Ultraviolet photoelectron spectroscopy study of the thermochromic phase transition in urethane-substituted polydiacetylenes

Threshold solid-state ionization energies determined from ultraviolet photoelectron spectra are reported for the thermochromic polydiacetylenes (PDAs) from the bis-ethyl- and bis-n-propyl urethanes of 5,7-dodecadiyn-1,12-diol (ETCD and PUDO, respectively) and the nonthermochromic 1,6-bis-p-toluenesulfonate of 2,4-hexadiyne-1,6-diol (PTS) at temperatures above and below the thermochromic phase transition. PDA-PTS has an ionization energy of 5.66 eV which does not change significantly as the temperature is raised above 140 degrees C. At 25 degrees C, PDA-ETCD and PDA-PUDO have threshold ionization energies of 5.65 and 5.51 eV, respectively. The ionization energies of these PDAs increase by approximately 0.34 eV as temperature is raised above 140 degrees C and returns to the lower values as temperature is reduced to 25 degrees C. The magnitude of the increase in ionization energy on heating to temperatures above the thermochromic transition is very close to the shift in energy of the electronic spectrum over the same temperature range. These observations suggest that the structural changes that take place in the course of the thermochromic transition are primarily associated with the valence band and are consistent with partial relief of mechanical strains.     

SCF-Xα MO studies of the x-ray spectra of CuO and ZnO

SCF-Xα scattered wave cluster MO calculations for the oxyanions CuO^?6₄ (D_4h symmetry) and ZnO^?6₄ (Td symmetry) yield results in good agreement with the X-ray photoelectron and X-ray emission spectra of CuO and ZnO, respectively. Agreement of the calculations with optical data is fair. Calculations of the valence electron and core electron hole states of these oxyanions support the assignment of photoelectron shakeup satellites to valence band to conduction band transitions. Calculated shakeup energies for the Cu2p core spectrum in CuO are 7.4 and 9.9 eV (cf. experimental values of 7.5 and 10.0 eV) while shakeup peaks in the valence region spectrum are predicted at 6.1 and 8.0 eV. (Cf. a broad peak with maximum at 8.1 eV observed experimentally.) The absence of intense low energy satellites in the spectra of ZnO is explained by the small amount of electron reorganization in the outer valence levels attendant upon hole formation.     

Transient photoconductivity of a polydiacetylene single crystal

Pulsed photoconductivity results are presented for single crystals of a fully-conjugated polymer, poly-2,4-hexadiyne-1,6-diol bis(p-toluene sulfonate). Carriers of both sign are observed and are found to have essentially identical action spectra with a photoconduction onset around 3 eV. Upper and lower limits could be placed on the charge carrier mobilities normal to the (011) crystallographic plain, which is roughly the polymer chain direction: 3 ? μ_e,h ? 10^?3 cm²/V s. The mobilities normal to the chain are about a factor of 10 less. The carrier lifetimes are about 0.5 μs for both holes and electrons but strongly dependent on the method of polymerization of the precursor monomer crystals.     

Energetics and dynamics of the interface of RuS2 and implications for photoelectrolysis of water

RuS₂ (E_G=1.3 eV), grown from liquid Bi, has proved to be a stable and fairly efficient photoanode for a potential assisted photoelectrolysis of water using visible and near infrared light. Its valence band has a quite pure d-character. Holes generated on d-states in the valence band lead to the formation of Ru-based surface states which induce interfacial coordination bonding. The average positive charge accumulated in these complexes determines the energetic position of the energy bands with the consequence that the band edges are shifted with application of an electrode potential until their position becomes stabilized by electron injection from the electrolyte. This conclusion is derived from capacity measurements performed in the region of light intensity limited and diffusion controlled anodic photocurrents (rotating disk experiments). The energetic limitation of RuS₂ photoelectrodes has mainly to be seen in the position of the surface states which are formed too high above the edge of the valence band. For the present this compound appears to be a very attractive model system for research on low photon energy photooxidation of water.     

