Flexible Transparent Conductive Electrodes: Unveiling Growth Mechanisms, Material Dimensions, Fabrication Methods, and Design Strategies.

Flexible transparent conductive electrodes (FTCEs) constitute an indispensable component in state-of-the-art electronic devices, such as wearable flexible sensors, flexible displays, artificial skin, and biomedical devices, etc. This review paper offers a comprehensive overview of the fabrication techniques, growth modes, material dimensions, design, and their impacts on FTCEs fabrication. The growth modes, such as the "Stranski-Krastanov growth," "Frank-van der Merwe growth," and "Volmer-Weber growth" modes provide flexibility in fabricating FTCEs. Application of different materials including 0D, 1D, 2D, polymer composites, conductive oxides, and hybrid materials in FTCE fabrication, emphasizing their suitability in flexible devices are discussed. This review also delves into the design strategies of FTCEs, including microgrids, nanotroughs, nanomesh, nanowires network, and "kirigami"-inspired patterns, etc. The pros and cons associated with these materials and designs are also addressed appropriately. Considerations such as trade-offs between electrical conductivity and optical transparency or "figure of merit (FoM)," "strain engineering," "work function," and "haze" are also discussed briefly. Finally, this review outlines the challenges and opportunities in the current and future development of FTCEs for flexible electronics, including the improved trade-offs between optoelectronic parameters, novel materials development, mechanical stability, reproducibility, scalability, and durability enhancement, safety, biocompatibility, etc.

Effect of Graphite Nanoplatelet Size and Dispersion on the Thermal and Mechanical Properties of Epoxy-Based Nanocomposites.

This study investigated the effect of graphite nanoplatelet (GNP) size and dispersion on the thermal conductivities and tensile strengths of epoxy-based composites. GNPs of four different platelet sizes, ranging from 1.6 to 3 µm, were derived by mechanically exfoliating and breaking expanded graphite (EG) particles using high-energy bead milling and sonication. The GNPs were used as fillers at loadings of 0-10 wt%. As the GNP size and loading amount increased, the thermal conductivities of the GNP/epoxy composites increased, but their tensile strengths decreased. However, interestingly, the tensile strength reached a maximum value at the low GNP content of 0.3% and thereafter decreased, irrespective of the GNP size. Our observations of the morphologies and dispersions of the GNPs in the composites indicated that the thermal conductivity was more likely related to the size and loading number of fillers, whereas the tensile strength was more influenced by the dispersion of fillers in the matrix.

NaCl-Assisted Temperature-Dependent Controllable Growth of Large-Area MoS2 Crystals Using Confined-Space CVD.

Due to its semiconducting nature, controlled growth of large-area chemical vapor deposition (CVD)-grown two-dimensional (2D) molybdenum disulfide (MoS2) has a lot of potential applications in photodetectors, sensors, and optoelectronics. Yet the controllable, large-area, and cost-effective growth of highly crystalline MoS2 remains a challenge. Confined-space CVD is a very promising method for the growth of highly crystalline MoS2 in a controlled manner. Herein, we report the large-scale growth of MoS2 with different morphologies using NaCl as a seeding promoter for confined-space CVD. Changes in the morphologies of MoS2 are reported by variation in the amount of seeding promoter, precursor ratio, and the growth temperature. Furthermore, the properties of the grown MoS2 are analyzed using optical microscopy, scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), and atomic force microscopy (AFM). The electrical properties of the CVD-grown MoS2 show promising performance from fabricated field-effect transistors. This work provides new insight into the growth of large-area MoS2 and opens the way for its various optoelectronic and electronic applications.

Bone Reconstruction Using Two-Layer Porcine-Derived Bone Scaffold Composed of Cortical and Cancellous Bones in a Rabbit Calvarial Defect Model.

In this study, we aimed to investigate the bone regeneration efficiency of two-layer porcine-derived bone scaffolds composed of cancellous and cortical bones in a rabbit calvarial defect model. Four circular calvaria defects were formed on cranium of rabbit and were filled with block bone scaffolds of each group: cortical bone block (Cortical group), cancellous bone block (Cancellous group), and two-layer bone block (2layer group). After 8 weeks, new bones were primarily observed in cancellous parts of the Cancellous and 2layer groups, while the Cortical group exhibited few new bones. In the results of new bone volume and area analyses, the Cancellous group showed the highest value, followed by the 2layer group, and were significantly higher than the Cortical group. Within the limitations of this study, the cancellous and two-layer porcine-derived bone scaffolds showed satisfactory bone regeneration efficiency; further studies on regulating the ratio of cortical and cancellous bones in two-layer bones are needed.

