Hydrogen bonds in lead halide perovskites: insights from ab initio molecular dynamics

Hydrogen bonds (HBs) play an important role in the rotational dynamics of organic cations in hybrid organic/inorganic halide perovskites, affecting the structural and electronic properties of the perovskites. However, the properties and even the existence of HBs in these perovskites are not well established. We investigate HBs in perovskites MAPbBr$_3$ (MA$^+$=CH$_3$NH$_3^+$), FAPbI$_3$ (FA$^+$= CH(NH$_2$)$_2^+$), and their solid solution (FAPbI$_3$)$_{7/8}$(MAPbBr$_3$)$_{1/8}$, using ab initio molecular dynamics and electronic structure calculations. We consider HBs donated by X-H fragments (X=N, C) of the organic cations and accepted by the halides (Y=Br, I), and characterize their properties based on pair distribution functions and on a combined distribution function of hydrogen-acceptor distance with donor-hydrogen-acceptor angle. By analyzing these functions, we establish geometric criteria for HB existence based on hydrogen-acceptor distance $d(H-Y)$ and donor-hydrogen-acceptor angle $\measuredangle(X-H-Y)$. The distance condition is defined as $d(H-Y)<0.3$ nm, for N-H-donated HBs, and $d(H-Y)<0.4$ nm for C-H-donated HBs. The angular condition is $135{^\circ}\le\measuredangle(X-H-Y)\le 180{^\circ}$ for both types of HBs. At the simulated temperature (350 K), the HBs dynamically break and form. We compute time correlation functions of HB existence and HB lifetimes, which range between 0.1 and 0.3 picoseconds at that temperature. The analysis of HB lifetimes indicates that N-H--Br bonds are relatively stronger than N-H--I bonds, while C-H--Y bonds are weaker. To evaluate the impact of HBs on vibrational spectra, we present the power spectra, showing that peaks associated with N-H stretching modes in perovskites are redshifted and asymmetrically deformed compared with the peaks of isolated cations.Comment: 19 pages, 9 figure

Atomic scale model and electronic structure of Cu2_2O/CH3_3NH3_3PbI3_3 interfaces in perovskite solar cells

Cuprous oxide has been conceived as a potential alternative to traditional organic hole transport layers in hybrid halide perovskite-based solar cells. Device simulations predict record efficiencies using this semiconductor, but experimental results do not yet show this trend. More detailed knowledge about the Cu$_2$O/perovskite interface is mandatory to improve the photoconversion efficiency. Using density functional theory calculations, here we study the interfaces of CH$_3$NH$_3$PbI$_3$ with Cu$_2$O to assess their influence on device performance. Several atomistic models of these interfaces are provided for the first time, considering different compositions of the interface atomic planes. The interface electronic properties are discussed on the basis of the optimal theoretical situation, but in connection with the experimental realizations and device simulations. It is shown that the formation of vacancies in the Cu$_2$O terminating planes is essential to eliminate dangling bonds and trap states. The four interface models that fulfill this condition present a band alignment favorable for photovoltaic conversion. Energy of adhesion, and charge transfer across the interfaces are also studied. The termination of CH$_3$NH$_3$PbI$_3$ in PbI$_2$ atomic planes seems optimal to maximize the photoconversion efficiency.Comment: 16 pages; 8 figures. Submitted to ACS Applied Materials & Interfaces. Published after changes not included her

Functionalization of gold nanostars with cationic ß-cyclodextrin-based polymer for drug co-loading and SERS monitoring

Gold nanostars (AuNSs) exhibit modulated plasmon resonance and have a high SERS enhancement factor. However, their low colloidal stability limits their biomedical application as a nanomaterial. Cationic ß-cyclodextrin-based polymer (CCD/P) has low cytotoxicity, can load and transport drugs more efficiently than the corresponding monomeric form, and has an appropriate cationic group to stabilize gold nanoparticles. In this work, we functionalized AuNSs with CCD/P to load phenylethylamine (PhEA) and piperine (PIP) and evaluated SERS-based applications of the products. PhEA and PIP were included in the polymer and used to functionalize AuNSs, forming a new AuNS-CCD/P-PhEA-PIP nanosystem. The system was characterized by UV–VIS, IR, and NMR spectroscopy, TGA, SPR, DLS, zeta potential analysis, FE-SEM, and TEM. Additionally, Raman optical activity, SERS analysis and complementary theoretical studies were used for characterization. Minor adjustments increased the colloidal stability of AuNSs. The loading capacity of the CCD/P with PhEA-PIP was 95 ± 7%. The physicochemical parameters of the AuNS-CCD/P-PhEA-PIP system, such as size and Z potential, are suitable for potential biomedical applications Raman and SERS studies were used to monitor PhEA and PIP loading and their preferential orientation upon interaction with the surface of AuNSs. This unique nanomaterial could be used for simultaneous drug loading and SERS-based detection

How Meaningful Is the Halogen Bonding in 1-Ethyl-3-methyl Imidazolium-Based Ionic Liquids for CO2 Capture?

