Laser-Induced Crystalline-Phase Transformation for Hematite Nanorod Photoelectrochemical Cells.

Generally, a high-temperature postannealing process is required to enhance the photoelectrochemical (PEC) performance of hematite nanorod (NR) photoanodes. However, the thermal annealing time is limited to a short duration as thermal annealing at high temperatures can result in some critical problems, such as conductivity degradation of the fluorine-doped tin oxide film and deformation of the glass substrate. In this study, selective laser processing is introduced for hematite-based PEC cells as an alternative annealing process. The developed laser-induced phase transformation (LIPT) process yields hematite NRs with enhanced optical, chemical, and electrical properties for application in hematite NR-based PEC cells. Owing to its improved properties, the LIPT-processed hematite NR PEC cell exhibits an enhanced water oxidation performance compared to that processed by the conventional annealing process. As the LIPT process is conducted under ambient conditions, it would be an excellent alternative annealing technique for heat-sensitive flexible substrates in the future.

Fabrication of flexible transparent Ag square-shaped mesh electrode by top-flat nanosecond laser ablation.

We report a facile top-flat square nanosecond (ns) laser direct writing ablation technique in a thin silver film substrate to fabricate the silver square-shaped cell structure of flexible transparent electrodes. Square silver cell structures feature smooth surface morphology, excellent edge definition, mechanical stability, strong adhesion to the substrate, and favorable resistance and transparency. In particular, this strategy enables fabrication of a high square-shaped cell areal density (ablated square cell to the total area) Ag mesh, substantially improving transparency ($ {\gt} {85}\% $>85%) without considerably sacrificing conductivity ($ {\lt} {5}\;\Omega \;{{\rm sq}^{ - 1}}$<5Ωsq-1 unit of resistance). Consequently, the proposed metallic square-shaped structure shows compatibility with a polyethylene naphthalate flexible substrate for silver-based wearable electronic devices without any protective layer over the electrodes.

Design and validation of a ten nanosecond resolved resistive thermometer for Gaussian laser beam heating.

Pulsed laser processing plays a crucial role in additive manufacturing and nanomaterial processing. However, probing the transient temperature field during the pulsed laser interaction with the processed materials is challenging as it requires both high spatial and temporal resolution. Previous transient thermometry studies have measured neither sub-100 µm spatial resolution nor sub-10 ns temporal resolution. The temperature field induced by Gaussian laser beam profiles has also not been accounted for. Here, we demonstrate a 9 ns rise time, 50 µm sized Pt thin-film sensor for probing the temperature field generated by a nanosecond pulsed laser on a semiconductor thin film. The measurement error sources and associated improvements in the thin film fabrication, sensor patterning, and electrical circuitry are discussed. We carried out the first experimental and theoretical analysis of spatial resolution and accuracy for measuring a Gaussian pulse on the serpentine structure. Transparent silica and sapphire substrates, as well as 7-45 nm insulation layer thicknesses, are compared for sensing accuracy and temporal resolution. Finally, the measured absolute temperature magnitude is validated through the laser-induced melting of the 40 nm thick amorphous silicon film. Preliminary study shows its potential application for probing heat conduction among ultrathin films.

Nanowire-on-Nanowire: All-Nanowire Electronics by On-Demand Selective Integration of Hierarchical Heterogeneous Nanowires.

Exploration of the electronics solely composed of bottom-up synthesized nanowires has been largely limited due to the complex multistep integration of diverse nanowires. We report a single-step, selective, direct, and on-demand laser synthesis of a hierarchical heterogeneous nanowire-on-nanowire structure (secondary nanowire on the primary backbone nanowire) without using any conventional photolithography or vacuum deposition. The highly confined temperature rise by laser irradiation on the primary backbone metallic nanowire generates a highly localized nanoscale temperature field and photothermal reaction to selectively grow secondary branch nanowires along the backbone nanowire. As a proof-of-concept for an all-nanowire electronics demonstration, an all-nanowire UV sensor was successfully fabricated without using conventional fabrication processes.

Facile fabrication of a superhydrophobic cage by laser direct writing for site-specific colloidal self-assembled photonic crystal.

Micron-sized ablated surface structures with nano-sized 'bumpy' structures were produced by femtosecond (fs) laser ablation of polytetrafluoroethylene (PTFE) film under ambient conditions. Upon just a single step, the processed surface exhibited hierarchical micro/nano morphology. In addition, due to the tribological properties of PTFE, polydimethylsiloxane (PDMS) could be replicated from the laser-ablated PTFE surface without anti-adhesive surface treatment. By controlling the design of the ablated patterns, tunable wettability and superhydrophobicity were achieved on both PTFE and PDMS replica surfaces. Furthermore, using fs laser ablation direct writing, a flexible superhydrophobic PDMS cage formed by superhydrophobic patterns encompassing the unmodified region was demonstrated for aqueous droplet positioning and trapping. Through evaporation-driven colloidal self-assembly in this superhydrophobic cage, a colloidal droplet containing polystyrene (PS) particles dried into a self-assembled photonic crystal, whose optical band gap could be manipulated by the particle size.

Low-cost facile fabrication of flexible transparent copper electrodes by nanosecond laser ablation.

Low-cost Cu flexible transparent conducting electrodes (FTCEs) are fabricated by facile nanosecond laser ablation. The fabricated Cu FTCEs show excellent opto-electrical properties (transmittance: 83%, sheet resistance: 17.48 Ω sq(-1)) with outstanding mechanical durability. Successful demonstration of a touch-screen panel confirms the potential applicability of Cu FTCEs to the flexible optoelectronic devices.

Vacuum-free, maskless patterning of Ni electrodes by laser reductive sintering of NiO nanoparticle ink and its application to transparent conductors.

We introduce a method for direct patterning of Ni electrodes through selective laser direct writing (LDW) of NiO nanoparticle (NP) ink. High-resolution Ni patterns are generated from NiO NP thin films by a vacuum-free, lithography-free, and solution-processable route. In particular, a continuous wave laser is used for the LDW reductive sintering of the metal oxide under ambient conditions with the aid of reducing agents in the ink solvent. Thin (∼ 40 nm) Ni electrodes of glossy metallic surfaces with smooth morphology and excellent edge definition can be fabricated. By applying this method, we demonstrate a high transmittance (>87%), electrically conducting panel for a touch screen panel application. The resistivity of the Ni electrode is less than an order of magnitude higher compared to that of the bulk Ni. Mechanical bending test, tape-pull test, and ultrasonic bath test confirm the robust adhesion of the electrodes on glass and polymer substrates.