- Email: [email protected]
Influence of electron-hole recombination on optical properties of boro-silicate glasses containing CdS quantum dots
Journal of Luminescence 181 (2017) 367–373
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Influence of electron-hole recombination on optical properties of boro-silicate glasses containing CdS quantum dots S.R. Munishwar, P.P. Pawar, R.S. Gedam n Department of Applied Physics, Visvesvaraya National Institute of Technology, Nagpur 440010, India
art ic l e i nf o
a b s t r a c t
Article history: Received 6 June 2016 Received in revised form 18 September 2016 Accepted 20 September 2016 Available online 2 October 2016
Semiconductor (CdS) quantum dots were grown in a glass matrix (SiO2-B2O3-ZnO-Na2O-K2O) by controlled heat treatment. Growth of CdS quantum dots were confirmed by optical absorption (UV–vis), photoluminescence (PL), micro-Raman spectroscopy, X-ray diffraction (XRD) and High resolution transmission electron microscopy (HRTEM). The structural modification occur in glass matrix were studied with Fourier transform infrared spectroscopy (FTIR). Clusters formation due to heat treatment was studied by field emission gun scanning electron microscopy (FEG-SEM). The observed experimental data shows that the CdS QDs size is very well controlled by single stage heat treatment. Size dependent blue shift in optical energy band gap and blue energy shift in PL intensity was observed with annealing time. The overall study of these glasses suggests these glasses are useful as an optical filters. & 2016 Elsevier B.V. All rights reserved.
Keywords: QDs XRD HRTEM PL
1. Introduction Semiconductor quantum dots (QDs) represent a general class of materials that are intermediate between bulk material and molecular species. Semiconductor quantum dots (QDs) with discrete electron and hole states exhibit quantum confinement effect therefore semiconductor quantum dots (QDs) exhibit size dependent molecular-like discrete electronic and optical properties [1]. Semiconductor Quantum dots are widely studied due to their unique electronic and optical properties compared to bulk materials [2]. The unique properties of semiconductor quantum dots (QDs) provide the possibility of applications in optoelectronic devices. Quantum dots are generally unstable and it is difficult to use these QDs for optoelectronic devices due to lack of proper solid matrix. Glass matrix provides the practical solution because of their good optical and mechanical properties. Glass is also chemically and thermally stable [3,4]. The glasses doped with Semiconductor QDs have attracted great interest due to their optical properties. The potential applications of these glasses are quantum dot lasers, LEDs, optical switches, color cut-off filters, photochromic glasses etc. The good efficiency of these devices can be established by the growth of quantum dots. The large surface to volume ratio of semiconductor quantum dots strongly affects the optical properties of devices mainly due to introducing surface polarization and surface states [1]. Quantum dots (QDs) are based on the element of group II–VI, III–V and IV–VI in the periodic table. n
Corresponding author. E-mail address: [email protected] (R.S. Gedam).
http://dx.doi.org/10.1016/j.jlumin.2016.09.045 0022-2313/& 2016 Elsevier B.V. All rights reserved.
Various QDs such as CdS, CdSe, PbS and PbSe were successfully embedded inside borosilicate, sodium silicate glasses [4,5]. It is reported that size of PbS QDs increases with increase in annealing time and the optical absorption and emission shows good agreement with size of QDs [6]. Since the emission wavelength can be controlled through the careful adjustment of temperature and pump intensity, the oxide glasses containing PbS QDs are suitable for active amplifying media over the wide wavelength region [7]. The optical properties of CdSe QDs in silicate glasses shows that PL peak position was independent of excitation wavelength because of size distribution of these QDs [8]. Glasses containing semiconductor quantum dots (QDs) from II-VI family such as CdS, CdSe developed for sharp cut-off color filter [4,9]. Thus the specific optical properties of semiconductor quantum dots glasses can be achieved by controlling the size and distribution of QDs in glasses. This growth and distribution of QDs in glass matrix is possible with controlled heat treatment. In this article, we have made successful attempt to grow the CdS semiconductor QDs in SiO2-B2O3-Na2O-K2O-ZnO glass system and characterize the glasses by DTA, HRTEM, FEG-SEM, EDX, RAMAN and Optical absorption and PL measurement were carried out for these glasses. The utility of these glasses as an optical filter has been verified.
