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Van der Waals (vdW) epitaxial growth of large-area and stable two-dimensional (2D) materials of high structural quality on crystalline substrates is crucial for the development of novel device technologies. 2D gallium monochalcogenides with low in-plane symmetry stand out among the layered semiconductor materials family for next-generation optoelectronic and energy conversion applications. Here, we demonstrate the formation of large-area, single crystal and optically active 2D monoclinic gallium telluride (m-GaTe) on silicon substrate via rapid thermal annealing induced phase transformation of vdW epitaxial metastable hexagonal gallium telluride (h-GaTe). Stabilization of multilayer h-GaTe on Si occurs due to the role of the first layer symmetry together with efficient GaTe surface passivation. Moreover, we show that the phase transformation of h-GaTe to m-GaTe is accompanied by the strain relaxation between Si substrate and GaTe. This work opens the way to the fabrication of single-crystal 2D anisotropic semiconductors on standard crystalline wafers that are difficult to be obtained by epitaxial methods.

The electronic properties of hydrogenated, spherical, Si/Ge and Ge/Si core-shell nanocrystals with a diameter ranging from 1.8 to 4.0 nm are studied within Density Functional Theory. Effects induced by quantum confinement and strain on the near-band-edge states localization, as well as the band-offset properties between Si and Ge regions, are investigated in detail. On the one hand, we prove that Si(core)/Ge(shell) nanocrystals always show a type II band-offset alignment, with the HOMO mainly localized on the Ge shell region and the LUMO mainly localized on the Si core region. On the other hand, our results point out that a type II offset cannot be observed in small (diameter less than 3 nm) Ge(core)/Si(shell) nanocrystals. In these systems, quantum confinement and strain drive the near-band-edge states to be mainly localized on Ge atoms inducing a type I alignment. In larger Ge(core)/Si(shell) nanocrystals, instead, the formation of a type II offset can be engineered by playing with both core and shell thickness. The conditions that favor the transition from a type I to a type II alignment for Ge(core)/Si(shell) nanocrystals are discussed in detail., 12 pages, 7 figures

The discovery of twisted bilayer graphene with tunable superconductivity has diverted great focus at the world of twisted van der Waals heterostructures. Here we propose a paradigm for bilayer materials, where covalent bonding replaces the van der Waals interaction between the layers. On the example of germanene-stanene bilayer, we show that such systems demonstrate fascinating topological properties and manifest giant capacitance effects of the order of C = 102μ F as well as dipole-like charge densities of q = 1 − 2 × 10−4μ C cm−2, showing promise for 2D ferroelectricity. The observed unique behaviour is closely linked to transverse strain-induced buckling deformations at the bilayer/substrate interface. In alternative GeSn bilayer structures with low twist angles the strain distortions trigger rich topological defect physics. We propose that the GeSn bilayer topology may be switched locally by a substrate-strain-induced electric fields. We demonstrate an approach to fabricate covalent bilayer materials, holding vast possibilities to transform applications technologies across solar, energy and optoelectronic sectors.

The study of intra and interlayer excitons in 2D semiconducting vdW heterostructures is a very hot topic not only from a fundamental but also an applicative point of view. Due to their strong light–matter interaction, Transition Metal Dichalcogenides (TMD) and group-III nitrides are particularly attractive in the field of opto-electronic applications such as photo-catalytic and photo-voltaic ultra-thin and flexible devices. Using first-principles ground and excited-state simulations, we investigate here the electronic and excitonic properties of a representative nitride/TMD heterobilayer, the AlN/WS2. We demonstrate that the band alignment is of type I, and low energy intralayer excitons are similar to those of a pristine WS2 monolayer. Further, we disentangle the role of strain and AlN dielectric screening on the electronic and optical gaps. These results, although they do not favor the possible use of AlN/WS2 in photo-catalysis, as envisaged in the previous literature, can boost the recently started experimental studies of 2D hexagonal aluminum nitride as a good low screening substrate for TMD-based electronic and opto-electronic devices. Importantly, our work shows how the inclusion of both spin-orbit and many-body interactions is compulsory for the correct prediction of the electronic and optical properties of TMD/nitride heterobilayers.

The study of intra and interlayer excitons in 2D semiconducting vdW heterostructures is a very hot topic not only from a fundamental but also an applicative point of view. Due to their strong light-matter interaction, Transition Metal Dichalcogenides (TMD) and group-III nitrides are particularly attractive in the field of opto-electronic applications such as photo-catalytic and photo-voltaic ultra-thin and flexible devices. Using first-principles ground and excited-state simulations, we investigate here the electronic and excitonic properties of a representative nitride/TMD heterobilayer, the AlN/WS2. We demonstrate that the band alignment is of type I, and low energy intralayer excitons are similar to those of a pristine WS2 monolayer. Further, we disentangle the role of strain and

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.

