Random Nanostructures Research Articles

Enhancing extraction efficiency of quantum dot light-emitting diodes introducing a highly wrinkled ZnO electron transport layer.

Light extraction efficiency is crucial for achieving highly efficient and bright quantum dot light-emitting diodes (QLEDs), and current efforts toward introducing light outcoupling nanostructures always require complicated procedures. An extremely simple and efficient method to introduce light outcoupling nanostructures in the ZnO electron transport layer (ETL) is demonstrated by adopting a certain heating rate during the annealing process. The ultimate device exhibits a current efficiency of 9.1 cd/A, giving a 50% efficiency improvement compared to the control device with a flat ZnO ETL. This arises from the increased light extraction efficiency induced by random nanostructures formed on a wrinkled ZnO ETL, which could also be modulated by adjusting the heating rate during the annealing process. This study not only provides a simple and efficient method to introduce light outcoupling nanostructures, but also shows ample room for further performance enhancement of QLEDs with the guideline of light extraction.

Fabrication of mechanically enhanced superhydrophobic surface using nanosecond laser-based high-throughput surface nanostructuring (nHSN)

Abstract In this work, we conduct a systematic experimental investigation of surface integrity for steel and aluminum alloys processed via nanosecond laser-based high-throughput surface nanostructuring (nHSN) process. nHSN surface morphology and surface chemistry are first experimentally characterized, and the results show that surface nanostructure with proper surface chemistry is generated due to the combined effect of chemical etching and attachment of functional groups of low surface energy. Desired surface wetting behavior is achieved using a proper silane reagent during the nHSN chemical immersion treatment phase, while surface nanostructure can be finely modulated by adjusting the laser processing parameters. nHSN nanostructures with fluorosilane chemistry exhibit strong capability to repel water, leading to superhydrophobicity achieved on two important engineering metal alloys, namely steel and aluminium alloys. It is further experimentally demonstrated that the microhardness and anti-corrosion performance of the nHSN surface can be significantly enhanced compared with the untreated surface, indicating that the nHSN process is very effective for enhancing the mechanical strength and corrosion resistance of these important metal alloys. These results show that nHSN simultaneously creates random nanostructures, attains desirable surface chemistry and enhances surface integrity over large-area metal alloy surfaces.

Nanosecond laser-based high-throughput surface nanostructuring (nHSN)

We present a novel nanosecond laser-based high-throughput surface nanostructuring (nHSN) process that can simultaneously create random nanostructures and attain desirable surface chemistry over large-area metal alloy surfaces. nHSN consists of two sequential steps: (1) nanosecond laser texturing (NLT) and (2) chemical immersion treatment (CIT). NLT step in water confinement (wNLT) does not generate topological patterns but preconditions the metal surface chemically and mechanically. Our analysis shows that surface nanostructuring results from a combined effect of chemical etching and attachment of functional groups during the CIT phase of nHSN. A proper silane reagent can be selected for the CIT phase to achieve the desired surface wetting behavior, while laser parameters can also be adjusted during the NLT phase to finely tune the nanostructuring mechanism. nHSN nanostructures with fluorosilane chemistry repel water, while those with cyanosilane chemistry attract water. Extreme wettability including superhydrophobicity and superhydrophilicity is assessed for multiple engineering metal alloys including aluminum, steel and titanium alloys. Compared with existing ultrashort laser-based surface-texturing methods, the nHSN laser scan time represents a significant improvement in processing efficiency and enables a practical throughput for large-area processing of engineering alloys.

A Bidirectional Deep Neural Network for Accurate Silicon Color Design.

