The improvements in random access memory that have driven many advances of the digital age owe much to the innovative application of physics and chemistry at the atomic scale.

Accordingly, a team led by UNL researchers has employed a Nobel Prize-winning material and common household chemical to enhance the properties of a component primed for the next generation of high-speed, high-capacity RAM.

The team, which published its findings in the Nov. 24 edition of the journal Nature Communications, engineered and tested improvements in the performance of a memory structure known as a ferroelectric tunnel junction.

The junction features a ferroelectric layer 100,000 times thinner than a sheet of paper, so thin that electrons can “tunnel” through it. This layer resides between two electrodes that can reverse the direction of its polarization — the alignment of positive and negative charges used to represent “0” and “1” in binary computing — by applying electric voltage to it.

The researchers became the first to design a ferroelectric junction with electrodes made of graphene, a carbon material only one atom thick. While its extreme conductivity makes graphene especially suited for small-scale electronics, the authors’ primary interest lay in how it accommodated nearly any type of molecule — specifically, ammonia — they placed between it and the ferroelectric layer.

A junction’s polarity determines its resistance to tunneling current, with one direction allowing current to flow and the other strongly reducing it. The researchers found that their graphene-ammonia combination increased the disparity between these “on” and “off” conditions, a prized outcome that improves the reliability of RAM devices and allows them to read data without having to rewrite it.

“This is one of the most important differences between previous technology that has already been commercialized and this emergent ferroelectric technology,” said Alexei Gruverman, a Charles Bessey Professor of physics who co-authored the study.

Ferroelectric materials naturally boast the quality of “non-volatility,” meaning they maintain their polarization — and can hence retain stored information — even in the absence of an external power source. However, the infinitesimal space between the positive and negative charges in a tunnel junction makes maintaining this polarization especially difficult, Gruverman said.

“In all memory devices, there is a gradual relaxation, or decrease, of this polarization,” he said. “The thinner the ferroelectric layer is, the more difficult it is to keep these polarization charges separate, as there is a stronger driving force in the material that tries to get rid of it.”

Gruverman said the team’s graphene-ammonia combination also shows promise for addressing this prevalent issue, significantly improving the stability of the junction’s polarization during the study.

Gruverman’s UNL co-authors included Haidong Lu and Dong Jik Kim, postdoctoral researchers in physics and astronomy; Alexey Lipatov, a postdoctoral researcher in chemistry; Evgeny Tsymbal, George Holmes University Professor of physics and astronomy; and Alexander Sinitskii, assistant professor of chemistry. The study was also authored by researchers from the University of Wisconsin-Madison and the Moscow-based Kurnakov Institute for General and Inorganic Chemistry.

The team’s research was conducted with the assistance of UNL’s Materials Research Science and Engineering Center — part of a nationwide network of MRSECs sponsored by the National Science Foundation — and also received support from the U.S. Department of Energy.

Nature Communications is the Nature Publishing Group’s multidisciplinary online journal of research in all areas of the biological, physical and chemical sciences.

A potential path to identify imperfections and improve the quality of nanomaterials for use in next-generation solar cells has emerged from a collaboration of University of Oregon and industry researchers.

To increase light-harvesting efficiency of solar cells beyond silicon’s limit of about 29 percent, manufacturers have used layers of chemically synthesized semiconductor nanocrystals. Properties of quantum dots that are produced are manipulated by controlling the synthetic process and surface chemical structure.

This process, however, creates imperfections at the surface-forming trap states that limit device performance. Until recently, improvements in production quality have relied on feedback provided by traditional characterization techniques that probe average properties of large numbers of quantum dots.

“We want to use these materials in real devices, but they are not yet optimized,” said co-author Christian F. Gervasi, a UO doctoral student.

In their study, detailed in the Journal of Physical Chemistry Letters, researchers investigated electronic states of lead sulfide nanocrystals. By using a specially designed scanning tunneling microscope, researchers created atomic-scale maps of the density of states in individual nanocrystals. This allowed them to pinpoint the energies and localization of charge traps associated with defects in the nanocrystal surface structure that are detrimental to electron propagation.

The microscope was designed in the lab of co-author George V. Nazin, a professor in the UO Department of Chemistry and Biochemistry. Its use was described in a previous paper in the same journal, in which Nazin’s lab members were able to visualize the internal structures of electronic waves trapped by external electrostatic charges in carbon nanotubes.

“This technology is really cool,” said Peter Palomaki, senior scientist for Voxtel Nanophotonics and co-author on the new paper. “When you really dig down into the science at a very fundamental level, this problem has always been an open-ended question. This paper is just the tip of the iceberg in terms of being able to understand what’s going on.”

The insight, he said, should help manufacturers tweak their synthesis of nanocrystals used in a variety of electronic devices. Co-author Thomas Allen, also a senior scientist at Voxtel, agreed. The project began after Allen heard Gervasi and Nazin discussing the microscope’s capabilities.

“We wanted to see what the microscope could accomplish, and it turns out that it gives us a lot of information about the trap states and the depths of trap states in our quantum dots,” said Allen, who joined Voxtel after completing the Industrial Internship Program in the UO’s Materials Science Institute. “The information will help us fine-tune the ligand chemistry to make better devices for photovoltaics, detectors and sensors.”

The trap states seen by the microscope in this project may explain why nanoparticle-based solar cells have not yet been commercialized, Nazin said.

“Nanoparticles are not always stable. It is a fundamental problem. When you synthesize something at this scale you don’t necessarily get the same structure for all of the quantum dots. Working at the atomic scale can produce large variations in the electronic states. Our tool allows us to see these states directly and allow us to provide feedback on the materials.”