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We report a non-invasive and multi-analytical physico-chemical method for the characterization of paper artworks, able to identify sizing, inks, and glues and to quantify oxidative degradation by-products. The proposed methodology is mostly carried out in situ by using easy-to-use and cheap portable instrumentation for the acquisition of multispectral images, punctual ultraviolet-visible-near infrared fiber-optics reflectance spectroscopy (FORS) and punctual fiber optics fluorescence spectroscopy. Further analytical information is provided by non-invasive in-situ sampling of paper surface by using soft latex sponges, making possible laboratory chromatographic and infrared spectroscopic analyses on the aqueous sponge extracts. The proposed diagnostics method was applied to two 17th century letters written by St Francis of Sales (1567-1622), collected at the Chigi Palace in the town of Ariccia (Italy). Results show an intense oxidative degradation of the letters, also localized in water spots, and the presence of carboxylic acids by-products. Analysis of FORS spectra provided the concentration of chromophores in the paper substrate. The diagnostic method allowed the identification of gelatin sizing, the presence of starch glue in specific areas of the letters and the type of ink used in the text. Our diagnostic approach aims to offer to conservator-restorer a characterization of a paper artwork, that can be applied to other, for a correct planning of conservation interventions.

During the centuries paper has been the most widely used writing support and therefore paper degradation is a major issue for cultural heritage conservation. The main component of paper is cellulose, one of the most abundant biomaterials on Earth. Cellulose oxidation is mainly responsible for the yellowing of the ancient samples, through the formation of optically active oxidized functional groups (chromophores). This results in severe chromatic deterioration of works of art on paper. In order to investigate the issue we applied several spectroscopic techniques (UV/Vis-IR-THz) interpreted by ab-initio theoretical computational simulations based on Density Functional theory (DFT) and Time-Dependent DFT (TDDFT) methods [1-4]. In this talk, we illustrate the experimental and theoretical methods and show their application to ancient work of art on paper. In particular, we show how to transform UV/Vis reflectance spectra of ancient samples into absorption spectra of cellulose fibers by using an improved version of the Kubelka-Munk theory suitable for strongly absorbing media. The procedure based on experimental and theoretical UV/Vis spectroscopy is particularly promising since it uses non-invasive and non-destructive measurements, which can be performed in-situ, allowing diagnostic analysis of delicate paper artifacts and the monitoring of their restoration interventions. We also present results on the state of degradation of the Leonardo Da Vinci's drawings including the famous self-portrait as well as other famous works of art on paper. [1] A Mosca Conte, O Pulci, MC Misiti, J ?ojewska, L Teodonio, C Violante and M Missori "Visual degradation in Leonardo da Vinci's iconic self-portrait: A nanoscale study" Appl. Phys. Lett.104, 224101-4 (2014). [2] M Peccianti, R Fastampa, AM Conte, O Pulci, C Violante, J ?ojewska, M Clerici, R Morandotti, M Missori "Terahertz absorption by cellulose: Application to ancient paper artifacts" Physical Review Applied 7 (6), 064019 (2017). [3] L Micheli, C Mazzuca, M Missori, L Teodonio, A Mosca Conte, O Pulci, L Arcadipane, S Dominijanni, A Palleschi, G Palleschi, S Iannuccelli, S Sotgiu, "Interdisciplinary approach to develop a disposable real time monitoring tool for the cleaning of graphic artworks. Application on "le Nozze di Psiche", Microchemical Journal 138, 369-378 (2018). [4] M Missori, D Pawcenis, J Bagniuk, A Mosca Conte, C Violante, MS Maggio, M Peccianti, O Pulci, J Lojewska, "Quantitative diagnostics of ancient paper using THz time-domain spectroscopy", Microchemical Journal 142, 54-61 (2018).

