Metal Patterning Process on Rigid and Flexible Substrates Using Nanoimprint Lithography and Resist Pattern Transfer Technique

Electrical and optical properties of nitrogen doped SnO2 thin films deposited on flexible substrates by magnetron sputtering

In this work, vertically aligned TiO2 nanowires (TiO2-NWs) and Ag nanoparticle (Ag-NP)-assisted and vertically aligned TiO2 nanowires (TAT-NWs) were deposited on glass and flexible polyethylene terephthalate (PET) substrates using the glancing angle deposition (GLAD) technique. The morphology and structural analysis of the samples manifest successful deposition of vertically aligned TiO2-NWs and TAT-NWs. The high-resolution transmission electron microscopy (HR-TEM) image of TiO2-NWs shows the polycrystalline nature. Further, the X-ray diffraction (XRD) result confirms the polycrystalline nature of both the TiO2-NW and TAT-NW samples. Besides, HR-TEM image confirms the presence of small crystal grains of Ag-NPs at the mid of the annealed TAT-NWs. It is evident from the selected area electron diffraction (SAED) analysis of TiO2-NWs and annealed TAT-NWs that the crystallinity of TiO2 present in the annealed TAT-NWs improves after annealing. The absorption spectrum analysis of TAT-NWs deposited on glass substrate shows enhances absorption peak in the visible region with a maximum peak at ~ 450 nm wavelength compared to the TiO2-NWs, which may be attributed to the surface plasmon resonance (SPR) effect of Ag-NPs. Further, it is interesting to observe that the TAT-NWs deposited on PET substrate show further absorption enhancement in the UV and visible regions. In addition, the photoluminescence (PL) analysis reveals that the band gap of TiO2-NWs is ~ 3.12 eV. Therefore, the proposed method of fabricating TAT-NWs on glass and flexible ITO-coated PET substrates using the GLAD technique may be applicable for developing novel photoanode for dye-sensitized solar cells (DSSCs) and other optoelectronic applications.

A low-cost design of millimeter-wave absorbing sheets is investigated. The absorber is composed of a perforated resistive film on a flexible polyethylene terephthalate (PET) substrate which is backed by a resistive ground plane. Broadband (60 GHz to 120 GHz) and wide-angle (up to 80°) absorption (> 50%) is achieved for a design with a square lattice of circular holes (diameter d = 5.5 mm and lattice period a = 7.5 mm) in the perforated resistive film of thickness t R = 100 μm and conductivity σ = 80 S/m.

This work investigates the endurance and performance of aluminum-doped zinc oxide (AZO) thin films fabricated on flexible polyethylene terephthalate (PET) substrates, providing new insights into their degradation linked to mechanical flexing and accelerated thermal cycling (ATC). The current study uniquely combines cyclic bending fatigue at 23 °C and 70 °C with ATC between 0 °C and 100 °C, simulating operational stresses in real-world environments, in contrast to previous research that has focused primarily on either isolated mechanical or thermal effects. The 425 nm thick films showed high transparency and conductivity, making them suitable materials for flexible electronics and optoelectronic devices. In this work, electrical resistivity, one of the most important performance parameters, was investigated after each mechanical or thermal cycle. The results indicate that mechanical cycling at high temperatures can drastically enhance the crack formation and electrical degradation, with an over 250% change in the electrical resistance (PCER) after 12,000 cycles at 70 °C and more than 300% after 500 thermal cycles. The highly deleterious effects of combined stressors on the structural integrity and electrical properties of AZO films are underlined by these observations. This study further suggests that the design of more robust AZO-based materials/coatings would contribute toward achieving better durability in flexible electronic applications. These findings also go hand in glove with the ninth goal of the United Nations’ Sustainable Development Goals, specifically Target 9.5: Enhance Research and Upgrade Industrial Technologies.

