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

Based on these Judd–Ofelt parameters, various radiative parameters such as radiative transition probabil-ities, radiative lifetime, branching ratios and stimulated emission cross-section

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, X4and

X6) have been obtained from optical absorption

measure-ments Based on these Judd–Ofelt parameters, various

radiative parameters such as radiative transition

probabil-ities, radiative lifetime, branching ratios and stimulated

emission cross-section for4G5/2 excited level of LaF3:Sm

nanocrystals were predicted

1 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,

opto-electronics, 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;

there-fore, the multiphonon relaxation of the excited states of

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 fluo-rides, LaF3 being an important material has received broad attention due to their potential applications in many fields of science and technique LaF3 unique nanostruc-tures, 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? ion-doped LaF3 nanocrystals fabricated by hydrothermal method We have used the J–O theory to determine

& Hoang Manh Ha

hoangmanhha@hus.edu.vn

1 Hanoi Architectural University, 10 Nguyen Trai,

Thanh Xuan, Hanoi, Vietnam

2 National University of Civil Engineering, 55 Giai Phong,

Hai Ba Trung, Hanoi, Vietnam

3 Center for Materials Science, Hanoi University of Science,

Vietnam National University, 334 Nguyen Trai,

DOI 10.1007/s10854-016-5603-1

absorption spectra of LaF3:Sm3? nanocrystals We have

also predicted radiative transition probabilities, branching

ratios, and radiative lifetimes for the4G5/2 excited state of

Sm3?ion in LaF3nanocrystals

2 Experimental

LaF3 nanocrystals doped with 1, 2, 3, 4 and 5 mol% of

Sm3? ions were prepared by hydrothermal method All

the chemicals used in our experiment, including

lan-thanum 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 min 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 1 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

precipi-tates were collected by centrifugation (6000 rpm) for

20 min 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

diam-eter of 1.3 cm and a thickness of 0.11 cm

Crystal structure of the synthesized samples was

char-acterized by an X-ray diffractometer SIMEMS D5005,

Bruker, Germany with Cu–Ka1 irradiation (k =

1.54056 A˚ ) The morphology of the samples was observed

by using a transmission electron microscopy Tecnai G220

FEI The optical absorption spectra were recorded in the

range of wavelength from 200 to 3000 nm using a

spec-trophotometer Cary-5000 Room temperature

photolumi-nescence (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

Fluo-rescence Spectrophotometer

3 Results and discussion

X-ray diffraction (XRD) patterns of pure LaF3and LaF3

:-Sm3? nanocrystals are presented in Fig.1 All the XRD peaks are unambiguously indexed to hexagonal phase with P3c1 space group of LaF3structure (JCPDS card no 32-0483) 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 were calculated to be a = 7.167 A˚ and c = 7.323 A˚ in good agreement with standard bulk values By applying Scherrer’s formula L½ ¼ 0:9k=ðbcoshÞ [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.2 shows a transmis-sion electron microscopy (TEM) image, high resolution (HR) TEM image and corresponding fast Fourier trans-formation (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.3 shows the optical absorption spectra of pure LaF3, and LaF3:5 %Sm3? samples for ultraviolet–visible–near infra-red (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 1 XRD patterns of LaF3nanocrystals doped with different Sm 3?

concentrations

1554 nm are assigned to the transitions from the ground

state6H5/2to various exited4D7/2,4D3/2,6P7/2,4L15/2,6P3/2,

6P5/2,4I13/2,4M15/2,6F11/2,6F9/2,6F7/2,6F5/2,6F3/2,6H15/2,

and6F1/2states 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 Table1 The

bonding parameter d is defined as d¼ 1 b

= b

 100 where the average value b¼ P

b

ð Þ=N, b ¼ mnc=ma, mncand

maare 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 d¼ 0:53 indicates that the nature of the

Sm3?-ligand bond in LaF3nanocrystals is ionic

Typical room-temperature PL spectra of undoped LaF3

and LaF3:5 %Sm3? nanocrystals are shown in Fig.4 As

seen from figure, undoped LaF3sample does not emit light

Whereas LaF3:5 %Sm3? sample exhibits emission

spec-trum with four dominant peaks at 560, 594, 639 and

705 nm corresponding to the emission transitions from the

4G5/2 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 electric-dipole (ED) transitions, whereas the other two transitions