THz time-domain spectroscopy (THz-TDS) is a suitable approach to study the state of preservation of cultural heritage made of organic materials in a non-destructive way. This technique highlights, in particular, the low-energy vibrational properties of these materials and the presence of the H-bonds between molecules [1]. We focus in particular on the study of modern artificially aged and ancient paper samples in order to determine in the THz spectra the distinctive features as a possible fingerprint of their state of aging and/or degradation. As water plays a significant role in the THz range, the behavior of THz spectra as a function of the hydration of samples was also investigated allowing discrimination between the spectral features induced by the simple presence of water and those due by the degradation of cellulose polymers in the paper. However, the use of the THz-TDS technique to establish the state of preservation of paper sheets poses significant challenges, in particular when measurements on low refractive index and thin samples, such as ancient documents, must be carried out [2]. We have successfully solved this problem by employing an efficient computational procedure to remove from the experimental signals the spurious interference effects generated by the Fabry-Pérot resonances in single freely standing paper sheets. The THz absorption spectra were then explained in terms of absorption peaks of the cellulose crystalline phase superimposed to a background contribution due to a disordered H-bonds network with a clear dependence from the degradation of samples [3]. DFT simulations confirmed that the spectral profile is composed of several peaks associated with long-range cellulose crystal phononic modes. A comparison between theoretical and experimental results also showed the important role of water in cellulose polymers dynamics at THz frequencies. An experimental and theoretical DFT-based study of alpha-lactose was also performed. This compound possesses a chemical structure similar to those of cellulose but the lack of disordered regions allows a precise measurement of vibrational THz peaks. Furthermore, the knowledge of hydrated and non-hydrated crystal structures allowed us to study the effect of water molecules on their vibrational properties. Due to the very importance of water in determining the THz behavior of organic material, we also focus our attention on the measurement and analysis of the dielectric response of pure water and chloride solutions in the range 0.2-1.6 THz. We were able to satisfactory represent the permittivity of solutions as the weighted sum of two single-parameter Debye relaxations describing clusters of correlated water molecules coexisting with poorly or uncorrelated single dipoles. Finally, since cultural heritage materials are in general complex and disordered systems, we have investigated the propagation of THz waves in disordered media with a refractive index varying on the scale of the wavelength. Results showed evidence of a resonant transport supported by finite-size scatterers that strongly affects the velocity of propagation of light and breaks the diffusion approximation. References 1. P.U. Jepsen, D.G. Cooke, M. Koch, Terahertz spectroscopy and imaging - modern techniques and applications, Laser Photonics Rev., 5 (1), 124-166 (2011). 2. R. Fastampa, L. Pilozzi, and M. Missori, Cancellation of Fabry-Perot interference effects in terahertz time-domain spectroscopy of optically thin samples, Phys. Rev. A., 95, 063831 (2017). 3. M. Missori, et al., Microchem. J., 142, 54-61 (2018).

The correlation between nanoscale morphology and charge injection rates at the interface between an organic semiconductor layer and a transparent metal oxide electrode was investigated by integrating molecular dynamics simulations with electronic structure calculations. The simulation approach proposed has been applied to the analysis of the hole injection mechanism at the interface between an amorphous layer of tris[(3-phenyl-1H-benzimidazol-1-yl-2(3H)-ylidene)-1,2-phenylene]Ir (DPBIC), a hole transport and emitter molecule, and the surface of indium tin oxide (ITO), a material commonly used as anode in OLEDs. The link between interface morphology and charge injection was investigated by implementing a two-step, top-down simulation approach. Namely, nanoscale molecular aggregation phenomena at the organic/electrode interface were first assessed by molecular dynamics simulations, mimicking different processing conditions, and followed by density functional theory calculations of the electronic coupling between molecular levels and the manifold of electrode states involved in the charge injection process. The correlation between structural parameters and electronic coupling suggests a significant role of specific molecule/electrode configurations on charge transport processes at the interface, resulting in a broad distribution of charge injection rates, and highlights the link between molecular structure, nanoscale aggregation and processing in the realization of heterointerfaces for efficient charge injection in organic electronic devices.

The accurate calculation of electronic excited states of large and electronically correlated biological chromophores in their complex protein environment still represents a challenge for quantum chemistry. Two ab initio techniques are recently emerging as candidates to correctly tackle this issue: Quantum Monte Carlo (QMC) calculations for the ground state geometry optimization, and Many Body Green's Function Theory (MBGFT) for exited state energies. In the present work we use the Variational Monte Carlo (VMC) to carry out structural optimizations and we present an extension of MBGFT to complex environments, using a Quantum Mechanics/Molecular Mechanics framework. This technique is applied to evaluate the optical properties of the retinal protonated Schiff base (RPSB) chromophore in rhodopsin, the protein responsible for dim light vision in the vertebrates. Vertical energies for the bright excitation of RPSB calculated solving the Bethe-Salpeter equation in gas phase and in the protein environment correspond to 2.19 eV and 2.58 eV, respectively. These data are in fair agreement with the available experimental findings. The comparison between these excitation energies and that obtained for a gas phase calculation on a retinal geometry obtained in the protein environment (2.03 eV) reveals the essential role of the protein field in the spectral tuning of the molecule. The proposed VMC/MBGFT methodologies can therefore be applied to obtain a reliable ab initio evaluation of optical properties of biological chromophores. © 2014 Elsevier B.V. All rights reserved.

