Investigation of Band-Offsets at Monolayer-Multilayer MoS2 Junctions by Scanning Photocurrent Microscopy

The thickness-dependent band structure of MoS2 implies that discontinuities in energy bands exist at the interface of monolayer (1L) and multilayer (ML) thin films. The characteristics of such heterojunctions are analyzed here using current versus voltage measurements, scanning photocurrent microscopy, and finite element simulations of charge carrier transport. Rectifying I-V curves are consistently observed between contacts on opposite sides of 1L-ML junctions, and a strong bias-dependent photocurrent is observed at the junction. Finite element device simulations with varying carrier concentrations and electron affinities show that a type II band alignment at single layer/multi-layer junctions reproduces both the rectifying electrical characteristics and the photocurrent response under bias. However, the zero-bias junction photocurrent and its energy dependence are not explained by conventional photovoltaic and photothermoelectric mechanisms, indicating the contributions of hot carriers.

💡 Research Summary

The paper investigates the electronic and optoelectronic behavior of junctions formed between monolayer (1L) and multilayer (ML) MoS₂, exploiting the well‑known thickness‑dependent band structure of this transition‑metal dichalcogenide. Because a single layer possesses a direct bandgap of ~1.8 eV while multilayer films have an indirect gap near 1.2 eV, the conduction‑band minimum and valence‑band maximum shift relative to each other, creating an intrinsic heterojunction when the two thicknesses meet.

Device fabrication involved exfoliating MoS₂ onto Si/SiO₂ substrates, patterning Ti/Au contacts so that some devices had both electrodes on the same thickness (either 1L or ML) and others spanned the 1L‑ML interface. Current‑voltage (I‑V) measurements showed almost linear behavior for same‑thickness contacts, but a pronounced rectifying characteristic when the contacts straddled the interface: forward bias (1L positive) yielded a large current, while reverse bias suppressed conduction. This asymmetry already hints at a type‑II band alignment, where electrons preferentially reside in the multilayer region and holes in the monolayer region.

Scanning photocurrent microscopy (SPCM) was then employed using a 532 nm laser. By raster‑scanning the beam across the device while sweeping the external bias, the authors mapped the spatial distribution of photocurrent. A strong, bias‑dependent photocurrent peak appeared precisely at the 1L‑ML boundary. Under forward bias the photocurrent increased dramatically; under reverse bias it nearly vanished. Spectral scans revealed a sharp rise near the 1.8 eV photon energy (consistent with the monolayer direct gap) but also a non‑negligible response at lower energies, indicating that conventional photovoltaic generation alone could not account for the observed zero‑bias signal.

To interpret these findings, the authors performed finite‑element simulations (COMSOL) of the device. The model treated the 1L and ML regions as separate 2D semiconductors with distinct electron affinities (χ) and doping concentrations. By setting χ₁L ≈ 4.0 eV and χ_ML ≈ 4.4 eV, the simulated band diagram displayed a type‑II offset: the conduction band of the multilayer lies below that of the monolayer, while the valence band of the monolayer sits above that of the multilayer. Carrier concentrations on the order of 10¹⁶ cm⁻³ were used to reproduce realistic carrier diffusion lengths. The simulated I‑V curves reproduced the experimentally observed rectification, and the bias‑dependent photocurrent distribution matched the SPCM maps when the type‑II alignment was assumed.

However, the zero‑bias photocurrent could not be reproduced by standard photovoltaic or photothermoelectric mechanisms in the model. The authors therefore propose that hot carriers generated by above‑bandgap photons contribute significantly. Hot electrons (or holes) retain excess kinetic energy before thermalizing, and the strong built‑in electric field at the type‑II interface can separate these carriers before they lose energy, yielding a measurable current even without external bias. This hypothesis is supported by the observed photocurrent persisting at photon energies below the monolayer direct gap, where only hot‑carrier processes could generate a response.

In summary, the work demonstrates that monolayer‑multilayer MoS₂ junctions naturally form type‑II heterojunctions, leading to rectifying electrical behavior and a bias‑tunable photocurrent localized at the interface. Moreover, the presence of a sizable zero‑bias photocurrent that cannot be explained by conventional mechanisms points to hot‑carrier dynamics as an additional, potentially exploitable effect. These insights broaden the understanding of thickness‑engineered 2D heterostructures and suggest new pathways for designing photodetectors, photovoltaics, and hot‑carrier devices based on transition‑metal dichalcogenides.