Aqueous synthesis of semiconductor nanocrystals

Abstract

Water is a natural medium for all forms of life. This is one reason why any solution-based techniques and processes proceeding in aqueous media are considered to be environmentally friendly and safe in comparison to others which demand, e.g. organic solvents or melts. In the field of the synthesis of colloidal semiconductor nanocrystals (NCs), which consist mainly of water insoluble II-VI and IV-VI compounds, the use of chemical precipitation reaction in aqueous media was historically the number one choice [1]. In the earlier work a commercially available colloidal silica sol (13 nm in diameter) was used as a carrier and stabilizer of the CdS [1] and ZnS [2] colloidal solutions as well as CdS-ZnS co-colloids [3]. The reported mean size of the NCs was relatively big, e.g. 37 nm diameter for CdS, as determined by fractional filtration through micropore filter, thus the optical spectra reported showed no serious deviations in comparison to the corresponding bulk materials [1]. Very soon the possibility of use of styrene/maleic acid anhydride copolymer (Brus group) or phosphates and polyphosphates (Henglein, Grätzel and Nozik groups) as stabilizers was recognised and colloidal (free of SiO2) solutions of CdS [4]-[6], ZnS [2], [7], PbS [5], Cd3P2 [8], Zn 3P2 [8], Cd3As2 [9], CdTe [10] and ZnTe [10] NCs were synthesized. Blue shifts of the absorption edge and the emission bands in comparison to the corresponding bulk materials were observed and reported. The theoretical background for this shift to higher energies/shorter wavelengths, the quantum confinement effect was introduced by brothers Efros [11] and Brus [12]. Further development of the stabilizing techniques included the use of short chelating peptides of general structure (g-Glu-Cys)n-Gly to control the nucleation and growth of CdS crystallites [13], micelles and vesicles [14] and finally the use of various short-chain thiols [15], [16]. © 2008 Springer-Verlag/Wien.