Karolina Z. Milowska

Stability and electronic structure of covalently functionalized graphene layers

We present exemplary results of extensive studies of mechanical, electronic, and transport properties of covalent functionalization of graphene monolayers (GML) with –NH2. We report new results of ab initio studies of covalent functionalization of GML with –NH2 groups up to 12.5% concentration. Our studies are performed in the framework of the density functional theory (DFT) and non-equilibrium Greens function (NEGF). We discuss the stability (adsorption energy), elastic moduli, electronic structure, band gaps, and effective electron masses as a function of the density of the adsorbed molecules. We also show the conductance and I(V) characteristic of these systems. Generally, the stability of the functionalized graphene layers decreases with the growing concentration of attachments and we determine the critical density of the molecules that can be chemisorbed on the surface of GLs. Because of local deformations of GLs and sp3 rehybridization of the bonds induced by fragments, elastic moduli decrease with increasing number of groups. Simultaneously, we observe that the functionalizing molecules stretch the graphenes lattice, the effect being more pronounced for higher concentration of adsorbed molecules. We find out that the GLs functionalization leads in many cases to the opening of the graphene band gap (up to 0.5302 eV for 12.5% concentration) and can be therefore utilized in graphene devices. The new HOMO and LUMO originate mostly from the impurity bands induced by the functionalization and they exhibit parabolic dispersion with electron effective masses comparable to ones in silicon or gallium nitride.

Electronic structure of graphene functionalized with boron and nitrogen

We present a theoretical study of the structural and electronic properties of graphene monolayer functionalized with boron and nitrogen atoms substituting carbon atoms. Our study is based on the ab initio calculations in the framework of the density functional theory. We calculate the binding energies of the functionalized systems, changes in the morphology caused by functionalization, and further the band gap energy as a function of the concentration of dopants. Moreover, we address the problem of possible clustering of dopants at a given concentration. We define the clustering parameter to quantify the dependence of the properties of the functionalized systems on the distribution of B/N atoms. We show that clustering of B/N atoms in graphene is energetically unfavorable in comparison to the homogenous distribution of dopants. For most of the structures, we observe a nonzero energy gap that is only slightly dependent on the concentration of the substituent atoms. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)