Micro-structured Arrays Research Articles

Wide-Spectrum Antireflective Properties of Germanium by Femtosecond Laser Raster-Type In Situ Repetitive Direct Writing Technique

A femtosecond laser raster-type in situ repetitive direct writing technique was used for the fabrication of anti-reflective microhole structures in Germanium (Ge) in the visible near-infrared range (300–1800 nm). This technique builds a layer of microstructured arrays on the surface of Ge, enabling Ge to exhibit excellent anti-reflective properties. The large-area micro-nanostructures of Ge were fabricated using femtosecond laser raster-type in situ repetitive direct writing. Ge microstructures are characterized by their structural regularity, high processing efficiency, high reproducibility, and excellent anti-reflective properties. Experimental test results showed that the average reflectance of the Ge microporous structure surface in the range of 300–1800 nm was 2.25% (the average reflectance of flat Ge was 41.5%), and the lowest reflectance was ~1.6%. This microstructure fabrication drastically reduced the optical loss of Ge, thus enhancing the photothermal utilization of Ge. The many nanoburrs and voids in the Ge microporous structure provided excellent hydrophobicity, with a hydrophobicity angle of up to 133 ± 2° (the hydrophobicity angle of flat Ge was 70 ± 2°). The high hydrophobicity angle allows for strong and effective self-cleaning performance. The femtosecond laser raster-type in situ repeatable direct writing technology has many desirable properties, including simplicity, high accuracy, flexibility, and repeatability, that make it one of the preferred choices for advanced manufacturing. The Ge micro-nanostructured arrays with excellent optical anti-reflective properties and hydrophobicity have become an attractive alternative to the current photo-thermal absorbers. It is expected to be used in many applications such as solar panels, photovoltaic sensors, and other optoelectronic devices.

Patterned immobilization of polyoxometalate-loaded mesoporous silica particles via amine-ene Michael additions on alkene functionalized surfaces

Polyoxometalates (POM) are anionic oxoclusters of early transition metals that are of great interest for a variety of applications, including the development of sensors and catalysts. A crucial step in the use of POM in functional materials is the production of composites that can be further processed into complex materials, e.g. by printing on different substrates. In this work, we present an immobilization approach for POMs that involves two key processes: first, the stable encapsulation of POMs in the pores of mesoporous silica nanoparticles (MSPs) and, second, the formation of microstructured arrays with these POM-loaded nanoparticles. Specifically, we have developed a strategy that leads to water-stable, POM-loaded mesoporous silica that can be covalently linked to alkene-bearing surfaces by amine-Michael addition and patterned into microarrays by scanning probe lithography (SPL). The immobilization strategy presented facilitates the printing of hybrid POM-loaded nanomaterials onto different surfaces and provides a versatile method for the fabrication of POM-based composites. Importantly, POM-loaded MSPs are useful in applications such as microfluidic systems and sensors that require frequent washing. Overall, this method is a promising way to produce surface-printed POM arrays that can be used for a wide range of applications.

Durability of micro diamond tools with different crystallographic planes

Micro diamond tools are indispensable for machining microstructured arrays. The cutting edge durability and consistency of micro diamond tools are the determinants of the microstructure quality and accuracy, in addition to the motion accuracy of the machine tool. A strength distribution model of the working area including the cutting edge and rake and flank faces was established considering diamond anisotropy and chip flow direction. Comprehensive wear resistances of micro diamond tools with different crystal orientation combinations were analyzed based on the model, and the wear prone areas of different tools were successfully predicted. The evolution processes of the sharpness and wear topography were monitored for every micro diamond tool in the micromachining experiments. The morphologies, profile errors and topological characteristic of the microstructures machined with different micro diamond tools with increasing cutting distance were analyzed. Finally, a conclusion was drawn that the wear resistances of the micro diamond tools in ascending order are Aγ{100}Aα{100}, Aγ{100}Aα{110}, Aγ{110}Aα{100}, and Aγ{110}Aα{110}. The three working areas of the Aγ{100}Aα{100} tool are prone to wear; in contrast, those of the Aγ{110}Aα{110}tool are resistant to wear. The tool wear of Aγ{100}Aα{110}is caused by flank face wear, and that of Aγ{110}Aα{100} is caused by rake face wear.

Subcellular Dynamic Immunopatterning of Cytosolic Protein Complexes on Microstructured Polymer Substrates.