Silicon nanostructure color has achieved unprecedented high printing resolution and larger color gamut than sRGB. The exact color is determined by localized magnetic and electric dipole resonance of nanostructures, which are sensitive to their geometric changes. Usually, the design of specific colors and iterative optimization of geometric parameters are computationally costly, and obtaining millions of different structural colors is challenging. Here, a deep neural network is trained, which can accurately predict the color generated by random silicon nanostructures in the forward modeling process and solve the nonuniqueness problem in the inverse design process that can accurately output the device geometries for at least one million different colors. The key results suggest deep learning is a powerful tool to minimize the computation cost and maximize the design efficiency for nanophotonics, which can guide silicon color manufacturing with high accuracy.

Plasmonically Engineered Textile Polymer Solar Cells for High-Performance, Wearable Photovoltaics.

A practically applicable type of wearable polymer solar cells (PSCs) is presented with the enhanced performance by exploiting simply embodied, plasmonic nanostructures on a commercially available textile platform of optically opaque, geometrically uneven, and physically permeable woven fabrics that are commonly not compatible with organic photovoltaics. On a conformable fabric substrate preferentially processed with organic/inorganic multilayers for both planarization and encapsulation, the fabrication of top-illuminated, inverted type of PSCs with a transparent top electrode consisting of optimized dielectric/metal/dielectric multilayers is conducted, where a nanostructure of disorderly distributed elliptical hemispheres is implanted at an opaque bottom silver electrode by spin-coated silica nanoparticles in advance of depositing this electrode. The nanostructured bottom electrode promotes the light trapping effect at wavelengths of the surface plasmon resonance, as well as reduces the electrical Ohmic loss, thereby achieving a device with the power conversion efficiency of ∼8.71% at the given plasmonic device, where a net improvement of the efficiency is ∼1.46% compared to the planar device comprising otherwise same constituent layers. Systematic studies on optical properties and associated photovoltaic performance in experiments, together with analytic numerical modeling, allow quantitative understanding of the underlying physics, providing optimal rules for tailoring random nanostructures to the textile PSCs in the context of high-performance wearable photovoltaics.

54‐3: Bright Quantum Dots LEDs Enabled by Imprinted Random Nanostructures

Super‐bright quantum dots LEDs (QLEDs) are achieved by imprinting speckle image holography structures inside the devices. Red QLEDs with imprinted structures can reach up to a luminance of 146,000 Cd/cm2 at 8V, 1.76 times to that of control devices with planar architecture, setting a new brightness record for all‐solution processed inverted red QLEDs.

Theory for the spin dynamics in ultrathin disordered binary magnetic alloy films: Application to cobalt-gadolinium

Abstract We present the theory of a dynamic variant of the non-local coherent potential approximation (DNLCPA) to investigate the spin dynamics in ultrathin magnetic cobalt-gadolinium random alloy films Co 1 - c Gd c n , n atomic monolayers being their thickness. This novel theoretical approach in the classical spin wave representation introduces the idea for the scattering potential of a defect as an operator built up from the phase matching of the spin dynamics on the defect site with the spin dynamics in an otherwise virtual crystal. The scattering potentials are then used to establish a dynamic formulation of the CPA employing the Dyson’s formalism. A mathematical approach is rigorously developed to solve analytically the Dyson T-matrix equation, and determine the unique physical solution for the configurationally averaged Green’s function propagator of the disordered system. The DNLCPA numerical calculations for the Co 1 - c Gd c n systems yield the characteristic eigenmodes and energy dispersion curves of the confined spin wave excitations and their corresponding lifetimes, for arbitrary alloy concentration and diverse film thickness. Our theoretical results for the lifetimes of spin waves depend strongly on their wave vectors, and are in general agreement with recent experimental data. The developed DNLCPA theoretical method is general and can be applied to compute the spin dynamics for any magnetic random binary alloy nanostructure.

