DSpace at VNU: Optical properties and Judd-Ofelt analysis of Sm ions in Lanthanum trifluoride nanocrystals

J Mater Sci: Mater Electron DOI 10.1007/s10854-016-5603-1 Optical properties and Judd–Ofelt analysis of Sm ions in Lanthanum trifluoride nanocrystals Hoang Manh Ha1 • Tran Thi Quynh Hoa2 • Le Van Vu3 • Nguyen Ngoc Long3 Received: 16 June 2016 / Accepted: 22 August 2016 Ó Springer Science+Business Media New York 2016 Abstract LaF3:samarium (Sm) nanocrystals have been prepared by hydrothermal method The nanocrystals were characterized by X-ray diffraction, transmission electron microscopy The absorption, luminescence spectra of LaF3:Sm samples were measured at room temperature By using Judd–Ofelt theory, intensity parameters (X2, X4 and X6) have been obtained from optical absorption measurements Based on these Judd–Ofelt parameters, various radiative parameters such as radiative transition probabilities, radiative lifetime, branching ratios and stimulated emission cross-section for 4G5/2 excited level of LaF3:Sm nanocrystals were predicted Introduction The study of rare-earth (RE) nanophosphors is currently an active research field in materials chemistry as these compounds have potential applications in optics, optoelectronics, optical communication, and biomedicine In comparison with oxide-based systems, fluorides possess very low vibrational energies, e.g., phonon energy in lanthanum trifluoride (LaF3) is about 350 cm-1; therefore, the multiphonon relaxation of the excited states of & Hoang Manh Ha hoangmanhha@hus.edu.vn Hanoi Architectural University, 10 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam National University of Civil Engineering, 55 Giai Phong, Hai Ba Trung, Hanoi, Vietnam Center for Materials Science, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam the rare earth ions doped will be minimal, resulting in a decrease of the non-radiative rate and a higher quantum efficiency of the luminescence Furthermore, the fluorides exhibit adequate thermal and environmental stability and therefore they are considered as ideal host materials for different luminescent lanthanide ions Among RE fluorides, LaF3 being an important material has received broad attention due to their potential applications in many fields of science and technique LaF3 unique nanostructures, such as nanowires, nanorods, nanosheets and nanoplates, have been successfully synthesized [1–8] These nanostructures exhibit various optical properties as efficient room temperature emission from the UV to the mid-IR LaF3 host matrix was doped with different RE ions such as Eu3? [1, 2, 5, 9, 10], Nd3? [3, 11], Ce3? [12], Er3? [13] and co-doped with Ce3? and Tb3? [6, 8, 14], Yb3? and Ho3? [15] Sm3? is one of the most popular RE ions, which is used extensively in optical devices The optical properties of Sm3? ions doped in many glasses have been studied in detail by using Judd–Ofelt (J–O) theory [16–23] In the existing literature there is very little work on the optical properties of Sm3? in LaF3 It must be emphasized that J–O analysis has been applied mainly to Sm3? ions doped in different glasses A few works carried out J–O analysis for Sm3? ion doped in some crystals [24, 25] To our knowledge, there is the only work of Leavitt et al [26] devoted to J–O analysis for Sm3? ion doped in LaF3 crystal Recently appeared a few works devoted to the J–O analysis of spectroscopic properties of Eu3? ions in polycrystalline powders [27] or nanocrystals [28] using emission spectra In this