Low Potential Barrier Research Articles

Transoid-to-Cisoid Conformation Changes of Single Molecules on Surfaces Triggered by Metal Coordination.

Conformational isomers are stereoisomers that can interconvert over low potential barriers by rotation around a single bond. However, such bond rotation is hampered by geometrical constraints when molecules are adsorbed on surfaces. Here, we show that the adsorption of 4,4′-bis(4-carboxyphenyl)-6,6′-dimethyl-2,2′-bipyridine molecules on surfaces leads to the appearance of prochiral single molecules on NiO(001) and to enantiopure supramolecular domains on Au(111) surfaces containing the transoid-molecule conformation. Upon additional Fe adatom deposition, molecules undergo a controlled interconversion from a transoid-to-cisoid conformation as a result of coordination of the Fe atoms to the 2,2′-bipyridine moieties. As confirmed by atomic force microscopy images and X-ray photoelectron spectroscopy measurements, the resulting molecular structures become irreversibly achiral.

Realization of vertical metal semiconductor heterostructures via solution phase epitaxy

The creation of crystal phase heterostructures of transition metal chalcogenides, e.g., the 1T/2H heterostructures, has led to the formation of metal/semiconductor junctions with low potential barriers. Very differently, post-transition metal chalcogenides are semiconductors regardless of their phases. Herein, we report, based on experimental and simulation results, that alloying between 1T-SnS2 and 1T-WS2 induces a charge redistribution in Sn and W to realize metallic Sn0.5W0.5S2 nanosheets. These nanosheets are epitaxially deposited on surfaces of semiconducting SnS2 nanoplates to form vertical heterostructures. The ohmic-like contact formed at the Sn0.5W0.5S2/SnS2 heterointerface affords rapid transport of charge carriers, and allows for the fabrication of fast photodetectors. Such facile charge transfer, combined with a high surface affinity for acetone molecules, further enables their use as highly selective 100 ppb level acetone sensors. Our work suggests that combining compositional and structural control in solution-phase epitaxy holds promises for solution-processible thin-film optoelectronics and sensors.

A theoretical study on the excited‐state intramolecular proton transfer mechanism of 4′‐dimethylaminoflavonol chemosensor

In this work, density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods are used to explore the excited‐state intramolecular proton transfer (ESIPT) mechanism of a novel system 4′‐dimethylaminoflavonol (DAF). By analyzing the molecular electrostatic potential (MEP) surface, we verify that the intramolecular hydrogen bond in DAF exists in both the S0 and S1 states. We calculate the absorption and emission spectra of DAF in two solvents, which reproduce the experimental results. By comparing the bond lengths, bond angles, and relative infrared (IR) vibrational spectra involved in the hydrogen bonding of DAF, we confirm the hydrogen‐bond strengthening in the S1 state. For further exploring the photoexcitation, we use frontier molecular orbitals to analyze the charge redistribution properties, which indicate that the charge transfer in the hydrogen‐bond moiety may be facilitating the ESIPT process. The constructed potential energy curves in acetonitrile and methylcyclohexane solvents with shortened hydrogen bond distances demonstrate that proton transfer is more likely to occur in the S1 state due to the lower potential barrier. Comparing the results in the two solvents, we find that aprotic polar and nonpolar solvents seem to play similar roles. This work not only clarifies the excited‐state behaviors of the DAF system but also successfully explains its spectral characteristics.

Equilibrium and Dynamic Surface Properties of Cationic/Anionic Surfactant Mixtures Based on Carboxylate Gemini Surfactant