In this work, ab initio Density Functional Theory calculations are performed to investigate the evolution of the electronic and optical properties of 2D Tellurium—called Tellurene—for three different allotropic forms (α-, β- and γ-phase), as a function of the number of layers. We estimate the exciton binding energies and radii of the studied systems, using a 2D analytical model. Our results point out that these quantities are strongly dependent on the allotropic form, as well as on the number of layers. Remarkably, we show that the adopted method is suitable for reliably predicting, also in the case of Tellurene, the exciton binding energy, without the need of computationally demanding calculations, possibly suggesting interesting insights into the features of the system. Finally, we inspect the nature of the mechanisms ruling the interaction of neighbouring Tellurium atoms helical chains (characteristic of the bulk and α-phase crystal structures). We show that the interaction between helical chains is strong and cannot be explained by solely considering the van der Waals interaction.

International audience; In this work, we study the structural and electronic properties of boron nitride bilayers sandwiched between graphene sheets. Different stacking, twist angles, doping, as well as an applied external gate voltage, are reported to induce important changes in the electronic band structure near the Fermi level. Small electronic lateral gaps of the order of few meV can appear near the Dirac points K. We further discuss how the bandstructures change applying a perpendicular external electric field, showing how its application lifts the degeneracy of the Dirac cones and, in the twisted case, moves their crossing points away from the Fermi energy. Then, we consider the possibility of co-doping, in an asymmetric way, the two external graphene layers. This is a situation that could be realized in heterostructures deposited on a substrate. We show that the co-doping acts as an effective external electric field, breaking the Dirac cones degeneracy. Finally, our work demonstrates how, by playing with field strength and p-n co-doping, it is possible to tune the small lateral gaps, pointing towards a possible application of C/BN sandwich structures as nano-optical terahertz devices.

In this work, we investigate by ab initio calculations and optical experiments the sensitivity of graphene quantum dots in their use as devices to measure the presence, and concentration, of heavy metals in water. We demonstrate that the quenching or enhancement in the optical response (absorption, emission) depends on the metallic ion considered. In particular, two cases of opposite behaviour are considered in detail: Cd2+, where we observe an increase in the emission optical response for increasing concentration, and Pb2+ whose emission spectra, vice versa, are quenched along the concentration rise. The experimental trends reported comply nicely with the different hydration patterns suggested by the models that are also capable of reproducing the minor quenching/enhancing effects observed in other ions. We envisage that quantum dots of graphene may be routinely used as cheap detectors to measure the degree of poisoning ions in water.

In this work, we investigate by ab initio calculations and optical experiments the sensitiv- ity of graphene quantum dots in their use as devices to measure the presence, and concentration, of heavy metals in water. We demonstrate that the quenching or enhancement in the optical response (absorption, emission) depends on the metallic ion considered. In particular, two test cases of opposite behaviour are considered: Cd 2+ , where we observe an increase in the optical response for increasing concentration, and Pb 2 whose emission spectra are quenched along the concentration rise. We envisage that quantum dots of graphene may be routinely used as cheap detectors to measure the degree of poisoning ions in water

The thermodynamic stability of hydroxylated graphane, that is, fully sp3 graphene derivatives coordinated with -H and -OH groups, has been recently demonstrated by ab initio calculations. Within the density functional theory approach, we investigate the electronic property modifications of graphane by progressive hydroxylation, that is, by progressively substituting -H with -OH groups. When 50% of graphane is hydroxylated, the energy bandgap reaches its largest value of 6.68 eV. The electronic affinity of 0.8 eV for graphane can widely change in the 0.28-1.60 eV range depending on the geometric configuration. Hydroxylated graphane has two interfaces with vacuum, hence its electron affinity can be different on each interface with the formation of an intrinsic dipole perpendicular to the monolayer. We envisage the possibility of using hydroxylated graphane allotropes with tunable electronic affinity to serve as interfacial layers in 2D material-based heterojunctions.

The scattering theory for two crossing metallic single-walled carbon nanotubes of finite length exposed to an electromagnetic field has been developed based on a synthesis of the quantum transport formalism and classical electrodynamics. The model of the point contact has been developed to be incorporated into the Hall\'en and Pocklington equations for crossing carbon nanotubes. The influence of the contact conductance and position as well as the angle between the tube axes on the tube polarizability has been analyzed in the range of 1 GHz to 10 THz. The physical mechanisms responsible for the electromagnetic interaction between crossing tubes with zero and nonzero intertube contact conductance are shown and discussed. The influence of the coupling between the tubes on the localized plasmon resonance in them has been demonstrated.

