Novel PbSe nanocrystals doped fluorogermanate glass matrix

Materials Science in Semiconductor Processing 34 (2015) 88–92

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Novel PbSe nanocrystals doped fluorogermanate glass matrix S. El-Rabaie a,n, T.A. Taha a, A.A. Higazy b a b

Physics and Engineering Mathematics Department, Faculty of Electronic Engineering, Menufiya University, Menouf 32952, Egypt Physics Department, Faculty of Science, Menufiya University, Shebin El-Koom 32511, Egypt

a r t i c l e in f o

Keywords: PbSe nanocrystals Germanate glasses Lead chalcogenides

abstract PbSe nanocrystals doped fluorogermanate glass has been prepared via thermal treatment method. The nanocrystals were characterized by optical absorption, X-ray diffraction (XRD) and transmission electron microscope (TEM) analysis. PbSe nanoparticles of average radii 4.3, 5.9, 7.0 and 7.8 nm with cubic and orthorhombic crystal structure were formed in the fluorogermanate glass matrix. Also, the effect of annealing temperature on the nanocrystal sizes was studied. The obtained optical energy gap, Eopt values show a decrease from 3.4 to 2.9 eV with increasing PbSe nanocrystal size. Raman spectroscopy confirmed the formation of PbSe nanoparticles by the appearance of surface and longitudinal optical phonons of PbSe. & 2015 Elsevier Ltd. All rights reserved.

1. Introduction Significant research effort has been devoted to developing lead chalcogenide nanocrystals (PbS, PbSe and PbTe) because of their potential application in photodetector, optical switches, laser materials, thermoelectric devices and solar cells [1–6]. Lead chalcogenides show strong quantum confinement effects due to its large Bohr radius compared with the II–VI compounds (CdTe, CdS and CdSe). PbSe has a band gap of 0.28 eV at room temperature and a Bohr radius exciton of 46 nm approximately [7–10]. Control and manipulation of size and morphology of crystalline nanomaterials are an interesting objective in modern material's science, chemistry, and physics. This high interest is motivated by the unique properties of nanomaterials with different sizes and morphologies associated with the nanoscale applications as photoelectric devices, drug delivery, sensors, filters, coatings, thermoelectrics and chemical catalysis [11–14]. Nanocrystals should be incorporated into chemically and mechanically

stable solid matrices and glasses to be suitable for practical applications [6,15]. Germanate glasses superior to other glasses by its high thermal, chemical and mechanical stability [16–18]. Oxyfluoride glasses have received great attention because it is characterized by extensive composition, low nonlinear refractive index and high content of doped semiconductor materials. They are promising host matrix for application in optics [19]. The addition of alkali fluorides into the glass system could increase their chemical durability, thermal stability and will decrease the harmful hygroscopic behavior which limits of their applications [20]. PbSe nanoparticles doped fluorogermanate glasses have not been prepared before. Here we describe the synthesis and characterization of PbSe nanocrystals doped high stability fluorogermanate glass matrix (10 NaF – 90 GeO2) which can be used for solar energy conversion. The structure of PbSe nanocrystals is studied by XRD, TEM, optical absorption, and Raman analysis. 2. Experimental

n

Corresponding author. Tel.: þ 002 01025309176; fax: þ 002 048 3660716. E-mail address: [email protected] (S. El-Rabaie). http://dx.doi.org/10.1016/j.mssp.2015.02.019 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

Our PbSe doped fluorogermanate glasses were prepared using starting powders with nominal compositions

