Red Fluorescence in Doped LaF_𝟑:Nd^𝟑+, Sm^𝟑+ Nanocrystals Synthesized by Microwave Technique

Abstract

Hexagonal shaped LaF₃ nanocrystals (NC) doped by Nd³⁺ and Sm³⁺ ions are synthesized using microwave technique. The prepared LaF₃ sample was characterized by XRD to confirm the average crystalline size of the particle is close to 20 nm (JCPDS standard card (32-0483) of pure hexagonal LaF₃ crystals). The Transmission Electron Microscope (TEM) analysis which indicates the size of the primary and secondary particle is in the range between 15 nm–20 nm. Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDAX) spectrometry have been carried out. The functional groups of the synthesized nanoparticles were confirmed by Fourier transform infrared spectroscopy (FTIR). The luminescent properties of the nanoparticles have been observed by excitation and emission spectra. Energy transfer from Nd³⁺ to Sm³⁺ has been observed. The transparency of the crystals has been confirmed using UV-VIS spectra. UV-Visible absorption spectrum indicates an energy gap of 4.9 eV and shows presence of wide transparency window. Non Linear Optical (NLO) properties of the synthesized nanoparticles have been studied. It has been found that Second Harmonic Generation (SHG) efficiency of LaF₃=Nd³⁺, Sm³⁺ is less than pure Potassium Dihydroxyl Phosphate (KDP) crystals.

1. Introduction

In recent years, the fields of luminescence and display materials have undergone a revival of sorts with the evolution to nanosized luminescent particles, driven primarily by an ever-increasing awareness of the unique physical and optical properties that differ from those of identical bulk materials as the size of the particle reduced to the nanometer region [1]. Studies on the luminescent properties of lanthanide-doped nanoparticles have attracted a great deal of interest since they are considered as potentially useful phosphors in lamps and display devices [2], components in optical telecommunication [3], active materials in lasers [4], new optoelectronics devices [5], up converters [6–8], magnetic resonance imaging (MRI) [9], and biological fluorescent labels [10–12]. LaF₃ nanocrystals are widely used in lubricants, additives in steel and metal alloy, electrode materials [13] chemical sensors, and biosensors [14]. LaF₃ possesses low phonon energy and adequate thermal and environmental stability [15], and hence is an excellent host matrix [16–18] for investigating luminescence properties. Nanoparticles of LaF₃, doped with lanthanide ions, are studied in the past for their luminescence properties [19–23]. In past, several investigations have been carried to study the optical properties of LaF₃:Nd³⁺ [24] for their possible applications in optoelectronics devices. In many circumstances, the use of microwave dielectric heating as a nonclassical energy source has been shown to dramatically reduce processing times, increase product yields, and enhance product purity or material properties compared to conventionally processed experiments and hence is favored here for synthesis of LaF₃: Nd³⁺, Sm³⁺ nanoparticles. Microwaveoven (Sharp Corporation) Sharp Carousel, made in Thailand, Model no R 210D operating voltage 230~240 V, 1.18 kW, current 5.2 A, with output power 800 W, operating at a frequency of 2.45 GHz equipped with an external autodialer temperature monitoring system was used for this purpose.

2. Experimental

2.1. Synthesis of Nanocrystals

LaF₃: Nd³⁺ and Sm³⁺ nanocrystals were synthesized by an aqueous route using microwave oven for heating in low power range. The method is simple and cost effective. Water soluble LaCl₃ + NdCl₃ + SmCl₃ (1 unit) and NH₄F (3 units) are mixed to obtain a solution in 1 : 3 molar proportion [25]. We prepared a 10 mL homogenous mixture (in deionized water) in a 100 mL beaker using 0.064 mol of LaCl₃ + NdCl₃ + SmCl₃, allowed a 10 mL solution of 0.576 mol NH₄F to drip into this solution uniformly through a funnel attached with a stopper to facilitate control of dripping and placed the whole set up inside a conventional microwave set at low power range (in on-off mode set at 30 sec) for around 30 min. The low power range setting largely helped us avoiding spill off of the solution. A white ultrafine crystalline precipitate identified as doped LaF₃ nanocrystals appears almost instantly having settled down to the bottom of the beaker. We stored this white precipitate in sealed tubes after washing it several times with deionized water, absolute methanol and acetone, and then drying it in microwave oven about 15 minutes. We subjected this specimen to detailed characterization and analysis.

