Energy-Transfer-Enhanced Emission and Quantum Sensing of VB- Defects in hBN-PbI2 Heterostructures

Spin defects in two-dimensional materials hold significant potential for quantum information technologies and sensing applications. The negatively charged boron vacancy (VB-) in hexagonal boron nitride (hBN) has attracted considerable attention as a quantum sensor due to its demonstrated sensitivity to temperature, magnetic fields, and pressure.1 However, its applications have thus far been limited by inherently dim photoluminescence (PL). By fabricating a van der Waals heterostructure with a sensitizing donor layer, lead iodide (PbI2), we effectively enhance the PL intensity from the VB- by 5-45x, while maintaining compatibility with other heterostructures and vdW optoelectronic platforms. The type-I band alignment at the heterojunction enables efficient exciton migration while suppressing back-electron transfer, and the strong spectral overlap between the PbI2 emission and defect absorption supports efficient fluorescence resonance energy transfer. Ab initio density functional theory (DFT) predicts a photon-ratcheting mechanism that boosts absorption and emission while maintaining magnetic resonance (ODMR) contrast through minimal hybridization. Experimentally, the heterostructure exhibits enhanced continuous-wave ODMR sensitivity and functions as a precise probe of external magnetic fields. This work establishes a proof-of-concept for amplifying weak defect signals in nanomaterials, highlighting a new strategy for engineering their optical and magnetic responses.

💡 Research Summary

In this work the authors address the long‑standing limitation of the negatively charged boron vacancy (V_B^–) in hexagonal boron nitride (hBN), namely its intrinsically weak photoluminescence (PL) that hampers high‑sensitivity quantum sensing. By integrating a thin lead‑iodide (PbI₂) layer beneath a neutron‑irradiated hBN layer that hosts V_B^– defects, they construct a four‑layer van der Waals heterostructure (hBN cap / hBN‑V_B^– / PbI₂ / hBN cap). Low‑temperature (4 K) photoluminescence mapping reveals a pronounced quenching of the PbI₂ free‑exciton emission (≈510 nm) precisely where it overlaps with the V_B^– emission region (≈780–820 nm). Simultaneously, the V_B^– PL intensity is amplified by a factor of 5 to 45, depending on excitation wavelength and local thickness, as quantified by differential intensity analysis that removes contributions from the PbI₂ self‑trapped exciton background.

The enhancement is attributed to efficient energy transfer from PbI₂ to the V_B^– defect. The authors demonstrate strong spectral overlap between the PbI₂ emission and the V_B^– absorption, enabling Förster‑type resonance energy transfer (FRET). Moreover, density‑functional theory (DFT) calculations show negligible electronic hybridisation between the two layers, preserving the defect’s spin density distribution and optical dipole moment. Consequently, PbI₂ acts as a “photon ratchet”: it absorbs a high‑energy photon (ω₁), emits a lower‑energy infrared photon (ω₂), and transfers the remaining energy (Ω = ω₁ − ω₂) to the V_B^– centre, which then radiates at its characteristic wavelength. This two‑photon nonlinear process is described by a second‑order susceptibility χ^(2) term and predicts roughly an order‑of‑magnitude PL boost, in line with the experimental 5–45× enhancement.

Crucially, the amplified PL does not degrade the quantum‑sensor functionality. Continuous‑wave optically detected magnetic resonance (cw‑ODMR) measurements show a markedly improved signal‑to‑noise ratio, allowing precise detection of external magnetic fields. The ODMR contrast and spin coherence times (T₂) remain essentially unchanged, confirming that the photon‑ratchet mechanism enhances optical absorption and emission without perturbing the defect’s spin Hamiltonian.

Structural analysis via high‑resolution transmission electron microscopy reveals nanoscale inhomogeneities in the PbI₂ layer, giving rise to a broad self‑trapped exciton (STE) emission (650–750 nm) that partially overlaps the V_B^– band. The authors carefully subtract this background to avoid overestimating the enhancement. Differential reflectance measurements further corroborate increased absorption in the heterostructure across energies below the V_B^– emission, especially above the PbI₂ bandgap, consistent with exciton renormalisation induced by the nearby hBN layer.

Overall, the paper establishes three intertwined design principles for boosting weak defect signals in two‑dimensional materials: (1) a type‑I band alignment that suppresses back‑electron transfer, (2) spectral overlap that enables efficient FRET, and (3) a nonlinear photon‑ratchet process that funnels excitation energy into the defect. By demonstrating a scalable, fabrication‑friendly van der Waals stack that works at both cryogenic and room temperature, the authors provide a proof‑of‑concept for engineering bright, high‑sensitivity quantum sensors based on 2D spin defects. The approach is readily extendable to other 2D hosts and defect species, opening a pathway toward integrated quantum photonic‑spin platforms for sensing, information processing, and optoelectronic applications.