Efficient Hydrogen Evolution by Mechanically Strained MoS₂ Nanosheets

We demonstrated correlations between mechanically bent tensile-strain-induced two-dimensional MoS₂ nanosheets (NSs) and their electrochemical activities toward the hydrogen evolution reaction (HER). The tensile-strain-induced MoS₂ NSs showed significantly steeper polarization curves and lower Tafel slopes than the strain-free ones, which is consistent with the simple d-band model. Furthermore, the mechanical strain increased the electrochemical activities of all the NSs toward the HER except those loaded with high MoS₂ mass. Mechanically bending MoS₂ NSs to induce tensile strain enables the production of powerful, efficient electrocatalysis systems for evolving hydrogen

Playing with Dimensions: Rational Design for Heteroepitaxial p–n Junctions

A design for a heteroepitaxial junction by the way of one-dimensional wurzite on a two-dimensional spinel structure in a low-temperature solution process was introduced, and it's capability was confirmed by successful fabrication of a diode consisting of p-type cobalt oxide (Co₃O₄) nanoplate/n-type zinc oxide (ZnO) nanorods, showing reasonable electrical performance. During thermal decomposition, the 30° rotated lattice orientation of Co₃O₄ nanoplates from the orientation of β-Co­(OH)₂ nanoplates was directly observed using high-resolution transmission electron microscopy. The epitaxial relations and the surface stress-induced ZnO nanowire growth on Co₃O₄ were well supported using the first-principles calculations. Over the large area, (0001) preferred oriented ZnO nanorods epitaxially grown on the (111) plane of Co₃O₄ nanoplates were experimentally obtained. Using this epitaxial p–n junction, a diode was fabricated. The ideality factor, turn-on voltage, and rectifying ratio of the diode were measured to be 2.38, 2.5 V and 10⁴, respectively

Playing with Dimensions: Rational Design for Heteroepitaxial p–n Junctions

A design for a heteroepitaxial junction by the way of one-dimensional wurzite on a two-dimensional spinel structure in a low-temperature solution process was introduced, and it's capability was confirmed by successful fabrication of a diode consisting of p-type cobalt oxide (Co₃O₄) nanoplate/n-type zinc oxide (ZnO) nanorods, showing reasonable electrical performance. During thermal decomposition, the 30° rotated lattice orientation of Co₃O₄ nanoplates from the orientation of β-Co­(OH)₂ nanoplates was directly observed using high-resolution transmission electron microscopy. The epitaxial relations and the surface stress-induced ZnO nanowire growth on Co₃O₄ were well supported using the first-principles calculations. Over the large area, (0001) preferred oriented ZnO nanorods epitaxially grown on the (111) plane of Co₃O₄ nanoplates were experimentally obtained. Using this epitaxial p–n junction, a diode was fabricated. The ideality factor, turn-on voltage, and rectifying ratio of the diode were measured to be 2.38, 2.5 V and 10⁴, respectively

Self-Seeded Growth of Poly(3-hexylthiophene) (P3HT) Nanofibrils by a Cycle of Cooling and Heating in Solutions

In spite of the recent successes in transistors and solar cells utilizing poly­(3-hexylthiophene) (P3HT) nanofibrils, systematic analysis on the growth kinetics has not been reported due to the lack of analytical tools. This study proposed a simple spectroscopic method to obtain the crystallinity of P3HT in solutions. On the basis of the analytical approach, we found that the crystallinity hysteresis upon temperature is a simple function of the solubility parameter difference (Δδ) between the P3HT and the solvents. When Δδ ≥ 0.7, a cooling (−20 °C)-and-heating (25 °C) process allowed the preparation of solutions including 1D crystal seeds dispersed in the solution. Simple coating of the seeded solutions completed the growth of the seeds into long nanofibrils at the early stage of the coating and thereby achieved almost 100% crystallinity in the resulting films without any postannealing process. The existence of PCBM for bulk-heterojunction (BHJ) solar cells did not affect the nucleation and growth of the nanofibrils during the cooling-and-heating process. The solar cells prepared from the solutions with Δδ ≥ 0.7 had solar conversion efficiencies higher than the conventional thermally annealed cells

Boron-Doped Peroxo-Zirconium Oxide Dielectric for High-Performance, Low-Temperature, Solution-Processed Indium Oxide Thin-Film Transistor

