Film Percolation Research Articles

Optical Characteristics of Silver Thin Films from Island to Percolation in the Ultra-Wide Infrared Spectral Range

Silver (Ag) thin films have garnered significant attention due to their unique optical properties. This paper systematically investigates the optical characteristics of Ag films prepared using the electron beam evaporation method. The investigation was conducted using spectroscopic ellipsometry and covers a broad wavelength range of 1679 nm to 36 µm (0.738–0.034 eV), spanning from near-infrared to far-infrared regions. Optical and dispersion models were developed to analyze the impacts of Ag nanostructures on the complex refractive indices, dielectric functions, and reflectance. The results indicate that Ag particles and coalescence films exhibit non-metallic and low absorption properties, while Ag percolation and continuous films present a typical Drude model. The reflectance of Ag films increases as the film coverage ratio increases, and it can reach close to 100% in continuous film. Additionally, a non-destructive, non-contact, and vacuum-free means of confirming the percolation threshold of Ag films was proposed based on the slope of the imaginary part curve. This work is useful to guide simulations and provide a basis for the applications of Ag films in different fields.

Quantitative PEEM and Raman Study of Nanorough Au SERS-Active Substrates for Molecular Sensing Applications

Surface-enhanced Raman scattering (SERS) is a well-established surface-sensitive technique for detecting trace amounts of molecular analytes. While the impact of surface singularities on plasmonic materials has been widely studied, the fabrication of cost-effective, efficient SERS substrates remains a challenge. In this paper, we present the study of large-area nanorough Au SERS-active substrates, elaborated by thermal evaporation deposition by photoemission electron microscopy (PEEM), a high-resolution near-field mapping technique, to access the statistical properties of the hot-spot distribution. We experimentally demonstrate that the near-field PEEM and far-field Raman statistical signatures of nanorough Au surfaces are quantitatively correlated when used for molecular sensing. The maximum of the SERS signal of thiophenol (TP) molecules diluted to 10–6 M is observed near the film percolation threshold for which the hot-spot density is maximum. Finally, SERS measurements from solutions of TP, crystal violet, and rhodamine B molecules at 10–8 M demonstrated the sensitivity of our substrates for molecular sensing.

High dielectric constant acrylic resin based percolative composite with acidified carbon nanotubes intercalation of MXene

It is critical for electromechanical polymer film to have excellent dielectric properties. In this research, two-dimensional MXene (Ti3C2Tx) nanosheets decorated with acidified carbon nanotubes (referred to here as CNT-COOH@Ti3C2Tx) were used as conductive fillers to prepare high dielectric constant acrylic resin elastomer (AE)-based percolative composite films. Physical blends of Ti3C2Tx and AE (Ti3C2Tx/AE) and CNT-COOH@Ti3C2Tx/AE composite films with different volume fraction fillers were prepared by solution casting method, and their percolation thresholds (fc) were 1.38 vol% and 1.51 vol%, respectively. At 102 Hz, the dielectric constant of 1.42 vol% CNT-COOH@Ti3C2Tx/AE is 120, which is 1.29 times that of 1.26 vol% Ti3C2Tx/AE and 30 times that of AE. At the same time, the dielectric loss of 1.42 vol% CNT-COOH@Ti3C2Tx/AE remains at a relatively low level (0.15 at 102 Hz). The significant increase in the dielectric constant of the composite is closely related to the miniature capacitor effect and interfacial polarization effect.

Enhanced dielectric and electrical properties of three-component PMMA–NaNbO3–starch percolative composite films

Enhanced dielectric and electrical properties of three-component PMMA–NaNbO3–starch percolative composite films

Rationalizing charge carrier transport in ternary organic solar cells

Ternary bulk heterojunction (BHJ) organic solar cells have energy offsets between multiple donors and acceptors. In such bi-continuous percolating films, electron carriers mainly transport in acceptor materials, and hole carriers typically transport in donor materials. Changing the third component of additional donors or acceptors is a common method to fine-tune the transport properties in ternary BHJs. Experimentally, although there are some empirical guidelines for the mobility evaluation, a clear charge transporting model has still not been fully established in multi-component BHJ films. Herein, we observed a general regularity about charge transport that the charge carriers have higher possibilities to transport in polymeric materials even if only low dosage (<10% in weight) polymeric materials are introduced for both the hole and electron cases. From the extended Su–Schrieffer–Heeger model, a polymer “bridge” assisted charge transport mode in small molecules is proposed after the energy offset exceeds the threshold (ΔEe > 0.15 eV). This work provides a perspective for fine tuning the charge transport property in high-performance ternary organic solar cells.

