Posted in | Microscopy | Nanoanalysis

Digging Atomic Force Microscope Tip Deep into a Material to Realize More Energy-Efficient Computer Chips

UConn scientists have reported in Proceedings of the National Academies of Science that when a familiar tool is used in a way it was never planned to be used, it paves the way for a completely new technique for exploring materials.

Someday, their particular findings could be put to use to develop far more energy-efficient computer chips; however, the new method could itself open up innovative discoveries in a wide range of fields.

Atomic force microscopes (AFM) pull an ultra sharp tip over materials, extremely close but without coming into contact with the surface. The tip feels where the surface is and detects the electric and magnetic forces generated by the material. A scientist can methodically pass it back and forth and map out a material’s surface properties in the same manner in which a surveyor methodically paces across a piece of land to map the territory. AFMs can offer a map of the protrusions, holes, and properties of a material at a scale thousands of times smaller compared to a grain of salt.

AFMs have been developed to explore surfaces. Mostly, the user tries really hard not to literally bump the material with the tip since the surface of the material could be damaged. However, at times, it does get damaged. A few years back, Yasemin Kutes, a graduate student, and Justin Luria, a postdoc, dug into their sample accidentally while exploring solar cells in materials science and engineering professor Brian Huey’s lab. Initially, they thought it was an irritating mistake and observed that the material’s properties seemed different when Kutes stuck the AFM’s tip deep into the ditch she had dug accidentally.

Although Kutes and Luria did not pursue it, James Steffes, another graduate student, was intrigued to have a closer look at the concept. He wondered, what would be the result of intentionally using an AFM’s tip like a chisel to dig into a material? Would it be in a position to map out the electrical and magnetic properties layer by layer, producing a 3D picture of the properties of the material in the same manner it mapped the surface in 2D? And will the properties be any different deep within a material?

The answers—reported by Steffes, Huey, and their colleagues in PNAS—are yes and yes. They dug into a bismuth ferrite (BiFeO3) sample—a room temperature multiferroic. Multiferroics are materials that have the ability to possess multiple magnetic or electric properties simultaneously. For instance, bismuth ferrite is antiferromagnetic—it reacts to magnetic fields, but does not exhibit a North or South magnetic pole on the whole—as well as ferroelectric; that is, it has switchable electric polarization. In general, such ferroelectric materials are formed of tiny sections known as domains. Each domain is similar to a cluster of batteries with all their positive terminals aligned in the same direction. The clusters on both sides of that domain will point toward a different direction. They are extremely valuable for computer memory since the computer can flip the domains, “writing” on the material, using electric or magnetic fields.

While reading or writing information on a piece of bismuth ferrite, usually, a materials scientist can only observe what happens on the surface. However, they would intend to observe what happens underneath the surface—gaining insights into that would enable the material to be engineered into highly efficient computer chips that operate faster and use less energy compared to the ones available at present. That could bring about a huge difference in the overall energy consumption of the society—already, 5% of the total electricity consumed in the United States is used up for operating computers.

In order to discover this, an AFM tip was used by Steffes, Huey, and the remaining team to meticulously dig through a bismuth ferrite film and map out the inside, piece by piece. They found it was feasible to map the individual domains all the way down, revealing patterns and properties that were not usually evident at the surface. At times, a domain was narrowed down until it disappeared or split into a y-shape, or merged with another domain. The interior of the material had never before been observed to this detail. It was indicative, similar to looking at a 3D CT scan of a bone when one had only been able to read 2D X-rays earlier.

Worldwide, there are something like 30,000 AFMs already installed. A big fraction of those are going to try [3D mapping with] AFM in 2019, as our community realizes they have just been scratching the surface this whole time.

Brian Huey, Materials Science and Engineering Professor, University of Connecticut

He also considers more labs will now purchase AFMs if 3D mapping is shown to work for their materials, and certain microscope manufacturers will start designing AFMs particularly for 3D scanning.

Steffes has subsequently graduated from UConn with his PhD and now works at GlobalFoundries, a computer chip maker. Researchers at Intel, muRata, and elsewhere are also fascinated with what the team discovered about bismuth ferrite, as they look for new materials to develop the next-generation computer chips. Meanwhile, Huey’s group is now using AFMs to dig into materials of all types, from concrete to bone to a wide range of computer components.

Working with academic and corporate partners, we can use our new insight to understand how to better engineer these materials to use less energy, optimize their performance, and improve their reliability and lifetime—those are examples of what materials scientists strive to do every day.

Brian Huey, Materials Science and Engineering Professor, University of Connecticut

This study was funded by the National Science Foundation, UConn, and the School of Engineering.

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