We developed a solution-processed indium oxide (In₂O₃) thin-film transistor (TFT) with a boron-doped peroxo-zirconium (ZrO₂:B) dielectric on silicon as well as polyimide substrate at 200 °C, using water as the solvent for the In₂O₃ precursor. The formation of In₂O₃ and ZrO₂:B films were intensively studied by thermogravimetric differential thermal analysis (TG-DTA), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FT IR), high-resolution X-ray diffraction (HR-XRD), and X-ray photoelectron spectroscopy (XPS). Boron was selected as a dopant to make a denser ZrO₂ film. The ZrO₂:B film effectively blocked the leakage current at 200 °C with high breakdown strength. To evaluate the ZrO₂:B film as a gate dielectric, we fabricated In₂O₃ TFTs on the ZrO₂:B dielectrics with silicon substrates and annealed the resulting samples at 200 and 250 °C. The resulting mobilities were 1.25 and 39.3 cm²/(V s), respectively. Finally, we realized a flexible In₂O₃ TFT with the ZrO₂:B dielectric on a polyimide substrate at 200 °C, and it successfully operated a switching device with a mobility of 4.01 cm²/(V s). Our results suggest that aqueous solution-processed In₂O₃ TFTs on ZrO₂:B dielectrics could potentially be used for low-cost, low-temperature, and high-performance flexible devices

Effects of Solution Temperature on Solution-Processed High-Performance Metal Oxide Thin-Film Transistors

Herein, we report a novel and easy strategy for fabricating solution-processed metal oxide thin-film transistors by controlling the dielectric constant of H₂O through manipulation of the metal precursor solution temperature. As a result, indium zinc oxide (IZO) thin-film transistors (TFTs) fabricated from IZO solution at 4 °C can be operated after annealing at low temperatures (∼250 °C). In contrast, IZO TFTs fabricated from IZO solutions at 25 and 60 °C must be annealed at 275 and 300 °C, respectively. We also found that IZO TFTs fabricated from the IZO precursor solution at 4 °C had the highest mobility of 12.65 cm²/(V s), whereas the IZO TFTs fabricated from IZO precursor solutions at 25 and 60 °C had field-effect mobility of 5.39 and 4.51 cm²/(V s), respectively, after annealing at 350 °C. When the IZO precursor solution is at 4 °C, metal cations such as indium (In³⁺) and zinc ions (Zn²⁺) can be fully surrounded by H₂O molecules, because of the higher dielectric constant of H₂O at lower temperatures. These chemical complexes in the IZO precursor solution at 4 °C are advantageous for thermal hydrolysis and condensation reactions yielding a metal oxide lattice, because of their high potential energies. The IZO TFTs fabricated from the IZO precursor solution at 4 °C had the highest mobility because of the formation of many metal–oxygen–metal (M-O-M) bonds under these conditions. In these bonds, the ns-orbitals of the metal cations overlap each other and form electron conduction pathways. Thus, the formation of a high proportion of M-O-M bonds in the IZO thin films is advantageous for electron conduction, because oxide lattices allow electrons to travel easily through the IZO

Highly Bendable Large-Area Printed Bulk Heterojunction Film Prepared by the Self-Seeded Growth of Poly(3-hexylthiophene) Nanofibrils

Applying conventional printing technologies to fabricate large-area flexible bulk heterojunction (BHJ) solar cells is of great interest. Achieving this task requires (i) large tolerance of the maximum photoconversion efficiency (PCE) to the film thickness, (ii) fast hole transport in both the thickness and lateral directions of the BHJ layer, and (iii) improved stability against bending and heat. This paper demonstrates that a P3HT:PCBM BHJ layer made of long P3HT nanofibrils of almost 100% crystallinity can be an excellent approach to achieve large-area printed solar cells. We applied a cool-and-heat (C&H) process with a P3HT/PCBM <i>m</i>-xylene solution to generate P3HT:PCBM nanofibril composite films. We found that the hole transport of the nanofibril composite was 2.6 times faster in the thickness direction and 6.5 times more conductive in the in-plane direction compared with conventionally annealed composites. The fast hole transport in the thickness direction led to negligible dependence of the PCE on the thickness of the composite layer. The improved conductivity in the in-plane direction prevented the sharp drop of the PCE as the active area increased. Taking advantage of the unique characteristics, we employed a roll-printing method to fabricate large-area unit solar cells in air. In addition, the curved contour path of the nanofibrils provided excellent stability against large bending strains, allowing the production of highly bendable organic solar cells