Calculated Barrier Height Research Articles

Thermal desorption and pyrolysis direct analysis in real time mass spectrometry of Nafion membrane

AbstractPerfluorosulfonic acid ionomer membranes have been widely used as proton conducting membranes in various electrochemical processes such as polymer electrolyte fuel cells and water electrolysis. While their thermal stability has been studied by thermogravimetry and analysis of low molecular weight products, their decomposition mechanism is little understood. In this study a newly developed methodology of thermal desorption and pyrolysis in combination with direct analysis in real time mass spectrometry is applied for Nafion membrane. An ambient ionization source and a high‐resolution time‐of‐flight mass spectrometer enabled unambiguous assignment of gaseous products. Thermal decomposition is initiated by side chain detachment above 350°C, which leaves carbonyls on the main chain at the locations of the side chains. Perfluoroalkanes are released above 400°C by main chain scission and their further decomposition products dominate above 500 °C. DFT calculation of reaction energies and barrier heights of model compounds support proposed decomposition reactions.

A Spiro-Ge-Heterocyclic Compound Formation via Germylenoid and Formaldehyde: A Theoretical Study

Addition reaction of the germylenoid H2GeLiF (R1) and formaldehyde HCHO (R2) leading to the formation of a spiro-Ge-heterocyclic compound was investigated theoretically by using the M062X and QCISD methods for the first time. The geometries of the stationary points along the potential energy surfaces of the addition reaction were optimized at the M062X/6-311+G(d,p) level at first, and then the single-point energies were calculated at QCISD/6-311++G(d,p) level. The addition reaction of H2GeLiF and HCHO firstly generated an oxagermacyclopropane (c-H2GeOCH2, P1) and then P1 can further react with HCHO along two possible pathways (paths II and III). At the end of path II, one spiro-Ge-heterocyclic product (P2) forms, which is the 2,4-dioxagermolane. At the end of path III, the other spiro-Ge-heterocyclic product (P3) forms, which is the 2,5-dioxagermolane. According to the calculated barrier heights we can conclude that the path II is more favorable than path III. Therefore the dominant reaction pathway of the addition reaction between H2GeLiF and HCHO is that firstly R1 + R2 → P1 and then P1 + R2 → P2. The computational studies suggest that this reaction model can provide new ideas for the synthesis of heterocyclic germanium compounds.

Macroscopic–microscopic calculations of fission potential surface of uranium isotopes in the three quadratic surfaces parametrization

Five-dimensional (5D) fission potential energy surfaces (PES) for uranium nuclei are investigated based on the macroscopic–microscopic Lublin–Strasbourg drop model in the three-quadratic-surface parametrization, and the heights of static fission barriers are obtained. Asymmetric and symmetric fission paths are presented on the 5D PES of 236U for different nuclear shapes. The calculated barrier heights, EA and EB, are quite consistent with the experimental data for all even–even nuclei of uranium isotopes, from 230U to 244U.

Role of nuclear surface tension coefficients in the isotopic dependence of fusion dynamics for neutron-rich colliding nuclei: New parametrization of fusion barriers

A theoretical approach is developed to study systematically the influence of the nuclear surface tension coefficient, γ, of the proximity formalism on the isotopic behavior of the fusion dynamics over a wide range of neutron-rich colliding systems with condition of 1 ≤ N/Z < 1.6 for their compound nuclei. Here six different versions of the surface energy coefficient (γ-PD03, γ-MS67, γ-MS66, γ-MN81-I, γ-MN81-II and γ-MN76) in the calculation of the nuclear potential based on the original proximity potential 1977are employed. It is found that the fusion barrier characteristics and fusion cross sections follow a linear isotopic dependence which is sensitive to the change in the nuclear surface tension between the reacting nuclei. The maximum sensitivity of the data to this technical parameter is obtained for the use of the γ-PD03 version. Further, it is shown that the sensitivity of the isotopic behavior of the fusion cross sections to the coefficient γ increases by decreasing the incident energy. After the calculation of fusion barrier heights and positions using the different versions of proximity potentials, we have searched for their parametrization. Firstly, we present a pocket formula for fusion barriers which is directly dependent on both the coefficient γ and the N/Z ratio of the isotopic systems. Our analysis for the studied isotopic systems reveals that the agreement with the empirical data is improved when we consider the dependence of the analytical parameterized formulas on this coefficient. Further, the fusion cross sections obtained by the present pocket formula are in good agreement with the corresponding experimental data. A comparison with other well known parameterized forms in the context of isotopic studies is also made.