Characterization and photocatalytic activity of Fe- and N-co-deposited TiO2 and first-principles study for electronic structure

Titanium dioxide (TiO₂), co-deposited with Fe and N, is first implanted with Fe by a metal plasma ion implantation (MPII) process and then annealed in N₂ atmosphere at a temperature regime of 400-600 °C. First-principle calculations show that the (Fe, N) co-deposited TiO₂ films produced additional band gap levels at the bottom of the conduction band (CB) and on the top of the valence band (VB). The (Fe, N) co-deposited TiO₂ films were effective in both prohibiting electron-hole recombination and generating additional Fe-O and N-Ti-O impurity levels for the TiO₂ band gap. The (Fe, N) co-deposited TiO₂ has a narrower band gap of 1.97 eV than Fe-implanted TiO₂ (3.14 eV) and N-doped TiO₂ (2.16 eV). A significant reduction of TiO₂ band gap energy from 3.22 to 1.97 eV was achieved, which resulted in the extension of photocatalytic activity of TiO₂ from UV to Vis regime. The photocatalytic activity and removal rate were approximately two-fold higher than that of the Fe-implanted TiO₂ under visible light irradiation.     

Determination of the interface band alignment of Mg2Si/4H-SiC heterojuction for potential photodetector application

The Mg₂Si/4H-SiC heterojunction was prepared by radio frequency (RF) magnetron sputtering technique. The binding energies of Mg 2p, Si 2p, and C 1s core levels and the maxima of valence band were measured by X-ray photoelectron spectroscopy (XPS). Using the optical bandgap of Mg₂Si (0.78 eV) and 4H-SiC (3.25 eV), the band offsets of valence band (VBO) and conduction band (CBO) at Mg₂Si/4H-SiC interface were identified as 1.47 and 1.00 eV, respectively. The band alignment was evaluated to be type-I band alignment. The Mg₂Si/4H-SiC heterojunction could be a promising candidate for the infrared (IR) photodetector.     

Excitons in polymerized diacetylenes

A simple exciton theory for the excited electronic states of poly-2,4-hexadiyn-1,6-diol bis(p-toluene sulphonate) is described. Configuration interaction between bond excitons, charge transfer excitons and two-exciton states is included in the theory. The ? = 0 levels are described and their parentage traced to levels in two model compounds, vinyl acetylene and cyclododecatrienetriyne. The 2 eV transition of the polymer is predicted to be a collective state of the bond and charge transfer excitons with no two-exciton component. The lowest triplet state is predicted to lie below the first singlet and to also have extensive charge transfer character.     

Semiconducting thin films of zinc selenide quantum dots

A novel chemical route for deposition of zinc selenide quantum dots in thin film form is developed. The deposited films are characterized with very high purity in crystallographic sense, and behave as typical intrinsic semiconductors. Evolution of the average crystal size, lattice constant, lattice strain and the optical properties of the films upon thermal treatment is followed and discussed. The band gap energy of as-deposited ZnSe films is blue-shifted by ≈0.50 eV with respect to the bulk value, while upon annealing treatment it converges to 2.58 eV. Two discrete electronic states which originate from the bulk valence band are observed in the UV-VIS spectra of ZnSe 3D quantum dots deposited in thin film form via allowed electronic transitions to the 1S electronic state arising from the bulk conduction band—appearing at 3.10 and 3.50 eV. The splitting between these two states is approximately equal to the spin-orbit splitting in the case of bulk ZnSe. The electronic transitions in the case of non-quantized annealed films are discussed in terms of the direct allowed band-to-band transitions with the spin-orbit splitting of the valence band of 0.40 eV. The effective mass approximation model (i.e., the Brus model) with the static relative dielectric constant of bulk ZnSe fails to predict correctly the size dependence of the band gap energy, while only a slight improvement is obtained when the hyperbolic band model is applied. However, when substantially smaller value for ε_r (2.0 instead of 8.1) is used in the Brus model, an excellent agreement with the experimental data is obtained, which supports some earlier indications that the quantum dots ε_r value could be significantly smaller than the bulk material value. The ionization energy of a deep donor impurity level calculated on the basis of the temperature dependence of the film resistivity is 0.82 eV at 0 K.     

