Ultrathin PbS Sheets by Two-Dimensional Oriented Attachment

Abstract

Controlling anisotropy is a key concept to generate complex functionality in advanced materials. For this, oriented attachment of nanocrystal building blocks, a self assembly of particles into larger single crystalline objects, is one of the most promising approaches in nanotechnology. We report here the 2D oriented attachment of PbS nanocrystals into ultra-thin single crystal sheets with dimensions on the micrometer scale. We found that this process is initiated by co-solvents which alter nucleation and growth rates during the primary nanocrystal formation and finally driven by dense packing of oleic acid ligands on {100} facets of PbS. The obtained nanosheets can be readily integrated in a photo-detector device without further treatment.Controlled assembly leading to anisotropic nanostructures poses a conceptual challenge in materials research. Penn and Banfield [1, 2] described crystal growth, in which oxide nanoparticles coalesce in well defined crystalline orientations. Their method of oriented attachment of nanocrystals is now one of the most favorable techniques to grow linear or zig-zag-type one-dimensional nanostructures. In addition to strong size quantization effects occurring in these structures, their big advantage is solution processability making them attractive candidates for optoelectronic and thermoelectric applications in low-cost integrated systems. One-dimensional assemblies of oriented attachment have been reported, and in most cases the anisotropy during self-assembly is caused by crystal planes with preferred reactivity and dipole moments in the crystallites. Systems with cubic crystal symmetry, however, like PbS and PbSe, where beautiful one-dimensional oriented attachment occurs, are somewhat more difficult to explain. Oriented attachment, in this case, should result in three-dimensional networks rather than one-dimensional structures. The common explanation assumes that despite the strict monodispersity of the samples inhomogeneities in the chemical composition of surface planes exist and result in dipole moments within the nanocrystals. On the other hand organic ligand molecules play a crucial role in such processes by capping nanoparticle surfaces selectively and may hinder, modify, or trigger an oriented attachment [3]. In this work we show that the formation of ordered and densely packed ligand surface layers of oleic acid on {100} PbS surfaces can drive the normally isotropic crystal growths into a two-dimensional oriented attachment of nanocrystals. Hereby the presence of chlorine containing co-solvents during the initial nucleation and growth process of the nanocrystals plays a prominent role.

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