Preparation and spectroscopic characterization of ... - UTSA Physics

JOURNAL OF APPLIED PHYSICS 105, 093105 共2009兲

Preparation and spectroscopic characterization of Nd3+ : Y2O3 nanocrystals suspended in polymethyl methacrylate Dhiraj K. Sardar,1,a兲 Sreerenjini Chandra,1 John B. Gruber,1 Waldemar Gorski,2 Maogen Zhang,2 and Jun Ho Shim2 1

Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas 78249-0697, USA 2 Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78249-0698, USA

共Received 17 December 2008; accepted 24 March 2009; published online 4 May 2009兲 We describe a method to fabricate polymethyl methacrylate 共PMMA兲, a polymeric host, in which nanocrystals of Nd3+ : Y2O3 are suspended. The spectroscopic properties of this material are analyzed using the standard Judd–Ofelt technique. The phenomenological Judd–Ofelt intensity parameters are used to calculate the radiative decay rates and the branching ratios of the 4F3/2 → 4IJ 共J = 9 / 2, 11/2, 13/2, and 15/2兲 intermanifold transitions. The room temperature fluorescence lifetime has been measured for the most intense 4F3/2 → 4I11/2 emission transition. Emission cross sections for the intense intermanifold transitions and peak emission cross sections for the intense inter-Stark transitions are also reported. Assignments to individual Stark levels of the 4IJ manifolds have been made and compared with the calculated splittings reported earlier. Finally, the spectroscopic properties of the Nd3+ : Y2O3 nanocrystals suspended in PMMA are compared with those of Nd3+ doped in various host materials. Detailed optical analysis led to favorable values of fluorescence lifetime and emission cross section for the 4F3/2 → 4I11/2 transition, which suggest that the Nd3+ : Y2O3 nanocrystals embedded in PMMA would have potential for various photonic applications including laser systems and optical communication devices. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3122300兴 I. INTRODUCTION

Single and ceramic crystals of trivalent rare-earth 共RE兲 ions doped into cubic yttrium oxide 共Y2O3兲 have attracted significant attention due to their potential as laser materials having high optical and thermal quality.1–8 However, the high melting point of Y2O3 has hindered the use of standard methodologies for growing large crystals of high optical quality.9 Walsh et al.10 showed that single-crystal Nd3+ : Y2O3 shows a great deal of potential for 0.88 ␮m lasing from the 4 F3/2 → 4I9/2 transition with pulsed Q-switch operation. However, they also pointed out that there is a limitation such as the lack of availability of good optical quality single crystal. Recently, the authors have compared the optical and spectroscopic properties of nanocrystals and polycrystalline ceramics of Nd3+ : Y2O3 and Er3+ : Y2O3 with their respective single-crystal counterparts.3,11,12 In recent years, luminescent polymers have drawn a great deal of attention due to their important technological applications. These polymers are found to be potential candidates for the development of light emitting diodes and organic based lasers.13 One of the major advantages of REdoped plastic laser host materials is their ability to be easily drawn into flexible optical fibers for various photonic applications, e.g., laser systems and optical communication devices such as polymer optical fiber amplifier and integrated waveguide.14,15 In this article we present the preparation and spectroscopic characterization of Nd3+ : Y2O3 nanocrystals suspended in a polymeric plastic host polymethyl methacrya兲

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0021-8979/2009/105共9兲/093105/8/$25.00

late 共PMMA兲, which is one of the best vinyl polymers for numerous photonic applications owing to its exceptional optical clarity, simple synthesis, good weather resistance, high mechanical strength, excellent dimensional stability, and resistance to laser damage.14–17 The optical and spectroscopic properties of Nd3+ : Y2O3 nanocrystals suspended in PMMA 共Nd3+ : Y2O3 / PMMA兲 are characterized by employing a standard Judd–Ofelt 共JO兲18,19 formalism. The JO analysis has been applied to the room temperature absorption spectrum of Nd3+ : Y2O3 / PMMA to determine the radiative decay rates and emission branching ratios of Nd3+ transitions from the 4F3/2 metastable manifold to the 4IJ lower-lying multiplet manifolds. The fluorescence lifetime has been measured for the 4F3/2 metastable state and compared with that obtained for Nd3+ in various hosts. We have determined the emission cross sections of the 4F3/2 → 4IJ 共J = 9 / 2, 11/2, and 13/2兲 intermanifold transitions and also the peak emission cross sections of the major inter-Stark transitions within the corresponding intermanifold multiplets. A detailed Stark energy level analysis for transitions within the corresponding multiplet manifolds has been performed as well. The JO intensity parameters are compared with those for Nd3+ ions doped in various hosts. A comprehensive study of the spectroscopic properties has been already performed by Gruber and co-workers1,2,20–23 on Nd3+ in single crystals of Y2O3 and Y3Al5O12. II. EXPERIMENTAL DETAILS A. Materials preparation

The Nd3+ : Y2O3 nanocrystals were prepared by a slow precipitation method from a homogenous solution of dis-

105, 093105-1

© 2009 American Institute of Physics

Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

J. Appl. Phys. 105, 093105 共2009兲

Sardar et al.

