On-Surface Self-Assembled Structural Transformation Induced by Schiff Base Reaction and Hydrogen bonds.

By utilizing scanning tunneling microscopy (STM), the self-assembled nanostructures of three characteristic aldehydes have been examined at the solution-solid interface. By introducing the active reactant 5-aminoisophthalic acid (5-AIPA), we succeeded in changing the self-assembled molecular structures through the condensation reaction and obtained the information on structural transformation in real time. The corresponding carboxyl conjugated derivatives were formed in situ and developed into the closely packed and ordered molecular architectures via hydrogen bonds at the solution-solid surface. The relevant simulations have been utilized to interpret the mechanisms of forming the nanostructures. The corresponding theoretical calculation is used to explain the reaction mechanism. Compared with the traditional ways, the on-surface condensation reaction in situ could not only provide a more convenient method for regulating the self-assembled architectures but also offer a promising strategy for building functional nanostructures and devices.

Gate-tunable negative refraction of mid-infrared polaritons.

Negative refraction provides a platform to manipulate mid-infrared and terahertz radiation for molecular sensing and thermal emission applications. However, its implementation based on metamaterials and plasmonic media presents challenges with optical losses, limited spatial confinement, and lack of active tunability in this spectral range. We demonstrate gate-tunable negative refraction at mid-infrared frequencies using hybrid topological polaritons in van der Waals heterostructures. Specifically, we visualize wide-angle negatively refracted polaritons in α-MoO3 films partially decorated with graphene, undergoing reversible planar nanoscale focusing. Our atomically thick heterostructures weaken scattering losses at the interface while enabling an actively tunable transition of normal to negative refraction through electrical gating. We propose polaritonic negative refraction as a promising platform for infrared applications such as electrically tunable super-resolution imaging, nanoscale thermal manipulation, enhanced molecular sensing, and on-chip optical circuitry.

Differentiation of Multiple Mechanical Stimuli by a Flexible Sensor Using a Dual-Interdigital-Electrode Layout for Bodily Kinesthetic Identification.

Human bodily kinesthetic sensing is generally complicated and ever-changing due to the diversity of body deformation as well as the complexity of mechanical stimulus, which is different from the unidirectional mechanical motion. So, there exists a huge challenge for current flexible sensors to accurately differentiate and identify what kind of external mechanical stimulus is exerted via analyzing digital signals. Here, we report a flexible dual-interdigital-electrode sensor (FDES) that consists of two interdigital electrodes and a highly pressure-sensitive porous conductive sponge. The FDES can precisely identify multiple mechanical stimuli, e.g., pressing, positive bending, negative bending, X-direction stretching, and Y-direction stretching, and convert them into corresponding current variation signals. Moreover, the FDES exhibits other exceptional properties, such as high sensitivity, stretchability, large measurement range, and outstanding stability, accompanied by simple structural design and low-cost processing simultaneously. Additionally, our FDES successfully identifies various complex activities of the human body, which lays a foundation for the further development of multimode flexible sensors.

Two-Dimensional Molecular Network Built from Hierarchy Self-Assembly of Perylene Bisimide Derivatives.

The structures of two-dimensional (2D) self-assembled molecular networks formed by a series of perylene bisimide (PBI) derivatives were investigated by scanning tunneling microscopy (STM). By introducing different functional groups to the PBI rings, we successfully built a different self-assembled molecular network on the liquid-solid interface. When the substituent is propanol, PBI is aligned in lines. When we introduced either an ester group or an amide group to the PBI compounds, they tended to form dimers and trimers. Especially, the PBI with the amide groups can form a 2D porous molecular network by hierarchy self-assembly. The 2D porous molecular network has a great potential to be the host molecule for the accommodation of a guest molecule, coronene (COR), and the structure of the 2D porous molecular network can be tuned by varying the concentration. The density functional theory calculations were also performed to disclose the mechanisms involved.