Caroline Celle

Flexible transparent conductive materials based on silver nanowire networks: a review

The class of materials combining high electrical or thermal conductivity, optical transparency and flexibility is crucial for the development of many future electronic and optoelectronic devices. Silver nanowire networks show very promising results and represent a viable alternative to the commonly used, scarce and brittle indium tin oxide. The science and technology research of such networks are reviewed to provide a better understanding of the physical and chemical properties of this nanowire-based material while opening attractive new applications.

Highly Flexible Transparent Film Heaters Based on Random Networks of Silver Nanowires

AbstractWe demonstrate a new concept for the fabrication of flexible transparent thin film heaters based on silver nanowires. Thanks to the intrinsic properties of random networks of metallic nanowires, it is possible to combine bendability, transparency and high heating performances at low voltage, typically below 12 V which is of interest for many applications. This is currently not possible with transparent conductive oxide technologies, and it compares well with similar devices fabricated with carbon nanotubes or graphene. We present experiments on glass and poly(ethylene naphthalate) (PEN) substrates (with thicknesses of 125 μm and extremely thin 1.3 μm) with excellent heating performances. We point out that the amount of silver necessary to realize the transparent heaters is very low and we also present preliminary results showing that this material can be efficiently used to fabricate photochromic displays. To our knowledge, this is the first report of metallic nanowire-based transparent thin film heaters. We think these results could be a useful approach for the engineering of highly flexible and transparent heaters which are not attainable by existing processes.

Improvement of the Seebeck coefficient of PEDOT:PSS by chemical reduction combined with a novel method for its transfer using free-standing thin films

A simple method for improving the Seebeck coefficient of PEDOT:PSS up to 161 μV K−1 is presented and combined with a new process for transferring thick (>10 μm) films of PEDOT:PSS onto substrates with various shapes, and in particular onto flexible substrates. These reduced transferred films have been used in combination with a nickel ethylenetetrathiolate coordination polymer to fabricate cheap and flexible heat flux sensors.

Work Function Tuning for High‐Performance Solution‐Processed Organic Photodetectors with Inverted Structure

Organic photodetectors with inverted structure are fabricated by solution process techniques. A very thin interfacing layer of polyethyleneimine leads to a homogenous interface with low work function. The devices exhibit excellent performances, in particular in terms of low dark current density, wide range linearity, high detectivity, and remarkable stability in ambient air without encapsulation.

Metallic Nanowire‐Based Transparent Electrodes for Next Generation Flexible Devices: a Review

Transparent electrodes attract intense attention in many technological fields, including optoelectronic devices, transparent film heaters and electromagnetic applications. New generation transparent electrodes are expected to have three main physical properties: high electrical conductivity, high transparency and mechanical flexibility. The most efficient and widely used transparent conducting material is currently indium tin oxide (ITO). However the scarcity of indium associated with ITOs lack of flexibility and the relatively high manufacturing costs have a prompted search into alternative materials. With their outstanding physical properties, metallic nanowire (MNW)-based percolating networks appear to be one of the most promising alternatives to ITO. They also have several other advantages, such as solution-based processing, and are compatible with large area deposition techniques. Estimations of cost of the technology are lower, in particular thanks to the small quantities of nanomaterials needed to reach industrial performance criteria. The present review investigates recent progress on the main applications reported for MNW networks of any sort (silver, copper, gold, core-shell nanowires) and points out some of the most impressive outcomes. Insights into processing MNW into high-performance transparent conducting thin films are also discussed according to each specific application. Finally, strategies for improving both their stability and integration into real devices are presented.

Synthesis and purification of long copper nanowires. Application to high performance flexible transparent electrodes with and without PEDOT:PSS

AbstractWe demonstrate the hydrothermal synthesis of long copper nanowires based on a simple protocol. We show that the purification of the nanowires is very important and can be achieved easily by wet treatment with glacial acetic acid. Fabrication of random networks of purified copper nanowires leads to flexible transparent electrodes with excellent optoelectronic performances (e.g., 55 Ω/sq. at 94% transparency). The process is carried out at room temperature and no post-treatment is necessary. Hybrid materials with the conductive polymer PEDOT:PSS show similar properties (e.g., 46 Ω/sq. at 93% transparency), with improved mechanical properties. Both electrodes were integrated in capacitive touch sensors.