report, we studied optical properties of Sm3? iondoped LaF3 nanocrystals fabricated by hydrothermal method We have used the J–O theory to determine intensity parameters by analyzing room-temperature 123 J Mater Sci: Mater Electron absorption spectra of LaF3:Sm3? nanocrystals We have also predicted radiative transition probabilities, branching ratios, and radiative lifetimes for the 4G5/2 excited state of Sm3? ion in LaF3 nanocrystals Experimental LaF3 nanocrystals doped with 1, 2, 3, and mol% of Sm3? ions were prepared by hydrothermal method All the chemicals used in our experiment, including lanthanum oxide (La2O3), samarium oxide (Sm2O3), ammonium fluoride (NH4F) and glycine (NH2CH2COOH) are of analytic grade without further purification In a typical synthesis, 0.977 g of La2O3 and 1.046 g Sm2O3 were dissolved in dilute HNO3, and then dissolved in 48 ml deionized water under stirring, resulting in the formation of a colorless solutions of La(NO3)3 and Sm(NO3)3, respectively Mix the two mentioned above solutions in accordance with the appropriate rate After that, 0.4504 g of glycine was added into the mixture solution with stirring for 30 to form lanthanum (samarium)–glycine complex Then, 0.667 g NH4Fwas dissolved in 50 ml deionized water and the obtained NH4F aqueous solution was slowly added dropwise to the above complex solution After vigorous stirring for h at 50 °C, the milky colloidal solution was obtained and poured into a Teflon-lined stainless steel autoclave, and then heated at 150 °C for 12 h After the autoclave was naturally cooled down to room-temperature, the precipitates were collected by centrifugation (6000 rpm) for 20 and washed with deionized water This filter washing process was repeated 10 times The final product was dried in air at 60 °C for 12 h For measurement of absorption spectra, LaF3:Sm3? nanopowders were mixed into KBr powders with ratio LaF3:KBr = 3:7, and then the mixed powders were pressed into pellets with diameter of 1.3 cm and a thickness of 0.11 cm Crystal structure of the synthesized samples was characterized by an X-ray diffractometer SIMEMS D5005, Bruker, Germany with Cu–Ka1 irradiation (k = ˚ ) The morphology of the samples was observed 1.54056 A by using a transmission electron microscopy Tecnai G2 20 FEI The optical absorption spectra were recorded in the range of wavelength from 200 to 3000 nm using a spectrophotometer Cary-5000 Room temperature photoluminescence (PL) of the samples was measured on a spectrofluorometer FL 3-22 Jobin–Yvon Spex using 450 W xenon arc lamp as the excitation source Luminescence lifetime was measured using a Varian Cary Eclipse Fluorescence Spectrophotometer 123 Results and discussion X-ray diffraction (XRD) patterns of pure LaF3 and LaF3:Sm3? nanocrystals are presented in Fig All the XRD peaks are unambiguously indexed to hexagonal phase with  space group of LaF3 structure (JCPDS card no 32P3c1 0483) with the following diffraction peaks: (002), (110), (111), (112), (202), (211), (300), (113), (004), (302), (221), (114), (222), (223), (304) and (410) No peaks of any other phases or impurities are detected The lattice parameters ˚ and c = 7.323 A ˚ in were calculated to be a = 7.167 A good agreement with standard bulk values By applying Scherrers formula ẵL ẳ 0:9k=bcoshị [29] to the (111) diffraction peak, the average crystallite sizes of pure LaF3 and LaF3:5 %Sm3? nanocrystals are estimated to be 27 and 16 nm, respectively Moreover, Fig shows a transmission electron microscopy (TEM) image, high