Coordinate Diagram Research Articles

Dual role of oxygen-related defects in the luminescence kinetics of AlN:Mn2.

This study presents the impact of temperature and pressure on AlN:Mn2+ luminescence kinetics. Unusual behavior of Mn2+ optical properties during UV excitation is observed, where a strong afterglow luminescence of Mn2+ occurs even at low temperatures. When the temperature increases, the contribution of the afterglow luminescence is further enhanced, causing a significant increase in the luminescence intensity. The observed phenomena may be explained by an energy diagram in which the ON-VAl complex in AlN:Mn2+ plays a key role. Hence the ON-VAl complex defect in AlN:Mn2+ plays a double role. When the ON-VAl defect is located close to Mn2+ ions, it is responsible for transferring excitation energy directly to Mn2+ ions. However, when the ON-VAl defect complex is located far from Mn2+ ions, its excited state level acts as an electron trap responsible for afterglow luminescence. Additionally, three models have been tested to explain the structure of the emission spectrum and the strong asymmetry between the excitation and emission spectra. From the most straightforward configuration coordinate diagram through the configuration coordinate diagram model assuming different elastic constants in the excited and ground-states ending by a model based on the Jahn-Teller effect. We proved that only the Jahn-Teller effect in the excited 4T1 electronic state with spin-orbit coupling could fully explain the observed phenomena. Finally, high-pressure spectroscopic results complemented by the calculations of Racah parameters and the Tanabe-Sugano diagram are presented.

Structural, luminescent properties and Judd-Ofelt analysis of CaMgSiO4:Eu3+ phosphor for solid state lighting

A series of Eu3+ doped calcium magnesium silicate (CaMgSiO4) phosphor samples were successfully prepared via solid state synthesis technique. Their structural and morphological studies were investigated by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) measurements. Phase structure and phase purity of sample were investigated by Rietveld refinement method. The vibrational and bonding behavior of silicate groups were confirmed by Fourier transform infrared (FTIR) and Raman spectroscopy. The luminescent properties of prepared samples were studied by photoluminescence (PL) and thermoluminescence (TL) characteristics. The emission spectra recorded under the excitation wavelength λex = 396 nm. The highest emission peak was observed at 595 nm which corresponds to the 5D0 → 7F1 transition of Eu3+ ions with selection rule ΔJ = 1. The concentration quenching and energy transfer mechanisms were elucidated by Blasse and Dexter's formula. The photometric parameters correlated color temperature (CCT), color rendering index (CRI) and color purity (CP) were computed using CIE chromaticity co-ordinate diagram. In addition, Judd-Ofelt (JO) parameters (Ω2, Ω4) and other derived radiative parameters were calculated using Judd-Ofelt theory from the PL emission spectra. Time resolved fluorescence spectra were carried out to determine the decay lifetime of the samples. The prepared samples are expected to be suitable candidates for the applications as solid state devices, lighting and optical displays.

Определение тимола с предварительным мицеллярно-экстракционным концентрированием

For the efficient preconcentration of azo compounds – products of the interaction of 4-nitrophenyldiazonium with thymol, a system 4-nitroaniline (4-NA) – NO2- – Triton X-100 – NaOH – ethanol has been proposed. The optimal conditions for the formation of micellarsaturated phases of the system under study have been established: 3.10-4 M 4-NA – 3.10-4 M NO2- – 5% Triton X-100 – 2.8 M NaOH – 10 vol. % С 2Н5ОН. A spectrophotometric study of the above system has been carried out. A linear dependence was built in the coordinates A (at λ max = 552 nm) vs с(thymol), which is described by an equation of the form A = f(c), A = 26291c + 0.02; R2 = 0.997. The range of the determined contents of thymol is (2·10-6 – 4·10-5) mol/l. A technique for the colorimetric determination of thymol in aqueous media (color channel G) has been developed. The intensity of the channel G chromaticity (IG) is linearly dependent on pc(thymol) in accordance with the equation IG = 54.2pc – 267, R 2 = 0; the lower limit of the determined contents of thymol is 1.10-6 mol/l, which is two times less than in the variant of its spectrophotometric determination. The profiles of petal diagrams in the color coordinates of the RGB CMYK model have been constructed; the dependences of their area (S) and perimeter (P) on the thymol concentration have been obtained (P: y = 278x – 10.13; R 2 = 0.97; S: y = 20182x – 87649, R 2 = 0.99).