The fascinating electronic and optoelectronic properties of freestanding graphene and the possible inclusion of novel two-dimensional (2D) systems in silicon-based electronics have driven the search for atomic layers consisting of other group-IV elements Si, Ge, Sn, and Pb, which form similar hexagonal lattices and are isoelectronic to graphene. The resulting 2D crystals silicene, germanene, stanene and plumbene, referred as Xenes, but also their functionalized counterparts, e.g. the hydrogenated sheet crystals, named as Xanes, silicane, germanane, and stanane, are in the focus of this review article. In addition, halogenated Xenes are investigated. The consequences of the larger atomic radii on the atomic geometry, the energetic stability, and possible epitaxial preparations are discussed. In the case of honeycomb atomic arrangements, the low-energy electronic excitations are ruled by almost linear bands. Spin–orbit coupling opens small gaps leading to Dirac fermions with finite effective masses. The linear bands give rise to an absorbance of the Xenes determined by the finestructure constant in the long-wavelength regime. While for vanishing photon energies the excitonic influence is still an open question, saddle-point excitons and excitons at M 0 van Hove singularities appear at higher frequencies. After opening substantial fundamental gaps by hydrogenation, the absorption edges of the Xanes, silicane, germanane, and stanane, are dominated by bound excitons with extremely large binding energies. Other chemical functionalizations, but also vertical electric fields, yield electronic structures ranging from topological to trivial insulators. Even a quantum spin Hall phase is predicted at room temperature. The topological character and the possible quantization of the spin Hall conductivity are studied versus gap inversion, chemical functionalization, and Rashba spin–orbit interaction. The drastic changes of the electronic properties of Xenes with chemical functionalization, interaction with the substrate, and external perturbations, open future opportunities for tailoring fundamental properties and, therefore, interesting applications in novel electronic and optoelectronic nanodevices.

Combining scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we demonstrate how to tune the doping of epitaxial graphene from p to n by exploiting the structural changes that occur spontaneously on the Ge surface upon thermal annealing. Furthermore, using first-principle calculations, we build a model that successfully reproduces the experimental observations. Since the ability to modify graphene electronic properties is of fundamental importance when it comes to applications, our results provide an important contribution toward the integration of graphene with conventional semiconductors.

An innovative method to quantitatively assess the state of degradation of paper in a non-destructive way has been developed by using THz time-domain spectroscopy (THz-TDS). The method relies on the observation that the hydrolytic and oxidative degradation of cellulose in paper is accompanied with rearrangement of the hydrogen bonds (H-bond) network of cellulose polymers. This gives clear fingerprints in the THz spectral profiles. THz-TDS is a suitable approach to study low-energy vibrational properties of biological materials in a non-destructive way. The THz photon energy range (about 1-40 meV) is particularly suitable to probe the H-bonds between molecules. However, the quantitative assessment of the state of preservation of artworks poses significant challenges, in particular when measurements on low refractive index and thin samples, such as ancient documents and drawings, must be carried out [1]. In the present study, we have successfully solved this problem obtaining a precise determination of the absorption by cellulose fibers in the 0.2-3.5 THz (7-117cm-1) range in single freely standing paper sheets. This has been achieved by developing a new procedure to remove from the experimental signals the spurious interference effects generated by the Fabry-Pérot resonances in the sheet. By using this approach, the THz absorption coefficient of cellulose fibers in ancient and modern samples artificially aged has been obtained. The THz absorption spectra are explained in terms of absorption peaks of the cellulose crystalline phase superimposed to a background contribution due to a disordered H-bonds network [2]. The complex evolution of the spectra as a function of natural and artificial aging is explained with a reduction up to 30% of the H-bond density in the cellulose polymer networks and a parallel increasing of the sample crystallinity evident in the peaks at about 2.1 and 3.1 THz. The comparison with density functional theory ab-initio calculations suggests that the observed THz spectral changes could be related not only to the recrystallization of amorphous regions, but also to the progressive degradation of the external surfaces of crystalline domains. These results demonstrate a significant potential in the non-destructive analysis of the structural modifications of cellulose and paper artifacts by aging, resulting, at a macroscopic scale, in a progressive weakening of the mechanical properties of paper [3]. [1] T. Trafela, M. Mizuno, K. Fukunaga, and M. Strlic, Appl. Phys. A, 111, 2013, 83-90. [2] E. Scarpellini, M. Ortolani, A. Nucara, L. Baldassarre, M. Missori, R. Fastampa, and R. Caminiti, J. Phys. Chem. C 120, 2016, 24088-24097. [3] L. Teodonio, M. Missori, D. Pawcenisc, J. ?ojewskac, F. Valle, Micron 91, 2016, 75-81.

