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Investigation of optical and structural properties of aqueous CdS quantum dots under gamma irradiation
Radiation Physics and Chemistry 166 (2020) 108476
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Investigation of optical and structural properties of aqueous CdS quantum dots under gamma irradiation
T
H. Alehdaghia,∗, E. Assara, B. Azadegana,∗∗, J. Baedia, A.A. Mowlavia,b a b
Department of Physics, Faculty of Science, Hakim Sabzevari University, Sabzevar, Iran International Center for Theoretical Physics (ICTP), Trieste, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical properties PL Quantum dots CdS Gamma Dosimeter
The optical properties of aqueous CdS quantum Dots (QDs) under gamma irradiation were systemically studied with dose of 0–20 kGy. The QDs synthesized by microwave assisted method with different size and concentration. By transmission electron spectroscopy (TEM) the size of particle was indicated about 3–4 nm. The results show that degrading of photoluminescence of QDs is strongly depended on concentration, so that the fastest degradation is corresponded to low concentration. In other side, redshift of PL peck and bandgap edge was strongly depend on the initial size of QDs, thus the smaller size had the largest redshift. The X-ray diffraction analysis did not show changes in crystalline structure after gamma irradiation. The FTIR results did not show changes after gamma irradiation, as well. It has been found that the main mechanism of the behavior of QDs under gamma irradiation is growth of CdS QDs. These results showed that CdS QDs have a high potential to use as gamma dosimeter.
1. Introduction In recent years, applications of quantum dots (QDs) semi-zero dimensional materials in various area have been accelerated (Alivisatos, 1996; Wu et al., 2003). II-VI semiconductors QDs such as chalcogenides are one of the most multi-applicable materials in light emitting diode (LED) (Caruge et al., 2008; Molaei et al., 2012; Shen et al., 2019; Sun et al., 2007), photovoltaic (Jiao et al., 2015; Rosiles-Perez et al., 2018), gas sensing (Nazzal et al., 2003), photodetectors (Cao et al., 2019). Decreasing size, in nano-scale, leads to quantum confinement effects that shifts the optical properties of semiconductors especially band-gap toward blue wavelength. By using this trend, photoluminescent QDs can show all visible lights (Qu and Peng, 2002). An increase in surfaceto-volume ratio is another factor in creating a good sensor in the nanoscale systems. Therefore, the optical properties of QDs can be a good parameter to sense the various environmental effects such as chemical sensing (Xia and Zhu, 2008; Zare et al., 2017), light sensing in photodiodes (Rauch et al., 2009) and even laser (Smirnov et al., 2018a, 2018b) or ionization radiation sensing. Ionization radiation can effect on QDs in several scenarios. For example, one of them can be studying of degradation of QDs after irradiation exposure. Another one is scrutiny of reaction time after gamma ray radiate to solution. Letant group detected gamma and alpha irradiation with detection of visible photon ∗
by the using of QDs (Letant and Wang, 2006). Nathan et al. studied the effect of 137Cs gamma irradiation with the doses ranging from 0 to 1.3 kGy on the photoluminescence (PL) of CdSe/ZnS quantum dots which synthesized by hot injection method (Withers et al., 2008). In a similar work, The PL properties of CdSe/ZnS QDs were studied by irradiation of 60Co source with different doses between 0 and 100 Gy (Stodilka et al., 2009). Gaur et al. studied PL properties of immobilized CdTe QDs in porous silicon with gamma irradiation dose up to 160 kGy (Gaur et al., 2013). There are a few reports where gamma irradiation was used as assister to synthesis or time reducer in polymerization (Bekasova et al., 2013; Chang et al., 2011; Raju et al., 2017; Yang et al., 2018). The main fact of using gamma irradiation is production of some radical in solution that increase the reaction speed in order to QDs synthesis (Ribeiro et al., 2015). However, there are low researches about effects of ionization irradiation on the optical properties of QDs especially water-soluble. A promising approach for synthesis of QDs is aqueous route is due to inexpensive, safe and easy to produce (Li et al., 2017; Zare et al., 2015) while in hot injection method need to high temperature with dangerous and toxic precursor, is a pivotal need (C˅apek et al., 2009; Parobek et al., 2018). Microwave assisted method is a fast and facile to synthesize aqueous colloidal QDs by which it can be have QDs just after a few minute (Karimipour and Molaei, 2016). With utilizing of the
Corresponding author. Corresponding author. E-mail addresses: [email protected] (H. Alehdaghi), [email protected] (B. Azadegan).
∗∗
https://doi.org/10.1016/j.radphyschem.2019.108476 Received 26 May 2019; Received in revised form 10 July 2019; Accepted 1 September 2019 Available online 04 September 2019 0969-806X/ © 2019 Elsevier Ltd. All rights reserved.