Luminescent properties and energy transfer mechanism from Tb3+ to Eu3+ in single-phase color-adjustable Sr3MgSi2O8:Eu3+,Tb3+ phosphor prepared by the sol-gel method

A series of single-phase color-adjustable Sr3MgSi2O8:Eu3+,2Tb3+ phosphors were synthesized with the application of sol-gel method. The crystal structure, luminescence characteristics, fluorescence lifetime, energy transfer mechanism and thermal stability of phosphors were investigated. The results show that the emission spectrum of Sr3−x−yMgSi2O8:xEu3+,yTb3+ compose both the 5D4→7F5 transition of Tb3+ and the 5D0→7F1, 5D0→7F2 transition of Eu3+ at the excitation wavelength of 238 nm. The optimal doping concentration of Tb3+ was determined to be 2% when concentration quenching effect was comprehensively considered. On the basis of the energy transfer of Tb3+→Eu3+, the emission peak intensity of Eu3+ increases and that of Tb3+ decreases with the rising Eu3+ concentration. The unidirectional energy transfer efficiency of Tb3+→Eu3+ reaches 89.4% when the Eu3+ doping concentration is 8%. On the basis of Dexter's theory, the major mechanism of energy transfer lies in the electric dipoles interaction between Tb3+ and Eu3+. With an activation energy of 0.309 eV, Sr2.95MgSi2O8:3%Eu3+,2%Tb3+ phosphors enjoys good thermal stability. Its color coordinate diagram indicates that the luminous colors of phosphors can be customized from red to orange, then to green with the changing doping concentrations of Eu3+ and Tb3+. Based on Sr3MgSi2O8:Eu3+,Tb3+ phosphors has a good matchability with ultraviolet and near-ultraviolet LED chips, so it can be considered that Sr3MgSi2O8:Eu3+,Tb3+ phosphors have good application potential in the field of ultraviolet and near-ultraviolet white light emitting diodes.

Walking with robot-generated haptic forces in a virtual environment: a new approach to analyze lower limb coordination

BackgroundWalking with a haptic tensile force applied to the hand in a virtual environment (VE) can induce adaptation effects in both chronic stroke and non-stroke individuals. These effects are reflected in spatiotemporal outcomes such as gait speed. However, the concurrent kinematic changes occurring in bilateral lower limb coordination have yet to be explored.MethodsChronic stroke participants were stratified based on overground gait speed into lower functioning (LF < 0.8 m/s, N = 7) and higher functioning (HF ≥ 0.8 m/s, N = 7) subgroups. These subgroups and an age-matched control group (N = 14, CG) walked on a self-paced treadmill in a VE with either robot-generated haptic leash forces delivered to the hand and then released or with an instrumented cane. Walking in both leash (10 and 15 N) and cane conditions were compared to pre-force baseline values to evaluate changes in lower limb coordination outcomes.ResultsAll groups showed some kinematic changes in thigh, leg and foot segments when gait speed increased during force and post-force leash as well as cane walking. These changes were also reflected in intersegmental coordination and 3D phase diagrams, which illustrated increased intersegmental trajectory areas (p < 0.05) and angular velocity. These increases could also be observed when the paretic leg transitions from stance to swing phases while walking with the haptic leash. The Sobolev norm values accounted for both angular position and angular velocity, providing a single value for potentially quantifying bilateral (i.e. non-paretic vs paretic) coordination during walking. These values tended to increase (p < 0.05) proportionally for both limbs during force and post-force epochs as gait speed tended to increase.ConclusionsIndividuals with chronic stroke who increased their gait speed when walking with tensile haptic forces and immediately after force removal, also displayed moderate concurrent changes in lower limb intersegmental coordination patterns in terms of angular displacement and velocity. Similar results were also seen with cane walking. Although symmetry was less affected, these findings appear favourable to the functional recovery of gait. Both the use of 3D phase diagrams and assigning Sobolev norm values are potentially effective for detecting and quantifying these coordination changes.

