Browsing by Author "Mejia-Lopez, Jose"

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    Charge transfer evidence in donor-acceptor single-walled carbon nanotubes filled with sexithiophene oligomers: Nanotube diameter dependence
    (2021) Chenouf, Jamal; Boutahir, Mourad; Rahmani, Abdelhai; Chadli, Hassane; Hermet, Patrick; Mejia-Lopez, Jose; Rahmani, Abdelali
    Encapsulation of photoactive organic molecules inside single-walled carbon nanotubes (SWNTs) appears to be of great interest in terms of high power conversion efficiency and long-term stability for a commercial application of organic solar cells (OSCs). In this paper, we report a charge transfer (CT) evidence in donor-acceptor SWNTs filled with Sexithiophene oligomers (6T) by Raman spectroscopy. To compute the optimal diameter and demonstrate the most stable structure of the hybrid systems with either a single 6T molecule encapsulated into SWNTs (6T@SWNTs), or two 6T chains encapsulated (6T-6T@SWNTs), we have performed structural geometry optimization on the hybrid encapsulated systems using a convenient Lennard-Jones (LJ) expression of the van der Waals (vdW) intermolecular potential. Combining the density functional theory (DFT), molecular mechanics, bond polarizability model, and the spectral moment method (SMM), we computed the polarized nonresonant Raman spectra of 6T molecule and SWNTs (metallic and semiconducting) before and after encapsulation. The influence of the encapsulation on the Raman-active modes of the 6T molecule and those of the nanotube (radial breathing modes and tangential modes) are analyzed. In particular, significant changes observed in the G-band wavenumber. The possibility (or not) of an eventual CT between the 6T oligomer and the nanotube in both hybrid systems (6T@SWNTs and 6T-6T@SWNTs) is discussed. We show that there is a dependence of the CT with respect to the diameter of SWNTs, the CT vanish with increasing diameter of the nanotubes. Our finding of CT behavior in the filled SWNTs with respect to SWNT diameter will provide a useful guidance for enhancing the performance of OSCs by SWNTs.
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    Convoluted Magnetoresistance and Magnetic Reversal Processes in Ni-Fe Segmented Cylindrical Nanodots with Tunable Size and Composition for Technological Applications
    (2023) Velasquez, Ever A.; Mazo-Zuluaga, Johann; Mejia-Lopez, Jose
    Due to their unique properties, bi-segmented systems are currently used in several technological applications such as sensing devices, high density magnetic data storage systems, spintronics and microelectromechanical components, among others. In this study, the magnetoresistance and magnetic properties of Ni-Fe bi-segmented cylindrical nanodots in a broad range of diameters and heights are discussed. The power of the First Principles approach, as considered in the density functional theory formulation, is used to study the structural and magnetic relaxation effects, and atomistic simulations, through the Fast Monte Carlo methodology, are employed to explore the magnetoresistance and magnetic behaviors of these systems. By means of the magnetic hysteresis and magnetoresistance signals, convoluted magnetization reversal schemes are discussed. These effects take place depending on the size of the Ni and Fe components as a result of the interplay among exchange interactions and size and shape effects induced by dipolar interactions. Since size has become an experimental controllable parameter, due to the enriched phenomena, the effects discussed in these bi-component systems are useful for the design and production of devices for technological applications with relevance beyond the observed in the more restricted single component systems.
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    Density Functional Theory Calculations of Optoelectronic Properties of Individual and Encapsulated Magnesium Porphyrin in Carbon Nanotubes for Organic Nanohybrid Solar Cells
    (2024) El Fatimy, Anass; Boutahir, Oussama; Rahmani, Abdelhai; Boutahir, Mourad; Mejia-Lopez, Jose; Termentzidis, Konstantinos; Rahmani, Abdelali
    This research discusses incorporating a single magnesium porphyrin molecule into a semiconducting single-walled carbon nanotube (SWCNT) for solar cell applications. Using density functional theory (DFT), the study examines the optical and electronic properties of the isolated magnesium porphyrin molecule and two configurations of the hybrid system. Results show structural stability due to charge transfer between the molecule and the nanotube. Different exchange-correlation functionals (GGA and HSE06) yield varied bandgap results, affecting light absorption. Integration of the molecule into the SWCNT reduces the bandgap. Encapsulation of the molecule influences absorption and stability under irradiation. These encapsulated systems exhibit type II heterojunction characteristics, making them promising for organic solar cells based on SWCNTs, offering potential for highly efficient solar cells.