Disposable non-invasive and compatible real time monitoring tool was developed in order to follow the cleaning process of paper artwork directly in situ. This tool was based on a biocompatible cleaning hydrogel coupled with flow electrochemical diagnostic tool, suitable to verify in situ and in a simple way the assessment of degradation of artwork and the efficiency of cleaning process. In this paper, the results obtained by applying this tool on a great format artwork with a lining as support, “Le Nozze di Psiche”, engraved by Diana Scultori, printed in 1613, are reported. This opera was affected by a structural and chromatic deterioration due to a strong oxidative degradation. Such deterioration was probably accelerated by the adhesive (a mixture of starch paste and animal glue) used in a previous lining intervention. In this case, the cleaning agents used are rigid hydrogels of Gellan gum, modified with hydrolytic enzymes. By using the flow sampling system, all materials removed by the gel was carried up to a thin layer cell containing a selective electrochemical biosensors, suitable to monitor both treatments, the cleaning process and the removal of lining. These were monitored, allowing understanding when both processes were completed, thus avoiding lengthy and unnecessary cleaning applications. The effectiveness of cleaning with Gellan gel was assessed quantitatively by using non-invasive optical reflectance spectroscopy by a portable instrumentation, elaborating data with an improved version of the Kubelka-Munk theory in order to recover the absorption coefficient of the cellulose fibers of “Le Nozze di Psiche”. The concentration of oxidized groups acting as chromophores was obtained by comparing the experimental optical absorption spectra to those simulated computationally by using TDDFT-based calculations. By following the cleaning with Gellan gel the results indicate a large decrease of the concentration of degradation product of cellulose. Moreover, chromatographic analysis were carried out in order to evaluate the amount of acid compound, produced during the aging and present on the graphic artwork, using the Gellan gel after cleaning step. The results obtained from the restoration of “Le Nozze di Psiche” have allowed the restorers to evaluate innovative methods for cleaning treatment of paper artworks with a highly specialized scientific-diagnostic approach

We perform a systematic {\it ab initio} study of the work function and its uniform strain dependence for graphene and silicene for both tensile and compressive strains. The Poisson ratios associated with armchair and zigzag strains are also computed. Based on these results, we obtain the deformation potential, crucial for straintronics, as a function of the applied strain. Further, we propose a particular experimental setup with a special strain configuration that generates only the electric field, while the pseudomagnetic field is absent. Then, applying a real magnetic field, one should be able to realize experimentally the spectacular phenomenon of the collapse of Landau levels in graphene or related two-dimensional materials., 9 pages, 7 figures; final version published in PRB

This chapter reviews the principles behind the theory and computation of interband transitions at surfaces. Particular emphasis is given to state-of-the-art ab-initio methodologies, and their application is illustrated through numerous practical examples in realistic surface systems. Following a simple introduction, we outline a general theoretical framework of surface spectroscopy, obtaining an expression for the surface response function that is appropriate for computation with ab-initio methods and for simulating various experimental spectroscopic techniques. We then revise the key ab-initio approaches, namely, density functional theory, Green's function-based many-body theory, and time-dependent density functional theory. The rest of the chapter presents examples of calculations at different levels of theory for various clean and modified surfaces of current interest, illustrating in each case the interplay between the surface geometry, electronic band structure, and interband excitations. Advanced concepts such as local field and excitonic effects at surfaces are highlighted. The chapter concludes with an outlook on challenges in the field and perspectives for future development.

Development of on-chip integrated carbon-based optoelectronic nanocircuits requires fast and non-invasive structural characterization of their building blocks. Recent advances in synthesis of single wall carbon nanotubes and graphene nanoribbons allow for their use as atomically precise building blocks. However, while cataloged experimental data are available for the structural characterization of carbon nanotubes, such an atlas is absent for graphene nanoribbons. Here we theoretically investigate the optical absorption resonances of armchair carbon nanotubes and zigzag graphene nanoribbons continuously spanning the tube (ribbon) transverse sizes from 0.5(0.4) nm to 8.1(12.8) nm. We show that the linear mapping is guaranteed between the tube and ribbon bulk resonance when the number of atoms in the tube unit cell is \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2N+4$$\end{document}2N+4, where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$N$$\end{document}N is the number of atoms in the ribbon unit cell. Thus, an atlas of carbon nanotubes optical transitions can be mapped to an atlas of zigzag graphene nanoribbons., The authors combine ab-initio density functional theory with tight-binding calculations to investigate the optical absorption resonances of armchair carbon nanotubes and zigzag graphene nanoribbons, and show that an atlas of carbon nanotubes optical transitions can be mapped to an atlas of optical resonances of zigzag graphene nanoribbons.

