Applications of UV-nanoimprint soft stamps in fabrication of single-frequency diode lasers

We show how to use a modified poly-dimethyl-siloxane (PDMS) soft stamp to reduce pattern deformation and residual layer thickness in soft UV-nanoimprint lithography. A soft stamp thinned with toluene reduces the residual layer of a resist by as much as 50% compared to an unthinned stamp. We apply the soft UV-nanoimprint to prepare nanopatterned waveguides for a single-frequency diode laser. This laser operates with a side-mode suppression ratio of 50 dB, which indicates that the patterns are precise and uniform over the whole imprint field. To the best of our knowledge, this is the first single-frequency laser fabricated by soft UV-nanoimprint technology.     

Strain Reduction in Selectively Grown CdTe by MBE on Nanopatterned Silicon on Insulator (SOI) Substrates

Silicon-based substrates for the epitaxy of HgCdTe are an attractive low-cost choice for monolithic integration of infrared detectors with mature Si technology and high yield. However, progress in heteroepitaxy of CdTe/Si (for subsequent growth of HgCdTe) is limited by the high lattice and thermal mismatch, which creates strain at the heterointerface that results in a high density of dislocations. Previously we have reported on theoretical modeling of strain partitioning between CdTe and Si on nanopatterned silicon on insulator (SOI) substrates. In this paper, we present an experimental study of CdTe epitaxy on nanopatterned (SOI). SOI (100) substrates were patterned with interferometric lithography and reactive ion etching to form a two-dimensional array of silicon pillars with ～250 nm diameter and 1 μm pitch. MBE was used to grow CdTe selectively on the silicon nanopillars. Selective growth of CdTe was confirmed by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Coalescence of CdTe on the silicon nanoislands has been observed from the SEM characterization. Selective growth was achieved with a two-step growth process involving desorption of the nucleation layer followed by regrowth of CdTe at a rate of 0.2 Å s^?1. Strain measurements by Raman spectroscopy show a comparable Raman shift (2.7 ± 2 cm^?1 from the bulk value of 170 cm^?1) in CdTe grown on nanopatterned SOI and planar silicon (Raman shift of 4.4 ± 2 cm^?1), indicating similar strain on the nanopatterned substrates.     

Nanopatterned Nafion Microelectrode Arrays for In Vitro Cardiac Electrophysiology

In this study, nanopatterned Nafion microelectrode arrays for in vitro cardiac electrophysiology are reported. With the aim of defining sophisticated Nafion nanostructures with highly ionic conductivity, fabrication parameters such as Nafion concentration and curing temperature are optimized. By increasing curing temperature and Nafion concentration, the replication fidelity of Nafion nanopatterns when copied from a polydimethylsiloxane master mold are controlled. It is also found that cross‐sectional morphology and ion current density of nanopatterned Nafion strongly depends on the fabrication parameters. To investigate this dependency, current‐voltage analysis is conducted using organic electrochemical transistors overlaid with patterned Nafion substrates. Nanopatterned Nafion is found to allow higher ion current densities than unpatterned surfaces. Furthermore, higher curing temperatures are found to render Nafion layers with higher ion/electrical transfer properties. To optimize nanopattern dimensions, electrical current flows, and film uniformity, a final configuration consisting of 5% nanopatterned Nafion cured at 65 °C is chosen. Microelectrode arrays (MEAs) are then covered with optimized Nafion nanopatterns and used for electrophysiological analysis of two types of induced pluripotent stem cell‐derived cardiomyocytes (iPSCs‐CMs). These data highlight the suitability of nanopatterned Nafion, combined with MEAs, for enhancing the cellular environment of iPSC‐CMs for use in electrophysiological analysis in vitro.     

