Efficiency and Mechanism of Catalytic Siloxane Exchange in Vitrimer Polymers: Modeling and Density Functional Theory Investigations.

Of late, siloxane-containing vitrimers have gained significant interest due to their fast dynamic characteristics over a reasonable temperature range (180-220 °C), making them well-suited for diverse applications. The exchange reaction pathway in the siloxane vitrimers is accountable for the covalent adaptive network, with the reaction's effectiveness being regulated by either organic or organometallic catalysts. However, directly studying the exchange reaction pathway in the bulk phase using experimental approaches is challenging because of the intricate and interconnected structure of these vitrimers. Here, we perform comprehensive density functional theory (DFT) and experimental investigations to discover the detailed catalytic efficacy of siloxane exchange and provide direction for the reaction process using a 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) catalyst. The calculated transition barrier energy and catalytic efficiency of hexamethyldisiloxane and dihydroxy-dimethylsilane exchange derived from the nudged elastic band with transition-state calculations strongly agree with the experimental findings. In addition, Fukui indices, along with partial charges, are employed to evaluate the nucleophilic and electrophilic behaviors of silanol and siloxane molecules. Our analysis revealed that by utilizing the Fukui indices of both the acid and the base, we can make an approximate estimation of the respective kinetics of the SN2 process in the siloxane exchange reaction mechanism. These findings establish a foundation for comprehending a crucial aspect of the exchange mechanism in siloxane vitrimer systems and could aid in the development of novel catalysts.

Enhanced Computational Study with Experimental Correlation on I-V Characteristics of Tantalum Oxide (TaOx) Memristor Devices in a 1T1R Configuration.

Memristors, non-volatile switching memory platform, has recently attracted significant interest, offering unique potential to enable the realization of human brain-like neuromorphic computing efficiency. Memristors also demonstrate excellent temperature tolerance, long-term durability, and high tunability with nanosecond pulses, making them highly attractive for neuromorphic computing applications. To better understand the material processing, microstructure, and property relationship of switching mechanisms in memristor devices, computational methodologies, and tools are developed to predict the I-V characteristics of memristor devices based on tantalum oxide (TaOx) resistive random-access memory (ReRAM) integrated with an n-channel metal-oxide-semiconductor (NMOS) transistor. A multiphysics model based on coupled partial differential equations for electrical and thermal transport phenomena is solved for the high- and low-resistance states during the formation, growth, and destruction of a conducting filament through SET and RESET stages. These stages effectively represent the migration of oxygen vacancies within an oxide exchange layer. A series of parametric studies and energy minimization calculations are conducted to determine probable ranges for key material and model parameters accounting for the experimental data. The computational model successfully predicted the measured I-V curves across various gate voltages applied to the NMOS transistor in the one transistor one resistance (1T1R) configuration.

Probing Interlayer Interactions and Commensurate-Incommensurate Transition in Twisted Bilayer Graphene through Raman Spectroscopy.

Twisted 2D layered materials have garnered much attention recently as a class of 2D materials whose interlayer interactions and electronic properties are dictated by the relative rotation/twist angle between the adjacent layers. In this work, we explore a prototype of such a twisted 2D system, artificially stacked twisted bilayer graphene (TBLG), where we probe, using Raman spectroscopy, the changes in the interlayer interactions and electron-phonon scattering pathways as the twist angle is varied from 0° to 30°. The long-range Moiré potential of the superlattice gives rise to additional intravalley and intervalley scattering of the electrons in TBLG, which has been investigated through their Raman signatures. Density functional theory (DFT) calculations of the electronic band structure of the TBLG superlattices were found to be in agreement with the resonant Raman excitations across the van Hove singularities in the valence and conduction bands predicted for TBLG due to hybridization of bands from the two layers. We also observe that the relative rotation between the graphene layers has a marked influence on the second order overtone and combination Raman modes signaling a commensurate-incommensurate transition in TBLG as the twist angle increases. This serves as a convenient and rapid characterization tool to determine the degree of commensurability in TBLG systems.

