Ensemble electron spin resonance of monolayers of molecular spins on surface
ORAL · Invited
Abstract
Recent advances in scanning tunnelling microscope-electron resonance spectroscopy (STM-ESR) have established atomic and molecular spins on surfaces as basic qubit systems for potential quantum information science and sensing technologies [1]. To complement these single spin techniques, we have recently developed a novel home-built ensemble electron spin resonance (ESR) spectrometer operating under ultra-high vacuum (UHV) environment [2].
A key element of our spectrometer is the in-house fabricated microstrip line resonator designed for X-band having a Cu(111) single crystal surface finish, which allows performing ESR characterization of in-situ prepared surface spins in a clean and well-defined environment. The resonator exhibits a moderate quality factor of a few hundreds, which allows us to perform both continuous-wave (CW) and pulsed measurement, down to 10 K with a flow cryostat.
Using this spectrometer, we demonstrate the detection sensitivity down to a monolayer (ML) of yttrium bis-phthalocyanine (YPc2) molecules. Combining our ESR characterization with STM, we investigate the morphology, electronic structure, and spin properties of one to multiple MLs of YPc2 on Cu(111), as well as on decoupling layers of zinc phthalocyanine (ZnPc). Our measurements reveal an unexpected paramagnetic spin quenching from multiple MLs of YPc2 when the first layer is in direct contact with Cu(111), while the spins from YPc2 are preserved when decoupled from Cu(111) with more than 5 ML of diamagnetic ZnPc. Density functional theory (DFT) simulations attribute this quenching to the combination of electron delocalization in π-conjugated systems and strong hybridization between YPc2 frontier orbitals and Cu(111) conduction electrons [3].
These results establish our home-built ESR platform as a sensitive tool for probing spin physics in molecular layers and heterostructures, and they highlight the critical role of interfacial hybridization in preserving surface-adsorbed molecular spins. Ongoing work explores alternative crystalline finishes and broader classes of paramagnetic molecules.
[1] Adv. Mater. 35, 2107534 (2022)
[2] Rev. Sci. Instrum. 95, 063904 (2024)
[3] Nanoscale 17, 22163 (2025)
A key element of our spectrometer is the in-house fabricated microstrip line resonator designed for X-band having a Cu(111) single crystal surface finish, which allows performing ESR characterization of in-situ prepared surface spins in a clean and well-defined environment. The resonator exhibits a moderate quality factor of a few hundreds, which allows us to perform both continuous-wave (CW) and pulsed measurement, down to 10 K with a flow cryostat.
Using this spectrometer, we demonstrate the detection sensitivity down to a monolayer (ML) of yttrium bis-phthalocyanine (YPc2) molecules. Combining our ESR characterization with STM, we investigate the morphology, electronic structure, and spin properties of one to multiple MLs of YPc2 on Cu(111), as well as on decoupling layers of zinc phthalocyanine (ZnPc). Our measurements reveal an unexpected paramagnetic spin quenching from multiple MLs of YPc2 when the first layer is in direct contact with Cu(111), while the spins from YPc2 are preserved when decoupled from Cu(111) with more than 5 ML of diamagnetic ZnPc. Density functional theory (DFT) simulations attribute this quenching to the combination of electron delocalization in π-conjugated systems and strong hybridization between YPc2 frontier orbitals and Cu(111) conduction electrons [3].
These results establish our home-built ESR platform as a sensitive tool for probing spin physics in molecular layers and heterostructures, and they highlight the critical role of interfacial hybridization in preserving surface-adsorbed molecular spins. Ongoing work explores alternative crystalline finishes and broader classes of paramagnetic molecules.
[1] Adv. Mater. 35, 2107534 (2022)
[2] Rev. Sci. Instrum. 95, 063904 (2024)
[3] Nanoscale 17, 22163 (2025)
*This work was supported by the Institute for Basic Science (Grant No. IBS-R027-D1).
–
Publication: 1. F. H. Cho et al., Rev. Sci. Instrum. 95, 063904 (2024)
2. S. Oh, F. H. Cho et al., Nanoscale 17, 22163 (2025)
Presenters
-
Franklin H Cho
- Center for Quantum Nanoscience, Ewha Womans University