High-resolution Thermal Micro-imaging Using Europium Chelate Luminescent Coatings

Micro-electronic devices often undergo significant self-heating when biased to their typical operating conditions. This paper describes a convenient ...

Argonne National Laboratory

When the dimension of materials decreases to mesoscale, their properties can change dramatically, depending on the boundary conditions imposed by the sample architecture including geometry, morphology, and hierarchical structures. Here we show that electrodeposition, a method for reducing materials from a solution onto a substrate, can provide a versatile pathway to tailor the architecture of mesostructures. Novel lead (Pb) structures ranging from nanowires, mesoparticles with octahedral, decahedral, and icosahedral shapes to porous nanowires, multipods, nanobrushes, and even snowflake-shaped structures were synthesized through systematically exploring electrodeposition parameters including reduction potentials, solution concentration, starting materials, supporting electrolytes, and surfactants.

Tuning the architecture of mesostructures by electrodeposition.

We report a mechanism of nonisothermal dendritic flux penetration in superconducting films. Our numerical and analytical analysis of coupled nonlinear Maxwell and thermal diffusion equations shows that dendritic flux pattern formation results from spontaneous branching of propagating flux filaments due to nonlocal magnetic flux diffusion and positive feedback between flux motion and Joule heating. The branching is triggered by a thermomagnetic edge instability, which causes stratification of the critical state. The resulting distribution of thermomagnetic microavalanches is not universal, because it depends on a spatial distribution of defects. Our results are in good agreement with experiments on Nb films.

Dendritic flux avalanches and nonlocal electrodynamics in thin superconducting films.

The constructive interference of surface plasmon polaritons (SPP) launched by nanometric holes allows us to focus SPP into a spot of high near-field intensity having subwavelength width. Near-field scanning optical microscopy is used to map the local SPP intensity. The resulting SPP patterns and their polarization dependence are accurately described in model calculations based on a dipolar model for the SPP emission at each hole. Furthermore, we show that the high SPP intensity in the focal spot can be launched and propagated on a Ag strip guide with a 250 x 50 nm2 cross section, thus overcoming the diffraction limit of conventional optics. The combination of focusing arrays and nano-waveguides may serve as a basic element in planar nano-photonic circuits.

Subwavelength focusing and guiding of surface plasmons.

Hydrothermal synthesis of Mn doped anatase (TiO2) nanoparticles using scrolled nanotubes of TiO2 and MnCl2 as the starting materials is described. Incorporation of Mn2+ ions on the substitutional sites was confirmed using X-ray absorption fine structure (FT-XAFS) while the oxidation state Mn(II) and coordination environment were determined using both electron paramagnetic resonance (EPR) and X-ray absorption near edge spectroscopy (XANES). Two different hyperfine couplings of 96 and 86 G were found using high-field (130 GHz) EPR reporting that Mn atoms occupy two distinct sites: one undercoordinated (reconstructed surface) and the other octahedral crystalline geometry (nanoparticle core), respectively. It was found that Mn atoms that occupy surface layers are weakly bound to the anatase lattice and can be easily leached using simple dialysis, while those incorporated in the nanoparticle core are bound more strongly and cannot be removed by dialysis. Light excitation EPR reveals that Mn ions incorporated in the surface layers participate in the charge separation, while those trapped deeply in the nanoparticle core do not show any photoactivity. Doping of the core of nanoparticles with Mn2+ ions, on the other hand, enables synthesis of optically transparent films having superparamagnetic behavior at room temperatures with a saturation magnetic moment of 1.23 microB per Mn atom.

Charge separation and surface reconstruction: a Mn2+ doping study.

We discuss surface enhanced Raman spectroscopy (SERS) structures aimed at providing robust and reproducible enhancements. The structures involve periodic arrays of gold nanospheres near silver film structures that may also be patterned. They enable one to excite Bloch wave surface plasmon polaritons (SPPs) that can also couple to local surface plasmons (LSPs) of the nanospheres, leading to the possibility of multiplicative enhancements. If the magnitude of the average electric field, /E/, between the particles is enhanced by g such that /E/ = g/E(0)/, /E(0)/ being the incident field, realistic finite-difference time-domain simulations show that under favorable circumstances g approximately equal 0.6 g(SPP) g(LSP), where g(SPP) and g(LSP) are enhancement factors associated with the individual components. SERS enhancements for the structures can be as high as O(g(4)) = 10(8).

