Controlled Self-Assembly of Nanoscale Superstructures in Phase-Change Ge-Sb-Te Nanowires.

Controlled growth of semiconductor nanowires with atomic precision offers the potential to tune the material properties for integration into scalable functional devices. Despite significant progress in understanding the nanowire growth mechanism, definitive control over atomic positions of its constituents, structure, and morphology via self-assembly remains challenging. Here, we demonstrate an exquisite control over synthesis of cation-ordered nanoscale superstructures in Ge-Sb-Te nanowires with the ability to deterministically vary the nanowire growth direction, crystal facets, and periodicity of cation ordering by tuning the relative precursor flux during synthesis. Furthermore, the role of anisotropy on material properties in cation-ordered nanowire superstructures is illustrated by fabricating phase-change memory (PCM) devices, which show significantly different growth direction dependent amorphization current density. This level of control in synthesizing chemically ordered nanoscale superstructures holds potential to precisely modulate fundamental material properties such as the electronic and thermal transport, which may have implications for PCM, thermoelectrics, and other nanoelectronic devices.

Perspective Toward Complementary & Alternative Medicines in the Prevention of COVID-19 Infection.

Background: Across the globe, people are seeking integrative and holistic measures to prevent coronavirus (COVID-19) infection in the form of complementary and alternative medicines (CAM) with or without conventional medicines. This study was done to know the extent of CAM use for COVID-19 prophylaxis and to know beliefs and attitudes of people related to CAM use in India. Methodology: A pretested and prevalidated questionnaire was circulated on social media. Participants, who completed the online form and gave voluntary consent, were included. The questionnaire included demographic details and questions related to CAM use, preferences with reasons, preparations used, perceived role of CAM in prevention, immunity boosting and side effects, sources of information, etc. Results: Out of 514 responses, 495 were analyzed. 47.07% of respondents were males and 52.93% were females. 66.9% were using CAM for COVID-19 prophylaxis. The association between age, gender, and profession with CAM use was statistically significant (P < 0.05). 41.1% reported CAM use in the past. 36.6% of CAM users were taking "Kadha" and 33% were using ayurvedic medicines. Other frequently used CAM preparations were chyavanprash, giloy, tulsi, ginger, pepper, cloves, honey, sudarshanghanvati, arsenic-30, lemon juice, cinnamon, steam inhalation, ashwagandha, swasarivati, coronil, and warm saline water gargles. 46.9% of the CAM users were on self-medication and 52.3% preferred CAM over allopathy. Conclusion: Complementary and alternative medicine utilization for COVID-19 prophylaxis is widespread and self-medication is prevalent. As no specific cure is available in conventional systems, people believe in traditional medicines more than conventional, yet confusion exists. There is a need of increasing awareness regarding side effects, drug-drug interactions, and self-medication.

A Universal Pick-and-Place Assembly for Nanowires.

With the introduction of techniques to grow highly functional nanowires of exotic materials and demonstrations of their potential in new applications, techniques for depositing nanowires on functional platforms have been an area of active interest. However, difficulties in handling individual nanowires with high accuracy and reliability have so far been a limiting factor in large-scale integration of high-quality nanowires. Here, a technique is demonstrated to transfer single nanowires reliably on virtually any platform, under ambient conditions. Functional nanowires of InP, AlGaAs, and GeTe on various patterned structures such as electrodes, nanophotonic devices, and even ultrathin transmission electron microscopy (TEM) membranes are transferred. It is shown that the versatility of this technique further enables to perform on-chip nano-optomechanical measurements of an InP nanowire for the first time via evanescent field coupling. Thus, this technique facilitates effortless integration of single nanowires into applications that were previously seen as cumbersome or even impractical, spanning a wide range from TEM studies to in situ electrical, optical, and mechanical characterization.

Real-time nanomechanical property modulation as a framework for tunable NEMS.

Phase-change materials (PCMs) can switch between amorphous and crystalline states permanently yet reversibly. However, the change in their mechanical properties has largely gone unexploited. The most practical configuration using suspended thin-films suffer from filamentation and melt-quenching. Here, we overcome these limitations using nanowires as active nanoelectromechanical systems (NEMS). We achieve active modulation of the Young's modulus in GeTe nanowires by exploiting a unique dislocation-based route for amorphization. These nanowire NEMS enable power-free tuning of the resonance frequency over a range of 30%. Furthermore, their high quality factors ([Formula: see text] > 104) are retained after phase transformation. We utilize their intrinsic piezoresistivity with unprecedented gauge factors (up to 1100) to facilitate monolithic integration. Our NEMS demonstrate real-time frequency tuning in a frequency-hopping spread spectrum radio prototype. This work not only opens up an entirely new area of phase-change NEMS but also provides a novel framework for utilizing functional nanowires in active mechanical systems.

