Flexible Polydimethylsiloxane Film Research Articles

Removable PDMS Films with Ultra‐Low AgNPs Content and Effective SERS Effect by N‐Octylamine Transfer and Immersion Deposition

AbstractThe short alkyl chain of N‐octylamine was demonstrated to be able to transfer silver nanoparticles (AgNPs) from the aqueous to toluene phase with a narrow average size lower than 10 nm via ligand exchange between N‐octylamine and the sodium citrate used as a reducing and stabilizing agent during AgNPs synthesis. Surface plasmon analysis by UV‐visible spectroscopy showed that the N‐octylamine concentration significantly influences the concentration and stability of AgNPs transferred to the toluene phase and the time required for such transfer. Then, the transferred silver nanoparticles were deposited on a polydimethylsiloxane (PDMS) substrate by drop‐casting (PDMS‐T‐50‐D) or immersion (PDMS‐T‐50‐I). XPS analysis demonstrated that the AgNPs are deposited at lower than 0.02 % atomic percentages through both deposition methods. The Raman signal and the noise by fluorescence signal were shown to be significantly influenced by the AgNPs deposition method on PDMS film. The trace content of AgNPs on the flexible PDMS films on both PDMS‐T‐50‐D and PDMS‐T‐50‐I substrates can eliminate the fluorescence signal of the analyte; however, only the PDMS‐T‐50‐I substrate demonstrated a remarkable SERS‐enhanced signal. These results could significantly contribute to the design of new flexible and removable SERS substrates with ultra‐low contents of noble metal NPs.

Unravelling the impact of carbon allotropes in flexible polydimethylsiloxane film towards self-powered triboelectric humidity sensor

The influence of surface charge on tribo-layer is a crucial parameter in improving the performance of triboelectric nanogenerators (TENG). Nevertheless, most researchers focus on the structure and design of TENG rather than increasing the surface charge density, which is the main parameter for energy harnessing in nanogenerators. Herein, we tweaked the surface charge density of polydimethylsiloxane (PDMS) via incorporating different types of carbon allotropes to shift the charge density (positive to negative and vice versa) for high output TENG. The PDMS/GO TENG device generated a voltage of 234 V; higher compared to that of bare PDMS and other nanocomposite films, which is attributed to the high surface charge of PDMS/GO confirmed via Kelvin probe force microscopic (KPFM) analysis. As a proof concept, we have demonstrated the use of PDMS/GO-based TENG device for self-powered humidity sensor.

Manipulating concentration quenching and thermal stability of Eu3+-activated NaYbF4 nanoparticles via phase transition strategy toward diversified applications

Red-emitting phosphors play a vital role in reinforcing the quality of phosphor-converted white light-emitting diodes. Unfortunately, concentration quenching and unsatisfied thermal stability are still the inevitable shortcomings of the red-emitting phosphors. Herein, we utilized phase transition to simultaneously tailor the concentration quenching and thermal stability of Eu3+-activated NaYbF4 (NaYbF4:xEu3+) nanoparticles, in order to overcome these limitations. With the increasing doping level, the resultant samples undergo a phase transition from cubic to hexagonal phase. Notably, the optimal doping content for Eu3+ in the NaYbF4 nanoparticles with cubic phase is 30 mol%. However, when the phase transition occurs, the concentration quenching takes place at a much higher doping concentration, i.e. the optimum content of Eu3+ in the structure is increased to about 80 mol%. Moreover, with elevating the Eu3+ content, the local coordination environment of Eu3+ in the NaYbF4 host lattice is changed from inversion (centrosymmetric) to non-inversion symmetry (non-centrosymmetric), which is further confirmed by the theoretical calculation results via Judd-Oflet theory. Furthermore, compared with cubic samples, the hexagonal NaYbF4:0.8Eu3+ nanoparticles show improved thermal stability. Importantly, utilizing hexagonal NaYbF4:0.8Eu3+ nanoparticles, a designed white light-emitting diode emits warm white light, with desired color coordinates (0.398,0.374), high color rendering index (86.3), and low correlated color temperature (3537 K). Besides, NaYbF4:xEu3+ nanoparticles and their flexible polydimethylsiloxane films exhibit potential applications in high-temperature optical anti-counterfeiting. This work may support a new perspective for tailoring the concentration quenching and thermal stability of various phosphors.

Engineered Microstructure Derived Hierarchical Deformation of Flexible Pressure Sensor Induces a Supersensitive Piezoresistive Property in Broad Pressure Range.

