- Email: [email protected]
ZnS quantum dots
Optical Materials 86 (2018) 545–549
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Structural and optical properties of upconversion CuInS/ZnS quantum dots Magdy Ali
a,b,∗
c
a
, Jehan El Nady , Shaker Ebrahim , Moataz Soliman
T
a
a
Department of Materials Science, Institute of Graduate Studies and Research, Alexandria University, 163 Horrya Avenue, El-Shatby, P.O. Box 832, Alexandria, Egypt European Egyptian Pharmaceutical Industries (EEPI), Km 25, Alexandria Cairo desert road, El Ameriya, Alexandria, Egypt Electronic Materials Department, Advanced Technology and New Materials Research Institute, City of Scientific Research and Technological Applications (SRTA-City), New Borg El-Arab City, P.O. Box 21934, Alexandria, Egypt b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Quantum dots CIS ZnS Chalcopyrite Excitation-independent Up-conversion Down-conversion
A facile one-pot method to synthesis CuInS/ZnS (CIS/ZnS) QDs was developed. The prepared CIS/ZnS QDs exhibited a high bright emission. Moreover, the upconversion photoluminescence (PL) of the CIS/ZnS QDs was explored. Interestingly, extraordinary excitation-independent emission for both up and down conversion fluorescence of the CIS/ZnS QDs were observed. Analysis of X-ray diffraction (XRD) of CIS/ZnS QDs showed chalcopyrite crystal structure. The high-resolution transmission electron Microscopy (HRTEM) images demonstrated crystalline CIS/ZnS QDs with spherical shape and average diameter sizes of 2.5 nm and 3.6 nm for CIS core and CIS/ZnS QDs core shell, respectively. The selected area electron diffraction (SAED) suggested that the prepared CIS/ZnS QDs are polycrystalline with about 0.32 nm lattice distance. The PL peaks position was almost fixed and exhibited a strong peak at about 640 nm for both up and down conversion emission with a linear relationship between the intensity of the PL emission peaks and excitation wavelengths.
1. Introduction Colloidal semiconductors quantum dots (QDs) have received a great attention as a result of their interesting optical and electrical properties [1,2]. QDs acquire a strong quantum confinement in the range size from 2 to 10 nm [3,4]. Accordingly, QDs have potential applications in many areas, for instance solar cells, optoelectronic, light emitting diode, sensors, and biological imaging [5–8]. Although, these unique properties, there is a toxicity issue related to the heavy metals, such as cadmium and lead [9,10]. In recent years there are focusing on the Cd-free of CuInS ∕ZnS (CIS/ZnS) QDs to avoid this problem [11]. It was found that the produced CIS/ZnS QDs with several methods demonstrated an excitation-dependent photoluminescence (PL) spectrum [12]. In addition, the photostability of CIS core was significantly enhanced by covering with a shell of ZnS [13]. Recent reports documented that the upconversion nanoparticles can emit light in the ultraviolet, visible or in the near-infrared region under excitation of longer wavelength [14,15]. The optical properties of such nanomaterials are inhibiting the light scattering and autofluorescence from biological tissue [16]. Therefore, these materials have potential in many areas such as invivoimaging [17]. Herein, the aim of this work is to synthesis a new facile strategy for CIS/ZnS QDs from an aqueous phase with a bright PL and high stability. Interestingly, those CIS/ZnS QDs demonstrates both excitation∗
independent upconversion and downconversion PL features. To the best of the authors’ knowledge, this is the first report demonstrates the excitation-independent downconversion and upconversion PL behavior for aqueous CIS/ZnS QDs. In addition, this excitation-independent exhibits a linear relationship between the intensity of the PL emission peaks and excitation wavelengths. Moreover, the proposed mechanisms of both excitation-independent downconversion and upconversion PL behavior is explored and discussed. 2. Experimental 2.1. Materials Indium (III) chloride tetrahydrate (InCl3.4H2O, 97%), copper (II) chloride (CuCl2, 99%), 3-mercaptopropionic acid (MPA, 99%), zinc acetate (Zn(OAc)2, 99.99%), and sodium hydroxide were purchased from Sigma-Aldrich. Sodium sulfide hydrate (Na2S.xH2O, 60–62%) and isopropanol (C3H8O, 99.5%) were obtained from ACROS Organics. 2.2. Synthesis of CIS and CIS/ZnS QDs To synthesis of CIS QDs with a Cu: In ratio of 1:6, an aqueous solution of InCl3 (0.20 mmol) was mixed with 0.2 mmol of MPA in 20 mL deionized water with a contentious stirring for 3 min. A 0.033 mmol of
Corresponding author. Institute of Graduate Studies and Research, Alexandria University, PO Box 832, 163 Horreya Avenue, El-Shatby, Alexandria, Egypt. E-mail address: [email protected] (M. Ali).