resolution (HR) TEM image and corresponding fast Fourier transformation (FFT) pattern with [0001] zone axis of pure LaF3 nanocrystals The measured lattice spacing in the HRTEM image is 0.36 nm which corresponds to the (110) plane of hexagonal LaF3 nanocrystals in good agreement with the result of XRD analysis For optical properties of the all samples, Fig shows the optical absorption spectra of pure LaF3, and LaF3:5 %Sm3? samples for ultraviolet–visible–near infrared (UV–Vis–NIR) region at room temperature It can be seen that the pure LaF3 is transparent in UV–VIS–NIR region except the wavelength range from 1370 to 1530 nm The absorption spectrum of the LaF3:5 %Sm3? exhibits several bands assigned to f–f transitions from the ground state to various excited states of Sm3? ions Namely, fifteen discrete absorption bands located at 344, 361, 374, 389, 400, 415, 462, 479, 947, 1088, 1242, 1389, 1421, 1484 and Fig XRD patterns of LaF3 nanocrystals doped with different Sm3? concentrations J Mater Sci: Mater Electron Fig a TEM image, b HRTEM image and c corresponding FFT pattern of LaF3 nanocrystals Fig The room-temperature absorption spectra of pure LaF3 and LaF3:5 %Sm3? samples in range of a 330–500 nm and b 900–1750 nm 1554 nm are assigned to the transitions from the ground state 6H5/2 to various exited 4D7/2, 4D3/2, 6P7/2, 4L15/2, 6P3/2, P5/2, 4I13/2, 4M15/2, 6F11/2, 6F9/2, 6F7/2, 6F5/2, 6F3/2, 6H15/2, and 6F1/2 states of Sm3? ions, respectively The absorption band wavenumbers for Sm3? ions doped in LaF3 nanocrystals and aquo-ions along with nephelauxetic ratio b and bonding parameter d are presented in Table The ÂÀ Á à bonding parameter d is defined as d ¼ À b =b  100 P where the average value b ẳ bị=N, b ¼ mnc =ma , mnc and ma are the wavenumbers of the corresponding transitions in LaF3:Sm3? nanocrystals and aquo-ions, respectively, and N  is the number of levels that are used to calculate b The value of d ¼ À0:53 indicates that the nature of the Sm3?-ligand bond in LaF3 nanocrystals is ionic Typical room-temperature PL spectra of undoped LaF3 and LaF3:5 %Sm3? nanocrystals are shown in Fig As seen from figure, undoped LaF3 sample does not emit light Whereas LaF3:5 %Sm3? sample exhibits emission spectrum with four dominant peaks at 560, 594, 639 and 705 nm corresponding to the emission transitions from the G5/2 excited state to the 6H5/2, 6H7/2, 6H9/2 and 6H11/2 states of the Sm3? ion, respectively As can be seen from the results of J–O analysis (presented below), the two transitions 4G5/2 ? 6H9/2 and 6H11/2 are purely electricdipole (ED) transitions, whereas the other two transitions G5/2 ? 6H5/2 and 6H7/2 contain contributions of both the ED and magnetic-dipole (MD) It is worth noting that all the emission lines have the same excitation spectra, which prove that all these lines possess the same origin Typical PLE spectrum monitored at 594 nm emission line of LaF3:5 %Sm3? nanocrystals is depicted in Fig It can be seen that the excitation spectrum totally coincides with the absorption one On the basis of these absorption and photoluminescence experimental observations, Luminescence decay of %Sm3? ions doped in LaF3 nanocrystals is shown in Fig As shown in the figure, the non-exponential decay curve has been fitted to the tri-exponential function: 123 J Mater Sci: Mater Electron Table Absorption transition wavenumbers for Sm3? ions doped in LaF3 nanocrystals mnc and aquo-ions ma along with nephelauxetic ratio b and bonding parameter d Transition 6H5/2? vnc (cm-1) va (cm-1) [32] b = vnc/va 29,070 29,100 0.999 27,701 27,700 26,738 26,750 0.9995 25,707 25,650 1.0022 25,000 24,950 1.002 24,096 24,050 1.0019 21,645 21,600 1.0021 20,877 20,800 1.0037 10,560 10,500 1.0057 9191 9200 0.999 8052 8000 1.0065 7047 6760 1.0425 D7/2 D3/2 P7/2 L15/2 P3/2 P5/2 I13/2 M15/2 F11/2 F9/2 F7/2 F5/2 ? 