Effects of strain on electronic and optic properties of holey two-dimensional C2N crystals

Abstract

A new two-dimensional material, the C2N holey 2D (C2N-h2D) crystal, has recently been synthesized. Here we investigate the strain effects on the properties of this new material by first-principles calculations. We show that the material is quite soft with a small stiffness constant and can sustain large strains ≥ 12%. It remains a direct gap semiconductor under strain and the bandgap size can be tuned in a wide range as large as 1 eV. Interestingly, for biaxial strain, a band crossing effect occurs at the valence band maximum close to a 8% strain, leading to a dramatic increase of the hole effective mass. Strong optical absorption can be achieved by strain tuning with absorption coefficient ∼ 10 6 cm −1 covering a wide spectrum. Our findings suggest the great potential of strain-engineered C2N-h2D in electronic and optoelectronic device applications.Since the discovery of graphene, two-dimensional (2D) materials have attracted tremendous interest due to their many fascinating properties.1-4 One current focus is to explore new 2D materials with suitable semiconducting bandgaps for device applications. Recently, such a new 2D crystal, C 2 N holey 2D (C 2 N-h2D) crystal, has been successfully synthesized.5 The material has a direct bandgap (with reported optical gap size around 2 eV), and a field-effect-transistor fabricated based on it shows a high on/off ratio of 10 7 , suggesting its great potential for applications in electronics and optoelectronics. 5-8For application purposes, it is crucial to have the ability to tailor electronic properties of the material. Strain has long been known as an effective mechanism for tuning properties of semiconductors. It is especially useful for low-dimensional systems since they can usually sustain much larger strains than their bulk crystals. In particular, it has been shown that 2D materials, such as graphene, MoS 2 , and phosphorene, have excellent mechanical flexibility (with critical strains ≥ 25%), [9][10][11][12][13] which makes strain an extremely powerful approach for engineering the properties of 2D materials. 14-19Motivated by the urgent need in understanding the physical properties of the newly discovered C 2 N-h2D material and by the great interest in engineering it for applications, in this work, we investigate the effects of biaxial and uniaxial strains on the electronic and optic properties of monolayer C 2 N-h2D crystals using first-principles calculations. We find that the material is quite flexible with a small stiffness constant and can withstand strains ≥ 12%. Under different types of strain, while still maintaining a direct bandgap, the gap size can be tuned in a wide range as large as 1 eV. More interestingly, for biaxial strain, due to different bonding characters of the bands, there is a switch of band ordering near the valence band maximum (VBM) at a critical strain < 8%, leading to a strain-induced dramatic increase of the hole effective mass. Despite its atomic thickness, this material shows fairly large optical absorption over most visible light s...

Ferromagnetism and Half‐Metallicity in Atomically Thin Holey Nitrogenated Graphene Based Systems

“…However, the maximum stress (12.02 GPa) occurs at 12 % biaxial tensile strain (elastic limit is 0.12). So, the elastic strain limit of the C N @C 2 N system is comparable to pure C 2 N (uniaxial strain 17 % and biaxial 12 %) systems . We calculated the mechanical properties of C N @C 2 N by calculating the Young's modulus ( Y ) and Poisson's ratio ( η ).…”

“…Thus, the monolayer of C N @C 2 N satisfies all the criteria (such as C 11 > |C 12 | and C 44 >0) to be mechanically stable. Our calculated Young's modulus (55.94 N m −1 ) and Poisson's ratio ( η =0.25) of C N @C 2 N are very comparable with the pure holey C 2 N system ( Y =71 N m −1 and η =0.28) . Thus, planar C N @C 2 N shows similar mechanical properties as pure C 2 N and thus can be synthesized …”

Tunable half-metallic magnetism in an atom-thin holey two-dimensional C2N monolayer

“…It shows a semiconducting phase with a direct bandgap ≈ 1.65 eV at the Γ point, in good agreement with the previous DFT results and the experimental value (∼ 1.96 eV). 20,26,27 The conduction band minimum (CBM) at the Γ point involves three nearly degenerate states, and one band has almost flat dispersion along the Γ-M direction (see figure 2(c)). This leads to a prominent van Hove singularity in the density of states (DOS) at the CBM.…”

“…The optimized lattice parameter that we obtain is 8.330Å, which agrees well with the experiment measurement and previous theoretical studies. 20,26,27,29,30 The structure involves three types of bonds, as indicated in figure 1(b). We find that C-N bond ≈ 1.339Å, C-C(1) bond ≈ 1.432Å, and C-C(2) bond ≈ 1.469Å.…”

“…[36][37][38][39] A previous computational study has shown that the critical strain of h2D-C 2 N monolayer is larger than 12%. 27 In figure 5(a), we plot the band structure of h2D-C 2 N monolayer without doping under a uniaxial 8% strain along the armchair direction. Compared with the unstrained band structure in figure 2(a), one observes that the initially degenerate states at the CBM split due to the reduced symmetry, and a more dispersed band is pushed below the flat band, forming the new CBM.…”