2. Experimental Glass system 36SiO2 15B2O3 15Na2O-9K2O-25ZnO with 3 wt % CdS were prepared by melt-quench technique. All the chemicals
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S.R. Munishwar et al. / Journal of Luminescence 181 (2017) 367–373
having used for this synthesis were of 99.99% purity. All the chemicals were weighed using Shimadzu analytical balance and then mixed in agate mortar and pestle up to 8 h in order to enhance the homogeneity. After mixing thoroughly, the mixture was kept in a platinum crucible and melted in a furnace at 1100 °C for 2 h. The glasses were quenched at room temperature in aluminum mould to get transparent glass. To remove the thermal stresses, quenched glasses were annealed at 350 °C for 3 h and allowed to cool up to room temperature. The prepared glasses were cut in a suitable shape and polished optically. The differential thermal analysis (DTA) measurement of glass samples was carried out using Hitachi TG/DTA7200. The glass transition temperature (480 °C) was noted from DTA analysis. These glass samples were heat treated with optimized single step heat treatment schedule at 500 °C for 10, 35, 60 and 80 h. Growth of quantum dots were confirmed by X-ray diffraction studies using PAnalytical X’Pert Pro. Raman scattering spectra were studied with micro Raman system from Jobin Yvon Horibra LABRAM-HR visible (400–1100 nm). Ar þ laser operating at 488.0 nm with high stability confocal microscope and 50X objective lens was used as a laser source to excite the glass samples in powder form. Fourier Transform Infrared (FT-IR) was carried out by using Thermo Scientific Nicolet iS5 Spectrometer with iD5 ATR Accessory. Morphology and elemental analysis of the samples were characterized by field emission-gun scanning electron microscopy (FEG-SEM) and energy dispersive X-ray analysis (EDAX) (JEOL JSM-7600F). The field-emission transmission electron microscope (FE-TEM, JEOL JEM-2100F) was used for determining the size of quantum dots (QDs) and lattice fringe observation. Optical absorption and PL spectra were recorded by JASCO V-670 Spectrophotometer and JASCO Spectroflurometer FP-8200 respectively.
Fig. 1. X-ray diffraction (XRD) patterns of glass samples. Table 2 Band gap and crystallite size of CdS QDs embedded in glass matrix from UV-Visible and XRD. Glass sample
Linear band gap (Egopt) eV
Crystallite size from Band Gap (nm)
Average crystallite size from XRD (nm)
G1 G2 G3 G4 G5
3.70 2.71 2.54 2.50 2.43
1.22 2.45 3.10 3.85 4.90
– 2.73 3.83 4.39 5.17
3. Results and discussion Fig. 1 shows X-ray diffraction (XRD) pattern of as made glass sample (G1) and samples heat treated at 500 °C for 10, 35, 60, 80 hours (i.e. G2, G3, G4, G5) respectively. For as made glass sample (G1), no diffraction peaks are seen in XRD pattern but the crystalline phases starts appearing with increase in annealing time for G2, G3 G4 and G5. Since the amorphous nature of glass matrix dominates the crystallinity of grown quantum dots, the XRD peaks appear with less intensity. The XRD pattern shows peaks corresponding to (101), (102), (110) and (103) planes (reference code 01–075-1545) at 2θ positions around 28.64°, 36.91°, 43.71° and 47.18° respectively which confirms the growth of hexagonal phases of CdS QDs in glass matrix. Crystallite size (D) in annealed glasses were determined by Scherrer formula and depicted in Table 2 D¼
ð0:94 λÞ ðβ cos θÞ
ð1Þ
Table 1 FTIR peak positions of glass samples for different heating time. G1
G2
720 – 920
708 – 926
G3
G4
G5
Assignments
692 692 692 Bending of B-O-B linkage [14] 875 875 880 Stretching vibration of BO4 [15] 1096 1082 1097 Stretching vibrations of B-O-Si linkage [16,17] 1230 1230 1370 1370 1370 Asymmetric stretching vibrations of B–O bonds [14] 1379 1387 1527 1527 1527 Asymmetric stretching modes of borate triangles BO3 and BO2O [18]
Fig. 2. Micro-Raman Spectra of CdS QDs nanocrystal embedded in glass matrix measured at room temperature with the excitation of 488 nm.
where λ is the wavelength, β is the full-width at half maximum (FWHM) of the peak and θ is Bragg's angle. Fig. 2 shows the room temperature (300 K) Raman spectra of the glasses. From this figure it is observed that no Raman peak is present for glass sample G1, but it starts appearing with increase in annealing time [10]. Since the electron- phonon interaction increases with the growth of crystallinity in the glass matrix [11], Raman peak starts growing roughly around 302.6 cm–1 for G2 and is more prominent for samples G3, G4 and G5 at positions 303.25 and 304.73 cm–1 and 306.2 cm 1 respectively. The observed Raman shift can be understood on the basis of inter sub band transition within the conduction band of CdS quantum dots [12]. It
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Fig. 3. FTIR spectra of glasses showing structural changes occur during heat treatment on glass matrix.
