Scientists Pattern Nanoscale Donuts into 2D Crystal to Control its Electrical and Optical Properties

Researchers from Rice University have been spending money on “donuts,” assisting a national laboratory in bringing about a revolution in electronics and, maybe, quantum computing.

Scientists from Oak Ridge National Laboratory and their coworkers, including theoretical scientists from Rice’s Brown School of Engineering, patterned nanoscale donuts into a two-dimensional crystal and realized a new level of control over its optical and electrical properties.

With scientists giving more attention to nanoscale materials for applications such as quantum information processing, a technique to customize them from the bottom up will render them more practical.

The group, including Rice materials theorist Boris Yakobson and graduate students Nitant Gupta and Henry Yu, has reported the study outcomes in Science Advances.

Oak Ridge researchers headed by David Geohegan, Kai Xiao, and Kai Wang first developed 2D tungsten disulfide through chemical vapor deposition on a flat substrate patterned with sharp steps and trenches and curved in a single direction. The crystal easily matched with the obstacles without any variation in its properties.

However, when the supporting surface was made into a dome or well shape—a “non-zero” Gaussian curvature in scientific terms—the developing crystal was forced to adjust by contracting or stretching. Tensile strain was added by an outward curve, which also minimized its electrical bandgap to alter its semiconducting properties. Compressive strain was added by an inward curve, rendering it a stronger insulator.

This was the inspiration for the donuts. Micron-scale rings with a height of 20 to 180 nm were imprinted into the substrate and pressurized the crystal to match with the rising and falling curves, thereby compressing it around each donut’s outer slope and stretching it in the hole.

As the material grows over the topography of a donut, it creates localized strain. Strain in 2D material is beneficial because it allows us to tune its electronic properties, so we can change the bandgap at different places.

Nitant Gupta, Graduate Student, Rice University

The mathematical models developed at Rice University demonstrated that most of the strain (and the change in bandgap) occurred at the centers of the donuts, less on the body, and least outside, as established by photoluminescent and Raman spectra measurements at Oak Ridge.

According to Xiao, one method to create arrays of “hot spots” for single-photon emitters—a component of quantum computers—is to apply highly localized strain. “You can engineer how much strain you impart to a crystal by designing objects for them to grow over,” he said.

The scientists found out that the donuts’ height had an impact not just on the strain inside and outside but also on the growth pattern of the crystal. They discovered that donuts with a height of 40 nm were the sweet spot for maximum strain in the hole. Above this height, strain was distributed beyond the bumps and altered the normal triangular growth pattern of the crystal, making the material to branch out.

Growth over donuts measuring180 nm resulted in multiple merging crystals isolated by grain boundaries. The crystals were broken into separate domains by the high curvature of the substrate, signifying a potential technique for controlling those boundaries and their electronic properties.

Strain also had an impact on phonons, the collective vibrational excitations in a material that have an impact on the conduction of heat, sound, and electricity. While phonon modes were softened by tensile strain, they were stiffened by compressive strain.

Astonishingly, the crystals developed more rapidly in the flat plains between donuts. The models developed at Oak Ridge revealed that the strain prompted additional nucleation sites between the obstacles. Moreover, upon removing the crystals from the substrate, the strain vanished and there was no trace of the trench or donut patterns.

According to Yu, the new study continues a collaboration that began with patterning 2D materials onto cones.

In that paper we mainly explored how topography like a cone can create grain boundaries, essentially breaking the material up into two pieces. Here, we’re mostly focusing on how the strain changes the bandgap or the properties, and how to avoid breaking up.

Henry Yu, Graduate Student, Rice University

Yakobson stated that the study will pave the way for computational models that enable researchers to estimate the characteristics of any 2D material on any kind of supporting surface.

An even greater challenge is to solve the inverse problem—that is, to determine the topography of the substrate, which would yield the specific pattern of strain desired for electronic, optical or magnetic functionality of the deposited atomic film.

Boris Yakobson, Materials Theorist, Rice University

Alexander Puretzky, Bernadeta Srijanto, Xufan Li, Masoud Mahjouri-Samani, Christopher Rouleau, Akinola Oyedele, Mina Yoon, Xiang Gao, and Gyula Eres of Oak Ridge; Zhili Hu of Nanjing University of Aeronautics and Astronautics, China; and Mengkun Tian of the University of Tennessee are the co-authors of the study.

Wang is a postdoctoral research associate, Xiao a staff scientist, and Geohegan leader of the Functional Hybrid Nanomaterials Group at the Center for Nanophase Materials Sciences at Oak Ridge. Yakobson is the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry at Rice.

The study was funded by the U.S. Department of Energy (DOE) Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division; DOE’s Center for Nanophase Materials Sciences; DOE’s National Energy Research Scientific Computing Center; and the Office of Naval Research.

A model by Rice University shows how compressive strain (blue) and tensile strain (red) form as a growing two-dimensional crystal conforms to a micron-scale “donut” pattern on a substrate. The strain makes the hole a semiconductor and a possible source of single-photon emission. (Video credit: Nitant Gupta/Henry Yu/Yakobson Research Group)