Structural Stability and Optical Properties of hexagonal and cubic CdSe Nanocrystals synthesized in MgO
- PDF / 181,991 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 16 Downloads / 174 Views
FF9.27.1
Structural Stability and Optical Properties of hexagonal and cubic CdSe Nanocrystals synthesized in MgO S.W.H. Eijt1, M.A. van Huis1*, P.E. Mijnarends1,2, B.J. Kooi3, M. Nanu4 1 Faculty of Applied Sciences, Delft University of Technology, Delft, The Netherlands 2 Physics Department, Northeastern University, Boston, MA 02115 3 Materials Science Centre, University of Groningen, Groningen, The Netherlands 4 Inorganic Chemistry, Faculty of Applied Sciences, Delft University of Technology, Delft, The Netherlands *present address: Kavli Institute of Nanoscience, Delft University of Technology, Delft, The Netherlands ABSTRACT We present a study of CdSe nanocrystals synthesized in MgO by precipitation of Cd and Se supersaturated solid solutions, created in MgO single crystals by ion implantation, in the temperature range between 300 oC and 1100 oC. For high-dose ion implantation, optical absorption spectroscopy revealed the presence of the ~1.8 eV CdSe semiconductor band-edge. Small sized nanocrystals adopt the rocksalt instead of the wurtzite structure because the former fits better in the MgO matrix and results in lower interface energies. A better understanding of these structural changes and optical properties is obtained from ab-initio total energy calculations on wurtzite, zincblende and rocksalt CdSe using the VASP pseudopotential code. The calculated electronic band structures are compared of zincblende CdSe, a direct band-gap semiconductor, and rocksalt CdSe, which has an indirect optical band-gap. INTRODUCTION Semiconductor nanocrystals have received considerable interest because of the strong variation of their optical and electronic properties in the size range below ~10 nm [1]. In the past decade, substantial progress has been achieved in the chemical preparation of monodisperse colloidal CdSe nanocrystals. Passivation of undesired surface states is achieved by capping with ligand molecules. Another passivation method is to embed nanocrystals into inorganic host matrices with a sufficiently large optical band-gap. This results in far more robust samples. Furthermore, if ion implantation is used to create the embedded nanocrystals, the manufacture of these nanocrystals can be straightforwardly integrated into microelectronic device processing. Important applications are currently in non-linear optical devices [2] and could be envisioned in the creation of novel types of thin-film solar cells [3]. Previous studies [4-6] have shown that it is feasible to generate CdSe nanocrystals inside MgO using co-implantation of the two ion species and subsequent thermal annealing steps. The recombination of Cd and Se atoms into CdSe clusters is enabled by their mobility obtained at elevated temperatures, and is driven by the gain in binding energy [2]. Transmission electron microscopy studies [4-6] revealed that large nanoclusters have the wurtzite crystal structure of bulk CdSe. The smallest nanoclusters (< 5 nm), however, adopt the denser rocksalt structure (a = 5.61 Å). Here, three CdSe lattice spacings closely f
Data Loading...