Radiation Physics and Chemistry 166 (2020) 108476
H. Alehdaghi, et al.
benefit, we can use them in a system, which senses to gamma irradiation and have a capacity to use as dosimeter. Herein, we studied, for the first time, effect of gamma irradiation on the physical and chemical properties of aqueous CdS QDs. The QDs were synthesized in facile and straightforward in aqueous solution by microwave irradiation in lower than 10 min. The PL and transmittance properties of QDs for different sizes and concentrations were studied. Transmission electron microscopy indicated the size of QDs about 3–4 nm. The crystalline and surface chemical properties before and after gamma irradiation were compared by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), respectively. 2. Materials and method All chemical were used with analytical grade without any purification. 3CdSO4.5H2O, Na2S2O3.2H2O and Thioglycolic acid (TGA) were purchased from Merck chemical company used for precursor of CdS QDs and capping agent, respectively. CdS QDs was synthesized the same as pervious reports (Alehdaghi et al., 2014; Karimipour and Molaei, 2016). Cd and S precursor was solved in DI water separately, with concentration of 5 mM and 50 mM, respectively. Then 12 mM TGA added to the solution of Cd that leads to milky solution. After that, both solutions were mixed together and pH was increased by ammonium to make clear the solution. Next, the solutions were put in a 500 W domestic microwave for different time to have various sizes. Finally, the QDs were extracted by centrifuge and solved in DI water. Micro tubes with 2 ml of QDs water-soluble were prepared and irradiated by 60Co source which produced two gammas of 1.33 and 1.17 MeV energies with irradiation dose rate of 1.85 Gy/s. For every experiments, one sample was selected without irradiation as a reference, which shows 0 Gy irradiation dose. This sample was in the same condition with irradiated samples. Optical transmittance measurement was performed by PerkinElmer, Lambda 25 and PL was measured by Avantes spectrometer (AvaSpec2048 TEC) with a high pressure Hg lamp with a 360 nm filter. IR spectra (FTIR) were recorded on a Shimadzu 435-U-04 spectrophotometer (KBr pellets). X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis were carried out by D8-Advance Bruker with Cu target and Tecnai-F-20, respectively.
Fig. 1. The transmission electron microscopy of CdS QDs (a), high-resolution transmission electron microscopy (b), the corresponding histogram size (c) and a schematic shows gamma ray decomposes water molecule to the derivation.
sample) to 1 kGy along with a little red shift in peak position. The significance change in PL is occurred at 20 kGy gamma dose, so that solution color, under excitation light, was changed from blue to green due to the red shift in PL maximum peak (from 474 nm to 554). In Fig. 2 (b) the transmittance spectra for 1 kGy and 20 kGy gamma doses are drawn. Here, the red shift after gamma irradiation exposure is evidently seen. There are 2 nm and 46 nm shift toward red wavelength for 1 kGy and 20 kGy gamma doses, respectively. It can be estimated that the gamma irradiation provides the required energy for growing of QDs. We also investigated the optical properties of larger-sized CdS QDs with increasing growth time of up to 7 min to examine the effect of size in exposure to different doses of gamma radiation (Smirnov et al., 2018b). As shown in Fig. 3 (a, b), the PL and transmittance is changed by different gamma doses. In comparison with small size of QDs, change in PL and transmittance is low. Exposure up to 1 kGy cannot create a large change in the peaks position of PL spectra while exposure of 20 kGy was leading to decrease of intensity by factor 1.5 and a little red shift. The absorption data confirms this change, so that there is no variation in the bandgap in 1 kGy gamma dose while for 20 kGy dose, decrement in the band gap is 0.04 eV equivalent to 0.1 nm increment in size (calculated using effective mass approximation eq. (1)).
