering, National Academy of Sciences of Ukraine, 03680 Kyiv, Ukraine G. V. Kurdyumov Institute for Metal Physics, National Academy of Sciences of Ukraine, 03680 Kyiv, Ukraine c Donetsk National University, 21021 Vinnytsia, Ukraine d Institute for Scintillation Materials, National Academy of Sciences of Ukraine, 61001 Kharkiv, Ukraine b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 May 2016 Received in revised form 16 February 2017 Accepted 18 February 2017 Available online 24 February 2017
Luminescence and absorption spectra of nanostructured carbon nitride (CNx)-films doped by EuCl3 were studied. It was found that the films with europium concentration equal to 4.5 at% and 10 at% placed on the glass substrate exhibit a luminescence spectrum specific to the EuCl3 compound but with a frequency shift depending on the rare-earth dopant primary concentration. Comparison of the luminescence spectra with theoretical calculations showed a considerable part of the coordination complexes [EuCl9]6entering into chemical bonding with the carbon nitride film matrix that directly impacts the covalency degree of the《Eu3 þ –Сl1-》bond. It was found that some of the complexes have vacancy-type defects but the number of the vacancies is small and could be well identified only in films with a higher europium concentration. & 2017 Elsevier B.V. All rights reserved.
Keywords: Carbon nitride films Europium Luminescence spectra Crystal field theory Nanostructured complex
1. Introduction In the last time, special attention has been paid to thin films synthesized on the basis of carbon and nitrogen due to the possibility of their wide applications in the hydrogen power engineering and production of electrochemical probes, functional sensor elements and tribological coatings. Despite a large number of experimental and theoretical activities, a number of problems, from production technology up to the features of the electronic structure of CNx compositions, remains to be unsolved [1–7]. Besides the obvious technological potential, carbon-nitride films are of interest for fundamental studies. It is due to the fact that CNx compounds are subjected to profound structural changes during the synthesis. Indeed, a number of topologically different structures (namely, amorphous, diamond-like, graphite-like, fullerene-like, nanocolumnar and mixed ones [8]) can be obtained depending of fabrication conditions. Implantation of rare-earth (RE) ions into the CNx-films during the growth process gives a possibility to determine the different phases and structural changes. In this case, the implanted rare-earth element acts as a probe. Indeed, optical and luminescence spectra are the most sensitive to n Corresponding author at: G. V. Kurdyumov Institute for Metal Physics, National Academy of Sciences of Ukraine, 03680 Kyiv, Ukraine. E-mail address: [email protected] (O.V. Gornostaeva).
http://dx.doi.org/10.1016/j.jlumin.2017.02.040 0022-2313/& 2017 Elsevier B.V. All rights reserved.
structural deformation of the nearest environment of rare-earth (RE) ions, to the formation of vacancies in the first coordination sphere, and to a ligand composition changes. Such a rare-earth doping will lead to the creation of new nanostructured carbonnitride materials with luminescent properties which may have practical applications. To use RE-ions as a probe of carbon nitride films we need a theoretical method for the RE-ion spectrum calculation which is sensitive to the crystal-field distortions at the RE-ion site. For this purpose, we have applied the Modified Crystal Field Theory (MCFT) [9,10]. This method allows us to calculate an electronic spectrum of a RE-ion, which is placed in a crystal matrix of an arbitrary symmetry and shape. From the experimental point of view, to observe the change of the spectrum after the rare-earth ion implementation in the CNx film, we should have a reference spectrum of a pure compound that serves as a doping. We chose EuCl3 as a referent compound because it demonstrates Eu-ion luminescence spectrum in a bulk state as well as in a CNx-matrix. The formation mechanism of the luminescence spectrum of CNx: EuCl3 can be restored with the help of the MCFT modeling and comparison with the spectrum of a pure EuCl3 sample. Furthermore, the Eu3 þ ion is convenient to use as a probe since the electronic structure of a free Eu3 þ ion [11,12] and the electronic structure of the europium ion in submicron and nano-scaled dispersed materials [13] are well understood.
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2. Experimental details and sample characterization In this work, nanostructured CNx (xo1) carbon-nitride films have been fabricated by reactive magnetron sputtering of the 40 mm graphite (99.9%) target in the argon and nitrogen atmosphere onto the cover glass substrates without catalysts. It is one of the widely used and prospective methods to obtain nanostructured carbon-nitride films [14]. The films were formed in different sputtering conditions, namely, power, cathode current, pressure, concentration of the plasma components, the substrate temperature, the distance from the substrate to the cathode and others. We applied substrates made of polished quartz glass with a roughness of about 1 nm. It gives opportunity for monitoring the complicated surface morphology of the obtained samples with an accuracy of 1 nm. The planar DC magnetron with a flat cathode and a ring anode was used for the film manufacturing. Sometimes to improve the morphological composition of the films, the manufacturing process includes annealing. Upon annealing, amorphous carbon, which appears as a byproduct in magnetron sputtering, disappears. The operating target was in the form of a graphite disk and the target surface was covered with powder EuCl3 under pressure. During the film deposition, a part of europium ions imbeds into the CNx films as EuCl3 clusters. Whereas some Eu3 þ ions are surrounded by amorphous carbon and other part stays in the form of europium metal particles. We note that only EuCl3 clusters demonstrate luminescence properties. We have investigated a series of samples with different Eu concentrations. The luminescence properties just of two representatives with Eu concentrations 4.5 at% and 10 at% will be presented. Note that the technology in question does not require high temperatures of substrates under coating process. In this sense, there is no need to use a heat-resistant material as a substrate and, hence, the range of possible materials for coating is expanding significantly. Technology conditions we used to create CNx-films in a nanocolumnar phase [15] were as follows, the target-to-substrate distance was 2.5–3.0 cm, discharge power during the film deposition did not exceed 20 W, discharge current on the magnetron target was 20–40 mA, cathode–anode voltage was 430–450 V, the substrate temperature was 300 °C, the gas pressure inside the magnetron chamber was 26 Pa. During the film deposition, the floating plasma potential, near the substrate, was about 21 V. The time of the film growth was 2 h. The nucleation process and dynamics of the growth of C-N nanostructure films were analyzed by the NTEGRA Aura Scanning Probe Microscope and the field emission scanning electron microscopy (FE-SEM) method with JEOL JSM-6490 LV. Relative amounts of carbon and nitrogen as well as the local chemical structure of the films were obtained with Mg Kα high resolution x-ray photoemission spectroscopy (HR-XPS) with KRATOS AXIS– 165 spectrometer and also with INCA Penta FETx3 spectrometer. The film thickness was estimated by the interference microscope MII-4, as well as with SEM micrographs. The nanocolumnar structure consists of nanocolumns vertically oriented relative to the substrate. The nanocolumns have an elongated cylindrical shape with averaged diameter 60–80 nm and penetrate the entire film thickness [15]. In Fig. 1(a) we can see the nanocolumnar structure doped by Eu3 þ ions. Micrograph of the surface of a CNx: EuCl3 (x E0.14) film with 4.76 at% of europium is shown in Fig. 1(b). The results of microstructure analysis are given in Table 1. Note that according to the microstructural data all doped samples contain some oxygen.
