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.
Icosahedral symmetry, which is not compatible with truly long-range order, can be found in many systems, such as liquids, glasses, atomic clusters, quasicrystals and virus-capsids. To obtain ...arrangements with a high degree of icosahedral order from tens of particles or more, interparticle attractive interactions are considered to be essential. Here, we report that entropy and spherical confinement suffice for the formation of icosahedral clusters consisting of up to 100,000 particles. Specifically, by using real-space measurements on nanometre- and micrometre-sized colloids, as well as computer simulations, we show that tens of thousands of hard spheres compressed under spherical confinement spontaneously crystallize into icosahedral clusters that are entropically favoured over the bulk face-centred cubic crystal structure. Our findings provide insights into the interplay between confinement and crystallization and into how these are connected to the formation of icosahedral structures.
Copper indium sulfide (CIS) quantum dots (QDs) are attractive as labels for biomedical imaging, since they have large absorption coefficients across a broad spectral range, size- and ...composition-tunable photoluminescence from the visible to the near-infrared, and low toxicity. However, the application of NIR-emitting CIS QDs is still hindered by large size and shape dispersions and low photoluminescence quantum yields (PLQYs). In this work, we develop an efficient pathway to synthesize highly luminescent NIR-emitting wurtzite CIS/ZnS QDs, starting from template Cu2‑x S nanocrystals (NCs), which are converted by topotactic partial Cu+ for In3+ exchange into CIS NCs. These NCs are subsequently used as cores for the overgrowth of ZnS shells (≤1 nm thick). The CIS/ZnS core/shell QDs exhibit PL tunability from the first to the second NIR window (750–1100 nm), with PLQYs ranging from 75% (at 820 nm) to 25% (at 1050 nm), and can be readily transferred to water upon exchange of the native ligands for mercaptoundecanoic acid. The resulting water-dispersible CIS/ZnS QDs possess good colloidal stability over at least 6 months and PLQYs ranging from 39% (at 820 nm) to 6% (at 1050 nm). These PLQYs are superior to those of commonly available water-soluble NIR-fluorophores (dyes and QDs), making the hydrophilic CIS/ZnS QDs developed in this work promising candidates for further application as NIR emitters in bioimaging. The hydrophobic CIS/ZnS QDs obtained immediately after the ZnS shelling are also attractive as fluorophores in luminescent solar concentrators.
A recent discovery in membrane research is the ability of styrene-maleic acid (SMA) copolymers to solubilize membranes in the form of nanodisks allowing extraction and purification of membrane ...proteins from their native environment in a single detergent-free step. This has important implications for membrane research because it allows isolation as well as characterization of proteins and lipids in a near-native environment. Here, we aimed to unravel the molecular mode of action of SMA copolymers by performing systematic studies using model membranes of varying compositions and employing complementary biophysical approaches. We found that the SMA copolymer is a highly efficient membrane-solubilizing agent and that lipid bilayer properties such as fluidity, thickness, lateral pressure profile, and charge density all play distinct roles in the kinetics of solubilization. More specifically, relatively thin membranes, decreased lateral chain pressure, low charge density at the membrane surface, and increased salt concentration promote the speed and yield of vesicle solubilization. Experiments using a native membrane lipid extract showed that the SMA copolymer does not discriminate between different lipids and thus retains the native lipid composition in the solubilized particles. A model is proposed for the mode of action of SMA copolymers in which membrane solubilization is mainly driven by the hydrophobic effect and is further favored by physical properties of the polymer such as its relatively small cross-sectional area and rigid pendant groups. These results may be helpful for development of novel applications for this new type of solubilizing agent, and for optimization of the SMA technology for solubilization of the wide variety of cell membranes found in nature.
