Electron tomography of prolamellar bodies and their transformation into grana thylakoids in cryofixed Arabidopsis cotyledons.

The para-crystalline structures of prolamellar bodies (PLBs) and light-induced etioplast-to-chloroplast transformation have been investigated via electron microscopy. However, such studies suffer from chemical fixation artifacts and limited volumes of 3D reconstruction. Here, we examined Arabidopsis thaliana cotyledon cells by electron tomography (ET) to visualize etioplasts and their conversion into chloroplasts. We employed scanning transmission ET to image large volumes and high-pressure freezing to improve sample preservation. PLB tubules were arranged in a zinc blende-type lattice-like carbon atoms in diamonds. Within 2 h after illumination, the lattice collapsed from the PLB exterior and the disorganized tubules merged to form thylakoid sheets (pre-granal thylakoids), which folded and overlapped with each other to create grana stacks. Since the nascent pre-granal thylakoids contained curved membranes in their tips, we examined the expression and localization of CURT1 (CURVATURE THYLAKOID1) proteins. CURT1A transcripts were most abundant in de-etiolating cotyledon samples, and CURT1A was concentrated at the PLB periphery. In curt1a etioplasts, PLB-associated thylakoids were swollen and failed to form grana stacks. In contrast, PLBs had cracks in their lattices in curt1c etioplasts. Our data provide evidence that CURT1A is required for pre-granal thylakoid assembly from PLB tubules during de-etiolation, while CURT1C contributes to cubic crystal growth in the dark.

Diffusivity and band offset analysis of a graphene interlayer at the back contact of a copper zinc tin sulphide solar cell.

Cu2ZnSnS4 (CZTS) is a promising 3rd generation solar cell absorber based on earth-abundant and nontoxic elements. However, the formation of detrimental MoS2 in the Mo/CZTS back contact interface hinders the overall performance due to poor band alignment in the back contact and a degraded interface. We propose that graphene can be a suitable candidate for the protective interlayer at the back contact to prevent the formation of amorphous MoS2 by blocking S diffusion. Using first principles calculations, we investigated the kinetics processes of S atom diffusion on and across the graphene plane with various defects considered. It was found that while an S atom can easily diffuse on graphene with a diffusion barrier of 0.355 eV, it is hard to diffuse across the graphene plane with or without defects, with diffusion barriers ranging from 1.19 eV through a double vacancy to 8.66 eV across pristine graphene. In addition, a band offset calculation using a local potential alignment method was performed to understand the role of graphene in hole transport. We discover that the interpolation of Hartree potential data can largely improve the stability of the band offset algorithm, comparable to a core level alignment method. The band offset calculation results show that the Fermi level of graphene is 0.664 eV higher than the valence band maximum of CZTS. Therefore, graphene is a benign interlayer in the back contact that can facilitate hole collection. Our calculations suggest that graphene is a promising protective interlayer.