- Email: [email protected]
Influence of ambient gas on the optical properties of CdS quantum dots prepared by plasma-liquid interactions
Author’s Accepted Manuscript Influence of ambient gas on the optical properties of CdS quantum dots prepared by plasma-liquid interactions M. Shariat, M. Karimipour, M. Molaei www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(18)30608-2 https://doi.org/10.1016/j.jlumin.2018.11.031 LUMIN16101
To appear in: Journal of Luminescence Received date: 5 April 2018 Revised date: 13 November 2018 Accepted date: 15 November 2018 Cite this article as: M. Shariat, M. Karimipour and M. Molaei, Influence of ambient gas on the optical properties of CdS quantum dots prepared by plasmaliquid interactions, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.11.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of ambient gas on the optical properties of CdS quantum dots prepared by plasma-liquid interactions M.Shariat*, M. Karimipour and M. Molaei Department of Physics, Faculty of Science, Vali-e-Asr University of Rafsanjan, Rafsanjan, Iran *Corresponding author: [email protected]
Abstract In this study CdS quantum dots were fabricated by injecting non-equilibrium plasma jet into the aqueous solution including Na2S2O3, CdSO4 and Thioglycolic acid (TGA) in the presence of an ambient gas such as nitrogen and oxygen gases. In order to investigate the structure, morphology, size and chemical nature of the prepared CdS nanocrystals in presence of ambient gas, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) were employed. The optical properties of CdS nanoparticles were characterized using UV-Vis absorption and photoluminescence (PL) spectroscopies. It was found that the presence of ambient gases especially oxygen, reduces the size and concentration of the synthesized nanoparticles, while the crystalline structure of these nanoparticles is not altered. It was also found that with the increase of flow rate of ambient gas, the PL emission intensity of quantum dots was reduced and the PL peak position as well as the absorption edge undergoes a blue shift. Introduction Most II-VI semiconductors quantum dots (QDs) such as CdS, ZnS and ZnSe are ideals for use in various fields of optics and electronics because of their appropriate energy gap of 1-3 eV.1 They are widely used in optoelectronic equipment such as diodes and optical sensors.2 The band gap of these materials is strongly dependent on their size in nano-scale which is known as quantum confinement effect 1,3. Cadmium sulfide is one of the most important semiconductors in the II-VI group and has a direct band gap of 2.42 eV at room temperature 4,5 in its bulk form. CdS nanocrystals (NCs) have been used for a variety of applications, including optical sensors2, photoelectronics6, solar cells7, and light-emitting diodes8 due to their unique electrical and optical properties. Due to its various applications, variety of methods including photochemical reactions9, chemical bath technique10,
1
microwave heating11,12, sol-gel process13 and ultrasonic irradiation14 have been employed for the production of CdS QDs. In recent decades, the plasma-liquid interaction (PLI) as an effective method has been employed to produce organic and inorganic semiconducting nanoparticles1519 . The advantages of this method are that it can be operated close to room temperature and it has very low power consumption and high controllability 18. In this method, the plasma is injected into the plasma-liquid interface at the atmospheric pressure. Plasma produces reactive chemical species including ions, electrons, radicals and photons. These energetic species, especially electrons can stimulate the chemical reactions in the solution interface and lead to the nanoparticles formation 20,21. Mussard et al. showed that in contact of the nonthermal plasma jet with a liquid, the energy and negative electric charges are transferred from plasma to the liquid surface, but they did not exactly specify which of the plasma species play an essential role in the transmission of energy and negative charges 22. Rumbach et al. using a novel optical method illustrated that the plasma electrons can be dissolved at the interface of plasma and liquid 23,24. They found that solvated electrons are the most important plasma species in advancing chemical reactions; in particular, the reduction of metal cations 25. It has been estimated that the solvated electrons at the liquid/plasma interface can penetrate prior to the reaction up to a depth of 2.5nm and the concentration of solvated electrons at the liquid interface has been evaluated to be about 1mM 24. Furthermore, the production and behavior of plasma-solvated electrons and the other reactive species strongly depends on the ambient gas that surrounds plasma jet in the contact with liquid25-28. Therefore, the chemical and physical processes that lead to the formation of nanoparticles in the plasma/liquid interface can easily be controlled by utilizing different reactive ambient gas such as oxygen and nitrogen. In this work, the synthesis of CdS nanoparticles is carried out by plasma injection into a solution surface in the presence of oxygen and nitrogen ambient gases. It was believed that electrons from the plasma jet are injected into the solution, helping to reduce the Cd2+ ions and form the CdS nanoparticles in solution.18 However, the role of plasma-injected solvated electrons on the synthesis of CdS NCs has not been clearly determined. The reactive oxygen and nitrogen species are generated when the plasma is formed in O2 and N2 ambient gases. These reactive species can scavenge the solvated electrons and reduce their concentration in liquid
2
25
. These processes may have a large effect on the synthesis and optical properties of the CdS nanoparticles fabricated by the PLI method.
METHODS AND MATERIALS: In this work, CdSO4 and Na2S2O3 were used as precursors of Cadmium and Sulfur, respectively, also Thioglycolic acid (TGA) was used as a capping agent. All of these materials were purchased from Merck Company and used without any purification. A 20cc aqueous solution was prepared from CdSO4, Na2S2O3 and TGA, respectively at the concentrations of 1, 4 and 2.5 mM. NaOH was used to adjust the solution pH to 8.5. The final transparent solution lied at the bottom of a glass vessel (three neck round bottom flask, 50ml) under non-thermal plasma jet treatment at atmospheric pressure.
Fig. 1. Sketch of the experimental setup for plasma–liquid interactions.
