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Role of planar buckling on the electronic, thermal, and optical properties of Germagraphene nanosheets

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Abstract

We report the electronic, the thermal, and the optical properties of a Germagraphene (GeC) monolayer taking into account buckling effects. The relatively wide direct band gap of a flat GeC nanosheet can be changed by tuning the planar buckling. A GeC monolayer has an sp2 hybridization in which the contribution of an s-orbital is half of the contribution of a p-orbital leading to stronger σ-σ bonds compared to the σ-π bonds. Increasing the planar buckling, the contribution of an s-orbital is decreased while the contribution of a p-orbital is increased resulting in a sp3-hybridization in which the σ-π bond becomes stronger than the σ-σ bond. As a result, the band gap of a buckled GeC is reduced and thus the thermal and the optical properties are significantly modified. We find that the heat capacity of the buckled GeC is decreased at low values of planar buckling, which is caused by the anticrossing of the optical and the acoustic phonon modes affecting phonon scattering processes. The resulting optical properties, such as the dielectric function, the refractive index, the electron energy loss spectra, the absorption, and the optical conductivity show that a buckled GeC nanosheet has increased optical activities in the visible light region compared to a flat GeC. The optical conductivity is red shifted from the near ultraviolet to the visible light region, when the planar buckling is increased. We can thus confirm that the buckling can be seen as another parameter to improve GeC monolayers for optoelectronic devices.

Introduction

Research on two-dimensional (2D) materials indicates a great potential for next-generation electronic and optical applications due to their rich physical characteristics and outstanding electronic properties [1], [2], [3], [4], [5]. However, some of the 2D materials such as graphene and silicene have a vanishing gap and are thus called gapless materials. The vanishing gap causes problems for applications using graphene-based electronic devices. Thus, many investigations have tried to search for other 2D materials [6], [7]. In recent years, there a lot of attention has been given to new 2D materials such as BN [8], MoS2 [9], BeO [10], [11], and GeC [12], which have a wider band gap and can be considered as semiconductor materials.

Theoretical investigations have reported that GeC monolayers are semiconductors and structurally stable [13], [14]. This has led researchers to study them intensively. In addition to computational analysis using density functional theory, experimental synthesis has been used to investigate the production of GeC monolayer as a possible 2D material. Various synthesis techniques, including plasma-enhanced chemical vapor deposition, activated reactive evaporation, and chemical vapor deposition can all be used to create germagraphene monolayers [15], [16], [17].

The band gap of a GeC monolayer is found to be 2.1eV (GGA) and 4.06eV using Heyd–Scuseria–Ernzerhof (HSE) hybrid functional at zero value for the buckling factor, i.e. a flat structure [18]. The valuable band gap of GeC indicating semiconducting properties can be further improved using several techniques in order to enhance its possible role in thermoelectric and optoelectronic applications. One may control the band gap of a fully hydrogenated GeC monolayer by biaxial strain or external electric field and a semiconductor–metal phase transition takes place at certain elongation caused by biaxial strain. The band gap has thus been enhanced to 3.49eV displaying photocatalytic characteristics for water splitting [19]. Likewise, the mechanical, electronic, and magnetic properties of a GeC monolayer can be modified through hydrogen or halogen passivation [20], [21] Doping a GeC monolayer could be considering as another technique to modify the band gap. For instance, F and C dopant atoms in a GeC monolayer disrupt the planar structure and a surface-functionalized GeC monolayer with low-buckling results [22]. With this type of doping the band gap is seen to vary from 2.8eV to 3.2eV in calculations using HSE.

In this work, we perform DFT calculations based on the Kohn–Sham formalism implemented in the Quantum espresso software package [23], [24]. In the calculations, we tune the buckling parameter to study the electronic, the thermal, and the optical properties of a GeC monolayer. The results show that the buckling effects can be considered as an alternative way for controlling its physical properties, such as the band gap, the thermal conductivity and the heat capacity.

The structure of the paper is as follows: Section 2 includes details of the computational methods, and Section 3 demonstrates the calculated electrical, the thermal, and the optical properties for a GeC monolayer with different degree of buckling. The last section, Section 4, is the conclusion.

Section snippets

Methodology

A 2 × 2 supercell of a GeC monolayer with equal number of Ge and C atoms is considered. The GeC structure is fully relaxed with high values of cutoffs for the plane-waves kinetic energy and the charge densities fixed at 1088.5eV, and 1.088×104eV, respectively [25]. In the relaxation process, the forces on the atoms are less than 10−5 eV/Å , where a dense Monkhorst–Pack grid with 18 × 18 × 1 is used. The distance between GeC monolayers is assumed to be 20Å  in the z-direction, which is long

Results

In this section, we show the obtained results for the electronic, the thermal and the optical properties of a GeC monolayer with different values for the planar buckling parameter, Δ. In addition, for the sake of comparison, we recalculate the physical properties of a flat GeC monolayer, Δ=0.0, and use them as reference points to compare to.

Conclusions

In summary, we have used density functional theory to study the properties of a Germagraphene, GeC, monolayer by considering the buckling effects. The GGA-PBE functionals with full potential augmented plane waves has been used in the calculations. The findings for the electronic properties indicate that planar buckling results in a tunable band gap, and the energy band gap decrease by increasing planar buckling. This is due to redistribution of orbital hybridization and the contribution ratio

CRediT authorship contribution statement

Nzar Rauf Abdullah: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Validation, Visualization, Writing – original draft, Writing – review & editing. Yousif Hussein Azeez: Data curation, Formal analysis, Validation, Visualization, Writing – review & editing. Botan Jawdat Abdullah: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the University of Sulaimani, Iraq and the Research center of Komar University of Science and Technology, Iraq . The computations were performed on resources provided by the Division of Computational Nanoscience at the University of Sulaimani.

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