Valley filters, accumulators, and switches induced in graphene quantum dots by lines of adsorbed hydrogen atoms

We present electronic structure and quantum transport calculations that predict conducting channels induced in graphene quantum dots by lines of adsorbed hydrogen atoms to function as highly efficient, experimentally realizable valley filters, accumulators, and switches. The underlying physics is an interesting property of graphene Dirac point resonances (DPRs) that is revealed here, namely, that an electric current passing through a DPR-mediated conducting channel in a given direction is carried by electrons of only one of the two graphene valleys. Our predictions apply to lines of hydrogen atoms adsorbed on graphene quantum dots that are either free standing or supported on a hexagonal boron nitride substrate.

Figure 1

(a) Two-terminal monolayer graphene quantum dot with armchair edges. Electron source and drain contacts are each composed of 46 semi-infinite 1D leads (orange wavy lines) that connect the quantum dot to the reservoirs. Hydrogen atoms (yellow disks) are placed on top of carbon atoms and divide the nanostructure into two equal parts. (b) Relaxed geometry of adsorbed hydrogen on graphene. (c) Fragment of the double hydrogen line on graphene. The double H line may be viewed as a series of $x$-oriented H-dimers; such paradimers on graphene are very stable [46].

Figure 2

(a),(b) Calculated two-terminal conductance $G$ [Eq. (2)] of graphene quantum dot in Fig. 1 with (G-H) and without (G) hydrogen lines at zero temperature for symmetric and asymmetric carbon on-site energies vs Fermi energy, respectively. Insets: Conductance fine structure. (c),(d): Band structures of zigzag graphene nanoribbon with lines of hydrogen and symmetric and asymmetric on-site energies of carbon atoms, respectively. States localized near the hydrogen lines (H bands) and other graphene states (G bands) are present.

Figure 3

(a)–(c) Calculated electron accumulation (i.e., current-induced electron population) $A_n^{K^{\prime }\left(K\right)}$ in valley $K^{\prime }\left(K\right)$ near H lines in Fig. 1 shown as red (blue) disks. (a) Symmetric case for $E_{\text{F}}=-0.0065\mathrm{eV}$. (b),(c) Asymmetric case for $E_{\text{F}}=0.0502$ and $-0.0638\mathrm{eV}$ [red and blue H bands in Fig. 2], respectively. Electrons flow in the positive $y$ direction. Disk diameters proportional to $A_n^{K^{\prime }\left(K\right)}$, but diameters in (c) are scaled down by factor 7 relative to those in (a) and (b). (d)–(f) Calculated square amplitudes (red) of representative electron H-band eigenstates of graphene nanoribbons near lines of hydrogen corresponding to the valley accumulations of (a)–(c), respectively. Ribbon unit-cell fragment is also shown. Inset: Rhombic Brillouin zone of graphene with Dirac points $K$ and $K^{\prime }$.