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ZnS quantum well structures prepared by aqueous route
Optical Materials 69 (2017) 23e29
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Tunable narrow emission in ZnS/CdS/ZnS quantum well structures prepared by aqueous route Hitanshu Kumar 1, Asha Kumari 1, Ragini Raj Singh* Department of Physics and Materials Science, Jaypee University of Information Technology, Waknaghat, Solan 173234, H.P., India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 February 2017 Received in revised form 30 March 2017 Accepted 4 April 2017
ZnS/CdS/ZnS quantum wells (QWs) with tunable narrow emission due to narrow size distribution were successfully synthesized by wet chemical method. QWs give high luminescence and hence important materials for recent nanotechnology applications such as optoelectronics and bioimaging. In this work we put forward a QW structure with extended potential barrier, where electron and holes are confining in the core of ZnS and ZnS/CdS QDs by creating a ZnS/CdS/ZnS potential well like structure. Multilayers break the continuity of structure, which gives the combined advantages of ZnS, CdS and ZnS/CdS. This well like structure of quantum dots gives highest possibility to confine the electron and holes inside core and enhance the optical properties with suitable shell thickness. Structural, morphological and optical properties were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV evisible absorption (UVevis) and photoluminescence (PL) spectroscopy. Quantum yield of these quantum dot structures was also measured. © 2017 Elsevier B.V. All rights reserved.
Keywords: Quantum dots Quantum wells XRD TEM Photoluminescence
1. Introduction Highly luminescent semiconductor QDs have several applications due to their high luminescence quantum efficiency, spectral tunability, and high chemical stability in the present production displays, biolabels and lasers. Semiconductors such as CdS and ZnS belong to IIeVI group are scientifically important for their prospective optical applications [1] and optoelectronic devices [2,3]. At room temperature ZnS and CdS possess energy band gaps about 3.7 eV and 2.4 eV respectively [9] and are excellent materials for optoelectronic applications in this spectral range. Frequently, scientists give more attention to QDs; core/shell QDs and multishell QDs because these classes of semiconductor nanocrystals will lead to a series of novel optical and electronic properties [4,5]. The core/ shell CdS/ZnS structures [6,7] and multilayer structures of CdS and ZnS, (CdS/ZnS/CdS) [8e11] have attracted much attention nowadays, as the optical properties can be enhanced and the band gap can be tuned by the surface chemical bonds and charge transfer in the surface/interface region of QDs and QW structures. Chemical or
* Corresponding author. E-mail addresses: [email protected], [email protected] (R.R. Singh). 1 First authors: Both the authors will be considered as first authors as contributed equally. http://dx.doi.org/10.1016/j.optmat.2017.04.009 0925-3467/© 2017 Elsevier B.V. All rights reserved.
physical process can be used to prepare such type of structures [12,13]. These structures have unique and enhanced physical properties due to the core/shell interface effects. Numerous investigations have focused on forming the quantum well (QW) by sandwiching the low band gap material in the well between the layers of quantum dots of higher band gap material viz. ZnS/PbS/ ZnS [14], CdS/HgS/CdS [15], ZnS/CdSe/ZnS [16]. Literature survey reveals that core/shell type of QDs or QWs formed by over coating of low band gap material with high band gap materials exhibit high photoluminescence intensity as a result of elimination of surface nonradiative recombination [17]. Coating of QD cores with an inorganic shell, made of a material with a higher band gap eliminate the negative effects of defect states. Here, in this work we propose a procedure for obtaining highly luminescent QDs based on charge carrier confinement engineering. We propose a novel core-multishell ZnS/CdS/ZnS QD structure, which gives proper confinement to exciton by means of the moderate shell thickness, will be sufficient for reliable protection of excited carriers from the environment but not greatly increasing the QD diameter. To synthesize ZnS/CdS/ZnS QW structure the size of core (ZnS/CdS) was kept fixed with variable shell thicknesses. The thickness of shell was varied by using different cappent ratio of 2mercaptoethanol i.e 1 ml and 2 ml. The charge, functionality and reactivity of the narrow band gap material (CdS) can be improved by its surface modification with wider band gap material (ZnS).
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Consequently enhance the PL emission due to localization of the e-h pair [18e20]. It implies that the wave function of electron and hole may be spatially separated in core and shell, reducing the probability of non-radiative decay in the surface states and trap sites [21]. One more interesting point of our work is that as we coat core ZnS/CdS with ZnS in presence of 2-mercaptoethanol, the introduction of ZnS can greatly passivate the CdS surface to protect it from oxidation and prevent CdS leaching in to the surrounding medium [22,23]. The highlight and novelty of our work is to form and analyze the structural, optical and surface properties of highly luminescent wurtzite ZnS/CdS/ZnS QWs synthesized at low temperature (35 ± 1 C). It has been found that the synthesis of ZnS/CdS/ZnS QW also possible by different method like pulse laser deposition, chemical method in an air atmosphere. Beside this, many authors have also been reported on ZnS/CdS/ZnS QWs but they have opted different synthesis methods and particles were of cubic structures [24]. A few authors [25,26] have been reported the wurtzite ZnS/ CdS/ZnS but methods were different or they were used physical synthesis method on high temperatures. On the contrary in this report we propose a simple and cost effective wet chemical method which is very effective method to form the wurtzite QWs with good size controllability. We have attempted to change the functional groups at the surface of prepared QDs which helps