A schematic of the plasma reactor is shown in Fig. 1. Here the non-equilibrium plasma was created by the configuration of dielectric-barrier discharge (DBD) jet. In this DBD jet, a quartz tube was applied as a dielectric with internal and external diameter of 2 and 4 mm, respectively. The voltage electrode was a 0.5mm diameter stainless steel rod located in the center of the quartz tube, and the ground electrode was a copper foil with a width of 10 mm that was wrapped around the outer surface of quartz tube, also its distance to the end of the tube was 15 mm. The plasma jet was driven by an AC power supply with a voltage of 6kV and a repetition frequency of 18 kHz. Pure helium gas (99.999%) was flown around the 3
stainless steel electrode inside the quartz tube as the plasma working gas. The flow rate of the helium gas was fixed by a mass flow controller at 2000 standard centimeter cubic per minute (SCCM). As shown in Fig. 1, the DBD plasma jet was placed inside the glass vessel so that the distance between nozzle and the solution surface be approximately 5mm. The oxygen and nitrogen as an ambient gas was fed into the vessel with different gas flow rate. The optimal plasma treatment time for all samples was 40 min. After plasma treatment, the solution was centrifuged and dried at room temperature and then the resulting powder was used for characterization.
Characterization methods: The electrical and optical characteristics of DBD plasma jet have been vastly studied in our previous work 18. It has been demonstrated that the power consumption for this plasma jet depends on the applied voltage and have a value of less than 5 Watt. It’s important to note that the plasma power consumption at the presence of various ambient gases does not change much. The structure, size and morphology of the prepared CdS QDs were studied using transmission electron microscopy (TEM, Zeiss EM900), high resolution TEM (HRTEM, Tecnai F20) and field emission scanning electron microscopy (FESEM, TESCAN MIRA3). Also, the X-ray diffraction (XRD) with the advanced d8 Bruker instrument was used for better understanding of the CdS NCs structure. The chemical nature and elemental analysis of the nanoparticles fabricated by PLI approach in the presence of various ambient gases were performed by energy dispersive X-ray spectroscopy (EDS) analysis. The optical properties including absorption and photoluminescence (PL) of QDs were investigated using an Avantes spectrometer (Avantes 2048) at room temperature. The wavelength of light that was used for exciting of the QDs in the PL measurement was 254 nm. Results and discussion: XRD was taken from the nanoparticles synthesized by the PLI method in different environmental gases, which is shown in Fig. 2. In all samples, the two main peaks are observed at 2 =28.4o and 47.7o angles, which are related to the diffraction of the crystalline planes (101) and (103), respectively. It emphasizes that the XRD pattern belongs to the crystallographic card No. JCPDS 41-1049 corresponding to
4
P63mc space group of hexagonal structure and lattice constants a=0.41 and c=0.67 nm.
Fig. 2. XRD pattern the synthesized CdS NCs.
This figure also indicates that the presence of ambient gas around the plasma jet does not affect the structure of the CdS nanoparticles. With the aid of the DebyeScherrer formula, the estimated size of NCs was calculated to be 15.5, 14.1 and 12.8 Å in absence of ambient gas, Nitrogen and oxygen gases, respectively (for more details, see Supplementary information). Therefore, the presence of reactive gas (oxygen and nitrogen) around the plasma plume decreases the size of the CdS NCs.
5
Fig. 3. TEM images and size distribution histograms of CdS NCs produced (a) and (b) without the presence of ambient gas, (c) and (d) with the presence of O2 gas.
Fig. 3 shows the TEM images and size distribution histograms of the CdS nanoparticles in the absence of any ambient gas and the present of oxygen gas. Fig 3 (a) and (c) show that synthesized nanoparticles have a spherical-like shape and most of them have a diameter of about 5 and 2.5 nm for ambient gas and oxygen gas, respectively. Inset of Fig. 3(a) shows HRTEM image of a CdS nanoparticle in ambient gas that appear to be spherical and crystalline with a lattice spacing of 6
approximately 0.31nm. This lattice spacing is consistent with XRD 101 hexagonal planes spacing. This means TEM actually confirmed XRD measurements.
Fig. 4. FESEM image of CdS nanoparticles in the (a) presence of nitrogen, (b) absence of ambient gas.
The FESEM images of the QDs produced in the presence of N2 gas with a flow rate of 100 SCCM and those produced in the absence of ambient gas are shown in Figures 4a and 4b, respectively. According to Fig. 4, the nanoparticles that were synthesized by the plasma method in N2 environment have a smaller average size (4.7nm) compared to those obtained in the absence of ambient gas (6.5nm). For both samples synthesized using N2 gas and ambient gas (no gas), the size estimation extracted from FESEM and TEM is larger than Debye-Scherrer estimation. This increase in the average size estimated from TEM and FESEM relative to the sizes determined from X-ray has observed in other studies.29 Some experimental and analysis points of XRD using Debye-Scherrer formula should be clarified. The crystallite size of samples is calculated for one preferential atomic plane which is not exactly the size of particles for poly-crystallite samples. Generally, the crystallite area corresponding to a specific atomic plane determines its impact on the diffraction pattern. For QDs, since one or two main peaks are usually observed in XRD pattern, it means that the borders of those specific atomic 7
planes are such extended over the area of a single particle that other diffraction peaks are not clearly observed. In this case, almost all the particle size is influenced by a single atomic plane and the contribution of other planes in texturing (mosaicity) of particle is negligible as they are not observable in XRD pattern as well. Thus in this case a rough estimation for QDs size can be obtained by Debye-Scherrer formula. In other word, it can be stated that the size of particle is at least greater or equal to Debye-Scherrer estimation. Therefore, in this work, FESEM size estimation is in well agreement with Debye-Scherrer calculation, meaning that Debye-Scherrer is not greater than FESEM sizes. More interestingly the overall size estimation trend from TEM, FESEM and Debye-Scherrer are consistent with each other such that with N2 or O2 ambient gas application for the synthesis, all of them TEM, FESEM and Debye-Scherrer estimation sizes decrease. Moreover, TEM, FESEM are 5nm, 6.5nm, respectively, in excellent agreement for no gas synthesized sample.
Fig. 5. EDS spectrum of the sample displayed in the (a) presence of nitrogen, (b) absence of ambient gas.
Fig.5 represents the elemental analysis of the same samples that were characterized with FESEM images. It reveals that atomic ratio of Cd/S in the nanoparticles produced in the presence of reactive ambient gas is 1/4.34 which is lower than those without ambient gas which has a ratio of 1/3.2. It can be attributed the solvated electrons that can lead to the reduction of Cd 2+ ions were decreased under a nitrogen environment. These figures also indicate that the CdS nanoparticles with high purity was fabricated by the plasma injection liquid method.
