Surface Chemistry of Lead Halide Perovskite Colloidal Nanocrystals.

ConspectusThe surface chemistry of lead halide perovskite nanocrystals (NCs) plays a major role in dictating their colloidal and structural stability as well as governing their optical properties. A deep understanding of the nature of the ligand shell, ligand-NC, and ligand-solvent interactions is therefore of utmost importance. Our recent studies have revealed that such knowledge can be harnessed following a multidisciplinary approach comprising chemical, structural, and spectroscopic analyses coupled with atomistic modeling. We show that specific surface terminations can be produced only by employing flexible and versatile syntheses that enable to work under desired conditions. In this Account, we first describe our studies aimed at synthesizing CsPbBr3 NCs with various surface terminations. These include CsPbBr3 NCs prepared under Br- and oleylamine-rich conditions, which feature a ligand shell composed of alkylammonium-Br species and a photoluminescence quantum yield (PLQY) of ∼90%. On the other hand, taking advantage of the inability of secondary amines to bind to the perovskite NCs surface, we could prepare cuboidal CsPbBr3 NCs bearing a Cs-oleate surface termination and a PLQY of 70% by employing oleic acid and secondary alkylamines. In the quest to identify ligands that can bind more strongly than oleates or primary alkylammonium ions to the surface of NCs already in the synthesis step, we used phosphonic acids as the sole ligands in the CsPbBr3 NCs synthesis, which yielded NCs with a truncated octahedron shape, high PLQY (∼100%), and a PbBr2-terminated surface passivated by hydrogen phosphonates and phosphonic acid anhydride. The surface chemistry and the stability of perovskite NCs were investigated via ad-hoc postsynthesis treatments. We found, for example, that reacting oleylammonium-Br-terminated NCs with stoichiometric amounts of neutral primary alkylamines (or their conjugated acids) led to a partial replacement of oleylammonium ions with new alkylammonium ions (following a deprotonation/protonation mechanism), which resulted in a boost of the PLQY (up to 100%) and of the NCs' colloidal stability. Similar results in terms of optical properties were achieved by treating Cs-oleate-terminated NCs with alkylammonium-carboxylate or quaternary ammonium-Br (namely, didodecyldimethylammonium-Br, DDA-Br) couples. Interestingly, when the native NCs are ligand exchanged with DDA-Br, the ligand shell is then composed of species not bearing any proton. This, in turn, enabled us to study the interaction of such NCs with a variety of ligands under completely aprotic conditions wherein these DDA-Br-capped NCs were basically inert. The only exceptions were carboxylic, phosphonic, and sulfonic acids that were capable of stripping surface DDA-Br couples. As a note, most studies on CsPbBr3 NCs to date have focused primarily on choosing ligands with specific anchoring groups rather than on tuning the length and type of alkyl chains, as this is time-consuming and requires a large number of syntheses. Our recent developments in the computational chemistry of colloidal NCs are expected to provide a pivotal role in this direction since they can be integrated with machine learning models to investigate with greater details the ligand-NC, ligand-ligand, and ligand-solvent interactions and ultimately find optimal candidates through the prediction of surfactant properties using high-throughput data sets.

Colloidal InAs Tetrapods: Impact of Surfactants on the Shape Control.

We have approached the synthesis of colloidal InAs nanocrystals (NCs) using amino-As and ligands that are different from the commonly employed oleylamine (OA). We found that carboxylic and phosphonic acids led only to oxides, whereas tri-n-octylphosphine, dioctylamine, or trioctylamine (TOA), when employed as the sole ligands, yielded InAs NCs with irregular sizes and a broad size distribution. Instead, various combinations of TOA and OA delivered InAs NCs with good control over the size distribution, and the TOA:OA volume ratio of 4:1 generated InAs tetrapods with arm length of 5-6 nm. Contrary to tetrapods of II-VI materials, which have a zinc-blende core and wurtzite arms, these NCs are entirely zinc-blende, with arms growing along the ⟨111⟩ directions. They feature a narrow excitonic peak at ∼950 nm in absorption and a weak photoluminescence emission at 1050 nm. Our calculations indicated that the bandgap of the InAs tetrapods is mainly governed by the size of their core and not by their arm lengths when these are longer than ∼3 nm. Nuclear magnetic resonance analyses revealed that InAs tetrapods are mostly passivated by OA with only a minor fraction of TOA. Molecular dynamics simulations showed that OA strongly binds to the (111) facets whereas TOA weakly binds to the edges and corners of the NCs and their combined use (at high TOA:OA volume ratios) promotes growth along the ⟨111⟩ directions, eventually forming tetrapods. Our work highlights the use of mixtures of ligands as a means of improving control over InAs NCs size and size distribution.

