Semiconductor colloidal quantum dots (CQDs) have attracted vast scientific and technological interest throughout the past three decades, due to the unique tuneability of their optoelectronic properties by variation of size and composition. However, the nanoscale size brings about a large surface-to-bulk volume ratio, where exterior surfaces have a pronounced influence on the chemical stability and on the physical properties of the semiconductor. Therefore, numerous approaches have been developed to gain efficient surface passivation, including a coverage by organic or inorganic molecular surfactants as well as the formation of core/shell heterostructures (a semiconductor core epitaxially covered by another semiconductor shell). This review focuses on special designs of core/shell heterostructures from the IV-VI and II-VI semiconductor compounds, and on synthetic approaches and characterization of the optical properties. Experimental observations revealed the formation of core/shell structures with type-I or quasi-type-II band alignment between the core and shell constituents. Theoretical calculations of the electronic band structures, which were also confirmed by experimental work, exposed surplus electronic tuning (beyond the radial diameter) with adaptation of the composition and control of the interface properties. The studies also considered strain effects that are created between two different semiconductors. It was disclosed experimentally and theoretically that the strain can be released via the formation of alloys at the core-shell interface. Overall, the core/shell and core/alloyed-shell heterostructures showed enhancement in luminescence quantum efficiency with respect to that of pure cores, extended lifetime, uniformity in size and in many cases good chemical sustainability under ambient conditions.

Controlling the synthesis of narrow band gap semiconductor nanocrystals (NCs) with a high-quality surface is of prime importance for scientific and technological interests. This Letter presents facile solution-phase syntheses of SnTe NCs and their corresponding core/shell heterostructures. Here, we synthesized monodisperse and highly crystalline SnTe NCs by employing an inexpensive, nontoxic precursor, SnCl2, the reactivity of which was enhanced by adding a reducing agent, 1,2-hexadecanediol. Moreover, we developed a synthesis procedure for the formation of SnTe-based core/shell NCs by combining the cation exchange and the Kirkendall effect. The cation exchange of Sn(2+) by Cd(2+) at the surface allowed primarily the formation of SnTe/CdTe core/shell NCs. Further continuation of the reaction promoted an intensive diffusion of the Cd(2+) ions, which via the Kirkendall effect led to the formation of the inverted CdTe/SnTe core/shell NCs.

Above band-edge photoexcitation of PbSe nanocrystals induces strong below band gap absorption as well as a multiphased buildup of bleaching in the 1Se1Sh transition. The amplitudes and kinetics of these features deviate from expectations based on biexciton shifts and state filling, which are the mechanisms usually evoked to explain them. To clarify these discrepancies, the same transitions are investigated here by double-pump-probe spectroscopy. Re-exciting in the below band gap induced absorption characteristic of hot excitons is shown to produce additional excitons with high probability. In addition, pump-probe experiments on a sample saturated with single relaxed excitons prove that the resulting 1Se1Sh bleach is not linear with the number of excitons per nanocrystal. This finding holds for two samples differing significantly in size, demonstrating its generality. Analysis of the results suggests that below band edge induced absorption in hot exciton states is due to excited-state absorption and not to shifted absorption of cold carriers and that 1Se1Sh bleach signals are not an accurate counter of sample excitons when their distribution includes multiexciton states.

The achievement of tunable optical properties across a wide spectral range, along with an efficient surface passivation of lead chalcogenide (PbSe) colloidal quantum dots (CQDs), has significant importance for scientific research and for technological applications. This paper describes two comprehensive pathways to tune optical activities of PbSe CQDs in the near-infrared (NIR, 0.75−1.4 μm) and the short-wave infrared (SWIR, 1.4−3 μm) ranges. A one-pot procedure enabled the growth of relatively large PbSe CQDs (with average sizes up to 14 nm) exploiting programmable temperature control during the growth process. These CQDs showed optical activity up to 3.2 μm. In addition, PbSe/PbS core/shell CQDs prepared by an orderly injection rate led to an energy red-shift of the absorption edge with the increase of the shell thickness, whereas a postannealing treatment further extended the band-edge energy toward the SWIR regime. A better chemical stability of the CQDs with respect to that of PbSe core CQDs was attained by shelling of PbSe by epitaxial layers of PbS, but limited to a short duration (<1 day). However, air stability of the relatively large PbSe as well as the PbSe/PbS CQDs over a prolonged period of time (weeks) was achieved after a postsynthesis chlorination treatment.

