Intrinsic luminescence of SrF2 nanoparticles

Author’s Accepted Manuscript Intrinsic luminescence of SrF2 nanoparticles T. Demkiv, M. Chylii, V. Vistovskyy, A. Zhyshkovych, N. Gloskovska, P. Rodnyi, A. Vasil’ev, A. Gektin, A. Voloshinovskii www.elsevier.com/locate/jlumin

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S0022-2313(17)30254-5 http://dx.doi.org/10.1016/j.jlumin.2017.05.036 LUMIN14760

To appear in: Journal of Luminescence Received date: 16 February 2017 Revised date: 19 April 2017 Accepted date: 13 May 2017 Cite this article as: T. Demkiv, M. Chylii, V. Vistovskyy, A. Zhyshkovych, N. Gloskovska, P. Rodnyi, A. Vasil’ev, A. Gektin and A. Voloshinovskii, Intrinsic luminescence of SrF2 nanoparticles, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2017.05.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.

Intrinsic luminescence of SrF2 nanoparticles T. Demkiv1, M. Chylii1*, V. Vistovskyy1, A. Zhyshkovych1, N. Gloskovska1, P. Rodnyi4, A. Vasil’ev3, A. Gektin2, A. Voloshinovskii1 1

Ivan Franko National University of Lviv, 8 Kyryla i Mefodiya Str., 79005 Lviv, Ukraine 2 Institute for Scintillation Materials, National Academy of Sciences of Ukraine, Nauka Ave. 60, 61001, Kharkiv, Ukraine 3 Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, 119991 Moscow, Russia 4 Saint-Petersburg State Polytechnical University, 29 Polytekhnicheskaya Str., 195251 Saint-Petersburg, Russia *

Corresponding author email: [email protected]

Abstract The influence of SrF2 nanoparticle size on their luminescence and kinetic properties under photo- and X-ray excitation was studied. The self-trapped exciton luminescence intensity was shown to reveal dependence on both the nanoparticle size and the range of excitation energies. The lowest sensitivity to the nanoparticle size is observed in the case of excitation in the region of optical exciton formation. The recombination of electrons with surface defects and non-radiative decay of excitons due to diffusion to the surface is found to be the determining factors of the luminescence quenching in the case of X-ray excitation.

Keywords Thermalization length, self-trapped exciton, nanoparticle, luminescence, X-ray.

1.

INTRODUCTION

Features of electromagnetic radiation interaction with nanoparticles attract a considerable interest of researchers because of the opportunity to obtain materials with new physical properties. Nanoparticles of semiconductor compounds when the exciton Bohr radius is comparable to the size of these nanoparticles exhibit

substantial changes in the exciton energy spectrum known as a manifestation of the quantum size effect [1]. The small size of nanoparticles contributes to the appearance of coherent excitons that interfere with each other what leads to a significant reduction of the luminescence decay time for free excitons [2,3]. In the case of impurity luminescence the influence of nanoparticle surface becomes a governing factor for luminescence parameters. It substantially results in the impurity luminescence decay [4], but in some cases the complex luminescence centers involving surface defects may be formed giving the new luminescence properties [5– 8]. Besides, the new radiative features in the case of recombination luminescence for inorganic dielectric nanoparticles appear. Among them we would to distinguish dependence of the nanoparticle luminescence properties on the ratio between the migration distances of electronic excitations and the nanoparticle sizes [9–13]. The aim of this work is to systematize the effects associated with the size dependence of recombination luminescence properties of fluoride nanoparticles. Previous studies for BaF2 and CaF2 nanoparticles [9,10] have shown that luminescence intensity of the self-trapped excitons decreases with the decrease of nanoparticle size. However, the dependencies of luminescence intensity on nanoparticle size are different for various mechanisms of exciton creation. Thus, under excitation in the region of optical exciton formation, luminescence intensity reveals the weakest dependence on the nanoparticle size, whereas excitation in the region of band-to-band transitions (where the excitation mechanism is a recombination one) results in much stronger dependence. The latter also concerns the case of X-ray quanta excitation, where the luminescence intensity depends strongly on the nanoparticle sizes. Difference in the size dependencies of the recombination luminescence intensity is explained by the losses during migration of free charge carriers to luminescence centers. One of the parameters which determines the intensity of recombination luminescence is the length of electronic excitation migration. This parameter, dependently on the electron kinetic energy, describes the mean free path or thermalization length of charge carriers. In particular, when thermalization length of the electrons is comparable to the nanoparticle size, electrons may reach the surface of the nanoparticles and relax non-radiatively there or even

