WSe2 bilayers reveal quantum photocurrent linked to single dopants

Researchers have, for the first time, directly visualized photocurrent hotspots centered on single dopants within bilayer tungsten diselenide, revealing a photovoltaic response at the nanometer scale. Using photoconductive atomic force microscopy, the team observed these nanometer-scale currents with opposite polarities depending on the dopant’s layer. Vertical WSe2/V:WSe2 homobilayer devices exhibited a macroscopic photocurrent that scales linearly with dopant concentration and a compensation voltage that is independent of illumination power and dopant density, a result contrasting with traditional bulk homojunction devices. These results establish dopant-defined point-like junctions as the elementary photovoltaic units in atomically thin homobilayers.

Vanadium-Doped WSe2 Bilayers Exhibit Point-Like Photovoltaic Junctions

Vanadium-doped tungsten diselenide (WSe2) bilayers exhibit localized photovoltaic responses centered on individual dopant atoms, a finding that redefines the scale at which light can be converted into electricity in two-dimensional semiconductors. Using photoconductive atomic force microscopy, researchers directly visualized these nanometer-scale photocurrent hotspots, revealing a photovoltaic effect previously thought to occur only across larger junctions. The study demonstrates a linear relationship between macroscopic photocurrent and dopant concentration in vertical WSe2/V:WSe2 homobilayer devices, a departure from the behavior observed in traditional bulk homojunctions.

This scaling suggests a fundamentally different mechanism for charge carrier generation and separation at the nanoscale, where conventional diffusion and depletion lengths are no longer applicable. The devices also exhibit a compensation voltage that remains constant regardless of both illumination power and dopant density. Photocurrent spectroscopy and modeling indicate that the vanadium dopants locally transform tightly bound excitons, electron-hole pairs, into separated charge states between the layers.

This process occurs within a remarkably confined region of approximately one nanometer, enabling efficient exciton dissociation. The team’s work builds on previous investigations into strong light-matter interactions in atomically thin heterostructures and expands the understanding of dimensionality-inhibited chemical doping in two-dimensional semiconductors. Funding for the research came from the Ministry of Education, Singapore, under AcRF Tier 2 (T2EP50124-0025) and Tier 1 (A-8001995-00-00), as well as the NRF-T-CRP-2025-0007 grant.

Photoconductive AFM Visualizes Nanometer-Scale Current Hotspots Around Single Dopants

Photoconductive atomic force microscopy has revealed that individual dopants within two-dimensional semiconductors can function as discrete photovoltaic junctions. This visualization indicates that the photovoltaic effect is not solely a bulk property, but can originate from the activity of single atoms incorporated into the semiconductor lattice. These photocurrent hotspots, centered on individual dopants, exhibit opposite current polarities depending on whether the dopant resides in the top or bottom layer of the bilayer structure.

WSe2/V:WSe2 Homobilayers Demonstrate Linear Photocurrent & Compensation Voltage

Researchers at the National University of Singapore have demonstrated a surprising photovoltaic mechanism within layered tungsten diselenide, revealing that individual dopant atoms can act as discrete junctions for converting light into electrical current. The work, detailed in Nature Nanotechnology, utilized photoconductive atomic force microscopy to visualize photocurrent originating from single vanadium dopants embedded within a WSe₂ bilayer. This direct observation challenges the conventional understanding of photocurrent generation, typically considered a bulk property, by pinpointing its origin at the nanometer scale.

This scaling behavior diverges from traditional bulk homojunction devices, suggesting a fundamentally different process governs charge separation in these atomically thin materials. As the researchers report, this counterintuitive result challenges established models of photovoltaic behavior.

Exciton Dissociation via Dopants Enables Efficient Charge Separation in Bilayers

The ability to harness light energy at the nanoscale depends on understanding how charge carriers separate within materials; recent work with tungsten diselenide bilayers demonstrates that this process can occur remarkably efficiently, localized to individual dopant atoms. Investigations into vertical homobilayer devices revealed a fundamental shift in how photocurrent is generated, moving away from traditional models of bulk behavior.

The data supporting the findings of this study are available within the article and its Supplementary Information. This level of control over charge separation at the atomic scale opens possibilities for designing novel, highly efficient nanoscale optoelectronic devices, potentially exceeding the limitations of current bulk materials.