A comparison of DLPNO-CCSD(T) and CCSD(T) method for the determination of the energetics of hydrogen atom transfer reactions

We have assessed the accuracy of the DLPNO-CCSD(T) method against the CCSD(T) method in determining the barrier heights and reaction energetics for a series of hydrogen atom transfer reactions. Our reaction list consists of both closed shell and open shell prototype systems. For closed shell systems, the difference in calculated barrier heights at DLPNO-CCSD(T)/aug-cc-pVnZ and CCSD(T)/aug-cc-pVnZ (n = D, T and Q) level of theory were found to be always less than ∼0.8 kcal mol−1 with a standard deviation of 0.23, 0.18 and 0.16 kcal mol−1 at aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets, respectively. The standard deviation of the same for the open shell systems were found to be 0.43, 0.79 and 0.91 kcal mol−1 at aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets, respectively. Furthermore, the study reveals that a modification is needed in the recommended value of the parameter “TcutPNO” while using the DLPNO-CCSD(T) method for reactions exhibiting multireference character.

Improvement of Ag(In,Ga)Se2/Mo Interface and Solar Cell Performance by Preselenization of the Mo Back Contact

In Ag(In,Ga)Se2 (AIGS) solar cells, a MoSe2 thin layer may provide a critical contact between the AIGS layer and the Mo layer. To test this, AIGS films were deposited on Mo-coated substrates without and with preselenization. With preselenization, the MoSe2 thickness in the AIGS samples ranged between 30 nm and 60 nm, while no MoSe2 layer was observed for the AIGS samples without preselenization. The c -axis of the MoSe2 layer was parallel to that of the Mo layer, with c = 12.5 A. The presence of MoSe2 in the AIGS films was confirmed by secondary ion mass spectrometry and preselenization exhibited no effect on the promotion of sodium diffusion in the AIGS films. Without preselenization, the AIGS and Mo had a Schottky contact with a calculated barrier height between 0.66 and 0.74 eV, based on the AIGS/Mo current–voltage curves. In contrast, the MoSe2 layer converted the AIGS and Mo contact to an ohmic type. Preselenization increased the solar cell conversion efficiency of AIGS to 9.3% (open-circuit voltage: 957.9 mV, short-circuit current: 18.4 mA/cm2, and fill factor: 52.9%). Additionally, the quantum efficiency was improved. In the absence of preselenization, the conversion efficiency was 5.2% (open-circuit voltage: 850.6 mV, short-circuit current: 13.1 mA/cm2, and fill factor: 46.7%).

Structural and optoelectronic characterization of Cu2CoSnS4 quaternary functional photodetectors

Al/p-Si/Cu2CoSnS4/Al quaternary functional semiconductor photodetectors showing solar detector properties were produced via sol-gel method. SEM, EDS and XRD techniques were used in the confirmation of the chemical composition of the photodiodes where nanoparticle like characteristics of Cu2CoSnS4 was seen. UV–vis spectroscopy was used in the investigation of optoelectronic characteristics. It was seen that photodiodes have a high absorption rate with minimum reflectance, bandgap energy was calculated as 1.19 eV. Current – time and current – voltage characteristics of the photodiodes revealed that photodiodes are sensitive to daylight; photodiodes present rectifying characteristics. Thermionic emission theory was used in the calculation of barrier height, ideality factor, photoresponse, photosensitivity, and linear dynamic rate characteristics. Capacitance – voltage, conductance – voltage, corrective capacitance – voltage, corrective conductance – voltage graphs were used to investigate the electrical properties of the Al/p-Si/Cu2CoSnS4/Al photodiodes. The electrical characteristics of the photodiodes reflect frequency dependent characteristics. Such a behaviour was found to be an indication of the existence of interface states. The density of interface (Dit) calculations revealed that the density of interface states strongly depends on AC signal frequency where diminished Dit was seen for increased signal frequency.

