UCL Chemistry

Spectroscopic Techniques

Inelastic Electron Tunnelling Spectroscopy (IETS)

IETS, which was developed in the 1960s, is an experimental tool for studying the vibrations of molecules adsorbed on metal oxides. It yields vibrational spectra of adsorbates with high resolution and sensitivity. In addition, using IETS, optically forbidden transitions may be observed as well. Combined with the spatial resolution of scanning tunnelling microscopy (STM), STM-IETS makes it possible to record vibrational spectra of individual atoms/molecules and hence provides STM with chemical specificity. The working principle of IETS is quite straightforward. For electrons tunnelling through molecules, apart from elastic tunnelling, there exist processes in which an electron loses its energy by exciting a vibration mode within the molecule. Since the maximum energy that can be lost in the inelastic process is eVbias, inelastic tunnelling takes place only when the tunnelling electron possesses energy eVbias > ħω (Figure 1a). The appearance of the new channel with inelastic process contributes to the increase of the conductance dI/dV for V > 0. The relation of I-V, dI/dV and d²I/dV² curves in which elastic tunnelling takes place are schematically shown in Figure 1b. Therefore, using STM and keeping the tip at fixed position on a surface, and by measuring the change in conductance (d²I/dV²) as function of bias voltage Vbias, one can record inelastic peaks in the spectra and assign them to different vibrational modes present on the surface.

Figure 1:  A tunneling electron emitted from a STM tip is trapped in a molecule-induced resonant state.  The electron is dissipated into the metal surface after a short time, during which the electron could excite a phonon or molecules vibrational mode with an energy quantum of ħω [1].

Here are some experimental results of single molecule vibrational spectroscopy of CO and N2 molecules adsorbed on a Cu(110) surface, performed at T = 5 K using our low-temperature scanning tunnelling microscopy (LT-STM) [2]. In the STM topographical images, the appearance of CO and N₂ molecules behave differently from each other at different bias voltages (Figure 2).

Figure 2:  20x20 nm² STM images (2.0 nA) of CO (solid circles) and N₂ (dash circles) co-adsorbed on Cu(110) at 5 K, (a) V_bias = 0.33 V and (b) V_bias = 3.27 V.

By recording d²I/dV² spectra on the clean Cu(110) surface, and on top of a CO and a N₂ molecule, respectively, the inelastic peaks corresponding to different vibrational modes present on the CO and N₂ co-adsorbed surface were clearly indentified (Figure 3 and 4).

Figure 3:  d²I/dV² spectrum taken over clean Cu(110) (solid line) and a CO molecule on Cu(110) (solid line with circles), showing the region of energy where the CO hindered rotation mode is observed at 5 K.

Figure 4:  d²I/dV² spectra taken over clean Cu(110) (darker line) and a N₂ molecule on Cu(110) (lighter line) at 5 K.  (a) Typical spectrum (positive bias) for a N₂ molecule in which an inelastic peak, which is at 265 mV, can be seen clearly.  (b) Region of the energy spectrum where the lower vibrational modes at bias voltages 11 and 36 mV are observed.

References

[1] T. Komeda, Prog. Surf. Sci. 78, 41 (2005).
[2] L. Leung, C. A. Muryn, G. Thornton, Surf. Sci. 566, 671 (2004).

X-Ray Photoelectron Spectroscopy (XPS)

X-ray Photoelectron Spectroscopy (XPS) is an easy and accurate technique to analyse surfaces by bombarding photons in the X-ray energy range in an ultra high vacuum (UHV) environment. The energy of a photon incident on a surface which is proportional to the frequency of the X-ray source, can be transferred to an electron in the sample. If this energy is greater than the binding energy of the electron within its atomic orbital, the electron will be emitted from the atom with a kinetic energy equal to the difference between the photon and binding energies. These photoelectrons are analysed by the X-ray photoelectron spectrometer which records the number of emitted photoelectrons with a specific kinetic energy. The quantity of the photoelectrons with the specific energy of interest is proportional to the concentration of the emitting atom in the sample. When photons of a fixed energy are used, a spectrum can be produced with peaks corresponding to atomic orbitals. Electrons in orbitals closer to the nucleus will have higher binding energy due to the greater effect of the nuclear positive charge. Thus, the kinetic energy of the photoelectrons ejected from the inner orbitals will be much lower than those from the outer atomic orbitals. As electrons have a limited ability to travel through solids, the analyser can only detect the photoelectrons which are emitted from atoms close to the surface. XPS can also be used for Auger Electron Spectroscopy (AES) if an Auger electron is emitted as a result of the irradiation of the sample.

Auger Electron Spectroscopy (AES)

Auger Electron Spectroscopy (AES) discovered by Pierre Auger and separately by Lise Meitner, is one of the most widely used techniques for determining the precise elemental composition of surfaces. The basic principles underlying the technique can be seen in figure (a) below, and the experimental setup, using the Low Energy Electron Diffraction (LEED) optics in retarding field analyser (RFA) mode can be seen in (b).

An incident photon (or electron) causes a core electron to be emitted by photoemission resulting in a hole or electron vacancy (i). This is followed by a process called autoionization, where this vacancy is filled by an electron transition from a level with lower binding energy (‘down’ electron) (ii). As a result of this there is a quantum of energy available, ΔE, equal to the difference in binding energies between the core hole and the down electron, which can be removed by a photon (X-ray fluorescence) or by transfer to a third electron (the Auger electron) which escapes into the vacuum (iii) with energy Ekin = EK - EL1 - EL2,3 - φ, where φ is the workfunction.