Coherent Interactions in One-Dimensional Topological Photonic Systems and Their Applications in All-Optical Logic Operation.

Topological photonics has become an active subfield of photonics analogous to the electronic counterpart, and the bulk-edge correspondence leads to robust topologically protected interfacial states. However, a single-topological interface mode with fixed energy cannot be easily manipulated, hindering its applications in optical devices. Here, we study coupled-waveguide arrays mapped to a one-dimensional Su-Schrieffer-Heeger system with two coupled topological interfaces. This configuration greatly increases device versatility and tunability while keeping the confinement of coupled-interface modes inherited from the topological properties nearly intact. Theoretically predicted oscillations between coupled interfaces is experimentally observed. The spatial and energetic isolation of the coupled interface states from the bulk modes is experimentally observed and theoretically confirmed by calculating the degree of localization of the eigenstates, which is found to be comparable to a single-interface state. Finally, a proof-of-principle, all-optical logic circuit is fabricated based on coupled interfaces, demonstrating its potential in assembling on-chip topological optical devices.

Generation of helical topological exciton-polaritons.

Topological photonics in strongly coupled light-matter systems offer the possibility for fabricating tunable optical devices that are robust against disorder and defects. Topological polaritons, i.e., hybrid exciton-photon quasiparticles, have been proposed to demonstrate scatter-free chiral propagation, but their experimental realization to date has been at deep cryogenic temperatures and under strong magnetic fields. We demonstrate helical topological polaritons up to 200 kelvin without external magnetic field in monolayer WS2 excitons coupled to a nontrivial photonic crystal protected by pseudo time-reversal symmetry. The helical nature of the topological polaritons, where polaritons with opposite helicities are transported to opposite directions, is verified. Topological helical polaritons provide a platform for developing robust and tunable polaritonic spintronic devices for classical and quantum information-processing applications.

Z2 Photonic Topological Insulators in the Visible Wavelength Range for Robust Nanoscale Photonics.

Topological photonics provides an ideal platform for demonstrating novel band topology concepts, which are also promising for robust waveguiding, communication, and computation applications. However, many challenges such as extremely large device footprint and functionality at short wavelengths remain to be solved which are required to make practical and useful devices that can also couple to electronic excitations in many important organic and inorganic semiconductors. In this letter, we report an experimental realization of Z2 photonic topological insulators with their topological edge state energies spanning across the visible wavelength range including in the sub-500 nm regime, which requires highly optimized nanofabrication. The photonic structures are based on deformed hexagonal lattices with preserved 6-fold rotational symmetry patterned on suspended SiNx membranes. The experimentally measured energy-momentum dispersion of the topological lattices directly shows topological band inversion by the swapping of the brightness of the bulk energy bands, and also the helical edge states when the measurement is taken near the topological interface. The robust topological transport of the helical edge modes in real space is demonstrated by successfully guiding circularly polarized light beams unidirectionally through sharp kinks without significant signal loss. This work paves the way for small footprint photonic topological devices working in the short wavelength range that can also be utilized to couple to excitons for unconventional light-matter interactions at the nanoscale.

Low-Power Switching through Disorder and Carrier Localization in Bismuth-Doped Germanium Telluride Phase Change Memory Nanowires.

One of the major problems with phase change memory (PCM) is the high current density required for the crystal-amorphous transformation via a melt-quench process. However, alternative low-energy pathways of amorphization via a defect-assisted process have also been proposed. Here, a defect-assisted amorphization pathway in Bi-doped GeTe nanowires is utilized to establish that carrier localization effects can significantly decrease the energy costs of amorphization. We demonstrate a strategy of doping GeTe nanowires with bismuth to engineer carrier localization effects via Fermi level/mobility edge tuning and increased atomic disorder. Enhanced carrier localization increases the carrier-lattice coupling, and therefore, the energy supplied to carriers via electrical pulses can be more efficiently extracted by the lattice to induce the critical bond distortions required for amorphization without an intermediate melting process. RESET (crystal to amorphous transition) current densities as low as ∼0.3 MA cm-2 are achieved for 8% Bi-doped GeTe nanowires, which is nearly a 3-fold reduction compared to undoped GeTe nanowires and is significantly less than GeTe thin film devices (∼50 MA cm-2). We demonstrate good reversibility of switching in the Bi-doped GeTe nanowires and also demonstrate the existence of intermediate resistance states which can be accessed by controlled electrical pulsing. The combination of low-power switching in conjunction with multiple resistance states indicates that doping strategies in PCM nanowires are beneficial for non-volatile memory and neuromorphic computing applications.

