Multiple Bending Cycles Research Articles

Scalable fabrication of flexible 3D PEDOT/rGO Scaffold for high-performance supercapacitors via a self-assembly method

Conducting polymers hold great prospects in energy storage, especially in the field of flexible energy storage, however, their broad applications are limited due to the poor processability, difficulty in structural design diversity and unsatisfactory performance when used alone. Herein, the processability of PEDOT in organic phase is significantly enhanced by surfactant modification via a self-assembly process, contributing to the convenient and low-cost construction of 3D PEDOT architecture. Importantly, the PEDOT and rGO can be introduced into the same porous structure synchronously and conveniently. Superior capacitive performance is shown for the porous PEDOT/rGO film benefits from the combination of PEDOT with excellent electrochemical stability, rGO with high conductivity and porous structure with abundant active sites and chambers for electrolyte storage. The porous PEDOT/rGO film can be used as a scaffold for the deposition of conductive polymers. For example, once PPy is deposited, a high specific capacitance of 407.8 F g-1 at a current density of 0.5 A g-1, accompanied by a prominent capacitance retention of 90% after 5000 cycles are achieved. When it is used to construct the symmetric supercapacitor (SSC), a high energy density of 52.9 Wh kg-1 at the power density of 200.2 W kg-1 is delivered, with a high capacitance retention of 85%. Moreover, it shows excellent mechanical flexibility upon multiple bending cycles, rendering great potential in portable or wearable energy storage devices.

Back Interface and Absorber Bulk Deep-Level Trap Optimization Enables Highly Efficient Flexible Antimony Triselenide Solar Cell.

The unique 1D crystal structure of Antimony Triselenide (Sb2Se3) offers notable potential for use in flexible, lightweight devices due to its excellent bending characteristics. However, fabricating high-efficiency flexible Sb2Se3 solar cells is challenging, primarily due to the suboptimal contact interface between the embedded Sb2Se3 layer and the molybdenum back-contact, compounded by complex intrinsic defects. This study introduces a novel Molybdenum Trioxide (MoO3) interlayer to address the back contact interface issues in flexible Sb2Se3 devices. Further investigations indicate that incorporating a MoO3 interlayer not only enhances the crystalline quality but also promotes a favorable [hk1] growth orientation in the Sb2Se3 absorber layer. It also reduces the barrier height at the back contact interface and effectively passivates harmful defects. As a result, the flexible Sb2Se3 solar cell, featuring a Mo-foil/Mo/MoO3/Sb2Se3/CdS/ITO/Ag substrate structure, demonstrates exceptional flexibility and durability, enduring large bending radii and multiple bending cycles while achieving an impressive efficiency of 8.23%. This research offers a straightforward approach to enhancing the performance of flexible Sb2Se3 devices, thereby expanding their application scope in the field of photovoltaics.

Carbon felt from acrylic dust bags as flexible EMI shielding layer and resistive heater

Carbon felt is widely used due to its lightweight and conductive nature. However, limited research has been conducted regarding the tension applied during its preparation. This study aims to explore the influence of different loading tensions during the carbonization process on the properties of acrylic-based carbon felts. Enhanced flexibility, lower shrinkage and improved mechanical and electrical properties were found for the samples obtained in the edge loading mode. Subsequently, the performance of carbon felts in electromagnetic interference (EMI) shielding and resistive heating was studied. The carbon felts achieved a notable EMI shielding effectiveness of 55 dB. The specific shielding effectiveness values are significantly higher than other reported materials due to their low density and high porosity. Moreover, the carbon felts displayed commendable electrical heating performance through high heating rates and superior heating efficiency. Lastly, the structural stability was assessed via self-designed bending experiments. Good stability of the resistance and heating behavior of the carbon felts across multiple bending cycles was observed. With low manufacturing costs, good stability and air permeability, the flexible carbon felt is expected to be widely used for barrier applications and wearable electronics.

