Enhancement of interlayer exchange coupling via intercalation in 2D magnetic bilayers: towards high Curie temperature.

Two-dimensional magnetic materials are considered as promising candidates for developing next-generation spintronic devices by providing the possibility of scaling down to nanometers. However, a low Curie temperature is a crucial problem for practical applications, being intimately related to weak interlayer exchange coupling. Here, by using density functional theory calculations, we show that interlayer exchange coupling can be enhanced by intercalating 3d transition metals (Sc to Zn) into a bilayer of CrI3 and NiI2. It is found that intercalated Ni and Cr atoms exhibit strong antiferromagnetic coupling with the CrI3 and NiI2 host layers, respectively. This enhances the ferromagnetic interlayer exchange coupling between the host layers by many folds compared to pristine CrI3 and NiI2 bilayers. Moreover, both intercalated compounds show out-of-plane magnetic anisotropy with half metallic nature, which makes them ideal candidates for spintronics applications. Thereby our work provides a rational approach to raise the Curie temperature of non-metallic two-dimensional magnets by intercalation.

Water dissociation and COOH formation on Fe modified Cu(100) surface: A density functional theory study.

Water splitting has emerged as a promising route for sustainable hydrogen production. In this research paper, adsorption and dissociation of H2O accompanied with dissociated constituents reactions with CO2 and CO have been investigated on Fe modified Cu(100) surface employing density functional theory (DFT) at GGA-PW91 level. The adsorption and other reactions carried out on Fe2-Cu(100) surfaces gave very promising results. The adsorption of H2O on Fe top of this surface occurs yielding Eads -1.73 eV, which highlights a favorable adsorption on the Fe-modified Cu(100) surface. The activation energy for the water splitting reaction is found to be 0.65 eV, suggesting a feasible pathway for hydrogen evolution. The process also accompanies reaction energy of -0.54 eV. Furthermore, the interaction between carbon dioxide (CO2) and the H-atom on the surface lead to the formation of COOH through surmounting an activation barrier of 1.09 eV. The final position of COOH gets further stabilization having exothermicity of -0.43 eV. Another route for COOH formation from CO + OH operates on the Cu(100) part of the surface with a small activation barrier of 0.14 eV through exothermic process of -0.29 eV, however, diffusion of CO and OH from Fe to Cu is energetically expensive. This study signifies the consumption of CO and CO2 in addition to water splitting giving birth to useful products.

Impact of fluorination on the energy level alignment of an F n ZnPc/MAPbI3 interface.

We have studied interactions at an interface between a Methylammonium Lead Iodide (MAPbI3) surface and zinc-phthalocyanine molecules with F substituting peripheral H (F n ZnPc; n = 4, 8, 12, and 16) by employing hybrid density functional theory (DFT) based simulations. These calculations show that F n ZnPc molecules form a stable interface with MAPbI3, whose binding strength is comparable to that of the un-substituted (ZnPc) case. As a consequence of fluorination, an increase in the ionization potential/electron affinity (i.e., a systematic lowering of molecular energy levels), as well as interfacial charge transfer, is observed whose magnitude depends upon the degree of fluorination. In contrast to the common belief of unfavorable hole transfer for excessive fluorination, our work unveils that the valence band offset remains favorable for all ranges of substitution (n); thus, hole transfer from MAPbI3 to F n ZnPc is facilitated while the electron transfer process is suppressed. This unusual behavior originates from the intermolecular interaction and substrate-to-molecule electron transfer at the heterojunction, which gradually suppresses the downward shift of F n ZnPc energy levels by increasing the value of n. Given the beneficial impacts of fluorination, such as hydrophobicity, our work provides valuable insight for exploiting stable F n ZnPc towards high-efficiency perovskite solar cells.

The impact of molecular orientation on carrier transfer characteristics at a phthalocyanine and halide perovskite interface.

We have studied the interface properties of metal phthalocyanine (MPc, M = Zn, Cu) molecules at a methylammonium lead iodide (MAPbI3) surface using density functional theory (DFT) based simulations. From the adsorption energies, the face-on orientation is found to have an order of magnitude stronger binding energy than the edge-on orientation, where CuPc binds a little stronger than ZnPc with its closer interfacial distance. Our detailed analysis of interface electronic structure suggests that the edge-on configuration possesses a large energy barrier for the hole transfer from MAPbI3 to MPc molecules. In contrast, the face-on configuration has no such barrier, facilitating the hole transfer, while at the same time the desirable alignment of the conduction band suppresses the electron-hole recombination. Therefore, the face-on configuration is clearly found to be more suitable for the photovoltaic process, in line with the experimental reports. Our work emphasizes the impact of MPc orientation upon perovskite solar cell efficiency besides other factors such as Pc thin film's mobility and morphology, and provides insightful guidance to efficient and stable hole transport layers.

Edge functionalized graphene nanoribbons with tunable band edges for carrier transport interlayers in organic-inorganic perovskite solar cells.

Organic based graphene nanoribbons (GNRs) can be good candidates as carrier extraction interlayers for organic/inorganic hybrid perovskite solar cells, owing to the possibility of tuning the band edge energy levels through varying the width and the type of edge functionalization. By using the density functional theory (DFT) method, the electronic structures of H or F edge functionalized armchair type GNRs on MAPbI3(001) are calculated. It is shown that the electronic structure of H- or F-passivated GNRs is almost undisrupted by the non-covalent interaction with the PbI2 surface layer of MAPbI3(001), thereby one can tune the width and edge chemistry of GNRs to enhance the carrier extraction or blocking. Especially all H-GNRs five to ten carbon atoms wide exhibit good matching for hole extraction, while F-GNRs require a specific width for electron extraction. Exploiting the unzipping synthesis of carbon nanotubes in the solution phase, our result provides a facile strategy for efficient carrier extraction.