Self-standing SnS nanosheet array: a bifunctional binder-free thin film catalyst for electrochemical hydrogen generation and wastewater treatment.

Hydrogen generation during wastewater treatment has remained a long-standing challenge for the environment preservation welfare. In the present work, we have fabricated a promising bifunctional thin film-based catalyst for hydrogen generation with concurrent wastewater treatment. The prepared catalyst film is a vertically oriented thin SnS (tin monosulfide) nanosheet array on a Ni-foam (SnS/NF) obtained via a solution process, demonstrating a promising electrocatalytic activity towards the generation of green H2 fuel at the cathodic side and the decomposition of urea waste at the anodic side. Notably, while assembling two identical electrodes as cathode and anode together with a reference electrode (i.e., SnS/NF∥SnS/NF vs. RHE assembly) in 1 M KOH aqueous electrolyte containing 0.33 M urea, the electrolyzer electrolyzed urea at a lower cell potential of 1.37 and 1.43 V (vs. RHE) to deliver a current density of 10 mA cm-2 and 50 mA cm-2, respectively, for the decomposition of urea at the anodic SnS/NF electrode and green hydrogen fuel generation at the cathodic SnS/NF electrode. This activity on electrocatalytic urea decomposition lies within the best performance to those of the previously reported sulfide-based and other catalytic materials. The promising catalytic activities of the SnS catalyst film are attributed to its combined effect of self-standing nanosheet array morphology and high crystallinity, which provides abundant active sites and a facile charge transfer path between the nanosheet arrays and the electrolyte. Thus, the present work offers a green avenue to the waste-urea treatment in water and sustainable hydrogen energy production.

Application of Titanium-Carbide MXene-Based Transparent Conducting Electrodes in Flexible Smart Windows.

Among various available materials used in transparent and flexible devices, MXenes are attracting attention as a brand-new candidate in this category. Ti3C2Tx MXene as a 2D material has exceptional properties, making it a potential material having numerous applications in different areas. Because of its high conductivity, it can be used in transparent conducting electrodes (TCEs). In this study, the MXenes etched by highly concentrated acid at 50 °C,were spin-coated on polyethylene terephthalate (PET) film and annealed at moderate temperatures up to 170 °C. The adhesion of MXene to PET was found to be remarkably improved by annealing. These TCEs exhibited a sheet resistance of ∼424 Ω/sq. and transmittance of ∼87%. The aging stability of MXene-coated PET films against oxidation under ambient conditions was studied up to 28 days and resistance change was found ∼30% during this period. The flexibility test showed low bending resistance change (∼1.5%) at 1000th cycle and cumulative resistance change of ∼20% at a bending radius of ∼3.9 mm after 1000 cycles. These transparent, flexible, and conducting electrodes were used to fabricate polymer dispersed liquid crystal (PDLC)-based flexible smart windows. The smart windows fabricated by curing PDLC mixture sandwiched between the MXene electrodes were also found flexible in ON/OFF states. The MXene-based flexible smart windows resulted in good opacity in the OFF state and high transparency in the ON state, exhibiting low threshold voltage <10 V and high transmittance ∼80% at 60 V. The flexible smart windows operated normally even at ∼4 mm bending radius.

NIR self-powered photodetection and gate tunable rectification behavior in 2D GeSe/MoSe2 heterojunction diode.

