Wafer-scale hBN crystalline films: Although wafer-scale polycrystalline films of insulating hexagonal boron nitride (hBN) can be grown, the grain boundaries can cause both scattering or pinning of charge carriers in adjacent conducting layers that impair device performance. Lee et al. grew wafer-scale single-crystal films of hBN by feeding the precursors into molten gold films on tungsten substrates. The low solubility of boron and nitrogen in gold caused micrometer-scale grains of hBN to form that coalesced into single crystals. These films in turn supported the growth of epitaxial wafer-scale films of graphene and tungsten disulfide. Science , this issue p. 817

The effective application of graphene and other 2D materials is strongly dependent on the industrial-scale manufacturing of films and powders of appropriate morphology and quality. Here, we discuss three state-of-the-art mass production techniques, their limitations, and opportunities for future improvement. The industrial application of two-dimensional (2D) materials strongly depends on the large-scale manufacturing of high-quality 2D films and powders. Here, the authors analyze three state-of-the art mass production techniques, discussing the recent progress and remaining challenges for future improvements.

We investigated the layer-number-dependent work function of MoS₂ nanoflakes by using Kelvin probe force microscopy (KPFM) to measure the surface potential. The work functions of as-prepared 1- to 6-layer MoS₂ nanoflakes were 5.15–5.39 eV and increased with increasing layer number. After annealing, the work functions of the nanoflakes decreased to 0.1–0.2 eV due to elimination of absorbed molecules on the surface. However, the work function of the edge region of the annealed flakes was relatively larger than that of the internal region. The charge carrier trapping by adsorbed molecules due to the polarity and the hydrophilicity of MoS₂ may cause a reduction in the work function of the annealed flakes compared with that for MoS₂ exposed to air. The dependence of the obtained work function of MoS₂ nanoflakes on the number of layers is essential to the formation of metal contacts for fabricating future MoS2-based devices.

Understanding and controlling the nanoscale transport of excitonic quasiparticles in atomically thin two-dimensional (2D) semiconductors are crucial to produce highly efficient nano-excitonic devices. Here, we present a nanogap device to selectively confine excitons or trions of 2D transition metal dichalcogenides at the nanoscale, facilitated by the drift-dominant exciton funneling into the strain-induced local spot. We investigate the spatiospectral characteristics of the funneled excitons in a WSe ₂ monolayer (ML) and converted trions in a MoS ₂ ML using hyperspectral tip-enhanced photoluminescence imaging with <15-nm spatial resolution. In addition, we dynamically control the exciton funneling and trion conversion rate by the gigapascal-scale tip pressure engineering. Through a drift-diffusion model, we confirm an exciton funneling efficiency of ∼25% with a significantly low strain threshold (∼0.1%), which sufficiently exceeds the efficiency of ∼3% in previous studies. This work provides a previously unexplored strategy to facilitate efficient exciton transport and trion conversion of 2D semiconductor devices.