3. Results To observe of size distribution and crystal nature of CdS QDs, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed. Fig. 1(a–c) shows TEM results for CdS QDs. The TEM image shows small particles (Fig. 1 a) with average size of 3.2 nm (Fig. 1 c). In HRTEM image, single crystalline nature of QDs is resolved so that d-spacing of 0.33 nm is seen (Fig. 1 b), corresponding to the (002) faces of hexagonal face of CdS (Geng et al., 2011). Fig. 1 (d) shows a schematic of gamma irradiation reaction to aqueous solution. When ionization ray interacts to water molecules produce there active radicals e-aq, H. and .OH and other products such as OH−, H2O2, O3, H2, O2, H+ and UV (Ismail et al., 2013; Ribeiro et al., 2015). Many reports emphasize, when ionization irradiation interact with a sample that is soluble in water, the main portion of degradation is due to the product created by gamma interaction with water molecules. Fig. 1 (d) is a schematic showing of the products that attack to CdS quantum dots. The PL spectra of QDs with size of 3.04 nm with different doses of gamma irradiation are shown in Fig. 2 (a). The black curve regarding to the reference and the witness sample which shows two peaks in 474 and 540 nm related to surface traps of CdS QDs with blue color (Mohsennia et al., 2015; Razzaq et al., 2014). According to the bandgap of reference sample (355 nm equivalent to 3.49 eV), which was obtained from transmittance spectra (Fig. 2 b), it can be clearly seen there is no bandgap emission for the samples. The intensities of the PL spectra increase with increasing the dose from 0 Gy (means the reference
E (x ) = E∞ (x ) +
h2 ⎛ 1 ⎞ 3.572 e 2 4.890 e 4 ⎜ ⎟ − − 2 2 μ (x ) 2d 2 ⎝ μ (x ) ⎠ ε (x ) d h ε (x )
(1)
where. E(x), E∞(x), h, d, μ(x), ϵ(x), and e is quantum dots band gap, bulk band gap, planck constant, quantum dot diameter, reduced mass, permittivity of the matter, elementary charge, respectively. Comparison of optical properties of two sizes of CdS QDs synthesized in the aqueous method indicates that the effect of gamma radiation on QDs is size-depended. This means that samples of various initial 2
Radiation Physics and Chemistry 166 (2020) 108476
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Fig. 2. PL spectra for the QDs solution with 3.04 nm exposed to different dose of gamma irradiation (a) and transmittance spectra of the QDs solution after dose of 1 kGy and 20 kGy (b).
relative increment in size is 0.23%, which means that after 20 kGy gamma dose, the QD grows to 3.74 nm. The Ostwald ripening process is one the most agent to grow QDs (Gu et al., 2008). As shown in Fig. 3 (c), the amount growth of the QDs depends strongly on the initial size of the QDs, especially in small size of about 3 nm. It means that smaller size of the QDs leads to more sensitivity for gamma dosimetry. To further investigate the impact of gamma irradiation on the QDs, the effect of concentration in addition to the effect of size was studied. After centrifugation, extracted solution with contents of the 3.34 nm QDs (at a concentration of 20 mg/cc) was diluted 10 and 50 times, i.e.,
sizes show different changes after gamma irradiation exposure. To confirm that the effects of gamma radiation on optical properties is size dependent, two other sizes of the QDs CdS were studied. It can be easily synthesized CdS QDs of different sizes by adjusting the time of microwave irradiation. The four aqueous solutions of CdS QDs with the size of 3.04, 3.34, 3.48 and 3.59 nm were exposed to 20 kGy gamma dose. Fig. 3 (c) shows the relative increase in QDs size of the samples versus their initial size which was obtained from bandgap data of QDs after gamma exposure and the related reference data using effective mass approximation. For example, for the QDs with the size of 3.04 nm, the
Fig. 3. The PL (a) and transmittance (b) properties of CdS QDs synthesized with time growth of 7 min in microwave for three different doses 0, 1 and 20 kGy. The bar graph of relative increase in QDs size of CdS versus the initial size of QDs (c). 3
Radiation Physics and Chemistry 166 (2020) 108476
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Fig. 4. The PL intensity (a, c) and transmittance spectra (b, d) of QDs before (reference sample) after 1 and 20 kGy gamma irradiation for different concentration 2 mg/cc (a, b) and 0.4 mg/cc (c, d). The relative decrease in PL intensity ((e), the left) and the relative increase in QDs size ((e), the right) for three different concentrations after 20 kGy gamma irradiation.
To further understand the effect of concentration on the PL intensity and the size of QDs, the relative decrease in PL intensity (the difference I −I between initial intensity and final intensity to initial intensity, i f ) Ii and the relative increase in QDs size, after exposure of 20 kGy, versus concertation were plotted in Fig. 4 (e). It is seen that for the concentration of 0.4 mg/cc, the relative decrease in PL intensity is 0.99, meaning that the intensity of PL is dramatically reduced by factor of 100. While for concentration of 20 mg/cc that is about 0.60 (PL intensity after gamma irradiation was changed to 40% of the initial intensity). The relative increase in QDs sizes are 0.27, 0.24 and 0.07 for concentrations of 0.4, 2 and 20 mg/cc, respectively. The trend of size change of QDs is very similar to PL intensity so that the amount of variation in QDs size decreases with increasing concentrations. These results propose for having a sensitive dosimeter to low dose, concentration of QDs solution should be low while high concentration solution of the QDs can leads to a dosimeter capable of sensing high dose. X-ray diffraction (XRD) measurements for drop-cast of QDs were conducted to study effect of gamma irradiation on the crystal structure. In Fig. 5, XRD patterns of two samples is shown that the black one is corresponding to the sample before gamma exposure and the red one is for the sample after 20 kGy gamma irradiation. There are two wide peaks at 27.9° and 47.0° for two samples before and after gamma irradiation. According to JCPDS file number 41.1049 (Alehdaghi et al., 2014; Molaei et al., 2012), these peaks are almost corresponded to the diffraction of (101) and (103) crystallographic planes that indicates hexagonal phase for CdS synthesized here. These two wide peaks consist of three peaks of (100), (002) and (101) for the first and (110), (103) and (112) for the second (Wang et al., 2016). With Debye Scherrer evaluation (eq. (2)) the full-width at half-maximum (FWHM) of peak (101) shows crystallite size of about 1 nm that it is smaller than the size earned by TEM due to low crystalline properties of QDs. Comparing of the XRD pattern of QDs before and after 20 kGy gamma irradiation showed there is no change in crystalline structure of QDs after gamma exposure. It means that gamma rays with 662 keV energy could not modify the CdS atoms and damage the crystal of the nanostructure. The crystallite size calculated by Debye Scherrer showed that gamma irradiation increases the size of QDs from 0.8 nm to 1.0 nm, which is consistent with transmittance spectrum in point of view of size increment after gamma irradiation.