Investigating the effect of laser modulation and input energy on bending mechanism and bending angle in the multi-pass laser forming process through real-time monitoring

Laser forming is a non-contact type process used for bending, spatial forming, and alignment of metallic components through a controlled application of laser energy. The process involves localized rapid heating and cooling, inducing thermal strains exceeding the elastic strain of the material resulting in plastic deformation. The process is broadly governed by the temperature gradient mechanism (TGM), buckling mechanism (BM), and upsetting mechanism (UM). However, in the case of multi-pass laser forming process, heat accumulation and variation in the flow stress because of consecutive thermal cycles and strain hardening due to multiple bending cycles makes the process highly complicated. While operating laser in modulated mode may be a wise choice to handle the heat accumulation, inter-pulse cooling cycles along with variation in peak and average power with change in duty cycle affects the bending mechanism and angle significantly. Therefore, in the present study, a real-time monitoring system was developed to investigate the bending mechanism for a multi-pass laser forming process carried out in laser modulated mode. Also, the effect of average power, peak power, heat accumulation, strain hardening on bending mechanism and bending angle were investigated. Keeping the peak power constant, variation in duty cycle from 100% to 40% resulted in reduced average energy and strain hardening was found to be dominant, decreasing bending angle per pass. On contrast, variation in duty cycle by keeping the average power constant resulted in heat accumulation dominating the bending mechanism.

Exfoliated Se nanoparticles decorated on MoS2/paper-based heterojunction as a flexible, self-powered and highly responsive photodetector

Self-powered heterostructure based photodetectors are of great research interest owing to their zero-power consumption, enhanced light-matter interaction, and the ability to respond in harsh environmental conditions. Selenium (Se) nanoparticles have been recently explored in photodetectors due to their high absorption coefficient and superior light-sensing ability. However, most reported Selenium based photoconductors utilize sophisticated device fabrication methodology, requiring regulated environmental conditions, elevated temperature and face flexibility issues. This work demonstrates a discrete(localized) heterojunction of exfoliated Selenium decorated on hydrothermally grown MoS2/paper-based high-performance, flexible, and self-powered photodetector. X-ray Diffraction and Raman Spectroscopy studies reveal the presence of hexagonal Se and the SEM image shows the uniformly grown MoS2 on cellulose paper. It also confirms the presence of Se clustered nanoparticles. The hybrid heterostructure device displays broadband response in the Vis-NIR region. Upon light illumination, a built-in field is generated due to the efficient separation of the charge carriers, making the fabricated device to operate at zero bias. The as-fabricated device exhibits superior responsivity of 0.14 A/W and 0.49 A/W and detectivity of 4.55 × 1011 Jones (intensity- 35.8 mW/cm2) and 5.01 × 1010 Jones (intensity- 30.08 mW/cm2) upon Vis and NIR irradiation, respectively, at no bias. This superior performance is asserted to the peak absorption of Se nanoparticles in the NIR region. The device also showed superior resistance to bending stress and retained its original characteristics even after multiple bending cycles (∼1050). Hence, with this facile device fabrication and simple device architecture strategy, this work paves the way for developing high-performance tensile photodetectors.

A flexible self-powered UV–Vis photodetector based on Successive Ionic Layer Adsorption and Reaction (SILAR) deposited CdS on asymmetric contacts

Self-powered photodetectors exhibiting broadband spectral responses have drawn significant attention owing to their zero-power consumption, high sensitivity, and enhanced light-matter interaction. However, considering the existing complex fabrication processes for self-powered devices, facile fabrication schemes that can be performed at room temperature and are scalable are highly desirable. This work demonstrates a metal-semiconductor-metal (MSM) based flexible, self-powered photodetector employing Successive Ionic Layer Adsorption and Reaction (SILAR) deposition of CdS directly on a paper substrate with interdigitated Asymmetric contacts, namely of pencil-drawn graphite and silver paste. Upon light illumination, a built-in field is generated due to the asymmetry in the fermi levels of the metal contacts, supporting efficient carrier separation. The as-fabricated device extends its absorbance from Ultraviolet (UV) to Visible (Vis) spectra, with the highest peak absorption around 520 nm. Further, the device exhibits a responsivity of 6.56 mA/W and 1500 mA/W and detectivity of 2.3 × 108 Jones and 5.66 × 1010 Jones when illuminated with Visible (intensity-0.0125 mW/cm2) and UV (intensity-2.50 mW/cm2) lights respectively. Without any external power, the device showed response times of 0.11 s and 1.42 s when subjected to UV and Vis irradiations. This detector also exhibited superior resistance to bending stress and retained its performance even after multiple bending cycles (∼1000). Hence, the facile, low-cost, and room-temperature SILAR deposition technique-assisted photodetector fabrication paves the way for developing other high-performance and flexible photodetectors for various optoelectronic applications.