Electroreflectance studies on the excitons in PTS and DCHD single crystals

In polydiacetylene single crystals of PTS [poly-2,4-hexadiyne-1,6-diol-bis(p-toluene sulfonate)] and of DCHD [poly-1,6-di(N-carbazolyl)-2, 4-hexadiyne] electric field modulated reflectance spectra in the range of the π-π* excitations of the polymer indicate considerable delocalization of the electrons along the chain. Shape and size of the excitonic electroreflectance spectra are determined by a large charge transfer component of the excitons. Evaluation of the spectra yields about 20% and 40% charge transfer with excitation to neighbouring π bonds in PTS and DCHD, respectively. Large differences occur in electroreflectance in the range of vibronic excitons pointing to pronounced coupling of the carbazole side group in DCHD to the π electrons of the chain. At higher energy, ?0.48 eV above the lowest exciton transition in both polymers a large electroreflectance signal is observed which is attributed to a band transition.     

The valence electronic excited states of trans-1,3-butadiene and trans,trans-1,3,5-hexatriene from generalized valence bond and configuration interact

Self-consistent ab initio generalized valence bond (GVB) and configuration interaction (Cl) calculations are presented for the ground and valence electronic excited states of trans-1,3-butadine and all trans-1,3,5-hexatrine. Previous workers have suggested that (all trans) polyenes exhibit a parity-forbidden valence excited state (2¹ A_g at an energy just below that of the first dipole-allowed (1¹ B_u) state. We find such valence excited electronic states for butadiene (ΔE = 7.06 eV) and hexatriene (ΔE = 5.87 eV), but in both cases the excitation energy is considerably higher than the dipole-allowed transitions (zero-zero transitions at 5.95 eV and 4.95 eV, respectively). The lower two triplet states are found at 3.35 eV and 5.08 eV for butadie and at 2.71 eV and 4.32 eV in hexatrine, in good agreement with experimental values (3.2–3.3 eV and 4.92 eV for butadiene and 2.66 eV and 4.1–4.2 eV for hexatrine). Considering the states formed by removing one electron from the π space we found ion states at 8.95 eV and 11.40 eV for butadiene and at 8.33 eV, 10.53 eV, and 11.60 eV for hexatriene, in godo agreement with experimental results (9.0 eV and 11.5 eV for butadiene and 8.45 eV, 10.43 eV and 11.6 eV for hexatriene).     

A First-Principles Calculation of Electronic Properties of LiNH<Subscript>2</Subscript> and NaNH<Subscript>2</Subscript>

Band spectra, densities of states, total and deformation densities of α-LiNH₂ and α-NaNH₂ are calculated from the first principles using the density functional method in the all-electron approximation. The upper valence band is formed mostly by nitrogen p-states with a small admixture of metal states, the lower conduction bands are formed by the states of all atoms in α-LiNH₂ and mainly by sodium and nitrogen states in α-NaNH₂. The bottom of the conduction band appears in both crystals in the center of the Brillouin zone. α-LiNH₂ exhibits indirect-gap transitions at the absorption edge and three valence band extrema at a short distance of ~0.15 eV from each other. The top of the valence band in α-NaNH₂ appears in the center of the Brillouin zone with the competing maximum at the lateral point at a distance of ~0.06 eV. The electron density distributions testify that polar covalent bonding occur inside the amide anion and ionic bonding occurs between the metal and the amide ion.     

Influence of ionic interfacial layers on electronic properties of Alq3/Si(100) interface

The electronic structures of Alq₃/Si(100), Alq₃/LiBr/Si(100), and Alq₃/KCl/Si(100) systems are presented in this report. Their energy level diagrams were prepared and discussed. The formation of the LiBr and KCl interfacial layers between an Alq₃ film and a Si(100) substrate results in a decrease of the energy barrier at the interface. The studies were carried out in situ in ultrahigh vacuum by ultraviolet photoelectron spectroscopy. Alq₃ as well as LiBr and KCl layers were vapour evaporated onto n‐type Si(100) crystal. The electron affinity of clean Si(100) surface was 4.0 eV, and the position of the valence band maximum was 0.7 eV below E_F. The energetic distance between the valence band maximum of Si(100) and the highest occupied molecular orbital level were 1.5, 2.6, and 2.2 eV, for the Alq₃/Si(100), Alq₃/LiBr/Si(100), and Alq₃/KCl/Si(100) systems, respectively.     

