Methylammonium Groups Research Articles

Enhanced piezoelectricity of MAPbI3 by the introduction of MXene and its utilization in boosting high-performance photodetectors.

Recently, perovskite photodetectors (PDs) have been risen to prominence due to substantial research interest. Beyond merely tweaking the composition of materials, a cutting-edge advancement lies in leveraging the innate piezoelectric polarization properties of perovskites themselves. Here, the investigation shows utilizing Ti3C2Tx, a typical MXene, as an intermediate layer for significantly boosting the piezoelectric property of MAPbI3 thin films. This improvement is primarily attributed to the enhanced polarization of the methylammonium (MA+) groups within MAPbI3, induced by the OH groups present in Ti3C2Tx. A flexible PD based on the MAPbI3/MXene heterostructure was then fabricated. The new device is sensitive to a wide range of wavelengths, displays greatly enhanced performances owing to the piezo-phototronic coupling. Moreover, the device is endowed with a greatly reduced response time, down to millisecond level, through the pyro-phototronic effect. The characterization shows applying a -1.2% compressive strain on the PD leads to a remarkable 102% increase of the common photocurrent, and a 76% increase for the pyro-phototronic current. The present work reveals how the emerging piezo-phototronic and pyro-phototronic effects can be employed to design high-performance flexible perovskite PDs. This article is protected by copyright. All rights reserved.

Thermal Effects and Halide Mixing of Hybrid Perovskites: MD and XPS Studies.

Thermal effects in organo-metal halide perovskites are studied by ab initio molecular dynamics (MD) simulations performed at effective temperatures of 293 and 383 K and by X-ray photoelectron spectroscopy (XPS). We find that the cause of thermal instability in this class of perovskites is the rotation of the methylammonium (MA) groups that destroy the rigid lattice of pure compounds (MAPbI3 and MAPbBr3). When the Pb-I lattice is initially distorted by partial replacement of the I with Cl or Br, this not only prevents formation of PbI2 seeds but also improves lattice flexibility and stability against the temperature-induced motion and rotation of MA groups.

Dynamic Disorder Restriction of Methylammonium (MA) Groups in Chloride-Doped MAPbBr3 Hybrid Perovskites: A Neutron Powder Diffraction Study.

The hybrid methylammonium (MA) lead halide MAPbX3 perovskites present an appealing optoelectronic behavior with applications in high-efficiency solar cells. The orientation of the organic MA units may play an important role in the properties, given the degrees of freedom for internal motion of MA groups within the PbX6 network. The present neutron powder diffraction study reveals the dynamic features of the MA units in the hybrid perovskite series MAPb(Br1-x Clx )3 , with x=0, 0.33, 0.5, 0.67, and 1. From difference Fourier maps, the H and C/N positions were located within the PbX6 lattice; the refinement of the crystal structures unveiled the MA conformations. Three different orientations were found to exist as a function of the chlorine content (x) and, therefore, of the cubic unit-cell size. These conformations are stabilized by H-bond interactions with the halide ions, and were found to agree with those reported from theoretical calculations.

Hybrid Organic-Inorganic Perovskites as Promising Substrates for Pt Single-Atom Catalysts.

Single-atom catalysts (SACs) combine the best of two worlds by bridging heterogeneous and homogeneous catalysis. The superior catalytic properties of SACs, however, can hardly be exploited without a suitable substrate. Here, we explore the possibility of using hybrid organic-inorganic perovskites as supporting materials for single transition-metal atoms. By means of first-principles calculations, we predict that single Pt atoms can be incorporated into methylammonium lead iodide surfaces by replacing the methylammonium groups at the outermost layer. The iodide anions at the surface provide potentially uniform anchoring sites for the Pt atoms and donate electrons, generating negatively charged Pt_{1}^{δ-} species that allow for preferential O_{2} adsorption in the presence of CO. Such Pt sites are able to catalyze CO oxidation and may also play a role in CO_{2} reduction. The fundamental understanding generated here will shed light on potential applications of hybrid perovskites in the field of (photo)catalysis.

Nanowells on silica particles in water containing long-distance porphyrin heterodimers.

