Transition Barrier Research Articles

First-Principles Investigations on the Semiconductor-to-Metal Phase Transition of 2D Si2Te3 for Reversible Resistive Switching.

Si2Te3 is attracting attention due to its compatibility with Si technology while still showing advantages as a two-dimensional layered material. Although recent experimental studies have observed the resistive switching process in Si2Te3-based memristors, the mechanism has not been clearly identified. In this study, first-principles density functional theory calculations are employed to understand the relationship between the phase transition of Si2Te3 and the reversible resistive switching of the Si2Te3-based memristor. Our calculation results show that although semiconducting Si2Te3 is energetically more stable than two metallic Si2Te3 phases (α and β), two metallic Si2Te3 can be energetically stabilized by excess holes. The enhanced energetic preference of two metallic Si2Te3 by excess holes is explained by the reduced occupation of antibonding states between Si and Te. Our study finds that the energy barrier for the phase transition between semiconducting Si2Te3 and α-metallic Si2Te3 varies significantly by excess charge carriers so the phase transition can be directly connected to the reversible resistive switching of the Si2Te3-based memristor under external bias. Our finding will serve as a cornerstone for optimizing the resistive switching process of the Si2Te3-based memristor.

Precarity and Struggles of Employment: A Case Study of Undergraduates in Hong Kong

Despite having acquired an education that should prepare them for the workforce, many fresh graduates are facing difficulties in securing gainful employment in Hong Kong. Drawing on the concept of “employment precarity”, this study examines the predicaments that fresh university graduates encounter in Hong Kong in relation to social identity transition and job-seeking barriers. Using Critical Discourse Analysis, the researchers adopt a qualitative methodology to examine the written reflections of eight undergraduate students at a university in Hong Kong. (1) The findings reveal that students experience frustration, anxiety and uncertainty during the transition, indicating a need for improvement in the current pedagogical policies. (2) Recommendations are offered for higher education policymakers and universities to help alleviate these hardships. New sets of pedagogical strategies are developed to mitigate graduates’ exposure to employment precarity and to enhance their employability.

Frequency regulation of Terahertz quantum cascade lasers by changing barrier thickness

Terahertz (THz) sources with different frequency are important to gas detection. THz quantum cascade lasers (QCLs) based on resonant phonon structure with different frequency emission by changing transition barrier thickness are hereby presented. By theoretically calculation, a 0.19 nm thickening change of transition barrier would cause an approximately 350 GHz regulation range in frequency, and is then confirmed by experimental results. The corresponding far field beam profile keeps almost unchanged. With the increase of the transition barrier thickness, the output power only decreases by 15%, but the threshold current density decreases 24%. This simple method contributes to a satisfactory gas-detection approach with a stability of relative high output power.

Mechano-/seeding-triggered crystal-to-crystal phase transition in luminescent switching cocrystals

Currently, there are only a few cases of both mechanical stimulation-induced and seeding-triggered fluorescence conversion in which the mechanism of phase transition remains less well investigated. Herein, the first report of a luminescent switch based on mechano-/seeding-triggered crystal-crystal phase transition in a cocrystal polymorphic system is present. Based on the structure analysis and theoretical calculations, the luminescent switching can be attributed to the orbital overlap of the chromophores and exciton coupling. Furthermore, a mechanism of the phase transition was proposed, in which the synthons undergo movements including slipping and twisting during the phase transition. The effects of heating, local mechanical stimulus, and seed contact on the transition were discussed. It was proposed that contacting seeds can lower the energy barrier of the phase transition by inducing the formation of intermediate transition states of synthons via molecular recognition. The energy barrier was found to be the key factor in such mechano-/seeding-triggered phase transition.