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    Dynamical Stability and Physical Properties of Fe Dihalide Nanowires
    (2023) Mejia-Lopez, Jose; Lopez-Moreno, Sinhue; Mazo-Zuluaga, Johann; Romero-Vazquez, Pricila Betbirai; Moran-Lopez, Jose Luis
    An extensive first-principles and atomistic Monte Carlo study on isolated FeX2 (X = F, Cl, Br, I) nanowires is presented. The structural properties of the FeX2 chains are determined and compared with their bulk structures. The results indicate that in the lowest energy configuration, the wires crystalize in a system that belongs to the space group P42/mmc (No. 131, Z = 2, point group D-4h(9)), with antiferromagnetic arrangement. The stability is determined by calculating the phonon frequencies in the whole Brillouin zone within the supercell approach. The relative stability of the periodic chains is also determined by calculating the elastic properties and comparing them with bulk cases. The band structure, the density of states, the magnetic properties, the anisotropy energy, and topological analysis, performed with the Quantum Theory of Atoms in Molecules approach, are also reported and discussed. The results support the idea that these FeX2 nanowire systems are promising materials for practical applications, like lithium-ion batteries.
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    Experimental and theoretical study of synthesis and properties of Cu2O/ TiO2 heterojunction for photoelectrochemical purposes
    (2023) Matamala-Troncoso, Felipe; Saez-Navarrete, Cesar; Mejia-Lopez, Jose; Garcia, Griselda; Rebolledo-Oyarce, Jose; Nguyen, Cuong Ky; MacFarlane, Douglas R.; Isaacs, Mauricio
    The Cu2O/TiO2 heterojunction is an attractive surface for its optoelectronic properties for developing catalysts, cells, and solar devices. However, the mechanisms involved in synthesizing an electrode using the Cu2O/TiO2 heterojunction can affect the surface properties and the surface/electrolyte interactions. In this work, we studied the formation mechanism of the Cu2O/TiO2 heterojunction by electrochemical deposition (ECD) of Cu2O mol-ecules on TiO2 nanoparticles previously deposited on a fluorine-doped thin oxide coated glass substrate (FTO). The photoelectrochemical properties of the Cu2O/TiO2/FTO electrode were characterized by XRD, FE-SEM, TEM, EDX, UV-vis diffuse reflectance spectroscopy (DRS), Raman spectroscopy, and electrochemical methods. Theoretical methods such as ab-initio density functional theory calculations and molecular dynamics simulations were used to understand the experimental results. The analysis carried out by theoretical methods allowed us to identify the initial steps of the formation mechanism of Cu2O molecules on TiO2 nanoparticles. Theoretical calculations demonstrated that forming a Cu2O nanowire-like network on the TiO2 nanoparticle matrix favors the charge transfer at the electrolyte/semiconductor interface, promoting the behavior of the electrode as a cathode. Finally, the Cu2O/TiO2/FTO electrode synthesized was used to perform the reduction photoelectrocatalyzed of nitrate to ammonia under illumination with a Xe-Hg arc lamp and applying-0.5 V bias potential (vs Ag/AgCl sat.) to evaluate the performance of the electrode as a cathode.
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    Force-constant model for the vibrational modes in black-phosphorene and phosphorene nanoribbons (PNRs)
    (2021) Boutahir, Oussama; Lakhlifi, Souhail; Abdelkader, Sidi Abdelmajid Ait; Boutahir, Mourad; Rahmani, Abdelhai; Chadli, Hassane; Mejia-Lopez, Jose; Rahmani, Abdelali
    We present in this paper, force constant model developed for Black phosphorene in order to reproduce the vibrationnal properties calculated from density functional theory. The results of this model are compared with the experimental data available from Raman spectroscopy measurements. Excellent agreement is obtained between calculation and Raman experiment. On the basis of the resulting force model, the non-resonant Raman spectra of a large number of armchair and zigzag phosphorene nanoribbons (PNRs) are calculated using the bond polarizability model in the framework of spectral moments method. We have found a good agreement with group theory concerning the number of the Raman-active modes of black phosphorene. We report the effect of the edge and width on the vibrational properties of PNRs by increasing the width, the main characteristic feature is dictated by A2g Raman active mode. It exhibits different characteristic for armchair and zigzag edges. The mode is upshifted under armchair edge and downshifted in the zigzag one. Moreover, we observe additional Raman modes as a function of the ribbon width and we propose an equation, ?min = A/L (A = 98 cm-1 nm), to estimate the PNRs width L from the knowledge of the lowest Raman frequency mode ?min.
