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Impact of the matrix on optical properties of nanocomposites with CdS QDs
Journal of Alloys and Compounds 823 (2020) 153811
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Impact of the matrix on optical properties of nanocomposites with CdS QDs S.V. Rempel a, b, *, Yu V. Kuznetsova a, I.D. Popov a, А.А. Rempel a, b, c a
Institute of Solid State Chemistry of the Ural Branch of the Russian Academy of Sciences, 91, Pervomaiskaya St., 620990, Ekaterinburg, Russia Ural Federal University Named After the First President of Russia B. N. Yeltsin, 19, Mira St., 620002, Ekaterinburg, Russia c Institute of Metallurgy of the Ural Branch of the Russian Academy of Sciences, 101, Amundsena St., 620016, Ekaterinburg, Russia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 August 2019 Received in revised form 16 December 2019 Accepted 10 January 2020 Available online 11 January 2020
In the present work the silica-based matrix effect on the luminescence properties of the nanocomposites which contain cadmium sulfide quantum dots (CdS QDs) was studied. Water alkaline solution of sodium silicates Na2O(SiO2)n, or liquid glass, or tetraethyl orthosilicate (TEOS) were used as simple models of a dielectric matrix. The synthesis of nanocomposites by sol-gel method using liquid glass and TEOS was carried out in two ways: mixing the prepared colloidal solution of nanoparticles with silica precursor or synthesis of nanoparticles directly in the matrix. The composite based on CdS QDs in a more complex matrix of silicate glass was also studied. The optical absorption and luminescence were studied in detail depending on the synthesis method. It was shown that various atomic defects contribute predominantly to the luminescence of QDs formed in different matrices, the matrix affects on the formation of the defects, therefore the luminescence range. In addition it was shown that method of synthesis affected the intensity of luminescence. © 2020 Elsevier B.V. All rights reserved.
Keywords: Quantum dots Matrix Luminescence Atomic defects Traps
1. Introduction In recent years, a huge amount of research has been devoted to the study and development of new methods for producing quantum dots (QDs). Practical applications and new possibilities to use them require QDs with various properties in different matrices. Significant interest is focused on QDs based on the semiconductor nanocrystals of AIIBVI compounds. Practical works, as a rule, are devoted to various methods of synthesis and characterization of QDs [1e14]. In theoretical works, attempts are made to find general patterns in the most interesting properties [15e18]. The luminescence range of semiconductor nanoparticles changes with size due to quantum confinement, which is the most important property of QDs. Unfortunately, the luminescence depends not only on the size or size distribution of nanocrystals, but also on a number of factors which affect the range and intensity of luminescence. One factor is an inability to control a stoichiometry during the synthesis of nanocrystals of the order of 2e5 nm, which
* Corresponding author. Institute of Solid State Chemistry of the Ural Branch of the Russian Academy of Sciences, 91, Pervomaiskaya st., 620990, Ekaterinburg, Russia. E-mail address: [email protected] (S.V. Rempel). https://doi.org/10.1016/j.jallcom.2020.153811 0925-8388/© 2020 Elsevier B.V. All rights reserved.
leads to appearance of atomic defects in the crystal structure. Atomic defects, such as vacancies, interatomic ions, etc., create local distortions of the crystal lattice. The energy levels of such defects are located within the band gap and can trap charge carriers in the semiconductors. Optical and other properties become dependent on the properties and concentration of defects because additional channels of radiative and nonradiative recombination thus appear. A number of works are devoted to the study of the predominant formation of defects in cadmium sulfide (CdS), energy position of these defects, and their influence on optical and electrical properties [19e22]. Another factor that affects luminescence is a tendency of nanocrystals to agglomerate. To maintain the stability, it is necessary to place QDs in the matrix or use organic/inorganic stabilizers. The authors of [23] studied the effect of anionic and cationic vacancies on the CdS surface on the adsorption of water molecules. Work [24] further suggests that surface ligands may play a role in assisting exciton relaxation. QDs properties synthesized by using different methods are compared in works [25,26]. The experimental results show that QDs synthesized in different matrices have different atomic defects. For example, the main defects of CdS nanoparticles in glass composites are cadmium vacancies (VCd). CdS nanocrystals in the colloid solution possess sulfur vacancy (VS), VCd, interstitial sulfur (IS), and cadmium atoms
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adsorbed on the surface (SCd) as defects [1]. Wherein, the luminescence intensity of QDs in aqueous colloidal solutions is lower than that in glass [10,11,27]. Thus, the matrix affects on the formation of the defects, therefore the luminescence range. However, it must be recognized that at present there is no clear understanding of the processes of formation and development of intrinsic defects of quantum dots, which are luminescence centers, during the formation of nanocrystals. This shows a lack of studies devoted to comparison of the effects of different matrixs on the properties of the obtained QDs. Disordered crystalline structures of CdS nanoparticles (average size between 3 and 9 nm) prepared by wet chemical synthesis were discovered and described in Ref. [28]. In the previous work the formation of distributed spherical CdS nanoparticles incorporated in matrix (by high temperature melting) were confirmed by high resolution focused ion beam scanning electron microscopy and SAXS results [10]. Schemes of the nanocomposite synthesis by using secondary heat treatment and shape of nanoparticle from WAXS and SAXS were presented in Ref. [29]. In the present work, the CdS QDs