We investigate systematically the influence of the nature of thiol-type capping ligands on the optical and structural properties of highly luminescent CdTe quantum dots synthesized in aqueous media, comparing mercaptopropionic acid (MPA), thioglycolic acid (TGA), 1-thioglycerol (TGH), and glutathione (GSH). The growth rate, size distribution, and quantum yield strongly depend on the type of surface ligand used. While TGH binds too strongly to the nanocrystal surface inhibiting growth, the use of GSH results in the fastest growth kinetics. TGA and MPA show intermediate growth kinetics, but MPA yields a much lower initial size distribution than TGA. The obtained fluorescence quantum yields range from 38% to 73%. XPS studies unambiguously put into evidence the formation of a CdS shell on the CdTe core due to the thermal decomposition of the capping ligands. This shell is thicker when GSH is used as ligand, as compared with TGA ligands.

In this work a colloidal approach to synthesize water-soluble CdSe quantum dots (QDs) bearing a surface ligand, such as thioglycolic acid (TGA), 3-mercaptopropionic acid (MPA), glutathione (GSH), or thioglycerol (TGH) was applied. The synthesized material was characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), UV-visible spectroscopy (UV-Vis), and fluorescence spectroscopy (PL). Additionally, a comparative study of the optical properties of different CdSe QDs was performed, demonstrating how the surface ligand affected crystal growth. The particles sizes were calculated from a polynomial function that correlates the particle size with the maximum fluorescence position. Curve resolution methods (EFA and MCR-ALS) were employed to decompose a series of fluorescence spectra to investigate the CdSe QDs size distribution and determine the number of fraction with different particle size. The results for the MPA-capped CdSe sample showed only two main fraction with different particle sizes with maximum emission at 642 and 686 nm. The calculated diameters from these maximum emission were, respectively, 2.74 and 3.05 nm.

Abstract:We conducted a comparative synthesis of water-soluble CdTe/CdS colloidal nanocrystalline semiconductors of the core/shell type. We prepared the CdS shell using two different methods: a one-pot approach and successive ionic layer adsorption and reaction (SILAR); in both cases, we used 3-mercaptopropionic acid (MPA) as the surface ligand. In the one-pot approach, thiourea was added over the freshly formed CdTe dispersion, and served as the sulfur source. We achieved thicker CdS layers by altering the Cd:S stoichiometric ratio (1:1, 1:2, 1:4, and 1:8). The Cd:S ratios 1:1 and 1:2 furnished the best optical properties; these ratios also made the formation of surface defects less likely. For CdTe/CdS obtained using SILAR, we coated the surface of three differently sized CdTe cores (2.17, 3.10, and 3.45 nm) with one to five CdS layers using successive injections of the Cd 2+ and S 2-ions. The results showed that the core size influenced the optical properties of the materials. The deposition of three to five layers over the surface of smaller CdTe colloidal nanocrystals generated strain effects on the core/shell structure.

In this work, we have synthesized CdTe quantum dots (QDs) dispersed in an aqueous medium at ambient temperature, and investigated their optical properties. Synthesis of CdTe QDs in the presence of simple amines removed the need for an additional energy source and inert atmosphere, in a simple and inexpensive experimental setup. The use of ammonia or hydrazine promoted nanoparticle growth by kinetic nanocrystal agglomeration in the initial growth stage. These weak electrolytes acted in the electrical double layer during the growth of the nanocrystals. A comparative study on the concentration of hydrazine in the reaction medium helped to investigate their role in nanocrystal growth. Substitution of hydrazine for ethylenediamine and other electrolytes like sodium chloride and ammonium chloride contributed to a better understanding of the mechanism that underlies the use of primary amines in the synthesis of CdTe. The synthesis conditions afforded the highest photoluminescence quantum yield for CdTe QDs prepared at room temperature (27.5%). Keywords: optical properties, semiconductors, chemical synthesis, luminescence, nucleation IntroductionNanocrystalline semiconductors with diameters smaller than the Bohr exciton radius of the material are called quantum dots (QDs). The physical properties of QDs depend heavily on nanocrystal size.1,2 The first syntheses of these materials date back to the 1980s. 3 QDs possess unique properties that distinguish them from other materials; e.g., nanocrystal size-dependent photoluminescence (PL) emission, narrow emission and absorption bands, elevated PL quantum yields, high PL intensity, good chemical stability, and resistance to photodegradation. 4 These characteristics make QDs applicable in an ever-growing number of fields, 5 such as technological areas, including solar cells, [5][6][7] LEDs, 8,9 photocatalytic processes, 10,11 and biomedical systems. 12 QDs can conjugate with biomolecules. Indeed, QDs have found application as biological markers in biomedical assays and in vivo imaging, [12][13][14][15][16][17] and as biosensors and sensors to detect small molecules and proteins. 18,19 A variety of experimental techniques are available to synthesize QDs, but the colloidal chemistry route stands out, this route affords matrix-free QDs and enables control of nanocrystal size, shape, and functional chemical surface. 20,21 According to the Lamer diagram, two main interdependent events underlie nanocrystal formation via the colloidal route: nucleation and growth. 22 Such events may stem from agglomeration of small clusters by means of kinetic processes and/or thermodynamically favored diffusion of monomers, as described by the classic colloid growth theory. 22,23 The colloidal chemistry method is flexible: it produces QDs in solvents with high boiling point, as in the case of organometallic synthesis, [24][25][26] or at 100 °C, in aqueous medium at normal pressure. 21,27 Although both methods have intrinsic advantages and disadvantages, 21,[24][25][26][27] authors have favored the s...