Confined growth of armchair MoS2 nanotubes at the 1-nm limit.

Atomically thin nanotubes of semiconducting transition metal dichalcogenides offer a platform for exploring quantum phenomena at the one-dimensional limit and for realizing nanoscale transistor channels. However, conventional syntheses produce only large-diameter (>10 nm), multiwalled tubes with uncontrolled chiralities. We report the synthesis of single-walled molybdenum disulfide (MoS2) nanotubes with diameters approaching 1 nm, achieved through spatially confined reactions inside boron nitride (BN) nanotubes. The confined geometry stabilizes otherwise inaccessible, highly strained MoS2 nanotubes, yielding structurally well-defined armchair configurations. Their bandgaps shrink systematically with decreasing diameter, in accordance with long-standing theoretical predictions. The insulating BN sheath simultaneously provides an intrinsic gate-all-around architecture, thereby promising access to truly nanoscale transistor channels.