Heat-induced morphological changes in silver nanowires deposited on a patterned silicon substrate

Metallic nanowires (NWs) are sensitive to heat treatment and can split into shorter fragments within minutes at temperatures far below the melting point. This process can hinder the functioning of NW-based devices that are subject to relatively mild temperatures. Commonly, heat-induced fragmentation of NWs is attributed to the interplay between heat-enhanced diffusion and Rayleigh instability. In this work, we demonstrated that contact with the substrate plays an important role in the fragmentation process and can strongly affect the outcome of the heat treatment. We deposited silver NWs onto specially patterned silicon wafers so that some NWs were partially suspended over the holes in the substrate. Then, we performed a series of heat-treatment experiments and found that adhered and suspended parts of NWs behave differently under the heat treatment. Moreover, depending on the heat-treatment process, fragmentation in either adhered or suspended parts can dominate. Experiments were supported by finite element method and molecular dynamics simulations.

In Situ TEM Characterization and Modulation for Phase Engineering of Nanomaterials

Solid-state phase transformation is an intriguing phenomenon in crystalline or noncrystalline solids due to the distinct physical and chemical properties that can be obtained and modified by phase engineering. Compared to bulk solids, nanomaterials exhibit enhanced capability for phase engineering due to their small sizes and high surface-to-volume ratios, facilitating various emerging applications. To establish a comprehensive atomistic understanding of phase engineering, in situ transmission electron microscopy (TEM) techniques have emerged as powerful tools, providing unprecedented atomic-resolution imaging, multiple characterization and stimulation mechanisms, and real-time integrations with various external fields. In this Review, we present a comprehensive overview of recent advances in in situ TEM studies to characterize and modulate nanomaterials for phase transformations under different stimuli, including mechanical, thermal, electrical, environmental, optical, and magnetic factors. We briefly introduce crystalline structures and polymorphism and then summarize phase stability and phase transformation models. The advanced experimental setups of in situ techniques are outlined and the advantages of in situ TEM phase engineering are highlighted, as demonstrated via several representative examples. Besides, the distinctive properties that can be obtained from in situ phase engineering are presented. Finally, current challenges and future research opportunities, along with their potential applications, are suggested.

Current density at failure of twinned silver nanowires

Silver nanowires have a wide range of potential applications in stretchable and transparent electronics due to their excellent electrical, mechanical, and optical properties. For a successful application in electronic devices, evaluating the electrical reliability of these nanowires is required. We have studied experimentally the behavior of current density at failure for penta-twinned silver nanowires with diameters between 53 and 173 nm, for 93 samples. The current densities at failure are widely scattered, have an average of 9.7 × 10⁷A cm^-2, and a standard deviation of 2.96 × 10⁷A cm^-2. Heat-transfer modeling is employed to explain the results, and Weibull statistics are used to quantify failure probabilities, thus offering guidelines for future designs based on these nanowires. The scatter observed in the measurements is attributed to surface-roughness variations among samples, which lead to local hot spots of high current density. These results quantify the Joule heating electrical reliability of silver nanowires and highlight the importance of heat transfer in increasing it.

Direct Atomic-Level Imaging of Zeolites: Oxygen, Sodium in Na-LTA and Iron in Fe-MFI

Zeolites are becoming more versatile in their chemical functions through rational design of their frameworks. Therefore, direct imaging of all atoms at the atomic scale, basic units (Si, Al, and O), heteroatoms in the framework, and extra‐framework cations, is needed. TEM provides local information at the atomic level, but the serious problem of electron‐beam damage needs to be overcome. Herein, all framework atoms, including oxygen and most of the extra‐framework Na cations, are successfully observed in one of the most electron‐beam‐sensitive and lowest framework density zeolites, Na‐LTA. Zeolite performance, for instance in catalysis, is highly dependent on the location of incorporated heteroatoms. Fe single atomic sites in the MFI framework have been imaged for the first time. The approach presented here, combining image analysis, electron diffraction, and DFT calculations, can provide essential structural keys for tuning catalytically active sites at the atomic level.

Mechanism of Heat-Induced Fusion of Silver Nanowires

Physical changes in arranged silver nanowires were monitored during progressive heating inside a transmission electron microscope. Using the in-situ experimental method, overall variation of silver nanowires and movement of the silver atoms could be assessed. The physical morphology of silver nanowires was rapidly transformed above 350 °C as they fused with each other, which led to extrusion of the silver atoms. Around 550 °C, silver nanowires were almost fused into one, filling a relatively large void between silver nanowires. However, above 575 °C, the united silver nanowire was completely cut off, starting from the region that was suspected to have defects. For the first time, the fusion of arranged silver nanowires and the configurational changes of silver atoms during heating were visualized, and the migration between silver atoms and the damage mechanism of silver nanowires were assessed. Moreover, the relationship of physical morphology and electrical property of silver nanowires according to the temperature were investigated using the ex-situ experimental method. As silver nanowires started to split at 300 °C, the electrical conductivity deteriorated greatly. Beyond 350 °C, the electrical conductivity was completely lost while silver nanowires disintegrated rapidly, and silver nanowires completely disappeared at 450 °C.

