Atomic arrangements and electrical properties are closely related. Think of phase change materials such as Ge-Sb-Te (GST) alloy: When the alloy is crystalline (amorphous), its resistance is low (high). Memory devices using this property are attractive because they are nonvolatile and easily scalable. In our group, we investigated nanostructural changes of working phase change memory devices in situ inside TEM, by observing the phase change response of GST nanobridges to electrical signals we applied to them. We find that reality is always more complicated than a simple picture you have in mind, that is when GST nanobridges undergo crystalline-to-amorphous changes, the phase change is often not pure, but rather mixed. This observation was possible because of the in situ TEM electrical measurement capability that allows one-to-one correlation between the electrical and structural properties. Ongoing project – in situ studies of GST nanobridges using the TEM electrical holder

In nano world, even mechanical properties – something you think is mundane – can be quite exotic and different from the bulk counterpart. Carbon nanotubes, stronger than steel, is a good example. State-of-art wind turbines and turbo engine blades employ nanotechnology for stronger, lighter, and longer life components. Thus, studying the mechanical properties of nanomaterials is important. What would be even more exciting is to watch how the nanomaterials respond under mechanical stresses in real time. How would defects be created and propagated when you squash or stretch a nanotube, for example? Would the mechanical response change drastically if the number of atoms involved is limited to only a few hundred atoms? Using in situ nanoindentation holder in TEM, our group is investigating how a nanostructure would deform under a mechanical stress induced by a diamond tip in situ.

Ongoing project – nanoindentation of gold nanoparticles

Dark field TEM images of a gold nanoparticle, before and after indented with a diamond tip. Twinning boundaries and other defects can be easily imaged in dark field where only a few diffracted spots are selected to form an image.

Because of its versatile functionality and ease of use, scanning probe microscopy (SPM) has been widely used to investigate local interactions in microscopic nature. Conductive atomic force microscopy (C-AFM) can provide local electronic signals conveniently, but the probe resolution of C-AFM has been limited by the tip geometry, which is defined by standard semiconductor technology.

We take advantage of nanoscale filament formation, which is previously reported to be small as a few atoms’ size, to improve the lateral resolution greatly. An atomic metal filament is formed in an insulating layer on the conventional AFM via resistive switching initiated by electrostatic potential. The filament on top of the AFM tip builds extremely small conductive path so that it demonstrates ~ 1 nm lateral resolution in C-AFM, which is more than 10 times finer than previous technology. Not limited on conductivity measurement, we also anticipate this atomic filament formation will be applicable for various scanning probe techniques. Potential applications would be high resolution magnetic probe and electrochemical probe, as well as bio-chemical sensing in molecular level.