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Layered transition-metal dichalcogenides (TMDs) constitute an emerging class of materials that provide researchers a platform to realize fundamental studies and to design promising optoelectronic devices. While defects are an almost unavoidable part of TMDs, they bring additional interesting properties absent in defect-free layers. Moreover, the controlled introduction of defects in TMDs makes it possible to tailor the electromagnetic properties of the materials. Here we report defect-induced properties of single-layer $\mathrm{Pd}{\mathrm{Se}}_{2}$ and demonstrate the emergence of magnetism at the nanoscale. Our first-principle calculations indicate that Se vacancies create in-gap defect states, which can be responsible for trapping of carriers. The complex square ${V}_{\mathrm{Pd}+4\mathrm{Se}}$ vacancy induces spin-polarized states with a total local magnetic moment of $2\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ per defect, making it possible to introduce magnetization into $\mathrm{Pd}{\mathrm{Se}}_{2}$ and therefore expand the family of two-dimensional (2D) magnets. The defect formation energies are much lower compared to many other TMD materials that can explain the presence of a large number of Se defects after mechanical exfoliation of $\mathrm{Pd}{\mathrm{Se}}_{2}$ layers, while the central location of the Pd atoms preserves them from exfoliation-induced defect formation. The negatively charged vacancies are prone to form and in many cases demonstrate spin-polarized states. The small diffusion barrier of ${\mathrm{V}}_{\mathrm{Se}}$ in 2D $\mathrm{Pd}{\mathrm{Se}}_{2}$ leads to an easy room-temperature migration, while ${\mathrm{V}}_{\mathrm{Pd}}$ demonstrates a high diffusion barrier preventing its spontaneous migration. As a guide for experimentalists, we simulate and characterize scanning tunneling microscope images in valence and conduction states and estimate the electron-beam energy for a controllable production of various defect patterns. These intriguing findings make $\mathrm{Pd}{\mathrm{Se}}_{2}$ an ideal platform to study defect-induced phenomena.

The fundamental properties of recently synthesized single- and bilayer ${\mathrm{PdSe}}_{2}$ are investigated using accurate many-body perturbation GW theory to quantitatively examine their electronic structure and explain the insufficiency of previously reported experimental and theoretical results. Including electron-hole interactions responsible for exciton formation, we solve the Bethe-Salpeter equation on top of the ${\mathrm{GW}}_{0}$ approximation to predict the optical properties. The fundamental quasiparticle band gaps of single- and bilayer ${\mathrm{PdSe}}_{2}$ are 2.55 and 1.89 eV, respectively. The optical gap of monolayer ${\mathrm{PdSe}}_{2}$ reduces significantly due to a large excitonic binding energy of 0.65 eV comparable to that of ${\mathrm{MoSe}}_{2}$, while an increase of the layer number decreases the excitonic binding energy to 0.25 eV in bilayer ${\mathrm{PdSe}}_{2}$. The giant band gap renormalization of \ensuremath{\sim}36--38% in the bilayer (BL) ${\mathrm{PdSe}}_{2}$/graphene heterostructure has a high impact on the construction of ${\mathrm{PdSe}}_{2}$-based devices and explains the experimentally observed band gap. The small value of the experimental optical gap of single-layer (SL) ${\mathrm{PdSe}}_{2}$ (1.3 eV) can be explained by the presence of Se vacancies, which can drop the Tauc-estimated optical gap to \ensuremath{\sim}1.32 eV. The absorption spectra of both mono- and bilayer ${\mathrm{PdSe}}_{2}$ cover a wide region of photon energy, demonstrating promising application in solar cells and detectors. These findings provide a basis for a deeper understanding of the physical properties of ${\mathrm{PdSe}}_{2}$ and ${\mathrm{PdSe}}_{2}$-based heterostructures.

Resonant x-ray Raman scattering is a strongly coherent process. The well defined phase relations between scattering channels through the core excited states localized at different atoms result in a ...

Near-edge x-ray-absorption fine-structure spectra of two fullerenes, ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70},$ have been simulated by means of density-functional theory techniques using ground-state Kohn-Sham orbitals, transition state potentials, or full core-hole potentials. The very good experimental agreement obtained when the full core hole explicitly is taken into account gives an indication of significant excitonic effects for the x-ray-absorption spectra of fullerenes.

