Current-Driven Hydrogen Desorption from Graphene: Experiment and Theory

Abstract

Electron-stimulated desorption of hydrogen from the graphene/SiC(0001) surface at room temperature was investigated with ultrahigh vacuum scanning tunneling microscopy and ab initio calculations in order to elucidate the desorption mechanisms and pathways. Two different desorption processes were observed. In the high electron energy regime (4-8 eV), the desorption yield is independent of both voltage and current, which is attributed to the direct electronic excitation of the C-H bond. In the low electron energy regime (2-4 eV), however, the desorption yield exhibits a threshold dependence on voltage, which is explained by the vibrational excitation of the C-H bond via transient ionization induced by inelastic tunneling electrons. The observed current independence of the desorption yield suggests that the vibrational excitation is a single-electron process. We also observed that the curvature of graphene dramatically affects hydrogen desorption. Desorption from concave regions was measured to be much more probable than desorption from convex regions in the low electron energy regime (∼2 eV), as would be expected from the identified desorption mechanism.

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“…2]. This is in conceptual agreement with a few simulations reporting the modulation of adsorption and desorption energies of hydrogen atoms on graphene by local curvatures [37,38]. The dynamic morphology transition of graphene ripples from convex to concave could be helped by, for example, thermal fluctuations [Fig.…”

“…Therefore, one would expect the hydrogen adatoms to either desorb from the surface or undergo local reconstruction via desorption followed by re-adsorption to form H clusters when the graphene ripples evolve from convex to concave. The local clustering structures are predicted to be energetically more stable with respect to hydrogen atoms being randomly adsorbed on the graphitic surfaces and have been observed using tunnelling microscopy [38][39][40][41].…”

Probing Franck–Condon-like Excitations in Anchoring of Phthalocyanine Molecules on Au(111)

“…Our theoretical analysis relies on the assumption that the most important vibronic interaction arises from a shift in the equilibrium structure of the C−H bonds of NiPC. 4 The long-distance migration of excited NiPCs prior to the anchoring, which can be up to 4 nm from the tip position (Figure 1c−e), indicates that the injected electron remains in the molecule for sufficient time to induce the structural change (Figure 5b). The injected electron by the Franck−Condon-like excitation (step 1) induces an atomic force on the NiPC initially in its ground state.…”

“…Figure b represents the proposed dehydrogenation process driven by the Franck–Condon-like excitation. Our theoretical analysis relies on the assumption that the most important vibronic interaction arises from a shift in the equilibrium structure of the C–H bonds of NiPC . The long-distance migration of excited NiPCs prior to the anchoring, which can be up to 4 nm from the tip position (Figure c–e), indicates that the injected electron remains in the molecule for sufficient time to induce the structural change (Figure b).…”

“…Manipulation of individual atoms and molecules using scanning tunneling microscopy (STM) has drawn wide interests owing to its unique ability to induce local mechanical motions and chemical reactions on a nanometer scale. − In contrast to the conventional photo or thermal-induced chemical/mechanical transitions, tunneling electrons play an important role in the STM manipulation. When a large number of electrons travels through atoms or molecules, inelastic tunneling electrons, a small fraction of the electrons, transfer the energy to the nuclei of atoms or molecules giving rise to the vibration, hopping, dissociation, and chemical reactions.…”