Room temperature CdSe Semiconductor nano-crystals
Nayan Mishraa V. D. Abrahama
aLaxmi Institute of Technology epartment of Physics, Oriental University, Indore , M. P. , India
Abstract
CdSe quantum dots of tunable size have been assembled on glass/ ITO substrates using a wet synthesis route in the presence of the cadmium and selenium precursors and surfactant. The physical properties of the quantum dots have been investigated using X-ray diffraction, electron microscopy and optical spectroscopy techniques. It is shown that strong near band edge luminescence can be obtained from these quantum dots. The luminescence behavior has been interpreted in the light of the quantum confinement effects.
1
1.0. Introduction:
Quantum dot structures based on II-VI semiconductors are presently of significant interest due to their advantageous linear and non linear optical properties. The electronic and optical properties of these quantum dots are size dependent [1-3]. These semiconductor quantum dots reveals considerable applications in tunable displays [4-5], light emitting diodes [6], optoelectronic devices [7-8], biological labels [9] etc.
CdSe is an important semiconductor of the group II-VI family. The scientific interest emerged are due to the tenability of its absorption edge over the entire visible spectrum by controlling the size. Synthesis of nano-crystalline CdSe performance in vacuum [10-11] or arrested precipitation in solution [12-14]. But the study on the deposition of CdSe quantum dots as thin films found to be not substantial. This paper reports the results on synthesis of quantum dot CdSe thin film (Q-CdSe) in an aqueous medium and their characterization.
2 Experimental Details:
2.1 Deposition of CdSe films
The Glass\Indium tin oxide coated substrates used for deposition of Q-CdSe film were first thoroughly cleaned in a detergent (Extron MA 01 MERCK) and then ultra-sonificated. The CdSe quantum dots were synthesized at room temperature in a reaction matrix containing sodium seleno sulphite, cadmium chloride and tri-ethanolamine in
1:3:1.37 molar ratios in triple distilled water. The pH of the bath was adjusted to 11 with the help of ammonia solution. The size of the CdSe nano-crystal was controlled by adding an appropriate concentration of mercapto-ethanol as surfactant in the bath. The value of surfactant to selenium atom in the synthesis bath was varied from 0 to 2.42.
2.2 Characterisation Techniques
X-ray diffraction patterns were recorded in the thin film grazing angle mode of XRD6000 (SHIMADZU, JAPAN) powder diffractometer, using CuKα line. Beam divergence was restricted with the help of 0.15 mm slit on the source side. Drive axis was 2θ for scan range between 200-650 covered in 10/minute with step size of 0.020 .The instrument propagation error in the d-value was ± 0.003 Å.
TEM characterization was carried out using Techni 20G2 Transmission Electron Microscope operated at 200 kV. The bright and dark field images were acquired by applying axial tilt and selecting a small portion of the brightest diffraction ring.
The HOMO-LUMO gap of the Q-CdSe films was determined from optical absorption studies with the help of UV-Vis spectrophotometer (model UVPC1601, SHIMADZU Corporation JAPAN) in the spectral range between 300-800 nm using a spectral bandwidth of 2 nm at room temperature.
Photoluminescence (PL) emission spectra for Q-CdSe films were recorded from a computer controlled rationing luminescence spectrophotometer LS55 (Perkin-Elmer Instruments, UK) with λ accuracy = ±1.0 nm and λ reproducibility = ± 0.5 nm. A tunable 20 kW pulse <10 ms from a Xenon discharge lamp was used as the excitation source for recording photoluminescence emission spectra. A gated photo multiplier tube was used as a detector. Prior to the PL experiments signal-to-noise ratio was adjusted to 500:1, using the Raman band of water with excitation at 350 nm.
3 Results and Discussion:
3.1 Structural Analysis
The result of XRD characterization of two Q-CdSe films sam1 and sam2 grown using surfactant to selenium ratio of 0 and 2.42 have been shown in fig 1.The as prepared CdSe film sam1 is dominated by reflections arising from (111), (220) and (311) planes at 2= 25.8 o, 42.25 o and 51o.