to accommodate these QDs in bio-application purposes. We have employed X-ray diffraction (XRD), UVeVis spectroscopy, photoluminescence spectroscopy (PL) and Transmission electron microscopy (TEM) techniques to characterize ZnS, CdS, CdS/ZnS and ZnS/CdS/ZnS core/ shell QDs. Synthesis and analysis of wurtzite ZnS/CdS/ZnS grown on low temperature is being presented here for the very first time. 2. Experimental details 2.1. Characterization Techniques Different characterization techniques were used to study the structural and optical properties. XRD has been performed for structural investigation and the patterns were recorded in a Shimadzu powder x-ray diffractometer using Cu Ka1 radiation. Absorbance spectra of prepared samples were recorded by using PerkinElmer Lambda2 UVeVis spectrophotometer. Photoluminescence (PL) emission spectra and Photoluminescence (PL) Excitation spectra of the samples were recorded by a computer controlled rationing luminescence spectrophotometer LS55 (Perkin-Elmer Instruments, UK) with l accuracy ¼ ±1.0 nm and l reproducibility ¼ ±0.5 nm. A 20 kW pulse <10 ms from a Xenon discharge lamp was used as the excitation source for recording photoluminescence emission spectra. A gated photo multiplier tube was used as a detector. Prior to the PL experiments signal-to noise ratio was adjusted to 500:1, using the Raman band of water with excitation at 220 nm. Transmission electron microscopy (Hitachi H-7500) has been performed as the finest proof for the particle size and particle size distribution. 2.2. Synthesis of ZnS/CdS and ZnS/CdS/ZnS quantum dots The reaction matrix employed in our study consisted of AR grade CdCl2, NH4Cl and thiourea in 1:1.5:3 M ratios for CdS and AR grade ZnSO4, (NH4)2SO4 and thiourea in 1:1.5:1.5 M ratios for ZnS. Seed growth method was employed for synthesis of ZnS/CdS core/ shell structures. In order to synthesize desired ZnS/CdS core/shell, firstly we prepared ZnS QDs as seed by solution growth method at 35 ± 1 C with fixed amount of 2-Mercaptoethanol. Complexing agent was used to control the release of metal ions (Zn2þ) and sulphur ions (S2) to produce the controlled homogeneous reaction. The reaction matrix was prepared in two parts. The initial
solutions were prepared from anhydrous zinc sulphate (ZnSO4) and ammonium sulphate ((NH4)2SO4) in 50 ml of water. Ammonia (NH3) was used as a complexing agent until the formation of clear metallic complexes and so the pH was kept at 8.5. Then thiourea CS (NH2)2 was added as the source of sulphur in the prepared solution. Immediately 2 ml of (5% solution) 2-mercaptoethanol was added as a surfactant to control the particle size of QDs and continuously stirred for 5 h. After synthesis prepared sample of ZnS were centrifuged (3500 RPM, 20 min) and washed several times with distilled water and dried. To synthesize ZnS/CdS core/shell QDs, a CdS shell layer was grown on ZnS core nanocrystals. The method of shell formation of CdS was same as seed ZnS QDs just the source for cadmium that is cadmium chloride (CdCl2) and respective complexing agent ammonium chloride (NH4Cl) have took place. As the additional step for the synthesis of core/shell structures inclusion of 0.1 g of prepared ZnS QDs was mandatory just before the addition of thiourea. After that required amount of 2-mercaptoethanol was added in solution for capping to obtain ZnS/CdS core/shell structures. For ZnS/CdS/ZnS QW formation, now ZnS/CdS (0.2 g) has been used as seed and the remaining method is same as opted for ZnS QD synthesis. The size of ZnS/CdS core was kept fixed and varied the shell thickness of ZnS using varying cappent ratio. When the reaction was completed, purification of all core/shell nanocrystals has been performed. Then core/shell QDs were dispersed in distilled water and store. The process flow for the synthesis of ZnS/ CdS/ZnS QDQWs is presented in Fig. 1. ZnS/CdS/ZnS1 has thicker shell of ZnS as compared to ZnS/CdS/ZnS2. 3. Results and discussions 3.1. Structural characterization Some parameters like selection of precursors and temperature plays an important role, these parameters affects the crystal structure in case of CdS and ZnS QDs and ultimately helps to obtained desired crystal structures of QDs and QWs [27]. X-ray diffraction (XRD) spectra of 2-mercaptoethanol capped CdS, ZnS/ CdS, ZnS, ZnS/CdS/ZnS1 and ZnS/CdS/ZnS2 QDs have been shown in Fig. 2. Spectra of prepared QDs is in good agreement with JCPDS411049 in CdS and JCPDS 12-0688 in ZnS QDs. The prominent peaks (002), (101), (200) and (110) were indexed to the wurtzite structure in case of CdS and their core/shell structures. The prominent peaks in ZnS and their core QDs are indexed to (0010), (108), and (206) for wurtzite structure. Although there are no specific reports are available on synthesis of wurtzite ZnS and CdS quantum dots but some reports are available related to formation of II-VI group semiconductor nanocrystals wurtzite structure to understand the effect of synthesis parameters on structure formation [28e30]. In case of ZnS/CdS and ZnS/CdS/ZnS prominent peaks are same with small shift in 2 theta position (Table 1); this shift is clear indication for core/shell QDs formation. The crystallite size of all quantum dots (D) has been calculated by the most prominent peaks using Scherrer's formula [27].
D ¼ kl=b cos q
(1)
The calculated crystallite size from Scherrer formula is 3.1 nm, 2.3 nm, 3.5 nm, 4.3 nm and 3.9 nm for ZnS, CdS, ZnS/CdS, ZnS/CdS/ ZnS1 and ZnS/CdS/ZnS2 respectively. As can be seen from the figure in ZnS/CdS/ZnS1 the diffraction peaks of CdS and ZnS overlap each other and have intense peak. Whereas in case of ZnS/CdS/ZnS2 peak intensities decreased because ZnS shell put constraints on ZnS/CdS core. As the shell thickness reduced or the size of shell reduced the core closed inside ZnS shell experience more compressive strain, due to this the intensity of peaks obtained in ZnS/CdS/ZnS2 get
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Fig. 1. Schematic representation of the synthesis procedure to obtain ZnS QDs, ZnS/CdS core/shell QDs and finally ZnS/CdS/ZnS QW structures.
reduced. Diffraction peaks used for calculation of crystallite size along with FWHM values are tabulated in Table 1.
