Polyoxometalates (POMs) containing localized spins have potential as molecular qubits for quantum computing applications. POMs may accommodate magnetic atoms such as V in their structures, resulting in novel molecules with promising magneto/electro-optical properties. However, for practical applications, molecular qubits need to be prepared in optically-addressable arrays which imposes unavoidable interactions with underlying supports and adjacent POMs. In particular, spin-lattice (phonon) coupling is an influential decoherence mechanism that remains insufficiently understood for supported molecular qubits.<br/>Herein, we synthesized POMs containing different numbers of V atoms and transferred them into the gas phase using electrospray ionization. Ion soft landing, a versatile surface modification technique, was used to deliver mass-selected POMs with predetermined V-composition to different self-assembled monolayer surfaces (SAMs) with controlled coverage. Conventional alkylthiol (HSAM), hydrophobic perfluorinated alkylthiol (FSAM), and hydrophilic carboxylic-acid (COOH-SAM) surfaces on gold were selected as well-defined model supports with which to characterize POM-substrate and POM-POM interactions. Infrared reflection absorption spectroscopy (IRRAS), scattering-type scanning near-field optical microscopy (s-SNOM), and density functional (DFT) theoretical calculations provide insight into the vibrational modes of supported POMs, how they are influenced by V-doping, and how they are perturbed by interaction with the HSAM, FSAM, and COOH-SAM surfaces. Spatially-resolved s-SNOM results also reveal the VPOM distribution on the SAM supports and the effect of surface coverage on the POM-SAM and POM-POM interactions. The electronic properties of the bare POMs are determined by negative ion photoelectron spectroscopy (NIPES) and cyclic voltammetry (CV), while the supported POMs are investigated by electrostatic force microscopy (EFM) and scanning Kelvin probe microscopy (KPFM). Our results provide fundamental insight into how POM-support and POM-POM interactions influence the vibrations of supported molecular qubits, which is central to increasing their coherence times for eventual applications.

Influence of substrate and intermolecular interactions on the vibrations of supported polyoxometalate spin qubits

ACS Spring 2022

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