Interpreting core-level spectra of oxidizing phosphorene: Theory and experiment

We combine ab initio density functional theory calculations with the equivalent cores approximation to determine core-level binding-energy shifts at phosphorus sites caused by oxidation of phosphorene. We find that presence of oxygen increases the core-level binding energies of P atoms and expect binding-energy shifts of up to 6 eV in highly defective geometries. We have identified likely binding geometries of oxygen that help to interpret the observed core-level photoemission spectra in samples at different stages of oxidation and allow us to determine the fractions of specific local geometries.

Figure 1

Ball-and-stick models of selected pristine, defective, and oxidized phosphorene structures. Structures (a)–(f) represent a defect in a phosphorene monolayer, and (g)–(j) are isolated fragments. The phosphorus atom, for which core-level binding-energy shifts are calculated, is highlighted in light green. Calculated core-level binding-energy shifts $\mathrm{\Delta }{E}_B^F$(P) for this specific P atom are given below each structure.

Figure 2

Schematic of the thermodynamic Born-Haber cycle used to determine core-level binding energies of P atoms in (a) pristine and (b) oxidized phosphorene. (c) Schematic illustration of the cause of the core-level binding-energy shift $\mathrm{\Delta }{E}_B^F$(oxidized P).

Figure 3

Phosphorus core-level spectra in few-layer phosphorene at different levels of oxidation. (a) Observed spectra of pristine samples under inert ${\mathrm{N}}_2$ atmosphere, given by the solid black line, are compared to spectra obtained after exposure to ambient conditions, shown by the dot-dashed blue line, or pure oxygen, shown by the dashed green line. Expected positions of the $2p_{3/2}$ peak of P in different chemical environments, specified in Fig. 1, are shown by the vertical dashed lines. $\mathrm{\Delta }{E}_B^F$ denotes the core-level binding-energy shift for a particular geometry. (b) Spectral decomposition of the feature in the $132–136$-eV binding-energy range in the sample exposed to ambient conditions into components attributed to different structures. The contribution of representative functional groups is described by rigidly shifting the spectrum of the pristine species.

Figure 4

Calculated core-level binding-energy shifts $\mathrm{\Delta }{E}_B^F$ at different phosphorus sites in oxidized phosphorene. Structures (a)–(c) are identical to the structure in Fig. 1, structures (d)–(f) are identical to the structure in Fig. 1, and structures (g)–(i) are identical to the structure in Fig. 1. ${\mathrm{P}}_0, {\mathrm{P}}_1$, and ${\mathrm{P}}_2$ indicate the specific phosphorus sites, for which the core-level binding-energy shifts are being calculated.