Scaling a van der Waals quantum Hall semiconductor

Indium selenide (InSe), a two-dimensional (2D) layered semiconductor material, have attracted much attention recently due to their high mobility and fascinating physical properties. Nevertheless, the preparation of few-layer InSe is limited to mechanical exfoliation which hinders their potential in the future applications. Recently reports of few-layer InSe synthesized via chemical vapor deposition (CVD) method failed to maintain high mobility characteristics of InSe. Here, we explored the controlled one-step synthesis of 2D InSe nanoflakes with chemical vapor transport (CVT) by appropriately slowing down the growth rate. Diverse growth routes were developed by using different transport agents. As-grown InSe nanoflakes with thickness down to monolayer was successfully obtained. Atomic-resolution ADF-STEM imaging revealed the high quality of the as-synthesized InSe nanoflakes. Encapsulated by BN at both sizes, the electrical transport of our InSe nanoflakes shows excellent performance with carrier mobilities larger than 1000 cm²V^-1s^-1 at room temperature and 5000 cm²V^-1s^-1 at 1.5 K, enabling us further observe the quantum Hall effect. This is the first time that a quantum Hall effect was observed in as-grown 2D semiconductor materials.