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Applications for THOR and qRICO technology

  • advanced Raman microscopy (THOR)
  • quantitative crystallographic orientation mapping of crystalline materials (qRICO)
  • Application for THOR technology

    Deep volumetric Raman imaging (dVRI) by THOR system

    Three‐dimensional (3D) confocal Raman mapping is one of the most promising techniques to study the chemical composition of complex organic and inorganic materials. However, quantitative volumetric Raman imaging is still a state‐of‐the‐art technique with few reported applications, mostly due to interference from molecular fluorescence, limited sample transparency at the laser excitation wavelength, complexity of chemometric analysis and low Raman scattering cross section of the chemical components. The molecular fluorescence of samples can be suppressed in several ways. However, the most proper and common techniques like Kerr‐gated Raman, ultraviolet Raman and near‐infrared (NIR) Fourier transform (FT) Raman are not well applicable to the diffraction limited deep volumetric Raman imaging (dVRI) due to the limitation connected with confocality or penetration depth. The most appropriate solution is the usage of NIR lasers (780, 785, and 830 nm) as excitation sources. For the sole purpose of suppressing sample fluorescence, Nd:YAG lasers with 1,064‐nm excitation wavelength are a very attractive solution; however, the Raman scattering cross section becomes usually too low for dVRI.
    In this situation, where the fluorescence of samples takes place, our THOR system operates with NIR laser with wavelength of 785 nm. However, sample transparency is limited, even when using a laser at 785‐nm excitation wavelength. It is important to mention that in‐depth Raman mapping is also limited by off‐axis laser refraction effects leading to Raman signal attenuation and decreased axial resolution. The usual way of addressing this problem is to increase the laser power and/or the exposure time. However, the latter leads to an unrealistic total time of 3D map acquisition (>3 days). Increasing the laser power, instead, overheats and burns the sample.

    With THOR system, we present an efficient solution to solve the previously discussed problems of dVRI of samples that manifest fluorescence and low transparency. This result was achieved by the development of a confocal Raman microscope with high Raman signal throughput, optimization of sample mapping method, and further chemometric hyperspectral data analysis based on nonnegative least squares.

    Wide Line SERS mapping by THOR system

    Surface‐enhanced Raman spectroscopy (SERS)‐based molecular detection at extremely low concentrations often relies on mapping of a SERS substrate. This yields a large number (>1000) of SERS spectra that can improve the limit of detection; however, the signal collection time is a major constraint. In THOR system, a wide line (WL) laser focusing technique aimed at fast mapping of SERS substrates. The WL technique enables acquisition of thousands of SERS spectra in a few seconds without missing any of the electromagnetic “hot spots” in the illuminated area. In addition, the SERS signal averaging across the line in the WL mode displays extremely high signal‐to‐noise ratios. The advantages of the WL technique for SERS‐based sensing are verified using different analyte molecules, that is, p‐coumaric acid and melamine. Results show that the limit of detection can be improved by one order of magnitude compared to results obtained using a common point-focusing Raman microscopes.

    Comparison of SERS microscope laser illumination modes. Here we present SERS spectra of BPE (10 µm concentration) measured on Au nanopillars substrate at point (black), DL line (red), and WL (blue) illumination modes; zoomed spectra in the range 1750–1950 cm−1 for SNR demonstration. All presented spectra were acquired at equal conditions: 0.2 mW µm−1 of laser intensity, laser wavelength 785 nm, exposure time 0.1 s, 10× magnification microscope objective. SNR was measured as signal divided on RMS noise.

    Raman line-focus microscopy with chemical decomposition

    A Raman line-focus mapping option was applied for fast simultaneous mapping of differently sized and shaped particles of nitrofurantoin monohydrate, revealing the appearance of multiple solid-state forms and the non-uniformity of this particle system during the complex dehydration process. This method provides an in-depth understanding of phase transformations and can be used to explain practical industrial challenges related to variations in the quality of particulate materials.