is also observed from Raman spectra that the width of longitudinal optical (LO) phonon intensity peak becomes narrow due to increase in size of CdS quantum dots in the glass matrix with increase in annealing time. This size dependence of Raman scattering spectra can be studied using full width half maximum (FWHM). The FWHM for the glass sample reduces from 13.38 cm–1 to 9.1 cm–1 with increase in size of the quantum dots [13]. Fig. 3 shows FTIR spectra of glasses and the bands related to these vibrations are mentioned in the Table 1. Fig. 4 (a and b) shows Field Emission Gun scanning electron microscopy (FEG-SEM) images of glass sample heat treated at 500 °C for 35 h. These FEG-SEM images give the impression of cluster formation of CdS semiconductor QDs due to Ostwald ripening. It is observed that glass becomes soft due to heat treatment which allows nanocrystallites to migrate and agglomerate to form a cluster in the interstitial positions of glass matrix. Fig. 4c shows EDAX pattern of the glass sample heat treated at 500 °C for 35 h. This pattern gives the chemical composition present in glass matrix. It is interesting to observe that the elements present in the glass matrix are very close to the chemical compositions used for glass synthesis. This confirms the homogeneous diffusion and growth of CdS nanocrystals in the glass matrix. The possible glass structure showing agglomeration of CdS QDs is shown in Fig. 5. It can be observed from the figure that the alkali ions (i.e. Na þ /K þ ) and CdS nano-clusters replaced in the interstitial positions of glass matrix after heat treatment which is supported by FTIR spectra (Fig. 3) due to the stretching vibration of BO4- unit of glass matrix. The HRTEM micrographs of nanocrystals formed in glass sample (G2) annealed at 500 °C for 35 hours are shown in Fig.6. Images captured at 20 nm scale shows that quantum dots are uniformly embedded in glass matrix which are spherical in shape in the range of 2 to 6 nm (Fig. 6(a) and (b)). FE-TEM image (Fig.6c) shows the lattice fringes which confirm the formation of CdS quantum dots in the glass matrix. The lattice spacing of 0.207 nm between adjacent lattice planes is corresponding to the distance between two (110) crystal planes. Inset in Fig. 6d shows the
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diffraction pattern which displays three diffused rings. These rings are assigned to CdS (101), (110) and (103) diffraction planes respectively. The indexing was done following the usual procedure of determining d-spacing of each ring and matching with powder XRD data. The SAED matches very well with reference code 01– 075-1545 indicating the consistency with XRD analysis. Fig. 7 shows the size distribution of grown CdS nanocrystals in the glass matrix heat treated at 500 °C for 35 h. This plot shows that the quantum dots present in glass matrix ranging from 2 to 6 nm and average diameter is 3.8 nm (i.e. radius R ¼ 1.9). The size of Bohr exciton radius (aB) is around 2.8 nm for bulk CdS [19]. Since R/aB is very less than one, quantum confinement is possible [20,21]. Fig. 8 shows images of as made glass and heat treated glass samples. It can be clearly seen from images that as made glass sample (G1) is clear and transparent. The glass sample heat treated at 500 oC for different time duration (i.e. G2, G3, G4, G5 for 10, 35, 60 and 80 hours respectively) turned dark yellowish with increase in annealing time. The change in color of glasses due to increase in size of QDs. The increase in the size of QDs with annealing time is confirmed by optical absorption spectra as shown in Fig. 9. The absorption of photon by quantum dots occurs if energy of photons exceeds the band gap. The decrease in size of semiconductor quantum dots (QDs) leads to quantum confinement therefore glass samples shows red shift in absorption edge. Glass sample G2 shows (Fig. 9) excitonic peak near the band edge which confirms the formation of small particle and their distribution in the matrix. The effective band gap energies of these glasses were calculated from the relation [22]. n αhν ¼ A hν Eg opt ð2Þ where Egopt is the optical band gap energy, A is constant which is different for different transitions, n ¼1/2 denotes direct allowed and n¼2 denotes indirect allowed transition. The size of QDs were calculated using Brus's equation [23,24] and depicted in Table 2: ¼ Eg ðbulkÞ þ Eopt g
2 ℏ2 Π 1 1 1:8e2 þ 2 4Πεεo r 2mo r me mh
ð3Þ
Where, Egopt is is the optical band gap energy, Eg is bulk energy band gap of CdS (2.42 eV), ε is dielectric coefficient for CdS (5.7), εo is permittivity of free space, me is effective mass of electron (0.19), mh is effective mass of hole (0.8), mo is mass of electron and r radius of CdS QDs. It is observed from this table that size of QDs increases with annealing time. The increase in size of QDs leads to decrease in energy band gap which confirms the quantum confinement effect. Fig. 10 shows the emission spectra of glasses. It is observed from this spectra that emission peak appears at 413 nm for all glasses (G1, G2, G3, G4 and G5) due to compositional impurities or lattice defects in glasses. Since the energy supplied (i.e. excited at 345 nm) to sample G1 is not sufficient, emission is not observed for this glass sample (Fig.10). Glass G2 excited at 345 nm, shows emission peaks at 436 nm and 556.5 nm respectively. The emission peak present at 436 nm (2.84 eV) is due to inter band as well as intra band transition at band-edge [25,26]which is very much close to the calculated optical band gap ( Egopt). This band-edge transition acknowledges the band gap value of glass sample ( G2) and also to other samples as depicted in Table 3. The emission peak at 556.5 nm for G2 appears due to formation of trapped electronic state near band edge. The pictorial representation of band edge transitions and trap-assisted electron hole recombination for all the sample is depicted in the Fig.11.
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Fig. 4. (a), (b) FEG-SEM images of glass sample and (c) EDAX spectra of glasses heat treated at 500 °C for 35 h.
Fig. 5. Schematic diagram showing compositional arrangement and agglomeration of CdS clusters at interstitial positions in glass matrix.
