Microstructural and optical properties of CdSe / CdS / ZnS core-shell-shell quantum dots

CdSe/CdS/ZnS core-shell-shell quantum dots (QDs) were synthesized by using a solution process. High-resolution transmission electron microscopy images and energy dispersive spectroscopy profiles confirmed that stoichiometric CdSe/CdS/ZnS core-shell-shell QDs were formed. Ultraviolet-visible absorption and photoluminescence (PL) spectra of CdSe/CdS/ZnS core-shell-shell QDs showed the dominant excitonic transitions from the ground electronic subband to the ground hole subband (1S(e)-1S3/2(h)). The PL mechanism is suggested; the carriers generated by the exciting high-energy photons in the shell region are relaxed to the bandedge states of the core region and recombined to emit lower-energy photons. The activation energy of the carriers confined in the CdSe/CdS/ZnS core-shell-shell QDs, as obtained from temperaturedependent PL spectra, was 200 meV. 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Introduction
Semiconductor core-shell quantum dots (QDs) have been particularly attractive because of interest in investigations of fundamental physics and their promising potential applications for electronic and optoelectronic devices, such as memory devices, light-emitting devices, and solar cells [1][2][3][4][5].The prospect of potential applications of core-shell QDs has driven extensive efforts to control precisely the core sizes and the shell thicknesses of the QDs because their optical and electronic properties significantly depend on these parameters [6][7][8].Among various kinds of core-shell QDs, CdSe/ZnS core-shell QDs [9][10][11][12][13][14][15][16] are one of the most intensively studied kinds; the CdSe core can cover the most of the visible spectrum depending on its size and the ZnS shell with its large bandgap can confine the carriers well.However, CdSe/ZnS core-shell QDs suffer from the defects due to a large lattice mismatch between CdSe and ZnS, and CdS middle shell with intermediate lattice constant was introduced to form CdSe/CdS/ZnS core-shell-shell QDs [17].The stable organic bistable memory devices were fabricated utilizing CdSe/ZnS core-shell QDs [18], and the luminance efficiency of the organic light-emitting devices was enhanced due to the insertion of the active layer containing CdSe/CdS/ZnS core-shell-shell QDs [19].Even though some studies concerning the formation and the basic physical properties of the CdSe/CdS/ZnS core-shell-shell QDs have been conducted [19][20][21][22][23], investigations on their interband transitions and activation energy have not been reported yet.Furthermore, studies on the interband transitions and activation energy of CdSe/CdS/ZnS core-shell-shell QDs are very important for improving the efficiencies of electronic and optoelectronic devices based on core-shell-shell QDs.
This paper presents data for the microstructural and optical properties of CdSe/CdS/ZnS core-shell-shell QDs.Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) measurements were performed to characterize the microstructural and the stoichiometric properties of the CdSe/CdS/ZnS core-shell-shell QDs.Absorption and temperature-dependent photoluminescence (PL) measurements were carried out in order to investigate the interband transitions and to determine the activation energy and the quantum efficiency in the CdSe/CdS/ZnS core-shell-shell QDs.
TEM measurements were performed using JEOL 2010EX and Philips Tecnai F20 with acceleration voltage of 200 kV.The samples for the cross-sectional TEM measurements were prepared by cutting and polishing with sandpaper to a thickness of approximately 10 µm, and were then argon-ion milled to electron transparency.The absorption spectra were measured by using a VARIAN Cary 100 Conc UV/vis spectrophotometer.The PL measurements were carried out using a 50-cm monochromator equipped with an RCA 31034 photomultiplier tube.The excitation source was the 4416-Å line of a He-Cd laser, and the laser power density was 10 mW/cm 2 .The sample temperature was controlled between 10 and 350 K by using a He displex system.The synthesis procedure of CdSe/CdS/ZnS core-shell-shell QDs was described elsewhere [20].The series of the reaction steps for the synthesis of CdSe/CdS/ZnS core-shell-shell QDs are schematically shown in Fig. 1, and described in detail in the followings.0.103 g of CdO, #253011 0.901 g of oleic acid and 20 ml of tri-n-octylamine were loaded into a three-necked flask.The mixture was degassed under vacuum to remove the oxygen and water at 150°C for 1 h.The flask was heated to 300°C by using a heating mantle under N 2 atmosphere, and then the 0.2 ml of TOP-Se solution (1M, Tri-n-octylphosphine base) was injected into the flask at 300°C.The mixture was stirred for 90 sec, and then 0.1 ml of 1-dodecanethiol and 3 ml of tri-noctylamine were added slowly into the flask at rate of 0.5 ml/min and held at 300°C for 40 min.To form the ZnS shell, 8 ml of Zn Oleate solution was added over 8 minutes in the flask containing CdSe/CdS core-shell QDs at 100°C, and then 0.56 ml of 1-dodecanethiol and 3 ml of tri-n-octylamine were added slowly to the flask at rate of 0.5 ml/min and held at 100°C for 30 min.The solution was workup by using ethanol to produce CdSe/CdS/ZnS core-shell-shell QDs. Figure 3 shows the linear UV-visible absorption spectrum of CdSe/CdS/ZnS core-shellshell QDs dispersed in toluene at room temperature.The dominant excitonic transitions from the ground electronic subband to the ground hole subband (1S(e)-1S 3/2 (h)) are clearly observed [23,24].Two shoulders, one at 538 nm and the other at 498 nm correspond (1S(e)-2S 3/2 (h)) and (1P(e)-1P 3/2 (h)), respectively [23,24].Three clearly resolved lowest excitonic transitions in the absorption spectrum, together with a narrow emission linewidth of the PL spectrum, indicate the high quality of the CdSe/CdS/ZnS core-shell-shell QDs [25].When the UV light with a wavelength of 365 nm is exposed to the CdSe/CdS/ZnS coreshell-shell QDs dispersed in toluene, after the CdSe/CdS/ZnS core-shell-shell QDs absorb the UV lights, the photons with a wavelength of 604 nm are emitted, as shown in Figs.4(a) and 4(b).The schematic diagram of the optical processes in the CdSe/CdS/ZnS core-shell-shell QDs is shown in Fig. 4(c).The CdSe/CdS/ZnS core-shell-shell QDs allow for a spectral separation of the absorption and emission by confining the two processes to different regions [26].The fundamental operation process begins with an absorption of the incident photon, in which high photon energies might create electron-hole pairs in the states predominantly located in the shells [27].Then, electrons and holes are relaxed rapidly to the band-edge states, which are confined within the CdSe core.Finally the exciton recombines radiatively, resulting in the emission of the photons with a different wavelength in comparison with that with incident photons [27].This carrier movement is in contrast with the case of CdSe/CdS/ZnO nanowires in which electrons move to the ZnO shell and holes move the CdSe core [28].Figure 5 shows a PL spectrum for the CdSe/CdS/ZnS core-shell-shell QDs at several temperatures.The dominant peak corresponds to the (1S(e)-1S 3/2 (h)).The peak energy is 2.056 eV at 10 K, and shifts to lower energy with increasing temperature, reaching about 1.968 eV at 300 K.The PL integrated intensity tends to decrease as the temperature increases.The PL quenching at higher temperatures is caused by an escape of carriers via a nonradiative recombination path [29].However, the integrated PL intensity significantly increases with increasing temperature from 100 to 200 K.This behavior is attributed to the presence of the barriers induced by the strain fields at the interfaces formed between the CdSe/CdS/ZnS core-shell-shell QDs [30].As shown in Fig. 6(a), the temperature dependence of the intensity can be fitted using the equation I = I 0 / {1 + C[exp(-E A /k B T)]} [31], where I 0 and C are constants and E A is an activation energy that corresponds to the barrier height for the non-radiative recombination path.The activation energy of the carriers confined in the CdSe/CdS/ZnS core-shell-shell QDs, determined by fitting integrated PL intensities as a function of the reciprocal temperature [31], is 200 meV.From the temperature-dependent PL spectra, the quantum efficiency (QE) for the CdSe/CdS/ZnS core-shell-shell QDs at 300 K is analyzed by using the equation of QE(300 K) ≈QE(0 K) × [I(300 K)/I(10 K)] [32].Because the non-radiative recombination processes for the CdSe/CdS/ZnS core-shell-shell QD is negligible at low temperatures, the QE at 10 K approaches 100% [33].The ratio [I(300 K)/I(10 K)] was estimated from the integrated PL intensities.These results demonstrate that the QE for the CdSe/CdS/ZnS core-shell-shell QDs at 300 K is estimated to be about 57%, as shown in Fig. 6(b).