Two-dimensional (2D) heterostructure with atomically sharp interface holds promise for future electronics and optoelectronics because of their multi-functionalities. Here we demonstrate gate-tunable rectifying behavior and self-powered photovoltaic characteristics of novel p-GeSe/n-MoSe2 van der waal heterojunction (vdW HJ). A substantial increase in rectification behavior was observed when the devices were subjected to gate bias. The highest rectification of ~ 1 × 104 was obtained at Vg = - 40 V. Remarkable rectification behavior of the p-n diode is solely attributed to the sharp interface between metal and GeSe/MoSe2. The device exhibits a high photoresponse towards NIR (850 nm). A high photoresponsivity of 465 mAW-1, an excellent EQE of 670%, a fast rise time of 180 ms, and a decay time of 360 ms were obtained. Furthermore, the diode exhibits detectivity (D) of 7.3 × 109 Jones, the normalized photocurrent to the dark current ratio (NPDR) of 1.9 × 1010 W-1, and the noise equivalent power (NEP) of 1.22 × 10-13 WHz-1/2. The strong light-matter interaction stipulates that the GeSe/MoSe2 diode may open new realms in multi-functional electronics and optoelectronics applications.

Supercapacitors based on Ti3C2Tx MXene extracted from supernatant and current collectors passivated by CVD-graphene.

An ultrahigh capacity supercapacitor is fabricated using a nano-layered MXene as an active electrode material, and Ni-foil is used as a current collector. The high-quality Ti3C2Tx obtained from supernatant during etching and washing processes improves the specific capacitance significantly. As another strategy, the surface of Ni-foil is engineered by coating chemical vapor deposition-grown graphene. The graphene grown directly on the Ni-foil is used as a current collector, forming the electrode structure of Ti3C2Tx/graphene/Ni. The surface passivation of the current collectors has a high impact on charge-transfer, which in turn increases the capacitance of the supercapacitors. It is found that the capacitance of the graphene-based supercapacitors is more than 1.5 times of the capacitance without graphene. A high specific capacitance, ~ 542 F/g, is achieved at 5 mV/s scan rate based on cyclic voltammetry analysis. Also, the graphene-based supercapacitor exhibits a quasi-rectangular form in cyclic voltammetry curves and a symmetric behavior in charge/discharge curves. Furthermore, cyclic stability up to 5000 cycles is confirmed with high capacitance retention at high scan rate 1000 mV/s. A reduced series resistance with a high limit capacitance is revealed by equivalent circuit analysis with the Nyquist plot.

WSe2 Homojunction p-n Diode Formed by Photoinduced Activation of Mid-Gap Defect States in Boron Nitride.

A single nanoflake lateral p-n diode (in-plane) based on a two-dimensional material can facilitate electronic architecture miniaturization. Here, a novel lateral homojunction p-n diode of a single WSe2 nanoflake is fabricated by photoinduced doping via optical excitation of defect states in an h-BN nanoflake upon illumination. This lateral diode is fabricated using a mechanical exfoliation technique by stacking the WSe2 nanoflake partially on the h-BN and Si substrates. The carrier type in the part of the WSe2 film on the h-BN substrate is inverted and a built-in potential difference is formed, ranging from 5.0 to 4.50 eV, which is measured by Kelvin probe force microscopy. The contact potential difference across the junction of p-WSe2 and n-WSe2 is found to be ∼492 mV. The lateral diode shows an excellent rectification ratio, up to ∼3.9 × 104, with an ideality factor of ∼1.1. A typical self-biased photovoltaic behavior is observed at the p-n junction upon the illumination of incident light, that is, a positive open-circuit voltage (Voc) is generated, that is, voltage obtained (at Ids = 0 V), and also a negative short-circuit current (Isc) is generated, that is, current obtained (at Vds = 0 V). The presence of built-in potential in the proposed homojunction diode establishes Isc and Voc upon illumination, which can be implemented for a self-powered photovoltaic system in future electronics. The proposed doping technique can be effectively applied to form planar homojunction devices without a photoresist for future electronic and optoelectronic applications.

Asymmetric electrode incorporated 2D GeSe for self-biased and efficient photodetection.

2D layered germanium selenide (GeSe) with p-type conductivity is incorporated with asymmetric contact electrode of chromium/Gold (Cr/Au) and Palladium/Gold (Pd/Au) to design a self-biased, high speed and an efficient photodetector. The photoresponse under photovoltaic effect is investigated for the wavelengths of light (i.e. ~220, ~530 and ~850 nm). The device exhibited promising figures of merit required for efficient photodetection, specifically the Schottky barrier diode is highly sensitive to NIR light irradiation at zero voltage with good reproducibility, which is promising for the emergency application of fire detection and night vision. The high responsivity, detectivity, normalized photocurrent to dark current ratio (NPDR), noise equivalent power (NEP) and response time for illumination of light (~850 nm) are calculated to be 280 mA/W, 4.1 × 109 Jones, 3 × 107 W-1, 9.1 × 10-12 WHz-1/2 and 69 ms respectively. The obtained results suggested that p-GeSe is a novel candidate for SBD optoelectronics-based technologies.

