Epitaxial CdSe-Au Nanocrystal Heterostructures by Thermal Annealing

ABSTRACT: The thermal evolution of a collection of heterogeneous CdSe−Au nanosystems (Au-decorated CdSe nanorods, networks, vertical assemblies) prepared by wet-chemical approaches was monitored in situ in the transmission electron microscope. In contrast to interfaces that are formed during kinetically controlled wet chemical synthesis, heating under vacuum conditions results in distinct and well-defined CdSe/Au interfaces, located at the CdSe polar surfaces. The high quality of these interfaces should make the heterostructures more suitable for use in nanoscale electronic devices.

FIGURE LEFT: Enlargement and flattening of the CdSe/Au interface (a-c). During heating at a high heating rate (2 K/s), the width of the interface grew by a factor of 3. Images are shown corresponding to temperatures of 150 °C (a), 250 °C (b), and 300 °C (c), respectively. Such an interface reconstruction enables the two crystal lattices to accommodate to each other to maximize the number of covalent bonds between them. various CdSe/Au interfaces with both the CdSe and the Au crystals in identifiable zone axes are shown (d-f) All configurations show the same epitaxial relationship, as indicated in the figure.

DENSsolutions Comments

Colloidal inorganic nanocrystals of semiconductors are promising materials in a variety of applications. For example, in photocatalysis and photovoltaics, the photoinduced generation of charge carriers in nanocrystals can lead to the oxidation/reduction of molecular species or to the generation of clean electrical energy, respectively. In both cases, the performance of the material depends strongly on its charge separation ability and on the construction of suitable nanocrystal-electrode interfaces.
The growth of metallic domains directly on the surface of the semiconductor nanocrystals can help to improve both factors. Thus the nature of the metal-semiconductor nanointerface and its influence on the local electronic structure of the nanorod have become subjects of particular interest in the past decade. Various efforts are devoted to improve the interface between metal and semiconductors.
Using the DENSsolutions heating system, the researchers are able to observe the In-Situ thermal evolution of heterogeneous CdSe-Au nanosystem. The low drift at elevated temperature allows structural and shape transformations in individual nanocrystals that could be imaged in real time with atomic resolution. The researchers showed that such treatment leads to heterostructures with better defined metal-semiconductor interfaces. The high quality of these interfaces should make the heterostructures more suitable for use in nanoscale electronic devices.