This chapter introduces the Hubbard model and its applicability as a corrective tool for accurate modeling of the electronic properties of various classes of systems. The attainment of a correct description of electronic structure is critical for predicting further electronic-related properties, including intermolecular interactions and formation energies. The chapter begins with an introduction to the formulation of density functional theory (DFT) functionals, while addressing the origin of bandgap problem with correlated materials. Then, the corrective approaches proposed to solve the DFT bandgap problem are reviewed, while comparing them in terms of accuracy and computational cost. The Hubbard model will then offer a simple approach to correctly describe the behavior of highly correlated materials, known as the Mott insulators. Based on Hubbard model, DFT+U scheme is built, which is computationally convenient for accurate calculations of electronic structures. Later in this chapter, the computational and semiempirical methods of optimizing the value of the Coulomb interaction potential (U) are discussed, while evaluating the conditions under which it can be most predictive. The chapter focuses on highlighting the use of U to correct the description of the physical properties, by reviewing the results of case studies presented in literature for various classes of materials.

Due to their unique optical properties, plasmonic materials are widely used in nonlinear optics, nanophotonics, optoelectronics, photocatalysis, biosensing, information storage, etc. Researchers usually need to know the detailed permittivity behavior at the vicinity of surface plasmons’ excitation wavelengths, which in turn are located near the zero points of the real part of the permittivity called epsilon-near-zero (ENZ). We hereby introduce a spectral fitting method to quickly obtain the materials' permittivity at the ENZ region and summarize the experiences of selecting dispersion models and optimizing model parameters. Specifically, we have made a detailed description of the optical constant fitting process for a series of plasmonic materials such as heavily doped semiconductors, transparent conductive oxides, organic conductive materials, two-dimensional materials, and sandwiched composites. Hopefully, to provide specific data and theoretical support for researchers in the field of photoelectric properties of plasmonic materials.

Plasmonic effect plays a significant role in many optoelectronic devices and enables various innovative applications. It has been widely studied in metallic materials, for example, Ag and Au, and later was expanded to transparent conductive oxides, etc. However, such plasmonic structures have limitations in many emerging optoelectronics including flexible optoelectronics, organic optoelectronics, and so on, due to their inorganic natures. In this manuscript, we discovered that the acid-modified highly conductive organic PEDOT:PSS film shows interesting plasmonic properties in the vis–NIR region and exhibits great potentials for activating surface plasmon polaritons. The dispersion curves of dielectric permittivity and optical constants of two modified PEDOT:PSS samples are obtained by inversion calculation of the spectroscopic ellipsometry data with the Drude–Lorentz dispersion model. The permittivity crossover wavelengths λC and the surface plasmon wavelengths λsp are found to be located squarely in the 650–900 nm range, which will enable future plasmonic device applications in the vis–NIR region. The activation of surface plasmon polaritons propagation mode of modified PEDOT:PSS is directly observed and confirmed by prism coupling experiments. In addition, further quantitative analysis revealed that our modified PEDOT:PSS samples have comparable abilities to generate, propagate, and confine surface plasmon polaritons as indium tin oxide (ITO). To the best of our knowledge, this is the first direct demonstration of an organic structure showing equivalent plasmonic properties to the inorganic ones. We believe it will open up much more possibilities for the optoelectronic devices, due to the flexibility, lightness, biological compatibility, and solution processability of the organic plasmonic materials.

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light absorption is a substantial problem that profoundly influences a wide range of disciplines. Whereas it is fundamentally restricted by the bandgap energy of the involved materials. Herein, we study the sub-bandgap light absorption in germanium films via Berreman mode (BE) and its enhancement through weak coupling to Fabry-Perot cavity mode. This enhancement is performed by integrating the semiconductor film into a microcavity structure and tune its resonance frequency to match the epsilon-near-zero (ENZ) wavelength of the film material in a planar multilayer structure. We ascertained that our approach of electric field confinement in the semiconductor layer could perform significant light absorption at large incidence angles. That provides a novel, general, and simple method to enhance the optical and optoelectronic responses of any ENZ material, especially semiconductors below their bandgap energies.