Introduction of defects in hexagonal boron nitride for vacancy-based 2D memristors.

Two-dimensional (2D) materials have become potential resistive switching (RS) layers to prepare emerging non-volatile memristors. The atomically thin thickness and the highly controllable defect density contribute to the construction of ultimately scaled memory cells with stable switching behaviors. Although the conductive bridge random-access memory based on 2D hexagonal boron nitride has been widely studied, the realization of RS completely relying on vacancies in 2D materials has performance superiority. Here, we synthesize carbon-doped h-BN (C-h-BN) with a certain number of defects by controlling the weight percentage of carbon powder in the source. These defects can form a vacancy-based conductive filament under an applied electric field. The memristor displays bipolar non-volatile memory with a low SET voltage of 0.85 V and shows a long retention time of up to 104 s at 120 °C. The response times of the SET and RESET process are less than 80 ns and 240 ns, respectively. The current mapping by conductive atomic force microscopy demonstrates the electric-field-induced current tunneling from defective sites of the C-h-BN flake, revealing the defect-based RS in the C-h-BN memristor. Moreover, C-h-BN with excellent flexibility can be applied to wearable devices, maintaining stable RS performance in a variety of bending environments and after multiple bending cycles. The vacancy-based 2D memristor provides a new strategy for developing ultra-scaled memory units with high controllability.

Flexible Nanoarchitectonics for Biosensing and Physiological Monitoring Applications.

Flexible and implantable electronics hold tremendous promises for advanced healthcare applications, especially for physiological neural recording and modulations. Key requirements in neural interfaces include miniature dimensions for spatial physiological mapping and low impedance for recognizing small biopotential signals. Herein, a bottom-up mesoporous formation technique and a top-down microlithography process are integrated to create flexible and low-impedance mesoporous gold (Au) electrodes for biosensing and bioimplant applications. The mesoporous architectures developed on a thin and soft polymeric substrate provide excellent mechanical flexibility and stable electrical characteristics capable of sustaining multiple bending cycles. The large surface areas formed within the mesoporous network allow for high current density transfer in standard electrolytes, highly suitable for biological sensing applications as demonstrated in glucose sensors with an excellent detection limit of 1.95µm and high sensitivity of 6.1mA cm-2 µM-1 , which is approximately six times higher than that of benchmarking flat/non-porous films. The low impedance of less than 1 kΩ at 1kHz in the as-synthesized mesoporous electrodes, along with their mechanical flexibility and durability, offer peripheral nerve recording functionalities that are successfully demonstrated in vivo. These features highlight the new possibilities ofournovel flexible nanoarchitectonics for neuronal recording and modulation applications.

High responsivity self-powered flexible broadband photodetector based on hybrid Selenium-PEDOT:PSS junction