ZnS shelling of I–III–VI2 nanocrystals (NCs) invariably leads to blue-shifts in both the absorption and photoluminescence spectra. These observations imply that the outcome of ZnS shelling reactions ...on I–III–VI2 colloidal NCs results from a complex interplay between several processes taking place in solution, at the surface of, and within the seed NC. However, a fundamental understanding of the factors determining the balance between these different processes is still lacking. In this work, we address this need by investigating the impact of precursor reactivity, reaction temperature, and surface chemistry (due to the washing procedure) on the outcome of ZnS shelling reactions on CuInS2 NCs using a seeded growth approach. We demonstrate that low reaction temperatures (150 °C) favor etching, cation exchange, and alloying regardless of the precursors used. Heteroepitaxial shell overgrowth becomes the dominant process only if reactive S- and Zn-precursors (S-ODE/OLAM and ZnI2) and high reaction temperatures (210 °C) are used, although a certain degree of heterointerfacial alloying still occurs. Remarkably, the presence of residual acetate at the surface of CIS seed NCs washed with ethanol is shown to facilitate heteroepitaxial shell overgrowth, yielding for the first time CIS/ZnS core/shell NCs displaying red-shifted absorption spectra, in agreement with the spectral shifts expected for a type-I band alignment. The insights provided by this work pave the way toward the design of improved synthesis strategies to CIS/ZnS core/shell and alloy NCs with tailored elemental distribution profiles, allowing precise tuning of the optoelectronic properties of the resulting materials.
Nanoscale cation exchange (CE) has resulted in colloidal nanomaterials that are
unattainable by direct synthesis methods. Aliovalent CE is complex and synthetically
challenging because the exchange ...of an unequal number of host and guest cations is
required to maintain charge balance. An approach to control aliovalent CE reactions is
the use of a single reactant to both supply the guest cation and extract the host
cation. Here, we study the application of GaCl
3
–L complexes L =
trioctylphosphine (TOP), triphenylphosphite (TPP), diphenylphosphine (DPP) as reactants
in the exchange of Cu
+
for Ga
3+
in
Cu
2–
x
S nanocrystals. We find that noncomplexed
GaCl
3
etches the nanocrystals by S
2–
extraction, whereas
GaCl
3
–TOP is unreactive. Successful exchange of Cu
+
for
Ga
3+
is only possible when GaCl
3
is complexed with either TPP or
DPP. This is attributed to the pivotal role of the
Cu
2–
x
S–GaCl
3
–L activated
complex that forms at the surface of the nanocrystal at the onset of the CE reaction,
which must be such that simultaneous Ga
3+
insertion and Cu
+
extraction can occur. This requisite is only met if GaCl
3
is bound to a
phosphine ligand, with a moderate bond strength, to allow facile dissociation of the
complex at the nanocrystal surface. The general validity of this mechanism is
demonstrated by using GaCl
3
–DPP to convert CuInS
2
into
(Cu,Ga,In)S
2
nanocrystals, which increases the photoluminescence quantum
yield 10-fold, while blue-shifting the photoluminescence into the NIR biological window.
This highlights the general applicability of the mechanistic insights provided by our
work.
Maximizing the utilization of noble metals is crucial for applications such as catalysis. We found that the minimum loading of platinum for optimal performance in the hydroconversion of
n
-alkanes ...for industrially relevant bifunctional catalysts could be reduced by a factor of 10 or more through the rational arranging of functional sites at the nanoscale. Intentionally depositing traces of platinum nanoparticles on the alumina binder or the outer surface of zeolite crystals, instead of inside the zeolite crystals, enhanced isomer selectivity without compromising activity. Separation between platinum and zeolite acid sites preserved the metal and acid functions by limiting micropore blockage by metal clusters and enhancing access to metal sites. Reduced platinum nanoparticles were more active than platinum single atoms strongly bonded to the alumina binder.
Better on the outside
One way to maximize the activity of scarce and costly noble metals in catalysis is to use small cluster or even single atoms on metal oxide supports. Bifunctional platinum and zeolite catalysts are used industrially in alkane hydroconversion. Cheng
et al
. show that the amount of platinum used could be reduced and isomer selectivity improved when the catalyst preparation created platinum clusters on the surface of zeolite or its alumina binder, rather than in the interior of the zeolite channels. Preparations that would have formed single-atom catalysts were less active because the platinum ions adsorbed on the alumina surface were more difficult to reduce. —PDS
Isomer selectivity in alkane hydroconversion improves when platinum clusters are located on rather than in zeolites.
