Fluorescent and nonlinear optical features of CdTe quantum dots
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- Umar, A.A., Reshak, A.H., Oyama, M. et al. J Mater Sci: Mater Electron (2012) 23: 546. doi:10.1007/s10854-011-0434-6
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The study of photoluminescence and nonlinear optical properties of red (emitted at 650 nm), yellow (emitted at 570 nm) and green (emitted at 530 nm) CdTe quantum dots (QD) spin coated on quartz substrate that had been prepared by changing the ratio between octadecylphosphonic acid and octadecence within 0.1:1–1:1 was carried out. Spectral width of the emission spectra indicates an enhancement with the increasing of QDs sizes, namely ca. 25, 28 and 50 nm for the QD size of 2.5, 3.5 and 5 nm, correspondingly. The entire QDs samples feature a spherical morphology with a relatively narrow size distribution. The optical second harmonic generation (SHG) stimulated by coherent bicolor treatment at 1,540 nm and its second harmonic generation was studied versus the laser light power density and incident angle.
The novel nanotechnologies require search of low-sized semicondcuting materials, in particularly Quantum Dots (QDs), which are small enough (at least below 10 nm) to exhibit nano-confined effects. One of promising applications of such intriguing properties is their optoelectronic and nonlinear optical features , determined by multi-photon absoprtion. The latter may be applied in 3D multi-photon microscopy, high sensitive tools and probes for studying of different biological and medical subjects have generated tremendous interest due to their unique optical properties. QDs are also highly efficient multi-photon absorbers that can be useful for three dimensional multi-photon microscopy and imaging, and their surfaces can be modified to conjugate biomolecules for selective targeting . Their multi-photon properties are determined by third-order nonlinear optical susceptibilities. The coexistence of the nonlinear optical and optoelectronic properties such as promising emission features opens a rare possiblity of their use as materials for optoelectronic and biophotonics [3, 4, 5].
The synthesis of CdTe QDs followed the Talapin method  with several modifications . Typical procedure for the preparation of red-luminescence CdTe QDs (650 nm) is as follow: 54 mg of cadmium acetate hydrate (CH3COO)2Cd.2H20 (Aldrich), 30 mg octadecylphosphonic acid (ODPA) (PCL Synthesis, USA) were dissolved in 10 mL of 1-octadecen, C18H36 (Aldrich) in the presence of 0.6 mL of oleic acid (OA) (WAKO Company) at 350 °C. If using this recipe, the final concentration of ODPA and OA in the reaction is 8.6 and 86 mM, respectively, which correspond to the ODPA to OA ratio of 0.1:1.
After that, at 300 °C, 1 mL of 7 mM room-temperature TOPTe solution was quickly injected into the solution. The reaction was started and continued during 5 min. Then, the reaction was quickly interrupted by removing the heat as well as by adding the hexane into the reaction.
The yellow (570 nm) and green (530 nm) CdTe QDs were prepared by simply changing the ratio between ODPA and OA within 0.5:1 … 1:1, correspondingly. Other chemical as well as the reaction condition were left unchanged.
The QDs were washed three times in hexane by centrifugation. A tiny drop of QDs solutions were then transferred on a TEM cooper grid for a microscopy analysis. The QDs thin film on quartz substrate was prepared by a spin-coating technique.
The PL and optical absorption properties of the CdTe QDs were characterized using Perkin Elmer LS 55 Photoluminescence and Perkin Elmer Lambda 900 UV/VIS/NIR Spectrometer, respectively. The TEM images were taken using CM 12 Philips TEM apparatus. The atomic force microscopy image was taken using Ntegra Prima AFM apparatus (Russia) with non-contact tapping mode operation.
3 Results and discussion
QDs with three different photoluminescence properties have been successfully obtained in the present approach via a simple modification on ODPA to OA surfactants ratio in the reaction. Unlike in most QDs preparation , in the present technique (at a certain ratio), the PL properties of the QDs was found to be less affected when the growth time of the QDs was varied . This fact inferred that the QDs size was relatively unchanged during the growth process as proven by the TEM analysis. This enables QD’s to be obtained with a stable and fine-controlled size for applications. After following a series of washing, a tiny drop of QDs solution was transferred on to a cooper grid for TEM analysis.
The nonlinear optical properties for the semiconducitng QDs are performed usually by optical poling which uses two coherent bicolor beams forming non-centrosymmetric grating, necessary for observation of the first order nonlinear optical effects described by third rank polar tensors similarly to the described in the Ref. . The initial optical treatment was performed by s-polarized beam generated by Er:glass (τ = 13 ns laser generating at 1,530 nm). The occurrence of the non-centrosymmetry was monitored by diffraction of the first maximum for the 5 mW He–Ne cw laser for up to 3 min. It was established that the process of the grating was very sensitive to the angle of the incident beams, their power densities and relative intensity maximum. One of the reasons for such enhancement may be caused by exisence of trapping levels.
Dependence of the SHG versus the power density of the fundamental Er:glass 13 ns laser beam and its doubled frequency is presented in the Fig. 4. The ratio between the fundamental and the doubled frequency beam was about 6:1. The effect exists only during the simultaneous bicolor treatment and disappears after several minutes.
The increasing of optically-induced output may be caused by the reduction of the one-electron second-order susceptibility correlating with the enhancement of the electron–hole density in the QDs  which correlated with the coherent acoustic phonons and with the next oscillatory modulation of the absorption cross-section. So, the photoinduced contribution of the phonon subsystem including anharmonic one is prevailing [12, 13]. The corresponding effects may be comparable with the effects observed in the organic materials , however these materials are more stable and may be comparable with the glasses .
Generally one can see that with increasing of the QDs nanosizes we observe the red shift of absorption and fluorescence together with the increasing of the photoinduced second harmonic generation. This fact allows to operate by the emitting efficiency of the CdTe QDs, their maximum spectral emission range and the forming of the grating. So it may be applied for laser triggered optoelectronic devices varying their generation wavelengths depending on the sizes of the QDs.
The observed effects may be considered like the particular cases of the photoinduced non-centrosymmetry in the initially disordered materials [16, 17, 18]. It is a consequence of light driven electron bunching and generation of strong static electric field on the bunch boundaries resulting in effective photon absorption and bonds breaking in these regions while the main part of a sample is almost transparent for the photons. From this reason applying of this approach to the quantum nanodots may be very fruitful because the process of the photoinduced energy transfer is principally different.
The CdTe QD were synthesized with sizes of 2.5, 3.5 and 5 nm corresponding to green, yellow and red emission at wavelengths 530, 570 and 650 nm, correspondingly.
The morphology of the QDs thin films exhibit significant differences each other probably due to a unique attachment characteristic possessed by the samples related to the nature of the surfactant capping agent (self-adhesive characteristic). The nonlinear optical properties for the semiconducitng QD were performed usually by optical poling which uses two coherent bicolor beams forming non-centrosymmetric grating necessary for observation of the first order nonlinear optical effects described by third rank polar tensors. The initial optical treatment was performed by s-polarized polarized beam generated by Er:glass 13 ns laser generating at 1,530 nm.