Fig. 5. XRD patterns for CdS QDs before and after 20 kGy gamma irradiation.
concentration of 2 and 0.4 mg/cc, respectively. Fig. 4 shows the optical properties of samples with 2 mg/cc (a, b) and 0.4 mg/cc (c, d) after radiation dose of 1 and 20 kGy. Fig. 4 (a, c) shows the PL spectra where it is seen that after exposure of 1 kGy, the PL intensity is increased, while after exposure of 20 kGy it is decreased. The variation in the intensity of PL in 20 kGy gamma dose is sensitive to concentration, with decreasing the concentration from 2 mg/cc to 0.4 mg/cc, the PL intensity is vanished in 20 kGy gamma dose. The transmittance of the samples with 2 mg/cc (Fig. 4 (b)) and 0.4 mg/cc (Fig. 4 (c)) with 1 kGy and 20 kGy gamma doses represents redshift, which means that the size of QDs is increased. This red shift is also seen in the PL peaks of the samples after gamma irradiation. The transmittance amount for QDs with concentration of 0.4 mg/cc in dose radiation of 20 kG, in visible region, is low (i.e. opaque) because of scattering of light by large particle and clusters. Fig. 5 (d) shows that in 20 kGy gamma dose the transmittance of QDs is reduced. By comparing of the PL and the transmittance of the samples in 20 kGy gamma dose, it can be concluded that degradation of QDs for lower concentration is more vigorous. 4
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Fig. 6. FT-IR spectrum of TGA, CdS QDs before and after 20 kGy gamma irradiation. The vertical dotted lines show the same absorbance for QDs. The inset shows TGA molecule.
D=
kλ βcosθ
(2)
where k, λ, β and θ is structural constant (about 0.9), X-ray wavelength (1.54 A°), FWHM of the peak and Brag angel, respectively. To order to study the effect of gamma irradiation on surface of QDs, FT-IR analysis of two samples were preformed (before and after gamma irradiation). As mentioned in the materials and method section, QDs were capped by TGA. In Fig. 6, FT-IR spectrum pure TGA, CdS QDs before and after 20 kGy gamma irradiation have been shown. There is a broad peak about 3000 cm−1 for pure TGA that it is attributed to –OH stretching vibration while this peak was shifted to 3405 cm−1 for QDs. The absorbance at 2676 cm−1 and 2566 cm−1 belongs to S-H vibration bands in pure TGA. The absence of this peak in QDs capped TGA shows that TGA molecules attached to CdS QDs. After TGA capping to QDs, in FT-IR spectrum a peak at 1564 cm−1 appears which is attributed to the antisymmetric of COO−. TGA molecule is seen at the bottom of Fig. 6. The comparing of FT-IR spectrum before and after gamma irradiation concludes that gamma ray cannot remove capping agent (TGA) in aqueous QDs. Therefore, the degradation of PL intensity is attributed to growing of QDs by Ostwald ripening mechanism with production of free radicals in water due to gamma irradiation.
4. Conclusion The optical properties of aqueous CdS QDs with different sizes and concentrations were studied for the first time. Our results showed that the effect of gamma irradiation on the QDs is size-depended so that with decreasing the size of CdS QDs, the effect of gamma irradiation on the QDs is increased. Concentration of QDs solution was another factor to determine strength of the effectiveness of gamma irradiation on the optical properties of QDs, low concentration leads to more sensitive. The XRD analysis showed that gamma irradiation could not damage the crystalline structure of QDs. The FTIR spectrum indicated that gamma irradiation was not able to remove thiol groups from QDs surface. The main mechanism proposed for the changes was the growth of QDs by Ostwald ripening by production of free radicals in water due to gamma irradiation. Our results suggest that aqueous CdS QDs is a good candidate for use as a dosimeter by selecting the appropriate concentration and size.
