Abstract
Using inelastic electron tunneling spectroscopy with the scanning tunneling microscope (STM-IETS) and density functional theory calculations (DFT), we investigated properties of a single H2 molecule trapped in nanocavities with controlled shape and separation between the STM tip and the Au (110) surface. The STM tip not only serves for the purpose of characterization, but also is directly involved in modification of chemical environment of molecule. The bond length of H2 expands in the atop cavity, with a tendency of dissociation when the gap closes, whereas it remains unchanged in the trough cavity. The availability of two substantially different cavities in the same setup allows understanding of H2 adsorption on noble metal surfaces and sets a path for manipulating a single chemical bond by design.
Highlights
An additional mode around 9 mV appears at the closest tip-substrate distance [bottom spectrum of Fig. 1(b)], which is assigned to the hindered translational mode of the trapped H2
Using the 3D rigid free rotor model, we found that the energy shift of the rotational mode from 43 to 41 meV corresponds to an expansion of the H-H bond length from 0.746 Å to 0.766 Å
Physical insights for understanding the enhanced spatial resolution of the Au rows and variations in energy of the hydrogen vibrational and rotational modes were obtained from density functional theory (DFT) calculations using the Vienna Ab-initio Simulation Package (VASP).[18,19,20]
Summary
It was found that the bond length of H2 slightly expands as the gap of the STM nano-cavity was decreased.[15] Obviously, further studies of the changes in the rotational and vibrational modes with the tip-substrate distance (i.e., the size of cavity) offer unique opportunities to perceive with precise control the alternation of intra- and inter-molecular bonds in different chemical environments. These results show that the binding energy and the structure of the H2 molecule are sensitive to the variation in the tip-substrate separation over the row of Au atoms.
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