Lowest Energy Point Research Articles

Atomlike interaction and optically tunable giant band-gap renormalization in large-area atomically thin MoS2

Coulomb interactions in atomically thin transition metal dichalcogenides can be dynamically engineered by exploiting the dielectric environment to control the optical and electronic properties. Here we demonstrate an optically tunable giant band-gap renormalization (BGR) $\ensuremath{\sim}1200$ and 850 meV from the edge of the conduction band and complete suppression of the exciton absorption in large-area single-layer (1L) and three-layer (3L) ${\mathrm{MoS}}_{2}$, respectively. The observed giant BGR is two orders of magnitude larger than that in the conventional semiconductors, and it persists for tens of ps. Strikingly, our results demonstrate photoinduced transparency at the electronic band gap using an intense optical field at room temperature. Exciton bleach recovery in 1L and 3L show a contrasting fluence-dependent response, demonstrating the layer-dependent optical tuning of exciton lifetime in a way that would be both reversible and real time. We find that the optical band gap (exciton resonance peak) shows a transient redshift followed by an anomalous blueshift from the lowest energy point as a function of the photo-generated carrier density. The observed exciton energy shift is analogous to atom-atom interactions, and it varies as a Lennard-Jones like potential as a function of the interexciton separation.

Neutron radiative capture cross section for sodium with covariance analysis

The neutron radiative capture cross sections measurement has been carried out for the $$^{23}$$ Na nucleus in the neutron energy region from 0.6 to 3.2 MeV using the neutron activation technique followed by off-line $$\gamma $$ -ray spectrometry. The measurement was made relative to the $$^{115}$$ In(n,n $$\prime $$ $$\gamma $$ ) $$^{115}\hbox {In}^{m}$$ reference monitor reaction cross section. The neutrons were produced via the $$^{7}$$ Li(p,n) $$^{7}$$ Be reaction. Detailed uncertainty propagation has been performed using the covariance analysis, and the measured cross sections are being reported with their uncertainties, covariance, and correlation matrix. The necessary corrections have been made for the low background neutron energy contribution, $$\gamma $$ -ray true coincidence summing, and self-attenuation process. The obtained neutron spectrum averaged cross sections of $$^{23}$$ Na(n, $$\gamma $$ ) $$^{24}$$ Na are discussed and compared with the existing cross sections data retrieved from the EXFOR database. EMPIRE-3.2 and TALYS-1.9 calculations were performed in order to determine the radiative capture cross section in this energy region. The present results are also compared with the evaluated nuclear data from ENDF/B-VIII.0, TENDL-2019, IRDFF-1.05, JENDL-4.0, and JEFF-3.3. The obtained cross section results are in good agreement with existing experimental data, evaluated libraries, and reaction models for the highest energy points (2.11 and 3.13 MeV), while the lowest-energy point at 0.61 MeV underestimates them.

Synthesis, characterization, computational studies and biological activities of Co(II), Ni(II) and Cu(II) complexes of 2-Amino-1,3,4-thiadiazole derivatives

Co(II), Ni(II), and Cu(II) complexes of 2-Amino-5-ethyl-1,3,4-thiadiazole (AET) and 2-Amino-5-(ethylthio)-1,3,4-thiadiazole (AEST) have been synthesized and characterized based on elemental analysis, magnetic susceptibility, infrared (4000–400 cm−1), mass spectrometry (ESI and MALDI), UV–Vis (200–1100 nm) and thermal analysis (TGA/DTA). Molar conductance measurements proved that [M(L)2(H2O)2]Cl2·H2O are electrolytic complexes where M represents Co, Ni, and Cu divalent metal ions. The geometrical isomerism of [M(L)2(H2O)2]2+ ions were investigated by DFT-B3LYP calculations incorporated in Gaussian09 package; it favored the all trans isomers due to having the lowest energy points on the potential energy surface. The outcome of DFT-B3LYP quantum mechanical calculations using 6-31G(d) basis set favor six-coordinate sites via a bidentate ligand through exo amino and adjacent endo thiadiazole nitrogen (N3) donors. These results were consistent with magnetic measurements combined with infrared and UV–Vis spectral interpretations. The predicted metal–ligand binding energies from B3LYP/6-31G(d) calculations follow the trend Cu2+>Ni2+>Co2+, in agreement with the Irving–Williams series. Both AET and AEST ligands and the synthesized complexes were screened for their antibacterial activity and the outcome was high antimicrobial activity of the complexes compared to the free ligands against one or more microbial species and in some cases (copper complexes) higher activity than standard drugs.

