Flexible Polycarbonate Substrates Research Articles

Uniaxial Strain Tuning of Upconversion Photoluminescence in Monolayer WSe2

Strain engineering is one of the leading mechanical ways to tune the optical properties of monolayer transition metal dichalcogenides among different techniques. Here, uniaxial strain is applied on exfoliated 1L‐WSe2 flakes transferred on flexible polycarbonate substrates to study the strain tuning of upconversion photoluminescence. It is demonstrated that the peak position of upconversion photoluminescence is redshifted by around 20 nm as the applied uniaxial strain increases from 0% to 1.17%, while the intensity of upconversion photoluminescence increases exponentially for the upconversion energy difference ranging from −155 to −32 meV. The linear and sublinear power dependence of upconversion photoluminescence is observed for different excitation wavelengths with and without uniaxial strain, suggesting the multiphonon‐assisted mechanism in one‐photon regime for the upconversion process. These results offer the potential to advance 2D material‐based optical upconversion applications in night vision, strain‐tunable infrared detection, and flexible optoelectronics.

Emergent magneto-inductance effect in permalloy thin films on flexible polycarbonate substrates at room temperature

Emergent inductance has attracted significant interest for its relevance in both interesting fundamental physics and practical applications in magnetic devices that demand miniaturization without compromising inductance. In this Letter, we report the discovery of a stepwise magnetic field-induced emergent magneto-inductance (EML) effect in Permalloy (Py) thin films deposited on polycarbonate (PC) substrates. Remarkably, Py/PC devices exhibit an exceptionally large inductance variation exceeding 1 μH at room temperature, and intriguingly, a sign reversal of inductance occurs around the zero magnetic field. The dependencies of the EML effect on frequency, step magnetic field changes, and film width can be explained from the theory based on the spin motive force driven by transient domain wall motion. This study opens up exciting avenues for advancing our understanding of emergent inductance in fundamental physics and paves the way for practical applications in flexible magnetic devices.

Scalable and low-cost fabrication of flexible WS2 photodetectors on polycarbonate

We present a low-cost and easy-to-implement technique to fabricate large-area WS2 photodetector devices onto transparent and flexible polycarbonate substrates. The method relies on the deposition of large-area (in the cm scale) thin films (~30 nm thick) of WS2 by a recently introduced abrasion-induced method. Interdigitated electrical contacts are then deposited by thermal evaporation through a shadow mask. The photodetectors present well-balanced performances with an good trade-off between responsivity (up to 144 mA/W at a source-drain voltage of 10 V and illumination power of 1 μW) and response time (down to ~70 µs) and a detectivity value of 108 Jones. We found that the devices perform very reversibly upon several illumination and straining cycles and we found a moderate device-to-device variation.

Simultaneously improved conductivity and adhesion of flexible AgNW networks via a simple hot lamination process

Silver nanowires (AgNWs) are remarkable components that may be able to replace indium tin oxide (ITO) as transparent electrodes. However, a common coating method can cause poor conductivity and adhesively because of the creation of randomly organized connections of Ag NWs. These problems should be overcome in AgNW networks for transparent electrodes through material and engineering approaches. In this work, novel AgNW based transparent conductive electrodes (TCEs) on a flexible polycarbonate (PC) substrate, with synchronously improved adhesivity and reduced resistivity, are prepared via hot lamination process. The sheet resistance of the treated AgNW networks decreases from 100 to 12.5 Ω/sq. at optical transmittance of 94.5%. Calculation of figure of merit (FOM) displays a high value of 187, which is superior to that obtained from the flexible ITO, carbon nanotubes, graphene, and other metal nanowires. Meanwhile, the sheet resistance is almost unchanged after 30 tape tests, suggesting an outstanding adhesion of AgNW networks to the flexible substrate. In addition, the treated AgNW networks show small bending radius of 3.0 mm, indicating good flexibility. These results illustrate a promising alternative to traditional transparent conductive films for wearable electronic devices.

Room temperature growth of half-metallic Fe3O4 thin films on polycarbonate by reactive sputtering: Heterostructures for flexible spintronics

Nowadays, there is a huge demand of bendable/stretchable electronic devices because of their remarkable capabilities. With such an aim and inclination towards flexible spintronics devices, half-metallic nanocrystalline Fe3O4 thin films with various thicknesses are grown on flexible polycarbonate (PC) substrates by reactive sputtering at room temperature. The structural analysis reveals that as the deposition time increases from ∼165 s to ∼1335 s, the thickness of the film also increases from 50 nm to 400 nm with the size of the grain from ∼12 nm to ∼32 nm, respectively. The X-ray photoelectron spectroscopy (XPS) shows that the films grown on PC substrates are the pure form of Fe3O4 nanostructures. The electrical and magnetic characteristics are studied to illustrate the furtherance of the original belongings of Fe3O4. The Verwey transition temperature (Tv) of ∼127 K, Ms value of ∼317 emu/cm3 and MR -8.3% under H ∥ Film plane below 60 kOe at 300 K for 200 nm thick Fe3O4 film on PC substrate were observed. Additionally, several bending experiments are executed to confirm the retention and adjustability of these properties for such heterostructures. These remarkable results show that Fe3O4/PC heterostructures can be a promising applicant for flexible spintronics.