were synthesized in three different matrices to study the influence of the matrix and synthesis conditions on the formation of internal defects and optical properties of QDs. 2. Experimental details Nanocomposites based on CdS nanoparticles were synthesized at room and high temperatures by using a sol-gel method and high temperature melting, respectively. CdS nanoparticles in a silicate glass matrix (A) ware synthesized by using a heat treatment at 580e610 C during 3e15 h (Table 1). The synthesis details of composite based on silicate matrix are described in Ref. [10]. Liquid glass Na2O(SiO2)n (B) and TEOS Si(C2H5O)4 (C) were used as a silicon oxide matrix to synthesize nanocomposites by a sol-gel method. The liquid glass was used as received. TEOS was diluted with ethanol (EtOH) and hydrolyzed with H2O containing 0.3% HCl as TEOS:EtOH:H2O ¼ 1:10:5. The synthesis of nanocomposites by sol-gel includes two steps: first premixing one of initial solutions (Na2S or CdCl2) with a silicon-containing precursor (liquid glass or hydrolyzed TEOS) and then adding second precursor (Table 1). In addition, the replacement silicate matrix with water one was made to study the effect of water on luminescence of CdS nanoparticles. To do this, colloid solutions of CdS nanoparticles stabilized by disodium salt of ethylenediaminetetraacetic acid (EDTA) or (3mercaptopropyl)trimethoxysilane (MPS) were mixed with liquid glass or TEOS (Table 1, samples B3, B4, C3) and dried. The composites based on liquid glass were stirred for 5 min and then dried at 60 C for 24 h. The samples of TEOS-based composites were stirred for 24 h at room temperature until complete drying. Optical absorption, reflectance and luminescent spectra were recorded with FS-5 spectrofluorometer (Edinburgh Instruments) in UV, visible and NIR ranges at ambient temperature. Absolute quantum yield of luminescence were measured using method described here [31]. The same spectrofluorometer equipped by integrating sphere coated by Polytetrafluoroethylene were used. From optical absorption the band gap (Eg) and the average size Table 1 Synthesis conditions of composites by high temperature method. Sample
Temperature, C
Time, h
Eg, eV
, nm
A0 A1 A2 A3
e 580 580 610
e 3 5 3
4.10 3.01 2.96 2.65
e 3.9 4.1 6.2
of CdS QDs for all nanocomposites were calculated (Tables 1 and 2). The CdS QDs synthesized by using a heat treatment at 580 C have band gap of about 3 nm and average size of about 4 nm depending from the time of the heat treatment. The CdS QDs synthesized by sol-gel method have the band gap ranging from 2.48 to 2.86 eV and the average size from 3.8 to 10.3 nm. The luminescence spectra of QDs synthesized in different matrix have maxima in range from 475 to 788 nm. 3. Results and discussion Precise determination of the position of specific energy levels and moreover the prediction of their formation are difficult due to the influence of many factors. In particular, the level of the same defect has a different position relative to the edges of the energy zones depending on the charge [18]. In addition, as the band gap changes, the levels of shallow and deep impurities shift differently. The uneven distribution of defects leads to a chaotic arrangement of the perturbing potential and fluctuations of energy levels inside the band gap. This results in different energy values for the same defect located in unequal local conditions. Fluctuations in the position of the levels of a defect cause broadening of the level inside the forbidden zone. With the formation of various types of defects, individual lines merge into a strip. The combination of such factors, alongside with the QDs size distribution, is the reason for the appearance of wide structure less luminescence bands in the spectra of QDs. The authors [1] systematically investigated the phase transition behavior of CdS QDs in glass composites and colloids as well as bulk CdS by pressure-tuned resonant Raman scattering. As a result, energy level diagrams for CdS-glass composite and colloids were made. This work demonstrates that the main defects in the CdSglass composites are VCd, whereas the CdS nanoparticles in colloids include a number of atomic defects as VCd, VS, IS and SCd. Based on this approach, the mechanisms of defect formation during synthesis in the matrices under consideration can be hypothesides. The luminescence spectra of QDs synthesized in glass with hightemperature synthesis are shown in Fig. 1. With increasing temperature and/or duration of heat treatment, particles grow and the luminescence maximum shifts to the long-wavelength region. The heat treatment mode at 580 C for 5 h was found to be the most preferred one. A further increase in heat treatment temperature and time is impractical since it leads to an increase in the size of quantum dots and a shift of the maximum to the long-wavelength region along with the decrease of luminescence intensity. The luminescence spectrum of the sample after the optimal heat treatment mode shows a wide structureless peak with a maximum of 605 nm, which indicates the participation of defects IS and VCd in the recombination of charge carriers. The luminescence decay dynamics of CdS QDs in the glass had been previously studied [30]. The luminescence decay curves can be approximated with sum of several exponential functions. This points out at the traps contribution to the luminescence. The luminescence intensity of CdS in
Table 2 Synthesis conditions of composites by a sol-gel method. Sample
Stabilizer
Synthesis scheme
Eg, eV
, nm
B1 B2 B3 B4 C1 C2 C3
e e EDTA MPS e MPS MPS
(Na2S þ LG)þCdCl2 (CdCl2 þ LG) þ Na2S (CdS-EDTA)sol þ LG (CdS-MPS)sol þ LG (Na2S þ TEOS)þCdCl2 (Na2S þ TEOS þ MPS)þCdCl2 (CdS-MPS)sol þ TEOS
2.60 2.51 2.48 2.86 2.71 2.67 2.63
5.9 8.4 10.3 3.8 4.7 5.1 5.5
S.V. Rempel et al. / Journal of Alloys and Compounds 823 (2020) 153811
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Fig. 1. The effect of heat treatment temperature and time on the optical absorption (a) and luminescence (b) of the composites based on the silicate glass. The gray dashed line shows the optical absorption edge of bulk CdS. The excitation wavelength was 385 nm.
