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Anatase TiO 2 is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron–hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier–Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy. Here the authors combine steady-state angle-resolved photoemission spectroscopy, ellipsometry and ultrafast two-dimensional ultraviolet spectroscopy to examine the role of many-body correlations in anatase TiO 2 , revealing the existence of strongly bound excitons in single crystals and nanoparticles.

Spectroscopic ellipsometry is a means of investigating optical and dielectric material responses. Conventional spectroscopic ellipsometry is subject to trade-offs between spectral accuracy, resolution, and measurement time. Polarization modulation has afforded poor performance because of its sensitivity to mechanical vibrational noise, thermal instability, and polarization-wavelength dependency. We combine spectroscopic ellipsometry with dual-comb spectroscopy, namely, dual-comb spectroscopic ellipsometry. Dual-comb spectroscopic ellipsometry (DCSE). DCSE directly and simultaneously obtains the ellipsometric parameters of the amplitude ratio and phase difference between s-polarized and p-polarized light signals with ultra-high spectral resolution and no polarization modulation, beyond the conventional limit. Ellipsometric evaluation without polarization modulation also enhances the stability and robustness of the system. In this study, we construct a polarization-modulation-free DCSE system with a spectral resolution of up to 1.2 × 10 −5  nm throughout the spectral range of 1514–1595 nm and achieved an accuracy of 38.4 nm and a precision of 3.3 nm in the measurement of thin-film samples. Spectroscopic ellipsometry is an established technique to characterize the optical properties of a material. Here, Minamikawa et al. combine the method with dual-comb spectroscopy, which allows them to obtain ellipsometric parameters including the phase difference between s-polarized and p-polarized light.

Understanding protein adsorption kinetics to surfaces is of importance for various environmental and biomedical applications. Adsorption of bovine serum albumin to various self-assembled monolayer surfaces including neutral and charged hydrophilic and hydrophobic surfaces was investigated using in-situ combinatorial quartz crystal microbalance with dissipation and spectroscopic ellipsometry. Adsorption of bovine serum albumin varied as a function of surface properties, bovine serum albumin concentration and pH value. Charged surfaces exhibited a greater quantity of bovine serum albumin adsorption, a larger bovine serum albumin layer thickness, and increased density of bovine serum albumin protein compared to neutral surfaces at neutral pH value. The quantity of adsorbed bovine serum albumin protein increased with increasing bovine serum albumin concentration. After equilibrium sorption was reached at pH 7.0, desorption of bovine serum albumin occurred when pH was lowered to 2.0, which is below the isoelectric point of bovine serum albumin. Our data provide further evidence that combinatorial quartz crystal microbalance with dissipation and spectroscopic ellipsometry is a sensitive analytical tool to evaluate attachment and detachment of adsorbed proteins in systems with environmental implications.

Glasses prepared by physical vapor deposition (PVD) are anisotropic, and the average molecular orientation can be varied significantly by controlling the deposition conditions. While previous work has characterized the average structure of thick PVD glasses, most experiments are not sensitive to the structure near an underlying substrate or interface. Given the profound influence of the substrate on the growth of crystalline or liquid crystalline materials, an underlying substrate might be expected to substantially alter the structure of a PVD glass, and this near-interface structure is important for the function of organic electronic devices prepared by PVD, such as organic light-emitting diodes. To study molecular packing near buried organic–organic interfaces, we prepare superlattice structures (stacks of 5- or 10-nm layers) of organic semiconductors, Alq3 (Tris-(8-hydroxyquinoline)aluminum) and DSA-Ph (1,4-di-[4-(N,N-diphenyl)amino]styrylbenzene), using PVD. Superlattice structures significantly increase the fraction of the films near buried interfaces, thereby allowing for quantitative characterization of interfacial packing. Remarkably, both X-ray scattering and spectroscopic ellipsometry indicate that the substrate exerts a negligible influence on PVD glass structure. Thus, the surface equilibration mechanism previously advanced for thick films can successfully describe PVD glass structure even within the first monolayer of deposition on an organic substrate.

Palladium diselenide (PdSe2), a new type of two-dimensional noble metal dihalides (NMDCs), has received widespread attention for its excellent electrical and optoelectronic properties. Herein, high-quality continuous centimeter-scale PdSe2 films with layers in the range of 3L–15L were grown using Chemical Vapor Deposition (CVD) method. The absorption spectra and DFT calculations revealed that the bandgap of the PdSe2 films decreased with the increasing number of layers, which is due to the enhancement of orbital hybridization. Spectroscopic ellipsometry (SE) analysis shows that PdSe2 has significant layer-dependent optical and dielectric properties. This is mainly due to the unique strong exciton effect of the thin PdSe2 film in the UV band. In particular, the effect of temperature on the optical properties of PdSe2 films was also observed, and the thermo-optical coefficients of PdSe2 films with the different number of layers were calculated. This study provides fundamental guidance for the fabrication and optimization of PdSe2-based optoelectronic devices.

