Mid-gap luminescence in copper (Cu)-doped semiconductor nanocrystals (NCs) involves recombination of delocalized conduction-band electrons with copper-localized holes. Silver (Ag)-doped semiconductor NCs show similar mid-gap luminescence at slightly (∼0.3 eV) higher energy, suggesting a similar luminescence mechanism, but this suggestion appears inconsistent with the large difference between Ag and Cu ionization energies (∼1.5 eV), which should make hole trapping by Ag highly unfavorable. Here, Ag-doped CdSe NCs (Ag:CdSe) are studied using time-resolved variable-temperature photoluminescence (PL) spectroscopy, magnetic circularly polarized luminescence (MCPL) spectroscopy, and time-dependent density functional theory (TD-DFT) to address this apparent paradox. In addition to confirming that Ag:CdSe and Cu:CdSe NCs display similar broad PL with large Stokes shifts, we demonstrate that both also show very similar temperature-dependent PL lifetimes and magneto-luminescence. Electronic-structure calculations further predict that both dopants generate similar localized mid-gap states. Despite these strong similarities, we conclude that these materials possess significantly different electronic structures. Specifically, whereas photogenerated holes in Cu:CdSe NCs localize primarily in Cu(3d) orbitals, formally oxidizing Cu to Cu, in Ag:CdSe NCs they localize primarily in 4p orbitals of the four neighboring Se ligands, and Ag is not oxidized. This difference reflects a shift from "normal" to "inverted" bonding going from Cu to Ag. The spectroscopic similarities are explained by the fact that, in both materials, photogenerated holes are localized primarily within covalent [MSe] dopant clusters (M = Ag, Cu). These findings reconcile the similar spectroscopies of Ag- and Cu-doped semiconductor NCs with the vastly different ionization potentials of their Ag and Cu dopants.
Nanoclusters are important prenucleation intermediates for colloidal nanocrystal synthesis. In addition, they exhibit many intriguing properties originating from their extremely small size lying between molecules and typical nanocrystals. However, synthetic control of multicomponent semiconductor nanoclusters remains a daunting goal. Here, we report on the synthesis, doping, and transformation of multielement magic-sized clusters, generating the smallest semiconductor alloys. We use Lewis acid-base reactions at room temperature to synthesize alloy clusters containing three or four types of atoms. Mass spectrometry reveals that the alloy clusters exhibit "magic-size" characteristics with chemical formula of ZnCdSe (x = 0-13) whose compositions are tunable between CdSe and ZnSe. Successful doping of these clusters creates a new class of diluted magnetic semiconductors in the extreme quantum confinement regime. Furthermore, the important role of these alloy clusters as prenucleation intermediates is demonstrated by low temperature transformation into quantum alloy nanoribbons and nanorods. Our study will facilitate the understanding of these novel diluted magnetic semiconductor nanoclusters, and offer new possibilities for the controlled synthesis of nanomaterials at the prenucleation stage, consequently producing novel multicomponent nanomaterials that are difficult to synthesize.
Doping semiconductor nanocrystals with magnetic transition-metal ions has attracted fundamental interest to obtain a nanoscale dilute magnetic semiconductor, which has unique spin exchange interaction between magnetic spin and exciton. So far, the study on the doped semiconductor NCs has usually been conducted with NCs with larger than 2 nm because of synthetic challenges. Herein, we report the synthesis and characterization of Mn(2+)-doped (CdSe)13 clusters, the smallest doped semiconductors. In this study, single-sized doped clusters are produced in large scale. Despite their small size, these clusters have semiconductor band structure instead of that of molecules. Surprisingly, the clusters show multiple excitonic transitions with different magneto-optical activities, which can be attributed to the fine structure splitting. Magneto-optically active states exhibit giant Zeeman splittings up to elevated temperatures (128 K) with large g-factors of 81(±8) at 4 K. Our results present a new synthetic method for doped clusters and facilitate the understanding of doped semiconductor at the boundary of molecules and quantum nanostructure.
