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Colloidal heteronanocrystals (HNCs) can be regarded as solution-grown inorganic-organic hybrid nanomaterials, since they consist of inorganic nanoparticles that are coated with a layer of organic ligand molecules. The hybrid nature of these nanostructures provides great flexibility in engineering their physical and chemical properties. The inorganic particles are heterostructured, i.e. they comprise two (or more) different materials joined together, what gives them remarkable and unique properties that can be controlled by the composition, size and shape of each component of the HNC. The interaction between the inorganic component and the organic ligand molecules allows the size and shape of the HNCs to be controlled and gives rise to novel properties. Moreover, the organic surfactant layer opens up the possibility of surface chemistry manipulation, making it possible to tailor a number of properties. These features have turned colloidal HNCs into promising materials for a number of applications, spurring a growing interest on the investigation of their preparation and properties. This critical review provides an overview of recent developments in this rapidly expanding field, with emphasis on semiconductor HNCs (e.g., quantum dots and quantum rods). In addition to defining the state of the art and highlighting the key issues in the field, this review addresses the fundamental physical and chemical principles needed to understand the properties and preparation of colloidal HNCs (283 references).

Colloidal CsPbX3 (X = Br, Cl, and I) perovskite nanocrystals (NCs) have emerged as promising phosphors and solar cell materials due to their remarkable optoelectronic properties. These properties can be tailored by not only controlling the size and shape of the NCs but also postsynthetic composition tuning through topotactic anion exchange. In contrast, property control by cation exchange is still underdeveloped for colloidal CsPbX3 NCs. Here, we present a method that allows partial cation exchange in colloidal CsPbBr3 NCs, whereby Pb2+ is exchanged for several isovalent cations, resulting in doped CsPb1–xMxBr3 NCs (M= Sn2+, Cd2+, and Zn2+; 0 < x ≤ 0.1), with preservation of the original NC shape. The size of the parent NCs is also preserved in the product NCs, apart from a small (few %) contraction of the unit cells upon incorporation of the guest cations. The partial Pb2+ for M2+ exchange leads to a blue-shift of the optical spectra, while maintaining the high photoluminescence quantum yields (>50%), sharp absorption features, and narrow emission of the parent CsPbBr3 NCs. The blue-shift in the optical spectra is attributed to the lattice contraction that accompanies the Pb2+ for M2+ cation exchange and is observed to scale linearly with the lattice contraction. This work opens up new possibilities to engineer the properties of halide perovskite NCs, which to date are demonstrated to be the only known system where cation and anion exchange reactions can be sequentially combined while preserving the original NC shape, resulting in compositionally diverse perovskite NCs.

Colloidal CdTe quantum dots prepared in TOP/DDA (trioctylphosphine/dodecylamine) are transferred into water by the use of amino− ethanethiol•HCl (AET) or mercaptopropionic acid (MPA). This results in an increase in the photoluminescence quantum efficiency and a longer exciton lifetime. For the first time, water-soluble semiconductor nanocrystals presenting simultaneously high band-edge photoluminescence quantum efficiencies (as high as 60% at room temperature), monoexponential exciton decays, and no observable defect-related emission are obtained.

MRI detectable and targeted quantum dots were developed. To that aim, quantum dots were coated with paramagnetic and pegylated lipids, which resulted in a relaxivity, r 1 , of nearly 2000 mM -1 s -1 per quantum dot. The quantum dots were functionalized by covalently linking rv 3-specific RGD peptides, and the specificity was assessed and confirmed on cultured endothelial cells. The bimodal character, the high relaxivity, and the specificity of this nanoparticulate probe make it an excellent contrast agent for molecular imaging purposes.

In this work we have investigated the temperature-dependence of the band-edge photoluminescence decay of efficiently luminescing organically capped CdSe quantum dots ͑QDs͒ with diameters ranging from 1.7 to 6.3 nm over a broad temperature range ͑1.3-300 K͒. The overall trend is similar for all the investigated sizes, consisting of different temperature regimes. The low-temperature regime ͑below ϳ50 K͒ is characterized by purely radiative decay and can be modeled by a thermal distribution between a lower dark and a higher bright exciton state, with a size-dependent energy separation ͑viz., from 0.7 to 1.7 meV͒ and dark exciton lifetime ͑viz., from 0.3 to 1.4 s for QDs ranging from 6.3 nm to 1.7 nm in diameter͒. Nonradiative relaxation processes become increasingly important above ϳ50 K until the temperature antiquenching regime is reached, leading to a decrease in the nonradiative contributions and photoluminescence intensity recovery above ϳ200 K.