3. Luminescence spectral studies Optical absorption spectra in the 190–1100 nm range were measured using the SPECORD 200 spectrophotometer (Analytik
Jenа, Germany). Luminescence spectra were obtained using the computer-controlled setup based on the grating monochromator. The samples were excited by the fourth harmonic (266 nm) of the YAG:Nd3 þ pulsed laser (NL202 model, EKSPLA, Lithuania). The closed-cycle optical helium cryostat CS204AE-FMX-1AL (Advanced Research Systems, USA) was used for the luminescence spectra measurements at 10 K. The laser beam was focused onto the sample by a quartz lens. The laser radiation was directed on the sample at an angle to the light axis of the monochromator, in order to minimize the lightexposing of the registration system. Luminescence of the sample was focused on the entrance slit of the MDR-23 monochromator using the capacitor, a spectral separation in the measured range reached 15 cm 1. The beam inside the monochromator spreads in a spectrum and fed onto the exit slit. Spectrum scanning on the output slit was carried out automatically during rotation of the spectral device dispersing element using a special mechanism and stepping motor. The photomultiplier Hamamatsu R9110 operated in photon counting mode was used as a photodetector. The spectral resolution in the measured range was 15 cm 1. An electrical signal from the PMT has been transmitted through the CAMAC system to a personal computer. Stepper motor control and luminescence spectra recording were carried out using the own development software Spectral measurement v.17. Resulting number of pulses on each wavelength were averaged by several measurements to minimize random errors. The technique of the time-correlated single photon counting was used to study the lifetime of the Eu3 þ ions excited states in the carbon nitride films. Decay curves were measured by the TimeHarp 260 NANO system and the single photon detector PMA 182 (PicoQuant, Germany). An excitation was induced by the fourth harmonic of the Nd:YAG pulsed laser. Fig. 2 shows the absorption spectra of carbon nitride films without europium under different sputtering conditions. An absorption edge of the films shifts depending on the current and the atmosphere of the sputtering. The absorption spectra of the CNx: EuCl3 films before and after annealing were also obtained. Annealing of the europium activated carbon nitride samples was conducted in a muffle furnace at a temperature Т ¼350 °С over 20 min. Process stabilization time of the furnace was about 5 min. Unfortunately, only two barely visible bands at 394 nm (7F0–5L6) and 465 nm (7F0–5D2) have been observed in both cases. Fig. 3 shows the luminescence spectra of the CNx: EuCl3 films before annealing with the concentration of dopants equal to 4.5 at% and 10 at%. Luminescence spectra at Т ¼300 K of the pure compound EuCl3, which was used for europium sputtering, are also shown for comparison. Fig. 3 shows that the spectra of doped and undoped samples have obvious differences in the intensities and splitting of lines. For example, the five lines of varying intensities, which are derived from the EuCl3 sample, are observed in the region of 14,000C14,600 сm 1 and correspond to the 5D0-7F4 transition (Fig. 3(a), green line). With increasing concentration of the EuCl3 target material in the CNxsamples these lines are inhomogeneously broadened and shifted toward shorter wavelengths. Conversely, observed lines in the region of 5 D0-7F1 transition (Fig. 3b) with increasing EuCl3 concentration are almost not shifted, but their intensity markedly increased. The most intensive line, which corresponds to the 5D0-7F2 transition under EuCl3 concentration increasing, divides onto two broadened lines with smaller intensities. Fig. 4 shows luminescence decay curves for the CNx: EuCl3 films before annealing for various electronic transitions. Typical lifetimes of excited states differ insignificantly. Lifetime for the sample with dopant concentration equals to 4.5 at% is τ0 ¼125 μs (Fig. 4(a)). Lifetimes for the sample with concentration of dopant
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Fig. 1. (a) Micrograph of the surface of a CNx: y EuCl3 (x E0.14; y E0.0048); (b) SEM-image of the nanocolumnar CNx: EuCl3 film. The studied areas in (b) are indicated by the magenta rectangles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Composition of a CNx: EuCl3 sample. Spectrum
C
N
O
Cl
Eu
Total
1 2 3 Mean values
77,28 76,64 78,74 77,55
13,04 12,51 12,21 12,59
1,95 2,46 1,84 2,08
3,21 2,85 2,98 3,01
4,52 5,54 4,23 4,76
100% 100% 100%
All the data are given in at%.
Fig. 2. Absorption spectra of carbon nitride films without europium chloride under the different sputtering conditions (discharge current and content of working gas).
equal to 10 at% are τ0 ¼141 μs (5D0 – 7F4, 5D0 – 7F2) and τ0 ¼ 136 μs (5D0 – 7F1) (Fig. 4(b)). The shape of the decay curves for both samples differs from the monoexponential one that can be associated with the concentration or other type of luminescence quenching of europium ions.
4. Calculations of free Eu3 þ spectra For reliable interpretation of europium-doped films spectra a series of calculations have been carried out: 1) free Eu3 þ spectra calculation; 2) Eu3 þ ion in EuCl3-monocrystal spectra calculations;
Fig. 3. Luminescence spectra of the CNx: EuCl3 films before annealing and those of the pure compound EuCl3 measured in different spectral ranges: a) 5D0 – 7F4, transition; b) 5D0 – 7F1, 5D0 – 7F2 transitions.
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Table 2 Experimental [12] and calculated energy levels of a free Eu3 þ ion. Term
J
S 2 ¼ S(Sþ 1)
L 2 ¼ L(L þ1)
Ecalc, cm 1
Eexp, cm 1
7
0 1 2 3 4 5 6 0 1 2 6 3 7 8 2 3 4 5 6 9 10 4 3
12 12 12 12 12 12 12 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
12 12 12 12 12 12 12 6 6 6 70 6 71 72 20 21 22 23 24 72 73 73 28
0 372 1027 1875 2850 3906 5012 17,242 18,886 21,275 23,650 24,109 24,702 25,654 26,053 26,199 26,255 26,276 26,290 26,465 27,054 27,335 29,890
0 370 1040 1890 2860 3910 4940 17,270 19,030 21,510 – 24,390 – – – – – – – – – 27,640 –
F0 F1 7 F2 7 F3 7 F4 7 F5 7 F6 5 D0 5 D1 5 D2 5 L6 5 D3 5 L7 5 L8 5 G2 5 G3 5 G4 5 G5 5 G6 5 L9 5 L10 5 D4 5 H3 7
Calculations were carried out with the given parameters Z¼ 63, Zeff ¼ 14.4622, Kr ¼ 20.92, while the difference between experimental and calculation energy levels did not exceed 1.5%.
two different particles. The spin-orbit interaction potential for ions with 4f n ′ electronic configuration has a common form n′ ^ VSO = ∑i = 1 ξ( ri )( li, si), where the single-electron constant is written as:
ξnl =
Z × Kr3 α2 . 3 2 n l( l + 1)( l + 1/2)
(1)
Here, n and l are principal and orbital quantum numbers, Z is the nuclear charge, Kr is a variational parameter that depends on the effective nuclear charge Zeff. A free Eu3 þ ion is described by 4f 6 electron configuration, which consists of 3003 microstates. Due to relativistic interactions a ground term 7F splits into seven multiplets, an excited term 5D splits into five multiplets [12]. The experimental and calculated energy levels of a free Eu3 þ ion are given in Table 2. Fig. 4. Luminescence decay curves of the CNx: EuCl3 films measured for various electronic transitions before annealing. The doping concentrations are (a) 4.5 at%; (b) 10 at%.
