Hybrid nanostructures based on gold nanoparticles and functional coordination polymers: Chemistry, physics and applications in biomedicine, catalysis and magnetism

We review the current understanding of the time scale and mechanisms associated with the change in spin state in transition metal-based spin crossover (SCO) molecular complexes. Most time resolved experiments, performed by optical techniques, rely on the intrinsic light-induced switching properties of this class of materials. The optically driven spin state transition can be mediated by a rich interplay of complexities including intermediate states in the spin state transition process, as well as intermolecular interactions, temperature, and strain. We emphasize here that the size reduction down to the nanoscale is essential for designing SCO systems that switch quickly as well as possibly retaining the memory of the light-driven state. We argue that SCO nano-sized systems are the key to device applications where the “write” speed is an important criterion.

https://www.mdpi.com/2079-4991/12/10/1742/pdf?version=1653033823

Dynamics of Spin Crossover Molecular Complexes

<h2>Summary</h2> Bistable molecules represent a potential miniaturization limit for high-density information technologies. However, molecules exhibit memory effect typically at very low temperatures. This is the case of spin-crossover (SCO) complexes, where the concept of a molecular memory has not been considered due to the fast spin-state interconversion of all previous systems. Breaking this principle, here we report a slow relaxation process found in an SCO iron-triazole polyanionic complex. Multiple experimental evidences confirm the opening of a thermal hysteresis upon solid dilution and even in liquid solution. Density functional theory (DFT) calculations reveal the origin of this unexpected phenomenon to the appearance of an energy barrier that slows down the spin-state relaxation processes at the molecular level. These results show how SCO molecules may store information at room temperature, opening unique opportunities for molecular data storage. 

/pdf/molecular-memory-near-room-temperature-in-an-iron-dctybct2.pdf

Molecular memory near room temperature in an iron polyanionic complex

In this paper, we report the chemical strategy followed to obtain, in a direct way, nanoparticles of the RbxMn[Fe(CN)6](x+2)/3·nH2O (RbMnFe) Prussian blue analogue with the aim of keeping the switching ability of this compound at the nanoscale. The switching properties come from a reversible electron transfer between the iron and manganese ions and depends on the rubidium content in the structure that has to be higher than 0.6. Despite the multifunctionality of this family of compounds and its interest in various applications, no systematic studies were performed to obtain well-defined nanoparticles. This paper relates to such an investigation. To draw relationship between size reduction, composition, and switching properties, a special attention was brought to the determination of the composition through elemental analysis and structure refinement of powder X-ray diffraction patterns together with infrared spectroscopy and elemental analysis. Several chemical parameters were explored to control both the size reduction and the composition following a direct synthetic approach. The results show that the smaller the particles, the lower the rubidium content. This observation might prevent the observation of switching properties on very small particles. Despite this antagonist effect, we achieved switchable particles of around 200 nm without any use of surfactant. Moreover, the size reduction is associated with the observation of the electron transfer down to 52% of rubidium in the nanoparticles against 64% in microparticles. This work is of particular interest in processing such nanoparticles into devices.

https://hal.science/hal-03564080/document

Rational Direct Synthesis of RbMnFe Nanoparticles (RbMnFe = RbxMn[Fe(CN)6](2+x)/3·nH2O Prussian Blue Analogue).

Spin crossover complexes are a route toward designing molecular devices with a facile readout due to the change in conductance that accompanies the change in spin state. Because substrate effects are important for any molecular device, there are increased efforts to characterize the influence of the substrate on the spin state transition. Several classes of spin crossover molecules deposited on different types of surface, including metallic and non-metallic substrates, are comprehensively reviewed here. While some non-metallic substrates like graphite seem to be promising from experimental measurements, theoretical and experimental studies indicate that 2D semiconductor surfaces will have minimum interaction with spin crossover molecules. Most metallic substrates, such as Au and Cu, tend to suppress changes in spin state and affect the spin state switching process due to the interaction at the molecule–substrate interface that lock spin crossover molecules in a particular spin state or mixed spin state. Of course, the influence of the substrate on a spin crossover thin film depends on the molecular film thickness and perhaps the method used to deposit the molecular film.

/pdf/the-influence-of-the-substrate-on-the-functionality-of-spin-2cnmdzu9.pdf

The Influence of the Substrate on the Functionality of Spin Crossover Molecular Materials

Spin Crossover (SCO) is a promising switching phenomenon when implemented in electronic devices as molecules, thin films or nanoparticles. Among the properties modulated along this phenomenon, optically induced mechanical changes are of tremendous importance as they can work as fast light-induced mechanical switches or allow to investigate and control microstructural strains and fatigability. The development of characterization techniques probing 2 nanoscopic behaviour with high spatio-temporal resolution allows to trigger and visualize such mechanical changes of individual nanoscopic objects. Here we use Ultrafast Transmission Electron Microscopy (UTEM) to precisely probe the length changes of individual switchable nanoparticles induced thermally by nanosecond laser pulses. This allows us to reveal the mechanisms of spin switching, leading to the macroscopic expansion of SCO materials. This study was conducted on individual pure SCO nanoparticles and SCO nanoparticles encapsulating gold nanorods that serve for plasmonic heating under laser pulses. Length changes are compared with time-resolved optical measurements performed on an assembly of these particles.

/pdf/photo-thermal-switching-of-individual-plasmonically-41m1nyvnbv.pdf

Yaowei Hu

Papers

Photo-Thermal Switching of Individual Plasmonically Activated Spin Crossover Nanoparticle Imaged by Ultrafast Transmission Electron Microscopy.