3.2. Transmission electron microscopy
Fig. 2. X-ray diffraction patterns of 2-mercaptoethanol capped CdS, ZnS, ZnS/CdS, ZnS/ CdS/ZnS1 and ZnS/CdS/ZnS2 QDs.
To know shape and size of samples TEM of prepared QDs, core/ shell QDs and QWs has been performed. The results are being presented in Fig. 3. TEM images of ZnS (Fig. 3(a)) and CdS (Fig. 3(b)) QDs show highly mono-dispersed nanoparticles with average sizes of 3.3 nm and 2.5 nm respectively. Fig. 3 (c) indicates the micrograph for ZnS/CdS core/shell QDs, still there is indication of the formation of mono-disperse QDs with 4.2 nm particle size. Similarly ZnS/CdS/ZnS2 QW (Fig. 3(d)) has 4.6 nm particle sizes and (Fig. 3(e)) shows histogram for confirmation of particle size with respect to Fig. 3(d). It was confirmed from (Fig. 3(e)) that ZnS/CdS/ ZnS2 QW contain maximum particles of size 4.6 nm. These figures clearly signify that there is no agglomeration of QDs even after the formation of QWs; it indicates the capability to obtain either the fine solution or even powder of uniformly distributed nanoparticles for various applications. Fig. 3(c) of wurtzite ZnS/CdS QDs is depicted from Fig. 3(c) where core (ZnS/CdS) has been surrounded by the shell (ZnS) as the electron density of the CdS and ZnS is almost similar it is difficult to differentiate between the layers, and Fig. 3(d) shows the formation of wurtzite ZnS/CdS/ZnS2 QW, in this case one can observe the different shades on a single QW this may be due to the presence of three different layers in a system with comparable electron densities and that is why it is difficult to differentiate between the layers of the QW. This result of wurtzite structure formation also supports our findings from XRD results. Whereas, the particle sizes obtained from TEM are very slightly greater than the size obtained from XRD analysis. The size obtained
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Table 1 Structural parameters of ZnS, CdS, ZnS/CdS, ZnS/CdS/ZnS1 and ZnS/CdS/ZnS2 quantum dot structures. Samples
2q (Degree)
FWHM (Degree)
Particle size (nm)
Shift in 2q (Degree)
ZnS CdS ZnS/CdS ZnS/CdS/ZnS1 ZnS/CdS/ZnS2
28.68 28.98 28.99 28.79 28.68
2.88 3.82 2.55 2.09 2.29
3.1 2.3 3.5 4.3 3.9
e e 0.31 0.20 0.31
by XRD and TEM has the correlation that the XRD size is usually equals or smaller than the size obtained by TEM [31]. There is no big disparity in the particle sizes obtained from different ways; also it is clearly vindicated from the micrographs that the as prepared nanoparticles are of great quality. 3.3. Optical studies 3.3.1. Absorbance spectroscopy We demonstrate the optical absorbance spectra of CdS, ZnS/ CdS, ZnS/CdS/ZnS1 and ZnS/CdS/ZnS2 QWs (Fig. 4). ZnS/CdS core have the absorption peak at 430 nm, which corresponds to core of 3.3 nm, which is highly blue shifted from the CdS. After formation of ZnS/CdS/ZnS1 shell, 1s peak has been red shifted from ZnS/CdS to ZnS/CdS/CdS1. Similar observations of red shift due to shell growth have been observed in other ZnS/CdS/Zns1, ZnS/CdS/ZnS2 systems [32e34]. On the formation of a ZnS shell on ZnS/CdS, the 1s peak red shifted due to the size increment of QW and exciton, remains “spatially direct”, as indicated by the presence of a welldefined 1s absorption maximum. As the shell thickness is further increased, we observe a sharp absorption peak (in ZnS/ CdS/ZnS2, ZnS/CdS/ZnS1) indicating a transition in only shell to shell localization characterized by the increased e-hþ overlap [35]. In Fig. 4 small shell thickness shows smaller shift and higher shell thickness shows larger shift. Among them some important reasons are increment in the effective volume of exciton; as the electronic levels come closer that accompanies an increment in the size; and increased leakage of the exciton into the shell [36,37]. 3.3.2. Photoluminescence spectroscopy Consequence of size; and shell thickness has been studied in this work. To identify the change in luminescence behavior the core that is ZnS/CdS (core/shell) and ZnS/CdS/ZnS QW structures have been studied and discussed on the basis of the model presented in Fig. 5. It is easy to understand from the figure that how the luminescence is changing as the ZnS shell thickness varies. The experimental findings resemble with the model. The emission spectra of CdS, ZnS/CdS and ZnS/CdS/ZnS QWs with different thicknesses of ZnS shell are presented in Fig. 6. A broad emission band for CdS with maximum at 546 nm is observed. It is also shifted to blue in comparison with the bulk CdS. The emission band at 430 nm corresponds to ZnS/CdS core shell QDs; this can be noted that the luminescence has been increased shifted to blue with respect to CdS. Further in ZnS/CdS/ZnS there is increment in luminescence with an efficient tunability. This is because the wave functions of electron and hole in ZnS/CdS/ZnS1 are delocalized and in ZnS/CdS/ ZnS2 the wave function of electron mostly localized in shell but hole's wave function localized preferably in core. Localization of electron-hole in the system can lead to the superior PL enhancement and fine tunability [38]. PL emission intensity was low when the thickness of ZnS shell was high at this point 1 ml capping agent has been used. On increasing the cappent to 2 ml shell thickness was decreased and PL emission intensity was increased and can be clearly illustrated from
the emission of ZnS/CdS/ZnS2. The thinnest achievable shell of ZnS possesses the largest band gap and therefore, produces the maximum confinement potential for excited charge carriers. Conversely the conduction and valence bands move towards their bulk energies, resulting in the decrease in the effective energy barrier. In other way if the shell thickness increased, it can be physically extend the tunneling length, necessary for electron and hole interaction with the external medium, which confines the photo-generated carriers inside the photo-luminescent core. CdS creates a smaller interfacial strain, but its potential barriers are weaker than as compared to ZnS. Therefore, an approach combining the benefit of both the high potential barrier of ZnS and lattice compatibility of CdS was proposed in this work [39]. The emission corresponds to the bulk donoreacceptor recombination of CdS. Surface states usually act as radiation less recombination centers. The ZnS shell passivates the vacancies that are presented on the surface of the CdS and reduces the nonradiative recombination sites. These are responsible for the luminescence enhancement. The lattice parameters of CdS differ from that of ZnS by 7%. About one-unitcell-thick coating layers can accommodate the lattice misfit between CdS and ZnS by elastic strain exclusively [24]. If the shell is thicker several order more than one-unit cell it can increase strain due to the lattice mismatch and could direct the formation of dislocations. Some reduction in the luminescent intensity is ascribed to the creation of misfit dislocations, which act as centers for radiationless recombination when the thickness of the shell increases [24]. On the other hand, with the increasing amounts of ZnS, the concentration of the CdS luminescence centers decreases which results in some of the reduction in the luminescence intensity. 3.3.3. Quantum yield measurement Quantum yield is the basic property for any luminescent material. Therefore, its measurement is an important step. Here we measured the quantum yield of our prepared samples by using relative method. Equation (2) is presenting the formula used for quantum yield calculation by relative method [40].
