Characterisation of Functional Materials on the Nanoscale Using Raman Imaging, SPM and TERS

It is crucial that functional materials, such as graphene and silicon carbide (SiC), are comprehensively characterised before they can be used in green energy applications (for example in lithium ion batteries) and as inverters for applications in automotive power control systems.

This article shows how Raman spectroscopy can be a very effective in the continuing development of functional materials. With a flexible spatial resolution—spanning from millimetre to sub-micrometre—Renishaw's inVia Raman microscope is a powerful and proven tool for the investigation of electronic properties, chemical properties, homogeneity, and strain in these materials.

Characterisation of SiC Wafers

SiC is a promising material for high power electronics. When compared to silicon-based devices, SiC devices can work at much higher temperatures and voltages, making them promising for use in the drive trains in electric vehicles. However, large-scale growth of SiC is challenging and wafers usually contain inclusions of different polytypes and other defects, which considerably affect the performance of the final devices.

A powerful tool for characterising SiC, Raman spectroscopy is capable of distinguishing between SiC polytypes quickly and easily. It can also detect defects and determine both strain and electronic properties. Here an inVia Raman microscope, using Renishaw’s proprietary technology, characterises an entire 2” wafer in less than an hour, at a resolution of 50 µm. More than 870,000 data points were collected.

This highlights a speed increase of an order of magnitude over the past few years, thereby making it possible to perform Raman analysis on the timescales needed for the quality control of wafers. Figure 1 shows a composite Raman image.

A composite Raman image. The blue image illustrates change in peak position of the FTO Raman mode. This can be directly related to stress in the wafer. Hexagonal strain features can be seen, allowing micropipes at the centre of dislocations to be detected. The high spatial and spectral resolutions of inVia are demonstrated by the ability to image small strain features. A 6H-SiC inclusion is highlighted in red. Regions with 540nm PL, corresponding to growth defects, are shown in yellow. Inset a) spectra from the different polytypes of SiC, highlights how easy it is to differentiate polytypes using the inVia Raman microscope.

Figure 1. A composite Raman image. The blue image illustrates change in peak position of the FTO Raman mode. This can be directly related to stress in the wafer. Hexagonal strain features can be seen, allowing micropipes at the centre of dislocations to be detected. The high spatial and spectral resolutions of inVia are demonstrated by the ability to image small strain features. A 6H-SiC inclusion is highlighted in red. Regions with 540nm PL, corresponding to growth defects, are shown in yellow. Inset a) spectra from the different polytypes of SiC, highlights how easy it is to differentiate polytypes using the inVia Raman microscope.

TERS Imaging on Graphene and Functionalised Carbon Nanotubes

Tip enhanced Raman spectroscopy (TERS) integrates scanning probe microscopy (SPM) and Raman measurements, and enables the collection of Raman data on the nano scale.

This important technique is suitable for studying chemical properties of nanomaterials because it makes it possible to analyze materials at the nm scale. This is not possible with micro Raman spectroscopy which is limited by diffraction to a lateral resolution of ~ 300nm.

Graphene and carbon nanotubes (CNTs) are two important nanomaterials, which are being targeted at various applications in the energy industry. In this analysis, the TERS technique was applied to these materials and used to evaluate their homogeneity. Figures 2 and 3 show the comparison between TERS and AFM imaging.

ERS and AFM imaging on graphene and functionalised CNTs. a). TERS (red) and far field Raman spectra of graphene (blue). These results demonstrate the enhancement in signal provided by TERS; b) TERS image showing sub diffraction limit changes in the G/2D ratio for a graphene sample. This ratio is used to estimate CVD graphene thickness. Here there are significant changes in the ratio on a 10nm length scale showing graphene variation on the nanoscale.

Figure 2. TERS and AFM imaging on graphene and functionalised CNTs. a). TERS (red) and far field Raman spectra of graphene (blue). These results demonstrate the enhancement in signal provided by TERS; b) TERS image showing sub diffraction limit changes in the G/2D ratio for a graphene sample. This ratio is used to estimate CVD graphene thickness. Here there are significant changes in the ratio on a 10nm length scale showing graphene variation on the nanoscale.

a) AFM image of a CNT cluster, this image allows the size of the cluster to be determined but does not provide any chemical information; b)TERS image highlighting the distribution of functionalized CNTs within the cluster (red regions contain more functionalised CNTs). Here it is clear the mixing of functionalised and non functionalised CNTs is not uniform in the cluster.

Figure 3. a) AFM image of a CNT cluster, this image allows the size of the cluster to be determined but does not provide any chemical information; b)TERS image highlighting the distribution of functionalized CNTs within the cluster (red regions contain more functionalised CNTs). Here it is clear the mixing of functionalised and non functionalised CNTs is not uniform in the cluster.

Conclusion

This article shows how Renishaw’s advanced and powerful inVia Raman microscope can be effectively used for determining a material’s chemical properties, from macroscale to nanoscale.

It also shows how Raman spectroscopy, a robust surface characterisation tool, is used to characterise SiC wafers, demonstrating its ability to identify defect regions, detect polytypes, evaluate stress, and inspect the homogeneity of a 2” SiC wafer.

TERS is an innovative technique that makes it possible to obtain chemical data from samples with nanometre resolution. This proves especially useful for studying graphene, CNTs, and other emerging carbon materials.

This information has been sourced, reviewed and adapted from materials provided by Renishaw - Raman Spectroscopy.

For more information on this source, please visit Renishaw - Raman Spectroscopy.

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