Effect of oxygen vacancy and Si doping on the electrical properties of Ta2O5 in memristor characteristics.

The resistive switching behavior in Ta2O5 based memristors is largely controlled by the formation and annihilation of conductive filaments (CFs) that are generated by the migration of oxygen vacancies (OVs). To gain a fundamental insight on the switching characteristics, we have systematically investigated the electrical transport properties of two different Ta2O5 polymorphs ([Formula: see text]-Ta2O5 and λ-Ta2O5), using density functional theory calculations, and associated vacancy induced electrical conductivity using Boltzmann transport theory. The projected band structure and DOS in a few types of OVs, (two-fold (O2fV), three-fold (O3fV), interlayer (OILV), and distorted octahedral coordinated vacancies (OεV)) reveal that the presence of OILV would cause Ta2O5 to transition from a semiconductor to a metal, leading to improved electrical conductivity, whereas the other OV types only create localized mid-gap defect states within the bandgap. On studying the combined effect of OVs and Si-doping, a reduction of the formation energy and creation of defect states near the conduction band edge, is observed in Si-doped Ta2O5, and lower energy is found for the OVs near Si atoms, which would be advantageous to the uniformity of CFs produced by OVs. These findings can serve as guidance for further experimental work aimed at enhancing the uniformity and switching properties of resistance switching for Ta2O5-based memristors.

Energy Harvesting Using ZnO Nanosheet-Decorated 3D-Printed Fabrics.

In this work, we decorated piezoresponsive atomically thin ZnO nanosheets on a polymer surface using additive manufacturing (three-dimensional (3D) printing) technology to demonstrate electrical-mechanical coupling phenomena. The output voltage response of the 3D-printed architecture was regulated by varying the external mechanical pressures. Additionally, we have shown energy generation by placing the 3D-printed fabric on the padded shoulder strap of a bag with a load ranging from ∼5 to ∼75 N, taking advantage of the excellent mechanical strength and flexibility of the coated 3D-printed architecture. The ZnO coating layer forms a stable interface between ZnO nanosheets and the fabric, as confirmed by combining density functional theory (DFT) and electrical measurements. This effectively improves the output performance of the 3D-printed fabric by enhancing the charge transfer at the interface. Therefore, the present work can be used to build a new infrastructure for next-generation energy harvesters capable of carrying out several structural and functional responsibilities.

A Researcher's Perspective on Unconventional Lab-to-Fab for 2D Semiconductor Devices.

Current silicon technology is on the verge of reaching its performance limits. This aspect, coupled with the global chip shortage, makes a solid case for steering our attention toward the accelerated commercialization of other electronic materials. Among the available suite of emerging electronic materials, two-dimensional materials, including transition metal dichalcogenides (TMDs), exhibit improved short-channel effects, high electron mobility, and integration into CMOS-compatible processing. While these materials may not be able to replace silicon at the current stages of development, they can supplement Si in the form of Si-compatible CMOS processing and be manufactured for tailored applications. However, the major hurdle in the path of commercialization of such materials is the difficulty in producing their wafer-scale forms, which are not necessarily single crystalline but on a large scale. Recent but exploratory interest in 2D materials from industries, such as TSMC, necessitates an in-depth analysis of their commercialization potential based on trends and progress in entrenched electronic materials (Si) and ones with a short-term commercialization potential (GaN, GaAs). We also explore the possibility of unconventional fabrication techniques, such as printing, for 2D materials becoming more mainstream and being adopted by industries in the future. In this Perspective, we discuss aspects to optimize cost, time, thermal budget, and a general pathway for 2D materials to achieve similar milestones, with an emphasis on TMDs. Beyond synthesis, we propose a lab-to-fab workflow based on recent advances that can operate on a low budget with a mainstream full-scale Si fabrication unit.

Materials Innovations for Quantum Technology Acceleration: A Perspective.