QYs ¼ QYr Is =Ir Ar =As
2 ns nr
(2)
where s refers to sample, r refers to reference (standard). IS and Ir referred to integrated intensity of sample reference respectively. nS and nr referred to refractive index of sample and reference respectively and As and Ar refers to absorbance at excitation wavelength for sample and reference respectively. We have chosen 40 -6- diamidino-2-phenyl-indole (DAPI) as reference because it is very common to measure yield and having emission around 400e550 nm. Emission peak positions for all the prepared quantum dots also lie in this range. Quantum yield of DAPI is 0.58 for [41]. Same solvent was taken for sample and reference. Quantum yield calculated for all samples along with PL peak position, intensity; integrated intensity and FWHM are presented in Table 2. It is concluded from Table 2 that CdS has low quantum yield as compare to ZnS/CdS. This is because shells provide stability to core in case of
H. Kumar et al. / Optical Materials 69 (2017) 23e29
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Fig. 3. TEM micrographs (a) ZnS QDs; (b) CdS QDs; (c) ZnS/CdS core/shell QDs; (d) for ZnS/CdS/ZnS2 with smallest shell thickness (Dimension of the outer box in inset figures is 5 nm 5 nm) and (e) Histogram of ZnS/CdS/ZnS2.
ZnS/CdS. In case of ZnS/CdS/ZnS1 although the quantum yield is low as compared to its core ZnS/CdS but low value of FWHM confirms that ZnS/CdS/ZnS1contain particle monodispersity. Similarly ZnS/CdS/ZnS2 have better quantum yield as compared to ZnS/CdS/
ZnS1 with narrower FWHM. So that it is concluded that ZnS/CdS/ ZnS2 has good fluorescence and quantum yield with better monodispersity and fine tenability with respect to CdS and ZnS/CdS QD structures.
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Fig. 4. Absorption spectra of CdS, ZnS/CdS, ZnS/CdS/ZnS1 and ZnS/CdS/ZnS2. Fig. 6. Photoluminescence spectra of CdS, ZnS/CdS, ZnS/CdS/ZnS1 and ZnS/CdS/ZnS2.
Table 2 Quantum yield along with emission peak position, intensity; integrated intensity and FWHM. Samples
PL peak position (nm)
Intensity
Intensity (integrated)
FWHM(nm)
QY
CdS ZnS/CdS ZnS/CdS/ZnS1 ZnS/CdS/ZnS2
546 430 505 505
250 610 790 910
560 882 368 625
70 70 34 28
0.40 0.70 0.32 0.41
well, which were assumed to act as centers for radiation less recombination. Furthermore, PL spectra show a strong emission in QW centered at 505 nm. The strong emission at 505 nm is from the charge transfer of electrons from CdS electron to ZnS holes by excitation energy. Quantum yield of these quantum dot structures has also been measured and results indicate about high monochromaticity in the quantum well structures with high yield. These ZnS/CdS/ZnS are a promising optical material in the applications to various transparent green optoelectronic devices and in imaging and diagnostics in medical field. Acknowledgements Fig. 5. Schematic of ZnS/CdS/ZnS QW structures with different ZnS shell thickness with corresponding PL behavior.