Optical characteristics of silicon recombination random nanostructures for solar cells

Abstract Material properties and surface morphology of nanostructures have impact on optical characteristics of nanostructures. More and more arrays of nanostructures are investigated to regulate and control surface radiation characteristics. However, the majority of the object surfaces are random rough surfaces in daily life, the radiation characteristics of which are widely used in various fields including remote sensing, photovoltaic utilization and thermal control. Therefore, the radiation characteristics of random rough surfaces are important for both academic research and practical applications. In this paper, a recombination random nanostructure consisting of conical structures and nanopores based on silicon substrate was prepared by plasma etching and chemical corrosion. The three-dimensional coordinate data of the recombination structure surface are obtained by atomic force microscopy, the radiation characteristics of the recombination random nanostructure is investigated by numerical calculation and experimental measurement, which has excellent antireflection characteristics over 300nm-1100nm. The influence of the incident angle and polarization state on the radiation characteristics are analysed.

Stabilizing color shift of tandem white organic light-emitting diodes

One drawback of white organic light-emitting diodes (WOLEDs) is the white angular dependence (WAD). Aimed to suppress the WAD of tandem WOLED to a negligible degree, we investigated the effect of optical resonance and probed various scattering structures. In a two-tandem configuration, the emissive layer physically remote from the reflective metal electrode is causing detrimental WAD. By combining an external nanoparticle based volumetric scattering film and an internal random nanostructure, it was possible to suppress the WAD of tandem WOLED to a value as low as 0.005, which is indistinguishable to human eyes.

Random antireflective nanostructuring on binary near-wavelength period gratings

Fresnel reflection losses have been reduced by the fabrication of random nanostructures on optical component surfaces, acting as an antireflective treatment. These structures have shown a broadband enhancement over a wide range of angle-of-incidence and polarization insensitivity. The random nanostructures were fabricated on pre-existing, near-wavelength period, fused silica binary gratings, with periods of 1.595 and 1.166 μm, respectively. The diffractive properties and the efficiency were measured and compared prior and postimplementation of the random nanostructures. The postprocessed gratings retained the diffractive properties of the original gratings, such as the propagating diffraction order angles, period, and the fill factor. A selectable multiwavelength He–Ne laser working at 594, 612, and 633 nm was used to test the gratings for both incident polarizations, S (TE) and P (TM). The data were collected at normal and Bragg incidence. The Fresnel reflection was suppressed by a factor of 10 postprocessing, resulting in a reflected intensity of 0.5% to 1.0% for 1.595-μm period grating and around 0.5% to 4% for 1.166-μm period grating. A 10% enhancement was observed for the combined transmission diffraction efficiency at S polarization for the 1.166-μm grating. Beam profiles of the diffracted light show no adverse changes due to the random nanostructuring on both the gratings. The results indicate that random nanostructures can be an antireflective treatment for pre-existing binary gratings, without adverse effects on the grating diffractive properties and beam profiles.

Analysis of light-extraction efficiency according to the size of random nanostructures for OLED

OLED emits light through the electroluminescence phenomenon (production of light by an organic com-pound in response to an electric current). Light generated in the organic material undergoes attenuation owing to reflection from the metal and light loss due to refraction while passing through the ITO and glass substrate, thereby reducing the efficiency and lifetime of the light extraction. In this study, the refraction of random nanostructures was used as a light-extraction technology to minimize light attenuation and extract more light outward. More-over, experiments were conducted to increase the light-extraction efficiency of the internal loss light with high light loss. To obtain optimal light-extraction conditions, a two-step simulation was performed according to the width and depth of the nanostructures. Based on the results, a sample glass-panel was fabricated, and the light-extraction efficiency was verified.

Impact of pore size, interconnections, and dynamic conductivity on the electrochemistry of vanadium pentoxide in well defined porous structures.

