Low‐Temperature Synthesis of Star‐Shaped PbS Nanocrystals in Aqueous Solutions of Mixed Cationic/Anionic Surfactants

Abstract

Obtaining control over the morphology of colloidal nanocrystals with well-defined shapes remains an important goal of modern materials chemistry, since shape-controlled nanocrystals are promising candidates as active components in a wide range of technological applications, and are model systems for the study of nanoscale shape-dependent properties. [1] In particular, the shape-controlled synthesis of colloidal nanocrystals of metal chalcogenide semiconductors, such as cadmium chalcogenides, [1a,b,2] lead chalcogenides, [3] Ag 2 S, [4] and MnS, [3b,5] has attracted intensive interest; in this regard, synthetic methods based on high-temperature (typically 120-360°C) reactions in organic solutions involving capping agents have most often been employed. Although this hightemperature organic solution approach has been demonstrated to be a promising method for achieving morphological control of semiconductor nanocrystals, low-temperature synthesis in aqueous solutions is highly desirable because it represents an environmentally benign and user-friendly approach, which may be considered to be a relatively green chemical alternative of practical significance. [6] However, the development of a facile, low-temperature synthesis of shape-controlled semiconductor nanocrystals in aqueous solutions remains a great challenge. It is to be noted that mixtures of various surfactants, including alkyl amines, alkyl acids, alkylphosphonic acids, and trioctyl phosphine oxide (TOPO), are frequently used as capping agents to tailor the crystal shape in high-temperature solution-phase syntheses. Recently, we have demonstrated that the low-temperature synthesis of morphologycontrolled one-dimensional (1D) BaXO 4 (where X = Cr, Mo, W) nanostructures and their hierarchical architectures can be readily achieved in reverse-micelle solutions of mixed cationic/anionic surfactants. [7] These results have inspired us to explore the low-temperature synthesis of semiconductor nanocrystals with tailored shapes in aqueous solutions of mixed cationic/anionic surfactants.As a p-p semiconductor with a narrow bandgap (0.41 eV) and a large exciton Bohr radius (18 nm), PbS shows extensive quantum-size effects in nanocrystalline form and has wideranging potential applications, such as in near-IR (NIR) communication, optical switches, thermal and biological imaging, photovoltaics, and solar cells. [3a,8] While much effort has been devoted to the size-controlled synthesis of PbS quantum dots with tunable optical properties, [9] there are only a few reports on the shape-controlled synthesis of high-quality colloidal PbS nanocrystals. [3a,b] Notably, PbS nanocrystals with a variety of novel shapes, including rod-based multipods, truncated octahedrons, cubes, and stars with six symmetric horns, were successfully produced by the thermal decomposition of a molecular precursor in hot phenyl ether solvent at temperatures varying from 140 to 250°C.[3a] The obtained star-shaped PbS nanocrystals (50-60 nm in size) represent novel nanoscale structures, but the...

New ways to synthesize lead sulfide nanosheets—substituted alkanes direct the growth of 2D nanostructures

“…This might be a texture effect as well, like with sheets, since the stars have a favorite orientation on the substrate -they are sitting with three of their arms on the substrate. We can assume that the arms are growing from {100} planes, 16,17 the planes parallel to the substrate would then be the {111} planes and have a higher chance to be detected by the XRD. After washing the product several times to remove excess ligand from the surface of the crystals the XRD intensities did not change ( Figure S3), which supports the notion that the arms grow in the <100> directions and the texture effect stems from the shape of the product.…”

Phase diagram of octapod-shaped nanocrystals in a quasi-two-dimensional planar geometry

“…[1][2][3][4][5][6][7][8] Such samples were used to experimentally study self-assembly and mesophase behavior. [6][7][8][9][10][11] Concurrently, new simulation techniques were developed to explain the experimentally observed phenomenology and to tackle the complex numerical problems that such investigations bring about.…”

Crystal-structure prediction via the Floppy-Box Monte Carlo algorithm: Method and application to hard (non)convex particles

“…[57][58][59] However, such methods have gained popularity in the physics and mathematics communities and show great promise for future studies, especially with the emergence of the graphics card as a powerful platform for particle simulations and image processing. [60][61][62] Moreover, there has been a marked increase in the ability to synthesize a stunning array of faceted (nonconvex) particles, 15,26,27,[31][32][33][34][35] as well as a large improvement in the level of control with which such particles can be prepared. 28 This has led to particular interest from the materials science community in these overlap algorithms to perform simulations on nanoparticle and colloid systems.…”

“…[21][22][23][24][25] Perhaps most remarkable of all is the level of control that has been attained over the synthesis of nonconvex, irregular, and even punctured particles. Branched colloids and nanocrystals, such as octapods [26][27][28][29][30] and tetrapods, 31,32 have been created, as well as other nonconvex shapes, e.g., nanostars 15,[33][34][35] and colloidal caps. [36][37][38] Moreover, there is a better understanding of the way to achieve phase behaviour that is dominated by entropic contributions in experimental systems.…”