Photocatalytic, superhydrophilic, self-cleaning TiO2 coating on cheap, light-weight, flexible polycarbonate substrates

Photocatalytic TiO2/SiO2 coatings with excellent superhydrophilic wettability were prepared on light-weight polycarbonate substrates for self-cleaning applications. The effect of distinct SiO2 concentrations (0–40%) in TiO2 on the morphology, wettability, UV–Vis transmittance, haze, and durability of the coatings was studied in detail. TiO2/SiO2 coatings prepared with 20% silica in TiO2 showed superhydrophilic wetting properties with a smooth and uniform morphology with more than 85% transmittance. The lower haze value of TiO2/SiO2 coatings confirms their clear optical appearance. A low-friction layer of fluoroalkylsilane was applied to the coatings to improve their mechanical durability. In addition, the hydrophobic-hydrophilic patterns of different areas were prepared to check their effect on the haze and wetting properties.

Efficiency enhancement of flexible OLEDs by using nano-corrugated substrates and conformal Ag transparent anodes

In flexible OLEDs (FOLEDs), the traditional ITO anode has disadvantages such as refractive-index mismatches among substrate and other functional layers, leads to light loss of nearly 80%, meanwhile, its brittle nature and lack in raw materials hinder its further applications. We investigated an efficient FOLED using a semi-transparent silver (Ag) anode, whereas the device was built on a nano-corrugated flexible polycarbonate (PC) substrate prepared by thermal nanoimprint lithography. The corrugations were well preserved on each layer of the device, both the micro-cavity effect and surface plasmon polariton (SPP) modes of light loss were effectively suppressed. As a result, the current efficiency of the FOLED using a conformal corrugated Ag anode enhanced by 100% compared with a planar Ag anode device, and enhanced by 13% with conventional ITO device. In addition, owing to the quasi-periodical arrangements of the corrugations, the device achieved broad spectra and Lambertian angular emission. The Ag anode significantly improved the bending properties of the OLED as compared to the conventional ITO device, leading to a longer lifetime in practical use. The proposed manufacturing strategy will be useful for fabricating nano corrugations on plastic substrate of FOLED in a cost-effective and convenient manner.

Cu2+‐doped ITO as a Novel Efficient, Transparent, and Fast‐Response Transducer for Ammonia Sensing

As a novel approach, the effect of Cu2+‐doped indium tin oxide (ITO) on a flexible polycarbonate substrate is considered as an ammonia sensor. The sensor was fabricated using spin‐coating and subsequent annealing at 160°C for 60 min. The constructed sensor morphology accomplished by surface composition was explored using scanning electron microscopy (SEM) and energy dispersiveX‐ray (EDX) spectroscopy. Using the new strategy, a flexible sensor for ammonia determination with a fast response time of less than 7 s and a recovery time of 8 s was achieved. Sensor characteristics, such as sensitivity, recyclability, response/recovery time, selectivity, stability, flexibility, and transmittance of the layers, were examined. The impedance results showed high sensitivity when the constructed sensor was exposed to NH3 concentrations in the range 5–1000 ppm. The results showed that doping ITO with Cu2+ imparted higher electronic charge density to the sensor surface and enhanced the sensitivity of the sensor by a factor of 352% in comparison with that of pure ITO. The sensitivity, fast response, and recovery time with low‐cost materials and deposition procedure suggest an effective and disposable ammonia sensor at room temperature (23°C).

Improved performance of transparent-conducting AZO/Cu/AZO multilayer thin films by inserting a metal Ti layer for flexible electronics.

In order to improve the performance of transparent conductive Al-doped ZnO (AZO)/Cu/Al-doped ZnO multilayer thin films, multilayer thin films of titanium-embedded AZO/Cu/AZO structures have been designed and deposited onto flexible polycarbonate substrates to develop an indium-free transparent flexible electrode. The effect of Ti layer on the structural, optical, and electrical properties of multilayer thin films was investigated. The experiments reveal that the AZO/Ti/Cu/AZO fabricated in argon-oxygen mixtures has the best performance (commutation quality factor of 422.0): resistivity of 3.85×10-5 Ω·cm, sheet resistance of 4.31 Ω/sq, carrier concentration of 1.65×1022 cm-3, mobility of 9.87 cm2/Vs, and acceptable luminous transmittance of above 82% in the visible range.