Enhancement of Antibacterial Performance of Silver Nanowire Transparent Film by Post-Heat Treatment

Silver nanomaterials (AgNMs) have been applied as antibacterial agents to combat bacterial infections that can cause disease and death. The antibacterial activity of AgNMs can be improved by increasing the specific surface area, so significant efforts have been devoted to developing various bottom-up synthesis methods to control the size and shape of the particles. Herein, we report on a facile heat-treatment method that can improve the antibacterial activity of transparent silver nanowire (AgNW) films in a size-controllable, top-down manner. AgNW films were fabricated via spin-coating and were then heated at different temperatures (230 and 280 °C) for 30 min. The morphology and the degree of oxidation of the as-fabricated AgNW film were remarkably sensitive to the heat-treatment temperature, while the transparency was insensitive. As the heat-treatment temperature increased, the AgNWs spontaneously broke into more discrete wires and droplets, and oxidation proceeded faster. The increase in the heat-treatment temperature further increased the antibacterial activity of the AgNW film, and the heat treatment at 280 °C improved the antibacterial activity from 31.7% to 94.7% for Staphylococcus aureus, and from 57.0% to 98.7% for Escherichia coli. Following commonly accepted antibacterial mechanisms of AgNMs, we present a correlation between the antibacterial activity and surface observations of the AgNW film.

Optoelectronic and Electrothermal Properties of Transparent Conductive Silver Nanowires Films

Silver nanowires (AgNWs) show promise for fabricating flexible transparent conductors owing to their excellent conductivity, high transparency, and good mechanical properties. Here, we present the fabrication of transparent films composed of AgNWs with diameters of 20-30 nm and lengths of 25-30 μm on polyethylene terephthalate substrates and glass slides substrates using the Meyer rod method. We systematically investigated the films' optoelectronic and electrothermal properties. The morphology remained intact when heated at 25-150 °C and the AgNWs film showed high conductivity (17.6-14.3 Ω∙sq-1), excellent transmittance (93.9-91.8%) and low surface roughness values (11.2-14.7 nm). When used as a heater, the transparent AgNW conductive film showed rapid heating at low input voltages owing to a uniform heat distribution across the whole substrate surface. Additionally, the conductivity of the film decreased with increasing bending cycle numbers; however, the film still exhibited a good conductivity and heating performances after repeated bending.

Investigation of Ag-TiO2 Interfacial Reaction of Highly Stable Ag Nanowire Transparent Conductive Film with Conformal TiO2 Coating by Atomic Layer Deposition

The atomic layer deposition (ALD) technique is applied to coat Ag nanowires (NWs) with a highly uniform and conformal TiO₂ layer to improve the stability and sustainability of Ag NW transparent conductive films (TCFs) at high temperatures. The TiO₂ layer can be directly deposited on Ag NWs with a surface polyvinylpyrrolidone (PVP) coat that acts a bed for TiO₂ seeding in the ALD process. The ALD TiO₂ layer significantly enhances the thermal stability at least 100 fold when aged between 200-400 °C and also provides an extra function of violet-blue light filtration for Ag NW TCFs. Investigation into the interaction between TiO₂ and Ag reveals that the conformal TiO₂ shell could effectively prevent Ag from 1D-to-3D ripening. However, Ag could penetrate the conformal TiO₂ shell and form nanocrystals on the TiO₂ shell surface when it is aged at 400 °C. According to experimental data and thermodynamic evaluation, the Ag penetration leads to an interlayer composed of mixed Ag-Ag₂O-amorphous carbon phases and TiO_2-x at the Ag-TiO₂ interface, which is thought to be caused by extremely high vapor pressure of Ag at the Ag-TiO₂ interface at a higher temperature (e.g., 400 °C).