Difference of Eu3+ luminescent properties in YOCl and YOBr oxyhalide hosts

The photoluminescence spectra and luminescence lifetimes of Eu3+-doped oxyhalides, YOX:Eu3+ (X = Cl or Br), and their temperature dependence on different halide species of mixed-anion coordinations were investigated and analyzed. In terms of the ionic and covalent nature of bonding, Eu3+ ions form different coordination polyhedra in the isostructural YOCl and YOBr hosts: a ninefold [Eu3+O4Cl5] and an eightfold [Eu3+O4Br4] polyhedra. The Judd–Ofelt Ω2 parameter for YOCl:Eu3+ takes a very large value (=8.81 × 10−20 cm2) due to the ninefold polyhedron with C4v symmetry. On the other hand, despite the same C4v symmetry, YOBr:Eu3+ shows a very small Ω2 parameter (=2.72 × 10−20 cm2) because of its structural similarity to the square antiprism polyhedron with D4 symmetry. The Ω4 parameters for YOX:Eu3+ are much larger than those for other Eu3+-doped oxides, possibly related to the covalency of halide anions, Cl− and Br−, showing an intense luminescence band (5D0 → 7F4) at around 700 nm. The Eu3+ ions in these YOX hosts were excitable by charge transfer bands in 270–280 nm regions. The relaxation pathways from the charge transfer states to the initial states for luminescence are discussed, using the configuration coordinate diagrams with the spectroscopic characterization.

Exciton Self-Trapping Dynamics in 1D Perovskite Single Crystals: Effect of Quantum Tunnelling.

We present experimental and theoretical investigations of the photophysics in the one-dimensional (1D) hybrid organic-inorganic perovskite (HOIP) white-light emitter, [DMEDA]PbBr4. It is found that the broadband-emission nature of the 1D perovskite is similar to the case of two-dimensional (2D) HOIP materials, exciton self-trapping (ST) is the dominant mechanism. By comprehensive spectroscopic investigations, we observed direct evidence of exciton crossing the energy barrier separating free and ST states through quantum tunnelling. Moreover, we consider the lattice shrinking mechanisms at low temperatures and interpret the ST exciton formation process using a configuration coordinate diagram. We propose that the energy barrier separating free and ST excitons is temperature-dependent, and consequently, the manner of excitons crossing it is highly dependent on the exciting energy and temperature. For excitons located at the bottom of the free excitonic states, the quantum tunnelling is the dominant channel to the ST states.

Judd-Ofelt, radiative properties of Sm3+ ions doped PVA+PEG complex polymer films from spectral analysis

A trivalent rare earth ion Sm3+ (1-5 mol %) doped Poly (vinyl alcohol) (PVA) and Poly (ethylene glycol) (PEG) complex polymer blends were synthesized by using solution casting technique. The spectroscopic characteristics behaviour and Judd-Ofelt (J-O) intensity parameters, oscillatory strengths and radiative properties of the prepared polymer blends were investigated with the help of optical absorption and photoluminescence spectral studies. Well defined UV-VIS-NIR optical absorption bands pertaining Sm3+ ions are observed in optical absorption spectrum and these bands has been assigned to their respective electronic transitions. The theoretical oscillatory strengths (f cal) and Judd-Ofelt (J-O) network framing theory were adopted to evaluate J-O intensity parameters Ω λ (λ=2,4,6) from the experimental oscillatory strengths (f exp). The covalent characteristic nature of the Sm3+ ions with surrounding ligands within the host polymer matrix was confirmed through bonding parameter (δ). The coordinated environment of Sm3+ ions and the key significant J-O intensity parameters are used for calculation of spectroscopic radiative properties. Photoluminescence spectrum is obtained and the highly intensified emission peak is observed for the transition 4G5/2→6H7/2 at 595 nm under 401 nm excitation wavelengths. The CIE colour coordinate diagram shows that the present complex polymer films incorporated with Sm3+ ions are acting as perfect and highly stable material for active reddish-orange emissions.

