A grand challenge in materials science is to design and build a material with specific and useful properties – which might be electronic, mechanical, magnetic or optical. Nanoscience provides new tools for achieving this goal, since shrinking the size of a material often unlocks new properties.
We can build nanomaterials with tailored properties by individually moving each atom to the correct location, but this is too slow for use in the real world. A more practical approach is self-assembly, a spontaneous process that creates many nanostructure all at once.
Image 1: A complicated nanostructure (a magnetic bit made of 12 Fe atoms) created by moving each atom into place in a scanning tunneling microscope, contrasted with simple structures (water droplets on glass) created by spontaneous self-assembly, where surface tension forms each droplet into a hemispherical shape.
Can we use simple self-assembly processes to build complex nanostructures?
During self-assembly, the atoms move according to physical principles that can be quite simple: minimizing the total energy, or choosing the pathway with the lowest energy barrier. The motivation for the research is to try to understand the physics underlying a self-assembly process, giving us a better chance of controlling it to produce specific types of nanostructures.
Image 2: Transmission electron microscopy modified to allow ultra high vacuum conditions and growth from reactive gases
Image 3: A heated sample with central thin area for imaging, and an example of movie frames recorded during an experiment
Transmission electron microscopy (TEM) movies recorded during growth can provide atomic level details that transform our understanding of the growth process. We need to set up the microscope so that we can record growth as it takes place. This example shows an experiment carried out in the TEM at IBM. The photos show the microscope and details of how a sample is prepared and loaded into the TEM on a cartridge. In the movie shown here we used a silicon sample, heated it to over 600oC then exposed it to digermane, a reactive gas that contains germanium. The gas molecules crack (decompose) on the silicon surface, leaving Ge atoms behind. The Ge atoms spontaneously self-assemble into small islands that are visible in the movie because of the strain fields around each island.
These nanoscale islands, or “quantum dots”, form because of strain. As the Ge atoms stack up directly above the Si atoms they are compressed too close together leading to a high energy state. The energy is lowered by forming small bumps of Ge on the surface where the atoms can relax further apart. The resulting QDs have useful electronic properties but using them in devices requires control of their shape, size and position. The movie shows that the process of “coarsening”, or exchange of Ge between QDs, dominates QD evolution during growth. A growth model developed from the measurements helped to understand island evolution and suggest conditions to optimize shape and size uniformity.
Image 4: The process of coarsening, and how control of nucleation sites can reduce its effects and lead to regularly sized QDs
This is one example of how electron microscopy can help to explain and control a growth process. Check out the tabs on the left for descriptions of other research projects.