Thanks to the ultrahigh flexibility of 2D materials and to their extreme sensitivity to applied strain, there is currently a strong interest in studying and understanding how their electronic properties can be modulated by applying a uniform or nonuniform strain. In this work, using density functional theory (DFT) calculations, we discuss how uniform biaxial strain affects the electronic properties, such as ionization potential, electron affinity, electronic gap, and work function, of different classes of 2D materials from X-enes to nitrides and transition metal dichalcogenides. The analysis of the states in terms of atomic orbitals allows to explain the observed trends and to highlight similarities and differences among the various materials. Moreover, the role of many-body effects on the predicted electronic properties is discussed in one of the studied systems. We show that the trends with strain, calculated at the GW level of approximation, are qualitatively similar to the DFT ones solely when there is no change in the character of the valence and conduction states near the gap.

International audience; Electronic properties and lattice dynamics of bulk ɛ-GaSe and one, two, and three tetralayers GaSe are investigated by means of density functional and density functional perturbation theory. The few-tetralayer systems are semiconductors with an indirect nature of the fundamental band gap and a Mexican-hat shape is observed at the top of the valence band. The phonon branch analysis reveals the dynamical stability for all systems considered together with the LO-TO splitting breakdown in two-dimensional systems. In-plane (E) and out-of-plane (A) zone-center lattice vibrations dominate the Raman and IR spectra.

Abstract By means of ab initio band structure methods and model Hamiltonians we investigate the electronic, spin and topological properties of four monopnictides crystallizing in bct structure. We show that the Weyl bands around a WP W1 or W2 possess a strong anisotropy and tilt of the accompanying Dirac cones. These effects are larger for W2 nodes than for W1 ones. The node tilts and positions in energy space significantly influence the DOS of single-particle Weyl excitations. The node anisotropies destroy the conventional picture of (anti)parallel spin and wave vector of a Weyl fermion. This also holds for the Berry curvature around a node, while the monopole charges are independent as integrated quantities. The pairing of the nodes strongly modifies the spin texture and the Berry curvature for wave vectors in between the two nodes. Spin components may change their orientation. Integrals over planes perpendicular to the connection line yield finite Zak phases and winding numbers for planes between the two nodes, thereby indicating the topological character. Graphical abstract

We present a first principles study of the stability, and of the electronic and optical properties of graphene with nitrogen doped vacancies. Moreover, we use the vacancies as anchoring sites for Mg, Zn, Pd al Pt atoms and vary the concentration of defects. Decoration of the defects with metal atoms produces semi-metallic systems for any studied size of the cell, with linear bands crossing at the Fermi level. The peculiar electronic properties of massless Dirac fermions in graphene are hence kept, although with anisotropic Fermi velocities. New sharp peaks appear in the optical conductivity in the visible range, thus strongly enhancing the optical response of graphene.

The discoloration of paper, due to the development of oxidized groups acting as chromophores in its chief component, cellulose, is responsible for severe visual degradation of works of art on paper. By adopting a diagnostic method based on in situ non-invasive optical reflectance spectroscopy and time-dependent density functional theory ab initio calculations, it is possible to describe and quantify the chromophores in cellulose fibers in a non-destructive way. In order to recover the absorption coefficient of cellulose fibers from reflectance measurements a specific approach based on the Kubelka-Munk theory was applied. The concentrations of carbonyl groups acting as chromophores were obtained by fitting the experimental optical absorption spectra to those simulated by using ab initio calculations. This method was applied for monitoring the restoration interventions of two great format engravings Le Nozze di Psiche and Gesu Cristo e l’adultera by Diana Scultori (1547-1612), as well as a contemporary artwork by Renato Guttuso, Bozzetto per Crocifissione (dated 1940). All artefacts were affected by chromatic deterioration due to a strong oxidation of the paper. Results quantified the decreasing of chromophores concentration after washing and reducing treatments evidencing the different behavior of the carbonyl groups as a function of the specific protocol performed.