6F3/2 ? 6H15/2 ? 6F1/2 b ¼ 1:0053 Fig The room-temperature PL spectra under 400 nm excitation of the pure LaF3 and LaF3:5 %Sm3? nanocrystals d ẳ 0:53 ionic bondingị Fig Luminescence decay curve of Sm3? ions doped in LaF3 nanocrystals under 400 nm excitation  t Itị ẳ A1 exp s1      t t ỵ A2 exp ỵ A3 exp s2 s3 with A1 ¼ 176 Ỉ 8; s1 ¼ 2:06 Ỉ 0:08 ms; A2 ẳ 341ặ 6; s2 ẳ 0:45 ặ 0:02 ms and A3 ẳ 7000 ặ 4000; s3 ẳ 0:024ặ 0:004 ms Average experimental lifetime,sexp , is found to be 1.2 ms The experimental oscillator strength fexp of an absorption transition from the ground state to an excited state is determined using the following formula [30], fexp ¼ 4:318  10À9 r aðmÞ dm, where a is molar extinction coefficient at wavenumber m (cm-1) The a(m) values can be calculated from absorbance A by using Lambert–Beer’s law According to the J–O theory, the calculated oscillator strength fcal of an induced electric-dipole transition from the ground state wJ to an excited state w0 J is given by [30]: Fig Typical PLE spectrum monitored at 594 nm emission line of LaF3:5 %Sm3? nanocrystals Absorption spectrum is shown for comparison 123 fcal ẳ 8p2 mcm n2 ỵ 2ị 3h2J ỵ 1ị 9n X k¼2;4;6    2 Xk  wJ U k w0 J  ð1Þ J Mater Sci: Mater Electron where n is the refractive index of the material, J is the total angular momentum of the ground state, Xk (k ¼ 2; and 6)  2 are the J–O intensity parameters and U k  are the doubly reduced matrix elements of the unit tensor operator calculated from intermediate coupling approximation for a transition wJ ! w0 J For LaF3 n is given by [31]: n2 ¼ where k is wavelength in micrometer The ỵ k21:5376k 0:08812 reduced matrix elements for Sm3? ion are independent on host matrix and are taken from the work of Carnall et al [32] The J–O intensity parameters Xk are determined by a standard-least squares fitting method, which gives the best fit between experimental and calculated oscillator strengths The calculated oscillator strengths are then obtained using Xk and Eq (2) The quality of the fit has been expressed by the root mean square (r.m.s.) deviation of oscillator strengths r [21, 22]: "PÀ r¼ fexp À fcal N Á2 #1=2 ð2Þ where N denotes the total number of excited energy levels used for least-square fit In the case of overlapping absorption bands, the matrix elements of each of the transitions contributing to the overlapping band can be summed up, because the reduced matrix elements possess additivity and they can act independently of each other The overlapping absorption band then is integrated as a whole [30] The experimental and calculated oscillator strengths, their r.m.s deviation and J–O intensity parameters for the LaF3:5 %Sm3? nanocrystals are presented in Table The J–O intensity parameters are calculated to be X2 ¼ 6:823  10À20 cm2 , X4 ¼ 9:055  10À20 cm2 and X6 ¼ 3:994  10À20 cm2 The r.m.s deviation of oscillator strengths r ¼ 0:993  10À6 It is noticed from Table that the 6H5/2 ? 6P3/2 transition in visible region and the 6H5/2 ? F7/2 transition in NIR region possess the highest fexp and fcal values The J–O intensity parameters Xk of %Sm3?