Optoelectronics of Multijunction Heterostructures of Transition Metal Dichalcogenides.

Among p-n junction devices with multilayered heterostructures with WSe2 and MoSe2, a device with the MoSe2-WSe2-MoSe2 (NPN) structure showed a remarkably high photoresponse, which was 1000 times higher than the MoSe2-WSe2 (NP) structure. The ideality factor of the NPN structure was estimated to be ∼1, lower than that of the NP structure. It is claimed that the NPN structure formed a thinner depletion region than that of the NP structure because of the difference of carrier concentrations of MoSe2 and WSe2. Hence, the built-in electric field was weaker, and the motion of the photocarriers was facilitated. These behaviors were confirmed experimentally from a photocurrent mapping analysis and Kelvin probe force microscopy. The work function depended on the wavelength of the illuminator, and quasi-Fermi level was estimated. The surface photovoltage on the MoSe2 region was higher than that on WSe2 because the lower bandgap of MoSe2 induces more electron-hole pair generation.

Visibility of hexagonal boron nitride on transparent substrates.

The high transmittance and low reflectance of monolayer hexagonal boron nitride (hBN) lead to its invisibility under white-light, causing serious troubles in the search, transfer, and fabrication of 2D material devices. In this work, we demonstrate enhancing the contrast of hBN on a transparent substrate by simulation and experimental observation, where the highest contrast is obtained by using a polymer-based interfacial layer on a polydimethylsiloxane (PDMS) substrate. The simulation result reveals that the contrast under short wavelength light is higher than that under long wavelength. To confirm this, the red-green-blue components are extracted from the optical color image. The blue component image shows an hBN flake clearly on the substrate, while the hBN flake fades on the green and red components. Moreover, the contrast on transparent substrates have only positive value, while opaque substrates cause both negative and positive contrast depending on the thickness of the interfacial layer. Thus, the high contrast (∼4.5%) of hBN on the PDMS substrate enables us to observe mono- and few-layer hBN flakes under white-light illumination by an optical microscope.

Effect of Ti3C2T x MXenes etched at elevated temperatures using concentrated acid on binder-free supercapacitors.

The effect of Ti3C2T x MXene etched at different temperatures (25 °C, 50 °C, and 80 °C) on the capacitance of supercapacitors without the use of conducting carbon-black or a binder was studied. The MXene etched using concentrated HCl acid (12 M)/LiF was used as an active electrode and Ni-foil as a current collector. It was observed that the elevated etching temperature facilitates the etching of the MAX phase and the exfoliation of MXene layers. However, this led to the formation of additional functional groups at the MXene surface as the temperature was increased to 80 °C. The specific capacitance of Ti3C2T x -based supercapacitors increased from 581 F g-1 for MXene etched at 25 °C to 657 F g-1 for those etched at 50 °C at the scan rate of 2 mV s-1. However, the specific capacitance reduced to 421 F g-1 as the etching temperature was increased to 80 °C at the same scan rate. The supercapacitors based on MXenes etched at the intermediate temperature (50 °C) exhibited higher specific capacitance in a wide range of scan rate, symmetry in charge/discharge curves, high cyclic stability at a scan rate of 1000 mV s-1 for up to 3000 cycles. The electrochemical impedance spectroscopy studies indicated low series resistance, reduced charge-transfer resistance, and decreased Warburg impedance for the supercapacitor based on the MXene etched at the intermediate temperature.

Polymer-dispersed liquid-crystal-based switchable glazing fabricated via vacuum glass coupling.

