Magnetism in dodecanethiol-capped gold nanoparticles: Role of size and capping agent

摘要

In gold nanoparticles (Au NPs) capped with dodecanethiol (DT), the authors report the observation of superparamagnetic blocking temperature T_B approx = 50 K in D approx = 5 nm NPs but only diamagnetism in 12 nm NPs. For T < T_B=50 K, the strong temperature dependence of coercivity H_c, saturation magnetization M_s, and exchange bias H_e (in the field-cooled sample) confirm the blocked state resembling ferromagnetism with H_c approx = 250 Oe, H_e approx =-40 Oe, and M_s approx =10~(-2) emu/g at 5 K. The observed electron magnetic resonance line shows expected shift, broadening, and reduced intensity below T_B. A magnetic moment μ approx = 0.006μ_B per Au atom attached to DT is determined using a model which yields M_s varying as 1 /D, with its source being holes in the 5d band of Au produced by charge transfer from Au to S atoms in DT.