Unraveling two distinct polymorph transition mechanisms in one n-type single crystal for dynamic electronics

Cooperativity is used by living systems to circumvent energetic and entropic barriers to yield highly efficient molecular processes. Cooperative structural transitions involve the concerted displacement of molecules in a crystalline material, as opposed to typical molecule-by-molecule nucleation and growth mechanisms which often break single crystallinity. Cooperative transitions have acquired much attention for low transition barriers, ultrafast kinetics, and structural reversibility. However, cooperative transitions are rare in molecular crystals and their origin is poorly understood. Crystals of 2-dimensional quinoidal terthiophene (2DQTT-o-B), a high-performance n-type organic semiconductor, demonstrate two distinct thermally activated phase transitions following these mechanisms. Here we show reorientation of the alkyl side chains triggers cooperative behavior, tilting the molecules like dominos. Whereas, nucleation and growth transition is coincident with increasing alkyl chain disorder and driven by forming a biradical state. We establish alkyl chain engineering as integral to rationally controlling these polymorphic behaviors for novel electronic applications.

Origin of the Anomalous Electrical Transport Behavior in Fe-Intercalated Weyl Semimetal Td -MoTe2.

Weyl semimetal Td -MoTe2 has recently attracted much attention due to its intriguing electronic properties and potential applications in spintronics. Here, Fe-intercalated Td -Fex MoTe2 single crystals (0 < x< 0.15 ) are grown successfully. The electrical and thermoelectric transport results consistently demonstrate that the phase transition temperature TS is gradually suppressed with increasing x. Theoretical calculation suggests that the increased energy of the Td phase, enhanced transition barrier, and more occupied bands in 1T' phase is responsible for the suppression in TS . In addition, a ρα -lnT behavior induced by Kondo effect is observed with x ≥ 0.08, due to the coupling between conduction carriers and the local magnetic moments of intercalated Fe atoms. For Td -Fe0.15 MoTe2 , a spin-glass transition occurs at ≈10 K. The calculated band structure of Td -Fe0.25 MoTe2 shows that two flat bands exist near the Fermi level, which are mainly contributed by the dyz and orbitals of the Fe atoms. Finally, the electronic phase diagram of Td -Fex MoTe2 is established for the first time. This work provides a new route to control the structural instability and explore exotic electronic states for transition-metaldichalcogenides.

Transition experiences of candidate attorneys in South African law firms

This study explored the school-to-work transition experiences of new candidate attorneys within South African law firms. The participants included a purposive voluntary sample of 13 candidate attorneys (female = 76.9%, black = 69.2%, mean age = 24.7 years). They completed semi-structured in-depth interviews on the barriers and facilitators of their transition experiences in the working world as new employees leaving university. Thematic analysis of the data identified the following school-to-work transition barriers: (i) adapting to the world of work; (ii) misaligned, high and unrealistic expectations; (iii) organisational culture; and (iv) candidate attorneys’ experience. The candidate attorneys identified (v) positive organisational socialisation, and (vi) coping strategies as school-to-work facilitators. The findings indicate a need for work induction support programmes and workplace culture changes to mitigate the barriers candidate attorneys experience when entering the world of work.

Transition practices for survivors of childhood cancer: a report from the Children's Oncology Group.

Pediatric healthcare systems must support childhood cancer survivors to optimize their transition to adult care. This study aimed to assess the state of healthcare transition services provided by Children's Oncology Group (COG) institutions. A 190-question online survey was distributed to 209 COG institutions to assess survivor services, including transition practices, barriers, and implementation of services aligned with the six core elements of Health Care Transition 2.0 from the US Center for Health Care Transition Improvement. Representatives from 137 COG sites reported on institutional transition practices. Two-thirds (66.4%) of site discharge survivors to another institution for cancer-related follow-up care in adulthood. Transfer to primary care (33.6%) was a commonly reported model of care for young adult-aged survivors. Site transfer at ≤ 18 years (8.0%), ≤ 21 years (13.1%), ≤ 25 years (7.3%), ≥ 26 years (12.4%), or when survivors are "ready" (25.5%). Few institutions reported offering services aligned with the structured transition process from the six core elements (Median = 1, Mean = 1.56, SD = 1.54, range: 0-5). The most prevalent barriers to transitioning survivors to adult care were perceived lack of late-effects knowledge among clinicians (39.6%) and perceived lack of survivor desire to transfer care (31.9%). Most COG institutions transfer adult-aged survivors of childhood cancer elsewhere for survivor care, yet few programs report delivering recognized standards for quality healthcare transition programming to support survivors. Development of best practices for survivor transition is needed to help promote increased early detection and treatment of late effects among adult survivors of childhood cancer.

