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ABSTRACT

We describe a method to synthesize optical quality thin and thick films of close packed CdSe nanocrystallites (quantum dots). The average dot size is tunable from -12-150 A in diameter with -3.5% rms size distribution. The identity of each particle and the high monodispersity of the sample are maintained in the films as revealed by small-angle x-ray scattering, transmission electron microscopy, and optical spectroscopy. We use small-angle x-ray diffraction patterns to obtain form factors for the individual dots and to generate radial distribution functions for the densely packed films. We observe the random close packing of dots in the solid state using transmission electron microscopy. Comparing optical spectra of particles close packed in a film with those of nanocrystallites dispersed in alkanes reveals similar optical resonances in absorption while the emission lineshape is modified and its position is red shifted. Differences in film emission are consistent with electronic energy transfer between close packed dots within the inhomogeneous distribution of the sample.

INTRODUCTION

Spatial confinement of electronic excitations in semiconductor quantum dots (QD) gives rise to discrete electronic transitions which shift to higher energy with decreasing particle size'. Refined synthetic routes to produce highly monodisperse samples of Il-VI semiconductor nanocrystallites have made it possible to determine the hexagonal crystal structure and to assign the quantized electronic states resolved in optical spectra of nanocrystallite samples 24. Previous investigations have been aimed at characterizing the structural and optical properties of the individual quantum dote. Synthesis of a close packed solid of semiconductor nanocrystallites presents new possibilities in exploring collective physical phenomena arising from the interaction between clusters in close proximity6 . A solid state structure provides a convenient medium for novel optical and electronic devices which exploit the unique properties of the individual dots as well as any collective effects 7. In this proceedings, we present the synthesis, structural characterization, and optical spectroscopy of quantum dot solids prepared from highly monodisperse samples of CdSe nanocrystallites.

EXPERIMENTAL

Preparation of Close-Packed Structures Samples of CdSe nanocrystallites with particle sizes tunable from -12-150 A in diameter and -3.5% rms size distribution were synthesized according to the method of Murray, et a?. The 219

Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society

preparation of these particles involves injecting a mixture of dimethyl cadmium and tri-octyl phosphine selenide into hot tri-octyl phosphine oxide and growing nucleated CdSe seeds to the desired particle size. Post-fabrication processing of the nanocrystallites using size selective precipitation relies on the size dependent stability of the particles in solvents and nonsolvents to further narrow the cluster size distribution 2"3. During this step the particles