We have used atomic layer-by-layer oxide molecular beam epitaxy to grow epitaxial thin films of La2—xCaₓCuO₄ with x up to 0.5, greatly exceeding the solubility limit of Ca in bulk systems (x ∼ 0.12). A comparison of the optical conductivity measured by spectroscopic ellipsometry to prior predictions from dynamical mean-field theory demonstrates that the hole concentration p is approximately equal to x. We find superconductivity with Tc of 15 to 20 K up to the highest doping levels and attribute the unusual stability of superconductivity in La2—xCaₓCuO₄ to the nearly identical radii of La and Ca ions, which minimizes the impact of structural disorder. We conclude that careful disorder management can greatly extend the “superconducting dome” in the phase diagram of the cuprates.

Electronic correlations play important roles in driving exotic phenomena in condensed matter physics. They determine low-energy properties through high-energy bands well-beyond optics. Great effort has been made to understand low-energy excitations such as low-energy excitons in transition metal dichalcogenides (TMDCs), however their high-energy bands and interlayer correlation remain mysteries. Herewith, by measuring temperature- and polarization-dependent complex dielectric and loss functions of bulk molybdenum disulphide from near-infrared to soft X-ray, supported with theoretical calculations, we discover unconventional soft X-ray correlated-plasmons with low-loss, and electronic transitions that reduce dimensionality and increase correlations, accompanied with significantly modified low-energy excitons. At room temperature, interlayer electronic correlations, together with the intralayer correlations in the c -axis, are surprisingly strong, yielding a three-dimensional-like system. Upon cooling, wide-range spectral-weight transfer occurs across a few tens of eV and in-plane p–d hybridizations become enhanced, revealing strong Coulomb correlations and electronic anisotropy, yielding a two-dimensional- like system. Our result shows the importance of strong electronic, interlayer and intralayer correlations in determining electronic structure and opens up applications of utilizing TMDCs on plasmonic nanolithrography. Electronic and interlayer correlations are expected to affect the electronic and optical properties of transition metal dichalcogenides. Here, by using spectroscopic ellipsometry in the broad energy range, the authors uncover new electronic transitions and correlated plasmons in bulk MoS 2 .

Multicomponent oxides, such as many minerals and high entropy oxides, show promise as materials for protection in extreme environments. Similar to other phononically dominated materials, the spectrum of vibrational carriers and phonon scattering heavily influences thermal transport in multi-cation oxides. In this work, we experimentally and computationally investigate the nature of phonon scattering and thermal transport in a series of single and multi-cation rare earth sesquioxides and zirconates. A reduction in thermal conductivity was observed from the single to multi-cation oxides, which is directly correlated to measured optical mode lifetimes. Via spectroscopic ellipsometry, we observe red shifting of the optical modes from local bonding distortion. Density functional theory calculation was used to evaluate how bonding distortions influence the phononic scattering rate observed through modal broadening and reduced thermal conductivity. Compared to single-cation oxides, the multi-cation oxides, especially those with larger cation size variance, exhibited lower effective coordination number and greater bond distortion. The authors observe a reduction in thermal conductivity when transitioning from single to multi-cation oxides, exploring the role of local bonding distortions in phonon scattering and thermal transport.

The tuning of structural, optical, and electrical properties of Al-doped ZnO films deposited by atomic layer deposition technique is reported in this work. With the increasing Al doping level, the evolution from (002) to (100) diffraction peaks indicates the change in growth mode of ZnO films. Spectroscopic ellipsometry has been applied to study the thickness, optical constants, and band gap of AZO films. Due to the increasing carrier concentration after Al doping, a blue shift of band gap and absorption edge can be observed, which can be interpreted by Burstein-Moss effect. The carrier concentration and resistivity are found to vary significantly among different doping concentration, and the optimum value is also discussed. The modulations and improvements of properties are important for Al-doped ZnO films to apply as transparent conductor in various applications.

ZnSe films with thickness of 50, 70 and 100 nm were prepared on Corning 7059 glass substrates at room temperature by applying continuous or periodically interrupted physical vapour deposition at very low deposition rates (0.2 and 0.5 nm s −1 ). Part of the films was annealed at 200 °C after deposition. Optical absorption spectra determined from spectroscopic ellipsometry data show direct allowed transitions, which indicate that the films have microcrystalline structure. This is in agreement with scanning electron microscopy and atomic force microscopy results. The spectroscopic ellipsometry results also show that the porosity of the films is strongly affected by both manner of deposition and deposition rate; the annealing causes some porosity decrease. Optical transmission spectra reveal that the transmittance of as-deposited films, obtained by periodically interrupted manner is significantly influenced by their porosity.