4. Conclusion Highly luminescent ZnS/CdS/ZnS QW with narrow PL emission were successfully synthesized by wet chemical method at 35 ± 1 C. The QDs were characterized by XRD, TEM, UVevis absorption spectroscopy and photoluminescence spectroscopy. The TEM images showed that the QW structures were wurtzite and monodispersed. Strong carrier confinement in luminescent core (ZnS/ CdS) using appropriate ZnS outer shell layer has been obtained and the outcomes reflect from the luminescence studies. Luminescence in the region of 430 nme550 nm was observed. The luminescence from CdS was enhanced by the complete ZnS shell, which may be the result of the decrease in the defects on the surface of the CdS
This work was supported by the SERB, DST, India grant [grant number SR/FTP/PS-032/2013]. We are very thankful for the financial aid to this work provided by Jaypee University of information Technology, Waknaghat, Solan, H.P., India. We also thank SAIF, Panjab University, Chandigarh for characterized our samples. References [1] Q. Shen, J. Kobayashi, L.J. Diguna, T. Toyoda, Effect of ZnS coating on the photovoltaic properties of CdSe quantum dot-sensitized solar cells, J. Appl. Phys. 103 (8) (2008) 084304. [2] Y. Shirasaki, G.J. Supran, M.G. Bawendi, V. Bulovi c, Emergence of colloidal quantum-dot light-emitting technologies, Nat. Photonics 7 (1) (2013) 13e23. [3] S. Kim, T. Kim, M. Kang, S.K. Kwak, T.W. Yoo, L.S. Park, S.W. Kim, Highly luminescent InP/GaP/ZnS nanocrystals and their application to white lightemitting diodes, J. Am. Chem. Soc. 134 (8) (2012) 3804e3809. [4] H.C. Youn, S. Baral, J.H. Fendler, Dihexadecyl phosphate, vesicle-stabilized and insitu generated mixed CdS and Zns semiconductor particles-preparation and utilization for photosensitized charge separation and hydrogen generation,
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Tunable narrow emission in ZnS/CdS/ZnS quantum well structures prepared by aqueous route Hitanshu Kumar 1, Asha Kumari 1, Ragini Raj Singh* Department of Physics and Materials Science, Jaypee University of Information Technology, Waknaghat, Solan 173234, H.P., India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 February 2017 Received in revised form 30 March 2017 Accepted 4 April 2017
ZnS/CdS/ZnS quantum wells (QWs) with tunable narrow emission due to narrow size distribution were successfully synthesized by wet chemical method. QWs give high luminescence and hence important materials for recent nanotechnology applications such as optoelectronics and bioimaging. In this work we put forward a QW structure with extended potential barrier, where electron and holes are confining in the core of ZnS and ZnS/CdS QDs by creating a ZnS/CdS/ZnS potential well like structure. Multilayers break the continuity of structure, which gives the combined advantages of ZnS, CdS and ZnS/CdS. This well like structure of quantum dots gives highest possibility to confine the electron and holes inside core and enhance the optical properties with suitable shell thickness. Structural, morphological and optical properties were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV evisible absorption (UVevis) and photoluminescence (PL) spectroscopy. Quantum yield of these quantum dot structures was also measured. © 2017 Elsevier B.V. All rights reserved.
Keywords: Quantum dots Quantum wells XRD TEM Photoluminescence
1. Introduction Highly luminescent semiconductor QDs have several applications due to their high luminescence quantum efficiency, spectral tunability, and high chemical stability in the present production displays, biolabels and lasers. Semiconductors such as CdS and ZnS belong to IIeVI group are scientifically important for their prospective optical applications [1] and optoelectronic devices [2,3]. At room temperature ZnS and CdS possess energy band gaps about 3.7 eV and 2.4 eV respectively [9] and are excellent materials for optoelectronic applications in this spectral range. Frequently, scientists give more attention to QDs; core/shell QDs and multishell QDs because these classes of semiconductor nanocrystals will lead to a series of novel optical and electronic properties [4,5]. The core/ shell CdS/ZnS structures [6,7] and multilayer structures of CdS and ZnS, (CdS/ZnS/CdS) [8e11] have attracted much attention nowadays, as the optical properties can be enhanced and the band gap can be tuned by the surface chemical bonds and charge transfer in the surface/interface region of QDs and QW structures. Chemical or
* Corresponding author. E-mail addresses: [email protected], [email protected] (R.R. Singh). 1 First authors: Both the authors will be considered as first authors as contributed equally. http://dx.doi.org/10.1016/j.optmat.2017.04.009 0925-3467/© 2017 Elsevier B.V. All rights reserved.
physical process can be used to prepare such type of structures [12,13]. These structures have unique and enhanced physical properties due to the core/shell interface effects. Numerous investigations have focused on forming the quantum well (QW) by sandwiching the low band gap material in the well between the layers of quantum dots of higher band gap material viz. ZnS/PbS/ ZnS [14], CdS/HgS/CdS [15], ZnS/CdSe/ZnS [16]. Literature survey reveals that core/shell type of QDs or QWs formed by over coating of low band gap material with high band gap materials exhibit high photoluminescence intensity as a result of elimination of surface nonradiative recombination [17]. Coating of QD cores with an inorganic shell, made of a material with a higher band gap eliminate the negative effects of defect states. Here, in this work we propose a procedure for obtaining highly luminescent QDs based on charge carrier confinement engineering. We propose a novel core-multishell ZnS/CdS/ZnS QD structure, which gives proper confinement to exciton by means of the moderate shell thickness, will be sufficient for reliable protection of excited carriers from the environment but not greatly increasing the QD diameter. To synthesize ZnS/CdS/ZnS QW structure the size of core (ZnS/CdS) was kept fixed with variable shell thicknesses. The thickness of shell was varied by using different cappent ratio of 2mercaptoethanol i.e 1 ml and 2 ml. The charge, functionality and reactivity of the narrow band gap material (CdS) can be improved by its surface modification with wider band gap material (ZnS).
