Quantum confinement effects of CdTe nanocrystals sequestered in TiO2 matrix: effect of oxygen incorporation

doi:10.1016/j.physe.2004.08.110

Physica E: Low-dimensional Systems and Nanostructures

Volume 25, Issue 4, January 2005, Pages 554-561

https://doi.org/10.1016/j.physe.2004.08.110 Get rights and content

Abstract

Thin films of TiO₂ with high volume fraction ∼(40–55%) and crystallite size ∼(6–40 nm) of CdTe nanoparticles had been prepared by rf magnetron sputtering from a composite TiO₂:CdTe target at room temperature and 373 K. A detailed optical properties of nanocrystalline CdTe:TiO₂ films as-deposited and after thermal treatment (⩽300 °C) are studied. The absorbance of the TiO₂ films with CdTe nanocrystallite dispersions depends both on the nanocrystallite size and volume fraction. The blue-shifts of the optical absorption edge concurrent with the CdTe nanocystal size reduction for as-deposited and after thermal treatment of nanocrystalline CdTe:TiO₂ thin films with respect to the bulk semiconductor agrees quite well with the strong quantum confinement theory. A slight deviation in absorption edge values than the predicted values from the strong quantum confinement model can be attributed to change in interplanar distance due to oxygen incorporation and inhomogeneous size distribution of CdTe nanocrystallites in these films.

Introduction

Quantum dots are nanometer-size semiconductor structures and represent one of the most rapidly developing areas of current semiconductor research. Size-dependent chemical reactivity [1], optical nonlinearity [2], efficient photo-electron emission [3] and melting point reduction [4] are some of the interesting properties exhibited by nanoparticles. Applications take advantage of high surface area and confinement effects, which lead to nanostructures with different properties than conventional materials, and which create opportunities for innovating principles of operation for devices and instruments.

Recently, the nanostructures of II–VI materials have attracted much attention for their great fundamental, experimental and applied interests. CdS [5], CdSe [6], and CdTe [7] are amongst the most studied cases. CdTe has a direct band gap of 1.5 eV at room temperature, and is a suitable material for electro-optical devices. Because of its large exciton Bohr diameter (15.0 nm), this semiconductor offers the possibility of studying quantum confined materials with small reduced cluster sizes (ratio of cluster size to exciton Bohr diameter) [7]. Amongst the various physical vapor deposition (PVD) techniques for the preparation of CdTe nanocrystalline composites in TiO₂ matrix, RF sputtering has been the most versatile and proven one since it enables the controlled distribution of nanoparticle size and volume over a wide range [8]. Moreover, using RF magnetron sputtering, the processing at relatively low temperatures can avoid the possible reaction between the matrix and the semiconductor nanoparticles. However, nanocomposites film having large volume fraction of semiconductor particles in insulating film have not been studied in detail. It is expected that due to enhanced surface-to-volume ratio of semiconductor, nanocrystals should show different properties varied over a large range.

In the present study, RF-sputtered TiO₂ thin films of thicknesses ∼500 nm deposited at two substrate temperatures, room temperature (RT) (300 K) and 373 K with high volume fraction (40–55%) of CdTe have been obtained. Generally, certain threshold values of volume fraction and crystallite size are imperative in order to observe quantum confinement effects. In the present work, systematic variation in CdTe volume fraction (and thus crystallite size) has been carried out and its effect on the shift of the optical absorption edge has been investigated. Post-deposition thermal treatment in the temperature range of 250–300°C has also been carried out in order to elucidate its role in modulating the optical absorption edge of the composite thin films. The choice of TiO₂ as an insulating matrix is based on the fact that TiO₂ being an optically transparent wide band gap oxide, the quantum size effects of CdTe nanocrystals embedded in TiO₂ can be studied over a wide energy range. The volume fraction of CdTe (V_f) in nanocrystalline CdTe:TiO₂ films were systematically varied by increasing the number of CdTe pellets on the TiO₂ target.

Section snippets

Experimental

The CdTe nanocrystallites embedded in TiO₂ matrix were prepared by RF magnetron sputtering from a composite target. The target consisted of TiO_₂ sintered disc of 2 inches diameter having a CdTe pellet of diameter 10 mm placed at its center. By varying the number of CdTe pellets on the TiO₂ target, the semiconductor concentration in the deposited film can be controlled to a desired value. TiO₂ and CdTe were co-sputtered with a RF power of 240–260 W in the presence of Ar gas at 0.02 mbar. The

Results and discussion

M1–M3 corresponds to nanocrystalline CdTe:TiO₂ films with varying number (1–3) of CdTe pellets on the TiO₂ target prepared at $T_{s} = RT$ while M4–M6 represents the corresponding films prepared at $T_{s} = 373 K$ , respectively. The different elemental compositional values obtained by EDS measurements for nanocrystalline CdTe:TiO₂ films before and after thermal treatment (250–300 °C) are listed in Table 1. As shown in Table 1, by increasing the number of CdTe pellets on the TiO₂ target and upon thermal

Conclusions

TiO₂ thin films with high volume fraction of CdTe dispersions could be realized by using RF magnetron sputtering from a composite CdTe:TiO₂ target. EDS measurements allowed determination of the composition of the samples. The films prepared at RT and 373 K were mainly amorphous in nature, however, few crystalline signatures were observed for high V_f CdTe films prepared at high T_s. Interplanar distance of CdTe:TiO₂ composite thin films were found to be larger for films prepared at high T_s due to

Acknowledgments

We thank the Director, National Physical Laboratory for permission to publish this research work. Acknowledgments are also due to Dr. (Mrs.) M. Kar and Mr. K.N. Sood for optical and EDS measurements, respectively.

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Quantum confinement effects of CdTe nanocrystals sequestered in TiO2 matrix: effect of oxygen incorporation

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

Opt. Commun.

Solid State Commun.

J. Mater. Res.

J. Phys. Chem.

Science

Phys. Rev. B

Semicond. Sci. Technol.

Quantum confinement effects of CdTe nanocrystals sequestered in TiO₂ matrix: effect of oxygen incorporation