Summary and conclusions
CdSe/CdS/ZnS core-shell-shell QDs were synthesized and their microstructural and optical properties were studied.HRTEM images and EDS profiles showed the formation of the stoichiometric CdSe/CdS/ZnS core-shell-shell QDs.Ultraviolet-visible absorption and PL spectra of CdSe/CdS/ZnS core-shell-shell QDs showed the (1S(e)-1S 3/2 (h)) dominant excitonic transitions together with (1S(e)-2S 3/2 (h)) and (1P(e)-1P 3/2 (h)) excitonic transitions.The temperature-dependent PL spectra showed that the peaks corresponding to the (1S(e)-1S 3/2 (h)) of the CdSe/CdS/ZnS core-shell-shell QDs shifted to a lower energy side with increasing temperature, the amount of shift being 88 meV between 10 and 300 K.The activation energy of the CdSe/CdS/ZnS core-shell-shell QDs, as determined from the temperature-dependent PL spectra was 200 meV.The QE of the CdSe/CdS/ZnS core-shellshell QDs at 300 K was approximately 57%.The present results help in improving the understanding of the microstructural and optical properties of CdSe/CdS/ZnS core-shell-shell QDs.

Fig. 4 .
Fig. 4. Images of (a) before and (b) after exposures of UV excitation wavelength of 365 nm for CdSe/CdS/ZnS core-shell-shell quantum dots.(c) Schematic diagram of the optical processes in the CdSe/CdS/ZnS core-shell-shell quantum dots.

Fig. 6 .
Fig. 6.(a) Integrated photoluminescence intensity as functions of the reciprocal temperature for CdSe/CdS/ZnS core-shell-shell quantum dots.Solid rectangles represent experimental data and the solid line indicates the fitting curve.(b) Quantum efficiency as functions of the temperature for CdSe/CdS/ZnS core-shell-shell quantum dots.