the glass matrix is higher than that in other matrices. This may be due, firstly, to a lower concentration of atomic defects after hightemperature annealing. Secondly, luminescence intensity depends on the stoichiometry of CdS. It was found that luminescence intensity increases with increasing Cd content [20]. During hightemperature synthesis, a larger number of sulfur atoms may volatilize and a tendency for Cd to prevail appears. Optical absorption and luminescence spectra of CdS QDs in liquid glass and TEOS depending on the synthesis method are shown in Figs. 2 and 3, respectively. The synthesis based on the scheme of preliminary mixing of the source of sulfur ions with liquid glass and the subsequent addition of cadmium ions (Table 2, sample B1) causes the formation of nanoparticles with a predominant defect IS. This is connected with the lack of cadmium ions during the particle formation due to the limited mobility of Cd2þions in viscous liquid glass. A luminescence band with maximum at 530 nm is observed, which is associated with recombination between IS and the valence band (Fig. 2, Sample B1). The average size of the particle synthesized by this method is about 5.9 nm. In the case of adding S2 to the solution along with Cd2þ(Table 2, sample B2), a significant amount of VS is formed during the nanoparticle growth. After an excitation, electrons pass from the IS level to the Vs level throught the nonradiative relaxation followed by radiative relaxation to VCd (900 nm) (Fig. 2, sample B2). A significant contribution of this peak is observed in the spectrum along with all the others. The biggest impact on the luminescence is
associated with recombination of VS to the valence band (788 nm), which confirms the predominant formation of VS defects. The average size of the QDs synthesized with this method is slightly larger. With a significant excess of absorption in comparison with the previous sample, the integral luminescence intensity of sample B2 is lower, which indicates the existence of additional passes of nonradiative relaxation. In the case of mixing the liquid glass with colloid solution stabilized by EDTA (Table 2, sample B3), CdS QDs are already formed and changing of the matrix (water to silica-based) does not affect the defects of atomic structure and the position of the luminescence maximum (Fig. 2, sample B3). However, the largest average size of the QDs about 10.3 nm is observed. The increase of the luminescence intensity in comparison with the initial QDs in an aqueous solution indicates the prevention of luminescence quenching due to the polar water molecules. In addition, a composite was synthesized by using colloid solution of CdS QDs stabilized by MPS. The shell based on MPS molecules probably allows the particles to be more evenly distributed in the silica-based matrix upon drying and prevents the particles from agglomerating. In this case, the minimum particle size and maximum luminescence intensity are maintained, wherein the emission mechanism is based on the predominant recombination of the IS electron and the VCd hole (633 nm) (Fig. 2, sample B4). The analysis of the results obtained for the composites based on colloid solutions of CdS QDs indicates the formation of different predominant defects during stabilization by EDTA and MPS. In the first case, the
Fig. 2. The optical absorption (a) and luminescence (b) of the composites based on the CdS nanoparticles in the liquid glass. The excitation wavelength was 380 nm.
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Fig. 3. The optical absorption (a) and luminescence (b) of the composites based on the CdS nanoparticles in TEOS. The excitation wavelength was 380 nm.
main contribution is made by the recombination between Vs and the valence band, in the second case, between the IS electron and the VCd hole. In both cases, VS makes a significant contribution to the luminescence. Thus, the band maximum is shifted to the longwavelength range in comparison with the synthesis methods of CdS QDs directly in the liquid glass matrix. The synthesis of the composite by using TEOS (Table 2) results in a sharp absorption edge, an exciton peak and the maximum luminescence intensity at a wavelength of 745 nm (transition from VS to valence band) (Fig. 3). All samples C show similar shape of the emission spectra and low luminescence intensity. A decrease of the concentration in luminescence centers associated with the dominant band causes weaker bands. In CdS/TEOS samples, the intensities of luminescence bands at 530 and 633 nm are reduced, which most likely causes the development of the exciton peak. Both peaks are related to the contribution of the IS defect. Probably, the concentration of IS in CdS QDs is significantly lower in TEOS than in other matrices. The absence of significant shift of the long-wave peak in all samples with the change of the particle size indicates the participation of deep defects in the luminescence. However, the exciton peak shifts to the long-wavelength region with an increase in the size of the QD. The highest absolute quantum yield (QY) of luminescence was measured for CdS nanoparticles in glass-CdS nanocomposites QY ¼ 0.22 ± 0.02. Measured QY was about 0.02 for nanocomposites based on liquid glass and obtained using sol-gel method. A comparison the range of the luminescence spectra of CdS QDs synthesized in the different matrices is presented in Fig. 4 and
Table 3 The effect of matrix on Eg and luminescence of CdS nanoparticles. Sample
Matrix
Eg, eV
Position of maximum, nm
A B C
silicate glass liquid glass TEOS
2.96 2.60 2.71
475
530 e
605 633 e
745
Table 3. The shifts of the luminescence maximum are determined by fluctuations in the positions of the defect levels due to the reasons discussed above. Thus, the experimental optical results show the evidence of non-identity of the structural defects formed in CdS QDs that produce the luminescence. Nevertheless, we can hypothesize that the trap centers participated in the recombination of charge carriers. It is reasonable to attribute luminescence band at 530 nm to the recombination of electrons on IS deep donor and holes in the valence band. The band at 605 (633) nm is associated with the recombination of electrons in IS and holes in VCd. The recombination between the electrons in VS and holes in the valence band is attributed to the peak of 745 (788) nm and the one between the electrons in VS and holes in VCd e to the luminescence band at 900 nm. The energy levels and contributions of each defects to luminescent spectra vary in different matrix. Main defects in glassCdS nanocomposite are VCd, Is, in TEOS/CdS nanocomposite e Is, VCd, Vs, in liquid glass-CdS nanocomposite e Is, VCd, Vs.
