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Register a new userSuppression of diffusion of hydrogen adatoms on graphene by effective adatom interaction
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Resonant graphene dopants, such as hydrogen adatoms, experience long-range effective interaction mediated by conduction electrons. As a result of this interaction, when several adatoms are present in the sample, hopping of adatoms between sites belonging to different sublattices involves significant energy changes. Different inelastic mechanisms facilitating such hopping -- coupling to phonons and conduction electrons -- are considered. It is estimated that the diffusion of hydrogen adatoms is rather slow, amounting to roughly one hop to a nearest neighbor per millisecond.
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The electronic transport of monolayer graphene devices is studied before and after emph{in situ} deposition of a sub-monolayer coating of osmium adatoms. Unexpectedly, and unlike all other metallic adatoms studied to date, osmium adatoms shift the charge neutrality point to more positive gate voltages. This indicates that osmium adatoms act as electron acceptors and thus leave the graphene hole-doped. Analysis of transport data suggest that Os adatoms behave as charged impurity scatterers, albeit with a surprisingly low charge-doping efficiency. The charge neutrality point of graphene is found to vary non-monotonically with gate voltage as the sample is warmed to room temperature, suggesting that osmium diffuses on the surface but is not completely removed.
We have performed density functional theory calculations of graphene decorated with carbon adatoms, which bind at the bridge site of a C--C bond. Earlier studies have shown that the C adatoms have magnetic moments and have suggested the possibility of ferromagnetism with high Curie temperature. Here we propose to use a gate voltage to fine tune the magnetic moments from zero to 1$mu_B$ while changing the magnetic coupling from antiferromagnetism to ferromagnetism and again to antiferromagnetism. These results are rationalized within the Stoner and RKKY models. When the SCAN meta-GGA correction is used, the magnetic moments for zero gate voltage are reduced and the Stoner band ferromagnetism is slightly weakened in the ferromagnetic region.
The interaction of two resonant impurities in graphene has been predicted to have a long-range character with weaker repulsion when the two adatoms reside on the same sublattice and stronger attraction when they are on different sublattices. We reveal that this attraction results from a single energy level. This opens up a possibility of controlling the sign of the impurity interaction via the adjustment of the chemical potential. For many randomly distributed impurities (adatoms or vacancies) this may offer a way to achieve a controlled transition from aggregation to dispersion.
We present first principles calculations of the exchange interactions between magnetic impurities deposited on (001), (110) and (111) surfaces of Cu and Au and analyze them, in particular, in the asymptotic regime. For the (110) and the (111) surfaces we demonstrate that the interaction shows an oscillatory behavior as a function of the distance, R, of the impurities and that the amplitude of the oscillations decays as 1/R^2. Furthermore, the frequency of the oscillations is closely related to the length of the Fermi vector of the surface states existing on these surfaces. Due to the asymmetry of the the surface states dispersion, the frequency of the oscillations becomes also asymmetric on the (110) surfaces, while on the Au(111) surface two distinct frequencies are found in the oscillations as a consequence of the Bychkov-Rashba splitting of the surface states. Remarkably, no long range oscillations of the exchange interaction are observed for the (001) surfaces where the surface states are unoccupied. When burying the impurities beneath the surface layer, oscillations mediated by the bulk states become visible.
In order to realize applications of hydrogen-adsorbed graphene, a main issue is how to control hydrogen adsorption/desorption at room temperature. In this study, we demonstrate the possibility to tune hydrogen adsorption on graphene by applying a gate voltage. The influence of the gate voltage on graphene and its hydrogen adsorption properties was investigated by electrical transport measurements, scanning tunneling microscopy, and density functional theory calculations. We show that more hydrogen adsorbs on graphene with negative gate voltage (p-type doping), compared to that without gate voltage or positive gate voltage (n-type doping). Theoretical calculations explain the gate voltage dependence of hydrogen adsorption as modifications of the adsorption energy and diffusion barrier of hydrogen on graphene by charge doping.
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