    Raman experiments resulted in a matrix that has two dimensions [t, S(ν)], where t corresponds to the temperature or time points, while S(ν) corresponds to Raman spectra. We registered 220 spectra from the laser line at each temperature/time point, and all spectra were grouped as follows: Mline = [t1(S(ν)1, S(ν)2, … S(ν)220), t2(S(ν)1, S(ν)2, … S(ν) 220), … tn(S(ν)1, S(ν)2, … S(ν) 220)], where n is the number of temperature/time points. In order to extract information about the concentration and spectral profiles of the studied model compounds, we used multivariate curve resolution (MCR) and non-negative least squares (NNLS) methodology. Both methods realized in THOR data analysis software package.

    MCR and NNLS decomposed results of hydrate and anhydrous solid-state forms of theophylline. (a) Optical image of TP MH at 25 °C with laser line illumination (white line) from the Raman microscope (b) concentration profile of solid-state forms during isothermal dehydration at 50 °C for 90 minutes starting from TP MH to TP AH form II, via TP MS1 and TP MS2 (c1–4) chemical concentration maps (cut out artefacts from 1.8 mm laser line) of TP MH (c1), TP MS1 (c2), TP MS2 (c3), TP AH form II (c4) in the particle during dehydration where the laser line is illuminated, (d1–4) Raman spectra for TP MH, TP MS1, TP MS2 and TP AH form II respectively, (e1–e4) area plots showing only the dehydration profiles of TP metastable intermediates at four isothermal conditions (45, 50, 60 and 65 °C) where the colour under the area plots matches the TP metastable intermediates as shown in the concentration profiles. The stable forms of TP were taken into consideration for the area plots.

    Application for qRICO technology

    Semiconductors and Microelectronics

    Solar cell manufactures have a keen interest in a qRICO device. Crystallographic orientation analysis is a critical step in the development of efficient solar cell panels. Existing analytical techniques (EBSD, XRF, etc.) for two-dimensional crystallographic analysis are too expensive and too limiting in their capabilities to be routinely used in an industrial environment. Similarly, strong needs exist in the semiconductor device and process monitoring market to deliver crystallographic analysis both in process development applications and also for quality control purposes. A qRICO device is uniquely suited for fulfilling the demands in these areas.

    Dislocations and Stress in Electronics

    Inhomogeneities and dislocations in heterostructured semiconductors with high levels of local heating in operation conditions can greatly reduce the lifetime of devices like LEDs and laser diodes. Therefore, nano- and microscale orientation mapping of such structures by qRICO provides important information in the development and quality control of devices.

    2D Materials

    qRICO has a great potential of quantitative orientation mapping in 2D materials. It is used for orientation determination and dislocation search of separate layers in multi layered 2D materials.

    Ceramics

    3D grain mapping is very important in ceramics technology, because properties such as fracture strength is strongly influenced by the statistical distribution of grain orientation and the grain boundary topology. Thus qRICO provides unique information in piezo-, magneto-, and ferroelectrics.

    Superhard Materials

    Abrasives, drilling tools, superhard transparent windows – all mentioned examples require the knowledge of crystallographic orientation of poly-crystalline surfaces.

    Crystalline Drugs and Proteins

    Raman microscopy is often used for chemical mapping of pharmaceutical materials. It has been shown that polarized Raman microscopy can be used to visualize particles on the surface of tablet formulations, however so far no quantitative information on the orientation of single particles has been provided. We demonstrated successful Raman measurements of a content nonuniformity in a tablet containing carbamazepine dihydrate (CBZD) and polyvinylpyrrolidone (PVP). Using multivariate curve resolution (MCR), these components were decomposed including the fluorescence background as shown in Figure on the left. Applying qRICO to the same area on the surface of the tablet, we obtained an orientation map of CBZD (monoclinic symmetry, C2h crystal class). These findings show the potential for qRICO to provide insight into crystal face functionality in pharmaceutical research as well as in materials science.

    (a) Chemical map obtained by qRICO device from the surface of a compacted tablet containing CBZD and PVP (2:1)

    (b) 2D-qRICO map showing the random orientation of CBZD particles on the surface of the tablet. The colors refer to the orientations as defined by the inverse pole figure (bottom right). qRICO was performed at a step size of 8 µm, map dimension 134 × 134 pixels or 1072 × 1072 µm, exposure time per step was 150 ms.