Fluidic self-assembly for MicroLED displays by controlled viscosity.

Displays in which arrays of microscopic 'particles', or chiplets, of inorganic light-emitting diodes (LEDs) constitute the pixels, termed MicroLED displays, have received considerable attention1,2 because they can potentially outperform commercially available displays based on organic LEDs3,4 in terms of power consumption, colour saturation, brightness and stability and without image burn-in issues1,2,5-7. To manufacture these displays, LED chiplets must be epitaxially grown on separate wafers for maximum device performance and then transferred onto the display substrate. Given that the number of LEDs needed for transfer is tremendous-for example, more than 24 million chiplets smaller than 100 µm are required for a 50-inch, ultra-high-definition display-a technique capable of assembling tens of millions of individual LEDs at low cost and high throughput is needed to commercialize MicroLED displays. Here we demonstrate a MicroLED lighting panel consisting of more than 19,000 disk-shaped GaN chiplets, 45 µm in diameter and 5 µm in thickness, assembled in 60 s by a simple agitation-based, surface-tension-driven fluidic self-assembly (FSA) technique with a yield of 99.88%. The creation of this level of large-scale, high-yield FSA of sub-100-µm chiplets was considered a significant challenge because of the low inertia of the chiplets. Our key finding in overcoming this difficulty is that the addition of a small amount of poloxamer to the assembly solution increases its viscosity which, in turn, increases liquid-to-chiplet momentum transfer. Our results represent significant progress towards the ultimate goal of low-cost, high-throughput manufacture of full-colour MicroLED displays by FSA.

Scattering of surface plasmon polaritons at a planar interface by an embedded dielectric nanocube.

We investigate scattering of surface plasmon polaritons (SPPs) at a planar metal-dielectric interface by a dielectric nanocube embedded in the metal layer using finite element method-based simulations. The scattering characteristics of the embedded nanocube, such as the scattering and absorption cross sections, far-field scattering patterns, reflectance, and transmittance, are calculated as functions of the wavelength of the incident SPP waves in the visible range. The main features of the characteristics are explained in connection with the excitation of plasmonic eigenmodes of the embedded nanocube. The most efficient scattering into waves propagating away from the metal surface, i.e., the radiating modes, occurs when a dipolar-like plasmonic mode is excited, whose eigenfrequency can be tuned by changing the edge length of the nanocube.

Babinet-inverted optical Yagi-Uda antenna for unidirectional radiation to free space.

Nanophotonics capable of directing radiation or enhancing quantum-emitter transition rates rely on plasmonic nanoantennas. We present here a novel Babinet-inverted magnetic-dipole-fed multislot optical Yagi-Uda antenna that exhibits highly unidirectional radiation to free space, achieved by engineering the relative phase of the interacting surface plasmon polaritons between the slot elements. The unique features of this nanoantenna can be harnessed for realizing energy transfer from one waveguide to another by working as a future "optical via".

Dry-spray deposition of TiO2 for a flexible dye-sensitized solar cell (DSSC) using a nanoparticle deposition system (NPDS).

TiO2 powders were deposited on indium tin oxide (ITO) coated polyethylene terephthalate (PET) substrates for application to the photoelectrode of a dye-sensitized solar cell (DSSC). In the conventional DSSC manufacturing process, a semiconductor oxide such as TiO2 powder requires a sintering process at higher temperature than the glass transition temperature (T(g)) of polymers, and thus utilization of flexible polymer substrates in DSSC research has been constrained. To overcome this restriction related to sintering, we used a nanoparticle deposition system (NPDS) that could produce a thin coating layer through a dry-spray method under atmospheric pressure at room temperature. The powder was sprayed through a slit-type nozzle having a 0.4 x 10 mm2 rectangular outlet. In order to determine the deposited TiO2 thickness, five kinds of TiO2 layered specimens were prepared, where the specimens have single and double layer structures. Deposited powders on the ITO coated PET substrates were observed using FE-SEM and a scan profiler The thicker TiO2 photoelectrode with a DSSC having a double layer structure showed higher energy efficiency than the single layer case. The highest fabricated flexible DSSC displayed a short circuit current density J(sc) = 1.99 mA cm(-2), open circuit voltage V(oc) = 0.71 V, and energy efficiency eta = 0.94%. These results demonstrate the possibility of utilizing the dry-spray method to fabricate a TiO2 layer on flexible polymer substrates at room temperature under atmospheric pressure.

Controlled Enhancement in Hole Injection at Gold-Nanoparticle-on-Organic Electrical Contacts Fabricated by Spark-Discharge Aerosol Technique.

We demonstrate that hole injection from a top electrode composed of Au nanoparticles (AuNPs) capped with a thick Au layer into an underlying organic semiconductor, N, N'-diphenyl- N, N'-bis-[4-(phenyl- m-tolyl-amino)-phenyl]-biphenyl-4,4'-diamine (DNTPD), is significantly enhanced compared to that in a control device whose top electrode is composed entirely of a thick Au layer. The fabrication of this organic hole-only device with the AuNP electrode is made possible by dry, room-temperature distribution of AuNPs onto DNTPD using a spark-discharge aerosol technique capable of varying the average diameter ( D̅) of the AuNPs. The enhancement in hole injection is found to increase with decreasing D̅, with the current density of a device with D̅ = 1.1 nm being more than 3 orders of magnitude larger than that of the control device. Intensity-modulated photocurrent measurements show that the built-in potentials of the devices with the AuNP electrode are smaller than that of the control device by as much as 0.68 V, indicating that the enhanced hole injection originates from the increased work functions of these devices, which in turn decreases the hole injection barrier heights. X-ray photoelectron spectroscopy reveals that the increased work functions of the AuNP electrodes are due to surface oxidation of the AuNPs resulting in AuN and Au3N. The degree of oxidation of the AuNPs increases with decreasing D̅, consistent with the D̅-dependencies of the hole injection enhancement and the built-in potential reduction.