Fig. 4. The effect of matrix on the optical absorption (a) and luminescence (b) of the CdS nanoparticles.
S.V. Rempel et al. / Journal of Alloys and Compounds 823 (2020) 153811
4. Conclusion In the present work, nanocomposites based on CdS nanoparticles were prepared in soft and hard conditions by using a solgel method and high temperature melting, respectively. The matrix significantly affects the structure and optical properties of CdS QDs. By using optical absorption and luminescence spectroscopy it was shown that the type and concentration of the structure defects varies depending on the matrix and synthesis conditions The location of the energy levels (a distance from the bands energy) and contributions of each defects to the luminescent spectra depend on the matrix and synthesis conditions. The most highest-energy peaks of luminescence were found for the nanocomposites synthesized in soft conditions by growing nanoparticles directly in the matrix according to the scheme (Na2S þ X) þ CdCl2, where X ¼ TEOS, liquid glass. The nanocomposite based on the silicate glass and synthesized in hard conditions demonstrates the highest intensity of luminescence in visible and IR ranges. Author contribution section Svetlana V. Rempel: Project administration, Funding acquisition, Methodology, Preparing the samples, Formal analysis, Writing - Original Draft preparation, Writing - Review & Editing. Yulia V. Kuznetsova: Preparing the samples, Investigation, Validation, Writing e Editing. Ivan D. Popov: Preparing the samples, Investigation, Validation, Writing e Editing. Andrey A. Rempel: Supervision, Conceptualization, Methodology, Formal analysis, Writing - Review & Editing, All authors wrote, read and approved the final manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Russian Foundation for Basic Research (project no. 17-03-01024). References [1] X.S. Zhao, J. Schroeder, P.D. Persans, T.G. Bilodeau, Resonant-Raman-scattering and photoluminescence studies in glass-composite and colloidal CdS, Phys. Rev. B 43 (15) (1991) 12580e12589. [2] C. Dey, A.R. Molla, M. Goswami, G.P. Kothiyal, B. Karmakar, Synthesis and optical properties of multifunctional CdS nanostructured dielectric nanocomposites, J. Opt. Soc. Am. B 31 (8) (2014) J1761eJ1770. [3] K. Bansal, F. Antolini, S. Zhang, L. Stroea, L. Ortolani, M. Lanzi, E. Serra, S. Allard, U. Scherf, I.D.W. Samuel, Highly luminescent colloidal CdS quantum dots with efficient near-infrared electroluminescence in light-emitting diodes, J. Phys. Chem. C 120 (2016) 1871e1880. [4] J. Kakati, P. Datta, Schottky junction UV photodetector based on CdS and visible photodetector based on CdS:Cu quantum dots, Optik 126 (2015) 1656e1661. [5] Y. Hua, G. Lib, S. Zong, J. Shi, L. Guo, Self-assembled nanohybrid of cadmium sulfide and calcium niobate: photocatalyst with enhanced charge separation for efficient visible light induced hydrogen generation, Catal. Today 315 (2018) 117e125.
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[6] L.V. Garcia, M.I. Mendivil, G. Garcia Guillen, J.A. Aguilar Martinez, B. Krishnan, D. Avellaneda, G.A. Castillo, T.K. Das Roy, S. Shaji, CdS thin films prepared by laser assisted chemical bath deposition, Appl. Surf. Sci. 336 (2015) 329e334. [7] O.K. Echendu, S.Z. Werta, F.B. Dejene, K.O. Egbo, Structural, vibrational, optical, morphological and compositional properties of CdS films prepared by a lowcost electrochemical technique, J. Alloy. Comp. 778 (2019) 197e203. [8] L. Huang, Z.L. Wei, F.M. Zhang, X.S. Wu, Electronic and optical properties of CdS films deposited by evaporation, J. Alloy. Comp. 648 (2015) 591e594. [9] B.A. Harruff, C.E. Bunker, Spectral Properties of AOT-protected CdS nanoparticles: quantum yield enhancement by photolysis, Langmuir 19 (2003) 893e897. [10] I.D. Popov, Y.V. Kuznetsova, S.V. Rempel, A.A. Rempel, Tuning of optical properties of CdS nanoparticles synthesized in glass matrix, J. Nanoparticle Res. 20 (3) (2018) 78. [11] S.V. Rempel, A.A. Podkorytova, A.A. Rempel, Concentration quenching of fluorescence of colloid quantum dots of cadmium sulfide, Phys. Solid State 56 (3) (2014) 549e552. [12] S. Rempel, A. Razvodov, M. Nebogatikov, E. Shishkina, V. Shur, A. Rempel, Sizes and fluorescence of cadmium sulfide quantum dots, Phys. Solid State 55 (3) (2013) 624e628. [13] Y.V. Kuznetsova, I.B. Dorosheva, S.V. Rempel, Synthesis and optical properties of nanocomposite based on CdS nanoparticles in a silicon oxide matrix, in: AIP Conference Proceedings, 2015, 020054. [14] S.V. Rempel, N.S. Kozhevnikova, N.N. Aleksandrova, A.A. Rempel, Fluorescent CdS nanoparticles for cell imaging, Inorg. Mater. 