8
Fig. 6. (a) PL emission, (b) Absorption spectra of the NCs synthesized in various flow rate of N2.
According to Fig. 6a, PL intensity of the CdS nanoparticles decreases with increase of N2 ambient gas flow rate. It also exhibits that PL peak positions were clearly shifted to the higher energy with the N2 gas concentration increase. This blue-shift is also observed in the absorption edge of the CdS nanoparticles produced in PLI approach (Fig. 6b).
Fig. 7. (a) PL emission, (b) Absorption spectra of the NCs synthesized in various flow rate of O2 gas.
9
Fig. 7 a, b illustrates the PL and the absorption spectra of the produced CdS nanoparticles in various amounts of oxygen. In comparison to nitrogen, the PL intensity of the QDs decreases more rapidly with the increase of the O2 gas flow rate. For example, the NCs PL intensity with nitrogen gas at the flow rate of 100 SCCM is almost three times higher than O2 gas presence with the same flow rate. From Fig. 7b, it is clear that the absorption edge of nanoparticles synthesized by the PLI method shifted to the blue wavelength region when the concentration of O2 gas around plasma jet was increased.
Fig. 8. Photoluminescence spectrum at 50 SCCM flow rate of N2 gas (black line) and O2 gas (red line).
10
Fig. 8 compares the PL peak of CdS QDs produced in the nitrogen and oxygen gas for more clarity. According to the literatures, this peaks show a considerable stokes shift comparing to the absorption edge showing the trap state emission of QDs that are related to the recombination of donor-acceptor pairs and the interstitial cadmium and sulfur vacancy30,31. Difference between these two sets of peaks shows that ambient gas has a huge impact on the optical properties of nanoparticles. As it is seen the ambient gas has impact on both crystal growth and defect enrichment of samples. The one synthesized at presence of N2 has roughly three times emission intensity compare to that synthesized at O2. Moreover, the former emission of O2 gas has a slight blue shift compare to the latter one. This may arise from the fact that O2 gas decreases the solvated electrons from plasma leading to decrease of Cd2+ reaction and growth rate. Moreover, it decreases Cd/S stoichiometry ratio (Fig. 5) which means both sulfur vacancies and Cd interstitials would decrease as donor-acceptor centers as the main source of broad emission of CdS QDs. The inset of Fig.8 shows the emission of QDs with an excitation UV lamp. CdS emission changes from blue to green and then green-yellow for ambient gas (no gas), nitrogen and oxygen gas, respectively.
Fig. 9. (a) Band gap and (b) size of the QDs synthesized at different amount of ambient gas.
The band gap and particle size of the CdS QDs from the absorption edges were estimated based on the effective mass approximation (EMA) 32,33. Fig. 9a describes the band gap of CdS NCs which rises with the increasing flow rate of O2 and N2 gas, and subsequently the size of the prepared QDs decreases at the presence of 11
reactive ambient gases (Fig. 9b). It also represents that in comparison with nitrogen gas, the oxygen gas has more impact on the band gap and particles size. In other words, the size of the nanoparticle synthesized in oxygen environment is smaller than those QDs synthesized in the presence of nitrogen gas with the same conditions. These results are consistent with the results of the XRD and FESEM measurements. Decreasing the size of CdS NCs may be attributed to the fact that reactive ambient gases scavenge the plasma-injected solvated electrons and reduce the concentration of solvated electrons in the solution. Cd2+ ions are reduced by plasma solvated electrons and contributed to the formation of CdS nanoparticles, also the electronegativity of oxygen is more than that of nitrogen and oxygen can quickly scavenge the plasma electrons by producing O2-. Thus reducing the concentration of those solvated electrons suppresses the nanoparticles formation. In addition, the decreasing concentration of the solvated electrons due to the presence of electronegative oxygen molecules not only reduces the size of the nanoparticles produced by the PLI method, but also reduces the concentration of the synthesized QDs in the final solution which leads to a decrease in the PL and absorption intensities. Therefore, the solvated electrons play a key role in the synthesis of CdS nanoparticles with the plasma injection method. Conclusion: Direct plasma injection into liquid was applied for synthesis of the CdS QDs in the presence of different ambient gases. XRD studies show the presence of reactive ambient gas had no significant effect on the hexagonal structure of the synthesized CdS nanoparticles, while the size of the nanoparticles was reduced with the presence of oxygen and nitrogen ambient gases. The HRTEM and FESEM images revealed that the nanoparticles had a spherical shape. Moreover, the absorption and PL spectra shifted to the lower wavelength region and the PL intensity decreased with increasing the amount of nitrogen around the plasma jet. The presence of little amount of O2 gas strongly decreased the PL intensity and clearly shifted the absorption and PL spectrum to the blue wavelengths. It means that the size and optical properties of the prepared CdS can easily be controlled by the concentration of ambient gas surrounding the plasma. The oxygen ambient gas has an extreme effect on the absorption edge and the position and intensity of PL emission peak due to the reducing of the plasma-injected electrons. Thus, our results provide convincing evidence for the plasma-solvated electrons are the most fundamental plasma specie to induce the chemical reactions, in particular the reduction of Cd2+ cations that leads to the CdS QDs formation. 12
Acknowledgment This work was supported by Iran National Science Foundation (INSF). References 1 2 3 4
5
6
7 8 9
10
11
12
13
14 15
16
