There are several common ways to set up an ATR-FTIR system to monitor
the IR spectrum of a thin nanofilm. Perhaps the most straightforward
method is to build the thin nanofilm on an external substrate and then
clamp the substrate on top of the IRE, sandwiching the thin nanofilm
between the substrate and the IRE. An IR absorbance spectrum of the
nanofilm can then be obtained as described above. While this method is
easy to implement, it does not allow for continuous monitoring of the thin
nanofilm during its creation. To acquire that type of data, it is necessary to
construct a flow cell on top of the IRE and build the thin nanofilm on the
surface of the IRE itself. Using this setup, the IR spectrum can be monitored continuously and changes that occur in the spectrum during the
creation of the thin film can be recorded.
For the purposes of determining the IR spectrum of a thin nanofilm as
described above, a horizontal ATR (HATR) setup is commonly employed.
In this setup, the IRE is a parallel-sided crystal plate with dimensions on
the order of 1 cm by 5 cm. For an IRE of this size, the IR beam is typically
reflected between 5 to 10 times at each surface, depending on the exact
dimensions of the crystal and the angle of incidence. The upper face of
the crystal is exposed to the thin film through a clamping method or a
flow cell setup, as described. The most common crystal materials used for
HATR setups are zinc selenide (ZnSe) and germanium. Diamond is a
more robust material that can be used as an ATR crystal, although its
higher cost is a drawback.
8.5.2 Reflection absorption IR spectroscopy
Another surface-sensitive IR spectroscopy method is reflection absorption infrared spectroscopy, or RAIRS. In RAIRS, infrared light is shone
onto a metal surface at a grazing angle of incidence. The vibrational
spectrum of molecules adsorbed onto the surface is obtained by comparing the intensity of the reflected light from a clean surface to one
covered by a thin film. The vibrational modes that are observable in
RAIRS are governed by a certain metal surface selection rule.
The selection rule states that only modes that have a component of their
transition dipole moment perpendicular to the metal surface appear in a
RAIRS spectrum. Hence, the appearance of a RAIRS spectrum indicates
that the transition dipole moments of specific functional groups in the
film molecules have a component parallel to the surface normal, and
combined with other structural data can provide information on the
ordering of the adsorbed layer.
CHAPTER 8: Surface Characterization and Imaging Methods
292
the IR spectrum of a thin nanofilm. Perhaps the most straightforward
method is to build the thin nanofilm on an external substrate and then
clamp the substrate on top of the IRE, sandwiching the thin nanofilm
between the substrate and the IRE. An IR absorbance spectrum of the
nanofilm can then be obtained as described above. While this method is
easy to implement, it does not allow for continuous monitoring of the thin
nanofilm during its creation. To acquire that type of data, it is necessary to
construct a flow cell on top of the IRE and build the thin nanofilm on the
surface of the IRE itself. Using this setup, the IR spectrum can be monitored continuously and changes that occur in the spectrum during the
creation of the thin film can be recorded.
For the purposes of determining the IR spectrum of a thin nanofilm as
described above, a horizontal ATR (HATR) setup is commonly employed.
In this setup, the IRE is a parallel-sided crystal plate with dimensions on
the order of 1 cm by 5 cm. For an IRE of this size, the IR beam is typically
reflected between 5 to 10 times at each surface, depending on the exact
dimensions of the crystal and the angle of incidence. The upper face of
the crystal is exposed to the thin film through a clamping method or a
flow cell setup, as described. The most common crystal materials used for
HATR setups are zinc selenide (ZnSe) and germanium. Diamond is a
more robust material that can be used as an ATR crystal, although its
higher cost is a drawback.
8.5.2 Reflection absorption IR spectroscopy
Another surface-sensitive IR spectroscopy method is reflection absorption infrared spectroscopy, or RAIRS. In RAIRS, infrared light is shone
onto a metal surface at a grazing angle of incidence. The vibrational
spectrum of molecules adsorbed onto the surface is obtained by comparing the intensity of the reflected light from a clean surface to one
covered by a thin film. The vibrational modes that are observable in
RAIRS are governed by a certain metal surface selection rule.
The selection rule states that only modes that have a component of their
transition dipole moment perpendicular to the metal surface appear in a
RAIRS spectrum. Hence, the appearance of a RAIRS spectrum indicates
that the transition dipole moments of specific functional groups in the
film molecules have a component parallel to the surface normal, and
combined with other structural data can provide information on the
ordering of the adsorbed layer.
CHAPTER 8: Surface Characterization and Imaging Methods
292