Tailoring of the plasmonic and waveguide effect in bulk-heterojunction photovoltaic devices with ordered,nanopatterned structures

Various nano-patterned bulk-heterojunction (BHJ) films with different diameters and pitches were fabricated by a stamping method to tailor the plasmonic effect. The nanopatterned BHJ active layers exhibit regular-ordered embossing structures, which were confirmed by the surface morphological analysis with SEM and AFM. The simulation results confirm that devices with nanopatterned BHJ film with a diameter/pitch of 265/400 nm exhibit a strong improvement in E-field distribution intensity due to the combination of the plasmonic and waveguide modes compared to devices without a nanopattern, with 150/400 nm, or with 265/800 nm, which led to increased J_SC and cell efficiency in J–V curves under solar light illumination. The optimized plasmonic effect plays an important role in the light harvesting of BHJ devices.     

An Efficient Triple-Tandem Polymer Solar Cell

We present an efficient triple-tandem polymer solar cell with identical poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6, 6)C₆₁ (PCBM) bulk heterojunction as the active layers and highly transparent Al (1 nm)/ MoO₃ (15 nm) as the intermediate layer. This intermediate layer is structurally smooth as characterized by atomic force microscopy. Although identical organic active layers are used to construct such triple-tandem cell, a tripled open-circuit voltage of 1.73 V and power conversion efficiency of 2.03% are obtained under simulated solar irradiation of 100 mW/cm² (AM1.5), demonstrating a viable technique for fabricating triple-tandem polymer cell with the intermediate layer of Al/MoO₃.     

Visualization of UV by Nanopatterned Down‐Shifting Materials Mimicking Human Retinal Cone Cells

The conversion of invisible ultraviolet (UV) light to visible light by down‐shifting (DS) materials has a variety of important applications in the fields of optoelectronics and photonics. The ability to control emission colors as a function of the wavelength of incident UV light would significantly advance scientific research and technological applications. A novel strategy for UV visualization is demonstrated that employs nanoimprint lithography combined with a sol–gel process. The principles of trichromacy of human vision are applied; three DS materials sensitive to three different ranges of UV light are nanopatterned to mimic the three types of cone cells in the human retina. Each DS material then emits a distinctive color that can be recognized by each type of cone cells for visualization. The nanopatterned structure significantly intensifies the light emission by Mie scattering and spatially separates the three DS materials, thereby minimizing unwanted optical interference among them. The deliberately designed triple‐nanopatterned DS materials exhibit various emission colors ranging from green, to orange, to pink depending on the wavelength of the incident UV light. The current work would contribute to the development of novel strategies for multicolor tunable emission that may lead to innovative applications.     

Photoluminescent Arrays of Nanopatterned Monolayer MoS2

Monolayer 2D MoS₂ grown by chemical vapor deposition is nanopatterned into nanodots, nanorods, and hexagonal nanomesh using block copolymer (BCP) lithography. The detailed atomic structure and nanoscale geometry of the nanopatterned MoS₂ show features down to 4 nm with nonfaceted etching profiles defined by the BCP mask. Atomic resolution annular dark field scanning transmission electron microscopy reveals the nanopatterned MoS₂ has minimal large‐scale crystalline defects and enables the edge density to be measured for each nanoscale pattern geometry. Photoluminescence spectroscopy of nanodots, nanorods, and nanomesh areas shows strain‐dependent spectral shifts up to 15 nm, as well as reduction in the PL efficiency as the edge density increases. Raman spectroscopy shows mode stiffening, confirming the release of strain when it is nanopatterned by BCP lithography. These results show that small nanodots (≈19 nm) of MoS₂ 2D monolayers still exhibit strong direct band gap photoluminescence (PL), but have PL quenching compared to pristine material from the edge states. This information provides important insights into the structure–PL property correlations of sub‐20 nm MoS₂ structures that have potential in future applications of 2D electronics, optoelectronics, and photonics.     

Gold nanorod enhanced organic photovoltaics: The importance of morphology effects

Organic photovoltaic devices with a 30% improvement in power conversion efficiency are achieved when gold nanorods (Au NR) are incorporated into the active bulk heterojunction (BHJ) layer. Detailed analysis of the system is provided through microscopy, device characterization, and spectroscopy, demonstrating that the enhancement effects are predominantly caused by induced morphology changes in the BHJ film rather than plasmonic effects. Wide angle X-ray diffraction provides evidence that the nanorods loaded into the BHJ film have an effect on polymer crystal orientation, leading to a systematic performance increase in the devices as a result of both internal and external efficiency improvements.     

Thick‐Film Organic Solar Cells Achieving over 11% Efficiency and Nearly 70% Fill Factor at Thickness over 400 nm

Thickness‐insensitive small molecule acceptors (SMAs) are still a great challenge for developing thick‐film organic solar cells (OSCs) towards practical use. Herein, two SMAs, MF1 and MF2, are designed and synthesized by employing a bifunctional end group with fluorine and methyl moieties. Combined with fused‐ring cores with alkyl side chains, both MF1 and MF2 exhibit ordered π–π stacking and high charge carrier mobilities in neat and blend films. The champion devices based on PM7:MF1 and PM7:MF2 deliver high power conversion efficiencies (PCEs) of 12.4% and 13.7%, and high fill factors (FFs) of 78.3% and 74.5%, respectively. With increasing active layer thickness, the FFs of the OSCs decrease relatively slowly, demonstrating the preferrable properties of MF1 and MF2 in terms of their thickness insensitivity, especially for MF1. As a result, the two thick‐film OSCs achieve over 11% PCEs at an active layer thickness over 400 nm (an FF close to 70% for PM7:MF1) and over 10% PCEs when the thickness is increased up to 500 nm. These are the highest PCEs among OSCs with such active layer thicknesses to date. This work reveals a molecular design strategy by reasonably combining fluorine and methyl together to simultaneously enhance charge carrier mobilities and fine‐tune the morphology, which is beneficial to achieve high‐performance thick‐film OSCs.     

Thermally Induced Flexo‐Type Effects in Nanopatterned Multiferroic Layers

The difficulty to generate and control large strain gradients in materials hinders the investigation and application of flexoelectricity and flexomagnetism. This work demonstrates that thermal expansion can be used to induce very large non‐uniform strains at the nanoscale, resulting in giant strain gradients at moderate temperatures. This is demonstrated in a nanopatterned multiferroic hybrid layer consisting of a regular array of ferromagnetic metallic nanocylinders embedded in a ferroelectric polymer matrix. The thermally‐induced strain gradients can fully depolarize the ferroelectric component, and modify the magnetization of the ferromagnetic component via flexoelectric and flexomagnetic effects, respectively. Finite‐element analysis provides a quantitative view on thermal expansion‐induced strains and strain gradients supporting the experimental findings. This work shows that nanoscale thermal strain engineering provides an additional degree of freedom to control electrical polarization and magnetization, which paves the way for the design and operation of novel functional devices and nanostructures.     

Segmentation of intra-retinal layers from optical coherence tomography images using an active contour approach

Optical coherence tomography (OCT) is a noninvasive, depth-resolved imaging modality that has become a prominent ophthalmic diagnostic technique. We present a semi-automated segmentation algorithm to detect intra-retinal layers in OCT images acquired from rodent models of retinal degeneration. We adapt Chan-Vese's energy-minimizing active contours without edges for the OCT images, which suffer from low contrast and are highly corrupted by noise. A multiphase framework with a circular shape prior is adopted in order to model the boundaries of retinal layers and estimate the shape parameters using least squares. We use a contextual scheme to balance the weight of different terms in the energy functional. The results from various synthetic experiments and segmentation results on OCT images of rats are presented, demonstrating the strength of our method to detect the desired retinal layers with sufficient accuracy even in the presence of intensity inhomogeneity resulting from blood vessels. Our algorithm achieved an average Dice similarity coefficient of 0.84 over all segmented retinal layers, and of 0.94 for the combined nerve fiber layer, ganglion cell layer, and inner plexiform layer which are the critical layers for glaucomatous degeneration.     

Resonant cavity-enhanced (RCE) photodetectors

The photosensitivity characteristics of resonant cavity-enhanced (RCE) photodetectors are investigated. The photodetectors were formed by integrating the active absorption region into a resonant cavity composed of top and bottom (buried) mirrors. A general expression for quantum efficiency for RCE photodetectors was derived taking the external losses into account. Drastic enhancement in quantum efficiency is demonstrated at resonant wavelengths for a high quality factor Q cavity with a very thin absorption layer. An improvement by a factor of four in the bandwidth-efficiency product for RCE p-i-n detectors is predicted. Molecular beam epitaxy grown RCE-heterojunction phototransistors (RCE-HPT) were fabricated and measured demonstrating good agreement between experiment and theory     

Toward two-dimensional self-organization of nanostructures using wafer bonding and nanopatterned silicon surfaces

The structure of ultrathin silicon layers obtained by molecular hydrophobic bonding is investigated. The twist and tilt angles between the two crystals are accurately controlled. The buried Si|Si interface is observed by transmission electron microscopy and by grazing incidence X-ray techniques. For low twist angle values (/spl psi/<5/spl deg/) plane view observations reveal well-defined dislocation networks. Cross-section observations give evidence that the dislocation networks are localized at the bonding interfacial plane with no threading dislocation. Grazing incidence small angle X-ray scattering measurements confirm the good quality of the bonding interface as well as the quality of the dislocation networks. Grazing incidence X-ray diffraction is also used and shows the long-range order of the periodic strain field in the silicon layer. It shows, especially, the interaction between the dislocations. X-ray reflectivity was employed and estimated that the interfacial thickness (i.e., thickness of the bonding) lower than 1 nm decreases when the twist angle increases. The nanopatterned surface is then investigated by scanning tunneling microscopy and X-ray methods. To validate these substrates for long-range order self-organization, the growth of Si and Ge quantum dots is finally achieved.     

A polysilicon contacted subcollector BJT for a three-dimensionalBiCMOS process

A polysilicon contacted subcollector (PCS) bipolar junction transistor (BJT) was fabricated using selective epitaxial growth (SEG) of silicon to form the active region. The fabrication is the first step in the development of a novel 3-D BiCMOS process. To study the efficacy of the polysilicon collector contact, three types of BJTs were fabricated and their collector resistances were compared. These were the PCS BJT, a BJT fabricated in SEG silicon grown from a shallow trench incorporating a shallow collector contact with a buried layer, and a BJT fabricated in the silicon substrate with a shallow collector contact but no buried layer. The PCS BJT exhibited the smallest collector resistance as well as excellent device characteristics, demonstrating its viability for a 3-D BiCMOS process     

Studies of magnetic microstructures with soft X-ray transmission microscopy

We review recent achievements of magnetic full-field soft X-ray transmission microscopy. This technique allows for an element-specific imaging of magnetic domain structures in thin films and nanopatterned elements. With a lateral resolution down to 15 nm, the ability to record images in external magnetic fields and the application of stroboscopic pump-and-probe schemes detailed insights into fundamental mechanisms of micromagnetism can be obtained which are relevant in the development of high density and ultrafast magnetic storage and sensor technologies.     

Tuning the Work Function of Polar Zinc Oxide Surfaces using Modified Phosphonic Acid Self‐Assembled Monolayers

Zinc oxide (ZnO) is regarded as a promising alternative material for transparent conductive electrodes in optoelectronic devices. However, ZnO suffers from poor chemical stability. ZnO also has a moderate work function (WF), which results in substantial charge injection barriers into common (organic) semiconductors that constitute the active layer in a device. Controlling and tuning the ZnO WF is therefore necessary but challenging. Here, a variety of phosphonic acid based self‐assembled monolayers (SAMs) deposited on ZnO surfaces are investigated. It is demonstrated that they allow the tuning the WF over a wide range of more than 1.5 eV, thus enabling the use of ZnO as both the hole‐injecting and electron‐injecting contact. The modified ZnO surfaces are characterized using a number of complementary techniques, demonstrating that the preparation protocol yields dense, well‐defined molecular monolayers.     

