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Register a new userStructural deformations of two-dimensional planar structures under uniaxial strain: The case of graphene
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Nektarios Lathiotakis
Publication date
2017
fields
Physics
and research's language is
English
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In the present work, a method for the study of the structural deformations of two dimensional planar structures under uniaxial strain is presented. The method is based on molecular mechanics using the original stick and spiral model and a modified one which includes second nearest neighbor interactions for bond stretching. As we show, the method allows an accurate prediction of the structural deformations of any two dimensional planar structure as a function of strain, along any strain direction in the elastic regime, if structural deformations are known along specific strain directions, which are used to calculate the stick and spiral model parameters. Our method can be generalized including other strain conditions and not only uniaxial strain. We apply this method to graphene and we test its validity, using results obtained from {it ab initio} Density Functional Theory calculations. What we find is that the original stick and spiral model is not appropriate to describe accurately the structural deformations of graphene in the elastic regime. However, the introduction of second nearest neighbor interactions provides a very accurate description.
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In recent years, organic-inorganic hybrid perovskites have attracted wide attention due to their excellent optoelectronic properties in the application of optoelectronic devices. In the manufacturing process of perovskite solar cells, perovskite films inevitably have residual stress caused by non-stoichiometry components and the external load. However, their effects on the structural stability and photovoltaic performance of perovskite solar cells are still not clear. In this work, we investigated the effects of external strain on the structural stability and optoelectronic properties of tetragonal MAPbI3 by using the first-principles calculations. We found that the migration barrier of I- ion increases in the presence of compressive strain and decreases with tensile strain, indicating that the compressive strain can enhance the structural stability of halide perovskites. In addition, the light absorption and electronic properties of MAPbI3 under compressive strain are also improved. The variations of the band gap under triaxial and biaxial strains are consistent within a certain range of strain, resulting from the fact that the band edge positions are mainly influenced by the Pb-I bond in the equatorial plane. Our results provide useful guidance for realizing the commercial applications of MAPbI3-based perovskite solar cells.
Intermetallic compounds possess unique atomic arrangements that often lead to exceptional material properties, but their extreme brittleness usually causes fracture at a limited strain of less than 1% and prevents their practical use. Therefore, it is critical for them to exhibit either plasticity or some form of structural transition to absorb and release a sufficient amount of mechanical energy before failure occurs. This study reports that the ThCr2Si2-structured intermetallic compound (CaFe2As2) and a hybrid of its structure (CaKFe4As4) with 2 {mu}m in diameter and 6 {mu}m in height can exhibit superelasticity with strain up to 17% through a reversible, deformation-induced, lattice collapse, leading to a modulus of resilience orders of magnitude higher than that of most engineering materials. Such superelasticity also can enable strain engineering, which refers to the modification of material properties through elastic strain. Density Functional Theory calculations and cryogenic nanomechanical tests predict that superconductivity in CaKFe4As4 could be turned on/off through the superelasticity process, before fracture occurs, even under uniaxial compression, which is the favorable switching loading mode in most engineering applications. Our results suggest that other members with the same crystal structure (more than 2500 intermetallic compounds), and substitution series based on them should be examined for the possibility of manifesting similar superelastic and strain-engineerable functional properties.
We present a systematic study of the Raman spectra of optical phonons in graphene monolayers under tunable uniaxial tensile stress. Both the G and 2D bands exhibit significant red shifts. The G band splits into two distinct sub-bands (G+, G-) because of the strain-induced symmetry breaking. Raman scattering from the G+ and G- bands shows a distinctive polarization dependence that reflects the angle between the axis of the stress and the underlying graphene crystal axes. Polarized Raman spectroscopy therefore constitutes a purely optical method for the determination of the crystallographic orientation of graphene.
The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC(0001) are investigated by Auger electron spectroscopy (AES) and depolarized Raman spectroscopy. The selection of the depolarized component of the scattered light results in a significant increase in the C-C bond signal over the second order SiC Raman signal, which allows to resolve submonolayer growth, including individual, localized C=C dimers in a diamond-like carbon matrix for AES C/Si ratio of $sim$3, and a strained graphene layer with delocalized electrons and Dirac single-band dispersion for AES C/Si ratio $>$6. The linear strain, measured at room temperature, is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. The magnitude of the compressive strain can be varied by adjusting the growth time at fixed annealing temperature.
Among the interfacial transport modulations to the LaAlO3/SrTiO3 (LAO/STO) heterostructure, mechanical strain has been proven to be an effective approach by growing the LAO/STO films on different substrates with varying lattice mismatches to STO. However, this lattice-mismatch-induced strain effect is static and biaxial, hindering the study of the strain effect in a dynamic way. In this work we realize dynamic and uniaxial strain to the LAO/STO oxide heterostructure at low temperature, through mechanical coupling from a magnetostrictive template. This anisotropic strain results in symmetry breaking at the interface and induces further splitting of the electronic band structure and therefore produces different conductivities along the x and y in-plane directions. In particular, we observe that along the strained direction the interface conductivity decreases by up to 70% under a tensile strain, while it increases by 6.8% under a compressive strain at 2 K. Also, it is revealed that the modulation on the interfacial transport property can be anisotropic, i.e., the resistance changes differently when an excitation current is parallel or perpendicular to the strain direction. This approach of strain engineering provides another degree of freedom for control of transport properties of oxide heterostructures and opens an additional way to investigate strain effects in materials science.
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