Vibrational spectroscopy by means of first-principles molecular dynamics simulations

Vibrational spectroscopy is one of the most important experimental techniques for the characterization of molecules and materials. Spectroscopic signatures retrieved in experiments are not always easy to explain in terms of the structure and dynamics of the studied samples. Computational studies are a crucial tool for helping to understand and predict experimental results. Molecular dynamics simulations have emerged as an attractive method for the simulation of vibrational spectra because they explicitly treat the vibrational motion present in the compound under study, in particular in large and condensed systems, subject to complex intramolecular and intermolecular interactions. In this context, first-principles molecular dynamics (FPMD) has been proven to provide an accurate realistic description of many compounds. This review article summarizes the field of vibrational spectroscopy by means of FPDM and highlights recent advances made such as the simulation of Infrared, vibrational circular dichroism, Raman, Raman optical activity, sum frequency generation, and nonlinear spectroscopies

Design, fabrication and characterization of three-dimensional chiral photonic crystals

In this thesis, we have investigated an exciting subclass of photonic nanostructures: Chiral three-dimensional photonic crystals. We have fabricated several high-quality structures by using direct laser writing and have characterized them by transmission experiments and numerical calculations. This material class shows a high potential for applications because of intense response to circularly polarized light

A computational study of cyclic peptides with vibrational circular dichroism

Cyclic peptides are a class of molecules that has shown antimicrobial potential. These are complex compounds to investigate with their large conformational space and multiple chiral centers. A technique that can be used to investigate both conformational preferences and absolute configuration (AC) is vibrational circular dichroism (VCD). To extract information from the experimental VCD spectra a comparison with calculated spectra is often needed and this is the focus of this thesis: the calculation of VCD spectra. The VCD spectra are very sensitive to small structural changes, and to accurately calculate the spectra, all important conformers need to be identified. The first part of this thesis has been to establish a reliable computational protocol using meta-dynamics to sample the conformational space and ab initio methods to calculate the spectra for cyclic peptides. Using our protocol, we have investigated if VCD alone can determine the AC of cyclic tetra- and hexapeptides. We show that it is possible to determine the AC of the cyclic peptides with two chiral centers while for the peptides with three and four chiral centers, VCD is at best able to reduce the number of possible ACs and further investigation with other techniques is needed. Further, we investigated four cyclic hexapeptides with antimicrobial potential. These peptides, in contrast to the ones used for validating the protocol, consist of several amino acids with long and positively charged side chains. For these peptides, a molecular dynamics based approach provided VCD spectra in better agreement with experiment than our protocol. Reasons for this may be the lack of atomistic detail in the solvent model used during the conformational search and insufficient description of dispersion interactions during the meta-dynamics simulation

Optimization of a perfect absorber multilayer structure by genetic algorithms

Background: An increasing interest has been recently grown in the development of nearly perfect absorber materials for solar energy collectors and more in general for all the thermophotovoltaic applications. Methods: Wide angle and broadband perfect absorbers with compact multilayer structures made of a sequence of ITO and TiN layers are here studied to develop new devices for solar thermal energy harvesting. Genetic Algorithms are introduced for searching the optimal thicknesses of the layers so to design a perfect broadband absorber in the visible range, for a wide range of angles of incidence from 0° to 50°, and for both polarizations. Results: Genetic Algorithms allow to design several optimized structures with 6, 8, and 10 layers reaching a very high average absorbance of 97%, 99% and 99.5% respectively together with a low hemispherical total emissivity (<20%) from 200 °C till 400 °C. Conclusions: The proposed multilayer structures use materials with high thermal stability, and high melting temperature, can be fabricated with simple thin film deposition techniques, appearing to have very promising applications in solar thermal energy harvesting

Theoretical Investigation of the Circularly Polarized Luminescence of a Chiral Boron Dipyrromethene (BODIPY) Dye

