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Proceedings Paper

Optical properties of II-VI semiconductor nanoclusters for use as phosphors
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Paper Abstract

The optical properties of both II-VI (direct gap) and type IV (indirect gap) nanosize semiconductors are significantly affected not only by their size, but by the nature of the chemical interface of the cluster with the embedding medium. This affects the light conversion efficiency and can alter the shape and position (i.e. the color) of the photoluminescence (PL). As the goal of our work is to embed nanoclusters into either organic or inorganic matrices for use as near UV, LED-excited phosphor thin films, understanding and controlling this interface is very important for preserving the high Q.E. of nanoclusters known for dilute solution conditions. We describe a room temperature synthesis of semiconductor nanoclusters which employs inexpensive, less toxic ionic precursors (metal salts), and simple coordinating solvents (e.g. tetrahydrofuran). This allows us to add passivating agents, ions, metal or semiconductor coatings to identical, highly dispersed bare clusters, post-synthesis. We can also increase the cluster size by heterogeneous growth on the seed nanoclusters. One of the most interesting observations for our II-VI nanomaterials is that both the absorbance excitonic features and the photoluminescence (PL) energy and intensity depend on the nature of the surface as well as the average size. In CdS, for example, the presence of electron traps (i.e Cd(II) sites) decreases the exciton absorbance peak amplitude but increases the PL nearly two-fold. Hole traps (i.e. S(II)) have the opposite effect. In the coordinating solvents used for the synthesis, the PL yield for d~2 nm, blue emitting CdSe clusters increases dramatically with sample age as the multiple absorbance features sharpen. Liquid chromatographic (LC) separation of the nanoclusters from other chemicals and different sized clusters is used to investigate the intrinsic optical properties of the purified clusters and identify which clusters are contributing most strongly to the PL. Both LC and dynamic light scattering, show that as the nanocluster concentration approaches 1 x 10-4M and above, a large loss in light emission occurs due to association or "clumping" of clusters. Overcoming this natural tendency toward aggregation may be the most significant technical obstacle to the use of nanoclusters in thin film phosphors.

Paper Details

Date Published: 5 November 2002
PDF: 16 pages
Proc. SPIE 4808, Optical Properties of Nanocrystals, (5 November 2002); doi: 10.1117/12.451976
Show Author Affiliations
Jess P. Wilcoxon, Sandia National Labs. (United States)
Paula Newcomer, Sandia National Labs. (United States)

Published in SPIE Proceedings Vol. 4808:
Optical Properties of Nanocrystals
Zeno Gaburro, Editor(s)

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