Electrical control of anisotropic and tightly bound excitons in bilayer phosphorene

Monolayer and few-layer phosphorene are anisotropic quasi-two-dimensional (quasi-2D) van der Waals (vdW) semiconductors with the linear-dichroic light-matter interaction and the widely tunable direct band gap in the infrared frequency range. Despite recent theoretical predictions of strongly bound excitons with unique properties, it remains experimentally challenging to probe excitonic quasiparticles due to the severe oxidation that occurs during device fabrication. In this study, we report the observation of strongly bound excitons and trions with highly anisotropic optical properties in intrinsic bilayer phosphorene, which are protected from oxidation by encapsulation with hexagonal boron nitride (hBN) in a field-effect transistor (FET) geometry. Reflection contrast and photoluminescence spectroscopy clearly reveal the linear-dichroic optical spectra from anisotropic excitons and trions in the hBN-encapsulated bilayer phosphorene. The optical resonances from the exciton Rydberg series indicate that the neutral exciton binding energy is over 100 meV even with the dielectric screening from hBN. The electrostatic injection of free holes enables an additional optical resonance from a positive trion (charged exciton) ∼30 meV below the optical band gap of the charge-neutral system. Our work shows exciting possibilities for monolayer and few-layer phosphorene as a platform to explore many-body physics and photonics and optoelectronics based on strongly bound excitons with twofold anisotropy.

Figure 1

hBN-encapsulated bilayer phosphorene in FET geometry. (a) Crystal structure of bilayer phosphorene. (b) Optical microscope image of the representative bilayer phosphorene device. (c) The schematic of the FET device in (b).

Figure 2

Optical spectra of the anisotropic exciton for the undoped system. (a) The polarization-resolved $\mathrm{\Delta }R/R$. (b) The polarization-resolved photoluminescence spectra with the unpolarized laser excitation at 1.96 eV.

Figure 3

Optical spectra of the anisotropic excitons with gate control. (a) and (b) Photoluminescence spectra with polarization along the armchair direction for the hole-doped (a) and electron-doped (b) cases. (c) Intensity of photoluminescence at peak energy as a function of polarization angle $\theta$. (d) and (e) $\mathrm{\Delta }R/R$ with polarization along the armchair direction for the hole-doped (d) and electron-doped (e) cases. (f) $\mathrm{\Delta }R/R$ at peak energy as a function of polarization angle $\theta$.

Figure 4

The gate control of exciton energy structure. (a) The schematic of the energy levels for excitons and trions. (b) The energy separation of optical resonances between $X_0^{\mathrm{triplet}}$ and $X_{+}$ with the function of $V_{\mathrm{B}}$.