-doped LaF3 nanocrystals were calculated with various set of levels and the results are shown in Table In general, the Xk parameters are highly dependent on the ligand field of RE ions In addition, the Xk values also depend on the nature of levels used in the fit [16, 17] It can be seen that the Xk values obtained when all the observed levels are used for fitting are more or less similar to those obtained using all the levels except in turn 4L15/2, 4I13/2, 4D7/2, 6P7/2, or 4M15/2 level In particular, the X2 has negative value when all levels except the group of levels 6F5/2 ? 6F3/2 ? 6H15/2 ? 6F1/2 are used in the fit The Xk values obtained by using only UV–Vis levels or only NIR levels for fitting differ to those obtained by using all the levels It is worth noting that the value of X2 parameter is strongly changed Among the Xk parameters, X2 is sensitive to the covalency, structural change and asymmetry of the ligand field around the Sm3? site [33, 34] For Sm3? ion doped in some glasses [16, 17], the value of X2 increases with increasing the covalency between an RE ion and the ligand field and reducing the symmetry of the ligand field around the Sm3? ion The high magnitude of X2 in the present work indicates the increase of covalent bonding and the decrease of the symmetry of Sm3? site in LaF3 nanocrystals Compared with the case of the Sm3?-doped LaF3 crystal, the Xk values in %Sm3?-doped LaF3 nanocrystals are much larger Furthermore, the J–O parameters obtained by least square fit are used to predict the radiative properties of 4G5/2 excited states of Sm3? ion The radiative transition probability AR ðwJ; w0 J Þ for a transition wJ ! w0 J is the sum of electric and magnetic-dipole transition probabilities [30] and can be calculated from the following equation [30]: AR ðwJ; w0 J Þ ẳ Aed ỵ Amd " # 64p4 m3 n n2 ỵ 2ị Sed ỵ n Smd ẳ 3h2J þ 1Þ ð3Þ where Sed and Smd are the electric and magnetic-dipole line strengths, respectively The total radiative transition probability AT ðwJ Þ is expressed as X sR ðwJ ÞÀ1 ¼ AT ðwJ Þ ¼ AR ðwJ; w0 J Þ ð4Þ w0 J À Á Then, the stimulated emission cross-section rem kp can be expressed as ! k4p rem kp ẳ 5ị AR wJ; w0 J Þ 8pcn2 Dkeff where kp is the peak wavelength and Dkeff is its effective line width found by dividing the area of the emission peak by its average height The radiative transition probabilities Aed , Amd , AR , radiative lifetime sR for 4G5/2 level were calculated using the obtained Xk parameters and are presented in Table From the values of radiative transition probabilities of Table 4, it is noted that 4G5/2 ? 6H7/2 transition has the highest radiative transition rate compared to the other transitions Hence this transition is very useful for laser emission The emission band position kp , effective band width Dkeff , calculated bR and experimental bexp branching À Á ratios and peak stimulated emission cross-section rem kp of the 4G5/2 level of Sm3? ion in LaF3 nanocrystals are shown in Table As seen from the table, the predicted branching ratio of 4G5/2 ? 6H7/2 transition gets a maximum value being 0.3762, whereas the measured branching ratio is 0.5560 The experimental values of bexp are reduced in order as 4G5/2 ? 