LaF₃: Nd³⁺ and Sm³⁺ nanocrystals were also prepared by using methanol in place of deionized water with the method given above.

3. Characterizations

Powder X-ray diffraction (XRD) measurements have been performed using a PANALYTICAL X’PERT PROMPD diffractometer model using Cu K_α radiation λ = 1.5405 A° with scanning rate of 2° per min in the 2Θ range from 0° to 80°. Transmission electron microscope (TEM) analysis has been carried out for different magnification by PHILIPS (CM 200), 0.24 nm resolution at an operating voltage of 200 kV. SEM images have been obtained using ZEISS ULTRA FESEM, with accelerating voltage from 0.1 kV to 30 kV at beam current up to 100 nA, resolution 0.8 nm at 30 kV, magnification 10x to 1000kx, and image structure down to 10 nm size. Energy dispersive X-ray (EDAX) spectrometry has been done using a Quanta 200 FEG scanning electron microanalysis instrument. The UV-visible spectrum of the samples was recorded in the spectral range of 200–800 nm using a double beam (Perkin Elmer Corp.) spectrophotometer. The fluorescence spectrum was measured on LS 45 luminescence spectrometer (Perkin Elmer Corp.) using a high energy pulsed Xenon source for excitation and FL Win Lab software. NLO studies for the measurements of SHG efficiency, are obtained through the crystalline powder sample by using Kurtz and Perry technique.

4. Result and Discussion

The XRD results are shown in Figure 1(b) which indicates that LaF₃:Nd³⁺, Sm³⁺ nanoparticles are well crystallized, and the patterns are in good agreement with hexagonal structure (space group: 𝑃₃𝑐𝑙 (165), cell = 0.7187×0.7187×0.735nm³, 𝛼=𝛽=90^∘,𝛾=120^∘) known for bulk LaF₃ (JCPDS card no. 32-0483) [26]. The standard formula has been used for the determination of 𝑎, 𝑏, and 𝑐 parameters for hexagonal structure 1𝑑²=43ℎ²+ℎ𝑘+𝑘²𝑎²+𝑐𝑙²²,(1) where ℎ, 𝑘, and 𝑙 are the Miller indices for the XRD peaks.

(a)

(b)

Figure 1 

X-ray diffraction pattern of (A) LaF3 nanocrystals in dewater (B) LaF3 nanocrystals in methanol (both undoped), (b) XRD pattern of LaF3:Nd3+, Sm3+ nanocrystals.

The calculated cell parameters 𝑎=𝑏=0.7126nm and 𝑐=0.7255nm for the LaF₃:Nd³⁺ Sm³⁺ nanoparticles are smaller than those of LaF₃ nanoparticles (𝑎=𝑏=0.7187nm and 𝑐=0.735nm). The decrease in the lattice parameters of LaF₃:Nd³⁺, Sm³⁺ nanoparticles can be attributed to the smaller radius of Nd³⁺ ion (0.99 nm) and Sm³⁺ ion (0.96 nm) as compared with that of La³⁺ ion (0.106 nm) [27–29]. This indicates that Nd³⁺ ions and Sm³⁺ ions are doped into the LaF₃ lattice and occupied the site of La³⁺ ions, with the formation of a LaF₃: Nd³⁺, Sm³⁺ solid solution. The broadening of diffraction peaks for LaF₃:Nd³⁺, Sm³⁺ nanoparticles is also shown by Figure 1, which reveals the nanocrystalline nature of the samples. According to Scherrer equation, 𝐷=0.90𝜆/𝛽cos𝜃, where 𝐷 is the average crystallite size, 𝜆 is the X-ray wavelength (0.15405 nm) 𝜃 and 𝛽 being the diffraction angle and full width at half maximum of an observed peak, respectively. After subtraction of the equipment broadening, the full width at half maximum (FWHM) of the strongest peak (111) at 2θ = 27.9° helps to calculate the average crystalline size of LaF₃:Nd³⁺, Sm³⁺ nanoparticles as 15–20 nm.

