Abstract
We studied nitric oxide (NO) molecules on Cu(110) and Cu(001) surfaces with low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). NO monomers on the surfaces are characterized by STM images reflecting $2{\ensuremath{\pi}}^{*}$ resonance states located at the Fermi level. NO is bonded vertically to the twofold short-bridge site on Cu(110) and to the fourfold hollow site on Cu(001). When NO molecules form dimers on the surfaces, the valence orbitals are modified due to the covalent bonding. We measured inelastic electron tunneling spectroscopy (IETS) for both NO monomers and dimers on the two surfaces, and detected characteristic structures assigned to frustrated rotation and translation modes by density functional theory simulations. Considering symmetries of valence orbitals and vibrational modes, we explain the intensity of the observed IETS signals in a qualitative manner.
Highlights
Chemical identification of individual molecules adsorbed onto surfaces is one of the most significant challenges for nanoscale investigations using scanning probe microscopes (SPM) such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM)
We studied nitric oxide molecules on Cu(110) and Cu(001) surfaces by a combination of STM experiments and density functional theory (DFT) calculations
We observed isolated nitric oxide (NO) monomers on Cu(001) for the first time, and found that they are bonded to the hollow site in an upright configuration
Summary
Chemical identification of individual molecules adsorbed onto surfaces is one of the most significant challenges for nanoscale investigations using scanning probe microscopes (SPM) such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Recent theoretical studies have demonstrated valid models for elastic and inelastic tunneling processes, and have successfully reproduced experimental STM-IET spectra [11,12,13,14,15,16,17,18,19,20]. The experimental observations were studied theoretically by electronic structure calculations for the role of vibrational excitations in the tunneling process. This enabled us to assign the characteristic peaks in IETS to specific adsorbate vibrational modes and to discuss “propensity rules” based on symmetry considerations of the electron-vibration coupling matrix elements
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