Nanojoint Formation between Ceramic Titanate Nanowires and Spot Melting of Metal Nanowires with Electron Beam

Construction of nanoarchitectures requires techniques like joint formation and trimming. For ceramic materials, however, it is extremely difficult to form nanojoints by conventional methods like merging. In this work, we demonstrate that ceramic titanate nanowires (NWs) can be joined by spot melting under electron beam (e-beam) irradiation (EBI). The irradiation fuses the contacted spot of titanate NWs yielding an intact nanojoint. Nanojoints with different morphologies can be produced. The joint structures consist of titanium dioxide (TiO₂) rutile, anatase, and titanate phases in the direction away from the e-beam melting spot. The titanate binds to anatase via a crystallographic matching coherent interface (the oxygen atoms at the interface are shared by the two phases) and the anatase solidly binds to the rutile joint. The resulting rutile joint is stable at high temperatures. Additionally, it is demonstrated that the heat production from EBI treated rutile can be utilized to break metal NWs (Ag, Cu, and Ni) apart by spot melting. The required e-beam intensity is considerably mild (75 pA/cm²) which allows visual access and control over the NW melting. Direct melting of Ag and Cu is not applicable under EBI due to their high thermal conductivity even with high current density (500 pA/cm²). Our findings reveal that ceramic nanojoint formation and spot melting at nanoscale are applicable if the properties of nanomaterials are understood and properly utilized.

Imaging the Atomic Position of Light Cations in a Porous Network and the Europium(III) Ion Exchange Capability by Aberration-Corrected Electron Microscopy

In the present work, ETS-10 microporous titanosilicate has been synthesized and its structure characterized by means of powder XRD and aberration corrected scanning transmission electron microscopy (C_s -corrected STEM). For the first time, sodium ions have been imaged sitting inside the 7-membered rings. The ion-exchange capability has been tested by the inclusion of rare earth metals (Eu, Tb and Gd) to produce a luminescent material which has been studied by atomic-resolution C_s -corrected STEM. The data produced has allowed unambiguous imaging of light atoms in a microporous framework as well as determining the cationic metal positions for the first time, providing evidence of the importance of advanced electron microscopy methods for the study of the local environment of metals within zeolitic supports providing unique information of both systems (guest and support) at the same time.

In situ nanojoining of Y- and T-shaped silver nanowires structures using femtosecond laser radiation

We report the in situ joining of spatially separated silver nanowires without additional filler material by controlled irradiation with femtosecond laser pulses. Nanojoining under these conditions arises from highly localized heat generation in the vicinity of the gap between adjacent silver nanowires. Melting, followed by the flow of silver into the gap, is optimized by adjusting the direction of laser polarization relative to gap geometry. Our results show that melting of silver occurs on both nanowires in the vicinity of the gap between the two components. Successful formation of a joint is found to be a function of the angle between the long axis of the nanowires and the gap distance. Finite element simulations show that the strong localized electric field generated by optical excitation determines the location and the morphology of the resulting bond. Light coupling and the resulting emission properties of these Y-shaped nanowire structures have been simulated and are compared to similar structures where the gap remains open. It is suggested that joined Y-shaped couplers will have a higher switching ratio between emitted nanowire ends than those occurring in open-gap structures. Nanojoining induced by localized heating under strong field excitation may enable the production of robust branched metal nanowire structures for optical applications.

A novel Co@Au structure formed in bimetallic core@shell nanoparticles

Core@shell Co@Au nanoparticles of around 8 nm have been produced by the inert gas condensation method, revealing for the first time that most of the nanoparticles exhibit an icosahedral shape in agreement with the theoretical prediction. Additionally, we report the existence of a novel morphology which consists of a Co icosahedron surrounded by fcc Au facets, reported here for the first time.

Metallic nanowire networks: effects of thermal annealing on electrical resistance

Metallic nanowire networks have huge potential in devices requiring transparent electrodes. This article describes how the electrical resistance of metal nanowire networks evolve under thermal annealing. Understanding the behavior of such films is crucial for the optimization of transparent electrodes which find many applications. An in-depth investigation of silver nanowire networks under different annealing conditions provides a case study demonstrating that several mechanisms, namely local sintering and desorption of organic residues, are responsible for the reduction of the systems electrical resistance. Optimization of the annealing led to specimens with transmittance of 90% (at 550 nm) and sheet resistance of 9.5 Ω sq(-1). Quantized steps in resistance were observed and a model is proposed which provides good agreement with the experimental results. In terms of thermal behavior, we demonstrate that there is a maximum thermal budget that these electrodes can tolerate due to spheroidization of the nanowires. This budget is determined by two main factors: the thermal loading and the wire diameter. This result enables the fabrication and optimization of transparent metal nanowire electrodes for solar cells, organic electronics and flexible displays.

Consistent melting behavior induced by Joule heating between Ag microwire and nanowire meshes

The melting behavior of an Ag microwire mesh induced by Joule heating was numerically investigated and compared with that of the corresponding Ag nanowire mesh with the same structure but different geometrical and physical properties of the wire itself. According to the relationship of melting current and melting voltage during the melting process, a similar repetitive zigzag pattern in melting behavior was discovered in both meshes. On this basis, a dimensionless parameter defined as figure of merit was proposed to characterize the current-carrying ability of the mesh. The consistent feature of figure of merit in both meshes indicates that the melting behavior of the Ag nanowire mesh can be predicted from the present results of the corresponding Ag microwire mesh with the same structure but made from a different wire (e.g., different size, different material) through simple conversion. The present findings can provide fundamental insight into the reliability analysis on the metallic nanowire mesh-based transparent conductive electrode.

