Nano-Optical Photodetectors Based on Two-Dimensional Materials

Abstract

The discovery of graphene in 2004 opened the door to the wonderful world of two-dimensional (2D) layered materials, and the properties and applications of these materials have been hot research topics ever since. The atomic-level thinness and layered structure of 2D materials give rise to extraordinary properties and enable novel functionalities, and they have exhibited great potential in various fields including electronics and optoelectronics. They are particularly promising for photodetection, and detectors from ultraviolet to terahertz wavelengths have been demonstrated based on these materials. However, low light absorption in 2D materials, which originates from their thin structure, has hindered their widespread application in photodetection. In this thesis, we demonstrate optical nanostructures that can significantly boost the interaction of light with 2D materials and thus improve their photodetection performance. Our focus is on infrared (IR) photodetectors which have applications in a wide range of areas that include biomedical and thermal imaging, telecommunication, spectroscopy, and many other modern technologies.

First, we present a hybrid plasmonic structure for enhancing the light absorption in graphene in the long-wave IR (LWIR) spectral region. This structure, consisting of a metallic bull's eye grating and optical nanoantennas, employs surface plasmon polaritons and localized surface plasmons to concentrate light into a monolayer graphene flake with sub-wavelength lateral extent. Optical simulations show that this plasmonic structure provides a 558-fold light absorption enhancement in graphene and a 32-fold enhancement in the detectivity of the LWIR photodetector. It is also found that integrating this structure with an optical cavity substrate further boosts the device performance.

Black phosphorus (bP), another 2D layered material with a narrow and direct bandgap of 0.31 eV, has great potential for IR optoelectronics. Nevertheless, the performance of bP-based photodetectors is limited by weak light absorption in bP, resulting from its thinness and optical anisotropy. In the next work, via optical simulations, we demonstrate hybrid plasmonic nanoantenna/optical cavity structures that boost the IR light absorption in multilayer bP through polarization conversion and light intensity enhancement. In a reciprocal manner, these nanostructures enhance the spontaneous emission from bP. Light absorption and emission enhancements of up to 185-fold and 18-fold, respectively, are achieved. Detectivity and electroluminescence efficiency of 2D material-based photodetectors and light-emitting diodes can be significantly enhanced employing these optical nanostructures.

Recently, platinum diselenide (PtSe2), a 2D noble-transition-metal dichalcogenide, has also been investigated for IR detection. However, wavelengths up to the short-wave infrared region have been the main focus of these studies. In the last work, we present LWIR photodetectors based on multilayer PtSe2. We utilise a TiO2/Au optical cavity substrate for enhancing the LWIR light absorption in PtSe2. Responsivity values of up to 54 mA/W are obtained at 8.35 um. In addition, these devices show a fast photoresponse with a time constant of 54 ns to white light illumination. This study reveals the potential of multilayer PtSe2 for fast and broadband photodetection from visible to LWIR wavelengths. It also highlights the key role of the substrate in the performance of 2D material-based IR photodetectors.