We present a theory of radiative resonant x-ray Raman scattering by solids and polymers beyond the Born-Oppenheimer approximation. The role of electron-phonon coupling of the intermediate core ex ...

Soft-x-ray emission spectra from C2H4/Cu(110) and C6H6/Cu(110) have been obtained for two excitation energies, resonant and nonresonant, and resolved in all three spatial components (x,y,z). The on ...

We explore the utility of density-functional theory ~DFT! in conjunction with the transition-potential ~TP! method to simulate x-ray-absorption spectra. Calculations on a set of small carbon-containing molecules and chemisorbed species show that this provides a viable option for obtaining excitation energies and oscillator strengths close to the experimental accuracy of core-valence transitions. Systematic variations in energy positions and intensities of the different spectra in the test series have been investigated, and comparison is made with respect to the static exchange-, self-consistent-field, and explicit electron-correlation methods. The choice between standard exchange-correlation functionals is shown to be of little consequence for the valence resonant, here p*, parts of the x-ray-absorption spectra, while the long-range behavior of presently available functionals is found not to be completely satisfactory for Rydberg-like transitions. Implementing a basis set augmentation technique, one finds that DFT methods still account well for most of the salient features in the near-edge x-ray-absorption spectra, save for the multielectron transitions in the near continuum, and for some loss of Rydberg structure. For clusters modeling surface adsorbates, the DFT transition potential method reproduces well the spectral compression and intensity reduction for the valence level absorption compared to the free phase, provided fairly large clusters are taken into account. While for near-edge x-ray-absorption fine-structure ~NEXAFS! spectra of free molecules the DFT-TP and Hartree-Fock/static exchange methods have complementary advantages, the DFT-TP method is clearly to be preferred when using clusters to simulate NEXAFS spectra of surface adsorbates. @S0163-1829~98!00136-2#

We present calculations of partial-channel photoionization cross sections and photoelectron spectra in the ultraviolet energy region of the carbonyl systems CO, Cr(CO)${}_{6}$, and CO/Cu${}_{14},$ the latter system modeling CO/Cu(100). The dependence on photon energy, including He I and He II spectra, and on the particular ionization channel has been investigated for the three systems by means of the independent-channel static exchange method. It is found that the method is able to reproduce all essential trends and features of the spectra as well as the energy dependence of the cross sections. The COCu${}_{14}$ model can be used to reproduce the organization of states in the spectra of CO/Cu(100) and for the photon energy dependence of the individual channel cross sections, while the relative cross sections between the different parts of the spectra can only be qualitatively assessed due to the limited representation of the metal density of states in the cluster model.

Tail excitation and Stokes shifts have strong influence on the appearance of the RIXS spectra, both depending crucially on frequency and form of the spectral functions of the incoming photons and on the vibrational progressions of the core-excited states. The band-gap generated spectra emerge as consequences of Stokes shifts when the absorption energies are detuned from the lowest unoccupied molecular orbital resonance. The polarization and angular dependences of RIXS in C{sub 60} are found to be comparatively weak, something which is rationalized by the highly degenerate electronic structure and the spherical shape of the molecule. The computer simulations in this work rest on transition moments and energies obtained by {ital ab} {ital initio} Hartree-Fock calculations in the full {ital I}{sub {ital h}} point-group symmetry.

X-ray fluorescence of condensed ${\mathrm{C}}_{60}$ has been recorded in high resolution using monochromatic synchrotron radiation excitation. Strong intensity modulation of constituent spectral features is observed with varying excitation energy up to 10 eV above threshold. The energy dependence is interpreted as due to resonant inelastic x-ray scattering, leading to symmetry selection rules governing the two-photon process in the fully symmetric molecule.

Near edge x-ray absorption fine-structure spectra of graphenes are calculated using hybrid density-functional theory with the equivalent core hole approximation, aiming to resolve the ongoing debat ...