Self-seeded growth of five-fold twinned copper nanowires: mechanistic study, characterization, and SERS applications

A comprehensive mechanistic study conducted on the formation mechanism of five-fold twinned copper nanowires by heating copper(I) chloride with oleylamine at 170 °C is presented. Electron microscopy and UV-visible absorption spectra are used to analyze the growth mechanism of copper nanowires. High-resolution transmission electron microscopy and selected-area electron diffraction are used to investigate the detailed structure of copper nanowires and nanoparticles, and a five-twinned structure is shown to exist in the copper nanowires and nanoparticles. Additionally, experiments have been performed to indirectly confirm that oleylamine preferentially adsorbs on the {100} facets of growing crystals. On the basis of the above results, the self-seeded growth of copper nanowires is confirmed. In the initial stage of reactions, copper nanoparticles with two distinctive sizes are formed. As the reaction proceeds, larger five-twinned copper nanoparticles serve as seeds for anisotropic crystal growth. Further, copper atoms generated from an Ostwald ripening process or reduction reactions of a copper(I) chloride-oleylamine complex continue to deposit and crystallize on the twin boundaries. Once the {110} planes are generated, oleylamine preferentially adsorbs on the newly formed {100} facets and then guides the formation of nanowires. The electrical resistivity of a single copper nanowire is measured to be 41.25 nΩ-m, which is of the same order of magnitude as the value of bulk copper (16.78 nΩ-m). Finally, an effective surface-enhanced Raman spectroscopy active substrate made of copper nanowire is used to detect the 4-mercaptobenzoic acid molecules.

Numerical analysis of the electrical failure of a metallic nanowire mesh due to Joule heating

To precisely examine the electrical failure behavior of a metallic nanowire mesh induced by Joule heating (i.e., melting), a previously developed numerical method was modified with regard to the maximum temperature in the mesh and the electrical resistivity of the nanowire. A sample case of an Ag nanowire mesh under specific working conditions was analyzed with highly accurate numerical results. By monitoring the temperature in the mesh, the current required to trigger the melting of a mesh segment (i.e., the melting current) could be obtained. The melting process of a mesh equipped with a current source during actual operation was predicted on the basis of the obtained relationship between the melting current and the corresponding melting voltage in the numerical melting process. Local unstable and stable melting could be precisely identified for both the current-controlled and voltage-controlled current sources in the present example.

Printed silver nanowire antennas with low signal loss at high-frequency radio

Silver nanowires are printable and conductive, and are believed to be promising materials in the field of printed electronics. However, the resistivity of silver nanowire printed lines is higher than that of metallic particles or flakes even when sintered at high temperatures of 100-400 °C. Therefore, their applications have been limited to the replacement of transparent electrodes made from high-resistivity materials, such as doped metallic oxides, conductive polymers, carbon nanotubes, or graphenes. Here we report that using printed silver nanowire lines, signal losses obtained in the high-frequency radio were lower than those obtained using etched copper foil antennas, because their surfaces were much smoother than those of etched copper foil antennas. This was the case even though the resistivity of silver nanowire lines was 43-71 μΩ cm, which is much higher than that of etched copper foil (2 μΩ cm). When printed silver nanowire antennas were heated at 100 °C, they achieved signal losses that were much lower than those of silver paste antennas comprising microparticles, nanoparticles, and flakes. Furthermore, using a low temperature process, we succeeded in remotely controlling a commercialized radio-controlled car by transmitting a 2.45 GHz signal via a silver nanowire antenna printed on a polyethylene terephthalate film.

Transformations of carbon adsorbates on graphene substrates under extreme heat

We describe new phenomena of structural reorganization of carbon adsorbates as revealed by in situ atomic-resolution transmission electron microscopy (TEM) performed on specimens at extreme temperatures. In our investigations, a graphene sheet serves as both a quasi-transparent substrate for TEM and as an in situ heater. The melting of gold nanoislands deposited on the substrate surface is used to evaluate the local temperature profile. At annealing temperatures around 1000 K, we observe the transformation of physisorbed hydrocarbon adsorbates into amorphous carbon monolayers and the initiation of crystallization. At temperatures exceeding 2000 K the transformation terminates in the formation of a completely polycrystalline graphene state. The resulting layers are bounded by free edges primarily in the armchair configuration.