Graphene Flake Self-Assembly Enhancement via Stretchable Platforms and External Mechanical Stimuli.

While the green production and application of 2D functional nanomaterials, such as graphene flakes, in films for stretchable and wearable technologies is a promising platform for advanced technologies, there are still challenges involved in the processing of the deposited material to improve properties such as electrical conductivity. In applications such as wearable biomedical and flexible energy devices, the widely used flexible and stretchable substrate materials are incompatible with high-temperature processing traditionally employed to improve the electrical properties, which necessitates alternative manufacturing approaches and new steps for enhancing the film functionality. We hypothesize that a mechanical stimulus, in the form of substrate straining, may provide such a low-energy approach for modifying deposited film properties through increased flake packing and reorientation. To this end, graphene flakes were exfoliated using an unexplored combination of ethanol and cellulose acetate butyrate for morphological and percolative electrical characterization prior to application on polydimethylsiloxane (PDMS) substrates as a flexible and stretchable electrically conductive platform. The deposited percolative free-standing films on PDMS were characterized via in situ resistance strain monitoring and surface morphology measurements over numerous strain cycles, with parameters extracted describing the dynamic modulation of the film’s electrical properties. A reduction in the film resistance and strain gauge factor was found to correlate with the surface roughness and densification of a sample’s (sub)surface and the applied strain. High surface roughness samples exhibited enhanced reduction in resistance as well as increased sensitivity to strain compared to samples with low surface roughness, corresponding to surface smoothing, which is related to the dynamic settling of graphene flakes on the substrate surface. This procedure of incorporating strain as a mechanical stimulus may find application as a manufacturing tool/step for the routine fabrication of stretchable and wearable devices, as a low energy and compatible approach, for enhancing the properties of such devices for either high sensitivity or low sensitivity of electrical resistance to substrate strain.

Nanoscale Controlled Oxidation of Liquid Metals for Stretchable Electronics and Photonics

AbstractLiquid metals based on gallium have attracted considerable attention for soft and bioelectronics, thanks to their excellent combination of stretchability and conductivity. Nevertheless, owing to their large surface tension, these materials are notoriously difficult to pattern and shape into thin continuous films, or nanoscale 2D architectures, hindering practical use in systems with reduced dimensions. Herein, thanks to fine control in both substrate surface state and oxidation dynamics, a process for producing stretchable gallium‐based conducting films with percolation down to 90 nm thickness is presented. By further combining this process with lithography, it is also demonstrated that the approach enables, for the first time, stable stretchable gallium‐based optical metasurfaces with tunable resonance in the infrared. It is shown that oxygen partial pressure during evaporation determines the initial film percolation via an interplay between oxidation and dewetting. With this approach, conducting films with relative resistance change as low as 3% over 50% strain, with an excellent stability over 15k cycles are also demonstrated. Tunable soft optical metasurfaces with sub‐micrometer feature sizes are also realized, paving the way toward a novel paradigm in soft electronics and photonics.

High performance flexible organic photomultiplication photodetector based on an ultra-thin silver film transparent electrode

Flexible and lightweight photomultiplication-type organic photodetectors (PM-OPDs) have attracted wide attention for their broad application prospects, especially in the field of wearable electronic products. However, the commonly used indium tin oxide (ITO) conductive anode is not conducive to realize high-performance flexible PM-OPDs due to its rigidity and fragility. Here, on the flexible polyethylene terephthalate (PET) substrate, we successfully fabricate highly sensitive poly 3-hexylthiophene:phenyl-C70-butyric acid methyl ester (P3HT:PC70BM, 100:1) based PM-OPDs using ultra-thin silver films as transparent anodes. Specifically, a 1 nm thick MoO3 layer is utilized as the wetting layer for facilitating the silver film percolation, and a 2 nm thick MoO3 layer, as the hole transport layer, is coated on top of the ultra-thin silver film before coating the P3HT:PC70BM film. The as-prepared flexible PM-OPDs based on the ultra-thin silver film exhibit the optimal external quantum efficiency (EQE) and responsivity (R) of 1.3 × 105% and 388.4 A W−1, respectively, under −15 V bias, which are 1.98 times and 2.15 times greater than those of the ITO anode based device. More importantly, the device has good flexibility with the EQE maintaining 70.6% of its initial value after bending 10 times, and 51.4% of its initial value after bending 1000 times. This work paves the way for developing flexible PM-OPDs as well as other flexible optoelectronic devices.

