Enhanced Electroluminescence Quantum Efficiency via Tunable 2D Built‐In Electric Fields

The built‐in electric field (BIEF) is of utmost importance for 2D optoelectronic devices, which are typically realized in heterostructures created through interfacial engineering. However, the limitations of the sharp field distribution, inevitable energy band discontinuity, and inflexible construction of 2D heterostructures restrict their application. In this work, a large‐scale, continuous, and controllable BIEF using monolayer W x Mo 1− x S 2 alloys with tunable composition gradients is constructed. Density functional simulations demonstrate that a BIEF can be formed within the whole alloy due to the charge distribution resulting from the composition gradients. Accordingly, a monolayer W x Mo 1− x S 2 alloy with a controllable composition gradient via two‐step chemical vapor deposition and confirmed the continuous tunability of the band structure and the variation in the composition gradient within the alloy is synthesized. Using the BIEF, 2D light‐emitting diodes (LEDs) based on W x Mo 1− x S 2 alloys and achieve strong electroluminescence emission are constructed. A large BIEF facilitates charge carrier migration and recombination, and the external quantum efficiency (over 0.62%) is more than 5 times that of small BIEF LEDs. The study not only provides a novel approach for the on‐demand design of a BIEF but also provides an opportunity for potential applications in optoelectronic devices.