The recognition mechanism of crizotinib on MTH1: influence of chirality on the bioactivity

ABSTRACTThe mechanism of how (S)-crizotinib or (R)-crizotinib selectively recognises MutT homolog 1 (MTH1) was investigated by molecular dynamics simulations and molecular mechanics/Poisson-Boltzmann surface area (MMPBSA) free energy calculations. Our studies find that (S)-crizotinib could cause much more conformation fluctuation on MTH1 protein than (R)-crizotinib. Compared with (R)-crizotinib, (S)-crizotinib, to a greater degree, inhibited the anti-correlated motion of the whole MTH1 protein, especially the binding site. The closure motion of the binding site was slightly blocked by (R)-crizotinib, which should be attributed to the steric hindrance of (R)-crizotinib on loop 1 and loop 3. The closure motion of the binding site was greatly promoted by (S)-crizotinib, which should be attributed to the hydrogen bond interaction of (S)-crizotinib on loop 1 and loop 3. The stabilised degree of (S)-crizotinib on the binding pocket in MTH1 was more than that of (R)-crizotinib. In the lowest energy point, the presence of (R)-crizotinib made the binding site of MTH1 slightly become smaller, while the presence of (S)-crizotinib made the binding site of MTH1 form a stable hydrogen bond. These supported the experimental finding that the activity of (S)-crizotinib is superior to the (R)-crizotinib activity (IC50 72 vs. 1375 nM). The free energy calculations also confirmed this point. This work could not only illuminate the mechanism of (S)-crizotinib or (R)-crizotinib selectively recognising MTH1, but also provide a guideline of the design of such chiral drugs.

Electric-field-induced strong enhancement of electroluminescence in multilayer molybdenum disulfide.

The layered transition metal dichalcogenides have attracted considerable interest for their unique electronic and optical properties. While the monolayer MoS2 exhibits a direct bandgap, the multilayer MoS2 is an indirect bandgap semiconductor and generally optically inactive. Here we report electric-field-induced strong electroluminescence in multilayer MoS2. We show that GaN–Al2O3–MoS2 and GaN–Al2O3–MoS2–Al2O3-graphene vertical heterojunctions can be created with excellent rectification behaviour. Electroluminescence studies demonstrate prominent direct bandgap excitonic emission in multilayer MoS2 over the entire vertical junction area. Importantly, the electroluminescence efficiency observed in multilayer MoS2 is comparable to or higher than that in monolayers. This strong electroluminescence can be attributed to electric-field-induced carrier redistribution from the lowest energy points (indirect bandgap) to higher energy points (direct bandgap) in k-space. The electric-field-induced electroluminescence is general for other layered materials including WSe2 and can open up a new pathway towards transition metal dichalcogenide-based optoelectronic devices.

ChemInform Abstract: Can a Photochemical Reaction Be Concerted? A Theoretical Study of the Photochemical Sigmatropic Rearrangement of But-1-ene.

MC-SCF computations at the 4-31 G level using a complete active space (CAS) of four orbitals demonstrate the existence of a concerted photochemical pathway for [1,2] and [1,3] alkyl sigmatropic shifts. The central feature of this concerted path is a conical intersection (e.g., a genuine crossing) between ground and excited state from which a fully efficient return to the ground state is possible. Thus the excited-state surface has no minium with zero gradient (i.e., a critical point) but only a singurality which corresponds to the lowest energy point of a conical intersection between ground and excited states

A CASSCF and CASPT2 Study on the Excited States of s-trans-Formaldazine

The low-lying excited states of s-trans-formaldazine (H2CN-NCH2) have been investigated using the complete active space self-consistent field (CASSCF) and the multiconfigurational second-order perturbation (CASPT2) methods. The vertical excitation energies have been calculated at the state-average CASSCF and multistate CASPT2 levels employing the cc-pVTZ basis set. The photodissociation mechanisms starting from the S1 state have been determined. The lowest energy points along the seams of surface intersections have been located in both the Franck-Condon region and the N-N dissociation pathway in the S1 state. Once the system populates the S1 state, in the viewpoint of energy, the radiationless decay via S1/S0(3) conical intersection followed by the N-N bond fission in the ground-state is more favorable in comparison with the N-N dissociation process in the S1 state. A three-surface crossing region (S1/T1/T2), where the S1, T1, and T2 states intersect, was also found. However, the intersystem crossing process via S1/T1/T2 is not energetically competitive with the internal conversion via S1/S0(3).

