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Raman Spectroscopy: Overview01:20

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The underlying principle of Raman spectroscopy is based on the interaction between light and matter, specifically molecules' inelastic scattering of photons. When a monochromatic beam of light, typically from a laser source, interacts with a sample, most scattered light has the same frequency as the incident light. This is known as Rayleigh scattering.
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Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
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The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
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A conventional Raman spectrophotometer includes a laser source, a sample holding system, a wavelength selector, and a detector.
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X-ray Crystallography02:18

X-ray Crystallography

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The size of the unit cell and the arrangement of atoms in a crystal may be determined from measurements of the diffraction of X-rays by the crystal, termed X-ray crystallography.
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Twist-Dependent Raman and Electron Diffraction Correlations in Twisted Multilayer Graphene.

Nikita Gupta1, Sunil Walia2,3, Umesha Mogera1

  • 1Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India.

The Journal of Physical Chemistry Letters
|March 20, 2020
PubMed
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Twisted multilayer graphene (tMLG) twist angles were analyzed using electron diffraction and Raman spectroscopy. A new turbostratic factor correlates angle distribution with material properties, revealing a maximum intensity ratio of 17.92.

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Area of Science:

  • Materials Science
  • Condensed Matter Physics
  • Nanotechnology

Background:

  • Twisted multilayer graphene (tMLG) presents tunable electronic and optical properties through controlled angular rotations.
  • Understanding the relationship between structural disorder and material properties in tMLG is crucial for its application.

Purpose of the Study:

  • To investigate the distribution and population of twist angles in turbostratic graphene systems.
  • To establish a correlation between structural parameters derived from electron diffraction and Raman spectroscopy.
  • To introduce and validate a 'turbostratic factor' as a descriptor for tMLG.

Main Methods:

  • Utilized selected area electron diffraction (SAED) to analyze twist angle distributions in graphene sextets.
  • Employed Raman spectroscopy to probe the collective vibrational behavior at the same locations as SAED.
  • Calculated a 'turbostratic factor' based on angular spacings in SAED patterns.

Main Results:

  • The turbostratic factor was found to be low for regions with low 2D to G intensity ratios (I2D/IG).
  • A rapid increase in the turbostratic factor was observed with increasing I2D/IG values, saturating at 60° for highly turbostratic systems.
  • The maximum achievable I2D/IG value was determined to be 17.92, correlating structural and spectroscopic data.

Conclusions:

  • The turbostratic factor effectively quantifies the degree of angular disorder in multilayer graphene.
  • A clear relationship exists between structural twist angles and the Raman intensity ratio (I2D/IG).
  • This study provides insights into optimizing the properties of twisted multilayer graphene for advanced applications.