AbstractThe equilibrium and dynamic surface properties of cationic/anionic surfactant mixtures of carboxylate gemini surfactant (CGS12) and quaternary ammonium salts with different alkyl chain lengths were investigated, and the synergistic properties and solubilization capacity toward phenanthrene of these mixtures were evaluated. Results show that all cationic/anionic surfactant mixtures exhibit negative interaction parameters, indicating the strong synergistic effects in the reduction of surface tension and the formation of micelles. When the mixing ratio is 1:1, the surface tension reaches the minimum value. Moreover, as the alkyl chain length increases, the interaction parameters become more negative. In addition, the thermodynamic parameters suggest that the formation of mixed micelles is exothermic with a positive change in entropy. The tendency of dynamic surface tension curves of these mixtures is similar to that of pure CGS12. At the same concentration, these mixtures exhibit a higher adsorption rate than pure surfactants, and a lower adsorption potential barrier than cationic surfactants. However, for the CGS12/stearyltrimethylammonium bromide mixture, the drop rate constant of surface tension is lower than that of either component. Compared with the pure surfactant solutions, the cationic/anionic surfactant mixtures exhibit higher solubilization capacity toward phenanthrene. As the length of the alkyl chain increases, the solubilization capacity of the mixed cationic/anionic surfactant increases.

Solvation effect on the ESIPT mechanism of 2-(4′-amino-2′-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine

The excited-state intramolecular proton transfer (ESIPT) dynamics of 2-(4′-Amino-2′-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine (AHPIP-c) has been studied by using density-functional theory and time-dependent density-functional theory method. Three kinds of different polar aprotic solvents, including acetonitrile (strong polar), tetrahydrofuran (weak polar) and methylcyclohexane (non-polar) have been chosen to explore solvent effects on these molecules. The calculated absorption and fluorescence spectra agree well with the experimental results for the three solvents and the dual fluorescence emission mechanism is well explained. The electron density ρ(r) and Laplacian ∇2ρ(r) at the bond critical point (BCP) have been calculated using the Atoms-In-Molecule (AIM) theory, which prove that the intramolecular hydrogen bond (O1H2⋯N3) exists in the S0 state. The geometric parameters and the infrared vibrational spectra in the OH stretching vibrational region have been calculated, which manifests the hydrogen-bond is strengthened in the S1 state. The molecular electrostatic potential surface and frontier molecular orbitals analysis demonstrate that the proton transfer prefer occurring on excited state because of the charge redistribution upon photo-excitation. The results of potential energy curves, further confirm that the proton transfer process is more likely to conduct in the S1 state due to the lower potential energy barrier than that in the S0 state. In addition, we also find that ESIPT reaction is more easily to occur as the solvent polarity decreases. Therefore, we believe that solvent effect could play an important role in controlling excited state behaviors of AHPIP-c molecules.

Improved breakdown strength and energy density of Al2O3/nano-ZrO2 composite films via enhanced interfacial repairing behavior

Taking Al as top electrode, a repairing layer with ultrahigh dielectric strength and resistivity was generated at the interface between sol-gel Al2O3 films and electrodes under the applied electric field. This repairing layer effectively restored the defects on the film surface and improved the dielectric strength. More importantly, it was found that adding ZrO2 nanoparticle into the matrix film could further enhance the dielectric strength, since the ZrO2 nanoparticle is conductive to forming the interfacial repairing layer. The analysis of ac conduction properties showed that the introduction of nano-ZrO2 resulted in a lower potential barrier and a longer hopping distance for ionic migration. In addition, the simulation result indicated that nano-ZrO2 addition induced a local region of high electric field that accelerated ionic migration. As a result of the two factors, the nanocomposite structure of Al2O3/nano-ZrO2 is capable to provide more ions for the formation of interfacial repairing layer. With the stronger repairing behavior, Al2O3/nano-ZrO2 composite films exhibited significantly enhanced breakdown strength of ~ 576 MV/m and energy density of 14.7 J/cm3, in contrast with 385 MV/m and 5.9 J/cm3 for pure Al2O3 films with Al electrodes.

The excited state hydrogen bond and proton transfer mechanism of a novel dye CS‐Azine

