Jun 4 2019
Headed by researchers at the Department of Energy’s Oak Ridge National Laboratory (ORNL), a group of scientists investigated the way atomically thin two-dimensional (2D) crystals grow on 3D objects and the way these crystals are stretched and strained by the curvature of those objects.
Strain-tolerant, triangular, monolayer crystals of WS2 were grown on SiO2 substrates patterned with donut-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). (Image credit: Christopher Rouleau/Oak Ridge National Laboratory, U.S. Dept. of Energy)
Reported in Science Advances, the latest findings highlight a new method in which strain can be directly engineered during the growth of atomically thin 2D crystals to develop single photon emitters for use in quantum information processing.
The researchers initially examined the growth of the flat crystals on substrates that were patterned with sharp trenches and steps. Over these flat obstacles, the crystals conformally grew up and down without altering their growth rates and characteristics which was quite unexpected.
Conversely, curvy surfaces meant that the crystals have to stretch as they grew on the substrate to retain their crystal structure. This development of 2D crystals into the third dimension provided a new, interesting opportunity.
You can engineer how much strain you impart to a crystal by designing objects for them to grow over. Strain is one way to make ‘hot spots’ for single photon emitters.
Dr Kai Xiao, Senior Staff Scientist, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory
Xiao, along with ORNL colleagues—postdoctoral researcher Kai Wang (currently at Intel) and David Geohegan—had conceived the study.
When 2D crystals grow conformally and perfectly over 3D objects, the strain can be potentially localized to produce high-fidelity arrays of single photon emitters. The material’s band gap is changed by compressing or stretching the crystal lattice. The band gap is the energy gap that exists between the conduction band and the valence band of electrons and predominantly determines the optoelectronic properties of a material.
Strain engineering will allow scientists to funnel charge carriers to reintegrate exactly where required in the crystal rather than at haphazard defect locations. The experimentalists customized curved objects to localize strain in the crystal, and then determined the ensuing shifts in optical characteristics. They eventually forced co-authors at Rice University—theorists Boris Yakobson, Henry Yu, and Nitant Gupta—to replicate and map the way the curvature promotes strain at the time of crystal growth.
At ORNL, both Xiao and Wang developed experiments along with Bernadeta Srijanto in order to study how 2D crystals are able to grow over lithographically patterned arrays of nanoscale shapes. Photolithography masks were initially used by Srijanto to safeguard specific areas of the surface of a silicon oxide during the light exposure, and she later etched away the surfaces that were exposed to light to leave behind vertically standing shapes, such as steps, cones, and donuts.
Along with another postdoctoral researcher Xufan Li which is currently at Honda Research Institute), Wang subsequently inserted the substrates inside a furnace in which vaporized tungsten oxide reacted with sulfur and eventually formed tungsten disulfide as monolayer crystals on the substrates. These monolayer crystals grew as a well-ordered lattice of atoms in flawless triangular tiles that become larger with time by introducing a sequential row of atoms to their external edges. The 2D crystals appeared to easily fold like paper over sharp trenches and tall steps, but growth over curved objects caused the 2D crystals to stretch to retain their perfect triangular shape.
The researchers discovered that with regards to single photon emitters, “donuts” measuring 40 nm in length were excellent candidates, because the crystals can consistently withstand the strain induced by them, and the highest strain was exactly in the donut “hole”, as determined by shifts in the Raman scattering and photoluminescence. In the coming days, arrays of various structures, including donuts, can perhaps be patterned anywhere that quantum emitters are preferred before growing the crystals.
Photoluminescence mapping was used by Wang and Alex Puretzky, a co-author at ORNL, to expose where exactly the crystals nucleated and how quickly the edges of the triangular crystals advanced as they grew over the donuts. After a detailed examination of the images, the team was surprised to find that while perfect shapes were maintained by the crystals, the crystals’ edges that had been strained by donuts grew more quickly.
To better describe this phenomenal acceleration, Puretzky created a model of crystal growth while colleague Mina Yoon performed first-principles calculations. The duo’s work demonstrated that strain can possibly induce defects on a crystal’s growing edge. Such defects can considerably increase the number of nucleation sites that seed the growth of crystals along an edge, enabling it to grow more quickly than before.
Conformity and curvature are the factors that force crystals to grow easily up and down over deep trenches and become strained by shallow donuts. Here, the example of gift wrapping can be considered. It is easy to wrap boxes because the paper can fold effortlessly to adapt to the shape. However, an unboxed mug or other irregularly-shaped objects with curves cannot be wrapped conformally (to prevent tearing the paper, one would need to stretch it like plastic wrap.)
Similarly, the 2D crystals stretch to conform to the curves of the substrate. However, the strain ultimately becomes excessive and the crystals crack to release the strain, as revealed by atomic force microscopy and other methods. Once the crystal splits, the growth of the still-strained material continues but in different directions for every new arm. Zhili Hu at Nanjing University of Aeronautics and Astronautics carried out phase-field simulations of crystal branching, and Mengkun Tian (formerly of the University of Tennessee) and Xiang Gao of ORNL examined the crystals’ atomic structure through scanning transmission electron microscopy.
The results present exciting opportunities to take two-dimensional materials and vertically integrate them into the third dimension for next-generation electronics.
Dr Kai Xiao, Senior Staff Scientist, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory
The researchers are now planning to investigate whether the performance of customized materials can be further improved by strain.
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. At the Center for Nanophase Materials Sciences, we’re developing tools that will allow us to probe these structures and their quantum information aspects.
Dr Kai Xiao, Senior Staff Scientist, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory
The paper is titled, “Strain tolerance of two-dimensional crystal growth on curved surfaces.”
Material growth and optical and structural characterizations were supported by the DOE Office of Science. These experiments were carried out at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility. The study also utilized resources of the National Energy Research Scientific Computing Center, which is also a DOE Office of Science User Facility. An Office of Naval Research grant supported the work at Rice University.