Atomic-scale intermolecular interaction of hydrogen with a single VOPc molecule on the Au(111) surface

Description


Using STM and AFM, we have shown that the potential created by the vanadyl-phthalocyanine molecule and the tip can trap hydrogen molecules (H2). STM topographic image showed that most of the VOPc molecules are absorbed on the herringbone kink sites of the reconstructed Au(111) surface, where they have the lowest adsorption energy. The molecules exhibit two different appearances, which can be identified as O-up and O-down with equal probability. The presence of hydrogen in the junction can be evidenced by random fluctuations of the tunneling current over time, a phenomenon called random telegraph noise. Further, the interaction with the hydrogen enables extremely high spatial resolution imaging, revealing sub-molecular features in the molecule that cannot be observed with a normal tip alone. Lastly, we evidenced that the presence of the hydrogen molecule can be directly measured by atomic force microscopy. When increasing the applied bias beyond a certain threshold the hydrogen is expelled from the junction. The insight gained in this study is expected to have wide-ranging implications from basic science to the engineering of hydrogen storage devices.

Abstract


Molecular dynamics of hydrogen molecules (H2) on surfaces and their interactions with other molecules have been studied with the goal of improvement of hydrogen storage devices for energy applications. Recently, the dynamic behavior of a H2 at low temperature has been utilized in scanning tunnelling microscopy (STM) for sub-atomic resolution imaging within a single molecule. In this work, we have investigated the intermolecular interaction between H2 and individual vanadyl phthalocyanine (VOPc) molecules on Au(111) substrates by using STM and non-contact atomic force microscopy (NC-AFM). We measured tunnelling spectra and random telegraphic noise (RTN) on VOPc molecules to reveal the origin of the dynamic behavior of the H2. The tunnelling spectra show switching between two states with different tunnelling conductance as a function of sample bias voltage and RTN is measured near transition voltage between the two states. The spatial variation of the RTN indicates that the two-state fluctuation is dependent on the atomic-scale interaction of H2 with the VOPc molecule. Density functional theory calculations show that a H2 molecule can be trapped by a combination of a tip-induced electrostatic potential well and the potential formed by a VOPc underneath. We suggest the origin of the two-state noise as transition of H2 between minima in these potentials with barrier height of 20–30 meV. In addition, the bias dependent AFM images verify that H2 can be trapped and released at the tip–sample junction.
 
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