leave the nanoparticles instead of recombination with luminescence centers. In this case, the size of nanoparticles in which there is a sharp decline in the intensity of recombination luminescence can determine the average length of electron thermalization. Study of recombination luminescence dependence of SrF2 nanoparticles will contribute to the systematization of regularities of high-energy quanta interaction with nanoscale crystals of fluoride MeF2 (Me = Sr, Ca, Ba), in particular to reveal electron thermalization length dependence on the nanoparticle material. In addition to the fundamental interest, SrF2 nanoparticles are also of applied interest. There are several publications where SrF2 crystals, doped with lanthanide ions are considered as potential scintillation materials [14,15].

2.

EXPERIMENT

SrF2 nanoparticles were synthesized by chemical co-precipitation. To obtain SrF2 nanoparticles, SrCl2·6H2O solution was added dropwise to NH4F solution with continuous stirring using a magnetic stirrer. To slow down the reaction, in order to obtain the particles of small size, ethanol was used. The basic equation of chemical reaction is: SrCl2 + 2NH4F  SrF2 + 2NH4Cl The mixture was centrifuged and a fine white precipitate was obtained as a result. The precipitate was washed using distilled water to neutral values of pH, and the final phase was washed well with acetone. The synthesized nanoparticles were placed in a vacuum chamber for further drying to the constant weight. Such the synthesis provides the obtaining of nanoparticles with the small enough sizes. In order to obtain SrF2 nanoparticles of different size, the nanoparticles were annealed at temperatures 200, 400, 600 and 800ºС. For X-ray diffraction studies the layer of SrF2 nanopowder was deposited on a substrate using X-ray amorphous glue. The diffraction patterns were obtained by the means of STOE STADI P diffractometer. The measuring of luminescence spectra and luminescence decay kinetics of SrF2 nanoparticles upon pulse X-ray excitation were

carried out using facility based on the LOMO MDR-2 monochromator. The facility allows to carry out the luminescence-kinetic measurements in a time range of 10–9– 10–6 s in 200–800 nm spectral region. The anode voltage of the X-ray tube was equal to 40 kV, average current – 100 mA, pulse duration – 2 ns. The luminescence studies of SrF2 nanocrystals upon photoexcitation were performed in SUPERLUMI station at HASYLAB (DESY, Hamburg) [16].

3.

RESULTS AND DISCUSSION

3.1.

X-ray diffraction studies of SrF2 nanoparticles X-ray diffraction patterns of unannealed and annealed at 200, 400, 600 and

800 ºC SrF2 nanoparticles are shown in Fig. 1. According to X-ray diffraction studies, all obtained SrF2 nanoparticles possess the CaF2 fluorite structural type with lattice space symmetry Fm ̅ m. The average sizes of nanoparticles evaluated using Scherrer’s formula are shown in Table 1.

Fig. 1. The X-ray diffraction patterns for unannealed and annealed at 200, 400, 600 and 800 ºC SrF2 nanopowders. Table 1. The size of SrF2 nanoparticles according to X-ray diffraction measurements.

