Growing 2D Crystals Over 3D Surfaces Could Create Quantum Emission

A team led by the Oak Ridge National Laboratory (ORNL) explored how 2D crystals grow over 3D objects and how the curvature of 3D objects can stretch and strain the 2D crystals. The team’s findings could lead to a strategy for engineering strain during the growth of crystals to fabricate single-photon emitters for quantum information processing.

When atomically thin 2D crystals were grown on substrates patterned with sharp steps and trenches, the crystals grew up and down these flat obstacles without changing their properties or growth rates. However, when the 2D crystals were grown on curvy surfaces, the crystals were forced to stretch as they grew in order to maintain their crystal structure. The researchers found that they could govern the amount of strain that was passed on to a crystal by controlling the topographic curvature of the substrate. Using strain engineering, the researchers could funnel charge carriers to recombine precisely where desired in the crystal, instead of at random defect locations. Stretching or compressing the crystal lattice changes the material’s bandgap, which largely determines its optoelectronic properties.

The ORNL team tailored curved objects to localize strain in the crystal and then measured the shift in optical properties that occurred as a result. Researchers at Rice University simulated and mapped how the curvature induced strain during crystal growth.

Strain-tolerant, triangular, monolayer crystals of WS₂ were grown on SiO₂ substrates patterned with doughnut-shaped pillars, as shown in scanning electron microscope (bottom) and atomic force microscope (middle) image elements. The curvature of the pillars induced strain in the overlying crystals that locally altered their optoelectronic properties, as shown in bright regions of photoluminescence (top). Courtesy of Christopher Rouleau/Oak Ridge National Laboratory, U.S. Dept. of Energy.

Further experiments at ORNL explored the growth of 2D crystals over lithographically patterned arrays of nanoscale shapes. The researchers used photolithography masks to protect certain areas of a silicon oxide (SiO₂) surface during exposure to light, and then etched away the exposed surfaces to leave vertically standing shapes, including doughnuts, cones, and steps. They inserted the substrates into a furnace where vaporized tungsten oxide and sulfur reacted to deposit tungsten disulfide (WS₂) on the substrates as monolayer crystals. The crystals grew as an orderly lattice of atoms in perfect triangular tiles that grew larger with time by adding row after row of atoms to their outer edges. While the 2D crystals seemed to effortlessly fold like paper over steps and trenches, when growing over curved objects they had to stretch to maintain their triangular shape.

The researchers believe that the 40-nm high doughnut shape could potentially be used for creating single-photon emitters because the crystals tolerated the strain induced by this shape in a reliable way. The maximum strain was in the “hole” of the doughnut, as measured by shifts in the photoluminescence and Raman scattering. In the future, the researchers said, arrays of doughnuts or other structures could be patterned anywhere that quantum emitters are desired; then crystals could be grown on those structures.

The researchers used photoluminescence mapping to reveal where the crystals nucleated and how fast each edge of the triangular crystal progressed as it grew over the doughnuts. After analyzing the images, they discovered that although the crystals maintained their perfect shapes, the edges of crystals that had been strained by doughnuts grew faster. To explain this acceleration, they developed a crystal growth model and conducted first-principles calculations. Their work showed that strain was more likely to induce defects on the growing edge of a crystal. These defects could multiply the number of nucleation sites that seed crystal growth along an edge, allowing it to grow faster than before.

Use of atomic force microscopy and other techniques revealed that when the strain became too great, the crystals split to release the strain. After the crystal cracked, growth of the still-strained material proceeded in different directions for each new arm. At Nanjing University of Aeronautics and Astronautics, researchers performed phase-field simulations of crystal branching. At ORNL, researchers analyzed the atomic structure of the crystals by scanning transmission electron microscopy.

Growing 2D crystals over an array of micron-scale “doughnuts” could lead to tailored materials for quantum information processing and other novel electronics, according to scientists at Oak Ridge National Laboratory and Rice University. Courtesy of Henry Yu.

“Strain is one way to make ‘hot spots’ for single-photon emitters,” said researcher Kai Xiao. “The results present exciting opportunities to take two-dimensional materials and vertically integrate them into the third dimension for next-generation electronics.”

Next the researchers will explore whether strain can enhance the performance of tailored materials. “We’re exploring how the strain of the crystal can make it easier to induce a phase change so the crystal can take on entirely new properties,” Xiao said. “At the Center for Nanophase Materials Sciences, we’re developing tools that will allow us to probe these structures and their quantum information aspects.”

The ability to lithographically design and synthetically engineer strain within 2D crystals on curved surfaces could enable tunable bandgaps, quantum phase transformations, and other emergent exotic properties for use of 2D materials in optoelectronics and quantum information science.

The research was published in Science Advances (https://doi.org/10.1126/sciadv.aav4028).

Atomistic simulation mimicking the growth of an MoS₂ monolayer on a curved substrate, by sequential addition of rows of atoms to the “growing edge” and relaxation with Stillinger-Weber potential. The atoms are colored by the level of biaxial strain with red indicating regions experiencing stretching, and blue indicating compression. Courtesy of Rice University.