Copper doping in II–VI semiconductor nanocrystals (NCs) has sparked enormous debate regarding the oxidation state of Cu ions and their hugely differing consequences in optoelectronic applications. The identity of a magnetically active Cu2+ ion or a magnetically inactive d10 Cu+ ion has generally been probed using optical techniques, and confusion arises from the spatial clutter that is part of the technique. One major probe that could declutter the data obtained from ensemble emission is single-particle fluorescence spectroscopy. In this work, using this very technique along with X-ray absorption spectroscopy probing the local environment of dopant ions, we study Cu-doped II–VI semiconductor NCs to find conclusive evidence on the oxidation state of Cu dopants and hence the mechanism of their emission. Detailed analysis of blinking properties has been used to study the single-particle nature of the NCs.

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Transition metal (TM) doping in pristine II−VI semiconductor quantum dots (QDs) is known to add several otherwise unavailable properties by introducing midgap states in the host material. Albeit being extensively investigated, the periodicity of the observed properties with respect to the electronic structure has not been attempted so far. In this work, we investigate CdS QDs doped with several different TM ions (Mn, Fe, Co, Ni, and Cu) using extended X-ray absorption fine structure spectroscopy to study dopant-induced structural perturbations and femtosecond transient absorption (TA) spectroscopy to study the ultrafast charge carrier dynamics. This provides solid evidence for the origin of magnetization in doped QDs that has been lacking despite extensive studies. Further, we demonstrate that the ionic radius and the dopant oxidation state play crucial roles in determining the dopant−anion bond lengths. Based on the investigation of the relaxation pathways of excited charge carriers using ultrafast TA spectroscopy, we hypothesize that there exists photoinduced switching between multiple oxidation states in some dopants.

Manganese (Mn) is one of the most studied transition metal dopants to alter the optical properties of host semiconductor nanocrystals (NCs). In most cases, the doped NCs are characterized by an invariant broad photoluminescence (PL) spectrum at 2.12 eV. The nature of this emission, although thought to be atomic-like, has revealed several dependencies on the host NCs in recent literature that facilitates the energy/charge transfer to a spin and orbital forbidden channel. In this work, we study this transfer as a function of the host band gap and spin–orbit coupling. We demonstrate that while the energy/charge transfer is facilitated by the energy difference between the band gap and Mn excitation energy, the high spin–orbit coupling allows transfer and back transfer of energy/charge, thus giving rise to a tunable higher energy transition even in successful doping of Mn. We use low-temperature PL and gated PL to demonstrate this phenomenon.

Efficient and environmentally benign visible light responsive materials have been sought after owing to their interesting applications such as visible light photocatalysis, visible light water splitting and visible light sensing. In this research study, the effect of co-doping on the absorption and electrical properties of ZnS quantum dots is studied. Upon co-doping of Fe and Cu into ZnS quantum dots, a new absorption band in the visible region is observed. Furthermore, these quantum dots show photoresponse in the visible region unlike their undoped counterparts that is only effective in the UV region, suggesting their utility in light sensing applications.