Manipulating interfacial hydrogens at palladium via STM

The Journal of Chemical Physics

2015

Past scanning tunneling microscopy (STM) experiments of H manipulation on Pd(111), at low temperature, have shown that it is possible to induce diffusion of surface species as well as of those deeply buried under the surface. Several questions remain open regarding the role of subsurface site occupancies. In the present work, the interaction potential of H atoms with Pd(111) under various H coverage conditions is determined by means of density functional theory calculations in order to provide an answer to two of these questions: (i) whether subsurface sites are the final locations for the H impurities that attempt to emerge from bulk regions, and (ii) whether penetration of the surface is a competing route of on-surface diffusion during depletion of surface H on densely covered Pd(111). We find that a high H coverage has the effect of blocking resurfacing of H atoms travelling from below, which would otherwise reach the surface fcc sites, but it hardly alters deeper diffusion energy barriers. Penetration is unlikely and restricted to high occupancies of hcp hollows. In agreement with experiments, the Pd lattice expands vertically as a consequence of H atoms being blocked at subsurface sites, and surface H enhances this expansion. STM tip effects are included in the calculations self-consistently as an external static electric field. The main contribution to the induced surface electric dipoles originates from the Pd substrate polarisability. We find that the electric field has a non-negligible effect on the H-Pd potential in the vicinity of the topmost Pd atomic layer, yet typical STM intensities of 1-2 VÅ(-1) are insufficient to invert the stabilities of the surface and subsurface equilibrium sites.

Section: Introductionmentioning

confidence: 99%

Diffusion of hydrogen interstitials in the near-surface region of Pd(111) under the influence of surface coverage and external static electric fields

The Journal of Chemical Physics

2015

“…Illustratively, in the STM-induced manipulation of hydrogen on/in Pd(111), 22,23 the potential energy topography is decisive in the dynamics description, as it was shown that a quasi-thermal mechanism promoted by NAC suffices to explain the H-transfer behaviour under a wide range of external parameters, e.g., tunneling bias and current intensity. 24 In the present work we calculate, by means of densityfunctional theory (DFT), the potential energy landscape along a one-dimensional adiabatic reaction path to describe the trans ↔ cis tautomerisation of the isolated porphycene molecules adsorbed on Cu(111). That, in conjunction with the electronic structure analysis for both tautomers in their equilibrium configurations, serves the double purpose of (i) extracting information from the STM images (e.g., tautomer recognition, molecule buckling, and other geometrical parameters) and (ii) addressing the degree of importance of the van der Waals (vdW) interactions in the system.…”

Section: Introductionmentioning

confidence: 99%

On the tautomerisation of porphycene on copper (111): Finding the subtle balance between van der Waals interactions and hybridisation

Novko

The Journal of Chemical Physics

2016

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We use density-functional theory (DFT) to analyse the interaction of trans- and cis-porphycene with Cu(111) and their interconversion by intramolecular H-transfer. This tautomerisation reaction is characterised by small values for the reaction energy and barrier, on the order of ∼0.1 eV, where the trans configuration is thermodynamically more stable upon adsorption according to the experiments [J. N. Ladenthin et al., ACS Nano 9, 7287-7295 (2015)]. To gain even a qualitatively correct description of this reaction at the DFT level, an accurate treatment of dispersion interactions and a careful choice of the exchange contribution are required in order to predict the subtle energetics. Analysis of the electronic structure shows that adsorption is contributed by a van der Waals (vdW) interaction, mainly responsible for stabilising the polyaromatic fragments, and by a significant charge redistribution localised between Cu and the unsaturated N atoms of the molecule central cavity. We find that different vdW functionals can produce qualitatively different electronic structures, while yielding small trans vs. cis energy differences. Unlike other functionals surveyed here, vdW-DF with PBE exchange satisfactorily reproduces not only the experimental energetics but also the scanning tunneling microscopy images. This gives us confidence that this functional achieves a reliable balance between the two mechanisms contributing to the adsorption of porphycene.

“…The term in parentheses describes the probability of excitation of the locally harmonic mode Q α as the ratio of upward to downward rates. As demonstrated elsewhere, ,, these rates are a function of the STM parameters (bias voltage U and current intensity I ), the temperature, and the intermode coupling. Note that eq is the low-bias, low-temperature limit of a more general equation (see the SI), such that only the first excited state of each mode contributes to the potential deformation.…”

mentioning

confidence: 59%

“…The driving force for STM-induced reactions on metallic surfaces is the nonadiabatic coupling (NAC) between the excited electrons of the substrate and the adsorbate internal vibrations. − Reactions involving large-amplitude motions require proper treatment of intermode couplings beyond the harmonic approximation. , A few theories have been proposed to account for anharmonic coupling in the NAC terms, either along the reaction path, − by introducing coupling terms with specific soft modes, − or by increasing dimensionality. − Although promising, these methods remain only tractable in a low-dimensional space, where only a few relevant degrees of freedom are active.…”

mentioning

confidence: 99%

“…The rates required in eq originate from the creation/annihilation of electron–hole pairs in the metal, which can be computed perturbatively using Fermi’s Golden Rule. In recent work, it was shown how the anharmonic coupling rates for all vibrational transitions i → j can be calculated as which is expressed here in a form also valid in the low-bias limit. , The Bose–Einstein distribution B (|ℏω ij |) ensures that the static NAC rates obey the detailed balance. The last term in the brackets describes the efficiency of the STM perturbation, where the step function Θ ensures that only vibrational states at energies below e | U | can be excited.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Intermode Coupling Drives the Irreversible Tautomerization in Porphycene on Copper(111) Induced by Scanning Tunnelling Microscopy

Novko

2017

J. Phys. Chem. Lett.

Self Cite

In this contribution, we develop a nonadiabatic theory that explains, from first-principles, the recently reported irreversible trans → cis tautomerization of porphycene on Cu(111) induced by a scanning tunnelling microscope at finite bias. The inelastic contribution to the STM current is found to excite a large number of skeletal vibrational modes of the molecule, thereby inducing a deformation of the potential energy landscape along the hydrogen transfer coordinate. Above a threshold bias, the stability of the tautomers is reversed, which indirectly drives the reaction via intermode coupling. The proposed potential deformation term accounts effectively for the excitation of all internal vibrational modes without increasing the dimensionality of the problem. The model yields information about reaction rates, explains the reaction irreversibility at low temperatures, and accounts for the presence of resonant processes.