8 research outputs found
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 IIIâV sem..
Evidence for the Band-Edge Exciton of CuInS2 Nanocrystals Enables Record Efficient Large-Area Luminescent Solar Concentrators
AbstractTernary IâIIIâVI2 nanocrystals (NCs), such as CuInS2, are receiving attention as heavyâmetalsâfree materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bioâimaging. The origin of the optical properties of CuInS2 NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokesâshifted and longâlived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviours in defectâfree NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by lowâbandgap NCs would be accompanied by increased reâabsorption losses. Here, by studying stoichiometric CuInS2 NCs, it is revealed for the first time the spectroscopic signatures predicted for the free bandâedge exciton, thus supporting the VBâstructure model. At very low temperatures, the NCs also show darkâstate emission likely originating from enhanced electronâhole spin interaction. The impact of the observed optical behaviours on LSCs is evaluated by Monte Carlo rayâtracing simulations. Based on the emerging device design guidelines, opticalâgrade largeâarea (30Ă30 cm2) LSCs with optical power efficiency (OPE) as high as 6.8% are fabricated, corresponding to the highest value reported to date for largeâarea devices
Role of the Crystal Structure in Cation Exchange Reactions Involving Colloidal Cu<sub>2</sub>Se Nanocrystals
Stoichiometric Cu<sub>2</sub>Se nanocrystals
were synthesized in
either cubic or hexagonal (metastable) crystal structures and used
as the host material in cation exchange reactions with Pb<sup>2+</sup> 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 Cu<sub>2</sub>Se nanocrystals to Pb<sup>2+</sup> cations led to the initial formation of PbSe unselectively
on the overall surface of the host nanocrystals, generating Cu<sub>2</sub>Se@PbSe core@shell nanoheterostructures. The formation of
such intermediates was attributed to the low diffusivity of Pb<sup>2+</sup> ions inside the host lattice and to the absence of preferred
entry points in cubic Cu<sub>2</sub>Se. On the other hand, in hexagonal
Cu<sub>2</sub>Se nanocrystals, the entrance of Pb<sup>2+</sup> ions
generated PbSe stripes âsandwichedâ in between hexagonal
Cu<sub>2</sub>Se domains. These peculiar heterostructures formed as
a consequence of the preferential diffusion of Pb<sup>2+</sup> ions
through specific (<i>a</i>, <i>b</i>) planes of
the hexagonal Cu<sub>2</sub>Se 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
Selective Cation Exchange in the Core Region of Cu<sub>2â<i>x</i></sub>Se/Cu<sub>2â<i>x</i></sub>S Core/Shell Nanocrystals
We studied cation exchange (CE) in
core/shell Cu<sub>2â<i>x</i></sub>Se/Cu<sub>2â<i>x</i></sub>S nanorods
with two cations, Ag<sup>+</sup> and Hg<sup>2+</sup>, 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 Cu<sub>2â<i>x</i></sub>S shell and reached the Cu<sub>2â<i>x</i></sub>Se
core, replacing first Cu<sup>+</sup> 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
Quantized Electronic Doping towards Atomically Controlled "charge-Engineered" Semiconductor Nanocrystals
"Charge engineering" of semiconductor nanocrystals (NCs) through so-called electronic impurity doping is a long-standing challenge in colloidal chemistry and holds promise for ground-breaking advancements in many optoelectronic, photonic, and spin-based nanotechnologies. To date, our knowledge is limited to a few paradigmatic studies on a small number of model compounds and doping conditions, with important electronic dopants still unexplored in nanoscale systems. Equally importantly, fine-tuning of charge engineered NCs is hampered by the statistical limitations of traditional approaches. The resulting intrinsic doping inhomogeneity restricts fundamental studies to statistically averaged behaviors and complicates the realization of advanced device concepts based on their advantageous functionalities. Here we aim to address these issues by realizing the first example of II-VI NCs electronically doped with an exact number of heterovalent gold atoms, a known p-type acceptor impurity in bulk chalcogenides. Single-dopant accuracy across entire NC ensembles is obtained through a novel non-injection synthesis employing ligand-exchanged gold clusters as "quantized" dopant sources to seed the nucleation of CdSe NCs in organic media. Structural, spectroscopic, and magneto-optical investigations trace a comprehensive picture of the physical processes resulting from the exact doping level of the NCs. Gold atoms, doped here for the first time into II-VI NCs, are found to incorporate as nonmagnetic Au + species activating intense size-tunable intragap photoluminescence and artificially offsetting the hole occupancy of valence band states. Fundamentally, the transient conversion of Au + to paramagnetic Au 2+ (5d 9 configuration) under optical excitation results in strong photoinduced magnetism and diluted magnetic semiconductor behavior revealing the contribution of individual paramagnetic impurities to the macroscopic magnetism of the NCs. Altogether, our results demonstrate a new chemical approach toward NCs with physical functionalities tailored to the single impurity level and offer a versatile platform for future investigations and device exploitation of individual and collective impurity processes in quantum confined structures