The emission peak 470.6 nm of the sample G3 shows red shift of 34.6 nm and decrease in intensity with respect to PL spectra of G2 which supports further increase the in size of the quantum dots as well as increase in defect density in the glass matrix. The peak for glass sample G3 appears at 570 nm also shows red shift of 13.5 nm with respect to PL spectra of G2. This decrease in PL intensity is due to increase in number of defects in the glass
matrix. The intensity of this peak is further reduced for sample G4 and G5 with increase in QDs size as well as defects in the glass matrix due to more annealing time. The deep traps in the glasses interact with both the conduction and valence band and serve then as recombination centers. Since the recombination is fast and non-radiative, luminescence quenching is observed beyond G2 [27]. Fig. 12 shows overlap of absorption and emission spectra of glass sample G2. The spectral overlap reveals that when the glass samples were excited with energy more than the band gap (i. e. Eexe ¼ 3.59 eV), it shows PL emission peak at band edge (i.e. 2.84 eV) which is nearly equal to the absorption-edge as observed in UV–vis (i.e. Eg ¼ 2.71 eV). The larger emission peak of PL spectra present at 2.23 eV compared with absorption-edge 2.71 eV (UV– vis) indicates that the present QDs had trap-state emission rather than band-edge emission. Trap-state emission is associated with electron transition between trap states and the conduction band or valence band and mostly appears at smaller energies (2.23 eV) than band-edge emission (2.84 eV). There are many possible trapstate emissions, each with different emission energy and all these emissions contribute to the PL of QDs, resulting in a relatively wide emission peak. The low energy peak present at 2.23 eV attributed to recombination of charge carriers in deep traps of surface defect states which is responsible for observed PL spectra and broadening of emission band inform the broad size distribution of quantum dots [28]. The broadening and size distributions of QDs can be
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Fig. 6. (a), (b), (c): HRTEM micrograph showing at different resolutions and (d) IFFT image and SAED pattern (inset) of glass sample annealed at 500 °C for 35 h.
Fig. 7. Histogram of distribution of CdS QDs in glass annealed at 500 °C for 35 hours.
Fig. 9. Optical absorption spectra showing decrease in band gap with increasing heating duration.
Fig. 8. Images of glass samples in a natural light showing growth of CdS QDs in glass matrix.
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Fig. 10. Emission spectra of CdS QDs in glass matrix showing recombination of electron-hole pairs due to quantum confinement.
Fig. 12. Spectral overlap of emission and absorption spectra of glass sample G2.
Table 3 PL emission peaks for excitation at 3.59 eV with their CIE color chromaticity coordinates. Glass Sample
PL Peak Position (nm)
G1 G2 G3 G4 G5
413 413 413 413 413
– 436 470 481.5 496
– 456.5 570 576 597
Band gap from PL spectra (eV)
CIE chromaticity Coordinates (x,y)
– 2.84 2.63 2.57 2.50
0.23, 0.35, 0.38, 0.36, 0.32,
0.32 0.49 0.40 0.34 0.28
Fig. 13. CIE chromaticity diagram showing color emitted by CdS QDs in a glass matrix as a function of their size.
4. Conclusion Fig. 11. Energy band diagram of glasses containing CdS QDs.
clearly noticed from the green color emitted by the sample G2 in UV light. This correlation between UV–vis and PL study supports the electronic transitions at band edge and transitions occur in QDs due to quantum confinement. Table 3 shows the emission peaks which arise due to bandedge (band gap) as well as trap-state transitions. Second emission peak present in all the glass samples (except G1) imply band edge transitions and comparable with the band gap evaluated from UVVisible spectra. Fig.13 shows CIE chromaticity diagram for glasses. It is observed from this figure that the glasses emit different colors due to change in the size of the quantum dots. The CIE chromaticity coordinates (x, y) for each sample are mentioned in the Table 3. The optical study shows shift in band edge of glasses due to the change in size of the CdS QDs therefore these glasses can be used as an optical filters.
The glasses containing CdS semiconductor were prepared by melt-quench technique. The CdS QDs were grown in the glass matrix with optimized single step heat treatment schedule. Structural modification occur in heat treated samples is confirmed by FTIR study. XRD pattern and micro-Raman spectra give the confirmation of CdS phases in the glass matrix. The blue shift in absorption edge with increase in annealing duration confirms the growth of QDs in glass matrix. The size of CdS QDs was calculated using XRD pattern and absorption spectra and found to be in the range of 3 to 6 nm. The average size of CdS QDs obtained from HRTEM shows good agreement with size calculated from XRD and absorption spectra. The PL spectra shows emission near band gap due to band gap transitions and other lower energy values because of electron-hole recombination when excitation energy supplied is more than band gap values. Thus PL spectra gives confirmation of size as well as traps related emission. Red shift in PL spectra and decrease in PL intensity gives quantum confinement effect as an
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effect of heat treatment time. Color co-ordinates obtained from CIE chromaticity diagram and color emitted by glass sample in UV light also confirm the growth of CdS QDs. The controlled homogeneous growth of CdS QDs in a glassy matrix are responsible for shifting of absorption and emission bands, therefore these glasses can be used as an optical filters in devices.
Acknowledgements We are very much thankful to Department of Science and Technology (DST) for financial support for scientific research. We would also like to express our sincere, hearty thanks to Dr. V. G. Sathe, UGC-DAE-CSR, Indore for providing characterization facility for this work.
