Observation of Graphite-Like and Diamond-Like Nanostructures in the Raman Spectra of Natural and Synthesized MoS2 Crystals with Small Carbon Additives
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2020-12-30 |
| Journal | Physics and Chemistry of Solid State |
| Authors | N. E. ŠŠ¾rnienko, A. P. Naumenko, L.M. Kulikov |
| Institutions | National Academy of Sciences of Ukraine, Taras Shevchenko National University of Kyiv |
| Analysis | Full AI Review Included |
Executive Summary
Section titled āExecutive SummaryāThis research successfully identifies and characterizes ultra-small diamond-like and graphite-like carbon nanostructures embedded within molybdenum disulfide (MoS2) crystals, leveraging advanced Raman spectroscopy techniques.
- Discovery of Ultra-Small Nanostructures: Diamond-like (D) and graphite-like (G) carbon nanostructures, estimated to be less than 1 nm in size, were observed for the first time in natural molybdenite and synthesized 2H-MoS2 single crystals.
- Phonon Quasizone Narrowing: The nanostructures exhibit record low D-band frequencies (1284 - 1312 cm-1) and high G(k) frequencies (1387 - 1402 cm-1), providing strong evidence for phonon quasizone narrowing due to their extremely small dimensions.
- Novel Spectral Analysis: A new numerical decomposition method was employed to reliably separate highly overlapping D and G(k) vibrational bands, revealing fine structure components (D(k), G(k), D(kā), G(kā)) associated with Brillouin zone (BZ) edges and middle parts.
- Structural Ordering Control: Increasing carbon content (from 0.5 wt.% to 1.0 wt.%) in synthesized MoS2(C) nanocrystallites significantly strengthens the D bands (diamond-like) and enhances the ordering of the G bands (graphite-like).
- Resonant Activation Effect: Resonant laser excitation (632.8 nm) induces a structural transformation (MoS2 ā MoO3), forming a MoS2 + MoO3 nanocomposite that actively promotes the formation and ordering of the internal carbon nanostructures.
- Mechanism Insight: The presence of D(kā) and G(kā) components indicates a crucial role for processes involving the doubling of elementary quasi-cell sizes, suggesting complex disordering mechanisms.
Technical Specifications
Section titled āTechnical Specificationsā| Parameter | Value | Unit | Context |
|---|---|---|---|
| MoS2(C) Carbon Content | 0.5 and 1.0 | wt.% | Synthesized nanocrystallites |
| MoS2(C) Crystallite Size (Average) | 3.9(2) | nm | Direction [013], X-ray analysis |
| MoS2(C) Crystallite Size (Average) | 9.1(6) to 9.4(6) | nm | Direction [110], X-ray analysis |
| Raman Excitation Wavelength 1 | 632.8 | nm | He-Ne laser, resonant with MoS2 exciton states |
| Raman Excitation Wavelength 2 | 488 | nm | Ar+ laser, near electron absorption edge |
| Diamond D Band Frequency (Low) | 1284 to 1312 | cm-1 | Observed in MoS2(C) NCs, indicates size less than 1 nm |
| Graphite G(k) Band Frequency (High) | 1387 and 1402 | cm-1 | Corresponds to BZ edges, indicates size less than 1 nm |
| MoS2 Fundamental Mode (E12g) | 383 | cm-1 | Natural 2H-MoS2 single crystal |
| MoS2 Fundamental Mode (A1g) | 408 | cm-1 | Synthesized 2H-MoS2 single crystal |
| MoO3 Resonance Lines | 662, 818, 991 | cm-1 | Formed under 632.8 nm resonant excitation |
| D Band FWHM (C=1 wt.%) | 62 | cm-1 | Indicates increased ordering compared to C=0.5 wt.% |
| G Band FWHM (C=1 wt.%) | 95 | cm-1 | Corresponds to graphite-like scale size of approximately 1 nm |
Key Methodologies
Section titled āKey MethodologiesāThe study relied on a combination of synthesis, structural characterization, and advanced spectral analysis to identify and quantify the embedded carbon nanostructures.
