Abstract:As the interface between human and machine becomes blurred, hydrogel incorporated electronics and devices have emerged to be a new class of flexible/stretchable electronic and ionic devices due to their extraordinary properties, such as softness, mechanically robustness, and biocompatibility. However, heat dissipation in these devices could be a critical issue and remains unexplored. Here, we report the experimental measurements and equilibrium molecular dynamics simulations of thermal conduction in polyacrylamide (PAAm) hydrogels. The thermal conductivity of PAAm hydrogels can be modulated by both the effective crosslinking density and water content in hydrogels. The effective crosslinking density dependent thermal conductivity in hydrogels varies from 0.33 to 0.51 Wm −1 K −1 , giving a 54% enhancement. We attribute the crosslinking effect to the competition between the increased conduction pathways and the enhanced phonon scattering effect. Moreover, water content can act as filler in polymers which leads to nearly 40% enhancement in thermal conductivity in PAAm hydrogels with water content vary from 23 to 88 wt %. Furthermore, we find the thermal conductivity of PAAm hydrogel is insensitive to temperature in the range of 25-40 • C. Our study offers fundamental understanding of thermal transport in soft materials and provides design guidance for hydrogel-based devices.

Black phase CsPbI 3 perovskites have emerged as one of the most promising materials for use in optoelectronic devices due to their remarkable properties. However, black phase CsPbI 3 usually possesses poor stability and involves a phase change process, resulting in an undesired orthorhombic (δ) yellow phase. Here, the enhanced stability of CsPbI 3 nanocrystals is achieved by incorporating the Cu 2+ ion into the CsPbI 3 lattice under mild conditions. In particular, the Cu 2+doped CsPbI 3 film can maintain red luminescence for 35 days in air while the undoped ones transformed into the nonluminescent yellow phase in several days. Furthermore, first-principles calculations verified that the enhanced stability is ascribed to the increased formation energy due to the successful doping of Cu 2+ in CsPbI 3 . Benefiting from such an effective doping strategy, the as-prepared Cu 2+ -doped CsPbI 3 as an emitting layer shows much better performance compared with that of the undoped counterpart. The turn-on voltage of the Cu 2+doped quantum-dot light-emitting diode (QLED) (1.6 V) is significantly reduced compared with that of the pristine QLED (3.8 V). In addition, the luminance of the Cu 2+ -doped QLED can reach 1270 cd/m 2 , which is more than twice that of the pristine CsPbI 3 QLED (542 cd/m 2 ). The device performance is believed to be further improved by optimizing the purification process and device structure, shedding light on future applications.

Acoustic metasurfaces that can manipulate and control sound waves at 2D subwavelength scales open new avenues to unusual applications, such as asymmetric transmission, super-resolution imaging, and particle manipulation. However, the long-standing goals of pushing frontier metamaterials research into real practice are still severely constrained by cumbersome configuration, large acoustic loss, and rigid structure of the existing metamaterials. An ultrathin metasurface (10-300 µm in thickness, up to ≈λ/650, λ the wavelength) that is capable of imparting sound wave with a nontrivial phase shift with high transmittance (>80%) in the range of 5-30 kHz is fabricated here. The metasurface is comprised of a porous network of soft polymer fiber/rigid beads that are physically equivalent to crosslinked spring-mass resonators. Moreover, the traditional paper-cutting art to carve the ultrathin metasurface into hollow-out patterns is incorporated, resulting in a variety of remarkable functions, including acoustic vortex, focusing, and super-resolution. The hollow-out patterning approach innovates the traditional one-step metadevice fabrication process into two separated steps: 1) fabrication of ultrathin metasurfaces; 2) hollow-out patterning of metasurfaces. The strategy opens an avenue to mass production of acoustic metadevices, shedding light on the applications of the metamaterials in acoustic cloaking, acoustic positioning, and particle manipulation.