Researchers from the Faculty of Chemistry and the Faculty of Materials Science, MSU along with foreign colleagues discovered that two-dimensional (2D) sheets of cadmium telluride are capable of naturally folding into nanoscrolls. This effect may be used in photonics and electronics. The study’s results have been published in the highly-rated Chemistry of Materials journal.
These are TEM images of 2D cadmium telluride sheers. Left: initial flat sheets. Right: sheets after folding. Upper right corner: a scheme of a folded sheet. (Image credit: Roman Vasiliev)
In the course of the research, the team concentrated on 2D semiconductor materials. Among others, they include graphene, 2D layers of molybdenum disulfide, phosphorene, and 2D perovskites that have lately garnered lots of attention from scientists. These materials are one atom thick crystals possessing 2D electronic properties. Researchers are certain that they may be used for the creation of new devices.
"We studied 2D cadmium telluride CdTe and discovered an unexpected effect of spontaneous folding of its ultrathin (only 1 nm) sheets that are also called colloidal quantum wells," said Roman Vasiliev, a co-author of the research, PhD of Chemical Sciences, and Associate Professor of the Faculty of Chemistry and the Faculty of Materials Science, MSU.
Colloidal quantum wells are the latest generation of colloidal quantum dots. Quantum dots are recognized by their luminescent properties and are used in commercial gadgets, such as TV sets. Quantum wells, a 2D type of quantum dots, are being explored today, but it is a known fact that they have very narrow luminescence bands, which is vital for bright color rendering in light-emitting devices.
The team examined the properties of 2D sheets of cadmium telluride by exchanging organic molecules attached to their surface and fortifying the stability of nanoparticles. So as to synthesize 2D cadmium telluride, the team used the colloidal technique and obtained them in a flask. To do so, the researchers started a reaction of getting cadmium telluride nanoparticles from an organic solvent in the presence of surfactants. By altering the conditions of the reaction they made the particles to grow into one nanometer thick sheets.
Initially, the researchers grew flat 2D sheets covered with oleic acid as a stabilizer. They managed to get sheets with length reaching hundreds of nanometers and thickness of exactly one nanometer. After that, the team began substituting the molecules of oleic acid with other organic molecules and examining shapes and sizes of the acquired nanoparticles, as well as their composition and crystal structure. At this stage, they made use of a transmission electron microscope from the MSU Shared Knowledge Center.
During the study, the team found out that when a particular type of stabilizers (thiols) is used, flat sheets of cadmium telluride unexpectedly fold into perfect scrolls. When attached to the surface of a sheet, thiol molecules grow in thickness by one monolayer (0.15 nm) and cause mechanical stresses making the sheet to fold in a specific crystallographic direction. The folding happens for all nanoparticles simultaneously, and the radius of the fold is identical for all nanostructures.
The study opens new prospects for manipulations with 2D materials and nanoparticles. The folding effect came as a surprise for us. It resembles the process of making origami, but in this case the sheets are one nanometer thick. Knowing how to change spatial shape of nanoparticles, we could use them in the manufacturing of optic materials with anisotropic behavior and polarized luminescence. We could create active light-emitting matrices for displays that would reduce energy consumption and increase brightness and intensity of various devices. Perhaps, we could also develop new nano-devices, for example, tube-shaped transistors. These interesting properties may be of use in new generations of light-emitting and sensor devices, as well as in optic and optoelectronic technologies and nanotechnologies.
Roman Vasiliev, Co-Author