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Influence of electron-hole recombination on optical properties of boro-silicate glasses containing CdS quantum dots S.R. Munishwar, P.P. Pawar, R.S. Gedam n Department of Applied Physics, Visvesvaraya National Institute of Technology, Nagpur 440010, India
art ic l e i nf o
a b s t r a c t
Article history: Received 6 June 2016 Received in revised form 18 September 2016 Accepted 20 September 2016 Available online 2 October 2016
Semiconductor (CdS) quantum dots were grown in a glass matrix (SiO2-B2O3-ZnO-Na2O-K2O) by controlled heat treatment. Growth of CdS quantum dots were confirmed by optical absorption (UV–vis), photoluminescence (PL), micro-Raman spectroscopy, X-ray diffraction (XRD) and High resolution transmission electron microscopy (HRTEM). The structural modification occur in glass matrix were studied with Fourier transform infrared spectroscopy (FTIR). Clusters formation due to heat treatment was studied by field emission gun scanning electron microscopy (FEG-SEM). The observed experimental data shows that the CdS QDs size is very well controlled by single stage heat treatment. Size dependent blue shift in optical energy band gap and blue energy shift in PL intensity was observed with annealing time. The overall study of these glasses suggests these glasses are useful as an optical filters. & 2016 Elsevier B.V. All rights reserved.
Keywords: QDs XRD HRTEM PL
1. Introduction Semiconductor quantum dots (QDs) represent a general class of materials that are intermediate between bulk material and molecular species. Semiconductor quantum dots (QDs) with discrete electron and hole states exhibit quantum confinement effect therefore semiconductor quantum dots (QDs) exhibit size dependent molecular-like discrete electronic and optical properties [1]. Semiconductor Quantum dots are widely studied due to their unique electronic and optical properties compared to bulk materials [2]. The unique properties of semiconductor quantum dots (QDs) provide the possibility of applications in optoelectronic devices. Quantum dots are generally unstable and it is difficult to use these QDs for optoelectronic devices due to lack of proper solid matrix. Glass matrix provides the practical solution because of their good optical and mechanical properties. Glass is also chemically and thermally stable [3,4]. The glasses doped with Semiconductor QDs have attracted great interest due to their optical properties. The potential applications of these glasses are quantum dot lasers, LEDs, optical switches, color cut-off filters, photochromic glasses etc. The good efficiency of these devices can be established by the growth of quantum dots. The large surface to volume ratio of semiconductor quantum dots strongly affects the optical properties of devices mainly due to introducing surface polarization and surface states [1]. Quantum dots (QDs) are based on the element of group II–VI, III–V and IV–VI in the periodic table. n
Corresponding author. E-mail address: [email protected] (R.S. Gedam).
http://dx.doi.org/10.1016/j.jlumin.2016.09.045 0022-2313/& 2016 Elsevier B.V. All rights reserved.
Various QDs such as CdS, CdSe, PbS and PbSe were successfully embedded inside borosilicate, sodium silicate glasses [4,5]. It is reported that size of PbS QDs increases with increase in annealing time and the optical absorption and emission shows good agreement with size of QDs [6]. Since the emission wavelength can be controlled through the careful adjustment of temperature and pump intensity, the oxide glasses containing PbS QDs are suitable for active amplifying media over the wide wavelength region [7]. The optical properties of CdSe QDs in silicate glasses shows that PL peak position was independent of excitation wavelength because of size distribution of these QDs [8]. Glasses containing semiconductor quantum dots (QDs) from II-VI family such as CdS, CdSe developed for sharp cut-off color filter [4,9]. Thus the specific optical properties of semiconductor quantum dots glasses can be achieved by controlling the size and distribution of QDs in glasses. This growth and distribution of QDs in glass matrix is possible with controlled heat treatment. In this article, we have made successful attempt to grow the CdS semiconductor QDs in SiO2-B2O3-Na2O-K2O-ZnO glass system and characterize the glasses by DTA, HRTEM, FEG-SEM, EDX, RAMAN and Optical absorption and PL measurement were carried out for these glasses. The utility of these glasses as an optical filter has been verified.
