Ultralight (<10 milligrams per cubic centimeter) cellular materials are desirable for thermal insulation; battery electrodes; catalyst supports; and acoustic, vibration, or shock energy damping. We present ultralight materials based on periodic hollow-tube microlattices. These materials are fabricated by starting with a template formed by self-propagating photopolymer waveguide prototyping, coating the template by electroless nickel plating, and subsequently etching away the template. The resulting metallic microlattices exhibit densities ρ ≥ 0.9 milligram per cubic centimeter, complete recovery after compression exceeding 50% strain, and energy absorption similar to elastomers. Young's modulus E scales with density as E ~ ρ(2), in contrast to the E ~ ρ(3) scaling observed for ultralight aerogels and carbon nanotube foams with stochastic architecture. We attribute these properties to structural hierarchy at the nanometer, micrometer, and millimeter scales.

Complex optical constants and interband transitions of a hybrid perovskite CsPbBr3 thin film measured by spectroscopic ellipsometry.

No abstract

Palladium diselenide (PdSe2), a new type of two-dimensional noble metal dihalides (NMDCs), has received widespread attention for its excellent electrical and optoelectronic properties. Herein, high-quality continuous centimeter-scale PdSe2 films with layers in the range of 3L–15L were grown using Chemical Vapor Deposition (CVD) method. The absorption spectra and DFT calculations revealed that the bandgap of the PdSe2 films decreased with the increasing number of layers, which is due to the enhancement of orbital hybridization. Spectroscopic ellipsometry (SE) analysis shows that PdSe2 has significant layer-dependent optical and dielectric properties. This is mainly due to the unique strong exciton effect of the thin PdSe2 film in the UV band. In particular, the effect of temperature on the optical properties of PdSe2 films was also observed, and the thermo-optical coefficients of PdSe2 films with the different number of layers were calculated. This study provides fundamental guidance for the fabrication and optimization of PdSe2-based optoelectronic devices.

No abstract

No abstract

Catastrophic vs. gradual collapse of thin-walled nanocrystalline Ni hollow cylinders as building blocks of micro-lattice structures Supporting Information Mechanical Testing Two series of hollow cylinders with different wall thickness were prepared for uniaxial compression tests. For the first set (thick series), the wall thickness was ~ 400-600 nm, the diameter was 45-50 m, and the height ranged from 31 to 64 m. In the second set (thin series), the wall thickness was ~150 nm, with a diameter of ~30 m and length of 39 m. Uniaxial compression tests on the thick structures were performed in the XP

Size-dependent plastic flow behavior is manifested in nanoindentation, microbending, and pillar-compression experiments and plays a key role in the contact mechanics and friction of rough surfaces. Recent experiments using a hard flat plate to compress single-crystal Au nano-pyramids and others using a Berkovich indenter to indent flat thin films show size scaling into the 100-nm range where existing mechanistic models are not expected to apply. To bridge the gap between single-dislocation nucleation at the 1-nm scale and dislocation-ensemble plasticity at the 1-m scale, we use large-scale molecular dynamics (MD) simulations to predict the magnitude and scaling of hardness H versus contact size ഞ c in nano-pyramids. Two major results emerge: a regime of near-power-law size scaling H Ϸ ഞ c ؊ exists, with MD Ϸ 0.32 compared with expt Ϸ 0.75, and unprecedented quantitative and qualitative agreement between MD and experiments is achieved, with H MD Ϸ 4 GPa at ഞc ‫؍‬ 36 nm and Hexpt Ϸ 2.5 GPa at ഞ c ‫؍‬ 100 nm. An analytic model, incorporating the energy costs of forming the geometrically necessary dislocation structures that accommodate the deformation, is developed and captures the unique magnitude and size scaling of the hardness at larger MD sizes and up to experimental scales while rationalizing the transition in scaling between MD and experimental scales. The model suggests that dislocation-dislocation interactions dominate at larger scales, whereas the behavior at the smallest MD scales is controlled by nucleation over energy barriers. These results provide a basic framework for understanding and predicting size-dependent plasticity in nanoscale asperities under contact conditions in realistic engineered surfaces.atomistics ͉ dislocation interactions ͉ plasticity ͉ size-effects ͉ surfaces I t is now well established that plastic flow behavior in metallic materials is size dependent, with flow stress (1, 2) or hardness (3) increasing with decreasing volume of material under load. Experimental studies of nanoindentation (4-6) and wire and thin-film bending (7-10) and tension (11-17) clearly elucidate the size effects at micrometer scales. The rational for size-dependent plasticity has been attributed to the need for ''geometrically necessary'' defect structures, typically dislocations, which accommodate the rapid strain gradients that must exist in many small-scale deformation conditions and the influence of these dislocations in hardening the material. Phenomenological strain-gradient-plasticity theories (18-21) and discrete dislocation models (22) both qualitatively exhibit the observed size effects under varying conditions. However, the conceptual bases of these models typically rely on interactions between preexisting, statistically generated, and geometrically necessary dislocations that are not thought to be applicable at nanometer scales where relatively few dislocations exist. Atomistic studies using molecular dynamics (MD) at the nanometer scale can exhibit size effects (23-26), but they are usually ass...