An Introduction To Raman Imaging and 2D Materials

HORIBA, the global leader in Raman microscopy and spectroscopy systems, provides an extensive array of instruments and solutions for applications across a broad range of scientific R&D and QC measurements. These instruments include a host of spectroscopy solutions, including Raman fluorescence, custom spectroscopy, and particle characterization.

Image Credit: ShutterShock/James A Isbell

In this article, HORIBA outlines the basic principles of applied Raman imaging with an emphasis on 2D materials and the importance of spectral resolution to image rendering. In addition, it will cover combined imaging by laser-excited photoluminescence and Raman scattering of two-dimensional crystals to reveal the spatially varying solid-state structure of these materials.

Image VS. Rendering and How a Spectral Image is Created

An image is usually generated by capturing light from a physical object. A camera is then used to project that light onto a focal plane where a sensor captures that reflected or transmitted light so that it can be projected or presented again.

A rendering is something like a drawing or graphic creation submitted to an audience and likely seen every day in newspapers, magazines, and websites.

A rendering comports more with Roman spectral imaging because it requires mapping from which spectra is acquired at different locations. From such spectra, users extract information from the data to plot a spatial variation of that information, which is called rendering.

When producing Raman images, it is vital to pay attention to the analysis and processing of the data to effectively communicate what was discovered about the material and what uses are trying to convey in Raman images.

Resolving different compounds by Raman spectroscopy for the purposes of mapping and then imaging them is a frequent way of generating a Raman image. Another way would be using chemometric tools, which incorporate the entire spectrum to differentiate the different sets of spectra or the different compounds that are present.

The Importance of Spectral Resolution for Raman Spectral Imaging

The spectral resolution is important in the instrumentation and the software when imaging strain, micro crystallinity and nano crystallinity.

Micro crystallinity and strain distribution cause the Raman band to shift and broaden into a lower wave number. Bands can even shift to lower energy, and those are associated with nano crystallinity. This combination of having the instrument’s spectral resolution and software having color-coded brackets allows imaging of the spatial variation of the band shape of just a single element.

When looking at an expanded scale with an understanding of the structure of the fabrication of devices, users can see various layers and some isolated grains of materials like nanocrystals and silicon. This is very important to an electrical engineer who works with these materials as it can affect the electronic properties of the device – the degree of micro crystallinity and strain and nano crystallinity.

For example, in a polysilicon test structure hyperspectral data set, users can see the distinction and thickness between substrate silicon, polysilicon, and the nanocrystal and silicon because of the spectral resolution of the instrument in conjunction with the ability to use the software almost as a grading spectral slit to produce the image.

Raman Imaging Based on Crystallinity

Ion implantation is the favored method of introducing dopants to semiconductors, and in silicon, that is normally boron, phosphorus, or arsenic. Take a single crystal of silicon as an example. With a single crystal compound, the Raman selection rules dictate that only the Brillouin zone center will be Raman active.

When the silicon has been implanted, it is also hit by very heavy arsenic at high energy, becoming amorphous. In other words, a crystal lattice has now been disrupted and broken. This is one of the reasons why it is carried out after implantation, as devices have to be annealed to restore the crystallinity and activate the device.

Using the spectral resolution of the spectrometer in conjunction with software tools that allow users to probe that band structure can render images that reveal a lot more than simply plotting the intensity of one particular peak or even using even some other tools.

Raman Imaging of 2D Molybdenum Disulfide

At least one of the areas of interest is the use of Raman in electronics. Raman imaging of 2D molybdenum disulfide can reveal the spatial variation and layer number and strain of these materials.

For example, in a conceptualized single-layer molybdenum disulfide transistor, there is a familiar drain-source and gauge structure with a monolayer of molybdenum disulfide being the active material for the semiconductor.

What is interesting about the material is that in bulk, it is an indirect band gap semiconductor, but when it comes to the fuel layer, it becomes a direct band gap semiconductor. It is electro-optically active; it functions not only as a semiconductor but also as an emitter.

In images of the Raman spectroscopy and imaging of these 2D crystals, there is a good correspondence of the Raman images with the optical properties and reflected light images, and Raman spectroscopy and imaging of 2D at low energy phonons is as good, if probably better, in being able to identify and differentiate layer numbers in these 2D crystals than the separation of bands at higher energy.

In the more complex cases, users can even distinguish which bands are correlated just from the shapes in the Raman images.

Combined Raman and Photo Luminescence Imaging of 2D Tungsten Disulfide

The way that HORIBA has set up its spectrometer, Raman and photoluminescent spectra can be obtained in the same single measurement.

By overlaying Raman and photoluminescence images, users are able to see spatial variations within spectral images that do not emerge or appear in the reflected light image, such as the phonon and electronic properties of a crystal that cannot be revealed in just simply the reflected white light image.

Images do not just present themselves by collecting the data. This requires a knowledge of spectroscopy, chemistry, or physics to apply spectral acquisition conditions and analysis tools used in the software to render images that reveal information about chemical bonding and solid-state structure.

This information has been sourced, reviewed and adapted from materials provided by HORIBA Scientific.

For more information on this source, please visit HORIBA Scientific.

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