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Stretchable conductive films were obtained by screen printing and thermal treatment of a homogenous ink comprising a thermally reducible silver formate complex, an acrylate monomer, and a radical i...

Although in-situ monitoring of electrical resistance of polymer-supported thin films during a mechanical test is frequently used, the impact of the magnitude of electric current on material behavior in such experiments remained unknown. Here, we report on cyclic straining of polymer-supported gold metallization lines under direct current of different magnitudes. Fatigue damage in form of localized extrusions becomes more pronounced with increasing current as a result of Joule heat and material softening. After a critical current value is reached, crack propagation results in a local decrease of line cross-section leading to thermal runaway and open circuit failure.

Cellulose fibers are a major industrial input, but due to their irregular shape and anisotropic material response, accurate material characterization is difficult. Single fiber tensile testing is the most popular way to estimate the material properties of individual fibers. However, such tests can only be performed along the axis of the fiber and are associated with problems of enforcing restraints. Alternative indirect approaches, such as micro-mechanical modeling, can help but yield results that are not fully decoupled from the model assumptions. Here, we compare these methods with nanoindentation as a method to extract elastic material constants of the individual fibers. We show that both the longitudinal and the transverse elastic modulus can be determined, additionally enabling the measurement of fiber properties in-situ inside a sheet of paper such that the entire industrial process history is captured. The obtained longitudinal modulus is comparable to traditional methods for larger indents but with a strongly increased scatter as the size of the indentation is decreased further.

The aim of the current research work is to understand how individual local microstructure features such as grain size, grain orientation, grain boundary characteristics, as well as the global parameters of the microstructure (texture, grain size distribution, twin boundary fraction) influence the process of fatigue damage initiation in thin films. Cyclic strain with an amplitude of 1% was applied to polymer-supported ultrafine grained gold and copper films allowing for observation of numerous independent localized plasticity events within a single macrosample. Detailed electron backscatter diffraction analysis of the microstructure in the vicinity of these plasticity events was performed to gain statistical information and elucidate robust correlations. On the local scale, the damage was initiated in large grains (>1 µm) in the vicinity of coherent twin boundaries specifically oriented with respect to the loading direction. When appropriate large grains were not present in the initial microstructure, mechanically induced grain coarsening preceded fatigue damage initiation. If the initial microstructure did not contain properly oriented coherent twin boundaries, then the local grain size of dynamically coarsened grains was the sole parameter controlling plastic slip localization. On the global scale, the major parameters of the microstructure influencing the fatigue damage initiation process are the fraction of twin boundaries, texture, and the width of the grain size distribution. Based on the presented results a clear strategy for improving the fatigue life of thin films is proposed.

Polymer supported metal thin films for flexible and stretchable applications need to have exceptional electro-mechanical behavior. Typically, brittle metal films thicker than 300 nm will fail at low strains. It will be demonstrated that with residual stress and microstructural tuning, a 500 nm thick Mo film on polyimide has a high crack onset strain and does not fail electrically after 1000 tensile straining cycles of 1% due to a novel zig–zag microstructure. The zig–zag microstructure imparts a crack deflecting behavior which ensures longer lifetimes. As a consequence, a high crack onset strain does not indicate better behavior under cyclic loading conditions.

Here we provide a systematic study of tensile deformation mechanisms in PdSi metallic glasses employing polymer-supported sputter deposited films. Due to the polymer substrate, global strain localization within one critical shear band is inhibited and thus thousands of different shear bands and cracks can be examined within a single tensile sample. Two distinctly different types of shear bands, allowed by the sample geometry, are observed and analyzed. It is shown that crack formation occurs through activation of multiple out-of-plane shear bands leading to local plasticity and neck-like shape of crack edges. The in-plane shear bands, which appear at higher applied strains, typically do not lead to crack formation and can accommodate both applied tensile strain and transverse compressive strain. The mechanism of effective strain hardening through de-activation of in-plane shear bands is demonstrated by means of quasi-in-situ studies. Shear banding is shown to be the fundamental deformation mechanism for different film thicknesses ranging from 25 to 1000 nm. Polymer-supported films represent a unique model material system which enables easy characterization of the tensile behavior of metallic glasses and provides an opportunity to gain new insights into the fundamental mechanisms of shear banding.