Smooth and nonswelling spherical silica particles with a diameter of 100 nm and an aminopropyl coating are soluble in water at pH 11, coagulate quickly at pH 3, and redissolve at pH 9. Electron microscopy as well as visible spectra of covalently attached porphyrins indicate the aggregation state of the particles. Long-chain alpha,omega-dicarboxylic acids with a terminal oligoethyleneglycol (=OEG)-amide group were attached in a second self-assembly step to the remaining amine groups around the porphyrins. Form-stable 2-nm wells were thus obtained and were characterized by fluorescence quenching experiments using the bottom porphyrin as a target. The one-dimensional diffusion of fitting quencher molecules along the 2-nm pathway took several minutes. Porphyrins with a diameter above 2 nm could not enter the form-stable gaps at all. Added tyrosine stuck irreversibly to the walls of the nanowells and prevented the entrance of quencher molecules, the OEG-headgroups fixated 2,6-diaminoanthraquinone. A ring of methylammonium groups was then fixed at the walls of the wells at a distance of 5 or 10 A with respect to the bottom porphyrin. 2,6-Disulfonatoanthraquinone was attached only loosely to this ring, but the exactly fitting manganese(III) meso-(tetraphenyl-4-sulfonato)porphyrinate (Mn(III) TPPS) was tightly bound. Transient fluorescence experiments showed a fast decay time of 0.2 ns for the bottom porphyrin, when the Mn(III) TPPS was fixated at a distance of 5 A. Two different dyes have thus been immobilized at a defined subnanometer distance in an aqueous medium.

Conformation of poly(l-glutamic acid) in bipolar amphiphiles with alkyl ammonium cations of different hydrophobicity

The conformation of the sodium salt of poly(l-glutamic acid) (P(Glu)) in solutions of the cationic bipolar amphiphile 1, 20-isosanediylbis(alkylammonium chloride) (C20(RA)2) with different alkyl head groups as a function of amphiphile concentration was investigated using circular dichroism (CD). RA included methylammonium (MA), ethylammonium (EA), propylammonium (PA), butylammonium (BA), and pentylammonium (PeA) cationic groups. The CD spectrum of each C20(RA)2 had a double minima corresponding to the α-helix of P(Glu), which was replaced by CD spectra with a single minimum at wavelengths larger than 222 nm as concentration increased. These changes in the CD spectrum were ascribed to the conformational change from random coil to α-helix and to aggregates of helices. In C20(EA)2 solutions, a step-like change in the CD intensity was observed at 222 nm as a function of the ratio of C20(RA)2 to P(Glu). At the step, the CD spectrum of the complete α-helix was observed. At 10–35 °C, an α-helix was induced in P(Glu) in the order: C20(EA)2>C20(MA)2>C20(PA)2>C20(BA)2>C20(PeA)2. This order was ascribed to the best fit of ethylammonium to the P(Glu) side chain.

Crystal structure of monomethylammonium tribromocadmate(II)

CH3NH3CdBr3 crystallizes in the space group Ccm21, a = 13.693(1) A, b = 7.906(1) A, c = 6.886(1) A, V = 745.46(15) A3, Z = 4, D x = 3.42 Mg m−3. The structure of the title compound is one-dimensional consisting of infinite chains made of face sharing CdBr6 octahedra. The methylammonium groups are situated in the space between chains. The cations and anions are joined by hydrogen bonding.

Methylammonium groups at the solid walls of nanometer-sized, water-filled monolayer gaps as binding sites for a tetraanionic porphyrin.

Long-chain hydrosulfides containing two secondary amide functions and either electron-poor or electron-rich carbon-carbon double bonds were self-assembled on gold surfaces around a flat-lying, octaanionic porphyrin. Rigid and reactive surface monolayers with 2 nm-wide, porphyrin-based gaps were thus obtained. The gold electrodes were then immersed in water, and the double bonds on the gaps' surfaces reacted with methylamine. It was added to the double bonds either by Michael addition or by bromination with hypobromite followed by methylamine substitution. Only the double bonds at the border of the gaps were accessible to methylamine dissolved in the bulk water volume and could react. The walls of the rigid membrane gaps now contained methylammonium groups at the sites of the double bonds in defined heights. A tetracationic copper(II) porphyrinate could not diffuse any more into the gap and did not quench the fluorescence of the octaanionic porphyrin on the bottom of the gap. A tetraanionic porphyrin, on the other hand, was fixated by the ring of ammonium groups. The bound porphyrin then acted as molecular cover for the gap with respect to ferricyanide transport from bulk water to the electrode. It was removed by raising the pH to a value of 12, where the methylammonium groups were neutralized to amines. Lowering the pH to 7 again and addition of more of the anionic porphyrin reclosed the gap.The porphyrin "cover" should be localized at distances of 8-10 and 20 A from the bottom porphyrin by multiple charge interactions. The 8-10 A distance is ideal for studies of photoinduced electron transfer between two porphyrin monomers of different redox potential. Furthermore it was found, that redox-active tyrosine could be trapped in the water volume above the porphyrin on gold.

