Историко-педагогический обзор детской беспризорности в Древней Руси и императорской России

Axially resolved microphotoluminescence mapping of semiconductor nanowires held in an optical tweezers reveals important new experimental information regarding equilibrium trapping points and trapping stability of high aspect ratio nanostructures. In this study, holographic optical tweezers are used to scan trapped InP nanowires along the beam direction with respect to a fixed excitation source and the luminescent properties are recorded. It is observed that nanowires with lengths on the range of 3–15 μm are stably trapped near the tip of the wire with the long segment positioned below the focus in an inverted trapping configuration. Through the use of trap multiplexing we investigate the possibility of improving the axial stability of the trapped nanowires. Our results have important implication for applications of optically assisted nanowire assembly and optical tweezers based scanning probes microscopy

Strain-Engineered Multilayer Epitaxial Lift-Off for Cost-Efficient III–V Photovoltaics and Optoelectronics

The efficient removal of epitaxially grown materials from their host substrate has a pivotal role in reducing the cost and material consumption of III–V solar cells and in making flexible thin-film devices. A multilayer epitaxial lift-off process is demonstrated that is scalable in both film size and in the number of released films. The process utilizes in-built, individually engineered epitaxial strain in each film to tailor the bending without the need for external layers to induce strain. Even without external support layers, the films retain good integrity after the lift-off, as evidenced by photoluminescence measurements. The films can be further processed into devices, demonstrated here with the fabrication of cm-scale solar cells using a three-layer lift-off process. Based on the included cost analysis, the solar cells are fabricated with a facile two-step process from the as-released films. The scalable multilayer lift-off process is highly cost-efficient and enables a 4-to-6-fold reduction in the cost with respect to the single-layer epitaxial lift-off process. The results are therefore significant for III–V photovoltaics and any other technologies that rely on thin-film III–V semiconductors

Strain-Engineered Multilayer Epitaxial Lift-Off for Cost-Efficient III–V Photovoltaics and Optoelectronics

The efficient removal of epitaxially grown materials from their host substrate has a pivotal role in reducing the cost and material consumption of III–V solar cells and in making flexible thin-film devices. A multilayer epitaxial lift-off process is demonstrated that is scalable in both film size and in the number of released films. The process utilizes in-built, individually engineered epitaxial strain in each film to tailor the bending without the need for external layers to induce strain. Even without external support layers, the films retain good integrity after the lift-off, as evidenced by photoluminescence measurements. The films can be further processed into devices, demonstrated here with the fabrication of cm-scale solar cells using a three-layer lift-off process. Based on the included cost analysis, the solar cells are fabricated with a facile two-step process from the as-released films. The scalable multilayer lift-off process is highly cost-efficient and enables a 4-to-6-fold reduction in the cost with respect to the single-layer epitaxial lift-off process. The results are therefore significant for III–V photovoltaics and any other technologies that rely on thin-film III–V semiconductors

Nanowires Grown on InP (100): Growth Directions, Facets, Crystal Structures, and Relative Yield Control

Growth of III–V nanowires on the [100]-oriented industry standard substrates is critical for future integrated nanowire device development. Here we present an in-depth analysis of the seemingly complex ensembles of epitaxial nanowires grown on InP (100) substrates. The nanowires are categorized into three types as vertical, nonvertical, and planar, and the growth directions, facets, and crystal structure of each type are investigated. The nonvertical growth directions are mathematically modeled using a three-dimensional multiple-order twinning concept. The nonvertical nanowires can be further classified into two different types, with one type growing in the ⟨111⟩ directions and the other in the ⟨100⟩ directions after initial multiple three-dimensional twinning. We find that 99% of the total nanowires are grown either along ⟨100⟩, ⟨111⟩, or ⟨110⟩ growth directions by {100} or {111} growth facets. We also demonstrate relative control of yield of these different types of nanowires, by tuning pregrowth annealing conditions and growth parameters. Together, the knowledge and controllability of the types of nanowires provide an ideal foundation to explore novel geometries that combine different crystal structures, with potential for both fundamental science research and device applications

Three-Dimensional in Situ Photocurrent Mapping for Nanowire Photovoltaics

Devices based upon semiconductor nanowires provide many well-known advantages for next-generation photovoltaics, however, limited experimental techniques exist to determine essential electrical parameters within these devices. We present a novel application of a technique based upon two-photon induced photocurrent that provides a submicrometer resolution, three-dimensional reconstruction of photovoltaic parameters. This tool is used to characterize two GaAs nanowire-based devices, revealing the detail of current generation and collection, providing a path toward achieving the promise of nanowire-based photovoltaic devices

Strong Amplified Spontaneous Emission from High Quality GaAs_1–<i>x</i>Sb_<i>x</i> Single Quantum Well Nanowires