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Consequently enhance the PL emission due to localization of the e-h pair [18e20]. It implies that the wave function of electron and hole may be spatially separated in core and shell, reducing the probability of non-radiative decay in the surface states and trap sites [21]. One more interesting point of our work is that as we coat core ZnS/CdS with ZnS in presence of 2-mercaptoethanol, the introduction of ZnS can greatly passivate the CdS surface to protect it from oxidation and prevent CdS leaching in to the surrounding medium [22,23]. The highlight and novelty of our work is to form and analyze the structural, optical and surface properties of highly luminescent wurtzite ZnS/CdS/ZnS QWs synthesized at low temperature (35 ± 1 C). It has been found that the synthesis of ZnS/CdS/ZnS QW also possible by different method like pulse laser deposition, chemical method in an air atmosphere. Beside this, many authors have also been reported on ZnS/CdS/ZnS QWs but they have opted different synthesis methods and particles were of cubic structures [24]. A few authors [25,26] have been reported the wurtzite ZnS/ CdS/ZnS but methods were different or they were used physical synthesis method on high temperatures. On the contrary in this report we propose a simple and cost effective wet chemical method which is very effective method to form the wurtzite QWs with good size controllability. We have attempted to change the functional groups at the surface of prepared QDs which helps to accommodate these QDs in bio-application purposes. We have employed X-ray diffraction (XRD), UVeVis spectroscopy, photoluminescence spectroscopy (PL) and Transmission electron microscopy (TEM) techniques to characterize ZnS, CdS, CdS/ZnS and ZnS/CdS/ZnS core/ shell QDs. Synthesis and analysis of wurtzite ZnS/CdS/ZnS grown on low temperature is being presented here for the very first time. 2. Experimental details 2.1. Characterization Techniques Different characterization techniques were used to study the structural and optical properties. XRD has been performed for structural investigation and the patterns were recorded in a Shimadzu powder x-ray diffractometer using Cu Ka1 radiation. Absorbance spectra of prepared samples were recorded by using PerkinElmer Lambda2 UVeVis spectrophotometer. Photoluminescence (PL) emission spectra and Photoluminescence (PL) Excitation spectra of the samples were recorded by a computer controlled rationing luminescence spectrophotometer LS55 (Perkin-Elmer Instruments, UK) with l accuracy ¼ ±1.0 nm and l reproducibility ¼ ±0.5 nm. A 20 kW pulse <10 ms from a Xenon discharge lamp was used as the excitation source for recording photoluminescence emission spectra. A gated photo multiplier tube was used as a detector. Prior to the PL experiments signal-to noise ratio was adjusted to 500:1, using the Raman band of water with excitation at 220 nm. Transmission electron microscopy (Hitachi H-7500) has been performed as the finest proof for the particle size and particle size distribution. 2.2. Synthesis of ZnS/CdS and ZnS/CdS/ZnS quantum dots The reaction matrix employed in our study consisted of AR grade CdCl2, NH4Cl and thiourea in 1:1.5:3 M ratios for CdS and AR grade ZnSO4, (NH4)2SO4 and thiourea in 1:1.5:1.5 M ratios for ZnS. Seed growth method was employed for synthesis of ZnS/CdS core/ shell structures. In order to synthesize desired ZnS/CdS core/shell, firstly we prepared ZnS QDs as seed by solution growth method at 35 ± 1 C with fixed amount of 2-Mercaptoethanol. Complexing agent was used to control the release of metal ions (Zn2þ) and sulphur ions (S2) to produce the controlled homogeneous reaction. The reaction matrix was prepared in two parts. The initial
solutions were prepared from anhydrous zinc sulphate (ZnSO4) and ammonium sulphate ((NH4)2SO4) in 50 ml of water. Ammonia (NH3) was used as a complexing agent until the formation of clear metallic complexes and so the pH was kept at 8.5. Then thiourea CS (NH2)2 was added as the source of sulphur in the prepared solution. Immediately 2 ml of (5% solution) 2-mercaptoethanol was added as a surfactant to control the particle size of QDs and continuously stirred for 5 h. After synthesis prepared sample of ZnS were centrifuged (3500 RPM, 20 min) and washed several times with distilled water and dried. To synthesize ZnS/CdS core/shell QDs, a CdS shell layer was grown on ZnS core nanocrystals. The method of shell formation of CdS was same as seed ZnS QDs just the source for cadmium that is cadmium chloride (CdCl2) and respective complexing agent ammonium chloride (NH4Cl) have took place. As the additional step for the synthesis of core/shell structures inclusion of 0.1 g of prepared ZnS QDs was mandatory just before the addition of thiourea. After that required amount of 2-mercaptoethanol was added in solution for capping to obtain ZnS/CdS core/shell structures. For ZnS/CdS/ZnS QW formation, now ZnS/CdS (0.2 g) has been used as seed and the remaining method is same as opted for ZnS QD synthesis. The size of ZnS/CdS core was kept fixed and varied the shell thickness of ZnS using varying cappent ratio. When the reaction was completed, purification of all core/shell nanocrystals has been performed. Then core/shell QDs were dispersed in distilled water and store. The process flow for the synthesis of ZnS/ CdS/ZnS QDQWs is presented in Fig. 1. ZnS/CdS/ZnS1 has thicker shell of ZnS as compared to ZnS/CdS/ZnS2. 3. Results and discussions 3.1. Structural characterization Some parameters like selection of precursors and temperature plays an important role, these parameters affects the crystal structure in case of CdS and ZnS QDs and ultimately helps to obtained desired crystal structures of QDs and QWs [27]. X-ray diffraction (XRD) spectra of 2-mercaptoethanol capped CdS, ZnS/ CdS, ZnS, ZnS/CdS/ZnS1 and ZnS/CdS/ZnS2 QDs have been shown in Fig. 2. Spectra of prepared QDs is in good agreement with JCPDS411049 in CdS and JCPDS 12-0688 in ZnS QDs. The prominent peaks (002), (101), (200) and (110) were indexed to the wurtzite structure in case of CdS and their core/shell structures. The prominent peaks in ZnS and their core QDs are indexed to (0010), (108), and (206) for wurtzite structure. Although there are no specific reports are available on synthesis of wurtzite ZnS and CdS quantum dots but some reports are available related to formation of II-VI group semiconductor nanocrystals wurtzite structure to understand the effect of synthesis parameters on structure formation [28e30]. In case of ZnS/CdS and ZnS/CdS/ZnS prominent peaks are same with small shift in 2 theta position (Table 1); this shift is clear indication for core/shell QDs formation. The crystallite size of all quantum dots (D) has been calculated by the most prominent peaks using Scherrer's formula [27].
