1. Experimental Station

The first CIRI beamline experimental station is the FT-IR microscope (Hyperion 3000, Bruker) coupled with the Vertex 80V (Bruker) spectrometer. Depending on sample characteristics and experiment requirements the measurements can be carried out with a synchrotron beam or standard Globar thermal source.

2. Measurement geometry

The microscope is equipped with a condenser (bottom) and objective (upper). The infrared beam can be directed by a mirror system to the condenser (transmission mode) or the objective (reflection mode). Figure 1 shows the scheme of the FT-IR microscope.

Figure 1. Scheme of FT-IR microscope. Source: Bruker

In transmission mode, the focused beam from the condenser after partial absorption goes through the sample to the objective and is directed to the detector (Figure 2).

Figure 2. Transmission mode

In reflection mode, the beam from the objective focused on the sample surface is partially absorbed, and the reflected part goes back to the objective and is directed to the detector (Figure 3). One part of the objective is used to sample illumination, and the second one to signal collecting. The reflection spectra have a different profile compared to classical transparent spectra due to to complex nature of absorption and reflection effects. The bands in reflective spectra have changed shapes, and relative intensity and occur inverted. The measured spectra needed further mathematical processing, including transformation like Kramers-Kroning.

Figure 3. Reflection mode

The transflection mode is a version of the reflection mode. The beam for the objective goes through the sample, reflects from the substrate, and goes back to the objective. In this case, the beam goes through the sample two times (Figure 4).

• Figure 4. Transflection mode

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• The ATR exploits the attenuated total reflection effect for the measurement in the sample contact with the conical, germanium crystal in the ATR objective (Figure 5). The pressure force range is 0.5 – 8 N. As a result of the interference of the incident wave and the reflected wave, a standing wave is formed, which propagates in the direction perpendicular to the boundary of the so-called evanescent wave. Since the penetration depth depends on the wavelength of the incident radiation, a correction to the intensity of the received signal has to be applied. The ATR mode is the Surface method. The limitation of the method is the possibility of damaging the sample or crystal.

Figure 5. ATR mode

3. Measurement condition

Mapping with MCT detector (mercury-cadmium-telluride detector): the study area during single measurements is limited by a mechanical aperture. Mapping with the MCT detector is a point-by-point analysis of the sample (Figure 6).

Figure 6. Mapping: a) limitation of area with aperture, b) point-by-point analysis

The aperture wheel has 12 apertures of different diameters (from 0.3 to 3.75 mm) which can be rotated in the beam path. The measurement spot diameter depends on the objective magnification. Table 1 provides examples of diameters for five different apertures.

Tabela 1. Examples of apertures and measurement spot diameters for different objectives

Analysis method	Magnification	Numerical aperture	Pixel size [μm]	Working distance [mm]	Spatial resolution [μm] for 1000 cm-1	Detector	Spectral range
Imagining
Transmission	3.5x	0.15	11.4	high	~41	FPA 64x64	4500-900  cm-1
	15x	0.40	2.7	24.0	~15
	20x	0.70	2.0	14.0	~9
	36x	0.52	1.1	10.4	~12
	40x	0.78	1.0	8.3	~8
	100x	0.80	0.4	1.8	~7.7
Transflection	3,5x	0.15	11.4	high	~41
	15x	0.40	2.7	24.0	~15
	20x	0.70	2.0	14.0	~9
	36x	0.52	1.1	10.4	~12
	40x	0.78	1.0	8.3	~8
	100x	0.80	0.4	1.8	~7.5
ATR	20x	0.60	2.0	in contact	~10	FPA 64x64	4500-900  cm-1
Mapping
Transmission	3.5x	0.15		high	~41	MCT 250 μm	5000-415 cm-1
	15x	0.40		24.0	~15
	20x	0.70		14.0	~9
	36x	0.52		10.4	~12
	40x	0.78		8.3	~8
	100x	0.80		1.8	~7,5
Transflection	3.5x	0.15		high	~41	MCT 250 μm	5000-415 cm-1
	15x	0.40		24.0	~15
	20x	0.70		14.0	~9
	36x	0.52		10.4	~12
	40x	0.78		8.3	~8
	100x	0.80		1.8	~7.7
ATR	20x	0.60		in contact	~10	MCT 250 μm	5000-415 cm-1

 

The standard MCT detector 1 mm dedicated to measurement with Globar source has a range of 12000-600 cm^-1, with a KBr beamsplitter of range 8000-350 cm^-1.

4. Sample requirements

The sample requirements are summarized in Table 3.