Microstructural transformation and thermo-mechanical improvement of quinary Bi–Sn–In–Ga–Zn solder bumps on a flexible PET substrate

Anti-reflection based on the mimicking of moth eye subwavelength structures (SWSs) has several advantages over conventionally used multilayer thin film antireflective coatings like broad angular and spectral responses, polarization insensitivity and improved stability. In this study, we present the low cost fabrication of moth eye based SWSs on flexible polyethylene terephthalate (PET) substrate using Roll-to-Roll Ultraviolet Nanoimprint Lithography (UV-NIL). We have used a novel method to create a roller mold with embedded nanopores that can be used for easy replication of SWSs without demolding issues. Pure aluminum foil was wrapped around an acrylic cylinder (O = 90mm) and a multistep anodizing and pore widening scheme was employed to create well-ordered and periodic anodized aluminum oxide (AAO) nanopores with different aspect ratios. Optical characterization of the antireflective PET film shows a transmittance over 92 %, an increase of 3% as compared to bare PET, while reflectance can be reduced from 5.3% to 0.09% in the range of visible light. It was observed that higher aspect ratio SWSs is more effective in reducing reflections at higher incident angles. Consequently, we have demonstrated an economically feasible method for fabricating large area polymer films that can be used to reduce light reflections which impose severe limitations on the performance of many optoelectronic devices like solar cells, photodetectors and flat display panels.

The first GaS nanosheet-based photodetectors are demonstrated on both mechanically rigid and flexible substrates. Highly crystalline, exfoliated GaS nanosheets are promising for optoelectronics due to strong absorption in the UV-visible wavelength region. Photocurrent measurements of GaS nanosheet photodetectors made on SiO2/Si substrates and flexible polyethylene terephthalate (PET) substrates exhibit a photoresponsivity at 254 nm up to 4.2 AW(-1) and 19.2 AW(-1), respectively, which exceeds that of graphene, MoS2, or other 2D material-based devices. Additionally, the linear dynamic range of the devices on SiO2/Si and PET substrates are 97.7 dB and 78.73 dB, respectively. Both surpass that of currently exploited InGaAs photodetectors (66 dB). Theoretical modeling of the electronic structures indicates that the reduction of the effective mass at the valence band maximum (VBM) with decreasing sheet thickness enhances the carrier mobility of the GaS nanosheets, contributing to the high photocurrents. Double-peak VBMs are theoretically predicted for ultrathin GaS nanosheets (thickness less than five monolayers), which is found to promote photon absorption. These theoretical and experimental results show that GaS nanosheets are promising materials for high-performance photodetectors on both conventional silicon and flexible substrates.

We systematically investigated the effect of a Ta buffer layer and external stress on the magnetic properties of magnetostrictive Fe81Ga19 films deposited on flexible polyethylene terephthalate (PET) substrates. The Ta buffer layers could effectively smoothen the rough surface of PET. As a result, the FeGa films grown on Ta buffer layers exhibit a weaker uniaxial magnetic anisotropy and lower coercivity, as compared to those films directly grown on PET substrates. By inward and outward bending the FeGa/Ta/PET samples, external in-plane compressive and tensile stresses were applied to the magnetic films. Due to the inverse magnetostrictive effect of FeGa, both the coercivity and squareness of hysteresis loops for FeGa/Ta films could be well tuned under various strains.

An electrochemical sensor was fabricated on a flexible polyethylene terephthalate (PET) substrate for the detection of cadmium sulfide (CdS), a heavy metal compound. The sensor consists of a working and reference electrode that were gravure printed using silver (Ag) ink on the PET substrate. The performance of the sensor was investigated by measuring electrical impedance spectroscopy (EIS) for varying concentrations of the CdS. From the EIS based response, an impedance change of 11 ± 1%, 23 ± 1%, 34 ± 2% and 50 ± 3% was observed for the 1 pM, 1 nM, 1 µM and 1 mM concentrations of CdS, respectively when compared to de-ionized (DI) water, thus demonstrating the potential of employing gravure printed electrochemical sensors for heavy metal detection applications.

The fascinating properties of transition metal dichalcogenides have been studied for a decade to broaden their promising applications as flexible electronics. However, conventional mass production of 2D materials is carried out at relatively high temperatures over 600 °C, which typical flexible substrates cannot withstand. For this reason, proper low-temperature synthesis methods must be developed. Here, we propose a fast and low-temperature synthesis method for metallic MoS2, which can be achieved on a flexible polyethylene terephthalate (PET) substrate. A precursor solution was prepared by a mixture of (NH4)2MoS4 powder and dimethylformamide. This solution was then coated on the silicon or PET substrate and an MoS2 film can be fabricated in unconstrained patterns such as lines or a continuous film at micro scale. The necessary laser power and exposure time were confirmed from systematic Raman spectroscopy measurements. The metallic behavior of the synthesized film was demonstrated by the electrical device fabrication and these excellent metallic properties show the feasibility of using MoS2 as a patternable electrode material on flexible substrates.