47 (3) (2011) 223e226. [15] K. Nishidate, T. Sato, Y. Matsukura, M. Baba, M. Hasegawa, T. Sasaki, Density functional electronic structure calculations for native defects and Cu impurities in CdS, Phys. Rev. B 74 (2006), 035210. [16] A. Carvalho, A. Tagantsev, S. Oberg, P.R. Briddon, N. Setter, Intrinsic defects in CdTe and CdZnTe alloys, Physica B 404 (2009) 5019e5021. [17] M.J.I. Khan, S. Babar, A. Nabi, A.M. Rana, M. Iqbal, S.U.R.J. Ahmad, Theoretical studies of optical properties of Cu doped rocksalt CdS, J. Alloy. Comp. 695 (2017) 3605e3611. [18] F.I. Michos, M.M. Sigalas, Computational study of the absorption spectrum of defected ZnS nanoparticles, J. Appl. Phys. 123 (2018) 161587. [19] H. Wolf, T. Krings, T. Wiehert, Generation of intrinsic defects in CdS:In by doping with Li atoms, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 63 (1e2) (1992) 240e243. [20] H. Okamoto, J. Matsuoka, H. Nasu, K. Kamiya, Effect of cadmium to sulfur ratio on the photoluminescence of CdS-doped glasses, J. Appl. Phys. 75 (1994) 2251. [21] L. Meng, Y. Li, B. Yao, Z.-H. Ding, G. Yang, R.-J. Liu, R. Deng, L. Liu, Mechanism of effect of intrinsic defects on electrical and optical properties of Cu2CdSnS4: an experimental and first-principles study, J. Phys. 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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Impact of the matrix on optical properties of nanocomposites with CdS QDs S.V. Rempel a, b, *, Yu V. Kuznetsova a, I.D. Popov a, А.А. Rempel a, b, c a
Institute of Solid State Chemistry of the Ural Branch of the Russian Academy of Sciences, 91, Pervomaiskaya St., 620990, Ekaterinburg, Russia Ural Federal University Named After the First President of Russia B. N. Yeltsin, 19, Mira St., 620002, Ekaterinburg, Russia c Institute of Metallurgy of the Ural Branch of the Russian Academy of Sciences, 101, Amundsena St., 620016, Ekaterinburg, Russia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 August 2019 Received in revised form 16 December 2019 Accepted 10 January 2020 Available online 11 January 2020
In the present work the silica-based matrix effect on the luminescence properties of the nanocomposites which contain cadmium sulfide quantum dots (CdS QDs) was studied. Water alkaline solution of sodium silicates Na2O(SiO2)n, or liquid glass, or tetraethyl orthosilicate (TEOS) were used as simple models of a dielectric matrix. The synthesis of nanocomposites by sol-gel method using liquid glass and TEOS was carried out in two ways: mixing the prepared colloidal solution of nanoparticles with silica precursor or synthesis of nanoparticles directly in the matrix. The composite based on CdS QDs in a more complex matrix of silicate glass was also studied. The optical absorption and luminescence were studied in detail depending on the synthesis method. It was shown that various atomic defects contribute predominantly to the luminescence of QDs formed in different matrices, the matrix affects on the formation of the defects, therefore the luminescence range. In addition it was shown that method of synthesis affected the intensity of luminescence. © 2020 Elsevier B.V. All rights reserved.
Keywords: Quantum dots Matrix Luminescence Atomic defects Traps
1. Introduction In recent years, a huge amount of research has been devoted to the study and development of new methods for producing quantum dots (QDs). Practical applications and new possibilities to use them require QDs with various properties in different matrices. Significant interest is focused on QDs based on the semiconductor nanocrystals of AIIBVI compounds. Practical works, as a rule, are devoted to various methods of synthesis and characterization of QDs [1e14]. In theoretical works, attempts are made to find general patterns in the most interesting properties [15e18]. The luminescence range of semiconductor nanoparticles changes with size due to quantum confinement, which is the most important property of QDs. Unfortunately, the luminescence depends not only on the size or size distribution of nanocrystals, but also on a number of factors which affect the range and intensity of luminescence. One factor is an inability to control a stoichiometry during the synthesis of nanocrystals of the order of 2e5 nm, which
* Corresponding author. Institute of Solid State Chemistry of the Ural Branch of the Russian Academy of Sciences, 91, Pervomaiskaya st., 620990, Ekaterinburg, Russia. E-mail address: [email protected] (S.V. Rempel). https://doi.org/10.1016/j.jallcom.2020.153811 0925-8388/© 2020 Elsevier B.V. All rights reserved.