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Investigation of optical and structural properties of aqueous CdS quantum dots under gamma irradiation
T
H. Alehdaghia,∗, E. Assara, B. Azadegana,∗∗, J. Baedia, A.A. Mowlavia,b a b
Department of Physics, Faculty of Science, Hakim Sabzevari University, Sabzevar, Iran International Center for Theoretical Physics (ICTP), Trieste, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical properties PL Quantum dots CdS Gamma Dosimeter
The optical properties of aqueous CdS quantum Dots (QDs) under gamma irradiation were systemically studied with dose of 0–20 kGy. The QDs synthesized by microwave assisted method with different size and concentration. By transmission electron spectroscopy (TEM) the size of particle was indicated about 3–4 nm. The results show that degrading of photoluminescence of QDs is strongly depended on concentration, so that the fastest degradation is corresponded to low concentration. In other side, redshift of PL peck and bandgap edge was strongly depend on the initial size of QDs, thus the smaller size had the largest redshift. The X-ray diffraction analysis did not show changes in crystalline structure after gamma irradiation. The FTIR results did not show changes after gamma irradiation, as well. It has been found that the main mechanism of the behavior of QDs under gamma irradiation is growth of CdS QDs. These results showed that CdS QDs have a high potential to use as gamma dosimeter.
1. Introduction In recent years, applications of quantum dots (QDs) semi-zero dimensional materials in various area have been accelerated (Alivisatos, 1996; Wu et al., 2003). II-VI semiconductors QDs such as chalcogenides are one of the most multi-applicable materials in light emitting diode (LED) (Caruge et al., 2008; Molaei et al., 2012; Shen et al., 2019; Sun et al., 2007), photovoltaic (Jiao et al., 2015; Rosiles-Perez et al., 2018), gas sensing (Nazzal et al., 2003), photodetectors (Cao et al., 2019). Decreasing size, in nano-scale, leads to quantum confinement effects that shifts the optical properties of semiconductors especially band-gap toward blue wavelength. By using this trend, photoluminescent QDs can show all visible lights (Qu and Peng, 2002). An increase in surfaceto-volume ratio is another factor in creating a good sensor in the nanoscale systems. Therefore, the optical properties of QDs can be a good parameter to sense the various environmental effects such as chemical sensing (Xia and Zhu, 2008; Zare et al., 2017), light sensing in photodiodes (Rauch et al., 2009) and even laser (Smirnov et al., 2018a, 2018b) or ionization radiation sensing. Ionization radiation can effect on QDs in several scenarios. For example, one of them can be studying of degradation of QDs after irradiation exposure. Another one is scrutiny of reaction time after gamma ray radiate to solution. Letant group detected gamma and alpha irradiation with detection of visible photon ∗
by the using of QDs (Letant and Wang, 2006). Nathan et al. studied the effect of 137Cs gamma irradiation with the doses ranging from 0 to 1.3 kGy on the photoluminescence (PL) of CdSe/ZnS quantum dots which synthesized by hot injection method (Withers et al., 2008). In a similar work, The PL properties of CdSe/ZnS QDs were studied by irradiation of 60Co source with different doses between 0 and 100 Gy (Stodilka et al., 2009). Gaur et al. studied PL properties of immobilized CdTe QDs in porous silicon with gamma irradiation dose up to 160 kGy (Gaur et al., 2013). There are a few reports where gamma irradiation was used as assister to synthesis or time reducer in polymerization (Bekasova et al., 2013; Chang et al., 2011; Raju et al., 2017; Yang et al., 2018). The main fact of using gamma irradiation is production of some radical in solution that increase the reaction speed in order to QDs synthesis (Ribeiro et al., 2015). However, there are low researches about effects of ionization irradiation on the optical properties of QDs especially water-soluble. A promising approach for synthesis of QDs is aqueous route is due to inexpensive, safe and easy to produce (Li et al., 2017; Zare et al., 2015) while in hot injection method need to high temperature with dangerous and toxic precursor, is a pivotal need (C˅apek et al., 2009; Parobek et al., 2018). Microwave assisted method is a fast and facile to synthesize aqueous colloidal QDs by which it can be have QDs just after a few minute (Karimipour and Molaei, 2016). With utilizing of the
Corresponding author. Corresponding author. E-mail addresses: [email protected] (H. Alehdaghi), [email protected] (B. Azadegan).
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https://doi.org/10.1016/j.radphyschem.2019.108476 Received 26 May 2019; Received in revised form 10 July 2019; Accepted 1 September 2019 Available online 04 September 2019 0969-806X/ © 2019 Elsevier Ltd. All rights reserved.
