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Manganese-doped cadmium selenium quantum dots (Mn-doped CdSe QDs) were successfully synthesized in the absence of trioctylphosphine (TOP) at various ripening temperatures and times. The TEM image shows the QDs with average particles size of 5.3nm with almost spherical shape. The optical properties of Mn-doped CdSe QDs were investigated using ultraviolet-visible and photoluminescence spectroscopy. The behavior of Mn-doped CdSe QDs was monitored using the red-shift characteristics in the UV-Vis absorption spectra, and their size variations were estimated by the quantum confinement theory. The PL emission spectra of Mn-doped CdSe QDs shows similar evolution behavior as in the absorption behavior. Quantum confinement allows tuning of the CdSe bandgap energy across the Mn excited-state energies. The origin of stokes shift were discovered.
© 2016 Optical Society of America
1. Introduction
Doping nanocrystals with transition metal ions such as Mn2+ produces diluted magnetic semiconductor that could be used in spintronic devices. This is due to the effect of giant Zeeman splitting effect which results from the exchange interaction between the transition metal ions and the electronic states of the host nanocrystals [1]. By incorporating Mn2+ into Quantum Dots (QDs), the paramagnetic impurity provides a means of coupling the optical and magnetic properties of these materials via sp-d exchange interaction [2]. As a result, the control of doping nanocrystals with transition metal ions has become fundamental interest in scientific research as well as in the application of spintronics.
Although doping with transition metal ions can impart remarkable magneto-optical properties to QDs, an undesirable side effect by energy transfer is also often leads to quenching of excitons to dopants [3]. The Mn doping into CdSe has been highly limited due to the self-purification. However, much effort has been made to realize Mn2+ doping in II-VI semiconductor in order to produce new materials. Self-assembled Mn2+-doped CdSe QDs have been prepared using physical method such as molecular beam epitaxy (MBE) [4]. Meanwhile, the synthetic challenge of incorporating Mn2+ within colloidal CdSe QDs has hindered investigation of photophysical phenomena [5]. Mikulec et al [6], has successfully synthesized Mn-doped CdSe QDs using specially designated precursor at high temperature pyrolysis. Similar attempts has also been made by Kwak et al [7] and Oluwafemi et al [8] to synthesized Mn-doped zinc blende using simpler precursor.
It has been shown that at the initial nucleation and growth stage, the size of nanocrystal is primarily determined by the precursor concentration, capping ligand and precursor anion types, and temperature [9, 10]. However, after nucleation and growth, ripening dominates between the nanoparticles. The final size of nanocrystals is thus determined by the temperature and the time period of ripening, as well as the colour of the emitting light [11, 12]. Hence, the precise control of ripening is a critical issue for the applications of CdSe.
Here, we report the ripening behavior of CdSe Mn-doped using inverse micelle technique. Quantum confinement effect is occurred as the size of QDs is tunable with ripening temperature and time.
2. Experimental
0.5g of Mn acetate, 0.5 g of CdO, 25 ml paraffin oil and 15 ml oleic acid were loaded in a three neck round bottom flask. The mixture was placed in the glove box at vacuum condition. The solution was heated to 160 °C and stirred until CdO was completely dissolved and a light yellowish homogeneous solution was obtained. Then, 0.079 g Se in 50 ml paraffin oil was carefully heated to 230 °C and stirred in another three neck round bottom flask. The solution colour turned to light orange and then became wine red. 5ml of Mn-Cd solution was then swiftly injected into the Se solution during rapid stirring. The temperature was dropped to 210 °C immediately after the injection, then rose back to 230 °C. The heating process was immediately stopped when the temperature reached 230 °C. 2 ml of CdSe-Mn doped solution were carefully injected out [12, 13]. Samples were reheated at 230, 240 and 250 °C for 15 and 30 min ripening time, respectively.
The absorption spectra of Mn-doped CdSe QDs were obtained by using UVIKON 923 Double Beam UV-vis spectrophotometer. The photoluminescence (PL) spectra were obtained using Perkin Elmer LS55 fluorescence spectrophotometer with Xe lamp excitation source. TEM images were acquired using transmission electron microscopy (TEM) LEO LIBRA operating at 120 kV.
3. Results and discussion
Figure 1 shows the TEM image of Mn-doped CdSe QDs. The image shows that the QDs are almost spherical in shape, compact and dense. Histogram in Fig. 1 (insert) exhibited narrow particles size distribution with average QDs size is 5.3 nm.
Figure 2 shows the absorption spectra of Mn-doped CdSe QDs at various reaction times and temperatures. As the temperature and reaction time increases, the absorption peaks are shifted to the longer wavelength. This indicates that the QDs size are increases with the increase of temperature and time.
