Plasmonic Hot Electron Induced Layer Dependent Anomalous Fröhlich Interaction in InSe

Abstract

InSe is one of the most promising two-dimensional (2D) materials for electronic and optoelectronic applications because of its favourable bandgap and superior electron mobility compared to other layered semiconductors. However, due to the polar nature of InSe, Fröhlich interaction plays an important role in electrical transport, which becomes more significant in reduced dimensionality. Until now, it is not yet known how the dimensionality influences the strength and nature of the Fröhlich polaronic effect in InSe. Here, we report on layer dependent anomalous Fröhlich interaction in InSe from bulk to monolayer with the aid of plasmonic hot electron doping. When excited near the localized surface plasmon resonance, plasmonic nanostructures produce highly energetic electrons (known as hot electrons), which can be captured by a semiconductor such as InSe at the interface. These electrons then couple to the polar optical phonons via the Fröhlich interaction in InSe. With the aid of the strong plasmonic field, the Fröhlich interaction enabling us to monitor the polar phonons in conventional Raman measurements. We prepared nanostructures with three different metals (Ag, Au, and Al) using nanosphere lithography on InSe to study the hot electron doping effect by means of Raman spectroscopy. A finite element method simulation was used to understand the coupling between the plasmonic nanostructures and InSe. We observed that the intensity of polar LO phonon modes initially increases gradually with decreasing layer number and then drops drastically from 7L to 6L, i.e. at the thickness where the transition from quasi-direct to indirect bandgap occurs at room temperature. Additionally, a gradual decrease of intensity of the polar modes with decreasing layer thickness below this transition point is observed, which is due to the increasing indirect bandgap nature of InSe suggesting reduced Fröhlich coupling. Our results shed light on fundamental understanding of Fröhlich interaction in InSe, which is crucial for electronic and optoelectronic applications of this promising 2D material.

Tip-Enhanced Raman Spectroscopy of 2D Semiconductors

“…Due to strong light matter interaction TMDC monolayers, when integrated with plasmonic antennas, show great potential via modulation of their optoelectronic properties [78]. Generated hot electrons from plasmonic nanostructures can be injected into the conduction band of these 2D semiconductors due to favorable Schottky barrier heights at the interface and a large oscillator strength of these hot electrons [22,79]. The concept of the doping induced structural phase change is illustrated in Figure 7a.…”

“…Both few layer BP and InSe based field effect transistors show comparable career mobility (10 3 cm 2 V À1 s À1 ) at RT suggesting they can outperform Si based CMOS at the scaling limits [20]. However, BP suffers from poor stability in air [21] and the polar nature of InSe leads to layer dependent Fröhlich interaction [22] and thus optical phonon limited career mobility.…”