SERS enhancements via periodic arrays of gold nanoparticles on silver film structures.

Photovoltaic (PV) schemes often encounter a pair of fundamentally opposing requirements on the thickness of semiconductor layer: a thicker PV semiconductor layer provides enhanced optical density, but inevitably increases the charge transport path length. An effective approach to solve this dilemma is to enhance the interface area between the terminal electrode, i.e., transparent conducting oxide (TCO) and the semiconductor layer. As such, we report a facile, template-assisted, and solution chemistry-based synthesis of 3-dimensional inverse opal fluorinated tin oxide (IO-FTO) electrodes. Synergistically, the photonic crystal structure possessed in the IO-FTO exhibits strong light trapping capability. Furthermore, the electrical properties of the IO-FTO electrodes are studied by Hall effect and sheet resistance measurement. Using atomic layer deposition method, an ultrathin TiO(2) layer is coated on all surfaces of the IO-FTO electrodes. Cyclic voltammetry study indicates that the resulting TiO(2)-coated IO-FTO shows excellent potentials as electrodes for electrolyte-based photoelectrochemical solar cells.

Three-dimensional photonic crystal fluorinated tin oxide (FTO) electrodes: synthesis and optical and electrical properties.

The cost-effective self-assembly of 80 nm Au nanoparticles (NPs) into large-domain, hexagonally close-packed arrays for high-sensitivity and high-fidelity surface-enhanced Raman spectroscopy (SERS) is demonstrated. These arrays exhibit specific optical resonances due to strong interparticle coupling, which are well reproduced by finite-difference time-domain (FDTD) simulations. The gaps between NPs form a regular lattice of hot spots that enable a large amplification of both photoluminescence and Raman signals. At smaller wavelengths the hot spots are extended away from the minimum-gap positions, which allows SERS of larger analytes that do not fit into small gaps. Using CdSe quantum dots (QDs) a 3-5 times larger photoluminescence enhancement than previously reported is experimentally demonstrated and an unambiguous estimate of the electromagnetic SERS enhancement factor of ≈10(4) is obtained by direct scanning electron microscopy imaging of QDs responsible for the Raman signal. Much stronger enhancement of ≈10(8) is obtained at larger wavelengths for benzenethiol molecules penetrating the NP gaps.

Self-assembled large Au nanoparticle arrays with regular hot spots for SERS.

The newly developed hydrogen sensor, based on a network of ultrasmall pure palladium nanowires sputter-deposited on a filtration membrane, takes advantage of single palladium nanowires' characteristics of high speed and sensitivity while eliminating their nanofabrication obstacles. However, this new type of sensor, like the single palladium nanowires, cannot distinguish hydrogen concentrations above 3%, thus limiting the potential applications of the sensor. This study reports hydrogen sensors based on a network of ultrasmall Cr-buffered Pd (Pd/Cr) nanowires on a filtration membrane. These sensors not only are able to outperform their pure Pd counterparts in speed and durability but also allow hydrogen detection at concentrations up to 100%. The new networks consist of a thin layer of palladium deposited on top of a Cr adhesion layer 1-3 nm thick. Although the Cr layer is insensitive to hydrogen, it enables the formation of a network of continuous Pd/Cr nanowires with thicknesses of the Pd layer as thin as 2 nm. The improved performance of the Pd/Cr sensors can be attributed to the increased surface area to volume ratio and to the confinement-induced suppression of the phase transition from Pd/H solid solution (α-phase) to Pd hydride (β-phase).

Networks of ultrasmall Pd/Cr nanowires as high performance hydrogen sensors.

We present a pulsed-magnet system that enables x-ray single-crystal diffraction in addition to powder and spectroscopic studies with the magnetic field applied on or close to the scattering plane. The apparatus consists of a single large-bore solenoid, cooled by liquid nitrogen. A second independent closed-cycle cryostat is used for cooling samples near liquid helium temperatures. Pulsed magnetic fields close to ~30 T with a zero-to-peak-field rise time of ~2.9 ms are generated by discharging a 40 kJ capacitor bank into the magnet coil. The unique characteristic of this instrument is the preservation of maximum scattering angle (~23.6°) on the entrance and exit sides of the magnet bore by virtue of a novel double-funnel insert. This instrument will facilitate x-ray diffraction and spectroscopic studies that are impractical, if not impossible, to perform using split-pair and narrow-opening solenoid magnets. Furthermore, it offers a practical solution for preserving optical access in future higher-field pulsed magnets.