FIG. 1. SEM image of the Nd3+ : Y2O3 nanoparticles embedded in the PMMA-CTAB matrix.

culations, the average Nd3+ concentration over the entire material is 0.65 at. %; this corresponds to an ion density of 6.39⫻ 1020 cm−3. B. Absorption and fluorescence measurements

After verifying the optical transparency of PMMA in the 300–1600 nm wavelength range, the room temperature absorption spectrum was taken on the Nd3+ : Y2O3 / PMMA film by an upgraded Cary model 14R spectrophotometer. The absorption spectrum is shown in Fig. 2; it covers the range from 400 to 850 nm and consists of eight Nd3+共4f 3兲 absorption bands centered around 437, 462, 542, 592, 626, 689, 745, and 820 nm that involve 16 multiplet manifolds 2S+1LJ. The spectrum was taken at 0.1 nm intervals and the spectral bandwidth was automatically maintained at about 0.05 nm for all measurements. Room temperature fluorescence spectra of the 4F3/2 4 → IJ 共J = 9 / 2, 11/2, and 13/2兲 transitions were taken by exciting the sample with 806 nm laser emission from a Spectra Physics Model 3900S cw Ti:sapphire laser and are shown in Fig. 3. The spectra were analyzed by a SPEX Model 1250M monochromator equipped with a liquid nitrogen cooled ger100 2

G7/2+4G5/2

Nd3+:Y2O3/PMMA

80

-1

solved 10% NdCl3, 90% YCl3, and urea. The chemicals are 99.99% pure and purchased from Sigma-Aldrich. More details about the synthesis can be found in Ref. 24. The morphology of Nd3+ : Y2O3 nanocrystals are characterized by a Hitachi S-5500 scanning transmission electron microscope 共STEM兲. X-ray diffraction analysis has been used to confirm the cubic structure of Y2O3. The elemental analyses of the Nd3+ : Y2O3 nanocrystals have been performed using energy dispersive x-ray analysis to determine the doping percentages of the Nd3+ ions. It has been reported by Turkevich et al.25 that the light scattering can affect the intensity of emission when the diameter of the nanoparticles exceeds 50 nm. Hence, in the case of our nanocrystals of diameter about 150–200 nm, there can be a noticeable impact of light scattering on the emission intensity. The incorporation of Nd3+ : Y2O3 nanocrystals into a polymer matrix has been prompted as a consequence of the unique properties of the matrix, such as reduced gas permeability, improved physical performance, and increased heat resistance. The formation of cluster aggregation might seriously affect the distribution of nanocrystals in PMMA. Nonetheless, polymers are lyophobic colloids, which require certain energy for their formation; therefore they are unstable and form aggregates. These effects can be minimized by adding surfactants that envelope the nanoparticles, thereby reducing their aggregation in polymer matrices.14,15,26,27 Surfactants 共surface active reagents兲 are amphiphilic molecules containing a hydrophilic head and a hydrophobic tail. The “head” is either a charged moiety such as a sulfate or a quaternary amine group or a polar moiety such as several glucose units or oligomers of ethylene glycol while the “tail” is generally one or more hydrocarbon chain共s兲.28 The surfactant used in our experiments was cetyl trimethyl ammonium bromide 共CTAB兲. It is a cationic surfactant that forms micelles in aqueous solutions. The predominant “hydrophobic” hydrocarbon chains in PMMA interact with the hydrophobic tail of CTAB, forming aggregates, and the rareearth 共RE兲 nanoparticles are expected to be trapped at these positions resulting in a uniform distribution of particles in the polymer-surfactant matrix. Therefore, the mutual aggregation of nanoparticles can be reduced in such complex material fabrication process. Figure 1 shows the SEM image of Nd3+ : Y2O3 nanocrystals embedded in a PMMA-CTAB matrix. PMMA pellets 共Sigma-Aldrich兲 were dissolved in a 3:1 ratio solution of chloroform and acetone. In 5 ml of this solution was added 10 mg of CTAB 共Sigma-Aldrich兲. Approximately, 50 mg of Nd3+ : Y2O3 nanocrystals were added into the stirred solution of PMMA and CTAB and sonicated to obtain a uniform distribution of nanoparticles. The homogeneous mixture was poured over a Teflon plate and allowed to dry at room temperature over a period of 2–3 days to obtain the Nd3+ : Y2O3 / PMMA sample in the form of a thin film of thickness of 0.5 mm. Glass slides have been avoided in our experiments due to the fact that glass being hydrophilic in nature may oppose the hydrophobic behavior of PMMA and CTAB and thus prevent the uniform distribution of Nd3+ : Y2O3 nanocrystals in PMMA. According to our cal-

Absorption Coefficient (cm )

093105-2

60

4

G11/2+2D3/2+2G9/2+2K15/2 2

40

4 2

P1/2

G9/2+4G7/2+2K13/2 4 2

20

H(2)11/2 4

H(2)9/2+4F5/2

S3/2+4F7/2

F9/2

0 400

500

600

700

800

Wavelength (nm)

FIG. 2. Room temperature absorption spectrum Nd3+ : Y2O3 / PMMA ranging between 400 and 850 nm.

of

Nd3+


in

093105-3

J. Appl. Phys. 105, 093105 共2009兲

Sardar et al.

