# CO2 Sensing Characteristics of 2H-MoS2-Coated D-Shaped Optical Fiber Sensors

## TL;DR

This study develops a highly sensitive CO2 sensor using a 2H-MoS2-coated D-shaped optical fiber with improved stability and performance.

## Contribution

A phase-engineered 2H-MoS2 layer with healed sulfur vacancies is introduced for enhanced optical gas sensing.

## Key findings

- The sensor showed excellent linearity (R2 > 0.99) for CO2 concentrations between 1000–10,000 ppm.
- Phase transition to 2H-MoS2 and sulfur vacancy healing improved sensor repeatability and stability.
- The strategy addresses drift issues in conventional gas sensors and enhances optical interaction.

## Abstract

In this study, a highly crystalline 2H (hexagonal)-phase MoS2 sensing layer with a precisely controlled crystal structure was realized through a combination of DC sputtering and sulfurization annealing processes, and subsequently integrated with a D-shaped optical fiber to develop a highly sensitive carbon dioxide (CO2) sensor. Conventionally sputtered MoS2 thin films often suffer from the presence of unstable metallic 1T (tetragonal) phases and a high density of sulfur vacancies, which significantly degrade sensor reversibility and long-term stability. Here, high-temperature annealing under a sulfur-rich atmosphere was employed to induce a complete phase transition from the metastable 1T phase to the stable semiconducting 2H phase, while simultaneously healing sulfur vacancies. Enhanced crystallinity was confirmed by Raman spectroscopy. The fabricated sensor exhibited excellent linearity (R2 > 0.99) and markedly improved repeatability over a CO2 concentration range of 1000–10,000 ppm. This significant performance enhancement is attributed to reversible charge transfer induced by sulfur vacancy passivation, which modulates the complex refractive index of the MoS2 layer and optimizes optical interaction with the evanescent field of the D-shaped fiber. The phase engineering and defect-healing strategy presented in this work effectively addresses the drift issues commonly observed in conventional electrical gas sensors and provides a crucial pathway toward the realization of high-performance optical gas sensors.