Crystal structure of polymeric carbon nitride and the determination of its process-temperature-induced modifications.

Based on the arrangement of two-dimensional 'melon', we construct a unit cell for polymeric carbon nitride (PCN) synthesized via thermal polycondensation, whose theoretical diffraction powder pattern includes all major features measured in x-ray diffraction. With the help of this unit cell, we describe the process-temperature-induced crystallographic changes in PCN that occur within a temperature interval between 510 and 610 °C. We also discuss further potential modifications of the unit cell for PCN. It is found that both triazine- and heptazine-based g-C3N4 can only account for minor phases within the investigated synthesis products.

Small-polaron based holograms in LiNbO3 in the visible spectrum.

Diffraction efficiency, relaxation behavior and dependence on pump-beam intensity of small-polaron based holograms are studied in thermally reduced, nominally undoped lithium niobate in the visible spectrum (λ = 488 nm). The pronounced phase gratings with diffraction efficiency up to η = (10.8 ± 1.0)% appeared upon irradiation by single ns-laser pulses (λ = 532 nm) and are comprehensively assigned to the optical formation of spatially modulated densities of small bound NbLi4+ electron polarons, NbLi4+:NbNb4+ electron bipolarons, and O⁻ hole polarons. A remarkable quadratic dependence on the pump-beam intensity is discovered for the recording configuration K || c-axis and can be explained by the electro-optic contribution of the optically generated small bound polarons. We discuss the build-up of local space-charge fields via small-polaron based bulk photovoltaic currents.

AgGaSe2 thin films grown by chemical close-spaced vapor transport for photovoltaic applications: structural, compositional and optical properties.

Thin films of chalcopyrite AgGaSe(2) have been successfully grown on glass and glass/molybdenum substrates using the technique of chemical close-spaced vapor transport. The high crystallinity of the samples is confirmed by grazing-incidence x-ray diffraction, scanning and transmission electron microscopy, and optical transmission/reflection spectroscopy. Here, two of the three expected direct optical bandgaps are found at 1.77(2) and 1.88(6) eV at 300 K. The lowest bandgap energy at 4 K is estimated to be 1.82(3) eV. Photoluminescence spectroscopy has further revealed the nature of the point defects within the AgGaSe(2), showing evidence for the existence of very shallow acceptor levels of 5(1) and 10(1) meV, and thus suggesting the AgGaSe(2) phase itself to exhibit a p-type conductivity. At the same time, electrical characterization by Hall, Seebeck and four-point-probe measurements indicate properties of a compensated semiconductor. The electrical properties of the investigated thin films are mainly influenced by the presence of Ag(2)Se and Ga(2)O(3) nanometer-scaled surface layers, as well as by Ag(2)Se inclusions in the bulk and Ag clusters at the layers' rear side.

Tunable optical transition in polymeric carbon nitrides synthesized via bulk thermal condensation.

Polymeric derivatives of dicyandiamide were synthesized via a bulk thermal condensation method, using a range of process temperatures between 400 and 610 °C. The obtained carbon nitride powders exhibit an optical transition in the UV-green range that has been assigned to the direct bandgap of a semiconductor-like material. Within this context, the apparent bandgap is linearly tunable with increasing process temperatures, showing a temperature coefficient of - 1.7(1) meV K(-1) between 2.5 and 3.0 eV. The obtained results show a predominant optical transition within the tri-s-triazine unit of the polymer, with a bathochromic shift originating from a gradually increasing degree of polymerization.

Hologram recording via spatial density modulation of Nb(Li)(4+/5+) antisites in lithium niobate.

Hologram recording is studied in thermally reduced, nominally undoped lithium niobate in the time domain from 10 ns to 100 s by means of intense ns pump laser pulses (λ = 532 nm) and continuous-wave probe light (λ = 785 nm). It is shown that mixed absorption and phase gratings can be recorded within 8 ns that feature diffraction efficiencies up to 23 % with non-exponential relaxation and lifetimes in the ms-regime. The results are explained comprehensively in the frame of the optical generation of a spatial density modulation of Nb(Li)(4+/5+) antisites and the related optical features, i.e. absorption as well as index changes mutually related via the Kramers-Kronig-relation. Implications of our findings, such as the electrooptical properties of small bound Nb(Li)(4+) polarons, the optical features of Nb(Li)(4+):Nb(Nb)(4+) bipolarons, Nb(Nb)(4+) free polarons and O-hole-polarons, the impact of light polarization of pump and probe beams as well as of the polaron density are discussed.

