Epitaxial growth of atomically thin two-dimensional crystals such as transition metal dichalcogenides remains challenging, especially for producing large-size transition metal dichalcogenides bilayer crystals featuring high density of states, carrier mobility and stability at room temperature. Here we achieve in epitaxial growth of the second monolayer from the first monolayer by reverse-flow chemical vapor epitaxy and produce high-quality, large-size transition metal dichalcogenides bilayer crystals with high yield, control, and reliability. Customized temperature profiles and reverse gas flow help activate the first layer without introducing new nucleation centers leading to near-defect-free epitaxial growth of the second layer from the existing nucleation centers. A series of bilayer crystals including MoS₂ and WS₂, ternary Mo_1−xW_xS₂ and quaternary Mo_1−xW_xS_2(1−y)Se_2y are synthesized with variable structural configurations and tunable electronic and optical properties. The robust, potentially universal approach for the synthesis of large-size transition metal dichalcogenides bilayer single crystals is highly-promising for fundamental studies and technological applications.

In order to further improve cell efficiency and reduce cost in achieving grid parity, a large number of PV manufacturing companies, universities and research institutes have been devoted to a variety of low-cost and high-efficiency crystalline Si solar cells. In this article, the cell structures, characteristics and efficiency progresses of several types of high-efficiency crystalline Si solar cells that have been in small scale production or are promising in mass production are presented, including passivated emitter rear cell, tunnel oxide passivated contact solar cell, interdigitated back contact cell, heterojunction with intrinsic thin-layer cell, and heterojunction solar cells with interdigitated back contacts. Both the industrialization status and future development trend of high-efficiency crystalline silicon solar cells are also pinpointed.

Transition from multi-layer to monolayer and sub-monolayer thickness leads to the many exotic properties and distinctive applications of two-dimensional (2D) MoS₂. This transition requires atomic-layer-precision thinning of bulk MoS₂without damaging the remaining layers, which presently remains elusive. Here we report a soft, selective and high-throughput atomic-layer-precision etching of MoS₂in SF₆+ N₂plasmas with low-energy (<0.4 eV) electrons and minimized ion-bombardment-related damage. Equal numbers of MoS₂layers are removed uniformly across domains with vastly different initial thickness, without affecting the underlying SiO₂substrate and the remaining MoS₂layers. The etching rates can be tuned to achieve complete MoS₂removal and any desired number of MoS₂layers including monolayer. Layer-dependent vibrational and photoluminescence spectra of the etched MoS₂are also demonstrated. This soft plasma etching technique is versatile, scalable, compatible with the semiconductor manufacturing processes and may be applicable for a broader range of 2D materials and intended device applications.

Plasma processing is effective in diverse modifications of nanoscale 2D-TMDC materials, owing to its uniquely controllable, effective and clean characteristics.