2. Experimental Glass system 36SiO2 15B2O3 15Na2O-9K2O-25ZnO with 3 wt % CdS were prepared by melt-quench technique. All the chemicals
368
S.R. Munishwar et al. / Journal of Luminescence 181 (2017) 367–373
having used for this synthesis were of 99.99% purity. All the chemicals were weighed using Shimadzu analytical balance and then mixed in agate mortar and pestle up to 8 h in order to enhance the homogeneity. After mixing thoroughly, the mixture was kept in a platinum crucible and melted in a furnace at 1100 °C for 2 h. The glasses were quenched at room temperature in aluminum mould to get transparent glass. To remove the thermal stresses, quenched glasses were annealed at 350 °C for 3 h and allowed to cool up to room temperature. The prepared glasses were cut in a suitable shape and polished optically. The differential thermal analysis (DTA) measurement of glass samples was carried out using Hitachi TG/DTA7200. The glass transition temperature (480 °C) was noted from DTA analysis. These glass samples were heat treated with optimized single step heat treatment schedule at 500 °C for 10, 35, 60 and 80 h. Growth of quantum dots were confirmed by X-ray diffraction studies using PAnalytical X’Pert Pro. Raman scattering spectra were studied with micro Raman system from Jobin Yvon Horibra LABRAM-HR visible (400–1100 nm). Ar þ laser operating at 488.0 nm with high stability confocal microscope and 50X objective lens was used as a laser source to excite the glass samples in powder form. Fourier Transform Infrared (FT-IR) was carried out by using Thermo Scientific Nicolet iS5 Spectrometer with iD5 ATR Accessory. Morphology and elemental analysis of the samples were characterized by field emission-gun scanning electron microscopy (FEG-SEM) and energy dispersive X-ray analysis (EDAX) (JEOL JSM-7600F). The field-emission transmission electron microscope (FE-TEM, JEOL JEM-2100F) was used for determining the size of quantum dots (QDs) and lattice fringe observation. Optical absorption and PL spectra were recorded by JASCO V-670 Spectrophotometer and JASCO Spectroflurometer FP-8200 respectively.
Fig. 1. X-ray diffraction (XRD) patterns of glass samples. Table 2 Band gap and crystallite size of CdS QDs embedded in glass matrix from UV-Visible and XRD. Glass sample
Linear band gap (Egopt) eV
Crystallite size from Band Gap (nm)
Average crystallite size from XRD (nm)
G1 G2 G3 G4 G5
3.70 2.71 2.54 2.50 2.43
1.22 2.45 3.10 3.85 4.90
– 2.73 3.83 4.39 5.17
3. Results and discussion Fig. 1 shows X-ray diffraction (XRD) pattern of as made glass sample (G1) and samples heat treated at 500 °C for 10, 35, 60, 80 hours (i.e. G2, G3, G4, G5) respectively. For as made glass sample (G1), no diffraction peaks are seen in XRD pattern but the crystalline phases starts appearing with increase in annealing time for G2, G3 G4 and G5. Since the amorphous nature of glass matrix dominates the crystallinity of grown quantum dots, the XRD peaks appear with less intensity. The XRD pattern shows peaks corresponding to (101), (102), (110) and (103) planes (reference code 01–075-1545) at 2θ positions around 28.64°, 36.91°, 43.71° and 47.18° respectively which confirms the growth of hexagonal phases of CdS QDs in glass matrix. Crystallite size (D) in annealed glasses were determined by Scherrer formula and depicted in Table 2 D¼
ð0:94 λÞ ðβ cos θÞ
ð1Þ
Table 1 FTIR peak positions of glass samples for different heating time. G1
G2
720 – 920
708 – 926
G3
G4
G5
Assignments
692 692 692 Bending of B-O-B linkage [14] 875 875 880 Stretching vibration of BO4 [15] 1096 1082 1097 Stretching vibrations of B-O-Si linkage [16,17] 1230 1230 1370 1370 1370 Asymmetric stretching vibrations of B–O bonds [14] 1379 1387 1527 1527 1527 Asymmetric stretching modes of borate triangles BO3 and BO2O [18]
Fig. 2. Micro-Raman Spectra of CdS QDs nanocrystal embedded in glass matrix measured at room temperature with the excitation of 488 nm.
where λ is the wavelength, β is the full-width at half maximum (FWHM) of the peak and θ is Bragg's angle. Fig. 2 shows the room temperature (300 K) Raman spectra of the glasses. From this figure it is observed that no Raman peak is present for glass sample G1, but it starts appearing with increase in annealing time [10]. Since the electron- phonon interaction increases with the growth of crystallinity in the glass matrix [11], Raman peak starts growing roughly around 302.6 cm–1 for G2 and is more prominent for samples G3, G4 and G5 at positions 303.25 and 304.73 cm–1 and 306.2 cm 1 respectively. The observed Raman shift can be understood on the basis of inter sub band transition within the conduction band of CdS quantum dots [12]. It
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Fig. 3. FTIR spectra of glasses showing structural changes occur during heat treatment on glass matrix.