We study the electronic properties of two-dimensional (2D) group-III nitrides BN, AlN, GaN, InN, and TlN by first-principles approaches. With increasing group-III atomic number, a decrease of the electronic gap from 6.7 eV to 0 eV takes place. 2D GaN and 2D InN in honeycomb geometry present a direct gap at Γ, while the honeycomb structures of BN and AlN tend to be indirect semiconductors with the valence band maximum at K. Alloying of the nitrides allows tuning the gap with cation composition. Interestingly, Inx Ga1-xN and Inx Tl1-xN alloys enable, with varying x, to construct type I or type II heterostructures. We demonstrate that it is possible to tailor the electronic and optical response from UV to IR. We suggest that 2D InGaN and InTlN heterostructures may efficiently harvest light and serve as building blocks for a future generation of III-V solar cells. Finally, 2D InTlN with a low In content is eligible as the emitter and detector for THz applications.

Using ab initio density functional theory the band structure and the dielectric function of a bct Cd3As2 crystal are calculated. We find a Dirac semimetal with two Dirac nodes k± near the Γ point on the tetragonal axis. The bands near the Fermi level exhibit a linear behavior. The resulting Dirac cones are anisotropic and the electron-hole symmetry is destroyed along the tetragonal axis. Along this axis the symmetry-protected band linearity only exists in a small energy interval. The Dirac cones seemingly found by ARPES in a wider energy range are interpreted in terms of pseudo-linear bands. The behavior as 3D graphene-like material is traced back to As p orbital pointing to Cd vacancies, in directions which vary throughout the unit cell. Because of the Dirac nodes the dielectric functions (imaginary part) show a plateau for vanishing frequencies whose finite value is proportional to the Sommerfeld fine structure constant but varies with the light polarization. The consequences of the anisotropy of the Dirac cones are highlighted for the polarization dependence of the infrared optical conductivity.

Paper is an important material for many applications. During the centuries, it has been the most widely used writing support and therefore paper degradation is a major issue for cultural heritage. The main component of paper is cellulose, one of the most abundant biomaterials on Earth. Cellulose oxidation is mainly responsible for the yellowing of the ancient samples, through the formation of chromophores. In order to investigate this issue and the chromophores’ role, we exploit the optical properties combining non-destructive experiments and theoretical calculations based on ab-initio techniques. In this paper, we illustrate the method and show its application to three ancient paper samples. The procedure we describe is a precious tool for cultural heritage preservation: indeed, the approach we present is based on non-invasive and non-destructive measurements that allows a microscopic understanding of cellulose-based artifacts degradation.

The magnetism of graphone, a single-side-hydrogenated graphene derivative, has been related to the localized and unpaired p-electrons associated with the unhydrogenated carbon atoms. In the present density functional theory study, the effects the adhesion to either Cu(111) or α-quartz (0001) surface on the magnetic properties of graphone have been investigated. The total magnetization of the graphone adsorbed to copper and quartz surface is reduced by four and two times, respectively, with respect to the isolated graphone. We have shown there is electronic charge transfer from surface towards three-fold coordinated C atoms of graphone, but the main role in the partial magnetism quenching is played by bond formation and the consequent electron pairing of p-electrons. The critical temperature has been investigated on the basis of the mean field theory to evaluate the stability of the magnetism at ordinary temperature. © 2015 Elsevier B.V. All rights reserved.

Spectral properties of chromophores are used to probe complex biological processes in vitro and in vivo, yet how the environment tunes their optical properties is far from being fully understood. Here we present a method to calculate such properties on large scale systems, like biologically relevant molecules in aqueous solution. Our approach is based on many body perturbation theory combined with quantum-mechanics/molecular-mechanics (QM/MM) approach. We show here how to include quasi-particle and excitonic effects for the calculation of optical absorption spectra in a QM/MM scheme. We apply this scheme, together with the well established TDDFT approach, to indole in water solution. Our calculations show that the solvent induces a redshift in the main spectral peak of indole, in quantitative agreement with the experiments and point to the importance of performing averages over molecular dynamics configurations for calculating optical properties.

The Institute for Complex Systems, in collaboration with the University of Rome "Sapienza" and Tor Vergata and the University of Sussex (UK), has recently developed a THz spectroscopy laboratory aimed to academic and industrial research. The access to the wide THz electromagnetic band (which lies between optical waves and microwaves in the electromagnetic spectrum) has been limited for a long time by the lack of cost-effective and practical intense sources. The recent development of technologies for broadband THz generation is changing this scenario allowing a rapidly evolving field in research, industry, and security applications. Fingerprinting of spectroscopic lines in the THz region helps identifying chemical or biochemical molecules. Imaging with THz waves can be used for pharmaceutical, security, or identification of substances such as drugs, explosives or weapons. THz spectroscopy and imaging is used to quality control and optimization of industrial manufacturing processes. As example, one can look for tiny variations or defects in polymer and plastic materials, characterize the quality and morphology of paper products, textiles and wood products. Other examples span from measuring the thickness of coatings, non-destructive diagnostics for historical art conservation, and last but not the least, quality control of chocolate products with THz imaging.

