Heterophase homojunction formation in atomically thin 2D layers is of great importance for next-generation nanoelectronics and optoelectronics applications. Technologically challenging, controllable transformation between the semiconducting and metallic phases of transition metal chalcogenides is of particular importance. Here, we demonstrate that controlled laser irradiation can be used to directly ablate PdSe2 thin films using high power, or trigger the local transformation of PdSe2 into a metallic phase PdSe2-x using lower laser power. Such transformations are possible due to the low decomposition temperature of PdSe2 compared to other 2D transition metal dichalcogenides. Scanning transmission electron microscopy is used to reveal the laser-induced Se-deficient phases of PdSe2 material. The process sensitivity to the laser power allows patterning flexibility for resist free device fabrication. The laser patterned devices demonstrate that a laser-induced metallic phase PdSe2-x is stable with increased conductivity by a factor of about 20 compared to PdSe2. These findings contribute to 2 the development of nanoscale devices with homojunctions and scalable methods to achieve structural transformations in 2D materials.

The solid progress in the study of a single two-dimensional (2D) material underpins the development for creating 2D material assemblies with various electronic and optoelectronic properties. We introduce an asymmetric structure by stacking monolayer semiconducting tungsten disulfide, metallic graphene, and insulating boron nitride to fabricate numerous red channel lightemitting devices (LEDs). All the 2D crystals were grown by chemical vapor deposition (CVD), which has great potential for future industrial scale-up. Our LEDs exhibit visibly observable electroluminescence (EL) at both 5.5 V forward and 7.0 V backward biasing, which correlates well with our asymmetric design. The red emission can last for at least several minutes, and the success rate of the working device that can emit detectable EL is up to 80%. In addition, we show that sample degradation is prone to happen when a continuing bias, much higher than the threshold voltage, is applied. Our success of using high-quality CVD-grown 2D materials for red light emitters is expected to provide the basis for flexible and transparent displays.

We have developed a facile and general method to passivate thin black phosphorus (BP) flakes with large-area high-quality monolayer hexagonal boron nitride (hBN) sheets grown by the chemical vapor deposition (CVD) method. In spite of the one-atom-thick structure, the high-quality CVD-grown monolayer hBN has proven to be useful to prevent the degradation of thin BP flakes exfoliated on substrates. Mechanically exfoliated BP flakes prepared on a Si substrate are covered by the monolayer hBN sheet to preserve (otherwise unstable) atomic layered BP flakes from degradation. The present technique can generally be applied to fabricating BP-based electronic devices with much easiness.

We study the atomic structure and dynamics of defects and grain boundaries in monolayer Pd2Se3 using annular dark field scanning transmission electron microscopy. The Pd2Se3 monolayers are reproducibly created by thermally induced phase transformation of few-layered PdSe2 films in an in situ heating holder in the TEM to promote Se loss. A variety of point vacancies, one-dimensional defects, grain boundaries (GBs), and defect ring complexes are directly observed in monolayer Pd2Se3, which show a series of dynamics triggered by electron beam irradiation. High mobility of vacancies leads to self-healing of point vacancies by migration to the edge and subsequent edge etching under beam irradiation. Specific defects for Pd2Se3 are stabilized by the formation of Se–Se bonds, which can shift in a staggered way to buffer strain, forming a wave-like one-dimensional defect. Bond rotations are also observed and play an important role in defect and grain boundary dynamics in Pd2Se3 during vacancy production. The GBs form in a meandering pathway and migrate by a sequence of Se–Se bond rotations without large-scale vacancy formation. In the GB corners and tilted GBs, other highly symmetric vacancy defects also occur to adapt to the orientation change. These results give atomic level insights into the defects and grain boundaries in Pd2Se3 2D monolayers.

2D crystals are typically uniform and periodic in‐plane with stacked sheet‐like structure in the out‐of‐plane direction. Breaking the in‐plane 2D symmetry by creating unique lattice structures offers anisotropic electronic and optical responses that have potential in nanoelectronics. However, creating nanoscale‐modulated anisotropic 2D lattices is challenging and is mostly done using top‐down lithographic methods with ≈10 nm resolution. A phase transformation mechanism for creating 2D striated lattice systems is revealed, where controlled thermal annealing induces Se loss in few‐layered PdSe2 and leads to 1D sub‐nm etched channels in Pd2Se3 bilayers. These striated 2D crystals cannot be described by a typical unit cells of 1–2 Å for crystals, but rather long range nanoscale periodicity in each three directions. The 1D channels give rise to localized conduction states, which have no bulk layered counterpart or monolayer form. These results show how the known family of 2D crystals can be extended beyond those that exist as bulk layered van der Waals crystals by exploiting phase transformations by elemental depletion in binary systems.

Lead Iodide (PbI 2 ) is a large bandgap 2D layered material that has potential for semiconductor applications. However, atomic level study of PbI 2 monolayer has been limited due to challenges in obtaining thin crystals. Here, we use liquid exfoliation to produce monolayer PbI 2 nanodisks (30-40 nm in diameter and > 99% monolayer purity) and deposit them onto suspended graphene supports to enable atomic structure study of PbI 2 . Strong epitaxial alignment of PbI 2 monolayers with the underlying graphene lattice occurs, leading to a phase shift from the 1 T to 1 H structure to increase the level of commensuration in the two lattice spacings. The fundamental point vacancy and nanopore structures in PbI 2 monolayers are directly imaged, showing rapid vacancy migration and self-healing. These results provide a detailed insight into the atomic structure of monolayer PbI 2 , and the impact of the strong van der Waals interaction with graphene, which has importance for future applications in optoelectronics.

Photo-switching behavior of individual organic molecules was imaged by annular dark-field scanning transmission electron microscopy (ADF-STEM) using a highly electron beam transparent graphene support. Photo-switching azobenzene derivatives with ligands at each ends containing single transition metal atoms (Pt) were designed (Pt complex), and the distance between the strong ADF-STEM contrast from the two Pt atoms in each Pt-complex is used to track molecular length changes. UV irradiation was used to induce photo-switching of the Pt complex on graphene, and we show that the measured Pt-Pt distances within isolated molecules decreases from ~2.1 nm to ~1.4 nm, indicative of a trans-to-cis isomerization. Light illumination of the Pt-complex on the graphene support also caused their diffusion out from initial clusters to the surrounding area of graphene, indicating that the light-activated mobilization overcomes the inter-molecular van der Waals interactions. This approach shows how individual isolated heavy metal atoms can be included as markers

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