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benefit, we can use them in a system, which senses to gamma irradiation and have a capacity to use as dosimeter. Herein, we studied, for the first time, effect of gamma irradiation on the physical and chemical properties of aqueous CdS QDs. The QDs were synthesized in facile and straightforward in aqueous solution by microwave irradiation in lower than 10 min. The PL and transmittance properties of QDs for different sizes and concentrations were studied. Transmission electron microscopy indicated the size of QDs about 3–4 nm. The crystalline and surface chemical properties before and after gamma irradiation were compared by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), respectively. 2. Materials and method All chemical were used with analytical grade without any purification. 3CdSO4.5H2O, Na2S2O3.2H2O and Thioglycolic acid (TGA) were purchased from Merck chemical company used for precursor of CdS QDs and capping agent, respectively. CdS QDs was synthesized the same as pervious reports (Alehdaghi et al., 2014; Karimipour and Molaei, 2016). Cd and S precursor was solved in DI water separately, with concentration of 5 mM and 50 mM, respectively. Then 12 mM TGA added to the solution of Cd that leads to milky solution. After that, both solutions were mixed together and pH was increased by ammonium to make clear the solution. Next, the solutions were put in a 500 W domestic microwave for different time to have various sizes. Finally, the QDs were extracted by centrifuge and solved in DI water. Micro tubes with 2 ml of QDs water-soluble were prepared and irradiated by 60Co source which produced two gammas of 1.33 and 1.17 MeV energies with irradiation dose rate of 1.85 Gy/s. For every experiments, one sample was selected without irradiation as a reference, which shows 0 Gy irradiation dose. This sample was in the same condition with irradiated samples. Optical transmittance measurement was performed by PerkinElmer, Lambda 25 and PL was measured by Avantes spectrometer (AvaSpec2048 TEC) with a high pressure Hg lamp with a 360 nm filter. IR spectra (FTIR) were recorded on a Shimadzu 435-U-04 spectrophotometer (KBr pellets). X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis were carried out by D8-Advance Bruker with Cu target and Tecnai-F-20, respectively.
Fig. 1. The transmission electron microscopy of CdS QDs (a), high-resolution transmission electron microscopy (b), the corresponding histogram size (c) and a schematic shows gamma ray decomposes water molecule to the derivation.
sample) to 1 kGy along with a little red shift in peak position. The significance change in PL is occurred at 20 kGy gamma dose, so that solution color, under excitation light, was changed from blue to green due to the red shift in PL maximum peak (from 474 nm to 554). In Fig. 2 (b) the transmittance spectra for 1 kGy and 20 kGy gamma doses are drawn. Here, the red shift after gamma irradiation exposure is evidently seen. There are 2 nm and 46 nm shift toward red wavelength for 1 kGy and 20 kGy gamma doses, respectively. It can be estimated that the gamma irradiation provides the required energy for growing of QDs. We also investigated the optical properties of larger-sized CdS QDs with increasing growth time of up to 7 min to examine the effect of size in exposure to different doses of gamma radiation (Smirnov et al., 2018b). As shown in Fig. 3 (a, b), the PL and transmittance is changed by different gamma doses. In comparison with small size of QDs, change in PL and transmittance is low. Exposure up to 1 kGy cannot create a large change in the peaks position of PL spectra while exposure of 20 kGy was leading to decrease of intensity by factor 1.5 and a little red shift. The absorption data confirms this change, so that there is no variation in the bandgap in 1 kGy gamma dose while for 20 kGy dose, decrement in the band gap is 0.04 eV equivalent to 0.1 nm increment in size (calculated using effective mass approximation eq. (1)).
