Oxidation-Induced Structure Transformation: Thin-Film Synthesis and Interface Investigations of Barium Disilicide toward Potential Photovoltaic Applications.

Barium disilicide (BaSi2) has been regarded as a promising absorber material for high-efficiency thin-film solar cells. However, it has confronted issues related to material synthesis and quality control. Here, we fabricate BaSi2 thin films via an industrially applicable sputtering process and uncovered the mechanism of structure transformation. Polycrystalline BaSi2 thin films are obtained through the sputtering process followed by a postannealing treatment. The crystalline quality and phase composition of sputtered BaSi2 are characterized by Raman spectroscopy and X-ray diffraction (XRD). A higher annealing temperature can promote crystallization of BaSi2, but also causes an intensive surface oxidation and BaSi2/SiO2 interfacial diffusion. As a consequence, an inhomogeneous and layered structure of BaSi2 is revealed by Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The thick oxide layer in such an inhomogeneous structure hinders further both optical and electrical characterizations of sputtered BaSi2. The structural transformation process of sputtered BaSi2 films then is studied by the Raman depth-profiling method, and all of the above observations come to an oxidation-induced structure transformation mechanism. It interprets interfacial phenomena including surface oxidation and BaSi2/SiO2 interdiffusion, which lead to the inhomogeneous and layered structure of sputtered BaSi2. The mechanism can also be extended to epitaxial and evaporated BaSi2 films. In addition, a glimpse toward future developments in both material and device levels is presented. Such fundamental knowledge on structural transformations and complex interfacial activities is significant for further quality control and interface engineering on BaSi2 films toward high-efficiency solar cells.

Tuneable fluidics within graphene nanogaps for water purification and energy storage.

Precise control of liquid-solid interactions within sub-micrometer spaces is critical to maximize the active surface areas in porous materials, yet is challenging because of the limited liquid penetration. Here we discover an effective, dry-climate natural plant-inspired approach to guide water into sub-micrometer graphene microwells (Sub-µGWs) and to tune the transition from the hydrophobic to superhydrophilic states. Dry plasma texturing of Sub-µGWs by graphene 'nano-flaps' which adjust the tilt and density upon controlled liquid evaporation leads to controlled and stable sub-micrometer-scale surface modification and variable wettability in a wide range. This effect helps capture Au nanoparticles on the Sub-µGW surfaces as a proof-of-principle water purification platform and tune the charge-storage capacity and frequency response of Sub-µGW-based supercapacitors without altering the Sub-µGW backbones. The outcomes may be extended into diverse materials and solutions thus opening new opportunities for next-generation devices, systems and applications.