We discuss state-of-the-art doping strategies for colloidal quantum dots, their principle, advantages and challenges in implementing the strategies.

Semiconducting materials uniformly doped with optical or magnetic impurities have been useful in a number of potential applications. However, clustering or phase separation during synthesis has made this job challenging. Recently the "inside out" diffusion doping was proposed to be successful in obtaining large sized quantum dots (QDs) uniformly doped with a dilute percentage of dopant atoms. Herein, we demonstrate the use of basic physical chemistry of diffusion to control the size and concentration of the dopants within the QDs for a given transition metal ion. We have studied three parameters; the bond strength of the core molecules and the diffusion coefficient of the diffusing metal ion are found to be important while the ease of cation exchange was not highly influential in the control of size and concentration of the single domain dilute magnetic semiconductor quantum dots (DMSQDs) with diverse dopant ions M (Fe, Ni, Co, Mn). Steady state optical emission spectra reveal that the dopants are incorporated inside the semiconducting CdS and the emission can be tuned during shell growth. We have shown that this method enables control over doping percentage and the QDs show a superior ferromagnetic response at room temperature as compared to previously reported systems.

Photophysical properties and exciton dynamics of CdS supraparticles (400 nm) composed of quantum dots of CdS (3–5 nm) have been investigated when they are conjugated with the graphene sheet. Favorable electron transfer from photoexcited CdS to graphene was confirmed from the quenching of CdS emission and ultrafast transient absorption spectroscopy. Ultrafast electron transfer (<150 fs) was found to take place from photoexcited CdS to the graphene matrix. The charge separation process was monitored after following the bleach recovery kinetics at the excitonic position of the supraparticle CdS. Ultrafast transient absorption spectroscopic studies showed enhanced stability of exciton and efficient charge separation in the CdS supraparticle–graphene composite as compared to pure CdS supraparticles. These charge delocalizations and ultrafast electron transfers in the CdS supraparticle–graphene composite have been reflected in the photocatalytic dye degradation. The dye degradation rate was observed to be much faster (0.15 min–1) in the CdS supraparticle–graphene composite system compared to that of the pure CdS supraparticle (0.03 min–1).