Fabricating flexible pressure sensors with high sensitivity in a broad pressure range is still a challenge. Herein, a flexible pressure sensor with engineered microstructures on polydimethylsiloxane (PDMS) film is designed. The high performance of the sensor derives from its unique pyramid‐wall‐grid microstructure (PWGM). A square array of dome‐topped pyramids and crossed strengthening walls on the film forms a multiheight hierarchical microstructure. Two pieces of PWGM flexible PDMS film, stacked face‐to‐face, form a piezoresistive sensor endowed with ultrahigh sensitivity across a very broad pressure range. The sensitivity of the device is as high as 383 665.9 and 269 662.9 kPa−1 in the pressure ranges 0–1.6 and 1.6–6 kPa, respectively. In the higher pressure range of 6.1–11 kPa, the sensitivity is 48 689.1 kPa−1, and even in the very high pressure range of 11–56 kPa, it stays at 1266.8 kPa−1. The pressure sensor possesses excellent bending and torsional strain detection properties, is mechanically durable, and has potential applications in wearable biosensing for healthcare. In addition, 2 × 2 and 4 × 4 sensor arrays are prepared and characterized, suggesting the possibility of manufacturing a flexible tactile sensor.

Near-Field Electrospinning for Three-Dimensional Stacked Nanoarchitectures with High Aspect Ratios.

Near-field electrospinning (NFES) was developed to overcome the intrinsic instability of traditional electrospinning processes and to facilitate the controllable deposition of nanofibers under a reduced electric field. This technique offers a straightforward and versatile method for the precision patterning of two-dimensional (2D) nanofibers. However, three-dimensional (3D) stacked structures built by NFES have been limited to either micron-scale sizes or special shapes. Herein, we report on a direct-write 3D NFES technique to construct self-aligned, template-free, 3D stacked nanoarchitectures by simply adding salt to the polymer solution. Numerical simulations suggested that the electric field could be tuned to achieve self-aligned nanofibers by adjusting the conductivity of the polymer solution. This was confirmed experimentally by using poly(ethylene oxide) (PEO) solutions containing 0.1-1.0 wt% NaCl. Using 0.1 wt% NaCl, nanowalls with a maximum of 80 layers could be built with a width of 92 ± 3 nm, height of 6.6 ± 0.1 μm, and aspect ratio (height/width) of 72. We demonstrate the 3D printing of nanoskyscrapers with various designs, such as curved "nanowall arrays", nano "jungle gyms," and "nanobridges". Further, we present an application of the 3D stacked nanofiber arrays by preparing transparent and flexible polydimethylsiloxane films embedded with Ag-sputtered nanowalls as 3D nanoelectrodes. The conductivity of the nanoelectrodes can be precisely tuned by adjusting the number of 3D printed layers, without sacrificing transmittance (98.5%). The current NFES approach provides a simple, reliable route to build 3D stacked nanoarchitectures with high-aspect ratios for potential application in smart materials, energy devices, and biomedical applications.

Simple fabrication method of flexible carbon nanotube electrodes using inkjet and transfer printing methods for dopamine detection

Modern light wearable electrical devices require flexible, conductive materials for the development of portable and/or wearable devices. In this study, a flexible carbon nanotube (CNT) electrode is prepared, and the conductive CNTs are deposited on a flexible polydimethylsiloxane (PDMS) substrate using a combination of inkjet printing and transfer printing methods. Aqueous CNT ink is printed on a commercial overhead projector film. The subsequent deposition of a thin PDMS layer and peeling the layer off resulted in the production of a flexible PDMS film with conductive CNT patterns. The thickness of the prepared electrode increased by ∼1.2 µm after every 10 prints, and the sheet resistance decreased rapidly from 14.7 MΩ/sq to 913 kΩ/sq after 20 and 50 prints, respectively. The construction of a simple foldable electrical circuit and its application to the electrochemical sensing of dopamine using flexible electrodes are demonstrated. These flexible electrodes can be easily fabricated while displaying designer functionality.

A flexible SERS-active film for studying the effect of non-metallic nanostructures on Raman enhancement.

