Patrick F. Flowers

Solution-processed copper–nickel nanowire anodes for organic solar cells

This work describes a process to make anodes for organic solar cells from copper-nickel nanowires with solution-phase processing. Copper nanowire films were coated from solution onto glass and made conductive by dipping them in acetic acid. Acetic acid removes the passivating oxide from the surface of copper nanowires, thereby reducing the contact resistance between nanowires to nearly the same extent as hydrogen annealing. Films of copper nanowires were made as oxidation resistant as silver nanowires under dry and humid conditions by dipping them in an electroless nickel plating solution. Organic solar cells utilizing these completely solution-processed copper-nickel nanowire films exhibited efficiencies of 4.9%.

Ethylenediamine Promotes Cu Nanowire Growth by Inhibiting Oxidation of Cu(111)

The synthesis of metal nanostructures usually requires a capping agent that is generally thought to cause anisotropic growth by blocking the addition of atoms to specific crystal facets. This work uses a series of electrochemical measurements with a quartz crystal microbalance and single-crystal electrodes to elucidate the facet-selective chemistry occurring in the synthesis of Cu nanowires. Contrary to prevailing hypotheses, ethylenediamine, a so-called capping agent in the synthesis of Cu nanowires, causes anisotropic growth by increasing the rate of atomic addition to (111) facets at the end of a growing nanowire relative to (100) facets on the sides of a nanowire. Ethylenediamine increases the reduction rate of Cu(OH)2- on a Cu(111) surface relative to Cu(100) by selectively inhibiting the formation of Cu oxide on Cu(111). This work demonstrates how studying facet-selective electrochemistry can improve the understanding of the processes by which atoms assemble to form anisotropic metal nanostructures.

High-speed, solution-coatable memory based on Cu–SiO2 core–shell nanowires

Printable electronics has the potential to drastically reduce the environmental and economic costs associated with the production of electronic devices, as well as enable rapid prototyping of circuits and their printing on demand, similar to what 3D printing has done for structural objects. A major barrier to the realization of printable computers that can run programs is the lack of a solution-coatable non-volatile memory with performance metrics comparable to silicon-based devices. Here we demonstrate a non-volatile memory based on Cu-SiO2 core-shell nanowires that can be printed from solution and exhibits on-off ratios of 106, switching speeds of 50 ns, a low operating voltage of 2 V, and operates for at least 104 cycles without failure. Each of these metrics is similar to or better than Flash memory (the write speed is 20 times faster than Flash). Memory architectures based on the individual memory cells demonstrated here could enable the printing of the more complex, embedded computing devices that are expected to make up an internet of things.

Emergence of winner-takes-all connectivity paths in random nanowire networks

Nanowire networks are promising memristive architectures for neuromorphic applications due to their connectivity and neurosynaptic-like behaviours. Here, we demonstrate a self-similar scaling of the conductance of networks and the junctions that comprise them. We show this behavior is an emergent property of any junction-dominated network. A particular class of junctions naturally leads to the emergence of conductance plateaus and a “winner-takes-all” conducting path that spans the entire network, and which we show corresponds to the lowest-energy connectivity path. The memory stored in the conductance state is distributed across the network but encoded in specific connectivity pathways, similar to that found in biological systems. These results are expected to have important implications for development of neuromorphic devices based on reservoir computing.Nanowire networks with memristive properties are promising for neuromorphic applications. Here, the authors observe the formation of a preferred conduction pathway which uses the lowest possible energy to get through the network and could be exploited for the design of optimal brain-inspired devices.

Fully printed memristors from Cu-SiO 2 core-shell nanowire composites

An area of printed electronics in which additional development is necessary is printable non-volatile memory, which will be essential for the development of fully printed RFID tags and sensors with integrated data storage.[1] An approach to making a printable memory is to utilize materials that exhibit resistive switching; devices based on this mechanism are often referred to as memristors.[2] However, existing fully-printed memories using memristors have properties that do not allow for practical application, with inadequate cycling endurance ( 10 μs), or short retention times ( 2 NWs). When coated from solution, these devices have modest switching voltages (2 V), a fast switching speed (50 ns), good endurance (>104 cycles), and data retention times (4 days) comparable to other fully printed memristors.[6] This work reports the fabrication and characteristics of a Cu-SiO 2 NW/ethylcellulose composite that is aerosol printed in a fully-printed memristor array.