Designing new materials for photocathodes

Free electron lasers (FELs) are state of the art in terms of generating light pulses. By using free electrons as lasing medium, FELs provide tunable radiation in the infrared to X-rays wavelength range and on the femtosecond timescale. Their use in biology, chemistry and materials science allows for the probing of dynamic processes at levels never reached before. In order to design the new generation of FELs, a bright and low emittance electron beam must be generated from the photocathode. For this reason widespread experimental and computational efforts are taking place worldwide in order to generate more efficient materials for photocathode applications. However, a clear understanding of the emission process and, in particular, how the process is influenced by the surface structure needs further investigation. In this work, a computational tool based on the well know three step model (3SM) for photoemission is presented. In its original formulation, the 3SM is based on the bulk electronic structure of materials. In this study it is extended to explicit models of the surfaces. This approach retains the simple chemical intuition and it allows the disentanglement of the effect of the atomic, electronic and chemical structure of the surface on the observed photoemission. This is achieved by using a layer-by-layer decomposition of the surface electronic structure that is calculated through reliable density functional theory (DFT) calculations. Test calculations on clean copper, silver and magnesium surfaces are reported in this thesis and compared to the measured quantum efficiency. The ability of the model to simulate the influence of surface modifications on the observed quantum efficiency are also reported: the adsorption of oxygen, hydrogen and cesium on the Mg (0001) surface and the presence of steps on the Ag (111) surface are discussed. Hydrogen and oxygen were selected because they are well known contaminants of photocathode surfaces and cesium was simulated because it is commonly used to enhance the quantum efficiency of photoemitting materials. The calculations allow the known effects of surface states and work function changes to be rationalised. Furthermore, the simulations of the adsorbate covered surfaces generated some counter intuitive results that can be explained by the model developed in this work. The computational tool presented in this thesis has already provided some insights on how surface chemistry and reconstruction influence the quantum efficiency of materials currently used in photocathodes and will be used in future studies to generate guidelines for the design of more efficient photocathodes.Open Acces

Disordered Rock-Salt Type Li2TiS3 as Novel Cathode for LIBs: A Computational Point of View

The development of high-energy cathode materials for lithium-ion batteries with low content of critical raw materials, such as cobalt and nickel, plays a key role in the progress of lithium-ion batteries technology. In recent works, a novel and promising family of lithium-rich sulfides has received attention. Among the possible structures and arrangement, cubic disordered Li(2)TiS(3) has shown interesting properties, also for the formulation of new cell for all-solid-state batteries. In this work, a computational approach based on DFT hybrid Hamiltonian, localized basis functions and the use of the periodic CRYSTAL code, has been set up. The main goal of the present study is to determine accurate structural, electronic, and spectroscopic properties for this class of materials. Li(2)TiS(3) precursors as Li(2)S, TiS(2), and TiS(3) alongside other formulations and structures such as LiTiS(2) and monoclinic Li(2)TiS(3) have been selected as benchmark systems and used to build up a consistent and robust predictive scheme. Raman spectra, XRD patterns, electronic band structures, and density of states have been simulated and compared to available literature data. Disordered rock-salt type Li(2)TiS(3) structures have been derived via a solid solution method as implemented into the CRYSTAL code. Representative structures were extensively characterized through the calculations of their electronic and vibrational properties. Furthermore, the correlation between structure and Raman fingerprint was established

Graded Parafermions

A graded generalization of the Z_k parafermionic current osp(1|2)/U(1) coset conformal field theory. The structure of the parafermionic highest-weight modules is analyzed and the dimensions of the fields of the theory are determined. A free field realization of the graded parafermionic system is obtained and the structure constants of the current algebra are found. Although the theory is not unitary, it presents good reducibility properties.Comment: 27 pages, LaTeX, 1 eps file. Typos correcte

CRYSTALpytools: A Python infrastructure for the Crystal code

CRYSTALpytools is an open source Python project available on GitHub that implements a user-friendly interface to the Crystal code for quantum-mechanical condensed matter simulations. CRYSTALpytools provides functionalities to: i) write and read Crystal input and output files for a range of calculations (single-point, electronic structure, geometry optimization, harmonic and quasi-harmonic lattice dynamics, elastic tensor evaluation, topological analysis of the electron density, electron transport, and others); ii) extract relevant information; iii) create workflows; iv) post-process computed quantities, and v) plot results in a variety of styles for rapid and precise visual analysis. Furthermore, CRYSTALpytools allows the user to translate Crystal objects (the central data structure of the project) to and from the Structure and Atoms objects of the pymatgen and ASE libraries, respectively. These tools can be used to create, manipulate and visualise complicated structures and write them efficiently to Crystal input files. Jupyter Notebooks have also been developed for the less Python savvy users to guide them in the use of CRYSTALpytools through a user-friendly graphical interface with predefined workflows to complete different specific tasks

