Tunable magnetic exchange interactions in manganese-doped inverted core/shell ZnSe/CdSe nanocrystals

Magnetic doping of semiconductor nanostructures is actively pursued for applications in magnetic memory and spin-based electronics. Central to these efforts is a drive to control the interaction strength between carriers (electrons and holes) and the embedded magnetic atoms. In this respect, colloidal nanocrystal heterostructures provide great flexibility via growth-controlled `engineering' of electron and hole wavefunctions within individual nanocrystals. Here we demonstrate a widely tunable magnetic sp-d exchange interaction between electron-hole excitations (excitons) and paramagnetic manganese ions using `inverted' core-shell nanocrystals composed of Mn-doped ZnSe cores overcoated with undoped shells of narrower-gap CdSe. Magnetic circular dichroism studies reveal giant Zeeman spin splittings of the band-edge exciton that, surprisingly, are tunable in both magnitude and sign. Effective exciton g-factors are controllably tuned from -200 to +30 solely by increasing the CdSe shell thickness, demonstrating that strong quantum confinement and wavefunction engineering in heterostructured nanocrystal materials can be utilized to manipulate carrier-Mn wavefunction overlap and the sp-d exchange parameters themselves.Comment: To appear in Nature Materials; 18 pages, 4 figures + Supp. Inf

Charged Polymer/Nanoparticle Mixtures: Monte Carlo Simulations

We used Monte-Carlo simulations to study the formation of complexes between charged polymers (or polyelectrolytes) with oppositely charged spherical nanoparticles. We presented the model, the Monte Carlo numerical method and investigated the effects of the ionic concentration of the solution, polyelectrolyte rigidity (or flexibility), linear charge density, and surface charge of the nanoparticles. Polyelectrolyte adsorption is controlled by several competing effects. On the one hand, rigidity and electrostatic repulsion force the polyelectrolyte to adopt extended conformations and limit the number of monomers which may be attached to the nanoparticles. On the other hand, electrostatic attractive interactions between the particle and the polyelectrolyte monomers force the chain to undergo a structural transition and collapse at the particle surface. By increasing the intrinsic rigidity, we observed a transition from disordered and strongly bound complexes to a situation where the polymer touches the particles over a finite length, while passing by the formation of a solenoid conformation. We found that the critical ionic concentration at which adsorption/desorption is observed rapidly increases with the increase of the nanoparticle surface charge density in good agreement with experimental data dealing with the formation of complexes between micelles and oppositely charged polyelectrolytes. Adsorption is also promoted by decreasing the chain stiffness or decreasing the salt concentration for a given chain length

Intramolecular motion in dibenzobarrelenephosphinyl radical: A single crystal EPR study at variable temperature

The g, 31P and 1H hyperfine tensors of the dibenzobarrelene phosphinyl radical, trapped in an X-irradiated single crystal of dibenzobarrelene phosphine, were estimated at 45 and 300 K. They indicate that among the three locations of the phosphinyl hydrogen expected from DFT calculations, only two are occupied at 40 K and that the third one remains practically vacant, even at 300 K. The temperature dependence of the EPR spectrum was simulated by assuming jumps between two P–H bond orientations (energy barrier 0.5 kcal mol−1) which correspond to the conformation of the PH2 moiety in the only rotamer present in the dibenzobarrelene phosphine crystal

Theoretical Investigation of the Anion Binding Affinities of the Uranyl Salophene Complexes

The uranyl salophene complex and its co-complexes with several anions (H[2]PO[4][-], HSO[4][-], NO[2][-], OH[-], Cl[-], F[-]) in the gas phase are investigated theoretically. Equilibrium geometries of relevant species and complexation-induced structural changes are discussed. The [13]C NMR chemical shifts calculated at the gas-phase optimized geometry agree very well with experimental liquid-phase results. The optimized geometry agrees also very well with available crystallographic data. This indicates that the gas-phase structures derived from theoretical calculations can be considered representative also for the condensed phase. For all anions, except H[2]PO[4][-], the calculated gas-phase binding energies correlate well with experimental Gibbs free energies of complexation. The possible role of the solvent in the case of H[2]PO[4][-] complexation is discussed

Analysing the chromium-chromium multiple bond using multiconfigurational quantum chemistry

This Letter discusses the nature of the chemical bond between two chromium atoms in different di-chromium complexes with the metal atoms in different oxidation states. Starting with the Cr diatom, with its formally sextuple bond and oxidation number zero, we proceed to analyse the bonding in some Cr(I)–Cr(I) XCrCrX complexes with X varying from F, to Phenyl, and Aryl. The bond distance in these complexes varies over a large range: 1.65–1.83 Å and we suggest explanations for these variations. A number of di-chromium complexes with bond distances around or shorter than 1.80 Å have recently been synthesized and we study one of these complexes, Cr2(diazadiene)2 and show how the Cr–Cr bond order is related to the oxidation number and the ligand bonding, factors that are all involved in the determination of the short Cr–Cr bond length: 1.80 Å. The discussion is based on the use of multiconfigurational wave functions, which give a qualitatively correct description of the electronic structure in these multiply bonded systems

Air-stable crystalline primary phosphines and germanes : synthesis and crystal structures of dibenzobarellenephosphine and tribenzobarellenegermane

The syntheses of dibenzobarellenephosphine and tribenzobarellenegermane are described; at room temperature these primary phosphines and germanes form air-stable crystals whose structures are reported together with that of tribenzobarellenemethane

Dynamic phenomena in barrelenephosphinyl radicals: a complementary approach by density matrix analysis of EPR spectra and DFT calculations

The paper shows the possibilities of the complementary use of the density matrix formalism for the simulation of the anisotropic EPR spectra and the DFT potential energy surface calculations to obtain a detailed picture of the motions of radical molecules. The combined approach is illustrated by a comparative EPR study of three phosphorus derivatives of barrelene. Three compounds were chosen as the model molecules for the observation of different temperature dependent dynamics of radical fragment. Each molecule based on the same barrelene skeleton has a different set of substituents which by influencing the local chemical environment are likely to modify the internal dynamics. The temperature dependent EPR spectra are simulated by means of the density matrix formalism and the geometry of radicals are calculated with DFT. The motion is described in terms of rotational barriers, DFT calculated energy profiles and hypothetical intramolecular distortions. These two approaches lead to a similar microscopic picture of the intramolecular radical motion