S. El-Rabaie et al. / Materials Science in Semiconductor Processing 34 (2015) 88–92

(mole %) of 10 NaF – 90 GeO2 and an additional 1 wt % of (PbO2 – Se) with analytic reagent NaF (99.9%), GeO2 (99.99%), PbO2 (99.99%) and Se (99.99%) as raw materials. Powder batches of  20 g were mixed very well to ensure homogeneity and preheated at 350 1C for 1 h; this allowed the PbO2 and Se to decompose and react with other batch constituents before melting. Then, the mixes were placed in a second furnace at 1200 1C for 70 min in a covered porcelain crucible. The melt was poured into two mild steel split mold to form glass disks with thickness 2 nm. We have added sodium fluoride into the glass raw as an aggregator and for decrease of glass viscosity. This facilitates stabilizing PbSe crystallite forming on the glass body and prevents their recurrent dissolution in the batch [21]. As a result of using covered crucible and sodium fluoride addition the well-known problem of selenium evaporation from the melts during high-temperature glass formation is easily solved for the present glass [22]. Further thermal treatment of the glass matrix was performed at 475, 500, 525 and 550 1C for 30 min. to enhance the diffusion of Pb2 þ and Se2  ions. As a result of thermal treatment PbSe quantum dots were formed in the glass matrix. The glass transition temperature, Tg, and the crystallization temperature, Tc, have been determined by DTA at a heating rate of 5 1C min  1. The DTA measurements were performed using Shimadzu DTA-50 in a platinum crucible and nitrogen flux with flow rate of 10 ml/min. Absorption spectra were recorded on a JASCO Corp., V-570, Rev. 1.00UV/vis/NIR spectrometer. The XRD analysis was performed by a Philips Pw 1373 X-ray diffractometer with Cu radiation (λ ¼1.54056 Å). The form and size of the particles were determined by (TEM) transmission electron microscopy (TEM, JEOL 2000FX) operated at the accelerating voltage of 80 kV. Raman spectra were measured with JASCO spectrometer (model, 600 FTIR- Raman's attachments, Japan).

3. Results and discussion 3.1. Differential thermal analysis (DTA) A deferential thermal analysis measurement (DTA) for as quenched sample is shown in Fig. 1. DTA allowed us to identify the glass transition temperature (Tg) and crystallization temperature (Tc). We have found the values of Tg and Tc are 440 and 604 1C, respectively. According to these result we have treated the produced samples at 475, 500, 525 and 550 1C for 30 min to grow different size of nanoparticles.

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Fig. 2. X-ray diffraction patterns of the glass sample heat-treated at 500 and 525 1C for 30 min.

3.2. X-ray diffraction analysis The structural information was obtained from the X-ray diffraction data. A typical XRD patterns of the PbSe QDs doped glass samples annealed at 500 and 525 1C for 30 min is shown in Fig. 2. The positions of the peaks in the X-ray diffraction patterns of the glass sample completely coincide with those in the patterns of bulk PbSe crystals. This suggests the formation of a cubic and orthorhombic mixed phase of PbSe [23]. Compared with the bulk PbSe, the diffraction peaks of PbSe QDs broaden due to the reduced particle size. The full width at half maximum (FWHM) of the XRD peaks is acquired to calculate the mean sizes of PbSe QDs using Debye–Sherrer's equation. The particle size can be inferred from the width of the diffraction peaks. However, the background scattering from the host reduces the dynamic range of the measurements [24]. The particle size was determined from the Scherrer formula [25]: D¼

0:89λ β cos θ

ð1Þ

Where D is the crystallite size, β is the half-width of diffraction peak in radians, λ is the X-ray wavelength and θ is the angle of diffraction. The particles radii calculated in this method was found to be 7.1 and 8 nm for the glass samples heat-treated at 500 and 525 1C for 30 min. 3.3. Transmission electron microscope analysis

Fig. 1. DTA curve of the as quenched glass sample.

PbSe nanoparticles mean radius obtained from transmission electron microscope (TEM) image for the glass sample heat-treated at 525 1C for 30 min was 7 nm (Fig. 3), which is close to that calculated from XRD pattern. From the TEM picture, it was quite clear that all crystallites of PbSe are spherical and uniformly distributed in the glass matrix.

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Fig. 3. TEM micrograph of the glass sample heat-treated at 525 1C for 30 min. The inset denotes the electron diffraction pattern of the PbSe crystal. Fig. 5. Temperature dependence of PbSe QDs size.

Fig. 4. Absorption spectra of PbSe QDs precipitated in fluorogermanate glasses.

