Fabrication and Characterization of a Light-Emitting Device Based on the CdS/ZnS Spherical Quantum Dots

: In this work, we focus on the colloidal quantum dot based light-emitting diodes (QD-LEDs) performance. First, we synthesize the spherical QDs with a CdS core that covered with a wider band gap II – VI semiconductor acting as a shell (ZnS). In order to synthesize this nano crystal QDs with structure of CdS/ZnS/CdS/ZnS, we use a reverse micelle process. These four-layer quantum well quantum dots (QWQDs) can generate the white light emission. The positional design of different layers i.e., core/shell QD emitters is a critical factor for white emissive devices. The blue emission generated by CdS core mixes with green/orange components originating from ZnS inner shell and creates an efficiency white light emission. Then, we fabricate a white-QDLED with a device structure of ITO/ ZnO / QD / CBP/ MoO 3 / Al films. A thin film of CdS/ZnS/CdS/ZnS QDs is deposited by electrostatically assembled colloidal QD solutions. The experimental results show that the emission spectra of QDs and current density through the LED are controlled by varying the particle sizes. The peaks of absorbance and Photoluminescence (PL) spectrums get the red shifted as the dot size increases. Furthermore, this QD-LED with a smaller nano particle layer has a higher current density.


Introduction
Recently, QD-LEDs have been extensively investigated due to their unique and fantastic optical properties of colloidal synthesized quantum dots (QDs) [1][2][3][4]. QD films exhibit easy color tenability over the entire visible spectrum, broad absorption spectrum and high photoluminescence quantum yield. These attractive properties and advantages make them to be promising for the light emitting applications [5][6][7][8][9]. Solid state white light emitting diodes (WLEDs) as the new generation of light sources can offer the chance to lower thermal resistance and longer lifetime [10,11]. Group II-VI metal chalcogenide semiconductors specially CdS, CdSe and ZnS have been of interest because they show high photoluminescence (PL) in the visible region. Emission from the QD-LEDs depends strongly on the size of the QDs and their composition [8]. Cadmium compounds QDs arising from the color-saturated photoluminescence (red, green, blue) have attracted more attention as a good potential utility of the design and fabrication of the QD-based WLEDs. CdS has the best PL and is also widely used in solid state applications [12][13][14][15]. ZnS is another important semiconductor to investigate. It is extensively studied as a luminescent material for optoelectronic application [5,12]. Many different deposition techniques have been utilized to affect device performance and improve the optical properties of LEDs [16][17][18]. Some authors reported that the core-shell type nanoparticles such as CdSe/ZnS [5] are good candidates for white emission QD-LEDs. In this type of nanostructures that are composed of two different semiconductors, first semiconductor is the core and the second one, with a wider band gap than the core, is the shell. Enhancing the light-extraction efficiency is one of the key issues for realizing highly efficient LEDs. The experimentally researchers introduced several features to improve this efficiency in AlGaN-based diodes [19]. It has been demonstrated that the bright and efficient QD-LEDs fabricated from thin and uniform CdSe/ZnS colloidal QD films show high optical quality [20] and have maximum luminance and current efficiency [5].
This study aimed to investigate experimentally the performance of the QWQD diodes. In the present work, CdS/ZnS/CdS/ZnS QDs are prepared by a reverse micelle process. In this nano structure, blue emission from CdS core combines by the green and red emissions from ZnS inner shell and this recombination products efficiently white light emitting. In the following, we fabricate a deposition of luminescent QDs film by electrostatically assembled colloidal QD solutions. The experimental results have shown that the QDs size can manage the absorbance and PL spectrums. Finally, we investigate the influences of the dot size on the current density and present how this parameter can control the current density through this six layer WLED.

Experimental
White light emitting diode based on the CdS/ZnS/CdS/ZnS QDs is prepared by following two synthetic protocols. First, we have to synthesize the colloidal QDs, then we fabricate the a QD-WLED with ITO/ ZnO / QD / CBP/ MoO3 / Al structure.

CdS/ZnS QDS Synthesis
At first, we need solutions of Cd (CH3 COO)2 .2H2 O, Na2 S and Zn (CH3 COO)2 precursors in water with concentrations 0.1M and 0.26M, respectively. Now, we solve the dioctyl sulfosuccinate sodium salt (AOT) in heptane to prepare 0.1M surfactant solution and then add it to solution of each precursor. It should be mentioned that the volume ratio = [ 2 ] [ ] must be the same for Cd 2+ , Zn 2+ and S 2solutions. CdS core is formed after 15-minute reaction of 70cc S 2with 12cc Cd 2+ micellar stock solutions. In order to generate CdS/ZnS core/shell QDs, we slowly add 20cc Zn 2+ , 12cc Cd 2+ and finally 50cc Zn 2+ solutions to CdS core [12]. Finally, the resulting QWQDs are washed with heptane and methanol and dispersed in methanol for characterization.

QDLED Fabrication
The FTO-coated glass substrates were cleaned in ultrasonic baths of detergent, de-ionized water, and acetone, then were immediately put it in the oven. After the above cleaning, fabrication of the QD-LEDs starts with a zinc oxide (ZnO) electron-transport layer formation by spin-coating method of zinc acetate solution onto the FTO and heating it at 300 ° C for 5 min. Then, a thin QDs film as the emissive layer is electrophoretically deposited in an electrophoretic deposition (EPD) process [21][22][23][24]. In EPD procedure, we applied 45 Vcm -1 electric field for 5 min on two ZnO-on-FTO substrates fixed in 4 mm apart from each other. After that, we transferred the samples into a vacuum chamber to deposit the dicarbazolebiphenyl (CBP) as a hole transporting layer, molybdenum oxide (MoO3) as a hole-injection layer, and an aluminum (Al) by sequential thermal evaporation. The same method is described in detail in Ref. [20]. A schematic of the device structure and cross-sectional view of the multilayer white-emitting QDLED is shown in Fig. 1. Our QD-LED structure was designed to reach efficient electron and hole injection from these electrodes to the QDs. We also required to effectively block electrons and holes passing through the QD layer. It is well known that the ZnO particles is highly advantageous for QLEDs with conventional structures owing to high electron mobility (2 × 10 -3 cm 2 V -1 s -1 ) and low temperature behavior.