4

Fluorescence Intensity (Arb. Units)

2.0

1.6

4

F3/2

I11/2

4

I9/2

light. These two conditions are well satisfied for the nanoparticles used in our experiments. The effective index of refraction can be obtained from the following expression:

3+

Nd :Y2O3/PMMA

4

F3/2

(10 at. % NdCl3) 4

F3/2

neff共x兲 = xnY2O3 + 共1 − x兲nPMMA ,

4

I13/2

共1兲

1.2

where x is the “filling factor” showing what fraction of space is occupied by the Nd3+ : Y2O3 nanoparticles in the PMMA medium. The value of x was found to be 0.28. The wavelength-dependent indices of refraction for PMMA and Y2O3 were determined using Sellmeier’s dispersion equation

0.8

(2 at. % NdCl3) 0.4

0.0 900

1000

1100

1200

1300

1400

1500

Wavelength (nm)

n2共␭兲 = A +

FIG. 3. Room temperature fluorescence spectra from the 4F3/2 metastable state for various doping concentrations of Nd3+ in Nd3+ : Y2O3 / PMMA.

manium detector and a reflection grating having 600 grooves/mm blazed at 1.0 ␮m. The fluorescence lifetime was taken for the Nd3+ 4F3/2 → 4I11/2 transition by pulsing the laser beam with a chopper at 110 Hz. The fluorescence decay for this transition was analyzed with a 560 MHz Tektronix oscilloscope 共model TDS 3054B兲 and found to be a single exponential. III. DATA ANALYSIS A. Judd–Ofelt analysis

Eight absorption bands, 2 P1/2, 4G11/2 + 2D3/2 + 2G9/2 + K15/2, 4G9/2 + 4G7/2 + 2K13/2, 2G7/2 + 4G5/2, 2H共2兲11/2, 4F9/2, 4 S3/2 + 4F7/2, and 2H共2兲9/2 + 4F5/2, have been identified in the room temperature absorption spectrum 共Fig. 2兲 and tabulated in Table I. These absorption bands were chosen to determine the phenomenological JO intensity parameters for Nd3+ : Y2O3 / PMMA. A brief outline of the JO analysis is given in the following paragraphs. More details regarding the theory and applications can be found in literature.29–35 According to the studies reported by Meltzer et al.,36,37 it is necessary to introduce an effective index of refraction neff when the RE nanoparticles occupy only a small fraction of the total volume of polymer matrix and when the average size of nanoparticles is much smaller than the wavelength of 2

B␭2 D␭2 + , ␭2 − C ␭2 − E

共2兲

where A, B, C, D, and E are Sellmeier’s coefficients for PMMA and Y2O3 obtained from Refs. 11 and 38, respectively. The effective indices of refraction were calculated by substituting the values of filling factor and indices of refraction for PMMA and Y2O3 in Eq. 共1兲. Those values obtained for the appropriate mean wavelengths of the Nd3+ absorption transition bands are given in Table I. The measured and calculated linestrengths of the chosen absorption bands listed in Table I were used to determine the JO parameters, radiative decay rates, radiative lifetimes, and branching ratios. The JO parameters ⍀2, ⍀4, and ⍀6 tend to vary from host to host with the local environment due to the charges and positions of the ligand ions.39,40 The spectroscopic quality factor, X = ⍀4 / ⍀6, for the Nd3+ : Y2O3 nanocrystals in PMMA was found to be 1.03, which varies from 0.54 to 1.82 for Nd3+ in different host materials.29,41–43 Table II shows the comparison of JO parameters and spectroscopic quality factors obtained for Nd3+ in various hosts.29,41–43 The JO parameters can be applied to calculate the emission linestrengths corresponding to transitions from the upper multiplet manifolds 2S+1LJ to the lower manifolds 2S⬘+1 LJ⬘⬘ of Nd3+共4f 3兲 in Nd3+ : Y2O3 / PMMA. Using these linestrengths, the radiative decay rates A共J → J⬘兲 can be calculated for transitions between the upper manifold 共J兲 and the corresponding lower-lying multiplet manifolds J⬘ by the expression

TABLE I. Values of refractive indices and measured and calculated absorption linestrengths of Nd3+ in Nd3+ : Y2O3 / PMMA at 300 K.

Transition 共from 4I9/2兲 4

F5/2 + 2H共2兲9/2 F7/2 + 4S3/2 4 F9/2 2 H共2兲11/2 4 G5/2 + 2G7/2 2 K13/2 + 4G7/2 + 4G9/2 2 K15/2 + 2G9/2 + 2D3/2 + 4G11/2 2 P1/2 4

¯␭ 共nm兲

neff

Smeas 共10−20 cm2兲

Scalc 共10−20 cm2兲

共⌬S兲2 共10−40 cm4兲

820.4 745.2 689.4 626.4 591.6 541.8 461.2 436.8

1.598 1.599 1.601 1.602 1.603 1.605 1.610 1.612

3.772 3.752 0.341 0.349 12.60 2.647 0.340 0.517

4.00 3.62 0.27 0.07 12.61 2.42 0.43 0.19

0.0539 0.0180 0.0050 0.0788 0.0002 0.0509 0.0076 0.1047


093105-4

J. Appl. Phys. 105, 093105 共2009兲

Sardar et al.

