In situ TEM study of edge reconstruction and evolution in monolayer black phosphorus

Edge-Confined Excitons in Monolayer Black Phosphorus

Quantum confinement of two-dimensional excitons in van der Waals materials via electrostatic trapping, lithographic patterning, Moiré potentials, and chemical implantation has enabled significant advances in tailoring light emission from nanostructures. While such approaches rely on complex preparation of materials, natural edges are a ubiquitous feature in layered materials and provide a different approach for investigating quantum-confined excitons. Here, we observe that certain edge sites of monolayer black phosphorus (BP) strongly localize the intrinsic quasi-one-dimensional excitons, yielding sharp spectral lines in photoluminescence, with nearly an order of magnitude line width reduction. Through structural characterization of BP edges using transmission electron microscopy and first-principles GW plus Bethe-Salpeter equation (GW-BSE) calculations of exemplary BP nanoribbons, we find that certain atomic reconstructions can strongly quantum-confine excitons resulting in distinct emission features, mediated by local strain and screening. We observe linearly polarized luminescence emission from edge reconstructions that preserve the mirror symmetry of the parent BP lattice, in agreement with calculations. Furthermore, we demonstrate efficient electrical switching of localized edge excitonic luminescence, whose sites act as excitonic transistors for emission. Localized emission from BP edges motivates exploration of nanoribbons and quantum dots as hosts for tunable narrowband light generation, with future potential to create atomic-like structures for quantum information processing applications as well as exploration of exotic phases that may reside in atomic edge structures.

Unconventional Self-Reconstructed Trimer-like Metal Zigzag Edge of 1T-Phase Transition Metal Dichalcogenides

Undercoordination-induced slight bond contraction of the pristine edge is a conventional edge self-reconstructed pattern in two-dimensional materials that generally cannot drive the edge into its ground state. Despite reports of unconventional edge self-reconstructed patterns of 1H-phase transition metal dichalcogenides (TMDCs), there have been no reports of sister 1T-phase TMDCs. Here, we predict an unconventional (2 × 1) edge self-reconstructed pattern for 1T-TMDCs by considering 1T-TiTe₂. A novel self-reconstructed trimer-like metal zigzag edge (TMZ edge) with one-dimensional metal atomic chains and Ti₃ trimers is uncovered. Its metal triatomic 3d orbital coupling leads to Ti₃ trimerization. This TMZ edge occurs in group IV, V, and X 1T-TMDCs and has an energetic advantage far beyond conventional bond contraction. The unique triatomic synergistic effect results in better catalysis of the hydrogen evolution reaction (HER) for the 1T-TMDCs than with commercial platinum-based catalysts. This study provides a new strategy for maximizing the HER catalytic efficiency of 1T-TMDCs by using atomic edge engineering.

Atomically Sharp, Closed Bilayer Phosphorene Edges by Self-Passivation

Two-dimensional crystals' edge structures not only influence their overall properties but also dictate their formation due to edge-mediated synthesis and etching processes. Edges must be carefully examined because they often display complex, unexpected features at the atomic scale, such as reconstruction, functionalization, and uncontrolled contamination. Here, we examine atomic-scale edge structures and uncover reconstruction behavior in bilayer phosphorene. We use in situ transmission electron microscopy (TEM) of phosphorene/graphene specimens at elevated temperatures to minimize surface contamination and reduce e-beam damage, allowing us to observe intrinsic edge configurations. The bilayer zigzag (ZZ) edge was found to be the most stable edge configuration under e-beam irradiation. Through first-principles calculations and TEM image analysis under various tilting and defocus conditions, we find that bilayer ZZ edges undergo edge reconstruction and so acquire closed, self-passivated edge configurations. The extremely low formation energy of the closed bilayer ZZ edge and its high stability against e-beam irradiation are confirmed by first-principles calculations. Moreover, we fabricate bilayer phosphorene nanoribbons with atomically sharp closed ZZ edges. The identified bilayer ZZ edges will aid in the fundamental understanding of the synthesis, degradation, reconstruction, and applications of phosphorene and related structures.

Nanoimaging of the Edge-Dependent Optical Polarization Anisotropy of Black Phosphorus

The electronic structure and functionality of 2D materials is highly sensitive to structural morphology, not only opening the possibility for manipulating material properties but also making predictable and reproducible functionality challenging. Black phosphorus (BP), a corrugated orthorhombic 2D material, has in-plane optical absorption anisotropy critical for applications, such as directional photonics, plasmonics, and waveguides. Here, we use polarization-dependent photoemission electron microscopy to visualize the anisotropic optical absorption of BP with 54 nm spatial resolution. We find the edges of BP flakes have a shift in their optical polarization anisotropy from the flake interior due to the 1D confinement and symmetry reduction at flake edges that alter the electronic charge distributions and transition dipole moments of edge electronic states, confirmed with first-principles calculations. These results uncover previously hidden modification of the polarization-dependent absorbance at the edges of BP, highlighting the opportunity for selective excitation of edge states of 2D materials with polarized light.

Electron transport properties of PtSe2 nanoribbons with distinct edge reconstructions

Edge reconstructions of two-dimensional (2D) materials play a central role in determining the electronic transport properties of nanodevices. However, it is not feasible to study the relationship between edge reconstruction and electronic properties using experimental methods because of the complexity of the experimental environment and the diversity of edge reconstruction. Herein, we have combined density functional theory (DFT) calculations and the nonequilibrium Green's function (NEGF) method to investigate the inner physical mechanism of platinum diselenide (PtSe₂) nanoribbons, revealing distinctive negative differential resistance (NDR) behaviors in different nanoribbons with various edge reconstructions. The armchair PtSe₂ nanoribbons with different edge reconstructions are all metallic, while the zigzag PtSe₂ nanoribbons are semiconducting when the ratio of Pt to Se atoms at the edge is 1 : 2. These results reveal the internal source of the difference in the electron transport properties of PtSe₂ nanoribbons with different edge reconstructions, providing new ideas for the design of novel multifunctional PtSe₂ semiconducting and conducting electronic nanodevices with NDR properties.

Edge reconstructions of two-dimensional (2D) materials play a central role in determining the electronic transport properties of nanodevices.