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John O | October 2017

New theory gives scientists greater control of properties of 2-D materials


Researchers at Pennsylvania State University (University Park, Pa.) have theorized a method for controlling the grain boundaries of 2-D materials, areas where clusters of atoms meet other clusters on a substrate, which would give scientists greater influence over the properties of the materials that they are creating.

 

 


A grain boundary forms when graphene growth advances past an apex on a conical bump; it terminates at the foot of the bump, where curvature is saddle-like. (Crespi Lab / Penn State)

 

 

According to a report from the university, this new theory could impact the ability of scientists to develop new materials with tuned properties, such as electrical or thermal conductivity, mechanical properties, or magnetism.

 

When 2-D materials are created, researchers deposit atoms onto a substrate and those atoms self-arrange into crystalline clusters called grains. As the grains expand, they connect with other clusters and the area that they meet is called the grain boundary. Until now, that boundary area has been arbitrary, but it has a significant impact on a material’s properties.

 

Researchers theorized that “by manipulating the underlying substrate, they could predetermine where the grain boundaries would begin and end, and make them line up in orderly positions. The key shapes were based on something called Gaussian curvature, a series of hemispherical bumps and dips on a substrate that resembles an egg carton.”

 

Calculations demonstrated that in graphene and molybdenum disulfide (two of the most widely studied 2-D materials) the growth would form grain boundaries in pre-determined places without generating folds by detaching from the substrate.

 

Dr. Vincent Crespi, who led the research, noted, “We didn't know we could have such fine control of grain boundaries, and so we didn't think about carefully studying the magnetic, thermal and electronic properties of grain boundaries with an eye toward creating 'grain boundary materials' whose properties are determined by a controlled distribution of specified grain boundaries."

 

The research was recently published in Nano Letters. The abstract stated:

 

“Grain boundaries in two-dimensional crystals are usually thought to separate distinct crystallites and as such they must either form closed loops or terminate at the boundary of a sample. However, when an atomically thin two-dimensional crystal grows on a substrate of nonzero Gaussian curvature, it can develop finite-length grain boundaries that terminate abruptly within a monocrystalline domain.

 

“We show that by properly designing the substrate topography, these grain boundaries can be placed at desired locations and at specified misfit angles, as the thermodynamic ground state of a two-dimensional (2D) system bound to a substrate.

 

“Compared against the hypothetical competition of growing defectless 2D materials on a Gaussian-curved substrate with consequential fold development or detachment from the substrate, the nucleation and formation of finite-length grain boundaries can be made energetically favorably given sufficient substrate adhesion on the order of tens of meV/Å2 for typical 2D materials.

 

“New properties specific to certain grain boundary geometries, including magnetism and metallicity, can thus be engineered into 2D crystals through topographic design of their substrates.”

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