Technology Description:

The FT-IR laboratory at UiO MiNaLab is equipped with a state-of-the-art Bruker IFS 125HR Fourier Transform spectrometer that measures IR light transmitted through a material. The technique is used primarily for chemical identification of molecules from IR-active vibrations of their atomic constituents. Absolute quantification is possible when used in combination with certain techniques, e.g., secondary ion mass spectrometry. FT-IR can also be used to investigate absorption arising from impurities (electronic or vibrational) and provides a contact-free probe for free-charge carriers. Notably, the IFS 125HR can achieve a spectral resolution of up to 0.0063 cm^-1, which is necessary for resolving closely spaced ro-vibrational lines in gas phase spectroscopy. The use of pre-aligned optical components, a permanently-aligned interferometer, and various sources, beam splitters, and detectors provides access to a wide spectral range: from the mid-IR (600 cm^-1=16.7 µm = 74.3 meV) up to the visible (12 000 cm^-1 = 833 nm = 1488 meV). Absorption originating from residual CO₂ and H₂O vapor in the instrument is suppressed by operating the spectrometer under vacuum (base pressure of < 0.02 hPa) or by purging the instrument with dry N₂ gas. The spectrometer is equipped with a Janis PTSHI-950-5 low vibration, pulse tube cryostat, which permits temperature-dependent measurements from room temperature down to 5.0 K with a typical stability of ±0.01 K. The cryostat is equipped with ZnSe windows that complement the spectral range provided by the available sources, beam splitters, and detectors.

Sources:

Far-infrared (FIR): water-cooled Hg lamp with range of 4 - 1 000 cm^-1.

Mid-infrared (MIR): water-cooled Globar source with range of 40 - 13 000 cm^-1.

Near-infrared (NIR) / visible (VIS): Tungsten lamp with range of 1 000 - 25 000 cm^-1.

Beam Splitters:

Multilayer Mylar: range of 30 - 650 cm^-1.

KBr: range of 450 - 4 800 cm^-1.

CaF₂: range of 1 850 - 14 000 cm^-1.

Quartz(II): range of 9500 - 25 000 cm^-1.

Detectors:

DLaTGS (room temperature cooled): one for the MIR / NIR (370 -12 000 cm^-1) and on for the FIR (10 - 700 cm^-1).

Mercury cadmium telluride (MCT) (Liquid nitrogen cooled): 600 - 12 000 cm^-1) and one broad-band (420 - 12 000 cm^-1).

Indium antimonide (InSb) (Liquid nitrogen cooled): 1 850 - 12 000 cm^-1.

Indium gallium arsenide (InGaAs) (room temperature cooled): high gain unit, 5 800 - 12 800 cm^-1.

Silicon (Si) photodiode (room temperature cooled): 9 000 - 25 000 cm^-1.

*Note: the usable spectral range is determined by the exact combination of source, beam splitter, and detector used during the measurement. The IFS 125HR at UiO has an upper limit of ~17 000 – 18 000 cm^-1 due to the use of gold mirrors.*

Technical Description:

Complete software control – Bruker’s OPUS software allows selection of the IR source, apertures, optical filters, sample channels, and detectors without breaking the instrument’s vacuum.

Dry N₂ purging – significantly reduces the amount of time required to vent and re-evacuate the instrument when swapping beam splitters.

Advanced optics – easily incorporate additional optical elements, such as polarizers or cut-off filters, into the beam path.

External illumination – sapphire windows mounted perpendicular to the IR beam path allows one to check the effect of external illumination on a material; e.g., the effect of above-bandgap illumination on charge carriers.

Specialized sample holders - various holders available to accommodate crystalline samples of different sizes (5 × 5 or 10 × 10 mm² preferred). Each holder can accommodate typically 6-8 samples, reducing the time required to exchange samples.

Temperature-dependent measurements – sample cooling/heating in He exchange gas between 5 and 295 K using a LakeShore Model 335 temperature controller.

Automatic data acquisition – custom-written LabVIEW code issues joint commands to the spectrometer, temperature controller, and sample positioner, allowing automated measurements of, e.g., multiple samples at pre-defined temperatures.

Sample Limitations – Currently, samples are limited to bulk single crystal samples or thin films deposited on IR transparent substrates. Measurements of powders or liquids are not possible with our setup. Measurements of gas-phase samples should be discussed in more detail between the user and the lab responsible.