A broad perspective of quantum technology state of the art is provided and critical stumbling blocks for quantum technology development are identified. Innovations in demonstrating and understanding electron entanglement phenomena using bulk and low-dimensional materials and structures are summarized. Correlated photon-pair generation via processes such as nonlinear optics is discussed. Application of qubits to current and future high-impact quantum technology development is presented. Approaches for realizing unique qubit features for large-scale encrypted communication, sensing, computing, and other technologies are still evolving; thus, materials innovation is crucially important. A perspective on materials modeling approaches for quantum technology acceleration that incorporate physics-based AI/ML, integrated with quantum metrology is discussed.

Memristive Field-Programmable Analog Arrays for Analog Computing.

The increasing interests in analog computing nowadays call for multipurpose analog computing platforms with reconfigurability. The advancement of analog computing, enabled by novel electronic elements like memristors, has shown its potential to sustain the exponential growth of computing demand in the new era of analog data deluge. Here, a platform of a memristive field-programmable analog array (memFPAA) is experimentally demonstrated with memristive devices serving as a variety of core analog elements and CMOS components as peripheral circuits. The memFPAA is reconfigured to implement a first-order band pass filter, an audio equalizer, and an acoustic mixed frequency classifier, as application examples. The memFPAA, featured with programmable analog memristors, memristive routing networks, and memristive vector-matrix multipliers, opens opportunities for fast prototyping analog designs as well as efficient analog applications in signal processing and neuromorphic computing.

Perry Syndrome with a Novel Mutation and a Rare Presentation: First Report from India.

Objective: To characterize the first patient of Perry syndrome reported from India. Methods: A 62-year-old gentleman presented with acute encephalopathy, hypercapnia, central hypoventilation, and seizures. He required ventilatory support for persistent respiratory failure even after the resolution of the encephalopathy. History revealed symptoms of orthostatic hypotension, episodes of shallow breathing, unsteadiness of gait, anxiety and depression, and significant weight loss for the previous two years. His mother and elder brother had succumbed to a similar illness. Investigations for neuromuscular diseases, including myasthenia and Pompes disease, were negative. Genetic tests for muscular dystrophies and myopathies, investigations for infectious, autoimmune, and para-neoplastic diseases were negative. Neuroimaging and electrophysiological studies were unremarkable. During his hospital stay, he developed rigidity and bradykinesia. Results: In view of the prominent respiratory failure, Parkinsonism, unexplained weight loss, and family history, he was tested for Perry syndrome. A heterozygous missense variation in Exon 2 of the DCTN1 gene that results in the substitution of Proline for Alanine at codon 45 (pA45P) was detected. This variant was not detected in his clinically unaffected brother. The clinical presentation and genetic test indicate Perry syndrome, a rare autosomal dominant fatal disease, which has never been reported from India. The patient improved with Levodopa and neurorehabilitation but eventually succumbed to his illness three years later. Conclusion: Perry syndrome, though rare, should be considered in the differential diagnosis of patients with a family history of Parkinsonism and central hypoventilation.

Shape memory polymer composites (SMPCs) using interconnected nanowire network foams as reinforcements.

Shape memory polymers (SMPs), although offer a suite of advantages such as ease of processability and lower density, lag behind their shape memory alloy counterparts, in terms of mechanical properties such as recovery stress and cyclability. Reinforcing SMPs with inorganic nanowires and carbon nanotubes (CNTs) is a sought-after pathway for tailoring their mechanical properties. Here, inorganic nanowires also offer the added advantage of covalently binding the fillers to the surrounding polymer matrices via organic molecules. The SMP composites (SMPCs) thus obtained have well-engineered nanowire-polymer interfaces, which could be used to tune their mechanical properties. A well-known method of fabricating SMPCs involving casting dispersions of nanowires (or CNTs) in mixtures of monomers and crosslinkers typically results in marginal improvements in the mechanical properties of the fabricated SMPCs. This is owed to the constraints imposed by the rule-of-mixture principles. To circumvent this limitation, a new method for SMPC fabrication is designed and presented. This involves infiltrating polymers into pre-fabricated nanowire foams. The pre-fabricated foams were fabricated by consolidating measured quantities of nanowires and a sacrificial material, such as (NH4)2CO3, followed by heating the consolidated mixtures for subliming the sacrificial material. Similar to the case of traditional composites, use of silanes to functionalize the nanowire surfaces allowed for the formation of bonds between both the nanowire-nanowire and the nanowire-polymer interfaces. SMPCs fabricated using TiO2nanowires and SMP composed of neopentyl glycol diglycidyl ether and poly(propylene glycol) bis(2-aminopropyl ether) (Jeffamine D230) in a 2:1 molar ratio exhibited a 300% improvement in the elastic modulus relative to that of the SMP. This increase was significantly higher than SMPC made using the traditional fabrication route. Well-known powder metallurgy techniques employed for the fabrication of these SMPCs make this strategy applicable for obtaining other SMPCs of any desired shape and chemical composition.