Analysis of protein–protein interactions in living cells by protein micropatterning is currently limited to the spatial arrangement of transmembrane proteins and their corresponding downstream molecules. Here, we present a robust and straightforward method for dynamic immunopatterning of cytosolic protein complexes by use of an artificial transmembrane bait construct in combination with microstructured antibody arrays on cyclic olefin polymer substrates. As a proof, the method was used to characterize Grb2-mediated signaling pathways downstream of the epidermal growth factor receptor (EGFR). Ternary protein complexes (Shc1:Grb2:SOS1 and Grb2:Gab1:PI3K) were identified, and we found that EGFR downstream signaling is based on constitutively bound (Grb2:SOS1 and Grb2:Gab1) as well as on agonist-dependent protein associations with transient interaction properties (Grb2:Shc1 and Grb2:PI3K). Spatiotemporal analysis further revealed significant differences in stability and exchange kinetics of protein interactions. Furthermore, we could show that this approach is well suited to study the efficacy and specificity of SH2 and SH3 protein domain inhibitors in a live cell context. Altogether, this method represents a significant enhancement of quantitative subcellular micropatterning approaches as an alternative to standard biochemical analyses.

Frost spreading and pattern formation on microstructured surfaces.

Frost is found in nature as a symphony of nucleation and heat and mass transport, cascading from angstroms to several meters. Here, we use laser-induced fluorescence microscopy to investigate the pattern formation of frost growth in experiments which tune the mesoscopic length scale by using microstructured pillar arrays as a frost condenser surface. By controlling the degree of surface supercooling and the amount of condensate, different modes of frost patterning are uncovered, ranging from complete surface coverage to fractal-looking and limited-coverage structures of spiky appearance.

Icephobic performance of superhydrophobic coatings: A numerical analysis

Understanding the mechanism of cloud-sized particles impact and freezing is crucial to find viable solutions to prevent ice accumulation on critical aerodynamic surfaces such as aircraft wing or nacelle. It has been reported that superhydrophobic surfaces (SHS) have promising anti-icing properties due to their excellent water-repellent characteristics. However, due to the complexity of the freezing phenomenon on superhydrophobic surfaces, the anti-icing performance of such surfaces has not been fully understood. A multi-region multiphase flow solver including phase change has been developed to model the icing of a micro-droplet as it impinges on a superhydrophobic substrate with a given thickness, texture, and solid material thermal properties. The Navier-Stokes equation expressing the flow distribution of the liquid and the gas, coupled with the volume of fluid (VOF) method for tracking the liquid-gas and liquid-solid interfaces, was solved numerically using the finite volume methodology. The superhydrophobic morphology is modeled through a series of micro-structured arrays with squared cross-sectional pillars. As such, the thermal contact resistance is inherently accounted by the inclusion of air pockets underneath the micro-droplet. Consequently, the direct effect of surface topology and thermal properties on droplet maximum spreading diameter, penetration to the surface asperities, contact time, and the freezing onset have been investigated. Finally, icephobicity of two similar SHSs with various thermal properties were compared.

Ultraprecision machining of microlens arrays with integrated on-machine surface metrology.

Microlens arrays (MLAs) are increasingly applied in high-end photonics and imaging systems. Advanced diamond turning machining with tool servo technique is a superior method of fabricating micro-structured arrays with sub-micrometer form accuracy and nanometer surface finish. This paper proposes an innovative framework for ultraprecision machining of MLA. The established metrology-integrated machining platform consists of a 3-axis ultraprecision turning machine and a nanometric interferometric probe. On-machine surface measurement enables the in situ inspection and characterization of MLA features while preserving the consistency between the machining and measurement coordinate. The dedicated surface error characterization method and 3D corrective machining strategy are also presented. An experimental study is carried out in order to prove the proposed MLA characterization's and 3D corrective machining's effectiveness in improving the MLA surface accuracy.

