Transition metal dichalcogenides such as MoS2 and WSe2 are layered semiconductors that remain semiconducting at a single three-atom-thick layer, making them candidate channel materials for post-silicon transistors and a versatile substrate for stacked photonic, ferroelectric and spintronic devices; the electronics case now rests on real scaled devices, while most of the exotic quantum claims remain first-principles theory.
Summary
Transition metal dichalcogenides (TMDs) are compounds of the form MX2, where M is a transition metal (Mo, W, V-alloyed Mo) and X a chalcogen (S, Se, Te). They form covalently bonded triple layers held together by weak van der Waals forces, so single layers can be isolated or grown and then stacked without lattice matching. Monolayers have a direct band gap, strong spin-orbit coupling and inequivalent K/K’ valleys, which is the origin of both the transistor interest and the valleytronics interest. The polytype matters as much as the chemistry: 2H is semiconducting, 1T’ metallic, and 3R stacking breaks inversion symmetry and produces out-of-plane sliding ferroelectricity in bilayers.
Beyond logic, TMDs are being used as a substrate-like platform. Transfer printing without lattice mismatch motivates many-junction photovoltaics, although a conservative TMD gap window of 1.0 to 2.1 eV caps the detailed-balance efficiency near 63.4% at 50 junctions under full concentration, against 84.5% for an unconstrained ladder, with an experimentally plausible five-junction ladder identified. Subwavelength TMD films generate polarisation-entangled photon pairs by spontaneous parametric down-conversion without bulk phase matching, and periodic poling is being used to escape the coherence-length limit on efficiency. Proximity engineering supplies further knobs: a ferromagnetic V2O3 monolayer induces about 10.41 meV of zero-field valley splitting in WSe2, equivalent to roughly 10 T, with a Curie temperature of 500 K in calculation. The deciding parameters across all of this are the same three: growth uniformity and defect density, contact and doping control, and stacking or twist-angle fidelity.
Evidence base
- Monolayer MoS2 nanoribbon transistors narrowed from hundreds of nanometres to ~30-40 nm showed median on-current density up ~42% and subthreshold swing down ~16%, with a champion device at 995 microamps per micron at 1 V drain and 2.5 V overdrive
- A conservative TMD bandgap window of 1.0-2.1 eV limits multijunction efficiency to ~63.4% under full concentration at N=50, versus 84.5% for unconstrained ladders, with radiative quality and optics dominating beyond five junctions
- Ferroelectric switching in CVD-grown 3R-stacked WSe2 bilayers, probed via a graphene ferroelectric FET, is governed by intrinsic structural defects and multi-domain kinetics
- Periodically poled TMDs were investigated to scale native polarisation-entangled photon generation beyond the material coherence length in subwavelength-thick films, without conventional phase matching
- First-principles calculations predict ~10.41 meV spontaneous valley splitting in WSe2 from a kagome V2O3 monolayer, equivalent to ~10 T, with a Curie temperature of 500 K
- RF-sputtered MoTe2 films show a structural crossover near 4.5 nm thickness, metallic 1T’ below and semiconducting 2H above, with orbital-charge conversion detected only in the 1T’ phase
- MoS2 and WS2 flakes were characterised on Si3N4 waveguides using the waveguide’s own broadband defect photoluminescence as an internal probe, with no external source
Open questions
- Do the ~42% on-current and ~16% subthreshold swing gains observed on narrowing MoS2 ribbons to 30-40 nm hold as median statistics across wafer-scale CVD material, rather than in champion devices on transferred flakes?
- Can the disorder and multi-domain kinetics that govern switching in CVD-grown 3R-WSe2 bilayers be suppressed enough for reproducible non-volatile ferroelectric operation?
- Do the calculated zero-field valley splittings (10.41 meV for V2O3/WSe2, 37.3 and 78.2 meV for Mo0.75V0.25Te2) survive experimental realisation at real interfaces, and at what temperature?
- Does periodic poling of TMDs raise SPDC conversion efficiency enough to compete with established phase-matched bulk crystals on brightness, not just on device thickness?
Assessment drafted 2026-08-31 from up to 18 KB sources using the technology-scorecard framework; scores are a draft read pending review.