Ru-Cu Nanoheterostructures for Efficient Hydrogen Evolution Reaction in Alkaline Water Electrolyzers.

Combining multiple species working in tandem for different hydrogen evolution reaction (HER) steps is an effective strategy to design HER electrocatalysts. Here, we engineered a hierarchical electrode for the HER composed of amorphous-TiO2/Cu nanorods (NRs) decorated with cost-effective Ru-Cu nanoheterostructures (Ru mass loading = 52 µg/cm2). Such an electrode exhibits a stable, over 250 h, low overpotential of 74 mV at -200 mA/cm2 for the HER in 1 M NaOH. The high activity of the electrode is attributed, by structural analysis, operando X-ray absorption spectroscopy, and first-principles simulations, to synergistic functionalities: (1) mechanically robust, vertically aligned Cu NRs with high electrical conductivity and porosity provide fast charge and gas transfer channels; (2) the Ru electronic structure, regulated by the size of Cu clusters at the surface, facilitates the water dissociation (Volmer step); (3) the Cu clusters grown atop Ru exhibit a close-to-zero Gibbs free energy of the hydrogen adsorption, promoting fast Heyrovsky/Tafel steps. An alkaline electrolyzer (AEL) coupling the proposed cathode and a stainless-steel anode can stably operate in both continuous (1 A/cm2 for over 200 h) and intermittent modes (accelerated stress tests). A techno-economic analysis predicts the minimal overall hydrogen production cost of US$2.12/kg in a 1 MW AEL plant of 30 year lifetime based on our AEL single cell, hitting the worldwide targets (US$2-2.5/kgH2).

Indium arsenide quantum dots: an alternative to lead-based infrared emitting nanomaterials.

Colloidal quantum dots (QDs) emitting in the infrared (IR) are promising building blocks for numerous photonic, optoelectronic and biomedical applications owing to their low-cost solution-processability and tunable emission. Among them, lead- and mercury-based QDs are currently the most developed materials. Yet, due to toxicity issues, the scientific community is focusing on safer alternatives. In this regard, indium arsenide (InAs) QDs are one of the best candidates as they can absorb and emit light in the whole near infrared spectral range and they are RoHS-compliant, with recent trends suggesting that there is a renewed interest in this class of materials. This review focuses on colloidal InAs QDs and aims to provide an up-to-date overview spanning from their synthesis and surface chemistry to post-synthesis modifications. We provide a comprehensive overview from initial synthetic methods to the most recent developments on the ability to control the size, size distribution, electronic properties and carrier dynamics. Then, we describe doping and alloying strategies applied to InAs QDs as well as InAs based heterostructures. Furthermore, we present the state-of-the-art applications of InAs QDs, with a particular focus on bioimaging and field effect transistors. Finally, we discuss open challenges and future perspectives.

Alloying Bi-Doped Cs2Ag1-xNaxInCl6 Nanocrystals with K+ Cations Modulates Surface Ligands Density and Photoluminescence Efficiency.

We show how, in the synthesis of yellow-emissive Bi-doped Cs2Ag1-xNaxInCl6 double perovskite nanocrystals (NCs), preventing the transient formation of Ag0 particles increases the photoluminescence quantum yield (PLQY) of the NCs from ∼30% to ∼60%. Calculations indicate that the presence of even a single Ag0 species on the surface of a NC introduces deep trap states. The PL efficiency of these NCs is further increased to ∼70% by partial replacement of Na+ with K+ ions, up to a 7% K content, due to a lattice expansion that promotes a more favorable ligands packing on the NC surface, hence better surface passivation. A further increase in K+ lowers the PLQY, due to both the activation of nonradiative quenching channels and a lower oscillator strength of the BiCl6→AgCl6 transition (through which PL emission occurs). The work indicates how a deeper understanding of parameters influencing carrier trapping/relaxation can boost the PLQY of double perovskites NCs.

Stable CsPbBr3 Nanoclusters Feature a Disk-like Shape and a Distorted Orthorhombic Structure.

