Self-Organization of CdSe Nanocrystallites into Three-Dimensional Quantum Dot Superlattices

Abstract

The self-organization of CdSe nanocrystallites into three-dimensional semiconductor quantum dot superlattices (colloidal crystals) is demonstrated. The size and spacing of the dots within the superlattice are controlled with near atomic precision. This control is a result of synthetic advances that provide CdSe nanocrystallites that are monodisperse within the limit of atomic roughness. The methodology is not limited to semiconductor quantum dots but provides general procedures for the preparation and characterization of ordered structures of nanocrystallites from a variety of materials.T h e engineering. of materials and devices --on the nanometer scale is of considerable current interest in electronics ( I ) , optics (2), catalysis (3), ceramics (4), and magnetic storage (5). Nanometer-sized crystallites (nanocrystallites) can display optical, electronic, and structural properties that often are not present in either isolated molecules or macroscopic solids. Well-defined ordered solids prepared from tailored nanocrystalline building blocks provide opportunities for optimizing properties of materials and offer possibilities for observing interesting and potentially useful new collective physical ~henomena. Uniform particles, whether atomic, molecular, or colloidal, organize to form ordered solids when attractive and repulsive . interparticle forces are properly balanced. Ordered colloids or "colloidal crystals" have attracted scientific attention for more than 50 vears and remain an active area of researkh (6). The inherent tendency for monodisperse lyophobic colloids to self-organize provides a general route to new nanostructured materials.Nanocrystallites of semiconductors have discrete electronic transitions that are tunable with size. Thev are often referred to as quantum dots (QDs) or artificial atoms. Their highly polarizable excited states are potentially useful in optoelectronic applica-have been predicted for three-dimensional (3D) ordered arrays (superlattices) of QDs (7). Two-dimensional arrays of substantially larger dots fabricated with the use of lithogr a~h v show that unusual electronic behav-. , ior results from dot interactions (8).In this reDort we demonstrate that suDerlattices of nanometer-sized QDs can be generated either in solution as the "crystallization" of a monodisperse colloid or at a solid or liquid interface as a thin, ordered superlattice of dots. In both cases, the size and spacing of the dots are controlled with a precision that is limited by atomic roughness. Assembly at an interface has the added benefit that both the superlattice and the individual dots have their crystallographic axes oriented with respect to the interface, presenting an opportunity to control and study optical anisotropy in the individual crystallites and in the ordered arrays.Nanocrystallites of CdSe are ideal building blocks for the formation of Q D superlattices. Svnthet ic methods ~roduce macroscopic quantities of single-domain wurtzite crystallites with low defect densities, a uniform shape (slightly p...

Self-assembly of shape-controlled nanocrystals and their in-situ thermodynamic properties

“…Several questions, however, must be considered: (1) how high does the annealing temperature need to be to remove the capping polymer while the particle shape is still preserved? ; (2) to what temperature is the particle shape still stable? ; (3) is there a temperature induced shape transformation due to surface diffusion?…”

“…The electric, optical, transport and magnetic properties of the structures depend not only on the characteristics of individual nanocrystals, but also on the coupling and interaction among the nanocrystals arranged with long-range translational and even orientational order. Research has successfully fabricated self-assembly passi6ated nanocrystal superlattices (NCSs) of metal [1], semiconductor [2] and oxide clusters [3], which are a new form of materials with fundamental interests and technological importance.…”

Synthesis and Properties of Biocompatible Water-Soluble Silica-Coated CdSe/ZnS Semiconductor Quantum Dots

“…3,4 The ability to control these parameters has a profound impact in materials science since it can be harnessed for engineering assemblies of nanometer scale units with novel characteristics. [5][6][7][8] At this point, the prominent focus has been the optical properties of these semiconductor nanocrystals. They are governed by strong quantum confinement effects and, therefore, are size dependent.…”

Semiconductor Nanorod Liquid Crystals

“…Since methods were first developed to encapsulate nanocrystals with a monolayer of protective surfactant (1,2 ), nanocrystals have been assembled as active components inside plastic electronics (3,4,5), and they have been assembled into dimers, trimers (6), and crystals of nanocrystals (7,8). In each case the nanocrystals are treated as a conventional polymer or biological macromolecule from the assembly point of view.…”