First-principles study of structural stability, magnetism, and hyperfine coupling in hydrogen clusters adsorbed on graphene

Using first-principles electronic-structure calculations, we studied the structural and magnetic properties of various hydrogen clusters, including hydrogen monomer, dimers, trimers, tetramers, and hexamers adsorbed on a graphene surface. The magnetic behaviors of such defective systems were shown to strongly depend on the geometrical configuration of hydrogen atoms. The stability of the structures was demonstrated to be dependent on two important factors: the distance between hydrogen atoms and the strength of exchange couplings between the defect-induced magnetic moments. For the magnetic structures, the electron spins populate the quasilocalized $p_z$-type states on specific carbon atoms. The presence of such quasilocalized $p_z$-type states was shown to yield relatively strong hyperfine couplings at particular carbon sites in the neighborhood of hydrogen atoms.

Figure 1

(Color online) (a) Top view and (b) side view of spin-density distribution around a hydrogen atom adsorbed on the graphene. The red and green (dark and light in the grayscale) isosurfaces correspond to spin-up and spin-down dominated contributions.

Figure 2

(Color online) (a) Total density of states as obtained for pure graphene and the monomer, (b) and (c) are the respective partial density of states for carbon and hydrogen atoms in monomer.

Figure 3

(Color online) (a) Excess and depletion charge contours. (b) Side view of isosurface for the excess (red) and depletion (green) charge distributions as obtained for monomer.

Figure 4

(Color online) Geometrical configurations of dimers (labeled D1, D2, D3, D4, D5, D6, and D7), trimers (labeled T1, T2, T3, and T4), tetramers (labeled Q1 and Q2), and hexamers (labeled S1 and S2). The yellow (shaded) areas denote the regions commonly shared between the spin triangles. The white points indicate the position of hydrogen atoms. The red filled circles indicate the carbon atoms of interest for hyperfine calculations.

Figure 5

(Color online) Spin-density distributions of magnetic dimers, trimers, and tetramers mapped on a plane close to the graphene surface. The dark points indicate the position of hydrogen atoms and the spin triangles are shown by white lines.

Figure 6

(Color online) (a) Comparison of (a) $E_a$ (b) C-H bond lengths, and (c) C-C bond lengths as obtained for the monomer, dimers, trimers, tetramers, and hexamers.