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John O | August 2017

Researchers develop technique for characterizing thermal property of 2-D material

researchers from the agency for science, technology, and research (a*star) in singapore have developed a simple technique for characterizing the thermal properties of crystalline molybdenum disulfide, a 2-d material that could be applied to a range of applications including energy storage, optoelectronics, and flexible electronic devices.


a*star researchers have characterized the thermal properties of mos2.
(wikimedia commons)


according to a report from a*star, the researchers worked with hexagonal molybdenum disulfide (mos2), one of a family of semiconducting transitional materials, which has already demonstrated electronic properties as well as surprising strength and flexibility.


“determining the thermal characteristics of mos2 is key to unlocking its astonishing properties, but its complex geometry and the many required calculations for phonons — the different vibrational modes of atoms in a crystal lattice — are a costly and time-consuming computational process,” the article explained.


researchers developed a numerical technique to reduce the number of calculations for accurately determining the material’s thermal expansion coefficient.


the article continued, “by deforming a crystal of mos2, the researchers determined the change in frequency for each phonon in the lattice structure, and by applying a numerical method, based on perturbation theory, to these altered frequencies; they were able to estimate the crystal’s thermal characteristics, known as the grüneisen parameters. these parameters were then used to calculate the thermal expansion coefficients for hexagonal mos2.”


the researchers believe that this method is applicable to other 2-d materials as well.


the research was recently published in physical review b. the abstract stated:


“using density-functional perturbation theory and the grüneisen formalism, we directly calculate the linear thermal expansion coefficients (tecs) of a hexagonal bulk system mos2 in the crystallographic a and c directions. the tec calculation depends critically on the evaluation of a temperature-dependent quantity ii(t), which is the integral of the product of heat capacity and γi(ν), of frequency ν and strain type i, where γi(ν) is the phonon density of states weighted by the grüneisen parameters.


“we show that to determine the linear tecs we may use minimally two uniaxial strains in the z direction and either the x or y direction. however, a uniaxial strain in either the x or y direction drastically reduces the symmetry of the crystal from a hexagonal one to a base-centered orthorhombic one. we propose to use an efficient and accurate symmetry-preserving biaxial strain in the xy plane to derive the same result for γ(ν).


“we highlight that the grüneisen parameter associated with a biaxial strain may not be the same as the average of grüneisen parameters associated with two separate uniaxial strains in the x and y directions due to possible preservation of degeneracies of the phonon modes under a biaxial deformation. large anisotropy of tecs is observed where the linear tec in the c direction is about 1.8 times larger than that in the a or b direction at high temperatures.


“our theoretical tec results are compared with experiment. the symmetry-preserving approach adopted here may be applied to a broad class of two lattice-parameter systems such as hexagonal, trigonal, and tetragonal systems, which allows many complicated systems to be treated on a first-principles level.”

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