3) simulation of the europium-doped films spectra as a result of chlorine ligand environment deformation. A method, described in detail in [16], was used to calculate an electronic spectrum of a free Eu3 þ ion. The spectrum calculation implies solving the Schrödinger equation within a single-configura^ ^ ^ tion approximation with the Hamiltonian H = H0 + V , where the ^ operator H0 represents kinetic and potential energies of inner and outer valence electrons in the field of nucleus with the charge Z as well as the interaction energy of inner and outer electrons with each ^ other. The perturbation potential V is the operator sum of the elec^ tron-electron ( VEE ) and a number of relativistic interactions of the valence electrons. Apart from the traditional spin-orbit interaction ^ ( VSO , which describes the interaction of a particle's spin with its own orbital moment) perturbation potential includes so-called Breit's terms proportional to α2 ( α = e2/ℏc is a fine structure constant) ^ ^ [17,18], namely spin-spin VSS , orbit-orbit V^LL interactions, and VSL term which describes the interaction of spin and orbital moments between
5. Calculations of the crystal-field energy levels of Eu3 þ ions in a [EuCl9]6- coordination complex by MCFT To calculate crystal terms, we have used the method called a Modified Crystal Field Theory (MCFT), an advanced crystal-field theory [19]. Within the framework of the MCFT, it is assumed that the potential ^ of the interaction with the crystal field (VCF ) is added to the perturbation potential of a single ion [20]. Complete orthonormal system of multielectron zero-approximation wave-functions Ψ(SLJMJ) is used as basic functions for a given electronic configuration. The wave functions of the terms Ψ(SLJMJ) are linear combinations of multi-electronic determinant functions Φ(1,2,…,n’) (the numbers designate the single-electron wavefunctions ψi(nlmlms) for each of the valence electrons). Hydrogen-like wave-functions with the effective parameter a¼Zeff /na0 (a0 is the Bohr radius) were taken as the ψi(nlmlms) functions. Matrix elements Vμν were numerically calculated simultaneously over the entire potential, while the secular equation has an order corresponding to the calculated multi-determinant functions dimension:
^ Vμν = Ψ SLJMJ V Ψ S′L′JMJ
(
)
(
)
(2)
R.Yu. Babkin et al. / Journal of Luminescence 186 (2017) 247–254
n′
Vμν − εδμν = 0, μ , ν = 1, …, C(2l + 1)(2s + 1)
251
(3)
It should be pointed out, that in the case of rare-earth (RE) ions, the valence f-electrons occupy inner shells, which are shielded by a completely filled 5s25p6 shell and therefore are weakly coupled to the ligand field. In other words, the multiplet splitting of the REion spectrum due to an intra-atomic spin-orbit interaction is almost identical for a free ion and an ion in a crystal field. It means that the effective nuclear charge of the RE-ion remains the same or weakly varies. Thus, the effective nuclear charge of a free ion should be used to calculate rare-earth spectra [21]. In contrast, the ligand charges are strongly shielded by complete 5s25p6 shells, their values are considered in the MCFT as variables and are reCF latively reduced to the formal oxidation state: qeff = qeff − σCF . It should be noted that a one-parameter spectrum description has significant disadvantage: it does not take into account the fact that the charge Zeff is not local, but is represented by an electron density in the near-nucleus space. Spatial charge distribution depends on the wave-function Ψ(2)(r,Zeff) state. In its turn, this fact is responsible for emergence of an additional contribution to the energy and should modify the value of the nuclear charge shielding by the electrons of filled shells. This disadvantage could be eliminated by introduction of an effective nuclear charge for each term Zterm = ktermZ¯eff as an adjustable parameter. From the first sight, it would increase the number of unknown parameters that complicates the task, but it is not always the case. Depending on the nature of the problem to be solved it is necessary to use levels located in different energy ranges. For example, the decisive role in investigating magnetic ions properties by electron paramagnetic resonance method (EPR) is played by low-lying levels of the base term in a range ∼104 cm 1. In this case, a good agreement between theory and experiment can be achieved by using the average value of the effective nuclear charge Z¯eff obtained for freeion levels [20,22–24]. At the same time, to describe absorption and luminescence spectra of a free ion, and even more, those of an ion positioned in a crystalline matrix, we need information about the state of excited levels. Transitions with absorption or emission of an energy quantum usually concern excited terms in addition to the basic term. Thus, for a more precise description of the optical spectra it makes sense to introduce additional parameters to characterize the excited levels. In this work, we deal with the luminescence spectra of a Eu3 þ ion in the EuCl3 compound, thus, for the calculation of the energy levels we introduced parameters to characterize all terms involved in the process of quantum transitions. The energy levels of a Eu3 þ ion placed in a coordination complex [EuCl9]6- of the EuCl3 compound were calculated using the MCFT approach. EuCl3 crystallizes in a hexagonal system, space group P63/m (No.176). The elementary cell contains two formula units Z¼ 2 (Fig. 5). The atoms occupy the following positions: Eu3 þ – 2с (1/3, 2/3, 1/4), Cl- – 6h (0.38911(23), 0.30174(23), 1/4). It is seen the x- and y-positions of the chlorine ions are free because the coordination complex has an ability to deform. At that, the primary symmetry can be preserved or lowered. The calculations were based on the structural data [25] with given Z¼63, Zeff ¼ 14.4622 and Kr parameters which were picked C1 is an up for each term (Table 3). The effective chlorine charge qeff 6adjustable (variable) parameter. For the [EuCl9] complex it is C1 qeff = − 0.6. The results of calculations are presented in Table 3. Obviously, within the EuCl3 crystal field the symmetry of the Eu3 þ ion decreases, thereby degenerate terms are split forming singlet and doublet states. For example, the distance between the lowest 7 F0 and the highest 7F6 levels of the term 7F increased to E150 cm 1 versus a free ion.
Fig. 5. Crystallographic structure of the EuCl3.
Table 3 Energy levels of Eu3 þ ion placed in the coordination complexa – [EuCl9]6-. No. Term Ecalc, cm 1 7
F0 F1 F2 7 F3 7 F4 7 F5 7 F6
1
7
2
7
3 4 5 6 7
5
D0 D1 5 D2
8
5
9 10 a
0 320 (d), 491 988 (d), 1069 (d), 1145 1847, 1849, 1893 (d), 1918 (d), 1924 2874, 2881 (d), 2891 (d), 2900 (d), 2912, 2918 3850, 3864 (d), 3898 (d), 3936, 3942, 3974 (d), 4001 (d) 4846, 4852 (d), 4872 (d), 4910, 4912, 4973 (d), 5059(d), 5168 (d) 17,286 19,034 (d), 19,084 21,516 (d), 21,526, 21,530 (d)
Kr 20.858 20.897 20.979 20.977 20.929 20.927 20.830 20.858 19.602 21.621
In brackets the letter d marked doublets.
We compared the calculated [EuCl9]6- energies (Table 2) with a luminescence spectrum of the EuCl3 sample (a green line in Fig. 3). It is seen that the real sample symmetry is lower than symmetry of EuCl3 with a predetermined crystallographic data [25]. This follows from the fact that the real spectrum demonstrates splitting doublets. It gives a reason to use an additional distortion to simulate the spectrum of the EuCl3 sample. The modeling showed possible reasons for the sample symmetry decreasing as slight shifts of chlorine ions in the (xy) plane. These shifts lead to an energy-levels order change and additional levels splitting (Fig. 6). Fig. 6 shows that the simulated levels are well correlated with observed 5 D0⇒7F0 and 5D0⇒7F1 transitions in a range 16,800C17,300 cm 1. It should be noted that transitions between 5D0⇒7F0 terms are prohibited or have low intensity. Transitions between levels 5 D0⇒7F3 in a range 15,390C15,410 cm 1 are not observed in the EuCl3 compound at all, but they appear in carbon nitride films, activated by europium with concentration 4.5 at% (red curve) and 10 at% (blue curve). In other words, this group of transitions in the EuCl3 compound has practically zero intensity. Transitions between levels 5D0⇒7F4 in a range 14,200C14,500 cm 1 agree with calculated levels worse than previous ones. An intensive signal observed in a range 16,250C16,380 cm 1, corresponds to the transition 5D0⇒7F2 and could be caused by Eu3 þ ions either in EuCl3 or Eu2О3 [26] compounds. The Eu2О3 compound can appear under sputtering process due to presence of a small amount of oxygen on the chemical film's content (see Table 1).
6. Eu3 þ ion in carbon nitride spectrum Comparing the luminescence spectra of carbon nitride films doped by europium (4.5 at% and 10 at%) with the spectrum of the base EuCl3 material which was used during the films synthesis (green curve) showed their general agreement (Fig. 6). However, significant differences between the spectra, as additional splitting of spectral lines and
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Fig. 6. Luminescence spectra of the EuCl3 compound (green curve), Eu3 þ activated carbon nitride films, with a concentration of Eu3 þ 4.5 at% (red curve) or 10 at% (blue curve). The MCFT calculated levels of Eu3 þ ion placed in the coordination complex [EuCl9]6- (black vertical lines).