Kortshagen, U. Nonthermal plasma synthesis of semiconductor nanocrystals. Journal of Physics D: Applied Physics 42, 113001 (2009). Costa-Fernández, J. M., Pereiro, R. & Sanz-Medel, A. The use of luminescent quantum dots for optical sensing. TrAC Trends in Analytical Chemistry 25, 207-218 (2006). Alivisatos, A. P. Semiconductor clusters, nanocrystals, and quantum dots. science 271, 933-937 (1996). Khajuria, S., Sanotra, S., Ladol, J. & Sheikh, H. N. Synthesis, characterization and optical properties of cobalt and lanthanide doped CdS nanoparticles. Journal of Materials Science: Materials in Electronics 26, 7073-7080 (2015). Xie, R.-H., Bryant, G. W., Lee, S. & Jaskólski, W. Electron-hole correlations and optical excitonic gaps in quantum-dot quantum wells: Tight-binding approach. Physical Review B 65, 235306 (2002). Bansal, A. K. et al. Highly luminescent colloidal CdS quantum dots with efficient near-infrared electroluminescence in light-emitting diodes. The Journal of Physical Chemistry C 120, 18711880 (2016). Romeo, N., Bosio, A., Canevari, V. & Podesta, A. Recent progress on CdTe/CdS thin film solar cells. Solar Energy 77, 795-801 (2004). Huang, Y., Duan, X. & Lieber, C. M. Nanowires for integrated multicolor nanophotonics. small 1, 142-147 (2005). Goto, F., Ichimura, M. & Arai, E. A new technique of compound semiconductor deposition from an aqueous solution by photochemical reactions. Japanese journal of applied physics 36, L1146 (1997). Ramaiah, K. S., Pilkington, R., Hill, A., Tomlinson, R. & Bhatnagar, A. Structural and optical investigations on CdS thin films grown by chemical bath technique. Materials chemistry and physics 68, 22-30 (2001). Molaei, M., Iranizad, E. S., Marandi, M., Taghavinia, N. & Amrollahi, R. Synthesis of CdS nanocrystals by a microwave activated method and investigation of the photoluminescence and electroluminescence properties. Applied Surface Science 257, 9796-9801 (2011). Molaei, M., Hasheminejad, H. & Karimipour, M. Synthesizing and investigating photoluminescence properties of CdTe and CdTe@ CdS core-shell quantum dots (QDs): a new and simple microwave activated approach for growth of CdS shell around CdTe core. Electronic Materials Letters 11, 7-12 (2015). Gonçalves, L. F., Kanodarwala, F. K., Stride, J. A., Silva, C. J. & Gomes, M. J. One-pot synthesis of CdS nanoparticles exhibiting quantum size effect prepared within a sol–gel derived ureasilicate matrix. Optical Materials 36, 186-190 (2013). Wang, G. et al. Preparation and characterization of CdS nanoparticles by ultrasonic irradiation. Inorganic chemistry communications 4, 208-210 (2001). Bahrami, Z., Khani, M. R. & Shokri, B. Cylindrical dielectric barrier discharge plasma catalytic effect on chemical methods of silver nano-particle production. Physics of Plasmas 23, 113501 (2016). Huang, X., Li, Y., Zhong, X., Rider, A. E. & Ostrikov, K. K. Fast microplasma synthesis of blue luminescent carbon quantum dots at ambient conditions. Plasma Processes and Polymers 12, 5965 (2015).
13
17 18 19 20
21
22 23
24 25 26
27 28 29
30
31
32
33
Meiss, S. A. et al. Employing plasmas as gaseous electrodes at the free surface of ionic liquids: deposition of nanocrystalline silver particles. ChemPhysChem 8, 50-53 (2007). Shariat, M., Karimipour, M. & Molaei, M. Synthesis of CdS Quantum Dots Using Direct Plasma Injection in Liquid Phase. Plasma Chemistry and Plasma Processing 37, 1133-1147 (2017). Yan, T. et al. Microplasma-chemical synthesis and tunable real-time plasmonic responses of alloyed Au x Ag 1− x nanoparticles. Chemical Communications 50, 3144-3147 (2014). Patel, J., Němcová, L., Maguire, P., Graham, W. & Mariotti, D. Synthesis of surfactant-free electrostatically stabilized gold nanoparticles by plasma-induced liquid chemistry. Nanotechnology 24, 245604 (2013). Mariotti, D., Patel, J., Švrček, V. & Maguire, P. Plasma–liquid interactions at atmospheric pressure for nanomaterials synthesis and surface engineering. Plasma Processes and Polymers 9, 1074-1085 (2012). Mussard, M. D. V. S., Foucher, E. & Rousseau, A. Charge and energy transferred from a plasma jet to liquid and dielectric surfaces. Journal of Physics D: Applied Physics 48, 424003 (2015). Rumbach, P., Xu, R. & Go, D. B. Electrochemical Production of Oxalate and Formate from CO2 by Solvated Electrons Produced Using an Atmospheric-Pressure Plasma. Journal of The Electrochemical Society 163, F1157-F1161 (2016). Rumbach, P., Bartels, D. M., Sankaran, R. M. & Go, D. B. The solvation of electrons by an atmospheric-pressure plasma. Nature communications 6, 7248 (2015). Rumbach, P., Bartels, D. M., Sankaran, R. M. & Go, D. B. The effect of air on solvated electron chemistry at a plasma/liquid interface. Journal of Physics D: Applied Physics 48, 424001 (2015). Rumbach, P., Witzke, M., Sankaran, R. M. & Go, D. B. Decoupling interfacial reactions between plasmas and liquids: charge transfer vs plasma neutral reactions. Journal of the American Chemical Society 135, 16264-16267 (2013). Reuter, S. et al. Controlling the ambient air affected reactive species composition in the effluent of an argon plasma jet. IEEE Transactions on Plasma Science 40, 2788-2794 (2012). Jablonowski, H. et al. Plasma jet's shielding gas impact on bacterial inactivation. Biointerphases 10, 029506 (2015). Wageh, S. & Maize, M. Structure and optical properties of capped and uncapped CdS nanoparticles prepared in aqueous medium. Journal of Materials Science: Materials in Electronics 25, 4830-4840 (2014). Mishra, S. K., Srivastava, R. K., Prakash, S., Yadav, R. S. & Panday, A. Structural, photoconductivity and photoluminescence characterization of cadmium sulfide quantum dots prepared by a co-precipitation method. Electronic Materials Letters 7, 31-38 (2011). Vigil, O., Riech, I., Garcia-Rocha, M. & Zelaya-Angel, O. Characterization of defect levels in chemically deposited CdS films in the cubic-to-hexagonal phase transition. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 15, 2282-2286 (1997). Brus, L. E. Electron–electron and electron‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state. The Journal of chemical physics 80, 4403-4409 (1984). Lippens, P. & Lannoo, M. Calculation of the band gap for small CdS and ZnS crystallites. Physical Review B 39, 10935 (1989).
