Effect of Metal Doping and Vacancies on the Thermal Conductivity of Monolayer Molybdenum Diselenide

It is well understood that defect engineering can give rise to exotic electronic properties in transition-metal dichalcogenides, but to this date, there is no detailed study to illustrate how defects can be engineered to tailor their thermal properties. Here, through combined experimental and theoretical approaches based on the first-principles density functional theory and Boltzmann transport equations, we have explored the effect of lattice vacancies and substitutional tungsten (W) doping on the thermal transport of the suspended molybdenum diselenide (MoSe₂) monolayers grown by chemical vapor deposition (CVD). The results show that even though the isoelectronic substitution of the W atoms for Mo atoms in CVD-grown Mo_0.82W₀₁₈Se₂ monolayers reduces the Se vacancy concentration by 50% compared to that found in the MoSe₂ monolayers, the thermal conductivity remains intact in a wide temperature range. On the other hand, Se vacancies have a detrimental effect for both samples and more so in the Mo_0.82W₀₁₈Se₂ monolayers, which results in thermal conductivity reduction up to 72% for a vacancy concentration of 4%. This is because the mass of the W atom is larger than that of the Mo atom, and missing a Se atom at a vacancy site results in a larger mass difference and therefore kinetic energy and potential energy difference. Furthermore, the monotonically increasing thermal conductivity with temperature for both systems at low temperatures indicates the importance of boundary scattering over defects and phonon-phonon scattering at these temperatures.