ZnTe nanocrystal formation and growth control on UV-transparent substrate

https://doi.org/10.1016/j.cplett.2010.09.035Get rights and content

Abstract

ZnTe nanocrystals (NCs) were successfully grown in a UV transparent, vitreous substrate synthesized by fusion and then annealed. The formation of dot structures, emitting in the UV-range, was investigated by optical absorption (OA), atomic force microscopy (AFM) and Raman spectroscopy. Dot growth was evidenced by an OA band red-shift with increasing annealing time. Average sizes of the ZnTe dots were determined using the effective mass fit model with OA spectra and comparing the results with estimates from AFM images. A UV-transparent PZABP vitreous matrix was used because it allowed ZnTe NC growth and displayed typical Raman active phonon modes.

Research highlights

NCs were synthesized by fusion and annealed post-growth to control average dot size. ► The samples displayed two distinct bands, one sensitive to heat treatment and the other with ZnTe bulk-like properties. ► The band near 390 nm underwent a red-shift as annealing time increased. This is a clear signature of quantum confinement in dots of increasing size. ► It is hoped that these results may stimulate further research into new substrates that not only work in visible and broad-spectrum UV ranges, but also allow NC growth based on semiconductors with desirable properties for optoelectronic device applications.

Introduction

The II–VI semiconductor family, based on zinc telluride (ZnTe), has a zincblend crystalline structure with an exciton Bohr radius of 5.2 nm [1]. Due to its wide energy gap (2.26 eV) at room temperature [2], [3], [4], [5], ZnTe is attractive for the manufacture of optoelectronic devices such as light emitting diodes, green-range laser diodes [6], [7], electrochemical solar photocells [8], [9] and efficient, phosphorus based displays. ZnTe has also attracted much attention because of its very high electro-optic coefficient and dielectric constant in the terahertz range – properties which can be advantageous as electro-optic field detectors [4], [6], [9], [10]. Until now, ZnTe nanostructures have been synthesized by different techniques such as thermal evaporation, hot wall evaporation, radio frequency cathodic evaporation, solvothermal processes, molecular beam epitaxy [2], [6], [11], [12], [13] and colloidal solutions [5], [14]. Most of these ZnTe structures have been obtained in the form of monocrystalline nanowires [6], [9] and, more recently, nanocrystals [5]. Interest in NC band-gap is mainly due to its quantum confinement effect which is strongly dependent on NC size [15]. This study shows, probably for the first time, clear evidence of ZnTe nanocrystal growth in a vitreous matrix synthesized by fusion.

Section snippets

Sample preparation

ZnTe NC growth was detected after lengthy manipulation of the PZABP matrix yielding a UV-transparent template with final nominal composition: 65P2O5·14ZnO·1Al2O3·10BaO·10PbO (mol%), with homogenized and weighed 1Te (wt.%). Standard sample preparation procedures were followed that included mixing and fusing powder at 1300 °C for 30 min and then quickly cooling the melt to room temperature. To obtain different sized NCs, the samples underwent annealing at 480 °C for different time periods (2, 4, 6

Optical absorption spectra

Figure 1 shows OA spectra from the pure, manipulated PZABP matrix (bottom line) and five samples containing ZnTe NCs with 480 °C post-growth annealing for 2, 4, 6 and 8 h. It is apparent that the PZABP substrate is transparent in the UV and visible ranges while ZnTe NCs absorb or emit light. Thus, this new matrix was essential to optically observe and control the growth of ZnTe NCs through OA spectra. It is possible to identify two distinct OA spectra bands for ZnTe dots grown in the Te doped

Conclusion

A new PZABP glass matrix was synthesized that allows ZnTe NC growth and is transparent for a wide range of electromagnetic radiation used in many experiments. NCs were synthesized by fusion and annealed post-growth to control average dot size. Sample properties were investigated by OA, AFM and Raman spectroscopy. An OA red-shift indicated a dot size increase proportional to annealing time. The samples displayed two distinct bands, one sensitive to heat treatment and the other with ZnTe

Acknowledgments

The authors are grateful for the financial support of the following Brazilian agencies MCT/CNPq, FAPEMIG, FAPESP and CAPES.

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