Annealing temperature, оС unannealed 200 400 600 800

The average size of nanoparticles, nm 20±5 30 45 65 85

3.2

Mechanism of SrF2 nanoparticle luminescence under high-energy excitation. The band gap width of SrF2 crystal is Eg = 11.3 eV [17]. SrF2 nanoparticles

excited by synchrotron radiation quanta in the region of optical exciton formation (10.2 eV) reveal luminescence spectra (Fig. 2) structurally similar to the spectra of their bulk analogues [18,19]. The spectra consist of asymmetric emission band with a maximum peaked at λ = 305 nm. Asymmetry of the luminescence band of SrF2 nanoparticles on long wave side can be caused by the luminescence of self-trapped excitons localized near lattice defects. Measurements of the luminescence spectra with time resolution at room temperature show that only a slow decay component of microsecond range is present. Such the decay time is characteristic of -component of exciton emission. A fast -component of the self-trapped exciton luminescence in SrF2 crystals is registered only at low temperatures [18]. It is suggested that the slow component of the self-trapped exciton radiation can be described on the base of F-Hpair recombination model and fast component in frame of Vk-center+е- model [20]. As shown in Fig. 2 luminescence intensity under excitation in the region of optical exciton formation decreases with decrease of SrF2 nanoparticle size. This is due to the fact that the role of surface/volume ratio increases for small nanoparticles, what leads to non-radiative decay of the self-trapped excitons on surface defects [4,9,10].

Fig. 2. Luminescence spectra of SrF 2 nanoparticles of different sizes (a) under excitation with quanta hν = 10.2 eV (λex = 120 nm). The curves: 1 – a = 85 nm, 2 – a = 65 nm, 3 – a = 20 nm. T = 300 K.

Dynamics of changes of the luminescence excitation spectrum of the selftrapped excitons in SrF2 nanoparticles depending on their size is given in Fig. 3. One can distinguish particular ranges in the excitation spectra based on energy relaxation mechanisms. When excitation quanta possess energies less than the band gap width (hν < Eg), a direct optical formation of free excitons occurs. They relax due to the electron-phonon interaction and self-trapped excitons are created. In the region of low-energy band-to-band absorption (Eg < hν <

, where

is a photon

multiplication threshold) where the energy of exciting quanta is not enough for photon multiplication processes, their absorption leads to the formation of free electrons and holes, which, after thermalization phase, create the self-trapped excitons. At higher energies of exciting quanta (hν ≥

) the rise of luminescence

intensity is observed in the excitation spectrum due to the processes of electronic excitation multiplication. As it is seen in Fig. 3, the intensity rise in the luminescence excitation spectra of SrF2 nanoparticles is observed at energies hν < 2Eg (2Eg = 22.6eV). This threshold of the luminescence intensity increase observed at = 21.9 eV we identify with the start of electronic excitation multiplication

processes based on the mechanism of secondary excitons formation [21,22]. The energy position of this threshold corresponds to sum of the band gap of SrF2 crystal (11.3 eV) and position of exciton absorption band (10.6 eV) (Fig. 3).

Fig. 3. Normalized luminescence excitation spectra for the band λem = 305 nm of SrF2 nanoparticles with the following mean sizes: 1 – a = 85 nm, 2 – a = 65 nm, 3 – a = 20 nm. T = 300 K.

In the excitation spectrum of large SrF2 nanoparticles (85 nm) dip at 10.6 eV is observed in the region of optical exciton formation. This pecularity is caused by the high absorption coefficient, what results in low penetration depth of light (10 nm) in the nanoparticle volume. Light absorption mainly in the near-surface layer, which is typical for the maximum of exciton absorption band, results in the significant losses of excitation energy due to the interaction of excitons with near-surface defects, what explains the origin of this dip. As the size of SrF2 nanoparticles decreases, the dip becomes less pronounced and for the smallest nanoparticles (Fig. 3, curve 3) it is not observed at all. The absence of this dip in small nanoparticles can be explained by comparability of the penetration depth of exciting light and nanoparticle size in the maximum and tails of exciton absorption band. In this case the luminescence intensity will be determinated by ratio between penetration depth or nanoparticle size

(depending on which of them is lower) to thickness of near-surface layer, and the luminescence excitation band would reproduce profile of the exciton absorption band. The quantitative change in form of absorption band located in the range of optical exciton creation can also be explained within model of the exciton diffusion to surface defects, assuming that all excitons anigilate nonradiatively in an infinitely thin near-surface layer where the rate of excitations quenching is infinitely high. So, we ignore the defective layer thickness, which amounts at least two lattice constants, depends on the shape, size, and method of nanoparticles preparing. For simple model of surface losses in semi-infinite crystal the excitation spectrum (h) can be written as [23]:  (h ) 