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Material Synthesis:
- MoS2(C) nanocrystallites were synthesized using low-temperature Chemical Vapor Deposition (CVD) combined with self-oscillating temperatures.
- Carbon additives were controlled at 0.5 wt.% and 1.0 wt.% to study concentration effects.
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Structural Characterization:
- High-precision X-ray investigations (full-profile method using WinCSD calculations) were used to determine unit cell parameters and crystallite sizes (e.g., 4 nm in [013] direction, 9 nm in [110] direction).
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Raman Spectroscopy (RS):
- RS was performed using two distinct laser excitations: 632.8 nm (He-Ne, resonant with MoS2 exciton states) and 488 nm (Ar+, near the electron absorption edge).
- The 632.8 nm excitation was specifically used to study resonant effects, including the laser-induced formation of the MoS2 ā MoO3 nanocomposite.
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Advanced Spectral Decomposition:
- Observed broadband carbon D and G bands were numerically decomposed into constituent spectral components (Lorentz or Gaussian forms).
- A cubic polynomial approximation was used to subtract the Broadband Electronic Background (BEB).
- The correctness of the decomposition was verified by matching the spectra of the 2nd derivatives (d2I/dv2) of the experimental and calculated vibrational bands, allowing reliable separation of highly overlapping D and G(k) components.
Commercial Applications
Section titled āCommercial ApplicationsāThe ability to control the formation and ordering of ultra-small carbon nanostructures within MoS2 matrices opens several avenues for advanced material engineering and device development.
- Advanced Catalysis: The MoS2-MoO3 nanocomposite, activated by resonant radiation, shows promise for enhanced photocatalytic decomposition of water for alternative energy (hydrogen and oxygen production) and decomposition of organic pollutants.
- Hybrid Nanomaterials: The findings support the development of hybrid nanomaterials (nanocomposites) involving semiconductor 2D graphene-like layers (MoS2) with embedded diamond-like structures, offering unique synergistic properties for new multifunctional devices.
- Optoelectronics and Quantum Devices: The established method for controlling electronic states and wave nonlinearity via resonant laser interaction provides a pathway to tune the electronic properties of MoS2 nanoparticles, crucial for next-generation optoelectronic and quantum sensing applications.
- Biomedical Technology: MoS2 and MoO3 nanostructures are already being explored for photothermal therapies, and the controlled synthesis of these carbon-doped structures could lead to more stable and efficient therapeutic agents.
- High-Strength Composites: The formation of sp3-hybridized diamond-like inclusions within the MoS2 matrix suggests potential for creating composites with enhanced mechanical strength and stability, particularly in harsh environments.
View Original Abstract
A comparative study of Raman spectra excited by laser radiation Ī»L = 632.8 nm and 488 nm of natural crystals of 2H-MoS2 and nanocrystallites MoS2 (C) containing 0.5 and 1.0 wt.% Carbon additives. A detailed numerical analysis of the shape of observed D and G bands was performed. The complication of the spectra of graphite-like and diamond-like structures with the appearance of additional spectral components at 1440-1500 cm-1 and 1230-1270 cm-1 as a result of doubling the size of the corresponding elementary quasi-cells are analyzed. It is shown that the frequencies of D-bands of diamond-like nanostructures 1297 Ć· 1302 cm-1 donāt depend on Ī»L in contrast to the change in the frequencies of the G (k)-bands. A significant effect of 632.8 nm resonant radiation on the electronic states and properties of MoS2 (C) NC was established. The strengthening of the D bands of the diamond-like structure and the ordering of the graphite structure with increasing carbon content in MoS2 (C) nanocrystals have been established. The change of spectral positions of D, G, and G (k) bands at strengthening the degree of disordering of a diamond- and graphite-like structures is considered. The influence of laser radiation on carbon structures is discussed.