Hexa-Graphyne: A Transparent and Semimetallic 2D Carbon Allotrope with Distinct Optical Properties

TL;DR

Hexa-graphyne (HXGY) is analyzed as a planar 2D carbon allotrope built from sp- and sp2-hybridized carbon in distorted hexagonal and rectangular rings. Using all-electron density functional theory with PBE and HSE06, phonon analysis, ab initio molecular dynamics, and RPA-based optics, the study demonstrates strong energetic, dynamical, and thermal stability while revealing a semimetallic electronic structure with delocalized frontier states. The material is mechanically very soft ($Y\approx 25.78\ \mathrm{N\,m^{-1}}$) and highly anisotropic in its electronic behavior, yet optically isotropic, with strong UV absorption, high IR reflectivity, and visible-light transparency; sharp Raman and IR signatures provide clear experimental fingerprints. Nanoribbons derived from HXGY show width- and edge-dependent electronic phases, enabling tunable nanoelectronic and optoelectronic functionalities, including potential for UV detectors and transparent coatings.

Abstract

Herein, we conduct a comprehensive investigation of Hexa-graphyne (HXGY), a planar carbon allotrope formed by distorted hexagonal and rectangular rings incorporating sp and sp$^2$-hybridized carbon atoms. First-principles calculations confirm its energetic, dynamical and thermal stability (up to at least 1000 K). Regarding its band structure, this material exhibits a semimetallic nature. It exhibits high mechanical compliance, with a Young's modulus approximately 13 times lower and a Poisson's ratio nearly 4 times higher than those of graphene. The optical response is marked by strong ultraviolet absorption, high infrared reflectivity, and pronounced transparency in the visible-light range. Raman and infrared spectra exhibit sharp and well-separated peaks, providing a clear signature of acetylenic linkage stretching vibrations. Nanoribbon structures derived from HXGY show distinct electronic behaviors depending on the edge termination type and width. These findings highlight the HXGY potential for nanoelectronic and optoelectronic applications.

Figures (10)

• Figure 1: (a) Optimized atomic structure of HXGY, showing the different C–-C bond lengths and bond angles. The hexagonal unit cell and lattice vectors are indicated by the black box. (b) Charge density difference, highlighting regions of charge accumulation and depletion.
• Figure 2: Phonon band structure and projected density of states (PDOS) of HXGY monoyer along the high-symmetry lines of the Brillouin zone, shown in the inset. The dynamical stability of the system is confirmed by the absence of complex frequencies (which would conventionally appear as negative frequencies in the plot).
• Figure 3: Total energy time evolution during AIMD simulations at initial temperatures of 300K (green) and 1000K (orange). The legend indicates the average temperatures for each simulation. The insets show the top and side views of the final atomic configurations.
• Figure 4: Variation of the elastic strain energy as a function of uniaxial and biaxial strain applied to the HXGY lattice vectors.
• Figure 5: (a) Electronic band structure and projected density of states (PDOS) of HXGY calculated at the PBE level. The horizontal gray dashed line indicates the Fermi level. Visual representations of (b) the highest occupied crystalline orbital (HOCO) and (c) the lowest unoccupied crystalline orbital (LUCO), with yellow and purple colors denoting different orbital phases.