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        Surface enhanced Raman spectroscopy (SERS) is a powerful analytical tool for analyzing 2D hot spot plasmonic materials. In addition, hotspots are critical in SERS because they are locations with strong localized electromagnetic (EM) fields and contribute little to the overall intensity of SERS.
        Research: Improved Raman spectroscopy by combining plasmons and excitons of a large area MoS2 monolayer. Image credit: ogwen/Shutterstock.com
        An article published in the journal Applied Surface Science reports the enhancement of Raman signals by plasmon and exciton binding between a large area molybdenum sulfide (MoS2) monolayer and plasmonic nanogrooves (NG). Instead of amplifying the SERS, the plasmon-exciton coupling amplifies the Raman signal at the excitation wavelength.
        The spectrally tuned NG is used to study the improvement of isolation and coupling, and its performance is investigated by improvement in the electromagnetic domain. Thus, the present work demonstrates the potential application of atomically thin two-dimensional (2D) materials embedded in nanostructures that exhibit uniform Raman signals in nanophotonics and materials science.
        Raman spectroscopy is a non-destructive chemical analysis technique that provides detailed information about chemical structure, polymorphism, crystallinity, and molecular interactions. It is based on the interaction of light with chemical bonds within a material.
        SERS is a sensitive technique that enhances the Raman scattering of molecules supported by some nanostructured materials. When the electromagnetic field is amplified, localized surface plasmons create “hot spots” that amplify the Raman signals.
        Resonant Raman spectroscopy (RSS) is an advanced method for studying vibrational bands in the group frequency region. The information obtained is similar to Fourier Transform Infrared (FTIR) and Raman studies.
        In RRS, when the excitation wavelength coincides with the exciton transition or is close to the absorption band of the analyte, the Raman signal is amplified, resulting in increased sensitivity and selectivity. However, RRS is highly dependent on the excitation wavelength, limiting analytes in Raman spectra.
       While SERS provides signal amplification that is superior to traditional Raman methods, Surface Enhanced Raman Spectroscopy (SERRS) provides greater chemical signal amplification through the coupling between plasmon and molecular exciton resonances.
        A strong coupling between a polariton and an exciton arises when the energy exchange rate (g) in the bound system is greater than the relaxation conditions for the polariton and exciton. A polariton-exciton coupled system requires an energy exchange rate that is 2g greater than γ and ĸ (γ and ĸ correspond to the emitter scattering rate and resonator loss, respectively) for energy to be switched between light and matter.
        Nanophotonics studies the behavior of light at the nanoscale and the interaction of nanosized objects with light. Nanophotonics typically includes metallic components that can transmit and focus light using surface plasmons.
        MoS2 is a two-dimensional material with a hexagonal layered structure consisting of covalently bonded molybdenum (Mo) and sulfur (S) atoms. The excellent optical and electronic properties make ultra-thin molybdenum disulfide attractive for low power optoelectronic applications.
        With the rapid development of various ultra-thin MoS2 devices, methods for determining the unique properties and easily identifying atomic-thickness MoS2 flakes are in high demand. Raman spectroscopy is a powerful non-destructive characterization tool that has been used to study various MoS2 crystal structures.
        In the present work, molten sodium molybdate (Na2MoO4) was used as a precursor and uniformly distributed over the film using a chemical vapor deposition (CVD) process. The resulting MoS2 monolayer films were used for the quantitative study of analytes with an enhanced Raman signal.
        While most previous work on SERRS used molecules, this study chose two-dimensional transition metal dichalcogenide (TMDC) materials as read targets due to their atomic thickness and inherent homogeneity. Therefore, large-area MoS2 monolayers were used as templates for quantifying Raman signal amplification.
        When the surface plasmon resonance is tuned by the exciton resonance, a characteristic anti-crossing is observed, which indicates strong or moderate coupling. In addition, the surface plasmon resonance can be tuned by changing the depth, width, and period of the gold (Au) NG. Therefore, 2D MoS2 was integrated into 1D (1D) gold (Au) NG to achieve plasmonic exciton interaction.
        Unlike previous studies that only focused on the SERS effect, the plasmon resonances in this work are designed to be related to MoS2 excitons rather than excitation wavelengths. In addition, coupled and decoupled samples were examined to quantify the Raman gain. Thus, thanks to the results of this work, hot spots of plasmon-exciton interaction, including molecules with low concentration and low-dimensional nanomaterials, are being investigated.
        Overall, plasmonic NG integrated into a 2D material provides a powerful platform for amplifying Raman signals. At an excitation wavelength of 532 nm, a quantitative analysis of the 80-fold enhancement of the Raman signal compared to the MoS2 signal on flat Au films confirmed the enhancement of the electromagnetic field due to the exciton-plasmon interaction.
        In addition, factors affecting the amplification of the electromagnetic field are introduced to explain the amplification of the Raman signal caused by the plasmon-exciton interaction. In addition, the samples demonstrated in this paper have great potential in nanophotonics and surface science.
        Yu, M.V. et al. (2022). Improved Raman spectroscopy by coupling plasmons and excitons of a large area MoS2 monolayer. Applied surface science. https://www.sciencedirect.com/science/article/pii/S0169433222022954?via%3Dihub
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        Bhavna Kaveti is a science writer from Hyderabad, India. She holds MSc and MD from the Vellore Institute of Technology, India. in organic and medicinal chemistry from the University of Guanajuato, Mexico. Her research work is related to the design and synthesis of bioactive molecules based on heterocycles, and she has experience in multi-step synthesis and multi-component synthesis. During her doctoral research, she worked on the synthesis of various peptidomimetic molecules based on linked and fused heterocycles, which are expected to have additional functionalization potential for biological activity. While writing dissertations and research papers, she explores her passion for scientific writing and communication.
        Caveti, Buffner. (September 10, 2022). Nanogrooves and 2D materials help enhance Raman signals. Azo Nano. Retrieved May 15, 2023 from https://www.azonano.com/news.aspx?newsID=39659.
        Caveti, Buffner. “Nanogrooves and 2D materials contribute to Raman signal amplification”. Azo Nano. May 15, 2023 .
        Caveti, Buffner. “Nanogrooves and 2D materials contribute to Raman signal amplification”. Azo Nano. https://www.azonano.com/news.aspx?newsID=39659. (As of May 15, 2023).
        Caveti, Buffner. 2022. Nanogrooves and 2D materials enhance the Raman signal. AZoNano, accessed 15 May 2023, https://www.azonano.com/news.aspx?newsID=39659.
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