2D materials analysis techniques and tools
Two-dimensional materials (2D) consist of low dimensional structures that show promise in electronics, communications, photodetectors, gas absorption, catalysis, and many other applications yet to be discovered. Graphene, a single layer of carbon atoms, is one of the more widely known 2D materials. However, several other materials are being studied for their use in creating next-generation consumer and industrial products.
For example, 2D boron nitride is being explored for its thermal and electrical conductivity properties for use as a dielectric, in solid-state electrolyte materials for fuel cells, and for specialized coatings. Molybdenum disulfide exhibits semiconducting behavior when manufactured as a single layer and is being studied for use in a flexible circuit that could be used as a bendable and wearable “smart” e-textile material. And metal-organic frameworks (MOFs) have strong absorption capacities as they interact with “guest” molecules, making them good candidates for gas storage and separation, heterogeneous catalysis, and supercapacitors.
2D materials atomic resolution
By their definition, two-dimensional materials are one atomic layer thick, or possibly in a stack of a few layers. Therefore, characterization of these materials requires analytical techniques capable atomic-level exploration.
As scientists investigate the detailed properties of 2D materials, they’re increasingly turning to high-resolution scanning transmission electron microscopes (STEMs) along with integrated differential phase contrast (iDPC) to get the atomic level information they need. They’re also using X-ray photoelectron spectrometers (XPS) in combination with Raman spectroscopy for detailed surface analysis.
X-ray photoelectron spectroscopy
Our NEXSA X-ray Photoelectron Spectrometer (XPS) System is unique in the XPS world in that it can concurrently apply molecular spectroscopy on the same spot using an integrated Raman spectrometer. Raman is particularly useful at characterizing graphene and other carbon allotropes, but also plays a complementary role for XPS.
XPS measures the energies that bind electrons to an atom’s nucleus when ejected by an external force such as X-rays. Since most electrons are absorbed back into the material, only the topmost photoelectrons can be measured, making XPS one of the most surface sensitive techniques available. In determining the kinetic energy of the ejected photoelectron, researchers can understand what elements are in the sample surface, the energy level that the electron occupies indicating its electronic configuration, and the influence of other elements bonded to the atom in question, indicating its chemical state such as oxidation.
Raman spectroscopy
Raman spectroscopy measures the transition of molecules when they are excited by light, known as the Raman effect. In observing the vibrational pattern of the sample in a spectrum of peaks at certain wavenumbers, scientists can identify the molecular composition of the material by recognizing the spectral peaks or by matching data from a spectral library. Raman has the advantage over other vibrational techniques in that it can detect the differences in a material’s physical structure, such as identifying polymorphic phases. Measuring the intensity of spectral peaks indicates the volume of the molecules for quantification, enabling researchers to determine layer thicknesses.
Transmission electron microscopy
Transmission electron microscopy requires samples prepared in thin layers in order to transmit the imaging electrons onto a detector. Nanomaterials such as MOFs are highly sensitive to electron energies that might destroy the sample or corrupt its original structure. Therefore, ultra-low energizing voltages need to be applied.
High-angle annular dark-field imaging (HAADF) is a commonly used technique for STEMs. It can produce high-quality images at extreme low dosages with no sample damage. However, it cannot determine lighter elements in the sample, limiting access to oxygen, carbon or nitrogen potentially found in nanomaterials. Our Spectra 200 and Spectra 300 high-resolution STEMs feature the Panther STEM detection system, which includes optimized mechanical alignment and detector geometry for multi-signal acquisition and mechanical alignment accuracy.
Extreme-low-dose imaging (166 e- / Å2) of the metal organic framework (MOF) UiO 66. A Spectra 300 was used in combination with iDPC to image atomic-level details in this highly dose-sensitive material. Image courtesy of Professor Y. Han, King Abdullah University of Science and Technology.
Integrated differential phase contrast (iDPC) imaging is used on Spectra STEMs for the study of magnetic and electrical properties and for optimized Z-contrast imaging from hydrogen to uranium. Combined with Panther STEM detection systems, iDPC imaging enables the complete characterization of 2D structures for samples such as MOFs that are typically damaged under short exposures to the electron beam.
As the global market for 2D materials continues to grow, scientists are turning to powerful microscopy techniques such as STEMs, iDPC, XPS, and Raman spectroscopy to determine the detailed properties of 2D materials. To learn more about how these characterization techniques can increase our understanding of 2D materials, please watch the on-demand webinar, “Coincident XPS and Raman analysis for 2D materials.”
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