First-Principles Calculations of the Phonon, Elastic, and Thermoelectric Properties of a Ti2CO2 Monolayer.

The phonon, elastic, and thermoelectric properties of Ti2CO2 are investigated by first-principles calculations. The dynamic and mechanical stabilities of Ti2CO2 are confirmed. The Ti2CO2 monolayer exhibits strong acoustic-optical coupling with the lowest optical frequency of 122.83 cm-1. The TA mode originates from the contribution of Ti(XY) vibrations and has the largest gruneisen parameter at the Γ point; the LA mode has the main contribution of O(XY) and Ti(XY) vibrations and has the lowest gruneisen parameter at the M point. The analysis of the phonon spectrum indicates that the vibration contributions from C, O, and Ti atoms are mainly located in the low-, middle-, and high-energy regions, respectively. The Seebeck coefficient and electronic conductivity increase with increasing carrier concentration under room temperature. The analysis of mechanical properties shows that Ti2CO2 possesses a larger Young's modulus and bending modulus, which has a better ability to resist deformation. Thermal properties are further investigated.

External Electric Field-Induced the Modulation of the Band Gap and Quantum Capacitance of F-Functionalized Two-Dimensional Sc2C.

The modulation of electronic properties and quantum capacitance of Sc2CF2 under a perpendicular external E-field was investigated using density functional calculations for the potential application of nanoelectronics and nanophotonics. Sc2CF2 has an indirect band gap of 0.959 eV without an E-field. Furthermore, it undergoes a semiconducting-metallic transition under a positive E-field and a semiconductor-insulator transition under a negative E-field. The application of the negative E-field makes Sc2CF2 have an indirect band gap. Sc-d, F-p, and C-p states are mainly responsible for the significant variation of the band gap. Sc2CF2 under an external E-field always keeps the character of a cathode material under the whole potential. Especially, Sc2CF2 under a negative external E-field is more suitable for the cathode material due to its much smaller |Qp|/|Qn| with much higher Qn. The charge analysis is further performed.

Pressure-Induced Modulation of Electronic and Optical Properties of Surface O-Functionalized Ti2C MXene.

Functionalized MXenes have gained increasing interest in the fields of thermoelectric materials, hydrogen storage, and so forth. In this work, pressure-induced band modulation and optical properties of the Ti2CO2 monolayer are investigated by using density functional theory with the hybrid (HSE06) functional. The calculation reveals that Ti2CO2 MXenes under pressure are stable because of the positive E coh. Ti2CO2 undergoes a semiconductor-to-metal phase transition at about 7 GPa. The metallization of Ti2CO2 mainly results from the Ti-d state. Research indicates that there exist strong interactions between Ti-d and C-p, and Ti-d and O-p states, which are further confirmed by the charge analysis. In addition, the absorption is enhanced in the visible region with increasing pressure. We also observed some new absorption peaks in the visible region.

Strain-Induced Band Modulation, Work Function, and QTAIM Analysis of Surface O-Functionalized Ti2C MXene.

Functionalized MXenes have wide applications in the fields of gas sensors, thermoelectric materials, and hydrogen storage. Strain-induced band engineering and the work function (WF) of Ti2CO2 MXene are investigated theoretically. The calculations reveal that Ti2CO2 MXenes are stable because of the negative E coh, and all the strains considered are within the elastic limit. For Ti2CO2 MXene, strain-induced blue shift of the Ti d state results in the transformation from a semiconductor to a metal. At about 4%, Ti2CO2 MXene transforms from an indirect band gap to a direct band gap. The decreased WF induced by the strain improves the power efficiency of Ti2CO2 MXene.

Theoretical investigation on vibrational spectra, first order hyperpolarizability and NBO analysis of 4-Phenylpyridinium hydrogen squarate.

The vibrational frequencies of 4-Phenylpyridinium hydrogen squarate (4PHS) in the ground state have been investigated by using B3LYP/6-311++G(d,p) level. The analysis of molecular structure, natural bond orbitals and frontier molecular orbitals was also performed. The IR spectra were obtained and interpreted by means of potential energies distributions (PEDs) using MOLVIB program. NBO analysis proved the presence of C-H⋯O and N-H⋯O hydrogen bonding interactions, which is consistent with the analysis of molecular structure. The dipole moments and first-order hyperpolarizability (ßtot) are calculated and are 5.856 D and 4.72×10(-30) esu, respectively. The high ßtot value and the low HOMO-LUMO energy gap (4.062eV) are responsible for the optical and electron-transfer properties of 4PHS molecule. The photoresponse-related results indicate that 4PHS molecule is an excellent organic candidate of photon-responsive materials.

N-NO2 bond dissociation energies in acetonitrile: an assessment of contemporary computational methods.

The assessment of the N-NO2 bond dissociation energies (BDEs) was performed by various calculating methods (B3LYP, B3PW91, B3P86, B1LYP, BMK, MPWB1K, PBE0, CBS-4M and M06-2X) at 6-311+G(2d,p) basis set. Compared with the experimental BDEs, the results show that BMK and B3P86 methods reproduce the experimental values well. The mean absolute deviations from the experimental values obtained by BMK and B3P86 methods were 0.5 and 1.5 kcal/mol, respectively. B3LYP, B3PW91, B1LYP, MPWB1K and PBE0 methods underestimated the homolytic N-NO2 BDEs. B3LYP, B3PW91, B1LYP, M06-2X, CBS-4M methods failed to provide an accurate description of N-NO2 BDEs for N-Nitrosulfonamide compounds and showed larger mean absolute deviations and maximum deviations. Further, substituent effect based on BMK/6-311+G(2d,p) method was analysis. Natural bond orbital analysis shows that there exist good linear correlations between E((2)) of lpN1→BD*(O1-N2) and Hammett constants and a better correlation between the BDEs and the second order stabilization energy E((2)) of lpN1→BD*(O1-N2).