Sensing the Spin State of Room-Temperature Switchable Cyanometallate Frameworks with Nitrogen-Vacancy Centers in Nanodiamonds

Abstract

Room-temperature magnetically switchable materials play a vital role in current and upcoming quantum technologies, such as spintronics, molecular switches, and data storage devices. The increasing miniaturization of device architectures produces a need to develop analytical tools capable of precisely probing spin information at the single-particle level. In this work, we demonstrate a methodology using negatively charged nitrogen vacancies (NV − ) in fluorescent nanodiamond (FND) particles to probe the magnetic switching of a spin crossover (SCO) metal−organic framework (MOF), [Fe(1, , where X = Eu and Dy (materials 2a and 2b, respectively), in response to heat, light, and electron beam exposure. We employ correlative light−electron microscopy using transmission electron microscopy (TEM) finder grids to accurately image and sense spin−spin interacting particles down to the single-particle level. We used surfacesensitive optically detected magnetic resonance (ODMR) and magnetic modulation (MM) of FND photoluminescence (PL) to sense spins to a distance of ca. 10−30 nm. We show that ODMR and MM sensing was not sensitive to the temperatureinduced SCO of Fe II in 1 as formation of paramagnetic Fe III through surface oxidation (detected by X-ray photoelectron spectroscopy) on heating obscured the signal of bulk SCO switching. We found that proximal FNDs could effectively sense the chemical transformations induced by the 200 keV electron beam in 1, namely, Ag I → Ag 0 and Fe II → Fe III . However, transformations induced by the electron beam are irreversible as they substantially disrupt the structure of MOF particles. Finally, we demonstrate NV − sensing of reversible photomagnetic switching, Fe III + (18-crown-6) ⇆ Fe II + (18-crown-6) + • , triggered in 2a and 2b by 405 nm light. The photoredox process of 2a and 2b proved to be the best candidate for roomtemperature single-particle magnetic switching utilizing FNDs as a sensor, which could have applications into next-generation quantum technologies.

Supramolecular Systems Linked to Metal‐Organic Frameworks

“…Negatively charged nitrogen-containing chemical entities have been exploited for the magnetic switching of a MOF structure composed of Fe Ag cyanoheterometallic [Fe(1,6naphthyridine)2(Ag(CN)2)2]. The presence of paramagnetic Fe III either as low-spin (LS, S = 0, no unpaired d-orbital electrons) and high-spin (HS, S = 2, 4 unpaired d-orbital electrons) states impacts in the detectability of nitrogen vacancy (NV -) in the form of optically detected magnetic resonance (ODMR) and magnetic modulation (MM) [98].…”