https://doi.org/10.1016/j.optmat.2018.10.058 Received 16 September 2018; Received in revised form 30 October 2018; Accepted 31 October 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Optical Materials 86 (2018) 545–549
M. Ali et al.
CuCl2 was mixed with 0.2 mmol of MPA in 20 mL deionized water with a contentious stirring for 3 min and then the pH of this solution was adjusted to 9.0 using 1N NaOH. Then, the CuCl2 solution was added to the InCl3 solution with a contentious stirring for 5 min. Afterward, the temperature of the mixed precursors was raised to 90 °C with stirring and this reaction was kept for 30 min under reflux condition. Finally, ZnS shell was formed by adding one mL of 0.04 M Zn(OAc)2 dropwise over 1 min with a continuous stirring for 5 min. This solution of QDs was maintained for 5 min at 90 °C where the ZnS shell was grown on the core of CIS QDs. The prepared CIS and CIS/ZnS core shell QDs solutions were collected. The colloidal QDs were precipitated with an excess of isopropanol. The precipitate was centrifuged at 6500 rpm for 5 min and the supernatant was decanted. This process was repeated for twice. 2.3. Characterization techniques The crystallography of the prepared QDs were investigated using XRD technique (X-ray 7000 Shimadzu-Japan, X-ray diffraction machine using copper characteristic wavelength = 1.54 A °). The 2θ angles of the diffractometer were scanned from 10° to 60° with a scan rate of 10°/ min. The morphological properties were studied using HRTEM. HRTEM images were collected by using (Jeol JEM 2100F microscope) at an accelerating voltage of 200 kV. Samples for HRTEM were prepared by dispersing the dried powder of the QDs in ethanol and allowing a drop to dry onto a 3 mm diameter of carbon coated fine copper grid. Subsequently, the grid was dried in air before imaging. Perkin Elmer LS 55 fluorescence spectrophotometer was used to study the downconversion and upconversion emission spectra of the prepared QDs. All the measurements were conducted at room temperature. Both of the excitation and emission slits were fixed at 10.0 nm. The scan rate of the monochromators was 500 nm/min. The excitation wavelengths were changed from 260 to 340 nm for downconversion and from 800 to 700 nm for upconversion.
Fig. 2. HRTEM images of CIS QDs (A, B), CIS/ZnS QDs (C, D) insets of (B, D) show the HRTEM image of an individual QDs and the inset in (C) is the SAED pattern of CIS/ZnS QDs.
slightly peaks position are shifted to higher 2 theta after the growth of ZnS shell. This suggests that the ZnS shell is formed around the CIS cores. Main peaks of (111), (202), and (311) planes of chalcopyrite CIS QDs are observed at 27.5°, 47.3°, and 54.8° for the CIS QDs and 28.1°, 47.5°, and 55.2°, respectively for the CIS/ZnS QDs. The characteristic peaks of the XRD patterns of these QDs are matched well with the XRD references (JCPDS 32-0339, CuInS2 and 10–0434, ZnS) [19–21]. It is observed that XRD peak intensities are increased with growth of the ZnS shell due to the enhancement of the crystallinity resulted from the diffusion of Zn2+ ions into the CIS core in the vacancies sites [22]. The morphologies and crystallinities of CIS and CIS/ZnS QDs are investigated by HRTEM as shown in Figure (2). The QDs are crystalline with a spherical shape. CIS and CIS/ZnS QDs have average diameters of 2.5 nm and 3.6 nm, respectively. The average diameter of core CIS QDs is increased by growing up ZnS shell. The average diameter of the ZnS shell is estimated from the difference between the average diameters of CIS and CIS/ZnS and is found around 1.1 nm. This supports the formation of ZnS outer shell on the CIS core surface. The inset in Fig. 2C shows a SAED ring pattern of CIS/ZnS QDs. The diffraction rings observed suggesting that the prepared CIS/ZnS QDs are polycrystalline with a 0.32 nm lattice distance [23]. These results confirm the successful formation of both CIS and CIS/ZnS QDs [24]. The growth of ZnS shell improves not only the PL efficiency of the CIS QDs but also their photostability. The CIS and CIS/ZnS QDs were stored on shelf in ambient condition without any additional protections. The PL spectra of the QDs samples are conducted to evaluate the stability of the CIS and CIS/ZnS QDs. As shown in Fig. 3 (A), the PL spectra of the fresh prepared core CIS sharply decreases within one month, which indicates that the CIS QDs without protection of ZnS shell is unstable. However, the PL spectrum of the CIS/ZnS QDs shows a nonsignificant decrease after 6 months as shown in Fig. 3 (B). It indicating that the ZnS shell around CIS core will protect it from the oxidizing. The high photostability of CIS/ZnS QDs promotes the potential in broad spectrum applications. The PL emissions behavior in I−III−VI QDs are completely different from II−VI QDs or III−V QDs [25]. The full width at halfmaximum (FWHM) of PL emissions for CIS/ZnS QDs are broad with large Stokes shift, suggesting that the recombination of excited