Serpentine-pattern effects on the biaxial stretching of percolative graphene nanoflake films

Stretchable strain sensors based on percolative arrangements of conducting nanoparticles are essential tools in stretchable electronics and have achieved outstanding performance. Introducing serpentine patterns for strain-sensing materials is a very effective method for enhancing stretchability with a quantified structural resistance through a simple, reliable, and facile approach. Here, we investigate serpentine-pattern effects in the electrical responses to biaxial stretching for percolative graphene-nanoparticle films. Graphene nanoplatelet films are applied to a stretchable substrate using a facile spray-coating technique, for a variety of serpentine pattern shapes, aspect ratios, pattern frequencies, and number of coatings. The electrical responses after applying biaxial stretching (x-axis and y-axis) are measured and analyzed for comparison. The serpentine patterns that would be suitable for stretchable electrodes, sensitive sensors, and highly stretchable sensors are then identified. This work demonstrates the advantage of using serpentine patterns for stretchable strain sensors and offers guidelines for selecting suitable pattern types for strain sensors in stretchable-electronics applications.

Ultrabroadband infrared near-field spectroscopy and imaging of local resonators in percolative gold films

Percolation processes are ubiquitous in nature and are responsible for many critical phenomena such as first-order phase transitions and infectious epidemic networks. The optical properties of a percolative medium can generally be captured by the effective medium approximation (EMA) when the degree of percolation and the properties of the constituent materials are properly addressed. However, the important local collective responses of nanoclusters in the deep subwavelength regime are often only phenomenologically addressed in the standard EMA formalism. A comprehensive method that measures local light–matter interactions and registers how the local responses influence global optical properties has yet to be established on a firm basis. In this paper, we use infrared nano-imaging/spectroscopy to investigate percolative gold films in the vicinity of the critical percolation threshold. We demonstrate experimentally and theoretically that the near-field spectra yield quantitative information of the characteristic length scale of the local gold clusters and their relative oscillator strengths. As a result, EMA analysis can be augmented with near-field nano-spectroscopy to yield better predictability of the far-field reflection spectrum at the corresponding spectral range.

A universal approach of modelling the dielectric behaviors of percolative composite ceramics and thin films

The topological microstructures and electrical conductivities of (1-x)BaTiO3/xNi0.5Zn0.5Fe2O4 (BTO/NZFO) composite ceramics and thin films both exhibit a classic percolative behavior at the percolation threshold and can be well interpreted under the framework of percolation theory. However, their permittivities are in accordance with the Kirkpatrick model, which is contradictory with the well-established percolation model as revealed in previous studies. To elucidate this interesting phenomenon both experimentally and theoretically, a universal approach of modelling the dielectric behaviors of percolative systems was proposed by using equivalent circuit concept. Based on our approach, it can be revealed that the impedance of the conductive phase is too small to play a predominant role in the dielectric performance of the composites, rendering the measured capacitance and apparent permittivity to be giant around the percolation threshold. Only when the conductivity of the conductive phase reaches a certain value does this behavior appear, which can be satisfactorily interpreted in light of our universal model.

Self-Powered Pressure- and Vibration-Sensitive Tactile Sensors for Learning Technique-Based Neural Finger Skin.

Finger skin electronics are essential for realizing humanoid soft robots and/or medical applications that are very similar to human appendages. A selective sensitivity to pressure and vibration that are indispensable for tactile sensing is highly desirable for mimicking sensory mechanoreceptors in skin. Additionally, for a human-machine interaction, output signals of a skin sensor should be highly correlated to human neural spike signals. As a demonstration of fully mimicking the skin of a human finger, we propose a self-powered flexible neural tactile sensor (NTS) that mimics all the functions of human finger skin and that is selectively and sensitively activated by either pressure or vibration stimuli with laminated independent sensor elements. A sensor array of ultrahigh-density pressure (20 × 20 pixels on 4 cm2) of interlocked percolative graphene films is fabricated to detect pressure and its distribution by mimicking slow adaptive (SA) mechanoreceptors in human skin. A triboelectric nanogenerator (TENG) was laminated on the sensor array to detect high-frequency vibrations like fast adaptive (FA) mechanoreceptors, as well as produce electric power by itself. Importantly, each output signal for the SA- and FA-mimicking sensors was very similar to real neural spike signals produced by SA and FA mechanoreceptors in human skin, thus making it easy to convert the sensor signals into neural signals that can be perceived by humans. By introducing microline patterns on the top surface of the NTS to mimic structural and functional properties of a human fingerprint, the integrated NTS device was capable of classifying 12 fabrics possessing complex patterns with 99.1% classification accuracy.

Edge-Selective Gas Detection Using Langmuir Films of Graphene Platelets.

