Fluorescent and nonlinear optical features of CdTe quantum dots

  • A. A. Umar
  • Ali H. Reshak
  • M. Oyama
  • K. J. Plucinski
Article

DOI: 10.1007/s10854-011-0434-6

Cite this article as:
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

Abstract

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.

1 Introduction

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 [1], 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 [2]. 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].

2 Experimental

The synthesis of CdTe QDs followed the Talapin method [6] with several modifications [7]. 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 [6], 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 [7]. 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.

Figure 1 demonstrates typical TEM images of the CdTe QD that prepared with the three reaction conditions, namely ODPA to OA ratio of 0.1:1, 0.5:1 and 1:1, that grown for 5 min each. As can be seen from the images, the entire QDs samples feature a spherical morphology with a relatively narrow size dispersion. For the case of the QDs prepared with ODPA to OA ratio of 1:1 (Fig. 1a), the value of average size was found to be equal to about 2.5 nm. Meanwhile, the average size for the samples prepared for the ratio of 0.5:1 (Fig. 1b) and 0.1:1 (Fig. 1c) were found to be equal to 3.5 and 5 nm, respectively.
Fig. 1

TEM images of CdTe QDs prepared with three different ODPA to OA ratio: a 1:1, b 0.5:1 and c 0.1:1 with growth time of 5 min. The QDs size in (a), (b) and (c) are ca. 2.5, 3.5 and 5 nm, respectively

Figure 2 shows typical optical absorption and photoluminescence spectra of the QDs prepared in the present study. From the results presented, it was established that the QDs gave three different photoluminescence peaks, namely 530, 570 and 650 nm, depending on the size of the QDs; the smaller size corresponds to the longer emission wavelength is. In reference to their corresponding photoluminescence and optical absorption (Fig. 2b) peaks, it can be concluded that the PL properties of the whole QDs samples follow a normal Stokes emission features with absence of up-conversion processes, which is typical for bigger NC system [8]. It was also established that the spectral width of the emission spectra indicates an increase 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 broadening in the spectral width when the size increases could be directly associated with the increasing in the electron–phonon interaction in bigger QDs system [9]. Quantum yield calculations of the QDs PL were ca. 70, 75 and 43% for the samples with green, yellow and red emission, correspondingly. The variation in quantum yield can be directly related to the effect of surfactant concentrations attachment onto the QDs surface that effectively improves the confinement effect in the QDs system. The higher the surfactant concentrations (here the ODPA to OA ratio) the increase the PL quantum yield is.
Fig. 2

Photoluminescence (a) and optical absorption (b) spectra of the corresponding CdTe QDs. The quantum yield calculation for the samples is a 70%, b 75% and c 43%

The as prepared QDs have been attached onto quartz substrate by using a spin coating technique for further optical characterizations, namely second harmonic generation and optical nonlinearities. We have examined the morphology of the QDs films on the surface using atomic force microscopy technique. The results are shown in Fig. 3. As can be seen from the images, 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). For example, the green QDs samples (Fig. 3a) that had been prepared using high ODPA to OA ratio (1:1), exhibits a hilly topography reflecting the formation of aggregates QDs structures. This could be as the results of high self-adhesive characteristic between the individual QDs due to the presence of high surfactant concentration so that, during the spin-coating process, they tended to aggregate each other. Meanwhile, very smooth QDs thin films morphology was obtained for the yellow QDs, namely prepared using ODPA to OA ratio of 0.5:1 (Fig. 3b) and moderate aggregate structures was obtained for the red QDs thin films (0.1:1, Fig. 3c). A much clearer QDs distribution on the surface can not be obtained at the moment due to a low resolution of the AFM machine.
Fig. 3

AFM images of the QDs thin films on quartz substrate; agreen (2.5 nm), byellow (3.5 nm) and red (5 nm)

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. [10]. 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.

In the Fig. 4, one can clearly see that the optimal gratings for the C sample was achieved at incident angle about 58°. For the red and green laser, the optimal values are achieved at about 40°. Moreover, for the larger QDs (red samples C), the intensity of the first diffraction maximum subtitle is larger.
Fig. 4

Dependence of the diffraction efficiency versus the incident angle of the bicolor beam on the QDs surfaces at fundamental to beam ratio about 6:1 and fundamental power density equal to about 1 GW/cm2

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 observed nonlinear optical effects show a correlation with the absorption and photoluminescence. In particularly, the more wavelengths shifted absorption and luminescence favor the higher output photoinduced SHG (see Fig. 5). As a consequence in our case, the samples with larger QDs sizes showing red emission possess higher nonlinear optical output and the formed grating is more pronounced.
Fig. 5

Dependence of the output SHG versus the power density of the fundamental laser

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 [11] 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 [14], however these materials are more stable and may be comparable with the glasses [15].

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.

4 Conclusions

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.

Copyright information

© Springer Science+Business Media, LLC 2011

Authors and Affiliations

  • A. A. Umar
    • 1
  • Ali H. Reshak
    • 2
    • 3
  • M. Oyama
    • 4
  • K. J. Plucinski
    • 5
  1. 1.Institute of Microengineering and NanoelectronicsUniversiti Kebangsaan MalaysiaSelangor D.EMalaysia
  2. 2.Institute of Physical Biology-South Bohemia UniversityNove HradyCzech Republic
  3. 3.School of Material EngineeringMalaysia University of PerlisKangarMalaysia
  4. 4.Department of Material Chemistry, Graduate School of EngineeringKyoto UniversityKyotoJapan
  5. 5.Electronic DepartmentMilitary University TechnologyWarsawPoland

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