Influence of ionic liquids microenvironment on the coupling reaction of epoxide and carbon dioxide: DFT and MD

Abstract Theoretical calculation is a powerful tool to uncover the reaction mechanism. Besides it, the theoretical calculation is expected to provide more helpful clues and play more important role for catalysis. Although the calculated barrier height could not be directly compared with the experimental measured activation energy, the range of barrier heights would be the same with that of experimental yields. It would be beneficial to estimating the experimental result and designing the efficient catalysts. For the coupling reaction of carbon dioxide with epoxides catalyzed by ionic liquids, both the interactions among ionic liquids and microenvironment of ionic liquids would affect the calculated barrier heights. They are considered by combination of density functional theory and molecular dynamics in this work. The reliability of calculated barrier heights is improved. More important, this method not only is suitable for the ionic liquids with the similar structures but also be applied to ionic liquids with different structures.

Conductance Deep-Level Transient Spectroscopy and Current Transport Mechanisms in Au|Pt|n-GaN Schottky Barrier Diodes

The current transport mechanisms in Au|Pt|n-GaN Schottky barrier diodes are investigated in a temperature range of 40–325 K. The calculated barrier height and ideality factor are 1.12 and 2.13 eV, respectively, and it is observed that the barrier height Φb increases and the ideality factor n decreases with temperature increase. The apparent barrier height and the ideality factor derived by using thermionic emission theory are found to be strongly temperature-dependent. The increase in barrier height with increasing temperature has been explained as an effect of barrier inhomogeneity. This behavior has been interpreted based on the assumption of a Gaussian distribution of barrier heights due to barrier height inhomogeneity at the interface between the metal and semiconductor. The abnormal behavior of all these parameters can be attributed to the presence of deep levels thermally activated. Conductance deep-level transient spectroscopy (CDLTS) results shows that the two deep-level defects are observed in as-grown sample with activation energies of E1 = 0.18 eV and HL1 = 0.87 eV.

Effect of illumination intensity on the characteristics of Co/Congo Red/p-Si/Al hybrid photodiode

In this study, Congo Red (CR) organic dye interlayer was covered by sping coating on p-Si semiconductor wafer and Co/CR/p-Si/Al photodiode was fabricated. The surface morphology of CR film was characterized by SEM measurement. Current–voltage (I–V) characteristics of the photodiode were obtained under different illumination intensities. The electrical, sensitivity, and photoresponsivity properties of Co/CR/p-Si/Al photodiode were investigated using the current–voltage and capacitance–voltage measurements. While the calculated barrier height (Φb) and ideality factor (n) were 0.58 eV and 1.52 under the dark, these values were 0.55 eV and 1.62 under the 400 mW cm−2, respectively. The exponent of photocurrent was found to be 1.10. This value indicates that the photodiode exhibited a linear photocurrent variation with power of the light. It was found that the interface-state intensity of the photodiode decreased from 1.64 × 1015 to 8 × 1012 eV−1 cm−2 under the 400 mW cm−2 light intensity. The rectification rate of photodiode (RR) of the photodiode was found under the dark and illumination. All experimental results indicate that the photodiode exhibits a photoconducting characteristic.

Dissociative nature of C(sp2)-N(sp3) bonds of carbazole based materials via conical intersection: simple method to predict the exciton stability of host materials for blue OLEDs: a computational study.

