Formation of Arrays of Free-Standing CdS Quantum Dots Using the Langmuir-Blodgett Technique

Abstract

Aggregation clusters of CdS quantum dots (QDs) on the highly ordered pyrolytic graphite substrate were investigated by atomic force microscopy and scanning tunneling microscopy. QDs were initially formed within Y-type Langmuir-Blodgett (LB) films of cadmium behenate. The LB matrix was then removed by annealing. To study the process of QD assembly, the density of QDs in the initial film was varied systematically by increasing the number of monolayers (MLs) 0 from 1 to 20 ML. It was found that, at a small LB ML number, only a small part of CdS molecules form into QDs by diffusion along the plane of LB layers. With an increase of the LB ML number, the interlayer diffusion arises that leads to almost full binding of CdS molecules into QDs. The individually standing QDs were formed on the substrate for 1 and 2 ML samples. The QD size distributions of the samples with a small ML number are broader than that predicted by the two-dimensional Lifshitz-Slezov diffusion model. With an increase of the LB matrix thickness, QDs begin to assemble into clusters, revealing a ribbon structure due to diffusion-limited aggregation, and for the 20 ML LB matrix, a submonolayer QD film arises. By analyzing the occurrence of QDs as a function of aggregation number, pair bondings were estimated to be about 34 and 62 meV for the 4 and 8 ML samples, respectively.