6H7/2 [ 6H5/2 [ 6H9/2 [ 6H11/2 It should be 123 J Mater Sci: Mater Electron Table The absorption transition wavenumbers mnc , refractive index n, experimental fexp and calculated fcal oscillator strengths, their r.m.s deviation r and J–O intensity parameters Xk for the LaF3:5 %Sm3? nanocrystals (the fcal values are obtained using Set A in Table 3) Transition 6H5/2? vnc (cm-1) n fexp (910-6) Matrix elements [32] U(2) U(4) U(6) fcal (910-6) 29,070 1.627 0.0005 0.0375 2.620 27,701 1.623 0.0001 0.0251 3.538 1.676 P7/2 L15/2 26,738 25,707 1.621 1.619 0 0.0016 0.0751 0.0060 3.179 0.815 2.229 0.163 25,000 1.617 0.1684 10.870 10.080 24,096 1.616 0.0263 2.061 1.515 21,645 1.611 0.0030 0.0228 1.262 0.673 20,877 1.610 0 0.0307 2.426 0.673 10,560 1.597 0.0006 0.0515 0.970 0.580 9191 1.596 0.0206 0.3413 4.173 3.703 8052 1.595 0.0020 0.1429 0.4301 5.823 6.323 7047 1.595 0.3715 0.4204 0.0043 11.650 11.640 D7/2 D3/2 P3/2 P5/2 I13/2 M15/2 F11/2 F9/2 F7/2 6 6 F5/2 ? F3/2 ? H15/2 ? F1/2 X2 ¼ 6:823  10À20 cm2 X4 ¼ 9:055  10À20 cm2 X6 ¼ 3:99410À20 cm2 Table J–O intensity parameters (Â10À20 cm2 ) derived from different set of levels for LaF3:5 %Sm3? nanocrystals and corresponding r.m.s deviation r (Â10À6 ) LaF3:5 %Sm3? nanocrystals r ¼ 0:887  10À6 X2 X6 r Order Set A: All observed levels 6.82 9.05 3.99 ±0.993 X4 [ X2 [ X6 6.82 9.06 3.99 ±1.017 X4 [ X2 [ X6 Set C: Except 4I13/2 6.82 9.06 3.99 ±1.020 X4 [ X2 [ X6 Set D: Except 4D7/2 6.78 9.10 3.96 ±0.940 X4 [ X2 [ X6 Set E: Except 6P7/2 6.76 9.11 3.95 ±0.993 X4 [ X2 [ X6 Set F: Except 4M15/2 6.76 9.12 3.95 ±0.888 X4 [ X2 [ X6 11.97 4.50 4.91 ±0.207 X2 [ X6 [ X4 1.0 0.5 1.5 – X6 [ X2 [ X4 LaF3:Sm3? crystal [26] 3? J–O parameters for LaF3:Sm Transition 4G5/2? crystals [26] are shown for comparison vnc (cm-1) n Matrix elements [22] U(2) U(4) U(6) Aed (s-1) Amd (s-1) AR (s-1) 6906 1.595 0.0001 0.0005 0.42 0.42 8354 1.596 0.0018 0.0003 0.0002 4.07 4.07 9578 1.596 0.0017 0.0002 6.29 1.15 7.44 10,482 1.597 0.0072 0.0017 32.92 3.75 36.67 10,989 1.598 0.0011 0.0001 4.96 5.53 10.49 11,086 1.598 0 0.0002 0.48 0.48 11,236 1.598 0.0010 0 4.30 4.30 12,579 1.599 0.0002 0.0018 7.97 7.97 14,184 1.601 0.0053 0.0021 71.86 71.86 15,649 1.602 0.0112 0.0067 0.0020 248.85 248.85 16,835 17,857 1.604 1.605 0.0001 0.0003 0.0086 0.0006 0.0089 244.66 19.18 13.37 16.11 258.03 35.29 F11/2 F9/2 F7/2 F5/2 F3/2 H15/2 F1/2 H13/2 H11/2 H9/2 H7/2 H5/2 AT ¼ 123 X4 Set B: Except 4L15/2 Set I: Only levels in NIR region Table The electric Aed , magnetic-dipole Amd transition probabilities, radiative transition probability AR and radiative lifetime sR for 4G5/2 level of Sm3? ion in LaF3 nanocrystals 1.194 P AR ¼ 685:87 sÀ1 sR ¼ Ầ1 T ¼ 1:46 ms J Mater Sci: Mater Electron Table Emission band position kp , effective band width Dkeff , calculated bR and experimental bexp branching ratios and peak stimulated À Á emission cross-section rem kp of the 4G5/2 level of Sm3? ion in LaF3 nanocrystals Transition4G5/2? kp (nm) Dkeff (nm) bR bexp À Á rem kp ðÂ10À22 cm2 Þ 705 19 0.1048 0.0392 4.82 639 16 0.3628 0.1848 13.40 594 11 0.3762 0.5560 14.40 560 0.0515 0.2200 2.20 H11/2 H9/2 H7/2 H5/2 noted that the contribution of the three main transitions G5/2 ? 6H5/2 (560 nm), 4G5/2 ? 6H7/2 (594 nm) and G5/2 ?6H9/2 (639 nm) to the total branching ratio is about 96 % The value of stimulated emission cross-section rem ðkp Þ for the 4G5/2 ? 6H7/2 transition is found to be 14.40 Â10À22 cm2 This large stimulated emission cross-section is attractive to low-threshold, high gain laser applications The values of rem ðkp Þ for the 4G5/2 emission transitions are in the order of 4G5/2 ? 