is also observed from Raman spectra that the width of longitudinal optical (LO) phonon intensity peak becomes narrow due to increase in size of CdS quantum dots in the glass matrix with increase in annealing time. This size dependence of Raman scattering spectra can be studied using full width half maximum (FWHM). The FWHM for the glass sample reduces from 13.38 cm–1 to 9.1 cm–1 with increase in size of the quantum dots [13]. Fig. 3 shows FTIR spectra of glasses and the bands related to these vibrations are mentioned in the Table 1. Fig. 4 (a and b) shows Field Emission Gun scanning electron microscopy (FEG-SEM) images of glass sample heat treated at 500 °C for 35 h. These FEG-SEM images give the impression of cluster formation of CdS semiconductor QDs due to Ostwald ripening. It is observed that glass becomes soft due to heat treatment which allows nanocrystallites to migrate and agglomerate to form a cluster in the interstitial positions of glass matrix. Fig. 4c shows EDAX pattern of the glass sample heat treated at 500 °C for 35 h. This pattern gives the chemical composition present in glass matrix. It is interesting to observe that the elements present in the glass matrix are very close to the chemical compositions used for glass synthesis. This confirms the homogeneous diffusion and growth of CdS nanocrystals in the glass matrix. The possible glass structure showing agglomeration of CdS QDs is shown in Fig. 5. It can be observed from the figure that the alkali ions (i.e. Na þ /K þ ) and CdS nano-clusters replaced in the interstitial positions of glass matrix after heat treatment which is supported by FTIR spectra (Fig. 3) due to the stretching vibration of BO4- unit of glass matrix. The HRTEM micrographs of nanocrystals formed in glass sample (G2) annealed at 500 °C for 35 hours are shown in Fig.6. Images captured at 20 nm scale shows that quantum dots are uniformly embedded in glass matrix which are spherical in shape in the range of 2 to 6 nm (Fig. 6(a) and (b)). FE-TEM image (Fig.6c) shows the lattice fringes which confirm the formation of CdS quantum dots in the glass matrix. The lattice spacing of 0.207 nm between adjacent lattice planes is corresponding to the distance between two (110) crystal planes. Inset in Fig. 6d shows the
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diffraction pattern which displays three diffused rings. These rings are assigned to CdS (101), (110) and (103) diffraction planes respectively. The indexing was done following the usual procedure of determining d-spacing of each ring and matching with powder XRD data. The SAED matches very well with reference code 01– 075-1545 indicating the consistency with XRD analysis. Fig. 7 shows the size distribution of grown CdS nanocrystals in the glass matrix heat treated at 500 °C for 35 h. This plot shows that the quantum dots present in glass matrix ranging from 2 to 6 nm and average diameter is 3.8 nm (i.e. radius R ¼ 1.9). The size of Bohr exciton radius (aB) is around 2.8 nm for bulk CdS [19]. Since R/aB is very less than one, quantum confinement is possible [20,21]. Fig. 8 shows images of as made glass and heat treated glass samples. It can be clearly seen from images that as made glass sample (G1) is clear and transparent. The glass sample heat treated at 500 oC for different time duration (i.e. G2, G3, G4, G5 for 10, 35, 60 and 80 hours respectively) turned dark yellowish with increase in annealing time. The change in color of glasses due to increase in size of QDs. The increase in the size of QDs with annealing time is confirmed by optical absorption spectra as shown in Fig. 9. The absorption of photon by quantum dots occurs if energy of photons exceeds the band gap. The decrease in size of semiconductor quantum dots (QDs) leads to quantum confinement therefore glass samples shows red shift in absorption edge. Glass sample G2 shows (Fig. 9) excitonic peak near the band edge which confirms the formation of small particle and their distribution in the matrix. The effective band gap energies of these glasses were calculated from the relation [22]. n αhν ¼ A hν Eg opt ð2Þ where Egopt is the optical band gap energy, A is constant which is different for different transitions, n ¼1/2 denotes direct allowed and n¼2 denotes indirect allowed transition. The size of QDs were calculated using Brus's equation [23,24] and depicted in Table 2: ¼ Eg ðbulkÞ þ Eopt g
2 ℏ2 Π 1 1 1:8e2 þ 2 4Πεεo r 2mo r me mh
ð3Þ
Where, Egopt is is the optical band gap energy, Eg is bulk energy band gap of CdS (2.42 eV), ε is dielectric coefficient for CdS (5.7), εo is permittivity of free space, me is effective mass of electron (0.19), mh is effective mass of hole (0.8), mo is mass of electron and r radius of CdS QDs. It is observed from this table that size of QDs increases with annealing time. The increase in size of QDs leads to decrease in energy band gap which confirms the quantum confinement effect. Fig. 10 shows the emission spectra of glasses. It is observed from this spectra that emission peak appears at 413 nm for all glasses (G1, G2, G3, G4 and G5) due to compositional impurities or lattice defects in glasses. Since the energy supplied (i.e. excited at 345 nm) to sample G1 is not sufficient, emission is not observed for this glass sample (Fig.10). Glass G2 excited at 345 nm, shows emission peaks at 436 nm and 556.5 nm respectively. The emission peak present at 436 nm (2.84 eV) is due to inter band as well as intra band transition at band-edge [25,26]which is very much close to the calculated optical band gap ( Egopt). This band-edge transition acknowledges the band gap value of glass sample ( G2) and also to other samples as depicted in Table 3. The emission peak at 556.5 nm for G2 appears due to formation of trapped electronic state near band edge. The pictorial representation of band edge transitions and trap-assisted electron hole recombination for all the sample is depicted in the Fig.11.
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Fig. 4. (a), (b) FEG-SEM images of glass sample and (c) EDAX spectra of glasses heat treated at 500 °C for 35 h.
Fig. 5. Schematic diagram showing compositional arrangement and agglomeration of CdS clusters at interstitial positions in glass matrix.