Over the last decade, molecules capable of emitting circularly polarized light have attracted growing attention for potential technological and biological applications. The efficiency of such devices depend on multiple parameters, in particular the magnitude and wavelength of the peak of emitted light, and also on the dissymmetry factor for chiral applications. In light of these considerations, molecular systems with tunable optical properties, preferably in the visible spectral region, are particularly appealing. This is the case of boron dipyrromethene (BODIPY) dyes, which exhibit large molecular absorption coefficients, have high fluorescence yields, are very stable, both thermally and photochemically, and can be easily functionalized. The latter property has been extensively exploited in the literature to produce chromophores with a wide range of optical properties. Nevertheless, only a few chiral BODIPYs have been synthetized and investigated so far. Using a recently reported axially chiral BODIPY derivative where an axially chiral BINOL unit has been attached to the chromophore unit, we present a comprehensive computational protocol to predict and interpret the one-photon absorption and emission spectra, together with their chiroptical counterparts. From the physico-chemical properties of this molecule, it will be possible to understand the origin of the circularly polarized luminescence better, thus helping to fine-tune the properties of interest. The sensitivity of such processes require accurate results, which can be achieved through a proper account of the vibrational structure in optical spectra. Methodologies to compute vibrationally-resolved electronic spectra can now be applied on relatively large chromophores, such as BODIPYs, but require more extensive computational protocols. For this reason, particular attention is paid in the description of the different steps of the protocol, and the potential pitfalls. Finally, we show how, by means of appropriate tools and approaches, data from intermediate steps of the simulation of the final spectra can be used to obtain further insights into the properties of the molecular system under investigation and the origin of the visible bands

Chirality, symmetry-breaking, and chemical substitution in multiferroics

Multiferroic materials attract significant attention due to their potential utility in a broad range of device applications. The inclusion of heavy metal centers in these materials enhances their magnetoelectric properties, yielding fascinating physical phenomena such as the Dzyaloshinskii–Moriya interaction, nonreciprocal directional dichroism, enhancement of spin-phonon coupling, and spin-orbit-entangled ground states. This dissertation provides a comprehensive survey of magnetoelectric multiferroics containing heavy metal centers and explores spectroscopic techniques under extreme conditions. A microscopic examination of phase transitions, symmetry-breaking, and structure-property relationships enhances the fundamental understanding of coupling mechanisms. In A2Mo3O8 (A = Fe, Zn, Ni, and Mn), we use optical spectroscopy to analyze the electronic properties and compare our findings with first principles electronic structure calculations. We find that Fe2+ ions in the A site create a many-body effect from a valence band due to screening of the local moment – similar to a Zhang-Rice singlet. These findings advance the understanding of unusual hybridization with orbital occupation and the structure-property relationships in various metal-substituted systems. In a chiral, polar magnet, Ni3TeO6, we explore toroidal geometry to complete the set of configurations and develop structure-property relations by combining magneto-optical spectroscopy and first-principles calculations. The formation of Ni toroidal moments is responsible for the largest effects near 1.1 eV - a tendency that is captured by our microscopic model and computational implementation. Furthermore, we demonstrate deterministic control of nonreciprocal directional dichroism in Ni3TeO6 across the entire telecom wavelength range. This discovery will accelerate the development of photonics applications that take advantage of unusual symmetry characteristics. In Co4B2O9 (B = Nb, Ta), we combine variable temperature infrared spectroscopy, lattice dynamics calculations, and several different models of spin-phonon coupling to reveal the mechanism of magnetoelectricity. We reveal a spin-phonon coupling in Co2+ shearing mode near 150 cm−1 with coupling constants of 3.4 and 3.4 cm−1 for Co4Nb2O9 and the Ta analog, respectively. These coupling constants derive from interlayer exchange interactions, which contain competing antiferromagnetic and ferromagnetic contributions. Comparison to other contemporary oxides shows that spin-phonon coupling in this family of materials is among the strongest ever reported, suggesting an origin for magnetoelectric coupling