TABLE II. Comparison of spectroscopic of Nd3+ doped in different hosts. Nanocrystalline Nd3+ : Y2O3 / PMMA

Parameters

a

Nd3+ : Y2O3 Nd3+ : Y2O3 共ceramic兲b 共crystal兲c Nd3+ : PMMA

d

Nd3+ : YAG Nd3+ : YAG Nd3+ : YAG 共crystal兲e 共ceramic兲f 共ceramic兲g

⍀2 共10−20 cm2兲 ⍀4 共10−20 cm2兲 ⍀6 共10−20 cm2兲

9.39 5.26 5.11

4.09 2.97 3.85

8.55 5.25 2.89

2.11 3.78 2.61

0.20 2.70 5.00

0.22 2.57 3.71

0.22 3.55 5.33

X = ⍀4 / ⍀6

1.03

0.77

1.82

1.45

0.54

0.69

0.67

Branching ratios

␤共 4F3/2 / 4I9/2兲 ␤共 4F3/2 / 4I11/2兲 ␤共 4F3/2 / 4I13/2兲 ␤共 4F3/2 / 4I15/2兲

0.439 0.466 0.090 0.005

0.398 0.497 0.099 0.005

0.465 0.451 0.080

0.370 0.500 0.130 0.003

0.379 0.512 0.104

0.373 0.514 0.110 0.002

Radiative lifetime

␶共 4F3/2兲共ms兲

0.350

0.354

0.632

0.259

0.316

0.243

Measured lifetime

␶共 4F3/2兲共ms兲

1.35

0.225

0.240

0.252

␴共 4F3/2 / 4I9/2兲 ␴共 4F3/2 / 4I11/2兲 ␴共 4F3/2 / 4I13/2兲 ␴共R1 → Z1兲 ␴共R1 → Y 2兲 ␴共R1 → X2兲

2.54 4.59 0.38 2.91 4.50 1.00

JO parameters

Spectroscopic quality factor

Emission cross sections 共10−20 cm2兲

a

This work. Reference 41. c Reference 29. d Reference 42. e Reference 29. f Reference 34. g Reference 43. b

A共J → J⬘兲 =

64␲4e2

n共n2 + 2兲2 Scalc共J → J⬘兲. 9 ¯3 3h共2J + 1兲␭

共3兲

The radiative lifetime ␶r of the excited state 4F3/2 共J = 3 / 2兲 is calculated by

␶r =

1

兺 A共J → J⬘兲

共4兲

,

where the sum is taken over all the lower-lying states J⬘. The fluorescence branching ratios ␤共J → J⬘兲 for transitions originating from the 4F3/2 manifold are determined from the radiative decay rates by using the following equation:

␤共J → J⬘兲 =

A共J → J⬘兲

兺 A共J → J⬘兲

= A共J → J⬘兲␶r ,

共5兲

Calculated values of the radiative decay rates, branching ratios, and the radiative lifetimes are given in Table II. Fluorescence branching ratio is a critical parameter in laser designing, because it can predict the possibility of attaining stimulated emission. B. Emission intensity analysis

Our experimental results of the fluorescence analysis strongly support the studies reported by Robin et al.44 According to them, the photoluminescence intensity increases

with increase in Nd3+ concentration and attains the peak value corresponding to an approximate concentration of 2 at. %, but decreases with further increase in concentration. Figure 3 shows the dependence of photoluminescence intensity on Nd3+ concentration in the 4F3/2 → 4IJ 共J = 9 / 2, 11/2, and 13/2兲 emission transitions. For the Nd3+ : Y2O3 / PMMA sample doped with nanocrystals synthesized out of 10 at. % NdCl3, the fluorescence intensity was found to be relatively weaker as shown in Fig. 3 obtained with the monochromator slits completely open. For another sample with nanocrystals synthesized out of 2 at. % NdCl3, the photoluminescence intensity was found to be much stronger for the 4F3/2 → 4IJ emissions with the monochromator slit width kept at 200 ␮m. The individual spectra of the stronger emission are shown in Figs. 4–6 representing transitions between many of the Stark levels of the corresponding manifolds. The room temperature fluorescence spectrum shown in Fig. 3 justifies the predicted values of branching ratios given in Table II. The 4F3/2 → 4I11/2 transition is more intense than the 4F3/2 → 4I9/2 and 4F3/2 → 4I13/2 transitions. On the other hand, the 4F3/2 → 4I9/2 transition is relatively stronger for the ceramic11,41 and single crystals44 of Nd3+ : Y2O3. Our result is supported by the fluorescence data obtained by Nash et al.45 for the nanocrystals of Nd3+ : Y2O3 and those when suspended in polymers such as epoxy and chitosan. The significantly small value of branching ratio 共0.0046兲 corresponding to the 4F3/2 → 4I15/2 emission 共given in Table II兲 justifies its


J. Appl. Phys. 105, 093105 共2009兲

Sardar et al.

0.0

870

900

915

930

945

X2 0.0 1300

960

I13/2

X1 R1 1325

1350

Wavelength (nm)

1375

X5

X4

R1

X7 R1

R1

R1

X6

R1 X3 R2 X6 R2 X7

R2

X4

X5

X3 R2

R2 885

4

F3/2

R1 X21

R1

0.5

X1

Z5 Z54

Z4

R2

R1

Z3 R2

Z2 Z1 R2

1.0

R2

Z3 R1

Z4 R2

0.5

4

I9/2 Fluorescence Intensity (Arb. Units)

F3/2

R2

R1 Z1 R1 Z2

1.0

4

4

R2


093105-5

1400

1425

1450

1475

Wavelength (nm)

FIG. 4. Room temperature 4F3/2 → 4I9/2 fluorescence emission of Nd3+ in Nd3+ : Y2O3 / PMMA.