AbstractIn this work, based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods, we theoretically explore the excited state behavior for a novel dye molecule 3,3′‐(5,5′‐((1E,1E′)‐hydrazine‐1,2‐diylidenebis (methanylylidene))bis(2‐morpholinothiazole‐5,4‐diyl))bis(4‐hydroxy‐2H‐chromen‐2‐one) (CS‐Azine). Coupling with atoms in molecules, we investigate the intramolecular dual hydrogen bonds of CS‐Azine system and verify the formation of them. Via study the primary bond distances, bond angle, and infrared vibrational spectra involved in hydrogen bonding moieties, we find O1‐H2···N3 and O4‐H5···N6 of CS‐Azine should be strengthened in the S1 state. When exploring the photo‐excitation process, we confirm that the charge redistribution around hydrogen bonding moieties reveals the tendency of ESIPT reaction. To further investigate whether single or double proton transfer occurs in the S1 state, we consider two kinds of reaction paths (ie, the stepwise and synergetic ESIPT reactions). And the constructed potential energy curves demonstrate that only the single proton transfer reaction should be the most supported in the S1 state from CS‐Azine to CS‐Azine‐SPT form due to the low potential energy barrier. This work not only clarifies the excited state behavior and mechanism about CS‐Azine system but also paves the way for further applying CS‐Azine dye in future.

Novel Co3O4 nanocrystalline chain material as a high performance gas sensor at room temperature

In this study, a chain structure consisting of single crystal Co3O4 nanoparticles were synthesized by a one-step thermal treatment method. The microstructure and morphology of the samples were characterized by XRD, FT-IR, TEM, XPS and EIS. It is found that Co3O4 materials display chain structure and feature 70 nm particles in the diameter size. Interestingly, the chain structure feature of our Co3O4 materials improves electrode conductivity and carrier transportation with respect to polycrystal Co3O4. When calcined at 550 °C (Co3O4-550), the resulting sample displays good gas sensing performance for NOx at room temperature. Its measured gas sensitive response is 52.1% for 100 ppm NOx, and detection limit is lower than 100 ppb. The outstanding NOx gas sensing performance of the Co3O4-550 may be attributed to its unique chain structure. Tightly stucking particles and lower potential barrier are very beneficial to improve electrical properties of Co3O4-550, which is helpful to enhance its gas sensing performance.

Theoretical explorations about the excited state behaviors for two novel high efficient ESIPT compounds

Two high efficient excited state intramolecular proton transfer (ESIPT) compounds (i.e., 3-(5-([1,1′-biphenyl]-4-yl)oxazol-2-yl)-4′-(N,N-diphenylamino)-[1,1′-biphenyl]-4-ol (1) and 4′-(N,N-diphenylamino)-3-(5-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)oxazol-2-yl)-5-methyl-[1,1′-biphenyl]-4-ol (2)) are explored theoretically. Based on DFT and time-dependent DFT (TDDFT) methods, we investigate the hydrogen bonding interactions and ESIPT mechanism. Via B3LYP/TZVP/IEFPCM (toluene) theoretical level, we reappear the experimental steady-state spectra, which demonstrate that the theoretical manner is reasonable and effective. Based on reduced density gradient (RDG) versus sign(λ2)ρ analyses, we confirm intramolecular hydrogen bond for both 1-enol and 2-enol. Investigating geometrical parameters and infrared (IR) vibrational spectra, we verify the O-H···N should be strengthened in the S1 state for 1-enol and 2-enol systems. Exploring frontier molecular orbitals (MOs) and charge density difference (CDD) maps, we find charge redistribution provides the tendency of ESIPT. The constructed potential energy curves demonstrate that the proton transfer should happen in the S1 state. Particularly, the low potential energy barriers of forward and backward ESIPT process for both 1 and 2 systems, the dynamical equilibrium could be verified, which means 1 and 2 systems should be potential for novel white light LEDs materials. This work not only explores and explains previous experimental phenomenon, but also makes a reasonable assignment about the ESIPT mechanism.

Theoretical insights into the excited state intramolecular proton transfer mechanism for a novel 4‐methoxy‐3‐hydroxyflavone system