Hybrid organic-inorganic composites offer solution processability and ease to construct opto-electronic devices with diverse functionalities, which are desirable for applications like flexible and printed electronics. Trigonal Selenium (t-Se) has been recently investigated for photodetector application, owing to its inherent superior optical properties such as high absorption coefficient and photoconductivity. However, most of the reported Selenium photodetectors use vapor transport based device fabrication techniques which require controlled environments and face scalability issues. In this work, high-performance photodetector based on Selenium-PEDOT:PSS hybrid junction is demonstrated using a facile sonication assisted mechanical mixing and drop-casting technique. XRD and Raman analysis reveals the existence of Selenium in trigonal crystal structure and FE-SEM images show clustered nanoparticles. Selenium is mixed with PEDOT:PSS to form bulk heterojunction, which enhances the electron-hole pair separation efficiency and results in improved device responsivity. Also, due to the favorable band offset between Selenium and PEDOT:PSS which results in built-in field, the photodetector operates in self-biased mode. The device exhibits broad spectral response in UV–Vis–NIR range with responsivities of 0.56 A/W, 66 mA/W and 1.363 A/W at 315 nm, 620 nm and 820 nm, respectively. Response times of 7.7 s, 9.4 s and 6.4 s and external quantum efficiencies (EQE) of 192.58%, 13.25% and 206.55% are obtained for UV, Vis and NIR spectra respectively. Further to validate the robustness, the device is subjected to multiple bending cycles (∼2500) and minimal degradation in performance is observed. Given the simple device architecture and facile device fabrication scheme, this work would enable promising approach for high performance flexible photodetectors. • Selenium, possesses high absorption co-efficient and high photoconductivity with optical bandgap of 1.2–1.5 eV. • However, t-Se has been less explored for photodetector applications. • This work reports a solution-processed hybrid-heterojunction based on Se-PEDOT:PSS for photodetection. • The detector exhibits spectral response from UV–Vis to NIR and significant performance at zero bias condition.

Critical Salt Loading in Flexible Poly(vinyl alcohol) Sensors Fabricated by an Inkjet Printing and Plasma Reduction Method.

We report a low-temperature inkjet printing and plasma treatment method using silver nitrate ink that allows the fabrication of conductive silver traces on poly(vinyl alcohol) (PVA) film with good fidelity and without degrading the polymer substrate. In doing so, we also identify a critical salt loading in the film that is necessary to prevent the polymer from reacting with the silver nitrate-based ink, which improves the resolution of the silver trace while simultaneously lowering its sheet resistance. Silver lines printed on PVA film using this method have sheet resistances of around 0.2 Ω/□ under wet/dry and stretched/unstretched conditions, while PVA films without prior treatment double in sheet resistance upon wetting or stretching the substrate. This low resistance of printed lines on salt-treated films can be preserved under multiple bending cycles of 0–90° and stretching cycles of 0–6% strain if the polymer is prestretched prior to inkjet printing.

Flame-Retardant, Highly Conductive, and Low-Temperature-Resistant Organic Gel Electrolyte for High-Performance All-Solid Supercapacitors.

Traditional liquid electrolytes are volatile, flammable, and easy to leak, which makes the energy storage device easy to burn and explode in the case of overcharge and short circuit. Here, by utilizing the active P-H bond of a flame retardant (DOPO) to graft onto the polymer chain, flame-retardant organic gel electrolytes were fabricated to address these issues. The gel electrolyte had good ionic conductivity of 4 mS cm-1 at 20 °C and good flame retardant ability. By changing the molar ratio of the monomers and the salt concentrations, the mechanical strength of the gel electrolyte could be adjusted (maximum stress≈28 KPa, maximum strain≈305 %). The transport mechanism of lithium ions in the gel polymer electrolyte was proposed. The gel electrolyte-assembled supercapacitor (SC) possessed better electrochemical properties than that of SC assembled by liquid electrolyte. Importantly, the gel-based SC remained basically unchanged under multiple bending cycles. Additionally, the gel electrolyte had good low-temperature tolerance (0.1 mS cm-1 at -40 °C). The gel electrolyte-assembled SC could work normally in the temperature range of -20 to 60 °C. The multiple advantages of gel electrolyte expand the applications in ionic conductor and energy storage devices.