In this work, the origin of the singlet and triplet exciton-induced degradation of host materials with C(sp2)-N(sp3) bonds around nitrogen (carbazoles, acridines, etc.), connecting donor and acceptor units, was unravelled using DFT and CASSCF methods. The results reveal that molecules (employed in OLEDs) with basic units containing C(sp2)-N(sp3) bonds (nitrogen connected to carbon in a triangular fashion) have a natural tendency to fragment at the C-N bond through an S1/S0 conical intersection (CI). The calculation of barrier heights, to reach a dissociation point, indicates that degradation via triplet states is kinetically less feasible (ΔGT1-TS* > 25 kcal mol-1) compared to that via the first singlet excited state (ΔGS1-TS* ∼7-30 kcal mol-1). However, the long lifetime of triplets (as compared to singlets) aids in the reverse intersystem crossing from triplet to singlet state for subsequent degradation. From the results and inference, ΔGS1-TS* and ΔES1-T1 are proposed to be the controlling factors for exciton-induced degradation of host materials with C(sp2)-N(sp3) bonds. Furthermore, multiple functionalization of carbazole moieties reveals that polycyclic aromatic systems employed as acceptor units of host materials are best suited for PhOLEDs as they will increase their lifetime due to the larger ΔGS1-TS* and ΔES1-T1. For TADF-based devices, materials with fused ring systems (with N(sp3) at the centre) in the donor unit are the most recommended ones based on the findings of this work, as they avoid the dissociative channel altogether. A negative linear correlation between ΔGS1-TS* and HOMO-LUMO gap is observed, which provides an indirect way to predict the kinetic stability of these materials in excitonic states. These initial results are promising for the future development of the QSAR-type approach for the smart design of host materials for long-life blue OLEDs.

Calculation of Chemical Reaction Barrier Heights by Multiconfiguration Pair-Density Functional Theory with Correlated Participating Orbitals.

The accurate description of reaction barrier heights is challenging for quantum mechanical methods due to the need for a balanced treatment of dynamic and static correlation energies because their importance varies during the course of a chemical reaction. While some regions of potential energy surfaces are well-described by a single-reference wave function or by Kohn-Sham density functional theory, in other cases a multireference treatment is needed. For systems with many active electrons, most accurate multireference methods have prohibitive computational scalings with system size. Multiconfiguration pair-density functional theory, MC-PDFT, is a more affordable multireference approach that computes the total electron correlation energy in a single step by using the multiconfiguration kinetic energy, density, and on-top pair density and an on-top density functional. In this work, we apply MC-PDFT to a benchmark database (DBH24/18) of 24 diverse reaction barrier heights. We explore the role of active space and basis set selection on the performance of MC-PDFT. We find that MC-PDFT is able to calculate reaction barrier heights with a similar accuracy to complete active space second order perturbation theory, CASPT2, but at a lower computational cost, and we find that MC-PDFT is less dependent on basis set selection than CASPT2.

Effect of cluster of protic pyrazolium ionic liquids or epoxides on the cycloaddition of CO2

The reaction mechanism for the coupling reaction of carbon dioxide with epoxides catalyzed by single-component protic pyrazolium ionic liquids has been extensively elucidated. However, the calculated barrier heights are not accurate enough to predict the catalytic results for a series of ionic liquids with similar structures. The cluster of protic pyrazolium ionic liquids is regarded as catalyst to consider the effect of interactions among ionic liquids on the barrier heights. Moreover, the ionic liquids or epoxides would also affect the barrier heights as the environmental factors, which is also considered by ONIOM method. When the environmental influence is involved, the accuracy is greatly improved. The analysis of interactions between ionic liquid and reactant indicates that the electrophilic attacking plays the more critical role than nucleophilic attacking.

