Ultraviolet and Infrared luminescent Au-rich nanostructure growth in SiO2 by burrowing and inverse Oswald ripening process

We study the evolution of nanoparticle morphology through successive stages when Au-Si bilayer on SiO₂ is irradiated with 500 keV Xe-ions and resulting luminescence in the UV, Visible and infrared range. An array of nanoscale island morphology is developed on the silica surface in the initial stage of evolution which undergoes gradual burrowing in the silica matrix accompanied by elongation of large ones in the direction of incident ions under cumulative ion irradiation. Burrowing is found to occur in order to minimize the surface free energy of the nanoparticles. Numerical simulation based on the unified thermal spike model shows formation of molten latent tracks due to ions energy release which drive the dewetting of the metal layer and further give mobility to nanoparticle atoms leading to burrowing in the later stage of evolution and elongation of large nanoparticles. Satellite nanoparticles are observed to grow around Au nanoparticles embedded in silica through nucleation of Au atoms dissolved in the matrix by incident ions. The varying diameters of the Au satellite nanoparticles seem to result in luminescence in the UV and infrared range. The observed structure may find application in surface enhanced Raman scattering, catalysis, and LEDs.

Light Emitting Spin Active Electronic States in Ultra-Thin Mn Doped CdSe Layered Nanosheets

The layered nanosheets exhibit a variety of physical and optical properties originating from amalgamation of intra- and inter- layer electronic interactions, which makes them promising materials for advanced devices with varsatile controlling channels. In particular, the dilute magnetic semiconductor multilayered nanosheets have promising optical, electrical and magnetic properties that have been less explored so far. Here, the spin permissible optical properties from solvothermally grown Mn doped CdSe (thickness ~2.26 nm) multilayered nanosheets are reported on. The presence of multi-phase magnetic orderings with a sharp ferromagnetic transition at temperature ~48 K pertinent to the stabilization and co-existence of Mn²⁺ and Mn³⁺ based local phases have been observed from the (Cd,Mn)Se layered nanosheets corroborating to the x-ray absorption near edge structure, electron paramagnetic resonance, Raman scattering and magnetic measurements. The optical absorption and photoluminescence (PL) studies at room temperature affirm wide array of optical properties in the visible regime corresponding to the band edge and intriguing dopant-phase mediated spin approved transitions. The circularly polarized magneto-PL and life time analysis exhibits the spin-polarized fast radiative transitions confirming the presence of spin-active electronic states.

High density nonmagnetic cobalt in thin films

Recently high density (HD) nonmagnetic cobalt has been discovered in a nanoscale cobalt thin film, grown on Si(111) single crystal. This form of cobalt is not only nonmagnetic but also superconducting. These promising results have encouraged further investigations of the growth of the nonmagnetic (NM) phase of cobalt. In the original investigation, the cobalt film had a natural cobalt oxide at the top. We have investigated whether the growth of HD NM cobalt layers in the thin film depends on (i) a capping layer on the cobalt film, (ii) the thickness of the cobalt film and (iii) the nature of the substrate on which the cobalt film is grown. The results of such investigations indicate that for cobalt films capped with a thin gold layer, and for various film thicknesses, HD NM cobalt layers are formed. However, instead of a Si substrate, when the cobalt films are grown on oxide substrates, such as silicon oxide or cobalt oxide, HD NM cobalt layers are not formed. The difference is attributed to the nature-crystalline or amorphous-of the substrate.

Evidence of Formation of Superdense Nonmagnetic Cobalt

Because of the presence of 3d transition metals in the Earth’s core, magnetism of these materials in their dense phases has been a topic of great interest. Theory predicts a dense face-centred-cubic phase of cobalt, which would be nonmagnetic. However, this dense nonmagnetic cobalt has not yet been observed. Recent investigations in thin film polycrystalline materials have shown the formation of compressive stress, which can increase the density of materials. We have discovered the existence of ultrathin superdense nonmagnetic cobalt layers in a polycrystalline cobalt thin film. The densities of these layers are about 1.2–1.4 times the normal density of Co. This has been revealed by X-ray reflectometry experiments, and corroborated by polarized neutron reflectometry (PNR) experiments. Transmission electron microscopy provides further evidence. The magnetic depth profile, obtained by PNR, shows that the superdense Co layers near the top of the film and at the film-substrate interface are nonmagnetic. The major part of the Co film has the usual density and magnetic moment. These results indicate the possibility of existence of nonmagnetic Co in the earth’s core under high pressure.