D ¼ kl=b cos q
(1)
The calculated crystallite size from Scherrer formula is 3.1 nm, 2.3 nm, 3.5 nm, 4.3 nm and 3.9 nm for ZnS, CdS, ZnS/CdS, ZnS/CdS/ ZnS1 and ZnS/CdS/ZnS2 respectively. As can be seen from the figure in ZnS/CdS/ZnS1 the diffraction peaks of CdS and ZnS overlap each other and have intense peak. Whereas in case of ZnS/CdS/ZnS2 peak intensities decreased because ZnS shell put constraints on ZnS/CdS core. As the shell thickness reduced or the size of shell reduced the core closed inside ZnS shell experience more compressive strain, due to this the intensity of peaks obtained in ZnS/CdS/ZnS2 get
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Fig. 1. Schematic representation of the synthesis procedure to obtain ZnS QDs, ZnS/CdS core/shell QDs and finally ZnS/CdS/ZnS QW structures.
reduced. Diffraction peaks used for calculation of crystallite size along with FWHM values are tabulated in Table 1.
3.2. Transmission electron microscopy
Fig. 2. X-ray diffraction patterns of 2-mercaptoethanol capped CdS, ZnS, ZnS/CdS, ZnS/ CdS/ZnS1 and ZnS/CdS/ZnS2 QDs.
To know shape and size of samples TEM of prepared QDs, core/ shell QDs and QWs has been performed. The results are being presented in Fig. 3. TEM images of ZnS (Fig. 3(a)) and CdS (Fig. 3(b)) QDs show highly mono-dispersed nanoparticles with average sizes of 3.3 nm and 2.5 nm respectively. Fig. 3 (c) indicates the micrograph for ZnS/CdS core/shell QDs, still there is indication of the formation of mono-disperse QDs with 4.2 nm particle size. Similarly ZnS/CdS/ZnS2 QW (Fig. 3(d)) has 4.6 nm particle sizes and (Fig. 3(e)) shows histogram for confirmation of particle size with respect to Fig. 3(d). It was confirmed from (Fig. 3(e)) that ZnS/CdS/ ZnS2 QW contain maximum particles of size 4.6 nm. These figures clearly signify that there is no agglomeration of QDs even after the formation of QWs; it indicates the capability to obtain either the fine solution or even powder of uniformly distributed nanoparticles for various applications. Fig. 3(c) of wurtzite ZnS/CdS QDs is depicted from Fig. 3(c) where core (ZnS/CdS) has been surrounded by the shell (ZnS) as the electron density of the CdS and ZnS is almost similar it is difficult to differentiate between the layers, and Fig. 3(d) shows the formation of wurtzite ZnS/CdS/ZnS2 QW, in this case one can observe the different shades on a single QW this may be due to the presence of three different layers in a system with comparable electron densities and that is why it is difficult to differentiate between the layers of the QW. This result of wurtzite structure formation also supports our findings from XRD results. Whereas, the particle sizes obtained from TEM are very slightly greater than the size obtained from XRD analysis. The size obtained
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Table 1 Structural parameters of ZnS, CdS, ZnS/CdS, ZnS/CdS/ZnS1 and ZnS/CdS/ZnS2 quantum dot structures. Samples
2q (Degree)
FWHM (Degree)
Particle size (nm)
Shift in 2q (Degree)
ZnS CdS ZnS/CdS ZnS/CdS/ZnS1 ZnS/CdS/ZnS2
28.68 28.98 28.99 28.79 28.68
2.88 3.82 2.55 2.09 2.29
3.1 2.3 3.5 4.3 3.9
e e 0.31 0.20 0.31
by XRD and TEM has the correlation that the XRD size is usually equals or smaller than the size obtained by TEM [31]. There is no big disparity in the particle sizes obtained from different ways; also it is clearly vindicated from the micrographs that the as prepared nanoparticles are of great quality. 3.3. Optical studies 3.3.1. Absorbance spectroscopy We demonstrate the optical absorbance spectra of CdS, ZnS/ CdS, ZnS/CdS/ZnS1 and ZnS/CdS/ZnS2 QWs (Fig. 4). ZnS/CdS core have the absorption peak at 430 nm, which corresponds to core of 3.3 nm, which is highly blue shifted from the CdS. After formation of ZnS/CdS/ZnS1 shell, 1s peak has been red shifted from ZnS/CdS to ZnS/CdS/CdS1. Similar observations of red shift due to shell growth have been observed in other ZnS/CdS/Zns1, ZnS/CdS/ZnS2 systems [32e34]. On the formation of a ZnS shell on ZnS/CdS, the 1s peak red shifted due to the size increment of QW and exciton, remains “spatially direct”, as indicated by the presence of a welldefined 1s absorption maximum. As the shell thickness is further increased, we observe a sharp absorption peak (in ZnS/ CdS/ZnS2, ZnS/CdS/ZnS1) indicating a transition in only shell to shell localization characterized by the increased e-hþ overlap [35]. In Fig. 4 small shell thickness shows smaller shift and higher shell thickness shows larger shift. Among them some important reasons are increment in the effective volume of exciton; as the electronic levels come closer that accompanies an increment in the size; and increased leakage of the exciton into the shell [36,37]. 3.3.2. Photoluminescence spectroscopy Consequence of size; and shell thickness has been studied in this work. To identify the change in luminescence behavior the core that is ZnS/CdS (core/shell) and ZnS/CdS/ZnS QW structures have been studied and discussed on the basis of the model presented in Fig. 5. It is easy to understand from the figure that how the luminescence is changing as the ZnS shell thickness varies. The experimental findings resemble with the model. The emission spectra of CdS, ZnS/CdS and ZnS/CdS/ZnS QWs with different thicknesses of ZnS shell are presented in Fig. 6. A broad emission band for CdS with maximum at 546 nm is observed. It is also shifted to blue in comparison with the bulk CdS. The emission band at 430 nm corresponds to ZnS/CdS core shell QDs; this can be noted that the luminescence has been increased shifted to blue with respect to CdS. Further in ZnS/CdS/ZnS there is increment in luminescence with an efficient tunability. This is because the wave functions of electron and hole in ZnS/CdS/ZnS1 are delocalized and in ZnS/CdS/ ZnS2 the wave function of electron mostly localized in shell but hole's wave function localized preferably in core. Localization of electron-hole in the system can lead to the superior PL enhancement and fine tunability [38]. PL emission intensity was low when the thickness of ZnS shell was high at this point 1 ml capping agent has been used. On increasing the cappent to 2 ml shell thickness was decreased and PL emission intensity was increased and can be clearly illustrated from
