Multimodal E-Textile Enabled by One-Step Maskless Patterning of Femtosecond-Laser-Induced Graphene on Nonwoven, Knit, and Woven Textiles.

Personal wearable devices are considered important in advanced healthcare, military, and sports applications. Among them, e-textiles are the best candidates because of their intrinsic conformability without any additional device installation. However, e-textile manufacturing to date has a high process complexity and low design flexibility. Here, we report the direct laser writing of e-textiles by converting raw Kevlar textiles to electrically conductive laser-induced graphene (LIG) via femtosecond laser pulses in ambient air. The resulting LIG has high electrical conductivity and chemical reliability with a low sheet resistance of 2.86 Ω/□. Wearable multimodal e-textile sensors and supercapacitors are realized on different types of Kevlar textiles, including nonwoven, knit, and woven structures, by considering their structural textile characteristics. The nonwoven textile exhibits high mechanical stability, making it suitable for applications in temperature sensors and micro-supercapacitors. On the other hand, the knit textile possesses inherent spring-like stretchability, enabling its use in the fabrication of strain sensors for human motion detection. Additionally, the woven textile offers special sensitive pressure-sensing networks between the warp and weft parts, making it suitable for the fabrication of bending sensors used in detecting human voices. This direct laser synthesis of arbitrarily patterned LIGs from various textile structures could result in the facile realization of wearable electronic sensors and energy storage.

Full-color micro-LED display with photo-patternable and highly ambient-stable perovskite quantum dot/siloxane composite as color conversion layers.

In this paper, we successfully fabricated color conversion layers (CCLs) for full-color-mico-LED display using a perovskite quantum dot (PQD)/siloxane composite by ligand exchanged PQD with silane composite followed by surface activation by an addition of halide-anion containing salt. Due to this surface activation, it was possible to construct the PQD surface with a silane ligand using a non-polar organic solvent that does not damage the PQD. As a result, the ligand-exchanged PQD with a silane compound exhibited high dispersibility in the siloxane matrix and excellent atmospheric stability due to sol-gel condensation. Based on highly ambient stable PQD/siloxane composite based CCLs, full-color micro-LED display has a 1 mm pixel pitch, about 25.4 pixels per inch (PPI) resolution was achieved. In addition, due to the thin thickness of the black matrix to prevent blue light interference, the possibility of a flexible display that can be operated without damage even with a bending radius of 5 mm was demonstrated.

Hierarchically Porous, Laser-Pyrolyzed Carbon Electrode from Black Photoresist for On-Chip Microsupercapacitors.

We report a laser-pyrolyzed carbon (LPC) electrode prepared from a black photoresist for an on-chip microsupercapacitor (MSC). An interdigitated LPC electrode was fabricated by direct laser writing using a high-power carbon dioxide (CO2) laser to simultaneously carbonize and pattern a spin-coated black SU-8 film. Due to the high absorption of carbon blacks in black SU-8, the laser-irradiated SU-8 surface was directly exfoliated and carbonized by a fast photo-thermal reaction. Facile laser pyrolysis of black SU-8 provides a hierarchically macroporous, graphitic carbon structure with fewer defects (ID/IG = 0.19). The experimental conditions of CO2 direct laser writing were optimized to fabricate high-quality LPCs for MSC electrodes with low sheet resistance and good porosity. A typical MSC based on an LPC electrode showed a large areal capacitance of 1.26 mF cm-2 at a scan rate of 5 mV/s, outperforming most MSCs based on thermally pyrolyzed carbon. In addition, the results revealed that the high-resolution electrode pattern in the same footprint as that of the LPC-MSCs significantly affected the rate performance of the MSCs. Consequently, the proposed laser pyrolysis technique using black SU-8 provided simple and facile fabrication of porous, graphitic carbon electrodes for high-performance on-chip MSCs without high-temperature thermal pyrolysis.

Highly Efficient and Air-Stable Heterostructured Perovskite Quantum Dot Solar Cells Using a Solid-State Cation-Exchange Reaction.