Coercive field enhancement in microstructured (La0.4Pr0.6)0.67Ca0.33MnO3 thin films

The perovskite material (La0.4Pr0.6)0.67Ca0.33MnO3 (LPCMO) has complex electronic and magnetic behavior based on phase competition between ferromagnetic metallic (FMM) and insulating phases with similar free energies. Experimental evidence has indicated that in-plane stress anisotropy influences these phases and can affect electronic and magnetic properties. Here we investigate the roles that both stress and shape anisotropies may play in controlling the coercive field of the material. LPCMO thin films of various thicknesses (20, 25, and 30 nm) were deposited on (110) NdGaO3 (NGO) substrates using pulsed laser deposition and the coercive fields were measured. Photolithography was then used to fabricate microstructured arrays of LPCMO on the NGO substrates for each of the films. The coercive fields of these arrays of LPCMO were compared to the behavior of the corresponding unpatterned LPCMO thin films across a range of temperatures. Microstructure arrays for the thicker (25 and 30 nm) films showed a substantial increase in the coercive field after forming the arrays, whereas a thinner film (20 nm) showed almost no change in the coercive field. Stress anisotropy continues to play a dominant role in the behavior of LPCMO thin films and dimensionality of the magnetic domains also influences the results. The films show 2D behavior when film thickness approaches the size of the critical radius for single-to-multidomain transitions. Making thicker films allows for 3D behavior and a role for shape anisotropy to influence the coercive fields.

Anti-Reflectance Optimization of Secondary Nanostructured Black Silicon Grown on Micro-Structured Arrays.

Owing to its extremely low light absorption, black silicon has been widely investigated and reported in recent years, and simultaneously applied to various disciplines. Black silicon is, in general, fabricated on flat surfaces based on the silicon substrate. However, with three normal fabrication methods—plasma dry etching, metal-assisted wet etching, and femtosecond laser pulse etching—black silicon cannot perform easily due to its lowest absorption and thus some studies remained in the laboratory stage. This paper puts forward a novel secondary nanostructured black silicon, which uses the dry-wet hybrid fabrication method to achieve secondary nanostructures. In consideration of the influence of the structure’s size, this paper fabricated different sizes of secondary nanostructured black silicon and compared their absorptions with each other. A total of 0.5% reflectance and 98% absorption efficiency of the pit sample were achieved with a diameter of 117.1 μm and a depth of 72.6 μm. In addition, the variation tendency of the absorption efficiency is not solely monotone increasing or monotone decreasing, but firstly increasing and then decreasing. By using a statistical image processing method, nanostructures with diameters between 20 and 30 nm are the majority and nanostructures with a diameter between 10 and 40 nm account for 81% of the diameters.

High Speed Surface Illuminated Si Photodiode Using Microstructured Holes for Absorption Enhancements at 900–1000 nm Wavelength

A surface-illuminated silicon photodiode with both high speed and usable external quantum efficiency from 900 to 1000 nm wavelength is highly desirable for intra/inter data center Ethernet communications, high performance computing, and laser radar application. Such Si photodiodes have the potential for monolithic integration to CMOS integrated circuits which can significantly reduce the cost of data transmission per gigabit below one US dollar. To overcome silicon’s intrinsic weakness of absorption in these wavelengths, photon-trapping microstructured hole arrays are etched into the silicon surface, and the operational wavelengths of a high-speed silicon PIN photodiode are extended to 1000 nm. In this paper, the design and fabrication of such photon-trapping structures integrated into all-silicon photodiodes with significantly reduced absorption layer thicknesses to achieve high external quantum efficiency and fast response are presented. Different designs and geometries of the submicron holes on the sil...

3D micro-structured arrays of Zn\u039f nanorods

The fabrication of nanostructures with controlled assembly and architecture is very important for the development of novel nanomaterial-based devices. We demonstrate that laser techniques coupled with low-temperature hydrothermal growth enable complex three-dimensional ZnO nanorod patterning on various types of substrates and geometries. This methodology is based on a procedure involving the 3D scaffold fabrication using Multi-Photon Lithography of a photosensitive material, followed by Zn seeded Aqueous Chemical Growth of ZnO nanorods. 3D, uniformly aligned ZnO nanorods are produced. The increase in active surface area, up to 4.4 times in the cases presented here, provides a dramatic increase in photocatalytic performance, while other applications are also proposed.