A novel 3‐hydroxyflavone derivative, 4‐methoxy‐3‐hydroxyflavone (4M3HF), is theoretically investigated regarding its excited state intramolecular proton transfer (ESIPT) mechanism based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods. By comparing the solvent polarity, we selected acetonitrile and toluene in this work. First, based on atoms in molecules (AIM) analyses, we verified the formation of intramolecular hydrogen bond of the 4M3HF structure. On investigating the geometrical parameters (i.e., bond lengths and bond angles), we found that the hydrogen bond strength should be enhanced in the S1 state. By calculating the infrared vibrational spectra, we confirmed the strengthening of the intramolecular hydrogen bond in the S1 state. We further studied the excitation process using the frontier molecular orbitals method, based on which we concluded that the charge redistribution in solvents facilitates the ESIPT tendency. For exploring the ESIPT behavior, we constructed the potential energy curves along with ESIPT paths in both solvents (acetonitrile and toluene). We found that low potential energy barriers facilitate the ESIPT process in the 4M3HF system, which was also verified by searching the transition state (TS) structure. Comparing these two solvents, we concluded that solvent effects play little role in the ESIPT reaction in the 4M3HF system. And we could successfully explain the previous experimental result.

High-Efficiency Planar Hybrid Perovskite Solar Cells Using Indium Sulfide as Electron Transport Layer

For a high-performance perovskite solar cell (PSC), the electron transport layer (ETL) plays a prominent role in the transportation of photogenerated charge carriers from the perovskite layer to electrode. Herein, indium sulfide (In2S3) nanosheets are synthesized by solvent-thermal method; In2S3 ETL is prepared by spin-spraying technique, and planar PSC is thus fabricated. The morphology observation indicates that the as-prepared In2S3 ETL is compact, smooth, pinhole-free, and with an optimal thickness of 40 nm. The photoelectrical characterization reveals that, compared to TiO2 ETL, the In2S3 ETL has lower electron trap-state density, lower potential barrier for electron injection from the perovskite layer, higher electron mobility and electron extraction ratio at the In2S3/perovskite interface, smaller contact resistance, and charge recombination. The planar device fabricated with CH3NH3PbI3 perovskite and In2S3 ETL gains an impressive power conversion efficiency (PCE) of 18.83%, and the photovoltaic de...

New insight into gas sensing property of ZnO nanorods and nanosheets

ZnO nanorods and nanosheets were synthesized via a facile hydrothermal route, and their gas sensing properties were tested to figure out the effect of surface morphologies on gas sensing performance. It was surprisely found that the ZnO nanorods exhibit quicker response and recovery speed towards ethanol, which may be ascribed to the better conductivity and lower potential barrier, whereas the nanosheets display larger ethanol gas response owing to its higher specific surface area.

Structurally Flexible Oxocarbenium/Borohydride Ion Pair: Dynamics of Hydride Transfer on the Background of Conformational Roaming.

We apply Born-Oppenheimer molecular dynamics to the practically significant [dioxane-H(+)-acetone][(C6F5)3B-H(-)] and [Et2O-H(+)-OCPr2][(C6F5)3B-H(-)] ion pair intermediates. Dynamics of hydride transfer in cation/anion ion pair takes place on the background of large-amplitude configurational changes. Geometry of oxocarbenium/borohydride ion pairs is flexible, meaning that we uncover significant actual structural disorder at a finite temperature. Therefore, although the starting structure can be fairly close to the configurational area of the hydride transfer transition state (TS) and despite a low potential energy barrier (ca. 1.5 kcal/mol, according to the literature), already at T ≈ 325 K the system can remain ignorant of the TS region and move round and about ("roam") in the configurational space for a period of time in the range between 10 and 100 ps. This indicates structural flexibility of oxocarbenium/borohydride ion pair on apparently a flat potential energy "landscape" of cation/anion interaction, and this has not been taken into consideration by the free energy estimations in static considerations made thus far. The difference between the dynamics-based representation of the system versus the static representation amounts to the difference between quasi-bimolecular versus unimolecular descriptions of the hydride transfer step.

Drain Current Stress-Induced Instability in Amorphous InGaZnO Thin-Film Transistors with Different Active Layer Thicknesses.