leads to appearance of atomic defects in the crystal structure. Atomic defects, such as vacancies, interatomic ions, etc., create local distortions of the crystal lattice. The energy levels of such defects are located within the band gap and can trap charge carriers in the semiconductors. Optical and other properties become dependent on the properties and concentration of defects because additional channels of radiative and nonradiative recombination thus appear. A number of works are devoted to the study of the predominant formation of defects in cadmium sulfide (CdS), energy position of these defects, and their influence on optical and electrical properties [19e22]. Another factor that affects luminescence is a tendency of nanocrystals to agglomerate. To maintain the stability, it is necessary to place QDs in the matrix or use organic/inorganic stabilizers. The authors of [23] studied the effect of anionic and cationic vacancies on the CdS surface on the adsorption of water molecules. Work [24] further suggests that surface ligands may play a role in assisting exciton relaxation. QDs properties synthesized by using different methods are compared in works [25,26]. The experimental results show that QDs synthesized in different matrices have different atomic defects. For example, the main defects of CdS nanoparticles in glass composites are cadmium vacancies (VCd). CdS nanocrystals in the colloid solution possess sulfur vacancy (VS), VCd, interstitial sulfur (IS), and cadmium atoms
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adsorbed on the surface (SCd) as defects [1]. Wherein, the luminescence intensity of QDs in aqueous colloidal solutions is lower than that in glass [10,11,27]. Thus, the matrix affects on the formation of the defects, therefore the luminescence range. However, it must be recognized that at present there is no clear understanding of the processes of formation and development of intrinsic defects of quantum dots, which are luminescence centers, during the formation of nanocrystals. This shows a lack of studies devoted to comparison of the effects of different matrixs on the properties of the obtained QDs. Disordered crystalline structures of CdS nanoparticles (average size between 3 and 9 nm) prepared by wet chemical synthesis were discovered and described in Ref. [28]. In the previous work the formation of distributed spherical CdS nanoparticles incorporated in matrix (by high temperature melting) were confirmed by high resolution focused ion beam scanning electron microscopy and SAXS results [10]. Schemes of the nanocomposite synthesis by using secondary heat treatment and shape of nanoparticle from WAXS and SAXS were presented in Ref. [29]. In the present work, the CdS QDs were synthesized in three different matrices to study the influence of the matrix and synthesis conditions on the formation of internal defects and optical properties of QDs. 2. Experimental details Nanocomposites based on CdS nanoparticles were synthesized at room and high temperatures by using a sol-gel method and high temperature melting, respectively. CdS nanoparticles in a silicate glass matrix (A) ware synthesized by using a heat treatment at 580e610 C during 3e15 h (Table 1). The synthesis details of composite based on silicate matrix are described in Ref. [10]. Liquid glass Na2O(SiO2)n (B) and TEOS Si(C2H5O)4 (C) were used as a silicon oxide matrix to synthesize nanocomposites by a sol-gel method. The liquid glass was used as received. TEOS was diluted with ethanol (EtOH) and hydrolyzed with H2O containing 0.3% HCl as TEOS:EtOH:H2O ¼ 1:10:5. The synthesis of nanocomposites by sol-gel includes two steps: first premixing one of initial solutions (Na2S or CdCl2) with a silicon-containing precursor (liquid glass or hydrolyzed TEOS) and then adding second precursor (Table 1). In addition, the replacement silicate matrix with water one was made to study the effect of water on luminescence of CdS nanoparticles. To do this, colloid solutions of CdS nanoparticles stabilized by disodium salt of ethylenediaminetetraacetic acid (EDTA) or (3mercaptopropyl)trimethoxysilane (MPS) were mixed with liquid glass or TEOS (Table 1, samples B3, B4, C3) and dried. The composites based on liquid glass were stirred for 5 min and then dried at 60 C for 24 h. The samples of TEOS-based composites were stirred for 24 h at room temperature until complete drying. Optical absorption, reflectance and luminescent spectra were recorded with FS-5 spectrofluorometer (Edinburgh Instruments) in UV, visible and NIR ranges at ambient temperature. Absolute quantum yield of luminescence were measured using method described here [31]. The same spectrofluorometer equipped by integrating sphere coated by Polytetrafluoroethylene were used. From optical absorption the band gap (Eg) and the average size Table 1 Synthesis conditions of composites by high temperature method. Sample
Temperature, C
Time, h
Eg, eV
A0 A1 A2 A3
e 580 580 610
e 3 5 3
4.10 3.01 2.96 2.65
e 3.9 4.1 6.2
of CdS QDs for all nanocomposites were calculated (Tables 1 and 2). The CdS QDs synthesized by using a heat treatment at 580 C have band gap of about 3 nm and average size of about 4 nm depending from the time of the heat treatment. The CdS QDs synthesized by sol-gel method have the band gap ranging from 2.48 to 2.86 eV and the average size from 3.8 to 10.3 nm. The luminescence spectra of QDs synthesized in different matrix have maxima in range from 475 to 788 nm. 3. Results and discussion Precise determination of the position of specific energy levels and moreover the prediction of their formation are difficult due to the influence of many factors. In particular, the level of the same defect has a different position relative to the edges of the energy zones depending on the charge [18]. In addition, as the band gap changes, the levels of shallow and deep impurities shift differently. The uneven distribution of defects leads to a chaotic arrangement of the perturbing potential and fluctuations of energy levels inside the band gap. This results in different energy values for the same defect located in unequal local conditions. Fluctuations in the position