Recyclability of Vitrimer Materials: Impact of Catalyst and Processing Conditions.

With sustainability at the forefront of material research, recyclable polymers, such as vitrimers, have garnered increasing attention since their introduction in 2011. In addition to a traditional glass-transition temperature (T g), vitrimers have a second topology freezing temperature (T v) above which dynamic covalent bonds allow for rapid stress relaxation, self-healing, and shape reprogramming. Herein, we demonstrate the self-healing, shape memory, and shape reconfigurability properties as a function of experimental conditions, aiming toward recyclability and increased useful lifetime of the material. Of interest, we report the influence of processing conditions, which makes the material vulnerable to degradation. We report a decreased crosslink density with increased thermal cycling and compressive stress. Furthermore, we demonstrate that shape reconfigurability and self-healing are enhanced with increasing compressive stress and catalyst concentration, while their performance as a shape memory material remains unchanged. Though increasing the catalyst concentration, temperature, and compressive stress clearly enhances the recovery performance of vitrimers, we must emphasize its trade-off when considering the material degradation reported here. While vitrimers hold great promise as structural materials, it is vital to understand how experimental parameters impact their properties, stability, and reprocessability before vitrimers reach their true potential.

N/O→B dative bonds supplemented by N-HN/HC hydrogen bonds make BN-cages an attractive candidate for DNA-nucleobase adsorption - an MP2 prediction.

The response of B12N12-nanocages towards DNA-nucleobases (adenine, guanine, cytosine, and thymine) is investigated using MP2 and DFT (M06-2X) levels of theory with the 6-311+G** basis set. Multiple BN-cage-nucleobase structures for each nucleobase emerged depending on the number of Lewis base centers of nucleobases. The main source of stability of these complexes is the N/O→B dative bond, where the N or O atom of nucleobases donates the lone-pair electron to one of the boron atoms of the nanocage. Nitrogen atoms of the BN-cage, adjacent to the B-site forming dative bond, act as a proton acceptor to form multiple (N-HN and N-HC) hydrogen bonds, where proton-donors NH and CH are part of nucleobases. MP2/6-311+G** adsorption energies are -43.1, -43.4 and -45.3 kcal mol-1 (B12N12-adenine), -37.1, -41.9 and -43.3 kcal mol-1 (B12N12-guanine), -41.3 and -43.4 (B12N12-cytosine), and -29.3 and -31.3 (B12N12-thymine). Similar adsorption energies were recorded for larger BN-fullerenes-nucleobases, namely B16N16 and B24N24. Changes in adsorption energies and structures of these nano-bio-hybrid materials in aqueous media are also discussed. Computationally cost-effective MP2 single point calculations at the M06-2X optimized geometries were found to be reliable in predicting adsorption energies. The effect of the BN-network and H-bonds on the adsorption process is assessed by comparing the results with simple BH3-nucleobase models. BSSE correction to the adsorption energy is not recommended.

Synthesis and Characterization of Biotene: A New 2D Natural Oxide From Biotite.