Since the discovery of surface enhanced Raman scattering (SERS), the choice of SERS-active materials has been limited mainly to metals, especially gold and silver in the visible spectrum. Although non-metals can also be SERS-active by forming nanostructures or composite structures with SERS-active materials, the mechanism behind it is still unclear and there is no perfect technique to study it. In this work, by constructing a SERS structure on a flexible polydimethylsiloxane film, we provide a way to study the effect of non-metallic nanostructures on Raman enhancement by attaching the above film onto flat and nanostructured surfaces. It was found that a nanoporous silicon surface contributes to an additional, up to five times, Raman enhancement. The pore depth and pore size also influence the observed Raman enhancement. These findings will help us not only to understand the mechanism of SERS involving non-metallic nanostructures, but also to design more efficient SERS structures for various applications.

A Multiple Controllable Sol–Gel Preparation of 2D Silica Lamellar Composites Based on Azoimidazolium Surfactants

As an important method in the fabrication of nanoscale materials, the sol–gel reaction needs to be improved with respect to its controllability, convenience of use, and composition extensibility. This improvement is primarily based on the photoresponsiveness of the azo bond and the liquid crystal analog, which both originated from the azoimidazolium head. Using a series of azoimidazolium surfactants, an enhanced sol–gel strategy is developed. The formation manipulation of samples includes UV light exposure on precursor solutions, spin coating, drop casting and brushing. The substrates selected include: glass, 3D printed hard plastic parts, rubber, a flexible polydimethylsiloxane film, and the regenerated silk fibroin film. Characterization of X‐ray diffraction, electronic microscopes, differential scanning calorimetry, and atomic force microscope is performed on the samples. The authors find that the orderliness and its anisotropy of 2D silica lamellar composites are adjusted with photo, thermal, and mechanical manipulations. A temperature related transition and recovery of composite structures are discovered. Further utilizations of this strategy include the fabrication of Au nanoparticles wrapped inside a silica thin layer using anion exchange chemistry and the convenient drop casting technique facilitated by the extra ordering tendency of multiple interactions on the head.

Highly transparent flexible polydimethylsiloxane films – a promising candidate for optoelectronic devices

AbstractIn the present work, a siloxane based flexible substrate for optoelectronic applications with improved flexibility and high temperature withstanding capacity has been fabricated and the details are reported. The structural and optical properties of the substrate were characterized and the optical transparency is found to be about 90% − 95% in the visible region. The substrate withstands temperatures up to 300 °C without significant degradation and has a low thermal coefficient of expansion. Fourier transform infrared spectral data show the materials in the prepared substrate and SEM studies show the surface and cross‐sectional characteristics of the substrate. It has good mechanical strength, can withstand a high breaking load of 7 N, has a average tensile strength of 48.86 MPa and high average tensile modulus of 127.32 and also possesses excellent surface smoothness and resistance to both chemical and moisture with better rigidity; it will be a long lasting alternative for flexible plastic substrates. The optical characteristics such as refractive index (n) and extinction coefficient (k) are also reported. © 2016 Society of Chemical Industry

Ultrasensitive self-powered pressure sensing system

Portable and flexible pressure sensors with highly sensitive and small size have great potential applications in areas such as wearable electronics, environmental monitoring, and medical equipment. Here, we demonstrate an integrated self-powered pressure sensing system made of a passive resistive pressure sensor and a triboelectric nanogenerator. Based on wrinkled and flexible polydimethylsiloxane films, the whole device is of sandwich structure with ultrahigh sensitivity to pressure (204.4kPa−1), which is more than one order of magnitude higher than all previously reported flexible pressure sensors. And our system exhibits a very low detection limit, rapid response time, and long-term stability. In addition, we built a self-powered, portable visualization system for semi-quantitative analysis of pressure, which can directly convert a pressure information to visual display.

Enhancement of power conversion efficiency in GaAs solar cells with dual-layer quantum dots using flexible PDMS film

This study demonstrates the high performance of GaAs solar cells with dual-layer CdS quantum dots (QDs) carried by flexible polydimethylsiloxane (PDMS) film. Several parameters were enhanced, including the short circuit current, fill factor, and power conversion efficiency, which were measured under white light illumination similar to the solar spectrum. The flexible PDMS film was fabricated using the spin-coating technique, and was then used to embed dual-layer CdS QDs. Different concentrations of QDs were applied in this design to test the optimal combination. The proposed scheme enhances power conversion efficiency by 22%, compared to a GaAs solar cell without CdS QDs. The photon down-conversion capability in the ultraviolet (UV) range and additional anti-reflection capability in the longer visible-IR range were verified through the external quantum efficiency.