2D Materials

Van der Waals heterostructures, with their unique properties arising from the weak interlayer coupling and strong in-plane bonding, offer exciting opportunities for the design of novel materials with tailored electronic, optical, and mechanical properties.

Graphene is foreseen for a handful of electronic and optoelectronic nano-devices. Making nano-devices out of graphene requires nanopatterning. Determining the quality of patterned graphene is essential and the detection of defects demands a sensitive chemical nano-characterization tool.

MXenes is the largest and fastest growing 2D materials. They have unique properties such as good conductivity and a hydrophilic surface. The control of nanoscale composition would ultimately allow for engineering properties locally, gaining more control over the 2D material-based systems.

Glasses are essential materials with a multitude of uses and many forms. In the area of optoelectronics there is an interest to modify the glass composition to favor the incorporation of lanthanide elements.

Visualizing the distribution of structural defects and functional groups present on the surface of two-dimensional (2D) materials such as graphene oxide challenges the sensitivity and spatial resolution of most advanced analytical techniques.

Atomic Force Microscopy (AFM) associated to Raman spectroscopy has proven to be a powerful technique for probing chemical properties at the nanoscale. TERS in liquids will bring promising results in in-situ investigation of biological samples, catalysis and electrochemical reactions.

2D materials are state-of-the-art in nano- and opto-electronics. Characterizing their structural properties with a non-destructive approach at the micron scale is very important. We demonstrate in this paper how LabRAM Soleil™ confocal Raman multimode microscope is the perfect tool for these materials characterization.

Many challenges remain before the promise of 2D materials is realized in the form of practical nano-devices. An information-rich, nanoscale characterization technique is required to qualify these materials and assist in the deployment of 2D material-based applications.

Both TEPL and TERS images are well correlated with AFM morphological images obtained simultaneously, and all are consistent in revealing the nature (number of layers) of MoS2 flakes. Upon deconvolution, the TEPL signal is even capable of revealing local inhomogeneities within a MoS2 flake of 100 nm size. Kelvin probe measurement supports TEPL and TERS measurements and adds to the power of such tip-enhanced combinative tools. TEOS characterization of 2D materials is likely to contribute to further deployment of these materials into commercial products through a better understanding of their electrical and chemical properties at the nanoscale.

Raman and photoluminescence spectroscopy reveal different aspects of the solid state structure of 2D materials. Raman and photoluminescence imaging performed simultaneously with one instrument reveals the spatial variation of the solid state structure and electronic properties of 2D crystals that is not revealed in reflected white light imaging. That ability should allow materials scientists to better design and fabricate electronic and optoelectronic devices based upon 2D crystals.

The use of TERS to reveal the defects density in the structure of CNTs is of interest for a better understanding of the electrical properties of the devices made with such nano-objects. Not only defects concentration but also local chirality changes from the different radial breathing modes, pressure effect and strain distribution can be studied at the single carbon nanotube level through TERS.

The two methods - Analysis of fingerprint modes (intralayer) and Analysis of low-frequency modes (interlayer) - give complementary results and allow the determination of the number of MoS2 layers. Method 2 (using low frequency modes) gives excellent contrast; however it does not show single layer regions (which is related to the nature of the modes, rising from interaction between at least two layers). Method 1 (using fingerprint modes) shows all the layers, but the contrast is poorer, particularly for higher numbers of layers. The best result can be obtained combining the two methods. All the measurements (low-frequency and fingerprint) were done using ultra-low frequency ULFTM filters which allow a high throughput measurement in a full Raman range, down to <10 cm-1.