Sorin Melinte

Hybrid supercapacitor-battery materials for fast electrochemical charge storage

High energy and high power electrochemical energy storage devices rely on different fundamental working principles - bulk vs. surface ion diffusion and electron conduction. Meeting both characteristics within a single or a pair of materials has been under intense investigations yet, severely hindered by intrinsic materials limitations. Here, we provide a solution to this issue and present an approach to design high energy and high power battery electrodes by hybridizing a nitroxide-polymer redox supercapacitor (PTMA) with a Li-ion battery material (LiFePO4). The PTMA constituent dominates the hybrid battery charge process and postpones the LiFePO4 voltage rise by virtue of its ultra-fast electrochemical response and higher working potential. We detail on a unique sequential charging mechanism in the hybrid electrode: PTMA undergoes oxidation to form high-potential redox species, which subsequently relax and charge the LiFePO4 by an internal charge transfer process. A rate capability equivalent to full battery recharge in less than 5 minutes is demonstrated. As a result of hybrids components synergy, enhanced power and energy density as well as superior cycling stability are obtained, otherwise difficult to achieve from separate constituents.

Roll up nanowire battery from silicon chips

Here we report an approach to roll out Li-ion battery components from silicon chips by a continuous and repeatable etch-infiltrate-peel cycle. Vertically aligned silicon nanowires etched from recycled silicon wafers are captured in a polymer matrix that operates as Li+ gel-electrolyte and electrode separator and peeled off to make multiple battery devices out of a single wafer. Porous, electrically interconnected copper nanoshells are conformally deposited around the silicon nanowires to stabilize the electrodes over extended cycles and provide efficient current collection. Using the above developed process we demonstrate an operational full cell 3.4 V lithium-polymer silicon nanowire (LIPOSIL) battery which is mechanically flexible and scalable to large dimensions.

Towards all-organic field-effect transistors by additive soft lithography.

Unconventional nanofabrication is attractive for organic electronics because of its potential impact in manufacturing low-cost electronics starting from soluble precursors that can be processed and patterned via a sustainable technology. So far, the major endeavor aimed at the development of organicbased devices has been through the design of new materials, novel synthetic procedures and purification methods, optimized conditions for thin film growth, and original methods for nanofabrication. In particular, a strong effort was devoted to the technological control of organic semiconductors in transistors. Yet, only a limited number of studies have focused on new approaches for low cost fabrication of electrodes and their integration with the organic materials. Successful examples of unconventional electrode manufacturing include stencil printing of Au nanoparticles, inkjet printing, Ag electroless plating followed by microcontact patterning, lamination, microtransfer printing of Ag nanoparticles, metal transfer printing, and soft lithography. Although inkjet printing is probably the most straightforward example of an additive process where both the electrodes and the active layers can be realized on the same platform, the fabrication of the electrodes and the active layers often relies on different processes, specifications, and platforms. Precisely, standard microfabrication approaches consisting of photolithography and/or electron-beam lithography followed by vacuum metallization are generally used for the source and drain definition, while wet methods (spin-coating, layer-by-

Giant low temperature heat capacity of GaAs quantum wells near Landau level filling nu = 1.

We report low temperature (T) heat capacity (C) data on a multiple-quantum-well GaAs/AlGaAs sample in the quantum Hall regime. Relative to its low field magnitude, C exhibits up to similar to 10(5)-fold enhancement near v = 1 where Skyrmions are the ground state of the confined two-dimensional electrons. We attribute the large C to a Skyrmion-induced, strong coupling of the nuclear spin system to the lattice. The data are consistent with the Schottky nuclear heat capacity of Ga and As atoms in the quantum wells, except at very low T where C vs T exhibits a remarkably sharp peak, suggestive of a phase transition in the electronic system.

Graphene-coated holey metal films: Tunable molecular sensing by surface plasmon resonance

We report on the enhancement of surface plasmon resonances in a holey bidimensional grating of subwavelength size, drilled in a gold thin film coated by a graphene sheet. The enhancement originates from the coupling between charge carriers in graphene and gold surface plasmons. The main plasmon resonance peak is located around 1.5 μm. A lower constraint on the gold-induced doping concentration of graphene is specified and the interest of this architecture for molecular sensing is also highlighted.

Spin polarization and g factor of a dilute GaAs two-dimensional electron system.