Study on charge transport properties of hexaazatriphenylene and its derivatives

The structure optimization and frequency calculation of hexaazatriphenylene, triphenylene and their derivatives were carried out at the level of B3LYP/6-31G**, and the most stable configurations were obtained. On the basis of the most stable geometries, the relationships of the total energy with rotation degrees between the dimers were studied, and the lowest energy point was obtained. Based on the semi-classical model of the charge transport, our results predict that hexaazatriphenylene molecules have the largest positive charge transport rate and the smallest negative charge transport rate. Introduction of SH, OH, OR and F groups make the positive charge transport to increase, and the negative charge transport to decrease. The length of alkoxy group has little effect on charge transport properties.

Intramolecular Charge Transfer in 4-Aminobenzonitriles Does Not Necessarily Need the Twist

In electron donor/acceptor species such as 4-(dimethylamino)benzonitrile (DMABN), the excitation to the S(2) state is followed by internal conversion to the locally excited (LE) state. Dual fluorescence then becomes possible from both the LE and the twisted intramolecular charge-transfer (TICT) states. A detailed mechanism for the ICT of DMABN and 4-aminobenzonitrile (ABN) is presented in this work. The two emitting S(1) species are adiabatically linked along the amino torsion reaction coordinate. However, the S(2)/S(1) CT-LE radiationless decay occurs via an extended conical intersection "seam" that runs almost parallel to this torsional coordinate. At the lowest energy point on this conical intersection seam, the amino group is untwisted; however, the seam is accessible for a large range of torsional angles. Thus, the S(1) LE-TICT equilibration and dual fluorescence will be controlled by (a) the S(1) torsional reaction path and (b) the position along the amino group twist coordinate where the S(2)/S(1) CT-LE radiationless decay occurs. For DMABN, population of LE and TICT can occur because the two species have similar stabilities. However, in ABN, the equilibrium lies in favor of LE, as a TICT state was found at much higher energy with a low reaction barrier toward LE. This explains why dual fluorescence cannot be observed in ABN. The S(1)-->S(0) deactivation channel accessible from the LE state was also studied.

Conical intersections and the nonadiabatic reactions H2O+O(3P)↔OH(A 2Σ+)+OH(X 2Π)

In this work the reaction O(3P)+H2O→OH(X 2Π)+OH(A 2Σ+) and the reverse reaction were studied. Π3–3Δ and Π3–3Σ+ conical intersection seams were found that tie two reactant-channel potential energy surfaces to the two excited state potential energy surfaces correlating with OH(X 2Π)+OH(A 2Σ+) products. Near their lowest energy points, which occur for collinear geometries, these accidental symmetry-allowed conical intersection seams have very similar geometries and are separated by <90 cm−1. This close proximity may reflect a three state conical intersection not required by symmetry. The conical intersection seams were shown to be accessible from both the reactant and product channels without a barrier, above any reaction endoergicity, and provide in the case of the reverse reaction for branching to either H2O+O(3P) or OH(X 2Π)+OH(X 2Π).

18]Annulene: a benzene-like S0/S1 conical intersection located with CASSCF + GVB

A further parallel between the planar [18]annulene and benzene has been demonstrated: the existence of a topologically equivalent S0/S1 conical intersection. The lowest energy point on the crossing has been optimized with the MMBV and CASSCF+GVB/4-31G methods. These calculations suggest that fluorescence might be observed from the S1 state of [18]annulene, and that this would disappear at higher vibrational excess energies as a route to efficient non-radiative decay via the conical intersection became accessible (the 'channel 3' mechanism for benzene).

