Light-Induced Inhibition of Photoluminescence Emission of Core/Shell Semiconductor Nanorods and Its Application for Optical Data Storage

In this work, we demonstrate that the photoluminescence emission of CdSe/CdS spherical core/rod like shell nanorods can be completely inhibited by using low-energy femtosecond laser pulses at 720 nm. A combined analysis of optical confocal microscopy images and transmission electron microscope micrographs reveals the presence of different power regimes in the nanocrystals photodegradation process. The photoluminescence inhibition of the nanorods is found to start at a power range in which no apparent structural damage occurs to the nanorods after irradiation. This suggests the presence of a photochemical transformation of the nanocrystals that is of potential interest for application in optical data storage. This is because in recording systems based on photochemical processes the photoexcited volume can be effectively confined within the diffraction-limited laser focus. We indeed demonstrate that the recorded mark can be scaled down to a few nanorods, or even to a single nanorod, without crosstalk between adjacent nanostructures separated by the optical resolution of the instrument used. Finally, the intensities required for inhibiting the emission of nanorods and for avoiding any peripheral thermal damage of the hosting polymer matrix are determined (200–360 GW/cm²), and the mechanism underlying the photochemical process is discussed

Mobility and Spatial Distribution of Photoexcited Electrons in CdSe/CdS Nanorods

The mobility and spatial distribution of photoexcited electrons in CdSe/CdS core/shell nanorods was studied using optical-pump THz-probe spectroscopy. Measurements were conducted on two samples, differing in rod length. After photoexcitation the hole localizes in the CdSe core within a picosecond, while the electron delocalizes around the core. Analysis of the THz mobility with a model of one-dimensional electron diffusion on a finite rod yields an electron delocalization of ∼25% into the CdS shell and a mobility of 700 cm²/(V s). This is one and a half times the mobility value for bulk CdS, which can be due to quantum confinement effects on electron–phonon scattering and electronic structure

Atomic Ligand Passivation of Colloidal Nanocrystal Films via their Reaction with Propyltrichlorosilane

Colloidal nanocrystal films of different materials (semiconductors, metals) and shapes (spheres and rods) were dipped in solutions of propyltrichlorosilane (PTCS) in acetonitrile. This process removed most of the surfactants covering the surface of the tested nanocrystals, leaving their surface either unpassivated or passivated with chlorine atoms, depending on their composition. PTCS was reactive toward most of the surfactants used in nanocrystal synthesis and therefore such a procedure could be applied to a large variety of materials. All samples were characterized with FTIR, XRD, and XPS measurements. In nanocrystal films, the reduction of the separation between the nanocrystals resulting from the removal of surfactants led to an enhancement in both dark and photocurrent. The surface of Au nanocrystals is left unpassivated by the reaction with PTCS, which makes the process potentially useful for applications in catalysis and plasmonics

Blue-UV-Emitting ZnSe(Dot)/ZnS(Rod) Core/Shell Nanocrystals Prepared from CdSe/CdS Nanocrystals by Sequential Cation Exchange

Great control over size, shape and optical properties is now possible in colloidal Cd-based nanocrystals, which has paved the way for many fundamental studies and applications. One popular example of such class of nanocrystals is represented by CdSe(spherical core)/CdS(rod shell) nanorods. These can be nearly monodisperse in size and shape and have strong and stable photoluminescence that is tunable in the visible range (mainly by varying the size of the CdSe core). The corresponding Zn-based core/shell nanorods would be good candidates for tunable emission in the blue-UV region. However, while the synthesis of ZnS nanocrystals with elongated shapes has been demonstrated based on the oriented-attachment mechanism, elongated ZnS shells are difficult to fabricate because the more common cubic phase of ZnS has a highly symmetric crystal structure. We report here a procedure based on a sequence of two cation exchange reactions, namely, Cd²⁺⇒Cu⁺ and then Cu⁺⇒Zn²⁺, by which we transform colloidal CdSe(core)/CdS(shell) nanorods first into into Cu₂Se/Cu₂S nanorods, which are then converted into blue-UV fluorescent ZnSe(core)/ZnS(shell) nanorods. The procedure transfers the morphological and structural information of the initial Cd-based nanorods to the Zn-based nanorods. Therefore, the final nanoparticles are made by a ZnSe dot embedded in a rod-shaped shell of wurtzite ZnS. Since in the starting Cd-based nanorods the size of the CdSe core and the length of the CdS shell can be well controlled, the same holds for the final Zn-based rods. In the second step of the exchange reaction (Cu⁺⇒Zn²⁺), a large excess of Zn²⁺ ions added over the Cu⁺ ions present in the Cu₂Se/Cu₂S nanorods is the key requisite to obtain bright, band-edge emission (with quantum yields approaching 15%) with narrow line widths (approaching 75 meV). In these ZnSe/ZnS nanorods, photogenerated carriers appear to be more confined in the core region compared to their parent CdSe/CdS nanorods

Control of Ionic Conductivity by Lithium Distribution in Cubic Oxide Argyrodites Li_6+<i>x</i>P_1–<i>x</i>Si_<i>x</i>O₅Cl