The transmission electron microscopy (TEM) image in Figure 2 shows that the particles are well separated from each other. The nanocrystals have a hexagonal shape and a particle size of 6–20 nm. When these nanocrystals are incorporated in the polymer matrix, these particles are so small that the Rayleigh scattering can be nearly neglected. The selected area electron diffraction (SAED) pattern in Figure 2 inset shows three strong diffraction rings corresponding to the (002), (111), and (300) reflections, which is in agreement with the hexagonal LaF₃ structure [30], suggesting that the original structure of LaF₃ is retained even after the modification. The results are that particle size from TEM measurements is in agreement with those obtained from XRD studies.

Figure 2 

TEM image of Nd3+, Sm3+ doped LaF3 nanocrystals (Inset: SAED spectra).

Figure 3 shows the particle morphology of the prepared nanocrystals by using SEM image of LaF₃: Nd³⁺ and Sm³⁺ nanocrystals under low magnification. The hexagonal nanosized LaF₃: Nd³⁺ and Sm³⁺ particles are seen as aggregates of microdimensions. These aggregates of nanocrystals of large dimensions and of assorted sizes have been observed.

Figure 3 

SEM image of Nd3+, Sm3+ doped LaF3 nanocrystals.

Figure 4 shows energy dispersive (EDAX) spectra of the synthesized LaF₃:Nd³⁺, Sm³⁺ nanoparticals. The peaks for the element, namely, lanthanide, neodymium, and samarium are clearly visible along with other trace (impurity) elements.

Figure 4 

EDAX image of doped Nd3+, Sm3+ LaF3 nanocrystals.

Figure 5 has shown FTIR spectrum of the LaF₃:Nd³⁺, Sm³⁺ nanocrystals. The characteristic absorption peaks have been observed in the range from 4000 cm⁻¹ to 500 cm⁻¹. The broad absorption band at about 3434 cm⁻¹ can be attributed to 𝜈_as (O–H) stretching and bending vibrations. The peaks at 2925 cm⁻¹ and 2853 cm⁻¹ can be attributed to 𝜈_as (C–H) group of the long alkyl chain [31]. The characteristic IR peaks located at 1632 cm⁻¹ could be assigned to 𝛿 (H₂O) bending vibrations from the residual absorbed water and 1439 cm⁻¹ can be assigned to the asymmetric (𝜈_as) and symmetric (𝜈_as) bending vibrations of 𝛿 (O–H) group from methanol. Other absorptions are obtained due to use of methanol and acetone while preparing the sample.

Figure 5 

FTIR spectrum of the from Nd3+, Sm3+ doped LaF3 nanocrystals.

The excitation (monitored at 654 nm) and emission (excited at 357 nm) spectra of LaF₃:Nd³⁺, Sm³⁺ nanocrystals are shown in Figure 6. The excited peak centered at around 327 nm corresponds to the transition ⁴𝐼_9/2→⁴𝐷_7/2 for Nd³⁺ ion and a transition of samarium ions from ⁶𝐻_5/2 to ⁵𝐼_5/2. The emission spectrum shows a typical band between 600 and 700 nm corresponding to the ⁴𝐼_9/2→⁴𝐹_9/2 transition of Nd³⁺ ions, with peak at 654 nm, corresponding to red coloured emission from the nanocrystals [32].

Figure 6 

Excitation (𝜆em=654 nm) and emission (𝜆ex=357 nm) spectra of LaF3 nanocrystals doped Nd3+, Sm3+.