Perovskite quantum dots (PQDs) have expanded the scalability of perovskite materials by their high crystallinity, band-gap tunability, and surface ligand-driven functionalities in the colloidal state across optoelectronics as well as photovoltaics. To improve PQD performance in applications, however, defect control has emerged as a major challenge given the increased PQD surface area. Herein, we have developed a heterostructured PQD solar cell by combining CsPbI3 and FAPbI3 (FA, formamidinium) PQD layers to introduce a multinary PQD layer based on a solid-state A-site cation-exchange strategy. A heterostructure, including the solid-state diffusion-driven multinary PQD layer, creates an internally graded heterojunction for more efficient charge extraction. The best PQD cell achieves a power conversion efficiency (PCE) of 16.07% with negligible hysteresis. Furthermore, this architecture offers significantly enhanced stability with reduction of trap-assisted recombination as compared to cells of a monocompositional PQD layer. The unencapsulated device retains a 96% PCE after 1000 h in ambient storage.

Angle-Insensitive Transmission and Reflection of Nanopatterned Dielectric Multilayer Films for Colorful Solar Cells.

Aesthetically appealing photovoltaic (PV) panels with colorful layers are used in numerous applications involving color matching with the surroundings. To develop a colored film for a PV system, appropriate optical properties such as high transparency and low angle sensitivity are necessary because the colored layers can reduce the efficiency of the PV system by causing variations in the transmittance and angle of incidence. Herein, we propose a facile fabrication method for bioinspired three-dimensional (3D) photonic crystal (PC) films that exhibit broad angle-insensitive transmission and reflection, for application in colorful PV. This structure, patterned on a sequentially stacked 11-layer film of SiO2 and TiO2, is fabricated via nanoimprint lithography and a one-step dry-etching process, without using a metal mask. The changes in transmission and reflection are observed via ultraviolet-visible spectroscopy and from the reflected images obtained under various angles. The transmittance dips of the 3D PC film shift by less than 10 nm in wavelength, for angles from 0 to 45°, indicating low angle dependency. In addition, the change in the observed color, with respect to the viewing position, is less in the fabricated film. Once the 3D PC film was added to a commercial PV cell, it exhibited a higher efficiency (approximately 6% upper) when compared to a cell with a one-dimensional PC film, during the duration of the experiment, from 0 to 30°. Thus, the proposed method demonstrates excellent potential for developing structural color films for achieving aesthetically appealing PV cells.

Direct-Contact Microelectrical Measurement of the Electrical Resistivity of a Solid Electrolyte Interface.

Because of its effectiveness in blocking electrons, the solid electrolyte interface (SEI) suppresses decomposition reactions of the electrolyte and contributes to the stability and reversibility of batteries. Despite the critical role of SEI in determining the properties of batteries, the electrical properties of SEI layers have never been measured directly. In this paper, we present the first experimental results of the electrical resistivity of a LiF-rich SEI layer measured using a direct-contact microelectrical device mounted in an electron microscope. Measurements show that the SEI layer exhibits high electrical resistivity (2.3 × 105 Ω·m), which is comparable with those of typical insulating materials. Furthermore, a combined technique of advanced analyses and first-principles calculations show that the SEI layer is mainly composed of amorphous LiF and a minute nanocrystalline Li2CO3 compound. The electronic origin responsible for the high resistivity of the SEI layer is elucidated by calculating the band structures of various Li xF compounds and interpreting their effects on the resistivity. This study explains why SEI can prevent the degradation of electrode materials and consumption of Li ions in the electrolyte and thus can be viewed as a stepping stone for developing highly stable and reversible batteries.

Conversion Reaction of Nanoporous ZnO for Stable Electrochemical Cycling of Binderless Si Microparticle Composite Anode.

Binderless, additiveless Si electrode design is developed where a nanoporous ZnO matrix is coated on a Si microparticle electrode to accommodate extreme Si volume expansion and facilitate stable electrochemical cycling. The conversion reaction of nanoporous ZnO forms an ionically and electrically conductive matrix of metallic Zn embedded in Li2O that surrounds the Si microparticles. Upon lithiation, the porous Li2O/Zn matrix expands with Si, preventing extensive pulverization, while Zn serves as active material to form Li xZn to further enhance capacity. Electrodes with a Si mass loading of 1.5 mg/cm2 were fabricated, and a high initial capacity of ∼3900 mAh/g was achieved with an excellent reversible capacity of ∼1500 mAh/g (areal capacity ∼1.7 mAh/cm2) beyond 200 cycles. A high first-cycle Coulombic efficiency was obtained owing to the conversion reaction of nanoporous ZnO, which is a notable feature in comparison to conventional Si anodes. Ex situ analyses confirmed that the nanoporous ZnO coating maintained the coalescence of SiMPs throughout extended cycling. Therefore, the Li2O/Zn matrix derived from conversion-reacted nanoporous ZnO acted as an effective buffer to lithiation-induced stresses from volume expansion and served as a binder-like matrix that contributed to the overall electrode capacity and stability.