6H7/2 [ 6H9/2 [ 6H11/2 [ 6H5/2, which are similar to the case of Sm3? ion doped in glasses [16–23] As seen from Table 4, the predicted radiative lifetime sR for 4G5/2 level of Sm3? ion in LaF3 nanocrystals is calculated to be 1.46 ms The decay of PL in 340-500 nm region under 400 nm excitation has been measured and the result is shown in Fig The average experimental lifetime was determined to be sexp ¼ 1:20 ms smaller than the predicted radiative lifetime sR The discrepancy between theoretical and experimental lifetimes can be attributed to non-radiative relaxation (multiphonon decay and energy transfer) The measured lifetime includes all relaxation processes (both radiative and non-radiative processes) and can be expressed as [30] sexp ẳ ỵ WMP ỵ WET sR 6ị where WMP is the rate of multiphonon relaxation, WET is the rate of energy transfer The WMP is proportional to expðÀaDE= hxÞ, where a is a positive host-dependent constant, DE is the energy gap to the next lower level and hx is the phonon energy of the host material [23, 35] In  the case of Sm3? ion doped in LaF3 nanocrystals, the energy gap DE between 4G5/2 level and the next lower level F11/2 is approximately 6900 cm-1 much larger than phonon energy of LaF3 (350 cm-1) Hence the multiphonon relaxation is negligible and the rate of energy transfer is given by WET ¼ 1 À sexp sR ð7Þ WET for 4G5/2 level of Sm3? ion in LaF3 nanocrystals is found to be 148 sÀ1 The luminescence quantum efficiency of the excited state g ¼ sexp =sR is equal to the ratio of the experimental lifetime to the radiative lifetime Quantum efficiency of the excited 4G5/2 state of Sm3? ion in LaF3 nanocrystals is calculated to be 82 % The energy transfer maybe occurs mainly through cross-relaxation The channels that could be responsible for cross-relaxation of Sm3? ions in LaF3 nanocrystals are (4G5/2 ? 6F5/2) ? (6H5/2 ? F11/2) and (4G5/2 ? 6F11/2) ? (6H5/2 ? 6F5/2) because the energy differences between these transitions are negligible In these cross-relaxation processes the energy transfers from the Sm3? ion in the excited 4G5/2 state to a near-by Sm3? ion in the ground 6H5/2 state Conclusion The LaF3 and LaF3:Sm3? nanocrystals have been synthesized hydrothermal method The nanocrystals possess  space group The lattice hexagonal structure with P3c1 ˚ and parameters were calculated to be a = 7.167 A ˚ c = 7.323 A Absorption, PL and PLE spectra related to Sm3? ion have been investigated in detail Judd–Ofelt theory has been applied to the absorption spectrum to estimate Xk (k = 2, 4, 6) intensity parameters The results show that X2 ¼ 6:823  10À20 cm2 , X4 ¼ 9:055  10À20 cm2 and X6 ¼ 3:994  10À20 cm2 with the r.m.s deviation of oscillator strengths r ¼ 0:993  10À6 Based on the obtained J–O intensity parameters, the radiative properties of the 5G5/2 excited level have been predicted It is found that 4G5/2 ? 6H7/2 (594 nm) transition has the highest radiative transition rate compared to the other transitions In addition, the 4G5/2 ? 6H7/2 transition has maximal experimental branching ratio bexp ẳ 0:5560ị and maximal stimulated emission cross-section À22 ðrem ðkp Þ ¼ 14:40  10 cm Þ Luminescence quantum efficiency of the excited 4G5/2 state of Sm3? ion in LaF3 nanocrystals is determined to be 82 % Acknowledgments The authors would like to express appreciation to Center for Materials Science, Faculty of Physics, Hanoi University of Science, Vietnam National University for material characterization References X Zhang, T Hayakawa, M Nogami, IOP Conf Ser Mater Sci Eng 1, 012021 (2009) X Yang, X Dong, J Wang, G Liu, J Alloys Compd 487, 298 (2009) 123 J Mater Sci: Mater Electron G Luo, C Xiaoxia, W Wei, P Bo, F Dianyuan, J Rare Earths 31, 645 (2013) Z Wang, C Liu, Y Wang, Z Li, J Alloys Compd 509, 1964 (2011) L.G Jacobsohn, K.B Sprinkle, C.J Kucera, T.L James, S.A Roberts, H Qian, E.G Yukihara, T.A DeVol, J Ballato, Opt Mater 33, 136 (2010) J.J Vela´zquez, V.D Rodrı´guez, A.C Yanes, J del-Castillo, J Me´ndez-Ramos, Opt Mater 34, 1994 (2012) R Qin, H Song, G Pan, X Bai, B Dong, S Xie, L Liu, Q Dai, Q Xuesong, X Ren, H Zhao, Cryst Growth Des 9, 1750 (2009) C.