redistribution of their intensities, indicate that the structure of the immediate environment of europium is significantly transformed. At first, there could be low-symmetrical distortions of the coordination complex [EuCl9]6-, secondly, it could be due to vacancy defects, i. e. loss of chlorine ions under the sputtering process, thirdly, there can be a substitution of chlorine ions by carbon, nitrogen, oxygen ions. The substitution affects the oxidation degree of the ligand atoms. In addition, the general broadening of the spectra with increasing dopant concentration indicates the presence of an amorphization, i.e. the loss of translational symmetry by EuCl3 nanoparticles and the loss of the local symmetry in the nearest and next-to-the-nearest environment of europium ions. Luminescence spectra of the films with the dopant concentration 4.5 at% and 10 at% contain quite a number of intensively weak lines and specific energy shifts. Aforementioned features do not fit into simple complexes distortions model. Their occurrence can be associated with the partial destruction of the [EuCl9]6- complex and formation of chemical bonds with carbon nitride structures. We have modeled a case of vacancy appearance in a coordination complex and the formation of chemical bonds with oxygen, nitride and carbon ions. In the first case, it looks like the removal of one chlorine ion from the plane of type I or type II (Cl(I) and Cl(II), respectively, in Fig. 7). In the second case, it could be caused by changing the oxidation state of one or more ligand ions. Simulation of the luminescence spectra of the CNx: EuCl3 films with different concentrations of europium shows the best agreement between the experimental and calculated levels achieved by changing the oxidation state of three Cl- ions which are in plane of type I. It is interesting that this effect is equivalent to simultaneous shift of the Clions in the direction from europium (i.e. the non-centrality effect [9,10]). Calculation data in the range of 16,500 cm 1C17,100 cm 1 are shown in Fig. 8 (magenta lines). Magenta arrows in Fig. 8 indicate the direction of shift of the calculated (black) lines under the influence of changes in the chlorine oxidation degree or the displacement of chlorine ions in plane of type I. Fig. 9 shows that the spectrum with the europium concentration of 10 at% contains additional, intensively weak, peaks (they are marked by orange and dark blue arrows). Peaks marked with orange arrows are caused by the formation of vacancies in coordination complexes. Peaks marked with dark blue arrows are originated by an
Fig. 7. Formation of vacancies in the nearest environment of europium. Different colors show two types of planes from which the Cl ion can be extracted. The vacancy in the plane of type II is shown separately. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Luminescence spectral lines of europium, which caused by displacement of chlorine from the plane of type I along the z-axis (magenta dash-dot lines). Black lines show calculated levels for the pure compound EuCl3 with the shielding factor k¼ 0.6 for all ligands.
additional shielding of Cl- charges due to a chemical bonding with the parent CNx matrix. But the total amount of the complexes with vacancies is negligible which follows from the low intensity of these peaks. It may be well identified only in films with a high
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of [EuCl9]6- complexes enters into a chemical connection with the carbon nitride film matrix CNx. It is reflected in the additional screening of the chlorine ions from the nitrogen, carbon, and even oxygen ions. Thus, there is an indirect change in the degree of 《Eu3 þ –Сl1-》covalency bonding, so there is a distinctive shift of the singlet energy 16,960 сm-1 of EuCl3 towards high energies. At last, it is proved that some of the [EuCl9]6 complexes have vacancy-type defects. But since the intensity of the related additional lines is small, the overall proportion of complexes with vacancies is negligible. It could be well identified only in films with a high concentration of europium. Acknowledgements
Fig. 9. Luminescence spectral lines of europium, which caused by shielding of one chlorine ion with factor k¼ 0.5 in the plane of type I (navy solid lines), and a vacancy in the plane of type I (orange dash dot lines).
concentration of europium (the sample with the concentration 10 at% in Fig. 9). The simulation result of these effects is shown in the form of orange and dark blue lines in Fig. 9. Selective nature of the bonding between the ligand atoms and CNх-matrix should lead to the fact that the specific doublet structure diffuses and takes a shape of a continuous broad line. Therefore experimental observations of complete blurring of the spectral doublet structure as well as a general shift of the singlet luminescence peak to higher energies in comparison with the spectrum of pure EuCl3 indicates that most of the coordination complexes [EuCl9]6 in films with concentration of dopant 4.5 at% and 10 at% are involved in chemical bonds with carbon nitride matrix. Note that the formation of chemical bonds between chlorine ions and nitrogen, carbon or oxygen ions, a small portion of which is present in the structures, causes an additional shielding of chlorine ions and therefore changes the degree of covalency of the《Eu3 þ –Сl1-》bond. This effect can be modeled by reducing a point charge of one or more of chlorine ions. Theoretical calculations show that in both cases additional shielding of the chlorine ions which belong to a plane of type I and type II causes a singlet energy shift from 16,960 сm 1 to higher energies in agreement with the experimental data (Fig. 8). Chlorine ions shielding from the plane of type II gives less appreciable result, meanwhile, doublet splitting increases in both cases. Finally, we note that the luminescence spectrum modeling have exhibited no signs of the Сl1- ligand substitution by of nitrogen, carbon or oxygen ions.
7. Conclusions Nanostructured hybrid carbon nitride films were synthesized by reactive magnetron sputtering of a graphite target in a pure nitrogen atmosphere on a glass substrate. EuCl3 compound was used as the rare-earth target. Studies of luminescence and light absorption spectra in the nanostructured carbon nitride (CNx)-films with Eu3 þ ions impurities showed that the films with concentration of europium 4.5 at% and 10 at% exhibit luminescence with a spectrum similar to EuCl3 but with frequency shifts depending on the starting concentration of a rare-earth doping impurities. Theoretical modeling of luminescence spectra was carried out using the modified crystal field theory. The influence of the coordination complex [EuCl9]6- distortions, vacancy defects and the degree of chlorine ions oxidation on the Eu3 þ ions energy levels has been investigated. Theoretical calculations and comparison with the reference substance EuCl3 show that a considerable part
The work was carried out within the framework of the project No. 53/15-Н of the NAS of Ukraine Program "Fundamental issues of creation of new nanomaterials and nanotechnologies" and was partially supported by the State Fund for Fundamental Research of Ukraine No. F71/73-2016. We are also grateful to Mikhail Belogolovskii for his useful comments. References [1] D. Marton, K.J. Boyd, J. W. Rabalais, Synthesis of carbon nitride, Int. J. Mod. Phys. B 9 (1995) 3527-3558. [2] L. Stagi, D. Chiriu, Structural and optical properties of carbon nitride polymorphs, Diam. Relat. Mater. 68, (2016) 84–92. [3] S. Fujita, H. Habuchi, Optical properties of graphitic carbon nitride films prepared by evaporation, Diam. Relat. Mater. 65 (2016) 83-86. [4] S. Merkinis, R. Gudaitis, V. Kopustinkas, S. Tamulevicius, K. Slapicas, Piezoresistive, optical and electrical properties of diamond like carbon and carbon nitride films, Diam. Relat. Mater. 19 (2010) 1249–1253. [5] F. Alibart, M. Lejeune, K. Zellama, M. Benlahsen, Effect of nitrogen on the optoelectronic properties of a highly sp2-rich amorphous carbon nitride films, Diam. Relat. Mater. 20 (2011) 409–412. [6] H. Ito, H. Araki, A. Wada, Sticking probability of CN radicals, Diam. Relat. Mater. 20 (2011) 355–358. [7] J.N. Hart, F. Claeyssens, N.L. Allan, P.W. May, Carbon nitride: ab initio investigation of carbon-rich phases, Phys. Rev. B 80 (2009) 174111–174113. [8] N. Hellgren, M.P. Johansson, E. Broitman, L. Hultman, J.-E. Sundgren, Role of nitrogen in formation of hard and elastic CNx thin films by reactive magnetron sputtering, Phys. Rev. B 59 (1999) 5162–5169. [9] E.S. Zhitlukhina, K.V. Lamonova, S.M. Orel, P. Lemmens, Yu. G. Pashkevich, Spin state transformations of a 3d ion in the pyramidal environment and under lattice distortions, J. Phys.: Condens. Matter. 19 (2007) 156215–156216. [10] K.V. Lamonova, E.S. Zhitlukhina, R. Yu Babkin, S.M. Orel, S.G. Ovchinnikov, Yu. G. Pashkevich, Intermediate-spin state of a 3d ion in the octahedral environment and generalization of the Tanabe-Sugano diagrams, J. Phys. Chem. A 115 (2011) 13596–13604. [11] K. Binnemans, Interpretation of europium(III) spectra, Chem. Rev. 295 (2015) 1–45. [12] W.C. Martin, R. Zalubas, L. Hagan, The rare-earth elements, Nat. Stand. Ref. Data Ser., NSRDS-NBS, 1978. [13] O. Viagin, A. Masalov, I. Ganina, Y. Malyukin, Mechanism of energy transfer in Sr2CeO4 : Eu3 þ phosphor, Opt. Mater. 31 (2009) 1808–1810. [14] J. Musil, P. Baroch, J. Vlček, J.G. Han, Reactive magnetron sputtering of thin films: present status and trends, Thin Solid Films 475 (2005) 208–218. [15] R. Shalaev, A. Ulyanov, A. Prudnikov, G. Shin, S. Yoo, V. Varyukhin, Phys. Status Solidi (a) 207 (2010) 2300. [16] R. Yu Babkin, K.V. Lamonova, S.M. Orel, Yu. G. Pashkevich, Determination of the effective nuclear charge for free ions of transition metals from experimental spectra, Opt. Spectrosc. 107 (2009) 9–15. [17] G. Breit, Dirac's equation and the spin-spin interactions of two electrons, Phys. Rev. 39 (1932) 616–624. [18] H.A. Bethe, E.E. Salpeter, Quantum Mechanics of One- and Two-Electron Atoms, Plenum Press, New York, 1977. [19] S. Sugano, Y. Tanabe, H. Kamimura, Multiplets of Transition-Metal Ions in Crystals, Academic Press, New York and London, 1970. [20] V.A. Shapovalov, E.S. Zhitlukhina, K.V. Lamonova, V.V. Shapovalov, M. Rafailovich, S.A. Schwarz, R. Jahoda, V.J. Reidy, S.M. Orel, Yu. G. Pashkevich, Multiminimum adiabatic potential in the single crystal normal spinel ZnAl2O4, doped by Cu2 þ ions, J. Phys.: Condens. Matter. 22 (2010) 245504–7. [21] J. C. Slater, Atomic shielding constants, Phys. Rev. 36 (1930) 57–64. [22] K. Lamonova, I. Ivanchenko, S. Orel, S. Paranchich, V. Tkach, E. Zhitlukhina, N. Popenko, Yu. Pashkevich, Spectroscopic evidence of spinel phase clustering in solid solutions Hg1 xCrxSe (0.03 r x r 0.1), J. Phys.: Condens. Matter. 21 (2009) 045603–9.
