Thermal conductivity of monolayer MoS2, MoSe2, and WS2: Interplay of mass effect, interatomic bonding and anharmonicity (1509.01391v1)

Abstract: Phonons are essential for understanding the thermal properties in monolayer transition metal dichalcogenides, which limit their thermal performance for potential applications. We investigate the lattice dynamics and thermodynamic properties of MoS2, MoSe2, and WS2 by first principles calculations. The obtained phonon frequencies and thermal conductivities agree well with the measurements. Our results show that the thermal conductivity of MoS2 is highest among the three materials due to its much lower average atomic mass. We also discuss the competition between mass effect, interatomic bonding and anharmonic vibrations in determining the thermal conductivity of WS2. Strong covalent W-S bonding and low anharmonicity in WS2 are found to be crucial in understanding its much higher thermal conductivity compared to MoSe2.

• The paper demonstrates that MoS₂ achieves the highest thermal conductivity (34.5 ± 4 W/mK) due to its lower atomic mass.
• It employs DFT and phonon dispersion methods to elucidate how interatomic bonding and anharmonicity affect thermal transport.
• Debye temperature calculations confirm that atomic mass and bonding strength crucially modulate phonon dynamics in 2D TMDs.

Overview of Thermal Conductivity in Monolayer MoS₂, MoSe₂, and WS₂

This paper presents a detailed investigation into the thermal conductivity and lattice dynamics of three significant monolayer transition metal dichalcogenides (TMDs): MoS₂, MoSe₂, and WS₂. The paper utilizes first-principles calculations to reveal the mechanisms driving the thermal properties of these materials, which are of considerable interest for applications in nanoelectronics and energy devices.

Methodological Approach

The authors employed density functional theory (DFT) using the Vienna ab initio simulation package (VASP) to paper the lattice structures and thermodynamic properties of these monolayers. Within this framework, the Perdew, Burke, and Ernzerhof (PBE) parameterization of the generalized gradient approximation (GGA) was used for the exchange-correlation functional. A substantial k-mesh grid ensured precision during structural relaxation. Phonon dispersions were calculated using Phonopy with harmonic interatomic force constants derived from density functional perturbation theory.

Key Results and Findings

1. Thermal Conductivity Measurements:
   • Among the materials studied, MoS₂ exhibits the highest thermal conductivity, primarily due to its lower average atomic mass. WS₂ follows MoS₂, with MoSe₂ showing the lowest thermal conductivity.
   • The experimental thermal conductivities, 34.5 ± 4 W/mK for MoS₂ and 32 W/mK for WS₂, align well with their calculated values, elucidating the predictive accuracy of the employed theoretical methods.
2. Influence of Interatomic Bonding and Anharmonicity:
   • WS₂ benefits from strong covalent W-S bonds and low anharmonicity, which enhance its thermal conductivity over MoSe₂ despite its larger mass. Conversely, MoSe₂'s weaker Mo-Se bonding and higher anharmonicity lead to lower thermal conductivity.
   • The Grüneisen parameter, indicative of anharmonicity, further corroborates these observations, with MoS₂ showing higher values than WS₂, implying more anharmonicity and stronger bonding in MoS₂.
3. Debye Temperature Calculation:
   • The Debye temperatures for MoS₂, MoSe₂, and WS₂ were calculated to be 262.3 K, 177.6 K, and 213.6 K, respectively. These values highlight the role of atomic mass and bonding strength—especially in MoS₂, where the low atomic mass significantly elevates the Debye temperature.
4. Phonon Spectra and Mode Analysis:
   • Phonon dispersions and density of states (DOS) analysis reveal that MoS₂ and WS₂ have robust phonon transport due to the significant overlap between metal d-states and chalcogen p-states, enhancing their heat dissipation capabilities.

Implications and Future Directions

This research contributes to a deeper understanding of phonon-dominated thermal conductivity in 2D TMDs. The results underscore the importance of atomic mass, interatomic bonding, and anharmonicity in modulating thermal transport properties. For future materials design, these insights can guide the engineering of TMDs through doping or strain engineering to optimize thermal management in nano-scaled devices. Further exploration could involve studying the influence of defects and external stimuli on the thermal properties for more robust applications.

Conclusion

The paper effectively contextualizes the thermal properties of MoS₂, MoSe₂, and WS₂ within a framework of lattice dynamics and bonding characteristics. By harmonizing theoretical predictions with experimental findings, this work lays a foundational understanding crucial for advancing TMD applications in modern nanotechnology.

Overall, the paper offers substantial clarity on the thermal behaviors of MoS₂, MoSe₂, and WS₂, bearing implications for their future utilization in electronic applications requiring efficient thermal management.