Highly-Stable Li4Ti5O12 Anodes Obtained by Atomic-Layer-Deposited Al2O3.

LTO (Li4Ti5O12) has been highlighted as anode material for next-generation lithium ion secondary batteries due to advantages such as a high rate capability, excellent cyclic performance, and safety. However, the generation of gases from undesired reactions between the electrode surface and the electrolyte has restricted the application of LTO as a negative electrode in Li-ion batteries in electric vehicles (EVs) and energy storage systems (ESS). As the generation of gases from LTO tends to be accelerated at high temperatures (40⁻60 °C), the thermal stability of LTO should be maintained during battery discharge, especially in EVs. To overcome these technical limitations, a thin layer of Al2O3 (~2 nm thickness) was deposited on the LTO electrode surface by atomic layer deposition (ALD), and an electrochemical charge-discharge cycle test was performed at 60 °C. The capacity retention after 500 cycles clearly shows that Al2O3-coated LTO outperforms the uncoated one, with a discharge capacity retention of ~98%. TEM and XPS analyses indicate that the surface reactions of Al2O3-coated LTO are suppressed, while uncoated LTO undergoes the (111) to (222) phase transformation, as previously reported in the literature.

Carbon Textile Decorated with Pseudocapacitive VC/Vx Oy for High-Performance Flexible Supercapacitors.

It is demonstrated that, via V2 O5 coating by low temperature atomic layer deposition and subsequent pyrolysis, ubiquitous cotton textile can readily turn into high-surface-area carbon textile fully decorated with pseudocapacitive Vx Oy /VC widely usable as electrodes of high-performance supercapacitor. It is found that carbothermic reduction of V2 O5 (C + V2 O5 → C' + VC + CO/CO2 (g)) leads to chemical/mechanical activation of carbon textile, thereby producing high-surface-area conductive carbon textile. In addition, sequential phase transformation and carbide formation (V2 O5 → Vx Oy → VC) occurred by carbothermic reduction trigger decoration of the carbon textile with redox-active Vx Oy /VC. Thanks to the synergistic effect of electrical double layer and pseudocapacitance, the supercapacitors made of the hybrid carbon textile exhibit far better energy density (over 30-fold increase) with excellent cycling stability than the carbon textile simply undergone pyrolysis. The method can open up a promising and facile way to synthesize hybrid electrode materials for electrochemical energy storages possessing advantages of both electrical double layer and pseudocapacitive material.

Nanospherical solid electrolyte interface layer formation in binder-free carbon nanotube aerogel/Si nanohybrids to provide lithium-ion battery anodes with a long-cycle life and high capacity.

Silicon anodes for lithium ion batteries (LiBs) have been attracting considerable attention due to a theoretical capacity up to about 10 times higher than that of conventional graphite. However, huge volume expansion during the cycle causes cracks in the silicon, resulting in the degradation of cycling performance and eventual failure. Moreover, low electrical conductivity and an unstable solid electrolyte interface (SEI) layer resulting from repeated changes in volume still block the next step forward for the commercialization of the silicon material. Herein we demonstrate the carbon nanotube (CNT) aerogel/Si nanohybrid structure for anode materials of LiBs via freeze casting followed by an RF magnetron sputtering process, exhibiting improved capacity retention compared to Si only samples during 1000 electrochemical cycles. The CNT aerogels as 3D porous scaffold structures could provide buffer volume for the expansion/shrinkage of Si lattices upon cycling and increase electrical conductivity. In addition, the nanospherical and relatively thin SEI layers of the CNT aerogel/Si nanohybrid structure show better lithium ion diffusion characteristics during cycling. For this reason, the Si@CNT aerogel anode still yielded a high specific capacity of 1439 mA h g-1 after 1000 charge/discharge cycles with low capacity fading. Our approach could be applied to other group IV LiB materials that undergo large volume changes, and also has promising potential for high performance energy applications.

