Triggering Cation Exchange Reactions by Doping

Cation exchange (CE) reactions have emerged as a technologically important route, complementary to the colloidal synthesis, to produce nanostructures of different geometries and compositions for a variety of applications. Here it is demonstrated with first-principles simulations that an interstitial impurity cation in CdSe nanocrystals weakens nearby bonds and reduces the CE barrier in the prototypical exchange of Cd2+ ions by Ag+ ions. A Wannier function-based tight binding model is employed to quantify microscopic mechanisms that influence this behavior. To support our model, we also tested our findings in a CE experiment: both CdSe and interstitially Ag-doped CdSe nanocrystals (containing 4% of Ag+ ions per nanocrystal on average) were exposed to Pb2+ ions at room temperature and it was observed that the exchange reaction proceeds further in doped nanocrystals. The findings suggest doping as a possible route to promote CE reactions that hardly undergo exchange otherwise, for example, those in III–V sem..

Influence of the Ion Coordination Number on Cation Exchange Reactions with Copper Telluride Nanocrystals

Cu2-xTe nanocubes were used as starting seeds to access metal telluride nanocrystals by cation exchanges at room temperature. The coordination number of the entering cations was found to play an important role in dictating the reaction pathways. The exchanges with tetrahedrally coordinated cations (i.e. with coordination number 4), such as Cd2+ or Hg2+, yielded monocrystalline CdTe or HgTe nanocrystals with Cu2-xTe/CdTe or Cu2-xTe/HgTe Janus-like heterostructures as intermediates. The formation of Janus-like architectures was attributed to the high diffusion rate of the relatively small tetrahedrally coordinated cations, which could rapidly diffuse in the Cu2-xTe NCs and nucleate the CdTe (or HgTe) phase in a preferred region of the host structure. Also, with both Cd2+ and Hg2+ ions the exchange led to wurtzite CdTe and HgTe phases rather than the more stable zinc-blende ones, indicating that the anion framework of the starting Cu2- xTe particles could be more easily deformed to match the anion framework of the metastable wurtzite structures. As hexagonal HgTe had never been reported to date, this represents another case of metastable new phases that can only be accessed by cation exchange. On the other hand, the exchanges involving octahedrally coordinated ions (i.e. with coordination number 6), such as Pb2+ or Sn2+, yielded rock-salt polycrystalline PbTe or SnTe nanocrystals with Cu2-xTe@PbTe or Cu2-xTe@SnTe core@shell architectures at the early stages of the exchange process. In this case, the octahedrally coordinated ions are probably too large to diffuse easily through the Cu2-xTe structure: their limited diffusion rate restricts their initial reaction to the surface of the nanocrystals, where cation exchange is initiated unselectively, leading to core@shell architectures.Comment: 11 pages, 7 figures in J. Am. Chem. Soc, 13 May 201

Alkyl phosphonic acids deliver CsPbBr3 Nanocrystals with high photoluminescence quantum yield and truncated octahedron shape

We devised a colloidal approach for the synthesis of CsPbBr3 nanocrystals (NCs) in which the only ligands employed are alkyl phosphonic acids. Compared to more traditional syntheses of CsPbBr3 NCs, the present scheme delivers NCs with the following distinctive features: (i) The NCs do not have cubic but truncated octahedron shape enclosed by Pb-terminated facets. This is a consequence of the strong binding affinity of the phosphonate groups toward Pb2+ ions. (II) The NCs have near unity photoluminescence quantum yields (PLQYs), with no need of postsynthesis treatments, indicating that alkyl phosphonic acids are effectively preventing the formation of surface traps. (III) Unlike NCs coated with alkylammonium or carboxylate ligands, the PLQY of phosphonate coated NCs remains constant upon dilution, suggesting that the ligands are tightly bound to the surface

Exploiting the Transformative Features of Metal Halides for the Synthesis of CsPbBr3@SiO2 Core\u2013Shell Nanocrystals

The encapsulation of colloidal lead halide perovskite nanocrystals within silica (SiO2) is one of the strategies to protect them from polar solvents and other external factors. Here, we demonstrate the overcoating of CsPbBr3 perovskite nanocrystals with silica by exploiting the anhydride-induced transformation of Cs4PbBr6 nanocrystals. CsPbBr3@SiO2 core\u2013shell nanocrystals are obtained after (i) a reaction between colloidal Cs4PbBr6 nanocrystals and maleic anhydride in toluene that yields CsPbBr3 nanocrystals and maleamic acid and (ii) a silica-shell growth around CsPbBr3 nanocrystals via hydrolysis of added alkoxysilanes. The reaction between Cs4PbBr6 nanocrystals and maleic anhydride is necessary to promote shell formation from alkoxysilanes, as demonstrated in control experiments. The best samples of as-prepared CsPbBr3@SiO2 nanocrystals consist of 3c10 nm single-crystal CsPbBr3 cores surrounded by 3c5\u20137 nm amorphous silica shell. Despite their core\u2013shell structure, such nanostructures are poor emitters and degrade within minutes of exposure to ethanol. The photoluminescence intensity of the core\u2013shell nanocrystals is improved by the treatment with a solution of PbBr2 and ligands, and their stability in ethanol is extended to several days after applying an additional silica growth step. Overall, the investigated approach outlines a strategy for making colloidal core\u2013shell nanocrystals utilizing the transformative chemistry of metal halides and reveals interesting insights regarding the conditions required for CsPbBr3@SiO2 nanocrystal formation