In this study, the initial electrical properties, positive gate bias stress (PBS), and drain current stress (DCS)-induced instabilities of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with various active layer thicknesses (TIGZO) are investigated. As the TIGZO increased, the turn-on voltage (Von) decreased, while the subthreshold swing slightly increased. Furthermore, the mobility of over 13 cm2·V−1·s−1 and the negligible hysteresis of ~0.5 V are obtained in all of the a-IGZO TFTs, regardless of the TIGZO. The PBS results exhibit that the Von shift is aggravated as the TIGZO decreases. In addition, the DCS-induced instability in the a-IGZO TFTs with various TIGZO values is revealed using current–voltage and capacitance–voltage (C–V) measurements. An anomalous hump phenomenon is only observed in the off state of the gate-to-source (Cgs) curve for all of the a-IGZO TFTs. This is due to the impact ionization that occurs near the drain side of the channel and the generated holes that flow towards the source side along the back-channel interface under the lateral electric field, which cause a lowered potential barrier near the source side. As the TIGZO value increased, the hump in the off state of the Cgs curve was gradually weakened.

Thermal decomposition and kinetics of coal and fermented cornstalk using thermogravimetric analysis

The thermal behavior and kinetics of Yiluo coal (YC) and the residues of fermented cornstalk (FC) were investigated in this study. The Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods were used for the kinetic analysis of the pyrolysis process. The results showed that the activation energy (Eα) was increased with the increase of the thermal conversion rate (α), and the average values of Eα of YC, FC and the blend (mYC/mFC = 6/4) were 304.26, 224.94 and 233.46 kJ/mol, respectively. The order reaction model function for the blend was also developed by the master-plots method. By comparing the Ea and the enthalpy, it was found that the blend was favored to format activated complex due to the lower potential energy barrier. Meanwhile, the average value of Gibbs free energy of the blend was 169.83 kJ/mol, and the changes of entropies indicated that the pyrolysis process was evolved from ordered-state to disordered-state.

Effect of 2D MoS2 and Graphene interfaces with CoSb3 nanoparticles in enhancing thermoelectric properties of 2D MoS2-CoSb3 and Graphene-CoSb3 nanocomposites

The approach of incorporating a secondary phase in the bulk thermoelectric (TE) material has proved to be beneficial for enhancing the thermoelectric performance. We have investigated the effect of the presence of two-dimensional (2D) materials (MoS2 or graphene) on the structural, electrical, and thermoelectric properties of CoSb3 nanocomposite, in which CoSb3 nanoparticles of sizes 20–50 nm are uniformly anchored on the surface of 2D-sheets of MoS2 or graphene. The presence of 2D nanosheets enhances TE power factor and figure of merit (ZT). Inclusion of graphene in CoSb3 causes large enhancement in power factor as a result of significantly high electrical conductivity and appreciable Seebeck coefficient. 2D graphene seems to work by providing extra carrier conduction channels along with a low interfacial potential barrier for charge transport. Homogeneously dispersed 2D-sheets of MoS2 in CoSb3 seem to cause interfacial modulation of charge carrier effective mass assisted by relatively larger interfacial barrier to result in significantly larger Seebeck coefficient and highly suppressed phonon conductivity, much more than graphene. The ZT value in both nanocomposites gets significantly enhanced in the entire studied temperature range of 300–700 K, the gain increasing with temperature over the CoSb3. Whereas CoSb3/graphene nanocomposites exhibit unusually high ZT at higher temperatures (550–700 K), the CoSb3/2D-MoS2 nanocomposites exhibit better performance (over graphene) in near room temperature range. The present study provides a possible strategy to enhance the conversion efficiency of various TE materials and has significant potential for waste heat recovery applications in various temperature ranges.

Effect of graphene nanofillers on the enhanced thermoelectric properties of Bi2Te3 nanosheets: elucidating the role of interface in de-coupling the electrical and thermal characteristics

In this report, we investigate the effect of graphene nanofillers on the thermoelectric properties of Bi2Te3 nanosheets and demonstrate the role of interface for enhancing the overall figure of merit (ZT) ∼ 53%. The enhancement in the ZT is obtained due to an increase in the electrical conductivity (∼111%) and decrease in the thermal conductivity (∼12%) resulting from increased conducting channels and phonon scattering, respectively at the interfaces between graphene and Bi2Te3 nanosheets. A detailed analysis of the thermal conductivity reveals ∼4 times decrease in the lattice thermal conductivity in contrast to ∼2 times increase in the electronic thermal conductivity after the addition of graphene. Kelvin probe measurements have also been carried which reveals presence of low potential barrier at the interface between graphene and Bi2Te3 nanosheets which assist the flow of charge carriers thereby, increasing the mobility of the carriers. Thus, our results reveals a significant decrease in the lattice thermal conductivity (due to the formation of interfaces) and increase in the electron mobility (due to conducting paths at the interfaces) strongly participate in deciding observed enhancement in the thermoelectric figure of merit.