The origin of the 500 nm luminescence band related to oxygen vacancies in ZrO2

In this paper, ion-beam-induced luminescence (IBIL) spectra of raw ZrO2 irradiated with 2 MeV H+ ions and photoluminescence (PL) spectra of annealed (200, 500 and 800 °C in air, nitrogen or oxygen) ZrO2 were investigated. The luminescence intensity decreased with increasing fluence, which indicated that the concentration of luminescence centers decreased during irradiation. The PL spectral results show that, with increasing partial pressure of oxygen during annealing, the intensity of luminescence decreases due to the decrease in oxygen vacancies. First-principles calculations were used to study the formation energy and transition level for oxygen vacancies (Vo), Ti-substituted Zr (Ti) and Ti adjacent to the nearest Vo (Ti-Vo) complex. The binding energies of Ti3+ and Vo + are 2.10 eV, which indicates that oxygen vacancies benefit the formation of Ti3+. The calculated configuration coordinate diagram and IBIL results indicated that Vo is not the luminescence origin. A new light-emitting structure of the Ti-Vo complex (FA center) structure was proposed based on our calculations and previous studies. The calculated results of the luminescence model match the PL and IBIL spectra remarkably. The emission band of ZrO2 at approximately 500 nm was assigned to the FA center or Ti3+ (the eg to t2g transition) adjacent to Vo+.

Solid state diffusion and amalgamating anionic exchange at a KNaSO4 phosphors activated with Eu3+ , Dy3+ and Sm3+ rare earth ions to enhance w-LED performance.

In present work, KNa(SO4 ) phosphors doped with different concentrations of rare earth Eu3+ , Sm3+ and Dy3+ ions (0.05, 0.1, 0.3, 0.5, 0.7, 1 mol%) were synthesized using a solid-state diffusion technique. Photoluminescence (PL) investigations were carried out for the whole range of Eu3+ , Sm3+ and Dy3+ -doped phosphors; rare earth ions that retained maximum PL intensity were selected for advanced anionic exchange. In the present investigation, phosphors KNa(SO4 ):Eu3+ (1 mol%), KNa(SO4 ):Dy3+ (0.5 mol%) and KNa(SO4 ):Sm3+ (0.3 mol%) had the highest PL intensity, and were therefore selected for further anionic substitution of sulphate anions with different concentrations of vanadate, phosphate, and tungstate anions, such as KNa(SO4 )1-x (MO4 )x : W (where W = Eu3+ 1 mol%, Dy3+ 0.5 mol% and Sm3+ 0.3 mol%; MO4 = PO4 , VO4 , WO4 ; and x = 0.1, 0.3, 0.5, 0.7, 1). Structural and molecular environments of the substituted phosphors were characterized individually using X-ray diffraction and Fourier transform infrared spectroscopy. In-depth morphological investigations of the prepared phosphors were undertaken using scanning electron microscopy. For the principal investigation on enhancement of white light-emitting diode (w-LED) performance, the PL properties of all the synthesized phosphors were studied analytically. Emission intensity ratios for KNa(SO4 ):Eu3+ 1 mol%, KNa(SO4 )0 (PO4 )1 :Eu 1 mol%, KNa(SO4 )0.9 (VO4 )1 :Eu 1 mol%, and KNa(SO4 )0.9 (WO4 )0.1 :Eu 1 mol% were 1:1.15:1.23:0.08. PL intensity ratios for the phosphors KNaSO4 :Dy 0.5 mol% and KNa(SO4 )0.9 (PO4 )0.1:Dy 0.5 mol% was 1:2. The ratio of PL intensity was 1:3.2:0.8 for KNa(SO4 ):Sm 1 mol%, KNa(SO4 )0.5 (PO4)0.5 :Sm 0.3 mol%, and KNa(SO4 )0.7 (VO4 )0.3 :Sm 0.3 mol% phosphors, respectively. Chromaticity investigations were carried out using Commission Internationale de l'Éclairage colour co-ordinate diagrams, which suggested that the prepared Eu3+ -doped and Sm3+ -doped phosphors would be prospective candidates for red and green LEDs, respectively, whereas Dy3+ -doped phosphors showed emission in the blue and yellow regions. The entire study indicated that amalgamation of anionic exchange at a KNaSO4 phosphor activated with Eu3+ , Dy3+ and Sm3+ rare earth ions could generate and enhance white light emission.

