Abstract:The introduction of a mobile and polarised organic moiety as a cation in three-dimensional lead-iodide perovskites brings fascinating optoelectronic properties to these materials. The extent and the timescales of the orientational mobility of the organic cation and the molecular mechanism behind its motion remain unclear, with different experimental and computational approaches providing very different qualitative and quantitative description of the molecular dynamics. Here we use ultrafast two-dimensional vibrational spectroscopy of methylammonium (MA) lead iodide, to directly resolve the rotation of the organic cations within the MAPbI3 lattice. Our results reveal two characteristic time constants of motion. Using ab-initio molecular dynamics simulations, we identify these as a fast (~300 fs) 'wobbling-ina-cone' motion around the crystal axis, and a relatively slow (~3 ps) jump-like reorientation of the molecular dipole with respect to the iodide lattice. The observed dynamics are essential for understanding the electronic properties of perovskite materials. TOC figure:

Three-dimensional lead-halide perovskites have attracted a lot of attention due to their ability to combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder resulting in sharp band edges and narrow distributions of the electronic energies. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic methylammonium (MA) cation orientation in a range of pure and mixed trihalide perovskite materials. For pure MAPbX (X = I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. This acceleration is surprising given the expected strengthening of the hydrogen bonds between the MA and the smaller halide anions, but can be explained by the increase in the polarizability with the size of halide. Much slower dynamics, up to partial immobilization of the organic cation, are observed in the mixed MAPb(ClBr) and MAPb(BrI) alloys, which we associate with symmetry breaking within the perovskite unit cell. The observed dynamics are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronic systems.

The dynamics of organic cations in metal halide hybrid perovskites (MHPs) have been investigated using numerous experimental and computational techniques because of their suspected effects on the properties of MHPs. In this Perspective, we summarize and reconcile key findings and present new data to synthesize a unified understanding of the dynamics of the cations. We conclude that theory and experiment collectively paint a relatively complete picture of rotational dynamics within MHPs. This picture is then used to discuss the consequences of structural dynamics for electron−phonon interactions and their effect on material properties by providing a brief account of key studies that correlate cation dynamics with the dynamics of the inorganic sublattice and overall device properties.M etal halide perovskites (MHPs) are enjoying considerable academic and industrial interest due to their high photovoltaic power conversion efficiencies (22% as of July 2017), 1 ease and low cost of production, and broad material tuneability. 2 Besides applications in photovoltaics, MHPs may find use as solar thermoelectric materials, 3 LEDs, lasers, 4 and nonvolatile memory. 5 The macroscopic properties of MHPs emerge from diverse microscopic phenomena, including crystal structure, 6 defects, cation disorder, 7 ion migration, 8 and spin−orbit coupling. 9 Linking these phenomena to macroscopic performance is challenging because of the complex interplay between them. In this Perspective, we will address cation dynamics, their interactions with lattice vibrations (phonons), and their hypothesized effects on device performance.MHPs share a common ABX 3 (perovskite) structure as shown in Figure 1. This consists of (A) an organic molecule (e.g., methylammonium (MA) or formamidinium (FA)) or a large inorganic atom (e.g., caesium), (B) a metal dication (e.g., Pb (II) or Sn (II) ) and, (X) halide anions (Cl, Br, I, or some combination thereof). The M and X species form a cornersharing octahedral framework. The charge-balancing A-site cation occupies the central cavity generated by this framework and has a strong effect on the MHP structure, quantified by the Goldschmidt tolerance factor. 10 A-site ions just above or below the optimum size may induce tilting of the octahedra within the BX 3 sublattice away from a cubic perovskite. Mixing of ions will lead to inhomogeneity in the local structure, which may result in coupling of cation motion and equilibrium distribution to more complex local lattice dynamics.

We introduce a novel method to perform nonlinear vibrational spectroscopy on nanoscale volumes. Our technique uses the intense near field of infrared nanoantennas to amplify the nonlinear vibrational signals of molecules located in the vicinity of the antenna surface. We demonstrate the capabilities of the method by performing infrared pump-probe spectroscopy and two-dimensional infrared spectroscopy on 5 nm layers of polymethylmetacrylate. In these experiments we observe enhancement factors of the nonlinear signals of more than 4 orders of magnitude. We discuss the mechanism underlying the amplification process as well as strategies for further increasing the sensitivity of the technique.

The design of nano-antennas is so far mainly inspired by radio-frequency technology. However, material properties and experimental settings need to be reconsidered at optical frequencies, which entails the need for alternative optimal antenna designs. Here a checkerboard-type, initially random array of gold cubes is subjected to evolutionary optimization. To illustrate the power of the approach we demonstrate that by optimizing the near-field intensity enhancement the evolutionary algorithm finds a new antenna geometry, essentially a split-ring/two-wire antenna hybrid which surpasses by far the performance of a conventional gap antenna by shifting the n=1 split-ring resonance into the optical regime.PACS numbers: 84.40. Ba, 73.20.Mf, 78.67.Bf Light-matter interaction, i.e. absorption and emission of light as well as the control of its spectral and directional properties, can be optimized by means of antennalike plasmonic nano structures [1, 2]. This is of immediate importance in diverse fields of research ranging from solar energy conversion [3], photocatalytic [4] and sensing applications [5] to single-particle manipulation [6,7] and spectroscopy [8] as well as quantum optics and communication [9][10][11][12].RF-antenna designs are usually optimized for thin, infinitely good conducting wires that only support surface currents and are typically fed by transmission lines connected by infinitely narrow gaps [13]. For antennas at optical frequencies the general operation conditions deviate substantially from such ideal behaviour: (i) Antenna wire diameters are comparable to the electromagnetic penetration depth into the wire material leading to volume currents [14]. In the case of noble metals, such wires therefore exhibit plasmon resonances in the visible spectral range resulting in a reduced effective wavelength of wire waves [15]. (ii) Feeding (excitation) of optical antennas is often achieved by focused laser beams or quantum emitters. (iii) high-frequency-related effects such as the 'kinetic inductance' become significant [16]. It can therefore not be taken for granted that RF-inspired antenna designs, like dipole [17], bow tie [18,19] and Yagi-Uda antennas [20,21], represent 'optimal' geometries also at optical frequencies, although they provide a reasonable performance.Evolutionary algorithms (EAs) find optimized solutions to highly complex non-analytic problems by creating subsequent generations of individuals coded by their respective genomes that compete for the right to pass on their properties, according to a fitness parameter [22]. These optimized solutions can then be analyzed to foster the understanding of underlying physical principles. Evolutionary optimization has successfully been applied in various fields of research, including pulse shape optimization in coherent control of chemical reactions [23] and field localization in plasmonic structures [24,25]. Furthermore, evolutionary optimization has been used to aid the development of radio-wave antennas [26,27]. First attempts to employ such met...