The Electro-Optical Performance of Silver Nanowire Networks

Insight into the morphological instability of metallic nanowires under thermal stress

HYPOTHESIS

Metallic nanowires, particularly polyol-grown silver nanowires, exhibit a morphological instability at temperatures significantly lower than their bulk melting point. This instability is commonly named after Rayleigh's description of the morphological instability of liquid jets, even though it has been shown that its quantitative predictions are not consistent with experimental measurements. In 1996, McCallum et al. proposed a description of the phenomenon assuming a solid wire lying on a substrate. It is assumed that the latter description depicts more accurately the reality.

EXPERIMENTS

Nanowires with varying diameters have been deposited on silicon wafers. Statistical analysis of their radius and the wavelength of their periodical instability have been performed.

FINDINGS

McCallum et al.'s model better aligns with experimental observations compared to Rayleigh's description. This validation provides a robust theoretical framework for enhancing the stability of nanowires, addressing a crucial aspect of their development.

Bridge percolation: electrical connectivity of discontinued conducting slabs by metallic nanowires

The properties of nanostructured networks of conductive materials have been extensively studied under the lens of percolation theory. In this work, we introduce a novel type of local percolation phenomenon used to investigate the conduction properties of a new hybrid material that combines sparse metallic nanowire networks and fractured conducting thin films on flexible substrates. This original concept could potentially lead to the design of a novel composite transparent conducting material. Using a complementary approach including formal analytical derivations, Monte Carlo simulations and electrical circuit representation for the modelling of bridged-percolating nanowire networks, we unveil the key relations between linear crack density, nanowire length and network areal mass density that ensure electrical percolation through the hybrid. The proposed theoretical model provides key insights into the conduction mechanism associated with the original concept of bridge percolation in random nanowire networks.

Flexible Transparent Electrodes Formed from Template-Patterned Thin-Film Silver

Template-patterned, flexible transparent electrodes (TEs) formed from an ultrathin silver film on top of a commercial optical adhesive - Norland Optical Adhesive 63 (NOA63) - are reported. NOA63 is shown to be an effective base-layer for ultrathin silver films that advantageously prevents coalescence of vapor-deposited silver atoms into large, isolated islands (Volmer-Weber growth), and so aids the formation of ultrasmooth continuous films. 12 nm silver films on top of free-standing NOA63 combine high, haze-free visible-light transparency (T ≈ 60% at 550 nm) with low sheet-resistance (R s ${\mathcal{R}}_s$ ≈ 16 Ω sq-1), and exhibit excellent resilience to bending, making them attractive candidates for flexible TEs. Etching the NOA63 base-layer with an oxygen plasma before silver deposition causes the silver to laterally segregate into isolated pillars, resulting in a much higher sheet resistance (R s ${\mathcal{R}}_{s}$  > 8 × 106 Ω sq-1) than silver grown on pristine NOA63 . Hence, by selectively etching NOA63 before metal deposition, insulating regions may be defined within an otherwise conducting silver film, resulting in a differentially conductive film that can serve as a patterned TE for flexible devices. Transmittance may be increased (to 79% at 550 nm) by depositing an antireflective layer of Al2O3 on the Ag layer at the cost of reduced flexibility.

First-principles study of mercaptoundecanoic acid molecule adsorption and gas molecule penetration onto silver surface: an insight for corrosion protection