Light transmissions through a subwavelength hole array in a thin metal film are characterized by resonance peaks in the transmission spectrum. In this work we study the surface polaritons (SPs) in perforated thin metal films by analyzing the dynamic behavior of the Poynting vectors based on a full-vector finite-difference time-domain approach. It is shown that each resonance peak in the transmission spectrum is caused by a collective generation of SPs in the form of a dynamic damping oscillator which oscillates in space and time. The energy of the incident light is transported between the upper and the lower metal-air surfaces during the spatial oscillations largely through the air holes. This energy transport mechanism prevails even when the metal-film thickness becomes as thin as 100 nm, under which circumstance the wave functions of the SPs, localized separately on the two metal-air surfaces when the metal film is thick, begin to strongly overlap with each other. The lifetimes of the damping oscillators are different for different resonance peaks in the transmission spectrum and remain distinguishable by the order of their temporal development.

Because of the generation of polaritons, which are quasiparticles possessing the characteristics of both photonics and electronics, active photonic materials offer a possible solution to transfer electromagnetic energy below the diffraction limit and further increase the density of photonic integrated circuits. A theoretical investigation of these exciting materials is, therefore, very important for practical applications. Four different kinds of polaritons have been studied in this thesis, (1) surface polaritons of negative-index-material cylindric rods, (2) exciton polaritons of semiconductor quantum dots, (3) localized plasmon polaritons of metallic nanoshells, and (4) surface plasmon polaritons of subwavelength hole arrays in thin metal films. All these types of polaritons were found to strongly affect the optical properties of the studied active photonic materials. More specifically, (1) for two-dimensional photonic crystals composed of negative-index-material cylindric rods, the coupling among surface polaritons localized in the rods results in dispersionless anti-crossing bands; (2) for three-dimensional diamond-lattice quantum-dot photonic crystals, the exciton polariton resonances lead to the formation of complete band gaps in the dispersion relationships; (3) for metallic nanoshells, the thickness of the metal shell strongly modifies the localized plasmon polaritons, and therefore influences the degree of localization of the electromagnetic field inside the metallic nanoshells; (4) for subwavelength hole arrays in thin metal films, high-order surface-polariton Bloch waves contribute significantly to the efficient transmission. To numerically simulate these active photonic materials, we introduced three approaches, (1) an extended plane-wave-based transfer-matrix approach for negative- index-material media, (2) a plane-wave method for semiconductor quantum-dot photonic crystals, and (3) an auxiliary-differential-equation finite-difference time- domain approach for semiconductor quantum-dot arrays. A brief perspective is also given at the end of this thesis.

Because of the generation of polaritons, which are quasiparticles possessing the characteristics of both photonics and electronics, active photonic materials offer a possible solution to transfer electromagnetic energy below the diffraction limit and further increase the density of photonic integrated circuits. A theoretical investigation of these exciting materials is, therefore, very important for practical applications. Four different kinds of polaritons have been studied in this thesis, (1) surface polaritons of negative-index-material cylindric rods, (2) exciton polaritons of semiconductor quantum dots, (3) localized plasmon polaritons of metallic nanoshells, and (4) surface plasmon polaritons of subwavelength hole arrays in thin metal films. All these types of polaritons were found to strongly affect the optical properties of the studied active photonic materials. More specifically, (1) for two-dimensional photonic crystals composed of negative-index-material cylindric rods, the coupling among surface polaritons localized in the rods results in dispersionless anti-crossing bands; (2) for three-dimensional diamond-lattice quantum-dot photonic crystals, the exciton polariton resonances lead to the formation of complete band gaps in the dispersion relationships; (3) for metallic nanoshells, the thickness of the metal shell strongly modifies the localized plasmon polaritons, and therefore influences the degree of localization of the electromagnetic field inside the metallic nanoshells; (4) for subwavelength hole arrays in thin metal films, high-order surface-polariton Bloch waves contribute significantly to the efficient transmission. To numerically simulate these active photonic materials, we introduced three approaches, (1) an extended plane-wave-based transfer-matrix approach for negative- index-material media, (2) a plane-wave method for semiconductor quantum-dot photonic crystals, and (3) an auxiliary-differential-equation finite-difference time- domain approach for semiconductor quantum-dot arrays. A brief perspective is also given at the end of this thesis.