CsPbBr3 nanoclusters have been synthesized by several groups and mostly employed as single-source precursors for the synthesis of anisotropic perovskite nanostructures or perovskite-based heterostructures. Yet, a detailed characterization of such clusters is still lacking due to their high instability. In this work, we were able to stabilize CsPbBr3 nanoclusters by carefully selecting ad hoc ligands (benzoic acid together with oleylamine) to passivate their surface. The clusters have a narrow absorption peak at 400 nm, a band-edge emission peaked at 410 nm at room temperature, and their composition is identified as CsPbBr2.3. Synchrotron X-ray pair distribution function measurements indicate that the clusters exhibit a disk-like shape with a thickness smaller than 2 nm and a diameter of 13 nm, and their crystal structure is a highly distorted orthorhombic CsPbBr3. Based on small- and wide-angle X-ray scattering analyses, the clusters tend to form a two-dimensional (2D) hexagonal packing with a short-range order and a lamellar packing with a long-range order.

ZnCl2 Mediated Synthesis of InAs Nanocrystals with Aminoarsine.

The most developed approaches for the synthesis of InAs nanocrystals (NCs) rely on pyrophoric, toxic, and not readily available tris-trimethylsilyl (or tris-trimethylgermil) arsine precursors. Less toxic and commercially available chemicals, such as tris(dimethylamino)arsine, have recently emerged as alternative As precursors. Nevertheless, InAs NCs made with such compounds need to be further optimized in terms of size distribution and optical properties in order to meet the standard reached with tris-trimethylsilyl arsine. To this aim, in this work we investigated the role of ZnCl2 used as an additive in the synthesis of InAs NCs with tris(dimethylamino)arsine and alane N,N-dimethylethylamine as the reducing agent. We discovered that ZnCl2 helps not only to improve the size distribution of InAs NCs but also to passivate their surface acting as a Z-type ligand. The presence of ZnCl2 on the surface of the NCs and the excess of Zn precursor used in the synthesis enable the subsequent in situ growth of a ZnSe shell, which is realized by simply adding the Se precursor to the crude reaction mixture. The resulting InAs@ZnSe core@shell NCs exhibit photoluminescence emission at ∼860 nm with a quantum yield as high as 42±4%, which is a record for such heterostructures, given the relatively high mismatch (6%) between InAs and ZnSe. Such bright emission was ascribed to the formation, under our peculiar reaction conditions, of an In-Zn-Se intermediate layer between the core and the shell, as indicated by X-ray photoelectron spectroscopy and elemental analyses, which helps to release the strain between the two materials.

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties.

Metal halide perovskites represent a flourishing area of research, which is driven by both their potential application in photovoltaics and optoelectronics and by the fundamental science behind their unique optoelectronic properties. The emergence of new colloidal methods for the synthesis of halide perovskite nanocrystals, as well as the interesting characteristics of this new type of material, has attracted the attention of many researchers. This review aims to provide an up-to-date survey of this fast-moving field and will mainly focus on the different colloidal synthesis approaches that have been developed. We will examine the chemistry and the capability of different colloidal synthetic routes with regard to controlling the shape, size, and optical properties of the resulting nanocrystals. We will also provide an up-to-date overview of their postsynthesis transformations, and summarize the various solution processes that are aimed at fabricating halide perovskite-based nanocomposites. Furthermore, we will review the fundamental optical properties of halide perovskite nanocrystals by focusing on their linear optical properties, on the effects of quantum confinement, and on the current knowledge of their exciton binding energies. We will also discuss the emergence of nonlinear phenomena such as multiphoton absorption, biexcitons, and carrier multiplication. Finally, we will discuss open questions and possible future directions.

Stable and Size Tunable CsPbBr3 Nanocrystals Synthesized with Oleylphosphonic Acid.

We employed oleylphosphonic acid (OLPA) for the synthesis of CsPbBr3 nanocrystals (NCs). Compared to phosphonic acids with linear alkyl chains, OLPA features a higher solubility in apolar solvents, allowing us to work at lower synthesis temperatures (100 °C), which in turn offer a good control over the NCs size. This can be reduced down to 5.0 nm, giving access to the strong quantum confinement regime. OLPA-based NCs form stable colloidal solutions at very low concentrations (∼1 nM), even when exposed to air. Such stability stems from the high solubility of OLPA in apolar solvents, which enables these molecules to reversibly bind/unbind to/from the NCs, preventing the NCs aggregation/precipitation. Small NCs feature efficient, blue-shifted emission and an ultraslow emission kinetics at cryogenic temperature, in striking difference to the fast decay of larger particles, suggesting that size-related exciton structure and/or trapping-detrapping dynamics determine the thermal equilibrium between coexisting radiative processes.