Flexo-Ferroelectricity and a Work Cycle of a Two-Dimensional-Monolayer Actuator.

Well recognized mechanical flexibility of two-dimensional (2D) materials is shown to bring about unexpected behaviors to the recently discovered monolayer ferroelectrics, especially those displaying normal, off-plane polarization. A "ferro-flexo" coupling term is introduced into the energy expression, to account for the connection of ferroelectricity and bending (strain gradient) of the layer, to predict and quantify its spontaneous curvature and how it affects the phase transitions. With InP as a chemically specific representative example, the first-principles calculations indeed reveal strong coupling ∼P·ϰ between the ferroelectric polarization (P) and the curvature of the layer (ϰ ≡ 1/r), having profound consequences for both mechanics and ferroelectricity of the material. Due to flexural relaxation, the spontaneous polarization and the transition barrier rise significantly, leading to large changes in the Curie temperature, coercive field, and domain wall width and energy, based on Monte Carlo simulations. On the other hand, the polarization switching, characteristic to ferroelectrics, does induce an overall layer bending, enabling a conversion of electrical signal to movement as an actuator; its possible work-cycles and maximum work-efficiency are briefly discussed.

Does the current electricity grid support a just energy transition? Exploring social and economic dimensions of grid capacity for residential solar photovoltaic in Sweden

Through the Just Energy Transition in the Green New Deal, the European Union has emphasized the importance of inequality related to the energy transition. Yet, inequalities related to the hosting capacity for residential solar PV has gotten limited attention. Using estimations of hosting capacity for residential solar PV we analyze how residential solar PV hosting capacity is distributed based on social and economic status in Sweden. We find that solar photovoltaic hosting capacity is not equally distributed and that households with a high socio-economic burden tend to have less available hosting capacity for residential solar PV than households with a low socio-economic burden. A distribution of solar photovoltaic systems that best utilizes the existing grid would therefore on average be unfavorable for households with a high socio-economically burden. These findings could present a barrier for a Just energy transition towards a climate neutral energy system, and lead to increased inequalities in cost and access to renewable energy technologies, especially for countries with lower hosting capacity levels. To reduce potential inequality in grid access, we propose that policies either support the adoption of many small solar PV systems to avoid hosting capacity issues, or support households that currently lacks physical access to the residential solar PV market by allowing investments in solar PV systems at another property.

IGFBP2 derived from PO-MSCs promote epithelial barrier destruction by activating FAK signaling in nasal polyps

The nasal polyps (NPs) microenvironment comprises multiple cell types, including mesenchymal stromal cells (MSCs). Insulin-like growth factor binding protein 2(IGFBP2) plays crucial roles in cell proliferation, differentiation and more. However, the role of NPs-derived MSCs (PO-MSCs) and IGFBP2 in NPs pathogenesis remains poorly defined. Herein, primary human nasal epithelial cells (pHNECs) and MSCs were extracted and cultured. Extracellular vesicles (EVs) and soluble proteins were isolated to investigate the role of PO-MSCs on epithelial-mesenchymal transition (EMT) and epithelial barrier function in NPs. Our data showed that IGFBP2, but not EVs from PO-MSCs (PO-MSCs-EVs), exhibited a crucial role in EMT and barrier destruction. Moreover, focal adhesion kinase (FAK) signaling pathway is necessary for IGFBP2 to exert its functions in human and mice nasal epithelial mucosa. Altogether, these findings may improve the current understanding of the role of PO-MSCs in NPs microenvironment and ultimately contribute to the prevention and treatment of NPs.

