Transient Absorption Spectroscopy of NbOI$_2$

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.

Figures (4)

• Figure 1: (a) Crystal structure of NbOI$_2$. (b) Device structure showing a NbOI$_2$ flake sandwiched between two hexagonal boron nitride (hBN) flakes on a quartz substrate. (c) Optical microscope image of the device used for optical measurements. (d) Schematic band structure of NbOI$_2$ showing the relevant excitonic transitions.
• Figure 2: (a) Differential reflectance of NbOI$_2$ as a function of probe delay and probe photon energy. (b) Peak differential reflectance as a function of probe photon energy, corresponding to the dashed horizontal line in (a). The red curve highlights the excitonic resonance, while the dashed curve represents a fit based on a shift and saturation of the exciton resonance. (c) Time-resolved differential reflectance at two probe photon energies corresponding to the positive and negative peaks. (d) Schematic illustration of the origin of the differential reflectance spectrum from a shifted and saturated excitonic absorption peak.
• Figure 3: (a) Peak differential reflectance magnitude of NbOI$_2$ as a function of pump fluence, measured with probe photon energies of 2.25 and 2.34 eV, respectively. (b) Representative time-resolved differential reflectance traces at various pump fluences, measured with a 2.34 eV probe. (c) ${n_0}/{n(t)} - 1$ calculated from the data as a function of probe delay for several high pump fluences. The red lines represent linear fits to the data at early delays. (d) $\gamma n_0$ extracted from the fits shown in (c) as a function of the injected carrier density $n_0$. The red line indicates a linear fit.
• Figure 4: (a) Differential reflectance of NbOI$_2$ as a function of sample orientation and probe delay. (b) Peak differential reflectance as a function of the angle between the crystal’s polar direction and the probe polarization. The dashed line represents a fit. (c) Time-resolved differential reflectance at selected sample orientations. (d) Measurement configuration showing the pump and probe polarizations (vertical and horizontal in the laboratory frame, respectively) and the definition of the angle $\theta$.