New calculations of five-dimensional fission barriers for actinide nuclei

New calculations of fission barrier heights in a five-dimensional (5D) deformation space for actinide nuclei have been performed based on a macro-microscopic (MM) model. To calculate potential energy surface (PES), we use the 5D generalized Lawrence shape (GLS) realistic parameterization as the shape description, the Lublin-Strasbourg Drop (LSD) model for the macroscopic energy and the folded-Yukawa model as the single-particle potential. A method to use two-center oscillator basis in calculating Hamiltonian matrix on any kind of shape description was presented. For the pairing energy, we use the SBCS model. Finally, Strutinsky method is used to calculate the microscopic correction energy. To find the fission barriers and fission paths, some concepts, such as basins, edges and neighbors from geography, are extended to identify the topographical structures on the 5D PES and we assume that the fission path follows the line of steepest descent. The fission barriers and double-humped paths of U and Pu isotopes are calculated.

Identification of stable configurations in the superhydrogenation sequence of polycyclic aromatic hydrocarbon molecules

Superhydrogenated polycyclic aromatic hydrocarbon (PAH) molecules have been demonstrated to act as catalysts for molecular hydrogen formation under interstellar conditions. Here we present combined thermal desorption mass spectrometry measurements and density functional theory calculations that reveal the most stable configurations in the superhydrogenation sequence of the PAH molecule coronene (C24H12). Specifically, the experiments demonstrate the presence of stable configurations of superhydrogenated coronene at specific hydrogenation levels of 2, 10, 14, 18, and 24 extra hydrogen atoms. Density functional theory calculations of binding energies and barrier heights explain why these configurations are particularly stable and provide new insights into the superhydrogenation process of PAH molecules under interstellar conditions. Furthermore, an experimental cross-section for the first hydrogen atom addition to the neutral coronene molecule of |$\sigma _\text{add} = 2.7^{+2.7}_{-0.9}$| × 10−2 Å2 is derived from the experimental hydrogenation data.

Effect of ionic liquids clusters microenvironment on cycloaddition reaction of carbon dioxide

The mechanism for the cycloaddition of carbon dioxide and epoxides catalyzed by ionic liquids has been elucidated in lots of theoretical literature. It is well known that the calculated barrier height could not be compared with the activation energy. However, it is expected that the sequence of activity estimated by barrier height could be consistent with the experimental result. Actually, the calculated results are not accurate enough to be reliable. One of possible reasons is attributed to the improper treatment for the solvent effect. The solvent effect induced by ionic liquids is not carefully considered or neglected at all in most of previous theoretical studies. In this work, ionic liquids are really included in the catalytic system to consider the solvent effect by the ONIOM method along with the molecular dynamics simulations. When the solvent effect is considered, the theoretical reliability is greatly improved. More important, this model is suitable not only for non-protic ionic liquids but also for protic ones and not only for imidazolium ionic liquids but also for pyrazolium ionic liquids. The solvent effect aroused by epoxides is also considered by the same model.

Structure, stability and conversions of tautomers and rotamers of azulene-based uracil analogue

In this work, ab initio CBS-QB3 procedures were used to study structure and stability of possible tautomers and rotamers of azulene-based uracil analogue (AZU) in the gas phase and in water. Similar to uracil (U), the di–keto form of the new system is the most stable structure relative to keto–enol and di-enol forms. The calculated barrier heights for proton transfer reactions are in the range of 26.1–41.9 kcal/mol, while the OH rotational energy barriers are less than 12 kcal/mol. The rotomerization process in azulene-based system is more difficult than in uracil. However, tautomerization illustrates the opposite trend. Zwitterion tautomers of AZU were also investigated. Calculations in water cause some reordering of the relative stability of azulene-based tautomers and affect the energy barriers of tautomerization.

Self-Assembly Mechanisms of Triblock Janus Particles.