Figure 3 shows the absorption spectra of Mn-doped CdSe QDs at 230 °C for different reaction times. As the reaction time increases, the locations of the first exciton appeared at 600, 606 and 609 nm with respective reaction time of 0, 15 and 30 mins. As the time increases, there are changes in slope of the temperature absorption curves where the slopes are steeper at longer reaction time and higher temperature.
Figure 4 shows the absorption spectra of Mn-doped CdSe QDs at 240 °C for different reaction times. As the reaction time increases, the locations of the first exciton appeared at 612, 615 and 617 nm with respective reaction time of 0, 15 and 30 mins. Like in Fig. 3, there are changes in slope of the curve where the slopes are steeper at longer reaction time.
Figure 5 shows the absorption spectra of Mn-doped CdSe QDs at 250 °C for different reaction times. As the reaction time increases, the locations of the first exciton appeared at 619, 621 and 624 nm with respective reaction times of 0, 15 and 30 mins. Since the excitons are shifted to the longer wavelength, this indicate that the QDs size are increases with the reaction time. Similar to Fig. 4, the slope of the curve are steeper at longer reaction time.
The size variation of Mn-doped CdSe QDs is estimated based on the effective mass approximation [14]:
where, d = diameter of Mn-doped CdSeEg(bulk) = energy band gap of a bulk semiconductor (1.74 eV for CdSe and 1.81 eV for Mn-doped CdSe)
Eg(dots) = energy band gap of QD (calculated from the absorption peak)
h = Planck’s constant
m* = effective mass of an exciton (9.12 x 10−32 kg for CdSe and 4.19 x 10−32 kg for Mn-doped CdSe QDs)
The size of the QDs can be determined by plugging the energy band gap, Eg(dots) of the QDs from UV-visible spectrometry and all other constant values into Eq. (1). Table 1 summarizes the results where the QDs samples have different quantum confinement effect as their radius varies.
Table 1. Optical band gap and QDs radius of CdSe Mn-doped QDs at various reaction times and temperatures
Figure 6 shows the emission spectra of Mn-doped CdSe QDs at various reaction times and temperatures. The emission spectrum behaves similar evolution as that of absorption spectra, i.e. emission peaks are shifted to higher energy with decreasing particle size. It is seen that the emission spectra for all samples show a narrow band edge emission without broad deep trap emission is observed. This indicates that the core (CdSe) was perfectly capped by the Mn shell structure.
Here, the PL peak energy is larger than the first-exciton absorption. This is achieved by using quantum confinement to tune the excitonic levels to below all Mn excited states (Fig. 7) [13].
This shows that unquenched excitonic emission despite the presence of Mn doping. This scenario is interesting and fundamentally important because the size-tunable emission can be retained upon doping and co-exist with strong dopant-exciton magnetic exchange coupling.
Figures 8-10 show the combined absorption and emission spectra of Mn-doped CdSe QDs at 230, 240 and 250 °C for different reaction times. The absorption and emission peaks are shifted to longer wavelength as the reaction time increases from 0 to 30 mins. It can be seen that there is some over-lapping area between the absorption and emission spectra, which results in self-absorption phenomena i.e. the part of emission was absorbed by itself. Thus, decreasing the PL intensity at the short wavelength. In addition, it is seen that the emission peaks shows a red shift with respect to the absorption spectra for all samples. This large shift, namely called Stokes shift, arise from Stokes effect. The Stokes shift principally occurs if either the top of the valence band is an optically passive state or if the electron and hole are in a triplet state. Photon absorption from the top of the valence band in such cases is not allowed and only possible from optically active state lying deeper in the valence band. The exciton, once formed after absorption, cannot decay to the top of the valence band by direct dipole transition. De-excitation eventually take place with the help of phonons, thus giving rise to red shifted photons. Table 2 lists the experimentally observed Stokes shift in QDs.
Table 2. Experimental Stokes shift in Mn-doped CdSe QDs
Conclusion
High quality zinc blend Mn-doped CdSe QDs with a almost spherical morphology and narrow size distribution were successfully synthesized using the inverse micelle technique. The absorption and PL spectra of ripened CdSe Mn-doped QDs samples shows the size of QDs can be tunable with different reaction temperatures and times. The quantum confinement effect can be observed as the increases in optical band gap of ripened samples as the reaction temperatures and times increases due to QDs coarsening. This allowed tuning the semiconductor band gap energy across the dopant excited state levels and overcome the problem of quenching of excitons which energy transfer to the dopants. The apparent experimental Stokes shift observed in emission and absorption spectra of ripened samples.
Acknowledgment
This work was financially supported by UM/MOHE-HIR research grant (UM.C/HIR/MOHE/EHG/12), UMRG (RP011C – 13AET) and Postgraduate Research Grant (PG084-2012B). The authors gratefully acknowledge the financial support.
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