Transition metal dichalcogenides (TMDCs) exhibit unique thermal properties due to their layered atomic structure and strong in-plane covalent bonding coupled with weak out-of-plane van der Waals interactions. Understanding thermal conductivity, phonon transport, and heat dissipation in these materials is critical for applications in nanoelectronics, optoelectronics, and thermoelectric devices. The thermal behavior of TMDCs is governed by phonon dynamics, which are influenced by factors such as layer thickness, anisotropy, and interfacial effects.

Phonon transport in TMDCs is highly anisotropic, with in-plane thermal conductivity significantly higher than out-of-plane conductivity. For example, monolayer MoS2 has an in-plane thermal conductivity ranging between 30-100 W/mK, while the cross-plane conductivity is orders of magnitude lower, typically below 2 W/mK. This anisotropy arises from the directional dependence of phonon dispersion and scattering mechanisms. In-plane phonons propagate efficiently due to strong covalent bonds, whereas out-of-plane phonons experience increased scattering due to weak interlayer coupling. The anisotropy ratio varies among different TMDCs, with WS2 showing slightly higher in-plane thermal conductivity than MoS2 due to differences in atomic mass and bond stiffness.

Layer-dependent effects play a crucial role in thermal transport. As the number of layers increases, the in-plane thermal conductivity of TMDCs generally decreases due to enhanced phonon-phonon scattering and interfacial resistance. Monolayer TMDCs exhibit higher thermal conductivity than their bulk counterparts because boundary scattering dominates in ultrathin layers, suppressing Umklapp scattering. For instance, the thermal conductivity of monolayer MoS2 can be up to three times higher than that of bulk MoS2, which typically ranges between 50-60 W/mK. The reduction in thermal conductivity with increasing layer count is attributed to the activation of additional phonon scattering channels, including layer-breathing modes and interlayer shear modes.

Comparisons with graphene and hexagonal boron nitride (hBN) highlight the distinct thermal properties of TMDCs. Graphene possesses exceptionally high in-plane thermal conductivity, often exceeding 2000 W/mK, due to its strong sp2 bonding and long phonon mean free paths. In contrast, TMDCs exhibit lower thermal conductivity because of their heavier atomic masses and more complex phonon dispersion relations. hBN, another layered material, has an in-plane thermal conductivity of approximately 300-500 W/mK, higher than most TMDCs but lower than graphene. The difference arises from hBN's lighter atomic constituents and reduced anharmonic scattering compared to TMDCs. However, TMDCs offer advantages in applications requiring moderate thermal conductivity combined with tunable electronic and optical properties.

Heat dissipation in TMDCs is influenced by phonon-boundary scattering, defect density, and substrate interactions. In supported samples, the thermal conductivity is often lower than in suspended layers due to phonon leakage into the substrate and increased scattering at the interface. For example, the thermal conductivity of monolayer MoS2 on a SiO2 substrate can be reduced by 30-50% compared to its suspended counterpart. The choice of substrate and interfacial adhesion strength significantly impacts heat dissipation, making interfacial engineering a key consideration for device design.

Measurement techniques such as time-domain thermoreflectance (TDTR) and Raman thermometry are widely used to characterize thermal properties of TMDCs. TDTR provides high-precision measurements of cross-plane thermal conductivity and interfacial thermal resistance by monitoring the decay of thermal waves induced by a pulsed laser. This technique is particularly useful for studying thin films and heterostructures. Raman thermometry, on the other hand, exploits the temperature-dependent shift of Raman-active modes to estimate in-plane thermal conductivity. By locally heating the sample with a laser and analyzing the resulting temperature gradient, researchers can extract thermal transport parameters with spatial resolution at the micrometer scale. Both techniques have been instrumental in uncovering layer-dependent and anisotropic thermal behavior in TMDCs.

The role of defects and strain in thermal transport cannot be overlooked. Point defects, vacancies, and grain boundaries act as phonon scattering centers, reducing thermal conductivity. For instance, sulfur vacancies in MoS2 can decrease thermal conductivity by up to 20% depending on their concentration. Strain engineering offers a pathway to modulate thermal properties; tensile strain typically softens phonon modes and reduces thermal conductivity, while compressive strain may have the opposite effect in certain regimes.

In summary, thermal conductivity and phonon transport in TMDCs are governed by their anisotropic structure, layer-dependent effects, and interfacial interactions. While their thermal performance is inferior to graphene and hBN, TMDCs provide a balance between thermal, electronic, and optical properties, making them suitable for diverse applications. Advances in measurement techniques and interfacial engineering will further enhance the understanding and optimization of heat dissipation in these materials.