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[23] K. Lamonova, B. Bekirov, I. Ivanchenko, N. Popenko, E. Zhitlukhina, V. Burkhovetskii, S. Orel, Yu. Pashkevich, Specific features of the temperature behavior of ESR spectra of Fe-doped mercury selenide, Low Temp. Phys. 40 (2014) 655–661. [24] O.V. Gornostaeva, K.V. Lamonova, S.M. Orel, Yu.G. Pashkevich, Magnetic properties of Ce3 þ ion in ironcontaining oxypnictide CeFeAsO, Low Temp.
Phys. 39 (2013) 343–350. [25] B. Morosin, Crystal structures of anhydrous rare-earth chlorides, J. Chem. Phys. 49 (1968) 3007–3012. [26] Yu Jinqiu, Cui Lei, He Huaqiang, Yan Shihong, Hu Yunsheng, Wu Hao, Raman spectra of R2O3 (RE ¼ Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y): laser-excited luminescence and trace impurity analysis, J. Rare Earths. 32 (2014) 1–4.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Formation mechanism of luminescence spectra of carbon nitride films doped by europium chloride CNx: EuCl3 R.Yu. Babkin a, K.V. Lamonova a, S.M. Orel a, A.M. Prudnikov a, Yu.G. Pashkevich a, O.V. Gornostaeva b,c,n, O.G. Viagin d, P.O. Maksimchuk d, Yu.V. Malyukin d a
O. O. Galkin Donetsk Institute for Physics and Engineering, National Academy of Sciences of Ukraine, 03680 Kyiv, Ukraine G. V. Kurdyumov Institute for Metal Physics, National Academy of Sciences of Ukraine, 03680 Kyiv, Ukraine c Donetsk National University, 21021 Vinnytsia, Ukraine d Institute for Scintillation Materials, National Academy of Sciences of Ukraine, 61001 Kharkiv, Ukraine b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 May 2016 Received in revised form 16 February 2017 Accepted 18 February 2017 Available online 24 February 2017
Luminescence and absorption spectra of nanostructured carbon nitride (CNx)-films doped by EuCl3 were studied. It was found that the films with europium concentration equal to 4.5 at% and 10 at% placed on the glass substrate exhibit a luminescence spectrum specific to the EuCl3 compound but with a frequency shift depending on the rare-earth dopant primary concentration. Comparison of the luminescence spectra with theoretical calculations showed a considerable part of the coordination complexes [EuCl9]6entering into chemical bonding with the carbon nitride film matrix that directly impacts the covalency degree of the《Eu3 þ –Сl1-》bond. It was found that some of the complexes have vacancy-type defects but the number of the vacancies is small and could be well identified only in films with a higher europium concentration. & 2017 Elsevier B.V. All rights reserved.
Keywords: Carbon nitride films Europium Luminescence spectra Crystal field theory Nanostructured complex
1. Introduction In the last time, special attention has been paid to thin films synthesized on the basis of carbon and nitrogen due to the possibility of their wide applications in the hydrogen power engineering and production of electrochemical probes, functional sensor elements and tribological coatings. Despite a large number of experimental and theoretical activities, a number of problems, from production technology up to the features of the electronic structure of CNx compositions, remains to be unsolved [1–7]. Besides the obvious technological potential, carbon-nitride films are of interest for fundamental studies. It is due to the fact that CNx compounds are subjected to profound structural changes during the synthesis. Indeed, a number of topologically different structures (namely, amorphous, diamond-like, graphite-like, fullerene-like, nanocolumnar and mixed ones [8]) can be obtained depending of fabrication conditions. Implantation of rare-earth (RE) ions into the CNx-films during the growth process gives a possibility to determine the different phases and structural changes. In this case, the implanted rare-earth element acts as a probe. Indeed, optical and luminescence spectra are the most sensitive to n Corresponding author at: G. V. Kurdyumov Institute for Metal Physics, National Academy of Sciences of Ukraine, 03680 Kyiv, Ukraine. E-mail address: [email protected] (O.V. Gornostaeva).
http://dx.doi.org/10.1016/j.jlumin.2017.02.040 0022-2313/& 2017 Elsevier B.V. All rights reserved.
structural deformation of the nearest environment of rare-earth (RE) ions, to the formation of vacancies in the first coordination sphere, and to a ligand composition changes. Such a rare-earth doping will lead to the creation of new nanostructured carbonnitride materials with luminescent properties which may have practical applications. To use RE-ions as a probe of carbon nitride films we need a theoretical method for the RE-ion spectrum calculation which is sensitive to the crystal-field distortions at the RE-ion site. For this purpose, we have applied the Modified Crystal Field Theory (MCFT) [9,10]. This method allows us to calculate an electronic spectrum of a RE-ion, which is placed in a crystal matrix of an arbitrary symmetry and shape. From the experimental point of view, to observe the change of the spectrum after the rare-earth ion implementation in the CNx film, we should have a reference spectrum of a pure compound that serves as a doping. We chose EuCl3 as a referent compound because it demonstrates Eu-ion luminescence spectrum in a bulk state as well as in a CNx-matrix. The formation mechanism of the luminescence spectrum of CNx: EuCl3 can be restored with the help of the MCFT modeling and comparison with the spectrum of a pure EuCl3 sample. Furthermore, the Eu3 þ ion is convenient to use as a probe since the electronic structure of a free Eu3 þ ion [11,12] and the electronic structure of the europium ion in submicron and nano-scaled dispersed materials [13] are well understood.