Exploiting Le Chatelier's principle for a one-pot synthesis of nontoxic HHogGNPs with the sharpest nanoscopic features suitable for tunable plasmon spectroscopy and high throughput SERS sensing

Le Chatelier's principle leads to the generation of nontoxic, stable and highly anisotropic HHogGNPs.

White-light emission by phonon assisted coherent mixing of excitons in Au8-CdS hybrid nanorods

Gold cluster (Au₈) coated CdS hybrid nanorods (HNRs), synthesized using a sonication assisted assembly route, exhibit phonon assisted coherent mixing of excitons. As observed from optical absorption, Raman scattering, x-ray diffraction and transmission electron microscopic studies, the Au₈ modulates the crystal-and electronic-structure of the CdS nanorods, effecting enhancement of exciton-phonon (e-p) interactions. The e-p interaction and entropy effect mediated phase matching of the excitonic transitions, leading-via cooperative and coherent mixing of the excitons' color-to the emission of white light, has been confirmed from room temperature and time resolved photoluminescence measurements.

Green silver nanoparticles for drug transport, bioactivities and a bacterium (Bacillus subtilis)-mediated comparative nano-patterning feature

Supramolecular hydrogel-capped non-toxic Ag NPs are effective for cellular drug transport and are potentially bioactive, which also leads to the formation of novel silver nanoparticles.

Regrowth of Ge with different degrees of damage under thermal and athermal treatment

In this report, the recrystallization of pre-damaged Ge samples is extensively investigated under steady-state thermal annealing and ultrafast thermal spike-assisted annealing generated by high-energy ions.

Formation of Monodispersed Films from Size-Selected Copper Nanoclusters

Deposition of size-selected metal nanoclusters on a substrate with very low kinetic energy helps to keep the clusters intact with respect to their shapes and sizes as compared to clusters in flight condition. Here we report formation of monodispersed films of size-selected copper nanoclusters (diameter -3 nm) that are produced in a magnetron based gas aggregation type source equipped with a quardrupole mass filter (QMF) to select sizes of clusters before landing. Transmission Electron Microscopy (TEM) study shows that the size-distributions of isolated islands peaks around the selected size of clusters and consequently the diffusion of these nano-scale islands is very low.

Structural and electrochemical analysis of a novel co-electrodeposited Mn2O3–Au nanocomposite thin film

In this work we report the fabrication of both pristine Mn₂O₃ and Mn₂O₃–Au composite thin films on an indium tin oxide (ITO) substrate by a one-step novel co-electrodeposition technique.

The effect of artificial grain boundaries on magneto-transport properties of charge ordered-ferromagnetic nanocomposites

Nanocomposites of charge ordered insulating Pr_0.67Ca_0.33MnO₃(PCMO) and ferromagnetic metallic La_0.67Sr_0.33MnO₃(LSMO) nanoparticles have been prepared by chemical synthesis.

A supramolecular hydrogel for generation of a benign DNA-hydrogel

A DNA-hydrogel, produced using calf-thymus DNA and a 2′,4′,6′-tri(4-pyridyl)pyridine based luminescent supramolecular hydrogel, can stabilize sunray mediated photochemically synthesized bio-compatible, luminous Ag-NPs.

Enhancement of photoluminescence emission and anomalous photoconductivity properties of Fe3O4@SiO2 core–shell microspheres

We report the successful synthesis of both pristine Fe₃O₄ and the Fe₃O₄@SiO₂ core@shell structure. The Fe₃O₄@SiO₂ core@shell sample shows enhanced photoluminescence compared to pristine Fe₃O₄, while reduced and negative photoconductivity is observed in the same sample.

Synthesis and Characterization of Mn-Doped ZnO Nanocrystals

We report the synthesis and characterization of several sizes of Mn-doped ZnO nanocrystals, both in the free-standing and the capped particle forms. The sizes of these nanocrystals could be controlled by capping them with polyvinylpyrollidone under different synthesis conditions and were estimated by X-ray diffraction and transmission electron microscopy. The absorption properties of PVP-capped Mn-doped ZnO exhibit an interesting variation of the band gap with the concentration of Mn. Fluorescence emission, electron paramagnetic resonance, and X-ray absorption spectroscopy provide evidence for the presence of Mn in the interior as well as on the surface of the nanocrystals.

Large spin diffusion length in an amorphous organic semiconductor

We directly measured a spin diffusion length (lambdas) of 13.3 nm in amorphous organic semiconductor (OS) rubrene (C42H28) by spin polarized tunneling. In comparison, no spin-conserved transport has been reported in amorphous Si or Ge. Absence of dangling bond defects can explain the spin transport behavior in amorphous OS. Furthermore, when rubrene barriers were grown on a seed layer, the elastic tunneling characteristics were greatly enhanced. Based on our findings, lambdas in single-crystalline rubrene can be expected to reach even millimeters, showing the potential for organic spintronics development.

