Morphological dependent exciton dynamics and thermal transport in MoSe2 films.

Thermal transport and exciton dynamics of semiconducting transition metal dichalcogenides (TMDCs) play an immense role in next-generation electronic, photonic, and thermoelectric devices. In this work, we synthesize distinct morphologies (snow-like and hexagonal) of a trilayer MoSe2 film over the SiO2/Si substrate via the chemical vapor deposition (CVD) method and investigated their morphological dependent exciton dynamics and thermal transport behaviour for the first time to the best of our knowledge. Firstly, we studied the role of spin-orbit and interlayer couplings both theoretically as well as experimentally via first-principles density functional theory and photoluminescence study, respectively. Further, we demonstrate morphological dependent thermal sensitive exciton response at low temperatures (93-300 K), showing more dominant defect-bound excitons (EL) in snow-like MoSe2 compared to hexagonal morphology. We also examined the morphological-dependent phonon confinement and thermal transport behaviour using the optothermal Raman spectroscopy technique. To provide insights into the nonlinear temperature-dependent phonon anharmonicity, a semi-quantitative model comprising volume and temperature effects was used, divulging the dominance of three-phonon (four-phonon) scattering processes for thermal transport in hexagonal (snow-like) MoSe2. The morphological impact on thermal conductivity (ks) of MoSe2 has also been examined here by performing the optothermal Raman spectroscopy, showing ks ∼ 36 ± 6 W m-1 K-1 for snow-like and ∼41 ± 7 W m-1 K-1 for hexagonal MoSe2. Our research will contribute to the understanding of thermal transport behaviour in different morphologies of semiconducting MoSe2, finding suitability for next-generation optoelectronic devices.

Large Area Vertically Oriented Few-Layer MoS2 for Efficient Thermal Conduction and Optoelectronic Applications.

Large area growth of MoS2 can show great advances in optoelectronic devices due to its unique optical and electronic properties. Here, we directly grow vertically oriented and interconnected few-layer MoS2 over 1 × 1 cm2 of p-type Si substrate using CVD technique. We report for the first time the thermal conductivity of vertically oriented few-layer (VFL) MoS2 using the optothermal Raman technique. The reduced phonon-defect scattering due to minimal defects and strains in VFL MoS2 results in excellent thermal conductivity of 100 ± 14 W m-1 K-1 at room temperature. The photoluminescence and DFT study confirm the semiconducting behavior of VFL-MoS2. The VFL-MoS2/Si photodiode shows high photoresponsivity of 7.37 A W-1 at -2.0 V bias under 0.15 mW cm-2 intensity of 532 nm laser. The enhanced light trapping and highly exposed edges of VFL MoS2 due to vertical orientation, formation of efficient p-n junction at the MoS2/Si interface and effective charge separation leads to the excellent performance of grown VFL-MoS2 for optoelectronic applications.