High-resolution Spectroscopic Ellipsometry Research Articles

Detection of two-dimensional small polarons at oxide interfaces by optical spectroscopy

Two-dimensional (2D) perovskite oxide interfaces are ideal systems to uncover diverse emergent properties, such as the arising polaronic properties from short-range charge–lattice interactions. Thus, a technique to detect this quasiparticle phenomenon at the buried interface is highly coveted. Here, we report the observation of 2D small-polarons at the LaAlO3/SrTiO3 conducting interface using high-resolution spectroscopic ellipsometry. First-principles investigations show that interfacial electron–lattice coupling mediated by the longitudinal phonon mode facilitates the formation of these polarons. This study resolves the long-standing question by attributing the formation of interfacial 2D small polarons to the significant mismatch between experimentally measured interfacial carrier density and theoretical values. Our study sheds light on the complexity of broken periodic lattice-induced quasi-particle effects and its relationship with exotic phenomena at complex oxide interfaces. Meanwhile, this work establishes spectroscopic ellipsometry as a useful technique to detect and locate optical evidence of polaronic states and other emerging quantum properties at the buried interface.

Unravelling a new many-body large-hole polaron in a transition metal oxide that promotes high photocatalytic activity

A many-body large polaron, which is important for both fundamental physics and technological applications, has been predicted to occur in bismuth vanadate (BiVO4). Herein, using a combination of high-resolution spectroscopic ellipsometry, X-ray absorption spectroscopy at the V L3,2- and O K-edges, and high-resolution X-ray diffraction supported by theoretical calculations, we reveal a new many-body large-hole polaron in W-doped BiVO4 films and the interplay of the large-hole polaron and indirect bandgap when determining the photocatalytic activity. With various W doping concentrations and temperatures, anomalous spectral weight transfers in the complex dielectric function are observed, revealing electronic correlations, particularly the on-site Coulomb interactions of O p (Upp) and V d (Udd), and screening in BiVO4. Due to the distortion of BiO8 dodecahedra and Udd, Bi 6 s is lifted to the top of the valance band, which results in the formation of an indirect bandgap and a large-hole polaron. The large-hole polaron is found to form as a localized midgap state, consisting of O p hybridized with the V d and Bi sp orbitals, and this is important when determining the high photocatalytic activity of BiVO4. Our results show the importance of the interplay among the charge, orbital, and lattice degrees of freedom in forming the many-body large-hole polaron, which improves the conductivity and results in a transition metal oxide with high photocatalytic activity.

Dual phases of crystalline and electronic structures in the nanocrystalline perovskite CsPbBr3

Inorganic perovskites have recently attracted much attention as promising new nanocrystalline materials that have interesting fundamental phenomena and great potential in several applications. Herein, we reveal unusual structural and electronic changes in nanocrystalline cesium lead bromide (CsPbBr3) as a function of temperature using high-resolution spectroscopic ellipsometry, high-resolution transmission electron microscopy and terahertz spectroscopy measurements supported by first-principles calculations. New dual phases of crystalline and electronic structures are observed due to the nanocrystalline nature of the material. Interestingly, a change in the electronic structure occurs below 150 K, and the rate at which the nanocrystal transitions from the tetragonal to orthorhombic phase is found to be nonlinear with temperature. Our results show the importance of the charge and lattice interplay in determining the dual phases and fundamental properties of nanocrystalline materials.

Three-Dimensional Resonant Exciton in Monolayer Tungsten Diselenide Actuated by Spin-Orbit Coupling.

The intricate features of many-body interactions and spin-orbit coupling play a significant role in numerous physical phenomena. Particularly in two-dimensional transition metal dichalcogenides (2D-TMDs), excitonic dynamics are a key phenomenon that promises opportunities for diverse range of device applications. Here, we report the direct observation of a visible-range three-dimensional resonant exciton and its associated charged exciton in monolayer tungsten diselenide, as compared to monolayer molybdenum disulfide. A comprehensive experimental study that includes high-resolution TEM, Raman, high-resolution spectroscopic ellipsometry over a wide temperature range down to 4 K, high-energy temperature, and excitation power-dependent photoluminescence spectroscopy has been conducted. It is supported by first-principles calculations to unravel the influence of spin-orbit coupling in the formation of the resonant exciton and to identify its in-plane and out-of-plane features. Furthermore, we study the impact of temperature and thickness on the spin-orbit coupling strength in 2D-TMDs. This work is crucial in creating a platform in the fundamental understanding of high-energy resonant exciton in layered two-dimensional systems and that such high-energy optoelectronic features make them an increasingly attractive candidate for novel electronic and optoelectronic applications particularly in the aspects of solar cells and light-emitting diodes via the manipulation of excitonic states.