Recent advances in the large-scale production of graphene have led to the availability of solution-processable platelets on the commercial scale. Langmuir-Schaefer deposition is a scalable process for forming a percolating film of graphene platelets, which can be used for electronic gas sensing. Here, we demonstrate the use of this deposition method to produce functional gas sensors, using a chemiresistor structure from commercially available graphene dispersions. The sensitivity of the devices and the repeatability of the electrical response upon gas exposure have been characterized. Raman spectroscopy and Kelvin probe force microscopy show doping of the basal plane using ammonia (n-dopant) and acetone (p-dopant). The resistive signal is increased upon exposure to both gases, showing that sensing originates from the change in the contact resistance between nanosheets. We demonstrate that Arrhenius fitting of desorption response potentially allows measurements of desorption process activation energies for gas molecules adsorbed onto the graphene nanosheets.

Collective magnetic response of inhomogeneous nanoisland FeNi films around the percolation transition

By using superconducting quantum interference device (SQUID) magnetometry, we investigated anisotropic high-field (H ≲ 7T) low-temperature (10 K) magnetization response of inhomogeneous nanoisland FeNi films grown by rf sputtering deposition on Sitall (TiO2) glass substrates. In the grown FeNi films, the FeNi layer nominal thickness varied from 0.6 to 2.5 nm, across the percolation transition at the d c ≃ 1.8 nm. We discovered that, beyond conventional spin-magnetism of Fe21Ni79 permalloy, the extracted out-of-plane magnetization response of the nanoisland FeNi films is not saturated in the range of investigated magnetic fields and exhibits paramagnetic-like behavior. We found that the anomalous out-of-plane magnetization response exhibits an escalating slope with increase in the nominal film thickness from 0.6 to 1.1 nm, however, it decreases with further increase in the film thickness, and then practically vanishes on approaching the FeNi film percolation threshold. At the same time, the in-plane response demonstrates saturation behavior above 1.5–2T, competing with anomalously large diamagnetic-like response, which becomes pronounced at high magnetic fields. It is possible that the supported-metal interaction leads to the creation of a thin charge-transfer (CT) layer and a Schottky barrier at the FeNi film/Sitall (TiO2) interface. Then, in the system with nanoscale circular domains, the observed anomalous paramagnetic-like magnetization response can be associated with a large orbital moment of the localized electrons. In addition, the inhomogeneous nanoisland FeNi films can possess spontaneous ordering of toroidal moments, which can be either of orbital or spin origin. The system with toroidal inhomogeneity can lead to anomalously strong diamagnetic-like response. The observed magnetization response is determined by the interplay between the paramagnetic- and diamagnetic-like contributions.

Conductive scanning probe microscopy of the semicontinuous gold film and its SERS enhancement toward two-step photo-induced charge transfer and effect of the supportive layer

The semicontinuous gold film, enabling various electronic applications including development of surface-enhanced Raman scattering (SERS) substrate, is investigated using conductive atomic force microscopy (CAFM) and Kelvin probe force microscopy (KPFM) to reveal and investigate local electronic characteristics potentially associated with SERS generation of the film material. Although the gold film fully covers the underlying silicon surface, CAFM results reveal that local conductivity of the film is not continuous with insulating nanoislands appearing throughout the surface due to incomplete film percolation. Our analysis also suggests the two-step photo-induced charge transfer (CT) play the dominant role in the enhancement of SERS intensity with strong contribution from free electrons of the silicon support. Silicon-to-gold charge transport is illustrated by KPFM results showing that Fermi level of the gold film is slightly inhomogeneous and far below the silicon conduction band. We propose that inhomogeneity of the film workfunction affecting chemical charge transfer between gold and Raman probe molecule is associated with the SERS intensity varying across the surface. These findings provide deeper understanding of charge transfer mechanism for SERS which can help in design and development of the semicontinuous gold film-based SERS substrate and other electronic applications.

Enhanced dielectric performance of PDMS-based three-phase percolative nanocomposite films incorporating a high dielectric constant ceramic and conductive multi-walled carbon nanotubes

Enhanced dielectric constant of PDMS incorporating chain-ball structured CCTO@MWCNT nanoparticles.

Spectral focusing of broadband silver electroluminescence in nanoscopic FRET-LEDs.