Table 3. Sample requirements

 

	Transmission	Reflection	Transflection	ART
Thickness	5-30 μm	<10 mm	<15 μm	<10 mm
Roughness	-	<1 μm	<1 μm for imaging	<5 μm
Thickness uniformity	High for FPA imaging	High for  FPA imaging	High for FPA imaging FPA	High for FPA imaging
Substrate	Transparent for IR	-	High reflective for IR (gold or different metal)	-
Maximal dimensions (w x l x t) with substrate	25 x 40 x 10 mm	25 x 40 x 10 mm	25 x 40 x 10 mm	25 x 40 x 10 mm

 

Additional information:

1. The most popular sample substrates for transmission measurements are barium fluoride and calcium fluoride. The range of transparency depends on the substrate thickness:

https://www.crystran.com/optical-materials/calcium-fluoride-caf2

https://www.crystran.com/optical-materials/barium-fluoride-baf2

2. In the case of thick sample measurement in the reflection mode significantly less amount of light reaches the detector compared to thin film measurement in the reflection mode. For samples with high roughness, most light is scattered, therefore sampling is possible the ATR, transflection, or transmission is suggested.

5. Measurement conditions

· Measurement takes place at room temperature, in the atmosphere

· Measurements on the hot stage (maximal temperature range -196 °C – 600 °C) with liquid nitrogen cooling - possible after prior consultation with scientists working on the beamline

· Available measurements with linearly polarized light with manually rotated polarizers

· Standard spectral resolution: 2 – 16 cm-1, for mapping maximally 0.5 cm-1.

6. Sample holder

The sample on the substrate is placed on a movable stage, the maximum range of the stage movement is 50x75 mm. The preferred sample size is no more than 20x40x1 cm. Measurements of samples with other dimensions must be previously consulted with scientists working on the line.

The second end station of the CIRI beamline is a neaSCOPE microscope (neaspec GmbH, Germany - now part of attocube systems AG, Germany). The microscope combines an atomic force microscope (AFM) and infrared spectroscopy (IR). The measurements can be performed with nanometric spatial resolution below the diffraction limit of IR light. Depending on the characteristics of the sample and measurement needs, it is possible to use techniques based on two different phenomena, i.e. near-field scattering (s-SNOM, nano-FTIR) and photothermal expansion (AFM-IR). s-SNOM is based on an asymmetric interferometer where the AFM tip and the sample are located in one of the interferometer arms. The light from the tip-sample arm is recombined with the reference beam at the detector. Interferometric detection based on varying the reference mirror position, thus the reference phase, allows for simultaneous recording of the phase and amplitude of the tip-scattered light, which relate to the local absorption and reflectivity, respectively. Modulating the reference phase and corresponding signal detection enables complete suppression of the scattering background. Thus, s-SNOM returns pure optical and chemical near-field maps free of mechanical artifacts. Optical signals are acquired simultaneously with AFM topography and mechanical phase. A typical IR light source in s-SNOM is a quantum cascade laser (QCL). Nano-FTIR spectroscopy is a variation of s-SNOM in which a single-wavelength source is replaced with a broadband source, e.g. synchrotron beam or difference frequency generation (DFG) laser. This modification allows hyperspectral imaging (i.e. collection of the complete spectrum at every pixel of the scanned area) with nanoscale spatial resolution determined by the tip apex size. Finally, the AFM-IR technique uses thermal expansion of the sample under IR illumination. In this phenomenon, the sample increases its volume due to the absorption of the IR light. The expansion of the sample volume is detected by the AFM tip, which increases its deflection amplitude due to mechanical interaction with the sample. The method is dedicated to soft materials, characterized by a higher coefficient of thermal expansion.

A summary of all techniques available at the s-SNOM/AFM-IR end station is presented below:

Phenomenon	Near-field scattering		Photothermal expansion
Illustration
Technique	s-SNOM imaging	Nano-FTIR spectroscopy	AFM-IR contact mode	AFM-IR tapping mode
Measurement type	imaging	spectroscopy	imaging and spectroscopy
Type of AFM mode	tapping		contact	tapping
Source of IR light	QCL	synchrotron	QCL (synchrotron in future)
Source type of emission	continuous wave (CW)		pulsed (kHz)
Detection mode	pseudoheterodyne (PH)	asymmetric interferometry	AFM tip deflection
Detector type	LN2-cooled MCT detector		AFM tip
Spectral range [cm-1]	925 - 1725	550 - 3200	890 - 1779
Spectral resolution [cm-1]	1	≥ 6,25	1
Sample requirements	nanoobjects or thin films of high thickness uniformity (< 200 nm, recommended) adsorbing light in the mid-infrared region and permanently mounted on flat substrates (roughness < 2 nm, recommended) – AFM compatible
Type of substrate	silicon, gold	gold, silicon	silicon, gold, mica*
Minimal sample thickness	ca. 5 nm		tens of nanometers
Sample + substrate dimensions (W x L x H) [mm]	max. 40 x 40 x 10 (10 x 10 x 1, recommended)
Signal collection depth	tens of nanometers		ca. 1 micron
Measurement environment	solid samples in the air (room temperature and standard pressure)

* mica shows low thermal expansion but may contribute to the AFM-IR spectrum due to strong absorption bands in the mid-IR range.