3. Results and discussion The crystal structures of CIS and CIS/ZnS QDs are confirmed by XRD spectroscopy. Fig. 1 shows the XRD patterns of the synthesized CIS and CIS/ZnS QDs. The broad peaks of CIS and CIS/ZnS QDs are observed due to their small sizes. Due to Debye – Scherrer theory, broadening in XRD peaks of crystals is caused by decrease in size of crystals [18]. A
Fig. 1. XRD patterns of CIS, CIS/ZnS QDs and JCPDS of bulk ZnS and CIS. 546
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Fig. 3. (A) PL spectra of freshly prepared core CIS QDs and after one month on shelf, (B) freshly prepared CIS/ZnS QDs and after six months on shelf.
assisted recombination non-radiative [30,31]. It was known that shorter wavelengths have high absorption coefficients [32,33]. This high absorption coefficient increases the PL intensity due to the dependence on the number of energetic absorbed photons. There are several mechanisms were proposed for the up-conversion process, such as excited-state absorption (ESA) and energy transfer upconversion (ETU) [34]. In general, upconversion process depends on sequential absorption of two photons and luminescence steps for shorter-wavelength with the presence of metastable excited state E1, which occupied with excited photons [35]. In the case of ESA, excitation earns successive absorption of pump photons by a single groundstate (GS) ion. The general energy diagram of the ESA process is shown in Fig. 6 for a simple three-level system [34]. Once the atoms are excited from the GS to the metastable E1 level, the same or different atoms are raised from E1 to higher state E2 causes the upconversion emission, before it these excited atoms decay to the ground state and another photon is emitted [17]. To illustrate the comparison between upconversion and downconversion PL spectra of CIS/ZnS QDs, Fig. 7(A) shows the PL peaks are at the same position with an excitation wavelength for both up and downconversion at 450 and 800 nm respectively. Fig. 7 (B) shows luminance color photographs of yellow QDs pristine (left), ruby QDs excited by 800 nm (middle) and purple QDs excited by 450 nm. As expected, the photographs of CuInS/ZnS QDs are yellow under visible light while they emit bright ruby fluorescence under 800 nm excitation and violet fluorescence under 450 nm. The explanation of unshifted of the PL emission peak positions as
electron−hole pair from donor-acceptor pair (DAP) may occurred within the intraband [26]. Hamanaka et al. reported that the major defects in the CIS QDs are normally InCu, VS and VCu. These site defects act as deep donor or acceptor and take part in the carrier recombination processes [27]. Fig. 4 demonstrates the down-conversion PL of CIS/ZnS QDs and as the excitation wavelength raised from 260 to 300 nm, the PL peaks position are almost fixed and exhibited strong peaks at about 640 nm. The inset of Figure A (B) shows the linear relationship between the PL intensity and the excitation wavelength with a correlation coefficient of 0.965. These results suggest that it may be possible to fabricate a UV radiation sensor from the prepared CIS/ZnS QDs. The down-conversion and up-conversion PL spectra of most reported CIS QDs are dependent on excitation wavelength. In which, the PL peaks position are shifted with a maximum intensity as the excitation wavelengths changed. On the other hand, the CIS QDs prepared in this study show an excitationindependent PL behavior for both up and down-conversion. The excitation-independent emission of the CIS indicates uniformity of both the size and the surface state [28]. Fig. 5 (A) illustrates the PL up-conversion spectra of CIS/ZnS QDs excited by long-wavelength in the range from 800 to 700 nm with maximum broad peaks at about 640 nm. Fig. 5 (B) shows the linear relationship between the PL intensity and the excitation wavelength with correlation coefficient (R2) = 0.987. Most attractively, the upconversion PL spectra show also an excitation-independent PL behavior similar to those of the down-conversion. This is attributed to the thermalization losses within the conduction band [29] or increased auger
Fig. 4. (A) Down-conversion PL spectra of CIS/ZnS QDs at various excitation wavelengths and (B) the emission intensity as a function of the excitation wavelength with (R2 = 0.965). 547
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Fig. 5. (A) Up-conversion PL spectra of the CIS/ZnS QDs at various excitation wavelengths and (B) the emission intensity as a function of the excitation wavelength with (R2 = 0.987).