Fig-1- Grazing angle XRD spectra of two Q-
CdSe films sam1 and sam2
All the observed peaks were carefully analyzed using a peak fit algorithm assuming Gaussian line shape. It was found that all the observed peaks matched with the zinc –blended structure of CdSe. The increase in the concentration of the surfactant during the synthesis of CdSe quantum dots had two fold effect on the observed XRD spectra .The line widths of XRD reflexes broadened and the higher angle reflexes also weakened in intensity .The cumulative effect of peak broadening and intensity weakening was manifested as over lapping of the higher angle reflexes at 2=42.25 o and 51o.Our XRD results thus clearly suggest that particle size of CdSe can be effectively controlled with the use of surfactant. The average particle sizes experimentally determined from the observed line widths using Scherer’s formula was found to be 13.2nm and 1.9nm for CKF and CK-VIII respectively. In fact the particle size was found to systematically decrease with increasing use of surfactant to selenium ratio in the growth matrix. Fig 2 represents the Fig-2. Observed dependence of particle size with
surfactant to selenium ratio during synthesis.
Fig-3(a) Dark field image and (b) the SAD pattern of Q-CdSe sample2 grown using surfactant to selenium ratio of 2.42.
dependent of particle size with the variation in the surfactant to selenium ratio. It is therefore concluded that the surfactant employed in the studies can effectively control the size of CdSe quantum dots. The smallest synthesized size of Q-CdSe was 1.9 nm.
Figure 3(a & b) show the dark field TEM image and the corresponding diffraction pattern of Q-CdSe sam2 sample. The selective area diffraction pattern (fig.2b) revealed the presence of zinc blended phase of CdSe. The average particle size determined from the TEM data was 1.6 nm which is in close agreement with XRD result
3.2. Optical Characterization:
3.2.1. Optical absorption studies:
The optical absorption spectra for CdSe samples sam1 (13nm), sam2 (2.1 nm) have been shown in fig4. The sharp rise in the absorption onsets correspond to the fundamental absorption edges at 580nm and 426nm for sam1 and sam2 respectively. The absorption edges in CdSe correspond to the transition 1S3/2 -1Se, which obviously shifted to blue side following reduction in the size of Q-CdSe.The observed blue shifted absorbance is in agreement with the quantum confinement effect in nano particles.
In addition to fundamental absorption onset, each of the CdSe sample exhibited a second absorption onset on the blue side. The second absorption onset in CdSe may be attributed to the spin orbit interaction giving rise to the transition labeled as 2S3/2-1Se.
3.2.2. Photoluminescence Studies
The Photoluminescence emission spectra of CdSe samples sam1 along with sam2 have been shown in figure 5 respectively. The excitation wavelengths were 640nm (sam1) and 415nm (sam2).The observed luminescence in sam1, peak at 770nm was rather weak and red shifted vis-à-vis the corresponding absorption edge 680nm. However, the PL emission increased dramatically for sam2. The observed PL line shape was deconvoluted assuming Gaussian shape and the results have also been shown in.
Fig-4 Optical absorption spectra for two Q-CdSe samples1 and sample2 deposited on ITO/glass substrate.
Fig-5- Photoluminescence emission spectra of two Q-CdSe films sam1and sam2 CdSe
Fig-5 PL spectra for two Q-CdSe samples1 and sample2 deposited on ITO/glass substrate.
Fig 5. The observed luminescence spectrum was found to consist of four band E1, E2, E3 and E4.The peak E1 positioned at 457 nm was slightly stoke shifted vis-a-vis the corresponding absorption edge by 31 nm. The peak E1 can therefore be associated with excitonic luminescence. The other side bands E2, E3 and E4 may originate due to the presence of surface defects or traps
Conclusions
CdSe quantum dots of tunable size can be grown as thin film using an aqueous root in the presence of the surfactant. The role of the surfactant in controlling the particle size has been demonstrated using XRD, TEM, optical absorbance and luminescence spectroscopy. The as synthesized Q-CdSe samples have been shown to grow with a preferred orientation of the zinc blended phase. The quantum confinement effect in Q-CdSe samples have been demonstrated by the observation of blue shifted absorption edge and PL emission. It is also shown that the quantum dots give intense excitonic luminescence
Acknowledgments
I gratefully acknowledge Dr N.P Lalla Dr. Mukul Gupta UGC-DAE CSR, Indore for providing facilities for TEM & XRD experiments.
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