Vapor-liquid assisted chemical vapor deposition of Cu₂X materials

Transition metal dichalcogenides (TMDs) are known for their layered structure and tunable functional properties. However, a unified understanding on other transition metal chalcogenides (i.e. M₂X) is still lacking. Here, the relatively new class of copper-based chalcogenides Cu₂X (X = Te, Se, S) is thoroughly reported. Cu₂X are synthesized by an unusual vapor-liquid assisted growth on a Al₂O₃/Cu/W stack. Liquid copper plays a significant role in synthesizing these layered systems, and sapphire assists with lateral growth and exfoliation. Similar to traditional TMDs, thickness dependent phonon signatures are observed, and high-resolution atomic images reveal the single phase Cu₂Te that prefers to grow in lattice-matched layers. Charge transport measurements indicate a metallic nature at room temperature with a transition to a semiconducting nature at low temperatures accompanied by a phase transition, in agreement with band structure calculations. These findings establish a fundamental understanding and thrust Cu₂Te as a flexible candidate for wide applications from photovoltaics and sensors to nanoelectronics.