Polymer-dispersed liquid crystals (PDLCs) exhibiting transmittance switching are utilized for preserving energy and protecting privacy. Here we prepared a typical PDLC mixture from a commercially available polymer and liquid crystal with nano-beads. A wire-bar coater was used to coat the PDLC mixture on indium-tin-oxide-coated glass, and a PDLC cell was assembled by coupling another glass in a vacuum, instead of a polymer film. Uniform glass-based PDLCs were fabricated successfully with an area of 15 × 15 cm2, while an injection process with capillary action was not available for this large-scale device fabrication. The switching behavior of the cells was characterized by ramping the AC voltage, and a transmittance change of ∼70% was measured. In a typical roll-to-roll process, only flexible polymer films have been used in lamination, in which deterioration in transparency occurs over the course of time, reducing the efficiency. In this study, to improve the optical properties, PDLC switchable glazing is fabricated directly onto glass substrates instead of onto plastic polymers. This PDLC switchable glazing, exhibiting low haziness and wide-angle vision, can be fabricated at a large scale by a vacuum-coupling process, with potential use as glass windows for energy-efficient buildings.

Quartz tuning fork based three-dimensional topography imaging for sidewall with blind features.

Atomic force microscopy has a tremendous number of applications in a wide variety of fields, particularly in the semiconductor area for the 3D-stacked device. Imaging three-dimensional (3D) structures with blind features has progressively become a critical technique. Recently, a 3D-atomic force microscopy (AFM) technique has been proposed to image 3D features, especially those having sharp apices, like silicon pillars. However, the scanning strategy has drawbacks, such as long scanning time, and unstable operation, based on the premature algorithm. Herein, an improved 3D-AFM algorithm is reported that overcomes the aforementioned problems by an intelligent 3D scanning algorithm that incorporates sidewall history tracking, troubleshooting for sharp sidewall and sticking, and reactive direction adjustment. The proposed algorithm enables the 3D imagery of ZnO nano-rods and silicon nano-pillars to be achieved by using a high aspect-ratio multiwall carbon nanotube-based AFM probe, without time-consuming disorientation. This study establishes a method to construct a 3D image of arbitrary shape in reduced scanning time.

Acrylate-assisted fractal nanostructured polymer dispersed liquid crystal droplet based vibrant colored smart-windows.

We have studied liquid crystals (LCs) and acrylate-assisted thiol-ene compositions to synthesize dye based colorful polymer dispersed liquid crystals (PDLCs) without using a photo-initiator for smart-windows applications. A typical PDLC mixture was prepared by mixing LCs with UV-curable monomers, which included triethylene glycol diacrylate (TEGDA), trimethylolpropane diallyl ether (TMPDE, di-functional ene monomer), trimethylolpropane tris(3-mercaptopropionate) (TMPTMP, a thiol as a cross-linker), and a dichroic dye. The ratios of the TMPDE/TMPTMP and the LCs/TEGDA showed significant effects in altering the properties of the UV-cured PDLCs. During the curing process, the monomers polymerize and led to the encapsulation of the LCs in the form of interesting fractal nanostructures by a polymerization induced phase separation process. The switching time, electro-optical properties, power consumption, and ageing of the fabricated PDLCs were investigated. It was possible to achieve a 70-80% contrast (ΔT) at a voltage difference of ∼70 V with a fast switching time (τ) as low as < 20 milliseconds (ms) and low power consumption. These PDLCs had a low threshold voltage that ranged between 10 and 20 V. The sustainability of the fabricated UV-cured PDLCs was analyzed for up to 90 days, and the PDLCs were observed to be stable.

Operation Protocols To Improve Durability of Protonic Ceramic Fuel Cells.

To develop reliable and durable protonic ceramic fuel cells (PCFCs), the impacts of the operation protocols of PCFCs on the cell durability are investigated through analyses of the main degradation mechanisms. We herein propose three appropriately designed control protocols, including cathode air depletion, shunt current, and fuel cell/electrolysis cycling, to fully circumvent the operating-induced degradation of PCFCs. For this purpose, anode-supported cells, comprised of a NiO-BaCe0.7Zr0.1Y0.1Yb0.1O3-δ anode, BaCe0.7Zr0.1Y0.1Yb0.1O3-δ electrolyte, and NdBa0.5Sr0.5Co1.5Fe0.5O5+δ-Nd0.1Ce0.9O2-δ composite cathode, are prepared, and their long-term performances are evaluated under a galvanostatic condition of 0.5 A·cm-2 at 650 °C. The cell voltages of the protected cells using the operation protocols to prevent performance degradation are stably maintained under the applied current density for more than 1200 h without any noticeable degradation, whereas the performance of the unprotected cell gradually decreased with time, and the decay ratio was 14.9% over 850 h. The significant performance decay of the unprotected cell is strongly associated with the cathode degradation phenomenon, which was caused by the water vapor continuously produced during the electrochemical reactions. Hence, the performance recovery of the PCFCs with the operation protocols is achieved by incrementally decreasing the cathode potential (close to a value of zero) to minimize the effect of high PH2O and PO2 during the PCFC operations.