Measuring Changes in Undergraduate Chemistry Students’ Reasoning with Reaction Coordinate Diagrams: A Longitudinal, Multi-institution Study

The Reaction Coordinate Diagram Inventory (RCDI) was developed to measure chemistry students’ thinking and confidence when interpreting the kinetic and thermodynamic information encoded within reaction coordinate diagrams (RCDs). The RCDI was designed based upon analyses of qualitative data collected in semi-structured interviews with general chemistry students or organic chemistry students to ensure all items and distractors were grounded in students’ ideas and reasoning. A longitudinal, multi-institution research study was conducted using the RCDI to measure changes in undergraduate chemistry students’ reasoning with RCDs, and their confidence in doing so, at three points in time: (1) end of second-semester general chemistry, (2) beginning of first-semester organic chemistry, and (3) end of first-semester organic chemistry. The three institutions participating in this longitudinal study taught organic chemistry using either a traditional functional group curriculum or a mechanisms approach. Findings include evidence that individual students performed significantly better with significantly higher confidence as they progressed through courses, but that some RCD misconceptions, such as incorrectly assuming reaction progress indicates a time unit, are persistent throughout. Although two of the institutions taught organic chemistry using a mechanisms curriculum and one institution used the traditional functional group approach, no evidence exists of significant differences among students’ RCDI total score and average confidence across institutions.

Theory of the Thermal Stability of Silicon Vacancies and Interstitials in 4H–SiC

This paper presents a theoretical study of the electronic and dynamic properties of silicon vacancies and self-interstitials in 4H–SiC using hybrid density functional methods. Several pending issues, mostly related to the thermal stability of this defect, are addressed. The silicon site vacancy and the carbon-related antisite-vacancy (CAV) pair are interpreted as a unique and bistable defect. It possesses a metastable negative-U neutral state, which “disproportionates” into VSi+ or VSi−, depending on the location of the Fermi level. The vacancy introduces a (−/+) transition, calculated at Ec−1.25 eV, which determines a temperature threshold for the annealing of VSi into CAV in n-type material due to a Fermi level crossing effect. Analysis of a configuration coordinate diagram allows us to conclude that VSi anneals out in two stages—at low temperatures (T≲600 °C) via capture of a mobile species (e.g., self-interstitials) and at higher temperatures (T≳1200 °C) via dissociation into VC and CSi defects. The Si interstitial (Sii) is also a negative-U defect, with metastable q=+1 and q=+3 states. These are the only paramagnetic states of the defect, and maybe that explains why it escaped detection, even in p-type material where the migration barriers are at least 2.7 eV high.

Energy transfer and the anti-thermal quenching behavior of Sr8MgCe(PO4)7:Tb3+ for temperature sensing

Novel color-adjustable phosphors Sr8MgCe(PO4)7:Tb3+ were synthesized by the solid-state reaction method. The X-ray diffraction was used to characterize the phase formation. The host shows blue emission under 310 nm excitation, ascribed to the 5d-4f transitions of Ce3+, while Sr8MgCe(PO4)7:xTb3+ phosphors exhibit the emission band of Ce3+ as well as the characteristic lines of Tb3+ under 310 nm excitation. The color variation was observed as the concentration of Tb3+ changes. Moreover, the host sensitizes the emission of Tb3+ through the energy transfer process, and the mechanism is the dipole–dipole interaction. In addition, the emission of the host declines with the increase of the temperature while that of Tb3+ increases. This anti-thermal quenching behavior was interpreted by the configurational coordinate diagram. In addition, the temperature sensing behavior based on the emission peaks of the host and Tb was studied in the range 298–523 K, and the largest sensitivity is 0.379% K−1. The results indicate that Sr8MgCe(PO4)7:Tb3+ is an attractive color-tunable phosphor for solid state lighting and temperature sensing.

