Figure 4.

Optical density (OD) of dispersions of pure α-NiS nanoparticles (A) and Au-NiS core–shell particles (B) at 400 and 1000 nm during the cooling and reheating of the respective samples. It can be seen that during cooling, the OD at 400 nm decreases drastically in the case of the pure particles. The process seems to be reversible and also shows a hysteresis. For the Au-NiS particles, this hysteresis also exists but the OD does not drop (it increases instead) until a temperature below 245 K is reached. In both cases, the OD at 1000 nm increases at lower temperatures while also displaying a hysteresis. 235 K was the lowest achievable temperature with the used setup limited by the used solvent toluene.