Two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) are receiving increasing attention due to their unique optical and electronic properties. Their possible applications include the production of transistors, photo detectors, light emitting diodes (LEDs) and photovoltaic cells. In order to produce high-quality devices, synthesis processes must be evaluated efficiently. Thus, non-destructive imaging techniques are required for monitoring crystal properties and features such as grain boundaries, layer number, defect density, doping and strain fields.
In our new application note, we present a series of measurements of CVD-grown mono-layer molybdenum disulfide (MoS2), which illustrate the advantages of correlative Raman, second harmonic generation (SHG) and photoluminescence (PL) microscopy for investigating TMDs. All measurements were performed at the same area of interest using a WITec alpha300 microscope equipped with a 532 nm laser for Raman and PL imaging and a picosecond-pulsed 1064 nm laser for SHG excitation.
Strain fields in the crystal were visualized by Raman and PL imaging, as both the frequency of the E2g Raman mode (upper left image) and the wavelength of the PL signal (lower left image) were red-shifted in the same areas. Rim effects around the border of the MoS2 flake were clearly visible in the PL image (lower left image), as well as in the image of the A1g Raman mode (see the attached application note).
SHG microscopy is sensitive to changes in crystal orientation and symmetry and visualized grain boundaries in the MoS2 flake (upper right picture). Additionally, polarization-dependent SHG measurements can identify the crystal orientation and reveal strain fields. To this end, the excitation polarization is rotated while recording the intensity of the SHG signal component that has the same polarization as the incident light. Polarization series were recorded in a fully automated manner at three positions of the MoS2 flake (lower right picture). The distinct patterns observed indicate different strain levels.
Correlative Raman, PL and SHG imaging yields complementary and consistent information for characterizing single-layer TMD crystals by visualizing features of the crystal structure, such as grain boundaries or strain fields, without damaging the sample.
For more details, including further pictures and references, please download our 2-page application note on correlative high-resolution imaging of MoS2.
Raman imaging is a non-destructive tool for evaluating the quality of 2D materials as strain, doping, defects and layer number can be assessed. These two large-area Raman images visualize defect density (top) and strain fields (bottom) in a CVD-grown graphene flake at high spatial resolution (100 nm per pixel). They were acquired using the fully automated Raman microscope alpha300 apyron equipped with a 532 nm laser for excitation and TrueSurface for focus stabilization.
The upper Raman image is color-coded according to the intensity of the D-band in the recorded Raman spectra. It visualizes crystal defects, as the D-band intensity depends on the defect density in the carbon lattice. The observed width of the fine structures is very close to the diffraction limited lateral resolution achievable with 532 nm excitation, demonstrating the microscope’s high performance.
The lower Raman image is color-coded according to the peak position of the 2D-band, which was quantified by a Pseudo-Voigt fit. The image visualizes local strain/doping effects, as the frequency of the 2D-band is influenced by local strain and, to a lesser extent, by doping.
These examples offer conclusive proof that with an advanced and highly sensitive system, Raman imaging alone can provide access to the finest details of graphene crystal properties.
We’re thrilled to share that ParticleScout, our automated microparticle analysis tool, has been chosen as a finalist for the 2021 Wiley Analytical Science Award!
This distinction from a neutral jury of researchers and industry representatives includes ParticleScout among six nominated products in the “Spectroscopy & Microscopy” category. Now, the readers of “GIT Labor-Fachzeitschrift”, “Imaging & Microscopy” and all users of the Wiley Analytical Science Portal can vote for their favorite product on the following page:
Voting is possible from July 1st until August 28th. Wish us luck, or even better, voice your support for ParticleScout, the revolutionary tool that uses Raman spectroscopy to characterize microparticles, including microplastics in the environment.
A paper just published in the Journal of Biophotonics describes how Raman spectoscopy can enable the detection of RNA viruses in human saliva. Dr. Amit Dutt's group at the Tata Memorial Centre, based in Mumbai, obtained raw data with a WITec Raman microscope and carried out statistical analysis to find a set of 65 Raman spectral features that positively identified the presence or absence of viruses in a sample.
The analysis could be performed in less than a minute without adding a reagent to increase the signal. Their signal set was able to achieve 92.5% sensitivity and 88.8% specificity.
“This conceptual framework to detect RNA viruses in saliva could form the basis for field application of Raman Spectroscopy in managing viral outbreaks, such as the ongoing COVID-19 pandemic,” said the researchers.
Increased capacity required to meet growing demand for Raman imaging microscopes
June 10th, 2020
WITec GmbH, the technology leader in correlative Raman microscopy, is expanding its headquarters building in Ulm.
The new addition will offer more space for production and quality assurance, laboratories for development and customer demonstrations or sample measurements, larger conference rooms and greater logistical capability. When complete, the new addition will double the amount of space available at WITec’s core location.
“It was time to expand,” says Joachim Koenen, Co-founder and Managing Director of WITec. “We made advanced Raman imaging much more accessible by simplifying operation through our user-interface and extensive automation. The scientific community and marketplace have reacted very positively to these developments. As more researchers put our instruments in their labs, the word got out, and now we need the increased production capacity.”
WITec’s current headquarters was completed in 2009. Its design reflects the modularity and versatility of the scientific instruments produced within. An open and light-filled interior provides an environment for employees that is conducive to the exchange of ideas. Research and development, sales, applications, production, marketing, administration, technical service and logistics teams all work together hand-in-hand. This greatly contributes to WITec’s vaunted technological agility and preserves its innovative spirit.
As described by Olaf Hollricher, Co-founder and Managing Director of R&D at WITec, “Our headquarters was purpose-built to encourage cross-departmental interaction, from development through shipping, and it’s had a very positive effect on the dynamics and performance of our company. The new expansion will allow us to scale up while adhering to this concept.”
Construction of the new addition will continue through 2020 with an anticipated opening toward the end of 2021.
- WITec New Building Expansion Rendering (3.7 MB)
- Press Release - New Building Expansion - English (DOCX) (442 KB)
- Press Release - New Building Expansion - German (DOCX) (444 KB)