Fig. 6. Schematic representation of processes that convert low energy photon pump sources 1 and 2 into higher energy output upconversion 3. Fig. 8. Schematic illustration of (a) band bending and (b) expanding the HOMO level.
the excitation wavelength changed is more complicated than illustrated in Fig. 7. Two proposed mechanisms to investigate such phenomena are expanding HOMO level and bending the energy band. In the case of bending the energy band, the functional groups on the surface of QDs with sufficient electron-rich such as COOH and OH act as n-type
semiconductor [36]. Hence, the valence energy bands of the QDs bend upwards as shown in Fig. 8 (A) [37]. Because of the electronegativity of the oxygen atom (3.44) is larger than that of carbon atom (2.55) more
Fig. 7. PL emission downconversion and upconversion the of CuInS/ZnS QDs with an excitation wavelengths 450 and 800 nm respectively and the inset shows photograph of CIS/ZnS under 450 nm. 548
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electrons are captivated to oxygen atom in the C]O group. The building up electron acceptors on the surface of n-type semiconductor generally leads to increase in band bending by accumulation of negatively charged on the surfaces. Consequently, the energy levels of HOMO and LUMO become broader and the energy difference between them is reduced [37–39]. In the expanding the HOMO level mechanism shown in Fig. 8 (B), the surfaces of the CIS/ZnS QDs are passivated by extensive amount of the electron-donating groups such as COOH etc. Such passivation layer on the surface of CIS/ZnS QDs originates the electrons donation to the QDs and leads to increase the electron density on the surface. As a result, the electron energy of the valance band is raised and expanded toward the conduction band. This is similar to the effect of the QDs size on the band gap [40]. Based on the two the suggested mechanisms, it is supposed that both up and down conversion PL emissions obtained from the lowest allowed singlet states. In this study, the intensities of the PL peaks increase with progressively excitation wavelengths.
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4. Conclusion In summary, CIS/ZnS QDs with excellent PL properties were appropriately synthesized using a one pot synthesis in water. The results for both up and down conversion PL of CIS/ZnS QDs were observed and found to be independent of the excitation wavelength, which is quite different from recently reported CIS/ZnS QDs. Meanwhile, the linear relationship for both up and down conversion was achieved and plotted. The excitation-independent PL of the CIS/ZnS QDs suggesting uniformity of both the size and the surface state. It was found that the CIS/ZnS QDs are polycrystalline with average size of 3.6 nm and 0.32 nm lattice distance. Furthermore, the mechanisms for the upconversion PL and independence of the excitation wavelength of CIS/ ZnS QDs were demonstrated. These CIS/ZnS QDs would provide great potential for a wide range of applications such as bioscience and energy technology. References [1] S. Myung, A. Solanki, C. Kim, J. Park, K.S. Kim, K.B. Lee, Adv. Mater. 23 (2011) 2221–2225. [2] M. Labeb, A.-H. Sakr, M. Soliman, T.M. Abdel-Fettah, S. Ebrahim, Opt. Mater. 79 (2018) 331–335. [3] C.X. Guo, H.B. Yang, Z.M. Sheng, Z.S. Lu, Q.L. Song, C.M. Li, Angew. Chem. Int. Ed. 49 (2010) 3014–3017. [4] X. Liu, Y. Zhang, T. Yu, X. Qiao, R. Gresback, X. Pi, D. Yang, Part. Part. Syst. Char. 33 (2016) 44–52. [5] S. Zhu, J. Zhang, C. Qiao, S. Tang, Y. Li, W. Yuan, B. Li, L. Tian, F. Liu, R. Hu, Chem.