Different ESIPT Mechanisms forAngular‐Shaped Quinacridone in Toluene and Dimethyl Formamide (DMF) Solvents: A Theoretical Study

Adopting density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods, we investigat and present two different excited‐state intramolecular proton transfer (ESIPT) mechanisms ofangular‐quinacridone (a‐QD) in both toluene and DMF,theoretically. Comparing the primary structural variations of a‐QD involved in the intramolecular hydrogen bond, we conclude that N1–H2⋯O3 should be strengthened in the S1state, which may facilitate the ESIPT process. Particularly, in toluene, the S1‐state‐stable a‐QD enol* could not be located because of the non‐barrier ESIPT process. Concomitantly, infrared vibrational spectral analysis further verified the stability of the hydrogen bond. In addition, the role of charge–transfer interaction has been addressed under the frontier molecular orbitals (MOs), which depicts the nature of the electronic excited state and supports the ESIPT reaction. The potential energy curves according to variational N1–H2 coordinate demonstrates that the proton transfer process should occur spontaneously in toluene; however, in DMF, a low potential energy barrier of 0.493 kcal/mol is needed to complete the ESIPT reaction. Although this barrier of 0.493 kcal/mol is too low to make an important impact on the ESIPT reaction, just because of the existence of barrier, ESIPT mechanisms in toluene and DMF are different.

An analogy study on ESIPT reaction for 3BHC sensor between polar DMF and nonpolar toluene

A comparison about excited state intramolecular proton transfer (ESIPT) mechanism of a new sensor 3-(1,3-benzothiazol-2-yl)-2-hydroxynaphthalene-1-carbaldehyde (3BHC) in polar solvent dimethylformamide (DMF) and nonpolar solvent toluene have been investigated within the framework of the time-dependent density functional theory (TD-DFT) method. The reproduced previous experimental absorption and emission spectra via our calculations reveals the reasonability of the DFT and TD-DFT theoretical level. The staple bond lengths, bond angles, and corresponding infrared vibrational spectra demonstrate that the intramolecular hydrogen bond of 3BHC should be strengthened in both polar DMF and nonpolar toluene. Two kinds of ESIPT mechanisms for different solvents have been put forward; there is a low potential barrier in the ESIPT process in the DMF solvent, whereas there is almost a nonbarrier for the ESIPT process in the toluene solvent. Hence, we could conclude that the ESIPT process of 3BHC sensor is more likely to occur in the nonpolar solvent upon the photoexcitation, based on which, the excited state behavior of 3BHC could be controlled.

Spin Dynamics in Ferromagnet/10-nm-Thick N-Type GaAs Quantum Well Junctions

Spin dynamics in several different types of ferromagnetic metal (FM)/10-nm-thick n-type GaAs quantum well (QW) junctions is studied by means of time-resolved Kerr rotation measurements. Compared with the MnGa/in-situ doped 10-nm-thick n-type GaAs QW junction, the spin lifetime of the MnGa/modulation-doped 10-nm-thick n-type GaAs QW junction is shorter by a factor of 5, consistent with the D’yakonov–Perel’ spin relaxation mechanism. Meanwhile, compared with the spin lifetime of the MnAs/in-situ doped 10-nm-thick n-type GaAs QW junction, the MnGa/in-situ doped 10-nm-thick n-type GaAs QW junction is of a spin lifetime longer by a factor of 4.2. The later observation is well explained by the Rashba effect in the presence of structure inversion asymmetry, which acts directly on photo-excited electron spins. We demonstrate that MnGa-like FM/in-situ doped 10-nm-thick n-type GaAs QW junctions, which possess relatively low interfacial potential barriers, are able to provide long spin lifetimes.