Flexible and reconfigurable radio frequency electronics realized by high-throughput screen printing of vanadium dioxide switches

Smart materials that can change their properties based on an applied stimulus are in high demand due to their suitability for reconfigurable electronics, such as tunable filters or antennas. In particular, materials that undergo a metal–insulator transition (MIT), for example, vanadium dioxide (VO2) (M), are highly attractive due to their tunable electrical and optical properties at a low transition temperature of 68 °C. Although deposition of this material on a limited scale has been demonstrated through vacuum-based fabrication methods, its scalable application for large-area and high-volume processes is still challenging. Screen printing can be a viable option because of its high-throughput fabrication process on flexible substrates. In this work, we synthesize high-purity VO2 (M) microparticles and develop a screen-printable VO2 ink, enabling the large-area and high-resolution printing of VO2 switches on various substrates. The electrical properties of screen-printed VO2 switches at the microscale are thoroughly investigated under both thermal and electrical stimuli, and the switches exhibit a low ON resistance of 1.8 ohms and an ON/OFF ratio of more than 300. The electrical performance of the printed switches does not degrade even after multiple bending cycles and for bending radii as small as 1 mm. As a proof of concept, a fully printed and mechanically flexible band-pass filter is demonstrated that utilizes these printed switches as reconfigurable elements. Based on the ON and OFF conditions of the VO2 switches, the filter can reconfigure its operating frequency from 3.95 to 3.77 GHz without any degradation in performance during bending.

Highly flexible and conductive nanoporous Ag as good substrate for flexible hybrid supercapacitors

Flexible electrode materials with high volumetric capacitance are promising to applications in portable and wearable capacitive storage devices. Here, we utilized the highly flexible and conductive NP-Ag substrate to successfully develop a flexible nanoporous Ag@Co(OH)2 (NP–Ag@Co(OH)2) thin film electrode. After 100 multiple bending cycles, the NP-Ag@Co(OH)2 electrode can keep the original nanostructures and flexibility. The flexible NP-Ag@Co(OH)2 electrode exhibits the high volumetric capacitance of 929 F cm−3 at a current density of 2 A cm−3, which is higher than many reported supercapacitor electrodes. The flexible NP-Ag@Co(OH)2 electrode exhibits good supercapacitor performance even under the bending state. Moreover, the as-assembled NP-Ag@Co(OH)2 symmetric supercapacitor device has high energy density of 20.07 mWh cm−3 at power density of 0.85 W cm−3. The device supercapacitor retains 75% capacitance of the initial value at 2 A cm−3 after 2000 cycles. This work provides an insight into the rational design of the highly flexible, high-performance supercapacitor electrodes with the potential application in portable or wearable energy storage devices.

Self-powered all weather sensory systems powered by Rhodobacter sphaeroides protein solar cells

Natural photosynthetic proteins can convert solar energy into electrical energy with close to 100% quantum efficiency, and there is increasing interest in their use for sustainable photoelectrochemical devices. The primary processes of photosynthesis remain operational and efficient down to extremely low temperatures, and natural photosystems exhibit a variety of self-healing mechanisms. Herein we demonstrate the use of an amphiphilic triblock copolymer, Pluronic F127, to fabricate a self-healing photosynthetic protein photoelectrochemical cell that operates optimally at sub-zero temperatures. A concentration of 30% (w/w) Pluronic F127 depressed the freezing point of an electrolyte comprising 50 mM ubiquinone-0 in aqueous buffer such that optimal device solar energy conversion was seen at −12 °C rather than at room temperature. Fabrication of the protein photoelectrochemical cells with flexible electrodes enabled the demonstration of self-healing of damage caused by repeated mechanical deformation. Multiple bending cycles caused a marked deterioration of the photocurrent response to around a third of initial levels due to damage to the gel phase of the electrolyte, but this could be restored to ~95% by simply cooling and rewarming the device. This self-recoverability of the electrolyte extended the operational life of the protein cell through a process that increased its photoelectrochemical output during the repair. Utility of the cells as components of a touch sensor operational across a wide temperature range, including freezing conditions, is demonstrated.

