Che-Min Chou

The formation of helical assemblies from achiral supramacromolecules has been a highly mysterious process, but is of interest for the generation of a variety of structures and properties. Herein, novel hierarchical superhelices are developed from self-assembly of achiral dendron-jacketed block copolymers (DJBCPs). The presence of focal asymmetry in both the stereochemistry of the dendrons (donors) and interactive binding sites of the copolymer chains (acceptors) plays a pivotal role in generating a spontaneous symmetry breaking through tailored donor–acceptor matching via hydrogen bonding. The focal asymmetry, on a scale of several nanometers, propagates to helical twists of the achiral DJBCP hierarchical assemblies over hundreds of nanometers, such that the primary molecular asymmetry sequentially triggers a secondary helical conformation of the dendron-jacketed blocks, then tertiary supramolecular helical assemblies and eventually quaternary superhelices. The helical twisting power of the superhelices is tunable using the dendron grafting ratio, indicating that the magnitude of helical twisting can be quantitatively adjusted from local to entire structures.

A n-Cu2O layer formed a high-quality buried junction with p-Cu2O to increase the photovoltage and thus to shift the turn-on voltage positively. Mott−Schottky measurements confirmed that the improvement benefited from a thermodynamic shift of the flat-band potentials. The obtained extremely positive onset potential, 0.8 V (vs. RHE) in n-Cu2O/AuAg/p-Cu2O, is comparable with measurements from water-reduction catalysts. The AuAg alloy sandwiched between the homojunction of n-Cu2O and p-Cu2O improved the photocatalytic performance. This alloy both served as an electron relay and promoted electron-hole pair generation in nearby semiconductors; the charge transfer between n-Cu2O and p-Cu2O in the sandwich structure was measured with X-ray absorption spectra. Acting as a plasmonic photosensitizer, the alloy increased the efficiency of conversion of solar energy to electric current. The mechanism of plasmonic energy transfer involved a direct transfer of the plasmonic hot carriers; the interfacial Schottky barrier height modulated the plasmonic hot electron transfer. This facile sandwich structure combines both electrical and optical functions of alloy nanoparticles into a single structure, which has implications for the design of efficient devices to harvest solar energy.

We study the percolation problem in a binary phase-separating polymer mixture. By well-designed experiments, we can delineate the percolation line on the phase diagram with sufficient accuracy. Our experiments show that the percolation thresholds start from the random percolation limit (Φ ∼ 0.15) located near spinodal point at T → T_c and then converge toward the geometric coalescence limit (Φ ∼ 0.36) with an increase in the quench depth. This apparent percolation difficulty comes about largely from the Rayleigh instability accompanied by large-amplitude, short-wavelength fluctuations during the spinodal decomposition at deeper quench depth. As a result, the broken “rigid” domains tend to pack closely, and the so-called droplet spinodal decomposition occurs. On the other hand, we observe that, between the selectively attractive walls, the surface-drying percolating phase will break up into droplets prematurely, thereby shifting its percolation line rather considerably. To our knowledge, such an effect is not yet predicted by theory or simulation.

Fibrillar networks and spherulite assemblies are the two most frequently observed textures in weak gelation of crystallizable linear polymers. We find such two textures in response to the kinetic distinction between instability/spinodal and metastability/nucleation of the polymer crystallization and prove the morphogenetic transition in between. Moreover, it comes as a surprise when such a transition exhibits a spinodal singularity that reveals a mean-field-like “mesoscopic” phase transition behavior.