Action Spectroscopy with STM

Abstract

STM-AS is a spectroscopic method capable of vibrational analysis of individual adsorbates on surfaces.

2.1 Principle

STM-AS is a spectroscopic method capable of vibrational analysis of individual adsorbates on surfaces [1]. It is well known that injection of tunneling electrons from an STM tip into adsorbates can excite their vibrational states via inelastic electron tunneling (IET) process, which is used as STM-IETS. Some of these excitations can induce motion and reactions of the adsorbates [2], such as lateral hopping, rotation, desorption, isomerization, and bond formation/scission (Fig. 2.1a). STM-AS measures the motion/reaction probability as a function of applied bias voltage showing remarkable increases near the bias voltages corresponding to the vibrational energies (Fig. 2.1c). Requiring the dynamics of adsorbates, STM-AS is complementary to STM-IETS which requires static behavior. STM-AS can also detect electronic excitations as well, but here the focus is on the more frequently employed usage of vibrational spectroscopy.

Fig. 2.1

Schematics of (a) vibrationally induced motion/reactions of adsorbates under the STM tip, (b) tunneling current as a function of time indicating motion/reactions of the adsorbates, and (c) resulting reaction yield as a function of sample bias voltage in which increases in reaction yield indicate vibrational energies

In STM-AS, the reaction yield Y (reaction rate per electron) as a function of applied bias V is measured as follows. The STM tip is positioned over a target analyte at a fixed tunneling gap followed by feedback loop off. The tunneling electrons are then injected, and the time required for a single reaction event is read out from an I-t plot in which a sudden change in current takes place at the moment of reaction (Fig. 2.1b). Y(V) is statistically determined from multiple trials of this procedure. The Y–V plot can be fitted by the formula representing Y(V) to determine the vibrational energies as well as the vibrational broadening, rate constant, and reaction order (number of electrons required for the reaction) [3]. The vibrational energies enable identification of the functional group and orientation of the adsorbate, and evaluation of the bond strength, in the same manner with other vibrational spectroscopies. Other parameters allow us to gain deeper insights into the microscopic elementary processes behind the reactions induced by IET.

2.2 Features

• Vibrational modes (or electronic states) of a single molecule can be detected.

• Vibrational modes inducing dynamic motion of adsorbates can be detected.

• Selection rule is different from IR, Raman, and STM-IETS.

• Sufficiently low temperature for stabilizing adsorbates is required.

• Insights into elementary processes behind the dynamics can be obtained.

2.3 Instrumentation

For stable measurement of STM-AS, thermal diffusion and desorption of adsorbates, and unintended additional adsorption of any species must be suppressed. Thermal drifting must also be suppressed so that the STM tip can be stably positioned over the analyte. Therefore, the STM system must be equipped with an UHV chamber and cooled to sufficiently low temperature (depending on the analyte). The tunneling current flowing at constant applied bias voltage is recorded by an STM controller or an external oscilloscope to detect the moment of reaction; no additional equipment is required for acquiring the spectra.

2.4 Applications

2.4.1 Fundamental Structure of an Isolated Water Dimer on Pt(111)

The potential of STM-AS as a vibrational spectroscopy has been demonstrated in a structural study of a water dimer on Pt(111) [4]. This simplest building block of water clusters is a useful model for exploring the nature of the hydrogen bond (H-bond) and water–solid interactions. STM-AS is suitable for analyzing this mobile molecule even at low temperatures, and for obtaining cluster-size-specific vibrational information from mixture of differently sized clusters.

Lateral hopping of a water dimer (Fig. 2.2a, b) was detected as a sudden decrease of tunneling current (Fig. 2.2c) at a fixed tunneling gap, enabling us to obtain the STM-AS (Fig. 2.2e). A theoretical fit represents the experimental data very well, which results in a reasonable and precise assignment of each vibrational signal (marked by arrows in Fig. 2.2e). In particular, the O–H stretch mode observed at 375 mV was absent in previously reported IRAS and HREELS studies, indicating the advantage of STM-AS in its sensitivity and selectivity.

Fig. 2.2

STM image of a H₂O dimer on Pt(111) (a) and that recorded before and after an intentionally induced lateral hopping (b). (c) Tunneling current measured over a H₂O dimer at fixed tunneling gap showing sudden decrease corresponding to lateral hopping. (d) A three-quarter view of the optimized “H-down” structure of a water dimer on Pt(111) obtained from a DFT calculation. (e) STM-AS of lateral hopping of H₂O and D₂O dimers on Pt(111). Red circles and blue squares represent the experimental results of STM-AS for H₂O and D₂O, respectively. Thick solid curves are best-fit spectra. Reprinted with the permission from Ref. [4]. Copyright 2014 American Chemical Society

The vibrational energies enable us to deduce the internal structure of the water dimer. Two DFT studies have concluded different optimized structures and different vibrational energies, and STM-AS shows good agreement in vibrational energies with one of the models. The structure of the water dimer observed by STM was thus concluded to be the so-called H-down model (Fig. 2.2d), where one of the water molecules interacts with the Pt substrate not through the oxygen lone pair but through an OH–Pt hydrogen bond. It was shown here that STM-AS has the capability of determining the internal structure of an isolated molecular cluster at the sub-nm scale, which is difficult even with simple STM imaging.

2.4.2 Dissociation Pathways of a Single Dimethyl Disulfide on Cu(111)

The elementary process of S-S bond dissociation of dimethyl disulfide (DMDS, Fig. 2.3a, b) was revealed [5]. STM-AS (Fig. 2.3d) shows that the reaction is induced by excitation of the C–H stretch mode ν(C–H), or the combination of ν(C–H) and S–S stretch mode ν(S–S). For the excitation of ν(C–H), the reaction order N was found to be 2 (Fig. 2.3c), which is usually interpreted as the reaction induced by double-quanta excitation of ν(C–H). However, the fitting analysis of STM-AS shows that this scenario cannot explain the STM-AS signal and the transition from N = 2 to N = 1 for the combination mode excitation. Instead, the bias dependence of N and Y can be consistently explained by assuming that the reaction is induced when one electron excites ν(C–H) and the other excites ν(S–S). Thus, the STM-AS measurement and fitting analysis reveal not only the vibrational modes that trigger the reaction, but also deeper insight into the mechanism of vibrationally induced reactions via IET, leading to finding a novel mechanism; specifically, a reaction is induced only when two vibrational modes are concurrently at excited states, whether excited by one or two electron(s).

Fig. 2.3

STM images of DMDS on Cu(111) before (a) and after (b) inducing the dissociation. (c) Reaction order N as a function of V for the DMDS dissociation determined by measuring the reaction rate as a function of tunneling current. (d) STM-AS for the DMDS dissociation. Red circles are experimental data. Curve B is a best fit of a fitting function Y(V); curve A with different parameters is shown for comparison. The inset is an STM-AS at negative sample bias. Reprinted with permission from Ref. [5]. Copyright 2014, American Institute of Physics