Ultra-wide bandwidth with enhanced microwave absorption of electroless Ni-P coated tetrapod-shaped ZnO nano- and microstructures.

A viable lightweight absorber is the current need for stealth technology as well as microwave absorption. Several microwave absorbers have been developed, but it is still a challenge to fabricate an absorber that facilitates microwave absorption in broad bandwidth or covers the maximum portion of the frequency range 2-18 GHz, the commonly used range for radar and other applications. Therefore, it is highly required to develop a wide bandwidth absorber that can provide microwave absorption in the most part of the frequency range 2-18 GHz while simultaneously being lightweight and can be fabricated in desired bulk quantities by the cost-effective synthesis methods. In this paper, an attempt has been made to design an ultra-wide bandwidth absorber with enhanced microwave absorption response by using nickel-phosphorus coated tetrapod-shaped ZnO (Ni-P coated T-ZnO). In the Ni-P coated T-ZnO absorber, ZnO acts as a good dielectric contributor, while Ni as a magnetic constituent to obtain a microwave absorbing composite material, which has favorable absorption properties. Ni-P coated ZnO nano-microstructures are synthesized by a simple and scalable two-step process. First, tetrapod-shaped ZnO (T-ZnO) structures have been grown by the flame transport synthesis (FTS) approach in a single step process and then they have been coated with Ni-P by an electroless coating technique. Their morphology, degree of crystallinity and existing phases were studied in detail by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques. The complex permittivity and permeability of the "as-fabricated" T-ZnO and Ni-P coated T-ZnO have been measured in the frequency range of 4-14 GHz and their microwave absorption properties are computed using the coaxial transmission-reflection method. The strongest reflection loss (RL) peak value of -36.41 dB has been obtained at a frequency of ∼8.99 GHz with coating thickness of 3.4 mm for the Ni-P coated T-ZnO sample with a broad bandwidth of 10.0 GHz (RL < -10 dB) in the frequency range of 4.0-14.0 GHz.

Direct Growth of Freestanding ZnO Tetrapod Networks for Multifunctional Applications in Photocatalysis, UV Photodetection, and Gas Sensing.

Growth of freestanding nano- and microstructures with complex morphologies is a highly desired aspect for real applications of nanoscale materials in various technologies. Zinc oxide tetrapods (ZnO-T), which exhibit three-dimensional (3D) shapes, are of major importance from a technological applications point of view, and thus efficient techniques for growth of different varieties of tetrapod-based networks are demanded. Here, we demonstrate the versatile and single-step synthesis of ZnO-T with different arm morphologies by a simple flame transport synthesis (FTS) approach, forming a network. Morphological evolutions and structural intactness of these tetrapods have been investigated in detail by scanning electron microscopy, X-ray diffraction, and micro-Raman measurements. For a deeper understanding of the crystallinity, detailed high-resolution transmission electron microscopic studies on a typical ZnO tetrapod structure are presented. The involved growth mechanism for ZnO tetrapods with various arm morphologies is discussed with respect to variations in experimental conditions. These ZnO-T have been utilized for photocatalytic degradation and nanosensing applications. The photocatalytic activities of these ZnO-T with different arm morphologies forming networks have been investigated through the photocatalytic decolorization of a methylene blue (MB) solution under UV light illumination at ambient temperature. The results show that these ZnO-T exhibit strong photocatalytic activities against MB and its complete degradation can be achieved in very short time. In another application, a prototype of nanoelectronic sensing device has been built from these ZnO-T interconnected networks and accordingly utilized for UV detection and H2 gas sensing. The fabricated device structures showed excellent sensing behaviors for promising practical applications. The involved sensing mechanisms with respect to UV photons and H2 gas are discussed in detail. We consider that such multifunctional nanodevices based on ZnO tetrapod interconnected networks will be of interest for various advanced applications.