Better Eyes for Seeing the Sub-Nano Details
The understanding and successful application of nanoscale materials and devices is critically dependent on implementation of new techniques for “seeing” the structural details that determine their behavior. For example, there is current interest in semiconductor “nanorods,” formed by chemical processes, which may be employed in the future in various types of optical sensing and photonics applications. But there are very few methods of determining the crystal structure and the crystal perfection of individual nanoscale objects. A CNS investigator has now developed a technique that uses a powerful scanning transmission microscope to determine the crystal structure of nanoscale objects with better than 1 nanometer spatial resolution. A 0.2 nanometer electron beam is scanned along the object (see Figure) and the electrons that are scattered in different directions at each location are collected and analyzed. This provides information on the local atomic arrangement, crystal structure, of the object. In the case of the cadmium selenide (CdSe) quantum rod shown in the Figure, which is ~50 nm long and ~6 nm in diameter, the conclusion is that the rod is composed of individual short pieces of CdSe more or less aligned to a common axis, but with very small misalignments between the individual elements of the nanorod. This indicates that these rods grow by smaller CdSe crystals joining together in the chemical solution rather than by CdSe molecules being attached to a single CdSe seed one by one. The misalignment due to this growth by aggregation can have a major impact on the electronic properties of the nanorod .
An electron microscope image of several cadmium selenide “nanorod.” The green spots indicate where measurements are made of the scattering of electrons which reveal the orientation of atomic
[CNS Investigator: J. Silcox]
For more information see:
Zhiheng Yu, Megan Hahn, Joaquin Calcines, Todd Krauss, and John Silcox, "Study of the Internal Structure of Individual CdSe Quantum Rods Using Electron Nanodiffraction" Applied Physics Letters, 86, 013101 (2005).