Maksym V. Kovalenko

Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut

Metal halides perovskites, such as hybrid organic–inorganic CH3NH3PbI3, are newcomer optoelectronic materials that have attracted enormous attention as solution-deposited absorbing layers in solar cells with power conversion efficiencies reaching 20%. Herein we demonstrate a new avenue for halide perovskites by designing highly luminescent perovskite-based colloidal quantum dot materials. We have synthesized monodisperse colloidal nanocubes (4–15 nm edge lengths) of fully inorganic cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I or mixed halide systems Cl/Br and Br/I) using inexpensive commercial precursors. Through compositional modulations and quantum size-effects, the bandgap energies and emission spectra are readily tunable over the entire visible spectral region of 410–700 nm. The photoluminescence of CsPbX3 nanocrystals is characterized by narrow emission line-widths of 12–42 nm, wide color gamut covering up to 140% of the NTSC color standard, high quantum yields of up to 90%, and radiative lifetimes in the range of 1–29 ns. The compelling combination of enhanced optical properties and chemical robustness makes CsPbX3 nanocrystals appealing for optoelectronic applications, particularly for blue and green spectral regions (410–530 nm), where typical metal chalcogenide-based quantum dots suffer from photodegradation.

Colloidal Nanocrystals with Molecular Metal Chalcogenide Surface Ligands

Colloidal Nanocrystal Compounds Colloidal nanocrystals have properties that fall between those of the individual atoms but also differ from bulk due to confinement effects. They can thus be thought of as analogs of atoms, and, like atoms, there is a desire to bond together neighboring particles, which will also affect their properties. During synthesis, organic ligands are used to prevent the colloidal nanocrystals from growing too large or agglomerating, but these ligands result in poor interparticle coupling and communication. Kovalenko et al. (p. 1417) show that chalcogenide complexes (compounds based on S, Se, or Te) can effectively link together neighboring particles. Upon gentle heating, the ligands can be converted from insulating to semiconducting without altering the chemistry of the nanocrystals. Chalcogenide-based ligands are used to link colloidal nanocrystals together and can be converted into semiconducting complexes. Similar to the way that atoms bond to form molecules and crystalline structures, colloidal nanocrystals can be combined together to form larger assemblies. The properties of these structures are determined by the properties of individual nanocrystals and by their interactions. The insulating nature of organic ligands typically used in nanocrystal synthesis results in very poor interparticle coupling. We found that various molecular metal chalcogenide complexes can serve as convenient ligands for colloidal nanocrystals and nanowires. These ligands can be converted into semiconducting phases upon gentle heat treatment, generating inorganic nanocrystal solids. The utility of the inorganic ligands is demonstrated for model systems, including highly conductive arrays of gold nanocrystals capped with Sn2S64– ions and field-effect transistors on cadmium selenide nanocrystals.

Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I)

Postsynthetic chemical transformations of colloidal nanocrystals, such as ion-exchange reactions, provide an avenue to compositional fine-tuning or to otherwise inaccessible materials and morphologies. While cation-exchange is facile and commonplace, anion-exchange reactions have not received substantial deployment. Here we report fast, low-temperature, deliberately partial, or complete anion-exchange in highly luminescent semiconductor nanocrystals of cesium lead halide perovskites (CsPbX3, X = Cl, Br, I). By adjusting the halide ratios in the colloidal nanocrystal solution, the bright photoluminescence can be tuned over the entire visible spectral region (410–700 nm) while maintaining high quantum yields of 20–80% and narrow emission line widths of 10–40 nm (from blue to red). Furthermore, fast internanocrystal anion-exchange is demonstrated, leading to uniform CsPb(Cl/Br)3 or CsPb(Br/I)3 compositions simply by mixing CsPbCl3, CsPbBr3, and CsPbI3 nanocrystals in appropriate ratios.

Band-like transport, high electron mobility and high photoconductivity in all-inorganic nanocrystal arrays

Flexible, thin-film electronic and optoelectronic devices typically involve a trade-off between performance and fabrication cost. For example, solution-based deposition allows semiconductors to be patterned onto large-area substrates to make solar cells and displays, but the electron mobility in solution-deposited semiconductor layers is much lower than in semiconductors grown at high temperatures from the gas phase. Here, we report band-like electron transport in arrays of colloidal cadmium selenide nanocrystals capped with the molecular metal chalcogenide complex In(2)Se(4)(2-), and measure electron mobilities as high as 16 cm(2) V(-1) s(-1), which is about an order of magnitude higher than in the best solution-processed organic and nanocrystal devices so far. We also use CdSe/CdS core-shell nanoparticles with In(2)Se(4)(2-) ligands to build photodetectors with normalized detectivity D* > 1 × 10(13) Jones (I Jones = 1 cm Hz(1/2) W(-1)), which is a record for II-VI nanocrystals. Our approach does not require high processing temperatures, and can be extended to different nanocrystals and inorganic surface ligands.