Chemical and electronic structures of liquid methanol from x-ray emission spectroscopy and density functional theory

The O Kalpha x-ray emission spectra of water clusters with different sizes and conformations embedded in a continuum medium are simulated. The calculations have successfully explained the experimen ...

The x-ray photoelectron spectra (XPS) of copper tetraazaporphyrin (CuTAP), used as a model of copper phthalocyanine(CuPc) are studied by theoretical simulations. The nature of the main and satellit ...

A fundamental problem of x-ray spectroscopy is the role of relaxation of the electronic subsystem in the field of the transient core hole. The main intention of the present study is to explore the dynamics due to core-hole relaxation in the whole time domain, and to find out how it is manifested in finite molecular systems in comparison with solids. A technique is developed based on a reduction of the Nozieres-De Dominicis equation to a set of linear algebraic equations. The developed time-dependent formalism is applied to a numerical investigation of a one-dimensional tight-binding model. The formation of the x-ray profiles is explored on the real time scale, and the role of interaction with the core hole, band filling, and the final-state rule are investigated for systems of different size. The fort-nation of spectra of the infinite translational invariant system is studied by extensions of the finite systems. We found that the dynamics of finite systems, like molecules, differs qualitatively from solids: Contrary to the latter the time lapse of the Nozieres-De Dominicis domain for finite systems is squeezed between the inverse bandwidth and the revival time, which is proportional to the system size. For small molecules this means that there is no time for a "Mahan-Nozieres-De Dominicis singularity" to develop. Comparison with the strict solution of the Nozieres-De Dominicis equation shows that the adiabatic approximation describes x-ray absorption and emission considerably better than the fast approximation. This explains the suppression of the relaxation effects in x-ray emission of, e.g., gas phase and surface adsorbed molecules, but also that these effects are essential for the absorption case. There is still a quantitative distinction between the adiabatic approximation and the strict approach, which becomes more important for larger systems. Adopting the so-called finite state rule by von Barth and Grossman also for molecules, an almost complete numerical agreement between this rule and the strict x-ray-absorption and emission profiles for systems of different sizes is obtained. The simulations indicate that the final-state rule correction is important mainly near the absorption edge and at the top of the emission band.

We have developed a formulation of resonant x-ray Raman scattering of molecules and solids based on the Mahan-Nozieres-De Dominicis model. A key step in the formulation is given by a reduction of the Keldysh-Dyson equations for the Green's function to a set of linear algebraic equations. This gave way for a tractable scheme that can be used to analyze the resonant x-ray scattering in the whole time domain. The formalism is used to investigate the role of core-hole relaxation, interference, band filling, detuning, and size of the scattering target. Numerical applications are per-formed with a one-dimensional tight-binding model.

Copper tetraazaporphyrin is used as a model for copper phthalocyanine in theoretical simulations of the features observed at the Cu ${L}_{2,3}$ near-edge x-ray absorption spectra. Theoretical results obtained by a density-functional theory approach provided interpretations of the different features observed experimentally, including the so-called white line and the satellite structures, leading to the conclusion that the solid phase arrangement exerts no significant influence on the shape of the Cu ${L}_{2,3}$ copper molecular spectrum. The calculations show that the unoccupied $3d$ levels are essentially located in the plane ${(3d}_{{x}^{2}{\ensuremath{-}y}^{2}})$ of the molecule with a small contribution that has an out-of-plane ${3d}_{{z}^{2}\ensuremath{-}{r}^{2}}$ and ${3d}_{xz,yz}$ character. The angular dependence of the copper unoccupied electronic levels could be used to confirm the orientation of the copper tetraazaporphyrin on the substrates.

A previously devised statistical model for the evaluation of solvation shifts of electron binding energies is generalized to photoelectron spectral functions. Solvation shifts, widths, and deviations from normality are given by the first, second, and third moments of the energy distribution, respectively. The total-energy distribution as well as the Coulomb and polarization contributions are explicitly calculated by Monte Carlo simulations, thereby obtaining the full band shape. Applications are performed for liquid water and sodium ions at infinite aqueous solution.