Argyrodite is a key structure type for ion-transporting materials. Oxide argyrodites are largely unexplored despite sulfide argyrodites being a leading family of solid-state lithium-ion conductors, in which the control of lithium distribution over a wide range of available sites strongly influences the conductivity. We present a new cubic Li-rich (>6 Li+ per formula unit) oxide argyrodite Li7SiO5Cl that crystallizes with an ordered cubic (P213) structure at room temperature, undergoing a transition at 473 K to a Li+ site disordered F4̅3m structure, consistent with the symmetry adopted by superionic sulfide argyrodites. Four different Li+ sites are occupied in Li7SiO5Cl (T5, T5a, T3, and T4), the combination of which is previously unreported for Li-containing argyrodites. The disordered F4̅3m structure is stabilized to room temperature via substitution of Si4+ with P5+ in Li6+xP1–xSixO5Cl (0.3 x < 0.85) solid solution. The resulting delocalization of Li+ sites leads to a maximum ionic conductivity of 1.82(1) × 10–6 S cm–1 at x = 0.75, which is 3 orders of magnitude higher than the conductivities reported previously for oxide argyrodites. The variation of ionic conductivity with composition in Li6+xP1–xSixO5Cl is directly connected to structural changes occurring within the Li+ sublattice. These materials present superior atmospheric stability over analogous sulfide argyrodites and are stable against Li metal. The ability to control the ionic conductivity through structure and composition emphasizes the advances that can be made with further research in the open field of oxide argyrodites

Bi₄O₄Cu_1.7Se_2.7Cl_0.3: Intergrowth of BiOCuSe and Bi₂O₂Se Stabilized by the Addition of a Third Anion

Layered two-anion compounds are of interest for their diverse electronic properties. The modular nature of their layered structures offers opportunities for the construction of complex stackings used to introduce or tune functionality, but the accessible layer combinations are limited by the crystal chemistries of the available anions. We present a layered three-anion material, Bi₄O₄Cu_1.7Se_2.7Cl_0.3, which adopts a new structure type composed of alternately stacked BiOCuSe and Bi₂O₂Se-like units. This structure is accessed by inclusion of three chemically distinct anions, which are accommodated by aliovalently substituted Bi₂O₂Se_0.7Cl_0.3 blocks coupled to Cu-deficient Bi₂O₂Cu_1.7Se₂ blocks, producing a formal charge modulation along the stacking direction. The hypothetical parent phase Bi₄O₄Cu₂Se₃ is unstable with respect to its charge-neutral stoichiometric building blocks. The complex layer stacking confers excellent thermal properties upon Bi₄O₄Cu_1.7Se_2.7Cl_0.3: a room-temperature thermal conductivity (κ) of 0.4(1) W/mK was measured on a pellet with preferred crystallite orientation along the stacking axis, with perpendicular measurement indicating it is also highly anisotropic. This κ value lies in the ultralow regime and is smaller than those of both BiOCuSe and Bi₂O₂Se. Bi₄O₄Cu_1.7Se_2.7Cl_0.3 behaves like a charge-balanced semiconductor with a narrow band gap. The chemical diversity offered by the additional anion allows the integration of two common structural units in a single phase by the simultaneous and coupled creation of charge-balancing defects in each of the units

Room Temperature Magnetically Ordered Polar Corundum GaFeO₃ Displaying Magnetoelectric Coupling

The polar corundum structure type offers a route to new room temperature multiferroic materials, as the partial LiNbO₃-type cation ordering that breaks inversion symmetry may be combined with long-range magnetic ordering of high spin <i>d</i>⁵ cations above room temperature in the <i>A</i>FeO₃ system. We report the synthesis of a polar corundum GaFeO₃ by a high-pressure, high-temperature route and demonstrate that its polarity arises from partial LiNbO₃-type cation ordering by complementary use of neutron, X-ray, and electron diffraction methods. In situ neutron diffraction shows that the polar corundum forms directly from AlFeO₃-type GaFeO₃ under the synthesis conditions. The <i>A</i>³⁺/Fe³⁺ cations are shown to be more ordered in polar corundum GaFeO₃ than in isostructural ScFeO₃. This is explained by DFT calculations which indicate that the extent of ordering is dependent on the configurational entropy available to each system at the very different synthesis temperatures required to form their corundum structures. Polar corundum GaFeO₃ exhibits weak ferromagnetism at room temperature that arises from its Fe₂O₃-like magnetic ordering, which persists to a temperature of 408 K. We demonstrate that the polarity and magnetization are coupled in this system with a measured linear magnetoelectric coupling coefficient of 0.057 ps/m. Such coupling is a prerequisite for potential applications of polar corundum materials in multiferroic/magnetoelectric devices

Room Temperature Magnetically Ordered Polar Corundum GaFeO₃ Displaying Magnetoelectric Coupling

The polar corundum structure type offers a route to new room temperature multiferroic materials, as the partial LiNbO₃-type cation ordering that breaks inversion symmetry may be combined with long-range magnetic ordering of high spin <i>d</i>⁵ cations above room temperature in the <i>A</i>FeO₃ system. We report the synthesis of a polar corundum GaFeO₃ by a high-pressure, high-temperature route and demonstrate that its polarity arises from partial LiNbO₃-type cation ordering by complementary use of neutron, X-ray, and electron diffraction methods. In situ neutron diffraction shows that the polar corundum forms directly from AlFeO₃-type GaFeO₃ under the synthesis conditions. The <i>A</i>³⁺/Fe³⁺ cations are shown to be more ordered in polar corundum GaFeO₃ than in isostructural ScFeO₃. This is explained by DFT calculations which indicate that the extent of ordering is dependent on the configurational entropy available to each system at the very different synthesis temperatures required to form their corundum structures. Polar corundum GaFeO₃ exhibits weak ferromagnetism at room temperature that arises from its Fe₂O₃-like magnetic ordering, which persists to a temperature of 408 K. We demonstrate that the polarity and magnetization are coupled in this system with a measured linear magnetoelectric coupling coefficient of 0.057 ps/m. Such coupling is a prerequisite for potential applications of polar corundum materials in multiferroic/magnetoelectric devices