Twist-Angle-Dependent Optoelectronics in a Few-Layer Transition-Metal Dichalcogenide Heterostructure.

Lattice matching has been supposed to play an important role in the coupling between two materials in a vertical heterostructure (HS). To investigate this role, we fabricated a heterojunction device with a few layers of p-type WSe2 and n-type MoSe2 with different crystal orientation angles. The crystal orientations of WSe2 and MoSe2 were estimated using high-resolution X-ray diffraction. Heterojunction devices were fabricated with twist angles of 0, 15, and 30°. The I- V curve of the sample with the twist angle of 0° under the dark condition showed a diodelike behavior. The strong coupling due to lattice matching caused a well-established p-n junction. In cases of 15 and 30° samples, the van der Waals gap was built because of lattice mismatching, which resulted in the formation of a potential barrier. However, when the light-emitting diode light of 365 nm (3.4 eV) was illuminated, it was possible for excited electrons and holes to jump beyond the potential barrier and the current flowed well in both forward and reverse directions. The effects of the twist angle were analyzed by spectral responsivity and external quantum efficiency, where it was found that the untwisted HS exhibited higher sensitivity under IR illumination, whereas the twisting effect was not noticeable under UV illumination. From photoluminescence and Raman spectroscopy studies, it was confirmed that the twisted HS showed a weak coupling because of the lattice mismatch.

Comparison of Electrical and Photoelectrical Properties of ReS2 Field-Effect Transistors on Different Dielectric Substrates.

As one of the newly discovered transition-metal dichalcogenides (TMDs), rhenium disulfide (ReS2) has been investigated mostly because of its unique characteristics such as the direct band gap nature even in bulk form, which is not prominent in other TMDs (e.g., MoS2, WSe2, etc.). However, this material possesses a low mobility and an on/off ratio, which restrict its usage in high-speed and fast switching applications. Low mobilities or on/off ratios can also be caused by substrate scattering as well as environmental effects. In this study, we used few-layer ReS2 (FL-ReS2) as a channel material to investigate the substrate-dependent mobility, current on/off ratio, Schottky barrier height (SBH), and trap density of states of different dielectric substrates. The hexagonal boron nitride (h-BN)/FL-ReS2/h-BN structure was observed to exhibit a high mobility of 45 cm2 V-1 s-1, current on/off ratio of about 107, the lowest SBH of about 12 mV at a zero back-gate voltage ( Vbg), and a low trap density of states of about 5 × 1013 cm-3. These quantities are reasonably superior compared to the FL-ReS2 devices on SiO2 substrates. We also observed a nearly 5-fold improvement in the photoresponsivity and external quantum efficiency values for the FL-ReS2 devices on h-BN substrates. We believe that the photonic characteristics of TMDs can be improved by using h-BN as the substrate and capping layer.

Visualizing Degradation of Black Phosphorus Using Liquid Crystals.

Black Phosphorus (BP) is an excellent material from the post graphene era due to its layer dependent band gap, high mobility and high Ion/Ioff. However, its poor stability in ambient poses a great challenge for its practical and long-term usage. The optical visualization of the oxidized BP is the key and the foremost step for its successful passivation from the ambience. Here, we have conducted a systematic study of the oxidation of the BP and developed a technique to optically identify the oxidation of the BP using Liquid Crystal (LC). It is interesting to note that we found that the rapid oxidation of the thin layers of the BP makes them disappear and can be envisaged by using the alignment of the LC. The molecular dynamics simulations also proved the preferential alignment of the LC on the oxidized BP. We believe that this simple technique will be effective in passivation efforts of the BP, and will enable it for exploitation of its properties in the field of electronics.