Quantum confinement in semiconductor nanowires is of contemporary interest. Enhancing the quantum efficiency of quantum wells in nanowires and minimizing intrinsic absorption are necessary for reducing the threshold of nanowire lasers and are promising for wavelength tunable emitters and detectors. Here, we report on growth and optimization of GaAs_1–<i>x</i>Sb_<i>x</i>/Al_1–<i>y</i>Ga_<i>y</i>As quantum well heterostructures formed radially around pure zinc blende GaAs core nanowires. The emitted photon energy from GaAs_0.89Sb_0.11 quantum well (1.371 eV) is smaller than the GaAs core, thus showing advantages over GaAs/Al_1–<i>y</i>Ga_<i>y</i>As quantum well nanowires in photon emission. The high optical quality quantum well (internal quantum efficiency reaches as high as 90%) is carefully positioned so that the quantum well coincides with the maximum of the transverse electric (TE01) mode intensity profile. The obtained superior optical performance combined with the supported Fabry–Perot (F–P) cavity in the nanowire leads to the strong amplified spontaneous emission (ASE). Detailed studies of the amplified cavity mode are carried out by spatial–spectral photoluminescence (PL) imaging, where emission from nanowire is resolved both spatially and spectrally. Resonant emission is generated at nanowire ends and is polarized perpendicular to the nanowire, in agreement with the simulated polarization characteristics of the TE01 mode in the nanowire. The observation of strong ASE for single QW nanowire at room temperature shows the potential application of GaAs_1–<i>x</i>Sb_<i>x</i> QW nanowires as low threshold infrared nanowire lasers

Polarization Tunable, Multicolor Emission from Core–Shell Photonic III–V Semiconductor Nanowires

We demonstrate luminescence from both the core and the shell of III–V semiconductor photonic nanowires by coupling them to plasmonic silver nanoparticles. This demonstration paves the way for increasing the quantum efficiency of large surface area nanowire light emitters. The relative emission intensity from the core and the shell is tuned by varying the polarization of the excitation source since their polarization response can be independently controlled. Independent control on emission wavelength and polarization dependence of emission from core–shell nanowire heterostructures opens up opportunities that have not yet been imagined for nanoscale polarization sensitive, wavelength-selective, or multicolor photonic devices based on single nanowires or nanowire arrays

Engineering Highly Interconnected Neuronal Networks on Nanowire Scaffolds

Identifying the specific role of physical guidance cues in the growth of neurons is crucial for understanding the fundamental biology of brain development and for designing scaffolds for tissue engineering. Here, we investigate the structural significance of nanoscale topographies as physical cues for neurite outgrowth and circuit formation by growing neurons on semiconductor nanowires. We monitored neurite growth using optical and scanning electron microscopy and evaluated the spontaneous neuronal network activity using functional calcium imaging. We show, for the first time, that an isotropic arrangement of indium phosphide (InP) nanowires can serve as physical cues for guiding neurite growth and aid in forming a network with neighboring neurons. Most importantly, we confirm that multiple neurons, with neurites guided by the topography of the InP nanowire scaffolds, exhibit synchronized calcium activity, implying intercellular communications via synaptic connections. Our study imparts new fundamental insights on the role of nanotopographical cues in the formation of functional neuronal circuits in the brain and will therefore advance the development of neuroprosthetic scaffolds

Scalable Bright and Pure Single Photon Sources by Droplet Epitaxy on InP Nanowire Arrays

High-quality quantum light sources are crucial components for the implementation of practical and reliable quantum technologies. The persistent challenge, however, is the lack of scalable and deterministic single photon sources that can be synthesized reproducibly. Here, we present a combination of droplet epitaxy with selective area epitaxy to realize the deterministic growth of single quantum dots in nanowire arrays. By optimization of the single quantum dot growth and the nanowire cavity design, single emissions are effectively coupled with the dominant mode of the nanowires to realize Purcell enhancement. The resonance-enhanced quantum emitter system boasts a brightness of millions of counts per second with nanowatt excitation power, a short radiation lifetime of 350 ± 5 ps, and a high single-photon purity with g(2)(0) value of 0.05 with continuous wave above-band excitation. Finite-difference time-domain (FDTD) simulation results show that the emissions of single quantum dots are coupled into the TM01 mode of the nanowires, giving a Purcell factor ≈ 3. Our technology can be used for creating on-chip scalable single photon sources for future quantum technology applications including quantum networks, quantum computation, and quantum imaging

Effect of Nanocrystalline Domains in Photovoltaic Devices with Benzodithiophene-Based Donor–Acceptor Copolymers

We have investigated the effects of thin-film morphology on the photovolatic performance for a series of donor–acceptor copolymers based on benzodithiophene donor and benzothiadiazole acceptor units. Photovoltaic devices incorporating polymer:fullerene blends show highest efficiencies (up to 6%) for those polymers exhibiting the least degree of crystallinity in X-ray diffraction patterns and a corresponding lowest surface roughness in thin films. We find that the existence of such crystalline domains in thin polymer films correlates well with spectral signatures of polymer chain aggregates already present in solution prior to casting of the film. Polymer solubility and casting conditions therefore appear to be crucial factors for enhancing efficiencies of photovoltaic devices based on such donor–acceptor copolymers. To examine why the presence of crystallite domains lowers device efficiencies, we measured exciton diffusion lengths by modeling the time-dependent photoluminescence from thin polymer films deposited on an exciton quencher layer of TiO₂. We find that exciton diffusion lengths in these materials are substantial (4–7.5 nm) and show some variation with polymer crystallinity. However, ultrafast (1 ps) quenching of the polymer emission from polymer:PCBM blends indicates that the vast majority of excitons rapidly reach the charge-dissociating interface, and hence exciton diffusion does not represent a limiting factor. We therefore conclude that the subsequent charge extraction and lifetimes must be adversely affected by the presence of crystalline domains. We suggest that the formed crystallites are too small to offer significant enhancements in long-range charge carrier mobility but instead introduce domain boundaries which impede charge extraction. For this class of materials, polymer designs are therefore required that target high solubility and chain entropy, leading to amorphous film formation