Synthesis, Characterization and Optical Properties of CaF₂ and Pb Doped CaF₂ Nanocrystals

Synthesis, Characterization and Optical Properties of CaF₂ and Pb Doped CaF₂ Nanocrystals

Sohan M. Chauhan and B. S. Chakrabarty

Department of Applied Physics, Faculty of Technology & Engineering, The Maharaja Sayajirao University of Baroda, Vadodara-390001, Gujarat, India

Abstract: Calcium Fluoride and Lead (Pb) doped Calcium Fluoride (CaF2) nanocrystals were synthesized by the co- precipitation method and characterized by powder X-ray diffraction (PXRD) as well as Scanning Electron Microscopy (SEM). Optical absorption (UV-Visible) and photoluminescence (PL) studies were carried out. The PXRD patterns show cubic structure of the synthesized nanocrystals. The morphological features studied by SEM revealed that nanocrystals were agglomerated but porous. The optical absorption spectrum gave a strong and prominent absorption peak at 235 nm. Energy band gap of 4.6 eV for Pb doped CaF2was calculated using Taucs plot from the optical absorption spectra. The energy band gap is higher for doped CaF2 as compared to pure CaF2 nanocrystals (2.9 eV) and increases with the doping content. The PL spectrum gives broad band emission at 345 nm along with a small but distinct emission at 482 nm.

Keywords: Nanocrystals, co-precipitation, optical properties, absorbance, energy band gap, luminescence.

INTRODUCTION

The structural, magnetic, optical, dielectric, thermal and other physical properties of materials in the nanometer domain are different from those of bulk materials due to their large surface area to volume ratio and quantum confinement effects [1]. Fluoride compounds are very attractive materials for many potential applications such as phosphors, photonics, display monitors, imaging, light amplification and precursors for transparent ceramic processing [2]. Among the alkali fluoride compounds, Calcium Fluoride CaF2 is an attractive material because of its high stability and non-hygroscopic behavior. CaF2 is a well known host for luminescent ions due to its high transparency in a broad wavelength range, low refractive index and low phonon energy. CaF2 is one of the fluorides with wide band gap and has a large scale transparency [3]. Recently, CaF2 has gained much interest as a laser material when doped with rare earth materials [4]. In references to previous work on the fluorides, it was observed that the properties of the material can be tuned by the doping of metals. However, the reports on synthesis of Pb-doped CaF2 nanocrystals are very rare.

EXPERIMENTAL

Synthesis of CaF2 and Pb doped CaF2 nanocrystals

In the present work, pure CaF2 nanocrystals and Pb (3 mol %) doped CaF2 nanocrystals were synthesized by co-precipitation method.

For synthesis of CaF2, stoichiometric quantities of CaCl2.6H2O and NH4F and were taken in a 250 ml beaker and dissolved in 100 ml distilled water. The mixture was stirred for 3 hours constantly to achieve homogeneity. During stirring, the transparent reaction mixture transforms into opaque white suspension gradually. After stirring for 3 hrs, the solution was centrifuged with ethanol for 15 min at 3000 rpm. White residue was obtained. The residue was washed thoroughly with absolute ethanol to remove the residual chloride and the ammonium ions, taken on a Petri dish and dried at room temperature. The final product was white powder. The Pb (3 mol %) doped CaF2 nanocrystals were synthesized using the same process.

Characterization

The XRD measurement was carried out using a XPert Pro PANalytical powder diffractometer with Cu-K radiation ( = 1.54060 Å) in the scan range 20-90º. The morphology of synthesized sample was studied using scanning electron microscopy on a machine of JEOL make MODEL JSM 5810 LV. The optical absorption spectrum was carried out on a Thermo Scientific Evolution 600 UV- Visible spectrometer in a range 200-900 nm. For UV- Visible characterization the sample was dissolved in absolute ethanol and the absorbance was taken as a function of wavelength. The PL spectrum was carried out on a JASCO 6300 spectrophotometer.

RESULT AND DISCUSSION

Powder X-ray diffraction (XRD) analysis

Figures 1(a) and 1(b) show the PXRD pattern of as-synthesized CaF2 and Pb-doped CaF2 nanocrystals. All the obtained diffraction peaks were indexed and assigned

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to a cubic phase of the fluorite type structure with space group Fm3m. The pattern was compared with JCPDS card no. 77-2245 and 75-0363. The diffraction patterns confirm the cubic structure of the synthesized nanocrystals. The displayed peaks correspond to (hkl) values (111), (200), (220), (311), (400), (331) and (422). The lattice constant () was calculated by using (hkl) values. The average value of lattice constant was found to be 5.4739 Å for Pb doped CaF2 and 5.46868 Å for undoped CaF2 sample. This small difference may be due to the fact that when Ca+2 is substituted by a Pb+2 ion, charge compensating F¯ ions enter the Fluorite (CaF2) structure in interstitial cubic sites and the electronic repulsion between F¯ ions leads to a net increase of the lattice parameter.