-C Mi, Z.-h Tian, B.-f Han, C.-b Mao, X Shu-kun, J Alloys Compd 525, 154 (2012) S Janssens, G.V.M Williams, D Clarke, J Appl Phys 109, 023506 (2011) 10 A.M Cross, P Stanley May, F.C.J.M van Veggel, M.T Berry, J Phys Chem C 114, 14740 (2010) 11 K Fukuda, N Kawaguchi, S Ishizu, T Yanagida, T Suyama, M Nikl, A Yoshikawa, Opt Mater 32, 1142 (2010) 12 M Yao, A.G Joly, W Chen, J Phys Chem C 114, 826 (2010) 13 V.D Rodrı´guez, J Del Castillo, A.C Yanes, J Me´ndez-Ramos, M Torres, J Peraza, Opt Mater 29, 1557 (2007) 14 Q Wang, Y You, R.D Ludescher, Y Ju, J Lumin 130, 1076 (2010) 15 J Pichaandi, F.C.J.M van Veggel, M Raudsepp, Appl Mater Interfaces 2, 157 (2010) 16 C.K Jayasankar, P Babu, J Alloys Compd 307, 82 (2000) 17 V Venkatramu, P Babu, C.K Jayasankar, T Troăster, W Sievers, G Wortmann, Opt Mater 29, 1429 (2007) 18 K.S.V Sudhakar, M Srinivasa Reddy, L Srinivasa Rao, N Veeraiah, J Lumin 128, 1791 (2008) 19 T Suhasini, J Suresh Kumar, T Sasikala, K Jang, H.S Lee, M Jayasimhadri, J.H Jeong, S.S Yi, L Rama Moorthy, Opt Mater 31, 1167 (2009) 123 20 J Suresh Kumar, K Pavani, T Sasikala, A Sreenivasa Rao, N.K Giri, S.B Rai, L Rama Moorthy, Solid State Sci 13, 1548 (2011) 21 C Madhukar Reddy, G.R Dillip, K Mallikarjuna, Sd Zulifiqar Ali Ahamed, B Sudhakar Reddy, B Deva Prasad Raju, J Lumin 131, 1368 (2011) 22 S Thomas, R George, S.N Rasool, M Rathaiah, V Venkatramu, C Joseph, N.V Unnikrishnan, Opt Mater 36, 242 (2013) 23 J Yang, B Zhai, X Zhao, Z Wang, H Lin, J Phys Chem Solids 74, 772 (2013) 24 W Zhou, Q Zhang, J Xiao, J.Q Luo, W Liu, H Jiang, S Yin, J Alloys Compd 491, 618 (2010) 25 P Van Do, V.P Tuyen, V.X Quang, N.T Thanh, V.T.T Ha, N.M Khaidukov, Y.-I Lee, B.T Huy, J Alloys Compd 520, 262 (2012) 26 R.P Leavitt, C.A Morrison, J Chem Phys 73, 749 (1980) 27 M Ferhi, C Bouzidi, K Horchani-Naifer, H Elhouichet, M Ferid, J Lumin 157, 21 (2015) 28 P Ghosh, A Kar, A Patra, J Appl Phys 108, 113506 (2010) 29 B.E Warren, X-ray Diffraction (Dover publications Inc, New York, 1990), p 253 30 C Gorller-Walrand, K Binnemans, in Handbook on the Physics and Chemistry of Rare Earths, ed by K.A Gschneidner, L Eyring (Elsevier, New York, 1998), Chapter 167, p 101 31 R Laiho, M Lakkisto, Philos Mag B 48, 203 (1983) (As cited in Handbook of optical materials, Marvin J Weber, CRC Press, Boca Raton, London, New York, Washington, DC, 2003) 32 W.T Carnall, P.R Fields, K Rajnak, J Chem Phys 49, 4412 (1968) 33 S Tanabe, T Ohayagi, N Soga, T Hanada, Phys Rev B 46, 3305 (1992) 34 H Ebendorff-Heidepriem, D Ehrt, M Bettinelli, A Speghini, J Non-Cryst, Solids 240, 66 (1998) 35 T Miyakawa, D.L Dexter, Phys Rev B 1, 2961 (1970) ... LaF3 :Sm3 ? nanocrystals and aquo -ions, respectively, and N  is the number of levels that are used to calculate b The value of d ¼ À0:53 indicates that the nature of the Sm3 ?-ligand bond in LaF3 nanocrystals. .. 6F3/2, 6H15/2, and 6F1/2 states of Sm3 ? ions, respectively The absorption band wavenumbers for Sm3 ? ions doped in LaF3 nanocrystals and aquo -ions along with nephelauxetic ratio b and bonding parameter... of La2O3 and 1.046 g Sm2 O3 were dissolved in dilute HNO3, and then dissolved in 48 ml deionized water under stirring, resulting in the formation of a colorless solutions of La(NO3)3 and Sm( NO3)3,