Single crystals from glass could expand high-tech materials

Materials scientists and physicists at Lehigh University have demonstrated a new method for making single crystals that could allow a wider range of materials to be used in microelectronics, solar energy devices and other high-technology applications. The researchers report their discovery in a paper in Scientific Reports.

According to the researchers, their breakthrough opens the way for glasses and other solid materials with disordered atomic structures to be made in a single-crystal form, like silicon, giving them the superior properties required in high-tech applications such as lasers and light-emitting diodes (LEDs).

Single crystals of silicon are grown through melting, said Himanshu Jain, one of the paper's four authors. But melting causes many other highly useful materials to decompose or change composition.

"The boundaries between the tiny crystals in polycrystalline materials are weak or bad links and give the materials undesirable properties," said Jain, professor of materials science and engineering at Lehigh. "A single crystal, having no boundaries, has superior properties. It is stronger mechanically in corrosive environments, it is electronically superior and it transmits light well."

Jain's group used a novel heating strategy to convert glass into a single crystal without it first having to pass through a gaseous or liquid phase, and without the creation of unwanted crystals or nuclei. "In this first unambiguous demonstration of an all-solid-state glass [to] crystal transformation, extraneous nucleation is avoided...via spatially localized laser heating and inclusion of a suitable glass former in the composition," the group wrote in the Scientific Reports paper.

The lead author of the paper, Dmytro Savytskii, is a research scientist in the department of materials science and engineering at Lehigh. The other authors are: Brian Knorr, an assistant professor of physics at Fairleigh Dickinson University who received his PhD in physics from Lehigh in 2014; and Volkmar Dierolf, distinguished professor and chair of Lehigh's physics department.

In a single crystal, said Jain, all the atoms of a material are arranged in a perfectly ordered 3D lattice structure. To make a single large crystal of silicon, scientists pull up a tiny seed of single-crystal silicon from melted silicon. The atoms from the melted silicon deposit on the seed in a lattice structure; depending on the speed at which the crystal is pulled, a small or large single crystal of silicon emerges as the melt cools.

In order to make a single crystal of an antimony-sulfide (Sb2S3) chalcogenide glass, Jain's group developed a contrasting method. To induce the formation of a crystal inside the glass, the group used a laser to heat the glass from an ambient temperature to a crystallization temperature well below the melting point of the glass. The researchers used electron diffraction and microscopy color mapping to detect the orientation of atomic configurations and to identify single crystallinity at different places in the sample.

The formation of the crystal, Jain notes, occurs as the solid glass is heating up and not, as is the case with silicon, when the melt is cooling. This is readily observed in scratches that run across the glass and through the single crystal formed by the laser. Any melting of the glass would smooth out and eliminate these scratches.

"Once we make the single-crystal line," said Jain, "we backtrack to get additional parallel single-crystal lines and eventually a single-crystal-layer surface on top of the glass. We can stitch these lines to convert the entire glass surface into a single crystal."

The group's goal, said Jain, is to apply enough heat so that the disorganized atoms in the glass organize into just one single crystal without triggering the nucleation of unwanted crystals. "We want just one nucleus to form. If we get multiple nuclei, we end up with a polycrystalline ceramic material with undesired properties."

To prevent extraneous nucleation, the group uses a focused laser to limit the volume of glass that is heated and permit only one nucleus to form. That nucleus is then quickly grown into a single crystal.

The group also came up with a second strategy that relies on glass of a predesigned composition of antimony, sulfur and iodine (Sb-S-I). As the single crystal begins to form and grow, iodine moves out of the crystal into the neighboring glass, where it acts to suppress nucleation.