Energy Level Alignment of a Zinc(II) Tetraphenylporphyrin Dye Adsorbed onto TiO₂(110) and ZnO(112̅0) Surfaces

Abstract

The electronic structure of the Zn(II)-5-(3,5-dicarboxyphenyl)-10,15,20-triphenylporphyrin dye (ZnTPP-Ipa), chemisorbed onto ZnO(112 j 0) and TiO 2 (110) single-crystal surfaces, has been investigated by means of density functional theory (DFT) and by electron spectroscopy methods in an ultra-high-vacuum environment. The core levels (Ti 2p and Zn 2p) as well as the valence band have been probed using X-ray and ultraviolet photoemission spectroscopies, whereas the conduction band has been evaluated from inverse photoemission spectroscopy. The calculated density of states for the gas phase molecule compares well to the experimentally determined electronic structure, allowing both a simple understanding of the adsorbate electronic properties and a direct determination of the ZnTPP-Ipa frontier orbitals with respect to the substrates' band edges.

Self-energy and excitonic effects in the electronic and optical properties ofTiO2crystalline phases

“…The optical gaps ͑3.0-3.2 eV͒ are larger than the DFT ones and always smaller than the electronic ones calculated with many-body corrections, due to excitonic effects, phonon-assisted indirect transitions, and temperature effects. When the calculated electronic gap at many-body level is properly compared 20 to photoemission results, 50,53,63 as it is done in the present paper, a quite good agreement can be found. Some calculations of the optical spectra of these polymorphs 22,30,34,56,59 have been performed at the randomphase approximation ͑RPA͒ level, but this description is inappropriate to correctly take into account the optical response of the material, as we show in the results section.…”

“…32-45͒ and have been reinvestigated more recently. [46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65] From the photoemission data on rutile 42,44,47,50,53,63 the electronic band gap, corresponding to the difference between the valence-band maximum ͑VBM͒ and the conduction-band minimum ͑CBM͒, has reported values of 3.3Ϯ 0.5 eV ͑Ref. 53͒ ͓ultraviolet photoemission spectroscopy ͑UPS͒ and inverse photoemission spectroscopy ͑IPES͔͒, 3.6Ϯ 0.2 eV ͑Ref.…”

Optical properties of tungsten trioxide from first-principles calculations

“…The HSE06 functional [44,45] and the ∆SCF method yield very similar results for both γ-WO 3 and simple cubic WO 3 , and they appear to moderately underestimate photoemission experiments; analogous findings were reported for rutile TiO 2 , whose gap computed with the HSE06 functional, 3.05 eV [51], underestimates the measured photoemission gap (3.3-3.6 eV [19,20] However additional, important effects need to be taken into account, before carrying out a meaningful comparison with experiment, e.g. spin-orbit (SO) effects and corrections to the computed gap due to electron phonon interaction.…”

“…Several measurements by UV-vis spectroscopy [16] and photoelectrolysis [17] yielded an indirect optical gap of 2.6 eV at room temperature(T), while Salje et al [18], who measured transmission spectra at room temperature, reported a direct gap of 2.58 eV. Similar to the case of TiO 2 [7,19,20], direct and inverse photoemission measurements of the fundamental gap of WO 3 led to a value much larger (0.6∼0.7 eV) [21,22] than that of its optical gap, and this difference cannot be accounted for by the exciton binding energy. We note that optical and photoemission experiments were both conducted on the phase stable at room T. On the theoretical side, a coherent and consistent interpretation of experiments has not yet been formulated and the level of theory necessary to describe photoemission and absorption experiments of WO 3 is yet unclear.…”

Cascade energy transfer versus charge separation in ladder-type oligo(p-phenylene)/ZnO hybrid structures for light-emitting applications

“…9,10 The electron affinity of ZnO is about 4.2 eV, which is larger than that of most common conjugated organic molecules. Therefore, the formation of a type-II energy level alignment with ZnO accepting an electron from the excited organic component is expected [10][11][12][13]15 resulting in nonradiative deactivation. 9,10 Such configuration, detrimental when targeting light-emitting applications, holds for many other conventional inorganic semiconductors as well.…”