Room-temperature tunnel magnetoresistance and spin-polarized tunneling through an organic semiconductor barrier

Electron spin-polarized tunneling is observed through an ultrathin layer of the molecular organic semiconductor tris(8-hydroxyquinolinato)aluminum (Alq3). Significant tunnel magnetoresistance (TMR) was measured in a Co/Al2O3/Alq3/NiFe magnetic tunnel junction at room temperature, which increased when cooled to low temperatures. Tunneling characteristics, such as the current-voltage behavior and temperature and bias dependence of the TMR, show the good quality of the organic tunnel barrier. Spin polarization (P) of the tunnel current through the Alq3 layer, directly measured using superconducting Al as the spin detector, shows that minimizing formation of an interfacial dipole layer between the metal electrode and organic barrier significantly improves spin transport.

Carrier-controlled ferromagnetism in transparent oxide semiconductors

The search for an ideal magnetic semiconductor with tunable ferromagnetic behaviour over a wide range of doping or by electrical gating is being actively pursued as a major step towards realizing spin electronics. A magnetic semiconductor having a high Curie temperature, capable of independently controlled carrier density and magnetic doping, is crucial for developing spin-based multifunctional devices. Cr-doped In(2)O(3) is such a unique system, where the electrical and magnetic behaviour-from ferromagnetic metal-like to ferromagnetic semiconducting to paramagnetic insulator-can be controllably tuned by the defect concentration. An explicit dependence of magnetic interaction leading to ferromagnetism on the carrier density is shown. A carrier-density-dependent high Curie temperature of 850-930 K has been measured, in addition to the observation of clear magnetic domain structures in these films. Being optically transparent with the above optimal properties, Cr-doped In(2)O(3) emerges as a viable candidate for the development of spin electronics.

Morphology and size dependent optical properties of CdS nanostructures

CdS nanoparticles with different sizes ranging from 2.5 nm to 300 nm and nanorods with aspect ratio -32 were synthesized by simple solvothermal process with a view to explore the effect of size and shape on the optical properties of these nanoforms. Solvent, temperature and the Cd source played important role in determining the morphologies and sizes of the nanocrystals. Comparative study of the optical properties of these nanoforms showed systematic changes in the optical absorption spectra with the reduction in particle size. Nanorods showed bulk like properties. Photoluminescence and Raman studies were carried out to explore the size and morphology dependent optical properties of the CdS nanoforms.

Synthesis and Optical Properties of CdS Nanoribbons

Rapid production of single crystalline CdS nanoribbons with hexagonal wurtzite phase has been achieved by thermal evaporation of CdS powder on Si wafers. The flow rate of the carrier (Ar) gas along with the synthesis temperature plays an important role in defining the size and shape of the CdS nanoribbons. Scanning electron and transmission electron microscopic observations revealed the nanoribbons to have a flat end as well as side surfaces which will make it ideal for optoelectronic devices such as nanolasers and light emitting diodes based on individual nanoribbons. The nanoribbons have widths within 200-400 nm and lengths approximately a few hundred micrometers. Room-temperature photoluminescence measurements show green emission centered at approximately 525 nm which may be ascribed to the near band edge emission. The Raman spectra of the CdS nanoribbons show peaks around 304, 609, 915, and 1220 cm(-1) corresponding to the first-, second-, third-, and fourth-order longitudinal optical phonon modes, respectively.

Electron and phonon confinement and surface phonon modes in CdSe-CdS core-shell nanocrystals

The optical and vibrational properties of bare and CdS shelled CdSe nanocrystalline particles are investigated. To confirm the formation of such nanocrystals in our samples we estimate their average particle sizes and size distributions using TEM measurements. From the line profile analysis of the HRTEM images the core-shell structure in the particles has been confirmed. The blue shift in the optical absorption spectra, analysed using theoretical estimates based on the effective bond order model, establishes the electron confinement in the nanoparticles. The main aim of this paper is to show the unique characteristics of the nanocrystals (which are absent in the corresponding bulk material), such as confinement of optical phonons and the appearance of surface phonons. Making use of the dielectric continuum model we are able to match the experimental and theoretical values of the frequencies of the surface phonons. We believe that our studies using optical probes provide further evidence for the existence of core-shell structures in CdSe-CdS type materials.