On the mechanism of the reaction CH(X 2Π)+N2(X 1Σ+g)→HCN(X 1Σ+) +N(4S ). I. A theoretical treatment of the electronic structure aspects of the intersystem crossing

The reaction CH(X 2Π)+N2(X 1Σ+g)→HCN(X 1Σ+) +N(4S) has been suggested as the initial step in the formation of ‘‘prompt’’ NO in flame fronts. Since the reaction is spin-forbidden an intersystem crossing must occur in the vicinity of the allowed crossing hypersurface of the lowest doublet and quartet potential energy surfaces. In this work the electronic structure aspects to this intersystem crossing are considered using multireference configuration interaction wave functions. The key to this treatment is a new implementation of a constrained analytic gradient search algorithm which is used to locate regions of the doublet–quartet crossing hypersurface. In those regions of nuclear coordinate space the spin–orbit coupling (matrix elements of HSO) is determined using the full microscopic Breit–Pauli spin–orbit interaction (that is both the spin–orbit and spin–other–orbit contributions are included). Included in this treatment are the largest configuration state function expansions, 700 000–900 000 terms, used to date to evaluate matrix elements HSO within the full Breit–Pauli approximation. Our conclusions are as follows. The lowest energy point on the doublet–quartet crossing hypersurface corresponds, approximately, to a C2v nuclear configuration in which the HC moiety has been inserted into a highly stretched N2 bond. The spin–orbit coupling in this region is approximately 12 cm−1. This region is estimated to be endoergic with respect to the reactant channel asymptote by approximately 7.5 kcal/mol. A second region of the crossing hypersurface corresponds to a perturbed nitrogen atom collinearly adjacent to the nitrogen side of the HCN moiety which is in its ground electronic state. The spin–orbit coupling in this region is considerably larger, approximately 44 cm−1. However, this region is estimated to be endoergic with respect to the reactant channel asymptote by over 50 kcal/mol. A qualitative Landau–Zener analysis of the electronic structure data provides the first computational evidence supporting the role of this reaction in the production of prompt NO in flame fronts.

Theory of Amorphous Semiconductors

Abstract A general discussion of the physics of amorphous semiconductors is presented. It is shown that corresponding amorphous and crystalline solids are strongly related, because of the common chemistry which determines local structure. The effects of disorder are then analyzed in detail. It is suggested that a Cohen-Fritzsche-Ovshinsky-type band overlap is unlikely to occur in covalent semiconductors with strong electron-phonon coupling. The vast experimental evidence for the existence of well-defined defects in amorphous materials is summarized. Tight-binding calculations of the lowest energy point defects to be expected in chalcogenide, pnictide and tetrahedrally bonded amorphous semiconductors are presented, including intimate charged pairs. The motion of the Fermi energy as a function of electron concentration is calculated for several important cases, particularly for amorphous-silicon-based alloys. It is concluded that an ordinary one-electron approach for the electronic structure of these important materials is inadequate.

Neutron total cross section for tritium

The neutron total cross sections for hydrogen, deuterium, and tritium have been measured with the neutron-time-of-flight technique. The tritium data span the neutron energy range from 60 keV to 80 MeV, with an overall systematic uncertainty which varies from 0.5% for energies below 17 MeV to 2% at the highest energy measured; the statistical uncertainties exceed these values only below 200 keV and above 7 MeV. The three-mole high-pressure gas samples were nearly isotopically pure. Excellent agreement is found with accepted cross sections for hydrogen and deuterium, and with previous Van de Graaff measurements for tritium except at the lowest-energy point at 290 keV. A minimum in the cross section near 600 keV (with a rise at lower energy) is newly observed. The cross section in the heretofore unexplored energy region between 7 and 14 MeV exhibits no structure, and thus no evidence for the existence of a bound fourneutron state. The extrapolated zero-energy cross section is found to be 1.70\ifmmode\pm\else\textpm\fi{}0.03 b, in sharp disagreement with previous thermal-energy observations, but in agreement with calculations which also yield the currently accepted coherent scattering length 3.73 fm. Agreement with a prediction from an analysis of $p$-$^{3}\mathrm{He}$ data is within 1% at the resonance peak near 3.5 MeV, but the data differ from this prediction by as much as 6% at lower and higher energies. Thus, within the framework of a suitable theoretical analysis, the present data might provide a measure of the departure from charge symmetry of the nuclear force.NUCLEAR REACTIONS Measured $\ensuremath{\sigma}({E}_{n})$ for $^{1}\mathrm{H}$, $^{2}\mathrm{H}$, 0.06-80 MeV; LINAC, NTOF technique, isotopic, high-pressure gas samples; tritium; four-nucleon system; four-neutron state; scattering length; charge symmetry; $R$ matrix.