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Abstract Under photon excitation, 2D materials experience cascading energy transfer from electrons to optical phonons (OPs) and acoustic phonons (APs). Despite few modeling works, it remains a long‐history open problem to distinguish the OP and AP temperatures, not to mention characterizing their energy coupling factor (G). Here, the temperatures of longitudinal/transverse optical (LO/TO) phonons, flexural optical (ZO) phonons, and APs are distinguished by constructing steady and nanosecond (ns) interphonon branch energy transport states and simultaneously probing them using nanosecond energy transport state‐resolved Raman spectroscopy. ΔTOP −APis measured to take more than 30% of the Raman‐probed temperature rise. A breakthrough is made on measuring the intrinsic in‐plane thermal conductivity of suspended nm MoS2and MoSe2by completely excluding the interphonon cascading energy transfer effect, rewriting the Raman‐based thermal conductivity measurement of 2D materials.GOP↔APfor MoS2, MoSe2, and graphene paper (GP) are characterized. For MoS2and MoSe2,GOP↔APis in the order of 1015and 1014W m−3K−1andGZO↔APis much smaller thanGLO/TO↔AP. Under ns laser excitation,GOP↔APis significantly increased, probably due to the reduced phonon scattering time by the significantly increased hot carrier population. For GP,GLO/TO↔APis 0.549 × 1016W m−3K−1, agreeing well with the value of 0.41 × 1016W m−3K−1by first‐principles modeling.
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Quantitative Defect Analysis in CVD‐Grown Monolayer MoS 2 via In‐Plane Raman Vibration
ABSTRACT The synthesis of two‐dimensional transition metal dichalcogenide (2D‐TMD) materials gives rise to inherent defects, specifically chalcogen vacancies, due to thermodynamic equilibrium. Techniques such as chemical vapor deposition (CVD), metal‐organic chemical vapor deposition (MOCVD), atomic layer deposition (ALD), flux growth method, and mechanical exfoliation produce large‐scale, uniform 2D TMD films, either in bulk or monolayers. However, defects on the film surface impact its quality, and it is necessary to measure defect density. The phonon confinement model indicates that the first‐order Raman band frequency shift depends on defect density. Monolayer Molybdenum disulfide (MoS2) exhibits three phonon dispersions at the Brillouin zone edge (M point): out‐of‐plane optical phonon vibration (ZO), in‐plane longitudinal optical phonon vibration (LO), and in‐plane transverse optical phonon vibration (TO). The LO and ZO modes overlap with Raman in‐plane vibration (𝐸12g) and Raman out‐of‐plane vibration (𝐴1g), respectively, causing peak broadening. In the presence of defects, the Raman 𝐸12gvibration energy decreases due to a reduced restoring force constant. The Raman 𝐴1gvibration trend is random, influenced by both restoring force constant and mass. The study introduces a quantitative defect measurement technique for CVD‐grown monolayer MoS2using Raman 𝐸12gmode, employing sequential data processing algorithms to reveal defect density on the film surface.
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- Award ID(s):
- 2435570
- PAR ID:
- 10553994
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Nano Select
- ISSN:
- 2688-4011
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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