Versatile chemical transformations of surface functional groups in 2D transition-metal carbides (MXenes) open up a new design space for this broad class of functional materials. We introduce a general strategy to install and remove surface groups by performing substitution and elimination reactions in molten inorganic salts. Successful synthesis of MXenes with O, NH, S, Cl, Se, Br, and Te surface terminations, as well as bare MXenes (no surface termination) was demonstrated. These MXenes show distinctive structural and electronic properties. For example, the surface groups control interatomic distances in the MXene lattice, and Tin+1Cn (n = 1, 2) MXenes terminated with Te2− ligands show a giant, (>18%) in-plane lattice expansion compared to the bulk TiC lattice. Nb2C MXenes exhibited surface-group-dependent superconductivity.

The practical applications of CsPbX nanocrystals (NCs) have been limited by their poor stability. Although much effort has been devoted to making core-shell nanostructures to enhance the stability of CsPbX NCs, it is still very difficult to coat CsPbX NCs with another material on a single-particle level. In this work, we report a facile one-pot approach to synthesize CsPbBr@SiO core-shell nanoparticles (NPs), in which each core-shell NP has only one CsPbBr NC. The formation process has been carefully monitored. It has been found that the formation rates, determined by reaction temperature, precursor species, pH value, etc., of both CsPbBr and SiO are critical for the successful preparation of core-shell NPs. Thanks to the protection of SiO shell, the product shows much higher long-term stability in humid air and enhanced stability against ultrasonication treatment in water than that of naked CsPbBr NCs. This work not only provides a robust method for the preparation of core-shell nanostructures but also sheds some light on the stabilization and applications of CsPbX NCs.

We report a novel CsX-stripping mechanism that enables the efficient chemical transformation of nonluminescent CsPbX (X = Cl, Br, I) nanocrystals (NCs) to highly luminescent CsPbX NCs. During the transformation, CsPbX NCs dispersed in a nonpolar solvent are converted into CsPbX NCs by stripping CsX through an interfacial reaction with water in a different phase. This process takes advantage of the high solubility of CsX in water as well as the ionic nature and high ion diffusion property of CsPbX NCs, and produces monodisperse and air-stable CsPbX NCs with controllable halide composition, tunable emission wavelength covering the full visible range, narrow emission width, and high photoluminescent quantum yield (up to 75%). An additional advantage is that this is a clean synthesis as CsPbX NCs are converted into CsPbX NCs in the nonpolar phase while the byproduct of CsX is formed in water that could be easily separated from the organic phase. The as-prepared CsPbX NCs show enhanced stability against moisture because of the passivated surface. Our finding not only provides a new pathway for the preparation of highly luminescent CsPbX NCs but also adds insights into the chemical transformation behavior and stabilization mechanism of these emerging perovskite nanocrystals.

The poor stability of CsPbX (X = Cl, Br, I) nanocrystals (NCs) has severely impeded their practical applications. Although there are some successful examples on encapsulating multiple CsPbX NCs into an oxide or polymer matrix, it has remained a serious challenge for the surface modification/encapsulation using oxides or polymers at a single particle level. In this work, monodisperse CsPbX/SiO and CsPbBr/TaO Janus nanoparticles were successfully prepared by combining a water-triggered transformation process and a sol-gel method. The CsPbBr/SiO NCs exhibited a photoluminescence quantum yield of 80% and a lifetime of 19.8 ns. The product showed dramatically improved stability against destruction by air, water, and light irradiation. Upon continuous irradiation by intense UV light for 10 h, a film of the CsPbBr/SiO Janus NCs showed only a slight drop (2%) in the PL intensity, while a control sample of unmodified CsPbBr NCs displayed a 35% drop. We further highlighted the advantageous features of the CsPbBr/SiO NCs in practical applications by using them as the green light source for the fabrication of a prototype white light emitting diode, and demonstrated a wide color gamut covering up to 138% of the National Television System Committee standard. This work not only provides a novel approach for the surface modification of individual CsPbX NCs but also helps to address the challenging stability issue; therefore, it has an important implication toward their practical applications.