1  e  kd , 1  kL

(1)

where L – diffusion length of exciton over the radiation lifetime; d – thinkness of nanoparticle; k – absorbtion coefficient. For exciton the contour of absorption band can be wtitten in form:  h  h 0 2  , k h   k 0 exp   2    2   

(2)

where k0 = 0.11 nm-1, h 0 = 10.4 eV and  =0.2 eV= E /2.35 ( E – full width at half maximum of exciton absorption band). All mentioned parameters of exciton absorption band for SrF2 crystal were taken from [24]. Best fitting of experimental luminescence excitation spectra in the range of exciton absorption band was obtained for L=4 nm (fig 4). Therefore we can take L=4 nm as rough estimation of the exciton diffusion length during radiation lifetime.

Fig. 4 Simulation of luminescence excitation spectrum using the rough model for surface quenching with L=4 nm and thickness d = 85 nm (blue), 65 nm (yellow) and 20 nm (green).

It is needed also to pay attention to the shift of fundamental absorption edge from 9.9 eV to 10.0 eV, which is visually observed when to compare the luminescence excitation spectra of the largest (85 nm) and the smallest (20 nm) SrF2 nanoparticles (Fig. 3). Such shift of the edge to the high-energy region could be considered as a manifestation of the quantum size effect. However, the size of excitations in SrF2 (self-trapped excitons) is less than 1 nm and is more than an order of magnitude less than the smallest size of investigated nanoparticles. Under these conditions, the quantum size effect cannot be observed. Therefore, the mentioned shift of fundamental absorption edge reflects only the fact that the small-size nanoparticles don’t absorb the entire flow of incident light. In other words, this shift is caused by the absence of saturation effects in the luminescence excitation spectra of small nanoparticles. Let us consider the features of luminescence excitation spectra dependently on the nanoparticle size for the above distinguished excitation ranges. As it is seen from Fig. 3 the exciton absorption band is the most non-sensitive to size reduction. It should be noted that luminescence intensity under excitation in this range also

depends significantly on the size of nanoparticles (see Fig. 2). There is roughly a fivefold decrease in luminescence intensity upon excitation in the range of exciton absorption when the nanoparticle size changes from the largest (85 nm) to the smallest ones (20 nm). Under direct optical exciton creation, energy losses occur almost exclusively as a result of self-trapped exciton interaction with near-surface defects while under excitation in the other ranges besides this type of luminescence decay the migration losses will also occur. Therefore, for other excitation regions this luminescence intensity dependence is stronger, that is well seen in Fig. 3, where excitation spectra are normalized on intensity in maximum of exciton absorption band. When nanoparticle size decreases the luminescence intensity under excitation in the band-to-band absorption range decreases relatively exciton absorption band. This demonstrates the presence of additional mechanisms of luminescence quenching for band-to-band absorption range. The most size-sensitive is the range of electron-hole pair creation Eg < hν <

. Here, luminescence intensity is significantly reduced at the transition

to the smallest nanoparticles of 20 nm size. Under irradiation with photons with energy of this range free electrons and holes are generated in the conduction and valence band, respectively. Hovewer, the kinetic energy of the electronic excitations is not enough for multiplication processes, thus their relaxation occurs due to electron-phonon scattering. The electron thermalization length may be of the order of 101-102 nm [9,25,26]. Holes, due to their lower mobility, are supposed to possess the thermalization length of a few nanometers [27]. Therefore, non-radiative losses during the migration of electronic excitations are defined by the thermalization parameters, mainly electronic ones. If the electron reaches the nanoparticle surface during thermalization process, it can relax non-radiatively with the participation of surface energy levels or leave the nanoparticle. In this case, the excitation energy is lost for luminescence. Such the processes determine approximately 30-fold decrease in the luminescence intensity in the case of band-to-band excitation in the range of Eg < hν <

going from large (85 nm) to small (20 nm) nanoparticles.

