Transient Absorption Spectroscopy of NbOI$_2$

TL;DR

This study investigates the ultrafast photocarrier dynamics of NbOI$_2$, revealing exciton lifetime, anisotropic response, and recombination mechanisms, which are crucial for its optoelectronic applications.

Contribution

First transient absorption spectroscopy analysis of NbOI$_2$, providing detailed insights into its exciton behavior and anisotropic photocarrier dynamics.

Findings

Exciton lifetime of several tens of picoseconds.

Density-dependent exciton annihilation and Auger recombination.

Pronounced in-plane anisotropy in transient optical response.

Abstract

NbOI$_2$ has recently emerged as a new van der Waals material combining semiconducting behavior with intrinsic in plane ferroelectricity and pronounced transport and optical anisotropy. However, its photocarrier dynamics remain largely unexplored. Here we report transient absorption spectroscopy of NbOI$_2$ using femtosecond pump probe reflectance measurements. A pronounced transient absorption feature is observed near the 2.34 eV excitonic resonance, arising from photocarrier induced excitonic energy shifts and saturation. The decay dynamics reveal an exciton lifetime of several tens of picoseconds and show density-dependent behavior consistent with exciton exciton annihilation and defect-assisted Auger recombination, yielding a rate of 0.4 cm$^2$ s$^{-1}$, which is comparable to that in monolayer transition metal dichalcogenides. Polarization resolved measurements further reveal a pronounced in-plane anisotropy in the transient response that follows the linear absorption anisotropy. These findings provide fundamental insight into photocarrier dynamics in NbOI$_2$ and establish key parameters for understanding and exploiting its optoelectronic behavior.