Ultrafast Photodoping and Plasmon Dynamics in Fluorine-Indium Codoped Cadmium Oxide Nanocrystals for All-Optical Signal Manipulation at Optical Communication Wavelengths

We show the ultrafast photodoping and plasmon dynamics of the near-infrared (NIR) localized surface plasmon resonance (LSPR) of fluorine-indium codoped cadmium oxide (FICO) nanocrystals (NCs). The combination of high temporal resolution and broad spectral coverage allowed us to model the transient absorption (TA) spectra in terms of the Drude model, verifying the increase in carrier density upon ultrafast photodoping. Our analysis also suggests that a change in carrier effective mass takes place upon LSPR excitation as a result of the nonparabolic conduction band of the doped semiconductor with a consequently high signal response. Both findings are combined in this new type of plasmonic material. The combination of large transmission modulation with modest pump powers and ultrafast recombination times makes our results interesting for all-optical signal processing at optical communication wavelengths. At the same time, our results also give insights into the physical mechanisms of ultrafast photodoping and LSPR tuning of degenerately doped semiconductor NCs

Exploiting the Transformative Features of Metal Halides for the Synthesis of CsPbBr3@SiO2 Core-Shell Nanocrystals

Lead halide perovskite (LHP) nanocrystals (NCs) are an emerging semiconductive material with a great potential for applications in optoelectronic devices such as photodetectors, solar cells, light-emitting diodes, etc.1 Such an interest in LHP NCs is motivated by their easy synthesis combined with tunable and bright photoluminescence (PL) and strong absorption.2 Despite their impressive optoelectronic properties, LHP NCs experience fast degradation when exposed to UV irradiation, high temperature, moisture, acidic or alkaline environments and polar solvents. Silica has emerged as the most promising material for LHP NCs stabilization.3 However, such enhanced stability is achieved with bulk silica which cannot be employed in technologies that require colloidal stability (e.g. inkjet printing). As a consequence, the community is moving to colloidally stable LHP NCs@SiO2 core@shell systems. The complexity of growing silica shells onto preformed LHP NCs arises from NCs degradation under the conditions needed to grow silica, i.e. the acidic or alkaline environments that catalyze growth. Interestingly, Baranov et al. stabilized CsPbBr3 NCs in an acidic environment through the reaction of the non-luminescent C4PbBr6 NCs with poly(maleic anhydride-alt-1-octadecene) (PMAO). In particular, the oleylamine capping ligands react with the polymer promoting the formation of the luminescent CsPbBr3 NCs and acidifying the reaction environment due to maleamic acid formation.4 In our study, we exploited the acidic environment produced by the reaction of maleic anhydride (MANH, the reactive monomer of PMAO) with the oleylamine ligand of Cs4PbBr6 to prepare CsPbBr3@SiO2 in presence of tetraethyl orthosilicate (TEOS). XRD showed the partial conversion of the Cs4PbBr6 into the CsPbBr3 NCs which was confirmed by their green emission. The CsPbBr3@SiO2 were further coated with SiO2 enhancing the stability towards polar solvents and removing the residual Cs4PbBr6 NCs. These results provide interesting insights onto the mechanism of silica shell formation. Namely, Both the acidic environment and the Cs4PbBr6 NCs as starting material are needed to prepare CsPbBr3@SiO2. [1] S. Tiam Tan, X. Li, H. Volkan Demir. Small 2019, 15, 1902079. [2] L. Protesescu, S. Yakunin, M. Bodnarchuk, F. Krieg, R. Caputo, C. Hendon, R. Xi Yang, A. Walsh, M. Kovalenko. Nano Lett. 2015, 15, 3692\u20133696. [3] Q. Zhang, B. Wang, L. Kong, Q. Wan, C. Zhang, Z. Li, X. Cao, M. Liu, L. Li. Nat. Comm. 2020, 11, 31. [4] D. Baranov, G. Caputo, L. Goldoni, Z. Dang, R. Scarfiello, L. De Trizio, A. Portone, F. Fabbri, A. Camposeo, D. Pisignano, L. Manna. Chem. Sci., 2020, 11, 3986-3995

Organic A‐Site Cations Improve the Resilience of Inorganic Lead‐Halide Perovskite Nanocrystals to Surface Defect Formation

Publication status: PublishedAbstractLead halide perovskite nanocrystals (LHP NCs) are generally prone to surface defect formation during the purification process, resulting in a reduced photoluminescence (PL) quantum yield. The purification of LHP NCs using antisolvents leads to the detachment of surface atoms and ligandsin the form of alkylammonium‐halides or A‐carboxylates (A: Cs, methylammonium, MA, or formamidinium, FA. Currently, intense research is being carried out to improve the surface stability of LHP NCs using various long‐chain organic ligands that strongly bind to the surface of NCs. Herein, the findings on the higher surface stability of hybrid (MAPbBr3 and FAPbBr3) LHP NCs compared to that of inorganic CsPbBr3 NCs against purification with polar antisolvents are reported. It is discovered that the CsPbBr3 NC surface stability can be enhanced (reaching that of hybrid perovskite NCs) by the incorporation of a small amount of MA or FA A‐site organic cations into their lattice, corroborated by the evidence that the PL quantum yield of mixed A‐cation CsxFA1‐xPbBr3 and CsxMA1‐xPbBr3 NCs remains unaffected after several purification cycles. It is hypothesized that this is attributed to hydrogen bonding between the organic A‐site cations and the neighboring halides on the surface. These findings are not only fundamentally important but are also expected to have wide implications in the field of metal‐halide perovskite optoelectronics.</jats:p