Optical contrast is a popular preliminary approach for determining the layer numbers in two-dimensional materials. However, it is rarely employed as a confirmation technique.
Study: Optical Microscope Based Universal Parameter for Identifying Layer Number in Two-Dimensional Materials. Image Credit: RomanenkoAlexey/Shutterstock.com
A significant disparity in optical contrast across different imaging systems exists, necessitating a system-independent assessment of optical contrasts. A recent study published in the journal ACS Nano addresses this issue by presenting a universal approach for quantifying the layer numbers in two-dimensional materials utilizing red−green−blue (RGB) and RAW optical images.
Two-Dimensional Materials: Overview and Significance
Two-dimensional materials are made up of one or very few atomic layers of substances that have fascinating optoelectronic characteristics. Two-dimensional materials have found several applications in sensing and quantum computing due to their exceptional features like high surface-to-volume ratio, surface charge, form, high degree of anisotropy, and tunable chemical activity.
Since the discovery of graphene, two-dimensional materials have attracted remarkable scientific interest. Their distinct optical, electrical, and mechanical qualities offer considerable promise as crucial components in innovative electronics and optoelectronics applications.
The atomic thickness and exposed large surface area of two-dimensional materials make them extremely designable and manipulable, resulting in a wide range of industrial applications.
Fabrication Methods of Two-Dimensional Materials
Various approaches may be used to manufacture two-dimensional materials. While these processes differ greatly, they may be categorized as either top-down procedures, in which the two-dimensional materials are extracted from a bulk source, or bottom-up techniques, in which the two-dimensional materials are produced to the appropriate specifications.
Mechanical exfoliation, a top-down technique, is the most prevalent method for producing high-quality flakes of two-dimensional materials. However, exfoliation creates irregularly dispersed flakes with different layer numbers over the substrates, requiring a simple mechanism for identifying layer numbers.
Identification of Layer Number in Two-Dimensional Materials
Various approaches, such as Raman spectrometry, photoluminescence (PL), atomic force microscopy (AFM), or optical contrast, are used to determine the layer number. Setups for Raman spectrometry, AFM, or PL need specialized gear and specific systems. As a result, typical layer identification techniques are slow and increase the cost of the operation, particularly for multilayered samples.
The optical contrast approach, on the other hand, requires just a basic optical microscope imaging apparatus, making it very efficient and cheap. Many previous studies have used optical microscope images to identify layer numbers in two-dimensional materials.
Variations in the strength of the substrates and the flake intensity of two-dimensional materials for separate red, green, and blue channels may be used to distinguish various layered areas. Until now, optical contrast has been employed solely for fast identification rather than definitive verification of layer numbers.
Highlights and Key Developments of the Current Study
This research aimed to find a simple and universal criterion for measuring layer number in two-dimensional materials. Using several imaging devices, the researchers examined physically exfoliated molybdenum disulfide (MoS2) particles in RGB and RAW formats.
A Fresnel-reflectance-based scanning model was employed to compute and validate the observed intensity ratio. Moreover, the researchers developed a MATLAB-based graphical user interface (GUI) that can quickly determine layer numbers in two-dimensional materials.
For RGB images, the slope of flake intensity versus substrate intensity is derived from optical pictures with increasing light power. The intensity slope specifies the layer numbers and is independent of the imaging system used.
In RAW images, intensity slopes and ratios are independent of the system and brightness. Thus, the intensity slope (for RGB) and the intensity ratio (for RAW) are universal characteristics for determining layer numbers. Although the RAW format is not supported by all imaging systems, it can check layer numbers using a single optical image, making it a quick and system-independent generic technique.
Future Outlook
The slope technique and GUI established in this work do not need specific equipment, making them suitable for use in any lab equipped with an optical microscope. Based on these discoveries, it is reasonable to expect that this approach may be applied to any two-dimensional material. A repository of slope and ratio values for various two-dimensional materials on various substrates can be built for quick identification of layer numbers.
This approach is also predicted to speed up the determination of layer numbers and minimize heterostructure production time, making two-dimensional materials ideal for use in various industrial applications.
Reference
Mondal, M. et al. (2022). Optical Microscope Based Universal Parameter for Identifying Layer Number in Two-Dimensional Materials. ACS Nano. Available at: https://pubs.acs.org/doi/10.1021/acsnano.2c04833
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