3.4. Optical absorption spectroscopy Room temperature absorption spectra of the glass containing PbSe quantum dots (QDs) are shown in Fig. 4. The glass synthesis and treatment procedure allows us to obtain samples of different QDs sizes. There are clear absorption bands related to PbSe QDs in the nearinfrared region, and their peak's position moved to the short-wavelength side (blue-shifted) compared to the location of the band-gap energy of the bulk material. The sizes of PbSe QDs were calculated by the equation below from the measured band-gap energies for the first absorption peak [26]. 2

Eg ðRÞ ¼ Eg ð1Þ þ

h

8mn R2

ð2Þ

Here, R is the effective radius of the QDs in nanometer, Eg(R) is the effective band-gap energy of PbSe QDs, while

(Eg(1)¼0.28 eV) is the band-gap energy of the bulk PbSe semiconductor, mnis the reduced effective mass of electron and hole (mn ¼0.035m0) [7]. The results showed that the mean radii of the QDs in glasses were 4.3, 5.9, 7.0 and 7.8 nm when heat-treated at 475, 500, 525 and 550 1C for 30 min, respectively. Besides, all obtained sizes are smaller than the Bohr radius (46 nm [7,10]) of PbSe excitons, which reflects strong confinement. The QD-containing glasses fabricated by the method described above should be advantageous for optical applications since the exciton transition covers technologically important wavelengths, ranging from as low as 0.86 mm to be as high as 1.97 mm, within the near-infrared spectral region. Fig. 5 shows the temperature dependence of PbSe nanoparticle sizes. The nanocrystals sizes increases with increasing the annealing temperature. By increasing the heat-treatment temperature, the growth of larger PbSe nanocrystals might be expected. The main reason for this behavior is the less compact structure of the host glass matrix (10 NaF – 90 GeO2) which leads to decreasing the distance between Pb2 þ and S2 ions as the annealing temperature increases. The values of the optical energy gap, Eopt were determined for the glass samples heat treated at 475, 500, 535 and 550 1C for 30 min(using Davis–Mott relation [27,28]) by extrapolation to zero absorption in the (αħω)2 versus ħω (photon energy) plots in (Fig. 6). αðωÞ ¼ B½ðℏω Eopt Þn =ℏω

ð3Þ

where B is a constant, Eopt is the optical band gap energy and n is a number which characterizes the transition process (n ¼1/2 for direct allowed transition). The obtained Eopt values show a decrease from 3.4 to 2.9 eV with increasing PbSe QDs size but with a slow rate (Fig. 7). The low rate of changing the band gap at these temperatures may be due to the reduction of intrinsic defects at the nanoparticles glass interface [29–32]. Then the obtained results imply that the increase in nanoparticle size with increasing annealing temperature leads to a decrease in the band gap energy.

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Fig. 6. The variation of (αħω)2 with photon energy for the glass samples heat-treated for 30 min.

Fig. 7. Size dependence of the optical band gap for the glass samples annealed at 475, 500, 525 and 550 1C for 30 min.

3.5. Raman spectra Raman spectra of the undoped and doped glass samples in the range from 100 to 1000 cm  1 are shown in Fig. 8. For the undoped sample, the bands 325, 396 and 450 cm–1 may be attributed to the symmetric valence vibrations of the Ge–O–Ge bonds in the completely polymerized network that is predominantly built by a combination of sixmembered rings made from GeO4 tetrahedra. The bands 510 and 580 cm–1 are attributed to the vibrations of threemembered rings made from GeO4 tetrahedra. The band at 642 cm–1 is also ascribed to the vibrations of the oxygen atoms in the three-membered rings made from GeO4

Fig. 8. Raman spectra of undoped and doped (annealed at 475 for 30 min) glass samples.

tetrahedra. The band at 753 cm–1 is related to Q2 structural units form; these are GeO4 tetrahedra with two nonbridge oxygen atoms [33]. The weak bands 881, 908 and 942 cm–1 are related to the TO/LO splitting of antisymmetric vibrations of the Ge–O–Ge bridges [34,35]. These bands shift to lower wavenumber in the doped glass sample. The distinct peak near 125 cm  1 is attributed to the surface phonon (SP) mode of PbSe [36]. The main feature in this spectrum at 140 cm  1 is attributed to LO(Γ) of PbSe [37]. The bands at 175 and 260 cm  1 were attributed to vibrational lines 2LO(X) and 2LO(Δ) of PbSe, respectively, but other anharmonic effects could contribute to the asymmetry of the LO(Γ) line at its higher-energy side [36,38].

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