14
PII: DOI: Reference:
S0022-2313(18)30608-2 https://doi.org/10.1016/j.jlumin.2018.11.031 LUMIN16101
To appear in: Journal of Luminescence Received date: 5 April 2018 Revised date: 13 November 2018 Accepted date: 15 November 2018 Cite this article as: M. Shariat, M. Karimipour and M. Molaei, Influence of ambient gas on the optical properties of CdS quantum dots prepared by plasmaliquid interactions, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.11.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of ambient gas on the optical properties of CdS quantum dots prepared by plasma-liquid interactions M.Shariat*, M. Karimipour and M. Molaei Department of Physics, Faculty of Science, Vali-e-Asr University of Rafsanjan, Rafsanjan, Iran *Corresponding author: [email protected]
Abstract In this study CdS quantum dots were fabricated by injecting non-equilibrium plasma jet into the aqueous solution including Na2S2O3, CdSO4 and Thioglycolic acid (TGA) in the presence of an ambient gas such as nitrogen and oxygen gases. In order to investigate the structure, morphology, size and chemical nature of the prepared CdS nanocrystals in presence of ambient gas, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) were employed. The optical properties of CdS nanoparticles were characterized using UV-Vis absorption and photoluminescence (PL) spectroscopies. It was found that the presence of ambient gases especially oxygen, reduces the size and concentration of the synthesized nanoparticles, while the crystalline structure of these nanoparticles is not altered. It was also found that with the increase of flow rate of ambient gas, the PL emission intensity of quantum dots was reduced and the PL peak position as well as the absorption edge undergoes a blue shift. Introduction Most II-VI semiconductors quantum dots (QDs) such as CdS, ZnS and ZnSe are ideals for use in various fields of optics and electronics because of their appropriate energy gap of 1-3 eV.1 They are widely used in optoelectronic equipment such as diodes and optical sensors.2 The band gap of these materials is strongly dependent on their size in nano-scale which is known as quantum confinement effect 1,3. Cadmium sulfide is one of the most important semiconductors in the II-VI group and has a direct band gap of 2.42 eV at room temperature 4,5 in its bulk form. CdS nanocrystals (NCs) have been used for a variety of applications, including optical sensors2, photoelectronics6, solar cells7, and light-emitting diodes8 due to their unique electrical and optical properties. Due to its various applications, variety of methods including photochemical reactions9, chemical bath technique10,
1
microwave heating11,12, sol-gel process13 and ultrasonic irradiation14 have been employed for the production of CdS QDs. In recent decades, the plasma-liquid interaction (PLI) as an effective method has been employed to produce organic and inorganic semiconducting nanoparticles1519 . The advantages of this method are that it can be operated close to room temperature and it has very low power consumption and high controllability 18. In this method, the plasma is injected into the plasma-liquid interface at the atmospheric pressure. Plasma produces reactive chemical species including ions, electrons, radicals and photons. These energetic species, especially electrons can stimulate the chemical reactions in the solution interface and lead to the nanoparticles formation 20,21. Mussard et al. showed that in contact of the nonthermal plasma jet with a liquid, the energy and negative electric charges are transferred from plasma to the liquid surface, but they did not exactly specify which of the plasma species play an essential role in the transmission of energy and negative charges 22. Rumbach et al. using a novel optical method illustrated that the plasma electrons can be dissolved at the interface of plasma and liquid 23,24. They found that solvated electrons are the most important plasma species in advancing chemical reactions; in particular, the reduction of metal cations 25. It has been estimated that the solvated electrons at the liquid/plasma interface can penetrate prior to the reaction up to a depth of 2.5nm and the concentration of solvated electrons at the liquid interface has been evaluated to be about 1mM 24. Furthermore, the production and behavior of plasma-solvated electrons and the other reactive species strongly depends on the ambient gas that surrounds plasma jet in the contact with liquid25-28. Therefore, the chemical and physical processes that lead to the formation of nanoparticles in the plasma/liquid interface can easily be controlled by utilizing different reactive ambient gas such as oxygen and nitrogen. In this work, the synthesis of CdS nanoparticles is carried out by plasma injection into a solution surface in the presence of oxygen and nitrogen ambient gases. It was believed that electrons from the plasma jet are injected into the solution, helping to reduce the Cd2+ ions and form the CdS nanoparticles in solution.18 However, the role of plasma-injected solvated electrons on the synthesis of CdS NCs has not been clearly determined. The reactive oxygen and nitrogen species are generated when the plasma is formed in O2 and N2 ambient gases. These reactive species can scavenge the solvated electrons and reduce their concentration in liquid
2
25
. These processes may have a large effect on the synthesis and optical properties of the CdS nanoparticles fabricated by the PLI method.
METHODS AND MATERIALS: In this work, CdSO4 and Na2S2O3 were used as precursors of Cadmium and Sulfur, respectively, also Thioglycolic acid (TGA) was used as a capping agent. All of these materials were purchased from Merck Company and used without any purification. A 20cc aqueous solution was prepared from CdSO4, Na2S2O3 and TGA, respectively at the concentrations of 1, 4 and 2.5 mM. NaOH was used to adjust the solution pH to 8.5. The final transparent solution lied at the bottom of a glass vessel (three neck round bottom flask, 50ml) under non-thermal plasma jet treatment at atmospheric pressure.
Fig. 1. Sketch of the experimental setup for plasma–liquid interactions.
A schematic of the plasma reactor is shown in Fig. 1. Here the non-equilibrium plasma was created by the configuration of dielectric-barrier discharge (DBD) jet. In this DBD jet, a quartz tube was applied as a dielectric with internal and external diameter of 2 and 4 mm, respectively. The voltage electrode was a 0.5mm diameter stainless steel rod located in the center of the quartz tube, and the ground electrode was a copper foil with a width of 10 mm that was wrapped around the outer surface of quartz tube, also its distance to the end of the tube was 15 mm. The plasma jet was driven by an AC power supply with a voltage of 6kV and a repetition frequency of 18 kHz. Pure helium gas (99.999%) was flown around the 3
stainless steel electrode inside the quartz tube as the plasma working gas. The flow rate of the helium gas was fixed by a mass flow controller at 2000 standard centimeter cubic per minute (SCCM). As shown in Fig. 1, the DBD plasma jet was placed inside the glass vessel so that the distance between nozzle and the solution surface be approximately 5mm. The oxygen and nitrogen as an ambient gas was fed into the vessel with different gas flow rate. The optimal plasma treatment time for all samples was 40 min. After plasma treatment, the solution was centrifuged and dried at room temperature and then the resulting powder was used for characterization.