Metal-free Inorganic Ligands for Colloidal Nanocrystals: S2–, HS–, Se2–, HSe–, Te2–, HTe–, TeS32–, OH–, and NH2– as Surface Ligands

All-inorganic colloidal nanocrystals were synthesized by replacing organic capping ligands on chemically synthesized nanocrystals with metal-free inorganic ions such as S(2-), HS(-), Se(2-), HSe(-), Te(2-), HTe(-), TeS(3)(2-), OH(-) and NH(2)(-). These simple ligands adhered to the NC surface and provided colloidal stability in polar solvents. The versatility of such ligand exchange has been demonstrated for various semiconductor and metal nanocrystals of different size and shape. We showed that the key aspects of Pearsons hard and soft acids and bases (HSAB) principle, originally developed for metal coordination compounds, can be applied to the bonding of molecular species to the nanocrystal surface. The use of small inorganic ligands instead of traditional ligands with long hydrocarbon tails facilitated the charge transport between individual nanocrystals and opened up interesting opportunities for device integration of colloidal nanostructures.

Prospects of Nanoscience with Nanocrystals

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Todays strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.

Low-threshold amplified spontaneous emission and lasing from colloidal nanocrystals of caesium lead halide perovskites

Metal halide semiconductors with perovskite crystal structures have recently emerged as highly promising optoelectronic materials. Despite the recent surge of reports on microcrystalline, thin-film and bulk single-crystalline metal halides, very little is known about the photophysics of metal halides in the form of uniform, size-tunable nanocrystals. Here we report low-threshold amplified spontaneous emission and lasing from ∼10 nm monodisperse colloidal nanocrystals of caesium lead halide perovskites CsPbX3 (X=Cl, Br or I, or mixed Cl/Br and Br/I systems). We find that room-temperature optical amplification can be obtained in the entire visible spectral range (440–700 nm) with low pump thresholds down to 5±1 μJ cm−2 and high values of modal net gain of at least 450±30 cm−1. Two kinds of lasing modes are successfully observed: whispering-gallery-mode lasing using silica microspheres as high-finesse resonators, conformally coated with CsPbX3 nanocrystals and random lasing in films of CsPbX3 nanocrystals.

Monodisperse Antimony Nanocrystals for High-Rate Li-ion and Na-ion Battery Anodes: Nano versus Bulk

We report colloidal synthesis of antimony (Sb) nanocrystals with mean size tunable in the 10-20 nm range and with narrow size distributions of 7-11%. In comparison to microcrystalline Sb, 10 and 20 nm Sb nanocrystals exhibit enhanced rate-capability and higher cycling stability as anode materials in rechargeable Li-ion and Na-ion batteries. All three particle sizes of Sb possess high and similar Li-ion and Na-ion charge storage capacities of 580-640 mAh g(-1) at moderate charging/discharging current densities of 0.5-1C (1C-rate is 660 mA g(-1)). At all C-rates (0.5-20C, e.g. current densities of 0.33-13.2 Ag(1-)), capacities of 20 nm Sb particles are systematically better than for both 10 nm and bulk Sb. At 20C-rates, retention of charge storage capacities by 10 and 20 nm Sb nanocrystals can reach 78-85% of the low-rate value, indicating that rate capability of Sb nanostructures can be comparable to the best Li-ion intercalation anodes and is so far unprecedented for Na-ion storage.

Highly Dynamic Ligand Binding and Light Absorption Coefficient of Cesium Lead Bromide Perovskite Nanocrystals.

Lead halide perovskite materials have attracted significant attention in the context of photovoltaics and other optoelectronic applications, and recently, research efforts have been directed to nanostructured lead halide perovskites. Collodial nanocrystals (NCs) of cesium lead halides (CsPbX3, X = Cl, Br, I) exhibit bright photoluminescence, with emission tunable over the entire visible spectral region. However, previous studies on CsPbX3 NCs did not address key aspects of their chemistry and photophysics such as surface chemistry and quantitative light absorption. Here, we elaborate on the synthesis of CsPbBr3 NCs and their surface chemistry. In addition, the intrinsic absorption coefficient was determined experimentally by combining elemental analysis with accurate optical absorption measurements. (1)H solution nuclear magnetic resonance spectroscopy was used to characterize sample purity, elucidate the surface chemistry, and evaluate the influence of purification methods on the surface composition. We find that ligand binding to the NC surface is highly dynamic, and therefore, ligands are easily lost during the isolation and purification procedures. However, when a small amount of both oleic acid and oleylamine is added, the NCs can be purified, maintaining optical, colloidal, and material integrity. In addition, we find that a high amine content in the ligand shell increases the quantum yield due to the improved binding of the carboxylic acid.