In the energy range, where the multiplication of electronic excitations takes place (hν >

), dependency of luminescence intensity on the nanoparticle size is

not so pronounced, but it is stronger compared to the excitation in the range of exciton absorption band. Such the feature is understandable since the multiplication of electronic excitations results in the formation of two types of excitations: secondary excitons and electrons in the conduction band with kinetic energy less than Eg. Luminescence of secondary excitons is weakly dependent on the size of nanoparticles like for optically created excitons. In contrast, the possibility for electrons to escape from nanoparticle during thermalization process significantly reduces their contribution to luminescence. Therefore, dependence of the luminescence intensity on the nanoparticle size in the region of photon multiplication is intermediate between exciton and low-energy band-to-band absorption cases. In this case, in SrF2 nanoparticles with the smallest size (20 nm) the photon multiplication band (hν > 21.9 eV) arises mainly due to the processes of photoelectron scattering with formation of secondary excitons (Fig. 3, curve 3). This confirms that the threshold of photon multiplication in SrF2 nanoparticles corresponds to the formation of secondary excitons, not electron-hole pairs.

3.3

X-ray excited luminescence intensity and decay kinetics of SrF2 nanoparticle The structure of luminescence spectra of SrF2 nanoparticles in the case of X-

ray excitation restores the structure of those under optical excitation (Fig. 2). Dependence of X-ray excited luminescence intensity of SrF2 nanoparticles on their size has a threshold form (Fig. 5b, curve 1): a sharp decline in the intensity is observed for nanoparticle sizes of 85-45 nm. Similar dependence was also observed for the other types of nanoparticles [6,9,11,21]. The smallest nanoparticles (a = 20 nm) reveal emission intensity over an order of magnitude lower compared to the largest ones (85 nm). In the case of excitation in the range of direct optical exciton creation this depencence is considerable weaker (Fig. 5b, curve 2). Possible reasons of X-ray excited luminescence decrease may be: (i) the resonance interaction of excitons with surface defects; (ii) the migration losses of excitation energy as a result of electron trapping on surface defects or electron escape outside the nanoparticle. The study of time parameters of X-ray excited luminescence kinetics can provide some information about mechanism of quenching.

Fig. 6 shows the kinetics of X-ray excited luminescence decay for SrF2 nanoparticles of different size and SrF2 single crystal. Decay curves in all cases are non-exponential with clearly resolved fast and slow components. The long decay time component (about 1.2 s) which is present in luminescence of nanoparticles is close to that in single crystal and corresponds to radiation lifetime of self-trapped excitons [28]. Short decay component can be caused by luminescence quenching in result of exciton migration to nanoparticle surface or in result of high density of the electronic excitations in the track. The decay time constant of short component decrease from 107 to 22 ns when size of nanoparticles is changed from 85 to 20 nm. There are more reasons to consider that short component is caused by quenching in result of exciton migration to nanoparticle surface, since the contribution of quenching caused by high density of the electronic excitations in the track should not depend on nanoparticle size. Besides the changes of exciton band form in luminescence excitation spectra (Fig. 4) are in agreement with exciton diffusion model of losses on surface. The decrease of X-ray excited luminescence intensity with decreasing of nanoparticle size correlates with the reduction of luminescence intensity of selftrapped excitons under the excitation with photons in the range of band-to-band absorption (Fig. 3), where the main losses of excitation energy occur due to the fact that free charge carriers reach the nanoparticle surface during thermalization process. In this case, the losses of excitation energy will depend on the ratio between the average thermalization length of charge carriers and nanoparticle size. Then, in those compounds, where the average thermalization length of electrons is smaller, the range of X-ray excited luminescence intensity decline would be observed at smaller sizes of nanoparticles. In Fig. 5 the dependencies of X-ray intensity on the nanoparticle size of MeF2 are compared. The results of studies of X-ray excited luminescence intensity dependence on the size of nanoparticles for BaF 2 and CaF2 are taken from [10,11,29]. As it can be seen from the figure the nanoparticle sizes, at which the region of sharp decline of X-ray excited luminescence intensity is observed, grow in the sequence CaF2  SrF2  BaF2. Therefore, the average thermalization length of electrons should also increase in this series of nanoparticles. This agrees well with

