Molecular Spins Achieve 10-Fold Coherence Boost on 2D Surfaces
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Scientists have long sought to control individual molecular spins directly on material surfaces, a capability crucial for advancing nanoscale sensing with atomic resolution. Xuankai Zhou, Yan-Tung Kong, and Cheuk Kit Cheung, all from the 13. Physikalisches Institut at the University of Stuttgart, alongside Guodong Bian from the University of Birmingham, Reda Moukaouine from the Wigner Research Centre for Physics, and King Cho Wong, report a breakthrough in achieving this goal by creating a hybrid molecular-2D architecture. Their research demonstrates stable, optically addressable spins anchored onto hexagonal boron nitride, overcoming limitations imposed by spin stabilisation depth and exhibiting robust performance from 4K to room temperature. Remarkably, the team achieved a Hahn-echo spin coherence time exceeding 100s at 4K , significantly better than in bulk organic crystals , and extended coherence to over 300s using dynamic decoupling, challenging the expectation of surface-induced spin degradation. This innovative approach, offering long coherence, optical addressability and interfacial versatility, paves the way for scalable and adaptable sensing platforms beyond the capabilities of conventional solid-state systems. However, stabilizing these spins has been limited by the finite depth at which they can be maintained. This breakthrough centres on a novel approach to Quantum sensing, utilising a combination of aromatic spin molecules, specifically pentacene, with the two-dimensional material hBN to construct a hybrid surface spin platform. The hBN serves multiple crucial functions, protecting the spin molecules from environmental degradation, suppressing unwanted charge transfer, promoting self-assembly of molecular arrays, and hosting surface defects that interact with and modify the pentacene energy levels. Furthermore, the researchers chemically tuned the molecules through deuteration, improving the coherence time by more than ten-fold. Under dynamic decoupling, coherence was prolonged to the intrinsic lifetime limit, exceeding 300 microseconds. This remarkable enhancement demonstrates the potential for long-lived quantum information storage and processing at the nanoscale. The work opens exciting possibilities for nanoscale magnetic resonance imaging, the direct observation of emergent quantum phases, and the development of novel quantum devices, all achievable with a platform that can be readily integrated into existing technologies. This innovative approach promises to revolutionise quantum sensing by bringing the sensor directly to the sample surface, unlocking unprecedented levels of precision and control.
Molecular Spin Anchoring onto Hexagonal Boron Nitride enables Experiments employed a meticulous approach, utilising photoluminescence spectroscopy to characterise Pc molecules on hBN at both 4 K and room temperature, revealing a characteristic molecular phonon mode denoted as ω0. The study detailed optically detected magnetic resonance (ODMR) spectroscopy to probe spin transitions within the Pc molecules, specifically identifying |Tx⟩, |Ty⟩ and |Ty⟩, |Tz⟩ transitions at 917MHz and 1433MHz respectively. A double-resonance scheme, involving the application of two microwave tones, accessed the |Tx⟩, |Tz⟩ transition, demonstrating precise control over spin manipulation. Researchers calculated spin-density distributions of Pc, revealing a shift towards the hBN plane, which increases zero-field splitting with optical dipole orientation aligned along the molecular Y axis. Polarization-resolved measurements determined the Pc molecular Y-axis orientation, and out-of-plane magnetic field dependence studies confirmed an edge-on configuration of the Pc molecules, evidenced by consistent behaviour of the |Tx⟩, |Ty⟩ and |Ty⟩, |Tz⟩ transitions. Hundreds of Pc ODMR spots were analysed, revealing a threefold symmetry in alignment relative to the underlying hBN lattice, demonstrating precise molecular positioning. To further enhance spin coherence, the team chemically tuned the molecules through deuteration, replacing hydrogen with deuterium, which improved T2 by over ten-fold. Dynamic decoupling pulse sequences, specifically a CPMG sequence, were implemented to protect spin coherence in the |Tx⟩, |Tz⟩ basis, prolonging coherence to the intrinsic lifetime limit, exceeding 300μs.
Molecular Spins Stabilised on Hexagonal Boron Nitride offer Chemical tuning of the molecule via deuteration improved coherence by over 10-fold, and dynamic decoupling extended coherence to exceed 300μs. Continuous-wave optically detected magnetic resonance (ODMR) measurements under ambient conditions revealed clear resonance features matching the energy separation between triplet sublevels, with the |Ty⟩↔|Tz⟩ and |Ty⟩↔|Tx⟩ transitions observed at 1433MHz and 917MHz, respectively. A double-resonance technique uncovered an additional resonance near 2350MHz, confirming access to all triplet sublevels, and the |Ty⟩↔|Tz⟩ transition exhibited a linewidth of approximately 5MHz. Measurements confirm remarkable photostability, enabling cw-ODMR measurements lasting for days under ambient conditions and months at 4 K, significantly exceeding the stability of typical thin-film molecular systems.
The team recorded a fast decay time of 2 hours and a slow decay exceeding two days for photon counts, while the cw-ODMR contrast remained visible throughout the measurement duration. Rabi oscillations were driven at the |Ty⟩↔|Tz⟩ transition, achieving 7% maximum contrast at room temperature, limited by background fluorescence from defective hBN and uncoupled molecules. Furthermore, AFM maps revealed ultrathin Pc clusters with a minimum thickness of 3nm, confirming the stabilisation of Pc spin states at the interface. Analysis of hBN surface conditions showed that heavy O2-plasma bombardment strongly increased the number of ODMR spots, while electron-irradiated samples exhibited only a few, and pristine hBN showed almost none. Researchers obtained zero-field axial and rhombic splitting parameters D and E of 1891MHz and 459MHz at room temperature, respectively, significantly larger than those typically found in organic host matrices. Vector-field mapping of spin transitions unequivocally demonstrated edge-on orientations of Pc molecules on the hBN flakes, corroborated by a Raman signature at 1533cm−1 observed near ODMR-active spots. Pentacene-hBN Interfaces Yield Long-Lived Spin Coherence at Room Scientists have demonstrated optically addressable spin sensors directly at material interfaces by combining pentacene molecules with hexagonal boron nitride (hBN). The authors acknowledge a limitation in potentially enhancing coupling strength by reducing hBN layer thickness, and suggest exploring the vast family of over 10,000 related aromatic molecules to extend the sensor concept. Future work could focus on scaling the molecular quantum system to larger wafers and integrating it with diverse device architectures, potentially entering the exchange-interaction regime with further optimization. 👉 More information 🗞 Optically Addressable Molecular Spins at 2D Surfaces 🧠 ArXiv: https://arxiv.org/abs/2601.19988 Tags: