Fabrication and chemical lift-off of sub-micron scale III-nitride LED structures.

Nanoscale light emitting diodes (nanoLEDs, diameter < 1 µm), with active and sacrificial multi-quantum well (MQW) layers epitaxially grown via metal organic chemical vapor deposition, were fabricated and released into solution using a combination of colloidal lithography and photoelectrochemical (PEC) etching of the sacrificial MQW layer. PEC etch conditions were optimized to minimize undercut roughness, and thus limit damage to the active MQW layer. NanoLED emission was blue-shifted ∼10 nm from as-grown (unpatterned) LED material, hinting at strain relaxation in the active InGaN MQW layer. X-ray diffraction also suggests that strain relaxation occurs upon nanopatterning, which likely results in less quantum confined Stark effect. Internal quantum efficiency of the lifted nanoLEDs was estimated at 29% by comparing photoluminescence at 292K and 14K. This work suggests that colloidal lithography, combined with chemical release, could be a viable route to produce solution-processable, high efficiency nanoscale light emitters.

Strain relaxation of InGaN/GaN multi-quantum well light emitters via nanopatterning.

Strain in InGaN/GaN multiple-quantum well (MQW) light emitters was relaxed via nanopatterning using colloidal lithography and top-down plasma etching. Colloidal lithography was performed using Langmuir-Blodgett dip-coating of samples with silica particles (d = 170, 310, 690, 960 nm) and a Cl2/N2 inductively coupled plasma etch to produce nanorod structures. The InGaN/GaN MQW nanorods were characterized using X-ray diffraction (XRD) reciprocal space mapping to quantify the degree of relaxation. A peak relaxation of 32% was achieved for the smallest diameter features tested (120 nm after etching). Power-dependent photoluminescence at 13 K showed blue-shifted quantum well emission upon relaxation, which is attributed to reduction of the inherent piezoelectric field in the III-nitrides. Poisson-Schrödinger simulations of single well structures also predicted increasing spectral blueshift with strain relaxation, in agreement with experiments.

Enhanced light extraction from free-standing InGaN/GaN light emitters using bio-inspired backside surface structuring.

Light extraction from InGaN/GaN-based multiple-quantum-well (MQW) light emitters is enhanced using a simple, scalable, and reproducible method to create hexagonally close-packed conical nano- and micro-scale features on the backside outcoupling surface. Colloidal lithography via Langmuir-Blodgett dip-coating using silica masks (d = 170-2530 nm) and Cl2/N2-based plasma etching produced features with aspect ratios of 3:1 on devices grown on semipolar GaN substrates. InGaN/GaN MQW structures were optically pumped at 266 nm and light extraction enhancement was quantified using angle-resolved photoluminescence. A 4.8-fold overall enhancement in light extraction (9-fold at normal incidence) relative to a flat outcoupling surface was achieved using a feature pitch of 2530 nm. This performance is on par with current photoelectrochemical (PEC) nitrogen-face roughening methods, which positions the technique as a strong alternative for backside structuring of c-plane devices. Also, because colloidal lithography functions independently of GaN crystal orientation, it is applicable to semipolar and nonpolar GaN devices, for which PEC roughening is ineffective.

Moth eye-inspired anti-reflective surfaces for improved IR optical systems & visible LEDs fabricated with colloidal lithography and etching.

Near- and sub-wavelength photonic structures are used by numerous organisms (e.g. insects, cephalopods, fish, birds) to create vivid and often dynamically-tunable colors, as well as create, manipulate, or capture light for vision, communication, crypsis, photosynthesis, and defense. This review introduces the physics of moth eye (ME)-like, biomimetic nanostructures and discusses their application to reduce optical losses and improve efficiency of various optoelectronic devices, including photodetectors, photovoltaics, imagers, and light emitting diodes. Light-matter interactions at structured and heterogeneous surfaces over different length scales are discussed, as are the various methods used to create ME-inspired surfaces. Special interest is placed on a simple, scalable, and tunable method, namely colloidal lithography with plasma dry etching, to fabricate ME-inspired nanostructures in a vast suite of materials. Anti-reflective surfaces and coatings for IR devices and enhancing light extraction from visible light emitting diodes are highlighted.

Mobile apps for mood tracking: an analysis of features and user reviews.

Many mood tracking apps are available on smartphone app stores, but little is known about their features and their users' experiences. To investigate commercially available mood tracking apps, we conducted an in-depth feature analysis of 32 apps, and performed a qualitative analysis of a set of user reviews. Informed by a widely adopted personal informatics framework, we conducted a feature analysis to investigate how these apps support four stages of selftracking: preparation, collection, reflection, and action; and found that mood tracking apps offer many features for the collection and reflection stages, but lack adequate support for the preparation and action stages. Through the qualitative analysis of user reviews, we found that users utilize mood tracking to learn about their mood patterns, improve their mood, and self-manage their mental illnesses. In this paper, we present our findings and discuss implications for mobile apps designed to enhance emotional wellness.

Printable nanostructured silicon solar cells for high-performance, large-area flexible photovoltaics.

Nanostructured forms of crystalline silicon represent an attractive materials building block for photovoltaics due to their potential benefits to significantly reduce the consumption of active materials, relax the requirement of materials purity for high performance, and hence achieve greatly improved levelized cost of energy. Despite successful demonstrations for their concepts over the past decade, however, the practical application of nanostructured silicon solar cells for large-scale implementation has been hampered by many existing challenges associated with the consumption of the entire wafer or expensive source materials, difficulties to precisely control materials properties and doping characteristics, or restrictions on substrate materials and scalability. Here we present a highly integrable materials platform of nanostructured silicon solar cells that can overcome these limitations. Ultrathin silicon solar microcells integrated with engineered photonic nanostructures are fabricated directly from wafer-based source materials in configurations that can lower the materials cost and can be compatible with deterministic assembly procedures to allow programmable, large-scale distribution, unlimited choices of module substrates, as well as lightweight, mechanically compliant constructions. Systematic studies on optical and electrical properties, photovoltaic performance in experiments, as well as numerical modeling elucidate important design rules for nanoscale photon management with ultrathin, nanostructured silicon solar cells and their interconnected, mechanically flexible modules, where we demonstrate 12.4% solar-to-electric energy conversion efficiency for printed ultrathin (∼ 8 µm) nanostructured silicon solar cells when configured with near-optimal designs of rear-surface nanoposts, antireflection coating, and back-surface reflector.

Normalized median fluorescence: an alternative flow cytometry analysis method for tracking human embryonic stem cell states during differentiation.

Human embryonic stem cells (hESCs) are a promising cell source for tissue engineering and regenerative medicine, but before they can be used in therapies, we must be able to accurately identify the state and progeny of hESCs. One of the most commonly used methods for identification is flow cytometry. Many flow cytometry applications use antibodies to detect the amount of antigen present on/in a cell. This allows for the identification of unique cell populations or the tracking of expression changes within a population during differentiation. The results are typically presented as a percentage of positively expressing cells (%Pos) for a marker of choice, relative to a negative control. However, this reporting term is vulnerable to distortion from outliers and inaccuracy from loss of information about the population's fluorescence intensity. In this article, we describe an alternate strategy that uses the normalized median fluorescence intensity (nMFI), in which the MFI of the stained sample is normalized to the MFI of the negative control, as the reporting term to more accurately describe a population of cells in culture. We observed that nMFI provides a more accurate representation for the quality of a starting population and comparing data of different experimental runs. In addition, we demonstrated that the nMFI is a more sensitive measure of pluripotent and differentiation markers expression changes during hESC differentiation into three germ layer lineages.

Temporal application of topography to increase the rate of neural differentiation from human pluripotent stem cells.

Human pluripotent stem cells (hPSCs) are a promising cell source for tissue engineering and regenerative medicine, especially in the field of neurobiology. Neural differentiation protocols have been developed to differentiate hPSCs into specific neural cells, but these predominantly rely on biochemical cues. Recently, differentiation protocols have incorporated topographical cues to increase the total neuronal yield. However, the means by which these topographical cues improve neuronal yield remains unknown. In this study, we explored the effect of topography on the neural differentiation of hPSC by quantitatively studying the changes in marker expression at a transcript and protein level. We found that 2 µm gratings increase the rate of neural differentiation, and that an additional culture period of 2 µm gratings in the absence of neurotrophic signals can improve the neural differentiation of hPSCs. We envisage that this work can be incorporated into future differentiation protocols to decrease the differentiation period as well as the biochemical signals added, thus generating hPSC-derived neural cells in a more cost effective and efficient manner.

Substrate topography and size determine the fate of human embryonic stem cells to neuronal or glial lineage.

Efficient derivation of neural cells from human embryonic stem cells (hESCs) remains an unmet need for the treatment of neurological disorders. The limiting factors for current methods include being labor-intensive, time-consuming and expensive. In this study, we hypothesize that the substrate topography, with optimal geometry and dimension, can modulate the neural fate of hESCs and enhance the efficiency of differentiation. A multi-architectural chip (MARC) containing fields of topographies varying in geometry and dimension was developed to facilitate high-throughput analysis of topography-induced neural differentiation in vitro. The hESCs were subjected to "direct differentiation", in which small clumps of undifferentiated hESCs were cultured directly without going through the stage of embryoid body formation, on the MARC with N2 and B27 supplements for 7 days. The gene and protein expression analysis indicated that the anisotropic patterns like gratings promoted neuronal differentiation of hESCs while the isotropic patterns like pillars and wells promoted the glial differentiation of hESCs. This study showed that optimal combination of topography and biochemical cues could shorten the differentiation period and allowed derivation of neurons bearing longer neurites that were aligned along the grating axis. The MARC platform would enable high-throughput screening of topographical substrates that could maximize the efficiency of neuronal differentiation from pluripotent stem cells.

Nanotopography/mechanical induction of stem-cell differentiation.

The interplay of biophysical and biochemical cues in the extracellular microenvironment regulate and control the cell fate of stem cells. Understanding the interaction between stem cells and the extracellular substrate will be crucial in controlling stem cell differentiation for regenerative medicine applications. One of the biophysical properties of the microenvironment is substrate topology, which has been demonstrated to be an important mediator of stem cell lineage regulation. Biomimetic microenvironment topology can be engineered by chemical patterning or physical patterning. The rapid advancements in nanofabrication techniques have enabled versatility in patterning types with controlled chemistries, geometries and sizes. The chapter will focus on discussing the effect on physical nanotopography on stem cell differentiation and the current theories on the topography/ mechanical force induction of stem cell differentiation possibly through integrin clustering, focal adhesion, cytoskeleton organization and the nuclear mechanosensing to sense and integrate these biophysical signals from the extracellular microenvironment.