Properties of TiO2 films deposited on flexible substrates using direct current magnetron sputtering and using high power impulse magnetron sputtering

In order to get rid of the dependence on expensive photolithography technology and related facilities, an economic and simple design and fabrication technology for digital microfluidics (DMF) is proposed. The electrodes pattern was generated by inkjet printing nanosilver conductive ink on the flexible Polyethylene terephthalate (PET) substrate with a 3D circuit board printer, food wrap film was attached to the electrode array to act as the dielectric layer and Teflon® AF was sprayed to form a hydrophobic layer. The PET substrate and food wrap film are low cost and accessible to general users. The proposed flexible DMF chips can be reused for a long time by replacing the dielectric film coated with hydrophobic layer. The resolution and conductivity of silver traces and the contact angle and velocity of the droplets were evaluated to demonstrate that the proposed technology is comparable to the traditional DMF fabrication process. As far as the rapid prototyping of DMF is concerned, this technology has shown very attractive advantages in many aspects, such as fabrication cost, fabrication time, material selection and mass production capacity, without sacrificing the performance of DMF. The flexible DMF chips have successfully implemented basic droplet operations on a square and hexagon electrode array.

Flexible GaN-based light-emitting diodes (LEDs) on polyethylene terephthalate (PET) substrates are demonstrated by Younghun Jung et al. in their Letter on pp. 421–423. The process uses commercial LEDs on patterned sapphire substrates, laser lift-off (LLO), wet etching for additional surface roughening, and mounting of the freestanding LED on a PET substrate. Electrical and optical properties from the free-standing LLO-LEDs mounted on the flexible PET substrates were characterized. This process, illustrated on the back cover, is highly practical and can be easily implemented in a manufacturing process to fabricate large-scale flexible GaN-based LEDs.

An electrochemical flexible biosensor composed of gold (Au), molybdenum disulfide nanoparticles (MoS2 NPs), and Au (Au/MoS2/Au nanolayer) on the polyethylene terephthalate (PET) substrate is developed to detect envelope glycoprotein GP120 (gp120), the surface protein of HIV-1. To fabricate the nanolayer on the PET substrate, Au is sputter coated on the flexible PET substrate and MoS2 NPs are spin coated on Au, which is sputter coated once again with Au. The gp120 antibody is then immobilized on this flexible electrode through cysteamine (Cys) modified on the surface of the Au/MoS2/Au nanolayer. Fabrication of the biosensor is verified by atomic force microscopy, scanning electron microscopy, and cyclic voltammetry. A flexibility test is done using a micro-fatigue tester. Detection of the gp120 is measured by square wave voltammetry. The results indicate that the prepared biosensor detects 0.1 pg/mL of gp120, which is comparable with previously reported gp120 biosensors prepared even without flexibility. Therefore, the proposed biosensor supports the development of a nanomaterial-based flexible sensing platform for highly sensitive biosensors with flexibility for wearable device application.

Reflection loss can cause harmful effects on the performance of optoelectronic devices, such as cell phones, notebooks, displays, solar cells, and light-emitting diode (LED) devices. In order to obtain broadband antireflection (AR) properties, many researchers have utilized surface texture techniques to produce AR subwavelength structures on the interfaces. Among the AR subwavelength structures, the moth-eye nanostructure is one of the most promising structures, with the potential for commercialization in the near future. In this research, to obtain broadband AR performance, the optimization of moth-eye nanostructures was first carried out using the finite difference time domain method within the spectral ranges of 400-800nm, including the optimization of shape, height, pitch, and residual layer thickness. In addition, the continuous production of moth-eye nanostructure array upon a flexible polyethylene terephthalate substrate was demonstrated by using the roll-to-roll ultraviolet nanoimprint lithography (R2R UV-NIL) process and anodic aluminum oxide mold, which provided a solution for the cost-effective fabrication of moth-eye nanostructure array. The AR performance of moth-eye nanostructure array obtained by the R2R UV-NIL process was also investigated experimentally, and good consistence was shown with the simulated results. This research can provide a beneficial direction for the optimization and cost-effective production of the moth-eye nanostructure array.