4

G5/2?6H5/2and6H7/2contain 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 5 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

5 %Sm3? ions doped in LaF3 nanocrystals is shown in Fig.6 As shown in the figure, the non-exponential decay curve has been fitted to the tri-exponential function:

Fig 2 a TEM image, b HRTEM image and c corresponding FFT pattern of LaF3nanocrystals

Fig 3 The room-temperature absorption spectra of pure LaF3and LaF3:5 %Sm3?samples in range of a 330–500 nm and b 900–1750 nm

IðtÞ ¼ A1exp  t

s1

þ A2exp  t

s2

þ A3exp  t

s3

with A1¼ 176  8; s1¼ 2:06  0:08 ms; A2¼ 341 6; s2

¼ 0:45  0:02 ms and A3¼ 7000  4000; s3¼ 0:024 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  109r a mð Þ 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 w0J0is given by [30]:

fcal¼ 8p

2mcm 3h 2Jð þ 1Þ

n2þ 2

9n

X

k¼2;4;6

Xk wJ U kw0J0 2

ð1Þ

Table 1 Absorption transition

wavenumbers for Sm3?ions

doped in LaF3nanocrystals m nc

and aquo-ions m a along with

nephelauxetic ratio b and

bonding parameter d



Fig 4 The room-temperature PL spectra under 400 nm excitation of

the pure LaF3and LaF3:5 %Sm 3? nanocrystals

Fig 5 Typical PLE spectrum monitored at 594 nm emission line of

LaF3:5 %Sm 3? nanocrystals Absorption spectrum is shown for

comparison

Fig 6 Luminescence decay curve of Sm 3? ions doped in LaF3 nanocrystals under 400 nm excitation

where n is the refractive index of the material, J is the total

angular momentum of the ground state, Xk(k¼ 2; 4 and 6)

are the J–O intensity parameters and U k2

are the doubly reduced matrix elements of the unit tensor operator

cal-culated from intermediate coupling approximation for a

transition wJ! w0J0 For LaF3 n is given by [31]: n2¼

1þ 1:5376k 2

k 2 0:0881 2 where k is wavelength in micrometer The

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 Xkare 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]:

r¼

P

fexp fcal

N

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

parame-ters for the LaF3:5 %Sm3? nanocrystals are presented in

Table2 The J–O intensity parameters are calculated to be

X2¼ 6:823  1020cm2, X4¼ 9:055  1020cm2 and

X6¼ 3:994  1020cm2 The r.m.s deviation of oscillator

strengths r¼ 0:993  106 It is noticed from Table2that

the6H5/2?6P3/2transition in visible region and the6H5/2?

6F7/2transition in NIR region possess the highest fexpand fcal

values The J–O intensity parameters Xk of 5 %Sm3?-doped

LaF3nanocrystals were calculated with various set of levels

and the results are shown in Table3 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 Xkvalues

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 levels6F5/2?6F3/2?6H15/2?6F1/2are 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 X2parameter

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 X2increases 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 LaF3nanocrystals Compared with the case of the Sm3?-doped LaF3crystal, the Xk values

in 5 %Sm3?-doped LaF3nanocrystals are much larger Fur-thermore, the J–O parameters obtained by least square fit are used to predict the radiative properties of4G5/2excited states

of Sm3?ion The radiative transition probability ARðwJ; w0J0Þ for a transition wJ! w0J0 is the sum of electric and mag-netic-dipole transition probabilities [30] and can be calculated from the following equation [30]:

ARðwJ; w0J0Þ ¼ Aedþ Amd

4m3

3h 2Jð þ 1Þ

n nð 2þ 2Þ2

9 Sedþ n3Smd

ð3Þ

where Sedand Smdare the electric and magnetic-dipole line strengths, respectively The total radiative transition prob-ability ATðwJÞ is expressed as

sRðwJÞ1¼ ATðwJÞ ¼X

w0J 0

ARðwJ; w0J0Þ ð4Þ

Then, the stimulated emission cross-section rem kp

can be expressed as

rem kp

 

4 p

8pcn2Dkeff

!