Observation of partial reduction of manganese in the lithium rich layered oxides, 0.4Li2MnO3-0.6LiNi1/3Co1/3Mn1/3O2, during the first charge.

Lithium-rich layered oxides show promise as high-energy harvesting materials due to their large capacities. However, questions remain regarding the large irreversible loss in capacities for the first charge-discharge cycle due to oxygen removal in lattices related to layered Li2MnO3. Herein we present detailed studies on Li-rich Mn-based layered oxides of 0.4Li2MnO3-0.6LiNi1/3Co1/3Mn1/3O2 (Li-rich NCM) electrochemically activated between 2.5 V and 4.3 or 4.7 V vs. Li+/Li. Electron energy loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) revealed unusual manganese reduction after the first charge up to a high voltage of 4.7 V. Moreover, the electronic structure did not fully recover to the original pristine of Mn4+ state after discharge. Interestingly, these phenomena were not limited to a single particle, but were observed across the entire electrode. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and electron dispersive spectra (EDS) also showed a dramatic decline in oxygen content with highly porous morphologies, associated with oxygen vacancy formation following oxidation of O2- ions to O2. Our analysis suggests that an unstable manganese valence state with severe defects due to oxygen vacancies may lead to large irreversible capacity loss during the first charge-discharge cycle of Li-rich layered oxides.

Thermal Cycling Behavior of Zinc Antimonide Thin Films for High Temperature Thermoelectric Power Generation Applications.

The zinc antimonide compound ZnxSby is one of the most efficient thermoelectric materials known at high temperatures due to its exceptional low thermal conductivity. For this reason, it continues to be the focus of active research, especially regarding its glass-like atomic structure. However, before practical use in actual surroundings, such as near a vehicle manifold, it is imperative to analyze the thermal reliability of these materials. Herein, we present the thermal cycling behavior of ZnxSby thin films in nitrogen (N2) purged or ambient atmosphere. ZnxSby thin films were prepared by cosputtering and reached a power factor of 1.39 mW m(-1) K(-2) at 321 °C. We found maximum power factor values gradually decreased in N2 atmosphere due to increasing resistivity with repeated cycling, whereas the specimen in air kept its performance. X-ray diffraction and electron microscopy observations revealed that fluidity of Zn atoms leads to nanoprecipitates, porous morphologies, and even growth of a coating layer or fiber structures on the surface of ZnxSby after repetitive heating and cooling cycles. With this in mind, our results indicate that proper encapsulation of the ZnxSby surface would reduce these unwanted side reactions and the resulting degradation of thermoelectric performance.

All-solution-processed PbS quantum dot solar modules.

A rapid increase in power conversion efficiencies in colloidal quantum dot (QD) solar cells has been achieved recently with lead sulphide (PbS) QDs by adapting a heterojunction architecture, which consists of small-area devices associated with a vacuum-deposited buffer layer with metal electrodes. The preparation of QD solar modules by low-cost solution processes is required to further increase the power-to-cost ratio. Herein we demonstrate all-solution-processed flexible PbS QD solar modules with a layer-by-layer architecture comprising polyethylene terephthalate (PET) substrate/indium tin oxide (ITO)/titanium oxide (TiO2)/PbS QD/poly(3-hexylthiophene) (P3HT)/poly(3,4-ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT : PSS)/Ag, with an active area of up to 30 cm(2), exhibiting a power conversion efficiency (PCE) of 1.3% under AM 1.5 conditions (PCE of 2.2% for a 1 cm(2) unit cell). Our approach affords trade-offs between power and the active area of the photovoltaic devices, which results in a low-cost power source, and which is scalable to larger areas.

One-Step Deposition of Photovoltaic Layers Using Iodide Terminated PbS Quantum Dots.

We present a one-step layer deposition procedure employing ammonium iodide (NH4I) to achieve photovoltaic quality PbS quantum dot (QD) layers. Ammonium iodide is used to replace the long alkyl organic native ligands binding to the QD surface resulting in iodide terminated QDs that are stabilized in polar solvents such as N,N-dimethylformamide without particle aggregation. We extensively characterized the iodide terminated PbS QD via UV-vis absorption, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), FT-IR transmission spectroscopy, and X-ray photoelectron spectroscopy (XPS). Finally, we fabricated PbS QD photovoltaic cells that employ the iodide terminated PbS QDs. The resulting QD-PV devices achieved a best power conversion efficiency of 2.36% under ambient conditions that is limited by the layer thickness. The PV characteristics compare favorably to similar devices that were prepared using the standard layer-by-layer ethandithiol (EDT) treatment that had a similar layer thickness.

