Redox-Governed Charge Doping Dictated by Interfacial Diffusion in Two-Dimensional Materials

TL;DR

This study reveals that charge doping in 2D materials via air and acid exposure is driven by electrochemical redox reactions involving oxygen and water, with molecular diffusion at interfaces playing a key role.

Contribution

It uncovers the mechanistic details of hole doping in 2D materials, emphasizing the role of interfacial diffusion and electrochemistry, which was previously controversial.

Findings

Doping is driven by redox reactions with oxygen and water molecules.

Molecular diffusion occurs through the nanoscopic space between 2D materials and substrates.

HCl-induced doping depends on dissolved O2 and pH, following the Nernst equation.

Abstract

Controlling extra charge carriers is pivotal in manipulating electronic, optical, and magnetic properties of various two-dimensional (2D) materials. Nonetheless, the ubiquitous hole doping of 2D materials in the air and acids has been controversial in its mechanistic details. Here we show their common origin is an electrochemical reaction driven by redox couples of oxygen and water molecules. Using real-time photoluminescence imaging of WS2 and Raman spectroscopy of graphene, we capture molecular diffusion through the 2D nanoscopic space between 2D materials and hydrophilic substrates, and show that the latter accommodate water molecules also serving as a hydrating solvent. We also demonstrate that HCl-induced doping is governed by dissolved O2 and pH in accordance with the Nernst equation. The nanoscopic electrochemistry anatomized in this work sets an ambient limit to material properties, which is universal to not only 2D but also other forms of materials.