II–VI semiconductor nanocrystals in thin films and colloidal crystals

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Abstract

Semiconductor nanocrystals whose optical properties are largely determined by the quantum confinement effect are currently being extensively studied in both physics and chemistry. Highly luminescent thiol-capped CdTe and HgTe nanocrystals with desirable sizes ranging from less than two to approximately 8 nm have been recently synthesized in aqueous solutions by a wet chemical route. They were used for the preparation of composite multilayer thin films by the layer-by-layer (LBL) deposition technique. Films containing luminescent nanocrystals were made both on planar substrates and on submicron-sized monodisperse polystyrene spheres. Alternatively, nanocrystals have been incorporated as cores into silica spheres of desirable sizes. Composite nanocrystal/silica and core-shell latex/nanocrystal spheres have been used as building blocks for 3-D colloidal photonic crystals.

Introduction

II–VI semiconductor nanocrystals are currently of great technological interest as emitting materials for thin film electroluminescence devices [1], [2], [3] and as optical amplifier media for telecommunication networks, [4], [5] because of their strong bandgap luminescence and size-dependent optical properties due to the quantum confinement effect [6]. The incorporation of luminescent semiconductor nanocrystals into photonic crystals [7], [8] has obtained considerable attention recently as a promising pathway to novel light sources with controllable spontaneous emission.

Thiol-capped CdTe nanocrystals with size-dependent bandgap luminescence in the visible spectral region have been synthesized in aqueous colloidal solutions [9], [10] and used for fabrication of light-emitting diodes (LEDs) with light generation in the range 530–650 nm [3], and for impregnation of colloidal photonic crystals [8], [11], [12]. Recent experiments have shown that the quantum efficiency of photoluminescence (PL) of water-soluble thiol-capped CdTe nanocrystals can be sufficiently enhanced through pH control [10] and proper size-selective fractionation [13], up to 25–30% at room temperature. HgTe nanocrystals with very strong (up to 50% quantum efficiency at room temperature) bandgap luminescence in the near-IR spectral range have also been recently synthesized in aqueous dispersions [14], [15]. They can find applications in the telecommunication industry [4], [5], which is based on fiber-optical networks operating in the windows of transparency at 1300 and 1550 nm.

In this paper, we review our recent efforts in processing CdTe and HgTe nanocrystal dispersions into high quality thin films with thicknesses controllable on the nanometer scale using the layer-by-layer (LBL) assembly method both on planar substrates and on colloidal polystyrene (PS) spheres. The LBL assembly is based on the alternating adsorption of oppositely charged species, such as positively and negatively charged polyelectrolyte pairs [16] or polyelectrolytes and nanoparticles [17], [18], [19]. It can be equally effectively applied to the coating of both macroscopically flat [16], [17] and non-planar (e.g. colloidal particles) [18], [19] surfaces. We also briefly introduce a method of incorporation of semiconductor nanocrystals into silica spheres. Finally, we present some examples of colloidal photonic crystals (also called artificial opals [20]) made from bare latex spheres, composite nanocrystal/silica and core-shell latex/nanocrystal spheres, and of the latex-based photonic crystals with CdTe nanocrystals electrophoretically deposited therein [21], [22], [23], [24], [25].

Section snippets

Experimental section

Aqueous colloidal solutions of CdTe nanocrystals capped by thioglycolic acid and HgTe nanocrystals capped by thioglycerol were prepared as described previously [10], [14]. Fig. 1 shows room temperature PL spectra of two series of thiol-stabilized CdTe (2–5 nm size range) and HgTe (3–8 nm) nanocrystals. The bandgap luminescence of CdTe and HgTe nanocrystals is easily tunable with particle size in the visible and near-IR spectral regions, respectively, due to the quantum confinement effect. Fig. 1

Results and discussion

Fig. 2 presents a schematic illustration of the LBL assembly of polyelectrolytes and nanocrystals on planar substrates and colloidal spheres. In both cases, the consecutive electrostatic adsorption of oppositely charged species leads to the formation of multilayer composite structures containing nanocrystals.

CdTe- and HgTe-containing films formed by LbL deposition on planar substrates have been investigated by microscopy and spectroscopically. The internal structure of the LBL nanoparticulate

Conclusions

We have shown that semiconductor nanocrystals, with their exciting optical properties, can be used for fabrication of complex and ordered structures: composite thin films on planar substrates and colloids, and composite 3-D colloidal photonic crystals. The demonstration of luminescence in the visible and near-IR spectral ranges from CdTe and HgTe nanocrystals included into a solid film configuration, which could be the precursor for many different types of devices, is significant in the context

Acknowledgements

We are grateful to A. Kornowski (University of Hamburg) and J. Ostrander (Oklahoma State University) for assistance with TEM, and to Dr M. Harrison from the Corning Research Center (Ipswich, UK) for the luminescence measurements on HgTe nanocrystals. A.L. Rogach and A.S. Susha have been supported in part by an INTAS-Belarus research grant 97–250. A.L. Rogach acknowledges support of the DFG-Schwerpunktprogramm “Photonic Crystals”. A.L. Rogach and N.A. Kotov acknowledge the support of the

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