Spectroscopic properties of ferroic superlattices

The interplay between charge, structure, magnetism, and orbitals leads to rich physics and exotic cross-coupling in multifunctional materials. Superlattices provide a superb platform to study the complex interactions between different degrees of freedom. In this dissertation, I present a spectroscopic investigation of natural and engineered superlattices including FexTaS2 and (LuFeO3)m/(LuFe2O4)1 under external stimuli of temperature and magnetic field as well as chemical substitution. Studying the phase transitions, symmetry-breaking, and complex interface interactions from a microscopic viewpoint enhances fundamental understanding of coupling mechanism between different order parameters and the exciting properties. In FexTaS2, we use optical spectroscopies to analyze the electronic properties. Strikingly, Fe intercalation dramatically changes the metallic character, revealing two separate free carrier responses in the Fe monolayer and TaS2 slabs, respectively. Signatures of chirality are deeply embedded in the electronic structures. These include a transition of electron density pattern from triangular to Kagome to honeycomb, a hole to electron pocket crossover at the K-point, and low energy excitations between spin split bands that cross the Fermi surface. These findings advance the understanding of intercalation and symmetry-breaking on the fundamental excitations in metallic chalcogenides, while at the same time, raise important questions about how the embedded metal monolayer affects vibrational properties due to the free-carrier response screened the infrared-active phonons. To address these issues, we extended this work using Raman scattering spectroscopy to reveal the vibrational properties. We particular focus on the coherent excitations in the Fe monolayer. The results reveal both in- and out-of-plane vibrational excitations at low frequencies in the intercalated Fe monolayer. Extending the measurements to other intercalated chalcogenides such as Cr1/3NbS2 and RbFe(SO4)2 reveals structural-property relations, which confirms the intercalated monolayer excitations are general and intrinsic. Furthermore, the intercalated monolayer excitations have a trend that depend upon the metal-metal distance, the size of the van der Waals gap, and the metal-to-chalcogenide slab mass ratio. A model for how mass ratio affects the frequencies of the monolayer excitations is developed as well,which excellently fits to our experimental trend. These findings suggest that external stimuli such as pressure and strain may be able to tune these intercalated monolayer excitations. In the (LuFeO3)m/(LuFe2O4)1 multiferroic superlattices (m= 3, 7, and 9), we combined optical spectroscopy, magnetic circular dichroism, and first-principle calculations to uncover the origin of high temperature magnetism and charge-ordering states in a site-specific manner. Analysis of the dichroic spectra reveals optical hysteresis loops for different Fe sites. The site-specific coercivity vs. temperature curves are extracted from the optical hysteresis, which demonstrates that bulk magnetism derives principally from the LuFe2O4 layers. Magnetism emanating from the LuFe2O4 layer becomes more robust as the (3, 1) to (7, 1) to (9, 1) series progresses - a trend that correlates with increasing Lu-layer distortion. To understand this relationship more deeply, we extract the spectral signature of the interface for the (LuFeO3)m/(LuFe2O4)1 series (m = 3, 7 and 9). While the overall contribution of spin-down channel excitations is persistent over the sequence, enhanced Lu-layer distortion at the interface increases the contribution of the Fe2+ to Fe3+charge-transfer excitation in the spin-up channel. This amplifies LuFe2O4 layer magnetization and pinpoints the role of Fe2+. Key to this discovery is the ability of magneto-optical spectroscopy to provide direct, microscopic, site-specific information about interface magnetism in a two-dimensional material with multiple magnetic centers. Comparison of the theoretically predicted magnetic circular dichroism with the experimental spectrum also establishes the non-polar self-doped structure as the precise charge-ordering arrangement within the LuFe2O4 layer of the (3, 1) superlattice, thus resolving controversy regarding the many different isoenergetic charge states. In addition to introducing a remarkably powerful and versatile spectroscopic decomposition technique for revealing microscopic spin and charge character at the interface of a multiferroic superlattice with many different iron centers in a site-selective manner, this work provides a pathway to link bulk and interface properties in other engineered materials

Soft x-ray spectroscopy of organic and organometallic molecules and polymers

In this thesis, two aspects of research in soft X-ray absorption spectroscopy chemistry are explored. The first objective was to measure the natural circular dichroism of small chiral organic molecules at soft X-ray wavelengths. The second objective was to characterize the electronic structure and spectra of a series of organometallic polymers. The goal of the first part of this thesis was to enhance the sensitivity of Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy to the intrinsic “handedness” of chiral organic molecules. The phenomenon of X-ray natural circular dichroism (XNCD) has been well described by theoreticians; however, there have been few successful measurements reported, mainly due to the weakness of the effect and the difficulty of preparing suitable samples. The fourth chapter of this thesis outlines the requirements for XNCD experiments and the efforts made to prepare appropriate samples. The goal of the second part was to use NEXAFS spectroscopy as an analytical technique for the elemental and chemical characterization of innovative materials based on organoiron compounds. The interpretation of transition metal compounds by NEXAFS spectroscopy is difficult due to complex interactions between the metal and its surroundings. Two approaches are commonly used; an atomic multiplet model and a covalent bonding model, which lead to conflicting spectral assignments. Earlier NEXAFS studies of metallocene complexes were found to be lacking as these two models were not adequately rationalized. Owing in part to greatly improved instrumental sensitivity and to efficient theoretical calculations, the interpretation of NEXAFS spectra for a series of metallocene and metal arene complexes was refined. Enhanced understanding of the spectroscopy of these compounds eventually contributed to the characterization of a series of organometallic polymeric materials.Underlining these studies is the remarkable complementarity of NEXAFS spectroscopy and chemistry. A comprehensive understanding of the chemistry of the samples examined in the measurement of XNCD is shown to be crucial for a successful advancement of this spectroscopy. In return, optimization of soft X-ray spectroscopy of metallocenes is demonstrated to remarkably benefit the understanding of the organometallic polymers