Hydrogen, the most abundant element in the universe, has been studied as a sustainable energy source for some time. However, understanding of the interaction of hydrogen with organic molecules at the atomic scale is still limited. The study of hydrogen molecules, the lightest molecule in nature, is an experimental challenge, as it is hard to localize on surfaces where it is mobile by diffusion even at the very low temperature of liquid helium. This makes imaging of a single hydrogen molecule very difficult using microscopic techniques capable of atomic resolution.
In previous studies using scanning tunneling microscopy (STM) and atomic force microscopy (AFM), random switching of hydrogen was observed. A phenomenological two-state model has been suggested to explain experimental results, but the exact physical origin of the hydrogen dynamics was still an open question. In the study published in RCS Advances on February 3, 2021, researchers at the IBS Center for Quantum Nanoscience at Ewha Womans University (QNS) successfully explained the origin of hydrogen dynamics by combining experimental and theoretical efforts.
“By understanding the hydrogen dynamics, we were able to explain how a hydrogen molecule interacts with an organic molecule at the atomic scale,” said Jungseok Chae of QNS. The molecule used in this work (vanadyl-phtalocyanine, VOPC) is a member of a large group of phtalocyanine molecular family. Chae continues, “This phthalocyanine family is a promising candidate for building block for the organic molecule based hydrogen storage or catalysts. The understanding of intermolecular interactions between hydrogen molecules and organic molecules at the atomic scale will provide new insight into the engineering of the molecular design of hydrogen storage technologies.”
In this work, the research team showed that the potential created by the interaction of a vanadyl-phthalocyanine molecule and the STM tip can trap hydrogen molecules. STM topographic imaging 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, researchers 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,” said Junoh Jung the first author of the paper.
The experiment was conducted on an ultra-high vacuum STM/AFM system at the base temperature of 8.5 K, which has the capability of performing STM and AFM measurements simultaneously. In the presence of hydrogen, the resolution of VOPc molecule at low bias voltage increases and tunneling spectrum on VOPc molecule changed to have unconventional spectral line shapes. Random telegraph noise measurements were obtained to understand hydrogen dynamics and the intermolecular interactions with VOPc. In addition, frequency shift signal produced by AFM functionality was monitored to measure the atomic force between hydrogen and the tip.
In conclusion Chae offered, “As the transition from fossil fuel to sustainable green energy accelerates, hydrogen is one of the most promising candidates for zero-carbon based energy resources. Further investigation of molecular interactions of hydrogen with different molecules composed of various atomic species would help to verify the interactions of hydrogen with molecules in more detail. Additional study of the interaction between hydrogen and various molecules at the atomic scale will provide more insight into improved molecular architecture of hydrogen-related technologies.”