Mechanical stability of Ag and Cu printed and evaporated metallization lines on polymer substrates is investigated by means of monotonic tensile and cyclic bending tests. It is shown that lines which demonstrate good performance during monotonic tests fail at lower strains during a cyclic bending tests. Evaporated lines with the grain size of several hundreds of nanometers have good ductility and consequently good stability during monotonic loading but at the same time they fail at low strains during cyclic bending. Printed lines with nanocrystalline microstructure, in contrast, demonstrate more intensive cracking during monotonic loading but higher failure strains during cyclic bending. Apart from the grain size effect, the effect of film thickness on the saturation crack density after cyclic bending is also demonstrated. Thinner films have higher crack density in accordance with the shear lag model.

Strong room-temperature grain coarsening in gold films on polyimide induced by cyclic uniaxial mechanical strain is demonstrated. Detailed electron backscatter diffraction analysis revealed that, in contrast to the predictions of shear-coupled grain boundary migration model, the grain coarsening is isotropic and coarsened grains do not exhibit any specific crystallographic orientations or misorientations to the neighboring grains. It is shown that a thermodynamic model where the driving force appears due to the difference in yield stresses between the grains with different sizes provides an adequate explanation of the experimental data.

Printing of metallic films has been preferred over vacuum technologies for roll-to-roll processes because of faster processing times and lower processing costs. Films can be produced by depositing inks containing suspended metallic particles within a solvent and then heating the films to both remove the solvent and sinter the particles. The resulting printed structure and electrical and mechanical behavior of the printed films has been studied to better understand their electro-mechanical response to loading and eventual brittle fracture. This study evaluated the electro-mechanical behavior of 1.25-μm printed Ag films using in situ resistance and in situ imaging methods. Digital image correlation was utilized with confocal laser scanning microscope images to better visualize crack initiation during tensile straining. This technique showed that cracks initiated earlier in the thicker areas of the film (crests) than in lower areas (troughs) because of a higher density of printing defects and the increased thickness.

Despite the large number of experiments demonstrating that grains in a metallic material can grow at room temperature due to applied mechanical load, the mechanisms and the driving forces responsible for mechanically induced grain coarsening are still not understood. Here we present a systematic study of room temperature grain coarsening induced by cyclic strain in thin polymer-supported gold films. By means of detailed electron backscatter diffraction analysis we were able to capture both the growth of individual grains and the evolution of the whole microstructure on the basis of statistical data over thousands of grains. The experimental data are reported for three film thicknesses with slightly different microstructures and three different amplitudes of cyclic mechanical loading. Although different kinds of grain size evolution with increasing cycle number are observed depending on film thickness and strain amplitude, a single model based on a thermodynamic driving force is shown to be capable to explain initiation and stagnation of grain coarsening in all cases. The main implication of the model is that the grains having lower individual yield stress are coarsening preferentially. Besides, it is demonstrated that the existence of local shear stresses imposed on a grain boundary is not a necessary requirement for room-temperature grain coarsening.

Metal films on polymer substrates are commonly used in flexible electronic devices and may be exposed to large deformations during application. For flexible electronics, the main requirement is to remain conductive while stretching and compressing. Therefore, the electro-mechanical behaviour of 200-nm-thick Cu films on polyimide with two different microstructures (as-deposited and annealed) were studied by executing in situ fragmentation experiments with x-ray diffraction, under an atomic force microscope, and with 4-point probe resistance measurements in order to correlate the plastic deformation with the electrical behaviour. The three in situ techniques clearly demonstrate different behaviours controlled by the microstructure. Interestingly, the as-deposited film with a bi-modal microstructure is more suited for flexible electronic applications than an annealed film with homogenous 1-µm-sized grains. The as-deposited film reaches a higher yield stress, with unchanged electrical conductivity, and does not show extensive surface deformation during straining.

The electro-mechanical performance of Mo thin films grown by dc magnetron sputter deposition on polyimide substrates was investigated. A series of 500 nm thick Mo films was synthesized at different discharge powers to evaluate the effect of deposition conditions on the structure–stress relationship and to correlate the intrinsic properties of the Mo films with their electro-mechanical response. Different in-situ fragmentation tests were performed to assess the crack morphology, the change in electrical resistance and the film stress during straining. A direct relation between the residual stress state of the Mo thin films and the discharge power was noticed as the stress changed from tensile to compressive with increasing discharge power. All Mo films showed brittle fracture when strained in tension and the critical crack onset strain correlated with the residual stress state. In-situ synchrotron diffraction experiments enabled the characterization of the fracture strength, which was unaltered by the discharge power and found to be approximately 1700 MPa for all Mo thin films studied.