3. Results To observe of size distribution and crystal nature of CdS QDs, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed. Fig. 1(a–c) shows TEM results for CdS QDs. The TEM image shows small particles (Fig. 1 a) with average size of 3.2 nm (Fig. 1 c). In HRTEM image, single crystalline nature of QDs is resolved so that d-spacing of 0.33 nm is seen (Fig. 1 b), corresponding to the (002) faces of hexagonal face of CdS (Geng et al., 2011). Fig. 1 (d) shows a schematic of gamma irradiation reaction to aqueous solution. When ionization ray interacts to water molecules produce there active radicals e-aq, H. and .OH and other products such as OH−, H2O2, O3, H2, O2, H+ and UV (Ismail et al., 2013; Ribeiro et al., 2015). Many reports emphasize, when ionization irradiation interact with a sample that is soluble in water, the main portion of degradation is due to the product created by gamma interaction with water molecules. Fig. 1 (d) is a schematic showing of the products that attack to CdS quantum dots. The PL spectra of QDs with size of 3.04 nm with different doses of gamma irradiation are shown in Fig. 2 (a). The black curve regarding to the reference and the witness sample which shows two peaks in 474 and 540 nm related to surface traps of CdS QDs with blue color (Mohsennia et al., 2015; Razzaq et al., 2014). According to the bandgap of reference sample (355 nm equivalent to 3.49 eV), which was obtained from transmittance spectra (Fig. 2 b), it can be clearly seen there is no bandgap emission for the samples. The intensities of the PL spectra increase with increasing the dose from 0 Gy (means the reference
E (x ) = E∞ (x ) +
h2 ⎛ 1 ⎞ 3.572 e 2 4.890 e 4 ⎜ ⎟ − − 2 2 μ (x ) 2d 2 ⎝ μ (x ) ⎠ ε (x ) d h ε (x )
(1)
where. E(x), E∞(x), h, d, μ(x), ϵ(x), and e is quantum dots band gap, bulk band gap, planck constant, quantum dot diameter, reduced mass, permittivity of the matter, elementary charge, respectively. Comparison of optical properties of two sizes of CdS QDs synthesized in the aqueous method indicates that the effect of gamma radiation on QDs is size-depended. This means that samples of various initial 2
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Fig. 2. PL spectra for the QDs solution with 3.04 nm exposed to different dose of gamma irradiation (a) and transmittance spectra of the QDs solution after dose of 1 kGy and 20 kGy (b).
relative increment in size is 0.23%, which means that after 20 kGy gamma dose, the QD grows to 3.74 nm. The Ostwald ripening process is one the most agent to grow QDs (Gu et al., 2008). As shown in Fig. 3 (c), the amount growth of the QDs depends strongly on the initial size of the QDs, especially in small size of about 3 nm. It means that smaller size of the QDs leads to more sensitivity for gamma dosimetry. To further investigate the impact of gamma irradiation on the QDs, the effect of concentration in addition to the effect of size was studied. After centrifugation, extracted solution with contents of the 3.34 nm QDs (at a concentration of 20 mg/cc) was diluted 10 and 50 times, i.e.,
sizes show different changes after gamma irradiation exposure. To confirm that the effects of gamma radiation on optical properties is size dependent, two other sizes of the QDs CdS were studied. It can be easily synthesized CdS QDs of different sizes by adjusting the time of microwave irradiation. The four aqueous solutions of CdS QDs with the size of 3.04, 3.34, 3.48 and 3.59 nm were exposed to 20 kGy gamma dose. Fig. 3 (c) shows the relative increase in QDs size of the samples versus their initial size which was obtained from bandgap data of QDs after gamma exposure and the related reference data using effective mass approximation. For example, for the QDs with the size of 3.04 nm, the
Fig. 3. The PL (a) and transmittance (b) properties of CdS QDs synthesized with time growth of 7 min in microwave for three different doses 0, 1 and 20 kGy. The bar graph of relative increase in QDs size of CdS versus the initial size of QDs (c). 3
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Fig. 4. The PL intensity (a, c) and transmittance spectra (b, d) of QDs before (reference sample) after 1 and 20 kGy gamma irradiation for different concentration 2 mg/cc (a, b) and 0.4 mg/cc (c, d). The relative decrease in PL intensity ((e), the left) and the relative increase in QDs size ((e), the right) for three different concentrations after 20 kGy gamma irradiation.
To further understand the effect of concentration on the PL intensity and the size of QDs, the relative decrease in PL intensity (the difference I −I between initial intensity and final intensity to initial intensity, i f ) Ii and the relative increase in QDs size, after exposure of 20 kGy, versus concertation were plotted in Fig. 4 (e). It is seen that for the concentration of 0.4 mg/cc, the relative decrease in PL intensity is 0.99, meaning that the intensity of PL is dramatically reduced by factor of 100. While for concentration of 20 mg/cc that is about 0.60 (PL intensity after gamma irradiation was changed to 40% of the initial intensity). The relative increase in QDs sizes are 0.27, 0.24 and 0.07 for concentrations of 0.4, 2 and 20 mg/cc, respectively. The trend of size change of QDs is very similar to PL intensity so that the amount of variation in QDs size decreases with increasing concentrations. These results propose for having a sensitive dosimeter to low dose, concentration of QDs solution should be low while high concentration solution of the QDs can leads to a dosimeter capable of sensing high dose. X-ray diffraction (XRD) measurements for drop-cast of QDs were conducted to study effect of gamma irradiation on the crystal structure. In Fig. 5, XRD patterns of two samples is shown that the black one is corresponding to the sample before gamma exposure and the red one is for the sample after 20 kGy gamma irradiation. There are two wide peaks at 27.9° and 47.0° for two samples before and after gamma irradiation. According to JCPDS file number 41.1049 (Alehdaghi et al., 2014; Molaei et al., 2012), these peaks are almost corresponded to the diffraction of (101) and (103) crystallographic planes that indicates hexagonal phase for CdS synthesized here. These two wide peaks consist of three peaks of (100), (002) and (101) for the first and (110), (103) and (112) for the second (Wang et al., 2016). With Debye Scherrer evaluation (eq. (2)) the full-width at half-maximum (FWHM) of peak (101) shows crystallite size of about 1 nm that it is smaller than the size earned by TEM due to low crystalline properties of QDs. Comparing of the XRD pattern of QDs before and after 20 kGy gamma irradiation showed there is no change in crystalline structure of QDs after gamma exposure. It means that gamma rays with 662 keV energy could not modify the CdS atoms and damage the crystal of the nanostructure. The crystallite size calculated by Debye Scherrer showed that gamma irradiation increases the size of QDs from 0.8 nm to 1.0 nm, which is consistent with transmittance spectrum in point of view of size increment after gamma irradiation.