We present a detailed model to study the nucleation of triblock Janus particles from solution. The Janus particles are modeled as cross-linked polystyrene spheres whose poles are patched with sticky alkyl groups and their middle band is covered with negative charges. To mimic the experimental conditions, solvent, counterions, and a substrate, on which the crystallization takes place, are included in the model. A many-body dissipative particle dynamics simulation technique is employed to include hydrodynamic and many-body interactions. Metadynamics simulations are performed to explore the pathways for nucleation of Kagome and hexagonal lattices. In agreement with experiment, we found that nucleation of the Kagome lattice from solution follows a two-step mechanism. The connection of colloidal particles through their patches initially generates a disordered liquid network. Subsequently, orientational rearrangements in the liquid precursors lead to the formation of ordered nuclei. Biasing the potential energy of the largest crystal, a critical nucleus appears in the simulation box, whose further growth crystallizes the whole solution. The location of the phase transition point and its shift with patch width are in very good agreement with experiment. The structure of the crystallized phase depends on the patch width; in the limit of very narrow patches strings are stable aggregates, intermediate patches stabilize the Kagome lattice, and wide patches nucleate the hexagonal phase. The scaling behavior of the calculated barrier heights confirms a first-order liquid-Kagome phase transition.

Influence of Ta2O5 Interfacial Oxide Layer Thickness on Electronic Parameters of Al/Ta2O5/p-Si/Al Heterostructure

We describe the impact of Ta2O5 interfacial oxide layer thickness (ranging from 100-350 nm) on electrical and structural properties of Al/Ta2O5/p-Si/Al Metal-Insulator-Semiconductor (MIS) Schottky barrier diodes using RF magnetron sputtering. We studied the Schottky barrier device parameters such as ideality factor, barrier height and series resistance and are evaluated from current-voltage (I-V) measurements. The barrier height and ideality factor values are significantly varying with Ta2O5 oxide layer thickness and found to be 0.58 eV, 2.35, 0.71 eV, 2.10 and 0.78 eV, 1.87 for 20, 40 and 60 nm, respectively. It was noticed that the calculated barrier height and ideality values for this prepared Al/Ta2O5/p-Si/Al MIS Schottky barrier diode were greatly improved than those conventional metal-semiconductor (MS) Schottky diodes. The XRD studies revealed that the 100-nm thickness film exhibited poor crystallinity whereas 200 and 350 nm thickness films showed improved crystallinity with orthorhombic phase of β-Ta2O5. The presence of this orthorhombic phase of β-Ta2O5 is confirmed with FTIR studies. To explore the structural transformations in Ta2O5 films with varying thicknesses, Raman spectroscopy was utilized. In addition, the improvement in Schottky diode parameters was correlated with the enhanced crystallinity noticed in XRD studies.

Temperature Dependent Electron Transport Properties of Gold Nanoparticles and Composites: Scanning Tunneling Spectroscopy Investigations.

Scanning tunneling spectroscopy (STS) is used for investigating variations in electronic properties of gold nanoparticles (AuNPs) and its composite with urethane-methacrylate comb polymer (UMCP) as function of temperature. Films are prepared by drop casting AuNPs and UMCP in desired manner on silicon substrates. Samples are further analyzed for morphology under scanning electron microscopy (SEM) and atomic force microscopy (AFM). STS measurements performed in temperature range of 33 °C to 142 °C show systematic variation in current versus voltage (I-V) curves, exhibiting semiconducting to metallic transition/Schottky behavior for different samples, depending upon preparation method and as function of temperature. During current versus time (I-t) measurement for AuNPs, random telegraphic noise is observed at room temperature. Random switching of tunneling current between two discrete levels is observed for this sample. Power spectra derived from I-t show 1/f2 dependence. Statistical analysis of fluctuations shows exponential behavior with time width τ ≈ 7 ms. Local density of states (LDOS) plots derived from I-V curves of each sample show systematic shift in valance/conduction band edge towards/away from Fermi level, with respect to increase in temperature. Schottky emission is best fitted electron emission mechanism for all samples over certain range of bias voltage. Schottky plots are used to calculate barrier heights and temperature dependent measurements helped in measuring activation energies for electron transport in all samples.