The effective g factor (g(*)) of a dilute interacting two-dimensional electron system is expected to increase with respect to its bare value as the density is lowered, and to eventually diverge as the system makes a transition to a ferromagnetic state. We report here measurements of g(*) in dilute (density 0.8 to 6.5x10(10) cm(-2)), high-mobility GaAs two-dimensional electrons from their spin polarization in a parallel magnetic field. The data reveal a surprising trend. While g(*) is indeed significantly enhanced with respect to the band g factor of GaAs, the enhancement factor decreases from about 6 to 3 as the density is reduced.

Surface Coating Mediated Swelling and Fracture of Silicon Nanowires during Lithiation

Surface passivation of silicon anodes is an appealing design strategy for the development of reliable, high-capacity lithium-ion batteries. However, the structural stability of the coating layer and its influence on the lithiation process remain largely unclear. Herein, we show that surface coating mediates the swelling dynamics and the fracture pattern during initial lithiation of crystalline silicon nanopillars. We choose conformally nickel coated silicon architectures as a model system. Experimental findings are interpreted based on a chemomechanical model. Markedly different swelling and fracture regimes have been identified, depending on the coating thickness and silicon nanopillar diameter. Nanopillars with relatively thin coating display anisotropic swelling similar to pristine nanopillars, but with different preferred fracture sites. As the coating thickness increases, the mechanisms become isotropic, with one randomly oriented longitudinal crack that unzips the core-shell structure. The morphology of cracked pillars resembles that of a thin-film electrode on a substrate, which is more amenable to cyclic lithiation without fracture. The knowledge provided here helps clarify the cycling results of coated nanosilicon electrodes and further suggests design rules for better performance electrodes through proper control of the lithiation and fracture.

Nanowire-decorated microscale metallic electrodes.

One of the challenging aspects of science and technology on a nanometer-scale is the precise three-dimensional control of nano-objects. Scanning probe microscopy manipulation, magneticor electric-field alignment and lithography-based techniques are only a few of the techniques that have been reported so far. Nevertheless, most of these techniques are still being developed and their integration for device fabrication represents a real challenge for the scientific community. Within this context, nanowires and nanotubes are of great interest because they lie between the macroscopic and atomic scales. The ability to fabricate andmanipulate such objects in a reliablemanner on a large scale will foster their use in electronic, photonic, and sensing applications. Templatebased methods have been successfully used for nanowire fabrication as they allow the realization of complex organic/ inorganic nanostructures. To date, nanoporous anodic alumina oxide (AAO) made by the electrochemical oxidation of aluminum has been extensively used because it provides a good platform for the development of various nanostructures. This interest originates from the fact that AAO membranes, having a high density of nanopores (up to 10 cm ), are easily produced over large areas with variable thicknesses. Moreover, a good chemical and mechanical stability combined with interesting electrical properties make AAO membranes good candidates for nanowire fabrication. However, the use of such nanostructures as passive or active components in emerging electronic devices requires smartly engineered arrays of nanowires with well defined position and pitch.

Highly Ordered Conjugated Polymer Nanoarchitectures with Three-Dimensional Structural Control

Conductive polymers are a class of materials with vast potential for tomorrows ultra-large-scale technologies as they combine structural and functional diversity with flexible synthesis and processing approaches. A missing component, with their subtle chemical structure, is reliable building at nanoscale. Here we report on the patterning of polyaniline, a prototypical conjugated polymer, with an unprecedented areal patterning order and density exceeding 0.25 teradot/inch(2). With template-confined growth, through platinum-surface-catalyzed polymerization of aniline, highly ordered arrays of distinct polyaniline nanowires are produced with a typical diameter <or=15 nm and aspect ratio higher than 20. Up-scaling is straightforward. Complex three-dimensional structural control is achieved through a direct pattern transfer via resist- and dose-modulated electron beam lithography. The morphology-modulated nanowires self-assemble in key-lock type architectures induced by the structure asymmetry and nonuniformity of the capillary forces associated with the re-entrant features.

Structural and electrical characterization of hybrid metal-polypyrrole nanowires.

We present here the synthesis and structural characterization of hybrid Au-polypyrrole-Au and Pt-polypyrrole-Au nanowires together with a study of their electrical properties from room temperature down to very low temperature. A careful characterization of the metal-polymer interfaces by transmission electron microscopy revealed that the structure and mechanical strength of bottom and upper interfaces are very different. Variable temperature electrical transport measurements were performed on both multiple nanowires-contained within the polycarbonate template-and single nanowires. Our data show that the three-dimensional Mott variable-range-hopping model provides a complete framework for the understanding of transport in polypyrrole nanowires, including nonlinear current-voltage characteristics and magnetotransport at low temperatures.