Nanoribbon‐Based Flexible High‐Performance Transistors Fabricated at Room Temperature

AbstractSi‐nanoribbon‐based high‐performance field‐effect transistors (FETs) with room temperature (RT)‐deposited dielectric are presented. The distinct feature of these devices is that the high‐quality SiNx dielectric deposition at RT, directly on the transfer‐printed nanoribbons, is compatible with most flexible substrates. The performance of these FETs (mobility ≈656 cm2 V−1 s−1 and on/off ratio >106) is on par with the highest performance of similar devices reported with high‐temperature processes, and significantly higher than devices reported with low‐temperature processes. The transfer and output characteristics of nanoribbon‐based field‐effect transistors under planar, tensile, and compressive bending and multiple bending cycles (100) show excellent mechanical stability of the devices as they retain performance. The device characteristics are also compared with the equivalent simulation data. The excellent response of nanoribbon‐based FETs and the fabrication compatibility with diverse flexible substrates makes the presented approach attractive for flexible electronics applications such as conformal tactile active matrix sensors for e‐skin, where high performance is needed.

Highly efficient flexible perovskite solar cells and their photo-stability

We report a simple method to fabricate highly efficient and robust flexible perovskite solar cells (F-PSCs) along with their photo-stability. The solar cells were prepared on indium tin oxide (ITO) coated polyethylene terephthalate substrates. The solar cells incorporated methylammonium lead iodide chloride (CH3NH3PbI3−xClx) perovskite as the light absorber, tin oxide (SnO2) as the electron transport layer (ETL), Spiro-MeOTAD as the hole transport layer and Ag as the top electrode. This structure exhibited a very high external quantum efficiency (EQE) of ~80% at 500 nm and a power conversion efficiency (PCE) of 12.7% ± 0.6% (active area 9.6 mm2) with multiple bending cycles showing no effect on the performance, even after 100 bending cycles. To evaluate their lifetime in the dark and under actual operating conditions, a systematic study was performed on the unsealed devices. The solar cells showed a halflife (T50) of about 340 h when stored in the dark in an ambient environment, but they degraded very rapidly in direct sunlight. The main reason behind their degradation was found to be the rapid degradation in their short circuit current density (Jsc). The same device design was processed to fabricate large area flexible solar modules with an area of 5 × 5 cm2 (active area 15.84 cm2) and the solar modules exhibited PCE of 5.6% ± 0.2%. Substantial loss of PCE in the modules compared to small area devices has been attributed to the larger series resistance of the longer ITO electrodes used in the modules.

Carbon nanotube conductive additives for improved electrical and mechanical properties of flexible battery electrodes

Flexible electronics are being pursued as replacements for rigid consumer electronic products such as smartphones and tablets, as well as for wearable electronics, implantable medical devices, and RFIDs. Such devices require flexible batteries with electrodes that maintain their electro-chemical performance during multiple bending cycles. These electrodes typically consist of an active battery material blend with a conductive additive and a binder. Whilst the choice of active battery material is typically dictated by the desired battery power and energy requirements, there is more freedom in changing the conductive additives to cope with strain induced during the bending of flexible batteries. Here we compare the mechanical and electrical properties of free standing cathodes using lithium cobalt oxide (LiCoO2) as the active material and 10–20 wt% of amorphous carbon powder (CP) or carbon nanotubes (CNTs) as conductive additives. We found that the CNT based electrodes showed less crack formation during bending and have a Young's modulus up to 30 times higher than CP electrodes (10 wt% loading). Further, the electrical resistance of pristine CNT electrodes is 10 times lower than CP electrodes (20 wt% loading). This difference further increases to a 28 times lower resistance for CNT films after 2000 bending cycles. These superior properties of CNT films are reflected in the electrochemical tests, which show that after bending, only the electrodes with 20 wt% of CNTs remain operational. This study therefore highlights the importance of the conductive additives for developing reliable flexible batteries.