New insights into cancer: MDM2 binds to the citrullinating enzyme PADI4

PADI4 is one of the human isoforms of a family of enzymes implicated in the conversion of arginine to citrulline. MDM2 is an E3 ubiquitin ligase which is crucial for down-regulation of degradation of the tumor suppressor gene p53. Given the relationship between both PADI4 and MDM2 with p53-signaling pathways, we hypothesized they may interact directly, and this interaction could be relevant in the context of cancer. Here, we showed their association in the nucleus and cytosol in several cancer cell lines. Furthermore, binding was hampered in the presence of GSK484, an enzymatic PADI4 inhibitor, suggesting that MDM2 could bind to the active site of PADI4, as confirmed by in silico experiments. In vitro and in silico studies showed that the isolated N-terminal region of MDM2, N-MDM2, interacted with PADI4, and residues Thr26, Val28, Phe91 and Lys98 were more affected by the presence of the enzyme. Moreover, the dissociation constant between N-MDM2 and PADI4 was comparable to the IC50 of GSK484 from in cellulo experiments. The interaction between MDM2 and PADI4 might imply MDM2 citrullination, with potential therapeutic relevance for improving cancer treatment, due to the generation of new antigens

The intrinsically disordered, epigenetic factor RYBP binds to the citrullinating enzyme PADI4 in cancer cells

14 p.-6 fig.-1 tab.RYBP (Ring1 and YY 1 binding protein) is a multifunctional, intrinsically disordered protein (IDP), best described as a transcriptional regulator. It exhibits a ubiquitin-binding functionality, binds to other transcription factors, and has a key role during embryonic development. RYBP, which folds upon binding to DNA, has a Zn-finger domain at its N-terminal region. By contrast, PADI4 is a well-folded protein and it is one the human isoforms of a family of enzymes implicated in the conversion of arginine to citrulline. As both proteins intervene in signaling pathways related to cancer development and are found in the same localizations within the cell, we hypothesized they may interact. We observed their association in the nucleus and cytosol in several cancer cell lines, by using immunofluorescence (IF) and proximity ligation assays (PLAs). Binding also occurred in vitro, as measured by isothermal titration calorimetry (ITC) and fluorescence, with a low micromolar affinity (~1 μM). AlphaFold2-multimer (AF2) results indicate that PADI4's catalytic domain interacts with the Arg53 of RYBP docking into its active site. As RYBP sensitizes cells to PARP (Poly (ADP-ribose) polymerase) inhibitors, we applied them in combination with an enzymatic inhibitor of PADI4 observing a change in cell proliferation, and the hampering of the interaction of both proteins. This study unveils for the first time the possible citrullination of an IDP, and suggests that this new interaction, whether it involves or not citrullination of RYBP, might have implications in cancer development and progression.This research was funded by Ministry of Science and Innovation MCIN/AEI/10.13039/501100011033/ and “ERDF A way of Making Europe” [PID2021-127296OB-I00 to AVC; and PDC2022-133952-I00 to EF]; by Instituto de Salud Carlos III co-funded by European Social Fund “Investing in your future” [CP19/00095 to CdJ] [PI22/00824 to MS and CdJ] [PI18/00394 to OA]; by Diputación General de Aragón [“Protein targets and Bioactive Compounds group” E45-20R to AVC, and “Digestive Pathology Group” B25-20R to OA], and by Consellería de Innovación, Universidades, Ciencia y Sociedad Digital (Generalitat Valenciana) [CAICO 2021/0135 to CdJ and JLN].Peer reviewe

Effect of Post-Synthesis Treatments on the Properties of ZnS Nanoparticles: An Experimental and Computational Study

This work deals with the characterization of ZnS NanoParticles (NP), prepared by precipitation employing thioacetamide as sulfur source at different reaction time length. The attention is focused on the modification induced on structural, surface and electronic properties of ZnS NP by post-synthesis treatments. These were aimed at removing from the samples surface adsorbed reactants, by washing or thermal treatments, both in air or vacuum. The effect of these parameters is followed by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Fourier Transform InfraRed (FTIR), gas-volumetric and ThermoGravimetric Analysis (TGA). Moreover, the effect of nanostructuration on the semiconducting material band gap is evaluated by Diffuse Reflectance UV-Vis (DR UV-Vis) spectroscopy. Density Functional Theory (DFT) calculations have been employed to clarify the role of the adsorbed reactants on the surface stability and to assess the relationship between particle size and band gap value

Effect of Post-Synthesis Treatments on the Properties of ZnS Nanoparticles: An Experimental and Computational Study

This work deals with the characterization of ZnS NanoParticles (NP), prepared by precipitation employing thioacetamide as sulfur source at different reaction time length. The attention is focused on the modification induced on structural, surface and electronic properties of ZnS NP by post-synthesis treatments. These were aimed at removing from the samples surface adsorbed reactants, by washing or thermal treatments, both in air or vacuum. The effect of these parameters is followed by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Fourier Transform InfraRed (FTIR), gas-volumetric and ThermoGravimetric Analysis (TGA). Moreover, the effect of nanostructuration on the semiconducting material band gap is evaluated by Diffuse Reflectance UV-Vis (DR UV-Vis) spectroscopy. Density Functional Theory (DFT) calculations have been employed to clarify the role of the adsorbed reactants on the surface stability and to assess the relationship between particle size and band gap value