Theoretical and experimental investigations of Co-Cu bimetallic alloys-incorporated carbon nanowires as an efficient bi-functional electrocatalyst for water splitting

Application of noble metal-free electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) during electrocatalytic water splitting is crucial for clean energy conversion and has drawn extensive attention. However, the development of highly active and low cost electrocatalysts is a considerable challenge. Herein, Co-Cu alloy nanoparticles-incorporated carbon nanowires electrocatalyst was synthesized and evaluated for both OER and HER. The nanomaterials were fabricated by facile electrospinning of sol-gel composed of cobalt acetate, copper acetate, and poly(vinyl alcohol) followed by calcination in an inert environment. Adjusting the composition of the metallic counterpart was found to significantly enhance electrochemical properties of the catalyst. Furthermore, the unique nanowire morphology and structural properties of incorporated Co-Cu alloy, the (Co0.95Cu0.05@CNWs) composition exhibits good electrocatalytic performance for both OER and HER in the alkaline medium. Physicochemical characterizations using X-ray diffraction, X-ray photoelectron spectroscope, scanning electron microscopy, and transmission electron microscopy have confirmed the formation of alloy structure and nanowire morphology. The optimum composition (Co0.95Cu0.05@CNWs) requires small overpotential, ɳ10 of ∼285 mV for oxygen evolution reaction (OER) and ∼160 mV for hydrogen evolution reaction (HER) with the corresponding Tafel slope of 92 mV dec−1 and 172 mV dec−1 versus the reversible hydrogen electrode, respectively. In addition, only negligible loss in activity was observed after 1000 cycles and prodeces cell voltage of 1.58 V at current of 10 mA/cm2 and 1.72 V at current density of 50 mA/cm2 in two electrode system. Density Functional Theory (DFT) calculations were employed to verify experimental results. Electronic density of states (DOS) results reveal an increase in electronic states near the Fermi level upon Co-Cu heterojunctioning with CNWs. This is indicative of improved catalytic activity and more favorable binding energies of HER and OER intermediates. Reaction coordinate diagrams for HER and OER were developed, which aided in identifying thermodynamically limiting steps. This work may provide a feasible approach for incorporating other transition metals to design low-cost and high-performance bifunctional electrocatalysts for overall water splitting.

Design and theoretical study of superlative quantum efficiency and thermal stability phosphor: The system of Sr9Eu(PO4)7

The rare breed of phosphor material that exhibits constantly strong luminescence without undergoing quenching are designed to improve the performance of the phosphor. The Eu3+ ion occupies the sites of a quadrangular bipyramid geometry (connected by O–P–O–Eu–O–P–O bonds) that serve to effectively isolate individual Eu3+ atoms from each other in Sr9La1−xEux(PO4)7 system. Accordingly, the photoluminescence (PL) intensity continued to grow up to 100 mol% concentration of Eu3+, and this gave a rise to the prolonged lifetime to 2.51 ms in parallel with preeminent quantum efficiency up to 90%. Temperature dependent PL measurements of Sr9Eu(PO4)7 phosphor possessed stable emission to 83% of its room temperature value at 423 K. Here, we propose a new configurational coordination diagram on the bases of shape of charge transfer state. Depending on this model, the PLE spectrum can provide approximate information that are important parameters in phosphor such as the quantum efficiency, thermal quenching activation energy and optimal concentration of active ion. By virtue of this model and the rare breed phosphor material that are reported for the first time, we could easily choose a Sr9Eu(PO4)7 crystal as potential red-emitting phosphors of the future replacement decisions for lighting.

Quasi-One-Dimensional Ising Models with Defects of the “Random Local Field” Type: the Imry–Ma Phase in Spaces with a Dimension Higher than the Lower Critical Dimensionality

The phase diagram in coordinates "temperature - concentration of defects" of quasi-one-dimensional Ising models with defects of the "random local field" type is investigated. The confrontation of the tendency to the emergence of the long-range order due to a weak interaction between one-dimensional spin chains and the tendency to the formation of the Imry-Ma phase in which the order parameter follows the fluctuations of the random field created by defects is studied. The possibility of the appearance of the Imry-Ma phase in a situation where the space dimension exceeds the lower critical dimension is shown. The question of the existence of the long-range order in the Ising model with random fields in space with the critical dimension dl = 2 is considered.