Recently, all-inorganic cesium lead halide (CsPbX 3 , X = Cl, Br, I) perovskite nanocrystals have drawn much attention because of their outstanding photophysical properties and potential applications. In this work, a simple and efficient solvothermal approach to prepare CsPbX 3 nanocrystals with tunable and bright photoluminescent (PL) properties, controllable composition, and morphology is presented. CsPbX 3 nanocubes are successfully prepared with bright emission high PL quantum yield up to 80% covering the full visible range and narrow emission line widths (from 12 to 36 nm). More importantly, ultrathin CsPbX 3 (X = Cl/Br, Br, and Br/I) nanowires (with diameter as small as ≈2.6 nm) can be prepared in a very high morphological yield (almost 100%). A strong quantum confinement effect is observed in the ultrathin nanowires, in which both the absorption and emission peaks shift to shorter wavelength range compared to their bulk bandgap. The reaction parameters, such as temperature and precursors, are varied to investigate the growth process. A white light-emitting device prototype device with wide color gamut covering up to 120% of the National Television System Committee standard has been demonstrated by using CsPbBr 3 nanocrystals as the green light source. The method in this study provides a simple and efficient way to prepare high-quality CsPbX 3 nanocrystals.The ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/adfm.201701121.light-emitting devices (LEDs), [3] fieldeffect transistors, [4] solar cells, [5] and so on. Although much progress has been made in the preparation of traditional colloidal semiconductor nanocrystals, the current synthetic strategies are still suffering from several challenges, including low yield, complex procedures, and toxic chemicals. It is thus highly desired to develop new types of semiconductor nanocrystals that possess excellent photoluminescent properties and can be prepared in a simple, reproducible, and cost-effective way. For this purpose, recently, a new family of perovskite semiconductor nanocrystals has proven to be a great candidate for optoelectronic applications due to their low cost, easy preparation, high reproducibility, and great photoluminescent properties. [6] For example, the organic-inorganic hybrid perovskite, i.e., CH 3 NH 3 PbX 3 (X = Cl, Br, I), have been used in solar cell and showed great performance. [7] However, the hybrid perovskite nanocrystals are highly sensitive to moisture and oxygen, resulting in low stability. Since the pioneered work by Kovalenko and co-workers in 2015, intensive research efforts have been devoted to studying the all-inorganic cesium lead halide (CsPbX 3 , X = Cl, Br, I) perovskite nanocrystals, which show outstanding photophysical properties, including composition-and size-controlled photoluminescent property, high PL quantum yield (PLQY), narrow emission widths, and short radiative lifetime. [8] The all-inorganic perovskite nanocrystals have been prepared through...

X-type ligands, for example, the pair of oleylamine (OAm) and oleic acid (OA), have been widely used to prepare CsPbX3 nanocrystals (NCs). However, the proton exchange between coordinated OAm and OA may induce the detachment of ligands, resulting in poor performance after cleaning or long-time storage. Herein, density functional theory calculations predict that primary amines (L-type ligands) can stabilize a PbBr x -rich surface and yield a trap-free material with fully delocalized valence band maximum and conduction band minimum states, which can significantly improve the photophysical properties and stability of CsPbBr3 NCs. Along this prediction, a room-temperature reprecipitation method using L-type ligands (OAm, n-octylamine, or undecylamine) as the sole capping ligand has been developed to synthesize high-quality CsPbBr3 NCs with near-unity photoluminescence quantum yield and dramatically improved stability against purification and water treatment. The enhancement can be attributed to the strong binding of unprotonated amines to lead atoms and the effective surface passivation provided by the resulted PbBr x -rich surface, which are highly consistent with the theoretical predictions. This work not only offers an approach to synthesize high-quality perovskite NCs but also provides an in-depth understanding of the surface modification of CsPbX3 NCs for practical applications.

No abstract

Fabricating CsPbX3-based heterostructures has proven to be a feasible way to tune their photophysical properties. Here, we report the successful fabrication of Janus CsPbX3/ZrO2 heterostructure nanocrystals (NCs), in which each CsPbX3 NC is partially covered by ZrO2. According to the band alignment, CsPbBr3/ZrO2 and CsPbI3/ZrO2 can be indexed as type I and type II composites, respectively. The type I composites display great enhancement in photoluminescence quantum yield (from 63 to 90%) and photoluminescence lifetime (from 12.9 to 66.1 ns) because of the charge carrier confinement and passivation effect provided by ZrO2. In contrast, the type II composites can be used in photocatalytic reduction of CO2 because electrons and holes are effectively separated and accumulated in ZrO2 and CsPbI3, respectively, under irradiation. Janus CsPbBr3/ZrO2 NCs showed a stability much higher than that of pristine CsPbBr3 against polar solvent treatment. A stable and highly efficient light-emitting device with luminous efficiency up to 55 lm W–1 is fabricated by using CsPbBr3/ZrO2 NCs as the green light source. This work may not only enrich the family of surface-passivated perovskite materials but also provide a good example for the rational design of specific composites in the metal halide perovskite field.