Anharmonic effects and temperature dependence of the lattice spectra of ferroelectric crystals A3B2C9

In the present work the Raman spectra of the family of ferroelectric crystals A3B2C9 (where A=Cs, Rb; B=Sb, Bi; C=Br) are given and the assignment of some high frequency bands is fulfilled. The conclusion about the possible differences of the structures of the crystals Rb3Bi2Br9 and Rb3Sb2Br9 from the well-known and thoroughly investigated structure of the Cs3Bi2Br9 crystal is made. It is shown that anharmonic interaction of vibrations can give rise to anomalous T-dependence of spectra in the low frequency region. For a crystal having methyl-ammonium (MA) groups the strong anharmonic effects can generate a very complicated structure around internal fundamental frequencies of the MA group which may be considered as a result of the phase transition effects.

Structural phase transition in single-crystal CH3NH3CdBr3: I. Experimental studies

Single crystals of CH3NH3CdBr3 (MACB) have been studied as a function of temperature by means of differential scanning calorimetry, X-ray diffraction and Raman scattering measurements. A first-order structural phase transition is evidenced by all three techniques at 170 K. The high-temperature phase (I) is hexagonal, with space group P63/mmc and Z=2 formula units per unit cell. The X-ray diffraction patterns of the low-temperature phase (II) are consistent with a complex orthorhombic unit cell with lattice parameters such as aII approximately=4ahex, bII approximately=2 square root 3ahex and cII approximately=3chex, i.e. corresponding to Z=96; however, it is shown that the system can be described in a simplified manner by means of an orthorhombic 'projection' unit cell onto the hexagonal plane with parameters such as ap approximately=ahex, bp approximately= square root 3ahex and cp approximately=3chex. The Raman data clearly indicate that the phase transition is of the order-disorder type, owing to the reorientational processes of the methylammonium groups.

Lattice dynamics and structural phase transitions in perovskite-type layer compounds. IV. Diffuse X-ray scattering in (CH3NH3)2CdCl4

For pt.III see ibid., vol.17, p.217 (1984). Phase transitions of (CH3NH3)2CdCl4 (MACC) have been investigated by means of diffuse X-ray scattering experiments. The diffuse scattering patterns observed in the disordered orthorhombic room temperature (ORT) and tetragonal low-temperature (TLT) phases have been interpreted through the hypothesis that disorder involves not only the methylammonium groups but also the CdCl6 octahedra perovskite-type layers. A simple model of planar disorder qualitatively accounts for the observed diffuse patterns. The data indicate that each layer can be considered as a mosaic of ordered 'clusters' involving about a hundred octahedra, and that disorder occurs between layers along the direction c perpendicular to the layer planes. On going into the monoclinic low-temperature (MLT) phase, the diffuse scattering disappears as expected for an ordered phase.

Lattice dynamics and structural phase transitions in perovskite-type layer compounds. II. The analysis of the temperature-dependent Raman spectra of (CH3NH3)2ClCl4

For pt.I see ibid., vol.16, p.1353 (1983). The low-frequency (0-500 cm-1) Raman spectra of (CH3NH3)2CdCl4 single crystal and of its deuterated homologues (CD3NH3)2CdCl4 and (CH3ND3)2CdCl4 have been recorded at various temperatures covering the domains of existence of the disordered orthorhombic room-temperature (ORT) and tetragonal low-temperature (TLT) phases. An assignment of the observed Raman-active lattice vibrations is proposed; it is based upon the observed polarisation selections, the comparison of the spectra of the various isotopic derivatives and of the spectra previously analysed for the monoclinic low-temperature phase. It is concluded that in the ORT and TLT phase disorder involves not only the orientation of the methyl ammonium groups but also the CdCl6 octahedra sublattice. The peculiar selection rules observed for a low-lying overdamped mode, assigned to oscillations of the rigid CdCl6 octahedra, are interpreted using the hypothesis of existence of short-range correlations. A new qualitative approach for the transition mechanism in this system is tentatively given.

Crystal structure determination of complexes of monomethylammonium chloride and mercury(II) chloride: CH3NH3HgCl3, (CH3NH3)2HgCl4, and CH3NH3Hg2Cl5

AbstractThe crystal structures have been determined of CH3NH3HgCl3, (CH3NH3)2HgCl4, and CH3NH3Hg2Cl5. In (CH3NH3)2HgCl4 the HgII atom is tetrahedrally coordinated by four Cl atoms with HgCl bond lengths of 2.464 to 2.478 Å. In the other two compounds the HgII atom is involved in two short covalent HgCl bonds, forming a pseudo HgCl2 molecule and two much longer bridging HgCl bonds. The methylammonium groups are connected by hydrogen bonds to the chlorine atoms. The nature of the hydrogen bonding scheme probably causes disorder of the methylammonium groups.

The heat capacity of methylammonium chromium alum

A lambda anomaly at 165 K has been observed in the heat capacity versus temperature curve for methylammonium chromium alum. The associated molar entropy change is approximately RIn3 and corresponds to cessation of rotation of the methylammonium groups.