The energy level scheme of synthesized nanocrystals is shown in Figure 7. Here, Nd³⁺ energy levels arise from the 4𝑓³ electronic configurations. The emission from the 4𝑓²5𝑑 higher lying states is in the UV range. Some transitions among 4𝑓³ levels of Nd³⁺ ion are spin-allowed, such as ⁴𝐼_9/2, ⁴𝐹_3/2,⁴𝐹_5/2, ⁴𝐹_7/2,⁴𝐹_9/2,⁴𝐺_7/2 or ⁴𝐹_3/2,⁴𝐺_7/2, ⁴𝐷_3/2 but this is not valid for the remaining ones. Neodymium has been recognized as one of the most efficient rare-earth luminescence centers in minerals, while its emission has been found only in the IR part of the spectrum. Nevertheless, it is well known that Nd³⁺ may also generate UV-visible luminescence in certain matrixes, for example, in laser (YAG-Nd). The Nd³⁺ UV and visible luminescence spectra consists of many narrow lines whose half-widths reach only several cm⁻¹. It has been found that the Nd³⁺ UV and visible luminescence depend on the excitation wavelength [33].

Figure 7 

Energy level scheme of LaF3:Nd3+, Sm3+ nanocrystals.

The observed Nd³⁺ UV and visible luminescence spectra are interpreted in the following way: the most intense lines originates from level ²𝐹_5/2 to ⁴𝐹_5/2 of Nd³⁺ resulting in UV emission, ²𝐻_9/2 (lines around 400 nm) to ⁴𝐹_9/2 (lines around 435 nm) and ²𝐻_11/2 (lines in the range 450–464 nm), to ⁴𝐺_5/2 (lines around 480 nm) and to ²𝐺_7/2 (lines around 488 nm). In minerals, such luminescence is mixed with other rare earth elements (REE) lines and hence is not detected by steady-state spectroscopy. Trivalent samarium (Sm³⁺) activated minerals usually display an intense luminescence spectrum with a distinct line structure in the red-orange part of the spectrum. The radiating term ⁴𝐺_5/2 is separated from the nearest lower level ⁶𝐹_11/2 by an energy interval of ∼7,500 cm⁻¹. This distance is too large compared to the energy of phonons capable to accomplish an effective nonradiative relaxation of excited levels and these processes do not significantly affect the nature of the spectra in minerals. Thus, all detected lines of the Sm³⁺ luminescence take place from one excited level and are usually characterized by a long decay time [34]. UV-visible range investigated from the absorption spectra shows that the cut-off wavelength 𝜆_max lies at 250 nm with a corresponding energy gap of 4.9 eV suggesting its use in optoelectronics devicesas shown in Figure 8.

Figure 8 

UV-Vis absorption spectrum of LaF3:Nd3+, Sm3+ nanocrystals.

It is observed that the measured relative SHG efficiency of LaF₃ doped Nd³⁺, Sm³⁺ in deionized water with KDP crystal is 0.281 and that of LaF₃ doped Nd³⁺, Sm³⁺ in methanol with KDP crystal is 0.513 by using Kurtz and Perry technique [35].

5. Conclusions

Nanocrystals of LaF₃:Nd³⁺, Sm³⁺ have been rapidly synthesized by chemical route using domestic microwave oven at low power range. Nanocrystals of hexagonal geometry with particle size varying from 15 nm to 20 nm have been located by TEM analysis which is in agreement with the average crystalline size obtained from XRD studies. SEM pattern shows dispersed particles with traces of aggregates; EDAX spectra confirm the elemental components in the nanocrystals. The FTIR spectrum has been used for identification of fundamental groups present in the material. The absorption edge in UV spectra is found at 250 nm (corresponding to 4.9 eV) and a wide window between 250 nm to 800 nm has been observed. Emission of red color (654 nm) has been observed for LaF₃:Nd³⁺, Sm³⁺ nanocrystals at an excitation wavelength of 357 nm. It has been found that relative SHG efficiency of LaF₃ = Ln³⁺ (Ln³⁺ = Nd³⁺, Sm³⁺) containing rare earth elements are less than pure KDP crystals.

Acknowledgments

Thanks are due the staff of TIFR, SAIF IIT Mumbai, IIT Madras, and ISFAL Punjab for providing experimental facilities. This work was partially supported by the University of Mumbai (Letter no. APD/237/70 OF 2010 dated 13/01/2010). S. G. Gaurkhede thanks Material research Lab, Birla College, for providing experimental facility and Bhavan’s College, Andheri (W), Mumbai for institutional support.