2D crystals conform to 3D curves create a constraint for quantum device engineering

A team led by scientists from the Oak Ridge National Laboratory of the Ministry of Energy explored how two-dimensional (2D) atomically thin crystals can grow on three-dimensional objects and how the curvature of these objects can stretch and soften the crystals. The results, published in Progress of science, indicate a strategy to directly constrain engineers in the growth of atomically thin crystals to produce single-photon emitters for quantum information processing.

The team first studied the growth of flat crystals on substrates with steep steps and trenches. Surprisingly, the crystals developed conformably between these flat obstacles without changing their properties or their growth rates. Curved surfaces, however, required the crystals to stretch as they grew to maintain their crystal structure. This growth of 2D crystals in the third dimension offered a fascinating opportunity.

"You can create all the tension you transmit to a crystal by designing objects on which it can grow," said Kai Xiao, who, along with his colleagues at ORNL, David Geohegan and postdoctoral researcher Kai Wang (now at Intel) have designed the study. "Constraint is a way to create" hot spots "for single photon emitters."

The conformal growth of perfect 2D crystals on 3D objects could help locate the constraint in order to create high fidelity networks of single photon emitters. The stretching or compression of the crystal lattice modifies the forbidden band of the material, the energy gap between the valence and electron conduction bands, which largely determines the optoelectronic properties of a material . Using strain engineering, researchers can channel charge carriers to recombine precisely where they want in the crystal instead of locating random defects. By adapting the curved objects to locate the stresses in the crystal, and then measuring the resulting changes in optical properties, the experimenters coerced the co-authors of Rice University – the theoreticians Henry Yu, Nitant Gupta and Boris Yakobson – to simulate and map the effect of curvature on crystal growth.

At ORNL, Wang and Xiao designed experiments with Bernadeta Srijanto to explore the growth of 2D crystals on lithographic patterned matrices of nanometric shapes. Srijanto first used photolithography masks to protect certain areas of a silicon oxide surface when exposed to light, then etched surfaces. exposed to leave vertically standing forms, including donuts, cones and steps. Wang and another postdoctoral researcher, Xufan Li (now at the Honda Research Institute), then inserted the substrates into an oven where vaporized tungsten oxide and sulfur reacted to deposit tungsten disulfide on substrates under form of monolayer crystals. The crystals grew as an ordered array of atoms in perfect triangular tiles that grew larger with time by adding a row after a row of atoms at their outer edges. While the two-dimensional crystals seemed to bend effortlessly like paper on high steps and sharp trenches, growth on curved objects forced the crystals to stretch to retain their triangular shape.

Scientists have discovered that "donuts" of 40 nanometer height were excellent candidates for single photon emitters, as the crystals could reliably tolerate the stress that they induced, and that the maximum stress was precisely in the "hole" of the donut, measured by the variations of photoluminescence. and Raman dispersing. In the future, networks of donuts or other structures could be configured wherever quantum emitters are desired before crystal growth.

Wang and ORNL co-author Alex Puretzky used photoluminescence mapping to reveal where the crystals were nucleated and how fast each edge of the triangular crystal progressed as it grew above the donuts. After a careful analysis of the images, they were surprised to discover that, even if the crystals kept their perfect shape, the edges of the crystals stretched by the donuts grew faster.

To explain this acceleration, Puretzky developed a crystalline growth model and her colleague Mina Yoon made the calculations according to the basic principles. Their work has shown that the strain is more likely to induce flaws on the growing edge of a crystal. These defects can multiply the number of nucleation sites at the origin of crystal growth along one edge, allowing it to grow faster than before.

The reason why crystals can easily grow in deep, deep trenches, but become tense by shallow donuts, is related to compliance and curvature. Imagine packing gifts. The boxes are easy to pack because the paper can bend to fit the shape. But an irregularly shaped object with curves, such as an unpacked cup, is impossible to pack properly (to avoid tearing the paper, you must be able to stretch it like a plastic film).

2D crystals also stretch to conform to the substrate curves. Eventually, however, the stress becomes too great and the crystals separate to release stress, atomic force microscopy and other revealed techniques. After cracking of the crystal, the growth of the material still under stress continues in different directions for each new arm. At the Nanjing University of Aeronautics and Astronautics, Zhili Hu performed simulations of the crystalline phase by phase field. Xiang Gao of ORNL and Mengkun Tian (formerly of the University of Tennessee) analyzed the atomic structure of crystals by transmission electron microscopy.

"The results offer exciting opportunities to take two-dimensional materials and vertically integrate them into the third dimension of next-generation electronics," Xiao said.

Researchers will then explore whether stress can improve the performance of custom-made materials. "We are studying how the crystal strain can facilitate the induction of a phase change, so that the crystal can acquire new properties," Xiao said. "At the Center for Nanophase Materials Science, we are developing tools that will enable us to analyze these structures and their aspects relating to quantum information."

The title of the article is "Stress tolerance for two-dimensional crystalline growth on curved surfaces."