Paper is an important material for many applications. During the centuries, it has been the most widely used writing support and therefore paper degradation is a major issue for cultural heritage. The main component of paper is cellulose, one of the most abundant biomaterials on Earth. Cellulose oxidation is mainly responsible for the yellowing of the ancient samples, through the formation of chromophores. In order to investigate this issue and the chromophores' role, we exploit the optical properties combining non-destructive experiments and theoretical calculations based on ab-initio techniques. In this paper, we illustrate the method and show its application to three ancient paper samples. The procedure we describe is a precious tool for cultural heritage preservation: indeed, the approach we present is based on non-invasive and non-destructive measurements that allows a microscopic understanding of cellulose-based artifacts degradation. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We investigate the optical properties of the tZt - penta -3,5- dieniminium cation, a simplified model for the protonated Schiff base of 11- cis retinal in rhodopsin, along the isomerization pathway by ab-initio calculations based on Many-Body Perturbation Theory using the GW method and the Bethe–Salpeter equation. Our calculations are carried out on a few significant CASSCF geometrical configurations of the isomerization minimal energy path taken from the literature. Our excitation energies are qualitatively in agreement with previous Quantum Monte Carlo and post-Hartree–Fock calculations. We also employ TDDFT based methods, and investigate the outcome of using different approximations and several exchange–correlation functionals.

Optical properties of aromatic chromophores are used to probe complex biological processes, yet how the environment tunes their optical properties is far from being fully understood. Here we present a method to calculate such properties on large-scale systems, like biologically relevant molecules in aqueous solution. Our approach is based on many-body perturbation theory combined with a quantum mechanics/molecular mechanics (QM/MM) approach. We include quasiparticle and excitonic effects for the calculation of optical absorption spectra in a QM/MM scheme. We apply this scheme, together with the well-established TDDFT approach, to indole in water solution. Our calculations show that the solvent induces a red shift in the main spectral peak of indole, in quantitative agreement with the experiments, and they point to the relevance of both the electrostatic and geometrical origin of the shift.

The electronic structure of isolated bis(phthalocyaninato) terbium(ill) molecules, a novel single-molecular-magnet (SMM), supported on the Cu(111) surface has been characterized by density functional theory and scanning tunneling spectroscopy. These studies reveal that the interaction with the metal surface preserves both the molecular structure and the large spin magnetic moment of the metal center. The 4f electron states are not perturbed by the adsorption while a strong molecular/metal interaction can induce the suppression of the minor spin contribution delocalized over the molecular ligands. The calculations show that the inherent spin magnetic moment of the molecule is only weakly affected by the interaction with the surface and suggest that the SMM character might be preserved.

The electronic, structural, and magnetic properties of Co-based low-dimensional nanostructures supported by Pt surfaces are investigated using computer simulations based on density functional theory. The effects of the local orientation of the magnetization, including the magnetic anisotropy energy, are accounted for within a noncollinear spin-density functional theory formalism where the spin-orbit interaction is described by fully relativistic ultrasoft pseudopotentials. The magnetic moments, the direction of the easy-magnetization axis, and the anisotropy energy are compared with available experimental and theoretical data, and are shown to be extremely sensitive to the local atomic environment of the Co adatoms. We argue that this sensitivity could be exploited to tailor the properties of magnetic devices by engineering the local environment and/or the operating conditions of nanomagnets.

We show how to include the spin-orbit coupling in electronic structure calculations of crystalline solids based on plane waves and ultrasoft pseudopotentials. The spin-orbit split electronic band structures of fcc-Au and of fcc-Pt are calculated by fully relativistic pseudopotentials derived from an atomic Dirac-like equation. The results are compared with accurate all-electron calculations available in the literature.

Using ab initio density functional theory the band structure and the dielectric function of a bct Cd3As2 crystal are calculated. We find a Dirac semimetal with two Dirac nodes k± near the Γ point on the tetragonal axis. The bands near the Fermi level exhibit a linear behavior. The resulting Dirac cones are anisotropic and the electron-hole symmetry is destroyed along the tetragonal axis. Along this axis the symmetry-protected band linearity only exists in a small energy interval. The Dirac cones seemingly found by ARPES in a wider energy range are interpreted in terms of pseudo-linear bands. The behavior as 3D graphene-like material is traced back to As p orbital pointing to Cd vacancies, in directions which vary throughout the unit cell. Because of the Dirac nodes the dielectric functions (imaginary part) show a plateau for vanishing frequencies whose finite value is proportional to the Sommerfeld fine structure constant but varies with the light polarization. The consequences of the anisotropy of the Dirac cones are highlighted for the polarization dependence of the infrared optical conductivity.