Cs3Cu4In2Cl13 Nanocrystals: A Perovskite-Related Structure with Inorganic Clusters at A Sites.

An effort to synthesize the Cu(I) variant of a lead-free double perovskite isostructural with Cs2AgInCl6 resulted in the formation of Cs3Cu4In2Cl13 nanocrystals with an unusual structure, as revealed by single-nanocrystal three-dimensional electron diffraction. These nanocrystals adopt a A2BX6 structure (K2PtCl6 type, termed vacancy ordered perovskite) with tetrahedrally coordinated Cu(I) ions. In the structure, 25% of the A sites are occupied by [Cu4Cl]3+ clusters (75% by Cs+), and the B sites are occupied by In3+. Such a Cs3Cu4In2Cl13 compound prepared at the nanoscale is not known in the bulk and is an example of a multinary metal halide with inorganic cluster cations residing in A sites. The stability of the compound was supported by density functional theory calculations that also revealed that its bandgap is direct but parity forbidden. The existence of the Cs3Cu4In2Cl13 structure demonstrates that small inorganic cluster cations can occupy A sites in multinary metal halides.

An Integrated Multilevel Analysis Profiling Biosafety and Toxicity Induced by Indium- and Cadmium-Based Quantum Dots in Vivo.

Indium phosphide quantum dots (QDs) have emerged as a new class of fluorescent nanocrystals for manifold applications, from biophotonics to nanomedicine. Recent efforts in improving the photoluminescence quantum yield, the chemical stability and the biocompatibility turned them into a valid alternative to well established Cd-based nanocrystals. In vitro studies provided first evidence for the lower toxicity of In-based QDs. Nonetheless, an urgent need exists for further assessment of the potential toxic effects in vivo. Here we use the freshwater polyp Hydra vulgaris, a well-established model previously adopted to assess the toxicity of CdSe/CdS nanorods and CdTe QDs. A systematic multilevel analysis was carried out in vivo, ex vivo, and in vitro comparing toxicity end points of CdSe- and InP-based QDs, passivated by ZnSe/ZnS shells and surface functionalized with penicillamine. Final results demonstrate that both the chemical composition of the QD core (InP vs CdSe) and the shell play a crucial role for final outcomes. Remarkably, in absence of in vivo alterations, cell and molecular alterations revealed hidden toxicity aspects, highlighting the biosafety of InP-based nanocrystals and outlining the importance of integrated multilevel analyses for proper QDs risk assessment.

Benzoyl Halides as Alternative Precursors for the Colloidal Synthesis of Lead-Based Halide Perovskite Nanocrystals.

We propose here a new colloidal approach for the synthesis of both all-inorganic and hybrid organic-inorganic lead halide perovskite nanocrystals (NCs). The main limitation of the protocols that are currently in use, such as the hot injection and the ligand-assisted reprecipitation routes, is that they employ PbX2 (X = Cl, Br, or I) salts as both lead and halide precursors. This imposes restrictions on being able to precisely tune the amount of reaction species and, consequently, on being able to regulate the composition of the final NCs. In order to overcome this issue, we show here that benzoyl halides can be efficiently used as halide sources to be injected in a solution of metal cations (mainly in the form of metal carboxylates) for the synthesis of APbX3 NCs (in which A = Cs+, CH3NH3+, or CH(NH2)2+). In this way, it is possible to independently tune the amount of both cations and halide precursors in the synthesis. The APbX3 NCs that were prepared with our protocol show excellent optical properties, such as high photoluminescence quantum yields, low amplified spontaneous emission thresholds, and enhanced stability in air. It is noteworthy that CsPbI3 NCs, which crystallize in the cubic α phase, are stable in air for weeks without any postsynthesis treatment. The improved properties of our CsPbX3 perovskite NCs can be ascribed to the formation of lead halide terminated surfaces, in which Cs cations are replaced by alkylammonium ions.

Colloidal Synthesis of Double Perovskite Cs2AgInCl6 and Mn-Doped Cs2AgInCl6 Nanocrystals.