absence in the entire 4F3/2 → 4IJ fluorescence spectrum. According to Chang,21 the intensity of this emission is less than 2% of that to the entire 4IJ multiplet in the case of singlecrystal Nd3+ : Y2O3. The fluorescence lifetime of the 4F3/2 metastable state is determined to be 1.35 ms, which could be affected by the low-index PMMA matrix in which the Nd3+ : Y2O3 nanoparticles are embedded. Table II gives a comparison of the measured and radiative lifetimes of the 4 F3/2 metastable state of Nd3+ doped in various host materials.29,41–43 Emission cross sections of the 4F3/2 → 4I9/2, 4F3/2 4 → I11/2, and 4F3/2 → 4I13/2 intermanifold transitions are obtained using the following equation:

and g共˜␯兲 is the line shape function as given by

␴共J,J⬘ ;˜␯兲 =

␭2 ␤共J,J⬘兲 g共˜␯兲, 8␲cn2 ␶J

4

I11/2

R1

F3/2

Y2 R2

Y3 R1

Y5 R2

R2

Y3

R1 Y1 R2 1040

1060

1080

1100

Y5 R1

Y4

Y5

R1 1120

R1

Y4 R2


Y1

Y2

4

0.5

0.0

冕

I共˜␯兲

where I共˜␯兲 is the intensity at ˜␯. The peak emission cross sections of the major interStark transitions 共R1 → Z1兲, 共R1 → Y 2兲, and 共R1 → X2兲 within the corresponding multiplet manifolds have been determined using the following expressions for a Gaussian line shape:

␴ p共R1 → Z1兲 =

␴ p共R1 → Y 2兲 =


␭ p2

␴ p共R1 → X2兲 =

冋册冋册

ln 2 2 4␲n c⌬˜␯ ␲ ␭ p2

1/2

ln 2 2 4␲n c⌬˜␯ ␲

and ␭ p2

冋册

ln 2 2 4␲n c⌬˜␯ ␲

A共R1 → Z1兲,

共8兲

1/2

A共R1 → Y 2兲,

共9兲

A共R1 → X2兲,

共10兲

1/2

where ␭ p is the wavelength at the peak position of the emission band, c is the speed of light, and ⌬˜␯ is the full width at half maximum linewidth. A共R1 → Z1兲 is the radiative transition rate from the lowest Stark level 共R1兲 of the 4F3/2 manifold to the lowest Stark level 共Z1兲 of the 4I9/2 manifold in Eq. 共8兲; A共R1 → Y 2兲 is the radiative transition rate from the lowest Stark level 共R1兲 of the 4F3/2 manifold to the second lowest Stark level 共Y 2兲 of the 4I11/2 manifold in Eq. 共9兲; and A共R1 → X2兲 is the radiative transition rate from the lowest Stark level 共R1兲 of the 4F3/2 manifold to the second lowest Stark level 共X2兲 of the 4I13/2 manifold in Eq. 共10兲. Expressions for the corresponding radiative transition rates are given as follows:

1140

W avelength (nm)

共7兲

,

I共˜␯兲d˜␯

共6兲

where ␭ is the wavelength at the peak emission, ˜␯ is the wave number, ␤共J , J⬘兲 is the fluorescence branching ratio for the transition from the upper manifold J to the lower manifold J⬘, ␶J is the radiative lifetime of the excited manifold J,

1.0

g共˜␯兲 =

A共R1 → Z1兲 =

共1 + e−⌬/kT兲␤共R1 → Z1兲␤共 4F3/2 → 4I9/2兲

␶共 4F3/2兲


,

共11兲

093105-6

J. Appl. Phys. 105, 093105 共2009兲

Sardar et al. TABLE III. Stark level analysis of the 4F3/2 metastable state.

2S+1

Lj

a

Trans.b

␭

c

I

d

Eexpt e 共cm−1兲

Ecalc f 共cm−1兲

⌬E g 共cm−1兲

4

I9/2

R2 → Z1 R2 → Z2 R1 → Z1 R1 → Z2 R2 → Z3 R2 → Z4 R1 → Z3 R2 → Z5 R1 → Z4 R1 → Z5

877.8 880.0 893.5 895.7 898.5 913.3 915.0 930.0 930.4 947.5

0.10 0.23 0.96 0.94 0.25 0.31 0.67 0.21 0.39 0.48

11 391 11 362 11 190 11 162 11 128 10 948 10 928 10 751 10 747 10 552

11 404 11 375 11 208 11 179 11 137 10 962 10 941 10 761 10 761 10 565

⫺13 ⫺13 ⫺18 ⫺17 ⫺9 ⫺15 ⫺14 ⫺10 ⫺14 ⫺13

4

I11/2

R2 → Y 1 R2 → Y 2 R1 → Y 1 R1 → Y 2 R2 → Y 3 R2 → Y 4 R2 → Y 5 R1 → Y 3 R1 → Y 4 R1 → Y 5 R1 → Y 5

1052.7 1056.8 1075.5 1079.6 1080.5 1095.5 1103.3 1104.6 1119.9 1127.7 1131.0

0.37 0.37 0.93 1.00 0.52 0.26 0.78 1.94 0.46 0.33 0.45

9 498 9 461 9 297 9 261 9 253 9 127 9 062 9 051 8 928 8 866 8 840

9 506 9 469 9 311 9 273 9 257 9 133 9 073 9 061 8 937 8 877 8 849

⫺8 ⫺8 ⫺14 ⫺13 ⫺4 ⫺6 ⫺11 ⫺10 ⫺10 ⫺11 ⫺9

4

I13/2

R2 → X1 R2 → X2 R1 → X1 R1 → X2 R2 → X3 R2 → X4 R2 → X5 R1 → X3 R2 → X6 R2 → X7 R1 → X4 R1 → X5 R1 → X6 R1 → X7

1318.4 1322.3 1354.3 1358.5 1367.7 1388.8 1404.6 1406.6 1408.5 1413.8 1427.9 1444.5 1449.6 1454.8