Flexible electronic eardrum

Flexible mechanosensors with a high sensitivity and fast response speed may advance the wearable and implantable applications of healthcare devices, such as real-time heart rate, pulse, and respiration monitoring. In this paper, we introduce a novel flexible electronic eardrum (EE) based on single-walled carbon nanotubes, poly-ethylene, and poly-dimethylsiloxane with micro-structured pyramid arrays. The EE device shows a high sensitivity, high signal-to-noise ratio (approximately 55 dB), and fast response time (76.9 μs) in detecting and recording sound within a frequency domain of 20–13,000 Hz. The mechanism for sound detection is investigated and the sensitivity is determined using the micro-structure, thickness, and strain state. We also demonstrated that the device is able to distinguish human voices. This unprecedented performance of the flexible electronic eardrum has implications for many applications such as implantable acoustical bioelectronics and personal voice recognition.

Freezing the polarization of CH3NH3PbI3 and CH3NH3PbI3-xClx perovskite films

Thin films of methylammonium lead halides, CH3NH3PbI3 and CH3NH3PbI3-xClx, were deposited onto symmetrical microstructured electrode arrays of gold or platinum on Si/SiO2 wafers. Polarization studies were carried out on perovskite films under vacuum in the dark. For poling, a constant voltage was applied to the samples while the temperature was cycled between 295 K and 4 K. The measured current densities depending on the temperature showed distinct characteristics relating strongly to the crystal phase and the dielectric properties of the perovskite films. Voltage sweeps were carried out at different scan rates at specific temperature intervals after poling. The polarization of the films due to the migration of iodide vacancies in direction of the blocking perovskite/metal interface was frozen almost up to room temperature. Charge carriers were only able to cross the blocking barrier and contribute to the current where the ions have accumulated during poling. All J-V curves showed hysteresis: inverted and regular hysteresis at room temperature and below, respectively. Inverted hysteresis originates from the slow accumulation of ions at the blocking barrier, while regular hysteresis arises from a distortion in the adjacent crystals which will be discussed.

Neutral- and Multi-Colored Semitransparent Perovskite Solar Cells.

In this review, we summarize recent works on perovskite solar cells with neutral- and multi-colored semitransparency for building-integrated photovoltaics and tandem solar cells. The perovskite solar cells exploiting microstructured arrays of perovskite “islands” and transparent electrodes—the latter of which include thin metallic films, metal nanowires, carbon nanotubes, graphenes, and transparent conductive oxides for achieving optical transparency—are investigated. Moreover, the perovskite solar cells with distinctive color generation, which are enabled by engineering the band gap of the perovskite light-harvesting semiconductors with chemical management and integrating with photonic nanostructures, including microcavity, are discussed. We conclude by providing future research directions toward further performance improvements of the semitransparent perovskite solar cells.

Enhanced Light Extraction From Green Quantum Dot Light-Emitting Diodes by Attaching Microstructure Arrayed Films

In this study, microstructure arrayed films, such as microlens and microprism arrayed sheets, were introduced to the front side of quantum dot light-emitting diodes (QLEDs) to enhance the extraction efficiency of the QLEDs without fabricating complex micro/nano-structures directly on the substrate of the QLEDs. Additionally, the optical properties of the microlens and microprism film-attached QLEDs were compared with those of conventional QLEDs (without microstructures). Conventional QLEDs were fabricated as a reference device with gradient-thick shell green emitting CdSe@ZnS/ZnS quantum dots (QDs). The microlens-attached QLEDs have a high current efficiency (CE) of 31.3 cd/A, an external quantum efficiency (EQE) of 7.9% at 7 V, and a bright luminescence value of 99 350 cd/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 10 V. The CE, EQE, and luminescence values of the microlens-attached QLEDs are 1.47, 1.41, and 1.57 times higher than those of conventional QLEDs, respectively. The EL spectra and the color coordinates of the QLEDs only changed very slightly by adding microstructured arrays to the QLEDs.

Stability analysis of chemically modified mRNA using micropattern-based single-cell arrays.

The measurement of mRNA turnover in living cells plays an important role in the search for stable mRNA constructs for RNA-based therapies. Here we show that automated time-lapse microscopy combined with micropatterned arrays allows for efficient high-throughput monitoring of fluorescent reporter protein expression at the single-cell level. The fluorescence time courses after mRNA transfection yield the distribution of individual mRNA expression and degradation rates within a population. We compare mRNA constructs with combinations of 5' and 3' UTR sequences and find a systematic broadening and shift towards longer functional half-lives for UTR stabilized mRNA. At the same time the life time distribution of the destabilized EGFP reporter protein was found to be constant and narrowly distributed. Using mathematical modeling, we show that mRNA functional life-time predicts the time-integrated protein level, i.e. the area under the curve (AUC) of mRNA translation. Our approach paves the way for quantitative assessment of hitherto unexplored mRNA functional life time heterogeneity, possibly predicated on multiple mRNA secondary structures and its dependence on UTR sequences.