Few inventions have shaped the world like the incandescent bulb. Edison used thermal radiation from ohmically heated conductors, but some noble metals also exhibit 'cold' electroluminescence in percolation films, tunnel diodes, electromigrated nanoparticle aggregates, optical antennas or scanning tunnelling microscopy. The origin of this radiation, which is spectrally broad and depends on applied bias, is controversial given the low radiative yields of electronic transitions. Nanoparticle electroluminescence is particularly intriguing because it involves localized surface-plasmon resonances with large dipole moments. Such plasmons enable very efficient non-radiative fluorescence resonance energy transfer (FRET) coupling to proximal resonant dipole transitions. Here, we demonstrate nanoscopic FRET-light-emitting diodes which exploit the opposite process, energy transfer from silver nanoparticles to exfoliated monolayers of transition-metal dichalcogenides. In diffraction-limited hotspots showing pronounced photon bunching, broadband silver electroluminescence is focused into the narrow excitonic resonance of the atomically thin overlayer. Such devices may offer alternatives to conventional nano-light-emitting diodes in on-chip optical interconnects.

Role of Sputter Deposition Rate in Tailoring Nanogranular Gold Structures on Polymer Surfaces.

The reproducible low-cost fabrication of functional polymer-metal interfaces via self-assembly is of crucial importance in organic electronics and organic photovoltaics. In particular, submonolayer and nanogranular systems expose highly interesting electrical, plasmonic, and catalytic properties. The exploitation of their great potential requires tailoring of the structure on the nanometer scale and below. To obtain full control over the complex nanostructural evolution at the polymer-metal interface, we monitor the evolution of the metallic layer morphology with in situ time-resolved grazing-incidence small-angle X-ray scattering during sputter deposition. We identify the impact of different deposition rates on the growth regimes: the deposition rate affects primarily the nucleation process and the adsorption-mediated growth, whereas rather small effects on diffusion-mediated growth processes are observed. Only at higher rates are initial particle densities higher due to an increasing influence of random nucleation, and an earlier onset of thin film percolation occurs. The obtained results are discussed to identify optimized morphological parameters of the gold cluster ensemble relevant for various applications as a function of the effective layer thickness and deposition rate. Our study opens up new opportunities to improve the fabrication of tailored metal-polymer nanostructures for plasmonic-enhanced applications such as organic photovoltaics and sensors.

Enhancing the Sensitivity of Percolative Graphene Films for Flexible and Transparent Pressure Sensor Arrays

Flexible and transparent pressure sensor arrays can find applications in many places such as touch panels, artificial skin, or human motion detection. However, conventional strain gauges are rigid and opaque and are not suitable for such applications. Graphene‐based percolative strain gauges can overcome these challenges but currently are still in the infancy of their development. In this work, the performance of graphene‐based percolative strain gauges is investigated and guidelines to improve the durability and sensitivity of graphene films as sensing elements are developed. It is found that the gauge factor depends on the initial resistance of the graphene film. For the same film resistance, it is found that graphene flake size and film morphology also play a role in determining the gauge factor. Increasing the flake–flake resistance through assembly of surfactant molecules between graphene flakes provides an additional route to enhance the gauge factor. Furthermore, encapsulating the percolative film in micrometer‐thin Poly(methyl methacrylate) does not disrupt the sensing process but significantly improves the sensor's durability. Finally, thus enhanced graphene strain gauges are integrated into flexible and transparent pressure sensor arrays that exhibit high reproducibility and sensitivity.

Out-of-Plane and In-Plane Magnetization Behavior of Dipolar Interacting FeNi Nanoislands around the Percolation Threshold

Magnetic properties of inhomogeneous nanoisland FeNi films were studied by SQUID magnetometry. The FeNi films with nominal thickness ranging from 0.6 to 2.0 nm were deposited by rf sputtering on Sitall glass substrates and covered by a protecting Al2O3 layer on the top. The SQUID data indicate pronounced irreversibility behavior for the out-of-plane temperature-dependent magnetization response (measured at H≃100 Oe) using zero-field cooling (ZFC) and field-cooled warming (FCW) after the applied dc magnetizing field Hm≃2 T for the FeNi samples with nominal thickness 1.1 nm ≲d≲1.8 nm, below the percolation threshold. The positive difference between the FCW and ZFC data identifies two irreversibility temperature scales, TB≈50 K and T⁎≈200 K, which can be associated with the superparamagnetic and superferromagnetic behavior in inhomogeneous nanoisland FeNi films, respectively. However, above the film percolation threshold, we observed a crossover from the out-of-plane to in-plane magnetization orientation. Here, the in-plane FCW-ZFC difference implies negative remanent magnetization response in the temperature range TB≲T≲T⁎. The observed magnetization properties can be associated with the presence of the superferromagnetic phase in self-assembled clusters of quasi-2D metallic magnetic FeNi nanoislands.