of the levels of a defect cause broadening of the level inside the forbidden zone. With the formation of various types of defects, individual lines merge into a strip. The combination of such factors, alongside with the QDs size distribution, is the reason for the appearance of wide structure less luminescence bands in the spectra of QDs. The authors [1] systematically investigated the phase transition behavior of CdS QDs in glass composites and colloids as well as bulk CdS by pressure-tuned resonant Raman scattering. As a result, energy level diagrams for CdS-glass composite and colloids were made. This work demonstrates that the main defects in the CdSglass composites are VCd, whereas the CdS nanoparticles in colloids include a number of atomic defects as VCd, VS, IS and SCd. Based on this approach, the mechanisms of defect formation during synthesis in the matrices under consideration can be hypothesides. The luminescence spectra of QDs synthesized in glass with hightemperature synthesis are shown in Fig. 1. With increasing temperature and/or duration of heat treatment, particles grow and the luminescence maximum shifts to the long-wavelength region. The heat treatment mode at 580 C for 5 h was found to be the most preferred one. A further increase in heat treatment temperature and time is impractical since it leads to an increase in the size of quantum dots and a shift of the maximum to the long-wavelength region along with the decrease of luminescence intensity. The luminescence spectrum of the sample after the optimal heat treatment mode shows a wide structureless peak with a maximum of 605 nm, which indicates the participation of defects IS and VCd in the recombination of charge carriers. The luminescence decay dynamics of CdS QDs in the glass had been previously studied [30]. The luminescence decay curves can be approximated with sum of several exponential functions. This points out at the traps contribution to the luminescence. The luminescence intensity of CdS in
Table 2 Synthesis conditions of composites by a sol-gel method. Sample
Stabilizer
Synthesis scheme
Eg, eV
B1 B2 B3 B4 C1 C2 C3
e e EDTA MPS e MPS MPS
(Na2S þ LG)þCdCl2 (CdCl2 þ LG) þ Na2S (CdS-EDTA)sol þ LG (CdS-MPS)sol þ LG (Na2S þ TEOS)þCdCl2 (Na2S þ TEOS þ MPS)þCdCl2 (CdS-MPS)sol þ TEOS
2.60 2.51 2.48 2.86 2.71 2.67 2.63
5.9 8.4 10.3 3.8 4.7 5.1 5.5
S.V. Rempel et al. / Journal of Alloys and Compounds 823 (2020) 153811
3
Fig. 1. The effect of heat treatment temperature and time on the optical absorption (a) and luminescence (b) of the composites based on the silicate glass. The gray dashed line shows the optical absorption edge of bulk CdS. The excitation wavelength was 385 nm.
the glass matrix is higher than that in other matrices. This may be due, firstly, to a lower concentration of atomic defects after hightemperature annealing. Secondly, luminescence intensity depends on the stoichiometry of CdS. It was found that luminescence intensity increases with increasing Cd content [20]. During hightemperature synthesis, a larger number of sulfur atoms may volatilize and a tendency for Cd to prevail appears. Optical absorption and luminescence spectra of CdS QDs in liquid glass and TEOS depending on the synthesis method are shown in Figs. 2 and 3, respectively. The synthesis based on the scheme of preliminary mixing of the source of sulfur ions with liquid glass and the subsequent addition of cadmium ions (Table 2, sample B1) causes the formation of nanoparticles with a predominant defect IS. This is connected with the lack of cadmium ions during the particle formation due to the limited mobility of Cd2þions in viscous liquid glass. A luminescence band with maximum at 530 nm is observed, which is associated with recombination between IS and the valence band (Fig. 2, Sample B1). The average size of the particle synthesized by this method is about 5.9 nm. In the case of adding S2 to the solution along with Cd2þ(Table 2, sample B2), a significant amount of VS is formed during the nanoparticle growth. After an excitation, electrons pass from the IS level to the Vs level throught the nonradiative relaxation followed by radiative relaxation to VCd (900 nm) (Fig. 2, sample B2). A significant contribution of this peak is observed in the spectrum along with all the others. The biggest impact on the luminescence is
associated with recombination of VS to the valence band (788 nm), which confirms the predominant formation of VS defects. The average size of the QDs synthesized with this method is slightly larger. With a significant excess of absorption in comparison with the previous sample, the integral luminescence intensity of sample B2 is lower, which indicates the existence of additional passes of nonradiative relaxation. In the case of mixing the liquid glass with colloid solution stabilized by EDTA (Table 2, sample B3), CdS QDs are already formed and changing of the matrix (water to silica-based) does not affect the defects of atomic structure and the position of the luminescence maximum (Fig. 2, sample B3). However, the largest average size of the QDs about 10.3 nm is observed. The increase of the luminescence intensity in comparison with the initial QDs in an aqueous solution indicates the prevention of luminescence quenching due to the polar water molecules. In addition, a composite was synthesized by using colloid solution of CdS QDs stabilized by MPS. The shell based on MPS molecules probably allows the particles to be more evenly distributed in the silica-based matrix upon drying and prevents the particles from agglomerating. In this case, the minimum particle size and maximum luminescence intensity are maintained, wherein the emission mechanism is based on the predominant recombination of the IS electron and the VCd hole (633 nm) (Fig. 2, sample B4). The analysis of the results obtained for the composites based on colloid solutions of CdS QDs indicates the formation of different predominant defects during stabilization by EDTA and MPS. In the first case, the
Fig. 2. The optical absorption (a) and luminescence (b) of the composites based on the CdS nanoparticles in the liquid glass. The excitation wavelength was 380 nm.