Fig. 5. XRD patterns for CdS QDs before and after 20 kGy gamma irradiation.
concentration of 2 and 0.4 mg/cc, respectively. Fig. 4 shows the optical properties of samples with 2 mg/cc (a, b) and 0.4 mg/cc (c, d) after radiation dose of 1 and 20 kGy. Fig. 4 (a, c) shows the PL spectra where it is seen that after exposure of 1 kGy, the PL intensity is increased, while after exposure of 20 kGy it is decreased. The variation in the intensity of PL in 20 kGy gamma dose is sensitive to concentration, with decreasing the concentration from 2 mg/cc to 0.4 mg/cc, the PL intensity is vanished in 20 kGy gamma dose. The transmittance of the samples with 2 mg/cc (Fig. 4 (b)) and 0.4 mg/cc (Fig. 4 (c)) with 1 kGy and 20 kGy gamma doses represents redshift, which means that the size of QDs is increased. This red shift is also seen in the PL peaks of the samples after gamma irradiation. The transmittance amount for QDs with concentration of 0.4 mg/cc in dose radiation of 20 kG, in visible region, is low (i.e. opaque) because of scattering of light by large particle and clusters. Fig. 5 (d) shows that in 20 kGy gamma dose the transmittance of QDs is reduced. By comparing of the PL and the transmittance of the samples in 20 kGy gamma dose, it can be concluded that degradation of QDs for lower concentration is more vigorous. 4
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Fig. 6. FT-IR spectrum of TGA, CdS QDs before and after 20 kGy gamma irradiation. The vertical dotted lines show the same absorbance for QDs. The inset shows TGA molecule.
D=
kλ βcosθ
(2)
where k, λ, β and θ is structural constant (about 0.9), X-ray wavelength (1.54 A°), FWHM of the peak and Brag angel, respectively. To order to study the effect of gamma irradiation on surface of QDs, FT-IR analysis of two samples were preformed (before and after gamma irradiation). As mentioned in the materials and method section, QDs were capped by TGA. In Fig. 6, FT-IR spectrum pure TGA, CdS QDs before and after 20 kGy gamma irradiation have been shown. There is a broad peak about 3000 cm−1 for pure TGA that it is attributed to –OH stretching vibration while this peak was shifted to 3405 cm−1 for QDs. The absorbance at 2676 cm−1 and 2566 cm−1 belongs to S-H vibration bands in pure TGA. The absence of this peak in QDs capped TGA shows that TGA molecules attached to CdS QDs. After TGA capping to QDs, in FT-IR spectrum a peak at 1564 cm−1 appears which is attributed to the antisymmetric of COO−. TGA molecule is seen at the bottom of Fig. 6. The comparing of FT-IR spectrum before and after gamma irradiation concludes that gamma ray cannot remove capping agent (TGA) in aqueous QDs. Therefore, the degradation of PL intensity is attributed to growing of QDs by Ostwald ripening mechanism with production of free radicals in water due to gamma irradiation.
4. Conclusion The optical properties of aqueous CdS QDs with different sizes and concentrations were studied for the first time. Our results showed that the effect of gamma irradiation on the QDs is size-depended so that with decreasing the size of CdS QDs, the effect of gamma irradiation on the QDs is increased. Concentration of QDs solution was another factor to determine strength of the effectiveness of gamma irradiation on the optical properties of QDs, low concentration leads to more sensitive. The XRD analysis showed that gamma irradiation could not damage the crystalline structure of QDs. The FTIR spectrum indicated that gamma irradiation was not able to remove thiol groups from QDs surface. The main mechanism proposed for the changes was the growth of QDs by Ostwald ripening by production of free radicals in water due to gamma irradiation. Our results suggest that aqueous CdS QDs is a good candidate for use as a dosimeter by selecting the appropriate concentration and size.
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