0.29 0.26 0.81 0.96 0.24 0.07 0.12 0.16 0.10 0.07 0.11 0.22 0.16 0.19

7 583 7 561 7 382 7 360 7 310 7 199 7 118 7 108 7 098 7 072 7 002 6 921 6 897 6 872

7 590 7 564 7 394 7 368 7 311 7 204 7 124 7 115 7 099 7 075 7 008 6 928 6 903 6 879

⫺7 ⫺3 ⫺12 ⫺9 ⫺1 ⫺5 ⫺6 ⫺7 ⫺1 ⫺3 ⫺6 ⫺7 ⫺6 ⫺7

a

Multiplet manifolds. Emission from Rn to Zn, Y n, and Xn Stark levels. Wavelength in nanometers. d Relative intensity. e Experimental energy of transition in vacuum wave numbers. f Calculated energy of transitions from Ref. 21. g Difference between the experimental and calculated energy values. b c

A共R1 → Y 2兲 =

共1 + e−⌬/kT兲␤共R1 → Y 2兲␤共 4F3/2 → 4I11/2兲

␶共 F3/2兲 4

,

共12兲 and A共R1 → X2兲 =

共1 + e−⌬/kT兲␤共R1 → X2兲␤共 4F3/2 → 4I13/2兲

␶共 4F3/2兲

,

共13兲 where ⌬ is the difference in energy between the Stark levels R1 and R2 of the 4F3/2 manifold, k is the Boltzmann constant, T is the absolute temperature, ␶ is the radiative lifetime of the 4 F3/2 manifold, ␤共R1 → Z1兲, ␤共R1 → Y 2兲, and ␤共R1 → X2兲 are

the branching ratios for the corresponding inter-Stark transitions, and ␤共 4F3/2 → 4I9/2兲, ␤共 4F3/2 → 4I11/2兲, and ␤共 4F3/2 → 4I13/2兲 are the branching ratios for the respective intermanifold transitions. The corresponding values of emission cross sections and peak emission cross sections are given in Table II. The detailed Stark splitting analysis has been performed for the 4F3/2 → 4IJ 共J = 9 / 2, 11/2, and 13/2兲 emission transitions and given in Table III. Experimental energy values of the Stark transitions are compared with those calculated by Chang21 for Nd3+ in bulk polycrystalline Nd3+ : Y2O3. The difference between the experimental and calculated values retains consistency except for very few transitions. Further-


093105-7

J. Appl. Phys. 105, 093105 共2009兲

Sardar et al.

more, a difference in energy values has been reported by Nash et al.45 for Nd3+ in Nd3+ : Y2O3 nanocrystals in their pure form and those embedded in the epoxy and chitosan polymers. IV. DISCUSSIONS AND CONCLUSION

We have developed and reported a methodology to fabricate thin films of Nd3+ : Y2O3 / PMMA. An in-depth spectroscopic analysis of this material has been performed following the JO model. The three phenomenological intensity parameters ⍀2, ⍀4, and ⍀6 and the spectroscopic quality factor X = ⍀4 / ⍀6 have been calculated for the Nd3+ : Y2O3 / PMMA sample. As shown in Table II, the ⍀␭ 共␭ = 2 , 4 , 6兲 parameters are considerably higher for the nanocrystalline sample than in the ceramic and the bulk. Boyer et al.46 explained that this behavior could be due to the fact that for the nanocrystalline sample a higher fraction of the Nd3+ ions is on the surface of the particles compared to the ceramic and bulk crystalline samples. Therefore, the average crystal field experienced by the ions in the nanocrystals will be different from that in the ceramic and bulk crystalline samples. Moreover, the adsorbed CO2 and water present as the contaminants on the surface of the nanoparticles could also vary the crystal field experienced by the Nd3+ ions. This reasoning also supports the consistent crystal-field energy level shifts between our sample and the bulk Nd3+ : Y2O3.21 As shown in Table II, the ⍀2 value of Nd3+ in Nd3+ : Y2O3 / PMMA is comparable to that obtained for the single crystalline Nd3+ : Y2O3.29 According to Krupke,29 the ⍀2 parameter is most sensitive to the local structure and composition and reflects the asymmetry of the local environment at the Nd3+ site, indicating that the presence of PMMA does not effectively alter the coordination environment of Nd3+ ions. Moreover, the spectroscopic quality factor is found to be slightly larger than that for the ceramic Nd3+ : Y2O3. Xu et al.42 determined the radiative lifetime of Nd3+ in neodymium octanoate doped in PMMA as 0.632 ms, which is approximately two times longer than what we obtained 共0.30 ms兲 for the Nd3+ : Y2O3 / PMMA sample. This discrepancy can be explained by the observations of Meltzer et al.36,37 According to them, the radiative lifetime of the electronic transitions of an ion embedded in a medium is proportional to the inverse of neff共neff2 + 2兲2 / 9. The effective indices of refraction calculated for the Y2O3 / PMMA medium 共given in Table I兲 are comparatively large with respect to those for the pure PMMA,38 and thereby giving rise to a reduced radiative lifetime for our sample, which falls in the same range of value reported by Kumar et al.41 for the ceramic Nd3+ : Y2O3. The fluorescence lifetime of 1.35 ms for the 4F3/2 4 → I11/2 transition of Nd3+ : Y2O3 / PMMA is much longer than that of ceramic Nd3+ : Y2O3.41 According to Boyer et al.,46 the particle size of nanocrystals can influence the emission lifetime. For example, their experimental results show that the 5D0 luminescence decay time for the nanocrystalline Eu3+ : Lu2O3 sample is about two times longer than that for the bulk sample.46 The significant enhancement in the emis-