We studied monoatomic linear carbon chains stabilised by gold nanoparticles attached to their ends and deposited on a solid substrate. We observe spectral features of straight chains containing from 8 to 24 atoms. Low temperature PL spectra reveal characteristic triplet fine-structures that repeat themselves for carbon chains of different lengths. The triplet is invariably composed of a sharp intense peak accompanied by two broader satellites situated 15 and 40 meV below the main peak. We interpret these resonances as an edge-state neutral exciton, positively and negatively charged trions, respectively. The time-resolved PL shows that the radiative lifetime of the observed quasiparticles is about 1 ns, and it increases with the increase of the length of the chain. At high temperatures a non-radiative exciton decay channel appears due to the thermal hopping of carriers between parallel carbon chains. Excitons in carbon chains possess large oscillator strengths and extremely low inhomogeneous broadenings., Comment: 23 pages, 5 figures

Highly oriented pyrolitic graphite (HOPG) is an inert substrate with a structural honeycomb lattice, well suited for the growth of two-dimensional (2D) silicene layer. It was reported that when Si atoms are deposited on HOPG surface at room temperature, they arrange in two configurations: silicene nanosheets and three dimensional clusters. In this work we demonstrate, by using scanning tunneling microscopy (STM) and Raman spectroscopy, that a third configuration stabilizes in the form of Si 2D nanosheets intercalated below the first top layer of carbon atoms. The Raman spectra reveal a structure located at 538 cm$^{-1}$ which we ascribe to the presence of sp$^2$ Si hybridization. Moreover, the silicon deposition induces several modifications in the graphite D and G Raman modes, which we interpret as an experimental evidence of the intercalation of the silicene nanosheets. The Si atom intercalation at room temperature takes place at the HOPG step edges and it detaches only the outermost graphene layer inducing a strong tensile strain mainly concentrated on the edges of the silicene nanosheets. Theoretical calculations of the structure and energetic viability of the silicene nanosheets, of the strain distribution on the outermost graphene layer and its influence on the Raman resonances support the STM and Raman observations., 18 pages, 9 figures, 2 tables

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).

We predict the stability of a graphenelike silicene sheet on one monolayer of aluminum oxide. We find that the honeycomb buckled structure of silicene is not broken upon interaction with one monolayer of ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ in the kagome geometry. As a consequence, the electronic band structure shows unperturbed cones with massless Dirac fermions embedded into ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$-derived bands. The heterostructure conserves the topological character of the freestanding silicene. The optical properties of the silicene/${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ bilayers clearly maintain the characteristic infrared universal limit of $\ensuremath{\pi}\ensuremath{\alpha}$. A quantized absorbance $N\ifmmode\cdot\else\textperiodcentered\fi{}\ensuremath{\pi}\ensuremath{\alpha}$ (with $N$ silicene layers) also occurs for stacking of multilayers.

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).

Using density-functional and many-body perturbation theory, we study the electronic, optical, and excitonic properties of indium nitride as single monolayer and bilayer in comparison with the bulk phase. We investigate the stable geometry for the monolayer, the graphenelike unbuckled honeycomb structure, and for the bilayer the AA' stacking geometry. We demonstrate that the quasiparticle and optical gaps, going from bulk to two-dimensional systems, open dramatically due to strong quantum confinement and reduced screening. Large excitonic effects, which survive at room temperature, are predicted. Our results suggest that low-dimensional InN is a promising material for optoelectronic devices in the visible to near-infrared spectral ranges varying with the number of atomic layers and light propagation.

We propose a characterization tool for studies of the band structure of new materials promising for the observation of topological phase transitions. We show that a specific resonant feature in the entropy per electron dependence on the chemical potential may be considered as a fingerprint of the transition between topological and trivial insulator phases. The entropy per electron in a honeycomb two-dimensional crystal of germanene subjected to the external electric field is obtained from the first principle calculation of the density of electronic states and the Maxwell relation. We demonstrate that, in agreement to the recent prediction of the analytical model, strong spikes in the entropy per particle dependence on the chemical potential appear at low temperatures. They are observed at the values of the applied bias both below and above the critical value that corresponds to the transition between the topological insulator and trivial insulator phases, while the giant resonant feature in the vicinity of zero chemical potential is strongly suppressed at the topological transition point, in the low temperature limit. In a wide energy range, the van Hove singularities in the electronic density of states manifest themselves as zeros in the entropy per particle dependence on the chemical potential., Comment: 8 pages, 5 figures; final version published in PRB

We report on Raman scattering of GaSe for thicknesses varying from bulk down to bilayer. The Raman spectra using 532 nm excitation show a strong dependence on layer thickness. The two out-of-plane modes of GaSe shift in opposite directions as the thickness increases. However, from pentalayer the Raman spectra of GaSe do not show any further change thus reaching bulk nature in terms of optical properties. We also perform ab initio density functional theory calculations for the geometry, the electronic band structure, and the Raman frequencies for bulk down to one monolayer. Good agreement with the experimental results is found, and we reproduce the trend, driven by the number of layers, in the Raman shifts. Our results present the first systematic approach to a layer dependent Raman study of GaSe and answer contradictions reported in the literature.

