Regulation of quantum spin conversions in a single molecular radical

In this work, we present real-time accurate detection of quantum spin conversions of a single molecular radical and realize their precise regulation by temperature, electric field, and magnetic field, using a sensitive single-molecule electrical method.
Published in Materials
Regulation of quantum spin conversions in a single molecular radical

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Spin is an intrinsic property of particles such as electrons. The development of devices based on logic operations and devices that use electronic spins to store and process information has promoted the innovation of electronic information technologies. In recent decades, with the advancement of experimental techniques, the research on the detection and regulation of spin states is gradually moving from the macroscopic to the nanoscale, and even to the single-spin level, which holds significant implications for practical applications of quantum spins. However, precise detection and regulation of single spins remain formidable challenges.

The single-molecule electrical platform holds distinct advantages in the research field of detecting and regulating spin states at the single-molecule level. Different from macroscopic statistics, the real-time feedback based on the conductance signals of individual molecules can intuitively offer a complete depiction of spin dynamics and simplify complex structural analysis.

In this work, an individual naphtho[1,2-c:5,6-c]bis([1,2,5]thiadiazole) derivative NTCPhN was sandwiched between nanogapped graphene electrodes via covalent amide bonds to construct stable graphene-molecule-graphene single-molecule junctions. At a temperature of 2 K, the current stability diagram of the device proved a single-electron transport behavior, and the inelastic electron tunneling spectroscopy was in excellent consistence with infrared and Raman spectra simulated theoretically. These results collectively proved the successful construction of a single radical molecular device.

Firstly, variable-temperature magnetism measurements of the bulky sample confirmed the formation of the open-shell singlet ground state and demonstrated thermally accessible triplet state characteristics of NTCPhN, with a relatively low singlet-triplet energy gap. Real-time current (I–t) traces of the device at different temperatures revealed three distinct conductance states, with the proportion of these states (reflecting the stability of the structure) varying with temperature. By combining the calculated transmission spectra and the structural stability order, we assigned the three conductance states to the closed-shell singlet, open-shell singlet and triplet electronic structures of NTCPhN. The lifetimes of spin states and the activation energies of state conversions were fitted based on the variable-temperature I–t data, revealing that an increase in temperature facilitates the conversion from closed-shell to open-shell states, in particular towards the open-shell triplet state.

The nanometer-scale gaps between graphene electrodes ensure a sufficiently strong external electric field (EEF). Employing the concept of electric field catalysis, we conducted It measurements on the device under different bias voltages, leading to the manipulation of the stability of singlet and triplet structures of NTCPhN using electrostatic fields. Experimental and theoretical findings consistently illustrated that the EEF can effectively reduce the potential energy difference between the closed-shell singlet and open-shell triplet states, thus facilitating the conversion from closed-shell singlet to open-shell triplet states.

Finally, we investigated the impact of the magnetic field on spin state conversions of a single NTCPhN. At low temperatures, the device exhibits a positive magnetoresistance (MR) effect, while both the control closed-shell molecules with similar structures and the incompletely cut graphene ribbons showed no significant MR. Furthermore, It measurements were performed at different magnetic fields, and the Gibbs free energy of activating spin state conversions was fitted. The results indicated that an enhanced magnetic field can promote the conversion from closed-shell to open-shell triplet states, while suppressing the conversion from closed-shell to open-shell singlet states.

Our findings demonstrated enormous potentials of single-molecule electrical methods in direct detection and regulation of spin states of radical molecules. By utilizing external stimuli such as temperature, bias voltages, and magnetic fields, the spin conversions can be efficiently regulated, providing a clear elucidation of the thermodynamic and kinetic principles governing the evolution of electronic structures. This novel strategy not only expands the detection capabilities of open-shell species, but also simplifies the complex structural analysis required for the macroscopic detection. More importantly, with further efforts to achieve stable manipulation of quantum spin states at room temperature, it will provide crucial CMOS-compatible technological supports for the development of practical molecular spin-based quantum information systems.

For more details, please refer to our paper “Regulation of quantum spin conversions in a single molecular radical” in Nature Nanotechnology (2024).

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