Room temperature spin crossover in molecular and polymeric materials

• Main Author: Bovo, Gianluca
• Other Authors: Stavrinou, Paul ; Bradley, Donal
• Published: Imperial College London 2015
• Subjects: 620.1
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.724130

Spin crossover materials represent a remarkable model of molecular spin switching, where a transition between low spin and high spin configurations can be reversibly triggered by suitable external stimuli and is often accompanied by a hysteresis of various physical properties, holding potential for application in memory devices and optical switches. This thesis work investigates the thermal spin crossover behaviour of a class of Fe(II)-triazole polymers where the chemical properties can be tailored to achieve solubility in common solvents and one mononuclear Fe(II) complex deemed suitable for thermal evaporation. A broad spectrum of experimental techniques including magnetic, optical, calorimetric and structural assessments are applied to characterise the spin crossover phenomena in powders, thick films and pellets, highlighting the impact of the molecular environment and the effects of the processing methods on the spin switching. Additionally, thermal expansion measurements with a capacitance dilatometer provide a macroscopic evaluation of the mechanical changes associated with the transitions, which mirror the expansion/contraction of the Fe coordination sphere at the molecular level. Solution processing and vacuum deposition of the materials further enables the preparation and study of stable thin films, better suited to device application, with optical characterisation confirming the retention of spin crossover thermochromic properties. As a first step to deploy the iron-polymers inside simple electronic devices, MIM structures (metal-insulator-metal) are fabricated and tested by impedance spectroscopy, showing the presence of a thermal hysteresis loop in the dielectric function around room temperature. We demonstrate that in addition to the previous studies on bulk powders, optical and dielectric bistability with sharp spin transitions can also be achieved for thin films devices, thus opening interesting new perspectives for the application of spin crossover polymeric and molecular materials.