Realizing a state of matter in two dimensions has repeatedly proven a novel route of discovering new physical phenomena. Van der Waals (vdW) materials have been at the center of these now extensive research activities. They offer a natural way of producing a monolayer of matter simply by mechanical exfoliation. This work demonstrates that the possible multiferroic state with coexisting antiferromagnetic and ferroelectric orders possibly persists down to the bilayer flake of NiI2. By exploiting the optical second-harmonic generation technique, both magnitude and direction of the ferroelectric order, arising from the cycloidal spin order, are successfully traced. The possible multiferroic state's transition temperature decreases from 58 K for the bulk to about 20 K for the bilayer. Our observation will spur extensive efforts to demonstrate multi-functionality in vdW materials, which have been tried mostly by using heterostructures of singly ferroic ones until now.

In nano-device applications using two-dimensional (2D) van der Waals materials, a heat dissipation through nano-scale interfaces can be a critical issue for optimizing device performances. By using a time-domain thermoreflectance measurement technique, we examine a cross-plane thermal transport through mono-layered ( n = 1) and bi-layered ( n = 2) WSe 2 flakes which are sandwiched by top metal layers of Al, Au, and Ti and the bottom Al 2 O 3 substrate. In these nanoscale structures with hetero- and homo-junctions, we observe that the thermal boundary resistance (TBR) is significantly enhanced as the number of WSe 2 layers increases. In particular, as the metal is changed from Al, to Au, and to Ti, we find an interesting trend of TBR depending on the WSe 2 thickness; when referenced to TBR for a system without WSe 2 , TBR for n = 1 decreases, but that for n = 2 increases. This result clearly demonstrates that the stronger bonding for Ti leads to a better thermal conduction between the metal and the WSe 2 layer, but in return gives rise to a large mismatch in the phonon density of states between the first and second WSe 2 layers so that the WSe 2 -WSe 2 interface becomes a major thermal resistance for n = 2. By using photoemission spectroscopy and optical second harmonic generation technique, we confirm that the metallization induces a change in the valence state of W-ions, and also recovers a non-centrosymmetry for the bi-layered WSe 2 .

A SrRuO3 thin film has been widely used as a metal electrode in electronic devices based on transition metal oxides, and hence it is important to understand its thermal transport properties to minimize a thermal degradation problem during the device operation. Using the time-domain thermoreflectance measurement technique, we investigate the cross-plane thermal conductivity of the SrRuO3 thin films with a thickness variation from 1 μm to 8 nm. We find that the thermal conductivity is reduced from about 6 W m−1 K−1 for the 1 μm thick film to about 1.2 W m−1 K−1 for the 8 nm thick film, and attribute this behavior to the boundary scattering of thermal carriers which originally have the mean free path of about 20 nm in a bulk state. Also, we observe a clear dip behavior of the thermal conductivity in the intermediate thickness around 30 nm which suggests an existence of a strong scattering source other than the film boundary. We explain this result by considering an additional interfacial scattering at the tetragonal-orthorhombic phase boundary which is formed during the strain relaxation with an increase of the film thickness.

Colloidal lead halide perovskites face challenges in practical optoelectronic applications due to the toxicity of lead (II). To overcome this issue, a novel approach using metal‐organic frameworks (MOFs) for recycling lead (II) management in perovskite materials, is developed. Through surface‐functionalization of MOF‐808 and pH tuning, the MOF achieves recyclable adsorption‐desorption of lead (II), confining the lead source within the MOF‐808 template or the perovskites. This controllable transformation between lead (II) chelation and perovskites prevents lead (II) leakage. Additionally, separated lead (II) ions from decomposed perovskite can be adsorbed by MOF‐808, facilitating recycling. The resulting MOF‐808‐EDTA@CsPbBr3 composite exhibits comparable optical performance to pure CsPbBr3 powder, but with improved stabilities, effectively preventing lead (II) leakage. Hierarchical desorption of lead (II) in the MOF‐808‐EDTA@Pb2+ complex enables reversible on/off fluorescence signal switching. Detached lead (II) from decomposed MOF‐808‐EDTA@CsPbBr3 composite can be re‐adsorbed by MOF‐808‐EDTA, enabling the construction of CsPbBr3 nanocrystals in a new cycle. This approach is economically attractive and environmentally friendly. This study addresses the challenge of lead (II) leakage and offers the potential for optical anti‐counterfeiting in perovskite materials with a sustainable approach.

We investigated the thermal transport properties of magnetic van der Waals materials, TMPS3 (TM = Mn, Ni, and Fe), using the time-domain thermoreflectance technique. We determined the cross-plane thermal conductivity, which turns out to be relatively low, i.e., about 1 W m−1 K−1 for all TMPS3 investigated. When compared with previous results of graphite and transition metal dichalcogenides (TMDs), thermal conductivity becomes smaller as it goes from graphite to TMDs to TMPS3, and the difference is larger at low temperature, e.g., around 50 K. From the Callaway model analysis, we could attribute the large thermal conductivity reduction for TMPS3, particularly at low temperature, to the phonon scattering from the boundary. We actually confirmed the existence of the large population of the stacking faults with the cross-sectional transmission electron microscopy image of MnPS3. This suggests that intrinsic or extrinsic stacking faults formed in van der Waals materials and their heterostructures can play an important role in reducing the cross-plane thermal conductivity as a source of the boundary scattering.