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...