549
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Structural and optical properties of upconversion CuInS/ZnS quantum dots Magdy Ali
a,b,∗
c
a
, Jehan El Nady , Shaker Ebrahim , Moataz Soliman
T
a
a
Department of Materials Science, Institute of Graduate Studies and Research, Alexandria University, 163 Horrya Avenue, El-Shatby, P.O. Box 832, Alexandria, Egypt European Egyptian Pharmaceutical Industries (EEPI), Km 25, Alexandria Cairo desert road, El Ameriya, Alexandria, Egypt Electronic Materials Department, Advanced Technology and New Materials Research Institute, City of Scientific Research and Technological Applications (SRTA-City), New Borg El-Arab City, P.O. Box 21934, Alexandria, Egypt b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Quantum dots CIS ZnS Chalcopyrite Excitation-independent Up-conversion Down-conversion
A facile one-pot method to synthesis CuInS/ZnS (CIS/ZnS) QDs was developed. The prepared CIS/ZnS QDs exhibited a high bright emission. Moreover, the upconversion photoluminescence (PL) of the CIS/ZnS QDs was explored. Interestingly, extraordinary excitation-independent emission for both up and down conversion fluorescence of the CIS/ZnS QDs were observed. Analysis of X-ray diffraction (XRD) of CIS/ZnS QDs showed chalcopyrite crystal structure. The high-resolution transmission electron Microscopy (HRTEM) images demonstrated crystalline CIS/ZnS QDs with spherical shape and average diameter sizes of 2.5 nm and 3.6 nm for CIS core and CIS/ZnS QDs core shell, respectively. The selected area electron diffraction (SAED) suggested that the prepared CIS/ZnS QDs are polycrystalline with about 0.32 nm lattice distance. The PL peaks position was almost fixed and exhibited a strong peak at about 640 nm for both up and down conversion emission with a linear relationship between the intensity of the PL emission peaks and excitation wavelengths.
1. Introduction Colloidal semiconductors quantum dots (QDs) have received a great attention as a result of their interesting optical and electrical properties [1,2]. QDs acquire a strong quantum confinement in the range size from 2 to 10 nm [3,4]. Accordingly, QDs have potential applications in many areas, for instance solar cells, optoelectronic, light emitting diode, sensors, and biological imaging [5–8]. Although, these unique properties, there is a toxicity issue related to the heavy metals, such as cadmium and lead [9,10]. In recent years there are focusing on the Cd-free of CuInS ∕ZnS (CIS/ZnS) QDs to avoid this problem [11]. It was found that the produced CIS/ZnS QDs with several methods demonstrated an excitation-dependent photoluminescence (PL) spectrum [12]. In addition, the photostability of CIS core was significantly enhanced by covering with a shell of ZnS [13]. Recent reports documented that the upconversion nanoparticles can emit light in the ultraviolet, visible or in the near-infrared region under excitation of longer wavelength [14,15]. The optical properties of such nanomaterials are inhibiting the light scattering and autofluorescence from biological tissue [16]. Therefore, these materials have potential in many areas such as invivoimaging [17]. Herein, the aim of this work is to synthesis a new facile strategy for CIS/ZnS QDs from an aqueous phase with a bright PL and high stability. Interestingly, those CIS/ZnS QDs demonstrates both excitation∗
independent upconversion and downconversion PL features. To the best of the authors’ knowledge, this is the first report demonstrates the excitation-independent downconversion and upconversion PL behavior for aqueous CIS/ZnS QDs. In addition, this excitation-independent exhibits a linear relationship between the intensity of the PL emission peaks and excitation wavelengths. Moreover, the proposed mechanisms of both excitation-independent downconversion and upconversion PL behavior is explored and discussed. 2. Experimental 2.1. Materials Indium (III) chloride tetrahydrate (InCl3.4H2O, 97%), copper (II) chloride (CuCl2, 99%), 3-mercaptopropionic acid (MPA, 99%), zinc acetate (Zn(OAc)2, 99.99%), and sodium hydroxide were purchased from Sigma-Aldrich. Sodium sulfide hydrate (Na2S.xH2O, 60–62%) and isopropanol (C3H8O, 99.5%) were obtained from ACROS Organics. 2.2. Synthesis of CIS and CIS/ZnS QDs To synthesis of CIS QDs with a Cu: In ratio of 1:6, an aqueous solution of InCl3 (0.20 mmol) was mixed with 0.2 mmol of MPA in 20 mL deionized water with a contentious stirring for 3 min. A 0.033 mmol of
Corresponding author. Institute of Graduate Studies and Research, Alexandria University, PO Box 832, 163 Horreya Avenue, El-Shatby, Alexandria, Egypt. E-mail address: [email protected] (M. Ali).