The emission peak 470.6 nm of the sample G3 shows red shift of 34.6 nm and decrease in intensity with respect to PL spectra of G2 which supports further increase the in size of the quantum dots as well as increase in defect density in the glass matrix. The peak for glass sample G3 appears at 570 nm also shows red shift of 13.5 nm with respect to PL spectra of G2. This decrease in PL intensity is due to increase in number of defects in the glass
matrix. The intensity of this peak is further reduced for sample G4 and G5 with increase in QDs size as well as defects in the glass matrix due to more annealing time. The deep traps in the glasses interact with both the conduction and valence band and serve then as recombination centers. Since the recombination is fast and non-radiative, luminescence quenching is observed beyond G2 [27]. Fig. 12 shows overlap of absorption and emission spectra of glass sample G2. The spectral overlap reveals that when the glass samples were excited with energy more than the band gap (i. e. Eexe ¼ 3.59 eV), it shows PL emission peak at band edge (i.e. 2.84 eV) which is nearly equal to the absorption-edge as observed in UV–vis (i.e. Eg ¼ 2.71 eV). The larger emission peak of PL spectra present at 2.23 eV compared with absorption-edge 2.71 eV (UV– vis) indicates that the present QDs had trap-state emission rather than band-edge emission. Trap-state emission is associated with electron transition between trap states and the conduction band or valence band and mostly appears at smaller energies (2.23 eV) than band-edge emission (2.84 eV). There are many possible trapstate emissions, each with different emission energy and all these emissions contribute to the PL of QDs, resulting in a relatively wide emission peak. The low energy peak present at 2.23 eV attributed to recombination of charge carriers in deep traps of surface defect states which is responsible for observed PL spectra and broadening of emission band inform the broad size distribution of quantum dots [28]. The broadening and size distributions of QDs can be
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Fig. 6. (a), (b), (c): HRTEM micrograph showing at different resolutions and (d) IFFT image and SAED pattern (inset) of glass sample annealed at 500 °C for 35 h.
Fig. 7. Histogram of distribution of CdS QDs in glass annealed at 500 °C for 35 hours.
Fig. 9. Optical absorption spectra showing decrease in band gap with increasing heating duration.
Fig. 8. Images of glass samples in a natural light showing growth of CdS QDs in glass matrix.
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Fig. 10. Emission spectra of CdS QDs in glass matrix showing recombination of electron-hole pairs due to quantum confinement.
Fig. 12. Spectral overlap of emission and absorption spectra of glass sample G2.
Table 3 PL emission peaks for excitation at 3.59 eV with their CIE color chromaticity coordinates. Glass Sample
PL Peak Position (nm)
G1 G2 G3 G4 G5
413 413 413 413 413
– 436 470 481.5 496
– 456.5 570 576 597
Band gap from PL spectra (eV)
CIE chromaticity Coordinates (x,y)
– 2.84 2.63 2.57 2.50
0.23, 0.35, 0.38, 0.36, 0.32,
0.32 0.49 0.40 0.34 0.28
Fig. 13. CIE chromaticity diagram showing color emitted by CdS QDs in a glass matrix as a function of their size.
4. Conclusion Fig. 11. Energy band diagram of glasses containing CdS QDs.
clearly noticed from the green color emitted by the sample G2 in UV light. This correlation between UV–vis and PL study supports the electronic transitions at band edge and transitions occur in QDs due to quantum confinement. Table 3 shows the emission peaks which arise due to bandedge (band gap) as well as trap-state transitions. Second emission peak present in all the glass samples (except G1) imply band edge transitions and comparable with the band gap evaluated from UVVisible spectra. Fig.13 shows CIE chromaticity diagram for glasses. It is observed from this figure that the glasses emit different colors due to change in the size of the quantum dots. The CIE chromaticity coordinates (x, y) for each sample are mentioned in the Table 3. The optical study shows shift in band edge of glasses due to the change in size of the CdS QDs therefore these glasses can be used as an optical filters.
The glasses containing CdS semiconductor were prepared by melt-quench technique. The CdS QDs were grown in the glass matrix with optimized single step heat treatment schedule. Structural modification occur in heat treated samples is confirmed by FTIR study. XRD pattern and micro-Raman spectra give the confirmation of CdS phases in the glass matrix. The blue shift in absorption edge with increase in annealing duration confirms the growth of QDs in glass matrix. The size of CdS QDs was calculated using XRD pattern and absorption spectra and found to be in the range of 3 to 6 nm. The average size of CdS QDs obtained from HRTEM shows good agreement with size calculated from XRD and absorption spectra. The PL spectra shows emission near band gap due to band gap transitions and other lower energy values because of electron-hole recombination when excitation energy supplied is more than band gap values. Thus PL spectra gives confirmation of size as well as traps related emission. Red shift in PL spectra and decrease in PL intensity gives quantum confinement effect as an
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effect of heat treatment time. Color co-ordinates obtained from CIE chromaticity diagram and color emitted by glass sample in UV light also confirm the growth of CdS QDs. The controlled homogeneous growth of CdS QDs in a glassy matrix are responsible for shifting of absorption and emission bands, therefore these glasses can be used as an optical filters in devices.
Acknowledgements We are very much thankful to Department of Science and Technology (DST) for financial support for scientific research. We would also like to express our sincere, hearty thanks to Dr. V. G. Sathe, UGC-DAE-CSR, Indore for providing characterization facility for this work.
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