HgS and HgS/CdS Colloidal Quantum Dots with Infrared Intraband Transitions and Emergence of a Surface Plasmon

HgS colloidal quantum dots (CQDs) are synthesized at room temperature using a dual-phase method. The HgS CQDs ranging from 3 to 15 nm exhibit air-stable n-doping and infrared intraband absorptions. For HgS CQDs of small sizes, the doping density is close to 2 electrons per dot, while for larger ones, their intraband absorption peaks shift to as far as 10 mu m and exhibit Lorentzian line shapes. Under reducing potentials, these long-wavelength absorption peaks increase in strength and blue shift. This behavior can be explained through a classical model of the local field, showing how the degenerate single-electron transitions shift to a frequency that is the quadratic mean of the individual transition and a surface plasmon coming from a number of oscillators. This indicates that the intraband absorption of large, n-doped HgS CQDs is therefore becoming a surface plasmon. The same synthetic method works for HgS/CdS core/shells. Encapsulating HgS in a CdS shell removes the natural n-doping of the HgS cores, resulting in an interband photoluminescence at 1.5 mu m with similar to 5% quantum yield. The n-doping partially recovers upon film formation, and increases in strength after ligand exchange and annealing. The core/shell greatly improves the thermal stability of the HgS cores, allowing an annealing temperature as high as 200 degrees C.