Stabilizing 2D Phosphorus Allotropes at Confined Heterointerfaces

<b>There is a significant interest in the synthesis of monolayer black phosphorus known as phosphorene for its unique in-plane anisotropy, which gives rise to a variety of anisotropic electrical and optical properties and therefore applications. The synthesis of phosphorene remains, however, challenging due to the extreme temperature and pressure requirements. Moreover, the degradation of phosphorene is inevitable when exposed to air without proper surface passivation. To circumvent these challenges, we have developed a new synthesis route to stabilize 2D allotropes of phosphorus at a heterointerface between graphene/Ge (110) via intercalation. The rationale behind the intercalation scheme is justified by the fact that nanobubbles within the graphene heterointerface creates an ultra-high local pressurized environment (&gt;1 GPa) which may provide a driving force to nucleate the black phosphorus phase. Furthermore, the graphene sheet itself can also protect the phosphorene layer from oxidation. In addition, the limited diffusivity of phosphorus into germanium and the anisotropic graphene/Ge (110) reconstructed heterointerface will further promote the formation of the desired 2D allotrope phase.</b><br/><b>Here, we explore and compare silicon phosphate, phosphoric acid, red phosphorus and black phosphorus as the starting phosphorus source for intercalating the graphene/Ge(110) heterointerface. In the case of the silicon phosphate source, we observed that phosphorus oxide (P2O5; the decomposition product of silicon phosphate) both doped and intercalated the graphene/Ge(110) heterointerface after annealing the source up to 950C. The existence of P2O5 was confirmed from the strong P and O signals in EDS mapping, especially within the graphene nanobubbles. We observed an up-shift of the G and 2D peak positions in our Raman measurements of the intercalated graphene which is indicative of charge transfer from graphene to P2O5. This was further confirmed by the appearance of a Raman peak around 1100 cm-1, which was induced by the symmetry bond stretching between P and non-bridging O in [PO4-] unit, which we further corroborated from Density Functional Theory (DFT) calculations and in XPS measurements of the relevant core-levels. </b><br/><b>To reduce the intercalated P2O5, we performed annealing studies in hydrogen at atmospheric pressures. After post-annealing, both the Raman peak at ~1100 cm-1, which was related to P2O5, and the P=O bond, in both O1s XPS spectrum, disappeared. Also, the O signal in EDS mapping was much weaker, suggesting the reduction of the intercalated P2O5. We correlate the structure of the 2D phosphorus (black phosphorus, red phosphorus, blue phosphorus etc. ) at the confined nanobubble at graphene/Ge (110) heterointerface through detailed Raman, synchrotron based micro/nano-FTIR and scanning tunneling microscopy. In addition, we will investigate the intercalation process with other phosphorus sources including red phosphorus, black phosphorus and phosphoric acid to understand the influence of phosphorus sources on the phase stabilized.</b>