A study of valence ionization of ammonia with synchrotron radiation

Angular distributions of photoelectrons from the three valence levels of molecular ammonia, 2a₁ with a binding energy (BE) of 27.7 eV, 1e (BE = 16.3 eV) and 3a₁ (BE = 10.9 eV) have been measured in the 30–130 eV photon energy range. Comparison is made in the case of the 3a₁ molecular orbital with the calculated atomic N 2p asymmetry parameters. Photoionization branching ratios have also been obtained in this photon energy range and good agreement has been found with previous quasiphotoionization measurements and with the one-center-expansion calculations of Cacelli et al. The satellite structure in the inner valence (2a₁) region has been investigated at 60 eV and compared with a number of energy and intensity calculations.     

First-principles study of structural,elastic, electronic and optical properties of perovskites XCaH3 (X = Cs and Rb) under pressure

The stability, structural parameters, elastic constants, electronic and optical properties of perovskites CsCaH₃ and RbCaH₃ were investigated by the density functional theory. The calculated lattice parameters are in agreement with previous calculation and experimental data. The energy band structures, density of states, born-effective-charge and Mulliken charge population were obtained. The perovskites CsCaH₃ and RbCaH₃ present a direct band gap of 3.15 eV and 3.27 eV at equilibrium. The top of the valence bands reflects the s electronic character for both structures. Furthermore, the absorption spectrum, refractive index, extinction coefficient, reflectivity, energy-loss spectrum, and dielectric function were calculated. The origin of the spectral peaks was interpreted based on the electronic structures. The static dielectric constant and refractive index are indeed, inverse proportional to the direct band gap.     

Rydberg or Valence? The Long‐Standing Question in the UV Absorption Spectrum of 1,1′‐Bicyclohexylidene

The electronic excited states of the olefin 1,1′‐bicylohexylidene (BCH) are investigated using multiconfigurational complete active space self‐consistent‐field second order perturbation theory in its multi‐state version (MS‐CASPT2). Our calculations undoubtedly show that the bulk of the intensity of the two unusually intense bands of the UV absorption of BCH measured with maxima at 5.95 eV and 6.82 eV in the vapor phase are due to a single ππ* valence excitation. Sharp peaks reported in the vicinity of the low‐energy feature in the gas phase correspond to the beginning of the π3s_R Rydberg series. By locating the origin of the ππ* band at 5.63 eV, the intensity and broadening of the observed bands and their presence in solid phase is explained as the vibrational structure of the valence ππ* transition, which underlies the Rydberg manifold as a quasi‐continuum.     

Spiro[4.4]nonatetraene and its Positive and Negative Radical Ions: Molectronic Structure Investigations

The electronic structure of spiro[4.4]nonatetraene 1 as well as that of its radical anion and cation were studied by different spectroscopies. The electron‐energy‐loss spectrum in the gas phase revealed the lowest triplet state at 2.98 eV and a group of three overlapping triplet states in the 4.5 – 5.0 eV range, as well as a number of valence and Rydberg singlet excited states. Electron‐impact excitation functions of pure vibrational and triplet states identified various states of the negative ion, in particular the ground state with an attachment energy of 0.8 eV, an excited state corresponding to a temporary electron attachment to the 2b₁ MO at an attachment energy of 2.7 eV, and a core excited state at 4.0 eV. Electronic‐absorption spectroscopy in cryogenic matrices revealed several states of the positive ion, in particular a richly structured first band at 1.27 eV, and the first electronic transition of the radical anion. Vibrations of the ground state of the cation were probed by IR spectroscopy in a cryogenic matrix. The results are discussed on the basis of density‐functional and CASSCF/CASPT2 quantum‐chemical calculations. In their various forms, the calculations successfully rationalized the triplet and the singlet (valence and Rydberg) excitation energies of the neutral molecule, the excitation energies of the radical cation, its IR spectrum, the vibrations excited in the first electronic absorption band, and the energies of the ground and the first excited states of the anion. The difference of the anion excitation energies in the gas and condensed phases was rationalized by a calculation of the Jahn‐Teller distortion of the anion ground state. Contrary to expectations based on a single‐configuration model for the electronic states of 1 , it is found that the gap between the first two excited states is different in the singlet and the triplet manifold. This finding can be traced to the different importance of configuration interaction in the two multiplicity manifolds.     

