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Researchers discover "new type of crystalline aggregate"

Researchers have discovered a “new type of crystalline aggregate,” in which small rod-shaped crystals are mutually attached according to a unique relationship.

This crystalline aggregate consists of rod-shaped nanocrystals of zinc oxide (ZnO). Zinc oxide is an excellent semiconductor、absolutely non-toxic and compatible with living organisms and can be employed in numerous applications, such as transparent electronics, light condensers, piezoelectronics, cosmetics, and others. In the crystalline aggregates described here, a large number of ZnO nanorods bristle from the central core like pencil-leads, ultimately taking a shape that resembles a sea urchin. Crystalline aggregates exist everywhere and can be used in many devices. For example, in the field of thin film technology, research is being carried out with the aim of creating ordered stacked layers of crystalline substances. In the natural world, astonishing control can be seen in the biomineralization process which produces elaborate crystalline aggregates such as those in Coccolithophores. Research on crystalline aggregates does not aim simply at understanding and imitating the mechanism of growth of crystals in the natural world. It is also extremely important for organizing substances as required by many applications, particularly in small regions where manipulation of the individual constituents is difficult.

The newly-discovered law in connection with the formation of this crystalline aggregate is based on a property of zinc oxide called polarity. This means that the two opposite ends of a ZnO rod-shaped crystal are positively electrically charged and negatively electrically charged respectively. Until now, it had been unknown that when ZnO rod-shaped crystals form aggregates, whether alignment of this polarization occurs or not. It is now discovered that, in one aggregate, all of the ZnO rods grew with a polarization orientation, so that all the positively charged ends point outward (and in this case, the negatively charged tips are fixed at the center), or vice versa. As another important fact, the two types of aggregates are mutually related by a unique and simple law of symmetry. If the respective ZnO rods in one aggregate are turned upside down, their polarity reverses, and that aggregate changes to the other type of aggregate. This unique relationship between crystalline aggregates was not known previously.

This new law of aggregation is important in the development of new properties. Although all of the crystal rods are the same (i.e., have identical physical properties), because the charges of their respective tips are different, different properties exist in aggregates with positive tips and those with negative tips. It is not possible to obtain the same properties with a single piece substance or a random assembly of crystal rods without considering these polarities.

A large number of other important substances exist which also possess polarity and form aggregates with similar shapes, for example, gallium nitride, which is used in light emitting diodes. Accordingly, it is possible that the property of natural polarity control is applicable to all such crystalline aggregates. This is a completely new and unprecedented phenomenon. High expectations are placed on the future development of applications as substances with unique properties which had not existed in the past.

Source: National Institute for Materials Science /...

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