A detailed investigation of the correlation between mechanical damage and the change in electrical resistance induced by cyclic tensile loading is performed for polymer-supported gold, copper, and printed silver films. Four distinct types of resistance response to mechanical loading are described and linked to the mechanical properties and microstructure of thin films. It is shown that significant topographical changes induced by cyclic loading can result in minor changes in the overall electrical resistance. Presented results show that an underestimation of mechanical damage can be made when only the electrical failure criterion is used for reliability characterization of polymer-supported metal films.

The coupling of electrical and mechanical property measurements for thin films is a relatively new area of research. It is particularly useful to know how electrical resistance could be altered with mechanical straining for flexible electronic applications. While coupled resistance measurements have been made in conjunction with tensile straining, a clear description of the measurement technique is rarely provided. Explained here is a four-point probe resistance measurement that has been incorporated into tensile straining grips for accurate and repeatable in situ resistance measurements. Also described is an analytical explanation of how to correct for constant resistances that do not change during the experiment. The new technique has been employed on Cu films on polyethylene terephthalate (PET) to illustrate its use and to discuss how resistance will recover after 24 h.

Systematic investigation of the effect of tensile and compressive cyclic bending strains on the mechanical reliability of inkjet printed and evaporated conductive silver lines on polyethylene naphthalate substrates is presented. With the help of a new bending test apparatus it is shown that cyclic tensile, compressive, and mixed tensile-compressive bending strains result in different amounts of induced mechanical damage in printed silver lines. In contrast, evaporated silver lines with the same geometry show no dependence on the type of strain. A detailed comparison of the fracture mechanisms in printed and evaporated silver is given using scanning electron microscopy and focused ion beam analysis.

Although it is evident that the formation of multiple through-thickness cracks in polymer-supported thin films leads to an increase of the electrical resistance, the attempts to quantify the dependence of resistance growth only based on the induced crack density have not yet been successful. In this paper a recently developed relationship, representing the resistance growth as a two-variable polynomial function of crack density and crack length, is utilized to analyze the crack patterns induced by monotonic and cyclic tensile loading of 250 nm thick Cu films with a 10 nm Cr adhesion layer on polyimide. It is demonstrated that by knowing the in-situ resistance during deformation and post-mortem linear density of induced cracks, it is possible to extract the effective crack lengths, a parameter which is often ignored during experimental characterization of the damage induced through mechanical loading. The described algorithm is not only much more cost-effective in comparison to time-consuming in-situ microscopy methods, it also reflects the reliability of the whole sample rather than of only selected surface areas.

The study of electromigration (EM) in metallisations for flexible thin film systems has not been a major concern due to low applied current densities in today's flexible electronic devices. However, the trend towards smaller and more powerful devices demands increasing current densities for future applications, making EM a reliability matter. This work investigates EM in 50 nm Au thin films with a 10 nm Cr adhesion layer on a flexible polyimide substrate at high current densities. Results indicate that EM does occur and could be used as a self-healing mechanism for flexible electronics., GRAPHICAL ABSTRACT

Graphical abstract, Highlights • Electromechanical properties of metal films are studied with in-situ AFM-4PP tensile straining. • AFM characterizes the surface damage and 4PP measures the resistance under tensile strain. • The difference between local film necking and through thickness cracks can be made. • It will be shown when the strain is removed, cracks bridge and the resistance decreases., In order to advance flexible electronic technologies it is important to study the electrical properties of thin metal films on polymer substrates under mechanical load. At the same time, the observation of film deformation and fracture as well as the stresses that are present in the films during straining are also crucial to investigate. To address both the electromechanical and deformation behavior of metal films supported by polymer substrates, in-situ 4 point probe resistance measurements were performed with in-situ atomic force microscopy imaging of the film surface during straining. The 4 point probe resistance measurements allow for the examination of the changes in resistance with strain, while the surface imaging permits the visualization of localized thinning and crack formation. Furthermore, in-situ synchrotron tensile tests provide information about the stresses in the film and show the yield stress where the deformation initiates and the relaxation of the film during imaging. A thin 200 nm Cu film on 23 μm thick PET substrate will be used to illustrate the combined techniques. The combination of electrical measurements, surface imaging, and stress measurements allow for a better understanding of electromechanical behavior needed for the improvement and future success of flexible electronic devices.