https://doi.org/10.1016/j.optmat.2018.10.058 Received 16 September 2018; Received in revised form 30 October 2018; Accepted 31 October 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Optical Materials 86 (2018) 545–549
M. Ali et al.
CuCl2 was mixed with 0.2 mmol of MPA in 20 mL deionized water with a contentious stirring for 3 min and then the pH of this solution was adjusted to 9.0 using 1N NaOH. Then, the CuCl2 solution was added to the InCl3 solution with a contentious stirring for 5 min. Afterward, the temperature of the mixed precursors was raised to 90 °C with stirring and this reaction was kept for 30 min under reflux condition. Finally, ZnS shell was formed by adding one mL of 0.04 M Zn(OAc)2 dropwise over 1 min with a continuous stirring for 5 min. This solution of QDs was maintained for 5 min at 90 °C where the ZnS shell was grown on the core of CIS QDs. The prepared CIS and CIS/ZnS core shell QDs solutions were collected. The colloidal QDs were precipitated with an excess of isopropanol. The precipitate was centrifuged at 6500 rpm for 5 min and the supernatant was decanted. This process was repeated for twice. 2.3. Characterization techniques The crystallography of the prepared QDs were investigated using XRD technique (X-ray 7000 Shimadzu-Japan, X-ray diffraction machine using copper characteristic wavelength = 1.54 A °). The 2θ angles of the diffractometer were scanned from 10° to 60° with a scan rate of 10°/ min. The morphological properties were studied using HRTEM. HRTEM images were collected by using (Jeol JEM 2100F microscope) at an accelerating voltage of 200 kV. Samples for HRTEM were prepared by dispersing the dried powder of the QDs in ethanol and allowing a drop to dry onto a 3 mm diameter of carbon coated fine copper grid. Subsequently, the grid was dried in air before imaging. Perkin Elmer LS 55 fluorescence spectrophotometer was used to study the downconversion and upconversion emission spectra of the prepared QDs. All the measurements were conducted at room temperature. Both of the excitation and emission slits were fixed at 10.0 nm. The scan rate of the monochromators was 500 nm/min. The excitation wavelengths were changed from 260 to 340 nm for downconversion and from 800 to 700 nm for upconversion.
Fig. 2. HRTEM images of CIS QDs (A, B), CIS/ZnS QDs (C, D) insets of (B, D) show the HRTEM image of an individual QDs and the inset in (C) is the SAED pattern of CIS/ZnS QDs.
slightly peaks position are shifted to higher 2 theta after the growth of ZnS shell. This suggests that the ZnS shell is formed around the CIS cores. Main peaks of (111), (202), and (311) planes of chalcopyrite CIS QDs are observed at 27.5°, 47.3°, and 54.8° for the CIS QDs and 28.1°, 47.5°, and 55.2°, respectively for the CIS/ZnS QDs. The characteristic peaks of the XRD patterns of these QDs are matched well with the XRD references (JCPDS 32-0339, CuInS2 and 10–0434, ZnS) [19–21]. It is observed that XRD peak intensities are increased with growth of the ZnS shell due to the enhancement of the crystallinity resulted from the diffusion of Zn2+ ions into the CIS core in the vacancies sites [22]. The morphologies and crystallinities of CIS and CIS/ZnS QDs are investigated by HRTEM as shown in Figure (2). The QDs are crystalline with a spherical shape. CIS and CIS/ZnS QDs have average diameters of 2.5 nm and 3.6 nm, respectively. The average diameter of core CIS QDs is increased by growing up ZnS shell. The average diameter of the ZnS shell is estimated from the difference between the average diameters of CIS and CIS/ZnS and is found around 1.1 nm. This supports the formation of ZnS outer shell on the CIS core surface. The inset in Fig. 2C shows a SAED ring pattern of CIS/ZnS QDs. The diffraction rings observed suggesting that the prepared CIS/ZnS QDs are polycrystalline with a 0.32 nm lattice distance [23]. These results confirm the successful formation of both CIS and CIS/ZnS QDs [24]. The growth of ZnS shell improves not only the PL efficiency of the CIS QDs but also their photostability. The CIS and CIS/ZnS QDs were stored on shelf in ambient condition without any additional protections. The PL spectra of the QDs samples are conducted to evaluate the stability of the CIS and CIS/ZnS QDs. As shown in Fig. 3 (A), the PL spectra of the fresh prepared core CIS sharply decreases within one month, which indicates that the CIS QDs without protection of ZnS shell is unstable. However, the PL spectrum of the CIS/ZnS QDs shows a nonsignificant decrease after 6 months as shown in Fig. 3 (B). It indicating that the ZnS shell around CIS core will protect it from the oxidizing. The high photostability of CIS/ZnS QDs promotes the potential in broad spectrum applications. The PL emissions behavior in I−III−VI QDs are completely different from II−VI QDs or III−V QDs [25]. The full width at halfmaximum (FWHM) of PL emissions for CIS/ZnS QDs are broad with large Stokes shift, suggesting that the recombination of excited
