Salt-Assisted Heteroepitaxy Growth of MoS₂/HOPG Heterostructures

The engineering of van der Waals heterostructures provides a powerful pathway to create materials with tailored properties for the next generation photonic and optoelectronic devices. The lift-off and stacking process to assemble heterostructures can introduce interfacial contaminants and structural defects that could degrade performance. To overcome this, we have developed a direct synthesis route that produces high quality as-grown MoS₂/HOPG heterostructures with an intrinsically clean interface via chemical vapor deposition (CVD). The preferential lateral growth of MoS₂ and the suppression of vertical growth were achieved through the introduction of a NaBr surfactant. Atomic force microscopy (AFM) revealed the topography and coverage of monolayers of MoS₂ on HOPG. Raman spectroscopy confirms the characteristic vibrational modes of MoS₂, wherein the frequency separation between the E’ and A’₁ modes is indicative of its monolayer nature. The analysis of temperature dependent phonon behavior revealed that the observed redshift of vibrational frequencies is attributed to the combined effects of lattice thermal expansion and anharmonicity. The MoS₂/HOPG heterostructure exhibits a significant suppression of its photoluminescence. The moiré superlattices of the MoS₂/HOPG heterostructure were characterized using torsional force microscopy. This work establishes a surfactant-mediated synthesis of high-quality van der Waals heterostructures with properties ideal for optoelectronic applications.