The formation mechanisms of colloidal magic-size semiconductor
nanostructures have remained obscure. Herein, we report the room temperature
synthesis of three species of ultrathin CdTe magic-size ...nanowires
(MSNWs) with diameters of 0.7 ± 0.1 nm, 0.9 ± 0.2 nm, and
1.1 ± 0.2 nm, and lowest energy exciton transitions at 373, 418,
and 450 nm, respectively. The MSNWs are obtained from Cd(oleate)
2
and TOP-Te, provided diphenylphosphine and a primary alkylamine
(RNH
2
) are present at sufficiently high concentrations,
and exhibit sequential, discontinuous growth. The population of each
MSNW species is entirely determined by the RNH
2
concentration
RNH
2
so that single species are only obtained at specific
concentrations, while mixtures are obtained at concentrations intermediate
between the specific ones. Moreover, the MSNWs remain responsive to
RNH
2
, interconverting from thinner to thicker upon RNH
2
decrease and from thicker to thinner upon RNH
2
increase. Our results allow us to propose a mechanism for the formation
and interconversion of CdTe MSNWs and demonstrate that primary alkylamines
play crucial roles in all four elementary kinetic steps (viz., monomer
formation, nucleation, growth in length, and interconversion between
species), thus being the decisive element in the creation of a reaction
pathway that leads exclusively to CdTe MSNWs. The insights provided
by our work thus contribute toward unravelling the mechanisms behind
the formation of shape-controlled and atomically precise magic-size
semiconductor nanostructures.
Glycerol is an attractive renewable building block for the synthesis of di‐ and triglycerols, which have numerous applications in the cosmetic and pharmaceutical industries. In this work, the ...selective etherification of glycerol to di‐ and triglycerol was studied in the presence of alkaline earth metal oxides and the data are compared with those obtained with Na2CO3 as a homogeneous catalyst. It was found that glycerol conversion increased with increasing catalyst basicity; that is, the conversion increases in the order: MgO<CaO<SrO<BaO. The best selectivity values for (di‐ + tri‐) glycerol (>90 % at 60 % conversion) are obtained over CaO, SrO, and BaO. For these catalysts no substantial acrolein formation was observed. Furthermore, at the start of the reaction mainly linear diglycerol was produced, whereas at higher conversion degrees branched diglycerol started to form. In another series of experiments different types of CaO materials were prepared. It was found that these CaO‐based materials not only differed in their surface area and number of basic sites, but also in their Lewis acid strength. Within this series the CaO material possessing the strongest Lewis acid sites had the highest catalytic activity, comparable to that of BaO, pointing towards the important role of Lewis acidity for this etherification reaction. Based on these observations a plausible alternative reaction scheme for glycerol etherification is presented, which considers the facilitation of the hydroxyl leaving process. Finally, the stability of the catalytic solids under study was investigated and it was found that colloidal CaO particles of about 50–100 nm can be spontaneously generated during reaction. Catalytic testing of these CaO colloids, after isolation from the reaction medium, revealed a very high etherification activity. Understanding the nature of these Ca‐based colloids opens new opportunities for investigating supported colloidal particle catalysts to take advantage of both their hetero‐ and homogeneous nature.
Calcium oxide and colloids in the act: Different basic oxides, such as MgO, CaO, SrO, and BaO, were investigated for the etherification of glycerol to di‐ and triglycerol. In particular, CaO turned out to be an interesting material and both basicity and Lewis acidity play an active role in glycerol etherification (see figure).
Semiconductor copper indium sulfide quantum dots are emerging as promising alternatives to cadmium- and lead-based chalcogenides in solar cells, luminescent solar concentrators, and deep-tissue ...bioimaging due to their inherently lower toxicity and outstanding photoluminescence properties. However, the nature of their emission pathways remains a subject of debate. Using low-temperature single quantum dot spectroscopy on core–shell copper indium sulfide nanocrystals, we observe two subpopulations of particles with distinct spectral features. The first class shows sharp resolution-limited emission lines that are attributed to zero-phonon recombination lines of a long-lived band-edge exciton. Such emission results from the perfect passivation of the copper indium sulfide core by the zinc sulfide shell and points to an inversion in the band-edge hole levels. The second class exhibits ultrabroad spectra regardless of the temperature, which is a signature of the extrinsic self-trapping of the hole assisted by defects in imperfectly passivated quantum dots.
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