In this work, the synthesis and characterization of ultrathin metal oxide, called biotene, using liquid-phase exfoliation from naturally abundant biotite are demonstrated. The atomically thin biotene is used for energy harvesting using its flexoelectric response under multiple bending. The effective flexoelectric response increases due to the presence of surface charges, and the voltage increases up to ≈8 V, with a high mechano-sensitivity of 0.79 V N-1 for normal force. This flexoelectric response is further validated by density functional theory (DFT) simulations. The atomically thin biotene shows an increased response in the magnetic field and thermal heating. The synthesis of two-dimensional (2D) metal-oxide biotene suggests a wealth of future 2D-oxide material for energy generation and energy harvesting applications.

Energy Harvesting Using Cotton Fabric Embedded with 2D Hexagonal Boron Nitride.

Continuous health monitoring through sensitive physiological signals (using a wearable device) is crucial for the early detection of heart diseases and breathing problems. Here, we have developed a flexible hBN/cotton hybrid device that can detect minor signals such as heartbeat and breathed-out air pressure. Systematic observation of the real-time motion sensing showed a peak-to-peak voltage output of ∼1.5 V for each heart rate pulse. The as-fabricated device showed a high voltage output of up to ∼10 V upon applying a pressure of ∼3 MPa. The FTIR results and DFT calculation suggested a chemical interaction between hBN and cellulose, giving rise to flat band characteristics and partially filled σ-bonding (sp2) hybridization. The atomic-scale chemical interface between atomically thin hBN and surface functional groups present on cotton resulted in charge localization and enhanced output voltage. An hBN/cotton hybrid device can bring new insights and opportunities to develop a self-charging and health-monitoring energy-harvesting cloth.

Two-dimensional cobalt telluride as a piezo-tribogenerator.

Two-dimensional (2D) materials have been shown to be efficient in energy harvesting. Here, we report the use of waste heat to generate electricity via the combined piezoelectric and triboelectric properties of 2D cobalt telluride (CoTe2). The piezo-triboelectric nanogenerator (PTNG) produced an open-circuit voltage of ∼5 V under 1 N force and the effect of temperature in the range of 305-363 K shows a four-fold energy conversion efficiency improvement. The 2D piezo-tribogenerator shows excellent characteristics with a maximum voltage of ∼10 V, fast response time, and high responsivity. Density functional theory was used to gain further insights and validation of the experimental results. Our results could lead to energy harvesting approaches using 2D materials from various thermal sources and dissipating waste heat from electronic devices.

Review of strategies toward the development of alloy two-dimensional (2D) transition metal dichalcogenides.

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted significant attention owing to their prosperity in material research. The inimitable features of TMDCs triggered the emerging applications in diverse areas. In this review, we focus on the tailored and engineering of the crystal lattice of TMDCs that finally enhance the efficiency of the material properties. We highlight several preparation techniques and recent advancements in compositional engineering of TMDCs structure. We summarize different approaches for TMDCs such as doping and alloying with different materials, alloying with other 2D metals, and scrutinize the technological potential of these methods. Beyond that, we also highlight the recent significant advancement in preparing 2D quasicrystals and alloying the 2D TMDCs with MAX phases. Finally, we highlight the future perspectives for crystal engineering in TMDC materials for structure stability, machine learning concept marge with materials, and their emerging applications.

Photoabsorption Imaging at Nanometer Scales Using Secondary Electron Analysis.

Optical imaging with nanometer resolution offers fundamental insights into light-matter interactions. Traditional optical techniques are diffraction limited with a spatial resolution >100 nm. Optical super-resolution and cathodoluminescence techniques have higher spatial resolutions, but these approaches require the sample to fluoresce, which many materials lack. Here, we introduce photoabsorption microscopy using electron analysis, which involves spectrally specific photoabsorption that is locally probed using a scanning electron microscope, whereby a photoabsorption-induced surface photovoltage modulates the secondary electron emission. We demonstrate spectrally specific photoabsorption imaging with sub-20 nm spatial resolution using silicon, germanium, and gold nanoparticles. Theoretical analysis and Monte Carlo simulations are used to explain the basic trends of the photoabsorption-induced secondary electron signal. Based on our current experiments and this analysis, we expect that the spatial resolution can be further improved to a few nanometers, thereby offering a general approach for nanometer-scale optical spectroscopic imaging and material characterization.