When ductile metal films on compliant polymer substrates are strained in tension catastrophic failure can be suppressed by the substrate, thus allowing for their use in flexible electronics and sensors. However, the charge carrying ductile films must be of an optimum thickness and microstructure for the suppression of cracking to occur. Studies of strained films on polymer substrates tend to have more emphasis on the electrical properties and thickness effects than on the film microstructure or deformation behaviour. To address both the electrical degradation and deformation behaviour of metal films supported by polymer substrates two types of combined electro-mechanical in-situ tests were performed. First, is a combination of in-situ resistance measurements with in-situ confocal scanning laser microscopy imaging of the film surface during cycling. The 4 point probe resistance measurements allow for the examination of the changes in resistance with strain, while the surface imaging permits the visualization of extrusion and crack formation. Second, is the combination of in-situ resistance with in-situ X-ray diffraction measurements of the film stresses during cycling. The combination of electrical measurements, surface imaging, and stress measurements allow for a complete picture of electro-mechanical behaviour needed for the improvement and future success of flexible electronic devices.

Electrical resistance of polymer-supported thin metal films subjected to cyclic tensile loading typically is expected to grow with the cycle number due to fatigue-induced damage. Here it is demonstrated that electrical resistance can also decrease with the cycle number. Using electron backscatter diffraction analysis, it is shown that significant strain-induced, room-temperature grain coarsening is responsible for the resistance decrease. Grain coarsening has a vital effect on the electrical stability of metal films in a low-cycle fatigue regime.

Substantial recovery (decrease) of electrical resistance during and after unloading is demonstrated for copper films on polyethylene terephthalate substrates subjected to a tensile strain with different peak values. Particularly, the films strained to 5% exhibit full resistance recovery after unloading despite clearly visible plastic deformation of the film. The recovery of electrical resistance in connection with the mechanical behavior of film/substrate couple is discussed with the help of in situ scanning electron microscopy and X-ray diffraction analysis.

Within this work, in situ tensile straining tests are employed to study the electromechanical properties of MoNbTaVW and AlCuCrTaTi high entropy alloy thin films which were synthesized by cathodic arc deposition on polyimide substrates at room temperature. The crack onset strains of MoNbTaVW and AlCuCrTaTi were observed to be around 2% and 2.5%, respectively. The sharp rise in resistance at strains exceeding the crack onset strain indicates a brittle behavior of both films, while the interfacial shear stress is higher for the MoNbTaVW film.

The use of flexible electronics has increased in recent years. In order to have robust and long lasting flexible displays and sensors, the combined electro-mechanical behavior needs to be assessed. The most common method to determine electrical and mechanical behavior of conductive thin films used in flexible electronics is the fragmentation test or uniaxial tensile straining of the film and substrate. When performed in situ, fracture and deformation behavior can be determined. The use of in situ electrical resistance measurements can be informative about the crack onset strain of brittle layers, such as transparent conductors, or the stretchability of metal interconnects. The combination of in situ electrical measurements with in situ X-ray or confocal laser scanning microscopy can provide even more information about the failure mechanisms of the material systems. Lattice strains and stresses can be measured with X-rays, while cracking and buckle delaminations can be studied with confocal laser scanning microscopy. These new combinations of in situ methods will be discussed as well as methods to quantify interfacial properties of conductive thin films on polymer substrates. The combined techniques provide valuable correlated electrical and mechanical data needed to understand failure mechanisms in flexible devices.

Tuning the flow of light by external fields is a challenging task for scientific studies and optical applications, but it is important in many applications such as switches, modulators, and slow wave structures. Here, new results are presented which demonstrate that this effectively can be achieved by external magnetic fields in one-dimensional photonic crystals made from semiconducting material. The advantage of using semiconducting material is the magnetic-field dependent dielectric function of the free charge carriers particularly where the magnetic field causes large and strongly varying contributions – near the plasma frequency and the cyclotron resonance frequency. The results of simulations on the basis of a multiple scattering method at infrared and microwave frequencies and of experiments on Indium Antimonide in the latter frequency regime confirm the tunability up to the extreme case from full transparency to opaqueness and vice versa.

