II-VI colloidal semiconductor nanocrystals (NCs), such as CdSe NCs, are often plagued by efficient nonradiative recombination processes that severely limit their use in energy-conversion schemes. While these processes are now well-known to occur at the surface, a full understanding of the exact nature of surface defects and of their role in deactivating the excited states of NCs has yet to be established, which is partly due to challenges associated with the direct probing of the complex and dynamic surface of colloidal NCs. Here, we report a detailed study of the surface of cadmium-rich zinc-blende CdSe NCs. The surfaces of these cadmium-rich species are characterized by the presence of cadmium carboxylate complexes (CdX) that act as Lewis acid (Z-type) ligands that passivate undercoordinated selenide surface species. The systematic displacement of CdX from the surface by N,N,N',N'-tetramethylethylene-1,2-diamine (TMEDA) has been studied using a combination of H NMR and photoluminescence spectroscopies. We demonstrate the existence of two independent surface sites that differ strikingly in the binding affinity for CdX and that are under dynamic equilibrium with each other. A model involving coupled dual equilibria allows a full characterization of the thermodynamics of surface binding (free energy, as well as enthalpic and entropic terms), showing that entropic contributions are responsible for the difference between the two surface sites. Importantly, we demonstrate that cadmium vacancies only lead to important photoluminescence quenching when created on one of the two sites, allowing a complete picture of the surface composition to be drawn where each site is assigned to specific NC facet locale, with CdX binding affinity and nonradiative recombination efficiencies that differ by up to two orders of magnitude.

Colloidal indium nitride nanocrystals (InN NCs) are stable heavily-doped nanomaterials, with as-prepared electron densities around ⟨N e⟩ ∼ 7.4 × 1020 cm–3, independent of size, making these attractive candidates for charge storage applications at the nanoscale. Unfortunately, many fundamental quantities that inevitably control the behavior of charges in InN NCs, such as the band potentials or the energy of the Fermi level, are currently unknown. Here, we report a direct and simple optical spectroscopic method that allows to quantify the charge storage capacity of colloidal InN nanocrystals. A size-independent, high volumetric capacitance (69 ± 4) F·cm–3 is found, underlying the potential of InN NCs as nanoscaled supercapacitors in energy harvesting and storage applications. Importantly, this study directly yields the band edge potentials and the charge-neutrality level of InN NCs as a function of NC size, positioning the conduction band potential of InN at about (1.13 ± 0.07) V vs Fc+/0 (ferrocenium/ferrocene), consistent with calculated estimates of bulk electron affinity values (E A ∼ 6 eV), and the charge-neutrality level (i.e., the Fermi level of pristine InN NCs) at (−0.59 ± 0.03) V vs Fc+/0. The apparent absence of quantum confinement on the energy of the conduction band potential for NC sizes where it should appear, dubbed here “quantum confinement resilience effect”, is discussed in terms of the nonparabolic band dispersion of InN.

Semiconductor nanocrystals are often characterized by complex excited-state dynamics which reflect the inhomogeneous character of ensemble of nanocrystals. A new hybrid inorganic–organic donor–acceptor system involving CdSe nanocrystals and paramagnetic nitronyl nitroxide free radicals is shown to lead to efficient Förster (dipolar) resonance energy transfer. This transfer process, which is monitored by steady-state and time-dependent photoluminescence quenching experiments, occurs on a time scale similar to that of the intrinsic recombination in CdSe nanocrystals, allowing to unravel some of the complexity associated with the excited-state photophysics of semiconductor nanocrystals. A Stern–Volmer formalism that can handle the multiexponential nature of the time-dependent excited-state kinetics of CdSe nanocrystals is developed, leading to excellent agreement between steady-state and time-dependent photoluminescence data when a log-normal distribution model is used for the intrinsinc recombination rate constant and a Poisson distribution for the number of bound quenchers per emitter.

Organic free radicals related to the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical are known as photoluminescence-quenchers when coupled to group II-chalcogenide colloidal quantum dots (QDs), but the mechanism responsible for this phenomenon has so far remained unresolved. Using a combination of time-resolved photoluminescence and transient absorption spectroscopies, we demonstrate that photoexcited colloidal CdSe QDs coupled to 4-amino-TEMPO undergo highly efficient reductive quenching, that is, hole transfer from the valence band of the quantum dot to the organic paramagnetic species. Interestingly, the process is shown to occur on a subpicosecond time scale for bound 4AT; such a large rate constant for the extraction of holes from photoexcited CdSe QD by a molecular species is rare and underlines the potential that TEMPO derivatives can play in mediating efficient redox processes involving colloidal CdSe QDs.

The k • p effective mass approximation (EMA) predicts large, nearly size-independent exciton oscillator strengths in quantum confined semiconductors. Yet, experimental reports have concluded that the total oscillator strength of the lowest-energy (1S 3/2 1Se) excitons in strongly confined CdSe NQDs is small and strongly size-dependent. Using the optical Stark effect, we show that the oscillator strength of the 1S 3/2 1Se excitonic absorption peak in CdSe NQDs follows the predictions of the EMA. These oscillator strengths enable helicity-selective unsaturated Stark shifts corresponding to femtosecond pseudo-magnetic fields exceeding 100 T.