Results and discussion
The proposed spherically grown QDs composes of two different emitter layers in which each layer can emit specific wavelengths in the visible range (RGB spectrum). In introduced nano structure QD, CdS as a core is coated by shell ZnS. The core CdS with band gap =2.58 can emit the blue spectrum and shell ZnS can emit both of the green and red spectrums. Hence, this novel type of QWQD achieves efficiently white light emission. The absorption and Photoluminescence spectra were investigated to explore the optical properties of theses QDs. Experimental results have shown that the dot size has central role in control of the output light properties.
The absorbance spectrums of WLED at different time after the synthesis are shown in Fig. 2.
The water-to-surfactant molar ratio is W=5. The absorbance spectrums show two peaks centered at ≅480 nm for core CdS and ≅525 nm for core/shell CdS/ZnS that exhibit slightly red shift with time passing. CdS/ZnS QDs samples have higher stability than CdS core and retain their PL properties for longer time. After time focusing, we investigate the QDs size dependence of the absorption and PL spectra. QWQDs emission can be tuned by varying the dot size, which is easily controlled by [25][26]. Particle size of dots in reverse micelle method decreases with the reducing of values as (1 + 15 ) 3 − 1 = 27•5 [12], thus it leads to a blue-shift of emission wavelength.  Fig. 3. At higher magnitudes of parameter, PL spectra of QDs shifts to right hand due to the increase of particle size.
The absorbance spectrums as a function of the for three different values of are shown in Fig. 4. This figure reveals the absorption spectra from two important observations: (a) absorbance decreases with the decrease in the core thickness; (b) the absorbance peak gets blue shifted as the core size decreases in which the peak at =481nm for =10 shifts to =484nm for =6 and =486nm for =3. Similar performance was achievable for CdS/ZnS core/shell absorbance spectrum (Fig. 5). As mentioned, increasing of values corresponds to a red-shift of emission wave length. A red shift of about 25 nm in the absorption peak was observed. The absorption spectra of CdS/ZnS shifted from 515 nm to 540 nm due to the increasing. Moreover, the absorbance of core/shell nanocrystals strongly depends on the thickness of the passivating shell. CdS QDs with the passivation of ZnS shell shows higher stability of the PL efficiency than a CdS core. Figs. 6 and 7 show the absorbance as a function of the emission wave length at two different thicknesses of the ZnS shell. First we synthesize the core/Shell CdS/ZnS, then we decrease or increase the shell thickness to prepare core/half shell ( Fig.  6(b)) or core/double shell (Fig. 7(b)). The change in absorbance peak is determined by the shell thickness and thus the dot size.  It should be mentioned that the defect emission of ZnS inner-shell can create both of the green emission peaked at ≅ 525 nm and orange emission at ≅580 nm [12,26]. So, in presented structure, blue emission from core CdS can combine with green and red emission from the shell ZnS and generates the white light.
We manage dot size to study the absorbance, PL intensities and chromaticity coordinates diagram of the output light [12,28]. Absorbance, PL intensity and CIE coordinates for QDs with W=6 and core/ double shell layer structure are shown in Fig. 8. As shown in this figure, water-to-surfactant molar ratio plays an important role in optical properties of QDs emission. In following, we demonstrate how parameter can manage the current density of QD-LED fabricating with thin and uniform CdS/ZnS/CdS/ZnS colloidal QD film.  = 6 exhibits that the QDLED with larger nano particle layer ( = 3) has a higher current density. Thus, the increases in the QD size result in increased current density. From the enlarged current density, we can conclude that QD layer with W=3 is superior to that for W=6 in our device structure. Finally, Fig. 10 presents the electroluminescence (EL) spectra obtained from the fabricated Cds/ZnS/CdS/ZnS QD-LED for = 3 . This spectrum has two distinct peaks; the blue emission peak at ≅ 450 nm due to CdS and a red emission peak at ≅585 nm due to ZnS, as we expected.

Conclusion
In summary, we report the white light emission QWQDs enable us to fabricate the WLED based on the thin and uniform CdS/ZnS/CdS/ZnS colloidal QD film. Two-layer CdS/ZnS as a core/shell QD structure result the white light emission in which the blue and green/orange emission originate from CdS core and ZnS shell, respectively. Experimental results are shown that the emission properties and the current density critically depend on the water-to-surfactant molar ratio or dots size. We find a blue shift of absorbance and PL spectrums and smaller current density for mentioned structure at larger dot size. Figure 1 Cross-section schematic of a white-QLED with a device structure of ITO/ ZnO / QD active layers/CBP/ MoO3/ Al lms. ZnO layer was prepared by spin coaing, QDs lm was formed by EPD technique and CBP, MoO3, and Al were deposited by sequential thermal evaporation on top of the ZnO/QD layers.   Absorbance spectrum for core CdS at various water-to-surfactant molar ratio, a) W=10 and b) W=6 and (c) W=3.

Figure 5
Absorbance spectrum for CdS/ZnS core/shell when W values decreases from 10, 6 to 3.       EL spectra for the fabricated QDLED.