ABSTRACT
Although the first material synthesized via an aqueous approach and thoroughly investigated was CdS, the most successful example of the cadmium chalcogenides is CdTe owing to its unique optical properties, e.g., strong photoluminescence (PL) in the visible region. CdTe NCs had been obtained directly in water first by Resch et al. through the reaction of cadmium perchlorate with sodium telluride in the presence of sodium hexametaphosphate as stabilizer [8]. Rajh et al. employed a mixed stabilizer system containing hexametaphosphate and 3-mercapto-l,2-propanediol (thioglycerol (TG)) for successful synthesis of CdTe NCs [9]. Later, optimization by Rogach et al. led to a synthesis of stable NCs with sizes from 1.3 to 2.4 nm in the sole presence of thiols, viz., 2-mercaptoethanol and 1-thioglycerol [10]. State-of-the-art preparation techniques enable facile aqueous synthesis of CdTe QDs capped by different thiols, possessing diameters of up to 6 nm and PL covering most part of the visible region extending to the near infrared (NIR) with quantum yields (QYs) of up to 70-80%, which can compete with the best organometallicaly prepared materials [11]. For instance, as-prepared NCs by microwave irradiation demonstrate PL QY as high as 82% and a narrow size distribution [12]. Here we would like to note that very high QYs reported should be considered very carefully, since the QY value is very much dependent on the method of determination, standards used, instrument conditions, and so on. A reliable and quite detailed procedure for QY measurement has been reported recently by Grabolle et al. [13]. Luminescence of CdTe NCs determines their
potential applicability in the fields of light emitting diodes (LEDs) fabrication [14,15], color conversion [16], energy scavenging [17,18], optical sensing [19] and bioimaging [20].Based on the synthesis of CdTe nanoparticles, approaches for obtaining some alloyed materials such as CdSeTe [21], CdHgTe [22-24], as well as core/shell structures such as CdTe/CdS [25-27], CdTe/ZnTe [28], and even core/shell/shell CdTe/ CdS/ZnS [29] have been developed. The obtaining of different core/shell and core/shell/shell structures directly in water reported during the last decade is a great step forward toward a superior quality of NCs accessible from an organometallic route. Interesting hybrid approaches for the synthesis of core/shell particles combining aqueous and organometallic techniques were developed recently. Thereby CdTe and CdHgTe cores transferred from water were coated by ZnS shell via the hot injection technique [30]. As a recent advance, a facile synthesis of 1D structures such as CdTe [31,32] and CdHgTe [33] nanorods, allowing control of their sizes and aspect ratios, should be mentioned. Nevertheless, we have to admit that the aqueous method usually does not permit direct efficient shape control of nanoparticles during their growth due to temperature limitations: temperature of around 100°C is not sufficient to overcome the energy barrier of transforming zinc blende to wurtzite structure. Thus, NCs synthesized exhibit as a rule an isotropic zinc blende cubic crystal structure and mainly spherical or quasi-spherical shape, since this shape is thermodynamically the most stable [34]. Therefore, shaped CdTe nanostructures, such as 1D wires [35,36], twisted ribbons [37], 2D sheets [38], 3D gel-like networks [39,40] are achievable as a rule via postpreparative self-organization of preformed QDs. Assembly approaches of aqueous colloidal CdTe QDs are discussed in Chapter 3.Mild synthetic conditions used in an aqueous synthesis often disable perfect structural localization of atoms in the crystal lattice of nanoparticles, normally achievable through the hot injection method. In order to improve crystal structure, size distribution and correspondingly optical properties of CdTe QDs, microwave irradiation has been successfully applied [12,25,29,41,42].In the following paragraphs, we will review in detail the following aspects of the aqueous synthesis of CdTe NCs: its synthetic protocol, growth mechanism, postpreparative treatment
of the particles, and obtaining some alloyed and core/shell structures based on the CdTe matrix. 2.2 Synthetic ProtocolThe typical synthetic protocol for obtaining aqueous thiol-capped CdTe NCs is a one-pot process consisting of three main steps: (1) Formation of Cd2+/thiol complexes by dissolving a Cd salt (usually Cd(ClO4)2 or CdCl2) in water followed by addition of an appropriate thiol with subsequent pH adjustment. Note, cysteamine stabilization does not require pH adjustment. (2) Injection of a Te2-source (H2Te gas, NaHTe) in a thoroughly de-aerated (through purging by an inert gas (Ar, N2)) Cd2+/thiol solution results in the formation of a CdTe precursor (often referred as a monomer). Hydrogen telluride is produced either by decomposition of Al2Te3 (see Fig. 2.1) or electrochemical generation, a detailed recipe is reported in ref. [11]. (3) Nucleation and growth of NCs by heating or microwave
irradiation, usually upon reflux under open air conditions.