sion lifetime with respect to the radiative lifetime is thought as mainly due to the characteristic sample morphology. Our experimental results are supported by the results obtained by Nash et al.45 for the pure nanocrystalline sample of Nd3+ : Y2O3 共2.48 ms兲 and for those when embedded in different polymers such as epoxy 共1.99 ms兲 and chitosan 共1.34 ms兲. According to Liang et al.,14,15 the inhomogenity of the PMMA matrix can cause variation in the spectroscopic parameters such as effective bandwidth, radiative lifetimes, etc. A comparison of the radiative lifetimes obtained by Liang et for various Er3+ transitions in al.,14,15 Er共DBM兲3共TPPO兲2 / PMMA and Er共DBM兲3Phen/ PMMA with those the authors obtained for Er3+ in Er3+ : YAlO3 共Ref. 47兲 justifies this statement. Radiative reabsorption can also be another factor for the significant enhancement in the measured lifetime. Due to the aggregates of nanocrystals that are randomly directed in the polymer-surfactant matrix as shown in Fig. 1, there is a possibility that some of them can reabsorb and fluoresce again, thereby enhancing the lifetime. In conclusion, we find that the spectroscopic parameters of Nd3+ : Y2O3 nanocrystals suspended in PMMA are comparable to those of the nanocrystalline, polycrystalline, and single crystalline samples of Nd3+ : Y2O3 and much larger than those for the ceramic and single crystalline samples of Nd3+ : YAG. The Stark energy values within the 4F3/2 → 4IJ 共J = 9 / 2, 11/2, and 13/2兲 emission transition are close to those reported for Nd3+ in single crystalline Y2O3. The detailed spectroscopic analysis along with the long lifetime of the 4F3/2 excited state and the many advantages of the PMMA host suggest that this material may have significant potential for laser operation at 1.079 ␮m, which might enable it to have many photonic applications. ACKNOWLEDGMENTS

This research was supported in part by the National Science Foundation Grant No. DMR-0602649 and the American Chemical Society: Petroleum Research Fund Grant No. PRF 43862-B6. The authors would like to thank Kelly L. Nash 共UTSA Laser Laboratory兲 for her insightful suggestions. The authors would also like to thank Paul Kruger for his help in making the sample holder. 1

N. C. Chang, J. B. Gruber, R. P. Leavitt, and C. A. Morrison, J. Chem. Phys. 76, 3877 共1982兲. 2 J. B. Gruber, R. P. Leavitt, C. A. Morrison, and N. C. Chang, J. Chem. Phys. 82, 5373 共1985兲. 3 J. B. Gruber, D. K. Sardar, K. L. Nash, and R. M. Yow, J. Appl. Phys. 102, 023103 共2007兲. 4 A. Lupei, V. Lupei, T. Taira, Y. Sato, A. Ikuse, and C. Gheroghe, J. Lumin. 102–103, 72 共2003兲. 5 J. Lu, K. Ueda, H. Yagi, T. Yanagitani, Y. Akiyama, and A. A. Kaminskii, J. Alloys Compd. 341, 220 共2002兲. 6 A. A. Kaminskii, M. Sh. Akcharia, V. I. Alahita, K. Ueda, K. Takachi, J. Lu, T. Uematsu, M. Musha, A. Shirikawa, V. Gabler, H. J. Eichler, H. Yagi, T. Yanagitani, S. N. Bagayev, J. Fernandes, and R. Baldu, Quantum Electronics Conference, IQEC-2002, June 22–27, 2002, Moscow. 7 J. Lu, M. Prabhu, J. Song, C. Li, J. Xu, K. Ueda, A. A. Kaminskii, H. Yagi, and T. Yanagitani, Appl. Phys. B: Lasers Opt. 71, 469 共2000兲. 8 G. A. Kumar, C. W. Chen, R. Riman, S. Chen, D. Smith, and J. Ballato, Appl. Phys. Lett. 86, 241105 共2005兲. 9 A. A. Kaminskii, K. Ueda, A. F. Konstantinova, H. Yagi, T. Yanagitani, A. V. Butashin, V. P. Orekhova, J. Lu, K. Takaichi, T. Uematsu, M. Musha, and A. Shirokava, Crystallogr. Rep. 48, 6 共2003兲.


093105-8 10

J. Appl. Phys. 105, 093105 共2009兲

Sardar et al.

