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Register a new userComment on Sticking of Hydrogen on Supported and Suspended Graphene at Low Temperature
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Dennis P. Clougherty
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The sticking probability of cold atomic hydrogen on suspended graphene calculated by Lepetit and Jackson [Phys. Rev. Lett. {bf 107}, 236102 (2011)] does not include the effect of fluctuations from low-frequency vibrations of graphene. These fluctuations suppress the sticking probability for low incident energies ($lesssim 15$ meV).
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The effect of a normal H2 impurity upon the radial thermal expansion (Ar) of SWNT bundles has been investigated in the interval T = 2.2-27 K using the dilatometric method. It is found that H2 saturation of SWNT bundles causes a shift of the temperature interval of the negative thermal expansion towards lower (as compared to pure CNTs) temperatures and a sharp increase in the magnitude of (Ar) in the whole range of temperatures investigated. The low temperature desorption of H2 from a powder consisting of bundles of SWNTs, open and closed at the ends, has been investigated.
Narrow gaps are formed in suspended single to few layer graphene devices using a pulsed electrical breakdown technique. The conductance of the resulting devices can be programmed by the application of voltage pulses, with a voltage of 2.5V~4.5V corresponding to an ON pulse and voltages ~8V corresponding to OFF pulses. Electron microscope imaging of the devices shows that the graphene sheets typically remain suspended and that the device conductance tends to zero when the observed gap is large. The switching rate is strongly temperature dependent, which rules out a purely electromechanical switching mechanism. This observed switching in suspended graphene devices strongly suggests a switching mechanism via atomic movement and/or chemical rearrangement, and underscores the potential of all-carbon devices for integration with graphene electronics.
We study the sticking rate of atomic hydrogen to suspended graphene using four different methods that include contributions from processes with multiphonon emission. We compare the numerical results of the sticking rate obtained by: (1) the loop expansion of the atom self-energy, (2) the non-crossing approximation (NCA), (3) the independent boson model approximation (IBMA), and (4) a leading-order soft-phonon resummation method (SPR). The loop expansion reveals an infrared problem, analogous to the infamous infrared problem in QED. The 2-loop contribution to the sticking rate gives a result that tends to diverge for large membranes. The latter three methods remedy this infrared problem and give results that are finite in the limit of an infinite membrane. We find that for micromembranes (sizes ranging 100 nm to 10 $mu$m), the latter three methods give results that are in good agreement with each other and yield sticking rates that are mildly suppressed relative to the lowest-order golden rule rate. Lastly, we find that the SPR sticking rate decreases slowly to zero with increasing membrane size, while both the NCA and IBMA rates tend to a nonzero constant in this limit. Thus, approximations to the sticking rate can be sensitive to the effects of soft-phonon emission for large membranes.
In a recent paper [arXiv:0804.3569], Takatoshi Nomura {it et al.} reported a structural phase transition near 150 K in LaOFeAs and used space group Cmma to describe their X-ray diffraction data. However, they did not discuss how their proposed structure compares with the early neutron study by Cruz {it et al.}[arXiv:0804.0795] where the low temperature structure of LaOFeAs was described by space group P112/n. This caused some confusion, suggesting that there may be some disagreement on the low temperature structure of LaOFeAs as evidenced by several inquiries that we received. Here we show that the proposed structures from x-ray and neutron diffraction are basically identical. The P2/c (i.e., P112/n) cell becomes the primitive cell of the Cmma cell when the z-coordinate of the oxygen and iron are assumed to be exactly 0 and 0.5 (these numbers were reported to be -0.0057 and 0.5006 in neutron study). Our first-principles total-energy calculations suggest that the oxygen and iron atoms prefer to lie on the z=0 and 1/2 plane, respectively, supporting Cmma symmetry. However it is more convenient to describe the structural distortion in the primitive P2/c cell which makes it easier to see the connection between the high (i.e., P4/nmm) and low temperature structures.
We present a theoretical study of the dynamics of H atoms adsorbed on graphene bilayers with Bernal stacking. First, through extensive density functional theory calculations, including van der Waals interactions, we obtain the activation barriers involved in the desorption and migration processes of a single H atom. These barriers, along with attempt rates and the energetics of H pairs, are used as input parameters in kinetic Monte Carlo simulations to study the time evolution of an initial random distribution of adsorbed H atoms. The simulations reveal that, at room temperature, H atoms occupy only one sublattice before they completely desorb or form clusters. This sublattice selectivity in the distribution of H atoms may last for sufficiently long periods of time upon lowering the temperature down to 0 C. The final fate of the H atoms, namely, desorption or cluster formation, depends on the actual relative values of the activation barriers which can be tuned by doping. In some cases a sublattice selectivity can be obtained for periods of time experimentally relevant even at room temperature. This result shows the possibility for observation and applications of the ferromagnetic state associated with such distribution.
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