Characterization methods: The electrical and optical characteristics of DBD plasma jet have been vastly studied in our previous work 18. It has been demonstrated that the power consumption for this plasma jet depends on the applied voltage and have a value of less than 5 Watt. It’s important to note that the plasma power consumption at the presence of various ambient gases does not change much. The structure, size and morphology of the prepared CdS QDs were studied using transmission electron microscopy (TEM, Zeiss EM900), high resolution TEM (HRTEM, Tecnai F20) and field emission scanning electron microscopy (FESEM, TESCAN MIRA3). Also, the X-ray diffraction (XRD) with the advanced d8 Bruker instrument was used for better understanding of the CdS NCs structure. The chemical nature and elemental analysis of the nanoparticles fabricated by PLI approach in the presence of various ambient gases were performed by energy dispersive X-ray spectroscopy (EDS) analysis. The optical properties including absorption and photoluminescence (PL) of QDs were investigated using an Avantes spectrometer (Avantes 2048) at room temperature. The wavelength of light that was used for exciting of the QDs in the PL measurement was 254 nm. Results and discussion: XRD was taken from the nanoparticles synthesized by the PLI method in different environmental gases, which is shown in Fig. 2. In all samples, the two main peaks are observed at 2 =28.4o and 47.7o angles, which are related to the diffraction of the crystalline planes (101) and (103), respectively. It emphasizes that the XRD pattern belongs to the crystallographic card No. JCPDS 41-1049 corresponding to
4
P63mc space group of hexagonal structure and lattice constants a=0.41 and c=0.67 nm.
Fig. 2. XRD pattern the synthesized CdS NCs.
This figure also indicates that the presence of ambient gas around the plasma jet does not affect the structure of the CdS nanoparticles. With the aid of the DebyeScherrer formula, the estimated size of NCs was calculated to be 15.5, 14.1 and 12.8 Å in absence of ambient gas, Nitrogen and oxygen gases, respectively (for more details, see Supplementary information). Therefore, the presence of reactive gas (oxygen and nitrogen) around the plasma plume decreases the size of the CdS NCs.
5
Fig. 3. TEM images and size distribution histograms of CdS NCs produced (a) and (b) without the presence of ambient gas, (c) and (d) with the presence of O2 gas.
Fig. 3 shows the TEM images and size distribution histograms of the CdS nanoparticles in the absence of any ambient gas and the present of oxygen gas. Fig 3 (a) and (c) show that synthesized nanoparticles have a spherical-like shape and most of them have a diameter of about 5 and 2.5 nm for ambient gas and oxygen gas, respectively. Inset of Fig. 3(a) shows HRTEM image of a CdS nanoparticle in ambient gas that appear to be spherical and crystalline with a lattice spacing of 6
approximately 0.31nm. This lattice spacing is consistent with XRD 101 hexagonal planes spacing. This means TEM actually confirmed XRD measurements.
Fig. 4. FESEM image of CdS nanoparticles in the (a) presence of nitrogen, (b) absence of ambient gas.
The FESEM images of the QDs produced in the presence of N2 gas with a flow rate of 100 SCCM and those produced in the absence of ambient gas are shown in Figures 4a and 4b, respectively. According to Fig. 4, the nanoparticles that were synthesized by the plasma method in N2 environment have a smaller average size (4.7nm) compared to those obtained in the absence of ambient gas (6.5nm). For both samples synthesized using N2 gas and ambient gas (no gas), the size estimation extracted from FESEM and TEM is larger than Debye-Scherrer estimation. This increase in the average size estimated from TEM and FESEM relative to the sizes determined from X-ray has observed in other studies.29 Some experimental and analysis points of XRD using Debye-Scherrer formula should be clarified. The crystallite size of samples is calculated for one preferential atomic plane which is not exactly the size of particles for poly-crystallite samples. Generally, the crystallite area corresponding to a specific atomic plane determines its impact on the diffraction pattern. For QDs, since one or two main peaks are usually observed in XRD pattern, it means that the borders of those specific atomic 7
planes are such extended over the area of a single particle that other diffraction peaks are not clearly observed. In this case, almost all the particle size is influenced by a single atomic plane and the contribution of other planes in texturing (mosaicity) of particle is negligible as they are not observable in XRD pattern as well. Thus in this case a rough estimation for QDs size can be obtained by Debye-Scherrer formula. In other word, it can be stated that the size of particle is at least greater or equal to Debye-Scherrer estimation. Therefore, in this work, FESEM size estimation is in well agreement with Debye-Scherrer calculation, meaning that Debye-Scherrer is not greater than FESEM sizes. More interestingly the overall size estimation trend from TEM, FESEM and Debye-Scherrer are consistent with each other such that with N2 or O2 ambient gas application for the synthesis, all of them TEM, FESEM and Debye-Scherrer estimation sizes decrease. Moreover, TEM, FESEM are 5nm, 6.5nm, respectively, in excellent agreement for no gas synthesized sample.
Fig. 5. EDS spectrum of the sample displayed in the (a) presence of nitrogen, (b) absence of ambient gas.
Fig.5 represents the elemental analysis of the same samples that were characterized with FESEM images. It reveals that atomic ratio of Cd/S in the nanoparticles produced in the presence of reactive ambient gas is 1/4.34 which is lower than those without ambient gas which has a ratio of 1/3.2. It can be attributed the solvated electrons that can lead to the reduction of Cd 2+ ions were decreased under a nitrogen environment. These figures also indicate that the CdS nanoparticles with high purity was fabricated by the plasma injection liquid method.