A Combined Experimental and Computational Study of Gas Sensing by Cu3SnS4 Nanoparticulate Film: High Selectivity, Stability, and Reversibility for Room Temperature H2S Sensing

AbstractSeveral binary sulfides are used for sensing of gases such as NH3, NO2, and H2, but not for H2S, especially at room temperature, because of the relative inactivity of metal sulfide surface bonds for this gas. The situation can be entirely different in the ternary case, however, due to the possible synergy of interactions involving dual cation surface chemistry. This is borne out by the present work wherein the Cu3SnS4 material, the Cu‐rich ternary sulfide used for the first time in the gas sensing context, not only senses H2S at room temperature but also remarkably does so with high selectivity and stability. Thus, a combined experimental and computer modeling study on the use of nanocrystalline orthorhombic Cu3SnS4 phase is reported for H2S sensing. The material shows sensitivity for a wide range of H2S concentrations (from 10 to 2000 ppm). The performance of the sensing device fabricated on Kapton substrate remains intact even after several days and multiple bending cycles. Importantly, these experimental findings are consistent with the results of density functional theory calculations for binding energies for different gases, namely, H2S, NO2, NH3, and CO, on Cu3SnS4 surface.

Flex-Mode Mechatronic Functionality of Lead Iodide Hybrid Perovskite Systems

The mechatronic functionality of lead iodide hybrid perovskite thin films grown on the flexible substrate is investigated via the study of current-perpendicular-to-plane charge transport modulation under flex-mode compressive and tensile strains (CS and TS) for multiple flexing cycles. It is shown that the transport is significantly, reversibly, and asymmetrically modulated. Typically, for a strain of 0.088% (0.23%), a remarkable current modulation of +196% (+393%) is achieved for compressive strain and −49% (−53%) for tensile strain at an applied potential of 1 V. For low levels of bending, the response is robust for a large number of bending cycles. The effects of the change of organic cation from methylammonium to formamidinium and the grain size on the response are also examined. A comparative study of the structural, morphological, and optical properties of the pristine sample and the samples subjected to multiple bending cycles is performed to understand and elucidate the possible mechanisms of the strain-induced changes in the transport properties.

Rational design and fabrication of highly transparent, flexible, and thermally stable superhydrophobic coatings from raspberry-like hollow silica nanoparticles

Multifunctional coatings with superhydrophobicity, high transparency, thermal stability, flexibility, and ultralow refractive index have been investigated for many years. They have promising applications in industries such as in electronic and optical devices, photonic materials, and templates for fabricating biological and chemical sensors. However, the relatively complex preparation technology of these coatings or difficult to possess these properties simultaneously are still the main factors that limit their wide application. In this paper, we report a facile atmospheric approach to create transparent multifunctional raspberry-like particulate coatings with a low refractive index, which were obtained via one-pot base-catalyzed sol-gel process using tetraethyl orthosilicate (TEOS) and 1H, 1H, 2H, 2H-perfluorooctyltrimethoxysilane (POTS) as co-precursors. The excellent superhydrophobicity, mechanical flexibility, self-cleaning property, thermal and chemical stability of the as-fabricated coatings were demonstrated. The refractive indices of coatings can be easily tuned at a range of 1.07–1.16. Particularly, the resulted samples on the K9 glasses exhibited superhydrophobicity with a water contact angle (WCA) of 162° when the scale ratio of the POTS and TEOS was 1.0. The superhydrophobicity of the as-prepared coatings could last for more than half a year under indoor condition, demonstrating the long stability of the superhydrophobicity. Furthermore, we demonstrated that this simple efficient method could be extended to different substrates, including K9 glass, Polyvinyl chloride (PVC), stainless steel, aluminum alloy, and gingko leaf, to achieve superhydrophobicity. Interestingly, the superhydrophobicty of the coatings transferred to superhydrophilicity (WCA < 5°) by calcination at 500 °C, which resulted in a good antifogging property. Moreover, the coatings were not sensitive to the strong acid (pH = 1) and kept their superhydrophobic state for a long time (over 80 min). The coatings on PVC retain their excellent water-repellency after multiple bending cycles. Also, these coatings produced a fair transparency consisting of more than a 2.0% increase in optical transmittance in the visible-near infrared spectral range. Our study is promising building blocks for large-area, mechanical flexible, ultralow-refractive-index, superhydrophobic and environment-resistant surfaces.