Emerging flat bands and spin polarization in nanodiamond island superlattices with varying carrier effective masses

For the first time, a set of regular 2D sp²/sp³ hybrid superlattices of nanodiamond islands confined between two graphene sheets (NDI-c2G) were proposed and explored using Density Functional Theory Periodic Boundary Conditions simulations. The fusion involves coronene and ovalene molecules with top and bottom graphene lattices, which can be treated with electron beam irradiation and atomic hydrogen at the nanodiamond site in three different configurations: (1 1 1)_CD, (0 0 0 1)_HD, and (2¯110)_HD. This creates distorted sp³ nanodiamond sublattices within the sp² graphene fragments, serving as regions with confined electrons, making the charge carriers’ effective masses heavier. The presence of spin-splitting valence spin-up and conduction spin-down flat bands near the Fermi level in cubic ((1 1 1)_CD-I and (1 1 1)_CD-II) and hexagonal ((2¯110)_HD)-II) superlattices resulted in magnetic moments of 3.99μ_B, 3.99μ_B, and 1.99μ_B per unit cell, respectively. Various configurations significantly impacted their electronic properties, resulting in spin-polarized and non-spin-polarized systems with flat bands near the Fermi level, exhibiting a semiconducting nature. The averaged spin moments per carbon atom were 0.16μ_B, 0.14μ_B, and 0.08μ_B, localized at the sp²/sp³ interfaces of the nanodiamond sites. Analysis of the electronic states, band structure, charge carrier effective masses, and spin density distribution suggests NDI-c2G shares similar electronic nature to strongly correlated twisted bilayer graphenes at small magic angles.