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2. Experimental details and sample characterization In this work, nanostructured CNx (xo1) carbon-nitride films have been fabricated by reactive magnetron sputtering of the 40 mm graphite (99.9%) target in the argon and nitrogen atmosphere onto the cover glass substrates without catalysts. It is one of the widely used and prospective methods to obtain nanostructured carbon-nitride films [14]. The films were formed in different sputtering conditions, namely, power, cathode current, pressure, concentration of the plasma components, the substrate temperature, the distance from the substrate to the cathode and others. We applied substrates made of polished quartz glass with a roughness of about 1 nm. It gives opportunity for monitoring the complicated surface morphology of the obtained samples with an accuracy of 1 nm. The planar DC magnetron with a flat cathode and a ring anode was used for the film manufacturing. Sometimes to improve the morphological composition of the films, the manufacturing process includes annealing. Upon annealing, amorphous carbon, which appears as a byproduct in magnetron sputtering, disappears. The operating target was in the form of a graphite disk and the target surface was covered with powder EuCl3 under pressure. During the film deposition, a part of europium ions imbeds into the CNx films as EuCl3 clusters. Whereas some Eu3 þ ions are surrounded by amorphous carbon and other part stays in the form of europium metal particles. We note that only EuCl3 clusters demonstrate luminescence properties. We have investigated a series of samples with different Eu concentrations. The luminescence properties just of two representatives with Eu concentrations 4.5 at% and 10 at% will be presented. Note that the technology in question does not require high temperatures of substrates under coating process. In this sense, there is no need to use a heat-resistant material as a substrate and, hence, the range of possible materials for coating is expanding significantly. Technology conditions we used to create CNx-films in a nanocolumnar phase [15] were as follows, the target-to-substrate distance was 2.5–3.0 cm, discharge power during the film deposition did not exceed 20 W, discharge current on the magnetron target was 20–40 mA, cathode–anode voltage was 430–450 V, the substrate temperature was 300 °C, the gas pressure inside the magnetron chamber was 26 Pa. During the film deposition, the floating plasma potential, near the substrate, was about 21 V. The time of the film growth was 2 h. The nucleation process and dynamics of the growth of C-N nanostructure films were analyzed by the NTEGRA Aura Scanning Probe Microscope and the field emission scanning electron microscopy (FE-SEM) method with JEOL JSM-6490 LV. Relative amounts of carbon and nitrogen as well as the local chemical structure of the films were obtained with Mg Kα high resolution x-ray photoemission spectroscopy (HR-XPS) with KRATOS AXIS– 165 spectrometer and also with INCA Penta FETx3 spectrometer. The film thickness was estimated by the interference microscope MII-4, as well as with SEM micrographs. The nanocolumnar structure consists of nanocolumns vertically oriented relative to the substrate. The nanocolumns have an elongated cylindrical shape with averaged diameter 60–80 nm and penetrate the entire film thickness [15]. In Fig. 1(a) we can see the nanocolumnar structure doped by Eu3 þ ions. Micrograph of the surface of a CNx: EuCl3 (x E0.14) film with 4.76 at% of europium is shown in Fig. 1(b). The results of microstructure analysis are given in Table 1. Note that according to the microstructural data all doped samples contain some oxygen.
3. Luminescence spectral studies Optical absorption spectra in the 190–1100 nm range were measured using the SPECORD 200 spectrophotometer (Analytik
Jenа, Germany). Luminescence spectra were obtained using the computer-controlled setup based on the grating monochromator. The samples were excited by the fourth harmonic (266 nm) of the YAG:Nd3 þ pulsed laser (NL202 model, EKSPLA, Lithuania). The closed-cycle optical helium cryostat CS204AE-FMX-1AL (Advanced Research Systems, USA) was used for the luminescence spectra measurements at 10 K. The laser beam was focused onto the sample by a quartz lens. The laser radiation was directed on the sample at an angle to the light axis of the monochromator, in order to minimize the lightexposing of the registration system. Luminescence of the sample was focused on the entrance slit of the MDR-23 monochromator using the capacitor, a spectral separation in the measured range reached 15 cm 1. The beam inside the monochromator spreads in a spectrum and fed onto the exit slit. Spectrum scanning on the output slit was carried out automatically during rotation of the spectral device dispersing element using a special mechanism and stepping motor. The photomultiplier Hamamatsu R9110 operated in photon counting mode was used as a photodetector. The spectral resolution in the measured range was 15 cm 1. An electrical signal from the PMT has been transmitted through the CAMAC system to a personal computer. Stepper motor control and luminescence spectra recording were carried out using the own development software Spectral measurement v.17. Resulting number of pulses on each wavelength were averaged by several measurements to minimize random errors. The technique of the time-correlated single photon counting was used to study the lifetime of the Eu3 þ ions excited states in the carbon nitride films. Decay curves were measured by the TimeHarp 260 NANO system and the single photon detector PMA 182 (PicoQuant, Germany). An excitation was induced by the fourth harmonic of the Nd:YAG pulsed laser. Fig. 2 shows the absorption spectra of carbon nitride films without europium under different sputtering conditions. An absorption edge of the films shifts depending on the current and the atmosphere of the sputtering. The absorption spectra of the CNx: EuCl3 films before and after annealing were also obtained. Annealing of the europium activated carbon nitride samples was conducted in a muffle furnace at a temperature Т ¼350 °С over 20 min. Process stabilization time of the furnace was about 5 min. Unfortunately, only two barely visible bands at 394 nm (7F0–5L6) and 465 nm (7F0–5D2) have been observed in both cases. Fig. 3 shows the luminescence spectra of the CNx: EuCl3 films before annealing with the concentration of dopants equal to 4.5 at% and 10 at%. Luminescence spectra at Т ¼300 K of the pure compound EuCl3, which was used for europium sputtering, are also shown for comparison. Fig. 3 shows that the spectra of doped and undoped samples have obvious differences in the intensities and splitting of lines. For example, the five lines of varying intensities, which are derived from the EuCl3 sample, are observed in the region of 14,000C14,600 сm 1 and correspond to the 5D0-7F4 transition (Fig. 3(a), green line). With increasing concentration of the EuCl3 target material in the CNxsamples these lines are inhomogeneously broadened and shifted toward shorter wavelengths. Conversely, observed lines in the region of 5 D0-7F1 transition (Fig. 3b) with increasing EuCl3 concentration are almost not shifted, but their intensity markedly increased. The most intensive line, which corresponds to the 5D0-7F2 transition under EuCl3 concentration increasing, divides onto two broadened lines with smaller intensities. Fig. 4 shows luminescence decay curves for the CNx: EuCl3 films before annealing for various electronic transitions. Typical lifetimes of excited states differ insignificantly. Lifetime for the sample with dopant concentration equals to 4.5 at% is τ0 ¼125 μs (Fig. 4(a)). Lifetimes for the sample with concentration of dopant
R.Yu. Babkin et al. / Journal of Luminescence 186 (2017) 247–254
249
Fig. 1. (a) Micrograph of the surface of a CNx: y EuCl3 (x E0.14; y E0.0048); (b) SEM-image of the nanocolumnar CNx: EuCl3 film. The studied areas in (b) are indicated by the magenta rectangles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Composition of a CNx: EuCl3 sample. Spectrum
C
N
O
Cl
Eu
Total
1 2 3 Mean values
77,28 76,64 78,74 77,55
13,04 12,51 12,21 12,59
1,95 2,46 1,84 2,08
3,21 2,85 2,98 3,01
4,52 5,54 4,23 4,76
100% 100% 100%
All the data are given in at%.
Fig. 2. Absorption spectra of carbon nitride films without europium chloride under the different sputtering conditions (discharge current and content of working gas).
equal to 10 at% are τ0 ¼141 μs (5D0 – 7F4, 5D0 – 7F2) and τ0 ¼ 136 μs (5D0 – 7F1) (Fig. 4(b)). The shape of the decay curves for both samples differs from the monoexponential one that can be associated with the concentration or other type of luminescence quenching of europium ions.
4. Calculations of free Eu3 þ spectra For reliable interpretation of europium-doped films spectra a series of calculations have been carried out: 1) free Eu3 þ spectra calculation; 2) Eu3 þ ion in EuCl3-monocrystal spectra calculations;
Fig. 3. Luminescence spectra of the CNx: EuCl3 films before annealing and those of the pure compound EuCl3 measured in different spectral ranges: a) 5D0 – 7F4, transition; b) 5D0 – 7F1, 5D0 – 7F2 transitions.
