Interface and matrix effects on light-switchable solid-state spin transitions

University Of Florida, Gainesville FL

Investigators

Abstract

Part 1: Non-Technical Summary. Using light to induce motion on a macroscopic scale is sometimes referred to as the direct conversion of light to work and is expected to lead to new technologies in areas ranging from artificial muscles to light energy harvesting to wireless machines. Spin transition solids are a class of materials known to change volume when exposed to certain stimuli, including light. This volume change alone can be used to cause motion, but the effects are greatly amplified by combining the spin transition solid in a composite with other materials. This project, supported by the Solid State and Materials Chemistry program at NSF, explores the fundamental materials chemistry questions associated with light induced volume changes when spin transition solids are combined in a matrix with other materials to form a composite. Theoretical models predict how the spin transition should change in a matrix, but experiments to verify these predictions are currently lacking, and the magnitude of matrix effects have not been quantified. Researchers at the University of Florida develop new synthetic methods to place well-defined particles of the spin transition solid in matrices of different materials enabling quantitative measurements of how the volume change associated with the spin transition is transmitted to and amplified by the supporting matrix. The project utilizes national scientific user facilities including the Advanced Photon Source at Argonne National Labs and the NSLS II at Brookhaven National Labs, and provides students training in these advanced technologies. Results of the project will enable better design of the next generation of materials used for directly converting light to work. The relationship of this project to these developing technologies is highlighted in a planned exhibit themed Creating Motion with Light, to be displayed at local education and public outreach forums. Part 2: Technical Summary The significant volume change accompanying alterations in metal-ligand bonding during a solid-state spin transition opens the prospect of harvesting these effects for mechanical actuator technology. These emerging applications require the spin transition material to physically couple to other material components, yet the material interface can influence the characteristics of a spin transition, especially at high surface to volume ratios characteristic of the nanoscale or mesoscale. This project, supported by the Solid State and Materials Chemistry program at NSF, quantifies matrix or interface attributes and their influence on spin transitions in mesoscale particles. Experimentally, the charge transfer induced spin transition (CTIST) of rubidium cobalthexacyanoferrate as the light-switchable core with isostructural but chemically distinct cyanometallate shells as surrounding matrix are studied. Theoretical models predict the elastic properties of the core are the key determinants of the order and kinetics of the phase transition. These properties can be affected by the stiffness of the shell, the relative lattice constants of the core and shell, and the shell thickness; all are parameters that can be synthetically altered. Light-induced and temperature-dependent phase behavior is monitored with X-ray diffraction and magnetometry to provide activation energies and information about the order and cooperativity of the transition as components are changed. The elastic properties are measured using nuclear inelastic scattering (NIS) and X-ray diffraction under pressure, and they are correlated with the phase change behavior to quantify the matrix effects. In parallel, the scope extends to chemically dissimilar matrices. These studies inform the subject area of light-induced mechanical actuation, potential applications of which are the use of light to directly perform work and new light energy harvesting schemes. The relationship of this project to these developing technologies is highlighted in a planned exhibit themed Creating Motion with Light, to be displayed at local education and public outreach forums. The research tasks are designed to be platforms for graduate and undergraduate student education, providing technical expertise and knowledge along with general skills needed to be competitive in materials chemistry related high-technology professions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.