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Figure 1(a): PXRD of CaF2 nanocrystals

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Figure 2: SEM image of (a) CaF2 and (b) Pb doped CaF2 nanocrystals.

Optical absorption

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Figure 3 shows the optical absorption spectrum of CaF2 and Pb-doped CaF2 nanocrystals. The absorption spectrum shows that characteristic absorption peak is present in the UV-region and also shows that by doping of the Pb, absorbance increases as compared to pure CaF2 nanocrystals. The prominent absorption peak observed in Pb doped CaF2 is at 235 nm.

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Figure 1(b): PXRD of Pb doped CaF2 nanocrystals

Wavelength ()

The crystallite size was calculated by Scherers formula. It was found to be 53 nm for CaF2 and 21.4 nm for Pb doped CaF2. It is observed that the crystallite size of CaF2 decreases when doped with Pb.

SEM analysis

Figure 2(a) and 2(b) shows a SEM image of as- prepared CaF2 and Pb doped CaF2 samples. The SEM image reveals the morphology of the synthesized nanocrystals. The CaF2 nanocrystals are agglomerated, fluffy but porous. The agglomeration ranges from few microns to tens of microns.

Figure 3: optical absorption spectrum of CaF2 and Pb-doped CaF2 nanocrystals

The energy band gap Eg was obtained from the optical absorption spectra by extrapolating the straight line plot of (h)2 versus (h) to the energy axis by the method proposed by Wood and Tauc [5]. It was found to be 2.9 eV (Figure 4(a)) for CaF2 and 4.6 eV (Figure 4(b)) for Pb- doped CaF2 nanocrystals. It is observed that doping of the Lead (Pb) ions in CaF2 tends to change the energy band gap of the material.

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Figure 4(a): Energy band gp plot of pure CaF2 nanocrystals.

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Figure 4(b): Energy band gap plot of CaF2: Pb nanocrystals.

Photoluminescence

Figure 5 shows the PL emission spectrum of pure and Pb-doped CaF2 for excitation at 235 nm.

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Pb doped CaF2

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Figure 5: PL emission spectrum of CaF2 and Pb-doped CaF2 nanocrystals excited at 235 nm.

Broad emission was observed at 345 nm along with a small but distinct emission peak at 482 nm, when the sample was excited at 235 nm. Pb-doped CaF2 nanocrystals show luminescence of pure CaF2 with a small decrease in intensity of the emission peak [6]. Both the pure as well as doped samples show exactly the same spectral characteristics. Hence, the emission can be attributed to the host lattice. The difference in intensity between the emission peaks may be either due to doping or due to different amounts of sample taken for measurement.

CONCLUSIONS

In summary, synthesis technique, co-precipitation was employed to make CaF2 and Pb-doped CaF2 nanocrystals. XRD measurement indicates that a cubic crystalline phase can be obtained on a nanometer scale. SEM result revealed that the nanocrystals were agglomerated but porous. The optical absorption spectrum showed a strong and prominent peak at 235 nm and it is observed that the energy band gap of CaF2 nanocrystals increases by doping of Pb ions. Photoluminescence spectra showed a broad emission at 345 nm with a blue emission peak at 482 nm.

REFERENCES

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3. S. W. S. McKeever, M. D. BVrown, R. J. Abbundi, H. Chan, V. K. Mathur (1986). Characterization of optically active sites in CaF2:Ce, Mn from optical spectra, J. Appl. Phys. 60, 2505-2510.

4. Petit V, Doualan JL, Camy P, Menard V, Moncorge R (2004). CW

   and tunable laser operation of Yb+3 doped CaF2, Appl. Phys. B., 78, 681.

5. Wood D. L, Tauc J. (1972). Weak absorption tails in amorphous semiconductors, Physical Review B., 5, 3144-3151.

6. H. F. Folkerts, M. A.Hamstra, G. Blasse (1995). The luminescence of Pb+2 in alkaline earth sulfates, Chem. Phy. Lett., 246, 135.