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Fig. 3. The optical absorption (a) and luminescence (b) of the composites based on the CdS nanoparticles in TEOS. The excitation wavelength was 380 nm.
main contribution is made by the recombination between Vs and the valence band, in the second case, between the IS electron and the VCd hole. In both cases, VS makes a significant contribution to the luminescence. Thus, the band maximum is shifted to the longwavelength range in comparison with the synthesis methods of CdS QDs directly in the liquid glass matrix. The synthesis of the composite by using TEOS (Table 2) results in a sharp absorption edge, an exciton peak and the maximum luminescence intensity at a wavelength of 745 nm (transition from VS to valence band) (Fig. 3). All samples C show similar shape of the emission spectra and low luminescence intensity. A decrease of the concentration in luminescence centers associated with the dominant band causes weaker bands. In CdS/TEOS samples, the intensities of luminescence bands at 530 and 633 nm are reduced, which most likely causes the development of the exciton peak. Both peaks are related to the contribution of the IS defect. Probably, the concentration of IS in CdS QDs is significantly lower in TEOS than in other matrices. The absence of significant shift of the long-wave peak in all samples with the change of the particle size indicates the participation of deep defects in the luminescence. However, the exciton peak shifts to the long-wavelength region with an increase in the size of the QD. The highest absolute quantum yield (QY) of luminescence was measured for CdS nanoparticles in glass-CdS nanocomposites QY ¼ 0.22 ± 0.02. Measured QY was about 0.02 for nanocomposites based on liquid glass and obtained using sol-gel method. A comparison the range of the luminescence spectra of CdS QDs synthesized in the different matrices is presented in Fig. 4 and
Table 3 The effect of matrix on Eg and luminescence of CdS nanoparticles. Sample
Matrix
Eg, eV
Position of maximum, nm
A B C
silicate glass liquid glass TEOS
2.96 2.60 2.71
475
530 e
605 633 e
745
Table 3. The shifts of the luminescence maximum are determined by fluctuations in the positions of the defect levels due to the reasons discussed above. Thus, the experimental optical results show the evidence of non-identity of the structural defects formed in CdS QDs that produce the luminescence. Nevertheless, we can hypothesize that the trap centers participated in the recombination of charge carriers. It is reasonable to attribute luminescence band at 530 nm to the recombination of electrons on IS deep donor and holes in the valence band. The band at 605 (633) nm is associated with the recombination of electrons in IS and holes in VCd. The recombination between the electrons in VS and holes in the valence band is attributed to the peak of 745 (788) nm and the one between the electrons in VS and holes in VCd e to the luminescence band at 900 nm. The energy levels and contributions of each defects to luminescent spectra vary in different matrix. Main defects in glassCdS nanocomposite are VCd, Is, in TEOS/CdS nanocomposite e Is, VCd, Vs, in liquid glass-CdS nanocomposite e Is, VCd, Vs.
Fig. 4. The effect of matrix on the optical absorption (a) and luminescence (b) of the CdS nanoparticles.
S.V. Rempel et al. / Journal of Alloys and Compounds 823 (2020) 153811
4. Conclusion In the present work, nanocomposites based on CdS nanoparticles were prepared in soft and hard conditions by using a solgel method and high temperature melting, respectively. The matrix significantly affects the structure and optical properties of CdS QDs. By using optical absorption and luminescence spectroscopy it was shown that the type and concentration of the structure defects varies depending on the matrix and synthesis conditions The location of the energy levels (a distance from the bands energy) and contributions of each defects to the luminescent spectra depend on the matrix and synthesis conditions. The most highest-energy peaks of luminescence were found for the nanocomposites synthesized in soft conditions by growing nanoparticles directly in the matrix according to the scheme (Na2S þ X) þ CdCl2, where X ¼ TEOS, liquid glass. The nanocomposite based on the silicate glass and synthesized in hard conditions demonstrates the highest intensity of luminescence in visible and IR ranges. Author contribution section Svetlana V. Rempel: Project administration, Funding acquisition, Methodology, Preparing the samples, Formal analysis, Writing - Original Draft preparation, Writing - Review & Editing. Yulia V. Kuznetsova: Preparing the samples, Investigation, Validation, Writing e Editing. Ivan D. Popov: Preparing the samples, Investigation, Validation, Writing e Editing. Andrey A. Rempel: Supervision, Conceptualization, Methodology, Formal analysis, Writing - Review & Editing, All authors wrote, read and approved the final manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Russian Foundation for Basic Research (project no. 17-03-01024). References [1] X.S. Zhao, J. Schroeder, P.D. Persans, T.G. Bilodeau, Resonant-Raman-scattering and photoluminescence studies in glass-composite and colloidal CdS, Phys. Rev. B 43 (15) (1991) 12580e12589. [2] C. Dey, A.R. Molla, M. Goswami, G.P. Kothiyal, B. Karmakar, Synthesis and optical properties of multifunctional CdS nanostructured dielectric nanocomposites, J. Opt. Soc. Am. B 31 (8) (2014) J1761eJ1770. [3] K. Bansal, F. Antolini, S. Zhang, L. Stroea, L. Ortolani, M. Lanzi, E. Serra, S. Allard, U. Scherf, I.D.W. Samuel, Highly luminescent colloidal CdS quantum dots with efficient near-infrared electroluminescence in light-emitting diodes, J. Phys. Chem. C 120 (2016) 1871e1880. [4] J. Kakati, P. Datta, Schottky junction UV photodetector based on CdS and visible photodetector based on CdS:Cu quantum dots, Optik 126 (2015) 1656e1661. [5] Y. Hua, G. Lib, S. Zong, J. Shi, L. Guo, Self-assembled nanohybrid of cadmium sulfide and calcium niobate: photocatalyst with enhanced charge separation for efficient visible light induced hydrogen generation, Catal. Today 315 (2018) 117e125.
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