High quality transparent conductive Ag-based barium stannate multilayer flexible thin films

Transparent conductive multilayer thin films of silver (Ag)-embedded barium stannate (BaSnO3) structures have been deposited onto flexible polycarbonate substrates by magnetron sputtering at room temperature to develop an indium free transparent flexible electrode. The effect of thicknesses of Ag mid-layer and barium stannate layers on optical and electrical properties were investigated, and the mechanisms of conduction and transmittance were discussed. The highest value of figure of merit is 25.5 × 10−3 Ω−1 for the BaSnO3/Ag/BaSnO3 multilayer flexible thin films with 9 nm thick silver mid-layer and 50 nm thick barium stannate layers, while the average optical transmittance in the visible range from 380 to 780 nm is above 87%, the resistivity is 9.66 × 10−5 Ω · cm, and the sheet resistance is 9.89 Ω/sq. The change rate of resistivity is under 10% after repeated bending of the multilayer flexible thin films. These results indicate that Ag-based barium stannate multilayer flexible thin films can be used as transparent flexible electrodes in various flexible optoelectronic devices.

Scatterometry for nanoimprint lithography

Angular scatterometry is used to characterize the nanostructure parameters of two samples: a high dielectric contrast ∼100-nm period Al wire-grid polarizer (WGP), and a low dielectric contrast ∼130-nm period photoresist grating on a flexible polycarbonate substrate; both fabricated by nanoimprint lithography. The zero-order diffraction (reflection) is monitored for a large incident angle range from 8° to 80°. For the WGP, four wavelengths (244-, 405-, 633-, and 982-nm) are used to study the dependence of the scatterometry parametric determination as a function of the sample pitch to wavelength ratio (p/λ: 0.41–0.1). A 4-nm thick native Al2O3 layer was added to the scatterometry simulation and dramatically improved the cross-correlations between results at the different wavelengths. For the photoresist samples, the scatterometry results at 405 nm are compared with atomic force microscopy measurements and the master grating structure. The scatterometry results are sensitive to inhomogeneity of the sample and show a capability for classifying different types of macroscopic defects.

Organic/Inorganic Hybrid Materials for Organic Electronic Application

Organic/inorganic stacks were deposited on flexible polycarbonate substrate using inductively coupled plasma chemical vapor deposition (ICP-PECVD) for permeation barrier application. The effects of deposition temperature, RF power, gas flow ratio, deposition pressure on film properties of surface roughness, water vapor transmission rate (WVRT) were investigated. Energy dispersive spectrum (EDS), atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to characterize the film characteristics of the stack layers. It was found that the surface roughness Ra was as low as is 0.25 nm. The WVRT values of the optimum barriers structures were 10-2g/m2 day (1 pair of stacks) and 4.8 x 10-5g/m2 day (4 pair of stacks). This result indicated that the permeation barrier films prepared by ICP-PECVD could be a promising candidate for flexible electronic applications.

Patterned alignment of nematic liquid crystals generated by inkjet printing of gold nanoparticles and emissive carbon dots on both flexible polymer and rigid glass substrates

ABSTRACTPatterned homeotropic alignment using nanoparticles (NPs) was achieved using inkjet printing. Two types of gold NPs, one smaller and one larger in core diameter (2 and 5 nm) capped with a monolayer of dodecanethiol, and emissive carbon dots with a core diameter of 2.5 nm featuring a mixed ligand shell of carboxylic acid groups and aliphatic hydrocarbon chains were tested on both rigid glass and flexible polycarbonate substrates. To define the director across the entire cell and not just in the NP-printed areas, alignment ‘underlayers’ were tested, and 30° obliquely evaporated SiOx as alignment ‘underlayer’ generally provided the best results with the highest quality of the homeotropic alignment as well as the best contrast at the boundary between printed and non-printed (i.e. homeotropic and planar) domains of the fabricated cells. We also report that the chemical nature of the nematic liquid crystal (LC) used, the number of layers printed and the composition of the nano-ink need to be adjusted to obtain pattern alignment devices that positively benefit from both the properties of the LC and the nanomaterial printed.

Direct and Efficient Preparation of Graphene Transparent Conductive Films on Flexible Poly Carbonate Substrate by Spray-Coating.