B. M. Walsh, J. M. McMahon, W. C. Edwards, N. P. Barnes, R. W. Equall, and R. L. Hutcheson, J. Opt. Soc. Am. B 19, 2893 共2002兲. 11 D. K. Sardar, D. M. Dee, K. L. Nash, R. M. Yow, and J. B. Gruber, J. Appl. Phys. 100, 123106 共2006兲. 12 J. B. Gruber, K. L. Nash, D. K. Sardar, U. V. Valiev, N. Ter-Gabrielyan, and L. D. Merkle, J. Appl. Phys. 104, 023101 共2008兲. 13 W. H. Wong, K. K. Liu, K. S. Chan, and E. Y. B. Pun, J. Cryst. Growth 288, 100 共2006兲. 14 H. Liang, B. Chen, and Z. Zheng, Phys. Status Solidi B 241, 3056 共2004兲. 15 H. Liang, Z. Zheng, B. Chen, Q. Zhang, and H. Ming, Mater. Chem. Phys. 86, 430 共2004兲. 16 L. Bian, X. Qian, J. Yin, Z. Zhu, and Q. Lu, Mater. Sci. Eng., B 100, 53 共2003兲. 17 X. Huang and W. J. Brittain, Macromolecules 34, 3255 共2001兲. 18 B. R. Judd, Phys. Rev. 127, 750 共1962兲. 19 G. S. Ofelt, J. Chem. Phys. 37, 511 共1962兲. 20 J. B. Gruber, M. E. Hills, T. H. Allik, C. K. Jayasankar, J. R. Quagliano, and F. S. Richardson, Phys. Rev. B 41, 7999 共1990兲. 21 N. C. Chang, J. Chem. Phys. 44, 4044 共1966兲. 22 R. P. Leavitt, J. B. Gruber, N. C. Chang, and C. A. Morrison, J. Chem. Phys. 76, 4775 共1982兲. 23 C. A. Morrison, R. P. Leavitt, J. B. Gruber, and N. C. Chang, J. Chem. Phys. 79, 4758 共1983兲. 24 J. B. Gruber, D. K. Sardar, K. L. Nash, R. M. Yow, W. Gorski, and M. Zhang, J. Appl. Phys. 101, 113116 共2007兲. 25 J. Turkevich, G. Garton, and P. C. Stevenson, J. Colloid Sci. 9, 26 共1954兲. 26 L. Pang, Y. Shen, K. Tetz, and Y. Fainman, Opt. Express 13, 44 共2005兲. 27 R. J. Hunter, Introduction to Modern Colloid Science 共Oxford University Press, Oxford, 1993兲. 28 O. Soderman, P. Stilbs, and W. S. Price, Concepts Magn. Reson. 23A共2兲, 121 共2004兲.

W. F. Krupke, Phys. Rev. 145, 325 共1966兲; IEEE J. Quantum Electron. QE-7, 153 共1971兲. 30 M. J. Weber, Phys. Rev. 171, 283 共1968兲. 31 M. J. Weber and T. E. Varitimos, J. Appl. Phys. 42, 4996 共1971兲. 32 J. A. Caird and L. G. DeShazer, IEEE J. Quantum Electron. 11, 97 共1975兲. 33 T. S. Lomheim and L. G. De Shazer, J. Appl. Phys. 49, 5517 共1978兲. 34 D. K. Sardar, R. M. Yow, J. B. Gruber, T. H. Allik, and B. Zandi, J. Lumin. 116, 145 共2006兲. 35 D. K. Sardar and P. D. Bella, Phys. Rev. B 55, 2859 共1997兲. 36 R. S. Meltzer, S. P. Feofilov, B. Tissue, and H. B. Yuan, Phys. Rev. B 60, R14012 共1999兲. 37 R. S. Meltzer, W. M. Yen, H. Zheng, S. P. Feofilov, M. J. Dejneka, B. Tissue, and H. B. Yuan, J. Lumin. 94–95, 217 共2001兲. 38 I. D. Nikolov and C. D. Ivanov, Appl. Opt. 39, 2067 共2000兲. 39 A. A. Kaminskii, Crystalline Lasers: Physical Process and Operating Schemes 共CRC, Boca Raton, FL, 1996兲. 40 R. C. Powell, Physics of Solid-State Laser Materials 共Springer-Verlag, New York, 1998兲. 41 G. A. Kumar, J. Lu, A. A. Kaminskii, and K. Ueda, IEEE J. Quantum Electron. 42, 643 共2006兲. 42 X. Xu, H. Ming, and Q. Zhang, Opt. Commun. 199, 369 共2001兲. 43 G. A. Kumar, J. Lu, A. A. Kaminskii, K. Ueda, H. Yagi, T. Yanagitani, and N. V. Unnikrishnan, IEEE J. Quantum Electron. 40, 747 共2004兲. 44 I. C. Robin, R. Kumaran, S. Penson, S. E. Webster, T. Tiedje, and A. Oleinik, Opt. Mater. 共Amsterdam, Neth兲 30, 835 共2008兲. 45 K. L. Nash, R. C. Dennis, J. B. Gruber, and D. K. Sardar, J. Appl. Phys. 105, 033102 共2009兲. 46 J. C. Boyer, F. Vetrone, J. A. Capobianco, A. Speghini, and M. Bettinelli, J. Phys. Chem. B 108, 20137 共2004兲. 47 D. K. Sardar, S. Chandrasekharan, K. L. Nash, and J. B. Gruber, J. Appl. Phys. 104, 023102 共2008兲. 29