New, energy-saving, efficient and cost-effective processing technologies such as 2D and 3D inkjet printing (IJP) for the production and integration of intelligent components will be opening up very interesting possibilities for industrial applications of molecular materials in the near future. Beyond the use of home and office based printers, "inkjet printing technology" allows for the additive structured deposition of photonic and electronic materials on a wide variety of substrates such as textiles, plastics, wood, stone, tiles or cardboard. Great interest also exists in applying IJP in industrial manufacturing such as the manufacturing of PCBs, of solar cells, printed organic electronics and medical products. In all these cases inkjet printing is a flexible (digital), additive, selective and cost-efficient material deposition method. Due to these advantages, there is the prospect that currently used standard patterning processes can be replaced through this innovative material deposition technique. A main issue in this research area is the formulation of novel functional inks or the adaptation of commercially available inks for specific industrial applications and/or processes. In this contribution we report on the design, realization and characterization of novel active and passive inkjet printed electronic devices including circuitry and sensors based on metal nanoparticle ink formulations and the heterogeneous integration into 2/3D printed demonstrators. The main emphasis of this paper will be on how to convert scientific inkjet knowledge into industrially relevant processes and applications.

For a comprehensive characterization of mechanical reliability of metallization layers on polymer substrates both electrical and mechanical degradation should be taken into account. Although it is evident that cracking of a conductive film should lead to electrical degradation, the quantitative relationship between the growth of electric resistance and parameters of the induced crack pattern has remained thus far unexplored. With the help of finite element modelling we were able to find an explicit and concise expression which shows that electrical resistance grows with the fourth order of the crack length and second order of the areal crack density. The discovered relationship was verified by comparison with the experimental results of tensile testing of polymer-supported thin metal films. Presented model is independent of the length scale and can be applied to films with different thicknesses as long as Ohm's law is valid. It is demonstrated that linear crack density is an ambiguous parameter which does not properly capture the development of a crack pattern. For the unambiguous characterization of the intensity of a crack pattern a universal dimensionless factor is proposed. Presented results show that there is a wide range of possible crack patterns which do not lead to electrical failure of a conductive film that can be used for failure-free design of flexible electronic devices.

The plane-wave expansion method (PWEM), the multiple-scattering method (MSM) and the 3D finite-difference time-domain method (FDTD) are applied for simulations of propagation of electromagnetic waves through 3D colloidal photonic crystals. The system investigated is not a “usual” artificial opal with close-packed fcc lattice but a dilute bcc structure which occurs due to long-range repulsive interaction between electrically charged colloidal particles during the growth process. The basic optical properties of non-close-packed colloidal PhCs are explored by examining the band structure and reflection spectra for a bcc lattice of silica spheres in an aqueous medium. Finite size effects and correspondence between the Bragg model, band structure and reflection spectra are discussed. The effects of size, positional and missing-spheres disorder are investigated. In addition, by analyzing the results of experimental work we show that the fabricated structures have reduced plane-to-plane distance probably due to the effect of gravity during growth.

In this work we show that structures consisting of a metal hole array (MHA) lying on top of a 2D photonic crystal (PhC) exhibit the extraordinary transmission effect. In contrast to single MHAs, the extraordinary transmission in such hybrid structures is due to the coupling of an incident wave to eigenmodes of the PhC. Thus, the spectral positions of the transmission peaks are defined by the spectral positions of the corresponding PhC eigenmodes. Our results provide a novel powerful tool to manipulate light on a subwavelength scale.

We present numerical simulations as well as experimental investigations of 1D photonic crystals (PhC) with intentionally introduced surface roughness. An 'experimental simulation' of the roughness was created by gluing alumina powder to both sides of each alumina plate in an alumina?air structure. Transmission experiments were performed on this 1D PhC at microwave frequencies. A 'red-shift' of the band edges observed in experiment and simulation is explained by an increase of the effective thickness of the alumina layers as the surface roughness becomes stronger. The influence of the features of the roughness becomes visible in the simulations only at wavelengths short enough to be of the order of the dimensions of these features. Then, the band edges are smeared and the residual transmission in the band gaps is increased. We show that the main effect responsible for the smearing of the band edges is the scattering of the wave on roughness features. Even for very strong roughness (40% of the plate thickness) the band gaps are not destroyed completely.