We show here the first colloidal synthesis of double perovskite Cs2AgInCl6 nanocrystals (NCs) with a control over their size distribution. In our approach, metal carboxylate precursors and ligands (oleylamine and oleic acid) are dissolved in diphenyl ether and reacted at 105 °C with benzoyl chloride. The resulting Cs2AgInCl6 NCs exhibit the expected double perovskite crystal structure, are stable under air, and show a broad spectrum white photoluminescence (PL) with quantum yield of ∼1.6 ± 1%. The optical properties of these NCs were improved by synthesizing Mn-doped Cs2AgInCl6 NCs through the simple addition of Mn-acetate to the reaction mixture. The NC products were characterized by the same double perovskite crystal structure, and Mn doping levels up to 1.5%, as confirmed by elemental analyses. The effective incorporation of Mn ions inside Cs2AgInCl6 NCs was also proved by means of electron spin resonance spectroscopy. A bright orange emission characterized our Mn-doped Cs2AgInCl6 NCs with a PL quantum yield as high as ∼16 ± 4%.

Surface Characterization of TiO2 Polymorphic Nanocrystals through 1H-TD-NMR.

Nanocrystals (NCs) surface characterization is a fundamental step for understanding the physical and chemical phenomena involved at the nanoscale. Surface energy and chemistry depend on particle size and composition, and, in turn, determine the interaction of NCs with the surrounding environment, their properties and stability, and the feasibility of nanocomposites. This work aims at extracting more information on the surface of different titanium dioxide polymorphs using 1H-TD-NMR of water. Taking advantage of the interaction between water molecules and titanium dioxide NCs, it is possible to correlate the proton transverse relaxation times ( T2) as the function of the concentration and the specific surface area (δp· Cm) and use it as an indicator of the crystal phase. Examples of three different crystals phase, rutile, anatase, and brookite, have been finely characterized and their behavior in water solution have been studied with TD-NMR. The results show a linear correlation between relaxivity ( R2) and their concentration Cm. The resulting slopes, after normalization for the specific surface, represent the surface/water interaction and range from 1.28 g m-2 s-1 of 50 nm rutile nanocrystals to 0.52 for similar sized brookite. Even higher slopes (1.85) characterize smaller rutile NCs, in qualitative accordance with the trends of surface energy. Thanks to proton relaxation phenomena that occur at the NCs surface, it is possible to differentiate the crystal phase and the specific surface area of titanium dioxide polymorphs in water solution.

Ab Initio Structure Determination of Cu2- xTe Plasmonic Nanocrystals by Precession-Assisted Electron Diffraction Tomography and HAADF-STEM Imaging.

We investigated pseudo-cubic Cu2- xTe nanosheets using electron diffraction tomography and high-resolution HAADF-STEM imaging. The structure of this metastable nanomaterial, which has a strong localized surface plasmon resonance in the near-infrared region, was determined ab initio by 3D electron diffraction data recorded in low-dose nanobeam precession mode, using a new generation background-free single-electron detector. The presence of two different, crystallographically defined modulations creates a 3D connected vacancy channel system, which may account for the strong plasmonic response of this material. Moreover, a pervasive rotational twinning is observed for nanosheets as thin as 40 nm, resulting in a tetragonal pseudo-symmetry.

Forging Colloidal Nanostructures via Cation Exchange Reactions.

Among the various postsynthesis treatments of colloidal nanocrystals that have been developed to date, transformations by cation exchange have recently emerged as an extremely versatile tool that has given access to a wide variety of materials and nanostructures. One notable example in this direction is represented by partial cation exchange, by which preformed nanocrystals can be either transformed to alloy nanocrystals or to various types of nanoheterostructures possessing core/shell, segmented, or striped architectures. In this review, we provide an up to date overview of the complex colloidal nanostructures that could be prepared so far by cation exchange. At the same time, the review gives an account of the fundamental thermodynamic and kinetic parameters governing these types of reactions, as they are currently understood, and outlines the main open issues and possible future developments in the field.

Role of the Crystal Structure in Cation Exchange Reactions Involving Colloidal Cu2Se Nanocrystals.

Stoichiometric Cu2Se nanocrystals were synthesized in either cubic or hexagonal (metastable) crystal structures and used as the host material in cation exchange reactions with Pb2+ ions. Even if the final product of the exchange, in both cases, was rock-salt PbSe nanocrystals, we show here that the crystal structure of the starting nanocrystals has a strong influence on the exchange pathway. The exposure of cubic Cu2Se nanocrystals to Pb2+ cations led to the initial formation of PbSe unselectively on the overall surface of the host nanocrystals, generating Cu2Se@PbSe core@shell nanoheterostructures. The formation of such intermediates was attributed to the low diffusivity of Pb2+ ions inside the host lattice and to the absence of preferred entry points in cubic Cu2Se. On the other hand, in hexagonal Cu2Se nanocrystals, the entrance of Pb2+ ions generated PbSe stripes "sandwiched" in between hexagonal Cu2Se domains. These peculiar heterostructures formed as a consequence of the preferential diffusion of Pb2+ ions through specific (a, b) planes of the hexagonal Cu2Se structure, which are characterized by almost empty octahedral sites. Our findings suggest that the morphology of the nanoheterostructures, formed upon partial cation exchange reactions, is intimately connected not only to the NC host material, but also to its crystal structure.