ARðwJ; w0J0Þ ð5Þ

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 Table4 From the values of radiative transition probabilities of Table4, 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 bRand experimental bexpbranching ratios and peak stimulated emission cross-section rem kp

 

of the 4G5/2 level of Sm3? ion in LaF3 nanocrystals are shown in Table5 As seen from the table, the predicted branching ratio of4G5/2?6H7/2transition gets a maximum value being 0.3762, whereas the measured branching ratio

is 0.5560 The experimental values of bexp are reduced in order as4G5/2?6H7/2[6H5/2[6H9/2[6H11/2 It should be

Table 2 The absorption transition wavenumbers mnc, refractive index n, experimental fexpand calculated fcaloscillator strengths, their r.m.s deviation r and J–O intensity parameters Xk for the LaF3:5 %Sm 3? nanocrystals (the fcalvalues are obtained using Set A in Table 3 )

Table 3 J–O intensity

parameters (10 20 cm 2 )

derived from different set of

levels for LaF3:5 %Sm3?

nanocrystals and corresponding

r.m.s deviation r (10 6 )

J–O parameters for LaF3:Sm 3? crystals [ 26 ] are shown for comparison

Table 4 The electric A ed ,

magnetic-dipole A md transition

probabilities, radiative transition

probability A R and radiative

lifetime s R for4G5/2level of

Sm3?ion in LaF3nanocrystals

Transition4G5/2? vnc(cm-1) n Matrix elements [ 22 ] Aed(s-1) Amd(s-1) AR(s-1)

U(2) U(4) U(6)

AT¼ PA

R ¼ 685:87 s 1 s R ¼ A 1

T ¼ 1:46 ms

noted that the contribution of the three main transitions

4G5/2?6H5/2 (560 nm), 4G5/2?6H7/2 (594 nm) and

4

G5/2?6H9/2(639 nm) to the total branching ratio is about

96 % The value of stimulated emission cross-section

remðkpÞ for the4G5/2?6H7/2transition is found to be 14.40

1022cm2 This large stimulated emission cross-section is

attractive to low-threshold, high gain laser applications The

values of remðkpÞ for the4G5/2emission transitions are in the

order of4G5/2? 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 Table4, the predicted radiative lifetime sR

for4G5/2 level of Sm3?ion in LaF3nanocrystals is

calcu-lated 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.6 The average experimental lifetime was

determined to be sexp¼ 1:20 ms smaller than the predicted

radiative lifetime sR The discrepancy between theoretical

and experimental lifetimes can be attributed to

non-radia-tive 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]

1

sexp

¼ 1

sR

where WMP is the rate of multiphonon relaxation, WET is

the rate of energy transfer The WMP is proportional to

expðaDE=hxÞ, where a is a positive host-dependent

constant, DE is the energy gap to the next lower level and

x is the phonon energy of the host material [23,35] In

the case of Sm3? ion doped in LaF3 nanocrystals, the

energy gap DE between4G5/2level and the next lower level

6F11/2 is approximately 6900 cm-1 much larger than

pho-non energy of LaF3 (350 cm-1) Hence the multiphonon

relaxation is negligible and the rate of energy transfer is

given by

WET ¼ 1

sexp

 1

sR

ð7Þ

WET for 4G5/2 level of Sm3? ion in LaF3 nanocrystals is

found to be 148 s1 The luminescence quantum efficiency

of the excited state g¼ sexp=sR is equal to the ratio of the

experimental lifetime to the radiative lifetime Quantum

efficiency of the excited4G state of Sm3? ion in LaF

nanocrystals is calculated to be 82 % The energy transfer maybe occurs mainly through cross-relaxation The chan-nels that could be responsible for cross-relaxation of Sm3? ions in LaF3nanocrystals are (4G5/2 ?6F5/2) ? (6H5/2?

6F11/2) and (4G5/2?6F11/2) ? (6H5/2?6F5/2) because the energy differences between these transitions are neg-ligible 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 ground6H5/2 state

4 Conclusion The LaF3and LaF3:Sm3? nanocrystals have been synthe-sized hydrothermal method The nanocrystals possess hexagonal structure with P3c1 space group The lattice parameters were calculated to be a = 7.167 A˚ and

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 X2¼ 6:823  1020cm2, X4¼ 9:055 

1020cm2 and X6¼ 3:994  1020cm2 with the r.m.s deviation of oscillator strengths r¼ 0:993  106 Based

on the obtained J–O intensity parameters, the radiative properties of the5G5/2excited 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 ratioðbexp¼ 0:5560Þ and maximal stimulated emission cross-section

ðremðkpÞ ¼ 14:40  1022cm2Þ 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.

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