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Table 2 Experimental [12] and calculated energy levels of a free Eu3 þ ion. Term
J
S 2 ¼ S(Sþ 1)
L 2 ¼ L(L þ1)
Ecalc, cm 1
Eexp, cm 1
7
0 1 2 3 4 5 6 0 1 2 6 3 7 8 2 3 4 5 6 9 10 4 3
12 12 12 12 12 12 12 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
12 12 12 12 12 12 12 6 6 6 70 6 71 72 20 21 22 23 24 72 73 73 28
0 372 1027 1875 2850 3906 5012 17,242 18,886 21,275 23,650 24,109 24,702 25,654 26,053 26,199 26,255 26,276 26,290 26,465 27,054 27,335 29,890
0 370 1040 1890 2860 3910 4940 17,270 19,030 21,510 – 24,390 – – – – – – – – – 27,640 –
F0 F1 7 F2 7 F3 7 F4 7 F5 7 F6 5 D0 5 D1 5 D2 5 L6 5 D3 5 L7 5 L8 5 G2 5 G3 5 G4 5 G5 5 G6 5 L9 5 L10 5 D4 5 H3 7
Calculations were carried out with the given parameters Z¼ 63, Zeff ¼ 14.4622, Kr ¼ 20.92, while the difference between experimental and calculation energy levels did not exceed 1.5%.
two different particles. The spin-orbit interaction potential for ions with 4f n ′ electronic configuration has a common form n′ ^ VSO = ∑i = 1 ξ( ri )( li, si), where the single-electron constant is written as:
ξnl =
Z × Kr3 α2 . 3 2 n l( l + 1)( l + 1/2)
(1)
Here, n and l are principal and orbital quantum numbers, Z is the nuclear charge, Kr is a variational parameter that depends on the effective nuclear charge Zeff. A free Eu3 þ ion is described by 4f 6 electron configuration, which consists of 3003 microstates. Due to relativistic interactions a ground term 7F splits into seven multiplets, an excited term 5D splits into five multiplets [12]. The experimental and calculated energy levels of a free Eu3 þ ion are given in Table 2. Fig. 4. Luminescence decay curves of the CNx: EuCl3 films measured for various electronic transitions before annealing. The doping concentrations are (a) 4.5 at%; (b) 10 at%.
3) simulation of the europium-doped films spectra as a result of chlorine ligand environment deformation. A method, described in detail in [16], was used to calculate an electronic spectrum of a free Eu3 þ ion. The spectrum calculation implies solving the Schrödinger equation within a single-configura^ ^ ^ tion approximation with the Hamiltonian H = H0 + V , where the ^ operator H0 represents kinetic and potential energies of inner and outer valence electrons in the field of nucleus with the charge Z as well as the interaction energy of inner and outer electrons with each ^ other. The perturbation potential V is the operator sum of the elec^ tron-electron ( VEE ) and a number of relativistic interactions of the valence electrons. Apart from the traditional spin-orbit interaction ^ ( VSO , which describes the interaction of a particle's spin with its own orbital moment) perturbation potential includes so-called Breit's terms proportional to α2 ( α = e2/ℏc is a fine structure constant) ^ ^ [17,18], namely spin-spin VSS , orbit-orbit V^LL interactions, and VSL term which describes the interaction of spin and orbital moments between
5. Calculations of the crystal-field energy levels of Eu3 þ ions in a [EuCl9]6- coordination complex by MCFT To calculate crystal terms, we have used the method called a Modified Crystal Field Theory (MCFT), an advanced crystal-field theory [19]. Within the framework of the MCFT, it is assumed that the potential ^ of the interaction with the crystal field (VCF ) is added to the perturbation potential of a single ion [20]. Complete orthonormal system of multielectron zero-approximation wave-functions Ψ(SLJMJ) is used as basic functions for a given electronic configuration. The wave functions of the terms Ψ(SLJMJ) are linear combinations of multi-electronic determinant functions Φ(1,2,…,n’) (the numbers designate the single-electron wavefunctions ψi(nlmlms) for each of the valence electrons). Hydrogen-like wave-functions with the effective parameter a¼Zeff /na0 (a0 is the Bohr radius) were taken as the ψi(nlmlms) functions. Matrix elements Vμν were numerically calculated simultaneously over the entire potential, while the secular equation has an order corresponding to the calculated multi-determinant functions dimension:
^ Vμν = Ψ SLJMJ V Ψ S′L′JMJ
(
)
(
)
(2)
R.Yu. Babkin et al. / Journal of Luminescence 186 (2017) 247–254
n′
Vμν − εδμν = 0, μ , ν = 1, …, C(2l + 1)(2s + 1)
251
(3)
It should be pointed out, that in the case of rare-earth (RE) ions, the valence f-electrons occupy inner shells, which are shielded by a completely filled 5s25p6 shell and therefore are weakly coupled to the ligand field. In other words, the multiplet splitting of the REion spectrum due to an intra-atomic spin-orbit interaction is almost identical for a free ion and an ion in a crystal field. It means that the effective nuclear charge of the RE-ion remains the same or weakly varies. Thus, the effective nuclear charge of a free ion should be used to calculate rare-earth spectra [21]. In contrast, the ligand charges are strongly shielded by complete 5s25p6 shells, their values are considered in the MCFT as variables and are reCF latively reduced to the formal oxidation state: qeff = qeff − σCF . It should be noted that a one-parameter spectrum description has significant disadvantage: it does not take into account the fact that the charge Zeff is not local, but is represented by an electron density in the near-nucleus space. Spatial charge distribution depends on the wave-function Ψ(2)(r,Zeff) state. In its turn, this fact is responsible for emergence of an additional contribution to the energy and should modify the value of the nuclear charge shielding by the electrons of filled shells. This disadvantage could be eliminated by introduction of an effective nuclear charge for each term Zterm = ktermZ¯eff as an adjustable parameter. From the first sight, it would increase the number of unknown parameters that complicates the task, but it is not always the case. Depending on the nature of the problem to be solved it is necessary to use levels located in different energy ranges. For example, the decisive role in investigating magnetic ions properties by electron paramagnetic resonance method (EPR) is played by low-lying levels of the base term in a range ∼104 cm 1. In this case, a good agreement between theory and experiment can be achieved by using the average value of the effective nuclear charge Z¯eff obtained for freeion levels [20,22–24]. At the same time, to describe absorption and luminescence spectra of a free ion, and even more, those of an ion positioned in a crystalline matrix, we need information about the state of excited levels. Transitions with absorption or emission of an energy quantum usually concern excited terms in addition to the basic term. Thus, for a more precise description of the optical spectra it makes sense to introduce additional parameters to characterize the excited levels. In this work, we deal with the luminescence spectra of a Eu3 þ ion in the EuCl3 compound, thus, for the calculation of the energy levels we introduced parameters to characterize all terms involved in the process of quantum transitions. The energy levels of a Eu3 þ ion placed in a coordination complex [EuCl9]6- of the EuCl3 compound were calculated using the MCFT approach. EuCl3 crystallizes in a hexagonal system, space group P63/m (No.176). The elementary cell contains two formula units Z¼ 2 (Fig. 5). The atoms occupy the following positions: Eu3 þ – 2с (1/3, 2/3, 1/4), Cl- – 6h (0.38911(23), 0.30174(23), 1/4). It is seen the x- and y-positions of the chlorine ions are free because the coordination complex has an ability to deform. At that, the primary symmetry can be preserved or lowered. The calculations were based on the structural data [25] with given Z¼63, Zeff ¼ 14.4622 and Kr parameters which were picked C1 is an up for each term (Table 3). The effective chlorine charge qeff 6adjustable (variable) parameter. For the [EuCl9] complex it is C1 qeff = − 0.6. The results of calculations are presented in Table 3. Obviously, within the EuCl3 crystal field the symmetry of the Eu3 þ ion decreases, thereby degenerate terms are split forming singlet and doublet states. For example, the distance between the lowest 7 F0 and the highest 7F6 levels of the term 7F increased to E150 cm 1 versus a free ion.
Fig. 5. Crystallographic structure of the EuCl3.
Table 3 Energy levels of Eu3 þ ion placed in the coordination complexa – [EuCl9]6-. No. Term Ecalc, cm 1 7
F0 F1 F2 7 F3 7 F4 7 F5 7 F6
1
7
2
7
3 4 5 6 7
5
D0 D1 5 D2
8
5
9 10 a
0 320 (d), 491 988 (d), 1069 (d), 1145 1847, 1849, 1893 (d), 1918 (d), 1924 2874, 2881 (d), 2891 (d), 2900 (d), 2912, 2918 3850, 3864 (d), 3898 (d), 3936, 3942, 3974 (d), 4001 (d) 4846, 4852 (d), 4872 (d), 4910, 4912, 4973 (d), 5059(d), 5168 (d) 17,286 19,034 (d), 19,084 21,516 (d), 21,526, 21,530 (d)
Kr 20.858 20.897 20.979 20.977 20.929 20.927 20.830 20.858 19.602 21.621
In brackets the letter d marked doublets.