Mechanochemical Activation of Anthracene [4+4] Cycloadducts

Controlled formation and breaking of weak chemical bonds is a versatile method for modifying the properties of materials. Anthracene [4+4] cycloadducts are a prime example that can be formed by light and opened by external forces. We address the theoretical description of mechanochemistry of these cycloadducts, where the standard constraint geometry simulates forces approach fails due to the lack of consideration of temperature. Explicit inclusion of external forces reveals the corresponding transition barriers that are clearly dominated by rupture of the [4+4] inter-anthracene bonds. Other bonds come into play at extremely large forces only, which cannot be expected to be reached under ambient conditions. The theoretical results are in line with the experimental rheology of [4+4]-linked anthracene polymers, which indicates reversible re-formation of [4+4] cycloaddition bonds with ultraviolet light after mechanochemical bond breaking due to applied shear stress.

Unveiling Cas8 dynamics and regulation of a transposon-encoded cascade-TniQ complex.

Csy (CRISPR-System Yersinia, or ‘Cascade’) is a class 1, type 1F CRISPR-Cas system which utilizes RNA-guided strand invasion to target viral DNA and designate it for cleavage by an external nuclease (Cas2/3). It has been recently shown, in Vibrio Cholerae, that there exists a variant of Cascade encoded in a transposon (Tn6677). This variant binds to a TniQ homodimer, resulting in a ‘Cascade-TniQ’ complex which, unlike the original Cascade that targets viral DNA for cleavage, recruits a heteromeric transposase responsible for integration of Tn6677. This integration process starts with binding of Cascade's crRNA to the DNA target sequence located in the bacterial genome, resulting in the formation of the ‘R Loop,’ this step is the focus of the current research. Both classical molecular dynamics (cMD) and enhanced sampling methods (GaMD) have been employed to better understand the dynamical state of the system in the RNA-only and the DNA-bound configurations. We have found evidence for a multi-state like behavior, with drastic differences in the state population and transition barriers upon DNA binding. Furthermore, it appears that the TniQ homodimer undergoes changes in large-scale conformations dependent on its level of intrinsic stability. This allowed us to make a tentative claim on a role of Zinc-coordination within TniQ's Zinc-finger binding domains. Tying our findings together with data from literature allowed us to make general comments on how the RNA-bound state and R Loop formation may be regulated. Hopefully, these findings will eventually allow for a more streamlined effort in re-engineering Cascade-TniQ for use in mammalian cells.

Thermodynamics and kinetics of the B-S transition in base-modified DNA.

Understanding changes in the mechanical properties of nucleic acids is important to understand many biological processes. DNA exhibits a conformational change from canonical B- to overstretched S-form, characterized by 70% elongation, at ∼60–70 pN. The structure of S-form DNA is still debated and also difficult to distinguish from force-induced melting and peeling of DNA. Here, we characterized the kinetics and thermodynamics of the B-S transition and also investigated the effect of base modifications on the transition using tricyclic cytosine tC, which forms hydrogen bonds with guanine and changes the base stacking properties of B-DNA. The B-S transition in 60 bp DNA occurs at ∼63.5 pN (at 1 M salt) and fraying of ∼20 bp precedes the cooperative B-to-S transition of the remaining ∼40 bp. The B-S transition force increases gradually with tC incorporation, whereas the extension and free energy of the transition decrease. We conclude that fewer bp undergo the cooperative B-S transition in tC-stabilized duplexes, due to more fraying of B-DNA at higher transition forces, consistent with the transition becoming faster. For the first time we characterize the kinetics of the cooperative B-to-S transition and it occurs significantly faster than the alternative melting transition, while the extension of B-DNA at the B-to-S transition barrier is almost half-way between B- and S-DNA, suggesting an elastic nature of this transition. By characterizing the free energies of tC-modified relative to unmodified oligos, we estimated that each tC stabilizes the duplex by ∼3 kBT. The approach of studying short synthetic oligonucleotides with modified bases opens up new opportunities for understanding the local structure of nucleic acids with base-pair resolution.

