Non-oxidized bare copper nanoparticles with surface excess electrons in air.

Copper (Cu) nanoparticles (NPs) have received extensive interest owing to their advantageous properties compared with their bulk counterparts. Although the natural oxidation of Cu NPs can be alleviated by passivating the surfaces with additional moieties, obtaining non-oxidized bare Cu NPs in air remains challenging. Here we report that bare Cu NPs with surface excess electrons retain their non-oxidized state over several months in ambient air. Cu NPs grown on an electride support with excellent electron transfer ability are encapsulated by the surface-accumulated excess electrons, exhibiting an ultralow work function of ~3.2 eV. Atomic-scale structural and chemical analyses confirm the absence of Cu oxide moiety at the outermost surface of air-exposed bare Cu NPs. Theoretical energetics clarify that the surface-accumulated excess electrons suppress the oxygen adsorption and consequently prohibit the infiltration of oxygen into the Cu lattice, provoking the endothermic reaction for oxidation process. Our results will further stimulate the practical use of metal NPs in versatile applications.

Identifying Defect-Induced Trion in Monolayer WS2 via Carrier Screening Engineering.

Unusually high exciton binding energies (BEs), as much as ∼1 eV in monolayer transition-metal dichalcogenides, provide opportunities for exploring exotic and stable excitonic many-body effects. These include many-body neutral excitons, trions, biexcitons, and defect-induced excitons at room temperature, rarely realized in bulk materials. Nevertheless, the defect-induced trions correlated with charge screening have never been observed, and the corresponding BEs remain unknown. Here we report defect-induced A-trions and B-trions in monolayer tungsten disulfide (WS2) via carrier screening engineering with photogenerated carrier modulation, external doping, and substrate scattering. Defect-induced trions strongly couple with inherent SiO2 hole traps under high photocarrier densities and become more prominent in rhenium-doped WS2. The absence of defect-induced trion peaks was confirmed using a trap-free hexagonal boron nitride substrate, regardless of power density. Moreover, many-body excitonic charge states and their BEs were compared via carrier screening engineering at room temperature. The highest BE was observed in the defect-induced A-trion state (∼214 meV), comparably higher than the trion (209 meV) and neutral exciton (174 meV), and further tuned by external photoinduced carrier density control. This investigation allows us to demonstrate defect-induced trion BE localization via spatial BE mapping in the monolayer WS2 midflake regions distinctive from the flake edges.

Gate-Tunable Magnetism via Resonant Se-Vacancy Levels in WSe2.

The confined defects in 2D van der Waals (vdW)-layered semiconductors can be easily tailored using charge doping, strain, or an electric field. Nevertheless, gate-tunable magnetic order via intrinsic defects has been rarely observed to date. Herein, a gate-tunable magnetic order via resonant Se vacancies in WSe2 is demonstrated. The Se-vacancy states are probed via photocurrent measurements with gating to convert unoccupied states to partially occupied states associated with photo-excited carrier recombination. The magneto-photoresistance hysteresis is modulated by gating, which is consistent with the density functional calculations. The two energy levels associated with Se vacancies split with increasing laser power, owing to the robust Coulomb interaction and strong spin-orbit coupling. The findings offer a new approach for controlling the magnetic properties of defects in optoelectronic and spintronic devices using vdW-layered semiconductors.

Epitaxial Single-Crystal Growth of Transition Metal Dichalcogenide Monolayers via the Atomic Sawtooth Au Surface.

Growth of 2D van der Waals layered single-crystal (SC) films is highly desired not only to manifest the intrinsic physical and chemical properties of materials, but also to enable the development of unprecedented devices for industrial applications. While wafer-scale SC hexagonal boron nitride film has been successfully grown, an ideal growth platform for diatomic transition metal dichalcogenide (TMdC) films has not been established to date. Here, the SC growth of TMdC monolayers on a centimeter scale via the atomic sawtooth gold surface as a universal growth template is reported. The atomic tooth-gullet surface is constructed by the one-step solidification of liquid gold, evidenced by transmission electron microscopy. The anisotropic adsorption energy of the TMdC cluster, confirmed by density-functional calculations, prevails at the periodic atomic-step edge to yield unidirectional epitaxial growth of triangular TMdC grains, eventually forming the SC film, regardless of the Miller indices. Growth using the atomic sawtooth gold surface as a universal growth template is demonstrated for several TMdC monolayer films, including WS2 , WSe2 , MoS2 , the MoSe2 /WSe2 heterostructure, and W1- x Mox S2 alloys. This strategy provides a general avenue for the SC growth of diatomic van der Waals heterostructures on a wafer scale, to further facilitate the applications of TMdCs in post-silicon technology.

Single-Crystalline Monolayer Graphene Wafer on Dielectric Substrate of SiON without Metal Catalysts.

Many scientific and engineering efforts have been made to realize graphene electronics by fully utilizing intrinsic properties of ideal graphene for last decades. The most technical huddles come from the absence of wafer-scale graphene with a single-crystallinity on dielectric substrates. Here, we report an epitaxial growth of single-crystalline monolayer graphene directly on a single-crystalline dielectric SiON-SiC(0001) with a full coverage via epitaxial chemical vapor deposition (CVD) without metal catalyst. The dielectric surface of SiON provides atomically flat and chemically inert interface by passivation of dangling bonds, which keeps intrinsic properties of graphene. Atomic structures with a clean interface, full coverage of single-crystalline monolayer, and the epitaxy of graphene on SiON were confirmed macroscopically by mapping low energy electron diffraction (LEED) and Raman spectroscopy, and atomically by scanning tunneling microscopy (STM). Both of measured and calculated local density of states (LDOS) exhibit a symmetric and sharp Dirac cone with a Dirac point located at a Fermi level. Our method provides a route to utilize a single-crystalline dielectric substrate for ideal graphene growth for future applications.