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.

Separation of epitaxial thin films on a growth substrate and transfer onto other materials for functional heterostructures have boosted the transformative impact on science and technology. However, this scheme has proved challenging in thin-film thermoelectrics but promises a vast range of applications beyond the limited device configurations of bulk thermoelectrics. Here, the high-quality Bi0.5Sb1.5Te3 (BST) epitaxial thin film on a sapphire substrate grown by spontaneous van der Waals epitaxy (vdWE) is exfoliated and transferred onto versatile materials, creating high-performance thermoelectric membranes. Unprecedented millimeter-size vdWE BST membranes are produced by etching a pseudomorphic Te monolayer on the surface of a sapphire substrate in dilute HF solution. The intact exfoliation and direct transfer for vdWE BST membranes maintain the high-quality crystallinity, resulting in the remarkable zT value (∼0.9 at 300 K). These results represent the realization of long-pursued but undemonstrated high-performance thin-film thermoelectrics, paving the way for the design and fabrication of arbitrarily shaped thermoelectric devices.

2D van der Waals magnetic semiconductors have emerged along with the possibilities of achieving an efficient gate tunability and a proximity effect with a high magnetic anisotropy compared with 3D counterparts. Little explored are multiple magnetic phases with a single crystallographic phase. Herein, the multiple magnetic phases in a Mn-doped SnS 2 single crystal with different doping concentrations using a one-step self-flux method are reported. Two ferromagnetic phases with a canted spin direction exist regardless of the Mn-doping concentration at up to 5 at%. Antiferromagnetism coexists with the ferromagnetic order and strengthens at high Mn-doping concentrations. A magnetoresistance measurement conducted on a 2 at% Mn-SnS 2 flake exhibits a positive-to-negative crossover with a value of as high as 50% and clear anisotropy, confirming the presence of ferromagnetic order in the material. By revealing multiple magnetic phases in Mn-doped SnS 2 , the study broadens the scope of state-of-the-art research on layered magnetic semiconductors.

The discovery of new families, beyond graphene, of two-dimensional (2D) layered materials has always attracted great attention. However, it has been challenging to artificially develop layered materials with honeycomb atomic lattice structure composed of multicomponents such as hexagonal boron nitride. Here, through the dimensional manipulation of a crystal structure from sp3-hybridized 3D-ZnSb, we create an unprecedented layered structure of Zintl phase, which is constructed by the staking of sp2-hybridized honeycomb ZnSb layers. Using structural analysis combined with theoretical calculation, it is found that the 2D-ZnSb has a stable and robust layered structure. The bidimensional polymorphism is a previously unobserved phenomenon at ambient pressure in Zintl families and can be a common feature of transition metal pnictides. This dimensional manipulation of a crystal structure thus provides a rational design strategy to search for new 2D layered materials in various compounds, enabling unlimited expansion of 2D libraries and corresponding physical properties.