Selective-Area Epitaxy of CdTe on CdTe/ZnTe/Si(211) Through a Nanopatterned Silicon Nitride Mask

We report here the use of molecular beam epitaxy (MBE) to achieve selective-area epitaxy (SAE) and coalescence of CdTe on nanopatterned substrates with 0.5-μm-pitch arrays of CdTe/ZnTe/Si(211) seeding areas, exposed through a silicon nitride mask. The nanopatterned substrate surface morphology, crystallinity, and chemical composition were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS) both before and after exposure to CdTe flux by MBE. We find a seven times wider (422) CdTe XRD rocking curve full-width at half-maximum (FWHM) for our patterned samples, when directly compared with unpatterned samples with identical growth and pregrowth conditions. We also observe that electron beam-induced carbon deposits can serve as a mask material for SAE of CdTe by MBE. Finally, we point to possible further improvements in patterned sample architectures of the future, for use in CdTe films on Si.     

Hollow Two‐Layered Chiral Nanoparticles Consisting of Optically Active Helical Polymer/Silica: Preparation and Application for Enantioselective Crystallization

This article reports for the first time a novel category of hollow organic@inorganic hybrid two‐layered nanoparticles (NPs), in which the inner layer is formed by optically active helical polyacetylene, and the outer layer by silica. Such NPs show remarkable optical activity and are successfully used for enantioselective crystallization. To prepare such NPs, n‐butyl acrylate undergoes radical polymerization to first form poly(n‐butyl acrylate) (PBA) cores two shells by catalytic polymerization of substituted acetylene and sol–gel approach of TEOS (tetraethyl orthosilicate), respectively. Removal of the PBA cores provides the expected hollow core/shell NPs. The intense dircular dichroism (CD) effects demonstrate that the hollow chiral NPs possess considerable optical activity, arising from the helical substituted polyacetylenes forming the inner layer. The hollow NPs are further used as chiral templates to induce enantioselective crystallization of racemic alanines, demonstrating the significant potential applications of the hollow chiral NPs in chiral technologies. Also of particular significance is the detailed process of the induced crystallization observed by TEM. The strategy for preparing the hollow hybrid chiral NPs should be highlighted since it combines free radical polymerization and catalytic polymerization with sol–gel process in a single system, by which numerous advanced materials will be accessible.     

双偏振双波长混合应变量子阱激光器

本文报道了以混合应变量子阱结构为有源区的激光器。对有源区分别采用体材料、匹配量子阱、压应变量子阱和混合应变量子阱结构的激光器进行了比较。混合应变量子阱激光器能同时工作于两种偏振模式，而且两种偏振模式的激射波长不同。结合实验结果，我们可以看出在混合应变量子阱结构中，从偏偏自发辐射谱峰值长差不能推断两种量子陆的能带填充效应大小。     

Flexible Polymer Transducers for Dynamic Recognizing Physiological Signals

Ferroelectric polymers are of interest as most promising electroactive materials. Flexible transducers from ferroelectric polymer thin film with underneath semiconducting polymer active layer for high sensitive and versatile detection of physiological signals are described. When attached directly on the wrist, the flexible transducers can distinguish the transient pulse waves non‐invasively and in situ, due to their fast response (milliseconds) and high sensitivity (down to several Pascal) to instantaneous change of blood pressure. High‐resolution picture of one pulse wave is available to provide two most common parameters for arterial stiffness diagnosis. The transducers are also suitable for dynamic recognizing physiological signals under both physical exercise and medicine treatment, demonstrating their enormous potential for warning the risk of cardiovascular disease, and evaluating the efficacy of heart medicines. The transducers are easy to carry around with an operating voltage of 1 V and the power consumption less than 1 μW. Thus, they are valuable for applications like electronic skin and mobile health monitoring.