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.

Concurrent self-assembly of RGB microLEDs for next-generation displays.

MicroLED displays have been in the spotlight as the next-generation displays owing to their various advantages, including long lifetime and high brightness compared with organic light-emitting diode (OLED) displays. As a result, microLED technology1,2 is being commercialized for large-screen displays such as digital signage and active R&D programmes are being carried out for other applications, such as augmented reality3, flexible displays4 and biological imaging5. However, substantial obstacles in transfer technology, namely, high throughput, high yield and production scalability up to Generation 10+ (2,940 × 3,370 mm2) glass sizes, need to be overcome so that microLEDs can enter mainstream product markets and compete with liquid-crystal displays and OLED displays. Here we present a new transfer method based on fluidic self-assembly (FSA) technology, named magnetic-force-assisted dielectrophoretic self-assembly technology (MDSAT), which combines magnetic and dielectrophoresis (DEP) forces to achieve a simultaneous red, green and blue (RGB) LED transfer yield of 99.99% within 15 min. By embedding nickel, a ferromagnetic material, in the microLEDs, their movements were controlled by using magnets, and by applying localized DEP force centred around the receptor holes, these microLEDs were effectively captured and assembled in the receptor site. Furthermore, concurrent assembly of RGB LEDs were demonstrated through shape matching between microLEDs and receptors. Finally, a light-emitting panel was fabricated, showing damage-free transfer characteristics and uniform RGB electroluminescence emission, demonstrating our MDSAT method to be an excellent transfer technology candidate for high-volume production of mainstream commercial products.

Distinguishing Zigzag and Armchair Edges on Graphene Nanoribbons by X-ray Photoelectron and Raman Spectroscopies.

Graphene nanoribbons (GNRs) have recently emerged as alternative 2D semiconductors owing to their fascinating electronic properties that include tunable band gaps and high charge-carrier mobilities. Identifying the atomic-scale edge structures of GNRs through structural investigations is very important to fully understand the electronic properties of these materials. Herein, we report an atomic-scale analysis of GNRs using simulated X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Tetracene with zigzag edges and chrysene with armchair edges were selected as initial model structures, and their XPS and Raman spectra were analyzed. Structurally expanded nanoribbons based on tetracene and chrysene, in which zigzag and armchair edges were combined in various ratios, were then simulated. The edge structures of chain-shaped nanoribbons composed only of either zigzag edges or armchair edges were distinguishable by XPS and Raman spectroscopy, depending on the edge type. It was also possible to distinguish planar nanoribbons consisting of both zigzag and armchair edges with zigzag/armchair ratios of 4:1 or 1:4, indicating that it is possible to analyze normally synthesized GNRs because their zigzag to armchair edge ratios are usually greater than 4 or less than 0.25. Our study on the precise identification of GNR edge structures by XPS and Raman spectroscopy provides the groundwork for the analysis of GNRs.

Investigation of Thermally Induced Degradation in CH3NH3PbI3 Perovskite Solar Cells using In-situ Synchrotron Radiation Analysis.

In this study, we employ a combination of various in-situ surface analysis techniques to investigate the thermally induced degradation processes in MAPbI3 perovskite solar cells (PeSCs) as a function of temperature under air-free conditions (no moisture and oxygen). Through a comprehensive approach that combines in-situ grazing-incidence wide-angle X-ray diffraction (GIWAXD) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) measurements, we confirm that the surface structure of MAPbI3 perovskite film changes to an intermediate phase and decomposes to CH3I, NH3, and PbI2 after both a short (20 min) exposure to heat stress at 100 °C and a long exposure (>1 hour) at 80 °C. Moreover, we observe clearly the changes in the orientation of CH3NH3+ organic cations with respect to the substrate in the intermediate phase, which might be linked directly to the thermal degradation processes in MAPbI3 perovskites. These results provide important progress towards improved understanding of the thermal degradation mechanisms in perovskite materials and will facilitate improvements in the design and fabrication of perovskite solar cells with better thermal stability.

In Situ Spectroscopic and Computational Studies on a MnO2-CuO Catalyst for Use in Volatile Organic Compound Decomposition.

In situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory calculations were conducted to demonstrate the decomposition mechanism of propylene glycol methyl ether acetate (PGMEA) on a MnO2-CuO catalyst. The catalytic activity of MnO2-CuO was higher than that of MnO2 at low temperatures, although the pore properties of MnO2 were similar to those of MnO2-CuO. In addition, whereas the chemical state of MnO2 remained constant following PGMEA dosing at 150 °C, MnO2-CuO was reduced under identical conditions, as confirmed by in situ NEXAFS spectroscopy. These results indicate that the presence of Cu in the MnO2-CuO catalyst enables the release of oxygen at lower temperatures. More specifically, the released oxygen originated from the Mn-O-Cu moiety on the top layer of the MnO2-CuO structure, as confirmed by calculation of the oxygen release energies in various oxygen positions of MnO2-CuO. Furthermore, the spectral changes in the in situ NEXAFS spectrum of MnO2-CuO following the catalytic reaction at 150 °C corresponded well with those of the simulated NEXAFS spectrum following oxygen release from Mn-O-Cu. Finally, after the completion of the catalytic reaction, the quantities of lactone and ether functionalities in PGMEA decreased, whereas the formation of C=C bonds was observed.

Investigation of out-of-plane structural properties of a graphene monolayer with gap-plasmons: mode-selective Raman enhancement and the influence of additional sp(3) type defects.

We report that Raman enhancements of a graphene monolayer sandwiched at the Au nanoparticle-Au thin film junction are different and can be attributed to the influence of a z-polarized incident field. Closer to the center of the junction, radial breathing like-mode (RBLM) shows dramatic Raman enhancement in terms of the coincidence between the z-polarized incident field formed at the junction and the RBLM phonon axis. The appearance of an additional D* peak can be identified and is attributed to the additional out-of-plane sp(3) type defect signal. Correlating I(D*)/I(D) with RBLM intensity variation further substantiates that the observed D* peak is ascribed to another out-of-plane structural defect signal.

Exploring the relative bending of a CVD graphene monolayer with gap-plasmons.

We report a spectroscopic indicator showing the bending of a chemical vapor deposition (CVD) graphene monolayer on Cu foil or an arbitrary substrate after transfer. Using a Au nanoparticle (NP)-graphene monolayer-Au thin film (TF) junction system, the Radial Breathing-Like Mode (RBLM) Raman signal from the sandwiched graphene monolayer is evidently observed by employing a local z-polarized incident field formed at the Au NP-Au TF junction. We also utilized the RBLM intensity as a quantitative tool with a wide dynamic range (∼300%) compared to the 2D peak width (∼35%) for determining the relative degree of bending on the Au TF substrate. The RBLM signal from the CVD graphene monolayer is anticipated to be used as a valuable marker in exploring out-of-plane directional properties.

Functional group-selective adsorption using scanning tunneling microscopy.

In this study, we selectively enhanced two types of adsorption of 3-mercaptoisobutyric acid on a Ge(100) surface by using the tunneling electrons from an STM and the catalytic effect of an STM tip. 3-Mercaptoisobutyric acid has two functional groups: a carboxylic acid group at one end of the molecule and a thiol group at the other end. It was found that the adsorption occurring through the carboxylic acid group was selectively enhanced by the application of electrons tunneling between an STM tip and the surface. Using this enhancement, it was possible to make thiol group-terminated surfaces at any desired location. In addition, via the use of a tungsten STM tip coated with a tungsten oxide (WO(3)) layer, we selectively catalyzed the adsorption through the thiol group. Using this catalysis, it was possible to generate carboxylic acid group-terminated surfaces at any desired location. This functional group-selective adsorption using STM could be applied in positive lithographic methods to produce semiconductor substrates terminated by desired functional groups.

Molecular seesaw: a three-way motion and motion-induced surface modification.

We introduce a three-way molecular motion which can be a suitable switching system in future molecule-based nanocircuits. A real-space investigation revealed that vinylferrocene adsorbs site-specifically on the Ge(100) surface and then shows a reversible tilting motion, similar to a seesaw. Unlike conventional molecular motions, it not only has three stable switching states at room temperature but also shows a motion-induced surface-structure modification, allowing surface-mediated signal transmission. Demonstrated STM-tip influence on the motion allows the feasibility of tip-induced manipulation.

STM tip catalyzed adsorption of thiol molecules at the nanometer scale.

The tungsten oxide covered tungsten (W) tip of a scanning tunneling microscope was found to act as a catalyst to catalyze the S-H dissociative adsorption of phenylthiol and 1-octanethiol molecules onto a Ge(100) surface. By varying the distance between the tip and the surface, the area of the tip-catalyzed adsorption could be controlled. We have found that the thiol headgroup is the critical functional group for this catalysis and the catalytic material is the tungsten oxide layer of the tip. This local tip-catalyzed adsorption may be used in positive lithographic methods to produce nanoscale patterning on semiconductor substrates.