Band structure and electronic properties of lithium azide (LiN3)

Ab initio Hartree–Fock band structure and molecular calculations have been performed to study the electronic structure of LiN₃ in a monoclinic C 2/m crystal structure. The total energy, band structure, density of states, and charge densities are computed. The calculated lattice energy (energy to separate the ions infinitely apart) of 8.6 eV agrees very well with 8.45 eV deduced from Madelung and London polarizability energies. The calculated split of the N 1s core bands of 5.0 eV compares favorably with the experimental X-ray photoelectron value of 4.4 eV. This good agreement is not contributed to crystalline environment effects as proposed in earlier MO studies of N where the best values obtained were 5.1, 5.8, and 6.3 eV, but to the quality of the nitrogen core basis set. The calculated valence density of states supports one of two competing interpretations that peak III observed in the X-ray photoelectron spectrum arises from contaminations or other extrinsic states.     

Elucidating linear and nonlinear optical properties of defect chalcopyrite compounds ZnX2Te4 (X=Al,Ga, In) from electronic transitions

We presented a theoretical study of electronic band structure of three compounds ZnAl₂Te₄, ZnGa₂Te₄ and ZnIn₂Te₄ using pseudo potential method within density functional theory. Calculated band structures show that all band gaps are direct with at Γ with values of 1.639eV for ZnAl₂Te₄, 1.026eV for ZnGa₂Te₄and 0.836eV ZnIn₂Te₄. The linear properties based on dielectric function and non-linear optical properties based on second harmonic generation (SHG) were computed. The origin of four critical points (peaks) determined from the second derivative of the imaginary part of the dielectric function is elucidated. The use of individual k-points and individual combination of valence and conduction bands dependent matrix of the dielectric function and the nonlinear optical susceptibility allowed to a precise determination of inter band optical transitions. Indeed, inter-band analysis shows the high intensity of non-linear effect compared to linear effect. Moreover, non-linear inter-band optical transitions involve lower valence bands and higher conduction bands.     

The flammability properties of copolyesters and copolycarbonates containing acetylenes

The flammability properties of copolyesters and copolycarbonates containing varying percentages of 2-butyne-1,4-diol and 2,4-hexadiyne-1,6-diol were studied by use of oxygen index and dynamic thermogravimetry. The char formation was found to increase with increasing polymer unsaturation. For nonconjugated acetylenes there is an increase in the oxygen index with increasing unsaturation in a polymer. In contrast, conjugated acetylenes dramatically decrease the oxygen index of a polymer system.     

Optical constants of electrochemically synthesized polyindole and poly(5-carboxilic acid indole)

The optical properties and optical constants of the polyindole and poly(5-carboxilic acid indole) conductive polymers synthesized and doped electrochemically with ClO ₄ ^? in acetonitrile solution were investigated by means of transmittance and reflectance spectra, in the wavelength range of 300–800 nm. Absorption band centered at 425 nm assigned to the direct allowed electron transition (π → π*) from valence band to the conduction band. The optical band gap, E _g , was determined out of the optical absorption spectra. The E _g increases from 2.17 eV for polyindole film to 2.40 eV for poly(5-carboxilic acid indole) polymer thin film, which is attributed to the effect of electron withdrawing carboxylic acid functional group on the growth of chain length of the polymer during the electropolymerization. The oscillator energy E ₀, dispersion energy E _d and other parameters were determined by the Wemple-DiDomenico method.