Owing to the hydrophobic property and heat-labile of flexible substances, it is difficult to prepare graphene transparent conductive films (TCFs) on flexible substrate in a direct and effective way. Here we prepared a good dispersion of water/graphene oxide (GO)/ethanol, and the fabrication of graphene TCFs on flexible poly carbonate (PC) substrate was made by spray deposition of water/GO/ethanol, followed by the reduction of hydriodic acid (HI) fuming method. It can be found that when ethanol was added to GO solution, the drying dynamics of the spraying solvent increased and the problem of wetting property of GO dispersion on the PC could be effectively resolved. HI acid vapour can achieve an effective reduction of the GO film. The reduction effect of HI acid fuming method is more effective in comparation with traditional HI acid immersed method. An increase in spraying concentration can lead to a rise in coverage degree of film and folding degree of surface. 1/500 mg/ml is a relatively appropriate concentration for spray-coating. The thickness of the film was controlled by adjusting the spraying volume of water/GO/ethanol dispersion. The graphene TCFs exhibit a sheet resistance of less than 15.3 kΩ/sq at 74% transmittance.

Focused Electron and Ion Beam Induced Deposition on Flexible and Transparent Polycarbonate Substrates.

The successful application of focused electron (and ion) beam induced deposition techniques for the growth of nanowires on flexible and transparent polycarbonate films is reported here. After minimization of charging effects in the substrate, sub-100 nm-wide Pt, W, and Co nanowires have been grown and their electrical conduction is similar compared to the use of standard Si-based substrates. Experiments where the substrate is bent in a controlled way indicate that the electrical conduction is stable up to high bending angles, >50°, for low-resistivity Pt nanowires grown by the ion beam. On the other hand, the resistance of Pt nanowires grown by the electron beam changes significantly and reversibly with the bending angle. Aided by the substrate transparency, a diffraction grating in transmission mode has been built based on the growth of an array of Pt nanowires that shows sharp diffraction spots. The set of results supports the large potential of focused beam deposition as a high-resolution nanolithography technique on transparent and flexible substrates. The most promising applications are expected in flexible nano-optics and nanoplasmonics, flexible electronics, and nanosensing.

Development of highly transparent Pd-coated Ag nanowire electrode for display and catalysis applications

Ag nanowire transparent electrode has excellent transmittance (90%) and sheet resistance (20Ω/sq), yet there are slight drawbacks such as optical haze and chemical instability against aerial oxidation. Chemical stability of Ag nanowires needs to be improved in order for it to be suitable for electrode applications. In our recent article, we demonstrated that coating Ag nanowires with a thin layer of Au through galvanic exchange reactions enhances the chemical stability of Ag nanowire films highly and also helps to obtain lower haze. In this study, coating of a thin Pd layer has been applied successfully onto the surface of Ag nanowires. A mild Pd complex oxidant [Pd(en)2](NO3)2 was prepared in order to oxidize Ag atoms partially on the surface via galvanic displacement. The mild galvanic exchange allowed for a thin layer (1–2nm) of Pd coating on the Ag nanowires with minimal truncation of the nanowire, where the average length and the diameter were 12.5μm and 59nm, respectively. The Pd-coated Ag nanowires were suspended in methanol and then electrostatically sprayed on flexible polycarbonate substrates. It has been revealed that average total transmittance remain around 95% within visible spectrum region (400–800nm) whereas sheet resistance rises up to 175Ω/sq. To the best of our knowledge, for the first time in the literature, Pd coating was employed on Ag nanowires in order to design transparent electrodes for high transparency and strong chemical resistivity against nanowire oxidation. The current Pd-coated Ag nanowires may render an excellent catalyst system for fuel cell applications, as well as in organic synthesis with relatively low costs since our approach enables the fabrication of these nanowires with a very thin layer of Pd. We believe that mesh form of Pd-coated Ag nanowires will coin a new catalyst concept to the related areas since their sheet conductivity is high enough, and also little amount of Pd displays a large surface area as thin layers.

Photonic crystal slabs in flexible organic light-emitting diodes

Photonic crystal slabs integrated into organic light-emitting diodes (OLEDs) allow for the extraction of waveguide modes and thus an increase in OLED efficiency. We fabricated linear Bragg gratings with a 460-nm period on flexible polycarbonate substrates using UV nanoimprint lithography. A hybrid organic–inorganic nanoimprint resist is used that serves also as a high refractive index layer. OLEDs composed of a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) polymer anode, an organic emission layer [poly(p-phenylene vinylene) (PPV)-derivative “Super Yellow”], and a metal cathode (LiF/Al) are deposited onto the flexible grating substrates. The effects of photonic crystal slab deformation in a flexible OLED are studied in theory and experiment. The substrate deformation is modeled using the finite-element method. The influence of the change in the grating period and the waveguide thickness under bending are investigated. The change in the grating period is found to be the dominant effect. At an emission angle of 20° a change in the resonance wavelength of 1.2% is predicted for a strain of 1.3% perpendicular to the grating grooves. This value is verified experimentally by analyzing electroluminescence and photoluminescence properties of the fabricated grating OLEDs.