3. Results and discussion The crystal structures of CIS and CIS/ZnS QDs are confirmed by XRD spectroscopy. Fig. 1 shows the XRD patterns of the synthesized CIS and CIS/ZnS QDs. The broad peaks of CIS and CIS/ZnS QDs are observed due to their small sizes. Due to Debye – Scherrer theory, broadening in XRD peaks of crystals is caused by decrease in size of crystals [18]. A
Fig. 1. XRD patterns of CIS, CIS/ZnS QDs and JCPDS of bulk ZnS and CIS. 546
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Fig. 3. (A) PL spectra of freshly prepared core CIS QDs and after one month on shelf, (B) freshly prepared CIS/ZnS QDs and after six months on shelf.
assisted recombination non-radiative [30,31]. It was known that shorter wavelengths have high absorption coefficients [32,33]. This high absorption coefficient increases the PL intensity due to the dependence on the number of energetic absorbed photons. There are several mechanisms were proposed for the up-conversion process, such as excited-state absorption (ESA) and energy transfer upconversion (ETU) [34]. In general, upconversion process depends on sequential absorption of two photons and luminescence steps for shorter-wavelength with the presence of metastable excited state E1, which occupied with excited photons [35]. In the case of ESA, excitation earns successive absorption of pump photons by a single groundstate (GS) ion. The general energy diagram of the ESA process is shown in Fig. 6 for a simple three-level system [34]. Once the atoms are excited from the GS to the metastable E1 level, the same or different atoms are raised from E1 to higher state E2 causes the upconversion emission, before it these excited atoms decay to the ground state and another photon is emitted [17]. To illustrate the comparison between upconversion and downconversion PL spectra of CIS/ZnS QDs, Fig. 7(A) shows the PL peaks are at the same position with an excitation wavelength for both up and downconversion at 450 and 800 nm respectively. Fig. 7 (B) shows luminance color photographs of yellow QDs pristine (left), ruby QDs excited by 800 nm (middle) and purple QDs excited by 450 nm. As expected, the photographs of CuInS/ZnS QDs are yellow under visible light while they emit bright ruby fluorescence under 800 nm excitation and violet fluorescence under 450 nm. The explanation of unshifted of the PL emission peak positions as
electron−hole pair from donor-acceptor pair (DAP) may occurred within the intraband [26]. Hamanaka et al. reported that the major defects in the CIS QDs are normally InCu, VS and VCu. These site defects act as deep donor or acceptor and take part in the carrier recombination processes [27]. Fig. 4 demonstrates the down-conversion PL of CIS/ZnS QDs and as the excitation wavelength raised from 260 to 300 nm, the PL peaks position are almost fixed and exhibited strong peaks at about 640 nm. The inset of Figure A (B) shows the linear relationship between the PL intensity and the excitation wavelength with a correlation coefficient of 0.965. These results suggest that it may be possible to fabricate a UV radiation sensor from the prepared CIS/ZnS QDs. The down-conversion and up-conversion PL spectra of most reported CIS QDs are dependent on excitation wavelength. In which, the PL peaks position are shifted with a maximum intensity as the excitation wavelengths changed. On the other hand, the CIS QDs prepared in this study show an excitationindependent PL behavior for both up and down-conversion. The excitation-independent emission of the CIS indicates uniformity of both the size and the surface state [28]. Fig. 5 (A) illustrates the PL up-conversion spectra of CIS/ZnS QDs excited by long-wavelength in the range from 800 to 700 nm with maximum broad peaks at about 640 nm. Fig. 5 (B) shows the linear relationship between the PL intensity and the excitation wavelength with correlation coefficient (R2) = 0.987. Most attractively, the upconversion PL spectra show also an excitation-independent PL behavior similar to those of the down-conversion. This is attributed to the thermalization losses within the conduction band [29] or increased auger
Fig. 4. (A) Down-conversion PL spectra of CIS/ZnS QDs at various excitation wavelengths and (B) the emission intensity as a function of the excitation wavelength with (R2 = 0.965). 547
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Fig. 5. (A) Up-conversion PL spectra of the CIS/ZnS QDs at various excitation wavelengths and (B) the emission intensity as a function of the excitation wavelength with (R2 = 0.987).