Absorption cross sections and number densities of electron and hole polarons in congruently melting LiNbO(3).

The number densities and absorption cross sections of both optically generated and reduction-induced small electron and hole polarons in LiNbO(3) are determined by means of time-resolved pump-multiprobe spectroscopy. The data are obtained for free (Nb(Nb)(4+)) and bound (Nb(Li)(4+)) electron polarons, bound Nb(Li)(4+):Nb(Nb)(4+) electron bipolarons, and bound O(-) hole polarons. The peak absorption cross sections are in the range of σ(pol)≈(4-14) × 10(-22) m(2), comparable to that for Fe(2+). In all cases the ratio of occupied to unoccupied polaronic sites is less than 10(-2).

Electron small polarons and bipolarons in LiNbO(3).

An overview of the properties of electron small polarons and bipolarons is given, which can occur in the congruently melting composition of LiNbO(3) (LN). Such polarons influence the performance of this important optical material decisively. Since coupling to the lattice strongly quenches the tunnelling of free small polarons in general, they are easily localized at one site even by weak irregularities of a crystal. The mechanism of their optical absorptions is thus shared with those of small polarons localized by binding to selected defects. It is shown that the optical properties of free electrons in LN as well as those bound to Nb(Li) antisite defects can be attributed consistently to small polarons. This is extended to electron pairs forming bipolarons bound to Nb(Li)-Nb(Nb) nearest neighbours in the LN ground state. On the basis of an elementary phenomenological approach, relying on familiar concepts of defect physics, the peak energies, lineshapes, widths of the related optical absorption bands as well as the defect binding energies induced by lattice distortion are analysed. A criterion universally identifying small polaron absorption bands in oxide materials is pointed out. For the bipolarons, the dissociation energy, 0.27 eV, derived from a corresponding study of the mass action behaviour, is shown to be consistent with the data on isolated polarons. Based on experience with simple O(-) hole small polaron systems, a mechanism is proposed which explains why the observed small polaron optical absorptions are higher above the peak energies of the bands than those predicted by the conventional theory. The parameters characterizing the optical absorptions are seen to be fully consistent with those determining the electrical conductivity, i.e. the bipolaron dissociation energy and the positions of the defect levels as well as the activation energy of mobility. A reinterpretation of previous thermopower data of reduced LN on the basis of the bipolaron model confirms that the mobility of the free polarons is activated by 0.27 eV. On the basis of the level scheme of the bipolarons as well as the bound and free polarons the temperature dependence of the electronic conductivity is explained. The polaron/bipolaron concept also allows us to account for the concentrations of the various polaron species under the combined influence of illumination and heating. The decay of free and bound polarons dissociated from bipolarons by intense short laser pulses of 532 nm light is put in the present context. A critical review of alternative models, being proposed to explain the mentioned absorption features, is given. These proposals include: single free polarons in the (diamagnetic) LN ground state, oxygen vacancies in their various conceivable charge states, quadpolarons, etc. It is shown why these models cannot explain the experimental findings consistently.

Insight to UV-induced formation of laser damage on LiB(3)O(5) optical surfaces during long-term sum-frequency generation.

Microscopic investigations of UV-induced formation of laser damage on LiB(3)O(5) optical surfaces during long-term sum-frequency generation (SFG) uncovers a significant growth of a SiO(2)-amorphous layer spatially limited to the illuminated area. The layer gives rise to a catastrophic break-down of the LiB(3)O(5)-output surface upon long-term laser operation even at intensities far below the laser-induced damage threshold. The interaction of UV laser light, LiB(3)O(5) surface and foreign atoms in the ambient atmosphere is discussed in the frame of a two-step process for surface-damage formation.

Evidence for two-path recombination of photoinduced small polarons in reduced LiNbO(3).

The recombination of photoinduced free Nb(4+)(Nb) and bound Nb(4+)(Li) small polarons toNb(4+)(Li): Nb(4+)(Nb) bipolarons is investigated in nominally pure, reduced LiNbO(3) single crystals by means of excited-state-absorption spectroscopy. We discovered a two-component decay of the light-induced absorption alpha(li)(t) for probe light at lambda=785 nm and moderate pump beam intensities (I(p) << 670 GW/m(2)). These experimental results give strong evidence for the existence of a two-path recombination of the photoinduced polarons. A corresponding model taking into account hopping charge transport and trapping is presented.