Evaluation of the CHARMM36m force field combined with the OPC water model for protein simulations.

The accuracy of atomistic molecular dynamics (MD) simulations depends on the accuracy of the used force field. For proteins many force fields have been developed so far, each in combination with a particular water model. Typically, combining a protein force field with a water model for which is has not been developed is not expected to yield reliable results. However, recently the combination of the CHARMM36m force field with the OPC water model was shown to accurately estimate the compactness and secondary structure content of intrinsically disordered proteins. Whether CHARMM36m+OPC yields similar accuracy for globular proteins as well has, however, not yet been evaluated systematically. Here, we benchmark this combination on a set of six different globular proteins. To this end, we performed 50 x 1 μs MD simulations per protein using CHARMM36m+OPC and, for comparison, the same simulations using the well-established Amber99SB-ILDN+TIP4P force field. We compared the generated ensembles by means of RMSD, radius of gyration and secondary structure content and also compared the conformational dynamics. We found that CHARMM36m+OPC generates less compact ensembles and shows higher barriers for conformational transitions. Furthermore, we compare ensembles of both force fields to experimental crystal structures, B-factors, and NMR chemical shifts. Here, we found that both force fields compare equally well to experimental data, with CHARMM36m+OPC ensembles agreeing with chemical shifts slightly better, whereas Amber99SB-ILDN+TIP4P ensembles better agree with crystal structures. Our results show that CHARMM36m+OPC and Amber99SB-ILDN+TIP4P yield similar accuracy for globular proteins. Combined with earlier results for disordered proteins, these findings suggest that CHARMM36m+OPC should provide good accuracy for a broad range of disordered and folded proteins as well.

Synergistic effects of chain extenders and natural rubber on PLA thermal, rheological, mechanical and barrier properties

Poly (lactic acid) (PLA) has high potential to replace conventional petroleum-based plastics in a wide range of applications. However, the high brittles, thermal degradation, low melt strength and low viscosities of PLA pose challenges for its processing and performance satisfied for industrial use. Previously, we have shown that natural rubber (NR) incorporated into PLA improved the melt strength and viscosity and thus the processability of biopolyesters. Chain extension is a practical and cost-effective method of improving PLA processability by enhancing the molecular weight and re-bonding degraded chains. We hypothesized that addition of epoxy-based chain extenders will work synergistically with natural rubber to improve the elongational viscosity and strain hardening of PLA. The thermal transition, thermal degradation, mechanical and moisture barrier properties were studied to provide a comprehensive view of the chain-extended and NR-modified PLA. The results shows that chain extender with higher functionality (Joncryl 4368) had a more significant effect on chain extending, branching and restoring of PLA. Although the chain extender or the NR alone reduced both the dynamic and steady shear viscosities and minimally affected the elongational viscosity of PLA, they together resulted in shear-thickening and strain-hardening of PLA, suggesting a synergistic effect of the chain extender and NR on improving PLA rheological performance. The improved elongational viscosity of PLA was comparable to film grade low density polyethylene (LDPE). Additionally, NR addition significantly reduced the water vapor permeability of PLA. The PLA with improved melt viscosity and moisture barrier are promising for film and flexible packaging applications.

Effect of particle anisotropy on the thermodynamics and kinetics of ordering transitions in hard faceted particles.