Using a combination of the density functional theory and tight-binding calculations, we study electronic and optical properties of asymmetric chevron-type graphene nanoribbons recently synthesized with atomic precision. We demonstrate that the low-energy optical selection rules in such infinite chiral ribbons are more reminiscent of those for zigzag rather than for armchair ribbons. It is also shown that, starting from about 25 nm long ribbons, the low-energy absorption and therefore the selection rules of infinitely long ribbons are well reproduced in the finite cluster approach. Hence, ribbons longer than 25 nm (about 28 unit cells) can be treated as infinitely long ones.

We review the behavior of the entropy per particle in various two-dimensional electronic systems. The entropy per particle is an important characteristic of any many body system that tells how the entropy of the ensemble of electrons changes if one adds one more electron. Recently, it has been demonstrated how the entropy per particle of a two-dimensional electron gas can be extracted from the recharging current dynamics in a planar capacitor geometry. These experiments pave the way to the systematic studies of entropy in various crystal systems including novel two-dimensional crystals such as gapped graphene, germanene and silicene. Theoretically, the entropy per particle is linked to the temperature derivative of the chemical potential of the electron gas by the Maxwell relation. Using this relation, we calculate the entropy per particle in the vicinity of topological transitions in various two-dimensional electronic systems. We show that the entropy experiences quantized steps at the points of Lifshitz transitions in a two-dimensional electronic gas with a parabolic energy spectrum. In contrast, in doubled-gapped Dirac materials, the entropy per particles demonstrates characteristic spikes once the chemical potential passes through the band edges. The transition from a topological to trivial insulator phase in germanene is manifested by the disappearance of a strong zero-energy resonance in the entropy per particle dependence on the chemical potential. We conclude that studies of the entropy per particle shed light on multiple otherwise hidden peculiarities of the electronic band structure of novel two-dimensional crystals., Comment: 33 pages,15 figures. Accepted for publication in JETP. arXiv admin note: text overlap with arXiv:1803.02083, arXiv:1712.09118, arXiv:1703.08962, arXiv:1512.08450

Slightly buckled, graphene-like honeycomb crystals made by silicon, silicene, or by other group-IV elements such as germanene and stanene represent atomically thin films, i.e., two-dimensional (2D) systems. The theoretical description of their optical properties suffers from three difficulties, (i) a thickness much smaller than the wavelength of light, (ii) their common modeling by superlattice arrangements with sufficiently large layer distances, and (iii) the inclusion of many-body effects. Here, the solutions of all problems are discussed. (i) The optical response of an individual honeycomb crystal is described by a tensor of 2D optical conductivities or dielectric functions, which are related to the optical response of the corresponding superlattice. (ii) The influence of such a sheet crystal on the transmittance, reflectance and absorbance of a layer system is described. (iii) Excitonic and quasiparticle effects are demonstrated to widely cancel each other. Silicene sheets are investigated in detail. As a consequence of the linear bands and Dirac cones the low-frequency absorbance is defined by the Sommerfeld finestructure constant. Van Hove singularities represented by critical points in the interband structure are identified at higher photon energies. Clear chemical trends along the row C \(\rightarrow \) Si \(\rightarrow \) Ge \(\rightarrow \) Sn are derived. The influence of multiple layers is studied for the cases of bilayer silicene and graphene.

The exotic electrodynamics properties of graphene come from the linearly dispersive electronic bands that host massless Dirac electrons. A similar behavior was predicted to manifest in freestanding silicene, the silicon counterpart of graphene, thereby envisaging a new route for silicon photonics. However, the access to silicene exploitation in photonics was hindered so far by the use of optically inappropriate substrates in experimentally realized silicene. Here we report on the optical conductivity of silicon nanosheets epitaxially grown on optically transparent Al2O3(0001) from a thickness of a few tens of nanometers down to the extreme two-dimensional (2D) limit. When a 2D regime is approached, a Dirac-like electrodynamics can be deduced from the observation of a low-energy optical conductivity feature owing to a silicene-based interfacing to the substrate. © 2018 American Chemical Society.

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.

In this paper, we present results concerning ab initio calculations of the optical properties of group IV two-dimensional sheets and compare them with the prediction of an analytical model for excitons in two dimensions, which we explicitly derive. This comparison helps to understand the physics at the basis of the strong many-body effects present in the optical spectra of this class of materials. Although it demonstrates the need of full ab initio calculations for an accurate description of the two-dimensional excitons, it validates, at the same time, this simple approach to estimate, through a density functional theory calculation of the two-dimensional electronic and optical properties, the binding energy and the radius of the excitons in this class of materials.