Considering the tortuous, random porous nanostructures existing in many battery electrodes, it is essential to understand electronic and ionic behaviors in such a confined nanoscale porous geometry in which electron and ion transports can change dynamically. Here, we have carefully designed three dimensional (3D) interconnected porous electrode structures and performed experiments to probe how the ion and electron transport is impacted within these controlled geometries. By using anodized aluminum oxide as a template, we were able to fabricate both 1D array electrodes and 3D electrodes with varying numbers of interconnections, utilizing vanadium oxide (V2O5) as the active material. We demonstrate that the inherent properties of the electrode material in combination with the structural properties of the electrodes can both positively and negatively impact electrochemical characteristics. Most notably, electrodes with seven interconnecting layers in their structure had 19.7% less capacity at 25C than electrodes with zero interconnecting layers, demonstrating the negative effect of interconnections combined with poor electronic conductivity of V2O5 upon lithiation beyond one Li insertion. These results indicate that a careful consideration of the material and structural properties is needed for the design of high performance battery systems.

Influence of laser parameters in surface texturing of Ti6Al4V and AA2024-T3 alloys

Laser texturing can be used for surface modification of metallic alloys in order to improve their properties under service conditions. The generation of textures is determined by the relationship between the laser processing parameters and the physicochemical properties of the alloy to be modified. In the present work the basic mechanism of dimple generation is studied in two alloys of technological interest, titanium alloy Ti6Al4V and aluminium alloy AA2024-T3.Laser treatment was performed using a pulsed solid state Nd: Vanadate (Nd: YVO4) laser with a pulse duration of 10 ps, operating at a wavelength of 1064 nm and 5 kHz repetition rate. Dimpled surface geometries were generated through ultrafast laser ablation while varying pulse energy between 1 µJ and 20 µJ/pulse and with pulse numbers from 10 to 200 pulses per spot. In addition, the generation of Laser Induced Periodic Surface Structures (LIPSS) nanostructures in both alloys, as well as the formation of random nanostructures in the impact zones are discussed.

Improved performance of deep-blue polymer light-emitting diodes by one-step coating self-assembly hole injection/transport nanocomposites with both the optical and electrical optimization

In this study we demonstrate an easy solution-processed highly efficient deep-blue polymer light-emitting diode (PLED) via a simple one-step coating of self-assembly hole injection/transport nanocomposites to achieve both a finer hole ohmic contact and an increased light outcoupling, which is the first time report about both the optical and electrical optimization without necessitating changes in the design or structure of the wide bandgap deep-blue PLEDs themselves. The contact angle and surface energy measurement results demonstrate that triazine-based hole injection molecules can vertically migrate towards the bottom PEDOT:PSS layer to obtain a stable minimum of free energy, resulting in an optimal top-to-bottom HOMO energy level arrangement and an improved hole mobility in deep-blue PLEDs. The random surface nanostructure was formed on top of the hole bilayer, leading to the enhancement of light outcoupling verified by transmittance, transmittance haze and light extraction efficiency. Furthermore, in order to explore the reasons of the hole light scattering formation process, a transient drying monitoring technique is applied to track the drying process of the nanostructure films, revealing this approach effectiveness by easily modifying mixing ratios for obtaining different light outcoupling abilities.

Photoacoustic ultrasound sources from diffusion-limited aggregates

Metallic diffusion-limited aggregate (DLA) films are well-known to exhibit near-perfect broadband optical absorption. We demonstrate that such films also manifest a substantial and relatively material-independent photoacoustic response, as a consequence of their random nanostructure. We theoretically and experimentally analyze the photoacoustic phenomena in DLA films and show that they can be used to create broadband air-coupled acoustic sources. These sources are inexpensive and simple to fabricate and work into the ultrasonic regime. We illustrate the device possibilities by building and testing an optically addressed acoustic phased array capable of producing virtually arbitrary acoustic intensity patterns in air.

Modeling of optimum light absorption in random plasmonic solar cell using effective medium theory

Random plasmonic nanostructures are very suitable candidates for light trapping in thin film solar cells because of their ability of efficient transportation and localization of light in a broad spectrum. In this work, besides the introducing of a novel structure of plasmonic thin-film solar cell, in which metal nanoparticles are randomly distributed through the photoactive layer of solar cell, we are presenting a new simple calculation method which can predict the behavior of plasmonic solar cells.To avoid the difficulty of analytical calculation and due to small size of constituents of the structure, we have used the effective medium theory to describe its optical properties. We have used a general description of effective dielectric function that can support each effective medium theory named spectral density theory, which takes into account the percolation of metal component and also interaction among inclusions. Using this method, the optimum values of nanoparticle's filling fraction for each wavelength within the active layer can be found where the solar cell can have the maximum absorption of light, thereupon the optimum external quantum efficiency.