Monte Carlo simulations were used to study the influence of particle aspect ratio on the kinetics and phase behavior of hard gyrobifastigia (GBF). First, the formation of a highly anisotropic nucleus shape in the isotropic-to-crystal transition in regular GBF is explained by the differences in interfacial free energies of various crystal planes and the nucleus geometry predicted by the Wulff construction. GBF-related shapes with various aspect ratios were then studied, mapping their equations of state, determining phase coexistence conditions via interfacial pinning, and computing nucleation free-energy barriers via umbrella sampling using suitable order parameters. Our simulations reveal a reduction of the kinetic barrier for isotropic-crystal transition upon an increase in aspect ratio, and that for highly oblate and prolate aspect ratios, an intermediate nematic phase is stabilized. Our results and observations also support two conjectures for the formation of the crystalline state from the isotropic phase: that low phase free energies at the ordering phase transition correlate with low transition barriers and that the emergence of a mesophase provides a steppingstone that expedites crystallization.

Covalent-like bondings and abnormal formation of ferroelectric structures in binary ionic salts.

In the Mooser-Pearson diagram, binary ionic compoundss form into nonpolar symmetrical structures with high coordination numbers, while wurtzite structures should appear in the covalent region. Their tetrahedral bonding configurations break the inversion symmetry, with polarizations almost unswitchable due to the high barriers of abrupt breaking and reformation of covalent bonds. Here, through first-principles calculations, we find some exceptional cases of highly ionic ferroelectric binary salts such as lithium halides, which may form into wurtzite structures with covalent-like sp3 bondings, and the origin of these abnormal formations is clarified. Their high polarizations induced by symmetry breaking are switchable, with much smoother switching pathway refrained from abrupt bond breaking due to the long-range feature of Coulomb interactions. These covalent-like ionic bondings do reduce not only their ferroelectric switching barriers but also the phase transition barriers between polar and nonpolar phases, rendering high performance in applications such as nonvolatile memory and energy storage.

From linear to a circular economy in the e‐waste management sector: Experience from the transition barriers in the United Kingdom

AbstractE‐waste generation is rising apace, and it is estimated that e‐waste production could reach 74.7 Mt by 2030. Hence, it is essential to develop tactics and methodologies for the formal management and treatment of e‐waste. The implementation of the concepts of a circular economy (CE) has been perceived as an efficient methodology for the efficacious management of such waste; however, the transition process from the linear economy to CE still involves several barriers. There are only a few studies that have integrated the concept of CE with e‐waste management for sustainability. This study will focus on the UK's household e‐waste management sector to identify and analyze the key barriers that impede the transition of the UK's e‐waste management sector to the CE model. For this purpose, a team of experts was formulated and their expertise was captured using the Decision‐making trial and evaluation laboratory (DEMATEL) methodology. The DEMATEL technique employs critical evaluation principles, measures the weights of the evaluation criteria, and helps in finding the most critical barriers through a visual structural model. The DEMATEL method also categorizes the factors into cause‐and‐effect groups and utilizes a visualization model to simplify the interdependence and intensity of the impacts of the various factors. The results revealed that the “Lack of processing technologies,” “Lack of returns and profits,” “Lack of expert knowledge,” “Lack of effective guidance and standards for the proper collection methods,” and “Lack of proper knowledge” are the most significant barriers obstructing the adoption of CE in the management of e‐waste in the United Kingdom. Thus, the government needs to take strong actions to give financial support in terms of incentives to the stakeholders for effective e‐waste handling.

Challenges and barriers in transition to circular economy business models

Circular Economy (EC) is a term that has not been studied fully yet and is attracting the growing interest of scholars. It is vital for our planet and each of our lives to take care of EC principles. In this process, the business sector plays a leading role. This process is challenging and requires understanding circular economy principles first, after analyzing linear business models and comparing them to the CBM (Circular Economy Business Model). Based on these, it will be easier for businesses to adopt any of them. The aim of the paper is to discuss these and give recommendations based on the literature, research papers, and publications available at this moment. Key Findings: The barriers to moving from BM to CBM are still strong, but business behavior shows that there are already some signs of a circular economy, which can be considered an important step for the planet and people’s lives; recommendations are given on how to help businesses to overcome obstacles on this difficult path.