Epitaxial 2D MoSe₂ (HfSe₂) Semiconductor/2D TaSe₂ Metal van der Waals Heterostructures

Molecular beam epitaxy of 2D metal TaSe₂/2D MoSe₂ (HfSe₂) semiconductor heterostructures on epi-AlN(0001)/Si(111) substrates is reported. Electron diffraction reveals an in-plane orientation indicative of van der Waals epitaxy, whereas electronic band imaging supported by first-principles calculations and X-ray photoelectron spectroscopy indicate the presence of a dominant trigonal prismatic 2H-TaSe₂ phase and a minor contribution from octahedrally coordinated TaSe₂, which is present in TaSe₂/AlN and TaSe₂/HfSe₂/AlN but notably absent in the TaSe₂/MoSe₂/AlN, indicating superior structural quality of TaSe₂ grown on MoSe₂. Apart from its structural and chemical compatibility with the selenide semiconductors, TaSe₂ has a workfunction of 5.5 eV as measured by ultraviolet photoelectron spectroscopy, which matches very well with the semiconductor workfunctions, implying that epi-TaSe₂ can be used for low-resistivity contacts to MoSe₂ and HfSe₂

Υπερταχεία μη γραμμική οπτική απόκριση υψηλών φουλλερενίων και endohedral φουλλερενίων

In this work we present experimental results related to the ultrafast electronic nonlinear optical response of higher fullerenes and their endohedral derivatives. The Optical Kerr Effect technique was employed for the determination of the third order susceptibility ( ) 3 χ and the second order hyperpolarizability γ of these materials. A laser pulse of 830 nm and 100 fs duration was applied for the study of the nonlinearity that the two C84 isomers D2 (IV) and D2d (II) and a C84 isomer mixture exhibit. The same technique was applied to the C82 and C92 cages and the Dy@C82, Dy2@C82 and Er2@C92 endohedral fullerenes, respectively. We determined the values of ( ) 3 χ and γ of those materials and we investigated the influence of the encapsulated metal atoms on the nonlinear optical response of the carbon cages. In addition, measurements were carried out at laser wavelengths of 900 nm and 1000 nm, where no resonant contributions are present as in the case of 830 nm

Heterostructured CoOx–TiO2 Mesoporous/Photonic Crystal Bilayer Films for Enhanced Visible-Light Harvesting and Photocatalysis

Heterostructured bilayer films, consisting of co-assembled TiO2 photonic crystals as the bottom layer and a highly performing mesoporous P25 titania as the top layer decorated with CoOx nanoclusters, are demonstrated as highly efficient visible-light photocatalysts. Broadband visible-light activation of the bilayer films was implemented by the surface modification of both titania layers with nanoscale clusters of Co oxides relying on the chemisorption of Co acetylacetonate complexes on TiO2, followed by post-calcination. Tuning the slow photon regions of the inverse opal supporting layer to the visible-light absorption of surface CoOx oxides resulted in significant amplification of salicylic-acid photodegradation under visible and ultraviolet (UV)–visible light (Vis), outperforming benchmark P25 films of higher titania loading. This enhancement was related to the spatially separated contributions of slow photon propagation in the inverse opal support layer assisted by Bragg reflection toward the CoOx-modified mesoporous P25 top layer. This effect indicates that photonic crystals may be highly effective as both photocatalytically active and backscattering layers in multilayer photocatalytic films

Visible Light Trapping against Charge Recombination in FeO_x–TiO₂ Photonic Crystal Photocatalysts

Tailoring metal oxide photocatalysts in the form of heterostructured photonic crystals has spurred particular interest as an advanced route to simultaneously improve harnessing of solar light and charge separation relying on the combined effect of light trapping by macroporous periodic structures and compositional materials’ modifications. In this work, surface deposition of FeOx nanoclusters on TiO2 photonic crystals is investigated to explore the interplay of slow-photon amplification, visible light absorption, and charge separation in FeOx–TiO2 photocatalytic films. Photonic bandgap engineered TiO2 inverse opals deposited by the convective evaporation-induced co-assembly method were surface modified by successive chemisorption-calcination cycles using Fe(III) acetylacetonate, which allowed the controlled variation of FeOx loading on the photonic films. Low amounts of FeOx nanoclusters on the TiO2 inverse opals resulted in diameter-selective improvements of photocatalytic performance on salicylic acid degradation and photocurrent density under visible light, surpassing similarly modified P25 films. The observed enhancement was related to the combination of optimal light trapping and charge separation induced by the FeOx–TiO2 interfacial coupling. However, an increase of the FeOx loading resulted in severe performance deterioration, particularly prominent under UV-Vis light, attributed to persistent surface recombination via diverse defect d-states

Visible Light Trapping against Charge Recombination in FeOx-TiO2 Photonic Crystal Photocatalysts

Tailoring metal oxide photocatalysts in the form of heterostructured photonic crystals has spurred particular interest as an advanced route to simultaneously improve harnessing of solar light and charge separation relying on the combined effect of light trapping by macroporous periodic structures and compositional materials' modifications. In this work, surface deposition of FeOx nanoclusters on TiO2 photonic crystals is investigated to explore the interplay of slow-photon amplification, visible light absorption, and charge separation in FeOx-TiO2 photocatalytic films. Photonic bandgap engineered TiO2 inverse opals deposited by the convective evaporation-induced co-assembly method were surface modified by successive chemisorption-calcination cycles using Fe(III) acetylacetonate, which allowed the controlled variation of FeOx loading on the photonic films. Low amounts of FeOx nanoclusters on the TiO2 inverse opals resulted in diameter-selective improvements of photocatalytic performance on salicylic acid degradation and photocurrent density under visible light, surpassing similarly modified P25 films. The observed enhancement was related to the combination of optimal light trapping and charge separation induced by the FeOx-TiO2 interfacial coupling. However, an increase of the FeOx loading resulted in severe performance deterioration, particularly prominent under UV-Vis light, attributed to persistent surface recombination via diverse defect d-states

Topological surface states in epitaxial ( Sn Bi 2 Te 4 ) n ( Bi 2 Te 3 ) m natural van der Waals superlattices

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Topological band crossings in epitaxial strained SnTe

International audienceEpitaxial SnTe (111) is grown by molecular-beam epitaxy on Bi 2 Te 3 substrates. Structural evaluation indicates that SnTe deviates from cubic due to in-plane compressive strain, which induces significant changes in the electronic band structure. More specifically, a pair of gapless crossings between the two uppermost valence bands occurs in k space along the out-of-plane Z direction of the Brillouin zone, associated with a band inversion, thus defining topological three-dimensional Dirac nodes. Combined first-principles calculations and angle-resolved photoelectron spectroscopy reveal an overtilted Dirac cone indicating that the crossing is a topological type-III Dirac node at the borders between type-I and type-II Dirac nodes