The strong quantum confinement effect in lead selenide (PbSe) colloidal quantum dots (CQDs) allows to tune the bandgap of the material, covering a large spectral range from mid-to near infrared (NIR). Together with the advantages of low-cost solution processability, flexibility and easy scale-up production in comparison to conventional semiconductors especially in the mid-to near infrared range, PbSe CQDs have been a promising material for infrared optoelectronic applications. In this study, we synthesized monodisperse and high purity PbSe CQDs and then demonstrated the photodetectors working at different wavelengths up to 2.8 µm. Our high quality PbSe CQDs show clear multiple excitonic absorption peaks. PbSe CQD films of different thicknesses were deposited on interdigitated platinum electrodes by a simple drop casting technique to make the infrared photodetectors. At room temperature, the high performances of our PbSe CQD photodetectors were achieved with maximum responsivity, detectivity and external quantum efficiency of 0.96 A/W, 8.13 × 10 9 Jones and 78% at 5V bias. Furthermore, a series of infrared LEDs with a broad wavelength range from 1.5 μm to 3.4 μm was utilized to demonstrate the performance of our fabricated photodetectors with various PbSe CQD film thicknesses.

In this work, a novel dynamic cation exchange driven by an anion exchange procedure was used for incorporating Ni dopants into the Pb-site of CsPbBr3 and CsPb­(BrCl)3 perovskite nanocrystals under ambient conditions. Ni doping has significant merits in tuning the optical properties and introducing magnetic functionality. Moreover, Ni doping provides chemical and photochemical stability to a host phase with consequent importance for solar cell applications. The present work describes a thorough investigation of the Ni incorporation mechanism via structural, compositional, and optical characterizations. The results indicated the essential need for cation–anion coexchange, enabling control of the Ni concentration from <1% to about 12%, with uniform distribution across the host lattice. In addition, the doping improved the photoluminescence quantum yields beyond those of the undoped nanocrystals. The observations were corroborated by the density functional theory (DFT), confirming that the incorporation of Ni is energetically favorable.

A multidisciplinary approach for the production and characterization of colloidal quantum dots, which show great promise for implementation in modern optoelectronic applications, is described. The approach includes the design and formation of unique core/shell structures with alloy-composed layers between the core and the shell. Such structures eliminate interfacial defects and suppress the Auger process, thus reducing the known fluorescence blinking and endowing the quantum dots with robust chemical and spectral stability. The unique design enables the generation and sustained existence of single and multiple excitons with a defined spin-polarized emission recombination. The studies described herein implement the use of single-dot magneto-optical measurements and optically detected magnetic resonance spectroscopy, for direct identification of interfacial defects and for resolving exciton fine structure. The results are of paramount importance for a fundamental understanding of optical transitions in colloidal quantum dots, with an impact on appropriate materials design for practical applications.

The interface in PbSe/PbS core/shell colloidal quantum dots (CQDs) is subject to strain forces due to a 3% crystallographic mismatch between the constituents. The strain profile in PbSe/PbS CQDs was simulated using the classical linear elasticity model, under the assumption of spherical-symmetric dot and isotropic materials. The derived strain profile was incorporated into a band structure calculation to evaluate the influence on the electronic band-edges of the core/shell CQDs. The electronic energy states evaluated were in close agreement with the absorption edges of various core/shell CQDs with different core diameters and shell thicknesses. Furthermore, the synthesized CQDs underwent thermal annealing at various temperatures, thereby creating the alloying interface; consequently, their absorption and photoluminescence spectra exhibited spectral red-shift compared with the untreated samples. The band gap energy red-shift was simulated by the theoretical model, including smoothing potential at the interface. Measurements of the photoluminescence decays indicated an extension of the radiative lifetime after a controlled annealing process, denoting removal of defect quenchers around the core–shell interface. Thus, the study suggests practical means for mitigating interface strain to leverage the quality of core/shell structures.