Density functional tight binding study of ß-Ga2O3: Electronic structure, surface energy, and native point defects.

A new parameter set to model monoclinic gallium oxide, ß-Ga2O3, with the density functional tight binding (DFTB) method is developed. Using this new parameter set, DFTB calculations of bulk electronic band structure, surface energy of low-index surfaces, and formation energy of native point vacancy defects are performed and compared with the state-of-the-art density functional theory (DFT) calculations using the advanced hybrid exchange correlation functional. DFTB calculates the bandgap energy of 4.87 eV around the Fermi energy with the conduction band approximately following the DFT study by Peelaers and Van de Walle [Phys. Status Solidi B 252, 828 (2015)]. The surface energies calculated feature the correct order of stability among low index surfaces with surface energies in semiquantitative agreement with Bermudez' report [Chem. Phys. 323, 193 (2006)]. Oxygen and gallium vacancy defect formation energies and respective transition levels calculated using DFTB with a new parameter set are in semiquantitative agreement with the previous DFT reports by Varley et al. and Zacherle et al. [Appl. Phys. Lett. 97, 142106 (2010); Phys. Rev. B 87, 235206 (2013)]. This new semiempirical parameter set for ß-Ga2O3, validated in bulk, surface, and point properties, would be useful for large spatiotemporal quantum chemical calculations regarding ß-Ga2O3.

Developing nanotube junctions with arbitrary specifications.

Experimentally synthesized carbon nanotube (CNTs) junctions (either single or with 2D/3D CNT network topology) are expected to have random orientation of defect sites (non-hexagonal rings) around the junction. This random and irregular nature of the junction topology and defect characteristics is expected to affect their strength and durability as well as impact the associated mesoscopic and macroscopic properties. On the contrary, theoretical and computational studies often investigate structure-property relationships of pristine and regular junctions of carbon nanostructures. In this study, we developed a computational framework to model a variety of junction structures between CNTs with arbitrary spatial (orientation and degree of overlap) and intrinsic (chirality) specifications. The developed computational model also has the ability to tune the degree of topological defects around the junction via a variety of defect annihilation approaches. Our method makes use of the primal/dual meshing concept, where the development and manipulation of the junction nodes occur using triangular meshes (primal mesh), which is eventually converted to its dual mesh (honeycomb mesh) to render a fully covalently bonded CNT junction. Here each carbon atom has 3 bonded neighbors (mimicking sp2 hybridization). Under a given set of CNT orientation, overlap and chirality specifications, the approach creates a number of CNT junction configurations with varying degrees of energetic stability, offering an opportunity to investigate the effect of topological arrangement of defects around the junction on mechanical, electrical and thermal properties. In addition, it is shown via few examples that the discussed methodology can easily be extended to create multi-junction nanotube clusters, multi-wall nanotube junctions, as well as true 3D random network structures.

In situ mechanical investigation of carbon nanotube-graphene junction in three-dimensional carbon nanostructures.

Hierarchically organized three-dimensional (3D) carbon nanotubes/graphene (CNTs/graphene) hybrid nanostructures hold great promises in composite and battery applications. Understanding the junction strength between CNTs and graphene is crucial for utilizing such special nanostructures. Here, in situ pulling an individual CNT bundle out of graphene is carried out for the first time using a nanomechanical tester developed in-house, and the measured junction strength of CNTs/graphene is 2.23 ± 0.56 GPa. The post transmission electron microscopy (TEM) analysis of remained graphene after peeling off CNT forest confirms that the failure during pull-out test occurs at the CNT-graphene junction. Such a carefully designed study makes it possible to better understand the interfacial interactions between CNTs and graphene in the 3D CNTs/graphene nanostructures. The coupled experimental and computational effort suggests that the junction between the CNTs and the graphene layer is likely to be chemically bonded, or at least consisting of a mixture of chemical bonding and van der Waals interactions.