Fabrication of Silk Microstructures Using Photolithography

ABSTRACTPrecise spatial patterns and micro and nanostructures of peptides and proteins have widespread applications in tissue engineering, bioelectronics, photonics, and therapeutics. Optical lithography using proteins provides a route to directly fabricate intricate, bio-friendly architectures rapidly and across a range of length scales. The unique mechanical strength, optical properties, biocompatibility and controllable degradation of biomaterials from silkworms offer several advantages in this paradigm. Here, we present the biochemical synthesis and applications of a “protein photoresist” synthesized from the silk proteins, fibroin and sericin. Using light-activated direct-write processes such as photolithography, we show how silk proteins can form high resolution, high fidelity structures in two and three dimensions. Protein features can be precisely patterned at sub-microscale resolution (µm) at the bench-top over macroscale areas (cm), easily and repeatedly with high-throughput. For instance, periodic, microstructured arrays can be patterned over large areas to form structurally induced iridescent patterns and functional opto-electronic structures. We further demonstrate how photocrosslinked protein micro-architectures can function for the spatial guidance of cells without use of cell-adhesive ligands as biocompatible and biodegradable scaffolds. The ease of biochemical functionalization, biocompatibility, as well as favorable mechanical properties and biodegradation of this silk biomaterial provide opportunities for otherwise inaccessible applications as sustainable, bioresorbable protein microdevices.

Microstructured graphene arrays for highly sensitive flexible tactile sensors.

A highly sensitive tactile sensor is devised by applying microstructured graphene arrays as sensitive layers. The combination of graphene and anisotropic microstructures endows this sensor with an ultra-high sensitivity of -5.53 kPa(-1) , an ultra-fast response time of only 0.2 ms, as well as good reliability, rendering it promising for the application of tactile sensing in artificial skin and human-machine interface.

Development of a versatile capacitive tactile sensor based on transparent flexible materials integrating an excellent sensitivity and a high resolution

A versatile capacitive tactile sensor based on transparent flexible materials is developed in a simple and low-cost fabrication process. The sensor shows an excellent sensitivity (S=2.05 N-1), and is highly sensitive to the load as low as about 3 mN. Moreover, it exhibits a prominent resolution. The excellent device performance is attributed to the creative design of polydimethylsiloxane (PDMS) polymer layer, used as the structural material of the sensor, in which each sensing section acting as a sensor unit is a concave square with hemispheric micro-structured PDMS arrays. Meanwhile, other sections without any PDMS arrays serving as perfect natural wall-barriers can make each sensor unit separated effectively.

Contact-controlled amoeboid motility induces dynamic cell trapping in 3D-microstructured surfaces

On flat substrates, several cell types exhibit amoeboid migration, which is characterized by restless stochastic successions of pseudopod protrusions. The orientation and frequency of new membrane protrusions characterize efficient search modes, which can respond to external chemical stimuli as observed during chemotaxis in amoebae. To quantify the influence of mechanical stimuli induced by surface topography on the migration modes of the amoeboid model organism Dictyostelium discoideum, we apply high resolution motion analysis in microfabricated pillar arrays of defined density and geometry. Cell motion is analyzed by a two-state motility-model, distinguishing directed cellular runs from phases of isotropic migration that are characterized by randomly oriented cellular protrusions. Cells lacking myosin II or cells deprived of microtubules show significantly different behavior concerning migration velocities and migrational angle distribution, without pronounced attraction to pillars. We conclude that microtubules enhance cellular ability to react with external 3D structures. Our experiments on wild-type cells show that the switching from randomly formed pseudopods to a stabilized leading pseudopod is triggered by contact with surface structures. These alternating processes guide cells according to the available surface in their 3D environment, which we observed dynamically and in steady-state situations. As a consequence, cells perform “home-runs” in low-density pillar arrays, crawling from pillar to pillar, with a characteristic dwell time of ∼75 s. At the boundary between a flat surface and a 3D structured substrate, cells preferentially localize in contact with micropillars, due to the additionally available surface in the microstructured arrays. Such responses of cell motility to microstructures might open new possibilities for cell sorting in surface structured arrays.