Colloidal nanocrystals electronic energy levels are determined by strong size-dependent quantum confinement. Understanding the configuration of the energy levels of nanocrystal superlattices is vital in order to use them in heterostructures with other materials. A powerful method is reported to determine the energy levels of PbS nanocrystal assemblies by combining the utilization of electric-double-layer-gated transistors and advanced ab-initio theory.

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 relative energy stability between Si(111)2 × 1 positive and negative buckling is still a puzzle. Both isomers possess comparable energies in Density Functional Theory (DFT) calculations and, hence, should occur with the same probability. This is in contrast to experimental findings, which show that the positive buckling is the observed structure at low and room temperature. Here we investigate the role of the exchange-correlation (xc) approximation in DFT functionals. We find that the particular choice of the xc functional does not change the relative stability between the Si(111)2 × 1 isomers. Moreover, we investigate the effect of donor atoms on ground-state and optical properties of both isomers, highlighting the differences with respect to the undoped case for several possible positions of the phosphorus (P) donor atoms. Considering the most stable position of P, relative stability between the isomers is not strongly affected by doping whereas geometric parameters are different with respect to the no doping case. The optical response bears some resemblance with the undoped case but the intensity of reflectance anisotropy peak is decreased for both isomers. © 2013 Elsevier B.V.

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.

The prediction of silicene [1], a 2D honeycomb crystal of silicon analogous to graphene, lead to intensive theoretical study [2]. Experimental realization has, on the one hand, been successful: although synthesis of free-standing silicene has remained elusive, various reports of silicene growth on Ag(110), Ag(111), ZrB2, and Ir(111) have appeared, culminating in the recent demonstration of a field-effect silicene-based transistor [3]. On the other hand, many of these reports have since been questioned as the strongly hybridized nature of the Si-substrate bonding apparently destroys the key electronic property (Dirac cone) [4]. In this presentation we report first principles calculations of the structure and optical properties of silicene-like nanoribbons reported to form on Ag(110) and as 2D overlayers on Ag(111). In the case of Si/Ag(110), several structural models have been proposed that suggest a Si-stabilized missing row reconstruction based on zigzag [5] or pentamer [6] chains. Refined experimental studies using grazing incidence x-ray diffraction (GIXD), scanning tunneling microscopy (STM), and surface optical spectra (reflectance anisotropy spectroscopy RAS and surface differential reflectance spectroscopy SDRS), are interpreted using density-functional theory (DFT) and confirm the correct structure as being a pentamer chain reconstruction [7], definitively ruling out silicene nanoribbon structures [8]. The case of Si/Ag(111) is complicated by the abundance of single and multilayer phases [9], and we attempt to cast light on the atomic geometry by interpreting recent experimental SDRS measurements [9,10] using ab initio DFT simulations of the full Si/Ag overlayer phase. The importance of including many-body effects in the optical spectra calculations is examined [11]. [1] G.G Guzman-Verri and L.C. Lew Van Yoon, Phys. Rev. B. 76, 075131 (2007) [2] See e.g. C. Grazianetti, E. Cinquanta and A. Molle, 2D Mater. 3, 012001 (2016) [3] L. Tao et al, Nature Nanotechnology 10 227 (2015) [4] R. Quhe et al, Sci. Rep. 4, 5476 (2014) [5] C. Hogan et al Phys. Rev. B 92, 115439 (2015). [6] J. I. Cerdá, et al, Nat. Commun. 7, 13076 (2016). [7] G. Prévot, C. Hogan, et al, Phys. Rev. Lett. 117, 276102 (2016); [8] P. De Padova, et al, APL 96, 261905 (2010); B. Aufray et al, APL 96, 183102 (2010); C. Lian and J. Ni, Physica B: Condensed Matter 407, 4695 (2012). [9] Y. Borensztein et al, Phys. Rev. B. 92, 155407 (2015) [10] Y. Borensztein, G. Prevot, L. Masson, Physical Review B 89, 245410 (2014); [11] C. Hogan, O. Pulci, D. Martin, A. Curcella, Y. Borensztein, in preparation.

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 the most widely used writing support due to the remarkable properties of its principal component - cellulose - one of the most abundant biomaterials present on Earth. However, due to the complexity of the material, an exhaustive picture of its degradation pathways is still missing. In this paper, we will present recent results and progresses obtained in the comprehension of the role of cellulose oxidation in the yellowing of ancient paper. Visible and ultraviolet spectra of cellulose in ancient paper samples and reference modern samples artificially aged have been interpreted with the aid of ab-initio Time-Dependent Density Functional Theory calculations. Through the comparison of measured and calculated absorption spectra, several oxidized forms of cellulose polymers, acting as chromophores, and responsible for ancient paper yellowing were identified. The relative concentration of ketones and aldehydic groups depends on the environmental conditions in which samples were stored along their life.