High-End FTIR Applications
Using VERTEX Vacuum Spectrometers

  • Connect

Characterization of Ultra-Thin Layers

Highest Sensitivity for Mono-Molecular Layers.

Spectroelectrochemistry with Rapid Scan

Monitoring of Fast Electrochemical Process Without Atmospheric Disturbance.

Further Applications

VERTEX 80v and VERTEX 70v

High-End Research Applications Using VERTEX Series Vacuum Spectrometers.

VerTera THz-Extension for VERTEX 80v

The world's first combined FTIR/cw THz Spectrometer.

UHV FTIR Spectroscopy

VERTEX Vacuum Spectrometers adapted to Customized Ultra-High Vacuum Apparatus.

Step Scan TRS Spectroscopy

VERTEX 80v Provides Highest Accuracy Using Time Resolved Stepwise Data Acquisition.

MIR Photoluminescence (PL)

Amplitude Modulated Step Scan.

Characterization of
Ultra-thin Layers

Ultra-thin layers on metal or dielectric substrates can be characterized in reflection mode using FTIR technique. Due to the surface selection rules on metal substrates, s-polarized light cannot interact with the adsorbate molecules independent of incident angles, whereas p-polarized light reaches the maxima of absorbance at grazing incidence angle. Therefore, ultra-thin layers on metal substrates will be measured with Grazing Incidence Reflection (GIR) or Infrared Reflection Absorption Spectroscopy (IRRAS) using an incidence angle of around 80°.

Things become less obvious changing to non-metallic substrates, since both p- and s-polarized light can be absorbed by the thin layers. The intensity of the absorption bands varies depending on the incidence angle. Bands can even switch from negative to positive and vice versa by changing the incidence angle or polarization. Therefore, to fully characterize a thin layer on dielectric substrates, measurements using at least two incidence angles and with both polarizations must be carried out and also transmittance can be a valuable approach.

Ultra-thin layers show typically very weak absorption bands in IRRAS spectra, e.g. ranging down to 10-3 au (absorbance unit) for a monomolecular layer with a thickness of few nanometers or even 10-5 au on a dielectric substrate. For such weak absorbance bands highest sensitivity of the instrument is required. In the right side picture IRRAS spectra of a self-assembled monolayer on Au substrate are compared. The blue spectrum shows the result measured in a VERTEX 70 purge spectrometer. The residual water vapor absorbance masks the weak sample absorbance bands in the finger print region. After applying the automatic water vapor and CO2 compensation function in OPUS software the red spectrum will be received.

Anyhow, only the result measured in a VERTEX 70v vacuum spectrometer (green spectrum) shows very smooth baseline, especially in the regions of atmospheric disturbance around 3700 cm-1, 2300 cm-1 and 1600 cm-1. Furthermore, the spectrum measured in vacuum spectrometer is a pure experimental result without subsequent mathematical data manipulation as done for the red spectrum.

Fig. 1: IRRAS spectra of a self-assembled monolayer on Au measured in VERTEX 70 purge and VERTEX 70v vacuum spectrometers.
click to enlarge

Further Information

© Copyright 2020 Bruker Optik GmbH