833wileyonlinelibrary.com fi lm solar cells, and smart windows. [ 4 ] The main disadvantage of ITO is its limited optical performance at very low sheet resistances. Moreover, the brittleness of the ceramic ITO fi lms can present a bottleneck in the fabrication of highly fl exible devices. [ 5 ] These disadvantages have motivated recent research efforts toward alternative material systems such as carbon nanotube [ 6 ] or silver nanowire (AgNW) networks, [ 7,8 ] metallized electrospun nanowires, [ 9,10 ] graphene layers, [ 11 ] ultrathin metal fi lms, [ 12 ] self-forming [ 13 ] or patterned metal grids. [14][15][16][17][18][19][20] Ideally, besides having very good electrical and optical performance, the new system should be low cost, fl exible and include direct patterning. The former two can be achieved by the additive solution-processing of silver nanowire networks that show remarkable fl exibility. [ 8 ] Depending on the application, this method however requires a post deposition structuring step. Direct patterning can be implemented with metal-wire grid electrodes when considering suitable printing technologies. While grids have been realized with nanoscale lines in several studies, the fabrication relied on subtractive multistep patterning methods such as imprinting, [ 14,15,20 ] lithography, [ 15 ] or evaporative self-assembly. [ 16 ] For microscale line widths, although not completely additive, an elegant method using selective laser sintering of a silver or nickel nanoparticle fi lm has been presented by Hong et al. [ 17 ] and Lee et al., [ 18 ] respectively. A direct ink writing approach of concentrated silver inks has been shown by Ahn et al., demonstrating linewidths around 5 µm. [ 19 ] TCE of very high performance have been demonstrated by electrospinning of polymer nanowires followed by the metal evaporation resulting in nanotrough networks of various metals. [ 9 ] A similar procedure was used to fabricate a network of copper wires about 1 µm in diameter that can be transferred onto a fi ner mesh of solution-deposited nanowires. [ 10 ] However, this interesting method is neither additive nor does it have the ability for direct patterning.Electrohydrodynamic (EHD) printing, the technique used in this work, has been applied as a viable additive and noncontact printing technique. Conventional additive printing methods such as screen printing or inkjet printing simply lack the resolution needed for invisible metal grid TCE applications.
Electrohydrodynamic NanoDrip Printing of High Aspect Ratio Metal Grid Transparent Electrodes
Atomically smooth hexagonal boron nitride (hBN) flakes have revolutionized two-dimensional (2D) optoelectronics. They provide the key substrate, encapsulant, and gate dielectric for 2D electronics while offering hyperbolic dispersion and quantum emission for photonics. The shape, thickness, and profile of these hBN flakes affect device functionality. However, researchers are restricted to simple, flat flakes, limiting next-generation devices. If arbitrary structures were possible, enhanced control over the flow of photons, electrons, and excitons could be exploited. Here, we demonstrate freeform hBN landscapes by combining thermal scanning-probe lithography and reactive-ion etching to produce previously unattainable flake structures with surprising fidelity. We fabricate photonic microelements (phase plates, grating couplers, and lenses) and show their straightforward integration, constructing a high-quality optical microcavity. We then decrease the length scale to introduce Fourier surfaces for electrons, creating sophisticated Moirépatterns for strain and bandstructure engineering. These capabilities generate opportunities for 2D polaritonics, twistronics, quantum materials, and deepultraviolet devices.
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