In this work, we show strong experimental evidence in favor of a proposed incorporation mechanism of hydrophobic semiconductor nanocrystals (or quantum dots, QDs) in monodisperse silica spheres (diameter ∼35 nm) by a water-in-oil (W/O) reverse microemulsion synthesis. Fluorescence spectroscopy is used to investigate the rapid ligand exchange that takes place at the QD surface upon addition of the various synthesis reactants. It is found that hydrolyzed TEOS has a high affinity for the QD surface and replaces the hydrophobic amine ligands, which enables the transfer of the QDs to the hydrophilic interior of the micelles where silica growth takes place. By hindering the ligand exchange using stronger binding thiol ligands, the position of the incorporated QDs can be controlled from centered to off-center and eventually to the surface of the silica spheres. The proposed incorporation mechanism explains how we can have high control over the incorporation of single QDs exactly in the middle of silica spheres. It is likely that the proposed mechanism also applies to the incorporation of other hydrophobic nanocrystals in silica using the same method. In conjunction with our findings, we were able to make QD/silica particles with an unprecedented quantum efficiency of 35%.

A high luminescence efficiency is an important property of colloidal quantum dots (QDs), and quantum yields higher than 90% have been reported for coreÀshell QDs. 1 High efficiencies are especially important for application of QDs as luminescent biolabels, 2 in QD lasers, 3 in spectral converters for warm white LEDs, 4,5 electroluminescent devices, 6 and solar concentrators. 7 Luminescence efficiencies are strongly temperature-dependent. 8 Extensive temperature-dependent luminescence studies for colloidal QDs have been conducted at cryogenic temperatures (0.3À300 K). 9À15 In this temperature region, interesting effects were observed, including a prolonged lifetime below 20 K related to brightÀdark state splitting, 11,16 thermally activated quenching due to surface defect states, 9,10,17 and temperature antiquenching assigned to a phase transition in the capping layer. 14,15 However, the luminescence properties of QDs above room temperature (RT) are hardly investigated, and yet, for most applications in luminescent devices, the working temperature is higher than 300 K. An interesting example is the recent application of QDs as color converters in warm-white LEDs, 18 in which QDs serve as narrow band red emitters under excitation with blue light from a (In,Ga)N LED. The narrow emission bandwidth renders QDs superior to classical phosphors based on broad band emission from luminescent ions. 19 In high-power LEDs for general lighting applications, the heat generated in the pÀn junction and phosphor converter layer leads to temperatures as high as 150À200°C in the layer applied on top of the blue diode. 20 To avoid these high temperatures, the QD phosphor layer can be placed in a more remote configuration. Still, temperatures in such a configuration are expected to be well above 50°C due to heat dissipation of the QDs themselves (excess energy from converting the blue into red light). Clearly, the quenching of QD luminescence at elevated temperatures is relevant for application of QDs in luminescent devices, and a better insight in the quenching behavior is needed.Despite its importance, research on luminescence temperature quenching above RT is very limited for QDs. It is theoretically expected for a QD to have a very high luminescence quenching temperature (T q ). Three generally accepted mechanisms for thermal quenching involve thermally activated crossover from the excited state to the ground state, multiphonon relaxation, and thermally activated photoionization. The first mechanism is generally depicted in a simple configurational coordinate diagram. 8,21 The energy difference between the minimum * Address correspondence to a.meijerink@uu.nl.Received for review July 18, 2012 and accepted September 14, 2012. Published online 10.1021/nn303217qABSTRACT Thermal quenching of quantum dot (QD) luminescence is important for application in luminescent devices. Systematic studies of the quenching behavior above 300 K are, however, lacking. Here, high-temperature (300À500 K) luminescence studies are reported for highly ef...