© 2018 American Chemical Society. We report on several parameters that can be used to describe the 1-ethyl-3-methyl-4,5-(X2)imidazolium cations (where X = H, Br, and I) within the Canongia-Lopez and Padua Force Field (CL&P) framework. Geometrical parameters like intramolecular distances and radial distribution functions are close to the experimental structure. Density values obtained with our force field are within the expected ones from CL&P calculations in related systems. This information is used to simulate through molecular dynamics the solubilization of CO2 by these ILs. For pure ILs, the addition of halides in position 4 and 5 promotes an enhanced hydrogen bond interaction at position 2 with the oxygen atoms in the anion. It is found that CO2 should be in the interstices of the anion-cation 3D network with longer distances than those found in other reports at ab initio levels, suggesting that halogen bond, if present, may be not the driving force interaction in these systems. T

How Meaningful Is the Halogen Bonding in 1‑Ethyl-3-methyl Imidazolium-Based Ionic Liquids for CO₂ Capture?

We report on several parameters that can be used to describe the 1-ethyl-3-methyl-4,5-(X₂)­imidazolium cations (where X = H, Br, and I) within the Canongia-Lopez and Padua Force Field (CL&P) framework. Geometrical parameters like intramolecular distances and radial distribution functions are close to the experimental structure. Density values obtained with our force field are within the expected ones from CL&P calculations in related systems. This information is used to simulate through molecular dynamics the solubilization of CO₂ by these ILs. For pure ILs, the addition of halides in position 4 and 5 promotes an enhanced hydrogen bond interaction at position 2 with the oxygen atoms in the anion. It is found that CO₂ should be in the interstices of the anion–cation 3D network with longer distances than those found in other reports at ab initio levels, suggesting that halogen bond, if present, may be not the driving force interaction in these systems. Therefore, it seems that CO₂ interacts linearly via an oxygen atom with the cation and with the anion through a π-stacking or hydrogen-bonded fashions. Solvation enthalpies compare well with the experimental data, thereby suggesting that halogenated ILs dissolve more efficiently in CO₂ than C₂C₁Im⁺ derivatives. This result suggests that halogenated ILs can be considered as reliable candidates for CO₂ capture

Hydrogen bonds in lead halide perovskites: insights from ab initio molecular dynamics

Hydrogen bonds (HBs) play an important role in the rotational dynamics of organic cations in hybrid organic/inorganic halide perovskites, thus affecting the structural and electronic properties of the perovskites. However, the properties and even the existence of HBs in these perovskites are not well established. In this study, we investigate HBs in perovskites MAPbBr3 (MA+=CH3NH3+), FAPbI3 (FA+= CH(NH2)2+), and their solid solution with composition (FAPbI3)7/8(MAPbBr3)1/8, using ab initio molecular dynamics and electronic structure calculations. We consider HBs donated by X-H fragments (X=N, C) of the organic cations and accepted by the halides (Y=Br, I), and characterize their properties based on pair distribution functions and on a combined distribution function of hydrogen-acceptor distance with donor-hydrogen-acceptor angle. By analyzing these functions, we establish geometrical criteria for HB existence based on hydrogen-acceptor (H—Y) distance and donor-hydrogen-acceptor angle (X—H—Y). The distance condition is defined as d(H-Y)<3 Å, for N-H-donated HBs, and d(H-Y)<4 Å for C-H-donated HBs. The angular condition is 135°<∡(X-H-Y)<180° for both types of HBs. A HB is considered to be formed when both angular and distance conditions are simultaneously satisfied. At the simulated temperature (350 K), the HBs dynamically break and form. We compute time correlation functions of HB existence and HB lifetimes, which range between 0.1 and 0.3 ps at that temperature. The analysis of HB lifetimes indicates that N-H—Br bonds are relatively stronger than N-H—I bonds, while C-H—Y bonds are weaker, with minimal influence from the halide and cation. To evaluate the impact of HBs on vibrational spectra, we present the power spectrum in the region of N-H and C-H stretching modes, comparing them with the normal mode frequencies of isolated cations. We show that the peaks associated with N-H stretching modes in perovskites are redshifted and asymmetrically deformed, while the C-H peaks do not exhibit these effects