${\mathrm{C}}_3{\mathrm{H}}_2$: A wide-band-gap semiconductor with strong optical absorption

Using first-principles calculations, we predict a new type of partially hydrogenated graphene system, ${\mathrm{C}}_3{\mathrm{H}}_2$, which turns out to be a semiconductor with a band gap of 3.56 eV. The bands are rather flat at the band edges and thus lead to a large density of states, which further results in strong optical absorption between the valence band and the conduction band. Particularly, it shows strong optical absorption at about 4.5 eV for the light polarized along the lines connecting the nearest unhydrogenated carbon atoms. Thus, the predicted ${\mathrm{C}}_3{\mathrm{H}}_2$ system may have potential applications for a polarizer as well as other high-efficiency optical devices in the near ultraviolet region.

Figure 1

(a) Periodic structure of a graphene lattice with six carbon atoms in a unit cell. ${\mathrm{C}}_3{\mathrm{H}}_2$ can be obtained by hydrogenating four carbon atoms in a unit cell. (b) First Brillouin zone and high-symmetry points for ${\mathrm{C}}_3{\mathrm{H}}_2$. The high-symmetry line $K\text{−}\mathrm{\Gamma }\text{−}M\text{−}K\text{−}{M}^{\prime }\text{−}{K}^{\prime }\text{−}\mathrm{\Gamma }\text{−}{M}^{\prime }$ is shown in green, along which the phonon spectra and the band structure will be calculated. (c) and (d) Initial structures of a ${\mathrm{C}}_3{\mathrm{H}}_2$ unit cell in the chair and boat configurations, respectively.

Figure 2

Phonon spectra of ${\mathrm{C}}_3{\mathrm{H}}_2$ along the high-symmetry line $K\text{−}\mathrm{\Gamma }\text{−}M\text{−}K\text{−}{M}^{\prime }\text{−}{K}^{\prime }\text{−}\mathrm{\Gamma }\text{−}{M}^{\prime }$.

Figure 3

(a) Electronic band structure of ${\mathrm{C}}_3{\mathrm{H}}_2$ along the high-symmetry line $K\text{−}\mathrm{\Gamma }\text{−}M\text{−}K\text{−}{M}^{\prime }\text{−}{K}^{\prime }\text{−}\mathrm{\Gamma }\text{−}{M}^{\prime }$. The Fermi level is set to be zero. (b) A magnified view of the band structure near the Fermi level. Two blue dashed lines touch the valence bands' maximum (VBM) and the conduction bands' minimum (CBM), respectively. An indirect band gap of 3.56 eV is labeled.

Figure 4

(a) Total electronic DOS of ${\mathrm{C}}_3{\mathrm{H}}_2$. (b) Orbital-projected electronic DOS for each atom in a ${\mathrm{C}}_3{\mathrm{H}}_2$ unit cell.

Figure 5

The $4\times 4$ supercell of ${\mathrm{C}}_3{\mathrm{H}}_2$. The blue/red balls are the hydrogenated C atoms with H atoms above/below the carbon plane, and the yellow balls are the unhydrogenated ${\mathrm{C}}_{2,5}$ atoms. Along the ${\mathrm{C}}_2\text{−}{\mathrm{C}}_5$ direction, the nearest unhydrogenated ${\mathrm{C}}_{2,5}$ atoms form a dimer. The dimers are circled by ellipses.

Figure 6

Optical properties of ${\mathrm{C}}_3{\mathrm{H}}_2$. (a) The imaginary part, ${\varepsilon }_2$, of the complex dielectric function and (c) the absorption coefficient with the light polarizations parallel and perpendicular to the line connecting the two C atoms in a dimer. Panels (b) and (d) are the magnified ${\varepsilon }_2$ and the absorption coefficient in the range of energy 0–8 eV, respectively.