Stochastic tunneling electrons driving small-molecular dissociations
A recent paper published in ACS Nano presents how to probe single-molecule dissociation of diatomic NO from NO-Co-porphyrin complexes by using scanning tunneling microscopy setups, and for the first time, unveils novel single-molecule dissociation mechanics by hot tunneling electrons. Under tunneling junctions of the scanning tunneling microscope, both positive and negative voltage pulses gave rise to dissociations of NO. The NO dissociation rate exhibits a power law dependency on tunneling current, indicating that the single-molecule chemical reaction is associated with multiple electron scatterings. Using first-principles thermodynamics analyses, Yong-Hyun Kim and, collaborators proposed that the dissociation is induced by inelastic energy transfer from stochastic tunneling electrons to molecular vibrations via molecular orbital resonances.
Probing chemical reactions at the single-molecule level is highly challenging, yet very powerful in that it provides fundamental insights into the control of chemical reactions. Axial coordination of diatomic molecules such as CO, NO, and O2 to metalloporphyrins has been extensively studied because of its key roles in dynamic processes of biological functions such as blood pressure control and immune response.
A research team led by Professor Yong-Hyun Kim in the Graduate School of Nanoscience and Technology and Professor Se-Jong Kahng at Korea University presents theoretical and experimental results of single molecule dissociations of NO from NO-CoTPP complexes and reveals novel single-molecule dissociation mechanics. By using scanning tunneling microscopy, they observed that the NO dissociation took place with voltage pulses higher (lower) than +0.68 (-0.74) V at 0.1 nA tunneling current. They also measured the NO dissociation rate at different tunneling currents for ± 0.8 and ± 1.0 V voltage pulses, and found a power law dependency of the reaction rate (R) on the tunneling current (I ); that is, R~IN, where N is the order of reaction.
To microscopically understand the NO dissociation mechanics, the KAIST theory team performed first-principles density functional theory (DFT) thermodynamics of the NO dissociation, including adsorption-desorption energetics, zero-point energy, vibrational free-energy at experimental temperature, and chemical potential of NO gas at the cryogenic ultrahigh vacuum conditions. Intuitively, the tunneling-induced NO dissociation should involve several time scales, i.e., femto-seconds of electron-phonon scattering, pico-seconds of phonon-phonon scattering (τ0), and nano-seconds of electron tunneling (τ1). Each time scale plays a distinct role in the tunneling-electron-induced single molecule dissociation mechanics. It is suggested that the reaction energy barrier ΔG for the NO dissociation should be overcome probabilistically by one- and two-electron scattering process for low (0.8 V) and high bias voltages (1.0 V), respectively. The KAIST team revealed that the multi-electron scattering rate with molecular vibrations should be represented with (τ0/τ1)N-1 by stochastically tunneling electrons. Therefore, it was concluded that the stochasticity of correlations in energy transfer gives rise to the observed power-law dependency in the dissociative reaction.
Yong-Hyun Kim, his postdoc Yun Hee Chang and his student Eui-Sup Lee were listed as the authors of this contribution published in ACS Nano in July of 2015 (ACS Nano, 2015, 9, 7722).
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