Growth of a dense gadolinium metal-organic framework on oxide-free silicon for cryogenic local refrigeration.
Certain magnetic molecules and molecular-based frameworks are considered as alternative materials for cryogenic cooling due to their large magneto-caloric effect. This potential is particularly appealing for applications requiring local refrigeration, but so far no study has been aimed at making thin films of efficient molecular coolers on a typical device substrate and evaluating their cooling potential. In this work, the growth of a dense gadolinium metal-organic framework on oxide-free silicon covered by either self-assembled or covalent monolayers with carboxylic end groups is explored. A continuous coverage with nanosized crystallites of the selected gadolinium formate framework is successfully formed on covalent monolayers obtained by hydrosilylation with either undecenoic acid or undecynoic acid. The thickness of the magnetocaloric deposit can be increased by increasing the growth time, as shown by the surface densities derived through determination of the film magnetic properties. An estimation of the cooling potential of the thicker film obtained shows that it would refrigerate a 2 m thick Si membrane from 5 to 0.8 K, by adiabatic demagnetization from 2 T to zero field. Altogether, this study demonstrates the potential of molecular cooler thin films to develop on-chip magnetic refrigeration at cryogenic temperatures.