the emission of ZnS/CdS/ZnS2. The thinnest achievable shell of ZnS possesses the largest band gap and therefore, produces the maximum confinement potential for excited charge carriers. Conversely the conduction and valence bands move towards their bulk energies, resulting in the decrease in the effective energy barrier. In other way if the shell thickness increased, it can be physically extend the tunneling length, necessary for electron and hole interaction with the external medium, which confines the photo-generated carriers inside the photo-luminescent core. CdS creates a smaller interfacial strain, but its potential barriers are weaker than as compared to ZnS. Therefore, an approach combining the benefit of both the high potential barrier of ZnS and lattice compatibility of CdS was proposed in this work [39]. The emission corresponds to the bulk donoreacceptor recombination of CdS. Surface states usually act as radiation less recombination centers. The ZnS shell passivates the vacancies that are presented on the surface of the CdS and reduces the nonradiative recombination sites. These are responsible for the luminescence enhancement. The lattice parameters of CdS differ from that of ZnS by 7%. About one-unitcell-thick coating layers can accommodate the lattice misfit between CdS and ZnS by elastic strain exclusively [24]. If the shell is thicker several order more than one-unit cell it can increase strain due to the lattice mismatch and could direct the formation of dislocations. Some reduction in the luminescent intensity is ascribed to the creation of misfit dislocations, which act as centers for radiationless recombination when the thickness of the shell increases [24]. On the other hand, with the increasing amounts of ZnS, the concentration of the CdS luminescence centers decreases which results in some of the reduction in the luminescence intensity. 3.3.3. Quantum yield measurement Quantum yield is the basic property for any luminescent material. Therefore, its measurement is an important step. Here we measured the quantum yield of our prepared samples by using relative method. Equation (2) is presenting the formula used for quantum yield calculation by relative method [40].
QYs ¼ QYr Is =Ir Ar =As
2 ns nr
(2)
where s refers to sample, r refers to reference (standard). IS and Ir referred to integrated intensity of sample reference respectively. nS and nr referred to refractive index of sample and reference respectively and As and Ar refers to absorbance at excitation wavelength for sample and reference respectively. We have chosen 40 -6- diamidino-2-phenyl-indole (DAPI) as reference because it is very common to measure yield and having emission around 400e550 nm. Emission peak positions for all the prepared quantum dots also lie in this range. Quantum yield of DAPI is 0.58 for [41]. Same solvent was taken for sample and reference. Quantum yield calculated for all samples along with PL peak position, intensity; integrated intensity and FWHM are presented in Table 2. It is concluded from Table 2 that CdS has low quantum yield as compare to ZnS/CdS. This is because shells provide stability to core in case of
H. Kumar et al. / Optical Materials 69 (2017) 23e29
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Fig. 3. TEM micrographs (a) ZnS QDs; (b) CdS QDs; (c) ZnS/CdS core/shell QDs; (d) for ZnS/CdS/ZnS2 with smallest shell thickness (Dimension of the outer box in inset figures is 5 nm 5 nm) and (e) Histogram of ZnS/CdS/ZnS2.
ZnS/CdS. In case of ZnS/CdS/ZnS1 although the quantum yield is low as compared to its core ZnS/CdS but low value of FWHM confirms that ZnS/CdS/ZnS1contain particle monodispersity. Similarly ZnS/CdS/ZnS2 have better quantum yield as compared to ZnS/CdS/
ZnS1 with narrower FWHM. So that it is concluded that ZnS/CdS/ ZnS2 has good fluorescence and quantum yield with better monodispersity and fine tenability with respect to CdS and ZnS/CdS QD structures.
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Fig. 4. Absorption spectra of CdS, ZnS/CdS, ZnS/CdS/ZnS1 and ZnS/CdS/ZnS2. Fig. 6. Photoluminescence spectra of CdS, ZnS/CdS, ZnS/CdS/ZnS1 and ZnS/CdS/ZnS2.
Table 2 Quantum yield along with emission peak position, intensity; integrated intensity and FWHM. Samples
PL peak position (nm)
Intensity
Intensity (integrated)
FWHM(nm)
QY
CdS ZnS/CdS ZnS/CdS/ZnS1 ZnS/CdS/ZnS2
546 430 505 505
250 610 790 910
560 882 368 625
70 70 34 28
0.40 0.70 0.32 0.41
well, which were assumed to act as centers for radiation less recombination. Furthermore, PL spectra show a strong emission in QW centered at 505 nm. The strong emission at 505 nm is from the charge transfer of electrons from CdS electron to ZnS holes by excitation energy. Quantum yield of these quantum dot structures has also been measured and results indicate about high monochromaticity in the quantum well structures with high yield. These ZnS/CdS/ZnS are a promising optical material in the applications to various transparent green optoelectronic devices and in imaging and diagnostics in medical field. Acknowledgements Fig. 5. Schematic of ZnS/CdS/ZnS QW structures with different ZnS shell thickness with corresponding PL behavior.
4. Conclusion Highly luminescent ZnS/CdS/ZnS QW with narrow PL emission were successfully synthesized by wet chemical method at 35 ± 1 C. The QDs were characterized by XRD, TEM, UVevis absorption spectroscopy and photoluminescence spectroscopy. The TEM images showed that the QW structures were wurtzite and monodispersed. Strong carrier confinement in luminescent core (ZnS/ CdS) using appropriate ZnS outer shell layer has been obtained and the outcomes reflect from the luminescence studies. Luminescence in the region of 430 nme550 nm was observed. The luminescence from CdS was enhanced by the complete ZnS shell, which may be the result of the decrease in the defects on the surface of the CdS
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