Modulation of opto-electronic properties of InSe thin layers: Via phase transformation

Phase engineering of two-dimensional (2D) materials offers unique opportunities for acquiring novel opto-electronic properties and allows for the searching of outstanding candidates for applications in opto-electronic detectors, sensors, catalysis, or phase-change memory devices. Here, we report the phase-transformation from β-InSe to γ-In₂Se₃, exploiting the thermal annealing route to trigger the process starting at 200 °C that expands the family of phase-change materials. The presence of γ-In₂Se₃ is solidly confirmed by the characteristic peaks in X-ray diffraction (XRD) and energy dispersive X-ray (EDX), and is quite stable at ambient condition, thus facilitating substantial application in phase-change memory devices. A Raman shift in the A′₁ mode from 225 cm^-1 to 230 cm^-1 further illustrates the phase transformation. Besides the photoluminescence (PL) peak of β-InSe, the ∼2 eV PL peak, ascribed to γ-In₂Se₃, is observed in the annealed nanosheet. The increased PL band gap of β-InSe as a function of annealing temperature during phase transformation was possibly affected by the suppressed interlayer coupling, as well as the planar quantum confinement of photo-excited carriers by the external surfaces of the sheets. The photodetector performance with respect to photocurrent, mobility, detectivity, responsivity, and external quantum efficiency was subsequently evaluated after thermal annealing, showing deteriorated optical performance. The present work proved that thermal annealing could induce the successful phase transformation, and adjusted the opto-electronic properties in some extent, providing useful information for processing 2D materials based nano-devices.