Fig. 6. Schematic representation of processes that convert low energy photon pump sources 1 and 2 into higher energy output upconversion 3. Fig. 8. Schematic illustration of (a) band bending and (b) expanding the HOMO level.
the excitation wavelength changed is more complicated than illustrated in Fig. 7. Two proposed mechanisms to investigate such phenomena are expanding HOMO level and bending the energy band. In the case of bending the energy band, the functional groups on the surface of QDs with sufficient electron-rich such as COOH and OH act as n-type
semiconductor [36]. Hence, the valence energy bands of the QDs bend upwards as shown in Fig. 8 (A) [37]. Because of the electronegativity of the oxygen atom (3.44) is larger than that of carbon atom (2.55) more
Fig. 7. PL emission downconversion and upconversion the of CuInS/ZnS QDs with an excitation wavelengths 450 and 800 nm respectively and the inset shows photograph of CIS/ZnS under 450 nm. 548
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electrons are captivated to oxygen atom in the C]O group. The building up electron acceptors on the surface of n-type semiconductor generally leads to increase in band bending by accumulation of negatively charged on the surfaces. Consequently, the energy levels of HOMO and LUMO become broader and the energy difference between them is reduced [37–39]. In the expanding the HOMO level mechanism shown in Fig. 8 (B), the surfaces of the CIS/ZnS QDs are passivated by extensive amount of the electron-donating groups such as COOH etc. Such passivation layer on the surface of CIS/ZnS QDs originates the electrons donation to the QDs and leads to increase the electron density on the surface. As a result, the electron energy of the valance band is raised and expanded toward the conduction band. This is similar to the effect of the QDs size on the band gap [40]. Based on the two the suggested mechanisms, it is supposed that both up and down conversion PL emissions obtained from the lowest allowed singlet states. In this study, the intensities of the PL peaks increase with progressively excitation wavelengths.
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4. Conclusion In summary, CIS/ZnS QDs with excellent PL properties were appropriately synthesized using a one pot synthesis in water. The results for both up and down conversion PL of CIS/ZnS QDs were observed and found to be independent of the excitation wavelength, which is quite different from recently reported CIS/ZnS QDs. Meanwhile, the linear relationship for both up and down conversion was achieved and plotted. The excitation-independent PL of the CIS/ZnS QDs suggesting uniformity of both the size and the surface state. It was found that the CIS/ZnS QDs are polycrystalline with average size of 3.6 nm and 0.32 nm lattice distance. Furthermore, the mechanisms for the upconversion PL and independence of the excitation wavelength of CIS/ ZnS QDs were demonstrated. These CIS/ZnS QDs would provide great potential for a wide range of applications such as bioscience and energy technology. References [1] S. Myung, A. Solanki, C. Kim, J. Park, K.S. Kim, K.B. Lee, Adv. Mater. 23 (2011) 2221–2225. [2] M. Labeb, A.-H. Sakr, M. Soliman, T.M. Abdel-Fettah, S. Ebrahim, Opt. Mater. 79 (2018) 331–335. [3] C.X. Guo, H.B. Yang, Z.M. Sheng, Z.S. Lu, Q.L. Song, C.M. Li, Angew. Chem. Int. Ed. 49 (2010) 3014–3017. [4] X. Liu, Y. Zhang, T. Yu, X. Qiao, R. Gresback, X. Pi, D. Yang, Part. Part. Syst. Char. 33 (2016) 44–52. [5] S. Zhu, J. Zhang, C. Qiao, S. Tang, Y. Li, W. Yuan, B. Li, L. Tian, F. Liu, R. Hu, Chem.
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