Abstract
This work demonstrates that black phosphorene, a two dimensional allotrope of phosphorus, has the potential to be an efficient photo-thermionic emitter. To investigate and understand the novel aspects we use a combined approach in which ab initio quantum simulation tools are utilized along with semiclassical description for the emission process. First by using density functional theory based formalism, we study the band structure of phosphorene. From the locations of electronic bands, and band edges, we estimate the Fermi level and work function. This leads us to define a valid material specific parameter space and establish a formalism for estimating thermionic electron emission current from phosphorene. Finally we demonstrate how the emission current can be enhanced substantially under the effect of photon irradiation. We observe that photoemission flux to strongly dominate over its coexisting counterpart thermionic emission flux. Anisotropy in phosphorene structure plays important role in enhancing the flux. The approach which is valid over a much wider range of parameters is successfully tested against recently performed experiments in a different context. The results open up a new possibility for application of phosphorene based thermionic and photo-thermionic energy converters.
Highlights
Research on layered black phosphorus initiated more than a century ago[1], exfoliation of phosphorene, an atomically thin two-dimensional (2D) material, from its layered bulk counterpart is experimentally achieved only recently[2,3]
We demonstrate for the first time the potential of black phosphorene as an efficient thermionic emitter and show that its performance can be further enhanced through photon irradiation
The analysis of atomic and electronic structures of black phosphorene are carried out using density functional theory (DFT), where the properties of a many-electron system can be determined by using spatially dependent electron density obtained from the self-consistent iterative solutions of Kohn-Sham equations[32]
Summary
This work demonstrates that black phosphorene, a two dimensional allotrope of phosphorus, has the potential to be an efficient photo-thermionic emitter. From the locations of electronic bands, and band edges, we estimate the Fermi level and work function This leads us to define a valid material specific parameter space and establish a formalism for estimating thermionic electron emission current from phosphorene. Our results for phosphorene band structure is used for comparing the DFT-based and TB-based electronic bands to tune the tight-binding parameters, and for obtaining the work function of the materials. The parameter β(ν) which effectively leads to the redistribution of the electrons to the higher energy states can be determined by equating the factor β(ν)Λ(ν) of the total electron flux (nt) available for the emission with absorbed incident photon flux as β(ν)Λ(ν)nt = (I0/hν) ⇒ β(ν)nt = α Part of this absorbed energy is (a fraction μ) consumed in lattice thermalization to increase its surface temperature while rest of it (fraction 1 − μ) is utilized in the process of electron emission.
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