8
Fig. 6. (a) PL emission, (b) Absorption spectra of the NCs synthesized in various flow rate of N2.
According to Fig. 6a, PL intensity of the CdS nanoparticles decreases with increase of N2 ambient gas flow rate. It also exhibits that PL peak positions were clearly shifted to the higher energy with the N2 gas concentration increase. This blue-shift is also observed in the absorption edge of the CdS nanoparticles produced in PLI approach (Fig. 6b).
Fig. 7. (a) PL emission, (b) Absorption spectra of the NCs synthesized in various flow rate of O2 gas.
9
Fig. 7 a, b illustrates the PL and the absorption spectra of the produced CdS nanoparticles in various amounts of oxygen. In comparison to nitrogen, the PL intensity of the QDs decreases more rapidly with the increase of the O2 gas flow rate. For example, the NCs PL intensity with nitrogen gas at the flow rate of 100 SCCM is almost three times higher than O2 gas presence with the same flow rate. From Fig. 7b, it is clear that the absorption edge of nanoparticles synthesized by the PLI method shifted to the blue wavelength region when the concentration of O2 gas around plasma jet was increased.
Fig. 8. Photoluminescence spectrum at 50 SCCM flow rate of N2 gas (black line) and O2 gas (red line).
10
Fig. 8 compares the PL peak of CdS QDs produced in the nitrogen and oxygen gas for more clarity. According to the literatures, this peaks show a considerable stokes shift comparing to the absorption edge showing the trap state emission of QDs that are related to the recombination of donor-acceptor pairs and the interstitial cadmium and sulfur vacancy30,31. Difference between these two sets of peaks shows that ambient gas has a huge impact on the optical properties of nanoparticles. As it is seen the ambient gas has impact on both crystal growth and defect enrichment of samples. The one synthesized at presence of N2 has roughly three times emission intensity compare to that synthesized at O2. Moreover, the former emission of O2 gas has a slight blue shift compare to the latter one. This may arise from the fact that O2 gas decreases the solvated electrons from plasma leading to decrease of Cd2+ reaction and growth rate. Moreover, it decreases Cd/S stoichiometry ratio (Fig. 5) which means both sulfur vacancies and Cd interstitials would decrease as donor-acceptor centers as the main source of broad emission of CdS QDs. The inset of Fig.8 shows the emission of QDs with an excitation UV lamp. CdS emission changes from blue to green and then green-yellow for ambient gas (no gas), nitrogen and oxygen gas, respectively.
Fig. 9. (a) Band gap and (b) size of the QDs synthesized at different amount of ambient gas.
The band gap and particle size of the CdS QDs from the absorption edges were estimated based on the effective mass approximation (EMA) 32,33. Fig. 9a describes the band gap of CdS NCs which rises with the increasing flow rate of O2 and N2 gas, and subsequently the size of the prepared QDs decreases at the presence of 11
reactive ambient gases (Fig. 9b). It also represents that in comparison with nitrogen gas, the oxygen gas has more impact on the band gap and particles size. In other words, the size of the nanoparticle synthesized in oxygen environment is smaller than those QDs synthesized in the presence of nitrogen gas with the same conditions. These results are consistent with the results of the XRD and FESEM measurements. Decreasing the size of CdS NCs may be attributed to the fact that reactive ambient gases scavenge the plasma-injected solvated electrons and reduce the concentration of solvated electrons in the solution. Cd2+ ions are reduced by plasma solvated electrons and contributed to the formation of CdS nanoparticles, also the electronegativity of oxygen is more than that of nitrogen and oxygen can quickly scavenge the plasma electrons by producing O2-. Thus reducing the concentration of those solvated electrons suppresses the nanoparticles formation. In addition, the decreasing concentration of the solvated electrons due to the presence of electronegative oxygen molecules not only reduces the size of the nanoparticles produced by the PLI method, but also reduces the concentration of the synthesized QDs in the final solution which leads to a decrease in the PL and absorption intensities. Therefore, the solvated electrons play a key role in the synthesis of CdS nanoparticles with the plasma injection method. Conclusion: Direct plasma injection into liquid was applied for synthesis of the CdS QDs in the presence of different ambient gases. XRD studies show the presence of reactive ambient gas had no significant effect on the hexagonal structure of the synthesized CdS nanoparticles, while the size of the nanoparticles was reduced with the presence of oxygen and nitrogen ambient gases. The HRTEM and FESEM images revealed that the nanoparticles had a spherical shape. Moreover, the absorption and PL spectra shifted to the lower wavelength region and the PL intensity decreased with increasing the amount of nitrogen around the plasma jet. The presence of little amount of O2 gas strongly decreased the PL intensity and clearly shifted the absorption and PL spectrum to the blue wavelengths. It means that the size and optical properties of the prepared CdS can easily be controlled by the concentration of ambient gas surrounding the plasma. The oxygen ambient gas has an extreme effect on the absorption edge and the position and intensity of PL emission peak due to the reducing of the plasma-injected electrons. Thus, our results provide convincing evidence for the plasma-solvated electrons are the most fundamental plasma specie to induce the chemical reactions, in particular the reduction of Cd2+ cations that leads to the CdS QDs formation. 12
Acknowledgment This work was supported by Iran National Science Foundation (INSF). References 1 2 3 4
5
6
7 8 9
10
11
12
13
14 15
16