Location-thinking, value-thinking, and graphical forms: combining analytical frameworks to analyze inferences made by students when interpreting the points and trends on a reaction coordinate diagram

Reaction coordinate diagrams (RCDs) are chemical representations widely employed to visualize the thermodynamic and kinetic parameters associated with reactions. Previous research has demonstrated a host of misconceptions students adopt when interpreting the perceived information encoded in RCDs. This qualitative research study explores how general chemistry students interpret points and trends on a RCD and how these interpretations impact their inferences regarding the rate of a chemical reaction. Sixteen students participated in semi-structured interviews in which participants were asked to interpret the points and trends along provided RCDs and to compare relative reaction rates between RCDs. Findings derived from this study demonstrate the diversity of graphical reasoning adopted by students, the impact of students’ interpretations of the x-axis of a RCD on the graphical reasoning employed, and the influence of these ideas on inferences made about reaction rate. Informed by analytical frameworks grounded in the resources framework and the actor-oriented model of transfer, implications for instruction are provided with suggestions for how RCDs may be presented to assist students in recognizing the critical information encoded in these diagrams.

Investigating first-year undergraduate chemistry students’ reasoning with reaction coordinate diagrams when choosing among particulate-level reaction mechanisms

Reaction coordinate diagrams (RCDs) are an important tool used to visualize the energetics of a chemical reaction. RCDs provide information about the kinetics of the reaction, the mechanism by which the reaction occurs, and the relative thermodynamic stability of the molecules in a reaction. Previous research studies have characterized student thinking about chemical kinetics, including their confusion in distinguishing between kinetics and thermodynamics. Semi-structured interviews were conducted with 44 students enrolled in a second-semester, first-year undergraduate chemistry course to elicit students’ ideas about surface features of RCDs and to examine how students connect those surface features to features of particulate-level reaction mechanisms. Students were provided both a gas-phase reaction and its accompanying RCD, and then they were asked to choose the particulate-level reaction mechanism that best corresponded to both the reaction and the RCD from among several possible particulate-level reaction mechanisms. Students were asked to explain their reasoning throughout the interview. Findings include students who chose the correct mechanism with appropriate reasoning, as well as students who chose the correct mechanism yet still expressed inaccurate ideas related to the surface features of RCDs and the concepts encoded within them. Students struggled to explain and reason with surface features such as peaks, valleys, and peak height. Moreover, students frequently found it difficult to identify meaningful connections between these surface features, the stoichiometry of the reaction, and the steps in a reaction mechanism. In addition, many students failed to mention important features of RCDs when describing their reasoning about the connections between particulate-level reaction mechanisms and RCDs. The implications for incorporating these research findings into teaching practices in first-year undergraduate chemistry contexts are discussed.

Multiple Hydrogen Bond-Induced Structural Distortion for Broadband White-Light Emission in Two-Dimensional Perovskites

Low-dimensional hybrid lead-halide perovskites with broadband white-light emission upon near-UV excitation have attracted immense scientific interest due to their potential application for the next...

Polymerization-Induced Microphase Separation with Long-Range Order in Melts of Gradient Copolymers.

In this work, we studied the question of whether it is possible to develop a one-step approach for the creation of microphase-separated materials with long-range order with the help of spontaneous gradient copolymers, i.e., formed during controlled copolymerization solely due to the large difference in the reactivity ratios. To that end, we studied the polymerization-induced microphase separation in bulk on the example of a monomer pair with realistic parameters based on styrene (S) and vinylpirrolydone (VP) by means of computer simulation. We showed that for experimentally reasonable chain lengths, the structures with long-range order start to appear at the conversion degree as low as 76%; a full phase diagram in coordinates (fraction of VP—conversion degree) was constructed. Rather rich phase behavior was obtained; moreover, at some VP fractions, order–order transitions were observed. Finally, we studied how the conversion degree at which the order–disorder transition occurs changes upon varying the maximum average chain length in the system.