干黏附碳纳米管垂直阵列的转移及其黏附性能评价

Vertically aligned carbon nanotube arrays(VACNTs) exhibit high aspect-ratios and excellent mechanical strength, and have attracted attention in the field of gecko-inspired dry adhesives. However, the poor interfacial bonding between the silicon substrate and the VACNTs, the hard and brittle silicon substrate which is suitable for the growth of VACNTs in chemical vapor deposition(CVD) technique limit the development and application of VACNTs as the gecko-inspired dry adhesive. To overcome these limitations we present a simple and cost-effective heating transfer method without other media-assisted. We combine this with thermal oxidation detachment to transfer the grown VACNTs onto flexible polycarbonate(PC) substrates. The transfer process provides two different VACNTs morphologies; a curly entangled segment in the bottom and an aligning end segment in the top. The bottom transfer maintains the top morphology of the pristine VACNTs, which differs from the flip-over process known as top transfer. This transfer process results in a double transferred VACNTs structure which is a new type of dry adhesive. By optimizing the transfer process, the interfacial bonding force between the VACNTs and PC substrates is nearly three times stronger than that of grown VACNTs on silicon substrates at 250°C. The shear adhesion strengths of the bottom- and top-transferred VACNTs were tested and analyzed. Then, the shear adhesion strength of the transferred VACNTs were tested and analyzed according to the difference between the morphology and the degree of graphitization on the top of VACNTs after the top/bottom transfer methods. The shear adhesion strength of the transferred VACNTs increased with the normal preload. The adhesion strength of the bottom-transferred VACNTs was approximately 5 N/cm2, whereas that of the top-transferred VACNTs was 2.8 N/cm2 under a 16 N/cm2 normal preload force. The bottom-transferred VACNTs showed better adhesion because of the increasing contact area and higher degree of graphitization on the top of the structure according to scanning electron microscope and Raman spectra. Our transfer method is nondestructive, fast and suitable for VACNTs with a height greater than 200 um. Moreover, this method is not restricted by the area of the VACNTs, and thus can improve their integrity and flexibility, and increase their application as gecko-inspired dry adhesives.

Highly transparent Au-coated Ag nanowire transparent electrode with reduction in haze.

Ag nanowire transparent electrode has excellent transmittance and sheet resistance, yet its optical haze still needs to be improved in order for it to be suitable for display applications. Ag nanowires are known to have high haze because of the geometry of the nanowire and the high light scattering characteristic of the Ag. In this study, a Au-coated Ag nanowire structure was proposed to reduce the haze, where a thin layer of Au was coated on the surface of the Ag nanowires using a mild [Au(en)2]Cl3 galvanic displacement reaction. The mild galvanic exchange allowed for a thin layer of Au coating on the Ag nanowires with minimal truncation of the nanowire, where the average length and the diameter were 13.0 μm and 60 nm, respectively. The Au-coated Ag nanowires were suspended in methanol and then electrostatically sprayed on a flexible polycarbonate substrate that revealed a clear reduction in haze with a 2-4% increase in total transmittance, sheet resistance ranges of 80-90%, and 8.8-36.8 Ohm/sq. Finite difference time domain simulations were conducted for Au-coated Ag nanowires that indicated a significant reduction in the average scattering from 1 to 0.69 for Au layer thicknesses of 0-10 nm.

Efficiency improvement of organic bifunctional devices by applying omnidirectional antireflection nanopillars

OLED/OPV tandem devices have attracted great interest due to their various functions in green technology and potential sensing applications. Here we report a novel concept to improve the luminous efficiency and energy conversion efficiency of tandem bifunctional devices by integrating antireflection structures consisting of irregular nanopillars as the capping layer. The nanopillars were made on flexible polycarbonate substrates by using a nanoimprint method with an anodized aluminum oxide template. The average reflectivity of the patterned polycarbonate substrate at visible wavelengths was reduced to 3.3%. With the antireflection structures, the luminous intensity for bifunctional devices was improved by 41.9% compared to a device with a flat capping layer. The power conversion efficiency was improved from 0.48% to 0.61%, a 27.0% enhancement at normal incidence.