Conduction/Valence Band Structures of 2D Semiconductors: Charge Carrier Induced Renormalization in Mono- and Bilayers

Two-dimensional transition-metal dichalcogenides (TMDs) such as MoS₂, MoSe₂, WS₂, and WSe₂ are key candidates for next-generation electronics, photonics, and quantum devices. Monolayer TMDs possess direct bandgaps and unique valley degrees of freedom, while bilayers enable bandgap tuning and excitonic phases under applied fields. The most fundamental descriptor of their opto-electronic behavior is the energy–momentum dispersion. While valence band structures are well accessible by angle-resolved photoemission spectroscopy (ARPES), conduction bands (CBs) — critical for electron transport, n-type doping, and optoelectronic transitions — remain experimentally unexplored, with existing knowledge largely based on theory or indirect optical measurements.
Recently, the Yoshida group (Chiba University) developed angle-resolved low-energy inverse photoelectron spectroscopy (AR-LEIPS), enabling the first direct CB mapping in organic semiconductors and halide perovskites. This project unites Humboldt-Universität's expertise in large-area TMD sample fabrication and microscopic ARPES with Chiba University’s advanced AR-LEIPS capabilities to deliver the first direct CB band-structure maps for mono- and bilayer TMDs.
Furthermore, TMD layers will be placed on substrates spanning a wide work-function range (≈2.3–5.5 eV) to allow charge-neutral measurements and a systematic analysis of substrate-induced doping. After quality checks in Berlin, samples will be shipped under inert conditions to Chiba for optimized surface preparation and AR-LEIPS measurements, yielding CB dispersions, effective masses, and bandgaps.
The collaboration will provide benchmark datasets for theory and device design. Reciprocal research stays, joint data analysis, and joint publications will train young researchers, strengthen the long-term Japan–Germany cooperation, and establish a transferable methodology for future 2D materials research.