Recently, 11-mercaptoundecanoic acid (MUA) molecule has attracted attention as a promising passivation agent of Ag nanowire (NW) network electrode for corrosion inhibition, but the underneath mechanism has not been elaborated. In this work, we investigate adsorption of MUA molecule on Ag(1 0 0) and Ag(1 1 1) surface, adsorption of air gas molecules of H2O, H2S and O2 on MUA molecular end surface, and their penetrations into the Ag surface using the first-principles calculations. Our calculations reveal that the MUA molecule is strongly bound to the Ag surface with the binding energies ranging from -0.47 to -2.06 eV and the Ag-S bond lengths of 2.68-2.97 Å by Lewis acid-base reaction. Furthermore, we find attractive interactions between the gas molecules and the MUA@Ag complexes upon their adsorptions and calculate activation barriers for their migrations from the outermost end of the complexes to the top of Ag surface. It is found that the penetrations of H2O and H2S are more difficult than the O2 penetration due to their higher activation barriers, while the O2 penetration is still difficult, confirming the corrosion protection of Ag NW network by adsorbing the uniform monolayer of MUA. With these findings, this work can contribute to finding a better passivation agent in the strategy of corrosion protection of Ag NW network electrode.

Electrical conductance of two-dimensional random percolating networks based on mixtures of nanowires and nanorings: A mean-field approach along with computer simulation

We have studied the electrical conductance of two-dimensional (2D) random percolating networks of zero-width metallic nanowires (a mixture of rings and sticks). We took into account the nanowire resistance per unit length and the junction (nanowire-nanowire contact) resistance. Using a mean-field approximation (MFA) approach, we derived the total electrical conductance of these nanowire-based networks as a function of their geometrical and physical parameters. The MFA predictions have been confirmed by our Monte Carlo (MC) numerical simulations. The MC simulations were focused on the case when the circumferences of the rings and the lengths of the wires were equal. In this case, the electrical conductance of the network was found to be almost insensitive to the relative proportions of the rings and sticks, provided that the wire resistance and the junction resistance were equal. When the junction resistance dominated over the wire resistance, a linear dependency of the electrical conductance of the network on the proportions of the rings and sticks was observed.

Self-assembly, alignment, and patterning of metal nanowires

Armed with the merits of one-dimensional nanostructures (flexibility, high aspect ratio, and anisotropy) and metals (high conductivity, plasmonic properties, and catalytic activity), metal nanowires (MNWs) have stood out as a new class of nanomaterials in the last two decades. They are envisaged to expedite significantly and even revolutionize a broad spectrum of applications related to display, sensing, energy, plasmonics, photonics, and catalysis. Compared with disordered MNWs, well-organized MNWs would not only enhance the intrinsic physical and chemical properties, but also create new functions and sophisticated architectures of optoelectronic devices. This paper presents a comprehensive review of assembly strategies of MNWs, including self-assembly for specific structures, alignment for anisotropic constructions, and patterning for precise configurations. The technical processes, underlying mechanisms, performance indicators, and representative applications of these strategies are described and discussed to inspire further innovation in assembly techniques and guide the fabrication of optoelectrical devices. Finally, a perspective on the critical challenges and future opportunities of MNW assembly is provided.

Optical Properties of Colloidal Silver Nanowires

Silver nanowires are used in many applications, ranging from transparent conductive layers to Raman substrates and sensors. Their performance often relies on their unique optical properties that emerge from localized surface plasmon resonances in the ultraviolet. To tailor the nanowire geometry for a specific application, a correct understanding of the relationship between the wire's structure and its optical properties is therefore necessary. However, while the colloidal synthesis of silver nanowires typically leads to structures with pentagonally twinned geometries, their optical properties are often modeled assuming a cylindrical cross-section. Here we highlight the strengths and limitations of such an approximation by numerically calculating the optical and electrical response of pentagonally twinned silver nanowires and nanowire networks. We find that our accurate modeling is crucial to deduce structural information from experimentally measured extinction spectra of colloidally synthesized nanowire suspensions and to predict the performance of nanowire-based near-field sensors. On the contrary, the cylindrical approximation is fully capable of capturing the optical and electrical performance of nanowire networks used as transparent electrodes. Our results can help assess the quality of nanowire syntheses and guide in the design of optimized silver nanowire-based devices.

