Origin of anomalous p-type conductivity in monolayer Fe-doped MoS2

TL;DR

This study uncovers that the anomalous p-type conductivity in Fe-doped monolayer MoS2 arises from specific defect associates involving Fe and S atoms, confirmed through microscopy, transport measurements, and first-principles calculations.

Contribution

It reveals the atomic structure and origin of p-type behavior caused by defect associates in Fe-doped MoS2, resolving a longstanding controversy.

Findings

Defect associates form a 3FeMo-S triangle structure.

P-type conductivity is verified by STM/STS measurements.

Similar defect structures and effects are found in Fe-doped WS2.

Abstract

Substitutional doping effectively modulates carrier polarity of semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) like MoS2. Although Fe doping typically induces n-type conductivity in monolayer MoS2, anomalous p-type behavior has also been experimentally reported, the origin of which remains unresolved. Here, we prove that this anomalous p-type conductivity originates from defect associates formed through interactions between Fe dopants and S atoms, which consists of three Fe substituting Mo (FeMo) point defects arranged into an equilateral triangle with a central S atom, denoted as 3FeMo-S associate. Its p-type effect is directly verified through scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS) measurement, in sharp contrast to the n-type behavior induced by isolated FeMo point defects, and the conclusion is further supported by electrical transport measurements and first-principles calculations. Similar 3FeW-S associates and their p-type doping effect are also identified in monolayer Fe-doped WS2. This work resolves a longstanding controversy and highlights the critical role of defect associates in modulating properties of 2D TMDs.