estimation of mean thermalization length of electrons electrons with initial kinetic energy Ee0 = 5 eV (Table 2) made using expression proposed in [12]: 2

l

3

 ~        Ee0   8 a B2  *  tanh  LO    Ee0  / ln  4 Ee0  , 9  m e m0   2k B T    LO    LO  ~ – mean thermalization length a B – Bohr radius,  – effective

2 e , LO

where le,LO

dielectric permittivity ( ~ 1   1   st1 ,   – optic dielectric permittivity,

 st

– static

dielectric permittivity), me* – effective mass of electron, m0 – mass of electron,  LO – energy of optical phonons, Ee0 – initial kinetic energy of electron. This may be the evidence of the fact that the main parameter determining the X-ray excited luminescence intensity of nanoparticles is the ratio between the average thermalization length of electrons and nanoparticle sizes. Thus, the range of nanoparticle sizes for which the luminescence intensity is significantly changed can be used for rough estimates of the photoelectron thermalization length. Table 2. Parameters of MeF2 crystals: m*/m0 – ratio of effective mass of electron to mass of electron, n – refraction index,  = n2 – optic dielectric permittivity, st – static dielectric permittivity, ̃ – effective dielectric permittivity ( ~ 1   1   st1 ), ħLO – energy of optical phonons, le,LO – mean thermalization length of electrons with initial kinetic energy Ee0 = 5 eV. m*/m0 CaF2 0,6655[30] SrF2 0,6029[30] BaF2 0,5823[30]

n 1,436 [31] 1,441 [34] 1,478 [34]

 2,063 2,076 2,184

st 6,81 [32] 6,51 [35] 7,33 [35]

̃ 2,960 3,047 3,111

ħLO, еВ le,LO, nm 0,0574 [33] 66.8 0,0464 [33] 96.8 0,0404 [33] 119.1

Fig. 5. Dependence of self-trapped exciton luminescence intensity on CaF 2, SrF2 and BaF2 nanoparticle size under X-ray excitation (curves 1) and under synchrotron excitation with quanta hν = 10.2 eV (curve 2).

Fig. 6. The curves of luminescence decay time kinetics under X-ray excitation of SrF2 single crystal (1) and nanoparticles with different size: 2 – a = 85 nm, 3 – a = 20 nm. T = 300 K.

4. CONCLUSIONS

Luminescence properties of SrF2 nanoparticles reproduce the regularities found for the other members of the MeF2 (Me = Ca, Ba) series. Self-trapped exciton luminescence intensity decreases with decreasing size of nanoparticles, but the rate of decrease depends on the mechanism of luminescence excitation. The smallest sensitivity to the size of nanoparticles is characteristic of excitation in the range of optical exciton creation (h < Eg). In the case of excitation in the range of low energy band-to-band absorption transitions (Eg < hν <

), luminescence is the most

sensitive to the nanoparticle size. Under excitation in the range of photon multiplication (hν >

) luminescence intensity dependence on the size of the

nanoparticles is intermediate. The range of sharp decline of X-ray excited luminescence intensity was revealed, which can correspond to the case when the electronic thermalization length exceeds the nanoparticle size. The sizes of nanoparticles associated with the range of rapid decrease in the X-ray intensity are increased in the series of CaF2  SrF2  BaF2. The range of self-trapped exciton luminescence intensity sharp variation can be used for rough estimates of the electron thermalization length.

Main mechanisms of quenching of X-ray excited exciton luminescence of SrF2 nanoparticles are related to (i) recombination of electrons with surface defects, especially if the size of nanoparticles and electron thermalization length is comparable and (ii) non-radiative decay of excitons due to diffusion to the surface of the nanoparticles.

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