Doping lanthanide ions into colloidal semiconductor nanocrystals is a promising strategy for combining their sharp and efficient 4f-4f emission with the strong broadband absorption and low-phonon-energy crystalline environment of semiconductors to make new solution-processable spectral-conversion nanophosphors, but synthesis of this class of materials has proven extraordinarily challenging because of fundamental chemical incompatibilities between lanthanides and most intermediate-gap semiconductors. Here, we present a new strategy for accessing lanthanide-doped visible-light-absorbing semiconductor nanocrystals by demonstrating selective cation exchange to convert precursor Yb-doped NaInS nanocrystals into Yb-doped PbInS nanocrystals. Excitation spectra and time-resolved photoluminescence measurements confirm that Yb is both incorporated within the PbInS nanocrystals and sensitized by visible-light photoexcitation of these nanocrystals. This combination of strong broadband visible absorption, sharp near-infrared emission, and long (>400 μs) emission lifetimes in a colloidal nanocrystal system opens promising new opportunities for both fundamental-science and next-generation spectral-conversion applications such as luminescent solar concentrators.
Magic-sized semiconductor clusters represent an exciting class of materials located at the boundary between quantum dots and molecules. It is expected that replacing single atoms of the host crystal with individual dopants in a one-by-one fashion can lead to unique modifications of the material properties. Here, we demonstrate the dependence of the magneto-optical response of (CdSe)13 clusters on the discrete number of Mn(2+) ion dopants. Using time-of-flight mass spectrometry, we are able to distinguish undoped, monodoped, and bidoped cluster species, allowing for an extraction of the relative amount of each species for a specific average doping concentration. A giant magneto-optical response is observed up to room temperature with clear evidence that exclusively monodoped clusters are magneto-optically active, whereas the Mn(2+) ions in bidoped clusters couple antiferromagnetically and are magneto-optically passive. Mn(2+)-doped clusters therefore represent a system where magneto-optical functionality is caused by solitary dopants, which might be beneficial for future solotronic applications.
Replacing a single atom of a host semiconductor nanocrystal with a functional dopant can introduce completely new properties potentially valuable for "solotronic" information-processing applications. Here, we report successful doping of colloidal CdSe quantum dots with a very small number of manganese ions-down to the ultimate limit of one. Single-particle spectroscopy reveals spectral fingerprints of the spin-spin interactions between individual dopants and quantum-dot excitons. Spectrally well-resolved emission peaks are observed that can be related to the discrete spin projections of individual Mn ions. In agreement with theoretical predictions, the exchange splittings are enhanced by more than an order of magnitude in these quantum dots compared to their epitaxial counterparts, opening a path for solotronic applications at elevated temperatures.
The impact of quantum confinement on the exchange interaction between charge carriers and magnetic dopants in semiconductor nanomaterials has been controversially discussed for more than a decade. We developed manganese-doped CdSe quantum well nanoribbons with a strong quantum confinement perpendicular to the c-axis, showing distinct heavy hole and light hole resonances up to 300 K. This allows a separate study of the s-d and the p-d exchange interactions all the way up to room temperature. Taking into account the optical selection rules and the statistical distribution of the nanoribbons orientation on the substrate, a remarkable change in particular of the s-d exchange constant with respect to bulk is indicated. Room-temperature studies revealed an unusually high effective g-factor up to ~13 encouraging the implementation of the DMS quantum well nanoribbons for (room temperature) spintronic applications.
Unlike in more heavily doped quantum dots, however, the MCD intensity at the first CdSe exciton shows a field-induced sign flip as the field strength is increased, reflecting competition between sp-d exchange and the intrinsic Zeeman splittings of comparable magnitude. Most unusually, the competition between these two effects leads to a large apparent shift in the first MCD peak maximum, which we show is attributable to a difference in sign of the intrinsic excitonic g factor between the first and second excitons. Finally, the sp-d and intrinsic contributions to the excitonic Zeeman splittings each exhibit unique magnetic-field and temperature dependencies, allowing the MCD spectra of undoped, singly doped, and bi-doped quantum dot sub-ensembles to be analyzed.
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