Eigenanalysis of morphological diversity in silicon random nanostructures formed via resist collapse

This paper demonstrates eigenanalysis to quantitatively reveal the diversity and capacity of identities offered by the morphological diversity in silicon nanostructures formed via random collapse of resist. The analysis suggests that approximately 10^115 possible identities are provided per 0.18-um^2 area of nanostructures, indicating that nanoscale morphological signatures will be extremely useful for future information security applications where securing identities is critical. The eigenanalysis provides an intuitive physical picture and quantitative characterization of the diversity of structural fluctuations while unifying measurement stability concerns, which will be widely applicable to other materials, devices, and system architectures.

Evolution of film morphology in polymer solar cells based on rough electrode substrates

Periodic and random nanostructures as light-trapping management are extremely promising strategies to largely enhance light harvesting and efficiency of solar cell. However, solution-processed films deposited on nanostructured substrates often bring out uncontrollable morphology and lead to deficient interface. In this work, evolved multilayer film morphologies based on nanostructured electrode substrates were investigated in polymer solar cells. It was demonstrated that the different degree of rough front electrodes would significantly influence the morphologies of the following hole transport layer, active layer, and rear electrode layer. The morphological and optical properties of each layer on different rough front electrodes were compared in order to demonstrate the possibly evolved mechanism. A double light-trapping structure with a nano-textured front electrode and micro-textured rear electrode was realized to obtain the high-performance device.

Optical nano artifact metrics using silicon random nanostructures.

Nano-artifact metrics exploit unique physical attributes of nanostructured matter for authentication and clone resistance, which is vitally important in the age of Internet-of-Things where securing identities is critical. However, expensive and huge experimental apparatuses, such as scanning electron microscopy, have been required in the former studies. Herein, we demonstrate an optical approach to characterise the nanoscale-precision signatures of silicon random structures towards realising low-cost and high-value information security technology. Unique and versatile silicon nanostructures are generated via resist collapse phenomena, which contains dimensions that are well below the diffraction limit of light. We exploit the nanoscale precision ability of confocal laser microscopy in the height dimension; our experimental results demonstrate that the vertical precision of measurement is essential in satisfying the performances required for artifact metrics. Furthermore, by using state-of-the-art nanostructuring technology, we experimentally fabricate clones from the genuine devices. We demonstrate that the statistical properties of the genuine and clone devices are successfully exploited, showing that the liveness-detection-type approach, which is widely deployed in biometrics, is valid in artificially-constructed solid-state nanostructures. These findings pave the way for reasonable and yet sufficiently secure novel principles for information security based on silicon random nanostructures and optical technologies.

Enhancement of two-photon photoluminescence and SERS for low-coverage gold films.

Electromagnetic field enhancement (FE) effects occurring in thin gold films 3-12-nm are investigated with two-photon photoluminescence (TPL) and Raman scanning optical microscopies. The samples are characterized using scanning electron microscopy images and linear optical spectroscopy. TPL images exhibit a strong increase in the level of TPL signals for films thicknesses 3-8-nm, near the percolation threshold. For some thicknesses, TPL measurements reveal super-cubic dependences on the incident power. We ascribe this feature to the occurrence of very strongly localized and enhanced electromagnetic fields due to multiple light scattering in random nanostructures that might eventually lead to white-light generation. Raman images exhibit increasing Raman signals when decreasing the film thickness from 12 to 6-nm and decreasing signal for the 3-nm-film. This feature correlates with the TPL observations indicating that highest FE is to be expected near the percolation threshold.