Ultra-thin broadband solar absorber based on stadium-shaped silicon nanowire arrays

This paper investigates how the dimensions and arrangements of stadium silicon nanowires (NWs) affect their absorption properties. Compared to other NWs, the structure proposed here has a simple geometry, while its absorption rate is comparable to that of very complex structures. It is shown that changing the cross-section of NW from circular (or rectangular) to a stadium shape leads to change in the position and the number of absorption modes of the NW. In a special case, these modes result in the maximum absorption inside NWs. Another method used in this paper to attain broadband absorption is utilization of multiple NWs which have different geometries. However, the maximum enhancement is achieved using non-close packed NW. These structures can support more cavity modes, while NW scattering leads to broadening of the absorption spectra. All the structures are optimized using particle swarm optimizations. Using these optimized structures, it is viable to enhance the absorption by solar cells without introducing more absorbent materials.

Core/Shell Ag/SnO2 Nanowires for Visible Light Photocatalysis

This study presents core/shell Ag/SnO2 nanowires (Ag/SnO2NWs) as a new photocatalyst for the rapid degradation of organic compounds by the light from the visible range. AgNWs after coating with a SnO2 shell change optical properties and, due to red shift of the absorbance maxima of the longitudinal and transverse surface plasmon resonance (SPR), modes can be excited by the light from the visible light region. Rhodamine B and malachite green were respectively selected as a model organic dye and toxic one that are present in the environment to study the photodegradation process with a novel one-dimensional metal/semiconductor Ag/SnO2NWs photocatalyst. The degradation was investigated by studying time-dependent UV/Vis absorption of the dye solution, which showed a fast degradation process due to the presence of Ag/SnO2NWs photocatalyst. The rhodamine B and malachite green degraded after 90 and 40 min, respectively, under irradiation at the wavelength of 450 nm. The efficient photocatalytic process is attributed to two phenomenon surface plasmon resonance effects of AgNWs, which allowed light absorption from the visible range, and charge separations on the Ag core and SnO2 shell interface of the nanowires which prevents recombination of photogenerated electron-hole pairs. The presented properties of Ag/SnO2NWs can be used for designing efficient and fast photodegradation systems to remove organic pollutants under solar light without applying any external sources of irradiation.

Tuning the electro-optical properties of nanowire networks

Conductive and transparent metallic nanowire networks are regarded as promising alternatives to Indium-Tin-Oxides (ITOs) in emerging flexible next-generation technologies due to their prominent optoelectronic properties and low-cost fabrication. The performance of such systems closely relies on many geometrical, physical, and intrinsic properties of the nanowire materials as well as the device-layout. A comprehensive computational study is essential to model and quantify the device's optical and electrical responses prior to fabrication. Here, we present a computational toolkit that exploits the electro-optical specifications of distinct device-layouts, namely standard random nanowire network and transparent mesh pattern structures. The target materials for transparent conducting electrodes of this study are aluminium, gold, copper, and silver nanowires. We have examined a variety of tunable parameters including network area fraction, length to diameter aspect ratio, and nanowires angular orientations under different device designs. Moreover, the optical extinction efficiency factors of each material are estimated by two approaches: Mie light scattering theory and finite element method (FEM) algorithm implemented in COMSOL®Multiphysics software. We studied various nanowire network structures and calculated their respective figures of merit (optical transmittance versus sheet resistance) from which insights on the design of next-generation transparent conductor devices can be inferred.

Analytical modeling of orientation effects in random nanowire networks

Films made from random nanowire arrays are an attractive choice for electronics requiring flexible transparent conductive films. However, thus far there has been no unified theory for predicting their electrical conductivity. In particular, the effects of orientation distribution on network conductivity remain poorly understood. We present a simplified analytical model for random nanowire network electrical conductivity that accurately captures the effects of arbitrary nanowire orientation distributions on conductivity. Our model is an upper bound and converges to the true conductivity as nanowire density grows. The model replaces Monte Carlo sampling with an asymptotically faster computation and in practice can be computed much more quickly than standard computational models. The success of our approximation provides theoretical insight into how nanowire orientation affects electrical conductivity.