Transmission Electron Microscope (TEM) of Au Single Crystal
Transmission electron microscope(TEM) image of a gold particle that is <100 nm (1 nm = 10-9 m) in thickness and about 1 micron in lateral dimensions. For comparison purposes, a human red blood cell is about 10 microns or 10000 nm in diameter. The particle in the picture is a single crystal with the (111) plane forming the large facet seen.
A single crystal of a material is one in which the periodic arrangement of atoms in a solid has the same orientation everywhere. In contrast a polycrystal is made up of a large number of single crystals called grains that are separated by grain boundaries. Synthesis of such small particles, single or polycrystalline, of controlled size and shape forms an active area in the field of nanomaterials. Such small nanoparticles have properties different from their bulk (e.g. a chunk of gold that is 1 cm x 1cm x 1 cm) counterparts due to quantum confinement effects and their very large surface to volume ratio. This change is physical properties with a reduction in size to the nanometer regime and the potential application of such differences is what makes nanoparticles scientifically interesting. These Au (111) platelets for example have an inherent tendency to attach to biomolecules such as Cystein, thereby enabling bio-functionalization. This makes them potentially interesting for bio-medical engineering applications.
Studying the structure and properties of particles of such size is not an easy task. It requires the use of advanced characterization techniques such as the Transmission Electron microscope, which has been used to image this particle.
This nanoparticle was synthesized by using simple chemical techniques in Dr. RaviShankar’s lab by his PhD student Viswanath. Typically, such particles are synthesized using capping agents, chemicals that preferentially inhibit the growth of certain crystal faces. In the above example, a capping agent could have been used to prevent growth on the flat face, the (111) face of the FCC crystal and therefore yield the crystal in the shape observed. Vishawanath and Dr. Ravishankar however managed to obtain these particles without using any such capping agents by simply adjusting the thermodynamics of the chemical reaction that yielded the particles. In addition to Au they have also managed to synthesize particles of Ag, Pt, Pd and hydroxyapatite (the mineral that forms the human bone).
For more information visit Dr. Ravishankar’s faculty page and homepage.