Design of Bistable Gold@Spin-Crossover Core-Shell Nanoparticles Showing Large Electrical Responses for the Spin Switching.

A simple chemical protocol to prepare core-shell gold@spin-crossover (Au@SCO) nanoparticles (NPs) based on the 1D spin-crossover [Fe(Htrz)2 (trz)](BF4 ) coordination polymer is reported. The synthesis relies on a two-step approach consisting of a partial surface ligand substitution of the citrate-stabilized Au NPs followed by the controlled growth of a very thin layer of the SCO polymer. As a result, colloidally stable core@shell spherical NPs with a Au core of ca. 12 nm and a thin SCO shell 4 nm thick, are obtained, exhibiting a narrow distribution in sizes. Differential scanning calorimetry proves that a cooperative spin transition in the range 340-360 K is maintained in these Au@SCO NPs, in full agreement with the values reported for pristine 4 nm SCO NPs. Temperature-dependent charge-transport measurements of an electrical device based on assemblies of these Au@SCO NPs also support this spin transition. Thus, a large change in conductance upon spin state switching, as compared with other memory devices based on the pristine SCO NPs, is detected. This results in a large improvement in the sensitivity of the device to the spin transition, with values for the ON/OFF ratio which are an order of magnitude better than the best ones obtained in previous SCO devices.