We compared the calculated [EuCl9]6- energies (Table 2) with a luminescence spectrum of the EuCl3 sample (a green line in Fig. 3). It is seen that the real sample symmetry is lower than symmetry of EuCl3 with a predetermined crystallographic data [25]. This follows from the fact that the real spectrum demonstrates splitting doublets. It gives a reason to use an additional distortion to simulate the spectrum of the EuCl3 sample. The modeling showed possible reasons for the sample symmetry decreasing as slight shifts of chlorine ions in the (xy) plane. These shifts lead to an energy-levels order change and additional levels splitting (Fig. 6). Fig. 6 shows that the simulated levels are well correlated with observed 5 D0⇒7F0 and 5D0⇒7F1 transitions in a range 16,800C17,300 cm 1. It should be noted that transitions between 5D0⇒7F0 terms are prohibited or have low intensity. Transitions between levels 5 D0⇒7F3 in a range 15,390C15,410 cm 1 are not observed in the EuCl3 compound at all, but they appear in carbon nitride films, activated by europium with concentration 4.5 at% (red curve) and 10 at% (blue curve). In other words, this group of transitions in the EuCl3 compound has practically zero intensity. Transitions between levels 5D0⇒7F4 in a range 14,200C14,500 cm 1 agree with calculated levels worse than previous ones. An intensive signal observed in a range 16,250C16,380 cm 1, corresponds to the transition 5D0⇒7F2 and could be caused by Eu3 þ ions either in EuCl3 or Eu2О3 [26] compounds. The Eu2О3 compound can appear under sputtering process due to presence of a small amount of oxygen on the chemical film's content (see Table 1).
6. Eu3 þ ion in carbon nitride spectrum Comparing the luminescence spectra of carbon nitride films doped by europium (4.5 at% and 10 at%) with the spectrum of the base EuCl3 material which was used during the films synthesis (green curve) showed their general agreement (Fig. 6). However, significant differences between the spectra, as additional splitting of spectral lines and
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Fig. 6. Luminescence spectra of the EuCl3 compound (green curve), Eu3 þ activated carbon nitride films, with a concentration of Eu3 þ 4.5 at% (red curve) or 10 at% (blue curve). The MCFT calculated levels of Eu3 þ ion placed in the coordination complex [EuCl9]6- (black vertical lines).
redistribution of their intensities, indicate that the structure of the immediate environment of europium is significantly transformed. At first, there could be low-symmetrical distortions of the coordination complex [EuCl9]6-, secondly, it could be due to vacancy defects, i. e. loss of chlorine ions under the sputtering process, thirdly, there can be a substitution of chlorine ions by carbon, nitrogen, oxygen ions. The substitution affects the oxidation degree of the ligand atoms. In addition, the general broadening of the spectra with increasing dopant concentration indicates the presence of an amorphization, i.e. the loss of translational symmetry by EuCl3 nanoparticles and the loss of the local symmetry in the nearest and next-to-the-nearest environment of europium ions. Luminescence spectra of the films with the dopant concentration 4.5 at% and 10 at% contain quite a number of intensively weak lines and specific energy shifts. Aforementioned features do not fit into simple complexes distortions model. Their occurrence can be associated with the partial destruction of the [EuCl9]6- complex and formation of chemical bonds with carbon nitride structures. We have modeled a case of vacancy appearance in a coordination complex and the formation of chemical bonds with oxygen, nitride and carbon ions. In the first case, it looks like the removal of one chlorine ion from the plane of type I or type II (Cl(I) and Cl(II), respectively, in Fig. 7). In the second case, it could be caused by changing the oxidation state of one or more ligand ions. Simulation of the luminescence spectra of the CNx: EuCl3 films with different concentrations of europium shows the best agreement between the experimental and calculated levels achieved by changing the oxidation state of three Cl- ions which are in plane of type I. It is interesting that this effect is equivalent to simultaneous shift of the Clions in the direction from europium (i.e. the non-centrality effect [9,10]). Calculation data in the range of 16,500 cm 1C17,100 cm 1 are shown in Fig. 8 (magenta lines). Magenta arrows in Fig. 8 indicate the direction of shift of the calculated (black) lines under the influence of changes in the chlorine oxidation degree or the displacement of chlorine ions in plane of type I. Fig. 9 shows that the spectrum with the europium concentration of 10 at% contains additional, intensively weak, peaks (they are marked by orange and dark blue arrows). Peaks marked with orange arrows are caused by the formation of vacancies in coordination complexes. Peaks marked with dark blue arrows are originated by an
Fig. 7. Formation of vacancies in the nearest environment of europium. Different colors show two types of planes from which the Cl ion can be extracted. The vacancy in the plane of type II is shown separately. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Luminescence spectral lines of europium, which caused by displacement of chlorine from the plane of type I along the z-axis (magenta dash-dot lines). Black lines show calculated levels for the pure compound EuCl3 with the shielding factor k¼ 0.6 for all ligands.
additional shielding of Cl- charges due to a chemical bonding with the parent CNx matrix. But the total amount of the complexes with vacancies is negligible which follows from the low intensity of these peaks. It may be well identified only in films with a high
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of [EuCl9]6- complexes enters into a chemical connection with the carbon nitride film matrix CNx. It is reflected in the additional screening of the chlorine ions from the nitrogen, carbon, and even oxygen ions. Thus, there is an indirect change in the degree of 《Eu3 þ –Сl1-》covalency bonding, so there is a distinctive shift of the singlet energy 16,960 сm-1 of EuCl3 towards high energies. At last, it is proved that some of the [EuCl9]6 complexes have vacancy-type defects. But since the intensity of the related additional lines is small, the overall proportion of complexes with vacancies is negligible. It could be well identified only in films with a high concentration of europium. Acknowledgements
Fig. 9. Luminescence spectral lines of europium, which caused by shielding of one chlorine ion with factor k¼ 0.5 in the plane of type I (navy solid lines), and a vacancy in the plane of type I (orange dash dot lines).
concentration of europium (the sample with the concentration 10 at% in Fig. 9). The simulation result of these effects is shown in the form of orange and dark blue lines in Fig. 9. Selective nature of the bonding between the ligand atoms and CNх-matrix should lead to the fact that the specific doublet structure diffuses and takes a shape of a continuous broad line. Therefore experimental observations of complete blurring of the spectral doublet structure as well as a general shift of the singlet luminescence peak to higher energies in comparison with the spectrum of pure EuCl3 indicates that most of the coordination complexes [EuCl9]6 in films with concentration of dopant 4.5 at% and 10 at% are involved in chemical bonds with carbon nitride matrix. Note that the formation of chemical bonds between chlorine ions and nitrogen, carbon or oxygen ions, a small portion of which is present in the structures, causes an additional shielding of chlorine ions and therefore changes the degree of covalency of the《Eu3 þ –Сl1-》bond. This effect can be modeled by reducing a point charge of one or more of chlorine ions. Theoretical calculations show that in both cases additional shielding of the chlorine ions which belong to a plane of type I and type II causes a singlet energy shift from 16,960 сm 1 to higher energies in agreement with the experimental data (Fig. 8). Chlorine ions shielding from the plane of type II gives less appreciable result, meanwhile, doublet splitting increases in both cases. Finally, we note that the luminescence spectrum modeling have exhibited no signs of the Сl1- ligand substitution by of nitrogen, carbon or oxygen ions.
7. Conclusions Nanostructured hybrid carbon nitride films were synthesized by reactive magnetron sputtering of a graphite target in a pure nitrogen atmosphere on a glass substrate. EuCl3 compound was used as the rare-earth target. Studies of luminescence and light absorption spectra in the nanostructured carbon nitride (CNx)-films with Eu3 þ ions impurities showed that the films with concentration of europium 4.5 at% and 10 at% exhibit luminescence with a spectrum similar to EuCl3 but with frequency shifts depending on the starting concentration of a rare-earth doping impurities. Theoretical modeling of luminescence spectra was carried out using the modified crystal field theory. The influence of the coordination complex [EuCl9]6- distortions, vacancy defects and the degree of chlorine ions oxidation on the Eu3 þ ions energy levels has been investigated. Theoretical calculations and comparison with the reference substance EuCl3 show that a considerable part
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