Kortshagen, U. Nonthermal plasma synthesis of semiconductor nanocrystals. Journal of Physics D: Applied Physics 42, 113001 (2009). Costa-Fernández, J. M., Pereiro, R. & Sanz-Medel, A. The use of luminescent quantum dots for optical sensing. TrAC Trends in Analytical Chemistry 25, 207-218 (2006). Alivisatos, A. P. Semiconductor clusters, nanocrystals, and quantum dots. science 271, 933-937 (1996). Khajuria, S., Sanotra, S., Ladol, J. & Sheikh, H. N. Synthesis, characterization and optical properties of cobalt and lanthanide doped CdS nanoparticles. Journal of Materials Science: Materials in Electronics 26, 7073-7080 (2015). Xie, R.-H., Bryant, G. W., Lee, S. & Jaskólski, W. Electron-hole correlations and optical excitonic gaps in quantum-dot quantum wells: Tight-binding approach. Physical Review B 65, 235306 (2002). Bansal, A. K. et al. Highly luminescent colloidal CdS quantum dots with efficient near-infrared electroluminescence in light-emitting diodes. The Journal of Physical Chemistry C 120, 18711880 (2016). Romeo, N., Bosio, A., Canevari, V. & Podesta, A. Recent progress on CdTe/CdS thin film solar cells. Solar Energy 77, 795-801 (2004). Huang, Y., Duan, X. & Lieber, C. M. Nanowires for integrated multicolor nanophotonics. small 1, 142-147 (2005). Goto, F., Ichimura, M. & Arai, E. A new technique of compound semiconductor deposition from an aqueous solution by photochemical reactions. Japanese journal of applied physics 36, L1146 (1997). Ramaiah, K. S., Pilkington, R., Hill, A., Tomlinson, R. & Bhatnagar, A. Structural and optical investigations on CdS thin films grown by chemical bath technique. Materials chemistry and physics 68, 22-30 (2001). Molaei, M., Iranizad, E. S., Marandi, M., Taghavinia, N. & Amrollahi, R. Synthesis of CdS nanocrystals by a microwave activated method and investigation of the photoluminescence and electroluminescence properties. Applied Surface Science 257, 9796-9801 (2011). Molaei, M., Hasheminejad, H. & Karimipour, M. Synthesizing and investigating photoluminescence properties of CdTe and CdTe@ CdS core-shell quantum dots (QDs): a new and simple microwave activated approach for growth of CdS shell around CdTe core. Electronic Materials Letters 11, 7-12 (2015). Gonçalves, L. F., Kanodarwala, F. K., Stride, J. A., Silva, C. J. & Gomes, M. J. One-pot synthesis of CdS nanoparticles exhibiting quantum size effect prepared within a sol–gel derived ureasilicate matrix. Optical Materials 36, 186-190 (2013). Wang, G. et al. Preparation and characterization of CdS nanoparticles by ultrasonic irradiation. Inorganic chemistry communications 4, 208-210 (2001). Bahrami, Z., Khani, M. R. & Shokri, B. Cylindrical dielectric barrier discharge plasma catalytic effect on chemical methods of silver nano-particle production. Physics of Plasmas 23, 113501 (2016). Huang, X., Li, Y., Zhong, X., Rider, A. E. & Ostrikov, K. K. Fast microplasma synthesis of blue luminescent carbon quantum dots at ambient conditions. Plasma Processes and Polymers 12, 5965 (2015).
13
17 18 19 20
21
22 23
24 25 26
27 28 29
30
31
32
33
Meiss, S. A. et al. Employing plasmas as gaseous electrodes at the free surface of ionic liquids: deposition of nanocrystalline silver particles. ChemPhysChem 8, 50-53 (2007). Shariat, M., Karimipour, M. & Molaei, M. Synthesis of CdS Quantum Dots Using Direct Plasma Injection in Liquid Phase. Plasma Chemistry and Plasma Processing 37, 1133-1147 (2017). Yan, T. et al. Microplasma-chemical synthesis and tunable real-time plasmonic responses of alloyed Au x Ag 1− x nanoparticles. Chemical Communications 50, 3144-3147 (2014). Patel, J., Němcová, L., Maguire, P., Graham, W. & Mariotti, D. Synthesis of surfactant-free electrostatically stabilized gold nanoparticles by plasma-induced liquid chemistry. Nanotechnology 24, 245604 (2013). Mariotti, D., Patel, J., Švrček, V. & Maguire, P. Plasma–liquid interactions at atmospheric pressure for nanomaterials synthesis and surface engineering. Plasma Processes and Polymers 9, 1074-1085 (2012). Mussard, M. D. V. S., Foucher, E. & Rousseau, A. Charge and energy transferred from a plasma jet to liquid and dielectric surfaces. Journal of Physics D: Applied Physics 48, 424003 (2015). Rumbach, P., Xu, R. & Go, D. B. Electrochemical Production of Oxalate and Formate from CO2 by Solvated Electrons Produced Using an Atmospheric-Pressure Plasma. Journal of The Electrochemical Society 163, F1157-F1161 (2016). Rumbach, P., Bartels, D. M., Sankaran, R. M. & Go, D. B. The solvation of electrons by an atmospheric-pressure plasma. Nature communications 6, 7248 (2015). Rumbach, P., Bartels, D. M., Sankaran, R. M. & Go, D. B. The effect of air on solvated electron chemistry at a plasma/liquid interface. Journal of Physics D: Applied Physics 48, 424001 (2015). Rumbach, P., Witzke, M., Sankaran, R. M. & Go, D. B. Decoupling interfacial reactions between plasmas and liquids: charge transfer vs plasma neutral reactions. Journal of the American Chemical Society 135, 16264-16267 (2013). Reuter, S. et al. Controlling the ambient air affected reactive species composition in the effluent of an argon plasma jet. IEEE Transactions on Plasma Science 40, 2788-2794 (2012). Jablonowski, H. et al. Plasma jet's shielding gas impact on bacterial inactivation. Biointerphases 10, 029506 (2015). Wageh, S. & Maize, M. Structure and optical properties of capped and uncapped CdS nanoparticles prepared in aqueous medium. Journal of Materials Science: Materials in Electronics 25, 4830-4840 (2014). Mishra, S. K., Srivastava, R. K., Prakash, S., Yadav, R. S. & Panday, A. Structural, photoconductivity and photoluminescence characterization of cadmium sulfide quantum dots prepared by a co-precipitation method. Electronic Materials Letters 7, 31-38 (2011). Vigil, O., Riech, I., Garcia-Rocha, M. & Zelaya-Angel, O. Characterization of defect levels in chemically deposited CdS films in the cubic-to-hexagonal phase transition. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 15, 2282-2286 (1997). Brus, L. E. Electron–electron and electron‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state. The Journal of chemical physics 80, 4403-4409 (1984). Lippens, P. & Lannoo, M. Calculation of the band gap for small CdS and ZnS crystallites. Physical Review B 39, 10935 (1989).
14