The O-PTIR microscope is the third end station of the CIRI beamline. This is the second technique developed in recent years for measurements of infrared (IR) images with spatial resolution below the IR light diffraction limit. The microscope has two light sources – a probe laser with a wavelength in the visible range (532 or 785 nm, both available at CIRI beamline) and an infrared radiation source - a pulsed, tunable QCL laser, or a synchrotron source with a wide wavelength range.
During the experiment the sample is simultaneously illuminated with colocalized visible laser light and infrared light. When the frequency of the IR wave matches the vibrational frequency of the molecules, absorption occurs, leading to photothermal expansion and associated with it reduction in the index of refraction in the illuminated area of the sample. As a result, the intensity and angular distribution of visible light reflected, scattered, and/or transmitted by the sample is altered. The obtained signal is used to generate the spectroscopic infrared spectrum. The photothermal response depends on many parameters related to the sample, including the thermal conductivity, heat capacity and density of the absorbing molecules, as well as the power of the probe laser and the power of the infrared source. The spatial resolution is limited only by the wavelength of the laser acting as the probe.
Apart from the elastic scattering of photons, which is used for detection of IR absorption, inelastic scattering (Raman effect) occurs at the same time, i.e. the formation of photons with altered energy. As an effect, the Stokes and anti-Stokes lines appear in the spectra symmetrically on both sides of the Rayleigh line, with respectively reduced and increased frequencies. The O-PTIR method allows, thanks to the use of a visible laser, to obtain both infrared and Raman signals at the same time.

Figure 1. Simplified principle of O-PTIR measurement. For the detection of absorption of long-wavelength IR radiation short-wavelenghth visible radiation is used, allowing to dramatically improve the spatial resolution of the technique.

In-depth description of the O-PTIR microscopy along with requirements and examples is available in an open-access paper here: https://pubs.aip.org/aip/app/article/9/9/091101/3312368/A-tutorial-on-optical-photothermal-infrared-O-PTIR

Measurement configurations

In the O-PTIR microscope the sample can be illuminated by IR light from the top through Cassegrain objective or from the bottom through optics with off-axis parabolic mirror (OAP) as a focusing element.

When the IR is being directed through the top objective, it co-propagates with the visible probe beam. Both beams are arranged to be collinear and illuminate the sample from the same side at the same spot. In this configuration only Cassegrain objective can be used.

When the IR is being directed through the OAP system, it is delivered to the sample in counter-propagate configuration, that is from the opposite direction to the probe beam. In this configuration any top objective can be used for measurements, however, due to better optical parameters, refractive 50x or 60x objectives are preferred.

In the O-PTIR the IR light is not being detected directly by any detectors. The microscope is equipped with two detectors for visible light. One photodiode is adapted to detect scattered or reflected from the sample through the top objective. This photodiode can be automatically swapped for avalanche photodiode (APD) for experiments with photo and heat sensitive samples that require very low laser powers. The second photodiode is used for detection of light transmitted through the sample. It is used for samples that have low reflectivity or are too thin to scatter enough of the probe light.

The separate method of measurements can be used for samples with a strong fluorescence response. In the widefield method, fluorescence imaging is utilized for the detection of IR absorption from a defined area of the sample. For the experiment the counter-prop microscope configuration is used. OAP is set out-of-focus so that the IR light illuminates an area of the sample instead of being focused at a single spot. The measurements are performed by acquiring a sequence of fluorescence images with and without the IR light coming to the sample. By comparing those images with each other it is possible to reconstruct an IR spectra. With this method it is possible to obtain over 260 thousand IR spectra with 8 cm^-1 resolution, in less than an hour.

Figure 2. Diagram of the optical configuration of the O-PTIR microscope.

Experiment setup, requirements and sample preparation

Measurements in the co-prop configuration do not require any sample preparation. The method has no requirements for sample thickness or the use of any specific substrates, especially if only single spectra from selected points on the sample are planned to be measured. The measurements are usually non‑destructive, requiring no contact between the objective and the sample. In case of samples with irregular surface, imaging mode is limited to only small areas. For light or heat sensitive samples, it is possible to conduct measurements at low laser powers using an avalanche photodiode.

Measurements in counter-prop configuration require: 

• Sample thickness of no more than 10 µm
• Calcium fluoride substrate only, 10 mm in diameter, 0.35 mm thick

Slides in these dimensions fit into a holder adapted to the measurement table in the O-PTIR microscope and allow for quick start of planned experiments. Other IR-permeable substrates with a diameter of up to 2 cm and a thickness of up to 1 mm are still suitable for measurements, however, the quality of resulting spectra will be lower.

Requirements for the measurements in the widefield mode are similar to counter-prop mode. For the measurement to be feasible, the presence of strong autofluorescence or fluorescence from dyes in the sample is necessary.

For O-PTIR measurements in aqueous environments the microscope is equipped with 60x water-dipping objective. The requirements are analogous as in case of counter-prop mode.