Colloidal Synthesis of Bipolar Off-Stoichiometric Gallium Iron Oxide Spinel-Type Nanocrystals with Near-IR Plasmon Resonance.

We report the colloidal synthesis of ∼5.5 nm inverse spinel-type oxide Ga2FeO4 (GFO) nanocrystals (NCs) with control over the gallium and iron content. As recently theoretically predicted, some classes of spinel-type oxide materials can be intrinsically doped by means of structural disorder and/or change in stoichiometry. Here we show that, indeed, while stoichiometric Ga2FeO4 NCs are intrinsic small bandgap semiconductors, off-stoichiometric GFO NCs, produced under either Fe-rich or Ga-rich conditions, behave as degenerately doped semiconductors. As a consequence of the generation of free carriers, both Fe-rich and Ga-rich GFO NCs exhibit a localized surface plasmon resonance in the near-infrared at ∼1000 nm, as confirmed by our pump-probe absorption measurements. Noteworthy, the photoelectrochemical characterization of our GFO NCs reveal that the majority carriers are holes in Fe-rich samples, and electrons in Ga-rich ones, highlighting the bipolar nature of this material. The behavior of such off-stoichiometric NCs was explained by our density functional theory calculations as follows: the substitution of Ga3+ by Fe2+ ions, occurring in Fe-rich conditions, can generate free holes (p-type doping), while the replacement of Fe2+ by Ga3+ cations, taking place in Ga-rich samples, produces free electrons (n-type doping). These findings underscore the potential relevance of spinel-type oxides as p-type transparent conductive oxides and as plasmonic semiconductors.

Selective cation exchange in the core region of Cu2-xSe/Cu2-xS core/shell nanocrystals.

We studied cation exchange (CE) in core/shell Cu2-xSe/Cu2-xS nanorods with two cations, Ag(+) and Hg(2+), which are known to induce rapid exchange within metal chalcogenide nanocrystals (NCs) at room temperature. At the initial stage of the reaction, the guest ions diffused through the Cu2-xS shell and reached the Cu2-xSe core, replacing first Cu(+) ions within the latter region. These experiments prove that CE in copper chalcogenide NCs is facilitated by the high diffusivity of guest cations in the lattice, such that they can probe the whole host structure and identify the preferred regions where to initiate the exchange. For both guest ions, CE is thermodynamically driven as it aims for the formation of the chalcogen phase characterized by the lower solubility under the specific reaction conditions.

Sn cation valency dependence in cation exchange reactions involving Cu(2-x)Se nanocrystals.

We studied cation exchange reactions in colloidal Cu(2-x)Se nanocrystals (NCs) involving the replacement of Cu(+) cations with either Sn(2+) or Sn(4+) cations. This is a model system in several aspects: first, the +2 and +4 oxidation states for tin are relatively stable; in addition, the phase of the Cu(2-x)Se NCs remains cubic regardless of the degree of copper deficiency (that is, "x") in the NC lattice. Also, Sn(4+) ions are comparable in size to the Cu(+) ions, while Sn(2+) ones are much larger. We show here that the valency of the entering Sn ions dictates the structure and composition not only of the final products but also of the intermediate steps of the exchange. When Sn(4+) cations are used, alloyed Cu(2-4y)Sn(y)Se NCs (with y ≤ 0.33) are formed as intermediates, with almost no distortion of the anion framework, apart from a small contraction. In this exchange reaction the final stoichiometry of the NCs cannot go beyond Cu0.66Sn0.33Se (that is Cu2SnSe3), as any further replacement of Cu(+) cations with Sn(4+) cations would require a drastic reorganization of the anion framework, which is not possible at the reaction conditions of the experiments. When instead Sn(2+) cations are employed, SnSe NCs are formed, mostly in the orthorhombic phase, with significant, albeit not drastic, distortion of the anion framework. Intermediate steps in this exchange reaction are represented by Janus-type Cu(2-x)Se/SnSe heterostructures, with no Cu-Sn-Se alloys.