Improvements in purification of silver nanowires by decantation and fabrication of flexible transparent electrodes. Application to capacitive touch sensors

Transparent flexible electrodes made of metallic nanowires, and in particular silver nanowires (AgNWs), appear as an extremely promising alternative to transparent conductive oxides for future optoelectronic devices. Though significant progresses have been made the last few years, there is still some room for improvement regarding the synthesis of high quality silver nanowire solutions and fabrication process of high performance electrodes. We show that the commonly used purification process can be greatly simplified through decantation. Using this process it is possible to fabricate flexible electrodes by spray coating with sheet resistance lower than 25 Ω sq⁻¹ at 90% transparency in the visible spectrum. These electrodes were used to fabricate an operative transparent flexible touch screen. To our knowledge this is the first reported AgNW based touch sensor relying on capacitive technology.

Sub-ppm Detection of Nerve Agents Using Chemically Functionalized Silicon Nanoribbon Field-Effect Transistors†

Organophosphorus compounds (OPs) represent one of the most important and lethal classes of chemical warfare agents (e.g. sarin, tabun, soman). Highly active volatile OPs are powerful inhibitors of acetylcholinesterase, which is a critical enzyme of the nervous system. The ease of manufacturing OPs based on inexpensive starting materials makes these agents a weapon of choice for terrorist attacks. Thus, the rapid sensing of these nerve agents has recently become an increasingly important research goal. Various approaches have been reported for the detection of these chemical warfare agents including colorimetric and fluorimetric spectroscopies, enzymatic assays, piezoelectric devices, single-walled carbon nanotube resistors and capacitors. However, these systems are plagued by limitations such as slow response time, moderate selectivity, operational complexity, or limited portability. Field-effect transistors (FET) based on nanomaterials such as semiconducting nanowires, nanoribbons, or carbon nanotubes have been recently explored for chemical and biological detection. Their high effectiveness is mainly ascribed to an extreme sensitivity to electrostatic changes at the surface of the semiconductor and/or modifications of the Schottky barrier at the semiconductor/metal interface. A charge generation in the vicinity of the semiconductor of a FET is known to alter the electrical properties of the device. Several research groups have independently developed a series of small-molecule fluorescent sensors for OPs detection. They investigated organic moieties reactive towards OPs by formation of a phosphate ester intermediate and subsequent intramolecular nucleophilic substitution, which led to an ammonium salt and thus charge formation. We thought monitoring this charge generation with a functionalized FET could be a particularly promising approach. Herein, we report the development of an OPs chemical sensor based on highly sensitive silicon nanoribbon field-effect transistors (SiNR-FETs) functionalized with compound 1 (Scheme 1).

Conductive-probe atomic force microscopy characterization of silicon nanowire

The electrical conduction properties of lateral and vertical silicon nanowires (SiNWs) were investigated using a conductive-probe atomic force microscopy (AFM). Horizontal SiNWs, which were synthesized by the in-plane solid-liquid-solid technique, are randomly deployed into an undoped hydrogenated amorphous silicon layer. Local current mapping shows that the wires have internal microstructures. The local current-voltage measurements on these horizontal wires reveal a power law behavior indicating several transport regimes based on space-charge limited conduction which can be assisted by traps in the high-bias regime (> 1 V). Vertical phosphorus-doped SiNWs were grown by chemical vapor deposition using a gold catalyst-driving vapor-liquid-solid process on higly n-type silicon substrates. The effect of phosphorus doping on the local contact resistance between the AFM tip and the SiNW was put in evidence, and the SiNWs resistivity was estimated.

High Gain and Fast Detection of Warfare Agents Using Back-Gated Silicon-Nanowired MOSFETs

The top-down fabrication of doped p-type silicon-nanowired (NW) arrays and their application as gas detectors is presented. After surface functionalization with 3-(4-ethynylbenzyl)-1, 5, 7-trimethyl-3-azabicyclo [3.3.1] nonane-7-methanol molecules, the wires were subjected to an organophosphorous simulant, and both static and dynamic measurements were performed. A current gain of 4 × 106 is obtained upon the detection of the subpart-per-million concentration of a nerve-agent simulant. This represents a four-decade improvement over previous demonstration based on nanoribbons, proving better sensing capabilities of NWs. Technology-computer-aided-design simulations before and after gas detection have been performed to gain insight into the physical mechanisms involved in the gas detection and to investigate the impact of the surface-to-volume ratio on sensor sensitivity.