A single-solenoid pulsed-magnet system for single-crystal scattering studies.

The unique ability of plasmonic nanostructures to guide, enhance, and manipulate subwavelength light offers multiple novel applications in chemical and biological sensing, imaging, and photonic microcircuitry. Here the reproducible, giant light amplification in multiscale plasmonic structures is demonstrated. These structures combine strongly coupled components of different dimensions and topologies that resonate at the same optical frequency. A light amplifier is constructed using a silver mirror carrying light-enhancing surface plasmons, dielectric gratings forming distributed Bragg cavities on top of the mirror, and gold nanoparticle arrays self-assembled into the grating grooves. By tuning the resonances of the individual components to the same frequency, multiple enhancement of the light intensity in the nanometer gaps between the particles is achieved. Using a monolayer of benzenethiol molecules on this structure, an average SERS enhancement factor <EF> ∼10⁸ is obtained, and the maximum enhancement in the interparticle hot-spots is ∼3 × 10¹⁰, in good agreement with FDTD calculations. The high enhancement factor, large density of well-ordered hot-spots, and good fidelity of the SERS signal make this design a promising platform for quantitative SERS sensing, optical detection, efficient solid state lighting, advanced photovoltaics, and other emerging photonic applications.

Plasmonic amplifiers: engineering giant light enhancements by tuning resonances in multiscale plasmonic nanostructures.

A major obstacle to the management of malignant glioma is the inability to effectively deliver therapeutic agent to the tumor. In this study, we describe a polymeric nanoparticle vector that not only delivers viable therapeutic, but can also be tracked in vivo using MRI. Nanoparticles, produced by a non-emulsion technique, were fabricated to carry iron oxide within the shell and the chemotherapeutic agent, temozolomide (TMZ), as the payload. Nanoparticle properties were characterized and subsequently their endocytosis-mediated uptake by glioma cells was demonstrated. Convection-enhanced delivery (CED) can disperse nanoparticles through the rodent brain and their distribution is accurately visualized by MRI. Infusion of nanoparticles does not result in observable animal toxicity relative to control. CED of TMZ-bearing nanoparticles prolongs the survival of animals with intracranial xenografts compared to control. In conclusion, the described nanoparticle vector represents a unique multifunctional platform that can be used for image-guided treatment of malignant glioma.

Convection-enhanced delivery and in vivo imaging of polymeric nanoparticles for the treatment of malignant glioma.

A new critical-current-by-design paradigm is presented. It aims at predicting the optimal defect landscape in superconductors for targeted applications by elucidating the vortex dynamics responsible for the bulk critical current. To this end, critical current measurements on commercial high-temperature superconductors are combined with large-scale time-dependent Ginzburg-Landau simulations of vortex dynamics.

Toward Superconducting Critical Current by Design.

The behavior of vortex matter in high-temperature superconductors (HTS) controls the entire electromagnetic response of the material, including its current carrying capacity. Here, we review the basic concepts of vortex pinning and its application to a complex mixed pinning landscape to enhance the critical current and to reduce its anisotropy. We focus on recent scientific advances that have resulted in large enhancements of the in-field critical current in state-of-the-art second generation (2G) YBCO coated conductors and on the prospect of an isotropic, high-critical current superconductor in the iron-based superconductors. Lastly, we discuss an emerging new paradigm of critical current by design-a drive to achieve a quantitative correlation between the observed critical current density and mesoscale mixed pinning landscapes by using realistic input parameters in an innovative and powerful large-scale time dependent Ginzburg-Landau approach to simulating vortex dynamics.

Ulrich Welp

Materials Science Division

Argonne National Laboratory

High-resolution Thermal Micro-imaging Using Europium Chelate Luminescent Coatings

Ulrich Welp

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Materials Science Division

Argonne National Laboratory

High-resolution Thermal Micro-imaging Using Europium Chelate Luminescent Coatings

Materials Science Division