Oxygen electronic screening and hybridization determining the insulator-metal transition of Eu1−xBaxTiO3

An insulator-metal transition (IMT) is a fascinating yet hotly debated phenomenon in condensed-matter physics. Here we report on the evolution of electronic structure across IMT as in $\mathrm{E}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x}\mathrm{Ti}{\mathrm{O}}_{3}$ as a function of $x$ ($x=0$ for ETO and $x=0.5$ for EBTO) and temperature comprehensively using high-resolution spectroscopic ellipsometry, soft x-ray absorption spectroscopy at Ti $L$ and O $K$ edges, and transport measurements. We observe different types of spectral weight transfers (SWTs), which are responsible for tuning optical band gaps, accompanied by dramatic changes in electrical conductivity. At room temperature, when $x$ increases, SWT occurs due to the high energy of O $2p$ to low-energy bands. Such a SWT increases Eu-O-Ti hybridization and enhances Drude response, which increases electrical conductivity in EBTO. Interestingly, upon cooling, SWT arises from low-energy to high-energy bands and opens up a gap of a low-energy oxygen band, which then suppresses electrical conductivity in EBTO. We argue that the unscreened electron-electron interaction in oxygen yields a metal-to-charge-transfer insulator-like transition. Our result reveals comprehensively the importance of oxygen screening effects and hybridizations in the IMT, and it provides insight into $\mathrm{E}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x}\mathrm{Ti}{\mathrm{O}}_{3}$.

Structural and optical properties analysis of MoS2 nanoflakes on quartz substrate as prepared by mechanical exfoliation

We study the structural and optical properties of MoS2 nanoflakes on quartz substrate as prepared by mechanical exfoliation method. The structural and morphological properties of MoS2 nanoflakes were characterized by SEM, EDS, and XRD, while the high-resolution spectroscopic ellipsometry (SE) with the photon energy of 1.27 to 6.53 eV is used to study its optical characteristics. As the results, SEM data shows that MoS2 appears to be nanoflakes covering around 25-35% on the surface of quartz substrate, and XRD spectra shows the dominant orientation along c-axis (002). Based on spectroscopic ellipsometry analysis, the average thickness of MoS2 nanoflakes is around 12 nm or about 6-8 layers. By using Tauc plot method, we confirm that MoS2 nanoflakes have a semiconductor characteristic with the optical bandgap as high as 1.68 eV. Our study shows the important of structural and optical properties of MoS2 nanoflakes that can be utilized for future optoelectronic devices and energy-harvesting purposes.

Dielectric-function analysis of metals for plasmonic-device application

We study the potential of various metals (Pt, Al, Cu and Ni) as plasmonic material used for devices by analyzing their complex permittivity and comparing with other metals. Metals were characterized by using high-resolution spectroscopic ellipsometry covering energy range of 0.5 to 6.5 eV. In fitting process, instead using Drude model, we used the combination of Lorentz model to describe optical properties of metals. The results show that each metal has unique different features of ε1 and ε2 in range of far-infrared to vacuum-ultraviolet. Also, the loss by interband transition is observable for some metals. Furthermore, the plasmonic quality-factor, which are related to electric-field enhancement and heat production generated by surface plasmon, of metal nanoparticle have been calculated and we found the optimum region of device application for each metal. From this study, Cu is promising metal working in near-infrared to visible area potentially to substitute noble Ag and Au. On the other hand, Al is the best metal to be applied as plasmonic device working in ultraviolet region. Moreover, enhancement of plasmonic quality-factor by changing geometry and environment of metal is also discussed. Our studies give an alternative of fundamental perspective for plasmonic-device development especially for energy-harvesting purposes.

Plasmon–exciton interaction and screening of exciton in ZnO-based thin film on bulk Pt as analyzed by spectroscopic ellipsometry

We study plasmon–exciton interaction in ZnO-based thin film on bulk Pt by using high resolution spectroscopic ellipsometry. ZnO films on quartz are used as reference. This study shows the strong electronic interactions between ZnO film and Pt by considering the significant suppression of exciton in ZnO film, in comparison to ZnO film on quartz. We found that plasmon in Pt are responsible to provide transferred electron for electronic blocking of exciton in ZnO film induce by spontaneous recombination from Pt. In the case of Cu doped ZnO film, we confirm screening effects on exciton and a localized interband transition for both systems (ZnO film on Pt and ZnO film on quartz). In Cu-doped ZnO film, electronic blocking of exciton by Pt plasmon is more pronounce rather than screening effect by interband transition. Our results show the importance of plasmon from substrate and doping to modify the optical properties of wide bandgap semiconductor.