Controlled assembly of CdSe/MWNT hybrid material and its fast photoresponse with wavelength selectivity.

We report the novel assembly method of CdSe quantum dot (QD)/pyridine/multi-walled carbon nanotube (CdSe-py-MWNT) hybrid material between electrodes using two-step dielectrophoresis (DEP). At the first step, we assembled the individual MWNT between electrodes by the DEP method. At the second step, the CdSe-py materials were assembled onto the MWNT by DEP method again, which enables site specific and density controlled assembly of QDs. As the photoresponse results, the recovery time of the device fabricated was about 250 times faster than that of a similar CdSe-py-SWNT device using a single-walled carbon nanotube (SWNT) instead of a MWNT. Moreover, it was demonstrated that the optoelectronic property of the device could be modulated by the size of CdSe NQD assembled on a MWNT. We characterized the material and the device by using SEM, TEM, absorption spectroscopy, and optoelectronic instruments.

Efficient fabrication of wafer scale thin film of individualized single-walled carbon nanotubes by dual-nozzle spin casting.

In this paper, a dual-nozzle spin casting method was proposed to form a thin film of individualized single-walled carbon nanotubes (SWNTs) at the wafer scale. Each nozzle simultaneously ejected the SWNT solution and methanol, respectively. During the ejection process, two solutions were mixed at the contacting end of the nozzles and then dropped onto the substrate. Functionalization of the wafer substrate with the amine group improved the uniformity of the SWNT thin film as well as the adhesion between the individualized SWNTs and the substrate. The best condition of the spin casting involved the substrate functionalization using 3-aminopropyltriethosilane aqueous solution with a concentration of approximately 10 mM and a deposition velocity of approximately 5000 rpm. The root-mean-square roughness of the fabricated SWNT layer over the wafer substrate was found to be 1.4-1.8 nm, which indicated that the resultant thin film was one or two layers of SWNTs. The wafer scale SWNT thin film formed by dual-nozzle spin casting can be further used for the mass production and high integration of the SWNT nanoelectronic devices.

Efficient electron transfer in functional assemblies of pyridine-modified NQDs on SWNTs.

Nanocrystal quantum dot (NQD)/single-walled carbon nanotube (SWNT) hybrid nanomaterials were synthesized, assembled into field effect transistors (FETs) via dielectrophoresis (DEP), and characterized optically and electronically. The pyridine moiety functioned as a short, noncovalent linker between the NQDs and SWNTs and allowed more efficient carrier transfer through the assemblies without deleteriously altering electronic structures. Photoluminescence studies of the resulting assemblies support an efficient carrier transfer process in CdSe-py-SWNTs unlike that of CdSe/ZnS-py-SWNTs. The use of DEP as a means of controlling the assembly process allowed the creation of a SWNT array containing densely packed CdSe NQDs across a 2 mum gap between electrodes. Observations and characterization of the photocurrent, resistivity, gate dependence, and optical properties of these systems suggest efficient electron transfer from photoexcited NQDs to SWNTs.

Deterministic fabrication of carbon nanotube probes using the dielectrophoretic assembly and electrical detection.

We report the controlled dielectrophoretic assembly for the deterministic fabrication of carbon nanotube (CNT) probes. Electrical detection was applied to the dielectrophoretic assembly of CNT probes. Dielectrophoretic manipulation with an ac electric field of 5 MHz was used to form the CNT bridge across oppositely aligned tungsten tips (W-tips). A dc electric field was simultaneously applied to monitor the direct current flowing through the gap. The detected nanocurrent reveals that the CNT bridge is formed between W-tips in real time. We compared current data with bundle diameter of CNT probes in field emission scanning electron microscopy (FE-SEM) images. As the number of assembled CNTs increased, current was increased. With the obtained linear relationship, the number of the attached CNTs can be estimated without confirmation of the FE-SEM image. This combined use of the current detection method with dielectrophoretic manipulation will provide a reliable process for the fabrication of CNT probes.