A Selective Cation Exchange Strategy for the Synthesis of Colloidal Yb³⁺-Doped Chalcogenide Nanocrystals with Strong Broadband Visible Absorption and Long-Lived Near-Infrared Emission

Doping lanthanide ions into colloidal semiconductor nanocrystals is a promising strategy for combining their sharp and efficient 4<i>f</i>–4<i>f</i> 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 Yb³⁺-doped NaInS₂ nanocrystals into Yb³⁺-doped PbIn₂S₄ nanocrystals. Excitation spectra and time-resolved photoluminescence measurements confirm that Yb³⁺ is both incorporated within the PbIn₂S₄ 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

Valence-Band Mixing Effects in the Upper-Excited-State Magneto-Optical Responses of Colloidal Mn²⁺-Doped CdSe Quantum Dots

We present an experimental study of the magneto-optical activity of multiple excited excitonic states of manganese-doped CdSe quantum dots chemically prepared by the diffusion doping method. Giant excitonic Zeeman splittings of each of these excited states can be extracted for a series of quantum dot sizes and are found to depend on the radial quantum number of the hole envelope function involved in each transition. As seven out of eight transitions involve the same electron energy state, 1S_e, the dominant hole character of each excitonic transition can be identified, making use of the fact that the <i>g</i>-factor of the pure heavy-hole component has a different sign compared to pure light hole or split-off components. Because the magnetic exchange interactions are sensitive to hole state mixing, the giant Zeeman splittings reported here provide clear experimental evidence of quantum-size-induced mixing among valence-band states in nanocrystals

Mid-Gap States and Normal vs Inverted Bonding in Luminescent Cu⁺- and Ag⁺‑Doped CdSe Nanocrystals

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(3<i>d</i>) orbitals, formally oxidizing Cu⁺ to Cu²⁺, in Ag⁺:CdSe NCs they localize primarily in 4<i>p</i> orbitals of the four neighboring Se^2– 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 [MSe₄] 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

Quantum Confinement-Controlled Exchange Coupling in Manganese(II)-Doped CdSe Two-Dimensional Quantum Well Nanoribbons

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 similar to 13 encouraging the implementation of the DMS quantum well nanoribbons for (room temperature) spintronic applications.