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2010 DIPC Highlight
Mixed-valency signature in
vibrational inelastic electron
tunneling spectroscopy
M. Alducin, D. Sánchez-Portal, A. Arnau, and N. Lorente
Physical Review Letters 104, 165423 (2010)
The unexplained vibrational inelastic electron tunneling spectroscopy (IETS) of O 2 on Ag(110) is
now solved. When semilocal density functional theory is corrected by including static intraatomic
correlations, the IETS simulations are in excellent agreement with the experiment. The
projected density of states revels that when adsorbed along the [001] direction, O 2 on Ag(110) is
a mixed-valent system, where a spin-polarized molecular orbital dominates the electronic structure
at the Fermi.
Inelastic electron tunneling spectroscopy (IETS) performed with the scanning tunneling microscope
(STM) has given us unprecedented insight into the chemical structure of matter on the atomic scale. In
the STM, electrons are tunneled through the vacuum gap between the tip and the substrate by applying
them a bias voltage. Locating the tip over a single acetylene molecule adsorbed on a copper surface,
Stipe, Rezaei and Ho showed that the tunneling electrons can give energy to the molecule and excite a
particular vibrational mode. Since then, many experiments have confirmed the STM-IETS as a unique technique
in which the vibrational spectra of individual adsorbates can be recorded by varying the tip-substrate
voltage and used it as a fingerprint to identify the adsorbed species.
In general, the IETS signal corresponds to an increase of the STM conductance just at the energy threshold
of the vibration because the excitation of a vibrational mode opens an additional channel for conduction.
Interestingly enough, experiments performed for oxygen molecules adsorbed on a silver substrate showed
that this picture, i.e., the conductance increase, does not apply in this case. Specifically, when the molecules
are chemisorbed along the [001] direction on Ag(110) instead of increasing, the current decreases
at the vibrational threshold of the stretching mode. These experiments had remained unexplained during
years, since all the most sophisticated IETS simulations done on this system had been unable to reproduce
the conductance decreases.
The IETS simulations performed by researchers at the DIPC that were published in Physical Review Letters
have solved this long-standing puzzle. When semilocal density functional theory is corrected by including
static intra-atomic correlations (DFT+U), the IETS simulations are in excellent agreement with the experiment.
The analysis of the projected density of states shows that intra-atomic correlation leads to the spin
splitting of the g orbital and the subsequent pinning of one of these orbitals to the Fermi level. The unforeseen
consequence of our calculations is that when adsorbed along the [001] direction, O 2 on Ag(110)
is a mixed-valent system in which a localized spin polarized orbital is at resonance with the Fermi level.
The corresponding electronic structure leads to IETS simulations where the experimental change in conductance
and also their spatial tip-position dependence are finally explained. The theoretical results give
unprecedented information on the electronic and vibrational symmetries at work in this system. Importantly,
these calculations also show that IETS can reveal important information about the electronic structure
and magnetic nature of chemisorbed molecules. In this respect, IETS arises as a promising tool to
characterize the otherwise elusive molecular mixed-valency regime. In this regime, electron correlations
are responsible of large charge fluctuations between the adsorbate and the substrate that may lead to
strong non-adiabatic effects dominating the adsorption dynamics.
Figure 1. Spin-up (positive) and spin-down (negative) projected density of states of O 2 chemisorbed
along the [001] direction. (a) Semilocal DFT (GGA) results. (b) Semilocal DFT+U (GGA+U) results.
STM operating in IETS mode arises as a promising tool to characterize
the otherwise elusive molecular mixed-valency regime.
Figure 2. Semilocal DFT+U simulations of the spatial
distributions of the conductance changes for the
O-O stretch mode (left )and the antisymmetric
O 2 /Ag mode (right).
The unforeseen consequence
of these calculations is that
when adsorbed along the
[001] direction, O 2 on Ag(110)
is a mixed-valent system.
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