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    Predicting the structure configuration and Raman analysis of caffeine molecules encapsulated into single-walled carbon nanotubes: Evidence for charge transfer
    (2022) Chenouf, Jamal; Boutahir, Mourad; Mejia-Lopez, Jose; Rahmani, Abdelhai; Fakrach, Brahim; Chadli, Hassane; Rahmani, Abdelali
    A new hybrid nano-system constituted of a single-walled carbon nanotube (SWNT) filled by caffeine (Caff) molecule(s) is proposed as a potential candidate for organic solar cells. The stability of this hybrid system with either a single or two Caff molecule(s) encapsulated into SWNTs has been investigated. In particular, the optimal SWNT diameter is discussed for each configuration. Raman spectra have been calculated using an approach combining the density functional theory, molecular mechanics, bond polarizability model and the spectral moment's method. We have analyzed the influence of the nanoconfinement on the Raman-active modes of Caff molecule and those of SWNT. The analysis of the nanoconfinement effect on the Raman active modes of Caff molecules and those of SWNTs (radial breathing mode (RBM) and tangential mode (TM)) confirm the structural stability and indicate an evidence for a charge transfer between the Caff molecules and the nanotubes.
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    Role of carbon nanotubes as an acceptor to enhance the photovoltaic performances of organic solar cells based on π-conjugated thiophene as a donor materials
    (2021) Boutahir, Mourad; Chenouf, Jamal; Mejia-Lopez, Jose; Rahmani, Abdelhai; Chadli, Hassane; Rahmani, Abdelali
    Filled semiconducting single-walled carbon nanotubes (SWNTs), with pi-conjugated polymer as a light harvester and charge transporter in the active layer, could play a key role in the development of more qualified organic solar cells (OSCs) in the sense of high energy conversion efficiency and long-term stability. In this paper,we used three computational approaches to investigate a series of oligothiophene (nT) (n = 2, 4 or 6) encapsulated inside SWNTs. The first approach is based on a combination of the density functional theory (DFT), molecular mechanics, bond polarizability model and the spectral moment's method (SMM) to compute the nonresonant Raman spectra of the encapsulated systems. We reported the optimal tube diameter allowing the nT encapsulation. The influence of the encapsulation on the radial breathing mode and G-band modes of the selected semiconducting SWNTs zigzag (11,0), labeled (NT11), is testified to the presence of the charge transfer (CT) in the nT@NT11 hybrid systems without specifying their direction. The second approach is based on the electronic and optical properties with DFT at the generalized gradient approximation. The CT and its direction in nT@NT11 hybrid systems is identified by electronic and optical calculations. The third approach is based on electronic transport properties with DFT in combination to nonequilibrium Green's function formalism. We have investigated the transmission spectra and current-voltage characteristics of nT@NT11 hybrid. It was observed that a strong correlation between transmission spectra and DOS near the Fermi level due to delocalization of electronic states, and the I-V characteristic exhibits interesting electronic transport properties. The results illustrate that the filled semiconducting SWNTs exhibiting type II heterojunctions are expected to be a good candidate as a light harvester and charge transporter in the active layer, which can contribute to developing highly efficient filled SWNT-based OSCs.
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    Short bandgap of porphyrin molecules (Py) filled in a semiconducting single-walled carbon nanotube (Py@NT17) for highly efficient organic photovoltaic cells
    (2023) El fatimy, Anass; Boutahir, Mourad; Fakrach, Brahim; Mejia-Lopez, Jose; Boutahir, Oussama; Rahmani, Abdelhai; Chadli, Hassane; Rahmania, Abdelali
    In this paper, the porphyrin (Py) molecule, which is filled in the semiconducting single-walled carbon nanotube (SWNT) known as CNT17, is investigated. We have calculated the optoelectronic characteristics of two configurations of the hybrid system with a single Py molecule filled into CNT17 (Py@CNT17 and Py2@CNT17) using density functional theory (DFT). According to the investigation's findings, a charge transfer occurred between the Py molecules and the CNT17 nanotube, confirming the stability of the structure. As determined by the electronic calculation, we discovered that the rise in the fermi energy position of CNT17 after encapsulation, suggests a CT connection between Py and CNT17.
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    Tuning Magnetic Order in CrI3 Bilayers via Moire Patterns
    (2022) Leon, Andrea M.; Velasquez, Ever A.; Caro-Lopera, Francisco; Mejia-Lopez, Jose
    Commensurable twisted bilayers can drastically change the magnetic properties of chromium trihalide layered compounds, which opens novel opportunities for tuning magnetic states through layer rotations. Here, a mathematical approach to obtain moire patterns in twisted hexagonal bilayers by performing a certain commensurable rotation theta over one layer is presented. To test the approach, moire structures with theta=21.79 degrees and 32.20 degrees in the phases R (3) over bar and C2/m of CrI3 are obtained via the related methodology. For comparison purposes, a non-shifted CrI3 structure is also considered. Electronic and magnetic properties of the so-obtained systems are computed by ab initio methodologies. Results show the presence of rotation-angle-dependent magnetic configurations and steep modifications of the dispersion bands due to variations in the nearest and next nearest distances among layers of Cr atoms. Modifications obtained from these commensurable rotations are discussed on the basis of competition among different energy contributions due to changes in the atomic neighborhood.